Gold-thiolate monolayer-protected nanoparticles as sorptive interfaces for microsensor arrays

An investigation of gold-thiolate monolayer-protected nanoparticles (MPN) as chemically sensitive interface materials in microsensor arrays for the determination of volatile organic compounds is described. The primary target application uses an MPN-coated chemiresistor (CR) arrays as the detector in a micro-scale gas chromatograph (muGC), where changes in the electrical resistance of MPN films accompanying reversible vapor sorption are monitored. Combining CR film resistance data with mass uptake data from similarly coated thickness-shear mode resonators (TSMR), the MPN-coated CR response mechanism has been elucidated. Focusing initially on an n-octane-thiolate MPN, a response model was derived from established theory of electron transport through insulated granular-metal films that allows predictions of MPN-coated chemiresistor (CR) responses from vapor-film partition coefficients, and vapor densities and dielectric constants. Apparent swelling efficiencies differed for vapors from different functional group classes. Attempts to extend this model to more highly functionalized MPNs were only partially successful and revealed that CR responses are affected by additional variables that were not amenable to modeling. The diversity of an array of MPN-coated CR sensors was shown to be greater than a similarly coated array of mass-sensitive TSMR sensors. Models based on linear solvation energy relationships (LSER) have also been applied to MPN-coated CR (TSMR) sensors for the first time. The MPN solvation coefficients, which describe the contributions of each type of non-bonding interaction to vapor sorption, were estimated and partition coefficients estimated with reasonable accuracy. The use of MPN-CR sensor arrays as low-dead-volume muGC detectors was also explored. A multi-vapor analysis using micromachined separation and pumping modules was demonstrated. It was discovered that reductions in device area over three orders of magnitude have no effect on CR sensitivities or detection limits. This translates into commensurate decreases in detector dead volume and the mass of sample required per analysis. To facilitate fabrication of densely packed CR arrays, an electron beam induced crosslinking (EBIC) method was applied successfully to an integrated array of four MPN-coated CRs. Patterned films showed increases in sensitivity and changes in selectivity that could be related to expected structural changes in the MPN ligands.