Fabrication and application of long strands of silicon nanowires as sensors for bovine serum albumin detection

Article abstract of DOI:10.1063/1.2123393

Strands of aligned silicon nanowires (SiNWs) with lengths over 2 mm and diameters of 35 nm were synthesized via a two-step controlled growth. The large dimensionality made it possible to fabricate an individual strand of SiNWs into an electrode for cyclic voltammetric detection of bovine serum albumin (BSA). The strand sensor with the immense surface area and electrical conductivity of SiNWs shows the highest sensitivity and widest linear range ever reported for BSA detection.

Full text of DOI:10.1063/1.2123393

Fabrication and application of long strands of silicon nanowires as sensors for bovine
serum albumin detection
Ming-Wang Shao, Hui Yao, Ming-Liang Zhang, Ning-Bew Wong, Yue-Yue Shan, and Shuit-Tong Lee
Citation: Applied Physics Letters 87, 183106 (2005); doi: 10.1063/1.2123393
View online: http://dx.doi.org/10.1063/1.2123393
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APPLIED PHYSICS LETTERS 87, 183106 ͑2005͒
Fabrication and application of long strands of silicon nanowires
as sensors for bovine serum albumin detection
Ming-Wang Shao, Hui Yao, Ming-Liang Zhang, and Ning-Bew Wonga͒
Center of Super-Diamond and Advanced Films (COSDAF) and Department of Biology and Chemistry,
City University of Hong Kong, Hong Kong SAR, China
Yue-Yue Shan and Shuit-Tong Leeb͒
Center of Super-Diamond and Advanced Films (COSDAF) and Department of Physics and Materials
Science, City University of Hong Kong, Hong Kong SAR, China
͑Received 28 June 2005; accepted 8 September 2005; published online 25 October 2005͒
Strands of aligned silicon nanowires ͑SiNWs͒ with lengths over 2 mm and diameters of 35 nm were
synthesized via a two-step controlled growth. The large dimensionality made it possible to fabricate
an individual strand of SiNWs into an electrode for cyclic voltammetric detection of bovine serum
albumin ͑BSA͒. The strand sensor with the immense surface area and electrical conductivity of
SiNWs shows the highest sensitivity and widest linear range ever reported for BSA detection.
© 2005 American Institute of Physics. ͓DOI: 10.1063/1.2123393͔
One-dimensional nanostructures are attractive for appli-
cations as ultrasensitive sensors.1 Among functional materi-
als, silicon occupies a special position because of its excel-
lent semiconducting, mechanical, and optical properties,
as well as compatibility with well-established silicon
technology.2–7 When functionalized with gold and a self-
assembled organic layer, the Si wafer may be useful for bio-
material applications, taking advantage of the bioinertness of
gold and the high surface density of the organic layer.8–10 For
sensing applications, silicon nanowires have a number of ad-
ditional advantages, such as easy surface modification with
various functional groups,11–13 a vast surface-to-volume ra-
tio, fast response, reproducibility, and good reversibility at
room temperature.14–16 The several studies14–16 demonstrat-
ing the sensing potential of silicon nanowires ͑SiNWs͒ all
exploited the current-voltage characteristics of a made
SiNW. While the sensitivity of a field-effect transistor-based
sensor is promising, its detection selectivity nevertheless de-
pends on the judicious surface functionality designed to
respond to the specific species targeted for detection. On the
other hand, the cyclic voltammetry ͑CV͒ method, widely
adapted in sensor design, not only offers detection with high
sensitivity but also the selectivity with relative ease, which
derives intrinsically from the species-dependent redox
potentials.
In this work, we combine the advantage of the CV
method with the salient features of one dimensional nanoma-
terials to demonstrate the further biosensing potential of
SiNWs with sensitivity and potential selectivity. We func-
tionalize millimeter-long aligned SiNWs strands with gold
and thioacetic acid to condition them as anchors for electro-
active species and as electrodes to mediate electron transfer
reactions in solution. The modified SiNWs strands provide
immense surface areas and thus high-density immobilization
sites for proteins as well as electrical conductivity to enable
sensitive protein detection via the CV method. Although pro-
teins can be detected in various ways,17–19 the linear range of
detection is still unsatisfactory.20 Moreover, the selectivity of
the sensors remains a challenging issue. Electrochemical
functionalization is particularly powerful for biosensing be-
cause it makes possible the detection of a variety of proteins
with different electroactive groups, such as many enzymes
and transport proteins. Additionally, different proteins could
be distinguished by differences in redox potentials, although
deconvolution may be necessary to resolve the overlapping
CV peaks.
The present work reports the first use of SiNWs as an-
choring electrodes to detect proteins via the CV method. In
addition, the use of strands of high-density SiNWs provides
huge surface area to immobilize analytes and enhanced
electrical conductivity. Consequently, compared to current
detection methods,17–19 the SiNWs strand-based electrode
shows a wide linear concentration range and extremely high
sensitivity of detection.
SiNW strands were prepared using the oxide-assisted
growth method21,22 via a simple thermal-evaporation of SiO
powder. Briefly, SiO powder ͑Aldrich, 325 mesh, 99.9%͒ on
an alumina boat was placed at the center of a horizontal
alumina tube mounted inside a high-temperature furnace,
and a Si wafer substrate was loaded near one end of the tube.
The system was evacuated to 1ϫ10−2 Torr, then a carrier
gas consisting of 95% Ar and 5% H2 was introduced and
maintained at 10 Torr under a constant flow rate of
150 sccm. The SiO powder was heated to 1250 °C at a rate
of 40 °C/min and maintained at that temperature for 10 min,
while the Si wafer was located in the zone of 900 °C. Then,
the furnace was cooled down to room temperature. The Si
wafer was removed, immersed in a 1% HF aqueous solution
for 30 s, washed with de-ionized water and ethanol several
times, and dried in air. The furnace was reloaded with new
SiO powder and the processed Si wafer replaced at the same
locations as before, so that they were at the same temperature
as in the first step. The furnace was maintained at 1250 °C
for 5 h, and finally cooled to room temperature.
The product collected from the Si wafer was etched with
5% HF aqueous solution for 30 min. It was then rinsed with
de-ionized water and immersed in 1ϫ10−3 M AuCl3 aque-
ous solution. After drying, the SiNW product was dipped in
thioacetic acid for 1 h, and then washed with ethanol and
a͒
Electronic mail: bhnbwong@cityu.edu.hk
Electronic mail: apannale@cityu.edu.hk
b͒
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0003-6951/2005/87͑18͒/183106/3/$22.50 87, 183106-1 © 2005 American Institute of Physics
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183106-2
Shao et al.
Appl. Phys. Lett. 87, 183106 ͑2005͒
FIG. 2. ͑a͒ TEM and high-resolution TEM ͑inset͒ images of a single silicon
¯
nanowire showing a diameter of 35 nm with the growth direction of ͓112͔;
and ͑b͒ TEM image of a silicon nanowire coated with Au nanoparticles.
͑220͒ crystal planes of silicon according to the JCPDS 27-
1402, thus indicating that the growth axis of the wire is along
FIG. 1. SEM images of as-prepared SiNWs showing: ͑a͒ a strand-like struc-
ture; and ͑b͒ lengths larger than 2 mm; and ͑c͒ wires growing parallel with
a diameter of ϳ35 nm; the EDX ͑inset͒ indicating Si:O atomic ratio of
57:43; and ͑d͒ aligned wires at the tip of strand.
¯
the ͓112͔ direction.
The growth of millimeter-long SiNW strands is the result
of restraining secondary nucleation. After the first step of
growth at 1250 °C for 10 min, many big and small silicon
nuclei were grown on the Si wafer. Then the Si wafer was
treated with 1% HF aqueous solution for 30 s, the smaller
silicon nuclei were removed and larger ones survived. In the
second step of growth, additional silicon atoms deposited
and grew on the larger nuclei instead of germinating
new nucleation sites due to energy obstacle, leading to the
formation of long SiNW strands.
de-ionized water for 10 min, respectively. The processed,
long SiNW strand was fabricated into the working electrode
on an insulating plastic substrate by connecting one end of
the strand to the electrical circuit using conducting glue.
25 mg bovine serum albumin ͑BSA͒ was dissolved in
100 mL 0.5% NaCl aqueous solution. A 10.0 mL phosphate
buffer solution ͑PBS͒ was added to the voltammetry cell. For
BSA measurements, potential scanning was performed in the
range of 0.6 to −0.4 V at a scan rate of 100 mV s−1. Cyclic
voltammetry was recorded with a Chi 660A electrochemical
workstation ͑CH Instruments, Chenhua Corp, Shanghai,
China͒. A conventional three-electrode system was employed
with a functionalized SiNW strand as the working electrode,
a saturated calomel electrode ͑SCE͒ as the reference elec-
trode, and a platinum electrode as the counter electrode. All
potentials reported in this work were referenced to the SCE.
The morphologies of the as-prepared products, as char-
acterized by scanning electron microscopy ͑SEM͒ with a
Philips XL 30 FEG SEM spectrometer, show a strand-like
structure ͑Fig. 1͒. The strand is about 0.5 mm in diameter
and more than 2 mm in length ͓Figs. 1͑a͒ and 1͑b͔͒. Each
strand consists of a high density of SiNWs with a diameter of
about 35 nm, grown almost parallel to one other ͓Fig. 1͑c͔͒.
The Electron dispersive x-ray ͑EDX͒ spectrum ͓Fig. 1͑c͒,
inset͔ shows the nanowires, on the average, to have an
atomic ratio of Si:O=57:43, consistent with the oxide-
assisted growth.21,22 The tip of the strand is shown in Fig.
1͑d͒, displaying the aligned SiNWs.
When the SiNWs are treated with 5% HF aqueous solu-
tion for 30 min, the outer amorphous silicon oxide is re-
moved and Si–H bond is formed. Gold atoms are reduced
and attached as nanoparticles to the hydrogen-terminated
SiNWs by simple dipping in AuCl3 aqueous solution, the
reaction of which was reported in our previous work.23 Fig-
ure 2͑b͒ shows the TEM image of a typical Au-modified
SiNW with a diameter of ϳ30 nm. It shows that the SiNW is
covered with Au nanoparticles with a diameter of 7–15 nm
and at a density of about 1012–13 cm−2. The gold surfaces can
be readily functionalized via reaction with thiol-containing
compounds under mild conditions.10,24 The thioacetic acid
has a thiol group at one chain end to connect with gold and
a carboxyl group at the other chain end to anchor the amino
groups of proteins, respectively.
The SiNW strand, after treatment with HF, AuCl3 and
thioacetic acid, was placed on an insulating plastic substrate.
One end of the strand was connected to a copper wire using
conducting glue. The electrode, 3 mm in length and 0.5 mm
in diameter, was used as a sensor to detect BSA via CV. As
BSA has no metal ions, its redox reaction is expected to take
place on the sulfur double bonds. The process may thus be
written as follows:25
Figure 2͑a͒ presents a transmission electron microscopy
͑TEM͒ image ͑recorded using a Philips CM300 FEG TEM͒
of a typical single SiNW with a diameter of 35 nm. The
selected area electron diffraction pattern of the SiNW ͓Fig.
2͑a͒, lower inset͔, taken along the zone axis of the ͓111͔
͑R − S = S − R͒solution → ͑R − S = S − R͒
͑R − S = S − R͒adsorbed + 2H+ + 2e
,
adsorbed
¯
direction, shows sharp diffraction spots due to the ͑220͒ and
↔ ͑R − SH − SH − R͒
.
adsorbed
¯
͑022͒ planes, revealing the single crystal nature of the wire.
High-resolution TEM image ͓Fig. 2͑a͒, upper inset͔ shows
that the wire consists of a single-crystal core of 20 nm with
few defects and an amorphous shell of 4–8 nm; and the
amorphous shell is silicon oxide while the core is silicon.
The core reveals lattice fringes with a separation of
When CV is employed, the mechanism of detection should
be the same, although the material of electrode is different.
Figure 3 shows the cyclic voltammograms of BSA in pH
7.0 PBS at the functionalized SiNW electrode with a concen-
tration of 0, 0.38, 0.63, and 1.08 ␮M. The voltammograms
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0.193 nm, which agrees with the interlayer spacing of the
exhibit good response and clear cathodic and anodic peaks.
141.214.17.222 On: Tue, 23 Dec 2014 04:50:02

183106-3
Shao et al.
Appl. Phys. Lett. 87, 183106 ͑2005͒
In summary, millimeter-long strands of SiNWs were pre-
pared via restraining secondary nucleation. The SiNWs on
each strand were aligned with a length larger than 2 mm and
an average diameter of 35 nm. The large dimensionality of
the strand makes it convenient to manipulate and fabricate
each strand directly into a sensor with proper surface func-
tionalization. The modified SiNW sensor can detect BSA via
CV, while the high-density Au nanoparticles and conductiv-
ity supplied by SiNWs give the sensor high sensitivity and
reproducibility.
This project is supported by the Research Grants Council
of the Hong Kong Special Administrative Region ͑Project
Nos. CityU 102403 and CityU 3/04C͒, China.
FIG. 3. Cyclic voltammograms of BSA in pH 7.0 PBS with a concentration
of 0, 0.38, 0.63, and 1.08 ␮M ͑the size of the loop increases with the
concentration͒. Scan rate: 100 mV s−1
.
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FIG. 4. Peak current vs concentration of BSA showing a linear range
beyond 1 ␮M.
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