Biomedical Engineering Reference
In-Depth Information
allow organometallic compounds [52, 53] or metal ions [54, 55] to be detected at
nanomolar concentrations, their sensitivities remain insuffi cient compared with fl uores-
cent europium chelate labels for which picomolar levels can be determined.
A promising route for maximizing the number of metal markers per binding event is
the use of a metal nanoparticle label, which is composed of thousands of metal atoms.
Colloidal gold is one of the best candidates for electrochemical immunosensing and
immunoassay because of its excellent redox activity. Murielle
et al.
fi rst reported a sen-
sitive electrochemical immunoassay based on a colloidal gold label that, after oxidative
gold metal dissolution in an acidic solution, was indirectly determined by anodic strip-
ping voltammetry (ASV) at a single-use carbon-based screen-printed electrode (SPE)
[56]. The method was evaluated for a non-competitive heterogeneous immunoassay of
an immunoglobulin G (IgG) and a concentration as low as 3
10
12
M was deter-
mined, which was competitive with colorimetric enzyme-linked immunosorbent assay
(ELISA) or with immunoassays based on fl uorescent europium chelate labels. Chu
et al.
reported a similar electrochemical immunoassay based on silver on colloidal gold
labels which, after silver metal dissolution in an acidic solution, was indirectly deter-
mined by ASV at a glassy-carbon electrode [57]. The method was evaluated for a non-
competitive heterogeneous immunoassay of an immunoglobulin G (IgG) as a model.
The anodic stripping peak current of silver depended linearly on the IgG concentration
over the range of 1.66 ng mL
1
to 27.25 ng mL
1
in a logarithmic plot. A detection
limit as low as 1 ng mL
1
(i.e. 6
10
12
M) human IgG was achieved.
A new method based on cyclic accumulation of gold nanoparticles has been pro-
posed for detecting human immunoglobulin G (IgG) by anodic stripping voltamme-
try [58]. The dissociation reaction between dethiobiotin and avidin in the presence of
biotin provides an effi cient means for the cyclic accumulation of gold nanoparticles
used for the fi nal analytical quantifi cation. The anodic peak current increases gradually
with the increasing accumulation cycles. Five cycles of accumulation are suffi cient for
the assay. The low background of the proposed method is a distinct advantage provid-
ing a possibility for determination of at least 0.1 ng/mL
1
human IgG.
An electrochemical immunoassay method based on Au nanoparticle-labeled immu-
nocomplex enlargement used to detect complement C-3 was reported by Zhou
et al.
[59]. When the aggregates formed from nano-Au-labeled goat-anti-human C-3 and
nano-Au-labeled rabbit-anti-goat IgG were immobilized on the electrode surface by
the sandwich method (antibody-antigen-aggregate), the electrochemical signal of the
electrode was enlarged greatly. The reported immunosensor could quantitatively deter-
mine complement C-3 in the range of 0.12, similar to
117.3 ng mL
1
, and the detec-
tion limit was 0.02 ng mL
1
.
Liao
et al.
reported an amplifi ed electrochemical immunoassay by autocatalytic dep-
osition of Au
3
onto gold nanoparticle labels [60]. By coupling the autocatalytic depo-
sition with square-wave stripping voltammetry, enlarged gold nanoparticles labeled on
goat anti-rabbit immunoglobulin G (GaRIgG-Au) and, thus, the rabbit immunoglob-
ulin G (RIgG) analyte, could be determined quantitatively. From a calibration graph
over a broad dynamic range of concentrations (1-500 pg mL
1
;
R
2
0.9975), a very
low detection limit was obtained. The limit was 0.25 pg mL
1
(1.6 fM), which is three
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