Yougen Hu

Preparation of large micron-sized monodisperse polystyrene/silver core–shell microspheres with compact shell structure and their electrical conductive and catalytic properties

Utilizing a simple improved electroless plating method, ca. 6 μm sized monodisperse polystyrene/silver (PS/Ag) core–shell microspheres with a complete, homogeneous and compact coverage of Ag nanoparticles layer were successfully prepared. In this approach, modified lightly cross-linked PS (LCPS) microspheres with a uniform diameter of 5.6 μm were used as templates. After the LCPS cores were removed by dissolving with dimethyl formamide (DMF), the outer silver shells assembled by amounts of Ag nanoparticles maintain good hollow spherical structure. The size and coverage degree of Ag nanoparticles on the PS/Ag microspheres could be easily tuned by changing the concentration of [Ag(NH3)2]+ ions in aqueous media. The electrical conductivity of the obtained PS/Ag core–shell microspheres changes from 3.76 × 104 S m−1 to 3.33 × 105 S m−1 as increasing the coverage density of Ag nanoparticles. Moreover, the catalytic assays indicate that the resultant monodisperse PS/Ag core–shell microspheres show excellent catalytic activity for the reduction of methylene blue (MB) by NaBH4. In particular, the corresponding hollow Ag spheres exhibit more outstanding catalytic activity due to their stable hollow structure and higher specific surface area.

Low cost and highly conductive elastic composites for flexible and printable electronics

Printable elastic conductive composites with high conductivity and flexibility have exciting applications in burgeoning flexible electronics. However, the low-cost fabrication of flexible conductors is still a great challenge, especially they can be directly printed on various substrates even including the wearable woven textiles, and give perfect printed patterns. Herein, we report a facile and low-cost strategy to fabricate directly printable elastic conductive composites with a conductivity of 4.12 × 104 S m−1 at a low silver loading of 42.88 wt%. The elastic conductor is simply comprised of core–shell polystyrene/silver (PS@Ag) hybrid conductive particles with an average size of 5.86 μm and a viscous polydimethylsiloxane (PDMS) matrix. The viscous mixture pastes can be easily fabricated into conductive films or directly printed on various substrates to obtain elastic conductive circuits with arbitrary geometries and high resolution by the screen printing method. The fabricated elastic PS@Ag/PDMS conductor exhibits high electrical conductivity and good electrical stability under vigorous cycles of mechanical deformations, such as bending, crimping and stretching. These merits make it a competitive candidate for flexible circuits and wearable electronic devices.

Highly Sensitive Flexible Pressure Sensor Based on Silver Nanowires-Embedded Polydimethylsiloxane Electrode with Microarray Structure

Flexible pressure sensors have attracted increasing research interest because of their potential applications for wearable sensing devices. Herein, a highly sensitive flexible pressure sensor is exhibited based on the elastomeric electrodes and a microarray architecture. Polydimethylsiloxane (PDMS) substrate, coated with silver nanowires (AgNWs), is used as the top electrode, while polyvinylidene fluoride (PVDF) as the dielectric layer. Several transfer processes are applied on seeking facile strategy for the preparation of the bottom electrode via embedding AgNWs into the PDMS film of microarray structure. The flexible pressure sensor integrates the top electrode, dielectric layer, and microarray electrode in a sandwich structure. It is demonstrated that such sensors possess the superiorities of high sensitivity (2.94 kPa-1), low detection limit (<3 Pa), short response time (<50 ms), excellent flexibility, and long-term cycle stability. This simple process for preparing such sensors can also be easily scaled up to construct pressure sensor arrays for detecting the intensity and distribution of the loaded pressure. In addition, this flexible pressure sensor exhibits good performance even in a noncontact way, such as detecting voice vibrations and air flow. Due to its superior performance, this designed flexible pressure sensor demonstrates promising potential in the application of electronic skins, as well as wearable healthcare monitors.

Flexible and Highly Sensitive Pressure Sensor Based on Microdome-Patterned PDMS Forming with Assistance of Colloid Self-Assembly and Replica Technique for Wearable Electronics

Flexible pressure sensors are one of the vital component units in the next generation of wearable electronics for monitoring human physiological signals. In order to improve the sensing properties of the sensors, we demonstrate flexible, tunably resistive pressure sensors based on elastic microstructured polydimethylsiloxane (PDMS) film via a simple, low-cost colloid self-assembly technology, which uses monodispersed polystyrene (PS) microspheres as monolayer and an ordered sacrificial template. The sensors exhibit high sensitivity of -15 kPa-1 under low pressure (<100 Pa), with fast response time (<100 ms), high stability over 1000 cycles of pressure loading/unloading, low-pressure detection limit of 4 Pa, and wide working pressure regime (<5 kPa) by optimizing the size of PS microspheres. Moreover, the multipixel arrays of the pressure sensor are fabricated to illustrate the sensing ability of space pressure distribution. The developed flexible pressure sensors are successfully used to detect human neck pulse, show great promise for monitoring human body motions, and have potential applications in wearable devices.

A low-cost, printable, and stretchable strain sensor based on highly conductive elastic composites with tunable sensitivity for human motion monitoring

Strain sensors with high stretchability, broad strain range, high sensitivity, and good reliability are desirable, owing to their promising applications in electronic skins and human motion monitoring systems. In this paper, we report a high-performance strain sensor based on printable and stretchable electrically conductive elastic composites. This strain sensor is fabricated by mixing silver-coated polystyrene spheres (PS@Ag) and liquid polydimethylsiloxane (PDMS) and screen-printed to a desirable geometry. The strain sensor exhibits fascinating comprehensive performances, including high electrical conductivity (1.65 × 104 S/m), large workable strain range (> 80%), high sensitivity (gauge factor of 17.5 in strain of 0%–10%, 6.0 in strain of 10%–60% and 78.6 in strain of 60%–80%), inconspicuous resistance overshoot (< 15%), good reproducibility and excellent long-term stability (1,750 h at 85 °C/85% relative humidity) for PS@Ag/PDMS-60, which only contains ∼ 36.7 wt.% of silver. Simultaneously, this strain sensor provides the advantages of low-cost, simple, and large-area scalable fabrication, as well as robust mechanical properties and versatility in applications. Based on these performance characteristics, its applications in flexible printed electrodes and monitoring vigorous human motions are demonstrated, revealing its tremendous potential for applications in flexible and wearable electronics.

Tailoring Size and Coverage Density of Silver Nanoparticles on Monodispersed Polymer Spheres as Highly Sensitive SERS Substrates

Silver nanoparticles (AgNPs) were deposited onto the monodispersed carboxylic polystyrene (CPS) spheres by an improved in situ reduction method. The size and coverage density of the AgNPs on the surface of CPS spheres could be easily tailored by tuning the concentrations of carboxylic functional groups and silver precursor. The morphologies and structures of the resulting CPS/Ag hybrid particles were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), UV-Vis-NIR spectrometer and X-ray photoelectron spectroscopy (XPS), etc. The surface enhanced Raman scattering (SERS) performances of the resulting uniform CPS/Ag hybrid particles were investigated using 4-aminobenzenethiol (4-ABT) as the probe molecule. The optimized CPS/Ag hybrid particles show high enhancement factor (EF) of 2.71×10(7) , low limit of detection (LOD) of 10(-10)  m and good reproducibility with relative standard deviation (RSD) of 9.64 %. The good SERS improvement properties demonstrate these hybrid particles could be employed as simple and effective substrates in the SERS spectroscopy.

CuCl2 and stainless steel synergistically assisted synthesis of high-purity silver nanowires on a large scale

High-purity silver nanowires (AgNWs) with an average width of 60 ± 5 nm and a typical length of 10–20 μm were synthesized conveniently at a relatively high AgNO3 concentration of 0.3 M through a CuCl2 and stainless steel co-mediated polyol reduction method.

Enhanced oxidation resistance and electrical conductivity copper nanowires–graphene hybrid films for flexible strain sensors

Copper nanowires (CuNWs) are extremely prone to oxidation, which greatly limits their practical applications, even though they are inexpensive, abundant and have high electrical conductivity. Herein, a facile and novel method is developed to alleviate the oxidation of CuNWs by embedding CuNWs into water-dispersible modified graphene (WGP) sheets to form a uniform hybrid conductive film using a simple vacuum filtration process. The CuNWs and WGP films possess electrical conductivity of 3.19 × 103 S m−1 and 5.6 × 103 S m−1, respectively. With the addition of WGP to form a CuNWs–WGP hybrid film, the electrical conductivity of the CuNWs is further improved to 3.02 × 104 S m−1 due to the synergistic effects of the CuNWs and WGP. The CuNWs–WGP hybrid films show excellent antioxidative stability even after exposure in air for 8 weeks without any obvious oxidation. Then, using this conductive hybrid film, a sandwich structured PDMS/CuNWs–WGP/PDMS flexible strain sensor was fabricated. The strain sensor exhibits good flexibility and stretchability with only less than 15% loss of electrical conductivity over 450 times mechanical bending, and was successfully used to monitor human motion, such as finger bending, swallowing and voice recognition.

Room-Temperature Nanowelding of a Silver Nanowire Network Triggered by Hydrogen Chloride Vapor for Flexible Transparent Conductive Films

High contact resistance between silver nanowires (AgNWs) is a key issue in widespread application of AgNW flexible transparent conductive films as a promising candidate to replace the brittle and expensive indium tin oxide. A facile, room-temperature nanowelding method of an AgNW network triggered by hydrogen chloride (HCl) vapor is demonstrated to reduce the sheet resistance of the AgNW network. Under the visible light, O2 and HCl vapor serving as an etching couple induced silver atoms to be transferred from the bottom AgNW at the junction to the top one, and then, these silver atoms epitaxially recrystallized at the contact position with the lattice of the top AgNW as the template, ultimately resulting in the coalescence of the junction between AgNWs. Polydimethylsiloxane (PDMS) was spin-coated onto the HCl-vapor-treated (HVT) AgNW network on the polyethylene terephthalate substrate to fabricate PDMS/HVT AgNW films. The fabricated film with low sheet resistance and high transmittance retained its conductivity after 4000 bending cycles. Furthermore, excellent heating performance, electromagnetic interference shielding effectiveness, and foldability were obtained in the PDMS/HVT AgNW film. Thus, the role of the simple nanowelding process is evident in enhancing the performance of AgNW transparent conductive films for emerging soft optoelectronic applications.

Adaptive Optical System for Retina Imaging Approaches Clinic Applications

We presented “A small adaptive optical system on table for human retinal imaging” at the 3rd Workshop on Adaptive Optics for Industry and Medicine. In this system, a 19 element small deformable mirror was used as wavefront correction element. High resolution images of photo receptors and capillaries of human retina were obtained. In recent two years, at the base of this system a new adaptive optical system for human retina imaging has been developed. The wavefront correction element is a newly developed 37 element deformable mirror. Some modifications have been adopted for easy operation. Experiments for different imaging wavelengths and axial positions were conducted. Mosaic pictures of photoreceptors and capillaries were obtained. 100 normal and abnormal eyes of different ages have been inspected.The first report in the world concerning the most detailed capillary distribution images cover ±3° by ± 3° field around the fovea has been demonstrated. Some preliminary very early diagnosis experiment has been tried in laboratory. This system is being planned to move to the hospital for clinic experiments.