The probe at the heart of the study, published in the journal Results in Chemistry, is a class of nanocrystal known as a ternary quantum dot, composed of silver, indium, and sulfur wrapped in a protective zinc sulfide shell, abbreviated AIS/ZnS. Quantum dots are semiconductor particles so small, typically just a few nanometers across, that their electrons are squeezed by quantum confinement, giving them optical properties that bulk materials simply cannot match. When excited by light, these particles re-emit it as bright fluorescence, and the color and intensity of that glow can be tuned by adjusting particle size and composition. Unlike the cadmium-based quantum dots used in earlier cetirizine sensors, which raise serious toxicity concerns of their own, the silver indium sulfide family is far more biocompatible, making it a safer candidate for environmental monitoring and even biomedical applications.

The Johannesburg team synthesized their AIS/ZnS quantum dots following established procedures, capping the nanocrystals with L-glutathione, a natural tripeptide, to keep them stable and water-soluble. The resulting particles measured roughly 2.34 nanometers in diameter, as calculated from X-ray diffraction data using the Scherrer equation, and adopted the tetragonal chalcopyrite crystal geometry characteristic of the AgInS2 lattice. Their optical credentials were impressive: an absolute quantum yield of 76.51 percent, meaning more than three-quarters of absorbed photons were re-emitted as fluorescence, and an average fluorescence lifetime of 171.13 nanoseconds, corresponding to a decay constant of 4.5 million per second. The dots glowed steadily across a wide pH range from 3 to 11, with remarkably stable intensity between pH 4 and 10, a crucial property for a sensor that must operate in real environmental waters whose acidity varies unpredictably.

The sensing principle is elegantly simple. When cetirizine molecules encounter the glowing quantum dots, the fluorescence dims, a phenomenon called quenching. The researchers excite the dots at 500 nanometers and watch the emission fade as drug concentration rises. Across a cetirizine range spanning 0.001 to 35.4 micrograms per milliliter, the Stern-Volmer relationship between fluorescence loss and drug concentration held with good linearity, yielding a quenching constant of 1.92 times ten to the fourth per molar. The calculated bimolecular quenching rate constant, 1.12 times ten to the eleventh per molar per second, exceeded the diffusion limit for dynamic collisional quenching, which told the team the mechanism was static: the drug and the dot form a stable, non-fluorescent ground-state complex before any light is even absorbed.

The sensitivity figures are where the work becomes genuinely remarkable. At low cetirizine concentrations between 1 and 10 nanograms per milliliter, the team determined a limit of blank of 0.276 nanograms per milliliter, a limit of detection of 0.553 nanograms per milliliter, and a limit of quantification of 1.68 nanograms per milliliter. Those numbers place the AIS/ZnS probe among the most sensitive cetirizine chemosensors ever reported, comfortably reaching nanogram-per-milliliter territory that conventional techniques like high-performance liquid chromatography and liquid chromatography mass spectrometry achieve only with costly instrumentation and highly trained operators. A double-logarithmic binding analysis revealed a one-to-one stoichiometry between quantum dot and drug molecule, with a binding constant of 2.3 times ten to the fourth per molar, confirming that each dot pairs with a single cetirizine molecule in the sensing event.

One of the most visually dramatic findings emerged at the highest drug concentration. At 35.4 micrograms per milliliter, the quantum dots aggregated, their emission band shifted to longer wavelengths, and the photoluminescence collapsed by 93.5 percent. Under a handheld 365-nanometer ultraviolet lamp, the mixture glowed a deep red, a change visible to the naked eye. Even more intriguingly, when the researchers mechanically agitated the solution, the fluorescence snapped back to its original position and intensity, demonstrating that the aggregation is fully reversible and rooted in physical, non-covalent interactions rather than chemical destruction of the probe. This reversible, aggregation-induced quenching suggests the platform could be reset and reused, an attractive feature for practical field deployment.

To understand exactly how cetirizine grips the quantum dot surface, the team turned to density functional theory, modeling the AgInS core on a chalcopyrite template and optimizing the drug, the dot, and their complex at the B3LYP level with the LANL2DZ basis set for the heavy silver and indium atoms and 6-31G(d) for the lighter elements. Because the zinc sulfide shell confines both electron and hole to the core in this type-I structure, the simplified core model captures the essential electronic physics. The calculations gave the quantum dot a bandgap of 1.92 electron volts, in close agreement with the experimental 0-0 transition at 2.09 electron volts, a strong validation of the computational approach. Cetirizine, by contrast, showed a much larger gap of 4.81 electron volts, with its highest occupied orbital localized on the ethoxy piperazinyl group and its lowest unoccupied orbital on the 4-chlorophenyl methylphenyl moiety, mapping out the drug’s electron-donating and electron-accepting regions.

The computed electrostatic potential map identified the ether and carboxylic oxygen atoms as the drug’s nucleophilic hotspots, while the carboxylic proton emerged as the principal electrophilic site. In the optimized complex, charge flowed from cetirizine into the quantum dot, with the indium and sulfur atoms acting as the main acceptors, and close contacts of 2.9 to 3.3 angstroms pointed to pi-pi stacking and van der Waals interactions rather than covalent bonding. The calculated binding energy of minus 33.81 kilojoules per mole confirmed a spontaneous physical association. These theoretical predictions dovetailed neatly with the experimental infrared spectra: new peaks appeared at 2924, 2854, and 1746 wavenumbers in the complex, the last attributed to electrostatic interaction between the drug’s carboxylate ions and electrophilic sites on the glutathione-capped dot surface, alongside a shift of the broad hydrogen-bonding band from 3294 to 3315 wavenumbers.

The researchers also ruled out the two most common false leads in fluorescence sensing. Because cetirizine’s absorption and excitation bands show no spectral overlap with the quantum dot’s fluorescence, both Förster resonance energy transfer and the inner filter effect could be excluded, leaving ground-state complexation and aggregation as the true quenching pathways. The absence of overlap also means the probe’s signal cannot be artificially inflated by the drug simply absorbing the excitation light, a pitfall that has undermined less carefully characterized sensors.

The broader significance of the work lies in its combination of affordability, safety, and molecular-level rigor. Cetirizine overdoses in humans are linked to confusion, tremor, drowsiness, and tachycardia, and improper disposal of the drug has turned it into an emerging aquatic pollutant, yet detecting it at environmentally relevant concentrations has traditionally demanded chromatography suites and expert analysts. A quantum dot probe that can be read with a standard spectrofluorometer, or potentially even by eye under ultraviolet light, democratizes that capability. By pairing spectroscopic evidence with density functional theory, the Johannesburg team has not only delivered the first reported AIS/ZnS fluorescent sensor for cetirizine in water but also established a template for how computation and experiment can jointly decode sensing mechanisms at the molecular scale. As pharmaceutical residues continue to accumulate in the world’s waterways, glowing nanocrystals that dim on cue may become one of environmental science’s most valuable early-warning systems.

Subject of Research: Fluorescence detection of the pharmaceutical pollutant cetirizine in water using AgInS/ZnS quantum dots supported by density functional theory modeling

Article Title: AgInS/ZnS quantum dots as an ultrasensitive cetirizine sensor: Spectroscopic and computational approaches

Article References: Ojo, N. D., Masiu, L., & Oluwafemi, O. S. (2026). AgInS/ZnS quantum dots as an ultrasensitive cetirizine sensor: Spectroscopic and computational approaches. Results in Chemistry, 31, Article 103926. https://doi.org/10.1016/j.rechem.2026.103926

Image Credits: AI Generated

DOI: 10.1016/j.rechem.2026.103926

Keywords: quantum dots, cetirizine, fluorescence sensing, water pollution, pharmaceutical contaminants, AgInS/ZnS, density functional theory, fluorescence quenching, nanosensors, antihistamines, environmental monitoring, ternary nanocrystals