Rapid and label-free optical detection of individual carbon air pollutant nanoparticulates in biomedical samples.

Carbonaceous particle exposure and air pollution in general lead to a multitude of adverse human health effects and pose multiple challenges in terms of exposure, risk and safety assessment. Highly desirable for fast screening are label-free approaches for detecting these particle types in biological or medical context. We report a powerful approach for detecting carbonaceous particles using photothermal pump-probe microscopy, which directly probes their strong light absorption. The principle and reliability of this approach is demonstrated by examining 4 different carbon black (CB) species modeling soot with diameters ranging from 13 to 500 nm. Our results show that the proposed approach is applicable to a large number of CB types as well as black carbon. As the particles show a strong absorption over a wide spectral range as compared to other absorbing species, we can image CB particles almost background free. Our pump-probe approach allows label-free optical detection and unambiguous localization of CB particles in (bio)fluids and 3D cellular environments. In combination with fluorescence microscopy, this method allows for simultaneous colocalization of CB with different cellular components using fluorophores as shown here for human lung fibroblasts. We further demonstrate the versatility of pump-probe detection in a flow cell.