Mechanistic insights into pH-sensitive photoluminescence of carbon dots: The role of carboxyl group

We present a mechanistic study of pH-sensitive photoluminescence (PL) in two deliberately designed systems of carbon dots (CDs), which are relatively poor and rich in carboxyl groups anchored on their surfaces, denoted CDs-COOH(p) and CDs-COOH(r), respectively. The underlying PL mechanisms for the two contrasting CD systems are revealed to be different. As for CDs-COOH(p), the pH response of PL exhibits an asymmetric volcano-shaped pattern featuring dynamic and static quenching under acidic and alkaline conditions, dominated by the effects of hydrogen bonding and non-emissive ground-state complex, respectively. As for CDs-COOH(r), however, the pH response exhibits an interesting sigmoid-shaped pattern featuring PL quenching under acidic conditions but PL enhancement under alkaline conditions, both of which become more pronounced with increasing photoexcitation energy, exhibiting a nearly symmetric trumpet-shaped pattern. Such patterns of PL response to acidity/alkalinity and photoexcitation energy can be understood in terms of the prominent effect of excited-state proton transfer that is coupled to the surface emissive centers of the carboxyl group and can be effectively modulated via pH-regulated protonation/deprotonation. Our comparative analyses of the pH-regulated surface-sensitive PL quenching/enhancement behaviors in the two CD systems allow for elucidating the different surface-state-controlled PL mechanisms, highlighting the specific role of carboxyl groups in the pH-sensitive PL of CDs. The mechanistic insights gleaned from this work would be useful for CDs-based applications such as luminescence, sensing, and bioimaging.