Basic Principles and Current Trends in Colloidal Synthesis of Highly Luminescent Semiconductor Nanocrystals

The principal methods for the synthesis of highly luminescent core–shell colloidal quantum dots (QDs) of the most widely used CdSe, CdS, ZnSe, and other A^IIB^VI nanocrystals are reviewed. One‐pot versus multistage core synthesis approaches are discussed. The noninjection one‐pot method ensures slow, controllable growth of core nanocrystals starting from magic‐size seed recrystallization, which yields defect‐free cores with strictly specified sizes and shapes and a high monodispersity. Subsequent injection of shell precursors allows the formation of gradient core–shell QDs with a smooth potential barrier for electrons and holes, without strains or interfacial defects, and, as a consequence, a luminescence quantum yield (QY) approaching 100 %. These general approaches can also be applied to semiconductor core–shell QDs other than A^IIB^VI ones to cover the broad spectral range from the near‐UV to IR regions of the optical spectrum, thus displacing fluorescent organic dyes from their application areas.     

Promising avenues of research in nanoscience: chemistry of semiconductor nanoparticles

The state-of-the-art in the field of research on semiconductor nanoparticles is analyzed; cadmium chalcogenide nanoparticles are considered in most detail. Emphasis is placed on the methods of synthesis and on control of the size, composition, and structure of semiconductor nanoparticles — “quantum dots”. The state of the surface plays a significant role in determining the properties of nanoparticles. Organized nanostructures comprised of quantum dots are considered. The properties of semiconductor nanoparticles are described. Prospects for applications of semiconductor nanomaterials are discussed. Dedicated to Academician V. I. Minkin on the occasion of his 70th birthday. __________ Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 811–836, April, 2005.     

Electrochemical detection of quantum dots by stabilization of electrogenerated copper species

The determination of quantum dots is of particular interest as they are frequently used as labels in electrochemical biosensors. In this paper, we describe a method for the detection of very low concentrations of quantum dots using the voltammetric response of copper in ammonia solution. Copper species electrogenerated on the electrode surface are stabilized by the nanoparticles, preventing their oxidation by dissolved oxygen, and a relationship between the concentration of the nanoparticles and the copper voltammetric response can thus be obtained. The reported method shows a linear range between 0.05 and 2 nM of quantum dots, with a limit of detection in the order of 9 × 10⁷ nanoparticles. This method could be employed to improve the detection limit of electrochemical biosensors using quantum dots as labels.     

Quantum Dot-mediated Photoproduction of Reactive Oxygen Species for Cancer Cell Annihilation

While semiconductor quantum dots produce little singlet oxygen, they may undergo Type I photoreactions to produce other reactive oxygen species (ROS) to kill cells. CdTe quantum dots coated with thioglycolic acid were used to test that possibility. Some thiol ligands were purposely removed to regenerate the surface electron traps that were passivated by the ligand. This allowed photoinduced electrons to dwell on the surface long enough to be gathered by nearby oxygen molecules to produce ROS. The photocytotoxicity of these quantum dots was tested on nasopharyngeal carcinoma cells. Photokilling was shown to be drug and light dose dependent. Using 0.6 μm quantum dots for incubation and 4.8 J cm⁻² for irradiation, about 80% of the cells were annihilated. These quantum dots promised to be potent sensitizers for photoannihilation of cancer cells.     

Optical Determination of Cholesterol in Milk with Molecularly Imprinted Polymer-Coated Quantum Dots

Fluorescent molecularly imprinted polymer-coated CdSe/ZnS quantum dots were prepared in an efficient one-step synthesis. Their application as fluorescent nanoparticles for the direct quantification of cholesterol in milk was characterized. The quantum dots were used as cores to produce fluorescence. The molecularly imprinted polymer shells provided specific binding sites for cholesterol. The system exhibited good linearity for cholesterol from 0.5 to 150?µg?mL^?1, a low detection limit of 0.15?µg?mL^?1, and acceptable reproducibility with a relative standard deviation of 4.2% for six replicates. The molecularly imprinted polymer-coated quantum dots were used to determine cholesterol in fortified milk. Recoveries were from 87.0 to 105.2% and a possible mechanism is proposed. The fluorescent molecularly imprinted polymer-coated quantum dots exhibited excellent selectivity and provide a simple, rapid, selective, and effective analytical approach.     

Photochromic Transformations of Spiro Compounds: 1. A New Approach to Determining the Photocoloration Quantum Yield

A new approach to determining a photocoloration quantum yield for photochromic compounds was considered. This approach is based on comparison between the calculated and experimental values of maximum absorbance A ^B _maxof a photocolored form upon monochromatic irradiation. Using spirooxazines as an example, the quantum yield of photocoloration was determined, and A ^B _maxwas examined as a function of a number of parameters that characterize the photocolored form (the quantum yields of photocoloring and photobleaching and the lifetime of the colored form). It was found that A ^B _maxnonlinearly increased with decreasing rate constant (<0.2 s^–1) of the thermal bleaching of spirooxazines.     

Synthesis and properties of bifunctional magnetic-optical nanomaterials: Fe2O3@Pdmaema-capped II–VI semiconductor quantum dots nanocomposites

A facile route to synthesize a new type of multifunctional nanocomposites is reported. Here, PDMAEMA (poly[2-(dimethylamino)ethyl] methacrylate) is a key macromolecule serving as a bridge between magnetic Fe2O3 nanoparticles and luminescent quantum dots. Both Fe2O3 nanoparticles and Ⅱ-Ⅵ semiconductor quantum dots with a narrow size distribution are synthesized through a two-phase thermal approach. Subsequently, the atom transfer radical polymerization (ATRP) technique was applied to prepare magnetic Fe2O3@PDMAEMA core-shell nanoparticles. The thickness of PDMAEMA shell can be easily controlled by adjusting the reaction time. Finally, the ligand exchange method was exploited to modify Ⅱ-Ⅵ quantum dot with amine-containing polymer of PDMAEMA, which led to quantum dot securely bound by Fe2O3@PDMAEMA core-shell nanoparticle to form a multifunctional nanocomposite. The resulting nanocomposite remains variable emission by tuning the Ⅱ-Ⅵ semiconductor type and particle size and shows Hc at 49 kA/m and Tb at 16 K from Fe2O3 nanoparticles. The self-assembled behavior for the resulting samples is also discussed.     

Excited State Relaxation and Stabilization of Hydrogen Terminated Silicon Quantum Dots

Silicon is the leading semiconductor material in microelectronic industry. Owing to the large surface to volume ratio, low-dimensional Si nanostructures, for instance, silicon quantum dots exhibit diverse electronic and optical properties. Passivating the surface of Si nanostructures by a suitable species is thereby required to stabilize and engineer the dot properties in different environment. Recent theoretical advances in the investigation of the excited state properties of silicon quantum dots (QDs) are reviewed in this article. The theoretical calculations reveal that the excited state relaxation is prevalent in hydrogenated silicon nanoparticles. Stokes shift due to structure relaxation in the excited state varies with the particle size. It is therefore desirable to minimize Stokes shift for the purpose of maximizing its quantum yield or efficiency in photoluminescence applications. Consequently, surface functionalization by a suitable species turns out to be the most effective avenue. Determination of proper passivating agent is of outmost importance to satisfy the practical necessity. All these intermingled factors are briefly addressed in this article.     

Fluorescent-Magnetic Nanostructures Based on Polymer-Quantum Dots Conjugates

Summary: In this study it is presented the synthesis and the characterization of Fluorescent-Magnetic Nanostructures based on polymer-quantum dots conjugates. Polyvinyl alcohol (PVA) was used as the capping-ligand for the preparation of Cd_xMn_1-xS semiconductor nanocrystals via aqueous colloidal chemistry. Different substitution ratios of Cd²⁺ by Mn⁺² ions were investigated aiming at the formation of stable nanoparticles with photo-luminescent and semi-magnetic properties. UV-visible spectroscopy (UV-vis), photoluminescence spectroscopy (PL), Electric Paramagnetic Resonance Spectroscopy (EPR), and transmission electron microscopy (TEM) were used to characterize the formation and the relative stability of Cd_xMn_1-xS nanoparticles. The results have showed the influence of the Mn²⁺partially replacing Cd²⁺ in the optical behavior of the quantum dots (QDs) produced. In addition, the Cd_xMn_1-xS QDs have evidenced luminescent and semi-magnetic properties. Thus, the biocompatible water-soluble polymer was effective as ligand for synthesizing and stabilizing QDs conjugates with properties allowing them to be potentially applied as imaging and labeling probes in the biomedical field.     

碳点的性质及其在化学发光分析中的研究进展

碳点是一种新型的荧光碳纳米颗粒,与传统的半导体量子点相比,碳点具有毒性低、生物相容性好、原料丰富廉价、光稳定性好等特点,得到引了化学、材料和生物等各领域科学家的高度关注。 本文介绍了碳点的基本概念和性质、探讨了碳点在化学发光领域的应用研究进展并进行了展望。     

Quantum dot–NBD–liposome luminescent probes for monitoring phospholipase A2 activity

In this paper we describe the fabrication and characterization of new liposome encapsulated quantum dot–fluorescence resonance energy transfer (FRET)-based probes for monitoring the enzymatic activity of phospholipase A_2. To fabricate the probes, luminescent CdSe/ZnS quantum dots capped with trioctylphosphine oxide (TOPO) ligands were incorporated into the lipid bilayer of unilamellar liposomes with an average diameter of approximately 100 nm. Incorporating TOPO capped quantum dots in liposomes enabled their use in aqueous solution while maintaining their hydrophobicity and excellent photophysical properties. The phospholipid bilayer was labeled with the fluorophore NBD C₆-HPC (2-(6-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)hexanoyl-1-hexa decanoyl-sn-glycero-3-phosphocholine). The luminescent quantum dots acted as FRET donors and the NBD dye molecules acted as FRET acceptors. The probe response was based on FRET interactions between the quantum dots and the NBD dye molecules. The NBD dye molecules were cleaved and released to the solution in the presence of the enzyme phospholipase A_2. This led to an increase of the luminescence of the quantum dots and to a corresponding decrease in the fluorescence of the NBD molecules, because of a decrease in FRET efficiency between the quantum dots and the NBD dye molecules. Because the quantum dots were not attached covalently to the phospholipids, they did not hinder the enzyme activity as a result of steric effects. The probes were able to detect amounts of phospholipase A₂ as low as 0.0075 U mL^?1 and to monitor enzyme activity in real time. The probes were also used to screen phospholipase A₂ inhibitors. For example, we found that the inhibition efficiency of MJ33 (1-hexadecyl-3-(trifluoroethyl)-sn-glycero-2-phosphomethanol) was higher than that of OBAA (3-(4-octadecyl)benzoylacrylic acid).     

<Emphasis Type="Italic">In silico</Emphasis> study of the atomic and electronic structure of quantum dots of the CdTe family doped with atoms of rare earth elements

An in silico study of semiconductor quantum dots of the CdTe family doped with atoms of rare earth elements is performed based of density functional theory. An ab initio computer design of quantum dots based on CdTe nanoparticles doped with Eu и Gd atoms is carried out. Partial densities of states of CdTe:Eu and CdTe:Gd quantum dots are calculated and analyzed. X-ray absorption near edge (XANES) spectra near the Eu K-, L₁-, and L₃- and Gd K-, L₁-, and L₃-edges of CdTe:Eu and CdTe:Gd quantum dots are calculated. The sensitivity of XANES spectroscopy for the verification of parameters of a nanosized atomic structure of quantum dots based on CdTe particles doped with atoms of rare earth elements and the determination of the local atomic structure around the atoms of rare earth elements in quantum dots is demonstrated.     

用于检测重金属离子的磁性荧光复合微球

采用静电逐层自组装的方法,首先将PSS和PAH聚电解质交替沉积在CaCO₃中空微球表面,然后将Fe₃O₄磁性纳米粒子与CdSe量子点负载在中空微球表面不同的聚电解质层中,制备出具有磁性和荧光双重功能的复合微球,并将其作为荧光离子探针,研究了其对Cu²⁺和Pb²⁺离子检测的灵敏度、选择性及可行性。结果表明,复合微球显示出良好的磁性和荧光性能,对Cu²⁺和Pb²⁺离子的检测具有较高的灵敏度和选择性。尤为重要的是,可通过磁分离的方法将微球快速地从待测液中回收,从而能够避免量子点对环境造成的二次污染。     

Development of Novel Quantum Dots as Fluorescent Sensors for Application in Highly Sensitive Spectrofluorimetric Determination of Cu2+

Abstract

Quantum dots (QDs), semiconductor particles that have all three dimensions confined to the nanometer length scale are a good choice for the detection of heavy metals in aqueous media. In this study, novel CdSe/CdS QDs modified by mercaptoethanol were synthesized for Cu²⁺ detection, which shows high selectivity and sensitivity towards Cu²⁺ in the presence of other biological metallic ions. The detection mainly depends on the binding of Cu²⁺ onto the surface of QDs resulting in a chemical displacement of Cd²⁺. The subsequent formation of CuSe, and its application in hair and tea samples was also performed successfully.     

Direct coating of quantum dots with silica shell

This paper describes a method for direct coating of fluorescent semiconductor nanoparticles with silica shell. The fluorescent semiconductor nanoparticles used were CdSe _x Te_1–x nanoparticles coated with ZnS and succeedingly surface-modified with carboxyl groups, or quantum dots (Q-dots). The Q-dots were silica-coated by performing sol–gel reaction of tetraethyl orthosilicate (TEOS) using NaOH as a catalyst in the presence of the Q-dots. Quasi-perfect Q-dots/silica core-shell particles were formed at 5.0 M H₂O and 4.0 × 10⁻⁴ M NaOH. Under these concentrations of H₂O and NaOH, the particle size of Q-dots/silica particles could be varied from 20.1 to 38.1 nm as the TEOS concentration increased from 2.5 × 10⁻⁴ to 50 × 10⁻⁴ M. The Q-dots/silica particles showed fluorescence as well as the uncoated Q-dots.     

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.     

Fluorescent determination of graphene quantum dots in water samples

This work presents a simple, fast and sensitive method for the preconcentration and quantification of graphene quantum dots (GQDs) in aqueous samples. GQDs are considered an object of analysis (analyte) not an analytical tool which is the most frequent situation in Analytical Nanoscience and Nanotechnology. This approach is based on the preconcentration of graphene quantum dots on an anion exchange sorbent by solid phase extraction and their subsequent elution prior fluorimetric analysis of the solution containing graphene quantum dots. Parameters of the extraction procedure such as sample volume, type of solvent, sample pH, sample flow rate and elution conditions were investigated in order to achieve extraction efficiency. The limits of detection and quantification were 7.5 μg L⁻¹ and 25 μg L⁻¹, respectively. The precision for 200 μg L⁻¹, expressed as %RSD, was 2.8%. Recoveries percentages between 86.9 and 103.9% were obtained for two different concentration levels. Interferences from other nanoparticles were studied and no significant changes were observed at the concentration levels tested. Consequently, the optimized procedure has great potential to be applied to the determination of graphene quantum dots at trace levels in drinking and environmental waters.     

Bottom-Up Synthesis,Dispersion and Properties of Rectangular-Shaped Graphene Quantum Dots

Carbon nanomaterials have attracted the attention of the scientific community for more than 30 years now; first with fullerene, then with nanotubes and now with graphene and graphene related materials. Graphene quantum dots (GQDs) are nanoparticles of graphene that can be synthesized following two approaches, namely top-down and bottom-up methods. The top-down synthesis used harsh chemical and/or physical treatments of macroscopic graphitic materials to obtain nanoparticles, while the second is based on organic chemistry through the synthesis of polycyclic aromatic hydrocarbons exhibiting various sizes and shapes that are perfectly controlled. The main drawback of this approach is related to the low solubility of carbon materials that prevents the synthesis of nanoparticles containing more than few hundreds of sp² carbon atoms. Here we report on the synthesis of a family of rectangular-shaped graphene quantum dots containing up to 162 sp² carbon atoms. These graphene quantum dots are not functionalized on their periphery in order to keep the maximum similarity with nanoparticles of pure graphene. We chose water with sodium deoxycholate surfactant to study their dispersion and their optical properties (absorption, photoluminescence and photoluminescence excitation). The electronic structure of the particles and of their aggregates are studied using Tight-Binding (TB). We observe that the larger particles ( GQD 3 and GQD 4 ) present a slightly better dispensability than the smaller ones, probably because the larger GQDs can accommodate more surfactant molecules on each side, which helps to stabilize their dispersion in water.     

以无机硫为原料制备硫化铅量子点及其表征

根据高温下快速成核低温下慢速生长的量子点制备原理, 采用胶体化学的方法成功制备了不同粒径的硫化铅半导体量子点. 这种方法的特点是以无味和低毒的硫化钠作为制备硫化铅量子点硫的前驱物, 因此这是一种量子点的绿色化学合成方法. 油酸作为稳定剂控制硫化铅的粒径. 采用X射线衍射和高分辨透射电镜表征了量子点的晶体结构、形貌和粒径, 采用可见-近红外吸收光谱研究了硫化铅量子点的量子尺寸效应. 通过降低油酸的添加量可以促进量子点的生长, 得到较大粒径量子点. 并探讨了量子点的生长机理.     

用于检测重金属离子的磁性荧光复合微球

采用静电逐层自组装的方法,首先将PSS和PAH聚电解质交替沉积在CaCO3中空微球表面,然后将Fe3O4磁性纳米粒子与CdSe量子点负载在中空微球表面不同的聚电解质层中,制备出具有磁性和荧光双重功能的复合微球,并将其作为荧光离子探针,研究了其对Cu2+和Pb2+离子检测的灵敏度、选择性及可行性。结果表明,复合微球显示出良好的磁性和荧光性能,对Cu2+和Pb2+离子的检测具有较高的灵敏度和选择性。尤为重要的是,可通过磁分离的方法将微球快速地从待测液中回收,从而能够避免量子点对环境造成的二次污染。