Fabrication of Fluorescent Nanodots by Self-Crosslinking Ufasomes of Conjugated Linoleic Acid and Their Unique Fluorescence Properties

Carbon dots (CDs), as a kind of carbon-based fluorescent nanodots (FNDs), not only retain the advantageous characteristics of carbon-based materials (e.g., low toxicity and biocompatibility) but also exhibit tunable fluorescence emission, low photobleaching, and undergo facile surface functionalization. Therefore, the prospect of applying these materials for analysis and detection, cell imaging, drug delivery, light-emitting devices, photocatalysis, biosensing, and cancer treatment is promising. Although the synthesis of carbon dots from green and renewable feedstocks as biomass carbon sources is possible, the controllability of the involved chemical reactions is poor, resulting in poor atom economy, low quantum yields, and, especially, extremely low yields of carbon dots. In addition, these disadvantages could lead to an increase in equipment requirements and could pose a safety risk because of the need for hydrothermal and solvothermal synthesis. Certain methods even require large amounts of acid/alkali, strong oxidants, or organic solvents, thereby complicating the post-processing process and generating waste and emissions. This research aimed to implement a new idea, namely to "fabricate" rather than "synthesize" carbon-based FNDs from a certain kind of natural and small unsaturated molecule with surface activity relying on a self-assembling and self-crosslinking strategy in lieu of traditional approaches that involve uncontrollable reactions with unknown mechanisms including pyrolysis, dehydrolysis, polyconcensation, and carbonization. In this context, conjugated linoleic acid (CLA) has been studied extensively in our laboratory, and was found to have the self-assembly and self-crosslink characteristics required by the above innovative strategy. This motivated us to adopt CLA as a new carbon source in this study. First, CLA self-assembles into unsaturated fatty acid liposomes (ufasomes) in an aqueous solution of pH 8.6 at ambient temperature (15-25 degrees C), and then, the initiator Ammonium persulfate is added to induce self-crosslinking of the ufasomes at 80 degrees C to obtain firm and uniform nanoparticles. On this basis, the possibility of using them as FNDs is investigated. Consequently, FNDs based on self-crosslinked ufasomes (SCU-FNDs) are prepared in high FND yield of 73.9% after dialysis, with a consistent particle size (17 nm), a degree of self-crosslink (DSC) of 75%, and emission of bluish green fluorescence excited at 320 nm. Furthermore the "fabrication" route provided a clear solution of FNDs that could be applied directly without separation and purification and with no wasteful emissions, which is therefore beneficial for large-scale preparation. The experimental results showed that the fluorescence intensities of the SCU-FNDs are positively correlated with both the surface carboxyl groups and DSC results. A reasonable explanation for the former relationship is the effect of the restricted geometry of the ufasomes on the accumulation of oxygen atoms at the surface of FNDs, whereas the latter could be explained by the confinement effect of the covalent crosslink on the motion of the hydrocarbon chain of the CLA molecules. The experimental results also showed that the SCU-FNDs have temperature-sensitive fluorescence properties, which is attributed to the motion of the residual hydrocarbon chain inside the SCU-FNDs even though they have been locally polymerized. The change in the fluorescence intensity of the FNDs as a function of the temperature was good in accordance with the linear relationship I/I-0 = -0. 00922T + 1.229 (R-2 = 0.99) in the range of 25-85 degrees C, which demonstrates the potential for preparing green and safe undoped FNDs for use as biocompatible and temperature-sensitive fluorescent probes.