Tethered polymer chains: surface chemistry and their impact on colloidal and surface properties

High Antifouling Performance of Weakly Hydrophilic Polymer Brushes: A Molecular Dynamics Study

The antifouling performance of polymer brushes usually improves with increasing hydrophilicity of the grafted polymer. However, in some cases, less hydrophilic polymers show comparable or better antifouling performance than do more hydrophilic polymers. We investigate the mechanism of this anomalous behavior using molecular dynamics (MD) simulations of coarse-grained (CG) models of weakly and strongly hydrophilic polymers. The antifouling performance is evaluated from the potential of mean force of a model protein. The strongly hydrophilic polymer exhibits a better antifouling performance than the weakly hydrophilic polymer when the substrate of the polymer brush is repulsive. However, when the substrate is sufficiently attractive, the weakly hydrophilic polymer brush becomes more effective than the strongly hydrophilic brush in a certain range of grafting density. This is because the weakly hydrophilic polymer chains form a tightly packed layer that prevents the adsorbate molecule from contacting the substrate. We also perform all-atom (AA) MD simulations for several standard polymers to examine the correspondence with the CG polymer models. The weakly hydrophilic CG polymer is found to be similar to poly[N-(2-hydroxypropyl)methacrylamide] and poly(2-hydroxyethyl methacrylate), both of which have a hydroxyl group in a monomer unit. The strongly hydrophilic CG polymer resembles zwitterionic poly(carboxybetaine methacrylate). A discussion referring to the adsorption free energies of proteins on surfaces calculated in previous AA MD studies suggests that the higher antifouling performance of less hydrophilic polymer brushes can be realized for various combinations of protein and surface.

Fabrication of Oriented Polycrystalline MOF Superstructures

The field of metal-organic frameworks (MOFs) has progressed beyond the design and exploration of powdery and single-crystalline materials. A current challenge is the fabrication of organized superstructures that can harness the directional properties of the individual constituent MOF crystals. To date, the progress in the fabrication methods of polycrystalline MOF superstructures has led to close-packed structures with defined crystalline orientation. By controlling the crystalline orientation, the MOF pore channels of the constituent crystals can be aligned along specific directions: these systems possess anisotropic properties including enhanced diffusion along specific directions, preferential orientation of guest species, and protection of functional guests. In this perspective, we discuss the current status of MOF research in the fabrication of oriented polycrystalline superstructures focusing on the specific crystalline directions of orientation. Three methods are examined in detail: the assembly from colloidal MOF solutions, the use of external fields for the alignment of MOF particles, and the heteroepitaxial ceramic-to-MOF growth. This perspective aims at promoting the progress of this field of research and inspiring the development of new protocols for the preparation of MOF systems with oriented pore channels, to enable advanced MOF-based devices with anisotropic properties.

Antifouling Coatings from Glassy Polyelectrolyte Complex Films

Coatings that prevent or decrease fouling are sought for many applications, including those that inhibit the attachment of organisms in aquatic environments. To date, antifouling coatings have mostly followed design criteria assembled over decades: surfaces should be well/strongly hydrated, possess low net charge, and maintain a hydrophilic character when exposed to the location of use. Thus, polymers based on ethylene glycol or zwitterionic repeat units have been shown to be highly effective. Unfortunately, hydrated materials can be quite soft, limiting their use in some environments. In a major paradigm shift, this work describes glassy antifouling films made from certain complexes of positive and negative polyelectrolytes. The dense network of electrostatic interactions yields tough materials below the glass transition temperature, Tg, in normal use, while the highly ionic character of these polyelectrolyte complexes ensures strong hydration. The proximity of equal numbers of opposite charges within these complexes mimics zwitterionic structures. Films, assembled layer-by-layer from aqueous solutions, contained sulfonated poly(ether ether ketone), SPEEK, a rigid polyelectrolyte that binds strongly to a selection of quaternary ammonium polycations. Layer-by-layer buildup of SPEEK and polycations was linear, indicating strong complexes between polyelectrolytes. Calorimetry also showed that complex formation was exothermic. Surfaces coated with these films in the 100 nm thickness range completely resisted adhesion of the common flagellate green algae, Chlamydomonas reinhardtii, which were removed from surfaces at a minimum applied flow rate of 0.8 cm s-1. The total surface charge density of adsorbed cations, determined with a sensitive radioisotopic label, was very low, around 10% of a monolayer, which minimized adsorption driven by counterion release from the surface. The viscoelastic properties of the complexes, which were stable even in concentrated salt solutions, were explored using rheology of bulk samples. When fully hydrated, their Tg values were observed to be above 75 °C.

Toward Effective and Adsorption-Based Antifouling Zipper Brushes: Effect of pH, Salt, and Polymer Design

The undesired spontaneous deposition and accumulation of matter on surfaces, better known as fouling, is a problematic and often inevitable process plaguing a variety of industries. This detrimental process can be reduced or even prevented by coating surfaces with a dense layer of end-grafted polymer: a polymer brush. Producing such polymer brushes via adsorption presents a very attractive technique, as large surfaces can be coated in a quick and simple manner. Recently, we introduced a simple and scalable two-step adsorption strategy to fabricate block copolymer-based antifouling coatings on hydrophobic surfaces. This two-step approach involved the initial adsorption of hydrophobic-charged diblock copolymer micelles acting as a primer, followed by the complexation of oppositely charged-antifouling diblock copolymers to form the antifouling brush coating. Here, we significantly improve this adsorption-based zipper brush via systematic tuning of various parameters, including pH, salt concentration, and polymer design. This study reveals several key outcomes. First of all, increasing the hydrophobic/hydrophilic block ratio of the anchoring polymeric micelles (i.e., decreasing the hydrophilic corona) promotes adsorption to the surface, resulting in the most densely packed, uniform, and hydrophilic primer layers. Second, around a neutral pH and at a low salt concentration (1 mM), complexation of the weak polyelectrolyte (PE) blocks results in brushes with the best antifouling efficacy. Moreover, by tuning the ratio between these PE blocks, the brush density can be increased, which is also directly correlated to the antifouling performance. Finally, switching to different antifouling blocks can increase the internal density or strengthen the bound hydration layer of the brush, leading to an additional enhancement of the antifouling properties (>99% lysozyme, 87% bovine serum albumin).

Molecular Dynamics Study of the Antifouling Mechanism of Hydrophilic Polymer Brushes

We perform all-atom molecular dynamics simulations of the adsorption of amino acid side-chain analogues on polymer brushes. The analogues examined are nonpolar isobutane, polar propionamide, negatively charged propionate ion, and positively charged butylammonium ion. The polymer brushes consist of a sheet of graphene and strongly hydrophilic poly(carboxybetaine methacrylate) (PCBMA) or weakly hydrophilic poly(2-hydroxyethyl methacrylate) (PHEMA). The effective interactions between isobutane and polymer chains are repulsive for PCBMA and attractive for PHEMA. Gibbs energy decomposition analysis shows that this is due to the abundance of water in the PCBMA brush, which increases the steric repulsion and decreases the Lennard-Jones attraction. The affinity of the hydrophilic analogues is low for both PCBMA and PHEMA chains, but the balance between the components of the Gibbs energy is different for the two polymers. The simulations are performed at several θ, where θ is the degree of overlap of polymer chains. The antifouling performance against the neutral analogues is better for PCBMA than for PHEMA in the low and high θ regimes. However, in the middle θ regime, the antifouling performance of PHEMA is close to or better than that of PCBMA. This is attributed to the formation of a dense layer of PHEMA on the graphene surface that inhibits direct adsorption of analogue molecules on graphene. The charged analogues do not bind to either the PHEMA or PCBMA brush irrespective of θ.

Rational Construction of Protein-Mimetic Nano-Switch Systems Based on Secondary Structure Transitions of Synthetic Polypeptides

The manipulation of the flexibility/rigidity of polymeric chains to control their function is commonly observed in natural macromolecules but largely unexplored in synthetic systems. Herein, we construct a series of protein-mimetic nano-switches consisting of a gold nanoparticle (GNP) core, a synthetic polypeptide linker, and an optically functional molecule (OFM), whose biological function can be dynamically regulated by the flexibility of the polypeptide linker. At the dormant state, the polypeptide adopts a flexible, random-coiled conformation, bringing GNP and OFM in close proximity that leads to the "turn-off" of the OFM. Once treated with alkaline phosphatase (ALP), the nano-switches are activated due to the increased separation distance between GNP and OFM driven by the coil-to-helix and flexible-to-rigid transition of the polypeptide linker. The nano-switches therefore enable selective fluorescence imaging or photodynamic therapy in response to ALP overproduced by tumor cells. The control over polymer flexibility represents an effective strategy to manipulate the optical activity of nano-switches, which mimics the delicate structure-property relationship of natural proteins.

Scalable Fabrication of Reversible Antifouling Block Copolymer Coatings via Adsorption Strategies

Fouling remains a widespread challenge as its nonspecific and uncontrollable character limits the performance of materials and devices in numerous applications. Although many promising antifouling coatings have been developed to reduce or even prevent this undesirable adhesion process, most of them suffer from serious limitations, specifically in scalability. Whereas scalability can be particularly problematic for covalently bound antifouling polymer coatings, replacement by physisorbed systems remains complicated as it often results in less effective, low-density films. In this work, we introduce a two-step adsorption strategy to fabricate high-density block copolymer-based antifouling coatings on hydrophobic surfaces, which exhibit superior properties compared to one-step adsorbed coatings. The obtained hybrid coating manages to effectively suppress the attachment of both lysozyme and bovine serum albumin, which can be explained by its dense and homogeneous surface structure as well as the desired polymer conformation. In addition, the intrinsic reversibility of the adhered complex coacervate core micelles allows for the successful triggered release and regeneration of the hybrid coating, resulting in full recovery of its antifouling properties. The simplicity and reversibility make this a unique and promising antifouling strategy for large-scale underwater applications.

Tip-Induced Nanopatterning of Ultrathin Polymer Brushes

Patterned, ultra-thin surface layers can serve as templates for positioning nanoparticlesor targeted self-assembly of molecular structures, for example, block-copolymers. This work investigates the high-resolution, atomic force microscopebased patterning of 2 nm thick vinyl-terminated polystyrene brush layers and evaluates the line broadening due to tip degradation. This work compares the patterning properties with those of a silane-based fluorinated self-assembled monolayer (SAM), using molecular heteropatterns generated by modified polymer blend lithography (brush/SAM-PBL). Stable line widths of 20 nm (FWHM) over lengths of over 20000 µm indicate greatly reduced tip wear, compared to expectations on uncoated SiO_x surfaces. The polymer brush acts as a molecularly thin lubricating layer, thus enabling a 5000 fold increase in tip lifetime, and the brush is bonded weakly enough that it can be removed with surgical accuracy. On traditionally used SAMs, either the tip wear is very high or the molecules are not completely removed. Polymer Phase Amplified Brush Editing is presented, which uses directed self-assembly to amplify the aspect ratio of the molecular structures by a factor of 4. The structures thus amplified allow transfer into silicon/metal heterostructures, fabricating 30 nm deep, all-silicon diffraction gratings that could withstand focused high-power 405 nm laser irradiation.

Self-assembly of colloidal metal-organic framework (MOF) particles

Self-assembly of colloidal particles into ordered superstructures enables the development of novel advanced materials for diverse applications such as photonics, electronics, sensing, energy conversion, energy storage, diagnosis, drug or gene delivery, and catalysis. Recently, polyhedral metal-organic framework (MOF) particles have been proposed as promising colloidal particles to form ordered superstructures, based on their colloidal stability, size-tunability, rich polyhedral shapes, porosity and multifunctionality. In this review, we present a comprehensive overview of strategies for the self-assembly of colloidal MOF particles into ordered superstructures of different dimensionalities, highlighting some of their properties and applications, and sharing thoughts on the self-assembly of MOF particles.

Tuning the thermodynamic, optical, and rheological properties of thermoresponsive polymer solutions via silica nanoparticle shape and concentration

HYPOTHESIS

The shape and quantity of hydrophilic silica nanoparticles (NPs) can be used to tune the microstructure, rheology, and stability of phase-separating polymer solutions. In thermoresponsive polymer systems, silica nanospheres are well-studied whereas anisotropic NPs have little literature precedent. Here, we hypothesize that NP shape and concentration lower the onset of rheological and turbidimetric transitions of aqueous poly(N-isopropyl acrylamide) (PNIPAM) solutions.

EXPERIMENTS

Differential scanning calorimetry (DSC), Fourier-transform infrared spectroscopy (FTIR), turbidimetry, and oscillatory rheology are utilized to examine interactions between NPs, PNIPAM, and water and to track changes in phase separation and rheological properties due to NP concentration and shape.

FINDINGS

NP addition reduces phase separation enthalpy due to PNIPAM-NP hydrogen bonding interactions, the degree to which depends on polymer content. While NP addition minorly impacts thermodynamic and optical properties, rheological transitions and associated rheological properties are dramatically altered with increasing temperature, and depend on NP quantity, shape, and polymer molecular weight. Thus NP content and shape can be used to finely tune transition temperatures and mechanical properties for applications in stimuli-responsive materials.

Investigation of the Effect of Molecular Weight, Density, and Initiator Structure Size on the Repulsive Force between a PNIPAM Polymer Brush and Protein

This paper focuses on the effect of degree of polymerization (N), density ( $\sigma$ ), and pattern size ( $x$ ) on the interaction force between a periodically patterned Poly(N-isopropylacrylamide) (PNIPAM) brush and protein. The hydrophobic interaction, the Van der Waals attractive force, and the steric repulsive force were expressed in terms of $N$ , $\sigma$ , and $x$ . The osmotic constant (k1) and the entropic constant (k2) were determined from the fit of the steric repulsive force to an experimentally obtained force distance curve. The osmotic constant was 0.105, and the entropic constant was 0.255. Using these constants, the steric repulsive force was plotted as a function of the separation distance(s) between the substrate and the protein. The forces were determined at a separation distance equal to 0.3 nm, where L0 is the equilibrium thickness of the PNIPAM brush. At this separation distance, the value of the steric repulsive force was much higher than the value of the sum of the hydrophobic interaction and the Van der Waals attractive force for large degree of polymerization ( $N>100$ ) and density ( $\sigma >0.2$ chains/nm2). However, the repulsive force was comparable to the sum of the hydrophobic interaction and the Van der Waals attractive force for a small degree of polymerization ( $N<100$ ) and density ( $\sigma =0.2$ ). Furthermore, the steric repulsive force was plotted as a function of pattern size $x$ . The plot indicated that the steric repulsive force becomes nearly zero for all degrees of polymerization and density when the value of the initiator structure size was less than 200 nm. In addition to the steric repulsive force, the lateral extension of the chains in the periodically patterned PNIPAM brush was calculated by scaling low and compared with the experimental data taken from previously published literatures. The polymer brush structure was modelled as if the immediate bare substrate is so wide that even a stretched polymer segment cannot reach to the next polymer brush structure. In such models, the value of the lateral extension was equal to the thickness of the homogenous brush. It was independent of the pattern size. However, when the polymer brush structure was modelled as if there is another polymer brush structure at a distance half of the size of the period, the lateral extension was found to be dependent on the size of the initiator structure size due to chain bridging. This was witnessed by the patterning of polymer brushes using the interferometric patterning of PNIPAM brushes and an atomic force microscopy imaging of the polymer brush structures both in air and in water. The polymer brush structure resolution in water was much lower than the resolution in air, which indicates the lateral extension of the polymer chains in water. For such kind of periodic polymer brush structures, the gap between them was calculated, and it was found dependent on the degree of polymerization, density, and initiator structure size.

Molecular dynamics study of the interactions between a hydrophilic polymer brush on graphene and amino acid side chain analogues in water

We perform all-atom molecular dynamics simulations of poly(2-hydroxyethyl methacrylate) (PHEMA) brushes in aqueous solutions of isobutane, propionamide, and sodium propionate. These solutes are side chain analogues to leucine, glutamine, and glutamic acid, respectively. We compute the Gibbs energy profile of the solute's adsorption to the polymer brush and decompose it into the contributions from the steric repulsion, van der Waals interaction, and Coulomb interaction to reveal the energetic origin of repulsion or attraction of the solute by the polymer brush. The Henry adsorption constant is the amount of adsorption normalized by the concentration in aqueous solution. We examine the dependence of this quantity on the grafting density and chain length. Our results suggest that the concurrent primary and ternary adsorption mechanism may be more important than previously expected when the solute is hydrophobic.

Compact Peptoid Molecular Brushes for Nanoparticle Stabilization

Controlling the interfaces and interactions of colloidal nanoparticles (NPs) via tethered molecular moieties is crucial for NP applications in engineered nanomaterials, optics, catalysis, and nanomedicine. Despite a broad range of molecular types explored, there is a need for a flexible approach to rationally vary the chemistry and structure of these interfacial molecules for controlling NP stability in diverse environments, while maintaining a small size of the NP molecular shell. Here, we demonstrate that low-molecular-weight, bifunctional comb-shaped, and sequence-defined peptoids can effectively stabilize gold NPs (AuNPs). The generality of this robust functionalization strategy was also demonstrated by coating of silver, platinum, and iron oxide NPs with designed peptoids. Each peptoid (PE) is designed with varied arrangements of a multivalent AuNP-binding domain and a solvation domain consisting of oligo-ethylene glycol (EG) branches. Among designs, a peptoid (PE5) with a diblock structure is demonstrated to provide a superior nanocolloidal stability in diverse aqueous solutions while forming a compact shell (∼1.5 nm) on the AuNP surface. We demonstrate by experiments and molecular dynamics simulations that PE5-coated AuNPs (PE5/AuNPs) are stable in select organic solvents owing to the strong PE5 (amine)-Au binding and solubility of the oligo-EG motifs. At the vapor-aqueous interface, we show that PE5/AuNPs remain stable and can self-assemble into ordered 2D lattices. The NP films exhibit strong near-field plasmonic coupling when transferred to solid substrates.

Advances in design and applications of polymer brush modified anisotropic particles

Current advancements in the creation of anisotropy in particles and their surface modification with polymer brushes have established a new class of hybrid materials termed polymer brush modified anisotropic particles (PBMAP). PBMAPs display unique property combinations, e.g., multi-functionality in multiple directions along with smart behavior, which is not easily achievable in traditional hybrid materials. Typically, anisotropic particles can be categorized based on three different factors, such as shape anisotropy (geometry driven), compositional anisotropy (functionality driven), and surface anisotropy (spatio-selective surface modification driven). In this review, we have particularly focused on the synthetic strategies to construct the various type of PBMAPs based on inorganic or organic core which may or may not be isotropic in nature, and their applications in various fields ranging from drug delivery to catalysis. In addition, superior performances and fascinating properties of PBMAPs over their isotropic analogues are also highlighted. A brief overview of their future developments and associated challenges have been discussed at the end.

Development of Antibacterial and Antifouling Innovative and Eco-Sustainable Sol-Gel Based Materials: From Marine Areas Protection to Healthcare Applications

Bacterial colonization of surfaces is the leading cause of deterioration and contaminations. Fouling and bacterial settlement led to damaged coatings, allowing microorganisms to fracture and reach the inner section. Therefore, effective treatment of surface damaged material is helpful to detach bio-settlement from the surface and prevent deterioration. Moreover, surface coatings can withdraw biofouling and bacterial colonization due to inherent biomaterial characteristics, such as superhydrophobicity, avoiding bacterial resistance. Fouling was a past problem, yet its untargeted toxicity led to critical environmental concerns, and its use became forbidden. As a response, research shifted focus approaching a biocompatible alternative such as exciting developments in antifouling and antibacterial solutions and assessing their antifouling and antibacterial performance and practical feasibility. This review introduces state-of-the-art antifouling and antibacterial materials and solutions for several applications. In particular, this paper focuses on antibacterial and antifouling agents for concrete and cultural heritage conservation, antifouling sol-gel-based coatings for filtration membrane technology, and marine protection and textile materials for biomedicine. In addition, this review discusses the innovative synthesis technologies of antibacterial and antifouling solutions and the consequent socio-economic implications. The synthesis and the related physico-chemical characteristics of each solution are discussed. In addition, several characterization techniques and different parameters that influence the surface finishing coatings deposition were also described.

Approaches to inhibit biofilm formation applying natural and artificial silk-based materials

The discovery of penicillin started a new era of health care since it allowed the effective treatment of formerly deadly infections. As a drawback, its overuse led to a growing number of multi-drug resistant pathogens. Challenging this arising threat, material research focuses on the development of microbe-killing or microbe repellent agents implementing such functions directly into materials. Due to their biocompatibility, non-immunogenicity and mechanical strength, silk-based materials are attractive candidates for applications in the biomedical field. Furthermore, it has been observed that silks display high persistency in their natural environment giving reason to suspect that they might be attractive candidates to prevent microbial infestation. The current review describes the process of biofilm formation on medical devices and the most common strategies to prevent it, divided into effects of surface topography, material modification and integrated additives. In this context, recent state of the art developments in the field of natural and artificial silk-based materials with microbe-repellant or antimicrobial properties are addressed. These silk properties are controversially discussed and conclusions are drawn as to which parameters will be decisive for the successful design of new bio-functional materials based on the blueprint of silk proteins.

One-dimensional necklace-like assemblies of inorganic nanoparticles: Recent advances in design, preparation and applications

One-dimensional (1D) necklace-like assembly of inorganic nanoparticles exhibits unique collective properties, which are critical to open up new and remarkable opportunities in the field of nanotechnology. This review focuses on the recent advances in the production of these types of assemblies employing two strategies: colloidal synthesis and self-assembly procedures. After a brief description of the forces guiding nanoparticles towards the assembly, the main features of both strategies are discussed. Examples of approaches, typically involved in colloidal synthesis, are highlighted. The peculiar properties of 1D nanostructures are strictly associated with the nanoparticle arrangement in the form of highly ordered assemblies, which are attained during the synthesis both in the solution and using a template, as well as under the action of an external force. The various 1D necklace-like structures, created through nanoparticle self-assembly, demonstrate aligned, oriented nanoparticle organization. Diverse nature, size and shape of preformed particles as building blocks, along with utilizing different linkers, templates or external field lead to fabrication of 1D chain nanostructures with properties responsible for their wide applications. The unique structure-property relationship, both in colloidal synthesis, and self-assembly, offers broad spectrum of 1D necklace-like nanostructure implementations, illustrated by their use in photonics, electronics, electrocatalysis, magnetics.

Exploiting the Structural Metamorphosis of Polymers to 'Wrap' Micron-Sized Spherical Objects

There is growing interest in developing methods to 'wrap' nano- and micron-sized biological objects within films that may offer protection, enhance their stability or improve performance. We describe the successful 'wrapping' of lectin-decorated microspheres, which serve as appealing model micron-sized objects, within cross-linked polymer film. This approach utilizes polymer chains able to undergo a structural metamorphosis, from being intramolecularly cross-linked to intermolecularly cross-linked, a process that is triggered by polymer concentration upon the particle surface. Experiments demonstrate that both complementary molecular recognition and the dynamic covalent nature of the crosslinker are required for successful 'wrapping' to occur. This work is significant as it suggests that nano- and micron-sized biological objects such as virus-like particles, bacteria or mammalian cells-all of which may benefit from additional environmental protection or stabilization in emerging applications-may also be 'wrapped' by this approach.

Layer-by-layer films of polysaccharides modified with poly(N-vinylpyrrolidone) and poly(vinyl alcohol)

N-grafted copolymers of chitosan (460 kDa) with poly(N-vinylpyrrolidone) (2.4 kDa) or poly(vinyl alcohol) (2.0 ​kDa) as side chains were synthesized. Depending on the polymer-to-chitosan mass ratio the degree of amino group substitution with side chains in chitosan backbone was varied in the range of 0.01-0.33. Layer-by-layer films consisted of copolymers and dextran sulfate as polyanion were obtained. Thickness, hydrophilicity, and morphology of the films were investigated using QCM, UV-vis spectrophotometry, AFM, and contact angle measurements. The obtained films show enhanced protein-repellent properties in fetal bovine serum medium. The mass of adsorbed proteins on LbL films based on copolymer with a degree of substitution of 0.2 decreases by 50 ​% compared to unmodified chitosan. Protein-repellent properties of copolymer-based films are common for LbL films of grafted chitosan copolymers and depend on hydrophilic side chain density on the surface.

Adsorption on Ligand-Tethered Nanoparticles

We use coarse-grained molecular dynamics simulations to study adsorption on ligand-tethered particles. Nanoparticles with attached flexible and stiff ligands are considered. We discuss how the excess adsorption isotherm, the thickness of the polymer corona, and its morphology depend on the number of ligands, their length, the size of the core, and the interaction parameters. We investigate the adsorption-induced structural transitions of polymer coatings. The behavior of systems involving curved and flat "brushes" is compared.

Unraveling the Mechanism and Kinetics of Binding of an LCI-eGFP-Polymer for Antifouling Coatings

The ability of proteins to adsorb irreversibly onto surfaces opens new possibilities to functionalize biological interfaces. Herein, the mechanism and kinetics of adsorption of protein-polymer macromolecules with the ability to equip surfaces with antifouling properties are investigated. These macromolecules consist of the liquid chromatography peak I peptide from which antifouling polymer brushes are grafted using single electron transfer-living radical polymerization. Surface plasmon resonance spectroscopy reveals an adsorption mechanism that follows a Langmuir-type of binding with a strong binding affinity to gold. X-ray reflectivity supports this by proving that the binding occurs exclusively by the peptide. However, the lateral organization at the surface is directed by the cylindrical eGFP. The antifouling functionality of the unimolecular coatings is confirmed by contact with blood plasma. All coatings reduce the fouling from blood plasma by 8894% with only minor effect of the degree of polymerization for the studied range (DP between 101 and 932). The excellent antifouling properties, combined with the ease of polymerization and the straightforward coating procedure make this a very promising antifouling concept for a multiplicity of applications.

Multi-stimuli-responsive aggregation of nanoparticles driven by the manipulation of colloidal stability

The capacity to control the dispersed or aggregated state of colloidal particles is particularly attractive for facilitating a diverse range of smart applications. For this reason, stimuli-responsive nanoparticles have garnered much attention in recent years. Colloidal systems that exhibit multi-stimuli-responsive behaviour are particularly interesting materials due to the greater spatial and temporal control they display in terms of dispersion/aggregation status; such behaviour can be exploited for implant formation, easy separation of a previously dispersed material or for the blocking of unwanted pores. This review will provide an overview of the recent publications regarding multi-stimuli-responsive microgels and hybrid core-shell nanoparticles. These polymer-based nanoparticles are highly sensitive to environmental conditions and can form aggregated clusters due to a loss of colloidal stability, triggered by temperature, pH and ionic strength stimuli. We aim to provide the reader with a discussion of the recent developments in this area, as well as an understanding of the fundamental concepts which underpin the responsive behaviour, and an exploration of their applications.

Biofouling-Inspired Growth of Superhydrophilic Coating of Polyacrylic Acid on Hydrophobic Surfaces for Excellent Anti-Fouling

In this work, we demonstrated effective adsorption of poly(acrylic acid) (PAA) in saline water on various hydrophobic substrates, ranging from polyethylene and polytetrafluoroethylene, to form densely packed monolayers with water contact angle as low as 6.5° in air. This was a result of the synergy of long-range hydrophobic interactions between individual PAA chains and hydrophobic surfaces and short-range hydrogen bonding between neighboring PAA chains, reminiscent of the interaction balance encountered in biofouling. The PAA monolayers adsorbed on hydrophobic surfaces showed the ultrahigh packing density of surface COOH groups of 4.8 nm^-2, which contributed to the surface superhydrophilicity and its stability against surface reconstruction during aging even at temperature higher than PAA glass transition. Further, conjugation of the adsorbed PAA monolayers with polyethylene glycol results in excellent antifouling with nearly zero adsorption of proteins.

Insights into colloidal nanoparticle-protein corona interactions for nanomedicine applications

Colloidal nanoparticles (NPs) have attracted significant attention due to their unique physicochemical properties suitable for diagnosing and treating different human diseases. Nevertheless, the successful implementation of NPs in medicine demands a proper understanding of their interactions with the different proteins found in biological fluids. Once introduced into the body, NPs are covered by a protein corona (PC) that determines the biological behavior of the NPs. The formation of the PC can eventually favor the rapid clearance of the NPs from the body before fulfilling the desired objective or lead to increased cytotoxicity. The PC nature varies as a function of the different repulsive and attractive forces that govern the NP-protein interaction and their colloidal stability. This review focuses on the phenomenon of PC formation on NPs from a physicochemical perspective, aiming to provide a general overview of this critical process. Main issues related to NP toxicity and clearance from the body as a result of protein adsorption are covered, including the most promising strategies to control PC formation and, thereby, ensure the successful application of NPs in nanomedicine.

Polymer brushes with reversibly tunable grafting density

We propose a novel class of responsive polymer brushes, where the effective grafting density can be controlled by external stimuli. This is achieved by using end-grafted polymer chains that have an affinity to the substrate. For sufficiently strong surface interactions, a fraction of chains condenses into a near-surface layer, while the remaining ones form the outer brush. The dense layer and the more tenuous outer brush can be seen as coexisting microphases. The effective grafting density of the outer brush is controlled by the adsorption strength and can be changed reversibly and in a controlled way as a response to changes in environmental parameters. The effect is demonstrated by numerical self-consistent field calculations and analyzed by scaling arguments. Since the thickness of the denser layer is about a few monomer sizes, its capacity to form a microphase is limited by the product of the brush chain length and the grafting density. We explore the range of chain lengths and grafting densities where the effect is most pronounced. In this range, the SCF studies suggest that individual chains inside the brush show large rapid fluctuations between two states that are separated by only a small free energy barrier. The behavior of the brush as a whole, however, does not reflect these large fluctuations, and the effective grafting density varies smoothly as a function of the control parameters.

Interaction of Proteins with a Planar Poly(acrylic acid) Brush: Analysis by Quartz Crystal Microbalance with Dissipation Monitoring (QCM-D)

We describe the preparation of a poly(acrylic acid) (PAA) brush, polymerized by atom transfer radical polymerization (ATRP) of tert-butyl acrylate (tBA) and subsequent acid hydrolysis, on the flat gold surfaces of quartz-crystal microbalance (QCM) crystals. The PAA brushes were characterized by Fourier transform infrared (FT-IR) spectroscopy, ellipsometry and water contact angle analysis. The interaction of the PAA brushes with human serum albumin (HSA) was studied for a range of ionic strengths and pH conditions by quartz-crystal microbalance with dissipation monitoring (QCM-D). The quantitative analysis showed a strong adsorption of protein molecules onto the PAA brush. By increasing the ionic strength, we were able to release a fraction of the initially bound HSA molecules. This finding highlights the importance of counterions in the polyelectrolyte-mediated protein adsorption/desorption. A comparison with recent calorimetric studies related to the binding of HSA to polyelectrolytes allowed us to fully analyze the QCM data based on the results of the thermodynamic analysis of the binding process.

Exploring precision polymers to fine-tune magnetic resonance imaging properties of iron oxide nanoparticles

The use of bio-polymers as stabilising agents for iron oxide-based negative magnetic resonance imaging (MRI) contrast agents has become popular in recent years, however the wide polydispersity of biologically-derived and commercially available polymers limits the ability to produce truly tuneable and reproducible behaviour, a major challenge in this area. In this work, stable colloids of iron oxide nanoparticles were prepared utilising precision-engineered bio-polymer mimics, poly(2-acrylamido-2-methylpropane sodium sulfonate) (P(AMPS)) polymers, with controlled narrow polydispersity molecular weights, as templating stabilisers. In addition to producing magnetic colloids with excellent MRI contrast capabilities (r₂ values reaching 434.2 mM^-1 s^-1 at 25 °C and 23 MHz, several times higher than similar commercial analogues), variable field relaxometry provided unexpected important insights into the dynamic environment of the hydrated materials, and hence their exceptional MRI behaviour. Thanks to the polymer's templating backbone and flexible conformation in aqueous suspension, nanocomposites appear to behave as "multi-core" clustered species, enhancing interparticle interactions whilst retaining water diffusion, boosting relaxation properties at low frequency. This clustering behaviour, evidenced by small-angle X-ray scattering, and strong relaxometric response, was fine-tuned using the well-defined molecular weight polymer species with precise iron to polymer ratios. By also showing negligible haemolytic activity, these nanocomposites exhibit considerable potential for MRI diagnostics.

Adsorption-induced co-assembly of hairy and isotropic particles

We use coarse-grained molecular dynamics simulations to study the behavior of polymer-tethered particles immersed in fluids of isotropic particles. Particles modified with weakly anchored, mobile ligands are considered. We discuss how the concentration of fluid particles affects the morphology of an isolated hairy particle. It is shown that hairy particles present different morphologies including typical core-shell, octopus-like and corn-like, depending on fluid-segment interactions and the fluid density. The mechanism of changes in the shape of hairy particles is explained. The reconfiguration of the polymer corona arises from adsorption of fluid particles "on chains". The adsorbed fluid particles form bridges between the chains. This causes the mobile ligands to merge into clusters on the core surface. A part of the core remains empty so the hairy particle becomes a Janus-like object. We also study co-assembly in mixtures of hairy and isotropic particles. Depending on the strength of fluid-segment interactions, hairy particles with fluid particles trapped inside their coronas remain isolated or form mixed clusters of different structures. The aggregation of hairy particles results from the formation of bridges between chains belonging to different cores by fluid particles.

Responsive Adsorption of N-Isopropylacrylamide Based Copolymers on Polymer Brushes

We investigate the adsorption of pH- or temperature-responsive polymer systems by ellipsometry and neutron reflectivity. To this end, temperature-responsive poly (N-isopropylacrylamide) (PNIPAM) brushes and pH-responsive poly (acrylic acid) (PAA) brushes have been prepared using the "grafting onto" method to investigate the adsorption process of polymers and its reversibility under controlled environment. To that purpose, macromolecular brushes were designed with various chain lengths and a wide range of grafting density. Below the transition temperature (LCST), the characterization of PNIPAM brushes by neutron reflectivity shows that the swelling behavior of brushes is in good agreement with the scaling models before they collapse above the LCST. The reversible adsorption on PNIPAM brushes was carried out with linear copolymers of N-isopropylacrylamide and acrylic acid, P(NIPAM-co-AA). While these copolymers remain fully soluble in water over the whole range of temperature investigated, a quantitative adsorption driven by solvophobic interactions was shown to proceed only above the LCST of the brush and to be totally reversible upon cooling. Similarly, the pH-responsive adsorption driven by electrostatic interactions on PAA brushes was studied with copolymers of NIPAM and N,N-dimethylaminopropylmethacrylamide, P(NIPAM-co-MADAP). In this case, the adsorption of weak polycations was shown to increase with the ionization of the PAA brush with interactions mainly located in the upper part of the brush at pH 7 and more deeply adsorbed within the brush at pH 9.

In Pursuit of Zero 2.0: Recent Developments in Nonfouling Polymer Brushes for Immunoassays

"Nonfouling" polymer brush surfaces can greatly improve the performance of in vitro diagnostic (IVD) assays due to the reduction of nonspecific protein adsorption and consequent improvement of signal-to-noise ratios. The development of synthetic polymer brush architectures that suppress adventitious protein adsorption is reviewed, and their integration into surface plasmon resonance and fluorescent sandwich immunoassay formats is discussed. Also, highlighted is a novel, self-contained immunoassay platform (the D4 assay) that transforms time-consuming laboratory-based assays into a user-friendly and point-of-care format with a sensitivity and specificity comparable or better than standard enzyme-linked immunosorbent assay (ELISA) directly from unprocessed samples. These advancements clearly demonstrate the utility of nonfouling polymer brushes as a substrate for ultrasensitive and robust diagnostic assays that may be suitable for clinical testing, in field and laboratory settings.

Polymer-functionalized polymer nanoparticles and their behaviour in suspensions

In concentrated suspensions of polymer-functionalized nanoparticles, the softness of the core nanoparticles has a crucial effect on the mechanical behaviour of the resulting colloidal gels.

Nanoparticle Adsorption on Antifouling Polymer Brushes

Polymer brushes have been widely used to functionalize surfaces and provide antifouling capabilities against proteins and cells. Many efforts have focused on methods for functionalization of antifouling polymer brush surfaces for interactions with specific cells, proteins, and bacteria, but none have focused on immobilizing nanoparticles (NPs) on these surfaces. This article demonstrates that both pristine NPs and protein-coated NPs can adsorb onto well-functioning antifouling polymer brush coatings formed from poly-l-lysine-graft-poly(ethylene glycol) (PLL-g-PEG) and methoxy PEG-thiol. The role of ionic strength in solution, substrate surface material, and NP surface charge in the interaction was investigated to explore the forces behind the interaction. The adsorption of different types of NPs onto the surface was studied, determining that polystyrene, gold, carbon black, and silica particles can adsorb onto PLL-g-PEG. We show that the approach can be applied in, and studied by, both surface plasmon resonance and fluorescence imaging and suggest its application as a means to study NP-protein interactions, such as the protein corona. NPs self-assembled at antifouling polymer brush surfaces provide a novel platform for both scientific studies and applications in biotechnology.

Stabilizing Colloidal Particles against Salting-out by Shortening Surface Grafts

A dramatic improvement is reported in the stability of colloidal particles when stabilizing surface grafts are systematically shortened from small polymers to single monomers. The colloidal dispersions consist of fluorinated latex particles, exhibiting a weak van der Waals attraction, with grafted steric layers of poly(ethylene glycol) (PEG) of different chain lengths. Using an effective salting-out electrolyte, Na₂CO₃, particle aggregates are detected above a threshold salt concentration that is independent of the particle concentration. The results are interpreted in terms of a sudden onset of nondispersibility of single particles, triggered by the solvent not completely wetting particle surfaces. By decreasing the PEG chain length, the threshold salt concentration is found to increase sharply. For grafts with just a single ethylene glycol group, dispersions remain stable up to exceedingly high concentrations of Na₂CO₃. However, on removal of the surface coverage altogether, the classical stability behavior of charge-stabilized dispersions is recovered. The behavior can be captured by a simple model that incorporates effective polymer-solvent interactions in the presence of an electrolyte.

Polymer Brushes Immersed in Two-Component Solvents with Pure Volume Exclusion: Effect of Solvent Molecular Shape

Polymer brushes have wide application in surface modification. We study dense, short polymer brushes immersed in a mixing solvent under athermal conditions using the classical density functional theory. The brush polymer is short so that the equilibrium behavior of the brush deviates far from the scaling laws for infinite brush chains. The excluded volume interaction is the only interaction in the system. We compare the excluded volume effect of solvent molecules of different shapes. Two types of mixing solvents are considered: solvent composed of linear oligomers and monomers, or that of spherical particles and monomers. The effects of grafting density, solvent molecular size, and solvent number density on the brush height, the density profiles, the relative excess adsorption, and the brush-solvent interface width are systematically analyzed. In the adsorption aspect, the spherical particles have stronger ability than the linear oligomers do to penetrate through the brush layer and gather at the substrate. In the screening aspect, the oligomers are more capable of screening the excluded volume interaction between the brush chains than the spherical particles. The brush-solvent interface width decreases monotonically with increasing oligomer length, but it has a minimum with the increasing spherical particle size. Our research differentiates the attractive-interaction-induced phenomenon and the volume-exclusion-induced phenomenon in dense brush systems and exhibits the difference in the antifouling properties of the brushes contacting solvent molecules of different shapes.

Design Principles for Thermoresponsive Core-Shell Nanoparticles: Controlling Thermal Transitions by Brush Morphology

In this feature article, we summarize our recent work on understanding and controlling the thermal behavior of nanoparticles grafted with thermoresponsive polymer shells. Precision synthesis of monodisperse superparamagnetic iron oxide nanocrystals was combined with irreversible dense grafting of nitrodopamide-anchored thermoresponsive polymer chains. We provide an overview of how the dense and stable grafting of biomedically relevant polymers, including poly(ethylene glycol), poly( N-isopropylacrylamide), polysarcosin, and polyoxazolines, can be achieved. This platform has made it possible for us to demonstrate that the polymer brush geometry, as defined by the nanoparticle core and relative polymer brush size, determines the thermal transitions of the polymer brush. We furthermore summarize our work on how the polymer shell transitions and nanoparticle aggregation can be tuned. With the independent variation of the core and the shell, we can optimize and precisely control the thermally controlled solubility of our system. Finally, our feature article gives examples relevant to current and future applications. We show how the thermal response of the shell influences the nanoparticle performance in biological fluids and interactions with proteins and cells, also under purely magnetic actuation of the nanoparticles through the superparamagnetic iron oxide core.

Scaling and Interactions of Linear and Ring Polymer Brushes via DPD Simulations

Single and double layers of polymer coated surfaces are investigated by means of Dissipative Particle Dynamics (DPD), focusing on the difference between grafted ring and linear chains. Several different surface coverages σ , as well as chain lengths N and brush separations D, are analyzed for athermal, i.e., good solvent, conditions. The size in the form of the radius of gyration R g , the shape as asphericity δ ∗ , and orientation β ∗ , as well as density profiles as functions of distance from grafting plane ρ ( z ) , are studied. The effect of an added bond repulsion potential to suppress bond crossing in DPD is analyzed. Scaling laws of R g and its components R g ⊥ and R g ∥ are investigated. We find R g ∝ N ν , ν = 0.588 for surface coverages below the overlap surface concentration σ ∗ . For σ > σ ∗ we find R g ⊥ ∝ N ν ⊥ , ν ⊥ ≅ 1 and R g ∥ ∝ N ν ∥ , ν ∥ = 1 / 2 of ring brushes with the standard DPD model and ν ∥ ≅ 2 / 5 with added bond repulsion. The σ dependence of the radius of gyration was found to be R g ∝ σ μ with μ = 1 / 3 for surface coverages grater than σ ∗ . The perpendicular component R g ⊥ scales independent of the bond repulsion potential as R g ⊥ ∝ σ μ ⊥ , μ ⊥ = 1 / 3 , whereas the scaling of the parallel component exhibits a topological repulsion dependence R g ∥ ∝ σ μ ∥ , μ ∥ = - 1 / 12 for standard DPD and μ ∥ = - 1 / 6 for bond repulsion.

In situ probing of switchable nanomechanical properties of responsive high-density polymer brushes on poly(dimethylsiloxane): An AFM nanoindentation approach

Nanomechanical characteristics of end grafted polymer brushes were studied by AFM based, colloidal probe nanoindentation measurements. A high-density polymer brush of poly(2-hydroxyethyl methacrylate) (PHEMA) was precisely prepared on the surface of a flexible poly(dimethylsiloxane) (PDMS) substrate oxidized in ultraviolet/ozone (UVO). Exposure times less than 10min resulted in laterally homogeneous oxidized surfaces, characterized by a SiO_x thickness ∼35nm and an increased modulus up to 9MPa, as shown by AFM nanoindentation measurements. We have demonstrated that a high surface density of up to ∼0.63chains/nm² of the well-defined PHEMA brushes can be grown from the surface of oxidized PDMS by surface-initiated atom transfer radical polymerization (SI-ATRP) from trimethoxysilane derivatives mixed-SAM. The reversible nanomechanical changes of PHEMA layer between extended (hydrated state) and collapsed (dehydrated state) chain upon immersing in selective and non-selective solvents were investigated by in situ AFM nanoindentation analysis in liquid environments. The elastic modulus derived from force-indentation curves obtained for swollen PHEMA grafted chains in water was estimated to be equal 2.7±0.2MPa, which is almost two orders of magnitude smaller than the modulus of dry PHEMA brush. Additionally, under cyclohexane immersion, the modulus of the PHEMA layer decreased by one order of magnitude, indicating a more compact chain packing at the PDMS surface.

Fluorescent Patterns by Selective Grafting of a Telechelic Polymer

The preparation of patterned ultrathin films (sub-10 nm) composed of end-anchored fluorescently labeled poly(methyl methacrylate) (PMMA) is presented. Telechelic PMMA was synthesized utilizing activator regenerated by electron transfer atom transfer radical polymerization and consecutively end-functionalized with alkynylated fluorescein by Cu-catalyzed azide-alkyne cycloaddition (CuAAC) "click" chemistry. The polymers were grafted via the α-carboxyl groups to silica or glass substrates pretreated with (3-aminopropyl)triethoxysilane (APTES). Patterned surfaces were prepared by inkjet printing of APTES onto glass substrates and selectively grafted with fluorescently end-labeled PMMA to obtain emissive arrays on the surface.

Controlling the Surface Properties of Binary Polymer Brush-Coated Colloids via Targeted Nanoparticles

This paper explores a novel mechanism for controlling the surface properties of polymer-coated colloids using targeted ("sticky") nanoparticles which attract monomers of certain polymer species. In our study, colloids are coated by two types of tethered polymer chains having different chemical properties. Attraction of nanoparticles to the monomers of one polymer type causes these polymer chains to contract toward the grafting surface, rendering the other type more exposed to the environment. Thus, the effective surface properties of the colloid are dominated by the intended polymer type. We use coarse-grained molecular dynamics (CGMD) simulation to demonstrate that introducing nanoparticles which interact preferentially with certain types of polymers makes it possible to switch between different surface properties of the colloid. This mechanism can in principle be exploited in drug delivery systems and self-assembly applications.

Layer-by-layer adsorption: Factors affecting the choice of substrates and polymers

The electrostatic layer-by-layer technique for fabrication of multi-layered structures of various sizes and shapes using flat and colloidal templates coupled with polyelectrolyte layer-forming materials has attracted significant interest among both academic and industrial researchers due to its versatility and relative simplicity of the procedures involved in its execution. Fabrication of the multi-layered structures using the electrostatic layer-by-layer method involves several distinct stages each of which holds great importance when considering the production of a high-quality product. These stages include selection of materials (both template and a pair of construction polyelectrolytes), adsorption of the first polyelectrolyte layer onto the selected templates, formation of the second layer comprised of the oppositely charged polyelectrolyte and guided by the interactions between the two chosen polyelectrolytes, and multi-layering, where a selected number of layers are produced, and which is conditioned by both intrinsic properties of the involved construction materials and external fabrication conditions such as temperature, pH and ionic strength. The current review summarises the most important aspects of each stage mentioned above and gives examples of the materials suitable for utilization of the technique and describes the underlying physics involved.

End Point Versus Backbone Specificity Governs Characteristics of Antibody Binding to Poly(ethylene glycol) Brushes

End-grafted poly(ethylene glycol) (PEG) brushes are widely used in order to suppress undesired protein adsorption to surfaces exposed to blood or other biological fluids. The specific adsorption of antibodies (Abs) to PEG brushes associated with PEG's antigenicity is drawing increasing attention because it can affect clinical applications. Here, the adsorption to PEG brushes of two Ab types, specifically binding the polymer backbone and the polymer endpoints, is structurally characterized by neutron reflectometry. The measurements yield volume fraction profiles of PEG and of the adsorbed Abs with sub-nanometer resolution perpendicular to the surface. For all brush parameters in terms of grafting density and polymerization degree, the Ab profiles clearly differ between backbone binders and endpoint binders. The adsorbed Ab amount per unit area is substantial for both Ab types and for all brush parameters investigated, even for dense brushes, which impose a considerable osmotic barrier to Ab insertion. The results therefore indicate that variation of brush parameters alone is insufficient to prevent undesired Ab adsorption. Instead, our work motivates further efforts in the search for nonantigenic brush chemistry.