NCC-based SERS substrate: fundamentals, preparation and applications.

Abstract

This study reports the isolation and characterization of nanocrystalline cellulose (NCC) produced from discarded cigarette filters (DCF). The DCF were processed into cellulose via ethanolic extraction, hypochlorite bleaching, alkaline deacetylation, and then converted into NCC by sulphuric acid hydrolysis. The morphological structures of the isolated NCC, established with TEM, showed that the nanocrystals were needle-like with length in the range of between 62–258 nm. FEGSEM showed the morphological transition of the micro-sized DCF to a self-assembled NCC, while EDX revealed the presence of Ti (as TiO2) in DCF, which was retained in the NCC. A NCC sample that was freeze-dried showed a specific surface area of ~8 m2/g. The crystallinity of the NCC film and the freeze-dried samples were ~97% and ~94%, respectively. Crystallite sizes of the freeze-dried (8.4 nm) and film (7.6 nm) samples correlated with the mean width (8.3 nm) of the NCC, observed with the TEM. The isolated NCC was used in the dual role as a reducing- and stabilizing agent in the formation of silver nanoparticles (AgNPs). By this method, a notable size variation of the synthesized AgNPs was found over the pH range of between 5–10, ranging from 4.61 nm at pH = 9 and increasing to 19.93 nm diameter at pH = 5. The size and yield of the AgNPs were also affected by the reaction time and concentration. The spherically shaped AgNPs induced a localized surface plasmon resonance (LSPR) at around 416 nm. The Ag content in the dry AgNPs was 81.9 wt%, which correlated with 82.1 wt% mass left at 600 ºC. Further analysis showed that the dry AgNPs were macroporous with reduced surface area and porosity upon calcination. The sensitivity of the AgNPs showed excellent surface-enhanced Raman scattering (SERS) of riboflavin. The limit-of-detection (LOD) for riboflavin, based on a signal-to-background ratio of 3:1, was found to be 3 x 10-7 M. The intensities of SERS signals increased with increase in concentration. In addition, clusters of AgNPs were synthesized with NCC isolated from Whatman cellulose filter paper, which behaved as a dual reducing- and stabilizing agent, and Stӧber silica (SiO2) provided a suitable anchoring surface. The synthesized nanocomposite (AgNPs/SiO2/NCC) was evaluated as a substrate for surface-enhanced Raman scattering (SERS) of malachite green (MG) and compared with AgNPs/NCC nanocomposite. The FTIR spectra of both nanocomposites showed a weak carbonyl band (1754 cm-1), indicating partial oxidation of the NCC. The UV-vis spectrum of AgNPs/NCC showed a narrow peak at 412 nm, characteristicof LSPR of monodispersed AgNPs. However, this peak was broad with a shoulder at 490 nm in the spectrum of the AgNPs/SiO2/NCC, which indicated clustering of the plasmonic nanoparticles. TEM micrographs showed that the plasmonic nanoparticles were monodispersed with a mean diameter of 19.5 nm in AgNPs/NCC, while they aggregated into clusters on SiO2 in AgNPs/SiO2/NCC resulting in an approximately 20 nm increase in the mean diameter of the SiO2. The SEM/EDX spectra and XRD diffractograms of the nanocomposites showed Ag as the predominant element. The SERS performance of the nanocomposites was evaluated by using MG as a probe, showed AgNPs/SiO2/NCC as a superior substrate with significant improvement in intensities of Raman peaks of MG and high sensitivity as the LOD was 0.9 nm, while AgNPs/NCC showed a LOD of 5.2 nm, based on a signal-to-background ratio of 3:1. This result underscores the huge contribution of SERS “hot spots” as AgNPs assembled into clusters in contrast to monodispersed AgNPs in the absence of SiO2.