Artículo 5 SimultaneousVoltammetricDetect

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RESEARCH PAPER
Simultaneous voltammetric detection of cadmium(II), arsenic(III),
and selenium(IV) using gold nanostar–modified screen-printed
carbon electrodes and modified Britton-Robinson buffer
Dingnan Lu1 & Connor Sullivan1 & Eric M. Brack2 & Christopher P. Drew2 & Pradeep Kurup1
Received: 19 February 2020 /Revised: 30 March 2020 /Accepted: 3 April 2020
# Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract
The present work reports a newly developed square wave anodic stripping voltammetry (SWASV) methodology using novel
gold nanostar–modified screen-printed carbon electrodes (AuNS/SPCE) and modified Britton-Robinson buffer (mBRB) for
simultaneous detection of trace cadmium(II), arsenic(III), and selenium(IV). During individual and simultaneous detection,
Cd2+, As3+, and Se4+ exhibited well-separated SWASV peaks at approximately − 0.48, − 0.09, and 0.65 V, respectively (versus
Ag/AgCl reference electrode), which enabled a highly selective detection of the three analytes. Electrochemical impedance
spectrum tests showed a significant decrease in charge transfer resistance with the AuNS/SPCE (0.8 kΩ) compared with bare
SPCE (2.4 kΩ). Cyclic voltammetry experiments showed a significant increase in electroactive surface area with electrode
modification. The low charge transfer resistance and high electroactive surface area contributed to the high sensitivity for
Cd2+ (0.0767 μA (0.225 μg L−1)−1), As3+ (0.2213 μA (μg L−1)−1), and Se4+ (μA (μg L−1)−1). The three analytes had linear
stripping responses over the concentration range of 0 to 100 μg L−1, with the obtained LoD for Cd2+, As3+, and Se4+ of 1.6, 0.8,
and 1.6 μg L−1, respectively. In comparison with individual detection, the simultaneous detection of As3+ and Se4+ showed peak
height reductions of 40.8% and 42.7%, respectively. This result was associated with the possible formation of electrochemically
inactive arsenic triselenide (As2Se3) during the preconcentration step. Surface water analysis resulted in average percent recov-
eries of 109% for Cd2+, 93% for As3+, and 92% for Se4+, indicating the proposed method is accurate and reliable for the
simultaneous detection of Cd2+, As3+, and Se4+ in real water samples.
Keywords Metals/heavymetals . Stripping analysis . Electroanalytical methods . Nanoparticles/nanotechnology . Simultaneous
detection
Introduction
Inorganic contaminants, such as cadmium, arsenic, and sele-
nium, pose a significant threat to human and ecological health
due to their high toxicity and persistence in the environment.
Cadmium is one of the most toxic heavy metals, arising from
industrial wastes, phosphate fertilizers, smelting, and fuel
combustion [1]. In the environment, Cd2+ is the dominant
species and can irreversibly accumulate in the kidney, liver,
lung, and pancreas of the human body, causing neurological,
gastrointestinal, and skeletal illnesses [2]. The maximum con-
taminant level (MCL), as set by the EPA for Cd2+ in drinking
water, is 5.0 μg L−1 [3]. Arsenic is a poisonous chemical that
is distributed widely in nature, ranked as twentieth in abun-
dance among the elements in the earth’s crust [4]. Inorganic
As3+, the most toxic form [4], is water-soluble and has been
proven to cause many human health problems such as lung
* Pradeep Kurup
Pradeep_Kurup@uml.edu
Dingnan Lu
Dingnan_lu@student.uml.edu
Connor Sullivan
Connor_Sullivan1@student.uml.edu
Eric M. Brack
Eric.m.brack.civ@mail.mil
Christopher P. Drew
Christopher.p.drewphd.civ@mail.mil
1 Department of Civil and Environmental Engineering, University of
Massachusetts Lowell, One University Ave., Lowell, MA 01854,
USA
2 US Army Combat Capabilities Development Command Soldier
Center (CCDC SC), General Greene Ave, Natick, MA 01760, USA
Analytical and Bioanalytical Chemistry
https://doi.org/10.1007/s00216-020-02642-4
http://crossmark.crossref.org/dialog/?doi=10.1007/s00216-020-02642-4&domain=pdf
mailto:Pradeep_Kurup@uml.edu
cancer and keratosis [5, 6]. The MCL of total arsenic allowed
in drinking water is 10.0 μg L−1 [3]. Selenium is an essential
element for animals, plants, and humans [7]. However, toxic
effects have been observed in specific concentration ranges,
and these effects can arise from an excess of this metalloid. Of
the different oxidation states in which selenium can be found
in nature (i.e., 4+, 6+, 0, and 2−), Se4+ is by far the most toxic
form [8]. Furthermore, Se4+ is the only electroactive form of
selenium [9]. The MCL of total selenium allowed in drinking
water is 50.0 μg L−1 [3]. To address the environmental and
health risks from these contaminants, it is important to devel-
op a quick, sensitive, and accurate method for the detection of
trace levels of Cd2+, As3+, and Se4+ in drinking water.
Various analytical methods have been developed to de-
tect different heavy metals in water samples, such as in-
ductively coupled plasma-optical emission spectrometry
(ICP-OES) [10–12], atomic absorption spectrometry
(AAS) [13–15], inductively coupled plasma-mass spec-
trometry (ICP-MS) [16, 17], and colorimetric and fluores-
cence spectroscopy [18–20]. Although these methods
have been proven to detect heavy metal accurately, some
of the technologies mentioned above have significant
drawbacks. For example, ICP-MS and ICP-OES require
traditional centralized laboratories, expensive instruments,
and highly trained technicians. Furthermore, AAS re-
quires an additional gas supply to stimulate the optical
radiation by gasified metal atoms. Fluorescence spectros-
copy method needs a chemosensor or a biosensor
equipped with a unique interaction mechanism for pro-
voking or quenching the fluorescence effect when
reacting with a specific heavy metal species, therefore
preventing detection of multiple metal analytes, simulta-
neously. Using anodic stripping voltammetry (ASV) for
heavy metal detection in water is an attractive proposition
due to the high sensitivity, low cost, and the potential
application for onsite, simultaneous detection. Based on
the comprehensive literature review, Cd2+, As3+, and Se4+
have been previously studied using ASV detection
methods [9, 21, 22]. For example, Chow et al. [22] used
a glutathione-modified gold electrode to perform electro-
chemical detection of Cd2+ and reported a limit of detec-
tion (LoD) of 5 nM. Toor et al. [21] successfully used an
Au/Fe3O4 nanocomposite–modified glassy carbon elec-
trode (GCE) to detect low levels of As3+ in water.
Segura et al. [7] evaluated the performance of the gold
nanoparticle (AuNP)–modified GCE on the detection of
Se4+ and obtained a LoD of 0.12 ppb. It can be found that
among all the electrode materials, gold-based electrodes
showed superior performances on ASV detection of vari-
ous heavy metal ions. In our previous study, nanoscale
gold particles with a unique spiked geometry, referred to
as gold nanostar (AuNS), have been proposed for im-
proved ASV detection of Hg2+ and Pb2+ relative to
traditional AuNPs with typically spherical geometry [23,
24]. In order to enlarge the applicability of the AuNS on
ASV detection of heavy metals, it is necessary to develop
new protocols for additional metal analytes, particularly
cadmium, arsenic, and selenium.
Simultaneous detection of multiple metal ions is one of the
major growing trends of electrochemistry. For example,
Ruecha et al. [25] used a graphene-polyaniline nanocomposite
electrode to detect Zn2+, Cd2+, and Pb2+ in water simulta-
neously. Mafa et al. [26] performed electrochemical co-
detection of As3+, Hg2+, and Pb2+ on a bismuth-modified ex-
foliated graphite electrode and achieved satisfactory detection
sensitivity. Lu et al. [27] modified a GCE with CuZrO3/
graphene nanocomposite and used the modifiedGCE to detect
Pb2+ and Cd2+ simultaneously. All of these studies showed the
great potential and attractive prospect of electrochemical de-
tection of multiple metal ions simultaneously. However, it
should be noted that most of the published co-detection stud-
ies only emphasized the fabrication of novel,sensitive elec-
trodematerials while commonly overlooked the importance of
having a suitable supporting electrolyte solution. In order to
conduct ASV co-detection of multiple heavy metal ions, a
universal supporting electrolyte must be developed in the first
place, which should ideally possess a low background noise, a
sizeable electrochemical window, and minimal environmental
hazards.
Based on the previously published studies, the peak
locations of Cd2+, As3+, and Se4+ usually appear in a
stripping voltammogram at three widely separated re-
gions: Cd2+ at a highly negative potential [28–30], As3+
at a potential relatively close to 0 V [31–33], and Se4+ at a
very positive potential [9, 34, 35]. Therefore, the most
critical characteristic of a supporting electrolyte for ASV
co-detection of Cd2+, As3+, and Se4+ is a broad electro-
chemical window to ensure the precise observation of
each ion’s stripping peak. To the best of our knowledge,
such a universal supporting electrolyte has not been re-
ported in the literature. A buffer solution, known as
Britton-Robinson buffer (BRB), was previously devel-
oped for colorimetric determination of [36]. This buffer
consists of phosphoric acid, acetic acid, and boric acid,
with an equal concentration of 0.04 M, providing a wide
buffering range from pH 2 to 12. Compared with chloride,
nitrate, and sulfate, present in commonly used supporting
electrolytes, the anions (i.e., acetate, phosphate, and bo-
rate) from BRB possess a low oxidizing ability, which can
theoretically reduce the potential oxidation of gold nano-
particle, thereby suppressing the formation of the unwant-
ed gold oxidation peak when using a gold-based electrode
to conduct ASV detection. Recently, the BRB has been
used in electrochemical detection of different pharmaceu-
tical compounds, such as atorvastatin [37], quinapril [38],
sulfamethoxazole [39], and tinidazole [40]. However,
Lu D. et al.
there is scarce information on using the BRB as the
supporting electrolyte for ASV detection of heavy metal
ions. As a result, ASV detection of metal ions in the BRB
or modified BRB (mBRB) is worthy of a detailed study.
The first objective of the present study is to evaluate the
feasibility of using the AuNS/SPCE (electrode) and mBRB
(electrolyte) on electrochemical single detection of Cd2+,
As3+, and Se4+. In the second part of this work, simultaneous
detection of Cd2+, As3+, and Se4+ will be performed and fur-
ther validated by comparing the detection performance with
the conventional analytical method (i.e., atomic absorption
spectrum). To the best of our knowledge, no attempt has been
reported previously for the co-detection of cadmium, arsenic,
and selenium using electrochemical detection method.
Therefore, this study highlights the potential application of
both AuNS/SPCE and mBRB for the simultaneous
voltammetric detection of heavy metals in water.
Experimental
Reagents and apparatus
All chemicals, purchased from Sigma-Aldrich, Inc., were
of analytical reagent grade and used without further puri-
fication. The mBRB solution was prepared by mixing
equal amounts of stock solution of 0.1 M of phosphoric
acid and acetic acid, resulting in a 0.1 M of pH 2.0 mBRB
solution. Boric acid was excluded from the original BRB
recipe since the original buffering capacity in the basic
range is unnecessary in this study. Furthermore, boric acid
has been shown in medical studies that its reproductive
toxicity can cause male animals to suffer persistent testic-
ular atrophy [41]. Therefore, removal of the boric acid
reduced environmental and health risks associated with
the test methodology.
All voltammetric experiments were performed using a
WaveNANO potentiostat (Pine Instruments, NC, USA) with
the AfterMath software. All pH measurements were per-
formed by a benchtop pH meter (AR15 Accumet™) with a
gel-filled polymer body combination pH probe. A UV-Vis
spectrophotometer (PerkinElmer LAMBDA 25) was used to
perform UV-Vis measurements. Transmission electron mi-
croscopy (TEM) images were obtained using the Philips
EM400 T Microscope. HGA graphite furnace/atomic absorp-
tion spectrum (AAS) (PerkinElmer PinAAcle 900T) was used
to validate the results obtained by using the proposed
voltammetric method. The X-ray diffraction (XRD) pattern
was obtained using Rigaku RU-300E Diffractometer with
Cu Kα radiation (λ = 0.1542 nm). The Fourier transform in-
frared spectroscopy (FTIR) spectra were recorded using a
NICOLET iS50 FT-IR with a scanning range of 500–
4000 cm−1.
Preparation of gold nanostar suspension
and electrode modification
The gold nanostars were synthesized using the follow-
ing procedure. A 0.1 M 4-(2-hydroxyethyl ) - l -
piperazineethanesulfonic acid (HEPES) solution was first pre-
pared, with the pH adjusted to 7.4 using 10M sodium hydrox-
ide. The auric chloride solution was prepared by adding 30μL
of 30% stock to 5 mL DI water, followed by heating in an oil
bath at a constant temperature of 160 °C for 10 min. After
heating, the chloroauric acid solution was capped and cooled
to room temperature. Next, 2 mL of HEPES was mixed with
3 mL of DI water, and then, 50 μL of the freshly prepared
auric chloride solution was added to it with mildly stirring.
After mixing, the solution was left undisturbed while the outer
spikes were forming. The solution turned light blue after 5 min
and then darkened to a blue-green color in about 15 min. The
final AuNS suspension was capped and stored in a dark
temperature-controlled chamber at 10 °C.
Screen-printed carbon electrodes (SPCEs) were used
throughout the complete experiment, and all the SPCEs were
obtained from Pine Instrumentation. The coating of the AuNS
was performed first by wetting the surface of the working
electrode with deionized water, following by pipetting the
required volume (22 μL) of the AuNS solution onto the sur-
face of the working electrode. The electrode was then placed
in an oven at 40 °C overnight to dry the working electrode
surface. After drying, the electrodes were removed from the
oven and allowed to cool before testing.
Titration study
Titration was performed to quantify the buffering capacity of
the mBRB. The experiments were performed by titrating
100 mL of the mBRB, made by mixing 50 mL of 0.1 M
phosphoric acid and 50 mL 0.1 M acetic acid, with 0.2 N
sodium hydroxide. pH measurements were made after the
addition of each 2 mL of alkali. The pH values were recorded
to plot the titration curve.
Square wave anodic stripping voltammetry
All voltammetric experiments were performed using square
wave anodic stripping voltammetry (SWASV). The SWASV
parameters were a deposition time of 180 s, a deposition po-
tential of − 0.9 V, a stripping voltage range from − 0.9 to 0.9 V,
an amplitude of 70 mV, a period of 20 ms, a step increment of
11 mV, and a sampling width of 5 ms. All measurements were
carried out in triplicate. The limit of detection (LoD) was
calculated using the following equation:
LoD ¼ 3σ=m ð1Þ
Simultaneous voltammetric detection of cadmium(II), arsenic(III), and selenium(IV) using gold...
where σ is the standard deviation of replicate blank samples
and m is the slope of the calibration curve.
Results and discussion
Characterization of AuNS and AuNS/SPCE
TEM images were obtained to characterize the morphology
and estimate the size of AuNS. As shown in the representative
images (see Fig. 1), the average tip-to-tip diameter, average
spike length, and average inner sphere diameter were approx-
imately 49 ± 14 nm, 16 ± 1 nm, and 23 ± 6 nm, respectively.
Furthermore, the number of spikes per nanostar ranged from 4
to 10. The XDR pattern of the obtained AuNS powder showed
four diffraction peaks at 38.3°, 44.1°, 64.7°, and 77.3°, respec-
tively. The observed four diffraction peaks (see Fig. 2a) were
associated with the (111), (200), (220), and (311) Bragg re-
flections, respectively, which are highly consistent with the
XRD pattern of the standard crystalline gold XRD (JCPDS
no. 04-0784), therefore confirming the crystallinenature of
the prepared AuNS. The highest diffraction intensity recorded
on the (111) plane suggests the predominant orientation
among other planes. Thus, the average AuNS size was esti-
mated based on the width of the (111) diffraction using the
Debye-Scherrer equation [42]. The calculated mean size of the
AuNS was found to be 40 nm, which is relatively in agree-
ment with the TEM observation. FTIR analysis was carried
out to demonstrate the presence of different organic functional
groups on HEPES, which together can precisely exert the
capping and stabilizing influence on the inner sphere of gold
nanoparticles and eventually lead to the formation of nano-
scale star-shaped crystalline gold. As shown in Fig. 2b, a weak
band was found at approximately 1731 cm−1, which is asso-
ciated with the presence of amide III on the HEPES molecule
[43]. A relatively intense band is located at 1622 cm−1, which
corresponds to the stretching vibration of the piperazine (1-(2-
pyridyl)) ring [44]. Two intense bands observed at 1153 and
1022 cm−1 are assigned to the S=O stretching and pyridine
breathing, respectively, and a sharp band at 661 cm−1 is
mainly associated with the O=S=O in-plane deformation
[44, 45]. It is worth mentioning that many early studies found
the amide groups (I–III) can bind to gold nanoparticles and
result in the stabilization of the bound gold nanoparticles [42,
43, 46–48]. Based on the obtained FTIR spectrum, it can be
found that due to the presence of amide III, the selected cap-
ping reagent of HEPES can impose a mild binding influence
on the initially formed inner sphere of gold nanoparticles. The
unique feature of the HEPES buffer allows the controlled
growth of spherical gold nanoparticles, and eventually the
formation of the AuNS.
The UV-Vis spectra performed for the colorimetric charac-
terization of the AuNS suspension are presented in Fig. 2c.
The significant absorbance peak at 719 nm was associated
with the outer spikes, and the relatively small absorbance peak
at 521 nm was associated with the inner spherical cores.
Eventually, the obtained AuNS showed a blue-green color
(see Fig. 2c), which was mainly associated with the significant
absorbance peak at 719 nm. As shown in Fig. 2d, electro-
chemical impedance spectrum (EIS) tests were performed to
investigate the behavior of AuNS/SPCE and SPCE in the
presence of a redox probe (i.e., 10 mM Fe(CN)6
3−/4−). As
the electrolyte resistance (RS) remained unchanged, the charge
transfer resistance (RCT) decreased significantly from 2.4 kΩ
(bare SPCE) to 0.8 kΩ (AuNS/SPCE). The differences ob-
served in RCT can be ascribed to the increased surface area
associated with the nanoscale AuNS coating on the working
electrode surface. To directly estimate the effective surface
area, cyclic voltammetry (CV) tests were performed in the
presence of the same redox probe as used in the EIS tests
(see Fig. 2e). Based on the Randle-Sevcik equation [49], the
effective surface areas of AuNS/SPCE and SPCE can be cal-
culated directly using the following equation:
A ¼ IP= 2:69� 105 � n3=2D1=20 v1=2C
� �
ð2Þ
where A is the effective electrode area (cm2), IP is the peak
current (A), D0 is the standard diffusion coefficient of
K3[Fe(CN)6] at 25 °C (cm
2/s), n is the number of electrons
transferred in the redox event, v is the scan rate (V/s), and C is
Fig. 1 Representative TEM images of AuNS
Lu D. et al.
the concentration of the redox species (mol (cm3)−1). The
calculated effective surface areas of AuNS/SPCE and SPCE
were 14.3 and 7.6 mm2, respectively, indicating the AuNS
coating could approximately double the effective surface area
of an unmodified SPCE electrode.
Characterization of mBRB
The titrant curve of mBRB (see Fig. 3) showed a pH buffering
range from approximately 1.80 to 8.00. The linear regression
line (R2 of 0.9921) indicated a strong correlation between the
actual titration curve and the linear regression trendline. It is
clear that the pKa values of phosphoric acid and acetic acid
evenly distributed on the pH buffering range. Compared with
that of the original BRB, the effective buffer range of mBRB is
narrower, due to the exclusion of boric acid. Furthermore, as the
concentrations of phosphoric acid and acetic acid are higher in
themBRB (i.e., 50mM) than in the original BRB (i.e., 40mM),
the slope of mBRB is more gradual than that of the original
BRB, showing a better buffering capacity of mBRB.
The AuNP/SPCE was used to evaluate the electrochemical
window of the selected supporting electrolytes, including
50 mM mBRB, 50 mM HCl, 50 mM KCl, and 50 mM
HAc/NaAc (pH 5.0) (see Fig. 4). The SWSAV technique
was used with the corresponding parameters listed in the
“Square wave anodic stripping voltammetry” section.
Compared with the obtained baselines from all supporting
electrolytes, the mBRB shows the broadest electrochemical
window from approximated 0.8 to − 0.8 V, with the lowest
background noise. Some unexpected noise peaks were ob-
served in the voltammograms obtained in KCl and HCl at
approximately 0.0 V, which are potentially associated with
the Au0 anodically oxidizing to Au1+ in the presence of high
chloride concentration (i.e., 0.05 M) [50, 51]. Furthermore,
obvious hydrogen evolution occurred during the
preconcentration step in HCl when having a deposition poten-
tial of − 0.9 V. For practical applications, the deposition po-
tential has to be reduced (i.e., less negative) to prevent the
perturbing hydrogen evolution when applyingAVS in a strong
acid like HCl, HNO3, and HClO4. However, the decrease of
Fig. 2 a XRD pattern of AuNS
power, b FTIR spectrum of
AuNS/SPCE, c UV-Vis spectra of
the AuNS suspension, d Nyquist
plots, and e CV voltammograms
obtained on AuNS/SPCE and
bare SPCE in the presence of
10 mM Fe(CN)6
3−/4− and 0.1 M
KCl
Simultaneous voltammetric detection of cadmium(II), arsenic(III), and selenium(IV) using gold...
the deposition potential will reduce the effectiveness of a
preconcentration step, thereby reducing the sensitivity of the
AVS detection of heavy metals. Acetate buffer (pH at approx-
imately 5.0) is one of the most commonly used buffer systems
for the simultaneous detection of multiple heavy ions [26, 30,
52]. As shown in Fig. 4d, the acetate buffer showed a flat and
wide-open voltammogram from − 0.9 to 0.5 V. Within this
potential segment, ions like Cd2+ and As3+ can clearly exhibit
their oxidation peaks. However, the oxidation peak of Se4+
will be covered. Therefore, due to the large electrochemical
window, small background noise, and the relatively low oper-
ation risk, mBRB was selected as the supporting electrolyte
for the following detection experiments.
Individual detection of Cd2+, As3+, and Se4+
Cd2+, As3+, and Se4+ were first detected individually using
AuNS/SPCE in 0.05 M of mBRB. Figure 5a shows the
SWASV responses of Cd2+ over a concentration range of 0–
100 μg L−1. Well-defined stripping peak, linearly proportional
to the spiked concentration of Cd2+, was observed at a poten-
tial of − 0.48 V (versus Ag/AgCl). The linear regression equa-
tion between peak current and analyte concentration is de-
scribed as Eq. (3), with a correlation coefficient of 0.996.
The limit of detection (LoD) was calculated to be
1.70 μg L−1 using Eq. 1. Next, the stripping peak of As3+
was located at − 0.1 V (versus Ag/AgCl), with relatively large
Fig. 4 Electrochemical window of selected supporting electrolyte solutions: a 50 mM mBRB, b 5 mM KCl, c 50 mM HCl, and d 50 mM HAc/NaAc
(pH 5.0) (electrode: AuNS/SPCE; SWASV parameter: deposition time of 180 s at − 0.9 V, stripping voltage range from − 0.9 to 0.9 V)
Fig. 3 Titration curves of the
original BRB (blue line) (adapted
from Britton and Robinson [36])
and the modified BRB (red
dashed line)
Lu D. et al.
peak height values when comparing with Cd2+. The responses
of As3+ from 0 to 100 μg L−1 are shown in Fig. 5b. The
equation of the linear regression is presented as Eq. (4), with
its correlation coefficients of 0.998. The LoD was calculated
tobe 0.49 μg L−1. The SWASV responses of Se4+ for the
concentrations of 0, 25, 50, 75, and 150 μg L−1 are shown
in Fig. 5c. The stripping peak of Se4+ was located at approx-
imately 0.65 V (versus Ag/AgCl). The linear regression equa-
tion is presented as Eq. (5), with the correlation coefficient of
0.996 and the LoD of 0.90 μg L−1. It should be noted that the
observed anodic stripping peak of Se4+ is built partially on the
left side of the gold anodic stripping peak at 0.85 V (versus
Ag/AgCl), which resulted in the asymmetric profile of the
Se4+ stripping peak [53].
i ¼ 0:0734 C þ 1:2828 ð3Þ
i ¼ 0:225 C−0:4522 ð4Þ
i ¼ 0:2213 C−4:8362 ð5Þ
Simultaneous detection of Cd2+, As3+, and Se4+
The obtained ASV responses for the co-detection of Cd2+,
As3+, and Se4+ with increasing concentrations under the opti-
mal parameters are shown in Fig. 6a. The proposed electrodes
(i.e., AuNS/SPCE) and supporting electrolyte (i.e., 0.05
mBRB) were applied successfully to the simultaneous deter-
mination of the three target heavy metal ions. Cd2+, As3+, and
Se4+ showed individual peaks at approximately − 0.48, −
0.09, and 0.65 V (versus Ag/AgCl), respectively. It is clear
that the responses of three ions were well-separated in the
stripping voltammogram. The stripping peaks of Cd2+ and
Se4+ were located at the far ends of the electrochemical win-
dow, with the As3+ stripping peak lying in between them. The
separations among the three heavy metal ions are clear and
large, which ensures the feasibility of simultaneous detection.
It is worth noting that an additional open electrochemical win-
dow is available from approximately 0 to 0.4 V, which may be
suitable for adding mercury(II) [54] as another target heavy
metal ion in the future.
In Fig. 6b, the calibration curves of the Cd2+, As3+, and
Se4+ co-detection were built from 0 to 100 μg L−1. For Cd2+,
the linear regression equation is described as Eq. (6), with a
correlation coefficient of 0.985 and a LoD of 1.62 μg L−1.
Comparedwith the individual detection of Cd2+, simultaneous
detection showed very similar results. Specifically, the slope
of 0.0767 μA (μg L−1)−1 obtained from simultaneous detec-
tion is slightly different from the individual detection result of
0.0734 μA (μg L−1)−1. In addition to the similar slopes ob-
tained in the individual and simultaneous detection, the LoDs
were also highly similar (i.e., 1.70 μg L−1 for individual de-
tection and 1.62 μg L−1 for simultaneous detection).
For As3+, the linear regression equation is described as
Eq. (7) with a correlation coefficient of 0.996 and a LoD
of 0.83 μg L−1. For Se4+, the linear regression equation is
presented as Eq. (8), with a correlation coefficient of
0.989 and a LoD of 1.57 μg L−1. It is worth mentioning
that, unlike Cd2+, both As3+ and Se4+ showed obvious
decreases in their stripping peak height during the simul-
taneous detection. Compared with the slopes obtained in
the individual and simultaneous detections, As3+ and Se4+
showed apparent reductions of 40.8% and 42.7%, respec-
tively. The decreased peak heights, observed in the As3+
and Se4+ co-detection, are likely due to the formation of
insoluble arsenic triselenide (As2Se3). To the best of our
knowledge, there is no previous research that performed
ASV detection of As3+ and Se4+, simultaneously.
Therefore, the potential mutual influence of As3+ and
Fig. 5 SWASV response of the AuNS-modified SPCE for the individual
detection of a Cd2+ over a concentration range of 0 to 100 μg L−1, bAs3+
over a concentration range of 0 to 100 μg L−1, and c Se4+ over a concen-
tration range of 0 to 150 μg L−1
Simultaneous voltammetric detection of cadmium(II), arsenic(III), and selenium(IV) using gold...
Se4+ was studied quantitatively to explain the observed
decreased voltammetric responses in the following sec-
tions.
i ¼ 0:0767 C þ 1:0335 ð6Þ
i ¼ 0:1331 C þ 0:5588 ð7Þ
i ¼ 0:1267 C−1:4412 ð8Þ
Evaluation of interaction between As3+ and Se4+
Figure 7a shows the significant differences in the observed
stripping peak height when comparing individual and simul-
taneous detection of 100 μg L−1 As3+ and Se4+. It was clear
that both As3+ and Se4+ had a decreased peak height during
co-detection. The average peak height of 100 μg L−1 of As3+
reduced from 21.73 ± 0.64 μA (individual) to 13.23 ±
0.25 μA (simultaneous), resulting in a peak height reduction
of 39.1%. Similarly, the average peak height of Se4+ reduced
from 17.34 ± 1.91 μA (individual) to 9.48 ± 0.37 μA (simul-
taneous), showing a peak height reduction of 45.3%. The re-
sults were highly consistent within the observations in the
“Simultaneous detection of Cd2+, As3+, and Se4+” section,
indicating the peak height reductions of As3+ and Se4+ have
no direct relationship with the presence of Cd2+. Furthermore,
Fig. 7b and c clearly show the stripping peaks of As3+ and
Se4+ have no remarkable change when they are co-existing
with Cd2+.
As mentioned in the “Simultaneous detection of Cd2+,
As3+, and Se4+” section, the possible cause of the peak height
reductions of As3+ and Se4+ was associated with the formation
of As2Se3. Since arsenic (0) tends to oxidize to arsenite triox-
ide (As2O3) in the air at room temperature, and similar reac-
tion to form arsenic triselenide can happen when zero-valent
selenium is available [55], the relevant mechanisms can be
described as follows:
As3þ þ 3e−→As0 ð8Þ
Se4þ þ 4e−→Se0 ð9Þ
2As0 þ 3Se0→As2Se3 ð10Þ
The resultant of As2Se3 is highly stable and insoluble [56],
which may cause the weakened stripping voltammetric re-
sponse of As3+ and Se4+. For a better understanding of the
observed peak height reductions, additional SWASVanalysis
was performed over the concentration range of 0 to
100 μg L−1 when As3+ and Se4+ are co-existing (see Fig. 8).
The standard addition analysis demonstrated the voltammetric
responses of As3+ and Se4+ still increased linearly with the
Fig. 6 a SWASV response of the
AuNS-modified SPCE for the si-
multaneous detection of
cadmium(II), arsenic(III), and
selenium(IV) over a concentra-
tion range of 0 to 100 μg L−. b
The calibration curves of
cadmium(II), arsenic(III), and
selenium(IV) simultaneous
detection
Lu D. et al.
concentration increase, which was consistent with the results
shown in the “Simultaneous detection of Cd2+, As3+, and
Se4+” section. It should be noted that there is scarce informa-
tion regarding the interaction between arsenic and selenium,
and we are the first study reporting the decreased stripping
peak heights during the co-detection of As3+ and Se−.
However, it is reasonable to consider that the resultant
As2Se3 is electrochemically inactive.
Potential interferences and real water sample analysis
Several common ionic species, typically present in ground
and surface water [57, 58], were tested to evaluate the poten-
tial interferences of the proposed detection method (see
Table 1 for full results), specifically bicarbonate (HCO3
−),
chlor ide (Cl−) , sulfate (SO4
2+) , i ron(III) (Fe3+) ,
magnesium(II) (Mg2+), and calcium(II) (Ca2+). Each species
was testedwith 50μg L−1 of Cd2+, As3+, and Se4+ in triplicate.
The average percent recovery (PR) and the relative standard
deviation (RSD) were determined. It can be found that the PRs
of each analyte during the different interference tests were
generally above 90%, except for the results of the bicarbonate
group (PR < 90%). In addition to the low PRs, the results of
RSD obtained from the bicarbonate group were slightly larger
than the results from other common ionic species. The obser-
vations mentioned above together indicated that bicarbonate
ion is a noticeable interference for this proposed detection
method. As a common ion in groundwater, bicarbonate can
react with acetic acid and phosphoric acid and consequently
affect the system’s pH and ionic strength [59] as follows:
NaHCO3 þ CH3COOH→CO2↑þ H2Oþ CH3COONa ð11Þ
NaHCO3 þ H3PO4→CO2↑þ H2Oþ Na2HPO4 ð12Þ
Two possible solutions to minimize the interference from
bicarbonate ion are (1) diluting water samples (especiallya
groundwater sample) to reduce the concentration of
Fig. 7 a SWASV response of the individual and simultaneous detection
of arsenic(III) and selenium(IV). b SWASV response of the individual
and simultaneous detection of cadmium(III) and selenium(IV). c SWASV
response of the individual and simultaneous detection of cadmium(III)
and arsenic(IV)
Fig. 8 SWASV response of the
simultaneous detection of
arsenic(III) and selenium(IV)
over a concentration range of 0 to
100 μg L−1
Simultaneous voltammetric detection of cadmium(II), arsenic(III), and selenium(IV) using gold...
bicarbonate ion and (2) increasing the concentrations of acetic
acid and phosphoric acid to enhance the pH buffering capacity
of the developed mBRB. Although the high concentration of
bicarbonate ion can exert a moderate interference on the pro-
posed method, the overall performance of the interference
tests was acceptable (average PR ≥ 90%).
Surface water was sampled and tested to evaluate the ana-
lytical performance for real-world sample analysis. The water
sample from the Merrimack River (Lowell, MA) was first
filtered and preserved using 1 to 2 mL of concentrated nitric
acid following the EPA standardmethod [60]. Before analysis,
the pH of the preserved water sample was readjusted from 2.0
to 7.0 using 10 N sodium hydroxide. Without metal ion addi-
tion, the water sample from theMerrimack River was found to
contain non-detectable As3+ and Se4+, and this result was ver-
ified by testing the same water sample using the AAS method
(see Table 2). However, a trace Cd2+ concentration of 1.9 ±
0.2μg L−1 was observed in theMerrimack River sample using
the proposed voltammetric method. This observation was
highly consistent with the result of 2.2 μg L−1 obtained using
the AAS detection method (graphite furnace mode). After
spiking of 10 μg L−1 Cd2+, the proposed method resulted in
a Cd2+ concentration of 12.0 ± 0.5 μg L−1, which again indi-
cated an approximately 2.0 μg L−1 of Cd2+ found in the col-
lected water sample. With simultaneous addition of all target
ions at 10, 20, and 50 μg L−1, the proposed detection method
resulted in average PRs of 109% for Cd2+, 93% for As3+, and
92% for Se4+, respectively. The results of surface water
analysis demonstrated that the proposed method is accurate
and reliable for the simultaneous detection of Cd2+, As3+, and
Se4+ in real-world surface water samples.
Reproducibility
The reproducibility of the developed detection method was
evaluated by measuring the voltammetric response of a solu-
tion consisting of 50μg L−1 of Cd2+, As3+, and Se4+. Based on
the results of five consecutive tests performed within 1 h, the
RSD of each metal analytes was 5.7, 4.4, and 7.5% for Cd2+,
As3+, and Se4+, respectively. Next, the peak heights of
50 μg L−1 of each ion obtained in five separate days resulted
in the RSD of 6.5, 7.1, and 9.0% for Cd2+, As3+, and Se4+,
respectively. Estimation of the storage tolerance for the devel-
oped AuNS suspension is of interest. To evaluate that, 10 mL
of AuNS suspension was kept in a clean glass vial at a
temperature-controlled dark chamber at 10 °C for 3 months.
After storage, the AuNS suspension was used directly to
fabricate SPCEs, and the peak heights of all the metal
analytes showed no significant reduction (difference less
than 1%). The obtained RSD was 8.8, 7.9, and 10.4%
for Cd2+, As3+, and Se4+, respectively. It is worth men-
tioning that after an additional 1 month of storage, the
AuNS suspension started to fade and exhibit some no-
ticeable dark particles, which was associated with the
long-term nanoscale collision and the following precip-
itation of the AuNS particles.
Table 2 Summary of surface water analysis
Spiked (μg L−1) Proposed method (μg L−1) AAS method (μg L−1) PRa (%)
Cd2+ As3+ Se4+ Cd2+ As3+ Se4+ Cd2+ As3+ Se4+
0 1.9 ± 0.2 < LoD < LoD 2.2 ± 0.1 0.0 0.0 86b N/Ac N/Ac
10 12.0 ± 0.5 9.0 ± 0.8 8.9 ± 1.4 12.4 ± 0.3 9.6 ± 0.2 9.9 ± 0.3 120 90 89
20 21.7 ± 1.6 19.2 ± 2.1 18.7 ± 1.3 20.8 ± 0.1 19.5 ± 0.4 18.7 ± 0.3 108 96 94
50 49.7 ± 2.9 47.6 ± 2.4 45.9 ± 3.0 50.4 ± 0.5 48.9 ± 0.8 49.2 ± 0.5 99 95 92
a PR values calculated by comparing the results of the proposed method with spiked levels
b PR of Cd2+ (w/o spike) calculated by comparing the results of the proposed method and AAS method
cNot applicable
Table 1 Summary of common
interference testing with spiked
50 μg L−1 of each target ions
Interference species Concentration (mg L−1) PRa of Cd2+ (%) PRa of As3+ (%) PRa of Se4+ (%)
HCO3
− 200 87 ± 13 83 ± 17 84 ± 10
Cl− 500 103 ± 8 91 ± 8 92 ± 9
SO4
2+ 500 101 ± 5 98 ± 6 94 ± 7
Fe3+ 10 93 ± 7 88 ± 3 91 ± 5
Mg2+ 200 92 ± 3 96 ± 4 93 ± 5
Ca2+ 200 96 ± 6 93 ± 7 91 ± 7
aAverage percent recovery (PR) expressed as mean ± RSD of triplicate measurements
Lu D. et al.
Comparison with previous studies
The detection performance of the proposed method was com-
pared with other previously reported simultaneous detection
studies, and results are presented in Table 3. Although the
LoD obtained in this work is not the lowest, it is significantly
lower than the EPA’s MCL of 5, 10, and 50 μg L−1 for
cadmium(II), arsenic (total), and selenium (total) in drinking
water, respectively [3]. It should be noted that the proposed
method is the first work that includes Se4+ as a target analyte
during simultaneous ASV detection. Furthermore, this pro-
posed method has the potential of including more heavy metal
ions, such as Hg2+, Pb2+, and Cu2+, as additional co-detection
analytes. In comparison, the present work demonstrates a re-
liable and versatile method for voltammetric detection of mul-
tiple heavy metal ions simultaneously.
Conclusion
The gold nanostar (AuNS)–modified screen-printed carbon
electrodes (SPCEs) and the modified Britton-Robinson buffer
(mBRB) were used successfully to perform simultaneous
voltammetric detection of cadmium(II), arsenic(III), and
selenium(IV). The detection results suggested that (1)
AuNS/SPCE can sensitively detect the three analytes, with
LoD for Cd2+, As3+, and Se4+ of 1.6, 0.8, and 1.6 μg L−1,
respectively; (2) 50 mM of pH 2.0 mBRB can provide a
smooth and large electrochemical window that ensures all
the ASV peaks to build up separately (peak location for
Cd2+, As3+, and Se4+ are at − 0.48, − 0.09, and 0.65 V versus
Ag/AgCl, respectively); (3) the peak heights of the three
analytes increase linearly with increasing concentration.
Furthermore, the results of simultaneous detection showed
that the response of Cd2+ is independent of the presence of
As3+ and Se4+. However, the co-existence of As3+ and Se4+
leads to the formation of electrochemically inactive arsenic
triselenide (As2Se3) during the preconcentration period and
consequently results in approximately 40% peak height reduc-
tions for both As3+ and Se4+. Furthermore, the interferences
and real water analyses showed that the proposed method
could conduct reliable detection of Cd2+, As3+, and Se4+ si-
multaneously in water.
Funding information The authors received financial support from the
U.S. Army Combat Capabilities Development Command – Soldier
Center (Contract No. W911QY-17-2-0004) and the U.S. National
Science Foundation Grant No. 1543042.
Compliance with ethical standards
No informed consent, human participants, and animals applicable.
Conflict of interest The authors declare that they have no conflict of
interest.
Disclaimer Any opinions, findings, and conclusions or recom-
mendations expressed in this manuscript are those of the authors
and do not necessarily reflect the views of the National Science
Foundation.
Table 3 Summary of past efforts on the simultaneous ASV detection of heavy metal ions
Electrode Electrolyte Target ions and LoD (μg L−1) Matrix analysis Ref
MF/GCEa 10 mM NH4Ac/HCl; 5 mM NH4SCN Cd
2+ (0.06); Pb2+ (0.02) Seawater [29]
AgREb 10 mM HNO3; 10 mM NaCl Cd
2+ (0.6); Pb2+ (0.08) Drinking water [28]
Bi-EGEc 0.1 M acetate buffer (pH 5) Pb2+ (0.1); As3+ (0.02); Hg2+ (0.1) Wastewater [26]
AuNPs/GCEd 0.1 M acetate buffer (pH 5) Cd2+(34); Pb2+ (62); Cu2+ (19); Hg2+ (60) – [52]
BiBEe 0.1 M acetate buffer (pH 5) Zn2+ (0.4); Cd2+ (0.1); Pb2+ (0.1) River water [30]
Gr/PANI/SPEf 1 M acetate buffer (pH 4.5) Zn2+ (1.0); Cd2+ (0.1); Pb2+ (0.1) Human serum [25]
Au-Gr-Cys/Bi/GCEg 0.1 M acetate buffer (pH 4.5) Cd2+ (0.1); Pb2+ (0.05) Spring water [61]
CuZrO3-Gr/GCEh 0.1 M acetate buffer (pH 4.6) Cd2+ (0.5); Pb2+ (0.1) Soil sample [27]
AuNS/SPCE 50 mM mBRB (pH 2.0) Cd2+ (1.6); As3+ (0.8); Se4+ (1.6) River water This work
aMF/GCE stands for mercury film–modified glassy carbon electrode
bAgRE stands for silver rotating electrode
c Bi-EGE stands for bismuth-modified exfoliated graphite electrode
dAuNPs/GC stands for gold nanoparticle–modified glassy carbon electrode
e BiBE stands for bismuth bulk electrode
f Gr/PANI/SPE stands for graphene-polyaniline-modified screen-printed electrode
gAu-Gr-Cys/Bi/GCE stands for gold nanoparticle-graphene-cysteine nanocomposite–modified bismuth film electrode
h CuZrO3/Gr/GCE stands for copper zirconium oxide-graphene nanocomposite–modified glassy carbon electrode
Simultaneous voltammetric detection of cadmium(II), arsenic(III), and selenium(IV) using gold...
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Simultaneous voltammetric detection of cadmium(II), arsenic(III), and selenium(IV) using gold...
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	Simultaneous...
	Abstract
	Introduction
	Experimental
	Reagents and apparatus
	Preparation of gold nanostar suspension and electrode modification
	Titration study
	Square wave anodic stripping voltammetry
	Results and discussion
	Characterization of AuNS and AuNS/�SPCE
	Characterization of mBRB
	Individual detection of Cd2+, As3+, and Se4+
	Simultaneous detection of Cd2+, As3+, and Se4+
	Evaluation of interaction between As3+ and Se4+
	Potential interferences and real water sample analysis
	Reproducibility
	Comparison with previous studies
	Conclusion
	References