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

Department of Chemistry, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong, China.
Langmuir (Impact Factor: 4.38). 04/2008; 24(6):2727-31. DOI: 10.1021/la703018p
Source: PubMed

ABSTRACT Water-soluble nanosized semiconductor CdS particles (quantum dots, QDs) were synthesized with a protective layer of covalently grafting linear thermally sensitive poly(N-isopropylacrylamide) chains. Reversible association and dissociation of these CdS particles can be induced via an alteration of the solution temperature. The formation and fragmentation of the QD aggregates of the CdS particles were systematically investigated by laser light scattering (LLS) and confirmed by transmission electron microscopy (TEM). There exists a hysteresis during one heating-and-cooling cycle. The CdS particles stabilized with shorter PNIPAM chains (Mn=15,000 g/mol) can associate to form larger and denser spherical aggregates with a much higher aggregation number than those grafted with longer PNIPAM chains (Mn=31,000 g/mol) in the heating process. The dissociation (fragmentation) in the cooling process has two stages: initially, the aggregates dissociate as the temperature decreases, and then, the fragmentation stops over a wider temperature range before complete dissociation. We attribute such a two-stage fragmentation to a balanced effect of inter- and intrachain hydrogen bonding as well as the hydrophobic interaction between PNIPAM chains and CdS particles.

  • [Show abstract] [Hide abstract]
    ABSTRACT: 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.
    Soft Matter 02/2014; 10(12):2008-15. · 4.15 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: 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.
    Journal of Colloid and Interface Science 06/2013; · 3.55 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: 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.
    Advances in Colloid and Interface Science 08/2014; · 8.64 Impact Factor

Full-text (2 Sources)

Available from
Jun 11, 2014