Line excitation SERS spectroscopy using laser-induced graphene coated with silver nanoparticles

We report a line excitation scheme for surface enhanced Raman scattering (SERS) using a graphene substrate deposited with silver nanoparticles (AgNPs). Direct laser writing was employed for producing laser-induced graphene (LIG) layer on polyimide and electrochemical reaction was used to grow AgNPs onto the surface of LIG. In addition to the local-field enhancement on the plasmonic hotspots, the graphene-based charge-transfer and molecular adsorption properties make further contributions to the SERS mechanisms. Compared with conventional point excitation, the line excitation scheme allows much extended dynamic range of the excitation laser power before reaching the damage threshold of the target molecules, much enhanced Raman signal due to the integration over the focusing line, and high reliability/repeatability of the SERS signals. This also reduces significantly the sensitivity of the SERS signals to the randomly distributed nanostructures. Thus, much enlarged excitation area/volume for Raman interactions enables high stability of the signals with multiplied intensity through integration over the focusing line, as compared with point excitation scheme under the same excitation power density. The line excitation scheme shows a detection limit of 100 ppb in tracing melamine in water, attaining an enhancement factor of larger than 100, as compared with point excitation.