Sajjad Ullah

Surface structure and reactivity study of phosphotungstic acid-nitrogenated ormosils

Supramolecular interactions between nitrogenated groups in hybrid ormosils bearing phosphotungstate nanocatalyst were used to tune the photocatalytical activity of these class-II hybrid materials obtained through sol–gel chemistry. Surface chemistry and morphology of the materials was studied by scanning electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy and water contact angle measurements. The photocatalytic efficiency of these hybrid films, measured by the degradation of crystal violet over-layer deposited on ormosils films, is higher for ormosils bearing neutral, more polar and less H-bonding nitrile groups than those bearing alkylamine/alkylammonium functionalities, despite the lower W/Si atomic ratio on the surface and lower tungsten percentage of the pure nitrile bearing ormosils. Such higher surface reactivity of the nitrile bearing ormosils is due to weaker interaction with the phosphotungstate and the lower activity of amine bearing ormosils is attributed to the competition among reversible photochromism and photocatalysis pathways in these materials.

Microwave-assisted synthesis of NaYF4:Yb3+/Tm3+ upconversion particles with tailored morphology and phase for the design of UV/NIR-active NaYF4:Yb3+/Tm3+@TiO2 core@shell photocatalysts

Coupling of TiO2 with rare earth (RE) metal-based upconversion particles (UCPs) is a promising strategy to develop near-infrared (NIR)-driven TiO2-based photocatalysts. This, however, requires the controlled synthesis of bright NIR-to-UV UCPs with tailored morphology, size and crystalline phase. Herein, we report a rapid, efficient and reproducible co-precipitation/microwave (MW)-assisted hydrothermal method for the preparation of NaYF4:Yb3+/Tm3+ UCPs which allows tailoring the morphology (spherical nanoparticles, microrods) and crystalline phase (cubic, hexagonal) and hence the optical response of the resulting UCPs by simply varying the MW treatment time or ethylenediaminetetraacetic acid (EDTA) concentration. The EDTA concentration strongly affects the size of the primary UCPs during the co-precipitation step which plays a decisive role in dictating the final particle morphology in the subsequent MW-assisted hydrothermal step. Based on microscopic and XRD analyses, a model for the growth of UCPs with different morphologies has been proposed. Pure hexagonal microrods with strong upconversion (UC) photoluminescence (PL) in the UV (345 nm, 361 nm) and visible (450 nm, 475 nm) regions are obtained using MW treatment times of 30–60 min and EDTA/RE3+ molar ratios above 1.5. The UCPs@TiO2 core@shell composite photocatalysts prepared by coating anatase TiO2 on the surface of UCPs using a sol–gel method showed good photocatalytic activity under both ultraviolet (UV) and NIR light. This efficient and reproducible co-precipitation/MW-assisted approach may be considered as a step forward towards the efficient harvesting of sunlight for photochemical and photoelectrical applications based on TiO2.

Eu(III)-coordination polymer sub-micron fibers: material for selective and sensitive detection of Cu2+ ions via competition between photoinduced electron transfer and energy transfer

In this article, we report a unique strategy based on the competition between photoinduced electron transfer (PET) and intramolecular energy transfer (ET) processes occurring among the components of guanine-based europium(III) coordination polymer sub-micron fibers (CPMFs) for the selective and sensitive detection of Cu2+. The novel CPMFs were synthesized via a hydrothermal method using guanine, europium ions (Eu3+) and 3,5-dinitrosalicylic acid (DNSA) as an auxiliary linking molecule that can sensitize the luminescence of Eu3+ ions via intramolecular ET. The photoluminescence (PL) emission signal from the CPMFs is found to be weak due to the existence of PET from guanine to DNSA. The PET process from guanine to DNSA prevents the intramolecular ET from DNSA to Eu3+ ions, leading to the quenching of the luminescence from the CPMFs. However, 11 times enhancement of the PL emission signal from the CPMFs is observed in the presence of Cu2+ ions (12 μM) which is attributed to the suppression of the PET process (or enhancement of ET) as a result of coordination of Cu2+ with guanine molecules. This forms the basis of Cu2+ ion detection and the prepared CPMFs exhibited excellent selectivity and good sensitivity toward Cu2+ ions up to the 1.42 μM detection limit. The study was also extended to real tap water samples, where spiking with Cu2+ ions led to a 10 times enhancement of the PL emission intensity from the CPMFs, though hardly any change was noted for the unspiked tap water sample. We envision that our CPMFs are potential candidates for application in ultrasensitive time-resolved fluorometric assays owing to their long luminescence lifetime, good dispersion and stability in solution.

Enhanced NIR-I emission from water-dispersible NIR-II dye-sensitized core/active shell upconverting nanoparticles

Recently, there has been a surge in research studies directed towards near-infrared (NIR) dye-sensitized upconverting nanoparticles (UCNPs) as they carry the prominent advantages of a broader absorption range and enhanced upconversion efficiency. Unfortunately, however, the UCNPs combined with the native form of NIR dye are of little use for biological imaging in the NIR-I or NIR-II window as the dye- sensitization process is mostly carried out in non-aqueous media. To overcome this shortcoming, we propose to employ a water-dispersible NIR-II dye (IR-1061) to sensitize core/active shell UCNPs and achieve sufficiently high upconversion quantum efficiency in aqueous media. We have particularly focused on achieving strong NIR-I emission rather than visible upconversion emission as the latter suffers from the problem of shallow tissue penetration depth. For this purpose, Pluronic F68-encapsulated water-dispersible IR-1061 dye was coupled with polyethyleneimine (PEI)-coated NaYF4:Tm3+/Yb3+@NaYF4:Yb3+ core/active shell UCNPs. We thus achieved a 283% enhancement in NIR-I emission (i.e. 800 nm emission of Tm3+ ion) from water-dispersible NIR-II dye-sensitized core/active shell UCNPs via doping of ytterbium ions (Yb3+) in the UCNP shell, which bridged the energy transfer from the dye to the UCNP core. Practically, in comparison with the native form of the dye, this water-dispersible dye can also efficiently harvest irradiation energy, which is nonradiatively transferred to Yb3+ ions in the shell and subsequently to Yb3+ ions in the core. The latter sensitizes Tm3+ ions positioned in the core, thus generating upconversion luminescence from the UCNPs. We envision that our water-dispersible NIR-II dye-sensitized core/active shell UCNPs are not only potential candidates for a broad spectrum of photonic applications but that they will also find new opportunities in several biological applications.

The Effect of Ormosil Matrix Composition and Alkaline Earth Metal Doping on the Photochromic Response of Ormosil-Phosphotungstate Films

In this study, polyoxometallate based hybrid photochromic materials were prepared by incorporating phosphotungstate anion, PW12O403−, (PW) in hybrid tetraethyl orthosilicate and (3-glycidyloxypropyl)trimethoxysilane TEOS-GPTMS derived organomodified silicates (Ormosil) matrices by sol-gel method and the resulting materials were used to prepare multilayer films by dip-coating method. The effect of alkaline earth metal cations doping and matrix composition (%GPTMS) on the photochromic response of the hybrid films was studied in details. GPTMS, after undergoing ring opening reaction, leads to the formation of chelating sites (diol and ether functionalities) which helps in anchoring of cations, which in turn interacts with phosphotungstate anions and favors their incorporation in the hybrid films. For a fixed concentration of GPTMS, the cation-phosphotungstate electrostatic interaction and hence the photochromic response of the films follow the order Mg2+ < Ca2+ < Sr2+ < Ba2+, thereby, indicating that larger cations interact more strongly with the heteropolyanions. The presence of these cations and GPTMS concomitantly leads to increased incorporation of phosphotungstic acid hydrate (HPW) in the films, resulting in a significant enhancement of the photochromic response.