Article pubs.acs.org/Macromolecules
RAFT-Polymers with Single and Multiple Trithiocarbonate Groups as Uniform Gold-Nanoparticle Coatings Bastian Ebeling and Philipp Vana* Institut für Physikalische Chemie, Georg-August-Universität, Tammannstr. 6, D-37077, Göttingen, Germany S Supporting Information *
ABSTRACT: The nanostructures of nanohybrids with goldnanocrystal cores and shells of N-isopropylacrylamide polymers containing single or multiple trithiocarbonate (TTC) groups were analyzed in detail. The polymers were synthesized in RAFT polymerizations and then grafted to gold nanoparticles (∼14 nm) from citrate reduction. Analysis via TEM revealed self-assembled hexagonal lattices of gold cores. The interparticle distances increased, when polymers with single TTC end groups and growing molar masses were employed as coating agents, allowing for a reference curve to be found. When using polymers containing multiple TTC groups along their backbone for the surface modification, the spacings between the gold cores were unexpectedly low and remained constant, independent of the polymers’ chain lengths and the number of trithiocarbonate groups, which showsbacked up by results from size-exclusion chromatographythat these macromolecules are wrapped around the gold nanoparticles with several gold−TTC junctions. Absolutely no cross-linking was observed for these novel and promising nanohybrids.
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surface15,16 (graf ting-f rom) or the nanoparticles can be coated with preprepared polymers by ligand-exchange (graf ting-to). The synthesis of gold nanoparticles by reduction of the gold salt with sodium citrate in water, often called the Turkevich method,17 is arguably the most often used reaction for producing relatively low-disperse spherical gold nanoparticles with a diameter of typically 10−20 nm. This size regime is ideally suited for plasmonic applications, because here the nanoparticles’ plasmon band is very narrow18 and the specific absorption coefficient is higher than for smaller nanoparticles.19 These nanoparticles are very well suited for the grafting-to approach, because of their highly reactive surface, which is due to electrostatic (remaining positive charge) and not steric stabilization, in contrast to other methods, which yield AuNP with covalently bound long-chain ligands (typically thiols). These ligands often cannot be replaced completely, even when the competitive ligands are added in high excess. After the Turkevich reaction, no other substances except for the sodium citrate are present in the resulting AuNP solution. Drawbacks of the reaction are the high sensitivity of the produced colloid, the restriction to water as the solvent, and the low AuNP content per water volume. The first two problems hereof can be overcome by encapsulation with polymers. Despite the long time that the synthesis by citrate reduction is known, the elucidation of the exact mechanism for the formation of the
INTRODUCTION
Gold nanoparticles (AuNP) have gained great importance in the fields of (bio)sensors,1,2 medical applications,3−6 catalysis, and others, and they are already made use of in many industrial applications (e.g., cosmetics, food packaging, lubricants). When AuNP possess diameters of over approximately 2 nm, their structure becomes crystalline (hence the common term “nanocrystals”)7 and they exhibit strong localized plasmonresonance absorption of visible light,8 leading to intensely redcolored dispersions. To maintain a high colloidal stability, which is a prerequisite for most of the potential applications, and possibly also to equip them with special functional properties at the same time, the nanoparticles’ surface is often modified with organic substances. The stabilization of such coated AuNP is probably primarily of sterical type (gold cores are shielded by the encapsulating substances so that their surfaces can no longer get into contact), but also the role of entropy can be considered (possible conformations of the capping agent in solution are lost in case of aggregation).9 Regarding both effects, polymers are the ideal candidates for capping agents, which is a main reason for the large number of studies10,11 conducted on the synthesis and properties of these core−shell nanohybrids,12 sometimes referred to as “hairy nanoparticles”. Different strategies exist for the synthesis of polymer-encapsulated gold nanoparticles: They can be directly produced in situ by reduction of Au3+ ions during the polymerization13 or in the presence of already produced polymers,14 the polymer can be grown from the nanoparticles’ © XXXX American Chemical Society
Received: April 26, 2013 Revised: May 16, 2013
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dx.doi.org/10.1021/ma4008626 | Macromolecules XXXX, XXX, XXX−XXX
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Figure 1. Binding motifs of TTC-group containing polymers on AuNP: (a) Polymer chains with a single terminal TTC group bind to the AuNP with the functionalized end (R: rests of the of the RAFT agents’ stabilizing groups). For macromolecules with multiple TTC groups, three different binding schemes are in principle possible: (b) The TTC groups bind to different AuNP which are thus interconnected and a network is formed. (c) The polymer chains are wrapped around the AuNP, forming loops linked with multiple sites. (d) The macromolecules couple only with a single TTC group. All illustrations are schematics not drawn to scale.
AuNP is still the topic of research.20−22 While citrate AuNP seem to be toxic,23 coverage with polymers reduces the nanoparticles’ toxicity.24 Reversible addition−fragmentation chain transfer (RAFT) polymerization25−28 is a reversible-deactivation radical polymerization technique which is well suited for producing polymers for grafting to gold surfaces.29,30 All common RAFT agent groups can be relatively easily converted into thiolsthe most prominent group for linkage to gold surfaces.31,32 This transformation is not even necessary in most cases because other sulfur-containing functional groups, such as the trithiocarbonate (TTC) group which is treated in this publication, chemisorb onto gold surfaces themselves33 and polymers with TTC groups have already been applied to cover gold surfaces34−39 and appear to be able to create even denser polymer layers on the surface than thiol-capped polymers.40 When the TTC group is in the middle of the polymer chain, Vshaped polymer brushes are formed by the adsorption.41,42 Polymers with multiple TTC groups incorporated along their main chain are formed when specially designed polytrithiocarbonates are employed as RAFT agents in a radical polymerization. (To be consistent with our other publications on this topic,43,44 the term “block” will be used to describe the homogeneous chain segments which are interconnected by TTC groups in the polymer chain throughout this article.) We have already examined this type of polymerization and the ongoing mechanism in detail elsewhere44 and could deduce the ideal distributions of blocks among the resulting macromolecules analytically.43 We showed that this method yields more homogeneous products compared with the conventional polycondensation reaction of end-group functionalized prepolymers, which would in this case be for example the reaction of polymers with terminal thiol groups and carbon disulfide. We conducted the here presented study in order to get more insight into the grafting of TTC-group containing polymers of different architectures to citrate AuNP. As polymer, poly-Nisopropylacrylamide (PNIPAm) was employed, which is watersoluble and can thus be brought into direct contact with the AuNP. It has already been used in several studies involving AuNP, in most cases because of its property to exhibit a lower critical solution temperature in aqueous solution. For example, it has been shown that citrate AuNP functionalized with PNIPAm undergo a color change when heated over a certain temperature and salt is added to the solution.45 First, we used a
series of structurally equivalent PNIPAm samples with a single TTC group at the terminal chain end, but with different chain lengths, in order to systematically evaluate their influence on the functionalization process, which, to the best of our knowledge, has not been studied yet. (These samples will be referred to as “conventional polymers/nanohybrids” in the following.) Then, we studied the behavior of PNIPAm samples with multiple TTC groups along the polymer backbone in the same functionalization reaction and explored the resulting nanohybrids. (These samples will be denoted “multiblock polymers/nanohybrids”.) We were especially intrigued by the question in which way these multiblock polymers and the AuNP would connect to each other. Figure 1 shows the different binding motifs which could in principle occur. While for the conventional nanohybrids only one binding motif is expected (Figure 1a), for the multiblock nanohybrids three reasonable binding schemes exist: The multiblock polymers could connect with their binding groups to different AuNP, forming a network (Figure 1b), they could be wrapped around the AuNP, linked to the same particle at multiple sites and forming loops on the nanoparticles’ surface (Figure 1c), or they could bind to the AuNP with only a single TTC moiety, leaving the rest unreacted (Figure 1d). Du et al.46,47 used TTC-group containing multiblock copolymers as capping agents in the synthesis of AuNP of lower sizes than those of citrate particles and did not observe interconnection of the thus synthesized AuNP, but it was not possible to elucidate if loops on the surfaces had formed. At the same time, it is known that AuNP are fused into a covalently bonded network by α−ω-dithiols with short alkyl chains.48 In the following, we will show that we were able to reveal the true binding motif, performing systematic analyses via transmission electron microscopy (TEM) and size-exclusion chromatography (SEC): Our findings indicate that the multiblock polymers are wrapped around the AuNP as shown in Figure 1c after being grafted to the citrate particles. The resulting materials constitute a novel class of nanocomposites of very high stability having well-defined and reactive surfaces, which may be able to undergo further RAFT polymerizations or to be otherwise functionalized.
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EXPERIMENTAL PART
Materials. N-Isopropylacrylamide (Aldrich, 97%) and 2,2′-azobis(isobutyronitrile) (AIBN, Akzo Nobel, 98%) were recrystallized twice B
dx.doi.org/10.1021/ma4008626 | Macromolecules XXXX, XXX, XXX−XXX
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from toluene/hexane (3:1) and methanol, respectively, and dried under high vacuum before use. 4,4′-Azobis(4-cyanovaleric acid) (ACVA, Aldrich, ≥98.0%), gold(III) chloride trihydrate (ABCR, >99.9%), sodium citrate tribasic dihydrate (Aldrich, ≥99%), octadecylamine (ODA, Aldrich, ≥99.0%), 2-(dodecylthiocarbonothioylthio)-2methylpropanoic acid (1, Aldrich, 98%), dimethylformamide (DMF, Acros Organics, 99.8%, extra dry), N,N-dimethylacetamide (DMAc, Sigma-Aldrich, ≥99.9%), and lithium bromide (Sigma-Aldrich, ≥99.9%) were used without further purification. Throughout all experiments, ultrapure (type I) water (resistivity 18.2 MΩ cm at 25 °C, total organic carbon
