American Journal of Anal yt ical Chemistry, 2011, 2, 484-490
doi:10.4236/ajac.2011.24058 Published Online August 2011 (http://www.SciRP.org/journal/ajac)
An Indirect Immunoassay for Detecting Antigen Based on
Fluorescence Resonance Energy Transfer
Peihui Yang1,2*, Shuguang Yao1, Wei Wei1, Jiye Cai1,2
1Department of Chemistry, Jinan University, Guangzhou, China
2Key Laboratory of Optoelectronic Information and Sensing Technologies of Guangdong Higher Education Institutes,
Jinan University, Guangzhou, China
Received March 4, 2011; revised April 29, 2011; accepted May 16, 2011
Abstract
An indirect immunoassay for detecting antigen was developed. It was based on fluorescence resonance en-
ergy transfer (FRET) and quenching of gold nanoparticles. Bovine serum albumin (BSA) was chosen as
model antigen. Fluorescein isothiocyanate (FITC) was attached to anti-BSA antibody (anti-BSA–FITC) as
FRET donor, while BSA was conjugated to gold nanoparticles (GNPs–BSA) as FRET acceptor. The forma-
tion of anti-BSA–BSA immunocomplex resulted in the FRET between anti-BSA–FITC and GNPs–BSA.
Thus, the fluorescence of FRET donor was quenched, and the decreasing fluorescence intensity responded
linearly to the concentration of acceptor within the linear range. The concentration of BSA we obtained ac-
cording to the stoichiometric ratio between BSA and GNPs. Following this approach, we were able to spe-
cifically detect BSA. The detection limit for BSA was 0.5 nM and the linear range of the assay was 2.9 - 43.5 nM.
It had been successfully applied to specific detection of BSA in serum samples.
Keywords: Anti-BSA Antibody, BSA, Gold Nanoparticles, FRET, Indirect Immunoassay
1. Introduction
Over the past few years, nanoprobes have attracted con-
siderable attention in bioassays, and have been exploited
in biodetection, biolabeling and biosensor development
[1-3]. As a new spectral analysis method, fluorescence
resonance energy transfer (FRET) has been widely used
to detection of protein [4-6] and nucleic acid [7], due to
the characteristics of easy operation, high-sensitivity and
fast detection. Surface energy transfer (SET) between
dye molecules and metal nanoparticles has gained inter-
est because this technique is capable of measuring dis-
tances nearly twice as far as FRET [8]. Gold nanoparti-
cles (GNPs) have been of great interest because of their
high extinction coefficient and broad absorption spec-
trum in visible region which is overlapped with the
emission wavelength of usual FRET donor. Both theo-
retical calculations and experimental studies have well
demonstrated that GNPs are a kind of “superquencher”
which can quench the fluorescence of a range of dyes
with extraordinarily high efficiency [9]. Based on this
superquenching effect, GNPs have been successfully
served as acceptor in FRET. Pihlasalo et al. reported a
new and highly sensitive method to detect protein con-
centrations which relying on protein adsorption and
quenching of fluorescently labeled protein. This method
is based on the competitive assay principle with nonco-
valent adsorption of sample protein and dipyrryl-
methene-BF2 530 labeled BSA on nanosized gold parti-
cles in a homogeneous assay format [10]. Mayilo et al.
reported the homogeneous sandwich immunoassay with
GNPs as fluorescence quenchers. A detection limit of
0.7 ng/mL was obtained for protein cardiac troponin T
(cTnT), which is the lowest value reported for a homo-
geneous sandwich assay for cTnT [11]. Biotin assay with
sensitivities in the micromolar range have also been re-
ported [12]. QDs are used in FRET because of several
advantages. Haldar et al. demonstrated a pronounced
effect on the photoluminescence quenching and shorten-
ing of decay time of CdSe QDs during interaction with
GNPs in a GNPs−BSA conjugated CdSe QD system [8].
Yang et al. have attempted to employ two FRET assem-
bles in one system, in which luminescent CdTe QDs as
FRET donors and GNPs serving as the acceptor with
chymotrypsin as the linking bridge [13].
The present work is based on fluorescence signal

P. H. YANG ET AL.485
changes which were induced by antigen. Little attention
has been reported to the indirect immunoassay method
for detecting antigen with gold nanoparticles acting as a
bridge and the quenching of gold nanoparticles to FRET
donor and antigen. Here, we demonstrated a FRET-based
assay system by antigen conjugated gold nanoparticles as
FRET acceptor and FITC attached the corresponding
antibody as FRET donor. By making use of BSA and
anti-BSA antibody as a model antibody–antigen system,
and combining quenching of gold nanoparticles with the
specific recognition between antigen and antibody, we
designed an indirect immunoassay for detecting antigen.
The design rationale for this assay method is illustrated
in Scheme 1.
2. Experimental
2.1. Materials and Methods
BSA with purity above 99% was purchased from
Sigma. The BSA was weighed out and was digested in
PBS buffer. The final concentration of the BSA was 1.3 ×
10–4 M with the molar extinction coefficient of 45900
M–1·cm–1 at 278 nm. Newborn calf serum was purchased
from Hangzhou Sijiqing Biological Engineering Materi-
als Co., Ltd. Anti-BSA antibody was purchased from
Beijing Dingguo Changsheng Biotech. Co., Ltd. FITC,
hemoglobin (purity ≥ 98%) and plasmin were purchased
from Sigma-Aldrich (St. Louis, MO, USA). Glutathione
oxidized was purchased from Sinopharm Chemical Re-
agent Co., Ltd. Chloroauric acid (1.0 × 10–2 M) was pur-
chased from Shanghai Richjoint Chemical Reagents Co.,
Ltd. Sodium citrate solution (1 vol%) was obtained from
Scheme 1. Diagram describing the procedure of FRET: (a)
Labeling anti-BSA antibody with FITC; (b) BSA was con-
jugated to GNPs and quenched by GNPs; (c) FRET oc-
curred upon addition of BSA conjugated GNPs to the an-
ti-BSA–FITC solution. As a result, the fluorescence of the
FITC was quenched by the nearby GNPs, resulting in an
obvious decrease of FITC fluorescence intensity.
Guangzhou Chemical Reagent Factory. Phosphate buffer
(PBS, 0.01 M, pH = 7.4) was prepared according to the
previously reported procedure [14].
All fluorescence spectra were recorded by a Cary Ec-
lipse fluorescence spectrophotometer (Varian, USA), and
absorption spectra were recorded by an UV1901 spec-
trometer (Rui Li Anal. Instr. Com., China). The TEM
images of each sample were obtained by a PHILIPS
TECNAI-10 transmission electron microscope.
ξ-potential measurements were carried out using a Zeta-
sizer3000 H S Malvern Zetasizer (Malvern Instruments
Inc., USA).
2.2. Preparation of Gold Nanoparticles (GNPs)
and Gold Nanorods (GNRs)
GNPs were prepared according to Frens method [15].
Briefly, 5 mL of 1.0 × 10–2 M chloroauric acid was di-
luted to 50 mL with doubled distilled water and brought
to boil. Next, 10 mL of 1% citric acid was added to the
solution. Refluxing of the solution continued until the
color of the boiling solution changes from dark purple to
red vine color. The image clearly shows the nearly
spherical shape of the particles having an average semi-
diameter of 16 ± 2 nm. The concentration of GNPs was
measured according to the absorbance at 520 nm, using
the molar extinction coefficient of 2.7 × 108 M–1·cm–1 [16].
GNRs were prepared using the reported procedure
[17]. TEM image shows as-prepared GNRs with aspect
ratios 4:1.The UV-Vis absorption spectra and TEM im-
age are consistent with the results of Nikoobakgt et al.
[17].
2.3. Labeling Anti-BSA Antibody with FITC
Anti-BSA antibody was labeled by a modification of the
procedure of reported method [18]. Briefly, anti-BSA
antibody (0.2 g/mL) was reacted with FITC (1 mg/mL)
in 0.1 M sodium bicarbonate buffer, for 4 h at 37°C. The
UV-Vis Spectra of various concentration of anti-BSA–
FITC were measured. The stoichiometry for the binding
of FITC to anti-BSA antibody was obtained by the
UV-Vis absorption peak of various concentration of an-
ti-BSA–FITC. The F/P (FITC/protein) ratio was calcu-
lated from the following expression: F/P ratio = 3.1 ×
A495 / [A280 – 0.31 × A495]. The results suggested that an
average of 3.5 molecules of FITC binding to one an-
ti-BSA antibody molecule.
2.4. Preparation of BSA Conjugated Gold Na-
noparticles (GNPs–BSA)
GNPs-BSA was prepared by mixing equal volumes 8.7 ×
10–9 M solution of GNPs with 3.1 × 10–5 M solution of
BSA in PBS buffer. The mixture was incubated at 37°C

P. H. YANG ET AL. 486
overnight.
2.5. Emission Spectrum Measurements
The quenching intensity of gold nanoparticles on BSA
was measured by a Cary Eclipse fluorescence spectro-
photometer at excitation wavelength of 280 nm and
emission wavelength ranging from 300 - 500 nm. Then,
the fluorescence intensity (F) was measured at emission
wavelength of 345 nm; F0 was the intensity of the sys-
tems without GNPs. The fluorescence quenching inten-
sity (ΔF) can be calculated from the following Eqs: ΔF =
F0 – F.
The FRET assays were performed using the fluores-
cence spectrophotometer. By excitation at 480 nm, emis-
sion spectra were in the range of 500 to 700 nm. Then,
the fluorescence intensity (F) of the donor (anti-
-BSA–FITC) was measured at emission wavelength of
522 nm; F0 was the intensity of the systems without the
acceptor (GNPs–BSA). The fluorescence quenching in-
tensity (ΔF) can be calculated from the following Eqs:
ΔF = F0 – F.
2.6. UV-Vis Spectroscopy
UV-Vis spectra were recorded by a UV1901 spectrome-
ter. All UV-Vis spectra were taken in a wavelength range
of 200 - 800 nm.
2.7. Transmission Electron Microscopy
Characterization
Transmission electron micrographs (TEM) were obtained
with a Philips TECNAI-10 microscope. The TEM sam-
ples were prepared by taking a solution sample and cast-
ing it onto a carbon-coated copper grid sample holder
followed by evaporation in air at room temperature.
2.8. Particle Size and Zeta Potential
Measurements
Zeta potential distribution was measured by the particle
characterizer (Malvern ZetaSizer Nano ZS, Malvern,
UK). Briefly, the samples were loaded into the capillary
zeta potential cell for measurement. This instrument also
includes the DLS (dynamic laser scattering) function, by
which the particle size distribution can be analyzed.
2.9. Assay of the BSA in Serum Samples
New-born calf serum was diluted about 100 times during
the disposal procedure. A test of BSA concentration was
analyzed under the optimum concentration of gold na-
noparticles and experimental conditions by standard ad-
dition method.
3. Results and Discussion
3.1. Preparation and Characterization of FRET
Acceptor
In the reported study [19], it was shown that gold
nanoparticles quenched BSA fluorescence mainly
through a static quenching mechanism. The stoichiome-
try for the binding of BSA to GNPs was confirmed by
the relationship between fluorescence quenching inten-
sity and the concentration of GNPs. At pH7.4, upon ad-
dition of GNPs, the fluorescence intensity of BSA at
345 nm decreased with the increase of the concentration
of GNPs. To further increase the concentration of GNPs
produced no obvious change on the fluorescence of BSA,
which indicated that BSA adsorption on GNPs reached
saturation. The stoichiometry for the binding of BSA to
GNPs was calculated by the concentration ratio of BSA
to GNPs, until the fluorescence of BSA had no obvious
change. Varying the concentration of BSA, an average of
3.60 × 103:1 stoichiometry for the binding of BSA to
GNPs was calculated (Table 1). The FRET acceptor was
prepared with the same ratio as BSA to GNPs described
above.
3.2. Spectra Characteristics of FRET System
FRET occurred upon the addition of GNPs–BSA to the
anti-BSA–FITC solution, GNPs–BSA and FITC were
closed due to the specific binding of anti-BSA antibody
and BSA. When GNPs conjugate was present, the fluo-
rescence of FITC was quenched by the FRET acceptor,
as is shown in Figure 1.
Table 1. The stoichiometry for the binding of BSA to GNPs.
nBSA(mol) n GNPs(mol) nBSA:n GNPs nBSA:n GNPs (mean)RSD (%)
1.1 × 10–7 3.0 × 10–11 3.70 × 103
2.0 × 10–7 5.7 × 10–11 3.53 × 103
3.3 × 10–7 9.3 × 10–11 3.53 × 103
3.9 × 10–7 10.7 × 10–11 3.63 × 103
3.60×103 1.7

P. H. YANG ET AL. 487
500 520 540 560 580
100
200
300
400
500
600
700
1
6
F
/nm
Figure 1. The process of FRET between anti-BSA–FTIC
and GNPs–BSA. Emission spectra of anti-BSA–FITC with
the increasing of GNPs–BSA, the concentration of anti-
-BSA–FITC was 0.120 mg/ml and that of GNPs–BSA was
0.9, 1.8, 2.7, 3.6, 4.5, 5.4×10–10 M, respectively.
3.3. Interaction Mechanism between FRET
Donors and FRET Acceptors
3.3.1. Interference of Nonspecific Substances
Interference experiments were performed to exclude
nonspecific interactions of biomolecules, in which GNPs
were incubated with hemoglobin (Hb), oxidized glu-
tathione (GSSG), lysozyme according to the ratio of
BSA to GNPs as the FRET donor, respectively. The re-
sults showed that the capability of recognition with an-
ti-BSA–FITC can be summarized as BSA ＞ Hb ＞
GSSG ＞ lyszyme ＞ GNPs (Figure 2).
The net surface charges of Hb and GSSG at pH 7.4 are
negative, while anti-BSA–FITC is positively charged.
0 15304560
0
40
80
120
160
F
Canalytes×10-9M
BSA
GNPs only
Hb
GSSG
Lysozyme
Figure 2. The process of FRET between anti-BSA–FTIC
and GNPs-analyte at 25°C and 0.01 M PBS, and pH value
was 7.4. The concentration of anti-BSA–FITC was 0.120 mg/ml
and that of GNP–analyte was 5.0×10–8 M.
Therefore, the distances between Hb, GSSG and an-
ti-BSA–FITC are shortened due to electrostatic interac-
tions to partially produce FRET, resulting in fluores
cence quenching to a certain extent. The reason why the
quenching effect of Hb is higher than GSSG is that the
Hb has structural similarities to BSA. However, posi-
tively charged lyszyme have little quenching effect.
These results indicated that FRET occurs after recogni-
tion between FRET donor and energy acceptor, resuling
in fluorescence quenching.
3.3.2. Characterizati on of Specific Recog ni ti on
Between FRET Donor and FRET Acceptor
To directly visualize the specific recognition between
FRET donor and FRET acceptor, the gold nanorods with
aspect ratios 4:1 (Figure 3) were conjugated with FRET
donor and incubated with FRET acceptor, and then
dropped cast on a copper grid and examined by TEM
[20]. During imaging, we observed a large amount of
nanoparticle–nanorod dimers as shown in Figure 4,
These nanoparticle dimers are believed to be formed
through specific recognition between antigen and anti-
body. The mean particle size and size distribution were
measured by particle size analyzer [21]. After specific
recognition between antigen and antibody, the average
diameter of gold nanoparticles and nanorods mixed sys-
tem was increased (Figure 5). These results further indi-
cate that FRET occurs after recognition between FRET
donor and FRET acceptor, resulting in fluorescence
quenching.
3.4. Indirect Assay of Antigen with FRET
System
Several parameters such as pH and reaction time need
further optimization for development of this FRET sys
400 500 600 700 800900100011001200
0.0
0.2
0.4
0.6
0.8
1.0
520nm
800nm
Abs
/nm
Figure 3. Absorbance spectra of prepared GNPs with as-
pect ratio 4:1.
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P. H. YANG ET AL. 488
Figure 4. TEM micrographs of (a) antigen-conjugated gold nanoparticles; and (b) FRET donor-conjugated gold nanorods:
and (c) nanoparticle-nanorod conjugate oligomers formed from the binding of antigen-conjugated gold nanoparticles and
gold nanorods nanoprobes wi th F RET donor .
(a) (b)
Figure 5. Hydrodynamic diameter distribution plots as determined by DLS measurements: (a) the size distribution of GNPs
and GNRs mixed system; (b) the size distribution of GNPs and GNRs mixed system after specific recognition between antigen
and antibody.
0.0 0.5 1.0 1.5 2.0 2.5
0
50
100
150
200
CGNPs-BSA×10-7M
0.12mg/ml
0.075mg/ml
0.038mg/ml
F
Figure 6. The effect of the concentration of donor on the
process of FRET.
tem in order to tailor reaction results. To understand the
effect of the concentration of FRET donor on the process
of FRET, we have studied optimal FRET donor concen-
tration. It was found that the quenching effect increased
as the concentration of FRET donor increasing (Figure
6), which may related to the immunogenicity of FRET
donor. At lower concentrations, the immunogenicity of
FRET donor was low, so, the quenching effect initiated
by energy transfer was weak. As the concentration of
FR-ET donor increased, the immunogenicity of FRET
donor increasing proportionally. Hence, the quenching
effect increased. The quenching effect reached a maxi-
mum when anti-BSA–FITC concentration was 0.12
mg/ml. Therefore, 0.12 mg/ml of GNPs–BSA was used
in the following research.
Since recognition of donor–acceptor requires some time,
the optimal incubating time was studied. For the quenching
of fluorescence intensity of anti-BSA–FITC by GNPs–
BSA, as shown in Figure 7, fluorescence quenching inten-
sity increased gradually at incubating time range from 0
min to 22 min and reached a plateau at 22 min. Thus, 25
min was chosen for the assay. The influence of temperature
was examined and it was found that temperature had a little
influence on the results in the temperature range of 15 to
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opyright © 2011 SciRes. AJAC

P. H. YANG ET AL.489
0 102030405
0
0
10
20
30
40
50
60
F
t/min
Figure 7. The effect of incubating time on the process of
FRET at 25°C and 0.01 M PBS, and pH value was 7.4. The
anti-BSA–FITC concentration was 0.12 mg/mL, and the
GNPs concentration was 5.0 × 10–8 M.
3.0 4.5 6.0 7.5 9.010.512.0
-10
0
10
20
30
40
50
F
pH
Figure 8. The effect of pH value on the process of FRET at
25°C and 0.01 M PBS, after 25 min of incubation. The an-
ti-BSA–FITC concentration was 0.12 mg/mL, and the GNPs
concentration was 5.0 × 10–8 M.
40°C. So, room temperature (25°C) was chosen for fol-
lowing work. The influence of the buffer systems and the
pH value on FRET was examined. The results showed pH
7.4 PBS buffer was the most suitable (Figure 8 ), which can
avoid antigen or antibody denaturation when solutions at
higher pH value or at lower pH value.
Under the above optimized conditions, the fluores-
cence quenching intensity is linearly proportional to BSA
concentration from 2.9 nM to 43.5 nM with a detection
limit of 0.5 nM, the linear regression equation was ∆F =
2.887 × CBSA + 7.007, with a correlation coefficient(R)
of 0.9935.
The serum was diluted about 100 times during the dis-
posal procedure (sample 1), the recovery test was done
by the standard addition method (n = 3). The recovery
values with their standard deviation are shown in Table
2. The average quenching intensity was 6.0%, which was
obtained from 10 replicate determinations of 4.3 × 10–8
M GNPs–BSA under the optimized conditions. The me-
thod recovery was calculated compared with three
known concentration levels. The recovery values are
shown in Table 2 (sample 2). The above results indi-
cated that the proposed method has a high repeatability
and precision.
4. Conclusions
We have demonstrated an indirect immunoassay based
on fluorescence resonance energy transfer. Antigen was
conjugated gold nanoparticles as acceptor, and FITC was
attached the corresponding antibody as FRET donor.
BSA and anti-BSA antibody were chosen as a model
antibody-antigen system. The formation of anti-BSA–
BSA immunocomplex resulted in the FRET between
donor and acceptor. The concentration of BSA is deter-
mined by quenching of fluorescently FRET acceptor.
This method has been successfully applied to determina
Table 2. Analytical results of serum samples and recovery.
samples Added (nM) Founda(nm) Recovery Recovery (mean)
0 33.50 ± 0.60 __
5.80 38.70 ± 0.54 97.67
20.30 57.10 ± 0.69 116.5
Sample 1
37.70 68.70 ± 0.52 92.45
102.2
5.80 6.07 ± 0.25 104.7
20.30 21.70 ± 0.91 106.9 Sample 2
37.70 35.90 ± 1.51 95.21
103.3

P. H. YANG ET AL. 490
tion of total serum protein in serum samples.
5. Acknowledgements
This project was supported by the Major State Basic Re-
search Development Program of China (973 Program)
(No. 2010CB833603) and the National Natural Science
Foundation of China (No. 21071064).
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