Periodic Porous Alloyed Au-Ag Nanosphere Arrays and Their Highly Sensitive SERS Performance with Good Reproducibility and High Density of Hotspots

Swiss roll nanoarrays for chiral plasmonic photocatalysis

Manipulating the chirality at nanoscale has drawn great attention among scientists, considering its pivotal role in various applications of current interest, including nano-optics, biomedicine, and photocatalysis. In this work, we delve into this arena by fabricating chiral Swiss roll nanoarray (SRNA) continuous films employing colloidal lithography. The technique permits the dimension of Swiss roll metamaterials to reduce to nanoscale, thus achieving chiroptical response (circular dichroism (CD)) in the visible region. The interplay between the CD signals and plasmon resonance modes is revealed through theoretical simulations, enabling a deep understanding of chiral plasmonic metamaterials. The polarization-sensitive photocatalytic activity of chiral SRNAs is investigated, noting a marked increase in the reaction rate when the chirality of SRNAs matches with the handedness of circularly polarized light (CPL). Notably, the SRNA continuous films based on substrate possess integration and reusability without complex recycling process, enhancing their practicality in applications like sensing and plasmonic nanochemistry, particularly toward polarization-dependent photocatalysis.

Simple and Rapid Monolayer Self-Assembly of Nanoparticles at the Air/Water Interface

Colloidal crystals and their two-dimensional (2D) monolayers, which have been commonly applied in nanosphere lithography, have the potential to revolutionize many engineering disciplines; however, current production techniques are hampered by a restricted preparation area, laborious procedures, and the need for advanced equipment. We propose a self-assembly-driven, simple, and low-cost method to prepare 2D colloidal nanosphere monolayers with excellent repeatability across wide regions. The self-assembly capability of colloidal polystyrene (PS) nanospheres at the air/water interface was utilized to transfer the assembled monolayers onto a substrate. This innovative method combines the advantages of methods that permit deposition at the air/water interface, such as Langmuir and drop coating, in order to deliver defect-free, simple-to-install, and simple-to-apply deposition across vast regions. Using field emission scanning electron microscopy and atomic force microscopy, the resultant coatings were characterized. The size of the nanospheres was reduced using an oxygen plasma etch process in an inductively coupled plasma reactive ion etching system, and the reflectance properties of the substrates for various nanosphere sizes were investigated. By evaporation of a thin gold capping layer on the templates, their optical properties were compared using surface-enhanced Raman scattering spectroscopy. This work has the potential to expand the use of nanosphere lithography by offering a simple and reproducible method that eliminates the need for complicated experimental setups and reduces the amount of material required for monolayer coating, thus lowering the cost.

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

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

Planting gold nanoflower for harvesting reproducible SERS substrate

Surface-enhanced Raman scattering (SERS) is an ultrasensitive analytical method which has been applied in many fields, and the reproducibility of the substrate is important for reliable SERS analysis. In present work, an innovative method inspired by the flower planting process is put forward to acquire gold nanoflower (AuNF) SERS substrate. Three steps (digging holes, sowing the gold nanoseeds and seeds grow into gold nanoflowers) are included in the substrate fabrication process, and the influence of preparing conditions (like reacting time and Na3Au(SO3)2 concentration) on the substrate morphology and SERS performance are investigated. The acquired AuNF substrate not only exhibits good SERS performance but also possesses excellent reproducibility while being used to detect the rhodamine 6G (R6G) molecular. The relative standard deviation (RSD) of Raman signals among substrates acquired in distinct batches (substrate-to-substrate) is as low as 6.67 %. Since the AuNF substrate is prepared by the wet chemistry route based on seed-mediated growth and there are no expensive reagents or complicated process used, the new process to obtain AuNF substrate is cost-effective and easy to scale up.

Lab-on-Fiber Sensors with Ag/Au Nanocap Arrays Based on the Two Deposits of Polystyrene Nanospheres

Surface-enhanced Raman spectroscopy (SERS) can boost the pristine Raman signal significantly which could be exploited for producing innovative sensing devices with advanced properties. However, the inherent complexity of SERS systems restricts their further applications in rapid detection, especially in situ detection in narrow areas. Here, we construct an efficient and flexible SERS-based Lab-on-Fiber (LOF) sensor by integrating Ag/Au nanocap arrays obtained by Ag/Au coating polystyrene nanospheres on the optical fiber face. We obtain rich "hot spots" at the nanogaps between neighboring nanocaps, and further achieve SERS performance with the assistance of laser-induced thermophoresis on the metal film that can achieve efficiency aggregation of detected molecules. We achieve a high Raman enhancement with a low detection limitation of 10-7 mol/L for the most efficient samples based on the above sensor. This sensor also exhibits good repeatability and stability under multiple detections, revealing the potential application for in situ detection based on the reflexivity of the optical fiber.

Flexible 3D Substrate of Ag Nanoparticle-Loaded Carbon Aerogels with Outstanding Surface-Enhanced Raman Scattering Performance

Surface-enhanced Raman scattering (SERS), an ultra-sensitive and non-destructive analytic technique, has attracted wide attention from the scientific community. Despite its rapid development, limited hotspots on the SERS substrates have restricted their potential in practical applications. Herein, we developed a facile method to fabricate a flexible three-dimensional (3D) SERS substrate composed of silver nanoparticles (Ag NPs)-loaded carbon aerogels (CAs). Such a flexible Ag NPs/CAs substrate exhibited numerous hotspots, which can facilely be adjusted not only by tuning the density of Ag NPs but also by controlling the bending degree of the flexible substrate. In addition, the influence of hotspots on the local electric field enhancement was investigated by theoretical calculations. Moreover, the 3D network structure of the CAs with a large specific surface area and strong adsorption ability can improve the capture of target molecules. Consequently, the optimal Ag NPs/CAs substrate has a low detection limit of 10^-12 M for rhodamine 6G molecules as well as good repeatability. Furthermore, based on the good performance of SERS detection of the Ag NPs/CAs substrate, it can also be practically used for the detection of thiram molecules on the surface of cherry tomatoes. Such a flexible 3D Ag NPs/CAs substrate has great potential for practical environmental monitoring applications.

A hybrid SERS sensing platform constructed by porous carbon/Ag nanoparticles for efficient imatinib detection in bio-environment

Surface enhanced Raman scattering (SERS) is a rapid and non-destructive spectral detection technique, and has been widely implemented on trace-level molecule detection. In this work, a hybrid SERS substrate constructed by porous carbon film and silver nanoparticles (PCs/Ag NPs) was developed and then used for imatinib (IMT) detection in bio-environment. The PCs/Ag NPs was prepared by direct carbonizing the gelatin-AgNO₃ film in the air atmosphere, and an enhancement factor (EF) of 10⁶ was achieved with R6G as the Raman reporter. Hereafter, this SERS substrate was used as the label-free sensing platform to detect the IMT in the serum, and the experimental results indicate that the substrate is conducive to eliminating the interference from the complex biological molecules in the serum, and the characteristic Raman peaks belonging to IMT (10^-4 M) are accurately resolved. Furthermore, the SERS substrate was used to trace the IMT in the whole blood, the trace of ultra-low concertation of IMT is rapidly discovered without any pretreatment. Thus, this work finally suggests that the proposed sensing platform provides a rapid and reliable method for IMT detection in the bio-environment and offers a potential for its application in therapeutic drug monitoring.

An anisotropic nanobox based core-shell-satellite nanoassembly of multiple SERS enhancement with heterogeneous interface for stroke marker determination

Herein, A novel gold-silver alloy nanobox (AuAgNB)@SiO₂-gold nanosphere (AuNP) nanoassembly based on core-shell-satellite structure is fabricated and applied to the surface-enhanced Raman scattering (SERS) detection of S100 calcium-binding protein B protein (S100B). It contains an anisotropic hollow porous AuAgNB core with rough surface, an ultrathin silica interlayer labeled with reporter molecules, and AuNP satellites. The nanoassemblies were systematically optimized by tuning the reporter molecules concentration, silica layer thickness, AuAgNB size, and the size and number of AuNP satellite size. Remarkably, AuNP satellites are adjacent to AuAgNB@SiO₂, developing AuAg-SiO₂-Au heterogeneous interface. With the strong plasmon coupling between AuAgNB and AuNP satellites, chemical enhancement from heterogeneous interface, and the tip "hot spots" of AuAgNB, the SERS activity of the nanoassemblies was multiply enhanced. Additionally, the stability of nanostructure and Raman signal was significantly improved by the silica interlayer and AuNP satellites. Eventually, the nanoassemblies were applied for S100B detection. It demonstrated satisfactory sensitivity and reproducibility with a wide detection range of 10 fg/mL-10 ng/mL and a limit of detection (LOD) of 1.7 fg/mL. This work based on the AuAgNB@SiO₂-AuNP nanoassemblies with multiple SERS enhancements and favorable stability demonstrates the promising application in stroke diagnosis.

Application of sensitive SERS plasmonic biosensor for high detection of metabolic disorders

Due to the importance of early detection of metabolic diseases in newborns, it is essential to measure organoacids; L-Tryptophan, Sebacic acid, and Glutaric acid in very low concentrations. Therefore, the necessity of the construction of a powerful nondestructive biosensor just like the surface-enhanced Raman scattering (SERS) sensor is demonstrated. Through the growth of silver dendritic nanostructures on different substrates like aluminum (Al), copper (Cu), indium tin oxide (ITO), and silicon (Si), a new SERS-based biosensor was developed. Because the Raman signal of molecules adsorbed on dendritic nanostructures is significantly increased, SERS biosensors based on these nanostructures can be used to detect very low concentrations of materials. In this study, first, the organoacid L-Lysine was detected up to a concentration of 10^-12 M, by using a biosensor based on Al, Cu, ITO, and Si substrates. Then, by comparing the results obtained from different substrates, the silicon substrate as the most successful substrate with the best results was used in the SERS biosensor to detect the organoacids, L-Tryptophan, Sebacic acid, and Glutaric acid up to a concentration of 10^-12 M. SEM imaging was used to characterize silver dendritic nanostructures on solid substrates. The successful performance of the SERS biosensor based on silver dendrites in this study promises to be effective in diagnostic applications such as cancer diagnosis (the limit of single molecular detection).

Controllable Preparation of Gold Nanocrystals with Different Porous Structures for SERS Sensing

Porous Au nanocrystals (Au NCs) have been widely used in catalysis, sensing, and biomedicine due to their excellent localized surface plasma resonance effect and a large number of active sites exposed by three-dimensional internal channels. Here, we developed a ligand-induced one-step method for the controllable preparation of mesoporous, microporous, and hierarchical porous Au NCs with internal 3D connecting channels. At 25 °C, using glutathione (GTH) as both a ligand and reducing agent combined with the Au precursor to form GTH-Au(I), and under the action of the reducing agent ascorbic acid, the Au precursor is reduced in situ to form a dandelion-like microporous structure assembled by Au rods. When cetyltrimethylammonium bromide (C₁₆TAB) and GTH are used as ligands, mesoporous Au NCs formed. When increasing the reaction temperature to 80 °C, hierarchical porous Au NCs with both microporous and mesoporous structures will be synthesized. We systematically explored the effect of reaction parameters on porous Au NCs and proposed possible reaction mechanisms. Furthermore, we compared the SERS-enhancing effect of Au NCs with three different pore structures. With hierarchical porous Au NCs as the SERS base, the detection limit for rhodamine 6G (R6G) reached 10^-10 M.

Effect of Au Nanoparticle Agglomeration on SERS Signal Amplification

An analyzed substance's signal intensity and detection sensitivity in surface-enhanced Raman spectroscopy (SERS) significantly depend on the size and agglomeration degree of nanoparticles (NPs) forming the enhancing structure. Structures were manufactured by aerosol dry printing (ADP), where NPs' agglomeration depends on printing conditions and additional particle modification techniques. The influence of agglomeration degree on SERS signal enhancement was studied in three types of printed structures using the methylene blue model molecule as an analyte. We demonstrated that the ratio between individual NPs and agglomerates in a studied structure strongly affects SERS signal amplification, and structures formed mainly from non-agglomerated NPs enhance the signal better. In this sense, aerosol NPs modified by pulsed laser radiation provide better results than thermally modified NPs, since in laser modification a larger number of individual NPs is observed due to the absence of secondary agglomeration effects in the gas stream. However, increasing gas flow may minimize the secondary agglomeration, since the time allotted for the agglomeration processes is reduced. In this paper, we show how different NPs' agglomeration tendencies influence SERS enhancement to demonstrate the process of using ADP to form inexpensive and highly efficient SERS substrates with huge application potential.

Nanowire-in-bowl-shaped piezoelectric cavity structure for SERS directional detection of nanoplastics less than 50 nm

The accurate detection of nanoplastics is crucial due to their harmful effects on the environment and human beings. However, there is a lack of detection methods for nanoplastics smaller than 50 nm. In this research, we successfully constructed an Ag/CuO nanowire (NW)/BaTiO₃@Polyvinylidene fluoride (PVDF) Bowl-shaped substrate with a nanowire-in-Bowl-shaped piezoelectric cavity structure that can modulate surface-enhanced Raman scattering (SERS) by the piezoelectric effect by the virtue of the tip effect of the CuO NW and light focusing effect of the Bowl-shaped cavity. Due to its unique nanowire-in-Bowl-shaped structure and piezoelectrically modifiable ability, nanoplastics less than 50 nm were successfully detected and quantitatively analyzed. We believe that the Ag/CuO NW/BaTiO₃@PVDF Bowl-shaped substrate can provide an efficient, accurate, and feasible way to achieve qualitative and quantitative detection of nanoplastics.

Fabrication of hierarchical β-Bi2O3/AuAg microspheres for sensitive, selective and rapid detection of environment pollutants by surface-enhanced Raman spectroscopy

In this paper, we report a novel surface-enhanced Raman spectroscopy (SERS) substrate based on hierarchical β-Bi2O3/Au2Ag2 microspheres for rapid, sensitive and selective detection of environment pollutants including o-dianisidine (o-diASD) and Hg2+ in environmental samples. The sheet-like β-Bi2O3 not only provides large specific surface areas for adsorption of molecules and AuAg, but also emerges as semiconductor matrix with chemical enhancement combined with AuAg with electromagnetic enhancement, making promising SERS activity. Particularly, the β-Bi2O3/Au2Ag2 shows high SERS performance for 4-mercaptobenzoic acid and TMB with minimum detectable concentration of 0.1 μg/L with enhancement factor of 3.1 × 107 and 6.3 × 107, respectively. The density functional theory simulations were further adopted to explain the high SERS activity and selectivity for o-diASD and TMB. Finally, the β-Bi2O3/Au2Ag2 was applied to direct detection of o-diASD, and indirect detection of Hg2+ by TMB marking in environmental samples. The linearity range of 0.5-200.0 and 0.2-500.0 μg/L with limit of detection of 0.2 and 0.07 μg/L for o-diASD and Hg2+ ions can be achieved, respectively. This method provides a novel strategy in designing and fabricating SERS substrates with high performance for rapid, sensitive and accurate analysis of environmental pollutants.

Combining physical vapor deposition structuration with dealloying for the creation of a highly efficient SERS platform

Nanostructured noble metal thin films are highly studied for their interesting plasmonic properties. The latter can be effectively used for the detection of small and highly diluted molecules by the surface-enhanced Raman scattering (SERS) effect. Regardless of impressive detection limits achieved, synthesis complexity and the high cost of gold restrict its use in devices. Here, we report on a novel two-step approach that combines the deposition of a silver-aluminum thin film with dealloying to design and fabricate efficient SERS platforms. The magnetron sputtering technique was used for the deposition of the alloy thin film to be dealloyed. After dealloying, the resulting silver nanoporous structures revealed two degrees of porosity: macroporosity, associated to the initial alloy morphology, and nanoporosity, related to the dealloying step. The resulting nanoporous columnar structure was finely optimized by tuning deposition (i.e., the alloy chemical composition) and dealloying (i.e., dealloying media) parameters to reach the best SERS properties. These are reported for samples dealloyed in HCl and with 30 atom % of silver at the initial state with a detection limit down to 10^-10 mol·L^-1 for a solution of rhodamine B.

Hollow and nanoporous Ag sub-microcubes as SERS substrates

In this work, Ag(NH₃)₂NO₃ microcubes are obtained by directly evaporating the silver ammonia complex solution. In Na₃Cit solution, each Ag(NH₃)₂NO₃ microcube can crumble into lots of Ag₃Cit sub-microcubes with uniform size, which serve as sacrificial templates to yield Ag sub-microcages. The as-obtained Ag cages possess ultra-high surface-enhanced Raman scattering (SERS) sensitivity towards R6G and thiram.

Evaluation of SERS activity for cosputtered Ag-ZnX@PS (X = O, S, Se) composites: Carrier density dependence

Ag-ZnX (X = O, S, Se) composites coated on polystyrene (PS) arrays (Ag-ZnO@PS, Ag-ZnS@PS, Ag-ZnSe@PS) were successfully fabricated by using cosputtering technology. We found that ZnX doping decreased the carrier densities of these composites compared to that of pure Ag@PS, which was due to redistribution of electrons between Ag and ZnX. Thus, the carrier density of Ag was decreased, and the surface plasmon resonance (SPR) of Ag was redshifted in the Ag-ZnX composites. As the redshift of the SPR of Ag induced a high SPR contribution to the surface-enhanced Raman scattering (SERS), the SPR and charge transfer (CT) contributions were simultaneously increased with increasing carrier density in the Ag-ZnX composites. This study opens a new path to designing metal-semiconductor composites with controllable carrier density. Regulation of the carrier density will be of great help in understanding SPR and CT contributions.

Surface Filtration in Mesoporous Au Films Decorated by Ag Nanoparticles for Solving SERS Sensing Small Molecules in Living Cells

For surface-enhanced Raman spectroscopy (SERS) sensing of small molecules in the presence of living cells, biofouling and blocking of plasmonic centers are key challenges. Here, we have developed a mesoporous Au (AuM) film coated with a Ag nanoparticles (NPs) as a plasmonic sensor (AuM@Ag) to analyze aromatic thiols, which is an example of a small molecule, in the presence of a living cell strain (e.g., MDA-MB-231) as a model living system. The resulting AuM@Ag provides 0.1 nM sensitivity and high reproducibility for thiols sensing. Simultaneously, the AuM@Ag film filters large biomolecules, preventing Raman signals from overlapping produced by large biomolecules. After analysis, the AuM@Ag film undergoes recycling by the full dissolution of the Ag-thiol layer and removal of thiols from AuM. Furthermore, fresh AgNPs are formed for further SERS analysis, which circumvents the Ag oxidation issue. The ease of the AgNPs deposition allows up to 12 cycles of on-demand recycling and sensing even after utilization as a sensor in multicomponent media without enhancement and sensitivity loss. The reported mesoporous film with surface filtering ability and prominent recycling procedure promises to offer a new strategy for the detection of various small molecules in the presence of living cells.

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

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

High-Throughput Surface-Enhanced Raman Scattering for Screening Chemical Sensor Candidates Enabled by Bipolar Electrochemistry

A variety of hydrothermal or electrochemical methods have been explored to prepare noble metal nanostructures as surface-enhanced Raman scattering (SERS) substrates. However, most of those metallic nanoarrays are structurally homogeneous, which makes it laborious to select the high-performance substrates for particular Raman sensing purposes. Here, a high-throughput SERS imaging strategy is demonstrated for the first time for screening chemical sensors with sub-nanomolar sensitivities. Bipolar electrochemistry was applied to generate Au or Au-Ag gradient nanoarrays with diverse chemical compositions, morphologies, and particle dimensions ranging from several nanometers to micrometers. The selected "hot-spots" on the Au-Ag alloy nanoarray exhibited a 660-fold enhancement in SERS intensity compared to those on the pure Au gradient nanoarray. The SERS screening of 4-aminothiophenol, 4-nitrothiophenol, and 4-mercaptobenzoic acid was carried out that provided a limit of detection (LOD) between 1 and 5 pM. The distinctive LODs among three thiophenolic Raman probes are ascribed to the differences in the affinity of the probe to the alloy, orientation of the metal-ligand monolayer, or plasmonic environment of the nanoarray surface. As a continuous, rapid, and cost-effective manner to fabricate transitional nanostructures and screen out SERS responsive sites, this method not only facilitates controllable synthesis of noble metal nanoarrays but has the potential to provide an alternative tool for ultrasensitive chemical sensing on a wide range of bimetallic substrates.

Mesoporous Gold Nanostructures: Synthesis and Beyond

Mesoporous metal nanostructures have offered multiple advantages that cannot be realized elsewhere. These materials have been attracting more research attention in catalysis and electrocatalysis owing to their functional structures and compositions. Of the various mesoporous metals available, mesoporous gold (mesoAu) nanostructures are of special interest in surface-enhanced Raman scattering (SERS) and related applications because of their strong electromagnetic field (localized surface plasmon resonance). In the last few decades, various synthesis strategies have been developed to prepare mesoAu nanostructures with controllable morphologies that exhibit fascinating physicochemical properties and increase applications in SERS, catalysis, and electrocatalysis. In this Perspective, we systematically summarize recent advances in synthesis and applications of mesoAu nanostructures. Four synthesis strategies, including dealloying, nanocasting, electrochemical deposition, and intermediate template, are discussed in detail. Moreover, physicochemical properties and promising applications of mesoAu nanostructures are presented. Finally, we describe current challenges and give a general outlook to explore further directions in synthesis and applications of mesoAu nanostructures.

Iridescent Colloidal Crystals Composed of SiO2 Porous Hollow Sphere for SERS Application

In recent years, the application of low-refractive-index materials in the optical field has attracted considerable attention due to it high transmittance and high optical sensitivity. In this study, we synthesized SiO₂ porous hollow spheres (SPHS) with an ultralow refractive index (n = 1.05) by using a templating method. Their refractive indices could be easily controlled from 1.05 to 1.08 by tuning the thickness of shell. In addition, a droplet coatings method is proposed for SPHS colloidal crystal (CC) by controlling the temperature and humidity. The SPHS CCs displayed distinct structural colors when the incident angle was adjusted and demonstrated high angular resolution. Moreover, the iridescent color changes could be observed with the naked eye. For surface-enhanced Raman scattering application, more analyte could be absorbed by the porous shells, and metal nanoparticles were coated on the SPHSs surface to increase the hot spot density for improving the SERS intensity.

Multifunctional biosensor constructed by Ag-coating magnetic-assisted unique urchin core porous shell structure for dual SERS enhancement, enrichment, and quantitative detection of multi-components inflammatory markers

The simultaneous, precise, and quantitative detection of multi-components inflammatory markers (IMs) in sepsis serum by surface-enhanced Raman scattering (SERS) remains a challenging problem. A novel, multifunctional biosensor with dual enrichment and enhancement was designed for the ultrasensitive and quantitative analysis of multi-components IMs. The biosensor contains SERS tags-unique urchin core/porous shell (CPS) structure modified with Raman reporters (RaRs), magnetic assist-Ag coated Fe₃O₄ magnetic nanoparticles (Ag MNPs) modified with internal standard (IS), and then aptamer (Apt) modification to form the sandwich structure (Ag MNPs/IMs/CPS). This multifunctional sensor used for IM_S detection has the following innovations: The intensity ratio I_RaRs/I_IS with Lg C_IMs present a good and wide linear relationship to achieve the simultaneous, precise, and quantitative detection of IM_S in serum; The detection results display ultrasensitivity, and the limit of detection (LOD) for CRP, IL-6, and PCT is 100 fg/mL, 0.1 fg/mL, and 1.0 fg/mL, which is lower than other detection techniques; The calculated data of clinical blood samples of sepsis by this SERS method is consistent with the hospital results, and can provide more compositional data of IMs. Thus, this combined approach developed a sensing platform for rapid screening, accurate evaluation, early warning, and diagnosis of sepsis.

Mapping the effects of physical and chemical reduction parameters on local atomic distributions within bimetallic nanoparticles

Bimetallic nanoparticles prove advantageous over their monometallic counterparts due to the tunable, hybrid properties that result from combining different atomic species in a controlled way. The favorable optical and catalytic properties resulting from AgAu nanoparticle formation have been widely attributed to the existence of Ag-Au bonds, the maximization of which assumes the formation of a homogeneous alloy. Despite the importance of atomic scale structure in these systems, synthetic studies are typically not paired with structural characterization at the atomic scale. Herein, a comprehensive synthetic exploration of physical and chemical reduction parameters of resulting nanoparticle products is complemented with thorough X-ray characterization to probe how these parameters affect atomic scale alloy distributions within AgAu nanoparticles. Presented evidence shows Ag is substantially underincorporated into nanoparticle constructs compared with solution Ag : Au ratios regardless of precursor : reductant ratio or volume of reductant added. Both Ag and Au exhibit significant local clustering, with Ag distributed preferentially towards the nanoparticle surface. Most significantly, the results of this investigation suggest that reduction parameters alone can affect the local alloy distributions and homogeneity within bimetallic nanoparticles, even when the ratio of metallic precursors remains constant. Overall, this investigation presents the ability to control alloy distributions using kinetics and provides new considerations for optimizing synthetic methods to produce functional bimetallic nanoparticles.

Long Spiky Au-Ag Nanostar Based Fiber Probe for Surface Enhanced Raman Spectroscopy

The detection performances of noble metal-based surface enhanced Raman spectroscopy (SERS) devices are determined by the compositions and geometries of the metal nanostructures, as well as the substrates. In the current study, long spiky Au-Ag alloy nanostars were synthesized, and both core diameters and spike lengths were controlled by Lauryl sulfobetaine concentrations (as the nanostructure growth skeleton). The long spiky star geometries were confirmed by transmission electron micrograph images. Elements energy dispersive spectrometer mapping confirmed that Au and Ag elements were inhomogeneously distributed in the nanostructures and demonstrated a higher Ag content at surface for potential better SERS performance. Selected synthesized spiky nanostars were uniformly assembled on multi-mode silica fiber for probe fabrication by silanization. The SERS performance were characterized using crystal violet (CV) and rhodamine 6G (R6G) as analyte molecules. The lowest detection limit could reach as low as 10^-8 M, with a 6.23 × 10⁶ enhancement factor, and the relationship between analyte concentrations and Raman intensities was linear for both CV and R6G, which indicated the potential qualitative and quantitative molecule detection applications. Moreover, the fiber probes also showed good reproducibility and stability in the ambient atmosphere.

A novel dealloying strategy for fabricating nanoporous Ag via ζ'-AgGa alloy

A mild strategy for fabricating nanoporous silver (np-Ag) pieces was reported via preparation of Ag-Ga alloys in relatively low temperature and subsequent electrochemical dealloying in nitric acid (HNO₃) aqueous solution. After selectively etching Ga out of the Ag-Ga alloy, a typical three-dimensional (3D) bicontinuous nanoporous structure with a pore size of ∼67.21-159.33 nm was observed. A series of studies have shown that the addition of sodium dodecyl sulfate (SDS) results in minimum pore size. The coarsen exponent (n) is 1.61, and the activation energy was calculated to be 27.04 kJ mol^-1. Theζ'-AgGa alloy prepared at low temperature can be used as the precursor for the preparation of fine np-Ags, and this method provides a new strategy for the industrial production of dealloyed np-Ag.

Defect engineering in semiconductor-based SERS

Semiconductor-based surface enhanced Raman spectroscopy (SERS) platforms take advantage of the multifaceted tunability of semiconductor materials to realize specialized sensing demands in a wide range of applications. However, until quite recently, semiconductor-based SERS materials have generally exhibited low activity compared to conventional noble metal substrates, with enhancement factors (EF) typically reaching 103, confining the study of semiconductor-based SERS to purely academic settings. In recent years, defect engineering has been proposed to effectively improve the SERS activity of semiconductor materials. Defective semiconductors can now achieve noble-metal-comparable SERS enhancement and exceedingly low, nano-molar detection concentrations towards certain molecules. The reason for such success is that defect engineering effectively harnesses the complex enhancement mechanisms behind the SERS phenomenon by purposefully tailoring many physicochemical parameters of semiconductors. In this perspective, we introduce the main defect engineering approaches used in SERS-activation, and discuss in depth the electromagnetic and chemical enhancement mechanisms (EM and CM, respectively) that are influenced by these defect engineering methods. We also introduce the applications that have been reported for defective semiconductor-based SERS platforms. With this perspective we aim to meet the imperative demand for a summary on the recent developments of SERS material design based on defect engineering of semiconductors, and highlight the attractive research and application prospects for semiconductor-based SERS.

Porous Au-Ag Nanoparticles from Galvanic Replacement Applied as Single-Particle SERS Probe for Quantitative Monitoring

Plasmonic nanostructures have raised the interest of biomedical applications of surface-enhanced Raman scattering (SERS). To improve the enhancement and produce sensitive SERS probes, porous Au-Ag alloy nanoparticles (NPs) are synthesized by dealloying Au-Ag alloy NP-precursors with Au or Ag core in aqueous colloidal environment through galvanic replacement reaction. The novel designed core-shell Au-Ag alloy NP-precursors facilitate controllable synthesis of porous nanostructure, and dealloying degree during the reaction has significant effect on structural and spectral properties of dealloyed porous NPs. Narrow-dispersed dealloyed NPs are obtained using NPs of Au/Ag ratio from 10/90 to 40/60 with Au and Ag core to produce solid core@porous shell and porous nanoshells, having rough surface, hollowness, and porosity around 30-60%. The clean nanostructure from colloidal synthesis exhibits a redshifted plasmon peak up to near-infrared region, and the large accessible surface induces highly localized surface plasmon resonance and generates robust SERS activity. Thus, the porous NPs produce intensely enhanced Raman signal up to 68-fold higher than 100 nm AuNP enhancement at single-particle level, and the estimated Raman enhancement around 7800, showing the potential for highly sensitive SERS probes. The single-particle SERS probes are effectively demonstrated in quantitative monitoring of anticancer drug Doxorubicin release.

Flexible Plasmonic Biosensors for Healthcare Monitoring: Progress and Prospects

The noble metal nanoparticle has been widely utilized as a plasmonic unit to enhance biosensors, by leveraging its electric and/or optical properties. Integrated with the "flexible" feature, it further enables opportunities in developing healthcare products in a conformal and adaptive fashion, such as wrist pulse tracers, body temperature trackers, blood glucose monitors, etc. In this work, we present a holistic review of the recent advance of flexible plasmonic biosensors for the healthcare sector. The technical spectrum broadly covers the design and selection of a flexible substrate, the process to integrate flexible and plasmonic units, the exploration of different types of flexible plasmonic biosensors to monitor human temperature, blood glucose, ions, gas, and motion indicators, as well as their applications for surface-enhanced Raman scattering (SERS) and colorimetric detections. Their fundamental working principles and structural innovations are scoped and summarized. The challenges and prospects are articulated regarding the critical importance for continued progress of flexible plasmonic biosensors to improve living quality.

A digital SERS sensing platform using 3D nanolaminate plasmonic crystals coupled with Au nanoparticles for accurate quantitative detection of dopamine

We report a digital surface-enhanced Raman spectroscopy (SERS) sensing platform using the arrays of 3D nanolaminate plasmonic crystals (NLPC) coupled with Au nanoparticles and digital (on/off) SERS signal analysis for the accurate quantitative detection of dopamine (DA) at ultralow concentrations. 3D NLPC SERS substrates were fabricated to support the optically dense arrays of vertically-stacked multi-nanogap hotspots and combined with Raman tag-conjugated Au nanoparticles for NLPC-based dual-recognition structures. We demonstrate that the 3D NLPC-based dual-recognition structures including Au nanoparticle-induced additional hotspots can enable more effective SERS enhancement through the molecular recognition of DA. For the accurate quantification of DA at ultralow concentrations, we conducted digital SERS analysis to reduce stochastic signal variation due to various microscopic effects, including molecular orientation/position variation and the spatial distribution of nanoparticle-coupled hotspots. The digital SERS analysis allowed the SERS mapping results from the DA-specific dual-recognition structures to be converted into binary "On/Off" states; the number of "On" events was directly correlated with low-abundance DA molecules down to 1 pM. Therefore, the digital SERS platform using the 3D NLPC-based dual-recognition structures coupled with Au nanoparticles and digital SERS signal analysis can be used not only for the ultrasensitive, accurate, and quantitative determination of DA, but also for the practical and rapid analysis of various molecules on nanostructured surfaces.

A biosensing method for the direct serological detection of liver diseases by integrating a SERS-based sensor and a CNN classifier

Direct serological detection, due to its clinical facility and testing economy, affords prominent clinical values to the early detection of cancer. Surface-enhanced Raman spectroscopy (SERS)-based sensors have shown great promise in realizing this form of detection. Detecting liver cancer early with such a form, especially in terms of monitoring the pathogenic progression from hepatic inflammations to cancer, is the most effective clinical path to reducing the mortality rate. However, the methodology investigation for this purpose remains a formidable challenge. We fabricated a SERS-based sensor, consisting of Au-Ag nanocomplex-decorated ZnO nanopillars on paper. The sensor has an analytic enhancement factor of 1.02 × 107, which is enough to sense the biomolecular information of liver diseases through direct serum SERS analysis. A convolutional neural network (CNN) classifier for recognizing serum SERS spectra was constructed by deep learning. Integrating this sensor with the CNN, we established an intelligent biosensing method and realized direct serological detection of liver diseases within 1 min. As a proof-of-concept, the method achieved a prediction accuracy of 97.78% on an independent test dataset randomly sampled from 30 normal controls, 30 hepatocellular carcinoma (HCC) cases, and 30 hepatitis B (HB) patients. The results suggest this method can be developed for detecting liver diseases clinically and is worthy of exploration as a means of liver cancer surveillance. The presented sensor holds potential for clinical translation to the direct serological detection of diseases.

Femtosecond laser patterned superhydrophobic/hydrophobic SERS sensors for rapid positioning ultratrace detection

Ultratrace molecular detections are vital for precancer diagnosis, forensic analysis, and food safety. Superhydrophobic (SH) surface-enhanced Raman scattering (SERS) sensors are regarded as an ideal approach to improve detection performance by concentrating analyte molecules within a small volume. However, due to the low adhesion of SH surfaces, the analyte droplet is prone to rolling, making it hard to deposit molecules on a predetermined position. Furthermore, the sediment with a very small area on the SH-SERS surface is difficult to be captured even with a Raman microscope. In this study, femtosecond laser fabricated hybrid SH/hydrophobic (SH/HB) surfaces are successfully applied to realize a rapid and highly sensitive SERS detection. By modulating dual surface structures and wetting behaviors, the analyte molecules can be enriched at the edge of HB pattern. This improves the convenience and speed of Raman test. On a hybrid SH/HB SERS substrate with a circular HB pattern at 300-µm-diameter, a femtomolar level (10^-14 M) of rhodamine 6G can be detected by using analyte volumes of just 5 µL. The SERS enhancement factor can reach 5.7×10⁸ and a good uniformity with a relative standard deviation of 6.98% is achieved. Our results indicate that the laser fabrication of hybrid SERS sensor offers an efficient and cost-effective approach for ultratrace molecular detection.

Surface plasmon resonance induced charge transfer effect on the Ag-ZnSe-PATP system

This work demonstrated the effect of charge transfer (CT) induced by metal surface plasmon resonance (SPR) on surface-enhanced Raman scattering (SERS). We designed an Ag-ZnSe nanostructure and introduced p-aminothiophenol (PATP) molecules to form an Ag-ZnSe-PATP system. The proposed method compensates for the CT difficulty in wide-band-gap semiconductors, which was initiated by the SPR of Ag. The Raman intensity is enhanced differently depending on the action of excitation light of different wavelengths. The concept of the CT degree was introduced to analyze this intriguing phenomenon. The system constructed in this work combines the electromagnetic enhancement mechanism and the chemical enhancement mechanism, which helps further understand the SERS mechanism and provides important references for SERS research on wide-band-gap semiconductors.

A facile wet-chemistry approach to engineer an Au-based SERS substrate and enhance sensitivity down to ppb-level detection

A two-dimensional flexible surface-enhanced Raman scattering (SERS) filter substrate provides an alternative strategy for the highly sensitive portable detection of various toxic molecules and biomaterials. Herein, we developed a solid-liquid interfacial reduction reaction to post-engineer a solid Au nanostructure surface on filter paper to improve the SERS effect. Among four reductants (ascorbic acid, l-dopamine, hydroquinone (HQ), and formaldehyde), HQ possessed a larger oxidation overpotential and facilitated homogeneous growth, forming small Au branch-structure nanoparticles from HAuCl4 solution. Due to the surface effect by exposing abundant -OH groups and intrinsic aromatic rings from TNA/HQ on nano-gold, the SERS effect on positively charged analytes near the plasmonic Au surface was enhanced, while forming a protective layer against severe water interruption. The resulting SERS substrate with branched nano-gold provided several SERS-enhanced sites, increased the enhancement by more than 6 times compared to original SERS sensing, and displayed a 1.4-7.4 × 105 analytical enhancement factor, which leads to a limit of detection down to several ppb. Less than 6% of deviation in the SERS intensity at different sensing sites was observed. We successfully improved the primary SERS substrate using a high overpotential reductant. Owing to its soft and flexible properties, the paper-based SERS substrate can be used conveniently in different sizes, pasting on curved materials, detecting additives in fish, and preventing the coffee-ring effect, showing high practicality and potential commercial value in the future.

Nanostructured Black Aluminum Prepared by Laser Direct Writing as a High-Performance Plasmonic Absorber for Photothermal/Electric Conversion

Utilizing the abundant and renewable solar energy to address the global energy shortage and water scarcity is promising. Great effort has been devoted to photothermal conversion for its typically full-spectrum utilization and high efficiency. Here, the coral-like micro/nanostructure was fabricated on an aluminum sheet by a facile laser direct writing technology. The nanocluster and microscale branches of corals endowed this black aluminum with broad-band plasmonic absorption and rapid heat transfer from the light absorption region to substrate. The black aluminum achieved ultrahigh solar absorbance of over 92.6% (>95.1% in the visible range) and excellent light heating ability (>90.6 °C under 1.0 sun). With good photothermal properties, this plasmonic absorber was used in a state-of-the-art eight-layer membrane distillation system, producing a water yield of up to 2.40 kg m^-2 h^-1 and a high solar conversion efficiency of 166.5% under 1-sun irradiation. Photothermal electricity was also achieved based on this system with a thermoelectric generator, with a water yield of 0.89 kg m^-2 h^-1 and a maximum electrical power output of 7.21 μW cm^-2 under 1.0 sun. Considering the excellent performance of the plasmon-enhanced black aluminum, this work provides an alternative and feasible route toward high-efficient utilization of the solar energy.

Fabrication of ZnO Nanocap-Ordered Arrays with Controllable Amount of Au Nanoparticles Decorated and Their Detection and Degradation Performance for Harmful Molecules

This paper mainly presents a facile and cost-effective method to achieve large-scale ZnO nanocap (ZnO NC)-ordered arrays with a controllable amount of Au nanoparticles (Au NPs) decorated on their surface. The preparation process includes the construction of polystyrene nanosphere (PS) mask, metal deposition, and annealing process. The Au NPs/ZnO NCs have apparent hierarchical structure. Interestingly, the size and number of Au NPs can be controlled by changing the time of Au deposition and the diameter of PSs. Moreover, the Au NP/ZnO NC arrays can be used as a substrate to detect harmful dye molecules based on surface-enhanced Raman scattering (SERS) effect, and show ultrahigh sensitivity with a limit of detection (LoD) of 10^-10 M for crystal violet (CV) molecules. In addition, the above substrate has achieved reusable detection due to their excellent photocatalytic degradation performance for harmful molecules. The finite difference time-domain (FDTD) simulation results have revealed that SERS "hot spots" are almost distributed at the junctions of Au NPs and ZnO NCs. The above results show that the composite substrates have a good prospect in practical applications in the future.

Flexible Surface-Enhanced Raman Scattering Chip: A Universal Platform for Real-Time Interfacial Molecular Analysis with Femtomolar Sensitivity

We propose and demonstrate a flexible surface-enhanced Raman scattering (SERS) chip as a versatile platform for femtomolar detection and real-time interfacial molecule analysis. The flexible SERS chip is composed of a flexible and transparent membrane and embedded plasmonic dimers with ultrahigh particle density and ultrasmall dimer gap. The chip enables rapid identification for residuals on solid substrates with irregular surfaces or dissolved analytes in aqueous solution. The sensitivity for liquid-state measurement is down to 0.06 molecule per dimers for 10^-14 mol·L^-1 Rhodamine 6G molecule without molecule enrichment. Strong signal fluctuation and blinking are observed at this concentration, indicating that the detection limit is close to the single-molecule level. Meanwhile, the homogeneous liquid environment facilities accurate SERS quantification of analytes with a wide dynamic range. The synergy of flexibility and liquid-state measurement opens up avenues for the real-time study of chemical reactions. The reduction from p-nitrothiophenol (PNTP) to p-aminothiophenol (PATP) in the absence of the chemical reducing agents is observed at liquid interfaces by in situ SERS measurements, and the plasmon-induced hot electron is demonstrated to drive the catalytic reaction. We believe this robust and feasible approach is promising in extending the SERS technique as a general method for identifying interfacial molecular traces, tracking the evolution of heterogeneous reactions, elucidating the reaction mechanisms, and evaluating the environmental effects such as pH value and salty ions in SERS.

Large-Scale Flexible Surface-Enhanced Raman Scattering (SERS) Sensors with High Stability and Signal Homogeneity

Flexible surface-enhanced Raman scattering (SERS) sensors have attracted great attention as a portable and low-cost device for chemical and bio-detection. However, flexible SERS sensors tend to suffer low signal spatial homogeneity due to the uneven distribution of active plasmonic nanostructures (hot spots) and quick degradation of their sensitivity due to low adhesion of hot spots and flexible substrates during fast sampling. Herein, a large-area (20 × 20 cm²) polyimide (PI)-based SERS sensor is exploited for trace detection with high signal homogeneity and stability. The SERS sensor is constructed from PI through in situ growth of silver and gold core-shell nanoparticles (Ag@Au NPs) based on chemical reduction and galvanic replacement processes. Benefiting from the abundant carboxyl groups on the surface-cleaved PI, densely and uniformly distributed Ag@Au NPs are successfully prepared on the film under ambient conditions. The high Raman enhancement factor (EF) (up to 1.07 × 10⁷) and detection capability with low nanomolar (10^-9 M) detection limits are obtained for this flexible SERS sensor. The uniform Raman signals in the random region show good signal homogeneity with a low variation of 8.7%. Moreover, the flexible SERS sensor exhibited superior efficiency and durability after storage for 30 days even after 500 cycles of mechanical stimuli (bending or torsion). The residue of pesticide thiram (tetramethylthiuram disulfide, TMTD) has been rapidly traced by direct sampling from the apple surface, and a sensitivity of 10 ng/cm² for TMTD was achieved. These findings show that the PI-based SERS sensor is a very strong candidate for broad and simple utilization of flexible SERS for both laboratory and commercial applications in chemical and biomolecule detections.

Fabrication and Applications of 3D Nanoarchitectures for Advanced Electrocatalysts and Sensors

For the last few decades, nanoscale materials and structures have been extensively studied and developed, making a huge impact on human sustainability. For example, the introduction of nanostructures has brought substantial development in electrocatalysts and optical sensing applications. However, there are still remaining challenges that need to be resolved to further improve their performance, reliability, and cost-effectiveness. Herein, long-range ordered 3D nanostructures and their design principles are introduced with an emphasis on electrocatalysts for energy conversion and plasmonic nanostructures for optical sensing. Among the various fabrication techniques, sequential solvent-injection-assisted nanotransfer printing is suggested as a practical fabrication platform for tunable long-range ordered 3D nanostructures composed of ultrahigh-resolution building blocks. Furthermore, the importance of understanding and controlling the 3D design parameters is discussed to realize more efficient energy conversion as well as effective surface-enhanced Raman spectroscopy analyses, suggesting new solutions for clean energy and healthcare issues.

Efficient and tunable shape selective synthesis of Ag/CeO2 nanostructures modified highly stable SERS substrate for ultrasensitive detection of pesticides on the surface of an apple

Detection of pesticide residues from fruits and vegetables is of significant importance to ensuring human health and environmental safety. An efficient and tunable shape-selective synthesis of Ag/CeO₂ nanostructures as an active flexible SERS substrate for the detection of thiram on an apple surface via a paste, peel off, and paste again process was performed. The well-controlled formation of silver assembled CeO₂ microspheres constituting nanospheres and nanospindles with an average size of approximately 56 and 32 nm with anisotropic structures has been confirmed through morphological and crystallographic analysis. Interestingly, CeO₂ (111) was strongly anchored in the Ag (111) matrix, which provides a more adequate pathway for rapid ion-electron transportation, as observed from the structural and chemical composition analysis. The detection of thiram on the surface of an apple using our proposed nanospindle SERS active substrate achieves a wide detection range from 10^-2 to 10^-9 M with a correlation coefficient of 0.9929 and a low detection limit of 27 nM at S/N = 3. In addition, the charge transfer mechanism between the Ag/CeO₂ nanostructures and thiram molecules has also been proposed. We believe that the present work could provide novel ways to develop SERS active substrates for highly efficient onsite detection of pesticides on fruits in the near future.

Synthesis of bimetallic silver-gold nanoparticle composites using a cellulose dope: Tunable nanostructure and its biological activity

Introducing functional metal nanoparticles (NPs) into flexible substrate is being increasingly attempted to expand their application. Here, we extend the synthesis of cellulose to its unmodified dope achieving freestanding nanocomposite decorated with bimetallic Ag-Au NPs through the one pot reaction. In the procedure, cellulose chain not only acts as a reducing agent but also a biocompatible support for NPs with a mean size of 7.9-9.7 nm. Meanwhile, changing the addition order of Ag⁺ and AuCl₄^- generated different atom arrangement in the bimetallic NPs. Moreover, the correlation of bioactivity to NP atom arrangement was studied. The result revealed that the nanocomposite containing NPs with an ultrathin Ag-rich outermost shell around an Au-rich core showed better bactericidal ability while lower cytotoxicity. In addition, the nanocomposite exhibited a sensitive SERS property for determination of R6G with a high enhancement factor of 10⁸.

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.

Controlled Fabrication of Flower-Shaped Au-Cu Nanostructures Using a Deep Eutectic Solvent and Their Performance in Surface-Enhanced Raman Scattering-Based Molecular Sensing

Controlled synthesis of anisotropic bimetallic nanostructures with tunable morphology is of great current interest for their applications in surface-enhanced Raman scattering (SERS), plasmonics, and catalysis. Despite huge effort that has been devoted so far, fabrication of bimetallic nanostructures with controlled morphology and size remained to be a great challenge, especially when their shapes are anisotropic. Here, we report a facile, one-step synthetic approach for the fabrication of anisotropic bimetallic gold-copper nanostructures (Au-Cu NSs) of the 200-300 nm size range, using choline chloride/urea (ChCl/urea)-based deep eutectic solvent (DES) as the soft template. A concentration of the CuCl₂ precursor in the reaction mixture was found to impact the reduction kinetics of the metal ions, directly affecting the final morphology of the Au-Cu nanostructures and elemental distributions in them. The fabricated anisotropic Au-Cu NSs revealed a high SERS signal for crystal violet (CV) molecules adsorbed at their surfaces, with the signal enhancement factor as high as 0.21 × 10⁶ and capacity of detecting CV molecules of concentrations as low as 10^-10 M in their aqueous solutions. The growth mechanism of the anisotropic bimetallic nanostructures in DES and their SERS performance has been discussed. The simple DES-assisted synthesis strategy presented in this work can be adopted for large-scale nonaqueous fabrication of other bimetallic nanostructures in a quite "greener" way.

A split-type structure of Ag nanoparticles and Al2O3@Ag@Si nanocone arrays: an ingenious strategy for SERS-based detection

Herein, we report a facile strategy of combined SERS measurements based on a split-type SERS substrate, which exhibits excellent SERS activity, detection signal reproducibility and chemical stability. The SERS substrate consists of an ordered Al2O3@Ag@Si nanocone array and Ag nanoparticles (Ag-NPs), both of which are fabricated individually. The Al2O3@Ag@Si nanocone array is obtained by ion-sputtering Ag on an Si nanocone array and then coating an ultrathin (∼2 nm) Al2O3 dielectric layer via atomic layer deposition (ALD). Ag-NPs are synthesized via the liquid phase method and then immersed in an organic solvent for liquid seal. For the SERS measurement, Ag-NPs are dispersed in a liquid containing the probe molecules and then, they self-assemble on the surface of the Al2O3@Ag@Si nanocones. Subsequently, the ultrathin Al2O3 dielectric layer separates Ag-NPs from the Ag@Si nanocones, forming massive gap-introduced hot spots. This substrate is sensitive to 1 pM Rhodamine R6G with an average enhancement factor of up to 109, exhibiting excellent SERS activity. Moreover, due to the protection of the Al2O3 dielectric layer and the organic solvent for the Ag@Si nanocones and Ag-NPs against oxidation, the split-type SERS substrate achieves an SERS signal with almost no attenuation after five months, indicating its good chemical stability.

Highly reliable SERS substrate based on plasmonic hybrid coupling between gold nanoislands and periodic nanopillar arrays

To improve both sensitivity and reliability, a hybrid SERS substrate of combining gold nanoislands (GNI) with periodic MgF₂ nanopillar arrays was successfully developed. SERS detection performance of the proposed substrates was evaluated in terms of enhancement effect, signal-to-noise ratio (SNR), linearity, reproducibility and repeatability, and compared with the performance of a conventional SERS substrate based on GNI. Experimental and simulation results presented that significant improvement of SERS intensity and SNR by more than 3 times and a notable reduction in relative standard deviation were obtained. We hope that the suggested SERS platform with unique advantages in sensitivity and reliability could be extended to point-of-care detection of a variety of biomolecular reactions.

Engineering of flexible granular Au nanocap ordered array and its surface enhanced Raman spectroscopy effect

Surface enhanced Raman spectroscopy (SERS) is a new and developing analytical technology in chemical and biological detection. However, traditional hard SERS substrates are struggling to meet the growing demand for flexible devices. In this work, we introduce a simple, cost-effective and large scale preparation route to form a flexible Au nanocap (AuNC) ordered array as SERS substrates via reactive ion etching (RIE) method and then Au deposition. We find RIE is an excellent method for nanoroughening the surface of polystyrene (PS) spheres. Such flexible SERS substrates exhibit high sensitivity and uniformity for detecting organic molecules. The finite-difference time-domain simulation results revealed that a strong electric field coupling effect existed not only in the gap site between the Au nanoparticles (AuNPs), but also in the connection position between the AuNCs and the single AuNP. This study not only offers a novel way for nanoroughening of PS spheres, but also acquires flexible and cheap SERS substrates for quick and sensitive detection of organic molecules.

Plasmonic nanosponges filled with silicon for enhanced white light emission

Plasmonic nanosponges are a powerful platform for various nanophotonic applications owing to extremely high local field enhancement in metallic nanopores. The filling of the nanopores with high-refractive index semiconductors (e.g. Si, Ge, GaP, etc.) opens up opportunities for the enhancement of nonlinear effects in these materials. However, this task remains challenging due to the lack of knowledge on the integration process of metal and high-index semiconductor components in such nanoobjects. Here, we investigate metal-dielectric nanoparticles fabricated from bilayer Si/Au films by the laser printing technique via a combination of theoretical and experimental methods. We reveal that these hybrid nanoparticles represent the Au sponge-like nanostructure filled with Si nanocrystallites. We also demonstrate that the Au net provides strong near-field enhancement in the Si grains increasing the white light photoluminescence in the hybrid nanostructures compared to uniform Si nanoparticles. These results pave the way for engineering the internal structure of the sponge-like hybrid nanoparticles possessing white light luminescence and control of their optical properties on demand.