Label-Free, Quantitative Imaging of MoS2 -Nanosheets in Live Cells with Simultaneous Stimulated Raman Scattering and Transient Absorption Microscopy.

Probing nanoparticle-cell interactions in vivo plays a key role in the research and development of nanomedicine and nano-biophotonics. However, simultaneously imaging nanoparticles and intracellular biomolecules in a label-free manner has been a challenge, especially for the majority of nanoparticles that do not emit one- or two-photon excited fluorescence. In this work, a strong optical transient absorption (TA) signal is observed for synthesized MoS2 nanosheets that could be used for sensitive and quantitative imaging of their intracellular distributions. Furthermore, a dual-modal microscopy technique is developed to simultaneously image MoS2 with TA microscopy and cellular lipids/proteins with stimulated Raman scattering (SRS) microscopy. This technique is based on precise pulse engineering and dual-phase lock-in detection. Its capability to quantify the uptake dynamics of MoS2 into live HeLa cells is demonstrated and the colocalization of MoS2 nanosheets with intracellular lipid rich organelles is observed. Different sized MoS2 nanosheets are found to have distinct uptake dynamics as well as transient photocarrier dynamics. These results indicate that dual-modal SRS/TA microscopy may provide a general solution for investigating a large variety of nanomaterials that absorb light but do not fluoresce.