Exosomes contain cell-and cell-state-specific cargos of proteins, lipids, and nucleic acids and play significant roles in cell signaling and cell−cell communication. Current research into exosome-based biomarkers has relied largely on analyzing candidate biomarkers, i.e., specific proteins or nucleic acids. However, this approach may miss important biomarkers that are yet to be identified. Alternative approaches are to analyze the entire exosome system, either by "omics" methods or by techniques that provide "fingerprints" of the system without identifying each individual biomolecule component. Here, we describe a platform of the latter type, which is based on surface-enhanced Raman spectroscopy (SERS) in combination with multivariate analysis, and demonstrate the utility of this platform for analyzing exosomes derived from different biological sources. First, we examined whether this analysis could use exosomes isolated from fetal bovine serum using a simple, commercially available isolation kit or necessitates the higher purity achieved by the "gold standard" ultracentrifugation/filtration procedure. Our data demonstrate that the latter method is required for this type of analysis. Having established this requirement, we rigorously analyzed the Raman spectral signature of individual exosomes using a unique, hybrid SERS substrate made of a graphenecovered Au surface containing a quasi-periodic array of pyramids. To examine the source of the Raman signal, we used Raman mapping of low and high spatial resolution combined with morphological identification of exosomes by scanning electron microscopy. Both approaches suggested that the spectra were collected from single exosomes. Finally, we demonstrate for the first time that our platform can distinguish among exosomes from different biological sources based on their Raman signature, a promising approach for developing exosome-based fingerprinting. Our study serves as a solid technological foundation for future exploration of the roles of exosomes in various biological processes and their use as biomarkers for disease diagnosis and treatment monitoring.

A wide variety of complex, multicomponent plasmonic nanostructures have been shown to possess Fano resonances. Here we introduce a remarkably simple planar nanostructure, a single metallic nanodisk with a missing wedge-shaped slice, that also supports a Fano resonance. In this geometry, the Fano line shape arises from the coupling between a hybridized plasmon resonance of the disk and a narrower quadrupolar mode supported by the edge of the missing wedge slice. As a consequence, both disk size and wedge angle control the properties of the resonance. A semianalytical description of plasmon hybridization proves useful for analyzing the resulting line shape.

β-Amyloid aggregates in the brain play critical roles in Alzheimer's disease, a chronic neurodegenerative condition. Amyloid-associated metal ions, particularly zinc and copper ions, have been implicated in disease pathogenesis. Despite the importance of such ions, the binding sites on the β-amyloid peptide remain poorly understood. In this study, we use scanning tunneling microscopy, circular dichroism, and surface-enhanced Raman spectroscopy to probe the interactions between Cu ions and a key β-amyloid peptide fragment, consisting of the first 16 amino acids, and define the copper-peptide binding site. We observe that in the presence of Cu, this peptide fragment forms β-sheets, not seen without the metal ion. By imaging with scanning tunneling microscopy, we are able to identify the binding site, which involves two histidine residues, His13 and His14. We conclude that the binding of copper to these residues creates an interstrand histidine brace, which enables the formation of β-sheets.

Surface plasmonic tweezers and electrostatic forces can be employed as complementary methods for trapping and detecting molecules with high sensitivity and selectivity. The hotspots—localized regions of highly concentrated electromagnetic fields with large gradients—give rise to both the plasmonic tweezer effect and the surface‐enhanced Raman scattering (SERS) effect. So naturally, combining plasmonic tweezers and SERS makes for an ideal label‐free method for trapping and detecting molecules. Here, the trapping effect of the plasmonic tweezer is demonstrated by using the unique graphene–Au pyramid hybrid platform. While very powerful, the force associated with plasmonic tweezers is a short‐range effect (<50 nm from the spot of peak intensity). The electrostatic force, on the other hand, has long‐range interaction extending to beyond micrometers, which can guide molecules toward the hotspots. The authors present experimental evidence showing the combination of plasmonic tweezers and electrostatic forces by using an integrated electrostatic cell. Using the combined platform, trapping of single molecules in dilute solution is observed. These observations indicate a new approach for enhancing SERS sensitivity. It also offers a realistic possibility for precisely positional control of biomolecules, allowing the study of the properties of single biomolecules.

Manipulation of biomolecules in aqueous solution has been a critical issue for the development of many biosensing techniques and biomedical devices. Electrostatic force is an effective method for increasing both sensitivity and selectivity of various biosensing techniques. In this study, we employed surface-enhanced Raman spectroscopy (SERS) as an in situ label-free method to monitor the motion of biomolecules driven by this manipulation technique. We present the results of a combined experimental and simulation study to demonstrate that electrostatic force could enhance SERS detection of molecules in aqueous solutions with respect to sensitivity and selectivity. In regards to sensitivity, we successfully observed the signature of single molecule addition to individual SERS hot spots, in the form of the stepwise increase of Raman signal with time. With regard to selectivity, we obtained discernible SERS signature of selected families of molecules from a mixture of other molecular families of higher concentration by driving the specifically charged or polarized molecules toward or away from the electrodes/SERS surface based on their charge state, polarizability, mass, and environment pH value. We further report the experimental results on how the key factors affect the selective attraction and repulsion motion of biomolecules.

Both the surface‐enhanced Raman spectroscopy (SERS) and coherent anti‐Stokes Raman spectroscopy (CARS) are widely used methods in the bio‐sensing field for improving the intensity of Raman scattering process. By combining the mechanisms of CARS (coherence and nonlinear process) and SERS (plasmon resonance), a multiplicative enhancement can be achieved through surface‐enhanced CARS (SECARS). Besides sensitivity, high specificity with wide spectral bandwidth is also preferred for bio‐sensing techniques but not well developed in SECARS setups reported in the literature. In this work, a broadband SECARS setup with high sensitivity and high spectral resolution is presented. Rhodamine 6G dye molecules and several biomolecules are used as the model system to benchmark the functionality of the SECARS system in terms of its sensitivity, the lowest detectable concentration, and the spectral resolution. Our setup rendered single‐molecule sensitivity with spectral resolution of <35 cm−1. More than 102 times stronger signal‐to‐noise ratio compared with that of SERS is observed with the detection limit being 10−9 m. Different from the SECARS systems in the literature, our setup employs a unique graphene‐Au pyramids hybrid platform. The graphene in this structure provides additional SERS enhancement and a bio‐compatible surface. This powerful technique could be instrumental in furthering the understanding of various chemical and biological processes. Copyright © 2017 John Wiley & Sons, Ltd.

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