Synthesis and characterization of thiol-functionalized carbon nanoparticles for superoxide anion radical detection

Functionalized carbon dot nanoparticles are at the forefront of biological applications of carbon nanoparticles (CNPs), particularly in bioimaging and biosensing. In this study, both functionalized and unfunctionalized CNPs were utilized as fluorescent probes in cancer cell lines and Drosophila melanogaster. A human T lymphocyte (Jurkat), the MCF 10A cell line, and the wild-type Oregon strains of D. melanogaster were used in the study. N-doped CNPs were synthesized solely from o-phenylenediamine (oPD CNPs), and thiol-functionalized CNPs were synthesized from o-phenylenediamine and L-cysteine (SH-CNPs), both via a microwave-assisted synthesis. The synthesized SH-CNPs show maximum excitation and emission intensity at 340 nm and 412 nm, respectively. UV-vis absorption spectra present absorption peaks at 238 nm, ∼279 nm, and ∼297 nm, which are attributed to π → π* and n → π* transition states in thiol-containing aromatic systems. CNPs were spherical and polydisperse, with an average size of 64 ± 29 nm. Chemical characterization revealed the presence of the -SH functional group, characterized by an infrared absorption band at ∼2550 cm^-1, alongside other functional groups (OH, CO, COO, CC, C-C, NH, NH₂) on the surface of both the CNPs and their core. X-ray photoelectron spectroscopy analysis of S 2p also shows the binding energy of ∼164 eV allotted to C-SH. Scanning Electron Microscope/Energy Dispersive Spectroscope reveals the weight percentage (wt%) of the elemental composition of SH-CNPs to be 62.9 ± 0.2%, 17.2 ± 0.2%, 14.7 ± 0.1%, and 5.2 ± 0.1% for carbon, nitrogen, oxygen, and sulfur, respectively. A significant difference in the intensity of the SH-CNPs fluorescence between larvae treated with hydrogen peroxide (H₂O₂) and larvae treated with potassium superoxide (KO₂), with the latter group showing reduced fluorescence intensity. Taken together, these results demonstrate that surface functionalization plays a critical role in modulating the biological response and sensing capability of carbon dot nanoparticles. While unfunctionalized oPD CNPs are effective for structural bioimaging, thiol-functionalized CNPs provide additional functional sensitivity to oxidative stress conditions.