**5. Conclusions**

Importance of HMIs quantification is stated followed by efficient methods and their drawbacks. No doubt that electrochemical and optical methods are promising alternatives. Because of the advantages such as user friendly, economical and competency, electrochemical and optical methods are further considered for the discussion. Different electrochemical techniques followed for the HMIs quantification are described with the fundamentals and reasoning in brief. Among them stripping voltammetric techniques are found to be efficient. Many electrochemical methods are reported to be capable of sensing HMIs down to ppb level and few can detect down to pico molar concentration. Another advantage of the electrochemical methods is the simultaneous analysis of multiple HMIs. Electrochemical methods are yet to be adopted for the field [spot] analysis. On the other hand, optical methods in particular colorimetric methods are metamorphosed to naked eye sensing of HMIs which will help in field analysis. PADs are coined to further simplify and miniaturize both electrochemical and optical methods. Alternatives for the often-used expensive color inducing materials such as gold, platinum and silver are still need to be explored in case of PADs for optical sensing of HMIs. It can be concluded that PESs are highly sensitive and selective but cannot be carried to field. Whereas PADs used for optical sensing are sensitive and can be used for field analysis are semiquantitative.
