Images of a frozen-hydrated and chemically fixed HeLa cell sample that was treated with Fe3O4@TiO2 nanocomposites for 30 min. The nanocomposites consist of Fe3O4@TiO2 nanoparticles and peptides. The size of the Fe3O4@TiO2 nanoparticles is 6–7 nm, measured using atomic force microscopy. The peptides are 11 amino acids long with an N-terminal 3,4-dihydroxyphenyacetic acid moiety. (a) Overview optical image of the frozen-hydrated sample obtained using a Nikon light microscope equipped with an Instec cold stage. A 750 µm × 350 µm area of this sample, indicated by the white rectangle in (a), was rapidly scanned by the BNP to gain an overview of the distribution of cells in this sample. (b) Sulfur (S) fluorescence map of the sample area indicated in (a), acquired using fly-scan mode (continuous motion in the horizontal direction) with 0.85 µm step size. The count level (minimum to maximum range in counts s−1) is 0–100. The S signal indicates the presence of the cells. A 30 µm × 18 µm area, indicated by the white rectangle in (b), encompasses a single cell. (c) High-resolution fluorescence maps of the cell selected in (b) showing the distributions of iron (Fe), titanium (Ti), phosphorus (P) and their overlay. This image was acquired using fly-scan mode (continuous motion in the horizontal direction) with a 50 nm step size and 100 ms dwell time per pixel. The count levels (minimum to maximum range in counts s−1) are 0–705 for Fe, 0–569 for Ti and 0–124 for P. While P shows a cell outline, and a more intense P concentration in the region of the cell nucleus, co-localized Ti and Fe pixels correspond to the distribution of nanocomposites. The fluorescence maps were created by performing peak area fitting for every pixel. (d) The summed spectrum of the whole map with major peaks labeled. While the Kα lines are labeled with element symbols only, the Kβ lines are indicated using `Kb' in the graph.