Azo Dye Meets Carbon Paste: New Electrode Detects Dopamine, Uric Acid and Paracetamol at Once

Chemists in India have turned a vividly colored molecule best known for its place in the dye world into the working heart of an electrochemical sensor capable of spotting three medically important compounds at the same time. In a study published in Discover Electrochemistry, researchers at Davangere University synthesized a novel quinoline-based azo dye and used it to modify an ordinary carbon paste electrode, producing a sensor that cleanly separates the oxidation signals of dopamine, uric acid, and paracetamol in a single measurement. The work matters because these three molecules routinely coexist in biological fluids, and distinguishing them electrochemically has long been a challenge for unmodified electrodes, whose overlapping peaks and sluggish electron transfer blur the picture.

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