Functionalized CdS quantum dots-based luminescence probe for detection of heavy and transition metal ions in aqueous solution

A dark-field light scattering platform for real-time monitoring of the erosion of microparticles by Co2+

Monitoring chemical reactions using tools of analytical chemistry is crucial for understanding the mechanisms of these reactions. In this study, leaf-like poly(p-phenylenediamine) (PpPD) microparticles were prepared with precisely tailored properties. The dark field microscopy protocol proved to be a powerful tool for studying the erosion of microparticles induced by Co(2+). Such a protocol can have the benefit of improving the understanding of reaction mechanisms and can help expand the applications of dark field microscopy. A possible mechanism was proposed to explain the experimental observations.

Kinetic fluorescence quenching of CdS quantum dots in the presence of Cu(II): chemometrics-assisted resolving of the kinetic data and quantitative analysis of Cu(II)

In this work, the kinetic fluorescence behavior of CdS quantum dots (QDs) in the presence of Cu(II) was investigated. In contrast to some other transition metal ions such as Ag(I), Ni(II), and Hg(II), a gradual red-shift in the emission spectrum of CdS QDs was observed for Cu(II) during the reaction course. More investigations revealed the existence of two chemical components in the recorded kinetic data in the presence of Cu(II). Multivariate curve resolution-alternating least squares (MCR-ALS) method was applied in order to extract pure emission spectra and time-dependent profiles of these two components at different concentrations of Cu(II). The results obtained from resolving the data by MCR-ALS got some information about the mechanism of the interaction between CdS QDs and Cu(II) ions which were in good agreement with those reported in the literature. Moreover, the multivariate method of analysis, partial least-squares (PLS) method, was used to develop a multivariate calibration model for quantitative analysis of Cu(II) using the entire kinetic data sets. The calibration and validation sets were created ranging from 0.02 to 1μM of Cu(II) and were successfully calibrated and predicted by the PLS model. This method allowed a sensitive determination of Cu(II) ions with a detection limit of 13nM based on three times of the standard deviation corresponding to PLS regression.

Visible light catalysis-assisted assembly of Ni(h)-QD hollow nanospheres in situ via hydrogen bubbles

Hollow spheres are one of the most promising micro-/nanostructures because of their unique performance in diverse applications. Templates, surfactants, and structure-directing agents are often used to control the sizes and morphologies of hollow spheres. In this Article, we describe a simple method based on visible light catalysis for preparing hollow nanospheres from CdE (E = Te, Se, and S) quantum dots (QDs) and nickel (Ni(2+)) salts in aqueous media. In contrast to the well-developed traditional approaches, the hollow nanospheres of QDs are formed in situ by the photogeneration of hydrogen (H2) gas bubbles at room temperature. Each component, that is, the QDs, metal ions, ascorbic acid (H2A), and visible light, is essential for the formation of hollow nanospheres. The quality of the hollow nanospheres depends on the pH, metal ions, and wavelength and intensity of visible light used. Of the various metal ions investigated, including Cu(+), Cu(2+), Fe(2+), Fe(3+), Ni(2+), Mn(2+), RuCl5(2-), Ag(+), and PtCl4(2-), Ni(2+) ions showed the best ability to generate H2 and hollow-structured nanospheres under visible light irradiation. The average diameter and shell thickness of the nanospheres ranged from 10 to 20 nm and from 3 to 6 nm, respectively, which are values rarely reported in the literature. Studies using high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), inductively coupled plasma-mass spectroscopy (ICP-AES), and steady-state and time-resolved spectroscopy revealed the chemical nature of the hollow nanospheres. Additionally, the hollow-structured nanospheres exhibit excellent photocatalytic activity and stability for the generation of H2 with a rate constant of 21 μmol h(-1) mg(-1) and a turnover number (TON) of 137,500 or 30,250 for CdTe QDs or nickel, respectively, under visible light irradiation for 42 h.

Bioinspired, direct synthesis of aqueous CdSe quantum dots for high-sensitive copper(II) ion detection

Luminescent CdSe semiconductor quantum dots (QDs), which are coated with a denatured bovine serum albumin (dBSA) shell, have been directly synthesized via a bioinspired approach. The dBSA coated CdSe QDs are ultrasmall (d < 2.0 nm) with a narrow size distribution and exhibit a strong green fluorescent emission at about 525 nm. They can be stored for months at room temperature and possess excellent stability against ultraviolet irradiation, high salt concentration, and a wide physiological range of pH. Systematic experimental investigations have shown the contribution of dBSA with free cysteine residues for both their effective ion chelating and surface passivating interactions during the formation and stabilization of CdSe QDs. The luminescent QDs are used for copper(II) ion detection due to their highly sensitive and selective fluorescence quenching response to Cu(2+). The concentration dependence of the quenching effect can be best described by the typical Stern-Volmer equation in a linearly proportional concentration of Cu(2+) ranging from 10 nM to 7.5 μM with a detection limit of 5 nM. As confirmed by various characterization results, a possible quenching mechanism is given: Cu(2+) ions are first reduced to Cu(+) by the dBSA shell and then chemical displacement between Cu(+) and Cd(2+) is performed at the surface of the ultrasmall metallic core to impact the fluorescence performance.

Photocatalytic hydrogen evolution from glycerol and water over nickel-hybrid cadmium sulfide quantum dots under visible-light irradiation

Natural photosynthesis offers the concept of storing sunlight in chemical form as hydrogen (H2), using biomass and water. Herein we describe a robust artificial photocatalyst, nickel-hybrid CdS quantum dots (Nih-CdS QDs) made in situ from nickel salts and CdS QDs stabilized by 3-mercaptopropionic acid, for visible-light-driven H2 evolution from glycerol and water. With visible light irradiation for 20 h, 403.2 μmol of H2 was obtained with a high H2 evolution rate of approximately 74.6 μmol h(-1)  mg(-1) and a high turnover number of 38 405 compared to MPA-CdS QDs (mercaptopropionic-acid-stabilized CdS quantum dots). Compared to CdTe QDs and CdSe QDs, the modified CdS QDs show the greatest affinity toward Ni(2+) ions and the highest activity for H2 evolution. X-ray photoelectron spectroscopy (XPS), inductively-coupled plasma atomic emission spectrometry (ICP-AES), and photophysical studies reveal the chemical nature of the Nih-CdS QDs. Electron paramagnetic resonance (EPR) and terephthalate fluorescence measurements clearly demonstrate water splitting to generate ⋅OH radicals. The detection of DMPO-H and DMPO-C radicals adduct in EPR also indicate that ⋅H radicals and ⋅C radicals are the active species in the catalytic cycle.

Cysteamine CdS quantum dots decorated with Fe3+ as a fluorescence sensor for the detection of PPi

A new sensitive and selective fluorescence sensor for the detection of pyrophosphate (PPi) in aqueous media based on the Fe(3+) decorated cysteamine CdS QDs ([Cys-CdS QDs]-Fe(3+)) was proposed. The presence of PPi can induce the fluorescence quenching of [Cys-CdS QDs]-Fe(3+) due to the high formation constants between the phosphate group and Fe(3+). Because the complex between Fe(3+) and PPi acts as an efficient quencher, the concentration of PPi can be evaluated by tracking the degree of fluorescence quenching. The fabricated sensor was optimized to obtain the best sensor selectivity and sensitivity. Under optimal conditions, a linear relationship between the fluorescence response and the concentration of PPi was established in the range of 0.5-10 μM. The limits of detection and quantitation for PPi were found to be 0.11 and 2.78 μM, respectively. Furthermore, the proposed sensor exhibited high selectivity toward PPi relative to other common anions. The proposed sensor was successfully applied to the detection of PPi in urine samples with satisfactory results.

Semicondutor quantum dots-based metal ion probes

Semiconductor quantum dots (QDs) exhibit unique optical and photophysical properties that offer significant advantages over organic dyes as optical labels for chemo/bio-sensing. This review addresses the methods for metal ion detection with QDs, including photoluminescent, electrochemiluminescent, photoelectrochemical, and electrochemical approaches. The main mechanisms of direct interaction between QDs and metal ions which lead to photoluminescence being either off or on, are discussed in detail. These direct interactions provide great opportunities for developing simple yet effect metal ion probes. Different methods to design the chemically-modified QD hybrid structures through anchoring metal ion-specific groups onto the surface of QDs are summarized. Due to the spatial separation of the luminescence center and analyte recognition sites, these chemically-modified QDs offer greatly improved sensitivity and selectivity for metal ions. Several interesting applications of QD-based metal ion probes are presented, with specific emphasis on cellular probes, coding probes and sensing with logic gate operations.

Low-cost and gram-scale synthesis of water-soluble Cu-In-S/ZnS core/shell quantum dots in an electric pressure cooker

We report an electric pressure cooker for large-scale synthesis of water-soluble Cu-In-S/ZnS core/shell quantum dots. Low-cost thioglycolic acid and sodium citrate were used as the dual stabilizers. ∼3 grams of quantum dots with a tunable emission from 545 to 610 nm and quantum yield up to 40% were obtained in a batch.

An exceptional artificial photocatalyst, Nih -CdSe/CdS core/shell hybrid, made in situ from CdSe quantum dots and nickel salts for efficient hydrogen evolution

A novel hybrid Nih -CdSe/CdS core/shell quantum dot is a simple and exceptional artificial photocatalyst for H2 production from 2-propanol aqueous solution. Studies on the nature of the artificial photocatalyst and mechanism for H2 production demonstrate that the synthetic strategy is general and the artificial photocatalyst holds promise for light capture, electron transfer, and catalysis at the surface of the Nih -CdSe/CdS core/shell quantum dots, leading to a self-healing system for H2 evolution in harmony.

A novel phosphorescence sensor for Co2+ ion based on Mn-doped ZnS quantum dots

N-acetyl-L-cysteine-capped Mn-doped ZnS quantum dots (QDs) were prepared by hydrothermal methods. It could emit phosphorescence at 583 nm with the excitation wavelength at 315 nm. The phosphorescence intensity of QDs could be quenched dramatically by increasing the concentration of Co(2+) ion. The novel phosphorescence sensor based on N-acetyl-L-cysteine-capped QDs was developed for detecting Co(2+) ion with a linear dynamic range of 1.25 × 10(-6) -3.25 × 10(-5) m. The limit of detection and RSD were 6.0 × 10(-8) m and 2.3%, respectively. Interference experiments showed excellent selectivity over numerous cations such as alkali, alkaline earth and transitional metal ions. The possible quenching mechanism was also examined by phosphorescence decays. The proposed phosphorescence method was further applied to the trace determination of Co(2+) ion in tap and pond water samples with recoveries of 97.75-103.32%.

A highly selective turn-on ATP fluorescence sensor based on unmodified cysteamine capped CdS quantum dots

Unmodified cysteamine capped nanocrystalline cadmium sulfide quantum dots (Cys-CdS QDs) were demonstrated as a selective turn-on fluorescence sensor for sensing adenosine-5'-triphosphate (ATP) in aqueous solution for the first time. The fluorescence intensity of the Cys-CdS QDs was significantly enhanced in the presence of ATP. In addition, the fluorescence intensity of the Cys-CdS QDs increased when increasing ATP concentrations. On the other hand, other phosphate metabolites and other tested common anions did not significantly alter the fluorescence intensity of the Cys-CdS QDs. In addition, this sensor showed excellent discrimination of pyrophosphate (PPi) from ATP detection. The proposed sensor could efficiently be used for ATP sensing at very low concentration with LOD of 17 μM with the linear working concentration range of 20-80 μM. The feasibility of the proposed sensor for determining ATP in urine samples was also studied, and satisfactory results were obtained.

Hydrothermal synthetic mercaptopropionic acid stabled CdTe quantum dots as fluorescent probes for detection of Ag⁺

Mercaptopropionic acid (MPA) capped CdTe quantum dots (QDs) with particle size 3 nm have been successfully synthesized in aqueous medium by hydrothermal synthesis method. And the effects of different metal ions on MPA capped CdTe QDs fluorescence were studied using fluorescence spectrometry. The results demonstrated that at the same concentration level, Ag(+) could strongly quench CdTe QDs fluorescence, and the other metal ions had little effect on CdTe QDs fluorescence except Cu(2+). On the basis of this fact, a rapid, simple, highly sensitive and selective method based on fluorescence quenching principle for Ag(+) detection in aqueous solution was proposed. Under optimal conditions, the quenched fluorescence intensity (F(0)-F) increased linearly with the concentration of Ag(+) ranging from 4 × 10(-7) to 32 × 10(-7)mol L(-1). The limit of detection for Ag(+) was 4.106 × 10(-8)mol L(-1). The obtained plot of F(0)/F versus [Ag(+)] was an upward curvature, concave towards the y-axis, rather than a straight line. The modified form of the Stern-Volmer equation was third order in Ag(+) concentration. According to the modified Stern-Volmer equation, it can be inferred that dynamic quenching and static quenching simultaneously occurred when Ag(+) interacted with MPA capped CdTe QDs. At the same time other factors might also influence the quenching process. Based on this study, hydrothermal synthesized MPA capped CdTe QDs with particle size 3 nm may be used as a novel fluorescence probe to quantificationally and selectively detect Ag(+).

Complex Förster energy transfer interactions between semiconductor quantum dots and a redox-active osmium assembly

The ability of luminescent semiconductor quantum dots (QDs) to engage in diverse energy transfer processes with organic dyes, light-harvesting proteins, metal complexes, and redox-active labels continues to stimulate interest in developing them for biosensing and light-harvesting applications. Within biosensing configurations, changes in the rate of energy transfer between the QD and the proximal donor, or acceptor, based upon some external (biological) event form the principle basis for signal transduction. However, designing QD sensors to function optimally is predicated on a full understanding of all relevant energy transfer mechanisms. In this report, we examine energy transfer between a range of CdSe-ZnS core-shell QDs and a redox-active osmium(II) polypyridyl complex. To facilitate this, the Os complex was synthesized as a reactive isothiocyanate and used to label a hexahistidine-terminated peptide. The Os-labeled peptide was ratiometrically self-assembled to the QDs via metal affinity coordination, bringing the Os complex into close proximity of the nanocrystal surface. QDs displaying different emission maxima were assembled with increasing ratios of Os-peptide complex and subjected to detailed steady-state, ultrafast transient absorption, and luminescence lifetime decay analyses. Although the possibility exists for charge transfer quenching interactions, we find that the QD donors engage in relatively efficient Förster resonance energy transfer with the Os complex acceptor despite relatively low overall spectral overlap. These results are in contrast to other similar QD donor-redox-active acceptor systems with similar separation distances, but displaying far higher spectral overlap, where charge transfer processes were reported to be the dominant QD quenching mechanism.

The synthesis of highly water-dispersible and targeted CdS quantum dots and it is used for bioimaging by confocal microscopy

Synthesis of a highly dispersed hydrophilic CdS nanocrystals and their use as fluorescence labeling for live cell imaging is reported here. By carefully manipulating the surface of CdS nanocrystals, the dispersions of CdS-MAA-PEI-FA nanocrystals with high photostability is prepared. The receptor-mediated delivery of folic acid conjugated quantum dots into folate-receptor-positive cell lines such as CBRH7919 liver cancer cells was demonstrated by confocal microscopy. In the future, the further modified CdS nanoparticles can be used for the tissue imaging in vivo studies.

Diethyldithiocarbamate functionalized CdSe/CdS quantum dots as a fluorescent probe for copper ion detection

A new fluorescent probe for copper ion detection is reported that it is based on the quenching of the fluorescence of the diethyldithiocarbamate (DDTC)-functionalized quantum dots (QDs) in the presence of copper ions. DDTC was bound to the QDs via the surface ligand exchange to form DDTC-QDs conjugates following the capping of 2-mercaptoacetic acid on the core-shell CdSe/CdS QDs. It was found that the fluorescence intensity of the conjugates was quenched after coordinated with Cu(2+). A linear relationship existed between the extent of quenching and the concentration of copper in the range of 0-100 μg L(-1), with a detection limit of 0.29 μg L(-1) (3σ). The DDTC-functionalized QDs showed excellent selectivity for Cu(2+) over other metal cations. The fluorescent probe was successfully used for the determination of copper in environmental samples.

Procedures for controlling the size, structure and optical properties of CdS quantum dots during synthesis in aqueous solution

We report an easy approach for the synthesis of CdS Quantum Dots (CdS QDs) with high luminescence and temporal stability through the reaction of Cd(2+) and S(2-) in the presence of mercaptoacetic acid (MAA) as a capping reagent in aqueous medium, under normal pressure and room temperature. The influence of several experimental variables, including temperature, pH, the Cd/S ratio and the Cd/MAA ratio, on the optical properties of the QDs obtained was studied systematically. The experimental results indicate that these variables play an important role in determining the size and state of the surface of the nanoparticles, and hence their luminescent properties and temporal stability. The general aspects of nanocrystal nucleation and growth in the synthesis of nanocrystals were studied. The best conditions for the synthesis of nanoparticles of high quality are also reported. The CdS nanocrystals obtained exhibited a narrow PL band, with reproducible room-temperature quantum yields.

Luminescence of polyethylene glycol coated CdSeTe/ZnS and InP/ZnS nanoparticles in the presence of copper cations

The use of click chemistry for quantum dot (QD) functionalization could be very promising for the development of bioconjugates dedicated to in vivo applications. Alkyne-azide ligation usually requires copper(I) catalysis. The luminescence response of CdSeTe/ZnS nanoparticles coated with polyethylene glycol (PEG) is studied in the presence of copper cations, and compared to that of InP/ZnS QDs coated with mercaptoundecanoic acid (MUA). The quenching mechanisms appear different. Luminescence quenching occurs without any wavelength shift in the absorption and emission spectra for the CdSeTe/ZnS/PEG nanocrystals. In this case, the presence of copper in the ZnS shell is evidenced by energy-filtered transmission electron microscopy (EF-TEM). By contrast, in the case of InP/ZnS/MUA nanocrystals, a redshift of the excitation and emission spectra, accompanied by an increase in absorbance and a decrease in photoluminescence, is observed. For CdSeTe/ZnS/PEG nanocrystals, PL quenching is enhanced for QDs with 1) smaller inorganic-core diameter, 2) thinner PEG shell, and 3) hydroxyl terminal groups. Whereas copper-induced PL quenching can be interesting for the design of sensitive cation sensors, copper-free click reactions should be used for the efficient functionalization of nanocrystals dedicated to bioapplications, in order to achieve highly luminescent QD bioconjugates.

A colorimetric assay method for Co2+ based on thioglycolic acid functionalized hexadecyl trimethyl ammonium bromide modified Au nanoparticles (NPs)

A simple, rapid and sensitive colorimetric assay method for detection of Co(2+) through thioglycollic acid (TGA) functionalized hexadecyl trimethyl ammonium bromide (CTAB) modified Au NPs has been discovered in our work. TGA functionalized CTAB modified Au NPs can be aggregated quickly in the presence of Co(2+) through a cooperative metal-ligand interaction. Transmission electron microscope (TEM), energy dispersive X-ray spectroscopy (EDS), and UV-vis spectra were used to characterize the Au NPs aggregation. The presence of Co(2+) is monitored by a colorimetric response of functionalized Au NPs, and had a detection limit of 3.0 × 10(-7) M. Moreover, the selectivity of this method has been investigated by comparing with other metal ions (Hg(2+), Na(+), Cu(2+), Cd(2+), Ba(2+), Pb(2+), Mn(2+), Ni(2+), Zn(2+), Fe(2+) and Fe(3+)).

Ultrasensitive copper(II) detection using plasmon-enhanced and photo-brightened luminescence of CdSe quantum dots

Here, we present a simple platform for the use of the enhanced emission of 16-mercaptohexadecanoic acid (16-MHA) capped CdSe quantum dots (QDs) as a probe for ultrasensitive copper(II) detection. In this study, the photoluminescence (PL) of the QDs was first enhanced by Ag nanoprisms which were self-assembled on Si surfaces and then further increased by photobrightening. Using this approach, the control and different analytes could be readily probed all on a single platform using fluorescence microscopy. The enhanced PL intensity of CdSe QDs was selectively quenched in the presence of Cu(2+), accompanied by the emergence of a new red-shifted luminescence band. The quenching mechanism was found to be due to a cation exchange mechanism as confirmed by X-ray photoelectron spectroscopy (XPS) measurements. Herein, we have demonstrated that this simple methodology can offer a rapid and reliable detection of Cu(2+) with a detection limit as low as 5 nM and a dynamic range up to 100 muM in a fixed fast reaction time of 5 min. The potential applications of this technique were tested in two ways, for mixed-ion solutions and in physiological fluids, and both experiments exhibited good selectivity toward Cu(2+).

Systematic study of the interaction of cobalt ions with different-sized CdTe quantum dots

Five sizes of water-dispersed CdTe quantum dots (QDs) stabilized by thioglycolic acid (TGA) with a high photoluminescence (PL) quantum yield were synthesized and a size dependent quenching of the fluorescence by cobalt ions was also observed. No matter for smaller or larger particles, obvious quenching effect was observed, and the fluorescence quenching of CdTe nanoparticles depended on the concentration of cobalt ions solution. However, CdTe QDs with different size showed dramatically different quenching efficiency, sensitivity, linear range and selectivity. With the increase of size, the quenching efficiency reduced correspondingly. The smallest particle was the most sensitive with the limit of detection for cobalt ions is 7.3 x 10(-9) molL(-1) Co(2+). For larger particles, the sensitivity was much lower, but the linear range was relatively wide, under optimal conditions, the quenched fluorescence intensity increased linearly with the concentration of cobalt ions ranging from 3.32 x 10(-8) to 3.62 x 10(-6) molL(-1). Besides, the influence on the fluorescence signal of foreign cations, including Ca(2+), Mg(2+), Ni(2+), Ba(2+), Zn(2+), Cu(2+), Fe(3+) and Ag(+) were also studied, results showed a high selectivity of the smaller QDs towards cobalt ions. According to Stern-Volmer-type equation, quenching of quantum dot luminescence was most effective for the smallest particles with the highest K(sv).