Surface modification, functionalization and …
Byrne, S.J. , Corr, S.A. , Rakovich, T.Y. , Gun'ko, Y.K., Rakovich, Y.P. , Donegan, J.F. , Mitchell, S., Volkov, Y., Optimisation of the synthesis and modification of CdTe quantum dots for enhanced live cell imaging , , 16, (28), 2006, p2896-2902
(b) Stabilization against aggregation
Here we present a facile one-pot method to prepare high-quality CdTe nanocrystals in aqueous phase. In contrast to the use of oxygen-sensitive NaHTe or H2Te as Te source in the current synthetic methods, we employ more stable sodium tellurite as the Te source for preparing highly luminescent CdTe nanocrystals in aqueous solution. By selecting mercaptosuccinic acid (MSA) as capping agent and providing the borate-citrate acid buffering solution, CdTe nanocrystals with high quantum yield (QY >70% at pH range 5.0–8.0) can be conveniently prepared by this method. The influence of parameters such as the pH value of the precursor solution and the molar ratio of Cd2+ to Na2TeO3 on the QY of CdTe nanocrystals was systematically investigated in our experiments. Under optimal conditions, the QY of CdTe nanocrystals is even high up to 83%. The biological application of luminescent MSA-CdTe to HEK 293 cell imaging was also illustrated.
To date, many approaches, including organic synthesis – and aqueous synthesis –, have been developed to prepare luminescent QDs. Although high-quality CdTe nanocrystals (NCs) can be prepared in organic phases, they are unable to be directly used in biosystems owing to the hydrophobility of these NCs. Several methods like ligand-exchange, encapsulated into a water-soluble shell and arrested precipitation in water have been used to transfer hydrophobic CdTe NCs to aqueous solution, but the PL QYs of them are normally impaired after they are subjected to this transferring process –. Compared with organic synthesis, aqueous synthesis has the advantages of simplicity, reproducibility, and less toxicity. However, in most of aqueous synthesis of thiol-capped CdTe NCs, either H2Te (a highly toxic and flammable gas) or NaHTe (an unstable compound for its spontaneous oxidation in the presence of oxygen) is utilized as the tellurium precursor, which usually needs an inert atmosphere during the synthesis –. Considering the complexity caused by inert atmosphere used above, it is very necessary to employ a stable Te source as the precursor when synthesizing CdTe NCs in an aqueous solution. On the other hand, the QYs of CdTe NCs synthesized in the aqueous phase was generally low (, . However, by properly selecting thiol stabilizers like glutathione , or mercaptopropionic acid (MPA) , the QYs of CdTe NCs prepared in the aqueous solution could be improved significantly (45%~67%). To further improve the PL QY of CdTe NCs, the post-treatment process like selective photochemical etching , , or ultrasonic irradiation has been employed by several groups. For instance, Bao et al. used photochemical etching approach to produce CdTe NCs with a substantial increase in PL QY up to ~85%, but normally more than 20 days' illumination under room light of low-pressure mercury-rare-gas discharge lamps was required to bring the fluorescence QY to this value . Furthermore, several new techniques like hydrothermal treatment –, microwave irradiation , , have been employed for rapid synthesis of high quality CdTe NCs in aqueous phase. Although the preparation of CdTe NCs can be achieved in a short time, and the QYs of them are also improved a lot than before –, , , highly active and toxic NaHTe or H2Te was still used as the Te source.