Ag Nanorods-Based Surface-Enhanced Raman Scattering: Synthesis, Quantitative Analysis Strategies, and Applications

Facile In-Situ photocatalytic reduction of AuNPs on multilayer Core-Shell Fe3O4@SiO2@PDA magnetic nanostructures and their SERS application

Surface-enhanced Raman scattering (SERS) is a promising analytical technique for the rapid, sensitive, and repeatable detection in various SERS application fields. Herein, a new type of potential magnetically recyclable SERS substrate was designed and rapidly synthesized via a facile three-step template method. First, the magnetic ferroferric oxide (Fe3O4) cores were prepared by a convenient solvothermal approach, and coated with a thin layer of silica by a sol-gel process in order to improve their stability in complicated environments. Next, the negatively charged polydopamine (PDA)/K6[SiW11VIVO40]·7H2O (PDA/SiW11V) outer shell was assembled upon the magnetic Fe3O4@SiO2 core-shell nanoparticles via a layer-by-layer sequential adsorption process using the stickiness of PDA. The SiW11V multilayer shell can be used as the subsequent photocatalytic reduction precursors for the in-situ loading of high-density gold nanoparticles (AuNPs), without any other organic additives. The AuNPs decorated multilayer core-shell Fe3O4@SiO2@PDA magnetic nanostructures were employed as a potential magnetically recyclable SERS substrate, and showed excellent SERS performance. Using crystal violet (CV) as a model target, the as-fabricated AuNPs modified multilayer core-shell Fe3O4@SiO2@PDA magnetic nanostructures SERS substrates exhibited significant enhancement, and pushed the detection limit down to 10-12 M. Aside from the ultrahigh sensitivity, these SERS substrates also possess an excellent reproducibility (relative standard deviation (RSD) ∼ 8.3%), long-term stability (75 days), and unique chemical stability capability in different organic solvents and different environments with pH ≤ 10. Furthermore, a real-life application is also performed by the detection of melamine in spiked milk solution using the as-prepared magnetic nanostructures SERS-active substrates (limit of detection (LOD), 10-8 M). These results highlight that the rational design and controllable synthesis of multifunctional magnetic SERS substrates is a promising strategy in many different application fields such as biosensing, photoelectrocatalysis, and medical diagnosis.

Highly Sensitive Detection of Formaldehyde by Laser-Induced Graphene-Coated Silver Nanoparticles Electrochemical Sensing Electrodes

Formaldehyde (HCHO) poses a grave threat to human health because of its toxicity, but its accurate, sensitive, and rapid detection in aqueous solutions remains a major challenge. This study proposes a novel electrochemical sensor composed of a graphene-based electrode that is prepared via laser induction technology. The precursor material, polyimide, is modified via the metal ion exchange method, and the detective electrode is coated with graphene and silver nanoparticles. And the special structure of graphene-coated Ag was demonstrated using scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM), and X-ray diffraction (XRD), Fourier transform infrared (FT-IR), and X-ray photoelectron spectroscopy (XPS) results show that graphene provides more sites for Ag NRs to be exposed and increases the surface area of contact between the solution and the detection object. In addition, differential pulse voltammetry (DPV) analysis exhibits high linearity over the HCHO concentration range from 0.05 to 5 μg/mL, with a detection limit of 0.011 μg/mL (S/N = 3). The Ag NPs in the electrochemical reaction will adsorb the intermediate •CO and •OH, catalyze their combination, and finally convert to CO2 and H2O, respectively. A microdetection device, specially designed for use with commercial micro-workstations, is employed to fully demonstrate the practical application of the electrode, which paves a way for developing formaldehyde electrochemical sensors.

SERS Immunosensors for Cancer Markers Detection

Early diagnosis and monitoring are essential for the effective treatment and survival of patients with different types of malignancy. To this end, the accurate and sensitive determination of substances in human biological fluids related to cancer diagnosis and/or prognosis, i.e., cancer biomarkers, is of ultimate importance. Advancements in the field of immunodetection and nanomaterials have enabled the application of new transduction approaches for the sensitive detection of single or multiple cancer biomarkers in biological fluids. Immunosensors based on surface-enhanced Raman spectroscopy (SERS) are examples where the special properties of nanostructured materials and immunoreagents are combined to develop analytical tools that hold promise for point-of-care applications. In this frame, the subject of this review article is to present the advancements made so far regarding the immunochemical determination of cancer biomarkers by SERS. Thus, after a short introduction about the principles of both immunoassays and SERS, an extended presentation of up-to-date works regarding both single and multi-analyte determination of cancer biomarkers is presented. Finally, future perspectives on the field of SERS immunosensors for cancer markers detection are briefly discussed.

Preparation and SERS performance of silver nanowires arrays on paper by automatic writing method

Silver nanowire ink was written on the surface of drawing paper by automatic writing method. Scanning electron microscopy was used to characterize the surface morphologies of the drawing paper before and after writing silver nanowires. The effects of fabrication parameters and measurement parameters on silver nanowires arrays were investigated. Crystal violet was selected as the probe molecule to study the SERS performance of silver nanowires arrays. The detection limit of crystal violet was as low as 10^-15 mol/L. The uniformity and repeatability of the arrays were also explored, and the relative standard deviation values were about 10%. Moreover, silver nanowires arrays were also relatively stable that SERS signals were still observed after ten weeks. Detection of the crystal violet residue was further achieved on the substrates by continuously pressing nine times. In addition, silver nanowires arrays were also applied to the quantitative analyses of 2, 2'-bipyridyl.

Exosome detection via surface-enhanced Raman spectroscopy for cancer diagnosis

As nanoscale extracellular vesicles, exosomes are secreted by various cell types, and they are widely distributed in multiple biological fluids. Studies have shown that tumor-derived exosomes can carry a variety of primary tumor-specific molecules, which may represent a novel tool for the early detection of cancer. However, the clinical translation of exosomes remains a challenge due to the requirement of large quantities of samples when enriching the cancer-related exosomes in biological fluids, the insufficiency of traditional techniques for exosome subpopulations, and the complex exosome isolation of the current commercially available exosome phenotype profiling approaches. The evolving surface-enhanced Raman scattering (SERS) technology, with properties of unique optoelectronics, easy functionalization, and the particular interaction between light and nanoscale metallic materials, can achieve sensitive detection of exosomes without large quantities of samples and multiplexed phenotype profiling, providing a new mode of real-time and noninvasive analysis for cancer patients. In the present review, we mainly discussed exosome detection based on SERS, especially SERS immunoassay. The basic structure and function of exosomes were firstly introduced. Then, recent studies using the SERS technique for cancer detection were critically reviewed, which mainly included various SERS substrates, biological modification of SERS substrates, SERS-based exosome detection, and the combination of SERS and other technologies for cancer diagnosis. This review systematically discussed the essential aspects, limitations, and considerations of applying SERS technology in the detection and analysis of cancer-derived exosomes, which could provide a valuable reference for the early diagnosis of cancer through SERS technology. STATEMENT OF SIGNIFICANCE: Surface-enhanced Raman scattering (SERS) has been applied to exosomes detection to obtain better diagnostic results. In past three years, several reviews have been published in exosome detection, which were narrowly focus on methods of exosome detection. Selection and surface functionalization of the substrate and the combination detection with different methods based on SERS will provide new strategies for the detection of exosomes. This review will focus on the above aspects. This emerging detection method is constantly evolving and contributing to the early discovery of diseases in the future.

Silver-Graphene Oxide Nanohybrids for Highly Sensitive, Stable SERS Platforms

Graphene oxide-silver nanoparticle nanohybrids were synthesized by simple reduction of the silver nitrate and graphene oxide (GO) mixture in water using the mild reducing agent ascorbic acid. The concentration of ascorbic acid was varied to verify the possible influence of the GO surface composition on the efficiency of the hybrid material as substrates for surface enhanced Raman spectroscopy (SERS). Furthermore, the composites were conditioned in ammonia solution or in potassium hydroxide diluted solution. For comparison, the graphene oxide-silver nanoparticle composite has been synthesized using the ammonia-treated GO. All materials were characterized using spectroscopic and microscopic methods including UV-Vis, infrared, and Raman spectroscopy and scanning electron microscopy. The SERS efficiency of the nanohybrids was tested using 4-aminothiophenol (PATP). The optimal synthesis conditions were found. Ammonia and potassium peroxide drop-casted on the composite changed the SERS properties. The sample treated with KOH showed the best SERS enhancement. The variation of the SERS enhancement was correlated with the shape of the UV-Vis characteristics and the surface structure of the composites.

Bioinspired protein corona strategy enhanced biocompatibility of Ag-Hybrid hollow Au nanoshells for surface-enhanced Raman scattering imaging and on-demand activation tumor-phototherapy

Silver-based hybrid nanoprobes for surface-enhanced Raman scattering (SERS) imaging show their tremendous potential for precise biological detection and mediated phototherapy. However, the severe toxicity induced by Ag to normal mammalian cells hinders its further application. Herein, we presented a versatile bioinspired protein corona strategy through assembling bovine serum albumin (BSA) protected Raman tag DTTC-conjugated Ag-hybrid hollow Au nanoshells (hollow AgAu-DTTC-BSA), which their silver ion release and reactive oxygen species (ROS) generation are significantly suppressed, enabling no damage to normal cells and tissues, but can be reactivated on-demand under laser-irradiation at the tumor site. These nanoshells could also produce strong localized surface plasmon resonance for efficient-stable photothermal effect and enhanced SERS activity under laser irradiation, approved by both theoretical and experimental calculations. Furthermore, the biocompatible hollow AgAu-DTTC-BSA could detect both primary tumor tissues and tiny liver metastases (~0.18 mm) in orthotopic/subcutaneous CT26 colon tumor-bearing mice models. We also demonstrate their excellent therapeutic efficacy for colorectal solid neoplasms by accurate SERS imaging-guided photothermal therapy, simultaneously assisted with toxic Ag ion and ROS. These results suggest that hollow AgAu-DTTC-BSA is promising imaging assisted photothermal agents for solid tumor theranostics and enhancing the potential of Ag-based nanoparticles for practical treatment.

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.