Coupling of Semiconductor Nanowires with Neurons and Their Interfacial Structure
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We report on the compatibility of various nanowires with hippocampal neurons and the structural study of the neuron–nanowire interface. Si, Ge, SiGe, and GaN nanowires are compatible with hippocampal neurons due to their native oxide, but ZnO nanowires are toxic to neuron due to a release of Zn ion. The interfaces of fixed Si nanowire and hippocampal neuron, cross-sectional samples, were prepared by focused ion beam and observed by transmission electron microscopy. The results showed that the processes of neuron were adhered well on the nanowire without cleft.
KeywordsNanowires Neurons Coupling Interfaces TEM
Semiconductor nanowires have high aspect ratio, high surface area, and single crystallinity and thus are ideal building blocks for many devices on a nanometer scale [1, 2]. Among these, nanowire-based neuron devices that can monitor or stimulate neurons on a submicron dimension with high sensitivity have been recently noticed for their great potential in neuroscience . To realize a nanowire-based neuron device, coupling of nanowires with neurons is essential. Previous studies have shown that the coupling of Si or GaP nanowires with neurons is feasible [4, 5]. However, other semiconductor nanowires that can be considered for neuron devices have not yet been investigated. Meanwhile, monitoring or stimulating of neurons is strongly dependent on the nature of the interfaces between them [6, 7]. For example, the electronic coupling strength between neurons and devices is primarily dependent on the distance between the membrane and the device surface [8, 9]. In fact, the weak coupling between neuron and devices due to the extracellular cleft is one of the major problems in neuron-electronic interfaces. Analysis of the interfacial structures is thus essential in the design of nanowire-based neuron devices as well as for understanding the signal transfer mechanism. In the present study, we investigate the coupling of group IV (Si, Ge and SiGe), III-V (GaN), and oxide (ZnO) semiconductor nanowires with hippocampal neurons that are believed to be involved in the general and spatial memory, and characterize the coupled interface via transmission electron microscopy (TEM). Our results indicate that IV and III-V semiconductor nanowires are compatible with the neurons, whereas oxide semiconductor nanowires are not compatible. Characterization of the coupled Si nanowire–neuron interfaces shows two layers comprised of a coupling modifier and natural oxides with a thickness of ~8 nm. No clefts were found at the interfaces.
Synthesis of Nanowires
We synthesized Si (a), SiGe (b), Ge (c), and GaN (d) nanowires on a (a–c) Si (111) and (d) c-plane sapphire substrates coated with (a–c) Au, and (d) Ni as a VLS catalyst by a conventional CVD process employing (a) silicon tetrachloride (SiCl4, Alfa, 99.999%) as a silicon source, (b) SiCl4 as a silicon source and germanium powder as the germanium source, (c) germanium tetrachloride (GeCl4, Alfa, 99.999%) as a germanium source, and (d) metallic Ga powder as a gallium source and ammonium gas as a nitrogen source [10–13]. The substrates were placed in the center of quartz tube, and powder sources were also placed at the near of substrates with a distance of 1 in. Carrier gas transfers the source precursor through a bubbler to the quartz reactor, and hydrogen and argon gas were used as diluent gases, which regulate the concentration of the mixture containing source gas and carrier gas. The temperature of the furnace was increased at a heating rate of 50 °C min −1 to 800 °C under flow of source and carrier gases and kept for 10 through 60 min and then cooled down to room temperature. ZnO nanowires were grown by a typical carbothermal reduction process. An equal amount of ZnO and graphite powders were mixed and transferred to an alumina boat inside the processing tube. The processing temperature varied from 800 to 950 °C . The all prepared nanowires were observed by a scanning electron microscopy.
The nanowires were dispersed in ethanol and laid on the Si wafer. After sterilization by ethanol and UV light, the surface of the nanowires was chemically modified by a poly-l-lysine (PLL) coating for cell adhesion. Hippocampal neurons were then cultured on the nanowires. Briefly, the hippocampal neurons were isolated from 16- to 18-day-old fetal Sprague–Dawley rats and incubated with 0.25% trypsin Hanks balanced salt solution (HBSS) at 37 °C for 15 min. Cells were then mechanically dissociated with fire-polished Pasteur pipettes by trituration and plated on prepared substrata in a 24-well-plate culture dish. Cells were maintained in Neurobasal/B27 medium containing 0.5 mMl-glutamine, 25 μM glutamate, 25 μM 2-mercaptoethanol, 100 units ml−1 penicillin, and 100 μg ml−1 streptomycin. 50,000 cells were incubated with a substrate deposited in a well of the 24-well plate at 37 °C in 5% CO2 incubator.
Results and Discussion
In the earlier mentioned characterization studies, no clefts, which might be caused by filled culture medium before drying, were found. In the previous characterizations of the interfaces between human embryonic kidney (HEK) cell and a Si field effect transistor (FET)  or cells on a SiO2 substrate , cleft with an average width of roughly 40 nm was observed, depending on the type of modifier. It is not clear why such clefts have not been observed in the present neuron–nanowires interfaces. It may due to the different growth behavior of the neurons on the nanostructured surfaces formed by the nanowires when compared to the flat FET surface  or the small contact area on a nanometer scale. Regardless of the mechanism, the neuron–nanowire couples may be advantageous for the development of neuron devices in terms of signal transfer and electronic coupling, since the clefts pose critical problems in relation to signal transfer and electronic coupling strength.
Many approaches can be considered for the fabrication of nanowire-based neuron devices, including coupling nanowire transistors to neurons [24, 25] and probing neurons with vertical nanowire array . In all of these cases, the signal is transferred through the interface. In this regard, the formation of tight-, very thin interfaces between nanowires and neurons would lend promise for monitoring and/or stimulating of neurons. Furthermore, as shown in Fig. 4a, the neurons can wrap the nanowires in a Ω shape or totally in the case of a vertical array. This aspect also is potentially advantageous for highly sensitive monitoring and/or stimulating of neurons, since and omega- or all-surround gating effect is expected, akin to advanced transistor structures .
In summary, we investigated the compatibility of various nanowires with hippocampal neurons. Si, Ge, SiGe, and GaN nanowires were found to be compatible to neurons under the present culturing conditions. However, ZnO nanowires are toxic to neurons as a result of the release of Zn ions from the nanowires. The interface of coupled Si nanowires and neurons shows no clefts and is comprised of a SiO2 layer of 4 nm and a PLL coating layer of 4 nm. Formation of omega-shaped, tightly bonded interfaces with very thin interfacial layers is promising for monitoring and/or stimulating neurons by nanowires.
This research was supported by a grant from the National Research Laboratory program (R0A-2007-000-20075-0) and Pioneer research program (2009-008-1529) for converging technology through the Korea Science and Engineering Foundation funded by the Ministry of Education, Science & Technology.
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