Covalent Linking of CdTe Nanocrystals to Amino‐Functionalized Surfaces (2005)

CdTe-modified silicon wafers: Preliminarily aminated substrates of various compositions and shapes are linked with water-soluble, high-luminescent CdTe nanocrystals through the formation of an amide bond between the carboxylic groups of mercaptoacid-capped nanocrystals and the amino groups of the substrates (see picture). The carbodiimide chemistry has proven to be a useful tool in organic chemistry for producing chemical bonds under mild conditions.1 It allows the formation of amide and ester bonds between carboxylic and amino or hydroxylic groups even at room temperature in a standard buffer medium. This procedure has already been successfully used for the preparation of nanocrystal (NC) conjugates with bioactive molecules2–6 as well as for the coupling of semiconductors7 and gold8–9 NCs. To apply this chemistry to the fabrication of conjugates of CdTe NCs with inorganic substrates, the surface chemistry of the substrates has to be adjusted. Several methods for the preparation of layered structures involving semiconductor NCs are known. Probably the most important technique is the layer-by-layer (LbL) assembly,10 in which alternatingly charged polyelectrolytes and NC layers are deposited making use of electrostatic interactions. Other possibilities include the modification of the surface in order to have specific chemical groups available for a chemical reaction. Among those, a well-known technique for the modification of a surface of an inorganic substrate is the treatment with functionalized silanes, thus, a “silanization”. This method was already used for the preparation of Au nanoparticle arrays on indium–tin oxide (ITO), glass and SiO2 surfaces,11–12 for the preparation of thiol-13 and hydrocarbon-14 functionalized glass spheres and for the direct electrostatic adsorption of CdSe NCs.15 Herein, we propose to combine and alter the above-mentioned methods in order to generate CdTe-NC-functionalized substrates. Carbodiimide compounds are used for the formation of an amide bond between the amino groups of the substrate surface (i.e., of “aminated” surfaces) and the carboxylic groups of the stabilizers of the CdTe NCs. The changing of the surface chemistry is performed by pretreating the substrate surface with amino-functionalized silanes. The CdTe nanocrystals were prepared by wet chemical means utilizing thiols as the size-regulating and stabilizing agents16–17 Briefly, freshly prepared H2Te gas was introduced to a deaerated aqueous solution containing Cd(ClO4)2 and 3-mercaptopropionic acid. During the reflux of this solution the CdTe nanocrystals grow and fractions emitting from green [approximately one hour of synthesis, size about 2–3 nm as determined by transmission electron microscopy (TEM), powder X-ray diffraction (p-XRD), and optical spectroscopy16] to the near-infrared (approximately seven days of synthesis, about 6–8 nm) are obtained. The postpreparative procedure of size-selective precipitation16 was applied to the crude solutions to isolate fractions with narrower size distributions of the NCs. The isolated fractions were redissolved in water at concentrations similar to those of the crude solutions (evaluated by comparison of the absorption at the first absorption maxima). Subsequently, appropriate fractions of CdTe NCs were used for the conjugation with various substrates such as glass, silica or silicon with different shapes of the surfaces. A general illustration of the binding of the NCs to aminated surfaces with the aid of carbodiimide as a mediator in the formation of the amide bond is shown in Figure 1. General scheme of the substrate amination and coupling chemistry (EDC/NHS=1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide hydrochloride and N-hydroxysuccinimide). The silicon wafers, the quartz and the glass slides were pretreated before conjugation in the following manner: the glass (Menzel–Glaser, Germany) and the quartz slides (Hellma Optic GmbH, Germany) as well as the silicon wafers (with a 400-nm SiO2 layer) were cleaned by sonication first in 0.1 M NaOH solution, then in methanol, and finally in pure water for 15 min each. To form an amino-terminated layer on their surface, the substrates were immersed in a 3 vol % solution of 3-(aminopropyl)-triethoxysilane (Fluka) in 95 % methanol (named below “3 APTES solution”) under sonication for 15 min followed by thorough rinsing with methanol. Drying was performed by a thermal treatment at 120 °C for half an hour in an oven. The details of the linking of the CdTe NCs to flat substrates are as follows. An appropriate amount (0.1–0.5 mL) of aqueous solutions of CdTe NCs was mixed with 0.5 mL of MES buffer (2-(N-morpholino)ethane sulfonic acid 0.1 mM, Sigma) and 8 mL of Milli-Q water. The preliminarily cleaned aminated substrate was dipped into this solution. Subsequently, 0.5 mL of a solution of 1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide hydrochloride (Sigma) 0.025 M and N-hydroxysuccinimide (Aldrich) 0.025 M (named below “EDC/NHS solution”) was added under vigorous stirring, which was continued for another 10 min. After conjugation, the slides were washed thrice in Milli-Q water and rinsed thoroughly in acetone and toluene and dried under vacuum conditions. Figure S1 (Supporting Information) shows optical spectra of the modified slides. The photoluminescence (PL) properties of the films prepared were found to be more stable in time as compared to analogously LbL-assembled monolayer films. Although the surface coverage estimated from the optical densities of the modified glass slides is roughly 100 % (i.e., one monolayer), the formation of islandlike multilayers could not be excluded.18 The method described is not limited to flat substrates but is of more generality as seen by our successful attempts to coat glass spheres of micron size by CdTe NCs. A dispersion (5 wt % in a 10 % solution of ethanol in water) of borosilicate glass microspheres (2±0.5 μm, Duke Sci. Co., USA) was treated with 0.1 M NaOH for 15 min under sonication and washed thrice in methanol by centrifugation. The treated microspheres were immersed in the 3 APTES solution under sonication for 15 min and washed thoroughly with methanol by centrifugation. The final methanol dispersion was heated up to the boiling point for one hour. After that, the microspheres were separated from the methanol, redispersed in 0.5 mL of MES buffer and subsequently 0.1 mL of an aqueous solution of CdTe NCs and 0.5 mL of the EDC/NHS solution were added to the dispersion under vigorous shaking, which was continued for another 10 min. The conjugation was terminated by the addition of an excess amount of 0.1 M glycine solution. The spheres were washed thrice with Milli-Q water and stored in an appropriate solvent such as water or dimethylformamide (DMF). In Figure 2 we display a fluorescence microscopy image of the glass spheres conjugated with the CdTe NCs. The persistency of the luminescence of the NCs is nicely seen. This luminescence is not altered for at least three months. The inset shows a scanning electron microscopy (SEM) image of one particle of the same sample providing an impression of the coverage of the spheres with nanoparticles. Fluorescence microscopy images of glass spheres conjugated with CdTe NCs. The inset shows a SEM image of one of those particles. Figure 3 shows some optical properties of conjugates of CdTe NCs with silica beads.19 For the generation of these conjugates we used CdTe NCs of two different sizes, namely 3–4 nm in diameter and 6–8 nm in diameter, respectively. The absorption spectra of the conjugates posses well-pronounced first electronic transitions and strong emissions. The position of the absorption and luminescence maxima relate well to the sizes of the NCs, in accordance with the size–quantization effect being operative in semiconductor nanoparticles of such sizes. It is mentioned that the larger particles exhibit an emission at around 800 nm which, thus, occurs at lower energies than all CdTe-NC-emissions reported before, opening the path to the near-infrared spectral region hardly accessible with other materials. Another peculiarity of the spectra shown in Figure 3 is the fact that we have managed to receive absorption spectra from monolayers of NCs,20 also an unprecedented result. Absorption and luminescence spectra of silica particles conjugated with CdTe NCs of two different sizes. For the preparation of the silica beads we used a well-known literature method.21 Tetraethyl orthosilicate (1.5 mL) was added to a solution of ammonium hydroxide (2 mL) in ethanol (50 mL) to prepare particles at around 100 nm. The reaction mixture was stirred for 24 hours to yield silica nanoparticles. The amino functionalization of the particles' surface was accomplished by the addition of 25 μL of 3 APTES to the reaction mixture and continuing stirring for an additional 24 hours. After this, the mixture was heated up to the boiling point for one hour. The cooled mixture was washed thrice with ethanol by centrifugation to give stable dispersion of amino-functionalized silica spheres. Before conjugation, the silica particles were transferred into DMF by consecutive centrifugation and adding of fresh portions of DMF. The conjugates of the CdTe NCs with the aminated silica beads have been prepared as follows. 0.05 mL of aqueous solutions of the CdTe NCs and 0.25 mL of the dispersion of the aminated silica particles in DMF were added to 0.70 mL of DMF. Then, 0.5 mL of the EDC/NHS solution (in DMF) was added at vigorous shaking. Shaking was continued for another 5–20 min. The conjugation was stopped by centrifugation of the silica particles. The conjugated silica particles were washed thrice with pure DMF and stored as dispersions in the dark. In summary, a new method for modifying substrate surfaces has been developed: preliminarily aminated substrates of various compositions and shapes are linked with water-soluble, high-luminescent CdTe nanocrystals through the formation of an amide bond between the carboxylic groups of mercaptoacid-capped nanocrystals and the amino groups of the substrates. We acknowledge Andreas Kornowski and Uwe Borchert for taking the SEM and the fluorescence microscopy images, respectively. This work was supported by the German Science Foundation priority program (DFG SPP 1072). Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2267/2005/z400516_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.