Silica Cladding of Ag Nanoparticles for High Stability and Surface-Enhanced Raman Spectroscopy Performance
Zhao et al. Nanoscale Research Letters
Silica Cladding of Ag Nanoparticles for High Stability and Surface-Enhanced Raman Spectroscopy Performance
Miaomiao Zhao 0
Hao Guo 0
Wenyao Liu 0
Jun Tang 0
Lei Wang 0
Binzhen Zhang 0
Chenyang Xue 0
Jun Liu 0
Wendong Zhang 0
0 Science and Technology on Electronic Test & Measurement Laboratory, North University of China , Taiyuan, Shanxi 030051 , China
For high-precision biochemical sensing, surface-enhanced Raman spectroscopy (SERS) has been demonstrated to be a highly sensitive spectroscopic analytical method and Ag is considered to be the best material for SERS performance. Due to the high surface activity of Ag nanoparticles, the high stability of Ag nanostructures, especially in moist environments, is one of the key issues that need to be solved. A method for silica (SiO2) cladding of Ag nanoparticles (NPs) is demonstrated here for high sensitivity and long-term stability when putted in aqueous solution. The chemically inert, transparent, hydrophilic, and bio-compatible SiO2 surface acts as the protection layer for the Ag nanoparticles, which can also enhance the Raman intensity to a certain extent. In our study, the Ag@SiO2 core-shell substrate can detect crystal violet solutions with molar concentrations down to 10−12 M. After 24 h of immersion, the reduction in Raman scattering intensity is about 85 % for sole Ag NP films, compared to 12 % for the Ag coated with a 10-nm SiO2 layer. This thickness was found to be optimum for Ag@SiO2 core-shell substrates with long-term stability and high SERS activity.
SERS; Ag@SiO2; Long-term stability; Layer thickness
Background
As a powerful spectroscopic technique, surface-enhanced
Raman spectroscopy (SERS) has shown promising
applications in surface adsorption, biochemical sensing, and
tracelevel analysis as a result of its high sensitivity, rapid
response, and the advantages of nondestructive detection
[
1–4
]. The mechanism for SERS is mainly attributed to the
electromagnetic field enhancement caused by the localized
surface plasmon resonance of noble metal nanoparticles
(NPs). For isolated metal particles, the electromagnetic
enhancement can reach up to 106–107, and when in
nanogaps (so-called hotspots), it can reach up to 1010–1011,
because of the electromagnetic coupling between the
neighboring metal NPs [
5, 6
].
Among noble metals, Ag is considered to be one of
the most promising candidates for SERS applications
due to its low loss in optical frequency and high
plasmonic efficiency, as well as its lower cost compared to
other noble metals [
7–10
]. However, Ag NPs suffer from
sulfur contamination, oxidation, and agglomeration in
water and the atmosphere, and the biological incompatibility
of Ag is obvious, all of which limit their practical
application.
Significant efforts have been devoted to improve the
chemical stability of Ag NPs, and core-shell nanostructures
are one of the most popular methods, which have been
reported in literatures [
11, 12
]. Ag NPs capped with Au
[13], graphene [
14
], and TiO2 [
15
] have been reported in
recent years. Ma et al. [
16
] prepared ultrathin (~1.5 nm)
Al2O3 films by the atomic layer deposition technique on
Ag nanorods that can maintain robust morphologies to a
temperature of 400 °C. Li et al. [
17
] reported the use of a
single-atom-thick monolayer of graphene for the protection
of Ag NPs that can function as a highly stable SERS
substrate for nearly 1 month with ambient aerobic exposure.
The capping thickness of the protection layers can be
well controlled by the fabrication technology, which
has greatly extended the application of Ag-based SERS
substrates in different fields. However, a defect of this
coating approach is the tremendous decrease in SERS
activity, which is caused by the coating layers that separate
the target molecules from the Ag NPs and by the possible
morphology changes of the Ag NPs engendered during the
coating process. Thus, it is vital to find ways to deposit
protective layers which can cap Ag NPs at relatively low
temperatures and to precisely control the coating thickness
to prohibit the reduction of SERS sensitivity, while still
thick enough to be robust towards moist environments.
In this study, Ag NPs were fabricated on 2-in. silicon
wafers with a sputtering and vacuum annealing process.
We employed inductively coupled plasma-enhanced
chemical vapor deposition (ICPECVD) to deposit ultrathin SiO2
layers that can cap the exposed surface of Ag NPs with a
deposition temperature of 60 °C. After deposition of the
SiO2 layer, the SERS performance, as well as the coating
influences on the stability of the Ag NPs in a water
environment, were investigated. It was found that an
ultrathin (10 nm) SiO2 layer was thick enough to
effectively control the distance between the particles to avoid
the agglomeration and oxidation even when immersed in
water for 15 days. Furthermore, we found that the
coreshell structure can improve the SERS perf (...truncated)