A dual turn-on–off fluorometric probe based on silver sulfide quantum dots for simultaneous assay of creatinine and calcium in complex matrices

Biomarkers like creatinine (CRE) and calcium ions (Ca²⁺) are vital for detecting several disorders like chronic kidney disease (CKD). Hypocalcemia affects bones, the heart, and other organs. The main limitations of traditional CRE/Ca²⁺ monitoring protocols in biological samples are their invasiveness and time consumption. The present work aims at developing a rapid, highly sensitive, and selective fluorescent probe for the simultaneous determination of CRE and Ca²⁺ in pure and complex samples. The probe is based on Ag₂S quantum dots (QDs) modified with polyethyleneimine and imidazole dicarboxylic acid. The interaction of the analytes with the modified Ag₂S QDs causes a quenching in their fluorescence intensity at <i>λ</i> _em of 485 nm (<i>λ</i> _ex: 240 nm) and 605 nm (<i>λ</i> _ex: 300 nm) for CRE and Ca²⁺, respectively. The system was characterized with high-resolution transmission electron microscopy, FTIR spectroscopy, dynamic light scattering, and zeta potential measurement. The influence of solution pH, incubation time, amount of modified QDs, and interfering species was investigated. The probe demonstrated a limit of detection of 0.48 and 0.45 μg mL^-1, a linear range of 0.7-9.0 and 0.5-4.0 μg mL^-1, and recovery values in the ranges of 93.8-98.4 and 94.2-103.6% for CRE and Ca²⁺, respectively. The developed system can help in the early diagnosis of several renal disorders.