Bias-enhanced optical pH response of group III-nitride nanowires.

We show that the photoluminescence intensity of GaN and InGaN nanowires in electrolytes sensitively responds to variations of the pH value and the applied bias. The realization of an electrochemical working point allows pH detection with a resolution better than 0.05 pH. The observed effects are attributed to bias-dependent nonradiative recombination processes competing with interband transitions. The results show that group III-nitride nanowires are excellently suited as nanophotonic pH sensor elements.

Iridium oxide microelectrode arrays for in vitro stimulation of individual rat neurons from dissociated cultures.

We present the first in vitro extracellular stimulation of individual neurons from dissociated cultures with iridium oxide (IrO(x)) electrodes. Microelectrode arrays with sputtered IrO(x) films (SIROF) were developed for electrophysiological investigations with electrogenic cells. The microelectrodes were characterized with scanning electron and atomic force microscopy, revealing rough and porous electrodes with enlarged surface areas. As shown by cyclic voltammetry and electrochemical impedance spectroscopy, the large surface area in combination with the good electrochemical properties of SIROF resulted in high charge storage capacity and low electrode impedance. Thus, we could transfer the good properties of IrO(x) as material for in vivo stimulation electrodes to multi-electrode arrays with electrode diameters as small as 10 mum for in vitro applications. Single rat cortical neurons from dissociated cultures were successfully stimulated to fire action potentials using single or trains of biphasic rectangular voltage-controlled stimulation pulses. The stimulated cell's membrane potential was simultaneously monitored using whole-cell current-clamp recordings. This experimental configuration allowed direct evaluation of the influence of pulse phase sequence, amplitude, and number on the stimulation success ratio and action potential latency. Negative phase first pulses were more effective for extracellular stimulation and caused reduced latency in comparison to positive phase first pulses. Increasing the pulse amplitude also improved stimulation reliability. However, in order to prevent cell or electrode damage, the pulse amplitude is limited to voltages below the threshold for irreversible electrochemical reactions at the electrode. As an alternative to increasing the amplitude, a higher number of stimulation pulses was also shown to increase stimulation success.