Another much explored category of sensors for heavy metal ion detection are the
electrochemical sensors. Tang et al. [60] reported a reusable electrochemical
biosensor based on a platform of ordered mesoporous carbon and self-doped
polyaniline nanofibers. DNA was immobilized on these platforms and in the
presence of Hg
2+ ions, the folding was induced which resulted in the increased
electrical signal. The conductive nanofibers efficiently transmit these signals
resulting in a highly sensitive detection with an LOD of 0.6 fM of Hg
2+ . Ghosh
et al. [61] have demonstrated a highly sensitive precise detector fabricated using
Au@BSA coated on Nylon-6 nanofibers which were already precoated with a green
emitting fluorophore fluorescein isothiocyanate. The Hg
2+ ions quench the red
emission completely and the same was observed as a signal in the sensor. The
authors claim that the sensor can sense as low as 80 counts of ions of mercury.
Kacmaz et al. [62] reported an optical chemical sensor device for Hg
2+ ion
detection using ethyl cellulose nanofibers. The nanofibers were fabricated with an
azomethine flouroionophore as an additive dye. In the presence of the heavy metal
ions the optical absorption spectrum of the fiber changes much more as compared to
thin films and it has been demonstrated to detect Hg
2+ ion concentration as low as
0.07 nM. Another novel approach by Parsaee [63] was to use bio-ultrasound
assisted fabrication of Au-NPs using algae and immobilized over PAN nanofibers.
The detection was based on colorimetry and fluorescence in presence of the dye
Rhodamine-B(RhB). The catalytic reduction of RhB by Au-amalgam causes a color
change which could be mapped to the amount of Hg
2+ present in the solution.
The LOD of this method was reported to be 1.10 nM of metal ion concentration.
Another interesting work by Sanchez-Calvo et al. [64], reports fabrication of paper
based electrochemical sensor for Hg
2+ detection. The electrode was modified by
AuNPs-carbon nanofibers and rGO-AuNPs which increased their selectivity and
sensitivity. While nanofibers provided larger surface area, the AuNPs with their
affinity towards Hg
2+ helped in more sensitive recordings. The sensor was able to
detect up to 30 nM with CNFs/AuNPs chemistry.
3.4 Detection of Cadmium
Cadmium is another heavy metal which has serious human health hazards and it
badly affects renal function and the respiratory tracts [65]. It is present not only in
industrial effluents, but also in the electronic wastes like mobile phones and their
batteries, computer circuit boards etc. [66]. Many researchers have reviewed the
detection of Cadmium using different techniques like SERS, colorimetry,
Fluorescence etc. [38, 39, 67].
Promphet et al. [68] have utilized graphene/polyaniline/polystyrene electrospun
nanoporous fibers modified electrode and a voltammetry study for simultaneous
determination of lead and cadmium. The developed electrochemical sensor reported
a detection limit of 3.30 µg/l and 4.43 µg L
−1 for Lead and Cadmium, respectively.
They also reported reusage of the electrode after simple washing up to ten times.
308
A. K. Srivastava et al.
electrochemical sensors. Tang et al. [60] reported a reusable electrochemical
biosensor based on a platform of ordered mesoporous carbon and self-doped
polyaniline nanofibers. DNA was immobilized on these platforms and in the
presence of Hg
2+ ions, the folding was induced which resulted in the increased
electrical signal. The conductive nanofibers efficiently transmit these signals
resulting in a highly sensitive detection with an LOD of 0.6 fM of Hg
2+ . Ghosh
et al. [61] have demonstrated a highly sensitive precise detector fabricated using
Au@BSA coated on Nylon-6 nanofibers which were already precoated with a green
emitting fluorophore fluorescein isothiocyanate. The Hg
2+ ions quench the red
emission completely and the same was observed as a signal in the sensor. The
authors claim that the sensor can sense as low as 80 counts of ions of mercury.
Kacmaz et al. [62] reported an optical chemical sensor device for Hg
2+ ion
detection using ethyl cellulose nanofibers. The nanofibers were fabricated with an
azomethine flouroionophore as an additive dye. In the presence of the heavy metal
ions the optical absorption spectrum of the fiber changes much more as compared to
thin films and it has been demonstrated to detect Hg
2+ ion concentration as low as
0.07 nM. Another novel approach by Parsaee [63] was to use bio-ultrasound
assisted fabrication of Au-NPs using algae and immobilized over PAN nanofibers.
The detection was based on colorimetry and fluorescence in presence of the dye
Rhodamine-B(RhB). The catalytic reduction of RhB by Au-amalgam causes a color
change which could be mapped to the amount of Hg
2+ present in the solution.
The LOD of this method was reported to be 1.10 nM of metal ion concentration.
Another interesting work by Sanchez-Calvo et al. [64], reports fabrication of paper
based electrochemical sensor for Hg
2+ detection. The electrode was modified by
AuNPs-carbon nanofibers and rGO-AuNPs which increased their selectivity and
sensitivity. While nanofibers provided larger surface area, the AuNPs with their
affinity towards Hg
2+ helped in more sensitive recordings. The sensor was able to
detect up to 30 nM with CNFs/AuNPs chemistry.
3.4 Detection of Cadmium
Cadmium is another heavy metal which has serious human health hazards and it
badly affects renal function and the respiratory tracts [65]. It is present not only in
industrial effluents, but also in the electronic wastes like mobile phones and their
batteries, computer circuit boards etc. [66]. Many researchers have reviewed the
detection of Cadmium using different techniques like SERS, colorimetry,
Fluorescence etc. [38, 39, 67].
Promphet et al. [68] have utilized graphene/polyaniline/polystyrene electrospun
nanoporous fibers modified electrode and a voltammetry study for simultaneous
determination of lead and cadmium. The developed electrochemical sensor reported
a detection limit of 3.30 µg/l and 4.43 µg L
−1 for Lead and Cadmium, respectively.
They also reported reusage of the electrode after simple washing up to ten times.
308
A. K. Srivastava et al.