Surface-Enhanced Raman Spectroscopy of Analytes in Blood

Abstract

Although Raman scattering has traditionally been considered a weak process, making analysis of low concentration analytes in complex matrices difficult, both methodological and instrumentation advances in the last couple decades have made Raman spectroscopy a viable and useful analytical tool.1,9 This is especially true for analyte species within aqueous environments because the Raman scattering cross-section of water is small; one particular example of a critical aqueous environment is analysis of and in blood. This review will analyze much of the literature related to Raman analysis in blood within the last 20 years, including normal Raman, surface-enhanced Raman, and spatially offset Raman analyses. The first section will focus on direct analysis of blood samples, including determining the age of deposited or donated blood and blood content within body fluid mixtures. The second section will discuss intrinsic Raman-based detection of small molecules and protein analytes within blood as well as extrinsic Raman detection of tumors. The last section will review the recent use of spatially offset Raman and surface-enhanced spatially offset Raman spectroscopy to analyze molecular analytes, tissue, bone, tumors, and calcifications, including in vivo analysis. This focal point closes with perspective on critical gaps and upcoming developments for Raman analysis in blood. Microfluidic sensing platforms facilitate parallel, low sample volume detection using various optical signal transduction mechanisms. Herein, we introduce a simple mixing microfluidic device, enabling serial dilution of introduced analyte solution that terminates in five discrete sensing elements. We demonstrate the utility of this device with on-chip fluorescence and surface-enhanced Raman scattering (SERS) detection of analytes, and we demonstrate device use both when combined with a traditional inflexible SERS substrate and with SERS-active nanoparticles that are directly incorporated into microfluidic channels to create a flexible SERS platform. The results indicate, with varying sensitivities, that either flexible or inflexible devices can be easily used to create a calibration curve and perform a limit of detection study with a single experiment. In current events, ricin has been discussed frequently because of letters sent to high-ranking government officials containing the easily extracted protein native to castor beans. Ricin B chain, commercially available and not dangerous when separated from the A chain, enables development of ricin sensors while minimizing the hazards of working with a bioterror agent that does not have a known antidote. As the risk of ricin exposure, common for soldiers, becomes increasingly common for civilians, there is a need for a rapid, real-time detection of ricin. To this end, aptamers have been used recently as an affinity agent to enable the detection of ricin in food products via surface-enhanced Raman spectroscopy (SERS) on colloidal substrates. One goal of this work is to extend ricin sensing into whole human blood; this goal required application of a commonly used plasmonic surface, the silver film-over-nanosphere (AgFON) substrate, which offers SERS enhancement factors of 106 in whole human blood for up to 10 days. This aptamer-conjugated AgFON platform enabled ricin B chain detection for up to 10 days in whole human blood. Principle component analysis (PCA) of the SERS data clearly identifies the presence or absence of physiologically relevant concentrations of ricin B chain in blood. Spectrophotometry and colorimetry experiments are common in high school and college chemistry courses. Previous work has demonstrated that handheld camera devices can be used to quantify the concentration of a colored analyte in solution in place of traditional spectrophotometric or colorimetric equipment. This paper extends this approach to an investigation of a mesogold mineral supplement. With the addition of free Google applications, the investigation provides a feasible, sophisticated lab experience, especially for teachers with limited budgets.