2,2,6,6-Tetramethylpiperidine-1-oxy-Oxidized Cellulose Nanofiber-Based Nanocomposite Papers for Facile In Situ Surface-Enhanced Raman Scattering Detection

In this study, we report a flexible transparent freestanding surface-enhanced Raman scattering (SERS) platform composed of 2,2,6,6-tetramethylpiperidine-1-oxy-oxidized cellulose nanofibers (TEMPO-CNF) and gold nanoparticles (AuNPs) such as nanospheres (AuNSs) and nanorods (AuNRs) for in situ chemical sensing of a real-world surface. The TEMPO-CNF/AuNP-based nanocomposites were fabricated using two-step filtration with a pure TEMPO-CNF solution and a TEMPO-CNF/AuNP mixture solution. We demonstrate that the TEMPO-CNF/AuNR nanocomposite reveals better SERS activity than the TEMPO-CNF/AuNS nanocomposite. The TEMPO-CNF/AuNR nanocomposite detected rhodamine 6G down to 10 nM with a high enhancement factor of 2.1 × 10⁷ and exhibited good SERS measurement reproducibility in the flexible/bent state. No significant change in SERS intensity was observed even after 1000 cycles of bending to nearly 90°. Importantly, the flexible transparent TEMPO-CNF matrix allows the TEMPO-CNF/AuNR nanocomposite to be tightly wrapped onto the surface of an agricultural product for in situ detection as well as to directly detect pesticide residues down to 60 ng/cm², which is much lower than the maximum residue level for food safety. This high-performance SERS substrate based on a flexible transparent nanopaper for rapid in situ detection has great potential in various practical applications such as food safety and environmental monitoring.