Light-Trapping SERS Substrate with Regular Bioinspired Arrays for Detecting Trace Dyes

Breath Analysis via Surface Enhanced Raman Spectroscopy

Breath analysis is increasingly recognized as a powerful noninvasive diagnostic technique, and a plethora of exhaled volatile biomarkers have been associated with various diseases. However, traditional analytical methodologies are not amenable to high-throughput diagnostic applications at the point of need. An optical spectroscopic technique, surface-enhanced Raman spectroscopy (SERS), mostly used in the research setting for liquid sample analysis, has recently been applied to breath-based diagnostics. This promising noninvasive diagnostic tool has been demonstrated for the identification of various diseases, including lung cancer, gastric cancer, and diabetes. The versatility of SERS has enabled the use of different diagnostic strategies and allowed for fast and accurate detection of small analytes in exhaled breath. In this review, we provide an overview of recent advances in SERS-based breath analysis, focusing on sensors for the detection of gases and volatile organic compounds (VOCs) in exhaled breath, and highlight generic strategies for sample preconcentration and methods for spectral analysis. We aim to provide an overview of the state of the art and inspiration for further SERS investigation of expiration.

Factors that Affect Quantification in Surface-Enhanced Raman Scattering

Surface-enhanced Raman scattering (SERS) is an analytical technique capable of detecting trace amounts of specific species. The uniqueness of vibrational signatures is a major advantage of SERS. This combination of sensitivity and specificity has motivated researchers to develop diverse analytical methodologies leveraging SERS. However, even 50 years after its first observation, SERS is still perceived as an unreliable technique for quantification. This perception has precluded the application of SERS in laboratories that rely on consistent quantification (for regulatory purposes, for instance). In this review, we describe some of the aspects that lead to SERS intensity variations and how those challenges were addressed in the 50 years of the technique. The goal is to identify the sources of variations in SERS intensities and then demonstrate that, even with these pitfalls, the technique can be used for quantification when factors such as nature of the substrate, experimental conditions, sample preparation, surface chemistry, and data analysis are carefully considered and tailored for a particular application.

A highly structured Au-grafted nanoporous gold for surface-enhanced Raman scattering detection of ferbam

The expansive potential of surface-enhanced Raman scattering (SERS) has been well-established; however, the primary bottleneck hindering its routine analytical and commercial implementation is the poor signal reproducibility and challenges in substrate fabrication. Thus, the current work attempts to synthesize a scalable and reproducible nanoporous gold (npAu) decorated with gold (Au) nanoparticles to generate a highly structured Au@npAu nanocomposite. The substrate fabrication completes via three distinct routes: i) selective dealloying to form npAu on the Au film, ii) the fast deposition (i-t = -0.8 V, t = 10.0 s) of Au atoms across the npAu surface, and finally iii) the precise growth control of the generated Au@npAu by a series of by oxidation-reduction cycles (-0.03 to -0.4 V for 80.0 segments at ν = 50.0 mVs-1). The simulations of the dealloyed npAu and the final Au@npAu nanocomposite showed that the reduced interparticle spacing and ligament size in the Au@npAu nanocomposite is crucial for forming abundant "hot spot" regions with highly concentrated electromagnetic fields. The Au@npAu substrate reproducibility was assessed on 400.0 sites for SERS spectral acquisition with a relative standard deviation of 9.22 %. Furthermore, the Au@npAu was checked under different preparation batches for intra- and inter-day analysis and storage for 20.0 days with good stability. Finally, the substrate was checked for direct SERS detection of ferbam residues with a 4.34 × 10-9 mol L-1 sensitivity and examined in real samples with satisfactory recoveries (97.63 ± 1.95%-99.16 ± 0.24 %). This work offers a promising avenue towards highly reproducible, scalable and universal Au@npAu SERS substrate fabrication in diverse SERS-related applications.

Au nanoparticles decorated β-Bi2O3 as highly-sensitive SERS substrate for detection of methylene blue and methyl orange

In this work, Au/Bi2O3 was synthesized by loading Au nanoparticles (NPs) onto β-Bi2O3 by a simple solution reduction method. β-Bi2O3 was synthesized by a precipitation-thermal decomposition procedure, which results in significantly improved SERS detection limits down to 10-9 M for methylene blue (MB) and 10-7 M for methyl orange (MO) as probe molecules, comparable to those reported for the best semiconductor SERS substrates. In particular, further deposition of Au NPs (5.20% wt%) onto β-Bi2O3 results in a two-order-of-magnitude enhancement in detection sensitivity, achieving a detection limit of 10-11 M for MB and 10-9 M for MO. Under ultraviolet/visible irradiation, the Au/Bi2O3 hybrids substrate exhibits superior self-cleaning ability due to its photocatalytic degradation ability which can be applied repeatedly to the detection of pollutants. The advanced composite substrate simultaneously achieved ultra-low mass loading of Au NPs, outstanding detection performance, good reproducibility, high stability and self-cleaning ability. The development strategy of low load noble metal coupled high performance semiconductor β-Bi2O3 to obtain nano-hybrid materials provides a method to balance SERS sensitivity, cost effectiveness and operational stability, and can be synthesized in large quantities, which is a key step towards commercialization and has good reliability prospects.

Honeycomb-Inspired Surface-Enhanced Raman Scattering Microarray for Large-Area Automated Testing of Urease in Saliva Samples

Surface-enhanced Raman scattering (SERS) technology, as an important analytical tool, has been widely applied in the field of chemical and biomedical sensing. Automated testing is often combined with biochemical analysis technologies to shorten the detection time and minimize human error. The present SERS substrates for sample detection are time-consuming and subject to high human error, which are not conducive to the combination of SERS and automated testing. Here, a novel honeycomb-inspired SERS microarray is designed for large-area automated testing of urease in saliva samples to shorten the detection time and minimize human error. The honeycomb-inspired SERS microarray is decorated with hexagonal microwells and a homogeneous distribution of silver nanostars. Compared with the other four common SERS substrates, the optimal honeycomb-inspired SERS microarray exhibits the best SERS performance. The RSD of 100 SERS spectra continuously collected from saliva samples is 6.56%, and the time of one detection is reduced from 5 min to 10 s. There is a noteworthy linear relationship with a R2 of 0.982 between SERS intensity and urease concentration, indicating the quantitative detection capability of the urease activity in saliva samples. The honeycomb-inspired SERS microarray, combined with automated testing, provides a new way in which SERS technology can be widely used in biomedical applications.

Biomimetic SERS substrate with silicon-mediated internal standard: Improved sensing of environmental pollutants and nutrients

Biomimetic materials with fascinating natural micro-nano surface structures offer a good choice for the simple fabrication of surface-enhanced Raman scattering (SERS) substrate. This study presented a novel sodium carboxymethylcellulose (NaCMC)-Ag biomimetic substrate which was fabricated through the reverse replication of micro-nano structures from cantaloupe peel. Particularly, silicon nanoparticles (Si NPs) were doped into this flexible biomimetic substrate in its fabrication process. Abundant electromagnetic "hotspots" could be effectively excited in this Ag densely covered matrix which maintained numerous protrusions as well as vertical and horizontal grooves. Specifically, the doped Si NPs exhibited a robust intrinsic Raman peak, which could be employed as an internal standard to calibrate the target signal. In this regard, the biomimetic substrate with the optimal electromagnetic enhancement and the quantitative calibration capabilities exhibited a high enhancement factor and a remedied linear relationship in the detection. After a perfect uniformity of signal was proved by the corrected SERS mapping, the biomimetic SERS substrate was finally utilized in the practical analysis of methylene blue (MB) and β-carotene with ultra-low limit of detection, highlighting its importance in practical detection scenarios.

Polystyrene microspheres with ultra-rough surfaces engineered using RIE technique and applied using SERS

In this study, we successfully fabricated two ultra-rough surfaces based on polystyrene (PS) microspheres by employing the reactive ion etching (RIE) technique. Elemental analysis confirmed a stable AlF3 composition of the structures of these surfaces. We proposed the mechanism of the formation of these surfaces and performed SERS-related tests; the prepared substrates exhibited excellent SERS performance.

Photochemical decoration of gold nanoparticles on MoS2 nanoflowers grafted onto the flexible carbon cloth as a recyclable SERS sensor for the detection of antibiotic residues on curved surfaces

Surface-enhanced Raman spectroscopy (SERS)-based flexible substrate has recently been demonstrated to be effective in detecting molecules on curved surfaces, however a suitable method for fabricating the flexible SERS substrate still remains a hurdle. In this paper, we fabricated a flexible SERS substrate by anchoring the plasmonic gold nanoparticles (Au-NPs) onto the hydrothermally grown flower-like molybdenum disulfide (MoS2) grafted onto carbon cloth (CC) via a facile photoreduction route. Benefitting from the abundant hotspots generation of the Au-NPs and photo-induced charge-transfer ability of MoS2, the constructed Au-NPs/MoS2/CC substrate exhibit a superior SERS sensing ability, excellent SERS enhancement factor, high flexibility and mechanical stability towards the nitrofurantoin (NFT) with an ultra-low detection limit of 10-11 M. As a trial for practical applications, the flexible substrate was used to detect NFT (10-4 M) in the curved surfaces of meat samples via swab technique. The ability of the flexible Au-NPs/MoS2/CC substrate to sustain the robust Raman signals of NFT even after recycling up to 4 cycles validated its reusability. The proposed flexible SERS substrate with reusable capability indicates its great potential in practical applications for the detection of target molecules on the curved surfaces.

Advances in lithographic techniques for precision nanostructure fabrication in biomedical applications

Nano-fabrication techniques have demonstrated their vital importance in technological innovation. However, low-throughput, high-cost and intrinsic resolution limits pose significant restrictions, it is, therefore, paramount to continue improving existing methods as well as developing new techniques to overcome these challenges. This is particularly applicable within the area of biomedical research, which focuses on sensing, increasingly at the point-of-care, as a way to improve patient outcomes. Within this context, this review focuses on the latest advances in the main emerging patterning methods including the two-photon, stereo, electrohydrodynamic, near-field electrospinning-assisted, magneto, magnetorheological drawing, nanoimprint, capillary force, nanosphere, edge, nano transfer printing and block copolymer lithographic technologies for micro- and nanofabrication. Emerging methods enabling structural and chemical nano fabrication are categorised along with prospective chemical and physical patterning techniques. Established lithographic techniques are briefly outlined and the novel lithographic technologies are compared to these, summarising the specific advantages and shortfalls alongside the current lateral resolution limits and the amenability to mass production, evaluated in terms of process scalability and cost. Particular attention is drawn to the potential breakthrough application areas, predominantly within biomedical studies, laying the platform for the tangible paths towards the adoption of alternative developing lithographic technologies or their combination with the established patterning techniques, which depends on the needs of the end-user including, for instance, tolerance of inherent limits, fidelity and reproducibility.

A Solar-Driven Oil-Water Separator with Fluorescence Sensing Performance

Presently, the separation of oil and water through functional membranes inevitably entails either inefficient gravity-driven processes or energy-intensive vacuum pressure mechanisms. This study introduces an innovative photothermal evaporator that uses solar energy to drive oil-water separation while concurrently facilitating the detection of Fe3+ in wastewater. First, by alkali delignification, small holes were formed on the side wall of the large size tubular channel in the direction of wood growth. Subsequently, superhydrophilic SiO2 nanoparticles were in situ assembled onto the sidewalls of the tubular channels. Finally, carbon quantum dots were deposited by spin-coating on the surface of the evaporator, paralleling the growth direction of the wood. During the photothermal evaporation process, the tubular channels with small holes in the side wall parallel the bulk water, which not only ensures the effective water supply to the photothermal surface but also reduces the heat loss caused by water reflux on the photothermal surface. The superhydrophilic SiO2 nanoparticles confer both hydrophilic and oleophobic properties to the evaporator, preventing the accumulation of minute oil droplets within the device and achieving sustained and stable oil-water separation over extended periods. These carbon quantum dots exhibit capabilities for both photothermal conversion and fluorescence transmission. This photothermal evaporator achieves an evaporation rate as high as 2.3 kg m-2 h-1 in the oil-water separation process, and it has the ability to detect Fe3+ concentrations in wastewater as low as 10-9 M.

A highly stable terbium(III) metal-organic framework MOF-76(Tb) for hydrogen storage and humidity sensing

The present study described the synthesis and characterization of MOF-76(Tb) for hydrogen storage and humidity sensing applications. The structure and morphology of as-synthesized material were studied using powder X-ray diffraction, scanning, and transmission electron microscopy. The crystal structure of MOF-76(Tb) consists of terbium(III) and benzene-1,3,5-tricarboxylate(-III) ions, one coordinated aqua ligand and one crystallization N,N´-dimethylformamide molecule. The polymeric framework of MOF-76(Tb) contains 1D sinusoidally shaped channels with sizes of 6.6 × 6.6 Å propagating along c crystallographic axis. The thermogravimetric analysis of the prepared material exhibited thermal stability up to 600 °C. At 77 K and pressure up to 20 bar; 0.6 wt.% hydrogen storage capacity for MOF-76(Tb) was observed. Finally, the humidity sensing measurements (water adsorption experiments) were performed, and the results indicate that MOF-76(Tb) is not a suitable material for moisture sensing applications.

Electric field-induced assembly of Au-Ag alloy nanoparticles into nano-reticulation for ultrasensitive SERS

This paper discusses a method for assembling Au-Ag alloy nanoparticles (NPs) using direct current (DC) electric field to fabricate highly active SERS substrates. Different nanostructures could be obtained by regulating the intensity and action time of DC electric field. Under the condition of 5mA*10 min, we obtained Au-Ag alloy nano-reticulation (ANR) substrate with excellent SERS activity (Enhancement factor on order of magnitude of 10⁶). ANR substrate has excellent SERS performance due to the resonance matching between its LSPR mode and excitation wavelength. The uniformity of the Raman signal on ANR is greatly improved than bare ITO glass. ANR substrate also has the ability to detect multiple molecules: ANR substrate can respectively detect Rh6G and CV molecules with a concentration as low as 10^-10 M and 10^-9 M and the Raman spectral intensity of the probe molecules on the surface of the ANR substrate has good linear correlation with the molecular concentration (R²> 0.95). In addition, ANR substrate can detect both thiram and aspartame (APM) molecules far below (thiram for 0.0024 ppm and APM for 0.0625 g/L) the safety standard, which demonstrate its practical application potential.

Fabrication of Silver Nanobowl Arrays on Patterned Sapphire Substrate for Surface-Enhanced Raman Scattering

The current article discusses surface-enhanced Raman spectroscopy (SERS) as a powerful technique for detecting molecules or ions by analyzing their molecular vibration signals for fingerprint peak recognition. We utilized a patterned sapphire substrate (PSS) featuring periodic micron cone arrays. Subsequently, we prepared a three-dimensional (3D) PSS-loaded regular Ag nanobowls (AgNBs) array using self-assembly and surface galvanic displacement reactions based on polystyrene (PS) nanospheres. The SERS performance and structure of the nanobowl arrays were optimized by manipulating the reaction time. We discovered that the PSS substrates featuring periodic patterns exhibited superior light-trapping effects compared to the planar substrates. The SERS performance of the prepared AgNBs-PSS substrates was tested under the optimized experimental parameters with 4-mercaptobenzoic acid (4-MBA) as the probe molecule, and the enhancement factor (EF) was calculated to be 8.96 × 10⁴. Finite-difference time-domain (FDTD) simulations were conducted to explain that the AgNBs arrays' hot spots were distributed at the bowl wall locations. Overall, the current research offers a potential route for developing high-performance, low-cost 3D SERS substrates.

Spiropyran-Based Soft Substrate with SPR, Anti-Reflection and Anti-NRET for Enhanced Visualization/Fluorescence Dual Response to Metal Ions

The photoluminescence of modified spiropyran on solid surfaces is poor, and the fluorescence intensity of its MC form is weak, which affects its application in the field of sensing. In this work, a PMMA layer containing Au nanoparticles and a spiropyran monomolecular layer are coated on the surface of a PDMS substrate with inverted micro-pyramids successively by means of interface assembly and soft lithography, and the overall structure is similar to insect compound eyes. The anti-reflection effect of the bioinspired structure, the SPR (surface plasmon resonance) effect of the Au nanoparticles and the anti-NRET (non-radiation energy transfer) effect of the PMMA isolation layer raise the fluorescence enhancement factor of the composite substrate vs. the surface MC form of spiropyran to 5.06. In the process of metal ion detection, the composite substrate can achieve both colorimetric and fluorescence response, and the detection limit for Zn²⁺ can reach 0.281 μM. However, at the same time, the lack of the ability to recognize specific metal ions is expected to be further improved by the modification of spiropyran.

Promising Mass-Productive 4-Inch Commercial SERS Sensor with Particle in Micro-Nano Porous Ag/Si/Ag Structure Using in Auxiliary Diagnosis of Early Lung Cancer

The construction of commercial surface enhanced Raman scattering (SERS) sensors suitable for clinical applications is a pending problem, which is heavily limited by the low production of high-performance SERS bases, because they usually require fine or complicated micro/nano structures. To solve this issue, herein, a promising mass-productive 4-inch ultrasensitive SERS substrate available for early lung cancer diagnosis is proposed, which is designed with a special architecture of particle in micro-nano porous structure. Benefitting from the effective cascaded electric field coupling inside the particle-in-cavity structure and efficient Knudsen diffusion of molecules within the nanohole, the substrate exhibits remarkable SERS performance for gaseous malignancy biomarker, with the limit of detection is 0.1 ppb and the average relative standard deviation value at different scales (from cm² to µm² ) is ≈16.5%. In practical application, this large-sized sensor can be further divided into small ones (1 × 1 cm² ), and more than 65 chips will be obtained from just one 4-inch wafer, greatly increasing the output of commercial SERS sensor. Further, a medical breath bag composed of this small chip is designed and studied in detail here, which suggested high-specificity recognition for lung cancer biomarker in mixed mimetic exhalation tests.

Advances in surface-enhanced Raman spectroscopy technology for detection of foodborne pathogens

Rapid control and prevention of diseases caused by foodborne pathogens is one of the existing food safety regulatory issues faced by various countries and has received wide attention from all sectors of society. The development of rapid and reliable detection methods for foodborne pathogens remains a hot research area for food safety and public health because of the limitations of complex steps, time-consuming, low sensitivity, or poor selectivity of commonly used methods. Surface-enhanced Raman spectroscopy (SERS), as a novel spectroscopic technique, has the advantages of high sensitivity, selectivity, rapid and nondestructive detection and has exhibited broad application prospects in the determination of pathogenic bacteria. In this study, the enhancement mechanisms of SERS are briefly introduced, then the characteristics and properties of liquid-phase, rigid solid-phase, and flexible solid-phase are categorized. Furthermore, a comprehensive review of the advances in label-free or label-based SERS strategies and SERS-compatible techniques for the detection of foodborne pathogens is provided, and the advantages and disadvantages of these methods are reviewed. Finally, the current challenges of SERS technology applied in practical applications are listed, and the possible development trends of SERS in the field of foodborne pathogens detection in the future are discussed.

Computational Investigation of Advanced Refractive Index Sensor Using 3-Dimensional Metamaterial Based Nanoantenna Array

Photonic researchers are increasingly exploiting nanotechnology due to the development of numerous prevalent nanosized manufacturing technologies, which has enabled novel shape-optimized nanostructures to be manufactured and investigated. Hybrid nanostructures that integrate dielectric resonators with plasmonic nanostructures are also offering new opportunities. In this work, we have explored a hybrid coupled nano-structured antenna with stacked multilayer lithium tantalate (LiTaO₃) and Aluminum oxide (Al₂O₃), operating at wavelength ranging from 400 nm to 2000 nm. Here, the sensitivity response has been explored of these nano-structured hybrid arrays. It shows a strong electromagnetic confinement in the separation gap (g) of the dimers due to strong surface plasmon resonance (SPR). The influences of the structural dimensions have been investigated to optimize the sensitivity. The designed hybrid coupled nanostructure with the combination of 10 layers of gold (Au) and Lithium tantalate (LiTaO₃) or Aluminum oxide (Al₂O₃) (five layers each) having height, h₁ = h₂ = 10 nm exhibits 730 and 660 nm/RIU sensitivity, respectively. The sensitivity of the proposed hybrid nanostructure has been compared with a single metallic (only gold) elliptical paired nanostructure. Depending on these findings, we demonstrated that a roughly two-fold increase in the sensitivity (S) can be obtained by utilizing a hybrid coupled nanostructure compared to an identical nanostructure, which competes with traditional sensors of the same height, (h). Our innovative novel plasmonic hybrid nanostructures provide a framework for developing plasmonic nanostructures for use in various sensing applications.

A Bioinspired Ag Nanoparticle/PPy Nanobowl/TiO2 Micropyramid SERS Substrate

In this paper, the micropyramid structure was transferred to the TiO₂ substrate by soft imprinting. Then, the PPy nanobowls were assembled onto the surface of the TiO₂ micropyramids through the induction of the PS template. Finally, a layer of Ag nanoparticles was deposited on the surface of PPy nanobowls to form a novel Ag nanoparticle/PPy nanobowl/TiO₂ micropyramid SERS substrate. Its structure is similar to the bioinspired compound eyes. This substrate exhibited excellent antireflection, ultra-sensitivity, excellent uniformity, and recyclability. The concentration of R6G molecules can be detected as low as 10^-9 mol/L, and the Raman enhancement factor can reach 3.4 × 10⁵. In addition, the excellent catalytic degradation performance of the substrate ensures recyclability. This work proves that the micropyramid structure can be applied to other SERS materials besides silicon by the above methods, which broadens the selection range of composite SERS materials.

LSPR Tunable Ag@PDMS SERS Substrate for High Sensitivity and Uniformity Detection of Dye Molecules

At present, the use of efficient and cost-effective methods to construct plasmonic surface-enhanced Raman scattering (SERS) substrates of high sensitivity, uniformity and reproducibility is still crucial to satisfy the practical application of SERS technology. In this paper, a localized surface plasmonic resonance (LSPR) tunable flexible Ag@PDMS substrate was successfully constructed by the low-cost bio-template-stripping method and magnetron sputtering technology. The theory proves that the local electromagnetic field enhancement and "hot spot" distribution is adjustable by modifying the size of the optical cavity unit in the periodicity nanocavity array structure. Experimentally, using rhodamine 6G (R6G) as the target analyte, the SERS performance of optimal Ag@PDMS substrate (Ag film thickness for 315 nm) was researched in detail, which the minimum detection limit was 10^-11 M and the enhancement factor was calculated as 8.03 × 10⁸, indicating its high sensitivity. The relative standard deviation (RSD) was calculated as 10.38%, showing that the prepared substrate had excellent electromagnetic field enhancement uniformity. At last, the trace detection of Crystal violet (CV, LOD = 10^-9 M) and the simultaneous detection of three common dyes (R6G, CV and Methylene blue (MB) mixture) were also realized. This result suggests that the SERS substrate has a good application prospect in the quantitative and qualitative detection of dye molecules.

Machine Learning-Driven 3D Plasmonic Cavity-in-Cavity Surface-Enhanced Raman Scattering Platform with Triple Synergistic Enhancement Toward Label-Free Detection of Antibiotics in Milk

The surface-enhanced Raman scattering (SERS) technique with ultrahigh sensitivity has gained attention to meet the increasing demands for food safety analysis. The integration of machine learning and SERS facilitates the practical applicability of sensing devices. In this study, a machine learning-driven 3D plasmonic cavity-in-cavity (CIC) SERS platform is proposed for sensitive and quantitative detection of antibiotics. The platform is prepared by transferring truncated concave nanocubes (NCs) to an obconical-shaped template surface. Owing to the triple synergistic enhancement effect, the highly ordered 3D CIC arrays improve the simulated electromagnetic field intensity and experimental SERS activity, demonstrating a 33.1-fold enhancement compared to a typical system consisting of Au NCs deposited on a flat substrate. The integration of machine learning and Raman spectroscopy eliminates subjective judgments on the concentration of detectors using a single feature peak and achieves accurate identification. The machine learning-driven CIC SERS platform is capable of detecting ampicillin traces in milk with a detection limit of 0.1 ppm, facilitating quantitative analysis of different concentrations of ampicillin. Therefore, the proposed platform has potential applications in food safety monitoring, health care, and environmental sampling.

Synthesis of Dimpled Particles by Seeded Emulsion Polymerization and Their Application in Superhydrophobic Coatings

Dimpled particles are synthesized through the seeded polymerization of fluoroacrylate and styrene on swelled polystyrene spheres. The morphologies of the particles can be controlled by the polymerization temperature, the amount of solvent swelling the seeds or the ratio of the fluoroacrylate monomer over styrene. Golf-ball-like particles with many small dimples on their surfaces are obtained at low polymerization temperatures or with a small amount of solvent. Particles with a large single dimple are formed at higher polymerization temperatures, with larger solvent amounts or a higher ratio of fluoroacrylate over styrene. The morphology formation mechanism of these dimpled particles is proposed and the application of these particles in the fabrication of superhydrophobic coatings is demonstrated.

A Novel Bio-Inspired Ag/3D-TiO2/Si SERS Substrate with Ordered Moth-like Structure

This paper reports a novel method to fabricate a bio-inspired SERS substrate with low reflectivity, ultra-sensitivity, excellent uniformity, and recyclability. First, double layers of polystyrene spheres with different particle sizes were assembled on the surface of a silicon wafer to act as a moth-like template. Second, through the template sacrifice method, the TiO₂ film with a three-dimensional moth-like eye structure was induced by the double-layer polystyrene spheres in the previous step, and its microscopic morphology showed a high degree of order. Finally, Ag nanoparticles were assembled on the TiO₂ film to form a bio-inspired SERS substrate. This ordered bio-inspired structure can not only reduce reflection, but also reinforce the uniformity of hotspot density, which helps to improve the sensitivity and uniformity of the Raman signal. This bio-inspired SERS substrate can detect R6G molecules at a concentration as low as 1.0 × 10^-10 mol/L, and its enhancement factor (EF) can reach 6.56 × 10⁶. In addition, the composite of Ag and TiO₂ can realize the photocatalytic degradation of R6G and then realize the recyclability of the SERS substrate.

Flexible surface-enhanced Raman scatting substrates: recent advances in their principles, design strategies, diversified material selections and applications

Surface-enhanced Raman scattering (SERS) is widely used as a powerful analytical technology in cutting-edge areas such as food safety, biology, chemistry, and medical diagnosis, providing ultra-fast, ultra-sensitive, nondestructive characterization and achieving ultra-high detection sensitivity even down to the single-molecule level. Development of Raman spectroscopy is strongly dependent on high-performance SERS substrates, which have long evolved from the early days of rough metal electrodes to periodic nanopatterned arrays building on solid supporting substrates. For rigid SERS substrates, however, their applications are restricted by sophisticated pretreatments for detecting solid samples with non-planar surfaces. It is therefore essential to reassert the principles in constructing flexible SERS substrates. Herein, we comprehensively review the state-of-the-art in understanding, preparing and using flexible SERS. The basic mechanisms behind the flexible SERS are briefly outlined, typical design strategies are highlighted and diversified selection of materials in preparing flexible SERS substrates are reviewed. Then the recent achievements of various interdisciplinary applications based on flexible SERS substrates are summarized. Finally, the challenges and perspectives for future evolution of flexible SERS and their applications are demonstrated. We propose new research directions focused on stimulating the real potential of SERS as an advanced analytical technique for commercialization.

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 10 ng/mL, and the linear range is 10-1000 ng/mL. Furthermore, the flexible SERS substrates were successfully employed to detect thiram residues on factual apple samples, and trace amount (1 ng/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.

Effect of synthesis time on plasmonic properties of Ag dendritic nanoforests

The effects of synthesis time on the plasmonic properties of Ag dendritic nanoforests on Si substrate (Ag-DNF/Si) samples synthesized through the fluoride-assisted galvanic replacement reaction were investigated. The Ag-DNF/Si samples were characterized using scanning electron microscopy, energy-dispersive X-ray spectroscopy, reflection spectroscopy, X-ray diffraction and surface-enhanced Raman spectroscopy (SERS). The prolonged reaction time led to the growth of an Ag-DNF layer and etched Si hole array. SEM images and variations in the fractal dimension index indicated that complex-structure, feather-like leaves became coral-like branches between 30 and 60 min of synthesis. The morphological variation during the growth of the Ag DNFs resulted in different optical responses to light illumination, especially those of light harvest and energy transformation. The sample achieved the most desirable light-to-heat conversion efficiency and SERS response with a 30 min growth time. A longer synthesis time or thicker Ag-DNF layer on the Si substrate did not have superior plasmonic properties.

Effective Enrichment of Plasmonic Hotspots for SERS by Spinning Droplets on a Slippery Concave Dome Array

Surface-enhanced Raman scattering (SERS) detection requires dense hotspots and a uniform distribution of analytes to obtain a stable signal with good repeatability. However, due to the coffee-ring effect on the hydrophilic substrate, and the difficulty of droplet manipulation on the superhydrophobic substrate, few substrates can ensure that the analytes are evenly distributed. In this work, we develop a method that can efficiently enrich plasmonic hotspots for SERS measurement on the superhydrophobic concave dome array (SCDA). The SCDA is formed by spraying hydrophobic silica nanoparticles onto a polydimethylsiloxane (PDMS) slab with a concave dome array that can physically confine the droplets and overcome the coffee-ring effect. During droplet evaporation, the SCDA is driven by a horizontal spinner, and the droplets spin on the SCDA, enabling the plasmonic nanoparticles to become closely packed to form the SERS hotspots. The limit of detection (LOD) of the dynamic-enriched SERS hotspots for crystal violet and methylene blue can reach up to 10−11 M. Moreover, the LOD for melamine in milk can reach 5 × 10−7 M, which is lower than the safety threshold defined by the Food and Drug Administration (FDA). Based on this SERS platform, an effective, low-cost, and simple method for SERS detection in analytical chemistry and food safety is highly expected.

Rapid and non-invasive surface-enhanced Raman spectroscopy (SERS) detection of chlorpyrifos in fruits using disposable paper-based substrates charged with gold nanoparticle/halloysite nanotube composites

Chlorpyrifos is one of the most widely used organophosphate insecticides in agricultural production. Nevertheless, the residues of chlorpyrifos in agricultural by-product seriously threaten human health. Thus, the ultrasensitive detection of chlorpyrifos residues in agri-food products is of great demand. Herein, an AuNP/HNT-assembled disposable paper SERS substrate was prepared by an electrostatic self-assembly method to detect chlorpyrifos residues. The AuNP/HNT paper substrate exhibited high SERS activity, good reproducibility, and long-term stability, which was successfully used for quantitative detection of chlorpyrifos; the detection limit reached 7.9 × 10^-9 M. For spiked apple samples the calculated recovery was 87.9% with a RSD value of 6.1%. The excellent detection ability of AuNP/HNT paper-based SERS substrate indicated that it will play an important role in pesticide detection in the future. AuNP/HNT assembled disposable paper SERS substrate was prepared by an electrostatic self-assembly method to detect chlorpyrifos residues in fruits.

Large-Area Monolayer Films of Hexagonal Close-Packed Au@Ag Nanoparticles as Substrates for SERS-Based Quantitative Determination

In this work, quasi-spherical, small-sized, citrate-stabilized, core-shell (CS)-structured Au_5.5@Ag_m nanoparticles (NPs) with Ag shells of controlled thicknesses (m = 0, 1.25, 3.25, and 5.25) were successfully synthesized by using Au NPs with sizes of 5.5 nm as seeds. The as-prepared Au@Ag NPs after the phase transfer process were further used for the fabrication of high-quality large-area monolayer films of hexagonal close-packed Au@Ag nanoparticles (LAMF-HCP-Au@Ag NPs) by our improved self-assembly at the interface of toluene-DEG containing a proper amount of water (10% v/v). Moreover, after transferring the as-prepared LAMF-HCP-Au@Ag NPs onto polydimethylsiloxane (PDMS) substrates (LAMF-HCP-Au@Ag NP@PDMS substrates), the resulting LAMF-HCP-Au@Ag NP@PDMS substrates can exhibit uniformity in the intensity of the surface-enhanced Raman scattering signals. Furthermore, taking LAMF-HCP-Au_5.5@Ag_5.25 NP@PDMS substrates as an example, they can achieve quantitative detection with high sensitivity for crystal violet (CV) and 4-aminothiophenol (4-ATP) in the range from 10^-12 to 10^-7 M and from 10^-13 to 10^-7 M, respectively. Also, their limit of detection (LOD) for CV and 4-ATP are 10^-12 and 10^-13 M, respectively. Especially, the LOD for CV can also be as low as 10^-13 M by extending the immersing time.

A Simple Polypyrrole/Polyvinylidene Fluoride Membrane with Hydrophobic and Self-Floating Ability for Solar Water Evaporation

The traditional hydrophobic solarevaporator is generally obtained through the modification of alkyl or fluoroalkyl on the photothermal membrane. However, the modified groups can easily be oxidized in the long-term use process, resulting in the poor salt resistance and stability of photothermal membrane. In order to solve this problem, a simple polypyrrole/polyvinylidene fluoride membrane, consisting of an intrinsic hydrophobic support (polyvinylidene fluoride) and a photothermal material (polypyrrole), was fabricated by ultrasonically mixing and immersed precipitation. This photothermal membrane showed good self-floating ability in the process of water evaporation. In order to further improve the photothermal conversion efficiency, a micropyramid structure with antireflective ability was formed on the surface of membrane by template method. The micropyramids can enhance the absorption efficiency of incident light. The water evaporation rate reached 1.42 kg m⁻² h⁻¹ under 1 sun irradiation, and the photothermal conversion efficiency was 88.7%. The hydrophobic polyvinylidene fluoride ensures that NaCl cannot enter into membrane during the evaporation process of the brine, thus realizing the stability and salt resistance of polypyrrole/polyvinylidene fluoride in 3.5%wt and 10%wt NaCl solution.

The Design and Optimization of an Anti-Reflection Coating and an Intermediate Reflective Layer to Enhance Tandem Solar Cell Photons Capture

We have theoretically demonstrated an efficient way to improve the optical properties of an anti-reflection coating (ARC) and an intermediate reflective layer (IRL) to enhance tandem solar cell efficiency by localizing the incident photons’ energy on a suitable sub-cell. The optimum designed ARC from a one-dimensional ternary photonic crystal, consisting of a layer of silicon oxynitride (SiON), was immersed between two layers of (SiO2); thicknesses were chosen to be 98 nm, 48 nm, and 8 nm, respectively. The numerical results show the interesting transmission properties of the anti-reflection coating on the viable and near IR spectrum. The IRL was designed from one-dimensional binary photonic crystals and the constituent materials are Bi4Ge3O12 and μc-SiOx: H with refractive indexes was 2.05, and 2.8, respectively. The numbers of periods were set to 10. Thicknesses: d1 = 62 nm and d2 = 40 nm created a photonic bandgap (PBG) in the range of [420 nm: 540 nm]. By increasing the second material thickness to 55 nm, and 73 nm, the PBG shifted to longer wavelengths: [520 nm: 630 nm], and [620 nm: 730 nm], respectively. Thus, by stacking the three remaining structures, the PBG widened and extended from 400 nm to 730 nm. The current theoretical and simulation methods are based on the fundamentals of the transfer matrix method and finite difference time domain method.

Surface-enhanced Raman scattering-based lateral flow immunoassay mediated by hydrophilic-hydrophobic Ag-modified PMMA substrate

Recently, it is urgent to ameliorate the accumulation and quantification performances of surface-enhanced Raman scattering-based lateral flow immunoassay (SERS-based LFIA) to promote its reliable clinical application. Herein, a smart hydrophilic-hydrophobic SERS-based LFIA strip was demonstrated by decorating Ag nanoplates with hydrophilic surface onto the specific regions of hydrophobic polymethylmethacrylate (PMMA) film with Raman internal standard (IS), which can unexpectedly inhibit the "coffee-ring phenomenon". The target analytes were consequently enriched in the SERS-active Ag regions by the hydrophobic PMMA, considerably endowing the strip with amended quantitative monitoring ability. Aided by immunoprobes of flower-shaped Ag nanoplates, a limit of detection as 10 pg/mL and an outstanding correlation coefficient value (R²) of 0.992 for carcinoembryonic antigen (CEA) were obtained by utilizing this SERS-based LFIA strip, which can be conducive to clinical monitoring and will broaden the field of vision for the point-of-care diagnostic technique.

Micro-Nano Machining TiO2 Patterns without Residual Layer by Unconventional Imprinting

Usually, the residual layer remains after patterning TiO2 sol. The existence of the TiO2 residual layer in the non-pattern region affects its application in microelectronic devices. Here, a simple method, based on room-temperature imprinting, to fabricate a residual-free TiO2 pattern is proposed. The thermoplastic polymer with Ti4+ salt was fast patterned at room temperature by imprinting, based on the different interfacial force. Then, the patterned thermoplastic polymer with Ti4+ salt was induced into the TiO2 lines without residual layer under the hydrothermal condition. This method provides a new idea to pattern metal oxide without residual layer, which is potentially applied to the gas sensor, the optical detector and the light emitting diode.

SERS Selective Enhancement on Monolayer MoS2 Enabled by a Pressure-Induced Shift from Resonance to Charge Transfer

As a newly emerging approach for surface-enhanced Raman spectroscopy (SERS), pressure-induced SERS (PI-SERS) has been attracting increasing interest for its applications in Raman signal enhancement at extreme conditions. However, how to efficiently realize the PI-SERS enhancement and elucidate the corresponding mechanism remain open questions. Herein, we demonstrate the PI-SERS enhancement up to 8.04 GPa using monolayer molybdenum disulfide (ML-MoS₂) as a SERS substrate and three organic molecules with similar energy levels but different symmetries as probes. The combined theory and experiment results show that a pressure-induced increase in the Fermi level of the ML-MoS₂ substrate and a decrease in the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) energy gap of probe molecules lead to a transition from the multiple resonance-related SERS enhancement to charge transfer (CT)-dominated PI-SERS selective enhancement, depending on the incident laser energy and the pressure applied. Such PI-SERS selective enhancement has been discussed in the framework of CT-induced strengthening of electron-phonon coupling, as well as a possible match of the structural symmetries between probe molecules and the substrate. This study provides deep insights into our understanding of PI-SERS enhancement, and the revealed mechanism can be extended to other molecules for SERS at extreme conditions.

A novel photothermal, self-healing and anti-reflection water evaporation membrane

For the solar water evaporation system, there are no reports on the self-healing support, which is crucial for the sustainable use of solar evaporation membrane. In this work, a self-healing hydrogel is prepared via free radical copolymerization with covalent cross-linking and coordination cross-linking and is used as a photothermal water evaporation support. The photothermal material is then introduced into the hydrogel by physically doping acetylene carbon black. At the same time, inverted micro-pyramids are fabricated on the surface of the hydrogel by soft imprint to increase the utilization efficiency of incident light. The water evaporation rate of the composite membrane can reach 1.58 kg m-2 h-1, and the breaking elongation is 1580%. It can self-heal after it is completely broken, and can still be stretched 3 times its original length. The design of this self-healing photothermal membrane will provide a new idea for its application in a harsh outdoor environment.