The Role of sp3 Defects in Eliciting 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 compositionally analogous molecules. Herein, we present a novel method for tuning the fluorescence response of single-walled carbon nanotubes (SWCNTs) towards dopamine (DA) and serotonin, two compositionally similar 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 towards these neurotransmitters, where DA significantly increased the fluorescence of the defect-induced SWCNTs (D-SWCNTs) six-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 DA’s fluorescence enhancement, while serotonin’s fluorescence decrease was attributed to enhanced solvent relaxation effects in the presence of defects. Importantly, the divergent optical response between DA and serotonin by D-SWCNTs via the introduction of defects was validated in complex biological environments, such as serum. Further, the generality of our approach was confirmed by the successful demonstrations of a divergent fluorescence response of D-SWCNT suspended by an additional dispersant, namely lipid–polyethylene glycol (PEG). This study offers promising avenues for the broad applicability of tailored surface functionalization of SWCNTs to achieve divergent responses towards compositionally similar molecules and advance innovative applications in sensing, imaging, and diagnostic technologies.