Tunable carbon dot-polymer interfaces for mechanistic and analytical control of plasmonic creatinine sensing

Creatinine is an essential biomarker for evaluating kidney function, yet the correlation between interfacial chemistry and plasmonic signal behavior in its optical quantification remains insufficiently understood. In this work, hybrid polymer-carbon dot (CD) films were employed as tunable interfacial models to elucidate how polymer polarity influences molecular adsorption and plasmonic response in label-free creatinine sensing based on localized surface plasmon resonance (LSPR). Hydrophobic poly (methyl methacrylate) (PMMA) and hydrophilic polyaniline (PANI) were employed as contrasting matrices to elucidate how polymer polarity influences molecular adsorption and near-field response. FTIR revealed polymer-CD interactions through hydrogen-bonding and dipolar interactions, while FESEM and AFM confirmed compact, stratified surfaces for PMMA-CD and porous, fibrous morphologies for PANI-CD. The PMMA-CD film exhibited strong linearity and a low detection limit of 29 μM with a sensitivity of 0.14 nm μM^-1, whereas the PANI-CD film displayed a broader but less sensitive dynamic range (LOD = 99 μM, 0.041 nm μM^-1). The PMMA-CD layer also demonstrated excellent selectivity toward creatinine over urea, albumin, ascorbic acid, and ammonia. These findings establish a mechanistic and analytically controlled framework that links interfacial polarity, molecular adsorption, and plasmonic field confinement, providing a rational basis for the design of non-enzymatic optical sensors relevant to biomedical diagnostics and creatinine monitoring.