Chemistry Reference
In-Depth Information
1.2.3 Colloidal Nanoparticles (NPs)
Functionalised colloidal gold and monodisperse latex are the historical sensing
reagents for lateral-
ow assays, which do not require a readout device for quali-
tative results [
146
]. Lateral-
fl
fl
ow tests employ antibody-conjugated gold NPs, which
have extinction coef
cients that are higher than common organic dyes. Inkjet
printers have been used to deposit the NPs for multiplex detection on
filter paper
[
111
]. Other NP-based assay studies focused on improving the sensitivity of current
lateral-
ow tests by using paper network platforms. The 2D nitrocellulose-based
networks enabled multistep processes to amplify the signal in immunoassays to
improve the limit of detection [
115
,
147
]. These cards contained reagents stored dry
and the assay was activated by wicking the sample in a single-user step. They were
capable of multistep processes such as delivering rinse buffers and signal amplifi-
fl
-
cation reagents to the capture zones. These devices were demonstrated by using
porous materials such as nitrocellulose and cellulose depending on the sensing
application. Other 2D paper networks involved integrating the inlets of a number of
lateral
ow assays [
112
] (Fig.
1.2
c), adopting folding techniques [
119
] and
microplate paper platforms [
120
]. NP-based detection has been demonstrated with
metabolites [
109
,
111
,
113
,
117
,
121
,
147
], bacterial agents [
112
] in disease
diagnosis such as HIV [
119
], malaria [
115
], tuberculosis [
120
] and in environ-
mental monitoring [
114
,
118
]. To multiplex the assay, monodisperse latex can be
coupled with
fl
fluorescent and coloured dyes, and para/magnetic components. For
example, conjugated with dark dyes, monodisperse latex particles exhibit high
contrast on nitrocellulose or paper. Additionally, latex particles can be utilised with
different colours or
fl
fl
uorescent dyes.
1.2.4 Chemiluminescence (CL)
CL is based on the emission of light generated by a chemical reaction. In the presence
of reactants A (luminol) and B (H
2
O
2
), and a catalyst or excited intermediate
(3-aminophthalate), light is produced along with side products. Peroxidase catalyses
the oxidation of luminol to 3-aminophthalate, and the decay of the excited state (
)to
a lower energy level results in light emission, which can be enhanced by using phenol
derivatives such as p-iodophenol. A typical example of CL is the glow stick, which is
based on the reaction of peroxide with a phenyl oxalate ester ([A] + [B]
◊
→
[products] + light). In rapid diagnostics, CL has been explored for the detection of
glucose and uric acid [
130
] and tumour markers [
132
,
148
]. Glucose and uric acid
assays were based on oxidase reactions coupled with chemiluminescence reactions of
a rhodanine derivative with the generated H
2
O
2
in an acidic medium [
130
]. Uric acid
was determined through a chemiluminescence reaction between the rhodanine
derivative (3-p-nitrylphenyl-5-(40-methyl-20-sulphonophenylazo) rhodanine) and
→
[
◊
]
