Sensitivity and Reusability of SiO2 NRs@ Au NPs SERS Substrate in Trace Monochlorobiphenyl Detection

Advancements in reusable SERS substrates for trace analysis applications

Surface Enhanced Raman Spectroscopy (SERS) technique is an effective analytical technique in which fingerprint information about analytes can be obtained, can provide detection limit performance at the single molecule level, and analyzes are performed in a single step without any intermediate steps. SERS technique offers additional benefits rather than other analytical techniques including high selectivity, ultrasensitive detection, uncomplicated protocols, in situ sampling, on-set capability and cost-effectiveness. As a result of the combination of developments in materials and nanotechnology science with the SERS analysis technique, this technique strengthens its use advantage day by day. The most important factor that limited the use of this technique was the fact that the solution containing the desired analyte(s) was dropped onto the SERS substrate and the same substrate could not be reused in subsequent analyses. To solve this problem, scientists have focused on developing reusable SERS substrates in recent years. In these studies, scientists basically used three SERS substrate cleaning applications (1) washing the SERS substrate with a suitable solvent that can elute the analyte from SERS surface after analysis, (2) cleaning the SERS substrate with catalytic degradation of analytes after analysis by modifying them with catalytic active materials and (3) Applying plasma cleaning procedure to SERS substrate after analysis and (4) applying adsorption and desorption procedure prior to SERS analysis. Herein, the aim of this review article is to evaluate the reusable SERS substrates-based methods based on their level of development and their potential to recycle. This review offers a coherent discussion on a wide range of sensing schemes employed in fabricating the SERS substrates. We utilized a critical approach in which elaborative examples were selected to highlight key shortcomings of various experimental configurations. In the same vein, there is a discussion of the advantages and limitations concerning the key instrumental advances and the expansion of the recent methods developed in this area.

GLAD Based Advanced Nanostructures for Diversified Biosensing Applications: Recent Progress

Glancing angle deposition (GLAD) is a technique for the fabrication of sculpted micro- and nanostructures under the conditions of oblique vapor flux incident and limited adatom diffusion. GLAD-based nanostructures are emerging platforms with broad sensing applications due to their high sensitivity, enhanced optical and catalytic properties, periodicity, and controlled morphology. GLAD-fabricated nanochips and substrates for chemical and biosensing applications are replacing conventionally used nanomaterials due to their broad scope, ease of fabrication, controlled growth parameters, and hence, sensing abilities. This review focuses on recent advances in the diverse nanostructures fabricated via GLAD and their applications in the biomedical field. The effects of morphology and deposition conditions on GLAD structures, their biosensing capability, and the use of these nanostructures for various biosensing applications such as surface plasmon resonance (SPR), fluorescence, surface-enhanced Raman spectroscopy (SERS), and colorimetric- and wettability-based bio-detection will be discussed in detail. GLAD has also found diverse applications in the case of molecular imaging techniques such as fluorescence, super-resolution, and photoacoustic imaging. In addition, some in vivo applications, such as drug delivery, have been discussed. Furthermore, we will also provide an overview of the status of GLAD technology as well as future challenges associated with GLAD-based nanostructures in the mentioned areas.

Recent advances in simultaneous detection strategies for multi-mycotoxins in foods

Mycotoxin contamination has become a challenge in the field of food safety testing, given the increasing emphasis on food safety in recent years. Mycotoxins are widely distributed, in heavily polluted areas. Food contamination with these toxins is difficult to prevent and control. Mycotoxins, as are small-molecule toxic metabolites produced by several species belonging to the genera Aspergillus, Fusarium, and Penicillium growing in food. They are considered teratogenic, carcinogenic, and mutagenic to humans and animals. Food systems are often simultaneously contaminated with multiple mycotoxins. Due to the additive or synergistic toxicological effects caused by the co-existence of multiple mycotoxins, their individual detection requires reliable, accurate, and high-throughput techniques. Currently available, methods for the detection of multiple mycotoxins are mainly based on chromatography, spectroscopy (colorimetry, fluorescence, and surface-enhanced Raman scattering), and electrochemistry. This review provides a comprehensive overview of advances in the multiple detection methods of mycotoxins during the recent 5 years. The principles and features of these techniques are described. The practical applications and challenges associated with assays for multiple detection methods of mycotoxins are summarized. The potential for future development and application is discussed in an effort, to provide standards of references for further research.

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

Surface-Enhanced Raman Scattering (SERS) is a powerful technology that provides abundant chemical fingerprint information with advantages of high sensitivity and time-saving. Advancements in SERS substrates fabrication allow Ag nanorods (AgNRs) possess superior sensitivity, high uniformity, and excellent reproducibility. To further promote AgNRs as a promising SERS substrate candidate to a broader application scope, oxides are integrated with AgNRs by virtue of their unique properties which endow the AgNRs-oxide hybrid with high stability and recyclability. Aside from SERS substrates fabrication, significant developments in quantitative analysis strategies offer enormous approaches to minimize influences resulted from variations of measuring conditions and to provide the reasonable data analysis. In this review, we discuss various fabrication approaches for AgNRs and AgNRs-oxide hybrids to achieve efficient SERS platforms. Then, we introduce three types of strategies which are commonly employed in chemical quantitative analysis to reach a reliable result. Further, we highlight SERS applications including food safety, environment safety, biosensing, and vapor sensing, demonstrating the potential of SERS as a powerful and promising technique. Finally, we conclude with the current challenges and future prospects toward efficient SERS manipulations for broader real-world applications.

Tuning Thermal Catalytic Enhancement in Doped MnO2-Au Nano-Heterojunctions

Sodium (Na)- and potassium (K)-doped δ-MnO₂, which presented different band gaps, were synthesized by a hydrothermal method. Then, uniform Au nanoparticles (NPs) were deposited on MnO₂ to form metal-semiconductor nano-heterojunctions (MnO₂-Au). By comparing their temperature-dependent thermal catalytic performances, p-aminothiophenol to p, p'-dimercaptoazobenzene conversion was used as proof-of-concept transformations. MnO₂-Au hybrid materials demonstrated better thermal catalytic performances relative to individual Au NPs. Meanwhile, K-doped MnO₂-Au, with a MnO₂ support displaying a narrower bandgap, displayed superior catalytic activities relative to Na-doped MnO₂-Au. To get the same catalytic performance by individual Au NPs, it can be ∼50 K less by Na-doped MnO₂-Au and ∼100 K less by K-doped MnO₂-Au. The enhancement is mainly attributed to the thermally excited electrons in MnO₂, which were transferred to Au NPs. The additional electrons in Au NPs increase the electron density and thus contribute to the improvement of thermal catalysis. Our findings show that the establishment of a nano-heterojunction formed by metal NPs on a semiconductor support has a significant impact on thermal catalysis, where a narrower band gap can facilitate thermally excited carriers and thus bring about better catalytic performances. Thus, the results presented here shed light on the design of a nano-heterojunction catalyst to approach reactions with superior performance under moderate conditions.

Robust, reproducible, recyclable SERS substrates: monolayers of gold nanostars grafted on glass and coated with a thin silica layer

We prepared and characterized recyclable surface enhanced Raman spectroscopy (SERS) active glass chips. Gold nanostars were grafted on properly functionalized glasses by means of electrostatic interactions and then they were coated with a silica layer of controllable thickness in the nanometer range. The SERS activity of the obtained substrates were tested in terms of reproducibility and homogeneity intra-samples and inter-samples from different batches using the Raman reporter as the model compound rhodamine 6G. The uncoated substrates were used as reference to evaluate the effect of silica spacers on SERS enhancement factors (EFs). The chemical route to obtain silica-coated SERS chips is described in detail, and the morphology and the optical response of substrates have been characterized. We demonstrate that SERS substrates coated with 1 nm silica conserve a good EF, and that the coating confers to the SERS platform an extreme robustness leading to reusability of the substrates.

Improved Thermal Stability of Graphene-Veiled Noble Metal Nanoarrays as Recyclable SERS Substrates

The ability to enhance the heat resistance of noble metals is vital to many industrial and academic applications. Because of its exceptional thermal properties, graphene was used to enhance the thermal stability of noble metals. Monolayer graphene-covered noble metal triangular nanoarrays (TNAs) showed excellent heat resistance, which could maintain their original triangular nanoarrays at high temperatures, whereas bare noble metal TNAs all agglomerate into spherical nanoparticles. On the basis of this mechanism, we obtained a universal recyclable surface-enhanced Raman scattering (SERS) substrate; after 16 cycles, the SERS substrate still worked well. The improvement of the heat resistance of noble metals by graphene has a great significance to the working reliability and service life of electronic devices and the single-use problem of traditional SERS substrates.

Synthesis of Ball-Like Ag Nanorod Aggregates for Surface-Enhanced Raman Scattering and Catalytic Reduction

In this work, ball-like Ag nanorod aggregates have been synthesized via a simple seed-mediated method. These Ag mesostructures were characterized by scanning electron microscope (SEM), transmission electron microscopy (TEM), ultraviolet-visible spectroscopy (UV-Vis), and X-ray diffraction (XRD). Adding a certain amount of polyvinyl pyrrolidone (PVP) can prolong its coagulation time. These Ag nanorod aggregates exhibit effective SERS effect, evaluated by Rhodamine 6G (R6G) and doxorubicin (DOX) as probe molecules. The limit of detection (LOD) for R6G and DOX are as low as 5 × 10^-9 M and 5 × 10^-6 M, respectively. Moreover, these Ag nanorod aggregates were found to be potential catalysts for the reduction of 4-nitrophenol (4-NP) in the presence of NaBH₄.