Cheng-an Tao

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.

Direct and label-free detection of cholic acid based on molecularly imprinted photonic hydrogels

Here we report the design and preparation of a novel self-reporting sensor for cholic acid, an important biological compound produced in hepatocytes. Traditional detection methods of cholic acid are mostly dependent on analytical equipment, and are either time-consuming or require a derivatization process. In this work, a new approach based on molecularly imprinted photonic hydrogels (IPHs) is described, by which direct, sensitive and label-free detection of cholic acid can be achieved without any derivatization treatment and expensive instruments. The unique 3D ordered porous hydrogels that reveal optical changes in responsive to cholic acid concentration were prepared by combining colloidal crystal templating with the molecular imprinting technique. Due to their special hierarchical porous structure, which consists of 3D-ordered interconnected macroporous arrays with nanocavities derived from molecular imprinting, the formed photonic hydrogels allow rapid and ultrasensitive detection of the target analyte. The interconnected macropores are favorable for the rapid transport of the analyte in the hydrogel, while the inherent high affinity of nanocavities distributed in thin hydrogel walls allows IPHs to recognize the analyte with high specificity.

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.

Cucurbit[8]uril as Building Block for Facile Fabrication of Well‐Defined Organic Crystalline Nano‐objects with Multiple Morphologies and Compositions

Cucurbit[n]urils (CB[n]) have great potential in material and medical applications due to their advantageous molecular recognition properties. Despite organic microcrystals being highly desirable in materials science and the medical industry, CB[n]-based micro- and nanocrystals have not been reported. A facile and efficient approach for producing CB[8]-based organic crystals with well-defined micro- and nanostructures is described, based on the unique host-guest chemistry of CB[8] macrocycle with small guest molecules. The described strategy allows fabrication of micro- and nanocrystals with multiple morphologies and compositions by simply adjusting the preparation conditions and the type of guest molecules. The mechanisms for the formation of the micro/nanocrystals are studied, and morphology-dependent optical and thermal properties typical of organic micro/nanocrystals are described. Additionally, attractive potentials of the prepared microcrystals are shown upon storing small molecules, and in optical applications. The molecular recognition abilities of CB[8] are highlighted in both its preparation process and potential application.

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.

A new strategy for selective detection of nitrated explosives based on a confinement effect of nanocavity

Based on the confinement effect of the CB[8] cavity, a new strategy for explosives detection was developed by using a naphthalene embedded CB[8] complex as receptor. It was found that, dependent on the electronic structures and size of the tested explosives, the trapping of such analytes can significantly influence the photophysical properties of the naphthalene in the CB[8] nanocavity, leading to the direct detection and discrimination of distinctively different groups of trace explosives in the vapor phase, especially including the challenging aliphatic nitro-organics. Control experiments were performed to show the different sensing behaviors between the common organic vapors and nitrate-based explosives, which made it easy to realize the discrimination between target analytes and interferents. Due to the surface attached sensing elements, very fast response and high sensitivity were found in this system. The performed experiments suggested that the naphthalene embedded CB[8]-based sensing protocol opened a new way to develop a new kind of explosive sensor to enable a richer identification of threats.