Sofian M. Kanan

Carbon Nanotube−Ionic Liquid Composite Sensors and Biosensors

A new composite electrode has been fabricated using multiwall carbon nanotubes (MWCNT) and the ionic liquid n-octylpyridinum hexafluorophosphate (OPFP). This electrode shows very attractive electrochemical performances compared to other conventional electrodes using graphite and mineral oil, notably improved sensitivity and stability. One major advantage of this electrode compared to other electrodes using carbon nanotubes and other ionic liquids is its extremely low capacitance and background currents. A 10% (w/w) loading of MWCNT was selected as the optimal composition based on voltammetric results, as well as the stability of the background response in solution. The new composite electrode showed good activity toward hydrogen peroxide and NADH, with the possibility of fabricating a sensitive biosensor for glucose and alcohol using glucose oxidase and alcohol dehydrogenase, respectively, by simply incorporating the specific enzyme within the composite matrix. The marked electrode stability and antifouling features toward NADH oxidation was much higher for this composite compared to a bare glassy carbon electrode. While a loading of 2% MWCNT showed very poor electrochemical behavior, a large enhancement was observed upon gentle heating to 70 degrees C, which gave a response similar to the optimum composition of 10%. The ease of preparation, low background current, high sensitivity, stability, and small loading of nanotubes using this composite can create new novel avenues and applications for fabricating robust sensors and biosensors for many important species.

Semiconducting Metal Oxide Based Sensors for Selective Gas Pollutant Detection

A review of some papers published in the last fifty years that focus on the semiconducting metal oxide (SMO) based sensors for the selective and sensitive detection of various environmental pollutants is presented.

Synthesis of high surface area monoclinic WO3 particles using organic ligands and emulsion based methods

Several synthetic approaches have been used to obtain nano-sized monoclinic WO3 (m-WO3) powders. All of these methods begin with a standard preparative method where H2WO4 is first generated by passing a Na2WO4 solution through a cation-exchange resin. It is shown that high surface area particles are produced by dripping the H2WO4 exiting from the ion-exchange column into a solution containing oxalate and acetate exchange ligands or alternatively, into a water-in-oil (w/o) based emulsion. In comparison to commercial WO3 powders, the surface area of the m-WO3 powders were higher by factors of 10 and 20 times when prepared in the presence of acetate/oxalate chelating agents and w/o emulsions, respectively. The much higher surface areas enable infrared spectroscopic identification of surface sites along with detection and monitoring of gaseous reactions and adsorbed species on the surface of this metal oxide. This is demonstrated with the adsorption of a nerve agent simulant, dimethyl methyl phosphonate. In general, little is known about the reactions of gaseous molecules on m-WO3 surfaces and the fabrication of high surface area m-WO3 particles will aid in gaining an understanding of the chemical processes occurring in WO3 based sensors.

Luminescent homoatomic exciplexes in dicyanoargenate(I) ions doped in alkali halide crystals. ‘Exciplex tuning’ by site-selective excitation and variation of the dopant concentration

Abstract The photoluminescence spectra of dicyanoargenate(I) ions doped in KCl host crystals show several ultraviolet and visible emission bands. Each emission band becomes dominant at a characteristic excitation wavelength; i.e. the energy of the emission can be tuned. Both the experimental and theoretical results suggest the formation of Ag–Ag bonded excimers and exciplexes between adjacent Ag(CN) 2 − ions in the host lattice. The experimental evidence includes the broadness, the absence of detailed structure, and the low band energies of the luminescent bands. Ab-initio and extended Huckel calculations show that the lowest unoccupied molecular orbital (LUMO) is bonding with respect to Ag–Ag bonds while the highest occupied molecular orbital (HOMO) is antibonding. Further, the calculations indicate the existence of exciplexes with shorter Ag–Ag bond distances, higher binding energies, and larger Ag–Ag overlap populations than the corresponding ground state oligomers. The results in this study give rise to a new optical phenomenon which is called ‘exciplex tuning’. Tuning of the emission over the 285–610 nm wavelength range has been achieved in KCl/Ag(CN) 2 − crystals by site-selective excitation and varying the Ag(CN) 2 − dopant concentration.

Dual WO3 based sensors to selectively detect DMMP in the presence of alcohols

A size selective approach to improving selectivity in semiconducting metal oxides (SMO) sensors was obtained by tailoring the architecture of WO(3) powders. The key for achieving high selectivity is based on using a dual sensor configuration where the response on a porous WO(3) powder sensor was compared to the response on a nonporous WO(3) powder sensor. Detection selectivity between methanol and dimethyl methylphosphonate (DMMP) is obtained because the access of a gas molecule in the interior pore structure of WO(3) is size dependent leading to a size dependant magnitude change in the conductivity of SMO sensor.

Infrared study of UV-irradiated tungsten trioxide powders containing adsorbed dimethyl methyl phosphonate and trimethyl phosphate

The photodecomposition of dimethyl methylphosphonate (DMMP) and trimethyl phosphate (TMP) adsorbed on monoclinic WO3 powders when irradiated by ultraviolet light (UV) in air, oxygen, and under evacuation was investigated using infrared spectroscopy (IR). The IR spectra show that DMMP decomposes into methyl phosphonate upon exposure to 254 nm UV for 2 h at room temperature in air. The same decomposition of DMMP occurs only at temperatures above 300°C without UV illumination. TMP differs from DMMP in that the photodecomposition product is not the same as the decomposition product obtained by heating above 300°C. Thermal decomposition leads to formation of a phosphate on the surface, whereas photodecomposition leads to the same adsorbed methyl phosphonate as found for the thermal or photodecomposition of DMMP. Since TMP does not contain a P-CH3 bond, the formation of a methyl phosphonate on the surface after UV illumination involves a mechanism where CH3 groups migrate from the methoxy group to the phosphorous central atom. No decomposition is observed at room temperature when DMMP or TMP adsorbed on WO3 is irradiated under vacuum or in nitrogen atmosphere. Therefore, the photodecomposition of either DMMP or TMP adsorbed on WO3 at room temperature does not involve a reaction with the lattice oxygen but rather a reaction with the oxygen radicals produced by the decomposition of ozone.

Luminescence properties of silver(I)-exchanged zeolite Y and its use as a catalyst to photodecompose carbaryl in the presence of natural organic matter

Silver-doped Y-type zeolites with different silver loadings were prepared and analyzed spectroscopically at various temperatures. Several emission bands were observed for each AgY sample. Each emission band becomes dominant over the others by selecting a suitable excitation wavelength, indicating the presence of different silver clusters in zeolite Y. The study shows that the presence of AgY catalysts increases the photodecomposition rate of carbaryl by 4–42 times in comparison to the catalyst-free system, with the rate dependent on the amount of silver that was loaded on the Y-type zeolites. The presence of different concentrations of Suwannee River natural organic matter (NOM) affected the photodegradation reaction of carbaryl by allowing it to proceed via two different pseudo-first-order pathways. However, the presence of the silver-doped zeolite catalysts makes the photodecomposition of carbaryl proceed via first-order kinetics.

Silver nanoclusters doped in X and mordenite zeolites as heterogeneous catalysts for the decomposition of carbamate pesticides in solution

Ag(I) nanoclusters doped in X and mordenite zeolites were prepared and analyzed using spectroscopy. Both experimental and theoretical studies of the prepared compounds show the presence of silver nanoclusters with various sizes and environments. The presence of Ag(I) nanoclusters doped in X and mordenite zeolites with high silver loadings enhanced the photodecomposition rate of carbofuran 215-and 184-fold, respectively, while the photodecomposition of carbaryl in the presence of these catalysts show room temperature rate constants that are 182-and 168-times faster than the photodecomposition of carbaryl in the absence of the silver-doped zeolite catalysts. The reaction rates were found to be dependent on the amount of silver loaded into the zeolite.

Properties of 2-, 3-, and 4-acetylpyridine substituted ruthenium(II) bis(bipyridine) complexes: substituent effect on the electronic structure, spectra, and photochemistry of the complex

Bis(2,2′-bipyridine) complexes of ruthenium(II) with 2-, 3-, and 4-acetylpyridine derivatives were synthesized and structurally characterized. The effect of changing the location of the pyridines acetyl substituent was studied experimentally and theoretically to clarify the effect of substituent position on the chemical behavior and photochemical properties of the complex. The substituent position on the heterocyclic-pyridine was found to strongly affect the chemical and photochemical properties of the complex. Variation of the position of the substituent, and thus ligand modification brought by as a consequence of this variation, offers possibilities to design complexes of desired structural and photochemical properties.

Heterogeneous Photocatalysis with Nanoclusters of D10 Metal Ions Doped in Zeolites

Nanoclusters of d10 metal ions, including silver, gold, and copper ions anchored in the zeolite host, have been prepared and characterized using various spectroscopic techniques, including luminescence, Raman, and FT-EXAFS, along with theoretical calculations. Analyses indicate the formation of metal ion oligomers. The encapsulated oligomers in various zeolite hosts have been used to decompose toxic nitric oxide and pesticides into innocuous products. The properties of the various zeolites are identified and the catalytic role of parameters such as composition and pore diameter are investigated. The role of the d10 metal ion clusters in the catalytic reactions is discussed and kinetic mechanisms for the photocatalytic reactions are proposed. GRAPHICAL ABSTRACT