Charge Polarization-Enhanced Graphene Biosensors for the Attomole Detection of miRNA

Due to graphene's structural and electrical properties, electrical biosensors made of this 2D material have drawn tremendous attention in the field of biosensing, enabling label-free, amplification-free, highly sensitive, and selective detection of diverse biological targets. However, the detection of biomolecules with minimal size and charge remains challenging due to the Debye electrostatic screening effect. This study introduces a surface chemistry treatment that employs fullerene derivatives to enhance charge transfer to the graphene biosensor interface, overcoming this limitation. Specifically, (1,2-methanofullerene C60)-61-carboxylic acid (MFCA) is used as a linker molecule, replacing the traditional 1-pyrenebutanoic acid succinimidyl ester (PBASE). This modification facilitates the movement of electrons from biomarkers, such as microRNA (miRNA), across the Debye screening layer through a charge attraction effect. This approach achieves a detection limit (LoD) as low as 1 aM for hsa-mir-125b miRNA, a critical biomarker for Alzheimer's disease, and this is an improvement of 2-3 orders of magnitude over previous methods. The enhanced sensitivity is attributed to the efficient electron transfer from miRNA to the graphene surface, demonstrated by density functional theory (DFT) calculation and control experiment with the PBASE linker. Further, this method is also applied in the detection of another miR-34a with an ultralow LoD of 1 aM, showing its generalizability. This work enables the application of charge polarization-enhanced electrical biosensors in the early-stage diagnosis of various diseases with ultrahigh sensitivity.