The research team, led by Sadia Batool and Afzal Shah of Quaid-i-Azam University in Islamabad together with colleagues at the Yongjiang Laboratory and the University of Nottingham Ningbo, built their detection platform on a glassy carbon electrode modified with two complementary nanomaterials: carboxyl-functionalized multiwalled carbon nanotubes and iron-doped zinc oxide nanoparticles. Carbon nanotubes are celebrated in electrochemistry for their exceptional conductivity, high specific surface area and strong affinity for aromatic pollutants, and the introduction of carboxyl groups improves their dispersibility while providing anchoring points for catalytic particles. Iron doping of zinc oxide, meanwhile, modifies the electronic structure of the semiconductor, introduces oxygen vacancies and multiplies the active sites available for charge transfer. Together, the two components create a synergistic scaffold in which the nanotubes conduct electrons rapidly and engage in pi-pi stacking with the aromatic rings of the dye, while the doped oxide particles contribute additional electroactive sites and improved charge carrier mobility.

Before any electrochemistry took place, the team subjected both materials to a battery of characterization techniques, including X-ray diffraction, X-ray photoelectron spectroscopy, field-emission scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, Fourier-transform infrared spectroscopy and ultraviolet-visible spectroscopy. These analyses confirmed that the Fe-doped zinc oxide adopted a hexagonal wurtzite structure with an average crystallite size of roughly 14 nanometers, an optical band gap of 2.99 electronvolts and no impurity phases, indicating that iron ions had been successfully incorporated into the zinc oxide lattice. The layered double hydroxide adsorbent, a nickel-copper variant with carbonate anions tucked between its brucite-like layers, displayed a rhombohedral crystal structure and a hierarchical morphology of ultrathin, interconnected nanosheets. High-resolution transmission electron microscopy resolved lattice spacings of 0.258 nanometers for the doped oxide and 0.242 nanometers for the hydroxide, matching the expected crystal planes and confirming high crystallinity in both materials.

Electrochemical testing began with cyclic voltammetry and electrochemical impedance spectroscopy using a ferricyanide redox probe. A bare glassy carbon electrode showed sluggish electron kinetics, with a large peak-to-peak separation of 110 millivolts. Adding the iron-doped zinc oxide narrowed that separation to 97 millivolts and raised the peak current, while the full hybrid of functionalized nanotubes and doped oxide delivered the highest current and the smallest separation, just 82 millivolts. When the electrodes were challenged with 50 micromolar Eosin Yellow in phosphate buffer, the hybrid electrode produced an oxidation peak between 0.85 and 0.95 volts versus the silver-silver chloride reference that was roughly 1.5 to 2 times larger than that of the oxide-only electrode. The researchers attribute this amplification to the large pi-conjugated system of the nanotubes, which stacks against the dye’s aromatic skeleton, combined with the expanded surface area and conductivity contributed by the doped semiconductor.

Optimization experiments revealed how finely tuned the sensing conditions needed to be. The anodic current peaked at pH 6, where proton-coupled electron transfer proceeds most favorably, and declined under both strongly acidic and alkaline conditions. Deposition time mattered as well: a brief five-second preconcentration step gave the maximum response, while longer deposition saturated the electrode surface and hindered mass transport. A deposition potential of 0.2 volts proved optimal, strengthening the electrostatic attraction between the anionic dye and the positively biased sensor surface. Scan rate studies showed that the oxidation of Eosin Yellow is irreversible and governed by a mixed adsorption-diffusion mechanism, with a log-log slope of 0.775 falling between the theoretical values for purely diffusive and purely adsorptive control, a behavior typical of bulky dye molecules that preconcentrate on the electrode yet still depend partly on mass transport.

The analytical figures of merit are striking. Under the optimized conditions, square wave voltammetry delivered a linear response across concentrations spanning four orders of magnitude, from 0.01 to 30 micromolar, with a limit of detection of 0.314 nanomolar and a limit of quantification of 1.045 nanomolar. Those detection limits place the sensor among the most sensitive electrochemical platforms reported for this dye, surpassing earlier carbon-nanopowder electrodes and rivaling nanosensor approaches based on phytosynthesized cerium oxide. The combination of simple drop-cast fabrication, inexpensive instrumentation and portability gives the platform a practical edge over mass spectrometry, high-performance liquid chromatography and fluorescence analysis, which demand costly equipment, skilled operators and extensive sample pretreatment, and are poorly suited to on-site monitoring of industrial effluents.

Detection alone, however, does not clean water. For remediation, the team turned to a nickel-copper layered double hydroxide synthesized by co-precipitation, a class of materials whose positively charged layers and high anion exchange capacity make them natural traps for anionic dyes. Batch experiments showed that the dye’s characteristic absorption peak at 517 nanometers faded steadily with contact time, reaching equilibrium within 55 to 60 minutes without any peak shift, evidence that adsorption rather than chemical degradation was doing the work. Increasing the adsorbent dose from 2 to 6 milligrams lifted removal efficiency from 70 to 92 percent, beyond which additional dosage brought little benefit as active sites became saturated or aggregated. Maximum adsorption, around 95 percent, occurred at pH 6, and pH-drift measurements placed the point of zero charge of the material near pH 6.9, confirming that electrostatic attraction between the positively charged hydroxide surface and the doubly negative dye anions drives uptake near neutral conditions.

Kinetic and thermodynamic analyses painted a consistent mechanistic picture. The adsorption data followed pseudo-second-order kinetics with a rate constant of 0.012 grams per milligram per minute, indicating chemisorption as the rate-controlling step, while intra-particle diffusion plots that failed to pass through the origin revealed a three-stage process: rapid surface adsorption, followed by diffusion of dye molecules into pores, and finally equilibration as all sites filled. The equilibrium data fit the Langmuir isotherm with a correlation coefficient of 0.99, pointing to monolayer adsorption on a homogeneous surface and a maximum capacity of 71.4 milligrams per gram, several times higher than manganese dioxide-graphene composites and comfortably exceeding zeolite Y. Thermodynamic parameters confirmed the process is spontaneous and endothermic, with negative free energy changes between minus 81.39 and minus 86.86 kilojoules per mole across 298 to 313 kelvin and a positive enthalpy of 79.23 kilojoules per mole. The proposed mechanism combines surface electrostatic adsorption with anion exchange, in which dye anions swap into the interlayer galleries and displace carbonate.

The broader significance of the work lies in its integration. Most studies address either the identification or the elimination of organic dyes; this one couples an ultrasensitive electrochemical sensor with a cheap, environmentally benign adsorbent in a single strategy, enabling real-time trace monitoring alongside bulk remediation. Such a pairing could allow treatment plants to detect fugitive dye releases the moment they occur and then deploy the layered double hydroxide to strip the contaminant before discharge. The authors caution that future work must extend the platform to real water matrices, demonstrate long-term reusability of both the sensor and the adsorbent, and integrate the two functions into a sequential flow-based system. Even so, the study demonstrates that thoughtfully engineered nanocomposites, a conductive carbon-oxide hybrid on one electrode and a bimetallic hydroxide in a beaker, can move dye pollution management from passive monitoring toward active, responsive control of water quality.

Subject of Research: Electrochemical detection and adsorptive removal of the xanthene dye Eosin Yellow from wastewater using nanomaterials

Article Title: Electrochemical detection of eosin yellow dye and its adsorptive removal from wastewater

Article References: Batool, S., Khan, K. A., Deng, B., Shah, S. M., & Shah, A. (2026). Electrochemical detection of eosin yellow dye and its adsorptive removal from wastewater. Discover Electrochemistry, 3(1), Article 79. https://doi.org/10.1007/s44373-026-00169-4

Image Credits: AI Generated

DOI: 10.1007/s44373-026-00169-4

Keywords: Eosin Yellow, electrochemical sensor, carbon nanotubes, Fe-doped ZnO, layered double hydroxide, adsorption, wastewater treatment, dye pollution, water quality, nanocomposite, Langmuir isotherm, chemisorption