A hybrid resistive pulse-optical detection platform for microfluidic experiments

Integrated In-Plane Nanofluidic Devices for Resistive-Pulse Sensing

Single-particle (or digital) measurements enhance sensitivity (10- to 100-fold improvement) and uncover heterogeneity within a population (one event in 100 to 10,000). Many biological systems are significantly influenced by rare or infrequent events, and determining what species is present, in what quantity, and the role of that species is critically important to unraveling many questions. To develop these measurement systems, resistive-pulse sensing is used as a label-free, single-particle detection technique and can be combined with a range of functional elements, e.g., mixers, reactors, filters, separators, and pores. Virtually, any two-dimensional layout of the micro- and nanofluidic conduits can be envisioned, designed, and fabricated in the plane of the device. Multiple nanopores in series lead to higher-precision measurements of particle size, shape, and charge, and reactions coupled directly with the particle-size measurements improve temporal response. Moreover, other detection techniques, e.g., fluorescence, are highly compatible with the in-plane format. These integrated in-plane nanofluidic devices expand the toolbox of what is possible with single-particle measurements.

Effects of off-axis translocation through nanopores on the determination of shape and volume estimates for individual particles

Resistive pulses generated by nanoparticles that translocate through a nanopore contain multi-parametric information about the physical properties of those particles. For example, non-spherical particles sample several different orientations during translocation, producing fluctuations in blockade current that relate to their shape. Due to the heterogenous distribution of electric field from the center to the wall of a nanopore while a particle travels through the pore, its radial position influences the blockade current, thereby affecting the quantification of parameters related to the particle's characteristics. Here, we investigate the influence of these off-axis effects on parameters estimated by performing finite element simulations of dielectric particles transiting a cylindrical nanopore. We varied the size, ellipsoidal shape, and radial position of individual particles, as well as the size of the nanopore. As expected, nanoparticles translocating near the nanopore wall produce increase current blockades, resulting in overestimates of particle volume. We demonstrated that off-axis effects also influence estimates of shape determined from resistive pulse analyses, sometimes producing a multiple-fold deviation in ellipsoidal length-to-diameter ratio between estimates and reference values. By using a nanopore with the minimum possible diameter that still allows the particle to rotate while translocating, off-axis effects on the determination of both volume and shape can be minimized. In addition, tethering the nanoparticles to a fluid coating on the nanopore wall makes it possible to determine an accurate particle shape with an overestimated volume. This work provides a framework to select optimal ratios of nanopore to nanoparticle size for experiments targeting free translocations.

Multi-resistive pulse sensor microfluidic device

Resistive pulse sensors have been used to characterise everything from whole cells to small molecules. Their integration into microfluidic devices has simplified sample handling whilst increasing throughput. Typically, these devices measure a limited size range, making them prone to blockages in complex sample matrixes. To prolong their life and facilitate their use, samples are often filtered or prepared to match the sample with the sensor diameter. Here, we advance our tuneable flow resistive pulse sensor which utilises additively manufactured parts. The sensor allows parts to be easily changed, washed and cleaned, its simplicity and versatility allow components from existing nanopore fabrication techniques such as glass pipettes to be integrated into a single device. This creates a multi-nanopore sensor that can simultaneously measure particles from 0.1 to 30 μm in diameter. The orientation and controlled fluid flow in the device allow the sensors to be placed in series, whereby smaller particles can be measured in the presence of larger ones without the risk of being blocked. We illustrate the concept of a multi-pore flow resistive pulse sensor, by combining an additively manufactured tuneable sensor, termed sensor 1, with a fixed nanopore sensor, termed sensor 2. Sensor 1 measures particles as small as 10 μm in diameter, whilst sensor 2 can be used to characterise particles as small as 100 nm, depending upon its dimensions. We illustrate the dual pore sensor by measuring 1 and 10 μm particles simultaneously.

A Tunable Three-Dimensional Printed Microfluidic Resistive Pulse Sensor for the Characterization of Algae and Microplastics

Technologies that can detect and characterize particulates in liquids have applications in health, food, and environmental monitoring. Simply counting the numbers of cells or particles is not sufficient for most applications; other physical properties must also be measured. Typically, it is necessary to compromise between the speed of a sensor and its chemical and biological specificity. Here, we present a low-cost and high-throughput multiuse counter that classifies a particle's size, concentration, and shape. We also report how the porosity/conductivity or the particle can influence the signal. Using an additive manufacturing process, we have assembled a reusable flow resistive pulse sensor capable of being tuned in real time to measure particles from 2 to 30 μm across a range of salt concentrations, i.e., 2.5 × 10^-4 to 0.1 M. The device remains stable for several days with repeat measurements. We demonstrate its use for characterizing algae with spherical and rod structures as well as microplastics shed from tea bags. We present a methodology that results in a specific signal for microplastics, namely, a conductive pulse, in contrast to particles with smooth surfaces such as calibration particles or algae, allowing the presence of microplastics to be easily confirmed and quantified. In addition, the shapes of the signal and of the particle are correlated, giving an extra physical property to characterize suspended particulates. The technology can rapidly screen volumes of liquid, 1 mL/min, for the presence of microplastics and algae.

A methodology for characterising nanoparticle size and shape using nanopores

The discovery and characterisation of nanomaterials represents a multidisciplinary problem. Their properties and applications within biological, physical and medicinal sciences depend on their size, shape, concentration and surface charge. No single technology can currently measure all characteristics. Here we combine resistive pulse sensing with predictive logistic regression models, termed RPS-LRM, to rapidly characterise a nanomaterial's size, aspect ratio, shape and concentration when mixtures of nanorods and nanospheres are present in the same solution. We demonstrate that RPS-LRM can be applied to the characterisation of nanoparticles over a wide size range, and varying aspect ratios, and can distinguish between nanorods over nanospheres when they possess an aspect ratio grater then two. The RPS-LRM can rapidly measure the ratios of nanospheres to nanorods in solution within mixtures, regardless of their relative sizes and ratios i.e. many large nanospherical particles do not interfere with the characterisation of smaller nanorods. This was done with a 91% correct classification of nanospherical particles and 72% correct classification of nanorods even when the fraction of nanorods in solution is as low as 20%. The methodology here will enable the classification of nanomedicines, new nanomaterials and biological analytes in solution.

Increased Flexibility in Lab-on-Chip Design with a Polymer Patchwork Approach

Nanofluidic structures are often the key element of many lab-on-chips for biomedical and environmental applications. The demand for these devices to be able to perform increasingly complex tasks triggers a request for increasing the performance of the fabrication methods. Soft lithography and poly(dimethylsiloxane) (PDMS) have since long been the basic ingredients for producing low-cost, biocompatible and flexible devices, replicating nanostructured masters. However, when the desired functionalities require the fabrication of shallow channels, the “roof collapse” phenomenon, that can occur when sealing the replica, can impair the device functionalities. In this study, we demonstrate that a “focused drop-casting” of h-PDMS (hard PDMS) on nanostructured regions, provides the necessary stiffness to avoid roof collapse, without increasing the probability of deep cracks formation, a drawback that shows up in the peel-off step, when h-PDMS is used all over the device area. With this new approach, we efficiently fabricate working devices with reproducible sub-100 nm structures. We verify the absence of roof collapse and deep cracks by optical microscopy and, in order to assess the advantages that are introduced by the proposed technique, the acquired images are compared with those of cracked devices, whose top layer, of h-PDMS, and with those of collapsed devices, made of standard PDMS. The geometry of the critical regions is studied by atomic force microscopy of their resin casts. The electrical resistance of the nanochannels is measured and shown to be compatible with the estimates that can be obtained from the geometry. The simplicity of the method and its reliability make it suitable for increasing the fabrication yield and reducing the costs of nanofluidic polymeric lab-on-chips.

Estimation of Shape, Volume, and Dipole Moment of Individual Proteins Freely Transiting a Synthetic Nanopore

This paper demonstrates that high-bandwidth current recordings in combination with low-noise silicon nitride nanopores make it possible to determine the molecular volume, approximate shape, and dipole moment of single native proteins in solution without the need for labeling, tethering, or other chemical modifications of these proteins. The analysis is based on current modulations caused by the translation and rotation of single proteins through a uniform electric field inside of a nanopore. We applied this technique to nine proteins and show that the measured protein parameters agree well with reference values but only if the nanopore walls were coated with a nonstick fluid lipid bilayer. One potential challenge with this approach is that an untethered protein is able to diffuse laterally while transiting a nanopore, which generates increasingly asymmetric disruptions in the electric field as it approaches the nanopore walls. These "off-axis" effects add an additional noise-like element to the electrical recordings, which can be exacerbated by nonspecific interactions with pore walls that are not coated by a fluid lipid bilayer. We performed finite element simulations to quantify the influence of these effects on subsequent analyses. Examining the size, approximate shape, and dipole moment of unperturbed, native proteins in aqueous solution on a single-molecule level in real time while they translocate through a nanopore may enable applications such as identifying or characterizing proteins in a mixture, or monitoring the assembly or disassembly of transient protein complexes based on their shape, volume, or dipole moment.

Resistive pulse sensing as particle counting and sizing method in microfluidic systems: Designs and applications review

Resistive pulse sensing is a well-known and established method for counting and sizing particles in ionic solutions. Throughout its development the technique has been expanded from detection of biological cells to counting nanoparticles and viruses, and even registering individual molecules, e.g., nucleotides in nucleic acids. This technique combined with microfluidic or nanofluidic systems shows great potential for various bioanalytical applications, which were hardly possible before microfabrication gained the present broad adoption. In this review, we provide a comprehensive overview of microfluidic designs along with electrode arrangements with emphasis on applications focusing on bioanalysis and analysis of single cells that were reported within the past five years.

Optimal voltage for nanoparticle detection with thin nanopores

Optimal voltages were found for particle detections, at which the current blockade ratio did not depend on surface charge density.