Plasmonic Core-shell Nanoparticles Research Articles

Near-field and far-field optical properties of magnetic plasmonic core-shell nanoparticles with non-spherical shapes: A discrete dipole approximation study

The combined optical and magnetic properties of magnetic-plasmonic core-shell nanoparticles (NPs) makes them ideal candidates for many applications in biomedical fields. Plasmonic properties of the shell gives rise to Surface Enhanced Raman Scattering (SERS) that can be utilized for sensitive detections, while magnetic properties are useful for magnetic separation and magnetic guided delivery. The plasmonic properties of the shell depends on both the size and shape of the core and shell, and this property, in principle, can be calculated using the Discrete Dipole Approximation (DDA) method. However, since the DDA is an approximation method, its accuracy to calculate the plasmonic properties of the shell, especially the near-field enhancement relevant to SERS, has not been examined carefully. We present a systematic test on the accuracy of the DDA to calculate the plasmonic properties in terms of both the extinction spectra and the near-field enhancement of the magnetic-plasmonic core-shell NPs. Accuracy of the DDA method was first investigated in comparison to Mie theory results for spherical core-shell NPs, since Mie theory gives the exact solution to spherical shaped particles. DDA calculations were further extended to core-shell nanoparticles with octahedral cores. We elucidate convergence of the DDA results by considering the effects of dipole distance and shell thickness in regard to the NP spectral properties. This work validates application of the DDA methods for calculating electrodynamic properties of core-shell NPs and highlights plasmonic properties of core-shell with non-spherical cores.

Effective medium theory for anisotropic media with plasmonic core-shell nanoparticle inclusions

.Core-shell building blocks have been found useful in recent years as inclusions, in the search for metamaterials with tailored properties. Either the core or the shell of these composite inclusions may be metallic, and the dielectric component may be both radially anisotropic and radially inhomogeneous. In tunable anisotropic metamaterials, the tuning may then be achieved through the host, the core, or some combination thereof. However a theoretical picture is harder to build. Here we propose an approach to an effective medium theory for such materials, valid in the quasi-static limit. The method proceeds first by homogenising the interior of complex particle, and then uses standard anisotropic effective medium methods to provide bulk effective homogenized parameters. By varying the degree of inhomogeneity in the core, shell and dielectric-metal material volume fractions, the technique can be used as a tool for the design of metamaterials with specifically engineered properties. We find that metamaterial properties can be readily tuned by reorienting the optical axis of the host (e.g., liquid crystal). In particular, there is a possibility of switching between hyperbolic and conventional anisotropic metamaterial properties by changing inclusion shell properties.

Preparation conditions effect on the physico-chemical properties of magnetic–plasmonic core–shell nanoparticles functionalized with chitosan: Green route

We study the effect of temperatures and salt concentrations (two different points) on the physico-chemical and cytotoxicity properties of the core–shell magnetic–plasmonicnanoparticles functionalized with chitosan. This nano-composite core–shell has been produced through a green, facile, one-pot method using the fungal species Fuzarium oxyporium. The protocol of synthesis has been recorded as a patent in the Egyptian Academy of Science with no:2209/2017. We have proved the involvement of chitosan present in the fungal cell wall in the process of biosynthesis in addition to the decoration of the outer most layer of the nanocomposite by the chitosan functional groups. Characterization of the nanoparticles under selected conditions was performed by UV/visible, Fourier transform infrared spectroscopy (FTIR), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), vibrating sample magnetometer (VSM), and the dynamic light scattering (DLS). Furthermore, the cytotoxicity of the biosynthesized nanoparticles has been investigated for four different cell lines to identify the best condition for the biomedical applications. The data reveals that the 80 °C and the higher iron salt concentration yield the least particle size <2 nm and the best PDI ∼ 0.2, and least cytotoxicity 6% against liver cancer cell line and zero against other studied cell lines, additionally, it shows the best physico-chemical properties.

Estimation of sensing characteristics for refractory nitrides based gain assisted core-shell plasmonic nanoparticles

Refractory transition metal nitrides such as zirconium nitride (ZrN), hafnium nitride (HfN) and titanium nitride (TiN) have emerged as viable alternatives to coinage metals based plasmonic materials, e.g., gold (Au) and silver (Ag). The present work assesses the suitability of gain assisted ZrN-, HfN- and TiN-based conventional core-shell nanoparticles (CCSNPs) and multilayered core-shell nanoparticles (MCSNPs) for refractive index sensing. We report that the optical gain incorporation in the dielectric layer leads to multifold enhancement of the scattering efficiency (Qsca), substantial reduction of the spectral full width at half maximum, and a higher figure of merit (FOM). In comparison with CCSNPs, the MCSNP system exhibits superior sensing characteristics such as higher FOM, ∼ 45% reduction in the critical optical gain, response shift towards the biological window, and higher degree of tunability. Inherent biocompatibility, growth compatibility, chemical stability and flexible spectral tuning of refractory nitrides augmented by superior sensing properties in the present work may pave the way for refractory nitrides based low cost sensing.

Tunable multiband metasurfaces based on plasmonic core–shell nanoparticles

Metasurfaces, which can act as wavelength-selective surfaces with multiple passbands or stopbands, have numerous applications, including modulating electromagnetic radiation, and transmitarray or reflectarray antennas. In this paper, the feasibility of such metasurfaces, using plasmonic core–shell nanoparticles, has been studied. By analyzing the polarizability of such particles, it is shown that they are capable of possessing two plasmonic resonances that can be tuned to specific wavelengths by adjusting their design parameters and the constitutive media. In addition, a passband exists in between these two bands. The surface susceptibilities of the metasurface are determined using “sparse approximation formulas.” Analytical scattering coefficients were compared and found to agree with numerical results obtained using full-wave simulation.

Enhanced T1 MRI contrast and fluorescence stability within a plasmonic core-shell nanoparticle

Enhanced T1 MRI contrast and fluorescence stability within a plasmonic core-shell nanoparticle

Tuning Enhancement Efficiency of Multiple Emissive Centers in Graphene Quantum Dots by Core-Shell Plasmonic Nanoparticles.

Graphene quantum dots (GQDs) are emerging luminescent nanomaterials for energy, bioimaging, and optoelectronic applications. However, unlike conventional fluorophores, GQDs contain multiple emissive centers that result in a complex interaction with external electromagnetic fields. Here we utilize core-shell plasmonic nanoparticles to simultaneously enhance and modulate the photoluminescence (PL) intensities and spectral profiles of GQDs. By analyzing the spectral profiles, we show that the emissive centers are highly influenced by the proximity to the metal particles. Under optimal spacer thickness of 25 nm, the overall PL displays a four-fold enhancement compared with a pristine GQD. However, detailed lifetime measurements indicate the presence of midgap states that act as the bottleneck for further enhancement. Our results offer new perspectives for fundamental understanding and new design of functional luminescent materials (e.g., GQDs, graphene oxide, carbon dots) for imaging, sensing, and light harvesting.

Tuning light concentration inside plasmonic core-shell nanoparticles during laser irradiation

Computer modeling was carried out of the intensity distributions of optical (laser) radiation with wavelengths in the range of 180–540nm concentrated inside spherical two-layered core-shell nanoparticles with the core radii in the range 10–30nm and shell thicknesses range 5–40nm during irradiation. Different metals and oxides are used for core and shell materials of nanoparticles. Novel effect of light localizing at the nanoscale inside spherical two-layered core-shell NPs has been established on the base of computer calculations in the frame of the theory of diffraction of electromagnetic radiation on multilayer sphere. Light intensity concentrates in shadow hemisphere of core-shell NPs for the selected values of nanoparticle sizes and radiation wavelengths. These results can be applied in nanophotonics for construction of novel plasmonic devices and photonic components, and for different applications of the core-shell nanoparticles.

A magneto-fluidic nanoparticle trapping platform for surface-enhanced Raman spectroscopy.

A microfluidic device utilizing magnetically activated nickel (Ni) micropads has been developed for controlled localization of plasmonic core-shell magnetic nanoparticles, specifically for surface enhanced Raman spectroscopy (SERS) applications. Magnetic microfluidics allows for automated washing steps, provides a means for easy reagent packaging, allows for chip reusability, and can even be used to facilitate on-chip mixing and filtration towards full automation of biological sample processing and analysis. Milliliter volumes of gold-coated 175-nm silica encapsulated iron oxide nanoparticles were pumped into a microchannel and allowed to magnetically concentrate down into 7.5 nl volumes over nano-thick lithographically defined Ni micropads. This controlled aggregation of core-shell magnetic nanoparticles by an externally applied magnetic field not only enhances the SERS detection limit within the newly defined nanowells but also generates a more uniform (∼92%) distribution of the SERS signal when compared to random mechanical aggregation. The microfluidic flow rate and the direction and strength of the magnetic field determined the overall capture efficiency of the magneto-fluidic nanoparticle trapping platform. It was found that a 5 μl/min flow rate using an attractive magnetic field provided by 1 × 2 cm neodymium permanent magnets could capture over 90% of the magnetic core-shell nanoparticles across five Ni micropads. It was also observed that the intensity of the SERS signal for this setup was 10-fold higher than any other flow rate and magnetic field configurations tested. The magnetic concentration of the ferric core-shell nanoparticles causes the SERS signal to reach the steady state within 30 min can be reversed by simply removing the chip from the magnet housing and sonicating the retained particles from the outlet channel. Additionally, each magneto-fluidic can be reused without noticeable damage to the micropads up to three times.

Optical Absorption Modeling of Plasmonic Organic Solar Cells Embedding Silica-Coated Silver Nanospheres

We numerically study plasmonic solar cells in which a square periodic array of core–shell Ag@SiO2 nanospheres (NSs) are placed on top of the indium tin oxide (ITO) layer using a 3D finite-difference time-domain (FDTD) method. We investigate the influence of various parameters such as the periodicity of the array, the Ag core diameter, the active layer thickness, the shell thickness, and the refractive index of the shell materials on the optical performance of the organic solar cells (OSC). Our results show that the optimal periodicity of the array of NSs is dependent on the size of Ag core NSs in order to maximize optical absorption in the active layer. A very thin active layer (<70 nm) and an ultrathin (<5 nm) SiO2 shell are needed in order to obtain the highest optical absorption enhancement. Strong electric field localization is observed around the plasmonic core–shell nanoparticles as a result of localized surface plasmon resonance (LSPR) excited by Ag NSs with and without silica shell. Embedding 50 nm Ag NSs with 1-nm-thick SiO2 shell thickness on top of ITO leads to an enhanced intrinsic optical absorption in a 40-nm-thick poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM) active layer by 24.7% relative to that without the NSs. The use of 1-nm-thick ZnO shell instead of SiO2 leads to an enhanced intrinsic absorption in a 40-nm-thick P3HT:PCBM active layer by 27%.

Nanoplasmonically-engineered random lasing in organic semiconductor thin films.

We demonstrate plasmonically nano-engineered coherent random lasing and stimulated emission enhancement in a hybrid gain medium of organic semiconductors doped with core-shell plasmonic nanoparticles. The gain medium is composed of a 300 ± 2 nm thin waveguide of an organic semiconductor, doped with 53 nm gold nanoparticle cores, isolated within silica shells. Upon loading the nanoparticles, the threshold of amplified spontaneous emission is reduced from 1.75 μJ cm-2× 102 for an undoped gain medium, to 0.35 μJ cm-2× 102 for a highly concentrated gain medium, and lasing spikes narrower than 0.1 nm are obtained. Most importantly, selection of silica shells with thicknesses of 10, 17 and 21 nm enables engineering of the plasmon-exciton energy coupling and consequently tuning of the laser slope efficiency. With this approach, the slope efficiency is increased by two times by decreasing the silica shell from 21 nm down to 10 nm, due to the enhancement of the localized electric field.

Polydopamine-Enabled Approach toward Tailored Plasmonic Nanogapped Nanoparticles: From Nanogap Engineering to Multifunctionality.

We present a platform strategy that offers diverse flexibility in tailoring the structure and properties of core-shell plasmonic nanoparticles with built-in nanogaps. Our results have demonstrated that polydopamine serves multiple concerted functions as a nanoscale spacer to afford controllable nanogap sizes, a redox-active coating to promote metal shell growth, and a reactive scaffold to exclusively lock molecular probes inside the nanogap for surface-enhanced Raman scattering (SERS). More interestingly, the universal adhesion of polydopamine on diverse colloidal substrates allows for customized synthesis of multishell plasmonic nanogapped nanoparticles (NNPs) and multifunctional hybrid NNPs containing different cores (i.e., magnetic nanoparticles), which are not readily accessible by conventional methods. Internally coupled plasmonic NNPs with broadly tunable spectroscopic properties, highly active SERS, and multifunctionality hold great promise for emerging fields, such as sensing, optoelectronics, and theranostics, as demonstrated by the ultrasensitive SERS detection and efficient photothermal killing of food-borne pathogens here.

Optical Fano Resonance in Self-Assembled Magnetic–Plasmonic Nanostructures

We proposed a bottom-up approach for the fabrication of magnetic–plasmonic nanostructures that exhibit tunable plasmon enhanced hots spots and Fano resonance behavior. The nanostructures are formed from the self-assembly of magnetic–plasmonic core–shell nanoparticles. The magnetic core enables magnetophoretic control of particles during assembly, while the plasmonic shell provides interesting and useful optical behavior. We demonstrate proof-of-concept using a combination of Monte Carlo analysis to predict self-assembly and full-wave computational analysis to study the optical properties of the assembled structures. Our analysis demonstrates that viable structures can be assembled using a magnetic template-assisted self-assembly protocol and that flexible tunability of the optical response can be achieved due to the strong sensitivity of nanogap hot spots and Fano resonance features on distinct geometric parameters and the surrounding medium. We demonstrate the self-assembly and Fano resonance response of...

Optimization of Optical Absorption of Colloids of SiO2@Au and Fe3O4@Au Nanoparticles with Constraints.

We study the optical response of monodisperse colloids of core-shell plasmonic nanoparticles and introduce a computational approach to optimize absorption for photothermal applications that require dilute colloids of non-interacting particles with a prescribed volume fraction. Since the volume fraction is held constant, the particle concentration is size-dependent. Optimization is achieved by comparing the absorption spectra of colloids as a function of particle size and structure. We demonstrate the approach via application to colloids of core-shell SiO2@Au and Fe3O4@Au nanoparticles with particle sizes that range from 5–100 nm and with the incident wavelength varying from 600–1200 nm. The absorption spectra are predicted using Mie theory and the analysis shows that there is a unique mix of parameters (core radius, shell thickness, wavelength) that maximize absorption, independent of the value of volume fraction. We show that lossy Fe3O4 cores produce a much broader absorption peak with much less sensitivity to variations in particle structure and wavelength than lossless SiO2 cores. This approach can be readily adapted to colloids of nanoparticles with arbitrary materials, shapes and structure using appropriate numerical methods to compute the absorption spectra. As such, it is useful for the rational design of colloids and process variables for a broad range of photothermal applications.

Plasmonic Enhancement of Biosolar Cells Employing Light Harvesting Complex II Incorporated with Core–Shell Metal@TiO2 Nanoparticles

Here the study of effects of plasmonic nanoparticles (PNPs) conjugated with natural extract light‐harvesting complex II (LHCII) is reported. Three types of core–shell metal@TiO2 PNPs with distinct surface plasmonic resonance are prepared. The plasmonic adsorption of the metal core provides strong photon capture and enhances the LHCII excitation through plasmon‐induced resonance energy transfer (PIRET). More efficient charge separation is facilitated at LHCII/TiO2 interface as revealed by quenching of the fluorescence and reduction of the fluorescence lifetime of LHCII after adsorbing on PNPs. Femtosecond transient absorption provides further conclusive proof for charge injection from excited LHCII into the TiO2 conduction band. The plasmonic effects are further demonstrated in the enhanced photovoltaic properties after incorporating the PNPs in LHCII‐sensitized solar cells built on a 3D TiO2 nanotree photoanode. The photocurrent is enhanced by all PNPs under illumination of the full solar spectrum and the selected wavelength windows around Soret and Q bands of LHCII trimer, and the “dark region” between them. This study reveals that the core–shell PNPs can enhance the LHCII‐sensitized solar cells based on multiple mechanisms, including enhancing light harvesting, promoting PIRET from PNP to LHCII, and facilitating charge injection from excited LHCII into TiO2.

Size- and Shape-Controlled Synthesis and Properties of Magnetic-Plasmonic Core-Shell Nanoparticles.

Magnetic-plasmonic core-shell nanomaterials offer a wide range of applications across science, engineering and biomedical disciplines. However, the ability to synthesize and understand magnetic-plasmonic core-shell nanoparticles with tunable sizes and shapes remains very limited. This work reports experimental and computational studies on the synthesis and properties of iron oxide-gold core-shell nanoparticles of three different shapes (sphere, popcorn and star) with controllable sizes (70 to 250 nm). The nanoparticles were synthesized via a seed-mediated growth method in which newly formed gold atoms were added onto gold-seeded iron oxide octahedrons to form gold shell. The evolution of the shell into different shapes was found to occur after the coalescence of gold seeds, which was achieved by controlling the amount of additive (silver nitrate) and reducing agent (ascorbic acid) in the growth solution. First principles calculation, together with experimental results, elucidated the intimate roles of thermodynamic and kinetic parameters in the shape-controlled synthesis. Both discrete dipole approximation calculation and experimental results showed that the nanopopcorns and nanostars exhibited red-shifted plasmon resonance compared with the nanospheres, with the nanostars giving multispectral feature. This research has made a great step further in manipulating and understanding magnetic-plasmonic hybrid nanostructures and will make important impact in many different fields.

Enhanced DSSCs efficiency via Cooperate co-absorbance (CdS QDs) and plasmonic core-shell nanoparticle (Ag@PVP).

This paper describes cooperate the co-absorbance (CdS QDs) and the plasmonic core-shell nanoparticles (Ag@PVP) of dye synthesized solar cells in which CdS QDs and Ag@PVP are incorporated into the TiO2 layer. Cooperative nanoparticles show superior behavior on enhancing light absorption in comparison with reference cells. Cooperated DSSC exhibits the best performance with the power conversion efficiency of 7.64% which is superior to that of the free–modified DSSC with the PCE of 5%. Detailed studies offer an effective approach to enhance the efficiency of dye synthesized solar cells.

Transfer of energy between a pair of molecules near a plasmonic core-shell nanoparticle: Tunability and sensing

Our model is applied to the calculation of interaction energy between a pair of dipolar molecules (point dipoles) in the vicinity of a nanoshell monomer with core-shell structure, based on the dipole quasi-electrostatic theory of classical electrodynamics and using the Drude and Maxwell-Garnett model. In other words, this work discusses the intermolecular energy transfer from a donor molecule to an acceptor molecule near a spherical nanoparticle that is important for practical applications like sensing. It is shown that the proximity of plasmonic nanoparticles can have a strong effect on the energy transfer between molecules. In addition to the influence of the size, composition, embedding medium, and the filling fraction of doped particles on the interaction energy, the contribution of the dipolar, quadrupolar, octupolar, hexadecapolar, triakontadipolar, and higher order multipole interactions is presented and analyzed. Briefly, we will show that it is possible to achieve enhanced energy transfer by manipulation of different parameters as mentioned above.

From tunable core-shell nanoparticles to plasmonic drawbridges: Active control of nanoparticle optical properties.

The optical properties of metallic nanoparticles are highly sensitive to interparticle distance, giving rise to dramatic but frequently irreversible color changes. By electrochemical modification of individual nanoparticles and nanoparticle pairs, we induced equally dramatic, yet reversible, changes in their optical properties. We achieved plasmon tuning by oxidation-reduction chemistry of Ag-AgCl shells on the surfaces of both individual and strongly coupled Au nanoparticle pairs, resulting in extreme but reversible changes in scattering line shape. We demonstrated reversible formation of the charge transfer plasmon mode by switching between capacitive and conductive electronic coupling mechanisms. Dynamic single-particle spectroelectrochemistry also gave an insight into the reaction kinetics and evolution of the charge transfer plasmon mode in an electrochemically tunable structure. Our study represents a highly useful approach to the precise tuning of the morphology of narrow interparticle gaps and will be of value for controlling and activating a range of properties such as extreme plasmon modulation, nanoscopic plasmon switching, and subnanometer tunable gap applications.

Wide-Range Tunable Fluorescence Lifetime and Ultrabright Luminescence of Eu-Grafted Plasmonic Core-Shell Nanoparticles for Multiplexing.

Wide-range, well-separated, and tunable lifetime nanocomposites with ultrabright fluorescence are highly desirable for applications in optical multiplexing such as multiplexed biological detection, data storage, and security printing. Here, a synthesis of tunable fluorescence lifetime nanocomposites is reported featuring europium chelate grafted onto the surface of plasmonic core-shell nanoparticles, and systematically investigated their optical performance. In a single red color emission channel, more than 12 distinct fluorescence lifetime populations with high fluorescence efficiency (up to 73%) are reported. The fluorescence lifetime of Eu-grafted core-shell nanoparticles exhibits a wider tunable range, possesses larger lifetime interval and is more sensitive to separation distance than that of ordinary Eu-doping core-shell type. These superior performances are attributed to the unique nanostructure of Eu-grafed type. In addition, these as-prepared nanocomposites are used for security printing to demonstrate optical multiplexing applications. The optical multiplexing experiments show an interesting pseudo-information "a rabbit in a well" and conceal the real message "NKU."