Synthesis of positively charged silver nanoparticles via photoreduction of AgNO3 in branched polyethyleneimine/HEPES solutions

Branched polyethyleneimine (BPEI) and 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) were used collaboratively to reduce silver nitrate under UV irradiation for the synthesis of positively charged silver nanoparticles. The effects of molar ratio of the ingredients and the molecular weight of BPEI on the particle size and distribution were investigated. The mechanism for the reduction of Ag+ ions in the BPEI/HEPES mixtures entails oxidative cleavage of BPEI chains that results in the formation of positively charged BPEI fragments enriched with amide groups as well as in the production of formaldehyde, which serves as a reducing agent for Ag+ ions. The resultant silver nanoparticles are positively charged due to protonation of surface amino groups. Importantly, these positively charged Ag nanoparticles demonstrate superior SERS activity over negatively charged citrate reduced Ag nanoparticles for the detection of thiocyanate and perchlorate ions; therefore, they are promising candidates for sensing and detection of a variety of negatively charged analytes in aqueous solutions using surface-enhanced Raman spectroscopy (SERS).     

SERS-activating effect of chlorides on borate-stabilized silver nanoparticles: formation of new reduced adsorption sites and induced nanoparticle fusion

Changes in morphology, surface reactivity and surface-enhancement of Raman scattering induced by modification of borate-stabilized Ag nanoparticles by adsorbed chlorides have been explored using TEM, EDX analysis and SERS spectra of probing adsorbate 2,2'-bipyridine (bpy) excited at 514.5 nm and evaluated by factor analysis. At fractional coverages of the parent Ag nanoparticles by adsorbed chlorides <0.6, the Ag colloid/Cl(-)/bpy systems were found to be constituted by fractal aggregates of Ag nanoparticles fairly uniform in size (10 +/- 2 nm) and SERS spectra of Ag(+)-bpy surface species were detected. The latter result was interpreted in terms of the presence of oxidized Ag(+) and/or Ag(n)(+) adsorption sites, which have been encountered also in systems with the chemically untreated Ag nanoparticles. At chloride coverages >0.6, a fusion of fractal aggregates into the compact aggregates of touching and/or interpenetrating Ag nanoparticles has been observed and found to be accompanied by the formation of another surface species, Ag-bpy, as well as by the increase of the overall SERS enhancement of bpy by factor of 40. The same Ag-bpy surface species has been detected under the strongly reducing conditions of reduction of silver nitrate by sodium borohydride in the presence of bpy. The formation of Ag-bpy is thus interpreted in terms of the stabilization of reduced Ag(0) adsorption sites by adsorbed bpy. The formation of reduced adsorption sites on Ag nanoparticle surfaces at chloride coverages >0.6 is discussed in terms of local changes in the work function of Ag. Finally, the SERS spectral detection of Ag-bpy species is proposed as a tool for probing the presence of reduced Ag(0) adsorption sites in systems with chemically modified Ag nanoparticles.     

Surface-enhanced Raman scattering of 4-aminobenzenethiol on silver nanoparticles substrate

Active surface-enhanced Raman scattering (SERS) silver nanoparticles substrate was prepared by multiple depositions of Ag nanoparticles on glass slides. The substrate is based on five depositions of Ag nanoparticles on 3-aminopropyl-trimetoxisilane (APTMS) modified glass slides, using APTMS sol–gel as linker molecules between silver layers. The SERS performance of the substrate was investigated using 4-aminobenzenethiol (4-ABT) as Raman probe molecule. The spectral analyses reveal a 4-ABT Raman signal enhancement of band intensities, which allow the detection of this compound in different solutions. The average SERS intensity decreases significantly in 4-ABT diluted solutions (from 10⁻⁴ to 10⁻⁶ mol L⁻¹), but the compound may still be detected with high signal/noise ratio. The obtained results demonstrate that the Ag nanoparticles sensor has a great potential as SERS substrate.     

Probing silver nanoparticles during catalytic H2 evolution

Employing silver nanoparticles from a recently developed synthesis [Evanoff, D. D.; Chumanov, G. J. Phys. Chem. B 2004, 108, 13948] and a well-studied probe molecule, p-aminothiophenol, we follow changes at the surface of the particles during the conditioning and eventually the catalytic production of hydrogen from water using strongly reducing radicals. Injection of electrons into the particles causes pronounced variations in the intensity of the surface enhanced Raman scattering (SERS) spectrum of the probe molecule. These spectral changes are caused by changes in the Fermi-level energy that are in turn caused by changes in the silver ion concentrations or in the pH, or by changes in electron density in the particle. This correlation highlights the effect of the chemical potential on the SERS enhancement at the end of the particles synthesis. The intensity of the SERS spectra increases in the presence of the silver ions when excitation at 514 nm is utilized. When the Ag(+) ions in the colloidal suspension are completely reduced by the radicals and the particles operate in the catalytic mode, the SERS spectrum is too weak to record, but it can reversibly be recovered upon the addition of Ag(+). The effect of pH on the SERS intensity is similar in nature to that of the silver ions but is complicated by the pKa of the aminothiol and the point of zero charge (pzc) of the particles. It is hypothesized that as the particles cross the pzc (around neutral pH) the electrostatic interaction between the protonated amine headgroup of the probe and the positively charged surface increases the density of probe molecules in the perpendicular orientation at the expense of a competing species. This conversion results in enhanced SERS signals and is observable during the preconditioning stage of the particles. Indeed, adsorption isotherms of the probe indicate the presence of two species. In analogous previous observations these two species have been attributed to perpendicular and flat adsorption orientations of the deprotonated probe molecule relative to the particle surface. However, preliminary density functional calculations on relevant prototypes raise the possibility that the two species may be the probe molecule and a cationic form produced by charge transfer in the ground state from the chemisorbed probe to the metal. These two forms of the probe have differing electronic structures and vibrational frequencies, with perhaps differing orientations relative to the surface. Whichever is the correct interpretation, a neutral molecule in a flat orientation or a radical cation, this species is easier to replace than the other in competitive adsorption by ethanethiol.     

Adsorbed state of p-mercaptobenzoic acid on silver nanoparticles

To exploit a gap mode plasmon in flocculates of metal nanoparticles most efficiently, the interaction between adsorbed chemical species and metal nanostructures were adjusted. We successfully formed flocculates of Ag nanoparticles (AgNPs) using electrostatic interaction between dissociated p-mercaptobenzoic acid (PMBA), protonated p-aminothiophenol (PATP) and their counter ions (Mⁿ⁺, X^m−), as well as van der Waals force between neutral PMBA, PATP and AgNPs. Detailed adsorbed state of PMBA and PATP in addition to trapped counter ions was characterized using enormous SERS enhancement in a flocculation method.     

Bacteria and bacteriophage inactivation by silver and zinc oxide nanoparticles

Microbial contaminants such as bacteria and viruses are of great concern in water. As nanotechnology continues to grow, understanding the interactions of nanoparticles with bacteria and viruses is important to protect public health and the environment. In this study, the effect of two commonly used nanoparticles, silver nanoparticles (AgNPs, average particle size=21 nm) and zinc oxide nanoparticles (ZnO NPs, average particle size=39 nm), on the growth of bacteria (Eschericia coli) and bacteriophages (MS2) were evaluated using a standard double agar layer (DAL) method and a turbidimetric microtiter assay. A 1-h prior exposure of MS2 to nanoparticles did not inactivate MS2 at the highest nanoparticle concentrations tested (5mg/L total Ag and 20 mg/L ZnO). No bacteriophage inactivation was observed in the presence of AgNPs, Ag(+)/AgNPs (50:50 in mass ratio) or Ag(+) ions, all at the total Ag concentration of 5mg/L. In a binary (bacteria-phages) system where the E. coli host was exposed to MS2 and nanoparticles simultaneously, the dynamic changes of active bacteria and MS2 phages during incubation demonstrated that exposure of AgNPs (5mg/L Ag) and ZnO NPs (20mg/L ZnO) increased the number of phages by 2-6 orders of magnitude. These results suggested that exposure of nanoparticles could greatly facilitate bacterial viruses like MS2 to infect the E. coli host.     

Substrates with discretely immobilized silver nanoparticles for ultrasensitive detection of anions in water using surface-enhanced Raman scattering

Positively charged silver nanoparticles, Ag [+], obtained by UV-assisted reduction of silver nitrate using branched poly(ethyleneimine) (BPEI) and 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) solutions as reducing agents, were immobilized on glass surfaces to produce substrates active in surface-enhanced Raman scattering (SERS). Negatively charged silver nanoparticles, Ag [-], synthesized via a modified citrate reduction method, were also investigated for comparison. At a sparse surface coverage of 30 nanoparticles/microm(2), substrates with immobilized Ag [+] showed increasing SERS sensitivity to a variety of anions in water in the order SO(4)(2-) < CN(-) < SCN(-) approximately ClO(4)(-), with corresponding binding constants of 10(5), 3.3 x 10(5), and 10(7) (for both SCN- and ClO(4)(-)) M(-1), respectively. This order followed the Hofmeister series of anion binding in water. Significantly, substrates with Ag [+] allowed limit of detection values of 8.0 x 10(-8) M (8 ppb) and 2.7 x 10(-7) M (7 ppb) for environmentally relevant perchlorate (ClO(4)(-)) and cyanide (CN(-)) anions, respectively. In contrast, substrates with immobilized Ag [-], even upon subsequent modification by a monolayer of BPEI for positive surface charge of the nanoparticles, showed a drastically lower sensitivity to these anions. The high sensitivity of substrates with Ag [+] for anion detection can be attributed to the presence of two types of functional groups, amino and amide, on the nanoparticle surface resulting from UV-assisted fragmentation of BPEI chains. Both amino and amide provide strong binding of anions with Ag [+] nanoparticles due to the synergistic effect through a combination of electrostatic, hydrogen bonding, and dispersive interactions.     

Gold nanoparticles assembling on smooth silver spheres for surface-enhanced Raman spectroscopy

A simple and cost-effective chemical method was introduced to assemble gold (Au) nanoparticles on smooth silver (Ag) spheres for realizing surface-enhanced Raman scattering (SERS) enhancement by the replacement reaction between chloroauric acid and Ag spheres. In addition, the Ag-Au core-shell spheres were fabricated when a certain amount of chloroauric acid was used in the reaction solution. We found that the Ag particles decorated with small Au nanoparticles demonstrated the strongest SERS enhancement, while Ag-Au core-shell spheres showed the weakest enhancement.     

Surface-enhanced Raman scattering spectroscopy of explosive 2,4-dinitroanisole using modified silver nanoparticles

2,4-Dinitroanisole (DNAN) is being used as a replacement for 2,4,6-trinitrotoluene (TNT) as a less-sensitive melt-cast medium explosive than TNT. In this paper, we studied the surface-enhanced Raman spectroscopy (SERS) analysis of DNAN using Ag nanoparticles (AgNPs) modified by L-cysteine methyl ester hydrochloride. Due to the formation of a Meisenheimer complex between DNAN and the modifier, the modified AgNPs can detect 20 μg/L (0.2 ng) and 0.1 mg/L (1 ng) DNAN in deionized water and aged tap water, respectively. Three other chemicals (L-cysteine, N-acetyl-L-cysteine, and L-cysteine ethyl ester hydrochloride) were used as AgNPs modifiers to study the mechanism of the SERS of DNAN. It was confirmed that the amino group of L-cysteine methyl ester hydrochloride was the active group and that the methyl ester group significantly contributed to the high SERS sensitivity of DNAN. In order to further test the mechanism of Meisenheimer complex formation, the effect of anions and cations present in natural water on the SERS of DNAN was studied. It was found that CO(3)(2-), Cl(-), and K(+) at 100 mg/L did not negatively affect the SERS of 10 mg/L DNAN, while SO(4)(2-), Na(+), Mg(2+), and Ca(2+) at 100 mg/L significantly quenched the SERS of 10 mg/L DNAN. The negative effect of the bivalent cations could be offset by SO(4)(2-).     

Surface-enhanced Raman scattering measurements on silver nanoparticles covered with differently formed platinum films

Gold and silver electromagnetic nanoresonators covered by a thin layer of platinum are often used to study adsorption of various molecules on “model platinum surfaces” with surface-enhanced Raman scattering (SERS) spectroscopy. In this contribution spectra of pyridine adsorbed on films formed from core–shell Ag@Pt and Ag@Ag–Pt nanoparticles and pure Pt or Ag nanoparticles were measured using a confocal Raman microscope. The SERS spectra of pyridine adsorbed on alloy Ag@Ag–Pt nanoparticles could not be obtained as a linear combination of spectra measured on pure Ag and Pt surfaces. In other words, for silver electromagnetic nanoresonators covered by platinum there is no simple correlation between the “quality” of the deposited Pt layer and the relative intensity of SERS bands characteristic for adsorbate interacting with silver. The SERS spectra accumulated from various places of a film formed from Ag@Pt or Ag@Ag–Pt nanoclusters may differ significantly. Using Ag@Pt nanoparticles with practically negligible amount of Ag on the surface (as per the stripping measurement), it is possible to record SERS spectrum in which the contribution characteristic for pyridine adsorbed on the Ag surface is well visible. It means that, even for macroscopic samples of core–shell Ag–Pt nanoparticles, averaging of many spectra measured at various locations of the sample should be carried out to characterize reliably their properties.     

Multiplexed analysis of silver(I) and mercury(II) ions using oligonucletide-metal nanoparticle conjugates

A colorimetric assay has been developed for the simultaneous selective detection of silver(I) and mercury(II) ions utilizing metal nanoparticles (NPs) as sensing element based on their unique surface plasmon resonance properties. In this method, sulfhydryl group modified cytosine-(C)-rich ssDNA (SH-C-ssDNA) was self-assembled on gold nanoparticles (AuNPs) to produce the AuNPs-C-ssDNA complex, and sulfhydryl group modified thymine-(T)-rich ssDNA (SH-T-ssDNA) was self-assembled on silver nanoparticles (AgNPs) to produce the AgNPs-T-ssDNA complex. Oligonucleotides (SH-C-ssDNA or SH-T-ssDNA) could enhance the AuNPs or AgNPs against salt-induced aggregation. However, the presence of silver(I) ions (Ag(+)) in the complex of ssDNA-AuNPs would reduce the stability of AuNPs due to the formation of Ag(+) mediated C-Ag(+)-C base pairs accompanied with the AuNPs color change from red to purple or even to dark blue. Moreover, the presence of mercury(II) ions (Hg(2+)) would also reduce the stability of AgNPs due to the formation of Hg(2+) mediated T-Hg(2+)-T base pairs accompanied with the AgNPs color change from yellow to brown, then to dark purple. The presence of both Ag(+) and Hg(2+) will reduce the stability of both AuNPs and AgNPs and cause the visible color change. As a result, Ag(+) and Hg(2+) could be detected qualitatively and quantitatively by the naked eye or by UV-vis spectral measurement. The lowest detectable concentration of a 5 nM mixture of Ag(+) and Hg(2+) in the river water was gotten by the UV-vis spectral measurement.     

Metal ion redox potential plays an important role in high-yield synthesis of monodisperse silver nanoparticles

A facile method is developed to prepare monodisperse silver nanoparticles in a much enhanced yield by adding a trace amount of Fe(3+) ions into the reaction of AgNO(3) with oleic acid and oleylamine. We propose that Fe(2+) ions speed up the nucleation and growth of silver nanoparticles due to the electron-transfer between Ag(+) and Fe(2+), in which Fe(2+) is from Fe(3+) reduced by oleylamine.     

Fabrication of flower-like silver nanoparticles for surface-enhanced Raman scattering

The flower-like silver nanoparticles have been synthesized by reducing silver nitrate (AgNO₃) with ascorbic acid (AA) as the reductant and polyvinyl pyrrolidone (PVP) as the capping agent under vigorous stirring. Such flower-like nanoparticles are aggregates of small nanoplates and nanorods. They were tested as substrates for the surface-enhanced Raman scattering (SERS), showing high sensitivity for detecting Rhodamine 6G (R6G) at a concentration as low as 10^-7 mol/L. It has been found that replacing mechanical stirring with ultrasound sonication would drastically change the particle morphology, from flower-like nanoparticles to well-dispersed smaller nanoparticles. Furthermore, when trace amounts of NaCl were added into the reagents, well-dispersed Ag nanoparticles formed even in vigorous stirring. These phenomena can be explained with the diffusion and reactant supply during nucleation and growth of Ag nanoparticles.     

Synthesis of AgcoreAushell bimetallic nanoparticles for immunoassay based on surface-enhanced Raman spectroscopy

Layered core-shell bimetallic silver-gold nanoparticles were prepared by coating Au layers over Ag seeds by a seed-growth method. The composition of Ag100-xAux particles can vary from x=0 to 30. TEM and SEM images clearly show that the bimetallic nanoparticles are of core-shell structure with some pinholes on the surface. Strong surface-enhanced Raman (SER) signals of thiophenol and p-aminothiophenol have been obtained with these colloids. It was found that the SERS activity of aggregated colloids critically depends on the molar ratio of Ag to Au. With the increase of the Au molar fraction, the SERS activity enhances first and then weakens, with the maximal intensity being 10 times stronger than that of Ag colloids. The AgcoreAushell nanoparticles were then labeled with monoclonal antibodies and SERS probes and used for immunoassay analysis. In the proposed system, antibodies immobilized on a solid substrate can interact with the corresponding antigens to form a composite substrate, which can capture reporter-labeled AgcoreAushell nanoparticles modified with the same antibodies. The immunoreaction between the antibodies and antigens was demonstrated by the detection of characteristic Raman bands of the probe molecules. AgcoreAushell bimetallic nanoparticles, as a new SERS active and biocompatible substrate, will be expected to improve the detection sensitivity of immunoassay.     

A surface-enhanced Raman scattering method for detection of trace glutathione on the basis of immobilized silver nanoparticles and crystal violet probe

Unsatisfactory sensitivity and stability for molecules with low polarizability is still a problem limiting the practical applications of surface-enhanced Raman scattering (SERS) technique. By preparing immobilized silver nanoparticles (Fe₃O₄/Ag) through depositing silver on the surface of magnetite particles, a highly sensitive and selective SERS method for the detection of trace glutathione (GSH) was proposed on the basis of a system of Fe₃O₄/Ag nanoparticles and crystal violet (CV), in which the target GSH competed with the CV probe for the adsorption on the Fe₃O₄/Ag nanoparticles. Raman insensitive GSH replaced the highly Raman sensitive CV adsorbed on the surface of Fe₃O₄/Ag particles. This replacement led to a strong decrease of the CV SERS signal, which was used to determine the concentration of GSH. Under optimal conditions, a linear response was established between the intensity decrease of the CV SERS signal and the GSH concentration in the range of 50–700 nmol L⁻¹ with a detection limit of 40 nmol L⁻¹. The use of a Fe₃O₄/Ag substrate provided not only a great SERS enhancement but also a good stability, which guarantees the reproducibility of the proposed method. Its use for the determination of GSH in practical blood samples and cell extract yielded satisfactory results.     

Visible‐light‐driven self‐cleaning SERS substrate of silver nanoparticles and graphene oxide decorated nitrogen‐doped titania nanotube array

Graphene oxide (GO) and silver nanoparticles (Ag NPs) sequentially decorated nitrogen‐doped titania nanotube array (N‐TiO₂ NTA) had been designed as visible‐light‐driven self‐cleaning surface‐enhanced Raman scattering (SERS) substrate for a recyclable SERS detection application. N‐TiO₂ NTA was fabricated by anodic oxidation and then doping nitrogen treatment in ammonia atmosphere, acting as a visible‐light‐driven photocatalyst and supporting substrate. Ag/GO/N‐TiO₂ NTA was prepared by decorating GO monolayer through an impregnation process and then depositing Ag NPs through a polyol process on the surface of N‐TiO₂ NTA, acting as the collection of organic molecule and Raman enhancement. The SERS activity of Ag/GO/N‐TiO₂ NTA was evaluated using methyl blue as an organic probe molecule, revealing the analytical enhancement factor of 4.54 × 10⁴. Ag/GO/N‐TiO₂ NTA was applied as active SERS substrate to determine a low‐affinity organic pollutant of bisphenol A, revealing the detection limit of as low as 5 × 10^?7 m . Ag/GO/N‐TiO₂ NTA could also achieve self‐cleaning function for a recycling utilization through visible‐light‐driven photocatalytic degradation of the adsorbed organic molecules. Ag/GO/N‐TiO₂ NTA has been successfully reused for five times without an obvious decay in accuracy and sensitivity for organic molecule detection. The unique properties of this SERS substrate enable it to have a promising application for the sensitive and recyclable SERS detection of low‐affinity organic molecules. Copyright © 2016 John Wiley & Sons, Ltd.     

Determination of mercury(II) by surface-enhanced Raman scattering spectroscopy based on thiol-functionalized silver nanoparticles

Silver nanoparticles (Ag NPs) modified with sodium 2-mercaptoethanesulfonate (mesna) exhibit strong surface-enhanced Raman scattering (SERS). Their specific and strong interaction with heavy metal ions led to a label-free assay for Hg(II). The covalent bond formed between mercury and sulfur is stronger than the one between silver and sulfur and thus prevents the adsorption of mesna on the surface of Ag NPs. This results in a decrease of the intensity of SERS in the presence of Hg(II) ions. The Raman peak at 795?cm^?1 can be used for quantification. The effect of the concentration of mesna, the concentration of sodium chloride, incubation time and pH value on SERS were optimized. Under the optimal conditions, the intensity of SERS decreases with increasing concentration of Hg(II). The decrease is linear in the 0.01 and 2?μmol L^?1 concentration range, with a correlation coefficient (R²) of 0.996 and detection limit (S/N?=?3) is 0.0024?μmol L^?1. The method was successfully applied to the determination of the Hg(II) in spiked water samples.

p-Benzosemiquinone radical anion on silver nanoparticles in water

This communication reports the SERS observation of p-benzosemiquinone radical anion, produced on reduction of p-benzoquinone by Ag nanoparticles at the metal-water interface. The species is positively identified by comparison of the SERS spectrum with the resonance Raman spectra of the radical anion in aqueous solution. This is a rare SERS observation of a radical intermediate formed by surface reaction on nanosize silver particles in solution.     

TiO2 films loaded with silver nanoparticles: control of multicolor photochromic behavior

Ag-TiO(2) films exhibiting multicolor photochromism were prepared by photoelectrochemical reduction of Ag(+) to Ag nanoparticles in nanoporous TiO(2) films under UV light. Color of the Ag-TiO(2) film, initially brownish-gray, changes under a colored visible light to the color of the light and reverts to brownish-gray under UV light. Their chromogenic properties were improved by simultaneous irradiation for Ag deposition with UV and blue lights to suppress the formation of anisotropic Ag particles. Nonvolatilization of a color image was also achieved by removing Ag(+) that was generated during the irradiation with a colored light. Once nonvolatilized, the image can be reproduced by UV light, even after the image is discolored under white light. This new effect evidenced that nanopores in the TiO(2) film determine the resonance wavelengths of the Ag particles, as their molds. In addition, solvatochromic behavior of the Ag-TiO(2) film proved that nanospaces left around the Ag nanoparticles affect the resonance wavelengths of the Ag particles.     

Surface-enhanced Raman scattering of EDOT and PEDOT on silver and gold nanoparticles

Surface-enhanced Raman scattering (SERS) spectra of the conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) and its monomer 3,4-ethylenedioxythiophene (EDOT) on Ag and Au nanoparticles presenting different morphologies and stabilizing agents have been obtained using the excitation radiation at 633 nm. The SERS spectra of the monomer and polymer are strongly dependent both on the metal and capping agent of the substrate. SERS spectra of EDOT on Au nanospheres indicates that adsorption occurs with the thiophene ring perpendicular to the metal surface. In contrast, polymerization takes place on the silver surface of Ag nanospheres. EDOT adsorption on Ag nanoprisms with polyvinylpyrrolidone (PVP) as capping agent occurs similarly to that observed on gold. Surface-enhanced resonance Raman scattering (SERRS) spectra of PEDOT on gold nanostars that present a thick layer of PVP show no chemical interaction of PEDOT with the metal surface; however, when PEDOT is adsorbed on citrate stabilized gold nanospheres, the SERRS spectra suggest that thiophene rings are perpendicular to the surface. Oxidation of PEDOT also is observed on Ag nanospheres. The investigation of the interface between PEDOT and metal surface is crucial for the development in polymer-based optoelectronic devices since this interface plays a crucial role in their stability and performance.