Water-based route to colloidal Mn-doped ZnSe and core/shell ZnSe/ZnS quantum dots

Multiple underlying images tuned by Mn-doped Zn-Cu-In-S quantum dots

In this study, ZnS capped Cu-In-S (ZCIS) quantum dots doped with Mn ions are synthesized by a thermal injection method, with luminescence covering almost the entire visible area. The large Stokes shift effectively inhibits the self-absorption effect under luminescence, and the quantum yield of ZCIS quantum dots increased from 38% to 50% after ZnS capping and further to 69% after doping with Mn. First, red-, yellow-, and blue-emitting quantum dots were synthesized and then, polychromatic ensembles were obtained by mixing the trichromatic quantum dots in a different ratio. Using the home-built inkjet printer, multilayered and multicolor mixed patterns were obtained for information pattern storage and multilayer pattern recognition and reading.

Triple strategy-enhanced immunochromatographic assay based on APCB and AIEFM for the ultrasensitive detection of AFM1

Aflatoxin M1 (AFM1) is highly toxic, widely distributed, and difficult to monitor, posing a serious threat to human health. Therefore, a highly sensitive, rapid, convenient, and low-cost detection method must be urgently established. In this study, a triple strategy-enhanced immunochromatographic assay (ICA) was developed to satisfy these detection requirements. First, a turn-on signal output mode of the fluorescence quenching ICA substituted the turn-off mode of the traditional ICA for sensitive response to trace AFM1, with the limit of detection (LOD) reduced by approximately 4.9-fold. Then, a novel Au and polydopamine (PDA) cogrowth chrysanthemum-like blackbody was prepared as the quenching probe to reduce the background signal. This probe combined the excellent properties of Au nanoparticles with PDA. Thus, its fluorescence quenching constant was higher than that of single Au and PDA nanoparticles by 25.8- and 4.9-fold, respectively. Furthermore, an aggregation-induced emission fluorescence microsphere with a 5.7-fold higher relative quantum yield than a commercial fluorescence microsphere was selected as the signal output carrier to improve the signal-to-noise ratio. The integration of the above triple strategies established a 53.4-fold sensitivity-enhanced fluorescence quenching ICA (LOD = 0.9 pg/mL) for detecting AFM1 in milk, providing a strong technical guarantee for the safety monitoring of milk products.

Environmentally friendly, highly efficient, and large stokes shift-emitting ZnSe:Mn2+/ZnS core/shell quantum dots for luminescent solar concentrators

In this study, heavy-metal-free orange light-emitting ZnSe:Mn²⁺/ZnS doped-core/shell (d-C/S) quantum dots (QDs) were synthesized using a nucleation doping strategy. To synthesize high quality d-C/S QDs with high photoluminescence (PL) quantum yield (QY), the Mn²⁺ concentration was optimized. The resulting ZnSe:Mn²⁺(5%)/ZnS d-C/S QDs showed a high PL QY of 83.3%. The optical properties of the synthesized QDs were characterized by absorption and PL spectroscopy. Their structural and compositional properties were studied by X-ray diffraction, transmission electron microscopy, and energy dispersive X-ray spectroscopy. After doping Mn²⁺ into a ZnSe core, the ZnSe:Mn²⁺/ZnS d-C/S QDs showed a large Stokes shift of 170 nm. The ZnSe:Mn²⁺/ZnS d-C/S QDs were embedded in a poly(lauryl methacrylate) (PLMA) polymer matrix and the ZnSe:Mn²⁺/ZnS-based polymer film was fabricated. The fabricated ZnSe:Mn²⁺/ZnS-PLMA film was highly transparent in the visible spectral region (transmittance > 83.8% for λ ≥ 450 nm) and it exhibited bright orange light under air mass (AM) 1.5G illumination using a solar simulator. The optical path-dependent PL measurement of the ZnSe:Mn²⁺/ZnS-PLMA film showed no PL band shift and minimal PL decrease under variation of excitation position. These results indicate that the highly efficient and large Stokes shift-emitting ZnSe:Mn²⁺/ZnS QDs are promising for application to luminescent solar concentrators.

Binary temporary photo-response of ZnSe:Mn/ZnS quantum dots for visible time-domain anti-counterfeiting

The development of multi-level anti-counterfeiting techniques is of great significance for economics and security issues, particularly the newly emerged temporal-domain techniques based on lifetime coding. However, the intricate reading methods required to obtain temporal-level information are inevitably cumbersome and expensive, which greatly limits the practical applications of these techniques. Herein, we report a novel, unclonable time-domain anti-counterfeiting strategy for the first time, which is achieved using photo-responsive ZnSe:Mn/ZnS quantum dots (QDs) with dynamic luminescence and can be authenticated by the naked eye. Through introducing electron traps and constructing cascade electron channels in the QDs, the binary temporary photo-response is tailored and manifested as distinctive response rates between the band-edge and Mn ⁴T₁-⁶A₁ transition emissions. Impressively, the generated photo-response is instantaneous, is capable of delayed recovery, and can be visibly detected under UV irradiation. The prospective use of colorless, nontoxic aqueous-phase ZnSe:Mn/ZnS QDs provides a new idea and important guidance for developing the next generation of multi-level anti-counterfeiting techniques without the need for complex time-gated decoding instrumentation.

Comparative Studies of Blue-Emitting Zinc Selenide Nanocrystals Doped with Ag, Cu, and Mg towards Medical Applications

Blue-emitting Ag(+)-, Cu(2+)-, and Mg(2+)-doped ZnSe nanoparticles (NPs) were successfully synthesized at 80 °C by the precipitation method by using mercaptopropionic acid (MPA) as a stabilizer. UV–visible and photoluminescence (PL) studies were applied to investigate their physicochemical properties. Their structural properties were confirmed by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) and transmission electron microscopy (TEM). The size of the ZnSe: X-capped MPA showed a strong relationship with dopant metals. The diameters of the Mg-doped ZnSe and the Cu-doped ZnSe were 22–24 nm, while the Ag-doped ZnSe was halved, at about 13 nm. The photoluminescence was within a wavelength range of 400–550 nm. In addition, the PL intensities, as well as the photoluminescence quantum yields, were in the order of the decreasing ionic radii of the dopant metals (ZnSe:Ag < ZnSe:Mg < ZnSe:Cu). Furthermore, through the interaction with lysine, the PL intensity of the ZnSe:X was changed. Interestingly, the capacity of the ZnSe:Mg for lysine was significantly higher than that of other dopant metals. Moreover, the toxicity of the ZnSe:Mg was relatively insignificant toward the hMSCs (about 80% cell viability at 320 ppm), compared to the transition-metal dopant. Therefore, the ZnSe:Mg material could have great potential for bioapplications.

Mn-Doped Quinary Ag-In-Ga-Zn-S Quantum Dots for Dual-Modal Imaging

Doping of transition metals within a semiconductor quantum dot (QD) has a high impact on the optical and magnetic properties of the QD. In this study, we report the synthesis of Mn²⁺-doped Ag-In-Ga-Zn-S (Mn:AIGZS) QDs via thermolysis of a dithiocarbamate complex of Ag⁺, In³⁺, Ga³⁺, and Zn²⁺ and of Mn(stearate)₂ in oleylamine. The influence of the Mn²⁺ loading on the photoluminescence (PL) and magnetic properties of the dots are investigated. Mn:AIGZS QDs exhibit a diameter of ca. 2 nm, a high PL quantum yield (up to 41.3% for a 2.5% doping in Mn²⁺), and robust photo- and colloidal stabilities. The optical properties of Mn:AIGZS QDs are preserved upon transfer into water using the glutathione tetramethylammonium ligand. At the same time, Mn:AIGZS QDs exhibit high relaxivity (r ₁ = 0.15 mM^-1 s^-1 and r ₂ = 0.57 mM^-1 s^-1 at 298 K and 2.34 T), which shows their potential applicability for bimodal PL/magnetic resonance imaging (MRI) probes.

Molecularly Imprinted Magnetic Fluorescent Nanocomposite-Based Sensor for Selective Detection of Lysozyme

A new strategy for the design and construction of molecularly imprinted magnetic fluorescent nanocomposite-based-sensor is proposed. This multifunctional nanocomposite exhibits the necessary optics, magnetism and biocompatibility for use in the selective fluorescence detection of lysozyme. The magnetic fluorescent nanocomposites are prepared by combining carboxyl- functionalized Fe3O4 magnetic nanoparticles with l-cysteine-modified zinc sulfide quantum dots (MNP/QDs). Surface molecular imprinting technology was employed to coat the lysozyme molecularly imprinted polymer (MIP) layer on the MNP/QDs to form a core-shell structure. The molecularly imprinted MNP/QDs (MNP/QD@MIPs) can rapidly separate the target protein and then use fluorescence sensing to detect the protein; this reduces the background interference, and the selectivity and sensitivity of the detection are improved. The molecularly imprinted MNP/QDs sensor presented good linearity over a lysozyme concentration range from 0.2 to 2.0 μM and a detection limit of 4.53 × 10-3 μM for lysozyme. The imprinting factor of the MNP/QD@MIPs was 4.12, and the selectivity coefficient ranged from 3.19 to 3.85. Furthermore, the MNP/QD@MIPs sensor was applied to detect of lysozyme in human urine and egg white samples with recoveries of 95.40-103.33%. Experimental results showed that the prepared MNP/QD@MIPs has potential for selective magnetic separation and fluorescence sensing of target proteins in biological samples.

Cadmium Selenide Quantum Dots for Solar Cell Applications: A Review

Quantum dot-sensitized solar cells (QDSSCs) are significant energy-producing devices due to their remarkable capability to growing sunshine and produce many electrons/holes pairs, easy manufacturing, and low cost. However, their power conversion efficiency (4%) is usually worse than that of dye-sensitized solar cells (≤12%); this is mainly due to their narrow absorption areas and the charge recombination happening at the quantum dot/electrolyte and Ti O 2 /electrolyte interfaces. Thus, to raise the power conversion efficiency of QDSSC, new counter electrodes, working electrodes, sensitizers, and electrolytes are required. CdSe thin films have shown great potential for use in photodetectors, solar cells, biosensors, light-emitting diodes, and biomedical imaging systems. This article reviews the CdSe nanomaterials that have been recently used in QDSSCs as sensitizers. Their size, design, morphology, and density all noticeably influence the electron injection efficiency and light-harvesting capacity of these devices. A detailed overview of the development of QDSSCs is presented, including their basic principles, the synthesis methods for their CdSe quantum dots, and the device fabrication processes. Finally, the challenges and opportunities of realizing high-performance CdSe QDSSCs are discussed and some future directions are suggested.

Novel synthesis of Mn: ZnSe@ZnS core-shell quantum dots based on photoinduced fluorescence enhancement

A novel Type-I Mn: ZnSe@ZnS core-shell quantum dots (QDs) was reported through a two-step procedure by using low-cost inorganic salts and naturalbiomacromolecule as raw materials. Based on a designed structure of L-cysteine-capped Mn: ZnSe QDs in aqueous media with the controllable surface, Mn: ZnSe@ZnS core-shell QDs were formed due to photoactive ions and defect curing under continuous constant light. The influences of experimental variables, including synthesis conditions of Mn: ZnSe QDs, different types and affecting factors of photo irradiation had been systematically investigated. Under the effect of photoinduced fluorescence enhancement, the photoluminescence (PL) intensity increases significantly by about 5-10 times after 1-3 h of UV irradiation. The position of the fluorescence peak was red-shifted by about 17 nm, emitting orange-red fluorescence. The photoluminescence quantum yield (PL QY) was markedly improved (up to 35%). The structure and morphology of Mn: ZnSe@ZnS core-shell QDs were also confirmed by Fourier transform infrared spectroscopy (FTIR), X-ray powder diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray spectroscopy (EDS) in detail. The mechanism of photoinduced fluorescence enhancement was attributed to L-cysteine allowed to release S^2- to form a ZnS shell, and the passivated surface non-radiative relaxation centers of Mn: ZnSe@ZnS QDs was successfully synthesized with highuniform size, excellent photoluminescence performance, and good stability, all ofwhichmakethemgood potential candidates for white LEDs, and biological labels.

Ultrabright fluorescent microsphere and its novel application for improving the sensitivity of immunochromatographic assay

Fluorescent microsphere (FM) is widely used as probe in immunochromatographic assay (ICA). However, the performance of conventional FM is limited because of the aggregation-caused quenching effect. Herein, we compared a kind of conventional FM (DMFFM, loading DMF) with novel aggregation-induced emission FM (AIEFM, loading TCBPE). The fluorescence intensity of DMFFM initially increased and then decreased as the concentrations of the loading DMF increased. The fluorescence intensity of AIEFM increased as the concentrations of the loading TCBPE increased and retained a high value. AIEFM was compared with two commercial FMs purchased from Ocean (OFM) and Merk (MFM). The maximum fluorescence intensity and relative quantum yield of AIEFM was approximately 5 and 4.5 times higher than those of two commercial FMs. We used the novel AIEFM as a probe to improve the sensitivity of ICA. When Escherichia coli O157:H7 was detected as the target, the limit of detection of ICA based on AIEFM, OFM and MFM were 3.98 × 10³ CFU/mL, 4.48 × 10⁴ and 2.78 × 10⁴ CFU/mL, respectively. The ICA of AIEFM had 11 and 7 times improvement in sensitivity compared with that of OFM and MFM. Our results could be used as a basis for novel probes in practical ICA applications.

Influence of doping ion, capping agent and pH on the fluorescence properties of zinc sulfide quantum dots: Sensing of Cu2+ and Hg2+ ions and their biocompatibility with cancer and fungal cells

Herein, a facile one-pot synthetic method was explored for the fabrication of glutathione capped Mn²⁺ doped‑zinc sulphide quantum dots (GSH-Mn²⁺-ZnS QDs) for both fluorescent detection of Cu²⁺ and Hg²⁺ ions and for fluorescence imaging of two cancer (RIN5F and MDAMB231) and fungal (Rhizopus oryzae) cells. Particularly, doping of Mn²⁺ into ZnS QDs nanocrystal structure resulted a great improvement in the fluorescence properties of ZnS QDs. The emission peak of undoped ZnS QDs was found at 447 nm, which is due to the large number of surface defects in the ZnS QDs nanostructures. Under identical conditions, there is a good linear relationship between the quenching of fluorescence intensity and analytes (Cu²⁺ and Hg²⁺ ions) concentration in the range of 0.005 to 0.2 mM and of 0.025 to 0.4 mM for Cu²⁺ and Hg²⁺ ions, respectively. The GSH-Mn²⁺-ZnS QDs exhibit least cytotoxicity against RIN5F and MDAMB231 cells, demonstrating the multifunctional applications in sensing of metal ions and biocompatibility towards cancer (RIN5F and MDAMB231) and fungal (Rhizopus oryzae) cells.

Highly fluorescent, color tunable and magnetic quaternary Ag–In–Mn–Zn–S quantum dots

We report a simple and effective synthesis of Mn : AIZS quantum dots exhibiting color-tunable photoluminescence emission and magnetic properties.

Stabilization of Lead-Tin-Alloyed Inorganic-Organic Halide Perovskite Quantum Dots

Recently, lead-tin-based alloyed halide perovskite quantum dots (QDs) with improved stability and less toxicity have been introduced. However, the perovskite QDs containing tin are still unstable and exhibit low photoluminescence quantum yields (PLQYs), owing to the presence of defects in the alloyed system. Here, we have attempted to introduce sulfur anions (S^2-) into the host lattice (MAPb_0.75Sn_0.25Br₃) as a promising route to stable alloyed perovskite QDs with improved stability and PLQY. In this study, we used elemental sulfur as a sulfur precursor. The successful incorporation of sulfur anions into the host lattice resulted in a highly improved PLQY (>75% at room temperature), which is believed to be due to a reduction in the defect-related non-radiative recombination centers present in the host lattice. Furthermore, we found that the emission property could be tuned between the bright green and cyan-bluish regions without compromising on color quality. This work invigorates the perovskite research community to prepare stable, bright, and color-tunable alloyed inorganic-organic perovskite QDs without compromising on their phases and color quality, which can lead to considerable advances in display technology.

Luminescent, low-toxic and stable gradient-alloyed Fe:ZnSe(S)@ZnSe(S) core:shell quantum dots as a sensitive fluorescent sensor for lead ions

In this paper, an aqueous-based approach is introduced for facile, fast, and green synthesis of gradient-alloyed Fe-doped ZnSe(S)@ZnSe(S) core:shell quantum dots (QDs) with intense and stable emission. Co-utilization of co-nucleation and growth doping strategies, along with systematic optimization of emission intensity, provide a well-controllable/general method to achieve internally doped QDs (d-dots) with intense emission. Results indicate that the alloyed ZnSe(S)@ZnSe(S) core:shell QDs have a gradient structure that consists of a Se-rich core and a S-rich shell. This gradient structure cannot only passivate the core d-dots by means of the wider band gap S-rich shell, but also minimizes the lattice mismatch between alloyed core-shell structures. Using this novel strategy and utilizing the wider band gap S-rich shell can obviously increase the cyan emission intensity and also drastically improve the emission stability against chemical and optical corrosion. Furthermore, the cytotoxicity experiments indicate that the obtained d-dots are nontoxic nanomaterials, and thus they can be considered as a promising alternative to conventional Cd-based QDs for fluorescent probes in biological fields. Finally, it is demonstrated that the present low-toxicity and gradient-alloyed core:shell d-dots can be used as sensitive chemical detectors for Pb²⁺ ions with excellent selectivity, small detection limit, and rapid response time.

Enhanced visible-light-driven photocatalytic activity of Au@Ag core–shell bimetallic nanoparticles immobilized on electrospun TiO2 nanofibers for degradation of organic compounds

In this work, Au@Ag core–shell nanoparticles (NPs) with variable Ag shell thickness were synthesized and immobilized on TiO₂ nanofibers (TNF).

Hydrothermal synthesis of highly luminescent blue-emitting ZnSe(S) quantum dots exhibiting low toxicity

Highly luminescent quantum dots (QDs) that emit in the visible spectrum are of interest to a number of imaging technologies, not least that of biological samples. One issue that hinders the application of luminescent markers in biology is the potential toxicity of the fluorophore. Here we show that hydrothermally synthesized ZnSe(S) QDs have low cytotoxicity to both human colorectal carcinoma cells (HCT-116) and human skin fibroblast cells (WS1). The QDs exhibited a high degree of crystallinity, with a strong blue photoluminescence at up to 29% quantum yield relative to 4',6-diamidino-2-phenylindole (DAPI) without post-synthetic UV-irradiation. Confocal microscopy images obtained of HCT-116 cells after incubation with the QDs highlighted the stability of the particles in cell media. Cytotoxicity studies showed that both HCT-116 and WS1 cells retain 100% viability after treatment with the QDs at concentrations up to 0.5g/L, which makes them of potential use in biological imaging applications.

A facile synthesis of alloyed Mn-doped ZnSeS nanoparticles using a modified selenium/sulfur precursor in a one-pot approach

We demonstrate a highly reactive modified Se/S precursor for the facile synthesis of alloyed ZnSeS:Mn nanoparticles in a one-pot approach.

Highly bright multicolour emission through energy migration in core/shell nanotubes

Multicolour photoluminescence was achieved in gadolinium-based core/shell nanotube structures via energy migration of Ce³⁺→Gd³⁺→Ln³⁺ and Ce³⁺→Ln³⁺ (Ln = Eu, Tb, Dy and Sm) in separated layers.

The facile one-step aqueous synthesis of near-infrared emitting Cu+ doped CdS quantum dots as fluorescence bioimaging probes with high quantum yield and low cytotoxicity

The water-soluble Cu⁺:CdS QDs with NIR emission and high PLQY were prepared in a N₂ atmosphere and employed as bioimaging probes for 3T3 cells.

A two-step method to synthesize water-dispersible Mn:ZnSe/ZnO core/shell quantum dots with pure dopant emission

Water-soluble Mn:ZnSe/ZnO core/shell quantum dots with pure dopant emission were prepared via a two-step method.

Novel aqueous synthesis methods for ZnTe/ZnSe and Mn2+-doped ZnTe/ZnSe Type-II core/shell quantum dots

In this paper, novel approaches for the synthesis of Type-II core/shell quantum dots (ZnTe/ZnSe QDs) and Mn²⁺-doped Type-II core/shell quantum dots (Mn : ZnTe/ZnSe QDs) with mercaptopropionic acid (MPA) as stabilizer were proposed.

DNA mediated assembly of quantum dot–protoporphyrin IX FRET probes and the effect of FRET efficiency on ROS generation

Quantum dot-protoporphyrin IX FRET probes are assembled through DNA hybridization and the efficiency of FRET and ROS generation was studied.

Manganese doped fluorescent paramagnetic nanocrystals for dual-modal imaging

In this work, dual-modal (fluorescence and magnetic resonance) imaging capabilities of water-soluble, low-toxicity, monodisperse Mn-doped ZnSe nanocrystals (NCs) with a size (6.5 nm) below the optimum kidney cutoff limit (10 nm) are reported. Synthesizing Mn-doped ZnSe NCs with varying Mn(2+) concentrations, a systematic investigation of the optical properties of these NCs by using photoluminescence (PL) and time resolved fluorescence are demonstrated. The elemental properties of these NCs using X-ray photoelectron spectroscopy and inductive coupled plasma-mass spectroscopy confirming Mn(2+) doping is confined to the core of these NCs are also presented. It is observed that with increasing Mn(2+) concentration the PL intensity first increases, reaching a maximum at Mn(2+) concentration of 3.2 at% (achieving a PL quantum yield (QY) of 37%), after which it starts to decrease. Here, this high-efficiency sample is demonstrated for applications in dual-modal imaging. These NCs are further made water-soluble by ligand exchange using 3-mercaptopropionic acid, preserving their PL QY as high as 18%. At the same time, these NCs exhibit high relaxivity (≈2.95 mM(-1) s(-1)) to obtain MR contrast at 25 °C, 3 T. Therefore, the Mn(2+) doping in these water-soluble Cd-free NCs are sufficient to produce contrast for both fluorescence and magnetic resonance imaging techniques.

Mercaptopropionic acid-capped Mn(2+):ZnSe/ZnO quantum dots with both downconversion and upconversion emissions for bioimaging applications

Doped quantum dots (d-dots) can serve as fluorescent biosensors and biolabels for biological applications. Our study describes a synthesis of mercaptopropionic acid (MPA)-capped Mn(2+):ZnSe/ZnO d-dots through a facile, cost-efficient hydrothermal route. The as-prepared water-soluble d-dots exhibit strong emission at ca. 580 nm, with a photoluminescence quantum yield (PLQY) as high as 31%, which is the highest value reported to date for such particles prepared via an aqueous route. They also exhibit upconversion emission when excited at 800 nm. With an overall diameter of around 6.7 nm, the d-dots could gain access to the cell nucleus without any surface decoration, demonstrating their promising broad applications as fluorescent labels.

Ce-doped YAG nanophosphor and red emitting CuInS2/ZnS core/shell quantum dots for warm white light-emitting diode with high color rendering index

In this work, we report the solvothermal synthesis of Ce-doped YAG (YAG:Ce) nanoparticles (NPs) and their association with a free-Cd CuInS2/ZnS (CIS/ZnS) core/shell QDs for application into white light emitting diode (WLED). 1500 °C-annealed YAG:Ce NPs and CIS/ZnS core/shell QDs exhibited intense yellow and red emissions band with maxima at 545 and 667 nm, respectively. Both YAG:Ce nanophosphor and CIS/ZnS QDs showed high photoluminescence quantum yield (PL QY) of about 50% upon 460 nm excitation. YAG:Ce nanophosphor layer and bilayered YAG:Ce nanophosphor-CIS/ZnS QDs were applied on blue InGaN chip as converter wavelength to achieve WLED. While YAG:Ce nanophosphor converter layer showed low color rendering index (CRI) and cold white light, bilayered YAG:Ce nanophosphor-CIS/ZnS QDs displayed higher CRI of about 84 and warm white light with a correlated color temperature (CCT) of 2784 K. WLED characteristics were measured as a function of forward current from 20 to 1200 mA. The white light stability of bilayered nanophosphor-QDs-based WLED operated at 200 mA was also studied as a function of operating time up to 40 h. Interestingly, CRI and CCT of such device tend to remain constant after 7 h of operating time suggesting that layer-by-layer structure of YAG:Ce phosphor and red-emitting CIS/ZnS QDs could be a good solution to achieve stable warm WLED, especially when high current density is applied.

Dual functional carbonaceous nanodots exist in a cup of tea

Water-soluble, highly photoluminescent carbonaceous nanodots were obtained from tea water and were applied to Hg²⁺ detecting and cell imaging.

A facile and highly sensitive probe for Hg(II) based on metal-induced aggregation of ZnSe/ZnS quantum dots

Sensitive and selective detection strategies for toxic heavy metal ions, which are rapid, cheap and applicable to environmental and biological fields, are of significant importance. As a result of specific interaction between thiol(s) used as ligands and heavy metal ions, the photoluminescence intensity of quantum dots (QDs) in PBS buffer solution was quenched and the aggregation of QDs was formed at the same time. Herein, we present water-soluble, low toxic QDs, ZnSe/ZnS, which were applied for ultrasensitive Hg(2+) ion detection with a low detection limit (2.5 nM). In addition, a model has been proposed to explain the aggregation of QDs in the presence of heavy metal ions such as Hg(2+) ions.

Surface ion engineering of Mn2+-doped ZnS quantum dots using ion-exchange resins

We report the engineering of surface ions present as defects in doped quantum dots (Qdots) following their synthesis. This was achieved by treating the Qdots with cation-exchange resin beads (CB). An aqueous dispersion of Mn(2+)-doped ZnS Qdots, when treated with different amounts of CB, resulted in two kinds of changes in the emission due to Mn(2+) ions. First, the intensity increased in the presence of a smaller amount of CB, to the extent of a doubled quantum yield. With increased CB as well as incubation time, the emission intensity decreased systematically, accompanied by an increasing blue shift of the peak emission wavelength. Electron spin resonance results indicated the removal of clusters of Mn(2+) present in the Qdots by the CB, which has been attributed to changes in the emission characteristics. Transmission electron microscopy studies revealed that for smaller amounts of CB there was no change in the particle size, whereas for greater amounts the particle size decreased. The results have been explained on the basis of the removal of Mn(2+) (and also Zn(2+)) ions present on the surfaces of Qdots in the form of clusters as well as individual ions.

Blue-UV-emitting ZnSe(dot)/ZnS(rod) core/shell nanocrystals prepared from CdSe/CdS nanocrystals by sequential cation exchange

Great control over size, shape and optical properties is now possible in colloidal Cd-based nanocrystals, which has paved the way for many fundamental studies and applications. One popular example of such class of nanocrystals is represented by CdSe(spherical core)/CdS(rod shell) nanorods. These can be nearly monodisperse in size and shape and have strong and stable photoluminescence that is tunable in the visible range (mainly by varying the size of the CdSe core). The corresponding Zn-based core/shell nanorods would be good candidates for tunable emission in the blue-UV region. However, while the synthesis of ZnS nanocrystals with elongated shapes has been demonstrated based on the oriented-attachment mechanism, elongated ZnS shells are difficult to fabricate because the more common cubic phase of ZnS has a highly symmetric crystal structure. We report here a procedure based on a sequence of two cation exchange reactions, namely, Cd(2+)⇒Cu(+) and then Cu(+)⇒Zn(2+), by which we transform colloidal CdSe(core)/CdS(shell) nanorods first into into Cu(2)Se/Cu(2)S nanorods, which are then converted into blue-UV fluorescent ZnSe(core)/ZnS(shell) nanorods. The procedure transfers the morphological and structural information of the initial Cd-based nanorods to the Zn-based nanorods. Therefore, the final nanoparticles are made by a ZnSe dot embedded in a rod-shaped shell of wurtzite ZnS. Since in the starting Cd-based nanorods the size of the CdSe core and the length of the CdS shell can be well controlled, the same holds for the final Zn-based rods. In the second step of the exchange reaction (Cu(+)⇒Zn(2+)), a large excess of Zn(2+) ions added over the Cu(+) ions present in the Cu(2)Se/Cu(2)S nanorods is the key requisite to obtain bright, band-edge emission (with quantum yields approaching 15%) with narrow line widths (approaching 75 meV). In these ZnSe/ZnS nanorods, photogenerated carriers appear to be more confined in the core region compared to their parent CdSe/CdS nanorods.