Jau-Ye Shiu

Nanofluidic system for the studies of single DNA molecules

Here, we describe a simple and low‐cost lithographic technique to fabricate size‐controllable nanopillar arrays inside the microfluidic channels for the studies of single DNA molecules. In this approach, nanosphere lithography has been employed to grow a single layer of well‐ordered close‐packed colloidal crystals inside the microfluidic channels. The size of the polymeric colloidal nanoparticles could be trimmed by oxygen plasma treatment. These size‐trimmed colloidal nanoparticles were then used as the etching mask in a deep etching process. As a result, well‐ordered size‐controllable nanopillar arrays could be fabricated inside the microfluidic channels. The gap distance between the nanopillars could be tuned between 20 and 80 nm allowing the formation of nanofluidic system where the behavior of a single λ‐phage DNA molecule has been investigated. It was found that the λ‐phage DNA molecule could be fully stretched in the nanofluidic system formed by nanopillars with 50 nm gap distance at a field of 50 V/cm.

Polymeric nanopillar arrays for cell traction force measurements

This paper reports the development of a novel force measurement device based on polymeric nanopillar arrays. The device was fabricated by a process combining nanosphere lithography, oxygen plasma treatment, deep etching and nano‐molding. Well‐ordered polymeric nanopillar arrays with various diameters and aspect ratios were fabricated and used as cell culture substrates. Cell traction forces were measured by the deflection of the nanopillars. Since the location of the nanopillars can be monitored at all times, this device allows for the measurement of the evolution of adhesion forces over time.

On chip sorting of bacterial cells using sugar-encapsulated magnetic nanoparticles

Here we describe an integrated microfluidic sorting device, which utilized sugar-encapsulated magnetic nanoparticles to separate a specific strain of bacteria from a mixture solution. In our system, microfluidic devices consisting of two inlets and an electromagnet or a permanent magnet have been constructed by a soft lithography process. The magnetic field generated by either the electromagnet or the permanent magnet was strong enough to attract the bacteria bound to magnetic nanoparticles to cross the stream boundary of the laminar flow. The sorting efficiency was found to depend on both the flow rate and the strength of the magnetic field. The maximum sorting efficiency was measured to be higher than 90% and the selectivity was almost 100%. Since this microfluidic sorting device was able to separate 103 bacterial cells within 1min, it could be used for pathogenic diagnose applications.

Fabrication of tunable superhydrophobic surfaces

Inspired by the water-repellent behavior of the micro- and nano-structured plant surfaces, superhydrophobic materials, with a water contact larger than 150 degree, have received a lot of research attentions recently. It has been suggested that contamination, oxidation and current conduction can be inhibited on such superhydrophobic surfaces, and the flow resistance in the microfluidic channels can also be reduced using super water-repellent materials. In order to prepare superhydrophobic materials, we have developed two simple approaches for fabricating tunable superhydrophobic surfaces using nanosphere lithography and plasma etching. In the first case, the polystyrene nanospheres were employed to create well-ordered rough surfaces covered by gold and alkylthiols. Using oxygen plasma treatment, the topmost surface area can be modified systematically, as the result the water contact angle on such surfaces can be tuned from 132 to 170 degree. The water contact angles measured on these surfaces can be modeled by the Cassie’s formulation without any adjustable parameter. In the second approach, thin films of Teflon were spin-coated on the substrate surfaces and treated by oxygen plasma. Superhydrophobic surfaces with water contact angle up to 170 degree were obtained by this approach. If the ITO glasses were used as the substrates, the hydrophobicity of the surface can be tuned by applying DC voltage. Water contact angle can be adjusted from 158 degree to 38 degree.

Revealing the spatial distribution of the site enhancement for the surface enhanced Raman scattering on the regular nanoparticle arrays

The spatial distribution of the site enhancement for the surface-enhanced Raman scattering (SERS) on the regular nanoparticle arrays has been investigated by the confocal Raman microscopy. It was found that the spatial distribution of the Raman signals on the well-ordered nanoparticle arrays was very inhomogeneous and concentrated on the defects of the nanoparticle arrays. The SERS signals were also observed to depend on the thickness of silver film and the defect density. It has been demonstrated that the number of SERS active sites can be increased ten folds by trimming the size of nanoparticles using oxygen plasma.

Investigation of the growth of focal adhesions using protein nanoarrays fabricated by nanocontact printing using size tunable polymeric nanopillars

Here we describe a simple approach to create various sizes of protein nanoarrays for the investigation of cell adhesion. Using a combination of nanosphere lithography, oxygen plasma treatment, deep etching and nanomolding processes, well-ordered polymeric nanopillar arrays have been fabricated with diameters in the range of 50-600 nm. These nanopillar arrays were used as stamps for nanocontact printing to create fibronectin nanoarrays, which were used to study the size dependent formation of focal adhesion. It was found that cells can adhere and spread on fibronectin nanoarrays with a fibronectin pattern as small as 50 nm. It was also found that the average size of focal adhesion decreased as the size of the fibronectin pattern was reduced.

High Density Addressable Protein and Cell Patterning via Switchable Superhydrophobic Microarrays

There have been increasing research interests in the development of novel nano or micro-patterning techniques to create arrays of functional biomolecules for miniaturized assays, which could be used in various biomedical applications such as biosensors, proteomic, immunoassays or drug screening. Several techniques have been demonstrated capable of writing or printing biomolecules with very high degree of spatial control including dip-pen lithography, nanopipet, inkjet printing, photolithography, micro contact printing etc. The serial writing techniques provide the individual addressability whereas the parallel printing processes offer easy and fast protein patterning. However, it remains difficult to create complex patterns, such as functional multicomponent protein arrays, in parallel with individual addressability. Here we describe a novel approach to fabricate functional multicomponent protein and cell arrays where the electrowetting effect was employed to convert a water-repellent superhydrophobic surface into a wettable one allowing fast and high density addressable protein and cell deposition on the otherwise protein- and cell-resistant superhydrophobic surface. It has been shown that each element on the switchable superhydrophobic microarray could be addressed individually and different types of functional biomolecules could be selectively deposited on the microarray. Such protein patterning technique offers several advantages over other techniques including parallel protein deposition with individual addressability, possibility for large-scale patterning and easy integration with microfluidic system. The facts that the superhydrophobic surfaces can be fabricated from many hydrophobic materials and the electrowetting works for most dielectric materials allow great flexibility in the selection of the surface materials for protein deposition.

Actively controlled self-assembly of colloidal crystals in microfluidic networks

Self-assembly is a commonly used strategy in synthesis and fabrication. One of the most economic routes for the fabrication of large ensembles of functional nanosystem is to utilize self-assembly to assemble building blocks such as colloids, nanotubes and nanowires. However, if the functional nanostructures are to be assembled across many length scales within the integrated system, it is necessary to develop new tools for large-scale assembly of nanostructures and manipulation of individual components. Here we report a simple approach to actively control the formation of the self-assembled colloidal crystals in the two-dimensional microfluidic networks. Utilizing a combination of electrocapillary forces and evaporation induced self-assembly, it is possible to actively control the self-assembly process of the colloidal nanoparticles to form colloidal crystals inside the two-dimensional microchannel networks. Using this approach, we can not only selectively fabricate the colloidal crystals in the desired channels, but we can also build colloidal crystals with different optical properties in different channels or in the same channel. This method is not limited to the fabrication of colloidal crystals. In general, it can be configured to produce other novel functional materials using self-assembly process when it is integrated with more sophisticated microfluidic system.

The Magnetic Behavior of Triangular Shaped Permalloy Nanomagnet Arrays

During the past few decades, the density of magnetic storage has been improved considerably. To increase the storage capacity, it is necessary to reduce the size of magnetic grains. However, as the domain size decreases, their thermal stability will also decrease, which results in the loss of magnetization. To overcome the limit imposed by such superparamagnetic behavior, lots of recent research attentions have been focused on the patterned magnetic media. To maximize the storage density, it is preferable to create periodical magnetic patterns, in which single-domain magnetic dots are well separated from each other. In this experiment, we have utilized nanosphere lithography to create large-area well-ordered two dimension arrays of permalloy (Ni 80 Fe 20 ) nanoparticles. Nanosphere lithography is an inexpensive, simple, parallel, and high throughput fabrication technique. We have employed monodisperse polystyrene beads with diameter of 650, 560, 440, 350, 280 nm to fabricate triangle-shaped permalloy (Ni 80 Fe 20 ) nano-arrays with lateral dimension in the region of 170∼90 nm, and thickness in the region of 10∼50 nm. The magnetic behavior of these triangle-shaped nanomagnet arrays have been investigated by longitudinal magnetic optic Kerr effect (LMOKE) and magnetic force microscopy (MFM). It was found that the coercivity of the permalloy nanoparticle arrays increases with decreasing the thickness of the nanoparticle. This can be attributed to the interface effect between the arrays and the substrate.