Surface-Enhanced Infrared Absorption of p-Nitrobenzoic Acid on Planar Silver Halide Fiber

Abstract

Surface-enhanced infrared absorption (SEIRA), recently discovered for trypsin on a planar silver halide surface, is now demonstrated for the small organic molecule, p-nitrobenzoic acid (pNBA). Evaporation of pNBA solutions in acetonitrile yields the acid dimer (confirmed by the unique IR signature of carboxylic acid dimers). The absorption changes for pNBA with quantity exhibit two regimes: "enhanced" and "regular". A plot of absorbance versus quantity reveals how SEIRA changes with distance from the surface. The surface orientation of pNBA has the long and short axes (unit cell) parallel to the surface (packing density/carbonyl peaks). "Slice" spectra differentiate between surface pNBA and other layers. Most of the numerous previous reports on pNBA actually refer to the p-nitrobenzoate anion probably generated through reaction of surface silver oxide with the acid.

p‐Nitrobenzoic Acid Adsorption on Nanostructured Gold Surfaces Investigated by Combined Experimental and Computational Approaches

“…This supports a process well known in the literature: the dimerization of carboxylic acids. [41] It must be noted that for the spectrum of p-NBA (383 ng cm À2 ) we do not observe a broad OH stretching band in the 2500-3300 cm À1 region (Figure 4 a) ) on the rough Au surface, although Osawa et al [3] did not observe the antisymmetric modes of p-NBA molecules chemisorbed on Ag film. It should be noted that in [33] COOH [35] 1680 1686 -1690 [3] -C =O 1642 ----str.…”

Molecule‐Enhanced Surface‐Enhanced Infrared Absorption Spectroscopy (MOSEIRA)

“…The alcohol peaks are initially strongly enhanced over that expected for amplification (due to reflections) and surface enhancement 2b. 3 Over periods of time that depend on the alcohol, the absorbance usually decreases, for some alcohols rather rapidly and for others gradually. For methanol and ethanol, the absorbance actually increased by 10–20 % in the first stage of evaporation denoting the formation of additional enhanced species, as explained below.…”

“…We introduced a new simple device for attenuated total‐reflection Fourier‐transform infrared (ATR–FTIR) spectroscopy,2 and more recently, using planar silver halide (AgX) fibers, discovered that surface‐enhanced infrared absorption spectroscopy (SEIRA) of small amounts could be carried out for the protein trypsin and p ‐nitrobenzoic acid 2. 3 To generalize the method, we chose to study thin films of simple alcohols and have also made a number of significant improvements in our experimental technique. We utilize the more sensitive mercury–cadmium–telluride (MCT) detector rather than a deuterated triglycine sulphate (DTGS) detector, and use 1 s scans rather than averaging over 300 such scans.…”

“…These changes allow us to exploit the control of sample size and distribution that is an inherent advantage of the smooth AgX element. Historically, SEIRA experiments have been carried out on rough metal surfaces such as silver or gold,4 but as we have noted elsewhere2b, 3, 5 the actual surface consists of a metal oxide or halide for which sample quantitation is not easy. It is easier on more uniform surfaces as shown by an ATR–FTIR study of functionalized sensor chips on a diamond element using averages of 120 scans 6…”

Surface‐Enhanced Infrared Spectroscopy

“…Ag and Au island films are the most commonly used SEIRA substrates, but in electrochemical experiments the platinum group metals are preferred. Enhanced absorption of the vibrational spectrum in the infrared region has also been reported on planar Ag halide fibers, showing 10-fold spectral amplification that could be due to the increased coupling of evanescent waves through an increased number of internal reflections [92,93]. The investigation and explanation through adequate SEIRA models of the abnormal infrared effects (AIREs) observed in the bands of the SEIRA spectra of molecules adsorbed on these metals and on Fe [34,91] are ongoing.…”