Ultrafast Trap State-Mediated Electron Transfer for Quantum Dot Redox Sensing

Quantum dots (QDs) conjugated to electron acceptor ligands are useful as redox sensors in applications ranging from chemical detection to bioimaging. We aimed to improve effectiveness of these redox-sensing QD conjugates, which depends on the initial charge separation and on the competing mechanisms of recombination, including luminescence and electron transfer to the conjugated redox molecules. In this study, ultrafast laser measurements were used to study the excited state dynamics in CdTe/CdS core/shell QDs with quinone/quinol acceptor (Q2NS) ligands attached to the surface (up to 40 per QD). Detailed analysis, along with computational modeling of the system, showed multiple electron-transfer pathways and identified an ultrafast electron transfer from a surface electron trap state to the quinone ligands (2-8 ps). We propose that this leads to high, redox-dependent, quenching efficiencies (98.7% with an average of 10 quinone/quinols on the surface). As only low populations of redox ligands are required, the colloidal properties of the QD are preserved, which allows for further functionalization. These new insights into the excited state properties and ultrafast charge transfer have important implications for fields exploring charge extraction from quantum dots, which range from bioimaging to solar energy technology.