Ultrafast Electron Transfer Research Articles

Ultrafast electron transfer at the interface of gold nanoparticles and methylene blue molecular adsorbates.

Due to their unique property of possessing localized surface plasmon resonance (LSPR), metal nanoparticles (MNPs) have drastically impacted many applications. For instance, local field enhancement through LSPRs and plasmonic hot electron transfer are known to enhance the efficiency of MNP-based photoreactions. Here, we report on the ultrafast electron transfer from gold nanoparticles (Au-NPs) to methylene blue (MB) molecular adsorbate using femtosecond pump-probe and steady-state absorption and emission spectroscopy techniques. Although the energy band alignment of the interface allows both dipole-dipole Förster resonance energy transfer (FRET) and charge transfer, because the MB emission intensity at the Au-NPs/MB nanocomposite decreased by a factor of ∼3.6, the FRET process was ruled out. Selective excitation of LSPRs at the Au-NPs/MB nanocomposite sample in pump-probe experiments led to the formation of the MB ground-state depletion and a positive induced absorption at wavelengths shorter than ∼500 nm, which was attributed to the shoulder of the MB- anion absorption. Furthermore, despite the fact that the concentration of Au-NPs in the nanocomposite sample is the same as that in the Au-NPs solution, the initial intensity of the LSPR depletion signal was about six times weaker than that in the Au-NPs sample. These observations suggest that electron transfer from excited Au-NPs to MB adsorbates took place on a time-scale that is shorter than the ∼50 fs experimental temporal resolution.

Ultrafast separation of multiexcitons within core/shell quantum dot hybrid systems.

We investigated the electron transfer processes in methylene blue-CdTe and methylene blue-CdTe/CdSe complexes by steady state and femtosecond transient absorption spectroscopy by selective excitation of the quantum dot (QD) moiety. The ultrafast electron transfer is accelerated by the shell growth due to the separation of the charge carriers and the resulting increase of electron density in the shell. Transmission electron microscope images show that the successive addition of shell material deforms the spherical QDs until they adopt a tetrapodal shape. The increased donor-acceptor distance in the tetrapodal CdTe/CdSe QDs leads to a slower electron transfer. Photon flux dependent transient absorption measurements indicate the separation of two electrons for the QDs with a thin shell and thus demonstrate that charge carrier multiplication can be directly utilized for increased charge transfer in this type of QD hybrid system.

Ultrafast charge transfer in a nonfullerene all-small-molecule organic solar cell: a nonadiabatic dynamics simulation with optimally tuned range-separated functional.

Herein, we have employed linear-response time dependent density functional theory (LR-TDDFT)-based nonadiabatic dynamics simulations to investigate the ultrafast charge transfer in a nonfullerene all-small-molecule donor-acceptor (D-A) system formed by a porphyrin small-molecule donor ZnP and a recently developed nonfullerene small-molecule acceptor 6TIC, during which the optimally tuned range-separated hybrid (OT-RSH) functional was adopted. In combination with static electronic structure calculations, several important conclusions were drawn. Firstly, the ZnP and 6TIC are more likely combined together non-covalently in parallel rather than in perpendicular to form ZnP-6TIC due to the much larger adsorption energies, i.e. -44.6 kcal mol-1vs. -25.2 kcal mol-1. Secondly, the excited state properties obtained by OT-RSH functionals seem more consistent with the experimental results compared to their untuned versions. Specifically, the energy of the lowest charge transfer (CT) state was predicted to be smaller than the lowest lying local excitation (LE) states using the OT-RSH functional-based LR-TDDFT calculations, which is beneficial for the charge transfer process that might be crucial for the high power conversion efficiency (PCE) achieved experimentally. In contrast, the untuned RS functionals all predict higher CT state energies, which is contradictory to the high PCE obtained in the experiment. Moreover, strong hybridization upon excitation between these states was revealed, which might be one of the reasons responsible for the high PCE observed in the experiment. Finally, ultrafast excited state relaxation can be completed within 500 fs due to the small energy gaps and the strong nonadiabatic couplings between these states, which is accompanied by ultrafast photoinduced electron transfer from ZnP to 6TIC and photoinduced hole transfer the other way around. The efficient charge transfer processes and the involvement of two charge generation channels might be another cause resulting in the excellent photovoltaic performance of ZnP-6TIC based OSCs. Our present work not only provides solid evidence for elucidating the underlying mechanism observed in previous experiments, but also suggests that the combination of OT-RSH functionals and LR-TDDFT-based nonadiabatic dynamics simulations might be a powerful tool for investigating the excited state dynamics of organic D-A systems, which is crucial for the theoretical design of novel OSCs with better performances in the future.

Development of an experimental apparatus to observe ultrafast phenomena by tender X-ray absorption spectroscopy at PAL-XFEL.

Understanding the ultrafast dynamics of molecules is of fundamental importance. Time-resolved X-ray absorption spectroscopy (TR-XAS) is a powerful spectroscopic technique for unveiling the time-dependent structural and electronic information of molecules that has been widely applied in various fields. Herein, the design and technical achievement of a newly developed experimental apparatus for TR-XAS measurements in the tender X-ray range with X-ray free-electron lasers (XFELs) at the Pohang Accelerator Laboratory XFEL (PAL-XFEL) are described. Femtosecond TR-XAS measurements were conducted at the Ru L3-edge of well known photosensitizer tris(bipyridine)ruthenium(II) chloride ([Ru(bpy)3]2+) in water. The results indicate ultrafast photoinduced electron transfer from the Ru center to the ligand, which demonstrates that the newly designed setup is applicable for monitoring ultrafast reactions in the femtosecond domain.

Switching from ultrafast electron transfer to proton transfer in excited drug-protein complexes upon biotransformation.

Photosensitization by drugs is directly related with the excited species and the photoinduced processes arising from interaction with UVA light. In this context, the ability of gefitinib (GFT), a tyrosine kinase inhibitor (TKI) used for the treatment of a variety of cancers, to induce phototoxicity and photooxidation of proteins has recently been demonstrated. In principle, photodamage can be generated not only by a given drug but also by its photoactive metabolites that maintain the relevant chromophore. In the present work, a complete study of O-desmorpholinopropyl gefitinib (GFT-MB) has been performed by means of fluorescence and ultrafast transient absorption spectroscopies, in addition to molecular dynamics (MD) simulations. The photobehavior of the GFT-MB metabolite in solution is similar to that of GFT. However, when the drug or its metabolite are in a constrained environment, i.e. within a protein, their behavior and the photoinduced processes that arise from their interaction with UVA light are completely different. For GFT in complex with human serum albumin (HSA), locally excited (LE) singlet states are mainly formed; these species undergo photoinduced electron transfer with Tyr and Trp. By contrast, since GFT-MB is a phenol, excited state proton transfer (ESPT) to form phenolate-like excited species might become an alternative deactivation pathway. As a matter of fact, the protein-bound metabolite exhibits higher fluorescence yields and longer emission wavelengths and lifetimes than GFT@HSA. Ultrafast transient absorption measurements support direct ESPT deprotonation of LE states (rather than ICT), to form phenolate-like species. This is explained by MD simulations, which reveal a close interaction between the phenolic OH group of GFT-MB and Val116 within site 3 (subdomain IB) of HSA. The reported findings are relevant to understand the photosensitizing properties of TKIs and the role of biotransformation in this type of adverse side effects.

Electron transfer kinetics in CdS/Pt heterojunction photocatalyst during water splitting

Noble metal cocatalysts have shown great potential in boosting the performance of CdS in photocatalytic water splitting. However, the mechanism and kinetics of electron transfer in noble-metal-decorated CdS during practical hydrogen evolution is not clearly elucidated. Herein, Pt-nanoparticle-decorated CdS nanorods (CdS/Pt) are utilized as the model system to analyze the electron transfer kinetics in CdS/Pt heterojunction. Through femtosecond transient absorption spectroscopy, three dominating exciton quenching pathways are observed and assigned to the trapping of photogenerated electrons at shallow states, recombination of free electrons and trapped holes, and radiative recombination of locally photogenerated electron-hole pairs. The introduction of Pt cocatalyst can release the electrons trapped at the shallow states and construct an ultrafast electron transfer tunnel at the CdS/Pt interface. When CdS/Pt is dispersed in acetonitrile, the lifetime and rate for interfacial electron transfer are respectively calculated to be ~5.5 ps and ~3.5 × 1010 s−1. The CdS/Pt is again dispersed in water to simulate photocatalytic water splitting. The lifetime of the interfacial electron transfer decreases to ~5.1 ps and the electron transfer rate increases to ~4.9 × 1010 s−1, confirming that Pt nanoparticles serve as the main active sites of hydrogen evolution. This work reveals the role of Pt cocatalysts in enhancing the photocatalytic performance of CdS from the perspective of electron transfer kinetics.

Amino-functionalized nitrogen-doped graphene quantum dots and silver-graphene based nanocomposites: Ultrafast charge transfer and a proof-of-concept study for bioimaging applications

Recent advances in nanotechnology have led to the use of graphene quantum dots (GQDs) for functional imaging and cancer therapy. Thanks to the physicochemical properties of graphene, GQDs have been functionalized and modified to improve their biological properties. In this study, polyethyleneimine functionalized nitrogen-doped graphene quantum dots (PEI N-doped GQDs) and silver nanoparticles/nitrogen-doped graphene quantum dots (AgNPs/PEI N-doped GQDs) nanocomposites have been developed. The PEI N-doped GQDs show a strong emission spectrum at maximum 446 nm with 356 nm excitation wavelength while AgNPs/PEI N-doped GQDs nanocomposites indicate weak fluorescence intensity at maximum 446 nm with 344 nm excitation wavelength. Based on the femtosecond transient absorption spectroscopy measurements, this emission quenching can be attributed that the excited state lifetime of PEI N-doped GQDs decreasing in the presence of AgNPs due to ultrafast electron transfer from PEI N-doped GQDs to AgNPs. It is also supported by theoretical results sensing ability of AgNPs/PEI N-doped GQDs nanocomposite is better than that of PEI N-doped GQDs. When lung adenocarcinoma cells (A549) were treated with GQDs, material-specific fluorescent signals were obtained from the cytoplasm of cells only treated with Ag/PEI N-doped GQDs, as compared with N-doped GQDs. Overall, this study shows that Ag/PEI N-doped GQDs are promising nanosystems for bioimaging applications.

Synthesis, electronic and photophysical properties of a bisacridinium-Zn(II) porphyrin conjugate

The synthesis of a novel bisacridinium-Zn(II) porphyrin is reported and its properties investigated via electrochemical, photophysical and computational studies. Cyclic voltammetry studies revealed a two-electron oxidation of the Zn(II) porphyrin and the simultaneous one electron reductions of the two acridiniums. Using absorption, emission and ultrafast transient absorption spectroscopies, the near total fluorescence quenching observed following excitation of either the acridinium or Zn(II) porphyrin units was assigned to ultrafast electron transfer (≤0.3 ps) leading to a reduced acridinium and an oxidized porphyrin unit in the bisacridinium-Zn(II) porphyrin conjugate. In addition, computational studies were found to complement experimental results, with calculations revealing two near degenerate HOMOs for the porphyrin.

An oligomeric semiconducting nanozyme with ultrafast electron transfers alleviates acute brain injury.

Artificial enzymes have attracted wide interest in disease diagnosis and biotechnology due to high stability, easy synthesis, and cost effectiveness. Unfortunately, their catalytic rate is limited to surface electron transfer, affecting the catalytic and biological activity. Here, we report an oligomeric nanozyme (O-NZ) with ultrafast electron transfer, achieving ultrahigh catalytic activity. O-NZ shows electron transfer of 1.8 nanoseconds in internal cores and 1.2 picoseconds between core and ligand molecule, leading to ultrahigh superoxidase dismutase–like and glutathione peroxidase–like activity (comparable with natural enzyme, Michaelis constant = 0.87 millimolars). Excitingly, O-NZ can improve the 1-month survival rate of mice with acute brain trauma from 50 to 90% and promote the recovery of long-term neurocognition. Biochemical experiments show that O-NZ can decrease harmful peroxide and superoxide via in vivo catalytic chain reaction and reduce acute neuroinflammation via nuclear factor erythroid-2 related factor 2–mediated up-regulation of heme oxygenase-1 expression.

Ultrafast Electron Transfer in Binary Nanoparticle Superlattices under High Pressure

Ultrafast Electron Transfer in Binary Nanoparticle Superlattices under High Pressure

Giant Shape-Persistent Tetrahedral Porphyrin System: Light-Induced Charge Separation.

Tetraphenylmethane appended with four pyridylpyridinium units works as a scaffold to self‐assemble four ruthenium porphyrins in a tetrahedral shape‐persistent giant architecture. The resulting supramolecular structure has been characterised in the solid state by X‐ray single crystal analysis and in solution by various techniques. Multinuclear NMR spectroscopy confirms the 1 : 4 stoichiometry with the formation of a highly symmetric structure. The self‐assembly process can be monitored by changes of the redox potentials, as well as by modifications in the visible absorption spectrum of the ruthenium porphyrin and by a complete quenching of both the bright fluorescence of the tetracationic scaffold and the weak phosphorescence of the ruthenium porphyrin. An ultrafast photoinduced electron transfer is responsible for this quenching process. The lifetime of the resulting charge separated state (800 ps) is about four times longer in the giant supramolecular structure compared to the model 1 : 1 complex formed by the ruthenium porphyrin and a single pyridylpyridinium unit. Electron delocalization over the tetrameric pyridinium structure is likely to be responsible for this effect.

Ultrafast Interface Charge Separation in Carbon Nanodot-Nanotube Hybrids.

Carbon dots are an emerging family of zero-dimensional nanocarbons behaving as tunable light harvesters and photoactivated charge donors. Coupling them to carbon nanotubes, which are well-known electron acceptors with excellent charge transport capabilities, is very promising for several applications. Here, we first devised a route to achieve the stable electrostatic binding of carbon dots to multi- or single-walled carbon nanotubes, as confirmed by several experimental observations. The photoluminescence of carbon dots is strongly quenched when they contact either semiconductive or conductive nanotubes, indicating a strong electronic coupling to both. Theoretical simulations predict a favorable energy level alignment within these complexes, suggesting a photoinduced electron transfer from dots to nanotubes, which is a process of high functional interest. Femtosecond transient absorption confirms indeed an ultrafast (<100 fs) electron transfer independent of nanotubes being conductive or semiconductive in nature, followed by a much slower back electron transfer (≈60 ps) from the nanotube to the carbon dots. The high degree of charge separation and delocalization achieved in these nanohybrids entails significant photocatalytic properties, as we demonstrate by the reduction of silver ions in solution. The results are very promising in view of using these “all-carbon” nanohybrids as efficient light harvesters for applications in artificial photocatalysis and photosynthesis.

Boosting Cascade Electron Transfer for Highly Efficient CO2 Photoreduction

Artificial photosynthesis converting carbon dioxide into chemical fuels with a high added value is a promising solution to both fossil fuel shortage/pollution and global climate change; however, the development of highly efficient photocatalysts toward this goal is still largely bereft of fresh ideas. Herein, we propose a “cascade electron transfer” strategy through spurring both interfacial and inner electron transfer rates for a 0D/2D photocatalyst of CsPbBr3/CuTCPP metal organic framework (MOF). Upon photoexcitation, the heterojunction structure with an appropriate band alignment facilitates an ultrafast interfacial electron transfer rate of 1.4 ps from CsPbBr3 segment to CuTCPP MOF and subsequent ultrafast internal electron transfer of 21 ps from CuTCPP ligand to the Cu node within the enlarged 2D framework. The efficient electron transfer ensures efficient charge separation favorable for photocatalytic reactions: The photocatalyst exhibits an outstanding yield of 47.2 μmol g−1 h−1 (CO and CH4 combined) superior to previous reports. Rational design of hierarchical heterojunctions with matching electronic bandgap not only expedites cascade charge transfer but also prevents holes from recombining with electrons or oxidizing the photocatalysts without the necessity of sacrificial reagents. This work thus provides useful insight for boosting photocatalytic efficacy from a dynamic perspective.

Unblocked intramolecular charge transfer for enhanced CO2 photoreduction enabled by an imidazolium-based ionic conjugated microporous polymer

Efficient solar energy-driven conversion of CO2 to valuable chemicals is challenging. Here we design an imidazolium-based ionic conjugated microporous polymer (ImI-CMP), which unblocks readily intramolecular charge transfer and triggers CO2 photoreduction. Under visible light irradiation, ImI-CMP incorporated with Co (II) species exhibits a high CO production rate of 2953 μmol g−1 h−1 and a turnover frequency of 30.8 h−1; these numbers are competitive to that of the best porous organic polymers. The mechanism studies reveal that two factors play key roles in the outstanding photocatalytic performance. First, the imidazolium motifs on the ImI-CMP enhance the activation of CO2. Second, π-conjugation structure and the built-in electric field allow ultrafast intramolecular photoinduced electron transfer in the ImI-CMP. This work provides a new strategy for designing high-performance organic photocatalysts for CO2 reduction by combining cationic imidazolium motifs and π-conjugation structures.

Photogenerated Spin-Correlated Radical Pairs: From Photosynthetic Energy Transduction to Quantum Information Science.

More than a half century ago, the NMR spectra of diamagnetic products resulting from radical pair reactions were observed to have strongly enhanced absorptive and emissive resonances. At the same time, photogenerated radical pairs were discovered to exhibit unusual electron paramagnetic resonance spectra that also had such resonances. These non-Boltzmann, spin-polarized spectra were observed in both chemical systems as well as in photosynthetic reaction center proteins following photodriven charge separation. Subsequent studies of these phenomena led to a variety of chemical electron donor-acceptor model systems that provided a broad understanding of the spin dynamics responsible for these spectra. When the distance between the two radicals is restricted, these observations result from the formation of spin-correlated radical pairs (SCRPs) in which the spin-spin exchange and dipolar interactions between the two unpaired spins play an important role in the spin dynamics. Early on, it was recognized that SCRPs photogenerated by ultrafast electron transfer are entangled spin pairs created in a well-defined spin state. These SCRPs can serve as spin qubit pairs (SQPs), whose spin dynamics can be manipulated to study a wide variety of quantum phenomena intrinsic to the field of quantum information science. This Perspective highlights the role of SCRPs as SQPs, gives examples of possible quantum manipulations using SQPs, and provides some thoughts on future directions.

Ultrafast Electron Transfer in a Self-Assembling Sulfonated Aluminum Corrole-Methylviologen Complex.

Photoinduced electron transfer systems can mimic certain features of natural photosynthetic reaction centers, which are crucial for solar energy production. Among other tetra-pyrroles, the versatile chemical and photophysical properties of corroles make them very promising donors applicable in donor-acceptor complexes. Here, we present a first comprehensive study of ultrafast photoinduced electron transfer in a self-assembling sulfonated aluminum corrole-methylviologen complex combining visible and mid-IR transient absorption spectroscopy. The noncovalent D-A association of the corrole-methylviologen complex has the great advantage that photoinduced charge separation becomes possible even though the back electron transfer (BET) rate is large. Initial forward electron transfer from corrole to methylviologen is observed on an ∼130 fs time scale. Subsequent back electron transfer takes place with τBET = (1.8 ± 0.5) ps, revealing very complex relaxation dynamics. Direct probing in the mid-IR allows us to unravel the back electron transfer and cooling dynamics/electronic reorganization. Upon tracing the dynamics of the methylviologen-radical marker band at 1640 cm-1 and the C═C stretching of corrole at around 1500 cm-1, we observe that large amounts of excess energy survive the back transfer, leading to the formation of hot ground state absorption. A closer examination of the signal after 300 ps, surviving the back transfer, exhibits a charge-separation yield of 10-15%.

Criteria for Assessing the Interlayer Coupling of van der Waals Heterostructures Using Ultrafast Pump-Probe Photoluminescence Spectroscopy.

van der Waals (vdW) heterostructures of transition metal dichalcogenides (TMDCs) provide an excellent paradigm for next-generation electronic and optoelectronic applications. However, the reproducible fabrications of vdW heterostructure devices and the boosting of practical applications are severely hindered by their unstable performance, due to the lack of criteria to assess the interlayer coupling in heterostructures. Here we propose a physical model involving ultrafast electron transfer in the heterostructures and provide two criteria, η (the ratio of the transferred electrons to the total excited electrons) and ζ (the relative photoluminescence variation), to evaluate the interlayer coupling by considering the electron transfer in TMDC heterostructures and numerically simulating the corresponding rate equations. We have proved the effectiveness and robustness of two criteria by measuring the pump-probe photoluminescence intensity of monolayer WS2 in the WS2/WSe2 heterostructures. During thermal annealing of WS2/WSe2, ζ varies from negative to positive values and η changes between 0 and 4.5 × 10-3 as the coupling strength enhanced; both of them can well characterize the tuning of interlayer coupling. We also design a scheme to image the interlayer coupling by performing PL imaging at two time delays. Our scheme offers powerful criteria to assess the interlayer coupling in TMDC heterostructures, offering opportunities for the implementation of vdW heterostructures for broadband and high-performance electronic and optoelectronic applications.

Ultrafast photoinduced electron transfer in o-aminobenzoate – d-Amino acid oxidase complex

Absorption spectrum of ortho-amino benzoate (oAB) – D-amino acid oxidase complex (ODC) displays a charge transfer (CT) band around 570 nm. The fluorescence decays of the second lowest electronic state of ODC were measured using a fluorescence up-conversion technique. The lifetimes were 100 fs and 130 fs at 500 nm and 514 nm fluorescence emission wavelengths, respectively. These ultrafast lifetimes are ascribed to the photoinduced electron transfer (PET) from the oAB to the excited isoalloxazine (Iso*) in the ODC. The PET mechanism in ODC was studied with Marcus-Kakitani-Mataga (MKM) theory. The protein structures were obtained by molecular dynamics simulation (MDS). Mean distances between Iso and oAB over 12,500 MDS snapshots were 0.60 in subunit A (Sub A) and 0.58 nm in Sub B. The calculated lifetimes at 514 nm-emission were 97 and 179 fs in Sub A and Sub B, while at 500 nm there were 97 and 101 fs, respectively. Solvent reorganization energy was quite different between Sub A and Sub B, which is main reason for difference in the calculated lifetimes. Electrostatic (ES) energies between the photoproducts of donor -acceptor, and net ES energies between the photoproducts and ionic groups in the protein were calculated with MDS structures. These ES energies were evaluated with charge densities of neutral radical of oAB produced after PET and anion radical of Iso obtained by semi-empirical molecular orbital (MO) method. The logarithmic PET rate did not display any clear dependence on Rc, despite the ultrafast scale of the PET rates. Most of the logarithmic PET rates in Sub A displayed in the inverted region of the standard free energy gap in the energy gap law. The transition dynamics among local excited states of Iso, CT state and the ion-pair state were discussed.

MnOx bound on oxidized multi-walled carbon nanotubes as anode for lithium-ion batteries

Nanocomposites of manganese oxides and carbonaceous materials with strong interfacial connection hold extraordinary potential as high-performance electrode materials for energy storage devices. However, the interfacial interaction between manganese oxides and carbon has not been sufficient elucidated despite it may affect the chemical composition and morphology of the electrode materials and thus impact on the electrochemical performance. Herein, through regulating the amount of oxygen-containing functional groups on the surface of multi-walled carbon nanotubes (MWCNTs), a series of MnOx/MWCNTs nanocomposites with different compositions and morphologies are successfully synthesized. The Mn–O–C bonds, which can act as an ultrafast electron transfer path, are formed in the MnOx/MWCNTs nanocomposites. The influence of the amount and type of oxygen-containing functional groups in MWCNTs on the formation of Mn–O–C bonds, the composition and morphology of the nanocomposites is revealed clearly. The nanocomposite with the highest number of Mn–O–C bonds in the prepared MnOx/MWCNTs, which feature a structure with the binary manganese oxides (38% MnO2 and 62% Mn3O4) nanosheets growing vertically and uniformly on the oxidized MWCNTs, give the best electrochemical properties with high capacity (1353.2 mAh g−1 at 0.1 A g−1), outstanding rate capability (384.9 mAh g−1 at 20 A g−1), and durable long-term cyclability (1000 cycles with 70% retention at 2 A g−1). The quantitative kinetic analysis indicates that over 60% of the charge storage is pseudocapacitive. The distinct interfacial design strategy presents a new route to synthesis transition metal oxides/carbon hybrids with efficient energy storage and conversion.

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

The prospect of aqueous polyethyleneimine-capped CdS quantum dots (QDs), in toxic metal-ion sensing, has been explored. Pb2+ 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 Pb2+ 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 × 1011 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.