Tomas Martan

In vivo optical detection of pH in microscopic tissue samples of Arabidopsis thaliana.

Minimally invasive in vivo measurement of pH in microscopic biological samples of μm or μl size, e.g. plant cells, tissues and saps, may help to explain complex biological processes. Consequently, techniques to achieve such measurements are a focus of interest for botanists. This paper describes a technique for the in vivo measurement of pH in the range pH5.0 to pH7.8 in microscopic plant tissue samples of Arabidopsis thaliana based on a ratiometric fluorescence method using low-loss robust tapered fiber probes. For this purpose tapered fiber probes were prepared and coated with a detection layer containing ion-paired fluorescent pH-transducer 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (c-HPTS). A fluorescence ratiometric approach was employed based on excitation at 415 nm and 450 nm and on the comparison of the fluorescence response at 515 nm. The suitability of tapered fiber probes for local detection of pH between 5.0 and 7.8 was demonstrated. A pH sensitivity of 0.15 pH units was achieved within the pH ranges 5.0-5.9 and 7.1-7.8, and this was improved to 0.04 pH units within the pH range 5.9-7.1. Spatial resolution of the probes was better than 20 μm and a time response within 15-20s was achieved. Despite the minute dimensions of the tapered fiber probes the setup developed was relatively robust and compact in construction and performed reliably. It has been successfully employed for the in vivo local determination of pH of mechanically resistant plant tissues of A. thaliana of microscopic scale. The detection of momentary pH gradients across the intact plant seems to be a good tool for the determination of changes in pH in response to experimental treatments affecting for example enzyme activities, availability of mineral nutrients, hormonal control of plant development and plant responses to environmental cues.

Theoretical Analysis and Preparation of Tapered Suspended Core Microstructure Fibers

Suspended core microstructured optical fibers (SC-MOFs) represent a special kind of microstructured fibers which can be employed for optical sensing. In this article one experimental SC-MOF and its tapers were numerically analyzed in order to determine if the overlap of evanescent waves with the fiber cladding holes can be increased by tapering. Power distributions of fundamental modes in the SC-MOF and its tapers were calculated by using a vectorial finite element method. The calculations have shown that the evanescent wave overlap (at a wavelength of 1550 nm) can be increased from 0.75% up to 26% by tapering of a section of the SC-MOF from an original diameter of 125 µm down to 31.25 µm. Fiber tapers designed on the basis of the numerical analysis were prepared by so-called ‘flame brush technique’ employing a fiber puller of own construction. Experiments with the prepared tapers have proved that the cladding holes remain sufficiently large for passing liquid and gas analytes through and that the core diameter of the untapered pigtails is large enough for low-loss coupling of optical beams into the tapered fiber.

Optical sensor based on sensitive polymer layer

In chemical, oil, and food industries, there are still higher requirements on miniaturization of optical sensors for a concentration measurement of gases e.g. a CO2, O2, and NH3. The paper deals with development of miniaturised optical sensor for an aqueous carbon dioxide measurement using a sensitive polymer layer. The optical sensor module consists of two parts, a remission sensor and a removable layered structure (with incorporated dyed polymer) which is closely placed on the surface of a remission sensor. A dyed polymer film is used as an optical-chemical transducer working on a principle of colour changes caused by a chemical reaction of an analyte and indicator dye. A novel remission sensor module was developed for an evaluation of the spectral absorption changes of sensitive polymer layer. The remission sensor module composed of LED diodes located in a central cavity of the sensor module and PIN diodes situated around the cavity. The LEDs emit light with optimised wavelengths and irradiate the polymer film. Light response (the changes of the spectral absorption) of the irradiated polymer film is detected by PIN diodes. A colour shift is further analyzed and evaluated by electronics without using a photometer.

Microstructure optical fibres for detection of gaseous analytes

This paper deals with a steering-wheel microstructure optical fibre designed prepared and tested for detection of gaseous analytes. The inner structure of the steering-wheel microstructure fibre consists of a thin silica core that is surrounded by three cladding holes. Numerical simulations showed that the evanescent wave of the guided fundamental mode at a wavelength of 1550 nm penetrates into the cladding holes. The calculated overlap of the evanescent wave of guided mode with the cladding holes of 0.78% can suitably be employed for gaseous analytes detection. The prepared steeringwheel microstructure fibre was experimentally tested for detection of toluene vapors flowing in the cladding holes of the fibre. It has been proved that this type of microstructure fibre can be used for detection of gaseous analytes such as toluene in nitrogen or in air in concentrations of about 0.1 mol.%.

Dependence of detected intensity of fluorescence of dye on optical fiber tapered tip diameter

The measurement of pH in small objects (cells, drops of liquid etc.) using optical fluorescence-based sensors on optical fiber tapers is one of the most widely used optical techniques. In these sensors the diameter of the taper can play important role for collecting fluorescence from tested samples. This paper presents results of experimental measurements of fluorescence intensity of dye sensitive to pH in a solution that is excited by a blue laser. The fluorescence of the dye is collected by a taper tip. The fiber tips were prepared from a graded-index fiber with a core diameter of 50 μm. Measurements with taper tips of different diameters have allowed us to estimated a limited tip diameter necessary for collecting any fluorescence form the dye solution on a level of about 5μm.

Preparation and characterization of bottle optical microresonators with circular and hexagonal cross-sections

Whispering-Gallery-Mode (WGM) optical microresonators have attracted growing attention because of their big potential for chemical and biological sensing. Recently, optical bottle microresonators have been fabricated from short sections of optical fibers. These double-neck bottle-shaped microresonators have some features distinguishing them from spherical microresonators. They support non-degenerate whispering-gallery modes that exhibit two well-separated spatial regions with enhanced field strength. The free spectral range (FSR) of such microresonators is about one order of magnitude smaller than that of microsphere resonators of equal diameters. It means that these microresonators have much longer optical path-lengths and can be employed for highly sensitive detection. The paper deals with the preparation and characterization of bottle optical microresonators fabricated from silica optical fibers. A simple 2D numerical modelling has been used to investigate basic spectral characteristics of microresonators with hexagonal cross-sections. Cylindrical bottle microresonators are prepared from optical fiber Corning SMF-28, while microresonators with hexagonal cross-section are prepared from an experimental hexagonal silica fiber. This novel type of bottle microresonators is expected to have much simpler mode structure than cylindrical bottle microresonators. There are compared two methods for the preparation of bottle microresonators in this paper. The first method is based on a combination of controlled local heating and tapering the fiber by the use of carbon dioxide laser Coherent GEM SELECT 50. The second method is based on weak tapering of the silica optical fiber with glass processing unit Vytran GPX-3400.

Preparation of spherical optical microresonators and their resonance spectra in air and gaseous acetone

This paper deals with the preparation of spherical silica whispering-gallery-mode (WGM) microresonators and with their resonance spectra measured in air and in acetone vapors. Spherical microresonators with a diameter ranging from 320 to 360 micrometers have been prepared by heating the tip of a silica fiber by a hydrogen-oxygen burner. Details of this preparation are shown on spherical and spheroidal microresonators. The prepared microspheres were excited by a fiber taper and their resonance spectra were measured and Q factors estimated. Changes in the resonance spectra of the microspheres due to their contact with acetone vapor heated to 55 °C or with liquid acetone have been observed. These changes are explained by interaction of acetone with silica and by temperature changes of the microspheres.

Silicon micro sensors as integrated readout platform for colorimetric and fluorescence based opto-chemical transducers

Opto-chemical transducer almost offers unlimited possibilities for detection of physical quantities. New technologies and research show a steady increasing of publications in the area of sensoric principles. For transfer to real world applications the optical response has to be converted into an electrical signal. An exceptional opto chemical transducer loses the attraction if complex and expensive instruments for analysis are requires. Therefore, the readout system must be very compact and producible for low cost. In this presentation, the technology platform as a solution for these problems will be presented. We combine micro structuring of silicon, photodiode fabrication, chip in chip mounting and novel assembly technologies for creation of a flexible sensor platform. This flexible combination of technologies allows fabricating a family of planar optical remission sensors. With variation of design and modifications, we are able to detect colorimetric, fluorescent properties of an sensitive layer attached on the sensor surface. In our sensor with typical size of 6mm x 6mm x 1mm different emitting sources based on LEDs or laser diodes, multiple detection cannels for the remitted light and also measurement of temperature are included. Based on these sensors we proof the concept by demonstrating sensors for oxygen, carbon dioxide and ammonia based on colorimetric and fluorescent changes in the transducer layer. In both configurations, LEDs irradiated the sensitive polymer layer through a transparent substrate. The absorption or fluorescence properties of dyed polymer are changed by the chemical reaction and light response is detected by PIN diodes. The signal shift is analyzed by using a computer controlled evaluation board of own construction. Accuracy and reliability of the remission sensor system were verified and the whole sensor system was experimentally tested in the range of concentrations from 50 ppm up to 100 000 ppm for CO2 and O2 Furthermore, we develop concepts to use the sensor also for interferometric detection of layer properties and the combination with capacitive structures on the surface. This allows detecting of thickness or refractive index variation of layers in future.