Nanopore
technology has been extensively investigated for analysis
of biomolecules, and a success story in this field concerns DNA sequencing
using a nanopore chip featuring an array of hundreds of biological
nanopores (BioNs). Solid-state nanopores (SSNs) have been explored
to attain longer lifetime and higher integration density than what
BioNs can offer, but SSNs are generally considered to generate higher
noise whose origin remains to be confirmed. Here, we systematically
study low-frequency (including thermal and flicker) noise characteristics
of SSNs measuring 7 to 200 nm in diameter drilled through a 20-nm-thick
SiN
x
membrane by focused ion milling.
Both bulk and surface ionic currents in the nanopore are found to
contribute to the flicker noise, with their respective contributions
determined by salt concentration and pH in electrolytes as well as
bias conditions. Increasing salt concentration at constant pH and
voltage bias leads to increase in the bulk ionic current and noise
therefrom. Changing pH at constant salt concentration and current
bias results in variation of surface charge density, and hence alteration
of surface ionic current and noise. In addition, the noise from Ag/AgCl
electrodes can become predominant when the pore size is large and/or
the salt concentration is high. Analysis of our comprehensive experimental
results leads to the establishment of a generalized nanopore noise
model. The model not only gives an excellent account of the experimental
observations, but can also be used for evaluation of various noise
components in much smaller nanopores currently not experimentally
available.
Nanopore technology
holds great promise for a wide range of applications
such as biomedical sensing, chemical detection, desalination, and
energy conversion. For sensing performed in electrolytes in particular,
abundant information about the translocating analytes is hidden in
the fluctuating monitoring ionic current contributed from interactions
between the analytes and the nanopore. Such ionic currents are inevitably
affected by noise; hence, signal processing is an inseparable component
of sensing in order to identify the hidden features in the signals
and to analyze them. This Guide starts from untangling the signal
processing flow and categorizing the various algorithms developed
to extracting the useful information. By sorting the algorithms under
Machine Learning (ML)-based versus non-ML-based, their underlying
architectures and properties are systematically evaluated. For each
category, the development tactics and features of the algorithms with
implementation examples are discussed by referring to their common
signal processing flow graphically summarized in a chart and by highlighting
their key issues tabulated for clear comparison. How to get started
with building up an ML-based algorithm is subsequently presented.
The specific properties of the ML-based algorithms are then discussed
in terms of learning strategy, performance evaluation, experimental
repeatability and reliability, data preparation, and data utilization
strategy. This Guide is concluded by outlining strategies and considerations
for prospect algorithms.
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