Figure 1. Scheme of preparing for self-assembled monolayers of DNA strands on gold surfaces coated on mica.Surface-enhanced Raman scattering (SERS) is a process in which the Raman scattering intensity of molecules adsorbed on certain rough metal surface, (e.g., Ag, Au, Cu) is enhanced by factors of 10 4 -10 6 compared to the intensity expected for unabsorbed molecules of a comparable concentration.1-3 These enormous sensitivity enhancement easily allowed adsorbates of a submonolayer coverage to be readily detected by Raman spectroscopy. The change in the structure of molecules can be followed by observing the changes in the SERS spectra of the adsorbed molecules.The study of electron transfer through DNA 4 and the development of novel DNA detection technologies 5 have been focused significantly on binding oligonucleosides to metal surfaces and colloids for the variety of important fundamental studies and applications. About ten years ago, Mirkin and coworkers 6-8 reported a new DNA detection technology based on the sequencespecific interactions of DNA-modified gold nanoparticles probes with a target DNA analyte. Because of these recent advances in using DNA to build a variety of functional materials, 9 an understanding of how DNA and its building blocks interact with surfaces on the molecular level would be crucial. However, only a few studies have generated pertinent structural information regarding the binding and conformation of oligonucleosides and their building blocks on gold surfaces.
10,11Hereby self-assembled monolayers of DNA strands on gold surface have been studied with SERS and data were compared with SERS spectra of oligonucleosides in aqueous gold nanoparticles solution.12 Based on this study, the coordination structures of the DNA strands on gold surfaces are proposed.
ExperimentalApproximately 13 nm diameter gold nanoparticles were prepared by the citrate reduction of HAuCl4 as described previously. 12 An 0.1 mL of 4% (w/v) HAuCl4 solution was added to a reflux of 40 mL of nanopure water while stirring, and then 1 mL of 1% (w/v) trisodium citrate solution was quickly added drop by drop while stirring, which resulted in a change of solution color from pale yellow to deep red. After the color change, the resulting mixture was boiled for additional five minutes, allowed to cool to room temperature and the final color of solution obtained was raspberry-red.Self-assembled monolayers of DNA strands on gold surface were prepared as described previously. 13 Gold nanoparticles modified with alkylthiol-capped DNA strands were prepared under the presence of specific target DNA strands (Figure 1). A sample spotted with the appropriate 13-nucleotide capture strands was self-assembled on gold surfaces coated on mica with a 0.6 M NaCl phosphate-buffered saline (PBS) buffer solution (10 mM of phosphate, pH 7) in a humidity chamber at room temperature. After 4 hours, the sample was washed four times with 0.6 M NaCl PBS buffer solution to remove nonspecifically bound target and was treated with a 0.6 M NaCl PBS ...