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

Abstract

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 modelling 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 functionalisation. These new insights into the excited state properties and ultra-fast charge transfer has important implications for fields exploring charge extraction from quantum dots, which range from bioimaging to solar energy technology.

Quantum Sensors To Track Total Redox-Status and Oxidative Stress in Cells and Tissues Using Electron-Paramagnetic Resonance, Magnetic Resonance Imaging, and Optical Imaging

“…The fluorescence of this redox sensor is also modulated by complex-I of the mitochondrial electron-transport chain in the presence of NADH. A similar redox sensor, based on QDs conjugated with quinone/quinol electron acceptor ligands, has been described recently by Harvie et al 48 Redox-status of cells, tissues, and body fluids is considered an important diagnostic marker and a therapeutic target for a variety of pathologies, associated with a disturbance of redoxsignaling in the organism (such as cancer, atherosclerosis, neurodegeneration, inflammation, diabetes, etc.). In this context, the development of redox-sensitive contrast probes and techniques for noninvasive imaging of cellular and tissue redox-status is crucial for the diagnosis and control of diseases in the course of their therapy.…”

Deciphering the role of quantum dot size in the ultrafast charge carrier dynamics at the perovskite–quantum dot interface

“…Strong effect of trap-states on the dynamics of charge carriers transfer in the systems containing semiconductor materials were previously reported. [43,[46][47] As we have already discussed, the ke and kh components reflect two processes: charge carriers diffusion within the PS domain, and transfer at the interface of both materials. Figure 2b shows that electron transfer rate constant is the same for the systems which contain QDs of 4.5 and 5.3 nm.…”

Influence of Surface Ligands on Charge-Carrier Trapping and Relaxation in Water-Soluble CdSe@CdS Nanorods

“…Hence, under certain circumstances, surface trapping of charges can also support the desired function and increase their availability to surface-proximal catalytic centers and/or hole scavengers. A similar concept has recently been introduced as trap-state mediated charge transfer [83,84]. Thus, deciding on an optimal surface ligand is a sophisticated task and affords detailed knowledge on the function determining mechanistic steps and the impact of the ligand on these.…”