Pattemed self-assembled monolayers on solid substrates are being investigated for applications in material science, microelectronics, and biotechnology.' A W-photolithographic method was recently reported for patterning alkanethiol self-assembled monolayers (SAMs) on Au and Ag surfaces with micrometer r e~o l u t i o n .~~~ It was shown that upon exposure to UV light, thiolate head groups are photooxidized to sulfoxide species. The oxidized sulfur species are weakly bound to the surface relative to thiolates and thus are easily displaced from the monolayer by immersion in a solution containing a second alkanethiol molecule." Although some of the oxidation products from the W photolysis procedure have been identified using X-ray photoelectron spectroscopy and laser desorption mass spectrometry, many questions still remain about the mechanism of the photooxidation reaction. In this report, we present surfaceenhanced Raman spectroscopy (SERS) results acquired from W-irradiated SAMs on Ag. We show that the photooxidation of the alkanethiol monolayer proceeds essentially by a twostep mechanism. First, W exposure causes C-S bond scission and subsequent desorption of the hydrocarbon chain. Second, the exposed sulfur is oxidized to sulfite and sulfate species.SERS was used to monitor the disappearance of Raman bands associated with S-C and C-C bonds of the n-alkanethiols and the appearance of bands associated with S-0 species. Because of the relatively high Raman cross sections for the C-S and S-0 vibrations, SERS was found to be a highly sensitive technique for characterizing the effects of W exposure on alkanethiol monolayers. SAMs comprised of n-alkanethiols [CH3(CH&SH, where n = 5 , 9, and 171 were formed on electrochemically roughened Ag substrate^.^ SER spectra6 from as-formed S A M s were qualitatively similar to those reported previously? indicating the presence of densely-packed, highquality monolayers. W exposure of a sample was performed in air without removing the sample from its initial position, (1) For example, see: Stenger, D. A.; Georger, J. H.; Dulcey, C. S.; Hickman, J. J.; Rudolph, A. S.; Nielsen, T. B.; McCort, S. M.; Calvert, J. M. Electrodes were polished with 0.05 p n grit alumina, rinsed with 18 MQ deionized water, and sonicated for 5 min to remove all traces of the powder. Electrochemical roughening was done in a 1.0 M NaC104 solution and consisted of three cycles from -0.5 to -1-0.6 V (vs Ag/AgCl) with the potential poised for 30 and 10 s at the negative and positive potential extremes, respectively. The electrode was sequentially rinsed with water and ethanol and incubated in a 1-5 mM alkanethiol-ethanol solution for a minimum of 24 h. 0002-7863/95/1517-9574$09.00/0 600 760 SO0 900 1000 11'00 Raman Shift (cml) Figure 1. SER spectra acquired from a hexanethiol S A M on Ag after total UV exposure times of 0, 1, 2, 3, 4, 5 , and 6 min. See text for description.ensuring that the identical sample area was probed following each exposure. The low-pressure Hg pen-lamp (-3 mW/cm2) used for irradiating ...

Hybrid bilayer membranes (HBMs), consisting of a lipid monolayer covering a self-assembled alkanethiol monolayer on a metal surface, are useful models for studying the structure and function of cell membranes. Surface-enhanced Raman spectroscopy (SERS) and reflection absorption infrared spectroscopy (RAIRS) are used to study HBMs with various alkanethiol and lipid components. Together, these two techniques clearly indicate that the lipid forms a well-ordered, non-interdigitated layer on the alkanethiol with the head groups of the lipids oriented away from the metal surface. Both techniques reveal that the formation of an HBM produces small changes in the alkanethiols, which are similar to those caused by a reduction in temperature, indicative of an increase in order. The small magnitude of the perturbations in the alkanethiol monolayer upon addition of the lipid layer will simplify the further study of HBMs.

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Anti-Stokes Raman spectra of 28 explosive materials were obtained with 1064-nm excitation using fiber-optic sampling and a dispersive spectrograph equipped with a charge-coupled device (CCD) array detector. By using a silicon CCD detector, anti-Stokes features could clearly be observed for the majority of samples from -250 to -1650 cm(-1). Using the fiber-optic probe, spectra were routinely obtained from samples positioned up to twelve meters from the spectrograph within 240 s. The utility of an anti-Stokes correction routine is demonstrated, which routine allowed anti-Stokes spectra measured with 1064-nm excitation to be successfully searched and identified against libraries of Stokes spectra obtained using a Fourier transform (FT) Raman system equipped with a 1064-nm Nd:YAG laser.

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