Thermoresponsive CdS@PNIPAM core–shell nanocomposite

Abstract

Design and synthesis of core–shell nanocomposites, with a shell of an organic material coating an inorganic nanocrystal core, have been thoroughly investigated due to their prospective applications in many fields, such as catalysis, photonics, electronics, optics, and biomedicine [1–6]. In the past decade, tremendous research efforts have been devoted to design nanoparticles with the surface coating layer sensitive to external stimuli, such as ionic strength, light, temperature, and pH value [7–10]. Poly(N-isopropylacrylamide) (PNIPAM) is a well-known thermo-sensitive polymer, exhibiting a lower critical solution temperature (LCST) at about 32 C in water, and it is one of the most studied smart polymers to modify nanoparticles [11–13]. Generally, there are two approaches to covalently attach polymer chains to a solid surface. One is a ‘‘grafting to’’ method in which end-functionalized polymers are adsorbed onto the nanoparticles surface after polymerization. The other is a ‘‘grafting from’’ method, which usually involves the immobilizing of initiators onto the nanoparticles surface followed by in situ surface-initiated polymerization to obtain nanocomposites coated with polymer [14–16]. The route is practicable for the polymerization reaction of various monomers without any specific functional groups, which is required in the ‘‘grafting to’’ method. Up to now, many strategies have been developed to obtain thermoresponsive core–shell nanocomposite materials consisting of a core and a thermoresponsive PNIPAM shell based on the ‘‘grafting from’’ method [17–19]. For the ‘‘grafting from’’ method to prepare nanocomposite materials containing PNIPAM shell, it is pre-requisite to make use of the interaction between the inorganic nanocrystals (NCs) surface and the functional groups of organic molecules, such as silica particles and a silane coupling agent. Because of the ease of subsequent modification with silane to seed the radical polymerization of N-isopropylacrylamide (NIPAM), inorganic/PNIPAM core– shell nanostructures containing silica particles or silicacoated particles (such as metal, metal oxide) as cores have been most extensively studied [6, 18, 20]. However, the practical applications of the method have been seriously restricted for the toxicity, economy, and instability of silane. Furthermore, a layer of silica itself, formed between the metal oxide or metal core and PNIPAM shell, is chemical inert and has no any significance on the property and structure of nanocomposite. Thus, more general approach for the encapsulation of inorganic nanoparticles with thermoresponsive PNIPAM remains a difficult challenge for materials scientists, which may be critical for the tuning of properties of these core–shell nanomaterials. Herein, we report a facile two-step route to prepare CdS@PNIPAM core–shell nanocomposite. Microemulsion system can be devised properly to synthesize a large variety of nanocrystals with different chemistries and properties and with low dispersity [4, 5, 21]. In our first step, nearly monodispersed CdS NCs with hydrophobic organic molecules absorbed on the surface were obtained in a microemulsion system under solvothermal condition. In the second step, Reversible addition and fragmentation transfer (RAFT) agent molecules were absorbed on the CdS NCs surface by ligand-exchange process followed by C. H. Li (&) Department of Chemistry, Zhengzhou University, Zhengzhou 450001, People’s Republic of China e-mail: lch@zzu.edu.cn