Optically induced aggregation by radiation pressure of gold nanorods on graphene for SERS detection of biomolecules

Radiation pressure is used to push gold nanorods on multilayer graphene and create hybrid active surfaces for surface-enhanced Raman spectroscopy (SERS) in liquid. As a proof of concept, ultrasensitive detection of bovine serum albumin is shown and the aggregation kinetics is studied as a function of the irradiation time. We compare the results on graphene with experiments on glass and gold surfaces. Optical aggregation on graphene occurs on time scales of 20 min, ca. 3.5 times slower than on glass. No stable aggregation is obtained on gold. We attribute the differences to the destabilization effect of the standing wave produced on the metallic substrates, due to their higher reflectivity, and to the reduced thermophoretic effects, related to the higher heat dissipation. Despite the slowdown of the aggregation kinetics, the usage of graphene as substrate offers manifold benefits: an almost negligible fluorescence background when using near-infrared light (785 nm), the absence of thermal absorption as well as the possibility to easily functionalize the surface to enhance the affinity with the analytes. Our results enlarge the spectrum of materials that can be used for optical aggregation and SERS detection of biomolecules, highlighting the importance of controlling the physical properties of the surfaces. Graphic abstract: [Figure not available: see fulltext.]