Influence of electrophoresis waveforms in determining stochastic nanoparticle capture rates and detection sensitivity

Electrophoretic capture and release of charged polystyrene particles at the opening of a membrane pore has been investigated to determine the optimal applied current waveform, iapp(t), for ensuring true stochastic counting rates and to improve detection sensitivity (i.e., Delta(counts per second)/Delta(particle concentration)). In capture and release detection, charged particles are electrophoretically driven to the opening of a small pore ( approximately 60 nm diameter) in a membrane; capture of a single particle at the pore opening at time tau is signaled by a decrease in the flux of a redox species (Fe(CN)(6)4-) through the pore. The captured particle is then released by applying an electrophoretic current in the opposite direction, and the process is repeated to acquire sufficient statistical data to determine the solution particle concentration (Cp) based on the relationship between Cp and the average particle counting rate (tauavg(-1)). Both tauavg(-1) and the method sensitivity are shown, for the detection of 90 nm diameter polystyrene particles, to depend strongly on the applied current waveform. The observed dependence is a consequence of the nonequilibrium distribution of particles in the analyte solution that results from electrostatic forces acting on the particle whenever iapp has a nonzero value. Stochastic capture rates corresponding to an initial uniform distribution of particles are more closely achieved using an applied current waveform that includes an equilibration period (iapp=0) prior to electrophoretic capture. An increase in particle detection sensitivity, relative to the previously reported value, results from this equilibration step.