Revolutionizing Detection: Amorphous Titanium Dioxide/Silver Nanosheets Enhance Sensitivity in Surface-Enhanced Raman Spectroscopy

Artist's impression of Raman spectroscopy © Shahriyar - stock.adobe.com

Surface-enhanced Raman spectroscopy (SERS) is a powerful, non-destructive analytical technique that enhances the Raman scattering effect, allowing for the detection of molecules at extremely low concentrations as compared to classical Raman spectroscopy. Traditional SERS substrates often rely on precious metals like silver (Ag) and gold (Au), which, despite their effectiveness, suffer from issues such as poor stability and lack of uniformity in size and shape (1). To address these challenges, the team of researchers have developed a novel SERS substrate combining amorphous titanium dioxide (a-TiO2) and silver (Ag) nanosheets. This innovative approach, which was published in Applied Spectroscopy, aims to harness the benefits of both semiconductor and metal properties to achieve superior Raman detection capabilities (2).

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This research was conducted by Lan Zhang, Shiying Wu, Tingting Zhang, and their colleagues at the Jiangsu Key Laboratory of Chemical Pollution Control and Resources Reuse, Nanjing University of Science and Technology, the Guangdong Provincial Key Laboratory of Microbial Culture Collection and Application, the State Key Laboratory of Applied Microbiology Southern China, and the Information Materials and Intelligent Sensing Laboratory of Anhui Province, among others (1).

A semiconductor SERS substrate is designed to amplify the Raman scattering signal of molecules adsorbed on its surface, thereby increasing sensitivity of molecular detection. Unlike traditional metal-based SERS substrates, which rely on the electromagnetic enhancement from local surface plasmon resonance, semiconductor substrates like titanium dioxide (TiO2) or zinc oxide (ZnO) utilize broad band gaps, chemical stability, and controlled structures to enhance Raman signals through a combination of electromagnetic and chemical enhancement mechanisms. The chemical enhancement in semiconductors arises from charge-transfer interactions between the substrate and the analyte molecules, leading to increased polarizability and larger scattering cross-sections. Incorporating silver nanoparticles with amorphous TiO2 nanosheets creates a hybrid substrate that offers the combined benefits of metal and semiconductor properties; this achieves greater sensitivity, stability, and uniformity in SERS applications.

The novel a-TiO2/Ag nanosheets were created using graphene oxide (GO) as a template. Titanium tetrachloride was used to prepare the TiO2 precursors, which were then calcined at different temperatures to form crystalline (c-TiO2) and amorphous (a-TiO2) nanosheets. These nanosheets were further combined with silver nanoparticles through electrostatic interaction to form the final 2D a-TiO2/Ag structure (2).

The team tested the new substrate using rhodamine 6G (R6G) and malachite green (MG) as probe molecules. The results showed an enhancement factor (EF) of up to 1 × 108, with detection limits for R6G and MG as low as 7.6 × 10-11 M and 3.8 × 10-10 M, respectively. Additionally, pollutants like 4-mercaptobenzoic acid (4-MBA) and alizarin red S (ARS) were detected at concentrations as low as 3.79 × 10-8 M and 3.82 × 10-8 M, respectively. These findings underscore the exceptional sensitivity and uniformity of the 2D a-TiO2/Ag nanosheet substrates (2).

The researchers employed a range of sophisticated instruments to analyze the morphology and composition of the substrates. High-resolution transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were used to capture detailed images of the nanosheets. A Raman spectrometer with a 532 nm argon ion excitation laser facilitated the measurement of Raman spectra, while a Zeta potentiometer and energy dispersive X-ray (EDX) spectroscopy system aided in characterizing the nanoparticles' dispersion and composition (2).

The development of 2D a-TiO2/Ag nanosheets represents a significant advancement in the field of SERS. These substrates not only demonstrate excellent sensitivity and reproducibility but are also straightforward to prepare in large quantities, making them highly practical for various applications. The study highlights the potential of these nanosheets to revolutionize SERS by providing a stable, uniform, and highly sensitive detection platform (2).

By addressing the inherent limitations of traditional SERS substrates, this study opens new avenues for the application of SERS in fields such as environmental monitoring, medical diagnostics, and material science. The innovative use of amorphous semiconductor nanostructures combined with precious metals showcases a promising future for enhanced molecular detection techniques.

References

(1) Strobbia, P.; Henegar, A. J.; Gougousi, T.; Cullum, B. M. Layered Gold and Titanium Dioxide Substrates for Improved Surface Enhanced Raman Spectroscopic Sensing. Appl. Spectrosc. 2016, 70 (8), 1375–1383. DOI: 10.1177/0003702816647964

(2) Zhang, L.; Wu, S.; Zhang, T.; et al. Two-Dimensional Amorphous Titanium Dioxide/Silver (TiO2/Ag) Nanosheets as a Surface-Enhanced Raman Spectroscopy Substrate for Highly Sensitive Detection. Appl. Spectrosc. 2024, 78 (3), 257–267. DOI: 10.1177/00037028231213099