Derek Ho

Shape-controlled synthesis of organolead halide perovskite nanocrystals and their tunable optical absorption

Hybrid organolead halide perovskites (CH3NH3PbI3) with polymorphic structures have been successfully synthesized by controlling their solubility in solvents with different polarities. Crystal formation stages of the perovskites have been demonstrated for the first time. Shape changes of such perovskites are accompanied by transition in their crystal structures and variation of optical properties. Herein, a new trigonal phase for CH3NH3PbI3 has been observed with a rod-like morphology. Photoemission study indicates a significant red shift in the perovskite nanoparticles, compared to that of the rod-like nanocrystals. This solvent-controlled formation of polymorphic phases provide an additional approach for controlling the optical properties of CH3NH3PbI3 for various optoelectronic applications.

A reaction-based near-infrared fluorescent sensor for Cu2+ detection in aqueous buffer and its application in living cells and tissues imaging

Copper (II) is one of the most of important cofactors for numerous enzymes and has captured broad attention due to its role as a neurotransmitters for physiological and pathological functions. In this article, we present a reaction-based fluorescent sensor for Cu2+ detection (NIR-Cu) with near-infrared excitation and emission, including probe design, structure characterization, optical property test and biological imaging application. NIR-Cu is equipped with a functional group, 2-picolinic ester, which hydrolyzes in the presence of Cu2+ with high selectivity over completed cations. With the experimental conditions optimized, NIR-Cu (5μM) exhibits linear response for Cu2+ range from 0.1 to 5μM, with a detection limit of 29nM. NIR-Cu also shows excellent water solubility and are highly responsive, both desirable properties for Cu2+ detection in water samples. In addition, due to its near-infrared excitation and emission properties, NIR-Cu demonstrates outstanding fluorescent imaging in living cells and tissues.

Reaction-Based Off-On Near-infrared Fluorescent Probe for Imaging Alkaline Phosphatase Activity in Living Cells and Mice

Alkaline phosphatases are a group of enzymes that play important roles in regulating diverse cellular functions and disease pathogenesis. Hence, developing fluorescent probes for in vivo detection of alkaline phosphatase activity is highly desirable for studying the dynamic phosphorylation in living organisms. Here, we developed the very first reaction-based near-infrared (NIR) probe (DHXP) for sensitive detection of alkaline phosphatase activity both in vitro and in vivo. Our studies demonstrated that the probe displayed an up to 66-fold fluorescence increment upon incubation with alkaline phosphatases, and the detection limit of our probe was determined to be 0.07 U/L, which is lower than that of most of alkaline phosphatase probes reported in literature. Furthermore, we demonstrated that the probe can be applied to detecting alkaline phosphatase activity in cells and mice. In addition, our probe possesses excellent biocompatibility and rapid cell-internalization ability. In light of these prominent properties, we envision that DHXP will add useful tools for investigating alkaline phosphatase activity in biomedical research.

Construction of an alkaline phosphatase-specific two-photon probe and its imaging application in living cells and tissues

Alkaline phosphatase (ALP) is a family of enzymes involved in the regulation of important biological processes such as cell differentiation and bone mineralization. Monitoring the activity of ALP in serum can help diagnose a variety of diseases including bone and liver diseases. There has been growing interest in developing new chemical tools for monitoring ALP activity in living systems. Such tools will help further delineate the roles of ALP in biological and pathological processes. Previously reported fluorescent probes has a number of disadvantages that limit their application, such as poor selectivity and short-wavelength excitation. In this work, we report a new two-photon fluorescent probe (TP-Phos) to selectively detect ALP activity. The probe is composed of a two-photon fluorophore, a phosphate recognition moiety, and a self-cleavable adaptor. It offers a number of advantages over previously reported probes, such as fast reaction kinetics, high sensitivity and low cytotoxicity. Experimental results also showed that TP-Phos displayed improved selectivity over DIFMUP, a commonly utilized ALP probe. The selectivity is attributed to the utilization of an ortho-functionalised phenyl phosphate group, which increases the steric hindrance of the probe and the active site of phosphatases. Moreover, the two-photon nature of the probe confers enhanced imaging properties such as increased penetration depth and lower tissue autofluorescence. TP-Phos was successfully used to image the endogenous ALP activity of hippocampus, kidney and liver tissues from rat.

The intersection of CMOS microsystems and upconversion nanoparticles for luminescence bioimaging and bioassays.

Organic fluorophores and quantum dots are ubiquitous as contrast agents for bio-imaging and as labels in bioassays to enable the detection of biological targets and processes. Upconversion nanoparticles (UCNPs) offer a different set of opportunities as labels in bioassays and for bioimaging. UCNPs are excited at near-infrared (NIR) wavelengths where biological molecules are optically transparent, and their luminesce in the visible and ultraviolet (UV) wavelength range is suitable for detection using complementary metal-oxide-semiconductor (CMOS) technology. These nanoparticles provide multiple sharp emission bands, long lifetimes, tunable emission, high photostability, and low cytotoxicity, which render them particularly useful for bio-imaging applications and multiplexed bioassays. This paper surveys several key concepts surrounding upconversion nanoparticles and the systems that detect and process the corresponding luminescence signals. The principle of photon upconversion, tuning of emission wavelengths, UCNP bioassays, and UCNP time-resolved techniques are described. Electronic readout systems for signal detection and processing suitable for UCNP luminescence using CMOS technology are discussed. This includes recent progress in miniaturized detectors, integrated spectral sensing, and high-precision time-domain circuits. Emphasis is placed on the physical attributes of UCNPs that map strongly to the technical features that CMOS devices excel in delivering, exploring the interoperability between the two technologies.

CMOS Time-Resolved, Contact, and Multispectral Fluorescence Imaging for DNA Molecular Diagnostics

Instrumental limitations such as bulkiness and high cost prevent the fluorescence technique from becoming ubiquitous for point-of-care deoxyribonucleic acid (DNA) detection and other in-field molecular diagnostics applications. The complimentary metal-oxide-semiconductor (CMOS) technology, as benefited from process scaling, provides several advanced capabilities such as high integration density, high-resolution signal processing, and low power consumption, enabling sensitive, integrated, and low-cost fluorescence analytical platforms. In this paper, CMOS time-resolved, contact, and multispectral imaging are reviewed. Recently reported CMOS fluorescence analysis microsystem prototypes are surveyed to highlight the present state of the art.

Broadband Efficiency-Enhanced Mutually Coupled Harmonic Postmatching Doherty Power Amplifier

The postmatching topology is an effective approach for broadening the bandwidth of a Doherty power amplifier (DPA). Its efficiency can be enhanced using a second-harmonic short-circuit network (SHSN), but at the expense of bandwidth. In this paper, the SHSNs with mutual coupling are proposed to achieve efficiency enhancement without sacrificing bandwidth. A broadband Doherty amplifier was designed and fabricated based on commercially available gallium nitride HEMT (Cree CGH 40006P) devices to validate the proposed technique. Under continuous-wave excitation, the measured results of the proposed DPA demonstrated that a wide bandwidth of 40% (1.8–2.7 GHz) can be maintained, with the 6-dB back-off drain efficiency between 47.5% and 54%. Using a wideband code-division multiple access 3GPP test signal with the peak-to-average power ratio of 6.6 dB, the adjacent channel power ratio over the entire frequency band is below −30 and −25 dBc at a 6-dB back-off and saturation, respectively.

Coupling coefficient range extension technique for broadband branch-line coupler

Abstract In a conventional branch-line coupler (BLC), high coupling coefficients require very narrow lines, which results in high fabrication complexity and reduction in power handling capacity. In addition, changes in multiple design parameters are required to realize arbitrary coupling coefficients. In this paper, we present the design, fabrication, and characterization of a novel broadband branch-line coupler that extends the range of arbitrary coupling coefficients. The proposed structure is also able to achieve different coupling through tuning only one design parameter, and therefore can be used as a tunable coupler. Closed-form design equations are presented for the synthesis of broadband BLCs based on the proposed structure. To validate the proposed structure and the corresponding design methodology, a variety of broadband BLCs with different coupling coefficients have been designed, fabricated, and measured. Measurements show that couplers with 6, 9, and 12 dB coupling coefficients provide wide fractional bandwidths of 49, 49, and 52%, respectively.

Coupling Coefficient Reconfigurable Wideband Branch-Line Coupler Topology With Harmonic Suppression

The need for a flexible coupler topology to support a wide variety of coupling coefficients and robust suppression of harmonics often results in severely increased insertion loss, system size, and fabrication complexity. In this paper, we propose a two-section branch-line coupler (BLC) topology that simultaneously achieves a wide bandwidth, reconfigurable coupling coefficient, and harmonic suppression. Closed-form equations and a design methodology are provided. To verify the proposed technique, a BLC prototype operating at 3 GHz was designed, fabricated, and measured. The circuit can work under four configurations providing the coupling coefficients of 4, 6, 8, and 10 dB. The prototype achieved a 15-dB harmonic suppression up to 7.5 GHz. The measured fractional bandwidth, defined by 1-dB amplitude imbalance and 5° phase stability, is 30% from 2.55 to 3.45 GHz, the widest of reported reconfigurable couplers.

Postmatching Doherty Power Amplifier With Extended Back-Off Range Based on Self-Generated Harmonic Injection

In this paper, an asymmetric drain biased postmatching Doherty power amplifier (DPA) using harmonic injection is proposed for further back-off extension. The injected harmonic components are generated by the two active devices. Aided by the proposed harmonic injection network, drain waveform amplitude modulation for both devices can be achieved at saturation, which results in enhanced saturated power for both carrier and peaking devices, while carrier back-off power remains unchanged. As a consequence, the back-off region is extended. Moreover, the power utilization factor of the asymmetric drain biased DPA is also improved. A DPA for wideband code-division multiple access systems was designed and fabricated based on commercially available gallium nitride HEMT (Cree CGH 40010F) devices to validate the proposed technique. Measured results of the proposed DPA demonstrated that operation at 10-dB back off is possible between 1.6 and 1.9 GHz, with efficiency better than 46%.