Research Progress Of Electrode Modifiers For Electrochemical Heavy Metal Ion Detection

Heavy metal ions cause negative effects on the environment and living organisms.Heavy metal contamination has become an important global concern as they are non-biodegradable and accumulated along with the food chain.The excessive heavy metal ions in the human body can cause cancer and abnormal mutation.The World Health Organization（WHO）Guideline for Drinking-water Quality（GDWQ）recommended the limits should be less than 3μg L^-1 for cadmium（Ⅱ）,10μg L^-1 for lead（Ⅱ）,1μg L^-1 for mercury（Ⅱ）,3,000μg L^-1 for zinc（Ⅱ）,and 2,000μg L^-1 for copper（Ⅱ）,respectively.The development of sensors for detecting and monitoring heavy metal ions has attracted tremendous attention.Spectroscopic-based techniques for heavy metal ions detection suffer from an expensive and professional operation,while electrochemical methods provide portability with high sensitivity and low cost.Most of the research studies reported that electrochemical methods could improve detection efficiency by modifying the electrode.In this review,the research progress of electrode modifiers is emphasized.First,the current research of electrochemical sensors utilized to determine heavy metal ions in aqueous solutions and real samples is reviewed.Firstly,the main sources and toxicity of heavy metal ions,such as lead（Ⅱ）,cadmium（Ⅱ）,mercury（Ⅱ）,zinc（Ⅱ）and copper（Ⅱ）,are introduced,followed by the introduction of conventional spectroscopy technology of heavy metal ion detection,including UV-Visible Spectroscopy,X-ray Fluorescence Spectroscopy,Atomic Absorption Spectroscopy,Atomic Fluorescence Spectrometry,Inductive Coupled Plasma-Mass Spectrometry,and Inductively Coupled Plasma-Atomic Emission Spectroscopy.Secondly,the varieties of electrochemical techniques focusing on voltammetric analysis are summarized.Thirdly,the current working electrodes for voltammetric analysis are classified according to the types of materials,including gold,platinum and stainless-steel electrodes,carbon-based electrodes,screen-printed electrodes,and boron-doped diamond electrodes.The last part of this review focuses on the chemically modified electrodes（CMEs）with sensitivity and selective application.In brief,CMCs were divided into inorganic materials,organic materials,organic-inorganic hybrid materials,and microorganisms.Finally,the research and application trends of electrochemical for heavy metal detection are presented as a conclusion.The trend of working electrodes is the development of environmental friendly electrodes with high precision and fast response to achieve a low detection limit.Therefore,this review could be useful for developing new and nontoxic electrode materials for heavy metal ion detection in the future.