Nickel is a member of the transition

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