Rapid Colorimetric Screening of Elevated Phosphate in Urine: A Charge-Transfer Interaction

A charge-transfer (CT) interaction between 1,3,5-trinitro-2,4-dimethylbenzene (TNX) and anionic phosphate is evaluated, yielding a high band electronic transfer interaction that can be observed as a distinct color change when phosphate is present in solution. The induced interaction was studied using ¹H NMR, UV–visible, and Fourier transform infrared spectroscopies. The stoichiometric determination of the interaction was divined by means of continuous variation, applying the Schaeppi–Treadwell method to calculate the binding constant (k). Furthermore, the effect of the polarity of solvents toward the generation of the CT interaction was examined, with multiple solvents considered. Complex deconstruction studies were undertaken, examining the effects of water on complex destruction and understanding the volumes needed to hinder the CT interaction potency. Specificity and selectivity of the CT interaction were also studied against other biologically relevant species (CH₃CH₂OH, Na⁺, K⁺, Ca2⁺, Cl^–, HCO₃–, F^–, CH₃COO^–, and SO₄^2–), assessing the capabilities of the assay to differentiate anionic species and counter cations that could act as interferences. The role of TNX concentration in CT formation was also analyzed, aiming to optimize the phosphate-sensing assay and improve its limit of detection. The sensing platform was subsequently used to study phosphate concentrations in urine samples to further understand its potential application in biomedical research. To validate the developed technique, urine samples were analyzed for their phosphate content with both the developed sensor and a validated vanadate–molybdate reagent. The results indicate that the sensing method is capable of accurately reporting elevated phosphate levels in urine samples in a rapid and sensitive manner, illustrating that the colorimetric test could be used as a prescreening test for conditions such as hyperphosphatemia or chronic kidney disease.