Multifaceted Interaction Studies between Carbon Dots and Proteins of Clinical Importance for Optical Sensing Signals

Carbon dots (CDs) are emerging as efficient optical probes. However, their application potential for clinical diagnosis has not been adequately explored. Herein, we examined the suitability of pyroglutamate CDs for detecting glucose, cholesterol, and alcohol in blood serum through their peroxidative activity in the respective enzyme-catalyzed reactions following fluorometric and colorimetric approaches. In buffer, the CD's fluorescence intensity (λ_ex 354nm) enhanced over 115% after interaction with the enzyme proteins due to different lifetime components on its surface. The enhancement was also linked to FRET with the proteins (λ_ex 274nm for TRP/TYR). The electrostatic interactions, as revealed from the zeta potential study, generated binding energy (ΔG, kcal/mol) in the range of -5.8 to -6.3 and greatly shifted the protein's secondary structure to β-strand contents. The CD's fluorescence in the blood serum medium was also enhanced where serum's particulate components contributed to the emission. All these subvert fluorescence emissions could be substantially cleaned for detection of peroxide generated in the enzymatic reaction by filtering the serum particulates and redox proteins prior to the addition of CDs to the reaction systems. The CD, however, could complement well in ABTS-based (absorbance at λ_max 414nm) colorimetric reaction in blood serum without introducing protein or particle separation steps for sensitive detection of peroxide. The limit of detection, dynamic range, and sensitivity discerned for peroxide in the glucose oxidase-catalyzed reaction system were 183 μM, 0.02-0.10 mM (R² = 0.98), and 0.2482 AU mM^-1, respectively. Overall, these findings will guide clinical application of the peroxidatic CDs to detect various analytes in blood serum following fluorometric- and colorimetric-based principles.