Changxu Lin

Metal–Organic Frameworks with a Three-Dimensional Ordered Macroporous Structure: Dynamic Photonic Materials†

Tuning MOFs: When a metal-organic framework (MOF) with an ordered three-dimensional macroporous structure is integrated into a film, the resulting materials have an additional optical element, which can be used as a general and effective signal transducer. This, in combination with the hierarchical pore structure, makes these films interesting dynamic photonic materials with potential applications in sensors.

Label-Free Colorimetric Detection of Trace Atrazine in Aqueous Solution by Using Molecularly Imprinted Photonic Polymers

Based on the combination of colloidal-crystal templating and a molecular imprinting technique, a sensor platform for efficient detection of atrazine in aqueous solution has been developed. The sensor is characterized by a 3D-ordered interconnected macroporous structure in which numerous nanocavities derived from atrazine imprinting are distributed in the thin wall of the formed inverse polymer opal. Owing to the special hierarchical porous structure, the molecularly imprinted polymer opals (or molecularly imprinted photonic polymer; MIPP) allow rapid and ultrasensitive detection of the target analyte. The interconnected macropores are favorable for the rapid transport of atrazine in polymer films, whereas the inherent high affinity of nanocavites distributed in thin polymer walls allows MIPP to recognize atrazine with high specificity. More importantly, the atrazine recognition events of the created nanocavities can be directly transferred (label-free) into a readable optical signal through a change in Bragg diffraction of the ordered macropores array of MIPP and thereby induce color changes that can be detected by the naked eye. With this novel sensory system, direct, ultrasensitive (as low as 10(-8) ng mL(-1)), rapid (less than 30 s) and selective detection of atrazine with a broad concentration range varying from 10(-16) M to 10(-6) M in aqueous media is achieved without the use of label techniques and expensive instruments.

Reactive photonic film for label-free and selective sensing of cyanide.

Three-dimensional ordered inverse-opal films bearing a reactive trifluoroacetyl group are successfully constructed. Through the specific reaction between cyanide and trifluoroacetyl, the photonic films can selectively detect sub-micromolar levels of cyanide by distinct structural color change. Labeled molecules are not necessary for the sensing mechanism.

Electrothermally driven structural colour based on liquid crystal elastomers

In this article, a new type of electrothermally driven photonic crystal based on liquid crystal elastomers (LCEs) was developed, and its optical properties (structural colour) driven by voltage were described. Graphite nanoparticles were spin-coated on glass-substrates and acted as an electrothermal conversion layer, on which the prepared LCE-based inverse opaline films were mounted. When voltage is applied on the fabricated system, the heat produced by the graphite layer will induce the deformation of the coated inverse opaline film and thus the electrothermally driven photonic system or structural colour is realized. We found that realignment behaviour existed when these films were first above their glass transition temperatures (Tg), and during this realignment process, the structural colour of weakly crosslinked inverse opaline films disappeared, probably due to the collapse of the periodically ordered porous structure. The threshold cross-linking density (Cx) for producing LCE-based inverse opalines with reversible response is 25 mol%. Interestingly, it is found that the treatment of the prepared photonic films by using silicone oil could reduce the threshold Cx to 15 mol%, and the fabrication of LCE-based inverse opaline with widely tunable optical properties is possible. When the temperature of the used electrothermal conversion layer is close to the nematic–isotropic (N–I) transition temperature (TNI) of the LCE films, the liquid crystal moieties in inverse opaline structure became isotropic, leading to rapid shift of the Bragg-diffraction peak and corresponding structural colour change. After turning off the voltage, they could regain to the initial state. With the decrease of the cross-linking density of the photonic-structured elastomers, the degree of Bragg-diffraction shift became larger. Remarkably, the response of these films stimulated by electric voltage is fast and the reversibility is perfect.

Hierarchically imprinted porous films for rapid and selective detection of explosives.

On the basis of the combination of colloidal and mesophase templating, as well as molecular imprinting, a general and effective approach for the preparation of hierarchically structured trimodal porous silica films was developed. With this new methodology, controlled formation of well-defined pore structures not only on macro- and mesoscale but also on microscale can be achieved, affording a new class of hierarchical porous materials with molecular recognition capability. As a demonstration, TNT was chosen as template molecule and hierarchically imprinted porous films were successfully fabricated, which show excellent sensing properties in terms of sensitivity, selectivity, stability, and regeneracy. The pore system reported here combines the multiple benefits arising from all length scales of pore size and simultaneously possesses a series of distinct properties such as high pore volume, large surface area, molecular selectivity, and rapid mass transport. Therefore, our described strategy and the resulting pore systems should hold great promise for various applications not only in chemical sensors, but also in catalysis, separation, adsorption, or electrode materials.

Poly(p‐phenylene ethynylene)s with Facially Amphiphilic Pendant Groups: Solvatochromism and Supramolecular Assemblies

Novel functionalized poly(p-phenylene ethynylene)s (PPEs) bearing facially amphiphilic cholic and deoxycholic acid units are synthesized by a Pd-catalyzed Sonogashira cross-coupling reaction. Some interesting properties, particularly their optical and self-assembly characteristics, are unraveled. The PPEs that carry bile acid substituents exhibit remarkable solvatochromism in a wide range of solvent systems, and judicious choice of the solvents can adjust the size and morphology of the formed nanoscale supramolecular aggregates. The incorporation of these naturally occurring building blocks can also impart biocompatibility to the conjugated system and stimulate the growth of living cells.

Pyrrole containing ionic liquid as tecton for construction of ordered mesoporous silica with aligned polypyrrole nanowires in channels

Two types of ionic liquid (IL) based surfactants bearing a terminal pyrrole moiety were synthesized, and used as both structure-directing agents and monomers to prepare mesostructured silica with densely packed pyrrole units within channels via a hydrothermal synthesis procedure. A systematic study was carried out to address the influence of the relative concentration of the IL and the type of head group and counter ions on the mesostructure. As a main result, it was found that both the prepared imidazolium- and pyridium-based surfactants displayed a significantly strong tendency towards formation of a highly ordered two-dimensional (2D) hexagonal mesostructure in a broad range of conditions with excellent reproducibility. In situpolymerization of the pyrrole groups closely packed in the central region of the formed silica pore channels led to the desired mesostructured silica with well-distinct aligned polypyrrole (PPy) molecular wires in channels, which are clearly visible under TEM after the removal of silica framework. It is found that, due to the spatial confinement of the silica framework, the encapsulated PPys are elongated and straight, leading to longer conjugation length.

An Efficient Route to Rapidly Access Silica Materials with Differently Ordered Mesostructures through Counteranion Exchange

Control via exchange: By using a new type of imidazolium-based gemini surfactant as the template, highly ordered cubic mesoporous silica material (MCM-48) was directly synthesized in a broad range of reactant compositions and under mild conditions (see scheme). Moreover, by a simple counterion exchange strategy, hexagonal (MCM-41) and lamellar (MCM-50) mesoporous silicas are also easily and efficiently accessible.

Confined Self-Assembly Approach to Produce Ultrathin Carbon Nanofibers

A surfactant containing a terminal carbon source moiety was synthesized and used simultaneously as both template molecule and carbon source. On the basis of this special structure-directing agent, an efficient strategy for producing uniform carbon nanowires with diameter below 1 nm was developed using a confined self-assembly approach. Besides the capability of producing ultralong and thin carbon wires inaccessible by the previously reported approaches, the method described here presents many advantages such as the direct use of residue iron complex as catalyst for carbonization and no requirement of conventional tedious infiltration process of carbon source into small channels. Different methods including SEM, TEM, XRD, Raman spectroscopy, and conductivity measurement were employed to characterize the formed ultrathin carbon nanofibers. Additionally, the described strategy is extendable. By designing an appropriate surfactant, it is also possible for the fabrication of the finely structured carbon network and ultrathin graphitic sheets through the construction of the corresponding cubic and lamellar mesostructured templates.

Electrothermally Driven Fluorescence Switching by Liquid Crystal Elastomers Based On Dimensional Photonic Crystals

In this article, the fabrication of an active organic-inorganic one-dimensional photonic crystal structure to offer electrothermal fluorescence switching is described. The film is obtained by spin-coating of liquid crystal elastomers (LCEs) and TiO2 nanoparticles alternatively. By utilizing the property of LCEs that can change their size and shape reversibly under external thermal stimulations, the λmax of the photonic band gap of these films is tuned by voltage through electrothermal conversion. The shifted photonic band gap further changes the matching degree between the photonic band gap of the film and the emission spectrum of organic dye mounting on the film. With rhodamine B as an example, the enhancement factor of its fluorescence emission is controlled by varying the matching degree. Thus, the fluorescence intensity is actively switched by voltage applied on the system, in a fast, adjustable, and reversible manner. The control chain of using the electrothermal stimulus to adjust fluorescence intensity via controlling the photonic band gap is proved by a scanning electron microscope (SEM) and UV-vis reflectance. This mechanism also corresponded to the results from the finite-difference time-domain (FDTD) simulation. The comprehensive usage of photonic crystals and liquid crystal elastomers opened a new possibility for active optical devices.