Time-Resolved Study of Site-Specific Corrosion in a Single Crystalline Silver Nanoparticle.

We followed over 24 h a corrosion process in monocrystalline triangular-shaped nanoparticles at a single-particle level by atomic force microscopy and optical spectroscopy techniques under ambient laboratory conditions. The triangular-shaped form of the particles was selected, because the crystallographic orientation of the particles is well defined upon their deposition on a substrate. We observed that the particles already start to alter within this time frame. Surprisingly, the corrosion starts predominantly from the tips of the particles and it creates within few hours large protrusions, which strongly suppress the plasmon character of the particles. These observations support the crystallographic model of these particles consisting of a high-defect hexagonal closed packed layer, and they could help material scientists to design more stable silver nanoparticles. Moreover, this described technique can be used to reveal kinetics of the corrosion in the nanoscale of other materials.

Fabrication of micro-patterned substrates for plasmonic sensing by piezo-dispensing of colloidal nanoparticles.

In this work we describe a very fast and flexible method for fabrication of plasmon-supporting substrates with micro-patterning capability, which is optimized for plasmonic sensing. We combined a wet chemistry approach to synthesize metallic nanoparticles with a piezo-dispensing system enabling deposition of nanoparticles on the substrates with micrometer precision. In this way, an arbitrary pattern consisting of 200 µm small spots containing plasmonic nanostructures can be produced. Patterns with various nanoparticles exhibiting different plasmonic properties were combined, and the surface density of the particles could be easily varied via their solution concentrations. We showed that under controlled conditions the dispensing process caused no aggregation of the particles and it enabled full transfer of the colloidal solutions onto the substrate. This is an important condition, which enables these substrates to be used for reliable plasmonic sensing based on monitoring the spectral shift of the nanoparticles. We demonstrated the functionality of such substrates by detection of small protein adsorption on the spots based on plasmon label-free sensing method.

Plasmonic Nanosensor Array for Multiplexed DNA-based Pathogen Detection.

In this research we introduce a plasmonic nanoparticle based optical biosensor for monitoring of molecular binding events. The sensor utilizes spotted gold nanoparticle arrays as sensing platform. The nanoparticle spots are functionalized with capture DNA sequences complementary to the analyte (target) DNA. Upon incubation with the target sequence, it will bind on the respectively complementary functionalized particle spot. This binding changes the local refractive index, which is detected spectroscopically as the resulting changes of the localized surface plasmon resonance (LSPR) peak wavelength. In order to increase the signal, a small gold nanoparticle label is introduced. The binding can be reversed using chemical means (10 mM HCl). It is demonstrated that multiplexed detection and identification of several fungal pathogen DNA sequences subsequently on one sensor array are possible by this approach.

Improving colloidal stability of silica nanoparticles when stored in responsive gel: application and toxicity study.

When silica nanoparticles (SiNP) are stored in aqueous solution, even for few hours, they have a tendency to form agglomerates and therefore adapt inhomogeneous structures. Here we present a very practical method to store SiNP in responsive hydrogel. We have confirmed that SiNP kept in the responsive hydrogel do not undergo through undesirable morphological changes and while in storage they maintain their excellent colloidal stability. The effect of SiNP hollowing (i.e. dissolution of the core of the particles that leaves empty cavity inside) was significantly inhibited in the hydrogel, which is a critical feature for any nano-medical applications (e.g. controlled drug release). To demonstrate the applicability of the hydrogel-storing concept within a biologically relevant context, in this work we have evaluated the toxicological effects of the responsive SiNP-gel formulation in a model in vitro (human cell line U87GM and hemocompatibility using red blood cells) and ex ovo (hen's egg test) experiments. Particles stored in the gel as well as the pure gel did not affect the hemocompatibility (hemolysis and erythrocyte aggregation) up to a concentration of 100 µg/mL. Furthermore, systemic injections into the blood circulation of the chick area vasculosa confirmed the biocompatibility in a more complex biological environment. All evaluated toxicological values (hemorrhage, thrombosis, vascular lysis, and lethality) were comparable with the negative control, and no differences in toxicological response could be observed between the SiNP stored in hydrogel and the control nanoparticles stored in the solution.

Localized surface plasmon resonance based biosensing.

INTRODUCTION: Bioanalytical sensing based on the principle of localized surface plasmon resonance experiences is currently an extremely rapid development. Novel sensors with new kinds of plasmonic transducers and innovative concepts for the signal development as well as read-out principles were identified. This review will give an overview of the development of this field. Areas covered: The focus is primarily on types of transducers by preparation or dimension, factors for optimal sensing concepts and the critical view of the usability of these devices as innovative sensors for bioanalytical applications. Expert commentary: Plasmonic sensor devices offer a high potential for future biosensing given that limiting factors such as long-time stability of the transducers, the required high sensitivity and the cost-efficient production are addressed. For higher sensitivity, the design of the sensor in shape and material has to be combined with optimal enhancement strategies. Plasmonic nanoparticles from bottom-up synthesis with a post-synthetic processing show a high potential for cost-efficient sensor production. Regarding the measurement principle, LSPRi offers a large potential for multiplex sensors and can provide a high-throughput as well as highly paralleled sensing. The main trends are expected towards optimal LSPR concepts which represent cost-efficient and robust point-of-care solutions, and the use of multiplexed devices for clinical applications.

A New Strategy for Silver Deposition on Au Nanoparticles with the Use of Peroxidase-Mimicking DNAzyme Monitored via a Localized Surface Plasmon Resonance Technique.

Peroxidase-mimicking DNAzyme was applied as a catalyst of silver deposition on gold nanoparticles. This DNAzyme is formed when hemin binds to the G-quadruplex-forming DNA sequence. Such a system is able to catalyze a redox reaction with a one- or two-electron transfer. The process of silver deposition was monitored via a localized surface plasmon resonance technique (LSPR), which allows one to record scattering spectrum of a single nanoparticle. Our study showed that DNAzyme is able to catalyze silver deposition. The AFM experiments proved that DNAzyme induced the deposition of silver shells of approximately 20 nm thickness on Au nanoparticles (AuNPs). Such an effect is not observed when hemin is absent in the system. However, we noticed non-specific binding of hemin to the capture oligonucleotides on a gold NP probe that also induced some silver deposition, even though the capture probe was unable to form G-quadruplex. Analysis of SEM images indicated that the surface morphology of the silver layer deposited by DNAzyme is different from that obtained for hemin alone. The proposed strategy of silver layer synthesis on gold nanoparticles catalyzed by DNAzyme is an innovative approach and can be applied in bioanalysis (LSPR, electrochemistry) as well as in material sciences.

Electrically Excited Plasmonic Nanoruler for Biomolecule Detection.

Plasmon-based sensors are excellent tools for a label-free detection of small biomolecules. An interesting group of such sensors are plasmonic nanorulers that rely on the plasmon hybridization upon modification of their morphology to sense nanoscale distances. Sensor geometries based on the interaction of plasmons in a flat metallic layer together with metal nanoparticles inherit unique advantages but need a special optical excitation configuration that is not easy to miniaturize. Herein, we introduce the concept of nanoruler excitation by direct, electrically induced generation of surface plasmons based on the quantum shot noise of tunneling currents. An electron tunneling junction consisting of a metal-dielectric-semiconductor heterostructure is directly incorporated into the nanoruler basic geometry. With the application of voltage on this modified nanoruler, the plasmon modes are directly excited without any additional optical component as a light source. We demonstrate via several experiments that this electrically driven nanoruler possesses similar properties as an optically exited one and confirm its sensing capabilities by the detection of the binding of small biomolecules such as antibodies. This new sensing principle could open the way to a new platform of highly miniaturized, integrated plasmonic sensors compatible with monolithic integrated circuits.

Hyperspectral imaging of plasmon resonances in metallic nanoparticles.

The spectroscopy of metal nanoparticles shows great potential for label-free sensing. In this article we present a hyper-spectral imaging system combined with a microfluidic system, which allows full spectroscopic characterization of many individual nanoparticles simultaneously (>50 particles). With such a system we were able overcome several limitations that are present in LSPR sensing with nanoparticle ensemble. We experimentally quantified (incorporating atomic force microscopy as well) the correlation between geometry, position of plasmon resonance (λPeak) and sensitivity of the particles (Sb=1.63λPeak-812.47[nm/RIU]). We were able to follow the adsorption of protein layers and determined their spatial inhomogeneity with the help of the hyperspectral imaging.

Plasmonic coupling and long-range transfer of an excitation along a DNA nanowire.

We demonstrate an excitation transfer along a fluorescently labeled dsDNA nanowire over a length of several micrometers. Launching of the excitation is done by exciting a localized surface plasmon mode of a 40 nm silver nanoparticle by 800 nm femtosecond laser pulses via two-photon absorption. The plasmonic mode is subsequently coupled or transformed to excitation in the nanowire in contact with the particle and propagated along it, inducing bleaching of the dyes on its way. In situ as well as ex situ fluorescence microscopy is utilized to observe the phenomenon. In addition, transfer of the excitation along the nanowire to another nanoparticle over a separation of 5.7 µm was clearly observed. The nature of the excitation coupling and transfer could not be fully resolved here, but injection of an electron into the DNA from the excited nanoparticle and subsequent coupled transfer of charge (Dexter) and delocalized exciton (Frenkel) is the most probable mechanism. However, a direct plasmonic or optical coupling and energy transfer along the nanowire cannot be totally ruled out either. By further studies the observed phenomenon could be utilized in novel molecular systems, providing a long-needed communication method between molecular devices.

Molecular plasmonics: light meets molecules at the nanoscale.

Certain metal nanoparticles exhibit the effect of localized surface plasmon resonance when interacting with light, based on collective oscillations of their conduction electrons. The interaction of this effect with molecules is of great interest for a variety of research disciplines, both in optics and in the life sciences. This paper attempts to describe and structure this emerging field of molecular plasmonics, situated between the molecular world and plasmonic effects in metal nanostructures, and demonstrates the potential of these developments for a variety of applications.

Sensoric potential of gold-silver core-shell nanoparticles.

The sensitivities of five different core-shell nanostructures were investigated towards changes in the refractive index of the surrounding medium. The shift of the localized surface plasmon resonance (LSPR) maximum served as a measure of the (respective) sensitivity. Thus, gold-silver core-shell nanoparticles (NPs) were prepared with different shell thicknesses in a two-step chemical process without the use of any (possibly disturbing) surfactants. The measurements were supported by ultramicroscopic images in order to size the resulting core-shell structures. When compared to sensitivities of nanostructures reported in the literature with those of the (roughly spherical) gold-silver core-shell NPs, the latter showed comparable (or even higher) sensitivities than gold nanorods. The experimental finding is supported by theoretical calculation of optical properties of such core-shell NP. Extinction spectra of ideal spherical and deformed core-shell NPs with various core/shell sizes were calculated, and the presence of an optimal silver shell thickness with increased sensitivity was confirmed. This effect is explained by the existence of two overlapping plasmon bands in the NP, which change their relative intensity upon change of refractive index. Results of this research show a possibility of improving LSPR sensor by adding an extra metallic layer of certain thickness.

Comparison of numerical methods in near-field computation for metallic nanoparticles.

Four widely used electromagnetic field solvers are applied to the problem of scattering by a spherical or spheroidal silver nanoparticle in glass. The solvers are tested in a frequency range where the imaginary part of the scatterer refractive index is relatively large. The scattering efficiencies and near-field results obtained by the different methods are compared to each other, as well as to recent experiments on laser-induced shape transformation of silver nanoparticles in glass.

Optical properties of micro-patterned silver nanoparticle substrates.

In this paper, we describe a novel technique for depositing metal nanoparticles (NPs) on a planar substrate whereby the NPs are micro-patterned on the surface by a simple stamp-printing procedure. The method exploits the attractive force between negatively charged colloidal metal NPs and positively-charged polyelectrolyte layers which have been selectively deposited on the surface. Using this technique, large uniform areas of patterned metal NPs, with different plasmonic properties, were achieved by optimisation of the stamping process. We report the observation of unusual fluorescence emission from these structures. The emission was measured using epifluorescence microscopy. Fluorescence lifetime behaviour was also measured. Furthermore, the mu-patterned NPs exhibited blinking behaviour under 469 nm excitation and the fluorescence spectrum was multi-peaked. It has been established that the fluorescence is independent of the plasmon resonance properties of the NPs. As well as optimising the novel NP mu-patterning technique, this work discusses the origin and characteristics of the anomalous fluorescence behaviour in order to characterise and minimise this unwanted background contribution in the use of metal NPs for plasmonic enhancement of fluorescence for optical biochip applications.

Enhancing the analytical performance of immunoassays that employ metal-enhanced fluorescence.

In this work, we used a model assay system (polyclonal human IgG-goat antihuman IgG) to elucidate some of the key factors that influence the analytical performance of bioassays that employ metal-enhanced fluorescence (MEF) using silver nanoparticles (NPs). Cy5 dye was used as the fluorescent label, and results were compared with a standard assay performed in the absence of NPs. Two sizes of silver NPs were prepared with respective diameters of 60 +/- 10 and 149 +/- 16 nm. The absorption spectra of the NPs in solution were fitted accurately using Mie theory, and the dipole resonance of the 149-nm NPs in solution was found to match well with the absorption spectrum of Cy5. Such spectral matching is a key factor in optimizing MEF. NPs were deposited uniformly and reproducibly on polyelectrolyte-coated polystyrene substrates. Compared to the standard assay performed without the aid of NPs, significant improvements in sensitivity and in limit of detection (LOD) were obtained for the assay with the 149-nm NPs. An important observation was that the relative enhancement of fluorescence increased as the concentration of antigen increased. The metal-assisted assay data were analyzed using standard statistical methods and yielded a LOD of 0.086 ng/mL for the spectrally matched NPs compared to a value of 5.67 ng/mL obtained for the same assay in the absence of NPs. This improvement of approximately 66x in LOD demonstrates the potential of metal-enhanced fluorescence for improving the analytical performance of bioassays when care is taken to optimize the key determining parameters.

Nanoparticle strategies for enhancing the sensitivity of fluorescence-based biochips.

This article describes strategies for achieving fluorescence enhancement in optical biochips. Two strategies are discussed: plasmonic enhancement, which is due to the localized surface plasmon resonance of metal nanostructures that are adjacent to the fluorescent labels in optical immunoassays; and the use of high-brightness silica nanoparticles as enhanced labels. We present a review of the state-of-the-art in both areas, including synthesis techniques for the metal and silica nanoparticles and the use of the nanoparticles in optical immunoassays. Data are presented that highlight the key design parameters which influence the level of enhancement and model assay data are presented that illustrate potential enhancements in assay performance.

Experimental and theoretical studies of the optimisation of fluorescence from near-infrared dye-doped silica nanoparticles.

There is substantial interest in the development of near-infrared dye-doped nanoparticles (NPs) for a range of applications including immunocytochemistry, immunosorbent assays, flow cytometry, and DNA/protein microarray analysis. The main motivation for this work is the significant increase in NP fluorescence that may be obtained compared with a single dye label, for example Cy5. Dye-doped NPs were synthesised and a reduction in fluorescence as a function of dye concentration was correlated with the occurrence of homo-Förster resonance energy transfer (HFRET) in the NP. Using standard analytical expressions describing HFRET, we modelled the fluorescence of NPs as a function of dye loading. The results confirmed the occurrence of HFRET which arises from the small Stokes shift of near-infrared dyes and provided a simple method for predicting the optimum dye loading in NPs for maximum fluorescence. We used the inverse micelle method to prepare monodispersed silica NPs. The NPs were characterised using dynamic light scattering, UV spectroscopy, and transmission electron microscopy (TEM). The quantum efficiency of the dye inside the NPs, as a function of dye loading, was also determined. The fluorescent NPs were measured to be approximately 165 times brighter than the free dye, at an optimal loading of 2% (w/w). These experimental results were in good agreement with model predictions.

Optimization of plasmonic enhancement of fluorescence on plastic substrates.

In this work, we report on the uniform deposition of tailored plasmonic coatings on polymer substrates and on the distance dependence of the plasmonic enhancement of a fluorescent dye. Silver, gold, and silver/gold alloy nanoparticles (NPs) with a range of diameters were synthesized using chemical techniques and characterized using UV-vis absorption spectroscopy, transmission electron microscopy (TEM), and atomic force microscopy (AFM). Reproducible polyelectrolyte (PEL) layers, which were deposited on plastic microwell plates using a layer-by-layer technique, served as both a stable and uniform substrate for deposition of the NPs as well as providing spacer layers of known thickness between the NPs and the fluorescent dye. A maximum enhancement factor of approximately 11 was measured for 60 nm diameter pure silver NPs, for a dye-NP separation of approximately 3 nm. A shift in the localized surface plasmon resonance (LSPR) wavelength as a function of the effective refractive index of the PEL layers was also observed, and the measured shifts show a similar trend with theoretical predictions. This work will contribute toward the rational design of optical biochip platforms based on plasmon-enhanced fluorescence.