Integrating electrochemical sensors in circular economy: biochar-film sensors based on paper industry waste for agri-food by-product valorization

Abstract

Sustainable nanostructured paper sensors (PS) have been developed to detect bioactive compounds in plant-derived agri-food by-products. These sensors comprise 100 % recycled fibers-paper and pulp industry-derived biochar and are produced using accessible, cost-effective technologies.

In detail, black liquor-derived biochar (BH) was nano-dispersed in water using an ultrasound-assisted liquid phase exfoliation-like approach, avoiding solvents. Self-standing BH-sensing nanofilms were formed directly on cellulosic membranes and integrated into stencil-printed complete electrochemical sensors manufactured on recycled paper. The biochar-based paper sensors (BH-PS) were optimized, characterized, and then employed to analyze bioactive phenols present in agri-food waste and derivatives, including cocoa and coffee husks, exhausted coffee powder, and olive leaves and artichoke production waste-based supplements.

BH-PS demonstrated robust performance, yielding dose-response curves for representative bio-compounds such as caffeic acid, catechin, chlorogenic acid, and oleuropein. These showed excellent linearities (R² ≥ 0.9946) and detection limits ranging from 0.03 to 0.6 µM. The sensors were successfully used to quantify biomolecules in agri-food wastes and derivatives, with results comparable to conventional photometric assays (r ≥ 0.99; relative error vs. AuNPs assay: -3 % to +14 %). The method produced quantitative and reproducible recoveries for all samples (97–114 %; RSD ≤ 13 %, n = 3). Finally, the superior sustainability of the BH-PS was validated using the White Analytical Chemistry framework, achieving a significantly higher score (94 %) compared to traditional colorimetric and chromatographic methods (60–80 %).

This work demonstrates a circular economy model, utilizing plant-derived waste exclusively to fabricate integrated paper sensors, that were then applied to determine high-value bioactive compounds in plant-derived agri-food by-products.