Silver-nanoparticles-loaded chitosan foam as a flexible SERS substrate for active collecting analytes from both solid surface and solution

A self-calibrating flexible SERS substrate incorporating PB@Au assemblies for reliable and reproducible detection

The precise quantitative analysis using surface-enhanced Raman spectroscopy (SERS) in an uncontrollable environment still faces a significant obstacle due to the poor reproducibility of Raman signals. Herein, we propose a facile method to fabricate a self-calibrating substrate based on a flexible polyvinyl alcohol (PVA) film comprising assemblies of Prussian blue (PB) and Au NPs (PB@Au) for reliable detection. PB cores were coated with an Au shell through simple electrostatic interaction, forming core-shell nanostructure PB@Au assemblies within the PVA film. The outer Au layer provided identical trends in enhancement for both the PB core and neighboring targets while PB cores served as an internal standard (IS) to correct signal fluctuations. The prevention of competitive adsorption on the metal surface between targets and ISs was achieved. The proposed PVA/PB@Au film exhibited enhanced stability of Raman signals after IS correction, resulting in improved spot-to-spot and batch-to-batch reproducibility with significantly reduced standard deviation (RSD) values from 11.42% and 25.02% to 4.43% and 9.39%, respectively. Simultaneously, a higher accuracy in the quantitative analysis of 4-mercaptobenzoic acid (4-MBA) and malachite green (MG) was achieved with fitting coefficient (R2) values improving from 0.9675 and 0.9418 to 0.9974 and 0.9832, respectively. Moreover, the PVA/PB@Au film was successfully applied to detect residual MG in real fish samples. This work opens up an avenue to improve the reproducibility of Raman signals for flexible SERS substrates in the detection of residues under various complex conditions.

Silver-nanoparticle-coated Fe3O4/chitosan core-shell microspheres for rapid and ultrasensitive detection of thiram using surface magnetic solid-phase extraction-surface-enhanced Raman scattering (SMSPE-SERS)

We report a surface magnetic solid-phase extraction-surface-enhanced Raman scattering (SMSPE-SERS) method based on silver-nanoparticle-coated Fe3O4/chitosan (Fe3O4/CS@Ag) microspheres as the substrate, and this method integrates all steps from sample pretreatment to detection. Fe3O4/CS was synthesized by a one-step solvothermal method in which chitosan (CS) was used as a surface modifier and adsorbent. Fe3O4/CS@Ag microspheres exhibit both adsorption ability and SERS activity. Therefore, we used the SMSPE-SERS method to detect pesticide residues on fruit peel. The procedures of capturing, separating and enriching pesticides, as well as detection, are all integrated. In addition, the SERS substrate allows label-free detection of thiram pesticide in both fruit peel and apple juice. Owing to the uniform distribution of Ag NPs and the adsorption ability of CS, the thiram-detection sensitivity was sufficiently high to detect the lowest concentration of 1.2 ng/cm2, which was significantly lower than the maximum thiram residue limit (7 μg/cm2) in fruits. The method was comparable to high-performance liquid chromatography with recovery ranging from 86.60 to 109.69 %.

Chitosan supported silver nanostructures as surface-enhanced Raman scattering sensor: Spectroscopic and density functional theory insights

In this work, nanostructures comprising silver nanoparticles supported on a wrinkled chitosan matrix (Ag@Ch) were successfully synthesized by a simple aging process at room temperature for four days through self-assembly. Chitosan, a natural polysaccharide was used as a support as well as a reducing agent for the formation of Ag nanostructures and the creation of hotspots for SERS activity. The fabricated Ag@Ch nanostructures were characterized by several spectroscopic techniques and were used as a surface-enhanced Raman scattering (SERS) substrate. The effect of wet, dry, and liquid samples on the SERS enhancement has been studied and was found to be effective for sensing Methylene blue, Crystal Violet, and p-Nitrophenol with detection limits of 3.8, 8.1, and 8.2 ppb respectively. The SERS enhancement of the Ag@Ch was attributed to the combination of both electromagnetic (EM) and chemical effects (CE). Density functional theory (DFT) calculations were used to explain the observed surface enhancement. Good agreement was observed between the experimental and simulated spectra.

Selective surface enhanced Raman detection and effective photocatalytic degradation of sulfonamides antibiotic based on a flexible three-dimensional chitosan/carbon nitride/silver substrate

The prevalence of sulfonamide residues in aquatic environments poses serious environmental risks, and the sensitive detection and effective degradation of sulfonamides have attracted widespread attention. Here, the environmentally friendly chitosan (CS)/carbon nitride (CN) with three-dimensional porous structure is fabricated by freeze-drying method, and subsequently a new bifunctional flexible substrate (CS/CN/Ag) is prepared by anchoring of small sized AgNPs (6 ∼ 12 nm) on CS/CN. Importantly, the CS/CN/Ag substrate shows high adsorption capacity (∼ 83.06%) for sulfamethoxazole (SMX) solution within 20 mins and the limit of detection can be as low as 7.46 × 10-9 mol·L-1 with an enhancement factor of 3.3 × 105. Also, the CS/CN/Ag substrate displays highly selective for surface-enhanced Raman spectroscopy (SERS) detection of sulfonamides and also shows excellent SERS response for SMX in hospital wastewater samples. In addition, the photocatalytic degradation efficiency of SMX could reach as high as 99.22% within 20 mins of irradiation and the CS/CN/Ag still maintains outstanding photocatalytic performance after six cycles. Moreover, the Ag content in the CS/CN/Ag substrate is only 2.35%, and also the CS/CN/Ag exhibits good uniformity, repeatability, recyclability and stability. Therefore, this flexible and cost-effectively substrate of CS/CN/Ag shows great potential for the simultaneous SERS detection and photocatalytic degradation of pollutants in actual wastewater samples.

Scalable fabrication of silver covered polyurethane nanofibers as flexible SERS nanosensors for aflatoxin detection

Flexible surface enhanced Raman spectroscopy (SERS) nanosensors, constructed by integration of plasmonic nanostructures with polymeric substrates, have received increasing research interests for recent decades. When compared to abundant works on optimization of the plasmonic nanostructures, the research involving the influence of polymeric substrates on analytical performance of resultant flexible SESR nanosensors is unexpectedly limited. Herein, the ultra-thin silver layer has been deposited on the electrospun polyurethane (ePU) nanofibrous membranes via vacuum evaporation to prepare the flexible SRES nanosensors. Interestingly, we find that the molecular weight and polydispersion index of synthesized PU play important roles in regulating the fine morphology of electrospun nanofibers, which in turns determine the Raman enhancement of resultant flexible SERS nanosensors. Specifically, the optimized SERS nanosensor, obtained by evaporating 10 nm silver layer on top of nanofibers derived from electrospinning of PU with a weight-average molecular weight of 140,354 and polydispersion index of 1.26, enables label-free detection of the carcinogen of aflatoxin down to 0.1 nM. Thanks to its scalable fabrication and good sensitivity, the current work opens new way for design of cost-effective flexible SERS nanosensors for environmental monitoring and food security applications.

Place & Play SERS: sample collection and preparation-free surface-enhanced Raman spectroscopy

The ability to perform sensitive, real-time, in situ, multiplex chemical analysis is indispensable for diverse applications such as human health monitoring, food safety testing, forensic analysis, environmental sensing, and homeland security. Surface-enhanced Raman spectroscopy (SERS) is an effective tool to offer the ability by virtue of its high sensitivity and rapid label-free signal detection as well as the availability of portable Raman spectrometers. Unfortunately, the practical utility of SERS is limited because it generally requires sample collection and preparation, namely, collecting a sample from an object of interest and placing the sample on top of a SERS substrate to perform a SERS measurement. In fact, not all analytes can satisfy this requirement because the sample collection and preparation process may be undesirable, laborious, difficult, dangerous, costly, or time-consuming. Here we introduce "Place & Play SERS" based on an ultrathin, flexible, stretchable, adhesive, biointegratable gold-deposited polyvinyl alcohol (PVA) nanomesh substrate that enables placing the substrate on top of an object of interest and performing a SERS measurement of the object by epi-excitation without the need for touching, destroying, and sampling it. Specifically, we characterized the sensitivity of the gold/PVA nanomesh substrate in the Place & Play SERS measurement scheme and then used the scheme to conduct SERS measurements of both wet and dry objects under nearly real-world conditions. To show the practical utility of Place & Play SERS, we demonstrated two examples of its application: food safety testing and forensic analysis. Our results firmly verified the new measurement scheme of SERS and are expected to extend the potential of SERS by opening up untapped applications of sensitive, real-time, in situ multiplex chemical analysis.

A flexible surface-enhanced Raman Spectroscopy chip integrated with microlens

A novel flexible Surface-enhanced Raman Spectroscopy (SERS) chip integrated with microlens was proposed and designed, which consisted of PDMS film, planoconvex microlens, and silver nanoparticles (AgNPs) monolayer, and was of high signal collection efficiency. The flexible PDMS film integrated with microlens was designed by optical simulation, and fabricated by optimized micromachining process. AgNPs monolayer were uniformly assembled on the other side of the PDMS film through a liquid-liquid interface self-assembly method to form SERS chip. The prepared chip revealed excellent SERS performance with a Raman enhancement factor of about 10⁷ and a signal variation of <11.5 %. The SERS chip was successfully utilized for in-situ detection of thiram residues on tomato skins, and its characteristic peaks could still be clearly distinguished when the concentration was down to 2.5 μM. It was shown that the proposed SERS chip was suitable for in-situ detection of a real sample on complex surface morphology and shown potential prospect in the fields of chemical and biomedical detections.

Rapid and reliable detection and quantification of organophosphorus pesticides using SERS combined with dispersive liquid-liquid microextraction

Rapid and reliable detection and quantification of pesticide residues in complex matrices by surface enhanced Raman spectroscopy (SERS) remain challenging due to the low level of target molecules and the interference of nontarget components. In this study, SERS was combined with dispersive liquid-liquid microextraction (DLLME) to develop a rapid and reliable method for the detection of organophosphorus pesticides (OPPs). In this method, DLLME was used to extract and enrich two representative OPPs (triazophos and parathion-methyl) from a liquid sample, and a portable Raman spectrometer was used to analyze the separated sediment using homemade gold nanoparticles colloids as enhancing substrates. The results showed that the developed method displayed good sensitivity and stability for the detection and quantification of triazophos and parathion-methyl with R² ≥ 0.98. The calculated limits of detection (LODs) in the simultaneous detection of triazophos and parathion-methyl were 2.17 × 10^-9 M (0.679 ppb) and 2.28 × 10^-8 M (5.998 ppb), and the calculated limits of quantification (LOQs) were 7.23 × 10^-9 M (2.26 ppb) and 7.62 × 10^-8 M (19.098 ppb), respectively. Furthermore, the developed SERS method was successfully applied to the detection of triazophos and parathion-methyl in apple juice with recoveries between 78.07% and 110.87% and relative standard deviations (RSDs) ≤ 2.06%. Therefore, the developed DLLME facilitated liquid SERS method exhibited good sensitivity and stability for the rapid detection and quantification of OPPs and had the potential to be applied to the rapid detection of OPPs in complex matrices.

An anti-scratch flexible SERS substrate for pesticide residue detection on the surface of fruits and vegetables

We propose an anti-scratch flexible surface-enhanced Raman scattering substrate with arrayed nanocavity microstructures fabricated by colloidal lithography. The nanocavity microstructure of the substrate can well protect the inner gold nanoparticles during wipe sampling. The prepared flexible substrate was able to detect 4-aminothiophenol (4-ATP) with a concentration down to 1 fM. Furthermore, the substrate was used to detect 6-BA residues on the surface of apples and bean sprouts through wipe sampling, which shows great potential in the field of rapid on-site detection, especially in the detection of pesticide residues on the surface of fruits and vegetables.

Plasmonic features of free-standing chitosan nanocomposite film with silver and graphene oxide for SERS applications

A scalable procedure of SERS substrates design was developed using a novel plasmonic structure based on a freestanding chitosan film, silver nanoparticles, and graphene oxide. Chitosan provides a uniform distribution of silver nanoparticles from a colloidal suspension and, therefore, a reproducible Raman signal from local areas of measurements of several tens of microns. The addition of graphene oxide (GO) to the colloidal solution of silver nanoparticles suppresses the tortuous background fluorescence signal from the analyte and leads to an increase in the signal-to-fluorescence background intensity ratio by up to 6 times as compared to structures without GO. The manufactured plasmonic polymer nanocomposite provides a detection limit of down to 100 pM for R6G using a laser wavelength of 532 nm through a portable ×10 objective. The high colloidal stability of GO in water and the use of an aqueous colloid of silver nanoparticles simplify the procedure for creating a substrate by applying the GO-silver composite on the surface of a chitosan film without a need to form a GO film. Therefore, our approach paves a promising avenue to provide more sensitive detection even for the fluorescent analytes with short-wavelength lasers (532, 633 nm) instead of IR (785, 1024 nm) and foster the practical application of the developed plasmonic composites on portable Raman spectrometers.

Flexible 2D nanocellulose-based SERS substrate for pesticide residue detection

Nanocellulose holds considerable promise as an effective surface-enhanced Raman scattering (SERS) substrate for sensitive detection of trace targets. Flexible and high-sensitivity two-dimensional (2D) SERS substrates based on nanocrystalline cellulose (CNC) film were successfully developed via self assembly of two plasma nanoparticles: gold nanoflowers (AuNFs) and silver-coated gold nanocubes (Au@AgNCs). The loading process allows the precise control of nanoparticle distribution density and uniformity on CNC film, which are closely related to the plasma coupling effect between particles. The obtained CNC/Au@AgNC flexible two-dimensional substrate could sensitively detect pesticide residues on apple surface, and the detection limits (LOD) of dimethoate and acetamiprid were 4.1 and 10.7 μg/L, respectively. In addition, Raman signal intensity showed a good linear relationship with pesticide concentration in the range of 10-100 μg/L, which provided great potential for high sensitivity and field detection of dangerous targets.

In-Situ Synthesis of Methyl Cellulose Film Decorated with Silver Nanoparticles as a Flexible Surface-Enhanced Raman Substrate for the Rapid Detection of Pesticide Residues in Fruits and Vegetables

The purpose of this study was to develop a flexible substrate methylcellulose-decorated silver nanoparticles (MC/Ag NPs) film and explore its application in fruits and vegetables by surface enhanced Raman spectroscopy (SERS) technology for rapid detection of pesticides. The performance of the MC/Ag NPs film substrate was characterized by Nile blue A (NBA), and the detection limit was as low as 10⁻⁸ M. The substrate also exhibited satisfactory Raman signal strength after two months of storage. The impressive sensitivity and stability were due to the excellent homogeneity of the silver nanoparticles that were grown in situ in the methylcellulose matrix, which generated “hot spots” between the silver nanoparticles without a large amount of aggregation, and resulted in the ultra-high sensitivity and excellent stability of the MC/Ag NPs film substrate. The MC/Ag NPs film substrate was used to detect thiram pesticides on tomato and cucumber peels, and the minimum detectable level of thiram was 2.4 ng/cm², which was much lower than the maximum residue level. These results indicate that the MC/Ag NPs film is sensitive to rapid detection of multiple pesticides in food.

Optimum fabrication parameters for preparing high performance SERS substrates based on Si pyramid structure and silver nanoparticles

In this work, we propose simple and inexpensive methods to prepare micro/nano hierarchical Surface-Enhanced Raman Scattering (SERS) substrates, in which pyramid structure is created by using anisotropic wet etching of a silicon wafer and a silver thin film is deposited on these pyramid arrays by thermal evaporation. The ensemble is then annealed at 450 °C for 2 hours to form silver nanoparticles (AgNPs). The sizes and density of the pyramids and AgNPs are optimized mainly by changing the etching temperature (60-80 °C), the thickness of the Ag-film (15-45 nm) and etching time (3-10 min). The ultraviolet visible (UV-Vis) absorbance spectra show that the AgNPs formed with the 30 nm-thick film exhibit the strongest plasmonic effect. Under these conditions, the spherical AgNPs with sizes of 42-48 nm are densely distributed on the silicon micro-pyramid array. The obtained SERS signal is the strongest at the pyramid base-edge size of 7-10 μm. The enhancement factor obtained from the abamectin probe molecules is as high as 1 × 106 and the SERS substrates enable the detection of abamectin concentrations as low as 5.7 × 10-9 M. Therefore, this work provides a novel SERS substrate structure that has a high potential for use in medicine and biotechnology or as a food security sensor.

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.

Preparation of Hydrophobic Film by Electrospinning for Rapid SERS Detection of Trace Triazophos

For real application, it is an urgent demand to fabricate stable and flexible surface-enhanced Raman scattering (SERS) substrates with high enhancement factors in a large-scale and facile way. Herein, by using the electrospinning technique, a hydrophobic and flexible poly(styrene-co-butadiene) (SB) fibrous membrane is obtained, which is beneficial for modification of silver nanoparticles (Ag NPs) colloid in a small region and then formation of more "hot spots" by drying; the final SERS substrate is designated as Ag/SB. Hydrophobic Ag/SB can efficiently capture heterocyclic molecules into the vicinity of hot spots of Ag NPs. Such Ag/SB films are used to quantitatively detect trace triazophos residue on fruit peels or in the juice, and the limit of detection (LOD) of 2.5 × 10^-8 M is achieved. Ag/SB films possess a capability to resist heat. As a case, 6-mercaptopurine (6MP) that just barely dissolves in 90 °C water is picked for conducting Ag/SB-film-based experiments.

Hydrophobic Paper-Based SERS Sensor Using Gold Nanoparticles Arranged on Graphene Oxide Flakes

Paper-based surface-enhanced Raman scattering (SERS) sensors have garnered much attention in the past decade owing to their ubiquity, ease of fabrication, and environmentally friendly substrate. The main drawbacks of a paper substrate for a SERS sensor are its high porosity, inherent hygroscopic nature, and hydrophilic surface property, which reduce the sensitivity and reproducibility of the SERS sensor. Here, we propose a simple, quick, convenient, and economical method for hydrophilic to hydrophobic surface modification of paper, while enhancing its mechanical and moisture-resistant properties. The hydrophobic paper (h-paper) was obtained by spin-coating diluted polydimethylsiloxane (PDMS) solution onto the filter paper, resulting in h-paper with an increased contact angle of up to ≈130°. To complete the h-paper-based SERS substrate, gold nanoparticles arranged on graphene oxide (AuNPs@GO) were synthesized using UV photoreduction, followed by drop-casting of AuNPs@GO solution on the h-paper substrate. The enhancement of the SERS signal was then assessed by attaching a rhodamine 6G (R6G) molecule as a Raman probe material to the h-paper-based SERS substrate. The limit of detection was 10 nM with an R2 of 0.966. The presented SERS sensor was also tested to detect a thiram at the micromolar level. We expect that our proposed AuNPs@GO/h-paper-based SERS sensor could be applied to point-of-care diagnostics applications in daily life and in spacecraft.

A review on recent advances in the applications of surface-enhanced Raman scattering in analytical chemistry

This review is focused on recent developments of surface-enhanced Raman scattering (SERS) applications in Analytical Chemistry. The work covers advances in the fabrication methods of SERS substrates, including nanoparticles immobilization techniques and advanced nanopatterning with metallic features. Recent insights in quantitative and sampling methods for SERS implementation and the development of new SERS-based approaches for both qualitative and quantitative analysis are discussed. The advent of methods for pre-concentration and new approaches for single-molecule SERS quantification, such as the digital SERS procedure, has provided additional improvements in the analytical figures-of-merit for analysis and assays based on SERS. The use of metal nanostructures as SERS detection elements integrated in devices, such as microfluidic systems and optical fibers, provided new tools for SERS applications that expand beyond the laboratory environment, bringing new opportunities for real-time field tests and process monitoring based on SERS. Finally, selected examples of SERS applications in analytical and bioanalytical chemistry are discussed. The breadth of this work reflects the vast diversity of subjects and approaches that are inherent to the SERS field. The state of the field indicates the potential for a variety of new SERS-based methods and technologies that can be routinely applied in analytical laboratories.

A functional Au array SERS chip for the fast inspection of pesticides in conjunction with surface extraction and coordination transferring

The fast inspection of the pesticide residues on fruits and vegetables requires the development of facile, sensitive and accurate methods. Surface enhanced Raman scattering (SERS) is a promising way to provide a fast inspection method, which requires significant improvements in the reproducibility and feasibility. In the present work, an SERS method was developed for the fast inspection of pesticides on fruit peels in conjunction with surface extraction and coordination transferring. In this new method, the residual pesticides were directly extracted from fruit peels with an appropriate extraction solution and then quantitatively transferred onto an organic solvent-compatible Au array SERS chip through the strong chemical interactions between the heteroatoms in the pesticides and the gold surface. The functional SERS chip was fabricated by the interfacial assembly of an Au array on a membrane, which produced dense and uniformly distributed SERS hot spots and enabled compatibility with random curvature surfaces and handheld Raman spectrometers. As a proof of concept, sulfur atoms containing thiram on apples were detected at a concentration as low as 5 ng cm-2 with good reproducibility (relative standard deviation lower than 10%). The strong interactions between the sulfur atoms and gold surface during the coordination transferring process were confirmed by the enhanced vibrations of the specified bands occurring in both the Raman and IR spectra. This surface extraction-coordination transferring-based method holds wide applicability for heteroatom-containing pesticides, as demonstrated by the detection of various S- and P-containing pesticides.