Turning Banana Peels Biowaste into High-Performance Sensors: A Sustainable Approach to Detecting Toxic Chromium(VI)

Chromium contamination is a growing environmental and health concern, primarily due to the stark difference in toxicity between its two common oxidation states: trivalent chromium (Cr(III)) and hexavalent chromium (Cr(VI)). While Cr(III) is relatively harmless and even essential in trace amounts for human metabolism, Cr(VI) is highly toxic, carcinogenic, and water-soluble, making it a persistent pollutant in soil and water systems. Industries such as electroplating, leather tanning, and pigment production contribute significantly to Cr(VI) contamination, necessitating accurate and sensitive detection methods to prevent ecological and health hazards.

Traditional detection techniques like atomic absorption spectrometry (AAS) and inductively coupled plasma mass spectrometry (ICP-MS) offer precision but are expensive, complex, and inaccessible for routine monitoring. This creates an urgent need for cost-effective, portable, and user-friendly sensors capable of detecting Cr(VI) at trace levels. A recent study published by NanoBio Research Lab, IIT Kharagpur, in Wiley's ChemistrySelect introduces Ultrasensitive and Selective Detection of Chromium(VI) Using Biowaste Derived Fluorescent Nanoprobes of Hydrothermally Synthesized Carbon-Dots

The Innovation: Carbon Dots from Banana Peel Biowaste

The study introduces an innovative solution: fluorescent carbon dots (CDs) synthesized from banana peel biowaste via hydrothermal carbonization. This approach not only addresses the challenge of Cr(VI) detection but also aligns with circular economy principles by repurposing agricultural waste into high-value nanomaterials.

The hydrothermal process involves drying, grinding, and treating banana peels at 130°C for four hours, followed by purification steps like dialysis and lyophilization. The result is carbon dots with an average size of 1.9 nm, a quantum yield of 5%, and strong fluorescence emission at 440 nm. These properties make CDs ideal candidates for developing sensitive and selective nanosensors.

Physicochemical and Optical Properties

The CDs exhibit a rich surface chemistry, including ─OH, ─COOH, and ─NH₂ functional groups, confirmed through FTIR analysis. These functionalities enhance aqueous stability and facilitate interactions with Cr(VI) ions. High-resolution TEM revealed a uniform spherical morphology with a narrow size distribution, while XRD patterns indicated an amorphous carbon structure with graphitic characteristics.

Optical characterization showed a broad UV–vis absorption band (250–350 nm) and excitation-dependent fluorescence behavior, typical of carbon dots. Despite a moderate quantum yield compared to other biowaste-derived CDs, the synthesized dots demonstrated exceptional performance in Cr(VI) detection.

Ultrasensitive and Selective Detection

The CDs function as fluorescent nanoprobes, exhibiting significant fluorescence quenching upon interaction with Cr(VI). This quenching effect remains highly selective even in the presence of other metal ions like Fe, Hg, and Pb, underscoring the sensor’s anti-interference capability. The detection limit achieved was an impressive 52 nM (2.7 ppb), far below the World Health Organization’s permissible limit of 900 nM for drinking water.

The Stern–Volmer analysis confirmed a strong linear correlation between Cr(VI) concentration and fluorescence quenching in the 2.5–30 μM range, ensuring reliable quantification. Such sensitivity enables early detection of Cr(VI) in environmental samples, industrial effluents, and even forensic applications.

Mechanism of Detection

X-ray photoelectron spectroscopy (XPS) provided insights into the sensing mechanism. The interaction between CDs and Cr(VI) involves chemisorption, facilitated by nitrogen-containing surface groups. Interestingly, Cr(VI) undergoes partial reduction to Cr(III) upon binding, driven by electron transfer from ─NH moieties on the CDs. This dual role of adsorption and reduction enhances the sensor’s selectivity and stability.

Sustainability and Future Prospects

This research exemplifies how nanotechnology can intersect with sustainability. By converting banana peel waste into functional nanomaterials, the study promotes resource efficiency and environmental stewardship. The developed sensor eliminates the need for high-end instrumentation, making Cr(VI) detection accessible and affordable.

Future directions include integrating these fluorescent CDs into portable devices for on-site monitoring, developing forensic tools for toxicology, and exploring applications in bioimaging and photocatalysis. The concept could also inspire similar approaches for detecting other hazardous substances using biowaste-derived nanomaterials.

Conclusion

The synthesis of carbon dots from banana peel biowaste marks a significant step toward sustainable nanotechnology. With a detection limit of 52 nM, high selectivity, and cost-effectiveness, these fluorescent nanoprobes offer a practical solution for monitoring Cr(VI) contamination. Beyond environmental sensing, this innovation opens avenues for industrial safety, forensic science, and healthcare, reinforcing the role of green chemistry in addressing global challenges.

Disclaimer: This article was originally authored by researchers from NanoBio Research Lab, collaborating with other labs and institutes. The original article was published in Wiley's ChemistrySelect.

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