ABSTRACT
Apart from sample considerations, it is also important to choose an appropriate test system and define the endpoints to be tested: cell death, membrane permeability, apoptosis, interference with respiration, and so on. Given the complexity of each of these elements, it is not surprising that there is a good deal of contradiction and controversy in the nanotoxicology literature. In this chapter we will focus on the most common types of fluorescent semiconductor nanoparticles (quantum dots or QDs) in their interactions with bacteria. Many fewer toxicity studies have been done with QDs in prokaryotes as compared with mammalian cells. As of this writing, there were just over 20 published peer-reviewed articles on the subject. We will therefore be able to summarize nearly all that is known about how QDs interact with bacterial cells, and illustrate the key variables that can cause disagreements among studies, even when performed using the same bacterial strain. To minimize the issues of QD variability, we will emphasize the most common materials and surface chemistries. We will begin with an overview of QD materials and the effects of composition on toxicity. We will then discuss ROS, the published results and the gaps in the literature, with a recommendation of which assays provide the most consistent and conclusive results. A section on photosensitization explains the ways in which singlet oxygen production may be maximized if desired. We then devote a section to the results in bacteria using three common types of QDs: core CdSe, core-shell CdSe/ZnS (or CdS), and CdTe. 6.2 Quantum Dots Cores, Shells, and CapsThe most common QD materials for biological use are made from cadmium and selenium, in the form of CdSe wurzite nanocrystals (“bare” or “core” QDs). These nanocrystals are nearly always overlaid with another semiconductor material, such as ZnS or CdS, to increase the intensity of the light emission; these are referred to as “core-shell” QDs and the outer shell may be no more than a single atomic monolayer. For CdSe, the most commonly used QD material, particle sizes of 2-6 nm show photoluminescence emission that spans the visible range; some red-shifting is seen with ZnS
capping relative to the original core emission (Fig. 6.1). CdTe is another commonly used material, which can be made to emit in nearly the same colors, although its smaller bandgap means that correspondingly sized particles will be redder.
