Thermochromic Printable and Multicolor Polymeric Composite Based on Hybrid Organic-Inorganic Perovskite

Hybrid organic-inorganic perovskites (PVKs) are among the most promising materials for optoelectronic applications thanks to their outstanding photophysical properties and easy synthesis. Herein, a new PVK-based thermochromic composite is demonstrated. It can reversibly switch from a transparent state (transmittance > 80%) at room temperature to a colored state (transmittance < 10%) at high temperature, with very fast kinetics, taking only a few seconds to go from the bleached to the colored state (and vice versa). X-ray diffraction, Fourier-transform infrared spectroscopy, differential scanning calometry, rheological, and optical measurements carried out during heating/cooling cycles reveal that thermochromism in the material is based on a reversible process of PVK disassembly/assembly mediated by intercalating polymeric chains, through the formation and breaking of hydrogen bonds between polymer and perovskite. Therefore, differently from other thermochromic perovskites, that generally work with the adsorption/desorption of volatile molecules, the system is able to perform several heating/cooling cycles regardless of environmental conditions. The color and transition temperature (from 70 to 120 °C) can be tuned depending on the type of perovskite. Moreover, this thermochromic material is printable and can be deposited by cheap techniques, paving the way for a new class of smart coatings with an unprecedented range of colors.

Pseudocatalytic Hydrogels with Intrinsic Antibacterial and Photothermal Activities for Local Treatment of Subcutaneous Abscesses and Breast Tumors

The intimate relationship between bacteria and tumors has triggered a lot of activities in the development and design of bioactive materials to concurrently respond to antitumor and antibacterial demands. Herein, a pseudocatalytic hydrogel (AM-I@Agar) with intrinsic antibacterial and photothermal activities, synthesized by incorporating prefabricated amylose-iodine nanoparticles into low-melting-point agarose hydrogel, is explored as a bioactive agent for local treatment of subcutaneous abscesses and breast tumors. The AM-I@Agar hydrogel depicts the ability of pseudocatalytic O₂ generation from H₂ O₂ to alleviate hypoxia. Meanwhile, the AM-I@Agar hydrogel exhibits temperature self-regulation features, beneficial for avoiding thermal injury during photothermal therapy owing to thermochromic properties. Upon local injection into a subcutaneous abscess, methicillin-resistant Staphylococcus aureus is effectively eliminated by the AM-I@Agar hydrogel, and complete skin recovery is achieved in 8 d, demonstrating much better antibacterial effects compared with penicillin, a small-molecule antibiotic. AM-I/5-FU@Agar hydrogel, obtained after loading 5-fluorouracil (5-FU), significantly inhibits tumors in both normal 4T1 tumor-bearing mice and MRSA-infected 4T1 tumor-bearing mice models via a synergistic photothermal-chemo effect, and shows treatment efficiency superior to that achieved with photothermal therapy or 5-FU alone. This work provides a concept for the design and development of bioactive agents for potential management of bacteria-associated cancer.

A Smart Photothermal Nanosystem with an Intrinsic Temperature-Control Mechanism for Thermostatic Treatment of Bacterial Infections

Photothermal therapy (PTT) has attracted extensive attention in disease treatments. However, conventional photothermal systems do not possess a temperature-control mechanism, which poses a serious risk to healthy tissues and/or organs due to inevitable thermal damage. Herein, a smart photothermal nanosystem with an intrinsic temperature-control mechanism for thermostatic treatment of bacterial infections is reported. The smart photothermal nanosystem is constructed by loading a thermochromic material into a hollow-structured silica nanocarrier, in which the thermochromic material is composed of naturally occurring phase-change materials (PCMs), a proton-responsive spirolactone, and a proton source. The resulting nanosystem shows strong near-infrared (NIR) absorption and efficient photothermal conversion in solid PCMs but becomes NIR-transparent when PCMs are melted upon NIR irradiation. Such an attractive feature can precisely regulate the photothermal equilibrium temperature to the melting point of PCMs, regardless of the variation in external experimental parameters. In contrast to conventional PTT with severe thermal damage, the reported smart photothermal nanosystem provides an internal protection mechanism on healthy tissues and/or organs, which remarkably accelerates the recovery of bacteria-infected wounds. The smart photothermal nanosystem is a versatile PTT platform, holding great promise in the safe and efficient treatment of bacterial infections and multimodality synergistic therapy.

Controlled Synthesis of Luminescent Xanthene Dyes and Use of Ionic Liquid in Thermochromic Reaction

In this study, we demonstrate six novel xanthene derivatives and their spectroscopic and chemical properties. The presented synthesis examination allowed us to obtain two different compounds during one step, with open and closed lactone rings substituted with different length alkyl chains. Increasing the reaction efficiency to 77% was obtained using the microwave-assisted method. Moreover, the modification of O-alkylation synthesis in an ecofriendly way using a ball mill led to achieving exclusively one opened ring product. All of the synthesized compounds showed different spectroscopic behaviors in comparison with the different organic dyes; the typical concentration quenching of luminescence was not observed. The relationship between the length of the alkyl chain and the time of luminescence decay is presented. Synthetized closed forms of dyes turned out to be promising leuco dyes. For the first time, an ionic liquid was used as a developer of synthesized xanthene derivatives (as leuco dyes), which led to obtaining an irreversible thermochromic marker.

Pigmented Structural Color Actuators Fueled by Near-Infrared Light

Cuttlefish can modify their body shape and both their pigmentary and structural colors for protection. This adaptability has inspired the development of appearance-changing polymers such as structural color actuators, although in most cases, the original shape has been confined to being flat, and pigmented structural color actuators have not yet been reported. Here, we have successfully created a pigmented structural color actuator using a cholesteric liquid crystal elastomer with a lower actuation temperature where both actuation and coloration (structural and pigmental) are tunable with temperature and NIR light. The shape, structural color, and absorption of the NIR-absorbing dye pigment of the actuator all change with temperature. Light can be used to trigger local in-plane bending actuation in flat films and local shape changes in a variety of 3D-shaped objects. A cuttlefish mimic that can sense light and respond by locally changing its appearance was also made to demonstrate the potential of pigmented structural color actuators for signaling and camouflage in soft robotics.

Silver Nanowire-Based Printable Electrothermochromic Ink for Flexible Touch-Display Applications

Flexible, lightweight, low-power, and low-cost displays are an active area of interest in the electronics community. In this work, we have developed a composite electrothermochromic material consisting of silver nanowires (Ag NWs) and thermochromic powders, which exhibits reversible color (phase) change during biasing due to Joule heating. A wide variety of color combinations are possible with suitable thermochromic material selection. We have formulated this composite material as a printable ink so that patterned deposition can be achieved in a single step. A low processing temperature of 100 °C makes the composite compatible with a wide range of flexible substrates such as paper and polyethylene terephthalate (PET). The material (encapsulated with polydimethylsiloxane (PDMS)) exhibits good flexibility and is observed to be functional after 10 000 bending cycles with <7% resistance change. We have fabricated a low-power seven-segment color display to show the material's suitability for practical display applications. We have also demonstrated that the same layer can function as a display and as a touch sensor because of its conducting and chromatic properties without additional active layers on top. The material is suitable for the fabrication of low-cost, flexible touch color displays for interactive electronic readers, digital posters, and flexible digital signboards.

Systematic Exploration of the Synthetic Parameters for the Production of Dynamic VO2(M1)

Thermochromic dynamic cool materials present a reversible change of their properties wherein by increasing the temperature, the reflectance, conductivity, and transmittance change due to a reversible crystalline phase transition. In particular, vanadium (IV) dioxide shows a reversible phase transition, accompanied by a change in optical properties, from monoclinic VO₂(M1) to tetragonal VO₂(R). In this paper, we report on a systematic exploration of the parameters for the synthesis of vanadium dioxide VO₂(M1) via an easy, sustainable, reproducible, fast, scalable, and low-cost hydrothermal route without hazardous chemicals, followed by an annealing treatment. The metastable phase VO₂(B), obtained via a hydrothermal route, was converted into the stable VO₂(M1), which shows a metal–insulator transition (MIT) at 68 °C that is useful for different applications, from energy-efficient smart windows to dynamic concrete. Within this scenario, a further functionalization of the oxide nanostructures with tetraethyl orthosilicate (TEOS), characterized by an extreme alkaline environment, was carried out to ensure compatibility with the concrete matrix. Structural properties of the synthesized vanadium dioxides were investigated using temperature-dependent X-ray Diffraction analysis (XRD), while compositional and morphological properties were assessed using Scanning Electron Microscopy, Energy Dispersive X-ray Analysis (SEM-EDX), and Transmission Electron Microscopy (TEM). Differential Scanning Calorimetry (DSC) analysis was used to investigate the thermal behavior.

Femtosecond Laser-Induced Vanadium Oxide Metamaterial Nanostructures and the Study of Optical Response by Experiments and Numerical Simulations

Surface patterning is a popular approach to produce photonic metasurfaces that are tunable when electro-optic, thermo-optic, or magneto-optic materials are used. Vanadium oxides (V_yO_x) are well-known phase change materials with many applications, especially when used as tunable metamaterial photonic structures. Particularly, VO₂ is a well-known thermochromic material for its near-room-temperature phase transition from the insulating to the metallic state. One-dimensional (1D) VO₂ nanograting structures are studied by numerical simulation, and the simulation results reveal that the VO₂ nanograting structures could enhance the luminous transmittance (T_lum) compared with a pristine flat VO₂ surface. It is worth mentioning that T_lum is also polarization-dependent, and both larger grating height and smaller grating periodicity give enhanced T_lum, particularly at TE polarization in both insulating (20 °C) and metallic (90 °C) states of VO₂. Femtosecond laser-patterned VO₂ films exhibiting nanograting structures with an average periodicity of ≈500-700 nm have been fabricated for the first time to enhance thermochromic properties. Using X-ray photoelectron spectroscopy, it is shown that at the optimum laser processing conditions, VO₂ dominates the film composition, while under extra processing, the existence of other vanadium oxide phases such as V₂O₃ and V₂O₅ increases. Such structures show enhanced transmittance in the near-infrared (NIR) region, with an improvement in NIR and solar modulation abilities (ΔT_NIR = 10.8%, ΔT_sol = 10.9%) compared with a flat VO₂ thin film (ΔT_NIR = 8%, ΔT_sol = 10.2%). The slight reduction in transmittance in the visible region is potentially due to the scattering caused by the imperfect nanograting structures. This new patterning approach helps understand the polarization-dependent optical response of VO₂ thin films and opens a new gateway for smart devices.

Mechanochemical Synthesis and Temperature-Dependent Optical Properties of Thermochromic (Ag1-x Cux )2 HgI4

Thermochromic materials are generally synthesized via high-temperature melting reaction or solution-based synthesis. Herein, all-inorganic thermochromic compounds of (Ag_1-x Cu_x )₂ HgI₄ were synthesized by solvent-free simple and scalable mechanochemical grinding at room temperature. Temperature-dependent electronic absorption spectroscopy along with DSC analysis confirmed the thermochromic events within these materials, and the phase transition temperature varied with solid solution compositions. The photoluminescence (PL) spectra is red-shifted with the increase in the Cu content in (Ag_1-x Cu_x )₂ HgI₄ (x=0-1).

Ionic-Liquid-Based Printable Materials for Thermochromic and Thermoresistive Applications

Smart materials exhibiting thermochromic and themoresistive properties based on the electroactive polymer poly(vinylidene fluoride) (PVDF) and the ionic liquid (IL) bis(1-butyl-3-methylimidazolium) tetrachloronickelate ([Bmim]₂[NiCl₄]) have been developed with different contents of [Bmim]₂[NiCl₄] (10, 20, and 40 wt %) within the polymer matrix. The morphology of the composites is studied, and the thermochromic and thermoresistive properties are evaluated. Independently of the IL content, the PVDF/[Bmim]₂[NiCl₄] composites present a porous morphology and thermochromic response, revealed by the color change of the composites from transparent to dark blue, attributed to the tetrahedral complex NiCl₄^2- formed after a dehydration process. Further, the electrical conductivity increases with increasing IL content and decreases with increasing temperature. It is also shown that the incorporation of the IL into the PVDF matrix leads to an increase in the electroactive β phase and a decrease in the degree of crystallinity and thermal stability with increasing [Bmim]₂[NiCl₄] content. The printability and applicability of the developed materials as sensors are also demonstrated.

Thermochromism from Ultrathin Colloidal Sb2 Se3 Nanowires Undergoing Reversible Growth and Dissolution in an Amine-Thiol Mixture

Liquid-based thermochromics can be incorporated into an arbitrarily shaped container and provide a visual map of the temperature changes within its volume. However, photochemical degradation, narrow temperature range of operation, and the need for stringent encapsulation processes are challenges that can limit their widespread use. Here, a unique solution-based thermochromic comprising ultrathin colloidal Sb₂ Se₃ nanowires in an amine-thiol mixture is introduced. The nanowires undergo reversible growth and dissolution with repeated cycles of heating and cooling between 20 and 160 °C, exhibiting intense and contrasting color changes during these processes. Furthermore, the transition temperature in which a change in color first appears can be continuously tuned over a range larger than 100 °C by introducing controlled amounts of Sn²⁺ . The colloidal nanowire dispersion in the amine-thiol mixture retains its thermochromic properties over hundreds of temperature cycles, continuous heating at 80 °C over months, and shelf life of up to 2 years in an open container under ambient conditions. To illustrate its utility as a robust liquid thermochromic, the nanowire solution is coated onto standard filter paper and its uses as a rewritable surface by thermal scribing, as well as an inexpensive means of visualizing the temperature distribution of an anisotropically heated block are demonstrated.

Synthesis and Irreversible Thermochromic Sensor Applications of Manganese Violet

An irreversible thermochromic material based on manganese violet (MnNH₄P₂O₇) is synthesized. The crystal phase, chemical composition, and morphology of the synthesized material are analyzed using X-ray diffraction, scanning electron microscopy coupled with energy-dispersive X-ray spectrometry, and Fourier-transform infrared spectroscopy. The absorption spectra of the synthesized material are obtained using a UV-Vis spectrometer, and the thermochromism exhibited by the powdered samples at high temperatures is also investigated. The as-synthesized manganese violet pigment consists of pure α-MnNH₄P₂O₇ phase. In addition, the synthesized pigment largely consists of hexagonal crystals with a diameter of hundreds of nanometers. On heating, the pigment simultaneously loses H₂O and NH₃ in two successive steps at approximately 330⁻434.4 °C and 434.4⁻527 °C, which correspond to the formation of an intermediate phase and of Mn₂P₄O₁₂, respectively. An overall mass loss of 14.22% is observed, which is consistent with the expected 13.79%. An irreversible color change from violet to white is observed after exposure of the synthesized manganese violet pigment at 400 °C for 30 min. This is attributed to the oxidation of ammonia to hydroxylamine, which then decomposes to nitrogen and water, or alternatively to the direct oxidation of ammonia to nitrogen. Furthermore, we demonstrate the potential application of synthesized manganese violet in the production of irreversible thermochromic paint by mixing with potassium silicate solution as a binder and deionized water as a solvent at a specific ratio. The thermochromic paint is then applied in fabrication of irreversible thermochromic sensors by coating it onto a steel plate surface. Finally, we show that manganese violet-based irreversible thermochromic sensors are able to detect temperatures around 400 °C by changing color from violet to white/milky.

Synthesis and Thermochromic Properties of Cr-Doped Al₂O₃ for a Reversible Thermochromic Sensor

An inorganic thermochromic material based on Cr-doped Al₂O₃ is synthesized using a solid-state method. The crystal structure, chemical composition, and morphology of the synthesized material are analyzed using X-ray diffraction, scanning electron microscopy coupled with an energy-dispersive X-ray spectrometer, and Fourier transform infrared (FT-IR) spectroscopy. The color performances of the synthesized material are analyzed using a UV-VIS spectrometer. Finally, the thermochromism exhibited by the powdered samples at high temperatures is investigated. The material exhibits exceptional thermochromic property, transitioning from pink to gray or green in a temperature range of 25–600 °C. The change in color is reversible and is dependent on the surrounding temperature and chromium concentration; however, it is independent of the exposure time. This novel property of Cr-doped Al₂O₃ can be potentially employed in reversible thermochromic sensors that could be used not only for warning users of damage due to overheating when the environmental temperature exceeds certain limits, but also for detecting and monitoring the temperature of various devices, such as aeronautical engine components, hotplates, and furnaces.

Thermochromic Infrared Metamaterials

An infrared artificial thermochromic material composed of a metamaterial emitter and a bimaterial micro-electro-mechanical system is investigated. A differential emissivity of over 30% is achieved between 623 K and room temperature. The passive metamaterial device demonstrates the ability to independently control the peak wavelength and temperature dependence of the emissivity, and achieves thermal emission following a super Stefan-Boltzmann power curve.