Kyung Lock Kim

Selective detection of aqueous nitrite ions by surface-enhanced Raman scattering of 4-aminobenzenethiol on Au.

In this work, we have devised a selective nitrite-ion detection method based on the surface-enhanced Raman scattering (SERS) of 4-aminobenzenethiol (4-ABT) on Au. This is possible because, firstly, SERS is a very surface-sensitive technique with monolayer detection capability, and secondly, the amine group of 4-ABT reacts readily with nitrites in acidic media, forming a diazonium group, which can subsequently form an azo bond by reacting with a variety of benzene derivatives. From the peak intensity of the diazonium group, the presence of nitrite ions above 20 μM can be identified readily. From the peak intensity of the azo moiety alone, it is even possible to detect nitrite ions at concentrations as low as 5 μM, without interference from other anions. This work clearly illustrates the usefulness of SERS in environmental science research.

Effect of volatile organic chemicals on surface-enhanced Raman scattering of 4-aminobenzenethiol on Ag: comparison with the potential dependence.

4-Aminobenzenethiol (4-ABT) is an unusual molecule in the sense that several distinct peaks whose counterparts are rarely found in the normal Raman spectrum are observed in its surface-enhanced Raman scattering (SERS) spectra. Their origin has been argued over recently as due to either a metal-to-adsorbate charge transfer or the formation of a photoreaction product such as dimercaptoazobenzene (DMAB). In an electrochemical SERS measurement, the intensities of the new peaks depended strongly not only on the excitation wavelength but also on the electrode potential. Interestingly, we observed a similar spectral variation even under ambient conditions by exposure of 4-ABT on Ag to volatile organic chemicals (VOCs) such as acetone and ammonia. Since acetone and ammonia barely react directly with 4-ABT, the effect of VOCs must be indirect, presumably associated with the movement of electrons between VOCs and the Ag substrate causing either an increase or a decrease in the surface potential of Ag. Based on the potential-dependent SERS data, the effect of acetone therefore appeared to correspond to an application of +0.15 V to the Ag substrate vs. a saturated Ag/AgCl electrode, while the effect of ammonia corresponded to the application of -0.45 V to Ag. We admit that much the same VOC effect could be observable if a photoproduct was formed immediately upon irradiation and the product was also subjected to a chemical enhancement mechanism. The Gaussian response of the peak intensities of the b(2)-type bands to applied potential, as well as to VOCs, dictated that the new peaks appearing in the SERS of 4-ABT have nothing to do with any electrochemical reaction. In addition, a separate preliminary work suggested that the b(2)-type bands are not at least due to a photoreaction product such as DMAB.

Effect of organic vapors and potential-dependent Raman scattering of 2,6-dimethylphenylisocyanide on platinum nanoaggregates

The surface-enhanced Raman scattering characteristics of 2,6-dimethylphenylisocyanide (2,6-DMPI) on Pt nanoaggregates, in ambient and electrochemical environments and in the presence of organic vapors, were examined and compared with those on Au nanoaggregates. Due to the exclusive adsorption via the isocyanide group, the NC stretching band was very susceptible to the measurement conditions although the ring associated bands showed negligible peak shifts. In ambient conditions, the peak shift of the NC stretching vibration on Pt (29 cm(-1)) was one half of that on Au (61 cm(-1)), suggesting that the electron donation capability of the isocyanide group to Au was greater than that to Pt. In the electrochemical environment, the NC stretching peak varied linearly with slopes of ∼42 and ∼36 cm(-1) V(-1) on Pt and Au, respectively. On the other hand, the NC stretching bands of 2,6-DMPI on Pt red-shifted by as much as 15 and 41 cm(-1), in the presence of acetone and ammonia, respectively, corresponding to the lowering of the surface potential of Pt nanoaggregates from +0.2 to -0.2 and -0.8 V, respectively. On Au nanoaggregates, however, acetone appeared to increase the surface potential of Au from +0.2 to +0.3 V, although ammonia decreased the surface potential from +0.2 to -0.4 V. Acetone must then act as an electron donor when interacting with Pt while it serves as an electron acceptor when interacting with Au, in agreement with an ab initio quantum mechanical calculation.

Effect of organic vapors on Au, Ag, and Au–Ag alloy nanoparticle films with adsorbed 2,6-dimethylphenyl isocyanide

The physicochemical properties of metallic substrates are affected by the environment in different ways. It is generally difficult to determine these effects because the molecules in the environment interact weakly with metallic substrates. In this work, we demonstrate that even the effect of volatile organic compounds (VOCs) can be identified by utilizing the surface-enhanced Raman scattering of isocyanide molecules. The NC stretching band of 2,6-dimethylphenyl isocyanide (2,6-DMPI) adsorbed on Au, for instance, is blueshifted by 6 cm(-1) under an acetone flow and is redshifted by 20 cm(-1) under an ammonia flow. The same band of 2,6-DMPI adsorbed on Ag and Au0.5Ag0.5 alloy films is, however, redshifted equally by 8 and 13 cm(-1) under acetone and ammonia flows, respectively. This indicates that although the surface plasmons of Au0.5Ag0.5 alloy nanoparticles are clearly distinct from those of Ag (as well as Au) nanoparticles, both Au0.5Ag0.5 and Ag nanoparticles show a similar response to VOCs. These observations led us to conclude that the outermost parts of Au-Ag alloy nanoparticles are enriched with Ag atoms and that only the surfaces of metal nanoparticles, and not the bulk material, are affected by VOCs.