Multimodal Enzyme-Carrying Suprastructures for Rapid and Sensitive Biocatalytic Cascade Reactions

Engineering Oxidase-Based Cascade Nanoreactors Design, Catalytic Efficiency, and Applications in Disease Monitoring

Inspired by the advantages of biological cascade catalytic systems, it has been devoted to the discovery of novel oxidase-based cascade catalytic systems for disease monitoring. However, the low stability, easy inactivation, and poor reproducibility of oxidase significantly limit their practical applications. Immobilization of the oxidase can be enabled to protect them from external mediators and improve catalytic efficiency and reproducibility. Notably, the substrate channels and spatial confinement play an essential role in the construction of immobilized cascade nanoreactors to enhance the overall activity. Moreover, nanozymes, a class of enzyme mimics, have not only enzyme-like activity but also high stability and tunable catalytic properties, which bolster the development of cascade nanoreactors. Herein, recent advances in the assembly of cascade reactors involving enzymes/nanozymes are described. The importance of substrate channeling and spatial distribution in regulating the catalytic efficiency of the nanoreactor is highlighted. Then, along with an in-depth discussion of the cascade biosensors for disease monitoring, the design and application of innovative devices based on these sensing principles are also summarized, including microfluidic systems, hydrogel-based platforms, and test paper technologies. Finally, challenges and prospects for cascade nanoreactors are briefly discussed and prospected.

Hydrogel-Assisted Robust Supraparticles Evolved from Droplet Evaporation

Supraparticles, formed through the self-assembly of nanoparticles, are promising contenders in catalysis, sensing, and drug delivery due to their exceptional specific surface area and porosity. However, their mechanical resilience, especially in dimensions spanning micrometers and beyond, is challenged by the inherently weak interactions among their constituent building blocks, significantly constraining their broad applicability. Here, we have exploited a robust supraparticle fabrication strategy by integrating hydrogel components into the assembly system and evaporating on the superamphiphobic surface. The resultant SiO2/SA (sodium alginate) supraparticles, achieved by evaporating a 15% volume fraction dispersion of SiO2 nanoparticles containing 18.46 mg/mL of sodium alginate and subsequently cross-linking with Ca2+, demonstrate mechanical robustness with a fracture force of 6.04 N, representing a mechanical strength enhancement of 60 times higher than that prior to the incorporation of the hydrogel component. The supraparticles maintain their original morphology after 30 min of ultrasonic treatment (200 W), demonstrating mechanical stability. This method exhibits generalizability, enabling the customization of supraparticles with various building blocks and hydrogel backbone materials. Based on such a methodology, we have synthesized enzyme-carrying supraparticles, further expanding the potential applications in intricate cascade reactions. The encapsulated glucose oxidase and horseradish peroxidase maintained their inherent reactivity, and such hydrogel-assisted robust supraparticles exhibited exceptional performance in accurate glucose assays, indicating great practical application in biocatalysis.

Droplet evaporation on super liquid-repellent surfaces: A controllable approach for supraparticle fabrication

Supraparticles are agglomerates of nano- and/or microparticles with sizes ranging from tens to hundreds of microns, making them more accessible for handling and recovery than the building blocks. Supraparticles not only inherit the properties and functions of primary particles but also exhibit characteristics such as high porosity, large specific surface area, and improved functionalities, which can be attributed to the synergism, coupling, and co-localization among the constituents. Therefore, supraparticles hold promising applications in catalysis, drug delivery, sensing, etc. Among the various synthesizing strategies, evaporating droplets on a liquid-repellent surface is proposed as an effective approach to fabricate supraparticles with unique structural features and functions. The boundary conditions of such droplet-confinement methods significantly drive the formation of supraparticles by reducing or avoiding the use of solvents or processing liquids, which further accelerates the development and utilization of supraparticles. This paper presents an overview of recent developments in the fabrication of supraparticles by evaporating droplets on liquid-repellent surfaces. The review focuses on the evaporation processes on lubricant and superhydrophobic surfaces, structural regulation, and applications of supraparticles. Finally, an outlook on the future directions of evaporation on liquid-repellent surfaces mediated supraparticle fabrication is presented.

Surface-Templated Polymer Microparticle Synthesis Based on Droplet Microarrays

Polymer microparticle synthesis based on the surface-templated method is a simple and environmentally friendly method to produce various microparticles. Unique particles with different compositions can be fabricated by simply annealing a polymer on a liquid-repellent surface. However, there are hurdles to producing particles of homogeneous sizes with large quantities and varying the shape of particles. Here, a new approach to synthesizing multiple polymer microparticles using micropatterns with wettability contrast is presented. Polymer microparticles are formed in two steps. First, a layer of poly(sodium-4-styrenesulfonate) is deposited on the hydrophilic regions by dipping and withdrawing this micropattern from a polymer solution, and an array of microdroplets is formed. A dewetting-inducing layer on the pattern is introduced, and then target polymer patches are sequentially generated on it. By annealing over Tg, the contact line of the target polymer patch is freely receded, creating a particle form. The size and shape of the microparticle can be controlled by varying the micropatterns. In addition, it is demonstrated that microparticles made of polymer blends or polymer/nanoparticle composite are easily produced. This versatile method offers the potential of surface-templated synthesis to tailor polymer microparticles with different sizes, shapes, and functionalities in various research and applications.

Poly(ferrocenylsilane)-Based Redox-Active Artificial Organelles for Biomimetic Cascade Reactions

Artificial organelles serve as functional counterparts to natural organelles, which are primarily employed to artificially replicate, restore, or enhance cellular functions. While most artificial organelles exhibit basic functions, we diverge from this norm by utilizing poly(ferrocenylmethylethylthiocarboxypropylsilane) microcapsules (PFC MCs) to construct multifunctional artificial organelles through water/oil interfacial self-assembly. Within these PFC MCs, enzymatic cascades are induced through active molecular exchange across the membrane to mimic the functions of enzymes in mitochondria. We harness the inherent redox properties of the PFC polymer, which forms the membrane, to facilitate in-situ redox reactions similar to those supported by the inner membrane of natural mitochondria. Subsequent studies have demonstrated the interaction between PFC MCs and living cell including extended lifespans within various cell types. We anticipate that functional PFC MCs have the potential to serve as innovative platforms for organelle mimics capable of executing specific cellular functions.

Reversible crowdedness of pH-responsive and host-guest active polymersomes: Mimicking µm-sized cell structures

The structure-function characteristics of isolated artificial organelles (AOs) in protocells are mainly known, but there are few reports on clustered or aggregated AOs. To imitate µm-sized complex and heterogeneous cell structures, approaches are needed that enable reversible changes in the aggregation state of colloidal structures in response to chemical, biological, and external stimuli. To construct adaptive organelle-like or cell-like reorganization characteristics, we present an advanced crosslinking strategy to fabricate clustered polymersomes as a platform based on host-guest interactions between azobenzene-containing polymersomes (Azo-Psomes) and a β-cyclodextrin-modified polymer (β-CD polymer) as a crosslinker. First, the reversible (dis)assembly of clustered Azo-Psomes is carried out by the alternating input of crosslinker and adamantane-PEG3000 as a decrosslinker. Moreover, cluster size dependence is demonstrated by environmental pH. These offer the controlled fabrication of various homogeneous and heterogeneous Azo-Psomes structures, including the size regulation and visualization of clustered AOs through a fluorescent enzymatic cascade reaction. Finally, a temperature-sensitive crosslinking agent with β-CD units can promote the coaggregation of Azo-Psomes mediated by temperature changes. Overall, these (co-)clustered Azo-Psomes and their successful transformation in AOs may provide new features for modelling biological systems for eukaryotic cells and systems biology.

Evaporation-driven Supraparticle Synthesis by Self-Lubricating Colloidal Dispersion Microdrops

The surface-templated evaporation-driven (S-TED) method that uses liquid-repellent surfaces has attracted considerable attention for its use in fabricating supraparticles of defined shape, size, and porosity. However, challenges in achieving mass production have impeded the widespread adoption of the S-TED method. To overcome this limit, we introduce an evaporation-driven "multiple supraparticle" synthesis by drying arrays of self-lubricating colloidal dispersion microdrops. To facilitate this synthetic method, a hydrophilic micropattern is prepared on a hydrophobic substrate as a template. During the removal of the substrate out of a dispersion, liquid drops are trapped and generate a microdrop array. To produce supraparticles, the contact lines of the trapped drops must be able to recede freely during evaporation. However, hydrophilic micropatterns induce strong contact line pinning for microdrops that hinders supraparticle formation. Herein, we solve this contradiction by employing an Ouzo-like colloidal dispersion, where we can control the wettability of the drop trapping domain. The self-lubrication effect provided by the Ouzo-like solution enables smooth movement of the drops' contact lines during evaporation, thereby resulting in the successful fabrication of supraparticle arrays even within the trapping domain. This strategy offers a promising and scalable approach for large-scale evaporation-driven supraparticle synthesis with a potential for extension to various primary colloidal particles, further broadening its applicability.

Synthesis of Novel Polymer-Assisted Organic-Inorganic Hybrid Nanoflowers and Their Application in Cascade Biocatalysis

This study reports on the synthesis of novel bienzyme polymer-assisted nanoflower complexes (PANF), their morphological and structural characterization, and their effectiveness as cascade biocatalysts. First, highly porous polyamide 6 microparticles (PA6 MP) are synthesized by activated anionic polymerization in solution. Second, the PA6 MP are used as carriers for hybrid bienzyme assemblies comprising glucose oxidase (GOx) and horseradish peroxidase (HRP). Thus, four PANF complexes with different co-localization and compartmentalization of the two enzymes are prepared. In samples NF GH/PA and NF GH@PA, both enzymes are localized within the same hybrid flowerlike organic-inorganic nanostructures (NF), the difference being in the way the PA6 MP are assembled with NF. In samples NF G/PAiH and NF G@PAiH, only GOx is located in the NF, while HRP is preliminary immobilized on PA6 MP. The morphology and the structure of the four PANF complexes have been studied by microscopy, spectroscopy, and synchrotron X-ray techniques. The catalytic activity of the four PANF was assessed by a two-step cascade reaction of glucose oxidation. The PANF complexes are up to 2-3 times more active than the free GOx/HRP dyad. They also display enhanced kinetic parameters, superior thermal stability in the 40-60 °C range, optimum performance at pH 4-6, and excellent storage stability. All PANF complexes are active for up to 6 consecutive operational cycles.

Controllable Enzyme Immobilization via Simple and Quantitative Adsorption of Dendronized Polymer-Enzyme Conjugates Inside a Silica Monolith for Enzymatic Flow-Through Reactor Applications

Although many different methods are known for the immobilization of enzymes on solid supports for use in flow-through applications as enzyme reactors, the reproducible immobilization of predetermined amounts of catalytically active enzyme molecules remains challenging. This challenge was tackled using a macro- and mesoporous silica monolith as a support and dendronized polymer-enzyme conjugates. The conjugates were first prepared in an aqueous solution by covalently linking enzyme molecules and either horseradish peroxidase (HRP) or bovine carbonic anhydrase (BCA) along the chains of a water-soluble second-generation dendronized polymer using an established procedure. The obtained conjugates are stable biohybrid structures in which the linking unit between the dendronized polymer and each enzyme molecule is a bisaryl hydrazone (BAH) bond. Quantitative and reproducible enzyme immobilization inside the monolith is possible by simply adding a defined volume of a conjugate solution of a defined enzyme concentration to a dry monolith piece of the desired size. In that way, (i) the entire volume of the conjugate solution is taken up by the monolith piece due to capillary forces and (ii) all conjugates of the added conjugate solution remain stably adsorbed (immobilized) noncovalently without detectable leakage from the monolith piece. The observed flow-through activity of the resulting enzyme reactors was directly proportional to the amount of conjugate used for the reactor preparation. With conjugate solutions consisting of defined amounts of both types of conjugates, the controlled coimmobilization of the two enzymes, namely, BCA and HRP, was shown to be possible in a simple way. Different stability tests of the enzyme reactors were carried out. Finally, the enzyme reactors were applied to the catalysis of a two-enzyme cascade reaction in two types of enzymatic flow-through reactor systems with either coimmobilized or sequentially immobilized BCA and HRP. Depending on the composition of the substrate solution that was pumped through the two types of enzyme reactor systems, the coimmobilized enzymes performed significantly better than the sequentially immobilized ones. This difference, however, is not due to a molecular proximity effect with regard to the enzymes but rather originates from the kinetic features of the cascade reaction used. Overall, the method developed for the controllable and reproducible immobilization of enzymes in the macro- and mesoporous silica monolith offers many possibilities for systematic investigations of immobilized enzymes in enzymatic flow-through reactors, potentially for any type of enzyme.