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Reversible Phase Transfer of (CdSe/ZnS) Quantum Dots between Organic and Aqueous Solutions
Denis Dorokhin,†,‡ Nikodem Tomczak,†,§ Mingyong Han,§, G. Julius Vancso†,*

David N. Reinhoudt,‡ Aldrik H. Velders,‡ and

Materials Science and Technology of Polymers and, ‡Supramolecular Chemistry and Technology, Faculty of Science and Technology and MESA Institute for Nanotechnology,University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands, §Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 3 Research Link, Singapore 117602, and Division of Bioengineering, National University of Singapore, Singapore 117576

ybrid organic inorganic composite materials find numerous applications in optoelectronics,1,2nanocomposites,3 biotechnology,4 and biodiagnostics.5 For luminescence applications, semiconductor nanocrystals (quantum dots, QDs) are materials with unique optical properties, most notably due to the QD’s size- and composition-tunable luminescence emission, broad absorption and narrow emission lines, and low photobleaching rates.6 Because of these attractive properties QDs are currently applied in fields ofoptoelectronics,7 10 biolabeling,11 15 and sensing.16 18 For application of QDs in biological sensing, or biolabeling, the nanoparticles must be dispersible in aqueous solution and in relevant biological buffers.19 Ligand exchange reactions have been widely used to modify the surface of quantum dots to tune the QD solubility11,20 as well as to provide functionality for further coupling tobiomacromolecules.21 Phase transfer of QDs between solvents with markedly different polarity can be also achieved by coating the QDs with amphiphilic molecules via electrostatic22 hydrophobic/hydrophobic,23,24 or host guest interactions.25 28 For some applications it is desirable to transfer the nanoparticles from the sensing medium and perform analysis in a new medium free from the interfering backgroundmolecules. Therefore there is a need to develop QD materials, which would undergo reversible phase transfer between solvents of markedly different polarity. This might be realized by employing stimulus responsive ligands, which upon external stimuli (temperature, pH, electric field) change their chemistry, and therefore the


ABSTRACT Ttrioctylphosphine oxide(TOPO) stabilized CdSe/ZnS quantum dots (QD) were modified with 6-

ferrocenyl-1-hexanethiol (FcHT) or 11-ferrocenyl-1-undecanethiol (FcUT) via ligand exchange. The presence of ferrocenyl thiol ligands on the surface of the QDs was proven by diffusion ordered NMR spectroscopy. Upon replacement of the initial TOPO ligand with ferrocene derivatives the emission of the QDs decreased. Phase transfer offerrocene-modified QDs from organic solvents into water was achieved by complexation reactions with cyclodextrin ( -CD). The QDs coated with ferrocene thiols are soluble in nonpolar solvents and are transferred into the aqueous phase upon formation of host guest complexes between the ferrocene units and the cavity of -CD. The reversibility of the phase transfer was probed by the addition ofnaphthalene and adamantane derivatives to the aqueous phase containing QD-[Fc-CD] adduct.
KEYWORDS: quantum dots · CdSe/ZnS · ferrocene · cyclodextrin · host guest complexes · solution phase transfer · luminescence quenching

cal properties of the QD surface, or by using ligands able to form supramolecular complexes with other molecules. Nanoscale materials with multiple ferrocene units have attractedattention owing to possible applications of such materials in sensing,17,29 31 catalysis,32,33 or in optoelectronic devices.34 Ferrocene-coated metal32,35 and silica nanoparticles,30,33 dendrimers functionalized at the periphery,31,36,37 and metal-selenide nanoclusters passivated with ferrocenyl ligands38 40 have been reported. However, to the best of our knowledge there are no reports describing...
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