# Photodynamic Properties of CdSe/CdS Quantum Dots in Intracellular Media
## 1 Introduction  
Fluorescent semiconductor nanocrystals or quantum dots (QDs) have become indispensable for biomedical research especially in cellular biology due to their unique photophysical and physicochemical properties. Among the optical properties of QDs that make them attractive for biological applications are size-dependent tunable absorption and emission in the visible and NIR regions, narrow emission bandwidth, broad absorption bands especially for UV excitation, and large single- and multi-photon absorption cross-sections [1; 2; 3; 4; 5; 6; 7; 8]. These unique optical properties originate from a combination of bulk semiconductor properties and quantum confinement effect. QDs are also exceptionally bright and photostable compared to fluorescent dyes and proteins [9; 10]. Fluorescent dyes usually consist of organic molecules that are easily and irreversibly degraded through photo bleaching by the light used to excite them, progressively emitting less light over time. A multitude of biological applications have exploited these unique properties and to date, QDs have demonstrated utility as probes and labels for biomedical applications, photodynamic agents, in vitro and in vivo fluorophores, and contrast agents for deep-tissue imaging and detection as well as long-term tracking of biological cells, agents for delivery of drugs, molecular-scale platforms for assembling energy transfer-based sensors, theranostic materials, light-harvesting arrays, agents for diagnosis and therapy of genetic disorders and diseases, to name a few [11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21]. The biological application of QDs is still increasing exponentially. Despite the demonstrated biological applications of QDs, some challenges remain. For instance, cytotoxicity is a major concern when CdSe/CdS QDs are used as agents for drug delivery and disease detection and therapy because although they are carriers, they also have integrated functionalities. On the other hand, the primary challenge when using QDs for cellular imaging is the effective seeding into the cells, since QDs are nanoparticles made of an inorganic core. This difficulty is further compounded by the limited knowledge about the extra- and intracellular processing of such QDs. This is partly due to the strong dependence of the physicochemical and biophysical properties of the nanoparticle when it interacts with a complex biological environment. Several techniques that enable single particle injection in single cells have been reported [22; 23; 24; 25; 26; 27; 28; 29; 30]. For instance, the patch clamp technique inserts a glass micropipette filled with electrolyte into a cell. This technique offers both a high signal-to-noise ratio and temporal resolution [22; 23]. Previous works have also reported single particle injection by inserting metal or carbon microelectrodes into the cells, with advantages similar to inserting a glass micropipette [24; 25; 26]. However, these techniques require that the micropipettes and microelectrodes are as small as possible to increase the spatial resolution and minimize the invasiveness of the measurement. The overall performance of this technique is further limited by the impedance of the interface between the microstructures and the cell interior, which also limits how small the microstructures can be [22; 23]. Recently, three-dimensional (3D) nanostructures such as nanotubes integrated on top of a nanoscale field-effect transistor [27; 28] and nanoelectrodes [29] were successfully used to penetrate the cell membrane and enable the delivery of single nanoparticles into single selected cells. The ability to selectively deliver single nanoparticles into single cells benefits many applications. However, these methods are complex and require precise equipment. These are also invasive, with the possibility of disrupting the cell nuclear envelope, which may lead to negative side effects [29]. Endocytosis, which is the passive uptake of nanoparticles, is a non-invasive method of delivering nanoparticles into cells [30].