Synthesis and characterization of monolayer -protected gold nanoparticles and their organoplatinum composites as vapor -sensitive microsensor interface materials

Organothiolate-monolayer-protected gold nanoparticles (MPNs) offer numerous advantages as vapor-sensitive interface materials in microfabricated chemiresistor (CR) arrays. Small quantities of gas-phase species partitioning into thin MPN films cause large changes in resistance. In these studies, the synthesis, characterization, and testing of MPNs, alone and as composites with organoplatinum charge-transfer complexes, are explored. A new single-phase MPN synthesis is described, which avoids persistent ionic contamination from the phase-transfer catalyst (PTC) necessary in the conventional dualphase synthesis. Residual PTC leads to a frequency-dependent current through the MPN film, over two hour stabilization times, and anomalous vapor responses. The new method retains the high yield, particle-size control, and thiolate-monolayer functional-group diversity associated with the conventional MPN synthesis, while producing materials of higher purity in less than three minutes. MPNs with several different thiolate-ligand functionalities (e.g., aromatic and ester) were prepared by this single-phase method and fully characterized. Composite films of n-octanethiolate MPNs with charge-transfer complexes of the general formula PtCl2(olefin)(pyridine) were then tested and found to selectively interact with gas-phase olefins. A spontaneously reversible Pt-olefin interaction occurs that is accompanied by a decrease in film resistance. Structurally analogous non-olefins invariably cause an increase in film resistance associated with MPN swelling. The mechanism for the conduction enhancement was elucidated from the 33 nm red-shift in the UV-vis absorbance spectrum of PtCl2(styrene)(pyridine) when titrated with styrene. Detection limit reductions, for examined olefins, by as much as 104-fold are achieved by use of these composite CR interface films. Tethering Pt-complexes to unsaturated MPN ligands was successful but did not provide the selectivity and sensitivity observed with the composite films. In a follow-on study, composite films of n-octanethiolate MPNs with PtCl2(pyridine)2 were shown to reversibly and selectively interact with pyridine vapor and its derivatives, including the environmental tobacco smoke markers 4-vinylpyridine and nicotine. UV-vis absorbance spectrum red-shifts confirm a charge-transfer interaction similar to that observed with the Pt-olefin MPN composites. Although the degree of selectivity is less than that observed with the olefins, the results support the use of organometallic additives in MPN films as a general means of enhancing sensitivity and selectivity.