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Detection of cadmium sulphide nanoparticles by using screen-printed electrodes and a
handheld device
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2007 Nanotechnology 18 035502
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INSTITUTE OF PHYSICS PUBLISHING
NANOTECHNOLOGY
Nanotechnology 18 (2007) 035502 (6pp)
doi:10.1088/0957-4484/18/3/035502
Detection of cadmium sulphide
nanoparticles by using screen-printed
electrodes and a handheld device
Arben Merkoçi1 , Luiz Humberto Marcolino-Junior2 ,
Sergio Marı́n1,3 , Orlando Fatibello-Filho2 and Salvador Alegret3
1
Institut Català de Nanotecnologia, Campus UAB, 08193 Bellaterra, Barcelona, Catalonia,
Spain
2
Laboratório de Bioanalı́tica, Departamento de Quı́mica, Universidade Federal de São Carlos,
Rod. Washington Luiz, km 235, CP 676, 13560-970-São Carlos/SP, Brazil
3
Grup de Sensors and Biosensors, Departament de Quı́mica, Universitat Autònoma de
Barcelona, 08193 Bellaterra, Catalonia, Spain
E-mail: arben.merkoci.icn@uab.cat
Received 21 September 2006, in final form 13 November 2006
Published 3 January 2007
Online at stacks.iop.org/Nano/18/035502
Abstract
A simple method based on screen-printed electrodes and a handheld
potentiostatic device is reported for the detection of water soluble CdS
quantum dots modified with glutathione. The detection method is based on
the stripping of electrochemically reduced cadmium at pH 7.0 by using
square wave voltammetry. Various parameters that affect the sensitivity of the
method are optimized. QD suspension volumes of 20 µl and a number of
around 2 × 1011 CdS quantum dots have been able to be detected. The
proposed method should be of special interest for bioanalytical assays, where
CdS quantum dots can be used as electrochemical tracers.
S Supplementary data files are available from
stacks.iop.org/Nano/18/035502
(Some figures in this article are in colour only in the electronic version)
as small fluorescent molecules like dyes etc. It is well
known, despite the high detection limits and well known assay
protocols, that enzymes suffer from stability problems and
high cost as well as difficulties or near impossibility to carry
out simultaneous detections. Fluorescent dye labels also are
expensive, photobleach rapidly, and the equipment required to
project the image of the dyes on an array surface is expensive
and unwieldy [5].
In principle nanoparticles provide a novel platform for
improving specific activity of a label as well as affinity to the
tracer molecules (DNA probes or other biomolecules) [6, 7].
Nanosized particles have a chemical behaviour similar to small
molecules and can be used as specific labels for DNA strands
or antibodies. Nanoparticles in general and quantum dots [8]
(QDs) in particular may be expected to be superior in several
ways. Compared to existing labels, nanoparticles in general
and QDs especially are more stable and cheaper. They allow
more flexibility, faster binding kinetics (similar to those in
1. Introduction
Inorganic compounds with nanometre dimensions—nanoparticles or quantum dots—are important because of their
photovoltaic, photoelectrochemical, and electroluminescent
applications including sensors and biosensors [1–4].
Recent advances in nanobiotechnology, and particularly
the development of functionalized nanoparticles combined
with advances in molecular biology research, provide the
impulsion for the present explosion in DNA sensors and
immunosensors among other fields. These devices are playing
a growing role in various analytical applications where an
accurate, low cost, fast and on-line measuring system is
required.
To improve the assay sensitivity and to achieve a better
and more reliable analysis there is a great demand for labels
with higher specific activity. The most used labels for
DNA and immunosensors to date have been enzymes as well
0957-4484/07/035502+06$30.00
1
© 2007 IOP Publishing Ltd Printed in the UK
Nanotechnology 18 (2007) 035502
A Merkoçi et al
a homogeneous solution), high sensitivity and high reaction
rates for many types of multiplexed assays, ranging from
immunoassays to DNA analysis.
Electrical methods for the detection of nanoparticles have
been extensively used. Park et al [9] reported a conductivitybased DNA detection method utilizing oligonucleotidefunctionalized Au nanoparticles that provides an alternative to
existing detection methods (see [6, 11] in the mentioned paper)
and presents a straightforward approach to high-sensitivity and
selectivity, multiplexed detection of DNA.
Although conventional optical probes based on the
fluorescence of nanoparticles have been successfully used,
those based on electrochemical properties are still under
research. The importance of electrochemical methods in
nanoparticle based bioassays is extensively revised in the
last review of Chem. Rev., dedicated to Nanostructures in
Biodiagnostics [10] where it is emphasized among other things
that ‘Electrical detection methods offer the possibility of
portable assays that could be used in a variety of point-of-care
environments’.
The use of quantum dots as electrochemical labels for
DNA and immunosensing has several advantages. The related
technique—stripping voltammetry—compared to optical ones
is cheaper, faster and easy to use in field analysis. Moreover,
it offers the possibility for simultaneous detection of several
biological molecules in the same sample and using a unique
sensor owing to the distinctive voltammetric wave produced by
different electrochemical tracers. An electrical immunoassay
coding protocol for the simultaneous measurements of multiple
proteins based on the use of different inorganic nanocrystal
tracers have been developed. The concept is demonstrated for a
simultaneous immunoassay of β 2-microglobulin, IgG, bovine
serum albumin, and C-reactive protein in connection with ZnS,
CdS, PbS, and CuS colloidal crystals, respectively [11]. The
designed assay based on the nanocrystal dissolving shows the
efficient coupling of the multiprotein electrical detection to the
amplification feature of electrochemical stripping transduction
yielding fmol detection limits.
The offered advantages along with the possibility
to be used in several biosensing systems based on
electrochemical techniques require the development of novel
nanoparticle detection strategies. Several works on DNA
or immunoanalysis based on gold nanoparticle [12–18] or
quantum dot [19] detection by using stripping techniques
have been reported. The majority of these electrochemical
methods have used chemical dissolutions of gold nanoparticles
(in a hydrobromic acid/bromine mixture) or quantum dots
(with nitric acid) followed by accumulation and stripping
analysis of the resulting metal ion solutions. The HBr/Br2
solution is highly toxic and therefore methods based on direct
electrochemical detection of gold nanoparticle tags have been
developed [20, 21] and even applied for DNA analysis [22].
Regarding the direct electrochemical detection of quantum dots
so as to achieve a full integration of DNA electrochemical
sensors there is still much work to be done.
The following work shows a simple method for the
direct detection of cadmium sulphide quantum dots in neutral
solution medium (pH ∼ 7.0) without the need for chemical
dissolution. It is based on dropping a few microlitres of CdS
QD suspension on the surface of a screen-printed electrode and
Figure 1. Upper part: image of the hand held device system used for
CdS QD detection. The principal components are the potentiostat,
the palmtop PC and the screen-printed electrodes (SPE). Lower part:
schematic diagram of the direct voltammetric detection of the CdS
QDs using an SPE.
the subsequent square wave voltammetry detection based on
the reduced cadmium formation and stripping, giving a well
shaped and sensitive analytical signal. The proposed method
can be easily extended for other quantum dots based on other
heavy metals, offering new opportunities for applications in
electrochemical DNA genosensors.
2. Experimental details
2.1. Apparatus
All voltammetric experiments were performed using a
PalmSens (Palm Instrument BV, Houten, The Netherlands) that
consists of a portable potentiostat interfaced with a palmtop PC
(155 mm × 85 mm × 35 mm) (see figure 1, upper part).
Electrochemical experiments were carried out using a
screen-printed electrode (SPE) (Palm Instrument BV, Houten,
The Netherlands). The screen-printed electrochemical cell
consists of a graphite working electrode (diameter 3 mm),
a graphite counter-electrode and a silver pseudo-reference
electrode. Electron microscopy (TEM) of CdS QDs was
performed on a JEOL JEM-2010.
2.2. Reagents
Synthesis of CdS QDs was carried out using Schlenk techniques under nitrogen. Chemicals and solvents were used
as received from Sigma-Aldrich: cadmium perchlorate, hexamethyldisilathiane (HMSDT), and tetramethylammoniumhydroxide (TMAH).
All solutions were prepared in doubly distilled water.
Potassium dihydrogenphosphate, phosphoric acid, and sodium
2
Nanotechnology 18 (2007) 035502
A Merkoçi et al
phosphate buffer solution, pH 7.0). After this, using the same
sensor, a determined volume of the QD suspension is added
and measurements (SWV) performed by using the ‘sample’
option. In this way the subtracted curve was obtained.
No special activation of the electrode surface was used for
further experiments with the same sensor (up to six or seven
measurements).
hydroxide were purchased from Sigma-Aldrich; hydrochloric
acid (37% m/m) was purchased from PanReac (Barcelona,
Spain). Appropriate dilutions from this stock solution were
also prepared in 0.1 mol l−1 phosphate buffer solution (pH 7.0)
prior to each set of measurements.
2.3. Preparation of CdS quantum dot suspension
The preparation method is based on arrested precipitation
of water dispersed cadmium with sulphide precursors [23].
According to this method 3.228 g glutathione as modifier
and 0.799 g CdCl2 were first dissolved in 176 ml water
and stirred during 5 min. Subsequently 8.5 ml TMAH
(tetramethylammoniumhydroxide) and 315 ml ethanol were
added and after 10 min the precursor solution was thoroughly
degassed. 0.738 ml HMDST is added quickly onto the
degassed precursor solution, giving a clear (slightly yellow)
colloidal solution of glutathione-coated CdS nanoparticles.
The mixture was magnetically stirred for 1 h and the
prepared particles were precipitated by adding tetrahydrofuran
(THF). One day later the supernatant was decanted and
the precipitate was dissolved in water/THF mixture and
precipitated again with THF to remove excessive reagents and
reaction by-products, respectively. Finally, the supernatant
liquid was decanted and the precipitate was dried under
vacuum (<1 mbar). The powdery CdS nanoparticles were
dissolved again in water, obtaining a clear colloidal solution.
A stock solution of 1.86 × 1019 CdS QDs ml−1 in 0.1 mol l−1
phosphate buffer solution (pH 7.0) was prepared and used in
further experiments.
SWV of cadmium ion solution, before CdS QD detection,
was previously studied by using the SPE. Two detection
methods were performed. According to the first one the
SPE was immersed into 20 ml of a 0.1 mol l−1 HCl solution
and the SWV performed following the classical procedure:
accumulation of cadmium ions under stirring conditions and
than stripping in a quiescent solution. The results obtained
(not shown) were similar to those reported by Palchetti et al
[24]. Besides this method we pushed the research to the
detection of small sample volume so as to achieve later on
detection of lower QDs solution volumes. According to
this second method a volume as low as 20 µl of cadmium
solution dropped onto the SPE surface has been able to be
detected. (Supporting information, figure 1S, available at
stacks.iop.org/Nano/18/035502). A linear range of response
from 10 to 1000 ppb with a detection limit of around 5 ppb
cadmium ion was obtained.
2.4. Electrochemical detection of CdS QDs
3.2. Detection of CdS QDs
The measurements were performed suspending a volume of
20 µl under the sensor stripped in the horizontal position, to
ensure electrical contact (complete circuit).
Each SPE was pretreated, before using, by applying
−1.1 V for 300 s, and then square wave voltammetric (SWV)
scans were carried out until a low and stable background was
obtained.
SWV experiments were performed to evaluate the
electrochemical behaviour of the screen-printed electrode
(SPE) for CdS QD detection. The proposed protocol (figure 1,
lower part) involves the introduction of CdS QDs on the surface
of the SPE (figure 1(A)). During this step a drop of 20 µl
containing an appropriate concentration of CdS QDs (ranging
from 0.5 to 14.0 × 1016 QDs ml−1 ) was suspended onto the
SPE for 60 s and a potential of 0 V was applied. The second
step was the accumulation step. In this step (figure 1(B))
a deposition potential of −1.1 V for 120 s was applied to
promote the electrochemical reduction of Cd2+ ions contained
in the CdS QD structure to Cd0 . After the accumulation
step, SWV was performed. During this step (figure 1(C)) the
potential was scanned from −1.1 to −0.7 V (step potential
10 mV, modulation amplitude 30 mV and frequency 15 Hz),
resulting in an analytical signal due to the oxidation of Cd0 .
After the SWV measurement the SPE was manually cleaned
(figure 1(D)) with a 0.1 mol l−1 phosphate buffer solution
(pH 7.0).
A blank subtraction method was performed. The blank
was measured using a separate blank solution (0.1 mol l−1
The direct detection of CdS QD suspension on SPE represents
a novel approach with special interest for electrochemical
characterization of QDs and further applications in biosensing
based on labelling.
During the deposition time, applying a potential of
−1.1 V, reduced cadmium, necessary for further stripping,
could have been obtained via two mechanisms (see figure 2).
3. Results and discussion
3.1. Detection of cadmium ion solution dropped onto SPE
surface
First mechanism. The cadmium ions in equilibrium with CdS
nanoparticles can be directly reduced according to
(CdS)Cd2+ + 2e → (CdS)Cd0 .
(1)
Second mechanism. The direct reduction of CdS nanoparticles:
CdS + 2H+ + 2e → Cd0 + H2 S
(2)
in analogy with the cathodic reduction of PbS nanoparticles in
water solution [25], that depends on solution pH. (This is also
confirmed by the pH effect studies. See the following sections.)
The CdS QDs are reduced in this way to Cd0 while applying
a potential of −1.1 V for 120 s. After this ‘preconcentration’
the redissolution of the Cd0 formed occurs, giving the SWV
response that depends on the number of CdS QDs.
To get further insight into the response mechanism of
CdS QD detection linear sweep voltammetry (LSV) was also
performed. The plot of the corresponding peak current versus
the square root of the scan rate (Supporting information
3
Nanotechnology 18 (2007) 035502
A Merkoçi et al
Figure 2. Schematic diagram of the CdS nanoparticle detection
mechanisms either through reduced cadmium formation in acidic
(left) or neutral (right) medium.
figure 2S, available at stacks.iop.org/Nano/18/035502) did not
show a reversible system with diffusion control of the cadmium
redox reaction. Instead a mixed phenomenon (probably
coming from QDs adsorption) could have occurred, which can
be supported by the obtained plot curvature. This is due also to
the rate of mass transport (diffusion) for CdS QD nanoparticle
redox systems, which seems to be slower than that of the
cadmium ions, similar to that for Fe2 O3 nanoparticles [26].
The effect of pH on the SWV of CdS QD suspension was
studied. The results obtained (figure 3(A)) show that the peak
current decreases on increasing the pH, being more significant
from pH 7.0 to 3.0. This general augmentation of response is
related to the increase of the reduced cadmium produced by an
enhanced CdS reduction by decreasing the pH (according to
equation (2)). The results obtained show that even at pH 7.0
the SWV signal was around half of that obtained at pH 3.0
and so sufficient for further analytical use. Moreover, the
use of pH 7.0 is of interest, taking into consideration future
applications in DNA sensing, being the usual medium pH in
hybridization procedures.
The SWV parameters were optimized so as to obtain the
highest peak signal for CdS QD detection. Figures 3(B)–
(E) show the optimization results for a 20 µl drop of QD
suspension at pH 7.0 phosphate buffer. A maximum response
at 25 Hz was obtained while changing the frequency from
15 to 35 Hz (figure 3(B)), being the modulation amplitude
maximum at 30 mV at an operation range from 10 to 40 mV
(figure 3(C)). The effect of deposition potential upon QD
detection was also studied. As expected, the current response
was increased by decreasing the deposition potential from
Figure 3. (A) Effect of pH; (B) SW frequency; (C) modulation
amplitude; (D) deposition potential; (E) deposition time to the SPE
response for CdS QDs (4.65 × 1017 mol l−1 ) in a solution of
0.1 mol l−1 phosphate buffer (pH 7.0). Square-wave voltammetric
scan with frequency of 25 Hz, step potential 10 mV and amplitude of
30 mV. Deposition potential of −1.1 V during 120 s.
Figure 4. Performance of ten different sensors during measurements
with a 14 × 1016 QDs ml−1 concentration suspension. Others
experimental conditions as in the figure 3.
−1.0 to −1.2 V. An unexpected decrease was obtained on
decreasing the potential to −1.3 V (see figure 3(D)). This QD
current decrease is related to an inhibition of the response at
4
Nanotechnology 18 (2007) 035502
A Merkoçi et al
Figure 5. Square-wave stripping voltammograms after blank subtraction for increasing concentration of QDs: (a) 0.5, (b) 2.0, (c) 4.7, (d) 7.4,
(e) 9.3, (f) 12.1, (g) 14.0 × 1016 QDs ml−1 . Also shown is the corresponding calibration plot (right) over the range 0.5–14.0 × 1016 QDs ml−1 .
The measuring solution was 0.1 mol l−1 phosphate buffer, pH 7.0. Square-wave voltammetric scan with frequency of 25 Hz, step potential
10 mV and amplitude of 30 mV. Deposition potential of −1.1 V for 120 s.
more negative potential values, probably due to an opposite
effect coming from glutathione. The increase of deposition
time better supports the QD detection due to the increase of the
quantity of the reduced cadmium coming from the cadmium
ions in equilibrium with CdS QDs at the given conditions.
The response of diverse SPEs to the same drop of
a 14 × 1016 CdS QD concentration suspension was also
studied (figure 4). The results obtained show an RSD of
6.73%. The stability of the response for the same SPE was
also studied (Supporting information figure 2S, available at
stacks.iop.org/Nano/18/035502). For this case, an RSD of
11.8% was obtained for up to six measurements and it was
almost doubled (22.2%) for four consecutive measurements.
These results show that better responses are obtained by using
one SPE for each measurement.
Figure 5 shows typical square wave stripping voltammograms after the blank subtraction for increasing concentration
of CdS QDs up to 1016 QDs ml−1 . The corresponding calibration plot including the error bars for a set of three parallel measurements is also shown. CdS QDs as low as 1016 ml−1 have
been possible to detect. Taking into consideration the CdS QD
drop volume introduced (20 µl), this corresponds to a detection
limit of around 2 × 1014 CdS QDs.
during which the electrochemical reduction of CdS QDs to Cd0
occurs. After the accumulation step SWV was performed by
scanning from −1.1 to −0.7 V, resulting in an analytical signal
due to the oxidation of Cd0 .
The analytical signal used for the CdS QDs quantification
comes from a mixed phenomenon detection which depends on
the medium pH. The analytical protocols have been optimized
to give results which display a wide, linear response range as
well as a CdS QD detection limit that is of interest for various
applications ranging from DNA analysis to immunoassays.
The proposed technique represents a lower cost alternative to
optical methods and will be of interest for fast screening as well
as in field analysis.
4. Conclusions
References
Acknowledgments
This work was financially supported by the Spanish
‘Ramón Areces’ foundation (project ‘Bionanosensores’) and
MEC (Madrid) (projects MAT2005-03553, BIO2004-02776,
Nanobiomed CONSOLIDER). The authors acknowledge the
skilful experience of Mrs Anna Puig in all the technical support
given to this work. Special acknowledgement to CAPES
(Brazil) for financial support of LHM and OF.
[1]
[2]
[3]
[4]
A direct detection technique for CdS QDs, that can be extended
for other similar QDs, that avoids the chemical dissolving as
in the previously reported electrochemical detection methods
is achieved. The hypothesis for the detection is explained
in the light of the experimental results obtained. Moreover,
the optimization of the detection procedure is achieved, which
will be of interest for further applications of the proposed
techniques in developing electrochemical biosensors.
The detection of CdS QDs is simple, low cost, and based
on a sensitive electrochemical method. It is based on the square
wave voltammetry of the CdS QD suspension dropped onto the
surface of a screen printed electrode. In the first step a drop
of 20 µl containing an appropriate concentration of CdS QDs
is introduced onto the surface of the SPE and maintained
for 60 s while applying a potential of 0 V. In the second
step a deposition potential of −1.1 V for 120 s was applied,
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