Coordinated Detection of Micro- and Nanoparticles using Tuneable Resistive Pulse Sensing and Optical Spectroscopy
Authors:   , Hauer Peter 
Published: November 2021

<p>The detection and characterisation of micro- and nanoscale particles has become increasingly important in many scientific fields, spanning from colloidal science to biomedical applications. Resistive Pulse Sensing (RPS) and its derivative Tuneable Resistive Pulse Sensing (TRPS), which both use the Coulter principle, have proven to be useful tools to detect and analyse particles in solution over a wide range of sizes. While RPS uses a fixed size pore, TRPS uses a dynamically stretchable pore in a polyurethane membrane, which has the advantages that the pore geometry can be tuned to increase the device's sensitivity and range of detection. The technique has been used to accurately determine the size, concentration and charge of many different analytes.  However, the information obtained using TRPS does not give any insight into the particle's composition. In an attempt to overcome this, an experimental technique was developed in order to obtain simultaneous, time-resolved, high-resolution optical spectra of particles passing through the pore. Due to the ordered and controllable fashion in which the particles are guided through the sensing region, this approach has an advantage over diffusion based optical techniques. The experimental setup for the coordinated electrical and optical measurements involves many underlying physical phenomena, e.g. microuidics, electrokinetic effects, and Gaussian beam optics. A significant proportion of this work was therefore devoted to the development and the optimisation of the experimental setup by adapting a commercial TRPS device and a spectrometer with an attached microscope. Methods to engineer the spot size of a Gaussian beam to account for the different pore diameters, and the development of algorithms to filter, analyse and coordinate the recorded data are essential to the technique.  The results using fluorescently labelled polystyrene particle sets with diameters from 190nm to 2 µm show that matching rates between the electrical and optical measurements of over 90% can repeatedly be achieved. Mixtures of particle species with similar diameters but with different fluorescent labels were used to demonstrate the technique's capability to characterise the analyte on a particle-by-particle basis and extend the information that can be obtained by TRPS alone. It was also shown that the data acquired with the electrical and optical measurements complement each other and can be used to better understand the TRPS technique itself. The influence of experimental parameters, such as the particle velocity, the beam size and the optical detection volume, on the intensity of the optical signals and the matching rates was studied intensively. These studies showed that the technique requires a careful experimental design to achieve the best results. Overall, the developed technique enhances the particle-by-particle specificity of conventional RPS measurements, and could be useful for a range of particle characterization and bio-analysis applications.  Alongside the experiments, semi-analytic modelling and simulations using the Finite Element Method (FEM) were used to understand the particle motion through the pores, to interpret the experimental data, and predict the optical signals. The models were also used to assist the design and the optimization of the experiments. The FEM models were implemented with increasing physical detail and show superior understanding of the TRPS signals compared to the semi-analytic model, which is conventionally used in the TRPS field. The physical phenomena considered included o -axis trajectories, particle-field interactions for both fluid and electric fields, and the non-homogeneous distribution of ions close to the charged membrane and particle interfaces. Several effects which have been observed experimentally could be explained, including the intrinsic pulse height distribution, the current rectification, and the occurrence of bi-phasic pulses, demonstrating the benefits of FEM methods for RPS.</p>