Abstract: Over the past few years, different neuron-targeted nanoparticles (NPs) were designed to deliver drugs to enhance neuron protection and recovery, and much progress was made in our understanding of the uptake mechanism and the related physicochemical properties. Physicochemical properties attracting much attentions in NP’s design and modification include particle size, surface hydrophobicity, and charge (Wohlfart et al., 2012). Despite many achievements in vitro, the in vivo efficacy of most NP modifications are still quite limited, especially in the central nervous system (CNS). In the CNS, the blood-brain barrier (BBB) shields neurons and non-neuronal cells in the brain tissue from being exposed to unwanted molecules through different mechanisms that regulate the exchange of molecules and ions. Because it prevents the entry of over 95% of small molecules and almost 100% of large molecules (Pardridge, 2007), the BBB is a key limitation for drug delivery to the brain. We have been studying for quite some time NPs’ passage across the blood retina barrier (BRB) in living animals – a suitable surrogate model of the BBB. Here, the passage  of fluorescent NP’s across the BRB can be visualized in the living rat with a confocal laser scanning microscope using the in vivo confocal neuroimaging technique (Sabel et al., 1997). The retina is the only brain tissue available for non-invasive in vivo microscopic imaging of CNS neurons. Although the BRB is more permeable than the BBB for some compounds and the trans-endothelial electrical resistance of the BRB is lower than BBB in vitro, the BRB and BBB are similar regarding the expression of efflux proteins and the permeability for many drugs. Regarding passage of NPs into brain tissue, the preliminary data suggest that the results from our BRB model are also valid for the situation at the BBB (You et al., 2019). Unlike the CNS, the peripheral nervous system is not protected by the BBB, but there is still a long and complex route to trace the fate of NPs in vivo, including interaction with blood components and peripheral organs (Figure 1). Here, NPs may serve as a tool for sustained release of drugs which would otherwise be metabolized or filtered out too quickly.  In this context it is important to consider the multiple in vivo interactions through physicochemical properties to advance our understanding of mechanism of action and NP design both for sustained release and passage across biological barriers.