A chloride ion nanosensor for time-resolved fluorimetry and fluorescence lifetime imaging

In this work, the first CdSe/ZnS quantum dot (QD) photoluminescence lifetime based chloride ion nanosensor is reported. The acridinium dication lucigenin was self-assembled on the surface of negatively charged mercaptopropionic acid capped QDs to achieve QD-lucigenin conjugates. Upon attachment, a drastic decrease of the photoluminescence lifetime of both QD nanoparticles and lucigenin is observed by virtue of a charge transfer mechanism. Since lucigenin is a chloride-sensitive indicator dye, the photoluminescence decay of QD-lucigenin conjugates changes by adding chloride ion. The photoluminescence lifetime of the QDs in the conjugate increases after reacting with Cl(-), but also shows a concomitant decrease in the lucigenin lifetime immobilized on the surface. The photoluminescence lifetime of QD-lucigenin nanosensors shows a linear response in the Cl(-) concentration range between 0.5 and 50 mM. Moreover, the ratio τ(ave)(QD)/τ(ave)(luc) can be used as an analytical signal since the lifetime ratio presents a linear response in the same Cl(-) concentration range. The system also shows good selectivity towards most of the main anions and molecules that can be found in biological fluids. These nanosensors have been satisfactorily applied for Cl(-) determination in simulated intracellular media with high sensitivity and high selectivity. Finally, we demonstrate the potential application of the proposed nanosensor in confocal fluorescence lifetime imaging (FLIM). These results show the promising application of the QD-lucigenin nanosensors in FLIM, particularly for intracellular sensing, with the invaluable advantages of the time-resolved fluorescence techniques.     

Temperature-dependent energy transfer in cadmium telluride quantum dot solids

Efficiently luminescing colloidal CdTe quantum dots (QDs) were used for the preparation of monodispersed and mixed size QD solids. Luminescence spectra and decay times of the QD emission were measured as a function of temperature to study energy transfer (ET) processes in the QD solids. In the luminescence decay curves of the emission of the largest QDs (acceptors), a rise time of the luminescence signal is observed due to energy transfer from smaller QDs. Both the rise time (a measure for the energy transfer rate) and the luminescence decay time lengthen upon cooling. This is explained by the decreased dipole strength of the excitonic emission of the QDs in the solid due to the presence of a singlet and a lower lying triplet level. Studies of energy transfer in heteronuclear QD solids reveal that single-step ET dominates.     

The effects of chemisorption on the luminescence of CdSe quantum dots

We report on the effects of Lewis bases and other ligands on radiative recombination in CdSe quantum dots (QDs) in several solvents. Long-chain primary amines are found to be the most efficacious capping agents for CdSe QDs in nonpolar solvents. Primary alkylamines are superior to secondary and tertiary alkylamines. The kinetics of chemisorption and desorption in less polar solvents, such as hexane or chloroform, are temperature controlled and obey a Langmuir isotherm. Mercaptan adsorption also obeys a Langmuir isotherm, and alkylmercaptans rapidly displace amines, leading to luminescence quenching. In more polar solvents, such as toluene, ligands desorb, leading to luminescence quenching. It is proposed that surface Cd vacancies function as nonradiative recombination centers. The adsorption of a Lewis base to the QD raises the surface vacancy energy close to, or above, the conduction band edge and eliminates electron capture by the surface vacancies. Solvent polarity has a strong effect on luminescence since the solvent determines the extent of ligand adsorption to the QD surface.     

Luminescence of Polyethylene Glycol Coated CdSeTe/ZnS and InP/ZnS Nanoparticles in the Presence of Copper Cations

The use of click chemistry for quantum dot (QD) functionalization could be very promising for the development of bioconjugates dedicated to in vivo applications. Alkyne–azide ligation usually requires copper(I) catalysis. The luminescence response of CdSeTe/ZnS nanoparticles coated with polyethylene glycol (PEG) is studied in the presence of copper cations, and compared to that of InP/ZnS QDs coated with mercaptoundecanoic acid (MUA). The quenching mechanisms appear different. Luminescence quenching occurs without any wavelength shift in the absorption and emission spectra for the CdSeTe/ZnS/PEG nanocrystals. In this case, the presence of copper in the ZnS shell is evidenced by energy‐filtered transmission electron microscopy (EF‐TEM). By contrast, in the case of InP/ZnS/MUA nanocrystals, a redshift of the excitation and emission spectra, accompanied by an increase in absorbance and a decrease in photoluminescence, is observed. For CdSeTe/ZnS/PEG nanocrystals, PL quenching is enhanced for QDs with 1) smaller inorganic‐core diameter, 2) thinner PEG shell, and 3) hydroxyl terminal groups. Whereas copper‐induced PL quenching can be interesting for the design of sensitive cation sensors, copper‐free click reactions should be used for the efficient functionalization of nanocrystals dedicated to bioapplications, in order to achieve highly luminescent QD bioconjugates.     

Assembly and separation of semiconductor quantum dot dimers and trimers

Repeated precipitation of colloidal semiconductor quantum dots (QD) from a good solvent by adding a poor solvent leads to an increasing number of QD oligomers after redispersion in the good solvent. By using density gradient ultracentrifugation we have been able to separate QD monomer, dimer, and trimer fractions from higher oligomers in such solutions. In the corresponding fractions QD dimers and trimers have been enriched up to 90% and 64%, respectively. Besides directly coupled oligomers, QD dimers and trimers were also assembled by linkage with a rigid terrylene diimide dye (TDI) and separated again by ultracentrifugation. High-resolution transmission electron micrographs show that the interparticle distances are clearly larger than those for directly coupled dots proving that the QDs indeed are cross-linked by the dye. Moreover, energy transfer from the QDs to the TDI "bridge" has been observed. Individual oligomers (directly coupled or dye-linked) can be readily deposited on a substrate and studied simultaneously by scanning force and optical microscopy. Our simple and effective scheme is applicable to a wide range of ligand stabilized colloidal nanoparticles and opens the way to a detailed study of electronic coupling in, e.g., QD molecules.     

基于自然尺寸分布的单一CdTe量子点样品中Eu(III)诱导的能量转移测定磷酸根离子

本文报道了一种用水溶性CdTe量子点传感磷酸根离子的发光方法. 该法是基于在具有自然尺寸分布的单一量子点样品中三价铕所诱导的能量转移. 实验表明, 三价铕离子能诱导表面带有负电荷的量子点发生簇集, 引发量子点间的能量转移, 导致显著的发光猝灭和发射位移. 当在上述体系中加入磷酸根离子时, 由于它与三价铕离子强烈的配位竞争瓦解了所形成的量子点簇集, 因此引起量子点起始发光的逐渐恢复. 该法具有高的灵敏度、大的发射位移和可测颜色变化等量子点发光优势, 用于环境水样中磷酸根离子的测定, 结果令人满意.     

The quenching of CdSe quantum dots photoluminescence by gold nanoparticles in solution

The photoluminescence (PL) of CdSe quantum dots (QD) in aqueous media has been studied in the presence of gold nanoparticles (NP) with different shapes. The steady state PL intensity of CdSe QD (1.5-2 nm in size) is quenched in the presence of gold NP. Picosecond bleach recovery and nanosecond time-resolved luminescence measurements show a faster bleach recovery and decrease in the lifetime of the emitting states of CdSe QD in the presence of quenchers. Surfactant-capped gold nanorods (NR) with aspect ratio of 3 and surfactant-capped and citrate-capped nanospheres (NS) of 12 nm diameter were used as quenchers in order to study the effect of shape and surface charge on the quenching rates. The Stern-Volmer kinetics model is used to examine the observed quenching behavior as a function of the quencher concentration. It was found that the quenching rate of NR is more than 1000 times stronger than that of NS with the same capping material. We also found that the quenching rate decreases as the length of the NR decreases, although the overlap between the CdSe emission and the NR absorption increases. This suggests that the quenching is a result of electron transfer rather than long-range (Forster-type) energy transfer processes. The quenching was attributed to the transfer of electron with energies below the Fermi level of gold to the trap holes of CdSe QD. The observed large difference between NR and NS quenching efficiencies was attributed to the presence of the [110] facets only in the NR, which have higher surface energy.     

Fluorescence resonance energy transfer in CdSe/ZnS-DNA conjugates: probing hybridization and DNA cleavage

Nucleic-acid-functionalized CdSe/ZnS quantum dots (QDs) were hybridized with the complementary Texas-Red-functionalized nucleic acid. The hybridization was monitored by following the fluorescence resonance energy transfer from the QDs to the dye units. Treatment of the QD/dye DNA duplex structure with DNase I resulted in the cleavage of the DNA and the recovery of the fluorescence properties of the CdSe/ZnS QDs. The luminescence properties of the QDs were, however, only partially recovered due to the nonspecific adsorption of the dye onto the QDs. Similarly, nucleic-acid-functionalized Au nanoparticles (Au NPs) were hybridized with the complementary Texas-Red-labeled nucleic acid. The hybridization was followed by the fluorescence quenching of the dye by the Au NPs. Treatment of the Au NP/dye DNA duplex with DNase I resulted in the cleavage of the DNA and the partial recovery of the dye fluorescence. The incomplete recovery of the dye fluorescence originated from the nonspecific binding of the dye units to the Au NPs. The nonspecific binding of the dye to the CdSe/ZnS QDs and the Au NPs is attributed to nonprotected surface vacancies in the two systems.     

Tuning electrochemical rectification via quantum dot assemblies

A novel approach to tuning electrochemical rectification using 2D assemblies of quantum dots (QDs) is presented. Asymmetric enhancement of the oxidation and reduction currents in the presence of the Fe(CN)(6)(3-/4-) redox couple is observed upon adsorption of QDs at thiol-modified Au electrodes. The extent of the electrochemical rectification is dependent on the average QD size. A molecular blocking layer is generated by self-assembling 11-mercaptoundecanoic acid (MUA) and an ultrathin film of poly(diallyldimethylammonium chloride) (PDADMAC) on the electrode. The polycationic film allows the electrostatic adsorption of 3-mercaptopropionic acid (MPA)-stabilized CdTe QDs, generating 2D assemblies with approximately 0.4% coverage. The QD adsorption activates a fast charge transfer across the blocking layer in which the reduction process is more strongly enhanced than the oxidation reaction. The partial electrochemical rectification is rationalized in terms of the relative position of the valence (VB) and conduction band (CB) edges with respect to the redox Fermi energy (ε(redox)). Quantitative analysis of the exchange current density obtained from electrochemical impedance spectroscopy demonstrates that the enhancement of charge transport across the molecular barrier is strongly dependent on the position of the QD valence band edge relative to ε(redox). The average electron tunneling rate constant through the QD assemblies is estimated on the basis of the Gerischer model for electron transfer.     

Langmuir-Blodgett thin films of quantum dots: synthesis, surface modification, and fluorescence resonance energy transfer (FRET) studies

We describe herein studies on as-prepared hydrophobic ZnS-CdSe quantum dots (QDs) at the air-water interface. Surface pressure-area (pi-A) isotherms have been used to study the monolayer behavior. Uniform, lamellar multilayer thin films of QDs were deposited by the Langmuir-Blodgett (LB) technique. The role of two different surfactant systems commonly employed in the synthesis of these QDs (trioctylphosphine oxide-octadecylamine (TOPO-ODA) system and trioctylphosphine oxide-tetradecylphosphonic acid (TOPO-TDPA) system) on the monolayer behavior and the quality of thin films produced has been investigated. The thin films were characterized by quartz crystal microgravimetry (QCM), contact angle measurements, fluorescence spectroscopy, and transmission electron microscopy (TEM). These QD films were further modified by an amphiphilic polymer, poly(maleic anhydride-alt-1-tetradecene) (PMA). The hydrophobic interaction between the polymers and the surfactants attached to the QDs drove the self-assembly process. The carboxylic acid functional groups in the polymer were also used to immobilize avidin. We have demonstrated a proof of concept for the biosensing strategy wherein the avidin-coated QD films attracted biotinylated gold nanoparticles, resulting in fluorescence resonance energy transfer (FRET) quenching of the thin films.     

Intermediates Produced in the Electron-Transfer Processes of Phenothiazine/Semiconductor Systems

Phenothiazine radical cation PTH(+*) and phenothiazine dication PTH(2+) produced by photoinduced electron transfer in phenothiazine/semiconductor systems have been studied by using ESR, UV-visible absorption, and fluorescence spectroscopic methods. It is found that the PTH(+*) was generated by electron transfer from (1)PTH* and (3)PTH* to the conduction band of the semiconductors besides PTH(+*) resulted in the photoionization of PTH as well as PTH(2+) produced by electron transfer from PTH(+*) to the conduction band of the semiconductors successively. Very significant supplementary information provided by UV-visible absorption, ESR, and resonance Raman spectra of PTH(+*) and PTH(2+), which were obtained by oxidation of PTH with lead tetraacetate, not only confirmed the two-step mono-electron-transfer mechanism proposed for interpretation of electron-transfer processes in PTH/semiconductor systems but also demonstrated that the two successive electron-transfer steps were corresponding to the removal of a p-electron at the nitrogen atom and a p-electron at the sulfur atom in the PTH molecule, respectively. Consequently, the nonplanar configuration of the PTH molecule changed into a planar configuration of the PTH(2+). Copyright 2000 Academic Press.     

First generation and characterization of the enol of glycine, H(2)N--CH=C(OH)(2), in the gas phase

The enol of glycine, H(2)N-CH&dbond;C(OH)(2), is generated in the gas phase by neutralization of the corresponding radical cation, which is available by dissociative electron ionization of isoleucine. Reionization approximately 0.6 micros later shows that the isolated enol (2) exists and does not isomerize to the significantly more stable glycine molecule, H(2)N--CH(2)--COOH (1); hence the intramolecular tautomerization 2-->1 must be associated with high barriers. The neutralization-reionization reactivity of 1(+*) further confirms that neutral glycine has a canonical structure (1) and is not a zwitterion. The unimolecular chemistry of 1(+*) is dominated by C--C bond cleavage to the immonium ion (+)H(2)NCH(2); in sharp contrast, 2(+*) primarily loses H(2)O. The ylide ion (+)H(3)N--CH(*)--COOH, an intermediate in the water loss from 2(+*), is found to readily equilibrate to 2(+*) prior to dissociation. Tautomers 1(+*) and 2(+*) differ in their charge-stripping behavior, with only 2(+*) forming a stable dication. The radical anions 1(-*) and 2(-*), formed by charge reversal of 1(+*) and 2(+*), respectively, dissociate extensively to (mainly) different closed-shell fragment anions. An important channel is H(*) loss; 1(-*) yields the carboxylate ion H(2)N--CH(2)--COO(-) whereas 2(-*) yields the enolate ion H(2)N--CH=C(OH)O(-).     

Energy/Hole Transfer Phenomena in Hybrid α‐Sexithiophene (α‐STH) Nanoparticle–CdTe Quantum‐Dot Nanocomposites

Considerable attention has been paid to hybrid organic–inorganic nanocomposites for designing new optical materials. Herein, we demonstrate the energy and hole transfer of hybrid hole‐transporting α‐sexithiophene (α‐STH) nanoparticle–CdTe quantum dot (QD) nanocomposites using steady‐state and time‐resolved spectroscopy. Absorption and photoluminescence studies confirm the loss of planarity of the α‐sexithiophene molecule due to the formation of polymer nanoparticles. Upon photoexcitation at 370 nm, a nonradiative energy transfer (73 %) occurs from the hole‐transporting α‐STH nanoparticles to the CdTe nanoparticles with a rate of energy transfer of 6.13×10⁹ s^?1. However, photoluminescence quenching of the CdTe QDs in the presence of the hole‐transporting α‐STH nanoparticles is observed at 490 nm excitation, which is due to both static‐quenching and hole‐transfer‐based dynamic‐quenching phenomena. The calculated hole‐transporting rate is 7.13×10⁷ s^?1 in the presence of 42×10^?8 M α‐STH nanoparticles. Our findings suggest that the interest in α‐sexithiophene (α‐STH) nanoparticle–CdTe QD hybrid nanocomposites might grow in the coming years because of various potential applications, such as solar cells, optoelectronic devices, and so on.     

Intermittent electron transfer activity from single CdSe/ZnS quantum dots

Electron transfer activity from excited single CdSe/ZnS core/shell quantum dots (QDs) to adsorbed Fluorescein 27 was studied by single QD fluorescence spectroscopy. In comparison with QDs, the QD-acceptor complexes showed a shorter average and broader distribution of QD emission lifetimes due to electron transfer to adsorbates. Large fluctuation of lifetimes in single QD/dye complexes was observed, indicating intermittent electron transfer activity from QDs.     

Water soluble quantum dot nanoclusters: energy migration in artifical materials

Energy migration in self-assembled, water soluble, quantum dot (QD) nanoclusters is reported. These spherical nanoclusters are composed of CdSe QDs bound together by pepsin, a digestive enzyme found in mammals. A structural model for the clusters is suggested, based on scanning transmission electron microscopy, as well as dynamic light scattering and small angle X-ray scattering. Cluster sizes range from 100 to 400 nm in diameter and show a close-packed interior structure. Optical characterization of the absorption and emission spectra of the clusters is reported, finding photoluminescence quantum yields of up to approximately 60% in water for clusters made from core-shell CdSe-ZnS QDs. Clusters prepared from two different size populations of CdSe QD samples (3 and 4 nm in diameter) demonstrate energy migration and trapping. Resonance energy transfer (RET), from small to large dots within the QD-pepsin cluster, is observed by monitoring the quenching of the small donor dot fluorescence along with enhancement of the large acceptor dot fluorescence.     

Visible‐Light Sensitization and Photoenergy Storage in Quantum Dot/Polyoxometalate Systems

Recently, the process by which energy is transferred from photoexcited semiconductor nanocrystals, called quantum dots (QDs), to other semiconductors has attracted much attention and has potential application in solar energy conversion (i.e., QD‐sensitized solar cells). Sensitization of wide band gap polyoxometalates (POMs) to visible light by using CuInS₂ QDs dispersed in an organic solution is demonstrated herein. Photoluminescence quenching and lifetime studies revealed efficient electron transfer from the CuInS₂ QDs to POMs, such as SiW₁₂O₄₀ and W₁₀O₃₂, that were hybridized with a cationic surfactant. CuInS₂ QDs function as an antenna that absorbs visible light and supplies electrons to the POMs to enable certain photocatalytic reactions, including noble‐metal‐ion reduction. The photoenergy storage capabilities of the QD‐POM system, in which electrons photogenerated in QDs by visible‐light excitation are trapped and accommodated by POMs to form reduced POM, are also demonstrated. Electrons stored in the POM can be later discharged through reductive reactions, such as oxygen reduction, in the dark.     

Beyond labels: A review of the application of quantum dots as integrated components of assays, bioprobes, and biosensors utilizing optical transduction

A comprehensive review of the development of assays, bioprobes, and biosensors using quantum dots (QDs) as integrated components is presented. In contrast to a QD that is selectively introduced as a label, an integrated QD is one that is present in a system throughout a bioanalysis, and simultaneously has a role in transduction and as a scaffold for biorecognition. Through a diverse array of coatings and bioconjugation strategies, it is possible to use QDs as a scaffold for biorecognition events. The modulation of QD luminescence provides the opportunity for the transduction of these events via fluorescence resonance energy transfer (FRET), bioluminescence resonance energy transfer (BRET), charge transfer quenching, and electrochemiluminescence (ECL). An overview of the basic concepts and principles underlying the use of QDs with each of these transduction methods is provided, along with many examples of their application in biological sensing. The latter include: the detection of small molecules using enzyme-linked methods, or using aptamers as affinity probes; the detection of proteins via immunoassays or aptamers; nucleic acid hybridization assays; and assays for protease or nuclease activity. Strategies for multiplexed detection are highlighted among these examples. Although the majority of developments to date have been in vitro, QD-based methods for ex vivo biological sensing are emerging. Some special attention is given to the development of solid-phase assays, which offer certain advantages over their solution-phase counterparts.     

Multi-analyte homogenous immunoassay based on quenching of quantum dots by functionalized graphene

We propose a homogenous multi-analyte immunoassay based on the quenching of quantum dot (QD) fluorescence by means of graphene. Two QDs with emission maxima at 636 and 607 nm were bound to antibodies selective for mouse or chicken immunoglobulins, respectively, and graphene functionalized with carboxylic moieties was employed to covalently link the respective antigen. The antibody-antigen interaction led graphene close enough to QDs to quench the QD fluorescence by resonance energy transfer. The addition of free antigens that competed with those linked to graphene acted as a “turn-on” effect on QD fluorescence. Fluorescence emitted by the two QDs could be recorded simultaneously since the QDs emitted light at different wavelengths while being excited at the same wavelength and proved to be linearly correlated with free antigen concentration. The developed assay allows measuring both antigens over 2–3 orders of magnitude and showed estimated limits of detection in the nanomolar range. This approach is thus a promising universal strategy to develop homogenous immunoassays for diverse antigens (cells, proteins, low-molecular-mass analytes) in a multi-analyte configuration.     

Aptamer-capped nanocrystal quantum dots: a new method for label-free protein detection

We demonstrate that aptamer-capped near-infrared PbS quantum dots (QDs) can detect a target protein based on selective charge transfer. The water-soluble QDs are synthesized with the thrombin-binding aptamer, which retains the secondary quadruplex structure necessary for binding to thrombin. These QDs have diameters of 3-6 nm and fluoresce around 1050 nm. When the aptamer-functionalized QD binds to its target, a fluorescence quenching occurs due to charge transfer from amine groups on the protein to the QD. Thrombin is detected within 1 min with a detection limit of approximately 1 nM. This selective detection is observed even in the presence of high background concentrations of interfering negatively or positively charged proteins, suggesting that aptamer-capped QDs could be useful for label-free protein assays.     

Quantum dot modified multiwall carbon nanotubes

A novel strategy for the fabrication of multiwall carbon nanotube-nanocrystal heterostructures is shown. Different quantum dots (QDs) with narrow size distributions were covalently coupled to carbon nanotubes (CNTs) and silica-coated CNTs in a simple, uniform, and controllable manner. The structural and optical properties of CNT/QD heterostructures are characterized by electron microscopy and photoluminescence spectroscopy. Complete quenching of the PL bands in both QD core and core/shell heterostructures was observed after adsorption to the CNTs, presumably through either carrier ionization or energy transfer. The deposition of a silica shell around the CNTs preserves the fluorescence properties by insulating the QD from the surface of the CNT.