Mixing Assisted “Hot Spots” Occupying SERS Strategy for Highly Sensitive In Situ Study

Synovial fluid research based on SERS and SERRS for enhanced detection of biomarkers in staged osteoarthritis

Surface-enhanced (resonance) Raman scattering (SER(R)S) can extremely enhance Raman intensity of samples, which is helpful for detecting synovial fluid (SF) that does not show Raman activity under normal conditions. In this study, SER(R)S spectra of SF from three different osteoarthritis (OA) stages were collected and analyzed for OA progress, finding that the content of collagen increased throughout the disease, while non-collagen proteins and polysaccharides decreased sharply at advanced OA stage accompanied by the increase of phospholipid. The spectral features and differences were enhanced by salting-out and centrifugation. Much more information on biomolecules at different OA stages was disclosed by using SERRS for the first time, these main trace components (β-carotene, collagen, hyaluronic acid, nucleotide, and phospholipid) can be used as potential biomarkers. It indicates that SERRS has a more comprehensive ability to assist SERS in seeking micro(trace) biomolecules as biomarkers and facilitating accurate and efficient diagnosis and mechanism research of OA.

A robust SERS calibration using a pseudo-internal intensity reference

Surface-enhanced Raman scattering (SERS) with high molecular sensitivity and specificity is a powerful nondestructive analytical tool. Since its discovery, SERS measurements have suffered from the vulnerability of calibration curve, which makes quantification analysis a great challenge. In this work, we report a robust calibration method by introducing a referenced measurement as the intensity standard. This intensity reference not only has the advantages of the internal standard method such as reflecting the SERS substrate enhancement, but also avoids the introduction of competing adsorption between target molecules and the internal standard. Based on the normalized calibration curve, the magnitude of the R6G concentration can be well evaluated from 10^-7 M to 10^-12 M. Furthermore, we demonstrate that this pseudo-internal standard method can also work well using a different type of molecule as the reference. This SERS calibration method would be beneficial for the development of quantitative SERS analysis.

Rapid Quantitative Detection of Voriconazole in Human Plasma Using Surface-Enhanced Raman Scattering

There is an increasing demand for rapid detection techniques for monitoring the therapeutic concentration of voriconazole (VRC) in human biological fluids. Herein, a rapid and selective surface-enhanced Raman scatting method for point-of-care determination of VRC in human plasma was developed via a portable Raman spectrometer. This approach has enabled the quantification of the VRC spiked into human plasma at clinical relevant concentrations. A gold nanoparticle solution (Au sol) was used as the SERS substrate, and the agglomerating conditions on its sensitivity were optimized. The method involves the formation of hot spots, and the signal of VRC molecules adsorbed on the surface of the SERS hot spot was amplified by 10⁵. The calibration curve was linear in the range of 0.02-10 ppm, with satisfactory repeatability. The limit of detection was as low as 12.3 ppb. The variation in VRC spectra over time on different substrates demonstrated good reproducibility. Notably, the salting-out extraction method developed in this study was rapid and suitable for the quantitation of drugs in biological samples. Compared with traditional methods, this approach allows for the point-of-care quantification of VRC directly in a complex matrix, which may open up new exciting opportunities for future use of the SERS technique in clinical applications.

Analysis of Sequential Micromixing Driven by Sinusoidally Shaped Induced-Charge Electroosmotic Flow

Multi-fluid micromixing, which has rarely been explored, typically represents a highly sought-after technique in on-chip biochemical and biomedical assays. Herein, we propose a novel micromixing approach utilizing induced-charge electroosmosis (ICEO) to implement multicomplex mixing between parallel streams. The variations of ICEO microvortices above a sinusoidally shaped floating electrode (SSFE) are first investigated to better understand the microvortex development and the resultant mixing process within a confined channel. On this basis, a mathematical model of the vortex index is newly developed to predict the mixing degree along the microchannel. The negative exponential distribution obtained between the vortex index and mixing index demonstrates an efficient model to describe the mixing performance without solving the coupled diffusion and momentum equations. Specifically, sufficient mixing with a mixing index higher than 0.9 can be achieved when the vortex index exceeds 51, and the mixing efficiency reaches a plateau at an AC frequency close to 100 Hz. Further, a rectangle floating electrode (RFE) is deposited before SSFE to enhance the controlled sequence for three-fluid mixing. One side fluid can fully mix with the middle fluid with a mixing index of 0.623 above RFE in the first mixing stage and achieve entire-channel mixing with a mixing index of 0.983 above SSFE in the second mixing stage, thereby enabling on-demand sequential mixing. As a proof of concept, this work can provide a robust alternative technique for multi-objective issues and structural design related to mixers.

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

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

Microfluidics and surface-enhanced Raman spectroscopy, a win-win combination?

With the continuous development in nanoscience and nanotechnology, analytical techniques like surface-enhanced Raman spectroscopy (SERS) render structural and chemical information of a variety of analyte molecules in ultra-low concentration. Although this technique is making significant progress in various fields, the reproducibility of SERS measurements and sensitivity towards small molecules are still daunting challenges. In this regard, microfluidic surface-enhanced Raman spectroscopy (MF-SERS) is well on its way to join the toolbox of analytical chemists. This review article explains how MF-SERS is becoming a powerful tool in analytical chemistry. We critically present the developments in SERS substrates for microfluidic devices and how these substrates in microfluidic channels can improve the SERS sensitivity, reproducibility, and detection limit. We then introduce the building materials for microfluidic platforms and their types such as droplet, centrifugal, and digital microfluidics. Finally, we enumerate some challenges and future directions in microfluidic SERS. Overall, this article showcases the potential and versatility of microfluidic SERS in overcoming the inherent issues in the SERS technique and also discusses the advantage of adding SERS to the arsenal of microfluidics.

In Situ Microfluidic SERS Chip for Ultrasensitive Hg2+ Sensing Based on I--Functionalized Silver Aggregates

Mercury(II) ions are causing serious environmental pollution and health damage. Developing a simple, rapid, and sensitive sensor for Hg²⁺ detection is of great significance. Herein, we demonstrate an I^--functionalized surface-enhanced Raman scattering (SERS) substrate for rapid and sensitive Hg²⁺ sensing on a highly integrated microfluidic platform. Based on the combination reaction between I^- and Hg²⁺, the Hg²⁺ sensing is achieved via the SERS intensity "turn-off" strategy, where HgI₂ precipitation is formed on an SERS substrate interface, dissociating the Raman reporters that coadsorbed with I^-. Owing to the strong binding constant between I^- and Hg²⁺, our I^--functionalized substrate demonstrates a very fast sensing response (∼150 s). Through reliable in situ SERS detection, a robust calibration curve between the "turn-off" signal and "lgC" is obtained in a broad concentration range of 10^-9 to 10^-13 M. Additionally, the detectable Hg²⁺ concentration can be as low as 1 fM. The good selectivity toward Hg²⁺ is also verified by testing about a dozen common metal ions in water, such as K⁺, Na⁺, Ca²⁺, Mg²⁺, and so forth. Furthermore, we apply the SERS sensor for real tap and lake water sample detection, and good recoveries of 113, 97, and 107% are obtained. With its advantages of high integration, simple preparation, fast response, high sensitivity, and reliability, the proposed I^--functionalized SERS sensor microfluidic chip can be a promising platform for real-time and on-site Hg²⁺ detection in natural water.

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

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

Three-dimensional porous SERS powder for sensitive liquid and gas detections fabricated by engineering dense "hot spots" on silica aerogel

A three-dimensional porous SERS powder material, Ag nanoparticles-engineered-silica aerogel, was developed. Utilizing an in situ chemical reduction strategy, Ag nanoparticles were densely assembled on porous aerogel structures, thus forming three-dimensional "hot spots" distribution with intrinsic large specific surface area and high porosity. These features can effectively enrich the analytes on the metal surface and provide huge near field enhancement. Highly sensitive and homogeneous SERS detections were achieved not only on the conventional liquid analytes but also on gas with the enhancement factor up to ∼10⁸ and relative standard deviation as small as ∼13%. Robust calibration curves were obtained from the SERS data, which demonstrates the potential for the quantification analysis. Moreover, the powder shows extraordinary SERS stability than the conventional Ag nanostructures, which makes long term storage and convenient usage feasible. With all of these advantages, the porous SERS powder material can be extended to on-site SERS "nose" applications such as liquid and gas detections for chemical analysis, environmental monitoring, and anti-terrorism.

DNA-assisted synthesis of Ortho-NanoDimer with sub-nanoscale controllable gap for SERS application

Nanostructure with precisely controllable narrow gap width remains a great challenge, especially at the sub-nanoscale level. Here, a versatile strategy named as DNA-assisted synthesis of ortho-nanodimer (DaSON) is proposed to fabricate Ag (Au) nanodimers with a uniform gap width from nanometers to angstroms. In such a strategy, two nanoparticles are constrained by the equilibrium state of the DNA hybridization and electrostatic repulsion to form zipper-like ortho-nanostructures with an extremely uniform gap whose width can be finely adjusted at nanoscale or sub-nanoscale by changing the DNA sequence and the surface charge of nanoparticles. The inherent strong electromagnetic coupling in the uniform sub-nanometer gap can generates an unparalleled SERS enhancement together with an extraordinary reproducibility. Compared with conventional DNA-based nano-gap fabrication strategy, the DaSON strategy enhances the SERS intensity for more than two orders of magnitude with a detection limit of 100 aM for DNA, and significantly improves the reproducibility in both labeled and label-free SERS sensing applications. Moreover, the DaSON strategy holds wide applicability for arbitrary kinds of DNA-modifiable nanoparticles. Therefore, we believe that the DaSON strategy provides an innovative method for the synthesis of nanostructures with controllable nanogaps and has a promising future in multiple fields including nanotechnology, catalysis and photonics.

Applications of surface-enhanced Raman spectroscopy in detection fields

Surface-enhanced Raman spectroscopy (SERS) is a Raman spectroscopy technique that has been widely used in food safety, environmental monitoring, medical diagnosis and treatment and drug monitoring because of its high selectivity, sensitivity, rapidness, simplicity and specificity in identifying molecular structures. This review introduces the detection mechanism of SERS and summarizes the most recent progress concerning the use of SERS for the detection and characterization of molecules, providing references for the later research of SERS in detection fields.

Ratiometric Sensing of Polycyclic Aromatic Hydrocarbons Using Capturing Ligand Functionalized Mesoporous Au Nanoparticles as a Surface-Enhanced Raman Scattering Substrate

The absorption behavior between plasmonic nanostructures and a target molecule plays key roles in effective surface-enhanced Raman scattering (SERS) detection. However, for analytes with low surface affinity to the metallic surface, e.g., polycyclic aromatic hydrocarbons (PAHs), it remains challenging to observe the enhanced Raman signal. In this work, we reported a ratiometric SERS strategy for sensitive PAH detection through the surface functionalization of 3D ordered mesoporous Au nanoparticles (meso-Au NPs). By employing mono-6-thio-β-cyclodextrin (HS-β-CD) as capture ligands, the hydrophobic molecules, e.g., anthracene, could be effectively absorbed on the meso-Au NP surface via a host-guest interaction. Besides, a hydrophobic slippery surface is used as a concentrator to deliver and enrich the Au/analyte droplets into a small area. Consequently, the detection limits of anthracene and naphthalene are down to 1 and 10 ppb. The improved SERS enhancement is mainly ascribed to the host-guest effect of HS-β-CD ligands, large surface area and high-density of sub-10 nm mesopores of Au networks, as well as the enrichment effect of hydrophobic slippery surface. Moreover, the HS-β-CD (480 cm^-1 band) could serve as an internal standard, leading to the ratiometric determination of anthracene ranging from 1 ppm to 1 ppb. The proposed surface modification strategy in combination with the hydrophobic slippery surface shows great potential for active capture and trace detection of persistent organic pollutants in real-world SERS applications.

A fully integrated hand-powered centrifugal microfluidic platform for ultra-simple and non-instrumental nucleic acid detection

Hand-powered centrifugal microfluidics combined with isothermal nucleic acid amplification testing (NAAT) have been one of the most promising rapid detection platforms in resource-limited settings. However, current hand-powered centrifuges still suffer from customized instrument-based operation and low rotation rate; and most isothermal NAAT were conducted with complicated reaction systems for DNA detection and required an additional step for RNA detection. Herein, we built a fully hand-powered centrifugal miniaturized NAAT platform inspired by buzzer toys, which embedded sample preparation, strand exchange amplification (SEA) and visual fluorescence detection together. The centrifugal disc was easily fabricated, and operated the mixing in 1 min by simply dragging the looped rope through it with a mean input force of 16.5 N, enabling its rotation rate reach 5000 rpm. In addition, SEA was an ultra-simple one-step DNA or RNA detection method initiated by Bst DNA polymerase and a pair of primers, and thus we took all its merits and integrate it into microfluidic systems firstly. Furthermore, taking Vibrio parahemolyticus as an example, the microfluidic platform achieved DNA or RNA detection within 1 h; and the detection limit of the microchip for artificially spiked oysters was 10³ CFU/g without cumbersome sample preparation, and reached to 10⁰ CFU/g after enrichment. Therefore, we provided an ultra-simple and non-instrumental microfluidic platform powered merely by hands, performing general potential in sample-to-answer NAAT for versatile pathogens in remote regions.

Breaking the Affinity Limit with Dual-Phase-Accessible Hotspot for Ultrahigh Raman Scattering of Nonadsorptive Molecules

For surface-enhanced Raman scattering (SERS) analysis, only analytes that can be absorbed spontaneously onto a noble metal surface can be detected effectively. Therefore, getting nonadsorptive molecules close enough to the surface has always been a key challenge in SERS analysis. Here absorbance measurements show that the liquid-interfacial array (LIA) does not adsorb or enrich benzopyrene (Bap) molecules, which lack effective functional groups that can interact with the noble metal surfaces. But the SERS intensity of 0.1 ppm Bap on the LIA is 10 times larger than that of 10 ppm Bap on traditional solid substrate, i.e., 3 orders of magnitude of enhancement. The LIA overcomes the restriction of affinity between Bap molecules and the metal surface, and the Bap molecules can easily enter nanogaps without steric hindrance. Furthermore, both adsorptive and nonadsorptive molecules were used to observe the SERS enhancement behavior on the LIA platforms. In multiple detection, competitive SERS signal changes could be observed between adsorptive and nonadsorptive molecules or between nonadsorptive and nonadsorptive molecules. A theoretical scheme was profiled for localized surface plasmon resonance (SPR) properties of the LIA. Finite difference-time domain (FDTD) simulation shows that the LIAs have biphasic and accessible asymmetric hotspots, and the electric field enhancement in the CHCl₃ (O) phase is approximately four times larger than that of the water (W) phase. In addition, the position and relative strength of the electromagnetic field depend on the spatial position of gold nanoparticles (GNPs) relative to the liquid-liquid interface (LLI), i.e., when the GNP dimer is completely immersed in a certain phase, the electromagnetic field enhancement of the CHCl₃ phase is approximately 7 times larger than that of the W phase. We speculate that dual-phase-accessible hotspots and the hydrophobic environment provided by CHCl₃ are two important factors contributing to successful detection of four common polycyclic aromatic hydrocarbons (PAHs) with a detection limit of 10 ppb. Finally, the LIA platform successfully realizes simultaneous detection of multiple PAHs in both plant and animal oils with good stability. This study provides a new direction for the development of high-efficiency and practical SERS technology for nonadsorptive molecules.

Controlling the Morphologies of Silver Aggregates by Laser-Induced Synthesis for Optimal SERS Detection

Controlling the synthesis of metallic nanostructures for high quality surface-enhanced Raman scattering (SERS) materials has long been a central task of nanoscience and nanotechnology. In this work, silver aggregates with different surface morphologies were controllably synthesized on a glass-solution interface via a facile laser-induced reduction method. By correlating the surface morphologies with their SERS abilities, optimal parameters (laser power and irradiation time) for SERS aggregates synthesis were obtained. Importantly, the characteristics for largest near-field enhancement were identified, which are closely packed nanorice and flake structures with abundant surface roughness. These can generate numerous hot spots with huge enhancement in nanogaps and rough surface. These results provide an understanding of the correlation between morphologies and SERS performance, and could be helpful for developing optimal and applicable SERS materials.

Manipulating "Hot Spots" from Nanometer to Angstrom: Toward Understanding Integrated Contributions of Molecule Number and Gap Size for Ultrasensitive Surface-Enhanced Raman Scattering Detection

Narrower gaps between metal nanoparticles (so-called "hot spots") in surface-enhanced Raman scattering (SERS) substrates contribute to stronger electromagnetic (EM) enhancement; however, the accompanying steric effect hinders analyte molecules entering hot spots to access the benefit. To comprehensively understand integrated contributions of the gap size and molecule number accommodated in hot spots and then optimize design of SERS substrates, the thermal shrinking method was employed to manipulate hot spots and the "hottest zone" was defined to evaluate the integrated contributions to SERS intensity of the two factors. In the conventional shrink-adsorption mode, the contributions of the molecule number and gap size are competitive when the gap width is comparable with the target molecule size, which leads to oscillating behavior of SERS intensity versus gap size, and it is analyte molecule size dependent. This result suggests that engineering hot spots should be target molecule directed to achieve ultrasensitive detection. In the proposed adsorption-shrink mode, the contributions of the molecule number and gap size are synergistic, which makes the detection ability of the adsorption-shrink mode attains a single-molecule (SM) level. Excellent performance of the adsorption-shrink SERS strategy benefits detection of trace level pollutants in complex environments. Detection ranges for contaminants with different metal affinity, such as thiram, malachite green (MG), and formaldehyde, are as low as parts per billion, even down to parts per trillion.

Tri-fluid mixing in a microchannel for nanoparticle synthesis

It is becoming more difficult to use bulk mixing and bi-fluid micromixing in multi-step continuous-flow reactions, multicomponent reactions, and nanoparticle synthesis because they typically involve multiple reactants. To date, most micromixing studies, both passive and active, have focused on how to efficiently mix two fluids, while micromixing of three or more fluids together (multi-fluid mixing) has been rarely explored. This study is the first on tri-fluid mixing in microchannels. We investigated tri-fluid mixing in three microchannel models: a straight channel, a classical staggered herringbone mixing (SHM) channel, and a three-dimensional (3D) X-crossing microchannel. Numerical simulations and experiments were jointly conducted. A two-step experimental process was performed to determine the tri-fluid mixing efficiencies of these microchannels. We found that the SHM cannot significantly enhance mixing of three streams especially for a Reynolds number (Re) higher than 10. However, the 3D X-crossing channel based on splitting-and-recombination (SAR) showed effective tri-mixing performance over a wide Re range up to 275 (with a corresponding flow rate of 1972.5 μL min^-1), thereby enabling high microchannel throughput. Furthermore, this tri-fluid micromixing process was used to synthesize a kind of Si-based nanoparticle. This achieved a narrower particle size distribution than traditional bulk mixing. Therefore, SAR-based tri-fluid mixing is an alternative for chemical and biochemical reactions where three reactants need to be mixed.

Combination of liquid-based column separations with surface-enhanced Raman spectroscopy

Surface-enhanced Raman spectroscopy is a constantly developing analytical method providing not only high-sensitive quantitative but also qualitative information on an analyte. Thus, it is reasonable that it has been tested as a promising detection method in column separations. Although its implementation in analytical separations is not widespread, some surprising results, like enormous signal enhancement and demonstrations of single-molecule identifications, proved in only a few special examples, indicate the potential of the method. The high detection sensitivity and selectivity would be of paramount importance in trace analyses of biologically relevant molecules in complex matrices. However, the combination of surface-enhanced Raman spectroscopy with column separation methods brings two principal issues. Interactions of analytes with metal substrates can cause deteriorations of separations and the detection can be affected by background electrolytes or elution agents. Thus, in principle, this review is on the experimental and methodological solutions to these problems. First, theoretical and practical aspects of Raman scattering, and excitation of surface plasmon in colloid suspensions of nanoparticles and on planar nanostructured substrates are briefly explained. Advances in experimental arrangements of on-line and at-line couplings with column liquid phase separation methods, including microfluidic devices, are described together with chosen analytical applications.