•The biosensor had wide linear range (10aM∼100pM) and low detection limit (1.35aM).•The biosensor could realize accurate detection of miRNA-377 in human serum samples.•The biosensor amplified electrochemical signal by embedding MB into G-quadruplexes.•MXene-Au had high electrical conductivity and massive active sites for DNA capture. MicroRNAs (miRNAs) are considered as potential biomarkers for early diagnosis and prognostic assessment of diabetic nephropathy. In this work, an electrochemical biosensor for ultrasensitive detection of miRNA-377 was constructed based on MXene-Au nanocomposites and G-quadruplex nano-amplification strategy. As a promising nanocarrier, taking advantage of leveraging synergistic effects between Au nanoparticles (AuNPs) and MXene nanosheet, MXene-Au nanocomposites exhibited excellent electronic conductivity and provided massive active sites for DNA capture probe immobilization by Au-S bonds. AuNPs modified with Guanine-rich sequence DNA detection probes were designed as signal amplification nano-labels. Specifically, by inducing the transition of Guanine-rich detection probes to G-quadruplex, the strong affinity interaction between methylene blue and G-quadruplex could not only reflect trace concentration information of miRNA-377, but also lead to the further enhancement of electrochemical signal (2.7-fold). As a result, this newly designed biosensor exhibited superior sensing performance with a wide linear range from 10 aM to 100 pM and the limit of detection was as low as 1.35 aM. Compared with biosensors based on other nanocomposites and reported miRNA-377 biosensors, the proposed sensing platform did not require any thermal cycling or reverse transcription process, meeting the miRNA sensing requirements of convenient, sensitive, specific and stable. Furthermore, the as-constructed biosensor also displayed good selectivity, which was applied to accurate detection of miRNA-377 in human serum samples with satisfactory sensitivity, suggesting the biosensor system has promising applications in biological researches and early clinical diagnosis for diabetic nephropathy. Display omitted .