Dynamic modulation of a surface-enhanced Raman scattering signal by a varying magnetic field

Surface-enhanced Raman scattering (SERS) signals are fundamental for spectroscopy applications. However, existing substrates cannot perform a dynamically enhanced modulation of SERS signals. Herein, we developed a magnetically photonic chain-loading system (MPCLS) substrate by loading magnetically photonic nanochains of Fe₃O₄@SiO₂ magnetic nanoparticles (MNPs) with Au nanoparticles (NPs). We achieved a dynamically enhanced modulation by applying an external stepwise magnetic field to the randomly dispersed magnetic photonic nanochains that gradually align in the analyte solution. The closely aligned nanochains create a higher number of hot spots by new neighboring Au NPs. Each chain represents a single SERS enhancement unit with both a surface plasmon resonance (SPR) effect and photonic property. The magnetic responsivity of MPCLS enables a rapid signal enhancement and tuning of the SERS enhancement factor.

Fine-tunable fluorescence quenching properties of core-satellite assemblies of gold nanorod-nanosphere: Application in sensitive detection of Hg2

In this work, we developed a simple, effective fluorescence method to detect Hg²⁺ by inhibiting core-satellite assemblies of gold nanorods (AuNRs) and gold nanospheres (AuNPs). The fluorescence of Rhodamine 6G (Rh6G), which was simply mixed with the nanoassemblies, was efficiently quenched by the inner filter effect (IFE). When the heterogenous core-satellite nanostructures were assembled, the corresponding local surface plasmon resonance (LSPR) absorption shifts and broadens which results in the increase of the spectral overlap between the emission peak and the absorption band and more efficient energy transfer from Rh6G to nanoparticles. Fluorescence quenching efficiency is related to the size and number density of satellite nanoparticles. It is interesting that the AuNR-AuNP assemblies with the moderate size and high density of AuNPs have the best fluorescence quenching efficiency. In the presence of Hg²⁺, p-aminothiophenol (p-ATP) breaks away from the surface of AuNRs and competitively bounds to Hg²⁺, resulting in a low yield of the AuNR-AuNP assemblies, which further leads to the decrease of fluorescence quenching efficiency. Under the optimum conditions, the limit of detection (LOD) for Hg²⁺ was 0.18 nM, with an excellent linear response from 0.6 to 800 nM. Interference experiment and real samples detection indicate that these nanosensors endowed with higher sensitivity and selectivity for the detection of Hg²⁺ in the real samples. Compared with the conventional Hg²⁺ detection techniques, this method based on Hg²⁺ induced inhibition of core-satellite AuNR-AuNP assemblies has better performance and is suitable for the detection of Hg²⁺.

Applications of magnetic nanoparticles in surface-enhanced Raman scattering (SERS) detection of environmental pollutants

Environmental pollution, a major problem worldwide, poses considerable threat to human health and ecological environment. Efficient and reliable detection technologies, which focus on the appearance of emerging environmental and trace pollutants, are urgently needed. Surface-enhanced Raman scattering (SERS) has become an attractive analytical tool for sensing trace targets in environmental field because of its inherent molecular fingerprint specificity and high sensitivity. In this review, we focused on the recent developments in the integration of magnetic nanoparticles (MNPs) with SERS for facilitating sensitive detection of environmental pollutants. An overview and classification of different types of MNPs for SERS detection were initially provided, enabling us to categorize the huge amount of literature that was available in the interdisciplinary research field of MNPs based SERS technology. Then, the basic working principles and applications of MNPs in SERS detection were presented. Subsequently, the detection technologies integrating MNPs with SERS that eventually were used for the detection of various environmental pollutions were reviewed. Finally, the advantages of MNP-basedSERS detection technology for environmental pollutants were concluded, and the current challenges and future outlook of this technology in practical applications were highlighted. The application of the MNPs-basedSERS techniques for environmental analysis will be significantly advanced with the great progresses of the nanotechnologies, optics, and materials.

A sensitive SERS quantitative analysis method for Ni2+ by the dimethylglyoxime reaction regulating a graphene oxide nanoribbon catalytic gold nanoreaction

The nanogold reaction between HAuCl₄ and trisodium citrate (TCA) proceeded very slowly at 60°C in a water bath. The as-prepared graphene oxide nanoribbons (GONRs) exhibited strong catalysis during the reaction to form gold nanoparticles (Au NPs) and appeared as a strong surface-enhanced Raman scattering (SERS) peak at 1616 cm^-1 in the presence of the molecular probe Victoria blue 4R (VB4r). With increase in GONR concentration, the SERS peak increased due to increased formation of Au NPs. Upon addition of dimethylglyoxime (DMG) ligand, which was adsorbed onto the GONR surface to inhibit GONR catalysis, the SERS peak decreased. When Ni²⁺ was added, a coordination reaction between DMG and Ni²⁺ took place to form stable complexes of [Ni (DMG)₂ ]²⁺ and the release of free GONR catalyst that resulted in the SERS peak increasing linearly. A SERS quantitative analysis method for Ni²⁺ was therefore established, with a linear range of 0.07-2.8 μM, and a detection limit of 0.036 μM Ni²⁺ .

AuNS@Ag core-shell nanocubes grafted with rhodamine for concurrent metal-enhanced fluorescence and surfaced enhanced Raman determination of mercury ions

Mercury ion (Hg²⁺) is a highly hazardous and widespread pollutant with bio-accumulative properties. Although the existing Hg²⁺ detection methods have high sensitivity and reliability, whereas there have few reports concerning bimodal detection for Hg²⁺ with one sensor. Toward this goal, a novel sensor based on rhodamine derivatives (RhD) grafted AuNS@Ag core-shell nanocubes (CSN) has been synthesized and shown the bimodal detection capabilities with metal enhanced fluorescence (MEF) and surface enhanced Raman scattering (SERS) for Hg²⁺. Herein, resultant CSN acts as the signal enhancing material; RhD was modified on the outside of the shell to ensure the signal sensitive of the CSN-RhD hybrids. In this work, we investigate the size- and shape-dependent SERS activity of plasmonic CSN comprised of AuNS as cores and Ag cuboids as shells. The SERS activity of CSN with spherical core was found to increase with the increasing thickness of the Ag cubic shell. Sequel, under an optimized condition, a display of strong MEF and SERS signals of the resulting mixtures with increasing of Hg²⁺ concentrations was observed. The proposed bimodal sensor showed excellent performances for Hg²⁺ along with wide linear range of 0.001-1000 ppm and 0.01-1000 ppm as well as the relatively low detection limit of 0.94 and 5.16 ppb for MEF and SERS assays, respectively. Furthermore, the ability of the sensor to detect Hg²⁺was also confirmed in adulterated milk samples.

Three-Dimensional SERS Substrates Formed with Plasmonic Core-Satellite Nanostructures

We demonstrate three-dimensional surface-enhanced Raman spectroscopy (SERS) substrates formed by accumulating plasmonic nanostructures that are synthesized using a DNA-assisted assembly method. We densely immobilize Au nanoparticles (AuNPs) on polymer beads to form core-satellite nanostructures for detecting molecules by SERS. The experimental parameters affecting the AuNP immobilization, including salt concentration and the number ratio of the AuNPs to the polymer beads, are tested to achieve a high density of the immobilized AuNPs. To create electromagnetic hot spots for sensitive SERS sensing, we add a Ag shell to the AuNPs to reduce the interparticle distance further, and we carefully adjust the thickness of the shell to optimize the SERS effects. In addition, to obtain sensitive and reproducible SERS results, instead of using the core-satellite nanostructures dispersed in solution directly, we prepare SERS substrates consisting of closely packed nanostructures by drying nanostructure-containing droplets on hydrophobic surfaces. The densely distributed small and well-controlled nanogaps on the accumulated nanostructures function as three-dimensional SERS hot spots. Our results show that the SERS spectra obtained using the substrates are much stronger and more reproducible than that obtained using the nanostructures dispersed in solution. Sensitive detection of melamine and sodium thiocyanate (NaSCN) are achieved using the SERS substrates.