Luminescent Materials and Devices Group
Development of nanophosphors with enhanced down and up-conversion luminescence efficiency for solar cells, synthesis of doped nanocrystals and quantum dots for enhanced blue luminescence for display devices, core-shell nanophosphors for solar spectrum modification, novel nanophosphors for LEDs and luminomagnetic nanophosphors for bio-related applications are the current thrust of the group. Some of the group’s activities are highlighted below:
Highly Luminescent–Paramagnetic Nanophosphor Probes for In Vitro High-Contrast Imaging of Human Breast Cancer Cells
Luminomagnetic nanomaterials that possess desirable properties in a single entity have attracted broad interest in recent years. For instance, nanomaterials with both luminescent and magnetic properties, either paramagnetic or ferromagnetic, can be used in a wide range of applications in biological systems such as bioimaging, magnetic resonance imaging (MRI), diagnostics, and therapeutics. Such nanomaterials can serve as luminescent markers, while at the same time being controlled by an external magnetic field. Fluorescence imaging through luminescent nanoparticles is an ideal, noninvasive tool to visualize biological processes at the molecular and cellular level, by probing specific biomolecules and their intermolecular interactions. Nanoparticles have emerged as one of the most promising tools for sensing, manipulation, and detection of biological systems. Their biocompatibility or cytotoxicity primarily depends on their size as well as molecular design. Significant research efforts have been carried out towards developing nanomaterial-based biolabels with controlled size and shape as effective alternatives to organic fluorophores to overcome some of their limitations. Among them, quantum dots have been widely explored for biological applications and are commercially available. Quantum dots used for biolabeling have a semiconductor nanocrystal core (CdSe or CdSeTe) that determines their color and an inorganic shell (ZnS) that improves brightness and stability apart from other constituents. However, quantum dots possess some major disadvantages such as fluctuating photoluminescence (PL) intensity, low quantum efficiency, and biotoxicity. Hence, the rapidly growing healthcare industry is now looking for the development of stable, intense, nontoxic, photoluminescent, biolabel nanomaterials for applications such as diagnostics, immunoassay, fluorescence imaging, and DNA recognition. Rare-earth nanophosphors, with sharp emission bands, low toxicity, and high selectivity in their emitting wavelength, relatively higher quantum yields (greater than 61%), lower photobleaching potential, and long luminescence lifetimes (up to several microseconds), are ideal for alternative biological fluorescence labels through an appropriate choice of color-center element. Until recently, europium-based yttrium oxide (Y2O3:Eu3+) has been a widely used red phosphor in a variety of optical display applications. Nowadays it has become a focus of research among many groups owing to its low toxicity, strong luminescence intensity, high quantum yield, high chemical stability, vacuum-ultraviolet transparency, and exceptional optical damage threshold.
We have successfully synthesized and characterized the non-agglomerated highly luminescent–paramagnetic ultrafine Y1.9O3:Eu0.13+ nanophosphor with millisecond PL lifetime by a modified sol–gel method, which can be produced on a large scale. We have demonstrated that the Y1.9O3:Eu0.13+ nanophosphor possesses the essential features required for in vitro high-contrast bioimaging applications. The nanophosphor has colloidal stability and optical transparency in water, highly efficient hypersensitive red emission of Eu3+ peaking at 610 nm (5D0 – 7F2) upon 246 nm UV light excitation with characteristically sharp spectral lines in the visible region, paramagnetic properties, and low cellular toxicity. Thus, this novel approach enables high-contrast cellular and tissue imaging with high sensitivity, magnetic tracking capability and low toxicity.
Figure 1. a) PL emission spectrum of Y1.9O3:Eu0.13+ nanophosphor recorded at 246 nm excitation showing a sharp, intense, hypersensitive red emission peak with maximum at 610 nm (5D0 – 7F2) at room temperature. The inset shows the color coordinates of red emission; x = 0.5941, y = 0.3039. b) TRPL decay profile of Y1.9O3:Eu0.13+ nanophosphor recorded at room temperature while monitoring the emission at 610 nm at an excitation wavelength of 246 nm. Figure 2. Room-temperature M (H) curve of Y1.9O3:Eu0.13+ nanophosphor. Figure 3. In vitro fluorescence microscopy images of T47D cells incubated with Y1.9O3:Eu0.13+ nanophosphor (50 µg mL-1) for 4 h. Sequential images show: I) phase contrast of T47D cells; II) an individual nucleus stained blue with 4′-6-diamidino-2-henylindole(DAPI); III) red fluorescence staining by Y1.9O3:Eu0.13+ nanophosphor; IV) overlapped images of blue DAPI and red Y1.9O3:Eu0.13+ nanophosphor; V) overlap of phase contrast, blue, and red, from (I–III), respectively; and VI) in vitro localized PL images of Y1.9O3:Eu0.13+ nanophosphor from (V). Inset: localized PL spectra taken from level cells (red).
Hybrid 2D Nanomaterials as Dual-Mode Contrast Agents in Cellular Imaging
The design of multifunctional nanofluids is highly desirable for biomedical therapy/cellular imaging applications. The emergence of hybrid nanomaterials with specifi c properties, such as magnetism and fluorescence, can lead to an understanding of biological processes at the biomolecular level. Various hybrid systems have been analyzed in the recent past for several possible biomedical applications. Carbon-based hybrid systems such as carbon nanotubes with various nanoparticles are being widely tested for their biological applications because of their ability to cross cell membranes and their interesting thermal and electrical properties. Graphene oxide (GO) is a fairly new graphene-based system with a 2D carbon honeycomb lattice decorated with numerous functional groups attached to the backbone: these functional groups make it an excellent platform for further attachment of nanoparticles and synthesis of hybrid materials. Cell viability studies on GO have been recently attempted, showing biocompatibility. Moreover, the intrinsic photoluminescence (PL) properties of GO can be utilized for cellular imaging. The large surface area and non-covalent interactions with aromatic molecules make GO an excellent system for biomolecular applications and drug attachment. Hybrid materials can also enable non-invasive imaging methods and diagnosis protocols by combining the unique properties of the individual system. Advances in nanotechnology have led to the development of hybrid versions of these nanoparticles, which can improve upon the low sensitivity of MRI by other techniques such as fl uorescence. Fluorescence allows bio-imaging with high speed and sensitivity. It has been established that a combination of magnetic and fluorescent imaging techniques with nanostructured systems will be beneficial for in vivo disease diagnosis and in vitro monitoring of living cells. However, the synthesis of highly luminescent biomaterials
using ferromagnetic/superparamagnetic Fe3O4 is a complicated development owing to the fluorescence quenching property of Fe3O4.
We have demonstrated a 2D hybrid nanostructure-based nanofluid that can be used as a contrast agent in a dual mode imaging process, and that allows one to easily combine two complementary techniques (T2 MRI and optical fl uorescence imaging) in cellular imaging. An interfacial energy transfer mechanism has been identifi ed for the PL of GO-F. The time-resolved spectroscopy measurements reveal nanosecond decay for hybrid GO-F fluid, indicating its potential applications in biological systems. The hybrid GO-F fluid showed good cell viability with different cancer cell lines. This nanofluid exhibited an enhanced thermal conductivity and the nanoparticles of GO-F were found to penetrate the cell cytoplasm, making it viable for intra-cellular magnetic hyperthermia applications. The surface functionalities in GO provide a good platform for large loading of aromatic drug molecules, thereby avoiding “drug burst” effects associated with bare SPIONs.
Figure 1. a) Schematic of the hybrid GO-F. Fe3O4 nanoparticles are covalently attached to the graphene plane through oxygen functionalities. b) Transmission electron microscopy (TEM) image of GO-F showing the Fe3O4 nanoparticles distributed throughout GO. c) Micro-Raman spectrum. Graphitic order and disorder (G and D) Raman modes are marked. Photograph: GO-F suspension in water. d) Room temperature magnetization curve of GO-F powder. The S-like M ( H ) loop shows the superparamagnetic nature of the GO-F powder . Figure 2 a) UV-vis absorption spectra of Fe3O4, GO, and GO-F fluids. b) Room temperature PL emission spectrum of Fe3O4 nanoparticles at 365 nm excitation. c) PL excitation spectrum at 416 nm emission of Fe3O4 nanoparticles. d) PL emission spectrum of GO-F nanofluid at 365 nm excitation. e) PL excitation spectrum at 416 nm emission of GO-F nanofluid. g) The color coordinate of the blue emission. h) PL emission spectrum of GO at 324 nm excitation. i) TRPL decay profile of GO-F nanofluid recorded at room temperature while monitoring the emission at 469 nm at an excitation wavelength of 371 nm. j) The lifetime data and the parameter generated by the exponential fitting. Figure 3 . a) In vitro fluorescence microscopy images of T47D cells treated with GO-F (50 μ g mL− 1 ) for 24 h. i–iii) Low magnification images of T47D cells: i) Phase contrast picture, ii) fluorescence images of GO-F, and iii) overlay of images (i) and (ii). iv–vi) High magnification images of an individual T47D cell: iv) phase contrast picture of an individual T47D cell, v) fluorescence image of GO-F, and vi) overlay of images (iv) and (v). The overlay of the phase contrast and fluorescence images clearly demonstrates the localization of GO-F in the cellular cytoplasm, suggesting its suitability for bioimaging. b) T2-weighted MR image showing strong T2 contrast in agarose phantoms. It shows alginate phantoms doped with different concentrations (as shown) of GO-F. The T2-weighted image was acquired in a 7T scanner with multi-slice multi-echo sequence. The T2 relaxivity was 297.06 mM−1 s−1 .
Fabrication and Electro-optic Properties of Multi-walled Carbon Nanotube Driven Novel Electroluminescent Lamp
We present a novel, cost-effective and facile technique, wherein multi-walled carbon nano-tubes (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 as shown in Figure . 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 understood as an internal field emission, intra-CNT impact excitation and the recombination of electrons and holes through the impurity states.
Figure . a) SEM images show morphology of ZnS phosphor, carbon nanotubes, ZnS+MWCNT blend and ZnS/MWCNT hybrid material after annealing b) a schematic of the ZnS/MWCNT hybrid material-based AC-EL smart lamp structure.
Rare-earth Free Yellow-Green Emitting NaZnPO4:Mn Phosphor for Lighting Applications
The development of innovative materials that convert longer wavelength UV to blue light (300–470 nm) and eventually into white-light could certainly help in replacing the use of mercury plasma with a less toxic alternative and would lead to higher photon conversion efficiency. We have focused on the development of mercury-free inexpensive phosphor materials that are eco-friendly with improved luminous efficacy, energy-saving, long-lifetime, and low-power consumption characteristics. A new rare-earth free phosphor, NaZnPO4:Mn2+ (NZP:Mn2+) with ultra-violet to visible absorption (300-470 nm), exceptional yellow-green (543 nm) broad-band photoluminescence (PL) and appreciable color co-ordinates (x=0.39, y=0.58) was identified. It has a crystal structure consisting of discrete PO4 tetrahedra linked by ZnO4 and NaO4 distorted tetrahedral such that three tetrahedra, one of each kind, share one corner. The XRD profile of the NZP:Mn2+ phosphor is shown in Figure . The presence of UV sensitive Zn-O-Zn bonds and their efficient energy transfer to Mn2+ ions resulted in brightest PL and external quantum yield of 63% at 418 nm. For all excitations, PL emission is centered at ~543 nm, which is attributed to spin forbidden d-d transition (4T1→6A1) of Mn2+ ions as shown in Figure . Our experiment demonstrated the possibility of producing relatively inexpensive UV -converted white-light emitting diodes for future. The novel phosphor could also be useful for many display and lighting devices. Compared to the commercial sulfide phosphor (P-20), our NZP:Mn2+ nanophosphor showed enhanced brightness and efficiencies and can be a favorable choice for incrementing white LED technology. The factors influencing the brightness, Mn2+ PL, reaction atmosphere, concentration of dopant etc. were studied in detail and optimized. Under this in-house activity a commercial UV LED (with 375 nm emission) was successfully coated with the present nanophosphor that showed the light emission very close to the ‘ideal white’ site of the chromaticity diagram.
Figure . (a) XRD profile of NaZnPO4:Mn phosphor with the inset showing the phosphor under room light and UV 370 nm excitations (b) Photoluminescence excitation spectrum of the NaZnP4:Mn phosphor monitored at 543 nm emission and (c) Bright yellow-green photoluminescence recorded under various excitations mentioned in the figure.
||Dr. Virendra Shanker, Chief Scientist
|| Dr. Santa Chawla, Senior Principle Scientist
Dr. D. Haranath, Senior Scientist
Dr. Bipin Kumar Gupta, Scientist
Sh. Rajgir Rai, SMA
|| Mr. Vineet Kumar, SPF
Mrs. Vanjula Kataria, SPF
Ms. Savvi Mishra, DST-JRF
Ms. Deepika Yadav, PGRPE
Mr. Dileep Dwivedi, M Tech Trainee
Mr. Pawan Kumar, JRF
Ms. Yogyata Agrawal, M. Phil. Trainee
Ms. Swati Baliyan, M Tech Trainee
Mr. Naveen Khichar, M Tech Trainee