Surface-enhanced Raman scattering (SERS) is a robust spectral analysis technique that has received significant attention owing to its exceptional application potential in different fields. This article discusses SERS and its applications in detail.
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What is SERS?
SERS can detect trace amounts of analytes and identify them depending on their distinct vibrational characteristics. The technique is used extensively to study biological cell systems, proteins, and deoxyribonucleic acid (DNA).
High suitability, sensitivity, and selectivity for different analytical systems is the most significant feature of SERS. SERS also does not require any complex pretreatment of samples.
The advent of nanotechnology has further driven the advancements in SERS technology. For instance, SERS signals can be amplified by several orders of magnitude in multiple nanomaterial systems to achieve single molecule detection.
The use of SERS has been expanded to newer applications, including interface and surface research, catalytic and electrochemical reactions, trace analysis, biomedical monitoring, and biological and chemical sensors, due to the development of nanomaterials.
Importance of SERS-Active Materials
SERS-active substrates are crucial for obtaining reproducible and accurate spectral information. Thus, the fabrication and design of high-performance SERS substrates are necessary to promote the development of SERS technology.
Several studies have been performed to develop multipurpose and multiperformance SERS-active substrates, ranging from coinage metals to semiconductor materials and transition metals.
Among all substrate materials, semiconductors have gained more attention in recent years owing to their exceptional optical, chemical, and physical properties, including high carrier mobility, good biocompatibility, and high chemical stability. Semiconductor materials also display good controllability during SERS fabrication.
Semiconductor-based SERS
Semiconductor materials that can be prepared easily and possess high SERS reproducibility and enhancement are the most suitable as SERS substrates. Semiconductor materials with SERS enhancement ability display excellent charge-transfer (CT) enhancement and catalytic ability. Moreover, the superior biocompatibility of these materials allows their use in bioscience applications.
Metal-semiconductor composite SERS, organic semiconductor-based SERS, and inorganic semiconductor-based SERS are the major types of semiconductor-based SERS-enhanced mechanisms.
Semiconductor arsenide and sulfide, single elemental semiconductor nanomaterials such as graphene, silver halides, and metal oxides have been primarily studied as SERS-active inorganic semiconductor materials.
Among them, metal oxides, such as zinc oxide (ZnO), copper(I) oxide (Cu2O), nickel oxide (NiO), and titanium dioxide (TiO2), were investigated extensively as SERS-active substrates.
For instance, crystalline TiO2 nanofibers in a three-dimensional (3D) nanonetwork demonstrated a high Raman enhancement of 1.3 × 106 with a crystal violet probe due to several mechanisms, including nanocluster, plasmonic hybridization, and nanogap.
Similarly, 20 nm ZnO nanocrystals displayed an enhancement factor (EF) of 103 with a 4-mercaptopyridine (4-Mpy) Raman probe, while alpha-ferric oxide (Fe2O3) nanocrystals enhanced the Raman signal of 4-Mpy with 104 EF. Both enhancements were attributed to the CT mechanism.
A very high Raman enhancement of 105 was also observed on Cu2O nanospheres owing to the static chemical enhancement, resonant chemical enhancement, and electromagnetic (EM) enhancement. In silver halides, the Raman enhancement is mostly attributed to the EM mechanism.
In recent years, single elemental semiconductors, such as germanium, silicon, and graphene, have demonstrated Raman enhancement based on the CT mechanism. For instance, the Raman enhancement effect has been observed on the monolayer graphene surface. Material size effect, EF, spectral profiles, surface defects, and chemical bonding are the key factors that impact the semiconductor-enhanced Raman scattering.
Organic films based on small molecular semiconductors (SMSs) can also offer unique advantages over their macromolecular and inorganic alternatives, including highly controllable and facile synthesis and film fabrication, fine-tuning of optoelectronic properties, and structural versatility.
Thus, organic semiconductor materials can improve SERS activity, and SERS can be used as an effective tool to understand the performance of organic electronic devices.
For instance, nanoscale organic semiconductor α,ω-diperfluorohexylquaterthiophene (DFH-4T) molecule possessing hydrophobic properties significantly enhanced the signal of the methylene blue probe molecule with an EF of 3.4 × 103. The SERS enhancement of the pristine organic film was attributed to the CT between the organic substrate and the molecule.
Metal-semiconductor composite materials were also developed as SERS-active substrates, as achieving robust SERS enhancement is difficult using simple semiconductor materials. Composite materials with two or more materials, such as silver/CuO, display significantly better performance and possess improved application value compared to single nanomaterials.
Applications of Semiconductor-based SERS
Semiconductor-based SERS can be used in several applications, including photoelectric characterization, redox biochemistry, small ion sensing and biosensing, and organic pollutant determination and detoxification.
For instance, SERS based on TiO2/molybdenum trioxide (MoO3) semiconductors can be used in electrochromic devices for interfacial characterization as these semiconductors can provide an excellent host lattice for ion intercalation devices such as lithium-ion batteries.
Recent Studies on Semiconductor-based SERS
In a study recently published in the journal Light: Science and Applications, researchers proposed a CT mechanism to investigate the relationships and changes between photoluminescence (PL) and SERS.
The results demonstrated that the Raman scattering can be enhanced and the SERS background can be reduced by modulating the interaction between the substrate and the probe molecules.
Conclusion
To summarize, semiconductor materials have shown significant potential as SERS-active substrates with large Raman EFs under optimized conditions, superior selectivity, spectral reproducibility, and excellent thermal and chemical stability.
However, more research is required for a better understanding of the enhancement mechanism of semiconductor-enhanced Raman scattering, as only a few molecules can be selectively enhanced by semiconductors, which limits the application of semiconductor-based SERS.
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References and Further Readings
Yang, B., Jin, S., Guo, S., Park, Y., Chen, L., Zhao, B., Jung, Y. M. (2019). Recent Development of SERS Technology: Semiconductor-Based Study. ACS Omega, 4, 23, 20101–20108. https://doi.org/10.1021/acsomega.9b03154
Yang, J., Han, D., Gao, M., Su, R., Hu, M., Quan, Y., Yao, J., Yang, S. (2020) Monitoring the charge-transfer process in a Nd-doped semiconductor based on photoluminescence and SERS technology. Light: Science and Applications, 9, 117. https://doi.org/10.1038/s41377-020-00361-0
Ozaki, Y., Zhao, B., Ji, W., Han, X. X. (2017). Semiconductor-Enhanced Raman Scattering: Active Nanomaterials and Applications. Nanoscale. https://www.researchgate.net/publication/312392404_Semiconductor-Enhanced_Raman_Scattering_Active_Nanomaterials_and_Applications
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