Surface-enhanced Raman scattering study of the adsorption of the anthraquinone pigment alizarin on Ag nanoparticles

FT-Raman and surface-enhanced Raman scattering (SERS) spectroscopy were applied in the vibrational characterization and study of the adsorption and acidity behavior of the highly fluorescent anthraquinone dye alizarin on Ag colloids prepared by chemical reduction with hydroxylamine hydrochloride. The SERS spectra were obtained at different conditions of pH, excitation wavelength and pigment concentration in order to deduce the adsorption mechanism of this molecule. On the basis of the results found we propose an adsorption model for alizarin, which has a different acidic behavior on the metal surface to that in solution. On the metal the deprotonation order of the OH groups changes with respect to the aqueous solution, the OH in position 1 being the first to be ionized instead of that in position 2 as occurs in solution. The two main alizarin forms identified on the metal surface correspond to the mono-and dianionic alizarin species

UV Raman Excitation Profiles of Imidazole, Imidazolium, and Water

ination of periods of low magnetic field strength, during which the radial Lorentz force is insufficient to retard the radial diffusion of ions and electrons in the plasma. In addition to increasing analyte line-to-background intensity ratios, the hotter plasma core associated with the unidirectional discharge should be useful in the analysis of refractory solid powder samples. Preliminary studies t4 have indicated that the graphite vapor theta-pinch plasma may be useful for the direct analysis of solid powders. The principal problem associated with the present unidirectional discharge system is the limitation to operation at 6 kV or less. This situation should be corrected by the use of fast-recovery diodes. Alternatively, a discharge circuit based on a distributed-element LC transmission line should be capable of producing unidirectional, nearly square-wave current pulses without the use of a diode shunt. A low-voltage prototype circuit has produced very encouraging results. 1. S. R. Goode and D. T. Pipes, Spectrochim. Acta 36B, 925 (1981). 2. G. J. Kamla and A. Scheeline, Anal. Chem. 58, 923 (1986). 3. G. J. Kamla and A. Scheeline, Anal. Chem. 58, 932 (1986). 4. R. J. Klueppel and J. P. Waiters, Spectrochim. Acta 35B, 431 (1980). 5. V. Majidi and D. M. Coleman, Appl. Spectrosc. 41, 200 (1987 Received 6 June 1987. * Author to whom correspondence should be sent. UV Raman Excitation Profiles of I N T R O D U C T I O N The recent extension of R a m a n spectral measurements into the UV spectral region has resulted in a number of R a m a n excitation profile studies of small molecules such as acetamide, N-methylacetamide, 1,2 acetonitrile, sulfate, nitrate, and perchlorate 3 and larger aromatic molecules such as benzene, 4-7 substituted benezene derivatives, 8'9 imidazole, imidazolium, 1° and aromatic amino acids. 1°-15 Other studies examined nucleic acids, 16-19 proteins, ~,2°-22 and polycyclic aromatic hydrocarbons such as pyrene. 23-2~ The motivations for these studies include establishing the principles of resonance enhancement, ~-~° exploring excited states of these species, and demonstrating the analytical utility of resonance Raman scattering for studying aromatics in complex systems such as aromatic amino acids in proteins and polycyclic aromatic hydrocarbons in coal liquid samples. 1°-~5,23-26 Often resonance R a m a n enhancements of six to seven orders of magnitude are observed with UV excitation, in comparison to results from visible wavelength excitation. The vibrational modes enhanced are generally totally symmetric and distort the molecule along directions of electron density changes between the ground and the resonant electronic excited state. H Preresonance excitation of derivatives such as sulfate, acetone, and ace

Simultaneous optical coherence tomography and confocal imaging for retinal investigations

Techniques for simultaneous display of two en-face images of the retina, an optical coherence tomography image and a confocal image in pixel to pixel correspondence are presented. Such images are obtained from the eye using a fiberised low coherence interferometer and a bulk confocal receiver sharing the same galvanometer scanner and interface optics directing light to the eye. The images have different depth resolution and complement each other in the process of system adjustment and measurement

Dynamic Electrowetting on Nanofilament Silicon for Matrix-Free Laser Desorption/Ionization Mass Spectrometry

Dynamic electrowetting on nanostructured silicon surfaces is demonstrated as an effective method for improving detection sensitivity in matrix-free laser desorption/ ionization mass spectrometry. Without electrowetting, silicon surfaces comprising dense fields of oriented nanofilaments are shown to provide efficient ion generation and high spectral peak intensities for deposited peptides bound to the nanofilaments through hydrophobic interactions. By applying an electrical bias to the silicon substrate, the surface energy of the oxidized nanofilaments can be dynamically controlled by electrowetting, thereby allowing aqueous buffer to penetrate deep into the nanofilament matrix. The use of electrowetting is shown to result in enhanced interactions between deposited peptides and the nanofilament silicon surface, with improved signal-to-noise ratio for detected spectral peaks. An essential feature contributing to the observed performance enhancement is the open-cell nature of the nanofilament surfaces, which prevents air from becoming trapped within the pores and limiting solvent penetration during electrowetting. The combination of nanofilament silicon and dynamic electrowetting is shown to provide routine detection limits on the order of several attomoles for a panel of model peptides. Following its introduction in the late 1980s, 1 matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) has emerged as a leading method for soft ionization of analyte molecules prior to mass determination, typically by time-of-flight (TOF) MS. The technique has been widely used for a range of biopolymers, which are typically cocrystallized with a UV-absorbing organic matrix to enhance their ionization efficiency. However, MALDI-MS is well-known to suffer from excessive matrix background signal for low molecular weight analytes, and thus its application has been limited to the study of relatively large biopolymers. Furthermore, MALDI-MS sensitivity is generally lower than that of electrospray ionization (ESI)-MS, with typical detection limits around 1 fmol, 2 although limits on the order of 10-100 amol can achieved using microstructured targets and optimized target preparation methods. [3][4][5] In 1999, Wei et al. reported the desorption/ionization on porous silicon (DIOS) technique, 6 in which an electrochemically etched porous silicon (pSi) surface serves to efficiently absorb UV laser energy during LDI-MS analysis, allowing effective biomolecular measurements in the absence of organic matrix. As a result, matrix interference typically encountered below ca. 500 m/z in MALDI-MS is eliminated, allowing biomolecules within this range to be accessed by the DIOS-MS technology. 1 In addition to smallmolecule analysis, 7 the technology has been applied to protein characterization. [8][9][10] Because the pSi targets can be readily functionalized, preferential binding of analytes based on affinity 11,12 or hydrophobic 13 interactions has been demonstrated. The high sensitivity combined with good tolerance to contaminants also makes DIOS-MS an attractive platform for forensics applications. 14 Detailed reviews of the DIOS-MS technique, including background and applications, have been presented in recent review papers. 15,16 Nanostructured pSi surfaces developed for DIOS-MS are prepared by galvanostatic etching of silicon, 6,17,18 resulting in a surface consisting of nanoscale pores which exhibit a closed-cell morphology. For the analysis of biopolymers containing hydrophobic domains, such as peptides, silylation of the pSi surface

Counting Single Chromophore Molecules for Ultrasensitive Analysis and Separations on Microchip Devices A C R e s e a r c h

Separations of 15 pM rhodamine 6G and 30 pM rhodamine B performed in a micromachined electrophoresis channel were detected by counting fluorescence bursts from individual molecules. The migration times, peak widths, and analyte concentrations were estimated from the number and the migration time distribution of the detected molecules. Concentration detection limits estimated at >99% confidence were 1.7 pM rhodamine 6G and 8.5 pM rhodamine B. The separations required <35 s and the relative migration time uncertainties were less than 2.0%. These are the lowest detection limits reported for microchip separation devices and the first example of single-chromophore molecular counting for detection of chemical separations. In this work, we report the use of single-molecule fluorescence detection by confocal microscopy for detecting separations of † Present address