High-power microwave 0.25-μm gate doped-channel GaN/AlGaNheterostructure field effect transistor

We report on the high-power performance of the 0.25-μm gate Doped-Channel GaN/AlGaN Heterostructure Field Effect Transistors (DC-HFETs). At a drain bias voltage of 18 V and drain bias current of 46 mA, these 100-μm wide devices exhibit high gain at 8.4 GHz with a power density reaching 1.73 W/mm. The devices also display high gain at moderate power over a wide range of frequencies. This high gain at high frequency is a result of an optimal doping level in the AlGaN layer that gives rise to a high sheet charge density while maintaining a high-channel electron mobility. These results demonstrate the excellent microwave power capability of the GaN/AlGaN based heterostructure field effect transistors     

Determining Protein Size Using an Electrochemically Machined Pore Gradient in Silicon

Porous silicon films displaying a distribution of pore dimensions can be generated by electrochemically etching silicon in aqueous ethanolic HF using an asymmetric electrode configuration. The median pore size and breadth of the size‐distribution in the film can be set by adjusting the HF concentration, current density, and position of the counter electrode relative to the silicon electrode. Films with pore sizes in the range of a few nanometers are used as size‐exclusion matrices to perform an on‐chip determination of macromolecule dimensions. The test molecule used in this study is bovine serum albumin (BSA). Optical reflectivity spectra of the thin porous Si films display distinctive shifts in the Fabry–Perot fringes in regions of the film where the pore dimensions are larger than a critical size, interpreted to be the characteristic dimensions of the protein. Gating of the protein in and out of the porous films is demonstrated by adjustment of the solution pH below and above the pI (isoelectric point) value, respectively.     

Photoluminescent Porous Si/SiO2 Core/Shell Nanoparticles Prepared by Borate Oxidation

A systematic study on the activation of photoluminescence from luminescent porous silicon nanoparticles (LPSiNPs) by oxidation in aqueous media containing sodium tetraborate (borax) is presented. The treatment promotes surface oxidation of the porous silicon skeleton and consequently generates an electronically passivated material. Photoluminescence is ascribed to quantum confinement effects and to defects localized at the Si‐SiO₂ interface, and the strong photoluminescence is attributed to passivation of nonradiative surface defects. The oxidation treatment (carried out at 20 °C) generates a gradual blue shift of the photoluminescence peak wavelength (from 800 nm to 630 nm), while the bandwidth remains relatively constant (≈210 nm). During the treatment period, the external quantum yield (λ_ex = 365 nm) of photoluminescence increases to a maximum value of 23% after 200 min, and then it decreases at longer treatment times. The decrease in photoluminescence intensity at longer times is attributed to degradation and dissolution of the nanoparticles, which is inhibited at higher nanoparticle concentrations or by addition of free silicic acid.     

Detection of nitric oxide and nitrogen dioxide with photoluminescent porous silicon

The visible photoluminescence of porous Si is quenched by nitric oxide and nitrogen dioxide to detection limits of 1.4 × 10(-)(3) and 5.3 × 10(-)(5) Torr, respectively (corresponding to 2 ppm and 70 ppb). At analyte partial pressures in the low milliTorr range, the photoluminescence quenching is partially reversible; recovery from nitrogen oxide exposure occurs on a time scale of minutes. For both NO and NO(2), the reversible photoluminescence quenching response fits a Stern-Volmer kinetic model. At higher partial pressures, quenching deviates from Stern-Volmer kinetics and some permanent loss of photoluminescence intensity occurs due to oxidation of the porous Si surface. Photoluminescence from porous Si is not quenched by nitrous oxide or carbon dioxide and only slightly quenched by carbon monoxide and oxygen.     

Two‐Photon In Vivo Imaging with Porous Silicon Nanoparticles

A major obstacle in luminescence imaging is the limited penetration of visible light into tissues and interference associated with light scattering and autofluorescence. Near‐infrared (NIR) emitters that can also be excited with NIR radiation via two‐photon processes can mitigate these factors somewhat because they operate at wavelengths of 650–1000 nm where tissues are more transparent, light scattering is less efficient, and endogenous fluorophores are less likely to absorb. This study presents photolytically stable, NIR photoluminescent, porous silicon nanoparticles with a relatively high two‐photon‐absorption cross‐section and a large emission quantum yield. Their ability to be targeted to tumor tissues in vivo using the iRGD targeting peptide is demonstrated, and the distribution of the nanoparticles with high spatial resolution is visualized.     

Enhanced Performance of a Molecular Photoacoustic Imaging Agent by Encapsulation in Mesoporous Silicon Nanoparticles

Photoacoustic (PA) imaging allows visualization of the physiology and pathology of tissues with good spatial resolution and relatively deep tissue penetration. The method converts near‐infrared (NIR) laser excitation into thermal expansion, generating pressure transients that are detected with an acoustic transducer. Here, we find that the response of the PA contrast agent indocyanine green (ICG) can be enhanced 17‐fold when it is sealed within a rigid nanoparticle. ICG encapsulated in particles composed of porous silicon (pSiNP), porous silica, or calcium silicate all show greater PA contrast relative to equivalent quantities of free ICG, with the pSiNPs showing the strongest enhancement. A liposomal formulation of ICG performs similar to free ICG, suggesting that a rigid host nanostructure is necessary to enhance ICG performance. The improved response of the nanoparticle formulations is attributed to the low thermal conductivity of the porous inorganic hosts and their ability to protect the ICG payload from photolytic and/or thermal degradation. The translational potential of ICG‐loaded pSiNPs as photoacoustic probes is demonstrated via imaging of a whole mouse brain.     

A Surface‐Charge Study on Cellular‐Uptake Behavior of F3‐Peptide‐Conjugated Iron Oxide Nanoparticles

Surface‐charge measurements of mammalian cells in terms of Zeta potential are demonstrated as a useful biological characteristic in identifying cellular interactions with specific nanomaterials. A theoretical model of the changes in Zeta potential of cells after incubation with nanoparticles is established to predict the possible patterns of Zeta‐potential change to reveal the binding and internalization effects. The experimental results show a distinct pattern of Zeta‐potential change that allows the discrimination of human normal breast epithelial cells (MCF‐10A) from human cancer breast epithelial cells (MCF‐7) when the cells are incubated with dextran coated iron oxide nanoparticles that contain tumor‐homing F3 peptides, where the tumor‐homing F3 peptide specifically bound to nucleolin receptors that are overexpressed in cancer breast cells.