An electro-active platform based on a single-mode integrated optical waveguide over-coated with a 13-nm indium tin
oxide film was developed for highly sensitive investigations on the kinetics of redox reactions from a sub-monolayer of
cytochrome-c proteins. Optical impedance spectra (with and without cytochrome-c proteins present in the spectroelectrochemical
flow-cell) were measured with the single-mode integrated optical waveguide for a 10-mV ac electric
potential modulation. Significant changes in the ac component of the optical baseline response were observed, and a new
analysis was developed to factor out the working electrode effects and deliver accurate results for the Faradaic process.
Faradaic current density and active surface coverage were reconstructed at several modulation frequencies. As small as
7x10-14 mole/cm2 of cytochrome-c proteins were detected under such electric potential modulation leading to a faradaic
current of about 200 nA/cm2. Such level of faradaic current is extremely difficult to be isolated by other electrochemical
techniques (e.g. electrical impedance measurements) due to the strong background created by an always present electric double layer. We were able to achieve those detection limits because the optical signal is immune to those events and can be tuned solely to the Faradaic process. This highly sensitive and accurate strategy of spectro-electrochemistry is proved powerful for measurements of extreme small amount of electro-active proteins and has the potential to be used in many other important electrochemical processes.
Indium tin oxide (ITO) is one of the most used conductive transparent oxide (CTO). Its electrical and optical properties under different environments are crucial to several applications. In this study, single-mode, broadband, integrated optical waveguide (IOW) technique was used to investigate the spectroscopic properties of ultra-thin (~ 13 nm) ITO films under electrical potential modulation. Optical absorbance changes under cyclic voltammetry at various probing wavelengths were measured. Electric potential for minimum absorbance against wavelength was analyzed, and we observed a clear linear correlation between them. Understanding the mechanisms behind this unusual spectroscopic change under potential modulation may have an important impact on several technological applications of ITO.
In this work, we experimentally investigated the effects of sodium chloride on the molar absorptivity and surface density of a submonolayer of chlorophyll a adsorbed onto hydrophilic and hydrophobic solid/liquid interfaces. Those investigations were made possible by a broadband spectroscopic platform based on single-mode, integrated optical waveguides, which allows for extremely sensitive spectroscopic detection of analytes immobilized at submonolayer levels. Chlorophyll a with a constant bulk concentration (1.4 μM) was dissolved in phosphate buffer solutions (7 mM) of neutral pH, but with different sodium chloride concentrations. For a buffer solution of 1 mM of sodium chloride, the measured surface density of chlorophyll a was 0.209 pmol/cm2 for a hydrophilic and 0.125 pmol/cm2 for a hydrophobic surface. For a phosphate buffer solution of 10 mM of sodium chloride, the measured surface density of chlorophyll a was 0.528 pmol/cm2 for a hydrophilic and 0.337 pmol/cm2 for a hydrophobic surface. Additionally, a hypsochromic shift of the Soret band was observed for the adsorbed pigment in correlation with an increase in buffer ionic strength. The adsorption of chlorophyll a onto different surfaces can play an important role to elucidate several processes found in nature and provide a rationale for bio-inspired new material technologies.
We report a technique to substantially boost the spectral bandwidth of a conventional waveguide grating coupler by using a solid immersion cylindrical lens at the aplanatic condition to create a highly anamorphic beam and reach a much larger numerical aperture, thus enhancing the spectral bandwidth of a free-space propagating optical beam coupled into a single-mode planar integrated optical waveguide (IOW). Our experimental results show that the broadband IOW spectrometer thus created almost doubles (94% enhancement) the coupled spectral bandwidth of a conventional configuration. To exemplify the benefits made possible by the developed approach, we applied the technique to the broadband spectroscopic characterization of a protein submonolayer; our experimental data confirm the enhanced spectral bandwidth (around 380-nm) and illustrate the potentials of the developed technology. Besides the enhanced bandwidth, the broadband coupler of the single-mode IOW spectrometer described here is more robust and user-friendly than those previously reported in the literature and is expected to have an important impact on spectroscopic studies of surface-adsorbed molecular layers and surface phenomena.
The development of molecular devices based on immobilized protein films is currently a very active area of research worldwide. Detailed structural and functional characterization of these films is a prerequisite to the rational development of deposition methods that product bioactive structures, but is a technically difficult challenge. Two recent thrusts in our work have been investigated of methods designed to create macroscopically ordered arrays of protein molecules, and development of new optical techniques to characterize the ensemble properties of these arrays.
We have developed a method for simple and highly sensitive detection of multivalent proteins using an optical waveguide sensor. The optical biosensor is based on optically tagged glycolipid receptors imbedded within a fluid phospholipid bilayer membrane formed on the surface of a planar optical waveguide. The binding of multivalent toxin initiates a fluorescence resonance energy transfer resulting in a distinctive spectral signature that is monitored by measuring emitted luminescence above the waveguide surface. The sensor methodology is highly sensitive and specific, and requires no additional reagents or washing steps. Demonstration of the utility of protein-receptor recognition using planar optical waveguides is shown here by the detection of cholera toxin.
The planar integrated optical waveguide (IOW) is an inherently sensitive geometry for attenuated total reflection (ATR) spectroscopy of interfacial samples. A major disadvantage that has limited its wider use is the difficulty of measuring broadband spectra. Due to the quantized nature of light propagation in a planar IOW, conventional grating and prism couplers are efficient only over a narrow (less than 5 nm) spectral range at a given launch angle. We have developed a multichannel spectrometer capable of measuring a broadband visible ATR spectrum at the surface of a single mode, planar waveguide. The bandwidth is greater than 150 nm, which makes it possible to measure spectra of very weakly absorbing molecular films. We have also developed an electrochemically- active, planar IOW (EA-IOW) that combines the information content of spectroelectrochemistry with the sensitivity of the single mode planar waveguide geometry. An evaluation of this device has demonstrated that highly sensitive spectroelectrochemistry of surface confined films can be performed; the estimated pathlength enhancement is ca. 4,000 relative to a transmission geometry.
In this paper, we present a chemical sensor based on the modification of an optical resonator: the optical path length of the resonant cavity is changed by the chemical in question, thus shifting its resonant frequency.
In this work we present three novel optical approaches towards the implementation of bio-and chemical sensors. First we describe an absorbance spectroscopic techniques developed on a waveguide platform that features a sensitivity enhancement of 4 orders of magnitude compared to the conventional transmission measurements of ultra-thin films. Next we show a waveguide Zeeman interferometric technique, which is based on the relative phase change between the TE and TM waveguide modes, applied as a sensor platform. Finally an external-cavity laser, a semiconductor laser combined with a single mode optical fiber and a Bragg grating reflector, was built to work as an active sensor where the analyte species were incorporated inside the resonant cavity to increase sensitivity.
We study the emission properties of various laser cavities under pulsed optical excitation of the active semiconducting conjugated polymer material. Physical origin, magnitude, and dynamics of optical gain in these novel active laser materials are discussed leading to a selection of suitable cavity configurations for laser applications. We demonstrate laser action for various planar and ring resonator configurations that can be achieved in the regimes of transient inversion and quasi stationary excitation of the polymer material pumping with femtosecond and nanosecond pulses, respectively.
A novel scheme of a laser-based chemical sensor has been examined. The scheme is based on the lasing frequency shift of a DBR laser as a result of refractive index change of the sensitive coating in the presence of chemicals in question. The applicability and advantages of different schemes are discussed. The results of preliminary experiments related to the construction and stability of an external cavity DBR laser and interferometric measurements of refractive index change are presented.
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