Chem. Commun. 48, 5913 (2012). - Lasun

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COMMUNICATION
Highly efficient SERS test stripsw
Downloaded by Jilin University on 22 May 2012
Published on 25 April 2012 on http://pubs.rsc.org | doi:10.1039/C2CC31604H
Ran Zhang,a Bin-Bin Xu,a Xue-Qing Liu,a Yong-Lai Zhang,a Ying Xu,a Qi-Dai Chen*a and
Hong-Bo Sun*ab
Received 3rd March 2012, Accepted 24th April 2012
DOI: 10.1039/c2cc31604h
We present a facile production approach to highly efficient
SERS test strips by physical vapor deposition of silver on paper,
which contains natural wrinkle and fibril structures. The SERS
test strips open the door to highly sensitive (e.g., 10 10 M) SERS
detection in a convenient fashion.
In recent years, surface enhanced Raman scattering (SERS)
spectroscopy combining molecular fingerprint specificity with
potential single-molecule sensitivity1 has seen thorough development in chemical2 and biological analysis.3 It is attractive for its
enhancement, which allows quick detection of low-concentration
analytes.4 For SERS detection, micronanostructured metallic
substrates that give rise to localized surface plasmon resonances
(LSPRs) are widely used in both theoretical research5 and
experimental analysis.6 To date, the strong interaction between
LSPR and analytes has been used to boost the efficiency of
various analytical techniques, especially for SERS.7 Currently,
the reported SERS substrates are generally based on various
metallic substrates, such as rough metallic surfaces,8 metal
colloidal solutions,9 fractal metal films,10 periodic nanostructures11
and so on. Generally, the SERS substrates could be readily
produced through complex procedures including classical
‘‘bottom–up’’ and ‘‘top–down’’ approaches,12 represented by
lithography13 and self-assembly,14 respectively. Despite the
fact that these SERS-active substrates produce tremendous
SERS signal enhancement, even single molecule detection,
their complicated synthesis or fabrication procedures make
them inconvenient in common analysis. Moreover, their high
cost and storage problems further limit their commercial uses,
especially for routine laboratory analysis and on-site analysis
in the common medical care and public safety fields.
a
State Key Laboratory on Integrated Optoelectronics,
College of Electronic Science and Engineering, Jilin University,
2699 Qianjin Street, Changchun, 130012, P. R. China.
E-mail: chenqd@jlu.edu.cn, hbsun@jlu.edu.cn;
Fax: +86-431-85168281; Tel: +86-431-85168281
b
College of Physics, Jilin University, 119 Jiefang Road, Changchun,
130023, P. R. China
w Electronic supplementary information (ESI) available: Experimental
details; SERS spectrum of p-MA; SERS spectrum of R6G excited by a
laser of 785 nm; SEM images of silver of different thicknesses; SERS
spectra of paper with silver thicknesses from 10 nm to 90 nm;
1360 cm 1 and 1507 cm 1 peak intensity relation; TEM of nanofibrils;
EX distribution simulations; SERS activity spectra; EDX spectrum
and maps of the silver covered paper; calculation of SERS enhancement factor. See DOI: 10.1039/c2cc31604h
This journal is
c
The Royal Society of Chemistry 2012
As a convenient analysis and detection medium, test strips
such as pH test strips, glucose test strips, urine test strips and
lipolysis test strips are widely used in both research fields and
daily life because of their distinct convenience and high
efficiency. Especially in various biological and chemical detections,
test strips also demonstrate many unique advantages including
low-cost, easy handling, portability, high sensitivity and less
analyte consumption. However, for SERS spectroscopy, there is
still a lack of generally available and highly efficient substrates,
just like test strips, which significantly limits its wide applications and makes SERS analysis an ‘‘in-lab-only’’ technique. At
present, to prepare and use a high-performance SERS substrate
in a similar manner to test strips, is still not only a dream but
also a scientific challenge.
Paper, which is widely used in our daily life, is considered to
be a cheap and flexible substrate. Nowadays, the use of paper
has been extended beyond its general applications, for instance
in electronic displays15 and sensors.16 Fang et al. demonstrated
the use of Ag/Au nanoparticle-coated filter paper in a variety of
SERS measurements.17 In this communication we propose
convenient and efficient SERS test strips by simple physical
vapor deposition (PVD) coating of a silver layer on paper for
the first time. With the help of our SERS test strips, sensitive
and reproducible SERS tests are realized in a low cost and facile
fashion. Taking advantage of the connatural hierarchical
micro/nanostructures of paper constructed from irregular
stacking of cellulose microfibers which have abundant nanofibrils
and wrinkles on the surface, a deposited silver nanolayer could be
arranged into a flexible and wrinkled plasmonic nanostructure,18
which therefore, forms a large-area SERS ‘‘hot spot’’. As a
representative illustration, Rhodamine 6G is used as a probing
molecule and the limit of detection (LOD) of our SERS test strip
is at the level of 10 10 M.
Based on our experiments, we find that the microfibers of
the paper play a main role in the formation of SERS ‘‘hot
spots’’. So we first screened 6 kinds of candidate papers,
including filter paper, napkin paper, sulfate paper, kraft paper,
printing paper and newspaper. Fig. 1 shows scanning electron
microscope (SEM) images of the different kinds of papers. We
find that the fibers of the filter paper, napkin paper and sulfate
paper are relatively pure and the fibers of the kraft paper,
printing paper and newspaper are filled with some fragments.
Interestingly, the microfiber networks of the filter paper and
napkin paper are very loose, which could render them with
abundant porosity to ensure a hydrophilic and soft performance
Chem. Commun., 2012, 48, 5913–5915
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Fig. 1 SEM images of different kinds of papers: (a) filter paper; (b) napkin
paper; (c) sulfate paper; (d) kraft paper; (e) printing paper and (f) newspaper.
The insets are the fiber structures of the corresponding papers. The scale
bars of the outset and inset panels are 100 and 5 mm, respectively.
for their lab and life usage, respectively. Sulfate paper, which is
produced from pure cellulose fibers by using various sulfurous
salts to extract the lignin from wood chips in large pressure
vessels, shows a relatively smooth surface. Despite these
papers showing distinct differences in surface topography,
magnified images (see the insets of each SEM image) show
that there are abundant wrinkles and hierarchical structures
on the microfiber surfaces, which contribute to a significant
roughness for SERS substrates.
We fabricated the SERS test strips by decorating a certain
thickness of silver on the rough surfaces of the papers by PVD.
The different surface topography would have considerable
influence on the formation of SERS ‘‘hot spots’’. The SERS
test strips were versatile materials for different molecules and
excitation wavelengths, and we took Rhodamine 6G (R6G)
excited by a laser of 514 nm as an example for this communication (the Raman spectrum of p-mercaptoaniline (p-MA) is
available in the ESIw, Fig. S1 and the SERS spectrum of R6G
excited by a laser of 785 nm is shown in the ESIw, Fig. S2).
Fig. 2a shows the Raman spectra tested over different kinds of
silver coated papers. Notably, all the listed Ag/papers show
strong SERS activity due to the hierarchical roughness derived
from the natural wrinkles of the microfibers.
To investigate the trace detection capabilities of the SERS
test strips, the Ag/printing paper substrate was used as a
Fig. 2 Raman spectra of (a) different kinds of Ag/papers, R6G 10 6 M;
(b) 0.1 nM of the R6G collected from the Ag/printing paper substrate;
(c) different concentrations of R6G, inset is a semi-log plot of the
concentration versus 1651 cm 1 peak intensity; (d) 25 different spots on
the same fiber, R6G 10 6 M.
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Chem. Commun., 2012, 48, 5913–5915
representative example for the detection of R6G with a
concentration of 0.1 nM. Obviously, even down to such a
low concentration, strong SERS signals could still be identified
clearly (Fig. 2b), indicating the excellent enhancement of our
Ag/paper substrates. To avoid any undesired influence from
the additives in the paper, the PVD silver coating layer should
cover the entire surface without any exposure. Fig. S3 (ESIw)
shows SEM images of paper coated with silver of different
thicknesses. We found that the fibers were entirely covered by
silver when the thickness reached 20 nm (monitored by the film
thickness meter). SERS spectra of paper substrates with silver
thicknesses ranging from 10 nm to 90 nm all showed great
enhancement, in which the highest enhancement was found at
50 nm of silver coating (ESIw, Fig. S4). Fig. 2c shows Raman
spectra for different concentrations of R6G. All the characteristic
bands of the R6G exhibited a monotonic decrease in intensity
with decreasing concentration. A semi-log plot of the concentration
versus the 1651 cm 1 peak intensity showed a monotonic increase
in the Raman intensity with increasing R6G concentration (inset of
Fig. 2c). The 1360 cm 1 and 1507 cm 1 peak intensity relations are
shown in Fig. S5 (ESIw). The SERS enhancement relied on ‘‘hot
spots’’ and they were often scattered in the normal substrate, which
brought a lot of troubles to the test and significantly reduced their
reproducibility. However, in the paper-based SERS strips, the
microfibers which contain abundant rough wrinkles cover
B80% of the paper surface. Such a high coverage of microfibers
made the Ag/paper substrate full of ‘‘hot spots’’, which imparted
superior SERS enhancement to the Ag/paper composites, thus they
could work as highly efficient SERS test strips. To test the
reproducibility of our SERS strips, 25 spots were randomly chosen
on one microfiber, and the SERS signals were almost consistent,
which confirmed the homogeneity (Fig. 2d).
To gain further insight into the mechanism of the high
SERS enhancement of our test strips, we systematically studied
the structure of the paper. It was known that the cellulose fibers
in paper were wood cells which are usually a few millimetres
long, and several to tens of micrometres wide depending on the
origin. On the surface of the fibers, there are secondary micro/
nanofibrils that form wrinkles, as shown in Fig. 3a and b.
The wrinkles distributed in an approximately parallel arrangement with an interval of about 500 nm and a height of about
100 nm (Fig. 3c).
Fig. 3 SEM image of (a) a fiber with wrinkles distributed in an approximately parallel arrangement with an interval of about 500 nm; (b) the silver
nanoparticles compactly accumulated on the wrinkles corresponding to the
rectangular region in (a); AFM image and the cross section of (c) wrinkles;
(d) nanofibrils.
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Published on 25 April 2012 on http://pubs.rsc.org | doi:10.1039/C2CC31604H
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Fig. 4 (a) SERS test strips simulated the commercial pH test strips
which were made from silver coated paper; (b) the flexible and wellencapsulated SERS test strip; (c) the concentration process of the
analyte dropped on the test strips.
From the magnified image (Fig. 3b), we could observe that
the silver nanoparticles compactly accumulated on the wrinkles,
which further increased the surface roughness. The wood fibers
would be mechanically separated; the TEM image of a single
microfiber shows nanofibrils with diameters between 20 nm and
100 nm (ESIw, Fig. S6). The height of the tightly arranged
nanofibrils was about 10 nm, as shown in the section profile of
the atomic force microscopy (AFM) image (Fig. 3d). The
electric field component EX distribution simulations of the
enhancements caused by the periodic wrinkles and nanofibril
structures were implemented by the finite-difference timedomain (FDTD) method (ESIw, Fig. S7), which showed
104 and 400 times enhancements to the SERS signal, respectively.
Compared with chemical silver plating and PVD of silver film on
glass (ESIw, Fig. S8), we find a tremendous contribution of the
cellulose fibers to the SERS intensity.
Fig. 4a and b shows photographs of our SERS test strips.
Similar to pH test strips, the silver test papers could be easily
tailored into strips. The test strips can remain active for 13.5 h,
and we can place them in an airtight N2 atmosphere for long
term storage (ESIw, Fig. S9 and 10). Elemental maps of Ag,
C and O show clear boundaries on paper with and without
silver coating, as well as the homogeneous Ag distribution
(ESIw, Fig. S11). Compared with other SERS substrates, our
SERS test strips also exhibit the advantages of flexibility,
which allows bending without any desquamation, and enrichment of analytes due to the hydrophobicity. Fig. 4c shows the
contact angles (CAs) of a R6G aqueous droplet during the
evaporation process. The starting CA was about 1201, indicating
the hydrophobic feature. With the evaporation of water, the
droplet became smaller and smaller without diffusion. In this
regard, the analyte (here R6G) could be enriched in a defined
region, which further lowered detection concentration and
improved its sensitivity significantly.
In conclusion, we have developed a facile method for the
production of SERS test strips by PVD coating of silver nanolayers on various kinds of paper, including filter paper, napkin
paper, sulfate paper, kraft paper, printing paper and newspaper.
This journal is
c
The Royal Society of Chemistry 2012
Typically, the wrinkles and nanofibrils of the natural fiber on
paper formed a rough surface with strong LSPR, which could
serve as SERS ‘‘hot spots’’. In addition, our SERS test strips
also showed unique advantages of flexibility and hydrophobicity.
All of these features made our SERS test strips a promising
platform for highly efficient SERS detection down to very low
analyte concentration (10 10 M). The novel SERS test strips
open the way for convenient SERS detection and exhibit broad
commercial prospects in the near future.
This work was supported by the National Science Foundation
of China (Grant Nos. 90923037, 61137001, 61008014 and
60978048), as well as the China Postdoctoral Science Foundation
(Grant No. 20110490156). The experimental help and useful
discussion with Mr. Wen-Yi Zhang is also greatly acknowledged.
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