Hofmeister series (HS), ion specific effect, or lyotropic sequence acts as a pivotal part in a number of biological and physicochemical phenomena, e.g., changing the solubility of hydrophobic solutes, the cloud points of polymers and nonionic surfactants, the activities of various enzymes, the action of ions on an ion-channel, and the surface tension of electrolyte solutions, etc. This review focused on how ion specificity influences the critical micelle concentration (CMC) and how the thermoresponsive behavior of surfactants, and the dynamic transition of the aggregate, controls the aggregate transition and gel formation and tunes the properties of air/water interfaces (Langmuir monolayer and interfacial free energy). Recent progress of the ion specific effect in bulk phase and at interfaces in amphiphilic systems and gels is summarized. Applications and a molecular level theoretical explanation of HS are discussed comprehensively. This review is aimed to supply a fresh and comprehensive understanding of Hofmiester phenomena in surfactants, polymers, colloids, and interface science and to provide a guideline to design the microstructures and templates for preparation of nanomaterials.

Hydrogel-based sensors serve as an ideal platform for developing personalized wearable electronics due to their high flexibility and conformability. However, the weak stretchability and inferior conductivity of hydrogels have severely restricted their large-scale application. Herein, a natural polymer-based conductive hydrogel integrated with favorable mechanical properties, good adhesive performance, and excellent fatigue resistance was fabricated via interpenetrating tannic acid (TA) into a chitosan (CS) cross-linked network in an acidic aqueous solution. The hydrogel was composed of a regular hierarchical porous structure, which was built by the hydrogen bonding between TA and CS. In addition, the hydrogels exhibited adjustable mechanical properties (maximum yield stress of 7000 Pa) and good stretchability (strain up to 320%). Benefiting from the abundant catechol groups of TA, the proposed hydrogels could repeatedly adhere to various material surfaces and could be easily peeled off without residue. Moreover, the hydrogel exhibited stable conductivity, high stretching sensitivity (gauge factor of 2.956), rapid response time (930 ms), and excellent durability (>300 cycles), which can be assembled as a strain sensor to attach to the human body for precise monitoring of human exercise behavior, distinguishing physiological signals, and recognizing speech. Furthermore, the prepared hydrogels also exhibited stable sensing performance to temperature. As a result, the hydrogels exhibited dual sensory performance for both temperature and strain deformation. It is anticipated that the incorporation of strain sensors and thermal sensors will provide theoretical guidance for developing multifunctional conductive hydrogels and pave a way for the versatile application of hydrogel-based flexible sensors in wearable devices and soft actuators.

Due to the unique flexibility and modifiability of hydrogels, hydrogel-based wearable sensors have drawn tremendous attention. However, traditional hydrogels rigidify or dehydrate at extreme temperatures because their included water freezes or evaporates, which greatly impedes the development and practical application of the hydrogel-based wearable sensor. Herein, a temperature-tolerant organohydrogel with self-healing properties, adhesiveness, plasticity, and high stretchability was designed by introducing the specific base pairs, adenine (A) and thymine (T), into the polyacrylamide network in a water–glycerol (Gly) binary solvent. The gelation process was mainly driven by the covalent cross-linking and the complementary base pairing of the double helix structure of DNA. The prepared organohydrogels exhibited a tensile strength of 35 kPa, a toughness of 667 kJ m–3, and were highly flexibile with a rupture elongation of 3870%. Moreover, the organohydrogel demonstrated an excellent adhesive performance toward diverse organic and inorganic substrates. The organohydrogel displayed a maximum peeling force and adhesion strength of organohydrogel to filter paper of 149 and 122 kPa, respectively. In addition, the organohydrogel presented a rapid self-healing efficiency, long-term moisture retention, and good conductivity, even at subzero temperatures (−20 °C), and can be assembled as a dual strain and thermal sensor to realize the dual-sensing. The organohydrogel strain sensor exhibited a higher sensing sensitivity [gauge factor (GF) = 11.99] over a broad strain range (∼660%) and long-term durability (>135 cycles) and can be attached to the human body to monitor human motion in real-time. Significantly, the organohydrogel still maintained its high strain sensitivity (GF = 9.76) even at a lower temperature. We envisage that this study will provide a theoretical guidance for the design and development of multifunctional conductive hydrogels with antifreezing and antidrying properties and extend the application of the hydrogel-based sensor in electronic skin, flexible control panel, wearable devices, and health monitoring in extreme environments.

Stretchable PA/CS polymer hydrogels with multiresponsiveness presented rapid self-healing behavior, and can be used for metal ions scavenging.

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Three 3D coordination polymers have been synthesized by using the 1,1′:4′,1′′-terphenyl]-2′,4,4′′,5′-tetracarboxylate acid and Cd(ii),Co(ii) and Mn(ii), the Cd(ii) complex shows selective sensing Al3+/Fe3+ over the mixed metal ions.

In this study, novel supramolecular ionogels with ultrahigh efficient gelation and robust mechanical properties were prepared by mixing 4′-para-phenylcarboxyl-2,2′:6′,2″-terpyridine (PPCT) and zinc ions (Zn2+) in ethylammonium nitrate (EAN). The microstructure of the ionogels was determined to be three-dimensional networks of fibrous aggregates. X-ray diffraction and Fourier-transform infrared spectroscopy measurements demonstrated that ionogel formation involve the following steps: the terpyridine rings of PPCT form an assembled unit via π–π interaction, the unit further aggregates to form fibers using Zn2+ via hydrogen bonding and Zn2+ coordination, and the fibers stretch and intertwine to form a cross-linked network to immobilize EAN by solvophobic interactions, electrostatic interactions, and van der Waals forces. Rheological results revealed that ionogels exhibited high mechanical strength with an elastic modulus and a yield stress of 50 000 and 900 Pa, respectively. The ionogels of PPCT and Zn2+ mixtures served as the precursors to produce zinc sulfide (ZnS) nanoparticles (NPs). The uniform 10 nm-sized ZnS exhibited higher surface area and higher peroxidase-like activity that can be used for sulfide ion (S2–) colorimetric sensing to detect S2– at a lower limit detection of 5.27 nmol·L–1. Furthermore, an innovative, green, and convenient approach has been developed to produce ZnS NPs, which are an environmentally friendly and sustainable candidate material in bioengineering technology, environmental protection, and food industries.

Self-healing hydrogels were prepared by mixing the difunctionalized polyethylene glycol (DF-PEG) and chitosan (CS) in water. Due to the formation of Schiff base bond between DF-PEG and CS, the gelation could be realized in several seconds. Determined by the SEM showed that the hydrogel was composed of porous network structure. The dynamic formation and dissociation of Schiff base bond (À C=NÀ ) between the aldehyde group of DF-PEG and amino group of CS initiated the excellent self-healing property and reversible pH responsivity of hydrogels. Additionally, the hydrogels had excellent ductility and toughness with the fracture strain and stress of 88.2 % and 12.1 kPa. The hydrogels exhibited excellent strain sensing performance, which can be fabricated as the flexible sensor for real-time monitoring the large and delicate human motions. As a result, it is expected that the obtained self-healing hydrogels will broaden the application of gels in wearable electronics.