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We have determined the parameters necessary to fabricate reproducible neuronal patterns which we are using to begin studying fundamental issues in developmental neurobiology. The addition of a beam homogenizer, as well as a new surface preparation, has enabled the routine production of reproducible, high-resolution (2−20 μm) organosilane patterns. The effects of surface preparation and beam dosage were monitored using X-ray photoelectron spectroscopy (XPS) and proof of patterning is provided by high-resolution imaging XPS. We also report the guidance of neuronal adhesion and neurite outgrowth and the creation of reproducibly defined circuits of embryonic (E18−19) rat hippocampal neurons using these patterned surfaces in vitro. We have achieved a >50% rate of pattern formation, and at times the rate approaches 90%. We are using these patterns to address the issue of how geometric pattern cues might be used to affect cell-to-cell communication and we report the preliminary results on the synaptic development of the hippocampal neurons using dual patch-clamp electrophysiology. We monitored neurite outgrowth and the emergence of both spontaneous and evoked synaptic activity for both patterned and unpatterned (control) hippocampal cultures. The results indicate the intriguing possibility that geometry itself may be a modulating or trophic factor for cell development.

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The thermodynamics and kinetics of electron transfer are examined as a function of aqueous electrolyte and temperature on self-assembled monolayers containing attached redox centers. The monolayere are formed by coadaorbing an alkanethiol with a pendant pyRu(NH& redox center with an alkanethiol with a terminal carboxylic acid onto clean gold surfaces. The formal potential of the attached redox centers tracks the formal potential of a solution analog in all electrolytes (1 M Na2S04, NaC1, NdOs, or NaClO4) and at all temperaturea (5-55 "C). No change in the monolayer structure is discernible in chronoamperometry or Tafel plots over the same range of conditions. The electron transfer kinetics are independent of the identity of the anion in the electrolyte. The temperature dependence of the Tafel plots conforms qualitatively with the predictions of the Marcus model. A comparison of three voltammetric methods (cyclic voltammetry, chronoamperometry, and ac impedance epectroscopy) confirms that one standard rate constant and one reorganization energy coneistently describes the electron transfer kinetics of the majority of the redox centers. IntroductionThe incorporation of redox centers into self-assembled thiolate monolayers on electrodes provides a useful probe for the behavior of the monolayer.l-12 The coverage of the redox centers, which is easily measured by cyclic voltammetry, yields a direct measure of the concentration of the electroactive thiol in the monolayer. The concentration of the electroactive thiol is especially useful for studying the formation of mixed monolayers or the exchange of thiols in the monolayer with thiols in solution. Ion motion through the monolayeP4 and ion pairings can be studied with respect to the position of the redox center in the monolayer.Similarly, the monolayer provides a useful means of controlling spacing between the electrode and the redox center. Electron transfer kinetics can be measured as a function of potential and temperature without the complications of maw transfer. In order for the kinetic measurementa to be meaningful, several criteria which

Mixed monolayers containing an electro active thiol, HS(CHJ1sCONHCHJ>yRu(NH3)sz+13+, and a diluent thiol, HS(CHz)ISCOOH,on gold electrodes are examined in acetonitrile electrolyte (0.1 M LiCIOJ as a function of overpotential and temperature. Kinetic and thermodynamic parameters from ca. -45 to +20°C are obtained by cyclic voltammetry and chronoamperometry. Reorganization energies obtained from Tafel plots and Arrhenius plots both yield a reorganization energy of 0.8 eV for the redox centers.