Characterization of island films as surface-enhanced Raman spectroscopy substrates for detecting low antitumor drug concentrations at single cell level.

Gold and silver vacuum-deposited island films were characterized by studying deposition variables such as film thickness, evaporation rate, and substrate temperature. For both metals, these parameters were correlated with the surface-enhanced Raman spectroscopy (SERS) effect and an increase in film thickness and low evaporation rates were shown to upshift the wavelength at maximum optical density (lambda max) and increase the optical density of the substrates. In contrast, pre- and postdeposition annealing of gold films led to the formation of substrates that exhibited a downshift of lambda max. Our spectral data also indicated that silver films are substrates that are more suited for SERS applications where high frequency visible excitations are used. Measurements on gold films classified them into two groups: thin Au films (10-50 A) well adapted for red excitations and thicker ones that are operative in the near infrared. SERS results, which were obtained from a single HL60 cell treated with micromolar drug quantities, placed on thin gold island films indicated that these island films could be future promising substrates for SERS imaging at the cellular level.