Phase Transfer of CdS Nanocrystals Mediated by Heptamine β

Article pubs.acs.org/Langmuir

Phase Transfer of CdS Nanocrystals Mediated by Heptamine β‑Cyclodextrin Nicoletta Depalo,*,†,‡ Roberto Comparelli,† Jurriaan Huskens,§ Manon J. W. Ludden,§ Andras Perl,§ Angela Agostiano,†,‡ Marinella Striccoli,† and M. Lucia Curri† †

IPCF-CNR Bari division, Via Orabona 4, Bari, 70126, Italy Department of Chemistry, University of Bari, Via Orabona 4, Bari, 70126, Italy § Molecular NanoFabrication group, MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands ‡

S Supporting Information *

ABSTRACT: A fundamental and systematic study on the fabrication of a supramolecularly assembled nanostructure of an organic ligand-capped CdS nanocrystal (NC) and multiple heptamine β-cyclodextrin ((NH2)7βCD) molecules in aqueous solution has been here reported. The functionalization process of presynthesized hydrophobic CdS NCs by means of (NH2)7βCD has been extensively investigated by using different spectroscopic and structural techniques, as a function of different experimental parameters, such as the composition and the concentration of CD, the concentration of CdS NCs, the nature of the NC surface capping ligand (oleic acid and octylamine), and the organic solvent. The formation of a complex based on the direct coordination of the (NH2)7βCD amine groups at the NC surface has been demonstrated and found responsible for the CdS NC phase transfer process. The amine functional group in (NH2)7βCD and the appropriate combination of pristine capping agent coordinating the NC surface and a suitable solvent have been found decisive for the success of the CdS NC phase transfer process. Furthermore, a layer-by-layer assembly experiment has indicated that the obtained (NH2)7βCD functionalized CdS NCs are still able to perform the host−guest chemistry. Thus, they offer a model of a nanoparticle-based material with molecular receptors, useful for bio applications.

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INTRODUCTION Supramolecular architectures including inorganic nanocrystals (NCs) have attracted increasing attention due to the unique size-dependent opto-electronic and chemical properties, as well as their potential applications in biological labeling, molecular recognition, magnetic recording, and electronic devices.1−4 In this perspective, fabrication of functional building blocks based on NCs has been extensively explored by the convenient use of cyclodextrins (CDs), that are cyclic oligosaccharides, which have been demonstrated able not only to complex a variety of small hydrophobic molecules in aqueous solution, but also to perform the enantioselective analysis of chiral compounds, with the possibilities of internal selectivity (i.e., inclusion-type) or external selectivity (due to functional groups on the hydrophilic rims). In particular, CDs and their derivatives have been increasingly employed to synthesize metal and semiconductor nanoparticles or to induce their surface modification or/and their phase transfer into aqueous solution, resulting in supramolecularly assembled nanostructures useful for biological and sensing applications.5−19 For example, the surface of citrate-stabilized gold NPs has been functionalized with SH-CD molecules, poly(ethylene glycol) (PEG) and a targeting © 2012 American Chemical Society

antibody, obtaining a new type of CD-coated gold nanoparticle carrier for targeted delivery of an anticancer drug.11 CDmodified luminescent semiconductor NCs (CdSe, CdSe@ZnS, and ZnO@MgO core−shell systems) have been proposed as a versatile chiroselective sensing platform of different amino acids or as fluorescent probes for the optical detection of pollutant polycyclic aromatic hydrocarbons, phenols, and inorganic anions and cations or as thermometers in aqueous solutions.12−17 Novel superparamagnetic nanocomposites have been variously produced in the presence of CDs and suggested as potential candidate drug carriers and bioseparators for biomedical applications.18,19 In addition, a variety of NC 2D/3D assemblies arranged onto suitably functionalized surfaces by host−guest molecular interactions have been extensively explored to build up hierarchical superstructures, useful for their integration into nanodevices.11,13,20−26 Received: February 21, 2012 Revised: May 14, 2012 Published: May 17, 2012 8711

dx.doi.org/10.1021/la3007469 | Langmuir 2012, 28, 8711−8720

Langmuir

Article

Here, a supramolecular assembly formed of modified βCD, namely, heptamine β-CD ((NH2)7βCD), and organic ligandcapped cadmium sulfide (CdS) NCs in aqueous solution is reported. In particular, this work aims at providing a systematic study of the mutual role played by specific experimental parameters in the phase transfer, from organic to aqueous medium, of the hydrophobic CdS NC by using (NH2)7βCD. A deep elucidation of the most favorable conditions, such as the type of CD (αCD, βCD, and (NH2)7βCD), as well as the dispersing organic solvent and surface ligand of CdS NCs, to achieve a successful phase transfer procedure, is presented. The dependence of the process on the NC and CD concentration has been also investigated. Complementary optical (UV−vis, PL, FT-IR, DLS, SPR) and structural (TEM) techniques have been used to investigate the mechanism of NC functionalization by (NH2)7βCD. For the first time, to the best of our knowledge, a relevant indication of the interaction between key parameters involved in the phase transfer performance is proposed. In particular, the careful selection of the suitable organic solvent and capping agent, which affect the NC−NC interaction in solution, has been demonstrated to result in a different mechanism according to the specific, (NH2)7βCD or αCD, NC surface modification. The overall results suggest that the obtained structures are formed of CdS NCs decorated by (NH2)7βCD molecules, with NH2- groups coordinating the NC surface, and, consequently, the CD cavity is left empty and available to guest molecules. The final structures are shown to effectively perform host− guest chemistry, and thus represent a potential functionalized building block, to use, either in solution or deposited onto a substrate, for molecular recognition or chiral separation of small biologically relevant molecules.

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hexane). The CdS NC concentration and diameter values were evaluated by using absorbance spectra and calculating the extinction coefficient as reported in the literature.33 Nanocrystal Ligand Exchange Procedure. A ligand exchange procedure was carried out in order to replace the pristine OLEA molecules coordinating the CdS NC surface with OCTA.31 After the extraction/precipitation procedure, the recovered OLEA capped CdS NCs were dispersed in the minimum amount of CHCl3 and subsequently mixed with a large excess (5:1) of previously degassed OCTA under N2 atmosphere. The new ligand molecules (OCTA) can replace the original ones (OLEA) in a mass action driven process. CHCl3 was allowed to evaporate under vacuum and the reaction mixture was stirred and heated at T = 60 °C for 24 h under N2. Finally, the NCs were precipitated by adding a nonsolvent (CH3OH), recovered by centrifugation, and finally redispersed in organic solvent (CHCl3 or hexane). Phase Transfer of Organic-Capped CdS Nanocrystals from Organic Solvent to Aqueous Solution. Three different types of CDs were tested in order to achieve an effective functionalization of the organic-capped CdS NCs and to transfer them in aqueous solution (αCD, βCD, and (NH2)7βCD). CD aqueous solutions were prepared, without any pH adjustment. Two distinct sets of experiments were performed by using both OLEA and OCTA capped CdS NCs, and varying the following parameters: (i) NC concentration (5 × 10−7 and 10−6 M), (ii) organic dispersing solvent (chloroform and hexane), and (iii) CD concentration (10−4 and 10−2 M). The overall tested experimental conditions are summarized in Table S1 (Supporting Information). The CdS NC concentration values refer to the concentration in the organic solvent, while the CD to the concentration in water, both calculated before phase transfer. The CdS NC phase transfer from organic to aqueous dispersing solvent was carried out according to the experimental procedure which we have previously investigated and successfully performed on the system formed of OLEA capped CdS NCs dispersed in hexane (10−6 and 5 × 10−7 M) and αCD water solution (10−2 M).20,34 Briefly, a mixture of organic-capped CdS NCs in organic solvent mixed with an equal volume of CD aqueous solution was vigorously stirred at room temperature. After stirring for 20 h, the organic top phase was separated from the aqueous one on the bottom. The aqueous phase was then centrifuged at 3000 rpm for 10 min, to obtain an optically clear NC suspension. The evaporation of organic solvent in aqueous solution after CdS NC extraction was achieved by keeping the samples under N2 stream for 10 min. The phase transfer efficiency of OCTA capped CdS NCs by (NH2)7βCD) molecules was evaluated by estimating three indirect parameters: depletion percentage, water-soluble fraction, and insoluble fraction (1, 2, 3). The data were obtained by the values of absorbance (A) at the wavelength corresponding to the first allowed electronic transition of CdS NCs in the absorption spectra according to the following equations:

EXPERIMENTAL SECTION

Materials. All chemicals were purchased with the highest purity available and used as received without further purification or distillation. Cadmium oxide (CdO, powder 99.5%), sulfur (S, powder 99.9999%), oleic acid (OLEA, technical grade 90%), octadecene (ODE, technical grade 90%), and octylamine (OCTA, technical grade 90%) were obtained from Aldrich. β-cyclodextrin (βCD) and αcyclodextrin (≥98%, αCD) were purchased from Fluka. Adamantylterminated poly(propylene imine) (PPI) dendrimer (with 16 adamantyl groups, G3-PPI-(Ad)16),27 βCD heptathioether,28 and heptamine β-cyclodextrin ((NH2)7βCD)29 were synthesized according to previously reported procedures. 2,2″([1,1′-Biphenyl]-4.4′-diyldi-2,1ethenediyl)bis-benzenesulfonic acid disodium salt (Stilbene 3) was purchased from Lambda Physik. All solvents used were of analytical grade and purchased from Aldrich. The CD aqueous solutions were prepared by using water obtained by Milli-Q Gradient A-10 system (Millipore, 18.2 MΩcm, organic carbon content ≤4 μg/L) and filtered by 0.45 μm nylon membrane filters (Whatman). Disposable dialysers tubes MWCO 3500 were supplied by Aldrich. Synthesis of Oleic Acid Capped CdS NCs. Organic ligandcapped CdS NCs were synthesized by using the method reported elsewhere.30−32 Briefly, a mixture of CdO, OLEA, and ODE was continuously stirred and heated at 300 °C under ambient condition. An ODE solution of elemental sulfur was swiftly injected and the growth allowed to proceed at 250 °C. CdS NCs having different average size were obtained varying the molar ratio between the precursors and the reaction time. In particular, NCs with mean diameters of 3.0 and 4.0 nm were synthesized starting with Cd:S molar ratios 2.5:1 and 5:1, respectively. The reaction time was 10 min for both tested sizes. The CdS NCs obtained were extracted from the reaction mixture by adding a chloroform/methanol mixture (CHCl3/ CH3OH = 1:1 V/V) followed by precipitation by adding ethanol. The obtained yellow powder was redispersed in organic solvent (CHCl3 or

Depletion Percentage (%) =

A CHCl3(t = 0h) − A CHCl3(t = 20h)

Water‐Soluble Fraction (%) =

A CHCl3(t = 0h)

AH2O(t = 20h) A CHCl3(t = 0h)

(1)

× 100 (2)

Insoluble Fraction (%) = Depletion Percentage − Water Soluable Fraction

(3)

All reported data are presented as mean values ± standard deviation obtained from five replicates. Monolayer Preparation for SPR Measurements. Gold substrates for SPR (BK7 glass/2−4 nm Ti/50 nm Au) were obtained from SSens B.V., Hengelo, The Netherlands. Gold substrates were cleaned by dipping them in ″piranha″ solution (conc. H2SO4 and 30% H2O2 in a 3:1 ratio: CAUTION! Piranha solution reacts violently) for 5 s. After thorough rinsing with Milli-Q water, they were placed for 10 min in absolute ethanol in order to remove the oxide layer. 8712

dx.doi.org/10.1021/la3007469 | Langmuir 2012, 28, 8711−8720

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Article

Multilayer Formation. Experiments were performed as described before for multilayers of dendrimers and CD gold nanoparticles.26 The CD self-assembled substrates were immersed into the solution of the adamantyl dendrimer for 10 min, followed by rinsing with water. The films were then immersed in the NC solution, followed by rinsing with water. A multilayer structure was formed by repeating both adsorption steps (Figure 8).

Subsequently, the substrates were left in a freshly prepared solution of βCD heptathioether (0.1 mM) for 16 h at 60 °C. The samples were subsequently rinsed three times with CHCl3, ethanol, and Milli-Q water. UV−vis and Emission Spectroscopic Investigation. The phase transfer of CdS NCs from organic to aqueous phase was monitored by means of UV−vis absorption spectroscopy, by using a Varian Cary 5 spectrophotometer. The photoluminescence (PL) spectra were recorded by using the Eclypse Spectrofluorimeter (Varian). The relative PL quantum yields of NCs in solution were estimated by using stilbene 3 as dye reference, and comparing the integrated PL intensity of the NCs and the dye, both recorded exciting solutions of the same absorbance (