Fabrication of gold-coated PDMS surfaces with arrayed triangular micro/nanopyramids for use as SERS substrates

Flexible PDMS-Based SERS Substrates Replicated from Beetle Wings for Water Pollutant Detection

The flexible surface-enhanced Raman scattering (SERS) sensor, which has the bionic 3D nanoarray structure of a beetle-wing substrate (BWS), was successfully prepared by replicated technology and thermal evaporation. The bionic structure was replicated with polydimethylsiloxane (PDMS) and then silver (Ag) nanoisland thin films were deposited by thermal evaporation. The deposition times and thicknesses (25-40 nm) of the Ag thin films were manipulated to find the optimal SERS detection capability. The Ag nanoisland arrays on the surface of the bionic replicated PDMS were observed by scanning electron microscope (SEM), X-ray diffraction (XRD), and contact angle, which can generate strong and reproducible three-dimensional hotspots (3D hotspots) to enhance Raman signals. The water pollutant, rhodamine 6G (R6G), was used as a model molecule for SERS detection. The results show that 35 nm Ag deposited on a PDMS-BWS SERS substrate displays the strongest SERS intensity, which is 10 times higher than that of the pristine BWS with 35 nm Ag coating, due to the excellent 3D bionic structure. Our results demonstrate that bionic 3D SERS sensors have the potential to be applied in wearable devices and sensors to detect biomolecules and environmental pollutants, such as industrial wastewater, in the future.

Study of the Fabrication of Gold Nanoparticle-Graphene-Arrayed Micro/Nanocavities as SERS Substrates Compared to Two Different Angles of Triangular Pyramid Tips

Surface-enhanced Raman scattering (SERS) has attracted attention because of its enormous potential to detect molecules with low concentrations. The method of fabricating SERS substrates is of great importance for improving the detection resolution. However, SERS substrates with different triangular pyramid tips fabricated by using the tip-based nanoindentation method has not been reported. Here, we prepared arrayed micro/nanocavities on copper-based graphene using the continuous indentation method with a Berkovich tip and a cube-corner tip, which have different face angles. Gold nanoparticles were then sputtered onto the graphene-copper micro/nanocavities to form the Au@GR@Cu micro/nanocavities SERS substrates. The substrates formed using the Berkovich tip and cube-corner tip were labeled B2-B9 and C2-C9, respectively, in which the numbers indicate the machining feed. Rhodamine 6G (R6G) was employed, and the Raman intensities of R6G on the differently arrayed Au@GR@Cu micro/nanocavities were measured. The Raman intensities of R6G were stronger on the pile-ups than on the inverted triangular pyramid cavities. The Raman intensities of R6G were highest on the C2 and B2 structures and lowest on the C9 and B9 structures. The Raman intensities of R6G on the arrayed Au@GR@Cu micro/nanocavities fabricated by the cube-corner tip were stronger than those on the arrayed Au@GR@Cu micro/nanocavities fabricated using the Berkovich tip with the same machining feed. In addition, the electric field intensity and distribution of the B9 and C9 arrayed Au@GR@Cu were simulated using Comsol software. Au@GR@Cu structures fabricated by the cube-corner tip were generated with higher electric field intensities. Furthermore, the relative standard deviations at 1362 cm^-1 of R6G were 6.19 and 6.62% on the C2 and C4 surfaces, respectively, showing good homogeneity. The SERS spectra of 10^-9 mol/L malachite green solution and 10^-6 mol/L carbaryl solution were recognized on the C1, C2, and C4 surfaces on day 1 and after 3 months, respectively. After storage at room temperature for 3 months, the reductions in the Raman intensities were less than 10%, indicating excellent stability. The results showed that the arrayed Au@GR@Cu micro/nanocavities fabricated using the cube-corner tip performed better than those fabricated using the Berkovich tip and exhibited excellent uniformity, availability, and stability, providing great potential for detecting pesticides at low concentrations.

Development of Gold Nanoparticle-Based SERS Substrates on TiO2-Coating to Reduce the Coffee Ring Effect

Hydrophilic surface-enhanced Raman spectroscopy (SERS) substrates were prepared by a combination of TiO₂-coatings of aluminium plates through a direct titanium tetraisopropoxide (TTIP) coating and drop coated by synthesised gold nanoparticles (AuNPs). Differences between the wettability of the untreated substrates, the slowly dried Ti(OH)₄ substrates and calcinated as well as plasma treated TiO₂ substrates were analysed by water contact angle (WCA) measurements. The hydrophilic behaviour of the developed substrates helped to improve the distribution of the AuNPs, which reflects in overall higher lateral SERS enhancement. Surface enhancement of the substrates was tested with target molecule rhodamine 6G (R6G) and a fibre-coupled 638 nm Raman spectrometer. Additionally, the morphology of the substrates was characterised using scanning electron microscopy (SEM) and Raman microscopy. The studies showed a reduced influence of the coffee ring effect on the particle distribution, resulting in a more broadly distributed edge region, which increased the spatial reproducibility of the measured SERS signal in the surface-enhanced Raman mapping measurements on mm scale.

Transparent and flexible AuNSs/PDMS-based SERS substrates for in-situ detection of pesticide residues

Herein, we developed a PDMS-based flexible SERS substrate modified with gold nanostars (AuNSs) for in-situ detection of pesticide residues on fruit surfaces. AuNSs with sharp branches were assembled on aminated PDMS membrane by chemical bonding effect. The AuNSs/PDMS substrate showed good signal uniformity, stability and sensitivity using the Raman signal molecule 4-MBA and the detection concentration of 4-MBA was as low as 10^-8 mol/L. Then the AuNSs/PDMS substrate was used to detect methyl parathion (MP) standard solution. A good linear relationship between SERS intensity at 1342 cm^-1 and MP concentration was established in the range of 4 μg/mL-100 μg/mL, and the limit of detection was down to 1.946 μg/mL. Moreover, the AuNSs/PDMS substrate was directly covered on the surface of MP contaminated apple, and the Raman laser was incident from the back of substrate to achieve in-situ detection of pesticide residues. The recovery ratio indicated that the fabricated SERS substrate was realized successfully in real sample detection. AuNSs/PDMS substrate eliminates the requirement for sample pre-processing steps before testing, which can be used as a method for rapid inspection of pesticide in agricultural products on site.

Hexagonal arrays of plasmonic gold nanopyramids on flexible substrates for surface-enhanced Raman scattering

Surface-enhanced Raman spectroscopy (SERS) with pyramidal gold nanostructures increases the signal of Raman active analytes, since hotspots form at the edges and tip of a nanopyramid under illumination. 2D hexagonal arrays of pyramidal nanostructures with a quadratic base are fabricated through cost-effective nanosphere lithography and transferred onto elastomeric polydimethylsiloxane. By making use of the {111} crystal plane of a silicon (100) wafer, an inverted pyramidal array is etched, which serves as the complementary negative for the gold nanostructures. Either a continuous gold thin-film with protruding pyramids or separate isolated nanopyramids are produced. Three basic fabrication strategies are presented. The SERS enhancement is verified by Raman mapping of 4-mercaptobenzoic acid (4-MBA) molecules. Fabrication on a flexible substrate paves the way for future applications on curved surfaces orin situtunable resonances.