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.     

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.     

A selective determination of copper ions in water samples based on the fluorescence quenching of thiol‐capped CdTe quantum dots

CdTe quantum dots (QDs) capped with different stabilizers, i.e. thioglycolic acid (TGA), 3‐mercaptopropionic acid (MPA) and glutathione (GSH) were investigated as fluorescent probes for the determination of Cu²⁺. The stabilizer was shown to play an important role in both the sensitivity and selectivity for the determination of Cu²⁺. TGA‐capped CdTe QDs showed the highest sensitivity, followed by the MPA and GSH‐capped CdTe QDs, respectively. The TGA‐ and MPA‐capped CdTe QDs were not selective for Cu²⁺ that was affected by Ag⁺. The GSH‐capped CdTe QDs were insensitive to Ag⁺ and were used to determine Cu²⁺ in water samples. Under optimal conditions, quenching of the fluorescence intensity (F₀/F) increased linearly with the concentration of Cu²⁺ over a range of 0.10–4.0 µg/mL and the detection limit was 0.06 µg/mL. The developed method was successfully applied to the determination of Cu²⁺ in water samples. Good recoveries of 93–104%, with a relative standard deviation of < 6% demonstrated that the developed simple method was accurate and reliable. The quenching mechanisms were also described. Copyright © 2015 John Wiley & Sons, Ltd.     

A highly selective and simple fluorescent sensor for mercury (II) ion detection based on cysteamine‐capped CdTe quantum dots synthesized by the reflux method

Cysteamine (CA)‐capped CdTe quantum dots (QDs) (CA–CdTe QDs) were prepared by the reflux method and utilized as an efficient nano‐sized fluorescent sensor to detect mercury (II) ions (Hg²⁺). Under optimum conditions, the fluorescence quenching effect of CA–CdTe QDs was linear at Hg²⁺ concentrations in the range of 6.0–450 nmol/L. The detection limit was calculated to be 4.0 nmol/L according to the 3σ IUPAC criteria. The influence of 10‐fold Pb²⁺, Cu²⁺ and Ag⁺ on the determination of Hg²⁺ was < 7% (superior to other reports based on crude QDs). Furthermore, the detection sensitivity and selectivity were much improved relative to a sensor based on the CA–CdTe QDs probe, which was prepared using a one‐pot synthetic method. This CA–CdTe QDs sensor system represents a new feasibility to improve the detection performance of a QDs sensor by changing the synthesis method. Copyright © 2014 John Wiley & Sons, Ltd.     

d‐penicillamine capped cadmium telluride quantum dots as a novel fluorometric sensor of copper(II)

d ‐penicillamine‐capped cadmium telluride quantum dots (DPA‐capped CdTe QDs) were synthesized as the new fluorescent semiconductor nanocrystal in aqueous solution. Fourier transmission infrared spectroscopy, X‐ray diffraction, transmission electron microscopy, ultraviolet‐visible and photoluminescence spectroscopy were used for characterization of the QDs. Based on the quenching effect of Cu²⁺ ions on the fluorescence intensity of DPA‐capped CdTe QDs, a new fluorometric sensor for copper(II) detection was developed that showed good linearity over the concentration range 5 × 10^–9–3 × 10^–6 m with the detection limit 0.4 × 10^–9 m . Owing to the strong affinity of the DPA to copper(II), the sensor showed appropriate selectivity for copper(II) compared with conventional QDs. The DPA‐capped CdTe QDs was successfully applied for determination of Cu²⁺ concentration in river, well and tap waters with satisfactory results. Copyright © 2013 John Wiley & Sons, Ltd.     

Molecularly imprinted electrochemiluminescence sensor based on ZIF-8 doped with CdSe quantum dots for the detection of trace estriol

A composite of the metal–organic framework compound ZIF-8 doped with CdSe quantum dots (QDs) with sensitive and stable luminescence was synthesized, and a molecularly imprinted electrochemiluminescence (ECL) sensor was constructed based on this composite. The ZIF-8@CdSe molecularly imprinted ECL sensor combines the high sensitivity of ECL and the high selectivity of molecular imprinting to realize the sensitive and specific detection of estriol. CdSe QDs and gold nanoparticles were encapsulated within ZIF-8 to obtain the ZIF-8@CdSe QDs/GNP (ZIF@CdSe/GNP) composite. Subsequently, the GNPs were further loaded on the surface of this composite to obtain the GNP/ZIF@CdSe/GNP composite. l -Cysteine was used to immobilize the GNP/ZIF@CdSe/GNP composite on the surface of a gold electrode to obtain the GNP/ZIF@CdSe/GNP-modified gold electrode. A molecularly imprinted polymer (MIP) film was prepared on the surface of the modified electrode by electropolymerization with o-phenylenediamine as the functional monomer and estriol as the template molecule. After elution, estriol could be specifically recognized by the cavities. The readsorption of estriol by the MIP can prevent the coreactant from reaching the electrode surface through the cavities, thereby weakening ECL. A good linear relationship existed between the ∆ECL and lg C of estriol concentrations of 1 × 10⁻¹⁴ to 1 × 10⁻⁹ mol·L⁻¹. The detection limit was as low as 8.9 × 10⁻¹⁶ mol·L⁻¹. The sensor was applied in the determination of estriol in serum samples with a recovery of 97.0–102%.     

Enhanced electrochemiluminescence from reduced graphene oxide–CdTe quantum dots for highly selective determination of copper ion

An electrochemiluminescence (ECL) sensor based on reduced graphene oxide–CdTe quantum dots (RGO–CdTe QDs) composites for detecting copper ion (Cu²⁺) was proposed. The ECL behaviours of the RGO–CdTe QD modified electrode were investigated with H₂O₂ as the co‐reactant. Quantitative detection of Cu²⁺ was realized as Cu²⁺ could effectively quench the ECL signal of the RGO–CdTe QDs. A wide linear range of 1.00 × 10^?14 to 1.00 × 10^?4 M (R = 0.9953) was obtained under optimized conditions, and a detection limit (S/N = 3) was achieved of as low as 3.33 × 10^?15 M. The proposed sensor also exhibited good stability and selectivity for the detection of copper ions. Finally, the analytical application of the proposed sensor was also evaluated using river water.     

Novel fluorescent sensor using molecularly imprinted silica microsphere‐coated CdSe@CdS quantum dots and its application in the detection of 2,4,6‐trichlorophenol from environmental water samples

In this study, a high fluorescence sensitivity and selectivity, molecularly imprinted nanofluorescent polymer sensor (MIP@SiO₂@QDs) was prepared using a reverse microemulsion method. 2,4,6‐Trichlorophenol (2,4,6‐TCP) was detected using fluorescence quenching. Tetraethyl orthosilicate (TEOS), quantum dots (QDs) and 3‐aminopropyltriethoxysilane (APTS) were used as cross‐linker, signal sources and functional monomer respectively. The sensor (MIP@SiO₂@QDs) and the non‐imprinted polymer sensor (NIP@SiO₂@QDs) were characterized using infra‐red (IR) analysis, X‐ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The selectivity of MIP@SiO₂@QDs was examined by comparing 2,4,6‐TCP with other similar functional substances including 2,4‐dichlorophenol (2,4‐DCP), 2,6‐dichlorophenol (2,6‐DCP) and 4‐chlorophenol (4‐CP). Results showed that MIP@SiO₂@QDs had better selectivity for 2,4,6‐TCP than the other compounds. Fluorescence quenching efficiency displayed a good linear response at the 2,4,6‐TCP concentration range 5–1000 μmol/L. The limit of detection (LOD) was 0.9 μmol/L (3σ, n = 9). This method was equally applicable for testing actual samples with a recovery rate of 98.0–105.8%. The sensor had advantages of simple pretreatment, good sensitivity and selectivity, and wide linear range and could be applied for the rapid detection of 2,4,6‐TCP in actual samples.     

Development of a quantum dot molecularly imprinted polymer sensor for fluorescence detection of atrazine

Atrazine is a common agricultural pesticide which has been reported to occur widely in surface drinking water, making it an environmental pollutant of concern. In the quest for developing sensitive detection methods for pesticides, the use of quantum dots (QDs) as sensitive fluorescence probes has gained momentum in recent years. QDs have attractive and unique optical properties whilst coupling of QDs to molecularly imprinted polymers (MIPs) has been shown to offer excellent selectivity. Thus, the development of QD@MIPs based fluorescence sensors could provide an alternative for monitoring herbicides like atrazine in water. In this work, highly fluorescent CdSeTe/ZnS QDs were fabricated using the conventional organometallic synthesis approach and were then encapsulated with MIPs. The CdSeTe/ZnS@MIP sensor was characterized and applied for selective detection of atrazine. The sensor showed a fast response time (5 min) upon interaction with atrazine and the fluorescence intensity was linearly quenched within the 2–20 mol L^?1 atrazine range. The detection limit of 0.80 × 10^?7 mol L^?1 is comparable to reported environmental levels. Lastly, the sensor was applied in real water samples and showed satisfactory recoveries (92–118%) in spiked samples, hence it is a promising candidate for use in water monitoring.     

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.     

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.     

A sensitive fluorescent sensor based on the photoinduced electron transfer mechanism for cefixime and ctDNA

Cefixime is a third generation orally administered cephalosporin that is frequently used as a broad spectrum antibiotic against various gram‐negative and gram‐positive bacteria. In this study, a simple and sensitive fluorescent sensor for the determination of the cefixime and ctDNA was established based on the CdTe:Zn²⁺ quantum dots (QDs). The fluorescence of CdTe:Zn²⁺ QDs can be effectively quenched by cefixime in virtue of the surface binding of cefixime on CdTe:Zn²⁺ QDs and the subsequent photoinduced electron transfer process from CdTe:Zn²⁺ QDs to cefixime, in particular, the high sensitivity of QDs fluorescence emission to cefixime at the micromole per liter level, which render the cefixime‐CdTe:Zn²⁺ QDs system into fluorescence “OFF” status, then turn on in the presence of ctDNA. Furthermore, the Fourier transform infrared (FTIR) spectra of characteristic bands of C–N and N–H groups of cefixime endow evidence for the interaction of cefixime with CdTe:Zn²⁺ QDs. The relative electrochemical behavior of the affinity of CdTe:Zn²⁺ QDs for cefixime and ctDNA reveals the potential molecular binding mechanism.     

H2O2‐ and pH‐sensitive CdTe quantum dots as fluorescence probes for the detection of glucose

A novel fluorescence assay system for glucose was developed with thioglycollic acid (TGA)‐capped CdTe quantum dots (QDs) as probes. The luminescence quantum yield of the TGA‐capped CdTe QDs was highly sensitive to H₂O₂ and pH. In the presence of glucose oxidase, glucose is oxidized to yield, gluconic acid and H₂O₂. H₂O₂ and H⁺ (dissociated from gluconic acid) intensively quenched the fluorescence of QDs. The experimental results showed that the quenched fluorescence was proportional to the glucose concentration within the range of 0.01–5.0 mm under optimized experimental conditions. Compared with most of the existing methods, this newly developed system possesses many advantages, including simplicity, low cost, high flexibility, and good sensitivity. Furthermore, no complicated chemical modification of QDs and enzyme immobilization was needed in this system. Copyright © 2012 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.     

Flow‐injection chemiluminescence of the luminol–potassium periodate system enhanced by TGA–capped CdTe quantum dots for the determination of theophylline

The chemiluminescence (CL) behaviour of the luminol–potassium periodate system enhanced by CdTe quantum dots capped with thioglycolic acid (TGA–CdTe QDs) was studied using kinetic experiments, CL spectra, UV–vis absorption spectra and fluorescence spectra. The production of oxygen‐containing reactant intermediates (O₂^?? and OH^?) in the present CL system was verified by CL. The possible CL mechanism was discussed in detail. Furthermore, theophylline (THP) was determined based on its enhancement of the CL intensity of the CdTe QDs–luminol–potassium periodate system coupled with a flow‐injection technique. Under these optimized conditions, the linear range was found to be from 1.0 × 10^?8 to 1.0 × 10^?5 g/mL with a detection limit of 2.8 × 10^?9 g/mL (3σ). The recoveries for the determination of THP in tablets were from 98.2 to 99.6%.     

Effects of N‐Acetyl‐L‐Cysteine‐Capped CdTe Quantum Dots on Bovine Serum Albumin and Bovine Hemoglobin: Isothermal Titration Calorimetry and Spectroscopic Investigations

The interactions of N‐acetyl‐L‐cysteine‐capped CdTe quantum dots (QDs) with bovine serum albumin (BSA) and bovine hemoglobin (BHb) were investigated by isothermal titration calorimetry (ITC), fluorescence, synchronous fluorescence, fluorescence lifetime, ultraviolet–visible absorption, and circular dichroism techniques. Fluorescence data of BSA–QDs and BHb–QDs revealed that the quenching was static in every system. While CdTe QDs changed the microenvironment of tryptophan in BHb, the microenvironment of BSA kept unchanged. Adding CdTe QDs affected the skeleton and secondary structure of the protein (BSA and BHb). The ITC results indicated that the interaction between the protein (BSA and BHb) and QDs‐612 was spontaneous and the predominant force was hydrophobic interaction. In addition, the binding constants were determined to be 1.19 × 10⁵ L mol^?1 (BSA–QDs) and 2.19 × 10⁵ L mol^?1 (BHb–QDs) at 298 K. From these results, we conclude that CdTe QDs have a larger impact on the structure of BHb than BSA.     

Luminescent and hydrophilic LaF3–polymer nanocomposite for DNA detection

Luminescent LaF₃–Ce³⁺/Tb³⁺ nanocrystals have been successfully prepared via a simple wet chemical technique. For the next bioapplication, these nanoparticles dispersed in cyclohexane have also been functionalized with poly(St‐co‐MAA), based on a designed oil‐in‐water microemulsion system. These polymer‐coated nanospheres are water‐soluble and bioconjugable. Unlike semiconductor quantum dots, the as‐prepared lanthanum fluoride nanocrystals possess non‐size‐dependent emissions and completely stable photocycles. With functionalized LaF₃ nanospheres as fluorescence probes, a fluorescence method was developed for the rapid quantitative analysis of DNA, due to the quenching effect of fluorescence by the DNA. Under optimum conditions, the fluorescence intensity was proportional to the concentration of the introduced DNA over the range 2.5–35 µg/mL for calf thymus DNA (ctDNA) and 2.5–30 µg/mL for fish sperm DNA (fsDNA), respectively. Copyright © 2008 John Wiley & Sons, Ltd.     

Sensitive determination of enoxacin in pharmaceutical formulations by its quench effect on the fluorescence of glutathione‐capped CdTe quantum dots

A sensitive and simple method for the determination of enoxacin (ENX) was developed based on the fluorescence quenching effect of ENX for glutathione (GSH)‐capped CdTe quantum dots (QDs). Under optimum conditions, a good linear relationship was obtained from 4.333 × 10^?9 mol?L^?1 to 1.4 × 10^?5 mol?L^?1 with a correlation coefficient (R) of 0.9987, and the detection limit (3σ/K) was 1.313 × 10^?9 mol?L^?1. The corresponding mechanism has been proposed on the basis of electron transfer supported by ultraviolet–visible (UV) light absorption, fluorescence spectroscopy, and the measurement of fluorescence lifetime. The method has been applied to the determination of ENX in pharmaceutical formulations (enoxacin gluconate injections and commercial tablets) with satisfactory results. The proposed method manifested several advantages such as high sensitivity, short analysis time, low cost and ease of operation. Copyright © 2015 John Wiley & Sons, Ltd.     

A novel fluorescent assay for oxytetracycline hydrochloride based on fluorescence quenching of water‐soluble CdTe nanocrystals

A novel assay for oxytetracycline hydrochloride (OTC) based on fluorescence quenching was developed from the interaction between functionalized cadmium telluride quantum dots (CdTe QDs) and OTC. Optimum conditions for the detection of OTC were found after investigating all factors. Under optimum conditions, luminescence of CdTe nanocrystals (λ_ex = 365 nm, λ_em = 562 nm) was quenched by OTC in a concentration‐dependent manner best described by a modified Stern‐Volmer type equation. Good linearity was obtained with a regression coefficient of 0.9999 in the range of 1.34 ~ 13.4 x 10^‐5 mol/L and a limit of detection of 3.08 x 10^‐7 mol/L. In addition, the quenching mechanism was also established. The results imply that the close proximity of OTC‐CdTe was driven by electrostatic attraction and the resulting effective electron transfer from OTC to QDs could be responsible for fluorescence quenching of CdTe‐QDs. Copyright © 2012 John Wiley & Sons, Ltd.     

Chemiluminescence sensor for sulfonylurea herbicide using molecular imprinted microspheres as recognition element

Uniform molecular imprinting microspheres were prepared using precipitation polymerization with thifensulfuron‐methyl (TFM) as template, acrylamide as functional monomer and ethylene glycol dimethacrylate as cross‐linker. TFM could be selectively adsorbed on the molecularly imprinted polymers (MIPs) matrix through the hydrogen bonding interaction and the adsorbed TFM could be sensed by its strikingly enhancing effect on the weak chemiluminescence (CL) reaction between luminol and hydrogen peroxide. On this basis, a novel CL sensor for the determination of TFM using MIPs as recognition elements was established. The logarithm of net CL intensity (ΔI) is linearly proportional to the logarithm of TFM concentration (C) in the range from 1.0 × 10^?9 to 5.0 × 10^?5 mol L^?1 with a detection limit of 8.3 × 10^?10 mol L^?1 (3σ). The results demonstrated that the MIP–CL sensor was reversible and reusable and that it could strikingly improve the selectivity and sensitivity of CL analysis. Furthermore, it is suggested that the CL enhancement of luminol–H₂O₂ by TFM might be ascribed to the enhancement effect of CO₂, which came from TFM hydrolysis in basic medium. Copyright © 2010 John Wiley & Sons, Ltd.