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A microfluidic pump and mixer design based on ac faradaic polarization is proposed. Unlike ac electrokinetic devices based on capacitive charging of the electrodes, the design yields a net electro-osmotic flow for high-conductivity electrolytes at high voltages and frequencies without producing gas bubbles or generating pH gradients. The average velocity, which can be more than an order of magnitude higher than that generated by the capacitive mechanism, has an exponential dependence on the voltage and increases monotonically at low frequencies. Vortices and net flows with linear velocities in excess of 1mm∕s are generated with orthogonal microfabricated planar electrodes based on the unique flow and polarization features of this new ac charging mechanism.

The motion of a suspension of erythrocytes (red blood cells, RBCs) in response to a high-frequency alternating current (AC) field in a microfluidic device is examined with parallel and orthogonal electrode configurations to delineate the various fundamental driving forces. Cell repulsion from the platinum electrodes due to electrode polarization interacting with cell membrane polarizations is observed to be the strongest force acting on the particles in the first few seconds of field application. We exploit this strong repulsion to concentrate the bioparticles between the microelectrodes to amplify multiparticle aggregation phenomenon and dielectrophoretic (DEP) manipulation in a small and well-characterized region within the microfluidic device. Secondary motions include RBC pearl chain formation along field lines due to particle polarization followed by classical dielectrophoretic motion of the chains across field lines to regions of weaker field. These are driven by far weaker dipole-dipole and field-dipole interactions than the preliminary electrode repulsions. RBC chain length and total aggregated cells are presented for a variety of AC frequencies and are significantly amplified by the electrode repulsion. Motion of particles away from the polarized electrode is found to be species- and age-sensitive and can stand by itself as a promising identification and separation mechanism. In a 0.1 S/m isotonic phosphate buffer saline medium, we observe the largest cell mobilities at an optimal frequency of approximately 1 MHz, corresponding to the inverse diffusion time across the double layer of the cell and across the electrode's polarized layer. This suggests that the dielectric responses of both particles and electrodes in the low MHz frequency range are mostly determined by normal electromigration of ions from the bulk to their interfaces. Sensitivity to RBC age and species suggests that the surface proteins and membrane ion channels can affect the capacitance of the interface to accommodate the ions from the bulk. Such surface ion accumulation and polarization mechanisms are different from the classical dielectric theories. The resonant frequency of electrode polarization at around 1 MHz falls between positive and negative dielectrophoretic resonant frequency peaks - suggesting that the double-layer polarization mechanism is a distinct and potentially important bioparticle manipulation tool.

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The role of nitrogen doping in the highly efficient oxygen photosensitization of carbon dots is underlined.

The high polarizability and dielectrophoretic mobility of single-walled carbon nanotubes (SWNT) are utilized to capture and detect low numbers of bacteria and submicron particles in milliliter-sized samples. Concentrated SWNT solutions are mixed with the sample and a high-frequency (>100 kHz) alternating current (AC) field is applied by a microelectrode array to enhance bulk absorption of the particles (bacteria and nanoparticle substitutes) by the SWNTs via dipole-dipole interaction. The same AC field then drives the SWNT-bacteria aggregates to the microelectrode array by positive-AC dielectrophoresis (DEP), with enhanced and reversed bacteria DEP mobility due to the attached SWNTs. Since the field frequency exceeds the inverse RC time of the electrode double layer, the AC field penetrates deeply into the bulk and across the electrode gap. Consequently, the SWNTs and absorbed bacteria assemble rapidly (<5 min) into conducting linear aggregates between the electrodes. Measured AC impedance spectra by the same trapping electrodes and fields show a detection threshold of 10(4) bacteria/mL with this pathogen trapping and concentration technique.

Positive ac dielectrophoresis (DEP) is used to rapidly align ensembles of CdSe semiconductor nanowires (NWs) near patterned microelectrodes. Due to their large geometric aspect ratio, the induced dipole of the wires is proportional to their conductivity, which can be drastically enhanced under super-band-gap illumination by several orders of magnitude, with a corresponding increase in the wire DEP mobility. This optical enhancement of conductivity occurs because of the generation of mobile electrons and holes and is verified by a photocurrent measurement. The linear nanowire alignment exhibits a high degree of fluorescent polarization anisotropy in both absorption and emission. An unexpected observation is a reversible, factor of ∼4, electric-field-induced, and frequency-dependent enhancement of the nanowire emission near 10Hz. Such illumination-sensitive, field-enhanced, and frequency-dependent alignment and emission phenomena of NWs suggest an electrical-optical platform for fabricating CdSe nanowire devices for polarization-sensitive photodetection and biosensing applications.