Two-color surface plasmon resonance nanosizer for gold nanoparticles

On the Strong Binding Affinity of Gold-Graphene Heterostructures with Heavy Metal Ions in Water: A Theoretical and Experimental Investigation

Minimum energy configurations in 2D material-based heterostructures can enable interactions with external chemical species that are not observable for their monolithic counterparts. Density functional theory (DFT) calculations reveal that the binding energy of divalent toxic metal ions of Cd, Pb, and Hg on graphene-gold heterointerfaces is negative, in contrast to the positive value associated with free-standing graphene. The theoretical predictions are confirmed experimentally by Surface Plasmon Resonance (SPR) spectroscopy, where a strong binding affinity is measured for all the heavy metal ions in water. The results indicate the formation of a film of heavy metal ions on the graphene-gold (Gr/Au) heterointerfaces, where the adsorption of the ions follows a Langmuir isotherm model. The highest thermodynamic affinity constant K = 3.1 × 107 L mol-1 is observed for Hg2+@Gr/Au heterostructures, compared to 1.1 × 107 L mol-1 and 8.5 × 106 L mol-1 for Pb2+@Gr/Au and Cd2+@Gr/Au, respectively. In the case of Hg2+ ions, it was observed a sensitivity of about 0.01°/ppb and a detection limit of 0.7 ppb (∼3 nmol L-1). The combined X-ray photoelectron spectroscopy (XPS) and SPR analysis suggests a permanent interaction of all of the HMIs with the Gr/Au heterointerfaces. The correlation between the theoretical and experimental results indicates that the electron transfer from the graphene-gold heterostructures to the heavy metal ions is the key for correct interpretation of the enhanced sensitivity of the SPR sensors in water.

Optimizing and Quantifying Gold Nanospheres Based on LSPR Label-Free Biosensor for Dengue Diagnosis

The localized surface plasmon resonance (LSPR) due to light-particle interaction and its dependence on the surrounding medium have been widely manipulated for sensing applications. The sensing efficiency is governed by the refractive index-based sensitivity (ηRIS) and the full width half maximum (FWHM) of the LSPR spectra. Thereby, a sensor with high precision must possess both requisites: an effective ηRIS and a narrow FWHM of plasmon spectrum. Moreover, complex nanostructures are used for molecular sensing applications due to their good ηRIS values but without considering the wide-band nature of the LSPR spectrum, which decreases the detection limit of the plasmonic sensor. In this article, a novel, facile and label-free solution-based LSPR immunosensor was elaborated based upon LSPR features such as extinction spectrum and localized field enhancement. We used a 3D full-wave field analysis to evaluate the optical properties and to optimize the appropriate size of spherical-shaped gold nanoparticles (Au NPs). We found a change in Au NPs' radius from 5 nm to 50 nm, and an increase in spectral resonance peak depicted as a red-shift from 520 nm to 552 nm. Using this fact, important parameters that can be attributed to the LSPR sensor performance, namely the molecular sensitivity, FWHM, ηRIS, and figure of merit (FoM), were evaluated. Moreover, computational simulations were used to assess the optimized size (radius = 30 nm) of Au NPs with high FoM (2.3) and sharp FWHM (44 nm). On the evaluation of the platform as a label-free molecular sensor, Campbell's model was performed, indicating an effective peak shift in the adsorption of the dielectric layer around the Au NP surface. For practical realization, we present an LSPR sensor platform for the identification of dengue NS1 antigens. The results present the system's ability to identify dengue NS1 antigen concentrations with the limit of quantification measured to be 0.07 μg/mL (1.50 nM), evidence that the optimization approach used for the solution-based LSPR sensor provides a new paradigm for engineering immunosensor platforms.

Performance enhancement of solar cells based on high photoelectric conversion efficiency of h-BN and metal nanoparticles

In this article, we propose a new type of CdTe thin-film solar cell based on a CdTe/CdS heterojunction. We used the finite difference time domain method to simulate the propagation of electromagnetic waves in the time domain under certain boundary conditions and the change in the absorption rate of cells when optimising the structure. The simulation shows that the light absorption rate of the cell is significantly enhanced after adding h-BN and metal particles to the proposed structure. Under the irradiation of standard light AM1.5 with the wavelength range of 300 nm to 1000 nm, presenting a 90% absorption bandwidth over 700 nm, and the average absorption rate is as high as 92.9%. The short-circuit current and open-circuit voltage are 30.98 mA/cm² and 1.155 V, respectively, and the photoelectric conversion efficiency (PCE) increases to 30.76%, which is an increase of 27.58% compared to the original PCE. The result shows that, after metal nanoparticles are embedded in the absorption layer of the cell, the free electrons on the surface of the metal particles oscillate under the action of light. The electromagnetic field is confined to a small area on the surface of the particles and is enhanced, which is beneficial for the absorption of light by the cells. This study provides a basis for theoretical research and feasible solutions for the manufacture of thin-film solar cells with a high absorption rate and high efficiency.

Nanoscale engineering of ring-mounted nanostructure around AAO nanopores for highly sensitive and reliable SERS substrates

Surface-enhanced Raman scattering (SERS) is recognized as one of the most favored techniques for enhancing Raman signals. The morphology of the SERS substrate profoundly affects molecular Raman spectra. This study aimed to construct a ring-mounted nanostructured substrate via liquid-liquid two-phase self-assembly incorporated with anodic aluminum oxide (AAO) membrane transfer techniques. High-density nanoparticles (NPs) assembled on AAO membranes were ascribed to reduce the diameters of the nanopores, with Au-Ag alloy NPs to regulate the dielectric constant so as to reveal the local surface plasmon resonance tunability. SERS engineered in this way allowed for the fabrication of a ring-mounted nanostructured substrate where the distribution density of NPs and dielectric constant could be independently fine-tuned. High SERS activity of the substrate was revealed by detecting the enhanced factor of crystal violet and rhodamine 6G molecules, which was up to 1.56 × 10⁶. Moreover, SERS of thiram target molecules confirmed the supersensitivity and repeatability of the substrate as a practical application. The results of this study manifested a low-cost but high-efficiency ring-mounted nanostructured SERS substrate that might be suitable in many fields, including biosensing, medical research, environmental monitoring, and optoelectronics.

A sandwich SERS detection system based on optical convergence and synergistic enhancement effects

In this research, we propose a novel microlens surface-enhanced Raman spectroscopy (SERS) substrate @ Au film detection system, which is shown to have excellent attributes. This scheme involves the construction of a PDMS plano-convex microlens SERS-active substrate in combination with an Au film. Due to the optical convergence from the microlens, the synergistic enhancement effects due to the Au film, and the "Au film-molecules-AgNPs" sandwich structure, an outstanding SERS performance is achieved. Multiple tests using a portable Raman spectrometer show that the optical convergence due to the microlens and the coupling effects contribute around 1.85× and 26.18× enhancement of the Raman signal, respectively. Even for objective lenses with different numerical apertures, simulations show that the microlens SERS substrate can further enhance the signal collection efficiency; this indicates that the detection scheme is universally applicable. Moreover, the microlens SERS substrate @ Au film system shows excellent time stability, and its Raman enhancement performance remains consistently above 98% of the original signal, even one week later. Our proposed system is simple to prepare, is low cost and has many potential practical applications, which include the detection of biochemical samples.

Self-assembled nano-Ag/Au@Au film composite SERS substrates show high uniformity and high enhancement factor for creatinine detection

Serum creatinine is a key biomarker for the diagnosis and monitoring of kidney disease. Rapid and sensitive creatinine detection is thus important. Here, we propose a high-performance nano-Ag/Au@Au film composite SERS substrate for the rapid detection of creatinine in human serum. Au nanoparticles (AuNPs) and Ag nanoparticles (AgNPs) with uniform particle size were synthesized by a chemical reduction method, and the nano-Ag/Au@Au film composite SERS substrate was successfully prepared via a consecutive layer-on-layer deposition using an optimized liquid-liquid interface self-assembly method. The finite element simulation analysis showed that due to the multi-dimensional plasmonic coupling effect formed between the AuNPs, AgNPs, and the Au film, the intensity of the local electromagnetic field was greatly improved, and a very high enhancement factor (EF) was obtained. Experimental results showed that the limit of detection (LOD) of this composite SERS substrate for rhodamine 6G (R6G) molecules was as low as 1 × 10^-13M, and the Raman EF was 15.7 and 2.9 times that of the AuNP and AgNP monolayer substrates respectively. The results of different batch tests and SERS mapping showed that the relative standard deviations of the Raman intensity of R6G at 612 cm^-1were 12.5% and 11.7%, respectively. Finally, we used the SERS substrate for the label-free detection of human serum creatinine. The results showed that the LOD of this SERS substrate for serum creatinine was 5 × 10^-6M with a linear correlation coefficient of 0.96. In conclusion, the SERS substrate has high sensitivity, good uniformity, simple preparation, and has important developmental potential for the rapid detection and application of disease biomarkers.

Highly Porous and Ultra-Lightweight Aero-Ga2O3: Enhancement of Photocatalytic Activity by Noble Metals

A new type of photocatalyst is proposed on the basis of aero-β-Ga2O3, which is a material constructed from a network of interconnected tetrapods with arms in the form of microtubes with nanometric walls. The aero-Ga2O3 material is obtained by annealing of aero-GaN fabricated by epitaxial growth on ZnO microtetrapods. The hybrid structures composed of aero-Ga2O3 functionalized with Au or Pt nanodots were tested for the photocatalytic degradation of methylene blue dye under UV or visible light illumination. The functionalization of aero-Ga2O3 with noble metals results in the enhancement of the photocatalytic performances of bare material, reaching the performances inherent to ZnO while gaining the advantage of the increased chemical stability. The mechanisms of enhancement of the photocatalytic properties by activating aero-Ga2O3 with noble metals are discussed to elucidate their potential for environmental applications.

Dielectric-Loaded Waveguides as Advanced Platforms for Diagnostics and Application of Transparent Thin Films

An alternative approach to classical surface plasmon resonance spectroscopy is dielectric-loaded waveguide (DLWG) spectroscopy, widely used in the past decades to investigate bio-interaction kinetics. Despite their wide application, a successful and clear approach to use the DLWGs for the one-step simultaneous determination of both the thickness and refractive index of organic thin films is absent in the literature. We propose here, for the first time, an experimental protocol based on the multimodal nature of DLWGs to be followed in order to evaluate the optical constants and thickness of transparent thin films with a unique measurement. The proposed method is general and can be applied to every class of transparent organic materials, with a resolution and accuracy which depend on the nature of the external medium (gaseous or liquid), the geometrical characteristics of the DLWG, and the values of both the thickness and dielectric constant of the thin film. From the experimental point of view, the method is demonstrated in a nitrogen environment with an accuracy of about 3%, for the special case of electroluminescent thin films of Eu³⁺β-diketonate complexes, with an average thickness of about 20 nm. The high value of the refractive index measured for the thin film with the Eu(btfa)₃(t-bpete) complex was confirmed by the use of a spectroscopic model based on the Judd-Ofelt theory, in which the magnetic dipole transition ⁵D₀ → ⁷F₁ (Eu³⁺) for similar films containing Eu³⁺ complexes is taken as a reference. The DLWGs are finally applied to control the refractive index changes of the organic thin films under UVA irradiation, with potential applications in dosimetry and monitoring light-induced transformation in organic thin films.

A versatile and compact surface plasmon resonance spectrometer based on single board computer

The widespread diffusion of low-cost but high-performance hardware is enhancing the realization of scientific equipment with features at the research laboratory level. In this paper, we demonstrate hardware implementation of a surface plasmon resonance compact device with high accuracy and measurement times appropriate for many applications. Image acquisition is realized by a Raspberry Pi single board computer with a camera module, and a Python code is used to process data. A flexible optical setup can work in two different configurations, namely, the inspection mode and angle resolved measurement mode. The inspection mode is used to precisely locate the light-emitting diode interrogation beam on the sample, avoiding uneven or faulty regions. The measurement mode allows us to monitor in real time the position of the minimum reflectivity with subpixel resolution. Performance tests show a resolution in the bulk refractive index of 4.9 × 10^-6 refractive index units for 10 s acquisition time.

Optical sizing of nanoparticles in thin films of nonabsorbing nanocolloids

We study an optical method to infer the size of nanoparticles in a thin film of a dilute nonabsorbing nanocolloid. It is based on determining the contribution of the nanoparticles to the complex effective refractive index of a suspension from reflectivity versus the angle of incidence curves in an internal reflection configuration. The method requires knowing only approximately the particles' refractive index and volume fraction. The error margin in the refractive index used to illustrate this technique was 2%. The method is applicable to sizing nanoparticles from a few tens of nanometers to about 200 nm in radius and requires a small volume of the sample, in the range of a few microliters. The method could be used to sense nanoparticle aggregation and is suitable to be integrated into microfluidic devices.