Polyfluoroalkyl Chain-Based Assemblies for Biomimetic Catalysis.

Amphiphobic fluoroalkyl chains are exploited for creating robust and diverse self-assembled biomimetic catalysts. Long terminal perfluoroalkyl chains (Cn F2n+1 with n=6, 8, and 10) linked with a short perhydroalkyl chains (Cm H2m with m=2 and 3) were used to synthesize several 1,4,7-triazacyclononane (TACN) derivatives, Cn F2n+1 -Cm H2m -TACN. In the presence of an equimolar amount of Zn2+ ions that coordinate the TACN moiety and drive the self-assembly into micelle-like aggregates, the critical aggregation concentration of polyfluorinated Cn F2n+1 -Cm H2m -TACN⋅Zn2+ was lowered by ∼1 order of magnitude compared to the traditional perhyroalkyl counterpart with identical carbon number of alkyl chain. When 2'-hydroxypropyl-4-nitrophenyl phosphate was used as the model phosphate substrate, polyfluorinated Cn F2n+1 -Cm H2m -TACN⋅Zn2+ assemblies showed higher affinity and catalytic activity, compared to its perhyroalkyl chain-based counterpart. Coarse-grained molecular dynamic simulations have been introduced to explore the supramolecular assembly of polyfluoroalkyl chains in the presence of Zn2+ ions and to better understand their enhanced catalytic activity.

Coordination-Induced Multivalent Self-Assembling Catalysts for Spectral Sensing Zn2+ with High Selectivity and Sensitivity.

The introduction of signal amplification to molecular spectral sensing systems is an intriguing topic in supramolecular analytical chemistry. In this study, click chemistry was used to generate a triazole moiety to bridge with a long hydrophobic alkyl chain (Cn) and another short alkyl chain (Cm) bearing a 1,4,7-triazacyclonane (TACN) group for efficiently generating a self-assembling multivalent catalyst, Cn-triazole-Cm-TACN·Zn2+ (n and m represent the carbon numbers of both alkyl chains, respectively; n = 16, 18, and 20; m = 2 and 6), to catalyze the hydrolysis of 2-hydroxypropyl-4-nitrophenyl phosphate (HPNPP) when Zn2+ was added. The triazole moiety introduced adjacent to the TACN group plays an important role in improving the selectivity of Zn2+ because the triazole moiety can participate in the coordination interaction between the Zn2+ and neighboring TACN group. The supplementary triazole complexing increases the space requirement for coordinated metal ions. This catalytic sensing system also shows high sensitivity, with a favorable limit of detection down to 350 nM, even if only UV-vis absorption spectra rather than more sensitive fluorescence techniques were used for signaling, and can be used to determine the concentration of Zn2+ in tap water, which demonstrates the practical application feasibility.

A Non-Hydrolysis Reaction-Based Imine for Fluorescence Response toward Al3+ Ions with Extremely High Selectivity.

Designing an imine-based fluorescent probe capable of greatly suppressing the tendency of intrinsic hydrolysis reaction is an attractive topic in the field of chemo-/biosensing. In this work, hydrophobic 1,1'-binaphthyl-2,2'-diamine containing two amine groups was introduced to synthesize probe R-1 bearing two imine bonds linked by two salicylaldehyde (SAs). The hydrophobicity of binaphthyl moiety and the unique clamp-like structure formed from double imine bonds and from ortho-OH on SA part make probe R-1 is able to function as an ideal receptor to coordinate with Al3+ ions, leading to the fluorescence originated from the complex rather than from the assumed hydrolyzed fluorescent amine is turned on. Further study revealed that, when Al3+ ions were introduced, both the hydrophobic binaphthyl moiety and the clamp-like double imine structure in the designed imine-based probe showed important contributions to suppress the intrinsic hydrolysis reaction, resulting in generating a stable coordination complex with an extremely high selectivity in fluorescence response.

Minimalistic Artificial Catalysts with Esterase-Like Activity from Multivalent Nanofibers Formed by the Self-Assembly of Dipeptides.

Imitating and incorporating the multiple key structural features observed in natural enzymes into a minimalistic molecule to develop an artificial catalyst with outstanding catalytic efficiency is an attractive topic for chemists. Herein, we designed and synthesized one class of minimalistic dipeptide molecules containing a terminal -SH group and a terminal His-Phe dipeptide head linked by a hydrophobic alkyl chain with different lengths, marked as HS-C n+1-His-Phe (n = 4, 7, 11, 15, and 17; n + 1 represents the carbon atom number of the alkyl chain). The His (-imidazole), Phe (-CO2 -) moieties, the terminal -SH group, and a long hydrophobic alkyl chain were found to have important contributions to achieve high binding ability leading to outstanding absolute catalytic efficiency (k cat/K M) toward the hydrolysis reactions of carboxylic ester substrates.

Modulating Dual Fluorescence Emissions in Imine-Based Probes to Distinguish D2O and H2O.

How to distinguish D2O and H2O and determine the trace H2O content in D2O solvent, by using molecule-based spectral probes, is an intriguing topic in analytical chemistry, yet considerably few examples remain up to now, likely due to the very similar physical/chemical properties between D2O and H2O. In this work, we found that both the hydrolysis reactions to release fluorescent amines and aggregation-induced emission (AIE) of imines, functioning as dual fluorescence signals to distinguish D2O and H2O, could be modulated by changing the imine structures. The hydrophobicity of imines showed an important contribution to the ability of modulating the hydrolysis reactions and AIE, demonstrating a significant difference on fluorescence signals in D2O and H2O solvents. Among all tested imines, probe 3, condensed from 2-naphthylamine and salicylaldehyde, was found to have the potential ability to act as an ideal candidate for probing the H2O content in D2O solvent, particularly in a low H2O content range, using the ratiomeric emission signals.

Reaction-based fluorescence probes for "turn on" sensing fluoride ions.

Introducing a weak covalent bond into an originally highly fluorescent molecule to create a non-fluorescent probe is able to provide a new way to detect some nucleophilic targets with enhanced sensitivity. Herein, this is the first time that a tetraphenylethene (TPE)-based probe (TPEONO2) bearing a p-nitrobenzenesulfonyl moiety for the sensing of F- ions in aqueous solution via a cleavage reaction of the sulfonyl ester bond to induce aggregation-induced emission (AIE) has been reported.

Enantioselective Dynamic Exchange Reactions of Imines.

Although dynamic reactions of imines have been extensively studied, the dynamic behaviors manipulated by chirality remain nearly unexplored. In this work, enantioselective amine exchange reactions were demonstrated as a first example via the reaction of enantiomeric chiral amines such as natural amino acids with a series of innovative axially chiral 1,1'-binaphthyl-2,2'-diamine (BNDA)-based imines that were prepared from the condensation reactions between BNDA and salicylaldehyde (SA) or its derivatives. This enantioselective dynamic behavior can be directly indicated by the degree of the fluorescence response of the R-configuration of imines to the d-enantiomer of chiral amine, because the released BNDA can serve as the fluorescence signal output when the amine exchange reaction occurs, which is far higher than the response to its l-enantiomer under identical experimental conditions. For the S-configuration of chiral imines, the fluorescence response is the opposite. The enantioselective exchange reaction can be tuned by altering the electron-withdrawing or electron-donating capability of the substituent at position 4 or 5 of the SA part of chiral imines. Not only o-OH groups in SA-based imines but also protic solvents used as reaction media were found to be important to the dynamic behavior at high rates.

A Multi-Catalytic Sensing for Hydrogen Peroxide, Glucose, and Organophosphorus Pesticides Based on Carbon Dots.

In this work, a facile one-pot hydrothermal route was employed to synthesize a series of fluorescent carbon dots (CDs) by using 20 natural amino acids, respectively, as the starting materials. It was found that the CDs synthesized using phenylalanine could possess the intrinsic peroxidase-like activity that could effectively catalyze a traditional peroxidase substrate like 3, 3', 5, 5'- tetramethylbenzidine (TMB) in the presence of H2O2 to produce a blue solution; thereby, a catalytic sensing system for H2O2 has been developed. On the basis of this catalytic reaction, together with the fact that glucose oxidase (GOx) can catalyze the hydrolysis of glucose to generate H2O2, a sensitive catalytic sensing system for glucose could be further established. Furthermore, based on this catalytic reaction, taken together with the two enzymatic catalytic systems of acetylcholinesterase (AChE) and choline oxidase (CHO), a highly sensitive multi-catalytic sensing system could be successfully developed for organophosphorus (OPs) pesticides such as dimethoate, DDVP, and parathion-methyl. Limit of detections (LODs) of H2O2 and glucose were estimated to be 6.5 and 0.84 µM, respectively. The limit of detection of the sub-nM level could be obtained for tested dimethoate, DDVP, and parathion-methyl OPs pesticides. The established sensing systems can exhibit good practical application performance in serum and several fruit samples.

Self-Assembled Multivalent Ag-SR Coordination Polymers with Phosphatase-Like Activity.

We show herein the phosphatase-like catalytic activity of coordination polymers obtained after adding Ag+ -ions to thiols bearing hydrophobic alkyl chains terminated with a 1,4,7-triazacyclononane (TACN) group. The subsequent addition of Zn2+ -ions to the self-assembled polymers resulted in the formation of multivalent metal coordination polymers capable of catalysing the transphosphorylation of an RNA-model compound (2-hydroxypropyl-4-nitrophenyl phosphate, HPNPP) with high reactivity. Analysis of a series of metal ions showed that the highest catalytic activity was obtained when Ag+ -ions were used as the first metal ions to construct the backbone of the coordination polymer through interaction with the -SH group followed by Zn2+ -ions as the second metal ions complexed by the TACN-macrocycle. Furthermore, it was demonstrated that the catalytic activity could be modulated by changing the length of the hydrophobic alkyl chain.

Facile access to versatile aza-macrolides through iridium-catalysed cascade allyl-amination/macrolactonization.

Direct access to benzo-fused aza-macrolides was successfully achieved through iridium-catalysed intermolecular decarboxylative coupling of vinylethylene carbonates with isatoic anhydrides under relatively mild reaction conditions. Notably, this reaction proceeded through sequential allyl-amination/macrolactonization upon extrusion of CO2. Moreover, favourable fluorescence properties could be observed in the title macrocyclic products.

Diverse applications of TMB-based sensing probes.

Extending the research on 3,3',5,5'-tetramethylbenzidine (TMB) and its derivatives in analytical chemistry is important, considering that TMB is widely used as an enzyme catalytic substrate. In this work, two TMB derivatives, TMBS and TMBB, were synthesized via a facile and one-step condensation reaction between the -NH2 group of TMB and the -CHO group of salicylaldehyde or benzaldehyde. Because at low pH the two Schiff base compounds can release TMB which can emit strong fluorescence, the probes could show dual-modal signal responses, fluorescence and UV-vis absorption, towards the pH. Practical applications of pH sensing in Chinese rice vinegar and lemon juice samples were successfully demonstrated. On the basis of these findings, a catalytic chromogenic reaction was developed to monitor the pH with the naked eye, too. Furthermore, considering the chemical equilibrium reaction between CO2 and H2O and that glucose oxidase (GOD) can catalyse the dehydrogenation and oxidation reaction of ß-d-glucose to produce gluconic acid, both of which can result in lowering the pH values of the two Schiff base systems, highly sensitive and selective dual-modal sensing systems for detecting CO2 and ß-d-glucose have also been successfully established. Therefore, the two synthesized TMB derivatives can demonstrate their robust application potential.

Hydrogelators of cyclotriveratrylene derivatives.

Developing cavity-based supramolecular hydrogels is in its infancy because not many such hydrogelators are available. Reported herein is our creation of rigid cavitand cyclotriveratrylene (CTV) based hydrogelators from the molecular backbones of CTVs that were in limited cases shown to form organogels. For doing so deprotonable -COOH or protonable -NH(2) was introduced as terminal group into the rigid and hydrophobic CTV backbones. We thus successfully obtained optically anisotropic supramolecular hydrogels from these new CTVs hydrogelators with excellent thermostability and high tolerance towards strong electrolytes. The obtained CTV-1 and CTV-2 hydrogels are luminescent and exhibit reversible gel-to-sol and sol-to-gel transitions upon pH variations. The success in creating CTV-1 and CTV-2 hydrogelators on the basis of the skeleton of a CTV-organogelator suggests that balancing the hydrophilic and hydrophobic characters of the ionic and hydrophobic moieties well in the gelator molecule is important for designing a promising hydrogelator.

Supramolecular aggregation/disaggregation-based molecular sensing: a review focused on investigations from China.

Supramolecular aggregation and disaggregation induced by external stimuli can impact the optical or electrical signals of the aggregates/constituting units (receptors). Therefore, manipulating supramolecular aggregation/disaggregation has recently been employed to construct novel and promising photoluminescence (PL)-based sensing and recognition systems. The sensing systems were capable of substantially enhancing the sensitivity, relying on cooperative interactions occurring in the assembly/disassembly processes (mostly operating in emission turned-on or emission-enhanced mode). This review focuses mainly on recent advances in the new emerging PL-based sensing platforms, based on manipulating the behaviours of supramolecular aggregation/disaggregation, including aggregation-induced emission (AIE), metallophilic interactions-related sensing (metallophilic interactions-induced aggregation/disaggregation), metal coordination polymers-related sensing, and other sensing systems involving supramolecular aggregation/disaggregation. In particular, those sensing systems developed by scientists in China are summarized and highlighted.

A ratiometric luminescent sensing of Ag+ ion via in situ formation of coordination polymers.

A ratiometric luminescent sensing of Ag(+) ion is developed via the Ag(I)-NCys coordination polymeric luminophore in situ formed in aqueous solution upon mixing Ag(+) ion with the designed fluorescent thiol ligand NCys.

Metal-metal-interaction-facilitated coordination polymer as a sensing ensemble: a case study for cysteine sensing.

A detailed investigation of the absorption and CD signals of Ag(I)-cysteine (Cys) aqueous solutions at buffered or varying pH has allowed us to suggest that coordination polymers are formed upon mixing Ag(I) and Cys bearing a Ag(I)-Cys repeat unit. The formation of the coordination polymers are shown to be facilitated by both the Ag(I)···Ag(I) interaction and the interaction between the side chains in the polymeric backbone. The former allows for an immediate spectral sensing of Cys with enantiomeric discrimination capacity with both high sensitivity and selectivity, and the contribution of the side-chain/side-chain interaction serves to guide extended sensing applications by means of modulating this interaction. With our preliminary data on the corresponding Cu(I)-Cys and Au(I)-Cys systems that exhibited similar spectral signals, we conclude that the M(I)-SR coordination polymers (M = Cu, Ag, or Au) could in general function as spectral sensing ensembles for extended applications. This sensing ensemble involves the formation of coordination polymers with practically no spectral background, thus affording high sensing sensitivity and selectivity.

In situ encapsulating silver nanocrystals into hydrogels. A "green" signaling platform for thiol-containing amino acids or small peptides.

We found that Ag(I) could serve as a "bridging agent" for triggering sodium carboxymethylcellulose (NaCMC) solution to form a hydrogel which enabled Ag nanocrystals to be in situ produced and encapsulated into the stable gel matrix by means of photoreduction; the conceptual system could act as a novel signaling platform for thiol-containing amino acids or small peptides, operating in a "self-assembly and disassembly" model.

Anion-triggered melamine based self-assembly and hydrogel.

We found that melamine in its protonated form could be triggered by oxoanions such as NO(3)(-), PO(4)(3-), ATP, and SO(4)(2-) to form superstructures and to gelate a large amount of water molecules, presumably resulting from the electrostatic and hydrogen bonding interactions operating in a synergistic manner; the gelation can be reversibly switched off/on by increasing/decreasing pH or temperature.

Specific Hg(2+)-mediated perylene bisimide aggregation for highly sensitive detection of cysteine.

Hg(2+) was found to specifically induce the aggregation of perylene bisimide in a "thymine-Hg(2+)-thymine" binding motif and the resultant aggregates showed a highly selective and sensitive turn-on fluorescence response for cysteine, with a detection limit down to 9.6 nM.

Enhanced saccharide sensing based on simple phenylboronic acid receptor by coupling to Suzuki homocoupling reaction.

Substantially enhanced monosaccharide fluorescent sensing in aqueous solutions using a simple phenylboronic acid receptor is achieved by coupling the classic strategy based on saccharide-boronic acid interaction with catalytic Suzuki homocoupling reaction.

A ligand-chirality controlled supramolecular hydrogel.

We report a novel example of ligand-chirality finely controlled in situ supramolecular hydrogel formation based on the coordination of phenylalanine (Phe) to Cu(II) with higher selectivity over other metal ions. As decreasing both enantiomeric excesses (ee%) of ligand Phe towards its D- and L-forms, the gelation ability of Phe-Cu(II) supramolecular metallogelator was found to be weakened and eventually disappeared, which likely resulted from the stereoselectivity of the ligand Phe. Intermolecular hydrogen bonding, hydrophobic and/or pi-pi stacking interactions were also found to be essential for forming the metallogel. We believe that the present work can open up a new entry for developing novel and promising chiral sensing and recognition platforms, i.e. visually sensing chiral molecules by naked eyes due to the feature of a sol-to-gel transition induced smartly by varying the ligand chirality.