Glutamate sensing with enzyme-modified floating-gate field effect transistors.

Neurotransmitter release is the key factor of chemical messaging in the brain. Fast, sensitive and in situ detection of single cell neurotransmitter release is essential for the investigation of synaptic transmission under physiological or pathophysiological conditions. Although various techniques have been developed for detecting neurotransmitter release both in vitro and in vivo, the sensing of such events still remains challenging. First of all, the amount of neurotransmitter released during synaptic transmission is unknown because of the limited number of molecules released and the fast diffusion and reuptake of these molecules after release. On the other hand, advances in microelectronic biosensor devices have made possible the fast detection of various analytes with high sensitivity and selectivity. Specifically, enzyme-modified field-effect (ENFET) devices are attractive for such applications due to their fast response, small dimensions and the possibility to integrate a large number of sensors on the same chip. In this paper, we present a floating-gate FET device coated with glutamate oxidase (GLOD) layer. The surface chemistry was optimized for maximal enzyme loading and long-term stability, and characterized by quartz crystal microbalance and colorimetric assays. Enzyme loading was largest on poly-L-lysin-based surfaces combined with glutaraldehyde. The surface chemistry showed excellent stability for at least one month in Tris buffers stored at 4 degrees C. A glutamate detection limit of 10(-7) M has been obtained with the GLOD-coated FET and our sensor proved to be selective to glutamate only. We show that this biosensor is a promising tool for the in vitro detection of glutamate and can be extended to other neurotransmitters.

Porosity inherent to chemically crosslinked polymers. Poly(N-vinylimidazole) hydrogels.

Swollen polymer networks exhibit multiscale pores filled with solvent. Such porosity, inherent to cross-linked polymers, determines some of their most relevant physical properties and applications. In this research, several samples of chemically crosslinked poly(N-vinylimidazole) were synthesized with the same permanent crosslinking density at two different conversions, and their inherent porosity was characterized on freeze-dried specimens by SEM, TEM and nitrogen physisorption. It was thus found that all of the samples showed pores, both on the nanometer and the micrometer scales, whose dimensions were mostly equal to or larger than the mesh size of the primary polymer network (22 nm) and whose volume and specific surface decreased with increasing conversion. Micropores have, in all cases, a very minor contribution. Samples synthesized with the largest comonomer concentrations show quasi-spherical mesopores (90 nm average diameter at any conversion) and macropores (from 5 to 10 microm with increasing conversion), whereas the mesopores of samples synthesized with the largest crosslinker ratios were channel-like (150 nm) and the macropores were interconnected contiguous voids (3 microm). Samples with intermediate compositions exhibit the lowest porosity due, mostly, to interconnected mesopores. The differences in shape were ascribed to the mechanism of phase separation, taking place during polymerization, even for samples that are transparent following polymerization. The inherent porosity is a significant source of spatial inhomogeneity, which contributes to the increase in turbidity. Light scattering decreases with increasing ionization when the degree of protonation is greater than 10%. An important consequence of the inherent porosity is that the degrees of swelling determined either gravimetrically or through size measurements are not equivalent.

Effect of gold adsorption on the conductive properties of cyclo-octasulfur microcrystals.

The formation of gold crystallites on the surface of S8 promotes diffusion of electrons and determines the conductive properties of the shell-core nanosystems. Conducting probe atomic microscopy and four-probe resistance measurements confirmed that Au/S8 shell-core systems exhibit electrical conductivity on the micro- as well as on the nanoscale in contrast to non-covered S8 crystals, which are insulating. The conductivity of Au/S8 systems on the microscale was measured to be 10+/-1 S cm(-1). In XPS measurements, a single peak at 163.6 eV was observed for bulk S8 whereas an additional peak corresponding to a binding energy of 161.4 eV appeared for S8 adsorbed on a Au substrate. This is interpreted to mean that a chemical reaction has taken place. A process which results in adsorption of uniform gold nanolayers on needle shaped or fibrous S8 crystallites is under investigation.

Electrically conductive 2D-PAN-containing surfaces as a culturing substrate for neurons.

In the present contribution we report on a novel route to synthesize 2D-polyaniline (2D-PAN) on sulfonated-poly(styrene) (SPS) templates by allowing first monomer assembly followed by chemical oxidation to achieve polymerization. We show that Aplysia neurons grown on 2D-PAN exhibit an unusual growth pattern and adhesion to this conducting substrate that is manifested by the formation of giant lamellipodia. The lamellipodial domains are characterized by small gap between the plasma membrane and the 2D-PAN substrate (ca. 30 nm) and actin rich skeleton resembling the skeleton of growth cones. This behavior is characteristic to uniform substrates containing only 2D-PAN. However, in patterned substrates containing additionally poly(L-lysine) Aplysia neurons prefer to extend new neurites on the poly(L-lysine) domains.

Two-dimensional poling of organic and polymer films for improved nonlinear-optical properties.

Nematic films doped with molecules of high nonlinear polarizability can exhibit two-dimensional asymmetry after electric-field poling along one direction only. Consequently, these films exhibit more nonzero components of the second-order susceptibility tensor than are obtained through usual poling techniques. These films can exhibit very high nonlinear optical coefficients (close to 10(-7) esu/cm(3)) and retain most of their nonlinearity one week after removal of the poling field.