Infrared and Raman spectral signatures of aromatic nitration in thermoplastic urethanes.

The spectral signatures of nitro attack of the aromatic portion of thermoplastic urethanes (TPU) were determined. Eight fragment molecules were synthesized that represent the nitrated and pristine methylenediphenyl section common to many TPUs. Infrared (IR) and Raman (785 nm illumination) spectra were collected and modeled using the B3LYP/6-31G(d)//B3LYP/6-31G(d) model chemistry. Normal mode animations were used to fully assign the vibrational spectra of each fragment. The vibrational assignment was used to develop a diagnostic method for aromatic nitro attack in thermoplastic urethanes. The symmetric NO(2) stretch coupled out of phase with the C-NO(2) stretch (1330 cm(-1)) was found to be free from spectral interferences. Spectral reference regions that enable correction for physical differences between samples were determined. The carbonyl stretch at 1700 cm(-1) was the best IR reference region, yielding a limit of quantitation (LOQ) of 0.66 +/- 0.02 g N/100 g Estane. Secondary IR reference regions were the N-H stretch at 3330 cm(-1) or the urethane nitrogen deformation at 1065 cm(-1). The reference region in the Raman was a ring stretching mode at 1590 cm(-1), giving an LOQ of 0.69 +/- 0.02 g N/100 g Estane. Raman spectroscopy displayed a larger calibration sensitivity (slope = 0.110 +/- 0.004) than IR spectroscopy (slope = 0.043 +/- 0.001) for nitration determination due to the large nitro Raman cross-section. The full spectral assignment of all eight molecules in the infrared and Raman is presented as supplemental material.

Microsphere lithography on hydrophobic surfaces for generating gold films that exhibit infrared localized surface plasmon resonances.

Evaporation induced self-assembly is an established method for producing close-packed two-dimensional sphere masks on hydrophilic surfaces such as glass. In sphere lithography, gold or silver is deposited over sphere masks to generate a film-over-nanospheres or a nanoprism array that can be used as a sensing surface in localized surface plasmon extinction and surface enhanced Raman spectroscopy experiments. Sphere lithography is less commonly used to prepare sphere masks on hydrophobic surfaces associated with infrared window materials, in part because it is challenging to find solvents with wetting and evaporation characteristics that are appropriate for such surfaces. This wetting challenge can be overcome with appropriate surfactants. However, surfactant residues are then left behind on the sensing surface. We report methods for depositing monolayer crystalline sphere masks onto CaF2 windows that minimize surfactant residue by either using ethanol as a volatile cosolvent that enhances wetting, or by increasing the concentration of colloid to compensate for the reduced attractions between spheres and surface. The rate of evaporation of solvents from the colloid drop is controlled by fixing the headspace partial pressure of ethanol and/or water.