Solid-State Nanopore Sensors for Nucleic Acid Analysis - Nanopores: Sensing and Fundamental Biological Interactions - page 25

1.3.7 Chemically Modified Solid-State Nanopore Sensors

Chemical modification of the surface of solid-state nanopores offers a method to
tailor the physical and chemical properties of the nanopore. Selective transport
through functionalized solid-state nanopore arrays was previously demonstrated by
varying nanopore size (thereby restricting biomolecule passage based on molecular
weight) [ 38 ], nanopore surface charge [ 17 ], and nanopore polarity [ 39 ] (to achieve
the selective transport of either hydrophobic or hydrophilic molecules). More
recently, focus has shifted to the attachment of specific recognition sequences or
tethered receptors in the nanopore for target specific molecular recognition. In drug
screening and medicine, such a technique provides a means for label-free, real-time
kinetic analysis of biomolecular interactions at the single molecule level including
protein-protein, protein-DNA and receptor-ligand interactions. In fact, Lee et al.
demonstrated that enantiomeric drug separations could be achieved using an antibody
functionalized nanoporous array [ 51 ]. Functionalized nanopore channels can also help
elucidate the mechanisms driving biological processes, including cell signaling and
regulation. Jovanovic-Talisman demonstrated that functionalized polycarbonate
nanoporous arrays can reproduce the selectivity of nuclear pore complexes (NPCs),
an essential component in the trafficking of specific macromolecules between the
cell nucleus and cytoplasm [ 41 ]. Proteins referred to as phenylalanine-glycine
(FG)-nucleoporins typically line the walls of NPCs and facilitate the transient binding
and passing of transport factors and their cargo-bound complexes, while restricting
the passage of proteins that fail to specifically bind to FG-nucleoporins [ 69 ].
Using nanopore channels of the correct dimensions coated with FG-nucleoporins,
Jovanovic-Talisman et al. were able to reproduce key features of nucleocytoplasmic
transport, selectively discriminating against control proteins in favor of transport
factors and transport factor cargo complexes. Kohli demonstrated that selective
permeation through synthetic nanoporous membranes could be achieved using DNA
hybridization as the selective transport mechanism [ 49 ]. In this specific example, a
gold nanoporous array was functionalized using hairpin DNA with a thiol substituent
at the 5 0 end allowing it to be covalently attached to the inside walls of the array. The
analyte of interest was 18 base long ssDNA which was either a perfect complement to
the loop of the hairpin or contained a single base mismatch. Using optical absorbance
methods, Kohli demonstrated that single nucleotide polymorphisms could be detected
using this chemically modified nanopore platform under optimal conditions.
Various strategies have been implemented to chemically modify solid-state
nanopores. Covalent attachment chemistries are generally preferred due to the
stability and high packing density of self assembled monolayers (SAM's) on well
prepared sufaces. A very common SAM preparation involves the reaction of
molecules with a sulfhydrl termination group (

SH) with Au surfaces to form
S-Au attachments to the surface. An extensive review on the formation of SAM's
on Au surfaces of varying curvatures is provided by Love et al. [ 55 ]. In many cases
however, the surface of the nanopore may be an insulating oxide or nitride (SiO 2 ,
Si 3 N 4 ,Al 2 O 3 ). In these cases a covalent attachment chemistry specific to this