Quercetin is a naturally occurring flavonoid found in fruits, vegetables, and grains, and it has attracted enormous attention for its antioxidant, anti-inflammatory, antibacterial, and cardiovascular protective effects. Those properties have made it a staple ingredient in pharmaceuticals and nutritional supplements around the world. But the same popularity has created a hidden environmental problem. Excessive intake or uncontrolled release of quercetin can induce adverse effects such as nephrotoxicity, and its polyhydroxyl structure allows it to bind strongly with metal ions, which can further increase its potential toxicity in environmental systems. Tracking this molecule in rivers, estuaries, and coastal waters has therefore become a genuine analytical priority.

Existing methods for measuring quercetin include high-performance liquid chromatography, UV-visible spectrophotometry, capillary electrophoresis, and electrochemical techniques. Electrochemical sensing stands out because it offers high sensitivity, fast response, low cost, and simple operation. Yet the literature is crowded with sensors that come with strings attached. Some achieve impressively low detection limits using complex nanocomposites, but their synthesis involves multiple procedures that hinder reproducibility and large-scale application. Polymer-modified electrodes often show narrow practical linear ranges, molecularly imprinted polymer sensors require template removal and multi-component assembly, and advanced bimetallic or ionic liquid platforms depend on specialized materials. The field has been waiting for a design that combines simple fabrication with reliable performance in messy real-world samples.

The new sensor tackles this challenge with a two-part strategy. The first component is MOF-79, a metal-organic framework constructed from titanium nodes and adenine ligands. Metal-organic frameworks are crystalline lattices in which metal centers are connected by organic linkers, producing structures with enormous internal surface areas, tunable pores, and well-defined architectures. Titanium-based MOFs in particular have demonstrated good chemical stability and promising electronic properties. The second component is gold, a metal prized in electrochemistry for its excellent conductivity, catalytic activity, and biocompatibility. Gold nanoparticles on their own tend to aggregate during synthesis or operation, which shrinks the active surface area and degrades sensing performance. The porous MOF-79 framework solves this by acting as a rigid scaffold that keeps the gold nanoparticles uniformly dispersed and prevents them from clumping.

The researchers synthesized the composite through a straightforward hydrothermal route. MOF-79 was first grown in a Teflon-lined autoclave from titanium dioxide, adenine, and an oxalic acid solution held at 70 degrees Celsius for 15 hours. The resulting solid was then stirred with chloroauric acid under alkaline conditions, reducing the gold precursor into metallic nanoparticles distributed across the framework. A drop of the suspended material was deposited onto a polished glassy carbon electrode, producing the finished Au@MOF-79/GCE sensor. Characterization by scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy confirmed that the crystalline framework survived the gold incorporation intact and that the gold had been reduced to its metallic state, with characteristic Au 4f signals appearing at 83.1 and 87.0 electron volts.

Electrochemical testing revealed exactly why the marriage of gold and MOF works so well. In cyclic voltammetry experiments, the Au@MOF-79 electrode produced the strongest redox peaks for quercetin among three electrodes tested, outperforming both a bare glassy carbon electrode and one modified with MOF-79 alone. The porous framework showed a strong affinity for quercetin, enriching the molecule near the electrode surface, while the loaded gold enhanced the detection capability by accelerating electron transfer. When the team varied the scan rate from 10 to 175 millivolts per second, the peak currents rose in strict proportion to the sweep speed, with a correlation coefficient of 0.999 for the anodic peak. The reduced peak-to-peak separation after gold incorporation pointed to faster, more reversible charge transfer, and the overall behavior indicated an adsorption-controlled process in which the porous structure gathers more quercetin onto the electrode surface.

The team also fine-tuned the operating conditions. Peak currents remained above 2 microamps across a broad pH range from 5.7 to 8.0, but the signal reached its maximum at pH 7.0, where it was amplified roughly 1.7-fold compared with other values. Using differential pulse voltammetry at this optimum, the sensor delivered a linear response to quercetin concentrations spanning from 3.04 times ten to the minus fifth to 1.196 times ten to the minus third molar, with a detection limit of 6.0 times ten to the minus sixth molar. That limit sits at the micromolar level, higher than the best noble-metal-plus-carbon systems reported in the literature, but the authors argue the trade-off is deliberate. Their electrode was designed with an emphasis on structural stability, reproducibility, and straightforward fabrication rather than ultra-low detection limits achieved through sophisticated architectures, and the wide linear range and stable response make the sensitivity sufficient for practical determination.

Durability and selectivity proved equally convincing. A chronoamperometric run lasting 2600 seconds produced a smooth curve without obvious fluctuation, demonstrating high short-term operational stability. In interference experiments, metal ions including lead, lithium, iron, calcium, and cobalt were introduced at concentrations equal to that of quercetin, and none of them significantly disturbed the electrochemical signal. The real test, however, came from the environment itself. The researchers collected seawater from the Ten Mile Golden Beach in Jinzhou, Liaoning Province, mixed it fifty-fifty with phosphate buffer, and spiked it with known quercetin concentrations. The sensor tracked the added quercetin with a linear calibration closely mirroring the one obtained in pure buffer, and recovery measurements showed relative standard deviations between 2.1 and 3.8 percent, indicating excellent feasibility and accuracy in a matrix notorious for its salt, dissolved organic matter, and competing ions.

The significance of the work extends beyond one molecule. It offers a practical design philosophy for MOF-based electrochemical sensors destined for complex aqueous environments: let a porous framework do the enrichment work, let well-dispersed metal nanoparticles do the electron-transfer work, and keep the synthesis simple enough that the whole thing can be rebuilt reliably. As pharmaceutical consumption continues to climb worldwide, the residues of those compounds are increasingly detected in coastal and marine waters, and monitoring tools that are cheap, robust, and field-deployable will only grow in importance. A gold-studded molecular sponge that shrugs off seawater and still reads quercetin accurately is a small electrode with a large implication: environmental pharmaceutical surveillance may soon be within reach of any laboratory that can mix a few reagents and polish a glassy carbon rod.

Subject of Research: An Au nanoparticle and MOF-79 composite electrochemical sensor for detecting quercetin in seawater

Article Title: The electrochemical sensor Au@MOF-79 toward the quercetin detection in seawater

Article References: Duan, F., Yang, H., Qiao, C., Chen, J., Lin, P., & Ge, Q. (2026). The electrochemical sensor Au@MOF-79 toward the quercetin detection in seawater. Discover Electrochemistry, 3(1), Article 44. https://doi.org/10.1007/s44373-026-00117-2

Image Credits: AI Generated

DOI: 10.1007/s44373-026-00117-2

Keywords: electrochemical sensor, quercetin, MOF-79, gold nanoparticles, metal-organic framework, seawater, differential pulse voltammetry, electrocatalysis, environmental monitoring, pharmaceutical residues, hydrothermal synthesis, glassy carbon electrode