Shakeel Khan | University of Karachi (original) (raw)

Papers by Shakeel Khan

Research paper thumbnail of Cellulase production from Aspergillus niger MS82: effect of temperature and pH

New Biotechnology, 2009

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Research paper thumbnail of New Antimicrobial Alkaloids from the Roots of Polyalthia longifolia var. pendula

Planta Medica, 2003

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Research paper thumbnail of Ferromagnetism and adiabatic to non-adiabatic switching process in La0.33Sr0.67Mn1−xFexO3 (0≤x≤0.02) manganite

Journal of Magnetism and Magnetic Materials, 2014

ABSTRACT We have systematically investigated structural, electrical and magnetic properties of Fe... more ABSTRACT We have systematically investigated structural, electrical and magnetic properties of Fe doped La0.33Sr0.67MnO3 manganites synthesized through solid-state reaction-route. All the samples are found to have rhombohedral crystal structure. The crystallite sizes obtained by XRD data are much smaller than the average grain size obtained by scanning electron microscope (SEM). Temperature dependent resistivity data were fitted using Mott׳s variable-range hopping (VRH) and small polaron hopping (SPH) models for obtaining different parameters. The adiabatic SPH conduction mechanism is followed almost for both samples in the absence of magnetic field but a switching from adiabatic to non-adiabatic SPH conduction mechanism is found in the presence of magnetic field. Temperature dependent magnetization (M–T) measurements confirm the decrease in Curie-temperature (TC) with Fe doping. Both the samples exhibited ferromagnetic behavior at 10 K and 300 K with a small hysteresis loop and low coercivity.

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Research paper thumbnail of Influence of Mn Doping on Structural, Optical, Dielectric and Electrical Properties of ZnO Nanostructures

Journal of Nanoengineering and Nanomanufacturing, 2012

ABSTRACT Nanocrystalline samples of Zn1-xMnxO (0 ≤ x ≤ 0.10) have been prepared by sol-gel route.... more ABSTRACT Nanocrystalline samples of Zn1-xMnxO (0 ≤ x ≤ 0.10) have been prepared by sol-gel route. The synthesized samples were characterized by X-ray diffraction (XRD), SEM, EDX, UV–Vis, temperature dependent resistivity and dielectric properties at room temperature in the frequency range of 42 Hz to 5 MHz. XRD studies exhibited wurtzite (hexagonal) crystal structure similar to the parent compound (ZnO) for all samples. The average crystallite size was calculated from XRD data lies within the range 23-27 nm. The SEM micrographs show the agglomeration of the nanoparticles. The UV–Vis. measurement shows reduction in the band gap by Mn doping upto 3% and further it increases for the higher doping of Mn. The dielectric constant (ε), loss tangent (tanδ) and ac conductivity (σac) were studied as a function of frequency and composition at room temperature. The experimental results showed that ε and tanδ decrease when the frequency increases while σac increases with variation in frequency. This behavior can be explained on the basis of space charge polarization according to Maxwell and Wagner’s two-layer model. Resistivity measurement shows semiconducting behavior for all samples.

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Research paper thumbnail of Temperature-dependent dielectric and magnetic properties of Mn doped zinc oxide nanoparticles

Materials Science in Semiconductor Processing, 2014

ABSTRACT Mn substituted zinc oxide (ZnO) nanoparticles with general formula Zn1−xMnxO (x=0.0, 0.0... more ABSTRACT Mn substituted zinc oxide (ZnO) nanoparticles with general formula Zn1−xMnxO (x=0.0, 0.03, 0.05 and 0.10) were synthesized through a sol-gel route. The result of x-ray diffraction (XRD) analysis confirms the presence of wurtzite crystal structure for all samples suggested that doped Mn ions are occupying regular Zn sites. Transmission electron microscopy (TEM) images exhibit the growth of particle size in the range of 30-50 nm. The results of dielectric measurement indicate that the parameters ε′, ε″, tanδ and σac are decreasing with the increase in frequency; whereas these parameters increase with the increase in temperature. Dielectric parameters are also found to increase with the increase in Mn concentration up to 5%, beyond this value the parameters become lower. The dielectric properties have been explained on the basis of space charge polarization according to Maxwell and Wagner’s two-layer model and the hopping between adjacent Mn2+ and Mn3+ ions. The analysis of temperature dependant magnetization data gives the idea about ferromagnetic (FM) exchange interaction.

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Research paper thumbnail of STRUCTURAL AND MAGNETIC PROPERTIES OF La 0.67 Sr 0.33 Mn 0.93 Zn 0.07 O 3 MANGANITE

International Journal of Modern Physics: Conference Series, 2013

ABSTRACT The structural and magnetic measurements have been carried out on La0.67Sr0.33Mn0.93Zn0.... more ABSTRACT The structural and magnetic measurements have been carried out on La0.67Sr0.33Mn0.93Zn0.07O3 (LSMZO) polycrystalline sample. The sample has been prepared through Sol-Gel route. X-ray diffraction (XRD) pattern confirms the single phase nature of the sample with orthorhombic crystal symmetry. Field dependent magnetization shows the hysteresis loop with very low coercive field and the magnetization does not saturate up to the field of 5000 gauss in magnitude.

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Research paper thumbnail of Structural and optical properties of gel-combustion synthesized Zr doped ZnO nanoparticles

Optical Materials, 2013

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Research paper thumbnail of Study of structural property of Co ferrite thin film grown by pulsed laser deposition technique

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Research paper thumbnail of Structural and dielectric properties of Mn doped copper oxide (CuO) nanostructure

ABSTRACT Undoped and Mn doped CuO nanocrystalline powder samples were prepared through standard s... more ABSTRACT Undoped and Mn doped CuO nanocrystalline powder samples were prepared through standard solid state reaction method. The crystal structures of the CuO nanoparticles were characterized by X-ray diffraction. Dielectric measurements were performed on samples as a function of frequency at room temperate to determine the dielectric behavior of the samples. XRD data exhibited the presence of monoclinic crystal structure similar to the parent compound in all samples, suggesting that doped Mn ions sit at the regular lattice sites. The average crystallite size, calculated using Scherrer formula from XRD data, is found within the range of 23-27 nm. The dielectric constant (��), imaginary part of dielectric constant (��) and loss tangent (tan�) were studied as a function of frequency and composition at room temperature. The dependence of dielectric ��������� ������ ��������� ��������������������������������in terms of hopping. This behavior can be explained on the basis of space charge pola��������������������������� ����� �����������-layer model.

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Research paper thumbnail of Study of the structural and dielectric properties of Bi2O3 and PbO addition on BiNbO4 ceramic matrix for RF applications

Journal of Materials Science: Materials in Electronics, 2011

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Research paper thumbnail of Magnetic and electrical properties of In doped cobalt ferrite nanoparticles

Journal of Applied Physics, 2012

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Research paper thumbnail of Adiabatic to non adiabatic change in conduction mechanism of Zn doped La0.67Sr0.33MnO3 perovskite

Journal of Alloys and Compounds, 2013

ABSTRACT We reported the results of electrical transport properties of La0.67Sr0.33Mn1−xZnxO3 (x ... more ABSTRACT We reported the results of electrical transport properties of La0.67Sr0.33Mn1−xZnxO3 (x = 0.1 and 0.2) bulk samples prepared by solid-state reaction route over a wide temperature range from 5 to 300 K in the presence of magnetic fields. Powder X-ray diffraction patterns at room temperature confirmed that both samples were formed in a single phase. Temperature dependent resistivity data were fitted using Mott’s variable-range hopping (VRH) model for a limited range of temperatures (from 235.5 to 300 K for 10% Zn doped and from 133 to 300 K for 20% Zn doped samples) to calculate the hopping distance (Rh) and the density of states near the Fermi level, [N(EF)]. It was found that all the parameters vary systematically with the increase in Zn concentration. Moreover, the temperature dependent resistivity data were also fitted using the small polaron hopping (SPH) model. The adiabatic SPH conduction is followed for 10% doped sample at all fields while a non-adiabatic SPH conduction is found to be active for 20% doped sample in all fields. This type of change in the conduction mechanism is accompanied by subtle electronically induced structural changes involving Mn–O–Mn and Zn–O–Mn bond angles and bond lengths. Thus, we suggest that the transport properties can be explained according to the additional delocalization of charge carriers induced by Zn doping.

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Research paper thumbnail of Study of structural, electrical and magnetic properties of Zn doped La0.67Sr0.33MnO3

Journal of Alloys and Compounds, 2012

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Research paper thumbnail of The effect of temperature and pH on the motility of Listeria sp. J. Islamic Acad. Sci

: The effect of pH of the medium and temperature of incubation was studied on motility of four st... more : The effect of pH of the medium and temperature of incubation was studied on motility of four strains of 2 Listeria species. All the strains were found to be non-motile at pH 4. The motility was observed at a pH range of 5.5 to 9.0. However, the optimum pH and temperature for motility was 7.0 and 22°C respectively. The strains NCTC 7973 and C-274 were, however, motile at pH 5.0. At pH 10, NCTC 5214 m, C-274 and NCTC 7973 were motile at 22°C with the exception of C-274 which was motile also at 35°C. NCTC 5214 m was motile at pH 11 only at 22°C whereas C-274 was motile at both 22 and 35°C. Key Words: Listeria, pH. 28 Microbiology INTRODUCTION The members of the genus Listeria are pathogenic to a very wide range of animals (6, 7) causing multiple clinical manifestations including abortion or still birth in pregnant farm animals (1, 8, 9, 12-17). Listeria that has previously been placed as a member of the family Corynebacteriaceae (2) has now been classified with "Erysipelothrix&q...

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Research paper thumbnail of Synthesis of alkyl catechols and evaluation of their antibacterial and cyfotoxic activity

Pakistan journal of pharmaceutical sciences

A series of potential biologically active mono-, di- and tetra- alkyl catechols were prepared thr... more A series of potential biologically active mono-, di- and tetra- alkyl catechols were prepared through Friedel- Crafts alkylation of catechol, and evaluated for their antibacterial and cytotoxic activity. The mono-substituted alkyl derivatives showed maximum antibacterial activity in vitro which increased with the increasing length of the alkyl chains. Primary screening results indicated that all the monoalkyl derivatives except 4- (2-octyl) catechol inhibited the growth of B. bronchoseptica and maximum zones of inhibition were observed in case of monohexyl catechols (both n- and 2-hexyl) and monobenzyl derivative. In case of Gram-negative organisms growth of Kl. pneumoniae and A. calcoaceticus was inhibited by several derivatives. Mono-3-octyl-, monononyl- and monobenzyl catechols markedly inhibited the growth of Kl. pneumoniae. Mono-2-heptyl catechol inhibited the growth of six Gram-negative bacteria. Minimum inhibitory concentration of six most active compounds of the series was d...

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Research paper thumbnail of In vitro antibacterial studies of some newly synthesized phenacyl-thiosemicarbazones

Pakistan journal of pharmaceutical sciences

Four different phenacyl-thiosemicarbazone compounds were studied for their antibacterial activity... more Four different phenacyl-thiosemicarbazone compounds were studied for their antibacterial activity against seventeen different species/strains. Of the four compounds under study, O-methoxy- phenacyl-thiosemicarbazone was found to be highly active against almost all the cultures under study. The other compounds were also found to possess antibacterial spectrum, but in very high concentrations for some of the cultures.

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Research paper thumbnail of Production of Xylanases and Cellulases by Aspergillus Fumigatus MS16 Using Crude Lignocellulosic Substrates

Pakistan Journal of Botany

Xylanolytic and cellulolytic potential of a soil isolate, Aspergillus fumigatus (MS16) was studie... more Xylanolytic and cellulolytic potential of a soil isolate, Aspergillus fumigatus (MS16) was studied by growing it on a variety of lignocellulosics, purified cellulose and xylan supplemented media. It was noted that carboxymethyl cellulose, salicin and xylan induce the production of endoglucanase, β-glucosidase and xylanase, respectively. The study revealed that Aspergillus fumigatus (MS16) co-secretes xylanase and cellulase in the presence of xylan; the ratio of the two enzymes was influenced by the initial pH of the medium. The maximum titers of xylanase and cellulase were noted at initial pH of 5.0. Relatively higher titers of both the enzymes were obtained when the fungus was cultivated at 35 o C. Whereas, cellulase production was not detected when the fungus was cultivated at 40 o C. The volumetric productivity (Q p) of xylanase was much higher than cellulases. The organism produced 2-3 folds higher titers of xylanase when grown on lignocellulosic materials in submerged cultivati...

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Research paper thumbnail of Nexus 344

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Research paper thumbnail of Comparative study on cellulase production

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Research paper thumbnail of Comparative studies on cellulase production

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Research paper thumbnail of Cellulase production from Aspergillus niger MS82: effect of temperature and pH

New Biotechnology, 2009

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Research paper thumbnail of New Antimicrobial Alkaloids from the Roots of Polyalthia longifolia var. pendula

Planta Medica, 2003

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Research paper thumbnail of Ferromagnetism and adiabatic to non-adiabatic switching process in La0.33Sr0.67Mn1−xFexO3 (0≤x≤0.02) manganite

Journal of Magnetism and Magnetic Materials, 2014

ABSTRACT We have systematically investigated structural, electrical and magnetic properties of Fe... more ABSTRACT We have systematically investigated structural, electrical and magnetic properties of Fe doped La0.33Sr0.67MnO3 manganites synthesized through solid-state reaction-route. All the samples are found to have rhombohedral crystal structure. The crystallite sizes obtained by XRD data are much smaller than the average grain size obtained by scanning electron microscope (SEM). Temperature dependent resistivity data were fitted using Mott׳s variable-range hopping (VRH) and small polaron hopping (SPH) models for obtaining different parameters. The adiabatic SPH conduction mechanism is followed almost for both samples in the absence of magnetic field but a switching from adiabatic to non-adiabatic SPH conduction mechanism is found in the presence of magnetic field. Temperature dependent magnetization (M–T) measurements confirm the decrease in Curie-temperature (TC) with Fe doping. Both the samples exhibited ferromagnetic behavior at 10 K and 300 K with a small hysteresis loop and low coercivity.

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Research paper thumbnail of Influence of Mn Doping on Structural, Optical, Dielectric and Electrical Properties of ZnO Nanostructures

Journal of Nanoengineering and Nanomanufacturing, 2012

ABSTRACT Nanocrystalline samples of Zn1-xMnxO (0 ≤ x ≤ 0.10) have been prepared by sol-gel route.... more ABSTRACT Nanocrystalline samples of Zn1-xMnxO (0 ≤ x ≤ 0.10) have been prepared by sol-gel route. The synthesized samples were characterized by X-ray diffraction (XRD), SEM, EDX, UV–Vis, temperature dependent resistivity and dielectric properties at room temperature in the frequency range of 42 Hz to 5 MHz. XRD studies exhibited wurtzite (hexagonal) crystal structure similar to the parent compound (ZnO) for all samples. The average crystallite size was calculated from XRD data lies within the range 23-27 nm. The SEM micrographs show the agglomeration of the nanoparticles. The UV–Vis. measurement shows reduction in the band gap by Mn doping upto 3% and further it increases for the higher doping of Mn. The dielectric constant (ε), loss tangent (tanδ) and ac conductivity (σac) were studied as a function of frequency and composition at room temperature. The experimental results showed that ε and tanδ decrease when the frequency increases while σac increases with variation in frequency. This behavior can be explained on the basis of space charge polarization according to Maxwell and Wagner’s two-layer model. Resistivity measurement shows semiconducting behavior for all samples.

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Research paper thumbnail of Temperature-dependent dielectric and magnetic properties of Mn doped zinc oxide nanoparticles

Materials Science in Semiconductor Processing, 2014

ABSTRACT Mn substituted zinc oxide (ZnO) nanoparticles with general formula Zn1−xMnxO (x=0.0, 0.0... more ABSTRACT Mn substituted zinc oxide (ZnO) nanoparticles with general formula Zn1−xMnxO (x=0.0, 0.03, 0.05 and 0.10) were synthesized through a sol-gel route. The result of x-ray diffraction (XRD) analysis confirms the presence of wurtzite crystal structure for all samples suggested that doped Mn ions are occupying regular Zn sites. Transmission electron microscopy (TEM) images exhibit the growth of particle size in the range of 30-50 nm. The results of dielectric measurement indicate that the parameters ε′, ε″, tanδ and σac are decreasing with the increase in frequency; whereas these parameters increase with the increase in temperature. Dielectric parameters are also found to increase with the increase in Mn concentration up to 5%, beyond this value the parameters become lower. The dielectric properties have been explained on the basis of space charge polarization according to Maxwell and Wagner’s two-layer model and the hopping between adjacent Mn2+ and Mn3+ ions. The analysis of temperature dependant magnetization data gives the idea about ferromagnetic (FM) exchange interaction.

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Research paper thumbnail of STRUCTURAL AND MAGNETIC PROPERTIES OF La 0.67 Sr 0.33 Mn 0.93 Zn 0.07 O 3 MANGANITE

International Journal of Modern Physics: Conference Series, 2013

ABSTRACT The structural and magnetic measurements have been carried out on La0.67Sr0.33Mn0.93Zn0.... more ABSTRACT The structural and magnetic measurements have been carried out on La0.67Sr0.33Mn0.93Zn0.07O3 (LSMZO) polycrystalline sample. The sample has been prepared through Sol-Gel route. X-ray diffraction (XRD) pattern confirms the single phase nature of the sample with orthorhombic crystal symmetry. Field dependent magnetization shows the hysteresis loop with very low coercive field and the magnetization does not saturate up to the field of 5000 gauss in magnitude.

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Research paper thumbnail of Structural and optical properties of gel-combustion synthesized Zr doped ZnO nanoparticles

Optical Materials, 2013

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Research paper thumbnail of Study of structural property of Co ferrite thin film grown by pulsed laser deposition technique

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Research paper thumbnail of Structural and dielectric properties of Mn doped copper oxide (CuO) nanostructure

ABSTRACT Undoped and Mn doped CuO nanocrystalline powder samples were prepared through standard s... more ABSTRACT Undoped and Mn doped CuO nanocrystalline powder samples were prepared through standard solid state reaction method. The crystal structures of the CuO nanoparticles were characterized by X-ray diffraction. Dielectric measurements were performed on samples as a function of frequency at room temperate to determine the dielectric behavior of the samples. XRD data exhibited the presence of monoclinic crystal structure similar to the parent compound in all samples, suggesting that doped Mn ions sit at the regular lattice sites. The average crystallite size, calculated using Scherrer formula from XRD data, is found within the range of 23-27 nm. The dielectric constant (��), imaginary part of dielectric constant (��) and loss tangent (tan�) were studied as a function of frequency and composition at room temperature. The dependence of dielectric ��������� ������ ��������� ��������������������������������in terms of hopping. This behavior can be explained on the basis of space charge pola��������������������������� ����� �����������-layer model.

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Research paper thumbnail of Study of the structural and dielectric properties of Bi2O3 and PbO addition on BiNbO4 ceramic matrix for RF applications

Journal of Materials Science: Materials in Electronics, 2011

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Research paper thumbnail of Magnetic and electrical properties of In doped cobalt ferrite nanoparticles

Journal of Applied Physics, 2012

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Research paper thumbnail of Adiabatic to non adiabatic change in conduction mechanism of Zn doped La0.67Sr0.33MnO3 perovskite

Journal of Alloys and Compounds, 2013

ABSTRACT We reported the results of electrical transport properties of La0.67Sr0.33Mn1−xZnxO3 (x ... more ABSTRACT We reported the results of electrical transport properties of La0.67Sr0.33Mn1−xZnxO3 (x = 0.1 and 0.2) bulk samples prepared by solid-state reaction route over a wide temperature range from 5 to 300 K in the presence of magnetic fields. Powder X-ray diffraction patterns at room temperature confirmed that both samples were formed in a single phase. Temperature dependent resistivity data were fitted using Mott’s variable-range hopping (VRH) model for a limited range of temperatures (from 235.5 to 300 K for 10% Zn doped and from 133 to 300 K for 20% Zn doped samples) to calculate the hopping distance (Rh) and the density of states near the Fermi level, [N(EF)]. It was found that all the parameters vary systematically with the increase in Zn concentration. Moreover, the temperature dependent resistivity data were also fitted using the small polaron hopping (SPH) model. The adiabatic SPH conduction is followed for 10% doped sample at all fields while a non-adiabatic SPH conduction is found to be active for 20% doped sample in all fields. This type of change in the conduction mechanism is accompanied by subtle electronically induced structural changes involving Mn–O–Mn and Zn–O–Mn bond angles and bond lengths. Thus, we suggest that the transport properties can be explained according to the additional delocalization of charge carriers induced by Zn doping.

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Research paper thumbnail of Study of structural, electrical and magnetic properties of Zn doped La0.67Sr0.33MnO3

Journal of Alloys and Compounds, 2012

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Research paper thumbnail of The effect of temperature and pH on the motility of Listeria sp. J. Islamic Acad. Sci

: The effect of pH of the medium and temperature of incubation was studied on motility of four st... more : The effect of pH of the medium and temperature of incubation was studied on motility of four strains of 2 Listeria species. All the strains were found to be non-motile at pH 4. The motility was observed at a pH range of 5.5 to 9.0. However, the optimum pH and temperature for motility was 7.0 and 22°C respectively. The strains NCTC 7973 and C-274 were, however, motile at pH 5.0. At pH 10, NCTC 5214 m, C-274 and NCTC 7973 were motile at 22°C with the exception of C-274 which was motile also at 35°C. NCTC 5214 m was motile at pH 11 only at 22°C whereas C-274 was motile at both 22 and 35°C. Key Words: Listeria, pH. 28 Microbiology INTRODUCTION The members of the genus Listeria are pathogenic to a very wide range of animals (6, 7) causing multiple clinical manifestations including abortion or still birth in pregnant farm animals (1, 8, 9, 12-17). Listeria that has previously been placed as a member of the family Corynebacteriaceae (2) has now been classified with "Erysipelothrix&q...

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Research paper thumbnail of Synthesis of alkyl catechols and evaluation of their antibacterial and cyfotoxic activity

Pakistan journal of pharmaceutical sciences

A series of potential biologically active mono-, di- and tetra- alkyl catechols were prepared thr... more A series of potential biologically active mono-, di- and tetra- alkyl catechols were prepared through Friedel- Crafts alkylation of catechol, and evaluated for their antibacterial and cytotoxic activity. The mono-substituted alkyl derivatives showed maximum antibacterial activity in vitro which increased with the increasing length of the alkyl chains. Primary screening results indicated that all the monoalkyl derivatives except 4- (2-octyl) catechol inhibited the growth of B. bronchoseptica and maximum zones of inhibition were observed in case of monohexyl catechols (both n- and 2-hexyl) and monobenzyl derivative. In case of Gram-negative organisms growth of Kl. pneumoniae and A. calcoaceticus was inhibited by several derivatives. Mono-3-octyl-, monononyl- and monobenzyl catechols markedly inhibited the growth of Kl. pneumoniae. Mono-2-heptyl catechol inhibited the growth of six Gram-negative bacteria. Minimum inhibitory concentration of six most active compounds of the series was d...

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Research paper thumbnail of In vitro antibacterial studies of some newly synthesized phenacyl-thiosemicarbazones

Pakistan journal of pharmaceutical sciences

Four different phenacyl-thiosemicarbazone compounds were studied for their antibacterial activity... more Four different phenacyl-thiosemicarbazone compounds were studied for their antibacterial activity against seventeen different species/strains. Of the four compounds under study, O-methoxy- phenacyl-thiosemicarbazone was found to be highly active against almost all the cultures under study. The other compounds were also found to possess antibacterial spectrum, but in very high concentrations for some of the cultures.

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Research paper thumbnail of Production of Xylanases and Cellulases by Aspergillus Fumigatus MS16 Using Crude Lignocellulosic Substrates

Pakistan Journal of Botany

Xylanolytic and cellulolytic potential of a soil isolate, Aspergillus fumigatus (MS16) was studie... more Xylanolytic and cellulolytic potential of a soil isolate, Aspergillus fumigatus (MS16) was studied by growing it on a variety of lignocellulosics, purified cellulose and xylan supplemented media. It was noted that carboxymethyl cellulose, salicin and xylan induce the production of endoglucanase, β-glucosidase and xylanase, respectively. The study revealed that Aspergillus fumigatus (MS16) co-secretes xylanase and cellulase in the presence of xylan; the ratio of the two enzymes was influenced by the initial pH of the medium. The maximum titers of xylanase and cellulase were noted at initial pH of 5.0. Relatively higher titers of both the enzymes were obtained when the fungus was cultivated at 35 o C. Whereas, cellulase production was not detected when the fungus was cultivated at 40 o C. The volumetric productivity (Q p) of xylanase was much higher than cellulases. The organism produced 2-3 folds higher titers of xylanase when grown on lignocellulosic materials in submerged cultivati...

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Research paper thumbnail of Nexus 344

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Research paper thumbnail of Comparative study on cellulase production

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Research paper thumbnail of Comparative studies on cellulase production

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