Surfactant-Assisted Förster Resonance Energy Transfer from a Quantum Dot Complex for Highly Stable White Light Emission.

Herein we report the fabrication of a surfactant modified quantum dot complex (S-QDC, having λem = 485 nm) nanocomposite (composed of cetyltrimethylammonium bromide surfactants and a zinc-quinolate complex attached ZnS quantum dot), the donor capability of S-QDC in Förster resonance energy transfer (FRET) with an acceptor organic molecule (λem = 573 nm), and finally their utilization in the FRET-based white light emission having features near to mid-day sunlight. The Förster distance, energy transfer efficiency, donor-acceptor distance, number of binding sites, and binding constant are evaluated to be 3.48 nm, 85.74%, 2.58 nm, 0.94, and 1.87 × 104 M-1, respectively, for the current electrostatically driven FRET pair. The solid polymer coated FRET pair composite emits white light having chromaticity color coordinates of (0.33, 0.33) and correlated color temperature of 5350 K and also shows long-term atmospheric white luminescence stability up to 30 days, photostability, and thermal stability with preservation of their pristine morphology.

Surfactant-mediated enhanced FRET from a quantum-dot complex for ratiometric sensing of food colorants.

Herein we report that a surfactant modified quantum dot-complex (S-QDC; with λem-515 nm) nanocomposite, as a donor fluorophore, exhibits enhanced Förster resonance energy transfer (FRET) efficiency to an acceptor organic dye (λem-576 nm) in comparison to only the QDC. The proposed S-QDC (consisting of a ZnS quantum dot, zinc quinolate inorganic complex and cetyltrimethylammonium bromide (CTAB) surfactant) provides the unique and selective ratiometric visual detection of organic dyes present as food colorants in commercial chili powder, tomato ketchup and mixed fruit jam. Notably, the S-QDC shows a limit of detection (LOD) as low as 2.2 nM in the linear range of 0.17-4.89 µM for food colorants. Furthermore, the present work will bring new possibilities to unravelling the chemistry among surfactants, inorganic complexes and quantum dots to make newer optical materials with futuristic scope of utilization ranging from optical sensors to light emitting devices.

Physical insights into the facilitation of an unprecedented complexation reaction on the surface of a doped quantum dot leading to white light generation.

Herein, we report a complexation reaction between Zn2+ ions present on the surface of an orange-red-emitting environmentally sustainable Mn2+-doped ZnS QD and a non-emitting copper quinolate (CuQ2) complex, which leads to the formation of a greenish blue-emitting surface zinc quinolate (ZnQ2) complex. The synchronous contribution of the surface ZnQ2 complex and Mn2+-doped ZnS QD is directed towards the generation of photostable bright white light (at λex - 355 nm) with chromaticity coordinates of (0.34, 0.42), color rendering index (CRI) of 71 and color-correlated temperature (CCT) of 5046 K. The ZnQ2 complexed Mn2+-doped ZnS QD is herein called as quantum dot complex (QDC). The excitation- and time-dependent tunability in emission, chromaticity, CRI and CCT of QDC revealed their futuristic applications in light-emitting devices with an anticipated color output. The current work also shows the catalytic behavior of Mn2+-doped ZnS QDs towards facilitating the formation of surface ZnQ2 from CuQ2, which is not feasible with regard to the reactivity of CuQ2 under normal conditions according to the Irving-William series. The rate of the reaction was observed to be first order with respect to CuQ2 at 20 °C, and the complexation constant for the formation of ZnQ2 was estimated to be 8.3 × 105 M-1. This is important for understanding the surface chemistry of metal chalcogenide QDs towards complexation reactions.

Luminescence Enhancement based Sensing of L-Cysteine by Doped Quantum Dots.

The interaction of a presynthesized orange emitting Mn2+ -doped ZnS quantum dots (QDs) with L-Cysteine (L-Cys) led to enhance emission intensity (at 596 nm) and quantum yield (QY). Importantly, the Mn2+ -doped ZnS QDs exhibited high sensitivity towards L-Cys, with a limit of detection of 0.4±0.02 µM (in the linear range of 3.3-13.3 µM) and high selectivity in presence of interfering amino acids and metal ions. The association constant of L-Cys was determined to be 0.36×105  M-1 . The amplified passivation of the surface of Mn2+ -doped ZnS QDs following the incorporation and binding of L-Cys is accounted for the enhancement in their luminescence features. Moreover, the luminescence enhancement-based detection will bring newer dimension towards sensing application.

The quantum dot-FRET-based detection of vitamin B12 at a picomolar level.

Herein we report the picomolar level detection of vitamin B12 (VB12) using orange-red emitting ligand-free Mn2+-doped ZnS quantum dots (QDs; λ em = 587 nm) in an aqueous dispersion. Sensing was achieved following the quenching of the luminescence of the Mn2+-doped ZnS QDs with an increasing concentration of VB12. The Stern-Volmer constant was determined to be 5.2 × 1010 M-1. Importantly, the Mn2+-doped ZnS QDs exhibited high sensitivity towards VB12, with a limit of detection as low as 1.15 ± 0.06 pM (in the linear range of 4.9-29.4 pM) and high selectivity in the presence of interfering amino acids, metal ions, and proteins. Notably, a Förster resonance energy transfer (FRET) mechanism was primarily proposed for the observed quenching of luminescence of Mn2+-doped ZnS QDs upon the addition of VB12. The Förster distance (R o) and energy transfer efficiency (E) were calculated to be 2.33 nm and 79.3%, respectively. Moreover, the presented QD-FRET-based detection may bring about new avenues for future biosensing applications.

Hue- and Chromaticity-Based Exploration of Surface Complexation-Induced Tunable Emission from Non-Luminescent Quantum Dots.

Herein we report the use of a hue parameter of HSV (Hue, Saturation and Value) color space-in combination with chromaticity color coordinates-for exploring the complexation-induced luminescence color changes, ranging from blue to green to yellow to white, from a non-luminescent Fe-doped ZnS quantum dot (QD). Importantly, the surface complexation reaction helped a presynthesized non-luminescent Fe-doped ZnS QD to glow with different luminescence colors (such as blue, cyan, green, greenish-yellow, yellow) by virtue of the formation of various luminescent inorganic complexes (using different external organic ligands), while the simultaneous blue- and yellow-emitting complex formation on the surface of non-luminescent Fe-doped ZnS QD led to the generation of white light emission, with a hue mean value of 85 and a chromaticity of (0.28,0.33). Furthermore, the surface complexation-assisted incorporation of luminescence properties to a non-luminescent QD not only overcomes their restricted luminescence-based applications such as light-emitting, biological and sensing applications but also bring newer avenues towards unravelling the surface chemistry between QDs and inorganic complexes and the advantage of having an inorganic complex with QD for their aforementioned useful applications.

Chemical Reactions Involving the Surface of Metal Chalcogenide Quantum Dots.

This invited feature article focuses on the chemical reactions involving the surface ions of colloidal quantum dots (Qdots). Emphasis is placed on ion-exchange, redox, and complexation reactions. The pursuit of reactions involving primarily the cations on the surface results in changes in the optical properties of the Qdots and also may confer new properties owing to the newly formed surface species. For example, the cation-exchange reaction, leading to systematic removal of the cations present on the as-synthesized Qdots, enhances the photoluminescence quantum yield. On the other hand, redox reactions, involving the dopant cations in the Qdots, could not only modulate the photoluminescence quantum yield but also give rise to new emission not present in the as-synthesized Qdots. Importantly, the cations present on the surface could be made to react with external organic ligands to form inorganic complexes, thus providing a new species defined as the quantum dot complex (QDC). In the QDC, the properties of Qdots and the inorganic complex are not only present but also enhanced. Furthermore, by varying reaction conditions such as the concentrations of the species and using a mixture of ligands, the properties could be further tuned and multifunctionalization of the Qdot could be achieved. Thus, chemical, magnetic, and optical properties could be simultaneously conferred on the same Qdot. This has helped in externally controlled bioimaging, white light generation involving individual quantum dots, and highly sensitive molecular sensing. Understanding the species (i.e., the newly formed inorganic complex) on the surface of the Qdot and its chemical reactivity provide unique options for futuristic technological applications involving a combination of an inorganic complex and a Qdot.

A two-target responsive reversible ratiometric pH nanoprobe: a white light emitting quantum dot complex.

Herein we report the use of a white light emitting quantum dot complex (comprising an orange emitting Mn2+-doped ZnS quantum dot and greenish-blue emitting zinc-quinolate complex) as a two-target responsive ratiometric reversible pH nanosensor in the physiological range of 6.5-10.3, following changes in their luminescence intensity ratio, color and chromaticity.

A White Light-Emitting Quantum Dot Complex for Single Particle Level Interaction with Dopamine Leading to Changes in Color and Blinking Profile.

The interaction of the neurotransmitter dopamine is reported with a single particle white light-emitting (WLE) quantum dot complex (QDC). The QDC is composed of yellow emitting ZnO quantum dots (Qdots) and blue emitting Zn(MSA)2 complex (MSA = N-methylsalicylaldimine) synthesized on their surfaces. Sensing is achieved by the combined changes in the visual luminescence color from white to blue, chromaticity color coordinates from (0.31, 0.33) to (0.24, 0.23) and the ratio of the exponents (αon /αoff ) of on/off probability distribution (from 0.24 to 3.21) in the blinking statistics of WLE QDC. The selectivity of dopamine toward ZnO Qdots, present in WLE QDC, helps detect ≈13 dopamine molecules per Qdot. Additionally, the WLE QDC exhibits high sensitivity, with a limit of detection of 3.3 × 10-9 m (in the linear range of 1-100 × 10-9 m) and high selectivity in presence of interfering biological species. Moreover, the single particle on-off bilking statistics based detection strategy may provide an innovative way for ultrasensitive detection of analytes.

Surface Complexed ZnO Quantum Dot for White Light Emission with Controllable Chromaticity and Color Temperature.

We report the formation of blue emitting Zn(MSA)2 complex on the surface of a yellow emitting ZnO quantum dot (Qdot)-out of a complexation reaction between N-methylsalicylaldimine (MSA) and ZnO Qdot. This led to formation of a highly luminescent, photostable, single-component nanocomposite that emits bright natural white light, with (i) chromaticities of (0.31, 0.38) and (0.31, 0.36), (ii) color rendering indices (CRI) of 74 and 82, and (iii) correlated color temperatures (CCT) of 6505 and 6517 K in their solution and solid phases, respectively. Importantly, the control over the chromaticity and CCT-depending upon the degree of complexation-makes the reported nanocomposite a potential new advanced material in fabricating cost-effective single-component white light emitting devices (WLED) of choice and design in the near future.

Gold Nanocluster and Quantum Dot Complex in Protein for Biofriendly White-Light-Emitting Material.

We report the synthesis of a biofriendly highly luminescent white-light-emitting nanocomposite. The composite consisted of Au nanoclusters and ZnQ2 complex (on the surface of ZnS quantum dots) embedded in protein. The combination of red, green, and blue luminescence from clusters, complex, and protein, respectively, led to white light generation.

Synchronous Tricolor Emission-Based White Light from Quantum Dot Complex.

Herein we report the generation of synchronous tricolor emission for a single wavelength excitation from a quantum dot complex (QDC). The single-component QDC was formed out of a complexation reaction, at room temperature, between ligand-free Mn(2+)-doped ZnS quantum dots (Qdots) and a mixture of two organic ligands (acetylsalicylic acid and 8-hydroxyquinoline). Furthermore, the tunability in chromaticity color coordinates, which is important for solid-state lighting, was achieved following the synthesis of QDC. Moreover, the photostable QDC emitted white light (λex 320 nm) with (0.30, 0.33) and (0.32, 0.32) chromaticity color coordinates in the liquid and the solid phases, respectively. Hence, the white light-emitting QDC may be a superior material for light-emitting applications.

Double channel emission from a redox active single component quantum dot complex.

Herein we report the generation and control of double channel emission from a single component system following a facile complexation reaction between a Mn(2+) doped ZnS colloidal quantum dot (Qdot) and an organic ligand (8-hydroxy quinoline; HQ). The double channel emission of the complexed quantum dot-called the quantum dot complex (QDC)-originates from two independent pathways: one from the complex (ZnQ2) formed on the surface of the Qdot and the other from the dopant Mn(2+) ions of the Qdot. Importantly, reaction of ZnQ2·2H2O with the Qdot resulted in the same QDC formation. The emission at 500 nm with an excitation maximum at 364 nm is assigned to the surface complex involving ZnQ2 and a dangling sulfide bond. On the other hand, the emission at 588 nm-with an excitation maximum at 330 nm-which is redox tunable, is ascribed to Mn(2+) dopant. The ZnQ2 complex while present in QDC has superior thermal stability in comparison to the bare complex. Interestingly, while the emission of Mn(2+) was quenched by an electron quencher (benzoquinone), that due to the surface complex remained unaffected. Further, excitation wavelength dependent tunability in chromaticity color coordinates makes the QDC a potential candidate for fabricating a light emitting device of desired color output.

Surface complexation reaction for phase transfer of hydrophobic quantum dot from nonpolar to polar medium.

Chemical reaction between oleate-capped Zn(x)Cd(1-x)S quantum dots (Qdots) and 8-hydroxyquinoline (HQ) led to formation of a surface complex, which was accompanied by transfer of hydrophobic Qdots from nonpolar (hexane) to polar (water) medium with high efficiency. The stability of the complex on the surface was achieved via involvement of dangling sulfide bonds. Moreover, the transferred hydrophilic Qdots--herein called as quantum dot complex (QDC)--exhibited new and superior optical properties in comparison to bare inorganic complexes with retention of the dimension and core structure of the Qdots. Finally, the new and superior optical properties of water-soluble QDC make them potentially useful for biological--in addition to light emitting device (LED)--applications.