Screening for Microbial Metal-Chelating Siderophores for the Removal of Metal Ions from Solutions

To guarantee the supply of critical elements in the future, the development of new technologies is essential. Siderophores have high potential in the recovery and recycling of valuable metals due to their metal-chelating properties. Using the Chrome azurol S assay, 75 bacterial strains were screened to obtain a high-yield siderophore with the ability to complex valuable critical metal ions. The siderophore production of the four selected strains Nocardioides simplex 3E, Pseudomonas chlororaphis DSM 50083, Variovorax paradoxus EPS, and Rhodococcus erythropolis B7g was optimized, resulting in significantly increased siderophore production of N. simplex and R. erythropolis. Produced siderophore amounts and velocities were highly dependent on the carbon source. The genomes of N. simplex and P. chlororaphis were sequenced. Bioinformatical analyses revealed the occurrence of an achromobactin and a pyoverdine gene cluster in P. chlororaphis, a heterobactin and a requichelin gene cluster in R. erythropolis, and a desferrioxamine gene cluster in N. simplex. Finally, the results of the previous metal-binding screening were validated by a proof-of-concept development for the recovery of metal ions from aqueous solutions utilizing C₁₈ columns functionalized with siderophores. We demonstrated the recovery of the critical metal ions V(III), Ga(III), and In(III) from mixed metal solutions with immobilized siderophores of N. simplex and R. erythropolis.

An efficient and selective flourescent chemical sensor based on 5-(8-hydroxy-2-quinolinylmethyl)-2,8-dithia-5-aza-2,6-pyridinophane as a new fluoroionophore for determination of iron(III) ions. A novel probe for iron speciation

A novel fluorescent chemical sensor for the highly sensitive and selective determination of Fe(3+) ions in aqueous solutions is prepared. The iron sensing system was prepared by incorporating 5-(8-hydroxy-2-quinolinylmethyl)-2,8-dithia-5-aza-2,6-pyridinophane (L) as a neutral Fe(3+)-selective fluoroionophore in the plasticized PVC membrane containing sodium tetraphenylborate as a liphophilic anionic additive. The response of the sensor is based on the strong fluorescence quenching of L by Fe(3+) ions. At pH 5.5, the proposed sensor displays a calibration curve over a wide concentration range from 6.0 × 10(-4) to 1.0 × 10(-7) M, with a relatively fast response time of less than 2 min. In addition to a high stability and reproducibility, the sensor shows a unique selectivity toward Fe(3+) ion with respect to common coexisting cations. The proposed fluorescence optode was applied to the determination of iron(III) content of straw of rice, spinach and different water samples. The fluorescent sensor was also used as a novel probe for Fe(3+)/Fe(2+) speciation in aqueous solution.

Iron(III) selective molecular and supramolecular fluorescent probes

Iron is one of the most important elements in metabolic processes, being indispensable for all living systems and therefore it is extensively distributed in environmental and biological materials. However, both its deficiency and excess from the normal permissible limit can induce serious disorders. Therefore, several analytical techniques have been adopted for the detection of iron. Among the various techniques used for its detection, the method based on fluorescent sensors has received considerable interest in recent years because of its ability to provide online monitoring of very low concentrations without any pre-treatment of the sample together with the advantages of spatial and temporal resolution. In this article, efforts have been made to review the various molecular and supramolecular fluorescent sensors that have been developed for the selective detection of iron(III).

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.

Determining subnanomolar iron concentrations in oceanic seawater using a siderophore-modified film analyzed by infrared spectroscopy

Iron is a bioactive trace element in seawater that regulates photosynthetic carbon dioxide drawdown and export from surface waters by phytoplankton in upward of 40% of the world's oceans. While autonomous sensor arrays are beginning to provide high-resolution data on temporal and spatial scales for some key oceanographic parameters, current analytical methods for iron are not amenable to autonomous platforms because of the need for user involvement and wet chemistry-based approaches. As a result, very large gaps remain in our understanding of iron distribution and chemistry in seawater. Here we present a straightforward nanostructure-based method to measure dissolved iron in natural seawater. The device comprises an iron-specific chelating biomolecule, desferrioxamine B (DFB), covalently immobilized on a mesoporous silica film. Changes in infrared spectral signatures of the immobilized DFB upon Fe(III) complexation provide an accurate and precise measure of iron on the surface of a chip exposed to seawater. The current system has a detection limit of approximately 50 pM for a 1-L sample at pH 1.7 and was used to measure dissolved iron in subarctic Pacific waters without interference from other elements in seawater. This technology provides a major step toward obtaining accurate iron measurements on autonomous research platforms.

Fluorescence-Based Siderophore Biosensor for the Determination of Bioavailable Iron in Oceanic Waters

With direct evidence that iron is the chemical limitation of phytoplankton growth, particularly in the Southern Ocean, it is increasingly important to develop new tools that provide direct measurement of the bioavailable iron fraction in oceanic waters. Here we report the development of a fluorescence quenching-based siderophore biosensor capable of the in situ measurement of this ultratrace Fe(III) fraction at ambient pH ( approximately 8). Parabactin was extracted from cultures of Paracoccus denitrificans. The purified siderophore was encapsulated within a spin-coated sol-gel thin film, which was subsequently incorporated in a flow cell system. The parabactin biosensor has been fully characterized for the detection of Fe(III) in seawater samples. The biosensor can be regenerated by lowering the pH of the flowing solution, thereby releasing the chelated Fe(III), enabling multiple use. The LOD of the biosensor was determined to be 40 pM, while for an Fe(III) concentration of 1 nM, a reproducibility with a RSD of 6% (n = 10) was obtained. The accuracy of the biosensing system has been determined through analysis of a certified seawater reference sample. Samples from the Atlantic Ocean have been analyzed using the parabactin biosensor providing a concentration vs depth profile for the bioavailable Fe(III) fraction in the 50 pM-1 nM range.

Fluorescence of pyoverdin in response to iron and other common well water metals

When the fluorescent siderophore pyoverdin (produced by Pseudomonas aeruginosa) binds to a metal ion the fluorescence changes. A pyoverdin solution (in 0.1 M acetate buffer, pH 5.0) was placed in the microwells of a 96-well plate and varying concentrations of the metal cations Al(3+), Ca(2+), Cu(2+), Fe(2+), Fe(3+), Mn(2+), Mg(2+), and Zn(2+) were added. The fluorescence of pyoverdin 60 sec after the addition of an equimolar concentration of metal indicated: (1) no change for Ca(2+), Fe(2+), Mn(2+), Mg(2+), and Zn(2+); (2) a small increase (109%) for Al(3+); (3) decreases in fluorescence for Cu(2+) (83%) and for Fe(3+) (66%). The fluorescence of pyoverdin 24 hr after the addition of equimolar metal indicated: 1) very little change for Ca(2+), Mn(2+), Mg(2+), and Zn(2+); 2) a very large (270%) increase in fluorescence due to Al(3+); 3) an increase (113%) due to Cu(2+); 4) large decreases in fluorescence for both Fe(2+) (15%) and Fe(3+) (0%). Thus, for an iron assay using a free solution of pyoverdin, even with a short (60 sec.) reaction time there can be interference due to Cu(2+), and interference due to high levels of Al(3+).

Fluorescent complex of pyoverdin with aluminum

When a pyoverdin (PV), (a siderophore) from Pseudomonas fluorescens, binds aluminum 1:1, its natural fluorescence almost doubles, whereas PV-Fe is non-fluorescent. Complex formation allows [Al] determination down to 1 mug/l. Fe(III) in the sample interferes with [Al] determination, but added after PV, improves the assay's performance. Ascorbic acid does not eliminate Fe(III) interference. PV-Al fluorescence could have analytical and toxicological applications.

Sol-gel encapsulated horseradish peroxidase: a catalytic material for peroxidation

This study addresses the viability of sol-gel encapsulated HRP (HRP:sol-gel) as a recyclable solid-state catalytic material. Ferric, ferric-CN, ferrous, and ferrous-CO forms of HRP:sol-gel were investigated by resonance Raman and UV-visible methods. Electronic and vibrational spectroscopic changes associated with changes in spin state, oxidation state, and ligation of the heme in HRP:sol-gel were shown to correlate with those of HRP in solution, showing that the heme remains a viable ligand-binding complex. Furthermore, the high-valent HRP:sol-gel intermediates, compound I and compound II, were generated and identified by time-resolved UV-visible spectroscopy. Catalytic activity of the HRP:sol-gel material was demonstrated by enzymatic assays by using I(-), guaiacol, and ABTS as substrates. Encapsulated HRP was shown to be homogeneously distributed throughout the sol-gel host. Differences in turnover rates between guaiacol and I(-) implicate mass transport of substrate through the silicate matrix as a defining parameter in the peroxidase activity of HRP:sol-gel. HRP:sol-gel was reused as a peroxidation catalyst for multiple reaction cycles without loss of activity, indicating that such materials show promise as reusable catalytic materials.