Formation of palladium(0) nanoparticles at microbial surfaces

The increasing demand and limited natural resources for industrially important platinum‐group metal (PGM) catalysts render the recovery from secondary sources such as industrial waste economically interesting. In the process of palladium (Pd) recovery, microorganisms have revealed a strong potential. Hitherto, bacteria with the property of dissimilatory metal reduction have been in focus, although the biochemical reactions linking enzymatic Pd(II) reduction and Pd(0) deposition have not yet been identified. In this study we investigated Pd(II) reduction with formate as the electron donor in the presence of Gram‐negative bacteria with no documented capacity for reducing metals for energy production: Cupriavidus necator, Pseudomonas putida, and Paracoccus denitrificans. Only large and close‐packed Pd(0) aggregates were formed in cell‐free buffer solutions. Pd(II) reduction in the presence of bacteria resulted in smaller, well‐suspended Pd(0) particles that were associated with the cells (called “bioPd(0)” in the following). Nanosize Pd(0) particles (3–30 nm) were only observed in the presence of bacteria, and particles in this size range were located in the periplasmic space. Pd(0) nanoparticles were still deposited on autoclaved cells of C. necator that had no hydrogenase activity, suggesting a hydrogenase‐independent formation mechanism. The catalytic properties of Pd(0) and bioPd(0) were determined by the amount of hydrogen released in a reaction with hypophosphite. Generally, bioPd(0) demonstrated a lower level of activity than the Pd(0) control, possibly due to the inaccessibility of the Pd(0) fraction embedded in the cell envelope. Our results demonstrate the suitability of bacterial cells for the recovery of Pd(0), and formation and immobilization of Pd(0) nanoparticles inside the cell envelope. However, procedures to make periplasmic Pd(0) catalytically accessible need to be developed for future nanobiotechnological applications. Biotechnol. Bioeng. 2010;107: 206–215. © 2010 Wiley Periodicals, Inc.     

Microbially supported synthesis of catalytically active bimetallic Pd-Au nanoparticles

Bimetallic nanoparticles are considered the next generation of nanocatalysts with increased stability and catalytic activity. Bio-supported synthesis of monometallic nanoparticles has been proposed as an environmentally friendly alternative to the conventional chemical and physical protocols. In this study we synthesize bimetallic bio-supported Pd-Au nanoparticles for the first time using microorganisms as support material. The synthesis involved two steps: (1) Formation of monometallic bio-supported Pd(0) and Au(0) nanoparticles on the surface of Cupriavidus necator cells, and (2) formation of bimetallic bio-supported nanoparticles by reduction of either Au(III) or Pd(II) on to the nanoparticles prepared in step one. Bio-supported monometallic Pd(0) or Au(0) nanoparticles were formed on the surface of C. necator by reduction of Pd(II) or Au(III) with formate. Addition of Au(III) or Pd(II) to the bio-supported particles resulted in increased particle size. UV-Vis spectrophotometry and HR-TEM analyses indicated that the previously monometallic nanoparticles had become fully or partially covered by Au(0) or Pd(0), respectively. Furthermore, Energy Dispersive Spectrometry (EDS) and Fast Fourier Transformation (FFT) analyses confirmed that the nanoparticles indeed were bimetallic. The bimetallic nanoparticles did not have a core-shell structure, but were superior to monometallic particles at reducing p-nitrophenol to p-aminophenol. Hence, formation of microbially supported nanoparticles may be a cheap and environmentally friendly approach for production of bimetallic nanocatalysts.     

Non-enzymatic palladium recovery on microbial and synthetic surfaces

The use of microorganisms as support for reduction of dissolved Pd(II) to immobilized Pd(0) nanoparticles is an environmentally friendly approach for Pd recovery from waste. To better understand and engineer Pd(0) nanoparticle synthesis, one has to consider the mechanisms by which Pd(II) is reduced on microbial surfaces. Escherichia coli, Shewanella oneidensis, and Pseudomonas putida were used as model organisms in order to elucidate the role of microbial cells in Pd(II) reduction under acidic conditions. Pd(II) was reduced by formate under acidic conditions, and the process occurred substantially faster in the presence of cells as compared to cell-free controls. We found no difference between native (untreated) and autoclaved cells, and could demonstrate that even a non-enzymatic protein (bovine serum albumin) stimulated Pd(II) reduction as efficiently as bacterial cells. Amine groups readily interact with Pd(II), and to specifically test their role in surface-assisted Pd(II) reduction by formate, we replaced bacterial cells with polystyrene microparticles functionalized with amine or carboxyl groups. Amine-functionalized microparticles had the same effect on Pd(II) reduction as bacterial cells, and the effect could be hampered if the amine groups were blocked by acetylation. The interaction with amine groups was confirmed by infrared spectroscopy on whole cells and amine-functionalized microparticles. In conclusion, bio-supported Pd(II) reduction on microbial surfaces is possibly mediated by a non-enzymatic mechanism. We therefore suggest the use of amine-rich biomaterials rather than intact cells for Pd bio-recovery from waste.     

Biological control of the size and reactivity of catalytic Pd(0) produced by Shewanella oneidensis

The interaction between Shewanella oneidensis MR-1 and the soluble metal Pd(II) during the reductive precipitation of Pd(0) determined the size and properties of the precipitated Pd(0) nanoparticles. Assessment of cell viability indicated that the bioreduction of Pd(II) was a detoxification mechanism depending on the Pd(II) concentration and on the presence and properties of the electron donor. The addition of H₂ in the headspace allowed S. oneidensis to resist the toxic effects of Pd(II). Interestingly, 25 mM formate was a less effective electron donor for bioreductive detoxification of Pd(II), since there was a 2 log reduction of culturable cells and a 20% decrease of viable cells within 60 min, followed by a slow recovery. When the ratio of Pd:cell dry weight (CDW) was below 5:2 at a concentration of 50 mg l⁻¹ Pd(II), most of the cells remained viable. These viable cells precipitated Pd(0) crystals over a relatively larger bacterial surface area and had a particle area that was up to 100 times smaller when compared to Pd(0) crystals formed on non-viable biomass (Pd:CDW ratio of 5:2). The relatively large and densely covering Pd(0) crystals on non-viable biomass exhibited high catalytic reactivity towards hydrophobic molecules such as polychlorinated biphenyls, while the smaller and more dispersed nanocrystals on a viable bacterial carrier exhibited high catalytic reactivity towards the reductive degradation of the anionic pollutant perchlorate.     

Biological activity and mechanical stability of sol-gel-based biofilters using the freeze-gelation technique for immobilization of Rhodococcus ruber

Biofilters with long lifetime and high storage stability are very important for bioremediation processes to ensure the readiness at the occurrence of sudden contaminations. By using the freeze-gelation technique, living cells can be immobilized within a mechanically and chemically stable ceramic-like matrix. Due to a freeze-drying step, the embedded microorganisms are converted into a preserved form. In that way, they can be stored under dry conditions, which comply better with storage, transport, and handling requirements. Thus, in contrast to other immobilization techniques, there is no need for storage in liquid or under humid atmosphere. The biological activity, mechanical strength, and the structure of the biologically active ceramic-like composites (biocers) produced by freeze gelation have been investigated by using the phenol-degrading bacteria Rhodococcus ruber as model organism. Samples of freeze-gelation biocers have been investigated after defined storage periods, demonstrating nearly unchanged mechanical strength of the immobilization matrix as well as good storage stability of the activity of the immobilized cells over several months of storage at 4 °C. Repeated-batch tests demonstrated further that the freeze-gelation biocers can be repeatedly used over a period of more than 12 months without losing its bioactivity. Thus, these results show that freeze-gelation biocers have high potential of being scaled up from laboratory test systems to applications in real environment because of their long bioactivity as well as mechanical stability.     

Continuous removal of Cr(VI) from aqueous solution catalysed by palladised biomass of Desulfovibrio vulgaris

Growth-decoupled cells of Desulfovibrio vulgaris NCIMB 8303 can be used to reduce Pd(II) to cell-bound Pd(0) (Bio-Pd(0)), a bioinorganic catalyst capable of reducing hexavalent chromium to less toxic Cr(III), using formate as the electron donor. Magnetic resonance imaging showed that Bio-Pd(0), immobilized in chitosan and agar beads, is distinguishable from the surrounding gel and is evenly dispersed within the immobilization matrix. Agar-immobilized Bio-Pd(0) and 'chemical Pd(0)' were packed into continuous-flow reactors, and challenged with a solution containing 100 microM Cr(VI) (pH 7) at a flow rate of 2.4 ml h(-1). Agar-immobilized chemical Pd(0) columns lost Cr(VI) reducing ability by 160 h, whereas columns containing immobilized Bio-Pd(0) maintained 90% reduction until 680 h, after which reduction efficiency was gradually lost.     

Metal binding by bacteria from uranium mining waste piles and its technological applications

Uranium mining waste piles, heavily polluted with radionuclides and other toxic metals, are a reservoir for bacteria that have evolved special strategies to survive in these extreme environments. Understanding the mechanisms of bacterial adaptation may enable the development of novel bioremediation strategies and other technological applications. Cell isolates of Bacillus sphaericus JG-A12 from a uranium mining waste pile in Germany are able to accumulate high amounts of toxic metals such as U, Cu, Pb, Al, and Cd as well as precious metals. Some of these metals, i.e. U, Cu, Pd(II), Pt(II) and Au(III), are also bound by the highly orderd paracrystalline proteinaceous surface layer (S-layer) that envelopes the cells of this strain. These special capabilities of the cells and the S-layer proteins of B. sphaericus JG-A12 are highly interesting for the clean-up of uranium contaminated waste waters, for the recovery of precious metals from electronic wastes, and for the production of metal nanoclusters. The fabricated nanoparticles are promising for the development of novel catalysts. This work reviews the molecular biology of the S-layer of the strain JG-A12 and the S-layer dependent interactions of the bacterial cells with metals. It presents future perspectives for their application in bioremediation and nanotechnology.     

Palladium bionanoparticles production from acidic Pd(II) solutions and spent catalyst leachate using acidophilic Fe(III)-reducing bacteria

The acidophilic, Fe(III)-reducing heterotrophic bacteria Acidocella aromatica PFBC^T and Acidiphilium cryptum SJH were utilized to produce palladium (Pd) bionanoparticles via a simple 1-step microbiological reaction. Monosaccharide (or intracellular NADH)-dependent reactions lead to visualization of intra/extra-cellular enzymatic Pd(0) nucleation. Formic acid-dependent reactions proceeded via the first slow Pd(0) nucleation phase and the following autocatalytic Pd(II) reduction phase regardless of the presence or viability of the cells. However, use of active cells (with full enzymatic and membrane protein activities) at low formic acid concentration (5 mM) was critical to allow sufficient time for Pd(II) biosorption and the following enzymatic Pd(0) nucleation, which consequently enabled production of fine, dense and well-dispersed Pd(0) bionanoparticles. Differences of the resultant Pd(0) nanoparticles in size, density and localization between the two bacteria under each condition tested suggested different activity and location of enzymes and membrane “Pd(II) trafficking” proteins responsible for Pd(0) nucleation. Despite the inhibitory effect of leaching lixiviant and dissolved metal ions, Pd(0) bionanoparticles were effectively formed by active Ac. aromatica cells from both acidic synthetic Pd(II) solutions and from the actual spent catalyst leachates at equivalent 18–19 nm median size with comparable catalytic activity.

     

Manufacture of stable palladium and gold nanoparticles on native and genetically engineered flagella scaffolds

The use of bacterial flagella as templates for the immobilization of Pd and Au nanoparticles is described. Complete coverage of D. desulfuricans flagellar filaments by Pd(0) nanoparticles was obtained via the H(2)-mediated reduction of Pd(NH3)4]Cl2 but similar results were not obtained using HAuCl4. The introduction of additional cysteine-derived thiol residues in the E. coli FliC protein increased Au(III) sorption and reduction onto the surface of the flagellar filament and resulted in the production of stabilized Au(0) nanoparticles of approximately 20-50 nm diameter. We demonstrate the application of molecular engineering techniques to manufacture biologically passivated Au(0) nanoparticles of a size suitable for catalytic applications.     

Bioreductive deposition of palladium (0) nanoparticles on Shewanella oneidensis with catalytic activity towards reductive dechlorination of polychlorinated biphenyls

Microbial reduction of soluble Pd(II) by cells of Shewanella oneidensis MR-1 and of an autoaggregating mutant (COAG) resulted in precipitation of palladium Pd(0) nanoparticles on the cell wall and inside the periplasmic space (bioPd). As a result of biosorption and subsequent bioreduction of Pd(II) with H2, formate, lactate, pyruvate or ethanol as electron donors, recoveries higher than 90% of Pd associated with biomass could be obtained. The bioPd(0) nanoparticles thus obtained had the ability to reductively dehalogenate polychlorinated biphenyl (PCB) congeners in aqueous and sediment matrices. Bioreduction was observed in assays with concentrations up to 1000 mg Pd(II) l(-1) with depletion of soluble Pd(II) of 77.4% and higher. More than 90% decrease of PCB 21 (2,3,4-chloro biphenyl) coupled to formation of its dechlorination products PCB 5 (2,3-chloro biphenyl) and PCB 1 (2-chloro biphenyl) was obtained at a concentration of 1 mg l(-1) within 5 h at 28 degrees C. Bioreductive precipitation of bioPd by S. oneidensis cells mixed with sediment samples contaminated with a mixture of PCB congeners, resulted in dechlorination of both highly and lightly chlorinated PCB congeners adsorbed to the contaminated sediment matrix within 48 h at 28 degrees C. Fifty milligrams per litre of bioPd resulted in a catalytic activity that was comparable to 500 mg l(-1) commercial Pd(0) powder. The high reactivity of 50 mg l(-1) bioPd in the soil suspension was reflected in the reduction of the sum of seven most toxic PCBs to 27% of their initial concentration.     

Characterization of ZnO nanoparticles grown in presence of Folic acid template

ABSTRACT: BACKGROUND: ZnO nanoparticles (grown in the template of folic acid) are biologically useful, luminescent material. It can be used for multifunctional purposes, e.g., as biosensor, bioimaging, targeted drug delivery and as growth promoting medicine. METHODS: Sol-gel chemical method was used to develop the uniform ZnO nanoparticles, in a folic acid template at room temperature and pH ~ 7.5. Agglomeration of the particles was prevented due to surface charge density of folic acid in the medium. ZnO nanoparticle was further characterized by different physical methods. RESULTS: Nanocrystalline, wurtzite ZnO particles thus prepared show interesting structural as well as band gap properties due to capping with folic acid. CONCLUSIONS: A rapid, easy and chemical preparative method for the growth of ZnO nanoparticles with important surface physical properties is discussed. Emphatically, after capping with folic acid, its photoluminescence properties are in the visible region. Therefore, the same can be used for monitoring local environmental properties of biosystems.     

Synthesis, characterization, and antibacterial activity of three palladium(II) complexes of tetracyclines

Pd(II) complexes with three antibiotics of the tetracycline family (tetracycline, doxycycline and chlortetracycline) were synthesized and characterized by elemental, thermogravimetric, and conductivity analyses, and infrared spectroscopy. The interactions between Pd(II) ions and tetracycline were investigated in aqueous solution by (1)H NMR. All the tetracyclines studied form 1:1 complexes with Pd(II) via the oxygen of the hydroxyl group at ring A and that of the amide group. The effect of the three complexes on the growth of bacterial strains sensitive and resistant to tetracycline was studied. The Pd(II) complex of tetracycline is practically as efficient as tetracycline in inhibiting the growth of two Escherichia coli (E. coli) sensitive bacterial strains and 16 times more potent against E. coli HB101/pBR322, a bacterial strain resistant to tetracycline. Pd(II) coordination to doxycycline also increased its activity in the resistant strain by a factor of 2.     

Surface Plasmon Tunability and Emission Sensitivity of Ultrasmall Fluorescent Copper Nanoclusters

Ultrasmall copper nanoparticles have been synthesized using copper(II) salt as precursor by hydrazine reduction in the presence of citric acid and cetyltrimethylammonium bromide facilitating the growth of stable copper nanoparticles with an average diameter of <2 nm. The corresponding surface plasmon resonances were monitored under variable microenvironments, and it is seen that these tiny copper nanoparticles form aggregates under stipulated reaction conditions. It is noted that ultrasmall copper nanoparticles do not exhibit any characteristic surface plasmon band in the visible region; rather, a continuous absorption is seen over the entire UV–vis region. However, a well-defined plasmon absorption band makes its appearance while the particles are aggregated in close-packed assembly. These results demonstrate that the maximum of surface plasmon resonance is red-shifted from that of isolated particles because of electromagnetic interaction between the particles. The aggregation process is manifested upon changes of pH, anionic surfactant, etc. and is not reversible, i.e., the aggregates could not be re-dispersed into ultrasmall particles. The effect of addition of electrolyte has been monitored to study the surface plasmon damping of the copper nanoparticles. The plasmonic sensitivity of the copper nanoparticle aggregates has been elicited by the determination of amino acid chain length with exquisite sensitivity because of enormous electromagnetic field at the junction of the particles in the aggregates. Interestingly, the as-synthesized ultrasmall copper nanoclusters exhibit excellent fluorescence properties with a narrow emission profile. The emission properties of these copper nanoclusters have been utilized as an indicator for selective and ultrasensitive detection of highly toxic Hg^II ions in water in the nanomolar detection limit.     

Supported imprinted nanospheres for the selective recognition of cholesterol

The preparation of innovative polymeric systems using molecular imprinting technology for application in extracorporeal blood purification is described. Membranes based on a methylmethacrylate-co-acrylic acid copolymer, produced through the phase inversion method, were modified introducing into their structure specific binding sites for cholesterol molecule by adding molecularly imprinted nanoparticles in the membrane matrix. Membranes prepared are intended to selectively remove cholesterol from the blood by using interactions at a molecular level, between the membrane/nanoparticles devices and the template, created during the preparation of polymers. Three polymeric systems in form of nanoparticles were prepared differing in the polymerisation solvent (a mixture of acetonitrile and ethanol (1:1) or pure ethanol), and the molar ratio between the functional monomer and the cross-linker (2.3:1 and 1:1). Two out of three of the prepared polymers showed a very good template rebinding capacity both in phosphate buffer solution (pH 6.9) and in ethanol. In particular the nanoparticles rebound 115.4 mg cholesterol/g polymer in buffer solution, and 57 mg cholesterol/g polymer in ethanol.

The deposition of the nanoparticles on the surface of the phase inversion membranes produced devices with interesting rebinding performances towards cholesterol in buffer solution: a specific recognition of 14.09 mg cholesterol/g system (membrane and nanoparticles) was detected, indicating maintained binding capacity of supported particles as well.     

In situ magnetic separation and immobilization of dibenzothiophene-desulfurizing bacteria

In situ cell separation and immobilization of bacterial cells for biodesulfurization were developed by using superparamagnetic Fe₃O₄ nanoparticles (NPs). The Fe₃O₄ NPs were synthesized by coprecipitation followed by modification with ammonium oleate. The surface-modified NPs were monodispersed and the particle size was about 13 nm with 50.8 emu/g saturation magnetization. After adding the magnetic fluids to the culture broth, Rhodococcus erythropolis LSSE8-1 cells were immobilized by adsorption and then separated with an externally magnetic field. The maximum amount of cell mass adsorbed was about 530 g dry cell weight/g particles to LSSE8-1 cells. Analysis showed that the nanoparticles were strongly absorbed to the surface and coated the cells. Compared to free cells, the coated cells not only had the same desulfurizing activity but could also be easily separated from fermentation broth by magnetic force. Based on the adsorption isotherms and Zeta potential analysis, it was believed that oleate-modified Fe₃O₄ NPs adsorbed bacterial cells mainly because of the nano-size effect and hydrophobic interaction.     

Enhanced hydrogenation catalyst synthesized by Desulfovibrio desulfuricans exposed to a radio frequency magnetic field

Desulfovibrio desulfuricans reduces Pd(II) to Pd(0)-nanoparticles (Pd-NPs) which are catalytically active in 2-pentyne hydrogenation. To make Pd-NPs, resting cells are challenged with Pd(II) ions (uptake), followed by addition of electron donor to promote bioreduction of cell-bound Pd(II) to Pd(0) (bio-Pd). Application of radiofrequency (RF) radiation to prepared 5 wt% bio-Pd catalyst (60 W power, 60 min) increased the hydrogenation rate by 70% with no adverse impact on selectivity to cis-2-pentene. Such treatment of a 5 wt% Pd/carbon commercial catalyst did not affect the conversion rate but reduced the selectivity. Lower-dose RF radiation (2–8 W power, 20 min) was applied to the bacteria at various stages before and during synthesis of the bio-scaffolded Pd-NPs. The reaction rate (μ mol 2-pentyne converted s^-1) was increased by ~threefold by treatment during bacterial catalyst synthesis. Application of RF radiation (2 or 4 W power) to resting cells prior to Pd(II) exposure affected the catalyst made subsequently, increasing the reaction rate by 50% as compared to untreated cells, while nearly doubling selectivity for cis 2-pentene. The results are discussed with respect to published and related work which shows altered dispersion of the Pd-NPs made following or during RF exposure.     

Bioaccumulation of palladium by Desulfovibrio fructosivorans wild-type and hydrogenase-deficient strains

Wild-type Desulfovibrio fructosivorans and three hydrogenase-negative mutants reduced Pd(II) to Pd(0). The location of Pd(0) nanoparticles on the cytoplasmic membrane of the mutant retaining only cytoplasmic membrane-bound hydrogenase was strong evidence for the role of hydrogenases in Pd(0) deposition. Hydrogenase activity was retained at acidic pH, shown previously to favor Pd(0) deposition.     

Local magnetism in palladium bionanomaterials probed by muon spectroscopy

Palladium bionanomaterial was manufactured using the sulfate-reducing bacterium, Desulfovibrio desulfuricansm, to reduce soluble Pd(II) ions to cell-bound Pd(0) in the presence of hydrogen. The biomaterial was examined using a Superconducting Quantum Interference Device (SQUID) to measure bulk magnetisation and by Muon Spin Rotation Spectroscopy (μSR) which is uniquely able to probe the local magnetic environment inside the sample. Results showed behaviour attributable to interaction of muons both with palladium electrons and the nuclei of hydrogen trapped in the particles during manufacture. Electronic magnetism, also suggested by SQUID, is not characteristic of bulk palladium and is consistent with the presence of nanoparticles previously seen in electron micrographs. We show the first use of μSR as a tool to probe the internal magnetic environment of a biologically-derived nanocatalyst material.     

Biogenic synthesis and photocatalysis of Pd-PdO nanoclusters reinforced hierarchical TiO2 films with interwoven and tubular conformations

Hierarchical nanocomposite films with Pd-PdO nanoparticles anchored uniformly on the inner surface of TiO2 nanotubes were achieved through a stepwise bioredox/artificial oxygenation approach by using the natural eggshell membrane (ESM) as a template. The Pd content ratio of Pd-PdO loading could be arbitrarily varied from 0 to 53 wt %, and the ESM-morphic nanocomposites Pd-PdO/TiO2 exhibited porous and multiphasic features, facilitating light transport and molecule accessibility to the active site during photocatalytic reactions. The photocatalytic activity of target nanocomposites was determined by the degradation of rhodamine B. The composites with a ratio of 10 wt % TiO2 (5 wt % Pd of Pd-PdO loading) presented a high degradation efficiency of 99.3% and showed good stability with a second run of about 95.3% and a third run of 94.6%. These composites with structural particularity and complexity are expected to find potential applications in various fields, such as photovoltaic devices, gas sensors, antistatic coating, dye-sensitized solar cells, etc.     

Iron specificity of a biosensor based on fluorescent pyoverdin immobilized in sol-gel glass

Two current technologies used in biosensor development are very promising: 1. The sol-gel process of making microporous glass at room temperature, and 2. Using a fluorescent compound that undergoes fluorescence quenching in response to a specific analyte. These technologies have been combined to produce an iron biosensor. To optimize the iron (II or III) specificity of an iron biosensor, pyoverdin (a fluorescent siderophore produced by Pseudomonas spp.) was immobilized in 3 formulations of porous sol-gel glass. The formulations, A, B, and C, varied in the amount of water added, resulting in respective R values (molar ratio of water:silicon) of 5.6, 8.2, and 10.8. Pyoverdin-doped sol-gel pellets were placed in a flow cell in a fluorometer and the fluorescence quenching was measured as pellets were exposed to 0.28 - 0.56 mM iron (II or III). After 10 minutes of exposure to iron, ferrous ion caused a small fluorescence quenching (89 - 97% of the initial fluorescence, over the range of iron tested) while ferric ion caused much greater quenching (65 - 88%). The most specific and linear response was observed for pyoverdin immobilized in sol-gel C. In contrast, a solution of pyoverdin (3.0 μM) exposed to iron (II or III) for 10 minutes showed an increase in fluorescence (101 - 114%) at low ferrous concentrations (0.45 - 2.18 μM) while exposure to all ferric ion concentrations (0.45 - 3.03 μM) caused quenching. In summary, the iron specificity of pyoverdin was improved by immobilizing it in sol-gel glass C.