Ionic Strength Impact on miR-141 Detection by bioFET

Detection of microRNAs (miRNAs), which are small noncoding RNAs whose deregulation is strongly associated with various diseases, represents a progressively more important issue in clinical diagnostics. Among other approaches, field-effect transistor biosensors (bioFETs), characterized by rapid, accurate, and label-free detection, provide powerful platforms to develop miRNA-devoted detection devices. Biosensing via bioFETs primarily depends on the ability of complementary probes, immobilized on electrodes, to specifically capture miRNAs and transduce the binding into a detectable electrical signal. A critical factor for bioFET performance is the ionic strength. On the one hand, it reduces electrostatic repulsion, facilitating the association between negatively charged strands; on the other hand, it increases charge screening, which affects the detection capabilities. The balance between these opposite effects remains poorly understood, limiting the optimization of assay conditions. Here, we investigate the impact of ionic strength on detecting the clinically relevant miRNA 141 (miR-141) by acquiring the electrical signals over time at progressively higher concentrations of target using a custom-made extended gate (EG) bioFET. We found that both the hybridization affinity and the detected charge of the target are slightly affected by the ionic strength, which essentially modulates the organization of the capturing layer. Optimal analytical performance is achieved at 200-300 mM ionic strength, providing useful insights into reliable label-free miRNA detection by bioFETs under different conditions and supporting their potential future application in clinical settings.