Venkataramanan Mahalingam

Microwave synthesis, photoluminescence, and photocatalytic activity of PVA-functionalized Eu3+-doped BiOX (X = Cl, Br, I) nanoflakes.

We report a facile microwave-assisted green synthetic route for colloidal poly(vinyl alcohol) (PVA)-coated europium (Eu(3+))-doped luminescent heavy metal bismuth oxyhalide (BiOX; X = Cl, Br, I) nanoflakes at low temperature and examine their structural, optical, and photocatalytic characteristics. PVA coating onto the surface of the nanoflakes endows them with hydrophilic nature. Both Eu(3+)-doped BiOCl and BiOBr nanoflakes exhibit strong optical properties related to Eu(3+) and Bi(3+) which are quenched in case of Eu(3+)-doped BiOI matrix. These results are supported by Eu(3+) photoluminescence lifetime values of 0.61 ms, 0.59 ms, and 8.9 μs, respectively. The former two matrices have quite similar crystal field environments as deduced from the asymmetric ratios of (5)D0 → (7)F2 (614 nm) and (5)D0 → (7)F1 (591 nm) transitions. In addition to possessing interesting photoluminescence properties, a comparison of the photocatalytic activity of Eu(3+)-doped BiOX (X = Cl, Br, I) nanoflakes, with corresponding estimated band gaps of 3.36, 2.74, and 1.67 eV has been evaluated using Rhodamine B (RhB) dye under visible light irradiation. The nanoflakes exhibited 100% dye degradation under visible light irradiation. Eu(3+)-doped BiOCl nanoflakes manifested higher photocatalytic efficiency compared to the other matrices following apparent first-order kinetics. Such a boost in efficiency is attributed to their high surface area to volume ratios, layered crystalline structures, indirect band gap nature, and ability to utilize broad bands in the solar spectrum.

Highly Selective and Sensitive Detection of Cu2+ Ions Using Ce(III)/Tb(III)-Doped SrF2 Nanocrystals as Fluorescent Probe

We report a green synthetic approach to the synthesis of water dispersible Ce(3+)/Tb(3+)-doped SrF2 nanocrystals, carried out using environment friendly microwave irradiation with water as solvent. The nanocrystals display strong green emission due to energy transfer from Ce(3+) to Tb(3+) ions. This strong green emission from Tb(3+) ions is selectively quenched upon addition of Cu(2+) ions, thus making the nanocrystals a potential Cu(2+) ions sensing material. There is barely any interference by other metal ions on the detection of Cu(2+) ions and the detection limit is as low as 2 nM. This sensing ability is highly reversible by the addition of ethylenediaminetetraacetic acid (EDTA) with the recovery of almost 90% of the original luminescence. The luminescence quenching and recovery cycle was repeated multiple times without much effect on the sensitivity. The study was extended to real world water samples and obtained similar results. In addition to the sensing, we strongly predict the small size and high luminescence of the Ce(3+)/Tb(3+)-doped SrF2 nanocrystals can be used for bioimaging applications.

A Highly Efficient UV–Vis–NIR Active Ln3+-Doped BiPO4/BiVO4 Nanocomposite for Photocatalysis Application

In this Article, we report the synthesis of Ln(3+) (Yb(3+), Tm(3+))-doped BiPO4/BiVO4 nanocomposite photocatalyst that shows efficient photocatalytic activity under UV-visible-near-infrared (UV-vis-NIR) illumination. Incorporation of upconverting Ln(3+) ion pairs in BiPO4 nanocrystals resulted in strong emission in the visible region upon excitation with a NIR laser (980 nm). A composite of BiPO4 nanocrystals and vanadate was prepared by the addition of vanadate source to BiPO4 nanocrystals. In the nanocomposite, the strong blue emission from Tm(3+) ions via upconversion is nonradiatively transferred to BiVO4, resulting in the production of excitons. This in turn generates reactive oxygen species and efficiently degrades methylene blue dye in aqueous medium. The nanocomposite also shows high photocatalytic activity both under the visible region (0.010 min(-1)) and under the full solar spectrum (0.047 min(-1)). The results suggest that the photocatalytic activity of the nanocomposite under both NIR as well as full solar irradiation is better compared to other reported nanocomposite photocatalysts. The choice of BiPO4 as the matrix for Ln(3+) ions has been discussed in detail, as it plays an important role in the superior NIR photocatalytic activity of the nanocomposite photocatalyst.

Sonication-responsive organogelation of a tripodal peptide and optical properties of embedded Tm3+ nanoclusters

The sonication induced organogelation of a tripodal peptide has been investigated. The tripodal peptide does not form a gel in organic solvents after heating, cooling and ageing. Ultrasound has been used as a source of energy to tune the molecular assembly and organogelation. CD, NMR and fluorescence spectroscopies indicate distinct structural changes before and after sonication. FE-SEM and AFM of the xerogels reveal the nanofibrillar morphology. Moreover, the sonication responsive organogelation has been used to fabricate novel lanthanide nanocrystal decorated fibers. This gelation selectively quenches the viable emissions from the nanocryatsls as well the FRET observed between Tm3+/Yb3+-doped LiYF4 nanocrystals and ketocyanine dye occurring near 550 nm without affecting the NIR emissions of the nanocrystals.

3,5-Dinitrobenzoic Acid-Capped Upconverting Nanocrystals for the Selective Detection of Melamine

In this Research Article, we report for the first time the use of upconverting nanoparticles to detect melamine up to nanomolar concentration. Detection of melamine is important as it is one of the adulterant in protein rich food products due to its high nitrogen content. In this work, we have shown how the electron deficient 3,5-dinitrobenzoic acid (DNB)-coated Er/Yb-NaYF4 nanocrystals can specifically bind to electron rich melamine and alter the upconverting property of the nanocrystals. This selective binding led to the quenching of the upconversion emission from the nanocrystals. The high selectivity is verified by the addition of various analytes similar in structure with that of melamine. In addition, the selective quenching of the upconversion emission is reversible with the addition of dilute acid. This process has been repeated for more than five cycles with only a slight decrease in the sensing ability. The study was also extended to real milk samples, where the milk adulterated with melamine quenches the emission intensity of the DNB coated NaYF4:Er/Yb nanocrystals, whereas hardly any change is noted for the unadulterated milk sample. The high robustness and the sharp emission peaks make Er(3+)/Yb(3+)-doped NaYF4 nanocrystals a potential melamine sensing material over other organic fluorophores and nanocrystals possessing broad emissions.

Methyl Oleate-Capped Upconverting Nanocrystals: A Simple and General Ligand Exchange Strategy To Render Nanocrystals Dispersible in Aqueous and Organic Medium

We report a simple and general ligand exchange strategy to functionalize the nanocrystals with both hydrophobic and hydrophilic ligands. This is achieved by first capping the Er/Yb-doped NaYF4 nanocrystals with a weak ligand such as methyl oleate and subsequently ligand exchanged with various organic ligands which can strongly coordinate to the surface of the nanocrystals. The method involves only a simple stirring or sonication of the nanocrystals dispersion with the ligands of interest. Dicarboxylic acids such as sebacic acid, adipic acid, succinic acid, and malonic acid-functionalized nanocrystals which are difficult to achieve via thermal decomposition method were easily prepared by this ligand exchange strategy. In addition, low boiling point ligands like hexanoic acid can easily be coated over the surface of the Er/Yb-doped NaYF4 nanocrystals. Both size and shape of the nanocrystals were preserved after the ligand exchange process. The methyl oleate-capped Er/Yb-doped NaYF4 nanocrystals display strong upconversion emission after ligand exchanged with hydrophobic and hydrophilic molecules. The high stability of the nanocrystals after ligand exchange process is verified by performing time-dependent luminescent measurements at different pH, buffers, etc.

Strong Single‐Band Blue Emission from Colloidal Ce3+/Tm3+‐Doped NaYF4 Nanocrystals for Light‐Emitting Applications

An intense single-band blue emission at λ=450 nm is observed from Tm(3+) ions through Ce(3+) sensitization, for the first time, in colloidal Ce(3+) /Tm(3+) -doped NaYF4 nanocrystals. The intense Tm(3+) emission through broad-band excitation is advantageous for developing luminescent nanocomposites because they can be easily incorporated into polymers. The composites can easily be coated over UV light-emitting diodes (LEDs) to develop phosphor-based blue LEDs.

Effect of Intrinsic Properties of Anions on the Electrocatalytic Activity of NiCo2O4 and NiCo2OxS4–x Grown by Chemical Bath Deposition

Electrochemical water (H2O) splitting is one of the most promising technologies for energy storage by hydrogen (H2) generation but suffers from the requirement of high overpotential in the anodic half-reaction (oxygen evolution), which is a four-electron process. Though transition-metal oxides and oxysulfides are increasingly researched and used as oxygen evolution electrocatalysts, the bases of their differential activities are not properly understood. In this article, we have synthesized NiCo2O4 and NiCo2OxS4–x by a chemical bath deposition technique, and the latter has shown better oxygen evolution performance, both in terms of stability and activity, under alkaline conditions. Comprehensive analysis through time-dependent cyclic voltammetry, microscopy, and elemental analysis reveal that the higher activity of NiCo2OxS4–x may be attributed to the lower metal–sulfur bond energy that facilitates the activation process to form the active metal hydroxide/oxyhydroxide species, higher electrochemically active surface area, higher pore diameter and rugged morphology that prevents corrosion. This work provides significant insights on the advantages of sulfur-containing materials as electrochemical precatalysts over their oxide counterparts for oxygen evolution reaction.

Ce3+ sensitized Ln3+-doped nanocrystals for sensing and light-emitting applications

The scientific quest towards lanthanide (Ln3+)-doped nanomaterials is continuously growing which is largely attributed to the unique optical properties resulting from their inner 4f electrons. The intra 4f-4f transitions from Ln3+ ions span a wide optical window (say from UV to near infrared). In addition to the generally observed Stokes emission, they exhibit interesting anti-Stokes emission known as upconversion process, where one or more low energy photons combined to produce high energy light. However, the Ln3+-doped nanocrystals suffer from low quantum efficiency due to forbidden nature of the intra 4f transitions. Our objective of the increase the luminescence quantum efficiency of Ln3+ ions via energy transfer. Both Ce3+ and Yb3+ are good sensitizers for enhancing the luminescence via Stokes and upconversion process, respectively. This talk will cover discussion on Ce3+/Ln3+ (Ln = Tm, Tb and Sm)-doped nanocrystals for developing blue and white light emitting materials and their use in the fabrication of transparent nanocomposites by incorporating them in polymer matrix. The energy transfer mechanism between Ce3+ and some of the Ln3+ ions which led to the production of white light emission is shown in the below image. In addition, the talk covers the use of Ce3+/Tb3+-doped SrF2 nanocrystals for the detection of Cu2+ ions at the nanomolar concentration. Finally, the talk will cover how the Ce3+ to Ln3+ energy transfer can be tuned via phase change by taking NaYF4 as host for the lanthanide ions. In fact, the study provides new insight that the energy transfer efficiency between Ce3+ and Ln3+ ions is higher in cubic phase NaYF4 nanocrystals compared to hexagonal phase nanocrystals due to difference in crystal field splitting of the 4f5d level of Ce3+ ions.