Abstract
Aiming to understand and enhance the capacity of carbon dots (CDs) to transport through cell membranes and target subcellular organelles—in particular, mitochondria—a series of nitrogen-doped CDs were prepared by the one-step microwave-assisted pyrolysis of citric acid and ethylenediamine. Following optimization of the reaction conditions for maximum fluorescence, functionalization at various degrees with alkylated triphenylphosphonium functional groups of two different alkyl chain lengths afforded a series of functionalized CDs that exhibited either lysosome or mitochondria subcellular localization. Further functionalization with rhodamine B enabled enhanced fluorescence imaging capabilities in the visible spectrum and allowed the use of low quantities of CDs in relevant experiments. It was thus possible, by the appropriate selection of the alkyl chain length and degree of functionalization, to attain successful mitochondrial targeting, while preserving non-toxicity and biocompatibility. In vitro cell experiments performed on normal as well as cancer cell lines proved their non-cytotoxic character and imaging potential, even at very low concentrations, by fluorescence microscopy. Precise targeting of mitochondria is feasible with carefully designed CDs that, furthermore, are specifically internalized in cells and cell mitochondria of high transmembrane potential and thus exhibit selective uptake in malignant cells compared to normal cells.
Highlights
Carbon dots (CDs), an emerging group of fluorescent nanomaterials with typical sizes of less than 10 nm, are similar to semiconductor quantum dots with respect to their unique optical and electrochemical properties while, more importantly, their preparation is environmentally friendly and they exhibit water solubility, chemical inertness, low to minimal toxicity and biocompatibility [1]
Drug delivery to mitochondria requires the development of mitochondriotropic drug delivery systems, which has been pursued by attaching mitochondriotropic ligands in various drug nanocarriers, such as liposomes, polymers, dendrimers or carbon dots [17,18,19,20,21,22,23]
Nitrogen-doped carbon dots (CDs) were produced by microwave-assisted polymerization, cyclization and pyrolysis of citric acid (CA) and ethylenediamine (EDA), according to already published protocols [47]
Summary
Carbon dots (CDs), an emerging group of fluorescent nanomaterials with typical sizes of less than 10 nm, are similar to semiconductor quantum dots with respect to their unique optical and electrochemical properties while, more importantly, their preparation is environmentally friendly and they exhibit water solubility, chemical inertness, low to minimal toxicity and biocompatibility [1] Due to their intrinsic fluorescence properties and their ability to encompass functional groups on their surfaces, a variety of applications ranging from sensing and catalysis [2,3,4] to drug and gene vectors [5,6] or for biological imaging [2,7,8,9] are being intensively studied. Drug delivery to mitochondria requires the development of mitochondriotropic drug delivery systems, which has been pursued by attaching mitochondriotropic ligands in various drug nanocarriers, such as liposomes, polymers, dendrimers or carbon dots [17,18,19,20,21,22,23]
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