A Membrane-Bound Synthetic Receptor that Promotes Growth of a Polymeric Coating at the Bilayer-Water Interface

"Two-in-one" core-shell nanozyme probes with double signal amplification for high-performing surface plasmon resonance immunosensing

Herein, a "two-in-one" Ag@Au core-shell nanozyme probe inducing double-signal amplification has been developed to significantly elevate the sensitivity of SPR sensors via sandwich immunoassay. The Ag@Au core-shell nanozyme with intrinsic peroxide-like activity was demonstrated to catalyze a polymerization reaction leading to formation of polyaniline, allowing further improvement of detection performance of SPR immunosensor. The method demonstrated here offers a universal strategy for enhanced SPR detection and further expands the application of nanozymes.

Combining Excellent Selectivity with Broad Target Scope: Biosensing with Arrayed Deep Cavitand Hosts

Simple macrocyclic water-soluble hosts such as cucurbiturils, cyclophanes, and calixarenes have long been used for biosensing via indicator displacement assays. Using multiple hosts and dyes in an arrayed format allows pattern recognition-based "chemical nose" sensing, which confers exquisite selectivity, even rivaling the abilities of biological recognition tools such as antibodies. However, a challenge in indicator displacement-based biosensing with macrocyclic hosts is that selectivity and scope are often inversely correlated: strong selectivity for a specific target can limit wide application, and broad scope sensing can suffer from a lack of selectivity between similar targets. This problem can be addressed by using water-soluble, self-folding deep cavitands as hosts. These flexible bowl-shaped receptors can be easily functionalized with different motifs at the upper and lower rim, and the large cavities can bind many different fluorescent dyes, causing either fluorescence enhancement or quenching upon binding.Cavity-based affinity is strongest for NMe₃⁺ groups such as trimethyl-lysine, and we have exploited this for the site-selective recognition of post-translational lysine methylations in oligopeptides. The host recognizes the NMe₃⁺ group, and by applying differently functionalized hosts in an arrayed format, discrimination between identical modifications at different positions on the oligopeptide is possible. Multiple recognition elements can be exploited for selectivity, including a defined, yet "breathable" cavity, and variable upper rim functions oriented toward the target.While the performance of the host/guest sensing system is impressive for lysine methylations, the most important advance is the use of multiple different sensing mechanisms that can target a broad range of different biorelevant species. The amphiphilic deep cavitands can both bind fluorescent dyes and interact with charged biomolecules. These non-cavity-based interactions, when paired with additives such as heavy metal ions, modulate fluorescence response in an indirect manner, and these different mechanisms allow selective recognition of serine phosphorylation, lysine acetylation, and arginine citrullination. Other targets include heavy metals, drugs of abuse, and protein isoforms. Furthermore, the hosts can be applied in supramolecular tandem assays of enzyme function: the broad scope allows analysis of such different enzymes as chromatin writers/erasers, kinases, and phosphatases, all from a single host scaffold. Finally, the indirect sensing concept allows application in sensing different oligonucleotide secondary structures, including G-quadruplexes, hairpins, triplexes, and i-motifs. Discrimination between DNA strands with highly similar structures such as G-quadruplex strands with bulges and vacancies can be achieved. Instead of relying on a single highly specific fluorescent probe, the synthetic hosts tune the fluorophore-DNA interaction, introducing multiple recognition equilibria that modulate the fluorescence signal. By applying machine learning algorithms, a classification model can be established that can accurately predict the folding state of unknown sequences. Overall, the unique recognition profile of self-folded deep cavitands provides a powerful, yet simple sensing platform, one that can be easily tuned for a wide scope of biorelevant targets, in complex biological media, without sacrificing selectivity in the recognition.

Thermoresponsive Arrays Patterned via Photoclick Chemistry: Smart MALDI Plate for Protein Digest Enrichment, Desalting, and Direct MS Analysis

Sample desalting and concentration are crucial steps before matrix-assisted laser desorption/ionization-mass spectrometry (MALDI-MS) analysis. Current sample pretreatment approaches require tedious fabrication and operation procedures, which are unamenable to high-throughput analysis and also result in sample loss. Here, we report the development of a smart MALDI substrate for on-plate desalting, enrichment, and direct MS analysis of protein digests based on thermoresponsive, hydrophilic/hydrophobic transition of surface-grafted poly(N-isopropylacrylamide) (PNIPAM) microarrays. Superhydrophilic 1-thioglycerol microwells are first constructed on alkyne-silane-functionalized rough indium tin oxide substrates based on two sequential thiol-yne photoclick reactions, whereas the surrounding regions are modified with hydrophobic 1H,1H,2H,2H-perfluorodecanethiol. Surface-initiated atom-transfer radical polymerization is then triggered in microwells to form PNIPAM arrays, which facilitate sample loading and enrichment of protein digests by concentrating large-volume samples into small dots and achieving on-plate desalting through PNIPAM configuration change at elevated temperature. The smart MALDI plate shows high performance for mass spectrometric analysis of cytochrome c and neurotensin in the presence of 1 M urea and 100 mM NaHCO₃, as well as improved detection sensitivity and high sequence coverage for α-casein and cytochrome c digests in femtomole range. The work presents a versatile sample pretreatment platform with great potential for proteomic research.

Selective Heavy Element Sensing with a Simple Host-Guest Fluorescent Array

A simple three component array of host-fluorophore complexes is capable of sensitive and selective discrimination of heavy metal ions, including lanthanide and actinide salts in aqueous solution. Instead of applying optical sensors that only use "single-mode" detection, i.e., coordination of the metal to a specific ligand and monitoring the change in emission of an appended fluorophore, we exploit a series of host-fluorophore complexes that are affected by the presence of small amounts of metal ions in aqueous solution in different ways. Variable host-metal and host-guest-metal interactions lead to both turn-on and turn-off fluorescence sensing mechanisms, enhancing the discriminatory properties of the array. The limit of detection for certain metals is as low as 70 nM, and highly similar metals such as lanthanides and actinides can be easily distinguished at low micromolar concentrations in complex salt mixtures.

Hybrid Model Membrane Combining Micropatterned Lipid Bilayer and Hydrophilic Polymer Brush

Substrate-supported planar lipid bilayers (SPBs) are being utilized as a versatile model system of the biological membrane. However, the proximity between the solid support and membrane limits utility of SPBs for the functional analyses of membrane proteins. Here, we present a model membrane that can enlarge the distance between the substrate surface and the membrane by combining a stable scaffold of polymerized lipid bilayer with a hydrophilic polymer brush. A micropatterned SPB was generated by the lithographic polymerization of diacetylene lipids and subsequent incorporation of natural (fluid) lipid bilayers. Hydrophilic polymer brush of poly-2-methacryloyloxyethyl phosphorylcholine (poly(MPC)) was formed on the surface of polymeric bilayer by the in situ atom transfer radical polymerization (ATRP) in aqueous solution, in the presence of embedded fluid lipid bilayers. A model membrane protein (Haloquadratum walsbyi bacteriorhodopsin: HwBR) could be reconstituted into the polymer brush-supported bilayers with significantly reduced immobile molecules. Furthermore, the polymer brush terminals could be functionalized by successively polymerizing MPC and 2-aminoethyl methacrylate (AMA). The reactive amine moiety of poly(AMA) enables to conjugate a wide range of biological molecules and surfaces to the membrane. The combination of micropatterned bilayer and polymer brush mimics the two- and three-dimensional structures of the biological membrane, providing a platform to assay membrane proteins in a truly biomimetic environment.

Using photo-initiated polymerization reactions to detect molecular recognition

Radical polymerization reactions initiated by light can be used to provide signal amplification in molecular binding assays.

Cell and Protein Recognition at a Supported Bilayer Interface via In Situ Cavitand-Mediated Functional Polymer Growth

Water-soluble deep cavitands embedded in a supported lipid bilayer are capable of anchoring ATRP initiator molecules for the in situ synthesis of primary amine-containing polymethacrylate patches at the water:membrane interface. These polymers can be derivatized in situ to incorporate fluorescent reporters, allow selective protein recognition, and can be applied to the immobilization of nonadherent cells at the bilayer interface.

Responsive inverse opal hydrogels for the sensing of macromolecules

Dual responsive inverse opal hydrogels were designed as autonomous sensor systems for (bio)macromolecules, exploiting the analyte-induced modulation of the opal's structural color. The systems that are based on oligo(ethylene glycol) macromonomers additionally incorporate comonomers with various recognition units. They combine a coil-to-globule collapse transition of the LCST type with sensitivity of the transition temperature toward molecular recognition processes. This enables the specific detection of macromolecular analytes, such as glycopolymers and proteins, by simple optical methods. While the inverse opal structure assists the effective diffusion even of large analytes into the photonic crystal, the stimulus responsiveness gives rise to strong shifts of the optical Bragg peak of more than 100 nm upon analyte binding at a given temperature. The systems' design provides a versatile platform for the development of easy-to-use, fast, and low-cost sensors for pathogens.

Anionic deep cavitands enable the adhesion of unmodified proteins at a membrane bilayer

An anionic self-folding deep cavitand is capable of immobilizing unmodified proteins and enzymes at a supported lipid bilayer interface, providing a simple, soft bioreactive surface that allows enzymatic function under mild conditions. The adhesion is based on complementary charge interactions, and the hosts are capable of binding enzymes such as trypsin at the bilayer interface: the catalytic activity is retained upon adhesion, allowing selective reactions to be performed at the membrane surface.

Probing cavitand-organosilane hybrid bilayers via sum-frequency vibrational spectroscopy

Quinoxaline cavitands (QxCav) are transferred by Langmuir-Schaefer method on self-assembled monolayers (SAMs) of octadecyltrichlorosilane (OTS) and N,N-dimethyl-N-octadecyl-3-aminopropyltrimethoxysilyl chloride (DMOAP) on fused silica substrates. The molecular architectures of both the hydrophobic SAMs templates and the hybrid cavitand-organosilanes bilayers at the solid-air interface are investigated and correlated by sum-frequency vibrational spectroscopy. The results show that QxCav are always in the closed vase configuration and orient with their principal axis normal to the substrates. The role of the alkyl chains density in the SAM templates on the QxCav transfer ratio is pointed out.

Selective cavitand-mediated endocytosis of targeted imaging agents into live cells

A water-soluble synthetic receptor molecule is capable of selective, controlled endocytosis of a specifically tagged target molecule in different types of living human cells. The presence of suitable choline-derived binding handles is essential for the molecular recognition and transport process, allowing selective guest transport and imaging of cancer cells.