Intramolecular D-A structure achieved excellent electrochemiluminescence of g-C3N4 originated from doping thiophene ring.

• 3D Th ing -g-C 3 N 4 had extended π-conjugated system and accelerated electron transfer. • 3D Th ing -g-C 3 N 4 exhibited excellent cathodic ECL performance under lower potential. • The constructed ECL system used dissolved O 2 as the coreactant. • Multiple HCR based cascade amplification strategy was constructed to detecte miRNA. The low electrochemiluminescence (ECL) efficiency and poor ECL stability of graphitic carbon nitride (g-C 3 N 4) under high excitation potential due to obvious electrode passivation seriously restrict its further wider application. Here, construction of intramolecular donor–acceptor (D-A) structure had proposed to well resolve above problems, which had been rarely reported. As a research model, thiophene ring (electron donor, D) with strong electron donating property was doped into the heptazine (electron acceptor, A) of 3D g-C 3 N 4. An intramolecular D-A structure of 3D Th ing -g-C 3 N 4 was obtained, which had extended π-conjugated system and high-speed intramolecular electron transfer. As a result, 3D Th ing -g-C 3 N 4 with intramolecular D-A structure exhibiting excellent cathodic ECL performance with endogenous dissolved O 2 as coreactant under lower potential. Based on the above ECL system, an ultrasensitive biosensor was assembled for microRNA-221 detection with multiple HCR based cascade amplification strategy. The target-like DNA obtained through the initial HCR process induced by target could trigger multiple HCR processes again to realize the effective amplification of the target. The constructed biosensor had excellent detection performance for microRNA-221 and the detection limit was calculated as 5.25 aM. In conclusion, construction of intramolecular D-A structure was a novel and effective way to improve the ECL performance of g-C 3 N 4 , and the newly proposed multiple HCR based cascade amplification strategy paved provided new opportunities for the detection of cancer marker in early cancer diagnosis. [ABSTRACT FROM AUTHOR]