Oxidation Of Silver Nanoparticles Research Articles

Unravelling Anion Solvation in Water‐Alcohol Mixtures by Single Entity Electrochemistry

AbstractSingle entity electrochemistry is employed to gain insights into ion solvation in solvent mixtures. To this end, the time required for the oxidation of individual indicator nanoparticles to sparingly soluble products is used to probe ionic diffusion, and hence gain new insights into the solvation properties of solvent mixtures. Herein, water‐ethanol or water‐methanol mixtures of different compositions are analyzed following this new approach, using silver nanoparticle oxidation in the presence of chloride and iodide as a complementary indicator reaction. For increasing concentrations of the bulkier alcohol molecules in the mixtures with water, an increasing content of alcohol molecules in the halide's solvation shell is detected by the observation of hindered halide diffusion. The extent of this solvent replacement is shown to scale with the charge density of the ions and the experimental results are rationalized with respect to literature‐derived thermodynamic data, highlighting the ability of single entity electrochemistry to explore solvation in solvent mixtures.

Protein-Bound Uremic Toxins Quantification by a Colorimetric Sensor Based on the Oxidation of Silver Nanoparticles

This work shows a colorimetric sensor based on albumin bound to citrate-capped silver nanoparticles. The sensor capability of detecting protein-bound uremic toxins, such as indoxyl sulfate and p-cresylsulfate, is demonstrated. These uremic toxins enhance the oxidation of citrate-capped silver nanoparticles by hydrogen peroxide, affecting the localized surface plasmon resonance and allowing the proposed colorimetric sensing method. The method exhibits a linear response for indoxyl sulfate and p-cresylsulfate concentrations ranging from 15 to 100 mg/L with resolutions of 0.56 mg/L and 0.41 mg/L and expanded uncertainties for a confidence level of 95% of 17.23 mg/L and 12.55 mg/L, respectively. Limits of detection and quantification of 5.7 mg/L and 19 mg/L for indoxyl sulfate and of 3.2 mg/L and 10.7 mg/L for p-cresylsulfate were obtained for p < 0.05. These characteristics of the colorimetric method allow for a distinction between total normal and total uremic blood concentrations, which reported levels are (0.54 ± 4.00) mg/L and (37.07 ± 26.50) mg/L for indoxyl sulfate and (1.87 ± 2.31) mg/L and (23.00 ± 16.90) mg/L for p-cresylsulfate. Besides, this novel sensor significantly reduces costs of analysis and facilitates the quantification of those toxins. The interaction between albumin and citrate-capped silver nanoparticles was also investigated by Raman spectroscopy.

AgY zeolite as catalyst for the selective catalytic oxidation of NH3

Ag-exchanged Y zeolites (Si/Al = 2.5; Ag/Al = 0.30–0.95) have been tested in the NH 3 –SCO reaction, the most promising method for the elimination of ammonia emissions, and deeply characterized before and after reaction by using a variety of techniques (XRD, TEM, UV–Vis, 109 Ag NMR, XAS spectroscopies). The most active centres for the NH 3 –SCO reaction are Ag 0 nanoparticles (NPs) formed under reduction conditions and both activity and selectivity to N 2 increase with the silver loading. The Ag 0 NPs are dramatically modified under reaction conditions, being most of them dispersed resulting in small clusters and even atomically Ag + cations, the latter accounting for around half silver atoms. The presence of water into the reaction feed promotes the dispersion and oxidation of silver nanoparticles, but the catalyst performance is only slightly affected. The results are fully consistent with the previously proposed i-SCR mechanism for NH 3 –SCO reaction on silver catalysts. • Silver nanoparticles formed in AgNaY zeolites under hydrogen reduction are the active sites for NH 3 –SCO reaction. • The NH 3 conversion and N 2 selectivity on AgNaY increases with the silver loading. • Silver species change during the course of the NH 3 –SCO reaction. • Silver clusters and atomically dispersed Ag + are identified after the NH 3 –SCO reaction.

Ag Nanoparticle-Induced Oxidative Dimerization of Thiophenols: Efficiency and Mechanism.

By oxidation of silver nanoparticles (AgNPs) in two ways: thermal oxidation (TO) in molecular oxygen and cool oxidation in oxygen plasma (i.e., oxygen plasma irradiation, OPI), the efficiency and mechanism of visible light-induced selective transformation of 4-aminothiophenol (PATP) to 4,4'-dimercaptoazobenzene (DMAB)on the surface of AgNPs was explored. On the basis of the evolution of surface-enhanced Raman scattering (SERS) spectrum of PATP (10-5 M in ethanol) with the oxidation time, it can be concluded that OPI could improve the selective transformation efficiency (η) effectively, by 87 times for only 2 s; whereas TO could improve η conditionally, increasing at first and then decreasing gradually to zero. The results imply that silver oxide is not the root cause of the increased η. Combined with the results of SERS of oxygen species on the surface of AgNPs processed by the above-mentioned two ways, superoxide (O2-) and electrophilic oxygen atoms (O-) are suggested to be responsible for this selective transformation. Our study deepens the understanding of the mechanism of plasmonic photocatalysis and the role of silver oxide in selection transformation of organic molecules.

Coupled Optical and Electrochemical Probing of Silver Nanoparticle Destruction in a Reaction Layer.

The oxidation of silver nanoparticles is induced to occur near to, but not at, an electrode surface. This reaction at a distance from the electrode is studied through the use of dark‐field microscopy, allowing individual nanoparticles and their reaction with the electrode product to be visualized. The oxidation product diffuses away from the electrode and oxidizes the nanoparticles in a reaction layer, resulting in their destruction. The kinetics of the silver nanoparticle solution‐phase reaction is shown to control the length scale over which the nanoparticles react. In general, the new methodology offers a route by which nanoparticle reactivity can be studied close to an electrode surface.

On-Chip Stochastic Detection of Silver Nanoparticles without a Reference Electrode.

We report the electrochemical detection of 20 nm silver nanoparticles at a chip-based microelectrode array (MEA) without the need for a conventional reference electrode. This is possible due to the system's open-circuit potential allowing the oxidation of silver nanoparticles in the presence of phosphate-buffered saline (PBS). The hypothesis is confirmed by modulating the open-circuit potential via addition of ascorbic acid in solution, effectively inhibiting the detection of silver nanoparticle events. Employing the reference-free detection concept, we observe a linear relationship between the nanoparticle impact frequency at the microelectrodes and the nanoparticle concentration. This allows for viable quantification of silver nanoparticle concentrations in situ. The presented concept is ideal for the development of simple lab-on-a-chip or point-of-use systems enabling fast and low-cost screening of nanoparticles.

Collision and Oxidation of Silver Nanoparticles on a Gold Nanoband Electrode

We report the use of gold nanoband electrodes ranging from 60 to 180 nm in width to study collision and oxidation of single Ag nanoparticles (NPs). The use of nanoscale electrodes has enabled the observation of unique single-NP collision responses indicating a strong electrode size effect when the critical dimension of the electrode (the bandwidth) is reduced to that of NPs. In addition to multipeak events, NP collision on a nanoband electrode displays reduced collision frequency, significantly higher probability of single-peak events, and fewer subpeaks. More importantly, the average charge transferred in a single-peak event is about 50% less than that of the first subpeak of a multipeak event. The reduced charge of single-peak collisions and the more frequent appearance on nanoelectrodes are strong evidence that NPs start to behave differently at the electrode/solution interface when the size of the electrode is reduced to be comparable to that of the NPs. The reduced charge is likely due to a weaker pa...

Managing Heart Failure at Home With Point-of-Care Diagnostics.

The objective of this paper is development of an inexpensive point-of-care sensor for detecting the primary heart failure marker peptide, NT-proBNP. The device technology is based on an antibody sandwich assay, but with three innovative aspects. First, chemical amplification is carried out via oxidation of silver nanoparticles (NPs) attached to signaling antibodies rather than by enzymatic amplification. The electrochemical method is faster and eliminates the need for long-term storage of enzymes. Second, the antibody sandwich is formed on mobile magnetic beads. This enhances the rate of mass transfer of the biomarker and the signaling antibody to the primary detection antibody, which is immobilized on the magnetic beads. Third, the sensor itself is fabricated on a paper platform with screen-printed electrodes. This coupled with assembly by simple paper folding, keeps the cost of the sensor low. Here, we report on two separate assays. The first is based on a simple biotin-streptavidin conjugate, which is a preliminary model for the antibody assay. The results indicate a detection limit of 2.1 pM of silver NPs and an assay time of 7 min. The actual NT-proBNP antibody assay takes somewhat longer, and the dynamic detection range is higher: 2.9–582 nM. On the basis of the results presented in this paper, we conclude that this inexpensive paper-based sensor represents a viable technology for point-of-care testing of NT-proBNP, but nevertheless several challenges remain prior to clinical implementation. These include attaining a lower detection limit and better reproducibility, and optimizing the device for human blood.

Use of Silver Nanoparticles in Treatment of Socially Significant Diseases

The effects of colloidal silver are determined by concentration, sizes and stability of the finely-divided nanoparticles. Studies of silver-based drugs acting against antibiotic-resistant microorganisms are of significant scientific and practical interest. Silver does not effect on the infection, but directly on the cell structure. The main disadvantages of the current nanosilver-based compositions include low aggregative stability, polydispersity and oxidation of silver nanoparticles during prolonged storage. The main attention should be paid to the preservation of activity and bioavailability of the active substance, and ideally the increase in efficiency through synergy with other components of the formulation. Scientists of different special fields should focus not only on the development of new technologies for production of new nanomedicines, but on the possible health and environmental side effects

Voltammetric Study of the Influence of Various Phosphate Anions on Silver Nanoparticle Oxidation.

The antibacterial properties of silver are strongly controlled by the redox couple of silver/silver(I). This work reports the influence of phosphate anions on silver nanoparticle oxidation, which is important given the abundance of phosphate species in biological systems. The three different species of anions were found to have a varying degree of influence on silver oxidation with the order PO43−>HPO42−>H2PO4−. It was found that in the presence of phosphate anions, the silver oxidation potential shifts to a less positive value, which indicated the increasing ease of the oxidation reaction of silver. Given that the interplay between silver and its cation is crucial to its antibacterial properties and significant concentrations of the HPO42− anion are present at biological pH (near neutral), it is essential that the influence of the dibasic anion (HPO42−) on silver oxidation dynamics be considered for biological systems.

The Influence of the Capping Agent on the Oxidation of Silver Nanoparticles: Nano‐impacts versus Stripping Voltammetry

The influence of capping agents on the oxidation of silver nanoparticles was studied by using the electrochemical techniques of anodic stripping voltammetry and anodic particle coulometry ("nano-impacts"). Five spherical silver nanoparticles each with a different capping agent (branched polyethylenimine (BPEI), citrate, lipoic acid, polyethylene glycol (PEG) and polyvinylpyrrolidone (PVP)) were used to perform comparative experiments. In all cases, regardless of the capping agent, complete oxidation of the single nanoparticles was seen in anodic particle coulometry. The successful quantitative detection of the silver nanoparticle size displays the potential application of anodic particle coulometry for nanoparticle characterisation. In contrast, for anodic stripping voltammetry using nanoparticles drop casting, it was observed that the capping agent has a very significant effect on the extent of silver oxidation. All five samples gave a low oxidative charge corresponding to partial oxidation. It is concluded that the use of anodic stripping voltammetry to quantify nanoparticles is unreliable, and this is attributed to nanoparticle aggregation.

In situ optical microspectroscopy of the growth and oxidation of silver nanoparticles in silica thin films on soda-lime glass

An in situ optical microspectroscopy investigation of the growth and oxidation of silver nanoparticles (NPs) embedded in SiO2 thin films deposited on soda-lime glass has been conducted in real time during thermal processing in air. Variation of Ag NP size is followed by fitting of surface plasmon resonance (SPR) with spectra calculated by Mie theory, and analysed concurrently with the time evolution of SPR peak intensity. The NP transformations appeared to be temperature and time dependent. Silver NPs were indicated to grow at relatively high temperatures (e.g. 600 °C) due to Ostwald ripening, followed by a plateau and a gradual decrease in size resulting in SPR vanishing due to oxidation. The three phases were well separated in time. Oxidation appeared dominant at lower temperatures (e.g. 400 °C) as indicated by a continuous decrease in Ag particle size. The product of Ag NP oxidation was revealed by photoluminescence spectroscopy as single Ag+ ions. Furthermore, the data indicated that: (i) Ag+ ions are formed during heat treatment under an Ag/Ag+ redox equilibrium; (ii) the ions diffuse from the SiO2 matrix towards the soda-lime substrate where they stabilize; and (iii) the continuous removal of these ions from the matrix is necessary in order for the equilibrium to be displaced towards oxidation.

In situ approaches to establish colloidal growth kinetics.

A technique based on the back scattering phenomenon of dynamic light scattering has been employed to monitor the kinetics of gold and platinum metal nanoparticle growth and silver nanoparticle oxidation as well as in the determination of particle sizes ranging from 1 to 200 nm in diameter. The systems were chosen to examine the applicability of dynamic light scattering to nanoresearch over a broad range of sizes as well as both metallic and nonmetallic systems. The advantages of this instrumentation over traditional instruments such as X-ray diffraction (XRD), transmission electron microscopy (TEM), and scanning electron microscopy (SEM) have been highlighted.

Self-Assembly of Monolayer-Coated Silver Nanoparticles on Gold Electrodes. An Electrochemical Investigation

Redox behaviour of an array of dodecanethiol-coated silver nanoparticles, self-assembled on the gold electrode surface by dithiol linkers, is dominated in aqueous electrolytes by a reversible faradaic process that has been attributed to the anodic oxidation of silver nanoparticles. The nanoparticle array may be switched between the oxidised and reduced states repeatedly without any significant loss of electroactive material, thus showing a remarkable stability under electrochemical conditions. The mainly capacitive high-frequency electrical response of the nanoparticle array/electrolyte interface is characterised by a low value of capacitance, typical of a self assembled monolayer (SAM) of long-chain alkanethiols on gold; it has been associated with the compact organic layer interposed between the nanoparticles and the gold substrate. At lower frequencies, the interface displays a poorer electrical behaviour with both capacitive and resistive elements, which was associated with the more disordered organic layer located on the outer side of the film.