Synthesis and Characterization of Resorcinarene-Encapsulated Nanoparticles
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by using resorcinol-derived
calixarenes (resorcinarenes4 ) as nanoparticle surfactants (see Figure 1). These macrocycles are appended by several hydrocarbon tails and are well suited for nanoparticle encapsulation. First, the resorcinarene headgroup is rimmed with multiple oxygen atoms for polydentate binding to the nanoparticle surface. Second, the relatively high surface area of the concave headgroup (ca. I nm diameter) decreases the surfactant/particle ratio and consequently the entropic cost of selfassembly.5 Finally, the tailgroups are spaced such that they are highly mobile and do not form tightly packed domains, thereby increasing the solubility of the encapsulated materials. These surfactant properties are demonstrated by stabilizing neutral gold nanoparticles of up to 20 nm in hydrocarbon solutions.
X
/
R =
~}-1 nm
+
_
_
R.R
X
3-20 nm Au nanocluster
1: R=H, X=CH2CH 3 2: R=CH 3, X=CH 2CH 3
3: R=CH 3, X=CH=CH 2 Figure 1. Encapsulation of Au nanoparticles by resorcinarenes 1-3.
59 Mat. Res. Soc. Symp. Proc. Vol. 581 ©2000 Materials Research Society
EXPERIMENT AND RESULTS6 Neutral gold nanoparticles were prepared in the gas phase by condensation of pure metal vapor and were captured by passage through surfactant solutions in mesitylene.' Resorcinarenes 1-4 were synthesized according to literature procedures' and were diluted to 0.1-1.2 mM. Aerosols of gold nanoparticles of controlled size ranges were generated using a distributed arc cluster source (DACS) 9 and bubbled directly into the resorcinarene solutions, which turned deeply purple upon capture. This procedure was noteworthy in at least two respects. First, we noted that the resorcinarenes were several times more efficient at capturing nanoparticles than dodecanethiol (C12SH) at equimolar surfactant concentrations; at 520 nm, the characteristic Mie resonance for isolated gold particles, solutions of 3.7±1.5 nm Au particles captured in 1.2 mM of 1 or 2 absorbed 5-6 times more strongly than solutions of particles captured in 1.2 mM C 12SH. Second, both large (>10 nm) and small (5 nm in diameter were no longer present, correlating the loss of color to the agglomeration and precipitation of the larger particles (see Figures 4b,c). This demonstrates the resorcinareneE' exceptional ability to prevent the larger nanoparticles from aggregating in solution. Although resorcinarenes 1-3 have shown potential for maintaining large Au nanoparticles in a dispersed state, attempts to isolate the encapsulated nanoparticles from solution were unsuccessful. Purification by repeated precipitation or by chromatography resulted in extensive degradation. Excessive dilution of the surfactant also led to the gradual precipitation of the resorcinareneencapsulated particles, suggesting facile chemical exchange. Appropriate modification of the resorcinarene headgroup can increase the robustness of the encapsulation shell, so that chargeneutral nanoparticles can be isolated and characterized as discrete chemical entities. To this end, we have recently synthesized tet
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