Performing tip-enhanced Raman spectroscopy in liquids

Many outstanding questions in biology and medicine require analytical tools that provide imaging and chemical information with high spatial resolution. Tip-enhanced Raman spectroscopy (TERS) has been shown to allow both topographic and label-free chemical information to be obtained with a lateral resolution of approximately 20-50 nm, but has been performed only in air or ultrahigh vacuum until now. Since most biological samples such as cells and tissues can only be studied in their active state if they are kept in aqueous buffers, TERS in liquids would be a crucial step towards nanoscale chemical analysis of living biological entities. For the first time, we introduce TERS experiments that have been performed with both the tip and sample completely immersed in water. We demonstrate that SiOx/Ag-coated AFM tips provide enhancement factors of > 10(4) with visible light irradiation and are robust enough to be used in water. Furthermore, the tips have been protected from contaminants by adsorbing a self-assembled monolayer (SAM) of ethanethiolate on their Ag surfaces. The protection layer caused the enhancement to drop by a factor of approximately 5, but successfully prevented the adsorption of analyte molecules, carbon, and other contamination to the tip. Furthermore, our experiments have shown that the formation of carbonaceous contamination by laser irradiation on the tip is dramatically slowed down when TERS experiments are performed in water. Finally, a proof-of-principle study on SAMs of thiophenolate on Au surfaces demonstrates the feasibility of performing TERS in liquids. Copyright (C) 2009 John Wiley & Sons, Ltd.