Nanoscience has led to advances in physics, materials science, and biology, thereby impacting on applications in electronics, photovoltaic devices, biosensing, and bioimaging [1, 2]. Therefore, increasing amounts of different nanomaterials can be found in our daily life, not only in electronics such as smartphones, tablets, and television screens, but also in wall paints, clothing, and cosmetics. This constant exposure of our body to nanomaterials, whether unwanted or intended, has fueled research on the interaction of nanoparticles (NPs) and cells, providing new insights into their cytotoxic effects and helping to improve the design of nanomaterials to minimize hazardous effects on human health [3,4,5,6]. In addition, development of nanomedicines has also occurred for therapeutic or diagnostic purposes. Their high surface-to-volume ratio, numerous functionalization possibilities, and particular optoelectronic and magnetic properties make nanomaterials attractive for use in applications such as theranostics and bioimaging [1]. However, to achieve efficient use in those applications, it is necessary to be able to control and manipulate the number of NPs within the cells during a certain time window. Therefore, knowledge about cell uptake, metabolism, and excretion of NPs must be obtained [7].

There are many studies about the uptake of colloidal NPs by cells, in particular in vitro, but also in vivo. These have mostly focused on the influence of parameters such as the size, shape, surface charge, and surface functionality of the NPs on their intracellular accumulation kinetics [8, 9]. However, many fewer studies have been carried out on what happens to the NPs once they are inside cells [10]. NPs may be, for example, intracellularly degraded [11], which in turn may affect their cytotoxic potential. Usually, the degradation products of NPs will result in increased cytotoxicity as compared with intact NPs, e.g., by releasing toxic ions [12,13,14]. There are also some reports about the excretion of NPs by exocytosis, in which, for example, a relationship with size was observed for transferrin-coated gold NPs with faster exocytosis rates for smaller NPs. However, their shape may also play a role, as rod-shaped NPs have been reported to exhibit faster clearance from cells than spherical-shaped NPs [7]. Similar to cell uptake, the type of functional groups on the surface of the NPs can influence their exocytosis [15,16,17]. Despite those examples, the total body of work on exocytosis is limited, and many fewer publications exist on the fate of internalized NPs [16, 18,19,20,21].

Before discussing several studies that included quantitative analysis of the fate of internalized NPs, we want to point out that a key point when studying the interaction of NPs with cells is the quantitative aspect [22]. It is of utmost importance to know how the amount of NPs per cell varies with time. This is of particular interest in the above-mentioned scenarios of an environmental spill or exposure of humans to nanomedicines. Long-term toxic effects will depend on the rate at which NPs can be cleared by cells. For this reason, fundamental quantitative studies on their fate are important. We thus explain, based on a few recent studies, how quantitative studies of the cell uptake and clearance of NPs can be conducted using mass spectrometry and fluorescence-based techniques. Based on these studies, we discuss the dependence of exocytosis on the size of the NPs, and the role that cell proliferation plays in the reduction of NPs per cell.