(Manoj Sivasubramaniapandian, PhD scholar, talks about resistive-pulse sensing, as applied to diagnostic applications. The writing is un-edited and open to comments)
Resistive
pulse sensing, as the name suggests, depends on the transient resistance changes
in proportion to the ion current as a micron to molecular-scale particle
transits a pore or a narrow conduit. The presence of the particle at the
constriction changes the current across the constriction, which is proportional
to the particle's size, charge, shape, and conductivity. Since Coulter (1953)
developed the technique for counting suspended particles in a fluid, the
technique has undergone rapid evolution. While the earlier approach employs a fixed-sized
pore that restricts the choice of particles significantly, Tunable Resistive Pulse
Sensing (TRPS) permits pore dimension tuning, thereby ensuring improved
analytical range and sensitivity.
Figure 1: a) Schematic of the
resistive pulse sensing setup, b) Current response as differently-sized particles
transit through a fixed-diameter pore / conduit, and c) Particles transiting a
tapered pore and the corresponding current response.
Figure 1: a) Schematic of the
resistive pulse sensing setup, b) Current response as differently-sized particles
transit through a fixed-diameter pore / conduit, and c) Particles transiting a
tapered pore and the corresponding current response.
The
forces acting on the particles in TRPS are predominantly electrophoretic,
electroosmotic, and fluidic. Parameters such as pore size, voltage and pressure
are precisely controlled to tweak the magnitude of these forces. Several theoretical
models aim to determine particles' size, charge, and concentration. However,
the current models suffer from over-simplification, and finite element
modelling of simple experimental observations proves challenging owing to the limited
understanding of the geometry and activation mechanism of the pore. The Maxwell
model is suitable for spherical particles, much smaller than the fixed pore
diameter. However, if the diameter is not uniform, as in a tapered pore, it
results in a non-linear electric field gradient. While Heins attempted to address
this non-linear gradient in resistance across the tapered pore, the Maxwell
model is often employed owing to its simplicity in determining the size and
position of the particle.
On
the other hand, the charge or zeta potential is derived from electrophoretic
mobility and depends only on the pulse duration and not the magnitude, thereby enabling
simultaneous deduction of size and charge. Also, the particle-by-particle
measurement approach allows for resolving multiple zeta potentials in a
subpopulation. Ideally, particle transport should be primarily electrophoretic
to measure the zeta potential. Therefore, it is essential that pressure, if
applied, remain low to measure the electrophoretic mobility reliably. Other factors
involving the solution property and the presence of charges along the pore wall
could contribute to uncertainties and may require passivation. In cases where a
surfactant is employed to aid particle-by-particle measurement, its effect on
the overall charge cannot be considered insignificant. Nevertheless, the assumptions
regarding the ionic charge distribution, homogenous electric field, and the transport
of particles along the central axis under uniform flow rate are not always valid.
Besides
all these factors influencing the measurement, the technique faces several practical
challenges: a) the need for a specialized membrane apparatus and pressure
module to precisely control particle translocation. The accessible smaller pore
ranges are limited, and changing pore geometries result in measurement
uncertainties. The most predominantly used equipment developed by Izon Science
Ltd., New Zealand, has the smallest available pore size of 40 nm, b) non-specific
binding or occlusion of pores with aggregated particles and complex biological
fluids. The blockage causes non-linear drift or drop in the baseline current
during measurements and is increasingly challenging with particles of size 100
nm or less. Therefore, it becomes critical to condition the membrane with the
coating agent, wetting agent, sodium azide and PBS (Reagent kit from Izon
Science Ltd., New Zealand) to reduce non-specific particle-membrane
interactions and c) the need for calibration. Calibration with commercially
prepared and certified (for size) monodispersed beads, in conjunction with the actual
measurements and under the same context, including the buffer components, the stretch
of the pore, pressure, and voltage is critical.
Despite
all these challenges, TRPS finds varied applications in environmental
monitoring, biomedical research, and clinical diagnosis, to name a few. One
such application is the time-dependent study of Blundell et al. to understand
protein interaction with carboxyl functionalized beads that exhibits a shift in
the zeta potential with time and temperature. Protein interaction and formation
of the poorly delimited hard and soft corona layers is a dynamic process. It
depends on the particle's physicochemical properties (size, charge, surface
functionalization, and curvature) and system complexity (including pH,
composition, circulation time, and pressure). A lot remains to learn about the
control and prediction of such interactions, and it could be the fingerprint to
investigate pathogenesis or drug delivery.