Functionalizing gold with single strand DNA: novel insight into optical properties via combined spectroscopic ellipsometry and nanolithography measurements

Abstract

We have studied the self-assembly of 22-base oligonucleotides bound by a short alkyl thiol linker (C₆-ssDNA) on flat Au films. The self-assembled monolayer (SAM) was modified by addition of a spacer (mercaptohexanol, MCH). Molecular depositions were monitored in situ by spectroscopic ellipsometry (SE). SAMs were characterized in a liquid environment by coupling SE (difference spectra method) with Atomic Force Microscope (AFM) measurements. We exploited the biofilm thickness obtained by AFM nanolithography and imaging to solve the refractive index/thickness correlation in optical measurements on ultrathin molecular layers. The combined SE/AFM analysis provided reliable estimates of the thickness and the refractive index of the biofilm in the NIR region (650–1300 nm) and revealed new aspects of DNA molecular organization: exposure to MCH leads to an increase of both film thickness and refractive index, which points to a reorganization of C₆-ssDNA film. We show that the contribution of the thiol/Au interface has to be included in the optical model to obtain a more reliable determination of the refractive index of the biofilm in a liquid. The careful, correlative characterization of the mixed C₆-ssDNA/MCH SAM represents a key step towards the optimization of a robust detection scheme based on helix–helix hybridization.