Safyan A. Khan

Mode-multiplexed waveguide sensor

Abstract Optical sensors enable quantification of analyte concentrations non-invasively without being affected from electromagnetic fields. The development of single-mode waveguides (WGs) is simple approach; however, limitations in the spatial distribution of the refractive index and the inability to sense multiple samples at the same time. Here, we demonstrate a multi-mode WG model and matrix inversion method (MIM) to improve spatial information in at least one dimension. This method is used to optimize and estimate three external stimulus properties in TE00, TE01 and TM00 multiplexed modes for a semi-triangular ring resonator configuration. The multi-mode WG and MIM may have applications in the development of biosensors for multi-analyte detection.

Fabrication of TiO2/Ag/Ag2O Nanoparticles to Enhance the Photocatalytic Activity of Degussa P25 Titania

A simple chemical reduction approach was used to synthesize Ag nanoparticles (NPs) over a reputed photocatalyst, Degussa P25 (TiO2). Silver doping extended the P25 absorption wavelength from the ultraviolet to the visible region. The synthesized silver NPs (Ag NPs) were of spherical shape and had an average size of ~4.6 nm. In the next stage, Ag NPs were partially oxidized by treatment with hydrogen peroxide. The resulting P25/Ag/Ag2O nanocomposites were characterized by X-ray powder diffraction, transmission electron microscopy, energy dispersive X-ray analysis, Brunauer–Emmett–Teller analysis, and UV-visible spectroscopy. The photocatalytic activities of the P25, P25/Ag, and P25/Ag/Ag2O catalysts were investigated for the degradation of non-biodegradable dyes, methylene blue and rhodamine 6G. The P25/Ag/Ag2O nanocomposite exhibited better photodegradation activity than P25, as well as the commonly used Ag3PO4, under visible light irradiation.

Synthesis, structures and photoluminescence properties of mixed ligand divalent metal–organic frameworks

Three new metal–organic frameworks [Co2(bmip)(bdc)2]·2DMF (1), [Cu(bmip)(bdc)]·6H2O (2) and [Zn2(bmip)(bdc)2(H2O)2] (3) have been prepared using a solvothermal process, derived from a flexible N,N′-donor spacer 1,3-bis(2-methylimidazolyl)propane (bmip) and 1,4-benzenedicarboxylic acid (H2bdc) ligands. Whereas compound 1 shows a three-dimensional (3D) framework, exhibiting a pcuα-Po network topology, compound 2 displays a two-dimensional (2D) network structure lying parallel to the plane (1 1 −1) and shows a dia topology. The 2D layers in 2 are further linked by hydrogen bonding that involves water solvate molecules, which in turn results in an infinite 3D hydrogen bonded framework. Compound 3 on the other hand shows a three-dimensional (3D) complicated framework with zigzag chains having a ThSi2 topology. The structural features of 1–3 display diverse variations from 3D, 2D and 3D frameworks, respectively, that are entirely different for each transition metal [Co(II), Cu(II), and Zn(II)] with the same combination of mixed ligands under similar reaction conditions. The physical and chemical properties of 1–3 were investigated by single crystal X-ray diffraction, elemental analysis, FTIR spectroscopy (FTIR), thermogravimetric analysis (TGA) and photoluminescence properties.

Optical scattering from graphene foam for oil imaging/sensing

Oil spill detection is crucial, from an environmental perspective and in view of the associated economic losses. Current optical oil sensing techniques, such as underwater microscopy and light scattering methods, mainly focus on detecting the properties of particles or organisms in water and often require costly equipment and sophisticated data processing. Recent studies on graphitic foam show its extraordinary pollutant absorbing properties, with high absorption weight ratios. Here we propose to produce a graphene foam based ultra-light material that changes its optical properties on absorbing oil species. The results demonstrate clear changes in optical transmission and scattering properties of graphene foam when exposed to various oils. The effective graphene foam sorbent can be easily integrated with optical fiber systems to detect the optical property variations and also to monitor oil presence/spillages remotely. Such sensors can also be used for underground oil exploration.

Transformation of Cadmium Tetracyanoquinodimethane (TCNQ) into a Cadmium Terephthalate Metal-Organic Framework

The transformation of cadmium 7,7,8,8-tetracyanoquinodimethane (TCNQ) into a cadmium terephthalate co-ordination polymer is reported, with the chemistry of this material elucidated using elemental analysis, X-ray photoelectron spectroscopy and synchrotron radiation single-crystal X-ray diffraction. A heptacoordinated CdII linear coordination polymer catena-poly[triaqua-(μ2-benzene-1,4-dicarboxylato-κO,O′)cadmium(ii)]hydrate (1) was isolated while attempting to recrystallize Cd(TCNQ)2. Density functional theory calculations for the oxidation of benzylic carbon attached to the cyano group provided evidence that the reaction pathway proposed herein is highly exergonic and thermodynamically plausible. This structure showed a distorted pentagonal bipyramidal geometry together with a symmetrical mononuclear unit in which each CdII ion is doubly bridged by a dicarboxylato anion. Owing to the softness and minute size of these crystals, this structure had to be elucidated using synchrotron radiation X-ray crystallography.