Temperature-induced aggregation of poly(N-isopropylacrylamide)-stabilized CdS quantum dots in water

PNIPAmx–PPO36–PNIPAmx thermo-sensitive triblock copolymers: chain conformation and adsorption behavior on a hydrophobic gold surface

The presence of the PNIPAm block is not a sufficient condition for the complex adsorption behavior of PNIPAm_x–PPO₃₆–PNIPAm_x triblock copolymers.

Multifaceted thermoresponsive poly(N-vinylcaprolactam) coupled with carbon dots for biomedical applications

A fluorescent thermoresponsive polymer consisting of poly(N-vinylcaprolactam) (PVCL) coupled with carbon dots (CDs) (PVCL-CDs) was synthesized by reacting a carboxyl-terminated PVCL derivative with CDs via N-hydroxysuccinimide and N-(3-(dimethylamino)propyl)-N-ethylcarbodiimide hydrochloride coupling. The temperature-dependent fluorescence properties of this material were studied for biomedical applications. Fluorescence quenching in PVCL-CDs was observed above the lower critical solution temperature (LCST) due to thermo-induced aggregation of the PVCL chains. This fluorescent thermoresponsive PVCL-CDs showed good biocompatibility and was demonstrated as a thermometer for sensing intracellular temperatures and also as a marker for bioimaging. In addition, PVCL-CDs showed a significant fluorescence turn-on response to proteins above the LCST, which allows for the utilization of this material in biosensors. Thus, PVCL-CDs, with its tuneable size, low cytotoxicity, good photostability, ease of bioconjugation, and resistance to metabolic degradation, is a novel material for biomedical applications.

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.

Thermoresponsive dual emission nanosensor based on quantum dots and dye labeled poly(N-isopropylacrylamide)

Hybrid nanoparticle based on quantum dots and dye labeled PNIPAM shows ratiometric changes in fluorescence emission upon temperature cycling between 25 °C and 45 °C.

Thermoresponsive PNIPAAM bottlebrush polymers with tailored side-chain length and end-group structure

We explore the phase behaviour, solution conformation, and interfacial properties of bottlebrush polymers with side-chains comprised of poly(N-isopropylacrylamide) (PNIPAAM), a thermally responsive polymer that exhibits a lower critical solution temperature (LCST) in water. PNIPAAM bottlebrush polymers with controlled side-chain length and side-chain end-group structure are prepared using a "grafting-through" technique. Due to reduced flexibility of bottlebrush polymer side-chains, side-chain end-groups have a disproportionate effect on bottlebrush polymer solubility and phase behaviour. Bottlebrush polymers with a hydrophobic end-group have poor water solubilities and depressed LCSTs, whereas bottlebrush polymers with thiol-terminated side-chains are fully water-soluble and exhibit an LCST greater than that of PNIPAAM homopolymers. The temperature-dependent solution conformation of PNIPAAM bottlebrush polymers in D2O is analyzed by small-angle neutron scattering (SANS), and data analysis using the Guinier-Porod model shows that the bottlebrush polymer radius decreases as the temperature increases towards the LCST for PNIPAAM bottlebrush polymers with relatively long 9 kg mol(-1) side-chains. Above the LCST, PNIPAAM bottlebrush polymers can form a lyotropic liquid crystal phase in water. Interfacial tension measurements show that bottlebrush polymers reduce the interfacial tension between chloroform and water to levels comparable to PNIPAAM homopolymers without the formation of microemulsions, suggesting that bottlebrush polymers are unable to stabilize highly curved interfaces. These results demonstrate that bottlebrush polymer side-chain length and flexibility impact phase behavior, solubility, and interfacial properties.

Facile synthesis of gold core-polymer shell responsive particles

The free adsorption of an end-functionalised weak polybase, poly dimethylaminoethyl methacrylate (pDMAEMA), on the surface of colloidal gold nanoparticles (AuNPs) as a route to produce a responsive core-shell nanoparticle is explored here. Optimal conditions for the physisorption of the polymeric chains onto the colloidal nanoparticles are explored. A dense coverage is facilitated by rapidly mixing the well solvated pH responsive homopolymer, at low pH, into a relatively poor solvent environment, at higher pH, containing a stable dispersion of charge-stabilised gold nanoparticles. The rapid pH change causes the polymer chains to concurrently collapse and adsorb onto the gold nanoparticles. In order to achieve sterically stable, monodisperse and responsive core shell nanoparticles, a crucial factor is the pH difference of the systems prior to their mixing. Once adsorbed, end-functional thiol groups on the adsorbed polymer chains can form more permanent covalent attachments with the core particles. Dynamic light scattering coupled with mobility data of pH titration experiments show that the core-shell particles exhibit a responsive character consistent with the observed potentiometric titration data of the polymer. The same particles demonstrate reversible aggregation when cycled between pH extremes. This is confirmed by shifts in the SPR peak of the corresponding UV-Vis absorption profile. The ease and flexibility of this strategy for core-shell particle production, coupled with the stability and responsiveness of the product, make this a promising colloidal coating mechanism.

Different kinetic pathways of early stage calcium-phosphate cluster aggregation induced by carboxylate-containing polymers

Acidic proteins are critical to biomineral formation, although their precise mechanistic function remains poorly understood. A number of recent studies have suggested a nonclassical mineralization model that emphasizes the importance of the formation of polymer-stabilized mineral clusters or particles; however, it has been difficult to characterize the precursors experimentally due to their transient nature. Here, we successfully captured stepwise evolution of transient CaP clusters in mineralizing solutions and studied the roles of functional polymers with laser light scattering (LLS) to determine how these polymers influence the stability of nanoclusters. We found that the polymer structure can alter CaP aggregation mechanisms, whereas the polymer concentration strongly influences the rate of CaP aggregation. Our results indicate that the ability of acidic biomolecules to control the formation of relatively stable nanoclusters in the early stages may be critical for intrafibrillar mineralization. More importantly, LLS provided information about the size and the structural evolution of CaP aggregates, which will help define the process of controlled biomineralization.

Influence of the length and grafting density of PNIPAM chains on the colloidal and optical properties of quantum dot/PNIPAM assemblies

Structural and optical characterization of water soluble, thermo-responsive quantum dot/poly(N-isopropyl acrylamide) (QD/PNIPAM) hybrid particles using fluorescence correlation spectroscopy (FCS) and time-correlated single photon counting (TCSPC) measurements performed at temperatures below and above the lower critical solution temperature (LCST) of PNIPAM is reported. By increasing the temperature above the LCST, the signature of the PNIPAM chain collapse covering the QDs is revealed by FCS measurements. Despite the significant structural change, the TCSPC measurements show that the fluorescence lifetimes remain of the same order of magnitude at T > LCST. Such QD/PNIPAM hybrid particles with water solubility and robust thermo-responsive behavior at physiologically relevant temperatures are potentially useful for (bio)molecular sensing and separation applications.

End-Grafted Polymer Chains onto Inorganic Nano-Objects

Organic/inorganic nanohybrid materials have attracted particular scientific and technological interest because they combine the properties of the organic and the inorganic component. Inorganic nanoparticles exhibit interesting electrical, optical, magnetic and/or catalytic properties, which are related with their nano-scale dimensions. However, their high surface-to-volume ratio often induces agglomeration and leads to the loss of their attractive properties. Surface modification of the inorganic nano-objects with physically or chemically end-tethered polymer chains has been employed to overcome this problem. Covalent tethered polymer chains are realized by three different approaches: the “grafting to”, the “grafting from” and the “grafting through” method. This article reviews the synthesis of end-grafted polymer chains onto inorganic nanoparticles using “controlled/living” polymerization techniques, which allow control over the polymer characteristics and the grafting density of the end-tethered polymer chains.

Preparation of soft hydrogel nanoparticles with PNIPAm hair and characterization of their temperature-induced aggregation

There exists a great number of publications concerning the synthesis of core-shell and/or hairy particles by means of controlled/living polymerization. Nevertheless, how to fabricate ultrafine nanosized hairy particles, especially polymeric soft hairy particles, remains a significant challenge. This paper presents a simple self-developed approach consisting of a two-step photoinduced polymerization of cross-linked polyacrylamide (CLPAM) soft hydrogel nanoparticles (5-10 nm in diameter) grafted with poly(N-isopropylacrylamide) (PNIPAm) chains. The architecture of such ultrafine soft water-swollen CLPAM@PNIPAm core/shell nanoparticles (20-35 nm in diameter) demonstrated very specific temperature sensitive behaviors. During heating a fast association process was observed at approximately 33-34 degrees C and the singular hairy particles with 34 nm diameters clustered into aggregates that were approximately 120 nm in diameter. Raising the temperature further, however, led to a decrease in size to about 100 nm at 45 degrees C. This behavior was attributed to the formation of hydrophobic shell layers accompanying the shrinkage of PNIPAm chains with chain polar transformations. With the contraction pressure produced by further shrinkage of the hydrophobic shell layers, the soft fully swollen PAM cores expelled water and diminished in size. During the cooling process, these contracted cores that were trapped in the aggregates gave rise to an early dissociation. The hydrophilic hairy CLPAM@PNIPAm particles are believed to be potentially useful as carriers to specific target regions, e.g., cells for controlled drug delivery and other smart biomaterial applications.