Fabrication Of Surface-enhanced Raman Scattering Substrates Research Articles

Developing highly reliable SERS substrates based on Ag grown on alumina nanomeshes anodized under 1 V for efficiently sensing Raman-active molecules

We have developed silver-nanostructures grown on anodic alumina nanomesh (AAN) films to create new-type substrates for surface-enhanced Raman scattering (SERS). AAN with uniform thin sidewall of ∼ 5 nm was fabricated by anodizing Al sheets at 1 V in 6% H3PO4 solution. Subsequent AC electrochemical deposition of silver created an array of nanoparticles or nano-islands depending on growth time. The particle-island transition is non-monotonic evolution, since metal growth and dissolution compete in AC electrodeposition process. Systematic SERS study on various Ag-AAN films with trial probes of adenine solutions reveals collective contribution of electron-plasma oscillation and surface area of silver nanostructures in Raman enhancements. SERS signals are primarily contributed by surface area under excitation wavelength of 532 nm (away from plasmonic resonance). The average correlation coefficient between the SERS intensity and surface area was 0.85, indicating robust correlation. This value was reduced to 0.61 under excitation wavelength of 633 nm (closer to plasmonic resonance). Furthermore, increased Ag-deposition reduced the relative standard deviation of SERS intensities and thus improved both the uniformity and quality consistency of SERS substrates. Therefore, fabrication of SERS substrate with larger Ag surface area under similar SERS enhancement factors is suggested for high throughput in commercial sectors.

Centrifugation-based separation of triangular silver nanoplates from multi-shaped colloidal silver nanoparticles for fabrication of surface-enhanced Raman-scattering substrates

Centrifugation-based separation of triangular silver nanoplates from multi-shaped colloidal silver nanoparticles for fabrication of surface-enhanced Raman-scattering substrates

The effect of size-asymmetry of plasmonic heterodimers in surface-enhanced Raman scattering

Assembling metal nanoparticles to provide strong electric field hot spots in their gap spaces for surface-enhanced Raman scattering (SERS) is a great challenge. Therefore, the optimization of assembly parameters is at the heart of design and fabrication of SERS substrates. Herein, we experimentally demonstrate the SERS activity of hierarchical assembled size-asymmetric heterodimers (HDs) of silver nanoparticles (AgNPs) to unveil the effect of size-asymmetry in SERS enhancement factors (EFs). The HDs show an optimum configuration in which maximum EF is achieved. To study the underlying physics of the plasmonic coupling in HDs, we use the discrete dipole approximation method. The theoretical results confirm that the effects of AgNPs on each other through their electric near-fields can be considere as the key point to generate strong hot spots in HDs for SERS.

Large-scale flexible metal-covered polymer nanopillar arrays as highly uniform and reproducible SERS substrates for trace analysis

While surface-enhanced Raman scattering (SERS) spectroscopy has been studied extensively due to its sensitivity to molecular vibration states, the fabrication of SERS substrates with homogeneous activity over a large area still remains difficult. Here, a facial fabrication of large-scale metal-covered polymer nanopillar arrays used as reliable SERS substrates was developed by nano-pouring process using porous alumina membranes. Our strategy integrates the advantages of low-cost production, high reproducibility and good biocompatibility because the flexible transparent polymer nanopillars are nondestructive to biomaterial surfaces. The as-prepared SERS substrates exhibit highly uniform Raman signals with low relative standard deviation values of 4% at 1169 cm−1 of CV and 8.1% at 1355 cm−1 of R6G, both with the concentration of 10−5 M. The results also show that more than 95% intensity within the limits of the ±10% deviation of the average intensity of signals, indicating excellent stability of SERS signals. In addition, the mechanically flexible substrates can be attached to surfaces with complex morphologies and the good transparency makes the excitation and collection of signals from the backside of the substrates possible, showing great potential in analytical chemistry, food safety, medical diagnostics and other practical SERS detections.

Statistical analysis of cuboctahedral silver nanocrystals Langmuir–Blodgett film SERS substrate

AbstractWith single‐molecule sensitivity, surface‐enhanced Raman scattering (SERS) has promised to be a powerful tool from chemical analysis to biosensors. However, problem with large signal fluctuations for reproducibility has impeded SERS from widespread application. The fabrication of SERS substrate with large and uniform enhancement in combination with an effective target analyte‐transfer process is crucial for applications of SERS for trace molecule detection. In this study, we performed laser‐scanning confocal mapping to characterize 3 SERS substrate samples. The substrate consists of a monolayer of cuboctahedral silver nanocrystals (AgNCs) made with Langmuir–Blodgett technique. The synthesis of the cuboctahedral AgNCs involves the use of polyvinylpyrrolidone (PVP) as a shape‐controlling agent that remains on the surface of the nanocrystals. Using PVP as an analyte, we investigate the temporal and spatial variation of PVP SERS signals from the cuboctahedral AgNC Langmuir–Blodgett film. Analyzing the enhancement probability distribution with a model allows quantitative characterization of the cuboctahedral AgNC SERS substrates. The results help to identify important SERS substrate characteristics that influence the performance of the SERS substrate.

Preparation and SERS performance of Au NP/paper strips based on inkjet printing and seed mediated growth: The effect of silver ions

Surface-enhanced Raman scattering (SERS) has been widely used in biomedical sensing with the advantages of high sensitivity and label-free. However, the fabrication of SERS substrates with good Raman activity, reproducibility, and low cost is still under development in practical applications. This paper presents a practicable method for fabricating Au NP/paper strips by using inkjet printing and seed mediated growth. Small gold seed synthesized by borohydride reduction was used as ink and printed on the filter paper. The printed gold seed grew in situ in the growth solution and formed the gold nanoparticle (Au NP)/paper strips. The fabricated paper strip was characterized by diffuse reflectance spectroscopy and scanning electron microscopy (SEM). The diffuse reflectance spectra indicated that the Au NP/paper strips had two local surface plasmon resonance (LSPR) peaks: the short one at around 540 nm and the long one located in the range of 640–840 nm. And the long LSPR peak firstly shifted to red then to blue with the increased concentrations of silver ions in growth solution. From the SEM images, the shape of grown Au NPs was diverse, including sphere, rod, ellipsoid, dimer, trimer, and big aggregates. We thought the short peak came from the LSPR of nanospheres and the transvers LSPR of rod and ellipsoid like particles, while the long peak mainly came from the plasmonic coupling of dimer along the inter-particle axis. The obtained Au NP/paper strip with the long peak located around 650 nm had the highest SERS activity, which could be attributed to the plasmon resonance induced local field enhancement and nanogap effect. Also, the SERS performance results indicated the printed SERS strips exhibited satisfied uniformity and stability, demonstrating the potential of Au NP/paper strip in real-world applications.

Fundamentals and applications of SERS-based bioanalytical sensing

Abstract Plasmonics is an emerging field that examines the interaction between light and metallic nanostructures at the metal-dielectric interface. Surface-enhanced Raman scattering (SERS) is a powerful analytical technique that uses plasmonics to obtain detailed chemical information of molecules or molecular assemblies adsorbed or attached to nanostructured metallic surfaces. For bioanalytical applications, these surfaces are engineered to optimize for high enhancement factors and molecular specificity. In this review we focus on the fabrication of SERS substrates and their use for bioanalytical applications. We review the fundamental mechanisms of SERS and parameters governing SERS enhancement. We also discuss developments in the field of novel SERS substrates. This includes the use of different materials, sizes, shapes, and architectures to achieve high sensitivity and specificity as well as tunability or flexibility. Different fundamental approaches are discussed, such as label-free and functional assays. In addition, we highlight recent relevant advances for bioanalytical SERS applied to small molecules, proteins, DNA, and biologically relevant nanoparticles. Subsequently, we discuss the importance of data analysis and signal detection schemes to achieve smaller instruments with low cost for SERS-based point-of-care technology developments. Finally, we review the main advantages and challenges of SERS-based biosensing and provide a brief outlook.

Immobilization of silver nanoparticles into POEGMA polymer brushes as SERS-active substrates

Surface-enhanced Raman scattering (SERS) has attracted a great deal of interest during the past four decades and emerged as an ultrasensitive optical technique for chemical and biomedical analysis. It is widely accepted that the facile fabrication of SERS substrates with high activity and good reproducibility is of crucial importance for their applications. Herein, we report on a fast and robust method for the synthesis and immobilization of silver nanoparticles (AgNPs) into poly(oligo(ethylene glycol) methacrylate) (POEGMA) brushes under mild conditions without using any reducing agents. POEGMA brushes of different chain lengths were synthesized directly on silicon wafers by surface-initiated atom transfer radical polymerization with various reaction time. X-ray photoelectron spectroscopy and field emission scanning electron microscope measurements indicated that the AgNPs were firmly and homogeneously embedded into POEGMA brushes. The resulting POEGMA–AgNP hybrid films were employed as SERS substrates for the detection of 4-aminothiophenol, giving rise to an enhancement factor of up to 1.9 × 106. The influence of the POEGMA's chain length on SERS performance was also investigated. Copyright © 2016 John Wiley & Sons, Ltd.

In situ fabrication of 3D Ag@ZnO nanostructures for microfluidic surface-enhanced Raman scattering systems.

In this work, we develop an in situ method to grow highly controllable, sensitive, three-dimensional (3D) surface-enhanced Raman scattering (SERS) substrates via an optothermal effect within microfluidic devices. Implementing this approach, we fabricate SERS substrates composed of Ag@ZnO structures at prescribed locations inside microfluidic channels, sites within which current fabrication of SERS structures has been arduous. Conveniently, properties of the 3D Ag@ZnO nanostructures such as length, packing density, and coverage can also be adjusted by tuning laser irradiation parameters. After exploring the fabrication of the 3D nanostructures, we demonstrate a SERS enhancement factor of up to ∼2 × 106 and investigate the optical properties of the 3D Ag@ZnO structures through finite-difference time-domain simulations. To illustrate the potential value of our technique, low concentrations of biomolecules in the liquid state are detected. Moreover, an integrated cell-trapping function of the 3D Ag@ZnO structures records the surface chemical fingerprint of a living cell. Overall, our optothermal-effect-based fabrication technique offers an effective combination of microfluidics with SERS, resolving problems associated with the fabrication of SERS substrates in microfluidic channels. With its advantages in functionality, simplicity, and sensitivity, the microfluidic-SERS platform presented should be valuable in many biological, biochemical, and biomedical applications.