Fabrication of silver nanostructure array patterns (SNAPs) on silicon wafer for highly sensitive and reliable SERS substrates

Metal nanostructure arrays with large amounts of nano-gaps are important for surface enhanced Raman scattering applications, though the fabrications of such nanostructures are difficult due to the complex and multiple synthetic steps. In this research, we report silver nanostructure array patterns (SNAPs) on silicon wafer, which is fabricated with semiconductor manufacturing technology, Cu2O electrochemistry deposition, and Ag In-situ oxidation-reduction growth. Benefiting from the dense and uniform distribution of Ag nanowires, the fabricated SNAPs demonstrate a very strong and uniform surface-enhanced Raman scattering (SERS) effect. The efficiency of SNAPs was investigated by using rhodamine 6G (R6G) dye as an analyte molecule. The results show that the minimum detectable concentration of R6G can reach as low as 10-11 M, and the Raman signals in the random region show good signal homogeneity with a low relative standard deviation (RSD) of 4.77 %. These results indicate that the SNAPs perform a great sensitivity and uniformity as a SERS substrate. Furthermore, we used the SNAPs substrate to detect antibiotic sulfadiazine. The main peaks in sulfadiazine Raman and vibration modes assignments were obtained and the quantitative analysis model was established by principal component analysis (PCA). The detection and application results of sulfadiazine indicate that the SNAPs substrate can be applied for trace detection of antibiotics. In addition, we have cited the application of the SNAPs substrate in anti-counterfeiting labels. These practical applications demonstrate that the fabricated SNAPs can potentially provide a way to develop low-cost SERS platforms for environmental detections, biomedicine analysis, and commodities anti-counterfeiting.

Positively-charged plasmonic nanostructures for SERS sensing applications

Surface-enhanced Raman (SERS) spectroscopy has been establishing itself as an ultrasensitive analytical technique with a cross-disciplinary range of applications, which scientific growth is triggered by the continuous improvement in the design of advanced plasmonic materials with enhanced multifunctional abilities and tailorable surface chemistry. In this regard, conventional synthetic procedures yield negatively-charged plasmonic materials which can hamper the adhesion of negatively-charged species. To tackle this issue, metallic surfaces have been modified via diverse procedures with a broad array of surface ligands to impart positive charges. Cationic amines have been preferred because of their ability to retain a positive zeta potential even at alkaline pH as well as due to their wide accessibility in terms of structural features and cost. In this review, we will describe and discuss the different approaches for generating positively-charged plasmonic platforms and their applications in SERS sensing.

Integration of ligands equipped with quaternary amines on plasmonic surfaces generates positively-charged nanomaterials suitable for electrostatically binding negatively-charged species paving the way for their application in SERS sensing.

Mechanical power driven SPME-SERS ultra-fast detection of illegal additives in aquaculture water

A dual-function (extraction and detection) porous silver fiber with high enhancement effect was constructed based on a convenient electrochemical etching method. The prepared silver fiber not only had high enrichment capacity and good Surface Enhanced Raman Spectroscopy (SERS) performance but also had good laser stability and uniformity. A strategy combining mechanical power and integration of solid phase extraction (SPME) and SERS detection was used. Driven by mechanical power, the analyte malachite green (MG) was enriched on the prepared silver fiber after 40 seconds, which can realize an ultra-fast and sensitive detection with a detection limit of 8.48 × 10-9 M. At the same time, this fiber can be regenerated after being treated with NaBH4. The silver fiber can be used for the detection of MG and CV after being immersed in NaBH4 solution for a few minutes. After 5 cycles of processing, the measurement signals of the silver fiber can reach 70% of the initial signals. The mechanical power driven SPME-SERS (MPD-SPME-SERS) integrated detection method can be used to analyse aquaculture water within 1 minute with a good linear relationship.

Enhancing the Activity of Silver Nanowire Membranes by Electrochemical Cyclic Voltammetry as Highly Sensitive Flexible SERS Substrate for On-Site Analysis

The development of high-quality flexible surface-enhanced Raman spectroscopy (SERS) substrates is crucial for developing rapid SERS analysis in situ. Silver nanowire membranes as novel flexible substrates could benefit from the high collection efficiency of analytes by wrapping complex surfaces or wiping the surfaces of samples. However, their low SERS performance impedes further applications of silver nanowire membranes in analyte detection. Herein, we report an ultra-high-sensitivity silver nanowire membrane synthesized by a simple and time-saving cyclic voltammetry (CV) method. After CV treatment, a part of the silver nanowires on the silver nanowire membrane turned into small nanoparticles and nanorods. This nanostructure’s reconstitution increased the analytical enhancement factor of silver nanowire membranes by 14.4 times. Scanning and transmission electron microscopy, UV-vis spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy were employed to investigate the transformation in the membrane nanostructure. The CV-treated substrates exhibited high surface-enhanced Raman activity and good temporal stability. The limits of detection (LODs) for p-aminothiophenol, crystal violet, tetramethylthiuram disulfide, sodium perchlorate, malachite green, fluoranthene, and potassium nitrate are 3.7 × 10⁻¹² M, 5.1 × 10⁻¹¹ M, 5.4 × 10⁻¹¹ M, 6.3 × 10⁻⁹ M, 0.00693 ng, 0.0810 ng, and 0.0273 ng on this substrate, respectively. Additionally, the developed substrate is feasible for the detection of crystal violet in real samples. These results certify that CV-treated substrates possess broad application prospects in on-site SERS analysis.

Combining microextraction methods with surface-enhanced Raman spectroscopy towards more selective and sensitive analyte detection by plasmonic metal nanoparticles

Raman signals of analytes can be enhanced on the surface of noble nanoparticles by generating SERS signals, which can be further enhanced using microextraction (ME) techniques.

Surface-enhanced Raman scattering-based detection of hazardous chemicals in various phases and matrices with plasmonic nanostructures

Surface-enhanced Raman scattering (SERS)-based sensors utilize the electromagnetic-field enhancement of plasmonic substrates with the chemical specificity of vibrational Raman spectroscopy to identify trace amounts of a wide variety of different target analytes while being minimally affected by photobleaching. However, despite many advantageous features of this method, SERS sensors, particularly for detecting hazardous chemicals, suffer from several limitations such as requirement of gigantic signal enhancement that is often poorly controllable, subtle change and degradation of the SERS substrate, consecutive fluctuation of the signal, the lack of reliable receptors for capturing targets of interest and the absence of general principles for detecting various chemicals in different phases and matrices. To overcome these limitations and for SERS sensors to find practical use, one must (1) acknowledge the characteristics of the matrices of target systems, (2) finely engineer and tune the receptors of the SERS sensor to properly extract the target analyte from the phase, and (3) implement additional mechanistic modifications to enhance the plasmonic signal. This minireview underlines the difficulties associated with different phases and a wide range of target analytes, and introduces the practical measures undertaken to overcome the respective difficulties in SERS-based detection of hazardous chemicals.

Fabrication of Ag-nanosheets-built micro/nanostructured arrays via in situ conversion on Cu2O-coated Si nanocone platform and their highly structurally-enhanced SERS effect

A controllable and flexible route is presented for the fabrication of Ag-nanosheets-built micro/nanostructural ordered arrays via in situ conversion on the Cu₂O-coated silicon nanocone (SNC) platform in the AgNO₃-contained solution. The platform is pre-prepared by the reactive ion etching of the organic colloidal monolayer-covered silicon wafer, Cu sputtering deposition and in situ oxidation. The obtained Ag micro/nanostructured array consists of nearly spherical and micro-sized particles, which are hexagonally arranged on the substrate. The spherical particles are built of the vertically standing and cross-linked nanosheets with about 30 nm in thickness. This Ag-nanosheets-built array shows high number density of the edges and nanogaps as well as the robust and homogeneous structure. Its formation is attributed to the in situ conversion reaction on the Cu₂O-coated SNC platform and the preferentially-oriented connection of Ag nanoparticles. Such Ag array has shown significantly higher surface enhanced Raman scattering (SERS) activity than the Ag nanoparticles' film-covered SNC array, with the enhancement factor up to 10⁷ and the detection limitation down to ∼1 ppt level to the test molecules 4-aminothiophenol, as well as the good reproducibility in measurements. This study not only presents a controllable and flexible fabrication route to the plasmonic micro/nanostructured arrays but also provides the highly efficient and the practical chips for the SERS-based devices.

Solid-phase microextraction: An appealing alternative for the determination of endogenous substances - A review

The determination of endogenous substances is of great significance for obtaining important biotic information such as biological components, metabolic pathways and disease biomarkers in different living organisms (e.g. plants, insects, animals and humans). However, due to the complex matrix and the trace concentrations of target analytes, the sample preparation procedure is an essential step before the analytes of interest are introduced into a detection instrument. Solid-phase microextraction (SPME), an emerging sample preparation technique that integrates sampling, extraction, concentration, and sample introduction into one step, has gained wide acceptance in various research fields, including in the determination of endogenous compounds. In this review, recent developments and applications of SPME for the determination of endogenous substances over the past five years are summarized. Several aspects, including the design of SPME devices (sampling configuration and coating), applications (in vitro and in vivo sampling), and coupling with emerging instruments (comprehensive two-dimensional gas chromatography (GC × GC), ambient mass spectrometry (AMS) and surface enhanced Raman scattering (SERS)) are involved. Finally, the challenges and opportunities of SPME methods in endogenous substances analysis are also discussed.

Facing Challenges in Real-Life Application of Surface-Enhanced Raman Scattering: Design and Nanofabrication of Surface-Enhanced Raman Scattering Substrates for Rapid Field Test of Food Contaminants

Surface-enhanced Raman scattering (SERS) is capable of detecting a single molecule with high specificity and has become a promising technique for rapid chemical analysis of agricultural products and foods. With a deeper understanding of the SERS effect and advances in nanofabrication technology, SERS is now on the edge of going out of the laboratory and becoming a sophisticated analytical tool to fulfill various real-world tasks. This review focuses on the challenges that SERS has met in this progress, such as how to obtain a reliable SERS signal, improve the sensitivity and specificity in a complex sample matrix, develop simple and user-friendly practical sensing approach, reduce the running cost, etc. This review highlights the new thoughts on design and nanofabrication of SERS-active substrates for solving these challenges and introduces the recent advances of SERS applications in this area. We hope that our discussion will encourage more researches to address these challenges and eventually help to bring SERS technology out of the laboratory.

Silver nanoparticles supported onto a stainless steel wire for direct-immersion solid-phase microextraction of polycyclic aromatic hydrocarbons prior to their determination by GC-FID

The authors describe a new coating for use in solid-phase microextraction (SPME). Silver nanoparticles (AgNPs) were prepared by using gallic acid or glucose as the reducing agents, and then supported onto a stainless steel wire that was previously coated with a silver mirror. Coating with AgNPs was performed by a layer-by-layer approach of up to eight cycles of consecutive deposition of AgNPs and the thiol linker 1,8-octanedithiol. This procedure allows proper control of the coating thickness. Thicknesses are 3.2 μm and 3.5 μm with AgNPs obtained with gallic acid and glucose, respectively. This is in agreement with theoretical estimations (3.8 μm). The fibers were used in the direct-immersion SPME-GC-FID determination of 16 polycyclic aromatic hydrocarbons (PAHs) from different waters. The performance of the method was compared to the one using polydimethylsiloxane fibers (100 μm), which is the most suitable commercial SPME fiber for PAHs. Despite the low thickness of the AgNP coatings (compared to PDMS), the analytical features of the method using the most adequate coating (AgNPs prepared with gallic acid) include: (a) limits of detection down to 0.6 ng·mL^-1; (b) intra-day, inter-day, and inter-fiber precisions (expressed as RSDs) lower than 22, 26 and 25%, respectively; and (c) an operational lifetime of ~150 extractions/desorption cycles. The analysis of various spiked environmental waters using these fibers resulted in adequate analytical performance. Graphical abstract Silver nanoparticle based coatings for solid-phase microextraction fibers were prepared by a layer-by-layer approach. They were used for determination of 16 PAHs in waters by gas chromatography. Limits of detection are < 14 μg·L^-1 and intra-day, inter-day, and inter-fiber precisions are <26%.

Combination of a graphene SERS substrate and magnetic solid phase micro-extraction used for the rapid detection of trace illegal additives

Surface enhanced Raman scattering (SERS) is an ultra-sensitive spectroscopy technique, which can provide rich structural information for a great number of molecules, while solid phase micro-extraction (SPME) is an efficient method for sample pretreatment in analytical chemistry, particularly in a micro-system.

Large-volume constant-concentration sampling technique coupling with surface-enhanced Raman spectroscopy for rapid on-site gas analysis

In this work, a portable large-volume constant-concentration (LVCC) sampling technique coupling with surface-enhanced Raman spectroscopy (SERS) was developed for the rapid on-site gas analysis based on suitable derivatization methods. LVCC sampling technique mainly consisted of a specially designed sampling cell including the rigid sample container and flexible sampling bag, and an absorption-derivatization module with a portable pump and a gas flowmeter. LVCC sampling technique allowed large, alterable and well-controlled sampling volume, which kept the concentration of gas target in headspace phase constant during the entire sampling process and made the sampling result more representative. Moreover, absorption and derivatization of gas target during LVCC sampling process were efficiently merged in one step using bromine-thiourea and OPA-NH₄⁺ strategy for ethylene and SO₂ respectively, which made LVCC sampling technique conveniently adapted to consequent SERS analysis. Finally, a new LVCC sampling-SERS method was developed and successfully applied for rapid analysis of trace ethylene and SO₂ from fruits. It was satisfied that trace ethylene and SO₂ from real fruit samples could be actually and accurately quantified by this method. The minor concentration fluctuations of ethylene and SO₂ during the entire LVCC sampling process were proved to be <4.3% and 2.1% respectively. Good recoveries for ethylene and sulfur dioxide from fruit samples were achieved in range of 95.0-101% and 97.0-104% respectively. It is expected that portable LVCC sampling technique would pave the way for rapid on-site analysis of accurate concentrations of trace gas targets from real samples by SERS.

Au@Ag core–shell nanoparticles with a hidden internal reference promoted quantitative solid phase microextraction-surface enhanced Raman spectroscopy detection

Structural representation of the SPME-SERS fiber with an internal reference and the SERS detection.

Porous silver coating fiber for rapidly screening organotin compounds by solid phase microextraction coupled with surface enhanced Raman spectroscopy

Rapidly screening organotin by solid phase microextraction coupled with surface enhanced Raman spectroscopy.

Nanoporous Gold Disks Functionalized with Stabilized G-Quadruplex Moieties for Sensing Small Molecules

We report label-free small molecule sensing on nanoporous gold disks functionalized with stabilized Guanine-quadruplex (G4) moieties using surface-enhanced Raman spectroscopy (SERS). By utilizing the unique G4 topological structure, target molecules can be selectively captured onto nanoporous gold (NPG) disk surfaces via π-π stacking and electrostatic attractions. Together with high-density plasmonic "hot spots" of NPG disks, the captured molecules produce a remarkable SERS signal. Our strategy represents the first example of the detection of foreign molecules conjugated to nondouble helical DNA nanostructures using SERS while providing a new technique for studying the formation and evolution of G4 moieties. The molecular specificity of G4 is known to be controlled by its unit sequence. Without losing generality, we have selected d(GGT)₇GG sequence for the sensing of malachite green (MG), a known carcinogen frequently abused illegally in aquaculture. The newly developed technique achieved a lowest detectable concentration at an impressive 50 pM, two orders of magnitude lower than the European Union (EU) regulatory requirement, with high specificity against potential interferents. To demonstrate the translational potential of this technology, we achieved a lowest detectable concentration of 5.0 nM, meeting the EU regulatory requirement, using a portable probe based detection system.

Highly effective and uniform SERS substrates fabricated by etching multi-layered gold nanoparticle arrays

Gold nanoparticle multilayers printed on silicon substrates layer by layer were etched by a gold etchant to form highly effective and uniform substrates for surface-enhanced Raman scattering (SERS). The performance of the SERS substrates was systematically studied by adjusting the number of nanoparticle layers and the etching time. The optimized enhancement factor (EF) and the detection limit of the substrates were determined to be 8.6 × 10(6) and 1 × 10(-12) M, respectively. The high EF and low detection limit were attributed to the high density of "hot-spots" and the facile accession of probe molecules to these spots. Moreover, the SERS substrates exhibited a nice uniformity with a small spot-to-spot variation and a good sample-to-sample reproducibility as well. The experimental results were supported by finite-difference time domain (FDTD) simulations. Our study suggests that low-cost, large-scale, and uniform SERS substrates with a high EF and low detection limit can be achieved by using bottom-up chemical methods.

Diethyldithiocarbamate (DDTC) induced formation of positively charged silver nanoparticles for rapid in situ identification of inorganic explosives by surface enhanced Raman spectroscopy

Diethyldithiocarbamate could induce the generation of positively charged silver nanoparticles for rapidin situdetection of the explosives with a portable Raman spectrometer.

The highly sensitive and facile colorimetric detection of the glycidyl azide polymer based on propargylamine functionalized gold nanoparticles using click chemistry

A highly sensitive and facile colorimetric sensor for the glycidyl azide polymer explosive was developed based on propargylamine functionalized gold nanoparticles using click chemistry.