Shuli Wang

Janus Si Micropillar Arrays with Thermal-Responsive Anisotropic Wettability for Manipulation of Microfluid Motions

In this paper, Janus micropillar array (MPA) with fore-aft controllable wettability difference was demonstrated. With two-step modification process, we successfully decorate the Janus pillar skeletons with wettability-switchable polymer brush on one side and hydrophilic self-assembled monolayer on the other. Owing to the switchable wettability of the polymer brush, the patterned surface could switch between anisotropic wetting and isotropic wetting at different temperatures, which gives the possibility of coupling the well-designed surface with microfluidic channel to manipulate the microfluid motion. Additionally, a further photothermal control of microfluid was also established based on the thermal-responsive Janus MPA through introducing infrared light to adjust the temperature of the microfluidic system. We believe that the thermal-responsive Janus micropillar arrays would provide a new strategy to control the flow and motion of fluids in microfluidic channels and show potential applications in the future microfluidic chips.

Morphology-Patterned Anisotropic Wetting Surface for Fluid Control and Gas–Liquid Separation in Microfluidics

This article shows morphology-patterned stripes as a new platform for directing flow guidance of the fluid in microfluidic devices. Anisotropic (even unidirectional) spreading behavior due to anisotropic wetting of the underlying surface is observed after integrating morphology-patterned stripes with a Y-shaped microchannel. The anisotropic wetting flow of the fluid is influenced by the applied pressure, dimensions of the patterns, including the period and depth of the structure, and size of the channels. Fluids with different surface tensions show different flowing anisotropy in our microdevice. Moreover, the morphology-patterned surfaces could be used as a microvalve, and gas-water separation in the microchannel was realized using the unidirectional flow of water. Therefore, benefiting from their good performance and simple fabrication process, morphology-patterned surfaces are good candidates to be applied in controlling the fluid behavior in microfluidics.

Nanotransfer printing of gold disk, ring and crescent arrays and their IR range optical properties

We demonstrate a facile method to fabricate gold plasmonic microstructures based on the combination of colloidal lithography and a nanotransfer printing method. Poly(dimethylsiloxane) PDMS hemisphere arrays were fabricated through colloidal lithography and used as a “stamp” for the nanotransfer printing. Three kinds of plasmonic microstructures, gold disk, ring and crescent arrays, were fabricated by transferring gold “ink” onto the PDMS stamp, then to the substrate based on covalent “glue”. By adjusting the pressure applied during the printing process, the diameter of the as-prepared gold disks and gold rings can be precisely controlled, and these plasmonic arrays all exhibited significant diameter dependent LSPR properties in the NIR or Mid-IR range. In addition, by obliquely depositing gold ink onto the PDMS stamp, a gold crescent array with asymmetrical geometry was also prepared on the substrate. Owing to the asymmetric structure of the gold crescents, the gold crescent array showed significant polarization dependent LSPR properties in the Mid-IR range. We believe that these as-prepared gold plasmonic microstructures could show promising potential for application as real-time, label-free plasmonic sensing platforms in the IR range.

Responsive etalon based on PNIPAM@SiO2 composite spacer with rapid response rate and excellent repeatability for sensing application

In this paper, we demonstrate a responsive etalon fabricated through combining colloidal lithography and surface-initiated atom-transfer radical polymerization (SI-ATRP). The responsive etalon is simply constructed with one responsive spacer sandwiched by two reflective layers, and the middle responsive spacer is constructed by grafting thermo-responsive poly(N-isopropylacrylamide) (PNIPAM) brushes on a SiO2 nanosphere array. The etalon possesses one single interference peak in the visible region, and the interference peak changes sensitively against the concentration of the external stimulant (water vapor) or the temperature of the system, owing to the responsiveness of the PNIPAM brush. Importantly, the as-prepared etalon shows a rapid response rate and excellent stability, and it is also handy to realize the miniaturization and integration of the responsive etalon based on a conventional micro-fabrication method. These features all make the as-prepared responsive etalon an attractive candidate for future sensing applications. We believe such responsive etalons are promising for the fabrication of smart photonic materials and optical sensors that may be useful in tissue engineering, medical diagnosis, public security, and biochip areas.

Ag nanoparticle/polymer composite barcode nanorods

We demonstrate a facile method combining colloidal lithography, selective ion-exchange, and the in situ reduction of Ag ions (Ag+) for the fabrication of multi-segmented barcode nanorods. First, polymer multilayer films were prepared by spin-coating alternating thin films of polystyrene and polyacrylic acid (PAA), and then multi-segmented polymer nanorods were fabricated via reactive ion etching with colloidal masks. Second, Ag nanoparticles (Ag NPs) were incorporated into the PAA segments by an ion exchange and the in situ reduction of the Ag+. The selective incorporation of the Ag NPs permitted the modification of the specific bars of the nanorods. Lastly, the Ag NP/polymer composite nanorods were released from the substrate to form suspensions for further coding applications. By increasing the number of segments and changing the length of each segment in the nanorods, the coding capacity of nanorods was improved. More importantly, this method can easily realize the density tuning of Ag NPs in different segments of a single nanorod by varying the composition of the PAA segments. We believe that numerous other coded materials can also be obtained, which introduces new approaches for fabricating barcoded nanomaterials.

Unidirectional Wetting of Liquids on “Janus” Nanostructure Arrays under Various Media

We report the unidirectional wetting behavior of liquids (water and oil) on Janus silicon cylinder arrays (Si-CAs) under various media (air, water, and oil). The Janus cylinders were prepared by chemical modification of nanocylinders with different molecules on two sides. Through adjusting surface energies of the modified molecules, the as-prepared surfaces could control the wetting behavior of different types of liquids under various media. We discuss the regularity systematically and propose a strategy for preparing anisotropic wetting surfaces under arbitrary media. That is, to find two surface modification molecules with different surface energies, one of the molecules is easy to be wetted by the liquid under the corresponding media, while the other one is difficult. Additionally, by introducing thermal-responsive polymer brushes onto one part of Janus Si-CAs, the surfaces show thermal-responsive anisotropic wetting property under various media. We believe that due to the excellent unidirectional wettability under various media, the Janus surfaces could be applied in water/oil transportation, oil-repellent and self-cleaning coatings, water/oil separation, microfluidics, and so on.

Facile fabrication of homogeneous and gradient plasmonic arrays with tunable optical properties via thermally regulated surface charge density

A facile strategy is reported for the electrostatic self-assembly of homogeneous and gradient plasmonic nanoparticle arrays with tunable interparticle distances and optical properties. The interparticle distance is dominated by the surface charge density of the substrate, which is tuned via thermal annealing according to the temperature-dependent molecular mobility of the polymer. Oxygen plasma is employed to endow neutral polystyrene (PS) films with sufficient charges, enabling subsequent electrostatic adsorption. The density of surface charges can be readily tuned via a thermal annealing step after plasma treatment, which is confirmed by quantitative analyses of oxygen and nitrogen using X-ray photoelectron spectroscopy. Afterwards, PS films with regulated charge densities reshape the double layers around nanoparticles to various degrees during the assembly, leading to tunable interparticle separations. UV-Vis spectroscopy reveals tunable plasmonic properties owing to the critical role of interparticle separation in plasmon coupling. Here such structures are demonstrated to act as wavelength-selective substrates for multiplexed acquisition of surface enhanced Raman scattering. Alternatively, by applying a temperature gradient in the annealing step, we create a macroscopic surface with a continuous gradient in plasmonic properties. Such a “plasmonic library” can be a promising material for fast screening of interparticle distance or extinction spectrum in specific applications on one single substrate.