Adsorption characteristics of 1,4-phenylene diisocyanide on gold nanoparticles: Infrared and Raman spectroscopy study

The adsorption characteristics of 1,4-phenylene diisocyanide (1,4-PDI) on colloidal Au particles have been investigated by means of various analytical techniques including infrared and Raman spectroscopy. Surface-enhanced infrared absorption spectroscopy as well as atomic force microscopy and quartz crystal microbalance clearly showed that 1,4-PDI should be adsorbed on gold via the carbon lone-pair electrons of one isocyanide group and that the pendent isocyanide group could react further with Au nanoparticles, suggesting that nanoarrayed electrodes could be fabricated using 1,4-PDI and Au nanoparticles. In the Au sol medium, the adsorption scheme was highly dependent on the concentration of 1,4-PDI. As a result, in the surface-enhanced Raman scattering (SERS) spectra, the relative peak intensity of the v(NC)(free) and v(NC)(bound) modes varied with a sigmoid shape with respect to the 1,4-PDI concentration. The inflection point (IP) seemed surprisingly to correspond to the monolayer-coverage limit. At lower concentration below the IP, 1,4-PDI appeared to be bonded exclusively to two different gold particles forming two Au-C bonds. The resulting fractal clusters were rather small-sized, however, probably due to the repulsive barrier between Au particles at lower concentration. In contrast, at higher concentration above the IP, a highly aggregated superstructure showing both the v(NC)(free) and v(NC)(bound) bands was readily formed. Altogether, the major driving force for Au nanoparticles to assemble large aggregates seemed to be the formation of two Au-C bonds per 1,4-PDI rather than the usual adsorbate-induced reduction of the electrical repulsive barrier between Au particles.