Electronic Supplementary Material
Zinc oxide nanotubes decorated with silver nanoparticles as an ultrasensitive
substrate for surface-enhanced Raman scattering
Ming Gao 1, 2, 3, Guozhong Xing 4, Jinghai Yang 1*, Lili Yang 1, Yongjun Zhang 1,
Huilian Liu 1, Hougang Fan 1, Yingrui Sui 1, Bo Feng 1, 2, 3,
Yunfei Sun 1, 2, 3,
Zhiqiang Zhang 1, Shanshan Liu 1,Sean Li 4 and Hang Song 2
1. Key Laboratory of
Functional Materials Physics and Chemistry of the Ministry of
Education, Jilin Normal University, Siping 136000, P. R. China
2. Key Laboratory of Excited State Processes, Changchun Institute of Optics, Fine
Mechanics and Physics, Chinese Academy of Sciences, Changchun 130033, P. R.
China
3. Graduate School of the Chinese Academy of Sciences, Beijing 100049, P. R. China
4. School of Materials Science and Engineering, The University of New South Wales,
Sydney, New South Wales 2052, Australia
*Corresponding author at: Institute of Condensed State Physics, Jilin Normal
University, Siping 136000, People’s Republic of China. Tel.: +86 434 3290009; fax:
+86 434 3294566. E-mail address: jhyang1@jlnu.edu.cn.
Estimation of the enhancement factor
To quantitatively investigate the SERS activity of our ZNT/Ag-NPs substrate
samples, the SERS enhancement factor is determined as follows:
EF = (ISERS/Ibulk)/(NSERS /Nbulk)
(1)
where Nbulk and NSERS are the number of molecules probed in the liquid sample and
on the ZNT/Ag-NPs substrate, respectively; Ibulk and ISERS are the corresponding
Raman intensities. In this case, the determination of the EF requires that the spectra
from adsorbed and bulk molecules are measured under the identical conditions.
To obtain a quantitative evaluation of EF, the Raman spectra of 4-MPY adsorbed
on ZNT/Ag-NPs substrate and 0.3 M 4-MPY solutions are measured to obtain the
intensity ratio of the spectra directly. The peak of the C=C stretching mode (1577
cm-1) in the Raman spectrum of 4-MPY adsorbed on ZNT/Ag-NPs substrate is
selected to calculate EF values. The ratio of the intensities (ISERS/Ibulk) is 156. The
determination of the EF needs to know the number of adsorbed molecules (NSERS) and
bulk molecules (Nbulk). The number of 0.3 M 4-Mpy solution (Nbulk) in the analyzed
volume was calculated with the methods described in the literature. [51] For the optical
configuration and microscope employed in this work, the confocal depth was 21 µm,
and the diameter of the laser beam spot was 1 µm. Thus, for the 0.3 M 4-MPY
solution, Nbulk is estimated to be 3.0×109 molecules. The observed Raman signal is
most likely from a monolayer of 4-MPY as the samples were rinsed profusely with
deionized water. Assuming that the saturated monolayer of 4-MPY molecules
oriented perpendicularly to the surface, the area occupied per molecule of 4-Mpy is
~7.0 × 10−
15
cm2. Thus, the maximum number of 4-MPY molecules on the
sub-monolayers of ZnO nanotubes under the sampling area is 1.1 × 106. Therefore,
the enhancement factor is ~4.2 × 105.
RSD
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Figure S1. RSD-SERS graph. (a) SERS spectra of 4-MPY collected on the
statistically selected 20 places of the optimized substrate and (b) the corresponding
RSD values.