Flexible Bacterial Cellulose Nanofiber-Based SERS Sensors with 3D High-Density Hot Spots for Direct Detection of Malachite Green on Curved Surfaces

Nanocellulose has become an attractive biodegradable material for surface-enhanced Raman scattering (SERS) sensors. However, the development of nanofibrillar cellulose-based SERS substrates that are simultaneously flexible, highly sensitive, uniform, and stable remains a great challenge. Herein, a highly sensitive, reliable, and flexible gold nanoparticle-decorated bacterial nanocellulose (AuNPs@BNC) SERS substrate with three-dimensional (3D) high-density hot spots was constructed by the combination of in situ chemical reduction and vacuum filtration. The homogeneous AuNPs were grown in situ on the surface of networked bacterial cellulose nanofibers to form 3D high-density SERS hotspots. In addition, the hydrophilic BNC with good adsorption and permeability properties could capture target molecules in the high-density hotspot region to further enhance the SERS performance. As a result, the 3D AuNPs@BNC SERS substrate not only demonstrates a high detection sensitivity of up to 1.0 × 10^-10 M and an enhancement factor of 1.8 × 10⁷ for methylene blue molecules but also shows excellent signal reproducibility (relative standard deviation (RSD) = 8.3%) and long-term storage stability (over two months). Furthermore, the flexible 3D AuNPs@BNC SERS substrate is also able to detect trace malachite green (MG) with a low detection limit of 1.0 × 10^-11 M and prominent signal homogeneity of RSD = 7.8%. Besides, hazardous MG residues on the surface of shrimp could be directly identified by using flexible AuNPs@BNC SERS substrates through a feasible wipe-and-peel method. The flexible, reliable, and stable AuNPs@BNC substrate is promising for direct, sensitive, and rapid on-site detection of toxic molecules on seafood.