Aspects of recent development of immunosensors - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 244

of fl uorescent detector molecules. Moreover, in recent years, quantum dots as the most
suitable fl uorescence labels have received increasing applications for developing fl uo-
rescence immunosensors due to their high fl uorescence quantum yield and sensitivity
to environmental changes upon binding proteins. Aoyagi et al. proposed a reagent-
less, regenerable, and portable optic immunosensor for the ultra-sensitive detection
of a model sample of IgG based on changes in fl uorescent intensity of fl uorescent
quantum dot-labeled protein A [138]. An antibody for leukemia cell recognition was
attached to the luminophore-doped nanoparticle through silica chemistry, yielding an
optical microscopy imaging technique for the identifi cation of leukemia cells [139].
Experimental results in this report showed that the new technique using the antibody-
coated luminophore nanoparticles could allow leukemia cells to be easily and clearly
identifi ed with high effi ciency.
Chemiluminescence sensors have also been extensively applied in routine clinical
analysis as well as biomedical research due to the advantages of no radioactive wastes,
simple instrumentation, low detection limit, and wide dynamic range [14, 140-144].
A chemiluminescent immunosensor for carbohydrate antigen 19-9 (CA19-9) was
described by Lin et al. , with CA19-9 immobilized on the cross-linked chitosan mem-
brane [141]. The decrease of the immunosensor chemiluminescent signal was propor-
tional to the CA19-9 concentration in the range 2.0-25 U mL 1 , with the detection limit
of 1.0 U mL 1 . Pandian et al. developed an automated chemiluminometric immunoassay
for the measurement of HCG [142]. It was demonstrated that the immunoassay might
facilitate exploration of HCG utility for Down syndrome screening, early pregnancy
detection, and differentiation of invasive from non-invasive trophoblastic disease. An
optical microbiosensor has been newly designed for the diagnosis of hepatitis C virus
(HCV) by using a novel photo-immobilization methodology based on a photo-activable
electro-generated polymer fi lm [143]. Herein, the immunosensor using optical fi ber
photochemically modifi ed was tested for the determination of anti-E 2 protein antibod-
ies through chemiluminescence reaction. Another published study presented the use of
electrogenerated luminol chemiluminescence in a homogeneous immunosensor, where
digoxin was labeled with luminol through a luminol-BSA-digoxin conjugate [144]. The
prepared chemiluminescence immunosensor in a competitive format was shown allow-
ing for the detection of free digoxin with the concentration as low as 0.3

g L 1 .

µ

9.3.3 Microgravimetric immunosensors
Microgravimetric immunosensors may incorporate high sensitivity of piezoelectric
response and high specifi city of antibody-antigen immunoreaction. The detection
principle of these devices is generally based on adsorbate recognition where the selec-
tive binding may cause the changes in mass loading and interfacial properties (i.e. vis-
coelasticity and surface roughness), which can be recognized by a corresponding shift
in the oscillation frequency [145-148]. Outstanding features of these sensors include
low cost, simple usage, high sensitivity, and real-time output. Microgravimetric immu-
nosensors have two kinds of sensing formats, gas phase and solution phase sensing.
The sensitivity to the mass change in air on the transducer surface is about 1 Hz ng 1