Electrochemically fabricated self-aligned 2-D silver/alumina arrays as reliable SERS sensors

Trends in Application of SERS Substrates beyond Ag and Au, and Their Role in Bioanalysis

This article compares the applications of traditional gold and silver-based SERS substrates and less conventional (Pd/Pt, Cu, Al, Si-based) SERS substrates, focusing on sensing, biosensing, and clinical analysis. In recent decades plethora of new biosensing and clinical SERS applications have fueled the search for more cost-effective, scalable, and stable substrates since traditional gold and silver-based substrates are quite expensive, prone to corrosion, contamination and non-specific binding, particularly by S-containing compounds. Following that, we briefly described our experimental experience with Si and Al-based SERS substrates and systematically analyzed the literature on SERS on substrate materials such as Pd/Pt, Cu, Al, and Si. We tabulated and discussed figures of merit such as enhancement factor (EF) and limit of detection (LOD) from analytical applications of these substrates. The results of the comparison showed that Pd/Pt substrates are not practical due to their high cost; Cu-based substrates are less stable and produce lower signal enhancement. Si and Al-based substrates showed promising results, particularly in combination with gold and silver nanostructures since they could produce comparable EFs and LODs as conventional substrates. In addition, their stability and relatively low cost make them viable alternatives for gold and silver-based substrates. Finally, this review highlighted and compared the clinical performance of non-traditional SERS substrates and traditional gold and silver SERS substrates. We discovered that if we take the average sensitivity, specificity, and accuracy of clinical SERS assays reported in the literature, those parameters, particularly accuracy (93-94%), are similar for SERS bioassays on AgNP@Al, Si-based, Au-based, and Ag-based substrates. We hope that this review will encourage research into SERS biosensing on aluminum, silicon, and some other substrates. These Al and Si based substrates may respond efficiently to the major challenges to the SERS practical application. For instance, they may be not only less expensive, e.g., Al foil, but also in some cases more selective and sometimes more reproducible, when compared to gold-only or silver-only based SERS substrates. Overall, it may result in a greater diversity of applicable SERS substrates, allowing for better optimization and selection of the SERS substrate for a specific sensing/biosensing or clinical application.

Silver-Based SERS Pico-Molar Adenine Sensor

Adenine is an important molecule for biomedical and agricultural research and applications. The detection of low concentration adenine molecules is thus desirable. Surface-enhanced Raman scattering (SERS) is a promising label-free detection and fingerprinting technique for molecules of significance. A novel SERS sensor made of clusters of silver nanostructures deposited on copper bumps in valleys of an etched silicon substrate was previously reported to exhibit a low and reproducible detection limit for a 10⁻¹¹ M neutral adenine aqueous solution. Reflection of laser illumination from the silicon surface surrounding a valley provides additional directions of laser excitation to adenine molecules adsorbing on a silver surface for the generation of enhanced SERS signal strength leading to a low detection limit. This paper further reports a concentration dependent shift of the ring-breathing mode SERS adenine peak towards 760 cm⁻¹ with decreasing concentration and its pH-dependent SERS signal strength. For applications, where the pH value can vary, reproducible detection of 10⁻¹² M adenine in a pH 9 aqueous solution is feasible, making the novel SERS structure a desirable pico-molar adenine sensor.

Effects of phase transformation on the ultraviolet optical properties of alumina clusters in aircraft plumes

Alumina particles experience phase transition as an undercooling process along the plume, during which the liquid alumina clusters transform into multiphase, and then into α phase. The phase transformation model was built by an improved diffusion limited aggregation (DLA) algorithm with monomers of stratified structure. The effects of phase transformation on the ultraviolet optical characteristics of alumina clusters were studied using the superposition T-matrix method (STMM). We found that the alumina clusters in phase transition had completely different optical properties compared with the fixed phase ones. Forward scattering, absorption efficiency and asymmetry parameter gradually decreased, whereas backward scattering, scattering efficiency, and single-scattering albedo gradually increased during the phase transformation process. Besides, multiphase alumina clusters were compared with the other two equivalent models, including the sphere model approximated by equivalent volume sphere (EVS) and the equivalent surface sphere (ESS) approaches and single-phase cluster model approximated by Maxwell-Garnett (MG) and Bruggeman (BR) approaches. Generally speaking, the optical properties of the single-phase cluster approximated by MG and BR approaches were relatively close to those of the real multiphase alumina cluster. Whereas the spheres approximated by EVS and ESS had great deviations, especially when the number of monomers in the cluster was 20, the relative error of scattering efficiency calculated by ESS was up to 52%. Therefore, approximate approaches for multiphase clusters should be chosen cautiously. Our results give further the understanding of the optical properties of alumina clusters. As the phase states are usually closely related to the plume radiation and burning process, these kinds of researches will be helpful to aircraft detection, identification, and other related fields.

Silver SERS Adenine Sensors with a Very Low Detection Limit

The detection of adenine molecules at very low concentrations is important for biological and medical research and applications. This paper reports a silver-based surface-enhanced Raman scattering (SERS) sensor with a very low detection limit for adenine molecules. Clusters of closely packed silver nanoparticles on surfaces of discrete ball-like copper bumps partially covered with graphene are deposited by immersion in silver nitrate. These clusters of silver nanoparticles exhibit abundant nanogaps between nanoparticles, where plasmonic coupling induces very high local electromagnetic fields. Silver nanoparticles growing perpendicularly on ball-like copper bumps exhibit surfaces of large curvature, where electromagnetic field enhancement is high. Between discrete ball-like copper bumps, the local electromagnetic field is low. Silver is not deposited on the low-field surface area. Adenine molecules interact with silver by both electrostatic and functional groups and exhibit low surface diffusivity on silver surface. Adenine molecules are less likely to adsorb on low-field sensor surface without silver. Therefore, adenine molecules have a high probability of adsorbing on silver surface of high local electric fields and contribute to the measured Raman scattering signal strength. We demonstrated SERS sensors made of clusters of silver nanoparticles deposited on discrete ball-like copper bumps with very a low detection limit for detecting adenine water solution of a concentration as low as 10⁻¹¹ M.

Grating-like SERS substrate with tunable gaps based on nanorough Ag nanoislands/moth wing scale arrays for quantitative detection of cypermethrin

Considering the complexity and high-consumption of the existing approaches to fabricate three-dimensional (3D) regular substrate templates, the scales of the moth wings with evenly-distributed nanoarrays were discovered to provide an ideal bioscaffold for metal silver (Ag) to decorate on to fabricate a flexible, highly-ordered, low-cost and large-scale Ag nanoislands/moth wing (Ag/MW) SERS-active substrate. The grating-like substrate with the optimal morphology of rough and hierarchical Ag nanoislands exhibited high enhancement factor (EF, ~4.16 × 10⁵), low detection limit (10^-10 M) to 4-aminothiophenol (4-ATP), outstanding signal uniformity (the relative standard deviations were less than 15%) and superior identification performance in the quantitative detection of pesticide cypermethrin. The three-dimensional finite-difference time-domain (3D-FDTD) method simulated the spatial distribution of the electric field intensity in the substrates with different morphologies, showing a potential strong enhancement of Raman signals in sub-10 nm gaps between two adjacent Ag nanoislands of different layers. These prominent SERS properties of novel Ag/MW SERS-active substrates suggest their potential value in rapid on-side biological and chemical sensing. Meanwhile, the highly-ordered nanoarrays of moth wings provide a new idea for the preparation of regular biomimetic nanomaterials.

Highly sensitive and scalable AAO-based nano-fibre SERS substrate for sensing application

Well-ordered periodic nanostructures are excellent substrates for many surface-enhanced Raman spectroscopy (SERS) applications. Conventional fabrication approaches such as high precision electron beam lithography or focused ion beam produce high resolution nano-features with great reproducibility at the expense of low throughput. In this work, a highly sensitive and scalable AAO-nano-fibre (ANF) SERS substrate is demonstrated by optimising the second anodisation time of the standard two-step anodisation of aluminium and performing an additional wet etching step on the resulting AAO substrate. The optimised ANF substrate exhibits SERS sensitivity that surpasses the AAO nanoholes and the metal-film-on-nanoparticles substrates. A detection limit of 0.1 nM is achieved with a signal-to-noise ratio of 2.6-3 using a low excitation power of 0.1 mW. The ANF substrate exhibits an enhancement factor of 9.28 × 106 and a standard deviation of no more than 8%. The results indicate that the highly sensitive and scalable ANF substrate is a promising substrate for commercial SERS application.

Controlled plasmon enhanced fluorescence by silver nanoparticles deposited onto nanotube arrays

Three-dimensional (3D) plasmonic nanostructures of porous alumina array (PAA) with silver nanoparticles (AgNPs) were prepared for enhancing fluorescence emission. In order to avoid fluorescence quenching effects and obtain clear fluorescence enhancement, the molecules were separated by using such 3D substrates, and the mean distance between the molecules and nanoparticles' surface can be easily controlled by changing the diameters of the PAA tube. It was found that the PAA tube with smaller size provides better fluorescence enhancement. Enhanced cross section, a new fluorescence enhanced factor, combined with the simulation of localized electromagnetic field enhancement was presented to understand the experimental results.

Probing substrate influence on graphene by analyzing Raman lineshapes

We provide a new approach to identify the substrate influence on graphene surface. Distinguishing the substrate influences or the doping effects of charged impurities on graphene can be realized by optically probing the graphene surfaces, included the suspended and supported graphene. In this work, the line scan of Raman spectroscopy was performed across the graphene surface on the ordered square hole. Then, the bandwidths of G-band and 2D-band were fitted into the Voigt profile, a convolution of Gaussian and Lorentzian profiles. The bandwidths of Lorentzian parts were kept as constant whether it is the suspended and supported graphene. For the Gaussian part, the suspended graphene exhibits much greater Gaussian bandwidths than those of the supported graphene. It reveals that the doping effect on supported graphene is stronger than that of suspended graphene. Compared with the previous studies, we also used the peak positions of G bands, and I2D/IG ratios to confirm that our method really works. For the suspended graphene, the peak positions of G band are downshifted with respect to supported graphene, and the I2D/IG ratios of suspended graphene are larger than those of supported graphene. With data fitting into Voigt profile, one can find out the information behind the lineshapes.

Probing 2D sub-bands of bi-layer graphene

Investigations of Raman spectra and surface enhanced Raman spectra (SERS) of supported and suspended bilayer graphene were realized.

Fabrication of nanowire network AAO and its application in SERS

In this paper, nanowire network anodized aluminum oxide (AAO) was fabricated by just adding a simple film-eroding process after the production of porous AAO. After depositing 50 nm of Au onto the surface, nanowire network AAO can be used as ultrasensitive and high reproducibility surface-enhanced Raman scattering (SERS) substrate. The average Raman enhancement factor of the nanowire network AAO SERS substrate can reach 5.93 × 106, which is about 14% larger than that of commercial Klarite® substrates. Simultaneously, the relative standard deviations in the SERS intensities are limited to approximately 7%. All of the results indicate that our large-area low-cost high-performance nanowire structure AAO SERS substrates have a great advantage in chemical/biological sensing applications.

Surface-enhanced Raman scattering of suspended monolayer graphene

The interactions between phonons and electrons induced by the dopants or the substrate of graphene in spectroscopic investigation reveal a rich source of interesting physics. Raman spectra and surface-enhanced Raman spectra of supported and suspended monolayer graphenes were measured and analyzed systemically with different approaches. The weak Raman signals are greatly enhanced by the ability of surface-enhanced Raman spectroscopy which has attracted considerable interests. The technique is regarded as wonderful and useful tool, but the dopants that are produced by depositing metallic nanoparticles may affect the electron scattering processes of graphene. Therefore, the doping and substrate influences on graphene are also important issues to be investigated. In this work, the peak positions of G peak and 2D peak, the I2D/IG ratios, and enhancements of G and 2D bands with suspended and supported graphene flakes were measured and analyzed. The peak shifts of G and 2D bands between the Raman and SERS signals demonstrate the doping effect induced by silver nanoparticles by n-doping. The I2D/IG ratio can provide a more sensitive method to carry out the doping effect on the graphene surface than the peak shifts of G and 2D bands. The enhancements of 2D band of suspended and supported graphenes reached 138, and those of G band reached at least 169. Their good enhancements are helpful to measure the optical properties of graphene. The different substrates that covered the graphene surface with doping effect are more sensitive to the enhancements of G band with respect to 2D band. It provides us a new method to distinguish the substrate and doping effect on graphene. PACS: 78.67.Wj (optical properties of graphene); 74.25.nd (Raman and optical spectroscopy); 63.22.Rc (phonons in graphene).

Layer-dependent morphologies of silver on n-layer graphene

The distributions of sizes of silver nanoparticles that were deposited on monolayer, bilayer, and trilayer graphene films were observed. Deposition was carried out by thermal evaporation and the graphene films, placed on SiO2/Si substrates, were obtained by the mechanical splitting of graphite. Before the deposition, optical microscopy and Raman spectroscopy were utilized to identify the number of the graphene layers. After the deposition, scanning electron microscopy was used to observe the morphologies of the particles. Systematic analysis revealed that the average sizes of the nanoparticles increased with the number of graphene layers. The density of nanoparticles decreased as the number of graphene layers increased, revealing a large variation in the surface diffusion strength of nanoparticles on the different substrates. The mechanisms of formation of these layer-dependent morphologies of silver on n-layer graphene are related to the surface free energy and surface diffusion of the n-layer graphene. The effect of the substrate such as SiO2/Si was investigated by fabricating suspended graphene, and the size and density were similar to those of supported graphene. Based on a comparison of the results, the different morphologies of the silver nanoparticles on different graphene layers were theorized to be caused only by the variation of the diffusion barriers with the number of layers of graphene.

Observation of strain effect on the suspended graphene by polarized Raman spectroscopy

We report the strain effect of suspended graphene prepared by micromechanical method. Under a fixed measurement orientation of scattered light, the position of the 2D peaks changes with incident polarization directions. This phenomenon is explained by a proposed mode in which the peak is effectively contributed by an unstrained and two uniaxial-strained sub-areas. The two axes are tensile strain. Compared to the unstrained sub-mode frequency of 2,672 cm-1, the tension causes a red shift. The 2D peak variation originates in that the three effective sub-modes correlate with the light polarization through different relations. We develop a method to quantitatively analyze the positions, intensities, and polarization dependences of the three sub-peaks. The analysis reflects the local strain, which changes with detected area of the graphene film. The measurement can be extended to detect the strain distribution of the film and, thus, is a promising technology on graphene characterization.

Growth and optical properties of silver nanostructures obtained on connected anodic aluminum oxide templates

Ag nanostructures are grown by AC electrodeposition on anodic alumina oxide (AAO) connected membranes acting as templates. Depending on the thickness of the template and on the voltage applied during the growth process, different Ag nanostructures with different optical properties are obtained. When AAO membranes about 1 μm thick are used, the Ag nanostructures consist in Ag nanorods, at the bottom of the pores, and Ag nanotubes departing from the nanorods and filling the pores almost for the whole length. When AAO membranes about 3 μm thick are used, the nanostructures are Ag spheroids, at the bottom of the pores, and Ag nanowires that do not reach the upper part of the alumina pores. The samples are characterized by angle resolved x-ray photoelectron spectroscopy, scanning electron microscopy and UV-vis and Raman spectroscopies. A simple NaOH etching procedure, followed by sonication in ethanol, allows one to obtain an exposed ordered array of Ag nanorods, suitable for surface-enhanced Raman spectroscopy, while in the other case (3 μm thick AAO membranes) the sample can be used in localized surface plasmon resonance sensing.

Plasmonic coupling of silver nanoparticles covered by hydrogen-terminated graphene for surface-enhanced Raman spectroscopy

We report on strong plasmonic coupling from silver nanoparticles covered by hydrogen-terminated chemically vapor deposited single-layer graphene, and its effects on the detection and identification of adenine molecules through surface-enhanced Raman spectroscopy (SERS). The high resistivity of the graphene after subjecting to remote plasma hydrogenation allows plasmonic coupling induced strong local electromagnetic fields among the silver nanoparticles to penetrate the graphene, and thus enhances the SERS efficiency of adenine molecules adsorbed on the film. The graphene layer protects the nanoparticles from reactive and harsh environments and provides a chemically inert and biocompatible carbon surface for SERS applications.