Sol–gel molecularly imprinted sensor selectively detects catechol in water

Chemists have built a catechol sensor that starts with a simple recipe: a catechol molecule dissolved in ethanol, a silicon-based crosslinker, a thiol-bearing silane monomer, and a drop of hydrochloric acid. Stirred for six hours at room temperature, the mixture forms a porous sol–gel polymer that is grown directly on a gold electrode, with catechol molecules trapped inside the developing silicate network. When the template molecules are later washed out with nothing more than distilled water, they leave behind cavities whose shape and chemical environment match catechol alone.

The central problem the device addresses is selectivity. Catechol is a toxic phenolic compound common in industrial wastewater, and it coexists in environmental samples with structurally near-identical isomers such as hydroquinone and resorcinol. These isomers share close stereochemical structures and redox potentials that sit near one another, which makes direct electrochemical determination on an unmodified electrode nearly impossible. Conventional analytical routes, including chromatography, fluorescence, spectrophotometry, and chemiluminescence, can measure catechol but often demand expensive instruments, lengthy procedures, or separation steps that add complexity and reagent cost. Electrochemical methods are attractive for their simplicity, sensitivity, and speed, but they need a surface that can tell catechol apart from its chemical siblings.

Molecularly imprinted polymers provide that discrimination by casting recognition cavities around a template molecule, but the team behind the new sensor argues that conventional free radical polymerization has drawbacks: complex procedures, limited stability, costly initiators, and the need for tightly controlled conditions such as a nitrogen atmosphere, elevated temperature, and long reaction times. Fluorescence-based imprinted sensors, meanwhile, can be thrown off by temperature, pH, light intensity, and sample turbidity, which undermines accuracy in colored or contaminated water. The sol–gel route avoids these issues by hydrolyzing and condensing silane precursors under mild conditions, producing a stable, porous silicate matrix with good mechanical and thermal properties.

The key design choice was the functional monomer. The researchers used 3-mercaptopropyltrimethoxysilane, whose thiol group bonds strongly to gold. As the sol–gel polymerization proceeds on the electrode, these thiol groups anchor the growing film directly to the gold surface through gold–sulfur interactions, forming a self-assembled layer that needs no additional binder. This matters because binders such as Nafion, commonly used to fix polymer films to electrodes, can impede electron transfer and degrade sensor performance. The crosslinker, tetraethyl orthosilicate, builds the silicate framework around the embedded catechol templates, and the whole fabrication proceeds at room temperature without specialized equipment.

Template removal was handled with unusual restraint. Instead of the methanol, acetone, or acetic acid washes typical of imprinted polymer protocols, the modified electrode was simply immersed in distilled water. Cyclic voltammetry in blank phosphate buffer tracked the catechol oxidation peak after each wash, and the peak current fell steadily over four 15-minute elution steps in 30-milliliter volumes until it reached a stable minimum, indicating near-complete template extraction. Fourier-transform infrared spectroscopy corroborated the electrochemical evidence: catechol-associated absorption bands near 1511, 1407, 1347, and 1309 wavenumbers diminished or vanished after elution, while the silicate band near 1065 wavenumbers stayed essentially unchanged, confirming that the polymer matrix itself remained intact on the electrode.

Characterization supported the intended architecture. Scanning electron microscopy showed a relatively uniform, coherent polymer coating across the gold surface, and energy-dispersive X-ray spectroscopy detected silicon, oxygen, sulfur, and carbon, consistent with a layer built from the two silane precursors. The authors are careful to note that EDX identifies elements but cannot directly confirm a specific gold–sulfur bond; that interaction is inferred from the known affinity of thiols for gold and the design of the modification strategy. Electrochemical tests drove the selectivity point home. A bare gold electrode responded nearly identically to catechol and hydroquinone, and raising the concentration of either isomer raised its current in parallel. The imprinted electrode, by contrast, gave a clearly higher response to catechol, and increasing hydroquinone concentration had no notable effect on its signal. Strikingly, the imprinted layer showed no response at all to potassium ferricyanide, a reversible redox probe that the bare electrode handled with its textbook behavior, suggesting the polymer film blocks access to species that do not fit its cavities.

Optimization experiments pinned down the fabrication conditions. Basic catalysis with ammonia failed outright, producing turbid precipitates and flaky, non-adhesive deposits, partly because catechol oxidizes rapidly under alkaline conditions and loses the structure it needs to act as a template. Among acid catalysts, 0.1 molar hydrochloric acid gave the best balance: weaker acid left the network incompletely formed, while 1 molar acid accelerated hydrolysis into a heterogeneous layer. Six hours for each of the sol preparation and film formation stages produced the most complete polymer and the sharpest discrimination, and the original precursor ratio of 0.2 milliliters of monomer to 0.6 milliliters of crosslinker outperformed a doubled formulation. Measurement conditions were optimized in parallel, with pH 7.5 phosphate buffer yielding the maximum peak current; the oxidation potential shifted linearly with pH at 0.0623 volts per pH unit, close to the theoretical Nernstian value and consistent with equal numbers of electrons and protons in the catechol oxidation. Scan rate analysis showed peak current scaling with the square root of scan rate, indicating that diffusion of catechol through the polymer network to the recognition sites is the rate-limiting step.

Quantitative performance was assessed by differential pulse voltammetry over catechol concentrations from 1 micromolar to 5 millimolar. Two linear ranges emerged, 10 to 100 micromolar and 100 to 3000 micromolar, with sensitivities of 0.00584 plus or minus 0.00017 and 0.00389 plus or minus 0.00032 microamperes per micromolar, respectively. The limit of detection was 4.2 micromolar by the three-sigma-over-slope method and 5 micromolar by experimental determination. Against previously reported imprinted catechol sensors, several of which reach lower detection limits, some as low as 0.004 micromolar, the new device does not win on raw sensitivity, but the authors argue it combines a relatively broad working range with a simple, low-cost fabrication route, molecular imprinting, water-only template removal, and no need for secondary nanomaterials or conductive binders.

The sensor also held up under repeated and extended use. After ten days stored in phosphate buffer, its response to 50 micromolar catechol had fallen by only 5 percent. Five independently fabricated electrodes measuring the same solution showed a relative standard deviation of 3.25 percent, and a single electrode reused across four sequential measurements, washed with distilled water between runs, gave a repeatability relative standard deviation of 1.12 percent, evidence that catechol adsorption and desorption in the cavities are reversible. Selectivity tests with the most structurally similar interferents, evaluated by differential pulse voltammetry at concentrations well above the analyte level, showed the sensor tolerates hydroquinone only up to three times the catechol concentration, phloroglucinol up to ten times, resorcinol up to fifteen times, phenol up to four hundred times, and potassium chloride up to a thousand times. In spiked tap water, recoveries were 95.3 percent for 50 micromolar catechol and 100.2 percent for 100 micromolar. The authors caution that broader applicability would require testing in more complex environmental or industrial wastewater matrices, but the demonstration suggests a practical, environmentally gentle route to imprinted electrochemical sensors for catechol and, by extension, similar interfering compounds in real samples.

Subject of Research: A sol–gel molecularly imprinted polymer electrochemical sensor on a gold electrode for selective catechol detection

Article Title: Design of a novel electrochemical sensor based on a sol–gel molecularly imprinted polymer for rapid and selective detection of catechol

Article References: Mashhadizadeh, M. H., & Khodami, P. (2026). Design of a novel electrochemical sensor based on a sol–gel molecularly imprinted polymer for rapid and selective detection of catechol. Results in Chemistry, 31, Article 104019. https://doi.org/10.1016/j.rechem.2026.104019

Image Credits: AI Generated

DOI: 10.1016/j.rechem.2026.104019

Keywords: catechol, electrochemical sensor, molecularly imprinted polymer, sol–gel, gold electrode, self-assembled monolayer, differential pulse voltammetry, selectivity, template removal, tap water analysis, phenolic compounds, limit of detection