Beyond the solubility ceiling: hydrotropic media enable electrochemical sensing of uric acid at true physiological...

Electrochemical sensors for uric acid (UA), a biomarker of gout, renal dysfunction, and cardiovascular disease, face a fundamental but unacknowledged validity problem: UA's aqueous solubility ceiling of ∼0.42 mM means that sensors calibrated in pure aqueous media either operate below the clinically relevant concentration range, or measure responses from saturated solutions where dissolved and particulate UA coexist. Neither regime faithfully represents the physiological reality, where UA remains fully dissolved at concentrations well above this limit, stabilized by the complex biomolecular matrix of biological fluids. Here we demonstrate that hydrotropic media replicate this matrix solubilization effect, breaking the aqueous solubility ceiling and enabling electrochemical UA sensing at genuinely physiological concentrations. Two hydrotropes, nicotinamide and sodium salicylate, were systematically evaluated by UV-visible spectrophotometry across 0-25% (w/v) at pH 7.4. Nicotinamide progressively enhanced UA solubility from 0.48 to 2.65 mM at 25%, driven by cooperative π-π stacking between its aromatic ring and the xanthine scaffold of UA, and hydrogen bonding at the amide group. Sodium salicylate showed an initial peak solubility of 1.84 mM at 5%, followed by monotonic decline to 0.51 mM at 25%, attributable to salicylate self-association, ionic-strength-driven salting-out, and viscosity-restricted diffusion. These contrasting behaviors were independently corroborated by electrochemical impedance spectroscopy and differential pulse voltammetry on bare screen-printed gold electrodes. A molecularly imprinted polythiophene sensor (UA-IPt) deployed in the optimized nicotinamide medium achieved a DPV sensitivity of 88.99 µA mM^-1, a linear range of 0.05-0.3 mM, i.e., directly spanning the salivary physiological window, an LOD of 0.014 mM, and 97-104% recovery in spiked human saliva. Furthermore, sensor measurements over an extended concentration range (0.40-0.80 mM) exhibited excellent linearity (R² = 0.99), demonstrating reliable electrochemical quantification beyond the experimentally determined aqueous solubility limit of uric acid. Critically, scan-rate studies confirmed that hydrotropic media preserve the diffusion-limited electron-transfer mechanism at the electrode interface, ensuring that enhanced performance reflects genuine solubility gains rather than mechanistic artifacts. This work establishes hydrotropic media as a broadly applicable strategy for restoring measurement validity in electrochemical sensors for sparingly soluble biomarkers, with direct implications for reliable point-of-care diagnostics.