No abstract

High-quality Fourier transform surface-enhanced Raman scattering (FT−SERS) of a dilute solution of nicotinamide adenine dinucleotide (NAD) was recorded on a roughened Au electrode. It was found that the detection limit could be lower as much as 3 orders of magnitude by using a near-infrared laser as the exciting source and an in-situ roughening technique. The SERS of NAD shows a strong potential dependence in the non-Faradaic regions. In regions of positive electrode potential, only the bands responsible for the adenine and nicotinamide moieties can be observed. In contrast, with a negative shift of the potential, several additional strong bands representing the ribose and phosphate moieties are also evident. Based upon the SERS behaviors of NAD in the regions of different potential, an adsorption mechanism for dilute NAD on the gold electrode was proposed. With this mechanism, the stacked NAD molecule is considered to be opened to some extent on an electric charged electrode. Specifically, under sufficiently negative potential, the NAD molecule appears to exist in a well-extended state on the gold electrode, leading to the tight adsorption of the entire NAD molecule on the electrode.

Near-infrared Fourier transform Raman spectroscopy was used to study the SERS behaviors of nicotinamide adenine dinucleotide (NAD) in a solution containing Glutamate dehydrogenase enzymes. It was found that the SERS spectra of NAD adsorbed on a gold electrode should be largely changed by addition of glutamate dehydrogenase (GDH) in the electrolyte solutions. The potential dependence of the SERS revealed that, in an adequate negative potential region, NAD adsorbed on a gold electrode can be combined with the enzyme to a certain extent. The SERS spectra of NAD/GDH exhibit some different properties from those shown for normal SERS spectra of NAD alone in two aspects: (1) The existence of the enzymes led to decrease or disappearance of several strong bands under open circuit conditions. The reductions of the SERS bands are quite consistent with the changes of normal Raman spectra of NAD in combination with some NAD-dependent dehydrogenase in solutions. The protonation of the adenine moiety was proposed to explain this SERS behavior. (2) Several new bands, some of which can be attributed to the backbone peptide of the enzyme, appeared and increased in intensity with a negative shift of the electrode potential. The appearance of the new bands was attributed to the close approach of the active site of the enzyme induced by the coenzyme preadsorbed on the surface.

Electron temperature is measured from time resolved hairpin resonator probe measurements in a pulsed capacitively coupled argon plasma at 400 mTorr. Effective collision frequency is related to the electron energy distribution via the effective conductivity and closes a system of equations that allow electron temperature to be determined. Results generally show good agreement with expected behavior and unfiltered time-resolved optical emission measurements.