High roughness gold nanoparticles/silver nanowires composites: Fabrication, characterization and ultrasensitive SERS detection towards Rhodamine B

Machine learning-integrated surface-enhanced Raman spectroscopy analysis of multicomponent dye mixtures

Surface-enhanced Raman spectroscopy (SERS) is broadly used in the detection and analysis of materials with its fingerprint-like specificity and high sensitivity. However, resembling signals of analytes highly affect the identification and assignment of spectra, which has become a long-term issue to be solved. In this study, various models of machine learning are utilized and compared to support data analysis of complex SERS spectra. Silver-coated gold core-shell nanocubes (Au@AgNCs) are optimized as SERS substrates for the detection of four common dyes - methylene blue (MB), crystal violet (CV), rhodamine B (RhB) and malachite green (MG). Independent principal component analysis (ICA) was utilized to isolate the signals from the SERS spectra of the dye mixtures, and the isolated signals were further classified by commonly used classification models including K Nearest Neighbors (KNN), Support Vector Machines (SVM), Random Forests (RF), and Convolutional Neural Networks (CNN). The results show that the CNN model achieved an accuracy of 98% in the classification of single dyes and an accuracy of 97% in the classification of dye mixtures, which is significantly better than other models. Based on these findings, we propose ICA combined with CNN-assisted SERS spectroscopy as an effective analytical tool for analyzing dye mixtures.

Serum metabolic fingerprinting on Ag@AuNWs for traumatic brain injury diagnosis

Accurate and rapid diagnosis of traumatic brain injury (TBI) is very important for high quality medical services. Nonetheless, the current diagnostic platform still has challenges in the rapid and accurate analysis of clinical samples. Here, we prepared a highly stable, repeatable and sensitive gold-plated silver core-shell nanowire (Ag@AuNWs) for surface-enhanced Raman spectroscopy (SERS) metabolic fingerprint diagnosis of TBI. The core-shell structure significantly enhanced SERS intensity and enables the direct detection of 10μl serum within seconds. The principal component analysis-linear discriminant analysis (PCA-LDA) and partial least squares-DA (PLS-DA) are used to evaluate the classification effect of this technology on TBI, respectively. The diagnosis accuracy rate of PCA-LDA and PLS-DA is 73.3% and 86.7% for diagnosing TBI, respectively. Consequently, the PLS-DA model is the optimal selection for distinguishing between the TBI and sham groups. This research will facilitate the application-oriented creation of novel materials with tailored structural designs and the formulation of innovative precision medical protocols in the imminent future.

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.

Effect of NiAl alloy microparticles deposited in flexible SERS substrates on the limit of detection of rhodamine B molecules

Flexible-SERS (FSERS) substrates were fabricated by depositing Ni64Al36(NiAl)-alloy-microparticles and/or spherical Ag-NPs (sizes of 10-40 nm) on recycled plastics, which had an aluminum layer on their surface. First, FSERS substrates made of Al + Ag-NPs and an area of 1 cm2 were used to detect rhodamine B (RhB) molecules. The limit-of-detection (LOD) for RhB was 8.35 × 10-22 moles (∼503 molecules), and the enhancement factor (EF) was 3.11 × 1015. After adding NiAl-microparticles to the substrate, the LOD decreased to 8.35 × 10-24 moles (∼5 molecules) and the EF was increased to 2.05 × 1017. Such EF values were calculated with respect to substrates made only with Al + NiAl-alloy (without Ag-NPs), which did not show any Raman signal. Other FSERS substrates were made with graphene-layer + Ag-NPs or graphene-layer + NiAl-alloy + Ag-Nps, and the best LOD and EF values were 8.35 × 10-22 moles and 6.89 × 1015, respectively. Overall, combining the Ag-NPs and NiAl-alloy microparticles allowed for the zeptomole detection of RhB. This was possible due to the formation of Ag aggregates around the alloy microparticles, which enhanced the number of hotspots. If no alloy is present in the FSERS substrates, the detection of RhB is lowered. Overall, we presented a low-cost FSERS substrate that does not require expensive Au films or Au-NPs (as previously reported) to detect RhB at the zeptomole level.

Aramid nanofiber membrane decorated with monodispersed silver nanoparticles as robust and flexible SERS chips for trace detection of multiple toxic substances

Aramid nanofibers (ANFs) as an innovative nanoscale building block exhibit great potential for novel high-performance multifunctional membranes attributed to their extraordinary performance. However, the application of aramid nanofibers in the field of surface enhanced Raman scattering (SERS) sensing has been rarely reported. In this work, aramid nanofibers derived from commercial Kevlar fibers were synthesized by a facile dimethyl sulfoxide/potassium hydroxide (DMSO/KOH) solution treatment. The monodispersed silver nanoparticle-decorated aramid nanofiber (m-Ag@ANF) membranes were constructed by an efficient vacuum filtration technique. Taking advantages of unique intrinsic properties of ANF, the m-Ag@ANF substrates exhibit good flexibility, excellent mechanical properties and prominent thermal stability. Besides, due to the abundance of positively charged amino-group on the ANF substrates, the negatively charged m-AgNPs were uniformly and firmly deposited on the surface of ANF substrate through electrostatic interactions. As a result, the optimal flexible m-Ag-9@ANF SERS substrate exhibits high sensitivity of 10-9 M for methylene blue (MB) and excellent signal reproducibility (RSD = 6.37 %), as well as outstanding signal stability (up to 15 days). Besides, the 2D Raman mapping and FDTD simulations further reveal prominent signal homogeneity and strong electric field distribution for flexible m-Ag-9@ANF SERS substrate. Finally, it is demonstrated that the flexible m-Ag-9@ANF SERS substrate can also be used for detection of toxic molecules on irregular surfaces by a feasible paste-and-read process. The m-Ag@ANF paper exhibits potential applications as a flexible, low-cost, robust and stable SERS sensing platform for trace detection of toxic materials.

Molecularly imprinted polymer-coated hybrid optical waveguides for sub-aM fluorescence sensing

The sensitivity of fluorescent sensors is crucial for their applications. In this study, we propose a molecularly imprinted polymer (MIP)-coated optical fibre-hybrid waveguide-fibre sensing structure for ultrasensitive fluorescence detection. In such a structure, the MIP coated-hybrid waveguide acts as a sensing probe, and the two co-axially connected optical fibres act as a highly efficient probing light launcher and a fluorescence signal collector, respectively. For the dual-layered waveguide sensing probe, the inner hybrid waveguide core was fabricated using a hollow quartz nanoparticle-hybridized polymer composite with a low refractive index, and the outer MIP coating layer possesses a high refractive index. Simulations showed that this dual-layer configuration can cause light propagation from the waveguide core to the MIP sensing layer with an efficiency of 98%, which is essential for detection. To validate this concept, we adopted a popular fluorescent dye, rhodamine B, to evaluate the sensing characteristics of the proposed system. We achieved an extremely low limit of detection of approximately 1.3 × 10-19 g ml-1 (approximately 0.27 aM).

Ag triangle nanoplates assembled on PVC/SEBS membrane as flexible SERS substrates for skin cortisol sensing

Surface-enhanced Raman scattering (SERS) based on rigid substrates has been widely used in biomedical detection due to its high sensitivity and specificity. However, the tedious operation steps for preparing SERS rigid substrates limited their applications in real-world detection. Compared with general rigid substrate, the flexible substrate has the advantages of simple preparation and easy portability, which are suitable for rapid, wearable and personalized detection in the field of point-of-care test. Herein, the flexible SERS substrates employing copolymer were fabricated and used for detection of skin cortisol, a biomarker for evaluating psychological stress in sweat. Silver triangle nanoplates with sharp corner were used as enhanced particles, and transferred to polyvinyl chloride/styrene-ethylene-butene-styrene copolymer (PVC/SEBS) film through three-phase interface self-assembly. By adjusting the size of silver nanoparticles, the ratio of PVC to SEBS in the polymer film, and the number of transfers of self-assembled silver films, the enhancement effect of the flexible SERS substrate was maximized. In addition, functionalization of the flexible SERS substrate with cortisol antibodies is used to achieve specific detection of cortisol on the skin surface. Under the optimal conditions, the Raman peak intensities at 1268 and 1500 cm-1 of the cortisol had a good linear relationship with the logarithm of its concentration in the range of 10-7 to 10-3 M, and the detection limits were 5.47 × 10-8 M and 5.51 × 10-8 M, respectively. The flexible silver triangle nanoplates SERS substrate constructed by self-assembly in the three-phase interface has the characteristics of good specificity and high sensitivity, which has potential for transdermal cortisol wearable detection, providing a feasible method for the rapid evaluating psychological stress level.

A novel and controllable SERS system for crystal violet and Rhodamine B detection based on copper nanonoodle substrates

Copper nanostructures have attracted more and more attention due to low preparation cost, similar thermal conductivity and optical characteristics to silver nanostructures. A novel macroscopic dendritic copper nanonoodles with the length of 3-50 mm prepared by solid-state ionics method at 10 μA direct current electric field (DCEF) using fast ionic conductor RbCu₄Cl₃I₂ films was reported. The surface-enhanced Raman scattering (SERS) performance of prepared copper nanonoodles was detected by crystal violet (CV) and rhodamine B (RB) aqueous solution as analyte molecules. The results present that the copper nanonoodles assembled by short-range order copper nanowires and the diameters of nanowires changed from 20 nm to 80 nm, many regularly arranged nanoparticles with the diameter from 5 to 10 nm existed on the prepared copper nanonoodles, which lead to the nanonoodles have high surface roughness. The copper nanonoodles contain only Cu element, no O element and the fractal dimension of copper nanonoodles is 1.355 because of macroscopic dendritic structures. The prepared copper nanonoodles composed of pure Cu have high surface roughness and the free electrons on the rough copper nanonoodles resonate with the atomic nuclei inside the copper nanonoodles to form a locally enhanced electromagnetic field under the excitation of incident light, so the limiting concentrations for CV and RB detected by the prepared copper nanonoodles are as low as 1 × 10^-11 mol/L and 1 × 10^-12 mol/L, respectively. The centimeter-scale copper nanonoodles with low limiting concentration of analyte molecules can be used to detect harmful food additives.

Surface-Enhanced Raman Scattering from Dye Molecules in Silicon Nanowire Structures Decorated by Gold Nanoparticles

Silicon nanowires (SiNWs) prepared by metal-assisted chemical etching of crystalline silicon wafers followed by deposition of plasmonic gold (Au) nanoparticles (NPs) were explored as templates for surface-enhanced Raman scattering (SERS) from probe molecules of Methylene blue and Rhodamine B. The filling factor by pores (porosity) of SiNW arrays was found to control the SERS efficiency, and the maximal enhancement was observed for the samples with porosity of 55%, which corresponded to dense arrays of SiNWs. The obtained results are discussed in terms of the electromagnetic enhancement of SERS related to the localized surface plasmon resonances in Au-NPs on SiNW's surfaces accompanied with light scattering in the SiNW arrays. The observed SERS effect combined with the high stability of Au-NPs, scalability, and relatively simple preparation method are promising for the application of SiNW:Au-NP hybrid nanostructures as templates in molecular sensorics.