1,720,982 research outputs found
Boronic acid-based sugar sensing by surface-enhanced Raman spectroscopy
No full textBoronic acid-based sugar sensing on gold nanosurfaces have been investigated by surface-enhanced Raman spectroscopy (SERS). SERS is a sensitive molecular sensing technique where highly selective and quantitative identification of analytes are possible via vibrational fingerprints. Nowadays, SERS has a wide range of applications in biological and chemical detection. Sensitive glucose detection is one of the challenging problems in the diagnosis and treatment of diabetes and obesity. Both accurate quantification of the entire class of sugars and individual components as glucose have been regarded important for medical applications. A variety of phenylboronic acids have been used for highly selective sugar detection where the cis-diol configuration of sugars can be recognized in the dehydration reaction with phenylboronic acids. Despite the significance of the binding of vicinal glycols with boronic acid on silver surfaces, no detailed and careful study on gold surfaces has yet been reported by Raman spectroscopy. Considering that phenylboronic acids adsorbed on silver surfaces have been reported to quantify the monosaccharides concentration, it would be worthy to investigate the efficient saccharide sensing by using
gold surfaces including monosaccharides and disaccharides. Gold nanosurfaces have a wide range of potential applications such as chemical sensing and biological labeling due to the stability, biocompatibility, and consistent and easy sample preparations. Two gold surfaces including gold colloidal nanoparticles and aminopropylsilane (APS) gold surfaces were synthesized, and surface adsorption of mercaptophenylboronic acid derivatives and chemical sensing of sugars were performed. Gold colloidal nanoparticles were synthesized by reducing the gold ions with citrate ions, and APS gold surfaces were prepared by immersing pre-treated APS glass slides on gold colloidal nanoparticles.
ii In this thesis work, I investigated the preparation of efficient gold substrates for sugar detection by SERS. We used mercaptophenylboronic acid (MPBA) derivatives which demonstrate distinguishable Raman peaks when they bind on the metal surface via mercapto group. By using MPBA molecules with mercapto group at two different positions (4-MPBA vs. 3-MPBA), I investigated sugar detection by SERS on sugar molecules including D-glucose, D-fructose, and D-sucrose.Maste
Intramolecular charge transfer dynamics of Michler's ketone and its analogues confined in reverse micelles
No full textIn this thesis research, intramolecular charge transfer dynamics of 4,4-bis(dimethylamino)benzophenone (Michler’s ketone; MK) and its analogues, 4,4-diaminobenzophenone (AMK) and 4,4-bis(diethylamino)benzophenone confined in the reverse micelles have been investigated by steady-state and time-resolved electronic spectroscopy. Michler’s ketone has been known to show the intramolecular charger transfer (ICT) character in the excited states in polar solvents. Although MK and its analogues have widely been used as a sensitizer in numerous photonic applications including dye-sensitized solar cells, the details of the ICT mechanisms in the excited state, which shows strong dependence on the kind of organic solvents, are not yet fully understood.
Bis(2-ethylhexyl) sulfosuccinate sodium salt (Aerosol OT; AOT) is an amphiphilic surfactant which forms self-assembled water-in-oil reverse micelles when dissolved in organic solvent in the presence of polar solvent. The shape and size of reverse micelles are strongly dependent on the molar ratio of polar solvent to surfactant. In this work, the AOT reverse micelles with methanol and acetonitrile cores were used and the hydrodynamic radii of the polar cores were estimated as 1.7 nm.
The steady-state absorption spectra of AMK, MK, and EMK in bulk solutions of methanol and acetonitrile shows that these MK analogues forms hydrogen bonding in methanol solution. The absorption bands in methanol appear broadened and slightly red-shifted compared to those observed in acetonitrile solutions. The steady-state emission spectra of MK analogues show strong solvent dependence. Compared to the ICT emission bands in methanol solution, the spectra obtained in acetonitrile solutions show further blue-shifted representing the additional formation of the twisted ICT state. On the other hand, the absorption and emission spectra of MK analogues obtained in the reverse micelles show almost no solvent dependence between the methanol and acetonitrile cores. Since the polar solvent core of the small reverse micelles include only small number of solvent molecules and the solvent dynamics in the reverse micelles are known as abnormally slow compared to the bulk, the exited state dynamics of MK analogues in the polar cores of the small reverse micelles appear almost invariant between the methanol and acetonitrile cores.
Time-resolved emission spectra of MK analogues in bulk solvents and the reverse micelles with methanol and acetonitrile cores have also been obtained by a time-correlated single-photon counting (TCSPC) technique. The ultrafast ICT and twisted ICT dynamics of MK analogues are not determined in TCSPC measurements with a time resolution of ~150 ps. However, the emission lifetimes of the MK analogues in bulk and reverse micelle samples represent the solvent-dependent excited state dynamics of MK analogues with the hindered twisting of amino (AMK), dimethylamino (MK), and diethylamino (EMK) groups in the confined cores of the small reverse micelles. The emission lifetimes of MK analogous in the reverse micelles shows a strong dependence on the electron donating group (amino vs. diethylamino vs. diethylamino), which clearly shows that twisted ICT processes occur in the polar apolar solution of acetonitrile. The quantum yield and transient absorption measurements with a faster (~50 fs) temporal resolution are further required to elucidate the detailed mechanisms of ICT and twisted ICT in the excited states, which is left for future explorations.Maste
Plasmon-enhanced spectroscopy on homogeneous nanostructures
No full textPlasmon-enhanced spectroscopy has been of great interest in many disciplines including physics, chemistry, biology, and materials science. Numerous experimental methods based on surface plasmon resonance (SPR) of metal nanoparticles have been developed for spectroscopic, biosensing, and imaging purposes, where large enhancements in the optical signals of Raman scattering, infrared absorption, and fluorescence are required. Strong local electric fields of metal nanoparticles induced by SPR excitation are inevitable in the large signal enhancements, which can be optimized in general by the shape, size, or composition of the metal nanoparticles. Fluorescence spectroscopy has been widely used in many research fields, where higher sensitivity is generally required to lower the detection limits to single molecule level, for example, and higher photostability is required for longer measurement times in biosensing and trace analysis of bioactive components in cells. However, it is considered challenging to find fluorophores with both high sensitivity and photostability. Alexa Fluor dyes widely used in biological imaging due to their high photostability and selective binding capability to biological active sites show relatively low quantum yields in aqueous solutions. Raman spectroscopy has also been adopted in numerous chemical and biological sensing as a non-invasive and non-destructive analysis technique. However, powerful structure-specific chemical information of Raman scattering in a wide vibrational frequency range is mostly limited by the infinitesimal Raman cross-sections which is about 106 times smaller than those of fluorescence signals. Thus, enormous signal enhancements by the SPRs of metal nanostructures may be inevitable for the applications of Raman spectroscopy with the low detection limits or biological samples of very low concentrations.
Metal-enhanced fluorescence (MEF) and surface-enhanced Raman scattering (SERS) have been considered indispensable for the wide application of fluorescence and Raman spectroscopy, respectively, in overcoming the disadvantages of these spectroscopic methods including the signal sensitivity, photostability, etc. Fluorescence of chromophore is significantly enhanced when it is located near metal nanoparticles or nanosurfaces due to locally enlarged electric fields around the metal nanoparticles and plasmon-coupled emission accompanying the energy transfer between the SPR of metal nanoparticles and the excited states of chromophore. The photostability of chromophore increases with the reduced photobleaching or shortened emission lifetime. Similarly, Raman cross-sections of analytes can be enormously enhanced up to 1014–1015 times when located near metal nanoparticles. Locally enlarged electric fields of metal nanoparticles and charge transfer effects of surface adsorbates are generally considered as the origin of enormous signal enhancements. Raman measurements at single particle or molecular levels have been reported with highly optimized metal nanostructures. Moreover, vibrational probe of SERS measurements conveys local chemical information of site-specific surface adsorption of adsorbates on metal nanosurfaces, which is often considered indispensable in chemical and biological analysis of surface adsorbates.
In this dissertation, the MEF and emission kinetics of dye molecules are studied with homogeneous metal nanosubstrates with well-controlled particle diameters and well-defined SPR bands by steady-state and time-resolved fluorescence spectroscopy such as time-correlated single photon counting (TCSPC). Optimal conditions for the MEF with the homogeneous metal nanosurfaces are investigated by changing the particle diameters of the metal nanoparticles or by mixing different-sized metal nanoparticles in various composition ratios to obtain insights into the development of more efficient fluorescent probes for bioimaging and sensing. The SERS spectra and surface adsorption geometry of aromatic amino acids and short peptides depending on the local environments and surface properties of metal nanoparticles are investigated to understand those of biological macromolecules such as proteins, DNA, and RNA composed of the basic units, which can be applied in biological applications of SERS.
The theoretical basis of MEF and SERS, and synthesis methods for metal colloidal solutions and homogeneous and composite metal colloidal nanosurfaces, and details of TCSPC and micro-Raman setup for fluorescence and Raman measurements are introduced in Chapter 1. In Chapter 2 and 3, the MEF and emission kinetics changes of 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM), rhodamine 700 (Rh700), 4-dimethylamino-4′-nitrobiphenyl (DNBP), and coumarin 343 (C343) on homogeneous silver nanosurfaces composed of a specific diameter of 60–220 nm are discussed. An improved semi-empirical model based on the finite-difference time-domain (FDTD) simulation and emission kinetics measurement results is introduced to analyze the mechanism of the MEF by the dipole and quadrupole SPR of the homogeneous silver nanosurfaces. In Chapter 4, the fluorescence enhancement and emission kinetics of DNBP with the composite silver nanosurfaces showing the dipole SPR of small silver nanoparticles and the quadrupole SPR of large silver nanoparticles in similar wavelengths to the emission of DNBP are compared with those with the silver nanosurfaces composed of only small or large silver nanoparticles. Based on the results in the Chapter 2–4, the optimal conditions for strong MEF with the homogeneous metal nanosurfaces are discussed. In Chapter 5, the differences in the pH-dependent SERS spectra and surface adsorption geometry of L-alanyl-L-tryptophan (Ala-Trp) with citrate-reduced (CT) and borohydride-reduced (BH) gold colloidal nanoparticles are discussed. The difference in the surface geometry of Ala-Trp on the CT and BH gold nanosurfaces are explored in relation to the surface zeta potential change of the gold colloidal nanoparticles depending on pH. In Chapter 6, the surface modifications of CT gold nanoparticles by using cetyltrimethylammonium bromide (CTAB) surfactant as an aggregation agent are introduced to increase the SERS activity of several hydroxyanthraquinones (HAQs). In Chapter 7, the changes in the surface adsorption geometry of L-tyrosine (L-Tyr) and short peptides containing L-Tyr such as Gly-L-Tyr depending on pH are discussed based on the pH-dependent SERS spectra with BH and CT silver colloidal nanoparticles. Vibrational assignments of L-Tyr for major Raman bands are corrected based on the comparison of pH-dependent SERS spectra of L-Tyr with those of tyramine (TRM) and 3-(4-hydroxyphenyl)propionic acid (HPPA) and the density functional theory (DFT) calculation results. The pH-dependent SERS spectra and surface adsorption geometry of a short peptide, glycyl-L-tyrosine (Gly-L-Tyr) are compared with those of its local structure L-Tyr.Docto
Ultrafast Chemical Reactions Probed by Time-Resolved Vibrational Spectroscopy
No full textA chemical reaction refers a series of atomic rearrangements between the reactant and product molecules, where the rearrangements of single chemical bonds occur on the ultrafast time scales of femtosecond and picoseconds. The chemical reactions are generally described by the energy difference between the reactant and products, activation barriers, reaction rates, etc. The reaction rates of a certain chemical reaction can be obtained by experimentally observe the concentration changes of reactant or product molecules. In order to measure the reaction rates of the ultrafast chemical reactions such as intramolecular proton transfer, ultrafast spectroscopic methods are inevitable where ultrashort laser pulses initiate the chemical reactions in the excited state and time-resolved spectroscopy measures the spontaneous concentration changes of reactants and/or products. The reaction dynamics study by the time-resolved spectroscopy would be beneficial in many important applications including artificial photosynthesis, dye-sensitized solar cell, or molecular optoelectronics. Simple photochemical reactions of excited state proton transfers and photoinduced charge transfers with subsequent structural changes in the electron donor or acceptor groups are considered as one of the fundamental processes in many chemical and biological systems. Therefore, the excited-state reaction dynamics probed by time-resolved spectroscopic measurements can be used to understand the ultrafast bond breaking and formation of reactant and product molecules, or the or the instantaneous structural changes accompanying various excited state photophysical processes.
In this thesis, transient absorption spectroscopy and femtosecond stimulated Raman spectroscopy (FSRS) has been adopted as the time-resolved electronic and vibrational probes, respectively. Time-resolved electronic and vibrational probes would be complementary to each other in analyzing the ultrafast photophysical and photochemical processes in the excited state.
The structural changes of 1,2-dihydroxyanthraquinone (alizarin) in dimethyl sulfoxide (DMSO) solution upon the excited-state intramolecular proton transfer (ESIPT) reaction have been studied by FSRS measurements. Previous time-resolved electronic measurements may provide the detailed excited-state dynamics upon the proton transfer. However, the detailed structural changes of alizarin or related molecules upon the ESIPT has not been reported. The ESIPT dynamics of alizarin of 70-80 fs has been observed from the vibrational modes of ν(C═C) and ν(C═O) in the singlet excited state, where the population changes and opposite peak shifts of these modes occurring with the common 70-80 fs time constant are compatible to the proposed transition state of six-membered ring structure with intramolecular hydrogen bonding between the carbonyl and adjacent hydroxyl group.
The ESIPT dynamics of alizarin has been updated by improving the temporal resolution of FSRS measurements with the pulse compression of actinic pump. The vibrational probe of ν(C═C) and ν(C═O) modes in the excited state have provided more detailed ESIPT dynamics (110 fs) of alizarin with the population transfer between the locally-excited (LE) and proton-transferred state, and the coherent oscillation signals in these vibrational probes show that the reaction coordinate of the ESIPT reaction is strongly coupled to several low-frequency vibrational modes of intramolecular hydrogen bonding. The solvent vibrational modes of DMSO, ν(S=O) and ν(CSC) are strongly changed upon the ESIPT of the solute alizarin. Although the vibrational modes of DMSO are inseparable from the nonlinear cross-phase modulation and long-lasting hot ground state Raman signals, the ν(S=O) and ν(CSC) modes of DMSO shows instantaneous (60-120 fs) increase in the “free (isolated)” or “aggregated (dimer)” bands indicating the solvation changes of DMSO upon the ESIPT of alizarin. The solvent vibrational modes of DMSO, ν(S=O) and ν(CSC), may “probe” the ultrafast chemical reactions of the solute indirectly since these modes are very sensitive to the instantaneous solvation changes resulting from the structural changes of the solute.
The photoinduced charge transfer dynamics of curcumin in DMSO solution has also been investigated by FSRS measurements. Curcumin is one of the well-known antioxidants, and shows ultrafast intramolecular charge transfer (ICT) in the excited stat, where the excited state lifetime and fluorescence quantum yield are strongly dependent on solvent polarity and hydrogen bonding with solvent. The vibrational modes of curcumin in the LE and charge-transferred (CT) states are separately observed from the FSRS measurements, where an ultrafast ICT (0.6-0.8 ps) and subsequent vibrational relaxation (6-9 ps) dynamics in the CT state have been retrieved. The ground-state vibrational modes of curcumin, ν8a and ν(C=C,C=O) and the solvent vibrational modes of DMSO, ν(CSC) and ν(S=O) appear strongly coupled to the ICT dynamics of curcumin, which supports the strong solvation interactions including the hydrogen bonding. Especially, the ν(CSC) and ν(S=O) modes of DMSO represent the ultrafast (20-50 fs) dynamics with the hydrogen-bonded species upon the ICT of curcumin. However, further explorations on the detailed spectral changes between the “free” and “hydrogen-bonded” species in the ν(CSC) and ν(S=O) mode of DMSO are required to explain the solvation changes of DMSO in the solvation shells with the ultrafast ICT dynamics of chromophores.
Lastly, up-to-date investigations and future directions for the solvation dynamics study with the ultrafast excited-state processes of chromophores are summarized. All the FSRS measurements described in the thesis are based on the stimulated Raman “gain” measurements, where the Raman pump centered at higher frequency and the Raman probe with the lower frequencies complete the stimulated Raman process. Similarly, the stimulated Raman “loss” measurements requires the Raman pump at lower and the Raman probe at higher frequencies. It has been known that the modulations of the Raman probe by the Stokes and anti-Stokes Raman processes in the stimulated Raman “loss” measurements are observed in the opposite contribution while the Stokes and anti-Stokes Raman signals are inseparable in the stimulated Raman “gain” measurements. We propose the possible separation of the long-lasting thermal signals in the solvent vibrational modes of DMSO (observed in the excited-state dynamics of alizarin and curcumin) by combining the stimulated Raman “gain” and “loss” measurements. The “hot” ground-state transitions of the ν(S=O) and ν(CSC) modes of DMSO by the anti-Stokes Raman process can be subtracted with some experimental control between the stimulated Raman “gain” and “loss” measurements. Similarly, the nonlinear cross-phase modulation artifacts can be minimized by the control experiment only with the solvent. Further experimental developments are required, however, for more accurate determination of the solvation dynamics of DMSO, which may indirectly “probe” the ultrafast chemical reaction dynamics of chromophores in the excited state including the proton and charge transfer.
In this thesis research, the ultrafast chemical reactions of intramolecular proton and charge transfers have been observed by time-resolved electronic and vibrational spectroscopy. FSRS presents numerous advantages in the reaction dynamics study in the excited state due to its high temporal and spectral resolutions and multimodal vibrational probes in a wide spectral range. The ultrafast chemical reaction dynamics in the excited state can also be probed by the instantaneous changes in the solvent vibrational modes of DMSO including the hydrogen bonding interactions when the strong solvation dynamics exists between the chromophore and solvent molecules.Docto
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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