Recycled Plastic Sensor Made of Bamboo Biochar and Copper Detects Toxic Herbicide Paraquat

Paraquat is one of the most widely used herbicides in the world, and also one of the most dangerous. Linked in the scientific literature to severe tissue damage, pulmonary fibrosis and an elevated risk of Parkinson’s disease, the compound has earned its reputation as an environmental contaminant that threatens aquatic ecosystems and human health alike. Yet despite its toxicity, paraquat remains largely unmonitored in many water systems, particularly in developing countries where agricultural use is intense. A research team at São Paulo State University (UNESP) in Brazil now reports a device that could change how such contamination is detected: a disposable, low-cost electrochemical sensor built from recycled plastic, a biopolymer-based conductive ink, and a nanocomposite of bamboo-derived biochar and copper nanoparticles.

The device, described in the journal Discover Electrochemistry, is a screen-printed sensor fabricated on a polyethylene terephthalate substrate, the same plastic used in beverage bottles. The researchers, led by Erika Y. Ito, Francisco C. Barreto, Martin K. L. Silva and Ivana Cesarino, cut adhesive stencils with a desktop cutting plotter, applied them to sanded PET sheets, and manually spread a conductive ink composed of graphite and sodium alginate, a natural polysaccharide extracted from brown seaweed. After drying at 50 degrees Celsius for 24 hours, the bare sensors were finished by drop-casting a suspension of the biochar-copper nanocomposite onto the working electrode. The entire fabrication process requires no sophisticated cleanroom equipment, which the authors argue is central to democratizing environmental chemistry.

The choice of materials reflects a deliberate sustainability strategy. Biochar, produced by heating biomass under oxygen-limited conditions, offers a high surface area, porosity and chemical stability, all of which make it an attractive electrode modifier. The bamboo biochar used in this study was pyrolyzed at 900 degrees Celsius for one hour. Copper nanoparticles, meanwhile, are prized in electroanalytical chemistry for their high conductivity, catalytic activity and comparatively low toxicity and cost relative to noble metals such as gold or silver. When combined, the two materials act synergistically: the porous carbon matrix adsorbs paraquat molecules, while the copper nanoparticles accelerate electron transfer across the electrode surface.

Characterization confirmed the composite’s structure. Field-emission scanning electron microscopy revealed an irregular, clustered surface morphology on the modified electrode, which the researchers note is advantageous because it increases the active surface area available for electrochemical reactions. Energy-dispersive X-ray spectroscopy showed the sensor surface was composed of 79.8 percent carbon, 14.0 percent oxygen and 4.8 percent copper, with the characteristic copper emission lines confirming successful nanoparticle incorporation. Fourier-transform infrared spectroscopy verified that the graphite and sodium alginate had combined effectively in the conductive ink.

Electrochemical testing exposed the mechanism by which the sensor detects paraquat. Cyclic voltammetry showed that the herbicide undergoes a two-step, quasi-reversible reduction: the paraquat dication first accepts one electron to form a radical cation, then a second electron to form the neutral species. The neutral form interacts strongly with the hydrophobic bamboo biochar matrix, adsorbing onto the electrode surface. Differential pulse voltammetry proved essential for resolving the two reduction steps cleanly, because it suppresses the capacitive charging currents that otherwise blur the signal. The team systematically optimized the measurement conditions, settling on an acetate buffer at pH 4.0, a scan rate of 40 millivolts per second, a 30-second accumulation step, and a pulse amplitude of 100 millivolts.

The composition of the nanocomposite itself proved critical. The researchers prepared composites containing 20, 30, 40 and 50 percent copper by weight relative to the biochar, and found that the 20 percent formulation delivered the sharpest and most intense reduction peak for paraquat. At higher copper loadings, peak intensity dropped and peaks broadened, a trend the authors attribute to nanoparticle agglomeration and mass transport constraints that reduce the effective electroactive area and block adsorption sites on the biochar. In direct comparisons, the bare sensor produced a cathodic peak current of 1.40 microamperes, the copper-only modification reached 3.1 microamperes, and the full biochar-copper composite achieved 4.8 microamperes, a 243 percent enhancement over the isolated biochar modifier.

Under the optimized conditions, the finished sensor detected paraquat linearly from 10 to 250 micromoles per liter, with a correlation coefficient of 0.988, a sensitivity of 0.0234 microamperes per micromole per liter, a detection limit of 7.65 micromoles per liter and a quantification limit of 25.48 micromoles per liter. Statistical tests confirmed that the copper incorporation significantly improved sensitivity relative to raw biochar alone. Perhaps more importantly, the nanocomposite platform maintained sharp, single-peak voltammetric profiles across the working range, whereas the unmodified sensor showed peak broadening and secondary electrochemical responses above 50 micromoles per liter. The composite also resisted surface saturation and passivation at high analyte concentrations, extending the usable dynamic range.

Real-world testing revealed both strengths and honest limitations. In tap water assays, sensitivity dropped by roughly 30.5 percent compared with pure electrolyte, a matrix effect attributed to inorganic ions and dissolved organic substances competing for active sites on the electrode. Interference studies with three electroactive pharmaceuticals commonly found in aquatic environments, levofloxacin, sulfamethoxazole and chloroquine, produced signal suppressions of 27 to 36 percent, indicating that the sensor cannot be considered fully selective against organic micropollutants at elevated concentrations. The device also showed a limited shelf life: signal fell by 53.3 percent after eight days of storage and 63.5 percent after 21 days, a decay the researchers trace to atmospheric oxidation of the zero-valent copper nanoparticles into semiconducting copper oxides. They recommend fabricating sensors shortly before use and suggest protective coatings or vacuum packaging as future remedies. Ultraviolet-visible spectrophotometry served as an independent reference technique to validate the electrochemical results.

The authors are candid that the detection limit of 7.65 micromoles per liter, equivalent to about 1.97 milligrams per liter, is far above the drinking-water thresholds enforced in the European Union and Brazil, which sit in the microgram-per-liter range. The sensor is therefore not positioned as a replacement for chromatographic techniques such as HPLC-MS/MS in routine compliance monitoring. Its intended niche is decentralized, on-site screening of acute contamination scenarios, such as agricultural runoff near application fields, industrial effluent discharges or accidental spills, where paraquat concentrations can temporarily reach the milligram-per-liter regime. The upper limit of the linear range, 64.3 milligrams per liter, comfortably covers both routine contamination and spill events, and higher concentrations can be handled with simple dilution. Coupling the sensor with solid-phase extraction or magnetic pre-concentration could eventually bridge the gap toward trace-level monitoring.

What sets the work apart is its quantitative commitment to green chemistry. Using the AGREE metric, which scores analytical methods against the twelve principles of green analytical chemistry on a scale from 0 to 1, the method achieved 0.71, comfortably above the 0.60 threshold generally associated with good environmental performance. The complementary ComplexGAPI assessment, which extends the evaluation to pre-analysis processes, confirmed the favorable profile while flagging areas for improvement: the off-line sampling approach, the chemical synthesis of copper nanoparticles using sodium borohydride and sodium dodecyl sulfate, and the need for specialized disposal of used sensors contaminated with paraquat residues. The researchers calculated that analyzing samples directly in the field with a portable potentiostat could raise the AGREE score to approximately 0.77, an estimated 8 percent improvement. They also propose biosynthesizing the copper nanoparticles with plant extracts as a greener alternative to sodium borohydride, whose hydrolysis can generate flammable hydrogen gas. Spent sensors were handled as hazardous chemical waste throughout the study and sent for controlled high-temperature incineration. Despite its constraints, the team concludes that the SD-PET/BC-CuNPs platform stands out as an efficient, eco-friendly and viable tool for on-site environmental monitoring of paraquat, and future work will target anti-oxidation coatings, molecular imprinting for selectivity, and smartphone-controlled potentiostats to bring fully automated, point-of-need water testing closer to reality.

Subject of Research: A green disposable electrochemical sensor based on bamboo biochar and copper nanoparticles for detecting the herbicide paraquat in water

Article Title: Biochar copper nanocomposite modified sensor for the green electroanalytical sensing of paraquat

Article References: Ito, E. Y., Barreto, F. C., Silva, M. K. L., & Cesarino, I. (2026). Biochar copper nanocomposite modified sensor for the green electroanalytical sensing of paraquat. Discover Electrochemistry, 3(1), Article 88. https://doi.org/10.1007/s44373-026-00176-5

Image Credits: AI Generated

DOI: 10.1007/s44373-026-00176-5

Keywords: paraquat, electrochemical sensor, biochar, copper nanoparticles, screen-printed electrode, green analytical chemistry, water monitoring, herbicide detection, recycled PET, sodium alginate, differential pulse voltammetry, environmental contamination