Detection and Diagnosis: Heavy Toxic Metals

Prof. Youzhi Li, from Hunan Agricultural University, Changsha, China and his research team have recently published, “Total concentrations and sources of heavy metal pollution in global river and lake water bodies from 1972 to 2017”. They discussed on heavy metals like lead (Pb) cause a severe threat to living beings because of its toxicity and bioaccumulation. Heavy metals like lead (Pb) cause a severe threat to living beings because of its toxicity and bioaccumulation. Amongst the heavy metals, lead (Pb) is the well-known one for its historical use in paints, petrol, and fuse joints in water pipes. Though the tradition of using Pb has been considerably decreased in recent years, Pb in drinking water still remains to be a problem for many societies around the world [1]. Hence, it is one of the supreme importance to monitor the levels of Pb in drinking water, so that contamination can be promptly identified and moderated. The World Health Organization (WHO) mentions a guideline limit of 10μgL⁻¹ of total Pb content in drinking water [2].

Figure 1. Effects of heavy metal toxicity (Ref: CDC).

For detection of trace of Pb concentrations, the conventional testing approaches that include atomic adsorption spectroscopy (AAS), spectrophotometry, and inductively coupled plasma mass spectroscopy (ICP-MS) etc. However, these methods require costly laboratory-based equipment and trained personnel to operate them. Among field-based techniques, the most promising candidates are anodic stripping voltammetry and ion-selective electrodes. ISEs (ion-selective electrodes) are particularly stimulating as they do not involve the use of the toxic dropping mercury electrode or the pretreatment of sample with bismuth which is required in anodic stripping voltammetry (ASV).Historically, ISEs remained afflicted by poor mechanical durability and comparatively high lower detection limits (LDL) their use in on-site detection techniques. The mechanical durability of the ISE was elevated with the development of the solid-contact ion selective electrode (SCISE), in which the conventional inner substantial solution is replaced with a conducting polymer. In addition, development in LDL of ISE was realized by using low activity internal solutions, formation of preconditioning protocols and transmembrane ionic flux control. Because of the solid-contact, controlling the activity of inner substantial solutions is not appropriate for SCISEs. Also, the LDL of SCISEs can be dropped by carefully regulatory preconditioning actions.

Usually, ion selective membranes are stimulated over conditioning process in which the membrane is saturated with the analytic upon exposure to a solution with high analytic concentration resulting in uptake of excess of analytic ions and also extraction of counter ions. This results are in relatively high LDLs. This restriction is overcome by using a second conditioning step in a low analytic concentration solution which eliminates the excess analytic and the extracted ions in the membrane. Even using the two-step conditioning process, grants some deficiencies at lower analytic concentrations. When the sample analytic concentration is higher than that in the solid-contact layer, analytic ions flux toward the solid-contact layer because of the concentration gradient. This in turn causes the reduction of analytic near the membrane surface results in the so-called “super-Nernstian” reaction.

The ISE develops nonresponsive to further changes in analytic concentration below the super-Nernstian level. To overcome these glitches, researchers concentrated on controlling the transmembrane ionic flux to advance the LDLs. Outdated methods for transmembrane ionic flux control is realized through the claim of an external current contrasting the direction of the ionic flux over the ion selective membrane. Here, the concentration of analytic in the sample is not exhausted and a stable understanding at very low concentrations is accomplished. Though using this technique has caused in the development of LDL by over 2 magnitude of extent, it is not a practical technique for on-site detection as the magnitude of current to be applied is often unknown. Furthermore, application of the wrong currents can cause ion flux problem, that is, ion flux from the internal filling solution to the sample, leading to model impurity and membrane leakage. A variant of ion selective electrodes is the ion selective field effect transistor (ISFET), the metal gate electrode of a field effect transistor is substituted with an ion selective membrane. ISFETs offer an in-built signal amplification feature and are perfect for on-site sensing. Though, ISFETs suffers from the same problems that affects the conventional ISE, such as high LDL, that further delay their use for monitoring of heavy metals.

Shishir et al. have developed the methods for on-site detection of heavy metals [3], recently, the uses of trans membrane. A dual sensing gated field effect transistor for the detection of Pb²⁺is a low cost and flexible method. The performance shows LDL enhancement over two orders compared to outdated one sensing gate ISFET method. The realization of LDL to 1 μgL⁻¹, well below WHO guideline limit concentrates the efficiency sensor applicable for on-site Pb²⁺detection [4]. They reveal a Pb detection limit of 1 μgL⁻¹ by means of the dual-gated structure. Their unique Pb sensor design is demonstrated by mathematically solving the Planck-Nernst-Poisson equations. Their study, being built on robust non-apt sensor FET technology, we consider that this sensor fulfills the real time requirements low detection limit, high sensitivity and nonfragile, facilitating their use as an on-site contamination prevention and more efficient when compared to the conventional laboratory-based analytical techniques.

Our SNB team recommended this research article to enrich our viewer’s knowledge about on heavy metals contamination in developing countries causes major health issues in younger generation. Detection of heavy metal in ground water increases day by day owing to the improper waste water treatment, land fill leakage and other human activities. Easy detection and diagnosis techniques are required for relevant action and prevention of ill effects. The above detection technique provides easy and instant result on the presence of heavy metals in the environment.

References

B.O. Anyanwu, et al., Toxics, 6 (2018).
Q. Zhou, et al., Glob. Ecol. Conserv. 22, 925 (2020).
S. Venkatesh, et al., Anal. Chem.90, 7399 (2018).
World Health Organization. Guidelines for Drinking-Water Quality, Vol. 1; World Health Organization, 2004.

Blog Written By

Dr. K. Vaithinathan

Department of Materials science and engineering

City University of Hong Kong, Hong Kong

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Dr. A. S. Ganeshraja

Dr. K. Rajkumar

Dr. S. Chandrasekar

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