Interfacing Langmuir-Blodgett and Pickering Emulsions for the Synthesis of 2D Nanostructured Films: Applications in Copper Ion Adsorption

This research focuses on developing a 2D thin film comprising a monolayer of silica nanoparticles functionalized with polyethyleneimine (PEI), achieved through a novel integration of Langmuir-Blodgett (L-B) and Pickering emulsion techniques. The primary aim was to create a nanostructured film that exhibits dual functionality: iridescence and efficient metal ion adsorption, specifically Cu(II) ions. The methodology combined L-B and Pickering emulsion polymerization to assemble and stabilize a nanoparticle monolayer at an oil/water interface, which was then polymerized under UV radiation to form an asymmetrically structured film. The results demonstrate that the film possesses a high adsorption efficiency for Cu(II) ions, with the enhanced mechanical durability provided by a reinforcing layer of polyvinyl alcohol/glycerol. The advantage of combining L-B and Pickering emulsion technology is the ability to generate 2D films from functional nanoparticle monolayers that are sufficiently sturdy to be deployed in applications. The 2D film's practical applications in environmental remediation were confirmed through its ability to adsorb and recover Cu(II) ions from aqueous solutions effectively. We thus demonstrate the film's potential as a versatile tool in water treatment applications owing to its combined photonic and adsorptive properties. This work paves the way for future research on the use of nanoengineered films in environmental and possibly photonic applications focusing on enhancing the film's structural robustness and exploring its broader applicability to other pollutants and metal ions.

Heteroepitaxial fabrication of binary colloidal crystals by a balance of interparticle interaction and lattice spacing

HYPOTHESIS

The colloidal epitaxy utilizing a patterned substrate is used to fabricate colloidal crystals of the same structure and lattice spacing with the substrate, which is an effective technique for creating desired nanoscale architectures. However, this technique has been mainly limited to a single-component system. The colloidal epitaxy is versatile if multicomponent colloidal crystals can be produced, which is inspired by our previous study regarding binary colloidal crystals (b-CCs) fabricated at the edge of single-component crystals.

EXPERIMENTS

We have examined various particle size combinations of binary colloidal mixture and substrates for heteroepitaxial growth of b-CCs. Colloidal crystallization was achieved through depletion attraction induced by added polymers.

FINDINGS

We demonstrated heteroepitaxial growth of b-CCs on the foreign colloidal crystals as the substrate. Under depletion attraction, deviation from equilibrium interparticle distance because of lattice mismatch between the substrate and epitaxial layers induces strain energy among the particles, yielding the b-CCs to attain minimum strain energy. Various types of b-CCs are created by adjusting the particle size ratio and polymer concentration. The heteroepitaxial growth technique enables the fabrication of complex multicomponent colloidal crystals that greatly facilitate versatile applications of the colloidal crystals.

Metallic Plasmonic Array Structures: Principles, Fabrications, Properties, and Applications

The vast development of nanofabrication has spurred recent progress for the manipulation of light down to a region much smaller than the wavelength. Metallic plasmonic array structures are demonstrated to be the most powerful platform to realize controllable light-matter interactions and have found wide applications due to their rich and tunable optical performance through the morphology and parameter engineering. Here, various light-management mechanisms that may exist on metallic plasmonic array structures are described. Then, the typical techniques for fabrication of metallic plasmonic arrays are summarized. Next, some recent applications of plasmonic arrays are reviewed, including plasmonic sensing, surface-enhanced spectroscopies, plasmonic nanolasing, and perfect light absorption. Lastly, the existing challenges and perspectives for metallic plasmonic arrays are discussed. The aim is to provide guidance for future development of metallic plasmonic array structures.

Analytical model for transmission dips in self-assembled two-dimensional colloidal crystals

Self-assembled two-dimensional (2D) colloidal crystals (CCs) are utilized in various optical devices, lasers, biosensors, and light harvesting applications. Optical design tuning capabilities, in terms of sphere refractive index and diameter size, can influence the optical characteristics for the close-packed single-layer or multilayer structures. Often transmission dips in 2D CCs are observed, which cannot be explained by Bragg diffraction as it does for 3D photonic crystals. In this work, an analytical attempt to accurately model the transmission dips observed in the 2D CCs optical spectra is presented, aiming to explain the origin of these dips. The formation of a broad dip was studied experimentally as well. A less than 1% mismatching error was found between experiment and theory for the two blaze peak positions as well as for the transmission intensity ratio. Finally, the 2D CCs were integrated in mesostructured solar cells as light trapping structures.

From colloidal particles to photonic crystals: advances in self-assembly and their emerging applications

Over the last three decades, photonic crystals (PhCs) have attracted intense interests thanks to their broad potential applications in optics and photonics. Generally, these structures can be fabricated via either "top-down" lithographic or "bottom-up" self-assembly approaches. The self-assembly approaches have attracted particular attention due to their low cost, simple fabrication processes, relative convenience of scaling up, and the ease of creating complex structures with nanometer precision. The self-assembled colloidal crystals (CCs), which are good candidates for PhCs, have offered unprecedented opportunities for photonics, optics, optoelectronics, sensing, energy harvesting, environmental remediation, pigments, and many other applications. The creation of high-quality CCs and their mass fabrication over large areas are the critical limiting factors for real-world applications. This paper reviews the state-of-the-art techniques in the self-assembly of colloidal particles for the fabrication of large-area high-quality CCs and CCs with unique symmetries. The first part of this review summarizes the types of defects commonly encountered in the fabrication process and their effects on the optical properties of the resultant CCs. Next, the mechanisms of the formation of cracks/defects are discussed, and a range of versatile fabrication methods to create large-area crack/defect-free two-dimensional and three-dimensional CCs are described. Meanwhile, we also shed light on both the advantages and limitations of these advanced approaches developed to fabricate high-quality CCs. The self-assembly routes and achievements in the fabrication of CCs with the ability to open a complete photonic bandgap, such as cubic diamond and pyrochlore structure CCs, are discussed as well. Then emerging applications of large-area high-quality CCs and unique photonic structures enabled by the advanced self-assembly methods are illustrated. At the end of this review, we outlook the future approaches in the fabrication of perfect CCs and highlight their novel real-world applications.

Hydrogel-Based Colloidal Photonic Crystal Devices for Glucose Sensing

Diabetes, a common epidemic disease, is increasingly hazardous to human health. Monitoring body glucose concentrations for the prevention and therapy of diabetes has become very important. Hydrogel-based responsive photonic crystal (PC) materials are noninvasive options for glucose detection. This article reviews glucose-sensing materials/devices composed of hydrogels and colloidal photonic crystals (CPCs), including the construction of 2D/3D CPCs and 2D/3D hydrogel-based CPCs (HCPCs). The development and mechanisms of glucose-responsive hydrogels and the achieved technologies of HCPC glucose sensors were also concluded. This review concludes by showing a perspective for the future design of CPC glucose biosensors with functional hydrogels.

Uniform two-dimensional crystals of polystyrene nanospheres fabricated by a surfactant-assisted spin-coating method with polyoxyethylene tridecyl ether

Spin-coated self-assemblies of colloidal particles have been developed recently as an attractive close-packed monolayer of the particles for a variety of applications, but they are limited by the small area of their monolayers, especially given their low uniformity and monolayer coverage on large-area substrates. We report several noteworthy characteristics of a close-packed monolayer of polystyrene nanospheres (PS NSs) fabricated using a simple and inexpensive spin-coating method with a PS NS suspension mixed using the nonionic surfactant polyoxyethylene (12) tridecyl ether (PEO-TDE). In our study, we show that the PEO-TDE surfactant offers excellent wettability, surface tension, and a slow solvent evaporation rate of the PS NS suspension, similar to the conventional surfactant Triton X-100. We demonstrate that the relatively high monolayer coverage with reduced defects is produced when introducing the PEO-TDE surfactant. Specifically, monolayer coverage of more than 95% on a Si substrate was achieved, which is much better than that with the typical Triton X-100, and is one of the highest coverage rates realized by a spin-coating method. This excellent uniformity of the PS NS monolayer with high monolayer coverage is mainly attributed to the relatively low viscosity of the PS NS suspension, even at high concentrations of PEO-TDE. Moreover, the PEO-TDE surfactant provides highly uniform monolayers on a large-scale glass substrate even for large-sized PS NSs. We also highlight the fact that the PEO-TDE surfactant has another advantage in that the spin-coating process of the PS NS suspension can be done under common ambient laboratory conditions, unlike those required for the highly toxic Triton X-100. We therefore conclude that PEO-TDE can be a useful surfactant during the fabrication of close-packed monolayers for various applications owing to its simple and straightforward control of PS NSs, its uniform and high surface coverage, and due to the safety of the fabrication process.

Acetylcholinesterase-functionalized two-dimensional photonic crystals for the detection of organophosphates

AChE-modified 2D-PC was developed for the easy and visual detection of organophosphates.

Debye ring diffraction elucidation of 2D photonic crystal self-assembly and ordering at the air–water interface

Diffraction intensities and Debye ring widths depend on the colloidal particle ordering of the 2D photonic crystals.

Facile Assembly of Large-Area 2D Microgel Colloidal Crystals Using Charge-Reversible Substrates

2D colloidal crystals (CCs) have important applications; however, the fabrication of large-area, high-quality 2D CCs is still far from being trivial, and the fabrication of 2D microgel CCs is even harder. Here, we have demonstrated that they can be facilely fabricated using charge-reversible substrates. The charge-reversible substrates were prepared by modification with amino groups. The amino groups were then protected by amidation with 2,2-dimethylsuccinic anhydride. At acidic pH, the surface charge of the modified substrate will change from negative to positive as a result of the hydrolysis of the amide bonds and the regeneration of the amino groups. 2D microgel CCs can be simply fabricated by applying a concentrated microgel dispersion on the modified substrate. The negatively charged surface of the substrate allows the negatively charged microgel spheres, especially those close to the substrate, to self-assemble into 3D CCs. With the gradual hydrolysis of the amide bonds and the charge reversal of the substrate, the first 111 plane of the 3D assembly is fixed in situ on the substrate. The resulting 2D CC has a high degree of ordering because of the high quality of the parent 3D microgel CC. Because large-area 3D microgel CCs can be facilely fabricated, this method allows for the fabrication of 2D CCs of any size. Nonplanar substrates can also be used. In addition, the interparticle distance of the 2D array can be tuned by the concentration of the microgel dispersion. Besides rigid substrates (such as glass slides, quartz slides, and silicon wafers), flexible polymer films, including polyethylene terephthalate and poly(vinyl chloride) films, were also successfully used as substrates for the fabrication of 2D microgel CCs.

Rapid electrostatics-assisted layer-by-layer assembly of near-infrared-active colloidal photonic crystals

Here we report a rapid and scalable bottom-up technique for layer-by-layer (LBL) assembling near-infrared-active colloidal photonic crystals consisting of large (⩾1μm) silica microspheres. By combining a new electrostatics-assisted colloidal transferring approach with spontaneous colloidal crystallization at an air/water interface, we have demonstrated that the crystal transfer speed of traditional Langmuir-Blodgett-based colloidal assembly technologies can be enhanced by nearly 2 orders of magnitude. Importantly, the crystalline quality of the resultant photonic crystals is not compromised by this rapid colloidal assembly approach. They exhibit thickness-dependent near-infrared stop bands and well-defined Fabry-Perot fringes in the specular transmission and reflection spectra, which match well with the theoretical calculations using a scalar-wave approximation model and Fabry-Perot analysis. This simple yet scalable bottom-up technology can significantly improve the throughput in assembling large-area, multilayer colloidal crystals, which are of great technological importance in a variety of optical and non-optical applications ranging from all-optical integrated circuits to tissue engineering.

Large-area 2D microgel colloidal crystals fabricated via benzophenone-based photochemical reaction

Large area, high quality 2D microgel CCs were fabricated by first assembling the microgel spheres into highly ordered 3D CCs, followed by fixing the first 111 plane of the 3D CC onto the substrate via a benzophenone-based photochemical reaction.