Applications of inverse opal photonic crystal hydrogels in the preparation of acid-base color-changing materials

Hydrogels are three-dimensional (3D) crosslinked network hydrophilic polymers that have structures similar to that of biological protein tissue and can quickly absorb a large amount of water. Opal photonic crystals (OPCs) are a kind of photonic band gap material formed by the periodic arrangement of 3D media, and inverse opal photonic crystals (IOPCs) are their inverse structure. Inverse opal photonic crystal hydrogels (IOPCHs) can produce corresponding visual color responses to a change in acid or alkali in an external humid environment, which has wide applications in chemical sensing, anti-counterfeiting, medical detection, intelligent display, and other fields, and the field has developed rapidly in recent years. In this paper, the research progress on fast acid-base response IOPCHs (pH-IOPCHs) is comprehensively described from the perspective of material synthesis. The technical bottleneck of enhancing the performance of acid-base-responsive IOPCHs and the current practical application limitations are summarized, and the development prospects of acid-base-responsive IOPCHs are described. These comprehensive analyses are expected to provide new ideas for solving problems in the preparation and application of pH-IOPCHs.

Enhanced and Extended Ophthalmic Drug Delivery by pH-Triggered Drug-Eluting Contact Lenses with Large-Pore Mesoporous Silica Nanoparticles

Drug-eluting contact lenses (DCLs) have attracted considerable attention as potential therapeutic ophthalmic drug delivery devices. In this study, we propose, fabricate, and investigate pH-triggered DCLs that are combined with large-pore mesoporous silica nanoparticles (LPMSNs). Compared to reference DCLs, LPMSN-laden DCLs can prolong the residence time of glaucoma drugs in an artificial lacrimal fluid (ALF) environment at pH 7.4. Additionally, LPMSN-laden DCLs do not require drug preloading and are compatible with current contact lens manufacturing processes. LPMSN-laden DCLs soaked at pH 6.5 exhibit better drug loading than reference DCLs due to their specific adsorption. The sustained and extended release of glaucoma drugs by LPMSN-laden DCLs was successfully monitored in ALF, and the drug release mechanism was further explained. We also evaluated the cytotoxicity of LPMSN-laden DCLs, and qualitative and quantitative results showed no cytotoxicity. Our experimental results demonstrate that LPMSNs are excellent nanocarriers that have the potential to be used as safe and stable nanocarriers for the delivery of glaucoma drugs or other drugs. pH-triggered LPMSN-laden DCLs can significantly improve drug loading efficiency and control prolonged drug release, indicating that they have great potential for future biomedical applications.

Probing Colloidal Assembly on Non-Axisymmetric Droplet Surfaces via Electrospray

Microparticles trapped on the surface of a sessile droplet interact via electrostatic and capillary forces. The assembly of colloids at a fluid-fluid interface is governed by particle size, surface chemistry, and contact line roughness. We created nonspherical droplets using surface energy patterning and delivered microparticles to the liquid-air interface with electrospray atomization. Using a water droplet as the target, the particle assembly was observed over time. We found that the underlying surface energy pattern significantly influenced the colloidal assembly and drove particles toward the center of the droplet. The particles were arranged into a single, non-close-packed cluster with local hexagonal ordering but left a clear region with very few particles near the contact line. This depletion region is attributed to long-range electrostatic repulsion from the photoresist used to create the surface energy pattern, which retained electric charge from the electrospray. To understand the effect of electrostatic interactions, we explored target droplets with dissimilar dielectric properties. Using patterned substrates and electrospray for particle deposition, we can harness the assembly of colloids at a fluid interface to build repeatable monolayer patterns.

Bioinspired Polypeptide Photonic Films with Tunable Structural Color

We report a new synthetic strategy of combining N-carboxyanhydride (NCA) chemistry and photonic crystals for the fabrication of polypeptide structural color films. Driven by surface-initiated ring-opening polymerization, the di-NCA derivative of l-cystine (Cys) is introduced to replicate the functionalized colloidal crystal templates and construct freestanding P(Cys) films with tunable structural color. Furthermore, the feasibility of preparing patterned polypeptide photonic films is demonstrated via template microfabrication. Because of the incorporation of l-glutamate (Glu) components, the P(Cys-co-Glu) co-polypeptide films are endowed with a visual color responsiveness toward pH changes. Additionally, the polypeptide photonic films show on-demand degradability. Given the large family of amino acid building blocks, this powerful and versatile approach paves the way for chemical derivatization of multifunctional peptide-based optical platforms.

Colloidal Self-Assembly Approaches to Smart Nanostructured Materials

Colloidal self-assembly refers to a solution-processed assembly of nanometer-/micrometer-sized, well-dispersed particles into secondary structures, whose collective properties are controlled by not only nanoparticle property but also the superstructure symmetry, orientation, phase, and dimension. This combination of characteristics makes colloidal superstructures highly susceptible to remote stimuli or local environmental changes, representing a prominent platform for developing stimuli-responsive materials and smart devices. Chemists are achieving even more delicate control over their active responses to various practical stimuli, setting the stage ready for fully exploiting the potential of this unique set of materials. This review addresses the assembly of colloids into stimuli-responsive or smart nanostructured materials. We first delineate the colloidal self-assembly driven by forces of different length scales. A set of concepts and equations are outlined for controlling the colloidal crystal growth, appreciating the importance of particle connectivity in creating responsive superstructures. We then present working mechanisms and practical strategies for engineering smart colloidal assemblies. The concepts underpinning separation and connectivity control are systematically introduced, allowing active tuning and precise prediction of the colloidal crystal properties in response to external stimuli. Various exciting applications of these unique materials are summarized with a specific focus on the structure-property correlation in smart materials and functional devices. We conclude this review with a summary of existing challenges in colloidal self-assembly of smart materials and provide a perspective on their further advances to the next generation.

Optical Sensing and Imaging of pH Values: Spectroscopies, Materials, and Applications

This is the first comprehensive review on methods and materials for use in optical sensing of pH values and on applications of such sensors. The Review starts with an introduction that contains subsections on the definition of the pH value, a brief look back on optical methods for sensing of pH, on the effects of ionic strength on pH values and pKa values, on the selectivity, sensitivity, precision, dynamic ranges, and temperature dependence of such sensors. Commonly used optical sensing schemes are covered in a next main chapter, with subsections on methods based on absorptiometry, reflectometry, luminescence, refractive index, surface plasmon resonance, photonic crystals, turbidity, mechanical displacement, interferometry, and solvatochromism. This is followed by sections on absorptiometric and luminescent molecular probes for use pH in sensors. Further large sections cover polymeric hosts and supports, and methods for immobilization of indicator dyes. Further and more specific sections summarize the state of the art in materials with dual functionality (indicator and host), nanomaterials, sensors based on upconversion and 2-photon absorption, multiparameter sensors, imaging, and sensors for extreme pH values. A chapter on the many sensing formats has subsections on planar, fiber optic, evanescent wave, refractive index, surface plasmon resonance and holography based sensor designs, and on distributed sensing. Another section summarizes selected applications in areas, such as medicine, biology, oceanography, bioprocess monitoring, corrosion studies, on the use of pH sensors as transducers in biosensors and chemical sensors, and their integration into flow-injection analyzers, microfluidic devices, and lab-on-a-chip systems. An extra section is devoted to current challenges, with subsections on challenges of general nature and those of specific nature. A concluding section gives an outlook on potential future trends and perspectives.

Mechanochromic and thermochromic shape memory photonic crystal films based on core/shell nanoparticles for smart monitoring

Shape memory photonic crystals (SMPCs) combining the main characteristics of shape memory materials and photonic crystals have drawn increasing research interest. In sharp contrast to traditional responsive photonic crystals, the temporary shape of SMPCs can be "frozen" and photonic configurations can be modulated by temperature. However, the large-scale fabrication of SMPCs still remains a big challenge, making the practical application difficult. Herein novel scalable SMPC films with both mechanochromic and thermochromic properties are reported. Unlike traditional template-based methods resulting in only a small size, SMPC films are fabricated by a facile hot-pressing method and post-photocuring technology to give large-area freestanding polymer films. The films are mechanically robust and flexible, featuring an excellent structural color which can be changed upon stretching, similar to the color change process of chameleons in response to the environment. The blue-shift of the reflection peak up to 120 nm can be observed when the film is stretched. The films can be reversibly stretched and recovered in 25 cycles without obvious changes in reflection spectra. The temporary shape accompanied by tremendous color changes in the corresponding SMPC films after mechanical stress induced hot programming could be simply fixed by cooling the structure below the glass transition temperature of the polymer matrix. Incorporated programmed optical properties could afterwards be erased by temperature, and initial optical properties could be fully restored. Based on the fully reversible programmable shape as well as optical properties, the investigated SMPC films are expected to be promising candidates for various potential applications, such as smart monitoring, sensors, anti-counterfeiting, and displays.

Ultrathin Photonic Polymer Gel Films Templated by Non-Close-Packed Monolayer Colloidal Crystals to Enhance Colorimetric Sensing

Responsive polymer-based sensors have attracted considerable attention due to their ability to detect the presence of analytes and convert the detected signal into a physical and/or chemical change. High responsiveness, fast response speed, good linearity, strong stability, and small hysteresis are ideal, but to gain these properties at the same time remains challenging. This paper presents a facile and efficient method to improve the photonic sensing properties of polymeric gels by using non-close-packed monolayer colloidal crystals (ncp MCCs) as the template. Poly-(2-vinyl pyridine) (P2VP), a weak electrolyte, was selected to form the pH-responsive gel material, which was deposited onto ncp MCCs obtained by controlled O₂ plasma etching of close-packed (cp) MCCs. The resultant ultrathin photonic polymer gel film (UPPGF) exhibited significant improvement in responsiveness and linearity towards pH sensing compared to those prepared using cp MCCs template, achieving fast visualized monitoring of pH changes with excellent cyclic stability and small hysteresis loop. The responsiveness and linearity were found to depend on the volume and filling fraction of the polymer gel. Based on a simple geometric model, we established that the volume increased first and then decreased with the decrease of template size, but the filling fraction increased all the time, which was verified by microscopy observations. Therefore, the responsiveness and linearity of UPPGF to pH can be improved by simply adjusting the etching time of oxygen plasma. The well-designed UPPGF is reliable for visualized monitoring of analytes and their concentrations, and can easily be combined in sensor arrays for more accurate detection.

Self-Reporting Colorimetric Analysis of Drug Release by Molecular Imprinted Structural Color Contact Lens

As a prospective ophthalmic drug delivery device, contact lenses attract a lot of attention because of the improved drug residence time and bioavailability. Herein, we proposed and fabricated a molecular imprinted structural color contact lens for sustained timolol release which could self-report the release process by color change. The specific recognition of target timolol by molecular imprinted sites can not only increase the loading amount and the residence time of the drug but also endow the structure color of lens remarkable blue shift with the accumulative release of timolol. The fascinating contact lens can be further used for controlling release of a large number of ophthalmic drugs and has high potential to be a new generation of functional contact lenses.

2D Photonic Crystal Hydrogel Sensor for Tear Glucose Monitoring

Photonic crystal (PC) materials have huge potentials as sensors for noninvasive and real-time monitoring glucose in tears. We developed a glucose-sensitive PC material based on monolayered colloidal crystals (MCCs). Polystyrene nanoparticles were first self-assembled into a highly ordered MCC, and this two-dimensional (2D) template was then coated by a 4-boronobenzaldehyde-functionalized poly(vinyl alcohol) hydrogel. Such a sensor efficiently diffracts visible light, whose structural color could change from red through yellow to green, as the glucose concentration altered from 0 to 20 mM, covering both tears' and bloods' physiological ranges. The sensor also represents a rapid response within 180 s at each titration of glucose, combining the characteristics of high accuracy and sensitivity in detecting the glucose concentration in tears, and this intelligent sensing material presents certain possibility for the frontier point-of-care glucose monitoring.

Near-Infrared Trigged Stimulus-Responsive Photonic Crystals with Hierarchical Structures

Stimuli-responsive photonic crystals (PCs) trigged by light would provide a novel intuitive and quantitative method for noninvasive detection. Inspired by the flame-detecting aptitude of fire beetles and the hierarchical photonic structures of butterfly wings, we herein developed near-infrared stimuli-responsive PCs through coupling photothermal Fe₃O₄ nanoparticles with thermoresponsive poly(N-isopropylacrylamide) (PNIPAM), with hierarchical photonic structured butterfly wing scales as the template. The nanoparticles within 10 s transferred near-infrared radiation into heat that triggered the phase transition of PNIPAM; this almost immediately posed an anticipated effect on the PNIPAM refractive index and resulted in a composite spectrum change of ∼26 nm, leading to the direct visual readout. It is noteworthy that the whole process is durable and stable mainly owing to the chemical bonding formed between PNIPAM and the biotemplate. We envision that this biologically inspired approach could be utilized in a broad range of applications and would have a great impact on various monitoring processes and medical sensing.

Hydrogel Based Sensors for Biomedical Applications: An Updated Review

Biosensors that detect and convert biological reactions to a measurable signal have gained much attention in recent years. Between 1950 and 2017, more than 150,000 papers have been published addressing the applications of biosensors in different industries, but to the best of our knowledge and through careful screening, critical reviews that describe hydrogel based biosensors for biomedical applications are rare. This review discusses the biomedical application of hydrogel based biosensors, based on a search performed through Web of Science Core, PubMed (NLM), and Science Direct online databases for the years 2000⁻2017. In this review, we consider bioreceptors to be immobilized on hydrogel based biosensors, their advantages and disadvantages, and immobilization techniques. We identify the hydrogels that are most favored for this type of biosensor, as well as the predominant transduction strategies. We explain biomedical applications of hydrogel based biosensors including cell metabolite and pathogen detection, tissue engineering, wound healing, and cancer monitoring, and strategies for small biomolecules such as glucose, lactate, urea, and cholesterol detection are identified.

Hierarchical photonic structured stimuli-responsive materials as high-performance colorimetric sensors

Hierarchical photonic structures in nature are of special interest because they can be used as templates for fabrication of stimuli-responsive photonic crystals (PCs) with unique structures beyond man-made synthesis. The current stimuli-responsive PCs templated directly from natural PCs showed a very weak external stimuli response and poor durability due to the limitations of natural templates. Herein, we tackle this problem by chemically coating functional polymers, polyacrylamide, on butterfly wing scales which have hierarchical photonic structures. As a result of the combination of the strong water absorption properties of the polyacrylamide and the PC structures of the butterfly wing scales, the designed materials demonstrated excellent humidity responsive properties and a tremendous colour change. The colour change is induced by the refractive index change which is in turn due to the swollen nature of the polymer when the relative humidity changes. The butterfly wing scales also showed an excellent durability which is due to the chemical bonds formed between the polymer and wing scales. The synthesis strategy provides an avenue for the promising applications of stimuli-responsive PCs with hierarchical structures.

Label-free and pH-sensitive colorimetric materials for the sensing of urea

This communication demonstrates a facile method for naked-eye detection of urea based on the structure color change of pH-sensitive photonic crystals. The insertion of urease provides excellent selectivity over other molecules. The detection of urea in different concentration ranges could be realized by changing the molar ratio between the functional monomer and cross-linker.

Ultrathin polymer gel-infiltrated monolayer colloidal crystal films for rapid colorimetric chemical sensing

The ultrathin polymer gel-infiltrated monolayer colloidal crystal film shows rapid, linear, reversible, and colorimetric responses to pH variations.

Two-dimensional photonic crystal chemical and biomolecular sensors

We review recent progress in the development of two-dimensional (2-D) photonic crystal (PC) materials for chemical and biological sensing applications. Self-assembly methods were developed in our laboratory to fabricate 2-D particle array monolayers on mercury and water surfaces. These hexagonal arrays strongly forward Bragg diffract light to report on their array spacings. By embedding these 2-D arrays onto responsive hydrogel surfaces, 2-D PC sensing materials can be fabricated. The 2-D PC sensors utilize responsive polymer hydrogels that are chemically functionalized to show volume phase transitions in selective response to particular chemical species. Novel hydrogels were also developed in our laboratory by cross-linking proteins while preserving their native structures to maintain their selective binding affinities. The volume phase transitions swell or shrink the hydrogels, which alter their 2-D array spacings, and shift their diffraction wavelengths. These shifts can be visually detected or spectrally measured. These 2-D PC sensing materials have been used for the detection of many analytes, such as pH, surfactants, metal ions, proteins, anionic drugs, and ammonia. We are exploring the use of organogels that use low vapor pressure ionic liquids as their mobile phases for sensing atmospheric analytes.

Strong anionic polyelectrolyte microgels

We report a facile synthesis of highly uniform poly(styrene sulfonic acid) microgels, which carry a strong polyelectrolyte group at every repeating unit.

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.

25th anniversary article: ordered polymer structures for the engineering of photons and phonons

The engineering of optical and acoustic material functionalities via construction of ordered local and global architectures on various length scales commensurate with and well below the characteristic length scales of photons and phonons in the material is an indispensable and powerful means to develop novel materials. In the current mature status of photonics, polymers hold a pivotal role in various application areas such as light-emission, sensing, energy, and displays, with exclusive advantages despite their relatively low dielectric constants. Moreover, in the nascent field of phononics, polymers are expected to be a superior material platform due to the ability for readily fabricated complex polymer structures possessing a wide range of mechanical behaviors, complete phononic bandgaps, and resonant architectures. In this review, polymer-centric photonic and phononic crystals and metamaterials are highlighted, and basic concepts, fabrication techniques, selected functional polymers, applications, and emerging ideas are introduced.