DNA Functionalized Quantum Dot Synthesis The cadmium telluride (CdTe)/cadmium sulfide (CdS) core/shell quantum dots (QDs) were synthesized in a two-part procedure [1]. First, the core nanoparticles were produced. Briefly, freshly prepared sodium hydrogen telluride (NaHTe) was reacted with cadmium nitrate (Cd(NO3)2) in the presence of 3-mercaptopropionic acid (MPA) overnight. MPA in this reaction was used to control the QD growth and also served as a capping agent for the nanoparticles. For a detailed description of the experimental procedure, please refer to the Lab Book section of this website. The synthesis yielded CdTe cores with an emission maximum at 480 nm, which corresponds to the emission profile reported in the literature (Figure 1). ﷯ Figure 1. Emission spectrum of the synthesized cores. In the next step, a CdS shell was grown around the CdTe cores as lysozyme specific aptamers were incorporated into these CdS shells. Briefly, CdTe cores prepared in the first step were incubated in the presence of cadmium chloride (CdCl2) (source of cadmium), MPA (source of sulfur), and lysozyme specific aptamers in an oven at 90 degrees Celsius for different times allowing the growth of the shells of different thickness. For a detailed description of the experimental procedure, please refer to the Lab Book section of this website. The thickness of the shell determines the emission color of QDs. The incubation times were optimized to yield CdTe/CdS core/shell QDs that exhibit emission colors that span the visible range between 550 and 630 nm (Figure 2). ﷯ Figure 2. Emission spectra of CdTe/CdS core/shell QDs with different shell thickness. The incubation times for the shell growth were (left to right) 20, 30, 40, 50, 60, 70 minutes. DNA is often linked to solid surfaces through a sulfur containing moiety that is covalently attached to the DNA strand at one end. For example, thiols form strong bonds with gold surfaces. In order for the DNA to stay attached to the surface during interactions with target molecules, the affinity of the DNA towards the surface of the QD must be greater than the affinity of the DNA towards the target. For the CdTe quantum dots used in this project, the strength of the Cd-S bond (ΔH = 208.4 kJ/mol) is similar to the strength of the Cd-O bond (ΔH = 235.6 kJ/mol). Thus, the affinity of a thiolated DNA molecule for the QD surface would be similar to the affinity of the capping agent, water, or other substances containing oxygen for the same surface. Therefore, these molecules could potentially displace the thiolated DNA from the surface. To avoid this issue, we used a different, one-step functionalization strategy, where a CdS shell was grown around previously formed CdTe cores while the first five nucleotides in the aptamer strand were embedded throughout the thickness of the shell, anchoring the DNA to the QD with multiple Cd-S bonds. This was achieved by introducing sulfur into the strand using phosphorothioate bonds (Figure 1) to connect these first five nucleotides in the strand; the rest of the sequence contained conventional phosphodiester bonds. With this method, we produced robust DNA functionalized core/shell QDs [1]. Furthermore, in the aptamer a spacer consisting of several thymine nucleotides followed the phosphorothioated region, separating this region from the lysozyme specific region of the DNA strand. The thymine spacer was introduced to ensure that the QD surface does not affect the active region of the aptamer and its affinity for lysozyme. For more information on the aptamer sequence, please refer to the Lab Book section of this website. ﷯ Figure 3. Phosphorothioated nucleotide. Functionalization of CdTe/CdS core/shell quantum dots with lysozyme specific aptamers was confirmed by Förster Resonance Energy Transfer (FRET) experiments. References: [1] Deng, Z.; Samanta, A.; Nangreave, J.; Yan, H.; Liu, Y. Robust DNA-Functionalized Core/Shell Quantum Dots with Fluorescent Emission Spanning from UV–vis to Near-IR and Compatible with DNA-Directed Self-Assembly. Journal of the American Chemical Society, 134, 2012, 17424.