Application of PM-IRRAS to study thin films on industrial and environmental samples.

Polarization modulation-infrared reflection absorption spectroscopy (PM-IRRAS) was employed to analyze two unique samples: (1) an industrially prepared alkoxysilane-pretreated aluminum alloy (AA6111) in the absence and presence of a ~600-nm-thick lubricant coating and (2) a chemical warfare agent simulant, triethyl phosphate (TEP), on glass. For the pretreated aluminum samples, PM-IRRAS spectra were analyzed for three distinct regions; the SiO stretching vibration around 1120 cm(-1), the NH(2) bending mode at ~1600 cm(-1) and the CH stretching region around 2900 cm(-1). Our results showed that increasing the curing temperature (from 55 to 100 °C) improved the overall extent of cross-linking within the siloxane network. In addition, the spectra of lubricant (top coating) and the underlying siloxane layer for the aluminum samples with lubricant were collected for the same sample. Our results show that the nature of the siloxane film remains intact and unaltered after deposition of the lubricant top-coat. For detection of TEP on glass, the band at 1268 cm(-1), corresponding to the P═O vibration, was monitored. A droplet of TEP solution in dichloromethane was deposited on glass. After solvent evaporation had occurred, the intensity of the P═O vibration band was used to construct calibration curves to determine the experimental limit of detection, which was found to be ~200 µg for TEP on glass.

Nitro-substituted arene sulfenyl chlorides as precursors to the formation of aromatic SAMs.

The formation of aromatic SAMs on Au(111) using three nitro-substituted arene sulfenyl chlorides (4-nitrophenyl sulfenyl chloride (1), 2-nitrophenyl sulfenyl chloride (2), and 2,4-dinitrophenyl sulfenyl chloride (3)) is studied. The formation of SAMs and their quality are investigated as a function of the position of the nitro substituent(s) on the aromatic ring. The modified surfaces are characterized by X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), polarization modulation infrared reflection absorption spectroscopy (PMIRRAS), and cyclic voltammetry (CV). The results show that all three compounds are deposited on Au within very short times. The corresponding coverages are determined using CV. However, only compound 1 forms stable, long-range, well-ordered SAMs. The 4-nitrophenyl thiolate is adsorbed nearly vertically on the Au surface. Compounds 2 and 3 both form lower-quality SAMs where the adsorbed nitro-phenyl thiolates are more tilted. These SAMs are less stable than the ones obtained with the 4-nitrosubsituted precursor and decompose with time, leaving only sulfur on the gold surface.

Non-contact detection of chemical warfare simulant triethyl phosphate using PM-IRRAS.

Polarization modulation-infrared reflection absorption spectroscopy (PM-IRRAS) was employed to detect the chemical warfare agent (CWA) simulant triethyl phosphate (TEP) on gold, as well as on US military paint, i.e., chemical agent resistant coating (CARC). The targeted CWAs (G and V-series nerve agents) are characterized by phosphoric group vibrations present in the 1200 cm(-1) region. TEP displays two prominent peaks at 1268 cm(-1) and 1036 cm(-1) corresponding to P=O and (P)-O-C vibrations, respectively. A droplet of TEP solution in cyclohexane was deposited on gold and CARC substrates and after solvent evaporation PM-IRRAS spectra were collected in the 1200 cm(-1) region. The integrated peak area of the PO and (P)OC vibrations was used to construct calibration curves and to determine the experimental limit of detection (LoD). In the case of gold as the substrate the estimated LoD of ~0.48 µg and 1.23 µg was obtained for the P=O and (P)-O-C vibrations, respectively. In the case of CARC, a LoD of 24 µg was determined. These detection limits are at least 3 orders of magnitude lower than the typical lethal dose of G and V-series nerve agents, demonstrating potential of PM-IRRAS for non-contact detection of these CWAs.

Electrochemical and STM studies of 1-thio-ß-D-glucose self-assembled on a Au(111) electrode surface.

In this study, a Au(111) electrode is functionalized with a monolayer of 1-thio-ß-D-glucose (ß-Tg), producing a hydrophilic surface. A monolayer of ß-Tg was formed on a Au(111) surface by either (1) potential-assisted deposition with the thiol in a supporting electrolyte or (2) passive incubation of a gold substrate in a thiol-containing solution. For each method, the properties of the ß-Tg monolayer were investigated using cyclic voltammetry (CV), differential capacitance (DC), and chronocoulometry. In addition, electrochemical scanning tunneling microscopy (EC-STM) was used to obtain images of the self-assembled monolayer with molecular resolution. Potential-assisted assembly of ß-Tg onto a Au(111) electrode surface was found to be complicated by oxidation of ß-Tg molecules. The EC-STM images revealed formation of a passive layer containing honeycomb-like domains characteristic of a formation of S(8) rings, indicating the S-C bond may have been cleaved. In contrast, passive self-assembly of thioglucose from a methanol solution was found to produce a stable, disordered monolayer of ß-Tg. Since the passive assembly method was not complicated by the presence of a faradaic process, it is the method of choice for modifying the gold surface with a hydrophilic monolayer.

Atomic force microscopy studies of a floating-bilayer lipid membrane on a Au(111) surface modified with a hydrophilic monolayer.

The surface of a gold electrode was functionalized with a hydrophilic monolayer of 1-thio-ß-D-glucose formed by spontaneous self-assembly. The Langmuir-Blodgett/Langmuir-Schaefer (LB/LS) method was then used to assemble a bilayer onto the modified Au(111) surface. The bilayer lipid membrane (BLM) was separated from the Au(111) electrode surface by incorporating the monosialoganglioside GM1 into the inner leaflet of a bilayer composed of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and cholesterol. To make the inner leaflet, monolayers of GM1/DMPC/cholesterol with mole ratios of 1:6:3, 2:5:3, and 3:4:3 were used. The outer leaflet was composed of a 7:3 mole ratio of DMPC/cholesterol. Because of the amphiphilic properties of GM1, the hydrophobic acyl chains were incorporated into the BLM, whereas the large hydrophilic carbohydrate headgroups were physically adsorbed to the Au(111) electrode surface, creating a "floating" BLM (fBLM). This model contained a water-rich reservoir between the BLM and the gold surface. In addition, because of the bilayer being physically adsorbed onto the support, the fluidity of the BLM was maintained. The compression isotherms were measured at the air/water interface to determine the phase behavior and optimal transfer conditions. The images acquired using atomic force microscopy (AFM) and the force-distance measurements showed that the structure of the fBLM evolved with increasing GM1 content from 10 to 30 mol %, undergoing a transition from a corrugated to a homogeneous phase. This change was associated with a significant increase in bilayer thickness (from ∼5.3 to 7.3 nm). The highest-quality fBLM was produced with 30 mol % GM1.