Multiwalled Carbon Nanotube/Carbon Paste‐Modified Graphite Sheet Electrode Using Uric Acid as a Naturally Occurring Redox‐Active Molecule for Aptamer‐Mediated Signal Transduction

This study examines the use of uric acid (UA), a naturally occurring redox‐active molecule, as a reporter for aptamer‐mediated electrochemical signal transduction. Five graphite‐sheet electrode configurations were compared, with the multiwalled carbon nanotube/carbon paste‐modified graphite sheet electrode (MWCNT/CP/GSE) providing the strongest overall UA response. The measured linear UA range was 50–1000 μM, with a blank‐derived limit of detection of 0.26 μM and a calculated limit of quantification of 0.87 μM. Standard‐redox‐probe cyclic voltammetry (CV) and electrochemical impedance spectroscopy, together with a nanotube‐free CV‐treatment control, provided complementary evidence for changes caused by the modification sequence. Four consecutive UA measurements on the same MWCNT/CP/GSE electrode showed no progressive current loss, while the interdevice relative standard deviation was 9.2% across three independently fabricated electrodes. Using the T5 testosterone‐binding aptamer, the UA oxidation current decreased over the tested 100–700 nM testosterone interval. Additional cortisol, estradiol, and progesterone time‐course controls showed no comparable progressive attenuation under the tested conditions. At 400 nM testosterone, mean peak‐current retention was 78.83%–82.34% across 50, 100, and 200 μM UA backgrounds. These results support UA‐mediated signal transduction in a controlled model system and show normalized responses of similar magnitude across the tested UA backgrounds. The present study establishes a proof of concept in buffer; physiological testosterone sensitivity and quantitative performance in sweat remain to be established.