Mechanistic Insights into Selective Sensing of Pb²⁺ in Water by Photoluminescent CdS Quantum Dots

Abstract

The prospect of aqueous polyethyleneimine-capped CdS quantum dots (QDs), in toxic metal-ion sensing, has been explored. Pb 2+ binds strongly to the surface of the QDs facilitating ultrafast electron transfer. As a result, severe (∼90%) PL quenching is observed. Hot electron transfer plays an important role in the quenching process, as is elucidated by anticorrelation between the magnitude of ground-state bleach of the QDs and the concentration of Pb 2+ ions, as well as the concurrent decrease in bleach rise time. A second major contribution is from electron transfer from conduction band edge, with a rate constant of 1.45 × 10 11 s −1 . Selectivity in this "turn-off" sensing process is governed by the exergonicity, quality of QD surface, nature of capping ligand, and its metal-ion binding properties. Engineering these factors is crucial for the development of QD-based selective and efficient metal-ion sensors.

Exciton Dynamics in Colloidal CdS Quantum Dots with Intense and Stokes Shifted Photoluminescence in a Single Decay Channel

“…39 Selective PL quenching by Pb 2+ has been attributed to a combination of energetics of electron transfer and affinities of metal ions for the capping agent. 47 Once again, complementary time-resolved spectroscopic techniques have been used to elucidate the exciton dynamics in these systems. With this background, the present work is an attempt to elucidate exciton dynamics in CdS QDs synthesized by a Cd(II) complex-based sol−gel methodology, the detail of which is to be presented in an upcoming publication.…”

Defect-Mediated Charge Carrier Recombination of Zinc–Silver–Indium Sulfide QDs in the Presence of Naphthoquinone Derivatives: An Insight into the Interfacial Surface Dynamics

“…This aspect has been discussed elaborately by analyzing the ultrafast transient data ( vide infra ). Therefore, the observation of destruction of some of the surface/trap states of QD in the presence of chemical inputs, though, is rare but not uncommon. , This has been observed on a few occasions by some other researchers for different QDs and different ligands. ,, Thus, the present observation essentially tells us that when there is a reduction in τ avg value, it does not always reflect the dynamic behavior of the quenching event, rather it may indicate preferential removal of surface/trap states of the QDs upon interaction with molecules such as naphthoquinone derivatives . The fact that the SV equation is found not to be satisfactory in explaining the fluorescence quenching data is clearly indicative of the fact that the surface states of QD are significantly perturbed in the presence of naphthoquinones due to QD–naphthoquinone static interactions .…”

“…This indicates the possibility of the presence of surface/trap states that might have originated from intra-band-gap transitions and thus has dominated the excitonic peak . Even for different excitation wavelengths, observation of no change in the full width at half-maxima (FWHM) of QD’s emission spectrum is also indicative of the fact that the origin of emission is most likely due to defect states rather than the presence of heterogeneity in the sample . It is to be noted that because of these defect states, there is a large Stoke’s shift of ∼200 nm, and a long fluorescence lifetime of ZAIS has been observed ( vide infra ). , We also note here that the optical properties of ZAIS mostly correspond to the alloyed nature due to the cationic diffusion of Zn 2+ ions (ZnS) into AgInS 2 .…”

Lysozyme protected Cu Nano-Cluster: A Photo Switch for the Selective Sensing of Iron (Fe2+)

“…70,77 Pure dynamic/collisional quenching can also lead to a non-linear Stern-Volmer plot but with downward curvature in some extreme/rare cases if some fluorophores are less accessible than others. 78,79 For this later case a modified Stern-Volmer plot [F0/(F0 -F) vs 1/[Q]] should results in a linear plot 70,79 which could not be seen in the present case (Figure S8). In our case we could see a very interesting sharp transition at ~10 nM which has not been seen earlier.…”

“…45,63,80 The initial high slope indicating the existence of static quenching as the kinetics can not be explained with diffusion controlled quenching only. 79 Involvment of static quenching is further confirmed by lifetime measurements. In static quenching a complex is formed at the ground state between the fluorophore and the quencher which does not affect the lifetime of the system.…”