Polyethylenimine-activated localized excitation in carbon dots: ECL enhancement via aromatic fragment modulation for ultrasensitive microRNA biosensing

This work identified the localized excited state of reactive aromatic fragments rather than conventional surface state of carbon core as the electrochemiluminescence (ECL) origin of carbon dots (CDs), which could be suppressed by the stacking of the aromatic fragments. Concretely, the electrochemical reaction would specifically induce the localized excitation of the reaction site coupled aromatic fragment, whose emission quantum yield was significantly reduced as a result of extending the electron delocalization via π - π stacking with other aromatic fragments. According to this case, the ECL efficiency was expected to be enhanced by minimizing the stacking of aromatic fragments in CDs. Herein, we reported the ECL enhancement of CDs by employing polyethyleneimine (PEI) to encapsulate CDs, which not only increased the ECL efficiency by 22.7% by reducing π - π stacking between aromatic fragments in CDs, but also enriched large amount of CDs, resulting in the formation of novel PEI-CDs nanocomposites. By employing high-performance PEI-CDs nanocomposites as signal probes in combination with synergistic hybridization-mediated strand displacement circuits (SH-SDC) for signal amplification, an ultrasensitive ECL biosensing platform was constructed for microRNA-21 (miR-21) analysis. This biosensing method for miR-21 determination presented good sensitivity, featuring a linear response within the concentration range of 1.0 fmol/L to 1.0 nmol/L and achieving an exceptionally low detection limit of 0.31 fmol/L. Significantly, this study not only proposed an effective strategy to retain ECL efficiency in CDs enrichment, but also revealed the nature of ECL excited state, thereby providing guidance for the design of CDs with strong ECL performance.