Fig. 1: Conceptual demonstration of PNRSS.

a A PNRSS strand. A PNRSS strand is composed of functional modules as described. The reaction section, the most critical module, contains one or more reactive sites forming a fixed reactant. b The measurement configuration. During PNRSS, an MspA nanopore serves to dock a streptavidin-tethered PNRSS strand. The reaction section (dark yellow) is located precisely at the pore restriction for optimum performance. c The design of PNRSS to probe a single-molecule reaction between Ni²⁺ and a dual guanine reactant. Two neighbouring guanines on the PNRSS strand cooperatively bind a Ni²⁺ ion. d A continuous trace containing different states of a PNRSS measurement. Initially, the pore was unoccupied (i) and an open pore current was reported. A PNRSS strand was then captured by the pore, causing an immediate drop of the blockage level to I_p (ii). After the placement of Ni²⁺, reversible switching between I_b and I_p was observed, which respectively demonstrate the state when the PNRSS strand was not bound (iii) or bound (iv) with a Ni²⁺. The final concentration of Ni²⁺ was 0.8 mM. e Density scatter plot of \(\triangle I\) vs. t_off. The colour scale represents the local density around each point. The density scatter plot was generated using the ggplot2 package of R. The histogram of \(\triangle I\) with a Gaussian fitting is on the right of the scatter plot. Results of 13,435 events are included. f Concentration dependence. The reciprocal of inter-event interval (\(1/{\tau }_{{{{{\mathrm{on}}}}}}\)) and the reciprocal of dwell time (\(1/{\tau }_{{{{{\mathrm{off}}}}}}\)) is plotted against [Ni²⁺]. The measurement was carried out as described in ‘Methods’. The PNRSS strand 13G/14G (Supplementary Table 1) was applied. Nickel sulfate was added to trans at the desired final concentration. Error bars = SD (N = 3).