Solid-state nanopores are widely acknowledged as tools with which to study local structure in biological molecules. Individual molecules are forced through a nanopore, causing a characteristic change in an ionic current that depends on the molecules' local diameter and charge distribution. Here, the translocation measurements of long (˜5-30 kilobases) single-stranded poly(U) and poly(A) molecules through nanopores ranging from 1.5 to 8 nm in diameter are presented. Individual molecules are found to be able to cause multiple levels of conductance blockade upon traversing the pore. By analyzing these conductance blockades and their relative incidence as a function of nanopore diameter, it is concluded that the smallest conductance blockades likely correspond to molecules that translocate through the pore in predominantly head-to-tail fashion. The larger conductance blockades are likely caused by molecules that arrive at the nanopore entrance with many strands simultaneously. These measurements constitute the first demonstration that single-stranded RNA can be captured in solid-state nanopores that are smaller than the diameter of double-stranded RNA. These results further the understanding of the conductance blockades caused by nucleic acids in solid-state nanopores, relevant for future applications, such as the direct determination of RNA secondary structure.
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