Surface Enhanced Raman Spectroscopy Hotspots Research Articles

Evaporation-induced self-assembly of gold nanorods on a hydrophobic substrate for surface enhanced Raman spectroscopy applications

The controllable assembly of plasmonic nanoparticles has developed as one of the most significant approaches for surface enhanced Raman spectroscopy (SERS) applications. This study developed a simple approach to improve a large-scale ordered assembly of gold nanorods (GNRs) by controlling the droplet evaporation mode on hydrophobic substrates. The hydrophobic substrate was efficiently produced by spin coating the silicone oil onto the glass slides and annealing them. The analyte molecule rhodamine (R6G) was employed as a surface-enhanced Raman scattering probe to demonstrate the potential effects of the synthesized arrays. This hydrophobic platform enables the concentration and delivery of analyte molecules into the surface enhanced Raman spectroscopy sensitive site while suppressing the coffee ring effect generated by the smooth contraction motion of the base contact radius of the droplet without any pinning. Thus, the limit of detection (LOD) of the R6G analyte was lowered to 10−10 M and the homogenous dispersion of surface enhanced Raman spectroscopy hotspots within the self-assembly reproducible surface enhanced Raman spectroscopy signal. This new method enables a broad range of packing patterns and mechanisms by changing the host nanoparticles in the dispersion.

Sustainable Surface-Enhanced Raman Substrate with Hexagonal Boron Nitride Dielectric Spacer for Preventing Electric Field Cancellation at Au-Au Nanogap.

Nanogaps between Au nanoparticles and Au substrates are the simplest systems that generate extremely high electric fields at hotspots for surface-enhanced Raman spectroscopy (SERS). However, the electric field cancellation at the hotspots in the systems can cause the reduction of Raman signal when two metallic materials are physically contacted due to the low concentration of analytes. Here, we propose an atomically thin hexagonal boron nitride (h-BN) shielding layer for Au substrates, which can be used as an insulating spacer to prevent electrical shorts at nanogaps. Experimental investigation of the SERS effect combined with theoretical studies by finite-difference time-domain simulations demonstrate that the Au NP/h-BN/Au substrate structure has excellent performance in electrical short prevention, thus facilitating ultrasensitive Raman detection. The outstanding chemical and thermal stability of h-BN allow the efficient recycling of the SERS substrate by protecting the Au surface during the removal of Au NPs and molecular analytes by chemical and thermal processes.

Gold nanoparticle incorporated inverse opal photonic crystal capillaries for optofluidic surface enhanced Raman spectroscopy.

Novel transducers are needed for point of care testing (POCT) devices which aim at facile, sensitive and quick acquisition of health related information. Recent advances in optofluidics offer tremendous opportunities for biological/chemical analysis using extremely small sample volumes. This paper demonstrates nanostructured capillary tubes for surface enhanced Raman spectroscopy (SERS) analysis in a flow-through fashion. The capillary tube integrates the SERS sensor and the nanofluidic structure to synergistically offer sample delivery and analysis functions. Inside the capillary tube, inverse opal photonic crystal (IO PhC) was fabricated using the co-assembly approach to form nanoscale liquid pathways. In the nano-voids of the IO PhC, gold nanoparticles were in situ synthesized and functioned as the SERS hotspots. The advantages of the flow-through SERS sensor are multifold. The capillary effect facilities the sample delivery process, the nanofluidic channels boosts the interaction of analyte and gold nanoparticles, and the PhC structure strengthens the optical field near the SERS hotspots and results in enhanced SERS signals from analytes. As an exemplary demonstration, the sensor was used to measure creatinein spiked in artificial urine samples with detection limit of 0.9 mg/dL.

Laser hybrid micro/nano-structuring of Si surfaces in air and its applications for SERS detection.

Surface enhanced Raman spectroscopy (SERS) has been widely investigated as an effective technique for low-concentration bio-chemical molecules detection. A rapid two-step approach to fabricate SERS substrates with high controllability in ambient air is developed. Dynamic laser ablation directly creates microgroove on the Si substrate. Meanwhile, nanoparticles are synthesized via the nucleation of laser induced plasma species and the air molecules. It configures the Si surface into four different regions decorated with nanoparticles at different sizes. With Ag film coating, these nanoparticles function as hotspots for SERS. Microsquare arrays are fabricated on the Si surface as large-area SERS substrates by the laser ablation in horizontal and vertical directions. In each microsquare, it exhibits quasi-3D structures with randomly arranged and different shaped nanoparticles aggregated in more than one layer. With Ag film deposition, uniform SERS signals are obtained by detecting the 4-methylbenzenethiol molecules. The SERS signal intensity is determined by the size and shape distributions of the nanoparticles, which depend on the laser processing parameters. With the optimal laser fluence, the SERS signals show a uniform enhancement factor up to 5.5 × 106. This provides a high-speed and low-cost method to produce SERS substrates over a large area.

Surface-Enhanced Raman Spectroscopy Hot-Spots on Ostwald Ripened Silver Nanoparticles Prepared by Galvanic Displacement

Silver particles grown on silicon in a galvanic displacement process undergo Ostwald ripening in which small particles merge into bigger ones. In this process the particles are shown by scanning electron microscopy to become smoother. Even though the surface area of the nanoparticles is halved on ripening, the Raman enhancement provided by them is at least five times higher than of the particles that did not undergo the ripening. In several regions, transmission electron microscopy (TEM) images show that the ripened particles resemble agglomerated colloids. In these regions the signal due to surface-enhanced Raman scattering (SERS) is greatly enhanced, suggesting that they are the SERS “hot spots”. The importance of boundaries formed between silver particles to the increased Raman signal is suggested.