Infrared molecular binding spectroscopy realized in sorbent coated microfabricated devices
2014
Sorbent materials are utilized in a range of analytical applications including coatings for preconcentrator devices,
chromatography stationary phases, and as thin film transducer coatings used to concentrate analyte molecules of interest
for detection. In this work we emphasize the use of sorbent materials to target absorption of analyte vapors and examine
their molecular interaction with the sorbent by optically probing it with infrared (IR) light. The complex spectral
changes which may occur during molecular binding of specific vapors to target sites in a sorbent can significantly aid in
analyte detection. In this work a custom hydrogen-bond (HB) acidic polymer, HCSFA2, was used as the sorbent.
HCSFA2 exhibits a high affinity for hazardous vapors with hydrogen-bond (HB) basic properties such as the G-nerve
agents. Using bench top ATR-FTIR spectroscopy the HFIP hydroxyl stretching frequency has been observed in the mid
wave infrared (MWIR) to shift by up to 700 wavenumbers when exposed to a strong HB base. The amount of shift is
related to the HB basicity of the vapor. In addition, the large analyte polymer-gas partition coefficients sufficiently
concentrate the analyte in the sorbent coating to allow spectral features of the analyte to be observed in the MWIR and
long wave infrared (LWIR) while it is sorbed to HCSFA2. These spectral changes, induced by analyte-sorbent
molecular binding, provide a rich signal feature space to consider selective detection of a wide range of chemical species
as single components or complex mixtures. In addition, we demonstrate an HCSFA2 coated microbridge structure and
micromechanical photothermal spectroscopy to monitor spectral changes when a vapor sorbs to HCSFA2. Example
ATR-FTIR and microbridge spectra with exposures to dimethylmethylphosphonate (DMMP – G nerve agent simulant)
and other vapors are compared. In a generic form we illustrate the concept of this work in Figure 1. The results of this
work provide the potential to consider compact detection systems with high detection fidelity.
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