Ag Colloids Research Articles

Optical and Surface-Enhanced Raman Scattering Properties of Black and Metallic Silver Micro/Nano Structures Fabricated on Cicada Wings by Silver Mirror Reaction.

Silver was deposited on cicada (Cryptotympana facialis) wings to fabricate black and metallic silver micro/nano structures on the wings (black-Ag wings and metallic-Ag wings) by the silver mirror reaction. The transparent cicada wings with close-packed nanopillar array structures were used as the substrates based on natural materials. As the silver mirror reaction proceeded, the color of the wing gradually changed from transparent to brown, then to black, and finally to metallic silver. A thin Ag film coated nanopillar array structure is seen in the black-Ag wing. The transmittance and reflectance of the black-Ag wing were nearly 0% and an average of about 4% in the wavelength range of 200-800 nm, respectively. Densely stacked colloidal Ag particles with an average diameter of 155 nm were observed on the surface of the metallic-Ag wing. The surface enhanced Raman scattering (SERS) signal intensities of Rhodamine 6G (R6G) dripped and dried on the black-Ag wing, thin Ag film sputtered on glass slides, and metallic-Ag wing increased in that order. In particular, the SERS signal intensities of R6G on metallic-Ag wing were greater than two orders of magnitude than those on the black-Ag wing and thin Ag film.

Detection and Characterization of Silver Nanostructures in Consumer Products.

Silver (Ag) is one among the few nanomaterials which are widely used across several consumer products. However, there is limited research on detection and characterization of Ag nanostructures in complex matrices such as consumer products. Most previous studies for analytical method development were based on Ag liquid formulations or with standard materials. In this study, a total of fifteen commercial products including dietary supplements, deodorants, fabric, skin protectants, and toothpastes that declare nano or colloidal Ag ingredients were investigated. To characterize the quantity, size, size distribution, and morphology of Ag nanoparticles used in the products, several analytical instrumental techniques such as inductively coupled plasma-mass spectroscopy (ICPMS), dynamic light scattering (DLS), differential centrifugal sedimentation (DCS), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and energy dispersive X-ray spectrometry (EDS) were employed. Study results showed that Ag nanoparticles were found in eleven of the fifteen investigated commercial products, where the majority of Ag nanoparticles were spherical and smaller than 50 nm. The advantages and limitations of size characterization techniques were discussed with respect to product type. A combination of characterization techniques was highly desired based on the product type and other ingredients used to confirm the presence of nanostructures in consumer products.

Fabrication and characterization of colloidal silver nanoparticle via photochemical synthesis

This study used polyvinylpyrrolidone (PVP) and 4-cyano-4′-pentylbiphenyl (5CB) as capping agents for batch and cyclic photochemical synthesis, respectively, to fabricate colloidal silver nanoparticles via ultraviolet irradiation of ethanol containing silver nitrate (AgNO3). The cyclic photochemical process offers colloidal silver particles with mean particle size (MPS) and particle-size distribution (PSD) that were reduced by approximately 14% and 49% versus a batch photochemical process. Relative to PVP, the MPS and PSD of as-prepared Ag capped with 5CB decreased from 4.2 ± 1.3 nm to 3.7 ± 0.8 nm. This infers that MPS and PSD of colloid Ag particle could be manipulated by different photochemical processes and capping agents. In addition, the transition temperature between anisotropic and isotropic phases of 5CB liquid crystal was reduced by only about 4% when the as-prepared silver nanoparticles capped with 5CB reached 1 wt%. This suggests that blending as-prepared colloidal Ag nanoparticle with liquid crystals of 5CB increases the sensitivity to electric field and the nematic phase remains a stable state.

Label‐free detection of wild mushrooms DNA based on surface‐enhanced Raman spectroscopy

AbstractDirect and label‐free detection of unmodified DNA has always been a great challenge for DNA analyses. Surface‐enhanced Raman spectroscopy (SERS) can provide structural fingerprinting of analytes with single‐molecules sensitivity, so it could be the best candidate for label‐free detection of DNA. Wild mushrooms, a very nutritious and delicious food in our lives, are widely distributed in Yunnan province. Herein, a simple and reliable method, based on SERS, was developed for label‐free detection of DNA from six wild mushrooms and three common edible mushrooms. Concentrated Ag colloid prepared by microwave heating method with high SERS activity was used to obtain high reproducible SERS spectra. SERS signatures and spectroscopic band assignments of these mushrooms DNA were analyzed. Principal component analysis was used to distinguish these nine mushrooms. This experiment provides a simple and reliable method to detect mushrooms DNA, which is useful for the study of mushrooms DNA. Also, this approach has great potential for species identification of wild mushrooms and provides a basis for learning the interaction between DNA and small molecules. To our best knowledge, it is the first SERS study on DNA from wild mushrooms.

Dynamic nano-Ag colloids cytotoxicity to and accumulation by Escherichia coli: Effects of Fe3+, ionic strength and humic acid

Released Ag ions or/and Ag particles are believed to contribute to the cytotoxicity of Ag nanomaterials, and thus, the cytotoxicity and mechanism of Ag nanomaterials should be dynamic in water due to unfixed Ag particle:Ag+ ratios. Our recent research found that the cytotoxicity of PVP-Ag nanoparticles is attributable to Ag particles alone in 3 hr bioassays, and shifts to both Ag particles and released Ag+ in 48 hr bioassays. Herein, as a continued study, the cytotoxicity and accumulation of 50 and 100 nm Ag colloids in Escherichia coli were determined dynamically. The cytotoxicity and mechanisms of nano-Ag colloids are dynamic throughout exposure and are derived from both Ag ions and particles. Ag accumulation by E. coli is derived mainly from extracellular Ag particles during the initial 12 hr of exposure, and thereafter mainly from intracellular Ag ions. Fe3+ accelerates the oxidative dissolution of nano-Ag colloids, which results in decreasing amounts of Ag particles and particle-related toxicity. Na+ stabilizes nano-Ag colloids, thereby decreasing the bioavailability of Ag particles and particle-related toxicity. Humic acid (HA) binds Ag+ to form Ag+-HA, decreasing ion-related toxicity and binding to the E. coli surface, decreasing particle-related toxicity. HA in complex conditions showed a stronger relative contribution to toxicity and accumulation than Na+ or Fe3+. The results highlighted the cytotoxicity and mechanism of nano-Ag colloids are dynamic and affected by environmental factors, and therefore exposure duration and water chemistry should be seriously considered in environmental and health risk assessments.

Highly Photosensitive Daguerreotypes and their Reproduction: Physico-chemical Elucidation of Innovative Processes in Photography Developed around 1840 in Vienna.

A physico-chemical elucidation of the first photographic technology that allowed manifold reproduction is presented. An etched daguerreotype manufactured around 1840 in Vienna, preserved by the Technisches Museum Wien, served as a case study. Surface analysis showed that the photographic process involved the formation of colloidal Ag nanoparticles with sizes of 30-120 nm with shell layers consisting of Ag2 O, Ag2 S, and some AgCl. This breakthrough photographic technique provided a hitherto unachieved high sensitivity because of various halogenide mixtures without the use of Hg. The image development consisted of the reduction of the Ag halides by H2 SO3 created by the hydrolysis of S2 Cl2 leading to the formation of Ag nanoparticles adjacent to the Ag nuclei of the latent image. The fixing of the image was performed either by KCN or by Na2 S2 O3 . The investigated plate exhibits etched areas with Ag2 O conversion layers and no Cl or S. The gum arabic use for etching preferentially wetted the exposed Ag nanoparticle regions so that unexposed areas could be etched by HNO3 .

Pulsed laser synthesis of highly active Ag-Rh and Ag-Pt antenna-reactor-type plasmonic catalysts.

Ag, Pt, and Rh monometallic colloids were produced via laser ablation. Separate Ag–Rh and Ag–Pt heterostructures were formed by mixing and resulted in groupings of Rh/Pt nanoparticles adsorbing to the concavities of the larger Ag nanostructures. The 400 nm Ag plasmonic absorption peak was slightly blue-shifted for Ag–Pt and red-shifted for Ag–Rh heterostructures. Catalytic activity for the reduction of 4-nitrophenol increased significantly for Ag–Pt and Ag–Rh compared to the monometallic constituents, and persisted at lower loading ratios and consecutive reduction cycles. The enhancement is attributed to the Rh and Pt nanoparticles forming antenna–reactor-type plasmonic catalysts with the Ag nanostructures.

A one-pot synthesis of colloidal Ag–Au nanoparticles with controlled composition

We report a one-pot synthesis method for a production of a stable Ag–Au bimetal colloidal solutions (Ag–Au hydrosols) by simply mixing disodium salt of ethylenediaminetetraacetic acid (Na2EDTA) with AgNO3 and HAuCl4 in aqueous alkaline medium at 80 °C. We found that Na2EDTA act not only as a reductant of metal ions and a potent stabilizer of metal nanoparticle size, but also as an effective agent for controlling the composition of the formed nanoparticles, owing to the high complexing affinity of Na2EDTA to silver ions. It was found that by controlling Na2EDTA concentration in the reaction mixture it is possible to control not only size and size distribution of a synthesized colloidal bimetallic Ag–Au nanoparticles, but bimetallic Ag–Au nanoparticles composition (Ag/Au ratio) as well.

Biodegradable polyurethane/graphene oxide scaffolds for soft tissue engineering: in vivo behavior assessment

In this work, 3D porous electrospun scaffolds based on DegraPol® (DP), a polyester urethane, with different concentrations of graphene oxide (GO) (0.0–2.0 wt%) were obtained. In order to produce scaffolds with increased porosity, solutions of DP and GO were co-electrospun with polyethylene oxide (PEO), which was subsequently removed. The electrospinning resulted in the formation of porous and fibrous scaffolds with superior elasticity. The colloidal Ag was successfully used as a novel method of scaffold sterilization. The results of the in vivo analysis showed scaffold degradation, absence of an inflammatory process and penetration of tissue cells in the scaffold.

Quantum Efficiency Enhancement Depending on the Thickness of p-GaN Spacer Layer in Localized Surface Plasmon-Enhanced Near-Ultraviolet Light-Emitting Diodes by Using Colloidal Silver Nanoparticles

We demonstrated the dependence on thickness of p-GaN spacer layer in the localized surface plasmons (LSPs)-enhanced near-ultraviolet light-emitting diodes (NUV-LEDs) by pneumatic spray process using colloidal silver (Ag) nanoparticles (NPs). The LSPs-enhanced NUV-LEDs with 10- and 20-nm-thick p-GaN spacer layer showed enhanced internal quantum efficiency (IQE) and reduced effective exciton lifetime by introducing the colloidal Ag NPs. The IQE of LSPs-enhanced NUV-LEDs with 10- and 20-nm-thick p-GaN spacer layer was increased by 18.8% and 24.2%, respectively. These results indicate that the spontaneous emission rate is increased by LSPs-excitons resonant coupling. However, the NUV-LEDs with 40- and 100-nm-thick p-GaN spacer layer showed decreased IQE and extended exciton lifetime due to the evanescent wave property of LSPs field from colloidal Ag NPs.

Comparative Study of the Adsorption of Thiol and Isocyanide Molecules on a Silver Surface by in Situ Surface-Enhanced Raman Scattering

We report an investigation of the adsorption of thiol and isocyanide molecules on colloidal Ag nanocubes through surface-enhanced Raman scattering (SERS). Specifically, we collect SERS spectra in situ at different time points from a mixture of Ag nanocubes and ligand molecules at a specific concentration. We demonstrate that 4-nitrothiophenol could readily bind to the Ag surface through strong thiol–Ag interaction. We also observe red shifts for the SERS peaks as the concentration decreases, suggesting a change to the molecular orientation relative to the surface. Likewise, 4-aminothiophenol also adsorbs onto Ag quickly, but gives much weaker SERS signals relative to 4-nitrothiophenol because the electron-donating amine group would retard the chemical enhancement of SERS. Different from thiols, 1,4-phenylene diisocyanide binds to Ag surface through a relatively weak, σ-donation bond. With an increase in concentration, molecules tend to adsorb on the nanocubes with the benzene ring tilting away from the su...

Surface Lattice Resonances in Self-Assembled Arrays of Monodisperse Ag Cuboctahedra.

Plasmonic metal nanoparticles arranged in periodic arrays can generate surface lattice plasmon resonances (SLRs) with high Q-factors. These collective resonances are interesting because the associated electromagnetic field is delocalized throughout the plane of the array, enabling applications such as biosensing and nanolasing. In most cases such periodic nanostructures are created via top-down nanofabrication processes. Here we describe a capillary-force-assisted particle assembly method (CAPA) to assemble monodisperse single-crystal colloidal Ag cuboctahedra into nearly defect-free >1 cm2 hexagonal lattices. These arrays are large enough to be measured with conventional ultraviolet-visible spectroscopy, which revealed an extinction peak with a Q-factor of 30 at orthogonal illumination and up to 80 at oblique illumination angles. We explain how the experimental extinction changes with different light polarizations and angles of incidence, and compare the evolution of the peaks with computational models based on the coupled dipole approximation and the finite element method. These arrays can support high Q-factors even when exposed to air, because of the high aspect ratio of the single-crystal nanoparticles. The observation of SLRs in a self-assembled system demonstrates that a high level of long-range positional control can be achieved at the single-particle level.

In-situ electrospun aligned and maize-like AgNPs/PVA@Ag nanofibers for surface-enhanced Raman scattering on arbitrary surface

Abstract An efficient electrospun aligned surface enhanced Raman scattering (SERS) and maize-like substrate of polyvinyl alcohol (PVA) composite and Ag colloid nanofibers decorated with thermal evaporated Ag nanoparticles (AgNPs) has been developed by taking advantage of electrostatic interactions. The synergistic effects of the evaporated AgNPs (niblets) and the Ag colloid in PVA (corncob) could arouse strong electromagnetic field between the lateral and vertical nanogaps which has been demonstrated by experiment and finite-different time-domain (FDTD) simulation. In this experiment, the aligned nanofibers possesses an excellent sensitivity by detection of crystal violet (CV) and malachite green (MG) molecule at low concentration. Moreover, the proposed flexible SERS sensor was measured with outstanding uniformity and reproducibility. We also carried out in-situ electrospinning on a curved surface to detect the mixture of Sudan I, CV and MG molecule, which demonstrates that flexible SERS sensor, has enormous potential in accurate and in-situ detection on the complex geometric structure.

Damage on Escherichia coli and Staphylococcus aureus using white light photoactivation of Au and Ag nanoparticles

Bactericidal efficiency of Au and Ag nanoparticles (NPs) is reported with and without photoactivation by white light. Au and Ag NPs were synthesized with an average size of 14±1.2nm and of 4.6±0.5nm, respectively. The size distribution of the Ag colloid was relatively wide. Less than 4% of these NPs were largely decahedral, which, based on numerical calculations, determined the position of the optical band. In contrast, the Au colloid had a narrow optical band; a concentration of 1.3μg/ml was determined by theoretical and experimental spectra. Ag and Au NPs showed a superficial charge of −35mV and +57mV due to the presence of the citrate ions and cetyltrimethylammonium bromide on their surface, respectively. The effect of the NPs concentration on the viability of Escherichia coli and Staphylococcus aureus strains was investigated. It was found that Ag NPs were more effective against E. coli than Au NPs, whereas Au NPs were more effective against S. aureus than Ag NPs. The induced damage to the bacteria by the NPs was evaluated by AFM. The images show that the bacterial cell wall was changed in shape and in surface roughness, being more noticeable in S. aureus than in E. coli. The bactericidal activity of the photoactivated Ag NPs was almost doubled for both bacteria, whereas for the Au NPs, no bactericidal enhancement was observed for either strain. This can be explained by the high efficiency of Ag NPs to absorb white light and the consequent creation of hot spots that contribute to kill the bacteria.

Biological Photonic Crystal-Enhanced Plasmonic Mesocapsules: Approaching Single-Molecule Optofluidic-SERS Sensing.

Surface-enhanced Raman scattering (SERS) sensing in microfluidic devices, namely optofluidic-SERS, suffers an intrinsic trade-off between mass transport and hot spot density, both of which are required for ultra-sensitive detection. To overcome this compromise, photonic crystal-enhanced plasmonic mesocapsules are synthesized, utilizing diatom biosilica decorated with in-situ growth silver nanoparticles (Ag NPs). In our optofluidic-SERS testing, 100× higher enhancement factors and greater than 1,000× better detection limit were achieved compared with traditional colloidal Ag NPs, the improvement of which is attributed to unique properties of the mesocapsules. First, the porous diatom biosilica frustules serve as carrier capsules for high density Ag NPs that form high density plasmonic hot-spots. Second, the submicron-pores embedded in the frustule walls not only create a large surface-to-volume ratio allowing for effective analyte capture, but also enhance the local optical field through the photonic crystal effect. Last, the mesocapsules provide effective mixing with analytes as they are flowing inside the microfluidic channel. The reported mesocapsules achieved single molecule detection of Rhodamine 6G in microfluidic devices and were further utilized to detect 1 nM of benzene and chlorobenzene compounds in tap water with near real-time response, which successfully overcomes the constraint of traditional optofluidic sensing.

Synthesis of silver and gold nanoparticles by pulsed laser ablation for nanoparticle enhanced laser-induced breakdown spectroscopy

Chemically pure colloidal suspensions of silver and gold nanoparticles were synthesized by nanosecond pulsed laser ablation of metal plates placed in the ultrapure water. The nanoparticles were analyzed by UV-Vis spectroscopy and electron microscopy. The absorption spectra of silver and gold nanoparticles were basically the same as that of the chemically prepared nanoparticles. The diameter of almost spherically shaped Ag and Au nanoparticles prepared by 40 mJ laser energy was in the range of approximately 20–100 and 20–50 nm, respectively. The microdrops of Ag and Au colloidal solution were deposited on the surface of soda-lime glass and copper to perform nanoparticle-enhanced laser-induced breakdown spectroscopy. The results showed that Au nanoparticles cause much higher spectral enhancement, from both glass and copper targets, as compared to that of Ag nanoparticles. For the given target, type of nanoparticle and laser fluence, the enhancement factor of various spectral lines of an element was not the same. Moreover, the enhancement factor found to decrease with an increase of laser fluence, which is explained in terms of the electric field reduction due to the flow of electrons between two adjacent nanoparticles.

Probing the interfacial transition of acetonitrile/AOT/n-heptane microemulsion through in situ silver colloid synthesis

Very recently, it was suggested that non-aqueous acetonitrile/AOT/n-heptane microemulsions may undergo a interfacial transition from reverse micelle (RM) to bi-continuous microemulsion (BMC) above an intermediate ws (= [acetonitrile]/[AOT]). Herein, we develop a unique strategy for probing interfacial transition of the microemulsion by using microemulsion as a template for synthesis of hydrophobic silver colloids. Precursors (silver nitrate) and reducing agents (ascorbic acid) are insoluble in n-heptane phase and thus, Ag colloids formation occur solely inside the acetonitrile fraction and therefore, the structural morphology of the Ag colloids provides valuable information regarding the structure of the template where Ag colloids formation occur. At low ws (≤ 1), synthesized silver colloids exhibits remarkably strong surface plasmon resonance (SPR) band but the strength of SPR remarkably drops at ws = 2 and almost vanishes at ws = 3. TEM measurements revealed the presence of isolated spherical monodispersed silver nanoparticles (AgNPs) of ∼6 nm diameter at lower ws and larger distribution of nanoparticle sizes at ws = 2. Most interestingly, interconnected Ag colloids packed channels were found at ws = 3 which directly shows footprint of the BMC nature of the microemulsion template. Thus, the characteristics of Ag colloids (i.e. size distribution and optical quality) provide valuable information about interfacial structure of the microemulsion and suggested that microemulsion switches from RM to BMC nature at an intermediate ws.

Incidence and persistence of silver nanoparticles throughout the wastewater treatment process

While the predicted or observed concentrations of Ag NPs in wastewater treatment plants (WWTPs) have ranged from μg/L to ng/L, there is still uncertainty with regards to the realistic concentration range of Ag NPs in WWTPs. In addition, the persistence, removal, and size of Ag NPs throughout WWTP process is also not well investigated, particularly in real operating conditions. In this study, the incidence and persistence of Ag NPs in the wastewater process were studied by using single particle inductively coupled plasma mass spectrometry (sp-ICP-MS). The incidence of Ag NPs was determined in samples collected at the influent and effluent of the conventional process, as well as reclaimed and backwash waters of the ultrafiltration (UF) system in a WWTP (Santa Barbara, CA), showing a concentration of 13.5, 3.2, 0.5 and 9.8 ng/L, respectively, with relative standard deviations (RSDs) < 5%. Total Ag concentration (Ag NP and Ag+) ranged from 40 to 70 ng/L, in line with lower predicted values. Most of the Ag NPs detected were below 100 nm, with a few above 100 nm in the conventional effluent. Biological and physical processes in the secondary treatment removed 76.3% of the colloidal Ag fraction, while with the tertiary treatment (UF) the WWTP achieved a removal of 96.3% of the colloidal fraction. Persistence of Ag NPs in various water matrixes, including a synthetic wastewater (SWW), was determined by spiking 300 ng/L of Ag NPs (40 nm) and monitoring the concentrations and size change for 15 days. The persistence of Ag NPs in suspension was Influent > Effluent > Reclaimed > SWW. Partial dissolution of NPs in all waters was observed from time 0 h. Although the current concentrations in the outlet flows from WWTP (effluent and reclaimed waters) were low, the presence of small and stable Ag NPs may raise ecotoxicological concerns via bioaccumulation.

Light-Driven Self-Healing of Nanoparticle-Based Metamolecules.

Metamolecules and crystals consisting of nanoscale building blocks offer rich models to study colloidal chemistry, materials science, and photonics. Herein we demonstrate the self-assembly of colloidal Ag nanoparticles into quasi-one-dimensional metamolecules with an intriguing self-healing ability in a linearly polarized optical field. By investigating the spatial stability of the metamolecules, we found that the origin of self-healing is the inhomogeneous interparticle electrodynamic interactions enhanced by the formation of unusual nanoparticle dimers, which minimize the free energy of the whole structure. The equilibrium configuration and self-healing behavior can be further tuned by modifying the electrical double layers surrounding the nanoparticles. Our results reveal a unique route to build self-healing colloidal structures assembled from simple metal nanoparticles. This approach could potentially lead to reconfigurable plasmonic devices for photonic and sensing applications.

Light‐Driven Self‐Healing of Nanoparticle‐Based Metamolecules

AbstractMetamolecules and crystals consisting of nanoscale building blocks offer rich models to study colloidal chemistry, materials science, and photonics. Herein we demonstrate the self‐assembly of colloidal Ag nanoparticles into quasi‐one‐dimensional metamolecules with an intriguing self‐healing ability in a linearly polarized optical field. By investigating the spatial stability of the metamolecules, we found that the origin of self‐healing is the inhomogeneous interparticle electrodynamic interactions enhanced by the formation of unusual nanoparticle dimers, which minimize the free energy of the whole structure. The equilibrium configuration and self‐healing behavior can be further tuned by modifying the electrical double layers surrounding the nanoparticles. Our results reveal a unique route to build self‐healing colloidal structures assembled from simple metal nanoparticles. This approach could potentially lead to reconfigurable plasmonic devices for photonic and sensing applications.