A switchable green emitting dye and its phenomenal properties: implications for the photoluminescence features of carbon dots

Abstract

New molecular fluorophores are constantly being discovered in post-synthetic mixtures of carbon dots (CDs), prompting researchers to elucidate their role in the optical properties of these nanomaterials. It has been reported that the green-emitting fluorophore that forms during the synthesis of popular citric acid/urea CDs is HPPT (4-hydroxy-1H-pyrrolo[3,4-c]pyridine-1,3,6(2H,5H)-trione). However, due to the low concentration of HPPT-like molecules within the structure of CDs, their actual binding and contribution to the optical properties of CDs has not so far been convincingly confirmed. In this joint experimental and quantum chemical study, we show that HPPT is a strong acid and only its anionic form, HPPT⁻, present in solutions of pH 0–10, is emissive. Next, its fluorescence can be switched off rapidly in strongly alkaline environments as a result of HPPT⁻ hydrolysis, leading to the opening of its pyrrole ring and formation of CDPC (3-carbamoyl-2,6-dihydroxypyridine-4-carboxylic acid), existing as the CDPC²⁻ dianion under these conditions. Eventually, we found that the ring opening hydrolysis is reversible and the green emission may be restored in acidic environments. The kinetics and mechanism of this hydrolysis were also revealed. The optical features of citric acid (CA)–urea CDs under various conditions were compared with a simpler CD system prepared by treating the CDs obtained from CA solely with HPPT⁻ (HPPT@CDs). Our results indicate the feasibility of the post-synthetic modification of HPPT⁻ present in the structures of CA–urea CDs and HPPT@CDs. Without HPPT⁻ they emit blue fluorescence only. Thus, this makes the nanosystem switch the PL emission colour reversibly from green to blue owing to the opening and closing of the pyrrole ring in HPPT-like molecules. More importantly, the latter process may be considered a first step toward genuine fine tuning of the PL emission colour from CDs. These findings are of general importance to the further development of citric acid-based CDs with tailored properties.