High-performance SERS substrates: Advances and challenges
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Introduction Surface-enhanced Raman spectroscopy (SERS) is a highly sensitive vibrational spectroscopy that allows for the detection of analytes at extremely low concentrations.1–6 In SERS, the Raman signal of the probe molecule is amplified through excitation of localized surface plasmon resonances (LSPR) of the substrate, usually in the form of metal particles or a roughened metal film. Consequently, the magnitude of SERS enhancement is crucially dependent upon the SERS substrate. Many years of research have been devoted to creating and optimizing SERS substrates in order to provide the largest enhancements possible.1,7–9 The fundamental metric for SERS activity is the enhancement factor (EF), which quantifies the increase in signal intensity (counts s–1 mW–1) per molecule. Commonly, EFs range from 103–107, with a few highly enhancing substrates achieving EFs >108.9–11 Over time, SERS substrates have had many iterations and forms, beginning with roughened silver electrodes,12 then moving onto silver and gold colloids.13,14 Later, as the SERS research
community continued to grow, metal films with various thicknesses and nanostructured features were investigated as possible substrates.14–17 More recently, combinations of both colloidal and film fabrication strategies have been developed to form SERS substrates.4,5,7,9,18–20 Comparison of various SERS substrates is challenging because rigorous characterization of substrates is frequently overlooked for reasons of complexity. This prevents the determination of analytical parameters such as sensitivity and reproducibility. When classifying a material as a reliable, highly enhancing SERS substrate, several factors need to be taken into consideration.6 First, an extinction or scattering spectrum of the substrate must be acquired and the LSPR modes identified. Second, SERS spectra should be collected for non-resonant molecules to verify that the enhancement is not being amplified by a resonance Raman effect, which can increase overall enhancement by 103–106.21 Third, thorough structural characterization of the substrate should be performed either by transmission electron microscopy (TEM), scanning electron microscopy
Bhavya Sharma, Department of Chemistry, Northwestern University; [email protected] M. Fernanda Cardinal, Department of Chemistry, Northwestern University; [email protected] Samuel L. Kleinman, Department of Chemistry, Northwestern University; [email protected] Nathan G. Greeneltch, Department of Chemistry, Northwestern University; [email protected] Renee R. Frontiera, Department of Chemistry, University of Minnesota; [email protected] Martin G. Blaber, Department of Chemistry, Northwestern University; [email protected] George C. Schatz, Department of Chemistry, Northwestern University; [email protected] Richard P. Van Duyne, Department of Chemistry, Northwestern University; [email protected] DOI: 10.1557/mrs.2013.161
© 2013 Materials Research Society
MRS BULLETIN • VOLUME 38 • AUGUST 2013 •
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