Water soluble SERS labels comprising a SAM with dual spacers for controlled bioconjugation

Self-assembled monolayers (SAMs) of Raman reporter molecules adsorbed to the surface of metal nanoparticles are labelling agents for the selective detection of biomolecules by surface-enhanced Raman scattering (SERS). Advantages of SAM-based SERS labels include maximum coverage with Raman reporter molecules combined with their uniform molecular orientation within the SAM as well as minimal co-adsorption of other molecules from the surrounding. Water solubility and stability together with the option for controlled bioconjugation are desired properties of SERS labels. We present an approach to synthesize hydrophilic SERS labels in which the colloidal particle is stabilized by two different ethylene glycols attached to the SAM. This guarantees water solubility, independent of the type of a particular Raman reporter molecule and its properties at the SAM/solvent interface. Our dual SAM design is based on two different ethylene glycol spacers covalently bound to the same type of reporter molecule: a short monoethylene glycol (MEG-OH) and a longer triethylene glycol (TEG-COOH) moiety. The terminal carboxy group of the TEG spacer allows subsequent conjugation to biomolecules such as antibodies. Controlled bioconjugation is possible by varying the stoichiometric ratio of both spacers. Finally, dual SAM SERS labels are used in immuno-SERS microscopy for selective imaging of prostate-specific antigen in the epithelium of biopsies from patients with prostate cancer.