Ultrahigh-density array of silver nanoclusters for SERS substrate with high sensitivity and excellent reproducibility

From Genotype to Phenotype: Raman Spectroscopy and Machine Learning for Label-Free Single-Cell Analysis

Raman spectroscopy has made significant progress in biosensing and clinical research. Here, we describe how surface-enhanced Raman spectroscopy (SERS) assisted with machine learning (ML) can expand its capabilities to enable interpretable insights into the transcriptome, proteome, and metabolome at the single-cell level. We first review how advances in nanophotonics-including plasmonics, metamaterials, and metasurfaces-enhance Raman scattering for rapid, strong label-free spectroscopy. We then discuss ML approaches for precise and interpretable spectral analysis, including neural networks, perturbation and gradient algorithms, and transfer learning. We provide illustrative examples of single-cell Raman phenotyping using nanophotonics and ML, including bacterial antibiotic susceptibility predictions, stem cell expression profiles, cancer diagnostics, and immunotherapy efficacy and toxicity predictions. Lastly, we discuss exciting prospects for the future of single-cell Raman spectroscopy, including Raman instrumentation, self-driving laboratories, Raman data banks, and machine learning for uncovering biological insights.

Raman Scattering Enhancement through Pseudo-Cavity Modes

Raman spectroscopy plays a pivotal role in spectroscopic investigations. The small Raman scattering cross-section of numerous analytes, however, requires enhancement of the signal through specific structuring of the electromagnetic and morphological properties of the underlying surface. This enhancement technique is known as surface-enhanced Raman spectroscopy (SERS). Despite the existence of various proposed alternatives, the approach involving Fabry-Pérot cavities, which constitutes a straightforward method to enhance the electromagnetic field around the analyte, has not been extensively utilized. This is because, for the analyte to experience the maximum electric field, it needs to be embedded within the cavity. Consequently, the top mirror of the cavity will eventually shield it from the external laser source. Recently, an open-cavity configuration has been demonstrated to exhibit properties similar to the classic Fabry-Pérot configuration, with the added advantage of maintaining direct accessibility for the laser source. This paper showcases how such a simple yet innovative configuration can be effectively utilized to achieve remarkable Raman enhancement. The simple structure, coupled with its inexpensive nature and versatility in material selection and scalability, makes it an ideal choice for various analytes and integration into diverse Raman apparatus setups.

Metal-Organic Framework-Enabled Trapping of Volatile Organic Compounds into Plasmonic Nanogaps for Surface-Enhanced Raman Scattering Detection

Utilizing electromagnetic hotspots within plasmonic nanogaps is a promising approach to create ultrasensitive surface-enhanced Raman scattering (SERS) substrates. However, it is difficult for many molecules to get positioned in such nanogaps. Metal-organic frameworks (MOFs) are commonly used to absorb and concentrate diverse molecules. Herein, we combine these two strategies by introducing MOFs into plasmon-coupled nanogaps, which has so far remained experimentally challenging. Ultrasensitive SERS substrates are fabricated through the construction of nanoparticle-on-mirror structures, where Au nanocrystals are encapsulated with a zeolitic imidazolate framework-8 (ZIF-8) shell and then coupled to a gold film. The ZIF-8 shell, as a spacer that separates the Au nanocrystal and the Au film, can be adjusted in thickness over a wide range, which allows the electric field enhancement and plasmon resonance wavelength to be varied. By trapping Raman-active molecules within the ZIF-8 shell, we show that our plasmon-coupled structures exhibit a superior SERS detection performance. A range of volatile organic compounds at the concentrations of 10-2 mg m-3 can be detected sensitively and reliably. Our study therefore offers an attractive route for synergistically combining plasmonic electric field enhancement and MOF-enabled molecular enrichment to design and create SERS substrates for ultrasensitive detection.

Prismatic Confinement Induces Tunable Orientation in Plasmonic Supercrystals

Throughout history scientists have looked to Nature for inspiration and attempted to replicate intricate complex structures formed by self-assembly. In the context of synthetic supercrystals, achieving such complexity remains a challenge due to the highly symmetric nature of most nanoparticles (NPs). Previous works have shown intricate coupling between the self-assembly of NPs and confinement in templates, such as emulsion droplets (spherical confinement) or tubes (cylindrical confinement). This study focuses on the interplay between anisotropic NP shape and tunable "prismatic confinement" leading to the self-assembly of supercrystals in cavities featuring polygonal cross sections. A multiscale characterization strategy is employed to investigate the orientation and structure of the supercrystals locally and at the ensemble level. Our findings highlight the role of the mold interface in guiding the growth of distinct crystal domains: each side of the mold directs the formation of a monodomain that extends until it encounters another, leading to the creation of grain boundaries. Computer simulations in smaller prismatic cavities were conducted to predict the effect of an increased confinement. Comparison between prismatic and cylindrical confinements shows that flat interfaces are key to orienting the growth of supercrystals. This work shows a method of inducing orientation in plasmonic supercrystals and controlling their textural defects, thus offering insight into the design of functional metasurfaces and hierarchically structured devices.

Constructing a Comprehensive Nanopattern Library through Morphological Transitions of Block Copolymer Surface Micelles via Direct Solvent Immersion

This study establishes a comprehensive library of nanopatterns achievable by a single block copolymer (BCP), ranging from spheres to complex structures like split micelles, flower-like clusters, toroids, disordered micelle arrays, and unspecified unique shapes. The ordinary nanostructures of polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) surface micelles deposited on a SiOx surface undergo a unique morphology transformation when immersed directly in solvents. Investigating parameters such as immersion solvents, BCP molecular weight, substrate interactions, and temperature, this work reveals the influence of these parameters on the thermodynamics and kinetics governing the morphology transformation. Additionally, the practical application of BCP nanopattern templates for fabricating metal nanostructures through direct solvent immersion of surface micelles is demonstrated. This approach offers an efficient and effective method for producing diverse nanostructures, with the potential to be employed in nanolithography, catalysts, electronics, membranes, plasmonics, and photonics.

Precision nanoengineering for functional self-assemblies across length scales

As nanotechnology continues to push the boundaries across disciplines, there is an increasing need for engineering nanomaterials with atomic-level precision for self-assembly across length scales, i.e., from the nanoscale to the macroscale. Although molecular self-assembly allows atomic precision, extending it beyond certain length scales presents a challenge. Therefore, the attention has turned to size and shape-controlled metal nanoparticles as building blocks for multifunctional colloidal self-assemblies. However, traditionally, metal nanoparticles suffer from polydispersity, uncontrolled aggregation, and inhomogeneous ligand distribution, resulting in heterogeneous end products. In this feature article, I will discuss how virus capsids provide clues for designing subunit-based, precise, efficient, and error-free self-assembly of colloidal molecules. The atomically precise nanoscale proteinic subunits of capsids display rigidity (conformational and structural) and patchy distribution of interacting sites. Recent experimental evidence suggests that atomically precise noble metal nanoclusters display an anisotropic distribution of ligands and patchy ligand bundles. This enables symmetry breaking, consequently offering a facile route for two-dimensional colloidal crystals, bilayers, and elastic monolayer membranes. Furthermore, inter-nanocluster interactions mediated via the ligand functional groups are versatile, offering routes for discrete supracolloidal capsids, composite cages, toroids, and macroscopic hierarchically porous frameworks. Therefore, engineered nanoparticles with atomically precise structures have the potential to overcome the limitations of molecular self-assembly and large colloidal particles. Self-assembly allows the emergence of new optical properties, mechanical strength, photothermal stability, catalytic efficiency, quantum yield, and biological properties. The self-assembled structures allow reproducible optoelectronic properties, mechanical performance, and accurate sensing. More importantly, the intrinsic properties of individual nanoclusters are retained across length scales. The atomically precise nanoparticles offer enormous potential for next-generation functional materials, optoelectronics, precision sensors, and photonic devices.

Tetragonally Packed Inverted Cylindrical Microdomains from Binary Block Copolymer Blends with Enhanced Hydrogen Bonding

Hexagonally packed (HEX) cylindrical microdomains can be obtained through the self-assembly of block copolymers (BCPs) with a moderately asymmetric volume fraction of one block (f), resulting in the formation of minor cylinders. However, for next-generation lithography and high-density memory devices, it is desirable to obtain densely and tetragonally packed inverted cylindrical microdomains, which are composed of the major block in the minor matrix. The inverted cylinders differ from conventional HEX cylinders, which consist of the minor block in the matrix of the major block. In this study, we achieved this objective by utilizing a binary blend of a polystyrene-b-poly(4-vinylpyridine) copolymer (S4VP) and polystyrene-b-poly(4-hydroxystyrene) copolymer (SHS), where the P4VP block exhibited a strong hydrogen bonding interaction with the PHS block. By carefully controlling the molecular weight ratio of S4VP and SHS as well as the blend composition, we successfully observed tetragonally packed inverted PS cylinders with a square cross-section at a volume fraction of PS of 0.69.

Imaging of Antiferroelectric Dark Modes in an Inverted Plasmonic Lattice

Plasmonic lattice nanostructures are of technological interest because of their capacity to manipulate light below the diffraction limit. Here, we present a detailed study of dark and bright modes in the visible and near-infrared energy regime of an inverted plasmonic honeycomb lattice by a combination of Au⁺ focused ion beam lithography with nanometric resolution, optical and electron spectroscopy, and finite-difference time-domain simulations. The lattice consists of slits carved in a gold thin film, exhibiting hotspots and a set of bright and dark modes. We proposed that some of the dark modes detected by electron energy-loss spectroscopy are caused by antiferroelectric arrangements of the slit polarizations with two times the size of the hexagonal unit cell. The plasmonic resonances take place within the 0.5-2 eV energy range, indicating that they could be suitable for a synergistic coupling with excitons in two-dimensional transition metal dichalcogenides materials or for designing nanoscale sensing platforms based on near-field enhancement over a metallic surface.

Recent Advances in Nanoparticle-Based Optical Sensors for Detection of Pesticide Residues in Soil

The excessive and unreasonable use of pesticides has adversely affected the environment and human health. The soil, one of the most critical natural resources supporting human survival and development, accumulates large amounts of pesticide residues. Compared to traditional spectrophotometry analytical methods, nanoparticle-based sensors stand out for their simplicity of operation as well as their high sensitivity and low detection limits. In this review, we focus primarily on the functions that various nanoparticles have and how they can be used to detect various pesticide residues in soil. A detailed discussion was conducted on the properties of nanoparticles, including their color changeability, Raman enhancement, fluorescence enhancement and quenching, and catalysis. We have also systematically reviewed the methodology for detecting insecticides, herbicides, and fungicides in soil by using nanoparticles.

Significantly increased Raman enhancement enabled by hot-electron-injection-induced synergistic resonances on anisotropic ReS2 films

Two-dimensional (2D) materials are an excellent platform for surface-enhanced Raman spectroscopy (SERS). However, a poor detection sensitivity hinders their practical application. Exciton resonance (μ_ex) can improve SERS significantly by lending intensity to nearby charge-transfer resonance. Coincidentally, for ReS₂, the enhanced μ_ex can be achieved through the injection of excited-state electrons which can adjust the energy band to the SERS detection range. Moreover, ReS₂ has strong anisotropic properties, which adds an additional dimension for SERS. Therefore, ReS₂ is an ideal candidate to realize highly sensitive anisotropic SERS. In this paper, the metallic T phase of ReS₂ is introduced to the semiconducting Td phase by phase engineering. The photoinduced electron tunneling from the T phase to the Td phase can tune exciton emissions to the visible region, which effectively facilitates the photoinduced charge transfer processes. With RhB as the probe molecule, the synergistic resonance effects improve the limit of detection to 10^-9 M with the enhancement factor up to about 10⁸. Meanwhile, the obtained ultrasensitive SERS substrates also show good uniformity, stability as well as unique anisotropy. Our results open a new perspective in the improvement of the SERS performance.

CNN-assisted SERS enables ultra-sensitive and simultaneous detection of Scr and BUN for rapid kidney function assessment

Kidney disease is highly prevalent and may result in severe clinical outcomes. Serum creatinine (Scr) and blood urea nitrogen (BUN) are the most widely used biomarkers for kidney function assessment, yet when measured alone, the result can be affected by a variety of parameters such as age, gender, protein consumption, etc. Measuring Scr and BUN simultaneously can eliminate most of the external influences and greatly improve the assessment of kidney function. In this study, a real-time kidney function assessment system based on dual biomarker detection was proposed. Scr and BUN were determined using surface-enhanced Raman scattering (SERS) within the concentration range of 10^-1 to 10^-6 M and 0.28 to 100 mg dl^-1, respectively. A one-dimensional convolutional neural network (1D-CNN) model was employed to quantitatively analyze the concentration of biomarkers from the SERS spectral measurements. Moreover, we simulated a variety of kidney health conditions with 16 groups of mixed Scr and BUN in serum. The proposed CNN-assisted SERS method was used to quantify both biomarkers and provide diagnostic results. The Au core-Ag shell nanoprobes provided ultra-sensitive SERS detection and the CNN model achieved excellent regression results with an R² of 0.9871 in the testing dataset. The system demonstrated a rapid and robust evaluation for the assessment of kidney function, providing a promising idea for medical diagnosis with the help of spectroscopy and deep learning methods.

Nanomaterials meet surface-enhanced Raman scattering towards enhanced clinical diagnosis: a review

Surface-enhanced Raman scattering (SERS) is a very promising tool for the direct detection of biomarkers for the diagnosis of i.e., cancer and pathogens. Yet, current SERS strategies are hampered by non-specific interactions with co-existing substances in the biological matrices and the difficulties of obtaining molecular fingerprint information from the complex vibrational spectrum. Raman signal enhancement is necessary, along with convenient surface modification and machine-based learning to address the former issues. This review aims to describe recent advances and prospects in SERS-based approaches for cancer and pathogens diagnosis. First, direct SERS strategies for key biomarker sensing, including the use of substrates such as plasmonic, semiconductor structures, and 3D order nanostructures for signal enhancement will be discussed. Secondly, we will illustrate recent advances for indirect diagnosis using active nanomaterials, Raman reporters, and specific capture elements as SERS tags. Thirdly, critical challenges for translating the potential of the SERS sensing techniques into clinical applications via machine learning and portable instrumentation will be described. The unique nature and integrated sensing capabilities of SERS provide great promise for early cancer diagnosis or fast pathogens detection, reducing sanitary costs but most importantly allowing disease prevention and decreasing mortality rates.

Silver Nanocrystal Array with Precise Control via Star-like Copolymer Nanoreactors

Silver nanocrystal arrays had attracted much attention due to the unique plasmonic effect of their ordered nanostructure and the synergy among adjacent nanocrystals. Conventional preparation methods had several limitations, such as high cost, harsh preparation conditions, and complicated influencing factors, which could not be employed to fabricate the nanocrystal arrays in highly controlled fashion. To solve these issues, we reported ordered arrays of different Ag nanocrystals with precise control prepared by utilizing amphiphilic star-like poly(4-vinylpyridine)-block-polystyrene diblock copolymers as nanoreactors synthesized by sequential atom transfer radical polymerization. Moreover, this unimolecular nanoreactor method based on star-like copolymers with stable and predesigned nanostructures was proved to be a universal approach to prepare other nanocrystal arrays. This strategy had low cost, simple process flow, wide applicability, and structural stability that could fabricate nanocrystal array with precise control and continuously prepare more complex nanostructure units in a large scale to meet different functions and applications.

Large Area Patterning of Highly Reproducible and Sensitive SERS Sensors Based on 10-nm Annular Gap Arrays

Applicable surface-enhanced Raman scattering (SERS) active substrates typically require low-cost patterning methodology, high reproducibility, and a high enhancement factor (EF) over a large area. However, the lack of reproducible, reliable fabrication for large area SERS substrates in a low-cost manner remains a challenge. Here, a patterning method based on nanosphere lithography and adhesion lithography is reported that allows massively parallel fabrication of 10-nm annular gap arrays on large areas. The arrays exhibit excellent reproducibility and high SERS performance, with an EF of up to 10⁷. An effective wearable SERS contact lens for glucose detection is further demonstrated. The technique described here extends the range of SERS-active substrates that can be fabricated over large areas, and holds exciting potential for SERS-based chemical and biomedical detection.

Review of Thin-Layer Chromatography Tandem with Surface-Enhanced Raman Spectroscopy for Detection of Analytes in Mixture Samples

In the real world, analytes usually exist in complex systems, and this makes direct detection by surface-enhanced Raman spectroscopy (SERS) difficult. Thin layer chromatography tandem with SERS (TLC-SERS) has many advantages in analysis such as separation effect, instant speed, simple process, and low cost. Therefore, the TLC-SERS has great potential for detecting analytes in mixtures without sample pretreatment. The review demonstrates TLC-SERS applications in diverse analytical relevant topics such as environmental pollutants, illegal additives, pesticide residues, toxic ingredients, biological molecules, and chemical substances. Important properties such as stationary phase, separation efficiency, and sensitivity are discussed. In addition, future perspectives for improving the efficiency of TLC-SERS in real sample detecting are outlined.

A super-hydrophobic perfluoropolyether coated polytetrafluoroethylene sheets substrate for detection of acetamiprid surface-enhanced Raman spectroscopy

In this paper, a hydrophobic substrate as concentrators including an inner layer of polytetrafluoroethylene (PTFE) and an outer layer covered a thin layer of perfluoropolyether (PFPE) was constructed to achieve a higher sensitivity for acetamiprid (AC) SERS detection. The condensation effect of the PTFE-PFPE hydrophobic substrate-induced aggregation of gold nanoparticles (Au NPs) result ''hot spots'' for SERS. The hydrophobic substrate is better reproducibility (RSD < 5%) compared with that on a conventional silicon wafer. A further application of the hydrophobic substrate was demonstrated by the detection of AC in tea samples within a detection range of 0.03 mg/L to 3 mg/L. The hydrophobic substrate eliminates the problem of solution diffusion to avoid the "coffee ring" effect (When a droplet adheres to a solid surface, the suspended molecular particles usually deposit on the edge of the droplet to form a ring).

Surface-Enhanced Raman Scattering Imaging of Cetylpyridinium Chloride Adsorption to a Solid Surface

Surface active agents (surfactants) have found a variety of critical technological applications, from helping infant lungs breathe to fugitive dust control at industrial sites. Surfactant molecules adsorb to an interface and facilitate a decrease in the surface free energy (interfacial tension) between two immiscible phases. However, a limited number of methods (e.g., holography and fluorescence microscopy) achieved visualization of surfactant molecule distribution in multiphase systems qualitatively. To probe the efficacy and/or adsorption density of surfactants at such interfaces quantitatively, we demonstrate here a direct observation of surfactant adsorption by surface-enhanced Raman scattering (SERS). This work details the development of a research platform to study surfactant adsorption using Raman imaging. The imaging and analysis were successfully benchmarked against conventional interfacial tension measurements and thermodynamic theory employed to estimate surfactant adsorption at equilibrium. This in situ Raman-based experimental method provides a platform to interrogate structure-function relationships that inform the design process for new surfactant species.

Strong and Elastic Membranes via Hydrogen Bonding Directed Self-Assembly of Atomically Precise Nanoclusters

2D nanomaterials have provided an extraordinary palette of mechanical, electrical, optical, and catalytic properties. Ultrathin 2D nanomaterials are classically produced via exfoliation, delamination, deposition, or advanced synthesis methods using a handful of starting materials. Thus, there is a need to explore more generic avenues to expand the feasibility to the next generation 2D materials beyond atomic and molecular-level covalent networks. In this context, self-assembly of atomically precise noble nanoclusters can, in principle, suggest modular approaches for new generation 2D materials, provided that the ligand engineering allows symmetry breaking and directional internanoparticle interactions. Here the self-assembly of silver nanoclusters (NCs) capped with p-mercaptobenzoic acid ligands (Na₄ Ag₄₄ -pMBA₃₀ ) into large-area freestanding membranes by trapping the NCs in a transient solvent layer at air-solvent interfaces is demonstrated. The patchy distribution of ligand bundles facilitates symmetry breaking and preferential intralayer hydrogen bondings resulting in strong and elastic membranes. The membranes with Young's modulus of 14.5 ± 0.2 GPa can readily be transferred to different substrates. The assemblies allow detection of Raman active antibiotic molecules with high reproducibility without any need for substrate pretreatment.

A Liquid Interfacial SERS Platform on a Nanoparticle Array Stabilized by Rigid Probes for the Quantification of Norepinephrine in Rat Brain Microdialysates

For the reliable determination of trace chemicals in the brain, we created a SERS platform based on a functionalized AuNPs array formed at a liquid/liquid interface in a uniform fashion over a large substrate area through ternary regulations for real-time quantification of trace norepinephrine (NE). The rigid molecule, 4-(thiophen-3-ylethynyl)-benzaldehyde (RP1) was designed and co-assembled at AuNPs with 4-mercaptophenylboronic acid (MPBA) to chemically define NE via dual recognition. Meanwhile, the rigid structure assembly of RP1 and MPBA efficiently fixed the interparticle gap, guaranteeing reproducible SERS analysis. Furthermore, the Raman peak of C≡C group in the silent region was taken as a response element to further improve the accuracy. Combined with microdialysis, this SERS platform was developed for in-the-field testing of NE in rat brain microdialysates following anxiety.

Thin film block copolymer self-assembly for nanophotonics

The nanophotonic engineering of light-matter interactions has profoundly changed research behind the design and fabrication of optical materials and devices. Metasurfaces-arrays of subwavelength nanostructures that interact resonantly with electromagnetic radiation-have emerged as an integral nanophotonic platform for a new generation of ultrathin lenses, displays, polarizers and other devices. Their success hinges on advances in lithography and nanofabrication in recent decades. While existing nanolithography techniques are suitable for basic research and prototyping, issues of cost, throughput, scalability, and substrate compatibility may preclude their use for many metasurface applications. Patterning via spontaneous self-assembly of block copolymer thin films offers an enticing alternative for nanophotonic manufacturing that is rapid, inexpensive, and applicable to large areas and diverse substrates. This review discusses the advantages and disadvantages of block copolymer-based nanopatterning and highlights recent progress in their use for broadband antireflection, surface enhanced Raman spectroscopy, and other nanophotonic applications. Recent advances in diversification of self-assembled block copolymer nanopatterns and improved processes for enhanced scalability of self-assembled nanopatterning using block copolymers are also discussed, with a spotlight on directions for future research that would enable a wider array of nanophotonic applications.

Surface-enhanced Raman scattering of DNA bases using frozen silver nanoparticle dispersion as a platform

Raman spectroscopy is a powerful method to characterize molecules in various media. Although surface-enhanced Raman scattering (SERS) is often employed to compensate for the intrinsically poor sensitivity of Raman spectroscopy, there remain serious tasks, such as simple preparations of SERS substrates, sensitivity control, and reproducible measurements. Here, we propose freezing as an efficient way to overcome these problems in SERS measurements using DNA bases as model targets. Solutes are expelled from ice crystals and concentrated in the liquid phase upon freezing. Silver nanoparticles (AgNPs) are also concentrated in the liquid phase to aggregate with Raman target analytes. The SERS signal intensity is maximized when the AgNP concentration exceeds the critical aggregation value. Freezing allows up to 5000 times enhancements of the SERS signal. Thus, an efficient SERS platform is prepared by simple freezing. The simultaneous detection of four DNA bases effectively eliminates variations of signal intensities and allows the reliable determination of concentration ratios.

A new and facile route to prepare gold nanoparticle clusters on anodic aluminium oxide as a SERS substrate

In the present study, a new SERS-active gold nanoparticle clusters having a flower-shape have been prepared easily on nano porous anodic aluminium oxide (AAO) by immersing it in auric chloride solution without any need for complex production steps. In this process, presented for the first time, the metallic aluminum which were released under the influence of chloride ions due to pitting corrosion act as a reducing agent, while gold ions were reduced onto the AAO layer based on the difference in standard reduction potentials between aluminum and gold. Gold nanoparticle clusters on AAO layer formed "hot spots" providing enhanced Raman signal. Optical microscope, SEM, EDX, AFM, and UV-vis spectrophotometer have been used to characterize the substrate. In order to demonstrate applicability of the method, label free SERS measurements of nitrate ion was performed on the proposed sensing platform. A high sensitivity with 1.03 ppm of limit of detection level and the enhancement factor of 2.9 × 10⁵ were obtained for nitrate ion. In addition, remarkable recoveries ranging from 98.4% to 106.8% were obtained for nitrate spiked into drinking water samples. The inter-day and intra-day precisions of the method as relative standard deviation (RSD) were determined as 3.3% and 5.2%, respectively. The sensor platform, developed using a facile method and a low-cost base material (aluminum), can be a good alternative for SERS based sensing applications.

Self-Assembled Ag Nanocomposites into Ultra-Sensitive and Reproducible Large-Area SERS-Active Opaque Substrates

This work describes a novel, one-shot strategy to fabricate ultrasensitive SERS sensors based on silver/poly(methyl methacrylate) (PMMA) nanocomposites. Upon spin coating of a dispersion of PMMA and silver precursor on N-doped silicon substrate, closely separated silver nanoparticles were self-assembled into uniform nanospheres. As a result, a thin hydrophobic PMMA layer embedded with Ag nanoparticles (AgNPs) was obtained on the whole silicon substrate. Consequently, a large-scale, reproducible SERS platform was produced through a rapid, simple, low-cost, and high-throughput technology. In addition, reproducible SERS features and high SERS enhancement factors were determined (SEF ~10¹⁵). This finding matches the highest SEF reported in literature to date (10¹⁴) for silver aggregates. The potential and novelty of this synthesis is that no reducing agent or copolymer was used, nor was any preliminary functionalization of the surface carried out. In addition, the AgNPs were fabricated directly on the substrate’s surface; consequently, there was no need for polymer etching. Then, the synthetic method was successfully applied to prepare opaque SERS platforms. Opaque surfaces are needed in photonic devices because of the absence of secondary back reflection, which makes optical analysis and applications easier.

Graphoepitaxy of Symmetric Six-Arm Star-Shaped Poly(methyl methacrylate)-block-Polystyrene Copolymer Thin Film

The authors perform directed self-assembly based on graphoepitaxy of symmetric six-arm star-shaped poly(methyl methacrylate)-block-polystyrene copolymer [(PMMA-b-PS)₆ ] thin film. The affinity between each block and the trench wall is adjusted by using polymer brushes or selective gold (Au) deposition. When the surface of the trench is strongly selective for the PMMA block, (n+0.75)L₀ thick (n is the number of the lamellae, L₀ is lamellar domain spacing) lamellae parallel to the trench wall are formed at each side, while nanotubes are formed away from the trench wall. However, for a trench grafted with PS brushes, nanotubes are formed beside (n+0.25)L₀ thick lamellar layers. By adjusting the trench width (W) and the affinity between the block and the wall, various dual nanopatterns consisting of lines and nanotubes are fabricated. Moreover, when the trench wall is selectively deposited by Au, asymmetric dual nanopattern is formed, where different numbers of lines exist on each side wall, while nanotubes are formed in the middle of the trench. The observed morphologies depending on the commensurability condition between W and L₀ are consistent with predictions by self-consistent field theory.

Nanostructure-Based Surface-Enhanced Raman Spectroscopy Techniques for Pesticide and Veterinary Drug Residues Screening

Pesticide and veterinary drug residues in food and environment pose a threat to human health, and a rapid, super-sensitive, accurate and cost-effective analysis technique is therefore highly required to overcome the disadvantages of conventional techniques based on mass spectrometry. Recently, the surface-enhanced Raman spectroscopy (SERS) technique emerges as a potential promising analytical tool for rapid, sensitive and selective detections of environmental pollutants, mostly owing to its possible simplified sample pretreatment, gigantic detectable signal amplification and quick target analyte identification via finger-printing SERS spectra. So theoretically the SERS detection technology has inherent advantages over other competitors especially in complex environmental matrices. The progress in nanostructure SERS substrates and portable Raman appliances will promote this novel detection technology to play an important role in future rapid on-site assay. This paper reviews the advances in nanostructure-based SERS substrates, sensors and relevant portable integrated systems for environmental analysis, highlights the potential applications in the detections of synthetic chemicals such as pesticide and veterinary drug residues, and also discusses the challenges of SERS detection technique for actual environmental monitoring in the future.

An Inverted Honeycomb Plasmonic Lattice as an Efficient Refractive Index Sensor

We present an efficient refractive index sensor consisting of a heterostructure that contains an Au inverted honeycomb lattice as a main sensing element. Our design aims at maximizing the out-of-plane near-field distributions of the collective modes of the lattice mapping the sensor surroundings. These modes are further enhanced by a patterned SiO₂ layer with the same inverted honeycomb lattice, an SiO₂ spacer, and an Au mirror underneath the Au sensing layer that contribute to achieving a high performance. The optical response of the heterostructure was studied by numerical simulation. The results corresponding to one of the collective modes showed high sensitivity values ranging from 99 to 395 nm/RIU for relatively thin layers of test materials within 50 and 200 nm. In addition, the figure of merit of the sensor detecting slight changes of the refractive index of a water medium at a fixed wavelength was as high as 199 RIU^-1. As an experimental proof of concept, the heterostructure was manufactured by a simple method based on electron beam lithography and the measured optical response reproduces the simulations. This work paves the way for improving both the sensitivity of plasmonic sensors and the signal of some enhanced surface spectroscopies.

Spectroscopy-Assisted Label-free Molecular Analysis of Live Cell Surface with Vertically Aligned Plasmonic Nanopillars

A generalized label-free platform for surface-selective molecular sensing in living cells can transform the ability to examine complex events in the cell membrane. While vertically aligned semiconductor and metal-semiconductor hybrid nanopillars have rapidly surfaced for stimulating and probing the intracellular environment, the potential of such constructs for selectively interrogating the cell membrane is surprisingly underappreciated. In this work, a new platform, entitled nano-PROD (nano-pillar based Raman optical detection), enables molecular recording by probing fundamental vibrational modes of membrane constituents of cells adherent on a large-area silver-coated silicon nanopillar substrate fabricated using a precursor solution-based nanomanufacturing process. It is shown that the nano-PROD platform sustains live cells in near-physiological conditions, which can be directly profiled using surface-enhanced Raman spectroscopy due to the confined electromagnetic field enhancement. The experimental results highlight the utility of the platform in probing specific cell surface markers for accurately recognizing the phenotypically identical prostate cancer cells, differing only in prostate-specific membrane antigen expression. Due to its tunability, nano-PROD has the promise to be readily extendable to other applications that can leverage its unique combination of nanoscale topographic features and molecular sensing capabilities, from stain-free cytopathology inspection to understanding spatio-mechanical regulation in membrane receptor function.

Metal-Immobilized Micellar Aggregates of a Block Copolymer from a Mixed Solvent for a SERS-Active Sensing Substrate and Versatile Dip Catalysis

Here, we have reported micellar aggregations of an amphiphilic block copolymer in mixed solvent and their subsequent use as a template for the fabrication of a very dense, tunable metal nanoparticle-decorated surface for SERS and flexible dip catalysis applications. A silver nanoparticle-immobilized layer on silicon substrates shows excellent SERS (surface-enhanced Raman scattering)-based sensing performance for model analyte rhodamine B up to 10^-6 M concentration with a well-defined calibration curve. Furthermore, a facile approach to the preparation of metal NP-immobilized BCP membranes as efficient dip catalyst for two model reactions (the reduction of nitrophenol and the Suzuki-Miyaura reaction of iodobenzene or 2,7-diiodofluorene with phenyl boronic acid) is also demonstrated. The Ag NP-decorated film exhibits high efficiency and extensive reusability in a prototype reaction such as the reduction of nitrophenol by sodium borohydride with a very high turnover number, >126 (for a single use), whereas the Pd NP-immobilized film also has a high, ∼100%, reaction yield and extensive reusability and applicable for different aromatic systems. This work provides a new platform for the design and synthesis of a functionalizable, flexible, and highly mechanically stable dip catalyst which is highly demanded in the catalytic production of value-added chemicals and environmental applications such as wastewater treatment.

Porous Hybrids Structure between Silver Nanoparticle and Layered Double Hydroxide for Surface-Enhanced Raman Spectroscopy

Silver nanoparticle (AgNP), in terms of antibacterial, catalytic, electronic, and optical applications, is an attractive material. Especially, when prepared to furnish sharp edge and systematic particle orientation on the substrate, AgNPs can take advantage of surface-enhanced Raman spectroscopy (SERS). In this research, we suggested a synthetic method to immobilize the AgNP on metal oxide by utilizing Ag-thiolate and layered double hydroxide (LDH) as precursor and template, respectively. The layer-by-layer structure of LDH and Ag-thiolate transformed through reductive calcination to metal oxide and AgNP array. Physicochemical characterization, including powder X-ray diffraction, N₂ adsorption–desorption, microscopies, and X-ray photoelectron spectroscopy, revealed that the AgNP with sufficient crystallinity and particle gap was obtained at relatively high calcination temperature, ~600 °C. UV-vis diffusion reflectance spectroscopy showed that the calcination temperature affected particle size and electronic structure of AgNP. The prepared materials were subjected to SERS tests toward 4-nitrothiophenol (4-NTP). The sample obtained at 600 °C exhibited 50 times higher substrate enhancement factor (SEF) than the one obtained at 400 °C, suggesting that the calcination temperature was a determining parameter to enhance SERS activity in current synthetic condition.

Spectral engineering of ultrathin germanium solar cells for combined photovoltaic and photosynthesis

In densely populated areas, ground mounted photovoltaic power plants compete with agriculture for cultivable land. Agrivoltaic systems allow the combination of these two forms of land use by deliberately designed light sharing. In this contribution, we present a spectrally selective solar cell, for use in agrivoltaic systems, greenhouses, and photo-bioreactors. Our concept benefits from a solar cell with a transmission spectrum which can be easily tuned for the specific absorption requirements of algae and plants. This is achieved by a Fabry-Perot-type multilayer resonator as a back reflector, which determines the transmission and absorption spectrum of the solar cell. We demonstrate the extent of how this transmission spectrum can be engineered by varying the layer thicknesses of the reflector and we show how the reflecting metal layers in the back reflector influence the transmission and photocurrent generation of the spectrally selective solar cell. Finally, we analyze the optical loss mechanisms of the solar cell layer stack to address further optimization potential. Our work offers a spectrally selective solar cell which can be easily adjusted for the requirements of combining photovoltaic and photosynthesis.

Solvent Quality-Mediated Regioselective Modification of Gold Nanorods with Thiol-Terminated Polymers

Modification of nanorods (NRs) with functional polymer ligands is of great significance to enhance their surface chemistry and prompt their applications in many fields (e.g., photothermal therapy, bioimaging, and catalysis). However, the regioselective modification of AuNRs still remains a great challenge. Herein, we introduce a facile yet versatile strategy to achieve the regioselective modification of AuNRs through a solvent quality-mediated strategy. By employing a poor solvent of the original ligand cetyltrimethylammonium bromide (CTAB) as the medium in the modification, polymer ligands would selectively graft onto the two ends of AuNRs, while polymer ligands would graft onto the entire surface when employing a good solvent. This strategy demonstrates good reproducibility and is applicable to both hydrophilic and hydrophobic polymer ligand modifications. Moreover, by combing our strategy with the preoccupation route, the two ends and sidewall of AuNRs modified by two different polymers form an "ABA"-type building block, which can further self-assemble into well-ordered superstructures. Our finding provides a new opportunity for multifunctionalization of NRs.

Ag-Nanoparticles@Bacterial Nanocellulose as a 3D Flexible and Robust Surface-Enhanced Raman Scattering Substrate

We present a well-designed, low-cost, and simple synthetic approach to realizing the hybrid composites of Ag nanoparticle-decorated bacterial nanocellulose (denoted as Ag-NPs@BNC) as a three-dimensional (3D) flexible surface-enhanced Raman scattering (SERS) substrate with ultrahigh SERS sensitivity, excellent signal reproducibility, and stability. The homogeneous Ag-NPs with high density were in situ grown on the networked BNC fibers by the controlled silver mirror reaction and volume shrinkage treatment, which created uniformly distributed SERS "hot spots" in the 3D networked hybrid substrate. Attributed to these unique 3D hot spots, the as-presented Ag-NPs@BNC substrates exhibited ultrahigh sensitivity and good spectral reproducibility. Moreover, the hydrophilic BNC exhibits good permeability and adsorption performances, which could capture the target molecules in the highly active hot spot areas to further improve the SERS sensitivity. As a result, not only dye molecules (rhodamine 6G) but also toxic organic pollutants such as 2-naphthalenethiol and thiram have been detected using the hybrid substrates as SERS substrates, with sensitivities of 1.6 × 10^-8 and 3.8 × 10^-9 M, respectively. The good linear response of the intensity and the logarithmic concentration revealed promising applications in the rapid and quantitative detection of toxic organic pollutants. Besides, this self-supported Ag-NPs@BNC substrate demonstrated good stability and flexibility for varied detection conditions. Therefore, the 3D networked, flexible, ultrasensitive, and stable Ag-NPs@BNC substrate shows potential as a versatile SERS substrate in the rapid identification of various organic molecules.

Large area Al2O3-Au raspberry-like nanoclusters from iterative block-copolymer self-assembly

In the field of functional nanomaterials, core–satellite nanoclusters have recently elicited great interest due to their unique optoelectronic properties. However, core–satellite synthetic routes to date are hampered by delicate and multistep reaction conditions and no practical method has been reported for the ordering of these structures onto a surface monolayer. Herein we show a reproducible and simplified thin film process to fabricate bimetallic raspberry nanoclusters using block copolymer (BCP) lithography. The fabricated inorganic raspberry nanoclusters consisted of a ∼36 nm alumina core decorated with ∼15 nm Au satellites after infusing multilayer BCP nanopatterns. A series of cylindrical BCPs with different molecular weights allowed us to dial in specific nanodot periodicities (from 30 to 80 nm). Highly ordered BCP nanopatterns were then selectively infiltrated with alumina and Au species to develop multi-level bimetallic raspberry features. Microscopy and X-ray reflectivity analysis were used at each fabrication step to gain further mechanistic insights and understand the infiltration process. Furthermore, grazing-incidence small-angle X-ray scattering studies of infiltrated films confirmed the excellent order and vertical orientation over wafer scale areas of Al₂O₃/Au raspberry nanoclusters. We believe our work demonstrates a robust strategy towards designing hybrid nanoclusters since BCP blocks can be infiltrated with various low cost salt-based precursors. The highly controlled nanocluster strategy disclosed here could have wide ranging uses, in particular for metasurface and optical based sensor applications.

Large area Al₂O₃–Au raspberry-like nanoclusters and other complex structures have been created by iterative block-copolymer self-assembly, paving the way to a new generation of on-demand metallic architectures.

Droplet array for open-channel high-throughput SERS biosensing

Open-channel and high throughput are two important aspects of clinical diagnosis, correlation biochemical analysis, cell culture techniques and food safety. Here, we propose the mini-pillar based array for open-channel and high-throughput SERS detection of miRNA. The polydimethylsiloxane (PDMS) mini-pillars are used as a high-throughput platform, which have good anchoring and aggregation effects on microdroplets, greatly reducing the amount of analytical solution and facilitate the homogeneous sample distribution after evaporation. The deposited gold nanorods (Au NRs) on the pillars with optimized diameter served as SERS-active substrate, can greatly improve the sensitivity of SERS signal compared to other planar substrates. On the open-channel biological chip, sensitive, simultaneous, and specific detection of breast cancer marker miRNA-1246 can be performed. In this mini-pillar array SERS system, the limit of detection (LOD) is 10^-12 M. The mini-pillar array shows enormous potential for open channel, high-throughput biomolecular detection, providing an opportunity for biomedical point-of-care testing (POCT) and drug screening.

Highly Deformable Transparent Au Film Electrodes and Their Uses in Deformable Displays

With emerging interest in foldable and stretchable displays, the need to develop transparent deformable electrode and interconnection is increasing. Even though metal films have been standard electrodes in conventional electronic devices due to their high conductivity and well-established process, they have never been used for transparent deformable electrodes. We present highly conductive transparent deformable Au film electrodes and use them to fabricate a foldable perovskite light-emitting diode (PeLED) and a biaxially stretchable alternating current electroluminescence (ACEL) display. We exhibit the formation of an ultrathin (6 nm) continuous Au film on an anisotropic conductive ultrathin film (ACUF) of amorphous carbon. The ultrathin Au film was first formed on an ACUF-coated Si wafer (4 in. scale) through metal evaporation and transferred to the polymer substrates by a simple and effective water-assisted delamination process. Then, a hybrid electrode (ACUF/ACUF/Au) was produced as the transparent deformable electrode. Complicated interconnections could be created by metal deposition through a mask. The electrical conductance of the hybrid electrode was not affected by the crack formation in the Au film during electrode folding, crumpling, and stretching. We reveal the reason why the hybrid electrode can maintain such excellent electrical stability under deformation.

High-sensitivity SERS based sensing on the labeling side of glass slides using low branched gold nanoparticles prepared with surfactant-free synthesis

Surface-enhanced Raman scattering (SERS) has become a more attractive tool for biological and chemical sensing due to having a great detection potential to extremely low concentrations of analyte. Here, we report high-sensitivity SERS detection of low branched gold nanoparticles which are produced by a surfactant-free synthesis method. The effects of the size and branches of nanoparticles on the SERS signal intensity were also investigated. Among the prepared nanoparticles, a new type of nanoparticle with small protrusions produced by using a very low concentration of silver ions (2 μM in final solution) achieved the best enhancement factor of ∼4 × 10⁵ for DTNB used as a probe molecule. SERS measurements were performed on the labeling side of microscope glass slides for the first time. The substrate exhibited a good reproducible SERS signal with a relative standard deviation (RSD) of 1.7%. SERS signal intensity obtained using the labelling side was three times larger compared to that obtained using bare glass. To validate the sensing platform, dopamine, an important modulatory neurotransmitter in the brain, was tested. The reported platform was able to achieve label-free detection of dopamine at picomolar and nanomolar concentration level in aqueous and fetal bovine serum (FBS) solution at pH 8.5 respectively. Due to its surfactant-free preparation and enhanced SERS-based sensing features, our reported platform represents a strong alternative to be used in SERS-based sensing applications.

High-sensitivity dopamine detection on aggregated low branched nanoparticles on labelling side of glass slide as a SERS based sensor.

Facile Synthesis of Ultrastable Fluorescent Copper Nanoclusters and Their Cellular Imaging Application

Copper nanoclusters (Cu NCs) are generally formed by several to dozens of atoms. Because of wide range of raw materials and cheap prices, Cu NCs have attracted scientists’ special attention. However, Cu NCs tend to undergo oxidation easily. Thus, there is a dire need to develop a synthetic protocol for preparing fluorescent Cu NCs with high QY and better stability. Herein, we report a one-step method for preparing stable blue-green fluorescent copper nanoclusters using glutathione (GSH) as both a reducing agent and a stabilizing agent. High-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS) and electrospray ionization mass spectrometer (ESI-MS) were used to characterize the resulting Cu NCs. The as-prepared Cu NCs@GSH possess an ultrasmall size (2.3 ± 0.4 nm), blue-green fluorescence with decent quantum yield (6.2%) and good stability. MTT results clearly suggest that the Cu NCs@GSH are biocompatible. After incubated with EB-labeled HEK293T cells, the Cu NCs mainly accumulated in nuclei of the cells, suggesting that the as-prepared Cu NCs could potentially be used as the fluorescent probe for applications in cellular imaging.

Novel SERS-based process analysis for label-free segmented flow screenings

In microfluidic segmented flow processes label-free analytical techniques like surface enhanced Raman spectroscopy (SERS) can reveal the chemical composition of the individual droplet contents. The SERS system developed in this work enables a simple connection to micro segmented flow processes through miniaturization. The concept is based on the parallelization of silver/polyacrylamide composite SERS spots on a carrier plate on which the segments are deposited. The transfer of the segments allows an easy connection to existing flow processes and provides optimal conditions for Raman measurements using miniaturized spectrometers. The preparation of the SERS polymer composite was optimized in terms of the silver content in the polymer matrix to obtain a high SERS signal. The performance and long-term stability of the polymer have been successfully demonstrated. The deamination of adenine with sodium nitrite to hypoxanthine was chosen as a case study to demonstrate the capability of the novel SERS-based process analysis. A sequence of approximately one hundred segments in combination with a gradient of the nitrite concentration (0 to 0.4 mol L-1) was generated at two temperatures. The concentration of adenine and hypoxanthine were determined by using a multivariate calibration model, since the Raman spectra of both substances are overlapping. It was shown that the conversion of adenine is increased with higher nitrite concentration and temperature. A conversion of 35% was obtained at 50 °C and a conversion of 60% at 80 °C, respectively.

In situ monitoring of silver adsorption on assembled gold nanorods by surface-enhanced Raman scattering

Self-assembly of metal nanocrystals is able to create a gap of sub-nanometer distance for concentrating incoming light by the strong coupling of surface plasmon resonance, known as a 'hot spot'. Although the plasmonic property of silver is better than other metals in the visible range, the superior Raman enhancement of silver compared to gold is still under debate. To provide direct evidence, in this work, we studied the silver adsorption on assembled gold nanorods (AuNRs) using in situ surface-enhanced Raman scattering (SERS) measurements. The self-assembled AuNR multimers were used as the SERS substrate, where the 4-mercaptophenol (MPh) molecules in our experiment played dual roles as both probe molecules for the Raman scattering and linking molecules for the AuNR assembly in a basic environment. Silver atoms were adsorbed on the surface of gold nanorod assemblies by reduction of Ag⁺ anions. The stability of the adsorbed silver was guaranteed by the basic environment. We monitored the SERS signal during the silver adsorption with a home-built in situ Raman spectroscope, which was synchronized by recording the UV-vis absorption spectra of the reaction solution to instantly quantify the plasmonic effect of the silver adsorption. Although a minor change was found in the plasmonic resonance wavelength or intensity, the measured SERS signal at specific modes faced a sudden increase by 2.1 folds during the silver adsorption. The finite element method (FEM) simulation confirmed that the silver adsorption corresponding to the plasmonic resonance variation gave little change to the electric field enhancement. We attributed the mode-specific enhancement mechanism of the adsorption of silver to the chemical enhancement from charge transfer (CT) for targeting molecules with a specific orientation. Our findings provided new insights to construct SERS substrates with higher enhancement factor (EF), which hopefully would encourage new applications in the field of surface-enhanced optical spectroscopies.

Reproducible and Bendable SERS Substrates with Tailored Wettability Using Block Copolymers and Anodic Aluminum Oxide Templates

Surface properties are essential for substrates exhibiting high sensitivity in surface-enhanced Raman scattering (SERS) applications. In this work, novel SERS hybrid substrates using polystyrene-block-poly(methyl methacrylate) and anodic aluminum oxide templates is presented. The hybrid substrates not only possess hierarchical porous nanostructures but also exhibit superhydrophilic surface properties with the water contact angle ≈0°. Such surfaces play an important role in providing uniform enhanced intensities over large areas (relative standard deviation ≈10%); moreover, these substrates are found to be highly sensitive (limit of detection ≈10^-12 m for rhodamine 6G (R6G)). The results show that the hybrid SERS substrates can achieve the simultaneous detection of multicomponent mixtures of different target molecules, such as R6G, crystal violet, and methylene blue. Furthermore, the bending experiments show that about 70% of the SERS intensities are maintained after bending from ≈30° to 150°.

Three-Dimensional Nanoporous Metal Structures from Poly(2-vinylpyridine)-block-Poly(4-vinylpyridine) Copolymer Thin Film

We fabricated 3D nanoporous metal structures from poly(2-vinylpyridine)-block-poly(4-vinylpyridine) copolymer (P24VP) thin film with vertically oriented lamellar nanodomains by coordinating corresponding metal precursors followed by reduction to metals. Although metal precursors are coordinated with both P2VP and P4VP blocks, the metal coordination power toward P4VP block is much greater than that toward P2VP block. Thus, most of the metal precursors are located in the P4VP block, while a few exist in the P2VP block. After the metal precursors were reduced to corresponding metals by reactive ion etching, metals located in P4VP regions became continuous main frames. However, metals in P2VP regions could not be continuous because of smaller amounts, resulting in nanoporous structures. Using these 3D nanoporous structures, we measured the electrocatalytic activity for hydrogen evolution reaction. 3D nanoporous platinum (Pt) showed enhanced catalytic activity compared with Pt flat film due to the large surface area. Moreover, 3D nanoporous Pt/cobalt bimetallic structures showed better catalytic activity than 3D nanoporous Pt structures.

Molecular-Imprinting-Based Surface-Enhanced Raman Scattering Sensors

Molecularly imprinted polymers (MIPs) receive extensive interest, owing to their structure predictability, recognition specificity, and application universality as well as robustness, simplicity, and inexpensiveness. Surface-enhanced Raman scattering (SERS) is regarded as an ideal optical detection candidate for its unique features of fingerprint recognition, nondestructive property, high sensitivity, and rapidity. Accordingly, MIP based SERS (MIP-SERS) sensors have attracted significant research interest for versatile applications especially in the field of chemo- and bioanalysis, showing excellent identification and detection performances. Herein, we comprehensively review the recent advances in MIP-SERS sensors construction and applications, including sensing principles and signal enhancement mechanisms, focusing on novel construction strategies and representative applications. First, the basic structure of the MIP-SERS sensors is briefly outlined. Second, novel imprinting strategies are highlighted, mainly including multifunctional monomer imprinting, dummy template imprinting, living/controlled radical polymerization, and stimuli-responsive imprinting. Third, typical application of MIP-SERS sensors in chemo/bioanalysis is summarized from both small and macromolecular aspects. Lastly, the challenges and perspectives of the MIP-SERS sensors are proposed, orienting sensitivity improvement and application expanding.

Retracted Article: Synergistic action of star-shaped Au/Ag nanoparticles decorated on AgFeO2 for ultrasensitive SERS detection of a chemical warfare agent on real samples

It is a great challenge to design and fabricate a cost-effective surface-enhanced Raman spectroscopy (SERS) substrate with excellent reproducibility and sensitivity for reliable environmental analysis. In this work, we have synthesized silver ferrite (AgFeO₂) interlinked with star-shaped gold/silver (Au/Ag) bimetallic (BM) nanoparticles (NPs) by a simple physical method for the effective detection of an acetylcholinesterase (AchE) inhibitor, paraoxon ethyl (PE). The successful construction of AgFeO₂@Au/Ag NPs was confirmed by UV-Vis spectroscopy, X-ray diffraction, field emission scanning electron microscopy, energy dispersive X-ray spectroscopy and Raman spectroscopy. The enhancement of the SERS signal is achieved by the synergistic effect of the charge transfer mechanism and electromagnetic mechanism. The Raman peak centered at 1357 cm^-1 was selected as an ideal peak for the quantitative analysis of PE. The AgFeO₂@Au/Ag NPs can detect PE down to 1 × 10^-8 M with a high analytical enhancement factor of 3.53 × 10⁶ and excellent uniformity, as determined randomly from 14 spots (relative standard deviation, RSD, <15%). The recovery values of PE in tap water and tomato juice were from 93.16% to 99.16%. All these results suggest that our proposed SERS substrate has promising potential for the detection of PE. The proposed simple strategy for PE detection by SERS using AgFeO₂@Au/Ag NPs paves the way for future reliable environmental analysis and real sample monitoring.

Self-assembly synthesis of Ag@PANI nanocomposites as a tandem enzyme utilizing a highly efficient label-free SERS method to detect saccharides

Surface Enhanced Raman Scattering (SERS), with good reproducibility and stability, is an analytical testing technique that uses metals or metal nanoparticles as detection substrates, and it can also be used to characterize the information from molecular vibration.

Magnetic Assembly Route to Construct Reproducible and Recyclable SERS Substrate

The fabrication of a uniform array film through assembly of colloidal building blocks is of practical interest for the integrated individual and collective functions. Here, a magnetic assembly route was put forward to organize monodisperse noble metal microspheres into a uniform array film for surface-enhanced Raman scattering (SERS) application, which demonstrated the integrated signal sensitivity of single noble metal microspheres and reproducibility of their assembled uniform array film. For this purpose, monodisperse multifunctional Fe3O4@SiO2@TiO2@Ag (FOSTA) colloidal microspheres as building blocks were successfully synthesized through a homemade ultrasonic-assisted reaction system. When used in SERS test, these multifunctional microspheres could firstly bind the analyte (R6G) from solution and then assembled into a uniform film under an external magnetic field, which exhibited high SERS detection sensitivity with good reproducibility. In addition, due to the TiO2 interlayer in FOSTA colloidal microspheres, the building blocks could be recycled and self cleaned through photocatalytic degradation of the adsorbed analyte for recycling SERS application.