Abstract
Surface-enhanced Raman spectroscopy (SERS), a marvel that uses surfaces to enhance conventional Raman signals, is proposed for a myriad of applications, such as diagnosis of diseases, pollutants, and many more. The substrates determine the SERS enhancement, and plasmonic metallic nanoparticles such as Au, Ag, and Cu have dominated the field. However, the last decades have failed to translate SERS prototypes into real-life applications. Irreproducibility on the SERS signal that stems from the roughened SERS substrates is the main causative factor for this observation. To mitigate irreproducibility several two-dimensional (2-D) substrates have been sought for use as possible alternatives. Application of 2-D graphene substrates in Raman renders graphene-enhanced Raman spectroscopy (GERS). This account used density functional theory (DFT) substantiated with experimental Raman to compare the enhancement capabilities of plasmonic Au nanoparticles (SERS), graphene substrate (GERS), and coupling of the two SERS and GERS substrates. The DFT also enabled the study of the SERS and GERS systems molecular orbital to gain insight into their mechanisms. The amalgamation of the SERS and GERS occurrence, i.e., graphene doped with plasmonic metallic substrates showed a pronounced enhancement and matched the Au-driven enhancement emanating from both electromagnetic and charge transfer SERS and GERS mechanisms.
Highlights
Surface-enhanced Raman spectroscopy (SERS) has been intensely studied in the last four decades in analytical chemistry and food safety (Nguyen et al, 2014)
The roughened plasmonic metallic amplification of the Raman signal is reliant on the metallic surface and, termed SERS, while the 2-D graphene-dependent enhancement is graphene-enhanced Raman spectroscopy (GERS)
The density functional theory (DFT) was used as a vehicle to study the SERS and GERS substrates at BP86 D3BJ def2-SVP or TZVP def2/j level of theory, and Figure 1 depicts ground state geometries of the clusters
Summary
Surface-enhanced Raman spectroscopy (SERS) has been intensely studied in the last four decades in analytical chemistry and food safety (Nguyen et al, 2014). Application of graphene and its variants in SERS is merited by its flexibility, lightweight, photocatalytic, (Nguyen et al, 2014), uniform Raman signal (Miao et al, 2019), stability, low bulk cost production, biocompatibility (e.g., graphene oxide) (Almohammed et al, 2019), electrical, optical, thermal conductivity, and mechanical applications, molecular orientation and edge defect contributing to charge transfer (Kim et al, 2014), and flatness (Ling et al, 2015). This paper studies the various SERS substrates, from the traditional roughened metals of Au NPs to the 2-D graphene. It reports a density functional theory (DFT) study corroborated with experimental Raman to understand the SERS enhancement mechanism.
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