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

Carbon Nanosphere-Based TiO2 Double Inverse Opals

Inverse opals (IOs) are intensively researched in the field of photocatalysis, since their optical properties can be fine-tuned by the initial nanosphere size and material. Another possible route for photonic crystal programming is to stack IOs with different pore sizes. Accordingly, single and double IOs were synthesized using vertical deposition and atomic layer deposition. In the case of the double IOs, the alternating use of the two preparation methods was successfully performed. Hydrothermally synthesized 326 and 458 nm carbon nanospheres were utilized to manufacture two different IOs; hence the name 326 nm and 458 nm IOs. Heat treatment removed the sacrificial template carbon nanospheres, and the as-deposited TiO2 crystallized upon annealing into nanocrystalline anatase form. Reflectance mode UV-visible spectroscopy showed that most IOs had photonic properties, i.e., a photonic band gap, and by the "slow" photon effect enhanced absorbance, except the 326 nm IO, even though it also had an increase in absorbance. The IOs were tested by photocatalytic degradation of Rhodamine 6-G under visible light. Photocatalytic experiments showed that the 458 nm IO was more active and the double IOs showed higher efficiency compared to monolayers, even if the less effective 326 nm IO was the top layer.

Layered self-assemblies for controlled drug delivery: A translational overview

Self-assembly is a prominent phenomenon observed in nature. Inspired by this thermodynamically favorable approach, several natural and synthetic materials have been investigated to develop functional systems for various biomedical applications, including drug delivery. Furthermore, layered self-assembled systems provide added advantages of tunability and multifunctionality which are crucial for controlled and targeted drug release. Layer-by-layer (LbL) deposition has emerged as one of the most popular, well-established techniques for tailoring such layered self-assemblies. This review aims to provide a brief overview of drug delivery applications using LbL deposition, along with a discussion of associated scalability challenges, technological innovations to overcome them, and prospects for commercial translation of this versatile technique. Additionally, alternative self-assembly techniques such as metal-phenolic networks (MPNs) and Liesegang rings are also reviewed in the context of their recent utilization for controlled drug delivery. Blending the sophistication of these self-assembly phenomena with material science and technological advances can provide a powerful tool to develop smart drug carriers in a scalable manner.

Convenient and Efficient Fabrication of Colloidal Crystals Based on Solidification-Induced Colloidal Assembly

The simple yet efficient and versatile fabrication of colloidal crystals was investigated based on the solidification-induced colloidal crystallization process with particle/water suspension as precursor. The resulting colloidal crystals were constituted by crystal grains with sizes ranging from several tens of micrometers to a few millimeters. Each of the grains had a close-hexagonal array of colloids, which endowed the bulk colloidal crystal powders with some specific optical properties. The freezing of water was shown as the major driving force to form colloidal crystal grains, which supersaturated the solution with nanoparticles and thus induced the formation and growth of colloidal crystal seeds. This process is intrinsically different from those conventional methods based on shearing force, surface tension, columbic interaction or magnetic interaction, revealing a new strategy to fabricate colloidal crystals in a convenient and efficient way.

A Two-Step Strategy for Fabrication of Biocompatible 3D Magnetically Responsive Photonic Crystals

Extremely stable and biocompatible 3D magnetically responsive photonic crystals (MRPCs) are successfully prepared in aqueous solution. Classic hydrothermal synthesis was applied for preparation of the Fe₃O₄@C core. Modified Stöber method was then employed for synthesis of the different size of Fe₃O₄@C@SiO₂. Unlike the traditional magnetic nanoparticles, the highly negative charged superparamagnetic nanospheres (SMNs), i.e., the double-shell structure Fe₃O₄@C@SiO₂ are capable of rapidly self-assembling into 3D MRPCs with full visible and various colors that can be periodically and reversibly tuned under different kinds of external magnetic fields (EMFs) within 1 s. The assembling behavior and mechanism of the 3D MRPCs under EMF were monitored and analyzed. The preparation is simple and the size of the SMN is easily controllable by adjusting the amount of catalyst. Compared with the previous works, the synthesized 3D MRPCs are hydrophilic, and exhibit extremely high stability after 6-month storage. To conclude, our study provides an effective two-step strategy for fabrication of biocompatible 3D MRPCs and it reveals great potentials in biological fields.

High-Throughput Acoustofluidic Self-Assembly of Colloidal Crystals

Colloidal crystals are encountered in a variety of energy-harvesting applications, where they serve as waveguides or filters for electromagnetic and electro-optic energy. Techniques such as electric or magnetic assembly are used to assemble colloidal crystals, but are limited by crystal size, yield, and throughput. This article demonstrates the continuous, high-throughput assembly of two-dimensional (2D)-colloidal crystals in an acoustofluidic flow cell. The device is fabricated using off-the-shelf components and does not require a clean-room access. An experimental state diagram shows how the fluid flow rate and voltage applied to the piezoelectric element in our device can tune the crystal microstructure. Highly ordered colloidal crystals are continuously assembled in less than a minute with a throughput yield of several hundred particles per minute using this device. The acoustically assembled ordered 2D crystals are immobilized using a UV-curable resin and extracted as ordered polymer-particle fibers, demonstrating the ability of using acoustic fields to assemble ordered structures embedded in bulk materials. Particle tracking is used to construct the cross-channel particle distribution to understand the effect of acoustic compression on colloidal crystal assembly. Microparticle image velocimetry data is compared to a theoretical transport model to quantify the effect fluid flow and acoustic trapping has on the colloidal crystal ensemble.

Self-Assembled Curved Macroporous Photonic Crystal-Based Surfactant Detectors

Surfactants are extensively used as detergents, dispersants, and emulsifiers. Thus, wastewater containing high-concentration surfactants discharged to the environment pose a serious threat to the ecosystem. Unfortunately, conventional detection methods for surfactants suffer from the use of sophisticated instruments and cannot perform detections for various surfactants by a single analysis. The article reports the development of simple and sensitive surfactant detection using doctor-blade-coated three-dimensional curved macroporous photonic crystals on a cylindrical rod. The photonic crystals exhibit different hydrophobicities at various angular positions after surface modification. The penetration of aqueous surfactant solutions in the interconnected macropores causes red-shift as well as reduction in amplitude in the optical stop bands, resulting in surfactant detection with visible readout. The correlation between the surface tension, as well as the solution-infiltrated angular position, and the concentration of aqueous surfactant solutions has also been investigated in this study.

Approaches to self-assembly of colloidal monolayers: A guide for nanotechnologists

Self-assembly of quasi-spherical colloidal particles in two-dimensional (2D) arrangements is essential for a wide range of applications from optoelectronics to surface engineering, from chemical and biological sensing to light harvesting and environmental remediation. Several self-assembly approaches have flourished throughout the years, with specific features in terms of complexity of the implementation, sensitivity to process parameters, characteristics of the final colloidal assembly. Selecting the proper method for a given application amidst the vast literature in this field can be a challenging task. In this review, we present an extensive classification and comparison of the different techniques adopted for 2D self-assembly in order to provide useful guidelines for scientists approaching this field. After an overview of the main applications of 2D colloidal assemblies, we describe the main mechanisms underlying their formation and introduce the mathematical tools commonly used to analyse their final morphology. Subsequently, we examine in detail each class of self-assembly techniques, with an explanation of the physical processes intervening in crystallization and a thorough investigation of the technical peculiarities of the different practical implementations. We point out the specific characteristics of the set-ups and apparatuses developed for self-assembly in terms of complexity, requirements, reproducibility, robustness, sensitivity to process parameters and morphology of the final colloidal pattern. Such an analysis will help the reader to individuate more easily the approach more suitable for a given application and will draw the attention towards the importance of the details of each implementation for the final results.