Tumor Spheroid Uptake of Fluorescent Nanodiamonds Is Limited by Mass Density: A 4D Light-Sheet Assay

Fluorescent nanodiamonds (FNDs) with nitrogen-vacancy centers are promising candidates for long-term biolabeling and biosensing applications due to their biocompatibility and unique optomagnetic properties. The employment of nanomaterials in cancer therapy and diagnostics requires a deep understanding of how nanoparticles (NPs) interact with the three-dimensional (3D) tumor environment. We developed the "Tumor-in-a-Tube" platform, using 4D light-sheet microscopy to explore the spatiotemporal dynamics of FNDs with 3D tumor spheroids. By monitoring the real-time NP sedimentation, spheroid penetration, and cellular uptake of FNDs and polystyrene nanoparticles (PNPs), we marked the impact of the NP mass density on their spheroid interaction. Unlike PNPs, higher-density FNDs underwent rapid sedimentation, which minimized their effective concentration and hindered the FND-spheroid interactions. This results in constrained intratumoral accumulation and size-independent uptake and penetration. Longer FND effective-exposure time promotes size-dependent cell uptake, verified by FND treatment on 2D monolayers. Nonetheless, FNDs exhibited good biocompatibility and long-term spheroid labeling, allowing for cell isolation from different spheroid layers. Our results suggest the need for NP effective-exposure-time calibration in comparative NP assays, in 3D static models. Overall, our platform provides a valuable tool for bridging the gap between 2D and 3D static models in NP assessment, drug delivery, toxicology profiling, and translational research.