Inhibition and enhancement of glucose oxidase activity in a chitosan-based electrode filled with silver nanoparticles

A chitosan-based electrode filled with silver nanoparticles (AgNPs) and glucose oxidase (GOD) was used as an enzyme electrode to investigate the effect of aging process of AgNPs on the GOD activity. Freshly prepared AgNPs inhibit the GOD activity, however, the inhibitory effect decreased with the increase of aging time. After aged for a period of time, AgNPs showed enhancement effect on the GOD activity. The effect of aging was studied by the measurements of Ag⁺ ions concentration, zeta (ζ) potential and X-ray photoelectron spectroscopy (XPS). And the results indicated that the concentration of Ag⁺ ions in the silver sol decreased during the aging period (i.e. Ag⁺ ions converted to more inert silver metal Ag⁰). The effect of AgNPs on the GOD activity can be changed by controlling the aging time of AgNPs. This research provides a new and simple approach to mediate AgNPs property, which is of great value in potential application of AgNPs in biosensors and nanoscale devices.     

A comparative study of the antibacterial mechanisms of silver ion and silver nanoparticles by Fourier transform infrared spectroscopy

In this study, we performed the first comparative study of the antibacterial mechanisms of silver ion (Ag⁺) and silver nanoparticles (AgNPs) on Escherichia coli (E. coli) using Fourier transform infrared (FTIR) spectroscopy. Through a thorough analysis of the FTIR spectra of E. coli after silver treatment in the spectral regions corresponding to thiol group, protein, lipopolysaccharide (LPS), and DNA, we were able to reveal a multifaceted antibacterial mechanism of silver at the molecular level for both Ag⁺ and AgNPs. Features of such mechanism include: (1) silver complexes with thiol group; (2) silver induces protein misfolding; (3) silver causes loss of LPS from bacterial membrane; (4) silver changes the overall conformation of DNA. Despite the similarities between Ag⁺ and AgNPs with respect to their antibacterial mechanisms, we further revealed that Ag⁺ and AgNPs display quite different kinetics for silver-thiol complexation and loss of LPS, with Ag⁺ displaying fast kinetics and AgNPs displaying slow kinetics. At last, we proposed a hypothesis to interpret the observed different behaviors between Ag⁺ and AgNPs when interacting with E. coli.     

Displacement-dispersive liquid-liquid microextraction coupled with graphite furnace atomic absorption spectrometry for the selective determination of trace silver in environmental and geological samples

A novel displacement-dispersive liquid-liquid microextraction method was developed for the selective determination of trace silver in complicated samples by graphite furnace atomic absorption spectrometry. This method involves two steps of dispersive liquid-liquid microextraction (DLLME). Firstly, copper ion reacted with diethyldithiocarbamate (DDTC) to form Cu-DDTC complex and extracted with DLLME procedure using carbon tetrachloride (extraction solvent) and methanol (dispersive solvent); then, the sedimented phase was dispersed into the sample solution containing silver ion with methanol and another DLLME procedure was carried out. Because the stability of Ag-DDTC is larger than that of Cu-DDTC, Ag⁺ can displace Cu²⁺ from the pre-extracted Cu-DDTC and thus the preconcentration of Ag⁺ was achieved. Potential interference from co-existing transition metal ions with lower DDTC complex stability was largely eliminated as they cannot displace Cu²⁺ from Cu-DDTC complex. The tolerance limits for the co-existing ions were increased by a long way compared with conventional DLLME. Under the optimal conditions, the limit of detection was 20 ng L⁻¹ (3σ) for silver with a sample volume of 5.0 mL, and an enhancement factor of 72 was achieved. The proposed method was successfully applied to determine of trace silver in some environmental and geological samples with satisfactory results.     

Aptamer-functionalized silver nanoparticles for scanometric detection of platelet-derived growth factor-BB

In this work, we reported a scanometric assay system based on the aptamer-functionalized silver nanoparticles (apt-AgNPs) for detection of platelet-derived growth factor-BB (PDGF-BB) protein. The aptamer and ssDNA were bound with silver nanoparticles by self-assembly of sulfhydryl group at 5′ end to form the apt-AgNPs probe. The apt-AgNPs probe can catalyze the reduction of metallic ions in color agent to generate metal deposition that can be captured both by human eyes and a flatbed scanner. Two different color agents, silver enhancer solution and color agent 1 (10 mM HAuCl₄ + 2 mM hydroquinone) were used to develop silver and gold shell on the surface of AgNPs separately. The results demonstrated that the formation of Ag core–Au shell structure had some advantages especially in the low concentrations. The apt-AgNPs probe coupled with color agent 1 showed remarkable superiority in both sensitivity and detection limit compared to the apt-AuNPs system. The apt-AgNPs system also produced a wider linear range from 1.56 ng mL⁻¹ to 100 ng mL⁻¹ for PDGF-BB with the detection limit lower than 1.56 ng mL⁻¹. The present strategy was applied to the determination of PDGF-BB in 10% serum, and the results showed that it had good specificity in complex biological media.     

Calix[2]furano[2]pyrrole and related compounds as the neutral carrier in silver ion-selective electrode

The performance of calix[2]furano[2]pyrrole and related compounds used as neutral carriers for silver selective polymeric membrane electrode was investigated. The silver ion-selective electrode based on calix[2]furano[2]pyrroles gave a good Nernstian response of 57.1 mV per decade for silver ion in the activity range 1×10⁻⁶ to 1×10⁻² M. The present silver ion-selective electrode displayed very good selectivity for Ag⁺ ion against alkali and alkaline earth metal ions, NH₄⁺, and H⁺. In particular, the present Ag⁺-selective electrode exhibited very low responses towards Hg²⁺ and Pb²⁺ ions. The potentiometric selectivity coefficients of the silver ion-selective electrode exhibited a strong dependence on the solution pH. In particular, the response of the electrode to the Hg²⁺ activity was greatly diminished at pH 2.5 compared to that at pH 5.0. Overall, the performance of the present silver ion-selective electrode based on the ionophore, calix[2]furano[2]pyrrole, is very comparable to that of the electrode prepared with the commercially available neutral carrier in terms of slope, linear range, and detection limits.     

Effect of ligand on the redox reactions of thallium metal clusters

Pulse radiolysis of an aqueous solution of mono-valent thallium ion and mixed solutions of Tl⁺/Ag⁺ in the presence of various amino polycarboxylic acids such as trans-1,2-diaminocyclohexane-N,N,N′,N′-tetraacetic acid (DCTA), diethylenetriaminepentaacetic acid (DTPA), N-(2-hydroxyethyl)ethylenediaminetriacetic acid (HEDTA) and triethylenetetraminehexaacetic acid (TTHA) has been carried out. Abnormal valence states of Tl ions were generated. It is concluded that DCTA, DTPA, HEDTA and TTHA decrease the redox potential of Tl ions in aqueous solutions. It was observed that the electron transfer from complexed Tl₂⁺ to Ag⁺ varied in the range 7.5 × 10⁸ to 1.0 × 10⁹, depending on the type of complexing ligand. Electron transfer from Tl₂⁺ to Ag⁺ lead to the formation of silver atoms, which agglomerate further to form silver colloid.     

A flow injection analysis system for monitoring silver (I) ion and iodine residuals in recycled water from recovery systems used for spaceflight

A laboratory-built flow injection analyzer is reported for monitoring the drinking water disinfectants silver (I) ion and iodine in water produced from NASA's water recovery system. This analyzer uses spectrophotometric detection with a custom made 10 cm optical flow cell. Optimization and interference studies are discussed for the silver (I) ion configuration. Subsequent results using the silver (I) configuration with minor modifications and alternative reagents gave promising results for iodine determinations as well. The estimated MDL values for Ag⁺ and I₂ are 52 μg L⁻¹ Ag⁺ and 2 μg L⁻¹ I₂; the mean percent recoveries were 104% and 96.2% for Ag⁺ and I₂ respectfully; and percent relative standard deviations were estimated at 1.4% for Ag⁺ and 5.7% for I₂. The agreement of this potentially multifunctional analyzer to reference methods for each respective water disinfectant is measured using Bland–Altman analysis as well as more traditional estimates.     

Flow injection method for the determination of silver concentration in drinking water for spacecrafts

A flow injection method has been developed for determination of silver. The method is based on a reduction reaction with sodium borohydride which leads to the formation of a colloidal species which is monitored at a wavelength of 390 nm.The reaction variables flow rate, sodium borohydride concentration and pH, which affect sensitivity, were investigated and their effects were established using a two-levels, three-factor experimental design. Further optimization of manifold variables (reaction coil and injection volume) allowed us to determine silver in the range 0.050-5.0 mg L⁻¹ with a minimum detectable concentration of 0.050 mg L⁻¹. Silver is added, as biocide, to drinking water for spacecrafts. The chemical species of silver, present in this kind of sample, were characterized by a procedure based on the selective retention of Ag⁺ onto a 2.2.2. cryptand based substrate followed by determination of the non-bound and bound (after elution) Ag⁺ by the FIA method. The method optimized was applied to a drinking water sample provided for the launch with the Automated Transfer Vehicle (ATV) module Jule Verne to the International Space Station (March 9, 2008).     

Ag selective electrode based on glassy carbon electrode covered with polyaniline and thiacalix[4]arene as neutral carrier

Potentiometric sensor based on glassy carbon electrode covered with polyaniline and neutral carrier, e.g. thiacalix[4]arene containing pyridine fragments in the substituents in the lower rim has been developed and applied for determination of Ag⁺ ions in the range from 1.0 × 10⁻² to 5.0 × 10⁻⁷ M with the response time of 12 s. The presence of thiacalixarene in the surface layer improves the reversibility and selectivity of the signal towards transient metal ions. The potentiometric selectivity coefficients were determined for various measurement conditions. As shown, the pH control and the use of NaF as a masking agent fully eliminate the interfering effect of Hg²⁺ and Fe³⁺ ions, respectively. The reaction of Ag⁺ with thiacalixarene was proved by the investigation of the extraction of picrate complexes of transient metals in the organic phase. The potentiometric sensor developed was successfully used for the potentiometric determination of silver sulfathiazole (Argosulfan™).     

Fast functionalization of silver decahedral nanoparticles with aptamers for colorimetric detection of human platelet-derived growth factor-BB

Aptamer-silver decahedral nanoparticles (Ag₁₀NPs-aptamer) based detection was developed for protein. Ag₁₀NPs were synthesized by photochemical method. The advantage of Ag₁₀NPs was its tolerance of NaCl which facilitates the functionalization of silver nanoparticles with all kinds of ssDNA. Attaching aptamers to Ag₁₀NPs could be achieved within 2 h, much faster than traditional methods. Human platelet-derived growth factor-BB (PDGF-BB) was used as a model protein to test the binding capacity of aptamers attached on Ag₁₀NPs. Our data showed that the aptamer-Ag₁₀NPs conjugates were successful in detecting human PDGF-BB. Furthermore, we developed an aptamer-Ag₁₀NPs conjugates-based colorimetric sensor to detect PDGF-BB. The results showed a linear relationship between PDGF-BB concentrations (5 ng mL⁻¹–200 ng mL⁻¹) and ΔOD with excellent detection specificity in serum. Therefore, the sensor based on aptamer-Ag₁₀NPs conjugates was highly effective and sensitive and had great promise for further development and applications.     

Investigation of the interaction between cobalt sulfide coatings and Ag(I) ions using cyclic voltammetry in KClO4 and NaOH aqueous solutions and X-ray photoelectron spectroscopy

Cobalt sulfide coatings have been investigated by means of cyclic voltammetry in 0.1 M KClO₄ and 0.1 M NaOH solutions and analyzed using X-ray photoelectron spectroscopy. They have been shown to contain CoS(OH), CoS and Co(OH)₂. After treating such Co sulfide coatings with AgNO₃ solution, their composition changes: both the cobalt and oxygen content decreases and Ag (up to 85 at%) appears in the coating as Ag₂S, Ag₂O and metallic Ag. Co(II) compounds react with Ag⁺ ions according to an exchange reaction [CoS+2Ag⁺+2H₂O→Ag₂S+Co(OH)₂+2H⁺]. In the course of the reaction of Co(OH)₂ with silver ions, a redox process occurs, giving metallic silver [Co(OH)₂+Ag⁺+H₂O→Ag°+Co(OH)₃+H⁺ or Co(OH)₂+Ag⁺→Ag°+CoO(OH)+H⁺]. Ag₂S reduction takes place at more positive potentials than Cu reduction; therefore sulfide layers of cobalt modified with silver ions, unlike unmodified ones, may be plated with Cu from both acid and alkaline electrolytes. Electronic Publication     

Paper test strip for silver ions detection in drinking water samples based on combined fluorometric and colorimetric methods

In this study, a portable silver ion (Ag⁺) sensor was fabricated based on a dual signal output system using black phosphorus quantum dots (BPQDs) as probes. It is the first work for Ag⁺ detection using paper test strip based on BPQDs. The color change of BPQDs paper sensor for the determination of Ag⁺ was easily identified by naked eye. BPQDs were synthesized from bulk black phosphorus (BP) by mechanical exfoliation combined with a solvothermal method. BPQDs exhibited blue fluorescence with a quantum yield of 8.82 %. The fluorescence of BPQDs can be quenched by Ag⁺, and the absorbance of BPQDs is increased with increasing Ag⁺ concentration. The mechanism of the interaction between BPQDs and Ag⁺ involving fluorescence quenching and bonding was investigated by experimental and computational methods. The detection limit of Ag⁺ was 1.56 μg/mL and 0.19 μg/mL using fluorometry and colorimetry methods, respectively. A portable visual sensor based on paper test strip was constructed for Ag⁺ detection using the colorimetric approach. The strategy was employed to determine Ag⁺ successfully in drinking water samples. Therefore, the proposed portable Ag⁺ sensor can be potentially utilized for the lab-free analysis of drinking water and even dietary samples.     

Colorimetric detection of kanamycin based on analyte-protected silver nanoparticles and aptamer-selective sensing mechanism

In this work, a novel colorimetric detection method for kanamycin (Kana), a widely used aminoglycoside antibiotic, has been developed using unmodified silver nanoparticles (AgNPs) as sensing probe. The method is designed based on the finding that the analyte (Kana) can protect AgNPs against salt-induced aggregation, and nucleic acid aptamers can decrease the risk of false positives through an aptamer-selective sensing mechanism. By use of the proposed method, selective quantification of Kana can be achieved over the concentration range from 0.05 to 0.6 μg mL⁻¹ within 20 min. The detection limit is estimated to be 2.6 ng mL⁻¹, which is much lower than the allowed maximum residue limit. Further studies also demonstrate the applicability of the proposed method in milk samples, revealing that the method may possess enormous potential for practical detection of Kana in the future.     

Effect of the polarity of silver nanoparticles induced by ionic liquids on facilitated transport for the separation of propylene/propane mixtures

The effect of ionic liquids on the formation of a partial positive charge on the surface of silver nanoparticle and its subsequent effect on facilitated olefin transport were investigated. Three different ionic liquids of 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM⁺BF₄⁻), 1-butyl-3-methylimidazolium triflate (BMIM⁺Tf⁻), and 1-butyl-3-methylimidazolium nitrate (BMIM⁺NO₃⁻) were employed to control the positive charge density of the surface of silver nanoparticles. The positive charge density of the silver nanoparticles, as characterized by the binding energy of the silver atom, was in the following order: BMIM⁺BF₄⁻/Ag ? BMIM⁺Tf⁻/Ag > BMIM⁺NO₃⁻/Ag. This order was consistent with the tendency of ionic liquids to form free ions. The best separation performance for the propylene/propane mixtures was a mixed gas selectivity of 17 and a permeance of 7 GPU through a composite membrane consisting of BMIM⁺BF₄⁻/Ag. A better separation performance for olefin/paraffin mixtures was observed with a higher positive charge density of the silver nanoparticles. It was therefore concluded that facilitated olefin transport was a direct consequence of the surface positive charge of the silver nanoparticles induced by ionic liquids.     

Electron Transfer in Peptides: On the Formation of Silver Nanoparticles

Some microorganisms perform anaerobic mineral respiration by reducing metal ions to metal nanoparticles, using peptide aggregates as medium for electron transfer (ET). Such a reaction type is investigated here with model peptides and silver as the metal. Surprisingly, Ag⁺ ions bound by peptides with histidine as the Ag⁺‐binding amino acid and tyrosine as photoinducible electron donor cannot be reduced to Ag nanoparticles (AgNPs) under ET conditions because the peptide prevents the aggregation of Ag atoms to form AgNPs. Only in the presence of chloride ions, which generate AgCl microcrystals in the peptide matrix, does the synthesis of AgNPs occur. The reaction starts with the formation of 100 nm Ag@AgCl/peptide nanocomposites which are cleaved into 15 nm AgNPs. This defined transformation from large nanoparticles into small ones is in contrast to the usually observed Ostwald ripening processes and can be followed in detail by studying time‐resolved UV/Vis spectra which exhibit an isosbestic point.     

Highly selective silver nanoparticles based label free colorimetric sensor for nitrite anions

Highly selective label free colorimetric sensor based on AgNPs stabilized by phenolic chelating ligand, N,N′-bis(2-hydroxybenzyl)-1,2-diaminobenzene (1), for NO₂⁻ anions has been developed. Addition of NO₂⁻ showed selective decolourisation of brownish yellow colour of 1-AgNPs with the detection limit of 10⁻⁷ M. Absorption studies showed the complete disappearance of 1-AgNPs peak at 426 nm due to the conversion of AgNPs to silver ions. The presence silver ions were confirmed by white precipitates of AgCl formation with NaCl. The interference studies confirmed the high selectivity of NO₂⁻ sensing in presence of anions as well as cations by 1-AgNPs. A linear relationship was observed between the change of absorption and concentration of NO₂⁻. The present approach could be performed at room temperature and ambient conditions. The practical applications of 1-AgNPs for selective sensing of NO₂⁻ in different water samples such as ground, river, pond and tap water have also been demonstrated.     

The Ag+-G interaction inhibits the electrocatalytic oxidation of guanine--a novel mechanism for Ag+ detection

The heavy metal ions-nucleobases interaction is an important research topic in environmental and biochemical analysis. The presence of the silver ion (Ag⁺) may influence the formation of oxidation intermediate and the electrocatalytic oxidation activity of guanine (G), since Ag⁺ can interact with guanine at the binding sites which are involved in the electrocatalytic oxidation reaction of guanine. According to this principle, a new electrochemical sensor for indirectly detecting Ag⁺ based on the interaction of Ag⁺ with isolated guanine base using differential pulse voltammetry (DPV) was constructed. Among the heavy metal ions examined, only Ag⁺ showed the strongest inhibitory effect on the electrocatalytic oxidation of guanine at the multi-walled carbon nanotubes modified glassy carbon electrode (CNTs/GC). And the quantitative study of Ag⁺ based on Ag⁺-G sensing system gave a linear range from 100 nM to 2.5 μM with a detection limit of 30 nM. In addition, this modified electrode had very good reproducibility and stability. The developed electrochemical method is an ideal tool for Ag⁺ detection with some merits including remarkable simplicity, low-cost, and no requirement for probe preparation.     

A surfactant‐free synthesis of the silica nanosphere‐supported ultrafine silver nanoparticles and their antibacterial effects

In this study, a facile, efficient, and surfactant‐free method to synthesize silica nanosphere‐supported ultrafine silver nanoparticles (AgNPs) (~2.5 nm) was developed, and their antibacterial effects were investigated. In the synthesis process, the hydrolysis of 3‐mercaptopropyltrimethoxysilane was adopted to provide thiol groups and in situ reduce Ag⁺ to Ag⁰ for ultrafine AgNPs formation on the surface of the silica nanosphere. Electron microscopy characterization of the complex formed revealed that the ultrafine AgNPs were not agglomerated and grow without any surfactants because there were no excess electrons transported from the shell to reduce the silver ions to silver atoms. The antibacterial effects of the supported ultrafine AgNPs with the surfactant‐free surface were evaluated against the Escherichia coli even at very low dosage. After incubation with 20 μg/mL silica‐supported AgNPs up to 120 min, 99.7% of the E. coli were inactivated, according to the bacterial viability measured by flow cytometry.     

Biosynthesis, purification and characterization of silver nanoparticles using Escherichia coli

The application of nanoscale materials and structures, usually ranging from 1 to 100 nanometers (nm), is an emerging area of nanoscience and nanotechnology. Nanomaterials may provide solutions to technological and environmental challenges in the areas of solar energy conversion, catalysis, medicine, and water-treatment. The development of techniques for the controlled synthesis of nanoparticles of well-defined size, shape and composition, to be used in the biomedical field and areas such as optics and electronics, has become a big challenge. Development of reliable and eco-friendly processes for synthesis of metallic nanoparticles is an important step in the field of application of nanotechnology. One of the options to achieve this objective is to use ‘natural factories’ such as biological systems. This study reports the optimal conditions for maximum synthesis of silver nanoparticles (AgNPs) through reduction of Ag⁺ ions by the culture supernatant of Escherichia coli. The synthesized silver nanoparticles were purified by using sucrose density gradient centrifugation. The purified sample was further characterized by UV–vis spectra, fluorescence spectroscopy and TEM. The purified solution yielded the maximum absorbance peak at 420 nm and the TEM characterization showed a uniform distribution of nanoparticles, with an average size of 50 nm. X-ray diffraction (XRD) spectrum of the silver nanoparticles exhibited 2θ values corresponding to the silver nanocrystal. The size-distribution of nanoparticles was determined using a particle-size analyzer and the average particle size was found to be 50 nm. This study also demonstrates that particle size could be controlled by varying the parameters such as temperature, pH and concentration of AgNO₃.     

A near-infrared fluorescent chemodosimeter for silver(I) ion based on an expanded porphyrin

An expanded porphyrin [26]hexaphyrin(1.1.1.1.1.1) was exploited as a fluorescent chemodosimeter for Ag⁺ ions with high sensitivity and selectivity via near-infrared luminescence above 900 nm, a region that is free from optical interference in the visible wavelength range induced by the commonly used matrix and other organic compounds. The association constant for the Ag⁺-porphyrin complexation was evaluated by spectroscopic titration method to be 7.24 × 10¹⁰ M⁻¹.