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

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

Tuning d-Orbital Electronic Structure via Au-Intercalated Two-Dimensional Fe3GeTe2 to Increase Surface Plasmon Activity.

While extensive research has been dedicated to plasmon tuning within non-noble metals, prior investigations primarily concentrated on markedly augmenting the inherently low concentration of free carriers in materials with minimal consideration given to the influence of electron orbitals on surface plasmons. Here, we achieve successful intercalation of Au atoms into the layered structure of Fe3GeTe2 (FGT), thereby exerting control over the orbital electronic states or structure of FGT. This intervention not only amplifies the charge density and electron mobility but also mitigates the loss associated with interband transitions, resulting in increased two-dimensional FGT surface plasmon activity. As a consequence, Au-intercalated FGT detects crystal violet molecules as a surface-enhanced Raman scattering substrate, and the detection lines are 3 orders of magnitude higher than before Au intercalation. Our work provides insight for further studies on plasmon effects and the relation between surface plasmon resonance behavior and electronic structures.

Observing π-Au Interaction between Aromatic Molecules and Single Au Nanodimers with a Subnanometer Gap by SERS.

Interface interaction between aromatic molecules and noble metals plays a prominent role in fundamental science and technological applications. However, probing π-metal interactions under ambient conditions remains challenging, as it requires characterization techniques to have high sensitivity and molecular specificity without any restrictions on the sample. Herein, the interactions between polycyclic aromatic hydrocarbon (PAH) molecules and Au nanodimers with a subnanometer gap are investigated by surface-enhanced Raman spectroscopy (SERS). A cleaner and stronger plasmonic field of subnanometer gap Au nanodimer structures was constructed through solvent extraction. High sensitivity and strong π-Au interaction between PAHs and Au nanodimers are observed. Additionally, the density functional theory calculation confirmed the interactions of PAHs physically absorbed on the Au surface; the binding energy and differential charge further theoretically indicated the correlation between the sensitivity and the number of PAH rings, which is consistent with SERS experimental results. This work provides a new method to understand the interactions between aromatic molecules and noble metal surfaces in an ambient environment, also paving the way for designing the interfaces in the fields of catalysis, sensors, and molecular electronics.

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

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

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

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

Near-Neighbor Electron Orbital Coupling Effect of Single-Atomic-Layer Au Cluster Intercalated Bilayer 2H-TaS2 for Surface Enhanced Raman Scattering Sensing.

It is difficult to perfectly analyze the enhancement mechanism of two-dimensional (2D) materials and their combination with precious metals as surface enhanced Raman scattering (SERS) substrates using chemical enhancement mechanisms. Here, we propose a new mentality based on the coupling effect of neighboring electron orbitals to elucidate the electromagnetic field enhancement mechanism of single-atom-layer Au clusters embedded in double-layer 2H-TaS2 for SRES sensing. The insertion of Au atoms into the 2H-TaS2 interlayer was verified by XRD, AFM, and HRTEM, and a SERS signal enhancement of 2 orders of magnitude was obtained compared to the pure 2H-TaS2. XPS and micro-UV/vis-NIR spectra indicate that the outer electrons of neighboring Au and 2H-TaS2 overlap and migrate from Au to 2H-TaS2. First-principles calculations suggest strong electronic coupling between Au and 2H-TaS2. This study offers insights into SERS enhancement in nonprecious metal compounds and guides the development of new SERS substrates.

Development of a MoS2/Ag NP Nanopocket to Trap Target Molecules for Surface-Enhanced Raman Scattering Detection with Long-Term Stability and High Sensitivity.

Surface-enhanced Raman scattering (SERS) substrates mostly achieve highly sensitive detection by designing various hot spots; however, how to guide molecules to hot spots and prevent them from leaving has not been thoroughly considered and studied. Here, a composite MoS2/Ag NP nanopocket detector composed of MoS2 covered with a Ag NP film was fabricated to develop a general SERS method for actively capturing target molecules into hotspots. A finite element method (FEM) simulation of the multiphysics model was used to analyze the distributions of electric field enhancements and hydrodynamic processes in solution and air of the MoS2/Ag NP nanopocket. The results revealed that covering MoS2 slowed the evaporation of the solution, extended the window period for SERS detection, and enhanced the electric field in comparison with the monolayer Ag NP film. Therefore, in the process of dynamic detection, the MoS2/Ag NP nanopocket can provide an efficient and stable signal within 8 min, increasing the high sensitivity and long-term stability of the SERS method. Furthermore, a MoS2/Ag NP nanopocket detector was applied to detect antitumor drugs and monitor hypoxanthine structural changes in serum, which demonstrated long-term stability and high sensitivity for SERS analysis. This MoS2/Ag NP nanopocket detector paves the way for developing the SERS method in various fields.

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

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

Extending Plasmonic Enhancement Limit with Blocked Electron Tunneling by Monolayer Hexagonal Boron Nitride.

Fabricating ultrasmall nanogaps for significant electromagnetic enhancement is a long-standing goal of surface-enhanced Raman scattering (SERS) research. However, such electromagnetic enhancement is limited by quantum plasmonics as the gap size decreases below the quantum tunneling regime. Here, hexagonal boron nitride (h-BN) is sandwiched as a gap spacer in a nanoparticle-on-mirror (NPoM) structure, effectively blocking electron tunneling. Layer-dependent scattering spectra and theoretical modeling confirm that the electron tunneling effect is screened by monolayer h-BN in a nanocavity. The layer-dependent SERS enhancement factor of h-BN in the NPoM system monotonically increases as the number of layers decreases, which agrees with the prediction by the classical electromagnetic model but not the quantum-corrected model. The ultimate plasmonic enhancement limits are extended in the classical framework in a single-atom-layer gap. These results provide deep insights into the quantum mechanical effects in plasmonic systems, enabling the potential novel applications based on quantum plasmonic.

Quantification of the Real Plasmonic Field Transverse Distribution in a Nanocavity Using the Vibrational Stark Effect.

Quantifying the real plasmonic field strength experimentally has been long pursued in expanding the applications related to plasmonic enhancement. However, it is still an enormous challenge to determine the inhomogeneous plasmonic field distribution. Here, self-assembled monolayers (SAMs) of 4-mercaptobenzonitrile (MBN) are sandwiched as a gap spacer in a nanoparticle-on-mirror (NPoM) structure, effectively forming ultrahigh field enhancement to observe Stark shifts of the chemical bond. Transverse position-dependent Stark shifts of ν(C═C) and ν(C≡N) in the individual nanocavity measured by surface-enhanced Raman scattering (SERS) experiment combined with the Stark tuning rate by density functional theory (DFT) simulation accurately revealed the inhomogeneous plasmonic field transverse distribution and quantified the transverse plasmonic field strength up to ∼1.9 × 109 V/m, which matches the value predicted by finite element method (FEM) simulation. This work deepens the insight into plasmon-based technologies and will coordinate high-resolution techniques such as tip-enhanced Raman spectroscopy (TESR) to reveal the real plasmonic field distribution.

Solvent-driven biotoxin into nano-units as a versatile and sensitive SERS strategy.

In recent years, marine biotoxins have posed a great threat to fishermen, human security and military prevention and control due to their diverse, complex, toxic and widespread nature, and the development of rapid and sensitive methods is essential. Surface-enhanced Raman spectroscopy (SERS) is a promising technique for the rapid and sensitive in situ detection of marine biotoxins due to its advantages of rapid, high sensitivity, and fingerprinting information. However, the complex structure of toxin molecules, small Raman scattering cross-section and low affinity to conventional substrates make it difficult to achieve direct and sensitive SERS detection. Here, we generate a large number of active hotspot structures by constructing monolayer nanoparticle films with high density hotspots, which have good target molecules that can actively access the hotspot structures using nanocapillaries. In addition, the efficient and stable signal can be achieved during dynamic detection, increasing the practicality and operability of the method. This versatile SERS method achieves highly sensitive detection of marine biotoxins GTX and NOD, providing good prospects for convenient, rapid and sensitive SERS detection of marine biotoxins.

Floating Ag-NPs@Cu-NW bundles fabricated on copper mesh for highly sensitive SERS detection of uric acid in pretreatment-free urine.

The rapid and sensitive detection of ultra-trace marker molecules from biological samples is of great significance for the wide application of surface-enhanced Raman spectroscopy (SERS) methods in clinical diagnosis and disease monitoring. However, the cumbersome biological sample processing procedures and the poor enrichment of target analytes in hot spots hinder the practical applications of SERS methods. In this paper, we synthesized a novel floating SERS substrate by a simple one-step oxidation process, annealing and in situ chemical etching to form Ag-NPs@Cu-NW bundles on copper mesh (CM). In particular, under spontaneous bottom-up capillary action, the pressure difference at different nanogaps drives uric acid molecules to actively enter hot spots, so that the Ag-NPs@Cu-NW bundle nanostructure with the advantages of a light weight CM is capable of preventing the common coffee-ring effect and enhancing the spatial enrichment of analytes. Therefore, this SERS substrate realizes highly sensitive detection of uric acid at a level of 50 nM in pretreatment-free urine. Currently, this portable, flexible, simple, fast and cost-effective SERS substrate has great potential for early screening and clinical diagnosis of diseases in different biofluids.

Insight into ultrasensitive and high-stability flocculation-enhanced Raman spectroscopy for the in situ noninvasive probing of cupping effect substances.

The aggregation of nanoparticles is the key factor to form hot spots for the flocculation-enhanced Raman spectroscopy (FLERS) method. However, the structure of flocculation is still not clear. It is therefore necessary to explore and analyze the aggregation process of nanoparticles more carefully, so as to realize a better application of FLERS. Here, we report the application of in situ liquid cell transmission electron microscopy (TEM) combined with an in situ high-speed camera to analyze the particle behaviors. The results showed that flocculation can exist stably and the gap between the nanoparticles in the flocculation always remained at 7-9 nm, which ensured the high stability and sensitivity of the FLERS method. We successfully applied FLERS to the in situ noninvasive probing of cupping effect substances. The results indicated the scientific principle behind the traditional Chinese medicine method to some extent, which thus provides a new and effective method for the in situ dynamic monitoring of biological systems.

Insight into the Heterogeneity of Longitudinal Plasmonic Field in a Nanocavity Using an Intercalated Two-Dimensional Atomic Crystal Probe with a ∼7 ŠResolution.

Quantitative measurement of the plasmonic field distribution is of great significance for optimizing highly efficient optical nanodevices. However, the quantitative and precise measurement of the plasmonic field distribution is still an enormous challenge. In this work, we design a unique nanoruler with a ∼7 Šspatial resolution, which is based on a two-dimensional atomic crystal where the intercalated monolayer WS2 is a surface-enhanced Raman scattering (SERS) probe and four layers of MoS2 are a reference layer in a nanoparticle-on-mirror (NPoM) structure to quantitatively and directionally probe the longitudinal plasmonic field distribution at high permittivity by the quantitative SERS intensity of WS2 located in different layers. A subnanometer two-dimensional atomic crystal was used as a spacer layer to overcome the randomness of the molecular adsorption and Raman vibration direction. Combined with comprehensive theoretical derivation, numerical calculations, and spectroscopic measurements, it is shown that the longitudinal plasmonic field in an individual nanocavity is heterogeneously distributed with an unexpectedly large intensity gradient. We analyze the SERS enhancement factor on the horizontal component, which shows a great attenuation trend in the nanocavity and further provides precise insight into the horizontal component distribution of the longitudinal plasmonic field. We also provide a direct experimental verification that the longitudinal plasmonic field decays more slowly in high dielectric constant materials. These precise experimental insights into the plasmonic field using a two-dimensional atomic crystal itself as a Raman probe may propel understanding of the nanostructure optical response and applications based on the plasmonic field distribution.

Intelligent and robust DNA robots capable of swarming into leakless nonlinear amplification in response to a trigger.

Nonlinear DNA signal amplification with an enzyme-free isothermal self-assembly process is uniquely useful in nanotechnology and nanomedicine. However, progress in this direction is hampered by the lack of effective design models of leak-resistant DNA building blocks. Here, we propose two conceptual models of intelligent and robust DNA robots to perform a leakless nonlinear signal amplification in response to a trigger. Two conceptual models are based on super-hairpin nanostructures, which are designed by innovating novel principles in methodology and codifying them into embedded programs. The dynamical and thermodynamical analyses reveal the critical elements and leak-resistant mechanisms of the designed models, and the leak-resistant behaviors of the intelligent DNA robots and morphologies of swarming into nonlinear amplification are separately verified. The applications of the designed models are also illustrated in specific signal amplification and targeted payload enrichment via integration with an aptamer, a fluorescent molecule and surface-enhanced Raman spectroscopy. This work has the potential to serve as design guidelines of intelligent and robust DNA robots and leakless nonlinear DNA amplification, and also as the design blueprint of cargo delivery robots with the performance of swarming into nonlinear amplification in response to a target automatically, facilitating their future applications in biosensing, bioimaging and biomedicine.

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

Highly sensitive surface-enhanced Raman spectroscopy (SERS) sensing not only depends on an active substrate with high density of hot spots, but also depends more on whether the molecules can effectively enter the hot spot region. In this paper, a new SERS detection method based on the nano nest model is developed to autonomously capture molecules into hot spots. The nano nest is composed of silver nanowires modified with gold nanoparticles (Ag NW@Au NPs), which not only form high density hot spots between particles or particles-wires, but also have a coupled electromagnetic field enhancement effect. The SERS detection method based nano nest actively traps molecules through the capillary stage, and makes the molecules move toward densely stacked small gaps (hot spots) by capillary action. The above method has been used to detect different kinds of molecules, such as pesticide residues, adenosine triphosphate in culture medium, and antibiotic residues in aquatic products. In addition, an in situ SERS monitoring of allergic reactions was also performed using nano nests with the feature of actively trapping molecules into the hot spots. This nano nest will be able to perform a direct monitoring of biochemical reactions, and more importantly, it can provide a new scheme for SERS detection.

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

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

Metal coordination induced SERS nanoprobe for sensitive and selective detection of histamine in serum.

Sensitivity and credibility detecting histamine (HA) as an important neurotransmitter in biofluids is of importance in analytical science and physiology. Surface-enhanced Raman spectroscopy (SERS) is able to realize the high sensitivity with single molecules level, but providing the high sensitivity for HA with a small cross section remains a challenge. Here we develop the metal complex-based SERS nanoprobe nitrilotriacetic acid-Ni2+ (NTA-Ni2+) combined with self-assemble Au NPs active substrates for sensitive detection of HA. The NTA-Ni2+ can capture the HA molecules close to Au NPs substrates and then amplify the Raman signals of HA owing to the formation of a complex of NTA-Ni2+-HA. The self-assemble Au film through the evaporation-driven method can provide the high-density hot spots substrate with high stability and reproducibility. The NTA-Ni2+ decorated Au NPs as nanoprobe responds to HA with 1 µM level of sensitivity. More importantly, the developed SERS nanoprobe composing of NTA-Ni2+ and self-assemble Au NPs can be utilized to detect and monitor the HA spiked into serum, indicating the potential prospect in analysis of HA in complex specimen.

Construction of Optimal SERS Hotspots Based on Capturing the Spike Receptor-Binding Domain (RBD) of SARS-CoV-2 for Highly Sensitive and Specific Detection by a Fish Model.

It is highly challenging to construct the best SERS hotspots for the detection of proteins by surface-enhanced Raman spectroscopy (SERS). Using its own characteristics to construct hotspots can achieve the effect of sensitivity and specificity. In this study, we built a fishing mode device to detect the receptor-binding domain (RBD) of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) at low concentrations in different detection environments and obtained a sensitive SERS signal response. Based on the spatial resolution of proteins and their protein-specific recognition functions, SERS hotspots were constructed using aptamers and small molecules that can specifically bind to RBD and cooperate with Au nanoparticles (NPs) to detect RBD in the environment using SERS signals of beacon molecules. Therefore, two kinds of AuNPs modified with aptamers and small molecules were used in the fishing mode device, which can specifically recognize and bind RBD to form a stable hotspot to achieve high sensitivity and specificity for RBD detection. The fishing mode device can detect the presence of RBD at concentrations as low as 0.625 ng/mL and can produce a good SERS signal response within 15 min. Meanwhile, we can detect an RBD of 0.625 ng/mL in the mixed solution with various proteins, and the concentration of RBD in the complex environment of urine and blood can be as low as 1.25 ng/mL. This provides a research basis for SERS in practical applications for protein detection work.

In-situ SERS readout strategy to improve the reliability of beta-galactosidase activity assay based on X-gal staining in shortening incubation times.

Beta-galactosidase (ß-gal) activity is closed related with senescence cells and aging-associated diseases, however, the traditional readout of ß-gal activity based on X-gal staining was limited to low sensitivity in short incubation times and false positives in long incubation times. Here, we expose the potential role of insoluble X-gal hydrolysates in causing false positives by diffusion pollution depending on organic medium and then propose the in-situ Surface-enhanced Raman spectroscopy (SERS) readout strategy to identify and locate ß-gal positive cells. By building the blue-white screening model and fabricating SERS-active needle sensor, the sensitive detection of ß-gal has been realized with the detection limit of less than 1 nmol L-1. The in-situ SERS readout strategy is proved to be necessary and feasible to improve the reliability of X-gal staining assay through shortening the time to a few hours. Moreover, its application was also preliminarily evaluated to analyse individual cells and tissues, which showed the well consistency for judgement of ß-gal activity cells at different times. Consequently, by improving reliability and reducing time consumption, this SERS readout strategy may be of great significance to promote the application of X-gal staining assay in biology and biomedicine.