Role of Oxygen Defects in Eliciting a Divergent Fluorescence Response of Single-Walled Carbon Nanotubes to Dopamine and Serotonin

Modulating the optical response of fluorescent nanoparticles through rational modification of their surface chemistry can yield distinct optical signatures upon the interaction with structurally related molecules. Herein, we present a method for tuning the fluorescence response of single-walled carbon nanotubes (SWCNTs) toward dopamine (DA) and serotonin, two structurally related monoamine-hydroxylated aromatic neurotransmitters, by introducing oxygen defects into (6,5) chirality-enriched SWCNTs suspended by sodium cholate (SC). This modification facilitated opposite optical responses toward these neurotransmitters, where DA distinctly increased the fluorescence of the defect-induced emission of SWCNTs (D-SWCNTs) 6-fold, while serotonin notably decreased it. In contrast, pristine, defect-free SWCNTs exhibited similar optical responses to both neurotransmitters. The underlying mechanisms for the divergent fluorescence response were found to be polydopamine (PDA) surface adsorption in the case of the fluorescence enhancement in response to DA, while the fluorescence decrease in response to serotonin was attributed to enhanced solvent relaxation effects in the presence of defects. Importantly, the divergent optical response of D-SWCNTs to DA and serotonin, via the introduction of defects, was validated in complex biological environments such as serum. Further, the generality of our approach was confirmed by the demonstrations of a divergent fluorescence response of D-SWCNTs suspended by an additional dispersant, namely lipid-polyethylene glycol (PEG). This study offers promising avenues for the broad applicability of surface functionalization of SWCNTs to achieve divergent responses toward structurally related molecules and advance applications in sensing, imaging, and diagnostic technologies.