NCC-based SERS substrate: fundamentals, preparation and applications.
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Date
2018
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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.
Description
Doctoral Degree. University of KwaZulu-Natal, Durban.