Surface-enhanced Raman Scattering Signal Research Articles

Direct Virus Gene Detection: A CRISPR/dCas9-Mediated Surface-Enhanced Raman Scattering Strategy with Enzyme-Catalyzed Signal Amplification.

The aberrant growth of cervical cells caused by the infection of human papillomavirus (HPV) may cause cervical cancer. In order to effectively prevent the occurrence of cervical cancer and for better follow-up treatment after surgery, a rapid and reliable detection method of HPV DNA is essential. Here, a surface-enhanced Raman scattering (SERS) detection method was developed based on the clustered regularly interspaced short palindromic repeats (CRISPR)/dCas9 technique and the enzyme catalysis amplification reaction, which achieved a simple and rapid detection of low-content HPV genes. The CRISPR/dCas9/sgRNA complex was anchored above a magnetic bead, which can precisely capture target DNA sequences, exhibiting high selectivity for HPV genes. When the biotinylated target DNAs exist, they can bridge a streptavidin-modified horse radish peroxidase (HRP) to the magnetic bead, producing an HRP-decorated conjugate. This conjugate allows an HRP-catalyzed reaction for its substrate (3,3',5,5'-tetramethylbenzidine, TMB). Gold nanostars with a silica shell exhibiting the lightning rod effect of SERS were employed to measure the SERS spectra of the oxidative product of TMB. Enzyme catalysis and SERS co-contribute to the SERS signal output, ensuring a high detection sensitivity. This method is a proof of concept for detecting HPV DNAs in a complex system. The current method can be applied to other target DNAs simply by changing the sgRNA sequence. Many superiors portend that the CRISPR/dCas9-based SERS method is promising for further clinical application.

Developing highly reliable SERS substrates based on Ag grown on alumina nanomeshes anodized under 1 V for efficiently sensing Raman-active molecules

We have developed silver-nanostructures grown on anodic alumina nanomesh (AAN) films to create new-type substrates for surface-enhanced Raman scattering (SERS). AAN with uniform thin sidewall of ∼ 5 nm was fabricated by anodizing Al sheets at 1 V in 6% H3PO4 solution. Subsequent AC electrochemical deposition of silver created an array of nanoparticles or nano-islands depending on growth time. The particle-island transition is non-monotonic evolution, since metal growth and dissolution compete in AC electrodeposition process. Systematic SERS study on various Ag-AAN films with trial probes of adenine solutions reveals collective contribution of electron-plasma oscillation and surface area of silver nanostructures in Raman enhancements. SERS signals are primarily contributed by surface area under excitation wavelength of 532 nm (away from plasmonic resonance). The average correlation coefficient between the SERS intensity and surface area was 0.85, indicating robust correlation. This value was reduced to 0.61 under excitation wavelength of 633 nm (closer to plasmonic resonance). Furthermore, increased Ag-deposition reduced the relative standard deviation of SERS intensities and thus improved both the uniformity and quality consistency of SERS substrates. Therefore, fabrication of SERS substrate with larger Ag surface area under similar SERS enhancement factors is suggested for high throughput in commercial sectors.

Dynamic modulation of a surface-enhanced Raman scattering signal by a varying magnetic field.

Surface-enhanced Raman scattering (SERS) signals are fundamental for spectroscopy applications. However, existing substrates cannot perform a dynamically enhanced modulation of SERS signals. Herein, we developed a magnetically photonic chain-loading system (MPCLS) substrate by loading magnetically photonic nanochains of Fe3O4@SiO2 magnetic nanoparticles (MNPs) with Au nanoparticles (NPs). We achieved a dynamically enhanced modulation by applying an external stepwise magnetic field to the randomly dispersed magnetic photonic nanochains that gradually align in the analyte solution. The closely aligned nanochains create a higher number of hot spots by new neighboring Au NPs. Each chain represents a single SERS enhancement unit with both a surface plasmon resonance (SPR) effect and photonic property. The magnetic responsivity of MPCLS enables a rapid signal enhancement and tuning of the SERS enhancement factor.

An array structures of nanoparticle-coupled hyperbolic metamaterials for efficient SERS sensing

Hyperbolic metamaterials (HMMs) have been employed in the field of surface-enhanced Raman scattering (SERS) for their properties to support large-wave vector bulk plasmon polariton (BPP) waves, whereas the excitation of BPP is challenging to a certain extent. In addition, with the advance of SERS, the substrate for energy utilization has aroused increasing attention. An HMM-Ag nanoparticles (Ag NPs) platform based on patterned sapphire substrate (PSS) array structure was proposed in this study. With Ag NPs as the external coupling structure, the generated large-wave vector scattered light excited the BPP in the HMM and extended the energy into the Ag NPs gap. On that basis, a hot surface with a high density of “hot spots” was formed. PSS with excellent light trapping capability was employed as a substrate, which improved the energy utilization of the platform. Furthermore, the good hydrophobicity of the nanoparticle array structure further amplified the SERS signal. Lastly, the excellent performance of the platform was experimentally verified, the detection limits of CV and R6G reached 10-10 M and 10-11 M, respectively. Accordingly, it is believed that the plasma platform has promising applications in high-sensitivity sensing and biochemical detection.

Nondestructive separation/enrichment and rolling circle amplification-powered sensitive SERS enumeration of circulating tumor cells via aptamer recognition

Nondestructive separation/enrichment and reliable detection of extremely rare circulating tumor cells (CTCs) in peripheral blood are of considerable importance in tumor precision diagnosis and treatment, yet this remains a big challenge. Herein, a novel strategy for nondestructive separation/enrichment and ultra-sensitive surface-enhanced Raman scattering (SERS)-based enumeration of CTCs is proposed via aptamer recognition and rolling circle amplification (RCA). In this work the magnetic beads modified with "Aptamer (Apt)-Primer" (AP) probes were utilized to specifically capture CTCs, and then after magnetic separation/enrichment, the RCA-powered SERS counting and benzonase nuclease cleavage-assisted nondestructive release of CTCs were realized, respectively. The AP was assembled by hybridizing the EpCAM-specific aptamer with a primer, and the optimal AP contains 4 mismatched bases. The RCA enhanced SERS signal nearly 4.5-fold, and the SERS strategy has good specificity, uniformity and reproducibility. The proposed SERS detection possesses a good linear relationship with the concentration of MCF-7 cells spiked in PBS with the limit of detection (LOD) of 2 cells/mL, which shows good potential practicality for detecting CTCs in blood with recoveries ranging from 100.56% to 116.78%. Besides, the released CTCs remained good cellular activity with the normal proliferation after re-culture for 48 h and normal growth for at least three generations. The proposed strategy of nondestructive separation/enrichment and SERS-based sensitive enumeration is promising for reliable analysis of EpCAM-positive CTCs in blood, which is expected to provide a powerful tool for analysis of extremely rare circulating tumor cells in complex peripheral blood for liquid biopsy.

Does the chemical contribution have a secondary role in SERS?

It is an established understanding that the electromagnetic contribution (plasmon-mediated enhancement of a laser and scattered local electromagnetic fields) is the main actor in surface enhanced Raman scattering (SERS), with the so-called chemical (molecule-related) contribution assuming only, if any, a supporting role. The conclusion of our comprehensive experimental resonant study of a broad range of nanosphere lithography based metallic substrates, with covalently attached 4-mercaptobenzoic acid monolayers used as a probe (standard molecules that are non-resonant in solution), is that this accepted understanding needs to be revised. We present a detailed resonant SERS study of metal-film-over-nanosphere (MFON) substrates that is done by both scanning the laser wavelength and tuning the plasmon response through the nanosphere diameter, which is varied from 500 to 900 nm. Far and local field properties are characterized through measures of optical reflectivity and SERS efficiency, respectively, and are supported by numerical simulations. We demonstrate that SERS intensity depends indeed on the electromagnetic mechanism, determined by the plasmonic response of the system, but we observe that it is also strongly defined by a chemical resonant contribution related to a metal-to-ligand electronic transition of the covalently bound probe molecule. Optimum amplification occurs when the plasmon modes intersect with the ligand-to-metal chemical resonance, contributing synergically both mechanisms together. Quite notably, however, the largest SERS signal is observed when the laser is tuned with the metal-to-ligand transition, and typically does not follow the wavelength dependence of the plasmon modes when varying the nanosphere size. The same general trend is observed for other nanosphere lithography based substrates, including sphere segment void cavities and hexagonally ordered triangular nanoparticles, using either Ag or Au as the plasmonic metal, and also with a commercial substrate (Klarite). Interestingly, this extensive comparative investigation shows in addition that MFON substrates are significantly better than these other studied plasmonic substrates in terms of Raman intensity and homogeneity. We conclude that a deep understanding of both electromagnetic and chemical mechanisms is necessary to fully exploit these substrates for analytical applications.

High-Speed Spectral Characterization of Single-Molecule SERS Fluctuations.

The concept of plasmonic "hotspots" is central to the broad field of nanophotonics. In surface-enhanced Raman scattering (SERS), hotspots can increase Raman scattering efficiency by orders of magnitude. Hotspot dimensions may range from a few nanometers down to the atomic scale and are able to generate SERS signals from single molecules. However, these single-molecule SERS signals often show significant fluctuations, and the concept of intense, localized, yet static hotspots has come into question. Recent experiments have shown these SERS intensity fluctuations (SIFs) to occur over an extremely wide range of timescales, from seconds to microseconds, due to the various physical mechanisms causing SERS and the dynamic nature of light-matter interaction at the nanoscale. The underlying source of single-molecule SERS fluctuations is therefore likely to be a complex interplay of several different effects at different timescales. A high-speed acquisition system that captures a full SERS spectrum with microsecond time resolution can therefore provide information about these dynamic processes. Here, we show an acquisition system that collects at a rate of 100,000 SERS spectra per second, allowing high-speed characterization. We find that while each individual SIF event will enhance a different portion of the SERS spectrum, including a single peak, over 10s to 100s of microseconds, the SIF events overall do not favor one region of the spectrum over another. These high-speed SIF events can therefore occur with relatively equal probability over a broad spectral range, covering both the anti-Stokes and the Stokes sides of the spectrum, sometimes leading to anomalously large anti-Stokes peaks. This indicates that both temporally and spectrally transient hotspots drive the SERS fluctuations at high speeds.

Neodymium doping MoS2 nanostructures with remarkable surface-enhanced Raman scattering activity

In recent years, the study of surface-enhanced Raman scattering (SERS) substrate on semiconductors has greatly broadened the application of SERS technology in different fields. Developing new practical semiconductor SERS substrates materials is still important work. Here, different concentrations of Nd-doped MoS2 were synthesized by a simple hydrothermal method. Compared with pure MoS2, doping Nd successfully enhanced the SERS signal. The Enhancement factor (EF) of the 2 at% Nd-doped MoS2 is up to ∼1.29 × 106 due to the energy level transition and charge transfer effect. High sensitivity SERS detection of bilirubin, dipterex, and bisphenol A (BPA) was realized, and the lowest detection concentration was 10−8 M. Moreover, 2 at% Nd doped MoS2 as SERS substrate can realize high sensitivity SERS detection of bilirubin, dipterex and bisphenol A (BPA), and the limit of detection (LOD) is 10−8 M, 10−9 M, and 10−9 M, respectively. Furthermore, 2 at% Nd-doped MoS2 SERS substrate has high uniformity and excellent reproducibility, which is beneficial for practical applications. This work not only provides an efficient technique to get sensitive, stable, and low-cost SERS substrates but also advantageous for many application scenarios.

Flexible SERS strip based on HKUST-1(cu)/biomimetic antibodies composite multilayer for trace determination of ethephon

A surface-enhanced Raman scattering (SERS) sensor based on the folding and assembly characteristics of the three-dimensional structure of paper fibers, the skeleton controllability of metal-organic framework materials (MOFs), and the morphology designability of plasmonic noble metal materials has been established for rapid on-site determination of ethephon in food. HKUST-1(Cu) was assembled onto a carbon-treated chromatographic paper matrix by electrodeposition, and its skeleton respiration and sponge effect were used to overcome the bottleneck problem of poor affinity of SERS substrate for target molecules. Further coupled with the targeted recognition specificity of biomimetic antibodies, a paper-based interface with high specificity of molecular sensitivity was constructed. A sandwich multi-stage progressive enhancement structure was designed to couple plasma pine branch-shaped silver material in situ at the interface to realize superposition and collaborative amplification of SERS signals. When the paper-based strip sensor was used to monitor ethephon, it demonstrated a linear range of 10−3 to 10 mg kg−1 and a detection limit of around 1.39 × 10−4 mg kg−1. The construction and application of the paper-based HKUST-1(Cu)/biomimetic antibodies/pine branch-shaped silver material sensor will provide technical means and theoretical support for the rapid and efficient identification of biological ripening agent residues in food with multi-level signal enhancement.

Influence of sandwich-type DNA construction strategy and plasmonic metal on signal generated by SERS DNA sensors

The DNA biosensors are powerful tools in the gene mutation or pathogens detection. That is why there are a lot of DNA detection strategies and methods. Here we present the insight on a slightly overlooked DNA detection technique, surface-enhanced Raman scattering (SERS). The present work is a summary of the influence of the plasmonic metal of the SERS substrate and strategy of the sandwich-type biosensor construction, simply the placement of the Raman reporter and mismatches, on the SERS signal enhancement. We found that, although in general there is an increase in the intensity of the SERS signal when the distance between the Raman scatterer and the SERS-active surface decreases, for this type of DNA SERS sensor a greater intensity of the measured Raman signal is usually observed when the Raman reporter is farther away from the plasmonic substrate. This is probably caused by a significant change in the hybridisation efficiency for the different structures of the sensor analysed due to some steric hindrances.

Peroxidase-like Nanozymes for Point-of-Care SERS Sensing and Wound Healing.

The emergence of nanozymes provides a potential method for combating multidrug-resistant bacteria resulted from the abuse of antibiotics. However, in nanozyme-catalyzed systems, few studies have addressed the actual hydrogen peroxide (H2O2) level involved in sterilization. Herein, we designed a high-efficiency peroxidase-mimicking nanozyme with surface-enhanced Raman scattering (SERS) property by assembling gold nanoparticles on single-layer Cu2+-C3N4 (AuNP-Cu2+-C3N4). The nanozyme effectively converts the low-active Raman reporter 3,3',5,5'-tetramethylbenzidine (TMB) into its oxidized form with H2O2, resulting in SERS signal changes, thereby achieving highly sensitive quantification of H2O2 with limit of detection as low as 0.60 μM. More importantly, the nanozyme can specifically catalyze H2O2 into antibacterial hydroxyl radicals. In vitro and in vivo evaluations demonstrate the remarkable antibacterial efficacy of the nanozyme/H2O2 combination against Staphylococcus aureus (up to 99.9%), which could promote wound healing in mice and allow point-of-care monitoring the amount of H2O2 participated in effective sterilization. This study not only displays great potential in combining multiple functionalities of nanomaterials for versatile bioassays but also provides a promising approach to design nanozymes for biomedical and catalytic applications.

A cell wall-targeted organic-inorganic hybrid nano-catcher for ultrafast capture and SERS detection of invasive fungi

Due to the extended culture period and various inconveniences in vitro culture, the detection of invasive fungi is rather difficult, leading to high mortality rates of the diseases caused by them. It is, however, crucial for clinical therapy and lowering patient mortality to quickly identify invasive fungus from clinical specimens. A promising non-destructive method for finding fungi is surface-enhanced Raman scattering (SERS), however, its substrate has a low level of selectivity. Clinical sample components can obstruct the target fungi's SERS signal on account of their complexity. Herein, an MNP@PNIPAMAA hybrid organic-inorganic nano-catcher was created by using ultrasonic-initiated polymerization. The caspofungin (CAS), a fungus cell wall-targeting drug, is used in this study. We investigated MNP@PNIPAMAA-CAS as a technique to rapidly extract fungus from complex samples under 3 s. SERS could subsequently be used to instantly identify the fungi that were successfully isolated with an efficacy rate of about 75%. The entire process took just 10 min. This method is an important breakthrough that might be advantageous in terms of the rapid detection of invasive fungi.

Optical response of plasmonic silver nanoparticles after treatment by a warm microwave plasma jet

This work investigates the effect of plasma treatment on the morphology and composition of 15 × 15 mm2 silver nanoparticle (70–80 nm) thin films. The silver nanoparticles are deposited onto thermal silica (SiO2/Si) substrates by spin-coating, then they are treated by an open-to-air microwave argon plasma jet characterized by a neutral gas temperature of 2200 ± 200 K. Scanning electron microscopy analysis reveals that the number of isolated nanoparticles in the film sample decreases after exposure to multiple jet passes, and that polygonal structures with sharp corners and edges are produced. Similar structures with much rounder edges are obtained after conventional thermal annealing at temperatures up to 1300 K. Based on localized surface plasmon resonance analysis in the range of 350–800 nm, the main extinction band of silver nanoparticles experiences a redshift after treatment with the plasma jet or with thermal annealing. Moreover, both treatments induce surface oxidation of the nanoparticles, as evidenced by x-ray photoelectron spectroscopy. However, only the plasma-exposed samples exhibit a significant rise in the surface-enhanced Raman scattering (SERS) signal of oxidized silver at 960 cm−1. 29 29 μm2 mappings of hyperspectral Raman IMAging (RIMA) and multivariate curve resolution analysis by log-likelihood maximization demonstrate that the SERS signal is controlled by large-scale micrometer domains that exhibit sharp corners and edges.

SERS determination of hydroxy-α-sanshool in spicy hotpot seasoning: The strategy to restrain the interference of capsaicin and its mechanism

Hydroxy-α-sanshool (α-SOH) is the principal ingredient responsible for the numbing sensation in spicy hotpot. However, utilizing surface-enhanced Raman scattering (SERS) to analyze the α-SOH in hotpot seasoning is challenging due to the significant interference of capsaicin (CAP). Therefore, two schemes were proposed to address CAP interference in hotpot seasoning, namely laccase-catalyzed conversion and metal–organic framework (MOF) interaction. Among them, Fe-BTC MOF exhibited significant anti-interference effect and the underlying mechanism is elucidated. The motion of CAP aromatic ring was constrained by steric hindrance and electrostatic interactions of Fe-BTC. Additionally, the interaction between CAP aromatic ring and conjugated triene group in α-SOH was quenched, enhancing the α-SOH SERS signal. The proposed method had a significant anti-interference effect on α-SOH quantification in the presence of CAP, significantly enhancing the α-SOH SERS signal in a range of 0.85 to 4.00 × 107. The linearity and reproducibility of the proposed hotpot seasoning testing method were also validated.

Factor analysis of the time series of SERS spectra reveals water arrangement and surface plasmon changes in Ag nanoparticle systems

The enhancement of Raman signals of molecules localized in the vicinity of plasmonic nanoparticles, known as surface-enhanced Raman scattering (SERS) effect, is strongly influenced by the selected excitation wavelength. The optimal excitation wavelength in SERS measurements is given by the position of the surface plasmon extinction (SPE) band of the studied system. Even a small change of the SPE band intensity, position and/or shape during the measurement may influence the SERS signal significantly. In this work, we prepared several systems of Ag nanoparticles, which were used for the demonstration how the information about SPE changes can be obtained by multivariate statistical analysis (factor analysis; FA) from SERS spectral sets, and employed in more precise and more comprehensive interpretation of the results. In non-aggregated Ag colloidal systems measured at the excitation wavelength of 445 nm, SPE band changes could be monitored by the analysis of water stretching vibration together with the vibrations in the fingerprint region. The FA of the water stretching band region was shown to provide unique information on both arrangement and disarrangement of water molecules in the vicinity of Ag NPs during the time evolution of these SERS active systems. In addition, the FA of the fingerprint region helped to monitor a rapid metalation of meso-tetrakis(N-methyl-4-pyridyl)porphine in etched SERS systems with Ag+ ions released from the NPs surface. In aggregated Ag colloidal systems measured at the excitation wavelength of 785 nm, the FA of SERS spectral sets enabled us to reveal the contribution of the 2nd electromagnetic enhancement to the overall SERS signal. The reliability of our conclusions was verified by comparing the results obtained from FA of SERS spectral sets with the data obtained from the parallel SPE measurements of the studied systems.

Electroactive and SERS-Active Ag@Cu2O NP-Programed Aptasensor for Dual-Mode Detection of Tetrodotoxin.

Dual-mode nanotags with noninterference sensing signals improved the detection accuracy and sensitivity for the applications of tetrodotoxin (TTX) monitoring. Electroactive and surface-enhanced Raman scattering (SERS)-active Ag@Cu2O nanoparticles (NPs) were fabricated and displayed two electrooxidation signals at -0.13 and 0.17 V, attributed to the oxidization process of Cu+ and Ag0, respectively. Ag@Cu2O NPs were also found to exhibit stronger SERS performances than individual Ag NPs. The dielectric Cu2O shell with a large dielectric constant inhibited the attenuation of electromagnetic (EM) waves of Ag NPs, which strengthened the EM fields for SERS enhancement. The electron transfer from Ag to Cu2O to 4-aminothiophenol (4-ATP) also contributed to the SERS performances. Ag@Cu2O NPs were modified by TTX aptamers and assembled with MXene nanosheets (NSs) due to the large surface, good conductivity, and inherent Raman properties. The assemblies showed two-peaked electrooxidation signals and prominent SERS activity. An electrochemical detection curve was established by using the total peak intensity at -0.13 and 0.17 V as detection signals, and a ratiometric SERS detection curve was developed by applying the intensity at 1078 cm-1 (4-ATP) as the detection signal and 730 cm-1 (MXene NSs) as the reference signal. An electrochemical and SERS signal-programed dual-mode aptasensor was proposed for accurate TTX detection, with the limits of detection of 31.6 pg/mL for the electrochemical signal and 38.3 pg/mL for the SERS signal. The rational design of plasmonic metal-semiconductor heterogeneous nanocomposites had important prospects in establishing a multimodal biosensing platform for the quantitative and accurate detection of analytes in complex systems.

Amorphous Nitrogen-Doped Carbon Nanocages with Excellent SERS Sensitivity and Stability for Accurate Identification of Tumor Cells.

The surface-enhanced Raman scattering (SERS) bioprobe's strategy for identifying tumor cells always depended on the intensity difference of the Raman signal compared with that of normal cells. Hence, exploring novel SERS nanostructure with excellent spectra stability, a high enhancement factor (EF), and good biocompatibility is a primary premise for boosting SERS signal reliability and accuracy of tumor cells. Here, high SERS EF (5.52 × 106) is acquired by developing novel amorphous nitrogen-doped carbon (NDC) nanocages (NCs), whose EF value was in a leading position among carbon-based SERS substrates. In addition, a uniform SERS signal was obtained on NDC NCs due to homogeneous morphology and size. The delocalized carbon-conjugated systems of graphitic-N, pyrrole-N, and pyridine-N with lone pair electrons increase the electronic density of states and reduce the electron localization function of NDC NCs, thereby promoting the charge transfer process. The electron-donor platform of the NDC NCs facilitates the thermodynamic process of charge transfer, resulting in multimode vibrational coupling in the surface complexes, which greatly amplifies the molecular polarizability. Importantly, the good biocompatibility and signal stability endow these NDC NC SERS bioprobes unique superiority in distinguishing tumor cells, and quantitative recognition of two triple-negative breast cancer cells based on SERS detection mode has been successfully realized.

Potential Regulation for Surface-Enhanced Raman Scattering Detection and Identification of Carotenoids.

Surface-enhanced Raman scattering (SERS) is often impaired by the limited affinity of molecules to plasmonic substrates. Here, we use carbon fiber microelectrodes modified with silver nanoparticles as a plasmonic microsubstrate with tunable affinity for enrichment and molecular identification by SERS. The silver nanoparticles self-assemble by electrostatic interaction with diamine molecules that are electrochemically grafted onto the surface of the microelectrodes. β-carotene and trans-β-Apo-8'-carotenal, producing similar resonant SERS spectra, are employed as model molecules to study the effect of electroenrichment and SERS screening for different electrode potentials. The data show that at a characteristic electrode potential, the low affinity of polyene chains without hydrophilic groups to the substrate can be overcome. Different potentials were applied to recognize the two types of carotenoids by their typical SERS signal, and the applicability of this strategy was further confirmed in the environment of a real cell culture. The results indicate that by regulating the potential, carotenoid molecules with a similar molecular structure can be selectively quantified and identified by SERS. The developed SERS-active microelectrode is expected to help the development of portable, miniaturized point-of-care diagnostic SERS sensors.

Ratiometric response to formaldehyde by 3D silver SERS substrate with polyoxometalate as internal label

For surface-enhanced Raman scattering (SERS) analytical technology, the rational design of efficient substrate and effective internal standard is the key issue to implement the reliable detection. In this paper, a novel three-dimensional (3D) silver(I)-polyoxometalate complex of Ag2[Ag(trz)2][H6CrMo6O24]·2H2O (Ag3CrMo6, trz = 1, 2, 4-triazole) has been designed and synthesized, which can be used for the quantitative detection of formaldehyde (FA). Structural analysis reveals that the [H6CrMo6O24]3- polyoxoanions link Ag+ by diverse Ag−O(Mo−O) bonds to form a stable 3D framework. When Ag3CrMo6 is exposed to FA, the Ag+ can be partially reduced to Ag0 species to become the label-free SERS active substrate and and present the enhanced signals ascribed to the Mo−O vibrations. Due to that the reduced Ag0 species is only close to Mo−O groups, the Raman signals for Cr−O bonds are almost constant and can be used as internal standards. Therefore, the IMo−O/ICr−O values can be used as ratiometric SERS signals to fulfill the elusive SERS quantitative analysis for FA with the detection limit of 0.05 ng/L. This work opens up an innovative way to the rational design of label-free SERS substrate based on polyoxoanions and silver ions, by which the quantitative detection can be realized via the ratiometric Raman signals.

Dual-Aptamer-Assisted Ratiometric SERS Biosensor for Ultrasensitive and Precise Identification of Breast Cancer Exosomes.

Due to the heterogeneity of breast cancer, its early accurate diagnosis remains a challenge. Exosomes carry abundant genetic materials and proteins and are ideal biomarkers for early cancer detection. Herein, a ratiometric surface-enhanced Raman scattering (SERS) biosensor for exosome detection was constructed using a regularly arranged Au@Ag nanoparticles/graphene oxide (Au@Ag NPs/GO) substrate with 4-nitrothiophenol (4-NTP) molecules as an internal standard. Aptamers of two overexpressed proteins (epithelial cell adhesion molecule and human epidermal growth factor receptor 2) were linked by a short complementary DNA with rhodamine X modified at the 3'-terminal to form V-shaped double-stranded DNA, which attached to the surface of Au@Ag NPs/GO substrate for the selective recognition of breast cancer cell-derived exosomes. In the presence of exosomes, a competitive reaction occurred, resulting in the formation of the V-shaped double-stranded DNA/exosomes complex, and the V-shaped double-stranded DNA separated from the SERS substrate. The SERS signal of rhodamine X on the V-shaped double-stranded DNA decreased with the concentration of exosomes increasing, whereas the SERS signal of 4-NTP on the substrate remained stable. The ratiometric SERS strategy provides huge electromagnetic enhancement and abundant DNA adsorbing sites on the GO layer, achieving a wide detection range of 2.7 × 102 to 2.7 × 108 particles/mL and an ultralow limit of detection down to 1.5 × 102 particles/mL, without the requirement of any nucleic acid amplification. Particularly, the proposed method has significant applications in early cancer diagnosis as it can accurately identify breast cancer cell-derived exosomes in clinical serum samples and can differentiate pancreatic cancer patients and healthy individuals.