Fluorescence "Turn-On" Enzyme-Responsive Supra-Amphiphile Fabricated by Host-Guest Recognition between γ-Cyclodextrin and a Tetraphenylethylene-Sodium Glycyrrhetinate Conjugate

Fluorescence detection of α-amylase based on a host-guest complex between a pyrene-derived amphiphile and γ-cyclodextrin

A fluorescence detection platform for α-amylase was developed by exploiting the host-guest inclusion complex formed between γ-cyclodextrin (γ-CD) and a pyrene-functionalized amphiphile (P10CG). Spectroscopic analysis revealed that the pyrene moiety was encapsulated within the γ-CD cavity in a 2:2 stoichiometric ratio, inducing characteristic excimer fluorescence. Upon enzymatic hydrolysis of γ-CD by α-amylase, the subsequent release of P10CG led to a marked decrease in excimer emission intensity. A quantitative linear relationship (R2 > 0.99) was observed between the monomer-to-excimer intensity ratio and α-amylase concentrations ranging from 1 to 5 U/mL, with a calculated detection limit of 0.135 U/mL (S/N = 3). The γ-CD/P10CG supramolecular system demonstrated satisfactory sensitivity and selectivity for α-amylase in both buffer solutions and diluted serum matrices, thus providing a potential sensing system for α-amylase in complex biological systems.

Preparation of dual-epitopes imprinted particles with γ-cyclodextrin host-guest interaction and reversible addition-fragmentation chain transfer strategy for cytochrome c collaborative recognition

A dual-epitopes imprinted strategy for cytochrome c selective recognition assisted with γ-cyclodextrin host-guest interaction via N-terminal and C-terminal epitope's simultaneous imprinting and reversible addition-fragmentation chain transfer (RAFT) polymerization was developed. N-terminal and C-terminal nonapeptides of Cyt c (GI-9 and AE-9) were used simultaneously as the epitope to achieve collaborative recognition for cytochrome c. As a supramolecule, γ-cyclodextrin can encapsulate the aromatic functional groups of amino acid residues to capture the peptide and improve the corresponding spatial orientation for epitope or cytochrome c recognition by host-guest interaction. After the γ-cyclodextrin modification and dual-epitopes immobilization, the imprinted polymer was synthesized by RAFT polymerization with 4-cyano-4-(phenyl-carbonothioylthio) pentanoic acid as a chain transfer agent. After the template removal, the obtained dual-epitopes imprinted particles showed well binding ability to AE-9 (26.50 mg·g-1, IF= 4.13), GI-9 (7.36 mg·g-1, IF= 2.18) and cytochrome c (79.56 mg·g-1, IF= 3.27). With the successive addition of RAFT agent, the imprinting factor rising of epitope peptide and cytochrome c further illustrated the regulation of imprinted polymer chains. The imprinted particles had the advantage for cytochrome c recognition compared to other proteins and good reusability with 82.60 % repeated reproduction rate after six cycles of adsorption and desorption. Furthermore, the selective recognition for cytochrome c in bovine serum proved its potentiality to be applied in complex biological samples. It indicated that the combination of dual-templates epitope imprinting, γ-CD host-guest interaction and RAFT polymerization provided an efficient method for collaborative protein recognition with well selectivity, reusability and stability.

An enzyme/pH dual-responsive supramolecular fluorescent vesicle with tunable size fabricated by europium complex and polypseudorotaxanes

The fluorescent vesicles based on lanthanide ions are considered as an ideal biomimetic optical nanoplatform for simulating biological processes of cell membrane. However, the accurately and controllably adjusting the size of vesicles based on lanthanides while ensuring their fluorescence performance and stability still remains a challenge. Herein, a dual-stimuli-responsive fluorescent supramolecular vesicle with tunable size has been designed based on host-guest interaction and coordinating aggregation. Europium complexes can be encapsulated within supramolecular assemblies by assembling with polypseudorotaxanes (PPRs), which are formed by F127 and carboxymethyl-β-cyclodextrin (CMCD) through host-guest interaction. The fluorescence properties of the europium complexes have been significantly enhanced by confining and shielding them within vesicles. Upon the addition of α-amylase and HCl, the fluorescence intensity of the vesicles will gradually and significantly quench as a result of CMCD degradation and dissociation of the europium complexes. This research presents a convenient method for regulating the size of lanthanide fluorescent vesicles, and the supramolecular vesicles obtained with multi-stimuli response are anticipated to be utilized in the diagnosis of relevant diseases and targeted drug delivery.

Sensitive detection of α-amylase based on host-guest inclusion system of γ-cyclodextrin and dansyl-derived diphenylalanine

A highly sensitive detection system for α-amylase was developed via host-guest complexation between γ-cyclodextrin and dansyl-modified diphenylalanine (FF-Dns). The host-guest inclusion of FF-Dns into the cavity of γ-CD in a HEPES buffer solution (10 mM, pH 7.4) significantly enhanced the fluorescence intensity, and the emission wavelength gradually shifted from 558 to 535 nm. The hydrolysis of γ-CD by the addition of α-amylase released FF-Dns, leading to the recovery of the fluorescence emission characteristics. Therefore, the FF-Dns/γ-CD host-guest complexation system can serve as a platform for the sensitive detection of α-amylase with good selectivity against potential interference. The limit of detection (LOD) of the system was 0.004 U/mL, with a linear working range of 0-6 U/mL. The detection assay was successfully applied in 0.1 % serum, achieving an LOD of 0.017 U/mL and a linear working range of 0-10 U/mL.

Enzyme-Responsive Supramolecular Self-Assembly in Small Amphiphiles

Enzyme-responsive molecular assemblies have recently made remarkable progress, owing to their widespread applications. As a class of catalysts with high specificity and efficiency, enzymes play a critical role in producing new molecules and maintaining metabolic stability in living organisms. Therefore, the study of enzyme-responsive assembly aids in understanding the origin of life and the physiological processes occurring within living bodies, contributing to further advancements across various disciplines. In this Review, we summarize three kinds of enzyme-responsive assembly systems in amphiphiles: enzyme-triggered assembly, disassembly, and structural transformation. Furthermore, motivated by the fact that biological macromolecules and complex structures all originated with small molecules, our focus lies on the small amphiphiles (e.g., peptides, surfactants, fluorescent molecules, and drug molecules). We also provide an outlook on the potential of enzyme-responsive assembly systems for biomimetic development and hope this Review will attract more attention to this emerging research branch at the intersection of assembly chemistry and biological science.

Expanding the scope of self-assembled supramolecular biosensors: a highly selective and sensitive enzyme-responsive AIE-based fluorescent biosensor for trypsin detection and inhibitor screening

Trypsin, a pancreatic enzyme associated with diseases like pancreatic cancer and cystic fibrosis, requires effective diagnostic tools. Current detection systems seldom utilize macrocyclic molecules and tetraphenyl ethylene (TPE) derivative-based supramolecular assemblies, known for their biocompatibility and aggregation-induced emission (AIE) properties, for trypsin detection. This study presents an enzyme-responsive, AIE-based fluorescence 'Turn-On' sensing platform for trypsin detection, employing sulfated-β-cyclodextrin (S-βCD), an imidazolium derivative of TPE (TPE-IM), and protamine sulfate (PrS). The anionic S-βCD and cationic TPE-IM formed a strongly fluorescent supramolecular aggregation complex in an aqueous buffer. However, PrS suppresses fluorescence because of its strong binding affinity with S-βCD. The non-fluorescent TPE-IM/S-βCD/PrS supramolecular assembly system exhibits trypsin-responsive properties, as PrS is a known trypsin substrate. Trypsin restores fluorescence in the TPE-IM/S-βCD system through the enzymatic cleavage of PrS, correlating linearly with trypsin catalytic activity in the 0-10 nM concentration range. The limit of detection is 10 pM. This work contributes to the development of self-assembled supramolecular biosensors using charged TPE derivatives and β-cyclodextrin-based host-guest chemistry, offering an innovative fluorescence 'Turn-On' trypsin sensing platform. The sensing system is highly stable under various conditions, selective for trypsin, and demonstrates potential for biological analysis and disease diagnosis in human serum. Additionally, it shows promise for the screening of trypsin inhibitors.

Enzyme-Instructed Host-Guest Assembly/Disassembly for Biomedical Applications

Compared with the normal assembly/disassembly approaches, enzyme-instructed host-guest assembly/disassembly strategies due to their superior biocompatibility and specificity for specific substrates, can more effectively and precisely release molecules at lesions for reflecting in vivo biological events. Specifically, due to the over-expression of enzymes in specific tissues, the assembly/disassembly processes can directly occur on the pathological sites (or regions of interest), thus these enzyme-instructed processes are widely and effectively used for disease treatment or precise bioimaging. Based on it, we introduce the concept and major strategies of enzyme-instructed host-guest assembly/disassembly, illustrate their importance in the diagnosis and treatment of diseases, and review their advances in biomedical applications. Further, the challenges of these strategies in the clinic and future tendencies are also prospected.

Self-Assembly of Cyclodextrin-Coated Nanoparticles:Fabrication of Functional Nanostructures for Sensing and Delivery

In recent years, the bottom-up approach has emerged as a powerful tool in the fabrication of functional nanomaterials through the self-assembly of nanoscale building blocks. The cues embedded at the molecular level provide a handle to control and direct the assembly of nano-objects to construct higher-order structures. Molecular recognition among the building blocks can assist their precise positioning in a predetermined manner to yield nano- and microstructures that may be difficult to obtain otherwise. A well-orchestrated combination of top-down fabrication and directed self-assembly-based bottom-up approach enables the realization of functional nanomaterial-based devices. Among the various available molecular recognition-based "host-guest" combinations, cyclodextrin-mediated interactions possess an attractive attribute that the interaction is driven in aqueous environments, such as in biological systems. Over the past decade, cyclodextrin-based specific host-guest interactions have been exploited to design and construct structural and functional nanomaterials based on cyclodextrin-coated metal nanoparticles. The focus of this review is to highlight recent advances in the self-assembly of cyclodextrin-coated metal nanoparticles driven by the specific host-guest interaction.

Noncovalent Bile Acid Oligomers as Facial Amphiphilic Antimicrobials

New antimicrobial agents are needed to address the ever-growing risk of bacterial resistance, particularly for methicillin- and vancomycin-resistant Staphylococcus aureus (S. aureus). Here, we report a class of bile acid oligomers as facial amphiphilic antimicrobials, which are noncovalently fabricated by cholic acid (CA) and deoxycholic acid (DCA) with polyamines (e.g., diamines, diethylenetriamine, spermidine, and spermine). The antibacterial activities of these bile acid oligomers (CA/polyamines and DCA/polyamines) against S. aureus become stronger with increasing the amine group numbers of polyamines without obviously enhanced cytotoxicity and skin irritation. DCA/spermine, entirely composed of natural products, exhibits the best antibacterial activity but the lowest cytotoxicity and the weakest skin irritation. All CA/polyamines and DCA/polyamines form well-ordered ribbon-like aggregates, collecting numerous facial amphiphilic structures to significantly enhance the interactions with bacterial membranes. In particular, the biogenic polyamines with more than two amine groups provide extra positively charged sites, hence facilitating the binding of bile acid oligomers to the negatively charged outer membrane of the bacteria via electrostatic interaction. This in turn promotes more oligomeric bile acid units that can be inserted into the membrane through hydrophobic interaction between bile acids and lipid domains. The noncovalently constructed and separable amphiphilic antimicrobials can avoid the long-term coexistence of microorganisms and antibacterial molecules in different acting modes. Therefore, the noncovalent bile acid oligomers, especially those with higher oligomerization degrees, can be a potential approach to effectively enhance antibacterial activity, improve environmental friendliness, and reduce bacterial drug resistance.

Red emitting fluorogenic dye as an efficient turn-on probe for milk allergen

Development of simple, fast and non-destructive technique such as fluorescence based method for the quantification of milk allergens in various dairy products is a highly rewarding task. In this contribution, a red emitting fluorogenic dye, quinaldine red (QR) is reported for the detection and quantification of a milk allergen, beta lactoglobulin (β-LG) in milk and whey matrices, utilizing its high selectivity and sensitivity towards β-LG. Detail spectroscopic investigation reveals that binding of QR to the hydrophobic calyx site of β-LG protein substantially reduces the torsional agility and propensity of TICT state formation of QR, rendering the dye highly fluorescent in nature. This enables estimation of β-LG with LOD 52.1(±0.9) nM in buffer solution and 0.21(±0.01) μM in 5 % bovine milk matrix respectively. Additionally, high selectivity and sensitivity, excellent repeatability, quick response, and emission in the biologically favorable red spectral region make QR based fluorometric quantification of β-LG a highly attractive choice. Finally, the estimated β-LG concentrations in milk and whey matrices from fluorometric titration and densitometry methods are found to match excellently with each other, suggesting potential of QR as an efficient turn-on fluorescent probe for the quantification of β-LG (milk allergen) in various dairy products.

Multicharged cyclodextrin supramolecular assemblies

Multicharged cyclodextrin (CD) supramolecular assemblies, including those based on positively/negatively charged modified mono-6-deoxy-CDs, per-6-deoxy-CDs, and random 2,3,6-deoxy-CDs, as well as parent CDs binding positively/negatively charged guests, have been extensively applied in chemistry, materials science, medicine, biological science, catalysis, and other fields. In this review, we primarily focus on summarizing the recent advances in positively/negatively charged CDs and parent CDs encapsulating positively/negatively charged guests, especially the construction process of supramolecular assemblies and their applications. Compared with uncharged CDs, multicharged CDs display remarkably high antiviral and antibacterial activity as well as efficient protein fibrosis inhibition. Meanwhile, charged CDs can interact with oppositely charged dyes, drugs, polymers, and biomacromolecules to achieve effective encapsulation and aggregation. Consequently, multicharged CD supramolecular assemblies show great advantages in improving drug-delivery efficiency, the luminescence properties of materials, molecular recognition and imaging, and the toughness of supramolecular hydrogels, in addition to enabling the construction of multistimuli-responsive assemblies. These features are anticipated to not only promote the development of CD-based supramolecular chemistry but also contribute to the rapid exploitation of these assemblies in diverse interdisciplinary applications.

pH-Triggered Transition from Micellar Aggregation to a Host-Guest Complex Accompanied by a Color Change

A pH-triggered transition from micellar aggregation to a host-guest complex was achieved based on the supramolecular interactions between calixpyridinium and pyrroloquinoline quinone disodium salt (PQQ-2Na) accompanied by a color change. Our design has the following three advantages: (1) a regular spherical micellar assembly is fabricated by the supramolecular interactions between calixpyridinium and PQQ-2Na at pH 6 in an aqueous solution, (2) increasing the pH can lead to a transition from micellar aggregation to a host-guest complex due to the deprotonation of calixpyridinium, and at the same time (3) increasing the pH can lead to a color change owing to the deprotonation of calixpyridinium and the complexation of deprotonated calixpyridinium with PQQ-2Na. Benefitting from the low toxicity of calixpyridinium and PQQ-2Na, this pH-induced transition from micellar aggregation to a host-guest complex was further studied as a controllable-release model.

Aggregation-induced emission luminogen based self-healing hydrogels fluorescent sensors for α-amylase

A self-healing hydrogel with a dual network was prepared through the host–guest recognition of acrylate γ-cyclodextrins with tetraphenylethylenes, and the fluorescence of hydrogel was enhanced in the presence of α-amylase.

Enzyme-Responsive Molecular Assemblies Based on Host-Guest Chemistry

Recent years have witnessed a growing interest in the design of enzyme-responsive molecular assemblies that hold appealing applications in the fields of disease-related sensing, imaging, and drug delivery. Cyclodextrins (CDs) are amylase-cleavable host molecules that can associate with surfactants, alkanes, alkyl amines, fatty alcohols, and aromatic compounds to form diverse supramolecular structures. In this work, we report a versatile supramolecular platform to construct enzyme-responsive nanosystems via host-guest interactions, in which complexation between CDs and surfactants eventually leads to the formation of a variety of nanostructures such as vesicles and microtubes. These supramolecular structures are capable of loading water-soluble molecules or functional nanoparticles, which can be actively released on-demand in the presence of α-amylase. This universal strategy to fabricate enzyme-responsive supramolecular systems was further demonstrated with a range of surfactants with anionic, cationic, and nonionic headgroups. Our results highlight a versatile platform for the exploration of biologically responsive self-assembly with potential applications as controlled-release systems and microrobots.