Optimizing disposable materials for point-of-use SERS sensing applications
The Raman effect, discovered in 1928, has emerged as a powerful analytical tool for the sensitive detection of diverse analytes, from small molecules to whole cells. However, the inherently weak nature of Raman scattering limits its direct application in trace-level analysis. Surface-enhanced Raman spectroscopy (SERS) has revolutionized this field by amplifying the Raman signal up to 10^10-fold through the use of plasmonic nanostructures.
The design and optimization of SERS-active substrates are critical for achieving the desired sensitivity, selectivity, and robustness in sensing applications. Recent advancements have focused on the use of commercially available, cost-effective, and disposable materials as SERS substrates. These substrates, including paper, fabrics, polymers, and silica-based materials, can be tailored to enhance the plasmonic effect, improve analyte capture, and enable point-of-use (POU) sensing in resource-limited settings.
Harnessing the power of plasmonic nanostructures
The Raman effect originates from the inelastic scattering of photons by matter, providing a unique molecular fingerprint. The probability of Raman scattering is inherently low, necessitating the use of powerful lasers and sophisticated instrumentation for detection. SERS overcomes this limitation by leveraging the localized surface plasmon resonance (LSPR) of metallic nanostructures, which can amplify the Raman signal by several orders of magnitude.
The key to efficient SERS is the optimization of the plasmonic nanostructures, which serve as “hotspots” for the enhancement of the electromagnetic (EM) field. Factors such as the size, shape, and arrangement of these nanostructures, as well as their dielectric environment, can significantly impact the SERS signal. Researchers have explored a wide range of nanostructures, including zero-dimensional (0D) nanoparticles, one-dimensional (1D) nanorods and nanowires, two-dimensional (2D) nanofilms and graphene-based materials, and three-dimensional (3D) porous structures, to engineer highly effective SERS substrates.
Disposable SERS substrates for point-of-use sensing
The development of SERS-based sensing platforms for POU applications has gained significant attention in recent years. Disposable SERS substrates offer several advantages, including cost-effectiveness, ease of use, and the ability to mitigate biofouling. These substrates can be fabricated using a variety of commercially available and biocompatible materials, such as paper, fabrics, polymers, and silica-based materials.
Paper-based SERS substrates
Paper-based SERS substrates leverage the inherent properties of cellulose, such as porosity, hydrophilicity, and mechanical strength, to facilitate the integration of plasmonic nanoparticles and surface engineering. These substrates are affordable, scalable, and user-friendly, making them particularly suitable for resource-limited settings. However, the autofluorescence of organic materials in paper can pose a challenge, which can be mitigated by using longer-wavelength lasers and appropriate optical filters.
Fabric-based SERS substrates
Fabrics, with their uniform interwoven fibers, durability, and enhanced surface area, have also been explored as SERS substrates. The coarse fibrous structure of plant-based fabrics can be modified by coating or trapping plasmonic nanoparticles to create SERS-active surfaces. Optimizing the microfluidic flow and surface hydrophobicity of fabrics can further enhance the sensitivity and repeatability of these substrates.
Polymer-based SERS substrates
Polymers, such as polyethylene terephthalate (PET), poly(methyl methacrylate) (PMMA), and polydimethylsiloxane (PDMS), have gained attention as SERS substrate materials due to their chemical inertness, disposability, cost-effectiveness, and optical transparency. The flexibility and conformability of these polymers enable their integration with irregular surfaces, while their transparency can suppress background fluorescence and improve light penetration.
Silica-based SERS substrates
Silica-based materials, including fused silica, glass, and porous silica, have also been explored as SERS substrates. These materials offer advantages such as high-temperature resistance, chemical inertness, rigidity, and biocompatibility, making them suitable for a wide range of applications. The ease of functionalization and homogeneous surface roughness of silica-based substrates can contribute to improved SERS performance.
Enhancing SERS performance through surface engineering
The performance of SERS substrates can be further optimized through surface engineering techniques, such as the incorporation of specific surface modifiers, bioreceptors, and bioconjugation strategies.
Surface modifiers for improved SERS
Surface modifiers, including metal-organic frameworks (MOFs), covalent organic frameworks (COFs), aerogels, and hydrogels, can be integrated with plasmonic nanostructures to enhance the EM field, improve analyte capture, and facilitate the incorporation of bioreceptors.
Bioreceptor-functionalized SERS substrates
The use of bioreceptors, such as antibodies, aptamers, enzymes, and polysaccharides, can enhance the specificity of SERS-based sensors by selectively capturing target analytes. The integration of bioreceptors with SERS substrates requires careful consideration of factors like bioreceptor stability, cross-reactivity, and the impact on SERS performance.
Bioconjugation strategies
Various bioconjugation approaches, including physical, chemical, and biological conjugation, can be employed to immobilize bioreceptors on SERS substrates. These strategies aim to optimize the analyte-substrate interactions, ensure uniform hotspot distribution, and enhance the overall SERS signal.
Integrating SERS with emerging technologies
The versatility of SERS-based sensing has led to its integration with complementary technologies, such as electrochemistry, microfluidics, and artificial intelligence (AI), to further advance its capabilities.
Electrochemistry-SERS (EC-SERS) hybrid systems
The integration of electrochemical sensors with SERS can enhance the sensitivity and selectivity of analyte detection. EC-SERS systems leverage the ability to control the surface chemistry and redox state of the SERS substrate, enabling efficient analyte capture and signal enhancement.
Microfluidic SERS platforms
The integration of SERS with microfluidic devices can facilitate the handling of small sample volumes, improve analyte transport, and enable real-time monitoring. Microfluidic SERS platforms can be particularly useful for on-site sensing and point-of-care applications.
AI-powered SERS analysis
The incorporation of AI and machine learning algorithms can simplify the design of SERS systems and enable automated spectral analysis. These computational techniques can assist in the optimization of SERS substrate parameters, pattern recognition, and quantitative analysis of target analytes.
Conclusion
The design and optimization of SERS-active substrates have been at the forefront of research, driven by the need for sensitive, selective, and user-friendly sensing solutions. The emergence of disposable SERS substrates, fabricated using commercially available and cost-effective materials, has opened new avenues for point-of-use sensing applications in fields such as environmental monitoring, food quality control, and biomedical diagnostics.
The integration of SERS with complementary technologies, such as electrochemistry, microfluidics, and artificial intelligence, has further expanded the capabilities of this powerful analytical tool. As the field of SERS continues to evolve, the development of robust, portable, and intelligent SERS-based sensing systems will play a crucial role in addressing real-world challenges and revolutionizing the way we approach analytical measurements.
