Abstract
Nanopore-based biosensors have attracted attention as highly sensitive microscopes for detecting single molecules in aqueous solutions. However, the ionic current noise through a nanopore degrades the measurement accuracy. In this study, the magnitude of the low-frequency noise in the ionic current through a silicon nitride nanopore was found to change depending on the metal ion species in the aqueous solution. The order of the low-frequency noise magnitudes of the alkali metal ionic current was consistent with the order of the adsorption affinities of the metal ions for the silanol surface of the nanopore (Li <Na <K < Rb <Cs). For the more adsorptive alkaline earth metal ions (Mg and Ca), the low-frequency noise magnitudes were as low as those for Li ions. This tendency, i.e., metal ions having a very high or low adsorption affinity causing a reduction in low-frequency noise, suggests that the low-frequency noise was induced by the exchange reactions between protons and metal ions occurring on the silanol surface. In addition, the low-frequency noise in the ionic current remained low even after replacing the CaCl2 aqueous solution with a CsCl aqueous solution, indicating that Ca ions continued being adsorbed onto silanol groups even after removing the aqueous solution.
Highlights
Nanopore-based biosensors have attracted attention as highly sensitive microscopes for detecting single molecules in aqueous solutions
We found that the magnitude of the low-frequency noise in the ionic current through a nanopore changes /I2) was depending on the metal ion attributed to the adsorption species affinity in of the the aqueous solution
This is likely attributed to the enhancement in the exchange reaction between alkali metal ions and protons on the silanol surface of the nanopore
Summary
Nanopore-based biosensors have attracted attention as highly sensitive microscopes for detecting single molecules in aqueous solutions. The magnitude of the low-frequency noise in the ionic current through a silicon nitride nanopore was found to change depending on the metal ion species in the aqueous solution. The fluctuation in the ionic current through a nanopore originates from the following proton exchange reaction of the silanol (SiOH) groups[10]: SiO− +H+ SiO–H (1) This model predicts the pH dependence of the magnitude of the low-frequency noise[10]. We report an unexplored phenomenon whereby the low-frequency noise depends on the protons and on the metal ions (M+) in the solution, and this phenomenon cannot be explained by the proton fluctuation model
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