The following information is provided to assist the reader to understand the technology described below and certain environments in which such technology can be used. The terms used herein are not intended to be limited to any particular narrow interpretation unless clearly stated otherwise in this document. References set forth herein may facilitate understanding of the technology or the background thereof. The disclosure of all references cited herein are incorporated by reference.
A number or currently available oxygen sensors are based upon an optical measurement of oxygen concentration. In such sensors, a chemical film which has emission (for example, fluorescence, phosphorescence and/or luminescence) properties that depend upon oxygen concentration is immobilized upon a support/probe. In general, after the film is placed in an excited state by application of energy thereto, the emission signal is at a maximum in the absence of oxygen (O2). O2 molecules reduce or quench the emission signal. The emission signal is inversely related to the concentration of O2 in an environment.
Sensors based upon nanotube or other nanostructures for various chemical species are known. However, attempts to develop a nanoelectronic O2 sensor, have required vacuum conditions and sometimes high temperature.
In one aspect, a method of detecting an analyte in an environment, includes: immobilizing at least one photoactive composition on nanostructures, the photoactive composition exhibiting emission that is sensitive to the analyte; applying electromagnetic radiation to the immobilized photoactive moiety for a period of time; measuring at least one response; and using the measured response (for example, during or after application of the electromagnetic energy) to determine the presence the analyte in the environment. The nanostructures can, for example, include carbon nanostructures. In a number of embodiments, the analyte is oxygen.
The measured response can, for example, be at least one of a spectroscopic change or an electrical property change. Each of a spectroscopic change and an electrical property change can, for example, be measured. In a number of embodiments, a change in at least one electrical property of the nanostructures is measured.
The photoactive composition can, for example, include at least one photoactive moiety selected from the group of a naphthalimide, a derivative of a naphthalimide. tropolonate, a derivative of tropolonate, perylene, a derivative of perylene, salophen, a derivative of salophen, anthraquinone, a derivative of anthraquinone, fluorene, a derivative of fluorene, benzimidazole a derivative of benzimidazole, benzimidazole-pyridine, a derivative of benzimidazole-pyridine, salicylamide, derivative of salicylamide, 2-hydroxyisophthalimidem, a derivative of 2-hydroxyisophthalimidem, beta-diketone, a derivative of beta-diketone, pyridine, a derivative of pyridine, bipyridine, a derivative of bypyridine, terpyridine, a derivative of terpyridine, phenanthridine, a derivative of phenanthridine, quinoline. a derivatives of quinoline, a phenol, a derivative of a phenol, and bis(oxazolinyl)pyridine, a derivative of bis(oxazolinyl)pyridine.
The photoactive composition can, for example, include at least one of a lanthanide cation chelated with a photoactive ligand, a transition metal complex, a conjugated polymer (that is, an electroactive or photoactive conjugated polymer), or photoactive inorganic nanoparticles. In the case of a lanthanide cation chelated with a photoactive ligand, the lanthanide cation can, for example, be a cation of Lanthanum (La), Cerium (Ce), Praseodymium (Pr), Neodymium (Nd), Promethium (Pm), Samarium (Sm), Europium (Eu), Gadolinium (Gd), Terbium (Tb), Dysprosium (Dy), Holmium (Ho), Erbium (Er), Thulium, (TM), Ytterbium (Yb), or Lutetium (Lu).
The photoactive moiety can, for example, be incorporated within an oligomer or a polymer. In a number of embodiments, the photoactive moiety is incorporated within an oligomer or a polymer, and the oligomer or the polymer includes oxygen donor atoms or oxygen donor groups.
The nanostructures can, for example, be supported upon a surface or a substrate. The surface can, for example, include SiO2 or a polymer. The surface can, for example, translucent.
The method can also include measuring at least one electrical property of nanostructures including no photoactive composition immobilized thereon in the environment to provide a reference measurement.
In another aspect, a system for detecting an analyte in an environment, includes: nanostructures including at least one photoactive composition immobilized on the nanostructures, the photoactive composition exhibiting emission that is sensitive to an analyte; at least one energy source to apply electromagnetic radiation to the immobilized photoactive moiety for a period of time; and at least one measurement system (for example, a sensor) to measure a response, the measured response being used to determine the presence the analyte in the environment.
The system can, for example, include at least one measurement system to measure a spectroscopic change and/or at least one measurement system to measure a change in an electrical property of the nanostructures.
In a further aspect, a composition include nanostructures having immobilized thereon a photoactive composition comprising at least one photoactive moiety and at least one lanthanide cation. The photoactive composition can, for example, include at least lanthanide cation chelated with a photoactive ligand. The lanthanide cation can, for example, be a cation of Lanthanum (La), Cerium (Ce), Praseodymium (Pr), Neodymium (Nd), Promethium (Pm), Samarium (Sm), Europium (Eu), Gadolinium (Gd), Terbium (Tb), Dysprosium (Dy), Holmium (Ho), Erbium (Er), Thulium, (TM), Ytterbium (Yb), or Lutetium (Lu). In a number of embodiments, the cation is Ln3+, Pr3+, Nd3+, Sm3+, Eu3+, Tb3+, Dy3+, Ho3+, Er3+, Tm3+ or Yb3+. In a number of embodiments, the photoactive moiety is incorporated within an oligomer or a polymer.
The technology described herein, along with the attributes and attendant advantages thereof, will best be appreciated and understood in view of the following detailed description taken in conjunction with the accompanying drawings.
As used herein and in the appended claims, the singular forms “a,” “an”, and “the” include plural references unless the content clearly dictates otherwise. Thus, for example, reference to “a photoactive moiety” includes a plurality of such photoactive moieties and equivalents thereof known to those skilled in the art, and so forth, and reference to “the photoactive moiety” is a reference to one or more such photoactive moieties and equivalents thereof known to those skilled in the art, and so forth.
It is demonstrated herein that electrically conductive nanostructures (for example, single-walled carbon nanotubes or SWNT) or networks decorated with (or having immobilized thereon) a photoactive composition, which can, for example, include chromophoric or photoactive groups or moieties show reversible bimodal (that is, spectroscopic (including absorption and luminescence) and chemiresistor sensitivity) towards an analyte such as oxygen (O2) gas at room temperature under ambient pressure. The photoactive composition exhibits emission that is sensitive to the analyte. The photoactive composition can for example, include an oligomer or a polymer. The chromophore or photoactive groups or moieties are entities that can be placed in an excited state upon application of electromagnetic radiation or electromagnetic energy (for example, UV light, visible light etc., depending upon the range of wavelength over which the photoactive group is excited) thereto to exhibit detectable photoluminescence.
A number of classes of photoactive compositions are suitable for use herein. For example, lanthanide cations (for example, trivalent lanthanide cations, which are sometimes abbreviated Ln3+) chelated with chromophoric ligands to sensitize energy transfer to the lanthanide ion can be used herein. Chromophoric ligands for lanthanides can, for example, be small or relatively low molecular weight molecules or oligomeric or polymeric host systems. Transition metal complexes (for example iridium (III) bis(1-pyrenyl-isoquinolinato-N,C′) acetylacetonate or platinum-tetraphenylporphyrin) are also suitable for use herein. Electroactive and photoactive conjugated polymers such as poly(p-phenylenevinylene) (PPV), poly(p-phenylene) (PPP), polythiophene (PT), polyfluorene (PF) and others also are suitable for user herein. Still further, photoactive inorganic nanoparticles (for example, CdSe, ZnS, InAs, ZnSe, etc) are suitable for user herein.
In the case of trivalent lanthanide cations chelated with chromophoric ligands, complexing one or more chromophores or a composition (for example, an oligomer or polymer) including one or more chromophores or photoactive groups with a lanthanide or lanthanoid ion stabilizes the excited state baseline. The lanthanide or lanthanoid series comprises the fifteen elements with atomic numbers 57 through 71 (including Lanthanum (La), Cerium (Ce), Praseodymium (Pr), Neodymium (Nd), Promethium (Pm), Samarium (Sm), Europium (Eu), Gadolinium (Gd), Terbium (Tb), Dysprosium (Dy), Holmium (Ho), Erbium (Er), Thulium, (TM), Ytterbium (Yb), and Lutetium (Lu). In a number of embodiment, one or more of the following lanthanide cations are used Ln3+, Pr3+, Nd3+, Sm3+, Eu3+, Tb3+, Dy3+, Ho3+, Er3+, Tm3+ or Yb3+.
In several representative examples, complexes of europium cations (Eu3+) were formed with a dendrimer including naphthalimide chromophore groups such as 1,8-naphthalimide chromophore groups and/or derivatives of 1,8-naphthalimide chromophore groups for use in the detection of O2 (or NOx).
However, any electromagnetic radiation/energy absorbing (for example, UV, visible or near-infrared-light absorbing) chromophore or photoactive groups are suitable for use in the present invention. Examples of suitable chromophore groups include, but are not limited to, other members of the naphthalimide family (including for example, 2,3-naphthalimide and 1,2-naphthalimide), 1,8-naphthalimides functionalized at the 4-position with, for example, an amino group, a nitro group, a bromo, an isothiocyanate group or a polyethylene glycol (PEG) group. Such naphthalimides sensitize luminescent lanthanides (emitting, for example, in the visible and in the near-infrared), offering versatility for applications. Still other suitable chromophore groups include, for example, tropolonate and derivatives, perylene and derivatives (for example, perylene dimide), salophen and derivatives, anthraquinone and derivatives, fluorene and derivatives, benzimidazole and derivatives, benzimidazole-pyridine and derivatives, salicylamide and derivatives, 2-hydroxyisophthalimide and derivatives, beta-diketone and derivatives, pyridine and derivatives, bipyridine and derivatives, terpyridine and derivatives, phenanthridine and derivatives, quinoline and derivatives, phenols and derivatives and bis(oxazolinyl)pyridine and derivatives.
The excitation of the photoactive groups immobilized upon the nanostructures changes at least one microelectronic or electrical property of the nanostructures. Likewise, the subsequent effect of the analyte upon the excited state of the photoactive groups (in addition to changing photoemission) changes at least one microelectronic or electrical property of the nanostructures in measurable manner. In that regard, the effect, which is proportional to the concentration of the analyte, can be measured in the form of, for example, resistance, conductance, impedance etc.
Many different oligomers and polymers (including, for example, dendrimers and dendrons) can be used in the present invention to decorate or coat the nanostructures with a polymer-chromophore-lanthanide complexes or other photoactive groups. In general, dendrimers are regular, highly branched polymers that can, for example, have a monodisperse tree-like or generational structure. PAMAM (poly(amidoamine)) dendrimers of any generation are representative examples of suitable oligomers or polymers. In a number of embodiments, polymers and/or oligomers for decorating or coating the nanostructures include oxygen donor atoms/groups for coordination of lanthanides, groups for covalent or other incorporation of photoactive moieties and/or other groups for attachment to the surface of carbon nanotubes. For example, PAMAM oligomers and polymers include internal amide groups capable of binding hard Lewis acid lanthanide ions through their oxygen atoms and include terminal primary amine groups for functionalization with suitable photoactive groups.
As used herein, the term “polymer” refers to a compound including a plurality of repeat or structural units. As known in the art, polymers can be formed via polymerization of one or multiple monomers. The term “oligomer” refers to polymers containing relatively few structural units
In addition to exhibiting an optical (emission quenching) response to the analyte O2 gas, a linear, electronic response to O2 gas in concentrations between, for example, 5 and 27% was observed for nanostructures having an immobilized photoactive composition thereon at room temperature and ambient pressure. The bimodal response of such materials can, for example, provide for inherent calibration of sensors including such materials through comparison of the two types of responses. However, each response mode can be used individually and independently to measure analyte/O2 concentration. For example, measurement of the change of only one or more electrical properties (for example, resistance, conductance, impedance, etc.) can be used to measure the analyte.
Chemically sensitive solid-state resistors (chemiresistors), field effect transistors and emission platforms based on a molecular system with room temperature O2 gas sensitivity have a broad range of applications to, for example, monitor oxygen levels in enclosed environments or as wearable personal safety devices. In NTFET (nanotube field-effect transistor) devices, one, for example, measures electrical current through carbon nanotubes under an applied gate voltage. In chemiresistor devices, a gate voltage is not applied. In both types of devices, electrical conductance (or resistance) of nanotubes changes upon exposure to chemical analytes, thereby providing a sensor signal. Depending on the semiconducting nature of the nanotubes, application of a gate voltage can provide amplification of the sensor signal. Nanotubes such as single-walled carbon nanotubes (SWNTs) have been heralded as an ideal candidate for incorporation into extremely small and low power devices because they demonstrate extreme environmental sensitivity, high electrical conductivity, and inherent compatibility with existing microelectronic fabrication techniques.
Detection of oxygen concentrations at ambient temperature and pressure via the devices, systems and methods herein is a substantial improvement over previous attempts to form nanoelectronic sensors for O2, which, as described above, have required vacuum conditions and high temperature. The nanoelectronic sensing systems described herein can readily be incorporated into, for example, low power, micro-electronic devices having uses in a broad range of applications in civilian and/or military arenas (for example, in wearable personal safety devices and ambient O2 sensors for enclosed environments such as aircraft, submarines, mines or spacecraft). Other applications for the nanoelectronic sensing systems described herein include clinical breath monitoring (breath-by-breath oximetry), providing a practical and low-cost biodiagnostic tool. For such an application and other applications, the small size, light weight, and low power requirements of the sensors and sensing systems described herein provide significant advantages (for example, for incorporation into a wearable mask).
A schematic representation of sensor system 10 of the present invention is set forth in
A semiconducting SWNT or network of SWNTs 20 (or other nanostructures) can, for example, be disposed upon a substrate 30 (for example, quartz) and contacted by two conductive (for example, metallic such as Au and/or Ti) electrodes representing a source (S) and a drain (D). In several studied representative embodiments, a photoactive layer 24 of a lanthanide-containing dendrimer molecule (Eu8) formed with a derivative of a PAMAM dendrimer that incorporated 1,8-naphthalimide groups and lanthanide (Eu) cations (as described above) was coated upon the SWNT network 20.
In single-walled carbon nanotubes, all carbon atoms are located on the surface where current flows, making a stable conduction channel that is extremely sensitive to a surrounding chemical environment. Nanotubes, including SWNT's, have the ability to change conductance in response to interaction with (for example, absorption of) different gases. This characteristic is, for example, implemented in system 10. Support 30 can, for example, be an optically transparent, insulating layer of quartz in a FET-configured or chemiresistor circuit.
In the embodiment illustrated, energy is transmitted through the optically transparent quartz support from an energy source 40. Energy can alternatively be transmitted to chromophore-containing polymeric (or other photoactive) layer 24 directly (from above, in the orientation of
Measurements made with devices or systems including random networks of SWNTs can be advantageous because random network devices are less prone to failure as a result of the large number of conduction pathways. Additionally, while random network devices may not provide information on individual nanotube response, as with singly isolated SWNT FETs, they possess an intrinsic averaging effect in that they remove nanotube-to-nanotube variation as a result of the combined response of the entire network. As an analyte comes into contact with the device surface, SWNT conductance is modified to produce a detection signal.
The sensors or sensor systems studied herein exhibit limited sensitivity to NO2. However, the inherent sensitivity of the SWNT network to low concentrations of NO2 provides an internal standard against false measurement—ultimately providing an advantage over current solid-state platforms. Moreover, the insensitivity of the sensors or sensor systems to CO2 illustrates potential of the molecular system as a component in sensor array architectures for monitoring O2 levels in ambient atmospheres.
The representative polymer-chromophore/nanostructure systems described herein (with or without lanthanide complexing) can be deposited on many different types of surfaces or substrates, allowing flexible design and addressing the requirements of a broad range of applications. Various polymer surfaces can, for example, be both flexible and transparent. Example of flexible, translucent polymers suitable for use as substrates include polyimide (PI) and polyethylene terephthalate (PET). Small size, light weight, low power, room temperature operation, bimodal (spectroscopic and electronic) operation modes, oxygen selectivity, versatile design, can be adapted to different types and formats of supports.
As described above, in several representative embodiments, the sensors or sensor systems include a combination of SWNT and a lanthanide-containing dendrimer molecule (Eu8) formed with a derivative of a PAMAM dendrimer that incorporate 1,8-naphthalimide groups and lanthanide (Eu) cations. Such representative embodiments and studies thereof are described in further detail below. The studied representative examples of SWNT-Eu8 systems exhibited bimodal (optical spectroscopic and electrical conductance) sensitivity to molecular O2 at room temperature and ambient pressure as described above. Using representative SWNT-Eu8 based chemiresistors, linear and reversible electronic response to environmentally relevant O2 concentrations (varying, for example, between 5-27%) were demonstrated without the use of vacuum instrumentation or high temperatures.
It has been hypothesized that SWNTs donate electronic density into the photodepleted ground state of an overlying chromophore during illumination with light. Without limitation to any mechanism, the observed combined electrical and steady state spectroscopic behavior of the Eu8-SWNT devices hereof fits such hypothesis, wherein the increased device conductance, and decreased SWNT S11 band absorbance during 365 nm illumination suggests that the photodepleted ground state of the Eu8 complex exerted an attraction towards SWNT valence band electrons during illumination with 365 nm light.
The sustained Eu8-SWNT photogenerated “ON-state conductance” after illumination with 365 nm light resembles the behavior of optoelectronic memory devices composed of polymer-decorated SWNTs. In such systems, a so-called metastable ON-state persists after photoexcitation through the excitonic (separated electron-hole pairs) filling of electron traps at the SiO2 surface. SiO2 electron traps include SiOH groups, water molecules hydrogen bonded to the device surface, and a variety of other defects. The abrupt increase in the network conductance immediately following termination of UV light has recently been described as a characteristic of photo-induced exciton separation and charge trap filling in polymer-decorated SWNT devices. Moreover, the observation of a slight decay in the photogenerated ON-state conductance of the Eu8-decorated SWNT networks is a consequence of the gradual recombination of spatially separated excitons.
In contrast to polymer-decorated SWNT optoelectronic devices, which typically require an externally applied gate voltage to restore the initial conductance under ambient conditions, the photogenerated ON-state of the studied representative Eu8-SWNT system was sensitive towards O2. Without limitation to any mechanism, a model based on the relative energy levels of the Eu8 electronic states, and electron traps in the quartz substrate, is described in
After illumination, a comparable decrease was observed in both the naphthalimide photoactive moiety and in the Eu3+ centered emission bands, while the lifetime of the T3 state was not strongly affected. This observation suggests that the electron traps are located close in energy to the S1 state of the Eu8 complex. In this configuration, the traps can accept energy from the S1 state (block arrow) and inhibit intersystem crossing (ISC) into the T3 state, thereby acting as an electronic bottleneck. The trap-induced inhibition of ISC serves to decrease the electronic population in both the T3 and Eu3+-AL, producing the observed decrease in emission intensities. Subsequently, a Coulomb attraction develops between the photo-depleted S0 state and electrons in the SWNT valence band, which effectively p-dopes the SWNT valence band. This phenomenon produces the increased Eu8-SWNT network conductance during UV-illumination, sustained metastable ON-state conductance after the termination of UV-illumination, and decreased absorbance in the SWNT S11 band.
O2 passivates quartz charge traps, such as SiOH, through the introduction of non-radiative relaxation pathways. Consequently, and without limitation to any mechanism, it is proposed that the introduction of O2 results in adsorption on the device surface and passivation of the electron traps through the addition of non-radiative pathways. The adsorption of O2 removes the electron bottleneck, increases ISC, and leads to the restoration of both the naphthalimide and Eu3+ centered emission band intensities. The increased lifetime of the Eu3+ centered transition after 365 nm illumination is a consequence of O2 desorption, which removes any O2-induced non-radiative pathways in the Eu3+ electronic structure. Finally, exciton recombination in the naphthalimide S0 state eliminates the Coulombic attraction between the Eu8 ground state holes and SWNT valence band electrons, which decreases the Eu8-SWNT network conductance and increases the absorption of the SWNT S11 band O2 exposure.
To summarize the proposed response mechanism (
In several studies, the quartz surface was modified to study how an intentional increase in the number of surface electron traps (SiOH groups and H2O) would affect the Eu8-SWNT sensitivity to UV light and O2 exposure in atmospheres of 0% and 43% relative humidity. The studies indicated that devices with increased trap sites behaved in a qualitatively similar manner, but the response to an hour-long exposure to O2 was consistently greater than 100% (see
Using the Eu8-SWNT devices in a chemiresistor configuration as illustrated in
During the 200 second O2 exposure periods the device response did not saturate, however the rate of change in the network conductance scaled with the concentration of O2.
The representative Eu8-SWNT systems showed comparable photoresponse with N2 as the carrier gas and demonstrated insignificant response to CO2 and NH3. The systems were not adversely affected by relative humidity (0-43% RH), and retained good O2 sensitivity even after storing the device in ambient conditions one week after the initial measurement. As common with many solid-state O2 sensors, sensitivity towards NO2 was observed. To identify false positives resulting from the presence of oxidizing species, system 10 included both a Eu8-SWNT network 20 and a bare SWNT network 20a immobilized upon a quarts support 30a (see
The insignificant sensitivity of system 10 towards CO2 and NH3, the identifiable response to an oxidizing species (NO2), and the comparable operation in N2 and humid atmospheres indicate that system 20 provides a low-temperature platform for monitoring O2 levels under ambient conditions. In the design of a field platform, a small reservoir 60 of inert gas such as, for example, N2 or Ar can be provided to purge the sample chamber during illumination with energy source 40 which can, for example, be a compact UV light source. Systems of the present invention can, for example, be used to measure the concentration of a gas analyte in a liquid by use of a gas permeable membrane between the liquid and the decorated nano structures.
1. Synthesis of G3-PAMAM-(1,8-naphthalimide). Poly(amidoamine) (PAMAM) dendrimer G(3)-NH2 (390.5 mg, 0.0565 mmol) and 1,8-naphthalic anhydride (447.9 mg, 2.26 mmol) were suspended in DMF (25.0 mL) and stirred at 95° C. for 48 hours under a nitrogen atmosphere. The compound was purified by dialysis using a regenerated cellulose membrane (Fisher; nominal MWCO 12,000-14,000) in DMSO for three days. The solution in the dialysis membrane was dried in a vacuum oven to yield the title product as a brown solid (517.6 mg, 72.4%); 1H NMR (300 MHz; CDCl3, δ): 8.23 (br s, 64H, Ar H), 7.97 (br s, 64H, Ar H), 7.69 (m, 60H, —NH), 7.49 (br s, 64H, Ar H), 4.12 (br s, 64H, —N(CO)2CH2—), 3.51 (br s, 64H, —NHCH2—), 3.19 (m, 56H, —NHCH2—), 2.90-2.40 (m, 60H, —NCH2CH2NH—), 2.27 (m, 120H, —NCH2CH2CO—), 1.93 (m, 120H, —COCH2—).
2. Synthesis of Eu8-G3-PAMAM-(1,8-naphthalimide), Eu8. The preparation of the dendrimer complex was adapted from a synthesis protocol that described previously for a parent dendrimer ligand. Cross, J. P., Lauz, M, Badger, P. D & Petoud, S. Polymetallic lanthanide complexes with PAMAM-naphthalimide dendritic ligands: luminescent lanthanide complexes formed in solution. J. Am. Chem. Soc. 126, 16278-16279 (2004). G3-PAMAM-(1,8-naphthalimide)32 (16.81 mg, 1.326×10−6 mol) was dissolved in DMSO (5.0 mL) and a solution containing 1.397 mM of Eu(NO3)3 solution in DMSO (7.593 mL, 1.061×10−5 mol) was added. The mixture was incubated for seven days. DMSO was then evaporated in a vacuum oven, and the residual solid was dissolved in 10.0 mL of DMF to obtain 1.40×10−4 M solution.
3. SWNT Device Fabrication. Optically transparent and electrically conductive SWNT network devices were fabricated and measured as previously described in Kauffman, D. R. & Star, A. Simultaneous spectroscopic and solid-state electronic measurement of single-walled carbon nanotube devices. J. Phys. Chem. C 112, 4430-4434 (2008). Briefly, commercially available SWNTs (available from Carbon Solutions, Inc. or Riverside, California: P2 SWNTs; reported purity 70-90 percent) were suspended in DMF via sonication without further purification. 1 in2× 1/16th in thick fused quartz (SiO2) plates (available from Quartz Scientific of Fairport Harbor, Ohio; reported specific resistance of 10×1018 Ohm/cm3 at 20° C.) served as substrates 30, and were cleaned prior to SWNT deposition with acetone, rinsed with water and dried under compressed air. After spraycasting the SWNT networks with a commercial air brush (Iwata) onto the heated quartz plates, Al tape and Ag paint were used to form the device electrodes. To create devices with two SWNT networks a cotton tipped applicator soaked in acetone was used to wipe clean a section of the spray-cast SWNT network. Two devices were created from the bisected SWNT network by individually connecting electrodes to each section with Al tape and Ag paint. Quartz plates with additional hydroxyl surface groups were created by soaking overnight in a 3:1 solution of concentrated H2SO4 and 30% H2O2 (piranha solution). Nanotube field-effect transistor (NTFET) devices consisted of interdigitated Au electrodes (10 μm pitch size) on a Si/SiO2 substrate; dilute suspensions of Carbon Solutions P2 SWNTs in DMF were dropcast onto heated devices to form the conduction channel.
4. Device Decoration and Measurement. SWNT devices were decorated as follows: the devices were heated to just above the solvent boiling temperature and 200 μL (4 μL for NTFETs) of a particular molecule was dropcast evenly onto the surface of the device. For NTFET devices, all measurements were conducted in ambient conditions with a drain-source bias voltage of 100 mV with two Keithley model 2400 SOURCEMETER® text instruments, available from Keithley Instruments, Inc. of Cleveland, Ohio, interfaced to LABVIEW® software available from National Instruments of Austin, Tex.
UV-vis-NIR absorption spectra were recorded with a Perkin Elmer Lambda 900 UV-vis-NIR spectrophotometer, and steady-state excitation and emission spectra, as well as the luminescence lifetime measurements were recorded using a custom designed JY Horiba Fluorolog-322 spectrofluorimeter and a Tektronix TDS model 754D oscilloscope. At least 1000 luminescence decay traces, each containing 50,000 points were average and treated to calculate the lifetimes ORIGIN® 7.0 software available from Originlab Corporation of Northampton, Mass. The reported lifetime for a particular excited state is the average of at least two independent measurements. For multi-exponential fittings, the amplitude of the major component was used as a criteria for isolating the values reported in Table 1; components with amplitudes less than 1% were discarded. The Eu3+-AL lifetimes were deconvoluted by subtracting the component from the triplet state (T3) lifetime. Time-resolved excitation and emission spectra of the Eu8 solutions were measured using a Varian Cary Eclipse spectrofluorimeter.
For the optically transparent SWNT devices, the UV-exposure and gas sensitivity measurements were performed in a custom built gas delivery chamber as described in Kauffman, D. R. & Star, A. Simultaneous spectroscopic and solid-state electronic measurement of single-walled carbon nanotube devices. J. Phys. Chem. C 112, 4430-4434 (2008) that was housed inside of the spectrometers for simultaneous electrical and optical measurements; the device conductance was measured at a bias voltage of 500 mV with a Keithley model 2400 SOURECEMETER interfaced to LABVIEW 7.1 software. The network conductance of two devices on a single quartz substrate was simultaneously measured at 500 mV with a Keithley 2602 SOURCEMETER and a Keithley 708A switching matrix using Zephyr data acquisition software. The atmosphere inside the chamber was controlled with flowing research grade gases at a constant flow rate of 1000 standard cubic centimeters per minute (SCCM); all gases were dry unless otherwise noted. Atmospheres of 43% RH were created by passing the gases over the headspace of sealed container of saturated K2CO3 solution; literature RH: 43.2±0.3% at 20° C. The UV lamp used for device illumination was a UVP, Inc. Model UVGL-55 hand held unit (365 nm; 250 μW/cm2). All measurements were conducted at room temperature and ambient pressure. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) were conducted on a Phillips XL30 FEG microscope operated at an accelerating voltage of 10 kV; samples were sputter coated with Pd prior to imaging to prevent charging of the insulating quartz substrate.
5. Solution Phase Behavior of Eu8.
6. Solid-State Behavior of Eu8. The relative emission intensity of solid-state Eu8 (dropcast onto a quartz substrate) was constant when cycled between atmospheres of pure O2 and Ar, highlighting a fundamental difference between the behavior of solid-state and solution phase samples. However, after illuminating the sample with 365 nm light for 30 minutes (in flowing Ar), the emission profile of the solid-state Eu8 developed a sensitivity towards O2, in which the intensity was decreased after illumination and partially restored under flowing O2. The observed behavior is interesting compared to the solution phase behavior of Eu8, and to the solid-state behavior of other O2 sensitive photoluminescent complexes. To further explore the solid-state behavior of Eu8, solutions were dropcast onto optically transparent and electrically conductive SWNT devices as described in Kauffman, D. R. & Star, A. Simultaneous spectroscopic and solid-state electronic measurement of single-walled carbon nanotube devices. J. Phys. Chem. C 112, 4430-4434 (2008).
Minimal changes were observed in the spectroscopic signatures of both the SWNT network and the overlying Eu8 layer after the devices were decorated, and separate field-effect transistor measurements of Eu8 decorated SWNT networks suggest that the dominant effect of the overlying Eu8 layer was to introduce charge scattering sites along the SWNT network.
7. Behavior of Eu8 Decorated SWNT Networks. Using simultaneous UV-vis-NIR absorbance spectroscopy and network conductance measurements on Eu8-SWNT devices, the underlying SWNT network was able to transduce changes in the electronic structure of the Eu8 layer during illumination with 365 nm light and exposure to pure O2 gas. After a 30 minute illumination period the device experienced a decrease in the first semiconducting SWNT absorption band, labeled S11 in
After a 30 minute exposure to 365 nm light, the device conductance decreased rapidly in the presence of O2 such that nine individually tested devices experienced a 60±10% decrease after a 200 second exposure to pure flowing O2, regardless of the initial network conductance. After an hour long exposure to pure O2, nearly 100% restoration of the S11 band absorbance and approximately 90% recovery of the initial device conductance was observe. This electrical behavior was also reproducible, but after 1 hour of O2 exposure, some devices demonstrated a larger than 100% response that possibly stemmed from increased defect sites in the quartz substrate. The absorbance change of the SWNT S11 band displayed a time dependency like that of the network conductance during O2 exposure, which indicates that the device response stems from perturbations in the electronic density of the SWNT valence band rather than a modification of the potential barriers at the interface between the SWNT network and device electrodes. Throughout these experiments, the intensity of the M11 transition remained constant. While the metallic component of the SWNT network may contribute to the electrical response of the SWNT devices, the finite Fermi-level electron density of metallic SWNTs (inset in
8. Luminescence Lifetimes. The presence of the Eu3+ ions in the Eu8-SWNT networks allowed a quantitative comparison between the luminescence lifetimes of the Eu8 triplet state and the Eu3+ acceptor level (abbreviated as T3 and Eu3+-AL, respectively). In essence, the presence of the Eu3+ emission serve as a spectroscopic beacon that can help identify the electronic processes that occur in the Eu8-SWNT system by analyzing the two states' luminescence lifetimes. The luminescence lifetimes of the Eu8 T3 and Eu3+-AL were measured before and after illumination with 365 nm light (in flowing Ar), and after the reintroduction of pure O2 gas. The data presented in Table 1 shows that the luminescence lifetime of the T3 state was not strongly affected by 365 nm illumination or the presence of O2. However, an increase in the lifetime of the Eu3+-AL after 365 nm illumination, was observed which began to decrease towards its initial value after 1 hour of O2 exposure.
Quartz contains defects such as SiOH groups, among others, on the surface that can act as electron traps. These functionalities were confirmed spectroscopically. Soaking quartz in a 3:1 solution of concentrated H2SO4 and 30% H2O2 at room temperature further hyrdroxylates the surface (H-quartz), and produces a larger number of SiOH functionalities. This treatment renders the quartz surface more hydrophilic, because additional water molecules can hydrogen bond to the surface.
The foregoing description and accompanying drawings set forth embodiments at the present time. Various modifications, additions and alternative designs will, of course, become apparent to those skilled in the art in light of the foregoing teachings without departing from the scope hereof, which is indicated by the following claims rather than by the foregoing description. All changes and variations that fall within the meaning and range of equivalency of the claims are to be embraced within their scope.
This application claims benefit of U.S. Provisional Patent Application Ser. No. 61/180,214, filed May 21, 2009 and U.S. Provisional Patent Application Ser. No. 61,251,165, filed Oct. 13, 2009, the disclosures of which are incorporated herein by reference.
Number | Date | Country | |
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61180214 | May 2009 | US | |
61251165 | Oct 2009 | US |