Different Concentrations Of Ag Research Articles

Antibacterial ability and osteogenic activity of porous Sr/Ag-containing TiO2 coatings

Implant-associated infection and poor osseointegration remains a major clinical challenge in Ti-based implant materials. A versatile strategy to endow Ti-based implants with long-term antibacterial ability as well as better osteogenic activity is highly desirable for high quality implantation. Strontium (Sr) has been shown to be a significant element to favor bone growth by promoting new bone formation and inhibiting bone resorption. In this study, a novel duplex-treatment technique encompassing magnetron sputtering with micro-arc oxidation is utilized to fabricate porous Sr/Ag-containing TiO2 coatings loaded with different concentrations of Ag and Sr. All coatings are porous with pore size less than 5 µm. Ag is primarily distributed homogeneously inside the pores, and the concentrations of Ag in Sr/Ag-containing TiO2 coatings with low and high Ag contents are 0.40 at.% and 0.83 at.% respectively. We have demonstrated that this kind of coating displays long-lasting antibacterial ability even up to 28 d due to the incorporation of Ag. Further, Sr/Ag-containing TiO2 coatings with optimum Ag and Sr contents revealed good cytocompatibility, enhanced osteoblast spreading and osseointegration, which stemmed primarily from the synergistic effect exerted by the porous surface topography and the bioactive element Sr. However, this study has also identified, for the first time, that proper addition of Ag would further facilitate osteogenic effects. Besides, Sr may be able to alleviate the potential cytotoxic effect of excessive Ag. Thus, integration of optimum functional elements Ag and Sr into Ti-based implant materials would be expected to expedite osseointegration while simultaneously sustaining long-term antibacterial activity, which would provide new insights for relevant fundamental investigations and biomedical applications.

Ag-coated polyurethane fibers membranes absorbed with quinary fatty acid eutectics solid-liquid phase change materials for storage and retrieval of thermal energy

The novel quinary fatty acid eutectic (CA-LA-MA-PA-SA) of capric acid, lauric acid, myristic acid, palmitic acid and stearic acid was successfully prepared with the mass ratio of 61.09/24.61/8.13/4.01/2.16. Thereafter, the innovative Ag-coated polyurethane (PU) fibers membranes with different concentrations of Ag, which were selected as a supporting material to adsorb the CA-LA-MA-PA-SA eutectics, were successfully fabricated through electrospinning followed by magnetron sputter. The energy dispersive X-ray confirmed that Ag nanoclusters were successfully deposited on the surface of PU fibers as a result of sputter coating. The observations of atomic force microscope indicated that the surface roughness of the PU fibers significantly increased with increase in coating time. The scanning electron microscope images demonstrated that the CA-LA-MA-PA-SA eutectics were uniformly distributed into the three-dimensional porous structures of uncoated and Ag-coated PU fibers membranes. Furthermore, the differential scanning calorimeter curves suggested that the CA-LA-MA-PA-SA/PU/Ag composites phase change materials (PCMs) possessed melting enthalpies about 110 kJ/kg and melting temperature around 17 °C. The absorption ratios of the CA-LA-MA-PA-SA eutectic in composite PCMs was approximately at 73.74%–83.18%. The investigation on thermal performance indicated that we achieved higher melting and freezing rates of the CA-LA-MA-PA-SA/PU/Ag composites PCMs by increasing coating time. In addition to this, after depositing Ag nanoparticles the melting and freezing times of composites PCMs were shortened to about 21%–65%.

Vacuum plasma sprayed coatings using ionic silver doped hydroxyapatite powder to prevent bacterial infection of bone implants.

Fast and efficient osseointegration of implants into bone is of crucial importance for their clinical success; a process that can be enhanced by coating the implant surface with hydroxyapatite (HA) using the vacuum plasma spray technology (VPS). However, bacterial infections, especially the biofilm formation on implant surfaces after a surgery, represent a serious complication. With ever-increasing numbers of antibiotic-resistant bacteria, there is great interest in silver (Ag) as an alternative to classical antibiotics due to its broad activity against Gram-positive and Gram-negative bacterial strains. In the present study, silver ions were introduced into HA spray powder by ion exchange and the HA-Ag powder was applied onto titanium samples by VPS. The Ag-containing surfaces were evaluated for the kinetics of the silver release, its antibacterial effect against Staphylococcus aureus as well as Escherichia coli, and possible cytotoxicity against human bone cells. The HA-Ag coatings with different concentrations of Ag displayed mechanical and compositional properties that fulfill the regulatory requirements. Evaluation of the Ag release kinetic showed a high release rate in the first 24 h followed by a decreasing release rate over the four subsequent days. The HA-Ag coatings showed no cytotoxicity to primary human bone cells while exhibiting antibacterial activity to E. coli and S. aureus.

Assessment of silver nanoparticles contamination on faba bean-Rhizobium leguminosarum bv. viciae-Glomus aggregatum symbiosis: Implications for induction of autophagy process in root nodule

The current study sought to explore the response of application of purified silver nanoparticles (Ag NPs) to soil grown with faba bean (Vicia faba L.) and inoculated either with Rhizobium leguminosarum bv. viciae ASU (KF670819) or Glomus aggregatum 14G or in combination. Ag NPs was synthesized and stabilized using PVP and characterized by UV–vis spectroscopy and the characteristic surface plasmon resonance band centered at 430nm. Also, the morphologies and structures of Ag NPs were characterized by X-ray diffraction and transmission electron microscopy and the size distribution ranged from 5 to 50nm. In the first experiment, the germination and seedling growth of faba bean plants were tested under different concentrations of Ag+ as AgNO3 and Ag NPs (100–900μgkg−1 soil). The germination declined by 40% when exposed to Ag NPs at concentration 800μgkg−1 soil, while the same level from Ag+ completely inhibited seed germination. In the second experiment the effect of a high concentration of Ag NPs (800μgkg−1 soil) on R. leguminosarum bv. viciae growth and G. aggregatum activity in soil and their symbiosis with faba bean was investigated. It was proved that Ag NPs considerably retarded the process of nodulation, nitrogenase activity, mycorrhizal colonization, mycorrhizal responsiveness and glomalin content. High concentration of Ag NPs (800μgkg−1 soil) resulted in detectable alterations including the intracellular deterioration of cytoplasmic components by means of autophagy and disintegration of bacteroids. These findings elucidate the mechanism of toxic action of Ag NPs which resulted in early senescence of root nodules.

Piezoelectric and opto-electrical properties of silver-doped ZnO nanorods synthesized by low temperature aqueous chemical method

In this paper, we have synthesized Zn1−xAgxO (x = 0, 0.03, 0.06, and 0.09) nanorods (NRs) via the hydrothermal method at low temperature on silicon substrate. The characterization and comparison between the different Zn1−xAgxO samples, indicated that an increasing Ag concentration from x = 0 to a maximum of x = 0.09; All samples show a preferred orientation of (002) direction with no observable change of morphology. As the quantity of the Ag dopant was changed, the transmittances, as well as the optical band gap were decreased. X-ray photoelectron spectroscopy data clearly indicate the presence of Ag in ZnO crystal lattice. A nanoindentation-based technique was used to measure the effective piezo-response of different concentrations of Ag for both direct and converse effects. The value of the piezoelectric coefficient (d33) as well as the piezo potential generated from the ZnO NRs and Zn1−xAgxO NRs was found to decrease with the increase of Ag fraction. The finding in this investigation reveals that Ag doped ZnO is not suitable for piezoelectric energy harvesting devices.

DNA logic gate based on metallo-toehold strand displacement.

DNA is increasingly being used as an ideal material for the construction of nanoscale structures, circuits, and machines. Toehold-mediated DNA strand displacement reactions play a very important role in these enzyme-free constructions. In this study, the concept of metallo-toehold was utilized to further develop a mechanism for strand displacement driven by Ag+ ions, in which the intercalation of cytosine–cytosine mismatched base pairs on the toeholds provides additional control by varying of the concentration of Ag+ ions. The characteristics of displacement reaction in response to different concentration of Ag+ ions are investigated by fluorescence spectral and non-denaturing polyacrylamide gel electrophoresis. The reaction can successfully occur when the concentration of Ag+ ions is suitabe; excess Ag+ ions block the reaction. Furthermore, the displacement reaction can be tuned and controlled most efficiently under the condition of two C:C mismatched base pairs placed on the six-nt toehold. Based on our research, a mechanism was developed to construct Boolean logic gate AND and OR by employing strand displacement reaction as a tool, Ag+ and Hg2+ as input.

Optical properties of Ag nanoclusters formed by irradiation and annealing of SiO2/SiO2:Ag thin films

• Fabrication of films through the Reactive Electron Beam deposition technique. • Perfect and reproducible Ag nanoclustered host matrix. • Potential technological applicability in thermoelectric devices. We have deposited five periodic SiO 2 /SiO 2 + Ag multi-nano-layered films on fused silica substrates using physical vapor deposition technique. The co-deposited SiO 2 :Ag layers were 2.7–5 nm and SiO 2 buffer layers were 1–15 nm thick. Total thickness was between 30 and 105 nm. Different concentrations of Ag, ranging from 1.5 to 50 molecular% with respect to SiO 2 were deposited to determine relevant rates of nanocluster formation and occurrence of interaction between nanoclusters. Using interferometry as well as in situ thickness monitoring, we measured the thickness of the layers. The concentration of Ag in SiO 2 was measured with Rutherford Backscattering Spectrometry (RBS). To nucleate Ag nanoclusters, 5 MeV cross plane Si ion bombardments were performed with fluence varying between 5 × 10 14 and 1 × 10 16 ions/cm 2 values. Optical absorption spectra were recorded in the range of 200–900 nm in order to monitor the Ag nanocluster formation in the thin films. Thermal annealing treatment at different temperatures was applied as second method to form varying size of nanoclusters. The physical properties of formed super lattice were criticized for thermoelectric applications.

Effect of Silver Concentration on Thermoluminescence Studies of (Cd0.95Zn.05)S Phosphors with Trap Depth Parameters Synthesized by Solid State Reaction Method

The present paper reports the synthesis and characterization of Ag+ doped (Cd0.95Zn.05)S phosphors. The effect of variable concentration of silver on thermoluminescence (TL) studies was investigated. The samples were prepared by solid state synthesis technique which is suitable for large scale production for phosphors. The starting materials used for sample preparation were CdS, ZnS, (Ag(NO3)3 and sodium chloride was used as a flux during synthesis. The prepared sample was characterized by X-ray diffraction technique (XRD). Sample shows hexagonal structure and the particle size calculated by Scherer’s formula. The prepared phosphors were also examined by thermoluminescence technique. For recording TL glow curve every time 2mg phosphor was irradiated by 254nm UV source and fixed the heating rate at 8 0 C s -1 . Sample shows wellresolved peak at 1020C for different concentration of Ag+. In addition, the effect of Ag+ concentration at fixed UV exposure time was studied. The effect of UV exposure time and dose versus intensity plot were studied. Sample shows linear response with dose and broaden peak with lower temperature shows the less stability and more fading in TL glow curve. Trapping parameters were calculated for every recorded glow curve. The heating rate effect on TL glow curve was also studied. The sample shows the shifting of TL glow curve towards to lower temperature side with increasing of heating rate. The kinetic parameters like trap depth, frequency factor were calculated by using the Peak shape method, which are discussed in details. The TL Glow curves were fitted in CGCD (computerized glow curve convolution deconvolution) technique & trapping parameters calculated.

Effects of ionization on the toxicity of silver nanoparticles to Japanese medaka (Oryzias latipes) embryos

Increase in the use of manufactured nanomaterials (NMs) has led to concerns about the environmental impacts. Especially, hazard of metal-based NMs is more severe due to ions released from surface by water quality parameters and physicochemical properties after entering into the water environment. However, little is known about the effects of ionization on the toxicity of metal-based NMs in the water environment. To address this question, we prepared the suspensions of silver nanoparticles (AgNP) at 25 μg L−1 containing different concentrations of Ag+ (5, 10, 20, 45, and 75% Ag+ to total Ag), and evaluated their toxicity to Japanese medaka (Oryzias latipes) embryos. Higher Ag+ ratios in the AgNP suspension, suggesting the lower number of particles, led to the higher adverse effects on embryos and sac-fries. In addition, histopathology analysis revealed that AgNPs penetrated through chorion of eggs and skin membrane, and were distributed into the tissues. The results imply that the ionization could decrease the toxicity of metal-based NMs in the water environment.

Optical and structural properties of CdS thin films grown by chemical bath deposition doped with Ag by ion exchange

In this work thin CdS films using glycine as a complexing agent were fabricated by chemical bath deposition and then doped with silver (Ag), by an ion exchange process with different concentrations of AgNO3 solutions. The CdS films were immersed in silver solutions using different concentrations during 1min for doping and after that the films were annealed at 200°C during 20min for dopant diffusion after the immersion on the AgNO3 solutions. The aim of this research was to know the effects of different concentrations of Ag on the optical and structural properties of CdS thin films. The optical band gap of the doped films was determined by transmittance measurements, with the results of transmittance varying between 35% and 70% up to 450nm in the electromagnetic spectra and the band gap varying between 2.31 and 2.51eV depending of the silver content. X-ray photoelectron spectroscopy was used to study the influence of silver on the CdS:Ag films, as a function of the AgNO3 solution concentration. The crystal structure of the thin CdS:Ag films was studied by the X-ray diffraction method and the film surface morphology was studied by atomic force microscopy. Using the ion exchange process, the CdS films’ structural, optical and electric characteristics were modified according to silver nitrate concentration used.

Ag Nanoparticle Embedded TiO2 Composite Nanorod Arrays Fabricated by Oblique Angle Deposition: Toward Plasmonic Photocatalysis

Using a unique oblique angle co-deposition technique, well-aligned arrays of Ag nanoparticle embedded TiO2 composite nanorods have been fabricated with different concentrations of Ag. The structural, optical, and photocatalytic properties of the composite nanostructures are investigated using a variety of experimental techniques and compared with those of pure TiO2 nanorods fabricated similarly. Ag nanoparticles are formed in the composite nanorods, which increase the visible light absorbance due to localized surface plasmon resonance. The Ag concentrations and the annealing conditions are found to affect the size and the density of Ag nanoparticles and their optical properties. The Ag nanoparticle embedded TiO2 nanostructures exhibit enhanced photocatalytic activity compared to pure TiO2 under visible- or UV-light illumination. Ag plays different roles in assisting the photocatalysis with different light sources. Ag can be excited and can inject electrons to TiO2, working as an electron donor under visible light. While under UV illumination, Ag acts as an electron acceptor to trap the photogenerated electrons in TiO2. Due to the opposite electron transfer direction under UV and visible light, the presence of Ag may not result in a greater enhancement in the photocatalytic performance.

Polymer–ionic liquid composite membranes for propane/propylene separation by facilitated transport

In this work we propose the use of novel PVDF-HFP/BMImBF4–Ag+ facilitated transport membranes to carry out the separation of olefin/paraffin gas mixtures. These membranes exhibit good thermal, chemical and mechanical stability, good separation properties and easy manufacturing. The effect of membrane composition was evaluated concluding that the membrane with 80% polymer-20% IL w/w shows the best compromise between separation properties and mechanical resistance. The separation performance of membranes containing different concentrations of Ag+ in the range 0–80% w/w at temperatures between 293 and 323K and different operational conditions has been evaluated. The facilitated transport membranes herein prepared provided very promising results when tested with 50/50%v/v C3H8/C3H6 mixtures obtaining C3H6 permeabilities up to 6630barrer and C3H8/C3H6 selectivities over 700, placing these membranes well above the Robeson upper bound for polymeric and inorganic membranes. Moreover, these membranes showed excellent stability during long-term experiments carried out for 10 days.

Optical properties of Ag doped ZnO nanocrystals prepared by hydrothermal and photodeposition method

Ag/ZnO nanocomposites have been synthesized by facile hydrothermal and photodeposition method. The effect of different concentration of Ag on the luminous intensity of ZnO was studied. The morphology, structure and optical properties of Ag/ZnO were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and photoluminescence (PL), respectively. The XRD patterns show that intensities of diffraction peaks of Ag/ZnO were enhanced. The weak diffraction peak at 38.28° can be assigned to Ag2O when the concentration of Ag increased to 0.09 M. PL results demonstrate that the UV luminous intensity of ZnO was significantly influenced by the concentration of Ag. The UV luminous intensity of Ag/ZnO nanocomposites increased by 11 times as compared with undoped ZnO when the concentration of Ag was 0.03 M due to the local surface plasma resonance effect of Ag nanoparticles.

In vitro bactericidal and TEM study of the interaction of Ag-modified titania with coliform bacteria

This research aimed to investigate in vitro photocatalytic bactericidal effect of Ag–TiO2 nanocomposite using Escherichia coli as a model organism. Highly dispersed, Ag–TiO2 nanocomposite is used with an average particle size of less than 20 nm. Bactericidal analysis was carried out in Luria Bertani medium on solid agar plates with various illumination time and different concentrations of Ag–TiO2 nanocomposite. Transmission electron microscopic analysis of bacterial section was used to detect the effect of irradiation of Ag–TiO2 nanocomposite on the ultra structure of the bacterial cell in order to reveal possible cellular damage. The mechanism underlying the action of photoexcited Ag–TiO2 nanocomposite on E. coli cell membrane is also evaluated. The results confirmed that E. coli cells after the contact with Ag–TiO2 nanocomposite were damaged since they showed membrane disorganization. This causes the enhanced level of membrane permeability leading to the build-up of Ag–TiO2 nanocomposite in the bacterial membrane and also cellular internalization of these nanoparticles.

Anisotropic silver nanoparticles as filler for the formation of hybrid nanocomposites

Prismatic and plate-like silver nanoparticles (Ag NPs) were synthesized according to the seed-mediated method. These particles were used as precursors for preparation of homogenous, transparent and colored Ag/poly(vinyl alcohol) (PVA) nanocomposite films with different concentrations of Ag by solution-casting technique. Optical and structural characterization of these nanocomposites includes UV–visible spectroscopy, X-ray diffraction (XRD), FTIR spectroscopy and SEM measurements. Further, the effect of embedded nanoparticles on the thermal properties of the PVA matrix was studied. The value of the glass transition temperature of polymer is found to increase after embedding Ag NPs. Comparison of thermal properties of pure PVA and nanocomposite films showed that the thermo-oxidative stability of polymer slightly increased in the presence of Ag NPs. Furthermore, the effect of the Ag NPs on the crystallinity of polymer was also observed.

Interactions of silver nanoparticles with the marine macroalga, Ulva lactuca

The marine macroalga, Ulva lactuca, has been exposed for 48h to different concentrations of Ag added as either silver nanoparticles (AgNP) or aqueous metal (AgNO(3)) and the resulting toxicity, estimated from reductions in quenching of chlorophyll-a fluorescence, and accumulation of Ag measured. Aqueous Ag was toxic at available concentrations as low as about 2.5μgl(-1) and exhibited considerable accumulation that could be defined by the Langmuir equation. AgNP were not phytotoxic to the macroalga at available Ag concentrations up to at least 15μgl(-1) and metal measured in U. lactuca was attributed to a physical association of nanoparticles at the algal surface. At higher AgNP concentrations, a dose-response relationship was observed that was similar to that for aqueous Ag recorded at much lower concentrations. These findings suggest that AgNP are only indirectly toxic to marine algae through the dissolution of Ag(+) ions into bulk sea water, albeit at concentrations orders of magnitude greater than those predicted in the environment.

Influence of antimicrobial finishes on the biodeterioration of cotton and cotton/polyester fabrics: Leaching versus bio-barrier formation

The influence of antimicrobial activity of two contemporary finishes, specifically a dispersion of colloidal silver (Ag) and 3-(trimethoxysilyl)-propyldimethyloctadecyl ammonium chloride (Si-QAC), on the degree of biodeterioration of 100% cotton (CO) fabric and fabric composed of a mixture of cotton and polyester (CO/PET) was studied. Ag was chosen for the leaching agent, while Si-QAC was used as the bio-barrier-forming agent. The biodeterioration of samples finished with different concentrations of Ag and Si-QAC was analysed from a standard soil burial test after 3, 6 and 12 days of exposure to soil microflora. SEM micrographs revealed intensive biodeterioration of the unfinished cellulose fibres, while the highly biologically resistant polyester fibres remained undamaged. A controlled release of Ag successfully inhibited biodeterioration of the cellulose fibres in the CO and CO/PET fabrics when its concentration reached a lethal, biocidal concentration. Contrary to the effects of Ag, the bio-barrier formation of Si-QAC on CO and CO/PET fabrics was insufficient to protect the cellulose fibres during longer periods of soil burial, irrespective of its concentration. Intensive chemical changes to the cellulose were clearly seen from the FT-IR spectra of all of the samples. The resistance of the polyester component to biodeterioration did not provide any significant protection for the cotton component in CO/PET fabric.

Lanthanum Telluride Nanowires: Formation, Doping, and Raman Studies

A new approach for the synthesis of freely dispersible one-dimensional (1D) lanthanum telluride nanowires (La2Te3 NWs) in the solution phase is reported. The process involves a reaction between tellurium nanowires (Te NWs) and lanthanum nitrate (La(NO3)3) at room-temperature. Te NWs act as templates for the formation of La2Te3 NWs. The aspect ratio of the as prepared La2Te3 NWs is the same as the parent Te NWs. Various microscopic and spectroscopic tools such as HRTEM, SEM, EDAX, XRD, XPS, Raman, UV−visible, and fluorescence were used for the characterization of the NWs. A new surface enhanced Raman active substrate was synthesized by doping silver in La2Te3 NWs. Surface enhanced Raman spectra were studied using crystal violet (CV) as the analyte. Raman features have been observed up to a concentration of 10−8 M of CV. Different concentrations of Ag in Ag-doped La2Te3 NWs were investigated. The monodispersity, homogeneity, and Raman enhancement properties of the NWs, synthesized through a simple solution ...

Three different fluorescent responses to transition metal ions using receptors based on 1,2-bis- and 1,2,4,5-tetrakis-(8-hydroxyquinolinoxymethyl)benzene

The dipod 1,2-bis(8-hydroxyquinolinoxymethyl)benzene (3) and tetrapod 1,2,4,5-tetrakis(8-hydroxyquinolinoxymethyl)benzene (5) have been synthesized through nucleophilic substitution of respective 1,2-bis(bromomethyl)benzene (2) and 1,2,4,5-tetra(bromomethyl)benzene (4) with 8-hydroxyquinoline (1). For comparison, 1,3,5-tris(8-hydroxyquinolinoxymethyl)benzene derivatives (7a and 7b) have been obtained. The complexation behavior of these podands towards Ag+, Co2+, Ni2+, Cu2+, Zn2+, and Cd2+ metal ions has been investigated in acetonitrile by fluorescence spectroscopy. The sterically crowded 1,2,4,5-tetrapod 5 displays unique fluorescence ‘ON–OFF–ON’ switching through fluorescence quenching (λmax 395 nm, switch OFF) with <1.0 equiv of Ag+ and fluorescence enhancement (λmax 495 nm, switch ON) with >3 equiv Ag+ and can be used for estimation of two different concentrations of Ag+ at two different wavelengths. The addition of Cu2+, Ni2+, and Co2+ metal ions to tetrapod 5 causes fluorescence quenching, i.e., ‘ON–OFF’ phenomena at λmax 395 nm for <10 μM (1 equiv) of these ions but addition of Zn2+ and Cd2+ to tetrapod 5 results in fluorescence enhancement with a gradual shift of λem from 395 to 432 and 418 nm, respectively. Similarly, dipod 3 behaves as an ‘ON–OFF–ON’ switch with Ag+, an ‘ON–OFF’ switch with Cu2+, and an ‘OFF–ON’ switch with Zn2+. The placement of quinolinoxymethyl groups at the 1,3,5-positions of benzene ring in tripod 7a–b leads to simultaneous fluorescence quenching at λmax 380 nm and enhancement at λmax 490 nm with both Ag+ and Cu2+. This behavior is in parallel with 8-methoxyquinoline 8. The rationalization of these results in terms of metal ion coordination and protonation of podands shows that 1,2 placement of quinoline units in tetrapod 5 and dipod 3 causes three different fluorescent responses, i.e., ‘ON–OFF–ON’, ‘ON–OFF’, and ‘OFF–ON’ due to metal ion coordination of different transition metal ions and 1, 3, and 5 placement of three quinolines in tripod 7, the protonation of quinolines is preferred over metal ion coordination. In general, the greater number of quinoline units coordinated per metal ion in 5 compared with the other podands points to organization of the four quinoline moieties around metal ions in the case of 5.

Response of catalase activity to Ag +, Cd 2+, Cr 6+, Cu 2+ and Zn 2+ in five tissues of freshwater fish Oreochromis niloticus

Catalase (CAT, EC 1.11.1.6) is an important enzyme in antioxidant defense system protecting animals from oxidative stress. Freshwater fish Oreochromis niloticus were exposed for 96 h to different concentrations of Ag +, Cd 2+, Cr 6+, Cu 2+ and Zn 2+, known to cause oxidative stress, and subsequently CAT activities in liver, kidney, gill, intestine and brain were measured. In vivo, CAT was stimulated by all metals except Ag + in the liver and the highest increase in CAT activity (183%) resulted from 1.0 mg Cd 2+/L exposure, whereas 0.5 mg Ag +/L exposure resulted in a sharp decrease (44%). In tilapia kidney, cadmium and zinc had no significant effects on CAT activity, whereas 0.1 mg Cr 6+/L exposure caused a decrease (44%). Cadmium and zinc did not significantly affect the CAT activity in gill; however, 0.5 mg Ag +/L exposure caused an increase (66%) and 1.5 mg Cr 6+/L exposure caused a decrease (97%) in CAT activity. All metals, except Cu 2+(41% increase), caused significant decreases in CAT activity in the intestine. In brain, 1.0 mg Zn 2+/L resulted in an increase in CAT activity (126%), while 1.5 mg Ag +/L exposure caused a 54% decrease. In vitro, all metals — except Ag + and Cu 2+ in kidney — significantly inhibited the CAT activity in all tissues. Results emphasized that CAT may be considered as a sensitive bioindicator of the antioxidant defense system.