Characterization of CdS nanoparticles in solutions of P(TFE-co-PVDF-co-Prop)/N,N-dimethylformamide

We report in here the preparation of CdS nanoparticles (Q-CdS) in N,N-dimethylformamide (DMF) and poly(tetrafluoroethylene-co-vinylidenefluoride-co-propylene) (PTFE-co-PVDF-co-Prop) by reaction of cadmium acetate and thiourea at room temperature. The formation and size evolution of Q-CdS particles were followed by optical absorption spectroscopy as a function of the aging process of the solutions. The obtained results indicated that not only the Q-CdS particles were more stable in DMF than in aqueous solutions, probably due to the interaction with the CHNO group, but also they were formed in smaller sizes. The particle size obtained in DMF was estimated in 4-6 nm against 10-20 nm in aqueous solution. The results also showed that the velocity of Q-CdS formation is lower when the reaction was carried out in PTFE-co-PVDF-co-Prop dissolved in DMF. The Q-CdS particles formed in this system were also less stable than in pure DMF, probably due to the interaction of the CHNO fragment and CF₂ of polymer chain or phase segregation.     

Electronic Properties of Q-CdS Clusters Stabilized by Adenine

Adenine-capped Q-CdS has been synthesized in an aqueous medium. IR spectroscopy indicates an interaction between Q-CdS and adenine through Cd²⁺. The amount of adenine controls the size of the clusters. A typical 5×10⁻³ mol dm⁻³ of adenine produces nanoclusters having the onset of absorption and an emission band at 2.8 and 2.35 eV, respectively. Adenine binds to the shallower traps and enhances the emission intensity of the 530-nm band without causing any shift in emission. Thermolysis of these colloids leads to the production of larger CdS clusters with changed electronic properties. Relaxation kinetics of charge carriers shows their average lifetime to increase with a decrease in particle size. Illumination of these particles does not lead to their photodissolution. This catalyst is, however, photoactive. The addition of indole causes the quenching of its emission. The photocatalytic oxidation of indole produces indigo with a quantum efficiency of 0.03.     

Photophysics and photocatalytic behavior of composite CdS-purine nanoparticles in the presence of certain indoles

The photophysics of purine-capped Q-CdS has been examined in the presence of certain indoles. The addition of indole does not modify electronic spectrum of purine-capped Q-CdS but it forms a fluorescing charge-transfer intermediate with illuminated CdS, which has an emissive peak at 495 nm. The intensity and the lifetime of this intermediate are enhanced initially with an increase in concentration of indole. In the presence of other indoles, the fluorescence is simply quenched in a dynamic process without forming any fluorescing intermediate. In contrast, emissive CT intermediate is not formed in the presence of indole or any of its derivatives with adenine-capped Q-CdS. In all the cases the quenching of fluorescence, monitored by steady state and time-resolved methods, follows the Stern-Volmer relationship and takes place with a bimolecular rate constant of approximately 10(10) dm(3)mol(-1)s(-1). Purine-capped Q-CdS sensitizes the reactions of the investigated indole(s)-O2 couple much more efficiently than adenine-capped Q-CdS. The differences in quenching of fluorescence and reactivity of holes between purine-capped Q-CdS and adenine-capped Q-CdS are explained by the difference in the binding of indole to the particle. In the case of purine-capped Q-CdS, specific channels for the binding of the solutes are created through the H-bond with the surface-capped purine.     

Synthesis of CdS nanoparticles in colloidal state and its possible interaction with tyrosine

The basic objective is to develop a simpler method of preparation of the colloidal CdS nanoparticles with greater stability and to study interaction with tyrosine molecules. Average size of the particles in the colloid is found to be about 3 nm as probed by transmission electron microscopy (TEM) and dynamic light scattering (DLS) measurements. Effect of both sulfide enriched CdS as well as Cd(2+) enriched CdS on tyrosine is investigated both through absorbance and emission spectroscopy. Quenching of tyrosine emission followed Stern-Volmer relation and was found to be independent of temperature, indicating possible static quenching. However, Forster transfer between tyrosine and CdS can be suspected to mimic static quenching in addition to the charge transfer complexes. In the presence of Cd(2+) enriched CdS nanoparticles, the emission of tyrosine in phosphate buffer shows typical spectral broadening and a long wavelength increase in fluorescence emission. This may be attributed to the sensitized emission of CdS itself.     

一种Q态纳米CdS的新型制备法——聚合物分散法

采用一种Q态CdS的新型制备法——聚合物分散法, 即用水溶性聚合物溶液作为分散剂, 用2-巯基乙醇(或十二硫醇)作表面修饰剂, 在聚合物网络中构筑出四种粒径的、单分散性的Q-CdS. 通过UV-Vis光谱和TEM考察了Q-CdS的粒径及分布情况, 并用FL光谱研究了不同尺寸的Q-CdS的荧光性能. 结果表明, 采用聚合物分散法可以方便、快捷地得到粒径小且分布窄的Q-CdS纳米粒子, 这些粒子在紫外光谱及荧光光谱上均表现出明显的量子尺寸效应.     

Preparation of aqueous CdS colloids in the presence of cadmium complexonates: effect of complexonates on the size of CdS nanoparticles

The results of a systematic study of the preparation of CdS colloids in aqueous solutions containing different Cd²⁺ complexonates are presented. The effects of the ratio of the reagents and the nature and concentration of various stabilizing surfactants and Cd²⁺ complexonates, including those of some sulfur-containing compounds, on the size of the colloidal particles have been studied. Thermodynamic calculation of the expected equilibrium size of the colloidal particles as a function of the solvent composition, taking into account the increase in the solubility of the CdS phase as the particle size decreases, has been performed. Comparison of the calculated results with the experimental data shows that the size of colloidal particles is determined to a great extent by kinetic factors of their growth rather than by thermodynamic factors. It has been established that when the size of colloidal particles is less than a critical value, their dissolution by adding strong compexing agents to the system does not result in a change in the observed mean-volume size of the particles.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1739–1746, September, 1995.The authors are grateful to A. L. Chuvilin (G. K. Boreskov Institute of Catalysis, SB of the RAS) for help in preparing the electron photomicrographs.The work was financially supported by the Russian Foundation for Basic Research (Project No. 93-03-4816).     

Conjugated chromophore near the quantum-confined cadmium sulfide cluster: quenched photoluminescence and enhanced two-photon absorption

Quenching effect of the photoluminescence of 1,2,4,5-tetrakis(4-pyridylvinyl)benzene in the presence of CdS colloids stabilized by inverse micelles was observed. The observed regularities of luminescence quenching by the quenchers of different size were studied. An increase in the two-photon absorption cross section (6.5 times higher) and in the two-photon-induced fluorescence intensity was observed for the composite solution when pumped by 740-nm laser irradiation. The results are in accord with theoretical prediction of enhancement of third-order optical nonlinearity of quantum-confined semiconductor.     

Surface chemistry of luminescent colloidal silicon nanoparticles

Luminescent colloidal silicon particles are obtained by burning silane and slowly etching the product with hydrogen fluoride. Depending on their size, the particles emit red-orange or blue-green light with decay times between 28 micros and <0.1 micros, respectively. The quantum yield of the red luminescence is found to be 35%, i.e., much higher than the 7% previously reported, and the yield of the blue-green luminescence is 18%. The luminescence of the colloidal particles is quenched upon the attack of their surface by free radicals, oxygen-centered radicals being more efficient than carbon-centered ones. It is concluded from the dependence of the luminescence wavelength on the etching time and the dependence of the luminescence lifetime on the wavelength that the mechanism of the photoluminescence undergoes a change with decreasing particle size. The red luminescent particles exhibit amphiphilic properties, such as unusual wetting phenomena. This effect is understood in terms of the existence of few polar groups on the otherwise nonpolar surface of the particles, possibly Si-OH groups, which also act as centers of the red luminescence.     

Q-CdS/聚合物纳米复合膜的制备与荧光性能

采用配位化学合成原理 ,分离制备出颗粒尺寸小于 10nm的单分散性的Q态CdS(Q CdS)纳米粒子 ,将Q CdS纳米粒子与聚合物复合成膜 ,制备出一系列Q CdS 聚合物纳米复合膜 .用紫外可见吸收光谱与透射电镜研究了纳米复合膜的量子尺寸效应和分散性 .通过荧光光谱探讨了不同聚合物基体材料和不同Q CdS含量的纳米复合膜的荧光发光性能 .结果表明 ,一方面这种以聚合物为基体的纳米复合膜 ,由于聚合物与Q CdS之间的相互作用 ,使纳米复合膜表现出与单一相组分完全不同的特征荧光发射峰 ;另一方面 ,随着纳米复合膜中Q CdS含量的不断增大 ,纳米复合膜的荧光强度不断增强 ,在一定浓度时达到最大值 .     

表面修饰的Q态纳米CdS的荧光性能研究

以硫醇为表面修饰剂,通过控制硫醇与Cd²⁺的比例,得到性能稳定的Q-CdS,而后将Q-CdS与聚合物通过共混复合成膜.通过荧光光谱研究了硫醇和聚合物对Q-CdS的表面修饰作用,研究发现硫醇的长碳链有效阻止了CdS粒子间的团聚,碳链的增加导致Q-CdS稳定性的增强,Q-CdS的激子发射峰强度增大,且这种表面修饰效应随硫醇加入量的增大而增强,在一定硫醇加入量时的激子荧光发射强度达到最大.由于介电局域效应,聚合物的加入使Q-CdS的表面态荧光发光强度呈数量级增大.另一方面,随着聚合物加入量的增加又会导致Q-CdS的表面钝化,缺陷减小,表面态荧光发射峰相对减弱,而激子发射峰却增强.     

Size-dependent self-organization of colloidal particles on chemically patterned surfaces

A study of the self-organization of colloidal particles during the evaporation of particle solutions on chemically patterned surfaces is presented. On a surface with hydrophilic and hydrophobic regions, colloidal particles form compact structures on the hydrophilic sites. When a colloidal solution containing a mixture of particles with a variation in size is used, the number density of each type of particle deposited on the hydrophilic islands after evaporation decreases with increasing particle size. This makes it possible to produce a concentration gradient of the particles on islands of different sizes. It is shown that this technique could allow for particle separation.     

Temperature tunability of size in CdS nanoparticles and size dependent photocatalytic degradation of nitroaromatics

Size tunability of thiophenol capped CdS nanoparticles (NPs) has been achieved by controlling the temperatures in situ. Synthesis at 5 degrees C produced stable particles of smallest size having narrow size distribution and high photoluminescence quantum yield. The photoluminescence of thiophenol capped CdS NPs was quenched by the nitroaromatic compounds. The Stern-Volmer constant of dinitrobenzene was about 15-fold higher than nitrobenzene, which indicates that introduction of nitro groups in the benzene ring increases the quenching efficiency. Further, the as-prepared CdS NPs were found to display size dependent photocatalytic activity towards degradation of nitroaromatics. The catalytic efficiency of CdS quantum particles was quintupled with decrease in particle size from 5.8 to 3.8 nm. An empirical equation has been derived to correlate the catalytic efficiency of the nanoparticles with the twin factors operating in the quantum confinement regime: (i) change in surface to volume ratio and (ii) shift in conduction band edge.     

Colloidal stability and photophysical characteristics of luminesent silica nanoparticles modified with various nitrogen/oxygen-containing trialkoxysilanes

Surface modification of the luminescent silica nanoparticles doped with Tb(III)–p-sulfonatothiacalix[ 4]arene complex was carried out using a series of nitrogen/oxygen-containing trialkoxysilanes. It was found that groups capable of nonspecific interactions on the surface of the nanoparticles cause a significant decrease in their colloidal stability. The chromophore moieties in the modifiers were found to quench the luminescence of the nanoparticles. The surface modification of the nanoparticles is responsible for the change in the mechanism of luminescence quenching in the presence of copper(II) ions due to decreased accessibility of luminophores to the quencher.     

Synthesis and characterization of poly(acrylic acid) stabilized cadmium sulfide quantum dots

Cadmium sulfide (CdS) nanoparticles (NPs) capped with poly(acrylic acid) (PAA) were prepared in aqueous solutions from Cd(NO3)2 and Na2S. Influence of the COOH/Cd ratio (0.8-12.5), reaction pH (5.5 and 7.5), and PAA molecular weight (2100 and 5100 g/mol) on the particle size, colloidal stability, and photoluminescence were investigated. A Cd/S ratio of <1 causes ineffective passivization of the surface with the carboxylate and therefore results in a red shift of the absorption band and a significant drop in photoluminescence. Therefore, the Cd/S ratio was fixed at 1.1 for all experiments studying the mentioned variables. PAA coating provided excellent colloidal stability at a COOH/Cd ratio above 1. Absorption edges of PAA-coated CdS NPs are in the range of 460-508 nm. The size of the NPs increases slightly with increasing PAA molecular weight and COOH/Cd ratio at pH 7.5. It is demonstrated that there is a critical COOH/Cd ratio (1.5-2) that maximizes the photoluminescence intensity and quantum yield (QY, 17%). Above this critical ratio, which corresponds to smaller crystal sizes (3.7-4.1 nm) for each reaction set, the quantum yield decreases and the crystal size increases. Moreover, CdS NPs prepared at pH 7.5 have significantly higher QY and absorb at lower wavelengths in comparison with those prepared at pH 5.5. Luminescence quenching has not been observed over 8 months.     

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.     

Amplified luminescence quenching of phosphorescent metal-organic frameworks

Amplified luminescence quenching has been demonstrated in metal-organic frameworks (MOFs) composed of Ru(II)-bpy building blocks with long-lived, largely triplet metal-to-ligand charge-transfer excited states. Strong non-covalent interactions between the MOF surface and cationic quencher molecules coupled with rapid energy transfer through the MOF microcrystal facilitates amplified quenching with a 7000-fold enhancement of the Stern-V?lmer quenching constant for methylene blue compared to a model complex.     

UV–vis photodegradation of dyes in the presence of colloidal Q-CdS

The interaction of the dyes Safranin-O (SO) and Orange II (OII) with aqueous colloidal Q-CdS clusters, which emit single fluorescence bands with maximum wavelengths at 481 nm (excitonic band) or 559 nm (trapped band), has been studied. This was carried out by monitoring both the photodegradation of the dye in the presence of the clusters and the quenching of the clusters fluorescence by the dyes. The photolysis experiments were carried out by excitation either at 520 nm (the wavelength at which the dyes, but not the clusters absorb light) or at 350 nm (the wavelength at which the clusters strongly absorb light, and the dyes have absorbance minima). At 520 nm, photodegradation of SO could be observed, which follows a first-order kinetics (for trapped-band clusters) and a second-order kinetics (for excitonic-band clusters). For the excitation wavelength of 350 nm, photodegradation of either of the dyes could not be observed. The Stern–Volmer plots for the quenching of the excitonic band-clusters fluorescence by SO show an upward curvature, pointing to the occurrence of more than one species acting as the fluorescence quencher, whereas the Stern–Volmer plots for the quenching of the trapped band-clusters fluorescence by SO are linear, indicating that only one species acts as a fluorescence quencher. Lambert–Beer type plots (absorbance vs. concentration) are linear for SO in water and in trapped band-clusters solutions, but a similar study of SO in excitonic band-clusters solution show the occurrence of a new band, which can be assigned to a ground-state dimer of the dye. The latter can be used to explain both the upward curvature of the Stern–Volmer plots and the second-order kinetics observed for SO photodegradation in the SO-excitonic band-clusters system. The Stern–Volmer plots for the quenching of both fluorescence bands by OII are linear.     

Properties and biocompatibility of colloid cadmium sulfide nanoparticles

The properties and cytotoxicity of aqueous colloidal solutions of cadmium sulfide CdS nanoparticles obtained by chemical condensation were explored. Dynamic light scattering and optical spectroscopy were used to study the hydrodynamic diameter, size distribution, zeta-potential and optical properties of the CdS nanoparticles in solution. The cytotoxicity of colloidal CdS nanoparticles at different concentrations was assessed on cultures of human dermal fibroblasts and the cancer HeLa line.     

Effect of colloidal particle size on adsorbed monodisperse and bidisperse monolayers

Coating hydrogel films or microspheres by an adsorbed colloidal shell is one synthesis method for forming colloidosomes. The colloidal shell allows control of the release rate of encapsulated materials, as well as selective transport. Previous studies found that the packing density of self-assembled, adsorbed colloidal monolayers is independent of the colloidal particle size. In this paper we develop an equilibrium model that correlates the packing density of charged colloidal particles in an adsorbed shell to the particle dimensions in monodisperse and bidisperse systems. In systems where the molar concentration in solution is fixed, the increase in adsorption energy with increasing particle size leads to a monotonic increase in the monolayer packing density with particle radius. However, in systems where the mass fraction of the particles in the adsorbing solutions is fixed, increasing particle size also reduces the molar concentration of particles in solution, thereby reducing the probability of adsorption. The result is a nonmonotonic dependence of the packing density in the adsorbed layer on the particle radius. In bidisperse monolayers composed of two particle sizes, the packing density in the layer increases significantly with size asymmetry. These results may be utilized to design the properties of colloidal shells and coatings to achieve specific properties such as transport rate and selectivity.     

Coherent Magnetic Resonance of Nanocrystal Quantum‐Dot Luminescence as a Window to Blinking Mechanisms

Blinking of colloidal nanocrystal quantum dots, random intermittency in the stream of photons emitted by single particles, has long commanded the curiosity of researchers. Why does the particle suddenly shut off, and what are the pathways to quench emission? Single‐particle microscopy is not the only way to approach these fundamental questions on the interaction of light and matter: time‐domain sub‐ensemble spectroscopies can also yield relevant information on microscopic electronic processes. We illustrate recent advances in pulsed optically detected magnetic resonance and highlight the conceptual relevance to unravelling mechanisms controlling intermittency on the single‐particle level. Magnetic resonance reveals two distinct luminescence quenching channels, which appear to be related to those previously surmised from single‐particle studies: a trapped charge‐separated state in which the exciton is quenched by dissociation and the particle remains neutral; and a charged state of the particle in which spin‐dependent Auger recombination quenches luminescence.