Inverse Opal Photonic Crystals as an Optofluidic Platform for Fast Analysis of Hydrocarbon Mixtures

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.

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

A non-invasive, at-home test for neonatal jaundice can facilitate early jaundice detection in infants, improving clinical outcomes for neonates with severe jaundice and helping to prevent the development of kernicterus, a type of brain damage whose symptoms include hearing loss, impairment of cognitive capacity, and death. Here a photonic sensor that utilizes color changes induced by analyte infiltration into a chemically functionalized inverse opal structure is developed. The sensor is calibrated to detect differences in urinary surface tension due to increased bile salt concentration in urine, which is symptomatic of abnormal liver function and linked to jaundice. The correlation between neonatal urinary surface tension and excess serum bilirubin, the physiologic cause of neonatal jaundice, is explored. It is shown that these non-invasive sensors can improve the preliminary diagnosis of neonatal jaundice, reducing the number of invasive blood tests and hospital visits necessary for healthy infants while ensuring that jaundiced infants are treated in a timely manner. The use of inverse opal sensors to measure bulk property changes in bodily fluids can be extended to the detection of several other conditions, making this technology a versatile platform for convenient point-of-care diagnosis.

Full-color natural rubber latex with a photonic nanostructure composite

As environmental problems increase, there is an urgent demand for new eco-friendly materials. Natural rubber latex (NRL) is a natural material extracted from rubber trees. But its dyeing process with chemical dyes might result in contamination and environmental degradation. Here, NRL is composited with a photonic crystal (PhC) structure by spin coating for the first time. The polymethyl methacrylate (PMMA) photonic nanostructure has been embedded into NRL to give it colors and provide it with optical functionalities. Colors of the composite could be designed and controlled by the sizes of the nanocolloids from 180 nm to 295 nm. The colors have strong stability under external stretching. The 3D natural rubber latex photonic crystal (NRLPC) is used as a responsive material to detect volatile organic compounds (VOCs) including formaldehyde, acetone, toluene, xylene and styrene. With its visual color appearance, biocompatibility and flexibility, NRLPC has promising potential in various sensing applications.

Sandwich-Like Sensor for the Highly Specific and Reproducible Detection of Rhodamine 6G on a Surface-Enhanced Raman Scattering Platform

Nonspecificity and low reproducibility are always the main challenges in surface-enhanced Raman scattering (SERS) detection, especially for testing real samples. In this study, we developed a sandwich-like sensor (AuA-pMIP) to detect rhodamine 6G (R6G) by integrating a porous molecularly imprinted polymer (pMIP) with a well-ordered AuNP array (AuA). To form a uniformly distributed hot spot, AuA was fabricated at an oil-water interface and was subsequently fixed between pMIP and a support slide. Finite-difference time-domain simulation indicated that the enhanced electric field covered a distance of ∼2 μm above the AuA, in which the pMIP provided effective mass-transfer channels and sufficient specific binding sites for target molecules. High specificity for AuA-pMIP in R6G detection was demonstrated by comparing the SERS performance of R6G on AuA-pMIP with that of its structural analogues on the same sensor. Remarkably, the stable sandwich-like structure allowed us to achieve a recyclable SERS sensor with high reproducibility. Finally, AuA-pMIP displayed excellent specificity and sensitivity toward R6G in a test based on a real orange juice sample. This study presents a promising method to achieve real sample testing on a SERS platform.

Colorimetric Ethanol Indicator Based on Instantaneous, Localized Wetting of a Photonic Crystal

Easy-to-use sensors for ethanol solutions have broad applications ranging from monitoring alcohol quality to combating underage drinking. Although there are a number of electronic and colorimetric sensors available for determining alcohol concentration, there is currently no device that can concurrently provide a prompt, well-defined, quickly recoverable readout and remain readily affordable. Here, we developed a field-ready, colorimetric indicator that provides a fast, clear identification of ethanol-water mixtures between 0 and 40% based on the discoloration of a wetted photonic crystal. We cooperatively exploit the iridescence and the geometrical gating in silica inverse opal films (IOFs), together with a fine-tuned surface chemistry gradient, to distinguish ethanol concentrations by their wettability patterns in the different segments of the IOFs. The resultant all-in-one colorimetric sensor delivers a striking and instantaneous optical response at an ethanol concentration as low as 5%. We further improve the ease of use by seamlessly integrating this colorimetric platform with drinking glassware (a glass stirrer and a vial). This research provides an optimal means for colorimetric ethanol detection and is a step toward the immersible sensing of diverse molecules (e.g., biomarkers) in aqueous solutions without expensive laboratory tests.

Construction of an Array of Photonic Crystal Films for Visual Differentiation of Water/Ethanol Mixtures

A photonic crystal film (PCF) which consists of a porous layered structure with a highly ordered periodic arrangement of nanopores has been used to differentiate between various mixtures of water and ethanol (EtOH). The refractive index difference between the wall (silica) of the empty nanopore and air which occupies it results in the structural color of the PCF. This color disappears when the nanopores are infiltrated by a liquid with a similar refractive index to silica (or silicon dioxide). The disappearance of the structural color provides a means to construct a colorimetric sensor to differentiate between various water/EtOH mixtures based on their wettability of the nanopores in the PCF. In this study, an array of silica-based PCFs was synthesized on a silicon substrate with a precise control of nanopore properties using the co-assembly/sedimentation method. Using this method, we benefitted from having different PCFs on a single substrate. Chemical coatings, neck angles, and film thicknesses on each PCF were the three factors used to adjust the wettability of the pores. Nanopore wetting by water/EtOH mixtures was studied in a systematic manner based on the three factors, and the findings were used to develop a sensor for visual differentiation of various water/EtOH mixtures. The final developed sensor consisting of an array of six PCFs was able to differentiate between seven different water/EtOH mixtures: W10, W20, W30, W40, W50, W60, and W70, in which W10 means 10% of water in EtOH.

Dyeing and Functionalization of Wearable Silk Fibroin/Cellulose Composite by Nanocolloidal Array

A wearable silk fibroin/cellulose composite is reported. It is structurally dyed and functionalized by embedding three-dimensional (3D) or two-dimensional poly(methyl methacrylate) and polystyrene nanocolloidal arrays to form opal and inverse opal silk methylcellulose photonic crystal films (SMPCF). The brilliant color of SMPCF is utilized for naked-eye detection of humidity and a trace amount (0.02%) of H₂O content in organic solvents. Volatile organic compounds gases of 5 types were detected. By alternately exposed to organic solvents of methanol, acetonitrile, acetone, ethanol, isopropanol, n-butanol, carbon tetrachloride, and toluene, 3D inverse opal SMPCF displayed an excellent sensing performance with instantaneously color changes from green to red. The organic solvent sensitive SMPCF are wearable by integrated into a rubber glove. This composite has the potential to be used in wearable real-time sensing materials.