Arsenic (As) is a highly toxic heavy metal that poses a significant threat to the environment and human life. Although many arsenic detection methods are available, a real-time detection process of arsenic in plants or accurate estimation in agricultural soil is not available. Recently, a group of researchers from the Singapore-MIT Alliance for Research and Technology (SMART), MIT’s research enterprise in Singapore, has developed a novel plant nanobionic sensor to detect and monitor As in real-time.
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To develop this nanobionic As sensor, scientists used the unique properties of nanomaterials, along with the natural properties of plants, such as pre-concentration and hyperaccumulation of arsenic. The research was published in the journal Advanced Materials on November 26th, 2020.
Why is Arsenic Considered a Global Threat?
Globally, around 100 million people have been affected by As. For example, the negative consequences of contamination have been widely reported in many countries worldwide, like Bangladesh and West Bengal. Arsenic is most commonly found in an aqueous environment and soil in two forms, namely, arsenate [As (V)] and arsenite [As (III)]. Scientists have reported that arsenite is more harmful than arsenate or organic arsenic compounds.
According to the US Environmental Protection Agency (EPA) and World Health Organization (WHO), the permissible level of arsenic is 10 μg/L. However, it is challenging to detect As levels since they are often present in irrigation water, drinking water, vegetables, fish, meat, milk, and cereals.
Humans exposed to arsenic undergo several health hazards, such as cardiovascular diseases, skin lesions, arsenicosis, and cancer. Additionally, As can also promote mutations, hematological problems, hyperkeratosis, amongst many other health hazards.
A high concentration of As in the soil occurs as a result of anthropogenic activities, such as mining and smelting. Soil contaminated with As is extremely harmful to plants, found to inhibit plant growth, as well as causing substantial losses in crop yield. Food plants have also been shown to absorb As from the spoil, resulting in health problems when consumed by humans.
Detection of Arsenic in Plants Using Conventional Methods
Arsenite is predominantly found in paddy soil in anaerobic conditions, which plants can effectively absorb via various mechanisms. To detect arsenic in environmental samples, scientists use a range of analytical methods, including laser-induced breakdown spectroscopy (LIBS), atomic absorption and fluorescence spectroscopy (AAS, AFS), and high-performance liquid chromatography (HPLC).
Although techniques like these are accurate and highly sensitive, the main drawback is the amount of time these processes take due to the involvement of several steps like regular field sampling, plant tissue digestion, and extraction, to name a few. Conventional methods are not suitable for field testing as sophisticated instruments and skilled technicians are required, adding further challenges.
The development of As sensors that are easy to operate, accurate, and cost-effective is crucial to protect the agricultural industry and ensure a safe environment.
Plant Nanobionic Optical Sensor to Detect Arsenic in Real-time
Researchers from the Disruptive and Sustainable Technologies for Agricultural Precision (DiSTAP) at SMART, led by Tedrick Thomas Salim Lew and Professor Chua Nam Hai have engineered a novel plant nanobionic optical sensor capable of detecting and monitoring levels of arsenic in the underground environment in real-time.
A pair of single-walled carbon nanotube (SWNT)-based near-infrared (NIR) fluorescent nanosensors were developed to detect arsenite through changes in their emission intensity. In other words, the optical nanosensor displays changes in the fluorescence intensity upon arsenic detection.
The main advantages of using SWNT are its fluorescing properties in the NIR region, far from chlorophyll autofluorescence. The fact that it does not photobleach is another benefit.
This plant nanobionic As sensor is associated with the corona phase molecular recognition (CoPhMoRe) technique involving an adsorbed heteropolymer phase on the SWNT surface, called a corona, which offers a synthetic molecular recognition site that can interact with the target molecules.
The As nanosensor was easily implanted within the leaf mesophyll plant tissues without any adverse effects on plant development, indicating that these nano-based sensors could offer a non-destructive method to monitor the internal dynamics of As uptake by plants root system from the soil.
In this method, living plants by themselves act as As detectors. The levels of As detected by the sensor are analyzed using portable and low-cost electronics like a portable Raspberry Pi platform fitted with a charge-coupled device camera, similar to that of a smartphone camera. Owing to these advancements, the newly developed nanobionic optical sensor is understood to be more advantageous than conventional techniques for As monitoring and detection.
Applications and Future of the Plant Nanobionic Arsenic Sensor
Scientists at SMART have claimed that the novel plant nanobionic As sensor has successfully detected As in rice and spinach plants. The research team also extended its application to a fern species, Pteris cretica, known to heavily accumulate arsenic. It has also been stated that this novel nanosensor can be applied to other plant species.
As this is the first successful living plant-based sensor for As detection, researchers are optimistic that this technology would be vital for agricultural research to promote food safety and environmental monitoring of this toxic metal.
References and Further Readings
Lew, T., Park, M., Cui, J. and Strano, M., (2021) Plant Nanobionic Sensors for Arsenic Detection. Advanced Materials, 33(1), p.2005683. Available at: https://doi.org/10.1002/adma.202005683
MIT News. (2020) SMART researchers engineer a plant-based sensor to monitor arsenic levels in soil. [Online] Available at: https://news.mit.edu/2020/plant-nanobionic-sensor-monitor-arsenic-levels-soil-1213\
Hooda, V., Verma, N., Gahlaut, A. and Gothwal, A., (2021) Reusable Enzymatic Strip for Detection of Arsenic. Brazilian Archives of Biology and Technology, 64. Available at: https://doi.org/10.1590/1678-4324-2021200132