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

The two clusters at the heart of this work are chemical cousins with a fascinating structural relationship. M44, formally Au24Ag20 and protected by twenty-four 4-tert-butylphenylacetylide ligands and two chlorides, consists of a hollow icosahedral Au12 kernel wrapped in a fullerene-like Ag20 inner shell and capped by twelve exterior gold atoms. M81, with the formula Au43Ag38, is essentially a dimer of M44: two Au12 units fuse through an additional gold atom to form an Au25 kernel, while the two Ag20 shells merge with the loss of two silver atoms to create an Ag38 shell, all further wrapped by eighteen gold atoms and ligands. This face-fusion architecture is distinct from the side-on biicosahedral fusion seen in related clusters such as Au20Ag32 and Au23. Kohn-Sham molecular orbital analysis reveals that M44 behaves as a superatom resembling Au25, whereas M81 shows molecular-like character reminiscent of Au38, giving the researchers a matched pair in which electronic structure alone dictates whether a cluster leans toward photoluminescence or photothermal behavior.

The synthetic route is elegant in its simplicity. A mixture of chloro(dimethylsulfide)gold(I) and silver acetate dissolved in dichloromethane and methanol is combined with the deprotonated acetylene ligand, forming a reddish-yellow Au-Ag-alkynyl polymeric intermediate within minutes. Reduction with tert-butylamine borane, followed by reaction in the dark for one to three days, yields the clusters. Rather than relying on reaction timing and direct crystallization as in earlier work, the team employed thin-layer chromatography to separate the two products, which appear as distinct bands and were unambiguously identified by ultraviolet-visible-near-infrared absorption spectroscopy. Reaction time biases the outcome: quenching at roughly twenty-four hours favors M44, while extending to thirty-six or seventy-two hours promotes M81 growth. This post-synthetic purification strategy makes both clusters accessible from a single pot, a practical advantage for scaling up.

Optically, the monomer-to-dimer transformation produces dramatic consequences. M44 shows absorption peaks at 365 nanometers, convoluted features at 450 and 505 nanometers, and a broad band at 640 nanometers with tailing into the near-infrared. M81 displays additional bands at 640 and 765 nanometers and extends its absorption tail into the NIR-II window beyond 1000 nanometers. Most strikingly, the dimerization boosts the molar extinction coefficient at the long-wavelength band by roughly sixty times, from about 850 to 50,000 per molar per centimeter, consistent with the stronger transition dipole of the rod-like M81 structure. Both clusters emit photoluminescence in the NIR-II region, peaking near 1080 nanometers for M44 and 1320 nanometers for M81, the latter being among the rarest emission depths achieved by atomically precise clusters, previously matched only by gold quantum rods.

The photoluminescence quantum yields, however, are deliberately low: 3 percent for M44 and 1.5 percent for M81 in deuterated chloroform solution. Herein lies the counterintuitive design principle behind the work. Strong electron-phonon coupling quenches radiative emission and instead channels the absorbed photon energy into lattice vibrations, releasing it as heat. Transient absorption spectroscopy confirmed the underlying excited-state dynamics, with approximately one-picosecond components attributed to internal conversion and intersystem crossing, and longer nanosecond components matching the emission lifetimes of roughly 300 nanoseconds for M81. The long-lived triplet states even enable singlet oxygen generation in M44, as confirmed by the diagnostic 1275-nanometer singlet-oxygen phosphorescence signal under oxygen purging, while M81’s emission energy of 0.89 electron volts falls just below the 0.97-electron-volt gap needed for triplet-to-singlet oxygen energy transfer, so no such signal appears.

Systematic photothermal measurements across varying concentrations and optical power densities revealed a crucial and often overlooked trade-off: photothermal conversion efficiency and maximum temperature rise are counteracting parameters. At a modest optical power density of 0.5 watts per square centimeter and low absorbance, M81 achieved its record 91 percent efficiency with a temperature rise of 11.2 degrees Celsius. Pushing the power density to 2.5 watts per square centimeter dropped the efficiency to 38 percent but drove the temperature rise to 35.4 degrees Celsius, reaching a solution temperature of nearly 60 degrees. At higher absorbance, M81 heated toluene to 96.8 degrees Celsius. M44 followed similar trends with a peak efficiency of 69 percent and temperature rises up to 58 degrees. Under 1064-nanometer excitation deep in the NIR-II window, M81 still produced a measurable temperature rise despite weak extinction there, one of the first demonstrations of direct NIR-II photothermy from atomically precise clusters.

The solid-state results are where the numbers become genuinely startling. Thin films of the clusters embedded in a polymethyl methacrylate matrix, irradiated at 2.5 watts per square centimeter, reached temperature increases of 226 degrees Celsius for M44 and 155 degrees for M81. Such extreme heating in a solid film is precisely what is needed for energy-harvesting devices, since converting light into electricity requires a large, localized temperature gradient rather than the rapid dissipation that occurs in dilute solutions. Crucially, ultraviolet-visible-near-infrared spectra recorded before and after irradiation in all three media, organic solution, aqueous suspension, and solid film, showed no discernible change, confirming that both clusters survive months of ambient storage and repeated intense heating without degradation. The team also encapsulated both clusters in the FDA-approved poloxamer Pluronic F-127, producing water-soluble supraparticles around 150 nanometers in diameter with photothermal efficiencies near 50 percent in water, a regime well suited to photothermal tumor therapy.

The culmination of the study is the first thermoelectric generator application of atomically precise nanoclusters. The researchers deposited a layer of clusters onto the hot side of a commercial bismuth telluride thermoelectric generator and irradiated it with the same 808-nanometer laser, deliberately omitting any external cooling on the cold side to demonstrate electricity generation under minimal, device-integrated conditions. The Seebeck effect, in which a temperature gradient across a thermoelectric material drives directional charge-carrier diffusion, converted the cluster-generated heat into stable open-circuit voltages of 146 millivolts for M44 and 127 millivolts for M81, sustained over 900 seconds of continuous irradiation. Cycling experiments showed the devices could be switched on and off repeatedly without measurable degradation, and control experiments on bare thermoelectric generators produced no detectable voltage, confirming that the clusters themselves are responsible for the effect. Thermal paste at the interface yielded comparable voltages, indicating that direct cluster deposition already provides efficient thermal coupling.

The broader implications extend well beyond the laboratory laser experiments. More than half of the total solar radiation arrives in the near-infrared, yet conventional solar energy harvesting captures predominantly the ultraviolet and visible portions. The absorption profiles of both clusters overlap substantially with the solar spectrum, suggesting that direct sunlight could in principle drive the same photo-thermo-electric cascade. Encapsulating the clusters within heat-resistant carbon or silica shells could allow operation at even higher temperatures under focused solar illumination. Perhaps most conceptually significant is the reframing of electron-phonon coupling, typically dismissed as an unwanted quenching mechanism, into a deliberate design principle for near-infrared photonic energy harvesting. Key challenges remain, including broadening the absorption further into the shortwave infrared, improving thermal coupling, and demonstrating long-term operational stability, but this work establishes atomically precise nanoclusters as functional device components rather than mere solution-phase chromophores, and points toward a future in which quantum-sized metal clusters help reclaim the invisible half of sunlight.

Subject of Research: Near-infrared photothermal conversion and photo-thermo-electric energy generation using atomically precise bimetallic gold-silver nanoclusters

Article Title: Near‐Unity Photothermal Conversion in Bimetallic M44 and M81 Nanoclusters Enables NIR‐Driven Photo‐Thermo‐Electricity Generation

Article References: Sardar, A., Mazumder, A., Ji, W., He, G., Wang, Y., Luo, L., Chen, S., & Jin, R. (2026). Near‐Unity Photothermal Conversion in Bimetallic M 44 and M 81 Nanoclusters Enables NIR‐Driven Photo‐Thermo‐Electricity Generation. Advanced Science, Article e78061. https://doi.org/10.1002/advs.78061

Image Credits: AI Generated

DOI: 10.1002/advs.78061

Keywords: atomically precise nanoclusters, gold-silver nanoclusters, photothermal conversion, near-infrared, thermoelectric generator, Seebeck effect, NIR-II photoluminescence, solar energy harvesting, electron-phonon coupling, bismuth telluride, photothermal therapy, nanomaterials