# Heavy Metal Ion Detection Platforms Based on a Glutathione Probe: A Mini Review
## 4 Concluding Remarks  
Heavy metal contamination is one of the major concerns in an ever-increasingly industrialized world. Broad applications of heavy metals in modern industries, agriculture, and medicines have led to their widespread distribution in the environment. The toxic effects of heavy metals have proven to be the source of many health risks. Some of them, such as lead and mercury, are considered to be highly hazardous to humans even at a trace level, and are ranked as priority metals that are of public health significance. Therefore, it is crucial to develop effective detection techniques that are user-friendly, affordable, fast, and sensitive for point-of-need detection and monitoring.  
Among various approaches to detect HMIs, GSH is gradually becoming a popular probe due to its strong affinity with several HMIs and its flexibility with the sensing methods and materials. With multiple functional groups of (-SH), (-COOH), and (-NH2) on it, GSH is a versatile probe capable of binding with several metal ions, either individually or together at the same time. By adjusting the testing conditions, such as the solution's pH value, GSH exhibited a tunable complexation ability to different HMIs, with maximum affinity for Pb\({}^{2+}\) at a pH of around 7. While most GSH-based HMI detection platforms were designed to specifically detect Pb\({}^{2+}\), targets other than Pb\({}^{2+}\) have also been reported using GSH probes under different conditions.  
When using GSH as a probe, the benefit is more than rich functional groups on its surface for HMI detection. A variety of sensing strategies can be realized, owing to GSH's compatibility with diverse sensor materials and signal transduction methods. This comprehensive review emphasizes the advances made in the development of novel optical, electrochemical, and other GSH-based sensors for the detection of toxic heavy metal ions. Based on their signal transduction strategies, the reported HMI sensors can be broadly classified into optical methods and electrical methods. Optical detection is typically used in coordination with noble metal NPs and semiconductor QDs, while electrochemical methods often use FETs and working electrodes as the transducers. Each has its strengths and weaknesses. The optical methods use a new batch of NPs or QDs for each test, avoiding any need to modify the detectors themselves, while electrochemical methods require a newly modified device for each test. Based on the review, electrochemical methods are quicker to obtain results than optical methods with comparable LODs, and optical methods are less susceptible to interferences than electrochemical methods.  
To make these systems suitable for real-world applications, such as environmental monitoring and biological testing, much effort is needed to overcome the challenges that these sensors currently face, such as assay robustness, qualitative and quantitative analysis, control of sample temperature and pH, user-friendliness, and cost-effectiveness. Further, for field-deployable applications, miniaturization of these sensors and detectors, and simplicity and automation in the measurement procedure, are additional challenges.  
Conceptualization, [PERSON] and J.W.; Methodology, [PERSON] and J.Z.; Software, [PERSON]; Validation, J.W.; Formal Analysis, [PERSON]; Investigation, [PERSON]; Resources, [PERSON] and J.Z.; Data Curation, X.S.; Writing-Original Draft Preparation, X.S.; Writing-Review & Editing, [PERSON] and J.W.; Visualization, [PERSON]; Supervision, [PERSON]; Project Administration, [PERSON]; Funding Acquisition, [PERSON] and J.Z.  
This research was funded by the University of Tennessee Initiative for PON/POC Nanobiosensing, Institute for a Secure and Sustainable Environment, the US NSF CPS/USDA NIFA (Grant no. 2017-67007-26150), the open research fund of the Key Laboratory of MEMS of the Ministry of Education, Southeast University, China, and the National Natural Science Foundation of China (Grant no. 51728502).  
The authors declare no conflict of interest.