Vapor detection enabled by self-assembled colloidal photonic crystals

Mild Sol-Gel Conditions and High Dielectric Contrast: A Facile Processing toward Large-Scale Hybrid Photonic Crystals for Sensing and Photocatalysis

Solution processing of highly performing photonic crystals has been a towering ambition for making them technologically relevant in applications requiring mass and large-area production. It would indeed represent a paradigm changer for the fabrication of sensors and for light management nanostructures meant for photonics and advanced photocatalytic systems. On the other hand, solution-processed structures often suffer from low dielectric contrast and poor optical quality or require complex deposition procedures due to the intrinsic properties of components treatable from solution. This work reports on a low-temperature sol-gel route between the alkoxides of Si and Ti and poly(acrylic acid), leading to stable polymer-inorganic hybrid materials with tunable refractive index and, in the case of titania hybrid, photoactive properties. Alternating thin films of the two hybrids allows planar photonic crystals with high optical quality and dielectric contrast as large as 0.64. Moreover, low-temperature treatments also allow coupling the titania hybrids with several temperature-sensitive materials including dielectric and semiconducting polymers to fabricate photonic structures. These findings open new perspectives in several fields; preliminary results demonstrate that the hybrid structures are suitable for sensing and the enhancement of the catalytic activity of photoactive media and light emission control.

Flory-Huggins VOC Photonics Sensor Made of Cellulose Derivatives

As a widespread air pollutant, volatile organic compounds (VOCs) are harmful to the human body's skin, nervous system, and respiratory system. Low-cost, extensive, and continuous detection of VOCs is of great significance to human health. We infiltrated and coated cellulose acetate on the inverse opal photonic crystal skeleton of methylcellulose-polyvinyl alcohol-graphene oxide to construct a degradable, high-toughness cellulose VOC sensor. Cellulose acetate enhances the response to VOCs and achieves a highly selective response to acetone vapor due to the smaller Flory-Huggins parameter with acetone. This work proposes a general, simple, easy-to-use, and highly selective photonic crystal VOC sensor development strategy. Calculated from the Flory-Huggins solution theory, a suitable polymer was selected to modify the inverse opal photonic crystal framework and achieve high selectivity detection.

Highly Selective Gas Sensor Based on Hydrophobic Silica Decorated with Trimethoxyoctadecylsilane

Trimethoxyoctadecylsilane (OTMS) was successfully used to decorate mesoporous silica with a self-assembly method to enhance the relative gas selectivity. A quartz crystal microbalance was employed to measure the gas-sensing properties. The content of OTMS was the crucial factor that greatly affected the adsorption capacity (q) of silica, which could be converted to relative selectivity (S) to study the sensing mechanism. With increasing OTMS content, q was far higher for small-molecule gases compared to volatile organic compounds (VOCs), which could be explained by the polarity of the bonding objects, and S reached a maximum value of 45.71%. When exposed to VOCs, S was always greater than 0 among the three alcohols. The sensing mechanisms of undecorated silica and OTMS-decorated silica were quite different; the three-state mechanism was proposed to explain the sensing mechanism of OTMS-decorated silica. When exposed to small-molecule gases, the atoms that bonded with carbon atoms on OTMS greatly influenced q. With increasing OTMS content, the bonding energy of OTMS with CO₂ was far less than that with other molecules, resulting in a relative selectivity as high as 38.69%. Furthermore, macroperformance and microproperties were combined in three-dimensional coordinates, which could be applied to predict the sensing performance of silica.

Modeling the Reflectance Changes Induced by Vapor Condensation in Lycaenid Butterfly Wing Scales Colored by Photonic Nanoarchitectures

Gas/vapor sensors based on photonic band gap-type materials are attractive as they allow a quick optical readout. The photonic nanoarchitectures responsible for the coloration of the wing scales of many butterfly species possessing structural color exhibit chemical selectivity, i.e., give vapor-specific optical response signals. Modeling this complex physical-chemical process is very important to be able to exploit the possibilities of these photonic nanoarchitectures. We performed measurements of the ethanol vapor concentration-dependent reflectance spectra of the Albulina metallica butterfly, which exhibits structural color on both the dorsal (blue) and ventral (gold-green) wing sides. Using a numerical analysis of transmission electron microscopy (TEM) images, we revealed the details of the photonic nanoarchitecture inside the wing scales. On both sides, it is a 1D + 2D structure, a stack of layers, where the layers contain a quasi-ordered arrangement of air voids embedded in chitin. Next, we built a parametric simulation model that matched the measured spectra. The reflectance spectra were calculated by ab-initio methods by assuming variable amounts of vapor condensed to liquid in the air voids, as well as vapor concentration-dependent swelling of the chitin. From fitting the simulated results to the measured spectra, we found a similar swelling on both wing surfaces, but more liquid was found to concentrate in the smaller air voids for each vapor concentration value measured.

Gas-Sensing Mechanism of Silica with Photonic Bandgap Shift

Two physical models, the replacing model and the filling model, have been proposed to explain the mechanism of the effective-refractive-index change of photonic crystals (PCs). Theoretically, the photonic bandgap (PBG) would shift when PCs are exposed to different environments, which leads to a change of the effective refractive index of PCs. However, the mechanism of effective-refractive-index change is distinct when PCs are placed in different gases or vapors. Here, silica PCs were successfully fabricated by a self-assembly method. The PBG of silica PCs exhibit similar redshifts in volatile organic compounds (VOCs) and in small-molecule gases. We propose a replacing model to elaborate silica PCs exposed to VOCs and a filling model to explain silica PCs exposed to small-molecule gases. Additionally, the redshift of PBG was transformed into the volume fraction of adsorbed gas in order to achieve selective sensing of silica PCs. The approach could be exploited for applications in sensing.

Instantaneous, Simple, and Reversible Revealing of Invisible Patterns Encrypted in Robust Hollow Sphere Colloidal Photonic Crystals

The colors of photonic crystals are based on their periodic crystalline structure. They show clear advantages over conventional chromophores for many applications, mainly due to their anti-photobleaching and responsiveness to stimuli. More specifically, combining colloidal photonic crystals and invisible patterns is important in steganography and watermarking for anticounterfeiting applications. Here a convenient way to imprint robust invisible patterns in colloidal crystals of hollow silica spheres is presented. While these patterns remain invisible under static environmental humidity, even up to near 100% relative humidity, they are unveiled immediately (≈100 ms) and fully reversibly by dynamic humid flow, e.g., human breath. They reveal themselves due to the extreme wettability of the patterned (etched) regions, as confirmed by contact angle measurements. The liquid surface tension threshold to induce wetting (revealing the imprinted invisible images) is evaluated by thermodynamic predictions and subsequently verified by exposure to various vapors with different surface tension. The color of the patterned regions is furthermore independently tuned by vapors with different refractive indices. Such a system can play a key role in applications such as anticounterfeiting, identification, and vapor sensing.

Self-assembly of a nano hydrogel colloidal array for the sensing of humidity

Traditional artificial opals are assembled from silica or polystylene colloidals which have poor swellability and a lower response to stimuli. A novel three-dimensional photonic crystal array sensor which has a high stability and desired structural colour was fabricated from the self assembly of nano hydrogel colloids. The nano hydrogel colloids were prepared by co-polymerisation of N-isopropylacrylamide, functional monomer acrylic acid and N-tert-butylacrylamide. The relative humidity from 20% to 100% could be detected rapidly via the reflection spectrum of the nano hydrogel colloidal array with a maximum amount of red shift of 24 nm. The response kinetics for humidity of the nano hydrogel colloidal array were investigated, and correspondingly, a rational response mechanism of the compactness of the close-packed structure caused by the swelling of the nano hydrogel colloidal array was discussed. The nano hydrogel colloidal array sensor presented good reversibility and can be reused for at least five rounds.

A simple and low-cost humidity sensor based on self-assembled three dimensional nanohydrogel colloidal array was prepared for humidity sensing.

Visual and reversible carbon dioxide sensing enabled by doctor blade coated macroporous photonic crystals

With significant impacts of carbon dioxide on global climate change, carbon dioxide sensing is of great importance. However, most of the existing sensing technologies are prone to interferences from carbon monoxide, or suffer from the use of sophisticated instruments. This research reports the development of reproducible carbon dioxide sensor using roll-to-roll compatible doctor blade coated three-dimensional macroporous photonic crystals. The pores are functionalized with amine groups to allow the reaction with carbon dioxide in the presence of humidity. The adsorption of carbon dioxide leads to red-shift and amplitude reduction of the optical stop bands, resulting in carbon dioxide detection with visible readout. The dependences of the diffraction wavelength on carbon dioxide partial pressure for various amine-functionalized photonic crystals and different humidities in the environment are systematically investigated. In addition, the reproducibility of carbon dioxide sensing has also been demonstrated in this research.

Label-Free Vapor Selectivity in Poly(p-Phenylene Oxide) Photonic Crystal Sensors

The lack of sensors for low cost, extensive, and continuous detection of vapor pollutants is a serious concern for health and safety in industrialized urban areas. Colorimetric sensors, such as distributed Bragg reflectors made of polymers, could achieve this task thanks to their low cost and easy signal transduction but are typically affected by low vapor permeability and lack of selectivity without chemical labeling. Here we demonstrate all-polymer Bragg multilayers for label-free selective detection of organic volatile compounds. The system exploits the ability of amorphous poly(p-phenylene oxide), PPO, to uptake large amount of guest molecules and to form cocrystalline phases with distinct optical properties. Bragg stacks embedding PPO active layers show selective colorimetric response to vapors of carbon tetrachloride and aromatic homologues, which can be revealed by the naked eye.

Condensation in Nanoporous Packed Beds

In materials with tiny, nanometer-scale pores, liquid condensation is shifted from the bulk saturation pressure observed at larger scales. This effect is called capillary condensation and can block pores, which has major consequences in hydrocarbon production, as well as in fuel cells, catalysis, and powder adhesion. In this study, high pressure nanofluidic condensation studies are performed using propane and carbon dioxide in a colloidal crystal packed bed. Direct visualization allows the extent of condensation to be observed, as well as inference of the pore geometry from Bragg diffraction. We show experimentally that capillary condensation depends on pore geometry and wettability because these factors determine the shape of the menisci that coalesce when pore filling occurs, contrary to the typical assumption that all pore structures can be modeled as cylindrical and perfectly wetting. We also observe capillary condensation at higher pressures than has been done previously, which is important because many applications involving this phenomenon occur well above atmospheric pressure, and there is little, if any, experimental validation of capillary condensation at such pressures, particularly with direct visualization.

A transparent silica colloidal crystal/PDMS composite and its application for crack suppression of metallic coatings

A silica colloidal crystal (SCC)-polydimethylsiloxane (PDMS) composite with a heterogeneous surface of silica and PDMS was prepared by spreading a premixed PDMS solution on the 3D structured SCCs and curing the solution in-situ. Although the SCCs had a light blue color, the obtained composite of SCC and PDMS, due to the close effective refractive indexes of the materials, was colorless and transparent; the UV-vis spectra indicated a negligible effect of the added SCC on the transmittance of the PDMS sheet (1% reduction). Interestingly, the transparent composite sheet became translucent under stress and became clear again when relaxed. It was found that the wrinkles formed on the surface under stress were responsible for the optical change; and, the formation of the wrinkles was ascribed to the rigid nature of the SCC layer embedded in PDMS. We had applied this SCC/PDMS composite as a substrate to support a thin gold film of nanoscale thickness and found that the embedded SCC layer worked well as a transitional interface for bonding materials of mismatched mechanical properties. The incorporation of SCC layer significantly suppressed the crack generation and propagation of the gold film. The results demonstrated a potential approach for fabricating compliant and crackfree metallic films on polymeric substrates.

Towards outperforming conventional sensor arrays with fabricated individual photonic vapour sensors inspired by Morpho butterflies

Combining vapour sensors into arrays is an accepted compromise to mitigate poor selectivity of conventional sensors. Here we show individual nanofabricated sensors that not only selectively detect separate vapours in pristine conditions but also quantify these vapours in mixtures, and when blended with a variable moisture background. Our sensor design is inspired by the iridescent nanostructure and gradient surface chemistry of Morpho butterflies and involves physical and chemical design criteria. The physical design involves optical interference and diffraction on the fabricated periodic nanostructures and uses optical loss in the nanostructure to enhance the spectral diversity of reflectance. The chemical design uses spatially controlled nanostructure functionalization. Thus, while quantitation of analytes in the presence of variable backgrounds is challenging for most sensor arrays, we achieve this goal using individual multivariable sensors. These colorimetric sensors can be tuned for numerous vapour sensing scenarios in confined areas or as individual nodes for distributed monitoring.

Individual vapour sensors often suffer from poor selectivity, which hinders their broad applicability. Here, Potyrailo et al. fabricate individual sensors inspired by the Morpho butterfly capable of selectively detecting vapours in mixtures and with a variable moisture background.

Exploration and exploitation of water in colloidal crystals

Water on solid surfaces is ubiquitously found in nature, in most cases due to mere adsorption from ambient moisture. Because porous structures have large surfaces, water may significantly affect their characteristics. This is particularly obvious in systems formed by separate particles, whose interactions are strongly influenced by small amounts of liquid. Water/solid phenomena, like adsorption, condensation, capillary forces, or interparticle cohesion, have typically been studied at relatively large scales down to the microscale, like in wet granular media. However, much less is known about how water is confined and acts at the nanoscale, for example, in the interstices of divided systems, something of utmost importance in many areas of materials science nowadays. With novel approaches, in-depth investigations as to where and how water is placed in the nanometer-sized pores of self-assembled colloidal crystals have been made, which are employed as a well-defined, versatile model system with useful optical properties. In this Progress Report, knowledge gained in the last few years about water distribution in such nanoconfinements is gathered, along with how it can be controlled and the consequences it brings about to extract new or enhance existing material functionalities. New methods developed and new capabilities of standard techniques are described, and the water interplay with the optical, chemical, and mechanical properties of the ensemble are discussed. Some lines for applicability are also highlighted and aspects to be addressed in the near future are critically summarized.

Physical deposition improved SERS stability of morphology controlled periodic micro/nanostructured arrays based on colloidal templates

An effective and inexpensive method is developed to fabricate periodic arrays by sacrificial colloidal monolayer template route by chemical deposition and further physical deposition. By a colloidal template induced precursor solution dipping strategy, different periodic arrays of semi-hollow sphere array, inverse opal with monolayer pore arrays and hole arrays are obtained under different conditions. After magnetron sputtering deposition, their morphologies are changed to novel micro/nanostructured arrays of honeycomb-shaped arrays, hollow cavity arrays, and regular network arrays due to multiple direction deposition of sputtering deposition and shadow effect. After coating a gold thin layer, these periodic micro/nanostructured arrays are used as SERS active substrates and demonstrate a very stable SERS performance compared with periodic arrays achieved by direct colloidal template-induced chemical deposition. Additionally, a honeycomb-shaped array displays better SERS enhancement than that of a hollow cavity array or a regular network array. After optimization of honeycomb-shaped arrays with different periodicities, an array with periodicity of 350 nm demonstrates much stronger SERS enhancement and possesses a low detection limit of 10(-11) M R6G. Such stable SERS performance is useful for practical application in portable Raman detecting devices to detect organic molecules.

Hybrid mesoporous colloid photonic crystal array for high performance vapor sensing

A hybrid mesoporous photonic crystal vapor sensing chip was developed by introducing fluorescent dyes into mesoporous colloidal crystals. The sensing chip was capable of discriminating various kinds of vapors, as well as their concentrations, according to their fluorescence and reflective responses to vapor analytes.

Toward bioinspired nanostructures for selective vapor sensing: diverse vapor-induced spectral responses within iridescent scales of Morpho butterflies

The iridescent colors of Morpho butterflies have captured scientific intrigue for over a century. However, only recently photonic structures of the wing scales of Morpho butterflies have inspired new ideas in the diverse areas of technology including sensing. In this study, we performed theoretical and experimental evaluation of vapor-induced reflectance changes of the Morpho scales. These experiments provided additional details of the origin and the magnitude of vapor response selectivity in these natural photonic nanostructures and facilitated our design and fabrication of highly selective biomimetic photonic nanostructures.

Photonic crystals for chemical sensing and biosensing

This Review covers photonic crystals (PhCs) and their use for sensing mainly chemical and biochemical parameters, with a particular focus on the materials applied. Specific sections are devoted to a) a lead-in into natural and synthetic photonic nanoarchitectures, b) the various kinds of structures of PhCs, c) reflection and diffraction in PhCs, d) aspects of sensing based on mechanical, thermal, optical, electrical, magnetic, and purely chemical stimuli, e) aspects of biosensing based on biomolecules incorporated into PhCs, and f) current trends and limitations of such sensors.

Discovery of the surface polarity gradient on iridescent Morpho butterfly scales reveals a mechanism of their selective vapor response

For almost a century, the iridescence of tropical Morpho butterfly scales has been known to originate from 3D vertical ridge structures of stacked periodic layers of cuticle separated by air gaps. Here we describe a biological pattern of surface functionality that we have found in these photonic structures. This pattern is a gradient of surface polarity of the ridge structures that runs from their polar tops to their less-polar bottoms. This finding shows a biological pattern design that could stimulate numerous technological applications ranging from photonic security tags to self-cleaning surfaces, gas separators, protective clothing, sensors, and many others. As an important first step, this biomaterial property and our knowledge of its basis has allowed us to unveil a general mechanism of selective vapor response observed in the photonic Morpho nanostructures. This mechanism of selective vapor response brings a multivariable perspective for sensing, where selectivity is achieved within a single chemically graded nanostructured sensing unit, rather than from an array of separate sensors.

Nanofluidic crystal: a facile, high-efficiency and high-power-density scaling up scheme for energy harvesting based on nanofluidic reverse electrodialysis

The great advances in nanotechnology call for advances in miniaturized power sources for micro/nano-scale systems. Nanofluidic channels have received great attention as promising high-power-density substitutes for ion exchange membranes for use in energy harvesting from ambient ionic concentration gradient, namely reverse electrodialysis. This paper proposes the nanofluidic crystal (NFC), of packed nanoparticles in micro-meter-sized confined space, as a facile, high-efficiency and high-power-density scaling-up scheme for energy harvesting by nanofluidic reverse electrodialysis (NRED). Obtained from the self-assembly of nanoparticles in a micropore, the NFC forms an ion-selective network with enormous nanochannels due to electrical double-layer overlap in the nanoparticle interstices. As a proof-of-concept demonstration, a maximum efficiency of 42.3 ± 1.84%, a maximum power density of 2.82 ± 0.22 W m(-2), and a maximum output power of 1.17 ± 0.09 nW/unit (nearly three orders of magnitude of amplification compared to other NREDs) were achieved in our prototype cell, which was prepared within 30 min. The current NFC-based prototype cell can be parallelized and cascaded to achieve the desired output power and open circuit voltage. This NFC-based scaling-up scheme for energy harvesting based on NRED is promising for the building of self-powered micro/nano-scale systems.