Preparation, characterization of Fe3O4at TiO2magnetic nanoparticles
and their application for immunoassay of biomarker of exposure
to organophosphorus pesticides
Xiao Zhanga, Hongbo Wangb, Chunming Yangb,nn, Dan Dua,n, Yuehe Linc
aKey Laboratory of Pesticide and Chemical Biology of Ministry of Education, College of Chemistry, Central China Normal University, Wuhan 430079, PR China
bCollege of Chemistry and Chemical Engineering, Hunan Normal University, Changsha 410081, PR China
cPacific Northwest National Laboratory, Richland, WA 99352, USA
a r t i c l e i n f o
Received 4 July 2012
Received in revised form
20 September 2012
Accepted 22 September 2012
Available online 12 October 2012
a b s t r a c t
Novel Fe3O4at TiO2magnetic nanoparticles were prepared and developed for a new nanoparticle-based
immunosensor for electrochemical quantification of organophosphorylated butyrylcholinesterase
(BChE) in plasma, a specific biomarker of exposure to organophosphorus (OP) agents. The Fe3O4at
TiO2 nanoparticles were synthesized by hydrolysis of tetrabutyltitanate on the surface of Fe3O4
magnetic nanospheres, and characterized by attenuated total reflection Fourier-transform infrared
spectra, transmission electron microscope and X-ray diffraction. The functional Fe3O4 at TiO2
nanoparticles were performed as capture antibody to selectively enrich phosphorylated moiety instead
of phosphoserine antibody in the traditional sandwich immunoassays. The secondary recognition was
performed by quantum dots (QDs)-tagged anti-BChE antibody (QDs-anti-BChE). With the help of a
magnet, the resulting sandwich-like complex, Fe3O4 at TiO2/OP-BChE/QDs-anti-BChE, was easily
isolated from sample solutions and the released cadmium ions were detected on a disposable
screen-printed electrode (SPE). The binding affinities were investigated by both surface plasmon
resonance (SPR) and square wave voltammetry (SWV). This method not only avoids the drawback of
unavailability of commercial OP-specific antibody but also amplifies detection signal by QDs-tags
together with easy separation of samples by magnetic forces. The proposed immunosensor yields a
linear response over a broad OP-BChE concentrations range from 0.02 to 10 nM, with detection limit of
0.01 nM. Moreover, the disposable nanoparticle-based immunosensor has been validated with human
plasma samples. It offers a new method for rapid, sensitive, selective and inexpensive screening/
evaluating exposure to OP pesticides and nerve agents.
& 2012 Elsevier B.V. All rights reserved.
Exposure to organophosphorus (OP) pesticides still remains a
major occupational health risk factor across the world (Pope, 1999;
Story et al., 2011). Developing a simple, rapid, sensitive and
definitive method for detection of low-dose OP exposure is desired.
Biomonitoring is an efficient approach for quantitatively evaluating
occupational exposure to OP pesticides and nerve agents (Zhang
et al., 2010; Tsatsakis et al., 2010). Following exposure, OP agents
readily interact with enzymes and proteins such as acetycholines-
terase (AChE) and butyrylcholinesterase (BChE) in the biological
matrix to produce four types of relevant biomarkers, including:
(1) cholinesterase (ChE) activity; (2) phosphorylated adducts;
(3) metabolites by hydrolysis; and (4) unbound free OP in fluids
(Wang et al., 2008a, 2008b).
Detection of metabolites and free OPs are not accurate due to
the high affinity of OPs to ChE and other proteins. These methods
are often performed by LC/GC-MS (Noort et al., 2002; Wessels
et al., 2003; Margariti et al., 2007) and lack of portability and real-
time results. Although ChE activity assay is simple and sensitive
and is a good biomarker to assess the exposure to OP (Ellman
et al., 1961; Worek et al., 1999), an important challenge with this
assay is the need of a baseline ChE for each individual before a
meaningful ChE activity change can be measured. Generally, an
average value of ChE activity from a large amount of people
serves as this baseline. However, considering the inter- and intra-
individuals variations in levels of ChE and the deviation methods
from different laboratories, this assay is not accurate and may
provide ambiguous results at subclinical exposure (less than 20%
enzyme inhibition). Our group developed a baseline-free method
based on the reactivation of phosphorylated AChE to evaluate OP
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Biosensors and Bioelectronics 41 (2013) 669–674
exposure, but it is not useful for all OP exposures in which the
phosphorylated enzymes can either spontaneously regenerated,
or quickly aged (Du et al., 2009).
Phosphorylated ChE adduct (OP-ChE) is a more effective and
sensitive biomarker for directly evaluation of OP exposure in the
absence of baseline (Sporty et al., 2010). Clinical measurement of
OP-ChE adducts biomarker shows great promise for early predic-
tions, including GC-MS (Polhuijs et al., 1997), LC-MS/MS (Noort
et al., 2006) and matrix-assisted laser desorption/ ionization time
of flight mass spectrometry (MALDT-TOF MS) (Li et al., 2007).
However, these methods have an inherent disadvantage such as
complicated and expensive analysis, lack of portability and real-
time results, and inconvenience for field applications. Immunoas-
say of OP-ChE adducts is an optional technology with high
sensitivity and selectivity but sometimes challenged with una-
vailability of OP-specific antibodies. To help address this issue,
zirconia or titania nanoparticles (ZrO2or TiO2) were performed as
selective sorbent to recognize the phosphorylation moiety based
on the strong chelation with phospho-moieties (Fang et al., 1997;
Li et al., 2008). Chen and Chen (2005) synthesized magnetic
titania particles (composite of Fe3O4, SiO2, and TiO2 with ill-
defined structure) with an aim to combine the magnetic property
of magnetite particles and affinity of TiO2toward phosphopep-
tides for fast enrichment of phosphopeptides. However, the
TiO2coated magnetic particles synthesized had the disadvantage
of nonspecific binding of nonphosphopeptides. This can be
explained by the material with ill-defined structure. Detection
methods for OP-AChE/OP-BChE adducts using nanoparticle-based
immunoassay for diagnosis of exposure to OP agents have also
been developed in our group (Liu et al., 2008; Du et al., 2011a; Lu
et al., 2011; Du et al., 2012). We further reported another
magnetic beads based-immunosensor for quantification of OP
exposure by simultaneous detection of BChE activity and total
amount of enzyme (Du et al., 2011b). This approach not only
eliminates the individual variation of BChE values but also avoids
the drawback of the scarce availability of OP-specific antibodies.
Here we present the first report on the development of Fe3O4
at TiO2magnetic nanoparticles-based disposable electrochemical
immunosensor with quantum dot (QD)-linked antibody for sen-
sitive and selective detection of OP-BChE adduct in human
plasma. Fe3O4at TiO2magnetic nanoparticles not only selectively
capture phosphorylated adduct by metal chelation but also
directly separate it from biological matrices by simply exerting
an external magnetic field.
2.1. Reagents and apparatus
Carboxyl functional Quantum Dots at 565 nm (QDs, 0.05 mM)
were purchased from BaseLine Chromtech (Tianjing, China).
Human BChE, paraoxon, bovine serum albumin (BSA), 1-ethyl-
3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC),
N-hydroxysuccinimide (NHS), 11-mercaptoundecanoic acid (MUA),
phosphate buffer saline (PBS), acetate buffer and Tween-20 were
purchased from Sigma/Aldrich. Anti-BChE antibody was purchased
from Abcam Inc (Cambridge, MA). Ferric nitrate nonahydrate
(Fe(NO3)3?9H2O) and ferrous sulfate heptahydrate (FeSO4?7H2O)
were purchased from Shanghai Chemical Reagent Co. (Shanghai,
China). Tetrabutyl titanate was purchased from Alfa Aesar (Massa-
chusetts, USA). SuperBlock T20 (TBS) blocking buffer and Bicincho-
ninic acid assay (BCA) kit were purchased from Thermo scientific
Electrochemical measurements were performed on CHI 660C
workstation (CH Instruments Co., Shanghai, China). A disposable
screen-printed electrode (SPE) consisting of a carbon working
electrode, a carbon counter electrode, and an Ag/AgCl reference
electrodewere purchased from
The magnetic processing platform was the product of Bio-Nano
Technology Co. (Shanghai, China). Transmission electron micro-
scopy (TEM) micrographs were performed on JEOL 2000 trans-
mission electron microscopy operating at 200 kV. Attenuated
total reflection Fourier-transform infrared spectra (ATR-FTIR)
were recorded on a Nexus 470 FTIR (Nicolet, USA) equipped with
an omni sampler over 32 scans. X-ray diffraction (XRD) patterns
were obtained using a Philips Xpert X-ray diffractometer using
CuKa radiation at l¼1.5418˚A. The Autolab SPR system (Eco
Chemie B.V., Netherlands) was used in this work. The sensor
chip with a 50 nm thick gold layer and a 5 nm titanium sublayer
as the adhesive layer on glass was attached to the prism using an
index-matching oil (nd
2.2. Preparation of Fe3O4at TiO2nanoparticles
(1) Fe(NO3)3?9H2O and FeSO4?7H2O (2:1) were dissolved in
100 mL double distilled water. During rapid stirring at room
temperature (25 1C), ammonia solution was added into the
mixture, which was then stirred vigorously for 60 min. Nitrogen
was passed continuously through the mixture during the reaction.
The resulting precipitates were separated by a magnet and
washed with distilled water and absolute ethanol. These magnetic
nanoparticles were then re-dispersed in ethanol. (2) TiO2nano-
particles were prepared by dissolving tetrabutyltitanate in abso-
lute ethanol to form a clear solution and then hydrolyzed by
hydrothermal method under acidic solution. (3) TiO2nanoparti-
cles were added into the generated magnetic nanoparticles
solution, which was adjusted to acidic. The mixture stirred
vigorously in water bath maintained at 80–90 1C for 2 h. After
separation/washing/re-dispersion with ethanol, the obtained pro-
duct was dried in oven.
2.3. Preparation of OP-BChE adduct
OP-BChE was prepared by mixing BChE (100 mL with paraoxon
(10 mL acetone containing 34 mg paraoxon) in double distilled
water and incubated overnight. The resulting solution was then
exhaustively dialyzed with water for 72 h to remove unbound
paraoxon and the outcome p-nitrophenoxy. Furthermore, the
enzyme activity was determined with Ellman assay until it was
completely inhibited. The resultant OP-BChE adduct was concen-
trated with ultrafiltration to a final volume of 1.0 mL and stored
at ?20 1C for future use. The protein concentrations of the BChE
and OP-BChE stock solution were determined to be 10 mM and
5 mM, respectively, by BCA method.
2.4. Preparation of QDs-anti-BChE conjugate.
Briefly, 200 mL of carboxyl functionalized QDs were dispersed
in 1.0 mL of 100 mM MES buffer at pH 5.2 and then mixed with
100 mM NHS and EDC solution for 30 min under vigorous shak-
ing. The resulting mixture was centrifuged for 3 min at 5000 rpm
to remove the supernatant solution, and then re-dispersed in
1.0 mL PBS. At room temperature, 60 mL of 1.0 mg/mL anti-BChE
was added and the mixture was shaken slowly at room temperate
for 2 h in the dark and kept overnight at 4 1C. The resulting
QDs-anti-BChE conjugate was collected by centrifugation at
15,000 rpm for 5 min. After removal of the supernatant solution,
the purified QDs-anti-BChE conjugate was achieved and re-
suspended in 1.0 mL PBS containing 1% BSA and stored at 4 1C.
X. Zhang et al. / Biosensors and Bioelectronics 41 (2013) 669–674
2.5. SPR measurements of immunoaffinities
The cleaned sensor chip was immersed in the 1.0 mM MUA
solution overnight and then activated by 50 mL of the mixture of
100 mM EDC and NHS in PBS. Afterward, 50 mL of 1.0 mg/mL anti-
BChE antibody was introduced onto the MUA self-assemble layer
to obtain an anti-BChE antibody modified SPR sensor. To inves-
tigate the interaction of Fe3O4at TiO2, antibody and OP-BChE on
sensor chip, 50 mL of the OP-BChE or BChE in PBS was injected
into the cuvette, and the solution was kept in contact with the
anti-BChE antibody immobilized sensor chip for association for
40 min. Then the solution was drained out with a peristaltic
pump and a 50 mL PBS solution was injected for the dissociation
measurement. Then 50 mL of the Fe3O4at TiO2suspension was
injected for another incubation of 30 min and washed with PBS.
These procedures were controlled with a self-edited semi-auto-
matic program sequence.
2.6. Immunoassay procedure
Two hundred microliter (1 mg) Fe3O4at TiO2nanoparticles were
dispersed in 800mL acetate buffer (pH 4.0) and 25mL aliquot of
Fe3O4 at TiO2 was then transferred into centrifuge tubes. 25mL
aliquot of OP-BChE or spiked samples, which was diluted to the
desired concentration with acetate buffer containing 0.5% BSA was
further introduced into each tube and vortexed for 30 min at room
temperature. The mixtures were washed four times and then
magnetically separated and re-dispersed in acetate buffer. The
addition of 50-fold dilution of the QDs-anti-BChE conjugate
(50mL) and incubation of 40 min with gentle shaking results in
the sandwich immunocomplex (Fe3O4at TiO2/OP-BChE/QDs-anti-
BChE). After magnetic separation and washing, the resulting ‘‘wet-
cake’’ complex was on the tube wall. 10 mL of 1.0 M HCl was added
into each tube by mixing for 5 min to release cadmium ions from
the captured QD labels, followed by addition of 50 mL of 0.2 M
acetate buffer (pH 4.5) containing 0.5 mg/mL of Bi. 50mL super-
natants were transferred to the SPE and detected by square wave
2.7. Electrochemical measurements
Each SPE was first pretreated electrochemically by cyclic
voltammetric scanning for 10 times at a potential range of
0–1.5 V in 50 mM PBS (pH 7.4). Electrochemical SWV measure-
ments were performed using an in situ plated Bi film formed on
the SPE by 2 min accumulation at ?1.4 V. Subsequent stripping
was carried out after a 2 s rest period from ?1.1 V to ?0.6 V, with
a step potential of 4 mV, an amplitude of 25 mV, and a frequency
of 15 Hz. A baseline correction of the resulting voltammogram
was performed with CHI software. The detection limit (DL) was
calculated by 3-fold standard deviation of the blank (3s).
3. Results and discussion
3.1. Mechanism of nanoparticles-based immunosensor for OP-BChE
As shown in Scheme 1, the principle of immunosensor for
detection of phosphorylated protein, the biomarker of OP-BChE, is
similar to traditional sandwich immunoassays except that the first
recognition was performed by Fe3O4at TiO2magnetic nanoparticles
for chelation with the phosphorylated moiety instead of phospho-
serine antibody. The secondary recognition was performed by
QD-tagged anti-BChE antibodies (QDs-anti-BChE). With the help of
the magnet, the sandwich-like complex was isolated from solutions.
The captured QDs were then dissolved by a drop of acid to release
cadmium ions and followed by electrochemical detection on the
screen printed electrode (SPE). Numerous studies have shown that
TiO2have strong affinity to phosphoric group. In this protocol, the
use of Fe3O4at TiO2magnetic nanoparticles not only surpasses the
drawback of the scarce commercial availability of OP-specific anti-
body but also directly separate the target analyte from complex
3.2. Characterization of Fe3O4at TiO2nanoparticles
Fig. 1A shows the transmission electron microscopy (TEM)
images of Fe3O4 at TiO2 nanoparticles. One can see that these
particles displayed well-defined monodispersion with average
size of 100 nm. Several nano-size TiO2particles (?5 nm) were
uniformly located on the outer shell of the nanospheres. Fig. 1B
presents Fourier transform infrared (FTIR) spectrum of Fe3O4at
TiO2nanoparticles. It can be seen that the band at 1700 cm?1is
associated with the stretching vibration of CQO bond of carboxyl
groups. Deformation of O–H band is observed at 1410 cm?1.
The peak at 1100 cm?1is attributed to stretching vibration
of the C–C. The peak at 632.3 and 568.2 cm?1attributed to
the Fe–O–Fe stretching band. The Ti–O–Ti vibration band appears
at 442.6 cm?1. These results indicate successful synthesis of
Fe3O4at TiO2nanoparticles. X-ray diffraction (XRD) pattern was
further used to confirm the generation of Fe3O4at TiO2nanopar-
ticles (Fig. 1C). It shows that the peaks at 2y values of 25.25,
48.13, 54.27, 63.021 are indexed to (101), (200), (105) and (204)
crystal planes of TiO2. Moreover, the peaks at 37.99, 54.27 and
63.021 are associated with the (311), (422), (420) crystal planes
3.3. Evaluation of immunoaffinities among Fe3O4at TiO2
nanoparticles, antibody and OP-BChE
Surface plasmon resonance (SPR) has been developed as a
sensitive tool for real-time monitoring of small changes in
refractive index near the sensor surface and antibody-antigen
specific recognition. In the current experiment, the immunoaffi-
nities among Fe3O4at TiO2nanoparticles, antibody and OP-BChE
were investigated by SPR signal intensity, as shown in Fig. 2A. The
anti-BChE antibody modified SPR sensor can recognize both
OP-BChE (a) and BChE (b) with SPR angle shifts of 249 m1and
187 m1, respectively. After attaching Fe3O4at TiO2in the second
Scheme 1. Schematic illustration of sandwich-like immunoassays.
X. Zhang et al. / Biosensors and Bioelectronics 41 (2013) 669–674
immunoreaction, the SPR response of recognizing OP-BChE
further increased as expected, while for BChE showed little
change in angle response. This result indicates that TiO2nano-
particles served as recognition antibody can selectively capture
OP-BChE due to the strong binding affinity to phosphoric group.
The immunoaffinities of Fe3O4at TiO2, OP-BChE and QDs-anti-
BChE antibody were further studied by square wave voltammetry
(SWV) measurements (Fig. 2B). Here, 1.0 nM nonphosphorylated
BChE, 1.0 nM OP (paraoxon) and 1% BSA were served as controls
to be simultaneously examined as the challenging proteins. One
can see a well-defined voltammetric peak (curve a) from 1.0 nM
OP-BChE (curve a). The current response is much higher that
from non-phophorylated BChE (curve b), OP (curve c) and BSA
(curve d), indicating effective immunoreactions between Fe3O4
at TiO2, OP-BChE and QDs-anti-BChE to form sandwich-like Fe3O4
at TiO2/OP-BChE/QDs-anti-BChE complex. Signals from these
control experiments might be ascribed to the nonspecific adsorp-
tion of QDs-anti-BChE to the electrode.
Fig. 1. (A) TEM images of Fe3O4at TiO2nanoparticles, (B) FTIR spectrum of Fe3O4at TiO2nanoparticles, and (C) XRD spectra of Fe3O4at TiO2nanoparticles.
Fig. 2. (A) SPR responses for recognizing of OP-BChE (a) and BChE (b) in sandwich immunoreactions. (B) SWV responses of the immunosensor for 1.0 nM OP-BChE (a),
1.0 nM nonphosphorylated BChE (b), 1.0 nM OP (c), and 1% BSA (d).
X. Zhang et al. / Biosensors and Bioelectronics 41 (2013) 669–674
3.4. Optimization of electrochemical immunoassay conditions.
The incubation time is one of the important parameters for
both capturing OP-BChE and QDs-anti-BChE. As shown in Fig. 3A,
the current response increased with increasing incubation time
and tend to a steady value after 30 min (curve a) and 40 min
(curve b), respectively, which was used for Fe3O4 at TiO2 and
QDs-anti-BChE to recognize OP-BChE in the sandwich immunoas-
say. Longer time could result in a large non-specific signal.
pH is another important issue for the immunoreactions.
As reported in literature, TiO2have amphoteric properties that
can react either as a Lewis acid or base depending on the pH of
the reaction solution (Nawrocki et al., 2004). In acidic solution,
TiO2with positively charged titanium atoms (Lewis acid) shows
high binding affinity to phosphate ions (Lewis base), suggesting
that high binding selectivity to phosphorylated proteins should
be achievable. It was found that the highest response was
achieved at pH 4.0 (Fig. 3B). Lower adsorption pH does not mean
the best condition since phosphate ions may be prone to bind
with hydrogen ions in strong acidic condition and also cause the
damage of protein. Therefore, pH 4.0 was selected for the
Nonspecific adsorption has a significant influence on immu-
noassay responses. It can be seen from Fig. 3C that obviously
negligible signals were observed when using 0 nM OP-BChE as
control. Although both the current responses of sample (1.0 nM
OP-BChE) and the control decreased due to the shield effect of
BSA block agent, the single-to-noise was improved greatly. There-
fore, 1% BSA was added to the synthesized QDs-anti-BChE con-
jugate and the minimization of nonspecific adsorption (control
signals) was achieved and tend to be stable after that.
The response signal of the immunosensor also depends on the
concentration of QDs-anti-BChE. Fig. 3D shows the current
responses of both 1.0 nM OP-BChE and 0 nM OP-BChE at different
dilutions of QDs-anti-BChE solution. Although the immunosensor
displayed much higher electrochemical currents at high QDs-
anti-BChE concentration (50-fold dilution), the nonspecific signal
was very high. It can be seen that the best result is achieved
at 50-fold dilution of QDs-anti-BChE antibody with the largest
3.5. Analytical performance for electrochemical immunoassay
Under optimal conditions, the proposed method is tested with
different concentrations of OP-BChE. As shown in Fig. 4A, well
defined voltammetric peaks (cadmium) were observed with
incubation of OP-BChE, and the current response increased with
the increase of OP-BChE concentration. The linear relationship is
obtained over the log[OP-BChE] concentration range from 0.02 to
10 nM, with the detection limit of 0.01 nM (Fig. 4B). Since the
average concentration of BChE in human plasma is 40–70 nM
(Bartels et al., 2000; Jun et al., 2009; Masson and Lockridge, 2010),
this method is sensitive enough to detect 0.01 nM OP-BChE,
which is less than 1% BChE inhibition. Such a detection limit is
comparable to that of mass-spectrometric analysis of organopho-
sphorylated cholinesterase adducts (Sporty et al., 2010). The
reported detection limit is also comparable to our previous result
of 0.02 nM adducts by using ZrO2nanoparticles as capture anti-
body (Du et al., 2011a; Wang et al., 2009). The advantage of
synthesized Fe3O4at TiO2magnetic nanoparticles includes that
they not only selectively capture phosphorylated adduct but also
Fig. 3. Effects of parameters on immunosensing responses. (A) Incubation time
including Fe3O4at TiO2capture (a) and QDs-anti-BChE recognition (b), (B) pH of
immunoreactions solution, (C) Nonspecific adsorption, and (D) concentration of
QD-anti-BChE conjugate. In the optimization of each parameter, other conditions
were fixed when single parameter was optimized, including 30 min of Fe3O4
at TiO2and 40 min of QDs-anti-BChE (50-fold dilution) were used to recognize
OP-BChE in pH 4.0, respectively.
Fig. 4. (A) SWV responses of the immunosensor with the increasing OP-BChE concentrations, where curves a–k corresponds to 0.01, 0.02, 0.05, 0.1, 0.2, 0.5, 1.0, 2.0, 5.0, 10,
and 20 nM, respectively, and (B) calibration curve.
X. Zhang et al. / Biosensors and Bioelectronics 41 (2013) 669–674
directly separate the OP-BChE from biological matrices by simply Download full-text
exerting an external magnetic field.
The reproducibility of the proposed method is evaluated by
intra- and inter-assay coefficients of variation (CVs). The intra-
assay precision is evaluated by analyzing one sample for six
replicate determinations on one electrode. The CVs of the intra-
assay were 3.6% and 6.2% at 1.0 and 10.0 nM OP-BChE, respec-
tively. Similarly, the inter-assay CVs on six electrodes were 4.0%
and 6.7% at 1.0 and 10.0 nM phosphorylated BChE, respectively,
indicating acceptable reproducibility.
A series of OP-BChE human plasma samples were used to test
the accuracy and practical viability of the electrochemical quan-
tification approach. These samples were prepared by spiking
different amounts of OP-BChE to human plasma. The result of
each sample was determined three times and summarized in
Table 1, which shows the recoveries are in the range of 92–105%,
indicating that the electrochemical immunosensing approach is
We have designed a Fe3O4 at TiO2 nanoparticles-based dis-
posable electrochemical immunosensor for sensitive detection of
phosphorylated BChE in plasma, a specific biomarker of OP
exposure. Although OP-specific antibody is a good captur-
ing agent for developing immunoassay methods to measure
OP-protein, the rare commercial availability of these specific
antibodies limits the development of antibody-based methods.
The novelty of this approach includes: (1) using Fe3O4at TiO2
nanoparticles as capturing agents for selective enrichment phos-
phorylated BChE due to their high affinities, avoiding the draw-
back of the scarce availability of OP-BChE antibodies; (2) using
magnetic nanoparticles for directly separating OP-BChE from
biological matrices by simply exerting an external magnetic field;
and (3) using QDs label as reporting signal for amplifying the
electrochemical response. Such a method has good selectivity,
high sensitivity and acceptable reproducibility, which shows
promising for developing a simple, portable and inexpensive
detector for quantifying and evaluating exposures to OP pesti-
cides and nerve agents.
This work was supported by the National Natural Science
Foundation of China (21075047) and the Special Fund for Basic
Scientific Research of Central Colleges (CCNU11C01002). Y. Lin
acknowledges the financial support by the CounterACT Program,
National Institutes of Health Office of the Director (NIH OD), and
the National Institute of Neurological Disorders and Stroke
(NINDS), Grant Number U01 NS058161. The contents of this
publication are solely the responsibility of the authors and do
not necessarily represent the official views of the NIH. Pacific
Northwest National Laboratory is operated by Battelle for US-DOE
under Contract DE-AC05-76RL01830.
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