Paper BI-WeA10
Characterization of DNA Monolayers on Gold using Sum Frequency Generation Spectroscopy

Wednesday, October 22, 2008, 4:40 pm, Room 202

We investigated a series of model monolayers of single stranded DNA (ssDNA) on gold using broadband femtosecond Sum Frequency Generation (SFG) Spectroscopy. SFG processes, involving a non-linear resonant responses produced by exciting vibrational modes of molecular bonds using overlapping IR and visible beams, are inherently interface-specific. The surface specificity of SFG, combined with polarization dependence, allows for the investigation of the ordering and orientation of molecules at surfaces in air and through bulk solutions. However, due to the difficulties associated with operating an SFG system in biological relevant spectral regions (such as the amide I and fingerprint), combined with the difficulties of interpreting vibrational spectra from complex biomacromolecules, there are few published examples of characterization of DNA films using SFG. Our goal is to create high quality vibrational SFG spectra of model monolayers of DNA on gold and to compare these spectra to results obtained from complementary surface spectroscopies that have been applied to DNA monolayers, such as XPS, FTIR, and NEXAFS. Preliminary results in the C-H stretching region show distinctive changes in the locations and relative intensities of peaks for a film of thiol-modified 5-mers of thymine (T5-SH) compared to thiol modified 25-mers (T25-SH) and unmodified thymine 5-mers (T5). Preliminary SFG spectra of ssDNA films in the Amide I region revealed changes in the locations and intensities of the major peaks for the T5 film compared to a film of unmodified adenine 5-mers, as well as compared to T5-SH and T25-SH films. Refinement and validation of SFG Spectroscopy as a tool for the characterization of DNA monolayers on gold could provide another method for examining the structure of these films, and potentially serve as a bridge for comparing these systems ex situ and in situ.