Semiconductor nanocrystals or quantum dots (QDs) possess very interesting optical properties that have certain advantages over the traditional fluorescent organic dyes as cell-labeling probes. QDs exhibit size-dependent optical properties such that the emission colour of QDs is strongly dependent on the size of the nanocrystals. In this project, cadmium selenide (CdSe) QDs were employed as cell labels targeting the membrane of NIH-3T3 mouse fibroblast cells. The QDs were first synthesised by the organometallic method, whereby a selenium precursor (selenium solution in trioctylphosphine) was injected into a hot solution of cadmium stearate in trioctylphosphine oxide and hexadecyl amine. During the nucleation and growth process, the QDs’ size changed. Thus, the emission colours of the QDs could be tuned by varying the growth time (0-5 mins) after the injections of the selenium precursor. However, QDs that were synthesised through this method were hydrophobic in nature and they suffered from photoluminescence decay in the presence of non-coordinating solvent. Therefore, silica coating was applied onto these QDs via the reverse microemulsion approach. This strategy provides water solubility to the QDs and also protects the QDs from photoluminescence decay. Silica coating can also allow the conjugation of lipid molecules onto the QDs’ surface by first introducing an amine group on the silica surface. The amine was then coupled with a lipid using the coupling agent, 3-(3-dimethylaminopropyl)-1-ethyl carbodiimide (EDC). The lipid conjugated QDs had the ability to label the membrane of the cells.

In conclusion, the recent advancement in nanobiotechnology allows the engineering of semiconductor nanoparticles for biological imaging. However, the main concern in this project is the use of cadmium-based material, which is toxic in nature, for biological applications. Thus, the future direction of this project is geared towards the synthesis of a less toxic or non-toxic QD system to replace the CdSe QDs.

Student : Ang Chung Yen

Supervisor : S. Tamil Selvan and Jackie Y. Ying (Institute of Bioengineering and Nanotechnology)