Fluorescence enhancement of modified silver nanoparticles

Surface enhanced fluorescence (SEF) effect of acridine orange fluorophore in the proximity of silver nanoparticles (NPs) has been investigated experimentally in the aqueous solution system. It was found that the SEF effect could be influenced by the distribution of the NPs and the separation between the fluorophore molecule and metal surface. The fluorescence enhancement was improved significantly when Ag NPs was capped with 4-Aminothiophenol (PATP) that was acted as an isolating layer between the metal surface and fluorophore molecules. The results suggest that a proper distribution of metallic NPs and proper separation between fluorophore molecule and the particle surface are important for obtaining an optimal SEF effect.     

Surface-enhanced infrared spectroscopic studies of the catalytic behavior of silver nanoparticles on a germanium substrate

The catalytic activity of silver nanoparticles (AgNPs) on a germanium substrate is reported. Para-nitrothiophenol (pNTP) that had been adsorbed on this substrate is converted to p-aminothiophenol (pATP) under very mild reaction conditions, such as simply soaking in water. The AgNPs may be formed either by physical vapor deposition or by electroless deposition from a solution of silver nitrate. Analogous reactions were not observed on copper nanoparticles on germanium or AgNPs on silicon or zinc selenide even though very slow conversion of pNTP to pATP was observed with Au nanoparticles (AuNPs) on Ge under controlled reaction conditions. The effects of factors that could influence the catalytic reaction were examined; these included the particle size of the AgNPs, reaction temperature, concentration and chemical nature of other ions present in the solution, the pH of the water, and the nature of the substrate. The reaction rate was approximately independent of the particle size for AgNPs between 50 and 150 nm in diameter. Increasing the temperature accelerates the reaction significantly; at temperatures above 40 °C, the adsorbed pNTP is completely converted by water within five minutes. Not surprisingly, the reaction rate was increased as the pH of the solution was decreased, as the reduction of each nitro group to an amino group requires six protons. The presence of Br(-) and I(-) ions accelerated the reaction to the point that even at 4 °C, the conversion of the nitro group was still observable, while solutions containing chloride ions had to be heated to 40 °C before their effect became apparent. Apparently, Br(-) and I(-) ions remove the oxide layer from the surface of the germanium substrate, facilitating transfer of electrons from the germanium to the nitro group of the pNTP.     

An optical tweezer-based study of antimicrobial activity of silver nanoparticles

Understanding and characterizing microbial activity reduction in the presence of antimicrobial agents can help in the design and manufacture of antimicrobial drugs. We demonstrate the use of an optical tweezer setup in recording the changes in bacterial activity with time, induced by the presence of foreign bodies in a bacterial suspension. This is achieved by monitoring the fluctuations of an optically trapped polystyrene bead immersed in it. Examining the changes in the fluctuation pattern of the bead with time provides an accurate characterization of the reduction in the microbial activity. Here, we report on the effect of addition of silver nanoparticles on bacterial cultures of Pseudomonas aeroginosa, Escherichia coli and Bacillus subtilis. We observe a decrease in the bacterial activity with time for the investigated bacterial samples. This method in our opinion, enables one to track changes in bacterial activity levels as a function of time of contact with the antibacterial agent with greater efficacy than traditional cell counting methods.     

Single-molecule spectroscopic study of enhanced intrinsic phycoerythrin fluorescence on silver nanostructured surfaces

In this paper, we report on steady-state and time-resolved single-molecule fluorescence measurements performed on a phycobiliprotein, R-phycoerythrin (RPE), assembled on silver nanostructures. Single-molecule measurements clearly show that RPE molecules display a 10-fold increase in fluorescence intensity, with a 7-fold decrease in lifetime when they are assembled on silver nanostructured surfaces, as compared to control glass slides. The emission spectrum of individual RPE molecules also displays a significant fluorescence enhancement on silver nanostructures as compared to glass. From intensity and lifetime histograms, it is clear that the intensities as well as lifetimes of individual RPE molecules on silver nanostructures are more heterogeneously distributed than that on glass. This single-molecule study provides further insight on the heterogeneity in the fluorescence intensity and lifetimes of the RPE molecules on both glass and SiFs surfaces, which is otherwise not possible to observe using ensemble measurements. Finite-difference time-domain calculations have been performed to study the enhanced near-fields induced around silver nanoparticles by a radiating excited-state fluorophore, and the effect of such enhanced fields on the fluorescence enhancement observed is discussed.     

Complementary surface-enhanced resonance Raman spectroscopic biodetection of mixed protein solutions by chitosan- and silica-coated plasmon-tuned silver nanoparticles

Silver nanoparticles with identical plasmonic properties but different surface functionalities are synthesized and tested as chemically selective surface-enhanced resonance Raman (SERR) amplifiers in a two-component protein solution. The surface plasmon resonances of the particles are tuned to 413 nm to match the molecular resonance of protein heme cofactors. Biocompatible functionalization of the nanoparticles with a thin film of chitosan yields selective SERR enhancement of the anionic protein cytochrome b(5), whereas functionalization with SiO(2) amplifies only the spectra of the cationic protein cytochrome c. As a result, subsequent addition of the two differently functionalized particles yields complementary information on the same mixed protein sample solution. Finally, the applicability of chitosan-coated Ag nanoparticles for protein separation was tested by in situ resonance Raman spectroscopy.     

Effect of oxidation on surface-enhanced Raman scattering activity of silver nanoparticles: a quantitative correlation

We quantitatively studied, using X-ray photoelectron spectroscopy (XPS), oxidation of substrate-immobilized silver nanoparticles (Ag NPs) in a wide range of conditions, including exposure to ambient air and controlled ozone environment under UV irradiation, and we correlated the degree of silver oxidation with surface-enhanced Raman scattering (SERS) enhancement factors (EFs). The SERS activity of pristine and oxidized Ag NPs was assessed by use of trans-1,2-bis(4-pyridyl)ethylene (BPE) and sodium thiocynate as model analytes at the excitation wavelength of 532 nm. Our study showed that the exposure of Ag NPs to parts per million (ppm) level concentrations of ozone led to the formation of Ag(2)O and orders of magnitude reduction in SERS EFs. Such an adverse effect was also notable upon exposure of Ag NPs under ambient conditions where ozone existed at parts per billion (ppb) level. The correlated XPS and SERS studies suggested that formation of just a submonolayer of Ag(2)O was sufficient to decrease markedly the SERS EF of Ag NPs. In addition, studies of changes in plasmon absorption bands pointed to the chemical enhancement as a major reason for deterioration of SERS signals when substrates were pre-exposed to ambient air, and to a combination of changes in chemical and electromagnetic enhancements in the case of substrate pre-exposure to elevated ozone concentrations. Finally, we also found UV irradiation and ozone had a synergistic effect on silver oxidation and thus a detrimental effect on SERS enhancement of Ag NPs and that such oxidation effects were analyte-dependent, as a result of inherent differences in chemical enhancements and molecular binding affinities for various analytes.     

Fluorescence blinking dynamics of silver nanoparticle and silver nanorod films

Fluorescence blinking of silver nanoparticle films is observed when illuminated with red light (635?nm). The observed power-law off-time distribution is attributed to random surface diffusion and subsequent agglomeration of atomic Ag leading to the formation of photo-active Ag nanoclusters. These nanoclusters can in turn diffuse randomly to form non-emitting Ag clusters after aggregation with another Ag species. This is revealed in the power-law on-time distribution. Silver oxides found on the surfaces of Ag nanostructures are important for photoblinking to take place since nanostructures with a protective layer of polymeric citrate or cetyltrimethylammonium bromide (CTAB) against atmospheric O(2) do not display obvious emission intermittency.     

A spectroscopic study on the interaction between gold nanoparticles and hemoglobin

The interaction between horse hemoglobin and gold nanoparticles was studied using optical spectroscopy. UV-vis and fluorescence spectra show that a spontaneous binding process occurred between hemoglobin and gold nanoparticles. The Soret band of hemoglobin in the presence of gold nanoparticles does not show significant changes, which proves that the protein retained its biological function. A shift to longer wavelengths appears in the plasmonic band of gold nanoparticles upon the attachment of hemoglobin molecules. Gold nanoparticles quench the fluorescence emission of tryptophan residues in the structure of hemoglobin. The Stern-Volmer quenching constant, the binding constant and the number of binding sites were also calculated. Thermodynamic parameters indicate that the binding was mainly due to hydrophobic interactions.     

Fluorescence enhancement using silver nanotriangle arrays

We describe the fabrication of silver nanotriangle array using angle resolved nanosphere lithography and utilizing the same for enhancing fluorescence. The well established nanosphere lithography is modified by changing the angle of deposition between the nanosphere mask and the beam of silver being deposited resulting in nanotriangles of varying surface area and density. The 470 nm plasmon resonance wavelength of the substrate was determined using minimum reflectivity method which closely matches with excitation wavelength of the fluorophore. Ten times enhancement in fluorescence emission intensity is obtained from fluorescein isothiocyanate coated on top of silver nanotriangle array separated by a spacer layer of poly vinyl alcohol as compared to glass. The enhanced fluorescence emission is attributed to the increase in local field enhancement.     

Shaped gold and silver nanoparticles

Advance in the synthesis of shaped nanoparticles made of gold and silver is reviewed in this article. This review starts with a new angle by analyzing the relationship between the geometrical symmetry of a nanoparticle shape and its internal crystalline structures. According to the relationship, the nanoparticles with well-defined shapes are classified into three categories: nanoparticles with single crystallinity, nanoparticles with angular twins, and nanoparticles with parallel twins. Discussion and analysis on the classical methods for the synthesis of shaped nanoparticles in each category are also included and personal perspectives on the future research directions in the synthesis of shaped metal nanoparticles are briefly summarized. This review is expected to provide a guideline in designing the strategy for the synthesis of shaped nanoparticles and analyzing the corresponding growth mechanism.     

Fluorescence lifetime imaging by using time-gated data acquisition

The use of the time gating technique for lifetime reconstruction in the Fourier domain is a novel technique. Time gating provides sufficient data points in the time domain for reliable application of the Fourier transform, which is essential for the time deconvolution of the system of the integral equations employed in the reconstruction. The Fourier domain telegraph equation is employed to model the light transport, which allows a sufficiently broad interval of frequencies to be covered. Reconstructed images contain enough information needed for recovering the lifetime distribution in a sample for any given frequency within the megahertz-gigahertz band. The use of this technique is essential for recovering time-dependent information in fluorescence imaging. This technique was applied in reconstruction of the lifetime distribution of four tubes filled with Rhodamine 6G embedded inside a highly scattering slab. Relatively accurate fluorescence lifetime reconstruction demonstrates the effectiveness and the potential of the proposed technique.     

Fluorescence lifetime imaging with near-field scanning optical microscopy

Near-field scanning optical microscopy (NSOM) is a high-resolution scanning probe technique capable of obtaining simultaneous optical and topographic images with spatial resolution of tens of nanometers. We have integrated time-correlated single-photon counting and NSOM to obtain images of fluorescence lifetimes with high spatial resolution. The technique can be used to measure either full fluorescence lifetime decays at individual spots with a spatial resolution of <100 nm or NSOM fluorescence images using fluorescence lifetime as a contrast mechanism. For imaging, a pulsed Ti:sapphire laser was used for sample excitation and fluorescent photons were time correlated and sorted into two time delay bins. The intensity in these bins can be used to estimate the fluorescence lifetime at each pixel in the image. The technique is demonstrated on thin films of poly(9,9'-dioctylfluorene) (PDOF). The fluorescence of PDOF is the results of both inter- and intrapolymer emitting species that can be easily distinguished in the time domain. Fluorescence lifetime imaging with near-field scanning optical microscopy demonstrates how photochemical degradation of the polymer leads to a quenching of short-delay intrachain emission and an increase in the long-delay photons associated with interpolymer emitting species. The images also show how intra- and interpolymer species are uniformly distributed in the films.     

Photo-induced growth of DNA-capped silver nanoparticles

We report the photo-induced nucleation and growth of silver nanoparticles in aqueous solution in the presence of DNA oligomers. An organic dye (Cy5) was used as a photosensitizer to initiate the nanoparticle growth upon illumination with 647 nm light. The formation of nanoparticles and growth kinetics were observed by extinction spectroscopy, dynamic light scattering, and transmission electron microscopy. Irradiation of the precursor solutions with light at the Cy5 absorption maximum triggered the instantaneous formation of spherical particles with a metallic core ~15 nm in diameter. Remarkably, the particles feature significantly larger effective hydrodynamic diameters (35 nm) in solution, indicative of a DNA ad-layer on the nanoparticle surface. Centrifugation experiments confirmed that DNA was inseparably associated with the nanoparticles and indicated that DNA oligomers adsorb onto the nanoparticle surface during growth, playing the role of a capping agent. The introduced method is a fast and facile way to prepare DNA-capped silver nanoparticles in a single growth step.     

Antimicrobial active silver nanoparticles and silver/polystyrene core-shell nanoparticles prepared in room-temperature ionic liquid

Uniform silver nanoparticles and silver/polystyrene core-shell nanoparticles were successfully synthesized in a room temperature ionic liquid, 1-n-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]·BF₄). [BMIM]·BF₄ plays a protective role to prevent the nanoparticles from aggregation during the preparation process. Transmission electron micrographs confirm that both silver nanoparticles and core-shell nanoparticles are regular spheres with the sizes in the range of 5–15 nm and 15–25 nm, respectively. The X-ray diffraction analysis reveals the face-centered cubic geometry of silver nanoparticles. The as-prepared nanoparticles were also characterized by Fourier transform infrared spectroscopy, Raman spectroscopy, UV–vis diffuse reflectance spectroscopy and X-ray photoelectron spectroscopy. In addition, antimicrobial activities against E. coli and S. aureus were studied and the results show that both silver nanoparticles and core-shell nanoparticles possess excellent antimicrobial activities. The antimicrobial mechanism of the as-prepared nanoparticles was discussed.     

Surface-enhanced fluorescence from silver fractallike nanostructures decorated with silver nanoparticles

Fluorescence emission of fluorophore molecules in the close vicinity of a nanostructured metal surface can be enhanced through a local electromagnetic field with the help of surface plasmon resonance. The fluorescence enhancement effect is very sensitive to the topography and dielectric property of the metal substrate. In the current work, metal substrates with complex structures, which are made of silver fractallike structures and nanoparticles (NPs), are prepared through electrochemical reduction followed by physical deposition. The surface-enhanced fluorescence of Rhodamine 6G monolayer molecules deposited on the prepared complex substrates are investigated with the laser spectroscopic technique. The experimental results show that the fractallike structure decorated with silver NPs presents stronger fluorescence enhancement, compared with silver NPs or pure silver fractallike structures.     

Amorphous silicon coatings with silver nanoparticles

We describe a plasma-ion sputtering technology for obtaining amorphous silicon coatings containing dispersed silver nanoparticles with average dimensions of 20–30 nm. Results of X-ray diffraction and electron-microscopic investigations of these coatings are presented, and a possible mechanism of silver nano-particle formation from 2- to 3-nm-sized nanoclusters is considered.     

Fluorescence lifetime spectroscopy for pH sensing in scattering media

Fluorescence lifetime spectroscopy in the presence of tissuelike scattering is demonstrated from measurements of phase and modulation ratio as a function of modulation frequency using a pH-sensitive dye, Carboxy Seminaphthofluorescein-1 (C-SNAFL-1). From the optical diffusion equation describing the propagation and generation of fluorescence within solutions of 0.5 microM C-SNAFL-1 containing 2.0% (by volume) of Intralipid as a scatterer, the values of the average lifetime of C-SNAFL-1 were determined as the solution pH varied between 5 and 9. Average lifetime values were found to match those measured using traditional phase-modulation measurement in nonscattering media. Furthermore, the robustness of the spectroscopic technique was demonstrated by conducting lifetime measurements at varying scatterer concentrations (1.5-3.0 vol % Intralipid). These results confirm the approach for analytical sensing in scattering media via fluorescence lifetime kinetics in order to track changes in analyte concentrations.     

Preparing silver nanoparticles in supercritical water

Silver (Ag) nanoparticles were prepared using Ag microparticles as precursors in supercritical water (SCW). The effect of reaction time on preparing Ag nanoparticles was investigated. The obtained products were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements. The results indicated that, with the increase of reaction time, Ag microparticles prepared by calcination method with the sizes > 2 μm remained after SCW treatment because of their high crystallinity, but those with the sizes < 1 μm were destroyed and formed many Ag nanoparticles due to the destructive effect of SCW.     

Functionalization of textile materials with silver nanoparticles

Nanoparticles gained much scientific attention in the past decade due to their specific surface chemistry and properties which are significantly different from that of the corresponding bulk materials. Silver nanoparticles were the most extensively studied and they can be already found in a wide range of commercially available products. This article reviews the latest developments in finishing of textile materials with differently synthesized silver nanoparticles. Although antimicrobial activity of functionalized textile materials was primarily discussed, the possibilities to apply silver nanoparticles as a colorant or antistatic agent were also evaluated. In addition, the effect of chemical and physico-chemical activation of fibers before loading of silver nanoparticles on their binding efficiency was considered. Taking into account the importance of environmental impact of novel technologies, potential environmental risks emerging from silver release during washing of textile materials have been also discussed.     

Synthesis and characterization of titania-coated silver nanoparticles

Silver particles of 70 +/- 10 nm mean diameter were coated with uniform layers of amorphous titania by the hydrolysis and polycondensation process using titanium(IV) n-butoxide in aqueous solution. Because of the large dielectric function of titania, a shift to the longer wavelengths was observed for the plasmon resonance maximum of coated silver nanoparticles. By changing the alkoxide to particles ratio in the reaction mixture, thickness ranging from 5 to 20 nm was obtained. Hydrothermal treatment performed on coated particles at 350 degrees C for 4 h converted amorphous titania to the anatase crystalline form. The anatase form was confirmed by X-ray powder diffraction. More red shift of the plasmon resonance was observed for the hydrothermally treated samples compared with the particles coated with amorphous titania.