Nickel worldwide as they contain too much

Nickel (Ni) is a metallic element which is a member of the transition
metal class and is widely applied in industrial settings. Due to its unique
physical and chemical properties, it is noted as
one of the most important transition metal ions in biological systems 52, exhibiting both vital and toxic effects. Its toxicology has been well documented 18, 20 and Ni2+ compounds have been
classified as group one human carcinogens 89.
 Moreover, it has been linked to serious
health problems including respiratory system cancer 16,
skin allergies,  dermatitis or nickel-eczema 50. Ni has been widely used in industrial
applications as pipes and fittings, electroplating, stainless steel and alloys 16. Considering its ever-growing consumption,
environmental pollution by nickel-containing
products remains prevalent. In particular, human exposure to nickel occurs
primarily through digestion and inhalation 20. The maximum allowable level for Ni in drinking water
is set at 0.1 mg L-1 60 by the World Health Organisation (WHO).

Cobalt is a
major constituent of vitamin, is important for humans and other living
organisms. Cobalt can be found in trace levels both in water and other
biological samples. A radioactive isotope of cobalt is an important use in the
treatment of cancer and other related diseases94. However, at high concentration cobalt have been
linked to variety of illnesses such as polycythemia, skin allergies, pulmonary
disorders, and other adverse reactions94. Blue silica gel has been ban in other countries
worldwide as they contain too much cobalt which leads to such illnesses. The
environment which is contaminated by cobalt become health hazard not only for
humans but for all the animals involved in the food chain, as such system to
monitor cobalt level in water is of necessity.

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Zinc like
nickel and cobalt is an essential metal for normal physiological processes,
however consuming an excess of zinc can be a threat to human health


1.1.1.  Stripping voltammetry techniques

Contemporary analytical
techniques such as flame 19, 31, electrothermal 7 and graphite furnace 97 atomic absorption spectrometry (AAS), neutron
activation analysis (NAA) 83, 85, x-ray fluorescence spectrometry, (XRF) 91, inductively coupled plasma optical emission
spectrometry (ICP-OES) 12, 95, and electrochemical techniques have been used for
heavy metals determination. These spectroscopic methods, however, are limited
by availability, ex-situ measurements and, expensive and complicated
instrumentation. In contrast, electrochemical approaches have recently gained
traction due to their high speed, good selectivity,
high sensitivity, and low instrumentation cost 75. Among these, stripping analysis remains the most

Anodic stripping voltammetry has widely been
recognized as one of the most effective tools for analysis of metals, with many
established works having been performed. Heavy metals, rare earth metals, and platinum group metals, among others, have been investigated to date. While
less popular, adsorptive cathodic stripping voltammetry (AdCSV) has been shown
to be an extremely effective electroanalytical technique for the trace level
determination of metal ions that have an interfacial adsorptive character onto
the working electrode surface 4. The technique relies on an accumulation step in
which the electrode surface is pre-concentrated with the analyte by simple non-electrolytic adsorption.
Ni2+ and Co2+, however,
remain the most widely applied application 3, 8, 14, 33, 35,
41, 47, 49, 69, 73, 78, 79, 82.

The development and application of carbon-based
electrodes have received considerable attention in recent years for the analysis of the redox activity of inorganic and
organic substances alike 46. Graphite 57, carbon-paste 10, 44, 84, edge-plane pyrolytic graphite 56, pencil graphite 21 and more recently hand-drawn pencil graphite
electrodes 22 have all been investigated. Among these, pencil
graphite electrodes (PGE) have received particular interest in stripping
analysis, owing to their many advantageous properties. Their good electrical conductivity,
lack of pre-treatment, low cost, availability and low background current 42 are particularly valuable and make it an attractive
alternative to the well-known
glassy-carbon and gold electrodes (GCE and GE) 75. Recently, a number of studies on the use of PGEs for the analytical determination of
different heavy metal ions (Zn2+, Cd2+
and Pb2+) 21, 34, 74, 75, antioxidants 42, nicotine 55, polycyclic aromatic hydrocarbons (PAHs) 43 and nucleic acids 28, 65, among others have been reported.



While less toxic working electrode
materials, such as antimony (Sb), bismuth (Bi), lead (Pb) and silver (Ag)
amalgam films have been suggested, electroanalysis of heavy metals at mercury-film
and hanging mercury drop electrodes remain extremely popular due to their
ability for amalgam formation in the pre-concentration step. Additionally, the
high overpotential of hydrogen evolution 5 and the possibility of a constantly renewable surface
are attractive features for its use. Hereby, the possibility of electrode
poisoning by pre-deposited matter may be
eliminated. To date, Hg-film electrodes coupled with stripping
techniques have been recognized as the most sensitive method for the determination
of heavy metals such as nickel. For AdSV detection of metal ions, separate
accumulation and deposition potentials have usually been employed.

1.1.2.  Graphene

Graphene is a single atomic layer of carbon atoms tightly
packed in a two-dimensional honeycomb
lattice which makes electricity flow very
quickly. This novel material is atomically thin, chemically inert, consists of
light atoms, and possesses a highly ordered structure
53. Graphene is the strongest ever material measured and has a
remarkable electrically, mechanical and thermally conductivity. These
remarkable properties make graphene the ideal support film for graphite pencil
electrodes. Since it was first produced in 2004, graphene has attracted attention
and has shown to significantly improve the sensitivity in various applications
due to rapid electron transfer 72 and high surface-to-volume ratio. Graphene has been
used in a wide range of electrochemical sensing devices to date. To the best of
our knowledge, only two studies have been conducted on graphene-modified
sensors for application in the AdSV detection of metal ions. Zhan et al. first reported a ?-cyclodextrin and chemically reduced graphene oxide (?-CD-rGO)
nanocomposite in order to investigate its electrochemical response towards Pb2+
detection 92. The study demonstrated the excellent adsorption
ability of ?-CD and
extraordinary electron conductivity of rGO
for electrochemical analysis of metal ions by both synergistic and electrolytic
electrode pre-concentration. Our research group further exhibited a Nafion graphene, dimethylglyoxime modified
glassy carbon electrode (NGr-DMG-GCE) for
Ni2+ detection by AdCSV. The work performed by Pokpas et al. was the first reported signal amplification approach of graphene for Ni2+
detection. It further showed a novel method for improved selectivity towards Ni2+
in the presence of Co2+ and Zn2+ 73. To date, no work has been investigated for graphene

modified sensors by AdSV in the
presence of metallic-films or alternative electrode materials such as
disposable pencil graphite.


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