A bioaccumulation study in red (Palmaria palmata) and green (Ulvasp.) seaweed has been carried out after exposure to different concentrations of citrate-coated titanium dioxide nanoparticles (5 and 25 nm) for 28 days. The concentration of total titanium and the number and size of accumulated nanoparticles in the seaweeds has been determined throughout the study by inductively coupled plasma mass spectrometry (ICP-MS) and single particle-ICP-MS (SP-ICP-MS), respectively. Ammonia was used as a reaction gas to minimize the effect of the interferences in the48Ti determination by ICP-MS. Titanium concentrations measured inUlvasp. were higher than those found inPalmaria palmatafor the same exposure conditions. The maximum concentration of titanium (61.96 ± 15.49 μg g−1) was found inUlvasp. after 28 days of exposure to 1.0 mg L−1of 5 nm TiO2NPs. The concentration and sizes of TiO2NPs determined by SP-ICP-MS in alkaline seaweed extracts were similar for both seaweeds exposed to 5 and 25 nm TiO2NPs, which indicates that probably the element is accumulated inUlvasp. mainly as ionic titanium or nanoparticles smaller than the limit of detection in size (27 nm). The implementation of TiO2NPs inUlvasp. was confirmed by electron microscopy (TEM/STEM) in combination with energy dispersive X-Ray analysis (EDX).Graphical abstractSupplementary InformationThe online version contains supplementary material available at 10.1007/s00604-023-05849-1.