Raman and Surface-Enhanced Raman Scattering Detection in Flowing Solutions for Complex Mixture Analysis

Abstract

Raman scattering provides a chemical-specific and label-free method for identifying and quantifying molecules in flowing solutions. This review provides a comprehensive examination of the application of Raman spectroscopy and surface-enhanced Raman scattering (SERS) to flowing liquid samples. We summarize developments in online and at-line detection using Raman and SERS analysis, including the design of microfluidic devices, the development of unique SERS substrates, novel sampling interfaces, and coupling these approaches to fluid-based chemical separations (e.g., chromatography and electrophoresis). The article highlights the challenges and limitations associated with these techniques and provides examples of their applications in a variety of fields, including chemistry, biology, and environmental science. Overall, this review demonstrates the utility of Raman and SERS for analysis of complex mixtures and highlights the potential for further development and optimization of these techniques.

Keywords: Raman; SERS; chromatography; electrophoresis; microfluidics; online detection; surface-enhanced Raman scattering.

Figure 1:

Illustration highlighting how flow-through SERS and Raman spectroscopy intersect, providing technology for a wide range of applications. Capillary Electrophoresis image adapted with permission from Reference (61) copyright 2014 Royal Society of Chemistry, Microfluidic devices images adapted with permission from Reference (23); copyright 2018 American Chemical Society, Liquid Chromatography image adapted with permission from Reference (108a); copyright 2018 American Chemical Society, and electrochemical SERS images adapted with permission from Reference (65); copyright 2022 American Chemical Society. Abbreviations: CT, charge transfer; EOF, electroosmotic flow; LC, liquid chromatography; RB, rhodamine B, R6G, rhodamine 6G; SERS, surface-enhanced Raman scattering; SPR, surface plasmon resonance. TAMRA, 5-carboxytetramethylrhodamine.

Figure 2

(i) Photograph of the fluidic system before integration (a). The assembled fluidic system with embedded SERS sensor array (b). SERS maps recorded from the sensor array (c) with the typical SERS spectra of BPE for each sensor (d). Average signal of the five sensors at the 1198 cm⁻¹ peak in microfluidic. Panel adapted with permission from Reference (23); copyright 2018 American Chemical Society. (ii) (a) COMSOL simulations (left panels) showing analyte concentration with corresponding wide-field fluorescence images (right panels) in the xy-plane are depicted at various ratios between sheath flow rate and capillary flow rate. (b) The heatmap shows the observed SERS intensity at each Raman shift as a function of acquisition time for R6G solution during an experiment. The SERS intensity profile of a single Raman band (1357 cm^–1) as a function of acquisition time is shown. Panel adapted with permission from Reference (58); copyright 2013 American Chemical Society. Abbreviations: SERS, surface enhanced Raman spectroscopy; BPE, trans-1,2-bis-(4-pyridyl)-ethylene; R6G, Rhodamine 6G.

Figure 3

(i) Schematic Illustration of electrokinetic pre-separation and molecularly imprinted trapping of charged PAEs on a portable interface for selective SERS recognition. Panel adapted with permission from Reference (70); copyright 2022 American Chemical Society. (ii) Diagram of polystyrene chip with embedded 150 μm capillary and 25 μm SERS-active electrode. The addition of a 250 μm PDMS channel over the top allows for solution flow and in situ experiments (a). The actual chip (b). A brightfield image showing the capillary and electrode in the microchannel (c). Panel adapted with permission from Reference (88); copyright 2022 American Chemical Society. Abbreviations: PAEs, phthalate plasticizers; SERS, surface enhanced Raman spectroscopy; PDMS, polydimethylsiloxane.

Figure 4:

Some results from on-line SERS experiments. (i) (a) An example LC-SERS chromatogram from a single injection of guanine, M hypoxanthine, 1 adenine and xanthine, eluted in order. (b) shows the UV chromatogram for each injection of individual analytes. Panel adapted with permission from Reference (106); copyright 2014 American Chemical Society. (ii) (a) Total photon LC-SERS chromatogram of raw SERS signal for a 20 min (6000 spectra) run of a tumor lysate. Each peak in the LC-SERS chromatogram corresponds to a Raman spectrum. (b). Background corrected total photon LC-SERS chromatogram. c) Example spectra from different elution times during the LC separation. Panel adapted with permission from Reference (109); copyright 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. Abbreviations: LC-SERS, liquid chromatography-surface enhanced Raman spectroscopy; UV, ultraviolet.

Figure 5.

(i) Design of the system used to separate a solution containing Hg²⁺ and Pb²⁺ ions. a) The capillary was functionalized with AuNPs coated in MBA, where the SERS spectra are collected at various points along the capillary (b). c) The carboxylate stretching region changes based on which metal ions were coordinated to the MBA. At the end of the capillary only Pb²⁺ ions were observed. Panel adapted with permission from Reference (115); copyright 2010 American Chemical Society (ii) (a) Heatmap of the SERS intensity during the CE separation of rhodamine isomers. b) Zoomed in image of the heatmap showing signal from each analyte is observed for 2s. c) The electropherogram constructed the 1357 cm⁻¹ Raman band intensity, highlighted by the red rectangle in (a). Panel adapted with permission from Reference (61); copyright 2014 The Royal Society of Chemistry. Abbreviations: AuNPs, gold nanoparticles; MBA, mercaptobenzoic acid; SERS, surface enhanced Raman spectroscopy; CE, capillary electrophoresis.