Ultrasmall Graphene Quantum Dots as Exceptionally Stable and Efficient Substrates for Graphene-Enhanced Raman Spectroscopy Detection of Dyes

Surface-enhanced Raman scattering (SERS) is a powerful tool for investigating the chemical fingerprint of a molecule at nanoscales, and it mainly relies on noble metal nanoparticles, with their own limitations. Herein, we have developed a metal-free SERS substrate with cost-effective, exceptionally stable, highly efficient, ultrasmall, and uniformly sized graphene quantum dots (GQDs) to detect different dye molecules through the utilization of the graphene-enhanced Raman scattering (GERS) technique. The ultrasmall GQDs (∼1.53 nm) are synthesized using a hydrothermal method, assisted by a tip sonicator, and are utilized as GERS substrates. The charge transfers between the GQDs and RhB are studied using the shift in UV absorption peak positions for GQDs and RhB. Furthermore, we have amplified their GERS impact by employing a straightforward, metal-free, and dry technique through Ar plasma treatment. The structure of GQDs is hence modified, and the impact of the plasma treatment is analyzed using the Raman D band to G band intensity ratio (I_D/I_G) and changes in the photoluminescence spectra. Different laser excitation wavelengths are employed to study the resonance in the GERS effect with RhB on GQDs. We also studied the effect of GQD sizes on the GERS efficiency. For an excitation of 532 nm, a significantly enhanced GERS effect is observed, with an enhancement factor (EF) of 1.52 × 10⁶, for the GQDs of the smallest size (∼1.53 nm). Further, Ar plasma-treated GQDs (Ar-GQDs) elevated the EF up to 2.41 times compared to that of the untreated GQDs, achieving a value of 3.67 × 10⁶ for 10^–8 M RhB, which is significantly higher than the reported values. Consequently, the primary contributor to the GERS effect is the presence of structural defects on the GQDs induced by Ar plasma treatment, highlighting the promising potential of GQDs as a material for use in dye detection applications.