Virendra Shanker

National Physical Laboratory - India, Old Delhi, NCT, India

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Publications (51)136.42 Total impact

  • RSC Advances 10/2014; 4(98):54936-54947. · 3.71 Impact Factor
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    ABSTRACT: a A new ternary oxide Gd 2 CaZnO 5 having interesting structural, mechanical, electronic and optical properties is synthesized and is studied in detail using density functional theory. The analysis revealed two polymorphs: orthorhombic and tetragonal; the orthorhombic phase was found to be the most stable structure under ambient conditions. A high-pressure (hydrostatic) phase transition to the tetrag-onal phase is predicted at about 4 GPa. This is one of very few reports that depict the phase transition of oxide materials under pressure. The calculated results are in agreement with the X-ray diffraction studies supported by Rietveld analysis. Analysis of the optical properties revealed both polymorphs to be direct-gap semiconductors with low dielectric constants. The calculated elastic constants of both phases satisfy the mechanical stability criteria. It is also identified that the half-filled 4f orbital of Gd induces a strong magnetic spin polarization in the host oxide lattice indicating that the material could be effectively used in versatile applications ranging from biomedical devices to light emitting diodes. Inorganic luminescent materials (also called "phosphors") have become the subject of renewed research and development due to the requirement of efficient color display systems for next generation lighting and display devices. 1–3 Especially, a family of oxide crystals doped with rare-earth ions have become candidate materials for such display devices due to their highly efficient emission properties which are attributed to the sharp f–f transitions in the visible spectrum. 4 For example, ternary oxides defined by R 2 AMO 5 with R = rare-earth, A = alkaline-earth and M = Cu or Zn atoms are found to have the desired luminescent properties for efficient color tunability. 5–10 This family of ternary oxides having large band gaps can serve as good luminescent hosts in accommodating rare-earth ions as dopants and thereby yielding effective luminescence with high quantum efficiency. For instance, the red-emitting Eu 3+ doped R 2 BaZnO 5 is reported to show the quantum efficiency of about 75%. 11,12 Recently, we have synthesized a new red-emitting Gd 2 CaZnO 5 phosphor doped with Eu 3+ , which has a strong absorption in the blue (~465 nm) spectral region. 13 This is significant as the conventional red-emitting Y 2 O 3 :Eu 3+ phosphor does not show any absorp-tion for blue (~465 nm) light. Thus, the Eu 3+ -doped Gd 2 CaZnO 5 phosphor could become a replacement for con-ventional red-emitting phosphors in futuristic white light emitting diodes. There seems to be a dearth of information about the struc-tural and fundamental properties of the R 2 AMO 5 oxides, though their luminescence properties have been well charac-terized. 13 A relationship between the lattice structure and the nature of the constituent atoms forming the lattice has yet to be established. In other words, R 2 BaZnO 5 can crystallize in either orthorhombic or tetragonal phases under ambient conditions depending on the nature of the cations; Dy 2 SrCuO 5 is orthorhombic with a space group of Pnma, (no. 62) 14 and Nd 2 BaZnO 5 is tetragonal with a space group of I4/mcm (no. 140). 15 This has prompted us to undertake a detailed
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    ABSTRACT: a A new ternary oxide Gd 2 CaZnO 5 having interesting structural, mechanical, electronic and optical properties is synthesized and is studied in detail using density functional theory. The analysis revealed two polymorphs: orthorhombic and tetragonal; the orthorhombic phase was found to be the most stable structure under ambient conditions. A high-pressure (hydrostatic) phase transition to the tetrag-onal phase is predicted at about 4 GPa. This is one of very few reports that depict the phase transition of oxide materials under pressure. The calculated results are in agreement with the X-ray diffraction studies supported by Rietveld analysis. Analysis of the optical properties revealed both polymorphs to be direct-gap semiconductors with low dielectric constants. The calculated elastic constants of both phases satisfy the mechanical stability criteria. It is also identified that the half-filled 4f orbital of Gd induces a strong magnetic spin polarization in the host oxide lattice indicating that the material could be effectively used in versatile applications ranging from biomedical devices to light emitting diodes. Inorganic luminescent materials (also called "phosphors") have become the subject of renewed research and development due to the requirement of efficient color display systems for next generation lighting and display devices. 1–3 Especially, a family of oxide crystals doped with rare-earth ions have become candidate materials for such display devices due to their highly efficient emission properties which are attributed to the sharp f–f transitions in the visible spectrum. 4 For example, ternary oxides defined by R 2 AMO 5 with R = rare-earth, A = alkaline-earth and M = Cu or Zn atoms are found to have the desired luminescent properties for efficient color tunability. 5–10 This family of ternary oxides having large band gaps can serve as good luminescent hosts in accommodating rare-earth ions as dopants and thereby yielding effective luminescence with high quantum efficiency. For instance, the red-emitting Eu 3+ doped R 2 BaZnO 5 is reported to show the quantum efficiency of about 75%. 11,12 Recently, we have synthesized a new red-emitting Gd 2 CaZnO 5 phosphor doped with Eu 3+ , which has a strong absorption in the blue (~465 nm) spectral region. 13 This is significant as the conventional red-emitting Y 2 O 3 :Eu 3+ phosphor does not show any absorp-tion for blue (~465 nm) light. Thus, the Eu 3+ -doped Gd 2 CaZnO 5 phosphor could become a replacement for con-ventional red-emitting phosphors in futuristic white light emitting diodes. There seems to be a dearth of information about the struc-tural and fundamental properties of the R 2 AMO 5 oxides, though their luminescence properties have been well charac-terized. 13 A relationship between the lattice structure and the nature of the constituent atoms forming the lattice has yet to be established. In other words, R 2 BaZnO 5 can crystallize in either orthorhombic or tetragonal phases under ambient conditions depending on the nature of the cations; Dy 2 SrCuO 5 is orthorhombic with a space group of Pnma, (no. 62) 14 and Nd 2 BaZnO 5 is tetragonal with a space group of I4/mcm (no. 140). 15 This has prompted us to undertake a detailed
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    ABSTRACT: Blue emitting BaMgAl10O17:Eu2+ (BAM) phosphor is indispensable for Plasma Display panel and lighting because of high luminescence efficiency. However, thermal degradation (annealing in air at 500–600 °C) of BAM (upto ∼30%) remains an intriguing problem for display industry worldwide. In the present study, a systematic approach is pursued to develop highly efficient BAM phosphor that exhibits least degradation, understand the role of Eu2+ site occupancy in such BAM phosphor and encapsulate individual phosphor grains with a shell of silica nanoparticles. The approaches lead to highly efficient BAM:Eu2+ phosphor that showed no degradation against thermal baking (annealing at 500 °C in air) for both UV and VUV radiation under UV and VUV excitation. An optimum solid state chemical route including precursor phases, dopant concentration, and thermal regimes has been evolved to develop BAM. Emission from Eu2+ occupying three different sites is identified with energetically stable anti Beevers Ross as the dominant contributor. Coating by nano sized amorphous silica sol with subsequent sintering lead to uniform silica shell. This nano silica layer also helps to enhance the luminescence from phosphor grains.
    Journal of Luminescence 11/2013; 143:173–180. · 2.14 Impact Factor
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    ABSTRACT: Present work focuses on the effective doping of multi-walled carbon nanotube (CNT) in the ZnS:Cu phosphor nano-composite and thereafter improvement in the optical performance of electroluminescent (EL) device due to increased local field effects. To facilitate doping of CNTs into the phosphor and decrease the operating voltage of the EL device, CNTs were shortened by milling and incorporated effectively using a flux assisted solid-state annealing reaction. Interestingly shorter the length of CNTs used; greater was the local field enhancement, improvement in brightness and efficiencies observed for the EL devices. When the field is applied, adequate charge carriers are tunneled into the ZnS:Cu system through the tips of the CNTs by forming high energy hot spots thus enhancing the local field. The improved device characteristics are due to field enhancement and effective transfer of energy from hot spots to copper activator by impact ionization. The detailed electrical characterization of the novel EL device along with its brightness measurements are presented by considering the hot electron injection model.
    Science of Advanced Materials 10/2013; 6:1-6. · 2.51 Impact Factor
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    ABSTRACT: There has been a stringent demand for blue ($450 to 470 nm) absorbing and red ($611 nm) emitting material systems in phosphor converted white light emitting diodes (WLEDs) available in the market. The conventionally used red-emitting Y 2 O 3 :Eu 3+ phosphor has negligible absorption for blue light produced by GaInN based LED chips. To address this issue, a new red-emitting Gd 2 CaZnO 5 :Eu 3+ (GCZO:Eu 3+) nanophosphor system having exceptionally strong absorption for blue ($465 nm) and significant red ($611 nm) photoluminescence is presented. This is attributed to a dominant f–f transition (5 D 0 / 7 F 2) of Eu 3+ ions, arising due to an efficient energy transfer from the Gd 3+ sites of the host lattice to Eu 3+ ions. The external quantum yield (QY) measured at 465 nm absorption and 611 nm emission revealed that the GCZO:Eu 3+ nanophosphor has better QY of 23% as compared to commercial Y 2 O 3 :Eu 3+ , which is <1%. X-ray diffraction and microscopy observations showed the nanocrystalline nature and slightly elongated morphology of the sample, respectively. While the energy dispersive X-ray analysis identified the chemical constituents of the GCZO:Eu 3+ nanophosphor, the color overlay imaging confirmed the substitution of Eu 3+ for Gd 3+ ions. As seen from the QY statistics it is highly anticipated that the multifold absorption at $465 nm would certainly improve the color rendering properties of existing WLEDs. White light emitting diodes (WLEDs) are the future of solid-state lighting because of their innumerable advantages starting from being energy efficient, economical, compact, durable to having high luminous efficiencies. The widely used WLED employs a yellow-emitting Y 3 Al 5 O 12 :Ce 3+ phosphor integrated to a blue-emitting GaInN diode, the combination of which produces white light. However, some problems creep in regarding the color output of the WLEDs having very high color temperature (>10 000 K) and low color rendering index (<60), the reason being that the red-emitting Y 2 O 3 :Eu 3+ phosphor employed in WLEDs has a negligible absorption in the blue (450–470 nm) region. The excitation spectrum of Y 2 O 3 :Eu 3+ phosphor has only one broad band centred at 248 nm, which is attributed to charge transfer between the Eu 3+ cations and surrounding anions. Its emission spectrum has the most prominent peak at 612 nm corresponding to 5 D 0 / 7 F 2 , as expected from Judd–Ofelt selection rules. 1–8 Hence, there is a need for a new red-emitting phosphor material with strong absorption under blue light to render better color temperature resulting in an ideal WLED source. It is known that inorganic phosphors have been broadly investigated in various aspects of lighting applications such as screens in eld emission displays, plasma display panel thin lm electroluminescent devices and smart displays etc., one of which is WLEDs. 9–13 Among these inorganic phosphors, owing to their chemical inertness and moisture resistance oxide phosphors have surpassed sulphide phosphors 14,15 in phosphor converted WLEDs. Oxide lattices
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    Deepika Yadav, Savvi Mishra, Virendra Shanker, D Haranath
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    ABSTRACT: a b s t r a c t We present a novel methodology to design a hybrid electroluminescent (EL) lamp by embedding carbon nanotubes (CNTs) inside the ZnS:Mn phosphor particles by conventional solid state diffusion technique. By doing so, the phosphor particles exhibited increase in EL brightness and efficiency at low operating voltages (<80 V AC). Interestingly, shorter the length of CNTs used, greater was the field enhancement effect and lower was the operating voltages to glow the EL lamps. The role of CNTs have been identified to form conductive paths inside the ZnS particle thereby triggering EL due to electron injection to lumi-nescent centers (Mn 2+) at nominal voltages. In addition, a detailed electrical characterization of the novel EL lamp along with its spectral energy distribution studies are presented. Ó 2013 Elsevier B.V. All rights reserved.
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    ABSTRACT: Present work focuses on the effective doping of multi-walled carbon nanotube (CNT) in the ZnS:Cu phosphor system and thereafter improvement in the optical performance of electroluminescent (EL) device due to the effect of increased local field. To facilitate doping of CNTs into the phosphor and decrease the operating voltage of the EL device, CNTs were shortened by milling and incorporated effectively using a flux assisted solid-state annealing reaction. Interestingly shorter the length of CNTs used, greater was the local field enhancement, brightness and efficiency observed for the EL devices. When the field is applied, adequate charge carriers are tunneled into the ZnS:Cu system through the tips of the CNTs by forming high energy hot spots thus enhancing the local field. The improved device characteristics are due to field enhancement without flowing of undesired current in the EL device and effective transfer of energy from hot spots to copper activator causing field-ionization. The detailed electrical characterization of the novel EL device along with its brightness measurements are also presented by considering the hot electron injection model.
    05/2013;
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    ABSTRACT: We report a high ($94%) yield synthesis of intrinsic zinc oxide (ZnO) nanocrystal powders having crys-tallite sizes in the range 13–35 nm using a novel gel-incineration method with inexpensive precursor salts and citric acid as chelating agent. The influence of various precursor chemicals on the nanocrystal-lite size, morphology and luminescent properties has been studied in detail. It was identified that the ZnO nanocrystals prepared using organic precursor resulted the smallest crystallite size as compared to inor-ganic precursors. Reaction temperature was optimized to be $900 °C by simultaneous thermogravimetric analysis and differential scanning calorimetry studies. Morphology and microstructure of the ZnO nano-crystals have been studied using a scanning electron microscopy. Analysis of photoluminescence excita-tion and emission spectra enabled us to calculate the band gap energy and defect analysis of as prepared ZnO nanocrystals respectively. The stability of ZnO nanocrystals in water has been verified on time scale and its potential use has been successfully demonstrated for security marker applications. Ó 2013 Elsevier B.V. All rights reserved.
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    ABSTRACT: Manganese-doped sodium zinc phosphate (NaZnPO4:Mn) phosphor with exceptional features having ultra-violet (UV) to visible absorption (300–470 nm), yellow-green (~�543 nm) broad-band photoluminescence (PL), and appreciable color co-ordinates (x =0.39, y=0.58) is reported. It has a crystal structure consisting of discrete PO4 tetrahedra linked by ZnO4 and NaO distorted tetrahedral such that three tetrahedra, one of each kind, share one corner. The presence of UV sensitive Zn-O-Zn bonds and their efficient energy transfer to Mn 2þ ions resulted in brightest PL and external quantum yield of 63% at 418 nm. Our experiment demonstrated the possibility of producing inexpensive white-light emitting devices for future.
    Applied Physics Letters 11/2012; 101:221905. · 3.52 Impact Factor
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    ABSTRACT: We present a novel, cost-effective and facile technique, wherein multi-walled carbon nanotubes (CNTs) were used to transform a photoluminescent material to exhibit stable and efficient electroluminescence (EL) at low voltages. As a case study, a commercially available ZnS:Cu phosphor (P-22G having a quantum yield of 65 ± 5%) was combined with a very low (∼0.01 wt%) concentration of CNTs dispersed in ethanol and its alternating current driven electroluminescence (AC-EL) is demonstrated. The role of CNTs has been understood as a local electric field enhancer and facilitator in the hot carrier injection inside the ZnS crystal to produce EL in the hybrid material. The mechanism of EL is discussed using an internal field emission model, intra-CNT impact excitation and the recombination of electrons and holes through the impurity states.
    Nanotechnology 11/2012; 23(43):435704. · 3.84 Impact Factor
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    ABSTRACT: There has been a stringent demand for blue (~450-470 nm) absorbing and red (~611 nm) emitting material system in phosphor converted white light emitting diodes (WLEDs) available in the market. Conventionally used red-emitting Y2O3:Eu3+phosphor has negligible absorption for blue light produced by GaInN based LED chip. To address this issue, a new red-emitting Gd2CaZnO5:Eu3+ (GCZO:Eu3+) nanophosphor system having exceptionally strong absorption for blue (~465 nm) and significant red (~611 nm) photoluminescence (PL) is presented. This is attributed to a dominant f-f transition (5D0\rightarrow7F2) of Eu3+ ions, aroused due to an efficient energy transfer from the Gd3+ sites of the host lattice to Eu3+ ions. X-ray diffraction and microscopy observations revealed the nanocrystalline nature and a bit elongated morphology of the sample respectively. While the energy dispersive x-ray analysis identified the chemical constituents of the GCZO:Eu3+ nanophosphor, the color overlay imaging confirmed the substitution of Eu3+ for Gd3+ ions. It is highly anticipated that the multifold absorption at ~465 nm would certainly improve the color rendering properties of existing WLEDs.
    08/2012;
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    ABSTRACT: Highly luminescent-paramagnetic nanophosphors have a seminal role in biotechnology and biomedical research due to their potential applications in biolabeling, bioimaging, and drug delivery. Herein, the synthesis of high-quality, ultrafine, europium-doped yttrium oxide nanophosphors (Y(1.9) O(3) :Eu(0.1) (3+) ) using a modified sol-gel technique is reported and in vitro fluorescence imaging studies are demonstrated in human breast cancer cells. These highly luminescent nanophosphors with an average particle size of ≈6 nm provide high-contrast optical imaging and decreased light scattering. In vitro cellular uptake is shown by fluorescence microscopy, which visualizes the characteristic intense hypersensitive red emission of Eu(3+) peaking at 610 nm ((5) D(0) -(7) F(2) ) upon 246 nm UV light excitation. No apparent cytotoxicity is observed. Subsequently, time-resolved emission spectroscopy and SQUID magnetometry measurements demonstrate a photoluminescence decay time in milliseconds and paramagnetic behavior, which assure applications of the nanophosphors in biomedical studies.
    Small 07/2012; 8(19):3028-34. · 7.82 Impact Factor
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    ABSTRACT: In biological fluids, nanoparticles are always surrounded by proteins. As the protein is adsorbed on the surface, the extent of adsorption and the effect on the protein conformation and stability are dependent on the chemical nature, shape, and size of the nanoparticle (NP). We have carried out a detailed investigation on the interaction of bovine serum albumin (BSA) with polyethyleneimine-functionalized ZnO nanoparticles (ZnO-PEI). ZnO-PEI was synthesized using a wet chemical method with a core size of ~3-7 nm (from transmission electron microscopy). The interaction of BSA with ZnO-PEI was examined using a combination of calorimetric, spectroscopic, and computational techniques. The binding was studied by ITC (isothermal titration calorimetry), and the result revealed that the complexation is enthalpy-driven, indicating the possible involvement of electrostatic interaction. To investigate the nature of the interaction and the location of the binding site, a detailed domain-wise surface electrostatic potential calculation was performed using adaptive Poisson-Boltzmann software (APBS). The result shows that the protein surface can bind the nanoparticle. On binding ZnO-PEI, the protein gets destabilized to some extent, as displayed by CD (circular dichroism) and FTIR (Fourier transform infrared) spectroscopy. Chemical and thermal denaturation of BSA, when carried out in the presence of ZnO-PEI, also indicated a small perturbation in the protein structure. A comparison of the enthalpy and entropy components of binding with those derived for the interaction of BSA with ZnO nanoparticles explains the effect of hydrophilic cationic species attached on the NP surface. The effect of the NP surface modification on the structure and stability of BSA would find useful applications in nanobiotechnology.
    Langmuir 07/2012; 28(30):11142-52. · 4.38 Impact Factor
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    ABSTRACT: In the present study, 11-mercaptoundecanoic acid-modified gold nanoparticles (∼7 nm) were conjugated with chloroquine to explore their potential application in cancer therapeutics. The anticancer activity of chloroquine-gold nanoparticle conjugates (GNP-Chl) was demonstrated in MCF-7 breast cancer cells. The MCF-7 cells were treated with different concentrations of GNP-Chl conjugates, and the cell viability was assayed using trypan blue, resulting in an IC(50) value of 30 ± 5 μg/mL. Flow cytometry analysis revealed that the major pathway of cell death was necrosis, which was mediated by autophagy. The drug release kinetics of GNP-Chl conjugates revealed the release of chloroquine at an acidic pH, which was quantitatively estimated using optical absorbance spectroscopy. The nature of stimuli-responsive drug release and the inhibition of cancer cell growth by GNP-Chl conjugates could pave the way for the design of combinatorial therapeutic agents, particularly nanomedicine, for the treatment of cancer.
    Colloids and surfaces B: Biointerfaces 03/2012; 95:195-200. · 4.28 Impact Factor
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    ABSTRACT: For the first time, we report a highly efficient, tunable and bright photoluminescence with external quantum yield up to $34% under 350 nm excitation from hydrophobic SiO 2 gel nanoparticles. This is achieved by suitably modifying the relative surface states of the gel network with trivalent (Al 3+ /Ga 3+) ions and emission from rare-earth (Eu 3+) dopant. This attractive property arises from the intrinsic separation of absorption and the emission energies due to a large Stokes shift and offers opportunities for the design of novel nano-phosphor based white light-emitting devices. Silicates and zeolites are well-known mesoporous materials having a wide range of applications 1,2 from catalysis, gas separation/sorption, and ion exchange to recently developed low-k materials 3,4 or zeolite-dye microlasers. 5 But unfortunately the efficient luminescence from these materials has not been investigated. Photoluminescence is one of the key properties by which the potential and applicability of a phosphor material, an important component of lighting and display technology, could be methodically estimated. 6,7 Current lighting technology uses UV-LEDs with triple-wavelength RGB phosphors or blue LEDs with a yellow-emitting YAG:Ce 3+ phosphor to produce white light. 8 In the latter case, there is a serious demand for the exploitation of advanced and novel yellow-emitting phosphors. At present, mixed luminescence from various sources has always been the route to white light, without other possibilities being advocated. Similarly single activator photoluminescence in a host lattice that emits multiple colours covering the entire visible part of the electro-magnetic spectrum has not yet been observed. Compared to those inorganic materials added with dye or transition metal ion activators, 9,10 intrinsic photoluminescence from a meso-porous silica (SiO 2) gel is noticeably weak. 11 Thereafter, some groups started serious efforts on sol–gel synthesis of optical silica glasses doped with rare-earth elements for lasing applications, but failed due to their negligible photoluminescence. 12 To overcome this, from Knobbe in 1993 to Boilot in 2010 many groups have been working on incorporation of rare-earth complexes 13 (instead of rare-earth ions) or luminescent nanocrystals into core–shell SiO 2 nanostructures, 14 and demonstrated many advanced practical applications. Significant research into the structural evolution of silica gels has been done in the last 20 years. 15 But almost no effort has been made to investigate and improve the intrinsic rare-earth ion photoluminescence from SiO 2 gel networks. 16 Since the optical properties of doped silica systems depend intimately on the local structure and bonding of dopant cations, a detailed understanding of these factors is important from a device engineering perspective. The introduction of the dopant acts as a perturbation of this well-studied system. Eu 3+ was selected as a representative rare-earth ion because its unique fluorescence prop-erties make it an ideal probe of local structure determination. As a research goal to pursue, we have successfully prepared highly lumi-nous hydrophobic gels and powders that have distinctive and tunable photoluminescence properties using Eu 3+ as a single activator. Herein, we report a new class of highly luminescent gels that individually emit from violet to deep-red luminescence with exposure to 350 nm UV light (Fig. 3). The advantages of the present protocol are numerous such as: 1) high surface-to-volume ratios with effective prevention of further aggregation of the nanoparticles (NPs), so as to retain a high luminescence yield; 2) the profitability of size-quantized, nanometre-sized hydrophobic SiO 2 particles with higher redox potential, 17 which in turn enhances the charge-transfer rates between the SiO 2 network and the activator (Eu 3+) ion; sometimes reducing the activator ion by host–activator interactions; 3) easier preparation than those obtain-able with usual NPs, which need further functionalization by silanes; 4) good dispersity and useful dimensions for possible coating over many substrates. In addition, we demonstrate that the SiO 2 :Eu 3+ NPs are excellent phosphors with high luminescence yields which could be useful for designing multi-colour-emitting novel light emitting diode (LED) structures including white LEDs. Highly luminescent, mesoporous and hydrophobic SiO 2 :Eu 3+ gels were prepared by a modified sol–gel technique 18 in the presence of a trivalent co-activator and thermolyzing the gels using supercritical solvothermal reaction in an autoclave (see ESI Fig. S1). The role of trivalent co-doping is to avoid clustering 19 and effectively disperse/ isolate Eu 3+ ions in the SiO 2 matrix; and the supercritical treatment increases the thermal diffusion of the activator ions and creates short-range crystallinity in the SiO 2 gel network. 16 The fine structural details of SiO 2 :Eu 3+ NPs were investigated by using electron microscopy. A typical scanning electron microscope
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    ABSTRACT: Graphene is an intriguing two-dimensional material, which could be modified for achieving tunable properties with many applications. Photoluminescence of graphene due to plasmonic emission is well-known, however, attempts to develop strong luminescent graphene have been difficult. Synthesis of a graphene-based material with a dual optical functionality, namely quenching the fluorescence of organic dyes while maintaining its own self-luminescence, is an interesting and challenging proposition. Here, we demonstrate this optical bifunctionality in a lattice-modified luminescent graphene, where europium(III) cations are complexed with graphene through oxygen functionalities. After excitation at 314 nm, a hypersensitive red emission is observed at 614 and 618 nm showing the complexation of europium(III) with graphene. We demonstrate dual functionality of this graphene by the quenching of luminescence of Rhodamine-B while displaying its own hypersensitive red emission. The decay lifetime observed through the time-resolved spectroscopy confirms its potential for applications in biosensing as well as optoelectronics.
    Nano Letters 11/2011; 11(12):5227-33. · 13.03 Impact Factor
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    ABSTRACT: Designing of bio-compatible nanomagnets with multiple functionalities receives immense scientific attention due to their potential applications in bio-labeling, medical diagnosis and treatment. Here we report the synthesis of Nickel (Ni) incorporated single-walled carbon nanotube (SWCNT) hybrid and bio-compatible bundles having interesting magnetic and photoluminescence (PL) properties. The SWCNT exhibits a high-crystallinity and it has an average diameter of ∼1.7 nm. Ni particles of 10-20 nm were incorporated within the SWCNT bundles. These hybrid bundles exhibit PL and it is attributed to the presence of delocalized π electrons and their recombination at the defective sites of SWCNT. Magnetic characterization revealed that the SWCNT/Ni hybrid bundle possesses a high (50 Oe) coercivity compared to bulk Ni and a long range ferromagnetic ordering at room temperature. MTT-assay has been conducted to study the cytotoxicity of these hybrid nanostructures.
    Journal of Colloid and Interface Science 10/2011; 362(2):311-6. · 3.55 Impact Factor
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    ABSTRACT: Nanoparticles exposed to biofluids become coated with proteins, thus making protein-nanoparticle interactions of particular interest. The consequence on protein conformation and activity depends upon the extent of protein adsorption on the nanoparticle surface. We report the interaction of bovine serum albumin (BSA) with gold nanostructures, particularly gold nanoparticles (GNP) and gold nanorods (GNR). The difference in the geometry and surface properties of nanoparticles is manifested during complexation in terms of different binding modes, structural changes, thermodynamic parameters, and the activity of proteins. BSA is found to retain native-like structure and properties upon enthalpy-driven BSA-GNP complexation. On the contrary, the entropically favored BSA-GNR complexation leads to substantial loss in protein secondary and tertiary structures with the release of a large amount of bound water, as indicated by isothermal calorimetry (ITC), circular dichroism (CD), and Fourier transform infrared (FTIR) and fluorescence spectroscopies. The esterase activity assay demonstrated a greater loss in BSA activity after complexation with GNR, whereas the original activity is retained in the presence of GNP. The formation of large assemblies (aggregates) and reduced average lifetime, as evidenced from dynamic light scattering and fluorescence decay measurements, respectively, suggest that GNR induces protein unfolding at its surface. The effect of temperature on the CD spectra of BSA-GNP was found to be similar to that of pristine BSA, whereas BSA-GNR shows distortion in CD spectra at lower wavelengths, strengthening the perception of protein unfolding. High binding constant and entropy change for BSA-GNR complexation determined by ITC are consistent with large surfacial interaction that may lead to protein unfolding. The present work highlights the differential response of a protein depending on the nature of the nanostructure and its surface chemistry, which need to be modulated for controlling the biological responses of nanostructures for their potential biomedical applications.
    Langmuir 06/2011; 27(12):7722-31. · 4.38 Impact Factor

Publication Stats

166 Citations
136.42 Total Impact Points

Institutions

  • 2003–2013
    • National Physical Laboratory - India
      • Division of Physics of Energy Harvesting (NPL)
      Old Delhi, NCT, India
  • 2011–2012
    • Bose Institute
      • Department of Biochemistry
      Calcutta, Bengal, India
    • Rice University
      • Department of Mechanical Engineering and Materials Science
      Houston, TX, United States
  • 2009–2011
    • Council of Scientific and Industrial Research (CSIR), New Delhi
      New Dilli, NCT, India
  • 2010
    • Council For Scientific And Industrial Research
      New Dilli, NCT, India