Compositional Elucidation of Carbon Nanoparticle Synthesis Using Mechanistic Insights and Its Relevance to Biosensing Applications

Abstract

Carbon dots are an emerging type of carbon nanomaterial lauded for their 3- 10 nm sizes, photoluminescent properties, simple synthesis from cost-effective materials, good water solubility, polar surface functionalities, and good biocompatibility. Consequently, they have found considerable use in the biomedical sciences as imaging agents, carriers for drug delivery, substrates for biofunctionalization, and transduction elements in biosensors. However, a growing body of literatures investigating the PL origins of the most applied subtypes of CDs, those synthesized from bottom-up reactions, have raised questions about what the byproducts of these reactions are and how they are impacting the observed properties. Such CDs are notorious for their compositional complexity which is often used to explain their poor reproducibility. With this information in mind, we developed a biosensor for the detection of extracted SARS-CoV-2 RNA that relied on the forced aggregation induced emission enhancement (AIEE) of CDs, conjugated with antisense oligonucleotides, (ASOs) when they bound with the viral RNA. We found that changing the dialysis conditions of the CD-ASO conjugate or removal of large particles before introducing them to the SARS-CoV-2 RNA significantly impacted sensor performance. Following this study, we attempted to develop a CD based biosensor for the detection of stress biochemical markers using some of the CDs used in the SARS-CoV-2 biosensor, we found that after extensive purification, the CDs would assemble into ordered polymeric structures whose morphologies depended on the solvent used. Confounded by our observations, but determined to push forward with sensor development, we switched our CDs to those made from glucosamine hydrochloride. Despite our CDs being spectroscopically similar to CDs reported by other groups, we observed, with electron and optical microscopy, ordered crystalline and amorphous morphologies uncharacteristic of traditional CDs. Rigorous reanalysis of the data resulted in our conclusion that the ordered morphologies are assembled from amphiphilic chained copolymers and by making simple changes to the reaction conditions, these morphologies can be adjusted. Though chained oligomers and polymers are known to form from these reactions, they are often ignored and have not been reported to assemble into ordered nano and supramolecular structures. Our findings imply amphiphilic chained copolymers play a much larger role in the observed bulk properties of CDs than previously thought and may serve as a explanation for many of the reported discrepancies and inconsistencies found throughout the CD literatures.