Nanocellulose-silver ensembles for ultrasensitive SERS: An investigation on the role of nanocellulose fibers in the generation of high-density hotspots

Large scale hotspot engineering is a significant approach for the development of highly efficient surface enhanced Raman scattering (SERS) platforms. Herein, 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-oxidized nanocellulose fiber (T-NCF) serves as a labyrinth for developing highly sensitive and stable silver-based SERS platform enabling single molecular level SERS detection of analytes. The SERS activity of 4-methylbenzenethiol (4-MBT) in silver nanoconstructs with dissimilar size and shape (denoted as Ag/NCF-I and Ag/NCF-II systems) synthesized by varying T-NCF to Ag+ ratio, exhibited femtomolar sensitivity regardless of their structural variation. A detailed investigation of the SERS performance of both systems with 4-MBT at extremely low concentration (10−15 M) is carried out with the help of large-area Raman intensity mapping in order to evaluate the role of T-NCF in Raman signal enhancement. The analytical enhancement factors (AEFs) for Ag/NCF-I and Ag/NCF-II are calculated to be 1.4 × 1012 and 4.8 × 1011, respectively. A mechanism of local enrichment of analytes is postulated anticipating the ability of flexible nanocellulose fibers to congregate AgNPs, resulting in induced plasmonic coupling of local electromagnetic fields and high-intensity hotspot generation. The potential of T-NCF in generating hotspots can be considered as an alternative strategy to develop standards with long-term colloidal stability and scale-up production of highly sensitive AgNP based plasmonic platforms. This investigation ascertains the potential of nanocellulose fibers in the development of a robust lithography-free SERS sensing platform with single molecule level sensitivity.