Abstract
Imaging techniques based on mass spectrometry or spectroscopy methods inform in situ about the chemical composition of biological tissues or organisms, but they are sometimes limited by their specificity, sensitivity, or spatial resolution. Multimodal imaging addresses these limitations by combining several imaging modalities; however, measuring the same sample with the same preparation using multiple imaging techniques is still uncommon due to the incompatibility between substrates, sample preparation protocols, and data formats. We present a multimodal imaging approach that employs a gold-coated nanostructured silicon substrate to couple surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS) and surface-enhanced Raman spectroscopy (SERS). Our approach integrates both imaging modalities by using the same substrate, sample preparation, and data analysis software on the same sample, allowing the coregistration of both images. We transferred molecules from clean fingertips and fingertips covered with plasticine modeling clay onto our nanostructure and analyzed their chemical composition and distribution by SALDI-MS and SERS. Multimodal analysis located the traces of plasticine on fingermarks and provided chemical information on the composition of the clay. Our multimodal approach effectively combines the advantages of mass spectrometry and vibrational spectroscopy with the signal enhancing abilities of our nanostructured substrate.
Highlights
Label-free imaging techniques are crucial for understanding biological mechanisms at a molecular level
Surface-enhanced Raman spectroscopy (SERS) boosts Raman sensitivity and specificity through electromagnetic enhancement provided by plasmon resonances in the metal substrate, that is, the Raman signals of molecules in the close vicinity of metallic nanostructures are amplified by several orders of magnitude, and through chemical enhancement when a charge-transfer mechanism in the metal−adsorbate complex is established.[8]
Gold and silver nanoparticles are popular for SERS detection, as they are stable in air and can be used over a wide range of laser wavelengths (400−1000 nm for Ag and 600−1200 nm for Au).[9−12] SERS is frequently used for body fluid analyses, such as pathogen detection and for imaging applications such as tumor margin determination,[10] single-cell analysis,[12] and even for revealing chemical information from latent fingermarks.[9]
Summary
Label-free imaging techniques are crucial for understanding biological mechanisms at a molecular level. They are used for investigating a wide range of issues such as plant-based renewable energy, microbiological assays, diseases (in clinical medicine), and even forensic specimens.[1−5] Vibrational spectroscopy and mass spectrometry imaging techniques are the most popular choices to and simultaneously map a wide range of molecules present in living organisms or frozen tissues.[1]. Raman imaging is frequently used for exploring the chemical composition of biological samples.[6] It offers high spatial resolution maps (down to ∼250 nm lateral resolution) with information about the molecular structure (secondary structure of proteins, saturation level of lipids, etc.)[7] but with limited sensitivity and specificity. Sample preparation for SERS imaging using metallic nanoparticles is complicated (nanoparticles need to be functionalized with labels and binding molecules for a specific target),[13,14] while label-free experiments often experience nanoparticle surface saturation.[15]
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