SEM images of the opposite sides of an AAO membrane before (A and B) and after (C and D) deposition of 200 nm of gold.

The ionic impedance at 3 kHz through the morpholino modified membrane prepared with the 1:7 ratio of amine:ester silanes. Different stages of DNA sensing are shown, where target and mutant are the complementary and noncomplementary 15-mer DNA oligomers, respectively. The measurements were performed in 10 µM KCl buffer and denaturing was achieved with 9.0 M urea.

Bode plots at different concentrations of KCl for an unmodified membrane (A) and a membrane on which surface ss-DNA (21-mer) was covalently immobilized via amine (for 1 hour)-gluteraldehyde chemistry (B). The Impedance at 10 kHz for membranes from A (■) and B () as functions of the KCl concentration measured as the impedance of free solution. The solid lines represent the best fits with R_el = 22 Ω and the concentration jump, ΔC = 1.2×10⁻⁴ M (black), and 8×10⁻⁴ M (red).

Bode plots for ionic conductance (at 10 µM of KCl) through the membranes before (black) and after (red) immobilization of ss-DNA (16-mer). Different relative concentrations of the amine and ester (A – G) result in different surface densities of DNA. The effect of DNA binding, shown in H, as the ratio (red squares) of the impedance at 3 kHz before (black triangles) and after DNA binding (blue circles), has the optimal amine/ester ratio for maximal effect near amine:ester = 1:7, as in D. Note that for the surface with maximum amount of amines, the impedance increases after DNA binding.

Bode plots at different concentrations of KCl for the membrane modified with aminosilane (for 1 hour) and activated by gluteraldehyde (A); the same membrane after immobilization of the morpholino (B) and after hybridizing with complementary target ss-DNA 15-mer (C). (D) The Impedance at 10 kHz for membranes of A (■) and B () and C () as functions of the KCl concentration measured as the impedance of free solution. The solid lines represent the best fits with R_el = 22 Ω and the surface charge densities, the concentration jump, ΔC = 4.8×10⁻⁴ M (red), and 9.5×10⁻⁴ M (green).

Figure 2A shows the frequency dependence of the absolute impedance, |Z_mem|, (Bode plot) for an unmodified membrane with different concentrations of electrolyte. The equivalent circuit for this setup and its relation to the membrane geometry is shown in Scheme 1. The ionic resistance of the pores, R_p, in parallel with the pores’ capacitance, C_p, is connected in series with the double layer capacitance, C_dl, of the electrodes and the electrical resistance of the electrodes, R_el. Placing the electrodes directly at the membrane eliminates the contribution to the resistance from solution outside the pores. This is an important feature because the surface charge effect reveals at low ionic concentrations when even small gaps between the electrodes and the pores can have comparable to the pore resistance. The chosen electrode geometry circumvents this undesired effect and leaves only the electronic resistance of the gold electrodes, which is still noticeable because of the small thickness and narrow reams of the resulting electrode mesh (see Figure 1). At high frequencies and high electrolyte concentrations, the membrane capacitance, C_p, is barely recognizable in the Bode plots (one can identify it at low electrolyte concentrations as a dip at high frequencies). The impedance of the double layer capacitance, C_dl, arises as a low frequency impedance rise inversely proportional to frequency. Its value has contributions from both, the Helmholtz layer, C_H, and the diffusive layer, C_dif,: (5)