Solid-State Nanopore Sensors for Nucleic Acid Analysis - Nanopores: Sensing and Fundamental Biological Interactions - page 17

concentrations ( 10 mM), pore conductivity was found to be much larger than bulk

approximations calculated using pore geometry. This conductance deviation was
attributed to Debye layer overlap in the pore (Debye length is comparable to or
larger than the pore radius). Multiscale simulations of ion transport through these
pores coupled with experimental results suggested the presence of fixed negative
charges on the pore wall, and a reduction of the ion mobility due to fixed charge and
ion proximity to the pore wall.
Consistent with Ho's results, Smeets et al. [ 81 ] found that in ~10 nm diameter
SiO 2 pores, the negative surface charge lining the pore walls dominates pore
conductivity at salt concentration below ~0.1 M. Interestingly, a variable surface
charge density in the pore was extracted as a function of electrolyte concentration in
these experiments. pH dependent studies performed by Wanunu et al. in Si 3 N 4
nanopores did not reveal any significant changes in the conductance with varying
pH [ 93 ]. Nanopores formed in PET membranes also exhibited a net negatively
charged surface at neutral and slightly basic pH's due to the deprotonation of
carboxylate groups on the pore walls. The average density of carboxylate groups
was estimated to be 1.5 groups/nm 2 [ 78 ]. In contrast to SiO 2 , TiO 2 only presented a
slightly negatively charged surface at neutral or slightly basic pH. Unlike the
variable surface charge observed by Smeets et al. in SiO 2 pores, TiO 2 nanopore
conductance saturated at very low electrolyte concentrations [ 65 , 68 ]. Nam et al.
extracted a surface charge density of ~0.005 mC/m 2 in TiO 2 pores, significantly
lower than the charge density observed in SiO 2 pores which is estimated at between
~25 and 50 mC/m 2 [ 65 ]. Nam suggested that this low charge density may be
responsible for the saturation in ionic conductivity at lower KCl concentrations
than expected.
In contrast to the aforementioned systems, Al 2 O 3 nanopores are expected to be
positively charged at neutral or slightly basic pH. The formation of hetero-phase
crystalline domains (in particular

g a

-phases) of varying bond lengths and co-
ordinations during electron beam irradiation impart a non-uniform charge density in
the pore.

and

-Al 2 O 3 both exhibit different points-of-zero-charge (pzc's),
estimated at pH 9.1 and pH 8.5 in monovalent salt solution [ 1 , 88 ]. In addition, the
Zeta potentials of these materials measured in pH 7.5 electrolyte are ~50 mV and
~25 mV respectively, [ 7 , 27 ] and thus these charged nanocrystalline domains are
expected to interact with ions and charged polymers in the pore differently [ 90 ].

a -Al 2 O 3 and g

1.3.4.3 Manipulating Surface Charge in Nanopores

As mentioned, precise control over phase transformations in Al 2 O 3 nanopore
systems by varying electron dose provides a novel method to engineer surface
charge at the nanopore/fluid interface via the electron beam. This is very interesting
from a materials perspective. Another approach for tuning surface charge in the
nanopore is through electrical mechanisms. Nam et al. embedded a TiN gate
electrode directly in the nanopore and showed current rectification by applying
potentials to the gate electrode [ 65 ]. This gating behavior was only observed at very