Probing the debye layer: capacitance and potential of zero charge measured using a debye-layer transistor

A high-throughput label-free nanoparticle analyser

Synthetic nanoparticles and genetically modified viruses are used in a range of applications, but high-throughput analytical tools for the physical characterization of these objects are needed. Here we present a microfluidic analyser that detects individual nanoparticles and characterizes complex, unlabelled nanoparticle suspensions. We demonstrate the detection, concentration analysis and sizing of individual synthetic nanoparticles in a multicomponent mixture with sufficient throughput to analyse 500,000 particles per second. We also report the rapid size and titre analysis of unlabelled bacteriophage T7 in both salt solution and mouse blood plasma, using just ~1 × 10⁻⁶ l of analyte. Unexpectedly, in the native blood plasma we discover a large background of naturally occurring nanoparticles with a power-law size distribution. The high-throughput detection capability, scalable fabrication and simple electronics of this instrument make it well suited for diverse applications.

Electrofluidic gating of a chemically reactive surface

We consider the influence of an electric field applied normal to the electric double layer at a chemically reactive surface. Our goal is to elucidate how surface chemistry affects the potential for field-effect control over micro- and nanofluidic systems, which we call electrofluidic gating. The charging of a metal-oxide-electrolyte (MOE) capacitor is first modeled analytically. We apply the Poisson-Boltzmann description of the double layer and impose chemical equilibrium between the ionizable surface groups and the solution at the solid-liquid interface. The chemically reactive surface is predicted to behave as a buffer, regulating the charge in the double layer by either protonating or deprotonating in response to the applied field. We present the dependence of the charge density and the electrochemical potential of the double layer on the applied field, the density, and the dissociation constants of ionizable surface groups and the ionic strength and the pH of the electrolyte. We simulate the responses of SiO(2) and Al(2)O(3), two widely used oxide insulators with different surface chemistries. We also consider the limits to electrofluidic gating imposed by the nonlinear behavior of the double layer and the dielectric strength of oxide materials, which were measured for SiO(2) and Al(2)O(3) films in MOE configurations. Our results clarify the response of chemically reactive surfaces to applied fields, which is crucial to understanding electrofluidic effects in real devices.

Nanoscale radiofrequency impedance sensors with unconditionally stable tuning

Impedance sensors perform an important role in a number of biosensing applications, including particle counting, sizing, and velocimetry. Detection of nanoparticles, or changes in, e.g., the interfacial Debye-Hückel layer, can also be performed using nanoscale impedance sensors. One method for monitoring changes in the local impedance is to use radiofrequency reflectometry, which when combined with an impedance-matched sensor can afford very high sensitivity with very large detection bandwidth. Maintaining sensitivity and dynamic range, however, requires continuous tuning of the impedance matching network. Here we demonstrate a dual feedback tuning circuit, which allows us to maintain near-perfect impedance matching, even in the presence of long-term drifts in sensor impedance. We apply this tuning technique to a nanoscale interdigitated impedance sensor, designed to allow the direct detection of nanoparticles or real-time monitoring of molecular surface binding. We demonstrate optimal performance of the nanoscale sensor and tuned impedance network both when modulating the concentration of saline to which the sensor is exposed and when electronically switching between sensors configured in a two-element differential array, achieving a stabilization response time of <20 ms.