Silver-coated magnetite-carbon core-shell microspheres as substrate-enhanced SERS probes for detection of trace persistent organic pollutants

Analysis and remediation of phthalates in aquatic matrices: current perspectives

Phthalic acid esters (PAEs) are high production volume chemicals used extensively as plasticizers, to increase the flexibility of the main polymer. They are reported to leach into their surroundings from plastic products and are now a ubiquitous environmental contaminant. Phthalate levels have been determined in several environmental matrices, especially in water. These levels serve as an indicator of plasticizer abuse and plastic pollution, and also serve as a route of exposure to different species including humans. Reports published on effects of different PAEs on experimental models demonstrate their carcinogenic, teratogenic, reproductive, and endocrine disruptive effects. Therefore, regular monitoring and remediation of environmental water samples is essential to ascertain their hazard quotient and daily exposure levels. This review summarises the extraction and detection techniques available for phthalate analysis in water samples such as chromatography, biosensors, immunoassays, and spectroscopy. Current remediation strategies for phthalate removal such as adsorption, advanced oxidation, and microbial degradation have also been highlighted.

Environmental photochemistry with thiol- and silica-modified plasmonic nanocomposites: SERS sensing of municipal solid waste and tannery waste leachate from groundwater

Mismanagement of obsolete solid waste generates a massive deteriorating effect on the environment. There is a high level of open trash disposal contaminating its neighboring water bodies. This despoliation trash causes an endangerment to the living environment. The waste management act is to hinder harmful effects on human beings, animals, plants, and their natural environment through the principles of waste prevention, waste processing, and waste disposal. Surface-enhanced Raman scattering (SERS) enhances the hazardous chemical sensing of environmental pollutants. To vigorously focus on the leaching of a couple of landfills in groundwater and surface water, an unusual combination of SERS-based poly vinyl thiol and silica-modified silver nanocomposites (PVT/SiO2@Ag NCs) was synthesized. The optical, crystalline, and structural properties of PVT/SiO₂@Ag NCs were described with UV-visible spectroscopy (UV-Vis), X-ray diffractometer (XRD), transmission electron microscope (TEM), and energy-dispersive X-ray analysis (EDX). The surface plasmon resonance (SPR) is detected at 403 nm from the PVT/SiO₂@Ag NPs. The average crystallite size of PVT/SiO₂ @ Ag NCs is estimated using the Scherrer formula as 11 nm. The calculated specific surface area (SSA), strains, and dislocation densities demonstrate the improved mechanical properties of the substrate. The well-separated spherical shape of NPs is also observed, and the composition of silica and sulfur element in addition of Ag was confirmed by EDAX. Negatively charged SiO₂ were bound strongly with the SH group and Ag NPs through electrostatic interaction mechanism as S-Ag-O-Si-O-Ag-S. SERS sensitivity is demonstrated by the prepared nanoparticles using an environmentally ignored leachate of municipal solid waste (MSW) and tannery waste (TW) landfill. PVT/SiO₂@Ag NCs has detected the presence of innards of MSW leachate viz., aromatic hydrocarbon, phenols, phthalates, and pesticide from the groundwater. Furthermore, the TW leachate compositions of benzenes, hydrocarbons, amines, and chromium VI were analytically identified. Also, the leaching of TW leachate was confirmed in the water samples referred. Hence, this study provides a novel SERS sensor of PVT/SiO₂@Ag NCs in the tile to detect and analyze environmentally ignored organic and inorganic compounds.

Selective detection of tartaric acid using amino acid interlinked silver nanoparticles as a colorimetric probe

A variety of biomolecules with different functional groups play critical roles in almost all the processes occurring in living cells. Interaction of metallic nanoparticles (NPs) with various biomolecules generates a layer of molecules on their surface, and this biomolecular rich layer formed on the NP surface is described as a "biomolecular corona". The physicochemical properties of the NPs, including size, adsorption affinity, and charge on the particles' surfaces are the major factors influencing the characteristics of this corona. The formation of various biomolecular corona has been studied well, whereas the amino acid corona is relatively new by exploring their stability. In the present study, a novel formation of an amino acid corona with a fundamental interaction mechanism for a selective detection procedure using a colorimetric platform has been proposed. Herein, amino acid-coated silver NPs (AgNPs) have been used as a template with spectroscopic (steady state UV-Vis, FTIR) and imaging (HR-TEM, DLS) techniques. Our findings demonstrated that among different amino acid coronas, glutathione (GSH) stabilized AgNPs show a rapid reaction with tartaric acid. The extent and thermodynamics of the formed complex between the GSH/AgNPs and tartaric acid have also been studied and this suggested that the complex formed is spontaneous and energy releasing in nature.

Evaluating the detection efficacy of advanced bimetallic plasmonic nanoparticles for heavy metals, hazardous materials and pesticides of leachate in contaminated groundwater

During the decomposition of trashes, leachate is created and leaching is gradually pollutes the surface and groundwater. Thus, the most severe ecological impact is the risk of ground water pollution because of collection of leachate from unlined insecure landfills. Due to the low biodegradable organic strength, irregular productivity and composition, the environmentally neglected landfill leachate treatment is challenging. This work was conducted on a synthetically effective bimetallic surface enhanced Raman spectroscopic (SERS) nanosensor by gold/silver-bimetallic nanoparticles (Au/Ag-NPs), and used for the specific detection of municipal solid waste (MSW) landfill leachate in groundwater. The optical study of Au/Ag-NPs led to reflections from Ag cores and small Au shells. The structural studies represent the FCC structure of Au/Ag-NPs. The core-shell nanocrevice NPs with particle size of 23 nm played an important role with plasmonic behaviour enhances the electromagnetic excitation to achieve SERS detection and plasmonic photocatalysis. Thus, obtained results clearly show that Au was successfully added to Ag-NPs, and its existence can also be confirmed by energy dispersive spectroscopy (EDAX). The prepared SERS based sensors have the potential to detect aromatic hydrocarbon, pesticides and heavy metals from environmentally ignored MSW landfill leachate. In general, the application of this new synergetic strategy of the photocatalytic degradation of leachate was irradiated by visible wavelength with the rate constant of 0.0036/min, 0.0047/min and 0.005/min by Ag-NPs, Au-NPs and Au/Ag-NPs respectively. Overall, this is the only study achieved efficiently with photocatalytic degradation and SERS detection of environmentally ignored real sample (leachate) to make pollutant free homeland aquifers.

Silver Nanoparticles Grown on Cross-Linked Poly (Methacrylic Acid) Microspheres: Synthesis, Characterization, and Antifungal Activity Evaluation

Silver nanoparticles (AgNPs) exert profound physicochemical, biological, and antimicrobial properties, therefore, they have been extensively studied for a variety of applications such as food packaging and cultural heritage protection. However, restrictions in their stability, aggregation phenomena, and toxicity limit their extensive use. Hence, the use of functional substrates that promote the silver nanoparticles’ growth and allow the formation of uniform-sized, evenly distributed, as well as stable nanoparticles, has been suggested. This study reports on the fabrication and the characterization of hydrophilic polymer spheres including nanoparticles with intrinsic antifungal properties. Poly (methacrylic acid) microspheres were synthesized, employing the distillation precipitation method, to provide monodisperse spherical substrates for the growth of silver nanoparticles, utilizing the co-precipitation of silver nitrate in aqueous media. The growth and the aggregation potential of the silver nanoparticles were studied, whereas the antifungal activity of the produced nanostructures was evaluated against the black mold-causing fungus Aspergillus niger. The produced structures exhibit dose-dependent antifungal activity. Therefore, they could potentially be employed for the protection and preservation of cultural heritage artifacts and considered as new agents for food protection from fungal contamination during storage.

Strategies for SERS Detection of Organochlorine Pesticides

Organochlorine pesticides (OCPs) embody highly lipophilic hazardous chemicals that are being phased out globally. Due to their persistent nature, they are still contaminating the environment, being classified as persistent organic pollutants (POPs). They bioaccumulate through bioconcentration and biomagnification, leading to elevated concentrations at higher trophic levels. Studies show that human long-term exposure to OCPs is correlated with a large panel of common chronic diseases. Due to toxicity concerns, most OCPs are listed as persistent organic pollutants (POPs). Conventionally, separation techniques such as gas chromatography are used to analyze OCPs (e.g., gas chromatography coupled with mass spectrometry (GC/MS)) or electron capture detection (GC/ECD). These are accurate, but expensive and time-consuming methods, which can only be performed in centralized lab environments after extensive pretreatment of the collected samples. Thus, researchers are continuously fueling the need to pursue new faster and less expensive alternatives for their detection and quantification that can be used in the field, possibly in miniaturized lab-on-a-chip systems. In this context, surface enhanced Raman spectroscopy (SERS) represents an exceptional analytical tool for the trace detection of pollutants, offering molecular fingerprint-type data and high sensitivity. For maximum signal amplification, two conditions are imposed: an efficient substrate and a high affinity toward the analyte. Unfortunately, due to the highly hydrophobic nature of these pollutants (OCPs,) they usually have a low affinity toward SERS substrates, increasing the challenge in their SERS detection. In order to overcome this limitation and take advantage of on-site Raman analysis of pollutants, researchers are devising ingenious strategies that are synthetically discussed in this review paper. Aiming to maximize the weak Raman signal of organochlorine pesticides, current practices of increasing the substrate's performance, along with efforts in improving the selectivity by SERS substrate functionalization meant to adsorb the OCPs in close proximity (via covalent, electrostatic or hydrophobic bonds), are both discussed. Moreover, the prospects of multiplex analysis are also approached. Finally, other perspectives for capturing such hydrophobic molecules (MIPs-molecularly imprinted polymers, immunoassays) and SERS coupled techniques (microfluidics-SERS, electrochemistry-SERS) to overcome some of the restraints are presented.

Development of affinity between target analytes and substrates in surface enhanced Raman spectroscopy for environmental pollutant detection

Environmental pollution has long been a social concern due to the variety of pollutants and their wide distribution, persistence and being detrimental to health. It is therefore necessary to develop rapid and sensitive strategies to trace and detect these compounds. Among various detection methodologies, surface enhanced Raman spectroscopy (SERS) has become an attractive option as it enables accurate analyte identification, simple sample preparation, rapid detection and ultra-high sensitivity without any interference from water. For SERS detection, an essential yet challenging step is the effective capture of target analytes onto the surface of metal nanostructures with a high intensity of enhanced electromagnetic field. This review has systematically summarized recent advances in developing affinity between targets and the surface of SERS substrates via direct adsorption, hydrophobic functional groups, boronate affinity, metal organic frameworks (MOFs), DNA aptamers and molecularly imprinted polymers (MIPs). At the end of this review, technical limitations and outlook have been provided, with suggestions on optimizing SERS techniques for real-world applications in environmental pollutant detection.

Characterization of the iron oxide phases formed during the synthesis of core-shell FexOy@C nanoparticles modified with Ag

Core-shell Fe_xO_y@C nanoparticles (NPs) modified with Ag were studied with x-ray diffraction, transmission electron microscopy, energy dispersive elemental mapping, Mössbauer spectroscopy, static magnetic measurements, and optical magnetic circular dichroism (MCD). Fe_xO_y@C NPs synthesized by the pyrolysis process of the mixture of Fe(NO₃)₃ · 9H₂O with oleylamine and oleic acid were added to a heated mixture of oleylamine and AgNO₃ in different concentrations. The final product was a mixture of iron oxide crystalline NPs in an amorphous carbon shell and Ag crystalline NPs. The iron oxide NPs were presented by two magnetic phases with extremely close crystal structures: Fe₃O₄ and γ-Fe₂O₃. Ag is shown to form crystalline NPs located very close to the iron oxide NPs. An assumption is made about the formation of hybrid Fe_xO_y@C-Ag NPs. Correlations were obtained between the Ag concentration in the fabricated samples, their magnetic properties and the MCD spectrum shape. Introducing Ag led to a approximately linear decrease of the NPs saturation magnetization depending upon the Ag concentration, it also resulted into the MCD spectrum shift to the lower light wave energies. MCD was also studied for the Fe₃O₄@C NPs synthesized earlier with the same one-step process using different heat treatment temperatures, and MCD spectra were compared for two series of NPs. A possible contribution of the surface plasmon excitation in Ag NPs to the MCD spectrum of the Fe_xO_y@C-Ag NPs is discussed.

ZIF-8-modified Au-Ag/Si nanoporous pillar array for active capture and ultrasensitive SERS-based detection of pentachlorophenol

A novel SERS substrate based on a zeolitic imidazolate framework-8 (ZIF-8) film-modified Au-Ag/Si nanoporous pillar array (ZIF-8/Au-Ag/Si-NPA) was successfully fabricated for pentachlorophenol (PCP) detection. The Au-Ag/Si-NPA was synthesized through immersion plating and replacement reaction on the Si-NPA, which was prepared by the hydrothermal etching. The ZIF-8 film was coated via layer-by-layer growth technique. The ZIF-8 film is nanoporous and its thickness can be controlled by varying the growing number, which can significantly influence the SERS performance of the substrate. The substrate with optimal ZIF-8 thickness exhibited an excellent SERS response to PCP molecules. The SERS enhancement factor reached up to 1.8 × 10⁷ and the detection limit was down to 10^-13 M. Moreover, the substrate showed good uniformity with a relative standard deviation (RSD) of 8.7% and good selectivity. The PCP detection is hardly interfered by the coexisting organic compounds. The high SERS performance may be due to the enrichment effect of the ZIF-8 film. The ZIF-8 film could capture and enrich the trace PCP molecules by electrostatic interaction between the negatively charged PCP^- and the positively charged ZIF-8. This work suggests that the ZIF-8/Au-Ag/Si-NPA substrate has potential application in SERS analysis of the polar organic pollutant detection in environmental media.

Surface enhanced Raman scattering substrate for the detection of explosives: Construction strategy and dimensional effect

Surface-enhanced Raman spectroscopy (SERS) technology has been reported to be able to quickly and non-destructively identify target analytes. SERS substrate with high sensitivity and selectivity gave SERS technology a broad application prospect. This contribution aims to provide a detailed and systematic review of the current state of research on SERS-based explosive sensors, with particular attention to current research advances. This review mainly focuses on the strategies for improving SERS performance and the SERS substrates with different dimensions including zero-dimensional (0D) nanocolloids, one-dimensional (1D) nanowires and nanorods, two-dimensional (2D) arrays, and three-dimensional (3D) networks. The effects of elemental composition, the shape and size of metal nanoparticles, hot-spot structure and surface modification on the performance of explosive detection are also reviewed. In addition, the future development tendency and application of SERS-based explosive sensors are prospected.

A Review of Chinese Raman Spectroscopy Research Over the Past Twenty Years

This paper introduces the major Chinese research groups in the fields of biomedicine, food safety, environmental testing, material research, archaeological and cultural relics, gem identification, forensic science, and other research areas of Raman spectroscopy and combined methods spanning the two decades from 1997 to 2017. Briefly summarized are the research directions and contents of the major Chinese Raman spectroscopy research groups, giving researchers engaged in Raman spectroscopy research a more comprehensive understanding of the state of Chinese Raman spectroscopy research and future development trends to further develop Raman spectroscopy and its applications.

A physical approach for the estimation of the SERS enhancement factor through the enrichment and separation of target molecules using magnetic adsorbents

The controllable synthesis of nanosized Fe₃O₄ (10–20 nm) encapsulated in different numbers of graphene layers (1–5 layers) (Fe₃O₄@DGL NPs) was realized through a facile and green hydrothermal reaction at a temperature as low as 200 °C.

SERS using two-photon polymerized nanostructures for mycotoxin detection

Improved chemical- and bio-sensing with Surface Enhanced Raman Spectroscopy (SERS) requires nanostuctures that can be flexibly designed and fabricated with different physical and optical properties. Here, we present nano-pillar arrays ranging from 200 nm to 600 nm as SERS substrates for mycotoxin detection that are fabricated by means of two-photon polymerization. We built a nominal shape and a voxel-based model for simulating the enhancement of the electric field of the nano-pillar arrays using the Finite-Difference Time-Domain (FDTD) method. A new model was built based on the Atomic Force Microscopy (AFM) data obtained from the fabricated nanostructures and introduced into a FDTD model. We demonstrated the enhancement behavior by measuring the Raman spectrum of Rhodamine B solutions. Both the simulations and experimental results suggest that the 200 nm nano-pillar array has the highest Enhancement Factor (EF). Besides, we determined the limit of detection of the 200 nm pillar array by performing Raman measurements on Rhodamine B solutions with different concentrations. The detection limit of our 200 nm nano-pillar array is 0.55 μM. Finally we discriminated 1 ppm deoxynivalenol and 1.25 ppm fumonisin b1 in acetonitrile solutions by our SERS substrate in combination with principal component analysis. This versatile approach for SERS substrates fabrication gives new opportunities for material characterization in chemical and biological applications.

Nano-pillar arrays are presented ranging from 200 nm to 600 nm as SERS substrates for mycotoxin detection, fabricated by two-photon polymerization. This versatile approach gives new opportunities for material characterization in chemical and biological applications.

Optimization of a hybrid plasmonic configuration: particle on a corrugated film and its SERS application

Hybrid SERS configurations, which combine manufactured metallic chips with nanoparticles, have emerged as powerful and promising SERS substrates because they not only provide cost-effective and high-yield manufacture, but also demonstrate excellent sensitivity and outstanding reproducibility. Herein, a plasmonic hybrid structure, a particle on an Au film over nanoparticles (particle-AuFON) configuration, was studied for SERS application. In a previous study, we constructed a hybrid substrate by grafting Au@Ag core–shell NPs onto the AuFON structure. In this study, the hybrid substrate is designed and simulated to optimize electromagnetic enhancement while also affording exceptional uniformity, repeatability and stability, which are essential factors in SERS applications. This hybrid substrate provides good SERS performance with a detection limit of 1 × 10⁻¹⁰ M, which is 100-fold improvement compared to AuFON substrate or Au@Ag NPs. The excellent signal enhancement originates from the hotspot improvement and densification, as visualized by the FDTD calculations. Additional hotspots were created at the gaps between the Au@Ag NPs and the AuFON, thus improving the density of hotspots. Moreover, the intensity of the hotspots was improved due to EM coupling between the original hotspots and additional hotspots. To validate the feasibility of this hybrid substrate in SERS-based detection, melamine was detected as an example. The detection limit was 10 nM, which was much lower than the maximum limit of melamine in infant formula (1 ppm) legislated by the governments of both the United States and China. A calibration curve was plotted between the SERS intensity and melamine concentration with a correlation coefficient of 0.98. This hybrid SERS substrate shows great potential in SERS-based sensing and imaging, as it provides high sensitivity and outstanding reproducibility with a simple fabrication procedure, facilitating the cost-effective and high-yield manufacture of SERS substrates.

A plasmonic hybrid structure of particles on a Au film over nanoparticles (particle-AuFON) configuration was studied for application in SERS. It showed great potential in SERS-based sensing and it provides outstanding uniformity, repeatability and stability.

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.

Bimetallic plasmonic Au@Ag nanocuboids for rapid and sensitive detection of phthalate plasticizers with label-free surface-enhanced Raman spectroscopy

Phthalate plasticizers (PAEs) are posing a serious threat to human health, so it is urgent to develop effective and reliable ways to detect the food additives PAEs sensitively. In this study, we have reported plasmonic bimetallic Au@Ag core-shell nanocuboids for the rapid and sensitive detection of PAEs in liquor samples with a label-free Surface-enhanced Raman Spectroscopy (SERS) strategy. Compared with single-element nanostructures, the bimetallic SERS platform can integrate two distinct functions into a single entity with unprecedented properties. Consequently, we synthesized Au@Ag nanocuboids (Au@Ag NCs) composed of a Au nanorod (Au NR) core and a Ag cuboid shell, which could produce richer and broader plasmonic resonance modes than Au NRs. It is obvious that the SERS signals of crystal violet (CV) and butyl benzyl phthalate (BBP) reached a maximum as the thickness of the Ag coating shell was in a certain threshold and there was a strong dependence of the Raman enhancement on the Ag cuboid shell-thickness. Based on the optimized size, the sensitivity and repeatability of Au@Ag NCs were evaluated with limits of detection (LODs) at around 10^-9 M both for BBP and diethylhexyl phthalate (DEHP). In addition, the SERS active substrate core-shell Au@Ag NCs can be used to detect BBP as low as 1.3 mg kg^-1 spiked into the liquor samples. Thereby, the unique bimetallic Au@Ag NCs showed a huge potential for the rapid and sensitive detection of PAEs in liquor samples.

Tunable Silver Nanoparticle Arrays by Hot Embossing and Sputter Deposition for Surface-Enhanced Raman Scattering

Surface-enhanced Raman scattering (SERS) spectroscopy has attracted a lot of attention over the past 30 years. Due to its extreme sensitivity and label-free detection capability, it has shown great potential in areas such as analytical chemistry, biochemistry, and environmental science. However, the major challenge is to manufacture large-scale highly SERS active substrates with high controllability, good reproducibility, and low cost. In this study, we report a novel method to fabricate uniform silver nanoparticle arrays with tunable particle sizes and interparticle gaps. Using hot embossing and sputtering techniques, we were able to batch produce the silver nanoparticle arrays SERS active substrate with consistent quality and low cost. We showed that the proposed SERS active substrate has good uniformity and high reproducibility. Experimental results show that the SERS enhancement factor is affected by silver nanoparticles size and interparticle gaps. Furthermore, the enhancement factor of the SERS signal obtained from Rhodamine 6G (R6G) probe molecules was as high as 1.12 × 107. Therefore, the developed method is very promising for use in many SERS applications.

Au Nanoparticles Deposited on Magnetic Carbon Nanofibers as the Ultrahigh Sensitive Substrate for Surface-Enhanced Raman Scattering: Detections of Rhodamine 6G and Aromatic Amino Acids

Surface-enhanced Raman scattering (SERS) is a unique spectroscopy that can offer high-sensitive detection for many molecules. Herein, the Au particles deposited on carbon nanofiber-encapsulated magnetic Ni nanoparticles (NPs) (Ni@CNFs@Au) have been successfully synthesized for SERS measurements. The Ni@CNFs@Au substrates have the advantages of a high SERS sensitivity and good magnetic response. The Ni@CNFs could be directly obtained from CO₂ hydrogenation on a Ni catalyst, which has been characterized as having rich carboxylic acid groups, graphitic structures, and a high surface area. The Ni@CNFs surface could effectively increase the density of hotspots during Au NP aggregation and influence the morphology of the Au nanostructures. The spherical shape, oval shape, and coral-like Au nanostructures were prepared on Ni@CNFs with various Au concentrations. Brunauer-Emmett-Teller, zeta potential, high-resolution transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy measurements were used to characterize the Ni@CNFs@Au samples. The Au NPs deposited on the Ni@CNFs presented a suitable oval shape, and an average size of ∼30-40 nm. The size allowed surprisingly ultrasensitive SERS detection of rhodamine 6G (R6G) with a resolution of approximately a single molecule under an excitation wavelength of 532 nm. Using 785 nm excitation, a low R6G concentration of ∼1 × 10^-14 M was detected. Moreover, the Ni@CNFs@Au substrates could be rapidly magnetically separated after adsorption. Phenylalanine and tyrosine amino acids, which are associated with the liver disease, were examined using SERS with the Ni@CNFs@Au substrate. Ultralow concentrations of ∼1 × 10^-11 M for phenylalanine and ∼1 × 10^-13 M for tyrosine were clearly measured. The Ni@CNFs@Au substrates exhibited applicability as excellent SERS detection platforms that combine high-sensitivity and rapid magnetic separation for various adsorption molecules.

Surface Modification of Magnetic Iron Oxide Nanoparticles

Functionalized iron oxide nanoparticles (IONPs) are of great interest due to wide range applications, especially in nanomedicine. However, they face challenges preventing their further applications such as rapid agglomeration, oxidation, etc. Appropriate surface modification of IONPs can conquer these barriers with improved physicochemical properties. This review summarizes recent advances in the surface modification of IONPs with small organic molecules, polymers and inorganic materials. The preparation methods, mechanisms and applications of surface-modified IONPs with different materials are discussed. Finally, the technical barriers of IONPs and their limitations in practical applications are pointed out, and the development trends and prospects are discussed.

Detection and Characterization of Antibiotic-Resistant Bacteria Using Surface-Enhanced Raman Spectroscopy

This mini-review summarizes the most recent progress concerning the use of surface-enhanced Raman spectroscopy (SERS) for the detection and characterization of antibiotic-resistant bacteria. We first discussed the design and synthesis of various types of nanomaterials that can be used as the SERS-active substrates for biosensing trace levels of antibiotic-resistant bacteria. We then reviewed the tandem-SERS strategy of integrating a separation element/platform with SERS sensing to achieve the detection of antibiotic-resistant bacteria in the environmental, agri-food, and clinical samples. Finally, we demonstrated the application of using SERS to investigate bacterial antibiotic resistance and susceptibility as well as the working mechanism of antibiotics based on spectral fingerprinting of the whole cells.

Semi-quantitative analysis of multiple chemical mixtures in solution at trace level by surface-enhanced Raman Scattering

Surface-enhanced Raman scattering (SERS) technology combines with chemometric method of principal component analysis (PCA) was used to calculate the composition of chemical mixtures in solution. We reported here that there exists composition discrepancy between molecules in solution and molecules adsorbed on Ag@Al₂O₃ nanorods substrates due to difference in adsorption kinetics of each component. We proposed here a way to calculate the adsorption kinetics factor for each component using a standard sample as the reference, with which one could correct the predictions given by PCA. We demonstrate the validity of this approach in estimating the compositions of mixtures with two, three and four components of 1, 4-Benzenedithiol, 2-Naphthalenethiol, 4-Mercaptobenzoic acid, and 4-Mercaptopyridine molecules, with acceptable errors. Furthermore, a general formula applied to more complex mixtures was proposed to calculate compositions in solution.

Vancomycin-modified Fe3O4@SiO2@Ag microflowers as effective antimicrobial agents

Nanomaterials combined with antibiotics exhibit synergistic effects and have gained increasing interest as promising antimicrobial agents. In this study, vancomycin-modified magnetic-based silver microflowers (Van/Fe3O4@SiO2@Ag microflowers) were rationally designed and prepared to achieve strong bactericidal ability, a wide antimicrobial spectrum, and good recyclability. High-performance Fe3O4@SiO2@Ag microflowers served as a multifunction-supporting matrix and exhibited sufficient magnetic response property due to their 200 nm Fe3O4 core. The microflowers also possessed a highly branched flower-like Ag shell that provided a large surface area for effective Ag ion release and bacterial contact. The modified-vancomycin layer was effectively bound to the cell wall of bacteria to increase the permeability of the cell membrane and facilitate the entry of the Ag ions into the bacterium, resulting in cell death. As such, the fabricated Van/Fe3O4@SiO2@Ag microflowers were predicted to be an effective and environment-friendly antibacterial agent. This hypothesis was verified through sterilization of Gram-negative Escherichia coli and Gram-positive methicillin-resistant Staphylococcus aureus, with minimum inhibitory concentrations of 10 and 20 μg mL-1, respectively. The microflowers also showed enhanced effect compared with bare Fe3O4@SiO2@Ag microflowers and free-form vancomycin, confirming the synergistic effects of the combination of the two components. Moreover, the antimicrobial effect was maintained at more than 90% after five cycling assays, indicating the high stability of the product. These findings reveal that Van/Fe3O4@SiO2@Ag microflowers exhibit promising applications in the antibacterial fields.

Silver coated magnetic microflowers as efficient and recyclable catalysts for catalytic reduction

Synthesis of highly-branched Fe₃O₄@SiO₂@Ag microflowers and their use as efficient and recyclable catalysts for catalytic reduction.

Polyethyleneimine-mediated seed growth approach for synthesis of silver-shell silica-core nanocomposites and their application as a versatile SERS platform

PEI-mediated seed growth approach for synthesis of high performance silver-shell silica-core nanocomposites.

Fabrication of Fe3O4@SiO2@Ag magnetic–plasmonic nanospindles as highly efficient SERS active substrates for label-free detection of pesticides

Magnetic–plasmonic nanospindles serve as SERS substrates with controllable aggregation due to steady enrichment of mass molecules nearby abundant hot spots.

Hotspots engineering by grafting Au@Ag core-shell nanoparticles on the Au film over slightly etched nanoparticles substrate for on-site paraquat sensing

Paraquat (PQ) pollutions are ultra-toxic to human beings and hard to be decomposed in the environment, thus requiring an on-site detection strategy. Herein, we developed a robust and rapid PQ sensing strategy based on the surface-enhanced Raman scattering (SERS) technique. A hybrid SERS substrate was prepared by grafting the Au@Ag core-shell nanoparticles (NPs) on the Au film over slightly etched nanoparticles (Au FOSEN). Hotspots were engineered at the junctions as indicated by the finite difference time domain calculation. SERS performance of the hybrid substrate was explored using p-ATP as the Raman probe. The hybrid substrate gives higher enhancement factor comparing to either the Au FOSEN substrate or the Au@Ag core-shell NPs, and exhibits excellent reproducibility, homogeneity and stability. The proposed SERS substrates were prepared in batches for the practical PQ sensing. The total analysis time for a single sample, including the pre-treatment and measurement, was less than 5min with a PQ detection limit of 10nM. Peak intensities of the SERS signal were plotted as a function of the PQ concentrations to calibrate the sensitivity by fitting the Hill's equation. The plotted calibration curve showed a good log-log linearity with the coefficient of determination of 0.98. The selectivity of the sensing proposal was based on the "finger print" Raman spectra of the analyte. The proposed substrate exhibited good recovery when it applied to real water samples, including lab tap water, bottled water, and commercially obtained apple juice and grape juice. This SERS-based PQ detection method is simple, rapid, sensitive and selective, which shows great potential in pesticide residue and additives abuse monitoring.

Dual-Mode Encoded Magnetic Composite Microsphere Based on Fluorescence Reporters and Raman Probes as Covert Tag for Anticounterfeiting Applications

Utilizing fluorescence reporters and SERS probes as the security labels, a series of dual-mode encoded magnetic composite microspheres with micrometer size was designed and prepared for anticounterfeiting applications. At first, the micro-meter-sized melamine formaldehyde microspheres with different fluorescence molecules (FMF) were prepared by precipitation polymerization, and then the magnetite composite microspheres (FMF/MNPs) were fabricated by direct immobilization of magnetic nanoparticles (MNPs) onto the surface of FMF microspheres. After deposition of Ag nanoparticles (Ag-NPs) onto FMF/MNPs microspheres, the SERS probes were absorbed onto the surface of Ag-NPs, and then a protection layer of silica was coated on the composite microspheres by Stöber method. The combination of different fluorescence reporters and SERS probes greatly increased the encoding complexity and volume for high-level anticounterfeiting. The structure of the dual-encoded FMF/MNPs/Ag-NPs/SiO2 composite microspheres was characterized by FESEM, TEM, FLS(fluorescence spectrometer), XRD, VSM, UV-vis and EDS. The embedded magnetic nanoparticles enable the composite microspheres to be quickly isolated from the marked latex paint by magnet at the concentration of as low as 1 ppm, and the covert tag information can be read out even from one composite microsphere. In addition, the covert security information in the marked coating film can be also read out in situ and the existence of the composite microspheres does not influence the visible appearance of the coating film. All the above outstanding properties will make these dual-mode encoded composite microspheres as advanced security tags for next-generation anticounterfeiting applications.

Orientation-dependent nanostructure arrays based on anisotropic silicon wet-etching for repeatable surface-enhanced Raman scattering

Repeatable fabrication of sensitive plasmonic substrates through a simple procedure has become a major challenge for SERS-based sensing and imaging. Herein, a new class of high-performance SERS substrates, including pyramid, ridged-hexagon, and quasi-triangle nanostructures, is successfully fabricated based on the nanosphere lithography technique and anisotropic wet etching. Using the wafer-scale Cr-hole array as the etching mask, cavity-templates of various configurations are fabricated by the orientation-dependent wet etching technique, from where the nanostructure arrays are finally peeled-off. The anisotropic wet etching on (100), (110), and (111) silicon wafers has been systematically studied at the nanoscale revealing the formation mechanism of these cavity-templates. The peeled-off nanostructure arrays provide high-density tips and/or gaps (about 2.5 × 10(7) mm(-2)) and thus facilitate the generation of "hot spots". The distribution of the electromagnetic field is visualized by the finite difference time domain calculation. And the calculation results are validated by SERS characterization. The SERS enhancement factors of these substrates are in the order of 10(6)-10(7), with the maximum enhancement factor of 1.32 × 10(7) yielded by the ridged-hexagon arrays. The proposed nanostructure arrays present excellent homogeneity and reproducibility (with the largest relative standard deviation of 16.43%) for the reason that the SERS-active substrates are peeled-off from an identical template. The cost-effective fabrication, high sensitivity, good homogeneity and well-performed reproducibility demonstrate that these orientation-dependent NSs are good candidates for SERS-based in vitro and in situ detection and biosensing.

Detection of dimethyl methylphosphonate by thin water film confined surface-enhanced Raman scattering method

It is important and necessary to effectively detect the chemical warfare agents, such as highly toxic never agent sarin. However, based on the surface-enhanced Raman scattering (SERS) effect, detection of nerve agent simulant dimethyl methylphosphonate (DMMP) which is weakly interacted with SERS-active substrate has been the most challenge for the routine SERS detection method. To overcome this challenge, we put forward a thin water film confined SERS strategy. Under the space-confinement of water film, Raman measurements are carried out in the water evaporation process. The subsequent water evaporation induces concentrating of the DMMP molecules, which are thus successfully restricted within the strong electromagnetic field enhanced area above the SERS substrates, leading to the enhancement of their Raman signals. This study provides a new way to achieve the efficient SERS-based detection of the target molecules weakly interacted with the metal substrates.

Highly Sensitive and Reproducible SERS Performance from Uniform Film Assembled by Magnetic Noble Metal Composite Microspheres

To realize highly sensitive and reproducible SERS performance, a new route was put forward to construct uniform SERS film by using magnetic composite microspheres. In the experiment, monodisperse Fe3O4@SiO2@Ag microspheres with hierarchical surface were developed and used as building block of SERS substrate, which not only realized fast capturing analyte through dispersion and collection under external magnet but also could be built into uniform film through magnetically induced self-assembly. By using R6G as probe molecule, the as-obtained uniform film exhibited great improvement on SERS performance in both sensitivity and reproducibility when compared with nonuniform film, demonstrating the perfect integration of high sensitivity of hierarchal noble metal microspheres and high reproducibility of ordered microspheres array. Furthermore, the as-obtained product was used to detect pesticide thiram and also exhibited excellent SERS performance for trace detection.

A rapid SERS method for label-free bacteria detection using polyethylenimine-modified Au-coated magnetic microspheres and Au@Ag nanoparticles

A rapid and efficient method for label-free SERS detection of bacteria in solution.

Synthesis of ketoxime-functionalized Fe3O4@C core–shell magnetic microspheres for enhanced uranium(vi) removal

Ketoxime-functionalized carbon coated iron oxide (Fe₃O₄@C–KO) was synthesized and characterized. The adsorption property of uranium was also investigated.

Recyclable SERS substrates based on Fe2O3–Ag hybrid hollow microspheres with crumpled surfaces

Fe₂O₃–Ag hybrid hollow microspheres with crumpled surfaces were prepared as SERS recyclable substrates for 4-aminobenzenethiol (4-ABT) and toxin protein ricin.

Sensitivity and Reusability of SiO2 NRs@ Au NPs SERS Substrate in Trace Monochlorobiphenyl Detection

Surface-enhanced Raman scattering (SERS) effect is quite preferred to detect trace pollutants, and reusable SERS substrate is of important practical value. In this research, a kind of effective SiO2 nanorods (NRs)@ Au nanoparticles (NPs) substrate was fabricated completely with physical methods, and it was quite sensitive so that 1 × 10(-6) M monochlorobiphenyl (CB) could be detected. Furthermore, congeners of CB could be detected by reusing this kind of SERS substrate, and the cleaning treatment between every two detections was very simple. The excellent performance of the reusable SERS substrate indicated its great application potential.