Deciphering the role of carbon quantum dots-metal nanocomposites on surface enhanced Raman scattering and photoinduced enhancement

Ag and Au nanoparticles (NP), along with carbon quantum dots (CQD), were synthesized using straightforward methods. Ag NP exhibit localized surface plasmon resonance (LSPR) at 445 nm and Au NP at 551 nm. The quantum yield of as-synthesized CQD in water has been found to be 0.46 at 350 nm excitation. Nanocomposites (Au-CQD, Ag-CQD, Ag–Au, and Ag–Au-CQD) were used as surface enhanced Raman scattering substrates for methylene blue (MB) detection. At 785 nm excitation, Ag-CQD and Ag–Au-CQD showed higher analytical enhancement factors (AEFs) of 1.1 ×104 and 2.5 ×104, respectively, compared to Ag–Au (0.7 ×104). At 514 nm, simultaneous excitation of Ag and Au LSPR improved AEFs, especially for Ag–Au (1.7 ×104) and Ag–Au-CQD (1.8 ×104). Detection limits reached 10−7 M for Au-CQD and 10−8 M for others at 785 nm, while all composites showed a uniform detection limit of 10−8 M at 514 nm. AEFs were reproducible with ∼2%–9% relative standard deviation. Furthermore, COMSOL Multiphysics was used to analyze electric field distribution and establish a strong correlation between theoretical EFs and experimental AEFs in Ag–Au and Ag–Au-CQD nanocomposites. Additionally, photoinduced enhanced Raman scattering (PIERS) studies under 445 nm illumination revealed enhanced Raman signals via photoinduced charge transfer from CQD to metal NP. The highest enhancement was observed in Ag–Au-CQD, followed by Ag-CQD and Au-CQD, while Ag–Au without CQD showed minimal effects. Thus, the dual-wavelength approach enhances PIERS performance for sensitive MB detection.