Temperature Sensors Integrated with an Electrochromic Readout toward Visual Detection

Recent advances in flexible multifunctional electrochromic devices

Electrochromism refers to the phenomenon in which certain materials undergo a redox reaction under an applied voltage or current, resulting in reversible changes in their optical properties and color appearance. Electrochromic devices (ECDs) show great potential in smart windows, anti-glare rear-view mirrors and displays due to the advantages of low energy consumption and simple control mechanisms. However, traditional ECDs are unfavorable for wearable and deformable optoelectronics due to the structural rigidity and limited functions. Thus, flexible ECDs integrating visual information with other advanced technologies can realize multifunctionality and further expand their application fields. This review first introduces the structure and recent development of flexible ECDs, followed by comprehensively summarizing the recent development of flexible multifunctional ECDs, including energy harvesting, energy storage, multicolor displays, and smart windows. Finally, the challenges, development trends and future prospects in flexible multifunctional ECDs are proposed and discussed. We hope that this review can guide and accelerate the development of flexible multifunctional ECDs in the new era of smart optoelectronics.

Configurable swellability of hydrogel microstructure for structural-color-based imaging concealment/encryption

Optical information concealment/encryption technologies are of great importance to structural color applications. Although a series of responsive materials have been developed for dynamic structural color, the shortcomings of the high-quality synthesis process, the complex controlling method, and the low-resolution capability limit their practical use. Herein, we proposed a novel strategy of humidity-driven structural-color-based imaging concealment/encryption by utilizing metal-hydrogel-metal (MHM) nanocavities with configurable swellablity response to humidity change. With varied exposure doses, multi-stage MHM nanocavities with swellable hydrogel interlayers are achieved, generating dynamic structural color covering the visible spectrum. We revealed that the swelling ratio of hydrogel microstructures can be gradually adjusted between 1.05 and 2.08 by varying the exposure dose. We demonstrated that a hydrogel-based structural color image can be concealed with humidity changes by configurating swellable and non-swellable hydrogel pixels together. Furthermore, we developed the double exposure method in which the first exposure can generate pixel arrays for the deceptive image and the second exposure can locally suppress the swellablity of certain pixels. This method can highlight hidden images in a moist state, demonstrating a powerful strategy for high-density optical information encryption.

Photo-, Thermo-, Electrochromic, Erasable Inkless Printing, Ions Detection, and UV Detector Properties of Viologen Compounds Based on Homomolybdate/Keggin POMs

Three POMs-based viologen compounds with different structures were successfully constructed under solvothermal and hydrothermal conditions, [Cu(1,4-cby)2(H2O)0.5(β-Mo8O26)0.5]·C3H7NO·3H2O (1), [H2(1,4-cby)2]·(β-Mo8O26) (2) (1,4-cby·Cl = 1-(4-carboxybenzyl)-4,4'-bipyridine chloride), [H2(1,4-cbyy)2]·(SiMo12O40) (3) (1,4-cbyy·Cl = 1-(4-cyanobenzyl)-4,4'-bipyridine chloride). Compound 1 is a structure with the number "eight-like" metal-organic chain with Cu as the nodes, and compounds 2 and 3 are fascinating structures connected by hydrogen bonding interactions. More importantly, compounds 1-3 exhibit a good response to both light and electricity and the thermal response of compound 1 was also studied. The reasons for the response of compounds 1-3 to external stimuli were analyzed through methods such as UV-Vis, EPR, and XPS. In addition, the transient photocurrent response results of compounds 1-3 are the same as those obtained from kinetic calculations. Meanwhile, the coated filter paper based on compound 3 has been successfully applied in erasable inkless printing and anti-counterfeiting, the test paper of 3 can also detect metal ions, and the films based on compounds 1-3 are a flexible and portable ultraviolet (UV) detector.