Flexible Bacterial\nCellulose Nanofiber-Based SERS\nSensors with 3D High-Density Hot Spots for Direct Detection of Malachite\nGreen on Curved Surfaces

Nanocellulose has become an attractive biodegradable\nmaterial for\nsurface-enhanced Raman scattering (SERS) sensors. However, the development\nof nanofibrillar cellulose-based SERS substrates that are simultaneously\nflexible, highly sensitive, uniform, and stable remains a great challenge.\nHerein, a highly sensitive, reliable, and flexible gold nanoparticle-decorated\nbacterial nanocellulose (AuNPs@BNC) SERS substrate with three-dimensional\n(3D) high-density hot spots was constructed by the combination of\nin situ chemical reduction and vacuum filtration. The homogeneous\nAuNPs were grown in situ on the surface of networked bacterial cellulose\nnanofibers to form 3D high-density SERS hotspots. In addition, the\nhydrophilic BNC with good adsorption and permeability properties could\ncapture target molecules in the high-density hotspot region to further\nenhance the SERS performance. As a result, the 3D AuNPs@BNC SERS substrate\nnot only demonstrates a high detection sensitivity of up to 1.0 ×\n10<sup>–10</sup> M and an enhancement factor of 1.8 ×\n10<sup>7</sup> for methylene blue molecules but also shows excellent\nsignal reproducibility (relative standard deviation (RSD) = 8.3%)\nand long-term storage stability (over two months). Furthermore, the\nflexible 3D AuNPs@BNC SERS substrate is also able to detect trace\nmalachite green (MG) with a low detection limit of 1.0 × 10<sup>–11</sup> M and prominent signal homogeneity of RSD = 7.8%.\nBesides, hazardous MG residues on the surface of shrimp could be directly\nidentified by using flexible AuNPs@BNC SERS substrates through a feasible\nwipe-and-peel method. The flexible, reliable, and stable AuNPs@BNC\nsubstrate is promising for direct, sensitive, and rapid on-site detection\nof toxic molecules on seafood.