Flexible Spectroscopy Substrate Research Articles

Laser Parallel Nanofabrication of Dual‐Au‐Nanoholes on Microsphere‐Lens‐Array for Polarization‐Dependent SERS

AbstractHigh‐efficient fabrication of well‐ordered nanostructures on flexible substrates is a challenge to surface‐enhanced Raman spectroscopy (SERS) for analytical fingerprinting trace‐detection. In this work, a laser parallel nanofabrication method is reported via incident‐angle‐dependent photonic nanojet ablating Au film on the bottom surface of microsphere‐lens‐array (MLA), for combination with formed dual Au‐nanoholes (MLA/2‐AuNHs) as the flexible SERS substrates. The process parameters, i.e., pulsed laser energy, MLA diameter, Au film thickness, and laser incident angle, are optimized for high finish‐quality of dual‐AuNHs with a diameter of ≈875 nm and a tip gap of ≈90 nm. The SERS substrate demonstrates the limit of detection down to 10−11 M for 4‐nitrobenzenethiol molecules by the multiple optical regulations, including self‐aligned focusing to the hotspots and directional antenna, the strongly localized surface plasmon resonances, and optical whispering‐gallery modes for resonance energy transfer. The flexible PDMS film supporting the rigid MLA holds the SERS performance of MLA/2‐AuNHs with high flexibility. Moreover, the MLA/2‐AuNHs SERS substrate exhibits differential responses to orthogonally polarized excitation, by which the background noises can be eliminated to improve the Raman spectrum. The present work opens new opportunities to fabricate dielectric‐metal flexible SERS substrates by laser parallel nanofabrication via MLA for Raman trace detection.

SERS with Flexible β-CD@AuNP/PTFE Substrates for In Situ Detection and Identification of PAH Residues on Fruit and Vegetable Surfaces Combined with Lightweight Network.

The detection of polycyclic aromatic hydrocarbons (PAHs) on fruit and vegetable surfaces is important for protecting human health and ensuring food safety. In this study, a method for the in situ detection and identification of PAH residues on fruit and vegetable surfaces was developed using surface-enhanced Raman spectroscopy (SERS) based on a flexible substrate and lightweight deep learning network. The flexible SERS substrate was fabricated by assembling β-cyclodextrin-modified gold nanoparticles (β-CD@AuNPs) on polytetrafluoroethylene (PTFE) film coated with perfluorinated liquid (β-CD@AuNP/PTFE). The concentrations of benzo(a)pyrene (BaP), naphthalene (Nap), and pyrene (Pyr) residues on fruit and vegetable surfaces could be detected at 0.25, 0.5, and 0.25 μg/cm2, respectively, and all the relative standard deviations (RSD) were less than 10%, indicating that the β-CD@AuNP/PTFE exhibited high sensitivity and stability. The lightweight network was then used to construct a classification model for identifying various PAH residues. ShuffleNet obtained the best results with accuracies of 100%, 96.61%, and 97.63% for the training, validation, and prediction datasets, respectively. The proposed method realised the in situ detection and identification of various PAH residues on fruit and vegetables with simplicity, celerity, and sensitivity, demonstrating great potential for the rapid, nondestructive analysis of surface contaminant residues in the food-safety field.

Wearable and Flexible Nanoporous Surface-Enhanced Raman Scattering Substrates for Sweat Enrichment and Analysis

Surface-enhanced Raman spectroscopy (SERS), with high sensitivity to a broad range of molecules, can detect molecular “fingerprints” in a complex substance and thereby offers a promising solution for noninvasive medical diagnostics and personal healthcare monitoring. The eventual realization of such applications relies on SERS substrates with dense and uniform hot spots, good chemical stability, and high mechanical durability. With these criteria in mind, we developed a flexible nanoporous SERS substrate via the in situ synthesis of gold nanostars (AuNSs) on an ion-track-etched polycarbonate membrane. The nanoporous SERS substrate can realize analyte enrichment and exhibit excellent Raman performance by taking the advantage of hot spots on AuNSs. The SERS substrate yields highly repeatable and uniform signals for analytes (e.g., methylene blue) over a wide concentration range from 10–4 to 10–13 M. The flexible SERS substrate even can work well after 2000 times bending and exhibit excellent stability for long-term use. It can be prepared on a large scale with a low-cost and simple fabrication process and can be used repeatedly after cleaning to reduce the use-cost further. On-body experiments prove that the sweat SERS substrate allows effective identification of sweat contents, such as lactic acid and uric acid (UA), and the monitoring of diet-induced variation in sweat UA. The potential of the wearable nanoporous SERS substrates in sweat analysis thus has been demonstrated, opening possibilities for autonomous and noninvasive medical health monitoring.

A facile method for in-situ detection of thiabendazole residues in fruit and vegetable peels using Surface-Enhanced Raman Spectroscopy

A flexible Surface-Enhanced Raman Spectroscopy (SERS) substrate based on silver nanoparticles encapsulated in an agar gel has been probed to detect the fungicide thiabendazole (TBZ) reaching a limit of detection (LOD) of 30 ng/cm2. In addition, a simple and sensitive strategy was employed for in-situ detection of TBZ on fruit and vegetable peels. For that purpose, peels of different fruits and vegetables were intentionally contaminated with different amount of TBZ, and the analyte was subsequently extracted within few seconds by gently rubbing the surface with the SERS substrate. The lowest value of TBZ detected on eggplant and green pepper peels was 50 ng/cm2. The values achieved for apple and pear peels were 0.20 μg/cm2 and 40 ng/cm2, respectively. On the other hand, for tomato and strawberry peels the lowest value achieved was 0.50 μg/cm2. The variation in sensitivity can be attributed to differences in the surface properties of the different peels. The above results show that this flexible SERS substrate can be further employed for the detection of contaminants in practical applications for food safety inspection.

Microcontact printing of gold nanoparticle at three-phase interface as flexible substrate for SERS detection of MicroRNA

The rigidity of traditional solid-state surface-enhanced Raman spectroscopy (SERS) substrate hampers their application in the curved structure for nonplanar surface test and in-situ detection. Traditionally, the flexible Raman substrates are often prepared by transferring printing of patterned nanoparticles on the flexible materials such as polymer, paper, etc. However，the replicate patterns are often produced by high-cost instruments. In this study, a low-cost and flexible SERS substrate is prepared by using a microcontact printing technology to transfer three-phase-assembled nanoparticles on a polydimethylsiloxane film, which can stabilize the assembled nanoparticles. Combining with the endonuclease Nt.BbvCI assisted amplification method, a SERS biosensor is constructed for microRNA 21 (miRNA 21) assay. This platform presents a wide dynamic range (100 fM ∼1 nM), achieving a fabulous sensitivity with limit of detection of 11.96 fM for miRNA 21. Furthermore, after being bent 90° for 50 times, the Raman intensity of the flexible substrate shows a negligible change. This versatile flexible substrate exhibits considerable potential for SERS analysis, which also opens a new avenue for preparing flexible devices.

Highly Sensitive Flexible SERS-Based Sensing Platform for Detection of COVID-19.

COVID-19 continues to spread and has been declared a global emergency. Individuals with current or past infection should be identified as soon as possible to prevent the spread of disease. Surface-enhanced Raman spectroscopy (SERS) is an analytical technique that has the potential to be used to detect viruses at the site of therapy. In this context, SERS is an exciting technique because it provides a fingerprint for any material. It has been used with many COVID-19 virus subtypes, including Deltacron and Omicron, a novel coronavirus. Moreover, flexible SERS substrates, due to their unique advantages of sensitivity and flexibility, have recently attracted growing research interest in real-world applications such as medicine. Reviewing the latest flexible SERS-substrate developments is crucial for the further development of quality detection platforms. This article discusses the ultra-responsive detection methods used by flexible SERS substrate. Multiplex assays that combine ultra-responsive detection methods with their unique biomarkers and/or biomarkers for secondary diseases triggered by the development of infection are critical, according to this study. In addition, we discuss how flexible SERS-substrate-based ultrasensitive detection methods could transform disease diagnosis, control, and surveillance in the future. This study is believed to help researchers design and manufacture flexible SERS substrates with higher performance and lower cost, and ultimately better understand practical applications.

Quantitative SERS sensing mediated by internal standard Raman signal from silica nanoparticles in flexible polymer matrix

Attributed to poor signal uniformity and external interference, ultrasensitive surface-enhanced Raman spectroscopy (SERS) still faces difficulties in the reliable and quantitative detection of trace molecules. Here, a facile Ag/Si/sodium carboxy methyl cellulose (NaCMC) film with internal standard (IS) was promoted for quantitative determination of thiram. The effects of preparation conditions on SERS activity of the film were systematically investigated and then a flexible SERS substrate with high sensitivity and uniformity was fabricated. The enhancement factor was calculated to be 1.12 × 106 and SERS mapping was recorded with a relative standard deviation value of 19.8% by utilizing 4-mercaptobenzoic acid (4-MBA) as target molecule. Additionally, the dominant contribution of the IS from encapsulated Si nanoparticles (NPs) was confirmed in the quantitative assay of 4-MBA and thiram, facilitating attractive fitting coefficients (R2) as 0.991 and 0.998. Besides that, the proposed flexible film was conducted to scrub trace thiram from the surfaces of apple, orange, and cucumber, resulting in recoveries of 89%, 94%, and 91%. A smart and facile quantitative SERS substrate was developed here for monitoring trace biochemical molecules, verifying its potential utilizations in monitoring pesticide residues.

Pressing solids directly into sheets of plasmonic nanojunctions enables solvent-free surface-enhanced Raman spectroscopy

Often chemical analysis of solid materials begins with dissolving the sample in a solvent but this is undesirable, particularly if the physical form is important. In principle surface-enhanced Raman spectroscopy (SERS) should allow detection of solid analytes and offers attomolar sensitivity combined with molecular specificity. SERS requires the target molecules to sit in plasmonic hot-spots, which are normally nanojunctions, just a few nm across. This means that solid samples normally need to be dissolved in a solvent so they can diffuse into the enhancing region. Here, we show that SERS spectra of picograms of solid analytes can be directly obtained by pressing them into a flexible SERS substrate with a dense field of exposed nanojunction hot-spots anchored on its surface. We demonstrate that this can be a powerful tool for straightforward and non-destructive forensic analysis of layered materials (crossing ink lines), solid explosives and illicit drugs as well as for studying previously intractable samples such as pharmaceutical co-crystals, whose important solid state structure is lost when they are dissolved.

Depositing a flexible substrate of triangular silver nanoplates onto cotton fabrics for sensitive SERS detection

A flexible and effective substrate for surface-enhanced Raman spectroscopy (SERS) detection has been developed by dip coating of triangular silver nanoplates (TSNPs) onto cotton fabrics. The morphology, chemical composition and crystalline structure of the coated cotton fabrics were characterized by scanning electron microscope (SEM), ultraviolet-visible spectroscopy (UV-vis), X-ray photoelectron spectroscopic (XPS) and X-ray diffraction (XRD), respectively. These results confirmed the presence and well dispersion of the TSNPs on the surface of cotton fabrics. p-aminothiophenol (PATP) has been used as the probe molecule to evaluate the performance of the coated cotton fabrics as a flexible SERS substrate, and the PATP in diluted solutions can be detected with the concentration as low as 10−8 M. The results indicate that the TSNPs-coated fabrics exhibit sensitive SERS signals with excellent reproducibility and stability. The flexible SERS substrate has been demonstrated to rapidly detect carbaryl pesticides on fruits.

Development of nanofibrillated cellulose coated with gold nanoparticles for measurement of melamine by SERS

The objective of this study was to develop nanofibrillated cellulose (NFC)-based substrate for rapid detection of melamine in milk by surface-enhanced Raman spectroscopy (SERS). NFC were served as a highly porous platform to load with gold nanoparticles (AuNPs), which can be used as a flexible SERS substrate with nanoscale roughness to generate strong electromagnetic field in SERS measurement. The NFC/AuNP substrate was characterized by UV–Vis spectroscopy and electron microscopy. Milk samples contaminated by different concentrations of melamine were measured by SERS coupled with NFC/AuNP substrate. The spectral data analysis was conducted by multivariate statistical analysis [i.e. partial least squares (PLS)]. Satisfactory PLS result for quantification of melamine in milk was obtained (R = 0.9464). The detection limit for melamine extracted from liquid milk by SERS is 1 ppm, which meets the World Health Organization’s requirement of melamine in liquid milk. These results demonstrate that NFC/AuNP substrate has improved homogeneity and can be used in SERS analysis for food safety applications.

Floating silver film: A flexible surface-enhanced Raman spectroscopy substrate for direct liquid phase detection at gas–liquid interfaces

In recent years, surface-enhanced Raman spectroscopy (SERS) has developed rapidly and is used for the detection of molecules and biomolecules in liquids. However, few studies have focused on SERS using a water surface as the substrate. A floating metal film on water is desirable for an enhanced SERS performance. In this work, silver nanoparticles (Ag NPs) encased in poly(vinylpyrrolidone) films (Ag-PVP films) were synthesized on the surface of an aqueous solution by room temperature electron reduction. A floating silver film on a water surface was thereby achieved and is reported for the first time. The synthesized Ag-PVP film is an excellent flexible substrate for SERS and has other potential applications. Using the floating silver film as a flexible SERS substrate, 10–11 M of 4-aminothiophenol, 10–6 M of riboflavin, 10–9 M of 4-mercaptobenzoic acid, 10–7 M of 4-mercaptophenol, and 10–7 M of 4-aminobenzoic acid are identified, demonstrating potential use for the floating substrate in the liquid-phase detection of molecules.