Inverse Opal Films for Medical Sensing: Application in Diagnosis of Neonatal Jaundice

Nanoporous Polystyrene Inverse Opal Materials with Optical Interference Properties for Label-Free Biosensing

Colloidal crystal nanomaterials have been proven to be valuable substrates for optical-based biosensing due to their ordered macroporous nanostructure and brilliant optical properties. In this work, silica colloidal crystal (SCC) thin films, as well as polystyrene-SCC composite films and inverse opal (IO) polystyrene films fabricated using SCC as templates, are investigated for their application as substrate materials in optical interferometric biosensors. The SCC films formed by the self-assembly of silica colloidal crystals have the most densely packed nano-3D structure, also known as the opal structure. IO films are fabricated by filling the opal pores of SCC with polystyrene and then removing the template, resulting in an interconnected nano-3D ordered macroporous structure, as indicated by the name inverse opal. The performance of the three materials was compared and discussed based on an ordered porous layer interferometry optical platform, focusing on refractive index response, protein adsorption response, and biomolecular interaction response. These results could potentially offer innovative material support for the advancement of label-free optical biosensors, which can be used for more biological/biochemical/biomolecular reaction monitoring studies.

Inverse opals with reactive surface chemistry as sensors for aqueous pollutants

Inverse opal colorimetric sensors operating on wetting transitions usually rely on physical differences of the infiltrating liquid. Here, we exploit a reactive surface chemistry that changes wettability upon binding of an analyte. Upon binding of Fe3+ to a Schiff base immobilized on the porous structure, the surface becomes more hydrophilic, triggering the infiltration of the structure and causing the structural color to disappear.

Achieving Functionality and Multifunctionality through Bulk and Interfacial Structuring of Colloidal-Crystal-Templated Materials

Over the past 25 years, the field of colloidal crystal templating of inverse opal or three-dimensionally ordered macroporous (3DOM) structures has made tremendous progress. The degree of structural control over multiple length scales, understanding of mechanical properties, and complexity of systems in which 3DOM materials are a component have increased substantially. In addition, we are now seeing applications of 3DOM materials that make use of multiple features of their architecture at the same time. This Feature Article focuses on the different properties of 3DOM materials that provide functionality, including a relatively large surface area, the interconnectedness of the pores and the resulting good accessibility of the internal surface, the nanostructured features of the walls, the structural hierarchy and periodicity, well-defined surface roughness, and relative mechanical robustness at low density. It provides representative examples that illustrate the properties of interest related to applications including energy storage and conversion systems, sensors, catalysts, sorbents, photonics, actuators, and biomedical materials or devices.

Structural Color Ionic Hydrogel Patches for Wound Management

Ionic hydrogels have attracted extensive attention because of their wide applicability in electronic skins, biosensors, and other biomedical areas. Tremendous effort is dedicated to developing ionic hydrogels with improved detection accuracy and multifunctionality. Herein, we present an inverse opal scaffold-based structural color ionic hydrogel with the desired features as intelligent patches for wound management. The patches were composed of a polyacrylamide-poly(vinyl alcohol)-polyethylenimine-lithium chloride (PAM-PVA-PEI-LiCl) inverse opal scaffold and a vascular endothelial growth factor (VEGF) mixed methacrylated gelatin (GelMA) hydrogel filler surface. The scaffold imparted the composite patches with brilliant structural color, conductive property, and freezing resistance, while the VEGF-GelMA surface could not only prevent the ionic hydrogel from the interference of complex wound conditions but also contribute to the cell proliferation and tissue repair in the wounds. Thus, the hydrogel patches could serve as electronic skins for in vivo wound healing and monitoring with high accuracy and reliability. These features indicate that the proposed structural color ionic hydrogel patches have great potential for clinical applications.

Evaporation-Induced Self-Assembly of Metal Oxide Inverse Opals: From Synthesis to Applications

Inverse opals (IOs) are highly interconnected three-dimensional macroporous structures with applications in a variety of disciplines from optics to catalysis. For instance, when the pore size is on the scale of the wavelength of visible light, IOs exhibit structural color due to diffraction and interference of light rather than due to absorption by pigments, making these structures valuable as nonfading paints and colorants. When IO pores are in an ordered arrangement, the IO is a 3D photonic crystal, a structure with a plethora of interesting optical properties that can be used in a multitude of applications, from sensors to lasers. IOs also have interesting fluidic properties that arise from the re-entrant geometry of the pores, making them excellent candidates for colorimetric sensors based on fluid surface tension. Metal oxide IOs, in particular, can also be photo- and thermally catalytically active due to the catalytic activity of the background matrix material or of functional nanoparticles embedded within the structure.

Evaporation-induced self-assembly of sacrificial particles has been developed as a scalable method for forming IOs. The pore size and shape, surface chemistry, matrix material, and the macroscopic shape of the IO, as well as the inclusion of functional components, can be designed through the choice of deposition conditions such as temperature and humidity, types and concentrations of components in the self-assembly mixture, and the postassembly processing. These parameters allow researchers to tune the optical, mechanical, and thermal transport properties of IOs for optimum functionality.

In this Account, we focus on experimental and theoretical studies to understand the self-assembly process and properties of metal oxide IOs without (bare) and with (hybrid) plasmonic or catalytic metal nanoparticles incorporated. Several synthetic approaches are first presented, together with a discussion of the various forces involved in the assembly process. The visualization of the deposition front with time-lapse microscopy is then discussed together with analytical theory and numerical simulations to determine the conditions needed for the deposition of a continuous IO film. Subsequently, we present high-resolution scanning electron microscopy (SEM) of assembled colloids over large areas, which provides a detailed view of the evolution of the assembly process, showing that the organization of the colloids is initially dictated by the meniscus of the evaporating suspension on the substrate, but that gradually all grains rotate to occupy the thermodynamically most favorable orientation. High-resolution 3D transmission electron microscopy (TEM) is then presented together with analysis of the wetting of the templating colloids by the matrix precursor to provide a detailed picture of the embedding of metallic nanoparticles at the pore–matrix interface. Finally, the resulting properties and applications in optics, wetting, and catalysis are discussed, concluding with an outlook on the future of self-assembled metal-oxide-based IOs.