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

Selective and sensitive detection of dimethyl phthalate in water using ferromagnetic nanomaterial-based molecularly imprinted polymers and SERS

To overcome the complicated pretreatment, low selectivity and low sensitivity detection associated with the detection of dimethyl phthalate (DMP), this study synthesized ferromagnetic nanomaterials that coupled with surface enhanced Raman scattering (SERS) and molecular imprinting polymers (MIPs). The pretreatment process can be simplified by ferromagnetic nanomaterials, then Fe3O4@SiO2@Ag@MIPs selectively adsorbing DMP can be achieved, and SERS can be applied for DMP detection with high sensitivity. As a control, the non-imprinted polymers (NIPs) Fe3O4@SiO2@Ag@NIPs were synthesized. Adsorption experiments results showed that the saturation adsorption amounts of Fe3O4@SiO2@Ag@MIPs is 36.74 mg/g with 40 mg/L DMP and Fe3O4@SiO2@Ag@NIPs is 17.45 mg/g. For DMP, Fe3O4@SiO2@Ag@MIPs have a greater affinity. In addition, after seven adsorption-desorption cycles the Fe3O4@SiO2@Ag@MIPs are reusable with approximately a 9.8 % loss in adsorption capacity. With an 8.7 × 10-9 M detection limit, DMP detection was performed by SERS, which revealed that the Raman intensities of the associated characteristic peak were linearly proportional to the DMP concentrations. As a result, the recovery rate of the testing artificial water varied from 87.9 % to 117 %. These outcomes show that the suggested technique for finding DMP in actual water samples is practical.

Facile fabrication of Au-Ag alloy nanoparticles/Ag nanowires SERS substrates with bimetallic synergistic effect for ultra-sensitive detection of crystal violet and alkali blue 6B

The bimetallic nanostructure of Au and Ag can integrate two distinct properties into a novel substrate compared to single metal nanostructures. This work presents a rapid and sensitive surface-enhanced Raman scattering (SERS) substrate for detecting illegal food additives and dyes of crystal violet (CV) and alkali blue 6B (AB 6B). Au-Ag alloy nanoparticles/Ag nanowires (Au-Ag ANPs/Ag NWs) were prepared by solid-state ionics method and vacuum thermal evaporation method at 5μA direct current electric field (DCEF), the molar ratio of Au to Ag was 1:18.34. Many 40 nm-140 nm nanoparticles regularly existed on the surface of Ag NWs with the diameters from 80 nm to 150 nm. The fractal dimension of Au-Ag ANPs/Ag NWs is 1.69 due to macroscopic dendritic structures. Compared with single Ag NWs, the prepared Au-Ag ANPs/Ag NWs substrates show superior SERS performance because of higher surface roughness, the SERS active of Ag NWs and bimetallic synergistic effect caused by Au-Ag ANPs, so the limit of detections (LOD) of Au-Ag ANPs/Ag NWs SERS substrates toward detection of CV and AB 6B were as low as 10-16mol/L and 10-9mol/L, respectively. These results indicate that Au-Ag ANPs/Ag NWs substrates can be used for rapid and sensitive detection of CV and AB 6B and have great development potential for detection of illegal food additives and hazardous substances in the fields of environmental monitoring and food safety.

Extralong hot-spots sensor for SERS sensitive detection of phthalate plasticizers in biological tear and serum fluids

Phthalate plasticizers (PAEs) illegally used in food pose a great threat to human health. A new and efficient sensing platform for the sensitive detection of the PAE residues in biological fluids needs to be designed and developed. Here, we report a simple and reliable surface-enhanced Raman spectroscopy (SERS) active platform with extralong hot spots of Au nanobipyramids@Ag nanorods (Au NBPs@Ag NRs) for the rapid and sensitive detection of PAEs in biological fluids. To achieve high activity, Au NBPs@Ag NRs with different shell lengths were fabricated by controlling the synthesis conditions, and the corresponding SERS properties were investigated by using crystal violet (CryV) and butyl benzyl phthalate (BBP). The experimental results showed that a longer shell length correlated to greater Raman activity, which was confirmed by finite-difference time-domain (FDTD) electromagnetic simulation. More importantly, the extralong hot spots of the Au NBPs@Ag NR SERS-active substrate showed excellent homogeneity and reproducibility for the CryV probe molecules (6.21%), and the detection limit was 10-9 M for both BBP and diethylhexyl phthalate (DEHP). Furthermore, through the standard addition method, an extralong hot spots SERS substrate could achieve highly sensitive detection of BBP and DEHP in serum and tears fluids, and the detection limit was as low as 3.52 × 10-8 M and 2.82 × 10-8 M. Therefore, the Au NBPs@Ag NR substrate with an extraordinarily long surface is efficient and versatile, and can potentially be used for high-efficiency sensing analysis in complex biological fluids.

Nanotechnology-based optical sensors for Baijiu quality and safety control

Baijiu quality and safety have received considerable attention owing to the gradual increase in its consumption. However, owing to the unique and complex process of Baijiu production, issues leading to quality and safety concerns may occur during the manufacturing process. Therefore, establishing appropriate analytical methods is necessary for Baijiu quality assurance and process control. Nanomaterial (NM)-based optical sensing techniques have garnered widespread interest because of their unique advantages. However, comprehensive studies on nano-optical sensing technology for quality and safety control of Baijiu are lacking. In this review, we systematically summarize NM-based optical sensor applications for the accurate detection and quantification of analytes closely related to Baijiu quality and safety. Furthermore, we evaluate the sensing mechanisms for each application. Finally, we discuss the challenges nanotechnology poses for Baijiu analysis and future trends. Overall, nanotechnological approaches provide a potentially useful alternative for simplifying Baijiu analysis and improving final product quality and safety.

A novel electrochemical aptasensor for ultrasensitive detection of herbicide prometryn based on its highly specific aptamer and Ag@Au nanoflowers

Herbicide prometryn has become a common pollutant in aquatic environments and caused adverse impacts on ecosystems. This study developed an ultrasensitive electrochemical aptasensor for prometryn based on its highly affinitive and specific aptamer and Ag@Au nanoflowers (Ag@AuNFs) for signal amplification. Firstly, this study improved the Capture-SELEX strategy to screen aptamers and obtained aptamer P60-1, which had a high affinity (Kd: 23 nM) and could distinguish prometryn from its structural analogues. Moreover, the typical stem-loop structure in aptamer P60-1 was found to be the binding pocket for prometryn. Subsequently, an electrochemical aptasensor for prometryn was established using multiwalled carbon nanotubes and reduced graphene oxide as electrode substrate, Ag@Au NFs as signal amplification element, and aptamer P60-1 as recognition element. The aptasensor had a detection range of 0.16-500 ng/mL and a detection limit of 60 pg/mL, which was much lower than those of existing detection methods. The aptasensor had high stability and good repeatability, and could specifically detecting prometryn. Furthermore, the utility of the aptasensor was validated by measuring prometryn in environmental and biological components. Therefore, this study provides a robust and ultrasensitive aptasensor for accurate detection for prometryn pollution.

Combining physical vapor deposition structuration with dealloying for the creation of a highly efficient SERS platform

Nanostructured noble metal thin films are highly studied for their interesting plasmonic properties. The latter can be effectively used for the detection of small and highly diluted molecules by the surface-enhanced Raman scattering (SERS) effect. Regardless of impressive detection limits achieved, synthesis complexity and the high cost of gold restrict its use in devices. Here, we report on a novel two-step approach that combines the deposition of a silver-aluminum thin film with dealloying to design and fabricate efficient SERS platforms. The magnetron sputtering technique was used for the deposition of the alloy thin film to be dealloyed. After dealloying, the resulting silver nanoporous structures revealed two degrees of porosity: macroporosity, associated to the initial alloy morphology, and nanoporosity, related to the dealloying step. The resulting nanoporous columnar structure was finely optimized by tuning deposition (i.e., the alloy chemical composition) and dealloying (i.e., dealloying media) parameters to reach the best SERS properties. These are reported for samples dealloyed in HCl and with 30 atom % of silver at the initial state with a detection limit down to 10^-10 mol·L^-1 for a solution of rhodamine B.

Effects of aging on environmental behavior of plastic additives: Migration, leaching, and ecotoxicity

Microplastics (MPs), an emerging pollutant, are of global concern due to their wide distribution and large quantities. In addition to MPs themselves, various additives within MPs (such as plasticizers, flame retardants, antioxidants and heavy metals) may also have harmful effects on the environment. Most of these additives are physically bound to plastics and can therefore be leached from the plastic and released into the environment. Aging of MPs in the actual environment can affect the migration and release of additives, further increasing the ecotoxicological risk of additives to organisms. This work reviews the functions of several commonly used additives in MPs, and summarizes the representative characterization methods. Furthermore, the migration and leaching of additives in the human environment and marine environment are outlined. As aging promotes the internal chain breaking of MPs and the increase of specific surface area, it in turn stimulates the release of additives. The hazards of additive exposure have been elucidated, and various studies from the laboratory have shown that more toxic additives such as phthalates and brominated flame retardants can disrupt a variety of biological processes in organisms, including metabolism, skeletal development and so on. Increase of MPs ecological risk caused by the leaching of toxic additives is discussed, especially under the effect of aging. This study presents a systematic summary of various functional and environmental behaviors of additives in plastics, using weathering forces as the main factor, which helps to better assess the environmental impact and potential risks of MPs.

Highly sensitive and selective photoelectrochemical aptasensing of di-2-ethylhexyl phthalate based on graphene quantum dots decorated TiO2 nanotube arrays

A photoelectrochemical (PEC) sensing platform for di-2-ethylhexyl phthalate (DEHP) was constructed using graphene quantum dots decorated TiO₂ nanotube arrays (GQDs-decorated TiO₂ NTs) as the transducer species and the anti-DEHP aptamer as the biological recognition element. GQDs were synthesized using the alkali-mediated hydrothermal method, and then anchored onto the TiO₂ NTs uniformly and intimately via pronounced electrostatic interaction. Coupling GQDs with TiO₂ NTs not only enhanced visible-light absorption, but promoted charge separation and transportation, exhibiting excellent photocurrent response, and PEC activity. Various means were conducted to explore morphologies, optical, structural and PEC properties of the materials. As an identification unit, the anti-DEHP aptamer molecules were immobilized on GQDs-decorated TiO₂ NTs using a cross-linking coupling method. The developed PEC sensing platform exhibits excellent sensing behavior for DEHP, and provides a low detection limit of 0.1 ng/L, high selectivity and stability. Meanwhile, its application in real environmental samples was evaluated and satisfying results were achieved. Thus, the established sensing platform provides a promising tool to detect DEHP in the environment.

Facile and robust fabrication of hierarchical Au nanorods/Ag nanowire SERS substrates for the sensitive detection of dyes and pesticides

Surface enhanced Raman spectroscopy (SERS) has emerged as a promising tool for the rapid and ultrasensitive recognition of trace amounts of environmental pollutants. Hierarchical SERS substrates usually show superior performance to single-component substrates but require complicated preparation protocols. Herein, a facile, robust and low-cost route for the fabrication of hierarchical SERS substrates has been reported, in which no complicated laborious protocols or sophisticated equipment is needed. In the hierarchical SERS substrate, Au nanorods were distributed onto the network of Ag nanowires through evaporation induced self-assembly. The density of the Au nanorods and Ag nanowires could be easily tailored by tuning the number of droplets of gold nanorod solution and the concentration of silver nanowire solution. The nanogaps formed between Au nanorods and Ag nanowires were able to induce a rich enhanced electromagnetic field area via localized surface plasmon resonances and surface plasmon polaritons to achieve amplification of the Raman signal. The as-prepared substrate showed high uniformity and was capable of identifying 10^-12 M rhodamine 6G, 10^-10 M thiram and 10^-10 M crystal violet, with correlation coefficients (R²) all higher than 0.98. This approach can be employed for the detection of trace dyes, pesticides or other environmental pollutants with high sensitivity and uniformity.

Surface-Enhanced Raman Sensing of Semi-Volatile Organic Compounds by Plasmonic Nanostructures

Facile detection of indoor semi-volatile organic compounds (SVOCs) is a critical issue to raise an increasing concern to current researchers, since their emissions have impacted the health of humans, who spend much of their time indoors after the recent incessant COVID-19 pandemic outbreaks. Plasmonic nanomaterial platforms can utilize an electromagnetic field to induce significant Raman signal enhancements of vibrational spectra of pollutant molecules from localized hotspots. Surface-enhanced Raman scattering (SERS) sensing based on functional plasmonic nanostructures has currently emerged as a powerful analytical technique, which is widely adopted for the ultra-sensitive detection of SVOC molecules, including phthalates and polycyclic aromatic hydrocarbons (PAHs) from household chemicals in indoor environments. This concise topical review gives updated recent developments and trends in optical sensors of surface plasmon resonance (SPR) and SERS for effective sensing of SVOCs by functionalization of noble metal nanostructures. Specific features of plasmonic nanomaterials utilized in sensors are evaluated comparatively, including their various sizes and shapes. Novel aptasensors-assisted SERS technology and its potential application are also introduced for selective sensing. The current challenges and perspectives on SERS-based optical sensors using plasmonic nanomaterial platforms and aptasensors are discussed for applying indoor SVOC detection.

Bimetallic Au-Ag on a Patterned Substrate Derived from Discarded Blu-ray Discs: Simple, Inexpensive, Stable, and Reproducible Surface-Enhanced Raman Scattering Substrates

A simple and reproducible surface enhanced Raman scattering (SERS) substrate of bimetallic gold-silver (Au-Ag) based on discarded a Blu-ray disc read only memory (BD-ROM) was developed by simply incorporating electrochemical (EC) treatment and chemical reaction. The resurfaced AgBD-ROM substrate (r-AgBD-ROM) was fabricated by EC treatment on a Ag film layer in BD-ROM (AgBD-ROM) to generate silver nanoparticles (AgNPs) on the indented pattern surface. Then, galvanic displacement reaction of Au and Ag was carried out to prepare the bimetallic Au-Ag structure (Au-r-AgBD-ROM). The suitable size and density as well as location of NPs on the surface can be tuned via EC treatment conditions to obtain highly active SERS performance. The SERS enhancement phenomenon on our developed substrate was studied by observing the location of the SERS hot spot obtained by Raman mapping. The developed SERS substrate offers excellent stability (90 days), good uniformity [6.14% relative standard deviation (RSD)], and reproducibility (3.79% of RSD). Moreover, this substrate can be used as a promising sensor for detecting acetaminophen, ibuprofen, and mefenamic acid. This finding suggests a simple and low-priced process, which potentially facilitates fabrication of highly sensitive SERS substrates for practical applications.

Quantitative SERS sensor based on self-assembled Au@Ag heterogeneous nanocuboids monolayer with high enhancement factor for practical quantitative detection

Accurate and rapid quantitative detection of pesticide and pollutant levels in the actual sample can aid in protecting food security, environmental security, and human health. A high Raman enhancement factor and good repeatability of the surface-enhanced Raman spectroscopy (SERS) substrates are favorable to quantitative analysis. Herein, a quantitative SERS sensor based on constructed self-assembled plasmonic Au@Ag heterogeneous nanocuboids (Au@Ag NCs) monolayer was developed. The sensor was used to quantitatively detect the trace pesticides extracted from pear surfaces and pollutants in fishpond water. Densely packed Au@Ag NCs fabricated into large-scale monolayer films were chemically functionalized using 4-methyl-thiobenzoic acid (4-MBA) at the organic/aqueous interface, in which plentiful nanogaps contribute to increase hotspots. Their sharp corners and edges make the sensor have high SERS performance through providing abundant "hot spots." The obtained optically SERS-based sensor with uniform liquid-state interfacial nanoparticle arrays appeared to have nice SERS performance and uniformity using crystal violet (CV) as a probe molecule. In particular, the proposed SERS sensor was applied for quantitative detection of thiabendazole (TBZ) extracted from pear surfaces and malachite green (MG) in fishpond water down to levels of 0.0105 nM and 0.87 nM for SERS assay respectively. As a result, our proposed SERS quantitative detection strategy is quite preferred to on-site analysis and supervision of contaminant in food samples.

Size-dependent surface enhanced Raman scattering activity of plasmonic AuNS@AgNCs for rapid and sensitive detection of Butyl benzyl phthalate

Phthalate plasticizers (PAEs) are heavily applied to plastic products and poses severe threat to human health. Herein, it is especially urgent to find a stable and reliable method for detecting PAEs. In this report, a Surface Enhanced Raman Scatting (SERS) strategy coupled with plasmonic core-shell Au nanospheres@Ag nanocubes (AuNS@AgNCs) as substrates were employed for the rapid and sensitive detection of Butyl benzyl phthalate (BBP) in liquor samples, and plasmonic core-shell AuNS@AgNCs tend to perform richer localized surface plasmon resonance (LSPR) than AuNS. In this work, different sizes AuNS@AgNCs comprised of Au nanospheres as core and Ag nanocubes as shells were synthesized. Based on this, we then investigated the SERS activity of BBP and crystal violet (CV) reached a maximum level when the thickness of Ag coating shell arrived in a threshold, and even very low signal of trace BBP dissolved in liquor sample can be detected in existence of the plasmonic AuNS@AgNCs active substrate of 50 nm. The sensitivity and repeatability of the optimized size AuNs@AgNCs have been estimated and limits of detection (LOD) was 10^-9 M for BBP. In addition, finite difference time domain (FDTD) electromagnetic simulations also performed in great agreement with our experimental results.

Plasmonic tunable Ag-coated gold nanorod arrays as reusable SERS substrates for multiplexed antibiotics detection

Antibiotic contaminants in aqueous media pose a serious threat to human and ecological environments. Therefore, it is necessary to develop robust strategies to detect antibiotic residues. For this purpose, a self-assembly and in situ electrochemical reduction method is utilized to tailor silver nanoparticles (AgNPs)-coated GNRs (AgNPs/GNRs) large-scale vertical arrays. These AgNPs/GNRs arrays exhibit outstanding surface-enhanced Raman scattering (SERS) activities because of abundant Raman hot-spots among the adjacent AgNPs and GNRs, but also excellent stability and reproducibility due to the close-packed arrayed nanostructure. These remarkable features validate this arrayed substrate for high-sensitivity 4-aminothiophenol analysis with a detection limit of 0.35 pM and self-cleaning via electrochemical stripping of the adsorbed analytes and AgNPs from the GNRs arrays, therefore realizing renewable SERS applications. Moreover, the distinct SERS performance of AgNPs/GNRs arrays is verified via the analysis of multiplexed antibiotics at tens of picomolar level and no apparent changes of SERS activities are observed when recyclability is explored. The result demonstrates that the proposed AgNPs/GNRs arrays provide a novel strategy for avoiding conventional, disposable SERS substrates, as well as expanding SERS applications for simultaneous sensing and stripping of environmental contaminants.

Bimetallic Ag-Au alloy nanocubes for SERS based sensitive detection of explosive molecules

We have fabricated Ag-Au alloy nanocubes using the galvanic replacement of silver nanocubes by aqueous HAuCl₄ and investigated their morphological, structural, compositional and optical properties. The inter-diffusion between silver and gold at 100 °C leads to the formation of Ag-Au alloy nanocubes with hollow interiors. A broad tuning of the surface plasmon resonance (SPR) wavelength from 624 nm to 920 nm is obtained with the varying volume of HAuCl₄. When excited at wavelength 785 nm, the bimetallic Ag-Au nanocubes with pinholes exhibit two-fold Raman intensity enhancement compared to pristine Ag nanocubes. The surface-enhanced Raman spectroscopy (SERS) substrate prepared with Ag-Au alloy nanocubes shows high-intensity enhancement factor of 1.9 × 10⁷ for 11.2 wt% Au content. The SERS-active Ag-Au alloy nanocubes substrates were exploited for the detection of two explosive molecules; p-nitrobenzoic acid (PNBA) and picric acid (PA). Remarkable detection sensitivity and ultra-low detection limit of 1.7 × 10^-14 M for PNBA and 4.1 × 10^-11 M for PA were obtained, demonstrating the very high SERS detection capabilities of the as-prepared substrate.

Metal Nanoparticles-Enhanced Biosensors: Synthesis, Design and Applications in Fluorescence Enhancement and Surface-enhanced Raman Scattering

Metal nanoparticles (NP) that exhibit localized surface plasmon resonance play an important role in metal-enhanced fluorescence (MEF) and surface-enhanced Raman scattering (SERS). Among the optical biosensors, MEF and SERS stand out to be the most sensitive techniques to detect a wide range of analytes from ions, biomolecules to macromolecules and microorganisms. Particularly, anisotropic metal NPs with strongly enhanced electric field at their sharp corners/edges under a wide range of excitation wavelengths are highly suitable for developing the ultrasensitive plasmon-enhanced biosensors. In this review, we first highlight the reliable methods for the synthesis of anisotropic gold NPs and silver NPs in high yield, as well as their alloys and composites with good control of size and shape. It is followed by the discussion of different sensing mechanisms and recent advances in the MEF and SERS biosensor designs. This includes the review of surface functionalization, bioconjugation and (directed/self) assembly methods as well as the selection/screening of specific biorecognition elements such as aptamers or antibodies for the highly selective bio-detection. The right combinations of metal nanoparticles, biorecognition element and assay design will lead to the successful development of MEF and SERS biosensors targeting different analytes both in-vitro and in-vivo. Finally, the prospects and challenges of metal-enhanced biosensors for future nanomedicine in achieving ultrasensitive and fast medical diagnostics, high-throughput drug discovery as well as effective and reliable theranostic treatment are discussed.

Highly sensitive detection of an antidiabetic drug as illegal additives in health products using solvent microextraction combined with surface-enhanced Raman spectroscopy

The combined SME-SERS approach realized the effective separation and sensitive detection of illegal drug additives spiked in different healthy products.