Time-resolved Optical Waveguide Research Articles

The analysis of time-resolved optical waveguide absorption spectroscopy based on positive matrix factorization

Time-resolved optical waveguide absorption spectroscopy (OWAS) makes use of an evanescent field to detect the polarized absorption spectra of sub-monomolecular adlayers. This technique is suitable for the investigation of kinetics at the solid/liquid interface of dyes, pigments, fluorescent molecules, quantum dots, metallic nanoparticles, and proteins with chromophores. In this work, we demonstrate the application of positive matrix factorization (PMF) to analyze time-resolved OWAS for the first time. Meanwhile, PCA is researched to compare with PMF. The absorption/desorption kinetics of Rhodamine 6G (R6G) onto a hydrophilic glass surface and the dynamic process of Meisenheimer complex between Cysteine and TNT are selected as samples to verify experimental system and analytical methods. The results are shown that time-resolved OWAS can well record the absorption/desorption of R6G onto a hydrophilic glass surface and the dynamic formation process of Meisenheimer complexes. The feature of OWAS extracted by PMF is dynamic and consistent with the results analyzed by the traditional function of time/wavelength-absorbance. Moreover, PMF prevents the negative factors from occurring, avoids contradicting physical reality, and makes factors more easily interpretable. Therefore, we believe that PMF will provide a valuable analysis route to allow processing of increasingly large and complex data sets.

Direct observation of back energy transfer in blue phosphorescent materials for organic light emitting diodes by time-resolved optical waveguide spectroscopy

We demonstrate a high-sensitive transient absorption technique for detection of excited states in an organic thin film by time-resolved optical waveguide spectroscopy. By using a laser beam as a probe light, we detect small change in the transient absorbance which is equivalent to 10-7 absorbance unit in a conventional method. This technique was applied to organic thin films of blue phosphorescent materials for organic light emitting diodes. We directly observed the back energy transfer from emitting guest molecules to conductive host molecules.

The application of principal component analysis and non-negative matrix factorization to analyze time-resolved optical waveguide absorption spectroscopy data

Time-resolved optical waveguide absorption spectroscopy (OWAS) makes use of an evanescent field to detect the polarized absorption spectra of sub-monomolecular adlayers. This technique is suitable for the investigation of kinetics at the solid–liquid interface of dyes, pigments, fluorescent molecules, quantum dots, metallic nanoparticles, and proteins with chromophores. In this work, we demonstrate for the first time the application of non-negative matrix factorization (NMF) to the analysis of time-resolved OWAS data, because NMF prevents the negative factors from occurring, avoids contradicting physical reality, and makes factors more easily interpretable. Meanwhile, principal component analysis (PCA) is researched for the comparison of NMF. Two-factor loadings and scores derived by PCA and NMF are consistent with the results analyzed by the function of time/wavelength-absorbance. Moreover, NMF avoids contradicting physical reality, and makes factors more easily interpretable. We believe that NMF will provide a valuable analysis route to allow processing of increasingly large and complex data sets.

Benefits and Limitations of Porous Substrates as Biosensors for Protein Adsorption

Porous substrates have gained widespread interest for biosensor applications based on molecular recognition. Thus, there is a great demand to systematically investigate the parameters that limit the transport of molecules toward and within the porous matrix as a function of pore geometry. Finite element simulations (FES) and time-resolved optical waveguide spectroscopy (OWS) experiments were used to systematically study the transport of molecules and their binding on the inner surface of a porous material. OWS allowed us to measure the kinetics of protein adsorption within porous anodic aluminum oxide membranes composed of parallel-aligned, cylindrical pores with pore radii of 10-40 nm and pore depths of 0.8-9.6 μm. FES showed that protein adsorption on the inner surface of a porous matrix is almost exclusively governed by the flux into the pores. The pore-interior surface nearly acts as a perfect sink for the macromolecules. Neither diffusion within the pores nor adsorption on the surface are rate limiting steps, except for very low rate constants of adsorption. While adsorption on the pore walls is mainly governed by the stationary flux into the pores, desorption from the inner pore walls involves the rate constants of desorption and adsorption, essentially representing the protein-surface interaction potential. FES captured the essential features of the OWS experiments such as the initial linear slopes of the adsorption kinetics, which are inversely proportional to the pore depth and linearly proportional to protein concentration. We show that protein adsorption kinetics allows for an accurate determination of protein concentration, while desorption kinetics could be used to capture the interaction potential of the macromolecules with the pore walls.

Study of Competitive Adsorption Behavior of Protein andMethylene Blue by Optical Waveguide Spectroscopy

We used time-resolved optical waveguide spectroscopy(OWGS) to in-situ measure visible absorption spectrum for methylene blue(MB) adsorbed on a hydrophilic glass waveguide from an aqueous solution containing proteins such as bovine serum albumin(BSA) and hemoglobin(Hb).The competitive adsorption of MB and the protein was detected at a solution pH lower than the isoelectric point of the protein.Since the competitive adsorption behavior of MB and the protein is very sensitive to the protein concentration,the protein content in the mixed solution was readily determined by time-resolved OWGS.In addition,a kinetic equation for the competitive adsorption of the two molecules was deduced based on the Langmuir adsorption isotherm.The best fit of the measured time course of the absorbance with the theoretical kinetic equation reveals that the number of MB molecules adsorbed on the surface exponentially decreases with time after reaching a maximum.A quasi-linear relationship between the fitting parameters and the protein concentration was also obtained.

Time-Resolved Optical Waveguide Spectroscopy for Studying Protein Adsorption Kinetics

A time-resolved evanescent wave absorption technique is employed in studying the adsorption of a mammalian protein (cytochrome c) from an electrolyte-free solution on a hydrophilic glass surface. The method combines the sensitivity of a fast-scan spectral analyzer and surface specificity of slab optical waveguide (SOWG) technique. While proteins are viewed to undergo an overall irreversible adsorption process brought about by structural changes that occur after the initial adsorption phase, we have used an SOWG technique to investigate the first stage of the adsorption process while no noticeable changes have taken place. Absorbance data within the first two seconds after contact fit well into the reversible adsorption model characteristic of Langmuir-type adsorption but the goodness-of-fit diminishes as the protein molecules spend more time on the surface.

In Situ Investigation of Coadsorption of Myoglobin and Methylene Blue to Hydrophilic Glass by Broadband Time-Resolved Optical Waveguide Spectroscopy

Recently, we have developed a broadband optical waveguide (OWG) spectrometer by using commercially available glass plates of tens of micrometers in thickness as the substrate-free multimode waveguides (Qi et al. Opt. Lett. 2002, 27, 2001-2003). The spectrometer having a bandwidth from 360 to 800 nm is capable of simultaneously detecting the Soret-band absorption of heme proteins and the visible absorption of organic dyes. In this article, the spectrometer was used to in situ investigate coadsorption of methylene blue (MB) and myoglobin from the mixed aqueous solution onto bare glass. Both MB and myoglobin in the mixed solution are positively charged, which makes them not only avoid the chemical interaction between each other but also easy to adsorb to hydrophilic glass. It was found that the coadsorption of MB and myoglobin occurred just in the early stage and the glass surface was finally occupied by myoglobin. The OWG spectroscopic investigation into the respective MB and myoglobin adsorptions shows that MB adsorption is reversible to some degree but that of myoglobin is irreversible. It reveals that the electrostatic binding of myoglobin to bare glass is stronger than the case of MB. Therefore, the adsorbed MB can be substituted by myoglobin. Moreover, via the electrostatic repulsion the tightly immobilized myoglobin prevents bulk MB from occupying the empty surface sites. It is the reason MB is absent from the hydrophilic glass coated with a submonolayer of myoglobin. In the article, we explained both the strong dimerization of MB at the interface and a slow decrease with time of the Soret-band absorbance after its maximum was reached. We also estimated the myoglobin coverage based on the waveguide theory. The study shows the distinguished applicability of the broadband OWG spectroscopy for in situ, real-time monitoring of the dye-protein coadsorption to silica from the mixed solution.

In situ monitoring of metal nanoparticle self-assembly on protein-functionalized glass by broadband optical waveguide spectroscopy

Broadband, time-resolved optical waveguide (OWG) spectroscopy has been used for in situ, real-time investigation into the self-assembly of metal nanoparticle monolayers. The OWG spectroscopy makes it possible to use the transverse electric (TE) and transverse magnetic (TM) modes to measure surface plasmon absorption of immobilized metal nanoparticles in two directions, parallel and normal to the waveguide surface. Therefore, this technique can provide direction-dependent information on the metal nanoparticles at the interface. In this paper, a 50-μm-thick glass plate was used as a slab waveguide and the kinetics of Au nanoparticle adsorption on a hemoglobin-functionalized glass substrate was examined in the early stage of self-assembly. The findings show that with the TE mode the surface plasmon resonance (SPR) behavior for immobilized Au nanoparticles is different from that with the TM mode.

A Kinetic Study of Cytochrome c Adsorption to Hydrophilic Glass by Broad-Band, Time-Resolved Optical Waveguide Spectroscopy

The adsorption process of cytochrome c from aqueous solution onto glass and its desorption process from glass into water were investigated by broad-band, time-resolved optical waveguide spectroscop...

Experimental evidence of the reversibility of the first stage of protein adsorption at a hydrophobic quartz surface near the isoelectric point

AbstractThe reversibility of the first stage of adsorption of zwitterionic cytochrome c on a hydrophobic quartz surface was investigated using time‐resolved slab optical waveguide (SOWG) absorption spectroscopy. Using a novel prism‐free broadband coupling approach, absorbance data were collected successfully at a 50 ms time interval during the first few seconds after solution–surface contact. Near the isoelectric point where the cytochrome c molecules possess a net charge of zero and hence cannot be influenced by an electric field, the speed at which adsorption proceeded was found to be dependent on cytochrome c concentration as well as on surface hydrophobicity. It was also observed that the degree of protein adsorption increased as the surface hydrophobicity was increased. Within 6 s the adsorption process appeared to be reversible, as revealed by extremely low chi‐squared values when the absorbance data were fitted into the reversible Langmuir‐type kinetic model. The standard Gibbs free energy of adsorption was also calculated from the absorbance data. Copyright © 2003 John Wiley & Sons, Ltd.

Study of Initial Adsorption Process of Hemoglobin to Glass Surface by Using Time-Resolved Slab Optical Waveguide(SOWG) Spectroscopy

Abstract A slab optical waveguide(SOWG) has been used to study the initial adsorption process of hemoglobin on glass surface using time-resolved absorption measurement of the evanescent wave. Absorbance data were taken at 50-ms time interval during the first 12 s after the measurement has started. Results showed that hemoglobin was adsorbed on glass and the data demonstrated excellent agreement with Langmuir-like model.

Lag phases in the adsorption of lysozyme to Si(Ti)O2 surfaces in the presence of sodium thiocyanate. Part I. Phenomenology

The adsorption kinetics of hen egg white lysozyme (HEWL) in the presence of NaSCN at pH 7.4 onto Si0.8Ti0.2O2 surfaces were measured by means of time-resolved optical waveguide lightmode spectroscopy (OWLS). The adsorption kinetics were characterized by the presence of a lag phase whose duration decreased with increasing lysozyme bulk concentration. This lag phase was followed by rapid growth of the interfacial protein film which ceased at a surface coverage close to that predicted for a monolayer. At bulk concentrations lower than about 50 µg cm-3, the maximum rate of adsorption was equal to the rate predicted for a transport-controlled process from the solution to the surface and at higher bulk concentrations such lag phases were no longer observed. Storage of lysozyme solution prior to adsorption reduced the duration of the lag phase, but analytical ultracentrifugation measurements showed the absence of significant aggregation even at bulk concentrations of 103 µg cm-3. These features are consistent with an adsorption process requiring the adsorption of a small critical nucleus preformed in solution followed by the completion of the adlayer by a mechanism akin to crystal growth. The size of this critical nucleus was estimated to be of the order of only a few lysozyme molecules.

Optical Characterization of a Ferroelectric Liquid Crystalline Polymer Studied by Time-Resolved Optical Waveguide Spectroscopy

Te static structures and switching dynamics of side chain ferroelectric liquid crystalline polymer (PSiOFLC) [Si(Me)(R)O] n (R= (CH 2 ) 10 OPhCO 2 PhCO 2 CH 2 CH(Me)(Et)) were investigated by using time-resolved optical waveguide spectroscopy (TROWS). A good alignment of the mesogenic side chains was obtained by a shearing procedure. The liquid crystalline phases of PSiOFLC were characterized by determining the dielectric tensor diagonals of PSiOFLC, in which the contribution of the main chain to the optical anisotropy could be neglected. The PSiOFLC layer in the S A phase could be treated as a uniaxial refractive index ellipsoid which aligned parallel to the substrate plane with bookshelf structure. The PSiOFLC layer in the S C* phase under the large electric field was also considered as a uniaxial ellipsoid model. The transient waveguide mode patterns of PSiOFLC were successfully obtained with a millisecond time resolution. During the reorientation, the smectic layer structure changed from bookshelf to chevron and then back to the bookshelf structure. The slowness of the switching time was attributed to a polymer effect in which the polymer backbones lying in the smectic layer plane act as a hook to restrict the motion of side chains due to the binding of the mesogenic moieties to the polymer backbone, resulting in the increase of viscosity.

Dynamics of the Reorientation of a Ferroelectric Liquid Crystal Under an Electric Field Studied by Time-Resolved Optical Waveguide Spectroscopy

The dynamic structure and molecular reorientation of a ferroelectric liquid crystal induced by an alternating electric field have been investigated by time-resolved optical waveguide spectroscopy (TROWS) in which the transient waveguide mode patterns were recorded as a function of both time and angle of incidence of a probe beam. A ferroelectric liquid crystal, 4-(2S, 3S)-[2-chloro-3-methylpentanoyloxy]-4′-octyloxybiphenyl (3M2CPOOB) was utilized, which showed a fast response to an external electric field in the order of a microsecond. Under a high electric field, it forms the so-called bookshelf structure in which the director of 3M2CPOOB is tilted by 22.5° away from the smectic layer normal in the SmC* phase. Time-resolved waveguide mode patterns were successfully obtained with a time-resolution of 1.0 μs which enabled us to determine the transient features of dielectric tensor of 3M2CPOOB molecules in three dimensional coordinate system. A model that the molecular reorientation coupled with the layer buckling and the rotation around the cone was proposed from the quantitative analysis of the TROWS data.

Time-resolved optical waveguide study of the reorientation in a nematic liquid crystal under applied electric field

The reorientation behavior of nematic liquid crystal molecules induced by an electric pulsed field was investigated by means of time-resolved optical waveguide spectroscopy (TROWS) which was developed to observe transient changes in dielectric constants and to measure the thickness of a thin dielectric medium. Time evolution of the waveguide mode patterns was successfully obtained by using TROWS, which records the reflectivity as both functions of the incident angle of laser light and the time from switching-on or -off of the external electric field. The transient dielectric tensor profile in the 4′-pentyl-4-cyanobiphenyl (5CB) layer both after switching-on and -off of the electric field could be precisely determined in a three-dimensional coordinate system by using Deuling’s theory which treats the orientational deformation induced by an external electric field. Theoretical analysis of TROWS data clearly indicated that half of the 5CB molecules rotate clockwise in the plane defined by the rubbing direction and the substrate normal, and the other half rotates counter-clockwise in the same plane. The counter rotation compensates the effect of birefringence observed in the direction parallel and perpendicular to the rubbing direction and the reorientation kinetics could be well described by the Ericksen–Leslie equation. Relaxation behavior from homeotropic to planar orientation could be treated as an orientation diffusion process by applying a Gaussian distribution function to the maximum tilting angle at the middle of the cell. We report the usefulness of TROWS which provided more detailed information about the dynamics of the 5CB molecules.

Time-resolved optical waveguide spectroscopy: Application to a study of the switching process of a nematic liquid crystal under the influence of an electric field

We report a novel optical spectroscopy, time-resolved optical waveguide spectroscopy. This technique was developed in order to investigate the transient behavior of the dielectric constants of thin dielectric medium, e.g., changes of the refractive indices induced by an external trigger such as light, electric field, etc. Optical guided waves propagate in a thin dielectric medium whose optical properties, in particular film thickness and anisotropic dielectric tensor diagonals in molecular and laboratory coordinates, can be evaluated with high accuracy. We employed this technique to a nematic liquid crystal (5CB) as a thin dielectric medium, and molecular reorientation on the switching process was observed through the recording of transient waveguide patterns.