Probing polyvinylpyrrolidone-passivated graphene oxide nanoflakes as contrast agents inside tissue-like phantoms via multimodal confocal microscopy.

Beside attractive electrical, thermal and mechanical properties, graphene oxide (GO) exhibits visible and near-infrared (NIR) photoluminescence (PL) and well-defined fingerprint Raman bands which are remarkable optical signatures to implement GO as new contrast agent for the visualization of cells or tissue, including cancer tumors. However, the biomedical use of GO as optical contrast agent is to some extent hindered by the intrinsic low emission efficiency especially at neutral pH. Herein, we successfully modulate the PL of GO nanoflakes in acidic and neutral medium by passivating them with polyvinylpyrrolidone (PVP), an amphiphilic and biocompatible polymer, thus improving the PL at pH relevant for biomedical applications. We demonstrate the potential of as-fabricated PVP-GO nanocomposites to operate as dual Raman-PL contrast agents inside tissue-like agarose-phantoms via scanning confocal Raman microscopy (CRM) under excitation at 532 nm. Super-resolution re-scan confocal microscopy (RCM) was further employed to investigate the distribution of PVP-GO inside biological phantoms at 3D level under three excitation lines (405, 488, and 561 nm). Finally, two-photon excited fluorescence lifetime imaging microscopy (TPE-FLIM) at 810 nm excitation reveals the ability of PVP-GO to serve as NIR-activatable contrast agent inside tissue-like phantom. Notably, PVP coating empowers GO nanoflakes not only with enhanced optical signature, but also with excellent dispersibility inside biological phantoms, thus offering improved labeling performance of as-designed imaging contrast agent.