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Open Access Publications from the University of California

Scholarly Works (12 results)

  • Thesis
  • Peer Reviewed
UCLA Electronic Theses and Dissertations (2016)

Surface Enhanced Raman Spectroscopy (SERS) is a surface-sensitive technique based on plasmon resonance, which can enhance Raman signal intensity by several orders of magnitude, enabling even detection of a single molecule. Material topics involving SERS contain manipulation of specific hot spots through nano-fabrication and enhancement of substrate sensitivity and affinity to interested molecules. The main objective of this research is to gain fundamental understanding of capability limit of SERS optical fiber technology through systematic experimental studies combined with simulations.

SERS fiber probe based on Au nanotriangle has been fabricated and factors affecting Raman signal through fiber are investigated. These factors include numerical aperture of objective lens, slit width of spectrometer, fiber length and SERS array size on fiber facet. Size of SERS array coated on fiber facet is one primary factor accounting for Raman signal loss, which is ultimately due to decrease in laser power density on the fiber end. The most fundamental optimization consideration for SERS fiber probe is a tradeoff between sensitivity and spatial resolution.

A novel Au pyramid/ring SERS platform with enhancement of spatial confinement and electric field is demonstrated for potential SERS fiber probe application through finite-difference time-domain (FDTD) simulation. The FDTD simulation work provides an alternative way to enhance Raman signal intensity while keep high spatial resolution for SERS fiber probe. Excitation of surface plasmon polaritons (SPPs) by circular Au rings and coupling of SPPs with Au pyramids are studied to improve electric field and overall SERS enhancement of the platform. Au pyramids combined with rings can achieve confinement of the electric field within a 700?700 nm region, which could provide higher spatial resolution for SERS fiber probe.

In addition, a practical way of patterning metallic nanostructures for achieving high SERS enhancement factors (EFs) and high hot spot density is demonstrated. By superimposing one layer Au triangle array on another to form multilayer triangle array, SERS signal can be enhanced by two orders of magnitude. The physical understanding of the SERS enhancement of multilayer triangle array is also developed through FDTD simulation. The drastic increased SERS EFs and hot spot density are due to an increase of amount of gaps formed between Au triangles and a decrease of the gap size.

The thesis also presents a serendipitous discovery of bilayer MoS2 spectral features when exploring MoS2 as a potential SERS enhancement substrate. CVD grown AA’ (60? stacking) and AB (0? stacking) stacked bilayer MoS2 have distinct properties in terms of resonance Raman and photoluminescence. Raman and photoluminescence spectra of AA’ and AB stacked bilayer MoS2 are obtained on Au nano-pyramid surfaces under strong plasmon resonance. A Raman peak named as “a” peak appears in AA’ stacked bilayer MoS2 resonance Raman spectra but not in AB stacked bilayer, a feature that can be used as Raman finger print to differentiate the two types of bilayer MoS2.

    Cover page: Engineering SERS platform in fiber application with a spectroscopic study of bilayer MoS2
    • Article
    • Peer Reviewed
    UCLA Previously Published Works (2017)
    Surface-enhanced Raman spectroscopy (SERS) is a surface-sensitive technique that can enhance the intensity of Raman signal by several orders of magnitude, enabling even the detection of single molecule. This work presents the experimental and theoretical studies of an optical fiber probe with nano-structured end-facet for bio-sensing applications via SERS. The factors affecting the intensity of Raman signal passing through the fiber probe are investigated. These factors include the numerical aperture of the objective lens, the slit width of the spectrometer, the fiber length, and the size of SERS nano-array. The Raman signal loss through fiber compared with optical microscope-based free-space Raman detection is estimated. To further enhance the SERS enhancement factor, a hybrid graphene/Au nano-triangle structure is transferred on the end-facet of the fiber probe to enable SERS. Superimposing graphene layer on Au nano-structure is found to be superior over bare Au nano-structure in terms of the detection sensitivity. Copyright © 2016 John Wiley & Sons, Ltd.
      Cover page: SERS optical fiber probe with plasmonic end‐facet
      • Article
      • Peer Reviewed
      UCLA Previously Published Works (2016)
      This work presents a suspended graphene ribbon device for electrostatic discharge (ESD) applications. The device structure was proposed and fabricated after careful design considerations. Compared to the conventional ESD devices such as diodes, bipolar junction transistors, and metal-oxide-semiconductor field effect transistors, the proposed device structure is believed to render several advantages including zero leakage, low parasitic effects, fast response, and high critical current density. A process flow was developed for higher yield and reliability of the suspended graphene ribbons. Direct current (DC) and transmission-line pulse (TLP) measurements were carried out to investigate the switching behavior of the device, which is crucial for ESD operation. DC measurements with a different configuration were used to assess the mechanical shape evolution of the graphene ribbon upon biasing. Finite Element Simulations were conducted and agreed well with the experimental results. Furthermore, the current carrying capability of non-suspended graphene ribbons was tested using TLP. It was found that the critical current density of graphene is higher than that of copper wires widely used as interconnects in integrated circuits (ICs).
        Cover page: The electro-mechanical responses of suspended graphene ribbons for electrostatic discharge applications
        • Article
        • Peer Reviewed
        UCLA Previously Published Works (2016)
        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.
          Cover page: Selective Manipulation of Molecules by Electrostatic Force and Detection of Single Molecules in Aqueous Solution
          • Article
          • Peer Reviewed
          UCLA Previously Published Works (2016)
          By using finite-difference time-domain simulation, we investigate the enhancement of local electrical field by combining localized surface plasmon (LSP) of Au nanopyramid array and surface plasmon polaritons (SPPs) focused by Au nanorings. We report on the systematic investigation of structural variables that could affect the coupling efficiency between SPPs and LSP to obtain optimal design. The study aims at finding the quasi-quantitative relation between several key-variables of the pyramid-ring combination and the overall surface enhanced Raman spectroscopy (SERS) enhancement at the center of the structure. We consider the following factors that could affect the interaction between SPPs and LSP: ring radius, number of rings, pyramid spacing, number of pyramids, and the relative position of ring center and pyramid array center. By adjusting these structural variables, high electric field enhancement of up to 140× and large volume of hot spots can be achieved. We also investigate the incorporation of the LSP-SPP hybrid structure onto the facet of optical fiber for SERS-based detection.
            Cover page: Coupling SPP with LSPR for Enhanced Field Confinement: A Simulation Study
            • Article
            • Peer Reviewed
            UCLA Previously Published Works (2020)
            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.
              Cover page: Ultrasensitive amyloid β-protein quantification with high dynamic range using a hybrid graphene-gold surface-enhanced Raman spectroscopy platform.
              • Article
              • Peer Reviewed
              UCLA Previously Published Works (2015)
              Prominent resonance Raman and photoluminescence spectroscopic differences between AA' and AB stacked bilayer molybdenum disulfide (MoS2) grown by chemical vapor deposition are reported. Bilayer MoS2 islands consisting of the two stacking orders were obtained under identical growth conditions. Resonance Raman and photoluminescence spectra of AA' and AB stacked bilayer MoS2 were obtained on Au nanopyramid surfaces under strong plasmon resonance. Both resonance Raman and photoluminescence spectra show distinct features indicating clear differences in interlayer interaction between these two phases. The implication of these findings on device applications based on spin and valley degrees of freedom will be discussed.
                Cover page: Spectroscopic Signatures of AA′ and AB Stacking of Chemical Vapor Deposited Bilayer MoS2
                • Article
                • Peer Reviewed
                UCLA Previously Published Works (2015)
                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 ∼10(10). 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(-10) M 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).
                  Cover page: Label-Free SERS Selective Detection of Dopamine and Serotonin Using Graphene-Au Nanopyramid Heterostructure
                  • Article
                  • Peer Reviewed
                  UCLA Previously Published Works (2018)
                  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.
                    Cover page: Surface enhanced Raman spectroscopy distinguishes amyloid Β‐protein isoforms and conformational states
                    • Article
                    • Peer Reviewed
                    Climate & Ecosystems (2016)
                    Shewanella putrefaciens W3-18-1 harbours two periplasmic nitrate reductase (Nap) gene clusters, NapC-associated nap-alpha (napEDABC) and CymA-dependent nap-beta (napDAGHB), for dissimilatory nitrate respiration. CymA is a member of the NapC/NirT quinol dehydrogenase family and acts as a hub to support different respiratory pathways, including those on iron [Fe(III)] and manganese [Mn(III, IV)] (hydr)oxide, nitrate, nitrite, fumarate and arsenate in Shewanella strains. However, in our analysis it was shown that another NapC/NirT family protein, NapC, was only involved in nitrate reduction, although both CymA and NapC can transfer quinol-derived electrons to a periplasmic terminal reductase or an electron acceptor. Furthermore, our results showed that NapC could only interact specifically with the Nap-alpha nitrate reductase while CymA could interact promiscuously with Nap-alpha, Nap-beta and the NrfA nitrite reductase for nitrate and nitrite reduction. To further explore the difference in specificity, site-directed mutagenesis on both CymA and NapC was conducted and the phenotypic changes in nitrate and nitrite reduction were tested. Our analyses demonstrated that the Lys-91 residue played a key role in nitrate reduction for quinol oxidation and the Asp-166 residue might influence the maturation of CymA. The Asp-97 residue might be one of the key factors that influence the interaction of CymA with the cytochromes NapB and NrfA.
                      Cover page: Functional roles of CymA and NapC in reduction of nitrate and nitrite by Shewanella putrefaciens W3-18-1
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