Fabrication of 2D and 3D Architectures with Silver Nanostructures Building Blocks and Studying Their Refractive Index Sensitivity

In this research project, a colloidal solution of silver nanocubes was synthesized and using these nanocubes as building blocks, 2D and 3D ordered structures on solid supports were fabricated to study their optical properties and refractive index sensitivities. The silver nanocubes were synthesized by the polyol reduction process while their 2D and 3D ordered structures were fabricated by Langmuir-Blodgett trough (LB). Atomic force microscopy (AFM) and scanning electron microscopy (SEM) were employed to investigate the size and shape of the nanostructures as well as the morphologies of 2D and 3D structures. UV-visible absorption spectroscopy was employed to explore their optical properties. Finally, 2D and 3D assemblies of silver nanocubes were employed to investigate their refractive index sensitivity (RIS). The SEM image showed silver nanocubes with nominal edge length of 80 nm. Extinction spectra of 2D and 3D ordered structures are different than those in a colloidal state. Intensity of the plasmon resonance modes is higher for the 3D assembly than that of the 2D assembly. A new band in the low energy region of the spectrum appears for the 3D assembly because of interparticle coupling of the plasmon resonance modes. 3D assembly showed a higher RIS (158.9/ RIU) than of the 2D assembly (150.3/RIU). However, nanocubes are less ordered in 2D substrate than its counterpart 3D. Such 2D and 3D assemblies of silver nanocubes (AgNCs) could be potential candidates for making refractive index-based sensors as well as promising surface-enhanced Raman scattering (SERS) active substrates.     

Modifying and Fine Controlling of Silver Nanoparticle Nucleation Sites and SERS Performance by Double Silicon Etching Process

Different forms of modified and well-controlled plasmonic silver nanoparticles (AgNPs) were synthesized by silver ion reduction process of porous silicon (PS). Fine control of PS surface morphology was accomplished by employing two etching processes: light-induced etching (LIE) and photo electrochemical etching (PECE). The idea was to prepare excellent and reproducible surface-enhanced Raman scattering (SERS) substrates with high enhancement performance. PS surface modification was employed to create efficient and nearly uniformly distributed AgNP hotspot regions with very high specific surface areas. Reproducibility deviation of no more than 5% and enhancement factor of 1.2 × 10¹⁴ were obtained by SERS measurements at very low, rhodamine 6G (R6G) dye, concentration 10^?15 M. The PS morphology SERS substrate was well discussed and analyzed using field emission scanning electron microscopy (FE-SEM), X-ray diffraction spectroscopy (XRD), and Raman measurements.     

Formation of Self-organized Silver Nanocup-Type Structures and Their Plasmonic Absorption

The present work reports on the formation of extremely low volume, silver nanocup-type structures on the surface by annealing of ultra-thin silver film on quartz in inert environment. Atomic force microscopy studies together with scanning electron microscopy confirmed the formation of Ag nanocup-type structures at the surface. A basic physical model for the formation of nanocups in terms of buckling and Oswald ripening due to surface-induced morphological instability and diffusional mass transport under thermal treatment is demonstrated. Surface plasmon resonance absorptions of nanocup structures are studied and preliminary experiment for observing the surface-enhanced Raman scattering of fullerene C₇₀ molecules has been shown.     

Seed-mediated Plasmon-driven Regrowth of Silver Nanodecahedrons (NDs)

We report the synthesis of silver nanodecahedrons (NDs) for extending the localized surface plasmon resonance (LSPR) of silver nanostructures from blue to green-orange (~590 nm), which will enable much wider application opportunities using common laser light sources. In our photo-assisted method, we use a light-emitting-diode (LED) to control regrowth of silver ND from precursor seeds. Highly uniform silver NDs are synthesized when the LED emission peak coincides with the LSPR peak of the seeds. A two-step process involving precursor self-transformation into silver nanoprisms and nanoplates, and subsequent photo-activated regrowth of silver NDs has been proposed. Surface-enhanced Raman scattering of silver NDs in different sizes has been studied, and the average enhancement factor for each size is estimated to be in the order of ~10⁶.     

Localized Surface Plasmon Resonance and Refractive Index Sensitivity of Metal–Dielectric–Metal Multilayered Nanostructures

The localized surface plasmon resonances of multilayered nanostructures are studied using finite difference time domain simulations and plasmon hybridization method. Concentric metal–dielectric–metal (MDM) structure with metal core and nanoshell separated by a thin dielectric layer exhibits a strong coupling between the core and nanoshell plasmon resonance modes. The coupled resonance mode wavelengths show dependence on the dielectric layer thickness and composition of core and outer layer metal. The aluminum-based MDM structures show lower plasmon wavelength compared with Ag- and Au-based MDM nanostructures. The calculated refractive index sensitivity (RIS) factor is in the order Ag–Air–Ag>Au–Air–Au>Al–Air–Al for monometallic multilayered nanostructures. Bimetallic multilayered nanostructures support strong and tunable plasmon resonance wavelengths as well as high RIS factor of 510 nm/refractive index unit (RIU) and 470 nm/RIU for Al–Air–Au and Ag-Air-Au, respectively. The MDM structures not only exhibit higher index sensitivity but also cover a wide ultraviolet–near-infrared wavelengths, making these structures very promising for index sensing, biomolecule sensing, and surface-enhanced Raman spectroscopy.     

Simultaneous Photoluminescence and SERS Observation of Nanodiamond at Laser Deposition on Noble Metals

The metal-modified luminescence and surface-enhanced Raman scattering (SERS) occurring near nanostructured surfaces of noble metals recently have been observed for different kinds of nanocrystals associated with the metal nanostructures. In the present work, the photoluminescence and Raman scattering of diamond nanocrystals of sizes 100 and 300 nm patterned on Ag and Au thin nanostructured films via laser accelerated deposition using a femtosecond laser are discussed. The laser accelerated deposition forms ordered periodical nanodiamond–metal nanostructures and allows adjusting the interaction between nanodiamond and metal by varying the laser acceleration parameters as well as by using different metals (Ag and Au), and varying the structure of the metal film. Correspondingly, the spectroscopic properties of the system determined by interaction between nanoparticles and metal are tuned. The enhancement of nanodiamond photoluminescence together with SERS of graphite fraction and disordered carbon of nanodiamonds are observed for nanodiamond–Ag structures at 488- and 532-nm excitations, while for the nanodiamond–Au structure some characteristic SERS effects are observed at 785-nm excitation. The mechanisms of enhancement are discussed considering the nanodiamond–metal interaction and laser acceleration effect on nanodiamond.     

Plasmonic Interaction Between Silver Nano-Cubes and a Silver Ground Plane Studied by Surface-Enhanced Raman Scattering

The plasmonic interaction between silver nano-cubes and a silver ground plane with and without a dielectric spacer is studied for surface-enhanced Raman scattering (SERS) for rhodamine 6G (R6G) molecules absorbed onto the silver nano-cubes. Experimental results show that the composite substrates made from silver nano-cubes and the silver ground plane produce a stronger SERS signal than by the cubes alone, due to the plasmonic interaction between the cubes and the film. Numerical simulation is used to verify the plasmonic enhancement of the composite substrate and is consistent with the experimental results. The lowest concentration of R6G molecules which can be detected with the composite substrate is about 10⁻¹¹ M with our setup.     

SERS Study of Histamine by Using Silver Film over Nanosphere Structure

Optical properties of histamine and l-histidine have been analyzed by using surface-enhanced Raman scattering (SERS). A silver film over nanosphere (AgFON) structure with 120-nm-thick silver film on polystyrene nanospheres 1,000?nm in diameter is fabricated by nanosphere lithography to enhance the Raman signal excited at the laser wavelength of 532?nm. Normal Raman spectrum and the SERS spectrum of histamine and l-histidine were compared. Further, vibration modes of these molecules were calculated by using density functional method. In the SERS experiment, we were able to measure the Raman spectrum with a histamine concentration as less as 100?pM. This sensitivity is higher than that from high-performance liquid chromatography.     

Peptide-Guided Surface-Enhanced Raman Scattering Probes for Localized Cell Composition Analysis

The ability to control the localization of surface-enhanced Raman scattering (SERS) nanoparticle probes in bacterial cells is critical to the development of analytical techniques that can nondestructively determine cell composition and phenotype. Here, selective localization of SERS probes was achieved at the outer bacterial membrane by using silver nanoparticles functionalized with synthetic hydrophobic peptides.     

Silver salts of aromatic thiols applicable as core materials of molecular sensors operating via SERS and fluorescence

We have developed dual-tagging sensors, operating via both Raman and fluorescence spectroscopy, composed of silver aromatic thiolates (AgSRs) modified with fluorescent organic dye for multiplex immunoassays. Owing to the photo-induced production of SERS-active Ag nanoparticles, AgSRs exhibit the surface-enhanced Raman scattering (SERS) spectra of corresponding thiols. The fluorescence dye-modified AgSRs were accordingly fabricated using dye-grafted polyelectrolytes during layer-by-layer deposition of cationic and anionic polyelectrolytes onto AgSRs. In the final stage, the tagging sensors assembled with either specific biotin group or specific antibodies (anti-h-IgG or anti-r-IgG) were employed to detect either streptavidin molecules or target antigens (h-IgG or r-IgG), respectively. Since numerous AgSRs can be used as the core materials, multiple bioassays are expected to be accomplishable using the present methodology. The fluorescence signal may be used as an immediate indicator of molecular recognition, while the SERS signals can be used subsequently as the signature of specific molecular interactions.     

Modification of a Silver Substrate for Advanced Spectro-Electrochemical Applications of SERR Spectroscopy

A substrate for surface-enhanced resonance Raman spectroscopy (SERRS) in the near-ultraviolet (UV) range is presented, extending the potential window for electrochemical applications. Silver nanoparticles were synthesized exhibiting a localized surface plasmon resonance at the excitation wavelength and adsorbed onto a template-stripped silver substrate, whereby the number of particles per unit area was controlled by the adsorption time. Any attempt to employ spectro-electrochemistry on these surfaces, however, was hampered by the anodic dissolution of silver at potentials higher than 300 mV vs. standard hydrogen electrode (SHE). In order to extend the potential window for electrochemistry and still being able to use the resonance effect from silver nanoparticles, a 5-nm thick gold layer was sputtered on top of the Ag/AgNPs substrate. Cyclic voltammetry measurements of cytochrome c (cc) were carried out showing that the electrochemical behavior of gold can extend the potential range of the composite surface significantly. Furthermore, a potentiostatic titration of cc on this substrate by SERRS demonstrated that the resonance Raman effect of silver nanoparticles with the Soret band of the heme had been maintained in the presence of the gold adlayer. The positions of the plasmon resonances measured by reflection spectroscopy method were confirmed by finite-difference time-domain simulations. Gold is the optimal substrate for electrochemistry, whereas silver is the optimal material for plasmonic applications. Combining both metals gives us a surface with good performance for electrochemical applications as well as an enhancement effect sufficient to study redox-active biomacromolecules such as cc.     

New Gold Nanoparticles Adhesion Process Opening the Way of Improved and Highly Sensitive Plasmonics Technologies

Gold nanostructures have very suitable physical properties for plasmonic applications but do not stick on glass substrates. One usually uses a chromium adhesion layer that gives good mechanical adhesion but quench the plasmon. We developed a new adhesion process that permits a covalent bonding between gold and glass thanks to an MPTMS molecular layer throughout nanolithography process. We demonstrate that this new adhesion layer allows an improvement of the optical properties of the gold nanoparticles as well as an essential improvement of their surface-enhanced Raman scattering performances.     

Tuning the Plasmon of Metallic Nanostructures: From Silver Nanocubes Toward Gold Nanoboxes

One of the most significant advances in nanoscience and nanotechnology was partially driven by plasmonic effect of some noble metal nanostructures with different shapes and sizes. By controlling the geometry of metal nanostructures, their surface plasmon resonance (SPR) peaks could be tuned from the visible to the near-infrared region with various applications in sensors, optoelectronic, nanomedicine, and specifically cancer therapy. In this study, we have prepared gold nanoboxes (NBs) using the galvanic replacement between Ag nanocubes (NCs) and aqueous gold solution. Ultraviolet visible (UVvis) spectroscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD), and transmitting electron microscopy (TEM) were used to characterize silver NCs and gold NBs. The primary silver NCs were synthesized by conventional polyol method at the presence of sodium sulfide to highly tune the shape and size of the NCs. Optimized cubic silver nanostructures were obtained at 90 μl of sodium sulfide injection into the solution. Moreover, the effect of quality of the cubic structure on the shape and uniformity of gold NBs was investigated. Gold NBs with hollow interior structure and SPR peak ranging from 480 to 800 nm were successfully obtained at different injection volumes of HAuCl₄ into the solution. It was demonstrated that increasing the volume of HAuCl₄ solution to about 3 mL can increase the pore number and size until the primary structure collapses into small pieces. It was also found that the concentration of gold NBs and the corresponding SPR peak intensities decrease due to pore size enhancement and decline of charge density on the surface of metal hollow nanostructures.     

Electron Energy Loss and One- and Two-Photon Excited SERS Probing of “Hot” Plasmonic Silver Nanoaggregates

We report electron energy loss spectroscopy (EELS) and one- and two-photon excited surface-enhanced Raman scattering (SERS) and hyper Raman studies on plasmonic silver nanoaggregates. By comparison with computations, EELS imaging reveals an inverse relationship between local field intensity in an optical experiment and electron energy loss intensity at energies corresponding to excitation wavelengths used for optical probing. This inverse relation exists independent on specific nanoaggregate geometries and is mainly controlled by the gap size between the particles forming the aggregate. The ratio between two- and one-photon excited SERS measured at different excitation wavelengths provides information about local fields in the hottest spots and their dependence on the photon energy. Our data verify experimentally the predicted increase of local optical fields in the hot spots with increasing wave lengths. The reported findings show new experimental ways to characterize local fields of plasmonic nanostructures. This is of particular importance for complex structures which are not easily approachable by computations.     

Facile synthesis of Raman active phospholipid gold nanoparticles

Gold nanoparticle-based surface-enhanced Raman scattering (SERS) probes have shown promise for disease detection and diagnosis. To improve their structural and functional stability for in vivo applications, we synthesized a colloidal SERS gold nanoparticle that encapsulates Raman molecules adsorbed on 60 nm gold with a nonthiol phospholipid coating. Transmission electron microscopy and Raman and UV spectroscopy validated its reproducibility and stability. This novel lipid-based SERS probe provides a viable alternative to the PEGylation and silica coating strategies.     

Size Dependence of Nanoparticle-SERS Enhancement from Silver Film over Nanosphere (AgFON) Substrate

The dependence of nanoparticle size on surface-enhanced Raman scattering (SERS) from silver film over nanospheres substrate is studied. For a range of nanosphere sizes from 430 to 1,500 nm, optimum SERS signal is obtained with a nanosphere size of 1,000 nm at an excitation wavelength of 532 nm. We have clarified the physical origin of this optimization in an unambiguious way as due to resonant plasmonic excitations from 3D finite-difference time-domain simulations, as well as with the assistance of UV-visible reflectance spectrum.     

Solvent-Induced Morphological Changes of Polyhedral Silver Nanoparticles in Epoxy Resin

The use of just one material as reducing and capping agent during the synthesis of noble metal nanoparticles is of great interest for potential applications. This paper reports a simple method to prepare polyhedral silver nanoparticles at 80 °C using an epoxy resin (Araldite 506) as both reducing and capping agent. The formation of metal nanoparticles was investigated by Fourier-transform infrared spectroscopy, atomic force microscopy, transmission electronic microscopy, high-resolution transmission electronic microscopy and UV-vis spectroscopy. The proposed mechanism for the reduction of silver ions involves radicals as precursors of ketones and other products originated by thermo-oxidation of the resin. The nucleation of small spherical and polyhedral seeds (～7 nm) of polycrystalline silver and twin planes lead to big polyhedral silver nanoparticles of average size, 68 nm. On the other hand, the polyhedral silver nanoparticles dispersed in toluene changed to prolate-like particles, and their dispersion in dimethyl-sulphoxide and formamide originated elongated polyhedrons and concave nanostructures, respectively. These structural changes lead to unusual solvent-induced optical properties. For instance, the polyhedral nanoparticles dispersed in toluene red-shifted their surface plasmon resonance from 425 to 540 nm, in dimethyl-sulphoxide the spectrum exhibited a peak at 418 nm and a shoulder at 520 nm, and for the silver nanoparticles in formamide a broad band with maximum peak at 420 nm was observed. It is showed that the solvent/resin system works itself as structure-directing agent of silver nanoparticles. These results open the doors to achieve silver nanostructures highly sensitive to the dielectric environment, an ideal condition for applications in colorimetric sensors of molecules of biological or chemical interest.     

A Study on the Morphology of the Silver Nanoparticles Deposited on the n-Type Porous Silicon Prepared Under Different Illumination Types

In this study, the influence of n-type porous Si (n-PS) morphology properties and the performance of Ag nanoparticles (AgNPs)/n-PS Raman substrate were investigated. Two kinds of n-PS morphology (macro n-PS and mud n-PS) structures were fabricated by laser-assisted etching (LAE) process and ordinary light-assisted etching (OLAE) process, respectively. A simple and cost-effective immersion plating process of n-PS in 0.01 M concentration of AgNO₃ for 16-min immersion time was used to synthesize AgNPs. The morphological properties of the deposited AgNPs on the macro n-PS layer showed that the deposition process is concentrated on the pore wall with a little density, while for mud n-PS, the AgNP layer is mainly composed of high-density uniformly distributed spherical particles located over the mud surface. Surface-enhanced Raman scattering (SERS) process of AgNPs/n-PS revealed strong dependence on the morphology and the density of AgNPs. Enhancement factor (EF) of Raman signal of AgNPs/mud n-PS substrate is three orders of magnitude higher than that of AgNPs/macro n-PS substrate of about 1.6 × 10¹¹ and 8.2 × 10⁸, respectively.     

A Sensitive SERS Quantitative Analysis Method for Amino Acids Using Ruhemann’s Purple as Molecular Probe in Triangle Nanosilver Sol Substrate

The stable silver nanotriangle (AgNT) sol was prepared by reduction of silver ions with sodium borohydride in the presence of H₂O₂ and sodium citrate. Under the action of NaCl, AgNTs were aggregated to a highly surface-enhanced Raman scattering (SERS) active substrate. In pH 6.0 Na₂HPO₄-NaH₂PO₄ buffer solution (PBS) at 80 °C water bath, ninhydrin reacted with amino acids to form blue-violet complex Ruhemann’s purple (RP), and the RP molecules adsorbed on the aggregated AgNT surfaces that exhibited a strong SERS peak at 657 cm^?1. The peak was linear to amino acid concentration in the range of 0.7–99.8 μmol/L, with a detection limit (DL) of 0.5 μmol/L. This SERS method was applied to detect amino acid in samples, with satisfactory results. In addition, the analytical system was also investigated by resonance Rayleigh scattering (RRS), absorption, and electron microscope techniques.     

Silver nanoparticles from Prosopis glandulosa and their potential application as biocontrol of Acinetobacter calcoaceticus and Bacillus cereus

In the present study the characterization and properties of silver nanoparticles from Prosopis glandulosa leaf extract (AgNPs) were investigated using UV–Vis spectroscopic techniques, energy dispersive X-ray spectrometers (EDS), zeta potential and dynamic light scattering. The UV–Vis spectroscopic analysis showed the absorbance peaked at 487 nm, which indicated the synthesis of silver nanoparticles. The experimental results showed silver nanoparticles had Z-average diameter of 421 nm with higher stability (?200 mV). The EDS analysis also exhibited presentation of silver element. Additionally, the different concentrations of AgNPs (25, 50, 75 and 100 mg/mL) showed antibacterial activity against Acinetobacter calcoaceticus and Bacillus cereus. Finally, AgNPs from leaf extracts of P. glandulosa may be used as an agent of biocontrol of microorganism of importance medical. However, further studies will be needed to fully understand the antimicrobial activity of silver nanoparticles obtain from P. glandulosa.