Size-dependent LCST transitions of polymer-coated gold nanoparticles: cooperative aggregation and surface assembly

Polymer brush-based nanostructures: from surface self-assembly to surface co-assembly

Surface structures play an important role in the practical applications of materials. The synthesis of polymer brushes on a solid surface has emerged as an effective tool for tuning surface properties. The fabrication of polymer brush-based surface nanostructures has greatly facilitated the development of materials with unique surface properties. In this review article, synthetic methods used in the synthesis of polymer brushes, and self-assembly approaches applied in the fabrication of surface nanostructures including self-assembly of polymer brushes, co-assembly of polymer brushes and "free" block copolymer chains, and polymerization induced surface self-assembly, are reviewed. It is demonstrated that polymer brush-based surface nanostructures, including spherical surface micelles, wormlike surface structures, layered structures and surface vesicles, can be fabricated. Meanwhile, the challenges in the synthesis and applications of the surface nanostructures are discussed. This review is expected to be helpful for understanding the principles, methods and applications of polymer brush-based surface nanostructures.

Effect of Thermal Stimulus on Kinetic Rehydration of Thermoresponsive Poly(diethylene glycol monomethyl ether methacrylate)-block-poly(poly(ethylene glycol) methyl ether methacrylate) Thin Films Probed by In Situ Neutron Reflectivity

The kinetic rehydration of thin di-block copolymer poly(diethylene glycol monomethyl ether methacrylate)-block-poly(poly(ethylene glycol) methyl ether methacrylate) (PO₂-b-PO₃₀₀) films containing two thermoresponsive components is probed by in situ neutron reflectivity (NR) with different thermal stimuli in the D₂O vapor atmosphere. The transition temperatures (TTs) of PO₂ and PO₃₀₀ blocks are 25 and 60 °C, respectively. After the one-step stimulus (rapid decrease in temperature from 60 to 20 °C), the film directly switches from a collapsed to a fully swollen state. The rehydration process is divided into four steps: (a) D₂O condensation, (b) D₂O absorption, (c) D₂O evaporation, and (d) film reswelling. However, the film presents a different rehydration behavior when the thermal stimulus is separated into two smaller steps (first decrease from 60 to 40 °C and then to 20 °C). The film first switches from a collapsed to a semiswollen state caused by the rehydrated PO₃₀₀ blocks after the first step of thermal stimulus (60 to 40 °C) and then to a swollen state induced by the rehydrated PO₂ blocks after the second step (40 to 20 °C). Thus, the kinetic responses are distinct from that after the one-step thermal stimulus. Both the time and extent of condensation as well as evaporation processes are significantly reduced in these two smaller steps. However, the final states of the rehydrated PO₂-b-PO₃₀₀ films are basically identical irrespective of the applied thermal stimulus. Thus, the final state of thermoresponsive di-block copolymer films is not affected by the external thermal stimuli, which is beneficial for the design and preparation of sensors or switches based on thermoresponsive polymer films.

Gold/Pentablock Terpolymer Hybrid Multifunctional Nanocarriers for Controlled Delivery of Tamoxifen: Effect of Nanostructure on Release Kinetics

Here, we describe the preparation and characterization of organic/inorganic hybrid polymer multifunctional nanocarriers. Novel nanocomposites of gold nanoparticles using pH-responsive coordination pentablock terpolymers of poly(ε-caprolactone)-b-poly(ethylene oxide)-b-poly(2-vinylpyridine)-b-poly(ethylene oxide)-b-poly(ε-caprolactone), bearing or not bearing partially quaternized vinylpyridine moieties, were studied. The template morphology of the coordination pentablock terpolymer at physiological pH ranges from crew-cut to multicompartmentalized micelles which can be tuned by chemical modification of the central block. Additionally, the presence of 2VP groups allows the coordination of gold ions, which can be reduced in situ to construct gold@polymer nanohybrids. Furthermore, the possibility of tuning the gold distribution in the micelles, through partial quaternization of the central P2VP block, was also investigated. Various morphological gold colloidal nanoparticles such as gold@core-corona nanoparticles and gold@core-gold@corona nanoparticles were synthesized on the corresponding template of the pentablock terpolymer, first by coordination with gold ions, followed by reduction with NaBH₄. The pentablock and gold@pentablock nanoparticles could sparingly accommodate a water-soluble drug, Tamoxifen (TAX), in their hydrophobic micellar cores. The nanostructure of the nanocarrier remarkably affects the TAX delivery kinetics. Importantly, the hybrid gold@polymer nanoparticles showed prolonged release profiles for the guest molecule, relative to the corresponding bare amphiphilic pentablock polymeric micelles. These Gold@pentablock terpolymer hybrid nanoparticles could act as a multifunctional theranostic nanoplatform, integrating sustainable pH-controlled drug delivery, diagnostic function and photothermal therapy.

Constrained thermoresponsive polymers - new insights into fundamentals and applications

In the last decades, numerous stimuli-responsive polymers have been developed and investigated regarding their switching properties. In particular, thermoresponsive polymers, which form a miscibility gap with the ambient solvent with a lower or upper critical demixing point depending on the temperature, have been intensively studied in solution. For the application of such polymers in novel sensors, drug delivery systems or as multifunctional coatings, they typically have to be transferred into specific arrangements, such as micelles, polymer films or grafted nanoparticles. However, it turns out that the thermodynamic concept for the phase transition of free polymer chains fails, when thermoresponsive polymers are assembled into such sterically confined architectures. Whereas many published studies focus on synthetic aspects as well as individual applications of thermoresponsive polymers, the underlying structure-property relationships governing the thermoresponse of sterically constrained assemblies, are still poorly understood. Furthermore, the clear majority of publications deals with polymers that exhibit a lower critical solution temperature (LCST) behavior, with PNIPAAM as their main representative. In contrast, for polymer arrangements with an upper critical solution temperature (UCST), there is only limited knowledge about preparation, application and precise physical understanding of the phase transition. This review article provides an overview about the current knowledge of thermoresponsive polymers with limited mobility focusing on UCST behavior and the possibilities for influencing their thermoresponsive switching characteristics. It comprises star polymers, micelles as well as polymer chains grafted to flat substrates and particulate inorganic surfaces. The elaboration of the physicochemical interplay between the architecture of the polymer assembly and the resulting thermoresponsive switching behavior will be in the foreground of this consideration.

Influence of Molecular Weight Distribution on the Thermoresponsive Transition of Poly(N-isopropylacrylamide)

A series of poly(N-isopropylacrylamide) (PNIPAm) homopolymers with narrow molecular weight distributions (MWDs) is prepared via photoinduced electron/energy transfer-reversible addition-fragmentation chain transfer (PET-RAFT) polymerization. The thermal transition temperature of these polymer samples is analyzed via turbidity measurements in water/N,N'-dimethylformamide mixtures, which show that the cloud point temperatures are inversely proportional to the weight average molecular weight (M_w ). Binary mixtures of the narrowly distributed PNIPAm samples are also prepared and the statistical parameters for the MWDs of these blends are determined. Very interestingly, for binary blends of the PNIPAm samples, the thermoresponsive transition is not only dependent on the M_w , which has been shown previously, but also on higher order statistical parameters of the MWDs. Specifically, at very high values of skewness and kurtosis, the polymer blends deviate from a single sharp thermoresponsive transition toward a broader thermal response, and eventually to a regime of two more distinct transitions. This work highlights the importance of in-depth characterization of polymer MWDs for thermoresponsive polymers.

Responsive Polymer Brush Design and Emerging Applications for Nanotheranostics

Responsive polymer brushes are a category of polymer brushes that are capable of conformational and chemical changes in response to external stimuli. They offer unique opportunities for the control of bio-nano interactions due to the precise control of chemical and structural parameters such as the brush thickness, density, chemistry, and architecture. The design of responsive brushes at the surface of nanomaterials for theranostic applications has developed rapidly. These coatings can be generated from a very broad range of nanomaterials, without compromising their physical, photophysical, and imaging properties. Although the use of responsive brushes for nanotheranostic remains in its early stages, in this review, the aim is to present how the systems developed to date can be combined to control sensing, imaging, and controlled delivery of therapeutics. The recent developments for such design and associated methods for the synthesis of responsive brushes are discussed. The responsive behaviors of homo polymer brushes and brushes with more complex architectures are briefly reviewed, before the applications of responsive brushes as smart delivery systems are discussed. Finally, the recent work is summarized on the use of responsive polymer brushes as novel biosensors and diagnostic tools for the detection of analytes and biomarkers.

Theoretical and Experimental Design of Heavy Metal-Mopping Magnetic Nanoparticles

Herein, we show a comprehensive experimental, theoretical, and computational study aimed at designing macromolecules able to adsorb a cargo at the nanoscale. Specifically, we focus on the adsorption properties of star diblock copolymers, i.e., macromolecules made by a number f of H-T diblock copolymer arms tethered on a central core; the H monomeric heads, which are closer to the tethering point, are attractive toward a specific target, while the T monomeric tails are neutral to the cargo. Experimentally, we exploited the adaptability of poly(2-oxazoline)s (POxs) to realize block copolymer-coated nanoparticles with a proper functionalization able to interact with heavy metals and show or exhibit a thermoresponsive behavior in aqueous solution. We here present the synthesis and analysis of the properties of a high molecular mass block copolymer featured by (i) a polar side chain, capable of exploiting electrostatic and hydrophilic interaction with a predetermined cargo, and (ii) a thermoresponsive scaffold, able to change the interaction with the media by tuning the temperature. Afterward, the obtained polymers were grafted onto iron oxide nanoparticles and the thermoresponsive properties were investigated. Through isothermal titration calorimetry, we then analyzed the adsorption properties of the synthesized superparamagnetic nanoparticles for heavy metal ions in aqueous solution. Additionally, we use a combination of scaling theories and simulations to link equilibrium properties of the system to a prediction of the loading properties as a function of size ratio and effective interactions between the considered species. The comparison between experimental results on adsorption and theoretical prediction validates the whole design process.

The effects of cononsolvents on the synthesis of responsive particles via polymerisation-induced thermal self-assembly

Responsive nanogels were synthesised via RAFT-mediated polymerisation-induced thermal self-assembly in cononsolvent mixtures of water and ethanol. The solvent mixture affected the particle size, tacticity and thermal properties.

Protecting Group Free Synthesis of Glyconanoparticles Using Amino-Oxy-Terminated Polymer Ligands

Glycomaterials display enhanced binding affinity to carbohydrate-binding proteins due to the nonlinear enhancement associated with the cluster glycoside effect. Gold nanoparticles bearing glycans have attracted significant interest in particular. This is due to their versatility, their highly tunable gold cores (size and shape), and their application in biosensors and diagnostic tools. However, conjugating glycans onto these materials can be challenging, necessitating either multiple protecting group manipulations or the use of only simple glycans. This results in limited structural diversity compared to glycoarrays which can include hundreds of glycans. Here we report a method to generate glyconanoparticles from unprotected glycans by conjugation to polymer tethers bearing terminal amino-oxy groups, which are then immobilized onto gold nanoparticles. Using an isotope-labeled glycan, the efficiency of this reaction was probed in detail to confirm conjugation, with 25% of end-groups being functionalized, predominantly in the ring-closed form. Facile post-glycosylation purification is achieved by simple centrifugation/washing cycles to remove excess glycan and polymer. This streamlined synthetic approach may be particularly useful for the preparation of glyconanoparticle libraries using automation, to identify hits to be taken forward using more conventional synthetic methods. Exemplar lectin-binding studies were undertaken to confirm the availability of the glycans for binding and show this is a powerful tool for rapid assessment of multivalent glycan binding.

Thermoresponsivity, Micelle Structure, and Thermal-Induced Structural Transition of an Amphiphilic Block Copolymer Tuned by Terminal Multiple H-Bonding Units

Constructing noncovalent interactions has been a benign method to tune the stimuli responsivity and assembled structure of polymers in solution; this is essential for controlling the functions and properties of stimuli-responsive materials. Herein, we demonstrate a novel supramolecular strategy to manipulate the cloud point (T_cp) and assembled structure of thermoresponsive polymers in solution by using H-bonding interactions. We use poly(lactide-co-glycolide)-b-poly(ethylene glycol)-b- poly(lactide-co-glycolide) (PLGA-PEG-PLGA) as a model thermoresponsive polymer and functionalize its chain terminals by the self-complementary quadruple H-bonding motif, 2-ureido-4[1H]-pyrimidinone (UPy). UPy end functionalization and increasing PLGA block length decrease the T_cp of copolymer. Both UPy- and nonfunctionalized copolymers form the spherical micelles at low temperature. They undergo the intermicellar aggregation and form large compound micelles during heating; this thermally induced structural transition causes the presence of T_cp. Due to the UPy-UPy H-bonding interactions, UPy end functionalization leads to more copolymer chains to associate in one micelle, thus, enhancing the hydrodynamic, gyration radii, core size, as well as the packing density of PLGA in micelle core and grafting density of PEG on core-shell interface. The decreased T_cp of UPy-functionalized copolymer stemmed from the stronger intermicellar attractions at high temperature. Furthermore, UPy-functionalized copolymers exhibit higher drug loading content, slower drug release rate, and better separation efficiency in removing the hydrophobic substances from water than PLGA-PEG-PLGA precursors.

Controlled Phase Behavior of Thermally Sensitive Poly(N-isopropylacrylamide/ionic liquid) with Embedded Au Nanoparticles

We have synthesized a series of poly(N-isopropylacrylamide/ionic liquid) with deposited Au nanoparticles. The size of the nanoparticle range was varied from 10 to 35 nm, and these were characterized by transmission electron microscopy analysis. Ionic liquids (IL) were chosen by varying the polymerizable unit to be both in cationic (allyl) and anionic (acrylate) moiety. One-pot polymerization was done with N-isopropylacrylamide and IL using ammonium persulphate as the initiator, to which were added already prepared Au NPs. These thermally sensitive composites formed, possessed reversible swelling/deswelling abilities in water, and demonstrated a reversible visible phase transition, which was detected by differential scanning calorimetric measurements. The lower critical solution temperature (LCST) showed dependency on the size of nanoparticles and the IL independently. It was seen that the LCST of PNIPAM-based composite films can be tuned from 32 °C to a range of 23-67 °C by choosing the desired Au NP size, its concentration and kind of IL.

Cooperativity Principles in Self-Assembled Nanomedicine

Nanomedicine is a discipline that applies nanoscience and nanotechnology principles to the prevention, diagnosis, and treatment of human diseases. Self-assembly of molecular components is becoming a common strategy in the design and syntheses of nanomaterials for biomedical applications. In both natural and synthetic self-assembled nanostructures, molecular cooperativity is emerging as an important hallmark. In many cases, interplay of many types of noncovalent interactions leads to dynamic nanosystems with emergent properties where the whole is bigger than the sum of the parts. In this review, we provide a comprehensive analysis of the cooperativity principles in multiple self-assembled nanostructures. We discuss the molecular origin and quantitative modeling of cooperative behaviors. In selected systems, we describe the examples on how to leverage molecular cooperativity to design nanomedicine with improved diagnostic precision and therapeutic efficacy in medicine.

Polymer@gold Nanoparticles Prepared via RAFT Polymerization for Opto-Biodetection

Colloidal gold nanoparticles (Au NPs) have been used in several biological applications, which include the exploitation of size- and shape-dependent Localized Surface Plasmon Resonance (LSPR) in biosensing devices. In order to obtain functional and stable Au NPs in a physiological medium, surface modification and functionalization are crucial steps in these endeavors. Reversible addition-fragmentation chain transfer (RAFT) polymerization meets this need offering the possibility of control over the composition and architecture of polymeric shells coating Au NPs. Furthermore, playing with a careful choice of monomers, RAFT polymerization allows the possibility to design a polymer shell with the desired functional groups aiming at Au based nanocomposites suitable for biorecognition and biotargeting. This review provides important aspects concerning the synthesis and optical properties of Au NPs as well as concepts of RAFT polymerization. Understanding these concepts is crucial to appreciate the chemical strategies available towards RAFT-polymer coated Au core-shell nanostructures, which are here reviewed. Finally, examples of applications in opto-biodetection devices are provided and the potential of responsive "smart" nanomaterials based on such structures can be applied to other biological applications.

Influence of Grafted Block Copolymer Structure on Thermoresponsiveness of Superparamagnetic Core-Shell Nanoparticles

The morphology and topology of thermoresponsive polymers have a strong impact on their responsive properties. Grafting onto spherical particles has been shown to reduce responsiveness and transition temperatures; grafting of block copolymers has shown that switchable or retained wettability of a surface or particle during desolvation of one block can take place. Here, doubly thermoresponsive block copolymers were grafted onto spherical, monodisperse, and superparamagnetic iron oxide nanoparticles to investigate the effect of thermal desolvation on spherical brushes of block copolymers. By inverting the block order, the influence of core proximity on the responsive properties of the individual blocks could be studied as well as their relative influence on the nanoparticle colloidal stability. The inner block was shown to experience a stronger reduction in transition temperature and transition enthalpy compared to the outer block. Still, the outer block also experiences a significant reduction in responsiveness due to the restricted environment in the nanoparticle shell compared to that of the free polymer state. The demonstrated pronounced distance dependence importantly implies the possibility, but also the necessity, to radially tailor polymer hydration transitions for applications such as drug delivery, hyperthermia, and biotechnological separation for which thermally responsive nanoparticles are being developed.

Comparison of RAFT derived Poly(vinylpyrolidone) verses Poly(oligoethyleneglycol methacrylate) for the Stabilization of Glycosylated Gold Nanoparticles

Carbohydrates dictate many biological processes including infection by pathogens. Glycosylated polymers and nanomaterials which have increased affinity due to the cluster glycoside effect, are therefore useful tools to probe function, but also as prophylactic therapies or diagnostic tools. Here, the effect of polymer structure on the coating of gold nanoparticles is studied in the context of grafting density, buffer stability and in a lectin binding assay. RAFT polymerization is used to generate poly(oligoethyleneglycol methacrylates) and poly(N-vinyl pyrolidones) with a thiol end-group for subsequent immobilization onto the gold. It is observed that poly(oligoethylene glycol methacrylates), despite being widely used particle coatings, lead to low grafting densities which in turn resulted in lower stability in biological buffers. A depression of the cloud point upon nanoparticle immobilization is also seen, which might compromise performance. In comparison poly(vinyl pyrolidones) resulted in stable particles with higher grafting densities due to the compact size of each monomer unit. The higher grafting density also enabled an increase in the number of carbohydrates which can be installed per nanoparticle at the chain ends, and gave increased binding in a lectin recognition assay. These results will guide the development of new nanoparticle biosensors with enhanced specificity, affinity and stability.

Co-operative transitions of responsive-polymer coated gold nanoparticles; precision tuning and direct evidence for co-operative aggregation

Responsive polymers and polymer-coated nanoparticles have many potential bio-applications with the crucial parameter being the exact temperature where the transition occurs. Chemical modification of hydrophobic/hydrophilic or ligand binding sites has been widely explored as a tool for controlling this transition, but requires the synthesis of many different components to achieve precise control. This study reports an extensive investigation into the use of blending (i.e. mixing) as a powerful tool to modulate the transition temperature of poly(N-isopropylacrylamide) (PNIPAM) coated gold nanoparticles. By simply mixing two nanoparticles of different compositions, precise control over the transition temperature can be imposed. This was shown to be flexible to all possible mixing parameters (different polymers on different particles, different polymers on same particles and different sized particles with identical/different polymers). Evidence of the co-operative aggregation of differently sized nanoparticles (with different cloud points) is shown using transmission electron microscopy; particles with higher cloud points aggregate with those with lower cloud points with homo-aggregates not seen, demonstrating the co-operative behaviour. These interactions, and the opportunities for transition tuning will have implications in the rational design of responsive biomaterials.

Glycosylated gold nanoparticle libraries for label-free multiplexed lectin biosensing

Glycan/lectin interactions drive a wide range of recognition and signal transduction processes within nature. However, their measurement is complicated or limited by the analytical tools available. Most technologies require fluorescently labelled proteins (e.g. microarrays) or expensive infrastructure (such as surface plasmon resonance). This also limits their application in biosensing, especially for low-resource settings, where detection of pathogens based on glycan binding could speed up diagnosis. Here we employ a library-oriented approach to immobilise a range of monosaccharides onto polymer-stabilised gold nanoparticles to enable rapid and high-throughput evaluation of their binding specificities with a panel of lectins. The red to blue colour shift upon gold nanoparticle aggregation is used as the output, removing the need for labelled protein, enabling compatibility with 96-well microplates. Furthermore, we demonstrate the use of a flatbed scanner (or digital camera) to extract biophysical data, ensuring that only minimal resources are required. Finally, linear discriminant analysis is employed to demonstrate how the glyconanoparticles can be applied as a multiplexed biosensor capable of identifying pathogenic lectins without the need for any infrastructure and overcoming some of the issues of lectin promiscuity.

Lower critical solution temperature (LCST) phase behaviour of an ionic liquid and its control by supramolecular host–guest interactions

Unique LCST phase behaviour of imidazolium-based ionic liquids is reported, which can be controlled by concentration, the choice of cation, anion and solvent, and by supramolecular complex formation. MD simulations provide insight into the molecular basis of this LCST phenomenon.

Exploring the influence of the poly(4-vinyl pyridine) segment on the solution properties and thermal phase behaviours of oligo(ethylene glycol) methacrylate-based block copolymers: the different aggregation processes with various morphologies

P(MEO₂MA-co-OEGMA)-b-P4VP copolymers with different lengths of P4VP segments exhibit diverse aggregation processes with various morphologies upon heating.

Multivalent Glycopolymer-Coated Gold Nanoparticles

Glycosylated noble metal nanoparticles are a useful tool for probing biological binding events due to their aggregation-induced color changes, particularly for lectins that have multiple binding sites. To overcome the challenges of colloidal instability, which leads to false-positive results, it is essential to add polymeric coatings to these particles. Here we describe a versatile, and reliable, approach to enable coating of gold nanoparticles using well-defined polymers, with carbohydrate end groups. This produces multivalent nanoparticles that are both colloidally stable, but still retain their rapid colorimetric responses to lectin binding.

One-step grafting of polymers to graphene oxide

The direct grafting of poly(N-isopropylacrylamide) to the basal plane of graphene oxide has been achieved in a single step: cleavage of the terminal thiocarbonylthio group on RAFT grown poly(N-isopropylacrylamide) reveals a reactive thiol that attacks the epoxides present across the surface of graphene oxide. The new composite material was characterised by a combination of SSNMR, FTIR, Raman, EDX, XPS, TGA and contact angle measurement; it shows enhanced thermal stability and solubility in water.

Microwave Heating of Poly(N-isopropylacrylamide)-Conjugated Gold Nanoparticles for Temperature-Controlled Display of Concanavalin A

We demonstrate microwave-induced heating of gold nanoparticles and nanorods. An appreciably higher and concentration-dependent microwave-induced heating rate was observed with aqueous dispersions of the nanomaterials as opposed to pure water and other controls. Grafted with the thermoresponsive polymer poly(N-isopropylacrylamide), these gold nanomaterials react to microwave-induced heating with a conformational change in the polymer shell, leading to particle aggregation. We subsequently covalently immobilize concanavalin A (Con A) on the thermoresponsive gold nanoparticles. Con A is a bioreceptor commonly used in bacterial sensors because of its affinity for carbohydrates on bacterial cell surfaces. The microwave-induced thermal transitions of the polymer reversibly switch on and off the display of Con A on the particle surface and hence the interactions of the nanomaterials with carbohydrate-functionalized surfaces. This effect was determined using linear sweep voltammetry on a methyl-α-d-mannopyranoside-functionalized electrode.

Aggregation kinetics and colloidal stability of functionalized nanoparticles

The functionalization of nanoparticles has primarily been used as a means to impart stability in nanoparticle suspensions. In most cases even the most advanced nanomaterials lose their function should suspensions aggregate and settle, but with the capping agents designed for specific solution chemistries, functionalized nanomaterials generally remain monodisperse in order to maintain their function. The importance of this cannot be underestimated in light of the growing use of functionalized nanomaterials for wide range of applications. Advanced functionalization schemes seek to exert fine control over suspension stability with small adjustments to a single, controllable variable. This review is specific to functionalized nanoparticles and highlights the synthesis and attachment of novel functionalization schemes whose design is meant to affect controllable aggregation. Some examples of these materials include stimulus responsive polymers for functionalization which rely on a bulk solution physicochemical threshold (temperature or pH) to transition from a stable (monodisperse) to aggregated state. Also discussed herein are the primary methods for measuring the kinetics of particle aggregation and theoretical descriptions of conventional and novel models which have demonstrated the most promise for the appropriate reduction of experimental data. Also highlighted are the additional factors that control nanoparticle stability such as the core composition, surface chemistry and solution condition. For completeness, a case study of gold nanoparticles functionalized using homologous block copolymers is discussed to demonstrate fine control over the aggregation state of this type of material.

Towards being genuinely smart: ‘isothermally-responsive’ polymers as versatile, programmable scaffolds for biologically-adaptable materials

Thermo-responsive polymers are of broad interest in a range of biomedical and biotechnological fields. This review summaries the use of ‘isothermal’ transitions where thermo-responsive polymers are re-programmed to respond to other stimuli, but with the same outputs, with the aim of making them ‘smarter’.

Thermoresponsive polymer coated gold nanoparticles: from MADIX/RAFT copolymerization of N-vinylpyrrolidone and N-vinylcaprolactam to salt and temperature induced nanoparticle aggregation

In the present contribution, we report the synthesis of thermoresponsive homo and statistical copolymers of N-vinylcaprolactam and N-vinylpyrrolidone and the corresponding responsive gold nanoparticles.

Siderophore-inspired nanoparticle-based biosensor for the selective detection of Fe3+

Au nanoparticles modified with catechol-functionalised polymers provide a strong optical response which is highly specific to Fe³⁺ ions at physiologically relevant concentrations.

A novel approach to synthesize polymers for potential photodynamic therapy: from benzenedinitrile to phthalocyanine

RAFT synthesized polymers with benzenedinitrile side chains were converted to phthalocyanine conveniently, providing a novel method for making PDT polymers.

Tuning thermoresponsive behavior of diblock copolymers and their gold core hybrids. Part 2. How properties change depending on block attachment to gold nanoparticles

Thermoresponsive diblock copolymers of di(ethylene glycol) methyl ether methacrylate (DEGMA) and oligo(ethylene glycol) methyl ether acrylate (OEGA) were synthesized by reversible addition-fragmentation chain transfer polymerization, allowing us to prepare diblocks with a thiol group at the desired chain end, and bond that block to a ~20 nm gold nanoparticle core. The cloud point and coil-globule transition window were measured by UV-vis spectroscopy. The gold core lowered the cloud point and narrowed the coil-globule transition window of all the diblock hybrids, but raised the cloud point of statistical copolymer hybrids that had similar cloud points. The extent of the change in the thermo-response properties of the hybrid diblock copolymers was more significant when the gold was bonded to the DEGMA block than the OEGA block. This block is less hydrophilic and sterically hindered than OEGA and may adsorb more effectively to the gold so that the hydration of the outer OEGA block is relatively unaffected by the Au core. This work indicates that diblock copolymers allow factors such as steric bulk and the effects on arrangement around a metal core to be effective tools for manipulating thermo-responsive properties that are not as significant with statistical copolymers.

To aggregate, or not to aggregate? considerations in the design and application of polymeric thermally-responsive nanoparticles

The aim of this review is to highlight some of the challenges in designing thermally responsive nanoparticles, where the responsivity is endowed by a responsive polymeric corona. A review of the literature reveals many contradictory observations upon heating these particles through their transition temperature. Indeed, both an increase in size due to aggregation and particle shrinkage have been reported for apparently similar materials. Furthermore, careful review of the literature shows that responsive nanoparticles do not have the same transition temperature or properties as their constituent polymers. These observations raise serious questions as to how to achieve the rational design of a responsive particle with a predictable and reproducible response. Here we highlight specific cases where conflicting results have been observed for spherical particles and put these results into the context of flat-surface grafted polymer brushes to explain the behaviour in terms of grafting density, curvature, chain end effects and the role of the underlying substrate. A better understanding of these observations should lead to the improved design of nanoparticles with real function and applications.

Controlled Delivery of Functionalized Gold Nanoparticles by pH-Sensitive Polymersomes

The present study reports on the development of composite gold nanoparticles (AuNPs)/polymersome formulations, based on pH-responsive biocompatible polymer vesicles integrating prefunctionalized AuNPs, doped with a hydrophobic model probe for improved multimodal drug delivery. The polymer vesicles were prepared from an amphiphilic pentablock terpolymer poly(ε-caprolactone)-b-poly(ethylene oxide)-b-poly(2-vinylpyridine)-b-poly(ethylene oxide)-b-poly(ε-caprolactone) (PCL-PEO-P2VP-PEO-PCL), consisting of a pH-sensitive and biodegradable P2VP/PCL membrane, surrounded by neutral hydrophilic PEO looping chains. Additionally, partial quaternization of the P2VP block has been performed to introduce cationic moieties. Water-dispersible AuNPs carrying a hydrophobic molecule were encapsulated in the hydrophilic aqueous lumen of the vesicles, and the release was monitored at pH conditions simulating physiological and tumor environments. The complex delivery of the cargos from these vesicles showed improved and controlled kinetics relative to the individual nanocarriers, which could be further tuned by pH and chemical modification of the membrane forming block.

Smart photothermal-triggered bilayer phase transition in AuNPs-liposomes to release drug

Novel thermosensitive liposomes with embedded Au nanoparticles (AuNPs) in the liposome bilayer were prepared by a combination method of film build and supercritical CO(2) incubation. These AuNPs-liposomes possess AuNPs that are embedded in the bilayer and a drug that is encapsulated in the central aqueous compartment. The AuNPs in the liposomes can strongly absorb light energy and efficiently convert the absorbed energy to heat. The localized heat induces a phase transition in the liposome bilayer and releases the drug. The drug release from the AuNPs-liposomes can be controlled by the irradiation time and AuNPs concentration in the AuNPs-liposomes at room temperature, where the AuNPs function as a nanoswitch for triggering drug release both spatially and temporally. The results suggest that drug release from the AuNPs-liposomes is due to a photothermic effect that induces phase transition of the liposomes rather than destruction of the liposome bilayer.