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.     

Fluorescent probe for detection of Cu2+ using core‐shell CdTe/ZnS quantum dots

Core‐shell CdTe/ZnS quantum dots capped with 3‐mercaptopropionic acid (MPA) were successfully synthesized in aqueous medium by hydrothermal synthesis. These quantum dots have advantages compared to traditional quantum dots with limited biological applications, high toxicity and tendency to aggregate. The concentration of Cu²⁺ has a significant impact on the fluorescence intensity of quantum dots (QDs), therefore, a rapid sensitive and selective fluorescence probe has been proposed for the detection of Cu²⁺ in aqueous solution. Under optimal conditions, the fluorescence intensity of CdTe/ZnS QDs was linearly proportional to the concentration of Cu²⁺ in the range from 2.5 × 10^–9 M to 17.5 × 10^–7 M with the limit of 1.5 × 10^–9 M and relative standard deviation of 0.23%. The quenching mechanism is static quenching with recoveries of 97.30–102.75%. Copyright © 2015 John Wiley & Sons, Ltd.     

Application of hybrid SiO2‐coated CdTe nanocrystals for sensitive sensing of Cu2+ and Ag+ ions

A new ion sensor based on hybrid SiO₂‐coated CdTe nanocrystals (NCs) was prepared and applied for sensitive sensing of Cu²⁺ and Ag⁺ for the selective quenching of photoluminescence (PL) of NCs in the presence of ions. As shown by ion detection experiments conducted in pure water rather than buffer solution, PL responses of NCs were linearly proportional to concentrations of Cu²⁺ and Ag⁺ ions < 3 and 7 uM, respectively. Much lower detection limits of 42.37 nM for Cu²⁺ and 39.40 nM for Ag⁺ were also observed. In addition, the NC quenching mechanism was discussed in terms of the characterization of static and transient optical spectra. The transfer and trapping of photoinduced charges in NCs by surface energy levels of CuS and Ag₂S clusters as well as surface defects generated by the exchange of Cu²⁺ and Ag⁺ ions with Cd²⁺ ion in NCs, resulted in PL quenching and other optical spectra changes, including steady‐state absorption and transient PL spectra. It is our hope that these results will be helpful in the future preparation of new ion sensors. Copyright © 2012 John Wiley & Sons, Ltd.     

Fluorescence quenching investigation on the interaction of glutathione‐CdTe/CdS quantum dots with sanguinarine and its analytical application

Water‐soluble glutathione (GSH)‐capped core/shell CdTe/CdS quantum dots (QDs) were synthesized. In pH 5.4 sodium phosphate buffer medium, the interaction between GSH‐CdTe/CdS QDs and sanguinarine (SA) was investigated by spectroscopic methods, including fluorescence spectroscopy and ultraviolet‐visible absorption spectroscopy. Addition of SA to GSH‐CdTe/CdS QDs results in fluorescence quenching of GSH‐CdTe/CdS QDs. Quenching intensity was in proportion to the concentration of SA in a certain range. Investigation of the quenching mechanism, proved that the fluorescence quenching of GSH‐CdTe/CdS QDs by SA is a result of electron transfer. Based on the quenching of the fluorescence of GSH‐CdTe/CdS QDs by SA, a novel, simple, rapid and specific method for SA determination was proposed. The detection limit for SA was 3.4 ng/mL and the quantitative determination range was 0.2–40.0 µg/mL with a correlation coefficient of 0.9988. The method has been applied to the determination of SA in synthetic samples and fresh urine samples of healthy human with satisfactory results. Copyright © 2013 John Wiley & Sons, Ltd.     

Labeling of BSA and imaging of mouse T‐lymphocyte as well as mouse spleen tissue by l‐glutathione capped CdTe quantum dots

l ‐glutathione capped highly fluorescent CdTe quantum dots (QDs) were prepared by an aqueous approach and used as fluorescent labels to link albumin bovine serum (BSA) and rat anti‐mouse CD4, which was expressed on mouse T‐lymphocyte and mouse spleen tissue. The sharp and narrow emission peaks showed that the as‐prepared QDs have desirable dispersibility, uniformity and good fluorescence properties. Both CdTe–BSA and CdTe–CD4 conjugates showed an enhancement of fluorescence intensity over that of bare CdTe QDs. The experimental result of gel electrophoresis confirmed the successful conjugation of CdTe–BSA and CdTe–CD4. The fluorescent microscopic images of CdTe–CD4 labeled mouse T‐lymphocyte cells and mouse spleen tissue were compared with that obtained from fluorescein isothiocyanate labeling. It was demonstrated that the CdTe QDs‐based probe exhibited much better photostability and fluorescence intensity than fluorescein isothiocyanate, showing a good application potential in the immuno‐labeling of cells and tissues. Copyright © 2009 John Wiley & Sons, Ltd.     

A simple FRET system using two‐color CdTe quantum dots assisted by cetyltrimethylammonium bromide and its application to Hg(II) detection

In this study, we developed a novel simple fluorescence resonance‐energy transfer (FRET) system between two‐color CdTe quantum dots (QDs) assisted by cetyltrimethylammonium bromide (CTAB). Mercaptopropionic (MPA)‐capped CdTe QDs serving as both donors and acceptors were successfully synthesized by changing the refluxing time in aqueous solution. CTAB micelles formed in water and minimized the distance between the donors and acceptors significantly by electrostatic interactions, improving FRET efficiency. Several factors that affected the fluorescence spectra of the FRET system were investigated. The prepared FRET system was feasible as an effective fluorescent probe to detect Hg(II) in aqueous solution. At pH 7.0, a linear relationship between the quenched fluorescence intensity of orange‐emitting acceptors (QDs_(A)) and Hg(II) concentration was acquired in the range 5–250 nmol/L with a detection limit of 1.95 nmol/L. The developed method showed excellent analytical performance for Hg(II) with high sensitivity and acceptable selectivity, reproducibility and stability. This finding indicated that the method has a promising potential application for environmental monitoring. This study demonstrated the great promise of QDs for expedient, low‐cost and high‐sensitivity detection of Hg(II).     

Spectroscopic investigations on the conformational changes of lysozyme effected by different sizes of N‐acetyl‐l‐cysteine‐capped CdTe quantum dots

The effect of N‐acetyl‐l ‐cysteine‐capped CdTe quantum dots (NAC‐CdTe QDs) with different sizes on lysozyme was investigated by isothermal titration calorimetry (ITC), enzyme activity assays, and multi‐spectroscopic methods. ITC results proved that NAC‐CdTe QDs can spontaneously bind with lysozyme and hydrophobic force plays a major role in stabilizing QDs–lysozyme complex. Multi‐spectroscopic measurements revealed that NAC‐CdTe QDs caused strong quenching of the lysozyme's fluorescence in a size‐dependent quenching manner. Moreover, the changes of secondary structure and microenvironment in lysozyme caused by the NAC‐CdTe QDs were higher with a bigger size. The results of enzyme activity assays showed that the interaction between lysozyme and NAC‐CdTe QDs inhibited the activity of lysozyme and the inhibiting effect was in a size‐dependent manner. Based on these results, we conclude that NAC‐CdTe QDs with larger particle size had a larger impact on the structure and function of lysozyme.     

Highly luminescent CdTe/CdS/ZnO core/shell/shell quantum dots fabricated using an aqueous strategy

To create core/shell/shell quantum dots (QDs) with high stability against a harmful chemical environment, CdTe/CdS QDs were coated with a ZnO shell in an aqueous solution. An interfaced CdS layer sandwiched between a CdTe core and ZnO shell provided relaxation of the strain at the core/shell interface since lattice parameters of CdS are intermediate between those of CdTe and ZnO. The photoluminescence (PL) peak wavelength of the core/shell/shell QDs was shifted from 569 to 615 nm by adjusting the size of CdTe cores and thickness of CdS and ZnO shells, along with the highest PL quantum yield of the core/shell/shell QDs reaching 80%, which implies promising applications in the field of biomedical labeling. Due to the decrease of surface defects, it was observed that PL lifetimes significantly increased at room temperature as follows: 29.6 34.2, and 47.5 ns for CdTe (537 nm), CdTe/CdS (555 nm) and CdTe/CdS/ZnO (581 nm) QDs, respectively. Copyright © 2012 John Wiley & Sons, Ltd.     

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.     

One‐pot synthesis and characterization CdTe:Zn2+ quantum dots and its molecular interaction with calf thymus DNA

Tremendous research efforts have been dedicated to fabricating high‐quality Zn‐doped CdTe quantum dots (QDs) for any potential biomedical applications. In particular, the correlation of issues regarding how QDs interact with DNA is of greatest importance. Herein, a pH‐responsive study of the interactions between CdTe:Zn²⁺ quantum dots with 4 different sizes and calf thymus DNA (ctDNA) was conducted using multispectroscopic techniques and electrochemical investigation. Fluorescence studies revealed that this interaction process is predominantly a static process and groove binding was the main binding mode for CdTe:Zn²⁺ QDs to ctDNA. The calculated negative values of enthalpy (?45.06 kJ mol^?1) and entropy (?133.62 J mol^?1 K^?1) with temperature changes indicated that the hydrogen bonds and van der Waals interactions played major roles in the reaction. Furthermore, circular dichroism spectroscopy and Fourier transform infrared spectrometry analyses indicate that the normal conformation of ctDNA is discombobulated by CdTe:Zn²⁺ QDs. In addition, the electrochemical behavior of the affinity of CdTe:Zn²⁺ QDs for ctDNA agreed well with the results obtained from fluorescence experiments. This study might be meaningful for understanding the molecular binding mechanism of QDs for DNA and provides a basis for QD‐labeled systems.     

Molecular spectroscopic studies on the interactions of rhein and emodin with thioglycolic acid‐capped core/shell CdTe/CdS quantum dots and their analytical applications

Water‐soluble thioglycolic acid (TGA)‐capped core/shell CdTe/CdS quantum dots (QDs) were synthesized. The interactions of rhein and emodin with TGA‐CdTe/CdS QDs were evaluated by fluorescence and ultraviolet‐visible absorption spectroscopy. Experimental results showed that the high fluorescence intensity of TGA‐CdTe/CdS QDs could be effectively quenched in the presence of rhein (or emodin) at 570 nm, which may have resulted from an electron transfer process from excited TGA‐CdTe/CdS QDs to rhein (or emodin). The quenching intensity was in proportion to the concentration of both rhein and emodin in a certain range. Under optimized conditions, the linear ranges of TGA‐CdTe/CdS QDs fluorescence intensity versus the concentration of rhein and emodin were 0.09650–60 µg/mL and 0.1175–70 µg/mL with a correlation coefficient of 0.9984 and 0.9965, respectively. The corresponding detection limits (3σ/S) of rhein and emodin were 28.9 and 35.2 ng/mL, respectively. This proposed method was applied to determine rhein and emodin in human urine samples successfully with remarkable advantages such as high sensitivity, short analysis time, low cost and easy operation. Based on this, a simple, rapid and highly sensitive method to determine rhein (or emodin) was proposed. Copyright © 2014 John Wiley & Sons, Ltd.     

High quantum yield and well‐dispersed quantum dots luminescent composite through sodium carboxymethyl starch

It is a challenging task to prepare well‐dispersed and highly luminescent quantum dots (QDs) powder and a new strategy is reported in this article. Sodium carboxymethyl starch (CMS‐Na) was employed in this work to prepare the QDs–starch composite. Ultraviolet (UV) light shows that the blank starches had no fluorescence, while the QDs‐starches were highly luminescent. Scanning electron microscopy (SEM) observation shows that the QDs–starch composite has the typical particle morphology with the diameter around 200 nm. Energy dispersive X‐ray spectroscopy (EDX) results show that there are intensive tellurium (Te) and cadmium (Cd) element signals. Combined fluorescent lifetime and steady‐state spectrometer show that the QDs–starch quantum yields (QYs) increase when the QDs loading increases from 1 × 10^?6 mol/g to 2 × 10^?6 mol/g, but when the loadings further increase, the QYs decrease slightly. For the red colour (λ_em = 660 nm) QDs, the QYs can reach to as high as 28.2%, and for the other colour QDs they can also have the QYs above 22%. Time‐resolved photobleaching experiments show that the fluorescent QDs–starch composite has a half‐decay time of 40.23 s. These results indicate that the CMS‐Na is a promising QDs dispersant to obtain high QY QD composites.     

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.     

Interaction and energy transfer studies between bovine serum albumin and CdTe quantum dots conjugates: CdTe QDs as energy acceptor probes

In this paper, a systematic investigation of the interaction of bovine serum albumin (BSA) with water‐soluble CdTe quantum dots (QDs) of two different sizes capped with carboxylic thiols is presented based on steady‐state and time‐resolved fluorescence measurements. Efficient Förster resonance energy transfer (FRET) was observed to occur from BSA donor to CdTe acceptor as noted from reduction in the fluorescence of BSA and enhanced fluorescence from CdTe QDs. FRET parameters such as Förster distance, spectral overlap integral, FRET rate constant and efficiency were determined. The quenching of BSA fluorescence in aqueous solution observed in the presence of CdTe QDs infers that fluorescence resonance energy transfer is primarily responsible for the quenching phenomenon. Bimolecular quenching constant (k_q) determined at different temperatures and the time‐resolved fluorescence data provide additional evidence for this. The binding stoichiometry and various thermodynamic parameters are evaluated by using the van ‘t Hoff equation. The analysis of the results suggests that the interaction between BSA and CdTe QDs is entropy driven and hydrophobic forces play a key role in the interaction. Binding of QDs significantly shortened the fluorescence lifetime of BSA which is one of the hallmarks of FRET. The effect of size of the QDs on the FRET parameters are discussed in the light of FRET parameters obtained.     

Graphene‐amplified electrogenerated chemiluminescence of CdTe quantum dots for H2O2 sensing

Electrogenerated chemiluminescence (ECL) of thiol‐capped CdTe quantum dots (QDs) in aqueous solution was greatly enhanced by PDDA‐protected graphene (P‐GR) film that were used for the sensitive detection of H₂O₂. When the potential was cycled between 0 and ?2.3 V, two ECL peaks were observed at ?1.1 (ECL‐1) and ?1.4 V (ECL‐2) in pH 11.0, 0.1 M phosphate buffer solution (PBS), respectively. The electron‐transfer reaction between individual electrochemically‐reduced CdTe nanocrystal species and oxidant coreactants (H₂O₂ or reduced dissolved oxygen) led to the production of ECL‐1. While mass nanocrystals packed densely in the film were reduced electrochemically, assembly of reduced nanocrystal species reacted with coreactants to produce an ECL‐2 signal. ECL‐1 showed higher sensitivity for the detection of H₂O₂ concentrations than that of ECL‐2. Further, P‐GR film not only enhanced ECL intensity of CdTe QDs but also decreased its onset potential. Thus, a novel CdTe QDs ECL sensor was developed for sensing H₂O₂. Light intensity was linearly proportional to the concentration of H₂O₂ between 1.0 × 10^?5 and 2.0 x 10^‐7 mol L^?1 with a detection limit of 9.8 x 10^?8 mol L^?1. The P‐GR thin‐film modified glassy carbon electrode (GCE) displayed acceptable reproducibility and long‐term stability. Copyright © 2012 John Wiley & Sons, Ltd.     

Size‐dependent temperature sensitivity of photoluminescence peak position of CdTe quantum dots

The size dependence of the temperature coefficient (sensitivity) of the photoluminescence (PL) peak position of CdTe quantum dots stabilized by thioglycolic acid in aqueous solution has been investigated. Temperature sensitivity increases as the average radius of CdTe quantum dots decraeases. This must be taken into account in the design of solar light concentrators and light‐emmiting diode‐monitors as well as other technologies in which a fine tuning of the light emission is important. Copyright © 2013 John Wiley & Sons, Ltd.     

The binding interaction between cadmium‐based,aqueous‐phase quantum dots with Candida rugosa lipase

As a promising biolabeling biomaterials, quantum dots (QDs) present a great potential. However, the toxicity of QDs to organisms has attracted wide attention. In our research, we introduced an in vitro method to study the molecular mechanisms for the structure and activity alterations of Candida rugosa lipase (CRL) with the binding of 3‐mercaptopropionic acid‐capped CdTe QDs. Multiple spectroscopic methods, isothermal titration calorimetry, and enzyme activity measurements were used in this paper. QDs statically quenched the intrinsic fluorescence of CRL with the quenching constant decreases from 2.46 × 10¹³ to 1.64 × 10¹³ L mol^?1 second^?1 (298 to 310 K). It binds to CRL through hydrophobic force with 1 binding site, unfolding and loosening the skeleton and changed its secondary structure. Rather than aggregating on the surface, it enters the pocket of the CRL to interact with Ser‐209 (2.43 Å) and the residues surrounding Ser‐209, making the catalytic triad more exposed. Furthermore, the activity of CRL was inhibited by approximately 15%. This work demonstrates that 3‐mercaptopropionic acid‐capped CdTe QDs may cause negative effects to CRL and obtains a molecular mechanism on QD‐induced toxicity to proteins in vitro.     

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.     

Graphite oxide‐dispersed CdTe quantum dots nanocomposite for flexible display luminescent membranes

A quantum dot (QD) dispersant material was prepared using graphite oxide (GO). Luminescent films were prepared using polyvinyl alcohol as the polymer matrix. First, water‐soluble CdTe QDs were prepared by wet chemistry and GO was synthesized using a modified Hummers method. X‐Ray diffraction tests showed that the GO reflection peak [001] was 11.9°, which indicates that the d‐spacing is 0.7431 nm; atomic force microscopy showed a GO thickness of 200 nm. Fourier transform infrared spectra showed vibrations at 1624 cm^?1 for the carbonyl groups, and 3260 cm^?1 for the GO samples; the ‐C–O vibration was at 1320 cm^?1 and ‐COOH, ?OH vibrations were at 950 cm^?1. Fluorescent tests showed that pH had an impact on the QD colloidal stability. GO was neutralized before use as the host media for the GO/QDs nanocomposite. The results proved that the resultant nanocomposite is promising for use in brightness enhancement films in flexible displays.     

Assembly of light‐emitting diode based on hydrophilic CdTe quantum dots incorporating dehydrated silica gel

Stable photoluminescence QD light‐emitting diodes (QD‐LEDs) were made based on hydrophilic CdTe quantum dots (QDs). A quantum dot‐inorganic nanocomposite (hydrophilic CdTe QDs incorporating dehydrated silica gel) was prepared by two methods (rotary evaporation and freeze drying). Taking advantage of its viscosity, plasticity and transparency, dehydrated silica gel could be coated on the surface of ultraviolet (UV) light LEDs to make photoluminescence QD‐LEDs. This new photoluminescence QD‐LED, which is stable, environmentally non‐toxic, easy to operate and low cost, could expand the applications of hydrophilic CdTe QDs in photoluminescence. Copyright © 2015 John Wiley & Sons, Ltd.