Yibo Gao

Structural dependence of silver nanowires on polyvinyl pyrrolidone (PVP) chain length

The effect of the chain length of polyvinyl pyrrolidone (PVP) on the structures of silver nanowires (AgNWs) is explored in this study. It was found in the experiments that PVP, when serving as a capping agent, has a great impact on the morphology and structure of AgNWs. By means of a series of experiments and the inquiry of the growth mechanism, the critical minimum PVP chain length for the successful formation of uniform nanowires was discovered, below which only nanoparticles or short nanorods can be obtained. Surprisingly, a core-shell structure of a nanowire with a polycrystal was observed when PVP with a very long chain length was employed in the processing.

Low-frequency tunable acoustic absorber based on split tube resonators

We demonstrate a high-efficiency tunable acoustic absorber for low frequencies (<500 Hz) with subwavelength thickness. The acoustic absorber is based on split tube resonators and could reach high-efficiency absorption at tunable resonance frequency with wavelength in air at least 30 times larger than its total thickness in simulations and experiments. The resonance frequency and high-efficiency absorption of the absorber are robust under oblique incidence even at large angles. The absorber could have potential applications for acoustic engineering due to its high structural stability, ease of fabrication, subwavelength thickness, and robust high-efficiency.

Real-time concentration monitoring in microfluidic system via plasmonic nanocrescent arrays

In this work, on-chip bio/chemical sensor was reported based on localized surface plasmon resonance of nanocrescent patterns fabricated via electron beam lithography. The nanocrescent arrays with different dimensional features exhibited controllable plasmonic properties in accordance with the simulation results based on the finite-difference time-domain model. The highest refractive index sensitivity of the fabricated samples was achieved to be ~699.2 nm/RIU with a figure of merit of ~3.1 when the two opposite crescents own a gap of ~43.3 nm. Such obtained plasmonic sensor was further integrated into the microfluidic system which can simply control the specific analyte concentrations via tuning the flow rate ratios between two injecting microstreams. Our method has successfully demonstrated the capability of the nanocrescent patterns as on-chip plasmonic bio/chemical sensor for real-time monitoring of dynamic concentrations in the microchannel.

Enhanced photochromic efficiency of transparent and flexible nanocomposite films based on PEO–PPO–PEO and tungstate hybridization

Nanocomposite hybrid films were prepared by depositing tungstate on PEO–PPO–PEO (EPE) templates under acidic conditions by a one-pot self-assembly sol–gel method. Uniform and transparent films based on this precursor were fabricated and easily manipulated using a facile casting method. Upon irradiation by sunlight, the film exhibits a rapid photochromic response that was reversible at room temperature. UV-Vis analysis revealed that folding of the –CH2–O– chains in the polymer and the larger tungstate clusters increased the red shift for the adsorption of visible light. The mechanism underlying the effect of hybridization of polyethylene oxide (PEO) on the enhanced photochromic effect was characterized by NMR, Raman and FTIR. The degree of folding of the –CH2–O– polymer chains was influenced by the presence of acid, which increased the oxygen coordination of WO6 and WO4 in the tungstate clusters. The characterized W–O bonding peaks shifted upon EPE incorporation, consistent with changes in oxygen coordination within the material. This weak coordination between tungstate and –CH2–O– improved the intervalence charge transfer (IVCT). Oxygen from EPE may enter the oxygen sites in the O–W–O clusters to produce an unbalanced electron state, resulting in oxygen defects, which has a critical effect on the enhancement of the photochromic response. In addition, this method provides protons via this weak coordination, thereby dramatically enhancing the efficiency of the hybrid film.

On-chip DNA preconcentration in different media conductivities by electrodeless dielectrophoresis

Electrodeless dielectrophoresis is the best choice to achieve preconcentration of nanoparticles and biomolecules due to its simple, robust, and easy implementation. We designed a simple chip with microchannels and nano-slits in between and then studied the trapping of DNA in high conductive medium and low conductive medium, corresponding to positive and negative dielectrophoresis (DEP), respectively. It is very important to investigate the trapping in media with different conductivities since one always has to deal with the sample solutions with different conductivities. The trapping process was analyzed by the fluorescent intensity changes. The results showed that DNA could be trapped at the nano-slit in both high and low conductive media in a lower electric field strength (10 V/cm) compared to the existing methods. This is a significant improvement to suppress the Joule heating effect in DEP related experiments. Our work may give insight to researchers for DNA trapping by a simple and low cost device in the Lab-on-a-Chip system.

Functionalized PDMS with Versatile and Scalable Surface Roughness Gradients for Cell Culture.

This manuscript describes a simple and versatile approach to engineering surface roughness gradients via combination of microfluidics and photopolymerization. Through UV-mediated polymerization, N-isopropylacrylamide with concentration gradients are successfully grafted onto PDMS surface, leading to diverse roughness degrees on the obtained PDMS substrate. Furthermore, the extent of surface roughness can be controllably regulated via tuning the flow rate ratio between the monomer solution and deionized water. Average roughness ranging from 2.6±0.7 nm to 163.6±11.7 nm has been well-achieved in this work. Such PDMS samples are also demonstrated to be capable of working as supporting substrates for controlling cell adhesion or detachment. Because of the different degrees of surface roughness on a single substrate, our method provides an effective approach for designing advanced surfaces for cell culture. Finally, the thermosensitive property of N-isopropylacrylamide makes our sample furnish as another means for controlling the cell detachment from the substrates with correspondence to the surrounding temperature.

Digital microfluidic programmable stencil (dMPS) for protein and cell patterning

Patterning biomolecules and cells on substrates is usually a prerequisite for biological analysis and cell studies. A stencil is a versatile tool for sample patterning owing to its reusability and easy operation but it lacks addressable fluid control and programmable change of pattern. Here, we combined the advantages of a microfluidic chip and stencil to design a digitally controlled microfluidic programmable stencil. The key design is automatic passive matrix addressing based on combined application of two types of elastomeric valves. These two valves have distinct actuation thresholds, typically 13.4 psi for a round valve and much higher than 13.4 psi for a rectangle valve. Different types of protein and cell pattern on a 2D substrate could be obtained by controlling the fluid addressing code based on the passive matrix addressing method. An automatic microsampler was also applied to facilitate fast sample selection and introduction in the patterning process. A successful protein and cell pattern was obtained which could be used for downstream analysis and study.

Control the Drying Configuration of Suspensions via Regulating the Surface Topologies for Surface-Enhanced Raman Scattering Optimization

In this work, the authors present an innovative method to efficiently control the drying configuration of gold nanoparticles (AuNPs) for optimization of surface-enhanced Raman scattering (SERS) performance with improved sensitivity and re-producibility. Via repeated grafting-casting processes, we have simply regulated the surface topologies of polydimethylsiloxane (PDMS) with average surface roughness ranging from 1.4nm to 651.3nm. The assembling configurations of AuNPs after completed evaporation of solvent have been systematically studied on substrates with different roughness degrees. Furthermore, we found that based on the suspended droplet drying method, the inter-gaps among AuNPs can be well optimized on rougher substrate than on flat PDMS. The SERS spectra based on diverse substrates are investigated and compared, with the best available results arising from the roughest PDMS substrate accompanied by suspended drying means. By introducing rhodamine 6G as the probe molecule, such convenient method enables the detection limit down to 10-14mol/L with linear relationship between the Raman intensity and analyte concentrations within 10-8-10-14mol/L. The experimental results confirm the superiorities of our proposed method for preparation of SERS substrate, and thus providing a straightforward avenue for future construction of efficient and sensitive platform in fields from environmental monitoring to biological sensors.

A valve-free 2D concentration gradient generator

We report a concentration gradient generator featuring valve-free and simultaneous two-dimensional (2D) concentration generation. Each analyte is first diluted by a 1D gradient generator to five different concentrations and then directed into different chambers to generate combinations of different concentrations. The chip consists of three layers, which realizes a 3D fluid path, forming a skywalk structure that allows two perpendicular channels to cross each other in different layers. In this way, this chip could get rid of the pneumatic actuated valves (PAVs) or complicated channel designs that are conventionally adopted in controlling the infusion of multi-analytes. We believe that this design, which makes the chip independent of cumbersome external apparatus in controlling multi-analytes infusion, may potentially benefit the process of making microfluidic chips a portable and cost-effective product.

Synergistic Effect of Sunlight Induced Photothermal Conversion and H 2 O 2 Release Based on Hybridized Tungsten Oxide Gel for Cancer Inhibition

A highly efficient photochromic hydrogel was successfully fabricated via casting precursor, which is based on amorphous tungsten oxide and poly (ethylene oxide)-block-poly (propylene oxide)-block-poly (ethylene oxide). Under simulated solar illumination, the hydrogel has a rapid and controlled temperature increasing ratio as its coloration degree. Localized electrons in the amorphous tungsten oxide play a vital role in absorption over a broad range of wavelengths from 400 nm to 1100 nm, encompassing the entire visible light and infrared regions in the solar spectrum. More importantly, the material exhibits sustainable released H2O2 induced by localized electrons, which has a synergistic effect with the rapid surface temperature increase. The amount of H2O2 released by each film can be tuned by the light irradiation, and the film coloration can indicate the degree of oxidative stress. The ability of the H2O2-releasing gels in vitro study was investigated to induce apoptosis in melanoma tumor cells and NIH 3T3 fibroblasts. The in vivo experimental results indicate that these gels have a greater healing effect than the control in the early stages of tumor formation.