Effect of copolymer composition of controlled (N-vinylcaprolactam/N-vinylpyrrolidone) statistical copolymers on formation, stabilization, thermoresponsiveness and catalytic properties of gold nanoparticles
Introduction
Metal nanoparticles [1], especially gold nanoparticles (AuNPs), have attracted much attention because of their unique electronic, optical and catalytic properties resulting in their wide application ranging from optoelectronics to biology [2], [3]. These properties strongly depend on the particle size and shape, the nature of the protecting organic shell and the interparticle distance [4]. AuNPs are colloidally unstable and are susceptible to irreversible aggregation. To stabilize them, they can be coated with small organic molecules such as citrates, surfactants, ligands or polymers. These organic molecules provide an additional tool to modulate physico-chemical properties. To this end, polymers have proved to be a material of choice due to their flexibility for tailoring specific uses and also for their greater long-term stabilizing properties compared to small molecules such as citrates. Notable examples include responsive polymers. Indeed, coating AuNPs with stimuli-responsive polymers is a convenient way to change reversibly the properties of AuNPs. Thermoresponsive polymers of LCST-type have been grafted on AuNPs without losing their thermoresponsiveness [5], [6], [7], [8], [9].
Such hybrid materials can be obtained through the grafting or adsorption of a thermoresponsive polymer around AuNPs. This confers to the obtained hybrid material not only a steric stability, but also a capacity to control the NPs properties (drug delivery, optical, catalytic…) in response to a change of temperature [10], [11], [12]. Hence different families of thermoresponsive polymers like poly(N-isopropylacrylamide) [13], [14], [15], [16], [17], [18], poly(methyl vinyl ether) [19], poly(N-acryloyl-N’-propyl piperazine) [20] and copolymers based on oligo(ethylene glycol) (meth)acrylate [21], [22], [23] or poly(N-vinyl caprolactam) (PVCL) [24], [25] were used to obtain such hybrids. The use of this last family is of special interest for the formation of hybrid nanoparticles presenting a sharp reversible response to temperature. Moreover, the cloud point temperature of PVCL and corresponding hybrid materials can be easily tuned either by adjusting polymer chain length or by changing its concentration [24].
PVCL belongs to the family of poly(N-vinyl lactams) like poly(N-vinylpyrrolidone) (PVP), which is not thermoresponsive but is one of the most commonly used polymers for the stabilization of NPs of different nature for diverse applications [26]. The structure of N-vinylcaprolactam (VCL) monomer only differs from N-vinyl pyrrolidone (VP) by the inclusion of two additional methylene groups. Reversible-deactivation radical polymerization has recently made it possible to synthesize copolymers of VCL and VP of controlled architecture and tunable thermoresponsive properties [25], [27], [28], [29]. Detrembleur and coworkers synthesized PVCL-b-P(VCL-stat-VP)-b-PVCL with adjustable hydration/dehydration properties by cobalt-mediated radical polymerization followed by a radical coupling reaction, and then studied their solution behavior [28], [29]. Hoogenboom et al. used reversible addition-fragmentation chain transfer (RAFT)-derived P(VCL-stat-VP) to stabilize preformed AuNPs surrounded by citrate via a ‘grafting to’ approach. The polymer-coated AuNPs showed a larger hydrodynamic size in comparison to pristine citrate@AuNPs. In presence of 0.1 M NaCl, a color change depending on the copolymer composition and ultimately on the cloud point temperature of the polymers was observed [25].
The objective of this paper is to study the influence of composition of narrowly dispersed VCL/VP statistical copolymers of controlled molar mass on the formation, stabilization and modulation of the properties of AuNPs. For this purpose, a series of VCL/VP statistical copolymers containing different amounts of VP (between 0 and 100%) was synthesized by RAFT/MADIX polymerization (MADIX standing for macromolecular design by interchange of xanthates). The effect of the copolymer composition was evaluated, as shown in Scheme 1, both on the growth of gold nanoparticles synthesized in the presence of the polymers (in situ route) or on the properties (stabilization, optical, catalytic...) of preformed nanoparticles coated by these polymers.
Section snippets
Materials
Sodium tetrachloroaurate dihydrate (NaAuCl4∙2H2O), tetrachloroauric acid trihydrate (HAuCl4, 3H2O) and sodium borohydride (NaBH4) were purchased from Aldrich and were used without further purification. 1,4-dioxane (Aldrich, 99%) was passed through basic alumina to remove BHT inhibitor prior to polymerization, N-vinylcaprolactam (VCL, Aldrich, 98%) and N-vinylpyrrolidone (VP, Aldrich, > 99%) were both distilled over CaH2 and stored under argon prior to use. 2,2′-azobisisobutyronitrile (AIBN,
Polymer synthesis and characterization
A series of statistical copolymers of VCL and VP of predetermined Mn (20000 g∙mol−1) and compositions was synthesized by RAFT/MADIX polymerization as previously described,[27] taking advantage of the adequate reactivity of xanthates towards N-vinyl monomers. Their thermal properties in aqueous solution were determined by differential scanning calorimetry (DSC). The main characteristics of the synthesized copolymers are summarized in Table 1 (see Fig. S1-S4).
The cloud point temperatures were
Conclusion
The stabilization of AuNPs based on PVCL enables the access to hybrid thermosensitive materials with modular properties.[24], [25] Though structurally close to PVP, studies on PVCL are much less prevalent. In order to compare the ability of these two families for the formation, stabilization and modulation of the properties of AuNPs, a series of VCL/VP statistical copolymers of controlled molar masses and low dispersities, containing variable amounts of VP (between 0 and 100%), was synthesized
CRediT authorship contribution statement
Fang Yin: Investigation, Formal analysis, Data curation. Hong Hanh Nguyen: Investigation, Formal analysis. Olivier Coutelier: Investigation, Formal analysis. Mathias Destarac: Investigation, Formal analysis. Nancy Lauth-de Viguerie: Project administration, Supervision, Writing - original draft, Writing - review & editing Jean-Daniel Marty: Conceptualization, Project administration, Supervision, Writing - original draft, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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2022, Colloids and Surfaces A: Physicochemical and Engineering AspectsCitation Excerpt :Moreover, even if signals remained visible from 20 °C to 55 °C, the measured intensity dropped down along with temperature. Indeed, upon heating partial dehydration hampered chain mobility, inducing a shift of signals towards lower fields and a loss of the signal intensity [36–39]. The peaks did not completely disappear, probably due to the presence of residual water inside the aggregates.