Simultaneous Sensing of H2 O, D2 O and HOD through Peroxo Vibrations.

Detection of HOD simultaneously in the presence of a mixture of H2 O and D2 O is still an experimental challenge. Till date, there is no literature report of simultaneous detection of H2 O, D2 O and HOD based on vibrational spectra. Herein we report simultaneous quantitative detection of H2 O, D2 O and HOD in the same reaction mixture with the help of bridged polynuclear peroxo complex in absence and presence of Au nanoparticles on the basis of a peroxide vibrational mode in resonance Raman and surface enhanced resonance Raman spectrum. We synthesize bridged polynuclear peroxo complex in different solvent mixture of H2 O and D2 O. Due to the formation of different nature of hydrogen bonding between peroxide and solvent molecules (H2 O, D2 O and HOD), vibrational frequency of peroxo bond is significantly affected. Mixtures of different H2 O and D2 O concentrations produce different HOD concentrations and that lead to different intensities of peaks positioned at 897, 823 and 867 cm-1 indicating H2 O, D2 O and HOD, respectively. The lowest detection limits (LODs) were 0.028 mole fraction of D2 O in H2 O and 0.046 mole faction of H2 O in D2 O. In addition, for the first time the results revealed that the cis-peroxide forms two hydrogen bonds with solvent molecules.

Dimension-Dependent Magnetic Behavior of Manganese-Cysteine Inorganic Complex Nanoparticles.

A cysteine-based complex of Mn2+ led to the formation of nanoparticles in aqueous medium under ambient conditions. The formation and evolution of the nanoparticles in the medium were followed by ultraviolet-visible light (UV-vis) spectroscopy, circular dichroism, and electron spin resonance spectroscopy that also revealed a first-order process. The magnetic properties of the nanoparticles isolated as solid powders exhibited strong crystallite and particle size dependence. At low crystallite size, as well as particle size, the complex nanoparticles showed superparamagnetic behavior similar to other magnetic inorganic nanoparticles. The magnetic nanoparticles were found to undergo a superparamagnetic to ferromagnetic transition, and then to paramagnetic transition with a gradual increase in either their crystallite size or particle size. The discovery of dimension-dependent magnetic property of inorganic complex nanoparticles may usher in a superior option for tuning the magnetic behavior of nanocrystals, depending on the component ligands and metal ions.

Surface Engineering of Quantum Dots for Self-Powered Ultraviolet Photodetection and Information Encryption.

We demonstrate fabrication of photodetectors in the UVC and UVA regions, based on surface engineering of Mn2+-doped ZnS Qdot. Mn2+-doped ZnS Qdot exhibited UVC detection with a responsivity of 0.3 ± 0.02 A·W-1 and detectivity of 1.7 ± 0.2 1011 Jones. Following this, the Qdot was surface modified with 8-hydroxyquinoline 5-sulfonic acid ligand, which resulted in the formation of a bluish green zinc quinolate complex (Zn(QS)2) at the Qdot surface (defined as the quantum dot complex, QDC) exhibiting overall white photoluminescence. The detector developed with QDC as the photoactive material exhibited a responsivity of 0.2 ± 0.02 A·W-1 and detectivity of 1.2 ± 0.2 1011 Jones in the UVA band. This shift in the detection band from UVC in Qdot to UVA in QDC, through the surface complexation mechanism, is a new approach for tuning spectral detection featured in this work. Besides, the self-powered response of both the detectors exhibited attractive photoelectric characteristics. The detectors were incorporated in a portable prototype to show their potential application toward selective UVC and UVA spectral detection. Additionally, the dual-mode emission of the QDC was used for data encryption and decryption.

A Ratiometric and Visual Sensing of Phosphate by White Light Emitting Quantum Dot Complex.

Ratiometric and visual sensing of phosphate by using a white light emitting quantum dot complex (WLE QDC) is reported herein. The WLE QDC comprised of Mn2+-doped ZnS quantum dot (with λem = 585 nm) and surface zinc quinolate (ZnQS2) complex (with λem= 480 nm). The limit of detection was estimated to be of 5.9 nM in the linear range of 16.6-82.6 nM. This was accomplished by monitoring the variations in the photoluminescence color, intensity ratio (I480/I585), chromaticity and hue of the WLE QDC in the presence of phosphate. The high selectivity and sensitivity of WLE QDC toward phosphate was observed. The chemical interaction of ZnQS2 (present in WLE QDC) with phosphate might have led to the observed specificity in photoluminescence changes. The presented WLE QDC was successfully employed for the quantification of phosphate in samples prepared using environmental water and commercial fertilizer.

Enhanced solid-state plasmon catalyzed oxidation and SERS signal in the presence of transition metal cations at the surface of gold nanostructures.

The effect of several metal cations (Mn2+, Co2+, Ni2+, Cu2+ and Zn2+) on the photochemical conversion of 4-aminothiophenol (4-ATP) into 4,4'-dimercaptoazobenzene (DMAB) is probed using surface enhanced Raman scattering (SERS). The coupling reaction is carried out on the surface of Au nanoparticles and Au nanorods using 532 nm and 632.8 nm laser excitations, respectively, in the absence and presence of metal cations. Here, we report that DMAB formation on the surface of Au nanostructures - when carried out in the solid state - is augmented significantly (by a factor of 1.98 to 4.07 and 3.34 to 5.74 for Au nanoparticle and Au nanorod substrates, respectively, and depending on the metal). Furthermore, the SERS signal is also markedly enhanced. Thus, the results underpin a new way of carrying out a photochemical reaction with a higher yield along with a higher SERS signal.

Complexation Reaction-Based Two-Dimensional Luminescent Crystalline Assembly of Atomic Clusters for Recyclable Storage of Oxygen.

In this work, we report storage of oxygen in two-dimensional (2D) crystalline nanosheets comprising luminescent gold nanoclusters (Au NCs). Complexation reaction between Au NCs (stabilized by l-phenylalanine and mercaptopropionic acid) and zinc ions led to the formation of crystalline assembly of Au NCs. The crystalline nature of the assembly of Au NCs was confirmed through transmission electron microscopy (TEM), high-resolution TEM, and selected area electron diffraction (SAED) analysis. Atomic force microscopy (AFM) analysis, in conjunction with field emission scanning electron microscopy (FESEM) analysis, confirmed the 2D nature of the assembly of the Au NCs. The 2D crystalline nanosheets formed out of reaction between Au NCs and Zn2+ were found to be of near-uniform thickness, with an average value of 3.8 ± 1.65 nm. These 2D nanosheets constituting of hierarchically organized Au NCs were further used for reversible storage of oxygen at ambient conditions of 20 °C and 20 bar pressure.

Boron Doped Carbon Dots with Unusually High Photoluminescence Quantum Yield for Ratiometric Intracellular pH Sensing.

Herein we report that boron doping in carbon dots results in increased photoluminescence (PL) quantum yield, which could be used for ratiometric intracellular pH sensing in cancer cell lines. Using a mixture of citric acid monohydrate, thiourea, and boric acid, microwave-assisted synthesis of boron doped blue emitting carbon dots (B-Cdots) with an average size of 3.5±1.0 nm was achieved. For B-Cdots, the maximum quantum yield (QY) was observed to be 25.8 % (11.1 % (w/w) H3 BO3 input concentration), whereas, the same was calculated to be 16.9 % and 11.4 % for Cdots (synthesized from citric acid monohydrate and thiourea only) and P-Cdots (phosphorus doped carbon dots; synthesized using citric acid monohydrate, thiourea and phosphoric acid) (11.1 % (w/w) H3 PO4 input concentration), respectively. The observed luminescence efficiencies as obtained from steady state and time-resolved photoluminescence measurements suggest an alternative emission mechanism due to boron/phosphorus doping in carbon dots. We furthermore demonstrated facile composite formation using B-Cdots and another carbon dots with orange emission in presence of polyvinyl alcohol (PVA), resulting in white light emission (0.31, 0.32; λex 380 nm). The white light emitting composite enabled ratiometric pH sensing in the aqueous medium and showed favorable uptake properties by cancerous cells for intracellular pH sensing as well.

Room-Temperature Delayed Fluorescence of Gold Nanoclusters in Zinc-Mediated Two-Dimensional Crystalline Assembly.

We report that complexation-reaction-mediated two-dimensional crystalline assembly of gold (Au14) nanoclusters (NCs) exhibits room-temperature delayed fluorescence at 605 nm, with an unprecedented long lifetime of 0.5 ms and an exceptionally high quantum yield of 19.1 ± 0.9%. Interestingly, the as-synthesized Au NCs had a very weak delayed fluorescence signal. The enhancement in delayed fluorescence of Au NCs upon formation of assembly has been attributed to the crystallization-induced structural rigidity, which restricted the nonradiative transitions and enhanced the excited-state lifetime. The attainment of crystalline organization was substantiated by electron diffraction analysis. A possible structure was established based on experimental results and computational optimizations. Atomic force microscopy revealed the formation of multilayered two-dimensional nanosheets with thickness of 2.44 ± 0.48 nm.

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.

Gold-Nanocluster-Embedded Mucin Nanoparticles for Photodynamic Therapy and Bioimaging.

Effective delivery of a photosensitizer with the ability to trace its eventual progress forms an important aspect in photodynamic therapy (PDT). Further, the delivery mechanism might require possessing the ability to traverse through the complex mucus barrier that offers retention of therapeutic molecules. In this work, gold nanocluster (Au NC)-embedded mucin nanoparticles were synthesized by a rapid green synthetic procedure for application as nanocarriers and to achieve image-guided PDT. The mucin-based nanocarrier exhibited excellent biocompatibility toward normal cells (HEK 293T). The photosensitizer methylene blue (MB) was loaded onto these Au NC-mucin nanoparticles (NPs). HeLa cancer cells were treated with MB-loaded Au NC-mucin nanoparticles under irradiation of 640 nm light. The cell viability assay revealed that the viability of HeLa cells was reduced to 50% after treatment with MB-loaded Au NC-mucin NPs under 640 nm irradiation. The luminescence exhibited by Au NCs in the nanocarrier was applied for tracking the delivery of MB inside the HeLa cells using confocal microscopy. The flow cytometry assays elucidated the mechanism of cell death.

The nature of binding of quinolate complex on the surface of ZnS quantum dots.

We report that the Z-type binding rather than X-type binding was favored when 8-hydroxyquinoline (HQ) reacted with presynthesized ZnS quantum dots (Qdots) to form surface zinc quinolinate complexes having a preferred stoichiometry of 1 : 2 (surface Zn2+ : HQ). Importantly, the higher solubility in polar solvents and high desorption coefficient (following Langmuir binding isotherm) of HQ-treated ZnS Qdot in DMSO solvent compared with those in methanol clearly indicated the favorable Z-type binding of HQ and thus the formation of surface octahedral ZnQ2 complex. Furthermore, the characteristics peaks in the 1H-nuclear magnetic resonance (NMR) spectrum of the desorbed species and the ligand density calculation of the surface complex (formed due to the reaction between HQ and ZnS Qdot) supported the octahedral ZnQ2 complex formation. Interestingly, the presence of dangling sulphide and the loss of planarity of ZnQ2 complex on the surface of ZnS Qdots (in turn gaining structural rigidity) may be the reasons for the Z-type binding of HQ. The specific binding might be the reason for superior optical properties and thermal stability of the surface ZnQ2 complex compared to the free ZnQ2 complex as such. The results can be considered important towards understanding the coordination chemistry of inorganic complex on the surface of Qdots and thus for their application potential.

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.

Zinc-Coordinated Hierarchical Organization of Ligand-Stabilized Gold Nanoclusters for Chiral Recognition and Separation.

Three-dimensional organization of d- or l-tryptophan and mercaptopropionic acid-stabilized gold nanoclusters has been achieved by complexation of ligands using zinc ions. Powder X-ray diffraction and transmission electron microscopy analyses substantiated the crystalline nature of the assembly of atomic nanoclusters. The hierarchical arrangement of the nanoclusters exhibited superior optical properties (namely, enhanced photoluminescence and excited-state lifetime) as compared to the non-assembled nanoclusters. Furthermore, photoluminescence of the crystalline assembly of nanoclusters served as a visual marker for chiral recognition of d and l enantiomers of tryptophan, with subsequent separation of the corresponding enantiomer. A theoretical structure based on various experimental observations has also been proposed herein. The mechanistic aspect of the chiral separation is proposed to have occurred through attachment of d or l-tryptophan to the coordinatively unsaturated zinc ions, thus forming super complexes. The degree of stabilization of the super complexes is dictated by a "three-point versus two-point" interaction between the enantiomers and the chiral selector.

An Interactive Quantum Dot and Carbon Dot Conjugate for pH-Sensitive and Ratiometric Cu2+ Sensing.

Herein we report the photoinduced electron transfer from Mn2+ -doped ZnS quantum dots (Qdots) to carbon dots (Cdots) in an aqueous dispersion. We also report that the electron transfer was observed for low pH values, at which the oppositely charged nanoparticles (NPs) interacted with each other. Conversely, at higher pH values the NPs were both negatively charged and thus not in contact with each other, so the electron transfer was absent. Steady-state and time-resolved photoluminescence studies revealed that interacting particle conjugates were responsible for the electron transfer. The phenomenon could be used to detect the presence of Cu2+ ions, which preferentially, ratiometrically, and efficiently quenched the luminescence of the Qdots.

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 Embedded Albumin Nanoparticles for Two-Photon Imaging of Cancer Cells Accompanying Drug Delivery.

Gold nanoclusters in albumin nanoparticles (nanovehicles) are used for single-photon and two-photon imaging of cancer cells following the delivery of doxorubicin through the nanovehicle. NIR excitation and emission wavelengths in the biological window (650-900 nm) make the nanovehicle an ideal potential platform for imaging guided drug delivery.

Luminescent carbon dots for logic operations in two phases.

Basic and higher integrated logic operations have been achieved by using luminescent carbon dots through interactions with metal ions and organic molecules in liquid dispersions as well as in the solid phase. The biphasic simple and complex logic systems could be potentially used for various analytical applications as well as for the detection of important elements in diverse environments.

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.

Hierarchical logic structures based on responsive fluorescent gold nanoclusters.

Logic with Gold Clusters. Gold nanoclusters can be used for the development of hierarchical logic structures. This is based on the reversible change in the fluorescence of the clusters at different pH of the medium, its temperature and metal ion concentration dependence.

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.