Chemistry Reference
In-Depth Information
H
2
O
2
[
131
]. Studies on the detection of tumour markers involved fabricating sand-
wich-type immunoassays with a typical luminol-H
2
O
2
chemiluminescence system
catalysed by Ag
0
NPs [
132
]. Detection of tumour markers was also achieved by
sandwich CL-ELISA with antibodies that were covalently immobilised on a chitosan
modi
ed paper zone through glutaraldehyde cross-linking [
148
]. Correlating the
concentration of the analyte and the peak intensity of the emitted light allowed
quantitative analysis.
1.2.5 Electrochemiluminescence (ECL)
This sensing mechanism is based on luminescence generated by electrochemical
reactions. When electrochemically generated intermediates undergo exergonic
reactions, they result in an electronically excited state. This state emits light upon
relaxation to a lower level state, and therefore it enables readouts without the
requirement for a photodetector. ECL has the advantages of both luminescence and
electrochemistry. An ECL sensor based on orange luminescence was demonstrated
through the detection of 2-(dibutylamino)-ethanol (DBAE) and nicotinamide ade-
nine dinucleotide by readouts of luminescence [
123
]. The principle of ECL sensor
was based on Ru bp
ðÞ
3
and DBAE. At the electrode, the amine was oxidised and
formed a radical cation [DBAE
▪
]
+
, followed by deprotonation to create a DBAE
▪
radical, which reduced Ru bpy
2
2
þ
3
emitted light at 620 nm while relaxing to the ground state [
149
]. This mechanism
served as a coreactant that was oxidised solely by the electrode. In ECL, the
electrochemical potential initiated and controlled the chemiluminescence reaction.
The electrodes were printed using screen printing and adding an ECL active
luminophore followed by drying. The substrates were laminated onto a Zensor
screen-printed electrode using an of
3
3
ðÞ
to an excited state. Eventually, Ru bpy
ðÞ
ce laminator. The cyclic voltammetry of Ru
2
3
was used to characterise the electrodes. After the lamination step, an
incision was made in the laminate layer. After the introduction of the sample to the
assay, the potential was stepped from 0 to 1.15 V for a short period to initiate the
ECL. The initiation can be achieved by shifting the potential to a level more
positive than the oxidation potential of the ruthenium complex. Chronoamperom-
etry was adopted to generate ECL since it provided control over the reaction time
(Fig.
1.2
d). The ECL readouts were taken with a camera phone housing to block the
ambient light. ECL was connected to the mobile phone battery to obtain a short
pulse of low voltage. In addition to these drawbacks, ECL required a photomulti-
plier tube, which was costly in a miniaturised form. The data was analysed based on
the red pixel intensity of the ECL emission, which was correlated with a calibration
curve, hence the analyte concentration. Other applications to date included sensing
tumour markers [
103
,
124
,
126
] and ions [
128
].
ðÞ
bpy
