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The structural color of photonic crystals is usually whitish and has low saturation due to multiple incoherent scattering effects, limiting its application in practical visual observation. Herein, by controlling the calcination time of PS@SiO 2 nanoparticles, we obtain hollow SiO 2 with different in situ generated carbon black contents and assemble them into photonic crystal inks (PCIs). PCIs have excellent saturation and high crystallinity due to the in-situ generated carbon black capable of absorbing incoherently scattered light without affecting particle assembly. PCIs are further converted into non-close-packed photonic crystal films with highly saturated structural colors by photopolymerization of resins. This photonic crystal film can be swollen by an ethanol-water mixture, resulting in structural color changes and shifts in reflection peaks. Moreover, the photonic crystal film can dynamically change from a dry opaque state to an ethanol-wetted translucent state. Such solvent-responsive photonic crystal films with highly saturated structural colors are expected to be used in alcohol sensors and color anti-counterfeiting labels. We introduce carbon black in situ in h-SiO 2 cores to suppress incoherent scattering and enable them to assemble into photonic crystal inks (PCIs) with high saturation structural colors, and the amount of generated in situ carbon black can easily adjust the saturation of the structural color of PCIs. PCIs are further converted into non-close-packed photonic crystal film with highly saturated structural colors by photopolymerization of resins, and this film can be applied to visually detect ethanol-water mixtures and color security labels. [Display omitted] • A series of high-quality h-SiO 2 with different in situ generated carbon black contents are synthesized. • C@h-SiO 2 can be assembled into PCIs through electrostatic repulsion and solvation forces. • PCIs have excellent saturation and high crystallinity. • PCIs can transform into non-close-packed photonic crystal films for solvent detection and anti-counterfeiting labels. [ABSTRACT FROM AUTHOR]

Excessive friction between hydrogels and biological tissue is a major risk factor that can lead to tissue damage and inflammation, limiting their application as biomaterials. Herein, we designed a photonic hydrogel film with an extremely low friction coefficient (COF∼1.71 × 10−3) by embedding free linear polyacrylamide (PAAm) polymer chains within a thermosensitive hydrogel matrix. This photonic hydrogel with brush structure possesses elastic deformation and reversible color change due to its excellent mechanical property. As proof-of-concept, gentamicin sulfate (GS) was selected as model of antibiotic drug, the structural color of material changes during the release of GS. Consequently, the cumulative release amount of the GS can be evaluated by measuring the reflection wavelength. These features make our strategy potentially useful in preparation of materials with low COF and extending the application of photonic hydrogels, including ocular drug delivery and monitoring systems, as well as articular tissue-mechanical sensors. Drawing inspiration from natural systems, we propose a novel method to create biomimetic photonic hydrogels with an exceptionally low surface friction coefficient. These hydrogels can be utilized in the development of ocular drug delivery systems and for real-time monitoring of drug release efficiency. Compared to traditional eye drops or ointments, this dressing with local sustained-release characteristics can better maintain the concentration and efficacy of the drug. These extremely low surface friction photonic hydrogels have the potential to be a promising candidate for novel drug carriers for eye diseases and other situations that require materials with extremely low friction coefficient. [Display omitted] • The photonic crystal prepared exhibits an extremely low friction coefficient (1.71 × 10−3). • The hydrogel possesses excellent mechanochromic and thermochromism properties. • The GS was released from the hydrogel followed a Korsmeyer-peppas model. • The material has the potential to serve as an ocular drug carrier and enable visualization of drug release efficiency. [ABSTRACT FROM AUTHOR]