Mei-Ling Zheng

Molecular engineering of chromophores for combined second-harmonic and two-photon fluorescence in cellular imaging

A series of chromophores with enhanced second- and third-order nonlinear optical properties were engineered for use in combined second-harmonic and two-photon fluorescence microscopy. Electron-accepting moieties imparted nonlinear optical properties to the chromophores. The electron-rich carbazole core served as a template towards one- or two-dimensional chromophores. More efficient acceptor groups (pyridinium, benzazolium, benzothiazolium) on the carbazole donor core resulted in improved second- and third-order nonlinear optical properties. A selection of these chromophores was tested in a cellular environment with a multimodal multiphoton microscope. The structural differences of the chromophores resulted in high selectivity for mitochondria or the nucleus in two-photon fluorescence and ranging from no signal to high selectivity for mitochondria in the SHG channel.

Femtosecond direct laser writing of gold nanostructures by ionic liquid assisted multiphoton photoreduction

Amino-terminated ionic liquid assisted multiphoton photoreduction (IL-MPR) was developed for the direct writing of subwavelength gold nanostructures in AuCl4- ions aqueous solution by femtosecond laser. It was revealed that the carbon chain length was crucial for morphology and size control of gold nanostructures. A 228 nm width of gold nanostructure, which was beyond the optical diffraction limit, was fabricated by the matching between IL and the power and scanning speed of the laser beam. The measured conductivity is of the same order as that of bulk gold. Furthermore, we successfully fabricated a U-shaped terahertz planar metamaterial whose spectral response is consistent with the theoretical expectation. The IL-MPR nanofabrication protocol is expected to play an important role in the fabrication of fine metallic micro/nanostructures for applications in microelectromechanical systems, nanoelectronics and nanophotonics.

High efficiency solar cell based on ZnO nanowire array prepared by different growth methods

A high efficiency solar cell based on ZnO nanowire (NW) arrays of different morphology and crystalline quality has been assembled and investigated. According to the basic source of the hydrothermal growing solution, ZnO NW was prepared by using (a) ammonia solution (N method), (b) hexamethylene tetramine (H method) and (c) alternating of N and H (NH method), respectively. The morphology and crystalline quality of the ZnO NW array have been characterized by using scanning electron microscopy, Raman spectroscopy and transmission electron microscopy. The CdS and CdSe nanoparticles were deposited on ZnO NW array, which is applied as a ZnO/CdS/CdSe core/interlay/shell in solar cell with a polysulfide electrolyte and a CoS counter electrode. The results indicate that the photovoltaic behaviour strongly depends on the morphology and crystalline quality of the ZnO NW array. We observe an increasing short-circuit current density of N < NH < H by using ZnO NWs prepared with different methods. The best power conversion efficiency of 2.81% and a short-circuit current of 14.6 mA cm−2 were obtained. The results would open up new avenues towards the potential applications in designing high efficiency quantum dot sensitized solar cells.

Novel carbazole-based two-photon photosensitizer for efficient DNA photocleavage in anaerobic condition using near-infrared light

Two novel carbazole derivatives, BMEPC and BMEMC, were designed, synthesized and first reported as two-photon photosensitizers for DNA photodamage, which showed efficient DNA photocleavage ability under near-infrared light exposure via a two-photon process in anaerobic condition.

Complementary chiral metasurface with strong broadband optical activity and enhanced transmission

We present the design and realization of ultra-thin chiral metasurfaces with giant broadband optical activity in the infrared wavelength. The chiral metasurfaces consisting of periodic hole arrays of complementary asymmetric split ring resonators are fabricated by femtosecond laser two-photon polymerization. Enhanced transmission with strong polarization conversion up to 97% is observed owing to the chiral surface plasmons resulting from mirror symmetry broken. The dependence of optical activity on the degree of structural asymmetry is investigated. This simple planar metasurface is expected to be useful for designing ultra-thin active devices and tailoring the polarization behavior of complex metallic nanostructures.

Comparison of Staining Selectivity for Subcellular Structures by Carbazole-Based Cyanine Probes in Nonlinear Optical Microscopy

Fluorescence probes are powerful tools for investigating molecular behavior in living tissues and cells. For optimum imaging and exploring the relationship between the molecular structure of the probe and its function in the biological cells, the probe should be subcellular target specific, water dispersible, luminescent, and biocompatible. 2] To date, a variety of fluorescence probes has been developed for staining subcellular structures, such as the cell membrane, the mitochondria 5] and DNA in the nucleus, selectively. The subcellular target-selective probe opens up the way to visualizing the molecular behavior more efficiently and specifically. Along with the development of such fluorescent probes, microscopy techniques are being rapidly developed for imaging biological samples. Such imaging techniques allow us, for example, to determine the subcellular location, mobility, and transport pathways of specific proteins. 8] Two-photon excited fluorescence (TPEF) microscopy, an established nonlinear microscopy technique, is attractive because of its intrinsic 3D resolution capability and its increased penetration depth into tissue. Accordingly, enhancing the fluorescence emission from probes for TPEF microscopy not only involves the consideration of selectivity and affinity to cellular targets 12] but also requires a large two-photon absorption cross section (d). However, most reported probes suffer from a lack of staining selectivity for subcellular structures, and small values for d. This necessitates high laser intensity and/or high fluorophore/ label concentration to obtain high contrast images, which is detrimental for biological specimens. 11] Therefore, there is a need to develop TPEF probes with excellent staining selectivity for visualizing the subcellular structures in living cells without the problems of water-dispersibility, photodamage, and cellular autofluorescence. Furthermore, the design and comparison of a series of probes is necessary better to understand the relationship between the molecular engineering of the probe and its selective staining or behavior in biological targets. In this study, we have investigated a series of cyanine probes in order to assess their staining selectivity for subcellular structures in a living cell for TPEF microscopy (Scheme 1).

Inverse opal hydrogel sensor for the detection of pH and mercury ions

An inverse opal hydrogel (IOH) sensor was constructed through colloidal templating photopolymerization of acrylic acid and pemaerythritol-triacrylate. Its dual responsive behaviours to pH and mercury ions (Hg2+) were demonstrated by detecting the shift of the diffraction wavelength. The diffraction wavelength of the IOH sensor was dramatically red-shifted when the pH was increased from 11 to 13, due to the swelling of the hydrogel. The shift of the diffraction wavelength can be directly observed by the naked eye through the colour change of the IOH. A fast response behaviour of the IOH sensor to pH was approximately completed within 3 s. Furthermore, carboxyl groups were used to detect Hg2+ as recognition groups. A low detection limit of 10 nM for Hg2+ was achieved in the optimized IOH sensor. The present work indicates the prospect of constructing multi-responsive IOH sensors using a single recognition group through the facile colloidal templating route.

Asymmetric microstructure of hydrogel: two-photon microfabrication and stimuli-responsive behavior

We propose an asymmetric three-dimensional (3D) multiphoton polymerization microfabrication method, and successfully demonstrate it through fabricating the size- and shape-controlled stimuli-responsive asymmetric hydrogel microcantilevers. The reversible ion-responsive hydrogel microcantilevers exhibit attractive and controllable bending behavior due to their asymmetric deformation to external ions. The reversible bending direction of microcantilevers completed within only 0.133 s when the surrounding environment was alternated from water to 1 M NaCl solution. Furthermore, the microcantilevers exhibited large total bending angles θT up to 32.9°. The bending behavior can be also strongly influenced by increasing the length of the microcantilevers. The 3D stimuli-responsive hydrogels were demonstrated to be promising for the application of microactuators and micromanipulators. This designable microfabrication technique and the stimuli-responsive behavior of asymmetric microstructure of hydrogel with versatility in the shape would be prospective for developing biomedical microdevices.

A water soluble initiator prepared through host–guest chemical interaction for microfabrication of 3D hydrogels via two-photon polymerization

Hydrogels with a precise 3D configuration (3D hydrogels) are required for a number of biomedical applications such as tissue engineering and drug delivery. Two-photon polymerization (TPP) is an advanced method to fabricate 3D hydrogels. However, TPP of 3D hydrogels has been challenged by the lack of TPP initiators with high efficiency in aqueous medium. In this study, a water soluble TPP initiator (WI) with high fabrication efficiency was prepared by combining hydrophobic 2,7-bis(2-(4-pentaneoxy-phenyl)-vinyl)anthraquinone (N) with a C2v symmetrical structure and 2-hydroxypropyl-β-cyclodextrins through host-guest chemical interaction. Both one and two-photon optical properties of WI have been investigated. In aqueous medium, WI showed a two-photon absorption cross-section of around 200 GM at the wavelength of 780 nm which was much higher compared with those of commercial initiators. The threshold energy of TPP for the resin with WI as a photoinitiator (the molar ratio of N in resin is 0.03%) was 8.6 mW. 3D hydrogels with a woodpile microstructure were further fabricated by using an average power of 9.7 mW and a scanning speed of 30 μm s-1.

3D hydrogels with high resolution fabricated by two-photon polymerization with sensitive water soluble initiators

Hydrogels with precise 3D configuration (3D hydrogels) are crucial for biomedical applications such as tissue engineering and drug delivery, which require the improvement of the spatial resolution on both the microscopic and the nanometric scale. In this study, a water soluble two-photon polymerization (TPP) initiator (WI) with high initiating efficiency was prepared by using a poloxamer (PF127) to encapsulate 2,7-bis(2-(4-pentaneoxy-phenyl)-vinyl)anthraquinone via a hydrophilic-hydrophobic assembly. The threshold energy for WI was 6.29 mW at a linear scanning speed of 10 μm s-1, which was much lower than those reported previously. A lateral spatial resolution of 92 nm was achieved as the resolution breakthrough of 3D hydrogels. Finally, the microstructure with high accuracy simulating the morphology of adenovirus was fabricated at the laser power close to the threshold energy of TPP, further demonstrating the ultrahigh resolution of 3D hydrogels.