Host-Guest Chemistry in Layer-by-Layer Assemblies Containing Calix[n]arenes and Cucurbit[n]urils: A Review

This review provides an overview of the synthesis of layer-by-layer (LbL) assemblies containing calix[n]arene (CA[n]) and cucurbit[n]uril (CB[n]) and their applications. LbL assemblies, such as thin films and microcapsules, containing selective binding sites have attracted considerable attention because of their potential use in separation and purification, sensors for ions and molecules, and controlled release. CA[n]-containing LbL films have been prepared using sulfonated CA[n] and cationic polymers to construct chemical sensors and molecular containers. CA[n]-containing LbL films deposited on the surface of a porous support are useful as ion-selective membranes that exhibit selective permeability to monovalent ions over multivalent ions. CB[n]s have been used as molecular glues for the construction of LbL films and microcapsules by taking advantage of the strong affinity of CB[n]s to aromatic compounds. CB[n]s form a stable 1:1:1 ternary complex with electron-rich and electron-deficient molecules in LbL films to stabilize the assemblies. CB[n]-containing LbL films can also be deposited on the surfaces of micro templates and nanopore membranes to construct microcapsules for controlled release and nanochannels for selective ion transport, respectively.

Constructing a Multiplexed DNA Pattern by Combining Precise Magnetic Manipulation and DNA-Driven Assembly

There is an urgent demand to construct multiplexed biomolecular patterns to obtain more biological information from a single experiment. However, with only limited reports focusing on defective top-down approaches, challenges remain to develop a bottom-up strategy for multiplexed patterning. To this end, a novel strategy has been proposed to fabricate multiplexed DNA patterns via macroscopic assembly through combined precise magnetic manipulation and DNA hybridization-driven self-assembly. Therefore, a multiplexed DNA pattern composed of glass fibers loaded with multiple specific strands of DNA was constructed, and its potential application in simultaneous detection of multiplex target DNA was demonstrated. Moreover, the fabricated multiplexed DNA pattern shows an erasable behavior because the hybridized DNA can be disassembled by strand displacement.

Photosensitive Layer-by-Layer Assemblies Containing Azobenzene Groups: Synthesis and Biomedical Applications

This review provides an overview of the syntheses of photosensitive layer-by-layer (LbL) films and microcapsules modified with azobenzene derivatives and their biomedical applications. Photosensitive LbL films and microcapsules can be prepared by alternate deposition of azobenzene-bearing polymers and counter polymers on the surface of flat substrates and microparticles, respectively. Azobenzene residues in the films and microcapsules exhibit trans-to-cis photoisomerization under UV light, which causes changes in the physical or chemical properties of the LbL assemblies. Therefore, azobenzene-functionalized LbL films and microcapsules have been used for the construction of photosensitive biomedical devices. For instance, cell adhesion on the surface of a solid can be controlled by UV light irradiation by coating the surface with azobenzene-containing LbL films. In another example, the ion permeability of porous materials coated with LbL films can be regulated by UV light irradiation. Furthermore, azobenzene-containing LbL films and microcapsules have been used as carriers for drug delivery systems sensitive to light. UV light irradiation triggers permeability changes in the LbL films and/or decomposition of the microcapsules, which results in the release of encapsulated drugs and proteins.

Construction of protein assemblies by host–guest interactions with cucurbiturils

Protein assembly is important in nature and bionics. Herein, we have reviewed the recent progress in protein assemblies induced by cucurbituril-based supramolecular interactions and their applications.

Supramolecular Control over Split-Luciferase Complementation

Supramolecular split-enzyme complementation restores enzymatic activity and allows for on-off switching. Split-luciferase fragment pairs were provided with an N-terminal FGG sequence and screened for complementation through host-guest binding to cucurbit[8]uril (Q8). Split-luciferase heterocomplex formation was induced in a Q8 concentration dependent manner, resulting in a 20-fold upregulation of luciferase activity. Supramolecular split-luciferase complementation was fully reversible, as revealed by using two types of Q8 inhibitors. Competition studies with the weak-binding FGG peptide revealed a 300-fold enhanced stability for the formation of the ternary heterocomplex compared to binding of two of the same fragments to Q8. Stochiometric binding by the potent inhibitor memantine could be used for repeated cycling of luciferase activation and deactivation in conjunction with Q8, providing a versatile module for in vitro supramolecular signaling networks.

A functional protein retention and release multilayer with high stability

Effective and robust interfacial protein retention lies at the heart of the fabrication of protein-based functional interfaces, which is potentially applicable in catalysis, medical therapy, antifouling, and smart devices, but remains challenging due to the sensitive nature of proteins. This study reports a general protein retention strategy to spatial-temporally confine various types of proteins at interfacial regions. The proteins were preserved in mesoporous silica nanoparticles embedded in covalently woven multilayers. It is worth noting that the protein retention strategy effectively preserves the catalytic capabilities of the proteins, and the multilayer structure is robust enough to withstand the bubbling catalytic reactions and could be repeatedly used due to conservation of proteins. The spatiotemporal retention of proteins could be adjusted by varying the number of capping layers. Furthermore, we demonstrate that the protein-loaded interfacial layers could not only be used to construct catalytic-active interfaces, but also be integrated as the power-generating unit to propel a macroscopic floating device.

Chain length dependent alkane/β-cyclodextrin nonamphiphilic supramolecular building blocks

In this work we report the chain length dependent behavior of the nonamphiphilic supramolecular building blocks based on the host-guest inclusion complexes of alkanes and β-cyclodextrins (β-CD). (1)H NMR, ESI-MS, and SAXS measurements verified that upon increasing the chain length of alkanes, the building blocks for vesicle formation changed from channel type 2alkane@2β-CD via channel type alkane@2β-CD to non-channel type 2alkane@2β-CD. FT-IR and TGA experiments indicated that hydrogen bonding is the extensive driving force for vesicle formation. It revealed that water molecules are involved in vesicle formation in the form of structural water. Upon changing the chain length, the average number of water molecules associated with per building block is about 16-21, depending on the chain length.

Insulin particles as building blocks for controlled insulin release multilayer nano-films

Insulin nanoparticles (NPs) were prepared by pH-shift precipitation and a newly developed disassembly method at room temperature. Then, an electrostatic interaction-based, layer-by-layer (LbL) multilayer film incorporating insulin NPs was fabricated with poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH), which is described herein as Si/(PAH/PAA)5(PAH/PAA-insulin NPs)n. The positively charged insulin NPs were introduced into the LbL film in the form of biocompatible PAA-insulin NP aggregates at a pH of 4.5 and were released in phosphate-buffered saline (pH7.4), triggered by changes in the charges of the insulin molecules. In addition, the insulin-incorporated multilayer was swollen because of the different ionic environment, leading also to insulin release. Eighty percent of the insulin was released from the LBL film in the first stage of 3h, and sustained release could be maintained in the second stage for up to 7 days in vitro, which is very critical for specific diabetic patients. These striking findings could offer novel directions to researchers in establishing insulin delivery systems for diabetic therapy and fabricating other protein nanoparticles applied to various biomedical platforms.

Mesoporous Silica Nanoparticles Coated by Layer-by-Layer Self-assembly Using Cucurbit[7]uril for in Vitro and in Vivo Anticancer Drug Release

Mesoporous silica nanoparticles (MSNs) are promising solid supports for controlled anticancer drug delivery. Herein, we report biocompatible layer-by-layer (LbL) coated MSNs (LbL-MSNs) that are designed and crafted to release encapsulated anticancer drugs, e.g., doxorubicin hydrochloride (DOX), by changing the pH or by adding competitive agents. The LbL coating process comprises bis-aminated poly(glycerol methacrylate)s (BA-PGOHMAs) and cucurbit[7]uril (CB[7]), where CB[7] serves as a molecular bridge holding two different bis-aminated polymeric layers together by means of host-guest interactions. This integrated nanosystem is tuned to respond under specific acidic conditions or by adding adamantaneamine hydrochloride (AH), attributed to the competitive binding of hydronium ions or AH to CB[7] with BA-PGOHMAs. These LbL-MSN hybrids possess excellent biostability, negligible premature drug leakage at pH 7.4, and exceptional stimuli-responsive drug release performance. The pore sizes of the MSNs and bis-aminated compounds (different carbon numbers) of BA-PGOHMAs have been optimized to provide effective integrated nanosystems for the loading and release of DOX. Significantly, the operating pH for the controlled release of DOX matches the acidifying endosomal compartments of HeLa cancer cells, suggesting that these hybrid nanosystems are good candidates for autonomous anticancer drug nanocarriers actuated by intracellular pH changes without any invasive external stimuli. The successful cellular uptake and release of cargo, e.g., propidium iodide (PI), in human breast cancer cell line MDA-231 from PI-loaded LbL-MSNs have been confirmed by confocal laser scanning microscopy (CLSM), while the cytotoxicities of DOX-loaded LbL-MSNs have been quantified by the Cell Counting Kit-8 (CCK-8) viability assay against HeLa cell lines and fibroblast L929 cell lines. The uptake of DOX-loaded LbL-MSNs by macrophages can be efficiently reduced by adding biocompatible hydrophilic poly(ethylene glycol) or CB[7] without destroying the capping. In vivo tumor-growth inhibition experiments with BALB/c nude mice demonstrated a highly efficient tumor-growth inhibition rate of DOX-loaded LbL-MSNs, suggesting that the novel type of LbL-MSN materials hold great potentials in anticancer drug delivery.

Supramolecularly oriented immobilization of proteins using cucurbit[8]uril

A supramolecular strategy is used for oriented positioning of proteins on surfaces. A viologen-based guest molecule is attached to the surface, while a naphthol guest moiety is chemoselectively ligated to a yellow fluorescent protein. Cucurbit[8]uril (CB[8]) is used to link the proteins onto surfaces through specific charge-transfer interactions between naphthol and viologen inside the CB cavity. The assembly process is characterized using fluorescence and atomic force microscopy, surface plasmon resonance, IR-reflective absorption, and X-ray photoelectron spectroscopy measurements. Two different immobilization routes are followed to form patterns of the protein ternary complexes on the surfaces. Each immobilization route consists of three steps: (i) attaching the viologen to the glass using microcontact chemistry, (ii) blocking, and (iii) either incubation or microcontact printing of CB[8] and naphthol guests. In both cases uniform and stable fluorescent patterns are fabricated with a high signal-to-noise ratio. Control experiments confirm that CB[8] serves as a selective linking unit to form stable and homogeneous ternary surface-bound complexes as envisioned. The attachment of the yellow fluorescent protein complexes is shown to be reversible and reusable for assembly as studied using fluorescence microscopy.