Three-Dimensional and Time-Ordered Surface-Enhanced Raman Scattering Hotspot Matrix

Highly sensitive SERS detection of melamine based on 3D Ag@porous silicon photonic crystal

The stability, reproducibility and engineering of SERS substrate faces a great challenge in melamine SERS assay. In this work, a simple, highly sensitive, stable and cost-efficient SERS detection platform for melamine was established based on its Raman fingerprints spectrum. The Ag@ porous silicon photonic crystal (Ag@PPC) was prepared as the 3D SERS substrate by electrochemical etching and magnetron sputter technology. The main influence factors for the preparation of SERS substrate were investigated in detail. The analytical enhancement factor of the 3D SERS substrate can reach to 2.6 × 108. The 3D SERS detection platform showed a wide linear detection range of 10-4∼10 mg L-1 and a low limit of detection of 0.1 μg L-1 for melamine. Moreover, such detection platform showed good stability, high reproducibility and high recovery rates for melamine. The 3D Ag@PPC SERS substrate can be easily prepared and engineered, displaying a great potential application in food safety field.

Single-atom oxide-decorated AuNPs for universal enhancement in SERS detection of pesticide residues

BACKGROUND

In the context of modern agriculture, the proliferation of chemical use calls for enhanced pesticide detection to safeguard food quality and public health. The development of accurate testing methodologies is imperative to mitigate the environmental impact of pesticides and ensure the integrity of ecosystems, thereby reflecting the pressing need for advancements in agricultural safety protocols. Therefore, the development of highly sensitive monitoring technology for detecting pesticide residues in agricultural products is necessary for safeguarding human health, ensuring food safety, and maintaining environmental sustainability.

RESULTS

Herein, a controllable surface charge on single tungsten atom-modified gold nanoparticles was used to create an electrostatic force with positively charged pesticide residues. Moreover, hydrogen bonds formed by single-atom sites can induce analyte-adsorbed nanoparticle aggregation, and the sizes of single-tungsten-atom-decorated AuNPs can maintain a gap between each other, resulting in improved SERS detection sensitivity through analyte enrichment at gold nanoparticle hotspots. In terms of the detection limits for pesticide residue analysis, we can effectively achieve an ultrahigh sensitivity of 0.1 ppb for acetamiprid, paraquat and carbendazim, which is among the best SERS sensitivities at the state of the art. For apple sample analysis, our work demonstrated good reproductivity (RSD<6 %) and a strong linear relationship (R2 ≥ 0.97) for 4 pesticide residues after optimizing the pretreatment process, which proves the enormous potential in quantitative analysis.

SIGNIFICANCE

Single-atom sites hotspot are firstly successfully achieved and uniformly dispersed between Au nanoparticle, which can effectively increase the sensitivity, keep stability of the Raman scattering signals and possess a significant improvement beyond that of undecorated hotspots when applied in pesticide residue detection. This method can be employed as a universal strategy to capture pesticide residues at hotspots for SERS detection.

Magnetic-responsive sensors based on polydopamine macromolecules for highly sensitive detection of trace food colorant residues

As a kind of unique biomimetic macromolecule, polydopamine (PDA) have prominent in-situ reduction ability and interfacial adhesion. In this work, combined with in-situ reduction ability of PDA and excellent magnetic response performance of nickel foam (NF), a strategy was designed to fabricate a series of NF@PDA@AgNPs as magnetic-responsive surface enhancement Raman scattering (SERS) substrates for highly sensitive Rhodamine B (RhB) detection in chili powder. With crystal violet (CV) as probe molecule, the detection limit of SERS substrate could achieve 10-10 M, and the enhancement factor was as high as to 2.22 × 107. In addition, the NF@PDA@AgNPs SERS substrates showed excellent magnetic separation efficiency, good SERS uniformity and storage stability. More importantly, these substrates could achieve highly efficient collection and sensitive detection of RhB residues in chili powder by magnetic adsorption method, and the detection of limit was as low as to be 10-6 g/g. These NF@PDA@AgNPs substrates would be a great prospect for rapid and efficient pernicious contaminant detection in the chemical and biological fields.

Recent progress in Surface-Enhanced Raman Spectroscopy detection of biomarkers in liquid biopsy for breast cancer

Breast cancer is the most commonly diagnosed cancer in women globally and a leading cause of cancer-related mortality. However, current detection methods, such as X-rays, ultrasound, CT scans, MRI, and mammography, have their limitations. Recently, with the advancements in precision medicine and technologies like artificial intelligence, liquid biopsy, specifically utilizing Surface-Enhanced Raman Spectroscopy (SERS), has emerged as a promising approach to detect breast cancer. Liquid biopsy, as a minimally invasive technique, can provide a temporal reflection of breast cancer occurrence and progression, along with a spatial representation of overall tumor information. SERS has been extensively employed for biomarker detection, owing to its numerous advantages such as high sensitivity, minimal sample requirements, strong multi-detection ability, and controllable background interference. This paper presents a comprehensive review of the latest research on the application of SERS in the detection of breast cancer biomarkers, including exosomes, circulating tumor cells (CTCs), miRNA, proteins and others. The aim of this review is to provide valuable insights into the potential of SERS technology for early breast cancer diagnosis.

A stable and flexible Au@Ag NPs/PVA SERS platform for thiram residue detection on rough surface

Flexible and transparent surface-enhanced Raman scattering (SERS) substrates have gained great attention in analysis field as they offer a fast, non-destructive, and highly sensitive platform for in-situ detection. In this work, we present a facile one-pot strategy for synthesizing gold-cored silver shell nanoparticles (Au@Ag NPs) in the polyvinyl alcohol (PVA) colloid. With no other reducing agents, PVA can serve as both reducing and stabilizing agents for forming Au@Ag NPs. Besides, PVA acts as a scaffold to maintain SERS "hot-spots" by preventing nanoparticle aggregation. By using this flexible Au@Ag NPs/PVA colloid, the analytes can be extracted from rough surfaces for SERS measurements with excellent sensitivity, repeatability and stability. The SERS activity of the Au@Ag NPs/PVA remained at 89.8% even after 120 days of storage at room temperature in sealed air atmosphere. The selective detection of thiram residues on the surface of fruits and vegetables was successfully achieved. The limits of detection for thiram residues on apple and tomato surfaces were measured to be 0.58 and 0.56 ng cm-2, respectively, with recovery rate ranging from 91% to 107%. This work demonstrates the immense application potential of SERS colloid platform in the fields of food safety and environmental analysis.

Surface-Enhanced Raman Spectroscopy: Current Understanding, Challenges, and Opportunities

While surface-enhanced Raman spectroscopy (SERS) has experienced substantial advancements since its discovery in the 1970s, it is an opportunity to celebrate achievements, consider ongoing endeavors, and anticipate the future trajectory of SERS. In this perspective, we encapsulate the latest breakthroughs in comprehending the electromagnetic enhancement mechanisms of SERS, and revisit CT mechanisms of semiconductors. We then summarize the strategies to improve sensitivity, selectivity, and reliability. After addressing experimental advancements, we comprehensively survey the progress on spectrum-structure correlation of SERS showcasing their important role in promoting SERS development. Finally, we anticipate forthcoming directions and opportunities, especially in deepening our insights into chemical or biological processes and establishing a clear spectrum-structure correlation.

Acoustic Levitation Synthesis of Ultrahigh-Density Spherical Nucleic Acid Architectures for Specific SERS Analysis

Controllably regulating the electrostatic bilayer of nanogold colloids is a significant premise for synthesizing spherical nucleic acid (SNA) and building ordered plasmonic architectures. We develop a facile acoustic levitation reactor to universally synthesize SNAs with an ultra-high density of DNA strands, which is even higher than those of various state-of-the-art methods. Results reveal a new mechanism of DNA grafting via acoustic wave that can reconfigure the ligands on colloidal surfaces. The acoustic levitation reactor enables substrate-free three-dimentional (3D) spatial assembly of SNAs with controllable interparticle nanogaps through regulating DNA lengths. This kind of architecture may overcome the plasmonic enhancement limits by blocking electron tunneling and breaking electrostatic shielding in dried aggregations. Finite element simulations support the architecture with 3D spatial plasmonic hotspot matrix, and its ultrahigh surface-enhanced Raman scattering (SERS) capability is evidenced by in situ untargeted tracking of biomolecular events during photothermal stimulation (PTS)-induced cell death process. For biomarker diagnosis, the conjugation of adenosine triphosphate (ATP) aptamer onto SNAs enables in situ targeted tracking of ATP during PTS-induced cell death process. Particularly, the CD71 receptor and integrin α3β1 protein on PL45 cell membrance could be well distinguished by label-free SERS fingerprints when using specific XQ-2d and DML-7 aptamers, respectively, to synthesize SNA architectures. Our current acoustic levitation reactor offers a new method for synthesizing SNAs and enables both targeted and untargeted SERS analysis for tracking molecular events in living systems. It promises great potentials in biochemical synthesis and sensing in future.

Ag supraparticles with 3D hot spots to actively capture molecules for sensitive detection by surface enhanced Raman spectroscopy

To achieve highly sensitive detection using surface-enhanced Raman spectroscopy (SERS), it is imperative to fabricate a substrate with a high density of hot spots and facilitate the entry of target molecules into these hot spot regions. However, steric hindrance arising from the presence of surfactants and ligands on the SERS substrate may impede the access of target molecules to the hot spots. Here, we fabricate non-close-packed three-dimensional (3D) supraparticles with high-density hot spots to actively capture molecules. The formation of 3D supraparticles is attributed to the minimization of free energy during the gradual contraction of the droplet. The numerous capillaries present in non-close-packed supraparticles induce the movement of target molecules into the hot spot region through capillary force along with the solution. The results demonstrate that the SERS enhancement effect of 3D supraparticles is at least one order of magnitude higher than that of multi-layered nanoparticle structures formed under natural drying conditions. In addition, the SERS performance of 3D supraparticles is evaluated with diverse target molecules, including antimicrobial agents and drugs. Hence, this work provides a new idea for the preparation of non-close-packed substrates for SERS sensitive detection.

'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.

Highly Sensitive Microarray Immunoassay for Multiple Mycotoxins on Engineered 3D Porous Silicon SERS Substrate with Silver Nanoparticle Magnetron Sputtering

A high-throughput, rapid, and highly sensitive surface-enhanced Raman spectroscopy (SERS) microarray for screening multiple mycotoxins has been developed on a three-dimensional silver nanoparticle porous silicon (3D AgNP-Psi) SERS substrate, which was easy to be engineered by electrochemical etching and magnetron sputtering technology. The etching current density, etching waveform, and target material for magnetron sputtering have been investigated to obtain an optimal 3D SERS substrate. The optimized 3D AgNP-Psi SERS substrate showed an enhancement factor of 2.3 × 107 at 400 mA/cm2 constant current density etching for 20 s and Ag target magnetron sputtering for 200 nm thickness on the surface of Psi. The simulation electric field distribution showed the near-field enhancement can reach 3× higher than that of AuNPs. A protein microarray has been designed to screen multiple mycotoxins by AuNP Raman tags and a competitive immunoassay protocol on the surface of the 3D SERS substrate. The SERS protein microarray displayed wide linear detection ranges of 0.001-100 ng/mL for ochratoxin A, 0.01-100 ng/mL for aflatoxin B1, 0.001-10 ng/mL for deoxynivalenol, along with pg/mL low limit of detection, good recovery rates, repeatability, and reproducibility. The 3D SERS protein microarray is easily engineered and has a great potential application in medicine, environment, and food industry fields.

Hierarchically Structured Nanocomposites via Mixed-Graft Block Copolymer Templating: Achieving Controlled Nanostructure and Functionality

Integrating inorganic and polymerized organic functionalities to create composite materials presents an efficient strategy for the discovery and fabrication of multifunctional materials. The characteristics of these composites go beyond a simple sum of individual component properties; they are profoundly influenced by the spatial arrangement of these components and the resulting homo-/hetero-interactions. In this work, we develop a facile and highly adaptable approach for crafting nanostructured polymer-inorganic composites, leveraging hierarchically assembling mixed-graft block copolymers (mGBCPs) as templates. These mGBCPs, composed of diverse polymeric side chains that are covalently tethered with a defined sequence to a linear backbone polymer, self-assemble into ordered hierarchical structures with independently tuned nano- and mesoscale lattice features. Through the coassembly of mGBCPs with diversely sized inorganic fillers such as metal ions (ca. 0.1 nm), metal oxide clusters (0.5-2 nm), and metallic nanoparticles (>2 nm), we create three-dimensional filler arrays with controlled interfiller separation and arrangement. Multiple types of inorganic fillers are simultaneously integrated into the mGBCP matrix by introducing orthogonal interactions between distinct fillers and mGBCP side chains. This results in nanocomposites where each type of filler is selectively segregated into specific nanodomains with matrix-defined orientations. The developed coassembly strategy offers a versatile and scalable pathway for hierarchically structured nanocomposites, unlocking new possibilities for advanced materials in the fields of optoelectronics, sensing, and catalysis.

Constructing a 3D interconnected network of Ag nanostructures for high-performance SERS detection of food coloring agents

The design and preparation of various effective three-dimensional (3D) silver nanostructures is a frontier area of research in the field of surface-enhanced Raman scattering (SERS). This paper demonstrates a simple and novel method for the preparation of a substrate, whose surface was covered by a 3D interconnected network of Ag nanostructures, and the resulting network structure surface is free of organic contaminants. The EDS measurements confirm the metallic nature of the formed 3D Ag nanonetwork substrate. Additionally, the influence of experimental parameters on the morphology of the 3D Ag nanonetwork was also investigated, such as reaction time, hydrofluoric acid concentration, silver nitrate concentration and sodium citrate concentration. The 3D Ag nanonetwork has good uniformity. Importantly, the 3D Ag nanonetwork substrate was used to accurately and reliably detect amaranth (AR) and sunset yellow (SY) in beverages, with the lowest detection limit of 3 and 0.1 μg L-1, respectively. Therefore, this substrate is expected to be a promising candidate for SERS detection and offers attractive potential for a wider range of applications.

Three-Phase Catassembly of 10 nm Au Nanoparticles for Sensitive and Stable Surface-Enhanced Raman Scattering Detection

Interfacial self-assembly with the advantage of providing large-area, high-density plasmonic hot spots is conducive to achieving high sensitivity and stable surface-enhanced Raman scattering (SERS) sensing. However, rapid and simple assembly of highly repeatable large-scale multilayers with small nanoparticles remains a challenge. Here, we proposed a catassembly approach, where the "catassembly" means the increase in the rate and control of nanoparticle assembly dynamics. The catassembly approach was dropping heated Au sols onto oil chloroform (CHCl3), which triggers a rapid assembly of plasmonic multilayers within 15 s at the oil-water-air (O/W/A) interface. A mixture of heated sol and CHCl3 constructs a continuous liquid-air interfacial tension gradient; thus, the plasmonic multilayer film can form rapidly without adding functional ligands. Also, the dynamic assembly process of the three-phase catassembly ranging from cluster to interfacial film formation was observed through experimental characterization and COMSOL simulation. Importantly, the plasmonic multilayers of 10 nm Au NPs for SERS sensing demonstrated high sensitivity with the 1 nM level for crystal violet molecules and excellent stability with an RSD of about 10.0%, which is comparable to the detection level of 50 nm Au NPs with layer-by-layer assembly, as well as breaking the traditional and intrinsic understanding of small particles of plasmon properties. These plasmonic multilayers of 10 nm Au NPs through the three-phase catassembly method illustrate high SERS sensitivity and stability, paving the way for small-nanoparticle SERS sensing applications.

Real-Time Monitoring of a Single Molecule in Sub-nanometer Space by Dynamic Surface-Enhanced Raman Spectroscopy

In biology and chemistry, the ultimate goal is to monitor single molecules without labels. However, long-term monitoring of label-free molecules remains a challenge. Here, on the basis of the photothermal effect of gold nanorods (GNRs), we developed a platform for monitoring of a single molecule employing surface-enhanced Raman spectroscopy (SERS). Laser re-irradiation forms 1.0 nm gaps between GNRs, allowing us to observe single crystal violet (CV) molecules blinking for up to 4 min with dynamic surface-enhanced Raman spectroscopy (D-SERS). Bianalyte experiments confirm single-molecule features at CV concentrations of 10-14 M. Combining density functional theory (DFT) calculations with a free CV molecule observed in millisecond D-SERS, we propose that CV molecules can be confined to sub-nanometer space and the orientation of an individual CV moving in the range of 50-90° can be dynamically captured by D-SERS. This will provide a novel idea for effective exploration of the temporal and spatial dynamic processes of different reactions.

Long-lived SERS Matrix for Real-Time Biochemical Detection Using "Frozen" Transition State

For the long-time tracking of biological events, maintaining the bioactivity of the analytes during the detection process is essential. Here, we show a versatile surface-enhanced Raman Scattering (SERS) platform, termed a superwettable-omniphobic lubricous porous SERS (SOLP-SERS) substrate. The SOLP-SERS substrate could generate a three-dimensional liquid "hotspots" matrix with an ultra-long lifetime (tens of days) by confining tiny amounts of liquids within the gaps between nanoparticles. Then, the analytes are trapped in the uniform liquid "hotspots", whose bioactivity can be well maintained over a long period of time during SERS detection. Limits of detection down to femtomolar levels were achieved for various molecules. More importantly, SERS signals were uniform within the substrate and remained stable for more than 30 days. As a proof-of-concept experiment, the dynamic detection of the polymerization of Aβ peptides into amyloids was monitored by the SOLP-SERS substrate within 48 h. Moreover, the exosomes secreted by breast cancer cells, an important biomarker of cancer, were also measured. These results demonstrate that the SOLP-SERS platform will provide new insights into the development of real-time biochemical sensors with ultrahigh sensitivity.

High-throughput, highly sensitive and rapid SERS detection of Escherichia coli O157:H7 using aptamer-modified Au@macroporous silica magnetic photonic microsphere array

The aim of this research was to develop a simple, rapid, sensitive, high-throughput detection method for foodborne Escherichia coli (E. coli) O157:H7 based on the aptamer-modified gold nanoparticles@macroporous magnetic silica photonic microsphere (Au@MMSPM). Such Au@MMSPM array system for E. coli O157:H7 not only integrated sample pretreatment with rapid detection, but also showed highly enhanced effect to develop a highly sensitive SERS assay. The established SERS assay platform gave a wide linear detection range (10-10⁶ CFU/mL) and low limit of detection (2.20 CFU/mL) for E. coli O157:H7. The whole analysis time including sample pretreatment and detection was 110 min. This SERS-based assay platform provided a new high-throughput, highly sensitive and fast detection technology for monitoring E. coli O157:H7 in real samples from the fields of food industry, medicine and environment.

An autofocusing method for dynamic surface-enhanced Raman spectroscopy detection realized by optimized hill-climbing algorithm with long time stable hotspots

In the manual dynamic surface-enhanced Raman spectroscopy (D-SERS) detection process, it is difficult to focus on sample drop due to the constantly changing hotspot and easy judgment method. In this paper, we proposed an automatic focusing method based on long time stable hotspot with aid of optimization of hill-climbing algorithm and achieved on a designed device. First, set up a high temperature accelerating evaporation process to obtain hotspot and then cool to a low temperature rapidly to maintain it. Then, the spectral intensity was used as a focus of feedback signal in optimized hill-climbing algorithm to drive the sample stage to move up and down to adjust the depth of the laser on the samples to realize automatic focusing. As a result, the hotspot can be maintained for 5 min, and the autofocusing result can be achieved within 9 s, while the sensitivity was improved with two orders of magnitude in D-SERS detection of crystal violet (CV) compared with manual focusing.

A hybrid plasmonic nanoprobe using polyvinylpyrrolidone-capped bimetallic silver-gold nanostars for highly sensitive and reproducible solution-based SERS sensing

Practical solution-based assays using surface-enhanced Raman spectroscopy (SERS) with portable instrumentation are currently of particular interest for rapid, efficient, and low-cost detection of analytes. However, current assays still have limited applicability due to their poor sensitivity and reproducibility. Herein, we demonstrate highly stable polyvinylpyrrolidone (PVP)-capped bimetallic silver-coated gold nanostars (BGNS-Ag-PVP) as a solution-based SERS nanoprobe that is capable of producing a strong, uniform, and reproducible SERS signal using a portable Raman instrument. The developed hybrid BGNS-Ag-PVP nanostructure shows tunable optical properties with improved SERS sensitivity and reproducibility as compared to gold nanostars. We have synthesized bimetallic nanoprobes BGNS-Ag-PVP having three different silvers, referred to as BGNS-Ag-PVP-1, BGNS-Ag-PVP-2, and BGNS-Ag-PVP-3. The SERS performance of BGNS-Ag-PVP was studied using methylene blue (Meb) as a probe molecule, and we achieved a detection limit of up to 10 nM indicating the high sensitivity of the solution-based SERS platform. The application of such bimetallic nanoparticles is demonstrated via the sensitive detection of the antithyroid drug methimazole (Mz) used as a model analyte system. We have achieved a detection limit of 1 nM for Mz spiked with human urine indicating three orders of magnitude lower than previously reported solution-based SERS detection methods. Furthermore, the SERS performance was reproducible over 3 months indicating excellent stability and repeatability. The result illustrates the potential of this solution-based SERS detection platform as a promising sensing tool for analytes such as illicit drugs, and biomarkers that have affinity to bind on nanoprobes.

Combining thin-film microextraction and surface enhanced Raman spectroscopy to sensitively detect thiram based on 3D silver nanonetworks

By coupling thin-film microextraction (TFME) with surface enhanced Raman scattering (SERS), a facile method was developed for the determination of thiram in the complex matrix (orange juice or grape peel). The substrate of TFME was made by self-assembling silver sol on the silicon wafer to form a three-dimensional (3D) silver nanonetwork structure, without adding any template, which was used for TFME and SERS detection, respectively. The substrate exhibits high reproducibility with a relative standard deviation of about 7.32 % in spot and spot SERS intensity. The SERS signal intensity at a shift of 1384 cm^-1 and the thiram concentration showed good linearity in the range of 0.01-5 µg/L and the linear correlation coefficient was 0.9912. The detection limit for thiram was found to be 0.01 µg/L. The TFME-SERS method was applied for the determination of thiram in fruit juice and the results were obtained very well. Therefore, this method is expected to play a role in the detection of trace pollutants.

Simultaneous, Label-Free and High-throughput SERS Detection of Multiple Pesticides on Ag@Three-Dimensional Silica Photonic Microsphere Array

Rapid identification and quantitative simultaneous analysis for multiple pesticide in real samples based on surface-enhanced Raman spectroscopy (SERS) is still a challenge because of sample complexity, reproducibility, and stability of SERS substrate. With use of colloidal silver nanoparticles loaded three-dimensional (3D) silica photonic microspheres (SPMs) array as the analytical platform, a SERS-based array assay for multiple pesticides was developed in this work. The silver nanoparticles were fixed into the gaps formed by the self-assembled nanospheres of the 3D SPMs to produce "hot spots", on which the Raman enhanced effect was up to 9.86 × 10⁷ and the maximum electric field enhancement effect reached to 9.75 times, ensuring the target pesticides on the surface of the SERS-substrate integrated SPM can be detected sensitively. Using 2,4-dichlorophenoxyacetic acid (2,4-D), glyphosate, and imidacloprid as the testing pesticides, the label-free and high-throughput SERS assay for simultaneous detection of the pesticides was established, giving good linear detection ranges (0.1-204.8 μg/mL for 2,4-D, 0.3-247.9 μg/mL for glyphosate, and 0.2-204.8 μg/mL for imidacloprid) and low detection limits (3.03 ng/mL for 2,4-D, 3.14 ng/mL for glyphosate, and 8.82 ng/mL for imidacloprid). The spiked recovery rates in the real samples were measured in the range of 82-112%, which was consistent with that of the classical standard methods. The label-free 3D SERS array analytical platform provides a powerful tool for high-throughput and low-cost screening of multiple pesticide residues in real samples.

Optimized Design and Preparation of Ag Nanoparticle Multilayer SERS Substrates with Excellent Sensing Performance

Nanoparticle multilayer substrates usually exhibit excellent SERS activity due to multi-dimensional plasmon coupling. However, simply increasing the layers will lead to several problems, such as complex manufacturing procedures, reduced uniformity and poor reproducibility. In this paper, the local electric field (LEF) characteristics of a Ag nanoparticle (AgNP) multilayer were systematically studied through finite element simulations. We found that, on the glass support, the LEF intensity improved with the increase in the layers of AgNPs. However, the maximum LEF could be obtained with only two layers of AgNPs on the Au film support, and it was much stronger than the optimal value of the former. To verify the simulation results, we have successfully prepared one to four layers of AgNPs on both supports with a liquid-liquid interface self-assembly method, and carried out a series of SERS measurements. The experimental results were in good agreement with the simulations. Finally, the optimized SERS substrate, the 2-AgNP@Au film, showed an ultra-high SERS sensitivity, along with an excellent signal uniformity, which had a detection ability of 1 × 10-15 M for the Rhodamine 6G (R6G) and a relative standard deviation (RSD) of 11% for the signal intensity. Our study provides important theoretical guidance and a technical basis for the optimized design and application of high-performance SERS substrates.

A Rapid Therapeutic Drug Monitoring Strategy of Carbamazepine in Serum by Using Coffee-Ring Effect Assisted Surface-Enhanced Raman Spectroscopy

Carbamazepine (CBZ) has a narrow therapeutic concentration range, and therapeutic drug monitoring (TDM) is necessary for its safe and effective individualized medication. This study aims to develop a procedure for CBZ detection in serum using coffee-ring effect assisted surface-enhanced Raman spectroscopy (SERS). Silver nanoparticles deposited onto silicon wafers were used as the SERS-active material. Surface treatment optimization of the silicon wafers and the liquid-liquid extraction method were conducted to eliminate the influence of impurities on the silicon wafer surface and the protein matrix. The proposed detection procedure allows for the fast determination of CBZ in artificially spiked serum samples within a concentration range of 2.5-40 μg·mL^-1, which matches the range of the drug concentrations in the serum after oral medication. The limit of detection for CBZ was found to be 0.01 μg·mL^-1. The developed method allowed CBZ and its metabolites to be ultimately distinguished from real serum samples. The developed method is anticipated to be a potential tool for monitoring other drug concentrations.

Au Nanorods on Carbon-Based Nanomaterials as Nanohybrid Substrates for High-Efficiency Dynamic Surface-Enhanced Raman Scattering

Gold nanorods (AuNRs) with different aspect ratios were prepared by the seed-mediated growth method and combined with three carbon-based nanomaterials of multiple dimensions (i.e., zero-dimensional (0D) carbon black (CB), one-dimensional (1D) carbon nanotubes (CNTs), and two-dimensional (2D) graphene oxide (GO)). The AuNR/carbon-based nanomaterial hybrids were utilized in dynamic surface-enhanced Raman scattering (D-SERS). First, cetyltrimethylammonium bromide (CTAB) was used to stabilize and coat the AuNRs, enabling them to be dispersed in water and conferring a positive charge to the surface. AuNR/carbon-based nanomaterial hybrids were then formed via electrostatic attraction with the negatively charged carbon-based nanomaterials. Subsequently, the AuNR/carbon-based nanomaterial hybrids were utilized as large-area and highly sensitive Raman spectroscopy substrates. The AuNR/GO hybrids afforded the best signal enhancement because the thickness of GO was less than 5 nm, which enabled the AuNRs adsorbed on GO to produce a good three-dimensional hotspot effect. The enhancement factor (EF) of the AuNR/GO hybrids for the dye molecule Rhodamine 6G (R6G) reached 1 × 10⁷, where the limit of detection (LOD) was 10^-8 M. The hybrids were further applied in D-SERS (detecting samples transitioning from the wet state to the dry state). During solvent evaporation, the system spontaneously formed many hotspots, which greatly enhanced the SERS signal. The final experimental results demonstrated that the AuNR/GO hybrids afforded the best D-SERS signal enhancement. The EF value for R6G reached 1.1 × 10⁸ after 27 min, with a limit of detection of 10^-9 M at 27 min. Therefore, the AuNR/GO nanohybrids have extremely high sensitivity as molecular sensing elements for SERS and are also very suitable for the rapid detection of single molecules in water quality and environmental management.

Preparation and SERS performance of silver nanowires arrays on paper by automatic writing method

Silver nanowire ink was written on the surface of drawing paper by automatic writing method. Scanning electron microscopy was used to characterize the surface morphologies of the drawing paper before and after writing silver nanowires. The effects of fabrication parameters and measurement parameters on silver nanowires arrays were investigated. Crystal violet was selected as the probe molecule to study the SERS performance of silver nanowires arrays. The detection limit of crystal violet was as low as 10^-15 mol/L. The uniformity and repeatability of the arrays were also explored, and the relative standard deviation values were about 10%. Moreover, silver nanowires arrays were also relatively stable that SERS signals were still observed after ten weeks. Detection of the crystal violet residue was further achieved on the substrates by continuously pressing nine times. In addition, silver nanowires arrays were also applied to the quantitative analyses of 2, 2'-bipyridyl.

Single-model multi-tasks deep learning network for recognition and quantitation of surface-enhanced Raman spectroscopy

Surface-enhanced Raman scattering (SERS) spectroscopy analysis has long been the central task of nanoscience and nanotechnology to realize the ultrasensitive recognition/quantitation applications. Recently, the blooming of artificial intelligence algorithms provides an edge tool to revolutionize the spectroscopy analysis, especially for multiple substances analysis and large-scale data handling. In this study, a single-model multi-tasks deep learning network is proposed to simultaneously achieve the qualitative recognition and quantitative analysis of SERS spectroscopy. The SERS spectra of two kinds of hypoglycemic drugs (PHE, ROS) and the corresponding mixtures are collected, respectively, with the concentration grade from 10^-4 M to 10^-8 M. Based on the SERS spectroscopy dataset, the loss functions and hyperparameters of the multi-tasks classifications model are optimized, and the recognition accuracies are tested by simulation experiments. It is demonstrated that the accuracy rates of qualitative and quantitative analysis can reach up to 99.0% and 98.4%, respectively. Moreover, the practical feasibility of this multi-tasks model is demonstrated by using it to achieve qualitative and quantitative analysis of PHE and ROS in complex serum matrix. Overall, this single-model multi-tasks deep learning network shows significant potential for the recognition and quantitation of SERS spectroscopy, which provides the algorithmic and experimental basis for large-scale and multiple substances SERS spectra analysis.

Surface-enhanced Raman scattering as a potential strategy for wearable flexible sensing and point-of-care testing non-invasive medical diagnosis

As a powerful and effective analytical tool, surface-enhanced Raman scattering (SERS) has attracted considerable research interest in the fields of wearable flexible sensing and non-invasive point-of-care testing (POCT) medical diagnosis. In this mini-review, we briefly summarize the design strategy, the development progress of wearable SERS sensors and its applications in this field. We present SERS substrate analysis of material design requirements for wearable sensors and highlight the benefits of novel plasmonic particle-in-cavity (PIC)-based nanostructures for flexible SERS sensors, as well as the unique interfacial adhesion effect and excellent mechanical properties of natural silk fibroin (SF) derived from natural cocoons, indicating promising futures for applications in the field of flexible electronic, optical, and electrical sensors. Additionally, SERS wearable sensors have shown great potential in the fields of different disease markers as well as in the diagnosis testing for COVID-19. Finally, the current challenges in this field are pointed out, as well as the promising prospects of combining SERS wearable sensors with other portable health monitoring systems for POCT medical diagnosis in the future.

A Metallic Niobium Nitride with Open Nanocavities for Surface-Enhanced Raman Spectroscopy

The construction of open hot-spot structures that facilitate the entry of analytes is crucial for surface-enhanced Raman spectroscopy. Here, metallic niobium nitride (NbN) three-dimensional (3D) hierarchical networks with open nanocavity structure are first found to exhibit a strong visible-light localized surface plasmon resonance (LSPR) effect and extraordinary surface-enhanced Raman scattering (SERS) performance. The unique nanocavity structure allows easy entry of molecules, promoting the utilization of electromagnetic hot spots. The NbN substrate has a lowest detection limit of 1.0 × 10^-12 M and a Raman enhancement factor (EF) of 1.4 × 10⁸ for contaminants. Furthermore, the NbN hierarchical networks possess outstanding environmental durability, high signal reproducibility, and detection universality. The remarkable SERS sensitivity of the NbN substrate can be attributed to the joint effect of LSPR and interfacial charge transport (CT).

Multistage nucleic acid amplification induced nano-aggregation for 3D hotspots-improved SERS detection of circulating miRNAs

Circulating miRNAs in the blood can regulate disease development and thus indicate disease states via their various expression levels. For these reasons, circulating miRNAs constitute useful biomarkers, and an approach to the accurate detection of circulating miRNAs is attractive in the diagnosis and treatment of diseases. However, methods for clinical detecting of circulating miRNA that take both sensitivity and practicality into account are still needed. Therefore, we aimed herein to solve some inherent problems in the actual detection using a robust surface-enhanced Raman scattering (SERS) platform with integrated nucleic acid amplification and nanoparticle aggregation to construct 3D hotspots for improving performance of analyzing circulating miRNAs. After target recognition and initial signal amplification by DNAzyme, we observed that release triggered an open hairpin DNA on gold nanoparticles (AuNPs), which then promote AuNP aggregation, causing the accumulation of a large number of hotspots in three-dimention. The SERS biosensor achieved a better performance than the sandwich-type separation detection, with a low detection limit (0.37 fM) and a broad linear range (1 fM–10 nM) in liquids. This SERS platform can be used as a powerful tool for the detection of circulating miRNAs, and it can be used to improve the sensitivity and accuracy of various clinical-disease diagnoses.

Recent Advances in the Use of Surface-Enhanced Raman Scattering for Illicit Drug Detection

The rapid increase in illicit drug use and its adverse health effects and socio-economic consequences have reached alarming proportions in recent years. Surface-enhanced Raman scattering (SERS) has emerged as a highly sensitive analytical tool for the detection of low dosages of drugs in liquid and solid samples. In the present article, we review the state-of-the-art use of SERS for chemical analysis of illicit drugs in aqueous and complex biological samples, including saliva, urine, and blood. We also include a review of the types of SERS substrates used for this purpose, pointing out recent advancements in substrate fabrication towards quantitative and qualitative detection of illicit drugs. Finally, we conclude by providing our perspective on the field of SERS-based drug detection, including presently faced challenges. Overall, our review provides evidence of the strong potential of SERS to establish itself as both a laboratory and in situ analytical method for fast and sensitive drug detection and identification.

1,4-Benzenedithiol-Bridged Nanogap-Based Individual Particle Surface-Enhanced Raman Spectroscopy Mechanical Probe for Revealing the Endocytic Force

1,4-Benzenedithiol (BDT)-bridged core-satellite assemblies, as surface-enhanced Raman spectroscopy (SERS) mechanical probes, can be employed for real-time monitoring of the dynamics of endocytic forces and the accompanying trajectory of nanoparticles during the endocytosis process. These mechanical probes exhibit good responses in terms of SERS intensity ratios while undergoing mechanical pressure. Force tracing and the accompanying trajectory of nanoparticles are resolved accurately to render the endocytosis process in live cells. Density functional theory simulation results further proved the sensing scheme due to the electrons transforming between BDT and gold nanoparticles. Furthermore, this SERS mechanical probe is a valid method to visualize endocytic forces at multiple locations and establish a direct criterion to discriminate between cancer cells and normal cells.

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.

Controlling the Shrinkage of 3D Hot Spot Droplets as a Microreactor for Quantitative SERS Detection of Anticancer Drugs in Serum Using a Handheld Raman Spectrometer

Quantitative measurement is one of the ultimate targets for surface-enhanced Raman spectroscopy (SERS), but it suffers from difficulties in controlling the uniformity of hot spots and placing the target molecules in the hot spot space. Here, a convenient approach of three-phase equilibrium controlling the shrinkage of three-dimensional (3D) hot spot droplets has been demonstrated for the quantitative detection of the anticancer drug 5-fluorouracil (5-FU) in serum using a handheld Raman spectrometer. Droplet shrinkage, triggered by the shaking of aqueous nanoparticle (NP) colloids with immiscible oil chloroform (CHCl₃) after the addition of negative ions and acetone, not only brings the nanoparticles in close proximity but can also act as a microreactor to enhance the spatial enrichment capability of the analyte in plasmonic sites and thereby realize simultaneously controlling 3D hot spots and placing target molecules in hot spots. Moreover, the shrinking process of Ag colloid droplets has been investigated using a high-speed camera, an in situ transmission electron microscope (in situ TEM), and a dark-field microscope (DFM), demonstrating the high stability and uniformity of nanoparticles in droplets. The shrunk Ag NP droplets exhibit excellent SERS sensitivity and reproducibility for the quantitative analysis of 5-FU over a large range of 50-1000 ppb. Hence, it is promising for quantitative analysis of complex systems and long-term monitoring of bioreactions.

Single-atom sites on perovskite chips for record-high sensitivity and quantification in SERS

Surface enhanced Raman scattering (SERS) is a rapid and nondestructive technique that is capable of detecting and identifying chemical or biological compounds. Sensitive SERS quantification is vital for practical applications, particularly for portable detection of biomolecules such as amino acids and nucleotides. However, few approaches can achieve sensitive and quantitative Raman detection of these most fundamental components in biology. Herein, a noble-metal-free single-atom site on a chip strategy was applied to modify single tungsten atom oxide on a lead halide perovskite, which provides sensitive SERS quantification for various analytes, including rhodamine, tyrosine and cytosine. The single-atom site on a chip can enable quantitative linear SERS responses of rhodamine (10^-6-1 mmol L^-1), tyrosine (0.06-1 mmol L^-1) and cytosine (0.2-45 mmol L^-1), respectively, which all achieve record-high enhancement factors among plasmonic-free semiconductors. The experimental test and theoretical simulation both reveal that the enhanced mechanism can be ascribed to the controllable single-atom site, which can not only trap photoinduced electrons from the perovskite substrate but also enhance the highly efficient and quantitative charge transfer to analytes. Furthermore, the label-free strategy of single-atom sites on a chip can be applied in a portable Raman platform to obtain a sensitivity similar to that on a benchtop instrument, which can be readily extended to various biomolecules for low-cost, widely demanded and more precise point-of-care testing or in-vitro detection.

ELECTRONIC SUPPLEMENTARY MATERIAL

Supplementary material is available for this article at 10.1007/s40843-022-1968-5 and is accessible for authorized users.

Three-dimensional surface-enhanced Raman scattering substrates constructed by integrating template-assisted electrodeposition and post-growth of silver nanoparticles

Three-dimensional (3D) plasmonic nano-arrays can provide high surface-enhanced Raman scattering (SERS) sensitivity, good spectral uniformity and excellent reproducibility. However, it is still a challenge to develop a simple and efficient method for fabrication of 3D plasmonic nano-arrays with high SERS performance. Here we report a facile approach to construct ordered arrays of silver (Ag) nanoparticles-assembled spherical micro-cavities using polystyrene (PS) sphere template-assisted electrodeposition and post-growth. The electrodeposited small Ag nanoparticles grow into bigger stable nanoparticles during the post-growth process, which could significantly improve the SERS sensitivity. The Ag nanoparticles-assembled 3D micro-cavity array provides much more hotspots in the excitation laser beam-covered volume than the two-dimensional counterpart. The relative standard deviation (RSD) of 612 cm^-1 peak of rhodamine 6G (R6G) was calculated to be 8%, and the RSD of the characteristic peak taken from substrates of different batches was less than 10%. The detectable lower concentration as low as 1 fM was achieved for an aqueous solution of R6G. Such SERS substrate also showed high sensitivity to thiram (fungicide) and paraquat (herbicide) in water with limits of detection of 0.067 nM and 2.5 nM respectively. Furthermore, it also demonstrated that SERS detection of pesticide residues on fruits can be realized, showing a potential application in rapid monitoring food safety.

Exploring Synergistic Effect of Capillary Force and Electrostatic Attraction towards the SERS Sensitivity of D-SERS

Constructing well defined nanostructure is the primary step realizing the SERS detection with high sensitivity. The solid substrate prepared by liquid-liquid interface self-assembly has been demonstrated with the controllable and...

Construction of Ag nanowire@Au nanoparticle nano nests with densely stacked small gaps for actively trapping molecules to realize diversity SERS detection

The SERS detection method based nano nest model actively traps molecules by the capillary action to detect different kinds of molecules, such as pesticide residues, bioenergy substances, antibiotic residues and allergic reactions.

Cancer Cell-Specific Enhanced Raman Imaging and Photothermal Therapeutic Effect Based on Reversibly pH-Responsive Gold Nanoparticles

Stimuli-responsive nanoparticles are favorable for improving the selective delivery and rational vocation that easily avoids the undesirable barriers or side effects, leading to a further improved therapeutic efficiency. Furthermore, multifunctional nanomaterials have been extensively developed as attractive candidates for theranostic reagents for cancer treatment. In this article, we developed reversibly pH-responsive gold nanoparticles (AuNPs) with an enhanced Raman scattering signal as well as an efficient photothermal effect and demonstrated their applications as a theranostic reagent for cancer treatment. Surfaces of these AuNPs were modified with mixed layers of Cy3-modified single-stranded DNA (ssDNA-Cy3) for Raman probing and a negative charge supply and cytochrome C (Cyt C) for pH-responsive charge inversion. This combination of pH-responsive ligands and Raman probes played an important role in inducing the assembly or disassembly of AuNPs corresponding to the neighboring pH, accompanied by an additional highly distinguished Raman signal intensity. An operative reversible response of the AuNPs to pH is endowed with the characteristic behavior of AuNPs with the cancerous cell's acidic microenvironment of low pH. The responsive aggregation of AuNPs in a lower pH medium provides highly amplified signals attributed to well-formed hot spots between the particle surfaces that deliver better Raman scattering signals. The acidic pH-responsive aggregation of the particles also provided efficient photothermal treatments using a long-wavelength laser light with the benefit of deeper penetration for cancer cells. In vitro experiments employing cancer cells and control normal cells well-demonstrated the specificity of the particles to cancer cells in terms of highly enhanced Raman imaging and therapeutic efficiency.

The rapid SERS detection of succinylcholine chloride in human plasma is based on the high affinity between quaternary ammonium salt structures

Succinylcholine chloride (SCC) is a common poison that threatens human life. At present, there is a lack of research on its on-site rapid detection methods. In this work, the use of gold nanorods as an enhanced substrate based on the high affinity between the quaternary ammonium salt structure can achieve rapid SERS detection of SCC in plasma. The long alkane chain structure of cetyltrimethylammonium bromide (CTAB) and the quaternary ammonium salt structure of SCC have a high molecular affinity, so that the target molecule can show a strong and obvious characteristic signal of SERS. Combined with a simple pretreatment method, acetonitrile is used as a protein precipitation agent to effectively remove matrix interference. The constructed SERS substrate can achieve the sensitive detection of 2 × 10^-8 M level of SCC in plasma samples and has high detection reproducibility. The entire pre-processing and testing process can be completed within 7 min, which can be used as an important technical basis for the preliminary identification of on-site SCC-related drug cases. The research results provide an effective solution for the establishment of SCC analysis strategies in complex matrices, and can provide new ideas for solving the problems of difficult identification of common poisons in the field and the lack of rapid detection methods on site.

3D hotspot matrix of Au nanoparticles on Au island film with a spacer layer of dithiol molecules for highly sensitive surface-enhanced Raman spectroscopy

Engineering of efficient plasmonic hotspots has been receiving great attention to enhance the sensitivity of surface-enhanced Raman scattering (SERS). In the present study, we propose a highly sensitive SERS platform based on Au nanoparticles (AuNPs) on Au island film (AuIF) with a spacer layer of 1,4-benzenedimethanethiol (BDMT). The three-dimensional (3D) hotspot matrix has been rationally designed based on the idea of employing 3D hotspots with a vertical nanogap between AuIF and AuNPs after generating large area two-dimensional hotspots of AuIF. AuNPs@BDMT@AuIF are fabricated by functionalizing BDMT on AuIF and then immobilizing AuNPs. The SERS performance is investigated with Rhodamine 6G as a probe molecule and the determined enhancement factor is 1.3 × 10⁵. The AuNPs@BDMT@AuIF are then employed to detect thiram, which is used as a fungicide, with a detection limit of 13 nM. Our proposed platform thus shows significant potential for use in highly sensitive SERS sensors.

A sandwich SERS detection system based on optical convergence and synergistic enhancement effects

In this research, we propose a novel microlens surface-enhanced Raman spectroscopy (SERS) substrate @ Au film detection system, which is shown to have excellent attributes. This scheme involves the construction of a PDMS plano-convex microlens SERS-active substrate in combination with an Au film. Due to the optical convergence from the microlens, the synergistic enhancement effects due to the Au film, and the "Au film-molecules-AgNPs" sandwich structure, an outstanding SERS performance is achieved. Multiple tests using a portable Raman spectrometer show that the optical convergence due to the microlens and the coupling effects contribute around 1.85× and 26.18× enhancement of the Raman signal, respectively. Even for objective lenses with different numerical apertures, simulations show that the microlens SERS substrate can further enhance the signal collection efficiency; this indicates that the detection scheme is universally applicable. Moreover, the microlens SERS substrate @ Au film system shows excellent time stability, and its Raman enhancement performance remains consistently above 98% of the original signal, even one week later. Our proposed system is simple to prepare, is low cost and has many potential practical applications, which include the detection of biochemical samples.

A Novel Multiplex Mycotoxin Surface-Enhanced Raman Spectroscopy Immunoassay Using Functional Gold Nanotags on a Silica Photonic Crystal Microsphere Biochip

A novel multiplex mycotoxin surface-enhanced Raman spectroscopy (SERS) immunoassay was established for the first time on different artificial antigen-modified silica photonic crystal microspheres (SPCMs), which can be integrated into a biochip array to achieve multiplex detection using corresponding antibody-functionalized gold nanoparticles (AuNPs) as the SERS nanotag. The unique optical structure of SPCMs is helpful to find the detection spots easily, accommodate a large amount of probe molecules, and enhance the Raman signal intensity. Such enhancement was confirmed by the simulation result, showing the electric field enhancing effect in SPCMs with AuNPs being 7 times. A competitive SERS immunoassay was established using antigen-modified SPCMs and mycotoxins to compete for binding antibody-functionalized SERS nanotags, displaying broad linear detection ranges of 0.001-0.1 ng/mL for aflatoxin B₁ (AFB₁), 0.01-10 ng/mL for ochratoxin A (OTA), and 0.001-0.1 ng/mL for zearalenone (ZEN) and low detection limits of 0.82 pg/mL for AFB₁, 1.43 pg/mL for OTA, and 1.00 pg/mL for ZEN. In the spiked cereal samples, recovery rates of the method were measured in the range of 70.35-118.04% for the three mycotoxins, which was in agreement with that of the traditional enzyme-linked immunosorbent assay method. The SERS immunoassay for mycotoxin detection also showed high specificity and good repeatability and reproducibility. The new microsphere-based SERS immunoassay biochip only requires a one-step reaction and overcomes the disadvantages of fluorescence and chemiluminescence background signals. The work paves the way for further developing SERS-based microsphere suspension arrays for new targets.

Trapping analytes into dynamic hot spots using Tyramine-medicated crosslinking chemistry for designing versatile sensor

HYPOTHESIS

Due to the intrinsic nature of the surface-enhanced Raman scattering (SERS), the detection of molecules with weak binding affinities toward metal substrates is critical for development of a universal SERS sensing platform. We hypothesized the physical trapping of small pesticide molecules for active hot spot generation using tyramine-mediated crosslinking chemistry and silver nanoparticles (Ag NPs) enhances SERS detection sensitivity.

EXPERIMENTS

Tyramine-mediated crosslinking chemistry for sensor application was validated by ultraviolet-visible absorption spectroscopy, scanning electron microscopy, dynamic light scattering, and Raman spectroscopy. SERS sensing platform using tyramine-mediated crosslinking reaction was systematically studied for detection of 1,4-dyethylnylbenzene as a model analyte. This sensor system was applied to detect two other pesticides, thiabendazole and 1,2,3,5-tetrachlorobenzene, which have different binding affinities toward metal surfaces.

FINDINGS

The SERS signal of 1,4-dyethylnylbenzene obtained using this sensor system was 3.6 times stronger than that obtained using the Ag colloidal due to the nanogap of approximately 1.3 nm within the generated hot spots. This sensor system based on tyramine-mediated crosslinked Ag NPs was evaluated as a promising tool to achieve a solution based sensitive detection of various pesticide molecules that cannot be adsorbed on the surfaces of typical SERS substrates such as metal nanoparticles.

Silicon Microchannel-Driven Raman Scattering Enhancement to Improve Gold Nanorod Functions as a SERS Substrate toward Single-Molecule Detection

The investigation of enhanced Raman signal effects and the preparation of high-quality, reliable surface-enhanced Raman scattering (SERS) substrates is still a hot topic in the SERS field. Herein, we report an effect based on the shape-induced enhanced Raman scattering (SIERS) to improve the action of gold nanorods (AuNRs) as a SERS substrate. Scattered electric field simulations reveal that bare V-shaped Si substrates exhibit spatially distributed interference patterns from the incident radiation used in the Raman experiment, resulting in constructive interference for an enhanced Raman signal. Experimental data show a 4.29 increase in Raman signal intensity for bare V-shaped Si microchannels when compared with flat Si substrates. The combination of V-shaped microchannels and uniform aggregates of AuNRs is the key feature to achieve detections in ultra-low concentrations, enabling reproducible SERS substrates having high performance and sensitivity. Besides SIERS effects, the geometric design of V-shaped microchannels also enables a "trap" to the molecule confinement and builds up an excellent electromagnetic field distribution by AuNR aggregates. The statistical projection of SERS spectra combined with the SIERS effect displayed a silhouette coefficient of 0.83, indicating attomolar (10^-18 mol L^-1) detection with the V-shaped Si microchannel.

Rapid Formation of Nanoclusters for Detection of Drugs in Urine Using Surface-Enhanced Raman Spectroscopy

We developed a method based on surface-enhanced Raman spectroscopy (SERS) and a sample pretreatment process for rapid, sensitive, reproducible, multiplexed, and low-cost detection of illegal drugs in urine. The abuse of new psychoactive substances (NPS) has become an increasingly serious problem in many countries. However, immunoassay-based screening kits for NPS are usually not available because of the lack of corresponding antibodies. SERS has a great potential for rapid detection of NPS because it can simultaneously detect multiple kinds of drugs without the use of antibodies. To achieve highly sensitive SERS detection of drugs, sodium bromide was first employed to induce the rapid formation of Ag nanoclusters by aggregating silver nanoparticles (AgNPs) in the extracted sample solution. SERS measurements were performed immediately after the sample pretreatment without incubation. The three-dimensional SERS hot spots were believed to form significantly within the nanoclusters, providing strong SERS enhancement effects. The displacement of citrate molecules on the surfaces of the AgNPs by bromide ions helped increase the adsorption of drug molecules, increasing their areal density. We demonstrated the simultaneous detection of two kinds of NPS, methcathinone and 4-methylmethcathinone, in urine at a concentration as low as 0.01 ppm.

Discriminatory Detection of ssDNA by Surface-Enhanced Raman Spectroscopy (SERS) and Tree-Based Support Vector Machine (Tr-SVM)

We report label-free detection of 86-base single-stranded DNA (ssDNA) gene segments by surface-enhanced Raman spectroscopy (SERS). The use of a slippery liquid infused porous (SLIP) membrane induced aggregation of 43 nm gold nanoparticles and ssDNA upon pin-free droplet evaporation. The combined SLIPSERS approach generates significant numbers of SERS hot-spots and enabled detection at the 100 nM level of mecA and intI1 gene segments-two genes of interest in the context of antibiotic resistance. Tree-based multiclass support vector machine (Tr-SVM) classifiers were built to discriminate SERS spectra of 12 different gene sequences obtained by SLIPSERS: mecA, intI1, as well as analogues of mecA and intI1, respectively, with 2-10 base mismatches, and two random sequences. The trained predictive Tr-SVM classifiers correctly identified each gene sequence with a prediction accuracy of ∼90%. This study illustrates a novel means for discriminatory label-free SERS detection of ssDNA enabled by Tr-SVM.