Dispersion, dose and stability of semiconductor quantum dot biomarkers

Abstract

Photoluminescent semiconductor nanoparticles or quantum dots have significant potential for medical imaging. For optimum performance however, the dispersion of the nanoparticulate material when suspended in delivery or incubation media, any transformation of the particles in the media, plus the nature and degree of uptake of the nanoparticles by a particular cell or organism all need to be understood. Analytical electron microscopy can play a vital role in assessing this complex inter‐relationship, and we discuss here specific methods developed for this type of analysis. First, we will review the in vitro cellular uptake of commercially available CdSe/ZnS quantum dot nanoparticles with a coating specifically targeted for endocytic uptake (Invitrogen QTracker 705), dispersed in cell culture media and exposed to human osteosarcoma (U‐2 OS) cells. We have examined these nanoparticles as‐dispersed in cell culture media (t = 0 h), after 1 hour exposure to cells and after a round of cell division (t = 24 h). Transmission electron microscopy (TEM) has been used to assess the dispersion state of the nanoparticles in media after rapidly freezing suspensions to avoid drying artefacts [1], and in exposed cells which have been fixed and resin embedded [2]. The resin‐embedded cells have been further examined using serial block face scanning electron microscopy (SBF‐SEM), which enables quantification of nanoparticle loaded organelles in whole cell volumes for quantitative correlation to imaging flow cell cytometry [2]. From this we have measured probability densities for the number of quantum dots per agglomerate when in cell culture media and following uptake by cells in vitro [3, 4 and Figure 1 a‐d]. Thus, we will discuss the agglomeration processes that occur both in suspension and during endocytosis. Second and looking forward, most commercially available semiconductor quantum dots currently contain cadmium although its health and environmental risks may limit exploitation. Thus, copper indium sulfide (CIS) quantum dots have been investigated as a potential replacement [5]. Aberration corrected STEM‐EELS has identified some elemental separation of Cu and In within individual quantum dots [Figure 1 e‐h], which may be the origin of an In‐Cu anti‐site defect state known to act as a donor in the radiative recombination pathway for chalcopyrite CIS quantum dots. We will report here on further analysis using a FEI Titan cubed Themis 300 G2 S/TEM to assess elemental distribution by STEM‐EDX. Such analysis will enable additional characterisation of core‐shell coatings (e.g. CIS/ZnS/ZnS:Al) designed to improve photo luminescent quantum yield while enhancing environmental stability of the particles.