Molecularly doped metals

Molecular modification enables CO2 electroreduction to methane on platinum surface in acidic media

Cu-based materials can produce hydrocarbons in CO2 electroreduction (CO2RR), but their stability still needs to be enhanced particularly in acidic media. Metallic Pt is highly stable in both acidic and alkaline media, yet rarely utilized in CO2RR, due to the competitive activity in hydrogen evolution reaction (HER). In this research, abundant thionine (Th) molecules are stably confined within Pt nanocrystals via a molecular doping strategy. The Pt surface is successfully modulated by these Th molecules, and thereby the dominant HER activity is converted to CO2RR activity. CO2 could be electroreduced to CH4 using organic molecule-modified Pt-based catalysts for the first time. Specifically, this composite catalyst maintains more than 100-hour stability in strong acid conditions (pH 1), even comparable to those state-of-the-art CO2RR catalysts. In-situ spectroscopic analysis and theoretical calculations reveal that the molecular modification can decrease the energy barrier for *COOH formation, and guarantee the sufficient local *H near Pt surface. Additionally, the *H derived from H2O dissociation is favorable for the *CO hydrogenation pathway towards *CHO, eventually leading to the formation of CH4. This strategy might be easily applied to microenvironment and interface regulation in other electrocatalytic reactions.

Entrapment of antimicrobial compounds in a metal matrix for crop protection

Agricultural yields are often limited by damage caused by pathogenic microorganisms, including plant-pathogenic bacteria. The chemical control options to cope with bacterial diseases in agriculture are limited, predominantly relying on copper-based products. These compounds, however, possess limited efficacy. Therefore, there is an urgent need to develop novel technologies to manage bacterial plant diseases and reduce food loss. In this study, a new antimicrobial agent was developed using a doping method that entraps small bioactive organic molecules inside copper as the metal matrix. The food preservative agent lauroyl arginate ethyl ester (ethyl lauroyl arginate; LAE) was chosen as the doped organic compound. The new composites were termed LAE@[Cu]. Bactericidal assays against Acidovorax citrulli, a severe plant pathogen, revealed that LAE and copper in the composites possess a synergistic interaction as compared with each component individually. LAE@[Cu] composites were further characterised in terms of chemical properties and in planta assays demonstrated their potential for further development as crop protection agents.

Design and development of molten metal nanomaterials using sonochemistry for multiple applications

Molten metals have prospective applications as soft fluids with unique physical and chemical properties, yet materials based on them are still in their infancy and have great potential. Ultrasonic irradiation of molten metals in liquid media induces acoustic cavitation and dispersion of the liquid metal into micrometric and nanometric spheres. This review focuses on the synthesis of mmetallic materials via sonochemistry from molten metals with low melting point (< 420 ᴼC): Ga, Hg, In, Sn, Bi, Pb, and Zn, which can be melted in organic or inorganic media or water and of aqueous solutions of metallic ions to form two immiscible liquid phases. Organic molecule entrapment, polymer solubilization, chiral imprinting, and catalyst incorporation within metals or metallic particles were recently developed to provide novel hybrid nanomaterials for several applications including catalysis, fuel cells, and biomass-to-biofuel conversion. In all cases where molten metal was sonicated in an organic solvent, in addition to a solid precipitant, an interesting supernatant was obtained that contained metal-doped carbon dots (M@C-dots). Some of these M@C-dots were found to exhibit highly effective antimicrobial activity, promote neuronal tissue growth, or have utility in lithium-ion rechargeable batteries. The economic feasibility and commercial scalability of molten metal sonochemistry attract fundamental interest in the reaction mechanisms, as the versatility and controllability of the structure and material properties invite exploration of various applications.

Metal nanoparticles entrapped in metal matrices

We developed synthetic methods for the doping of metals (M) with metallic nanoparticles (NPs). To the best of our knowledge - unlike oxides, polymers and carbon-based supports - metals were not used so far as supporting matrices for metallic NPs. The composites (denoted M1-NPs@M2) comprise two separate phases: the metallic NPs (the dopant) and the entrapping 3D porous metallic matrix, within which the NPs are intimately held and well dispersed. Two different general synthetic strategies were developed, each resulting in a group of materials with characteristic structure and properties. The first strategy uses pre-prepared NPs and these are entrapped during reductive formation of the metallic matrix from its cation. The second strategy is in situ growth of the doped metallic NPs within the pre-prepared entrapping metallic matrix. These two methods were developed for two types of entrapping metallic matrices with different morphologies: porous aggregated metallic matrices and metallic foams. The leading case in this study was the use of Pt as the NP dopant and Ag as the entrapping matrix, using all of the four combinations - entrapment or growth within aggregated Ag or Ag foam matrices. Full physical and chemical properties analysis of these novel types of materials was carried out, using a wide variety of analytical methods. The generality of the methods developed for these bi-metallic composites was investigated and demonstrated on additional metallic pairs: Au NPs within Ag matrices, Pd NPs within Ni matrices and Ir-NPs within a Rh matrix. As the main application of metallic NPs is in catalysis, the catalytic activity of M1-NPs@M2 is demonstrated successfully for entrapped Pt within Ag for reductive catalytic reactions, and for Pd within Ni for the electrocatalytic hydrogen oxidation reaction.

Nanoengineered chiral Pt-Ir alloys for high-performance enantioselective electrosynthesis

The design of efficient chiral catalysts is of crucial importance since it allows generating enantiomerically pure compounds. Tremendous efforts have been made over the past decades regarding the development of materials with enantioselective properties for various potential applications ranging from sensing to catalysis and separation. Recently, chiral features have been generated in mesoporous metals. Although these monometallic matrices show interesting enantioselectivity, they suffer from rather low stability, constituting an important roadblock for applications. Here, a straightforward strategy to circumvent this limitation by using nanostructured platinum-iridium alloys is presented. These materials can be successfully encoded with chiral information by co-electrodeposition from Pt and Ir salts in the simultaneous presence of a chiral compound and a lyotropic liquid crystal as asymmetric template and mesoporogen, respectively. The alloys enable a remarkable discrimination between chiral compounds and greatly improved enantioselectivity when used for asymmetric electrosynthesis (>95 %ee), combined with high electrochemical stability.

Catalysis with Silver: From Complexes and Nanoparticles to MORALs and Single-Atom Catalysts

Silver catalysis has a rich and versatile chemistry now expanding from processes mediated by silver complexes and silver nanoparticles to transformations catalyzed by silver metal organic alloys and single-atom catalysts. Focusing on selected recent advances, we identify the key advantages offered by these highly selective heterogeneous catalysts. We conclude by offering seven research and educational guidelines aimed at further progressing the field of new generation silver-based catalytic materials.

Enzymes in a golden cage

We describe a general method for the entrapment of enzymes within bulk metallic gold. This is a new approach for the immobilization of enzymes on metals, which is commonly carried out by 2D adsorption or covalent biding, that is, the enzyme is in contact with the metal at a specific contact zone of the enzyme, while most of the rest of it remains exposed to the environment. The 3D metallic encaging of the enzymes is quite different: the enzyme is in contact with the metallic cage walls all around it and is well protected inside. The porous nature of the metallic matrix enables substrate molecules to diffuse inside, reach the active site, and let product molecules diffuse out. The generality of the approach was proven by the successful entrapment of five enzymes representing different classes and different bio- and medical applications: l-asparaginase (Asp), collagenase, horseradish peroxidase (HRP), laccase and glucose oxidase (GOx). GOx–gold conjugates have been of particular interest in the literature. The main challenge we had to solve was how to keep the enzyme active in the process of gold-synthesis from its cation – this required careful tailoring of reaction conditions, which are detailed in the paper. The gold entrapped enzymes gain thermal stability and protectability against harsh conditions. For instance, we could keep Asp alive at the extreme pH of 13, which normally kills the enzyme instantly. The entrapped enzymes obey the Michaelis–Menten kinetics, and activation energies were determined. Good recyclability for eight cycles was found. Multi-enzymatic reactions by combinations of the off-the-shelf bioactive enzyme@gold powders are possible, as demonstrated for the classical detection of GOx activity with HRP. Detailed material characterization and proposed mechanisms for the 3D protectability of the enzymes are provided. The new enzyme immobilization method is of wide potential uses in medicine, biotechnology, bio-fuel cells and enzymatic (electro)sensing applications.

We describe a general method for the entrapment of enzymes within bulk metallic gold.

Encoding Chiral Molecular Information in Metal Structures

The concept of encoding molecular information in bulk metals has been proposed over the past decade. The structure of various types of molecules, including enantiomers, can be imprinted in achiral substrates. Typically, to encode metals with chiral information, several approaches, based on chemical and electrochemical concepts, can be used. In this Minireview, recent achievements with respect to the development of such materials are discussed, including the entrapment of chiral biomolecules in metals, the chiral imprinting of metals, as well as the combination of imprinting with nanostructuring. The features and potential applications of these designer materials, such as chirooptical properties, enantioselective adsorption and separation, as well as their use for asymmetric synthesis will be presented. This will illustrate that the development of molecularly encoded metal structures opens up very interesting perspectives, especially in the frame of chiral technologies.

Affecting an Ultra-High Work Function of Silver

An ultra-high increase in the WF of silver, from 4.26 to 7.42 eV, that is, an increase of up to circa 3.1 eV is reported. This is the highest WF increase on record for metals and is supported by recent computational studies which predict the potential ability to affect an increase of the WF of metals by more than 4 eV. We achieved the ultra-high increase by a new approach: Rather than using the common method of 2D adsorption of polar molecules layers on the metal surface, WF modifying components, l-cysteine and Zn(OH)₂ , were incorporated within the metal, resulting in a 3D architecture. Detailed material characterization by a large array of analytical methods was carried out, the combination of which points to a WF enhancement mechanism which is based on directly affecting the charge transfer ability of the metal separately by cysteine and hydrolyzed zinc(II), and synergistically by the combination of the two through the known Zn-cysteine finger redox trap effect.

Metallic Conductive Luminescent Film

We report a solution for the challenge of having luminescence and metal conductivity from the same material. The fabrication of a hybrid metal-conductive luminescent film that manifests this dual property is described: the conductivity arising from a continuous gold thin film structure and luminescence originating from the embedded fluorescent emitters (nanoparticles of silica-coated CdSe/CdS quantum dots (QD/SiO₂ NPs)). The embedding of the QD/SiO₂ NPs is performed via a self-templating gold electroless process. The presence of the insulating silica layer on the QDs avoids quenching and enables luminescence, while still allowing plasmonic coupling of the QDs, as observed by luminescence lifetime analysis and by surface-enhanced Raman scattering. The potential applications of this special dual functionality are demonstrated by its used as a temperature probe: Passing current (heating the gold thin film) affects the emission intensity and induces a spectral red-shift of the QD/SiO₂ NPs. All properties of this metal-conductive luminescent film required the special embedding architecture and are not observed with simple adsorption of QD/SiO₂ NPs on a continuous Au film.

Random peptide mixtures entrapped within a copper-cuprite matrix: new antimicrobial agent against methicillin-resistant Staphylococcus aureus

The emergence of global antibiotic resistance necessitates the urgent need to develop new and effective antimicrobial agents. Combination of two antimicrobial agents can potentially improve antimicrobial potency and mitigate the development of resistance. Therefore, we have utilized metal molecular doping methodology whereby antimicrobial random peptides mixture (RPMs) are entrapped in a bactericidal copper metal matrix. The copper/RPM composite exhibits greater antimicrobial activity toward methicillin-resistant Staphylococcus aureus (MRSA) than either copper or RPMs alone. Our findings indicate that this bactericidal antimicrobial biomaterial could be utilized to efficiently eradicate antibiotic-resistant pathogenic bacteria for health, agricultural and environmental applications.

Entrapment of Drugs within Metallic Platinum and Their Delivery

Platinum has been a widely used metal for a variety of implanted medical devices, because of its inertness, low corrosion rate, high biocompatibility, high electric conductivity, and good mechanical stability. A highly desirable property still in need to be addressed is the tailoring of drug-delivery ability to that metal. This is needed in order to treat infections due to the process of implanting, to treat postoperation pain, and to prevent blood clotting. Can Pt itself serve as a delivery matrix? A review on metallic implants (Lyndon, J. A.; Boyd, B. J.; Birbilis, N. Metallic implant drug/device combinations for controlled drug release in orthopaedic applications. J. Control. Release 2014, 179, 63-75) proposes that "Metals themselves can be used for delivering pharmaceutics" but adds that "there has been no current research into [that] possibility" despite its advantages. Here we present a solution to that challenge and show a new method of using an inert metal as a 3D matrix from within which entrapped drug molecules are released. This new type of drug-delivery system is fabricated by the methodolodgy of entrapment of molecules within metals, resulting in various drugs@Pt. Specifically the following drugs have been entrapped and released: the pain-killer and platelet-inhibitor nonsteroidal anti-inflammatory drugs (NSAIDs) ibuprofen and naproxen, the antibiotic ciprofloxacin, and the antiseptic chlorhexidine. The delivery profiles of all biocomposites were studied in two forms, powders and pressed discs, showing, in general, fast followed by slow first order release profiles. It is shown that the delivery kinetics can be tailored by changing the entrapment process, by applying different pressures in the disc preparation, and by changing the delivery temperature. The latter was also used to determine the activation energy for the release. Full characterization of the metallic biomaterials is provided, including X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray (EDAX), thermogravimetric analysis (TGA), and surface area/porosity analysis.

Silver encapsulated copper salen complex: efficient catalyst for electrocarboxylation of cinnamyl chloride with CO2

An active catalyst, [Cu]@Ag composite, was synthesized for the first time and used as a cathode for electrocarboxylation of cinnamyl chloride with CO₂. β,γ-Unsaturated carboxylic acids were obtained with excellent yield and moderate selectivity. Moreover, reasonable yields and selectivities of carboxylic acids were also achieved with several allylic halides and aryl halides.

An active catalyst, [Cu]@Ag composite, was synthesized for the first time and used as a cathode for electrocarboxylation of cinnamyl chloride with CO₂.

Fine-tuning of the metal work function by molecular doping

Fine tuning of the metal work function (WF) in the range of 1 eV by 3D molecular doping of metals.

A pure magnetite hydrogel: synthesis, properties and possible applications

A magnetite-only hydrogel was prepared for the first time by weak base mediated gelation of stable magnetite hydrosols at room temperature. The hydrogel consists of 10 nm magnetite nanoparticles linked by interparticle Fe-O-Fe bonds and has the appearance of a dark-brown viscous thixotropic material. The water content in the hydrogel could be up to 93.6% by mass while volume fraction reaches 99%. The material shows excellent biocompatibility and minor cytotoxic effects at concentrations up to 207 μg mL^-1. The gel shows excellent sorption capacity for heavy metal adsorption such as chrome and lead ions, which is 225% more than the adsorption capacity of magnetite nanoparticles. Due to thixotropic nature, the gel demonstrates mechanical stimuli-responsive release behavior with up to 98% release triggered by ultrasound irradiation. The material shows superparamagnetic behavior with a coercivity of 65 emu g^-1 at 6000 Oe. The magnetite gels prepared could be used for the production of magnetite aerogels, magnetic drug delivery systems with controlled release and highly efficient sorbents for hydrometallurgy.

Sustained release from a metal - Analgesics entrapped within biocidal silver

Matrices for sustained release of drugs have been based on polymers, biomaterials and oxides. The use of the major family of metals as matrices for sustained release is, to the best of our knowledge, unknown. In this context we describe a new family of bio-composites for sustained release of drugs, namely analgesic drugs entrapped within metallic silver. Synthetic methodologies were developed for the preparation of ibuprofen@Ag, naproxen@Ag, tramadol@Ag and bupivacaine@Ag composites. Detailed kinetic analysis of the release of the drugs from within the metal, is provided, demonstrating that metals can indeed serve as reservoirs for drug release. The metal in our case acts not only as a drug releasing source, but also as an antibacterial agent and this property of the composites was studied. Unexpectedly, it was found that the entrapment of the analgesics within silver, dramatically enhances the growth inhibition activity of wild type Pseudomonas aeruginosa, exceeding by far the inhibition activity of the separate components. A mechanism for this interesting observation is provided. The strong antimicrobial activity combined with the analgesic activity open the road for future applications of these materials as dual-purpose components in wound treatment.

Automating multi-step paper-based assays using integrated layering of reagents

We describe a versatile and simple method to perform sequential reactions on paper analytical devices by stacking dry pullulan films on paper, where each film contains one or more reagents or acts as a delay layer. Exposing the films to an aqueous solution of the analyte leads to sequential dissolution of the films in a temporally controlled manner followed by diffusive mixing of the reagents, so that sequential reactions can be performed. The films can be easily arranged for lateral flow assays or for spot tests (reactions take place sequentially in the z-direction). We have tested the general feasibility of the approach using three different model systems to demonstrate different capabilities: 1) pH ramping from low to high and high to low to demonstrate timing control; 2) rapid ready-to-use two-step Simon's assays on paper for detection of drugs of abuse utilizing a 2-layer stack containing two different reagents to demonstrate the ability to perform assays in the z-direction; and 3) sequential cell lysing and colorimetric detection of an intracellular bacterial enzyme, to demonstrate the ability of the method to perform sample preparation and analysis in the form of a spot assay. Overall, these studies demonstrate the potential of stacked pullulan films as useful components to enable multi-step assays on simple paper-based devices.

An antibacterial copper composite more bioactive than metallic silver

Although known for its biocidal activity, copper is still not considered as a viable alternative to silver in many of its biocidal applications, mainly because it is generally considered to be a milder antibacterial metal. As copper is much cheaper than silver (1/100), it is potentially more accessible to the health and hygiene needs of third-world countries, to large volume consumer products, and to large-scale agricultural and water treatment needs. Therefore, enhancing the biocidal efficacy of copper is a sought-after goal. We report a method for achieving this goal: by entrapping molecules of the biocidal agent chlorhexidine (CH) within a metallic copper metal powder, using a new materials methodology, the antibacterial efficacy of copper towards two model nosocomial opportunistic bacteria - the Gram-negative Pseudomonas aeruginosa and the Gram-positive Staphylococcus epidermidis- is enhanced to provide a powerful antibacterial agent exceeding the activity of silver. ICP-MS elemental analysis and UV-spectroscopy indicated that the enhanced bactericidal effects of the synthesized composite, CH@Cu, are associated with the sustained release of both copper ions and CH, giving rise to synergistically enhanced activity.

Entrapment of a pyridine derivative within a copper–palladium alloy: a bifunctional catalyst for electrochemical reduction of CO2 to alcohols with excellent selectivity and reusability

A highly stable and recyclable catalyst, [PYD]@Cu–Pd, was demonstrated to have dual activity for electro-reduction of CO₂ in aqueous solution.

Electroless Functionalization of Silver Films by Its Molecular Doping

We present a methodology which by far extends the potential applications of thin conductive silver films achieved by an electroless molecular doping process of the metal with any of the endless functional molecules that the large library of organic molecules offer. The resulting metallic films within which the molecule is entrapped--molecule@Ag--carry both the classical chemical and physical properties of silver films, as well as the function of the entrapped molecule. Raman measurements of the organic molecules from within the silver films provide the first spectroscopic observations from within silver, and clearly show that entrapment, a three-dimensional process, and adsorption, a two-dimensional process, on silver films are distinctly different processes. Three organic molecules, the cationic Neutral red, the anionic Congo red, and the antibacterial agent chlorhexidine digluconate (CH), were used to demonstrate the generality of this method for various types of molecules. We studied the sensitivity of the film conductivity to the type of the molecule entrapped within the film, to its concentration, and to temperature. Dual functionality was demonstrated with CH@Ag films, which are both conductive and have prolonged and high antibacterial activity, a combination of properties that has been unknown so far.

Entrapment of a chiral cobalt complex within silver: a novel heterogeneous catalyst for asymmetric carboxylation of benzyl bromides with CO2

A novel way to accommodate heterogeneous catalysis, CO₂fixation and asymmetric synthesis on one catalyst is reported.

Entrapment of alkaloids within silver: from enantioselective hydrogenation to chiral recognition

An organically doped silver catalyst was synthesized for enantioselective electrohydrogenation and chiral recognition with high efficiency and remarkable recycle times.