WATER PHASE TRANSFER OF ORGANIC CAPPED CdS NANOCRYSTALS MEDIATED BY CYCLODEXTRINS

Colloidal semiconductor nanocrystals are receiving considerable attention as novel luminescent biological probes due to their unique optical properties,. The chemically synthesized nanoparticles are small in size, very bright and photostable. The advantages with respect to conventional fluorophores consist mainly in narrow band edge emissions, tunable with the nanocrystal size, broad absorption spectra and high solubility in most organic solvents. To be effectively applied in medical and biological applications, nanocrystals must be soluble in water. In this work high quality CdS nanocrystals (NCs) were synthesized in non-coordinating solvents by using oleic acid as capping agents [1]. Colloidal chemistry synthetic route offer a control close to atomic layer resolution over particle size and shape, thanks to the presence of surfactant ligands able to effectively coordinate the nanocrystal surface. The prepared CdS nanocrystals are highly luminescent with tunable emission ranging from UV-blue up to green region of visible spectra [2]. The long hydrocarbon chain of oleic acid is responsible for the nanoparticle hydrophobicity, so the as synthesized CdS NCs are insoluble in water matrices. Different procedures are used to to transfer nanoparticles from organic phase to water [3], recently a new method exploiting Cyclodextrins (CD) host guest chemistry has been reported for Ag and iron oxide clusters [4]. CD are donut-shaped cyclic oligosaccharides consisting of several gluocopyranose units, able to form host-guest complexes by including various molecules in their cavities. We applied this water transfer procedure to blue emitting CdS NCs. Experimental parameters such as CD and/or NC concentration, size and sample aging in water on the optical properties of luminescent nanoparticles were investigated. The obtained results show an effective phase transfer from organic phase to aqueous. The spectra recorded on the aqueous solution show that CDs do not cause any NC aggregation or anyway affect CdS NC absorbance properties, which remain stable in time (Fig. 1). The complexation likely involves the insertion of the apolar moiety of the oleic acid molecule into the CD cavity, while the polar group still co-ordinates the NC surface. The complexation is due to host guest mechanism ascribable to Van der Waals and hydrophobic interactions, although hydrogen bonding and steric effects also play a significant role. Studies are in progress to achieve 3D supramolecular architectures of such nanocrystals applying the host-guest chemistry of CDs[5, 6]: two routes are viable: the former involves the preparation of polyelectrolyte layer assemblies containing modified CD deposited onto proper substrate. The polyelectrolyte-CD layer could be used in this case as a template for organic capped NCs, in order to immobilize them in multiple layers by dipping the polyelectrolyteCD film directly in the NCs organic solution. The latter route aims to exploit the interactions between CD-CdS NC in solution phase with the layer of bare polyelectrolyte layer deposited onto a suitable substrate. Both of these approaches can be very appealing in the preparation of devices devoted to a wide range of application varying from (bio)sensing to optoelectronics.