Immunochromatographic dipstick assay format using gold nanoparticles labeled protein-hapten conjugate for the detection of atrazine

Article abstract of DOI:10.1021/es070194j

The present study describes a lateral-flow-based dipstick immunoassay format using a novel hapten-protein-gold conjugate for the rapid screening of atrazine in water samples. The immunoassay is based on the competitive inhibition, in which a newly develop

Full text of DOI:10.1021/es070194j

Environ. Sci. Technol. 2007, 41, 5028-5036
rainwater, the large scale application of this pesticide resulted
Immunochromatographic Dipstick
Assay Format Using Gold
Nanoparticles Labeled
in the contamination of drinking water and soil, causing
serious health risks even at very low (sub ppb) levels (2-3).
Alarmingly, the presence of atrazine has been detected in
water and soil at concentrations far exceeding its permissible
limit of 3 ppb (4). To control and contain such a hazardous
pollutant, a rapid method of detection of this organic toxin
is urgently required. A number of well established analytical
methods such as thin-layer chromatography (5), gas chro-
matography (6), high-performance liquid chromatography
Protein-Hapten Conjugate for the
Detection of Atrazine
J A S D E E P K A U R , † K . V I K A S S I N G H , †
R O B I N B O R O , K . R . T H A M P I , ‡
M A N O J R A J E , †
(
7), and gas chromatography-mass spectrometry (8), have
†
been widely used for the detection and quantification of
pesticides at the trace levels. However, these methods of
detection of pesticides are complex, time-consuming, and
require costly and bulky instrumentation (9). Immunoassay-
based detection techniques are being developed as an
alternative to detect these toxic chemicals in environmental
samples (10-16). These assays, apart from being highly
specific, exhibit the desired sensitivity and accuracy for the
detection of these low molecular weight contaminants
present in our environment.
G R I S H C . V A R S H N E Y , † A N D
C . R A M A N S U R I * , †
Institute of Microbial Technology, Sector 39-A, Chandigarh
1
60 036, India, and Laboratory for Photonics and Interfaces,
ISIC, EPFL, CH-1015 Lausanne, Switzerland
The present study describes a lateral-flow-based dipstick
immunoassay format using a novel hapten-protein-
gold conjugate for the rapid screening of atrazine in water
samples. The immunoassay is based on the competitive
inhibition, in which a newly developed hapten-protein-
gold conjugate competes with the free antigen present in the
sample, for the limited antibody binding sites available at
test zone on dipstick membrane, housed in a plastic cartridge.
The tracer used as the detection reagent was prepared
by first conjugating hapten (a derivative of atrazine) molecules
to a carrier protein (bovine serum albumin) via its surface
lysine residues and then linking colloidal gold nanoparticles
to the hapten-protein conjugate via cysteine residues of
the carrier protein. The developed conjugate showed a
high level of stability as it did not show any significant loss
of activity even after 8 weeks of storage at ambient
conditions. The color developed due to conjugate, based
on competitive inhibition approach, is correlated with the
concentration of atrazine sample. The sensitivity of the
developed dipstick was enhanced by gold nanoparticles,
as an amplification tag, presenting detection limit of atrazine
in standard water samples down to 1.0 ppb level. The kit
could serve as a rapid screening methodology for visual
screening of atrazine contamination of water samples
within 5 min of analysis time, and, when coupled with a
portable colorimeter, as an inexpensive semi-quantitative
assay. The method reported can be useful for screening a
large number of pesticides samples in a very short time
in the field.
In recent years, the developments of immunoassay in
“dipstick format” for the determination of pesticides are
gaining importance because of their low cost and rapid
screening capabilities (17-19). These assay formats are based
on the principle of competitive inhibition immunoassay
approach, where an anti-pesticide antibody is immobilized
on the nitrocellulose membrane. The amount of free antigen
(
pesticides) present in the sample competes with the
conjugate to bind with the available limited antibodies
binding sites. The color developed due to the reaction of
immuno-conjugate with some suitable substrate is correlated
with the concentration of the sample. These dipstick im-
munoassay formats follow the principles of dot-blot im-
munoassay procedural schemes that require longer assay
time, and therefore are not suitable for field applications. In
this paper, we report for the first time a lateral-flow-based
dipstick immunoassay using a novel hapten-protein-gold
conjugate for the determination of atrazine in water samples.
The developed dipstick system is quite simple, rapid, and
sensitive enough for pesticide detection. It takes less than 5
min to complete the measurement of atrazine in a sample,
and therefore is highly useful for field applications. The use
of this newly developed hapten-protein-gold conjugate as
a tracer molecule has improved the stability of the dipstick.
The tracer trapped on a releasing matrix inside the dipstick
module showed no loss of its activity even after 10 weeks of
storage at 4 °C. Recently, testing by competitive immuno-
chromatography is gaining importance, and being widely
used for the rapid detection of various haptens, such as
narcotics, clinical preparations, and pesticides, etc. (20, 21).
This technique is based on the distribution of molecules in
a biphasic system. In this assay, a specific antibody to test
compound is fixed on the surface of a nitrocellulose
membrane in the test zone and remains stationary during
the analysis. The mobile phase, which contains a test sample
and a tracer, moves toward the test zone and competes with
the available binding sites of antibodies coated on test zone.
The binding of tracer to specific antibodies leads to the
formation of a complex which can be evaluated visually and
is inversely proportional to antigenic concentration. The
kinetics of the process of an antigen binding to its antibody
in an immunochromatographic method is higher than in
the conventional dot assay (22).
1. Introduction
Atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triaz-
ine), because of its selectivity against broadleaf weeds and
annual grasses, is one of the most widely used herbicides for
combating weeds in corn, sugarcane, and sorghum crops
(
1). Since atrazine readily adsorbs into groundwater and
*
Corresponding author telephone: +91-172-2636680; fax: +91-
1
72-2690585; e-mail: raman@imtech.res.in.
†
‡
Institute of Microbial Technology.
Laboratory for Photonics and Interface.
5
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10.1021/es070194j CCC: $37.00

2007 American Chemical Society
Published on Web 06/12/2007

FIGURE 1. Reaction mechanism of hapten-protein-gold conjugate formation. Haptens are shown to link with the carrier protein via its
surface lysine residues while colloidal gold particles are linked to the carrier protein through its cysteine residues. Inset shows the
computer-generated model of the conjugate. In the model, disulfide bonds (yellow-green) of the protein (represented in sky blue color)
are linked to gold nanoparticles (purple) while surface lysine groups (royal blue) of the protein are linked to carboxylated hapten (mustard).
2
.2 Preparation of Nanogold Particles. Monodispersed
2
. Materials and Methods
(
∼28 nm) colloidal gold was prepared by slight modification
2.1 Reagents. Hapten-specific antibodies (anti-atrazine
of Frens method (24). A 200 mL solution of 0.01% tetra-
chloroauric acid (gold chloride) in Milli-Q water was taken
in a 500 mL Erlenmeyer flask. The solution was brought to
boiling point on a hot plate. A 4 mL portion of 1% trisodium
citrate solution was rapidly added to the boiling gold chloride
solution. The solution was allowed to boil for 10 min until
it developed a typical bright wine red color of colloidal gold.
The average particle size of colloidal gold particles was
determined using a Zetasizer particle analyzer (Malvern
Instruments) and estimated to be approximately 28 nm.
antibodies) were generated in our lab by immunizing young
New Zealand white rabbits with standard immunization and
purification protocols as described earlier (23). Bovine serum
albumin (BSA), dicyclohexyl carbodiimide (DCC), N-hydroxy-
succinimide (NHS) ester, hydroxylamine-HCl 3-mercapto-
propionic acid, 2-mercaptoethanol, tetrachloroauric acid, and
polyethylene glycol (PEG) were purchased from Sigma
Chemical Co., USA. Technical grade atrazine was purchased
from Supelco, USA. Peroxidase (POD) and 3,3′,5,5′-tetram-
ethylbenzidine (TMB) substrate were obtained from Ban-
galore-Genei, India. All chemicals, reagents, and solvents
used in this study were of high purity analytical grade. Buffers
were made in Milli-Q water.
2.3 Synthesis of Hapten-Protein-Gold Conjugate. For
preparing a hapten-protein-gold conjugate, mercaptopro-
pionic acid derivative of atrazine (MPAD) was first mixed
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FIGURE 2. Schematic diagram of an immunochromatographic dipstick employed in the described assay showing cross-reaction and assay
format.
FIGURE 3. Schematic of enlargement of gold nanoparticles by using gold enhancer.
with a carrier protein (BSA) at 1:50 molar ratio of protein and
hapten by using a procedure reported earlier (23). For this,
aliquot of this supernatant was mixed with the protein
solution (10 mg/mL in borate buffer, pH 9.0) to make a final
volume of 1 mL. The conjugate was then passed through a
P10 gel filtration column. Fractions with maximum protein
concentrations were pooled and checked for the final
concentration of hapten-protein with a spectrophotometer
(Shimadzu 1601A) at 280 nm. Gold particles were labeled
50 µmol of atrazine derivative was first mixed in 1 mL of
dimethylformamide (DMF) along with 125 µmol of dicyclo-
hexyl carbodiimide (DCC) and 125 µmol of N-hydroxysuc-
cinimide (NHS) ester. The mixture was incubated for 4 h and
then centrifuged to remove the urea precipitate. A 150 µL
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FIGURE 4. Effect of various buffers on antibody coating on dipstick
membrane: (a) 50 mmol phosphate, pH 7.2; (b) 50 mmol phosphate
+
3% methanol, pH 7.2; (c) phosphate buffer saline, pH 7.2; (d)
phosphate buffer saline + 3% methanol, pH 7.2; (e) 50 mmol
phosphate + 150 mmol NaCl, pH 7.2. All samples were detected by
a 40 nmol gold-hapten conjugated BSA.
with protein-hapten conjugate by first cleaving the disulfide
bonds of the protein using 2-mercaptoethanol to generate
sulfhydryl groups on protein which were used to link with
the colloidal gold particles for making a stable hapten-
protein-gold conjugate. For this, a solution of protein-
hapten conjugate (1 mg/mL) made in phosphate buffer (pH
7
.0, 50 mM) was mixed with 2-mercaptoethanol (25 mM)
and kept for incubation under mild shaking overnight at
°C. After incubation, the conjugate was dialyzed against
4
water for 48 h and stored at 4 °C. 1 mg protein was added
dropwise into 2 mL colloidal gold solution under mild stirring
condition and the mixture was incubated overnight at 4 C.
The conjugate was washed three times with 10 mM Tris
containing 3% BSA under centrifugation at 12000 rpm for 30
min. The loose pellet was resuspended in the same buffer
and its OD was brought to app. 5.0 at 520 nm. The conjugate
was refrigerated at 4 C. The reaction mechanism of hapten-
protein-gold conjugate formation is illustrated in Figure 1.
A computer-generated model of the hapten-protein-gold
conjugate is shown in the inset. The model, generated using
“Amber-8” software, highlights the binding of carrier protein
(
BSA) to carboxylated hapten (MPAD) via its surface lysine
residues. All the lysine residues of protein having an
accessibility g100 were selected for binding. The hapten-
protein conjugate is further shown to link with gold nano-
particles through the protein’s cysteine residues.
FIGURE 5. Hydrodynamic diameter of colloidal gold particles as
measured by dynamic light scattering system (a) before enhancement
and (b) after enhancement treatment with a solution of 0.01% gold
chloride in 0.4 mM hydroxylamine for 2 min at RT.
2.4 Immunochromatographic Dipstick and Assay Pro-
cedure. A lateral-flow-based dipstick kit was developed using
a nitrocellulose (NC) membrane on which an anti-pesticide
antibody is immobilized at the test line of the detection zone.
Test lines were made with 1 mg/mL anti-atrazine antibody
solution dispensed with an Easy Printer (Advance Microde-
vices, Ambala, India) at 0.7 µL per line on NC membrane
supplied with laminated card. After antibody deposition,
membranes were dried at RT in a clean air chamber (Laminar
hood) for 2 h. The sample pads having glass fiber lining were
soaked in hapten-protein-gold conjugate solution with OD
g 3.0 and then dried in a laminar hood at RT for 4 h. After
assembling sample and adsorbent pads on laminated cards
having NC membrane, the cards were cut into strips. A layout
of dipstick showing the dipstick schematic and assay format
is shown in Figure 2. The strips were housed in a plastic
housing and stored in sealed bags with desiccant before their
usage. For the sample analysis, 50 µL of sample solution
made in PBS containing 1% Tween-20 as detergent was added
into the sample well of the dipstick. The sample along with
the tracer trapped in sample pad moved through the
nitrocellulose membrane where these two different molecules
react competitively for the available binding sites of the anti-
atrazine antibodies coated on the membrane. The intensity
of color developed (reversibly) due to presence of tracer
correlates with the amount of analyte present in the sample.
The observed color due to binding of colloidal gold reagent
at the site of test line is inversely proportional to sample
concentration.
The intensity of signal generated at the test line of the
dipstick due to competitive inhibition of atrazine samples
with tracer was measured by a hand-held scanner (an
indigenously developed reflectometer) at 657 nm in the
reflection mode. The signal values, relative to the standard
atrazine solutions, were fitted to a logistic function using a
standard software package (Origin). The signal intensity “I”
0
was normalized as %B/B , according to the following
normalization equation:
%
B/B ) I - I /(I - I ) × 100
0
ex
0
ex
0
where I is the normal signal intensity, I is signal intensity
at zero concentration of the atrazine, and Iex is value at the
excess of the atrazine.
2.5 Sensitivity Enhancement of Dipstick with Gold
Nanoparticles. The sensitivity of colloidal gold particles
labeled with hapten-protein conjugates bound to antibodies
on the nitrocellulose strips of dipstick was enhanced by using
a mixture of gold chloride and hydroxylamine. The nitrocel-
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analyzer (Malvern Instruments) as well as by absorption
spectrum that showed maxima at 525 nm.
Gold-protein complexes, in general, are formed by
electrostatic attraction between the negatively charged gold
particle and the positively charged protein molecules (amine
groups present at the surface of the proteins are primarily
responsible for the positive charge on the protein molecule).
These types of protein-gold complexes are usually not stable
for a long time due to weak interaction forces applicable
between them. Also, for making hapten-protein conjugates,
the amine groups of the carrier protein (BSA) are used for
binding with carboxylated haptens (MPAD). By utilizing some
of the surface amine groups, the protein has become less
positive, and hence unsuitable for binding with negatively
charged colloidal gold particles. In order to overcome the
loss of amine groups in the conjugate, we have attempted
to use the cysteine residues of the protein to link with the
gold nanoparticles via thiol-gold dative bonds which are
very stable, in general. In the present study, we demonstrate
for the first time the synthesis of a highly stable hapten-
protein-gold conjugate used as the detection reagent for
the immunochromatographic dipstick application. The con-
jugate was made by first conjugating hapten (MPAD, a
derivative of atrazine) to a carrier protein (BSA) via its surface
lysine residues and then linking colloidal gold nanoparticles
to hapten-protein conjugate via cysteine residues of the
carrier protein (Figure 1). The reduced protein forms a stable
complex with gold particles through its thiol linkage.
3.2. Effect of Buffers on Antibody Coating on the Dip-
stick Membrane. The use of various agents to make a sharp
and dense capture line of anti-atrazine antibodies on NC
membrane was studied by dispensing 1 µL/line of antibody
solution using a microdispenser (Easy Printer, Advance
Microdevices, Ambala, India). The effect of various buffers
((a) 50 mmol phosphate, pH 7.2 (b) 50 mmol phosphate +
3% methanol, pH 7.2 (c) phosphate buffer saline, pH 7.2 (d)
phosphate buffer saline + 3% methanol, pH 7.2, and (e) 50
mmol phosphate + 150 mmol NaCl, pH 7.2) on the coating
of antibody on dipstick membrane is depicted in Figure 4.
It was observed that the saline buffers with 150 mmol NaCl
(lines c, d, and e) could not produce a sharp line of protein
on NC membrane since the ionic strength of the buffer
interferes with electrostatic interactions essential for protein
binding. It is therefore important to determine the optimum
buffer conditions that will result in a sufficient concentration
of capture protein in the solution. It was observed that by
adding a small percentage of methanol (3%) in the buffer
gave considerable improvement in antibodies coating on
NC membrane, giving a sharp line to improve the perfor-
mance of the immunoassay (line b). The use of a copre-
cipitating agent, such as alcohol, helps to rewet the membrane
and reduces any static charge on it. Partial destabilization
of the protein with such a coprecipitating agent may also
lead to the exposure of more-hydrophobic groups that are
normally hidden within the protein structure. The increase
in protein hydrophobicity resulting from the use of such
coprecipitating agents improved the protein binding on NC
membrane, which produced a sharp band of protein on NC
membrane.
FIGURE 6. Effect of gold-enhancer on improvement of visibility of
color of test line on NC membrane (a) before and (b) after
enhancement. Tracer dilutions (1:50; 1:200; 1:800, and 1:3200) were
prepared in PBS.
lulose strips were treated with the mixture of 0.01% gold
chloride in double distilled water and 0.4 mM hydroxylamine
hydrochloride (NH OH-HCl) for 2 min at RT and rinsed
2
with Milli-Q water. A schematic diagram of gold enhancement
process increasing the size of gold nanoparticles is shown in
Figure 3. Dynamic light scattering measurements of the native
and enhanced colloidal gold nanoparticles before and after
treatment with enhancer (mixture of gold chloride and
hydroxylamine) were performed in solution phase by using
a Zetasizer particle analyzer (Malvern Instruments).
2.6 Competitive Inhibition Enzyme Linked Immun-
osorbent Assay (ELISA). ELISA plates were coated with
different concentrations of anti-atrazine antibody made in
carbonate buffer (0.05 M, pH 9.6) by incubating the plates
overnight at 4 °C. After blocking with 10% skim milk, free
atrazine at different concentrations mixed with HRP-labeled
MPAD (25) were added into the plates, and incubated for 2
h at RT. The competition was also checked with MPAD, used
as free antigen. After washing the plates thoroughly, the color
was developed by the addition of TMB substrate (100 µL/
well) and absorbance was recorded at 450 nm using ELISA
reader (Microdevices, USA).
3
. Results and Discussion
3.1 Characterization of the Hapten-Protein-Gold Con-
jugate. The present immunochromatographic detection of
atrazine in water samples is based on the competitive
inhibition, in which a newly developed hapten-protein-
gold conjugate (tracer) competes with the free antigen present
in the sample, for the limited antibody binding sites available
at test zone on dipstick membrane. In most cases, the
immunochromatography is carried out using a complex of
protein with gold nanoparticles as a detecting agent. The
advantages of gold nanoparticles as a marker in immuno-
chromatographic detection are their ease of preparation of
required size, monodispersity in aqueous phase, and also
making conjugates with biomolecules to be used as detector
reagents. However, a considerable drawback of complexes
with colloidal gold particles with biomolecules is their
insufficient stability (21).
3.3 Effect of Blocking Agents to Reduce the Non-
specific Binding. Nitrocellulose membranes are naturally
hydrophobic. Because of the hydrophobicity of the mem-
brane, nonspecific interaction of a hydrophobic analyte with
the membrane can generate significant nonspecific signal.
This is more relevant with the detection of pesticide
molecules, which usually are nonpolar in nature. Also, the
size of pesticide molecules present in the sample is too low
to flow across the membrane along with tracer in immu-
nochromatography dipstick assay. The selection of a suitable
blocking agent for the dipstick application for pesticide
For making a stable hapten-protein-gold conjugate,
we first prepared gold nanoparticles using a standard method
(
24) with slight modification. Gold nanoparticles prepared
by this method resulted monodispersed colloidal particles
of 28 ( 5 nm size. This was confirmed by a Zetasizer particle
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FIGURE 7. Competitive inhibition between hapten-protein-gold conjugate and free antigen with antibody-coated test line of dipstick.
Antibody lining was made on the NC membrane by using different concentrations of antibody in PBS solution. SD values are indicated
as error bar (n ) 3).
FIGURE 8. Reactivity of anti-MPAD antibody with atrazine and hapten (MPAD) in direct ELISA format. The competition was done between
free antigens at different concentrations between 0.01 and 1200 µg/L and labeled MPAD-HRP conjugate at 1:8000 dilution against binding
on 0.5 µg/mL coated anti-MPAD antibody. SD values are indicated as error bar (n ) 3).
molecules is the critical part of its development. Ideally, the
blocking materials should be free from interference with the
interaction of antibodies and antigens at the capture line,
where such interference can substantially reduce the signal
intensity of the test. A range of blocking reagents such as
polymers (PVP, PVA, and PEG), surfactant (Tween 20, Triton-
X), and protein (BSA) were tested to demonstrate the
minimum nonspecific binding in dipstick applications. A
solution of 2% PVA mixed with 5% skim milk showed the
best results in terms of nonspecific binding in dipstick
applications (comparative data not shown). Protein mol-
ecules (2-5% BSA), in general, help to reduce the levels of
nonspecific background signal in various immunoassay
formats such as dot assay, ELISA, etc. However, they also
reduce the lateral wicking rate of the test system in dipstick
3.4 Sensitivity Enhancement of Dipstick with Gold
Nanoparticles. The quantification of immunoassays is
generally achieved by measuring the specific activity of a
label. The observation of larger Au nanoparticles is beneficial
for the visualization or quantification of the target molecules
on a nitrocellulose membrane used in dot-blot or dipstick
immunoassays. However, to increase the labeling efficiency,
smaller Au nanoparticles are usually used to prepare the
conjugate. Silver enhancement, also called autometallog-
raphy, is a commonly used enhancement approach for
improving the sensitivity of any assay using colloidal gold
nanoparticles (27). In this process, silver is deposited on the
gold surface. Because the metal particle acts as a catalyst,
the plating is highly specific. Various silver salts have been
used to increase the size of gold particles for giving better
visual effect or usefulness in electron microscopy. However,
the distribution of particle sizes after the silver enhancement
(
26), and hence are not recommended as suitable blocking
materials for dipstick applications.
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visible even at 1:800 dilution of tracer (Figure 6a). However,
test line visibility was drastically improved by using gold
enhancer as an amplification tag. The test line band became
quite visible even at 1:3200 tracer dilution (Figure 6b).
Au nanoparticle, as an amplification tag, is therefore an
important aspect to enhance the sensitivity of immunoassay
in dipstick format where gold-protein complex is employed
as a detector reagent.
3.5 Assay Development. The immunochromatographic
detection of atrazine in water samples is based on the
competitive inhibition approach where the pesticide in water
samples and hapten-protein-gold conjugate (tracer) com-
pete with the limited binding sites of available antibody on
the test line of the dipstick. The hapten-protein conjugate
is labeled with colloidal gold particles for one-step qualitative
assay development. No additional steps are needed for the
detection, as required in the case of enzymes where substrate
is added for the color development (18). Figure 7 shows the
competitive inhibition between hapten-protein-gold con-
jugate and free antigen present in standard samples with
antibody coated on test line of dipstick. Different concentra-
tions of antibody were selected to make the test line.
Concentrations of antibody as low as 0.45 µg/line showed a
good inhibition with standard atrazine samples, presenting
IC50 values ∼1.4 ppb. A hand-held scanner in the reflection
mode measured the intensity of the signal generated at the
test line. Data were obtained on duplicates of each standard
solution in three different experiments (n ) 3). The signal
values, relative to the standard atrazine solutions, were fitted
to a logistic function using a standard software package
(
Origin).
Standard ELISA was employed using the same anti-
atrazine antibody (0.5 µg/well) coated on a microtiter plate
Nunc Maxisorp) to compare the dipstick results for atrazine
(
detection. ELISA results showed a good inhibition with
standard atrazine samples, presenting IC50 values around
0.15 ppb (Figure 8). Data show a good agreement with dipstick
data. The differences in sensitivity limits between results
from ELISA and dipstick could be justified because of several
factors which may combine to determine the difference in
performance between dipstick and ELISA results. The higher
binding capacity of the antibodies on polystyrene microtiter
plate in comparison to that on NC membrane, and also
making use of a sophisticated ELISA reader for signal
detection contribute to obtain the higher sensitivity of ELISA.
The test line coated with the above antibody concentration
was used in a single-step lateral-flow colloidal gold-based
assay system. The inhibition of binding is given by a lack of
color formation at the test line where the capture antibody
is coated; images of the sticks were taken using a digital
camera (Nikon S3). The cross-reactivity of atrazine and some
of its analogues (a, atrazine; b, Simazin; c, Prometon: and
d, Terbutryn), as tested with anti-atrazine antibody coated
immunochromatographic dipstick, is shown in Figure 9.
Samples were added to the sample well and the response
was recorded using a digital color camera after 3 min of
sample addition. Anti-atrazine antibody showed very high
reactivity with atrazine in comparison to other atrazine
analogues. Similar results were obtained by ELISA assay for
confirming the cross-reactivity of atrazine analogues.
Figure 10 shows the lateral-flow dipstick assay performed
for detection of atrazine in aqueous samples using hapten-
protein-gold conjugate. Anti-atrazine antibody was used as
capture antibody immobilized on the nitrocellulose mem-
brane (1 µL/line) at a concentration of 1 mg/mL (test line).
Calibration curve was obtained using standard atrazine
solutions in the concentration range of 0 to 500 ppb. The
inhibition of tracer binding to the immobilized antibody is
quite evident even at <1.0 ppb and no binding of conjugate
is observed above 100 ppb. A relatively low coefficient of
FIGURE 9. Cross-reactivity of immunochromatographic dipstick
coated with anti-atrazine antibody with atrazine and its analogues:
(a) atrazin, (b) simazin, (c) prometon, and (d) terbutryn. Stock solutions
of samples were prepared in methanol (1 mg/mL) and further diluted
in PBS buffer for assay.
is not uniform because post staining with silver can get
oxidized which may reverse the silver staining.
In the present study, we have studied the signal en-
hancement of colloidal gold particles by using a solution of
gold chloride enhancer. Figure 3 shows the schematic
diagram of the gold nanoparticles’ enlargement with gold
chloride and hydroxylamine. The hydrodynamic diameter
of gold nanoparticles increased to around 310 nm, as
confirmed by a Zetasizer particle analyzer (Figure 5a and b).
Hydroxylamine used in the enhancer solution is thermo-
3
+
dynamically capable of reducing Au ions to bulk metal
3
+
(
28), where all the added Au ions go into the production
of larger gold particles by making contact with gold on NC
membrane, causing enlargement of the gold nanoparticles.
Treating colloidal gold-labeled hapten-protein conjugate
linked to antibody coated NC membrane with 0.01% gold
chloride and 0.4 mM hydroxylamine for 2 min at RT resulted
in increased visibility of the test line color due to enlargement
of the gold particles. The result suggests that by dipping
nitrocellulose membrane in a solution of 0.01% gold chloride
in 0.4 mM hydroxylamine for 2 min at RT effectively improved
the visual effect of test line of the dipstick (Figure 6a and b).
Without employing an enhancer tag, the test lines were not
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FIGURE 10. Sensitivity of immunochromatographic dipstick for atrazine samples. Samples spiked with different concentrations of atrazine
were added to the sample well and the response was recorded using a digital color camera after 3 min of sample addition.
support under the ISCB programme. R.B. is a pre-doctoral
student supported by a fellowship from CSIR, India. Authors
J.K. and K.V.S. had equal contributions to this work.
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(
(
(
(
FIGURE 11. Stability test of developed gold-protein-hapten
conjugate. The dipsticks were subjected to tracer solution (hapten-
gold-protein conjugate) after 2, 4, 6, and 8 weeks of their storage
in ambient conditions (room temperature in a desiccator).
7) Hernandez, F.; Hidalgo, C.; Sancho, J. V.; Lopez, F. J. Coupled-
column liquid chromatography applied to the trace level
determination of triazine herbicides and some of their me-
tabolites in water samples. Anal. Chem. 1998, 70, 3322-3328.
variation (CV) among three inhibition assays with dipstick
was observed with not much difference in color intensity, as
observed by hand-held scanner. The kit could therefore serve
as a rapid and reproducible screening test for visual screening
of atrazine contamination of water samples, and, when
coupled with a portable colorimeter, as an inexpensive
semiquantitative assay.
The newly synthesized colloidal gold-labeled hapten-
protein conjugate showed high stability and longer shelf life
while trapped in the sample pad of the dipstick at ambient
storage condition (Figure 11). The conjugate did not show
any significant loss of its activity and color intensity even
after 8 weeks of its use in the dipstick assay format. This was
attributed to strong dative bonds between the gold and the
protein molecules.
(8) Panshin, S. Y.; Carter, D. S.; Bayless, E. R. Analysis of Atrazine
and Four Degradation Products in the Pore Water of the Vadose
Zone, Central Indiana. Environ. Sci. Technol. 2000, 34, 2131-
2
137.
(
9) Slobodnik, J.; Oztezkizan, O.; Lingeman, H.; Brinkman, U. A.
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water samples on graphitized carbon and Empore-activated
carbon disks and on-line coupling to octadecyl-bonded silica
analytical columns. J. Chromatogr., A 1996, 750, 227-238.
(
10) Franck, M.; Kolar, V.; Eremin, S. A. Enzyme immunoassays for
s-triazine herbicides and their application in environmental
and food analysis. Anal. Chim. Acta 1995, 311, 349-356.
11) Thurman, E. M.; Meyer, M.; Pomes, M.; Schwab, A. P. Enzyme-
linked immunosorbent assay compared with gas chromatog-
raphy/mass spectrometry for the determination of triazine
herbicides in water. Anal. Chem. 1990, 62, 2043-2048.
(
(
12) Wittman, C.; Hock, B. Analysis of atrazine residues in food by
an enzyme immunoassay. J. Agric. Food Chem. 1993, 41, 1421-
The proposed dipstick assay seems to be a fast and reliable
field test for a preliminary qualitative/semiquantitative
screening of atrazine samples. The real samples can be
analyzed without any pre-enrichment or cleanup treatment.
The developed dipstick shows good accuracy, acceptable
precision, higher sensitivity, and longer shelf life.
1
425.
(13) Baumner, A. J.; Schmid, R. D. Fiber optic chemical sensors and
Biosenors. Biosens.s Bioelectron. 1998, 13, 519-529.
(14) Svitel, J.; Dzgoev, A.; Ramanathan, K.; Danielsson, B. Surface
plasmon resonance based pesticide assay on a renewable
biosensor surface using a reversible concavalin-A monosac-
charide interaction. Biosens. Bioelectron. 2000, 15, 411-415.
(
15) Aeppli, R. E.; Bargetzi, M. J.; Binz, H. Diagnosis screening of
multiple antigen-antibody reactions in a new single assay on
nitrocellulose: the line immunobinding assay (LIBA). J. Im-
munol. Methods 1989, 120, 93-98.
Acknowledgments
We thank Mr. Nalini Kant of M/s Microdevice, Ambala (India)
for the technical support. The Swiss Development Corpora-
tion (SDC) Switzerland and Department of Biotechnology
(
16) Giersch, T. A New Monoclonal Antibody for the Sensitive
Detection of Atrazine with Immunoassay in Microtiter Plate
and Dipstick Format. J. Agric. Food Chem. 1993, 41, 1006-1011.
(
DBT), India are gratefully acknowledged for the financial
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(
(
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17) Mosiello, L.; Gremisini, C.; Segre, L.; Spano, M. Dipstick
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received April 24, 2007. Accepted May 1, 2007.
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