ELECTROCHEMICAL DETECTION OF CARBON DIOXIDE USING A CARBOHYDRATE BASED COORDINATION POLYMER

Abstract

An electrochemical sensor for an analyte is provided. The electrochemical sensor includes CDMOF-2. The CDMOF-2 is capable of binding reversibly to CO2 as an analyte, thereby quantitatively detecting the analyte in a mixture. The CDMOF-2 is formed from reaction of γ-cyclodextrin with RbOH in the presence of methanol.

Description

BACKGROUND

1. Technical Field

The present disclosure relates to methods for preparing metal organic frameworks as sensors for the quantitative detection of analytes, particularly CO₂.

2. Description of Related Art

The detection of carbon dioxide within mixtures of gases has proven difficult owing to the presence typically of competing oxygen, carbon monoxide and water vapor. It stands to reason, therefore, that the clear benefits of having robust and inexpensive devices to provide a quantitative analysis of CO₂ concentrations in admixture with other gases provides more than enough impetus for the continued development of such devices. Much of the present sensing technology depends largely upon spectroscopic methods that become unreliable when the mixture of gases contains spectroscopically similar resonances. The complication and expense of fabricating the necessary devices for the detection of CO₂ in applications where they could be most useful makes these devices highly sought after in their own right. For example, in the medical arena, new sensing technologies could improve human health by enabling the analysis of human breath when a patient is showing clinical signs of hypercapnia. Likewise, in the field of occupational safety, the buildup of CO₂ emissions in the form of “blackdamp” in the mining and petroleum industries can be hazardous, particularly as we progress more and more toward coal liquefaction and the extraction of natural gas from shale oil deposits. Here, we describe a method to detect carbon dioxide within CO₂/N₂ and CO₂/air mixtures using a recently described metal organic framework⁵ (MOF) composed of cyclodextrin (CD) and alkali metal (Group1A) cations.

Smaldone et al. (Angew. Chem., Int. Ed. 2010, 49:8630-8634) reported of a new cyclodextrin derived material called CDMOF-2 that exhibits strong but reversible binding of carbon dioxide. Though the authors were able to crudely demonstrate a colorimetric response of CDMOF-2 in the presence of CO₂ by taking advantage of the unique chemistry that occurs within this highly porous material (Gassensmith et al., J. Am. Chem. Soc. 2011, 133:153121-5315), this response is by no means sufficient for practical quantitative analysis. This highly porous material belongs to a rapidly growing family of MOFs that are highly crystalline materials that are well structured chemically with building blocks—typically clusters of metal ions (components of the nodes) and rigid organics ligands (components of the extendable structural frameworks). Important features of MOFs are their (i) highly ordered nanoporosity, (ii) their large internal surface area, and (iii) the possibility of modifying their organic ligands post-synthetically. Accordingly, MOFs have been evaluated as potential nanoporous materials for applications in chemical separations, gas adsorption heterogeneous catalysis, ion exchange, drug delivery, ionic conduction, and sensing. More specifically, MOFs are receiving a lot of attention as a method for CO₂ sequestration and colorimetric sensing.

No instances of conductance-based MOFs for CO₂ detection have been reported in the literature. The need for an electrochemical means of sensing CO₂ comes, in part, from an emerging environmental requirement to monitor concentrations at and near high volume, emission point sources, and from the limitations in present state of the art technologies. Although chemiresistive metal oxides and semiconducting field effect transistors have been thoroughly investigated, they have a limitation to their application—namely, from reaction with ambient oxygenated species absorbed on the oxide surface. In order to circumvent this limitation, semiconducting oxide sensors usually operate at temperatures in excess of 200° C. As an alternative, MOFs have been shown to be an excellent choice for sensing analytes at relatively low temperatures.

The majority of previously-designed MOFs for the selective uptake of CO₂ have not exhibited reversible chemisorption desorption of carbon dioxide.

BRIEF SUMMARY

In a first aspect, an electrochemical sensor for an analyte is provided. The electrochemical sensor includes CDMOF-2.

In a second aspect, a method of quantitatively detecting an analyte in a medium is provided. The method includes the step of contacting a CDMOF-2 with the medium.

These and other features, objects and advantages of the present invention will become better understood from the description that follows. In the description, reference is made to the accompanying drawings, which form a part hereof and in which there is shown by way of illustration, not limitation, embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, objects and advantages other than those set forth above will become more readily apparent when consideration is given to the detailed description below. Such detailed description makes reference to the following drawings.

FIG. 1 depicts an exemplary schematic diagram illustrating the equilibrium proposed to exist during the chemisorption of CO₂ by CDMOF-2, expressed in the context of the structural formula of one of the four repeating maltosyl units present in a single CD torus. The [(Rb⁺)₄(CD)]₆ unit in which the six CD rings forming the sides of the cube are portrayed in different colors, wherein [(Rb⁺)₄(CD)]_6n is shown assembled in a planar fashion. The C and O atoms in the framework are depicted in different colors; the Rb atoms are depicted in larger purple color; the O atoms of the hydroxyl group available for CO₂ binding are depicted in red.

FIG. 2A depicts an exemplary impedance spectrum of a pristine CDMOF-2 sample following exposure to methanol vapor at room temperature.

FIG. 2B depicts an exemplary impedance spectrum of a CO₂-infused CDMOF-2 sample following exposure to methanol vapor at room temperature.

FIG. 3 depicts an exemplary plot showing the cyclic change of conductivity of a CDMOF-2 sample following sequential CO₂ sorption and desorption.

FIG. 4 depicts an exemplary exponential scale plot of average conductivity values in CDMOF-2 samples after their exposure for 5 min to CO₂ gas that was diluted with N₂ in various concentrations.

While the present invention is amenable to various modifications and alternative forms, exemplary embodiments thereof are shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description of exemplary embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the invention as defined by the embodiments above and the claims below. Reference should therefore be made to the embodiments and claims herein for interpreting the scope of the invention.

DETAILED DESCRIPTION

The compositions and methods now will be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all permutations and variations of embodiments of the invention are shown. Indeed, the invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. These embodiments are provided in sufficient written detail to describe and enable one skilled in the art to make and use the invention, along with disclosure of the best mode for practicing the invention, as defined by the claims and equivalents thereof.

Likewise, many modifications and other embodiments of the compositions and methods described herein will come to mind to one of skill in the art to which the invention pertains having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of skill in the art to which the invention pertains. Although any methods and materials similar to or equivalent to those described herein can be used in the practice or testing of the present invention, the preferred methods and materials are described herein.

Moreover, reference to an element by the indefinite article “a” or “an” does not exclude the possibility that more than one element is present, unless the context clearly requires that there be one and only one element. The indefinite article “a” or “an” thus usually means “at least one.”

As used herein, “about” means within a statistically meaningful range of a value or values such as a stated concentration, length, molecular weight, pH, sequence identity, time frame, temperature or volume. Such a value or range can be within an order of magnitude, typically within 20%, more typically within 10%, and even more typically within 5% of a given value or range. The allowable variation encompassed by “about” will depend upon the particular system under study, and can be readily appreciated by one of skill in the art.

As used herein, “CD-MOF” or “CDMOF” refers to γ-cyclodextrin-based metal-organic framework comprising a plurality of metal cations and a plurality of γ-cyclodextrin molecules. The compound referred to as “CD-MOF-2” or “CDMOF-2” comprises γ-cyclodextrin-based metal-organic framework comprising a plurality of metal cations and a plurality of γ-cyclodextrin molecules, wherein the metal cations are Rb⁺ cations. The compound referred to as “CA-CDMOF-2” is a carbonic acid derivative of CDMOF-2 formed by covalent binding of CO₂ to at least one non-coordinated free primary hydroxyl group present in the γ-cyclodextrin portion of CDMOF-2.

Overview

Applicants developed a new electrochemical sensor and method for detecting CO₂ using CDMOF-2, which is a γ-cyclodextrin-Rb metal oxide framework (MOF) complex formed from reacting γ-cyclodextrin and RbOH in the presence of methanol. The free primary alcohol hydroxyl groups present in the CDMOF-2 can reversibly bind to CO₂ to form carbonic acid derivatives of CDMOF-2. The “as synthesized” CDMOF-2 that exhibits high proton conductivity in pore filling methanolic media displays a dramatic decrease in its ionic conductivity on binding CO₂. This fundamental property has been exploited to create ratiometric sensor capable of measuring CO₂ concentrations quantitatively even in the presence of water and ambient oxygen.

Compositions and Methods of Synthesis

CDMOF-2 can be prepared by reaction of γ-cyclodextrin [γ-CD] and rubidium hydroxide (RbOH) at room temperature in aqueous methanol (or ethanol). γ-CD is a cyclic oligosaccharide composed of eight D-glucopyranosyl residues linked 1,4 to each other. Rb⁺ cations bind with γ-CD tori by coordinating to some of the ring oxygen atoms together with some of the secondary and primary hydroxyl groups at C-2, C-3 and C-6 on the glucopyranosyl rings. The coordination sphere round a particular Rb⁺ cation is satisfied by eight oxygen atoms from four different γ-CD tori. This coordination geometry gives rise to a unit cell for CDMOF-2 (FIG. 1, panel (i)), comprised of six CD tori and 24 Rb⁺ cations forming a cubic cage inside of which there exists a ˜17 Å diameter void with two kinds of windows—a large circular one (windows) of diameter 7.8 Å and a smaller triangular-shaped one (windows) that is 4.2 Å from the apex to the opposite side of the triangle. Body centered cubic (bcc) close packing of the (γ-CD) unit cells produces a framework with the larger circular windows and the smaller triangular-shaped windows aligned, respectively, along the 100 and 111 axes in the crystal. ¹³C NMR into the extended framework of CDMOF-2 results in covalent bonding of the CO₂ to the non-coordinated free primary hydroxyl groups, forming carbonic acid (CA) functions on the γ-CD tori to yield CA-CDMOF-2 (FIG. 1, (panel (ii)). For details of the synthesis and characterization of CDMOF-2, see U.S. Pat. No. 9,085,460 to Stoddart et al., entitled, “NANOPOROUS CARBOHYDRATE FRAMEWORKS AND THE SEQUESTRATION AND DETECTION OF MOLECULES USING THE SAME”, the contents of which is incorporated by reference in its entirety.

Methods of CO₂ Detection Using CDMOF-2

Generally, MOFs containing hydroxyl functional group in their frameworks have been found to release protons into their nanopores or nanochannels with relatively low activation energies and thereby exhibit proton conduction. In a systematic investigation of the ionic conductivity of CDMOF-2, the ‘as synthesized’ version of CDMOF-2 and a CO₂ gas-infused species, CA-CDMOF-2, with methanol or n-hexane as a pore filling solvent was evaluated. Based on the greater acidity of carbonic acids relative to that of the primary alcohols, the CO₂-infused CA-CDMOF-2 was expected to show higher conductivity than pristine CDMOF-2. Surprisingly, CDMOF-2 exhibits a ˜550-fold higher conductivity compared to that exhibited by CA-CDMOF-2 (see FIG. 2 and Table 1). The conductivity of pristine CDMOF-2 was measured to be ˜4.8 μScm⁻¹ while CA-CDMOF-2 is only 9 nScm⁻¹.

TABLE 1
Conductivities of CDMOF-2 samples before and after exposure
to gas phase CO2 for 5 min in various CO2 concentrations.
CO2 conc.	σ (nScm−1)
(%)a	Sample 1	Sample 2	Sample 3	Ave.	Ratio
0	4740	4569	5095	4801	550
10	1069	1129	1248	1149	130
20	228	198	223	216	24
30	115	99	110	108	12
40	63	49	57	57	6.5
60	21	24	20	22	2.5
90	11	12	10	11	1.2
100	9	9	8	9	1.0
aThe concentration of the CO2 gas was controlled by mass flow controller, diluting with N2 gas.

Without the claimed subject matter being bound or limited by any particular theory, this observation is tentatively attributed to the blockage of the hydrophilic, triangular-shaped windows with carbonates as the reaction proceeds. These windows are the sole location of all uncomplexed, primary alcohol functions and, consequently, the place where carboxylation occurs primarily.

An attractive property of CDMOF-2 is the high degree of reversibility found in the chemisorption of CO₂. This reversibility, which occurs rapidly under very mild conditions, liberating the sequestered CO₂, is important in the continuous reusability of CDMOF-2. To this end, the cyclic changes in conductivity by a pellet of CDMOF-2 as a result of undergoing multiple chemisorption/desorption experiments of CO₂ were conducted. A pellet sample was prepared and the sample was exposed to CO₂ gas (99.8%, bone dry) for chemisorption, tested and then heated at ˜80° C. for desorption and tested again. The conductivity of pristine CDMOF-2, which initially showed a high value, was decreased by a factor of ˜550-fold after CO₂ sorption, but was once again reinstated after CO₂ desorption. This process is completely reversible (FIG. 3) over many iterations with no degradation in performance.

These experiments suggests that this MOF acts as a sensor for CO₂ in the presence of air, from which no effort was made to exclude extraneous water vapor or oxygen. With these results in hand, the device was tested to determine if it displays ratiometric responses to the amount of CO₂ present in a mixture of gases. Thus, the MOF was tested and the conduction responses at various CO₂ gas concentrations were recorded. The conductivity dropped down sharply (FIG. 4) as the CO₂ concentration was in creased. The slope shows a rough exponential decay as the concentration is decreased with better sensitivity in the low CO₂ concentration regime, compared with that at high concentrations. Based on this observation, the sensitivity of the MOF-sensor will be affected by several factors including: (i) the reactivity of carboxylation, (ii) the diffusion rate of CO₂ gas into pellets, (iii) the pellet thicknesses, and (iv) the exposure time of the sample to CO₂ gas. Based on the assumption that the reaction rate of carboxylation would be much faster than the CO₂ diffusion rate, the slope was used as a criterion of sensitivity in low CO₂ concentration region, which is expected to be steep when the pellet thickness is thinner and/or CO₂ exposure time is longer. Thin pellets or films will be more sensitive as a real time reporter. Accordingly, CDMOF-2 comprises a crystalline matrix preferably having a pellet or film form.

Applications

In a first aspect, an electrochemical sensor for an analyte is provided. The electrochemical sensor includes CDMOF-2. In a first respect, the analyte comprises CO₂. In a second respect, CDMOF-2 is prepared according to the process that includes two steps. The first step includes forming an aqueous mixture of γ-cyclodextrin and RbOH. The second step includes vapor-diffusing methanol into the aqueous mixture. In a further elaboration of the first aspect, the CDMOF-2 includes being electrically contacted with indium-coated copper wire and conductive silver epoxy. In a third respect of the first aspect, the CO₂ resides in a medium. In a further elaboration of the third respect of the first aspect, the medium includes a gas phase selected from air, air/H₂O mixture, air/O₂ mixture and CO₂/N₂ mixture. In some respects, the medium includes a gas phase consisting of CO₂/N₂ mixture. In some respects, the sensor displays a ratiometric response to an amount of CO₂ present in the gas phase. In some respects, the amount of CO₂ present in the gas phase ranges from about 0% to about 100% CO₂. In some respects, the CDMOF-2 displays a conductivity decrease of ˜550-fold after CO₂ sorption.

In a second aspect, a method of quantitatively detecting an analyte in a medium is provided. The method includes the step of contacting a CDMOF-2 with the medium. In some respects, the analyte includes CO₂. In some respects, the medium includes a gas phase selected from air, air/H₂O mixture, air/O₂ mixture and CO₂/N₂ mixture. In some respects, the medium includes a gas phase consisting of CO₂/N₂ mixture. In some respects, the electrochemical sensor is made according to a process that includes several steps. The first step includes forming an aqueous mixture of γ-cyclodextrin and RbOH and vapor-diffusing methanol into the aqueous mixture, wherein the CDMOF-2 is formed. The second step includes electrically contacting the CDMOF-2 with indium-coated copper wire and conductive silver epoxy. In some respects, the sensor displays a ratiometric response to an amount of CO₂ present in the gas phase. In this respect, the amount of CO₂ present in the gas phase ranges from about 0% to about 100% CO₂. In this respect, the CDMOF-2 displays a conductivity decrease of ˜550-fold after CO₂ sorption.

EXAMPLES

The invention will be more fully understood upon consideration of the following non-limiting examples, which are offered for purposes of illustration, not limitation.

Example 1

Materials and Methods

Materials.

All solvents and reagents were obtained from commercial sources (Sigma Aldrich, Wacker Chemical, Cambridge Isotopes, Airgas) and were used without further purification. Anhydrous MeOH (99.8%, Aldrich) was used after further drying with vigorously dehydrated zeolite 4A in moisture-free, argon-charged glove box. For the preparation of CDMOF-2, first, γ-cyclodextrin (1 mmol) and RbOH (8 mmol) were dissolved in deionized H₂O (20 mL). After the solution had been filtered through a 13-mm syringe filter (0.45 m PTFE membrane) into pre-washed borosilicate culture tubes (16×150 mm), MeOH (ca. 50 mL) was allowed to vapor-diffuse into this gaseous solution over a period of two weeks. Eventually, colorless cubic single crystals were obtained and they were washed with MeOH twice prior to measuring the ionic conductivity. Pellets for conductivity measurements were electrically contacted using indium-coated copper wire (diameter=0.25 mm, Arcor) and conductive silver epoxy (type A and B, Chemtronix).

Instrumentation.

Dilution of CO₂ gas with N₂ was carried out using a mass flow controller (MKS instruments, M100B Mass-Flo® and Type 247 Power Supply/Readout for MKSMFC). Impedance spectra of the samples were recorded at ca. 50 mV in the frequency range of 0.1-2×10⁶ by using a 1286 Electrochemical interface-equipped SI1260 Impedance/Gain Phase analyzer (Solartron Analytical). Measurements were performed at room temperature (ca. 296K).

REFERENCES

• Gassensmith J J, Kim J Y, Holcroft J M, Farha O K, Stoddart J F, Hupp J T, Jeong N C. “A metal-organic framework-based material for electrochemical sensing of carbon dioxide,” J. Am. Chem. Soc. 136:8277-82 (2014).

All of the patents, patent applications, patent application publications and other publications recited herein are hereby incorporated by reference as if set forth in their entirety.

The present invention has been described in connection with what are presently considered to be the most practical and preferred embodiments. However, the invention has been presented by way of illustration and is not intended to be limited to the disclosed embodiments. Accordingly, one of skill in the art will realize that the invention is intended to encompass all modifications and alternative arrangements within the spirit and scope of the invention as set forth in the appended claims.

Claims

1. An electrochemical sensor for an analyte, comprising CDMOF-2.

2. The electrochemical sensor of claim 1, wherein the analyte comprises CO2.

3. The electrochemical sensor of claim 1, wherein CDMOF-2 is prepared according to the process comprising: forming an aqueous mixture of γ-cyclodextrin and RbOH; andvapor-diffusing methanol into the aqueous mixture.

4. The electrochemical sensor of claim 1, further comprising the CDMOF-2 being electrically contacted with indium-coated copper wire and conductive silver epoxy.

5. The electrochemical sensor of claim 2, wherein the CO2 resides in a medium.

6. The electrochemical sensor of claim 5, wherein the medium comprises a gas phase selected from air, air/H2O mixture, air/O2 mixture and CO2/N2 mixture.

7. The electrochemical sensor of claim 5, wherein the medium comprises a gas phase consisting of CO2/N2 mixture.

8. The electrochemical sensor of claim 7, wherein the sensor displays a ratiometric response to an amount of CO2 present in the gas phase.

9. The electrochemical sensor of claim 8, wherein the amount of CO2 present in the gas phase ranges from about 0% to about 100% CO2.

10. The electrochemical sensor of claim 1, wherein the CDMOF-2 displays a conductivity decrease after CO2 sorption.

11. A method of quantitatively detecting an analyte in a medium, comprising: contacting a CDMOF-2 with the medium.

12. The method of claim 11, wherein the analyte comprises CO2.

13. The method of claim 11, wherein the medium comprises a gas phase selected from air, air/H2O mixture, air/O2 mixture and CO2/N2 mixture.

14. The method of claim 11, wherein the medium comprises a gas phase consisting of CO2/N2 mixture.

15. The method of claim 11, wherein the electrochemical sensor is made according to a process comprising: forming an aqueous mixture of γ-cyclodextrin and RbOH,vapor-diffusing methanol into the aqueous mixture,wherein the CDMOF-2 is formed; andelectrically contacting the CDMOF-2 with indium-coated copper wire and conductive silver epoxy.

16. The method of claim 15, wherein the sensor displays a ratiometric response to an amount of CO2 present in the gas phase.

17. The method of claim 15, wherein the amount of CO2 present in the gas phase ranges from about 0% to about 100% CO2.

18. The method of claim 15, wherein the CDMOF-2 displays a conductivity decrease after CO2 sorption.