Enhanced dissolution of silver nanoparticles in a physical mixture with platinum nanoparticles based on the sacrificial anode effect.

A strategy to reduce implant-related infections is the inhibition of the initial bacterial implant colonization by biomaterials containing silver (Ag). The antimicrobial efficacy of such biomaterials can be increased by surface enhancement (nanosilver) or by creating a sacrificial anode system for Ag. Such a system will lead to an electrochemically driven enhanced Ag ion release due to the presence of a more noble metal. Here we combined the enlarged surface of nanoparticles (NP) with a possible sacrificial anode effect for Ag induced by the presence of the electrochemically more noble platinum (Pt) in physical mixtures of Ag NP and Pt NP dispersions. These Ag NP/Pt NP mixtures were compared to the same amounts of pure Ag NP in terms of cell biological responses, i.e. the antimicrobial activity against Staphylococcus aureus and Escherichia coli as well as the viability of human mesenchymal stem cells (hMSC). In addition, Ag NP was analyzed by ultraviolet-visible (UV-vis) spectroscopy, cyclic voltammetry, and atomic absorption spectroscopy. It was found that the dissolution rate of Ag NP was enhanced in the presence of Pt NP within the physical mixture compared to a dispersion of pure Ag NP. Dissolution experiments revealed a fourfold increased Ag ion release from physical mixtures due to enhanced electrochemical activity, which resulted in a significantly increased toxicity towards both bacteria and hMSC. Thus, our results provide evidence for an underlying sacrificial anode mechanism induced by the presence of Pt NP within physical mixtures with Ag NP. Such physical mixtures have a high potential for various applications, for example as antimicrobial implant coatings in the biomedicine or as bactericidal systems for water and surface purification in the technical area.

Enhanced antibacterial performance of ultrathin silver/platinum nanopatches by a sacrificial anode mechanism.

The development of antibacterial implant surfaces is a challenging task in biomaterial research. We fabricated a highly antibacterial bimetallic platinum (Pt)/silver(Ag) nanopatch surface by short time sputtering of Pt and Ag on titanium. The sputter process led to a patch-like distribution with crystalline areas in the nanometer-size range (1.3-3.9 nm thickness, 3-60 nm extension). Structural analyses of Pt/Ag samples showed Ag- and Pt-rich areas containing nanoparticle-like Pt deposits of 1-2 nm. The adhesion and proliferation properties of S. aureus on the nanopatch samples were analyzed. Consecutively sputtered Ag/Pt nanopatches (Pt followed by Ag) induced enhanced antimicrobial activity compared to co-sputtered Pt/Ag samples or pure Ag patches of similar Ag amounts. The underlying sacrificial anode mechanism was proved by linear sweep voltammetry. The advantages of this nanopatch coating are the enhanced antimicrobial activity despite a reduced total amount of Ag/Pt and a self-limited effect due the rapid Ag dissolution.

Deciphering the Surface Composition and the Internal Structure of Alloyed Silver-Gold Nanoparticles.

Spherical bimetallic AgAu nanoparticles in the molar ratios 30:70, 50:50, and 70:30 with diameters of 30 to 40 nm were analyzed together with pure silver and gold nanoparticles of the same size. Dynamic light scattering (DLS) and differential centrifugal sedimentation (DCS) were used for size determination. Cyclic voltammetry (CV) was used to determine the nanoalloy composition, together with atomic absorption spectroscopy (AAS), energy-dispersive X-ray spectroscopy (EDX) and ultraviolet-visible (UV/Vis) spectroscopy. Underpotential deposition (UPD) of lead (Pb) on the particle surface gave information about its spatial elemental distribution and surface area. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM) were applied to study the shape and the size of the nanoparticles. X-ray powder diffraction gave the crystallite size and the microstrain. The particles form a solid solution (alloy) with an enrichment of silver on the nanoparticle surface, including some silver-rich patches. UPD indicated that the surface only consists of silver atoms.

Electrochemistry at single bimetallic nanoparticles - using nano impacts for sizing and compositional analysis of individual AgAu alloy nanoparticles.

The increasing interest in producing bimetallic nanoparticles and utilizing them in modern technologies sets the demand for fast and affordable characterization of these materials. To date Scanning Transmission Electron Microscopy (STEM) coupled to energy dispersive X-ray spectroscopy is usually used to determine the size and composition of alloy nanoparticles, which is time-consuming and expensive. Here electrochemical single nanoparticle analysis is presented as an alternative approach to infer the particle size and composition of alloy nanoparticles, directly in a dispersion of these particles. As a proof of concept, 14 nm sized Ag0.73Au0.27 alloy nanoparticles are analyzed using a combination of chronoamperometric single nanoparticle analysis and cyclic voltammetry ensemble studies. It is demonstrated that the size, the alloying and the composition can all be inferred using this approach. Thus, the electrochemical characterization of single bimetallic alloy nanoparticles is suggested here as a powerful and convenient complement or alternative to TEM characterization of alloy nanoparticles.

Operando Studies of the Electrochemical Dissolution of Silver Nanoparticles in Nitrate Solutions Observed With Hyperspectral Dark-Field Microscopy.

Since nanoparticles are frequently used in commercial applications, there is a huge demand to obtain deeper insights into processes at the nanoscale. Especially, catalysis, chemical and electrochemical reaction dynamics are still poorly understood. Thus, simultaneous and coupled opto-and spectro-electrochemical dark-field microscopy is used to study in situ and operando the electrochemically driven dissolution mechanism of single silver nanoparticles in the presence of nitrate ions as non-complexing counter-ions, herein. Hyperspectral imaging is used to probe the intrinsic localized surface plasmon resonance of individual silver nanospheres before, during and after their electrochemical oxidation on a transparent indium tin oxide (ITO) electrode. Furthermore, optical video imaging was performed for additional information. Based on the complete loss of spectral information and intensity, a dissolution of the particles during the reaction was concluded. This way it is revealed that the dissolution of individual particles proceeds over several seconds, indicating a hindrance by the nitrate ions. Only electrochemical analysis does not provide this insight as the measured current does not allow distinguishing between successive fast dissolution of one particle after another or slow dissolution of several particles in a concerted manner. For comparison, experiments were performed in the presence of chloride ions. It was observed that the silver chloride formation is an instantaneous process. Thus, it is possible to study and define the reaction dynamics on the single nanoparticle level in various electrochemical systems and electrolyte solutions. Accordingly, operando opto- and spectro-electrochemical studies allow us to conclude, that the oxidation of silver to solvated silver cations is a kinetically slow process, while the oxidation to silver chloride is fast. We propose this approach as a new method to study electrocatalyst materials, their transformation and degradation under operando conditions.