Smartphone‐Based Mercury Detection System - 180D-FW-2024/Knowledge-Base-Wiki GitHub Wiki

Introduction

Mercury contamination in water poses severe health risks, including kidney damage and neurological disorders due to its toxic nature and bioaccumulation in the food chain.

Traditional detection methods, such as Atomic Absorption Spectroscopy (AAS), Inductively Coupled Plasma Mass Spectrometry (ICP-MS), and Atomic Fluorescence Spectroscopy (AFS), are accurate but have significant limitations. They require complex sample preparation, expensive equipment, and trained personnel, making them inaccessible for rapid, on-site detection, particularly in developing regions.

Furthermore, mercury's impact on the environment is profound, affecting aquatic ecosystems globally. The contamination is especially problematic in regions with high industrial activity and artisanal small- scale gold mining, where mercury is often released into water bodies. Therefore, there is a critical need for a detection system that is both effective and accessible.

Basic Overview of Mercury Contamination and its Effects

How Does Mercury Reach Water?

While it is potentially toxic and dangerous, mercury is actually a naturally occurring substance; volcano eruptions and other natural processes routinely emit stored mercury into Earth's atmosphere, where it may then be absorbed into bodies of water. Mercury can later leave these bodies of water through evaporation, which creates a cycle where mercury alternates between land, air, and water. For millions of years, this "mercury cycle" has prevented mercury from accumulating to dangerous levels in Earth's waters.

Why is Mercury Contamination an Issue Now?

Rapid industrialization in the past few centuries has thrown the previously mentioned mercury cycle off balance; apart from natural processes, mercury may also be emitted into the atmosphere through industrial processes like coal burning, waste incineration, and mining. The scale of industrial mercury emissions is such that a 2023 Harvard study found that total atmospheric mercury has increased sevenfold since the 16th century. This massive increase in atmospheric mercury has rapidly increased mercury levels in water, which in turn has significantly increased the risk of human mercury poisoning when eating fish (more on that in the next section) or drinking water in certain areas. Should industrial pollution continue as it has, more and more mercury will be released into the atmosphere and mercury levels in Earth's waters will continue to rise, affecting not only humans but also marine life.

What Happens to Mercury in the Ocean, and How Does it Affect Fish?

Of the significant amounts of mercury in Earth's oceans, a large chunk is converted by bacteria into methylmercury through a process called methylation. Methylmercury makes up 95% of the mercury found in fish, and is extremely toxic to humans if not consumed in moderation. However, although mercury levels in the ocean are rising, small, non-predatory fish are still and will likely remain safe to eat without fear of mercury poisoning. They accumulate methylmercury both by absorbing it from seawater that passes through their gills and by eating plankton that have themselves accumulated mercury, but the volume of mercury they end up storing in their tissues is not nearly enough to cause humans harm. Eating too much mercury through fish is only truly a concern when considering predator species, with species near the top of the food chain having the most mercury. This is because any mercury stored within the tissue of a consumed fish gets transferred to the fish that consumed it, creating a pyramid where top predators store large amounts of mercury within their tissue.

As a result of this "mercury pyramid," even small increases to mercury levels in Earth's oceans can have a profound effect both on marine life and what fish are safe to eat, especially with regards to those fish near the top of the food chain. Mercury exposure has been found to weaken fish livers, hurt reproduction, alter behavior, or even kill fish—all of these results could have disastrous consequences on the ocean environment, and act as a motivator for more thorough testing of mercury levels in water as well as decreased mercury pollution from industrial plants.

Contextual Challenges and Emergence of Smartphone-Based Sensing

Detecting mercury often relies on advanced laboratory methods, which are difficult to use in areas with limited resources, although they are highly accurate; they require complex sample preparation, costly instrumentation, and skilled people. These problems get worse in remote places that lack consistent power supply, specialized equipment, and the financial resources to support traditional lab setups. Recognizing these challenges, researchers have increasingly tended to use smartphones for detecting contaminants. Smartphones offer built-in cameras, computing power, and internet access, which makes it easier to analyze samples and share data. By linking colorimetric assays (color-based tests) with smartphone cameras, users can rapidly detect mercury levels on-site with only basic training. This portable and low-cost approach could lower the gap between precise lab methods and practical field testing, and makes it much faster and wider monitoring of mercury pollution.

New Mercury Detection System

To address these challenges, a novel, cost-effective smartphone-based mercury detection system utilizing a plasmonic colorimetric assay has been developed.

This method detects mercury in water samples by utilizing gold nanoparticles (Au NPs) that change color in the presence of mercury (II) ions. The system's mobile application calculates and displays the mercury concentration based on the captured images, providing immediate results suitable for on-site detection.

The design of the opto-mechanical attachment used with the smartphone, as shown in Figure 1, includes two LEDs (523 nm green and 625 nm red) powered by button cells, illuminating the sample and control solutions placed in cuvettes. The transmitted light through these cuvettes is captured by the smartphone camera through an external lens, which ensures uniform illumination and accurate imaging. This attachment is lightweight and integrates seamlessly with the smartphone, enabling portable and convenient mercury detection.

The system employs a dual-color dual-cuvette colorimetric detection method, utilizing two light sources: a green LED at 523 nm, which serves as a stable reference, and a red LED at 625 nm, significantly affected by the aggregation of Au NPs. The green LED illuminates the bottom half while the red LED illuminates the top half of each cuvette, preventing crosstalk between the lights.

This dual-light-source approach enhances the accuracy and sensitivity of the detection by minimizing errors and effectively distinguishing between the sample and control signals. This setup enables a two-color ratiometric method, where the green and red transmission signals are compared to quantify mercury concentration.

Mechanism of Mercury Detection

The schematic, in Figure 2a, shows how mercury (II) ions (Hg2+) cause the aggregation of gold nanoparticles (Au NPs) in the presence of NaCl, leading to a color change used for detection.

Initially, aptamers bind to mercury(II) ions (Hg²⁺) present in the water sample, leaving the Au NPs exposed. In the presence of sodium chloride (NaCl), the exposed Au NPs aggregate due to the ionic interactions. This aggregation process induces a visible color change from red to blue or purple. The color changes are used to quantify the mercury concentration in parts per billion (ppb).

The captured image on a smartphone under dual-wavelength illumination is shown in Figure 2b, where the control (Ctrl) and sample cuvettes are illuminated by red (625 nm) and green (523 nm) LEDs. The distance between the illumination spots ensures accurate measurement.

The image processing steps are detailed in Figure 2c, where the application separates the red and green channels to compute the green-to-red (G/R) ratio, normalizing the sample signal (G/R_S) against the control signal (G/R_C) to quantify mercury concentration. This dual-color approach enhances the accuracy and sensitivity of mercury detection.

Application Interface and Functionality

The smartphone application offers a user-friendly interface for conducting mercury detection tests and viewing results. Users can start a new test, calibrate the device, view contamination maps, and access instructions from the main menu.

The application guides users through capturing a transmission image of the sample and control cuvettes using the attached optical reader. It processes the image to compute green-to-red signal ratios, converting these ratios into mercury concentration levels using a pre-stored calibration curve.

Results are displayed on the screen and stored with time stamps and GPS coordinates for detailed tracking. Users can upload results to a secure server for spatiotemporal mapping, showing variations in mercury levels across different locations and times. The application's main menu, calibration, image processing, result display, and mapping features are illustrated in Figure 3.

Specificity Tests

Specificity tests evaluated the system's ability to detect mercury (II) ions amidst other metal ions like Fe3+, Ca2+, Cu2+, and Pb2+. Results indicated that the assay predominantly responds to mercury (II) ions with minimal interference from other metals, demonstrating high specificity for mercury detection in Figure 5.

Mapping of Mercury Concentration in Water Samples in California

To validate the effectiveness of our smartphone-based mercury detection system, they conducted field tests at over 50 different locations in California, including city tap water sources, rivers, lakes, and coastal areas. Water samples were collected in sterile containers and tested on-site using the system. The same protocol for sample preparation and detection, as detailed earlier, was followed to ensure consistency and accuracy across all tests.

Data from each test, including the mercury concentration levels, GPS coordinates, and time stamps, were recorded using the smartphone application. The collected data was then uploaded to a secure server for spatiotemporal analysis. A total of 150 samples were tested, with mercury concentrations ranging from undetectable levels to above the WHO guideline value of 6 ppb.

The results of the field testing are presented in Figure 6. The spatiotemporal map shows the distribution of mercury concentrations across different locations. Higher mercury levels were observed in coastal areas, particularly in the San Francisco Bay, likely due to industrial discharge and other anthropogenic activities. In contrast, city tap water samples generally showed undetectable levels of mercury, indicating effective municipal water treatment processes.

The successful application of the novel detection system in diverse field settings highlights its potential as a valuable tool for environmental monitoring. Its portability, cost-effectiveness, and ease of use make it particularly suitable for resource-limited settings, enabling widespread and continuous monitoring of mercury contamination. Future developments, such as incorporating additional sensors and improving data analytics, could further enhance its capabilities and impact.

Conclusion

In summary, this study introduced a cost-effective, smartphone-based mercury detection system using a plasmonic colorimetric assay. This innovative system integrates a lightweight opto-mechanical attachment and a custom- developed Android application to enable rapid digital quantification of mercury concentration in water samples. The main advantages of the system include its portability, ease of use, and rapid on-site detection, making it particularly suitable for resource-limited settings. However, it requires initial calibration and is influenced by ambient light conditions. Future research should focus on enhancing robustness against varying environmental conditions, expanding detection capabilities to other contaminants, and improving data analytics for comprehensive environmental monitoring. Collaboration with public health organizations could help implement this technology more broadly, facilitating widespread and continuous monitoring of mercury contamination.