The use of cell‐mediated chemistry is an emerging strategy that exploits the metabolic processes of living cells to develop biomimetic materials with advanced functionalities and enhanced biocompatibility. Here, a concept of a cell‐mediated catalytic process for forming protective nano‐shells on individual probiotic cells is demonstrated. This process is leveraged by the cell environment to induce oxidative polymerization of phenolic compounds, and simultaneously these phenolic polymers assemble to form nano‐coatings around individual cell surfaces. The detailed analysis reveals that the oxidation process is triggered by an essential nutrient (manganese) of the probiotic cells, which significantly increases the oxidation rate of phenolic compounds. The phenolic coatings, encapsulating each cell in nanometre scale, demonstrate excellent biocompatibility and biodegradability. Additionally, the in situ encapsulated probiotic cells display an improved gastric tolerance of up to ≈1.4 times higher than the native cells and enhanced adhesion as high as ≈1.6 times onto a model of intestinal epithelial cells. Finally, the coated probiotic cells exhibit a high antioxidant activity as an advanced feature. Overall, this method provides a unique approach to improve the probiotic delivery using the cell machinery to engineer encapsulating nanocoatings with protective benefits and new functionalities.

Tumor penetration of nanoparticles is crucial in nanomedicine, but the mechanisms of tumor penetration are poorly understood. This work presents a multidimensional, quantitative approach to investigate the tissue penetration behavior of nanoparticles, with focuses on the particle size effect on penetration pathways, in an MDA‐MB‐231 tumor spheroid model using a combination of spectrometry, microscopy, and synchrotron beamline techniques. Quasi‐spherical gold nanoparticles of different sizes are synthesized and incubated with 2D and 3D MDA‐MB‐231 cells and spheroids with or without an energy‐dependent cell uptake inhibitor. The distribution and penetration pathways of nanoparticles in spheroids are visualized and quantified by inductively coupled plasma mass spectrometry, two‐photon microscopy, and synchrotron X‐ray fluorescence microscopy. The results reveal that 15 nm nanoparticles penetrate spheroids mainly through an energy‐independent transcellular pathway, while 60 nm nanoparticles penetrate primarily through an energy‐dependent transcellular pathway. Meanwhile, 22 nm nanoparticles penetrate through both transcellular and paracellular pathways and they demonstrate the greatest penetration ability in comparison to other two sizes. The multidimensional analytical methodology developed through this work offers a generalizable approach to quantitatively study the tissue penetration of nanoparticles, and the results provide important insights into the designs of nanoparticles with high accumulation at a target site.