Advances in imaging-assisted sensing techniques for heavy metals in water: Trends, challenges, and opportunities

https://doi.org/10.1016/j.trac.2019.115758Get rights and content

Highlights

  • This review analyzes advances over colorimetric sensor materials for heavy metals.

  • Advances over colorimetric readouts (e.g., smartphones) are evaluated.

  • Deliberations are made on commercial colorimetric kits and mobile apps for sensing.

  • Performance evaluation are made for their translation into field-deployable systems.

  • Integration of microfluidic and smartphones has been realized with chromogenic probes.

Abstract

Heavy metal pollution of water resources is of worldwide concern, stimulating the development of breakthroughs in detection and remedy technologies. In the digital age, imaging solutions have been applied to almost every sector of civil society, from health and manufacturing to diagnostics, defense, and personal security. The next generation of monitoring sensors and systems for water quality and pollutants is developing at a rapid pace. The present review discusses various aspects of imaging-assisted quantification of heavy metals in water, including transduction techniques (colorimetric and photoluminescence), sensor materials (organic linkers, metal nanoparticles, carbonaceous & semiconductor quantum dots, etc.), the roles of equipment and readout tools (color charts, image scanners, digital cameras, mobile phones, etc.), substrates (organic and inorganic), and sampling methods. To the best of our knowledge, no such efforts have been made previously to critically and comprehensively analyze various aspects on the new imaging-based technologies in the environmental sector.

Section snippets

Introduction: heavy metals and metalloids

The contamination of natural water resources by heavy metals and metalloids is a controversial subject due to the hazardous effects on human health through the disturbance of nutritional, ecological, and environmental systems. The cause of this contamination is the relatively high density of metal ions, which exhibit toxicity on accumulation. Under the Priority List of Hazardous Substances, jointly prepared by the Agency for Toxic Substances and Disease Registry and the US Environmental

Colorimetric sensors

Colorimetric detection is one of the most frequently employed signal transduction techniques. Although colorimetric judgment was performed initially with the naked eye, while in these days various readout devices have been introduced to maximize efficiency. Despite significant advancements, the technique remains at a nascent stage with respect to sensitivity and selectivity relative to fluorescent techniques. Further, this approach is based on either change in intensity or shifts in the

Roles of materials in imaging-based quantification

Over the years, various materials have been studied for imaging-based quantification of heavy metal ions, especially in a water matrix. In this section, we assign the materials to four major categories based on molecular structure and properties: organic ligands, metal nanoparticles, carbon quantum dots, and semiconductor quantum dots. Their roles in sensing various heavy metals discussed and summarized in Fig. 1. These two principles of colorimetric sensing were addressed by manipulating

Readout tools utilized in imaging-based quantification of heavy metals: working principle, advantages, and problems

Since ancient times, light has been a source of fascination. As a form of energy, its interactions with the world have been of great benefit to humanity. Human eyes are natural spectators of light's behavior, including reflectance, emittance, and scattering of the visible part of the electromagnetic spectrum. The spectral information captured processed by the brain and interpreted in terms of color, intensity, and depth perception. Advancements in colorimetric sensing with the aid of ubiquitous

Microfluidics in imaging-based sensors

Paper-based analytical devices make use of wicking properties of paper without requiring external power sources or equipment. Microfluidics is a promising strategy for the development of affordable and field-deployable sensors. One such candidate is the “lab on a chip” μPAD for sensing and detection applications [72]. The principle of μPADs is to configure paper substrates into two regions, i.e., hydrophobic barriers and hydrophilic channels. The primary advantage of microfluidics is control of

Role of substrates in imaging-based sensor

The efficacy of an imaging-based colorimetric sensor is determined not only by choice of materials (e.g., chemo-responsive dyes, organic ligands, metal nanoparticles, and quantum dots) but also the selection of substrate type and materials (e.g., the geometry of flow path and immobilization technique) [75]. Many substrates for colorimetric sensors are available, based on their nature and structure. The essential characteristics of substrates include optical transparency (owing to high

Mobile applications and kits for general water parameters quantification

The research domain of smartphones has grown considerably over the years, owing to their minicomputer operating systems and miniaturized hardware (e.g., cameras and antennas for networking). This section offers detailed insights into mobile apps used for the visualization of colorimetric assays of environmental pollutants, especially heavy metals. Correlation of the color intensity (e.g., RGB and grayscale) to heavy metal concentration is utilized to quantify heavy metal contamination. The

Performance comparison and future outlook

The development of a sensor system for heavy metals in natural waters remains a challenge in terms of various criteria such as effectiveness, efficiency, sensitiveness, selectiveness, user-friendliness, affordability, and field applicability. It is essential for understanding the fundamental role of sensor materials, imaging tools, substrates, software platforms, and sampling methods to realize such categories of sensing systems. Colorimetric sensors are one of the most promising options in

Conclusions

Worldwide concerns regarding the pollution of heavy metals in natural water resources have stimulated technological breakthroughs in the monitoring and removal of such targets. In the current era of rapidly advancing computational intelligence, digital solution methods employed in almost every sector of civil society, including health, manufacturing, diagnostics, defense, and personal security. Monitoring and sensor systems that take advantage of these technologies have also been proposed and

Acknowledgments

PD and AT acknowledge the financial support received from Department of Science and Technology under SEED grant (No.: SP/YO/126/2017) and DST-INSPIRE fellowship, respectively. RJ acknowledges the CSIR-Senior Research Fellowship by the Council of Scientific & Industrial Research (CSIR), India. Support and encouragement by Director, CSIR-CSIO is also acknowledged. KHK acknowledges support made in part by grants from the National Research Foundation of Korea funded by the Ministry of Science, ICT

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