Nitrogen and Sulfur Doped Carbon Dots from Amino Acids for Potential Biomedical Applications

Nitrogen (N-) and sulfur (S-) doped carbon dots (CDs) were synthesized in a single step in a few min, 1-4 min via microwave technique from five different types of amino acids viz. Arginine (A), Lysine (L), Histidine (H), Cysteine (C), and Methionine (M). These amino acid derived N- and/or S- doped CDs were found to be in spherical shapes with 5-20 nm particle size range determined by Transition Electron Microscope (TEM) images and Dynamic Light Scattering (DLS) measurements. Thermal degradation, functional groups, and surface potential of the CDs were determined by Thermogravimetric Analysis (TGA), FT-IR spectroscopy, and zeta potential measurements, respectively. Although the zeta potential value of Cysteine derived CD (C-CD) was measured as -7.45 +/- 1.32 mV, the zeta potential values of A-CD, L-CD, H-CD, and M-CD particles were measured as +2.84 +/- 0.67, +2.61 +/- 1.0, +4.10 +/- 1.50 and+2.20 +/- 0.60 mV, respectively. Amongst the CDs, C- CDs was found to possess the highest quantum yield, 89%. Moreover, the blood compatibility test of CDs, determined with hemolysis and blood clotting tests was shown that CDs at 0.25 mg/mL concentration, CDs has less than 5% hemolysis ratio and higher than 50% blood clotting indexes. Furthermore, A-CD was modified with polyethyleneimine (PEI) and was found that the zeta potential values was increased to +34.41 +/- 4.17 mV (from +2.84 +/- 0.67 mV) inducing antimicrobial capability to these materials. Minimum Inhibition Concentration (MIC) of A-CD dots was found as 2.5 mg/mL whereas the PEI modified A-CDs, A-CD-PEI was found as 1 mg/mL against Escherichia coli ATCC 8739 (gram -) and Staphylococcus aureus ATCC 6538 (gram +) bacteria strains signifying the tunability of CDs.