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Photonic crystal has become a powerful technique for regulating electromagnetic waves due to its excellent features, such as structure designability and dynamic adjustability; in addition, it has great application value. In this paper, we visualize colors on the Ostwald color model and analyze the color differences of each group of color patches via the minimum color difference model. The manipulation law of particle size, temperature, and humidity on the color change of photonic crystal hydrogel is verified by these color analyses. [ABSTRACT FROM AUTHOR]

The cylindrical surface with Bragg-enhanced Fresnel reflectivity constructed from non-close-packed photonic crystals (NPCs) was designed for spectral selectivity and optical gain, which broke through the limitations of traditional Fresnel curved mirror with lowest central reflectivity and inability to perform spectral selectivity, and was conducive to expending the application of Fresnel mirror in the fields of signal transmission, optical measurement and instrument design. [Display omitted] According to the Fresnel theory, the reflectivity intensity of spherical and cylindrical convex surfaces decreases from their edge to center, and it is noteworthy and interesting for optical gain to study the enhancement of center reflectance. In this paper, a polydimethylsiloxane (PDMS) − encapsulated cylindrical non-closed-packed photonic crystals (NPCs) composite with Bragg-enhanced Fresnel reflectance was designed for spectral selectivity and optical gain. Theoretically and experimentally, the periodically ordered structure of NPCs achieved high-reflection of light in photonic bandgap and high-transmission in other bands, which enhanced Fresnel reflectivity of the convex center to specific bands. Furtherly, the cylindrical NPCs hydrogel with stretchability was applied for the dynamic tuning of optical signals. The reflection peak of the PDMS-encapsulated cylindrical NPCs composite blue-shifted from 608 nm to 413 nm with 50 % tensile strain and achieved a rapid transition of structural color from orange to blue-violet in 60 cycles. The new kind of photonic crystals composite for optical gain and spectral selection broke through the limitations of traditional Fresnel curved mirrors with the lowest central reflectivity and inability to perform spectral selectivity, and have great significance and application prospects in fields of signal transmission, optical measurement, and instrument design. [ABSTRACT FROM AUTHOR]

• Hydrogel sensor based on two-dimensional PS colloidal crystal array is developed. • The sensor achieves affinity by the interaction between polymer ligand and target. • The sensor shows high response, selectivity in buffers or body fluids. • It could be controlled by "temperature switch", exhibiting reusability to target It is significant to detect lysozyme both in fundamental research and in clinical practice, since its concentration in body fluid is associated with many diseases. In this study, a new kind of sensor for rapid screening the lysozyme was fabricated based on the two-dimensional (2D) polystyrene colloidal crystal array together with the optimized hydrogel containing acrylamide-based functional groups. Distinct hydrogel volume shrink was observed due to the hydrogel crosslink induced by the functional ligand–protein interactions while the protein bound to the sensor. Therefore, the analyte concentration could be accessed by of the lattice spacing of the 2D colloidal crystal array, which was detected by a change in the Debye diffraction ring diameter. The lysozyme concentration could be easily monitored by measuring the diameter of the Debye diffraction ring in the pathophysiologic range. The particle spacing of 2D colloidal crystal hydrogel sensor increased to about 22 nm while lysozyme concentration increased from zero to 1.0 mg/mL, meanwhile the structural color of the sensor blue shifted from red to yellow. The detection limit was confirmed as 1.38 × 10−3 mg/mL. The selective response to other common proteins in body fluids was also exhibited, indicating the great selective response properties to lysozyme. In addition, the 2D colloidal crystal hydrogel sensor was reusable to detect the lysozyme solution reversibly due to its thermal sensitivity. Moreover, the determination of lysozyme in artificial tears and human urine samples by this sensor demonstrated its feasibility in potential clinical analysis diagnosis. [ABSTRACT FROM AUTHOR]

[Display omitted] • A pH-responsive discoloration photonic crystal sensor was proposed for amniotic fluid recognition. • The sensor showed the high accuracy and sensitivity, rapid response and visual detection. • Compared with pH test paper, a noticeable color difference and extraordinary accuracy were obtained. In this study, we proposed a pH-responsive photonic crystal hydrogel sensor (PCHs) to conveniently and accurately recognize the amniotic fluid from urine of parturient women, which was constituted by three-dimensional polymethyl acrylamide (PMMA) array and optimized hydrogel. PMMA array was self-assembled by 180 nm monodispersed nanoparticles with vertical sedimentation method. The functional hydrogel containing N-acryloyl phenylalanine (APA), acrylamide (AM) and N, N'-methylene bis(acrylamide) (BIS) as co-monomer and crosslinking agent respectively, was photo-polymerized by the precursor under 365 nm UV light. This hydrogel significantly shrank or swelled corresponding to the acidity and alkalinity of the aqueous solution, which resulted in both the reflection peak position (λ max) of the prepared sensor and its structural color changing with the pH value. The λ max of PCHs redshifted from 514 nm to 606 nm in 60 s while the pH value of testing solution increased from 4.5 to 8.0, which exactly covered the pH value of amniotic fluid and urine. Correspondingly, its structure color gradually changed from pale-green to red ensuring their naked-eye determination. Furthermore, an extremely high sensitivity as 26.3 nm/pH of Δ λ max was obtained which was linearly related to pH value. The prepared sensor possessed steady accuracy and extraordinary sensitivity compared with the pH test strip which was generally used in amniorrhexis diagnosis nowadays besides the advantages of the low cost, simple operation, rapid response and visual detection mode. The reliability of this PCHs was further confirmed by the real sample detection which relative standard deviation was below 5.0 %, suggesting its potential clinical applications. [ABSTRACT FROM AUTHOR]

As a typical antibiotic that is harmful to the health of human beings and widely used in animal husbandry, oxytetracycline (OTC) has a potential threat to the food safety. Therefore, the design of portable sensors for efficiently monitoring trace amounts of OTC in foods is vital to maintain safe food supply in our daily life. Herein, a two-dimensional (2D) molecularly imprinted photonic crystal hydrogel (MIPCH) sensor for the detection of OTC was fabricated by combining the photonic crystal (PC) method and molecular imprinting technique. In this regard, OTC was used as imprinted template to form the MIPCH sensor with specific binding sites during polymerization. After the removal of template, the obtained MIPCH possessed the specific nanocavities that were complementary to target compound, OTC. In addition, the response of the MIPCH to tetracyclines (TCs) in aqueous solution was monitored through a readable change of Debye diffraction ring, which is related to the particle spacing variation of MIPCH in response to the target molecules. As the concentration of OTC increased from 0 to 60 μM, the particle spacing of MIPCH sensor increased to about 94 nm and the structural color redshifted from blue to red. More importantly, the detection of OTC in milk sample by this portable, cost-effective MIPCH sensor has also been achieved at the same range of OTC. With the increasing concentration of OTC in milk sample, the particle spacing of MIPCH sensor increased about 92 nm and the structural color of MIPCH changed from blue through green to orange. Image 1 • A novel 2D molecularly imprinted PC was developed for the fast screening of trace level residues of OTC in milk. • The detection could be realized by naked eyes through observing the diffraction ring or structural color. • The sensor also showed advantages in easy preparation and specificity towards the targets. [ABSTRACT FROM AUTHOR]

[Display omitted] • Colorful cellulose nanocrystal (CNC) – polyaniline (PANI) films were fabricated. • The color of CNC-PANI films changed with humidity, pH and organic solvents. • An environmental monitoring alarm with conductivity changes was demonstrated. • An electrochromic device using CNCs for data encryption was developed. Cellulose nanocrystals (CNCs), a renewable material extracted from plants, algae, and bacteria, form iridescent films with chiral nematic order. Here, we report stimuli-responsive CNC / polyaniline (PANI) composite films for use in optical and electrochemical sensing. CNC-PANI and CNC-glucose-PANI films with bright structural color were obtained by an optimized self-assembly process. The films responded to humidity, pH and organic solvents with a change in the reflection wavelength, change in visible structural color, and changes in conductivity. A two-layer, conductive CNC-PANI film was prepared and incorporated into a circuit, where it could respond to stimuli with a change in color and increased conductivity. Moreover, a PANI electrochromic device (ECD) with CNC films was fabricated for information encryption. With voltages up to 1.5 V, the color of the ECD changed from green to blue within 1 s, and the CNC pattern hidden inside appeared with good cyclability. The ECD can also be used for anti-counterfeiting due to the chiral nematic CNC pattern. This novel iridescent CNC-PANI film demonstrates the potential of visible electrochemical multi-sensing using natural, sustainable materials. [ABSTRACT FROM AUTHOR]

[Display omitted] • A large-scale amorphous photonic crystal (APC) array was cast by a robotic arm. • The APC arrays had non-iridescent structural colors and can be used in painting. • Molecularly imprinted APC sensors group was fabricated to detect explosives. • A qualitative and semi-quantitative result was obtained via color analysis methods. Leveraging advances in nanoscience and intelligent technology to engineer large-scale and stimuli-response amorphous photonic crystals (APCs) for use in inexpensive and portable sensing system for the detection of explosives. Polymethyl methacrylate (PMMA) colloidal microspheres with uniform particle size and good mono-dispersion were used to prepare APCs by spraying method. The structural color of APCs was angle-independent and changed with different microspheres size. Meanwhile, a robotic arm was designed to achieve an automatic and large-scale preparation of APCs, and this more accurate and cost-effective method not only provides an automatic process for large-scale APCs pattern painting in chemical industry, but also gives chemical sensors an access to be smart devices in life. Then 2,4,6-trinitrotoluene (TNT), 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) and cyclotetramethylenete-tranitramine (HMX) molecularly imprinted APCs (MIAPCs) were fabricated in the same way. The results showed that the response time of MIAPCs was 10 s, and the limit of detection was 0.01 mM with NaOH swollen. Specially, by using the more convenient HSB (Hue-Saturation-Brightness) and CIE (Commission Internationale de L'Eclairage) color analysis methods, combined with principal component analysis and pattern recognition methods, a sensor group with higher selectivity and sensitivity than a single sensor was constructed. A qualitative and semi-quantitative result was realized by responding to the changes of structural color before and after. Compared with traditional PhCs sensor, the APCs sensor has angle-independent structural color, which provides more accurate results and simplifies the detection based on the color analysis. This novel large-scale APCs sensor represents a potential of highly selective and real-time on-site visual explosive detection. In the meanwhile, the automatic APCs casting also offers chemical materials an access to combining with intelligent technology. [ABSTRACT FROM AUTHOR]

• A rapid self-assembly method was proposed to prepare the 3D-photonic crystal (PhC). • The effect of microspheres, heating and nitrogen blowing have been also studied. • Using this method, a p-nitrophenol-molecular imprinted PhC sensor was prepared. To overcome the long period of three-dimension photonic crystal colloidal array preparation, a rapid self-assembly method is proposed. The effect of microspheres diameter and concentration, heating and nitrogen blowing have been also studied. According to the results, when the microspheres suspension concentration is 0.2 wt% and heating temperature of 70 °C, a highly regular 3D face-centered cubic colloidal crystal array structure self-assembles on the gas-liquid interface within 3 min. Then using this method, a p-nitrophenol-molecular (p-NP) imprinted photonic crystal sensor with high selectivity and sensitivity is prepared. The p-NP imprinted sensor presents twice signal intense than the non-imprinted sensor towards the target, and the structural color changes from blue to green. Meanwhile, the sensor responds in 3 min and can be reversible more than 9 times. This rapid self-assembly method provides a suitable way for large-area colloidal photonic crystal preparation with higher efficiency and good repeatability, which has a high value for practical applications. [ABSTRACT FROM AUTHOR]