The appeal of biochar lies in its remarkable combination of properties. It is cheap, abundant, and derived from renewable waste biomass such as agricultural residues, wood waste, sewage sludge, and even coffee grounds. Its three-dimensional hierarchical pore structure, spanning micropores below two nanometers, mesopores between two and fifty nanometers, and macropores above fifty nanometers, creates an enormous internal surface area for capturing molecules. Its surface is naturally decorated with oxygen-containing functional groups such as hydroxyl and carboxyl moieties that bind analytes through hydrogen bonding, electrostatic attraction, and pi-pi interactions. Yet pristine biochar has a critical weakness: its electrical conductivity is often too low for high-performance electrochemical sensing, and it lacks sufficient catalytically active sites. Hybridization solves this problem by pairing biochar with conductive or catalytic partners.

The review details how the choice of synthesis route fundamentally shapes the final material. Slow pyrolysis, operating at heating rates of 0.1 to 5 degrees Celsius per minute and temperatures between 300 and 600 degrees Celsius, maximizes biochar yield and produces more ordered carbon structures. Fast pyrolysis, with heating rates up to 200 degrees per minute, favors liquid and gaseous products instead. Microwave-assisted pyrolysis delivers rapid, uniform heating and often yields higher porosity, while hydrothermal carbonization processes wet biomass in water at 180 to 250 degrees Celsius under pressure, producing hydrochar rich in oxygen functional groups but lower in conductivity. Pyrolysis temperature is the single most influential parameter: higher temperatures boost carbonization, conductivity, and surface area, but strip away the very surface groups that enable analyte interaction, forcing researchers to strike a careful balance.

Once synthesized, biochar becomes a scaffold for nanoscale building blocks of different dimensionalities. Zero-dimensional metal and metal oxide nanoparticles, including gold, silver, platinum, copper oxide, and nickel oxide, anchor onto biochar surfaces to introduce catalytic sites and accelerate electron transfer. One-dimensional structures such as carbon nanotubes and nanowires act as conductive bridges between isolated domains, reducing charge-transfer resistance. Two-dimensional materials, including graphene, reduced graphene oxide, MXenes, and metal-organic frameworks, add high porosity and broad adsorption areas. Heteroatom doping with nitrogen, sulfur, or phosphorus modulates the electronic structure and creates defect sites that enhance electrocatalytic performance. The review stresses that these components are not interchangeable: each contributes a distinct function, and rational selection should be guided by the dominant analytical bottleneck of the target pollutant rather than by simply maximizing conductivity.

Translating these materials into working sensors requires equally careful electrode engineering. The authors survey five main strategies. Layer-by-layer assembly offers precise control over film thickness and the spatial distribution of active sites, though excessive multilayers can increase interfacial resistance. Electrochemical deposition grows catalytic nanostructures directly on the electrode with excellent electrical contact, allowing particle density and morphology to be tuned through deposition potential and time. Homogeneous pre-mixed casting remains the most common laboratory method, dispersing biochar hybrids in solvent with polymeric binders before drop-casting. Screen-printing and 3D printing promise scalable manufacturing of disposable electrodes and complex architectures with programmable porosity. Finally, bulk modification of carbon paste electrodes distributes the electroactive material throughout the electrode volume, allowing a fresh surface to be regenerated by simple polishing when fouling occurs, an advantage for continuous monitoring in chemically complex matrices.

The sensing mechanism itself follows an elegant sequence. Contaminants first diffuse from the bulk solution into the porous biochar network, where they are enriched through adsorption interactions. The hybrid components then accelerate electron-transfer kinetics by providing conductive pathways and electrocatalytically active sites, converting the accumulated analyte into amplified electrochemical signals with improved sensitivity and lower detection limits. This capture-to-transduction logic is illustrated by striking examples from the recent literature. A walnut-shell biochar electrode coated with electropolymerized poly-tyrosine simultaneously detected cadmium, lead, copper, and mercury ions in water and soil samples. Oxygen-vacancy-engineered nickel cobalt oxide nanorods grown on coconut-shell biochar achieved highly sensitive lead detection, with the hybrid interface dramatically outperforming either component alone.

Pesticide sensing showcases a second design principle. For electroactive compounds such as carbendazim, a biochar and reduced graphene oxide nanocomposite couples local analyte enrichment with improved signal transport, with the hybrid producing higher oxidation currents than either material individually. For weakly responsive pesticides like malathion, catalytic activation becomes essential: a ternary nanocomposite of graphitic carbon nitride, copper oxide, and derived biochar exhibited the smallest peak separation, lowest charge-transfer resistance, and highest stripping current among tested electrodes. A porous biochar decorated with nano-zero-valent iron detected glyphosate through a combination of porous diffusion, electrostatic attraction, and hydrogen bonding, with the iron domains serving as electron-transfer-active centers. The authors note, however, that real-matrix validation, fouling resistance, and repeated-use stability remain underexamined across much of this literature.

Pharmaceuticals and emerging contaminants push the field toward matrix-reliability engineering. A sludge-derived biochar and graphite conductive ink, printed on transparency sheets using nail polish as a binder, detected the antibiotic imipenem in environmental samples, with biochar loading optimized as a genuine design variable that lowered charge-transfer resistance. A 3D-printed electrode combining acrylonitrile butadiene styrene waste, graphite, and rice husk biochar monitored levofloxacin in river and supply water after simple dilution. For plastic-associated chemicals, a zinc oxide and biochar nanohybrid detected bisphenol A in water, while a magnetite-activated biochar sensor targeted tetrabromobisphenol A. PFAS and microplastics remain frontier challenges: biochar can capture these persistent pollutants effectively, but converting that capture into a measurable electrochemical signal requires recognition layers and transduction elements that are only beginning to be developed. Early work has shown biochar electrodes detecting roughly 100-nanometer polystyrene microplastics, hinting at what may become possible.

Looking ahead, the review identifies biochar-based conductive inks as a potential game-changer. A biochar ink developed for malathion detection produced a current response roughly ten times higher than graphite-based electrodes, attributed to greater active-site availability, enhanced interfacial charge transfer, and higher hydrophilicity. If conductivity, dispersion stability, and printability can be reliably optimized, such inks could offer a sustainable alternative to commercial graphene and carbon nanotube formulations for screen-printed and flexible devices. The authors also call for artificial intelligence and machine learning to predict how feedstock, pyrolysis conditions, and modification strategies shape material properties, and for computational screening of hybrid compositions before synthesis. The central challenges remain reproducibility, given the intrinsic variability of biochar, and the gap between laboratory demonstrations and field deployment in wastewater, agricultural runoff, and seawater. But the trajectory is clear: a material once valued simply for making soil fertile may soon stand guard over the world’s water, one electron at a time.

Subject of Research: Biochar-based hybrid nanocomposites for electrochemical sensing of environmental contaminants

Article Title: Advanced functional biochar-based hybrid nanocomposites for electrochemical sensing platforms in environmental monitoring

Article References: Poorahong, S., Naorungroj, S., Jesadabundit, W., Thangphatthanarungruang, J., Promphet, N., Darayen, J., Wonsawat, W., Boctor, J. N. T., Murphy, D. V., Ozer, T., Chailapakul, O., & Jampasa, S. (2026). Advanced functional biochar-based hybrid nanocomposites for electrochemical sensing platforms in environmental monitoring. Discover Electrochemistry, 3(1), Article 72. https://doi.org/10.1007/s44373-026-00158-7

Image Credits: AI Generated

DOI: 10.1007/s44373-026-00158-7

Keywords: biochar, electrochemical sensors, nanocomposites, environmental monitoring, heavy metals, pesticides, PFAS, graphene, metal-organic frameworks, conductive ink, pyrolysis, water pollution