Patent Application
20230358677
The present invention generally relates to systems and methods of measuring local protonic species; and more particularly to systems and methods of using photoacids and photobases to quantify the local activity of protonic species.
Photoacids and photobases are molecules that, through absorption of light via an electronic transition alter their acidity, and perform reversible excited-state proton transfer (ESPT) to or from another species, thus causing a change in acidity. Acidity can be measured using an acid dissociation constant (also known as acidity constant, or acid-ionization constant, Ka), and/or pKa (the base-10 logarithm of Ka, pKa=−log10 Ka).
Measurements of localized pH and activity of protonic species are important in various areas of electrochemistry, from corrosion to bio-electrochemistry and electrocatalysis, and various biological processes. Different techniques are available to perform these measurements and offer possibilities in terms of spatial and temporal resolution, sensitivity, and precision. Measurements of localized pH and activity of protonic species can be performed using scanning probe techniques (such as, scanning electrochemical microscopy (SECM), scanning ion conductance microscopy (SICM), scanning ion-selective electrode technique (SIET)), laser (confocal) fluorescence microscopy, rotating ring-disc electrode (RRDE) voltammetry, and infrared spectroscopy, among others.
Many embodiments are directed to systems and methods for measurement and sensing of local pH and activity of protonic species using reversible excited-state photoacids and photobases.
One embodiment includes a measurement system comprising: a confocal microscope; and an electrochemical cell comprising an electrode submerged in an electrolyte comprising a photochemical compound; wherein a change in a species concentration in the electrolyte at the electrode changes a fluorescent signal of the photochemical compound such that the confocal microscope detects the fluorescent signal change to measure the concentration of the species; wherein the photochemical compound comprises a photoacid or a photobase; wherein the species is selected from the group consisting of: OH−, H+, a proton acceptor, a proton donor, a dissolved inorganic carbon, formate, acetate, glycine, and phosphate; and wherein the measured concentration signal has a time resolution of less than one second, and a spatial resolution of less than one micron.
In a further embodiment, the photoacid or the photobase is a ratiometric fluorescent dye and the fluorescent signal is independent of the photoacid or the photobase concentration.
In an additional embodiment, a base-10 logarithm of an acid dissociation constant (pKa) of the photoacid in ground-state is greater than 14.
In another embodiment, the photoacid is selected from the group consisting of: 8-aminopyrene-1,3,6-trisulfonic acid trisodium salt (APTS), 8-anilinonaphthalene-1-sulfonic acid sodium salt, 5-((2-aminoethyl)amino)naphthalene-1-sulfonic acid sodium salt, 5-aminonaphthalene-1-sulfonic acid sodium salt (NS—NH2), 6-bromo-5-aminonaphthalene-1-sulfonic acid sodium salt, and any combinations thereof.
In a further embodiment again, the photoacid comprises APTS and DHPDS, and the measured concentration signal is pOH ranging from 0 to 8.
In a yet further embodiment, the photoacid comprises APTS, and the measured concentration signal is pH ranging from 0 to 4.
In another embodiment again, the photoacid comprises 5-((2-aminoethyl)amino)naphthalene-1-sulfonic acid sodium salt, and the measured concentration signal is pOH ranging from 0 to 6.
In yet another embodiment, the photoacid comprises 9-hydroxyphenanthrene-3,10-disufonic acid disodium salt, and the species is a dissolved inorganic carbon, formate, acetate, or a proton acceptor.
In yet another embodiment again, the photoacid comprises 1-hydroxypyrene, and the species is a dissolved inorganic carbon.
In another additional embodiment, the photoacid comprises 6-bromo-5-aminonaphthalene-1-sulfonic acid sodium salt, and the species is a proton acceptor; wherein the detection occurs from a triplet electronic excited state.
In another embodiment again, the measured concentration signal has a spatial resolution from 250 nm to one micron.
In another yet embodiment, the confocal microscope is selected from the group consisting of: a confocal laser scanning microscope, a laser confocal scanning microscope, a fluorescence confocal laser scanning microscope.
Another embodiment further comprises a gas chamber in contact with the electrode and the electrode is a gas diffusion electrode.
In a further yet embodiment, gaseous carbon dioxide is fed through the gas chamber and reacts with OH− to form bicarbonate and carbonate anions, resulting in a decrease in OH− concentration.
In a further embodiment again, an applied current at the electrode induces carbon dioxide reduction reactions that generate OH−; wherein an increase in applied current density results in a decrease in pOH.
In another further embodiment, the current density ranges from 0 mA/cm2 to 200 mA/cm2 in magnitude.
In yet another further embodiment, the gas diffusion electrode comprises a macro-porous gas diffusion layer, a hydrophobic microporous layer, and a catalyst.
In an additional further embodiment, the gas diffusion electrode comprises a surface with a plurality of trenches.
In a yet further embodiment, the plurality of trenches has an irregular pattern with a width ranging from 5 microns to 30 microns.
In a further yet embodiment, a pOH inside the plurality of trenches is lower than the gas diffusion electrode surface.
Another embodiment includes a method for measuring pOH comprising:
In an additional embodiment again, the photoacid or the photobase is a ratiometric fluorescent dye and the fluorescent signal is independent of the photoacid or the photobase concentration.
In another yet embodiment, a base-10 logarithm of an acid dissociation constant (pKa) of the photoacid in ground-state is greater than 14.
In yet another further embodiment, the photoacid is selected from the group consisting of: 8-aminopyrene-1,3,6-trisulfonic acid trisodium salt (APTS), 8-anilinonaphthalene-1-sulfonic acid sodium salt, 5-((2-aminoethyl)amino)naphthalene-1-sulfonic acid sodium salt, 5-aminonaphthalene-1-sulfonic acid sodium salt (NS—NH2), 6-bromo-5-aminonaphthalene-1-sulfonic acid sodium salt, and any combinations thereof.
In a further embodiment again, the photoacid comprises APTS and DHPDS, and the measured concentration signal is pOH ranging from 0 to 8.
In another embodiment again, the photoacid comprises APTS, and the measured concentration signal is pH ranging from 0 to 4.
In an additional further embodiment, the photoacid comprises 5-((2-aminoethyl)amino)naphthalene-1-sulfonic acid sodium salt, and the measured concentration signal is pOH ranging from 0 to 6.
In a further yet embodiment, the photoacid comprises 9-hydroxyphenanthrene-3,10-disufonic acid disodium salt, and the species is a dissolved inorganic carbon, formate, acetate, or a proton acceptor.
In yet another embodiment, the photoacid comprises 1-hydroxypyrene, and the species is a dissolved inorganic carbon.
In a further yet embodiment, the photoacid comprises 6-bromo-5-aminonaphthalene-1-sulfonic acid sodium salt, and the species is a proton acceptor; wherein the measurement occurs from a triplet electronic excited state.
In an additional embodiment again, the measured pOH has a spatial resolution from 250 nm to one micron.
In yet another further embodiment again, the confocal microscope is selected from the group consisting of: a confocal laser scanning microscope, a laser confocal scanning microscope, a fluorescence confocal laser scanning microscope.
In a further embodiment, the electrochemical cell further comprises a gas chamber in contact with the electrode and the electrode is a gas diffusion electrode.
In another yet embodiment again, gaseous carbon dioxide is fed through the gas chamber and reacts with OH− to form bicarbonate and carbonate anions, resulting in a decrease in OH− concentration.
In another additional embodiment, an applied current at the electrode induces carbon dioxide reduction reactions that generates OH−; wherein an increase in applied current density results in a decrease in pOH.
In a yet further embodiment, the current density ranges from 0 mA/cm2 to 200 mA/cm2 in magnitude.
In yet another embodiment, the gas diffusion electrode comprises a macro-porous gas diffusion layer, a hydrophobic microporous layer, and a catalyst.
In a further additional embodiment, the gas diffusion electrode comprises a surface with a plurality of trenches.
In another further embodiment, the plurality of trenches has an irregular pattern with a width ranging from 5 microns to 30 microns.
In a further yet embodiment again, the pOH inside the plurality of trenches is lower than the gas diffusion electrode surface.
Additional embodiments and features are set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the specification or may be learned by the practice of the disclosure. A further understanding of the nature and advantages of the present disclosure may be realized by reference to the remaining portions of the specification and the drawings, which forms a part of this disclosure.
The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.
The description will be more fully understood with reference to the following figures, which are presented as exemplary embodiments of the invention and should not be construed as a complete recitation of the scope of the invention, wherein:
Turning now to the drawings and data, systems and methods for local pH measurement and sensing using reversible excited-state photoacids and photobases are described. In many embodiments, various reversible excited-state weak photoacids can be used as dynamic fluorescent sensors of the local OH− activity. In several embodiments, various reversible excited-state weak photobases can be used as dynamic fluorescent sensors of the local protonic activity. The photoacids and photobases can be used in combination with confocal fluorescent microscopy for measuring local OH− and H+ activity, activity of protonic species including (but not limited to) phosphate, glycine, dissolved inorganic carbon (DIC) species such as bicarbonate and carbonate, and/or species such as acetate, formate. The base-10 logarithm of an acid dissociation constant (pKa) of a weak photoacid in ground-state is greater than about 14. The base-10 logarithm of a base dissociation constant (pKb) of a weak photobase in ground-state is greater than about 14. Examples of weak photoacids for local OH− activity measurement include (but are not limited to) 8-aminopyrene-1,3,6-trisulfonic acid trisodium salt (APTS), 8-anilinonaphthalene-1-sulfonic acid sodium salt, 5-((2-aminoethyl)amino)naphthalene-1-sulfonic acid sodium salt, 5-aminonaphthalene-1-sulfonic acid sodium salt (NS—NH2), and 6-bromo-5-aminonaphthalene-1-sulfonic acid sodium salt. Various embodiments include sodium salt of the photoacid molecules. As can be readily appreciated, any types of salt (such as lithium, potassium, N-tetrabutylammonium) of the photoacids can be applied in accordance with various embodiments with the invention.
Photoacids and photobases are molecules that, through absorption of light via an electronic transition, perform reversible excited-state proton transfer (ESPT) to or from another species. Reversibility means that photoacids and/or photobases can be used for sustained light-to-ionic power conversion. In aqueous environments, ESPT can occur among others to and from OH− and H+. Therefore, the presence of OH− can quench the photoluminescence intensity of a weak photoacid while the presence of H+ can quench the photoluminescence of a weak photobase.
In many embodiments, various reversible excited-state weak photoacids or moderate photobases can be used as probes for measuring local OH− activity. In several embodiments, various moderate photoacids or weak photobases can be used as probes for measuring local H+ activity. Under equilibrium conditions, the OH− activity can be connected to the pH and pOH values with pOH=−log(aOH−) and pH=pKw−pOH≈14−pOH. Knowing the OH− activity also allows to determine the local pOH and pH values under equilibrium conditions. However, under non-equilibrium steady-state conditions, pH and pOH are unrelated.
Several embodiments use various reversible excited-state weak photoacids as fluorescent sensors for measuring the local OH− activity using confocal fluorescent microscopy. A confocal microscope can measure the local fluorescent signal of a probe time-resolved and spatially-resolved in three dimensions. As the fluorescent signal of photoacids is sensitive to the OH− activity, it can create three-dimensional, time-resolved maps of the local OH− activity with resolution on the micrometer scale. Photoacids such as APTS have additional advantage that they are ratiometric. Ratiometric fluorescent dye' refers to a probe with a spectrum that exhibits at least two distinct peaks with intensities changing in opposite directions upon varying the parameter to which the probe is sensitive. By evaluating the ratio between the peak intensities, the observed signal becomes independent of the local dye concentration. For APTS, its fluorescent spectrum has two distinct peaks whose intensity changes in different directions upon change of the OH− activity. By calculating the ratio of the signals integrated over the two distinct peaks, the signal becomes independent of the APTS concentration.
Systems and methods for measuring the local OH− activity have spatial resolution on the micron-scale and can create OH− activity maps on all planes in the three-dimensional space. It also allows mapping of the OH− activity around and in complex geometrical features, such as inside trenches on a surface.
The accessible pOH range of APTS is approximately from about 0 to about 3, which corresponds to pH of about 11 to about 14 under equilibrium conditions. Furthermore, APTS is a dual photoacid. In addition to OH− activity, APTS can also detect H+ activity under acidic conditions with a pH between about −4 and about 0 with a dynamic excited-state mechanism, as illustrated for disodium 9-hydroxyphenanthrene-3,10-disufonate in
In many embodiments, weak photoacids can be used for sensing and mapping local pOH (or pH) in the pOH range between about 0 and 8 and/or at current densities from about 0 mA/cm2 to about −200 mA/cm2. In several embodiments, weak photoacids can be used to map the local pOH value around gas diffusion electrodes (GDEs) for CO2 reduction (CO2R). The micrometer-scale morphology of a carbon dioxide reduction GDE can affect the mass transport properties and the local CO2R performance. GDEs are porous electrodes coated with a catalyst that can create a three-phase interphase between solid (catalyst), liquid (electrolyte) and gas (CO2) and allow to reduce CO2 with much higher rates compared to a conventional setup. CO2 undergoes reactions with the electrolyte and causes an increase in hydroxide anions (OH−) activity. The local activity of OH−, represented by the pOH value, around a GDE in contact with an aqueous electrolyte is an important parameter that governs the catalytic activity and CO2R selectivity. The combination of weak photoacids and confocal microscopy can map the inhomogeneous CO2 flow through the GDE via local pOH imaging with time- and three-dimensional spatial micrometer-scale resolution. Furthermore, upon applying an electrical current to the GDE, three-dimensional pOH maps around an operating GDE for CO2 reduction can be created, which enables the observance of a decrease in pOH for increasing current density. The local pOH value inside trenches on the GDE surface can also be measured and mapped, which shows that the pOH inside trenches is lower than on the GDE surface.
In certain embodiments, confocal microscopy including (but not limited to) confocal laser scanning microscopy (CLSM), laser confocal scanning microscopy (LCSM), fluorescence confocal laser scanning microscopy, can be used to create maps of the local pOH around a copper GDE by combining various ratiometric fluorescent dyes including (but not limited to) DHPDS and APTS. The dyes may have different sensing mechanisms. The local pOH decreases when current is applied due to the creation of OH− as a byproduct of CO2R. The pOH is lower inside microtrenches of the GDE compared to the electrode surface and decreases further as trenches become narrower due to enhanced trapping of OH−. Multiphysics simulations correlate well with measurements and reveal that the decreased pOH inside microcavities in the surface of a CO2R GDE leads to locally enhanced selectivity towards multicarbon (C2+) products. Narrow microstructures on the length scale of about 5 μm in a GDE surface can serve as local CO2R hotspots.
Photoacids and photobases are molecules that, through absorption of light via an electronic transition, perform reversible excited-state proton transfer (ESPT) to or from another species, and undergo a reversible change in acidity (pKa). Reversibility means that photoacids/photobases can be used for sustained light-to-ionic power conversion, which differs from stoichiometric photoacid or photobase generators that are used in photolithography and undergo irreversible proton-transfer reactions. Reversible ESPT from photoacids has been exploited for high spatiotemporal sensing of local pH during electrochemical reactions, to study proton-coupled reaction mechanisms, to trigger processes in biological organisms relevant to energy transduction, and in the design of artificial light-driven proton pumps. (See, e.g., Leenheer, A. J.; et al., J. Electrochem. Soc. 2012, 159, H752-H757; Fuladpanjeh-Hojaghan, B.; et al., Angew. Chemie Int. Ed. 2019, 58 (47), 16815-16819; Pande, N.; et al., J. Phys. Chem. Lett 2020, 11 (17), 7042-7048; Welch, A. J.; et al., J. Phys. Chem. C 2021, 125 (38), 20896-20904; Dempsey, J. L.; et al., J. Am. Chem. Soc. 2010, 132 (47), 16774-16776; Haghighat, S.; et al., J. Phys. Chem. B 2016, 120 (5), 1002-1007; Shafaat, O. S.; et al., Chem Bio Chem 2016, 17 (14), 1323-1327; Xie, X.; et al., Nat. Chem. 2014, 6 (3), 202-207; White, W.; et al., J. Am. Chem. Soc. 2017, 139 (34), 11726-11733; White, W.; et al., Joule 2018, 2 (1), 94-109; Wang, L.; et al., Adv. Mater. 2019, 31 (36), 1903029; Schulte, L.; et al., Joule 2021, 5, 1-15; Luo, S.; et al, Energy Environ. Sci. 2021, 14, 4961-4978; the disclosures of which are herein incorporated by reference.) An important consideration for these applications is that ESPT is chemically specific. This means that ESPT in aqueous environments can at least occur to and from H2O(l), from H+(aq), and to O−(aq), with rates dictated by the laws of mass action and mass transfer. These continuity of mass processes enable water to serve as a protonic semiconductor.
ESPT occurs due to a change in the energetics of a protic bond, which more broadly influences the Brønsted-Lowry acidity of the photoacid/photobase. This change in acidity is quantified through measurement of the acid dissociation equilibrium constant, Ka, which changes in the excited state to K*a. Knowledge of the excited-state pKa value, pK*a, is important for predicting ESPT behavior. (See, e.g., Ireland, J. F.; et al., In Advances in Physical Organic Chemistry; Academic Press, 1976; Vol. 12, pp 131-221; Shizuka, H.; Acc. Chem. Res. 1985, 18 (5), 141-147; Arnaut, L. G.; et al., J. Photochem. Photobiol. A Chem. 1993, 75 (1), 1-20; Tolbert, L. M.; et al., Acc. Chem. Res. 2002, 35 (1), 19-27; Agmon, N.; J. Phys. Chem. A 2005, 109 (1), 13-35; the disclosures of which are herein incorporated by reference.) Overall excited-state processes that occur for a hypothetical photoacid, PAH, are as follows,
where B stands for the proton acceptor. When B=H2O(l), Kq=K*a, and when B=OH−(aq), Kq=(K*b)−1, where K*b stands for the excited-state base dissociation equilibrium constant and is related to K*a as
with Kw being the autoprotolysis equilibrium constant of water and is equal to 10−14 at room temperature.
For the purpose of the disclosure, the values are reported as pX. pX equals to “−log X” or “−log aX”, where aX stands for the activity of species X. In some instances, the general equilibrium constant K, can be used for photoacids and photobases, without specifying Ka for photoacids, and Kb for photobases. For example, pK* can represent pK*a value for photoacids and/or pK*b value for photobases.
As the ESPT process is chemically specific, choosing a photoacid or a photobase for a particular application requires accurate quantification of its pK*. One way to approximate pK* is by adding the approximate value of ΔpK* to its ground-state pK value. Values for ΔpK* can be approximated based on the Förster cycle square scheme, via an analysis derived by Förster using the first law of thermodynamics.
However, for weak photoacids and/or photobases, the ground state pK value is greater than about 14, and often greater than about 20. Because of this fundamental difference, the Förster cycle analysis cannot be used to approximate pK* values for weak photoacids and/or photobases. It is difficult to quantify acidity of weak photoacids and photobases with less acidic (basic) excited states (pK* greater than about 7) accurately in water.
Another method for determining pK* involves acid-base titrations with steady-state photoluminescence detection, and data analysis using the Henderson-Hasselbalch equation with adjustment for excited-state lifetimes. (See, e.g., Ireland, J. F., et al., In Advances in Physical Organic Chemistry; Academic Press, 1976; Vol. 12, pp 131-221; the disclosure of which is incorporated herein by reference.) This analysis assumes that the excited state reaches quasi-equilibrium, meaning that protonated and deprotonated excited-state species each persist long enough to speciate according to pK*. Strong photoacids/photo bases, whose pK* less than about 0, often reach excited-state chemical quasi-equilibrium, because of their rapid near-molecular-vibration-limited rate of ESPT with water (forward direction of Reaction 1 with B=H2O(l)) and relatively longer excited-state lifetimes on the nanosecond timescale (forward direction of Reaction 2). However, the excited-state chemical quasi-equilibrium state may not occur for weak photoacids (photobases) due to slow rates of excited-state proton transfer. Many chromophoric protic molecules are weak photoacids/photobases, meaning that pK* greater than about 7, such that ESPT with water, whose pKb=0 and whose conjugate acid (H+(aq)) has pKa=0, is moderately thermodynamically unfavorable and as such there is a significant kinetic barrier to proton transfer that greatly slows ESPT. When the excited-state lifetime of weak photoacids/photobases is too short and/or pK* is too large, excited-state chemical quasi-equilibrium cannot be reached, thus reported pK* values are inaccurate.
Given the inadequacy of thermodynamic analyses, such as the ones based on the Förster cycle square scheme and the Henderson-Hasselbalch equation, kinetic analyses should be considered in order to accurately quantify pK*a values of weak photoacids. While a combination of short excited-state lifetime and weak photoacidity/photobasicity prohibits ESPT from occurring between photoacids/photobases and water, it also provides an opportunity to study ESPT with other proton-accepting/donating quenchers, even in liquid water. On the contrary, such experiments are more difficult to perform with strong photoacids/photobases due to competing ESPT with water. ESPT quenching of organic or inorganic photoacids with proton acceptors or photobases with proton donors has been previously reported. (See, e.g., Weller, A.; et al., Phys. Chemie 1954, 58 (10), 849-853; Weller, A.; Phys. Chemie 1957, 61 (8), 956-961; Weller, A. Prog. React. Kinet. 1961, 1, 187-214; Förster, T., Naturwissenschaften 1949, 36 (6), 186-187; Förster, T.; et al., Chem. Phys. Lett. 1975, 34 (1), 1-6; Gafni, A.; et al., Chem. Phys. Lett. 1978, 58 (3), 346-350; Laws, W. R.; et al., J. Phys. Chem. 1979, 83 (7), 795-802; Wan, P.; et al., J. Org. Chem. 1983, 48 (6), 869-876; Chattopadhyay, N.; et al., J. Photochem. 1987, 38, 301-309; Chattopadhyay, N.; J. Photochem. Photobiol. A Chem. 1989, 48 (1), 61-68; Chattopadhyay, N.; J. Photochem. Photobiol. A Chem. 1995, 88 (1), 1-4; Turro, C.; et al., J. Am. Chem. Soc. 1995, 117 (35), 9026-9032; Hicks, C.; Coord. Chem. Rev. 2001, 211 (1), 207-222; Stewart, D. J.; Proc. Natl. Acad. Sci. U.S.A. 2013, 110 (3), 876-880; Pines, E.; et al., Chem. Phys. Lett. 1997, 281 (4-6), 413-420; Mohammed, O. F.; et al., Chem. Phys. 2007, 341 (1-3), 240-257; Adamczyk, K.; et al., Science 2009, 326 (5960), 1690-1694; Adamczyk, K.; et al., Isr. J. Chem. 2009, 49, 217-225; Munitz, N.; et al., Isr. J. Chem. 2009, 49, 261-272; the disclosures of which are herein incorporated by reference.) Several of the prior studies focused specifically on weak photoacids or photobases, a few used the dynamic Stern-Volmer quenching analysis, and a few analyzed a series of proton-accepting quenchers each with a known pK value to obtain a suite of data that describes the dependence of the rate constant for ESPT on the proton-accepting ability, pK, of the quencher. When the driving force for such proton-transfer reactions is large and unfavorable, meaning ΔGo>>0, a linear free energy relationship exists, as observed by Brønsted. (See, e.g., Brønsted, J. N.; et al., Phys. Chem. 1924, 108, 185; the disclosure of which is herein incorporated by reference.) When the driving force is small, meaning that the pK of the proton-accepting/donating quenchers is similar to the pK of the molecule of interest, non-linear free energy relationships exist due to a non-zero activation free energy (ΔG‡), as observed by Eigen. (See, e.g., Eigen, M.; Pure Appl. Chem. 1963, 6 (1), 97-116; Eigen, M.; Angew. Chemie Int. Ed. English 1964, 3 (1), 1-19; Eigen, M.; Faraday Soc. 1965, 39, 7-15; the disclosures of which are herein incorporated by reference). This is because, like electron-transfer reactions, proton-transfer reactions with small driving forces, meaning ΔGo≈0, do not exhibit a linear dependence between ΔG‡and ΔGo. Non-linear free energy relationships based on the semiempirical Rehm-Weller model for excited-state electron-transfer reactions, which include a steady-state approximation, have been applied to analyze data for ground-state and excited-state proton-transfer reactions between photoacids and water. (See, e.g., Solntsev, K. M.; et al., J. Phys. Chem. A 2004, 108 (40), 8212-8222; the disclosure of which is herein incorporated by reference.) Analogous semiempirical models for proton transfer, developed by Marcus and Cohen, Agmon and Levine, Arnaut and Formosinho, and Kiefer and Hynes, have been used to analyze data for ESPT between photoacids/photobases and various proton acceptors/donors. (See, e.g., Marcus, R. A.; J. Phys. Chem. 1968, 72 (3), 891-899; Cohen, A. O.; et al., J. Phys. Chem. 1968, 72 (12), 4249-4256; Agmon, N.; et al., Chem. Phys. Lett. 1977, 52 (2), 197-201; Agmon, N.; Int. J. Chem. Kinet. 1981, 13 (4), 333-365; Arnaut, L. G.; et al., J. Phys. Chem. 1988, 92 (3), 685-691; Kiefer, P. M.; et al., J. Phys. Chem. A 2002, 106 (9), 1834-1849; Kiefer, P. M.; et al., J. Phys. Chem. A 2002, 106 (9), 1850-1861; the disclosures of which are herein incorporated by reference). Each of these studies used ultrafast spectroscopy to quantify rate constants for ESPT, and in no cases were weak photoacids studied.
Many embodiments implement steady-state photoluminescence spectroscopy coupled to models for interpretation of excited-state electron-transfer dynamics to accurately quantify pK*a of a weak photoacid based on analysis of its excited-state proton-transfer dynamics. Several embodiments use liquid water and aqueous hydroxide as proton-accepting quenchers of excited-state photoacids. Stern-Volmer quenching analysis is appropriate to extract rate constants for excited-state proton transfer from a weak photoacid to a series of proton-accepting quenchers. Analysis of the data by Marcus-Cohen bond-energy-bond-order theory yields an accurate value for pK*a of a weak photoacid. The method can be broadly accessible because it only uses steady-state photoluminescence spectroscopy.
Accurate knowledge of pK* values for weak photoacids/photobases can be useful for applications including (but not limited to) direct sensing of proton-accepting or proton-donating species in water via dynamic ESPT quenching. In certain embodiments, quantifying pK* of a weak photoacid/photobase can provide benefit to ocean deacidification via carbon capture, where ESPT can impart direct protonation/deprotonation of bicarbonate. Moreover, protonation of anionic bicarbonate can also increase activities and/or vary selectivity for electrochemical CO2 reduction by overcoming Coulombic repulsion with a negatively polarized electrode. Additionally, accurate knowledge of pK* values enables specific control over direct protonation/deprotonation of amino acid residues in proteins that influence processes, such as opening and/or closing of ion-selective channels, which will allow the processes to have larger per photon quantum yields in comparison to typical processes where light is used to alter bulk pH under steady-state conditions.
Electronic absorption spectra obtained via acid-base titration of an aqueous solution of the sodium salt of 5-amino-1-naphthalenesulfonic acid (NS—NH2) suggest that the protonation state of ground-state NS—NH2 is unchanged over the pH/Ho range of 15.0 (strongly alkaline) to 6.8 (weakly acidic). This implies that NS—NH2 exhibits pKa greater than about 17, which is consistent with pKa values of analogous aromatic amines, and not sulfonate groups. The sulfonate groups do not to participate in proton-transfer reactions over the intended pH range, but they may enhance desired water solubility and attenuate undesired aggregation. Steady-state photoluminescence spectra obtained over the same pH range vary in intensity, which suggests that a pH-dependent reaction(s) occurred in the excited state, most likely ESPT with the aromatic amine group on excited-state NS—NH2*. An erroneous method to determine a value of the excited-state pKa (pK*a pseudo) of about 12.34±0.02, as the mean±standard deviation from two independent measurements, is performed by simultaneous global analysis of both single-wavelength steady-state photoluminescence spectroscopy datasets to the Henderson-Hasselbalch equation,
where θ is the normalized photoluminescence intensity,
Because the Henderson-Hasselbalch equation is derived for proton-transfer reactions at thermal and chemical equilibrium, it is agnostic to whether the ESPT acceptor species are H2O(l) or OH−(aq), but it does require that the system has reached excited-state quasi-equilibrium. Table 1 below lists ground-state and excited-state acidities of aromatic-amine-based photoacids.
Observed rate constants can be determined for excited-state proton transfer from weak photoacids to a series of proton-accepting quenchers. For most aromatic amines, whose pKa values are quite basic and excited-state lifetimes are about 1-10 ns, acid-base titration with photoluminescence detection and data analysis using the Henderson-Hasselbalch equation result in pK*a pseudo about 12, irrespective of ground-state pKa value, which ranges from about 14 to about 21. The disparity in the size of the ranges of pKa and pK*a pseudo values suggest that analyses used to determine at least one of them may be inaccurate. The simplicity and ubiquity of Henderson-Hasselbalch analysis for ground-state proton-transfer reactions suggest that ground-state pKa values are accurate but that application of the Henderson-Hasselbalch equation to ESPT processes with NS—NH2* may be erroneous due to insufficient time to reach excited-state quasi-equilibrium. This implies that steady-state photoluminescence spectroscopy data are influenced by the rate of ESPT (forward direction of Reaction 1) in relation to the rate of excited-state deactivation (forward direction of Reaction 2) and that ESPT can be thought of as a quenching process of NS—NH2* by OH−(aq), suggesting that the physically different Stern-Volmer analysis may be more accurate than Henderson-Hasselbalch analysis.
The Henderson-Hasselbalch equation is mathematically identical to the Stern-Volmer equation when the quencher is assumed to be OH−(aq) for photoacids (and H+(aq) for photobases),
where aOH
and KSV,OH
To decipher contributions from static and dynamic quenching processes between NS—NH2* and OH−, time-resolved photoluminescence spectroscopy measurements with nanosecond time resolution are conducted, and data are best fit to a single decaying exponential function. Changes in observed excited-state lifetime are clear, thus supporting that dynamic quenching is operative, while changes in initial photoluminescence intensity are less clear and could not be quantified accurately due to signal convolution by the instrument response function, thus providing little insight into whether static quenching was operative. Excited-state lifetimes of NS—NH2* in strongly alkaline solutions (pH greater than about 12) could not be measured, because the forward direction of Reaction 1 is faster than instrument time resolution, due to mass action and large aOH
To further support the hypothesis that pK*a pseudo values derived from steady-state photoluminescence spectroscopy data are kinetically gated by ESPT with OH−(aq) and photoacid excited-state lifetime, pK*a pseudo values are determined for several aromatic amine photoacids with different excited-state lifetimes. These data are then best fit to the following equation, which should be appropriate assuming the reasonable assumption that this series of molecules exhibits similar encounter-controlled diffusion-limited bimolecular quenching rate constants with OH−(aq), that the rate of the backward direction of Reaction 1 is smaller than rate of return of the deprotonated excited-state photoacid to its electronic ground state, and that a constant pKw value of 14 is sufficient over a small range of ionic strengths,
pK*
a pseudo=−log τo,PAH*+(−log kq,ss,OH
A semilogarithmic plot of pK*a pseudo versus excited-state lifetime yields an average k+q,ss,OH
Given the support for the dynamic nature of the excited-state quenching process, it is important to realize that use of the Stern-Volmer equation to quantify bimolecular quenching rate constants is only accurate when the quenching process is essentially unidirectional, and therefore irreversible in relation to the excited-state lifetime of the deprotonated excited-state photoacid, i.e. NS—NH−*. Conversely, reversible bidirectional ESPT occurs when the rate of the backward direction of Reaction 1 is larger than rate of return of NS—NH−* to its electronic ground state, which necessitates that the Stern-Volmer equation be modified to include rate constants for ESPT in the forward and backward directions (Reaction 1) and excited-state lifetimes for both protonated and deprotonated excited-state species. In the case of NS—NH2 at pH values as large as 15, no definitive spectroscopic signals due to NS—NH−* and/or NS—NH− are observed within the detection limit of the time-resolved photoluminescence and transient absorption spectroscopy systems (time resolution of ˜1 ns). Moreover, time-correlated single photon counting photoluminescence spectroscopy (time resolution of about 200 ps) and ultrafast transient absorption spectroscopy (time resolution of about 1 ps) are performed at pH 7 and pH 14 and observed excited-state lifetimes for NS—NH2* are extracted from these data. The observed excited-state lifetime at pH 7 is consistent with that measured using nanosecond time-resolved photoluminescence spectroscopy data, while the observed excited-state lifetime at pH 14 is faster than the time resolution of the instrument, suggesting that ESPT is essentially a unidirectional and irreversible process. Also, since pKa>20 for NS—NH2, ground-state proton transfer from water to NS—NH− to regenerate NS—NH2 is expected to occur on the sub-picosecond timescale, which is faster than the instrument time resolution. Hence, the observed transient absorption signals are due to NS—NH−.
Assuming that photoluminescence quenching occurs by ESPT from NS—NH2* to OH−(aq), the data support that when aOH
where KSV,OH
Table 2 lists driving-force-dependent rate constants for direct excited-state proton transfer from NS—NH2* to various proton-accepting quenchers.
The series of data in the presence of two proton-accepting quenchers, i.e. OH− and B. To further support that a dynamic Stern-Volmer quenching process is operative, numerical simulations are performed using a chemical kinetics model that included experimentally derived rate constants and fluorimeter light intensity in the presence and absence of proton-accepting quenchers and varied excited-state lifetime of NS—NH−*. Notably for TNS—NH−*<0.3 ns, titration behavior is nearly independent of TNS—NH−*, suggesting ESPT to be essentially unidirectional and irreversible, further validating use of Equation 7 to accurately determine rate constants for ESPT.
Several embodiments provide accurate quantification of excited-state Brønsted-Lowry acidity and reorganization energy of weak photoacids using Marcus-Cohen bond-energy-bond-order (BEBO) theory. Data for this series of proton-accepting quenchers analyze the dependence of driving force on ESPT quenching rate constants and assess the validity of Marcus normal region behavior, as has been performed numerous times for excited-state electron transfer and proton-coupled electron transfer. Like excited-state electron-transfer quenching in polar solvents, ESPT is proposed to proceed through formation of an excited-state encounter complex between reactants prior to proton transfer, but unique to ESPT is dissociation of an excited-state encounter complex between products after proton transfer. Assume that return of the excited-state encounter complexes to their respective electronic ground states does not occur and that the final step is not rate determining. These assumptions lead to the following equation for the observed quenching rate constant, k+q,corr,i, which is dominated by the slowest reaction step in the forward direction after taking into consideration pre-equilibrium of preceding steps,
where
are equilibrium constants for formation of an excited-state encounter complex (EC) between reactants (R) and products (P), respectively, which each kinetically involve bimolecular encounter-controlled processes (k+EC,R and k−EC,P) and thermodynamically are dictated by effects due to electrostatics, sterics, and statistical entropic considerations.
is the equilibrium constant for the unimolecular proton-transfer step, with rate constants of k+PT and k−PT, and are parameters that are needed in order to accurately apply theories of driving-force-dependent kinetic processes.
The overall change in the standard Gibbs free energy for the three-step excited-state proton-transfer reaction sequence can be written as follows,
ΔGo=2.303RT(pK*a,NS—NH
ΔGo=ΔGEC,Ro+ΔGPTo+(−ΔGEC,Po)=2.303RTpKEC,R+ΔGPTo−2.303RTpKEC,P (10)
ΔGo=2.303RT(pK*a,NS—NH
where ΔGEC,Ro, ΔGPToand −ΔGEC,Po are the standard Gibbs free energy differences for the individual reaction steps in the excited-state proton-transfer reaction sequence. Kinetic data (Equation 8) and thermodynamic data (Equation 11) are analyzed using Marcus-Cohen bond-energy-bond-order (BEBO) theory, which is based on transition-state theory and results in the following relationship between ΔGPT‡ and ΔGPTo,
where A is the pre-exponential frequency factor (s−1), ΔGPT‡ is the standard Gibbs free energy of activation for the proton-transfer step, and ΔGo,PT‡ is ΔGPT‡ when ΔGPTo=0. Semilogarithmic plots of the observed effective rate constants kq,corr,i for each proton-accepting quencher, i, versus the quencher pKa using values from Table 2 are best fit by fixing the encounter-controlled rate constants to an approximate diffusion-limited value, k+EC,R=k−EC,P=3×1010 M−1 s−1, A=4.1×1011 s−1, and a=5.4 Å. This Marcus-Cohen BEBO analysis yielded for NS—NH2* a “true” pK*a=11.7±0.1 and ΔGo,PT‡=0.080±0.005 eV.
Marcus-Cohen BEBO theory is based on a localized reaction with strong electronic overlap such that bond order, and thus bond energy, dominate the driving-force dependence of the rate constant. This theory generally results in values of ΔGo‡ that are smaller than those for electron-transfer reactions, which are often assumed to be less localized. The driving-force dependence of less localized reactions is dominated by reorganization of nuclei, with a total reorganization energy, λ, that equals 4ΔGo,ET‡ under the typical Marcus quadratic relationship between the standard Gibbs free energy difference and the free energy of activation. The analysis seems appropriate using Marcus-Cohen BEBO theory, because ΔGo,PT‡<0.125 eV for ESPT from photoacids like those studied herein, yet ΔGo,ET‡>0.25 eV for electron-transfer reactions using similar molecules and solvents.
In some embodiments, the “true” pK*a of NS—NH2* experimentally derived is 11.7±0.1, compared to pK*a pseudo=12.34±0.02 obtained by best fitting the photoluminescence spectroscopy data shown in
Henderson-Hasselbalch analysis of steady-state photoluminescence spectroscopy data from weak photoacids can result in erroneous excited-state pKa values, which are influenced by the excited-state lifetime of the photoacid. Many embodiments provide methods and systems to accurately quantify pK*a of weak photoacids and photobases. Via dynamic Stern-Volmer quenching analysis of data obtained for unidirectional excited-state proton-transfer reactions, observed rate constants for excited-state proton transfer to a series of proton-accepting quenchers are quantified. Rate constants abstracted assuming a steady-state approximation are analyzed using Marcus-Cohen BEBO theory in conjunction with measured ground-state pKa values of the quenchers to deduce the excited-state pKa of the photoacid. The extracted value of pK*a is about 11.7±0.1.
Moderate photoacids and photobases can be used in combination with confocal fluorescent microscopy for measuring local OH− and H+ activity, activity of protonic species including (but not limited to) phosphate, glycine, dissolved inorganic carbon (DIC) species such as bicarbonate and carbonate, and/or species such as acetate, formate.
In some embodiments, sensors are based on excited-state proton-transfer reactions from electronic triplet excited states. Most sensors based on excited-state proton-transfer reactions from their electronic singlet excited state are limited in their proton/hydroxide/DIC sensing ability due to the short electronic singlet excited-state lifetimes of organic molecules, which is typically on the order of tens of nanoseconds. These lifetimes restrict the proton/hydroxide/DIC concentration sensing ability of these molecules to concentrations of a few millimolar or more. The sensing ability of these molecules can be enhanced to a concentration range of a few micromolar to as low as tens of nanomolar by incorporating heavy atoms in these molecules. Upon photoexcitation, the strong spin-orbit coupling due to the heavy atoms in these molecules induces an intersystem crossing to the electronic triplet excited states, which can typically last for more than microseconds to a few milliseconds. The long triplet excited-state lifetimes enable the detection of very slow excited-state proton-transfer reactions due to the slow mass action kinetics at low proton/hydroxide/DIC concentrations, thus making it possible to quantify the local proton/hydroxide/DIC concentrations on the micromolar-nanomolar scale. If the triplet excited state in these molecules is emissive at room temperature or at low temperatures via phosphorescence, a direct relation can be derived between the intensity of emission from the protonated form of the triplet excited state of the sensor and the local pH or pOH or the DIC concentration. If the triplet excited state is not emissive in these molecules, then the fluorescence emission intensity from the singlet excited state can be indirectly used to quantify the local pH or pOH or the DIC concentration, if there is an attainment of steady-state for the intersystem crossing reaction between the singlet and the triplet excited states of the protonated form of the sensor. In both the abovementioned cases, the setup will be similar to the one described for sensors based on excited-state proton-transfer reactions from their singlet excited states. The yield for intersystem crossing and phosphorescence in such sensors can be increased by (i) incorporating a heavy atom or a carbonyl group in the structure of the molecules, (ii) introducing salts of heavy atoms in the system, (iii) increasing rigidity (e.g., as an aggregate, in a host-guest cage, in a polymer, in the solid-state), or (iv) photoexcitation of a transition metal coordination complex that is used as a triplet sensitizer and subsequent triplet energy transfer to form the triplet excited state of the sensor molecule.
Example of molecule that can be used under this mechanism include sodium 6-bromo-5-aminonaphthalene-1-sulfonate:
wherein Br and/or SO3Na can be on any position of the phenyl ring.
In some embodiments, sensors based on parallel excited-state intermolecular and intramolecular proton-transfer reactions from electronic singlet excited states (
Examples of molecule that can be used under this mechanism:
can be excited below 350 nm.
can be excited using 400 nm light.
Storing renewable energy effectively for long time periods is important in order to reach negative CO2 emissions. The concept of electrochemical CO2 reduction (CO2R) is compelling because it enables the storage of renewable energy in the form of chemical bonds. CO2R establishes CO2 and water (H2O) as the primary sustainable feedstocks to form useful chemicals and fuels, thereby closing the carbon cycle. The electrochemical CO2R process is complex and there are many challenges that need to be overcome before this technology is energy-efficient and selective enough to be commonly used at an industrial scale.
One promising approach is the use of gas diffusion electrodes (GDEs). This type of electrode addresses the problem of mass transfer limitations that are encountered in conventional catalytic setups by delivering CO2 in the gas phase to a catalyst in contact with a liquid electrolyte. Most GDEs include a macro-porous gas diffusion layer topped with a hydrophobic microporous layer that is then subsequently coated with a catalyst. The catalyst can be wetted by a thin layer of aqueous electrolyte to provide ionic conductivity but not limit CO2 transport to the surface. This setup allows for increased current densities by more than one order of magnitude over conventional setups. For a conventional setup in which two electrodes are placed in CO2-saturated electrolyte, mass transport to the cathode limits the rate of CO2 consumption and hence, the maximum current density magnitude may not exceed 30 mA/cm2. In addition, tailored GDEs can achieve current density magnitudes larger than about 1 A/cm2.
Various parameters influence the activity and selectivity of a GDE. The interplay between these operating parameters can dictate the CO2R performance. The choice of catalyst material can influence the system's performance. One common CO2R catalyst is copper, because its moderate binding energy to CO as a reaction intermediate allows for its further reduction to desirable higher-order carbon products such as ethanol, propanol, ethylene, and acetate. (See, e.g., R. Kortlever, et al., J. Phys. Chem. Lett. 2015, 6, 4073-4082; K. Kuhl, et al., Energy Environ. Sci. 2012, 5, 7050-7059; S. Nitopi, et al., Chem. Rev. 2019, 119, 7610-7672; the disclosures of which are incorporated herein by reference.) Furthermore, the influence of various parameters (such as the composition and structure of the GDE, i.e. pore size, hydrophobicity, surface depositions or the presence of microstructures) on the selectivity and activity of CO2R, may not be well understood well. (See, e.g., B. Kim, et al., J. Power Sources 2016, 312, 192-198; L.-C. Weng, et al., Phys. Chem. Chem. Phys. 2018, 20, 16973-16984; P. Jeanty, et al., J. CO2 Util. 2018, 24, 454-462; E. R. Cofell, et al., ACS Appl. Mater. Interfaces 2021, 13, 15132-15142; the disclosures of which are incorporated herein by reference.) Other parameters that may affect GDE activity and selectivity include the applied potential, the properties of the electrolyte (e.g. the constituent ions and the viscosity), the cell configuration, and the choice of ion-exchange membranes. In addition to the aforementioned parameters, the local activities of hydroxides, aOH
For a GDE at open circuit, CO2 may diffuse through the macro-porous and microporous layers into the electrolyte where it rapidly reacts with OH− to form bicarbonate and carbonate anions. This decreases the local OH− concentration and thereby increases the pOH in the electrolyte. Due to the participation of CO2 in these buffer reactions, the ability to map the spatially and time-resolved pOH around a GDE enables to assess the local concentration of CO2 in the electrolyte around a GDE.
At a non-zero current, a portion of the CO2 molecules is reduced to form products such as carbon monoxide, formic acid, methane or ethylene, among others. During this non-equilibrium process, one OH− is created, or one buffer species is deprotenated for each electron involved. The same holds true for the competing hydrogen evolution reaction (HER) that needs to be suppressed to maximize the CO2R yield. As a result, at sufficiently high current densities, the pOH may decrease. This effect is in competition with an increase in pOH caused by unreduced CO2 molecules that undergo reactions to form bicarbonate and carbonate anions. As a result, a low observed pOH indicates the presence of CO2 reduction activity.
In addition, the pH and pOH can impact the reactivity and selectivity of CO2R. High pH (corresponding to low pOH) may suppress the parasitic HER and shift the CO2R selectivity towards C2+ products. (See, e.g., Z. Zhang, ACS Energy Lett. 2020, 5, 3101-3107; X. Liu, et al., Nat. Commun. 2019, 10, 32; K. Yang, et al., J. Am. Chem. Soc. 2019, 141, 15891-15900; L. Wang, et al., ACS Catal. 2018, 8, 7445-7454; the disclosures of which are herein incorporated by reference.) The reason for the latter may be that OH− actively suppresses the creation of single carbon products (Ci) and hydrogen molecules (H2), while it does not affect the C2+ current density. This leads to an increased Faradaic efficiency (FE) towards C2+ products at a given current density.
pOH in the vicinity of an operating GDE is an important parameter to optimize the performance of a GDE. pOH may vary as a function of distance from the electrode surface. In addition, it also varies in the plane parallel to the surface if there are any inhomogeneities such as microcavities present on the surface. Measuring techniques that can map pOH around an operating GDE with high resolution in three dimensions are desired.
Previous efforts towards this end have included theoretical studies of the pH around GDEs performing CO2R. (See, e.g., L.-C. Weng, et al., Phys. Chem. Chem. Phys. 2018, 20, 16973-16984; N. Nesbitt, et al., J. Phys. Chem. C. 2021, 125, 24, 13085-13095; S. Suter, et al., Energy Environ. Sci. 2019, 12, 1668-1678; the disclosures of which are herein incorporated by reference.) Experimental approaches have involved scanning electrochemical microscopy (SECM), surface-enhanced Raman (SERS), electrochemical atomic force microscopy (EC-AFM) and surface-enhanced infrared spectroscopy (SEIRAS). (See, e.g., A. Botz, et al., Angew. Chem., Int. Ed. 2018, 57, 12285-12289; S. Dieckhöfer, et al., Chem.—Eur. J. 2021, 27, 5906-5912; X. Lu, et al., J. Am. Chem. Soc. 2020, 142, 15438-15444; N. Nesbitt, et al., J. Electrochem. Soc. 2021, 168, No. 044505; M. C. O. Monteiro, et al., Curr. Opin. Electrochem. 2021, 25, No. 100649; N. C. Rudd, et al., Anal. Chem. 2005, 77, 6205-6217; the disclosures of which are herein incorporated by reference.) While these techniques are powerful and can reach spatial resolutions on the nanometer scale, they do not have the ability to map the operando pH/pOH of an entire macroscopic sample in three spatial dimensions.
Many embodiments combine confocal microscopy and weak photoacids in mapping local pOH and/or pH in micrometer scale and in three dimensions. Several embodiments implement fluorescent confocal laser scanning microscopy (CLSM) in the pH measurement. CLSM enables time-resolved measurements in three spatial dimensions. The spatial resolution of this technique can reach about 250 nm under ideal conditions. The time resolution can vary from microseconds to several seconds depending on the size and spatial resolution of the frame of interest. The combination of time resolution with sub-micrometer spatial resolution in three spatial dimensions with high accuracy enables pOH-mapping in accordance with many embodiments. In various embodiments, it enables mapping of the local pOH value under operating conditions over a wide current density scale, from 0 to about 200 mA/cm2 in magnitude; or greater than about 200 mA/cm2 in magnitude. Mapping the operando pOH in three dimensions with high resolution enables probing the pOH within microstructures on a sample surface. The methods enable measuring the operando pOH within cavities in the surface of various samples including (but not limited to) electrodes, working electrodes of an electrochemical cell, and gas diffusion electrodes. Measuring the pOH of a GDE surface can correlate the microstructure geometry of a GDE with its CO2R performance.
The ratiometric fluorescent photoacid dye DHPDS is sensitive to pH values between 6 and 10. The sensing mechanism of the photoacid DHPDS involves proton-transfer reactions in its electronic ground state to perturb its absorption spectrum. This is the mechanism used by fluorescent pH indicators in biological studies at near-neutral-pH conditions. With DHPDS, the local pH at the GDE surface may increase from pH 6.8 to greater than 10 as the magnitude of the current density is increased from 0 to −28 mA/cm2 in 100 mM KHCO3 electrolyte. In several embodiments, the pH inside trenches 5-20 μm wide in the GDE surface locally increases more than on the GDE surface. In a number of embodiments, the pH increases as the trench width diminishes which indicates that narrow trenches exhibit higher CO2R activity than wider trenches and planar surfaces. Weak photoacids, such as APTS, do not undergo proton-transfer reactions in its electronic ground state at near-neutral-pH and alkaline-pH conditions. Instead, the fluorescent signal of APTS is altered via quenching of its thermally equilibrated electronic excited state by direct proton transfer to aqueous OH− as discussed above. Aqueous dissolved inorganic carbon species do not interfere with this process. The aromatic amine form of APTS can be used as a probe for the pOH value and is sensitive to pOH values between about 0 and about 2.8, compared to pOH 4-8 for DHPDS. Several embodiments combine DHPDS with APTS for local pOH sensing, which is able to cover a pOH range from 0 to 8 (with a gap between 2.8 and 4) and investigate operating GDEs under current densities as large in magnitude as −200 mA/cm2. Some embodiments explore the influence of different bicarbonate concentrations in the electrolyte and of different microstructure geometries.
The electrochemical cell comprises of an electrolyte chamber with two perpendicular electrolyte inlets and outlets and is open at the top which allows the water immersion objective of the confocal microscope to be immersed into the electrolyte. The reference electrode, counter electrode, and GDE (working electrode) are immersed in an aqueous electrolyte. The GDE is in contact with a gas chamber that allows the flow of gaseous CO2. The GDE can be made of a Sigracet 22 BB carbon paper substrate covered with 300 nm Cu as well as a carbon black, graphite and Nafion coating. The reference electrode can be a silver/silver chloride electrode, and the counter electrode can be a Pt electrode. For experiments at constant current densities, a leakless silver/silver chloride reference electrode and a platinum mesh counter electrode are submerged into the electrolyte.
The enlargement (
Some embodiments use APTS in the absence of electrical current to investigate the diffusion of CO2 through a GDE. For this, 1 M KOH electrolyte (pOH 0) with 100 μM APTS can be used. The fluorescence signal is mapped in the plane perpendicular to the electrode surface as a function of time with one frame captured approximately every four seconds. After one minute of continuous measurements, a 10 standard cubic centimeters per minute (SCCM) CO2 gas stream is fed into the gas chamber of the electrochemical cell and the change in pOH in the electrolyte can be observed. The measurements are performed both with and without circulating the electrolyte at a rate of 6 mL/min.
In several embodiments, both dyes DHPDS and APTS can be used to map the pOH around an operating GDE performing CO2R at current densities between about 0 mA/cm2 and about −200 mA/cm2. Measurements are performed in the plane parallel to the electrode surface, at about 20 μm above the surface, at the surface, and at 20 μm below the surface inside a trench. A CO2-saturated aqueous KHCO3 solution with KHCO3 at concentrations of about 100 mM, 200 mM and 400 mM spiked with about 100 μM DHPDS or about 200 μM APTS is used as the electrolyte and pumped through two perpendicular inlets at a rate of about 6 mL/min. The CO2 gas flow through the gas chamber is set to about 10 SCCM. The electrolyte with dilute dye is removed from the cell and replaced after each measurement. Every measurement is performed at least three times. The CO2R performance of equivalent copper on carbon paper GDEs is tested with gas chromatography.
Certain embodiments show the local pOH in the electrolyte surrounding a GDE changed upon exposure to CO2 at open circuit. The pOH measurement combines time resolution with the capability to spatially resolve the local pOH inside inhomogeneities in the surface of a GDE. CO2 reacts with OH− and water molecules in the electrolyte to form bicarbonate and carbonate anions. This increases the pOH which becomes an indicator for the CO2 diffusion pattern.
The experiment is repeated under an electrolyte flowrate of 6 mL/min as shown in
CO2 diffusion through homogeneous GDE substrates made of laminated polytetrafluoroethylene (PTFE) with pore sizes ranging between 0.1 to 0.2 μm and 0.45 μm, are reported.
Gas chromatography for different current densities can be performed on copper GDEs to evaluate the CO2R performance. H2 production dominates for low current density magnitudes. As the magnitude of the current density increases, more CO2R products can be seen, and the selectivity shifts towards C2+ products. Ethylene for current densities of −50 mA/cm2 or higher in magnitude and ethanol for <200 mA/cm2 can be observed.
Combining the two pOH-dependent fluorescent ratiometric dyes, DHPDS and APTS, enables to cover the pOH range from 0 to 8. In the context of CO2R, a low local pOH under operation indicates high CO2R activity and is desirable because high pH can help suppress the parasitic HER and favor the formation of C2+ products.
For a current density of J equals to about 0 mA/cm2, the pOH equals 7.2 everywhere (pOH of CO2-saturated 100 mM KHCO3). When the current is non-zero, the local pOH decreases because OH− is created as a byproduct of CO2R. The pOH is lower at the electrode surface than 20 μm above the surface because CO2 is reduced at the electrode surface and for the pOH to decrease at +20 μm, OH− has to diffuse away from the surface. Due to electrolyte flow, a concentration gradient is created. Furthermore, the pOH inside the trench is lower than at the surface. This can be seen in the panels for current densities of −2 mA/cm2 and −20 mA/cm2.
The surface morphology of copper does not change during CO2R experiments, confirmed by SEM as well as EDS measurements for samples before and after CO2R. Potassium deposits may appear on the sample after CO2R that originate from KHCO3 molecules in the electrolyte, but the copper catalyst doesn't change in appearance, neither in the trenches nor on the planar electrode surface.
In many embodiments, the pOH sensitivity of the photoacid DHPDS can be extended to pOH from about 2.5 to about 8, excited at about 405 nm (laser power 1.2%, gain 100) and about 485 nm (laser power 2%, gain 80) and the emission is collected between 495 nm and 835 nm. The extended pOH range can be achieved by excitation of the absorption peak of DHPDS around 405 nm. With the new laser settings, the calibration curve of DHPDS can be fitted with a sigmoidal curve:
The pOH sensitivity of APTS in a basic environment is from about 0.5 to about 2.5, excited at about 405 nm (laser power 2%) and at about 448 nm (laser power 0.3%) and the emission is collected at about 460 nm-550 nm (gain 13) and about 570 nm-840 nm (gain 25). This sigmoidal calibration curve of APTS:
By combining DHPDS and APTS with the settings described, a pOH sensitivity between 0.5 and 8 with no gap can be achieved.
Further, APTS can also be used to sense the local pH value in acidic environments between pH about 0.5 and 2.5. In acidic pH, APTS may not work via quenching of its thermally equilibrated electronic excited state by direct proton transfer to aqueous OH− but rather undergoes a proton-transfer reaction in its electronic ground state to perturb its absorption spectrum, similar to DHPDS. The absorption spectrum of APTS in acidic pH exhibits two peaks which would make it a candidate for a ratiometric dye, however, one of these peaks is at a wavelength below 405 nm which is the lowest wavelength that can be excited by most commercially available confocal microscopes. Therefore, to measure spatially resolved maps of the local pH value with most confocal microscopes, only one peak can be excited and APTS can be used as a non-ratiometric dye. When it is excited at 405 nm (laser power 2%) and 448 nm (laser power 0.3%) and the emission is collected at 460 nm-550 nm (gain 13), a linear calibration curve results with a pH sensitivity between 0.5 and 2.5. The linear fit curve takes the form
where E stands for the emission signal.
These photoacid calibration measurements are performed with a Leica Stellaris 5 upright confocal microscope with a HC FLUOTAR L 25x/0.95 W VISIR water immersion objective.
Although specific embodiments of systems and apparatuses are discussed in the following sections, it will be understood that these embodiments are provided as exemplary and are not intended to be limiting.
Chemicals used for experiments are reagent grade and are used without any further purification, unless otherwise specified. The following chemicals are used as purchased: hydrochloric acid (36.5-38% w/v), potassium hydroxide (86%), glycine (>99%), L-proline (>99%), trifluoroethanol (>99%) potassium phosphate (>98%), sodium acetate (>99%), potassium chloride (>99%), phosphoric acid (85% w/w), 5-amino-1-naphthalenesulfonic acid, sodium salt (NS—NH2) (>98%), 5-((2-aminoethyl)amino)naphthalene-1-sulfonic acid, sodium salt (>98%), 8-anilinonaphthalene-1-sulfonic acid, sodium salt (>98%). Ultrahigh purity deionized water is used to make all solutions. Potassium bicarbonate (KHCO3, 99.95%), potassium hydroxide (KOH), CO2 gas (research grade, 99.999%), 6,8-dihydroxy-1,3-pyrenedisulfonic acid (DHPDS, ≥97.0%), 8-Aminopyrene-1,3,6-trisulfonic acid trisodium salt (APTS, ≥96.0%), Sigracet 22 BB carbon paper, laminated PTFE membrane filters (pore size 0.1-0.2 μm and 0.45 μm), copper (99.999%), Pt mesh (99.9%, 0.0726 mm diameter wires), leakless Ag/AgCl reference electrode. All materials were used without further modification.
8-Aminopyrene-1,3,6-trisulfonic acid, trisodium salt is synthesized and purified or purchased from Millipore Sigma. With the compound dissolved in deuterated methanol, 1H NMR and 13C NMR spectra are recorded (500 MHz for 1H NMR and 125 MHz for 13C NMR). Spectral information is as follows: 1H NMR (500 MHz, CD3OD) δ 8.19 (s, 1H), 8.43 (d, 1 H), 8.93 (d, 1H), 9.06 (d, 1 H), 9.16 (d, 1H), 9.31 (s, 1H); 13C NMR (125 MHz, CD3OD) δ 113.47, 116.29, 117.98, 120.61, 122.84, 123.47, 124.64, 126.40, 126.64, 127.48, 130.02, 130.56, 134.71, 135.09, 140.99, 144.54.
AMOD dual electron beam deposition system (Angstrom Engineering), NOVA NanoSEM 450 scanning electron microscope confocal with an Oxford Instrument's Xmax 80 mm2, Oakton 5+ pH meter, Denver Instruments Ultra Basic pH meter, Zeiss LSM 710 confocal microscope with a WN Achroplan 63x water immersion objective (numerical aperture 0.9), Biologic SP-200 potentiostat, Masterflex 77120-62 pump, CO2 gas flow controller (Alicat Scientific), SRI-8610 gas chromatograph.
Copper gas diffusion electrodes are fabricated by electron-beam deposition of 300 nm copper on Sigracet 22BB carbon paper. An AMOD dual electron beam deposition system (Angstrom Engineering) is used. 300 nm of Cu is deposited on the microporous layer of the carbon paper substrate at a rate of 2 Å/s with a rotating substrate holder. The deposited samples are spray-coated, first with a solution of one part deionized water, one part isopropyl alcohol and 2.5 mg carbon black per mL of solution, then with a solution of one part deionized water, one part isopropyl alcohol and 0.5 mL of 5 weight % Nafion per mL of solution. Both coatings are applied from a distance of eight centimeters for one second. The samples are then dried overnight in vacuum.
A NOVA NanoSEM 450 scanning electron microscope is used to capture images of the samples. The spot size is set to three and the acceleration voltage to 15 kV. To identify if copper is present inside trenches, energy dispersive X-ray spectroscopy (EDS) is performed with an Oxford Instrument's Xmax 80 mm2 in the aforementioned SEM with a spot size of five.
The electrochemical cell used for pOH imaging with confocal microscopy is designed to be compatible with the confocal microscope. Because a water-immersion objective is used, the electrolyte chamber needs to be open at the top. This means that the cell is oriented horizontally, otherwise the electrolyte would spill. The working distance of the water immersion objective is 1.7 mm. In order to allow the objective to be placed this close to the GDE surface, the cell operates without ion-exchange membranes. The cell is 3D-printed, the surfaces are sanded. A rubber gasket is placed in between the bottom gas chamber part and the top electrolyte chamber part for sealing. A hole where the GDE is placed connects the gas and electrolyte chambers, which is circular and has a surface area of 0.2 cm2. For experiments with applied current, a leakless Ag/AgCl reference electrode is used, and a Pt mesh is dipped into the electrolyte as the counter electrode. The electrolyte chamber, including the tubes used for pumping, holds approximately 10 mL of electrolyte when the objective is immersed into it.
Several embodiments use the steady-state ultraviolet-visible electronic absorption and photoluminescence spectroscopy for characterization. Equal amounts of aromatic amine photoacids are portioned into five separate 100 mL volumetric flasks. For the non-proton-accepting quencher experiments, the five flasks are filled to the volumetric marker with aqueous solutions of 1 M HCl, 1 M KOH, 2 M KOH, 3 M KOH, and 4 M KOH. The aqueous 1 M KOH solution is titrated with the aqueous 1 M HCl solution to reach the desired pH values from 14 to 6, and the H+ activities are measured using a pH meter (pH/Ion 510). For pH values more alkaline than 14, the pH meter does not accurately report H+ activity and therefore the Hammett acidity scale is used to approximate H+ activity. Electronic absorption spectra and photoluminescence spectra are acquired using absorbance and fluorescence spectrometer(s), using a square-bottom quartz cuvette (1 cm path length) at room temperature. Electronic absorption spectra are baselined on a spectrum of deionized water and were corrected for scattering of light. Photoluminescence spectra are recorded by exciting with 310 nm light, slit widths for the excitation and emission monochromators were set to 5 nm, and the PMT detector voltage is set to 600 V. Photoluminescence spectra are corrected for changes in solution refractive index, inner filter effects and the wavelength-dependent correction factor of the fluorimeter. For experiments that use non-OH− proton-accepting quenchers, equal amounts of the quencher are added to the dry volumetric flask intended to form the acidic and basic aqueous photoacid solutions, to prevent changes in total quencher concentration during the titration, and titrations are performed similarly to those in the absence of added proton-accepting quenchers. Although the total quencher concentration is kept constant throughout the titration, the activity of the deprotonated quencher species varied as a function of solution pH according to Equation 3. Ionic strength is matched at the most alkaline pH values; however, it is experimentally difficult to maintain the same ionic strength throughout the titration. Normalized photoluminescence intensities as a function of solution pH are obtained by dividing the corrected photoluminescence intensities at the wavelength of maximum photoluminescence to the maximum corrected photoluminescence intensity at the same wavelength. Maximum photoluminescence intensity is obtained for solutions at near neutral pH (aOH−≈10−7), which is considered to be a reasonable approximation of the photoluminescence intensity in the absence of OH− quencher required for use in the Stern-Volmer analysis.
Some embodiments use nanosecond time-resolved photoluminescence and transient absorption spectroscopies. Photoacid samples with near-neutral pH values obtained by acid-base titration are placed in a 1 cm pathlength quartz cuvette. Time-resolved spectra are acquired using a custom nanosecond spectroscopy system and the excitation source is the 355 nm pulse from the frequency-tripled 1064 nm fundamental line of an Nd:YAG laser. The laser power is measured prior to measurement using a thermal power sensor head to be 0.7 mJ per pulse. Emitted light passes through focusing optics and a filter wheel equipped with a long-pass filter to filter out stray laser light, enters a monochromator, and the intensity is measured by a photomultiplier tube housed by a 5-stage photomultiplier tube housing removed from LKS60 laser flash photolysis spectrometer. For transient absorption spectroscopy measurements, the probe beam is generated by a xenon arc lamp bulb (150 W, 18 V, 6200 K) and an optical chopper, frequency-locked to the pump beam, is used to reduce unnecessary illumination of samples. The sample is placed at a 45° angle with respect to the pump and probe beams. The detector system is powered by a high-voltage power supply electrically biased at 700-900 V (PS325/2500V-25W) and the resulting signal is digitized by an oscilloscope terminated at 50 Ohms. Data is acquired from 700 nm to 360 nm in 10 nm steps by averaging up to 4000 shots at a 10 Hz repetition rate. Before and after time-resolved measurements, a full electronic absorption spectrum is measured from 1100 nm to 200 nm and compared to the spectrum before pulsed-laser experiments to qualitatively ensure minimal dye degradation.
DHPDS and APTS are each ratiometric dyes that report on the local activity of OH−, because their spectra exhibit at least two distinct peaks that change in intensity in different directions upon altering the pH or pOH. The ratio of emission from either dye is determined by calculating the ratio of the signals from both peaks. It is independent of the local dye concentration, within the concentration ranges used in the experiments. To determine calibration curves of the ratio of emission as a function of pOH, aqueous solutions of known pOH are prepared. For this, aqueous stock solutions of KOH and HCl were diluted with pure water. The pOH of the solutions is confirmed by measuring the pH separately with two different pH meters (and calculating the pOH using pOH =14−pH). Both pH meters are calibrated with buffer solutions at pH 4, pH 7, and pH 10 before use. In the prepared solutions with known pOH, 100 μM DHPDS or 100 μM APTS is diluted from a stock solution. A laser beam scans the sample solutions line by line over the center of a liquid droplet, which is repeated three times and the average ratio of emission is calculated and correlated with the known pOH. For both dyes, the ratio of emission is plotted as a function of pOH and best fit to a sigmoidal function:
In the case of DHPDS, three distinct peaks in the absorption spectra are important for the calibration curve, one for each of the doubly protonated (R—(OH)2), monoprotonated (R—(OH)(O−)), and doubly deprotonated (R—(O−)2) states of the dye in its electronic ground state. Because DHPDS is a strong photoacid (pK*a≈0), excitation of either protonation state of DHPDS at near-neutral-pH conditions results in emission from a deprotonated form of the thermally equilibrated electronic excited state of the dye. This is because the kinetics for excited-state proton transfer are significantly faster than the excited-state lifetime of the dye and thus the thermally equilibrated electronic excited-state of the dye reaches chemical quasi-equilibrium. DHPDS is excited separately using 458 nm laser light (100% maximum power) and 488 nm laser light (20% maximum power) with the pinhole set to 70.1 μm and the gain set to 800 for each channel. Emitted light is detected in the wavelength interval of 505-754 nm separately for each excitation wavelength, and thus the emission ratio is the ratio between the signals collected from the two excitations. That emission ratio data as a function of pOH fits well to the nonideal Henderson-Hasselbalch equation, also known as the Hill equation,
with n the Hill coefficient ideality factor, pK*a obs an effective excited-state pKa, and pH=14−pOH, to obtain the calibration curve. The main contributor to the nonideal behavior of the titration data is likely the two pKa values for DHPDS, which is well supported by the observation of two isosbestic points in the titration data.
In the case of APTS in aqueous alkaline environments, two distinct peaks in the fluorescence spectra are important for the calibration curve, one for each of the protonated (R—NH2) and deprotonated (R—NH—) states of the dye in its thermally equilibrated electronic excited state. Because APTS is a very weak acid (pKa>14), deprotonation of its electronic ground state requires pH>14. Thus, only the protonated electronic ground state of APTS as the aromatic amine can be excited. However, in the presence of a large concentration of OH−, the thermally equilibrated electronic excited state can be quenched via proton transfer to OH− to form the deprotonated electronic excited state. Thus, emission can be observed from either protonation state, as the amine (R—NH2*) or the aminide (R—NH−*). As such, APTS is excited using 458 nm laser light (100% power) with the pinhole set to 57.1 μm and the gain set to 800. Emitted light is detected separately in the wavelength intervals of 480-550 nm and 551-754 nm, and thus the emission ratio is the ratio between the signals collected in the two emission wavelength ranges. That emission ratio data as a function of pH is best fit to the nonideal nonlinear dynamic Stern-Volmer equation and OH− as the quenching species,
with KSV,OH
When performing measurements with the dye DHPDS in the current density range between 0 and −20 mA/cm2, no significant decrease in photoluminescence intensity was observed. Some loss of fluorescent signal is observed for cathodic current densities of 100 mA/cm2 in magnitude, or larger, but in our application the signal of DHPDS saturates for current densities larger in magnitude than 20 mA/cm2 so this effect is not relevant herein.
However, for APTS shows a decrease in photoluminescence intensity for current density magnitudes>80 mA/cm2 and we used APTS to investigate the operation of a GDE with current densities as high as 200 mA/cm2 in magnitude. To understand this effect, the stability of APTS under different conditions is evaluated. A decrease in photoluminescence intensity from APTS with and without light and for solutions with pH 7.1 and pH 3 is seen, where the degradation effect is more pronounced for the pH 3 solution. Furthermore, a significant change is observed in color of a 10 mM APTS stock solution that is exposed to −100 mA/cm2 for 30 minutes, from bright green to brown, that persisted for days without indication of reverting back to its original form (
Irrespective, calibration curves of fresh APTS solutions with different APTS concentrations, as well as a curve that was measured with an APTS stock solution that was exposed to −100 mA/cm2 for 5 minutes, are nearly identical, suggesting that APTS is suitable to use as a pOH sensor even in the presence of significant degradation to a less emissive product. This can be explained by the ratiometric nature of APTS as a pOH sensor.
For each of DHPDS and APTS, the dye is dissolved in solution and placed under a Zeiss LSM 710 confocal microscope with a WN Achroplan 63x water immersion objective dipped into the solution. A laser beam scans the sample line by line with the settings described above. For both dyes, the pOH is calculated with the calibration curves obtained in Equations S1 and S2.
Experiments to visualize CO2 diffusion through a porous GDE are performed with the electrochemical cell described above. The pOH is resolved with the dye APTS according to the above explained procedure. 1 M KOH (pOH 0) is chosen as the electrolyte, so an expected increase in pOH upon exposure to CO2 can be detected with APTS. Experiments are performed with carbon paper GDEs prepared as described above, both at locations with a trench present and at locations without a trench present. In addition, laminated PTFE substrates with different pore sizes, coated with 300 nm Cu in the same way as carbon paper, are investigated. Experiments are carried out both with and without electrolyte flow through two perpendicular inlet tubes at a rate of 6 mL/min. Measurements are performed in the plane perpendicular to the GDE surface by scanning the laser line by line and moving the stage in the z-direction. The measuring speed is adjusted such that capturing one frame takes four to five seconds. The experiment is conducted as a time-series. A CO2 gas stream of 10 SCCM through the gas chamber along the back of the GDE was turned on after 1 minute of continuous measurements. This time point was later defined as t=0 s.
Experiments to map the pOH around an operating GDE performing CO2 reduction are performed with the electrochemical cell and the GDE described above. Aqueous electrolytes of different KHCO3 concentrations are used (100 mM, 200 mM and 400 mM). Before each experiment, the electrolyte is bubbled with 30 SCCM CO2 gas for at least 30 minutes. The pH is monitored with a pH meter and bubbling is continued until the pH stabilized. This ensured that the electrolyte is saturated with CO2. All experiments are conducted with both DHPDS and APTS dyes. The dye is dissolved in the CO2-saturated electrolyte: DHPDS at a concentration of 100 μM to investigate current densities smaller in magnitude than −20 mA/cm2, APTS at a concentration of 200 μM for 100 mM KHCO3 electrolyte/300 μM for 200mM and 400 mM KHCO3 electrolyte, to investigate current densities larger in magnitude than −20 mA/cm2. The electrochemical cell is assembled with a Cu GDE, Ag/AgCl leakless reference electrode and Pt mesh counter electrode. The cell is placed under the confocal microscope and the electrolyte chamber is filled with the prepared electrolyte. All experiments are conducted with electrolyte flow (6 mL/min through two perpendicular inlets) and with a gas stream of 10 SCCM CO2 through the gas chamber of the electrochemical cell. To determine the series resistance of the cell filled with electrolyte, potentiostatic electrochemical impedance spectroscopy (PEIS) is performed before each experiment. This allowed to perform an 85% IR electronic compensation of the electrochemical potential. A trench that is approximately 20 μm wide was identified on the GDE surface. The stage is positioned such that the focal point of the objective was 20 μm below the GDE surface inside the trench. A constant current is applied with the potentiostat. Measuring under galvanostatic conditions enables a constant flux of ions between electrodes. The system is allowed to reach steady state for 15 seconds, then a frame is captured in the x-y plane as described above. The speed is set to three such that taking one image takes approximately 45 seconds. The same procedure is repeated for the focal point being at the GDE surface and 20 μm above the surface for various different current densities. In between each measurement, the electrolyte is removed and replaced to introduce fresh, unused dye. All measurements are conducted at least three times.
An electrochemical cell optimized for use with gas chromatography is used for product detection during the performance of CO2R experiments with copper on carbon paper GDEs. A leakless Ag/AgCl electrode served as reference electrode and a platinum mesh as counter electrode. An anion exchange membrane (AGC, Selemion AMV) is used to separate cathode and anode. The gas chamber takes the form of a serpentine channel at the back of the GDE. The cell is sonicated before each experiment for at least 40 minutes and rinsed thoroughly after each experiment. 100 mM KHCO3 saturated with CO2 is pumped through the catholyte and anolyte chambers at a rate of 6.3 mL/min. CO2 is fed into the gas chamber at a rate of 10 SCCM. A flow meter placed before and after the cell is used to ensure that there were no gas leaks. The gas coming from the cell is returned to a electrolyte bath as a precaution in case of electrolyte breakthrough through the GDE. From there, the gas is sent through a vapor trap to a gas chromatograph. Chronopotentiometry experiments (constant current) are carried out at −10, −50, −100 and −200 mA/cm2 with a potentiostat. Before each experiment, potetiostatic electrochemical impedance spectroscopy (PEIS) is carried out to measure the resistance of the cell. This allowed to compensate the electrochemical potential by 85% with IR compensation.
6-bromo-5-aminonaphthalene-1-sulfonate can be synthesized following the protocol described below. The threads of a 20 mL scintillation vial was taped with Teflon tape. To this vial, sodium 5-aminonaphthalene-1-sulfonate (111.5 mg, 0.5 mmol), N-bromosuccinimide (86 mg, 0.5 mmol), and mandelic acid (12 mg, 20 mol %) were added. The solids were then dissolved in a solution of H2O:MeCN (1:1 v/v, 4 mL) and stirred at room temperature for 24 h. The reaction was then transferred to a separatory funnel, quenched with saturated NaHCO3, and extracted with EtOAc. The aqueous layer was collected and concentrated under reduced pressure and the remaining solids were triturated with MeOH. The resultant mixture was then filtered, and the filtrate was collected and absorbed onto Celite under reduced pressure. The resulting powder was purified using RediSep Gold Normal-Phase Silica Columns (MeOH/DCM 0:1-1:1 gradient, TLC Rf=0.47 in MeOH/DCM 1:3) and the fractions were collected and concentrated under reduced pressure to yield 20.6 mg of a pale orange solid (14%). 1H NMR (500 MHz, CD3OD) δ 8.17 (dd, J=2.85, 0.94 Hz, 1 H), 8.15 (s, 1 H), 8.11 (dd, J=9.23, 0.57 Hz, 1 H), 7.53 (d, J=9.24 Hz, 1 H), 7.45 (dd, J=8.41, 7.39 Hz, 1 H). ESI-TOF calculated for C10H7BrNO3S− [M-Na]−301.9, 299.9, found 301.8, 299.7.
As can be inferred from the above discussion, the above-mentioned concepts can be implemented in a variety of arrangements in accordance with embodiments of the invention. Accordingly, although the present invention has been described in certain specific aspects, many additional modifications and variations would be apparent to those skilled in the art. It is therefore to be understood that the present invention may be practiced otherwise than specifically described. Thus, embodiments of the present invention should be considered in all respects as illustrative and not restrictive.
As used herein, the singular terms “a,” “an,” and “the” may include plural referents unless the context clearly dictates otherwise. Reference to an object in the singular is not intended to mean “one and only one” unless explicitly so stated, but rather “one or more.”
As used herein, the terms “approximately,” and “about” are used to describe and account for small variations. When used in conjunction with an event or circumstance, the terms can refer to instances in which the event or circumstance occurs precisely as well as instances in which the event or circumstance occurs to a close approximation. When used in conjunction with a numerical value, the terms can refer to a range of variation of less than or equal to ±10% of that numerical value, such as less than or equal to ±5%, less than or equal to ±4%, less than or equal to ±3%, less than or equal to ±2%, less than or equal to ±1%, less than or equal to ±0.5%, less than or equal to ±0.1%, or less than or equal to ±0.05%.
Additionally, amounts, ratios, and other numerical values may sometimes be presented herein in a range format. It is to be understood that such range format is used for convenience and brevity and should be understood flexibly to include numerical values explicitly specified as limits of a range, but also to include all individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly specified. For example, a ratio in the range of about 1 to about 200 should be understood to include the explicitly recited limits of about 1 and about 200, but also to include individual ratios such as about 2, about 3, and about 4, and sub-ranges such as about 10 to about 50, about 20 to about 100, and so forth.
The current application claims the benefit of and priority under 35 U.S.C. § 119 (e) to U.S. Provisional Patent Application No. 63/332,540 entitled “Reversible Excited-State Weak Photoacids and Photobases as Dynamic Fluorescence Sensors of Local OH− and H+ Activity” filed Apr. 19, 2022, U.S. Provisional Patent Application No. 63/484,978 entitled “Direct Observation of the Local Microenvironment in Inhomogeneous CO2 Reduction Gas Diffusion Electrodes via Versatile pOH Imaging” filed Feb. 14, 2023. The disclosures of U.S. Provisional Patent Application No. 63/332,540, U.S. Provisional Patent Application No. 63/484,978 is hereby incorporated by reference in its entirety for all purposes.
This invention was made with government support under Grant No. DE-SC0021266 awarded by the Department of Energy. The government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 63332540 | Apr 2022 | US | |
| 63484978 | Feb 2023 | US |
