Biogenic platinum and palladium nanoparticles as new catalysts for the removal of pharmaceutical compounds

Revisiting the mechanistic pathways for bacterial mediated synthesis of noble metal nanoparticles

Synthesis and application of reliable nanoscale materials is a progressive domain and the limelight of modern nanotechnology. Conventional physicochemical approaches for the synthesis of metal nanoparticles have become obsolete owing to costly and hazardous materials. There is a need to explore alternative, cost-effective and eco-friendly strategies for fabrication of nanoparticle (NPs). Green synthesis of noble metal nanoparticles has emerged as a promising approach in the last decade. Elucidation of the molecular mechanism is highly essential in the biological synthesis of noble metal nanoparticles (NPs) for the controlled size, shape, and monodispersity. Moreover, mechanistic insights will help to scale up the facile synthesis protocols and will enable biotransformation of toxic heavy metals hence also providing the detoxification effects. Therefore, the current review article has primarily targeted the mechanisms involved in the green synthesis of metal NPs, which have been reported during the last few years. Detailed mechanistic pathways have highlighted nitrate reductase as a principle reducing agent in the bacterial mediated synthesis and stabilization of NPs. Furthermore, we have highlighted the potential implications of these mechanisms in bioremediation and biomineralization processes, which can play a critical role in biogeochemical cycling and environmental impacts of heavy metals. We anticipate that this review article will help researchers to address the challenges of bioremediation and modern nanotechnology.

Microbial synthesis of Pd-Pt alloy nanoparticles using Shewanella oneidensis MR-1 with enhanced catalytic activity for nitrophenol and azo dyes reduction

Bimetallic nanoparticles (NPs) often exhibit improved catalytic performance due to the electronic and spatial structure changes. Herein, a novel green biosynthesis method for Pd-Pt alloy NPs using Shewanella oneidensis MR-1 was proposed. The morphology, size and crystal structure of Pd-Pt alloy NPs were studied by a suite of characterization techniques. Results showed Pd-Pt alloy NPs were successfully synthesized inside and outside the cell. The biosynthesized Pd-Pt alloy NPs were polycrystalline and face-centered-cubic structure with the particle size ranged from 3-40 nm. Furthermore, the catalytic experiment demonstrated that the Pd-Pt alloy NPs exhibited the highest performance for the catalytic reduction of nitrophenol and azo dyes compared with the as-synthesized Pd and Pt monometallic NPs. This enlarged catalytic activity resulted from the synergistic effect of Pd and Pt element. Thereby, this paper provided a simple biosynthesis method for producing bimetallic alloy nanocatalyst with superior activity for contaminant degradation.

Synthesis and applications of biogenic nanomaterials in drinking and wastewater treatment

The continuous increase in water pollution by various organic & inorganic contaminants has become a major issue of concern worldwide. Furthermore, the anthropogenic activities for the manufacturing of various products have boosted this problem manifold. To overcome this serious issue, nanotechnology has initiated to explore various proficient strategies to treat waste water in a more precise and accurate way with the support of various nanomaterials. In recent times, nanosized materials have proved their applicability to provide clean and affordable water treatment technologies. The exclusive features such as high surface area and mechanical properties, greater chemical reactivity, lower cost and energy, efficient regeneration for reuse allow the nanomaterials perfect for water remediation. But the conventional routes of synthesis of nanomaterials encompass the involvement of hazardous and volatile chemicals; therefore the use of nanomaterials further creates the secondary pollution. This issue has intrigued the scientists to develop biogenic pathways and procedures which are environmentally safer and inexpensive. It has led to the new trends that involve developing bio-inspired nano-scale adsorbents and catalysts for the removal and degradation of a wide range of water pollutants. Carbohydrates, proteins, polymers, flavonoids, alkaloids and several antioxidants obtained from plants, bacteria, fungi, and algae have proven their effectiveness as capping and stabilizing agents during manufacture of nanomaterials. Application of biogenic nanomaterials for waste water treatment is relatively newer but rapidly escalating area of research. In the present review, promises and challenges for the synthesis of various biogenic nanomaterials and their potential applications in waste water treatment and/or water purification have been discussed.

Overview of microbes based fabricated biogenic nanoparticles for water and wastewater treatment

Treatment of toxic and emerging pollutants (T&EPs) is increasing the threats to the survival of conventional wastewater treatment (WWTs) technologies. The high installation and operational costs of advanced treatment technologies have shifted the research interest to the development of economical and reliable technology for management of T&EPs. Thus, recently biogenic nanoparticles (BNPs) fabricated using microbes/microorganisms are getting tremendous research interest due to their unique properties (i.e. high specific surface area, desired morphology, catalytic reactivity) for the biodegradation and biosorption of T&EPs. In addition, BNPs can be manufactured using metal contaminated water which indicates a hidden potential for resource recovery and utilization. Therefore, the present study discusses the adsorptive and catalytic performance of BNPs in the removal of T&EPs from water (W) and wastewater (WW). In addition, inspired by the superior performance of BNPs in advance WWT, a model of BNPs based WWT resource recovery and utilization process is also proposed. Finally, main issues i.e. mass production, leaching, poisoning/toxicity, regeneration, reusability and fabrication costs and process optimization are discussed which are main hinders in the transfer of BNPs based WWT technologies from laboratory to commercial scale.

From biotechnology principles to functional and low-cost metallic bionanocatalysts

Chemical, physical and mechanical methods of nanomaterial preparation are still regarded as mainstream methods, and the scientific community continues to search for new ways of nanomaterial preparation. The major objective of this review is to highlight the advantages of using green chemistry and bionanotechnology in the preparation of functional low-cost catalysts. Bionanotechnology employs biological principles and processes connected with bio-phase participation in both design and development of nano-structures and nano-materials, and the biosynthesis of metallic nanoparticles is becoming even more popular due to; (i) economic and ecologic effectiveness, (ii) simple one-step nanoparticle formation, stabilisation and biomass support and (iii) the possibility of bio-waste valorisation. Although it is quite difficult to determine the precise mechanisms in particular biosynthesis and research is performed with some risk in all trial and error experiments, there is also the incentive of understanding the exact mechanisms involved. This enables further optimisation of bionanoparticle preparation and increases their application potential. Moreover, it is very important in bionanotechnological procedures to ensure repeatability of the methods related to the recognised reaction mechanisms. This review, therefore, summarises the current state of nanoparticle biosynthesis. It then demonstrates the application of biosynthesised metallic nanoparticles in heterogeneous catalysis by identifying the many examples where bionanocatalysts have been successfully applied in model reactions. These describe the degradation of organic dyes, the reduction of aromatic nitro compounds, dehalogenation of chlorinated aromatic compounds, reduction of Cr(VI) and the synthesis of important commercial chemicals. To ensure sustainability, it is important to focus on nanomaterials that are capable of maintaining the important green chemistry principles directly from design inception to ultimate application.

Bioreduction of azo dyes was enhanced by in-situ biogenic palladium nanoparticles

Biogenic nanoparticles are promising materials for their green synthesis method and good performance in stimulation on reduction of environmental contaminants. In this study, Pd(0) nanoparticles (bio-Pd) were generated by Klebsiella oxytoca GS-4-08 in fermentative condition and in-situ improved the azo dye reduction. The bio-Pd was mainly located on cell membrane with a size range of 5-20 nm by TEM and XRD data analyses. Anthraquinone-2-disulfonate (AQS) greatly increased the reduction rate of Pd(II) with a reduction efficiency as high as 96.54 ± 0.23% in 24 h. The quinone respiration theory, glucose metabolism and the biohydrogen pathway were used to explain the enhancement mechanism of the in-situ generated bio-Pd on azo dye reduction. These results indicate that the in-situ generated bio-Pd by K. oxytoca strain is efficient for azo dye reduction without complex preparation processes, which is of great significance for the removal and subsequent safe disposal of hazardous environmental compounds.

Anaerobic biodegradation of pharmaceutical compounds: New insights into the pharmaceutical-degrading bacteria

Antibiotics and hormones are among the most concerning trace contaminants in the environment. Therefore, the present work aimed to identify anaerobic microorganisms with the ability to remove pharmaceutical products (PhPs) belonging to these two classes (ciprofloxacin, 17β-estradiol and sulfamethoxazole) under different anaerobic conditions, and to elucidate the bio-removal mechanisms involved. Ciprofloxacin was efficiently biodegraded under both nitrate- and sulfate-reducing conditions reaching a PhP removal superior to 80%, whereas 17β-estradiol was only biodegraded under nitrate-reducing conditions reaching a removal of 84%. No biodegradation of sulfamethoxazole was observed. In nitrate-reducing conditions the ciprofloxacin-degrading community was composed of Comamonas, Arcobacter, Dysgonomonas, Macellibacteroides and Actinomyces, genera while Comamonas and Castellaniella were the main bacteria present in the 17β-estradiol-degrading community. In sulfate-reducing conditions the community was mainly composed by bacteria affiliated to Desulfovibrio, Enterococcus and Peptostreeptococcus. Interestingly, the PhP under study were biodegraded even in the absence of additional carbon source, with 85% of ciprofloxacin removed under sulfate-reducing conditions and 62% and 83% of ciprofloxacin and estradiol removed, respectively, under nitrate-reducing conditions. This work provides new insights into anaerobic bioremediation of PhP and novel PhP-degrading bacteria.

Platinum nanoparticles in nanobiomedicine

Oxidative stress-dependent inflammatory diseases represent a major concern for the population's health worldwide. Biocompatible nanomaterials with enzymatic properties could play a crucial role in the treatment of such pathologies. In this respect, platinum nanoparticles (PtNPs) are promising candidates, showing remarkable catalytic activity, able to reduce the intracellular reactive oxygen species (ROS) levels and impair the downstream pathways leading to inflammation. This review reports a critical overview of the growing evidence revealing the anti-inflammatory ability of PtNPs and their potential applications in nanomedicine. It provides a detailed description of the wide variety of synthetic methods recently developed, with particular attention to the aspects influencing biocompatibility. Special attention has been paid to the studies describing the toxicological profile of PtNPs with an attempt to draw critical conclusions. The emerging picture suggests that the material per se is not causing cytotoxicity, while other physicochemical features related to the synthesis and surface functionalization may play a crucial role in determining the observed impairment of cellular functions. The enzymatic activity of PtNPs is also summarized, analyzing their action against ROS produced by pathological conditions within the cells. In particular, we extensively discuss the potential of these properties in nanomedicine to down-regulate inflammatory pathways or to be employed as diagnostic tools with colorimetric readout. A brief overview of other biomedical applications of nanoplatinum is also presented.

Extracellular Synthesis and Characterization of Gold Nanoparticles Using Mycobacterium sp. BRS2A-AR2 Isolated from the Aerial Roots of the Ghanaian Mangrove Plant, Rhizophora racemosa

Through the use of genomes that have undergone millions of years of evolution, marine Actinobacteria are known to have adapted to rapidly changing environmental pressures. The result is a huge chemical and biological diversity among marine Actinobacteria. It is gradually becoming a known fact that, marine Actinobacteria have the capability to produce nanoparticles which have reasonable sizes and structures with possible applications in biotechnology and pharmacology. Mycobacterium sp. BRS2A-AR2 was isolated from the aerial roots of the mangrove plant Rhizophora racemosa. The Mycobacterium was demonstrated for the first time ever to produce AuNPs with sizes that range between 5 and 55 nm. The highest level absorbance of the biosynthesized AuNPs was typical for actinobacterial strains (2.881 at 545 nm). The polydispersity index was measured as 0.207 in DLS and the zeta potential was negatively charged (- 28.3 mV). Significant vibration stretches were seen at 3314, 2358, 1635 and 667 cm^-1 in FT-IR spectra. This demonstrated the possible use of small aliphatic compounds containing -COOH, -OH, -Cl and -NH₂ functional groups in the stabilization of the AuNPs. The effect of the biosynthesized AuNPs on HUVEC and HeLA cell lines was measured at 48 h. IC₅₀ values were determined at 3500 µg/ml concentration for HUVEC and HeLA cell lines at 45.25 and 53.41% respectively.

Activating electrochemical catalytic activity of bio-palladium by hybridizing with carbon nanotube as "e- Bridge"

Nano metal catalysts produced by bacteria has received increasing attention owing to its environmental friendly synthesis route. However, the formed metal nanoparticles are associated with poorly conductive cells and challenged to be electrochemically applied. In this study, Palladium (Pd) nanoparticles were synthesized by Shewanella oneidensis MR-1. We demonstrated the limitation of palladized cells (Pd-cells) serving as electro-catalysts can be relieved by hybridizing with the conductive carbon nanotubes (Pd-cells-CNTs hybrid). Compared to the Pd-cells, the electrochemical active surface area of Pd in Pd-cells-CNTs10 (the ratio of Pd/CNTs is 1/10 w/w) were dramatically increased by 68 times to 20.44 m²·g^-1. A considerable enhancement of electrocatalytic activity was further confirmed for Pd-cells-CNTs10 as indicated by a 5-fold increase of steady state current density for nitrobenzene reduction at -0.55 V vs Ag/AgCl. These results indicate that the biogenetic palladium could has been an efficient electro-catalyst but just limited due to lacking an electron transport path (e ^- Bridge). This finding may also be helpful to guide the way to electrochemically use other biogenetic metal nano-materials.

Palladized cells as suspension catalyst and electrochemical catalyst for reductively degrading aromatics contaminants: Roles of Pd size and distribution

The palladized cell (Pd-cell) could be used as an efficient catalyst in catalyzing the degradations of a wide variety of environmental contaminants. Nevertheless, when the Pd NPs associate with the bacteria, the catalytic activity likely significantly affected by the biomass. Quantitative indicators that characterize of Pd-cell are necessary and little attention has been paid to investigate how the catalytic efficiency of Pd-cell is affected by the size and distribution of Pd NPs. To fill this gap, we explored the roles of the above-mentioned key factors on the performance of Pd-cell in catalyzing the degradations of two aromatic contaminants (nitrobenzene and p-chlorophenol) in two commonly used scenarios: (1) using Pd-cell as suspended catalyst in solution and (2) using Pd-cell as electrocatalyst directly coated on electrode. In scenario (1), the relationship of exposing area to Pd particle size and distribution factors was established. Based on theoretical estimation and catalytic performance analysis, the results indicated that adjusting the exposing area to a large value (9.3 ± 0.1 × 10⁵ nm² mg^-1 Pd) was extremely effective for improving the catalytic activity of Pd-cell used as a suspension catalyst. In scenario (2), our results showed that the best electrocatalytic performances were achieved on the electrode decorated with Pd-cells with the largest NP size (54.3 ± 16.4 nm), which exerted maximum electrochemical active surface area (10.6 m² g^-1) as well as favorable conductivity. The coverage of deposited Pd NPs (>95%) on the cell surface played a crucial role in boosting the conductivity of biocatalyst, thus determining the possibility of Pd-cell as an efficient electrocatalyst. The findings of this study provide a guidance for the synthesis and application of Pd-cell, which enables the design of Pd-cell to be suitable for different catalysis systems with high catalytic performance.