Preparation of a SERS substrate and its sample-loading method for point-of-use application

Large-area and low-cost SERS substrates based on a gold-coated nanostructured surface fabricated on a wafer-scale

This paper demonstrates a method to fabricate plasmonic nanostructures over a large area that can be implemented as SERS substrates. The proposed method comprises batch processes such as spin coating, reactive ion etching, and thin metal deposition. These processes can be performed on large wafers, resulting in large numbers of SERS substrates in a single run. The effects of different process parameters were studied to optimize the performance of the SERS substrates. The study of sensitivity on the optimized SERS substrates was conducted using the SERS-active molecule pMBA. The SERS substrates thus fabricated were able to detect molecule concentrations as low as 100 nM. The SERS substrates were also evaluated for uniformity across the sample and for sample-to-sample reproducibility. Finally, the SERS substrates were applied to demonstrate label-free detection of organophosphorous pesticides – paraoxon ethyl and paraoxon methyl.

A simple and novel fabrication process for fabricating a uniform and reproducible SERS substrate over a large area.

AgNPs decorated 3D bionic silicon nanograss arrays pattern with high-density hot-spots for SERS sensing via green galvanic displacement without additives

Surface-enhanced Raman scattering (SERS) sensing has always been considered as a kind of high-efficiency analysis technique in different areas. Herein, we report a AgNPs decorated 3D bionic silicon (Si) nanograss SERS substrate with higher sensitivity and specificity by green galvanic displacement. The Si nanograss arrays are directly grown on a Si substrate via catalyst-assisted vapor–liquid–solid (VLS) growth and subsequent plasma interaction. AgNPs were rapidly immobilized on Si nanograss arrays without any organic reagents, and avoiding the interference signal of additives. The AgNPs decorated 3D bionic silicon nanograss arrays not only possess a larger specific surface area (loading more reporter molecules), but also provide a potential distribution and arrangement for plentiful hot spots. Using Rhodamine 6G (R6G) as a probe molecule, the prepared SERS substrates exhibited great potential for high-sensitivity SERS sensing, and pushed the limit of detection (LOD) down to 0.1 pM. A higher Raman analytical enhancement factor (AEF, 3.3 × 10⁷) was obtained, which was two magnitudes higher than our previous Ag micro–nano structures. Additionally, the practicality and reliability of our 3D bionic SERS substrates were confirmed by quantitative analysis of the spiked Sudan I in environmental water, with a wide linear range (from 10⁻¹⁰ M to 10⁻⁶ M) and low detection limit (0.1 nM).

The Si nanograss arrays are directly grown on Si substrate via catalyst-assisted VLS growth and subsequent plasma interaction. AgNPs were rapidly immobilized on Si nanograss arrays for SERS sensing, without any organic reagents and additives.

Fabrication of uniform arrays of silver nanoparticles on silicon by electrodeposition in ethanol solution and their use in SERS detection of difenoconazole pesticide

Surface-Enhanced Raman Scattering (SERS) is a technique currently widely used in the identification and quantification of organic and biological molecules at low concentrations, in which an important application is the detection of pesticide residues in food. To accomplish this task, SERS substrates with high Raman enhancement factor and good reproducibility are required. One of the most commonly used SERS substrates is the SERS substrate made of silver nanoparticles immobilized on a solid substrate. In this report we first present the results of electrochemical deposition of silver nanoparticles on the silicon surface using ethanol electrolyte solution. Thanks to both factors, electrochemical deposition (instead of electroless) and ethanol electrolyte (instead of aqueous), under optimal conditions, on the surface of silicon a monolayer of silver nanoparticles grew, which are uniform in shape and size and are arranged very close to each other with nanometer separation. Next we report on the use of fabricated arrays of silver nanoparticles in the role of a SERS substrate. To test the performance of the SERS substrate, the probe molecules used were molecules of difenoconazole, a well-known fungicide. Results showed that difenoconazole could be detected with a detection limit of 0.023 ppm (5.6 × 10-8 M).

SERS substrates fabricated with star-like gold nanoparticles for zeptomole detection of analytes

This work presents the design of substrates for Surface Enhanced Raman Scattering (SERS) using star-like gold nanoparticles which were synthesized using a wet chemical method and functionalized with 1-dodecanethiol. This molecule allowed us to obtain a spacing of ∼2.6 nm among gold stars, which promoted the generation of SERS hotspots for single molecule detection. The gold nanoparticles were deposited on silicon substrates or on gold coated silicon substrates by using the Langmuir-Blodgett method which permitted the zeptomole detection of Rhodamine B (total moles per laser spot area). The Raman enhancement factor (EF) achieved for this level of detection was 10(12), and was obtained on the SERS substrate fabricated with the configuration: Si/Au film/Au nanoparticles. Raman spectra of the molecules TWEEN 20 and p-terphenyl were also measured in order to elucidate the effect of the molecule's length on the enhancement factor. According to these results, our SERS substrates without the gold film are useful for a minimum detection level of ∼10(-14) moles of analytes with sizes equal to or less than 1.3 nm and ∼10(-18) moles of analytes with the gold film (total moles per sample).

Polymer-mediated formation and assembly of silver nanoparticles on silica nanospheres for sensitive surface-enhanced Raman scattering detection

To impart a desired optical property to metal nanoparticles (NPs) suitable for surface-enhanced Raman scattering (SERS) applications, it is crucial to assemble them in two or three dimensions in addition to controlling their size and shape. Herein, we report a new strategy for the synthesis and direct assembly of Ag NPs on silica nanospheres (AgNPs-SiNS) in the presence of poly(ethylene glycol) (PEG) derivatives such as PEG-OH, bis(amino)-PEGs (DA-PEGs), and O,O'-bis(2-aminopropyl)PEG (DAP-PEG). They exhibited different effects on the formation of Ag NPs with variable sizes (10-40 nm) and density on the silica surface. As the molecular weight (MW) of DA-PEGs increased, the number of Ag NPs on the silica surface increased. In addition, DAP-PEG (MW of 2000), which has a 2-aminopropyl moiety at both ends, promoted the most effective formation and assembly of uniform-sized Ag NPs on a silica surface, as compared to the other PEG derivatives with the same molecular weight. Finally, we demonstrated that AgNPs-SiNS bearing 4-fluorobenzenethiol on its surface induced the strong SERS signal at the single-particle level, indicating that each hybrid particle has internal hot spots. This shows the potential of AgNPs-SiNS for SERS-based sensitive detection of target molecules.

Aggregation kinetics of SERS-active nanoparticles in thermally stirred sessile droplets

The aggregation kinetics of silver nanoparticles in sessile droplets were investigated both experimentally and through numerical simulations as a function of temperature gradient and evaporation rate, in order to determine the hydrodynamic and aggregation parameters that lead to optimal surface-enhanced Raman spectroscopic (SERS) detection. Thermal gradients promote effective stirring within the droplet. The aggregation reaction ceases when the solvent evaporates forming a circular stain consisting of a high concentration of silver nanoparticle aggregates, which can be interrogated by SERS leading to analyte detection and identification. We introduce the aggregation parameter, Γa ≡ τ(evap)/τ(a), which is the ratio of the evaporation to the aggregation time scales. For a well-stirred droplet, the optimal condition for SERS detection was found to be Γ(a,opt) = kc(NP)τ(evap) ≈ 0.3, which is a product of the dimerization rate constant (k), the concentration of nanoparticles (cNP), and the droplet evaporation time (τ(evap)). Near maximal signal (over 50% of maximum value) is observed over a wide range of aggregation parameters 0.05 < Γa < 1.25, which also defines the time window during which trace analytes can be easily measured. The results of the simulation were in very good agreement with experimentally acquired SERS spectra using gas-phase 1,4-benzenedithiol as a model analyte.

Reduction of CuO butterfly wing scales generates Cu SERS substrates for DNA base detection

We prepare three-dimensional Cu plasmonic structures via a reduction of CuO photonic crystals replicated from butterfly wing scales. These Cu superstructures with high purity provide surface-enhanced Raman scattering (SERS) substrates for the label-free detection of DNA bases down to a micromolar level, which is achieved for the first time on Cu and even comparable to the detection-sensitivity for DNA bases on some Ag substrates. The generation of such superstructures has provided a substantial step for the biotemplated SERS substrates with high sensitivity, high reproducibility, and ultra-low cost to detect biomolecules, and presented affordable high-quality routine SERS consumables for corresponding biolaboratories.

Surface-enhanced Raman spectroscopy (SERS): progress and trends

Surface-enhanced Raman spectroscopy (SERS) combines molecular fingerprint specificity with potential single-molecule sensitivity. Therefore, the SERS technique is an attractive tool for sensing molecules in trace amounts within the field of chemical and biochemical analytics. Since SERS is an ongoing topic, which can be illustrated by the increased annual number of publications within the last few years, this review reflects the progress and trends in SERS research in approximately the last three years. The main reason why the SERS technique has not been established as a routine analytic technique, despite its high specificity and sensitivity, is due to the low reproducibility of the SERS signal. Thus, this review is dominated by the discussion of the various concepts for generating powerful, reproducible, SERS-active surfaces. Furthermore, the limit of sensitivity in SERS is introduced in the context of single-molecule spectroscopy and the calculation of the 'real' enhancement factor. In order to shed more light onto the underlying molecular processes of SERS, the theoretical description of SERS spectra is also a growing research field and will be summarized here. In addition, the recording of SERS spectra is affected by a number of parameters, such as laser power, integration time, and analyte concentration. To benefit from synergies, SERS is combined with other methods, such as scanning probe microscopy and microfluidics, which illustrates the broad applications of this powerful technique.

Localized oblique-angle deposition: Ag nanorods on microstructured surfaces and their SERS characteristics

In this paper, we demonstrate a simple and convenient method of depositing Ag nanorods on a substrate inside a standard evaporation chamber with the substrate resting on a leveled stage. Microstructuring the substrate prior to the deposition imparts a large incidence angle (>70°) between the collimated vapor atoms and the local surface normal, which is essential to induce the shadowing effect. Thereby, a localized oblique-angle deposition (LOAD) is realized, forming nanorods selectively on the steep sidewalls of surface microcavities patterned via standard photolithography and silicon dry etching. We also demonstrate that these nanorods can boost SERS activity of the underlying substrate and thus perform comparable to those fabricated via advanced patterning techniques or conventional OAD whereby the entire substrate has to be tilted with respect to the incident vapor atoms. Our results suggest the viability of decorating microchannel sidewalls with SERS-active nanorods for integrated sample processing and SERS detection.

Electroless silver plating of the surface of organic semiconductors

The integration of nanoscale processes and devices demands fabrication routes involving rapid, cost-effective steps, preferably carried out under ambient conditions. The realization of the metal/organic semiconductor interface is one of the most demanding steps of device fabrication, since it requires mechanical and/or thermal treatments which increment costs and are often harmful in respect to the active layer. Here, we provide a microscopic analysis of a room temperature, electroless process aimed at the deposition of a nanostructured metallic silver layer with controlled coverage atop the surface of single crystals and thin films of organic semiconductors. This process relies on the reaction of aqueous AgF solutions with the nonwettable crystalline surface of donor-type organic semiconductors. It is observed that the formation of a uniform layer of silver nanoparticles can be accomplished within 20 min contact time. The electrical characterization of two-terminal devices performed before and after the aforementioned treatment shows that the metal deposition process is associated with a redox reaction causing the p-doping of the semiconductor.

A review on the fabrication of substrates for surface enhanced Raman spectroscopy and their applications in analytical chemistry

This work reviews different types of substrates used for surface-enhanced Raman scattering (SERS) that have been developed in the last 10 years. The different techniques of self-assembly to immobilize metallic nanoparticles on solid support are covered. An overview of SERS platforms developed using nanolithography methods, including electron-beam (e-beam) lithography and focused ion beam (FIB) milling are also included, together with several examples of template-based methodologies to generate metallic nano-patterns. The potential of SERS to impact several aspects of analytical chemistry is demonstrated by selected examples of applications in electrochemistry, biosensing, environmental analysis, and remote sensing. This review shows that highly enhancing SERS substrates with a high degree of reliability and reproducibility can now be fabricated at relative low cost, indicating that SERS may finally realize its full potential as a very sensitive tool for routine analytical applications.

The size control of silver nanocrystals with different polyols and its application to low-reflection coating materials

The size of silver nanocrystals in polyol synthesis can be simply controlled by tuning the viscosity of the reaction medium such as ethylene glycol, 1,2-propanediol, 1,4-butanediol and 1,5-pentanediol. We found that a higher viscose medium (1,5-pentanediol) led to monodispersed smaller particles thanks to the slow addition of silver atoms into the nuclei. Size-controlled silver nanocrystals of 30 nm were obtained in a viscosity controlled medium of 1,5-pentanediol to synthesize a low refractive index filler by coating with silica and subsequent etching of the silver core. The coated low-reflection layer from the hollow silica nanoparticles on polyethylene terephthalate (PET) film can greatly reduce the reflection of the PET film from 10% to 2% over the entire visible region.

Electroless deposition of silver onto silicon as a method of preparation of reproducible surface-enhanced Raman spectroscopy substrates and tip-enhanced Raman spectroscopy tips

A simple method for the production of silver nanoparticles on a silicon substrate that is suitable for surface-enhanced Raman spectroscopy (SERS) is presented. The method is based on spontaneous reduction of Ag(+) ions by elemental silicon. The oxide layer is removed from the surface of a silicon disk by etching with dilute HF that is present in the same dilute solution of silver nitrate that is used to form the silver nanoparticles. By controlling the concentrations of HF and AgNO(3), the morphology of the deposited silver nanostructures can be varied dramatically. The reproducibility of SERS measurements for substrates produced with a given concentration of HF and AgNO(3) is good (relative standard deviation approximately 10%). The application was extended to coating the tips of silicon cantilevers designed for atomic force microscopy (AFM) with silver nanoparticles to permit measurements of tip-enhanced Raman spectra (TERS). The feasibility of TERS measurements with AFM tips prepared in this way is demonstrated.