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Surface enhanced Raman spectroscopy (SERS) holds great promise in biosensing because of its single-molecule, label-free sensitivity. We describe here the use of a graphene-gold hybrid plasmonic platform that enables quantitative SERS measurement. Quantification is enabled by normalizing analyte peak intensities to that of the graphene G peak. We show that two complementary quantification modes are intrinsic features of the platform, and that through their combined use, the platform enables accurate determination of analyte concentration over a concentration range spanning seven orders of magnitude. We demonstrate, using a biologically relevant test analyte, the amyloid β-protein (Aβ), a seminal pathologic agent of Alzheimer’s disease (AD), that linear relationships exist between (a) peak intensity and concentration at a single plasmonic hot spot smaller than 100 nm, and (b) frequency of hot spots with observable protein signals, i.e. the co-location of an Aβ protein and a hot spot. We demonstrate the detection of Aβ at a concentration as low as 10(−18) M after a single 20 μl aliquot of the analyte onto the hybrid platform. This detection sensitivity can be improved further through multiple applications of analyte to the platform and by rastering the laser beam with smaller step sizes.

Large-scale close-packed two-dimensional (2D) colloidal crystal with high coverage is indispensable for various promising applications. The Langmuir-Blodgett (LB) method is a powerful technique to prepare 2D colloidal crystals. However, the self-assembly and movement of microspheres during the whole LB process are less analyzed. In this study, we clarify the crucial impact of hydrophilicity of the microspheres on their self-assembly in the LB process and on the properties of the prepared 2D colloidal crystals. The characteristic surface pressure-area isotherms of the microspheres have been analyzed and adjusted by only counting the quantity of the microspheres on the water surface, which leads to more accurate results. The critical surface pressures for hydrophilic and hydrophobic microspheres are about 61 and 46 mN/m, respectively. The decrease of the surface hydrophilicity of microspheres facilitates their self-assembly on the water surface, which further leads to higher coverage and less defects of the 2D colloidal crystals. A coverage of as high as 97% was obtained using hydrophobic microspheres. Entropy and intersphere capillary forces drive the self-assembly and transportation of the microspheres, respectively. Caused by the diffraction of visible light, opposite contrasts at local adjacent regions on the surface of the 2D colloidal crystals have been observed. The understanding of self-assembly of the microspheres during the LB process paves the way to fabricate the high-quality 2D colloidal crystals for various applications such as photonic papers and inks, stealth materials, biomimetic coatings, and related nanostructures.

Surface-Enhanced Raman Scattering (SERS) is used to differentiate two colon cancer cell line HCT 116, that is, to distinguish a TP53 gene knockout cell line (p53 -/-) from a wild type (p53 +/+). A label-free graphene/gold nanopyramid based SERS platform, combined with the multivariate analysis: principal component analysis, is used to profile live, dead, and burst colon cancer cells suspended in simulated body fluid (SBF). The graphene sheet permits SERS hotspot identification and provides a chemical enhancement for the biological constituents. This study found that a unique fingerprint exists for three different states of the cell, burst, live, and dead, which were used to differentiate the p53 +/+ and p53 -/- cell lines. Perceptron with Pocket Algorithm was also coupled with PCA to demonstrate an average of 81% sensitivity and 97% specificity in separating the two cell lines. The demonstration of single gene differentiation shows the great applicable potential of this SERS graphene hybrid platform for cancer diagnosis.

The orientation dependence of the Raman spectral features of individual protein/biomolecules is studied using surface-enhanced Raman spectroscopy (SERS). Large variation in spectral features mainly in term of peak intensity is observed from small proteins/peptides. We aim to address the question of whether the spectral features of SERS are uniquely determined by the type of protein/molecules or are influenced prominently by factors more than the identity of the molecules such as orientation of molecules relative to the substrate surface. The standard deviation in the intensity of individual Raman peaks diminishes for protein size larger than 13 amino acids. Secondary structure of protein (such as protein-protein interaction) remains unchanged regardless of protein orientation. Numerical simulation studies corroborate the experimental observation in that the SERS spectral features of biomedically relevant protein (of larger than 13 amino acids in size, which represent all human protein types) are not affected by the orientation of amino acids randomly dispersed on SERS-active surfaces. These findings are instrumental to understanding the exceedingly high (label-free) specificity when SERS is used in identifying proteins/peptides as can be found in numerous publications from different research groups in both in vivo and in vitro analyses. It was noted that the spectral position of all Raman peaks assignable to the various amino acids are independent of molecule orientation even though their intensities do vary.

Amyloid β‐protein (Aβ) self‐association is one process linked to the development of Alzheimer's disease (AD). Aβ peptides, including its most abundant forms, Aβ40 and Aβ42, are associated with the two predominant neuropathologic findings in AD, vascular and parenchymal amyloidosis, respectively. Efforts to develop therapies for AD often have focused on understanding and controlling the assembly of these two peptides. An obligate step in these efforts is the monitoring of assembly state. We show here that surface‐enhanced Raman spectroscopy (SERS) coupled with principal component analysis (PCA) readily distinguishes Aβ40 and Aβ42. We show further, through comparison of assembly dependent changes in secondary structure and morphology, that the SERS/PCA approach unambiguously differentiates closely related assembly stages not readily differentiable by circular dichroism spectroscopy, electron microscopy, or other techniques. The high discriminating power of SERS/PCA is based on the rich structural information present in its spectra, which comprises not only on interatomic resonances between covalently associated atoms and hydrogen bond interactions important in controlling secondary structure, but effects of protein orientation relative to the substrate surface. Coupled with the label‐free, single molecule sensitivity of SERS, the approach should prove useful for determining structure activity relationships, suggesting target sites for drug development, and for testing the effects of such drugs on the assembly process. The approach also could be of value in other systems in which assembly dependent changes in protein structure correlate with the formation of toxic peptide assemblies.

We report systematic transient characterization of a graphene ribbon (GR) used as an interconnect for electrostatic discharge (ESD) protection of future integrated circuits. A large set of GR wires (around 6000) with varying and practical dimensions were fabricated using the chemical vapor deposition method and characterized by transmission line pulsing (TLP) and very fast TLP (VFTLP) measurements. Comprehensive TLP and VFTLP testing with varying pulse rise time ( $t_{r})$ and duration ( $t_{d})$ was performed across a wide temperature range ( $T = -30/+110~^{\circ }\text{C}$ ). Measurement-based statistics reveal the relationship between ESD capability of GR wires and the wire length ( $L$ ), width ( $W$ ), and number of graphene layers, as well as ESD pulse shapes and operation temperature.

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.

Mammalian tissues calcify with age and injury. Analogous to bone formation, osteogenic cells are thought to be recruited to the affected tissue and induce mineralization. In the heart, calcification of cardiac muscle leads to conduction system disturbances and is one of the most common pathologies underlying heart blocks. However the cell identity and mechanisms contributing to pathological heart muscle calcification remain unknown. Using lineage tracing, murine models of heart calcification and invivo transplantation assays, we show that cardiac fibroblasts (CFs) adopt an osteoblast cell-like fate and contribute directly to heart muscle calcification. Small-molecule inhibition of ENPP1, an enzyme that is induced upon injury and regulates bone mineralization, significantly attenuated cardiac calcification. Inhibitors of bone mineralization completely prevented ectopic cardiac calcification and improved post injury heart function. Taken together, these findings highlight the plasticity of fibroblasts in contributing to ectopic calcification and identify pharmacological targets for therapeutic development.

A strongly confined and enhanced electromagnetic (EM) field due to gap-plasmon resonance offers a promising pathway for ultrasensitive molecular detections. However, the maximum enhanced portion of the EM field is commonly concentrated within the dielectric gap medium that is inaccessible to external substances, making it extremely challenging for achieving single-molecular level detection sensitivity. Here, a new family of plasmonic nanostructure created through a unique process using nanoimprint lithography is introduced, which enables the precise tailoring of the gap plasmons to realize the enhanced field spilling to free space. The nanostructure features arrays of physically contacted nanofinger-pairs with a 2 nm tetrahedral amorphous carbon (ta-C) film as an ultrasmall dielectric gap. The high tunneling barrier offered by ta-C film due to its low electron affinity makes an ultranarrow gap and high enhancement factor possible at the same time. Additionally, its high electric permittivity leads to field redistribution and an abrupt increase across the ta-C/air boundary and thus extensive spill-out of the coupled EM field from the gap region with field enhancement in free space of over 103 . The multitude of benefits deriving from the unique nanostructure hence allows extremely high detection sensitivity at the single-molecular level to be realized as demonstrated through bianalyte surface-enhanced Raman scattering measurement.

© 2015 American Chemical Society. Ultrasensitive detection and spatially resolved mapping of neurotransmitters, dopamine and serotonin, are critical to facilitate understanding brain functions and investigate the information processing in neural networks. In this work, we demonstrated single molecule detection of dopamine and serotonin using a graphene-Au nanopyramid heterostructure platform. The quasi-periodic Au structure boosts high-density and high-homogeneity hotspots resulting in ultrahigh sensitivity with a surface enhanced Raman spectroscopic (SERS) enhancement factor ∼1010. A single layer graphene superimposed on a Au structure not only can locate SERS hot spots but also modify the surface chemistry to realize selective enhancement Raman yield. Dopamine and serotonin could be detected and distinguished from each other at 10-10M level in 1 s data acquisition time without any pretreatment and labeling process. Moreover, the heterostructure realized nanomolar detection of neurotransmitters in the presence of simulated body fluids. These findings represent a step forward in enabling in-depth studies of neurological processes including those closely related to brain activity mapping (BAM).

A graphene-Au nano-pyramid hybrid system that enables label-free single molecule detection is demonstrated. The bio-compatible graphene-based SERS platform boosts a high density of hot spots with local SERS enhancement factor over 10(10) . We demonstrate that graphene can play a key role in quantitative study of SERS mechanisms, and can also serve as a promising building block in SERS active structures especially for biosensor applications.

The unique properties of graphene when coupled to plasmonic surfaces render a very interesting physical system with intriguing responses to stimuli such as photons. It promises exciting application potentials such as photodetectors as well as biosensing. With its semimetallic band structure, graphene in the vicinity of metallic nanostructures is expected to lead to non-negligible perturbation of the local distribution of electromagnetic field intensity, an interesting plasmonic resonance process that has not been studied to a sufficient extent. Efforts to enhance optoelectronic responses of graphene using plasmonic structures have been demonstrated with rather modest Raman enhancement factors of less than 100. Here, we examine a novel cooperative graphene-Au nanopyramid system with a remarkable graphene Raman enhancement factor of up to 10(7). Experimental evidence including polarization-dependent Raman spectroscopy and scanning electron microscopy points to a new origin of a drastically enhanced D-band from sharp folds of graphene near the extremities of the nanostructure that is free of broken carbon bonds. These observations indicate a new approach for obtaining detailed structural and vibrational information on graphene from an extremely localized region. The new physical origin of the D-band offers a realistic possibility of defining active devices in the form of, for example, graphene nanoribbons by engineered graphene folds (also known as wrinkles) to realize edge-disorder-free transport. Furthermore, the addition of graphene made it possible to tailor the biochemical properties of plasmonic surfaces from conventional metallic ones to biocompatible carbon surfaces.