Highly effective separation of semiconducting carbon nanotubes verified via short-channel devices fabricated using dip-pen nanolithography.

We have verified a highly effective separation of semiconducting single-walled carbon nanotubes (sc-SWNTs) via statistical analysis of short-channel devices fabricated using multipen dip-pen nanolithography. Our SWNT separation technique utilizes a polymer (rr-P3DDT) that selectively interacts with and disperses sc-SWNTs. Our devices had channel lengths on the order of 300-500 nm, with an average of about 3 SWNTs that directly connected the source-drain electrodes. A total of 140 SWNTs were characterized, through which we have observed that all of the SWNTs exhibited semiconducting behavior with an average on/off current ratio of ~10(6). Additionally, we have characterized 50 SWNTs after the removal of rr-P3DDT, through which we have again observed semiconducting behavior for all of the SWNTs with similar electrical characteristics. The relatively low average on-conductance of 0.0796 µS was attributed to the distribution of small diameter SWNTs in our system and due to the non-ohmic Au contacts on SWNTs. The largely positive threshold voltages were shifted toward zero after vacuum annealing, indicating that the SWNTs were doped in air. To the best of our knowledge, this is the first time numerous SWNTs were electrically characterized using short-channel devices, through which all of the measured SWNTs were determined to be semiconducting. Hence, our semiconducting single-walled carbon nanotube sorting system holds a great deal of promise in bringing forth a variety of practical applications in SWNT electronics.

Dip-pen nanolithography of electrical contacts to single-walled carbon nanotubes.

This paper discusses a method for the direct patterning of Au electrodes at nanoscale resolution using dip-pen nanolithography, with proof-of-concept demonstrated by creating single-walled carbon nanotube devices. This technique enables insight into three key concepts at the nanoscale: using dip-pen nanolithography as an alternative to electron-beam lithography for writing contacts to carbon nanotubes, understanding the integrity of contacts and devices patterned with this technique, and on a more fundamental level, providing a facile method to compare and understand electrical and Raman spectroscopy data from the same isolated carbon nanotube. Electrical contacts to individual and small bundle single-walled carbon nanotubes were masked by an alkylthiol that was deposited via dip-pen nanolithography on a thin film of Au evaporated onto spin-cast, nonpercolating, and highly isolated single-walled carbon nanotubes. A wet Au etching step was used to form the individual devices. The electrical characteristics for three different single-walled carbon nanotube devices are reported: semimetallic, semiconducting, and metallic. Raman analysis on representative devices corroborates the results from AFM imaging and electrical testing. This work demonstrates a technique for making electrical contact to nanostructures of interest and provides a platform for directly corroborating electrical and optical measurements. The merits of using dip-pen nanolithography include flexible device configuration (such as varying the channel length and the number, size, and orientation of contacts), targeted patterning of individual devices with imaging and writing conducted in the same instrument under ambient conditions, and negligible damage to single-walled carbon nanotubes during the fabrication process.

Substituent effects in pentacenes: gaining control over HOMO-LUMO gaps and photooxidative resistances.

A combined experimental and computational study of a series of substituted pentacenes including halogenated, phenylated, silylethynylated and thiolated derivatives is presented. Experimental studies include the synthesis and characterization of six new and six known pentacene derivatives and a kinetic study of each derivative under identical photooxidative conditions. Structures, HOMO-LUMO energies and associated gaps were calculated at the B3LYP/6-311+G**//PM3 level while optical and electrochemical HOMO-LUMO gaps were measured experimentally. The combined results provide for the first time a quantitative assessment of HOMO-LUMO gaps and photooxidative resistances for a large series of pentacene derivatives as a function of substituents. The persistence of each pentacene derivative is impacted by a combination of steric resistance and electronic effects as well as the positional location of each substituent. Silylethynyl-substituted pentacenes like TIPS-pentacene possess small HOMO-LUMO gaps but are not the longest lived species under photooxidative conditions, contrary to popular perception. A pentacene derivative with both chlorine substituents in the 2,3,9,10 positions and o-alkylphenyl substituents in the 6,13 positions is longer lived than TIPS-pentacene. Of all the derivatives studied, alkylthio- and arylthio-substituted pentacenes are most resistant to photooxidation, possess relatively small HOMO-LUMO gaps and are highly soluble in a variety of organic solvents. These results have broad implications for the field of organic molecular electronics where OFET, OLED, and other applications can benefit from highly persistent, solution processable pentacene derivatives.

Toxicity of CdSe Nanoparticles in Caco-2 Cell Cultures.

BACKGROUND: Potential routes of nanomaterial exposure include inhalation, dermal contact, and ingestion. Toxicology of inhalation of ultra-fine particles has been extensively studied; however, risks of nanomaterial exposure via ingestion are currently almost unknown. Using enterocyte-like Caco-2 cells as a small intestine epithelial model, the possible toxicity of CdSe quantum dot (QD) exposure via ingestion was investigated. Effect of simulated gastric fluid treatment on CdSe QD cytotoxicity was also studied. RESULTS: Commercially available CdSe QDs, which have a ZnS shell and poly-ethylene glycol (PEG) coating, and in-house prepared surfactant coated CdSe QDs were dosed to Caco-2 cells. Cell viability and attachment were studied after 24 hours of incubation. It was found that cytotoxicity of CdSe QDs was modulated by surface coating, as PEG coated CdSe QDs had less of an effect on Caco-2 cell viability and attachment. Acid treatment increased the toxicity of PEG coated QDs, most likely due to damage or removal of the surface coating and exposure of CdSe core material. Incubation with un-dialyzed in-house prepared CdSe QD preparations, which contained an excess amount of free Cd2+, resulted in dramatically reduced cell viability. CONCLUSION: Exposure to CdSe QDs resulted in cultured intestinal cell detachment and death; cytotoxicity depended largely, however, on the QD coating and treatment (e.g. acid treatment, dialysis). Experimental results generally indicated that Caco-2 cell viability correlated with concentration of free Cd2+ ions present in cell culture medium. Exposure to low (gastric) pH affected cytotoxicity of CdSe QDs, indicating that route of exposure may be an important factor in QD cytotoxicity.

Large scale 3D vertical assembly of single-wall carbon nanotubes at ambient temperatures.

We demonstrate three-dimensional directed assembly of single-wall carbon nanotubes (SWNT) into porous alumina nanotemplates on silicon substrates by means of electrophoresis and dielectrophoresis at ambient temperatures. Assembled SWNT provided an interconnection between the surface and base of the nanotemplate. I-V measurements clearly show that the connection between silicon and SWNT is established inside the templates. This technique is particularly useful for large scale, rapid, 3D assembly of SWNT over centimeter square areas under mild conditions for nanoscale electronics applications.

Reusable, reversibly sealable parylene membranes for cell and protein patterning.

The patterned deposition of cells and biomolecules on surfaces is a potentially useful tool for in vitro diagnostics, high-throughput screening, and tissue engineering. Here, we describe an inexpensive and potentially widely applicable micropatterning technique that uses reversible sealing of microfabricated parylene-C stencils on surfaces to enable surface patterning. Using these stencils it is possible to generate micropatterns and copatterns of proteins and cells, including NIH-3T3 fibroblasts, hepatocytes and embryonic stem cells. After patterning, the stencils can be removed from the surface, plasma treated to remove adsorbed proteins, and reused. A variety of hydrophobic surfaces including PDMS, polystyrene and acrylated glass were patterned using this approach. Furthermore, we demonstrated the reusability and mechanical integrity of the parylene membrane for at least 10 consecutive patterning processes. These parylene-C stencils are potentially scalable commercially and easily accessible for many biological and biomedical applications.

Cell and protein compatibility of parylene-C surfaces.

Parylene-C, which is traditionally used to coat implantable devices, has emerged as a promising material to generate miniaturized devices due to its unique mechanical properties and inertness. In this paper we compared the surface properties and cell and protein compatibility of parylene-C relative to other commonly used BioMEMS materials. We evaluated the surface hydrophobicity and roughness of parylene-C and compared these results to those of tissue culture-treated polystyrene, poly(dimethylsiloxane) (PDMS), and glass. We also treated parylene-C and PDMS with air plasma, and coated the surfaces with fibronectin to demonstrate that biochemical treatments modify the surface properties of parylene-C. Although plasma treatment caused both parylene-C and PDMS to become hydrophilic, only parylene-C substrates retained their hydrophilic properties over time. Furthermore, parylene-C substrates display a higher degree of nanoscale surface roughness (>20 nm) than the other substrates. We also examined the level of BSA and IgG protein adsorption on various surfaces and found that surface plasma treatment decreased the degree of protein adsorption on both PDMS and parylene-C substrates. After testing the degree of cell adhesion and spreading of two mammalian cell types, NIH-3T3 fibroblasts and AML-12 hepatocytes, we found that the adhesion of both cell types to surface-treated parylene-C variants were comparable to standard tissue culture substrates, such as polystyrene. Overall, these results indicate that parylene-C, along with its surface-treated variants, could potentially be a useful material for fabricating cell-based microdevices.

Generation of static and dynamic patterned co-cultures using microfabricated parylene-C stencils.

Many biological processes, such as stem cell differentiation, wound healing and development, involve dynamic interactions between cells and their microenvironment. The ability to control these dynamic processes in vitro would be potentially useful to fabricate tissue engineering constructs, study biological processes, and direct stem cell differentiation. In this paper, we used a parylene-C microstencil to develop two methods of creating patterned co-cultures using either static or dynamic conditions. In the static case, embryonic stem (ES) cells were co-cultured with fibroblasts or hepatocytes by using the reversible sealing of the stencil on the substrate. In the dynamic case, ES cells were co-cultured with NIH-3T3 fibroblasts and AML12 hepatocytes sequentially by engineering the surface properties of the stencil. In this approach, the top surface of the parylene-C stencil was initially treated with hyaluronic acid (HA) to reduce non-specific cell adhesion. The stencil was then sealed on a substrate and seeded with ES cells which adhered to the underlying substrate through the holes in the membrane. To switch the surface properties of the parylene-C stencils to cell adhesive, collagen was deposited on the parylene-C surfaces. Subsequently, a second cell type was seeded on the parylene-C stencils to form a patterned co-culture. This group of cells was removed by peeling off the parylene-C stencils, which enabled the patterning of a third cell type. Although the static patterned co-culture approach has been demonstrated previously with a variety of methods, layer-by-layer modification of microfabricated parylene-C stencils enables dynamic patterning of multiple cell types in sequence. Thus, this method is a promising approach to engineering the complexity of cell-cell interactions in tissue culture in a spatially and temporally regulated manner.

Three-dimensional assembly of single-walled carbon nanotube interconnects using dielectrophoresis.

We present a hybrid approach that combines top-down fabrication with bottom-up directed assembly for making single-walled carbon nanotube (SWNT) based three-dimensional interconnects. The SWNTs are assembled using dielectrophoresis at room temperature on a microfabricated 3D platform. The two-terminal resistance of the assembled SWNTs at 10 Vpp assembly voltage is approximately 545 Ω. Simulation of the dielectrophoretic assembly is carried out to understand the behavior of the SWNTs during assembly. Encapsulation of these devices using a conformal pinhole-free parylene layer resulted in a decrease of the total resistance.