In-situ observation of nanowire growth from luminescent CdTe nanocrystals in a phosphate buffer solution.

One-dimensional (1D) structures of metal and semiconductor materials, often referred to as nanowires, have become a focus of interdisciplinary research in recent years both as a model system for studies of 1D quantum confinement effects and also because of their potential applications in electronics and photonics. A number of direct synthetic methods for the production of nanowires of controlled composition, uniform thickness, and variable aspect ratios has been developed, including the use of hard (alumina membrane) and soft (rodlike micelle) templates, seed-mediated growth in solution, and a catalyst-mediated phase-separation approach. An alternative approach to obtain wires makes use of the assembly of nanoparticles preformed from colloidal suspensions. In a recent paper, Kotov and co-authors showed the feasibility of the spontaneous organization of CdTe nanocrystals (NCs) into crystalline nanowires, during a several-day aging process, which was promoted by the removal of stabilizer ligands through an intermediate methanol washing step. We present here further details about the formation and properties of CdTe nanowires, which were found to grow much faster in a standard physiological phosphate buffered solution. Our results include the in situ observation of the growth process using a confocal microscope. Luminescence and Raman spectroscopy data evidence the occurrence of recrystallization processes during the nanowire formation. CdTe NCs—building blocks for the nanowires—were synthesized in aqueous solution, using thioglycolic acid as stabilizer, following a previously reported approach. Postpreparative size-selective precipitation was applied to the crude solution of NCs to remove the species that had not reacted and separate the strongly emitting fractions of NCs with a narrow size distribution (photoluminescence quantum yield of 20–25%). The selected fractions were dissolved in water, providing stock solutions of CdTe NCs of two different sizes (i.e. , 2.4 nm and 3.5 nm in diameter) with a concentration of 0.05m referred to Te. The CdTe nanowires were obtained by addition of the stock solution of the CdTe NCs to the standard physiological phosphate buffered solution at pH 7.2, followed by incubation at 4 8C in 8-well chambered borosilicate coverslips (Nunc, USA) at ambient conditions for a period of up to 24 h. Incubation at low temperatures was performed to minimize the chances of a possible bacterial contamination, which might be important for potential biomedical applications. However, it was also found that an increase of the incubation temperature to 37 8C did not significantly alter either the dynamics or the pattern of the nanowire formation. As the self-assembled CdTe nanowires retain the luminescence of the NC building blocks, their formation in the chambered coverslips could be followed in situ with a confocal microscope. Different volume ratios of the CdTe NC stock solution and the phosphate buffer were tested, namely, 1:1, 1:5, 1:10, 1:50, 1:100, and 1:1000. Extremely diluted solutions precluded the formation of the nanowires, and highly concentrated solutions did not allow the observation of the wires due to a very strong background luminescence signal. The optimum volume ratio was found to be 1:50, which was used in all further experiments. Figure 1 shows typical confocal images of several nanowires assembled from the red-emitting (3.5-nm size) and green-emitting (2.4-nm size) CdTe NCs. The growth of the nanowires started after 45–60 minutes of incubation; “nucleation patches” emerged (Figure 1a) and continued to grow progressively, with the maximum number and length (up to 70– 80 mm) of observed nanowires being reached after 6–8 h. The nanowires assembled from red-emitting NCs often formed branching structures, bundles, and 3D-resolved networks (Figure 1b–d). Green-emitting NCs, in general, yielded mainly isolated nanowires (Figure 1e), which were less stable in time and more subject to spontaneous aggregation (Figure 1 f). Prolonged incubations (18 h and more) resulted in the formation of dark precipitates, which confirmed the presumed mechanism that the nanowire assembly occurs as a result of a partial destabilization of the CdTe NC ligand shells. At the conditions of our experiments, the destabilization is apparently promoted by the reduction of the electrostatic repulsion of CdTe NCs, due to the electrolytic properties of the phosphate buffer solution. The choice of proper NC concentrations allowed reasonably slow growth rates and, thus, a controllable nanowire formation. Noteworthy, the formation of nanowires was significantly inhibited by the addition of 0.1–3% v/w bovine serum albumin protein, which played the role of an additional capping ligand for the CdTe NCs in solution because of the residual sulfur groups. Once formed in the buffer solution, the nanowires retained their luminescence for at least 10–12 h. This result is in good agreement with our recently reported data on the compatibility of luminescent CdTe NCs with phosphate buffered saline solutions. [a] Dr. A. L. Rogach Department of Physics and CeNS Ludwig-Maximilians-Universit"t M#nchen, 80799 Munich (Germany) Fax: (+49)89-2180-3441 E-mail : [email protected] [b] Dr. N. Gaponik Institute of Physical Chemistry, University of Hamburg 20146 Hamburg (Germany) [c] Dr. Y. P. Rakovich, Prof. J. F. Donegan Physics Department Trinity College Dublin 2 (Ireland) [d] Dr. Y. Volkov, Dr. S. Mitchell, Prof. D. Kelleher Dublin Molecular Medicine Centre and Department of Clinical Medicine Trinity College Dublin 8 (Ireland) Supporting information for this article is available on the WWW under http://www.chemphyschem.org or from the author.