Concluding Remarks : Surface enhanced Raman scattering

Surface-enhanced Raman scattering spectroscopy monitoring and degradation of organic pollutants using a novel nanowire

A multifunctional Ag/AlOOH nanowires (ANW) composite substrate was constructed, which not only accomplishes highly sensitive detection of organic dye molecules, but also has excellent performance in the degradation of pollutants. The ANW in the Ag/ANW substrate possesses a high aspect ratio, which extends the distribution area of Ag and enables a large number of hot spots on the active substrate. Additionally, due to the abundant OH groups on the ANW, there is an increased number of anchor sites for adsorbed metal ions in the Ag/ANW compound, thus contributing to the enhancement and degradation of molecules. Moreover, the constructed multifunctional Ag/ANW nanocomplexes also show great promise for practical applications, providing a reference for the detection and degradation of contaminants.

A simple and efficient alginate hydrogel combined with surface-enhanced Raman spectroscopy for quantitative analysis of sodium nitrite in meat products

Sodium nitrite is a commonly used preservative and color protectant in the food industry. Conventional analytical methods are highly susceptible to food matrix interference, time-consuming and costly. In this study, the ion cross-linking method was employed to prepare alginate hydrogel substrates, and phenosafranin was chosen as a single-molecule probe to analyze sodium nitrite. Our investigation centered on elucidating the effects of alginate and cross-linking ion concentrations on Raman signal characteristics. The optimal Raman response was observed in the precursor solution with 1% sodium alginate and 0.1 mol L-1 cross-linking ions. The relative standard deviations (RSDs) of the feature peaks from the three substrate batches ranged from 1.22% to 16.30%, attesting the robustness and consistency of the substrates. The signal reduction of the substrates after a four-week storage period remained below 10%, indicating that the substrates had good reproducibility and stability. The limits of detection (LODs) for sodium nitrite in extracts from cured meat, luncheon meat, and sliced ham were determined to range from 3.75 mg kg-1 to 8.11 mg kg-1, with low interference from the food matrix. The support vector machine algorithm was utilized to train and predict the data, which proved to be more accurate (98.6%-99.8% recovery) than the traditional linear regression model (81.9%-112.7% recovery) in predicting the spiked samples. The application of hydrogel-based surface-enhanced Raman spectroscopy (SERS) substrates for nitrite detection in food, combined with machine learning for regression prediction in data processing, collectively augmented the potential of SERS technology in the field of food analysis.

'When is a hotspot a good nanospot' - review of analytical and hotspot-dominated surface enhanced Raman spectroscopy nanoplatforms

Substrate development in surface-enhanced Raman spectroscopy (SERS) continues to attract research interest. In order to determine performance metrics, researchers in foundational SERS studies use a variety of experimental means to characterize the nature of substrates. However, often this process would appear to be performed indiscriminately without consideration for the physical scale of the enhancement phenomena. Herein, we differentiate between SERS substrates whose primary enhancing structures are on the hundreds of nanometer scale (analytical SERS nanosubstrates) and those whose main mechanism derives from nanometric-sized gaps (hot-spot dominated SERS substrates), assessing the utility of various characterization methods for each substrate class. In this context, characterization approaches in white-light spectroscopy, electron beam methods, and scanning probe spectroscopies are reviewed. Tip-enhanced Raman spectroscopy, wavelength-scanned SERS studies, and the impact of surface hydrophobicity are also discussed. Conclusions are thus drawn on the applicability of each characterization technique regarding amenability for SERS experiments that have features at different length scales. For instance, while white light spectroscopy can provide an indication of the plasmon resonances associated with 10 s-100 s nm-scale structures, it may not reveal information about finer surface texturing on the true nm-scale, critical for SERS' sensitivity, and in need of investigation via scanning probe techniques.

Smart optical sensing of multiple antibiotic residues from wastewater effluents with ensured specificity using SERS assisted with multivariate analysis

Surface-enhanced Raman spectroscopy offers great potential for rapid and highly sensitive detection of pharmaceuticals from environmental sources. Herein, we investigated the feasibility of label-free sensing of antibiotic residues from wastewater effluents with high specificity by combining with multivariate analysis. Highly ordered silver nanoarrays with ∼34 nm roughness have been fabricated using a cost-effective electroless deposition technique. As-fabricated Ag arrays showed superior LSPR effects with an enhancement factor of 8 × 107. Excellent reproducibility has also been noticed with RSD values within 11%, whilst the sensor showed good stability and reusability characteristics for being used as a low-cost and reusable sensor. SERS studies demonstrated that antibiotics-spiked wastewater effluents can be detected with high efficiency in a label-free method. The molecular fingerprint bands of antibiotics such as sulfamethoxazole, sulfadiazine, and ciprofloxacin were well analyzed in effluent, tap, and deionized water. It has been found that antibiotics can be detected near picomolar levels; meanwhile, liquid chromatography-mass spectrometry (LC-MS) exhibited a detection limit within nanomolar concentrations only. Furthermore, the specificity of SERS sensing has been further analyzed using a multivariate analysis method, principal component analysis followed by linear discriminant analysis (PCA-LDA); which showed prominent discrimination to distinguish each antibiotic residue from wastewater effluents. The current study presented the potential of Ag nanoarray sensors for rapid, highly specific, and cost-effective analysis of pharmaceutical products for environmental remediation applications.

Improving Sensitivity and Reproducibility of Surface-Enhanced Raman Scattering Biochips Utilizing Magnetoplasmonic Nanoparticles and Statistical Methods

Surface-enhanced Raman scattering (SERS) technology has been widely recognized for its remarkable sensitivity in biochip development. This study presents a novel sandwich immunoassay that synergizes SERS with magnetoplasmonic nanoparticles (MPNs) to improve sensitivity. By taking advantage of the unique magnetism of these nanoparticles, we further enhance the detection sensitivity of SERS biochips through the applied magnetic field. Despite the high sensitivity, practical applications of SERS biochips are often limited by the issues of stability and reproducibility. In this study, we introduced a straightforward statistical method known as "Gaussian binning", which involves initially binning the two-dimensional Raman mapping data and subsequently applying Gaussian fitting. This approach enables a more consistent and reliable interpretation of data by reducing the variability inherent in Raman signal measurements. Based on our method, the biochip, targeting for C-reactive protein (CRP), achieves an impressive detection limit of 5.96 fg/mL, and with the application of a 3700 G magnetic field, it further enhances the detection limit by 5.7 times, reaching 1.05 fg/mL. Furthermore, this highly sensitive and magnetically tunable SERS biochip is easily designed for versatile adaptability, enabling the detection of other proteins. We believe that this innovation holds promise in enhancing the clinical applicability of SERS biochips.

Smart Ratiometric SERS Nanoprobe for Real-Time Monitoring Hydrogen Peroxide in Living Cells during NADH Treatment Associated with Ferroptosis

Studying the oxidative stress, especially the reactive oxygen species (ROS) response of ferroptosis, is crucial for the diagnosis and treatment of cancer based on ferroptosis. However, reliable quantitative analysis of intracellular ROS in cancer treatment for drug screening is still a challenge. Herein, a superior ratiometric SERS nanoprobe was developed for in situ, real-time, and highly sensitive detection of content variation of H2O2 within living cells. The SERS nanoprobe was prepared by coassembly of the internal standard molecule p-mercaptobenzonitrile and the reporter molecule p-mercaptophenylboronic acid on the surface of gold nanoparticles, used for synergistic calibration and detection of H2O2, which enables reliable detection of the true content of intracellular H2O2 without the interference of other substances in cells. Based on the nanoprobe, we found that the level of intracellular H2O2 of cancer cells was increased after the nicotinamide adenine dinucleotide (NADH) treatment, with a dose-dependence to the concentration of NADH. High doses of NADH (above 20 mM) can induce cell death by means of ferroptosis associated with the level elevation of intracellular lipid hydroperoxides. This study highlights the potential of the SERS nanoprobe for tracking content variation of cellular H2O2 and understanding its roles in screening new anticancer drugs.

"On-site" analysis of pesticide residues in complex sample matrix by plasmonic SERS nanostructure hybridized hydrogel

BACKGROUND

Surface-enhanced Raman spectroscopy (SERS) has been extensively used in biomedical and food safety detection due to its advantages of label-free, in situ and fingerprint spectrum. However, it is challenging to develop an excellent SERS substrate that possesses all three of these characteristics including sensitivity, repeatability and stability.

RESULTS

In this work, a specific sodium alginate hydrogel flexible SERS substrate encapsulated gold-silver core-shell nanoparticles (Au@Ag NPs) was developed to address the aforementioned issue. The Au@Ag NPs with SERS "hot spot" structure were evenly dispersed in the hydrogel, which achieved the direct and high efficiency detection of the pesticide residues from complex sample matrix. Taking thiram as objective, this SERS substrates exhibit high sensitivity (detection limit of approximately 1 × 10-10 mol/L), excellent stability (maintain above 78.35 % of SERS activity after 7 weeks) and outstanding repeatability (RSD in one substrate as low as 3.56 %). Furthermore, the flexible hydrogel SERS substrates can be used to analyze a variety of small molecules in real samples (juices, vegetables and fruits), without the need for a laborious pretreatment process.

SIGNIFICANCE

In light of the aforementioned benefits, the functional flexible hydrogel SERS substrates present a reliable platform for the accurate and on-site measurement of chemical contaminants from complex samples.

Exploring Reliable and Efficient Plasmonic Nanopatterning for Surface- and Tip-Enhanced Raman Spectroscopies

Surface-enhanced Raman scattering (SERS) is of growing interest for a wide range of applications, especially for biomedical analysis, thanks to its sensitivity, specificity, and multiplexing capabilities. A crucial role for successful applications of SERS is played by the development of reproducible, efficient, and facile procedures for the fabrication of metal nanostructures (SERS substrates). Even more challenging is to extend the fabrication techniques of plasmonic nano-textures to atomic force microscope (AFM) probes to carry out tip-enhanced Raman spectroscopy (TERS) experiments, in which spatial resolution below the diffraction limit is added to the peculiarities of SERS. In this short review, we describe recent studies performed by our group during the last ten years in which novel nanofabrication techniques have been successfully applied to SERS and TERS experiments for studying bio-systems and molecular species of environmental interest.

Comparative Study of SERS-Spectra of NQ21 Peptide on Silver Particles and in Gold-Coated "Nanovoids"

The NQ21 peptide has relatively recently attracted attention in the biomedical sphere due to its prospects for facilitating the engineering of the HIV1 vaccine and ELISA test. Today, there is still a need for a reliable and fast methodology that reveals the secondary structure of this analyte at the low concentrations conventionally used in vaccines and immunological assays. The present research determined the differences between the surface-enhanced Raman scattering (SERS) spectra of NQ21 peptide molecules adsorbed on solid SERS-active substrates depending on their geometry and composition. The ultimate goal of our research was to propose an algorithm and SERS-active material for structural analysis of peptides. Phosphate buffer solutions of the 30 µg/mL NQ21 peptide at different pH levels were used for the SERS measurements, with silver particles on mesoporous silicon and gold-coated "nanovoids" in macroporous silicon. The SERS analysis of the NQ21 peptide was carried out by collecting the SERS spectra maps. The map assessment with an originally developed algorithm resulted in defining the effect of the substrate on the secondary structure of the analyte molecules. Silver particles are recommended for peptide detection if it is not urgent to precisely reveal all the characteristic bands, because they provide greater enhancement but are accompanied by analyte destruction. If the goal is to carefully study the secondary structure and composition of the peptide, it is better to use SERS-active gold-coated "nanovoids". Objective results can be obtained by collecting at least three 15 × 15 maps of the SERS spectra of a given peptide on substrates from different batches.

Layer-by-layer coating of natural diatomite with silver nanoparticles for identification of circulating cancer protein biomarkers using SERS

Surface-enhanced Raman scattering (SERS) is an emerging spectroscopy technique for detecting and characterizing chemical or biological structures in the vicinity of plasmonic nanostructures. Colloidal, solid, and flexible nanostructures are widely used in SERS experiments to enhance the Raman intensity. The nanostructure used in SERS is one of the main influencing parameters and a growing research area. Fabrication of simple and cheap SERS substrates with a high enhancement factor is desired. In this study, we fabricated a reproducible, cheap, and flexible SERS active strip by coating natural diatomite (biosilica) with silver nanoparticles (AgNPs) using the layer-by-layer assembly method and the fabricated strip is used for the label-free identification of circulating cancer protein biomarkers. SERS active strips were fabricated having different numbers of AgNP layers on natural diatomite and comprehensive characterization techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), and UV/vis absorption spectrophotometry were used. SERS activities of the strips depending on the number of layers were evaluated using 4-aminothiophenol (4-ATP) and rhodamine 6G (Rh6G) molecules. We found that the SERS intensity is strongly dependent on the number of AgNP layers, with the maximum SERS intensity obtained from the strip with 5 layers of AgNPs, having a 2.0 × 105 enhancement factor. The strip with the highest SERS activity was used for the label-free identification of circulating cancer protein biomarkers (HER2, CA15-3, PSA, MUC4, and CA27-29). The results demonstrate that the fabricated strip can help in the effective label-free identification of circulating protein biomarkers and open new directions for SERS-based label-free biosensing applications.

Material design, development, and trend for surface-enhanced Raman scattering substrates

Surface-enhanced Raman scattering (SERS) is a powerful and non-invasive spectroscopic technique that can provide rich and specific chemical fingerprint information for various target molecules through effective SERS substrates. In view of the strong dependence of the SERS signals on the properties of the SERS substrates, design, exploration, and construction of novel SERS-active nanomaterials with low cost and excellent performance as the SERS substrates have always been the foundation and the top priority for the development and application of the SERS technology. This review specifically focuses on the extensive progress made in the SERS-active nanomaterials and their enhancement mechanism since the first discovery of SERS on the nanostructured plasmonic metal substrates. The design principles, unique functions, and influencing factors on the SERS signals of different types of SERS-active nanomaterials are highlighted, and insight into their future challenge and development trends is also suggested. It is highly expected that this review could benefit a complete understanding of the research status of the SERS-active nanomaterials and arouse the research enthusiasm for them, leading to further development and wider application of the SERS technology.

Dual-clamped One-Pot SERS-based Biosensors for Rapid and Sensitive Detection of SARS-CoV-2 Using Portable Raman Spectrometer

Rapid and sensitive diagnostics of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is of utmost importance to control the widespread coronavirus disease 2019 (COVID-19) upsurge. This study demonstrated a novel one-pot surface-enhanced Raman scattering (SERS) based immunoassay to detect SARS-CoV-2, without any washing process using a portable Raman spectrometer. The SERS-immune assay was designed using a regular digital versatile disk (DVD) substrate integrated with Raman reporter labeled silver nanoparticles for double clamping effects. The disks were molded to form nanopillar arrays and coated with silver film to enhance the sensitivity of immunoassay. The SERS platform demonstrated a limit of detection (LoD) up to 50 pg mL^-1 for SARS-CoV-2 spike protein and virus-like-particle (VLP) protein in phosphate buffer saline within a turnaround time of 20 minutes. Moreover, VLP protein spiked in untreated saliva achieved an LoD of 400 pg mL^-1, providing a cycle threshold (Ct) value range of 30-32, closer to reverse transcription-polymerase chain reaction (RT-PCR) results (35-40) and higher than the commercial rapid antigen tests, ranging from 25-28. Therefore, the developed one-pot SERS based biosensor exhibited highly sensitive and rapid detection of SARS-CoV-2, which could be a potential point-of-care platform for early and cost-effective diagnosis of the COVID-19 virus.

Surface enhanced Raman scattering active substrate based on hydrogel microspheres for pretreatment-free detection of glucose in biological samples

Determining glucose in biological samples is tedious and time-consuming due to sample pretreatment. The sample is usually pretreated to remove lipids, proteins, hemocytes and other sugars that interfere with glucose detection. A surface-enhanced Raman scattering (SERS) active substrate based on hydrogel microspheres has been developed to detect glucose in biological samples. Due to the specific catalytic action of glucose oxidase (GOX), the high selectivity of detection is guaranteed. The hydrogel substrate prepared by microfluidic droplets technology protects the silver nanoparticles from the surrounding environment and improves the stability and reproducibility of the assay. In addition, the hydrogel microspheres have size-adjustable pores that selectively allow small molecules to pass through. The pores block the entry of large molecules, such as impurities, enabling glucose detection through glucose oxidase etching without sample pretreatment. This hydrogel microsphere-SERS platform is highly sensitive and enables reproducible detection of different glucose concentrations in biological samples. The use of SERS to detect glucose provides clinicians with new diagnostic methods for diabetes and a new application opportunity for SERS-based molecular detection techniques.

Ultrasensitive enhanced Raman spectroscopy by hybrid surface-enhanced and interference-enhanced Raman scattering with metal-insulator-metal structures

High-sensitivity, reproducible, and low-cost substrate has been a major obstacle for practical sensing application of surface-enhancement Raman scattering (SERS). In this work, we report a type of simple SERS substrate which is composed of metal-insulator-metal (MIM) structure of Ag nanoisland (AgNI)-SiO₂-Ag film (AgF). The substrates are fabricated by only evaporation and sputtering processes, which are simple, fast and low-cost. By combining the hotspots and interference-enhanced effects in AgNIs and the plasmonic cavity (SiO₂) between AgNIs and AgF, the proposed SERS substrate shows an enhancement factor (EF) of 1.83 × 10⁸ with limit of detection (LOD) down to 10^-17mol/L for rhodamine 6 G (R6G) molecules. The EFs are ∼18 times higher than that of conventional AgNIs without MIM structure. In addition, the MIM structure shows excellent reproducibility with relative standard deviation (RSD) less than 9%. The proposed SERS substrate is fabricated only with evaporation and sputtering technique and the conventionally used lithographic methods or chemical synthesis are not required. This work provides a simple way to fabricate ultrasensitive and reproducible SERS substrates which show great promise for developing various biochemical sensors with SERS.

A highly sensitive surface-enhanced Raman scattering sensor with MIL-100(Fe)/Au composites for detection of malachite green in fish pond water

Concerns about food safety have been arisen due to the improper use of chemicals in aquaculture. Malachite green (MG) has attracted attention because of its illegal usage and its potential negative impacts on the environment and public health. Surface-enhanced Raman scattering (SERS) platforms coupled with different SERS substrates have been employed for rapid analysis of MG residues in food. However, the most commonly used SERS substrates were non-reusable and showed limited detection sensitivity. In this study, a novel SERS substrate with a good recyclability and a high sensitivity was prepared by electrostatically assembling together a metal-organic framework material called materials of institute lavoisie-100(Fe) (MIL-100(Fe)) and Au NPs. The lowest detectable concentration of MG was 10^-13 M based on the optimal substrate. The SERS sensor was applied for the detection of the trace MG in fish pond water, which was accomplished with the correlation coefficients R² = 0.991-0.996 in a concentration range of 10^-6-10^-13 M. Moreover, MIL-100(Fe)/Au was recycled at least five times, realizing a "detection to degradation", showing great potential for food contamination monitoring due to its distinguished performance.

Fabrication of a metal organic framework (MOF)-modified Au nanoparticle array for sensitive and stable SERS sensing of paraquat in cereals

A high-performance Au@MIL-101/PMMA/DT surface-enhanced Raman scattering (SERS) substrate was fabricated for sensitive and stable detection of paraquat by self-assembling metal organic framework-modified Au nanoparticles (Au@MIL-101) on a poly(methyl methacrylate) (PMMA) film and then immobilizing the formed substrate onto a duct tape (DT). The highly closely packed Au@MIL-101 array provided intensive hotspots for SERS sensing. The MIL-101 layer modified on the surface of Au nanoparticles could absorb paraquat to the electromagnetic enhancement area of Au nanoparticles. The DT on the bottom made the substrate smoother, which is beneficial for achieving a more stable detection performance. As a result, the constructed substrate exhibited outstanding uniformity with relative standard deviations of 9.47% and storage stability for 2 months. For detecting paraquat, the substrate showed a low detection limit of 7.1 × 10^-9  M (1.83 µg/kg) and wide linear range from 10^-8 to 10^-2  M. Furthermore, the substrate showed good detection performance in real cereal samples with desirable recovery rates from 91.57% to 102.32%.

SERS detection of thiram using polyacrylamide hydrogel-enclosed gold nanoparticle aggregates

The development of sensitive and long-term signal-stable plasmonic substrates is vital to the in-field application of the surface-enhanced Raman spectroscopy (SERS) technique. The colloidal gold nanoparticles (AuNPs) system is commonly used in SERS detection, but it shows less signal stability and reproducibility due to the uncontrollable aggregation of nanoparticles by adding aggregating agents in SERS detection. In this study, we developed a new SERS detection platform based on polyacrylamide hydrogel-enclosed plasmonic gold nanoparticle aggregates (PAH-AuANs). In the system, the formation of PAH can rapidly stabilize the gold nanoparticle aggregates, avoiding the over-aggregation or precipitation of AuNPs. With the PAH concentration in the range of 6-10 % and AuNPs at the concentration of 0.2 nM, the resulting PAH-AuNAs platform exhibited both sensitive SERS activity and excellent SERS signal stability. The relative standard deviation of the 4-MBA probe SERS signal collected from the PAH-AuNAs platform was lower than 3 %. The limit of detection for the pesticide thiram was down to 0.38 μg/L with a handheld Raman spectrometer. Moreover, the procedure for preparing the PAH-AuNAs platform was easy to handle, offering a new strategy for in-field detection of environmental contaminants with a handheld Raman spectrometer in the future.

Functionalized silver nanoparticles for SERS amplification with enhanced reproducibility and for ultrasensitive optical fiber sensing in environmental and biochemical assays

Plasmonic sensors have broad application potential in many fields and are promising to replace most bulky sensors in the future. There are various method-based chemical reduction processes for silver nanoparticle production with flexible structural shapes due to their simplicity and rapidity in nanoparticle fabrication. In this study, self-assembled silver nanoparticles (Ag NPs) with a plasmon peak at 424 nm were successfully coated onto -NH2-functionalized glass and optical fiber sensors. These coatings were rapidly produced via two denaturation reactions in plasma oxygen, respectively, and an APTES ((3-aminopropyl)triethoxysilane) solution was shown to have high strength and uniformity. With the use of Ag NPs for surface-enhanced Raman scattering (SERS), excellent results and good stability with the detection limit up to 10-10 M for rhodamine B and 10-8 M for methylene blue, and a signal degradation of only ∼20% after storing for 30 days were achieved. In addition, the optical fiber sensor with Ag NP coatings exhibited a higher sensitivity value of 250 times than without coatings to the glycerol solution. Therefore, significant enhancement of these ultrasensitive sensors demonstrates promising alternatives to cumbersome tests of dye chemicals and biomolecules without any complicated process.

Determination of 5-Hydroxymethylfurfural (5-HMF) in milk products by surface-enhanced Raman spectroscopy and its simulation analysis

5-Hydroxymethylfurfural (5-HMF) is a useful indicator of thermal damage degree and freshness of milk. It is of great importance to develop simple, rapid and accurate analytical methods for the sensitive detection of 5-HMF in milk and milk-based products. In this work, surface-enhance Raman spectroscopy (SERS) was used for rapid determination of 5-HMF in processed cheese by colloidal Au nanoparticles (AuNPs) substrate synthesized by the classical solvothermal reduction method. Density functional theory (DFT) calculations were carried out to determine the vibration assignments of 5-HMF and the surface enhancement effect of AuNPs substrate. The results found that a good linear response on the AuNPs substrate for 5-HMF in the concentration range of 0.1-75 mM was established with the detection limit of 75 μM (S/N = 3). Furthermore, the present method could be applied to the determination of 5-HMF in a cheese real sample which revealed its promising application in food safety and analysis.

Molecular Plasmonics with Metamaterials

Molecular plasmonics, the area which deals with the interactions between surface plasmons and molecules, has received enormous interest in fundamental research and found numerous technological applications. Plasmonic metamaterials, which offer rich opportunities to control the light intensity, field polarization, and local density of electromagnetic states on subwavelength scales, provide a versatile platform to enhance and tune light-molecule interactions. A variety of applications, including spontaneous emission enhancement, optical modulation, optical sensing, and photoactuated nanochemistry, have been reported by exploiting molecular interactions with plasmonic metamaterials. In this paper, we provide a comprehensive overview of the developments of molecular plasmonics with metamaterials. After a brief introduction to the optical properties of plasmonic metamaterials and relevant fabrication approaches, we discuss light-molecule interactions in plasmonic metamaterials in both weak and strong coupling regimes. We then highlight the exploitation of molecules in metamaterials for applications ranging from emission control and optical modulation to optical sensing. The role of hot carriers generated in metamaterials for nanochemistry is also discussed. Perspectives on the future development of molecular plasmonics with metamaterials conclude the review. The use of molecules in combination with designer metamaterials provides a rich playground both to actively control metamaterials using molecular interactions and, in turn, to use metamaterials to control molecular processes.

Leveraging Nanomechanical Perturbations in Raman Spectro-Immunoassays to Design a Versatile Serum Biomarker Detection Platform

While immunoassays are pivotal to medical diagnosis and bioanalytical chemistry, the current landscape of public health has catalyzed an important shift in the requirements of immunoassays that demand innovative solutions. For example, rapid, label-free, and low-cost screening of a given analyte is required to inform the best countermeasures to combat infectious diseases in a timely manner. Yet, the current design of immunoassays cannot accommodate such requirements as constraint by accumulative challenges, such as repeated incubation and washing, and the need of two types of antibodies in the sandwich format. To provide a potential solution, herein, a plasmonic Raman immunoassay with single-antibody, multivariate regression, and shift-of-peak strategies, coined as the PRISM assay, for serum biomarkers detection, is reported. The PRISM assay relies on Raman reporter-antibody conjugates to capture analytes on a plasmonic substrate. The ensuing nanomechanical perturbations to vibration of Raman reporters induce subtle but characteristic spectral changes that encode rich information related to the captured analytes. By fusing Raman spectroscopy and chemometric analysis, both Raman frequency shift- and multivariate regression models for sensitive detection of biomarkers are developed. The PRISM assay is expected to find a wide range of applications in clinical diagnosis, food safety surveillance, and environmental monitoring.

Sensitive and handy detection of pesticide residue on fruit surface based on single microsphere surface-enhanced Raman spectroscopy technique

HYPOTHESIS

Surface-enhanced Raman spectroscopy (SERS) has become an emerging and reliable tool for detecting pesticide residues due to its high sensitivity, fast testing speed and easy sample handling. SERS active substrates are the key to achieve efficient and sensitive detection. However, for the most widely used noble metal nanoparticles, there are problems of high noble metal nanoparticle usage and random aggregation. The micron-scale Raman spot is focused on multiple randomly aggregated nanoparticles during the test, resulting in poor reproducibility. Therefore, the development of micron-scale cost-effective SERS substrates with good reproducibility and simple detecting method is of great significance in practical detection.

EXPERIMENTS

Through deposition of silver nanoparticles (Ag-NPs) by chemical reduction on the surface of monodisperse sulfonated polystyrene (SPS) microspheres, micron-sized PS@Ag-NPs core-shell microspheres were prepared with excellent SERS activity. After that, two simple protocols (Method I and Method II) were explored for the determination of thiram on apple epidermis.

FINDINGS

Based on our developed strategy of the single microsphere SERS technique, we successfully fabricated uniform PS@Ag-NPs substrate with high SERS activity and excellent detection sensitivity. The single microsphere SERS technique possesses the capability of anti-dilutability and the utilization of ultra-low PS@Ag-NPs microsphere dosage, realizing qualitative and quantitative detection of thiram on apple with detection limits far below the standard stipulated by China and the European Union.

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.

Dimensional Design for Surface-Enhanced Raman Spectroscopy

Surface-enhanced Raman spectroscopy (SERS) is a vibrational spectroscopy technique that enables specific identification of target analytes with sensitivity down to the single-molecule level by harnessing metal nanoparticles and nanostructures. Excitation of localized surface plasmon resonance of a nanostructured surface and the associated huge local electric field enhancement lie at the heart of SERS, and things will become better if strong chemical enhancement is also available simultaneously. Thus, the precise control of surface characteristics of enhancing substrates plays a key role in broadening the scope of SERS for scientific purposes and developing SERS into a routine analytical tool. In this review, the development of SERS substrates is outlined with some milestones in the nearly half-century history of SERS. In particular, these substrates are classified into zero-dimensional, one-dimensional, two-dimensional, and three-dimensional substrates according to their geometric dimension. We show that, in each category of SERS substrates, design upon the geometric and composite configuration can be made to achieve an optimized enhancement factor for the Raman signal. We also show that the temporal dimension can be incorporated into SERS by applying femtosecond pulse laser technology, so that the SERS technique can be used not only to identify the chemical structure of molecules but also to uncover the ultrafast dynamics of molecular structural changes. By adopting SERS substrates with the power of four-dimensional spatiotemporal control and design, the ultimate goal of probing the single-molecule chemical structural changes in the femtosecond time scale, watching the chemical reactions in four dimensions, and visualizing the elementary reaction steps in chemistry might be realized in the near future.

A Dual-Modal Single-Antibody Plasmonic Spectro-Immunoassay for Detection of Small Molecules

Small molecules play a pivotal role in regulating physiological processes and serve as biomarkers to uncover pathological conditions and the effects of therapeutic treatments. However, it remains a significant challenge to detect small molecules given the size as compared to macromolecules. Recently, the newly emerging plasmonic immunoassays based on surface-enhanced Raman scattering (SERS) offer great promise to deliver extraordinary sensitivity. Nevertheless, they are limited by the intrinsic SERS intensity fluctuations associated with the SERS uncertainty principle. The single transducer that relies on the intensity change is also prone to false signals. Additionally, the prevailing sandwich immunoassay format proves less effective towards detecting small molecules. To circumvent these critical issues, a dual-modal single-antibody approach that synergizes both the intensity and shift of the peak-based immunoassay with Raman enhancement, coined as the INSPIRE assay, is developed for small molecules detection. With two independent transduction mechanisms, it allows better prediction of analyte concentration and attenuation of signal artifacts, providing a new and robust strategy for molecular analysis. With a proof-of-concept demonstration for detection of free T4 and testosterone in serum matrix, the authors envision that the INSPIRE assay could be expanded for a wide spectrum of applications in biomedical diagnosis, discovery of new biopharmaceuticals, food safety, and environmental monitoring.

Label-free and ultrasensitive SERS detection of pesticide residues using 3D hot-junction of a Raman enhancing montmorillonite/silver nanoparticles nanocomposite

Montmorillonite (MMT) coated with roughened noble metal nanoparticles are novel hybrid nanocomposite with a wide range of applications including agriculture, materials science and biomedical engineering. Herein, we developed a hybrid nanocomposite (MMT/AgNPs) based on MMT coated with silver nanoparticles (AgNPs), which can be used as a cost-effective and efficient surface-enhanced Raman spectroscopy (SERS) substrate for the detection of pesticides in fruits and vegetables. MMT itself is negatively charged and can be assembled with positively charged AgNPs through electrostatic interactions. Moreover, MMT has a layered 2D structure that possesses a large surface area, which can load a large number of AgNPs to form more SERS hotspots for the ultrasensitive measurement. SERS performance of the MMT/AgNPs nanocomposite was tested by 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB) and the substrate can obtain the strongest SERS enhancement effect with the volume ratio of MMT/AgNPs of 1 : 10. These substrates were applied in the measurement of thiram in apples and spinach samples by SERS. Detection limits of pesticide molecules of 5.0 × 10^-8 M and 1.0 × 10^-7 M in apples and spinach, respectively, were obtained. Most importantly, MMT nanosheets are a robust platform that allowed AgNPs to be evenly and thoroughly distributed and stabilized over the substrate, improving the repeatability and stability of SERS detection. These results reveal that the MMT/AgNPs nanocomposites are suitable substrates for the real-world SERS analysis of pesticide and other contaminants in complex food matrices.

Simultaneous and ultra-sensitive SERS detection of SLPI and IL-18 for the assessment of donor kidney quality using black phosphorus/gold nanohybrids

Due to the global challenge of donor kidney shortage, expanding the pool of deceased donors has been proposed to include expanded criteria donors. However, the lack of methods to precisely measure donor kidney injury and predict the outcome still leads to high discard rates and recipient complications. As such, evaluation of deceased donor kidney quality is critical prior to transplantation. Biomarkers from donor urine or serum provide potential advantages for the precise measure of kidney quality. Herein, simultaneous detection of secretory leukocyte peptidase inhibitor (SLPI) and interleukin 18 (IL-18), two important kidney injury biomarkers, has been achieved, for the first time, with an ultra-high sensitivity using surface enhanced Raman scattering (SERS). Specifically, black phosphorus/gold (BP/Au) nanohybrids synthesized by depositing Au nanoparticles (NPs) onto the BP nanosheets serve as SERS-active substrates, which offer a high-density of inherent and accessible hot-spots. Meanwhile, the nanohybrids possess biocompatible surfaces for the enrichment of target biomarkers through the affinity with BP nanosheets. Quantitative detection of SLPI and IL-18 were then achieved by characterizing SERS signals of these two biomarkers. The results indicate high sensitivity and excellent reproducibility of this method. The limits of detection reach down to 1.53×10^-8 mg/mL for SLPI and 0.23×10^-8 mg/mL for IL-18. The limits of quantification are 5.10×10^-8 mg/mL and 7.67×10^-9 mg/mL for SLPI and IL-18. In addition, simultaneous detection of these biomarkers in serum was investigated, which proves the feasibility in biologic environment. More importantly, this method is powerful for detecting multiple analytes inheriting from excellent multiplexing ability of SERS. Giving that the combined assessment of SLPI and IL-18 expression level serves as an indicator of donor kidney quality and can be rapidly and reproducibly conducted, this SERS-based method holds great prospective in clinical practice.

Facile fabrication of Ag@C@C8 nanoparticles as a SERS substrate and their environmental applications

Carbon-coated silver (Ag@C) nanoparticles were prepared by a one-step hydrothermal synthesis method, the surface was modified with chlorodimethyloctylsilane (C8) to generate C8 functionalized Ag@C@C8 nanoparticles with long-term stability and high sensitivity.

Ag Nanoislands Modified Carbon Fiber Nanostructure: A Versatile and Ultrasensitive Surface-Enhanced Raman Scattering Platform for Antiepileptic Drug Detection

A high-efficiency surface-enhanced Raman scattering (SERS) detection method with ultra-high sensitivity has been widely applied in drug component detection to optimize the product quality verification standards. Herein, a controllable strategy of sputtering Ag nanoislands on carbon fiber (C-fiber) via magnetron sputtering technology was proposed to fabricate a versatile Ag-C-fiber SERS active substrate. A wide range of multi-level electromagnetic enhancement “hot spots” distributed on Ag-C-fiber nanostructures can efficiently amplify Raman signals and the experimental enhancement factor (EEF) value was 3.871 × 106. Furthermore, substantial “hot spots” of large-scale distribution guaranteed the superior reproducibility of Raman signal with relative standard deviation (RSD) values less than 12.97%. Limit of detection (LOD) results indicated that when crystal violet (CV) is employed as probe molecule, the LOD was located at 1 × 10−13 M. By virtue of ultra-sensitivity and good flexibility of the Ag-C-fiber nanotemplate, Raman signals of two kinds of antiepileptic drugs called levetiracetam and sodium valproate were successfully obtained using an SERS-based spectral method. The Ag-C-fiber SERS detection platform demonstrated a good linear response (R2 = 0.97486) in sensing sodium valproate concentrations in the range of 1 × 103 ng/μL−1–1 ng/μL. We believe that this reliable strategy has potential application for trace detection and rapid screening of antiepileptic drugs in the clinic.

Reliable SERS detection of pesticides with a large-scale self-assembled Au@4-MBA@Ag nanoparticle array

The fabrication of sensitive and reliable interfacial plasmonic platform for measuring chemical contaminants in various phases is an exciting topic in the food industry and for environment monitoring. In this study, a high-performance surface-enhanced Raman spectroscopy (SERS) analytic platform was developed through self-assembly of the gold@4-mercaptobenzoic acid@silver nanoparticles (Au@4-MBA@Ag NPs) at the cyclohexane/water interface. By addition of the inducer ethanol, the Au@4-MBA@Ag NPs in aqueous phase was effectively migrated to the biphasic interface, forming a large-scale close-packed nanoparticle array. The average gap between adjacent nanoparticles was smaller than 3 nm, where intensive SERS "hot spots" were created for high-sensitive detection. Furthermore, using the sandwiched 4-MBA molecule as the internal standard to correct the Raman signal fluctuations, the point-to-point and batch-to-batch reproducibility of Au@4-MBA@Ag array were improved with lower relative standard deviation (RSD) values of 8.84% and 14.97%, respectively, and pesticides (thiram and thiabendazole) analysis in both aqueous and organic phases were achieved with higher accuracy (R² of 0.986 and 0.990) as compared with those without 4-MBA correction (R² of 0.867 and 0.974). The high-throughput fabrication of the self-assembled nanoparticle array is a promising approach for development of a sensitive and reliable SERS platform for chemical contaminants monitoring in multiphase.

Surface-enhanced Raman scattering (SERS) spectroscopy on localized silver nanoparticle-decorated porous silicon substrate

Surface-enhanced Raman scattering (SERS) spectroscopy is a rapid and non-destructive optical detection method that has been applied in various applications. Recently, three-dimensional (3D) substrate-based silicon nanostructures have been widely used as SERS substrates due to their high detection sensitivity, repeatability, and reusability. This paper uses a simple and low-cost electroless etching deposition process to generate silver nanoparticle-decorated porous silicon (Ag-PS) substrates. We propose a contact deposition process to generate localized Ag-PS (LocAg-PS) for SERS analysis. Due to the hydrophilic LocAg-PS pad on the hydrophobic PS background, the sample droplets self-aligned to the predefined LocAg-PS pads and condensed into a higher local concentration for high sensitivity SERS detection without extensive search for the hot spot. The effects of critical fabrication parameters and SERS analysis on the LocAg-PS surface were evaluated.

Stable SERS substrate based on highly reflective metal liquid-like films wrapped hydrogels for direct determination of small molecules in a high protein matrix

The study of the interaction between small molecules and proteins is important. Surface-enhanced Raman spectroscopy (SERS) is suitable for such applications since it has the power of detecting a molecule based on its intrinsic nature and without labeling. Herein, the MeLLFs@PAAG SERS substrate supporting highly reflective metal liquid-like films (MeLLFs) with polyacrylamide hydrogels (PAAG) has high-density "hot spots" to provide excellent SERS activity. The MeLLFs@PAAG formed by AgNPs only has less than 15% SERS activity loss when stored in the air for more than three weeks. By using rhodamine 6G (R6G) as a model analyte, the AgNPs based MeLLFs@PAAG SERS substrate exhibits an enhancement factor (EF) as high as 8.0 × 10⁶, a limit of detection (LOD) of 76.8 pM (S/N = 3). Also, the formed PAAG provided a 3D molecular network to orderly secure the assembled nanoparticles (NPs), which not only improves the stability of NPs but also shields the Raman signal of proteins as high as 45 g/L allowing the direct determination of the binding rate of human serum albumin (HSA) and doxorubicin (DOX). A binding rate of about 70% was detected, which is consistent with previous reports. Thus, proposed the MeLLFs@PAAG SERS substrate can be used as a promising candidate for SERS measurement in complex biological samples.

Recyclable Ag-Deposited TiO2 SERS Substrate for Ultrasensitive Malachite Green Detection

An ultrasensitive Ag-deposited TiO₂ flower-like nanomaterial (FLNM) surface-enhanced Raman scattering (SERS)-active substrate is synthesized via a hydrothermal method, and Ag nanoparticles (NPs) are deposited through electron beam evaporation. Malachite green (MG), which is widely used in aquaculture, is employed to assess the surface-enhanced Raman scattering (SERS) properties of TiO₂/Ag FLNMs. They exhibit ultrasensitivity (limit of detection (LOD) of MG reaches 4.47 × 10^-16 M) and high reproducibility (relative standard deviations (RSDs) are less than 13%); more importantly, the TiO₂/Ag FLNMs are recyclable, as enabled by their self-cleaning function due to TiO₂ photocatalytic degradation. Their recyclability is achieved after three cycles and their potential application is examined in the actual system. Finite difference time domain (FDTD) simulations and the charge-transfer (CT) mechanism further prove that the excellent SERS properties originate from localized surface plasmon resonance (LSPR) of Ag NPs and the coupling field between Ag and TiO₂ FLNMs. Therefore, TiO₂/Ag FLNMs show promising application in aquaculture.

Biosensing Using SERS Active Gold Nanostructures

Surface-enhanced Raman spectroscopy (SERS) has become a powerful tool for biosensing applications owing to its fingerprint recognition, high sensitivity, multiplex detection, and biocompatibility. This review provides an overview of the most significant aspects of SERS for biomedical and biosensing applications. We first introduced the mechanisms at the basis of the SERS amplifications: electromagnetic and chemical enhancement. We then illustrated several types of substrates and fabrication methods, with a focus on gold-based nanostructures. We further analyzed the relevant factors for the characterization of the SERS sensor performances, including sensitivity, reproducibility, stability, sensor configuration (direct or indirect), and nanotoxicity. Finally, a representative selection of applications in the biomedical field is provided.

Revealing the Hemispherical Shielding Effect of SiO2@Ag Composite Nanospheres to Improve the Surface Enhanced Raman Scattering Performance

Many studies widely used SiO₂@Ag composite nanospheres for surface enhanced Raman scattering (SERS), which mainly contributes to electromagnetic enhancement. In addition to experiments, previous simulations mostly adopted a two-dimensional model in SERS research, resulting in the three-dimensional information being folded and masked. In this paper, we adopted the three-dimensional model to simulate the electric field distribution of SiO₂@Ag composite nanospheres. It is found that when the Ag nanoparticles are distributed densely on the surface of SiO₂ nanospheres, light cannot pass through the upper hemisphere due to the local surface plasmon resonance (LSPR) of the Ag nanoparticles, resulting in the upper hemisphere shielding effect; and if there are no Ag nanoparticles distributed densely on the surface of SiO₂ nanospheres, the strong LSPR cannot be formed, so the incident light will be guided downward through the whispering gallery mode of the spherical structure. At the same time, we designed relevant experiments to synthesize SiO₂@Ag composite nanosphere as SERS substrate and used Rhodamine 6G as a probe molecule to study its SERS performance. This design achieved a significant SERS effect, and is very consistent with our simulation results.

Synthesis of bimetallic core-shelled nanoparticles modified by 2-mercaptoethanol as SERS substrates for detecting ferbam and thiabendazole in apple puree

Modification of surface-enhanced Raman spectroscopy (SERS) substrates with thiol ligands is an emerging approach in enhancing the stability and sensitivity of metal substrates due to their good affinity with metals such as Au, Ag, and Cu. Thus, in the current study, 2-mercaptoethanol was used to modify the surface of silver-coated gold nanoparticles to develop a novel SERS substrate for the rapid assessment of fungicide residues in fruit samples. Results showed that the substrate could achieve the detection of ferbam and thiabendazole residues in apple puree with limits of detection of approximately 0.0042 and 0.0064 ppm, high coefficients of determination of 0.9946 and 0.9968, good recoveries ranging from 80 to 105 and 81 to 107% and relative standard deviations of 3.5-7.5 and 3.8-7.9 %, respectively. Therefore, the substrate developed could potentially be utilised to assess other toxic agrochemicals in future.

In-situ surface-enhanced Raman scattering based on MTi20 nanoflowers: Monitoring and degradation of contaminants

Real-time and in-situ monitoring of chemical reactions has attracted great attention in many fields. In this work, we in-situ monitored the photodegradation reaction process of methylene blue (MB) by Surface enhanced Raman scattering (SERS) technique. An effective and versatile SERS platform assembled from MoS₂ nanoflowers (NFs) and TiO₂ nanoparticles (NPs) was prepared successfully. The optimized MoS₂/TiO₂ substrate (MTi₂₀) exhibits not only an ultra-high SERS response but also the excellent catalytic degradation performance to the contaminant MB, which provided a new material for real-time and in-situ monitoring the photodegradation process. Experiments prove that the detection limit is as low as 10^-13 M, and degradation rate is as high as 97.2% in 180 s, respectively. And the activity of the substrate kept in the air for 90 days is almost unchanged. Furthermore, as a practical SERS substrate, MTi₂₀ can also detect trace amounts of other harmful substances including malachite green (MG), bisphenol A (BPA) and endosulfan. Thus, this study come up with a new orientation at the real-time and in-situ monitoring of photocatalytic reaction and may be applied in environmental monitoring and food security fields in the future.

Fabrication of antireflective silver-capped tin oxide nano-obelisk arrays as high sensitive SERS substrate

Hybrid noble metal-semiconductor oxide nanostructures often provide unique and synergetic functionalities that are highly desirable in various practical applications. However, the fabrication of such systems with desired functionalities using cost-effective techniques is still a great challenge. In this work, we report a facile route for the preparation of novel Ag/SnO₂ nano-obelisk arrayed thin films on silicon substrates by spray pyrolysis and thermal evaporation techniques. The prepared samples exhibited broadband antireflectance in both UV and visible regions attributed to the refractive index gradient and scattering provided by the nano-obelisk arrays. The localised surface plasmon resonance of silver nanocaps further enhanced the light absorption contributing to the antireflective property of the hybrid system. Ag/SnO₂ nano-obelisk arrayed thin film exhibited excellent SERS performance with an enhancement factor of 1.13 × 10⁸ with a limit of detection value of 10^-12 M for the trace detection of R6G dye. In addition, Ag/SnO₂ nano-obelisk arrayed thin film based SERS substrate exhibited good homogeneity across the measured spots and outstanding stability which are essential for quantitative field analysis. The results indicate that the Ag/SnO₂ nano-obelisk arrayed thin films are efficient SERS substrates with the merits of having the ease of production, high sensitivity and stability for various practical sensing applications.

Detection of organic dyes by surface-enhanced Raman spectroscopy using plasmonic NiAg nanocavity films

This work presents the NiAg nanocavity film for the detection of organic dyes by surface-enhanced Raman spectroscopy (SERS). Nanocavity films were prepared by colloidal lithography using 518-nm polystyrene spheres combined with the electrochemical deposition of Ni supporting layer and Ag nanoparticles homogeneous SERS-active layer. The theoretical study was modelled by finite-difference time-domain (FDTD) simulation of electromagnetic field enhancement near the nanostructured surface and experimentally proven by SERS measurement of selected organic dyes (rhodamine 6G, crystal violet, methylene blue, and malachite green oxalate) in micromolar concentration. Furthermore, the concentration dependence was investigated to prove the suitability of NiAg nanocavity films to detect ultra-low concentrations of samples. The detection limit was 1.3 × 10^-12, 1.5 × 10^-10, 1.4 × 10^-10, 7.5 × 10^-11 mol·dm^-3, and the standard deviation was 20.1%, 13.8%, 16.7%, and 19.3% for R6G, CV, MB, and MGO, respectively. The analytical enhancement factor was 3.4 × 10⁵ using R6G as a probe molecule. The principal component analysis (PCA) was performed to extract the differences in complex spectra of the dyes where the first and second PCs carry 42.43% and 31.39% of the sample variation, respectively. The achieved results demonstrated the suitability of AgNi nanocavity films for the SERS-based detection of organic dyes, with a potential in other sensing applications.

Fabrication of Au Nanoparticle Arrays on Flexible Substrate for Tunable Localized Surface Plasmon Resonance

In this work, Au nanoparticle (AuNP) arrays on shape memory polyurethane (SMPU) substrates serve as flexible materials for tunable localized surface plasmon resonance (LSPR). AuNP arrays prepared by diblock copolymer self-assembly are transferred from rigid silicon wafers onto flexible SMPU substrates with ultrasonic treatment rather than peeling off directly. The resultant AuNP array SMPU films have excellent mechanical properties and stable thermodynamic properties. The LSPR arising from AuNP arrays is increased by negative bending on SMPU substrates, whereas the LSPR is decreased by positive bending. Besides, upon uniaxial tension, the vertical LSPR is increased first then decreased, whereas the parallel LSPR is similar, resulting in the overall LSPR of AuNP arrays being increased first and then decreased with the mechanical uniaxial tension of SMPU. Moreover, the resultant AuNP array SMPU films exhibit excellent flexibility, stability, and homogeneity in practical surface-enhanced Raman scattering (SERS) application. This approach of incorporating AuNP arrays on SMPU substrates for tuning plasmonic properties have great potential applications in SERS, fluorescence enhancement, and newly optoelectronic materials.

Improve optical properties by modifying Ag nanoparticles on a razor clam SERS substrate

Irregular substrates are inappropriate for enhancing surface enhanced Raman scattering (SERS) due to their poor performances in terms of uniformity, enhancement performance, and polarization characteristics. However, in this work, we purposely employed a natural biological razor clam material with messy and irregular structures to improve the SERS. The rough surface was achieved by magnetron sputtering Ag nanoislands on the prism layer of the razor clams, and the Ag nanoparticles were treated using the method of oil-water interface self-assembly to form relatively uniform structures. Compared to the substrate without Ag nanoparticles, the presented substrate has better reproducibility, polarization-independence, and higher SERS intensity, and the detect limitation of R6G can be decreased from 10^-12 M to 10^-18 M. The ultrasensitive detection of thiram gives our structures potential for high sensitivity biosensors.

Silver-Coated Disordered Silicon Nanowires Provide Highly Sensitive Label-Free Glycated Albumin Detection through Molecular Trapping and Plasmonic Hotspot Formation

Glycated albumin (GA) is rapidly emerging as a robust biomarker for screening and monitoring of diabetes. To facilitate its rapid, point-of-care measurements, a label-free surface-enhanced Raman spectroscopy (SERS) sensing platform is reported that leverages the specificity of molecular vibrations and signal amplification on silver-coated silicon nanowires (Ag/SiNWs) for highly sensitive and reproducible quantification of GA. The simulations and experimental measurements demonstrate that the disordered orientation of the nanowires coupled with the wicking of the analyte molecules during the process of solvent evaporation facilitates molecular trapping at the generated plasmonic hotspots. Highly sensitive detection of glycated albumin is shown with the ability to visually detect spectral features at as low as 500 × 10^-9 m, significantly below the physiological range of GA in body fluids. Combined with chemometric regression models, the spectral data recorded on the Ag/SiNWs also allow accurate prediction of glycated concentration in mixtures of glycated and non-glycated albumin in proportions that reflect those in the bloodstream.

A Programmable DNA-Silicification-Based Nanocavity for Single-Molecule Plasmonic Sensing

Plasmonic nanocavities are highly desirable for optical sensing because of their singular ability to confine light into deep subwavelength volumes. Yet, it remains profoundly challenging to fabricate structurally resilient nanocavities with high fidelity, and to obtain direct, noninvasive visualization of the plasmonic hotspots within such constructs. Herein, highly precise and robust nanocavities, entitled DNA-silicified template for Raman optical beacon (DNA-STROBE), are engineered by using silicified DNA scaffolds for spatial organization of discrete plasmonic nanoparticles. In addition to substantially enhancing structural stability and chemical inertness, DNA silicification significantly improves nanogap control, resulting simultaneously in large and controlled local electromagnetic field enhancement. The ultrasmall mode volume of the DNA-STROBE constructs promotes single-molecule occupancy enabling surface-enhanced Raman spectroscopy (SERS) observations of single-molecule activity even at elevated background concentration, significantly relaxing the restrictive pico- to nanomolar molecular concentration condition typically required for such investigations. Additionally, leveraging super-resolution SERS measurements allows noninvasive and diffraction-unlimited spatial profiling of otherwise unresolvable plasmonic hotspots. The highly programmable and reproducible nature of the DNA-STROBE, coupled with its quantitative label-free molecular readouts, provides a versatile platform with applications across the spectrum of nanophotonics and biomedical sciences.

A colorful approach towards developing new nano-based imaging contrast agents for improved cancer detection

Providing physicians with new imaging agents to help detect cancer with better sensitivity and specificity has the potential to significantly improve patient outcomes. Development of new imaging agents could offer improved early cancer detection during routine screening or help surgeons identify tumor margins for surgical resection. In this study, we evaluate the optical properties of a colorful class of dyes and pigments that humans routinely encounter. The pigments are often used in tattoo inks and the dyes are FDA approved for the coloring of foods, drugs, and cosmetics. We characterized their absorption, fluorescence and Raman scattering properties in the hopes of identifying a new panel of dyes that offer exceptional imaging contrast. We found that some of these coloring agents, coined as "optical inks", exhibit a multitude of useful optical properties, outperforming some of the clinically approved imaging dyes on the market. The best performing optical inks (Green 8 and Orange 16) were further incorporated into liposomal nanoparticles to assess their tumor targeting and optical imaging potential. Mouse xenograft models of colorectal, cervical and lymphoma tumors were used to evaluate the newly developed nano-based imaging contrast agents. After intravenous injection, fluorescence imaging revealed significant localization of the new "optical ink" liposomal nanoparticles in all three tumor models as opposed to their neighboring healthy tissues (p < 0.05). If further developed, these coloring agents could play important roles in the clinical setting. A more sensitive imaging contrast agent could enable earlier cancer detection or help guide surgical resection of tumors, both of which have been shown to significantly improve patient survival.

Film wrap nanoparticle system with the graphene nano-spacer for SERS detection

Film wrap nanoparticle system (FWPS) is proposed and fabricated to perform SERS effect, where the Ag nanoparticle was completely wrapped by Au film and the double-layered graphene was selected as the sub-nano spacer. In this system, the designed nanostructure can be fully rather than partly used to generate hotspots and absorb probe molecules, compared to the nanoparticle to nanoparticle system (PTPS) or nanoparticle to film system (PTFS). The optimal fabricating condition and performance of this system were studied by the COMSOL Multiphysics. The simulation results show that the strongly large-scale localized electromagnetic field appears in the whole space between the Ag nanoparticle and Au film. The experimental results show that the FWPS presents excellent sensitivity (crystal violet (CV): 10^-11 M), uniformity, stability and high enhancement factor (EF: 2.23×10⁸). Malachite green (MG; 10^-10 M) on the surface of fish and DNA strands with different base sequence (A, T, C) were successfully detected. These advanced results indicate that FWPS is highly promising to be applied for the detection of environmental pollution and biomolecules.