pH-sensitive ligand for luminescent quantum dots

We developed a strategy to switch the luminescence of semiconductor quantum dots with chemical stimulations. It is based on the photoinduced transfer of either energy from CdSe-ZnS core-shell quantum dots to [1,3]oxazine ligands or electrons from the organic to the inorganic components. The organic ligands incorporate a dithiolane anchoring group, an electron-rich indole, and a 4-nitrophenylazophenoxy chromophore in their molecular skeleton. Their adsorption on the surface of the quantum dots results in partial luminescence quenching. Electron transfer from the indole fragment to the nanoparticles is mainly responsible for the decrease in luminescence intensity. Upon addition of base, the [1,3]oxazine ring of the ligands opens to generate a 4-nitrophenylazophenolate chromophore, which absorbs in the range of wavelengths where the quantum dots emit. This transformation activates an energy-transfer pathway from the excited nanoparticles to the ligands. In addition, the oxidation potential of the ligand shifts in the negative direction, improving the efficiency of electron transfer. The overall result is a decrease in the luminescence quantum yield of 83%. Addition of acid also opens the [1,3]oxazine ring of the ligands. However, the resulting 4-nitrophenylazophenol does not absorb in the visible region and cannot accept energy from the excited nanoparticles. Furthermore, the oxidation potential shifts in the positive direction, lowering the electron-transfer efficiency. In fact, the luminescence quantum yield increases by 33% as a result of this transformation. These changes are fully reversible and can be exploited to probe the pH of aqueous solutions from 3 to 11. Indeed, our sensitive quantum dots adjust their luminescence in response to variations in pH within this particular range of values. Thus, our general design strategy can eventually lead to the development of pH-sensitive luminescent probes for biomedical applications based on the unique photophysical properties of semiconductor quantum dots.     

Photoluminescence of cadmium selenide quantum dots in polymer solutions

The effect of solvent on the photoluminescence of cadmium selenide quantum dots stabilized by oleic acid is examined with the example of two organic solvents: toluene and THF. It is found that THF favors desorption of ligands and formation of surface defects to a greater extent than toluene; as a result, the maximum of the photoluminescence band shifts to the red spectral region and its intensity decreases. The addition of polymers to the solution of quantum dots causes changes in the efficiency of photoluminescence and in the kinetics of its quenching. In the range of low concentrations (≤2 wt %) of quantum dots in polymer solutions, the intensity of luminescence first grows and then passes through a maximum and decreases. This effect may be explained both by the increasing number of surface defects and by quenching via energy transfer to polymers, especially in the case of polymers containing aromatic groups.     

Förster resonance energy transfer in nanoclusters of InP@ZnS colloidal quantum dots with dodecylamine ligand shells

Förster resonance energy transfer between InP@ZnS hydrophobic colloidal quantum dots of two different sizes has been studied in the closely packed nanoclusters formed spontaneously in an organic solvent upon the addition of a precipitating solvent. The quantum dots had a core@shell structure and were stabilized by dodecylamine ligands.     

Photoinduced enhancement in the luminescence of hydrophilic quantum dots coated with photocleavable ligands

In search of strategies to photoactivate the luminescence of semiconductor quantum dots, we devised a synthetic approach to attach photocleavable 2-nitrobenzyl groups to CdSe-ZnS core-shell quantum dots coated with hydrophilic polymeric ligands. The emission intensity of the resulting nanostructured constructs increases by more than 60% with the photolysis of the 2-nitrobenzyl appendages. Indeed, the photoinduced separation of the organic chromophores from the inorganic nanoparticles suppresses an electron-transfer pathway from the latter to the former and is mostly responsible for the luminescence enhancement. However, the thiol groups anchoring the polymeric envelope to the ZnS shell also contribute to the photoinduced emission increase. Presumably, their photooxidation eliminates defects on the nanoparticle surface and promotes the radiative deactivation of the excited quantum dots. This effect is fully reversible but its magnitude is only a fraction of the change caused by the photocleavage of the 2-nitrobenzyl groups. In addition, these particular quantum dots can cross the membrane of model cells and their luminescence increases by ~80% after the intracellular photocleavage of the 2-nitrobenzyl quenchers. Thus, photoswitchable luminescent constructs with biocompatible character can be assembled combining the established photochemistry of the 2-nitrobenzyl photocage with the outstanding photophysical properties of semiconductor quantum dots and the hydrophilic character of appropriate polymeric ligands.     

Aqueous synthesis of mercaptopropionic acid capped ZnSe QDs and investigation of photoluminescence properties with metal doping

An aqueous-based green route has been demonstrated for the preparation of ZnSe quantum dots (QDs) and doping of transition metals in the presence of thiol mercaptopropionic acid (MPA) as growth moderator by refluxing at 100 ​°C. The structure and morphology of synthesized ZnSe quantum dots have been investigated by using X-ray diffraction studies (XRD), Ultraviolet–Visible spectroscopy (UV–vis), Fourier Transform Infrared Spectroscopy (FTIR) and Photoluminescence (PL) Spectroscopy. XRD studies indicate the structure of the quantum dots is cubic with diameters in the range of 4–5 ​nm. Fourier Transform Infrared (FTIR) studies proves that MPA ligands were bound strongly on the ZnSe nanocrystal surface as thiolate. The band gap energy (E_g) was calculated to be 3.8 ​eV which is blue shifted from the standard value of bulk band gap (2.60–2.70eV. Photoluminescence spectra shows broad emission value ranging between 400 and 700 ​nm due to surface defects which has been reduced by doping with transition metals (Fe, Co, Ni, Cd) in aqueous medium. The effective passivation of trap luminescence is done by doping leading to increase in photoluminescence quantum yield specifically with nickel and cadmium doped ZnSe QDs.     

CdTe量子点荧光法测定阳离子表面活性剂

合成了巯基乙酸(TGA)保护的水溶性发光CdTe量子点,并考察了此探针在阳离子表面活性剂十六烷基三甲基溴化铵(CTMAB)中的发光行为。根据观察到的发光猝灭效应,建立了一种简单的测定阳离子表面活性剂的方法。考察了CdTe量子点的浓度、体系酸度、反应时间及共存物质等对测定的影响。在最佳条件下,CdTe量子点发光强度与CTMAB的浓度分别在6×10-7~9.0×10-6mol/L和1.2×10-5~3.8×10-5mol/L范围内分段成线性关系。该方法用于水样的阳离子表面活性剂的测定,回收率为97%~102%。     

Luminescence of Hybrid Nanostructures of InP@ZnS Colloidal Quantum Dots and <Emphasis Type="Italic">meso</Emphasis>-Tetra(3-pyridyl)porphyrin Molecules

The formation of hybrid nanostructures consisting of InP@ZnS colloidal quantum dots and mesotetra(3-pyridyl)porphyrin molecules adsorbed on the quantum dots has been studied. In such nanostructures, strong quenching of quantum dot luminescence and an increase in the emission intensity of porphyrin are observed due to nonradiative resonance energy transfer from colloidal quantum dots to porphyrin.     

Molecular dynamics simulation of the structure of mixed ligand shells stabilizing cadmium selenide nanoparticle surfaces with different curvatures

A full-atomic molecular dynamics simulation has been performed for a ligand shell of colloidal cadmium selenide quantum dots. Trioctylphosphine, trioctylphosphine oxide, octadecylphosphonic acid, and hexadecylamine have been used as ligands. For a mixture of the two former ligands, the effect of surface curvature on the fraction of surface ions of quantum dots bonded to ligands has been studied. It has been shown that, for particles with radii of 1.9 and 4.5 nm, every second and approximately third cadmium atom, respectively, is bonded to trioctylphosphine oxide. Partial introduction of octadecylphosphonic acid and hexadecylamine may increase the fraction of bonded surface atoms by more than two times.     

pH-sensitive quantum dots

We have designed organic ligands able to adsorb on the surface of CdSe-ZnS core-shell quantum dots and switch the luminescence of the inorganic nanoparticles in response to hydroxide anions. These compounds incorporate a [1,3]oxazine ring within their molecular skeleton, which reacts with the nucleophilic hydroxide anion to generate a 4-nitrophenylazophenolate chromophore. The chromogenic transformation activates an energy transfer pathway from the quantum dot to the adsorbed chromophores. As a result, the luminescence intensity of the coated nanoparticles decreases significantly in the presence of hydroxide anions. In fact, this mechanism can be exploited to probe the pH of aqueous solutions. Indeed, an increase in pH from 7.1 to 8.5 translates into a 35% decrease in the luminescence intensity of the sensitive quantum dots. Thus, our operating principles for luminescence switching can efficiently transduce a chemical stimulation into a change in the emissive response of semiconductor nanoparticles. In principle, this protocol can be extended from hydroxide anions to other target analytes with appropriate adjustments in the molecular design of the chromogenic ligands. It follows that luminescent chemosensors, based on the unique photophysical properties of semiconductor quantum dots, can eventually evolve from our design logic and choice of materials.     

Influence of amines and alkanethiols on the spectral and luminescent properties of InP@ZnS colloidal quantum dots

It has been shown that the precursors (long-chain amines and alkanethiols) used in liquid-phase colloidal synthesis of InP@ZnS quantum dots exert a significant influence on their spectral and luminescent properties. It has been found that dodecylamine and 1-dodecanethiol facilitate obtaining the particles with a narrow luminescence band and a low quantum yield, whereas oleylamine, 1-octanethiol, and 1,6-hexanedithiol stimulate the formation of the quantum dots with broad and intense luminescence. Conditions have been found under which the narrowing of the emission band to 46.3 nm becomes possible.     

Incorporation of quantum dots into a silica matrix using a compatible precursor

A method is developed for one-stage incorporation of cadmium-sulfide quantum dots synthesized in the presence of mercaptosuccinic acid into a silica matrix formed from a precursor containing ethylene-glycol residues—tetrakis(2-hydroxyethyl) orthosilica. This precursor has not previously been applied for this purpose. It is more compatible with diverse substances than tetraethoxysilane, which is traditionally used. Moreover, it is advantageous in its unlimited solubility in water; release of ethylene glycol, which does not precipitate quantum dots, rather than alcohol upon hydrolysis; and the feasibility of performing the sol-gel process at any pH value in a range of 2–10 without the addition of acid or alkali and without heating. When the precursor (50 wt %) is added to a dispersion of quantum dots, the system is transformed into a gel in as little as a few minutes. The synthesized hybrid materials are optically transparent. Therewith, the quantum dots incorporated into the silica matrix exhibit luminescence, with their spectral characteristics remaining almost unchanged.     

基于量子点的电致化学发光免疫传感器研究进展

电致化学发光集成了发光和电化学分析的优点,在生物传感分析方面具有广泛的应用前景.量子点因其独特的性质成为电致化学发光的三大发光体系之一.本文综述了近年来基于量子点的电致化学发光免疫传感器的种类及其信号放大技术,并就相关研究发展方向和趋势作了初步展望.     

In situ ligand exchange of thiol-capped CuInS2/ZnS quantum dots at growth stage without affecting luminescent characteristics

An aliphatic thiol ligand of CuInS(2)/ZnS core/shell quantum dots is replaced with a hydroxyl-terminated thiol ligand by utilizing 'on-off state' of ligands during growth stage of the quantum dots. After the ligand-exchange, negligible differences were observed on both photoluminescence spectrum and luminescent quantum efficiency. The reason for the high retention of luminescent efficiency comes from no local agglomeration and no surface deterioration of QDs. It is also observed that 70% of initial ligands are exchanged by the replacing ligand, determined by FT-IR and (1)H NMR. The proposed method provides the quantum dots with an excellent dispersibility in polar solvents, supported by identical luminescence decay characteristics of the QDs.     

On Stabilization of Colloidal Quantum Dots of Cadmium Selenide in the Presence of Octadecylphosphonic Acid

Classical molecular dynamics has been used to study the stabilization of colloidal quantum dots of CdSe with n-octadecylphosphonic acid molecules in combination with different auxiliary ligands, such as trioctylphosphine, trioctylphosphine oxide, and hexadecylamine. The effects of different ligands that may be formed due to interactions of n-octadecylphosphonic acid with CdO and Se in an initial mixture have been considered. It has been shown that, among these ligands, the stabilizing effect increases with a rise in the charge of a ligand per n-octadecyl chain. The role of the auxiliary ligands, e.g. octadecene that devoid of functional groups, has been studied.     

Observation of enhanced energy transfer in individual quantum dot-oligophenylene vinylene nanostructures

The temporal and spectral properties of luminescence from individual CdSe quantum dot-oligophenylene vinylene nanostructures (single quantum dots with conjugated organic ligands coordinated to the surface) are profoundly modified relative to blended films of the same components. These kinds of composite quantum dot-conjugated organic systems have attracted significant interest as a way to improve efficiency in photovoltaic device applications. By direct functionalization of the dot surface with the conjugated organic ligands, we realize a significant enhancement in energy transfer and luminescence stability.     

表面配体对CdSe量子点发光性质的影响

本文采用热注入法合成了以油胺/油酸为表面配体的、粒径均一的CdSe量子点(CdSe QDs)。调节表面配体交换中辛硫醇与CdSe QDs的比例,研究了表面配体对CdSe QDs光致发光及电致化学发光性质的影响,并提出了CdSe QDs的发光模型。结果表明,辛硫醇表面配体显著影响CdSe QDs的带边发射和深能级陷阱发射,因而导致CdSe QDs光致发光强度的显著降低,以及电致化学发光强度的增加。上述结果为进一步提高量子点的发光性能提供了依据。     

Computer Simulation of the Adsorption of meso-Tetra(3-Pyridyl)porphyrin Dye on the Surface of Colloidal CdSe Quantum Dots

A classical all-atom molecular dynamics simulation has been used to study the adsorption of mesotetra( 3-pyridyl)porphyrin dye on colloidal quantum dots of cadmium selenide. The competition between dye molecules and ligands covering the quantum dots has been investigated by the examples of trioctylphosphine, trioctylphosphine oxide, octadecylphosphonic acid, and hexadecylamine. It has been shown that the dye is strongly retained by the surface and is not washed out with a solvent either from a free surface or in the presence of trioctylphosphine in the shell. The strength of the adsorption of the dye is equal to that of trioctylphosphine. At the same time, it gives up its place on the surface to stronger ligands, octadecylphosphonic acid and hexadecylamine.     

CdTe荧光量子点与苯胺之间相互作用的研究

以巯基乙酸为稳定剂在水相中合成了CdTe荧光量子点,并使之与苯胺(Aniline)耦联.通过红外光谱分析,证实CdTe荧光量子点同Aniline的耦合主要是通过量子点周围巯基乙酸的—COOH与苯胺的—NH2形成的氢键实现的.将CdTe-苯胺、CdTe-丙氨酸(Alanine)耦合物荧光光谱的强度进行对比,发现苯环的存在可以极大地增强耦合物的荧光强度.     

Hydrophilization of CdSe quantum dots with surfactants

A procedure has been developed for hydrophilization of CdSe semiconductor quantum dots with anionic, cationic, and nonionic surfactants. Depending on the type of a surfactant, these particles may carry positive, negative, or no charges. In spite of the universality of the proposed procedure, the hydrophilization with specific surfactants must be carried out at specific concentrations of reagents and medium pH values. Under optimum conditions, the transfer of the quantum dots to water substantially increases the quantum yield of their luminescence. Conditions have been found under which the luminescence properties of the hydrophilic particles remain almost unchanged for a long time period.     

Specifics of the spectral and luminescent properties of ensembles of colloidal quantum dots

Using colloidal solutions of ZnS-shell indium phosphide quantum dots with two average sizes of 2.1 and 3.0 nm and a size distribution variance of 10%, it has been shown that the luminescence and the luminescence excitation spectra of the colloidal quantum dots substantially depend on the wavelength of exciting light and the detection wavelength, respectively, with both the relationships being nonlinear in character, which may indicate the bimodal type of the size distribution function. Similar measurements for CdSe colloidal quantum dots with an average particle size of d_av = 2.5 nm and a variance of 6% have shown that the effect of dependence of the luminescence peak position on the excitation wavelength is manifested to a much lesser extent.