Synthesis Methods and Optical Sensing Applications of Plasmonic Metal Nanoparticles Made from Rhodium, Platinum, Gold, or Silver.

The purpose of this paper is to provide an in-depth review of plasmonic metal nanoparticles made from rhodium, platinum, gold, or silver. We describe fundamental concepts, synthesis methods, and optical sensing applications of these nanoparticles. Plasmonic metal nanoparticles have received a lot of interest due to various applications, such as optical sensors, single-molecule detection, single-cell detection, pathogen detection, environmental contaminant monitoring, cancer diagnostics, biomedicine, and food and health safety monitoring. They provide a promising platform for highly sensitive detection of various analytes. Due to strongly localized optical fields in the hot-spot region near metal nanoparticles, they have the potential for plasmon-enhanced optical sensing applications, including metal-enhanced fluorescence (MEF), surface-enhanced Raman scattering (SERS), and biomedical imaging. We explain the plasmonic enhancement through electromagnetic theory and confirm it with finite-difference time-domain numerical simulations. Moreover, we examine how the localized surface plasmon resonance effects of gold and silver nanoparticles have been utilized for the detection and biosensing of various analytes. Specifically, we discuss the syntheses and applications of rhodium and platinum nanoparticles for the UV plasmonics such as UV-MEF and UV-SERS. Finally, we provide an overview of chemical, physical, and green methods for synthesizing these nanoparticles. We hope that this paper will promote further interest in the optical sensing applications of plasmonic metal nanoparticles in the UV and visible ranges.

Influences of Plasma Plume Length on Structural, Optical and Dye Degradation Properties of Citrate-Stabilized Silver Nanoparticles Synthesized by Plasma-Assisted Reduction.

Citrate-capped silver nanoparticles (Ag@Cit NPs) were synthesized by a simple plasma-assisted reduction method. Homogenous colloidal Ag@Cit NPs solutions were produced by treating a AgNO3-trisodium citrate-deionized water with an atmospheric-pressure argon plasma jet. The plasma-synthesized Ag@Cit NPs exhibited quasi-spherical shape with an average particle diameter of about 5.9-7.5 nm, and their absorption spectra showed surface plasmon resonance peaks at approximately 406 nm. The amount of Ag@Cit NPs increased in a plasma exposure duration-dependent manner. Plasma synthesis of Ag@Cit NPs was more effective in the 8.5 cm plume jet than in the shorter and longer plume jets. A larger amount of Ag@Cit NPs were produced from the 8.5 cm plume jet with a higher pH and a larger number of aqua electrons, indicating that the synergetic effect between plasma electrons and citrate plays an important role in the plasma synthesis of Ag@Cit NPs. Plasma-assisted citrate reduction facilitates the synthesis of Ag@Cit NPs, and citrate-capped nanoparticles are stabilized in an aqueous solution due to their repulsive force. Next, we demonstrated that plasma-synthesized Ag@Cit NPs exhibited a significant degradation of methylene blue dye.

Nitrate Capture Investigation in Plasma-Activated Water and Its Antifungal Effect on Cryptococcus pseudolongus Cells.

This research investigated the capture of nitrate by magnesium ions in plasma-activated water (PAW) and its antifungal effect on the cell viability of the newly emerged mushroom pathogen Cryptococcus pseudolongus. Optical emission spectra of the plasma jet exhibited several emission bands attributable to plasma-generated reactive oxygen and nitrogen species. The plasma was injected directly into deionized water (DW) with and without an immersed magnesium block. Plasma treatment of DW produced acidic PAW. However, plasma-activated magnesium water (PA-Mg-W) tended to be neutralized due to the reduction in plasma-generated hydrogen ions by electrons released from the zero-valent magnesium. Optical absorption and Raman spectra confirmed that nitrate ions were the dominant reactive species in the PAW and PA-Mg-W. Nitrate had a concentration-dependent antifungal effect on the tested fungal cells. We observed that the free nitrate content could be controlled to be lower in the PA-Mg-W than in the PAW due to the formation of nitrate salts by the magnesium ions. Although both the PAW and PA-Mg-W had antifungal effects on C. pseudolongus, their effectiveness differed, with cell viability higher in the PA-Mg-W than in the PAW. This study demonstrates that the antifungal effect of PAW could be manipulated using nitrate capture. The wide use of plasma therapy for problematic fungus control is challenging because fungi have rigid cell wall structures in different fungal groups.

Structural and Optical Sensing Properties of Nonthermal Atmospheric Plasma-Synthesized Polyethylene Glycol-Functionalized Gold Nanoparticles.

Polyethylene glycol-functionalized gold nanoparticles (Au@PEG NPs) were prepared by a simple plasma-assisted method without additional reducing chemicals. After irradiating tetrachloroauric acid (HAuCl4) and polyethylene glycol (PEG) in aqueous medium with an argon plasma jet, the gold precursor transformed into an Au@PEG NP colloid that exhibited surface plasma resonance at 530 nm. When the plasma jet entered the water, additional reactive species were induced through interactions between plasma-generated reactive species and aqueous media. Interaction of the gold precursor with the plasma-activated medium allowed the synthesis of gold nanoparticles (AuNPs) without reductants. The plasma-synthesized Au@PEG NPs had a quasi-spherical shape with an average particle diameter of 32.5 nm. The addition of PEG not only helped to stabilize the AuNPs but also increased the number of AuNPs. Au@PEG NP-loaded paper (AuNP-paper) was able to detect the degradation of rhodamine B, therefore, indicating that AuNP-paper can act as a surface-enhanced Raman scattering platform. Dye degradation by plasma treatment was investigated by optical absorption and Raman spectroscopy. The method proposed for the fabrication of Au@PEG NPs is rapid, low-cost, and environment-friendly and will facilitate the application of plasma-synthesized nanomaterials in sensors.

Optical Sensing Properties of ZnO Nanoparticles Prepared by Spray Pyrolysis.

We studied the optical sensing properties of ZnO nanoparticles prepared by spray pyrolysis. To investigate their optical sensing performance, we incubated peptides on ZnO nanoparticles. The photoluminescence (PL) peak intensity of peptides on the ZnO nanoparticles was higher than that of peptides on the ZnO film or on the glass plate. This observed PL enhancement is attributed to the optical confinement of ZnO nanoparticles. The low-temperature spectra displayed a strong exciton emission peak with multiple sidebands, attributed to the bound exciton and its longitudinal optical phonon sidebands. The strong exciton emission is thought to be the combined effect of optical confinement due to the nanoparticle geometry, reduction of defect emission by thermal annealing, and reduction of non-radiative relaxation at low temperatures.

Analysis of the antimicrobial effects of nonthermal plasma on fungal spores in ionic solutions.

The antimicrobial efficiency of reactive species-based control strategies is significantly affected by the dynamics of reactive species in the biological environment. Atmospheric-pressure nonthermal plasma is an ionized gas in which various reactive species are produced. The various levels of antimicrobial activity may result from the dynamic interaction of the plasma-generated reactive species with the environment. However, the nature of the interaction between plasma and environments is poorly understood. In this study, we analyzed the influence of the ionic strength of surrounding solutions (environment) on the antimicrobial activity of plasma in relation to the plasma-generated reactive species using a model filamentous fungus, Neurospora crassa. Our data revealed that the presence of sodium chloride (NaCl) in the background solution attenuated the deleterious effects of plasma on germination, internal structure, and genomic DNA of fungal spores. The protective effects of NaCl were not explained exclusively by pH, osmotic stability, or the level of reactive species in the solution. These were strongly associated with the ionic strength of the background solution. The presence of ions reduced plasma toxicity, which might be due to a reduced access of reactive species to fungal spores, and fungal spores were inactivated by plasma in a background fluid of nonionic osmolytes despite the low level of reactive species. Our results suggest that the surrounding environment may affect the behavior of reactive species, which leads to different biological consequences regardless of their quantity. Moreover, the microbicidal effect of plasma can be synergistically regulated through control of the microenvironment.

Optical and structural properties of Al-ZnO nanocomposites.

The optical and structural properties of aluminium-doped zinc oxide (AZO) films were investigated by photoluminescence (PL) spectroscopy, X-ray diffraction (XRD), and scanning electron microscopy. Pure zinc oxide (ZnO) and AZO composite films were deposited using vacuum evaporation method. The films exhibited different morphologies and crystallinity depending on the Al-doping. The SEM micrographs showed that a granular and compact structure could be seen for the ZnO film, while a nanoleaf structure with relatively porous nature was observed for the AZO composite film. The XRD patterns indicated that the crystalline growth orientation would be significantly affected by addition of Al. Compared with pure ZnO, the XRD peak intensity of the AZO composite was stronger and the line-width was narrower. Two-probe resistivity measurements showed that the AZO composites could be used as transparent conducting materials. The PL spectra revealed that the PL intensities of the AZO composites were stronger than that of the pure ZnO. The PL enhancement might be ascribed to the surface plasmon resonance of metal nanoclusters within the composite. Another possible reason of the PL enhancement would be the metal-induced crystallization caused by doping Al to ZnO matrix.

Optical and structural properties of plasma-treated ZnO nanostructures.

We studied the effect of plasma treatment on the structural and optical properties of ZnO nanostructures prepared by chemical bath deposition in an aqueous solution of Zn(NO3)2 and hexamethylenetetramine. The room-temperature photoluminescence (PL) spectrum of the as-grown ZnO nanostructures exhibited two emission bands due to exciton emission and defect emission. After treating with hydrogen plasma, the treated ZnO nanostructures exhibited stronger exciton emission than the as-grown, untreated ZnO nanostructures in their respective cathodoluminescence and PL spectra. The low-temperature PL spectrum of the hydrogen plasma-treated ZnO nanostructures showed a strong exciton emission at 3.34 eV, attributing to the bound exciton and its longitudinal optical-phonon sidebands. The strong exciton emission is thought to be due to the combined effect of exciton emission enhancement by defect passivation and optical confinement resulting from nanostructure geometry.

Optical and structural properties of nanobiomaterials.

In this review, the optical and structural properties of biomaterials are discussed. First, we demonstrate the optical and structural properties of natural and plasma-treated DNA, using UV-visible absorption, circular dichroism (CD), and Raman spectroscopy. Fluorescence and lasing action in the dye-doped DNA-surfactant complex are also explained. Additionally, nanomaterial-based DNA detection and DNA-templated nanomaterial growth are described. Next, we discuss protein folding studies utilizing fluorescence, CD, and nuclear magnetic resonance (NMR) spectroscopy. From the CD spectra of alpha-chymotrypsin (CT), we estimate the composition of a-helices and the beta-sheets, and random coils in the CT. 1H NMR spectroscopy is used to investigate the thermal effect on the refolding of CT in the presence of an ionic liquid. Finally, we explain the numerical simulation method used for studying the optical properties of biomaterials. Applications of the Monte-Carlo method in photodynamic therapy, skin tissue optics, and bioimaging are described.

Optical and structural properties of Ag:Ta2O5 nanocomposites.

In this paper, we investigated the optical and structural properties of the Ag:Ta2O5 nanocomposites. The nanocomposite films were prepared by magnetron co-sputtering of Ta2O5 with Ag chips. The transmission electron microscopy images clearly revealed that Ag nanoparticles were successfully produced by post-thermal annealing of the deposited films. The reflection spectra exhibited the surface plasmon resonance behavior. The photoluminescence (PL) spectra indicated that the Ag:Ta2O5 nanocomposites exhibited larger PL intensity than pure Ag or pure Ta2O5. The PL enhancement is thought to be due to the surface plasmon resonance of Ag nanoparticles. From the spatial intensity distributions that were obtained by solving Maxwell equations using the three-dimensional, finite-difference time-domain method, we found that there was strong confinement of optical field near the Ag nanoparticle. This optical confinement effect makes it possible to exhibit the enhanced PL.

Optical properties of Ag hemisphere-like nanoparticles.

We investigated the optical properties of Ag hemisphere-like nanoparticles. The nanoparticles were prepared by using magnetron sputtering and post-thermal annealing. The scanning-electron microscopy images reveal that hemisphere-like nanoparticles were successfully produced. The transmission spectra exhibit the surface plasmon resonance effect. From the spatial intensity distributions obtained by solving Maxwell equations using the three-dimensional, finite-difference time-domain method, we found that there is strong confinement of the optical field near the nanoparticle. This optical confinement makes it easy to exhibit the plasmon resonance effect.

Finite-difference time-domain numerical simulation study on the optical properties of silver nanocomposites.

The effects of the nanoparticle geometry and the host matrix on the optical properties of silver (Ag) nanocomposites were investigated. The spatial intensity distribution and absorption spectra were obtained by solving Maxwell equations using the finite-difference time-domain method. Local enhancement of the optical field was produced near the surface of the Ag nanoparticle. As the nanoparticle size increased, the plasmon-induced absorption increased and the surface plasmon resonance (SPR) wavelength of the Ag nanocomposite was redshifted. As the nanoparticle geometry was transformed from a sphere to an ellipsoid, two plasmon peaks appeared and their spectral spacing became larger with increasing the aspect ratio. The effects of the nanoparticle size and the anisotropic geometry on the optical properties of the Ag nanocomposites can be described by the Maxwell-Garnett theory and the Drude model. From the absorption spectra of the Ag nanocomposites with five different host matrices (SiO2, Al2O3, ZnO, ZrO2, and TiO2), it was found that the SPR wavelength of the Ag nanocomposite was redshifted with increasing the refractive index of the host matrix.

Influence of ZnO nanorod morphology on optical confinement--finite-difference time-domain numerical simulation study.

We studied the influence of nanorod (NR) morphology on the optical confinement. In order to understand the optical field confinement by the ZnO NR, we obtained the spatial intensity distribution inside/outside the NR by solving Maxwell equations using the finite-difference time-domain numerical simulation. The hexagonal cylinder-shaped NR exhibits a strong confinement and the circular cylinder-shaped NR shows also similar confinement effect. Meanwhile, the rectangular cylinder-shaped NR, the tapered NR, and the NR with sharp cone show a weak confinement of optical field as compared to that of the hexagonal cylinder-shaped NR. Next, as the rod length and/or the rod diameter increase, the high intensity region increases. This suggests that longer nanorod will exhibit more efficient lasing action.

Effects of seed layers on structural, morphological, and optical properties of ZnO nanorods.

We studied the effects of seed layers on the structural and optical properties of ZnO nanorods. ZnO and Ag-doped ZnO (ZnO:Ag) seed layers were deposited on glass substrates by magnetron co-sputtering. ZnO nanorods were grown on these seed layers by the chemical bath deposition in an aqueous solution of Zn(NO3)2 and hexamethyltetramine. SEM micrographs clearly reveal that ZnO nanorods were successfully grown on both kinds of seed layers. The XRD patterns indicate that crystallization of ZnO nanorods is along the c-axis. Meanwhile, the packing density and the vertical alignment of the ZnO nanorods on the ZnO seed layer are better than those of the ZnO nanorods on ZnO:Ag. The enhanced growth of nanorods is thought to be due to the fact that the ZnO layer exhibits a higher crystalline quality than the ZnO:Ag layer. According to the low-temperature photoluminescence spectra, the ZnO nanorods on the ZnO seed layer show a narrow strong ultraviolet emission band centered at 369 nm, while those on ZnO:Ag exhibit multiple bands. These results are thought to be related with the crystallinity of ZnO nanorods, the morphologies of ZnO nanorods, and the reflectivities of seed layers. More detailed studies for clarification of the seed layer effect on the growth of ZnO nanorods are desirable.

Microstructure of femtosecond laser-induced grating in amorphous silicon.

The femtosecond laser-induced grating (FLIG) formation and crystallization were investigated in amorphous silicon (a-Si) films, prepared on glass by plasma-enhanced chemical-vapor deposition. Probe-beam diffraction, micro-Raman spectroscopy, atomic force microscopy, scanning electron microscopy, and transmission electron microscopy were employed to characterize the diffraction properties and the microstructures of FLIGs. It was found that i) the FLIG can be regarded as a pattern of alternating a-Si and microcrystalline-silicon (microc-Si) lines with a period of about 2microm, and ii) efficient grating formation and crystallization were achieved by high-intensity recording with a short writing period.