Hydrazide–hydrazone derivatives have been utilized in molecular switches, sensing, metallo assemblies, drugs and other applications. The hydrazide–hydrazone functional group contains the –CO–NH–NCH– moiety that yields the hydrazide–hydrazone group its physical, chemical and biochemical properties; however, the application in security protection has not been explored before. Herein, the 2-(ortho-), 3-(meta-), 4-(para-) substituted pyridine conjugated to 2,6-dihydroxybenzoic acid, which containing hydrazide–hydrazone as photoswitch linker were designed and synthesized (3a–3c). Interestingly, when irradiating the ortho-substituted pyridine of molecule 3a with a light of 365 nm, obvious emission intensity enhancements were detected. However, 3a–3c before irradiation and 3b–3c after irradiation show no fluorescence. UV-vis, fluorescence, ¹H NMR and Fourier transform infrared spectra and density functional theory calculations were carried out, which revealed that the hydrazide–hydrazone derivative of 3a could undergo a characteristic E to Z photoisomerization after light irradiation for 365 nm. The Z isomer of 3a may have additional intramolecular hydrogen bonds to restrict the motions of the molecule, thus increasing the fluorescence intensity of 3a. The experimental and molecular calculation results agreed and thus demonstrated the first example of invisible ink based on the hydrazide–hydrazone motif.

We report a novel fluorescent bioprobe, tetraphenylethylene-Phe-Asp-Gly-Glu-Ala (TPE-FDGEA), and its self-assembly behavior, photophysical properties, and biocompatibility. The hydrogelator TPE-FDGEA exhibited aggregation-induced emission characteristics, which facilitated imaging of PC-3 human prostate cancer cells, thereby demonstrating the utility of such fluorescent probes for specific labeling of target cells in vitro.

Conductive self-healing hydrogels and related soft sensor devices are gaining extensive attention from academia to industry because of their impacts on the lifetime and ergonomic design of artificial skins and soft robotics, as well as health monitoring systems. However, so far the development of such a material has been limited considering performance and availability. In this work, we developed composite hydrogels of acrylamide, polyacrylamide, dialdehyde-functionalized poly(ethylene glycol) and conductive carbon black through an interpenetrating polymer network strategy. After optimizing the composition ratio, the resultant hydrogel exhibited self-healing reversibility mechanically and electrically when cut and self-healed. We used ¹H NMR and FT-IR spectroscopy to determine the self-healing mechanism of the system, thus demonstrating that the cooperative effect of the dynamic covalent and noncovalent interactions contributes to the self-healing capability of the gel. Rheology, scanning electron microscopy and light-emitting diode circuits were carried out to examine its macroscopic and microscopic properties, making it possible to apply in soft and conformable electronics.

Smart materials that adapt to various stimuli, such as light, hold immense potential across many fields. Photoresponsive molecules like azobenzenes, which undergo E–Z photoisomerization when exposed to light, are particularly valuable for applications in smart coatings, light-controlled adhesives, and photoresists in semiconductors and integrated circuits. Despite advances in using azobenzene moieties for stimuli-responsive adhesives, the role of push–pull electronic effects in regulating reversible adhesion remains largely unexplored. In this study, we investigate for the first time photo-controlled hydrogel adhesives of azobenzene with different push–pull electronic groups. We synthesized the monomers 4-methoxyazobenzene acrylate (ABOMe), azobenzene acrylate (ABH), and 4-nitroazobenzene acrylate (ABNO₂), and examined their effects on reversible adhesion properties. By incorporating these azobenzene monomers into acrylamide, dialdehyde-functionalized poly(ethylene glycol), and [3-(methacryloylamino)propyl]-trimethylammonium chloride, we prepared ABOMe, ABH, and ABNO₂ ionic hydrogels. Our research findings demonstrate that only the ABOMe ionic hydrogel exhibits reversible adhesion. This is due to its distinct transition state mechanism compared to ABH and ABNO₂, which enables efficient E–Z photoisomerization and drives its reversible adhesion properties. Notably, the ABOMe ionic hydrogel reveals an outstanding skin adhesion strength of 360.7 ± 10.1 kPa, surpassing values reported in current literature. This exceptional adhesion is attributed to Schiff base reactions, monopole–quadrupole interactions, π–π interactions, and hydrogen bonding with skin amino acids. Additionally, the ABOMe hydrogel exhibits excellent reversible self-healing capabilities, significantly enhancing its potential for injectable medical applications. This research underscores the importance of integrating multifunctional properties into a single system, opening new possibilities for innovative and durable adhesive materials.