Multifunctional photonic microobjects with asymmetric response in radial direction and their anticounterfeiting performance.

There are few explorations that have integrated multiple properties into photonic microobjects in a facile and controlled manner. In this work, we present a straightforward method to integrate different functions into individual photonic microobject. Droplet-based microfluidics was used to produce uniform droplets of an aqueous dispersion of monodispersed SiO2 nanoparticles (NPs). The droplets evolved into opal-structured photonic microballs upon complete evaporation of water. After infiltration of an aqueous solution of acrylamide (AAm) and acrylic acid (AAc) monomers into the interstices among SiO2 NPs, opal-structured SiO2 NPs/pAAm-co-AAc hydrogel composite photonic microballs were obtained upon UV irradiation. Afterwards, a wet etching process was introduced to etch the microballs in a controlled manner, yielding individual photonic microball composed of an SiO2 NPs/pAAm-co-AAc composite opal core and a neat pAAm-co-AAc shell. The pendant carboxylic acid groups in the skeleton of the hydrogel matrix were further utilized to react with positively charged compounds, such as Ruthenium compound containing fluorescent polymers. The resulting photonic microobjects eventually featured with localized stimulus-responsive properties and multiple colors under different modes. The multifunctional photonic microobjects were discovered to have fivefold of anticounterfeiting properties when used as building blocks for anticounterfeiting structures and may have other potential applications.

Dimerized Nonfused Electron Acceptor Based on a Thieno[3,4-c]pyrrole-4,6-dione Core for Organic Solar Cells.

In this work, the first dimerized nonfused electron acceptor (NFEA), based on thieno[3,4-c]pyrrole-4,6-dione as the core, has been designed and synthesized. The dimerized acceptor and its single counterpart exhibit similar energy levels but different absorption spectra due to their distinct aggregation behavior. The dimerized acceptor-based organic solar cells (OSCs) demonstrate a higher power conversion efficiency of 11.05%, accompanied by enhanced thermal stability. This improvement is attributed to the enhancement of the short-circuit current density and fill factor, along with an increase in the glass transition temperature. Characterizations of exciton dynamics and film morphology reveal that a dimerized acceptor-based device possesses an enhanced exciton dissociation efficiency and a well-established charge transport pathway, explaining its improved photovoltaic performance. All these results indicate that the dimerized NFEA as a promising candidate can achieve efficiency-stability-cost balance in OSCs.

Droplet Microfluidics-Assisted Fabrication of Shape Controllable Iron-Alginate Microgels with Fluorescent Property.

Droplet-based microfluidics-assisted fabrication of alginate microgels has extensive applications in biomaterials, biomedicines, and related fields. This approach is typically achieved by crosslinking droplets of an aqueous solution of sodium alginate with various divalent and trivalent ions, such as Ca2+, Ba2+, Sr2+, etc. Despite the exceptional features exhibited by bulk alginate hydrogels when using iron ions as the crosslinking reagent, including stimulus responsiveness and complex chemistry, no attention has been given to studying the fabrication of Fe-alginate microgels through droplet microfluidics. In this work, a facile method is presented for fabricating Fe-alginate microgels using single emulsion droplets as templates and an off-chip crosslinking technique to solidify the droplets. The morphologies of the resulting microgels can be systematically adjusted by manipulating different parameters such as viscosities and ionic strength of the collecting solutions. It should be noted that these resulting microgels undergo a color change from light brown to dark brown due to presumed self-oxidation of iron ions within their skeleton structure. Furthermore, these Fe-alginate microgels are functionalized by decorating them with a positively charged linear polymer via electrostatic interactions to impart them with stable fluorescent property. These functionalized Fe-alginate microgels may find potential applications in drug delivery carriers and biomimetic structures.

Hydrogels for Chemical Sensing and Biosensing.

The development and synthesis of hydrogels for chemical and biosensing are of great value. Hydrogels can be tailored to its own physical structure, chemical properties, biocompatibility, and sensitivity to external stimuli when being used in a specific environment. Herein, hydrogels and their applications in chemical and biosensing are mainly covered. In particular, it is focused on the manner in which hydrogels serve as sensing materials to a specific analyte. Different types of responsive hydrogels are hence introduced and summarized. Researchers can modify different chemical groups on the skeleton of the hydrogels, which make them as good chemical and biosensing materials. Hydrogels have great application potential for chemical and biosensing in the biomedical field and some emerging fields, such as wearable devices.

Correction: Controlled evaporation-induced phase separation of droplets containing nanogels and salt molecules.

[This corrects the article DOI: 10.1039/D2RA04585K.].

Controlled evaporation-induced phase separation of droplets containing nanogels and salt molecules.

Droplets without protection from surfactants or surfactant-like objects normally experience merging or a coalescence process since it is thermodynamically favored. However, division or replication of droplets is thermodynamically unfavored and comparably more difficult to realize. Herein, we demonstrate that a population of droplets that are composed of nanogels and salt spontaneously undergo a separation process under a slow solvent evaporation condition. Each individual droplet underwent changes in size, shape and eventually developed into two domains, which was caused by the screening effect due to the increased salt concentration as a result of solvent evaporation. The two domains gradually separated into nanogel-rich and salt-rich parts. These two parts eventually evolved into nanogel aggregates and branched structures, respectively. This separation was mainly due to the salting out effect and dewetting. Comparison studies indicate that both the nanogels and salt are indispensable ingredients for the phase separation. These discoveries may have profound applications in the fields of biomimetics and offer new routes for self-replication systems.

Microfluidics-Assisted Fabrication of Dual Stopband Photonic Microcapsules and Their Applications for Anticounterfeiting.

The assembly of two different kinds of colloidal particle-based photonic structures into an individual micro-object can achieve multifunctionality. In this study, core-shell photonic microcapsules with dual structural colors and photonic stop bands were prepared through a standard microfluidic technique. Photocurable resin suspension of silica nanoparticles and an aqueous suspension of nanogels were used as shell and core parts of microcapsules, respectively. The structural colors of shells and cores can be tuned by adjusting the concentrations of silica nanoparticles and soft nanogels in their corresponding suspensions. The individual microcapsules possess two distinct stop bands when the two suspensions are combined appropriately. Remarkably, the color information of the core part cannot be directly viewed at a macroscopic level (such as visual inspection) but can be detected at a microscopic scale (such as optical microscopy observation). The color information hidden enables the capability for information encryption and has potentially critical applications in anti-counterfeiting, display, and other fields.

Osmosis manipulable morphology and photonic property of microcapsules with colloidal nano-in-micro structure.

Full visible spectrum photonic droplets and consequent microcapsules with nano-in-micro structure were prepared through microfluidic technique. Photo-curable resin and suspension of monodispersed soft nanogels were used as shell and core of the microcapsules, respectively. Upon UV irradiation, the droplets can be subsequently transformed into photonic microcapsules with an ultrathin polymeric shell. The shell thickness of the photonic microcapsules was found to be approximately 700 nm. Due to the ultrathin shell and soft core, the photonic microcapsules with nano-in-micro structure display dramatic changes both in shapes and photonic property under the impact of osmosis effect or temperature stimulus. Typically, the shell and core parts of nano-in-micro structure could respectively undergo a size expansion/even rupture and a size decrease/buckling under hypotonic and hypertonic condition. Correspondingly, the peak value of the reflection spectra of the microcapsules showed a redshift and blue shift, respectively. The mechanism to the structure and optical properties variation involves the osmotic pressure induced the volume-fraction change of the nanogel-based photonic dispersion and the shell buckling of the core/shell microcapsules.

Microcapsules with Distinct Dual-Layer Shells and Their Applications for the Encapsulation, Preservation, and Slow Release of Hydrophilic Small Molecules.

Microcapsules with two distinct layers of shells were fabricated using an approach combining microfluidics and photopolymerization. Unlike conventional microcapsules with a single shell, a fluorinated oil layer was introduced between the lumen and the outer polymer shell. The fluorinated oil layer significantly suppresses the leakage of the encapsulated ingredients in the lumen and consequently gives the microcapsules remarkable slow release capability for hydrophilic small molecule-based payloads, such as Rhodamine 6G. The release period of Rhodamine 6G can be up to 4 months when using a photocurable resin as the shell material, and the release of Rhodamine 6G can be regulated via the osmolality of the incubation solution for porous hydrogel microcapsules. Even under maximum hypotonic conditions, the release period of Rhodamine 6G in the hydrogel microcapsules is at least 10 days. The slow release capability can be significantly enhanced (6 weeks or longer) by increasing the thicknesses of the hydrogel shell and fluorinated oil layer.

Responsive Photonic Hydrogel-Based Colorimetric Sensors for Detection of Aldehydes in Aqueous Solution.

In this work, we present a fast and efficient strategy for the preparation of responsive photonic hydrogels for aldehyde sensing by combining the self-assembly of monodisperse carbon-encapsulated Fe3O4 nanoparticles (NPs) and in situ photopolymerization of polyacrylamide (PAM) hydrogels. The responsive photonic hydrogels exhibit structural color variation after being treated with formaldehyde aqueous solution, which can be attributed to the chemical reaction between the amide groups in the hydrogels and the formaldehyde. We have also shown that the photonic hydrogels can be used to determine the concentration of formaldehyde and to differentiate aldehydes through a facile reflection spectra shift and color change. This study provides a facile strategy for the visualized determination of aldehyde in aqueous solution.

Microfluidics Fabrication of Self-Oscillating Microgel Clusters with Tailored Temperature-Responsive Properties Using Polymersomes as "Microreactors".

Poly(N-isopropylacrylamide)-based microgel clusters were successfully prepared using polymersomes as "microreactors", which were fabricated through microfluidics. The clusters were formed from the cross-linking reaction between ruthenium/amino group dual functionalized poly(N-isopropylacrylamide) microgels and linear poly(N-isopropylacrylamide)-r-(N-acryloxysuccinimide)-based polymer linkers under neutral pH conditions. By simply adjusting the ratio of N-isopropylacrylamide to N-acryloxysuccinimide in the polymer cross-linkers, the internal structures of the clusters can be controlled; hence, the temperature response of the clusters can be regulated. It was demonstrated that these different microgel clusters showed various degrees of chemomechanical oscillations when the clusters were exposed to a catalyst-free solution containing Belousov-Zhabotinsky reaction substrates.

Controlled Synthesis of Uniform, Micrometer-Sized Ruthenium-Functionalized Poly(N-Isopropylacrylamide) Gel Particles and their Application to the Catalysis of the Belousov-Zhabotinsky Reaction.

Ruthenium-functionalized poly(N-isopropyl acrylamide)-based chemically oscillating microgels with diameters between 1 and 6 µm are synthesized by a modified precipitation polymerization approach. It is found that the initial amount of N-isopropyl acrylamide (NIPAAm) can significantly affect the final sizes of the microgels. 2.5 g of initial NIPAAm results in microgels with maximum average diameter of ≈6 ± 0.5 µm. Making use of their fluorescence due to their ruthenium contents and their larger sizes compared to microgels prepared using other traditional methods, the impact of changes in the NaBrO3 concentrations on their microscopic behavior is studied using a combination of fluorescence microscopy and dynamic light scattering techniques. When increasing the concentration of NaBrO3 in a solution, the microgels first experience a decrease in size followed by aggregation that leads to the loss of colloidal stability. Finally, the redox potential behavior and optical performance of the Belousov-Zhabotinsky reaction catalyzed by these microgels are studied by electrochemical and spectroscopic means.

Microfluidic fabrication of polymersomes enclosing an active Belousov-Zhabotinsky (BZ) reaction: Effect on their stability of solute concentrations in the external media.

Core/shell double emulsions were fabricated using glasscapillary based microfluidic techniques. Poly(butadiene) 46-bpoly(ethylene oxide)30 in mixture with cyclo-hexane/chloroform were contained as the shell part of droplets, whose core part was the full 1,4-cyclohexadiene based Belousov-Zhabotinsky reaction solution of unknown osmolality. The droplets were collected in solutions of both low and relatively high concentrations of salt. This resulted in the respective increase or decrease of the core part diameter. In both cases, after an incubation period, the droplets eventually evolved into polymer vesicles. In solutions with low concentration of salt, the droplets evolved into polymer vesicles after the evaporation of the vola-tile solvent contained in the shell part. Due to the dewetting of the shell and core parts，droplets in solutions of relatively high salt concentration evolved into polymer vesicles only after three days of incubation. The dewetted shell part displayed crescent-moon-shapes with different curvatures. The final diameter of the vesicles differed from the diameter of the initial core droplets. We demonstrate that vesicles with unknown osmolality core parts are formed in both solutions of very low or relatively high concentration of salt; furthermore, we also demonstrate that they follow different formation pathways. In the appropriate conditions, the vesicles experienced a form of "collapsing" behavior due to the activity of the entrapped chemical reaction.

Microfluidic fabrication of shape-tunable alginate microgels: effect of size and impact velocity.

We report on a capillary-based microfluidic platform for the fabrication of non-spherical sodium alginate microgels. The sodium alginate droplets were crosslinked off-chip in a mixture of barium acetate and glycerol solution. Novel morphologies such as tear drop, lamp-like, mushroom-like, double-dimpled and bowl-like microgels were fabricated by controlling the size, impact velocity (at the crosslinking solution/oil interface), and concentration of sodium alginate solution. We monitored the microscale deformation process in situ at the interface and proposeed a deformation mechanism resulting in unique morphologies. Additionally, we constructed microgel superstructures by assembling the non-spherical alginate microgels to spherical poly(N-isopropylacrylamide) (pNIPAAm) microgels via electrostatic interaction.

Fabrication of shape controllable Janus alginate/pNIPAAm microgels via microfluidics technique and off-chip ionic cross-linking.

A novel method to fabricate shape controllable alginate/pNIPAAm complex microgels is reported. Monodisperse alginate/pNIPAAm droplets are created via microfluidics and cross-linked in different concentrations of hot glycerol/barium acetate water solutions. By changing the initial droplet size and glycerol concentration of the collecting solution, the resultant microgel shape and surface details can be systematically tuned. High-speed imaging is used to visualize and explain the microgel formation process.

Uniform core-shell photonic crystal microbeads as microcarriers for optical encoding.

We demonstrate a rapid and robust method to fabricate uniform core-shell photonic crystal (PC) microbeads by the microfluidic and centrifugation-redispersion technique. Colored crystalline colloidal arrays (CCAs) were first prepared through centrifugation-redispersion approach by self-assembly of polystyrene-poly(N-isopropylacrylamide) (PS-PNIPAm) core/shell nanoparticles (NPs). Different from the conventional NPs (e.g., charged PS or PNIPAm NPs), PS-PNIPAm NPs could easily self-assemble into well-ordered CCAs by only one purification step without laborious pretreatment (e.g., dialysis or ion exchange) or slow solvent-evaporation process. The CCAs is then encapsulated into a transparent polymer shell with functional groups (e.g., copolymer of ETPTA and butyl acrylate (BA)), triggering the formation of core-shell PC microbeads which can be used as optical encoding microcarriers. Importantly, this technique allows us to produce core-shell PC microbeads in a rapid and robust way, and the optical reflections of the PC microbeads appear highly stable to various external stimuli (e.g., temperature, pH value, and ionic strength) relying on the features of the CCAs core and protection of the polymer shell. Moreover, various probe biomolecules (e.g., proteins, antibodies, and so on) can be easily linked on the surface of the core-shell PC microbeads owing to the hydrophilic modification induced by the hydrolysis of BA on the microbead surface, enabling the multiplex biomolecular detection.

Janus photonic crystal microspheres: centrifugation-assisted generation and reversible optical property.

A new strategy to prepare core/shell Janus photonic crystal (PC) microspheres with reversible optical spectrum property is demonstrated. The microfluidic technique was employed to generate the uniform core/shell PC microspheres containing nanogels aqueous suspension. Under centrifugal force, the nanogel particles homogeneously dispersed in the core of microspheres would aggregate in the half of the microspheres, leading to Janus PC microspheres with varied reflection spectra at the different side of the spheres. More interestingly, such Janus structure of PC microspheres and their reflection spectrum were significantly reversible when the centrifugation was employed and removed alternatively. In addition, due to the soft and thermal-responsive nature of the building blocks (e.g., nanogels), Janus structures and optical properties of the PC microspheres are highly influenced by the temperature, centrifugal speed, and time, providing the other parameters on the manipulation of properties of the PC microspheres. This strategy provides a new concept for the preparation of Janus PC microspheres with tunable structures and optical properties, which will find potential applications in the field of sensors, optical devices, barcodes, etc.

Multiresponsive hydrogel photonic crystal microparticles with inverse-opal structure.

Hydrogel photonic crystal microparticles (HPCMs) with inverse-opal structure are generated through a combination of microfluidic and templating technique. Temperature and pH responsive HPCMs have firstly been prepared by copolymerizing functional monomers, for example, N-isopropylacrylamide (NIPAm) and methacrylic acid (MAA). HPCMs not only show tunable color variation almost covering the entire wavelength of visible light (above 150 nm of stop-band shift) by simply tailoring temperature or pH value of the solution, but also display rapid response (less than 1 min) due to the small volume and well-ordered porous structure. Importantly, the temperature sensing window of the HPCMs can be enlarged by controlling the transition temperature of the hydrogel matrix, and the HPCMs also exhibit good reversibility and reproducibility for pH response. Moreover, functional species or particles (such as azobenzene derivative or magnetic nanoparticles) can be further introduced into the hydrogel matrix by using post-treatment process. These functionalized HPCMs can respond to the UV/visible light without significantly influencing the temperature and pH response, and thus, multiresponsive capability within one single particle can be realized. The presence of magnetic nanoparticles may facilitate secondary assembly, which has potential applications in advanced optical devices.

Microfluidic fabrication and thermoreversible response of core/shell photonic crystalline microspheres based on deformable nanogels.

Soft photonic crystals (PC) are more appealing due to the responsiveness of the building blocking-deformable nanoparticles to the external stimuli. In this report, we demonstrate, for the first time, the generation of soft core/shell PC microspheres through a combination of a microfluidic technique, encapsulation of well-ordered temperature responsive polymer nanogels suspension, and photopolymerization of a transparent shell resin. This strategy not only ensures the monodispersity of core/shell PC microspheres, but also precisely controls their size, shell thickness, and optical properties by simply adjusting the flow rate ratio and mass fraction of the nanogels. More interestingly, the intensity of the reflection spectra of the crystalline nanogel arrays in the core can be modulated reversibly by controlling the shell thickness or the temperature. As a result of their symmetric structure, the resulting PC microspheres exhibited excellent structural colors and photonic band gaps for normal incident light independent of the position on the spherical surface. Multifunctional PC microspheres can also be generated by simply dispersing functional species together with the nanogels. This core/shell PC microsphere with tunable shell thickness and reversible thermoresponse could be significant for potential applications in the fields of chemical/biological sensors, display, encoding, and optical switching.

Construction of multifunctional photonic crystal microcapsules with tunable shell structures by combining microfluidic and controlled photopolymerization.

Holey photonic crystal (PC) microcapsules are generated through a combined technique of microfluidic- and controlled-photopolymerization. This versatile route allows the fine tuning of shell structure (from well-ordered nanoporous to single-hole structures with tailored hole size) by etching or by varying UV light intensity, and endowing the PC microcapsules with multifunctional properties.