Controlling the Nanoscale Gaps on Silver Island Film for Efficient Surface-Enhanced Raman Spectroscopy

Highly flexible copper tape decorated with Ag nanoarrays as ultrasensitive SERS platforms for multi-hazardous pollutant sensing

A highly flexible and cost-effective copper tape decorated with silver nanoparticles (Cu-TAg) has been developed for surface-enhanced Raman spectroscopy (SERS) sensing of multi-hazardous environmental pollutants. Highly ordered and spherical-shaped silver nanoarrays have been fabricated using a low-cost thermal evaporation method. The structural, morphological, and optical properties of Cu-TAg sensors have been studied and correlated to the corresponding SERS performances. The size of nanoparticles has been successively tuned by varying the deposition time from 5 to 25 s. The nanoparticle sizes were enhanced with an increase in the evaporation time. SERS investigations have revealed that the sensing potential is subsequently improved with an increase in deposition time up to 10 s and then deteriorates with further increase in Ag deposition. The highest SERS activity was acquired for an optimum size of ~ 37 nm; further simulation studies confirmed this observation. Moreover, Cu-TAg sensors exhibited high sensitivity, reproducibility, and recycling characteristics to be used as excellent chemo-sensors. The lower detection limit estimation revealed that it can sense even in the pico-molar range for sensing of rhodamine 6G and methylene blue. The estimated enhancement factor of the sensor is found to be 9.4 × 107. Molecular-specific sensing of a wide range of pollutants such as rhodamine 6G, alizarin red, methylene blue, butylated hydroxy anisole, and penicillin-streptomycin is demonstrated with high efficiencies for micromolar spiked samples. Copper tape functionalized with Ag arrays thus demonstrated to be a promising candidate for low-cost and reusable chemo-sensors for environmental remediation applications.

Fabrication of high quality electrochemical SERS (EC-SERS) substrates using physical vapour deposition

Screen-printed electrodes (SPEs) are an efficient and inexpensive substrate for electrochemical surface-enhanced Raman spectroscopy (EC-SERS) studies. Traditionally, the working electrode of the SPE is modified with either a colloidal paste of metal nanoparticles or an electrodeposited metallic film. These methods can be time-consuming and often produce non-uniform nanostructured films. Physical vapour deposition (PVD) is presented in this work as an efficient and effective alternative method for the production of SERS-active SPEs. SPEs coated with silver thin films via PVD show consistent and strong EC-SERS enhancement for the detection of two probe molecules, 4-aminothiophenol (p-ATP) and 5-(pyridine-4-yl)-1,3,4-oxadiazole-2-thiol (PYOT). The EC-SERS signal intensity for p-ATP and PYOT had coefficients of variation of 8.2% and 5.5%, respectively. More generally, these substrates show promise for reliable enhancement of molecules spanning diverse analytical applications.

Surface-Enhanced Raman Scattering and Fluorescence on Gold Nanogratings

Surface-enhanced Raman scattering (SERS) spectroscopy is a sensitive sensing technique. It is desirable to have an easy method to produce SERS-active substrate with reproducible and robust signals. We propose a simple method to fabricate SERS-active substrates with high structural homogeneity and signal reproducibility using electron beam (E-beam) lithography without the problematic photoresist (PR) lift-off process. The substrate was fabricated by using E-beam to define nanograting patterns on the photoresist and subsequently coat a layer of gold thin film on top of it. Efficient and stable SERS signals were observed on the substrates. In order to investigate the enhancement mechanism, we compared the signals from this substrate with those with photoresist lifted-off, which are essentially discontinuous gold stripes. While both structures showed significant grating-period-dependent fluorescence enhancement, no SERS signal was observed on the photoresist lifted-off gratings. Only transverse magnetic (TM)-polarized excitation exhibited strong enhancement, which revealed its plasmonic attribution. The fluorescence enhancement showed distinct periodic dependence for the two structures, which is due to the different enhancement mechanism. We demonstrate using this substrate for specific protein binding detection. Similar periodicity dependence was observed. Detailed theoretical and experimental studies were performed to investigate the observed phenomena. We conclude that the excitation of surface plasmon polaritons on the continuous gold thin film is essential for the stable and efficient SERS effects.

Near-Field Enhancement and Polarization Selection of a Nano-System for He-Ne Laser Application

In this paper, we focus on transmission behavior based on the single aperture with a scatter. Both the near-field enhancement and polarization selection can be achieved numerically with a proposed nano-system under He-Ne laser wavelength. The nano-system consists of an Ag antenna, a wafer layer, an Ag film with an aperture and a dielectric substrate. Numerical results show that the near-field enhancement is related to the FP-like resonance base on surface plasmon polaritons (SPPs) in the metal-isolator-metal (MIM) waveguide for transverse magnetic (TM) polarization. The near-field optical spot is confined at the aperture export with a maximal electric intensity 20 times the value of the incident field for an antenna length of 430 nm. The transmission cutoff phenomenon for transverse electric (TE) polarization is because the transmission is forbidden for smaller aperture width. High extinction ratios of 9.6×10-8 (or 70.2 dB) and 4.4×10-8 (or 73.6 dB) with antenna lengths of 130 nm and 430 nm are achieved numerically with the nano-system. The polarization selective property has a good angular tolerance for oblique angles smaller than 15°. The spectral response is also investigated. We further demonstrate that the nano-system is applicable for another incident wavelength of 500 nm. Our investigation may be beneficial for the detection of polar molecules or local nano polarized nanosource.