Probing the “Dark” Fraction of Core–Shell Quantum Dots by Ensemble and Single Particle pH-Dependent Spectroscopy

Abstract

The optical properties of core-shell CdSe-ZnS quantum dots (QDs) are characterized by complex photophysics leading to difficulties in interpreting quantitative measurements based on QD emission. By comparing the pH dependence of fluorescence of single QDs to that of an ensemble, we have been able to propose a molecular scale model of how QD surface chemical and physical processes are affected by protons and oxygen. We show that the connection between the ensemble fluorescence intensity and the single QD fluorescence properties such as dark fraction, blinking, particle brightness and a multi-exponential fluorescence lifetime decay is not trivial. The ensemble fluorescence intensity is more weakly dependent on pH than the single particle fluorescence which, together with fluorescence lifetime analysis, provided evidence that the dark fraction of QDs emits photons with low quantum efficiency and long lifetime. We uncovered two surface-dependent mechanisms that affected the fluorescence emission: an immediate physical effect of charges surrounding the QD and an irreversible chemical effect from reaction of the H+ and O2 with the QD shell surface. These results will have important implications for those using QD-based fluorescence lifetime imaging as well as for proper implementation of these probes for quantitative cellular imaging applications.

Excitation wavelength dependence of the photoluminescence quantum yield and decay behavior of CdSe/CdS quantum dot/quantum rods with different aspect ratios

“…19,20 For QDs and QDQRs, the size of the spectroscopic key parameter ΦPL 21 is particularly relevant as this value provides also a direct measure for particle and shell quality. [22][23][24] Low ΦPL of QDs and QDQRs indicate insufficient surface passivation and the formation of energy states within the band gap that can act as trapping sites for photogenerated charge carriers. Within the last decades, the question if QDs and QDQRs show an excitation wavelength (λexc) dependence of ΦPL, 21 has been addressed by several groups with controversial results.…”

Cadmium-free quantum dots as time-gated bioimaging probes in highly-autofluorescent human breast cancer cells

“…Our experimental setup consists of a Picoquant Microtime 200 scanning confocal microscope, and has been previously described. 13 Briefly, in this experiment, a 485 nm pulsed laser is reflected from a dichroic mirror (500DCXR, Chroma, Bellows Falls, VT) through a water-immersion objective (UPlanApo 60×/1.20w, Olympus, Tokyo, Japan) and focused to a diffraction-limited spot. The emitted light is collected by the same objective, passes through the same dichroic, a 100 μm pinhole, and finally an emission filter (605/55m, Chroma) before being detected onto an avalanche photodiode (PDM series, MPD, Bolzano, Italy).…”

Targeting breast cancer cells with a CuInS2/ZnS quantum dot-labeled Ki-67 bioprobe

“…When QDs are used in in vitro or in vivo experiments, they are present in complex biological environments that may directly or indirectly affect the colloidal and optical stability of the QD formulation (32)(33)(34). Thus, the aqueous colloidal stability of QDs is of concern for use in long-term in vivo imaging applications, including for tumor detection and labeling.…”