that due to the chemical effect. According to Garcia-Vidal and
Pendry’s model, the smaller metallic particles give higher
enhancement but the enhancement is almost the same if the
particles’ diameter is less than 15 nm.27 In this work, Ag
nanoparticles with an average diameter of 15 nm play an
important role in the ultrasensitivity.
The b-silver vanadate might also play a key role in such an
ultrahigh enhancement factor. As b-silver vanadate is a
layered compound, the Ag nanoparticle might shift to a
proper location because the laser beam is focused strongly
on an extremely small object,28,29 which is favorable to the
surface-enhanced factor.
In conclusion, a new and universal method for preparing a
substrate for surface-enhanced Raman spectroscopy with
ultrasensitive detection is proposed. The diameters of the silver
nanoparticles are 15 nm on average. By applying this method,
strong SERS signals were obtained with concentrations of Rh
6G, TNT, BSA and CV down to 1 Â 10À16, 1 Â 10À15, 1 Â
10À15 and 1 Â 10À16 M respectively. This ultrasensitive SERS
might facilitate the measurement of a wider range of analytes
at lower concentration and with a short detection time.
Financial support from the National Natural Science Foun-
dation of China (20571001, 20704001), the Education Depart-
ment (No. 2006KJ006TD) of Anhui Province, the Research
Grants Council of Hong Kong SAR (No. CityU 101807) and
the Anhui Provincial Natural Science Foundation (070414185)
is appreciated.
Fig. 2 SERS spectra on individually immobilized Ag nanoparticles:
(a) 1.0 Â 10À16 M Rh 6G methanol solution; (b) 1.0 Â 10À15 M TNT
solution; (c) 1.0 Â 10À15 M BSA solution; and (d) 1.0 Â 10À16 M CV
methanol solution.
estimated as 10–11 orders of magnitude. These peaks are
strong though the concentration of Rh 6G is much lower than
the detection limits previously reported.2
The detection and identification of trace trinitrotoluene
(TNT) is a problem of great practical interest. The SERS of
this method may possess the sensitivity required for such
applications.16 The ring breathing mode at ca. 1050 cmÀ1
,
together with the –NO2 deformation mode near 1350 cmÀ1
and the bands at 1270 cmÀ1, can be used as a ‘‘fingerprint’’ for
the detection of TNT. There are other peaks, which are quite
in accordance with the previous reference.17 Fig. 2b demon-
strates that these three SERS bands can be detected in con-
centration ranges down to 1 Â 10À15 M.
Notes and references
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scattering process associated with chemical interaction be-
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This journal is The Royal Society of Chemistry 2008
Chem. Commun., 2008, 2310–2312 | 2311