Figure 1. Scheme of nanopore-based single-molecule detection.

Voltage is applied using a pair of electrodes that provide an electrochemical circuit with the solution. The voltage (V) drives ions through the nanopore, which results in a measurable steady-state ‘open pore’ current (I). When a biomolecule in the top chamber diffuses into the pore, it blocks the ion pathway of the pore and the flux of ions is reduced, resulting in a negative spike that signals each molecule. Under the same applied voltage conditions, the rate of spike events has a linear correspondence with the macromolecular concentration. Therefore, miRNA duplexes can be counted and quantified by measuring their rate of passage through the pore. A real current trace is shown for illustration, as well as three parameters that are typically quantified in nanopore experiments. δI and t_d are used to identify the target blocks, while δt is used to calculate the rate of block occurrence, which is the inverted value of δt.

δI: Current block amplitude; δt: Interval between adjacent blocks; I: Current; t_d: Block duration; V: Voltage.