Figure 1. a) Scheme of microwave-assisted synthesis of CN photoelectrode. b) Thermal image for temperature distributions of CN MW580 on FTO by microwave heating. c) FTIR and d) LDI-TOF mass spectra of CN MWT (T = 400 to 580 °C).

Electrochemically generated chemiluminescence (ECL) has attracted significant interest over decades, ranging from fundamental studies of highly efficient electron-tophoton interconversion to practical bioassays. Nonetheless, the ECL efficiency of most emitters is low, which greatly hampers further development. Herein, we report a highly robust carbon nitride film with an unusually boosted ECL efficiency (2256 times higher than that of the reference Ru(bpy)3Cl2/K2S2O8. Double inoculation, which provided the primary interaction of carbon nitride with the substrate and the succedent growth, played a crucial role in preparation. The improved ECL efficiency was ascribed to few pinholes suppressing futile co-reagent reduction, maintenance of more orbitdelocalized heptazine subunit improving ECL kinetics, and transparency avoiding selfabsorption. As a result of the exceptionally high ECL efficiency, an ultrasensitive visual DNA biosensor by the naked eye was further successfully developed.

The development of electrochemiluminescence (ECL) emitters of different colors with high ECL efficiency (ΦECL) is appealing yet challenging for ultrasensitive multiplexed bioassays. Herein, we report the synthesis of highly efficient polymeric carbon nitride (CN) films with fine-tuned ECL emission from blue to green (410, 450, 470, and 525 nm) using the precursor crystallization method. More importantly, naked eye-observable and significantly enhanced ECL emission was achieved, and the cathodic ΦECL values were ca. 112, 394, 353, and 251 times those of the aqueous Ru­(bpy)3Cl2/K2S2O8 reference. Mechanism studies showed that the density of surface-trapped electrons, the associated nonradiative decay pathways, and electron–hole recombination kinetics were crucial factors for the high ΦECL of CN. Based on high ΦECL and different colors of ECL emission, the wavelength-resolved multiplexing ECL biosensor was constructed to simultaneously detect miRNA-21 and miRNA-141 with superior low detection limits of 0.13 fM and 25.17 aM, respectively. This work provides a facile method to synthesize wavelength-resolved ECL emitters based on metal-free CN polymers with high ΦECL for multiplexed bioassays.