Surface-Enhanced Raman Scattering of Sulfo-Cyanine NHS Esters Adsorbed on Gold Nanoparticles

Cyanine dyes are widely used in various techniques of bioimaging. The development of new types of optical tags with cyanine dyes combined with plasmonic nanoparticles is of great interest. In this work, surface-enhanced Raman scattering (SERS) study of optical behavior of cyanines adsorbed on gold nanoparticles in suspensions is presented to predict the optical response of corresponding tags. Two dyes, sulfo-cyanine 5.5 NHS ester and sulfo-cyanine 7.5 NHS ester were considered as they are suitable for the first transparency window of biological tissues. Gold nanoparticles of anisotropic shape were used as a plasmonic substrate. The assignment of bands in SERS spectra of both dyes was performed based on DFT calculations of vibrational frequencies of the molecules. The adsorption of dyes on the nanoparticles with different surface charges was studied. Spectral changes showed that a reorientation of the molecules occurs and reactive NHS ester tail is more accessible sterically in the case of a positive surface charge. The effect of aggregation-induced agent on the SERS response of sulfo-cyanines was also studied. Addition of chloride anions led to about five-fold increase of SERS intensity for sulfo-cyanine 5.5 NHS ester.