Synthesis of highly luminescent and biocompatible CdTe/CdS/ZnS quantum dots using microwave irradiation: a comparative study of different ligands

We compared the effects of several ligands frequently used in aqueous synthesis, including L‐cysteine, L‐cysteine hydrochloride, N‐acetyl‐L‐cysteine (NAC), glutathione and 3‐mercaptopropionic acid, for microwave synthesis of CdTe quantum dots (QDs) in a sealed vessel with varied temperatures and times, and then developed a rapid microwave‐assisted protocol for preparing highly luminescent, photostable and biocompatible CdTe/CdS/ZnS core–multishell QDs. The effects of molecular structures of these ligands on QD synthesis under high temperatures were explored. Among these ligands, NAC was found to be the optimal ligand in terms of the optical properties of resultant QDs and reaction conditions. The emission wavelength of NAC‐capped CdTe QDs could reach 700 nm in 5 min by controlling the reaction temperature, and the resultant CdTe/CdS/ZnS core–multishell QDs could achieve the highest quantum yields up to 74% with robust photostability. In addition, the effects of temperature, growth time and shell–precursor ratio on shell growth were examined. Finally, cell culturing indicated the low cytotoxicity of CdTe/CdS/ZnS core–multishell QDs as compared to CdTe and CdTe/CdS QDs, suggesting their high potential for applications in biomedical imaging and diagnostics. Copyright © 2014 John Wiley & Sons, Ltd.     

Aqueous synthesis of CdTe/CdS/ZnS quantum dots and their optical and chemical properties

In this paper, we described a strategy for synthesis of thiol‐coated CdTe/CdS/ZnS (core–shell–shell) quantum dots (QDs) via aqueous synthesis approach. The synthesis conditions were systematically optimized, which included the size of CdTe core, the refluxing time and the number of monolayers and the ligands, and then the chemical and optical properties of the as‐prepared products were investigated. We found that the mercaptopropionic acid (MPA)‐coated CdTe/CdS/ZnS QDs presented highly photoluminescent quantum yields (PL QYs), good photostability and chemical stability, good salt tolerance and pH tolerance and favorable biocompatibility. The characterization of high‐resolution transmission electron microscopy (HRTEM), X‐ray powder diffraction (XRD) and fluorescence correlation spectroscopy (FCS) showed that the CdTe/CdS/ZnS QDs had good monodispersity and crystal structure. The fluorescence life time spectra demonstrated that CdTe/CdS/ZnS QDs had a longer lifetime in contrast to fluorescent dyes and CdTe QDs. Furthermore, the MPA‐stabilized CdTe/CdS/ZnS QDs were applied for the imaging of cells. Compared with current synthesis methods, our synthesis approach was reproducible and simple, and the reaction conditions were mild. More importantly, our method was cost‐effective, and was very suitable for large‐scale synthesis of CdTe/CdS/ZnS QDs for future applications. Copyright © 2010 John Wiley & Sons, Ltd.     

Aqueous synthesis of highly stable CdTe/ZnS Core/Shell quantum dots for bioimaging

In this work, we report the synthesis, characterization and biological application of highly stable CdTe/ZnS (cadmium tellurite/zinc sulphide) Core/Shell (CS) quantum dots (QDs) capped with mercaptosuccinic acid (MSA). The CS QDs were synthesized using a simple one‐pot aqueous method. The synthesized CdTe/ZnS CS QDs were found to exhibit excellent stability even 100 days after preparation and also showed better photoluminescence quantum yield (PLQY) of about 50% compared with that of only CdTe QDs which was nearly 12%. The formation of the CdTe/ZnS CS was confirmed by high‐resolution transmission electron microscopy (HR‐TEM), and Fourier transform infra‐red (FTIR) and X‐ray diffraction (XRD) analyses. Further, on extending our study towards bioimaging of E. coli cells using the QDs samples, we found that CdTe/ZnS CS QDs showed better results compared with CdTe QDs.     

Photoluminescence properties of hybrid SiO2‐coated CdTe/CdSe quantum dots

Hybrid SiO₂‐coated CdTe/CdSe quantum dots (QDs) were prepared using CdTe/CdSe QDs prepared by hydrothermal synthesis. A CdSe interlayer made CdTe/CdSe cores with unique type II heterostructures. The hybrid SiO₂‐coated CdTe/CdSe QDs revealed excellent photoluminescence (PL) properties compared with hybrid SiO₂‐coated CdTe QDs. Because of the existence of spatial separations of carriers in the type II CdTe/CdSe core/shell QDs, the hybrid QDs had a relatively extended PL lifetime and high stability in phosphate‐buffered saline buffer solutions. This is ascribed to the unique components and stable surface state of hybrid SiO₂‐coated CdTe/CdSe QDs. During the stabilization test in phosphate‐buffered saline buffer solutions, both static and dynamic quenching occurred. The quenching mechanism of the hybrid QDs was not suited with the Stern–Volmer equation. However, the relative stable surface of CdTe/CdSe QDs resulted in lower degradation and relative high PL quantum yields compared with hybrid SiO₂‐coated CdTe QDs. As a result, hybrid SiO₂‐coated CdTe/CdSe QDs can be used in bioapplications. Copyright © 2013 John Wiley & Sons, Ltd.     

聚乙二醇介导鹅掌楸悬浮细胞与CdSe／ZnS量子点纳米颗粒共孵育的互作特征

利用纳米材料介导的药物靶向治疗和动物细胞转基因等相关研究,日益受到人们的关注．但植物因存在细胞壁的障碍,无论原位还是离体细胞培养条件下,利用纳米技术进行基因转移均存在很大难度．因此设想,如通过纳米颗粒材料物理尺寸的改变和表面化学修饰,能改变纳米颗粒与植物细胞壁界面上的生物物理或生物化学特征,从而有利于纳米颗粒材料穿越植物细胞壁进入植物细胞,将对推动纳米技术在植物转基因领域中的应用产生重要意义．根据以上设想,研究了不同的共孵育时间和温度等条件下,杂交鹅掌楸的胚性悬浮细胞与经不同表面化学修饰的CdSe／ZnS纳米颗粒之间相互作用过程的细胞生物学特征,以及CdSe／ZnS量子点的细胞毒性．结果表明,在共孵育后3h以内,激光共聚焦显微镜和电子扫描显微镜下,均可观察到经表面后修饰带正电荷的CdSe／ZnS纳米颗粒．同时,胞吞进入细胞内部的表面携带正电荷的CdSe／ZnS纳米颗粒的量明显与共培养时间、温度有明显的依赖关系,表明它们可以通过细胞的液相胞吞作用进入杂交鹅掌楸细胞内,且不影响细胞的活性;而表面带负电荷的CdSe／ZnS纳米颗粒则主要聚集在细胞外壁附近．在培养溶液中添加20％（质量比）聚乙二醇,可进一步提高鹅掌楸细胞胞吞CdSe／ZnS纳米颗粒的量和减轻CdSe／ZnS纳米颗粒的细胞毒性．本研究表明,以表面携带正电荷的CdSe／ZnS量子点纳米材料作为基因载体,在植物悬浮细胞的转基因研究和应用中具有广泛的前景．     

Quenching of Fluorescence from CdSe/ZnS Nanocrystal QDs Near Copper Nanoparticles in Aqueous Solution

Significant quenching of fluorescence from CdSe/ZnS nanocrystal quantum dots (QDs) coated with mercaptoundecanoic ligands has been realized by copper nanoparticles (NPs). (a) Static quenching in the electrostatic association between the CdSe/ZnS QDs and cetyltrimethylammonium bromide-coated Cu NPs and (b) dynamic quenching of the same nanocrystals by polyvinylpyrrolidone-coated Cu NPs were studied. In both cases, the quenching of fluorescence from the CdSe/ZnS nanocrystals is sensitive to nanomolar concentrations of the copper NPs, and the quenching efficiency increases as spectral overlap between the CdSe/ZnS emission and the copper nanoparticle absorption increases. This suggests that the observed quenching is a result of energy transfer processes. These findings open new avenues for the utilization of Cu NPs in energy transfer-based applications.     

Synthesis and characterisation of quantum dots coupled to mycolic acids as a water-soluble fluorescent probe for potential lateral flow detection of antibodies and diagnosis of tuberculosis

This work explores the potential use of cadmium-based quantum dots (QDs) coupled to mycolic acids (MAs) as a fluorescent probe to detect anti-MA antibodies which are biomarkers for tuberculosis (TB). The use of free MAs as antigens for the serodiagnosis of TB is known but has not been developed into a point of care test. This study focuses on the synthesis, solubility, and lateral flow of QDs coupled to MAs. Water-soluble CdSe/ZnS QDs capped with l -cysteine were synthesised and covalently coupled to MAs via amide linkages to form a water-soluble fluorescent probe: MA-CdSe/ZnS QDs. The MA-CdSe/ZnS QDs showed broad absorption bands and coupling, confirmed by the presence of amide bonds in the Fourier-transform infrared (FTIR) spectrum, resulting in a blue shift in fluorescence. Powder X-ray diffraction (XRD) revealed a shift and increase in the number of peaks for MA-CdSe/ZnS QDs relative to the L-cys-CdSe/ZnS QDs, suggesting that coupling changed the crystal structure. The average particle size of MA-CdSe/ZnS QDs was ~3.0 nm. Visual paper-based lateral flow of MA-CdSe/ZnS QDs was achieved on strips of nitrocellulose membrane with both water and membrane blocking solution eluents. The highly fluorescent MA-CdSe/ZnS QDs showed good water solubility and lateral flow, which are important properties for fluorescence sensing applications.     

Biofabrication of ZnS:Mn luminescent nanocrystals using histidine,hexahistidine, and His-tagged proteins: A comparison study

The ubiquitous hexahistidine purification tag has been used to conjugate proteins to the shell of CdSe:ZnS quantum dots (QDs) due to its affinity for surface-exposed Zn²⁺ ions but little attention has been paid to the potential of His-tagged proteins for mineralizing luminescent ZnS nanocrystals. Here, we compare the ability of free histidine, a His tag peptide, His-tagged thioredoxin (TrxA, a monomeric protein), and N- and C-terminally His-tagged versions of Hsp31 (a homodimeric protein) to support the synthesis of Mn-doped ZnS nanocrystals from aqueous precursors under mild conditions of pH (8.2) and temperature (37 °C). We find that: (1) it is possible to produce poor quality QDs when histidine is used at high (8 mM) concentration; (2) an increase in local histidine concentration through repetition of the amino acid as a His tag decreases the amount of needed reagent ≈10-fold and improves optical properties; (3) fusion of the same His tag to TrxA allows for ZnS:Mn QDs mineralization at micromolar concentrations; and (4) doubling the local hexahistidine concentration by exploiting Hsp31 dimerization further improves nanocrystal luminescence with the brightest particles obtained when His tags are spatially co-localized at the Hsp31 N-termini. Although hexahistidine tracts are not as efficient as combinatorially selected ZnS binding peptides at QD synthesis, it should be possible to use the large number of available His-tagged proteins and the synthesis approach described herein to produce luminescent nanoparticles whose protein shell carries a broad range of functions.     

Biomimetic, mild chemical synthesis of CdTe-GSH quantum dots with improved biocompatibility

Multiple applications of nanotechnology, especially those involving highly fluorescent nanoparticles (NPs) or quantum dots (QDs) have stimulated the research to develop simple, rapid and environmentally friendly protocols for synthesizing NPs exhibiting novel properties and increased biocompatibility. In this study, a simple protocol for the chemical synthesis of glutathione (GSH)-capped CdTe QDs (CdTe-GSH) resembling conditions found in biological systems is described. Using only CdCl(2), K(2)TeO(3) and GSH, highly fluorescent QDs were obtained under pH, temperature, buffer and oxygen conditions that allow microorganisms growth. These CdTe-GSH NPs displayed similar size, chemical composition, absorbance and fluorescence spectra and quantum yields as QDs synthesized using more complicated and expensive methods.CdTe QDs were not freely incorporated into eukaryotic cells thus favoring their biocompatibility and potential applications in biomedicine. In addition, NPs entry was facilitated by lipofectamine, resulting in intracellular fluorescence and a slight increase in cell death by necrosis. Toxicity of the as prepared CdTe QDs was lower than that observed with QDs produced by other chemical methods, probably as consequence of decreased levels of Cd(+2) and higher amounts of GSH. We present here the simplest, fast and economical method for CdTe QDs synthesis described to date. Also, this biomimetic protocol favors NPs biocompatibility and helps to establish the basis for the development of new, "greener" methods to synthesize cadmium-containing QDs.     

Nanoparticle targeting to neurons in a rat hippocampal slice culture model

We have previously shown that CdSe/ZnS core/shell luminescent semiconductor nanocrystals or QDs (quantum dots) coated with PEG [poly(ethylene glycol)]-appended DHLA (dihydrolipoic acid) can bind AcWG(Pal)VKIKKP₉GGH₆ (Palm1) through the histidine residues. The coating on the QD provides colloidal stability and this peptide complex uniquely allows the QDs to be taken up by cultured cells and readily exit the endosome into the soma. We now show that use of a polyampholyte coating [in which the neutral PEG is replaced by the negatively heterocharged CL4 (compact ligand)], results in the specific targeting of the palmitoylated peptide to neurons in mature rat hippocampal slice cultures. There was no noticeable uptake by astrocytes, oligodendrocytes or microglia (identified by immunocytochemistry), demonstrating neuronal specificity to the overall negatively charged CL4 coating. In addition, EM (electron microscopy) images confirm the endosomal egress ability of the Palm1 peptide by showing a much more disperse cytosolic distribution of the CL4 QDs conjugated to Palm1 compared with CL4 QDs alone. This suggests a novel and robust way of delivering neurotherapeutics to neurons.     

Synthesis of functionalized amphiphilic polymers for coating quantum dots

Quantum dots (QDs) need to be attached to other chemical species if they are to be used as biomarkers, therapeutic agents or sensors. These materials also need to disperse well in water and have well-defined functional groups on their surfaces. QDs are most often synthesized in the presence of ligands such as trioctylphosphine oxide, which render the nanoparticle surfaces hydrophobic. We present a complete protocol for the synthesis and water solubilization of hydrophobic CdSe/ZnS QDs using designer amphiphilic polymeric coatings. The method is based on functionalization of an anhydride polymer backbone with nucleophilic agents. Small functional groups, bulky cyclic compounds and polymeric chains can be integrated into the coating prior to solubilization. We describe the preparation of acetylene- and azide-functionalized QDs for 'click' chemistry. The method is universal and applicable to any type of nanoparticle stabilized with hydrophobic ligands able to interact with the alkyl chains in the coating in water.     

Enhancement of cell internalization and photostability of red and green emitter quantum dots upon entrapment in novel cationic nanoliposomes

Two quantum dots (QDs), a green emitter, CdSe and a red emitter, CdSe with ZnS shell are encapsulated into novel liposomes in two different formulations including cationic liposomes. Quantum dots have proven themselves as powerful inorganic fluorescent probes, especially for long‐term, multiplexed imaging and detection. Upon delivery into a cell, in endocytic vesicles such as endosomes, their fluorescence is quenched. We have investigated the potential toxic effects, photophysical properties and cell internalization of QDs in new formulation of liposomes as an in vitro vesicle model. Entrapment of QDs into liposomes is brought about with a decrease in their intrinsic fluorescence and toxicities and an increase in their photostability and lifetime. The biomimetic lipid bilayer of liposomes provides high biocompatibility, thereby enhancing the effectiveness of fluorescent nanoparticles for biological recognition in vitro and in vivo. The prepared lipodots could effectively prevent QDs from photo‐oxidation during storage and when exposed to ultraviolet (UV) light. Moreover, the flow cytometry of HEK 293 T cells showed that the cell internalization of encapsulated QDs in (DSPC/CHO/DOPE/DOAB) liposome is enhanced 10 times compared with non‐encapsulated QD (bare QDs).     

Solubilization and bio-conjugation of quantum dots and bacterial toxicity assays by growth curve and plate count

Quantum dots (QDs) are fluorescent semiconductor nanoparticles with size-dependent emission spectra that can be excited by a broad choice of wavelengths. QDs have attracted a lot of interest for imaging, diagnostics, and therapy due to their bright, stable fluorescence. QDs can be conjugated to a variety of bio-active molecules for binding to bacteria and mammalian cells. QDs are also being widely investigated as cytotoxic agents for targeted killing of bacteria. The emergence of multiply-resistant bacterial strains is rapidly becoming a public health crisis, particularly in the case of Gram negative pathogens. Because of the well-known antimicrobial effect of certain nanomaterials, especially Ag, there are hundreds of studies examining the toxicity of nanoparticles to bacteria. Bacterial studies have been performed with other types of semiconductor nanoparticles as well, especially TiO(2), but also ZnO and others including CuO. Some comparisons of bacterial strains have been performed in these studies, usually comparing a Gram negative strain with a Gram positive. With all of these particles, mechanisms of toxicity are attributed to oxidation: either the photogeneration of reactive oxygen species (ROS) by the particles or the direct release of metal ions that can cause oxidative toxicity. Even with these materials, results of different studies vary greatly. In some studies the Gram positive test strain is reportedly more sensitive than the Gram negative; in others it is the opposite. These studies have been well reviewed. In all nanoparticle studies, particle composition, size, surface chemistry, sample aging/breakdown, and wavelength, power, and duration of light exposure can all dramatically affect the results. In addition, synthesis byproducts and solvents must be considered. High-throughput screening techniques are needed to be able to develop effective new nanomedicine agents. CdTe QDs have anti-microbial effects alone or in combination with antibiotics. In a previous study, we showed that coupling of antibiotics to CdTe can increase toxicity to bacteria but decrease toxicity to mammalian cells, due to decreased production of reactive oxygen species from the conjugates. Although it is unlikely that cadmium-containing compounds will be approved for use in humans, such preparations could be used for disinfection of surfaces or sterilization of water. In this protocol, we give a straightforward approach to solubilizing CdTe QDs with mercaptopropionic acid (MPA). The QDs are ready to use within an hour. We then demonstrate coupling to an antimicrobial agent. The second part of the protocol demonstrates a 96-well bacterial inhibition assay using the conjugated and unconjugated QDs. The optical density is read over many hours, permitting the effects of QD addition and light exposure to be evaluated immediately as well as after a recovery period. We also illustrate a colony count for quantifying bacterial survival.     

Highly sensitive electrochemical detection of potential cytotoxicity of CdSe/ZnS quantum dots using neural cell chip

Cell chip was recently developed as a simple and highly sensitive tool for the toxicity assessment of various kinds of chemicals or nano-materials. Here, we report newly discovered potential cytotoxic effects of CdSe/ZnS quantum dots (QDs) on intracellular redox environment of neural cancer cells at very low concentrations which can be only detected by cell chip technology. Green (2.1 nm in diameter) and red (6.3 nm in diameter) QDs capped with cysteamine (CA) or thioglycolic acid (TA) were found to be toxic at 100 μg/mL when assessed by trypan blue and differential pulse voltammetry (DPV). However, in case of concentration-dependent cytotoxicity, toxic effects of TA-capped QDs on human neural cells were only measured by DPV method when conventional MTT assay did not show toxicity of TA-capped QDs at low concentrations (1-10 μg/mL). Red-TA QDs and Green-TA QDs were found to decrease electrochemical signals from cells at 10 μg/mL and 5 μg/mL, respectively, while cell viability decreased at 100 μg/mL and 50 μg/mL when assessed by MTT assay, respectively. The relative decreases of cell viability determined by MTT assay were 15% and 11.9% when cells were treated with 5-50 μg/mL of Red-TA QDs and 5-30 μg/mL of Green-TA QDs, respectively. However, DPV signals decreased 37.5% and 39.2% at the same concentration range, respectively. This means that redox environment of cells is more sensitive than other components and can be easily affected by CdSe/ZnS QDs even at low concentrations. Thus, our proposed neural cell chip can be applied to detect potential cytotoxicity of various kinds of molecular imaging agents simply and accurately.     

CdTe/ZnS quantum dots as fluorescent probes for ammonium determination

Novel CdTe/ZnS quantum dot (QD) probes based on the quenching effect were proposed for the simple, rapid, and specific determination of ammonium in aqueous solutions. The QDs were modified using 3‐mercaptopropionic acid, and the fluorescence responses of the CdTe/ZnS QD probes to ammonium were detected through regularity quenching. The quenching levels of the CdTe/ZnS QDs and ammonium concentration showed a good linear relationship between 4.0 × 10^?6 and 5.0 × 10^?4 mol/L; the detection limit was 3.0 × 10^?7 mol/L. Ammonium contents in synthetic explosion soil samples were measured to determine the practical applications of the QD probes and a probable quenching mechanism was described. Copyright © 2015 John Wiley & Sons, Ltd.     

Facile synthesis and photophysical characterization of luminescent CdTe quantum dots for Forster resonance energy transfer based immunosensing of staphylococcal enterotoxin B

Aqueous phase synthesis of CdTe quantum dots (QDs) with surface functionalization for bioconjugation remains the best approach for biosensing and bioimaging applications. We present a facile aqueous phase method to prepare CdTe QDs by adjusting precursor and ligand concentrations. CdTe QDs had photoluminescence quantum yield up to ≈33% with a narrow spectral distribution. The powder X‐ray diffraction profile elucidated characteristic broad peaks of zinc blende cubic CdTe nanoparticles with 2.5–3 nm average crystalline size having regular spherical morphology as revealed by transmission electron microscopy. Infra‐red spectroscopy confirmed disappearance of characteristic absorptions for –SH thiols inferring thiol coordinated CdTe nanoparticles. The effective molar concentration of 1 : 2.5 : 0.5 respectively for Cd²⁺/3‐mercaptopropionic acid/HTe^– at pH 9 ± 0.2 resulted in CdTe quantum dots of 2.2–3.06 nm having band gap in the range 2.74–2.26 eV respectively. Later, QD₅₂₃ and QD₆₀₁ were used for monitoring staphylococcal enterotoxin B (SEB; a bacterial superantigen responsible for food poisoning) using Forster resonance energy transfer based two QD fluorescence. QD₅₂₃ and QD₆₀₁ were bioconjugated to anti‐SEB IgY antibody and SEB respectively according to carbodiimide protocol. The mutual affinity between SEB and anti‐SEB antibody was relied upon to obtain efficient energy transfer between respective QDs resulting in fluorescence quenching of QD₅₂₃ and fluorescence enhancement of QD₆₀₁. Presence of SEB in the range 1–0.05 µg varied the rate of fluorescence quenching of QD₅₂₃, thereby demonstrating efficient use of QDs in the Forster resonance energy transfer based immunosensing method by engineering the QD size. Copyright © 2012 John Wiley & Sons, Ltd.     

Characterization and cancer cell targeted imaging properties of human antivascular endothelial growth factor monoclonal antibody conjugated CdTe/ZnS quantum dots

High luminescence quantum yield water‐soluble CdTe/ZnS core/shell quantum dots (QDs) stabilized with thioglycolic acid were synthesized. QDs were chemically coupled to fully humanized antivascular endothelial growth factor₁₆₅ monoclonal antibodies to produce fluorescent probes. These probes can be used to assay the biological affinity of the antibody. The properties of QDs conjugated to an antibody were characterized by ultraviolet and visible spectrophotometry, fluorescent spectrophotometry, sodium dodecyl sulfate–polyacrylamide gel electrophoresis, transmission electron microscopy and fluorescence microscopy. Cell‐targeted imaging was performed in human breast cancer cell lines. The cytotoxicity of bare QDs and fluorescent probes was evaluated in the MCF‐7 cells with an MTT viability assay. The results proved that CdTe/ZnS QD–monoclonal antibody nanoprobes had been successfully prepared with excellent spectral properties in target detections. Surface modification by ZnS shell could mitigate the cytotoxicity of cadmium‐based QDs. The therapeutic effects of antivascular endothelial growth factor antibodies towards cultured human cancer cells were confirmed by MTT assay. Copyright © 2014 John Wiley & Sons, Ltd.     

Preparation and Biological Effect of Nucleotide-Capped CdSe/ZnS Quantum Dots on Tetrahymena thermophila

In this paper, we described the preparation and characterization of different types of modified CdSe/ZnS quantum dots (QDs) and explored the biological effects of QDs with different surface modifications on the whole growth of unicellular protozoan Tetrahymena thermophila BF(5) using a thermal activity monitor air isothermal microcalorimeter. Our results demonstrated that adenosine 5'-monophosphate (AMP) showed stronger interaction with QDs than other types of nucleotide. AMP-QDs could stimulate the growth of T. thermophila while mercaptoacetic acid-capped CdSe/ZnS quantum dots inhibited it. In addition, the population density determination and fluorescence imaging of T. thermophila BF(5) also confirmed the results obtained from microcalorimetry. It is believed that this approach will provide a more convenient methodology for the kinetics and thermodynamics of microorganism when coexisting with QDs in real time, and all of which are very significant to understanding the effect of QDs to organism.     

Influence of quantum dots on the aromaticity of thiosalicylic acid

When ligands are coordinated to quantum dots (QDs), the ring current of the ligand strongly influences the applications of the QDs, for example in solar cell technology. The Raman spectrum of the ligand can be used to probe and identify ions or measure ion concentrations. Here, we investigated, using a theoretical method, the aromaticities and Raman spectra of CdTe, CdSe, and CdS QDs coordinated with thiosalicylic acid ligands. We found that the aromaticity of the benzene ring in free thiosalicylic acid increased when it was used as a QD ligand. The ring currents of the benzene rings in the CdTe–ligand, CdSe–ligand, and CdS–ligand systems were stronger than the ring current of the benzene ring in free thiosalicylic acid; in other words, the QDs influence the ring current—they enhance the electron transfer rate of the benzene ring. We also discovered that the CdTe–ligand and CdSe–ligand systems have stronger ring currents than the CdS–ligand system. The high electronegativity and vacant d orbital of the sulfur atom influence the ring current of the ligand in the CdS–ligand system. Further, the Raman spectrum of free thiosalicylic acid was different from the spectra of the ligands in the QD–ligand systems: the Raman spectra of COO^? in each QD–ligand system was enhanced compared with that of the COO^? in free thiosalicylic acid.

Spontaneous charge transfer between zinc tetramethyl-tetra-2,3-pyridinoporphyrazine and CdTe and ZnS quantum dots

Zinc tetramethyl-tetra-2,3-pyridinoporphyrazine (ZnTmtppa(-2)) gets reduced to the ZnTmtppa(-3) species on interaction with CdTe QDs capped with 2-mercaptoethanol (2-ME) or thioglycolic acid (TGA) and ZnS QDs capped with 2-ME. The interaction occurs without photolysis. The fluorescence of the QDs is quenched by ZnTmtppa resulting in large quenching constants. Binding of ZnTmtppa to QDs occurs with two molecules of the former binding to the latter.