Photothermal Enhancement in Core-Shell Structured Plasmonic Nanoparticles

Plasmonic nanoparticles (NPs) with photothermal effects can be exploited as efficient heat sources in various applications. Here, the photothermal properties in core-shell structured plasmonic NPs, including metal/silica NP, silica/metal NP, and metal/silica/metal NP, are investigated. Compared with bare metal NPs, the core-shell plasmonic NPs not only exhibit extremely agile tunability in the surface plasmon resonances but also show considerably enhanced photothermal effects in terms of the maximum temperature rise. For metal/silica NPs and metal/silica/metal NPs, the SiO₂ shells function as effective thermal-protective layers for enhanced photothermal effect. For silica/metal NPs, the SiO₂ core and the metal shell show uniform temperature rise. These findings are essential for applying the core-shell structured plasmonic NPs on photothermal imaging, nanofluidics, etc.     

Plasmonic Perovskite Solar Cells Utilizing Au@SiO<Subscript>2</Subscript> Core-Shell Nanoparticles

The role of Au@SiO₂ core-shell nanoparticles on optical properties of perovskite solar cells has been explored using both the theoretical computations and the experiments. A quasi-static model is used to study the surface plasmon resonances (SPRs) of Au@SiO₂ core-shell nanospheres. Au@SiO₂ core-shell nanoparticles, with varying shell thickness and core radius, were assumed to be embedded in methylammonium lead triiodide (CH₃NH₃PbI₃) perovskite active layer. Enhanced absorption in the active layer is obtained due to the near-field plasmonic effect of the embedded core-shell nanoparticles. Theoretical modelling shows that a shell thickness of 1 nm and core diameter of 20 nm provide absorption enhancement in the orange-red region of the electromagnetic spectrum. Experiments performed using ～20-nm-sized Au@SiO₂ core-shell nanoparticles (with a shell thickness of ～1 nm) clearly demonstrate the enhanced absorption and the resulting enhancement in photocurrent due to the plasmonic effects. An efficiency enhancement of over 18 % is obtained for the best plasmonic perovskite solar cell containing Au@SiO₂ nanoparticles in Au@SiO₂-TiO₂ weight ratio of ～1 %. Incident photon-to-current conversion efficiency (IPCE) data also showed enhancement in photocurrent for the plasmonic device. The quasi-static modelling approach provides a good correlation between theory and experiment.     

Analytical Modeling Based on Modified Effective Medium Theories for Optical Properties of Photovoltaic Material-Incorporated Plasmonic Nanoparticles

Theoretical guidance on the optical properties of plasmonic nanoparticles (NPs) is of significant importance in tremendous numbers of fields like photovoltaics. The incorporation of plasmonic NPs into photovoltaic material can promote optical absorption either via the excitation of localized surface plasmon resonance (LSPR) modes or due to multiple light scattering. Since most fabrication techniques for the incorporation of NPs into photovoltaic material result in a random array of NPs with various sizes, numerical simulations based on solving the Maxwell equations are computationally expensive and prohibitively slow for this large number of NPs. Therefore, in this paper, based on modified effective medium theories, taking into account finite size of NPs, size dispersion for NPs, extrinsic dynamic effect, and intrinsic confinement effect, fast and cost-effective analytical modeling, considering both LSPR and scattering effects, is presented to obtain the optical properties of photovoltaic material incorporated by spherical NPs with nonuniform size and random distribution. Then, by means of presented analytical modeling, considering reasonably low and high volume fractions of NPs in addition to small and large size of NPs, the effect of different parameters of embedded NPs into organic and inorganic photovoltaic materials is explored.

     

Vapor Phase Deposition, Structure, and Plasmonic Properties of Polymer-Based Composites Containing Ag–Cu Bimetallic Nanoparticles

Nanocomposite (NC) thin films with noble metal nanoparticles embedded in a dielectric material show very attractive plasmonic properties due to dielectric and quantum confinement effects. For single component nanoparticles (NPs), the plasmon resonance frequency can only be tuned in a narrow range. Much interest aroused in bimetallic nanoparticles (BNPs), however many wet chemical approaches do not allow large variation of the NP alloy composition and filling factor. Here, we report a vapor phase co-deposition method to produce polymer–metal NCs with embedded Ag_1 − x Cu _x alloy particles. The method allows production of NPs with controlled alloy composition (x), metal filling (f) and nanostructure in a protecting Teflon AF matrix. The nanostructure size and shape were characterized by transmission electron microscope. Energy dispersive X-ray spectroscopy was used to determine x and f. The optical properties and the position of surface plasmon resonances were studied by UV–Vis spectroscopy. The plasmon resonances can be tuned over a large range of the visible spectrum associated with the change in x, f, and nanostructure. For low filling factors and small particle sizes, only one resonance peak was observed. This is attributed to enhanced miscibility at the nanoscale. Double plasmon resonances were seen for larger particle sizes in accord with phase separation expected from the bulk phase diagram and were explained in terms of the formation of core-shell structures with Cu core and Ag shell. Changes upon annealing at 200 °C are also reported.     

Enhanced Optical Absorption and Spectral Photocurrent in a-Si:H by Single- and Double-Layer Silver Plasmonic Interfaces

Single and double plasmonic interfaces consisting of silver nanoparticles embedded in media with different dielectric constants including SiO₂, SiN_x, and Al:ZnO have been fabricated by a self-assembled dewetting technique and integrated to amorphous silicon films. Single plasmonic interfaces exhibit plasmonic resonances whose frequency is red-shifted with increasing particle size and with the thickness of a dielectric spacer layer. Double plasmonic interfaces consisting of two different particle sizes exhibit resonances consisting of double minima in the transmittance spectra. The optical extinction of a-Si:H deposited on these interfaces is broadened into the red indicating higher absorption and/or scattering at wavelengths higher than those typically absorbed by a-Si:H without plasmonic interfaces. While the photocurrent shows an overall decrease for the samples with the interfaces, significant enhancement of photocurrent is observed near the low-energy edge of the bandgap (600–700 nm). These results correlate well with the broadened extinction spectra of the interfaces and are interpreted in terms of enhanced absorption in that region.     

Magnetic–Plasmonic FePt@Ag Core–Shell Nanoparticles and Their Magnetic and SERS Properties

Magnetic–plasmonic FePt@Ag core–shell nanoparticles (NPs) with different Ag shell thicknesses were successfully synthesized using a seed-mediated method. They presented not only localized surface plasmon resonance in the visible region, but also superparamagnetic behavior at room temperature. When normalized by the weight of FePt, the saturation magnetization of the FePt@Ag NPs was found to be higher than that of FePt NPs, suggesting that the Ag shell effectively passivated the FePt NP surfaces, avoiding the direct interaction between the FePt core and surface capping ligands that typically forms a magnetically dead layer in FePt NPs. Despite the high colloidal stability and the small size of the FePt@Ag NPs, the NPs were easily separated using a permanent magnet. The surface enhanced Raman scattering (SERS) activity of the FePt@Ag NPs was then examined using thiophenol as a Raman reporter molecule and was found to be equivalent to that of Ag NPs. Moreover, the SERS activity of the FePt@Ag NPs was enhanced when a magnetic field was applied during the preparation of the SERS substrate (FePt@Ag NP film). These FePt@Ag NPs hold promise as dual-functional sensing probes for environmental and diagnostic applications.     

Plasmon Features of Coinage Metal Nanoparticles Supported on Zeolites

The plasmonic features in the optical response of coinage metal nanoparticles supported on different type of zeolites were studied. The shifts in the plasmon frequency were analyzed for Cu, Ag, and Au nanoparticles in mordenite, β-zeolite, and Y-zeolite. It was shown experimentally that the resonance energy is sensitive both to type of zeolite structure and counter-cation of zeolite, as well as to annealing temperature and chemical composition of zeolite, their SiO₂/Al₂O₃ molar ratio. A theoretical framework was employed to identify physical mechanism for this sensitivity. Within a simple model, the width of the absorption window identified in the imaginary part of the bulk dielectric function of the different metals was seen to play the important role in establishing the range of the plasmon energies available. In terms of an effective dielectric function, the composite medium was fully described by the complex dielectric function of the metal involved, the dissipation-free dielectric function of the zeolite matrix, and the filling fraction which relates the volume of metal inclusions as a fraction of the total sample volume. The sensitivity of the optical spectra is understood in terms of variations in both the dielectric response of the zeolite matrix as well as nanoparticle size.     

Optimization of Nanoparticle Size for Plasmonic Enhancement of Fluorescence

This paper reports on the enhancement of fluorescence that can result from the proximity of fluorophores to metallic nanoparticles (NPs). This plasmonic enhancement, which is a result of the localized surface plasmon resonance at the metal surface, can be exploited to improve the signal obtained from optical biochips and thereby lower the limits of detection. There are two distinct enhancement effects: an increase in the excitation of the fluorophore and an increase in its quantum efficiency. This study focuses on the first of these effects where the maximum enhancement occurs when the NP plasmon resonance wavelength coincides with the fluorophore absorption band. In this case, the excitation enhancement is proportional to the square of the amplitude of the electric field. The scale of the enhancement depends on many parameters, such as NP size and shape, metal type, and NP–fluorophore separation. A model system consisting of spherical gold/silver alloy NPs, surrounded by a silica spacer shell, to which is attached a fluorescent ruthenium dye, was chosen and the dependence of the fluorescence enhancement on NP diameter was investigated. Theoretical calculations, based on Mie theory, were carried out to predict the maximum possible enhancement factor for spherical NPs with a fixed composition and a range of diameters. Spherical NPs of the same composition were fabricated by chemical preparation techniques. The NPs were coated with a thin silica shell to overcome quenching effects and the dye was attached to the shell.     

In-Plane and Out-of-Plane Plasmons in Random Silver Nanoisland Films

Effective permittivity of closely spaced random nanoparticles supported by a substrate has been calculated using a modified Yamaguchi’s model (MYM) which involves the exact expression of a local field outside a metal nanoparticle (NP) along with the effective-medium approach. Pulsed laser deposition has been used to deposit silver nanoisland films on SiO₂ substrates. In-plane and out-of-plane plasmonic responses have been calculated using MYM for various filling fractions and the results are compared with those obtained from spectroscopic ellipsometry. Distinct features of out-of-plane and in-plane plasmons are observed with an spectroscopic ellipsometer and their behavior is supported by the present theoretical investigation. The comparison of the effective dielectric constants of the films obtained from ellipsometry data with those calculated using MYM shows uniaxial optical anisotropy in our case. The calculated morphological parameters (filling fraction, aspect ratio, and average particle size) using MYM are also found to be consistent with those obtained from FESEM images.     

Theoretical Investigation of Plasmonic Properties of Quantum-Sized Silver Nanoparticles

Plasmonic nanoparticles (NPs) like silver (Ag) strongly absorb the incident light and produce enhanced localized electric field at the localized surface plasmon resonance (LSPR) frequency. Enormous theoretical and experimental research has focused on the plasmonic properties of the metallic nanoparticles with sizes greater than 10 nm. However, such studies on smaller sized NPs in the size range of 3 to 10 nm (quantum-sized regime) are sparse. In this size regime, the conduction band of the metal particles discretizes, thus altering plasmon properties of the NPs from classical to the quantum regime. In this study, plasmonic properties of the spherical Ag NPs in size range of 3 to 20 nm were investigated using both quantum and classical modeling to understand the importance of invoking quantum regime to accurately describing their properties in this size regime. Theoretical calculations using standard Mie theory were carried out to monitor the LSPR peak shift and electric field enhancement as a function of the size of the bare plasmonic nanoparticle and the refractive index (RI) of the surrounding medium. Comparisons were made with and without invoking quantum regime. Also, the optical properties of metallic NPs conjugated with a chemical ligand using multi-layered Mie theory were studied, and interesting trends were observed.

     

Third-Order Nonlinear Optical Response of Layered Composites Materials Containing Gold Nanoparticles

Nonlinear third-order susceptibility \(\chi _{\text {eff}}^{(3)}\) of composites materials having alternated layers of dielectric and plasmonic nanostructures of gold nanoparticles was theoretically studied using the effective medium theory and the degenerate electron gas model. Real and imaginary parts of χeff(3) for the proposed composite material were calculated for the spectral region around the plasmon resonance of gold nanoparticles. The results reveal an enhanced nonlinear optical response compared with the obtained one for individual layers, as well as a reversal signal of \(\chi _{\text {eff}}^{(3)}\) for different volume fraction conditions.     

Optical and Thermal Enhancement of Plasmonic Nanofluid Based on Core/Shell Nanoparticles

The plasmonic effect is introduced in solar thermal areas to enhance light harvest and absorption. The optical properties of plasmonic nanofluid are simulated by finite difference time domain (FDTD) method. Due to the excitation of localized surface plasmon resonance (LSPR) effect, an intensive absorption peak is observed at 0.5 μm. The absorption characteristics are sensitive to particle size and concentration. As the particle size increases, the absorption peak is broadened and shifted to longer wavelength. The absorption of SiO₂/Ag plasmonic nanofluid is improved gradually as the volume concentration increases, especially in the UV region. The absorption edge is shifted from 0.6 to 1.0 μm as the volume concentration increases from 0.001 to 0.01. The thermal simulation of suspended SiO₂/Ag nanoparticle shows a uniform temperature rise of 17.91 K under solar irradiation (AM 1.5), while under the same condition, the temperature rises in Ag nanoparticle and Al nanoparticle are 11.12 and 5.39 K, respectively. The core/shell plasmonic nanofluid exhibits a higher photothermal performance, which has a potential application in photothermal areas. A higher temperature rise can be obtained by improving the incident light intensity or optical absorption properties of nanoparticles.     

Optimization of the Field Enhancement and Spectral Bandwidth of Single and Coupled Bimetal Core–Shell Nanoparticles for Few-Cycle Laser Applications

We have theoretically studied and optimized the field enhancement and temporal response of single and coupled bimetal Ag/Au core–shell nanoparticles (NPs) with a diameter of 160 nm and compared the results to pure Ag and Au NPs. Very high-field enhancements with an amplitude reaching 100 (with respect to the laser field centered at 800 nm) are found at the center of a 2-nm gap between Ag/Au core–shell dimers. We have explored the excitation of the bimetal core–shell particles by Fourier transform-limited few-cycle optical pulses and identified conditions for an ultrafast plasmonic decay on the order of the excitation pulse duration. The high-field enhancement and ultrafast decay makes bimetal core–shell particles interesting candidates for applications such as the generation of ultrashort extreme ultraviolet radiation pulses via nanoplasmonic field enhancement. Moreover, in first experimental studies, we synthesized small bimetal Ag/Au core–shell NPs and compared their optical response with pure Au and Ag NPs and numerical results.     

Grating-based surface plasmon resonance detection of core-shell nanoparticle mediated DNA hybridization

In this report, we have investigated enhanced surface plasmon resonance (SPR) detection of DNA hybridization using gold core - silica shell nanoparticles in localized plasmonic fields. The plasmonic fields were localized by periodic linear gratings. Experimental results measured for hybridization of 24-mer single-stranded DNA oligomers suggest that core-shell nanoparticles (CSNPs) on gratings of 400 nm period provide enhanced optical signatures by 36 times over conventional thin film-based SPR detection. CSNP-mediated DNA hybridization produced 3 times larger angular shift compared to gold nanoparticles of the same core size. We have also analyzed the effect of structural variation. The enhancement using CSNPs was associated with increased surface area and index contrast that is combined by improved plasmon coupling with localized fields on gratings. The combined approach for conjugated measurement of a biomolecular interaction on grating structures is expected to lower the limit of detection to the order of a few tens of fg/mm(2).     

Synthesis,Characterization, and Functionalization of Hybrid Au/CdS and Au/ZnS Core/Shell Nanoparticles

Plasmonic nanoparticles are an attractive material for light harvesting applications due to their easily modified surface, high surface area and large extinction coefficients which can be tuned across the visible spectrum. Research into the plasmonic enhancement of optical transitions has become popular, due to the possibility of altering and in some cases improving photo-absorption or emission properties of nearby chromophores such as molecular dyes or quantum dots. The electric field of the plasmon can couple with the excitation dipole of a chromophore, perturbing the electronic states involved in the transition and leading to increased absorption and emission rates. These enhancements can also be negated at close distances by energy transfer mechanism, making the spatial arrangement of the two species critical. Ultimately, enhancement of light harvesting efficiency in plasmonic solar cells could lead to thinner and, therefore, lower cost devices. The development of hybrid core/shell particles could offer a solution to this issue. The addition of a dielectric spacer between a gold nanoparticles and a chromophore is the proposed method to control the exciton plasmon coupling strength and thereby balance losses with the plasmonic gains. A detailed procedure for the coating of gold nanoparticles with CdS and ZnS semiconductor shells is presented. The nanoparticles show high uniformity with size control in both the core gold particles and shell species allowing for a more accurate investigation into the plasmonic enhancement of external chromophores.     

A Study of the Surface Plasmon Resonance of Silver Nanoparticles by the Discrete Dipole Approximation Method: Effect of Shape,Size, Structure,and Assembly

The surface plasmon resonance (SPR) of silver nanoparticles (AgNPs) was studied with the discrete dipole approximation considering different shapes, sizes, dielectric environments, and supraparticles assemblies. In particular, we focused our simulations on AgNPs with sizes below 10 nm, where the correction of silver dielectric constant for intrinsic size effects is necessary. We found that AgNPs shape and assembly can induce distinctive features in the extinction spectra and that SPR is more intense when AgNPs have discoid or flat shapes and are embedded in a dielectric shell with high refractive index. However, the SPR loses much of its distinctive features when size effects and stabilizing molecules induce significant broadening of the extinction bands that is often observed in the case of thiolated AgNPs smaller than about 5 nm. These results are useful indications for in situ characterization and monitoring of AgNPs synthesis and for the engineering of AgNPs with new plasmonic properties.     

Broadband Tunable and Double Dipole Surface Plasmon Resonance by TiO2 Core/Ag Shell Nanoparticles

A design of a TiO₂ core and Ag shell spherical nanoparticle is theoretically presented. The nanoparticles display double dipole plasmonic resonance peaks: one located at the ultraviolet range, the other is widely tunable from the visible to the near infrared region. The tunability can be easily controlled by varying the sizes of the core and the shell. The near field patterns of the double plasmonic resonance peaks are analyzed, and the dipole resonance modes for those two peaks are confirmed for the suitable core–shell sizes.     

Controlling the thickness of polymeric shell on magnetic nanoparticles loaded with doxorubicin for targeted delivery and MRI contrast agent

The polymeric functionalization of superparamagnetic iron oxides nanoparticles is developed for cancer targeting capability and magnetic resonance imaging. Here the nanoparticles (NP) are decorated through the adsorption of a polymeric layer around the particle surface for the formation of core-shell. The synthesized magnetic nanoparticles (MNPs) are conjugated with fluorescent dye, targeting ligand, and drug molecules for improvement of target specific diagnostic and possible therapeutics applications. In this investigation doxorubicin was loaded into the shell of the MNPs and release study was carried out at different pH. The core-shell structure of magnetic NP coated chitosan matrix was visualized by TEM observation. The cytotoxicity of these magnetic NPs is investigated using MTT assay and receptor mediated internalization by HeLa and NIH3T3 cells are studied by fluorescence microscopy. Moreover, compared with T₂-weighted magnetic resonance imaging (MRI) in the above cells, the synthesized nanoparticles are showed stronger contrast enhancements towards cancer cells.     

The advance anticancer role of polymeric core-shell ZnO nanoparticles containing oxaliplatin in colorectal cancer

We evaluated the activity of core-shell ZnO nanoparticles (ZnO-NPs@polymer shell) containing Oxaliplatin via polymerization through in vitro studies and in vivo mouse models of colorectal cancer. ZnO NPs were synthesized in situ when the polymerization step was completed by co-precipitation. Gadolinium coordinated-ZnONPs@polymer shell (ZnO-Gd NPs@polymer shell) was synthesized by exploiting Gd's oxophilicity (III). The biophysical properties of the NPs were studied using powder X-ray diffraction (PXRD), Fourier transforms infrared spectroscopy, Ultraviolet-visible spectroscopy (UV-Vis), field emission electron microscopy (FESEM), transmission electron microscopy (TEM), atomic force microscopy, dynamic light scattering, and z-potential. (3-(4,5-Dimethylthiazol-2-yl)−2,5-diphenyltetrazolium bromide) (MTT) was used to determine the antiproliferative activity of ZnO-Gd-OXA. Moreover, a xenograft mouse model of colon cancer was exerted to survey its antitumor activity and effect on tumor growth. In the following, the model was also evaluated by histological staining (H-E; Hematoxylin & Eosin and trichrome staining) and gene expression analyses through the application of RT-PCR/ELISA, which included biochemical evaluation (MDA, thiols, SOD, CAT). The formation of ZnO NPs, which contained a crystallite size of 16.8 nm, was confirmed by the outcomes of the PXRD analysis. The Plate-like morphology and presence of Pt were obtained in EDX outcomes. TEM analysis displayed the attained ZnO NPs in a spherical shape and a diameter of 33 ± 8.5 nm, while the hydrodynamic sizes indicated that the particles were highly aggregated. The biological results demonstrated that ZnO-Gd-OXA inhibited tumor growth by inducing reactive oxygen species and inhibiting fibrosis, warranting further research on this novel colorectal cancer treatment agent.     

Nanoparticle-based biocompatible and targeted drug delivery: characterization and in vitro studies

Paclitaxel nanoparticles (PAX NPs) prepared with the size of 110 ± 10 nm and ζ potential of -40 ± 3 mV were encapsulated in synthetic/biomacromolecule shell chitosan, dextran-sulfate using a layer-by-layer self-assembly technique. Zeta potential measurements, analysis of X-ray photoelectron spectroscopy, and scanning electron microscopy confirmed the successful adsorption of each layer. Surface modifications of these core-shell NPs were performed by covalently conjugating with poly(ethylene glycol) (H(2)N-PEG-carboxymethyl, M(w) 3400) and fluorescence labeled wheat germ agglutinin (F-WGA) to build a biocompatible and targeted drug delivery system. 32% of PAX was released from four bilayers of biomacromolecule assembled NPs within 8 h as compared with >85% of the drug released from the bare NPs. Moreover, high cell viability with PEG conjugation and high binding capacity of WGA-modified NPs with Caco-2 cells were observed. This biocompatible and targeted NP-based drug delivery system, therefore, may be considered as a potential candidate for the treatment of colonic cancer and other diseases.