Boron Precursor-Dependent Evolution of Differently Emitting Carbon Dots.

Attention has been directed toward electron-deficient boron doping in carbon dots (CDs) with the expectation of revealing new photophysical aspects in accordance with varying amounts of boron content. It has been emphatically shown that boron uptake in CDs varies with different boron precursors evolving altered emissive CDs. Boron doping in CDs causes definite surface defect due to the generation of electron-deficient states. Modified hydrothermal treatment of a mixture of ascorbic acid (AA) and different boron precursor compounds (borax/boric acid/sodium borate/sodium borohydride) produces different kinds of boron-doped CDs (BCDs). These BCDs (<6 nm) differ in size, emission maxima (∼15 nm), and fluorescence intensity but carry unchanged excitation maxima (365 nm). These differences are related to the nature of boron precursor compounds. The most fluorescing BCD (quantum yield ≈ 5%) is identified from the borax-mediated reaction and is used for the detection of Fe(III) on a nanomolar level in water via the fluorescence "Turn Off" phenomenon. Again, Fe(III)-infested CD solution regains its lost fluorescence, with AA paving the way for nanomolar level AA detection from the same pot. The proposed method has been tactfully made interference free for the quantitative measure of Fe(III) and AA in real samples. Furthermore, new photophysical properties of the CDs with variable boron contents supplement information that is hitherto unknown. Theoretical calculations also justify the observed optical behavior of the as-synthesized BCDs. The calculations describe the variable amount of boron doping-related huge charge polarization within the carbon surface, leading to the formation of surface defects. Thus, subsequent electronic transition-related red shift in the absorption spectrum authenticates experimental findings.

Metal Bromide Controlled Interfacial Aromatization Reaction for Shape-Selective Synthesis of Palladium Nanostructures with Efficient Catalytic Performances.

Herein, the effect of diverse metal bromides for the shape evolution of palladium nanostructures (Pd NS) has been demonstrated. Aromaticity-driven reduction of bromopalladate(II) is optimized to reproducibly obtain different Pd NS at the water/organic layer interface. In this soft interfacial strategy, a redox potential driven reaction has been performed, forming the thermodynamically more stable (>10(4) -fold) PdBr4 (2-) precursor from PdCl4 (2-) by adding extra metal bromides. In the process, the reductant, Hantzsch dihydropyridine ester (DHPE), is aromatized. Interestingly, alkali metal bromides devoid of coordination propensity exclusively evolve Pd nanowires (Pd NWs), whereas in the case of transition metal bromides the metal ions engage the 'N' donor of DHPE at the interface, making the redox reaction sluggish. Hence, controlled Pd nanoparticles growth is observed, which evolves Pd broccolis (Pd NBRs) and Pd nanorods (Pd NRs) at the interface in the presence of NiBr2 and CuBr2 , respectively, in the aqueous solution. Thus, the effect of diverse metal bromides in the reaction mixture for tailor-made growth of the various Pd NS is reported. Among the as-synthesized materials, the Pd NWs stand to be superior catalysts and their efficiency is almost 6 and 2.5 times higher than commercial 20 % Pd/C in the electrooxidation of ethanol and Cr(VI) reduction reaction by formic acid, respectively.

Intriguing cysteine induced improvement of the emissive property of carbon dots with sensing applications.

A simple fluorometric technique has been adopted for cysteine (Cys) sensing in alkaline medium down to the nM level. The huge fluorescent signal of the solution is a consequence of fluorescent carbon dots (CDs) produced in situ from modified hydrothermal (MHT) reaction between Cys and dopamine (DA). It has been observed that the inherent fluorescence of DA is drastically quenched in alkaline solution. Cys can selectively rescue the fluorescence of DA. Thus, Cys determination in a straightforward way, but only to a micro molar (10(-7) M i.e. 0.1 µM) level is possible through such fluorescence enhancement. Sensitive Cys determination remains associated with the in situ generated CDs, but the external addition of pre-formed CDs to Cys solution fails miserably towards Cys detection. However, CDs prepared from the Cys-DA system in alkaline solution admirably increase the limit of detection (LOD) of Cys at least two orders higher (10(-9) M) than that observed without hydrothermal technique i.e., without CDs. This method finds applications for Cys determination in biological samples and pharmaceutical preparations.

Mn oxide-silver composite nanowires for improved thermal stability, SERS and electrical conductivity.

Redox transformation reaction between aqueous AgNO3 and Mn(CH3COO)2 at low temperature (∼80 °C) has been adopted for industrial-scale production of uniform Ag-MnOOH composite nanowires for the first time. Varying amounts of incorporated Ag in the composite retain the 1D morphology of the composite. Nanowires upon annealing evolve Ag-MnO2 nanocomposites, once again with the retention of the parental morphology. Just 4 % of silver incorporation in the composite demonstrates metal-like conducting performance from the corresponding semiconducting material. Transition of MnO2 to Mn2O3 to Mn3O4 takes place upon heat treatment in relation to successive increase in Ag concentrations in the nanowires. The composites offer resistance to the observed oxide transformation. This is evidenced from the progressive increase in transition temperature. In situ Raman, ex situ thermal and XRD analysis corroborate the fact. The composite with 12 % Ag offers resistance to the transformation of MnO2, which is also verified from laser heating. Importantly, Ag nanoparticle incorporation is proved to offer a thermally stable and better surface enhanced Raman scattering (SERS) platform than the individual components. Both the Ag-MnOOH and Ag-MnO2 nanocomposites with 8 atomic % Ag show the best SERS enhancement (enhancement factor ∼10(10)). The observed enhancement relates to charge transfer as well as electromagnetic effects.

Intriguing manipulation of metal-enhanced fluorescence for the detection of Cu(II) and cysteine.

Commercially available salicylaldehyde, in alkaline medium, exhibits strong fluorescence after one hour of UV exposure in the presence of Ag(I) . The phenolic group of salicylaldehyde is converted into the quinone form under alkaline conditions in the presence of AgNO3 , resulting in aggregated Ag(0), which causes approximately 250 times fluorescence enhancement of the in situ produced quinone. Such high silver-enhanced-fluorescence (SEF) is selectively quenched by cysteine, arginine, histidine, methionine, and tryptophan. In contrast to the other amino acids, ageing brings selectivity of the cysteine-induced quenching effect. Interestingly, Cu(II) is found to be the only metal ion that exclusively regenerates the lost fluorescence. Thus, quenching and recovery of fluorescence (Turn Off/On) can be used for the selective and sensitive detection of cysteine as well as Cu(II) ions in one pot. Alteration of the electric field density around the fluorophore (lightening rod effect) and scattering/absorption cross-section have been proposed to account for the Off/On fluorescence.

Redox-responsive copper(I) metallogel: a metal-organic hybrid sorbent for reductive removal of chromium(VI) from aqueous solution.

Herein, we report a new strategy to remove toxic Cr(VI) ion from aqueous solution using metal-organic hybrid gel as sorbent. The gel could be easily synthesized from the commercially available organic ligand 2-mercaptobenzimidazole (2-MBIm) and copper(II) chloride in alcoholic medium. The synthesis involves one-electron reduction of Cu(II) to Cu(I) by 2-MBIm, and then gel formation is triggered through Cu(I)-ligand coordination and extensive hydrogen-bonding interactions involving the "-NH" protons (of 2-MBIm ligand), solvent molecules, and chloride ions. The gel shows entangled network morphology. Different microanalytical techniques (FTIR, powder XRD, FESEM, TEM, rheology etc.) have been employed for complete characterizations of the gel sample. Both Cu(I) (in situ formed) and Cl(-) ions trigger the gel formation as demonstrated from systematic chemical analyses. The gel also exhibits its stimuli-responsive behavior toward different interfering chemical parameters (pH, selective metal ions and anions, selective complexing agents, etc.). Finally the gel shows its redox-responsive nature owing to the distinguished presence of Cu(I) metal centers throughout its structural backbone. And this indeed helps in the effective removal of Cr(VI) ions from aqueous solution. Reduction of Cr(VI) to Cr(III) ions and its subsequent sorption take place in the gel matrix. The reductive removal of Cr(VI) has been quantitatively interpreted through a set of different kinetic measurements/models, and the removal capacity of the gel matrix has been observed to be ∼331 mg g(-1) at pH ∼ 2.7, which is admirably higher than the commonly used adsorbents. However, the capacity decreases with the increase in pH of the solution. The overall removal mechanism has been clearly demonstrated. Again, the gel could also be recycled. Thus, the low-cost and large-scale fabrication of the redox-active metallogel makes it an efficient matrix for the toxic ion removal and hence indicates the high promise of this new generation hybrid material for environmental pollution abatement.

Morphology controlled synthesis of SnS2 nanomaterial for promoting photocatalytic reduction of aqueous Cr(VI) under visible light.

A mild, template free protocol has been demonstrated for SnS2 nanoflake formation at the gram level from SnCl2 and thioacetamide (TAA). The SnS2 nanoflakes congregate to nanoflowers and nanoyarns with variable TAA concentrations. BET measurements reveal that the synthesized nanomaterials are highly porous having very high surface area, and the nanoflower has higher surface area than the nanoyarn. The synthesized nanomaterial finds application for promoting photoreduction of extremely toxic and lethal Cr(VI) under visible light irradiation due to their porous nature. The nanoflowers photocatalyst is proved to be superior to nanoyarn due to the increased surface area and higher pore volume. It was also inferred that increased pH decreased the reaction rate. The present result suggests that the morphology-dependent photoreduction of Cr(VI) by SnS2 nanomaterial under visible light exposure will endorse a new technique for harvesting energy and purification of wastewater.

Selective dopamine chemosensing using silver-enhanced fluorescence.

Condensation product of salicylaldehyde and 1,3 propylenediamine becomes a diiminic Schiff base, which is oxidized by AgNO3 in alkaline solution, and in turn, stable Ag(0) is produced at room temperature. Under this condition, the solution exhibits intense silver nanoparticle enhanced fluorescence (SEF) with the λ(em) at 412 nm. Dopamine is selectively detected down to the nanomolar level via exclusive fluorescence quenching of the SEF. Dopamine-infested solution regains the fluorescence [i.e., SEF in the presence of Hg(II) ions]. Thus dopamine and Hg(II) in succession demonstrate "turn off/on" fluorescence due to the change in the scattering cross section of Ag(0) and gives a quantitative measure of dopamine in real samples. The proposed method is free from interferences of common biocompetitors.

Photoproduced fluorescent Au(I)@(Ag2/Ag3)-thiolate giant cluster: an intriguing sensing platform for DMSO and Pb(II).

Synergistic evolution of fluorescent Au(I)@(Ag2/Ag3)-thiolate core-shell particles has been made possible under the Sun in presence of the respective precursor coinage metal compounds and glutathione (GSH). The green chemically synthesized fluorescent clusters are giant (∼600 nm) in size and robust. Among all the common water miscible solvents, exclusively DMSO exhibits selective fluorescence quenching (Turn Off) because of the removal of GSH from the giant cluster. Again, only Pb(II) ion brings back the lost fluorescence (Turn On) leaving aside all other metal ions. This happens owing to the strong affinity of the sulfur donor of DMSO for Pb(II). Thus, employing the aqueous solution containing the giant cluster, we can detect DMSO contamination in water bodies at trace level. Besides, a selective sensing platform has emerged out for Pb(II) ion with a detection limit of 14 × 10(-8) M. Pb(II) induced fluorescence recovery is again vanished by I(-) implying a promising route to sense I(-) ion.

Green synthesis of highly fluorescent Au(I)@Ag2/Ag3-thiolate core-shell particles for selective detection of cysteine and Pb(II).

Giant Au(I)@Ag2/Ag3-thiolate clusters with strong fluorescence (λex 400 nm, λem 564 nm, and quantum yield 8.3%) have been prepared in aqueous medium from glutathione and corresponding precursor salts at neutral pH under sunlight. An intriguing synergism between Au(I)core and Ag(0)shell imparts long-term stability to the fluorescent giant clusters (synthesized with a chemically green method) due to drifting of electron density towards core from shell. The strong fluorescence is selectively damped by cysteine (Cys) leaving aside all other essential amino acids ("Turn Off"). This quenched fluorescence is restored again on introducing Pb(II) ions in the system ("Turn On"). No other metal ion can cause such a recovery of the lost fluorescence. Such "Turn Off" and "Turn On" fluorescence helps in the design of a selective Cys as well as Pb(II) sensor in one pot. Detection of Cys and Pb(II) down to 5 × 10(-8) M and 15 × 10(-8) M, respectively, is possible following the present strategy.

Aggregation of nitroaniline in tetrahydrofuran through intriguing H-bond formation by sodium borohydride.

The participation of sodium borohydride (NaBH4) in hydrogen bonding interactions and transient anion radical formation has been proved. Thus, the properties of NaBH4 are extended beyond the purview of its normal reducing capability and nucleophilic property. It is reported that ortho- and para-nitroanilines (NAs) form stable aggregates only in tetrahydrofuran (THF) in the presence of NaBH4 and unprecedented orange/red colorations are observed. The same recipe with nitrobenzene instead of nitroanilines (NAs) in the presence of NaBH4 evolves a transient rose red solution due to the formation of a highly fluorescent anion radical. Spectroscopic studies (UV-vis, fluorescence, RLS, Raman, NMR etc.) as well as theoretical calculations supplement the J-aggregate formation of NAs due to extensive hydrogen bonding. This is the first report where BH4(-) in THF has been shown to support such an aggregation process through H-bonding. It is further confirmed that stable intermolecular hydrogen bond-induced aggregation requires a geometrical match in both the nitro- and amino-functionalities attached to the phenyl ring with proper geometry. On the contrary, meta-nitroaniline remains as the odd man out and does not take part in such aggregation. Surprisingly, Au nanoparticles dismantle the J-aggregates of NA in THF. Explicit hydrogen bond formation in NA has been confirmed experimentally considering its promising applications in different fields including non-linear optics.

Synthesis of highly fluorescent silver clusters on gold(I) surface.

Evolution of fluorescence from a giant core-shell particle is new and synergistic, which requires both gold and silver ions in an appropriate ratio in glutathione (GSH) solution. The formation of highly fluorescent Ag(2)/Ag(3) clusters on the surface of Au(I) assembly results in giant Au(I)(core)-Ag(0)(shell) water-soluble microparticles (~500 nm). Here, Au(I) acts as the template for the generation of fluorescent Ag clusters. The presence of gold under the synthetic strategy is selective, and no other metal supports such synergistic evolution. The core-shell particle exhibits stable and static emission (emission maximum, 565 nm; quantum yield, 4.6%; and stroke shift, 179 nm) with an average lifetime of ~25 ns. The drift of electron density by the Au(I) core presumably enhances the fluorescence. The positively charged core offers unprecedented long-term stability to the microparticles in aqueous GSH solution.

Fabrication of porous ß-Co(OH)2 architecture at room temperature: a high performance supercapacitor.

A facile, cost-effective, surfactant-free chemical route has been demonstrated for the fabrication of porous ß-Co(OH)2 hierarchical nanostructure in gram level simply by adopting cobalt acetate as a precursor salt and ethanolamine as a hydrolyzing agent at room temperature. A couple of different morphologies of ß-Co(OH)2 have been distinctly identified by varying the mole ratio of the precursor and hydrolyzing agent. The cyclic voltammetry measurements on ß-Co(OH)2 displayed significantly high capacitance. The specific capacitance obtained from charge-discharge measurements made at a discharge current of 1 A/g is 416 F/g for the Co(OH)2 sample obtained at room temperature. The charge-discharge stability measurements indicate retention of specific capacitance about 93% after 500 continuous charge-discharge cycles at a current density of 1 A g(-1). The capacitive behavior of the other synthesized morphology was also accounted. The nanoflower-shaped porous ß-Co(OH)2 with a characteristic three-dimensional architecture accompanied highest pore volume which made it promising electrode material for supercapacitor application. The porous nanostructures accompanied by high surface area facilitates the contact and transport of electrolyte, providing longer electron pathways and therefore giving rise to highest capacitance in nanoflower morphology. From a broad view, this study reveals a low-temperature synthetic route of ß-Co(OH)2 of various morphologies, qualifying it as supercapacitor electrode material.

Green synthesis and reversible dispersion of a giant fluorescent cluster in solid and liquid phase.

A water-soluble highly fluorescent silver cluster on Au(I) surface has been synthesized with green chemistry under sunlight. The evolution of the silver cluster is synergistic, demanding gold and glutathione. The fluorescent Au(I)core-Ag(0)shell particles are huge in size and at the same time they are robust. That is why they become a deliverable fluorescing solid upon drying. Again, the giant particles run into common water miscible solvents. As a result, the fluorescence intensity increases to a great extent without any alteration of emission maxima. In this respect, acetone has been found to be the best-suited solvent. To have a universal applicability of the fluorescent clusters, the particles in the water pool of a reverse micelle have been prepared to transfer the particles into different water immiscible solvents. The comparatively lower fluorescence intensity of the particles has been ascribed to a space confinement effect. Finally, giant-cluster-impregnated yellow-orange fluorescent polymer film and fluorescent cotton wool, as well as paper substrate, have been prepared. The antibacterial activity of the fluorescent particle has also been tested involving modified cotton wool and paper substrate for Gram-negative and -positive Escherichia coli and Staphylococcus aureus, respectively.

Large-scale solid-state synthesis of Sn-SnO2 nanoparticles from layered SnO by sunlight: a material for dye degradation in water by photocatalytic reaction.

Phase pure spherical Sn-SnO2 nanoparticles (∼ 50 nm) in gram level have been synthesized from well-defined SnO microplates (∼ 2.0 µm) using focused solar irradiation. The first step of the reaction involves simple stirring of a strong NaOH solution with fine SnCl2·2H2O powder. Precipitated blue black microplates of SnO are finally transformed into high band gap Sn-SnO2 nanoparticles with sunlight. During the solid-state photodecomposition of microplates, spherical SnO2 nanoparticles along with tiny Sn(0) particles are evolved simultaneously. Tiny Sn(0) particles, improved surface area, stability toward adverse environmental conditions, and inherited negative surface charge electrostatically stabilize the Sn-SnO2 particle rendering it excellent water dispersible. The presence of Sn(0) nanoparticles in spherical SnO2 nanoparticles improves the charge (electrons and holes) separation efficiency. Then, the as-prepared particles selectively invite cationic dye molecules to the particle surface due to negative surface charge and degrade the dyes at a faster rate under UV light.

Diiminic Schiff bases: an intriguing class of compounds for a copper-nanoparticle-induced fluorescence study.

The condensation products of salicylaldehyde and different diamines constitute an important class of diiminic Schiff bases (DSBs). This class of compounds has been rediscovered as reducing as well as capping agents under UV irradiation. UV irradiation of alkaline DSB solutions in the presence of water-soluble copper salts has been employed to produce copper nanoparticles (CuNPs). Intriguing CuNP-stimulated fluorescence behavior of the solution has been observed. Depending upon the nature of the spacer in between two iminic bonds, fluorescence enhancement or quenching is observed. Such surprising fluorescence contrast has been ascribed to far-field radiation and lossy surface waves.

Selective and sensitive recognition of Cu2+ in an aqueous medium: a surface-enhanced Raman scattering (SERS)-based analysis with a low-cost Raman reporter.

In the present study, surface-enhanced Raman spectra of a bifunctional Raman reporter, 2-mercaptobenzimidazole, has been found to be responsive exclusively towards Cu(2+) ions while the reporter remains anchored on the Au nanoparticle surface. Thus a specific Cu(2+)-ion-detection protocol emerges. The simplicity, sensitivity, and reproducibility of the method allow routine and quantitative detection of Cu(2+) ions. An interference study involving a wide number of other metal ions shows the procedure to be uniquely selective and analytically rigorous. A theoretical study was carried out to corroborate the experimental results. Finally, the method is promising for real-time assessment of Cu(2+) ions in aqueous samples and also has the ability to discriminate Cu(I) and Cu(II) ions in solution.

Electrochemical aspects of coinage metal nanoparticles for catalysis and spectroscopy.

Down scaling bulk materials can cause colloidal systems to evolve into microscopically dispersed insoluble particles. Herein, we describe the interesting applications of coinage metal nanoparticles (MNPs) as colloid dispersions especially gold and silver. The rich plasmon bands of gold and silver in the visible range are elaborated using the plasmon resonance and redox potential values of grown metal microelectrode (GME). The gradation of their standard reduction potential values (E 0), as evaluated from the Gibbs free energy change for bulk metal, is ascribed to the variation in their size. Also, the effect of nucleophiles in the electrolytic cell with metal nanoparticles (MNPs) is described. The nucleophile-guided reduction potential value is considered, which is applicable even for bulk noble metals. Typically, a low value (as low as E 0 = +0.40 V) causes the oxidation of metals at the O2 (air)/H2O interface. Under this condition, the oxidation of noble metal particles and dissolution of the noble metal in water are demonstrated. Thus, metal dissolution as a function of the size of metal nanoparticles becomes eventful and demonstrable with the addition of a surfactant to the solution. Interestingly, the reversal of the nobility of gold (Au) and silver (Ag) microelectrodes at the water/electrode interface is confirmed from the evolution of normal and inverted 'core-shell' structures, exploiting visible spectrophotometry and surface-enhanced Raman scattering (SERS) analysis. Subsequently, the effect of the size, shape, and facet- and support-selective catalysis of gold nanoparticles (NPs) and the effect of incident photons on current conversion without an applied potential are briefly discussed. Finally, the synergistic effect of the emissive behaviour of gold and silver clusters is productively exploited.

A galvanic replacement reaction and the Kirkendall effect in the room-temperature synthesis of tubular NiSe2: a nanozyme catalyst with peroxidase-like activity.

The synthesis of nickel selenide nanostructures under ambient conditions remains fascinating, aesthetically beautiful, and energy efficient, as most reported methods involve high-temperature techniques. In this work, we have reported the wet chemical synthesis of NiSe2 nanostructures at room temperature. The approach starts with nickel nanowires (NiNW) and selenous acid as active ingredients. Upon the incubation of NiNW in selenous acid, zero-valent metallic nickel gradually oxidised with the successive deposition of nano-selenium, a reductive product, over the pristine NiNW surface. This thermodynamically controlled galvanic replacement reaction (GRR) is favourably governed by the reduction potential values of the Ni2+/Ni and SeO32-/Se redox couples. Moreover, the selenium nanoparticles over the NiNW surface and the oxidized Ni2+ underneath then interplay during inward and outward diffusion. The different diffusivities of the elements/ions cause the generation of void interiors, thus resulting in tubular nanostructures. Therefore, both the GRR and nanoscale Kirkendall effect jointly remain engaged, resulting in the formation of hollow NiSe2 nanotubular structures. Then, we ably exploit this heterogeneous chalcogenide nanostructure material as an artificial enzyme for peroxidase mimics. This provides a method for the naked-eye detection of peroxide in solution. The peroxidase activity was selectively restrained in the presence of glutathione. Hence, a colourimetric assay was simultaneously developed for the selective detection of this biothiol. The intrinsic nanozyme activity of the substrate is hitherto unknown and can, hence, be explored further with other nanostructured nickel selenide materials.

Redox-switchable superhydrophobic silver composite.

Unique packaging of Ag(2)O on the surface of polycrystalline AgCl allows fabrication of a new useful, superhydrophobic composite material. This pure inorganic material with surface porosity of submicrometer aperture size fabricates air pockets, which make the composite material superhydrophobic. The new material behaves like lotus leaves, butterfly wings, or water strider's leg in relation to superhydrophobicity. Visible light induces photoreduction of solid Ag(2)O surface layer and generates Ag(0), making the composite surface superhydrophilic. Reoxidation of Ag(0) on the composite surface gives back the hydrophobicity that represents the redox-switchable wetting property of the material.