The feasibility of using quartz nanopipets as simple and cost-effective Coulter counters for calibration-less quantitation and sizing of nanoparticles by resistive pulsing sensing (RPS) was investigated. A refined theory was implemented to calculate the size distribution of nanoparticles based on the amplitude of resistive pulses caused by their translocation through nanopipets of known geometry. The RPS provided diameters of monodisperse latex nanoparticles agreed within the experimental error with those measured by using scanning electron microscopy (SEM), dynamic light scattering (DLS), and nanoparticle tracking analysis (NTA). The nanopipet-based counter, by detecting individual nanoparticles, could resolve with similar resolution as SEM mixtures of monodisperse nanoparticles having partially overlapping size distributions, which could not be discriminated by DLS or NTA. Furthermore, by calculating the hydrodynamic resistance of the nanopipets and consequently the volume flow through the tip enabled for the first time the calibration-less determination of nanoparticle concentrations with nanopipets. The calibration-less methodology is applied to sizing and quantitation of inactivated poliovirus of ~26 nm diameter, which is the smallest size spherical shape virus ever measured by resistive pulse sensing.

13We discuss representative electrochemical nanopore sensing strategies, highlighting their 14 underlying theoretical principles, and limitations.

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Unlike the overwhelming majority of nanopore sensors that are based on the measurement of a transpore ionic current, here we introduce a potentiometric sensing scheme and demonstrate its application for the selective detection of nucleic acids. The sensing concept uses the charge inversion that occurs in the sensing zone of a nanopore upon binding of negatively charged microRNA strands to positively charged peptide-nucleic acid (PNA) modified nanopores. The initial anionic permselectivity of PNA-modified nanopores is thus gradually changed to cationic permselectivity, which can be detected simply by measuring the nanoporous membrane potential. A quantitative theoretical treatment of the potentiometric microRNA response is provided based on the Nernst-Planck/Poisson model for the nanopore system assuming first order kinetics for the nucleic acid hybridization. An excellent correlation between the theoretical and experimental results was observed, which revealed that the binding process is focused at the nanopore entrance with contributions from both in pore and out of pore sections of the nanoporous membrane. The theoretical treatment is able to give clear guidelines for further optimization of potentiometric nanopore-based nucleic acid sensors by predicting the effect of the most important experimental parameters on the potential response.

We propose a calibration‐free resistive pulse sensing (RPS) methodology based on nanopipets to follow the size and concentration changes of nanoparticles during size‐selective separation processes. The study was performed with monodisperse latex nanoparticles and their mixtures that were subjected to separation procedures such as filtration through track‐etch membranes of various pore diameters as well as centrifugal filtration. The RPS measurements were found to be suitable to determine both the recovery of the nanoparticles after filtration as well as subtle changes in their size distribution. Filtration through track‐etch membranes preserved the mean size of the nanoparticles while their recovery was affected by the size of the pore and coexistence of larger nanoparticle fractions in the sample. Centrifugal filtration resulted in the appearance of aggregates.

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