Since a number of medical conditions require simultaneous treatment and diagnostics, the field of molecular therapeutics has recently turned to multifunctional approaches allowing for both therapy and biomedical imaging. A number of such molecular and nanoformulations are combined with fluorophores that allow for imaging of the delivery pathways of the drug in the visible. This is optimal for in-vitro or ex-vivo work, however, can not be utilized well in-vivo. Thus there is a need in nanoformulations optimized for both in-vitro and in-vivo studies. Nanocarbons, possessing intrinsic stable fluorescence in the visible and near-IR stand out as candidates for such complex application.

In this work, we for the first time produce a biocompatible nanocarbon quantum dots (NCQDs) that exhibit two-color emission both in visible and NIR possess a capability for biological pH sensing. These NCQDs show the crystalline graphitic structure in TEM and average sizes of c.a. 5 nm beneficial for cellular internalization. As opposed to related structures such as graphene oxide and other graphene derivatives NCQDs show high quantum yield in green (~500 nm) of 68.1% calculated via a comparative method using Coumarin-153 as a basis. Near-IR emission at ~860 nm shows lower relative quantum yield but is located in the water window with reduced absorption and lower autofluorescence backgrounds providing a promising potential route for in-vivo studies. Emission of NCQDs also depends on pH of the surrounding medium. The change in pH of as-prepared NCQDs from 2.71 to 8.2 yields an increase of fluorescence intensity up to ~60%. The actual spectral signature of NCQD emission also changes with pH: upon spectral decomposition of the broad emission feature the green component (470-514 nm) shows an increase at higher pH with respect to the red component (846-888 nm). Biocompatibility of NCQDs is verified by the MTT assay showing a low cytotoxic response. As potential drug delivery agents, NCQDs also show efficient cellular internalization assessed via in-vitro imaging in green. Additionally, pH-dependent shifts of the spectral features allow differentiating between acidic cancerous and neutral healthy exocellular environments allowing to use NCQDs for cancer detection. Therefore, our results indicate that NCQDs have a significant potential in bio-applications because of their capacity for dual color green/near-IR imaging for in-vitro/in-vivo studies, pH sensitivity, water solubility, low cytotoxicity and high capacity for cellular internalization.