Voltage-controlled metal binding on polyelectrolyte-functionalized nanopores

Abstract

Most of the research in the field of nanopore-based platforms is focused on monitoring ion currents and forces as individual molecules translocate through the nanopore. Molecular gating, however, can occur when target analytes interact with receptors appended to the nanopore surface. Here we show that a solid state nanopore functionalized with polyelectrolytes can reversibly bind metal ions, resulting in a reversible, real-time signal that is concentration dependent. Functionalization of the sensor is based on electrostatic interactions, requires no covalent bond formation, and can be monitored in real time. Furthermore, we demonstrate how the applied voltage can be employed to tune the binding properties of the sensor. The sensor has wide-ranging applications and, its simplest incarnation can be used to study binding thermodynamics using purely electrical measurements with no need for labeling.

Figure 1

Schematic representation of the electrochemical configuration and the reversible binding of cupric ions on the chitosan/PAA nanopipette.

Figure 2

Monitoring of the functionalization of a nanopipette with chitosan/PAA. Solutions: pH 7 (0.1M KCl, 10mM Tris HCl), pH 3 (0.1M KCl, 10mM phosphate/citrate). Nanopipette filled with 0.1M KCl, 10mM Tris HCl (pH 7).

Figure 3

Variation of the rectification coefficient vs. numbers of chitosan/PAA layers deposited at pH 3 and 7. Solution: 0.1M KCl, 10mM phosphate/citrate buffered to the desired pH. Nanopipette filled with 0.1M KCl, 10mM Tris HCl (pH 7).

Figure 4

pH response of a bare nanopipette (black squares) and chitosan/PAA nanopipette (red triangles). Measurements were carried out in a 0.1M KCl solution, buffered with 10 mM phosphate/citrate to the desired pH. Error bars were calculated from at least four different pH measurements with the same nanopipette. Nanopipette filled with 0.1M KCl, 10mM Tris HCl (pH 7).

Figure 5

Variation of the rectification coefficient after recycling of the nanopipette. Cu²⁺ concentration: 100 μM (pH =7). The sensor was regenerated by immersing the sensor into a pH=3 solution for 1 minute. Nanopipette filled with 0.1M KCl, 10mM Tris HCl (pH 7).

Figure 6

Response of the chitosan/PAA nanopipette to various concentration of Cu²⁺ in 0.1 M KCl, 10mM Tris HCl, pH=7. The inset shows the linear fit (R: 0.997). The ratio I_s/I_b was calculated from the negative peaks of a sinusoidal waveform of 500 mV amplitude and 5 Hz frequency.

Figure 7

Role of the waveform on the Cu²⁺ detection by a chitosan/PAA functionalized nanopipette. a) Cartoon depicting the role of electrophoresis on the interaction of cupric ions with a nanopipette b) Output current, the arrow indicates the addition of Cu²⁺ ions (final concentration in solution 150 μM). No change is detected while applying a positive voltage, while an immediate response occurred upon switching to a negative potential which caused a variation on the following positive step.