Acousto-optic surface-plasmon resonance measurements of thin films on gold

In this work a highly sensitive optical technique was used to study the adsorption of thin organic layers onto a gold film from water, in situ and in real-time. Optical excitation of a surface-plasmon resonance (SPR) was used to probe the gold/water interface. The use of an acousto-optic tunable filter then provides a differential technique for monitoring the SPR position in optical wavelength. This allows optical changes at the metal/liquid interface to be measured as the adsorption of thin organic layers occurs. Adsorption of a poly(ethylene glycol) monododecyl ether surfactant $({\mathrm{C}}_{\mathrm{12}}{\mathrm{E}}_{\mathrm{8}})$ and a 30-mer DNA oligonucleotide with a mercaptohexyl group at the $5^{\prime }$ -phosphate end (DNA-SH) onto gold from water were examined. Conventional, angle-dependent reflectivity measurements taken on the same system provided complementary SPR data, allowing the sensitivity of the two techniques to be compared.