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Growing concerns regarding the health and environmental effects of nanomaterial production have led many leading researchers within these fields to transition their conventional synthesis methods to more environmentally friendly production methods. Similar eco-friendly solutions have been proposed for the development of green electrochemical sensing platforms.
Nanomaterials and Electrochemical Sensors
Electrochemical sensors are powerful and highly sensitive analytical tools that are associated with several favorable characteristics, including simple design, rapid response rates, simple preparation time, self-contained portability, and overall inexpensive cost requirements.
When designed and developed for biomedical and biological applications, advanced nanomaterials such as graphene, conducting polymers (CPs) and various metal nanoparticles have enhanced sensing capacity of these devices, while simultaneously maintaining low costs and easy fabrication processes.
Growing Health Concerns
Despite the desirable advantages associated with these novel nanomaterials, recent advancements within the field of nanotoxicology have shown that exposure to these nanomaterials can lead to adverse health and environmental effects.
In addition to the nanoparticles themselves, the preparation methods required to manufacture these materials often involve the use of several hazardous chemicals, some of which can include sulfuric acids, hydrofluoric acid, hydrochloric acids, and organic solvents.
Organic solvents, for example, are highly volatile and toxic chemicals that are considered to be a severe problem for human and environmental health.
Regardless of what industry nanoparticles are being produced for, these concerns continue to exist directly and indirectly.
Green Substrate Materials
Several different substrates have been proposed as solutions to overcome the health concerns surrounding electrochemical sensor production. Some of the natural and non-hazardous materials that have shown promising results include paper, clay, zeolites and biowaste-based sensors.
Paper-based sensors
As a substrate, paper has several advantages, including:
- Easy foldability
- Flexibility
- Lightweight
- Biocompatible
- Hydrophilic
- Low-cost
- Highly modifiable surface area
To date, various electrochemical paper-based analytical devices (ePADs) have been developed and demonstrated their unique potential in future biomedical applications.
During the drug development process, for example, ePADs are widely accepted tools that are capable of performing rapid, selective and sensitive analyses of both active and inactive pharmaceutical ingredients.
More recently, a disposable ePAD for the detection of ascorbic acid, which is an antioxidant that increases both physical and chemical stability of pharmaceutical formulations, has been introduced.
The technology behind this sensor is attributed to a dispersion based on carbon black nanoparticles that improve its electrochemical sensing capabilities. Moreover, the carbon black nanoparticles allow for the over-potential for ascorbic acid oxidation to be reduced, improving the overall sensitivity of the sensor to detect this antioxidant.
Overall, this specific ePAD is reliable for controlling the quantity and quality of ascorbic acid present in tested products while simultaneously only requiring a single drop to be used during the analysis process.
Clay-Based Sensors
The chemical weathering process of hydrous aluminosilicates results in the production of various forms of clay minerals.
Clay minerals are associated with several advantages that can enhance the performance of electrochemical sensors, some of which include excellent thermal and mechanical stabilities, elevated surface area, biocompatibility and low cost.
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One of the most studied clay minerals that has been used for this purpose includes halloysite nanotubes (HNTs), which are a type of kaolin material that possesses dioctahedral 1:1 clay structure and nanotube shape.
Some of the unique properties associated with HNTs include:
- A hollow nanotubular structure
- Superior hydrophilicity and thermal stability capabilities
- Tunable surface chemistry that is advantageous for electrochemical sensing applications
Green Solvents
In addition to using alternative substrate materials in the move towards green electrochemical sensors, several studies have investigated how the replacement of traditional hazardous solvents with green solvents during the production process can be achieved.
Compared with organic solvents, which are highly volatile and toxic, ionic liquids (ILs) are negligibly volatile and biocompatible. ILs exhibit excellent chemical and thermal stability characteristics, inherent catalytic properties, and high ionic conductivity, all of which are crucial during the production of electrochemical sensing materials.
Another type of green solvent that has been introduced into the production of electrochemical sensors includes deep eutectic solvents (DESs).
The synthesis of DESs is achieved by mixing hydrogen bond acceptors (HBAs), particularly quaternary ammonium salts, with hydrogen bond donors (HBDs), such as alcohols, amines or carbohydrates.
This reaction results in the production of eco-friendly solvents that are non-toxic, biodegradable and inexpensive to produce.
DESs support has already been successfully incorporated into the production of various electrochemical sensing applications, some of which include the electrodeposition of alloys, metals, semiconductors and polymers.
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DESs have also been used during the synthesis of various nanomaterials, including shape-controlled nanoparticles, DNA/RNA architectures, electrodeposited thin films and hierarchically porous carbon.
Conclusion
Recent advancements within the world of material sciences have allowed the synthesis strategies responsible for the production of electrochemical sensors to realistically move towards a green and economical future.
In addition to replacing the actual components of the sensor, such endeavors have also found that green and eco-friendly solvents have a similar or comparable efficiency of traditional organic solvents.
This progress supports the requirements needed in the transition towards green electrochemical sensors.
References and Further Reading
Maduraiveeran, G., Sasidharan, M., & Ganesan, V. (2018). Electrochemical sensor and biosensor platforms based on advanced nanomaterials for biological and biomedical applications. Biosensors and Bioelectronics 103; 113-129. doi:10.1016/j.bios.2017.12.031
Kalambate, P. K., Rao, Z., Dhanjai, Wu, J., Shen, Y., et al. (2020). Electrochemical (bio) sensors go green. Biosensors and Bioelectronics 163. doi:10.1016/j.bios.2020.112270
Antonacci, A., Scognamiglio, V., Mazzaracchio, V., Caratelli, V., et al. (2020). Paper-Based Electrochemical Devices for the Pharmaceutical Field: State of the Art and Perspectives. Frontiers in Bioengineering and Biotechnology. doi:10.3389/fbioe.2020.00339
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