Exploring Surface-Enhanced Raman Spectroscopy (SERS)

Surface-enhanced Raman spectroscopy (SERS) is an altered version of Raman spectroscopy commonly used in life sciences for cell analysis. It has also been utilized to detect chemical residues for agricultural, pharmaceutical, and forensic purposes. A laser is used throughout SERS experiments to irradiate the samples, and their plasmonic response (the difference in wavelengths between incoming and outgoing waves) is measured to generate a spectroscopic output.

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SERS offers a greater signal-to-noise ratio than ordinary Raman spectroscopy, so it is useful in single-molecule detection or detecting analytes in low concentrations. Improvement of the spectroscopic signal-to-noise ratio can be accomplished by using SERS substrates that create “hot spots”—local areas of high signal intensity—via unique morphological features.

Examples of these features commonly seen include nano-wrinkled surfaces or metallic nanoparticles. Gold (Au) and Silver (Ag) nanoparticles are popular due to their plasmons resonating at frequencies similar to those commonly used lasers, such as visible and NIR.

Even though SERS substrates commonly contain expensive materials, plasma cleaning can eliminate contaminants and analytes without damaging the substrates. This enables the reuse of SERS substrates and reduces operational costs.

Harrick Plasma’s plasma cleaners can be utilized in various steps of the SERS substrate fabrication process. This article will examine how plasma treatments clean, hydrophilize, and modify the surface morphologies of SERS substrates. It will also investigate how several plasma treatment parameters can affect the sensitivity of SERS substrates, improve performance and detection, and determine how Harrick Plasma’s products are used to make SERS substrates.

Hydrophilizing SERS Substrates

SERS substrates need to maintain structural integrity to prevent artifacts while being measured. Their structural integrity can be improved through sufficient layer adhesion throughout the substrate.

SERS substrates typically consist of lower support material, a polystyrene or metallic nanoparticles layer, and a layer of analyte in solution (for example, R6G, pesticides, or cells).

The support material on the bottom, such as a Si wafer, enables the substrate to be handled and moved. The nanoparticle layer heightens the analyte layer's plasmonic output, which holds the sample to be studied. The analyte must adhere well to the nanoparticles to benefit from their enhanced plasmonic response.

Plasma treatment is commonly used to improve adhesion between substrate layers by increasing their hydrophilicity. For example, Wen et al. plasma-treated a gold-plated silicon wafer before depositing gold nanoparticles. Due to the plasma treatment, the angle of water contact of the Au-plated silicon wafer lowered by a factor of 2 (from 79.6° to 40.2°). The decreased contact angle correlates with increased hydrophilicity, so the resulting layer of gold nanoparticles showed good adherence to the Au-plated silicon wafer.

Several other SERS studies have made use of the hydrophilizing characteristics of plasma treatment:

• Liu et al.’s SERS substrates contained a plasma-treated TEM grid, a silanization layer of 3-aminopropyltriethoxysilane (APTES), and a subsequent arrangement of polyaniline-coated gold nanorods.
• Pandya et al. plasma-treated silica fibers to give them hydrophilic capabilities before covering them with polystyrene microspheres via nanosphere lithography.
• Purwidyantri et al.’s SERS substrates consisted of a plasma-treated ITO/glass substrate, polystyrene nanospheres installed through nanosphere lithography, an adhesion layer of either Al₂O₃ or Cr, and a layer of gold.

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Cleaning SERS Substrates to Improve Detection

 SERS substrates exhibiting organic contamination display signal background levels and limited measurement repeatability. This is a particular problem during the study of low concentrations of analytes. Researchers commonly remove undesirable contaminations through plasma cleaning of the SERS substrates before an experiment:

• Henderson et al. eliminated surface contamination from an Ag-nanorod array utilizing argon plasma. The nanorod array was utilized to improve the Raman spectroscopy signal of pneumoniae. Kawaguchi et al. also used argon plasma cleaning to eliminate contamination from Ag-nanoparticle SERS substrates.
• Tran et al. plasma-cleaned the nanostructured substrates before detecting rhodamine 6G (R6G) and the pesticide paraxon.

Cleaning SERS Substrates for Reuse

Reusing SERS substrates has a financial benefit, as it allows their expensive components, like Au and Ag, to be used effectively. However, the substrates require thorough cleaning before reuse to avoid cross-contamination.

Plasma cleaning is often used to remove self-assembled monolayer (SAM) samples from SERS substrates before the study of new samples: Lu et al. made use of oxygen plasma to remove p-aminothiophenol (PATP) from a substrate that contained silver nanoparticles. The SERS substrate was reused after plasma treatment to discover methylene blue, rhodamine 6G, and other dyes.

Negri et al. eliminated 1-propanethiol (C3SH) from Ag-nanorod SERS substrates by utilizing argon plasma. The cleaned substrates were subsequently coated with 1,2-bis(4-pyridyl)ethylene (BPE) for additional SERS studies.

Short (≤ 4 min) Ar plasma treatments preserved approximately 60% of the BPE SERS signal intensity on reused substrates compared to the BPE signal from unused substrates. In addition, argon plasma did not cause oxidization of the silver nanorods, which is a common cause of SERS signal reduction.

Modifying Surface Morphologies of SERS Substrates

SERS substrates have an excellent signal-to-noise ratio. One cause of this is their distinct surface morphology. As Yu et al. described, adding components with sharp corners or edges can improve a substrate's plasmonic response. These characteristics promote a large density of “hot spots” with combined signals that are several orders of magnitude stronger than those from typical substrates.

A method commonly used to create these characteristics involves wrinkling a polydimethylsiloxane (PDMS) layer with plasma treatment. Throughout the O₂ plasma treatment of a stretched PDMS layer, a hydrophilic silica (SiO₂) film develops on the surface. The difference in Young’s moduli of the silica film and the PDMS causes the silica to become nano-wrinkled when the applied strain is removed.

Following this process, Li et al. and Zhang et al. sputter-coated the nano-wrinkled PDMS sheet with silver nanoparticles. The substrates in both studies were flexible, transparent, and robust during repeat folding. These features offer exceptional SERS detection of pesticide residues on produce. Alternatively to silver nanoparticles, polystyrene-capped nanobricks can be created on the wrinkled PDMS, as performed by Chen et al.

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Optimization of Plasma Treatment Parameters

The ideal plasma treatment parameters for SERS substrates depend on the substrate material. Potential substrate chemistry and morphology alterations must be considered when selecting a plasma gas, gas flow rate, and treatment time.

Singh et al. investigated the effects of various plasma gases (Ar, N­₂, O₂) on an inexpensive graphene oxide (GO) SERS substrate. All of the plasma gases lowered the GO's Raman peak intensities; this was seen more readily in the first 10 seconds of treatment for Ar and N2, in contrast to the first 10 seconds of O2, where no change was seen. 

Regardless of the gas, the Raman peak intensities were reduced for exposure times greater than 10s. Plasma treatment gases also affected the SERS enhancement factor (EF) when searching for the dye Rhodamine B.

When compared to N₂ and 10% H₂/90% Ar, pure Ar created the highest EF for Rhodamine B, no matter the dye's concentration.

Capaccio et al. noted that SERS substrates that contain silver should not be plasma treated with O₂ or air because of the resulting oxidation and loss of plasmonic activity.

Hosomi et al. explored the effects of Ar plasma on silver nanoparticle (Ag-NP) SERS substrates. Hosomi noted that Ar plasma treatments of less than 15 seconds were required to maintain the plasmonic activity of Ag-NP substrates. In addition, Ar treatments that lasted longer than 10 seconds resulted in undesirable morphological changes in the silver nanoparticles.

Jeong et al. used oxygen plasma under various conditions to treat hollow porous gold-nanoshell (HPAuNS) SERS substrates. The nanoshells contained Au nanoparticles adsorbed onto polystyrene colloids.

At O₂ plasma treatment times greater than three minutes or at high (30 sccm) O₂ flow rates, the Au nanoparticles became sintered at their contact points, resulting in poorly defined interfaces and a reduction in the substrates' SERS sensitivity at wavelengths > 800 nm. Jeong recommends short O2 plasma treatments at low flow rates to maintain these substrates' SERS response.

References and Further Reading

Hydrophilizing SERS Substrates: Articles by Harrick Plasma Users

• Liu, Y., Yue, S., Wang, Y. N., Wang, Y., & Xu, Z. R. (2020). “A multicolor-SERS dual-mode pH sensor based on smart nano-in-micro particles”. Sensors and Actuators, B: Chemical, 310. 10.1016/j.snb.2020.127889
• Pandya, A. H., Kumaradas, J. C., & Douplik, A. (2019). “Miniature optical fiber sensors using surface enhanced Raman spectroscopy (SERS) for remote biochemical sensing”. Journal of Biomedical Photonics and Engineering, 5(1). 10.18287/JBPE19.01.010301
• Purwidyantri, A., Hsu, C. H., Yang, C. M., Prabowo, B. A., Tian, Y. C., & Lai, C. S. (2019). “Plasmonic nanomaterial structuring for SERS enhancement”. RSC Advances, 9(9), 4982–4992. 10.1039/c8ra10656h
• Wen, P., Yang, F., Ge, C., Li, S., Xu, Y., & Chen, L. (2021). “Self-assembled nano-Ag/Au@Au film composite SERS substrates show high uniformity and high enhancement factor for creatinine detection”. Nanotechnology, 32(39). 10.1088/1361-6528/ac0ddd

Cleaning SERS Substrates to Improve Detection: Articles by Harrick Plasma Users

• Henderson, K. C., Sheppard, E. S., Rivera-Betancourt, O. E., Choi, J. Y., Dluhy, R. A., Thurman, K. A., Winchell, J. M., & Krause, D. C. (2014). “The multivariate detection limit for Mycoplasma pneumoniae as determined by nanorod array-surface enhanced Raman spectroscopy and comparison with limit of detection by qPCR”. Analyst, 139(24), 6426–6434. 10.1039/c4an01141d
• Tran, M., Roy, S., Kmiec, S., Whale, A., Martin, S., Sundararajan, S., & Padalkar, S. (2020). “Formation of size and density controlled nanostructures by galvanic displacement”. Nanomaterials, 10(4). 10.3390/nano10040644

Cleaning SERS Substrates for Reuse: Articles by Harrick Plasma Users

• Lu, G., Li, H., Wu, S., Chen, P., & Zhang, H. (2012). “High-density metallic nanogaps fabricated on solid substrates used for surface enhanced Raman scattering”. Nanoscale, 4(3), 860–863. 10.1039/c1nr10997a
• Negri, P., Marotta, N. E., Bottomley, L. A., & Dluhy, R. A. (2011). “Removal of surface contamination and self-assembled monolayers (SAMs) from silver (Ag) nanorod substrates by plasma cleaning with argon”. Applied Spectroscopy, 65(1), 66–74. 10.1366/10-06037

Modifying Surface Morphologies of SERS Substrates: Articles by Harrick Plasma Users

• Chen, Y., Yin, H., Sikdar, D., Liu, H., Zhu, Q., Yao, G., Qi, H., & Gu, N. (2020). “Multiscale Patterned Plasmonic Arrays for Highly Sensitive and Uniform SERS Detection”. Advanced Materials Interfaces, 7(17). 10.1002/admi.202000248
• Li, X., Li, L., Wang, Y., Hao, X., Wang, C., Yang, Z., & Li, H. (2023). “Ag NPs@PDMS nanoripple array films as SERS substrates for rapid in situ detection of pesticide residues”. Spectrochimica Acta – Part A: Molecular and Biomolecular Spectroscopy, 299. 10.1016/j.saa.2023.122877
• Zhang, H., Zhao, N., Li, H., Wang, M., Hao, X., Sun, M., Li, X., Yang, Z., Yu, H., Tian, C., & Wang, C. (2022). “3D Flexible SERS Substrates Integrated with a Portable Raman Analyzer and Wireless Communication for Point-of-Care Application”. ACS Applied Materials and Interfaces, 14(45), 51253–51264. 10.1021/acsami.2c12201

Optimization of Plasma Treatment Parameters: Articles by Harrick Plasma Users

• Capaccio, A., Sasso, A., & Rusciano, G. (2022). “Feasibility of SERS-Active Porous Ag Substrates for the Effective Detection of Pyrene in Water”. Sensors, 22(7). 10.3390/s22072764 
• Hosomi, K., Takahiro, K., Nishiyama, F., & Yokoyama, S. (2019). “Plasma-induced recovery of plasmonic sensitivity of aged silver nanoparticles to ethanol vapor and plasma exposure-time dependence”. Thin Solid Films, 673, 52–56. 10.1016/j.tsf.2019.01.018
• Jeong, S., Kim, M. W., Jo, Y. R., Kim, N. Y., Kang, D., Lee, S. Y., Yim, S. Y., Kim, B. J., & Kim, J. H. (2019). “Hollow Porous Gold Nanoshells with Controlled Nanojunctions for Highly Tunable Plasmon Resonances and Intense Field Enhancements for Surface-Enhanced Raman Scattering”. ACS Applied Materials and Interfaces, 11(47), 44458–44465. 10.1021/acsami.9b16983
• Singh, N. S., Mayanglambam, F., Nemade, H. B., & Giri, P. K. (2022). “Plasma-Treated Graphene Surfaces for Trace Dye Detection Using Surface-Enhanced Raman Spectroscopy”. ACS Applied Nano Materials, 5(5), 6352–6364. 10.1021/acsanm.2c00445

This information has been sourced, reviewed and adapted from materials provided by Harrick Plasma.

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