BACKGROUND

Sulfonated and perfluorosulfonic acid (PSA) polymers, such as the commercially available compounds Amberlyst® and Nafion®, respectively, have been of long-standing interest as sustainable solid acid catalyst alternatives to toxic, non-reusable homogeneous acid catalysts in the production of high-volume specialty chemicals such as methyl tert-butyl ether (MTBE) and Bisphenol-A. As the demand for portable energy increases, these materials are also attracting renewed attention as heterogeneous biodiesel synthesis catalysts in place of homogenous alkali for renewable energy applications. These materials eliminate the extensive work-up procedures required to introduce acidity into more typical zeolite and mixed metal oxide catalysts, yet retain the advantage of clean product separation afforded by heterogeneous catalysts. In addition, these materials have exhibited significant activity in the presence of fatty acid impurities, which are common in low-cost vegetable oil feedstock. PSA polymer catalysts can be used a broad range of technologically important organic synthesis reactions.

The present investigators recently demonstrated that PSA polymer membranes can be used as both solid acid catalysts and durable scaffold materials in the preparation of optical sensors. Earlier work resulted in the discovery that by immobilizing a dihydroxy-benzene reagent, into a PSA polymer membrane; a controlled organic synthesis occurs and a selective product of this reaction (e.g., a flavan in the case of acetone and various quinones in the case of anhydrides) is found to produce a visible spectrum color change which could be employed to measure target molecule concentrations to less than parts per billion (ppb). This approach was utilized to develop portable optical sensing devices for real-time, non-invasive analyses in medical diagnostics, occupational settings, and environmental testing.

A key drawback, however, to the use of solid acid polymers as catalysts is that they can be readily deactivated by the presence of water as well as salt impurities present in the environment. Without being bound by theory, it is believed that cations such as K+and Ca++can exchange with protons in the polymer and reduce or ultimately eliminate polymer acidity. The same super-acidic nature that makes these materials highly active catalysts also results in their ready de-protonation by salts. Furthermore, water incorporation can dilute local acidity within the membrane. For example, an ambient relative humidity less than 15% has been found sufficient to completely deactivate an 1100 Equivalent Weight sample of Nafion for acetone condensation with resorcinol in an optical sensing device of the prior art. Furthermore, not only catalyst de-activation but proton conductivity may be completely disrupted due to cation exchange of protons with impurities such as K+and Ca++present in the environment. Such behavior has limited the use of PSA polymer catalysts to toxic and flammable organic solvents or to high temperatures, and has also prevented commercial realization in real environments of the previously developed optical sensing elements for portable, real time gas analysis. In particular, exhaled breath for medical diagnostic purposes has relative humidity levels that approach 100% and a method has yet to be developed to provide accurate real-time trace gas analyses in such an environment. Further, although detection and measurement of formaldehyde concentrations in dry environments is possible, it would be desirable to test for presence and amount of formaldehyde in humid environments, such as in fuel cell effluent where concentration is a measure of efficiency loss in the fuel cell.

Hence, there remains a need in the art for PSA polymer catalysts capable of retaining catalytic activity in humid environments. Further, there is a need in the art for portable non-invasive optical sensors capable of reliably measuring concentration of volatile organic compounds in high-humidity and/or high-salt environments associated with medical diagnostics and fuel cell efficiency determinations.

SUMMARY

Accordingly, one broad embodiment of the invention provides PSA polymer membranes reformulated to maintain their catalytic activity even in high-humidity and potentially high-salt environments. Further embodiments provide optical sensing devices comprising certain inventive PSA membranes designed to produce a color-shift reaction product in the presence of target compounds in humid environments. Specific clinical and environmental applications of optical sensing device according to the invention designed to detect, for example, acetone in exhaled human breath and formaldehyde in fuel cell effluent are also described.

In particular, an embodiment directed to a method of preserving the catalytic activity of a solid sulfonic acid catalyst polymer in a humid environment is provided. Broadly characterized the method comprises immobilizing an organic acid in the solid sulfonic acid catalyst polymer, wherein the pKa of the immobilized acid is greater than the pKa of the solid sulfonic acid catalyst polymer.

Another embodiment provides optical sensors for detecting target compounds in a humid environment. The optical sensor comprises a PSA polymer catalyst membrane comprising an immobilized organic acid having a pKa greater than the pKa of the PSA polymer membrane, and an immobilized organic reagent. The organic reagent is capable of reacting with the target compound to produce a color shifted product such that exposure of the optical sensor to the target compound in a humid environment produces a detectable color shift on the optical sensor. The optical sensor may be fabricated into a portable optical sensing device. Specific embodiments with utility in medical diagnostics, detection of environmental contamination, and analysis of fuel cell efficiency are disclosed.

A further embodiment provides a real-time non-invasive method for estimating blood glucose concentration in a human based on a calculated concentration of acetone in exhaled breath of the human in accordance with particular aspects of the invention. The method comprises: providing a PSA membrane comprising immobilized organic reagent and at least one immobilized organic acid having a pKa greater than the pKa of the PSA polymer membrane, the immobilized reagent selected such that a reaction occurs between the organic reagent and acetone to produce a detectable color-shifted product; exposing the PSA polymer membrane to a volume of exhaled breath; measuring concentration of a product of the reaction based on a detected color shift; calculating the concentration of acetone in the volume of exhaled breath from the concentration of product measured; and correlating the concentration of acetone to a standard blood glucose concentration. The inventive methods, sensors and membranes may be used as part of a treatment regimen for diabetic patients in the monitoring of blood glucose levels in the patient across a time frame and adjusting the patient's insulin levels accordingly.

These and other aspects and embodiments are described more clearly and will be further understood by reference to the Detailed Description. Figures and Examples are provided herein to illustrate particular aspects and features of embodiments of the invention, and should not be construed as limiting of the scope of the invention as defined by the instant claims.

All references (e.g., printed publications such as books, papers, patents, patent applications, catalogs, databases) are incorporated herein by reference. In the event of a conflict or inconsistency, the present specification, as modified by any amendments thereto, shall control.

