Synthesis and Spectral Studies of CdTe–Dendrimer Conjugates
© to the authors 2009
Received: 1 April 2009
Accepted: 5 May 2009
Published: 22 May 2009
In order to couple high cellular uptake and target specificity of dendrimer molecule with excellent optical properties of semiconductor nanoparticles, the interaction of cysteine-capped CdTe quantum dots with dendrimer was investigated through spectroscopic techniques. NH2-terminated dendrimer molecule quenched the photoluminescence of CdTe quantum dots. The binding constants and binding capacity were calculated, and the nature of binding was found to be noncovalent. Significant decrease in luminescence intensity of CdTe quantum dots owing to noncovalent binding with dendrimer limits further utilization of these nanoassemblies. Hence, an attempt is made, for the first time, to synthesize stable, highly luminescent, covalently linked CdTe–Dendrimer conjugate in aqueous medium using glutaric dialdehyde (G) linker. Conjugate has been characterized through Fourier transform infrared spectroscopy and transmission electron microscopy. In this strategy, photoluminescence quantum efficiency of CdTe quantum dots with narrow emission bandwidths remained unaffected after formation of the conjugate.
Rapid advances in nanotechnology and nanoscience have spurred interests in developing a variety of nanostructured materials. In this context, semiconductor nanoparticles (also known as quantum dots) are the most promising ones due to their high photochemical stability and size-tunable photoluminescence . Quantum dots (QDs), in particular, have potential applications in optoelectronics, biosensing and biolabelling, etc. [2, 3] Recently, integration of these QDs with biological macromolecules greatly expands the impact of optical imaging, sensing and also of therapeutic strategies [4, 5]. The binding of different categories of molecules to QDs has been studied by optical methods to elucidate binding mechanism, because the surface of nanoparticles (NPs) affects the electronic states, which, in turn, influence the photoluminescence (PL) emission of QDs . In our earlier reports, the binding of amino acids, DNA bases, biological relevant metal ions, enzyme and peroxynitrite (PN), a powerful biological oxidant with semiconductor QDs was investigated, and a quantitative correlation was established therein [7–11]. However, the binding of dendrimer with QDs is of special interest as immobilization of semiconductor nanocrystal onto dendrimer has great implication in the field of material science in view of amalgamation of excellent luminescent properties of semiconductors with varied functionalities of dendrimer molecules. PAMAM Dendrimers are synthetic spherical macromolecules with a well-defined surface, comprising a core, branching sites and a large number of terminal groups . The biomimetic properties and low cytotoxicity of dendrimer molecules make them potentially useful for many biological applications such as gene transfection, diagnostics, drug delivery as well as nanoscale building blocks [13, 14]. The molecules are small enough to pass into the cell membrane and can be used to deliver substances such as drugs, genetic materials or chemical markers right into the cells . Thus, CdTe–Dendrimer conjugates can act as new luminescent multi-functional nanostructured materials. To achieve binding specificity and targeting ability, QDs can be linked to monoclonal antibodies, peptides, oligonucleotides, small inhibitor or hydrophilic segment (such as polyethylene glycol [PEG]). It is expected that the self-assembly of the dendrimer molecules onto NPs provide a route to modifying the NPs for targeted imaging of cancer cells . In general, assembly process of NPs largely relies on noncovalent interactions. However, the drawback is the inherent instability of these conjugates under varied environmental parameters, such as low pH, higher temperature, ionic strength, etc. Thus, an alternative pragmatic approach to preparing QD assemblies is interfacing through covalent binding [8, 17, 18]. This method of functionalizing QDs is simple and can avoid complicated synthesis and characterization of the intermediate products of QDs, when reactions are performed on the QD surfaces. A number of reports have been published in which biological molecules have been attached onto the surface of QDs. However, optimization of conjugation process is a prerequisite for the use of QDs in biomedical applications.
In the present work, the interactions between CdTe QDs and dendrimers of different generations have been studied fluorimetrically. The binding constantsKSVandKband binding stoichiometry of the complex (n) have been determined. Significant differences on the values of binding constants for dendrimers of different generation suggest that generation has strong influence upon the interaction between CdTe QDs and dendrimer. We, further, demonstrate a simple method of the preparation of CdTe–Dendrimer conjugates through covalent binding using Glutaric dialdehyde (G) linker and describe their optical properties. Semiconductor–dendrimer conjugate, thus synthesized, represents a hybrid material in which fluorescence of semiconductors convolutes with the biomimetic properties of dendrimer, which is ideally suited for various biomedical applications such as fluorescence imaging and probing of biological systems.
L-Cysteine hydrochlorides and cadmium nitrate tetrahydrate were purchased from Merck, Germany. The starburst dendrimers (PAMAM) of generation 2.0, 4.0, 5.0 (NH2terminated) and glutaraldehyde were obtained from Sigma Aldrich, Germany. Telluric acid (H2TeO4, 2H2O) and sodium borohydrate (NaBH4) were purchased from BDH.
Synthesis of CdTe Quantum Dots and CdTe–Dendrimer Conjugate
CdTe QDs were synthesized following our method as reported earlier [19, 20]. An aqueous solution of Cd2+ ion (4.68 × 10−2 M) and L-cysteine (11.70 × 10−2 M) was prepared, and pH was adjusted to 11.2–11.8 (using Jenway 3345 ion meter). Then, NaHTe solution was added under nitrogen atmosphere. The resultant solution was refluxed at 100 °C. The as-prepared CdTe QD solutions were further diluted 10 times, and concentrations were so chosen that absorbance was kept <0.1 to avoid self-absorbance effects. The aqueous solution of 3.47 × 10−7 M PAMAM dendrimer of Generation 2, 4 or 5 was prepared, and few microlitres (10–50 μL) of these solutions were added to the different sets of 3 mL solution of CdTe QDs. In a typical synthesis of CdTe–Dendrimer conjugate, 3 mL aqueous solution of CdTe NPs (5 × 10−6 M) was mixed with 50 μL of glutaraldehyde (10% in water) and 1 mL dendrimer (7.0 × 10−6 M). The solution was allowed to react at room temperature for 24 h and dialyzed against the phosphate buffer.
UV-Vis absorption and photoluminescence (PL) spectra of nanoparticles solution were recorded on Shimadzu UV-1601PC and Perkin Elmer LS-55 luminescence spectrometer respectively. Size distribution of CdTe QDs was determined by dynamic light scattering spectrophotometer (Model DLS—nanoZS, Zetasizer, Nanoseries, Malvern Instruments). Samples were filtered several times through a 0.22-μm Millipore membrane filter prior to recording measurements. The Fourier transform infrared spectroscopy (FTIR) spectra were recorded with Perkin Elmer, Spectrum GX equipment using KBr pellet with a resolution of 2 cm−1. Transmission electron microscopy (TEM) was carried out on JEOL-2010 with acceleration voltage of 200 kV. A drop of as-prepared solution of CdTe–Dendrimer conjugate was placed on a carbon-coated copper grid of 300 meshes and dried before putting it on to the TEM sample chamber.
Results and Discussion
The as-synthesized CdTe QDs were stable and highly luminescent . The FWHM (full-width at half maximum) of the PL spectrum is 36 nm, which suggests narrow size distribution. The average size of CdTe QDs was found to be 3.2 nm as determined from TEM images, which is in good agreement with the size (3.0 nm) determined from the correlation of particle size and optical band gap. Further, average size and size distribution of CdTe QDs obtained from UV spectra and TEM image were also supported by DLS histogram (dav = 3.4 nm with narrow size distribution).
Binding constants and binding capacity of dendrimer with CdTe QDs
Binding capacity (n)
Effect of solvent polarity on Stern-Volmer (Ksv) constant
Ksv(Dendrimer G5.0) 107 M−1
In summary, binding constants (KsvandKb) and binding capacity (n) of dendrimer to CdTe QDs have been determined fluorimetrically on the basis of noncovalent interaction. Further, dendrimer molecules are attached to the QDs through covalent binding using Glutaraldehyde. The successful assembly of QDs and dendrimer with desired functionality has significant implications in material research and demands most extensive inquiries into the luminescent electronic and chemical properties of these unique building blocks as they are incorporated into new and functional nanostuctured materials. This approach may also be applicable for conjugation with other semiconductor nanoparticles. The combination of the spectroscopic characteristics of the nanocrytal with biomolecular function of dendrimer molecule can potentially make high impact on current biomedical technologies and possibly in nanoelectronics, microphotonics and related fields.
Transmission electron microscopy was carried out at Electron Microscopy Facility in Saha Institute of Nuclear Physics, Kolkata.
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