Surface Plasmon Resonance Wavelength Research Articles

Dependent scattering and plasmon coupling in concentrated suspensions of optically hard nanoparticles

This study establishes the accuracy and efficacy of the recently developed radiative transfer with reciprocal transactions (R2T2) method for quickly simulating radiation transfer through concentrated thick suspensions of optically hard nanoparticles featuring a large mismatch in refractive and/or absorption indices compared with their surrounding medium. Concentrated suspensions of optically hard nanoparticles exhibit strong light scattering and dependent scattering effects including both near-field interactions among particles and interferences of scattered waves in the far-field. Concentrated suspensions of metallic nanoparticles also exhibit plasmon coupling effect that leads to widening of absorption peak and red-shift in the peak surface plasmon resonance wavelength. However, predicting these complex interactions between EM waves and particles in thick and concentrated suspensions by explicitly solving Maxwell's equations is computationally intensive, if not impossible. Conventional solutions like Lorenz–Mie theory combined with independent scattering approximation do not account for dependent scattering and plasmon coupling. Furthermore, the dense medium radiative transfer theory is a far-field approximation that does not account for near-field effects, leading to significant errors in predictions, as illustrated in this study. By contrast, the R2T2 method's predictions showed excellent agreement with the solutions of Maxwell's equations obtained using the superposition T-matrix method for thin films containing optically hard particles. The method also rigorously accounted for multiple scattering as well as plasmon coupling in thick concentrated suspensions. These results could facilitate the design of plasmonic suspensions used in various energy and environmental applications.

Numerical demonstration of an efficient up-conversion enhancement strategy by plasmon excitations in a gap-mode nanocavity

Lanthanide-doped up-conversion nanoparticles (UCNPs) can be widely used as near-infrared absorbing photoactive materials in bioimaging and photovoltaic devices. However, the low absorption cross section and low luminescence quantum rate of UCNPs significantly limit its use. Therefore, a hexagonal gold pillar nano-array structure consisting of UCNPs, gold pillar arrays, gold film and silica layer is proposed. By optimizing the structural parameters, the surface plasmon resonance wavelengths are simultaneously matched with the 808 nm excitation wavelength and 450 nm emission wavelength of the UCNPs. The gap-mode nanocavity confines the incident excitation radiation to nanophotonic hotspots with greatly high field strengths, thus theoretically proposed an efficient strategy to enhance the up-conversion luminescence. The plasmonic structure investigated in this study combines the advantages of ease of processing, high absorption, low loss and good controllability of its localized surface plasmon resonance. It provides a new solution for up-conversion enhancement design of gap-mode nanocavities.

Influence of initial temperature of water analyte on the readings of optical sensors based on surface plasmon resonance

The analysis of additional aspects of thermal influence on the sensory effect of the three-layer optical structure “prism - gold nanolayer - water” was continued. Here, the influence of the initial temperature of the “gold nanolayer - water” system on the surface plasmon resonance wavelength change is studies. It was established that the change in the value of the sensor effect along the wavelength of the surface plasmon resonance in the range of initial temperatures from 20 °C to 25 °C is insignificant. With an increase in the initial temperature to ∼100 °C, the change in the surface plasmon resonance wavelength can reach 0.75%, and is fixed experimentally.

High sensitive optical fiber SPR sensor for label-free detection of Staphylococcus aureus

Optical fiber bacterial sensors have attracted attention in many fields due to their unique advantages. However, it is still difficult to promote its practicality which is mainly subject to their low sensitivity, inferior stability, and reliability. This work reported here an optical fiber surface plasmon resonance (SPR) biosensor enhanced by gold nanoparticles (AuNPs)/Au film coupling for label-free detection the Staphylococcus aureus (S. aureus), while both the gold substrates were successfully biofunctionalized with goat anti-human immunoglobulin G (IgG) as the probe to capture and detect the bacteria. In the experiment, the refractive index（RI） sensitivity of the Multimode-Singlemode-Multimode optical fiber (MSMOF) coated with Au film was improved by 19 % after the AuNPs were homogeneously decorated on the Au film surface. To detect S. aureus by the sensor, the SPR wavelength variation in the transmission spectrum was recorded, and its limit of detection (LOD) was demonstrated experimentally to be about 50 CFU/mL and was obtained theoretically to be 1.18 CFU/mL during its concentration range of 50–108 CFU/mL. Also, it shows great linearity in the range of 102-108 CFU/mL and the sensor exhibits excellent specificity and stability. This kind of SPR-enhanced optical fiber biosensor is fully promising for its applications in biomedicine, food safety, pharmaceuticals, etc.

Modelling plasmonic nanoparticles: Unveiling the optical torque potential with a dynamic driven damped harmonic oscillator model and width dependent effects

Linearly polarised light induces torque on anisotropic plasmonic nanoparticles, which allows for the manipulation of their orientation. In this study, we calculate the torque induced on a plasmonic nanocuboid by modelling the surface plasmon resonance (SPR) as a driven damped harmonic oscillator. Using numerical simulations, we demonstrate the model’s accuracy in predicting the torque induced for narrow nanocuboids. We also investigate the effects of varying the nanocuboid’s width on the torque and demonstrate that the torque behaviour exhibits drastic changes, with the formation and modification of additional modes, particularly at the transverse SPR wavelength. As the torque arises from the movement of charges, we utilize charge distribution profiles to explain this shift in torque behaviour.

ZnO-Au Hybrid Metamaterial Thin Films with Tunable Optical Properties

ZnO-Au nanocomposite thin films have been previously reported as hybrid metamaterials with unique optical properties such as plasmonic resonance properties and hyperbolic behaviors. In this study, Au composition in the ZnO-Au nanocomposites has been effectively tuned by target composition variation and thus resulted in microstructure and optical property tuning. Specifically, all the ZnO-Au nanocomposite thin films grown through the pulsed laser deposition (PLD) method show obvious vertically aligned nanocomposite (VAN) structure with the Au nanopillars embedded in the ZnO matrix. Moreover, the average diameter of Au nanopillars increases as Au concentration increases, which also leads to the redshifts in the surface plasmon resonance (SPR) wavelength and changes in the hyperbolic behaviors of the films. As a whole, this work discusses how strain-driven tuning of optical properties and microstructure resulted through a novel Au concentration variation approach which has not been previously attempted in the ZnO-Au thin film system. These highly ordered films present great promise in the areas of sensing, waveguides, and nanophotonics to name a few.

Enhancement of NaYF<sub>4</sub>:Yb<sup>3+</sup>/Er<sup>3+</sup> up-conversion luminescence based on anodized alumina template

Up-conversion nanoparticle (UCNP) can collect near-infrared (NIR) light and convert it into visible light. Therefore, UCNP has potential applications in fields such as biomedicine, anti-counterfeiting, and solar cells. However, the efficiency of traditional UCNP in the above-mentioned fields is relatively low, greatly limiting its use in related fields. Therefore, enhancing the up-conversion luminescence intensity of up-conversion nanoparticles is particularly important and urgently needed. In this work, anodic alumina templates are used to enhance the luminescence intensity of up-conversion nanocrystals. NaYF<sub>4</sub>:Yb<sup>3+</sup>, Er<sup>3+</sup>with a diameter of 35 nm is prepared by using co-precipitation method. Single pass AAO templates with pore size and pore spacing of 88 nm and 100 nm are prepared by using two-step anodization method. Finally, NaYF<sub>4</sub>:Yb<sup>3+</sup>, Er<sup>3+</sup>/AAO composite structures are formed by using spin coating method. The red green light emission intensity of NaYF<sub>4</sub>:Yb<sup>3+</sup>, Er<sup>3+</sup>/AAO sample can increase 4.4 and 9.0 times that of NaYF<sub>4</sub>:Yb<sup>3+</sup>, Er<sup>3+</sup>/Al reference sample, respectively. The enhancement mechanism is explored by using the finite difference time domain method, and the results show that the primary source of enhancement is the localized surface plasmon resonance effect of the pores in the anodic alumina template. At the same time, the relationship between the up-conversion luminescence intensity of NaYF<sub>4</sub>:Yb<sup>3+</sup>, Er<sup>3+</sup>/AAO sample and the incident angle is investigated. The experimental results show that as the incident angle increases, the luminescence intensity of the red and green light of NaYF<sub>4</sub>:Yb<sup>3+</sup>, Er<sup>3+</sup>/AAO samples first decrease and then increase. Due to the coupling of the local surface plasmon resonance with the excitation wavelength and emission wavelength, the up-conversion luminescence intensity of the sample can be affected. The relationship of AAO channel enhancement factor with incident angle at excitation wavelength and emission wavelength is studied by using the finite difference time domain method. The results indicate that as the incident angle increases, the enhancement factor at the excitation wavelength decreases, while the enhancement factor at the emission wavelength increases after being illuminated at an incident angle of 15°. Therefore, when the incident angle is less than 20°, the electric field intensity at 980 nm dominates, but when it is greater than 20°, the electric field intensity at 540 nm and 650 nm takes precedence. The above results provide a reference for putting them into practical applications in the fields of anti-counterfeiting and solar cells.

Advanced Integration of Glutathione-Functionalized Optical Fiber SPR Sensor for Ultra-Sensitive Detection of Lead Ions.

It is crucial to detect Pb2+ accurately and rapidly. This work proposes an ultra-sensitive optical fiber surface plasmon resonance (SPR) sensor functionalized with glutathione (GSH) for label-free detection of the ultra-low Pb2+ concentration, in which the refractive index (RI) sensitivity of the multimode-singlemode-multimode (MSM) hetero-core fiber is largely enhanced by the gold nanoparticles (AuNPs)/Au film coupling SPR effect. The GSH is modified on the fiber as the sensing probe to capture and identify Pb2+ specifically. Its working principle is that the Pb2+ chemically reacts with deprotonated carboxyl groups in GSH through ligand bonding, resulting in the formation of stable and specific chelates, inducing the variation of the local RI on the sensor surface, which in turn leads to the SPR wavelength shift in the transmission spectrum. Attributing to the AuNPs, both the Au substrates can be fully functionalized with the GSH molecules as the probes, which largely increases the number of active sites for Pb2+ trapping. Combined with the SPR effect, the sensor achieves a sensitivity of 2.32 × 1011 nm/M and a limit of detection (LOD) of 0.43 pM. It also demonstrates exceptional specificity, stability, and reproducibility, making it suitable for various applications in water pollution, biomedicine, and food safety.

Multiwavelength SERS of Magneto-Plasmonic Nanoparticles Obtained by Combined Laser Ablation and Solvothermal Methods.

The present study introduces a novel method for the synthesis of magneto-plasmonic nanoparticles (MPNPs) with enhanced functionality for surface-enhanced Raman scattering (SERS) applications. By employing pulsed laser ablation in liquid (PLAL) to synthesize plasmonic nanoparticles and wet chemistry to synthesize magnetic nanoparticles, we successfully fabricated chemically pure hybrid Fe3O4@Au and Fe3O4@Ag nanoparticles. We demonstrated a straightforward approach of an electrostatic attachment of the plasmonic and magnetic parts using positively charged polyethylenimine. The MPNPs displayed high SERS sensitivity and reproducibility, and the magnetic part allowed for the controlled separation of the nanoparticles from the reaction mixture, their subsequent concentration, and their precise deposition onto a specified surface area. Additionally, we fabricated alloy based MPNPs from AgxAu100-x (x = 50 and 80 wt %) targets with distinct localized surface plasmon resonance (LSPR) wavelengths. The compositions, morphologies, and optical properties of the nanoparticles were characterized by using transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), UV-vis spectroscopy, and multiwavelength Raman spectroscopy. A standard SERS marker, 4-mercaptobenzoic acid (4-MBA), validated the enhancement properties of the MPNPs and found an enhancement factor of 2 × 108 for the Fe3O4@Ag nanoparticles at 633 nm excitation. Lastly, we applied MPNP-enhanced Raman spectroscopy for the analysis of the biologically relevant molecule adenine and found a limit of detection of 10-7 M at 785 nm excitation. The integration of PLAL and wet chemical methods enabled the relatively fast and cost-effective production of MPNPs characterized by high SERS sensitivity and signal reproducibility that are required in various fields, including biomedicine, food safety, materials science, security, and defense.

Fabrication of 3D popcorn-like Ag microstructures film array substrate: SERS and catalytic property

Surface-enhanced Raman scattering (SERS) has tremendous significance in the fields of ultrasensitive detection and real-time in situ monitoring in catalysis. However, it remains a challenge to simultaneously achieve higher-quality SERS and excellent plasmonic catalytic properties in a nano/microstructure substrate under random excitation light. Here, we propose the design of a three-dimensional (3D) popcorn-like silver (Ag) microstructure (MS) with diameter of 1.2 ± 0.1 µm and a unique rough surface topography, realize a wide surface plasmon resonance (SPR) band ranging from 300 nm to 1100 nm, approximating a full spectrum. The popcorn-like Ag MSs with full-spectrum property effectively broaden the spectral overlap between the SPR band and wavelength of excitation light or absorption of probe molecules. Moreover, the 3D popcorn-like Ag MSs were used to construct a monolayer film array substrate on silicon wafer with a good homogeneity over large areas by an interfacial self-assembly method. The monolayer Ag MSs film array substrate can provide abundant “hot spots” and present excellent SERS performance for sensitive detection of Rhodamine 6G (Rh6G) with a low detection limit of 10−9 M. The single-layer Ag MSs film array with superior SERS activity is utilized for in situ monitoring the plasmonic catalytic kinetics of 4-nitrothiophenol (4-NTP) dimerized into 4, 4′-dimercaptoazobenzene (DMAB) under 532 nm and 785 nm excitation, respectively. The mechanism on plasmonic catalytic reaction of the popcorn-like Ag MSs substrate is systemically investigated by experimental and theoretical simulations aspects. The Ag MSs film array substrate will offer a new strategy to further explore applications of SERS in sensors, photocatalysis, and bio/chemical analysis.

Surface-Enhanced Raman Scattering Studies of Au-Ag Bimetallic Nanoparticles with a Tunable Surface Plasmon Resonance Wavelength Synthesized by Picosecond Laser Irradiation

Here, we present a simple and green method of preparing Au-Ag bimetallic nanoparticles (NPs) with a tunable surface plasmon resonance (SPR) wavelength by using picosecond laser irradiation. Au-Ag alloy NPs have been produced by irradiating the solutions containing respective metallic salts in a polyvinyl alcohol (PVA) matrix using a picosecond laser in a single-step process. The SPR wavelength of the Au-Ag bimetallic NPs is observed to be shifted/changed with the Au-Ag concentration and the laser irradiation parameters. The Au-Ag NPs embedded in the PVA matrix are advantageous for Surface-Enhanced Raman scattering (SERS) applications. The estimated enhancement factors (EFs) were observed to vary as a function of conditions of the Au-Ag bimetallic alloy NPs synthesis and also on the concentration of Au at a fixed input fluence of irradiation. The SERS active platforms of Au-Ag bimetallic NPs showed EFs as high as of the order of 108 for Crystal Violet (CV) dye samples at nano molar concentrations. The present study demonstrates a simple, single-step, and green method that fabricates Au-Ag alloy-based nanocomposites suitable for SERS investigations with significantly higher orders of EFs.

Direct Observation of In-Focus Plasmonic Cargos via Breaking Angular Degeneracy in Differential Interference Contrast Microscopy.

Breaking the angular degeneracy arising from the 2-fold optical symmetry of plasmonic anisotropic nanoprobes is critical in biological studies. In this study, we propose differential interference contrast (DIC) microscopy-based focused orientation and position imaging (dFOPI) to break the angular degeneracy of single gold nanorods (AuNRs). Single in-focus AuNRs (39 nm × 123 nm) within a spherical mesoporous silica shell were characterized with high throughput and produced distinct doughnut-shaped DIC image patterns featuring two lobes in the peripheral region, attributed to the scattering contribution of the AuNRs with large scattering cross sections. Interestingly, rotation of the lobes was observed in the focal plane for a large AuNR (>100 nm) tilted by more than ∼20° from the horizontal plane as the rotational stage was moved by 10° in a rotational study. From the rotation-dependent characteristic patterns, we directly visualized counterclockwise/clockwise rotations without the angular degeneracy at the localized surface plasmon resonance wavelength. Therefore, our dFOPI method can be applied for in vivo studies of important biological systems. To validate this claim, we tracked the three-dimensional rotational behavior of transferrin-modified in-focus AuNRs during clathrin-mediated endocytosis in real time without sacrificing the temporal and spatial resolution. In the invagination and scission stage, one or two directed twist motions of the AuNR cargos detached the AuNR-containing vesicles from the cell membrane. Furthermore, the dFOPI method directly visualized and revealed the right-handed twisting action along the dynamin helix in dynamin-catalyzed fission in live cells.

A dual-signal output plasmonic sensor based on glucose oxidase-triggered etching of triangular silver nanoprism for sensitive detection of fumonisins

A dual-signal output plasmonic sensor based on glucose oxidase (GOx)-triggered etching of triangular silver nanoprisms (TAgNPRs) was developed for detecting fumonisins (FBs). In the sensor, glucose oxidase (GOx)-FB1 conjugation was used as competitive antigens that compete with FBs to bind with anti-FBs monoclonal antibody (McAb) coated in microplate well. GOx catalyzed glucose to produce hydrogen peroxide (H2O2) which acted as an oxidant to etch TAgNPRs. As a result, the wavelength of localized surface plasmon resonance (LSPR) peak blue shifted along with a color change of the solution. The amount of FBs was correlated to the wavelength shift of LSPR peak and the color change of the solution from light blue to violet in a linear range of 0.1–500 ng/mL. The colorimetric and LSPR detection limits were 0.1 ng/mL and 0.06 ng/mL respectively. The sensitivity of the dual-signal output sensor is better than those of aggregation plasmonic sensor and magnetoimmunosensor.

Localize surface plasmon resonance of silver nanoparticles using Mie theory

In this work, the optical properties of silver nanoparticles (AgNPs) were explored using Mie theory compared with the experimental AgNPs using the chemical reduction method. Mie’s theory is suited for accurately evaluating the scattering, absorption, and extinction cross-sections of spherical AgNPs. Therefore, the wavelength of localized surface plasmon resonance (LSPR) in the optical spectra of the spherical AgNPs was calculated. The experimental AgNPs have a spherical shape with an average particle size of 30 nm. In addition, the crystalline structure of AgNPs was found to be cubic with fcc structure with lattice constant a= 4.155 Å. Moreover, the excitation of the LSPR of the AgNPs was simulated using the finite-difference time-domain (FDTD) method at different wavelengths to explore the LSPR phenomena. The results show that spherical AgNPs are the most widely used materials in biosensors, biomedicine, optoelectronic devices, and solar cells due to their surface plasmon resonances in the visible spectrum region.

Comparison of Gold Nanoparticles Prepared Using Monobasic Sodium Citrate or Sodium Borohydride for Neomycin Determination in Saliva after Solid-Phase Extraction (SPE) on a Molecularly Imprinted Polymer (MIP)

Two distinct spherical gold nanoparticles (AuNPs) were compared for the spectrophotometric determination of neomycin in saliva. The AuNPs were produced using AuCl3 and monobasic sodium citrate (in water bath at 100 °C) under magnetic stirring (AuNPs-citrate) and using HAuCl4 and NaBH4, at room-temperature under mechanical agitation in a commercial reactor (AuNPs-H). Both AuNPs were spherical with diameters of 7.7 nm (AuNPs-H) and 26.1 nm (AuNPs-citrate) and the maximum wavelength of the localized surface plasmon resonance (LSPR) bands were at 511 nm (AuNPs-H) and 529 nm (AuNPs-citrate). Equivalent spectral extinctions were found despite the fact the large difference in concentrations of AuNPs in dispersions: 4.2 × 10−9 mol L−1 for the AuNPs-H and 8.7 × 10−11 mol L−1 for the AuNPs-citrate. Both AuNPs interacted with aminoglycosides (AMG), affecting intensity of the LSPR band as the concentration of AMG increased. The response of the AuNPs-H was more sensitive toward AMG covering the following ranges: 0.6–600 µg L−1 (gentamicin), 7.3–550 µg L−1 (neomycin) and 14–520 µg L−1 (kanamycin). AuNPs-H optical response was more robust in function of the pH with AuNPs-citrate response only observed in acid solution, favoring electrostatic interaction with AMG. Catalytic activity of AuNPs-H, in reducing the 4-phenolate ion, presented a higher rate constant (4.3 × 10−3 s−1) and was used as analytical probe to determine neomycin in saliva after solid phase extraction with a commercially available AMG imprinted polymer enabling quantification to 0.36 μg of the analyte.

Numerical study of a D-shaped optical fiber SPR biosensor for monitoring refractive index variations in biological tissue via a thin layer of gold coated with titanium dioxide

This study aims to explore the numerical analysis of the impact of integrating titanium oxide (TiO2) into a D-shaped optical fiber biosensor based on surface plasmon resonance (SPR). A thin layer of gold (Au) is applied to the flat section of the fiber, which is also coated with a thin layer of titanium dioxide (TiO2). The behavior and performance of the proposed biosensor for use in biological environments are evaluated using the finite element method (FEM). The optical response of SPR-based biosensors is highly dependent on the analyzed medium, enabling the detection of pathogenic cells and abnormalities in biological tissues. This provides high sensitivity and selectivity, as well as real-time detection accuracy and speed. In this study, the biosensor is incorporated into a biological medium with a refractive index that varies with wavelength. A series of simulations have been conducted to plot the spectra of transmissions, absorptions, and dielectric losses obtained in the output of the sensor instrument. From these spectra, the corresponding surface plasmon resonance (SPR) wavelength (λSPR) within the visible-near-infrared band can be determined. Taking into account the various parameters that influence plasmonic interactions, the biosensor's performance parameters, in particular sensitivity and refractive index resolution have been optimized. Our results show that the presence of the TiO2 layer improves the performance of the proposed sensor and offers the possibility of adjusting the resonance wavelength (λSPR). In addition, our proposed sensor can achieve a better resolution of 7.50×10-6[RIU] in 1.34-143 range of analyte refractive index, which notably exceeds that of current technologies. This opens up new prospects in the field of chemical and biological detection.

Influence of Au on the nucleation of Ag nanoparticles in GeO2-PbO glasses and characterization of their ultrafast third-order nonlinear responses within the plasmon resonance region

Germanate (GeO2-PbO) glasses doped with AgNO3 and co-doped with AgNO3 and Au2O3 were synthesized via melting-quenching technique. Silver nanoparticles were nucleated within the sample doped only with AgNO3 and its plasmon resonance was characterized. The influence of the addition of Au2O3 on the silver nanoparticles nucleation and on the spectroscopic properties of the produced composite materials were investigated using TEM, STEM and UV–Vis spectroscopy. Using the nonlinear ellipse rotation technique, the pure electronic contribution to the samples nonlinear refraction and absorption were characterized within the visible range, including the localized surface plasmon (LSP) resonance wavelength of the metallic nanoparticles. The real and imaginary parts of the third-order nonlinear susceptibility of the glass, composite samples and of the individual nanoparticles were also obtained. In addition, all-optical switching figures of merit of the samples were also calculated. From these results, the dispersion curves of the electronic contribution to the third-order susceptibility of the metallic nanoparticles were obtained and two figures of merit for all-optical switching of the composite materials were calculated. The studied samples are potential candidates for all-optical switching applications with exception to the one with the highest volume filling factor, whose linear and nonlinear absorption coefficients associated to the local field factor deplete the material performance around the LSP resonance wavelength. As far as we know nonlinear ellipse rotation technique was not used before to evaluate the influence of metallic nanoparticles on the nonlinear optical properties of glasses.

Smartphone-enabled colorimetric visual quantification of highly hazardous trivalent chromium ions in environmental waters and catalytic reduction of p-nitroaniline by thiol-functionalized gold nanoparticles

High-efficiency sensing systems for extremely hazardous chromium (Cr(III)) ions are important due to their detrimental effects on human health and the environment. We employed a spectrophotometric method combined with a smartphone (red, green, and blue (RGB) color ratio)-based detection platform to realize the quick, visually quantifiable in situ detection of Cr(III) ions using surface plasmon resonance (SPR)-aided colorimetry. For optical sensing nanoprobes, we synthesized the 2-Mercapto-5-methyl-1,3,4-thiadiazole (MMT)-modified gold nanoparticles (MMT-AuNPs) using a wet chemical method. By way of a coordination reaction, the Cr(III) ions induce the as-prepared MMT-AuNPs to aggregate and subsequently change the SPR wavelength band. The freshly synthesized MMT-AuNPs exhibited a wine-red color. While Cr(III) ions interact with the MMT-AuNPs, the color of the latter evolved from wine red to purple, thus facilitating visual monitoring. The SPR-relevant color change allowed the quantitative sensing of Cr(III) ions in the range of 40–128 nM, with the limit of detection of 6.93 nM when employing the spectrophotometric method and 12.4 nM when using the smartphone RGB color ratio. Furthermore, we developed the spectrophotometric technique that used the smartphone RGB color ratio for on-site analysis of Cr(III) ions in environmental water samples, indicating the possibility of its chemo-sensing applications for portable quantitative detection devices. Additionally, the catalytic performance of the MMT-AuNPs was demonstrated by the reduction of p-nitroaniline in the presence of sodium borohydride. It was interestingly unveiled that the MMT-AuNPs showed outstanding catalytic performance with a catalytic rate constant of 6.31 × 10−3 s−1.

Silver-based SERS substrates fabricated using a 3D printed microfluidic device.

The detection of harmful chemicals in the environment and for food safety is a crucial requirement. While traditional techniques such as GC-MS and HPLC provide high sensitivity, they are expensive, time-consuming, and require skilled labor. Surface-enhanced Raman spectroscopy (SERS) is a powerful analytical tool for detecting ultralow concentrations of chemical compounds and biomolecules. We present a reproducible method for producing Ag nanoparticles that can be used to create highly sensitive SERS substrates. A microfluidic device was employed to confine the precursor reagents within the droplets, resulting in Ag nanoparticles of uniform shape and size. The study investigates the effects of various synthesis conditions on the size distribution, dispersity, and localized surface plasmon resonance wavelength of the Ag nanoparticles. To create the SERS substrate, the as-synthesized Ag nanoparticles were assembled into a monolayer on a liquid/air interface and deposited onto a porous silicon array prepared through a metal-assisted chemical etching approach. By using the developed microfluidic device, enhancement factors of the Raman signal for rhodamine B (at 10-9 M) and melamine (at 10-7 M) of 8.59 × 106 and 8.21 × 103, respectively, were obtained. The detection limits for rhodamine B and melamine were estimated to be 1.94 × 10-10 M and 2.8 × 10-8 M with relative standard deviation values of 3.4% and 4.6%, respectively. The developed SERS substrate exhibits exceptional analytical performance and has the potential to be a valuable analytical tool for monitoring environmental contaminants.

Facile synthesis of silver-rhenium nanoparticles with plasmonic and mesoporous properties

Cetyltrimethylammonium bromide (CTAB) capped rhenium (Re0@CTA(aq)) NPs were prepared by using chemical reduction method at room temperature in aqueous solution. The resulting Re0@CTA(aq) sol was utilized as sacrificial template and silver ions (Ag+) for the synthesis of silver-rhenium bimetallic NPs (Ag0-Re0@CTA(aq)) under metal displacement galvanic cell reaction in aqueous solution with CTAB. Surface Plasmon resonance (SPR) band intensity and wavelength maxima of Plasmon band of Ag0-Re0@CTA(aq) depends on the concentration of CTAB. The Plasmon band was blue shifted from 434, 439, 519, 535, 465, and 445 nm, respectively, for 0.0, 0.75, 1.0, 1.25, 1.5, and 2.0 mM of CTAB. Oxidation states, crystalline nature, elemental composition, surface morphology, shape, size, and textural properties of as-prepared silver-rhenium were determined with X-ray photoelectron spectrometry (EDX), X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), and nitrogen adsorption-desorption isotherm (BET). Fourier transform infrared spectrometry (FTIR), 1H NMR, 13C NMR, and gas chromatography-mass spectrometry (GC-EIMS) analysis were used to establish the capping properties of CTAB. Application of FTIR, 1H NMR, 13C NMR and GC-EIMS analysis shows that the CTAB consistently remains adsorbed on Ag0-Os0 NPs. Optical properties of plasmonic nanomaterials can be tuned by using a suitable concentration of CTAB surfactant.