Macroscale colloidal noble metal nanocrystal arrays and their refractive index-based sensing characteristics

Colloidal noble metal nanocrystals are promising for a large number of optical and biotechnological applications. Many practical applications require the formation of large-area, high-density, and uniformly distributed metal nanocrystal arrays on various substrates, to overcome the limitations brought by the instability of colloidal metal nanocrystal solutions and the high cost of single-particle spectroscopy characterizations. A method is developed for directly depositing colloidal metal nanocrystals, including Au nanospheres, Au nanorods, Au nanobipyramids, and (Au core)/(Ag shell) nanorods, from their solutions onto different substrates. The resultant nanocrystal arrays are relatively uniform and dense, with the peak extinction value of a single layer reaching 0.3. Their areas are up to 10 cm by 10 cm and can be further increased if larger-size containers are utilized. The refractive index sensitivities are studied for Au nanorod arrays supported on glass slides, mesoporous silica and titania films, and capped with different molecules. Au nanorods deposited on mesoporous titania films are found to exhibit the highest index sensitivities, comparable to those of the same nanorod sample in solutions. It is expected that this approach will greatly facilitate plasmonic applications that require large-area arrays of noble metal nanocrystals. A facile, low-cost approach for the formation of noble metal nanocrystal arrays on different substrates at high densities and in areas as large as 10 cm by 10 cm is demonstrated. The effects of the capping manner and supporting substrate on the refractive index sensing characteristics of the metal nanocrystal arrays are studied.