Enantioselective cytotoxicity of ZnS:Mn quantum dots in A549 cells

Chirality strongly influences many biological properties of materials, such as cell accumulation, enzymatic activity, and toxicity. In the past decade, it has been shown that quantum dots (QDs), fluorescent semiconductor nanoparticles with unique optical properties, can demonstrate optical activity due to chiral ligands bound on their surface. Optically active QDs could find potential applications in biomedical research, therapy, and diagnostics. Consequently, it is very important to investigate the interaction of QDs capped with chiral ligands with living cells. The aim of our study was to investigate the influence of the induced chirality of Mn‐doped ZnS QDs on the viability of A549 cells. These QDs were stabilized with D‐ and L‐cysteine using a ligand exchange technique. The optical properties of QDs were studied using UV–Vis, photoluminescence (PL), and circular dichroism (CD) spectroscopy. The cytotoxicity of QDs was investigated by high content screening analysis. It was found that QDs stabilized by opposite ligand enantiomers, had identical PL and UV–Vis spectra and mirror‐imaged CD spectra, but displayed different cytotoxicity: QDs capped with D‐cysteine had greater cytotoxicity than L‐cysteine capped QDs.     

Tunable excitation properties of ZnCdS:Mn/ZnS quantum dots for cancer imaging

Water‐soluble ZnS:Mn quantum dots (QDs) were synthesized using a hydrothermal method with 3‐mercaptopropionic acid as stabilizer. The optical properties of ZnS:Mn QDs were thoroughly investigated by tuning the doping concentration of Mn²⁺ and the Zn/S precursor ratio, to obtain an optimal parameter for QDs with excellent fluorescence characteristics. ZnS:Mn QDs excited at only one wavelength, however, which seriously limited their further application. Here, a trace Cd ion was doped into a ZnS host, resulting in QD excitation covering a wide adjustable waveband. Furthermore, when a ZnS shell was coated onto the surface of the ZnCdS:Mn QDs, photoluminescence intensity and stability were further enhanced. After coupling with an anti‐CK 19 antibody, the ZnCdS:Mn/ZnS core/shell QDs were able to function by labeling cancer cells, indicating that they could be considered as a suitable bio‐probe for cells and tissue imaging.     

Direct and indirect immunolabelling of HeLa cells with quantum dots

With excellent optical properties, quantum dots (QDs) have been made as attractive molecular probes for labelling cells in biological research. In this study high‐quality CdSe QDs prepared in a paraffin–oleic acid system were used as fluorescent labels in direct and indirect detection of carcinoembryonic antigen (CEA), a cancer marker expressed on the surface of HeLa cells. The primary antibody (Ab) (rabbit anti‐CEA8) and secondary Ab (goat anti‐rabbit IgG) were covalently linked to carboxyl‐functioned CdSe QDs, and both the QDs–antibody and QDs–IgG probes were successfully used to label HeLa cells. The present study demonstrates the practicability of CdSe QDs as an attractive type of fluorescent labels for biological applications such as protein probes and cell imaging. Copyright © 2008 John Wiley & Sons, Ltd.     

Cu‐doped quantum dots: a new class of near‐infrared emitting fluorophores for bioanalysis and bioimaging

Transition metal ion‐doped quantum dots (QDs) exhibit unique optical and photophysical properties that offer significant advantages over undoped QDs, such as larger Stokes shift to avoid self‐absorption/energy transfer, longer excited‐state lifetimes, wider spectral window, and improved chemical and thermal stability. Among the doped QDs emitters, Cu is widely introduced into the doped QDs as novel, efficient, stable, and tunable optical materials that span a wide spectrum from blue to near‐infrared (NIR) light. Their unique physical and chemical characteristics enable the use of Cu‐doped QDs as NIR labels for bioanalysis and bioimaging. In this review, we discuss doping mechanisms and optical properties of Cu‐doped QDs that are capable of NIR emission. Applications of Cu‐doped QDs in in vitro biosensing and in in vivo bioimaging are highlighted. Moreover, a prospect of the future of Cu‐doped QDs for bioanalysis and bioimaging are also summarized.     

Aqueous synthesis of highly luminescent surface Mn2+‐doped CdTe quantum dots as a potential multimodal agent

Mn²⁺‐doped CdTe quantum dots (QDs) were synthesized directly via a facile surface doping strategy in aqueous solution. The best optical property emerged when the added amount of Mn²⁺ was 5% compared to Cd²⁺ in the CdTe nanoparticles and the reaction temperature was 60 °C. The fluorescence and magnetic properties of the QDs were studied. The as‐prepared Mn²⁺‐doped CdTe QDs have high quantum yield (48.13%) and a narrow distribution with an average diameter of 3.7 nm. The utility of biological imaging was also studied. Depending on the high quantum yield, cells in culture were illuminated and made more distinct from each other compared to results obtained with normal QDs. They also have a prominent longitudinal relaxivity value (r₁ = 4.2 mM^?1s^?1), which could indicate that the Mn²⁺‐doped CdTe QDs can be used as a potential multimodal agent for fluorescence and magnetic resonance imaging. Copyright © 2014 John Wiley & Sons, Ltd.     

Near‐Infrared,Heavy Metal‐Free Colloidal “Giant” Core/Shell Quantum Dots

“Giant” core/shell quantum dots (g‐QDs) are a promising class of materials for future optoelectronic technologies due to their superior chemical‐ and photostability compared to bare QDs and core/thin shell QDs. However, inadequate light absorption in the visible and near‐infrared (NIR) region and frequent use of toxic heavy metals (e.g., Cd and Pb) are still major challenges for most g‐QDs (e.g., CdSe/CdS) synthesized to date. The synthesis of NIR, heavy metal‐free, Zn‐treated spherical CuInSe₂/CuInS₂ g‐QDs is reported using the sequential cation exchange method. These g‐QDs exhibit tunable NIR optical absorption and photoluminescence (PL) properties. Transient fluorescence spectroscopy shows prolonged lifetime with increasing shell thickness, indicating the formation of quasi type‐II band alignment, which is further confirmed by simulations. As a proof‐of‐concept, as‐synthesized g‐QDs are used to sensitize TiO₂ as a photoanode in a photoelectrochemical (PEC) cell, demonstrating an efficient and stable PEC system. These results pave the way toward synthesizing NIR heavy metal‐free g‐QDs, which are very promising components of future optoelectronic technologies.     

Near Infrared,Highly Efficient Luminescent Solar Concentrators

The fabrication of a low reabsorption emission loss, high efficient luminescent solar concentrator (LSC) is demonstrated by embedding near infrared (NIR) core/shell quantum dots (QDs) in a polymer matrix. An engineered Stokes shift in NIR core/shell PbS/CdS QDs is achieved via a cation exchange approach by varying the core size and shell thickness through the refined reaction parameters such as reaction time, temperature, precursor molar ratio, etc. The as‐synthesized core/shell QDs with high quantum yield (QY) and excellent chemical/photostability exhibit a large Stokes shift with respect to the bare PbS QDs due to the strong core‐to‐shell electrons leakage. The large‐area planar LSC based on core/shell QDs exhibits the highest value (6.1% with a geometric factor of 10) for optical efficiency compared to the bare NIR QD‐based LSCs and other reported NIR QD‐based LSCs. The suppression of emission loss and the broad absorption of PbS/CdS QDs offer a promising pathway to integrate LSCs and photovoltaic devices with good spectral matching, indicating that the proposed core/shell QDs are strong candidates for fabricating high efficiency semi‐transparent large‐area LSCs.     

Synthesis of near‐infrared‐emitting CdTeSe and CdZnTeSe quantum dots

We exploited the synthesis of near‐infrared (NIR) emitting ternary‐alloyed CdTeSe and quaternary‐alloyed CdZnTeSe quantum dots (QDs) with rod and tetrapod morphologies, which have tunable emission in the NIR electromagnetic spectrum. The morphologies of the QDs depended strongly on their growth kinetics, probably due to the coordinating ligands used in the preparation. Using oleic acid, stearic acid and hexadecylamine as ligands and keeping the same reaction parameters, QDs with tetrapod and rod morphologies were created. Not only had the capping ligands influenced the morphologies of QDs, but also they influenced the optical properties of QDs. The molar ratios of Cd/Zn and Te/Se upon preparation were adjusted for investigating the effect of composition on the properties of resulting QDs. By varying the composition of QDs, the photoluminescence (PL) wavelength of QDs was tuned from 650 nm to 800 nm. To enhance PL efficiency and stability, QDs were coated with a CdZnS shell. As NIR PL has numerous advantages in biological imaging detection, these QDs hold great potential for application. Copyright © 2012 John Wiley & Sons, Ltd.     

Characterization of the interaction of a mono‐6‐thio‐β‐cyclodextrin‐capped CdTe quantum dots–methylene blue/methylene green system with herring sperm DNA using a spectroscopic approach

Novel, water‐soluble CdTe quantum dots (QDs) capped with β‐cyclodextrin (β‐CD) and ~ 4.0 nm in diameter were synthesized in aqueous solution, and characterized using transmission electron microscopy (TEM). A fluorescence‐sensing system based on the photoinduced electron transfer (PET) of (mono‐6‐thio‐β‐CD)–CdTe QDs was then designed to measure the interaction of phenothiazine dyes [methylene blue (MB) and methylene green (MG)] with herring sperm DNA (hsDNA). This fluorescence‐sensing system was based on a fluorescence “OFF–ON” mode. First, MB/MG adsorbed on the surface of (mono‐6‐thio‐β‐CD)–CdTe QDs effectively quenches the fluorescence of (mono‐6‐thio‐β‐CD)–CdTe QDs through PET. Then, addition of hsDNA restores the fluorescence intensity of (mono‐6‐thio‐β‐CD)–CdTe QDs, because hsDNA can bind with MB/MG and remove it from the as‐prepared (mono‐6‐thio‐β‐CD)–CdTe QDs. In addition, detailed reaction mechanisms of the (mono‐6‐thio‐β‐CD)–CdTe QDs–MB/MG–hsDNA solution system were studied using optical methods, by comparison with the TGA–CdTe QDs–MB/MG–hsDNA solution system. Copyright © 2014 John Wiley & Sons, Ltd.     

Deposition of CdS,CdS/ZnSe and CdS/ZnSe/ZnS shells around CdSeTe alloyed core quantum dots: effects on optical properties

In this work, we synthesized water‐soluble L ‐cysteine‐capped alloyed CdSeTe core quantum dots (QDs) and investigated the structural and optical properties of deposition of each of CdS, CdS/ZnSe and CdS/ZnSe/ZnS shell layers. Photophysical results showed that the overcoating of a CdS shell around the alloyed CdSeTe core [quantum yield (QY) = 8.4%] resulted in effective confinement of the radiative exciton with an improved QY value of 93.5%. Subsequent deposition of a ZnSe shell around the CdSeTe/CdS surface decreased the QY value to 24.7%, but an increase in the QY value of up to 49.5% was observed when a ZnS shell was overcoated around the CdSeTe/CdS/ZnSe surface. QDs with shell layers showed improved stability relative to the core. Data obtained from time‐resolved fluorescence measurements provided useful insight into variations in the photophysical properties of the QDs upon the formation of each shell layer. Our study suggests that the formation of CdSeTe/CdS core/shell QDs meets the requirements of quality QDs in terms of high photoluminescence QY and stability, hence further deposition of additional shells are not necessary in improving the optical properties of the core/shell QDs. Copyright © 2015 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.     

A simple and sensitive label‐free fluorescence sensing of heparin based on Cdte quantum dots

A rapid, simple and sensitive label‐free fluorescence method was developed for the determination of trace amounts of an important drug, heparin. This new method was based on water‐soluble glutathione‐capped CdTe quantum dots (CdTe QDs) as the luminescent probe. CdTe QDs were prepared according to the published protocol and the sizes of these nanoparticles were verified through transmission electron microscopy (TEM), X‐ray diffraction (XRD) and dynamic light scattering (DLS) with an average particle size of about 7 nm. The fluorescence intensity of glutathione‐capped CdTe QDs increased with increasing heparin concentration. These changes were followed as the analytical signal. Effective variables such as pH, QD concentration and incubation time were optimized. At the optimum conditions, with this optical method, heparin could be measured within the range 10.0–200.0 ng mL^?1 with a low limit of detection, 2.0 ng mL^?1. The constructed fluorescence sensor was also applied successfully for the determination of heparin in human serum. Copyright © 2015 John Wiley & Sons, Ltd.     

Sodium 4‐mercaptophenolate capped CdSe/ZnS quantum dots as a fluorescent probe for pH detection in acidic aqueous media

Development of the fluorescent pH detection method is promising due to the sensitivity, easy operation, and low‐cost, etc. However, traditional organic fluorophores have still some disadvantages such as the tedious preparation and purification as well as low photostability and water solubility, which limits the rapid detection application. Semiconductor quantum dots (QDs) have recently risen to prominence as an alternative for organic fluorophores in fluorescence analysis by virtue of their convenient synthesis and superior optical properties. In this study, we report on sodium 4‐mercaptophenolate functionalized CdSe/ZnS QDs (denoted as ^?OPhS‐QDs), which can serve as a selective “on–off” fluorescence probe for aqueous media pH. ^?OPhS‐QDs exhibit strong fluorescence in near neutral medium. As a Lewis organic base, ^?OPhS‐ moieties on QDs surface easily binds to proton under acidic conditions to yield 4‐mercaptophenol capped QDs (i.e. HOPhS‐QDs), which acts as an efficient hole trapper. As a result, the QDs photoluminescence (PL) is switched off. Under optimal conditions, the present probe exhibits a good linear relationship between fluorescence response and pH values in the pH range 3.0–5.2. Furthermore, the present probe exhibits a high selectivity for proton over other common cations and has been successfully used for pH detection in real water samples.     

Highly luminescent hybrid SiO2‐coated CdTe quantum dots: synthesis and properties

Novel hybrid SiO₂‐coated CdTe quantum dots (QDs) were created using CdTe QDs coated with a hybrid SiO₂ shell containing Cd²⁺ ions and a sulfur source via a sol–gel process in aqueous solution. Aqueous CdTe QDs with tunable emitting color created through a reaction between cadmium chloride and sodium hydrogen telluride was used as cores for the preparation of hybrid SiO₂‐coated CdTe QDs. In our experiments we found that the surface state of the cores and preparation conditions that affect the formation of the hybrid SiO₂ shell also greatly affect photoluminescence of the hybrid SiO₂‐coated CdTe QDs. The generation of CdS‐like clusters in the vicinity of the CdTe QDs, caused the quantum size effect of the QDs to be greatly reduced, which changes photoluminescence properties of the hybrid QDs fundamentally. Namely, the novel hybrid SiO₂ shell played an important role in generating a series of specific optical properties. In addition, the novel hybrid SiO₂ shell can be created if no CdTe QD is added. In order to gain an insight into the inter structure of the hybrid shell, we characterized the hybrid SiO₂‐coated CdTe QDs using X‐ray diffraction analysis and discuss the formation mechanism of such a hybrid structure. This work is significant because the novel hybrid SiO₂‐coated CdTe QDs with its excellent properties can be used in many applications, such as biolabeling and optoelectronic devices. Copyright © 2013 John Wiley & Sons, Ltd.     

Significant improvement in photoluminescence of ZnSe(S) alloyed quantum dots prepared in high pH solution

In this paper, we described a simple approach for aqueous synthesis of highly luminescent ZnSe(S) alloyed quantum dots (QDs) in the presence of 3‐mercaptopropionic acid as stabilizers using zinc chloride and NaHSe as precursors. The synthesis conditions were systematically investigated. We observed that the pH value of the Zn precursor solution had significant influence on the optical properties and the structure of the as‐prepared ZnSe(S) QDs. The optimal pH value and molar ratio of Zn²⁺ to HSe^? were 12.0 and 25 : 1 respectively. Under the optimal conditions, we prepared highly photoluminescent ZnSe(S) QDs at up to 31% quantum yield (compared with Rhodamine 6G). The characterization of HRTEM and XRD showed that the ZnSe(S) QDs had good monodispersity and nice crystal structure. The fluorescence life time spectra demonstrated that ZnSe(S) QDs had a long lifetime in contrast to fluorescent dyes. Compared with the currently used organometallic approach, our method was ‘green’, the reaction condition was mild and the as‐prepared ZnSe(S) QDs were water‐soluble. More importantly, our method was low cost, and was very suitable for large‐scale synthesis of highly luminescent ZnSe(S) QDs for the future applications. Copyright © 2009 John Wiley & Sons, Ltd.     

CdSe and CdSe/CdS core–shell QDs: New approach for synthesis,investigating optical properties and application in pollutant degradation

In this work, CdSe quantum dots (QDs) were synthesized by a simple and rapid microwave activated approach using CdSO₄, Na₂SeO₃ as precursors and thioglycolic acid (TGA) as capping agent molecule. A novel photochemical approach was introduced for the growth of CdS QDs and this approach was used to grow a CdS shell around CdSe cores for the formation of a CdSe/CdS core–shell structure. The core–shells were structurally verified using X‐ray diffraction, transmission electron microscopy and FTIR (Fourier‐transform infrared (FTIR)) spectroscopy. The optical properties of the samples were examined by means of UV–Vis and photoluminescence (PL) spectroscopy. It was found that CdS QDs emit a broad band white luminescence between 400 to 700 nm with a peak located at about 510 nm. CdSe QDs emission contained a broad band resulting from trap states between 450 to 800 nm with a peak located at 600 nm. After CdS shell growth, trap states emission was considerably quenched and a near band edge emission was appeared about 480 nm. Optical studies revealed that the core–shell QDs possess strong ultraviolet (UV) ? visible light photocatalytic activity. CdSe/CdS core–shell QDs, showed an enhancement in photodegradation of Methyl orange (MO) compared with CdSe QDs.     

Luminescent gelatin nanospheres by encapsulating CdSe quantum dots

Quantum dots (QDs) have been encapsulated within gelatin nanoparticles (GNPs), which gives GNPs fluorescent properties and improves the biocompatibility of QDs. Hydrophilic CdSe QDs were produced through thermodecomposition following the ligand‐exchange method, and were then encapsulated in GNPs. The results of high‐resolution transmission electron microscopy and transmission electron microscopy show that CdSe QDs and QDs‐encapsulated GNPs (QDs‐GNPs) have average diameters of 5 ± 1 and 150 ± 10 nm, respectively. Results of both high‐resolution transmission electron microscopy and confocal laser scanning microscopy indicate that CdSe QDs are successfully encapsulated within GNPs. The QDs‐GNPs have distinctive fluorescent properties with maximum emission at 654 nm, with a 24 nm red‐shift comapred with hydrophilic mercaptoundecanoic acid (MUA)‐modified QDs. In addition, an in vitro cytotoxicity test shows that QDs‐GNPs do not have any toxic effect on cells. It is expected that QDs‐GNPs might be an excellent candidate as a contrast agent in bio‐imaging. Copyright © 2013 John Wiley & Sons, Ltd.     

Core–shell structured CdTe/CdS@SiO2@CdTe@SiO2 composite fluorescent spheres: Synthesis and application for Cd2+ detection

Core–shell structured quantum dot (QD)–silica fluorescent nanoparticles have attracted a great deal of attention due to the excellent optical properties of QDs and the stability of silica. In this study, core–shell structured CdTe/CdS@SiO₂@CdTe@SiO₂ fluorescent nanospheres were synthesized based on the Stöber method using multistep silica encapsulation. The second silica layer on the CdTe QDs maintained the optical stability of nanospheres and decreased adverse influences on the probe during subsequent processing. Red‐emissive CdTe/CdS QDs (630 nm) were used as a built‐in reference signal and green‐emissive CdTe QDs (550 nm) were used as a responding probe. The fluorescence of CdTe QDs was greatly quenched by added S^2?, owing to a S^2?‐induced change in the CdTe QDs surface state in the shell. Upon addition of Cd²⁺ to the S^2?‐quenched CdTe/CdS@SiO₂@CdTe@SiO₂ system, the responding signal at 550 nm was dramatically restored, whereas the emission at 630 nm remained almost unchanged; this response could be used as a ratiometric ‘off–on’ fluorescent probe for the detection of Cd²⁺. The sensing mechanism was suggested to be: the newly formed CdS‐like cluster with a higher band gap facilitated exciton/hole recombination and effectively enhanced the fluorescence of the CdTe QDs. The proposed probe shows a highly sensitive and selective response to Cd²⁺ and has potential application in the detection of Cd²⁺ in environmental or biological samples.     

Atomic Layer Deposition of CdS Quantum Dots for Solid‐State Quantum Dot Sensitized Solar Cells

Functioning quantum dot (QD) sensitized solar cells have been fabricated using the vacuum deposition technique atomic layer deposition (ALD). Utilizing the incubation period of CdS growth by ALD on TiO₂, we are able to grow QDs of adjustable size which act as sensitizers for solid‐state QD‐sensitized solar cells (ssQDSSC). The size of QDs, studied with transmission electron microscopy (TEM), varied with the number of ALD cycles from 1‐10 nm. Photovoltaic devices with the QDs were fabricated and characterized using a ssQDSSC device architecture with 2,2',7,7'‐tetrakis‐(N,N‐di‐p methoxyphenylamine) 9,9'‐spirobifluorene (spiro‐OMeTAD) as the solid‐state hole conductor. The ALD approach described here can be applied to fabrication of quantum‐confined structures for a variety of applications, including solar electricity and solar fuels. Because ALD provides the ability to deposit many materials in very high aspect ratio substrates, this work introduces a strategy by which material and optical properties of QD sensitizers may be adjusted not only by the size of the particles but also in the future by the composition.     

Tuning optical properties and optical rotation of 3‐mercaptopropionic acid capped organic–inorganic hybrid perovskites

The emission wavelength of organic–inorganic hybrid perovskite quantum dots (QDs) can be tuned by controlling reaction time relevant to the halide exchange. It is because halide exchange with different time would lead to different molar ratio of halides in perovskite QDs such as Cl and Br. Here, to research the ligand's effect on the halide exchange, this work synthesized 3‐mercaptopropionic acid (MPA)‐capped CH₃NH₃PbBr_xCl_3‐x QDs. It was found that SH^? of MPA appeared to inhibit the halide exchange during the reation. Moreover, although the MPA‐capped CH₃NH₃PbBr_xCl_3‐x QDs did not contain the chiral centre, they exhibit the optical rotation. This may provide a method for chirality manipulation of perovskite.