Rational Design and Application of a Redox-Active, Photoresponsive, Discrete Metallogelator

Anticounterfeiting Feature of a Writable and Self-Erasable Ni(II)-Metallogel Pad via Fluorescent "Turn-On" Detection of Cyanide

A Schiff base 5-(2-hydroxy-3-methoxybenzylidieneamino)-1-H-imidazole-4-carboxamide (HL) comprising multibinding sites has been synthesized with the aim of fabricating a supramolecular gel. The gelator HL was characterized by FT-IR, 1H & 13C NMR, and ESI-MS techniques and also formed a [Ni(L)2] complex. The gelation property of HL was investigated with various metal ions, wherein Ni(II) selectively forms a mechanically and thermally stable supramolecular metallogel (MG) in the presence of a triethylamine base in DMF-MeOH media. Characterization of MG was accomplished with different spectro-analytical techniques such as FT-IR, ESI-MS, powder-XRD, SEM, rheological investigations, UV/vis, and fluorescence. The gelator HL displays moderate emission upon addition of Ni2+ and gives "turn-off" fluorescence output by forming the complex [Ni(L)2] (MG) due to the chelation-enhanced quenching of fluorescence (CHEQ). Job plot and ESI-MS data suggested a 2:1 stoichiometry between HL and Ni(II) in MG. Further, MG exhibited highly selective and ultrasensitive "turn-on" fluorescence signaling with CN- in the background presence of several cations and anions. The limit of detection (LoD) of MG was determined to be 6.9 × 10-9 M for CN- using the fluorescence technique. Notably, MG behaves as a fluorescent writable pad material explicitly with CN- under 365 nm UV light but not under ordinary light and the fluorescent text is self-erased after 15 min. Hence, MG can be used as a metallogel pad in the presence of CN- to communicate secret messages. Overall, the present work explores the fabrication of a thermo- and mechanostable Ni(II)-metallogel (MG), which selectively and ultrasensitively detects CN- both in the solution phase and in the gel form, wherein MG behaves as a writable and self-erasable pad with anticounterfeiting features for practical applications.

Recent Advances in Stimuli-Responsive Metallogels

Recently, stimuli-responsive supramolecular gels have received significant attention because their properties can be modulated through external stimuli such as heat, light, electricity, magnetic fields, mechanical stress, pH, ions, chemicals and enzymes. Among these gels, stimuli-responsive supramolecular metallogels have shown promising applications in material science because of their fascinating redox, optical, electronic and magnetic properties. In this review, research progress on stimuli-responsive supramolecular metallogels in recent years is systematically summarized. According to external stimulus sources, stimuli-responsive supramolecular metallogels, including chemical, physical and multiple stimuli-responsive metallogels, are discussed separately. Moreover, challenges, suggestions and opportunities regarding the development of novel stimuli-responsive metallogels are presented. We believe the knowledge and inspiration gained from this review will deepen the current understanding of stimuli-responsive smart metallogels and encourage more scientists to provide valuable contributions to this topic in the coming decades.

Tuning the through-space charge transfer emission in triarylborane and triarylamine functionalized dipeptide organogels

We report herein a new class of either carbazolyl or BMes₂ (Mes = mesityl) group functionalized Boc-Lys(Z)-Phe-OMe (Z = carbobenzyloxy) dipeptides-Boc-Lys(Z)-Phe-C₅-carbazolyl (N2) and Boc-Lys(Z)-Phe-C₆-BMes₂ (B2). Both of the compounds are able to gel in several common aromatic solvents at low concentration. The photophysical studies reveal the existence of intense through space charge transfer interaction between the donor and acceptor units in the B2 and N2 based dual-component supramolecular organogels. Furthermore, by tuning the B2 : N2 ratios in the binary gels, both the maximum emission wavelength and the morphologies of the dual-component gels can be effectively modulated.

Ultrasound-Induced Amino Acid-Based Hydrogels With Superior Mechanical Strength for Controllable Long-Term Release of Anti-Cercariae Drug

Stimulus-responsive hydrogels are significantly programmable materials that show potential applications in the field of biomedicine and the environment. Ultrasound as a stimulus can induce the formation of hydrogels, which exhibit the superior performance of different structures. In this study, we reported an ultrasound-induced supramolecular hydrogel based on aspartic acid derivative N,N'-diaspartate-3,4,9,10-perylene tetracarboxylic acid imide, showing superior performance in drug release. The results show that the driving force of this ultrasonic induced hydrogel could be attributed to hydrogen bonding and π-π interaction. The rheological and cytotoxicity test illustrate excellent mechanical properties and biocompatibility of the hydrogel. The anti-Schistosoma japonicum cercariae (CC) drug release results show large drug loadings (500 mg/ml) and long-term release (15 days) of this hydrogel. This study demonstrates that this hydrogel may serve as a slow-release platform for anti-CC.

Multi-responsive organo- and hydrogelation based on the supramolecular assembly of fluorocarbon- and hydrocarbon-containing hybrid surfactants

HYPOTHESIS

Novel photoresponsive hybrid surfactants, in which a combination of perfluoroalkyl and alkyl chains and cationic head groups are connected via azobenzene moieties, are excellent candidates for assembling low-molecular-weight organogels (LMOGs) with reversibly switchable viscoelasticities triggered by external stimuli.

EXPERIMENTS

The structure-composition-property relationships of gels assembled with the hybrid surfactants were investigated by UV-vis and NMR spectroscopy, SEM, XRD, and rheology.

FINDINGS

Hybrid surfactants containing perfluorohexyl chains with more than six carbons gelled in a variety of organic solvents at concentrations of less than a few percent. In particular, compositions with the perfluorooctyl and somewhat shorter hydrocarbon chains (C1-C4) gelled in both organic solvents and water. The gellable solvent species can be well grouped according to their solubility parameters, suggesting that gelation properties can be predicted from the chemical structure of the surfactant. Mechanical and structural investigations revealed that gel viscoelasticity can be reversibly altered by applying photo, shear, and heat stimuli, which is achieved through the formation and deformation of lamella-like molecular aggregates. The multi-responsive gelation and facile molecular design of the present hybrid surfactants will expand the fields in which fluorinated LMOGs can be applied.

Supramolecular chiroptical switches

Recent progress in chiroptical switches including on/off, amplification, and inversion of the chiral signals such as ECD and CPL in supramolecular assemblies is shown.

Photoresponsive Zn2+-specific metallohydrogels coassembled from imidazole containing phenylalanine and arylazopyrazole derivatives

Stimuli-responsive supramolecular gels and metallogels have been widely explored in the past decade, but the fabrication of metallogels with reversible photoresponsive properties remains largely unexplored.

Mechanistic Insights into the Self-Assembly of an Acid-Sensitive Photoresponsive Supramolecular Polymer

The supramolecular polymerization of an acid‐sensitive pyridyl‐based ligand (L₁) bearing a photoresponsive azobenzene moiety was elucidated by mechanistic studies. Addition of trifluoroacetic acid (TFA) led to the transformation of the antiparallel H‐bonded fibers of L₁ in methylcyclohexane into superhelical braid‐like fibers stabilized by H‐bonding of parallel‐stacked monomer units. Interestingly, L₁ dimers held together by unconventional pyridine–TFA N⋅⋅⋅H⋅⋅⋅O bridges represent the main structural elements of the assembly. UV‐light irradiation caused a strain‐driven disassembly and subsequent aggregate reconstruction, which ultimately led to short fibers. The results allowed to understand the mechanism of mutual influence of acid and light stimuli on supramolecular polymerization processes, thus opening up new possibilities to design advanced stimuli‐triggered supramolecular systems.

Bioorganometallic ferrocene-tripeptide nanoemulsions

We demonstrate an effective strategy to prepare highly stable nanoemulsions using ferrocene-modified tripeptides. Compared with traditional nanoemulsions, bioorganometallic peptide nanoemulsions are appealing for a number of reasons, including long-term and outstanding thermal stability, redox activity and biocompatibility. The formed nanoemulsions could remain stable for more than four months at room temperature, which is the highest stability reported so far for peptide and protein emulsifiers. The phase behaviour and size distribution of the emulsions could be precisely tailored by altering the temperature, solvent ratio and redox state of the ferrocene moiety. In this process, we observed a unique enthalpy-driven phase transition from nanoemulsions to hydrogels, which could be attributed to the competition between the interfacial free energy and the association energy among the self-assembling peptides. Moreover, we could impart catalytic activity to the nanoemulsions through rationally altering the sequence of the tripeptides. The structurally tunable, functional bioorganometallic nanoemulsions offer new opportunities in many areas including drug delivery, and the food and cosmetic industries.

A label-free and universal platform for the construction of an odd/even detector for decimal numbers based on graphene oxide and DNA-stabilized silver nanoclusters

Molecular logic devices with different functions can perform various tasks in the areas of biological molecule detection, disease diagnosis, multivariate analysis, and bioimaging. Herein, a series of logic circuits based on silver nanoclusters (AgNCs)/graphene oxide (GO) are constructed to execute nonarithmetic functions, including 3-, 4-, and 5-bit odd/even checking. The resulting devices can differentiate between even and odd decimal numbers in the range from 0 to 31. Moreover, the devices can be expanded to operate with wider ranges of numbers when more inputs are added. The signal reporter is structured using AgNCs and GO, preventing laborious modification of biomolecules. The designed DNA-based logic nanodevices share the same DNA platform and a constant threshold value, showing great potential for application in information processing at the molecular level. Additionally, these devices can stably carry out their logic operations in a biological matrix, indicating that the AgNC/GO-based system can operate in a complicated biological environment. Given the biocompatibility and design flexibility of DNA, this study provides novel outcomes towards the development of label-free intelligent nanodevices. This may open a new path for the application of AgNCs/GO in molecular logic circuits and fluorescence imaging.

A ferrocene-based organogel with multi-stimuli properties and applications in naked-eye recognition of F− and Al3+

Ferrocene-based gelator F-6/chloroform gel exhibited multi-stimuli responsiveness, through which ‘naked eye’ recognition of both Al³⁺ and F⁻ was realized.

Amino acid-based amphiphilic hydrogels: metal ion induced tuning of mechanical and thermal stability

A phenylalanine based gelator was found to form a hydrogel in phosphate buffer solution. Its mechanical properties are influenced by a range of metal ions.

Photoresponsive self-healing supramolecular hydrogels for light-induced release of DNA and doxorubicin

An azobenzene-containing cyclic dipeptide PAP-DKP-Lys is a photoresponsive low-MW hydrogelator. The gelation process can be triggered with temperature, pH, light, and ionic strength. The resulting self-healing gels can encapsulate dsDNA or an anticancer drug doxorubicin, and release them in a light-dependent manner.

Carbon-Nanotube-Mediated Electrochemical Transition in a Redox-Active Supramolecular Hydrogel Derived from Viologen and an l-Alanine-Based Amphiphile

A two-component hydrogelator (16-A)2 -V(2+) , comprising an l-alanine-based amphiphile (16-A) and a redox-active viologen based partner (V(2+) ), is reported. The formation the hydrogel depended, not only on the acid-to-amine stoichiometric ratio, but on the choice of the l-amino acid group and also on the hydrocarbon chain length of the amphiphilic component. The redox responsive property and the electrochemical behavior of this two-component system were further examined by step-wise chemical and electrochemical reduction of the viologen nucleus (V(2+) /V(+) and V(+) /V(0) ). The half-wave reduction potentials (E1/2 ) associated with the viologen ring shifted to more negative values with increasing amine component. This indicates that higher extent of salt formation hinders reduction of the viologen moiety. Interestingly, the incorporation of single-walled carbon nanotubes in the electrochemically irreversible hydrogel (16-A)2 -V(2+) transformed it into a quasi-reversible electrochemical system.

Ferrocene-containing non-interlocked molecular machines

Ferrocene is chemically robust and readily functionalized which enables its facile incorporation into more complex molecular systems. This coupled with ferrocene's reversible redox properties and ability to function as a “molecular ball bearing” has led to the use of ferrocene as a component in wide range of non-interlocked synthetic molecular machine systems.

A photoswitchable diarylethene heterodimer for use as a multifunctional logic gate

A photoswitchable diarylethene heterodimer was prepared and its applications to logic gates were successfully demonstrated.

Recent progress in ferrocene- and azobenzene-based photoelectric responsive materials

Ferrocene- and azobenzene-based derivatives are commonly used photoelectric responsive materials and possess wide range of applications.

Selective aliphatic/aromatic organogelation controlled by the side chain of serine amphiphiles

Structural modifications in the side chain of N-Fmoc-l-serine amphiphiles induce the selective gelation of either aliphatic or aromatic hydrocarbon solvents.