The molecule at the center of the study, 5-[(Z)-1,3-thiazol-2-yldiazenyl]quinolin-8-ol, abbreviated TDQ, belongs to the azo dye family, compounds defined by their signature nitrogen-nitrogen double bond, the azo group, which gives them their intense colors. Azo dyes have historically dominated textiles, leather, paints, and printing, but researchers have increasingly explored them in solar cells, optical systems, and even pharmacology, where azo-bearing heterocycles show antibacterial, antiviral, and antifungal activity. Quinoline, the second structural element of TDQ, brings its own electrochemical pedigree: the redox behavior of quinoline derivatives has attracted sustained research interest, making the pairing of a quinoline scaffold with a thiazole-linked azo bridge a logical candidate for electrode modification.

Synthesis of the dye followed a classic diazotization-coupling route performed under carefully controlled cold conditions. The team dissolved 2-aminothiazole in concentrated hydrochloric acid and rapidly chilled the mixture in an ice-salt bath to roughly 0 to 5 degrees Celsius before adding nitrosylsulfuric acid to generate the diazonium salt. After two hours of stirring at the same low temperature, the diazonium solution was combined with an alkaline solution of 8-hydroxyquinoline, with the pH held between 5 and 6 throughout the coupling step. Thin-layer chromatography tracked the reaction to completion, and the resulting dark red solid was filtered, washed, dried, and recrystallized from ethanol, delivering a 70 percent yield with a melting point of 95 to 96 degrees Celsius.

Structural confirmation relied on a battery of spectroscopic techniques. Infrared spectroscopy revealed the hydroxyl stretch of the quinoline group at 3382 inverse centimeters, aromatic carbon-hydrogen stretches at 2929, carbon-nitrogen and carbon-sulfur vibrations at 1602 and 778, and, critically, the azo linkage at 1454 inverse centimeters. Proton nuclear magnetic resonance in DMSO-d6 showed the expected hydroxyl singlet near 9.0 ppm alongside aromatic signals between 6.82 and 7.37 ppm. Ultraviolet-visible spectra recorded in DMSO, DMF, ethanol, and acetone displayed sharp absorptions at 414, 396, 386, and 409 nanometers respectively, with DMSO giving the strongest response, while mass spectrometry produced a molecular ion peak at m/z 257.3, consistent with the formula C12H8N4OS.

To build the sensor, the researchers ground one milligram of TDQ with graphite powder and silicon oil in a 70:30 ratio for half an hour, then packed the paste into a 3-millimeter electrode cavity. Electrochemical benchmarking against a bare carbon paste electrode used the standard potassium ferrocyanide redox probe in 1 M potassium chloride. The bare electrode showed weak redox peaks and large peak separations, hallmarks of slow electron transfer and a limited active surface. The TDQ-modified version, by contrast, delivered markedly higher peak currents at reduced overpotential, and calculations based on the Randles-Sevcik equation put the electroactive surface area at 0.04526 square centimeters, well above the 0.0267 square centimeters of the unmodified paste.

Impedance spectroscopy reinforced that story. Nyquist plots showed a semicircle at high frequencies whose diameter reflects charge transfer resistance: the bare electrode registered 234.82 ohms, while the TDQ-modified electrode dropped to 177.47 ohms, indicating faster interfacial electron transport and more efficient ion pathways. Bode plots added a capacitive dimension, with the modified electrode displaying more negative phase angles at low frequencies, a sign of stronger charge retention and better energy storage behavior. The authors note that this combination of lower resistance and enhanced capacitance makes the dye-modified surface attractive not only for sensing but potentially for energy storage applications as well.

With the platform validated, the team turned to the three target analytes in 0.2 M phosphate buffer. pH optimization across the range 5.8 to 7.4 showed peak currents rising to a maximum at pH 7.0, the physiological value, which was adopted for all subsequent experiments. Scan rate studies then probed the underlying mechanism: for dopamine, a log-log slope of 1.012 indicated an adsorption-controlled process, while uric acid’s slope of 0.370 pointed to diffusion control, and paracetamol’s slope of 0.474 likewise suggested diffusion-dominated behavior. Concentration studies from 10 to 100 micromolar produced calibration curves with correlation coefficients of 0.9979 for dopamine, 0.9982 for uric acid, and 0.98 for paracetamol.

The detection limits, calculated as three times the standard deviation divided by the calibration slope, came out at 6.86 micromolar for dopamine, 10.28 micromolar for uric acid, and 15.93 micromolar for paracetamol, with corresponding quantification limits of 21.15, 34.29, and 42.58 micromolar. The most striking result, however, came from simultaneous detection. In cyclic voltammetry, the modified electrode resolved three distinct oxidation peaks at 0.265 volts for dopamine, 0.554 volts for uric acid, and 0.875 volts for paracetamol, giving peak-to-peak separations of 0.289 and 0.321 volts. Differential pulse voltammetry sharpened the discrimination further, with separations of 0.227 and 0.362 volts, allowing all three compounds to be identified and quantified in a single run.

Practical durability and real-world performance rounded out the evaluation. Across five identically prepared electrodes, the relative standard deviation was just 0.85 percent, and the sensor retained 94 percent of its original dopamine response after twenty days of laboratory storage. When the team applied the standard addition method to a commercially available dopamine hydrochloride injection diluted into buffer, recoveries of spiked samples ranged from 97 to 102 percent, demonstrating that the sensor can deliver accurate measurements in a genuine pharmaceutical matrix rather than only in idealized solutions.

The broader significance lies in the simplicity and economy of the approach. Carbon paste electrodes are cheap, easy to prepare, and easy to refresh, and a milligram-scale dye modifier transforms their otherwise mediocre electrochemistry into a selective, stable sensing platform. Because dopamine imbalances are linked to Parkinson’s disease, Alzheimer’s disease, schizophrenia, and Huntington’s disease, uric acid levels flag kidney failure, pneumonia, and metabolic disorders, and paracetamol monitoring underpins safe use of one of the world’s most common analgesics, a single low-cost electrode that reads all three simultaneously could find a home in clinical labs, pharmaceutical quality control, and point-of-care diagnostics. The Davangere team’s work adds a quinoline-azo dye to the growing toolbox of molecular modifiers, showing that sometimes the fastest route to better biosensing runs through the chemistry of color.

Subject of Research: Electrochemical detection of dopamine, uric acid, and paracetamol using a quinoline-azo dye modified carbon paste electrode

Article Title: Electrochemical sensing of dopamine, uric acid, and paracetamol using a quinoline-azo dye modified carbon paste electrode

Article References: Paalaplara, K. H., Chinnagiri, K. T., Chinnagiri, R. T., & Thippandegowdru, R. M. (2026). Electrochemical sensing of dopamine, uric acid, and paracetamol using a quinoline-azo dye modified carbon paste electrode. Discover Electrochemistry, 3(1), Article 42. https://doi.org/10.1007/s44373-026-00127-0

Image Credits: AI Generated

DOI: 10.1007/s44373-026-00127-0

Keywords: electrochemical sensor, azo dye, quinoline, carbon paste electrode, dopamine, uric acid, paracetamol, cyclic voltammetry, differential pulse voltammetry, electrocatalysis, detection limit, pharmaceutical analysis

Writing in the journal Discover Electrochemistry, Annapoorna S. Bhat and Rajesh N. Hegde describe the construction of a carbon paste electrode modified with a zinc oxide–cerium dioxide nanocomposite, a material combination that dramatically boosts the electrode’s sensitivity to the dye. The work addresses a persistent analytical challenge: tartrazine appears in real foods alongside sugars, salts, vitamins and other colorants, and regulators such as the European Union’s Scientific Committee for Food and the Joint FAO/WHO Expert Committee on Food Additives set the acceptable daily intake at 7.5 milligrams per kilogram of body weight. Detecting whether products stay within that ceiling requires instruments capable of measuring vanishingly small concentrations without being fooled by the surrounding chemical clutter.

The team’s approach began with the synthesis of the nanocomposite itself. Using a straightforward co-precipitation method, the researchers combined solutions of cerium nitrate and zinc nitrate with ammonium oxalate, stirring the mixture for eight hours at room temperature before filtering, washing, drying and calcining the solid at 400 degrees Celsius for five hours. The result was an agglomerated structure of spherical particles and block-like shapes, which scanning electron microscopy and energy-dispersive X-ray analysis confirmed contained both zinc and cerium phases. Ultraviolet-visible spectroscopy added further confirmation, revealing characteristic absorption bands at 340 nanometers for the composite and 230 nanometers for cerium-oxygen charge-transfer transitions, while Fourier-transform infrared spectroscopy identified the distinctive ZnO stretching vibration at 482 inverse centimeters and the Ce-O vibration at 514 inverse centimeters.

Why pair these two particular oxides? The answer lies in their complementary electronic personalities. Zinc oxide is a wide-bandgap semiconductor prized for its high electron-transfer capability and thermal stability, and it furnishes an abundance of surface active sites where tartrazine molecules can adsorb. Cerium dioxide, meanwhile, is celebrated for its redox behavior, driven by surface defects and oxygen vacancies that shuttle charge with unusual efficiency. Together, the authors report, the composite synergistically increases surface area, accelerates electron transport and stabilizes the electrode, turning an ordinary carbon paste into a far more capable sensing platform.

The evidence for that enhancement is quantitatively clear. By cycling the bare and modified electrodes in a potassium ferricyanide solution and applying the Randles–Sevcik equation, the team calculated that the effective surface area grew from 0.016 square centimeters for the unmodified paste to 0.024 square centimeters after modification, a 50 percent increase. When tartrazine was introduced, the contrast was even more striking. In a pH 5.5 phosphate buffer, the unmodified electrode oxidized the dye at 0.948 volts with a weak peak current of 2.230 microamperes, whereas the nanocomposite-modified electrode delivered a current of 3.228 microamperes, roughly 1.44 times larger, indicating markedly faster electron transfer at the composite surface. The voltammograms showed only an oxidation peak and no reduction peak on the reverse scan, marking the process as electrochemically irreversible.

Optimizing the sensor required the researchers to fine-tune several experimental variables. The pH of the supporting electrolyte proved critical: scanning across buffers from pH 4.5 to 6.5, they found the strongest oxidation signal at pH 5.5, with the peak potential shifting linearly to less positive values as pH rose. The slope of that relationship, 34.8 millivolts per pH unit, pointed to the direct participation of a single proton in the oxidation mechanism. The proportion of nanocomposite in the paste also mattered. Testing loadings from 2.5 to 12.5 percent, the team observed a steady rise in signal up to 10 percent, where the greatest number of electroactive sites became available. Beyond that point, however, the particles began to agglomerate, obstructing electron transfer and dragging the current back down, so the 10 percent formulation was selected for all subsequent measurements.

Scan-rate studies fleshed out the kinetics of the process. As the scan rate climbed from 0.01 to 0.15 volts per second, the oxidation current grew in direct proportion to the rate itself, a signature of adsorption-controlled electron transfer. Applying Laviron’s analysis to the relationship between peak potential and the logarithm of scan rate, the researchers determined that approximately one electron, calculated as 1.32, participates in the oxidation of tartrazine, a finding consistent with a proton-coupled single-electron mechanism on the composite surface.

The analytical performance of the optimized electrode is the heart of the study. Using square-wave voltammetry, the team established a linear relationship between peak current and tartrazine concentration spanning 0.15 to 5 micromolar in the pH 5.5 buffer. From four independent calibration curves, with a standard deviation of 0.0106 and a slope of 0.3679, they calculated a limit of detection of 0.086 micromolar and a limit of quantification of 0.289 micromolar, figures that compare favorably with modified electrodes reported elsewhere in the literature. Selectivity was tested against common food-borne interferents, including sodium, potassium and calcium ions and ascorbic acid. The metal ions left the response essentially untouched, with recoveries between 3.7 and 4.5 percent deviation and relative standard deviations of 2 to 7 percent, while only elevated concentrations of ascorbic acid nudged the peak current slightly.

Crucially, the sensor was not confined to buffer solutions. The researchers applied it to two retail products purchased from local grocery shops in India, an orange-flavored instant drink mix and a custard powder, neither of which declares its tartrazine content on the label. Using the standard-addition method, they quantified 0.71 milligrams per liter of tartrazine in the drink mix and 0.82 milligrams per liter in the custard powder, with recoveries ranging from 91 to 110 percent and relative standard deviations between 1.8 and 8.7 percent across four measurements. Interestingly, an additional oxidation peak between 0.6 and 0.65 volts revealed the presence of Sunset Yellow, a second synthetic dye, in both samples, in agreement with the products’ ingredient declarations and demonstrating the electrode’s ability to flag multiple colorants in a single sweep.

The sensor also proved durable. Five measurements taken with freshly renewed electrode surfaces within a single day yielded a relative standard deviation of just 2.49 percent for the peak current, and over five consecutive days of testing the electrode retained 97.2 percent of its initial response, indicating robust storage stability. Taken together, the results sketch a practical vision for food safety monitoring: a simple, low-cost electrode that can be prepared from graphite powder, paraffin oil and a modest quantity of nanocomposite, then deployed with a portable electrochemical analyzer to check dye levels in minutes rather than hours. The authors conclude that the method provides a rapid and efficient route to tartrazine determination, one that could complement or even replace chromatographic techniques such as high-performance liquid chromatography in quality-control laboratories where affordability and speed matter as much as precision.

Subject of Research: Voltammetric detection of the synthetic food dye tartrazine using a ZnO–CeO2 nanocomposite-modified carbon paste electrode

Article Title: Development of ZnO–CeO2 modified carbon paste electrode for the voltammetric detection of tartrazine

Article References: Development of ZnO–CeO2 modified carbon paste electrode for the voltammetric detection of tartrazine. (n.d.). https://doi.org/10.1007/s44373-026-00155-w

Image Credits: AI Generated

DOI: 10.1007/s44373-026-00155-w

Keywords: tartrazine, food dye, ZnO–CeO2 nanocomposite, carbon paste electrode, voltammetry, electrochemical sensor, food safety, square-wave voltammetry, E102, nanomaterials, limit of detection, commercial food samples