A calcium alginate sponge with embedded gold nanoparticles as a flexible SERS substrate for direct analysis of pollutant dyes

Silver microplasma-engineered nanoassemblies on periodic nanostructures for SERS applications

This research aimed to enhance the performance of surface-enhanced Raman scattering (SERS) substrates through the implementation of periodic nanostructures, effectively increasing surface area and uniformity. The approach involved a two-step process: initially, magnetron sputtering was employed to minimize the Raman background signal from the polymer substrate, and subsequently, the microplasma nanoparticle coating method was utilized to augment the presence of silver nanoparticles (AgNPs) for enhancing SERS efficacy. The outcome revealed several key findings: a coefficient of variation (CV) of approximately 8% for individual substrates (3 × 3 cm2), a CV of 6% between different fabrication batches, and a sustained signal strength of 85% over a storage period exceeding two months in a moisture-proof enclosure, thus meeting commercial product standards. Moreover, the substrate demonstrated a limit of detection of 8.4 × 10-7 M (306.5 ppb) for malachite green under non-resonance Raman excitation conditions along with an impressive enhancement factor of 2.69 × 106, establishing it as a high-performance and stable SERS substrate.

A simple and efficient alginate hydrogel combined with surface-enhanced Raman spectroscopy for quantitative analysis of sodium nitrite in meat products

Sodium nitrite is a commonly used preservative and color protectant in the food industry. Conventional analytical methods are highly susceptible to food matrix interference, time-consuming and costly. In this study, the ion cross-linking method was employed to prepare alginate hydrogel substrates, and phenosafranin was chosen as a single-molecule probe to analyze sodium nitrite. Our investigation centered on elucidating the effects of alginate and cross-linking ion concentrations on Raman signal characteristics. The optimal Raman response was observed in the precursor solution with 1% sodium alginate and 0.1 mol L-1 cross-linking ions. The relative standard deviations (RSDs) of the feature peaks from the three substrate batches ranged from 1.22% to 16.30%, attesting the robustness and consistency of the substrates. The signal reduction of the substrates after a four-week storage period remained below 10%, indicating that the substrates had good reproducibility and stability. The limits of detection (LODs) for sodium nitrite in extracts from cured meat, luncheon meat, and sliced ham were determined to range from 3.75 mg kg-1 to 8.11 mg kg-1, with low interference from the food matrix. The support vector machine algorithm was utilized to train and predict the data, which proved to be more accurate (98.6%-99.8% recovery) than the traditional linear regression model (81.9%-112.7% recovery) in predicting the spiked samples. The application of hydrogel-based surface-enhanced Raman spectroscopy (SERS) substrates for nitrite detection in food, combined with machine learning for regression prediction in data processing, collectively augmented the potential of SERS technology in the field of food analysis.

Ultralight sponge made from sodium alginate with processability and stability for efficient removal of microplastics

Due to natural agents and human activities, large quantities of microplastics enter the marine environment. As an emerging pollutant, MPs have attracted worldwide attention and become a great challenge in recent years. Sodium alginate is a kind of natural polysaccharide with non-toxic, stability, and low cost. In this study, sodium alginate sponge was prepared by secondary freeze-drying technology. Alginate sponge contains a large number of hydrophilic groups; thus, alginate sponge has super water-absorbed (the water absorption rate range from 1193-5232%). Meanwhile, the alginate sponge has high porosity of 81.93% and excellent mechanical properties. The removal efficiency of 100 mg·L-1 microplastics by alginate sponge reached up to 92.3%. The 1 mg·L-1 and 10 mg·L-1 microplastics can be completely absorbed in 27 h and 60 h, respectively. The adsorption mechanism of microplastics adsorbed onto alginate sponge included intra-particle diffusion, hydrogen bonds interactions, and π-π interactions. In addition, the adsorption of MPs loaded Cu2+/Na+ by sponge in complex aqueous environments is still significant. This study expands the development prospect of sodium alginate sponge materials in the field of water treatment and provides a new green approach for the removal of microplastics.

Synthesis of recyclable SERS platform based on MoS2@TiO2@Au heterojunction for photodegradation and identification of fungicides

Surface-enhanced Raman spectroscopy (SERS) substrates based on metal/semiconductors have attracted much attention due to their excellent photocatalytic activity and SERS performance. However, they generally exhibit low light utilization and photocatalytic efficiencies. Herein, molybdenum disulfide coated titanium dioxide modified with gold nanoparticles (MoS₂@TiO₂@Au) as a heterojunction-based recyclable SERS platform was fabricated for the efficient determination of fungicides. Results showed that the MoS₂@TiO₂@Au platform could rapidly degrade 90.7% crystal violet in 120 min under solar light irradiation and enable reproducible and sensitive SERS analysis of three fungicides (methylene blue, malachite green, and crystal violet) and in-situ monitor of the photodegradation process. The platform could also be reused five times due to the unique integrated merits of the MoS₂@TiO₂@Au heterojunction. Meanwhile, experiments in determining methylene blue in prawn protein solution achieved a limit of detection of 1.509 μg/L. Therefore, it is hoped that this work could expand detection applications of photocatalytic materials.

Green Synthesis of Three-Dimensional Au Nanorods@TiO2 Nanocomposites as Self-Cleaning SERS Substrate for Sensitive, Recyclable, and In Situ Sensing Environmental Pollutants

In this work, a simple, low-cost, green, and mild method for the preparation of three-dimensional nanocomposite materials of gold nanorods (Au NRs)@TiO₂ is reported. The surface of Au NRs was coated with TiO₂ in situ reduction at room temperature without a complicated operation. The synthetic Au NRs@TiO₂ nanocomposites were used as surface-enhanced Raman spectroscopy (SERS) active substrates for the reusable and sensitive detection of environmental pollutants. The results showed that the pollutants on Au NRs@TiO₂ nanocomposites have higher SERS activity and reproducibility than those on the Au NR substrate without the presence of TiO₂. Moreover, the SERS substrate can be readily recycled by UV-assisted self-cleaning to remove residual analyte molecules. Malachite green (MG) and crystal violet (CV) were used as examples to demonstrate the feasibility of the proposed sensor for the sensitive detection of environmental pollutants. The results showed that the limit of detections (LODs) were 0.75 μg/L and 0.50 μg/L for MG and CV, respectively, with the recoveries ranging from 86.67% to 91.20% and 83.70% to 89.00%. Meanwhile, the SERS substrate can be easily regenerated by UV light irradiation. Our investigation revealed that within three cycles, the Au NRs@TiO₂ substrates still maintained the high SERS enhancement effect that they showed when first used for SERS detection. These results indicated that the method can be used to detect MG and CV in really complex samples. Due to the high sensitivity, reusability, and portability and the rapid detection property of the proposed sensor, it can have potential applications in the on-site detection of environmental pollutants in a complex sample matrix.

Facial Fabrication of Large-Scale SERS-Active Substrate Based on Self-Assembled Monolayer of Silver Nanoparticles on CTAB-Modified Silicon for Analytical Applications

Surface-enhanced Raman spectroscopy (SERS) has been proven to be a promising analytical technique with sensitivity at the single-molecule level. However, one of the key problems preventing its real-world application lies in the great challenges that are encountered in the preparation of large-scale, reproducible, and highly sensitive SERS-active substrates. In this work, a new strategy is developed to fabricate an Ag collide SERS substrate by using cetyltrimethylammonium bromide (CTAB) as a connection agent. The developed SERS substrate can be developed on a large scale and is highly efficient, and it has high-density “hot spots” that enhance the yield enormously. We employed 4-methylbenzenethiol(4-MBT) as the SERS probe due to the strong Ag–S linkage. The SERS enhancement factor (EF) was calculated to be ~2.6 × 10⁶. The efficacy of the proposed substrate is demonstrated for the detection of malachite green (MG) as an example. The limit of detection (LOD) for the MG assay is brought down to 1.0 × 10⁻¹¹ M, and the relative standard deviation (RSD) for the intensity of the main Raman vibration modes (1620, 1038 cm⁻¹) is less than 20%.

Detection of organic dyes by surface-enhanced Raman spectroscopy using plasmonic NiAg nanocavity films

This work presents the NiAg nanocavity film for the detection of organic dyes by surface-enhanced Raman spectroscopy (SERS). Nanocavity films were prepared by colloidal lithography using 518-nm polystyrene spheres combined with the electrochemical deposition of Ni supporting layer and Ag nanoparticles homogeneous SERS-active layer. The theoretical study was modelled by finite-difference time-domain (FDTD) simulation of electromagnetic field enhancement near the nanostructured surface and experimentally proven by SERS measurement of selected organic dyes (rhodamine 6G, crystal violet, methylene blue, and malachite green oxalate) in micromolar concentration. Furthermore, the concentration dependence was investigated to prove the suitability of NiAg nanocavity films to detect ultra-low concentrations of samples. The detection limit was 1.3 × 10^-12, 1.5 × 10^-10, 1.4 × 10^-10, 7.5 × 10^-11 mol·dm^-3, and the standard deviation was 20.1%, 13.8%, 16.7%, and 19.3% for R6G, CV, MB, and MGO, respectively. The analytical enhancement factor was 3.4 × 10⁵ using R6G as a probe molecule. The principal component analysis (PCA) was performed to extract the differences in complex spectra of the dyes where the first and second PCs carry 42.43% and 31.39% of the sample variation, respectively. The achieved results demonstrated the suitability of AgNi nanocavity films for the SERS-based detection of organic dyes, with a potential in other sensing applications.

The current state of the art of plasmonic nanofibrous mats as SERS substrates: design, fabrication and sensor applications

Surface-enhanced Raman scattering (SERS) is a widely used analytical tool that allows molecular fingerprint-based ultra-sensitive detection through an enhanced electromagnetic field generated by plasmonic metal nanoparticles (MNPs) by virtue of their localized surface plasmon resonance (LSPR). Although significant progress has been made in the design and fabrication of a variety of SERS substrates, MNP-decorated electrospun nanofibrous (NF) mats have attracted much attention due to their unique nanoscale structural and functional properties. This review focuses on the current state of the art in the fabrication of plasmonic NF mats with the main focus on the pre-mix, in situ, and ex situ approaches. The characteristic functional advantages and limitations of these strategies are also highlighted, which might be helpful for the research community when adopting a suitable approach. The potential of these plasmonic NF mats as a SERS-active optical sensor substrate, and their performance parameters such as the limit of detection, analytical range, and enhancement factor, and real-world applications are also discussed. The summary and futuristic discussion in this review might be of significant value in developing plasmonic NF mat-based SERS-active point-of-care diagnostic chips for a wide range of applications.

Core-satellite assemblies and exonuclease assisted double amplification strategy for ultrasensitive SERS detection of biotoxin

In this work, core-satellite assemblies and exonuclease assisted double amplification strategy is developed to produce surface-enhanced Raman scattering (SERS) biosensor towards ultrasensitive detection of biotoxin. In the presence of target molecules, the exonuclease III (Exo III) assisted efficient recycling amplification provides an excellent pathway for the fabrication of core-satellite SERS sensor. Briefly, the proposed strategy includes the following double amplifications: (i) Exo III induced target-related signal amplification; (ii) core-satellite assemblies assisted formation of SERS "hot-spots" induced signal amplification. To show the applicability of the suggested strategy, the detection of ochratoxin A (OTA), one of the most toxic and widely distributed biotoxin, is demonstrated as an example. The results show that the limit of detection (LOD) of OTA is 0.83 fg mL^-1 (S/N = 3). On the basis of the DNA aptamer induced specific target recognition, hence our sensing strategy is easy to be expended to the ultrasensitive detection of other targets, e.g., DNAs, RNAs, and other molecules that have corresponding DNA aptamers.

Cryostructuring of Polymeric Systems. 52. Properties, Microstructure and an Example of a Potential Biomedical Use of the Wide-Pore Alginate Cryostructurates

Wide-pore cryostructurates were prepared via freezing sodium alginate aqueous solutions with subsequent ice sublimation from the frozen samples, followed by their incubation in the ethanol solutions of calcium chloride or sulfuric acid, rinsing, and final drying. Such sequence of operations resulted in the calcium alginate or alginic acid sponges, respectively. The swelling degree of the walls of macropores in such matrices decreased with increasing polymer concentration in the initial solution. The dependence of the degree of swelling on the cryogenic processing temperature had a bell-like character with a maximum for the samples formed at -20 °C. According to 1H NMR spectroscopy, the content of mobile (non-frozen) water in the frozen water-sodium alginate systems also depended on the initial polymer concentration and freezing temperature. The cryostructurates obtained did not lose their integrity in water, saline, in an acidic medium at pH 2 for at least three weeks. Under alkaline conditions at pH 12 the first signs of dissolution of the Ca-alginate sponge arose only after a week of incubation. Microbiological testing of the model depot form of the antibiotics entrapped in the Ca-alginate cryostructurate demonstrated the efficiency of this system as the antibacterial material.