The gold standard for pesticide residue analysis remains tandem mass spectrometry, whether coupled to gas or liquid chromatography. These techniques deliver exceptional sensitivity, selectivity, and accuracy, but they come with real costs: laborious sample preparation, large volumes of organic solvent, and dependence on specialized laboratory instrumentation that cannot travel to a farm field or a market stall. Complex matrices such as traditional Chinese medicines make things worse, because co-extracted pigments, volatile oils, saponins, and alkaloids interfere with ionization efficiency and degrade quantitative reliability. Fluorescence-based assays have emerged as a faster, simpler alternative, but many existing probes suffer from a critical weakness: elevated concentrations of organic solvents, such as the acetonitrile or methanol used in QuEChERS sample preparation, inflate background fluorescence and erode detection accuracy.

A team of Chinese researchers reporting in Frontiers in Sensors has now built a fluorescent sensor designed specifically to survive that solvent problem. The platform, which the authors call the AZ NPs-ACh sensor, is constructed through the coordination assembly of gold-silver nanoclusters with zinc ions. The synthesis is deliberately simple: bovine serum albumin reduces gold and silver precursors into stable, highly fluorescent nanoclusters, with the protein’s sulfur atoms forming coordination bonds that prevent metal aggregation. Histidine and cysteine residues within the albumin then bind zinc ions through stable zinc-nitrogen bonds, producing a metal-protein complex that serves as the fluorescent sensing element. The researchers optimized the gold-to-silver precursor ratio at 4:1, and characterized the resulting particles by HAADF-STEM, confirming that gold, silver, nitrogen, and zinc are co-localized within the structure. Dynamic light scattering showed the parent nanoclusters at roughly 5 to 10 nanometers, while the assembled sensor particles average about 400 nanometers.

Optically, the sensor emits at 690 nanometers when excited at 310 nanometers, a Stokes shift exceeding 380 nanometers. That large separation between excitation and emission is more than a technical footnote: it prevents spectral overlap with the spontaneous luminescence of biological samples and makes the probe suitable for messy, real-world specimens. The sensor maintains consistent optical responses across physiological pH from 6.0 to 7.4 and temperatures from 30 to 37 degrees Celsius, a stability window that tracks the conformational integrity of the albumin scaffold. Under strong alkaline conditions, electrostatic repulsion between albumin molecules can irreversibly disrupt the protein’s tertiary structure, and thermal denaturation can expose and aggregate the nanoclusters, so the observed stability window makes physical sense.

The solvent tolerance is the headline feature. In methanol-water mixtures across varying solvent ratios, and in pure water, methanol, ethanol, and ethyl acetate, the sensor retains more than 95 percent of its fluorescence intensity. That means analysts can run detection directly in the extraction solutions of complex samples without transferring to a specific solvent, dramatically simplifying pretreatment. Fifteen-day stability assays showed the assembled sensor is markedly more stable than bare gold-silver nanoclusters, an improvement the authors attribute to the zinc-nitrogen coordination bonds suppressing nanocluster aggregation and fluorescence quenching. Selectivity testing against common metal ions and amino acids showed a significant response only to hydrogen peroxide, indicating strong analytical specificity in realistic sample environments.

The sensing chemistry hinges on what hydrogen peroxide does to those zinc-nitrogen bonds. Infrared spectroscopy revealed a stretching vibration band attributable to the zinc-nitrogen bond in the low wavenumber region, and its intensity diminished markedly after hydrogen peroxide treatment. X-ray photoelectron spectroscopy showed the zinc 2p3/2 peak deconvolving into two new components at approximately 1026 and 1048 electron volts, consistent with a transformation of the zinc coordination environment from nitrogen ligands to oxygen ligands. The structural reorganization quenches the 690-nanometer emission. Under a 365-nanometer ultraviolet lamp, the fluorescence fades visibly from bright to dim as hydrogen peroxide concentration rises, and quantitative measurements on a microplate reader confirmed good linearity between fluorescence change and hydrogen peroxide concentration from 0 to 10 millimolar, with a linear calibration range of 5 to 100 micromolar, a detection limit of 1.4 micromolar, and a quantification limit of 4.7 micromolar. The response is fast, with significant quenching within five minutes and a stable signal out to sixty minutes.

Converting that hydrogen peroxide response into pesticide detection requires an enzymatic cascade. Acetylcholinesterase hydrolyzes acetylcholine to choline, and choline oxidase then oxidizes the choline to generate hydrogen peroxide, which the sensor captures as a fluorescent signal. The researchers validated the cascade carefully: working solutions containing only one or two of the three components produced no detectable hydrogen peroxide and no fluorescence quenching, ruling out non-specific interactions and background interference. Optimization experiments settled on 0.8 units per milliliter for both enzymes and 0.5 molar acetylcholine, with a 30-minute sensor incubation and a signal that reaches steady state within 20 minutes after incubation ends. Because organophosphorus pesticides inhibit acetylcholinesterase, the presence of a pesticide suppresses the entire cascade, starving the sensor of hydrogen peroxide and preserving its fluorescence. The degree of quenching therefore reports pesticide concentration indirectly but quantitatively.

Using triazophos, a broad-spectrum organophosphate banned in numerous countries over toxicity and persistent residue concerns, the team demonstrated a linear response from 2 to 200 nanograms per milliliter, with a detection limit of 0.69 nanograms per milliliter and a quantification limit of 2.0 nanograms per milliliter. Spiking and recovery tests on dried Chenpi, a traditional Chinese medicine made from aged tangerine peel, confirmed accuracy in a genuinely difficult matrix, and ten batches of Chenpi samples validated method applicability. Rapid ice-bath extraction minimized interference from endogenous matrix components during sample preparation. The linear working range of 1 to 200 nanograms per milliliter cited for real samples comfortably covers regulatory maximum residue limits for many commodities.

The most practically striking element is the paper-based format. The researchers immobilized the sensing material on a paper substrate that remains stable between 20 and 50 degrees Celsius and holds its fluorescence for at least three days when stored dry and protected from light. Dropping a reaction solution onto the sensing zone and illuminating it with a 365-nanometer portable ultraviolet lamp, the kind of inexpensive device sold as a counterfeit-detection pen, produces a naked-eye fluorescence gradient that decreases as hydrogen peroxide, and by extension the pesticide signal, increases. This enables semi-quantitative screening of pesticide residues directly in farm fields and markets, by non-specialists, without any laboratory infrastructure.

The work establishes what the authors describe as a new paradigm for convenient visual monitoring of pesticide residues, by coupling solvent-tolerant nanocluster chemistry with a paper-device design and an enzyme-linked amplification strategy. For regulators, herbal medicine producers, and food safety inspectors, the appeal is obvious: a probe that tolerates the very solvents needed to extract pesticides from complex matrices, emits in a range readable by eye under cheap illumination, and detects triazophos at sub-nanogram-per-milliliter levels. Larger-scale validation across additional matrices and pesticide species will determine how broadly the platform travels, but the core demonstration, that a coordination-assembled albumin nanocluster can deliver reliable fluorescence in the messy solvents of real sample prep, addresses one of the most persistent obstacles standing between laboratory fluorescence assays and genuine field deployment.

Subject of Research: A solvent-tolerant fluorescent nanocluster sensor for detecting organophosphorus pesticide residues via acetylcholinesterase-hydrogen peroxide signaling

Article Title: Universal solvent-adaptive fluorescent AZ NPs-ACh sensor for reliable visual detection of organophosphorus pesticides via AChE-H2O2 signaling

Article References: Hu, H., Zhang, Y., Wang, L., Wu, Y., Sheng, K., Zeng, Z., Yuan, S., Jin, Y., Ma, L., Tian, J., & Wang, B. (2026). Universal solvent-adaptive fluorescent AZ NPs-ACh sensor for reliable visual detection of organophosphorus pesticides via AChE-H2O2 signaling. Frontiers in Sensors, 7, Article 1756022. https://doi.org/10.3389/fsens.2026.1756022

Image Credits: AI Generated

DOI: 10.3389/fsens.2026.1756022

Keywords: organophosphorus pesticides, fluorescent sensor, gold-silver nanoclusters, acetylcholinesterase, hydrogen peroxide, triazophos, paper-based sensor, traditional Chinese medicine, food safety, zinc coordination, on-site detection, choline oxidase