Localized surface plasmon resonance of single silver nanoparticles studied by dark-field optical microscopy and spectroscopy

Application of nanoplasmonic biosensors based on nanoarrays in biological and chemical detection

Technological innovation, cost effectiveness, and miniaturization are key factors that determine the commercial adaptability and sustainability of sensing platforms. Nanoplasmonic biosensors based on nanocup or nanohole arrays are attractive for the development of various miniaturized devices for clinical diagnostics, health management, and environmental monitoring. In this review, we discuss the latest trends in the engineering and development of nanoplasmonic sensors as biodiagnostic tools for the highly sensitive detection of chemical and biological analytes. We focused on studies that have explored flexible nanosurface plasmon resonance systems using a sample and scalable detection approach in an effort to highlight multiplexed measurements and portable point-of-care applications.

Nanoplasmonic biosensors: Theory, structure, design, and review of recent applications

Nanoplasmonic biosensing shows an immense potential to satisfy the needs of the global health industry - low-cost, fast, and portable automated systems; highly sensitive and real-time detection; multiplexing and miniaturization. In this review, we presented the theory of nanoplasmonic biosensing for popular detection schemes - SPR, LSPR, and EOT - and underline the consideration for nanostructure design, material selection, and their effects on refractometric sensing performance. Later, we covered the bottom-up and top-down nanofabrication methods for nanoplasmonic biosensors. Subsequently, we reviewed the recent examples of nanoplasmonic biosensors over a wide range of clinically relevant analytes in the diagnosis and prognosis of a wide range of diseases and conditions such as biomarker proteins, infectious bacteria, viral agents. Finally, we discussed the challenges of nanoplasmonic biosensing toward clinical translation and proposed strategic avenues to be competitive against current clinical detection methods. Hopefully, nanoplasmonic biosensing can realize its potential through successful demonstrations of clinical translation in the upcoming years.

LSPR based on-chip detection of dengue NS1 antigen in whole blood

The development of a biosensor for rapid and quantitative detection of the dengue virus continues to remain a challenge. We report a lab-on-chip device that combines membrane-based blood plasma separation and a localized surface plasmon resonance (LSPR) based biosensor for on-chip detection of dengue NS1 antigen from a few drops of blood. The LSPR effect is realized by irradiating UV-NIR light having a spectral peak at 655 nm onto nanostructures fabricated via thermal annealing of a thin metal film. We study the effect of the resulting metal nanostructures on the LSPR performance in terms of sensitivity and limit of detection, by annealing silver films at temperatures ranging from 100 to 500 °C. The effect of annealing temperature on the nanostructure size and uniformity and the resulting optical characteristics are investigated. Further, the binding between non-targeted blood plasma proteins and NS1-antibody-functionalized nanostructures on the LSPR performance is studied by considering different blocking mechanisms. Using a nanostructure annealed at 200 °C and 2X-phosphate buffer saline with 0.05% Tween-20 as the blocking buffer, from 10 μL of whole blood, the device can detect NS1 antigen at a concentration as low as 0.047 μg mL-1 within 30 min. Finally, we demonstrate the detection of NS1 in the blood samples of dengue-infected patients and validate our results with those obtained from the gold-standard ELISA test.

Electron Beam Infrared Nano-Ellipsometry of Individual Indium Tin Oxide Nanocrystals

Leveraging recent advances in electron energy monochromation and aberration correction, we record the spatially resolved infrared plasmon spectrum of individual tin-doped indium oxide nanocrystals using electron energy-loss spectroscopy (EELS). Both surface and bulk plasmon responses are measured as a function of tin doping concentration from 1-10 atomic percent. These results are compared to theoretical models, which elucidate the spectral detuning of the same surface plasmon resonance feature when measured from aloof and penetrating probe geometries. We additionally demonstrate a unique approach to retrieving the fundamental dielectric parameters of individual semiconductor nanocrystals via EELS. This method, devoid from ensemble averaging, illustrates the potential for electron-beam ellipsometry measurements on materials that cannot be prepared in bulk form or as thin films.

Citrate-silver nanoparticles and their impact on some environmental beneficial fungi

Colloidal suspensions of silver nanoparticles (AgNPs) with surface modified by capping with citrate ions were synthesized by chemical reduction method. Transmission and Scanning Electron Microscopy as well as darkfield Optical Microscopy provided information on the nanoparticle morphology, with fine symmetrical grains and log-normal fitted size distribution. Small Angle X-ray Scattering method allowed theoretical confirmation of colloidal silver nanoparticle fine granularity, based on measurements in the native fluid sample. UV–Vis spectrophotometry allowed studying the Localized Surface Plasmon Resonance band versus the stability of the citrate-AgNP sample after storage and after UV-C exposure. The colloidal AgNP impact on Phanerochaete chrysosporium environmental microorganisms was studied by specific biochemical investigations. Silver released from the colloidal suspension of AgNPs was supposed to induce changes in some antioxidant enzymes and in some enzymes of Krebs’ cycle. Catalase activity was moderately changed (an increase with over 50%) as well as superoxide dismutase activity, while the diminution of the activities of four dehydrogenases synthesized in the fungus mycelium was emphasized also: a decrease with about 60% for malate dehydrogenase, with over 50% for isocitrate dehydrogenase and succinate dehydrogenase and with about 40% for alpha-ketoglutarate dehydrogenase. These findings suggested the nano-toxicological issues of citrate-AgNPs impact on the environmental beneficial microorganisms.

Effect of Ar Gas Pressure on LSPR Property of Au Nanoparticles: Comparison of Experimental and Theoretical Studies

In this study, the thin films were produced by using pulsed laser deposition (PLD) technique from gold (Au) nanoparticles deposited on two kinds of substrates under different argon (Ar) gas pressure. Microscope glass slides and silicon (100) wafers were used as amorphous and crystal substrates. The films were deposited under 2 × 10⁻³ mbar, 1 × 10⁻² mbar, 2 × 10⁻² mbar argon (Ar) ambient gas pressure. Effect of the background gas pressure on the plasma plume of the ablated Au nanoparticles was investigated in details. Morphology of Au nanoparticle thin films was investigated by means of atomic force microscopy (AFM) technique. Absorption spectra of Au nanoparticles were examined by using UV-Vis spectrometry. Extinction spectra of Au nanoparticles were calculated by using metallic nano particles boundary element method (MNPBEM) simulation programme. Both experimental spectra and simulation data for Au nanoparticles were obtained and compared in this work. It was concluded that they are also in good agreement with literature data. The measurements and the simulation results showed that localized surface plasmon resonance (LSPR) peaks for Au nanoparticles were located in the near infrared region (NIR) because of the larger size of the disk-like shape of Au nanoparticles, and the near-field coupling between Au nanoparticles. It was demonstrated that as the ambient gas (Ar) pressure was increased, the size and the density of Au nanoparticles on the substrate were decreased and the LSPR peak shifts toward the short wavelength region in the spectrum. This shift has been explained by the changes in the morphology of produced thin films.

Photothermally Triggered Endosomal Escape and Its Influence on Transfection Efficiency of Gold-Functionalized JetPEI/pDNA Nanoparticles

Plasmonic nanoparticles for drug delivery have attracted increasing interest over the last few years. Their localized surface plasmon resonance causes photothermal effects on laser irradiation, which allows for delivering drugs in a spatio-temporally controlled manner. Here, we explore the use of gold nanoparticles (AuNP) as carriers for pDNA in combination with pulsed laser irradiation to induce endosomal escape, which is currently considered to be one of the major bottlenecks in macromolecular drug delivery on the intracellular level. In particular, we evaluate nanocomplexes composed of JetPEI (polyethylenimine)pDNA and 10 nm AuNP, which do not exhibit endosomal escape by themselves. After incubating HeLa cells with these complexes, we evaluated endosomal escape and transfection efficiency using low- and high-energy laser pulses. At low laser energy heat is produced by the nanocomplexes, while, at higher laser energy, explosive vapour nanobubbles (VNB) are formed. We investigated the ability of heat transfer and VNB formation to induce endosomal escape and we examine the integrity of pDNA cargo after inducing both photothermal effects. We conclude that JetPEI/pDNA/AuNP complexes are unable to induce meaningful transfection efficiencies because laser treatment causes either dysfunctionality of the cargo when VNB are formed or forms too small pores in the endosomal membrane to allow pDNA to escape in case of heating. We conclude that laser-induced VNB is the most suitable to induce effective pDNA endosomal escape, but a different nanocomplex structure will be required to keep the pDNA intact.

High sensitivity plasmonic biosensor based on nanoimprinted quasi 3D nanosquares for cell detection

Quasi three-dimensional (3D) plasmonic nanostructures consisting of Au nanosquares on top of SU-8 nanopillars and Au nanoholes on the bottom were developed and fabricated using nanoimprint lithography with simultaneous thermal and UV exposure. These 3D plasmonic nanostructures were used to detect cell concentration of lung cancer A549 cells, retinal pigment epithelial (RPE) cells, and breast cancer MCF-7 cells. Nanoimprint technology has the advantage of producing high uniformity plasmonic nanostructures for such biosensors. Multiple resonance modes were observed in these quasi 3D plasmonic nanostructures. The hybrid coupling of localized surface plasmon resonances and Fabry-Perot cavity modes in the quasi 3D nanostructures resulted in high sensitivity of 496 nm/refractive index unit. The plasmonic resonance peak wavelength and sensitivity could be tuned by varying the Au thickness. Resonance peak shifts for different cells at the same concentration were distinct due to their different cell area and confluency. The cell concentration detection limit covered a large range of 5 × 10(2) to 1 × 10(7) cells ml(-1) with these new plasmonic nanostructures. They also provide a large resonance peak shift of 51 nm for as little as 0.08 cells mm(-2) of RPE cells for high sensitivity cell detection.

Real-time monitoring of oxidative etching on single Ag nanocubes via light-scattering dark-field microscopy imaging

A low-cost and easily-conducted light-scattering dark-field microscopy imaging (iDFM) technique for real-time and in situ monitoring of the oxidative etching on a single Ag nanocube was presented by using an ordinary dark-field microscopy system, which provides an alternative approach to study the behaviors of metal nanoparticles in chemical reactions and biological processes at the single nanoparticle level.

Strong fluorescence emission localized at a tapered silver-plated sub-wavelength pore

Up to 200 times fluorescence enhancement was observed inside a tapered silver-plated sub-wavelength pore.

Detection of silver nanoparticles in cells by flow cytometry using light scatter and far-red fluorescence

The cellular uptake of different sized silver nanoparticles (AgNP) (10, 50, and 75 nm) coated with polyvinylpyrrolidone (PVP) or citrate on a human derived retinal pigment epithelial cell line (ARPE-19) was detected by flow cytometry following 24-h incubation of the cells with AgNP. A dose dependent increase of side scatter and far red fluorescence was observed with both PVP and citrate-coated 50 nm or 75 nm silver particles. Using five different flow cytometers, a far red fluorescence signal in the 700-800 nm range increased as much as 100 times background as a ratio comparing the intensity measurements of treated sample and controls. The citrate-coated silver nanoparticles (AgNP) revealed slightly more side scatter and far red fluorescence than did the PVP coated silver nanoparticles. This increased far red fluorescence signal was observed with 50 and 75 nm particles, but not with 10 nm particles. Morphological evaluation by dark field microscopy showed silver particles (50 and 75 nm) clumped and concentrated around the nucleus. One possible hypothesis to explain the emission of far red fluorescence from cells incubated with silver nanoparticles is that the silver nanoparticles inside cells agglomerate into small nano clusters that form surface plasmon resonance which interacts with laser light to emit a strong far red fluorescence signal. The results demonstrate that two different parameters (side scatter and far red fluorescence) on standard flow cytometers can be used to detect and observe metallic nanoparticles inside cells. The strength of the far red fluorescence suggests that it may be particularly useful for applications that require high sensitivity. © Published 2013 Wiley-Periodicals, Inc.

Effect of treatment media on the agglomeration of titanium dioxide nanoparticles: impact on genotoxicity, cellular interaction, and cell cycle

The widespread use of titanium dioxide (TiO2) nanoparticles in consumer products increases the probability of exposure to humans and the environment. Although TiO2 nanoparticles have been shown to induce DNA damage (comet assay) and chromosome damage (micronucleus assay, MN) in vitro, no study has systematically assessed the influence of medium composition on the physicochemical characteristics and genotoxicity of TiO2 nanoparticles. We assessed TiO2 nanoparticle agglomeration, cellular interaction, induction of genotoxicity, and influence on cell cycle in human lung epithelial cells using three different nanoparticle-treatment media: keratinocyte growth medium (KGM) plus 0.1% bovine serum albumin (KB); a synthetic broncheoalveolar lavage fluid containing PBS, 0.6% bovine serum albumin and 0.001% surfactant (DM); or KGM with 10% fetal bovine serum (KF). The comet assay showed that TiO2 nanoparticles induced similar amounts of DNA damage in all three media, independent of the amount of agglomeration, cellular interaction, or cell-cycle changes measured by flow cytometry. In contrast, TiO2 nanoparticles induced MN only in KF, which is the medium that facilitated the lowest amount of agglomeration, the greatest amount of nanoparticle cellular interaction, and the highest population of cells accumulating in S phase. These results with TiO2 nanoparticles in KF demonstrate an association between medium composition, particle uptake, and nanoparticle interaction with cells, leading to chromosomal damage as measured by the MN assay.

Material-selective surface chemistry for nanoplasmonic sensors: optimizing sensitivity and controlling binding to local hot spots

Optical sensors utilizing the principle of localized surface plasmon resonance (LSPR) offer the advantage of a simple label-free mode of operation, but the sensitivity is typically limited to a very thin region close to the surface. In bioanalytical sensing applications, this can be a significant drawback, in particular since the surface needs to be coated with a recognition layer in order to ensure specific detection of target molecules. We show that the signal upon protein binding decreases dramatically with increasing thickness of the recognition layer, highlighting the need for thin high quality recognition layers compatible with LSPR sensors. The effect is particularly strong for structures that provide local hot spots with highly confined fields, such as in the gap between pairs of gold disks. While our results show a significant improvement in sensor response for pairs over single gold disks upon binding directly to the gold surface, disk pairs did not provide larger signal upon binding of proteins to a recognition layer (already for around 3 nm thin layers) located on the gold. Local plasmonic hot spots are however shown advantageous in combination with directed binding to the hot spots. This was demonstrated using a structure consisting of three surface materials (gold, titanium dioxide, and silicon dioxide) and a new protocol for material-selective surface chemistry of these three materials, which allows for controlled binding only in the gap between pairs of disks. Such a design increased the signal obtained per bound molecule by a factor of around four compared to binding to single disks.