March 10, 2010 / Vol. 8, No. 3 / CHINESE OPTICS LETTERS 309
Tagged molecule induced nanoparticle aggregation:
Raman reporter-labeled immuno-Au
aggregate as immuno-sensor
Chunyuan Song (yyySSS), Zhuyuan Wang (ÍÍÍ), Ruohu Zhang (ÜÜÜeeemmm),
Jing Yang ( ¬¬¬), Xuebin Tan (!!!ÆÆÆRRR), and Yiping Cui (www²²²)
∗
Advanced Photonics Center, Southeast University, Nanjing 210096, China
∗
E-mail: cyp@seu.edu.cn
Received July 14, 2009
A novel structure with high surface enhanced Raman scattering (SERS) activity and bio-specificity as a
SERS-based immuno-sensor (named as Raman reporter-labeled immuno-Au aggregate) is demonstrated
and employed for protein detection. In each fabrication process, the features of those aggregates are
obtained and characterized by ultraviolet-visible (UV-Vis) extinction spectra, transmission electron mi-
croscopy (TEM) images, scanning electron microscopy (SEM) pictures, and SERS spectra. Experimental
results indicate that proper amounts of the reporter molecules can result in the moderate aggregation mor-
phologies of gold nanoparticles. Compared with the previously reported method using Raman reporter-
lab eled immuno-Au nanoparticles, more sensitive SERS-based protein detection is realized with this novel
immuno-sensor.
OCIS co des: 160.4236, 290.5860, 300.6450.
doi: 10.3788/COL20100803.0309.
An immuno-sensor is a compact analytical device incor-
porating an antibody, an antigen or its fragment, either
integrated within or intimately associated with a phys-
iochemical transducer
[1]
. A large number of immuno-
sensors based on the specific antibody-antigen interaction
have been develop ed and provide a sensitive and selective
tool for the estimation of proteins. Thereinto, surface
enhanced Raman scattering (SERS) based immunoassay
as a new immunoassay technique with higher sensitivity
is known as a potent detection means for protein de-
termination, which has been demonstrated to have the
ability to detect picomole to femtomole concentrations
[2]
.
Typically, this technique often makes use of the immune
recognition of the specific binding between the undeter-
mined proteins and the immuno-sensor named Raman
reporter-labeled immuno-Au nanoparticles. The sample
protein can be determined qualitatively or quantitatively
by SERS signal produced by the reporter
[3−8]
.
Though the distance dependent properties of gold
nanoparticles have been explored to enhance the sen-
sitivity of immuno-sensors
[1]
, it has been well known
that the respondence ability of SERS-based immunoas-
say depends strongly on the morphology (e.g., the size,
shape, or aggregation) of the gold nanoparticles
[9,10]
. No
Raman signal of reporter could be detected when small
gold nanoparticles are used as a SERS substrate
[11]
.
Krug et al. found that the isolated nanoparticle with av-
erage dimension at 25 nm was insufficient to enhance the
Raman signals of the reporter absorbed on the surface of
nanoparticle. Lamentedly, bigger gold nanoparticles can
result in higher SERS activity, but their stability may de-
crease significantly with their size increasing when they
are modified with Raman reporter
[12,13]
. Thus, many re-
searchers have to carry out the SERS based-immunoassay
using some small gold nanoparticles as immuno-sensor,
and some additional activation treatments such as silver
stain must be executed to obtain a high signal-to-noise
ratio (SNR) of the SERS signal
[14−20]
.
More recently, many researches make it clear that
moderate aggregates of novel metal nanoparticles can
generate a remarkable surface enhancement effect and
provide feasibility to solve those problems mentioned
above. It is well known that a surprising enhanced signal
obtained at the junctions b etween nanoparticles of the
aggregates is 2– 40 times stronger than that obtained
at an isolated nanoparticle
[21−25]
. Actually, the gold
nanoparticle aggregates with a certain controlled size
and high stability can be induced absolutely by adjust-
ing the tagged molecules
[26−28]
. So, the reporter-labeled
gold aggregates induced by tagged molecule with rich
“hot spots” may have a stronger SERS activity with-
out the sacrifice of high stability. It is assumed that with
such novel immuno-sensor, the immunoassay can not only
have no use for additional activation treatments of small
nanoparticles, but also avoid the instability aroused by
big nanoparticles. In this letter, a novel structure with
high SERS activity and bio-specificity named Raman
reporter-labeled immuno-Au aggregate is demonstrated
and employed as an immuno-sensor for SERS-based pro-
tein detection successfully.
The reagents used in our experiment are given as
follows. Hydrogen tetrachloroaurate(I II) trihydrate
(HAuCl
4
·3H
2
O), 4-mercaptobenzoic acid (4MBA), and
poly-L-lysine were purchased from Sigma. Trisodium cit-
rate (Na
3
C
6
H
5
O
7
·2H
2
O) was obtained from Sinopharm
Chemical Reagent Co., Ltd. Human antigen IgG, goat
anti-human IgG antibody, rat antigen IgG, and bovine
serum albumin (BSA) were purchased from Nanjing
KeyGEN Biotechnology Co., Ltd. Deionized water
(18 MΩ/cm) was used throughout the whole course of
this experiment. The following buffer solutions were
used: borate buffer solution (BBS, 2 mmol/L, pH = 9),
tris buffer solution (TBS, 10-mmol/L tris, 150-mmol/L
NaCl, pH = 7– 8), and TBS/0.1% tween buffer
1671-7694/2010/030309-04
c
° 2010 Chinese Optics Letters