Gold Rods Research Articles

Influence of experimental conditions on localized surface plasmon resonances measurement by electron energy loss spectroscopy

Scanning transmission electron microscopy (STEM) combined with electron energy loss spectroscopy (EELS) has become a standard technique to map localized surface plasmon resonances with a nanometer spatial and a sufficient energy resolution over the last 15 years. However, no experimental work discussing the influence of experimental conditions during the measurement has been published up to now. We present an experimental study of the influence of the primary beam energy and the collection semi-angle on the plasmon resonances measurement by STEM-EELS. To explore the influence of these two experimental parameters we study a series of gold rods and gold bow-tie and diabolo antennas. We discuss the impact on experimental characteristics which are important for successful detection of the plasmon peak in EELS, namely: the intensity of plasmonic signal, the signal to background ratio, and the signal to zero-loss peak ratio. We found that the primary beam energy should be high enough to suppress the scattering in the sample and at the same time should be low enough to avoid the appearance of relativistic effects. Consequently, the best results are obtained using a medium primary beam energy, in our case 120 keV, and an arbitrary collection semi-angle, as it is not a critical parameter at this primary beam energy. Our instructive overview will help microscopists in the field of plasmonics to arrange their experiments.

Potentiometric Solid-Contact Ion-Selective Electrode for Determination of Thiocyanate in Human Saliva.

A new solid-contact potentiometric ion-selective electrode for the determination of SCN− (SCN-ISE) has been described. Synthesized phosphonium derivative of calix[4]arene was used as a charged ionophore. The research included selection of the ion-selective membrane composition, determination of the ISEs metrological parameters and SCN-ISE application for thiocyanate determination in human saliva. Preparation of the ISEs included selection of a plasticizer for the ion-selective membrane composition and type of the electrode material. The study was carried out using ISE with liquid internal electrolyte (LE-ISE) and solid-contact electrodes made of glassy carbon (GC-ISE) and gold rods (Au-ISE). The best parameters were found for GC sensors for which the ion-selective membrane contained chloroparaffin as a plasticizer (S = 59.9 mV/dec, LOD = 1.6 × 10−6 M). The study of potentiometric selectivity coefficients has shown that the thiocyanate-selective sensor could be applied in biomedical research for determination of SCN− concentration in human saliva. The accuracy of the SCN− determination was verified by testing 59 samples of volunteers’ saliva by potentiometric sensors and UV-Vis spectrophotometry as a reference technique. Moreover, SCN− concentrations in the smokers’ and non-smokers’ saliva were compared. In order to investigate the influence of various factors (sex, health status, taken medications) on the thiocyanate level in the saliva, more extensive studies on a group of 100 volunteers were carried out. Additionally, for a group of 18 volunteers, individual profiles of SCN− concentration in saliva measured on a daily basis for over a month were collected.

Differential dynamic microscopy to measure the translational diffusion coefficient of nanorods

We explore differential dynamic microscopy (DDM) as a low angle scattering technique to determine the translational diffusion coefficients of gold rod shaped nanoparticles. The method is tested using five differently sized nanorods, and compared with results obtained from polarized dynamic light scattering. For the rods studied here, the method of DDM may be a more robust technique as obtaining the translational diffusion coefficient is more straightforward. Results obtained from DDM are then used as an input to fitting depolarized dynamic light scattering data for the determination of the rotational diffusion coefficient. The measured diffusion coefficients are compared with theoretical predictions based on rod sizes.

Enhancement of circular dichroism in epsilon-near-zero chiral hyperbolic metamaterials

We are presenting a theoretical study of light transmission through a slab of hyperbolic metamaterial composed of gold rods, with chiral inclusions, thus possessing both hyperbolic and chiral properties. 4 × 4 transfer matrix formalism was used for solution of a wave transmission through a slab of a chiral hyperbolic material. We have shown that circular dichroism (CD) can be enhanced by several orders of magnitude in epsilon-near-zero (ENZ) mode when the diagonal component of the permittivity tensor corresponding to the normal to interface coordinate tends to zero. The necessary condition of the enhancement is a non-zero light incidence angle. This effect results from a considerable wavelength shortening in normal direction that increases the interaction between light and matter. We have shown that the increase of the incidence angle leads to the increase of dichroism in vicinity of ENZ wavelength range and the dichroism can change sign in this resonance area. The dichroism becomes huge at large incidence angles (θ > 30°) and for each incidence angle the maximal enhancement is observed within a certain wavelength range (in vicinity of ϵ-near-zero). In opposite to discussion of the enhancement of the pseudochirality reported previously in the literature, our study relates to the true chirality. We expect these results to be of interest to researchers engaged in chemical composition of organic substances by retrieval of effective parameters from measurements of the CD.

Biotin self-assembled monolayer for impedimetric genosensor for direct detection of HIV-1

This study was performed to prove the feasibility of a direct impedimetric DNA genosensor by using biotin self-assembled monolayer (SAM). The sensor was composed of a gold rod with a SAM of biotin-modified 19-base oligonucleotide probe (5´-AAA GAT TAA TCC TGG G -3´,) for HIV-1. The electrode modification was characterized by Surface-Enhancement Raman Spectroscopy (SERS), cyclic voltammetry and the hybridization with the complementary oligonucleotide was monitored by impedimetric technique. SERS was used to confirm the linking of Biotin-DNA on the surface of the Au electrode. The coverage was 89% of the surface and a concentration of complementary oligonucleotides of 2.5 10−12 molL−1 could be detected.

Target DNA detection of human papilloma virus-16 E7 gene by capture-target-reporter sandwich on interdigitated electrode sensor

Human papilloma virus (HPV) affects predominantly the genital area, which includes vagina, cervix, penis, vulva scrotum, rectum and anus. Among 100 types of HPV, 14 types are considered to cause the risky cancer. The gene HPV-16 E7 is responsible for the development of cancer with the infected women. Earlier identification of this gene sequence avoids the cancer progression. The targeted HPV-16 E7 sequence was sandwiched by capture and reporter sequences on the carbodiimidazole-modified interdigitated electrode (IDE) surface. Target sequence at 100 f. was paired to the capture sequence immobilized on IDE sensing surface. To this surface, different concentrations of reporter sequence with and without gold rod (GNR) were evaluated. In both cases the detections were attained 1 aM by the reporter sequence pairing and with GNR increments in current were found. This enhancement was found to be 1000 folds, considering the condition was revealed in the absence of reporter. This sandwich detection strategy of capture-target-reporter sequences for HPV-16 detection on the IDE sensing surface helps to diagnose the association of cervical cancer.

Quantitative Measurement of the Optical Cross Sections of Single Nano-objects by Correlative Transmission and Scattering Microspectroscopy.

The scattering and absorption of light by nano-objects is a key physical property exploited in many applications, including biosensing and photovoltaics. Yet, its quantification at the single object level is challenging and often requires expensive and complicated techniques. We report a method based on a commercial transmission microscope to measure the optical scattering and absorption cross sections of individual nano-objects. The method applies to microspectroscopy and wide-field image analysis, offering fine spectral information and high throughput sample characterization. Accurate cross-section determination requires detailed modeling of the measurement, which we develop, accounting for the geometry of the illumination and detection as well as for the presence of a sample substrate. We demonstrate the method on three model systems (gold spheres, gold rods, and polystyrene spheres), which include metallic and dielectric particles, spherical and elongated, placed in a homogeneous medium or on a dielectric substrate. Furthermore, by comparing the measured cross sections with numerical simulations, we are able to determine structural parameters of the studied system, such as the particle diameter and aspect ratio. Our method therefore holds the potential to complement electron microscopy as a simpler and cost-effective tool for structural characterization of single nano-objects.

Structure-insensitive switchable terahertz broadband metamaterial absorbers

A nearly structural-insensitive switchable broadband terahertz metamaterial absorber is proposed, which consists of several gold rods and a vanadium dioxide (VO2) ground plane separated by a dielectric spacer. By controlling the temperature of VO2, state of the absorber can be switched from “on” to “off” in the frequency range of 1.116–1.564 THz. The physical mechanism of the absorber is given based on the wave-interference theory and field distributions. Interestingly, the proposed absorber shows a similar performance for random arrangements of a given number of rods on the top layer, which could greatly reduce the requirement of fabrication accuracy.

Surface-plasmon-based photonic crystal fibers for high-bandwidth filter realization

In this paper, we propose two novel and simple designs for photonic-crystal-fiber-based polarization filters, which consist of a gold rod and gold ring at the center. The performance characteristics of both the designs are analyzed by using the finite element method. Surface plasmon resonance is induced through the coupling mechanism when the phase-matching condition is achieved. Bandwidth performance of the gold rod model is evaluated by varying the diameter of the two small air holes near the fiber core, whereas the same is done for the gold ring model by varying the thickness of the gold ring. From the simulation results, it can be inferred that the proposed filter structure achieved high bandwidth up to 1000 nm over the communication bands.

Strong nonlinear optical response from ZnO by coupled and lattice-matched nanoantennas

We show the effective generation of second harmonic generated light in thin zinc oxide (ZnO) films by using double-resonant plasmonic nanoantennas. The designed structure consists of three gold rods with two localized surface plasmon resonances at ω and 2ω. Two of the nanoantennas are designed to be resonant for the fundamental frequency ω in order to provide a strong localization of light. The third antenna lies in between the two fundamental antennas separated by a small gap (≈30nm). Due to a strong overlap between the second harmonic resonance at 2ω and the fundamental resonance, the overall far-field radiation is significantly increased. Second harmonic generation spectroscopy measurements show an enhancement by a factor of nine compared to the emission from single dipole rods. Additionally, by optimizing the lattice constant for the nanoantenna arrays, the overall second harmonic response from the ZnO was significantly increased by a factor as large as 70, which is a great improvement for comparable plasmonic nanoantennas on thin zinc oxide.

Monitoring amyloid-β proteins aggregation based on label-free aptasensor

Amyloid-β oligomers (AβO) are important diagnostic markers for Alzheimer's disease (AD) and potential therapeutic targets for treating it. Herein, a simple and label-free electrochemical biosensor is presented for the specific recognition of AβO based on the binding of these biomarkers to ssDNA aptamer receptors. The ssDNA aptamer self-assembles to gold rod electrodes through Au-S interactions and binds specifically to AβO. The novel aptasensor shows a wide linear concentration range from 0.1 to 500 nM with a low detection limit of 0.03 nM as derived from electrochemical impedance spectroscopy invesastigations. Furthermore, owing to the high selectivity among Aβ species, this label-free sensor was used to monitor the process of Aβ protein aggregation, which was validated by atomic force microscope analysis. In addition, the aptasensor can be used to detect AβO in artificial CSF with satisfying accuracy. To our knowledge, it is first label-free aptasensor for an AβO assay based on EIS that works in artificial CSF and can be used for monitoring Aβ protein aggregation. These features, together with its easy fabrication, operation convenience, and effective regeneration, make our new aptasensor a promising tool for the early detection of AD and drug development by monitoring Aβ plaques degradation.

In‐Situ covalent synthesis of gold nanorods on GO surface as ultrasensitive Raman probe

In this paper, using thiolated graphene oxide (GO‐O‐SH) as substrate, gold nanorods (AuNRs) covalently linked to the GO surface by in‐situ seed growth method were first reported. The as‐prepared composites were characterized by UV–vis spectrum, transmission electron microscopy (TEM), scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FT‐IR). Experimental results indicated that the introduction of short flexible organic chain between GO and AuNRs contributed to the homogenous synthesis of gold rods, and uniform gold nanorods with aspect ratio within 3~8 were covalently linked to the surface of GO with high stability and yield. The strategy represented an outstanding improvement in comparison to the traditional route for fabricating GO@AuNRs composites. Furthermore, based on coupling of the two nanomaterials, the composites could act as high sensitive Raman probe with limit of detection (LOD) reaching 1 × 10−12 M.

Analytical Ultracentrifugation: Nanoparticle Gradient Materials by Centrifugation (Small 50/2018)

In article number 1803518, Helmut Cölfen and co-workers use analytical ultracentrifugation to fabricate tailored nanoparticle gradient materials. Gradients of gold spheres, gold rods, and spherical iron oxide nanoparticles are generated by sedimentation of the nanoparticles in a thermoreversible gelatin matrix. The materials can be simulated in advance and the gradients are detected online by the optical systems of the centrifuge to produce desired property gradients.

Modeling of electrochemical properties of potential-induced defects in butane-thiol SAMs by using artificial neural network and impedance spectroscopy data

Electrochemical impedance spectroscopy (EIS) is commonly used in studying solid–liquid interfacial properties. In this work, the electrochemical behavior of potential-induced defects in self-assembled CH3(CH2)3SH monolayer (SAM) electrodeposited on the surface of a mono-crystalline gold rod has been experimentally investigated and theoretically modeled. An equivalent electrical circuit with mixed kinetic and charge transfer is applied to analyze the impedance of the experimental data of the interface. The results are compared to those obtained with a second model calculation based on an artificial neural network (ANN) for predicting the electrochemical properties of the same interface. The proposed approach defines the equivalent circuit elements for the interface under study. The latter is based on adjusting the electrical parameters by using a detailed methodology proposed to build up the equivalent circuit. We illustrated the experimental and predicted numerical simulation of the corresponding model. Numerical simulation of the neural network model shows the impact of using equivalent circuits to describe the features of the materials under study. The results demonstrate a high performance in predicting and optimizing the equivalent circuit. The advantage of our adopted model appears by comparing our model results and experimental data analysis.

Fluorescence enhancement by a dark plasmon mode

We investigate the fluorescence properties of colloidal quantum dots attached to gold rod nanoantennas. These structures are fabricated by a two step electron beam lithography process in combination with a chemical linking method. By varying the nanoantenna length, the plasmon modes of the nanoantennas are successively tuned through the emission band of the quantum dots. We observe a pronounced fluorescence enhancement both for short and long nanoantennas. These findings can be attributed to the coupling of the quantum dots to bright and dark plasmon modes, respectively.

Biomolecular Corona Dictates Aβ Fibrillation Process.

Amyloid beta (Aβ), which forms toxic oligomers and fibrils in brain tissues of patients with Alzheimer's disease, is broadly used as a model protein to probe the effect of nanoparticles (NPs) on oligomerization and fibrillation processes. However, the majority of the reports in the field have ignored the effect of the biomolecular corona on the fibrillogenesis of the Aβ proteins. The biomolecular corona, which is a layer composed of various types of biomolecules that covers the surface of NPs upon their interaction with biological fluids, determines the biological fates of NPs. Therefore, during in vivo interaction of NPs with Aβ protein, what the Aβ actually "sees" is the human plasma and/or cerebrospinal fluid (CSF) biomolecular-coated NPs rather than the pristine surface of NPs. Here, to mimic the in vivo effects of therapeutic NPs as antifibrillation agents, we probed the effects of a biomolecular corona derived from human CSF and/or plasma on Aβ fibrillation. The results demonstrated that the type of biomolecular corona can dictate the inhibitory or acceleratory effect of NPs on Aβ1-42 and Aβ25-35 fibrillation processes. More specifically, we found that the plasma biomolecular-corona-coated gold NPs, with sphere and rod shapes, has less inhibitory effect on Aβ1-42 fibrillation kinetics compared with CSF biomolecular-corona-coated and pristine NPs. Opposite results were obtained for Aβ25-35 peptide, where the pristine NPs accelerated the Aβ25-35 fibrillation process, whereas corona-coated ones demonstrated an inhibitory effect. In addition, the CSF biomolecular corona had less inhibitory effect than those obtained from plasma.

Coupled Plasmon Resonances and Gap Modes in Laterally Assembled Gold Nanorod Arrays

Abstract The assembly of metal nanocrystals offers a flexible method for creating new materials with tunable, size-dependent optical properties. Here we study the lateral assembly of gold nanorods into arrays, which leads to strong colour changes due to surface plasmon coupling. We also demonstrate the first example of gap modes in colloid systems, an optical mode in which light waves propagate in the channels between the gold rods. Such modes resonate at wavelengths which strongly depend on the gap width and length.

One-Stage Synthesis of Gold Hydrosol with Nanoparticles of Desired Shape

The effect of borohydride concentration on the synthesis of gold nanoparticles in solutions of chloroauric acid, cetyltrimethylammonium bromide, and ascorbic acid in the absence of seeds has been studied systematically. Variations in the concentration of NaBH4 allow one to obtain particles of different sizes and shapes. A method has been developed for the one-stage synthesis of large pentagonal gold rods (the average length and thickness are 550 ± 135 and 71.2 ± 11.6 nm, respectively) with a high yield using borohydride in an ultra-low (≤5 × 10–8 mol/L) concentration. The resulting particles have been characterized using optical spectroscopy, scanning and transmission electron microscopy (including high-resolution technique), and electron diffraction.

Gain-modulated plasmonic Rabi oscillations of coupled nanocomplex

Strong coupling in nanostructures can bring intriguing optical phenomena such as ultrafast Rabi oscillation—periodical energy exchange phenomenon between two modes. Rabi splitting appears in the frequency-domain spectra for strong coupling system. However, in metallic nanosystems the time-domain Rabi oscillations are hard to be observed because the plasmon lifetime is limited by the heavy ohmic losses. Here we report a theoretical investigation of surface plasmon coupling behaviour of two gold nanorods with one being a core-shell rod filled with a gain material and find the periodic energy exchange phenomenon which recalls the concept of Rabi oscillation. The gain material-cored gold-shell structure dipolar mode hybridizes with the solid gold rod quadrupolar mode to form the Fano resonances. Energy exchange between the two rods happens through the near field coupling. Two approaches, to prolong plasmon lifetime by increasing the gain efficiency and to increase Rabi oscillation frequency by increasing the coupling strength, are suggested to increase the Rabi oscillation cycles. Our results offer a way to achieve unique control of light at the nanoscale and further to explore plasmonic Rabi oscillation phenomena in plasmonic nanosystems.

Understanding How Acoustic Vibrations Modulate the Optical Response of Plasmonic Metal Nanoparticles.

Measurements of acoustic vibrations in nanoparticles provide an opportunity to study mechanical phenomena at nanometer length scales and picosecond time scales. Vibrations in noble-metal nanoparticles have attracted particular attention because they couple to plasmon resonances in the nanoparticles, leading to strong modulation of optical absorption and scattering. There are three mechanisms that transduce the mechanical oscillations into changes in the plasmon resonance: (1) changes in the nanoparticle geometry, (2) changes in electron density due to changes in the nanoparticle volume, and (3) changes in the interband transition energies due to compression/expansion of the nanoparticle (deformation potential). These mechanisms have been studied in the past to explain the origin of the experimental signals; however, a thorough quantitative connection between the coupling of phonon and plasmon modes has not yet been made, and the separate contribution of each coupling mechanism has not yet been quantified. Here, we present a numerical method to quantitatively determine the coupling between vibrational and plasmon modes in noble-metal nanoparticles of arbitrary geometries and apply it to silver and gold spheres, shells, rods, and cubes in the context of time-resolved measurements. We separately determine the parts of the optical response that are due to shape changes, changes in electron density, and changes in deformation potential. We further show that coupling is, in general, strongest when the regions of largest electric field (plasmon mode) and largest displacement (phonon mode) overlap. These results clarify reported experimental results and should help guide future experiments and potential applications.