Quantum dots (QDs) are now used extensively for labeling in biomedical research due to their unique photoluminescence behavior, involving size-tunable emission color, a narrow and symmetric emission profile and a broad excitation range [1]. Uncoated QDs made of CdTe core are toxic to cells because of release of Cd2+ ions into the cellular environment. This problem can be partially solved by encapsulating QDs with polymers, like poly(N-isopropylacrylamide) (PNIPAM) or poly(ethylene glycol) (PEG). Based on biological compatibility, fast response as well as pH, temperature and magnetic field dependent swelling properties, hydrogel nanospheres has become carriers of drugs, fluorescence labels, magnetic particles for hyperthermia applications and particles that have strong optical absorption profiles for optical excitation. The toxicity of uncoated QDs are known; however, there have been a very limited number of studies specially designed to assess thoroughly the toxicity of nanosphere encapsulated QDs against QD density and dosing level.
In this work, we present preliminary studies of biological effects of a novel QD based nanomaterial system on Escherichia coli (E. coli) bacteria. Cadmium chalcogenide QDs provide the most attractive fluorescence labels in comparison with routine dyes or metal complexes. Nanospheres on the other hand are the most commonly used carriers of fluorescence labels for fluorescence detection. The integration of fluorescent QDs in nanospheres therefore provides a new generation of fluorescence markers for biological assays. Hydrogels based on PNIPAM is a well known thermoresponsive polymer that undergoes a volume phase transition across the low critical solution (LCST) [2]. Therefore, the inherent temperature-sensitive swelling properties of PNIPAM offer the potentiality to control QD density within the nanospheres. In the present work, E. coli growth was monitored as E. coli served as a representation of how cells might respond in the presence of hydrogel encapsulated QDs in their growth environment. The present work describes the successful encapsulation of CdTe QDs in PNIPAM gel network. Microgel encapsulated QDs were synthesized by first preparing PNIPAM microspheres with cystaminebisacrylamide as a crosslinker and CdTe QDs capped with a stabilizer. The CdTe QDs were bonded into PNIPAM microgels through the replacement of CdTe's stabilizer inside PNIPAM microspheres. Growth curves were generated for E. coli growing in 20 mL of LB media containing hydrogel encapsulated QD nanospheres (400 nm diameter) at relatively higher (0.5mg/mL) and lower (0.01mg/mL) concentration of solution. From the growth curves, there was no evidence at lower concentration (0.01mg/mL) that the hydrogel encapsulated QDs prevent the microbial cells from growing but at higher concentration (0.5mg/mL), microbial growth was inhibited. Transmission Electron Microscopy (TEM) was used to characterize QD size and density inside the hydrogel nanospheres. Scanning Electron Microscopy (SEM) was used to observe size and morphology of the hydrogel particles. Further investigation is going on cell growth response at different QD density and to evaluate the limiting hydrogel concentration for different QD densities.
