Morphology-Controlled Coating of Colloidal Particles with Silica: Influence of Particle Surface Functionalization.

We present a general, convenient, and efficient synthetic concept for the coating of colloidal particles with a silica (SiO2) shell of well-defined and precisely controlled morphology and porosity. Monodisperse submicroscopic polystyrene (PS) particles were synthesized via two-stage emulsifier-free emulsion polymerization and subsequent swelling polymerization, enabling selective particle surface modification by the incorporation of ionic (methacrylic acid, MAA) or nonionic (hydroxyethyl methacrylate, HEMA or methacrylamide, MAAm) comonomers, which could be proven by zeta potential measurements as well as by determining the three-phase contact angle of the colloidal particles adsorbed at the air-water and n-decane-water interface. The functionalized particles could be directly coated with silica shells of variable thickness, porosity, and controlled surface roughness in a seeded sol-gel process from tetraethoxysilane (TEOS), leading to hybrid PS@silica particles with morphologies ranging from core-shell (CS) to raspberry-type architectures. The experimental results demonstrated that the silica coating could be precisely tailored by the type of surface functionalization, which strongly influences the surface properties of the colloidal particles and thus the morphology of the final silica shell. Furthermore, the PS cores could be easily removed by thermal treatment, yielding extremely uniform hollow silica particles, while maintaining their initial shell architecture. These particles are highly stable against irreversible aggregation and could be readily dried, purified, and redispersed in various solvents. Herein we show a first example of coating semiconducting CdSe/ZnS nanocrystals with smooth and spherical silica shells by applying the presented method that are expected to be suitable systems for applications as markers in biology and life science by using fluorescence microscopy methods, which are also briefly discussed.

Utilizing stretch-tunable thermochromic elastomeric opal films as novel reversible switchable photonic materials.

In this work, the preparation of highly thermoresponsive and fully reversible stretch-tunable elastomeric opal films featuring switchable structural colors is reported. Novel particle architectures based on poly(diethylene glycol methylether methacrylate-co-ethyl acrylate) (PDEGMEMA-co-PEA) as shell polymer are synthesized via seeded and stepwise emulsion polymerization protocols. The use of DEGMEMA as comonomer and herein established synthetic strategies leads to monodisperse soft shell particles, which can be directly processed to opal films by using the feasible melt-shear organization technique. Subsequent UV crosslinking strategies open access to mechanically stable and homogeneous elastomeric opal films. The structural colors of the opal films feature mechano- and thermoresponsiveness, which is found to be fully reversible. Optical characterization shows that the combination of both stimuli provokes a photonic bandgap shift of more than 50 nm from 560 nm in the stretched state to 611 nm in the fully swollen state. In addition, versatile colorful patterns onto the colloidal crystal structure are produced by spatial UV-induced crosslinking by using a photomask. This facile approach enables the generation of spatially cross-linked switchable opal films with fascinating optical properties. Herein described strategies for the preparation of PDEGMEMA-containing colloidal architectures, application of the melt-shear ordering technique, and patterned crosslinking of the final opal films open access to novel stimuli-responsive colloidal crystal films, which are expected to be promising materials in the field of security and sensing applications.

Fully reversible shape transition of soft spheres in elastomeric polymer opal films.

Core-interlayer-shell (CIS) beads featuring noncross-linked hard cores were used to prepare large and well-defined elastomeric opal films with remarkably distinct iridescent reflection colors. The matrix of the opal films was cross-linked by UV-irradiation after compression molding of the CIS beads mixed with a bifunctional monomer. Stress-induced deformation of the embedded PS cores lead to hexagonally arranged spheroid oblates with an aspect ratio of 2.5. Optical characterization shows that bead deformation provokes a tremendous photonic band gap shift of about 160 nm. Fully reversible shape transition from the spheroid oblates back to the spherical beads and hence full recovery of the original photonic band gap can be achieved.

Mechanochromic and thermochromic shape memory photonic crystal films based on core/shell nanoparticles for smart monitoring.

Shape memory photonic crystals (SMPCs) combining the main characteristics of shape memory materials and photonic crystals have drawn increasing research interest. In sharp contrast to traditional responsive photonic crystals, the temporary shape of SMPCs can be "frozen" and photonic configurations can be modulated by temperature. However, the large-scale fabrication of SMPCs still remains a big challenge, making the practical application difficult. Herein novel scalable SMPC films with both mechanochromic and thermochromic properties are reported. Unlike traditional template-based methods resulting in only a small size, SMPC films are fabricated by a facile hot-pressing method and post-photocuring technology to give large-area freestanding polymer films. The films are mechanically robust and flexible, featuring an excellent structural color which can be changed upon stretching, similar to the color change process of chameleons in response to the environment. The blue-shift of the reflection peak up to 120 nm can be observed when the film is stretched. The films can be reversibly stretched and recovered in 25 cycles without obvious changes in reflection spectra. The temporary shape accompanied by tremendous color changes in the corresponding SMPC films after mechanical stress induced hot programming could be simply fixed by cooling the structure below the glass transition temperature of the polymer matrix. Incorporated programmed optical properties could afterwards be erased by temperature, and initial optical properties could be fully restored. Based on the fully reversible programmable shape as well as optical properties, the investigated SMPC films are expected to be promising candidates for various potential applications, such as smart monitoring, sensors, anti-counterfeiting, and displays.

Ultrafast assembly of nanoparticles to form smart polymeric photonic crystal films: a new platform for quick detection of solution compositions.

Photonic crystals (PCs) are an important subset of photonic materials with specific optical properties, which can be utilized for structural color printing, anti-counterfeiting technologies, chemical sensors and so on. However, the fabrication of scalable, high-quality and uniform photonic crystal films at room temperature still remains a big challenge. Herein, a fast, energy efficient and scalable process is reported for the first time. A high-quality polymeric photonic crystal film can be fabricated from the uniform core/shell particle slurry within several seconds by a calendering process. The obtained crystalline structure can be rapidly captured by photo-curing, and the resultant PC films show extremely strong iridescent tunable structural colors. Because the as-designed PC film matrix is sensitive to solutions with different solubility parameters, a prototype demo sensor is firstly set up for quick detection of the composition of the alcohol/H2O mixture as a model of white spirits, which has the feature of reversible and linear quantitative sensing performance. In addition, the as-prepared PC film is further developed as an inexpensive test strip showing quick detection of ethanol/octane mixtures (possessing different solubility parameters) as a model of ethanol gasoline. This facile, scalable and energy efficient fabrication procedure is exceedingly promising for high-throughput production, showing great potential for industrialization of PC sensors and detectors. The combination of uniform particles and a dispersion medium can be potentially designed for different stimuli responsive systems, which is beneficial for applications ranging from sensing, anti-counterfeiting, to some special optical devices.

Revealing Invisible Photonic Inscriptions: Images from Strain.

Photonic structural materials have received intensive interest and have been strongly developed over the past few years for image displays, sensing, and anticounterfeit materials. Their "smartness" arises from their color responsivity to changes of environment, strain, or external fields. Here, we introduce a novel invisible photonic system that reveals encrypted images or characters by simply stretching, or immersing in solvents. This type of intriguing photonic material is composed of regularly arranged core-shell particles that are selectively cross-linked by UV irradiation, giving different strain response compared to un-cross-linked regions. The images reversibly appear and disappear when cycling the strain and releasing it. The unique advantages of this soft polymer opal system compared with other types of photonic gels are that it can be produced in roll to roll quantities, can be vigorously deformed to achieve strong color changes, and has no solvent evaporation issues because it is a photonic rubber system. We demonstrate potential applications together with a fabrication procedure which is straightforward and scalable, vital for user take-up. Our work deepens understanding of this rubbery photonic system based on core-shell nanospheres.

Thermo-cross-linked elastomeric opal films.

An efficient and convenient thermal cross-linking protocol in elastomeric opal films leading to fully reversible and stretch-tunable optical materials is reported. In this study, functional monodisperse core-shell particles were arranged in a face-centered cubic (fcc) lattice structure by a melt flow process. A problem up to now was that un-cross-linked films could not be drawn fully reversibly and hence lost their optical and mechanical performance. After thermal cross-linking reaction, the obtained films can be drawn like rubbers and the color of their Bragg reflection changes because of controlled lattice deformation, which makes the cross-linked films mechanochromic sensors. Different techniques were developed for the cross-linking of the films a posteriori, after their preparation in the melt flow process. A photo-cross-linking approach was reported earlier. This study now deals with a very efficient thermo-cross-linking approach based on the chemistry of hydroxyl- and isocyanate-functionalities that form urethane bridges. The focus of the present work is the mechanism and efficiency of this cross-linking process for elastomeric opal films with excellent mechanical and optical properties.