Steinmetz L Bourquin J Barosova H Haeni L Caldwell J Milosevic A Geers C Bonmarin M Taladriz-Blanco P Rothen-Rutishauser B Petri-Fink A 2020 Evaluating nanomaterial uptake and association by cells is relevant for in vitro studies related to safe-by-design approaches, nanomedicine or applications in photothermal therapy. However, standard analytical techniques are time-consuming, involve complex sample preparation or include labelling of the investigated sample system with e.g. fluorescent dyes. Here, we explore lock-in thermography to analyse and compare the association trends of epithelial cells, mesothelial cells, and macrophages exposed to gold nanoparticles and multi-walled carbon nanotubes over 24 h. The presence of nanomaterials in the cells was confirmed by dark field and transmission electron microscopy. The results obtained by lock-in thermography for gold nanoparticles were validated with inductively coupled plasma optical emission spectrometry; with data collected showing a good agreement between both techniques. Furthermore, we demonstrate the detection and quantification of carbon nanotube-cell association in a straightforward, non-destructive, and non-intrusive manner without the need to label the carbon nanotubes. Our results display the first approach in utilizing thermography to assess the carbon nanotube amount in cellular environments. https://sonar.ch/global/documents/40067 eng info:eu-repo/semantics/altIdentifier/doi/10.1039/d0nr03330h info:eu-repo/semantics/altIdentifier/pmid/32789375 Nanoscale. - 2020 General Materials Science Rapid and sensitive quantification of cell-associated multi-walled carbon nanotubes. http://purl.org/coar/resource_type/c_6501