Surface-enhanced Raman Spectroscopy (SERS)

Harrick Plasma → Applications → Alter Surface Chemisty → Surface-enhanced Raman Spectroscopy (SERS)

What is Surface-enhanced Raman Spectroscopy (SERS)?

Surface-enhanced Raman spectroscopy (SERS) is a modified form of Raman spectroscopy often used in the life sciences for cell analysis. SERS has also been used to detect chemical residues for pharmaceutical, agricultural, and forensic purposes. During SERS experiments, samples are irradiated with a laser and their plasmonic response (wavelength difference between incoming and outgoing waves) is measured, generating a spectroscopic output.

SERS provides a higher signal-to-noise ratio than ordinary Raman spectroscopy, making it useful for single molecule detection or detection of analytes in low concentrations. To improve the spectroscopic signal-to-noise ratio, SERS substrates create “hot-spots” — local regions of high signal intensity —via special morphological features. Common examples of these features include nano-wrinkled surfaces or metallic nanoparticles. Silver (Ag) and Gold (Au) nanoparticles are popular choices because their plasmons resonate at frequencies near those of commonly used lasers (visible, NIR).

Although SERS substrates often consist of expensive materials, plasma cleaning can remove contamination and analytes from the substrates without damaging them. This allows SERS substrates to be reused and lowers operational costs.

Harrick Plasma’s plasma cleaners have been used in numerous steps of the SERS substrate fabrication process. Below, you will discover how plasma treatments hydrophilize, clean, and modify the surface morphologies of SERS substrates. Additionally, you will learn how various plasma treatment parameters affect the sensitivity of SERS substrates to improve detection and performance.

How are Harrick Plasma’s products used to make SERS substrates?

Hydrophilizing SERS substrates

SERS substrates must maintain their structural integrity to prevent artifacts during measurement. This structural integrity can be enhanced by providing good adhesion between the various layers of the substrate. SERS substrates often consist of a bottom support material, a layer of metallic or polystyrene nanoparticles, and a layer of analyte in solution (ex: R6G, pesticides, cells). The bottom support material (such as a Si wafer) allows the substrate to be handled and transported. The nanoparticle layer increases the plasmonic output of the analyte layer. Lastly, the analyte layer contains the sample being studied. The analyte must adhere well to the nanoparticles to receive their enhanced plasmonic response.

Plasma treatment is often used to improve the adhesion between substrate layers by increasing their hydrophilicity. For example, Wen et. al. plasma treated a gold-plated silicon wafer before depositing gold nanoparticles. As a result of plasma treatment, the water contact angle of the Au-plated silicon wafer decreased by nearly a factor of 2 (from 79.6° to 40.2°). Since a decreased contact angle correlated with an increased hydrophilicity, the subsequent layer of gold nanoparticles adhered well to the Au-plated silicon wafer.

Several other SERS studies have relied on the hydrophilizing capabilities of plasma treatment:

• Liu et. al.’s SERS substrates consisted of a plasma-treated TEM grid, a silanization layer of 3-aminopropyltriethoxysilane (APTES), and then an arrangement of polyaniline-coated gold nanorods.
• Pandya et. al. plasma treated silica fibers to make them hydrophilic before covering them with polystyrene microspheres via nanosphere lithography.

Purwidyantri et. al.’s SERS substrates were made from a plasma-treated ITO/glass substrate, polystyrene nanospheres deposited via nanosphere lithography, an adhesion layer of either Cr or Al₂O­₃, and finally a layer of gold.

Cleaning SERS substrates to improve detection

Organic contamination on SERS substrates causes signal background levels and limits measurement reproducibility. This is particularly problematic when studying analytes which have low concentrations. Researchers often remove unwanted contamination by plasma cleaning their SERS substrates prior to an experiment:

• Henderson et. al. removed surface contamination from an Ag-nanorod array using argon plasma. The nanorod array was used to enhance the Raman spectroscopy signal of pneumoniae. Argon plasma cleaning was also used by Kawaguchi et. Al. to remove contamination from Ag-nanoparticle SERS substrates.

Tran et. Al. plasma cleaned their nanostructured substrates prior to detecting rhodamine 6G (R6G) and the pesticide paraxon. 

Cleaning SERS substrates for reuse

Reusing SERS substrates is financially beneficial, allowing their expensive components (Au, Ag) to be put to maximum use. However, the substrates must be thoroughly cleaned before reuse to avoid contamination from previous samples.

Plasma cleaning is commonly used to remove self-assembled monolayer (SAM) samples from SERS substrates prior to studying new samples:

• Lu et. al. used oxygen plasma to remove p-aminothiophenol (PATP) from a substrate containing silver nanoparticles. After plasma treatment, the SERS substrate was reused to detect methylene blue, rhodamine 6G, and other dyes.

Negri et. al. removed 1-propanethiol (C₃SH) from Ag-nanorod SERS substrates using argon plasma. The cleaned substrates were then coated with 1,2-bis(4-pyridyl)ethylene (BPE) for further SERS studies. Short (≤ 4 min) Ar plasma treatments preserved about 60% of the BPE SERS signal intensity on reused substrates compared to the BPE signal from previously unused substrates. Additionally, argon plasma did not oxidize the silver nanorods, avoiding a common cause of SERS signal reduction.

Modifying surface morphologies of SERS substrates

One cause of SERS substrates’ excellent signal-to-noise ratio is their distinct surface morphology. As discussed in Yu, et. al., the plasmonic response of a substrate can be enhanced by adding components with edges or sharp corners. These features cause a large density of “hot-spots”, whose combined signals are many orders of magnitude stronger than those from ordinary substrates.

A common method to create such features involves wrinkling a polydimethylsiloxane (PDMS) layer using plasma treatment. During O₂ plasma treatment of a stretched PDMS layer, a hydrophilic silica (SiO₂) film forms on the surface. Due to the difference in Young’s moduli of the silica film and the underlying PDMS, the silica becomes nano-wrinkled upon removing the applied strain. After performing this process, both Li et. al. and Zhang et. al. sputter coated the nano-wrinkled PDMS sheet with silver nanoparticles. The substrates in both studies were transparent, flexible, and robust under repeated folding. These qualities allowed excellent SERS detection of pesticide residues on fruits and vegetables. As an alternative to silver nanoparticles, polystyrene-capped nanobricks can be fabricated on the wrinkled PDMS, as performed by Chen et. al.

Optimization of plasma treatment parameters

Optimal plasma treatment parameters for SERS substrates depend on the substrate material. Possible changes in substrate chemistry and morphology must be considered when choosing a plasma gas, treatment time, and gas flow rate.

• Singh et. al. explored the effect of various plasma gases (Ar, N­₂, O₂) on a cost-effective graphene oxide (GO) SERS substrate. All three plasma gases reduced the Raman peak intensities of the GO; this effect was more pronounced during the first 10 seconds of treatment for Ar and N₂, whereas no change was seen during the first 10 seconds of O₂ For exposure times longer than 10s, longer plasma treatment times (using any gas) reduced the Raman peak intensities. Plasma treatment gases also affected the SERS enhancement factor (EF) when detecting the dye Rhodamine B. Compared to N₂ and 10% H₂/90%Ar, pure Ar produced the highest EF for Rhodamine B, regardless of the dye concentration.
• Hosomi et. al. investigated the effect of Ar plasma on silver nanoparticle (Ag-NP) SERS substrates. As noted by Capaccio et. al., SERS substrates containing silver should not be plasma treated with O₂ or air due to resultant oxidation and loss of plasmonic activity. To preserve the plasmonic activity of Ag-NP substrates, Hosomi observed that short (<15s) Ar plasma treatments were required. Additionally, Ar treatments longer than 10 seconds caused unwanted morphological changes in the silver nanoparticles.
• Jeong et. al. used oxygen plasma under various conditions to treat hollow porous gold-nanoshell (HPAuNS) SERS substrates. These nanoshells consisted of Au nanoparticles adsorbed onto polystyrene colloids. At long O₂ plasma treatment times (> 3min) or high (30 sccm) O₂ flow rates, the Au nanoparticles became sintered at their contact points, creating poorly defined interfaces and reducing the SERS sensitivity of the substrates at wavelengths > 800 nm. Jeong therefore recommended short O₂ plasma treatments at low flow rates to preserve the SERS response of these substrates.