Catalytic Biosensors from Complex Coacervate Core Micelle (C3M) Thin Films

Interactions between Hyaluronic Acid and Chitosan by Isothermal Titration Calorimetry: The Effect of Ionic Strength, pH, and Polymer Molecular Weight

Hyaluronic acid (HA)/chitosan (CHI) complex coacervates have recently gained interest due to the pH-dependent ionization and semiflexibility of the polymers as well as their applicability in tissue engineering. Here, we apply isothermal titration calorimetry (ITC) to understand the apparent thermodynamics of coacervation for HA/CHI as a function of the pH, ionic strength, and chain length. We couple these ITC experiments with the knowledge of the charge states of HA and CHI from potentiometric titration to understand the mechanistic aspects of complex formation. Our data demonstrate that the driving force for the complex coacervation of HA and CHI is entropic in nature and this driving force decreased with increasing ionic strength. We also observed a decrease in the stoichiometry for ion-pairing with increasing ionic strength, which we suggest is a consequence of the changing degree of ionization for HA at higher ionic strengths. An increase in the strength of interactions with pH was hypothesized to also be a result of changes in the degree of ionization of HA, though stronger interactions were observed at the lowest pH tested, likely due to contributions from hydrogen bonding between HA and CHI.

Do ethoxylated polymeric coacervate micelles respond to temperature similarly to ethoxylated surfactant aggregates?

HYPOTHESIS

Ethoxylated complex coacervate core micelles (C3Ms), formed by the electrostatic coacervation of a charge-neutral diblock copolymer and an oppositely charged homopolymer, exhibit morphology governed by molecular packing principles. Additionally, this morphology is temperature-dependent, leading to transitions similar to those observed in classical ethoxylated surfactant aggregates.

EXPERIMENTS

To explore the thermal effects on the size and morphology of C3Ms, we employed dynamic light scattering (DLS), small-angle X-ray scattering (SAXS), and cryogenic transmission electron microscopy (cryo-TEM). These techniques were applied to C3Ms formed by copolymers with varying poly(ethylene oxide) (EO) lengths.

FINDINGS

Increasing the temperature-induced a transition from spherical to elongated aggregates, contingent on the EO block length. This morphological transition in EO-containing C3Ms parallels the behavior of classical ethoxylated surfactant aggregates. Despite the fundamental differences between hydrophobically driven and electrostatic coacervate micelles, our findings suggest that similar molecular packing principles are universally applicable across both systems. Our results offer valuable insights for predicting the structural properties of these coacervate platforms, which is crucial for envisioning their future applications.

Polyphenol-stabilized coacervates for enzyme-triggered drug delivery

Stability issues in membrane-free coacervates have been addressed with coating strategies, but these approaches often compromise the permeability of the coacervate. Here we report a facile approach to maintain both stability and permeability using tannic acid and then demonstrate the value of this approach in enzyme-triggered drug release. First, we develop size-tunable coacervates via self-assembly of heparin glycosaminoglycan with tyrosine and arginine-based peptides. A thrombin-recognition site within the peptide building block results in heparin release upon thrombin proteolysis. Notably, polyphenols are integrated within the nano-coacervates to improve stability in biofluids. Phenolic crosslinking at the liquid-liquid interface enables nano-coacervates to maintain exceptional structural integrity across various environments. We discover a pivotal polyphenol threshold for preserving enzymatic activity alongside enhanced stability. The disassembly rate of the nano-coacervates increases as a function of thrombin activity, thus preventing a coagulation cascade. This polyphenol-based approach not only improves stability but also opens the way for applications in biomedicine, protease sensing, and bio-responsive drug delivery.

Combining and concentrating nanocelluloses for cryogels with remarkable strength and wet resilience

Cellulose cryogels are promising eco-friendly materials that exhibit low density, high porosity, and renewability. However, the applications of these materials are limited by their lower mechanical and water resistance compared to petrochemical-based lightweight materials. In this work, nanocelluloses were functionalized with cationic and anionic groups, and these nanomaterials were combined to obtain strong and water-resilient cryogels. To prepare the cryogels, anionic and cationic micro- and nanofibrils (CNFs) were produced at three different sizes and combined in various weight ratios, forming electrostatic complexes. The complex phase was concentrated by centrifugation and freeze-dried. Porous and open cellular structures were assembled in all compositions tested (porosity >90 %). Compressive testing revealed that the most resistant cryogels (1.7 MPa) were obtained with equivalent amounts of negatively and positively charged CNFs with lengths between 100 and 1200 nm. The strength at this condition was achieved as CNF electrostatic complexes assembled in thick cells, as observed by synchrotron X-ray tomography. In addition to mechanical strength, electrostatic complexation provided remarkable structural stability in water for the CNF cryogels, without compromising their biodegradability. This route by electrostatic complexation is a practical strategy to combine and concentrate nanocelluloses to tailor biodegradable lightweight materials with high strength and wet stability.

Assessing the Structure and Equilibrium Conditions of Complex Coacervate Core Micelles by Varying Their Shell Composition and Medium Ionic Strength

Complex coacervates result from an associative phase separation commonly involving oppositely charged polyelectrolytes. When this associative interaction occurs between charged-neutral diblock copolymers and oppositely charged homopolymers, a nanometric aggregate called a complex coacervate core micelle, C3M, is formed. Recent studies have addressed the issue of their thermodynamic or kinetic stability but without a clear consensus. To further investigate this issue, we have studied C3Ms formed by the combination of poly(diallyldimethylammonium) and copolymer poly(acrylamide)-b-poly(acrylate) using different preparation protocols. Dynamic light scattering and small-angle X-ray scattering measurements suggest that these structures are in an equilibrium condition because the aggregates do not vary with different preparation protocols or upon aging. In addition, their stability and structures are critically dependent on several parameters such as the density of neutral blocks in their shell and the ionic strength of the medium. Decreasing the amount of copolymer in the system and, hence, the density of neutral blocks in the shell results in an increase in the aggregate size because of the core growth, although their globular shape is retained. On the other hand, larger clusters of micelles were formed at higher ionic strengths. Partially replacing 77% of the copolymer with a homopolymer of the same charge or increasing the ionic strength of the system (above 100 mmol L-1 NaCl) leads to a metastable state, after which phase separation is eventually observed. SAXS analyses reveal that this phase separation above a certain salt concentration occurs due to the coagulation of individual micelles that seem to retain their individual globular structures. Overall, these results confirm earlier claims that equilibrium C3Ms are achieved close to 1:1 charge stoichiometry but also reveal that these conditions may vary at different shell densities or higher ionic strengths, which constitute vital information for envisioning future applications of C3Ms.

High-Throughput Screening of Streptavidin-Binding Proteins in Self-Assembled Solid Films for Directed Evolution of Materials

Evolution has shaped the development of proteins with an incredible diversity of properties. Incorporating proteins into materials is desirable for applications including biosensing; however, high-throughput selection techniques for screening protein libraries in materials contexts is lacking. In this work, a high-throughput platform to assess the binding affinity for ordered sensing proteins was established. A library of fusion proteins, consisting of an elastin-like polypeptide block, one of 22 variants of rcSso7d, and a coiled-coil order-directing sequence, was generated. All selected variants had high binding in films, likely due to the similarity of the assay to magnetic bead sorting used for initial selection, while solution binding was more variable. From these results, both the assembly of the fusion proteins in their operating state and the functionality of the binding protein are key factors in the biosensing performance. Thus, the integration of directed evolution with assembled systems is necessary to the design of better materials.

Enzymatic cascade reaction in simple-coacervates

The design of enzymatic droplet-sized reactors constitutes an important challenge with many potential applications such as medical diagnostics, water purification, bioengineering, or food industry. Coacervates, which are all-aqueous droplets, afford a simple model for the investigation of enzymatic cascade reaction since the reactions occur in all-aqueous media, which preserve the enzymes integrity. However, the question relative to how the sequestration and the proximity of enzymes within the coacervates might affect their activity remains open. Herein, we report the construction of enzymatic reactors exploiting the simple coacervation of ampholyte polymer chains, stabilized with agar. We demonstrate that these coacervates have the ability to sequester enzymes such as glucose oxidase and catalase and preserve their catalytic activity. The study is carried out by analyzing the color variation induced by the reduction of resazurin. Usually, phenoxazine molecules acting as electron acceptors are used to characterize glucose oxidase activity. Resazurin (pink) undergoes a first reduction to resorufin (salmon) and then to dihydroresorufin (transparent) in presence of glucose oxidase and glucose. We have observed that resorufin is partially regenerated in the presence of catalase, which demonstrates the enzymatic cascade reaction. Studying this enzymatic cascade reaction within coacervates as reactors provide new insights into the role of the proximity, confinement towards enzymatic activity.

Synthetic Protocell as Efficient Bioreactor: Enzymatic Superactivity and Ultrasensitive Glucose Sensing in Urine

It is believed that membraneless cellular condensates play a critical role in accelerating various slow and thermodynamically unfavorable biochemical processes. However, the exact mechanisms behind the enhanced activity within biocondensates remain poorly understood. Here, we report the fabrication of a high-performance integrated cascade bioplatform based on synthetic droplets for ultrasensitive glucose sensing. Using a horseradish peroxidase (HRP) and glucose oxidase (GOx) cascade pair, we report an unprecedented enhancement in the catalytic activity of HRP inside the synthetic membraneless droplet. Liquidlike membraneless droplets have been prepared via multivalent electrostatic interactions between adenosine triphosphate (ATP) and poly(diallyldimethylammonium chloride) (PDADMAC) in an aqueous medium. Compartmentalized enzymes (GOx/HRP@Droplet) exhibit high encapsulation efficiency, low leakage, prolong retention of activity, and exceptional stability toward protease digestion. Using an HRP@Droplet composite, we have shown that the enzymatic reaction within the droplet follows the classical Michaelis-Menten model. Our findings reveal remarkable enhancement in the catalytic activity of up to 100- and 51-fold for HRP@Droplet and GOx/HRP@Droplet, respectively. These enhanced activities have been explained on the basis of increased local concentrations of enzymes and substrates, along with altered conformations of sequestered enzymes. Furthermore, we have utilized highly efficient and recyclable GOx/HRP@Droplet composite to demonstrate ultrasensitive glucose sensing with a limit of detection of 228 nM. Finally, the composite platform has been exploited to detect glucose in spiked urine samples in solution and filter paper. Our present study illustrates the unprecedented activity of the compartmentalized enzymes and paves the way for next-generation composite bioreactors for a wide range of applications.

Functional enzyme-polymer complexes

The use of biological enzyme catalysts could have huge ramifications for chemical industries. However, these enzymes are often inactive in nonbiological conditions, such as high temperatures, present in industrial settings. Here, we show that the enzyme PETase (polyethylene terephthalate [PET]), with potential application in plastic recycling, is stabilized at elevated temperature through complexation with random copolymers. We demonstrate this through simulations and experiments on different types of substrates. Our simulations also provide strategies for designing more enzymatically active complexes by altering polymer composition and enzyme charge distribution.

Engineered and native enzymes are poised to solve challenges in medicine, bioremediation, and biotechnology. One important goal is the possibility of upcycling polymers using enzymes. However, enzymes are often inactive in industrial, nonbiological conditions. It is particularly difficult to protect water-soluble enzymes at elevated temperatures by methods that preserve their functionality. Through atomistic and coarse-grained molecular dynamics simulations that capture protein conformational change, we show that an enzyme, PETase (polyethylene terephthalate [PET]), can be stabilized at elevated temperatures by complexation with random copolymers into nanoscale aggregates that do not precipitate into macroscopic phases. We demonstrated the efficiency of the method by simulating complexes of random copolymers and the enzyme PETase, which depolymerizes PET, a highly used polymer. These polymers are more industrially viable than peptides and can target specific domains on an enzyme. We design the mean composition of the random copolymers to control the polymer–enzyme surface contacts and the polymer conformation. When positioned on or near the active site, these polymer contacts can further stabilize the conformation of the active site at elevated temperatures. We explore the experimental implications of this active site stabilization method and show that PETase-random copolymer complexes have enhanced activity on both small molecule substrates and solid PET films. These results provide guidelines for engineering enzyme–polymer complexes with enhanced enzyme functionality in nonbiological environments.

Responsive Pickering Emulsions Stabilized by Frozen Complex Coacervate Core Micelles

Frozen complex coacervate core micelles (C3Ms) were developed as a class of particle stabilizers for Pickering emulsions. The C3Ms are composed of a core of electrostatically interacting weak polyelectrolytes, poly(acrylic acid) (pAA) and poly(dimethylaminopropylacrylamide) (pDMAPAA), surrounded by a corona of water-soluble and surface active poly(N-isopropylacrylamide) (pNiPAM). Mixing parameters of the two polymer solutions, including pH, mixing method, charge ratio, and salinity of the medium, were carefully controlled, leading to monodisperse, colloidally stable C3Ms. A combination of dynamic light scattering and proton nuclear magnetic resonance experiments showed that the C3Ms gradually disassembled from a dynamically frozen core state in pure water into free polyelectrolyte chains above 0.8 M NaCl. Upon formulation of dodecane-in-water emulsions, the frozen C3Ms adsorb as particles at the droplet interfaces in striking contrast with most of the conventional micelles made of amphiphilic block copolymers which fall apart at the interface. Eventually, increasing the salt concentration of the system triggered disassembly of the C3Ms, which led to emulsion destabilization.

Regulated Polyelectrolyte Nanogels for Enzyme Encapsulation and Activation

Polyelectrolyte (PE) nanogels consisting of cross-linked PE networks integrate the advanced features of both nanogels and PEs. The soft environment and abundant intrinsic charges are of special interest for enzyme immobilization. However, the crucial factors that regulate enzyme encapsulation and activation remain obscure to date. Herein, we synthesized cationic poly (dimethyl aminoethyl methacrylate), PDMAEMA, nanogels with well-defined size and cross-link degrees and fully investigated the effects of different control factors on lipase immobilization. We demonstrate that the cationic PDMAEMA nanogels indeed enable efficient and safe loading of anionic lipase without disturbing their structures. Strong charge interaction achieved by tuning pH and larger particle size are favorable for lipase loading, while the enhanced enzymatic activity demands nanogels with smaller size and a moderate cross-link degree. As such, PDMAEMA nanogels with a hydrodynamic radius of 35 nm and 30% cross-linker fraction display the optimal catalytic efficiency, which is fourfold of that of free lipase. Moreover, the immobilization endows enhanced enzymatic activity in a broad scope of pH, ionic strength, and temperature, demonstrating effective protection and activation of lipase by the designed nanogels. Our study validates the crucial controls of the size and structure of PE nanogels on enzyme encapsulation and activation, and the revealed findings shall be helpful for designing functional PE nanogels and boosting their applications for enzyme immobilization.

The effects of protein charge patterning on complex coacervation

The complex coacervation of proteins with other macromolecules has applications in protein encapsulation and delivery and for determining the function of cellular coacervates. Theoretical or empirical predictions for protein coacervates would enable the design of these coacervates with tunable and predictable structure-function relationships; unfortunately, no such theories exist. To help establish predictive models, the impact of protein-specific parameters on complex coacervation were probed in this study. The complex coacervation of sequence-specific, polypeptide-tagged, GFP variants and a strong synthetic polyelectrolyte was used to evaluate the effects of protein charge patterning on phase behavior. Phase portraits for the protein coacervates demonstrated that charge patterning dictates the protein's binodal phase boundary. Protein concentrations over 100 mg mL-1 were achieved in the coacervate phase, with concentrations dependent on the tag polypeptide sequence covalently attached to the globular protein domain. In addition to shifting the binodal phase boundary, polypeptide charge patterning provided entropic advantages over isotropically patterned proteins. Together, these results show that modest changes of only a few amino acids in the tag polypeptide sequence alter the coacervation thermodynamics and can be used to tune the phase behavior of polypeptides or proteins of interest.

Direct Simulations of Phase Behavior of Mixtures of Oppositely Charged Proteins/Nanoparticles and Polyelectrolytes

We use direct simulations of particle-polyelectrolyte mixtures using the single chain in mean field framework to extract the phase diagram for such systems. At high charges of the particles and low concentration of polymers, we observe the formation of a coacervate phase involving the particles and polyelectrolytes. At low particle charges and/or high concentration of polymers, the mixture undergoes a segregative phase separation into particle-rich and polymer-rich phases, respectively. We also present results for the influence of particle charge heterogeneity on the phase diagram.

Protein Vesicles Self-Assembled from Functional Globular Proteins with Different Charge and Size

Protein vesicles can be synthesized by mixing two fusion proteins: an elastin-like polypeptide (ELP) fused to an arginine-rich leucine zipper (Z_R) with a globular, soluble protein fused to a glutamate-rich leucine zipper (Z_E). Currently, only fluorescent proteins have been incorporated into vesicles; however, for protein vesicles to be useful for biocatalysis, drug delivery, or biosensing, vesicles must assemble from functional proteins that span an array of properties and functionalities. In this work, the globular protein was systematically changed to determine the effects of the surface charge and size on the self-assembly of protein vesicles. The formation of microphases, which included vesicles, coacervates, and hybrid structures, was monitored at different assembly conditions to determine the phase space for each globular protein. The results show that the protein surface charge has a small effect on vesicle self-assembly. However, increasing the size of the globular protein decreases the vesicle size and increases the stability at lower Z_E/Z_R molar ratios. The phase diagrams created can be used as guidelines to incorporate new functional proteins into vesicles. Furthermore, this work reports catalytically active enzyme vesicles, demonstrating the potential for the application of vesicles as biocatalysts or biosensors.

On Complex Coacervate Core Micelles: Structure-Function Perspectives

The co-assembly of ionic-neutral block copolymers with oppositely charged species produces nanometric colloidal complexes, known, among other names, as complex coacervates core micelles (C3Ms). C3Ms are of widespread interest in nanomedicine for controlled delivery and release, whilst research activity into other application areas, such as gelation, catalysis, nanoparticle synthesis, and sensing, is increasing. In this review, we discuss recent studies on the functional roles that C3Ms can fulfil in these and other fields, focusing on emerging structure-function relations and remaining knowledge gaps.

Tunable Polymeric Scaffolds for Enzyme Immobilization

The number of methodologies for the immobilization of enzymes using polymeric supports is continuously growing due to the developments in the fields of biotechnology, polymer chemistry, and nanotechnology in the last years. Despite being excellent catalysts, enzymes are very sensitive molecules and can undergo denaturation beyond their natural environment. For overcoming this issue, polymer chemistry offers a wealth of opportunities for the successful combination of enzymes with versatile natural or synthetic polymers. The fabrication of functional, stable, and robust biocatalytic hybrid materials (nanoparticles, capsules, hydrogels, or films) has been proven advantageous for several applications such as biomedicine, organic synthesis, biosensing, and bioremediation. In this review, supported with recent examples of enzyme-protein hybrids, we provide an overview of the methods used to combine both macromolecules, as well as the future directions and the main challenges that are currently being tackled in this field.

Polyelectrolyte Multilayers Enhance the Dry Storage and pH Stability of Physically Entrapped Enzymes

Polyelectrolyte multilayers (PEMs) are attractive materials for immobilizing enzymes due to their unique ionic environment, which can prevent unfolding. Here, we demonstrated that the stability to dry storage and elevated pH were significantly enhanced when negatively charged nitroreductase (NfsB) was embedded in a PEM by depositing alternating layers of the enzyme and polycation (PC) onto porous silica particles. The PC strength (i.e., pK_a) and the surface charge of the film were varied to probe the effects that internal and surface chemistry had on the pH stability of the entrapped NfsB. All films showed enhanced activity retention at elevated pH (>6), and inactivation at reduced pH (<6) similar to NfsB in solution, indicating that the primary stabilizing effect of immobilization was achieved through ionic interactions between NfsB and the PC and not through changes to the surface charge of the NfsB. Additionally, films that were stored dry at 4 °C for 1 month retained full activity, while those stored at room temperature lost 30% activity. Remarkably, at 50 °C, above the NfsB melting temperature, 40% activity was retained after 1 month of dry storage. Our results suggest that internal film properties are significantly more important than surface charge, which had minor effects on activity. Specifically, immobilization with the weak PC, poly(l-lysine), increased the optimal pH and the activity of immobilized NfsB (which we attribute to greater permeability), relative to immobilization with the strong PC, poly(diallyldimethylammonium chloride). However, NfsB was leached from the PLL film to a greater extent. Overall, these observations demonstrate that internal ionic cross-linking is key to the stabilizing effects of PEMs and that the pH response can be tuned by controlling the number of cross-links (e.g., by changing the strength of the PC). However, this may be at the cost of reduced loading, illustrating the necessity of simultaneously optimizing enzyme loading, internal ionic cross-linking, and substrate transport.

Charge Matters: Electrostatic Complexation As a Green Approach to Assemble Advanced Functional Materials

We report on electrostatically complexed materials bearing advanced functions that are not possible for other assemblies. The fundamentals of electrostatic association between oppositely charged polyelectrolytes and colloidal particles are introduced together with the conditions needed for complexation, including those related to ionic strength, pH, and hydration. Related considerations allow us to control the properties of the formed complexes and to develop features such as self-healing and underwater adhesion. In contrast to assemblies produced by typical hydrophobic and chemical interactions, electrostatic complexation leads to reversible systems. A state-of-the-art account of the field of electrostatically complexed materials is provided, including those formed from biomolecules and for salt-controlled rheology, underwater adhesiveness, and interfacial spinning. Finally, we present an outlook of electrostatic complexation from the colloidal chemistry perspective.