IOP PUBLISHING NANOTECHNOLOGY Nanotechnology 19 (2008) 475101 (8pp) doi:10.1088/0957-4484/19/47/475101 Transfection of aqueous CdS quantum dots using polyethylenimine Hui Li 1 , Wei-Heng Shih 1 , Wan Y Shih 2 , Linyi Chen 3 , S-Ja Tseng 4 and Shiue-Cheng Tang 4 1 Department of Materials Science and Engineering, Drexel University, Philadelphia, PA 19104, USA 2 School of Biomedical Engineering, Science, and Health Systems, Drexel University, Philadelphia, PA 19104, USA 3 College of Life Science, National Tsing Hua University, Hsinchu 30013, Taiwan 4 Department of Chemical Engineering and Brain Research Center, National Tsing Hua University, Hsinchu 30013, Taiwan Received 3 July 2008, in final form 29 September 2008 Published 29 October 2008 Online at stacks.iop.org/Nano/19/475101 Abstract In this study, we have examined the transfection of aqueous CdS quantum dots (QDs) in the cytoplasm of PC12 neuronal cells using polyethylenimine (PEI) as carrier. The CdS QDs were prepared using a unique aqueous synthesis method, at 5 nm in size and capped with 3-mercaptopropyltrimethoxysilane (MPS). They exhibited a quantum yield of 7.5% and a zeta potential of −25 mV. With PEI they formed complexes by electrostatic attraction. At PEI/QD number ratios of >100, the PEI–QD complexes obtained exhibited a saturated size of about 24 nm and a zeta potential of about 15 mV. Confocal microscopy showed that PEI–QD complexes of a PEI/QD number ratio of 200 were successfully internalized and uniformly distributed inside the cells, indicating that the PEI–QD complexes were able to rupture the vesicles to enter the cytoplasm without aggregation. In addition, we showed that the presence of the PEI did not reduce the photoluminescence of the QDs and only mildly reduced the mitochondrial activity of the transfected cells—with no apparent reduction at a PEI/QD ratio of <40 to about 30% reduction at a PEI/QD number ratio of 200. (Some figures in this article are in colour only in the electronic version) 1. Introduction Quantum dots (QDs) are increasingly used as a photolumi- nescent marker for bioimaging applications [1–3]. Due to their superior brightness and photostability as compared to organic fluorophores, QDs offer great potential for intracellular single-molecule imaging [4, 5]. However, due to lack of an effective means to successfully deliver QDs into cytoplasm without aggregation, so far QDs have been mostly used in extracellular imaging applications [6–8]. Although strategies have been explored to deliver QDs into cells, each method has its shortcomings. While endocytosis of QDs is an easy and spontaneous process, it results in sequestration of most QDs in the endosomal compartment where they are unavailable for subsequent intracellular assays. Transfection via cationic liposomes and electroporation allow for parallel delivery of QDs in the cells simultaneously with endosomal escape, but the internalized QDs are prone to aggregation instead of dispersion. Cytoplasmic microinjection into single cells yields an aggregate-free dispersion of QDs inside the cell with nuclear exclusion, but requires each cell to be individually manipulated [9]. Many researchers have tried conjugating QDs with peptides [10], DNA [11], and nanogels [12] for intracellular QD delivery and labeling by transfection. For example, the Tat, a typical membrane-permeable carrier peptide, was conjugated with thiol-capped CdTe QDs to form Tat-QDs, which enhanced the intracellular delivery of QDs [13]. However, dynamic confocal imaging studies indicated that the internalized Tat- QDs were tethered to the inner vesicle surfaces and were trapped in cytoplasmic organelles. It was also found that Tat- QDs strongly bound to cellular membrane structures such as filopodia and that large QD-containing vesicles were released from the tips of filopodia by vesicle shedding [14]. Previously it has been shown that polyethylenimine (PEI) is an effective transfection medium in gene delivery [15, 16]. 0957-4484/08/475101+08$30.00 © 2008 IOP Publishing Ltd Printed in the UK 1