Chemistry Reference
In-Depth Information
Fig. 1.2
Sensors utilised for point-of-care testing,
a
Colorimetric detection of heavy metals,
b
Electrochemical sensing of glucose, lactate and uric acid,
c
Antibody conjugated gold NP
detection of Pseudomonas aeruginosa and Staphylococcus aureus,
d
ECL emission from a paper-
based device at different analyte concentrations,
e
Fluorescent sensing for measuring bacterial
growth. Adapted from Ref. [
69
] with permission from The Royal Society of Chemistry
either usage of serum/plasma samples or a red blood cell
filter. Despite its drawbacks,
colorimetric sensing is widely used, and it can also be quanti
ed by a handheld reader
[
141
,
142
] or a smartphone camera [
72
,
143
].
1.2.2 Electrochemical Sensors
Electrochemistry and ion-selective electrodes have been explored widely due to
their well-known principle of operation and maturity. Traditionally, electrochemical
sensors have three electrodes: a counter electrode, a working electrode, and a
reference electrode. In paper-based assays, carbon ink was used for the counter and
the working electrode, whereas silver/silver chloride ink was used for the fabrica-
tion of the reference electrode. The reaction zones comprised this multiple-electrode
mechanism (Fig.
1.2
b). Electrochemical sensors allowed the detection of glucose
[
94
,
95
], lactate [
94
,
95
], uric acid [
94
], cholesterol [
95
], tumour markers [
101
],
dopamine [
106
] and drugs [
107
]. Other studies described heavy metal sensors for
environmental monitoring [
94
,
96
,
99
,
100
]. As compared to colorimetric reagents,
the fabrication of electrochemical sensors required an additional deposition step of
conductive inks on the paper matrix. All these electrodes and electronic wires were
screen printed using graphite and silver inks, respectively. When the sample was
introduced to the device, it was wicked up into the sensing zones and the amper-
ometric measurement was initiated via a glucometer. Alternatively, gold can be
sputtered on the paper matrix through a shadow mask for the deposition of elec-
trodes [
107
]. Such electrodes can be characterised by cyclic and square wave
voltammetry, and chronoamperometry. In contrast to colorimetric reagents, elec-
trochemical sensors respond within seconds and have sensitivities down to nM
[
145
]. Electrochemical detection is also independent of the ambient light and is less
prone to interference from the colour/deteriorations of substrate. Although the
attributes of electrochemical detection such as maturity and suitability for minia-
turisation are attractive, the requirement for a readout device increases its com-
plexity and the cost per test.
