The feasibility of using tunable magnetic nanoparticles embedded in cylindrical hydrogel materials as a flow regulator via thermo-mechanical gating is studied within microfluidic channels. Ferromagnetic nanoparticles (Fe3O4) encapsulated within a thermo-sensitive polymer network (-poly(N-isopropylacrylamide) (PNIPAM)) was polymerized inside 300 µm diameter micro-capillary tubes. An oscillating magnetic field range 20–125 Oe, (100–1000 kHz) was used to induce heat and control the valving action. Valving action was effectively regulated by modulating the magnetically responsive PNIPAM networks (MPNIPAM) and thereby physically regulating the harmonics (swelling and shrinking) of the polymer monolith inside the microchannel. Magnetic properties in terms of saturation magnetization, remanence and coercivity of the designed system have been extracted for data accuracy. The optimum concentration of NIPAM monomer in the polymer matrix and the embedded nanoparticles yield ∼80% volume shrinkage inside the microchannel, which is close to the undoped PNIPAM system, without compromising the oscillating field induced heating. Very importantly, the oscillating field-actuated de-swelling response time is ∼3 s, which is significantly faster than the thermal actuation, and in addition the microvalve exhibits a faster response time compared with the macrovalve (MPNIPAM monolith inside 1500 µm diameter channel). The enhanced shrinkage rate and the actuation efficiency might be ideal for many biomedical applications, including synergistic application of heat and sustained releasing capability of chemotherapeutic agents.
By using femtosecond laser pulses to ablate microsized targets that are dispersed in liquid media, nanocrystalline Cr(2+)-doped ZnSe nanowires have been successfully fabricated for the first time. The phase and stoichiometries of the original materials are preserved while the sizes are reduced down to 30-120 nm for these nanowires. X-ray diffraction results show that the products are nanocrystalline ZnSe with cubic sphalerite structure. Scanning electron microscope results indicate that the products be ZnSe nanowires. The nanowires are usually 30-120 nm in diameter and several tens of micrometers in length. Photoluminescence of the nanocrystalline Cr(2+)-doped ZnSe nanowires shows strong emission at around 2000-2500 nm under excitation of 1300-2250 nm wavelength at room temperature. By using the Cr(2+)-doped ZnSe multiple nanowires as the gain medium, mid-infrared oscillation at 2194 nm has been established. The oscillation wavelength of the multiple nanowires laser is 150 nm shifted to shorter wavelengths in comparison with that of microsized powder random laser.
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