Atomic Layer Deposition to Fine-Tune the Surface Properties and Diameters of Fabricated Nanopores

Abstract

Atomic layer deposition of alumina enhanced the molecule sensing characteristics of fabricated nanopores by fine-tuning their surface properties, reducing 1/f noise, neutralizing surface charge to favor capture of DNA and other negative polyelectrolytes, and controlling the diameter and aspect ratio of the pores with near single Ångstrom precision. The control over the chemical and physical nature of the pore surface provided by atomic layer deposition produced a higher yield of functional nanopore detectors.

Figure 1

I-V curves of ion-sculpted nanopores (both with diameters ~14 nm) in 1 M KCl at pH 8.0 (circles) and at pH 2.0 (squares), without (open circles and squares) and with (filled circles and squares) coating by ALD of 3 nm of Al₂O₃. The current is normalized to the maximum current (I_max) observed at –600 mV. Only the uncoated nanopore (open symbols) exhibited rectification, strongly at pH 8.0 (circles), less strongly at pH 2.0 (squares). The uncoated pore's cation selectivity was 94.5% at pH 8.0 and 80.1% at pH 2.0; the coated pore's cation selectivity was 50.5% at pH 8.0, and 48.7% at pH 2.0. Inset (axes as in main figure): I-V curves of an ion-sculpted nanopore, at pH 8.0 (solid line) and at pH 2.0 (dashed line) but after overcoating 3 nm Al₂O₃ with 3 nm silica (see Methods). Final pore diameter, ~17 nm. Cation selectivity ~57.5% at pH 8; 49.0% at pH 2.0.

Figure 2

(a) Power spectra of ion-sculpted nanopores without (upper curve) and with (lower curve) ALD of 3 nm Al₂O₃ (both pore diameters ~10 nm). Both measurements in buffered 1 M KCl, pH 8.0, at 200 mV. Note the 1/f fitting (dotted line). (b) Noise level (at 10 Hz where 1/f noise dominates) increases with applied voltage level for the nanopore without coating (circles). The data are fitted by S_I = A₀*Voltage^1.92 (curve). On the other hand, 1/f noise is not significant at all voltage levels for the ALD nanopore (squares).

Figure 3

Translocation of bacteriophage lambda DNA (48.5 kbp) through nanopores (diameters ~ 15 nm). (a) An Al₂O₃ coated ion beam sculpted nanopore: final diameter ~15 nm, length ~40 nm. Each current blockage event represents a single DNA molecule passing through the pore. Two such events are enlarged from the several-second recording (arrows) and displayed in large scale. (b) An Al₂O₃ coated FIB pore: final diameter ~15 nm, length ~250 nm. The DNA translocates in similar time duration but causes smaller current blockage because of greater pore length of the FIB-coated pore. The enhanced portion of some blockages (within the dashed elipses) reflects a portion of the translocating DNA molecule that is folded on itself, such that two strands of the double-helix occupy the nanopore simultaneously. Translocation was driven by a 300 mV voltage bias.

Figure 4

Transmission electron microscopy (JEM-100CXII) images of several pores before (top row) and after (bottom row) deposition of Al₂O₃ coatings by atomic layer deposition. (Left) Even after 500 layers of Al₂O₃ coating, a square-shaped FIB pore (a) retains its square shape after its open area is reduced by ~9-fold (b). (Center) A ~21.6 nm diameter ion beam sculpted nanopore (c) was coated with 70 layers of Al₂O₃ to produce a ~4.8 nm nanopore (d). Note that the diameter of the original FIB pore (white arrows in c) from which the central nanopore has been sculpted has also been decreased (d) by the deposition of Al₂O₃. (Right) A ~7.1 nm diameter ion beam sculpted nanopore (e) was coated with 24 layers of Al₂O₃ to produce a ~2.0 nm nanopore (f).