Flexible Surface-enhanced Raman Scattering Research Articles

Machine Learning-Assisted Pesticide Detection on a Flexible Surface-Enhanced Raman Scattering Substrate Prepared by Silver Nanoparticles

Access to clean water is a pressing challenge affecting millions of lives and the aquatic body of the Earth. Sensitive detection of pollutants such as pesticides is particularly important to address this challenge. This study reports eco-friendly preparation of the surface-enhanced Raman scattering (SERS) substrate for machine learning-assisted detection of pesticides in water. The proposed SERS platform was prepared on a copy paper by reducing a silver salt using the extract of a natural plant, Cedrus libani. The fabricated SERS platform was characterized in detail using scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. The high-density formation of silver nanoparticles with an average diameter of 41 nm on the surface of the paper enabled detection of analytes with a nanomolar level sensitivity. This SERS capability was used to collect Raman signals of four different pesticides in water: myclobutanil, phosmet, thiram, and abamectin. Raman spectra of the pesticides are highly complex, challenging accurate determination of the pesticide type. To overcome this challenge and distinguish pesticides, machine learning (ML) approach was used. The ML-mediated detection of harmful pesticides on a versatile, green, and inexpensive SERS platform appears to be promising for environmental applications.

Controllable fabrication of silver-deposited polyurethane acrylate nanopillar array film as a flexible surface-enhanced Raman scattering (SERS) substrate with high sensitivity and reproducibility.

Acontrollable method for fabricating flexible surface-enhanced Raman scattering (SERS) substrates is demonstratedby depositing silver onto a flexible nanopillar array film. The flexible nanopillar array film was cost-effectively prepared by replicating an anodic aluminum oxide (AAO) template with UV-curable polyurethane acrylate (PUA) over a large area. Then, the deposition of silver was done by an Ar-assisted thermal evaporation. In the deposition process, the partial pressure of Ar was optimized because it has a significant influence on the SERS intensity through the microstructural changes of silver deposited on PUA nanopillars. In addition, the increase in the nanopillar diameter and height enhanced the SERS intensity obtained at 785-nm excitation because of the increased number of hot spots. However, the agglomeration of Ag-deposited nanopillars, which is caused by high aspect ratios, negatively affected the SERS performance in terms of intensity and standard deviation. The optimized Ag-deposited nanopillar array film with nanopillar diameters and heights of 80nm and 200nm exhibited excellent SERS sensitivity and signal reproducibility with stable mechanical flexibility. For application in food and biomedical analysis, it was used for detecting saccharin and peptide and showed a good linear relationship between the SERS intensity and concentration. These findings demonstrate the suitability of our method for the controllable fabrication and optimization of flexible SERS substrates with high sensitivity and reproducibility.

A robust, flexible adhesive tape-based SERS substrate fabricated by polymer etching and subsequent Au coating on the exposed SiO2 nanosphere monolayer

With the rapid development of trace detection, high-performance flexible surface-enhanced Raman scattering (SERS) substrates have enjoyed steady growth of interest. In this paper, a facile method to improve the robustness of the flexible SERS substrate via the synergistic effect of rigid SiO2 nanospheres and flexible tape was demonstrated for the first time. In detail, the spin-coated SiO2 nanosphere monolayer was transferred from the host silicon wafer into the tape by peeling-off process, followed by O2 plasma etching of tape polymer to expose the nanospheres, and final Au coating to form plentiful SERS “hotspots”. The as-prepared SERS sample shows a detection limit of Rhodamine 6G (R6G) down to 10−10 M and can afford a 500 times bending-releasing cyclic test. Our research provides a promising strategy to prepare robust SERS substrates which exhibit good potential in practical molecule detection on curved surfaces.

In Situ Collection and Rapid Detection of Pathogenic Bacteria Using a Flexible SERS Platform Combined with a Portable Raman Spectrometer.

This study aims to develop a simple, sensitive, low-cost, environmentally friendly and flexible surface-enhanced Raman scattering (SERS) platform, combined with a portable Raman spectrometer, for the rapid and on-site SERS detection of bacteria. Commercial tobacco packaging paper (TPP) with little background interference was used as a loading medium that effectively adsorbed Au nanoparticles and provided sufficient “hot spots”. This Au-tobacco packaging paper (Au-TPP) substrate used as a flexible SERS platform can maximize sample collection by wiping irregular surfaces, and was successfully applied to the on-site and rapid detection of pathogenic bacteria. Raman fingerprints of pathogenic bacteria can be obtained by SERS detection of spiked pork using wipeable Au-TPP, which verifies its value in practical applications. The results collected by SERS were further verified by polymerase chain reaction (PCR) results. It showed several advantages in on-site SERS detection, including accurate discrimination, simple preparation, easy operation, good sensitivity, accuracy and reproducibility. This study indicates that the established flexible SERS platform has good practical applications in pathogenic bacterial identification and other rapid detections.

Ag nanocubes monolayer-modified PDMS as flexible SERS substrates for pesticides sensing.

A new approach is presented to fabricate flexible surface-enhanced Raman scattering (SERS) substrate of Ag nanocubes monolayer-modified polydimethylsiloxane (Ag NCs/PDMS) through a powerful three-phase interface self-assemble method. The morphologies and crystal structures were characterized by scanning electron microscopy and X-ray diffraction. The self-assembled Ag NCs/PDMS substrate exhibited high SERS activity and good signal homogeneity, which was successfully used for quantitative detection of thiram; the detection limit reached 10ng/mL, and the linear range is 10-1000ng/mL. Furthermore, the flexible SERS substrates were successfully employed to detect thiram residues on factual apple samples, and trace amount (1ng/cm2) of thiram residues was detected on apple peels. The excellent SERS detection ability of self-assembled Ag NCs/PDMS substrate indicated that it will play an important role in pesticide detection in the future.

Facile fabrication of wafer-scale flexible SERS substrate via cryo-transferring SiO2 nanosphere multilayer into waterborne polyurethane (WPU) film

Preparing flexible Surface-Enhanced Raman Scattering (SERS) substrates with close-packed nanosphere monolayer has attracted considerable interest. Regrettably, although nanosphere (NS) multilayer can be much more easily assembled at a large area than monolayer, it has not yet been employed in the SERS field. Herein, we report the facile fabrication of wafer-scale flexible SERS substrate with NS multilayer for the first time. In detail, we cryo-transferred SiO2 NS multilayer which was easily spin-coated on host Si substrate completely into waterborne polyurethane (WPU) film, followed by air plasma etching to remove part of WPU to make only one layer of embedded SiO2 NS exposed. Cauliflower-like Au nanostructure with abundant “hot spots” was formed on NS crowns by e-beam evaporation, finally enabling the 2-inch flexible SERS substrate with a rhodamine 6G (R6G) detection limit of 10-9 M and the corresponding enhancement factor (EF) of ∼108.

Inkjet-printed paper-based surface enhanced Raman scattering (SERS) sensors for the detection of narcotics

Recent advances in inkjet-printing of advanced materials have provided a versatile platform for the rapid development and prototyping of sensor devices. We have recently demonstrated inkjet-printed surface enhanced Raman scattering (SERS) sensors on flexible substrates for the detection of variety of small molecules [Tay et al. in Front Chem 9:680556 (2021); Tay et al. in J Raman Spectrosc 52:563 (2020)]. These flexible SERS sensors have many advantages for performing point-of-sampling testing, among them liquid or aerosol filtration and swabbing capabilities. These simple sampling and separation features make these inkjet-printed paper-based sensors ideal for field applications. SERS detection of molecules with poor binding affinity towards the plasmonic surfaces of the sensors tends to be inefficient. A surface functionalization approach has been applied to SERS sensors to improve the molecule affinity and hence their detection sensitivity. In this paper, we investigate the optimization of SERS sensor fabrication to achieve optimal performance. Three performance criteria: diffuse reflectance, SERS background intensity from the as-printed blank sensors and SERS performance of sensors exposed to the benzenethiol reporter molecule, are characterized carefully to derive the optimal inkjet-printing conditions for producing the best performing SERS sensors. Additionally, we demonstrate the use of a simple potassium iodide functionalization scheme to improve the detection sensitivity for narcotics such as fentanyl by two orders of magnitude.Graphical abstract

Toward Wearable Biosensor with Ag Coated Quartz Fabric as Flexible Surface-Enhanced Raman Scattering Substrate

Toward Wearable Biosensor with Ag Coated Quartz Fabric as Flexible Surface-Enhanced Raman Scattering Substrate

Toward Wearable Biosensor with Ag Coated Quartz Fabric as Flexible Surface-Enhanced Raman Scattering Substrate

Toward Wearable Biosensor with Ag Coated Quartz Fabric as Flexible Surface-Enhanced Raman Scattering Substrate

Metal-free and flexible surface-enhanced Raman scattering substrate based on oxidized carbon cloth

Flexible surface-enhanced Raman scattering (SERS) substrates show great potential application in environmental, biological and technological sensors. However, most of flexible SERS substrates are designed based on depositing or embedding metal (Ag, Au, Cu) particles on or into polymer supports or woven fibers without SERS activity, which suffer from complex manipulation processes and high cost. In this work, oxidized carbon cloth fabricated by chemical oxidation of commercial carbon cloth used as a metal-free and highly flexible SERS substrate with detection of limit for Rhodamine 6G down to below 10−8 M is reported. The SERS sensitivity of oxidized carbon cloth is improved by more than 3 order magnitude than untreated carbon cloth. By combining experimental analyses and theoretical calculations, it is proved that rich oxygen-containing functional groups on carbon cloth surface introduced by chemical oxidation leads to enhanced charge transfer (chemical enhancement) between oxidized carbon cloth and adsorbed molecules, which is responsible for the improved SERS performance. This work may pave the way for metal-free and flexible SERS substrate design and provides valuable insights for carbon-based SERS substrate.

Gold Nanopolyhedron-Based Superlattice Sheets as Flexible Surface-Enhanced Raman Scattering Sensors for Detection of 4-Aminothiophenol

Gold Nanopolyhedron-Based Superlattice Sheets as Flexible Surface-Enhanced Raman Scattering Sensors for Detection of 4-Aminothiophenol

Transparent, Flexible Plasmonic Ag NP/PMMA Substrates Using Chemically Patterned Ferroelectric Crystals for Detecting Pesticides on Curved Surfaces.

Transparent and flexible surface-enhanced Raman scattering (SERS) substrates have attracted much interest for the detection of probe molecules on the curved surfaces of real samples, but a facile route to fabricate such substrates is still lacking. Herein, we present a rationally designed, high-performance flexible SERS substrate fabricated using a simple drop and peel-off technique for the ultrasensitive detection of pesticides. The proposed SERS substrate consists of a polymethyl methacrylate (PMMA) film anchored with plasmonic silver nanoparticles (Ag NPs), which are photoreduced using chemically patterned ferroelectric templates. The photoreduced Ag NPs extracted onto the PMMA film offer strong electromagnetic enhancement and produce intensive hotspots for the effective enhancement of the Raman signal. They provide superior SERS performance for the detection of parathion (PT) and fenitrothion (FNT) at trace-level concentrations of 10-9 M and 10-10 M with excellent enhancement factors in the order of 108 and 109, respectively. Moreover, the Ag NP/PMMA SERS substrate has good spot-to-spot uniformity and batch-to-batch reproducibility with the reservation of high detection sensitivity even after the mechanical deformation of bending and torsion up to 50 cycles. The multiplex detection ability is also investigated for the simultaneous detection of PT and FNT. To ensure the practical feasibility, the in-situ, real-time detection of PT and FNT on the curved surfaces of tomato and lemon using a fiber-coupled Raman probe is performed with limits of detection of 4.24 × 10-8 M and 2.74 × 10-9 M. The proposed Ag NP/PMMA flexible SERS substrate possesses unique features, such as easy fabrication through a simple, economical, rapid process, and facilitates straightforward implementation of in-situ SERS detection on curved fruit/vegetable surfaces.

In situ photochemical deposition of Ag nanoparticles on polyester fiber membranes as flexible SERS substrates for sensitive detection of sodium saccharin in soft drinks

Flexible surface-enhanced Raman scattering (SERS) substrates, specifically paper-based SERS substrates, are generally employed in many practical applications. However, paper-based SERS substrate quantitation is still challenging because of the inferior uniformity originating from the irregular morphology of the paper. The present study synthesized Ag nanoparticles (NPs) via the in situ photochemical reduction of Ag+ ions on a titanium dioxide (TiO2) film at the polyester fiber membranes (PFMs) surface. This generated many “hot spots” that presented significant SERS activity with an enhancement factor of 2.1 × 109. Sodium saccharin (SS) bonded to the Ag NPs surface via the carbonyl oxygen and ring nitrogen atoms. A SERS sensor with photochemically deposited Ag NPs was prepared for the sensitive and rapid detection of SS in soft drinks with a limit of detection of 0.3 mg/L. The recovery rate ranged from 93% to 103%, indicating that this method is reliable in actual sample detection with simple pretreatment steps and can be potentially applied for the on-site detection of food additives for food safety.

Flexible Surface-Enhanced Raman Scattering Chip: A Universal Platform for Real-Time Interfacial Molecular Analysis with Femtomolar Sensitivity.

We propose and demonstrate a flexible surface-enhanced Raman scattering (SERS) chip as a versatile platform for femtomolar detection and real-time interfacial molecule analysis. The flexible SERS chip is composed of a flexible and transparent membrane and embedded plasmonic dimers with ultrahigh particle density and ultrasmall dimer gap. The chip enables rapid identification for residuals on solid substrates with irregular surfaces or dissolved analytes in aqueous solution. The sensitivity for liquid-state measurement is down to 0.06 molecule per dimers for 10-14 mol·L-1 Rhodamine 6G molecule without molecule enrichment. Strong signal fluctuation and blinking are observed at this concentration, indicating that the detection limit is close to the single-molecule level. Meanwhile, the homogeneous liquid environment facilities accurate SERS quantification of analytes with a wide dynamic range. The synergy of flexibility and liquid-state measurement opens up avenues for the real-time study of chemical reactions. The reduction from p-nitrothiophenol (PNTP) to p-aminothiophenol (PATP) in the absence of the chemical reducing agents is observed at liquid interfaces by in situ SERS measurements, and the plasmon-induced hot electron is demonstrated to drive the catalytic reaction. We believe this robust and feasible approach is promising in extending the SERS technique as a general method for identifying interfacial molecular traces, tracking the evolution of heterogeneous reactions, elucidating the reaction mechanisms, and evaluating the environmental effects such as pH value and salty ions in SERS.

Large-Scale Flexible Surface-Enhanced Raman Scattering (SERS) Sensors with High Stability and Signal Homogeneity

Flexible surface-enhanced Raman scattering (SERS) sensors have attracted great attention as a portable and low-cost technique for chemical and bio-detecting. However, flexible SERS sensors tend to suffer low signal spatial homogeneity due to their uneven active plasmonic nanostructures (hot-spots) distribution, and quick degradation of SERS sensors' sensitivity due to low adhesion of hot-spots and flexible substrates during fast sampling. Herein, a large area (20×20 cm2) polyimide (PI) based SERS sensor is exploited for trace detection with high signal homogeneity and stability. The SERS sensor is constructed from PI through in situ growth of silver and gold core-shell nanoparticles (Ag@Au NPs) based on chemical reduction and galvanic replacement process. Benefiting from the abundant carboxyl groups on the surface cleavaged PI, densely and uniformly distributed Ag@Au NPs are successfully prepared on the film under ambient conditions. The high Raman enhancement factor (up to 1.07 × 107) and down detection capability of nanomolar (10-9 M) detection limits are obtained for this flexible SERS sensor. The uniform Raman signals in the random region show well signal homogeneity with a low variation of 8.7%. Moreover, the flexible SERS sensor exhibited superior efficiency durability after storage 30 days even after mechanical stimuli (bending or torsion) 500 cycles. The pesticide thiram (TMTD) residue has been rapidly traced by direct sampling from the apple surface, and a sensitivity of 10 ng/cm2 for TMTD was achieved. These findings show that the PI-based SERS sensor is a very strong candidate for broad and simple utilization of flexible SERS for both laboratory and commercial applications in chemical and biomolecule detections.

Ag‐Nanoparticles‐Decorated Ge‐Nanowhisker Grafted on Carbon Fiber Cloth as Flexible and Effective SERS Substrates

AbstractThree‐dimensional (3D) flexible surface enhanced Raman scattering (SERS) substrates of silver nanoparticles (Ag‐NPs) decorated Germanium nanowhiskers (Ge‐NWHKs) grafted on carbon fiber cloth (CFC) (denoted as Ag‐NPs@Ge‐NWHKs@CFC) are constructed via chemical vapor deposition growth of high‐density Ge‐NWHKs on CFC and then assembly of Ag‐NPs on the Ge‐NWHKs by galvanic displacement. Ordered 3D framework of Ge‐NWHKs grafted flexible CFC impels the formation of large amounts of Ag‐NPs with homogenous distribution via spontaneous reduction of Ag+ ions. Thus, the Ag‐NPs@Ge‐NWHKs@CFC SERS substrates present ultra‐high sensitivity, good reproducibility, and high flexibility. This SERS sensor has achieved a detection limit of 1 pM for Rhodamine 6G and 0.1 nM for thiram respectively. The as‐fabricated SERS substrates show promising potential for applications in rapid detection of trace organic pollutants in the aquatic environment.

Bioinspired Disordered Flexible Metasurfaces for Human Tear Analysis Using Broadband Surface-Enhanced Raman Scattering.

Flexible surface-enhanced Raman scattering (SERS) has received attention as a means to move SERS-based broadband biosensing from bench to bedside. However, traditional flexible periodic nano-arrangements with sharp plasmonic resonances or their random counterparts with spatially varying uncontrollable enhancements are not reliable for practical broadband biosensing. Here, we report bioinspired quasi-(dis)ordered nanostructures presenting a broadband yet tunable application-specific SERS enhancement profile. Using simple, scalable biomimetic fabrication, we create a flexible metasurface (flex-MS) of quasi-(dis)ordered metal–insulator–metal (MIM) nanostructures with spectrally variable, yet spatially controlled electromagnetic hotspots. The MIM is designed to simultaneously localize the electromagnetic signal and block background Raman signals from the underlying polymeric substrate—an inherent problem of flexible SERS. We elucidate the effect of quasi-(dis)ordering on broadband tunable SERS enhancement and employ the flex-MS in a practical broadband SERS demonstration to detect human tear uric acid within its physiological concentration range (25–150 μM). The performance of the flex-MS toward noninvasively detecting whole human tear uric acid levels ex vivo is in good agreement with a commercial enzyme-based assay.

3D Ultrasensitive Polymers-Plasmonic Hybrid Flexible Platform for In-Situ Detection.

This paper introduces a three-dimensional (3D) pyramid to the polymers-plasmonic hybrid structure of polymethyl methacrylate (PMMA) composite silver nanoparticle (AgNPs) as a higher quality flexible surface-enhanced Raman scattering (SERS) substrate. Benefiting from the effective oscillation of light inside the pyramid valley could provide wide distributions of 3D “hot spots” in a large space. The inclined surface design of the pyramid structure could facilitate the aggregation of probe molecules, which achieves highly sensitive detection of rhodamine 6G (R6G) and crystal violet (CV). In addition, the AgNPs and PMMA composite structures provide uniform space distribution for analyte detection in a designated hot spot zone. The incident light can penetrate the external PMMA film to trigger the localized plasmon resonance of the encapsulated AgNPs, achieving enormous enhancement factor (~). After undergoes mechanical deformation, the flexible SERS substrate still maintains high mechanical stability, which was proved by experiment and theory. For practical applications, the prepared flexible SERS substrate is adapted to the in-situ Raman detection of adenosine aqueous solution and the methylene-blue (MB) molecule detection of the skin of a fish, providing a direct and nondestructive active-platform for the detecting on the surfaces with any arbitrary morphology and aqueous solution.

Purine-blended nanofiber woven flexible nanomats for SERS-based analyte detection.

Flexible and free-standing polymeric fibers, coupled with metal nanoparticles, present an excellent opportunity for highly sensitive surface-enhanced Raman scattering (SERS) spectroscopy-based detection platforms. Such host matrices are prepared by only a few preferred methods, followed by induction of metal nanoparticle aggregation in a protected environment to afford functional SERS platforms. We provide an interesting advance in this area by choosing a thiol-modified purine (L)-polymer blend, obtained through the electrospinning process, for directed decoration with gold nanoparticles. The described SERS purine-nanomat substrate enables the detection of uric acid, an important indicator of gout, preeclampsia, cardiovascular and kidney diseases, in aqueous solution up to 100 nM, with good stability and high sensitivity, offering superiority over existing techniques in terms of sensitivity and cost-effectiveness.

Toward Flexible Surface-Enhanced Raman Scattering (SERS) Sensors for Point-of-Care Diagnostics.

Surface‐enhanced Raman scattering (SERS) spectroscopy provides a noninvasive and highly sensitive route for fingerprint and label‐free detection of a wide range of molecules. Recently, flexible SERS has attracted increasingly tremendous research interest due to its unique advantages compared to rigid substrate‐based SERS. Here, the latest advances in flexible substrate‐based SERS diagnostic devices are investigated in‐depth. First, the intriguing prospect of point‐of‐care diagnostics is briefly described, followed by an introduction to the cutting‐edge SERS technique. Then, the focus is moved from conventional rigid substrate‐based SERS to the emerging flexible SERS technique. The main part of this report highlights the recent three categories of flexible SERS substrates, including actively tunable SERS, swab‐sampling strategy, and the in situ SERS detection approach. Furthermore, other promising means of flexible SERS are also introduced. The flexible SERS substrates with low‐cost, batch‐fabrication, and easy‐to‐operate characteristics can be integrated into portable Raman spectroscopes for point‐of‐care diagnostics, which are conceivable to penetrate global markets and households as next‐generation wearable sensors in the near future.