DE202006011421U1 - Fiber optic sensor for pH measurement, has sensor head with area covered with thin sensitive layer of copolymer, where layer has high refractive index and forms micro environment for coloring material and reaches refractive index of core - Google Patents

Abstract

The sensor has a segment of a multi-mode light-wave cable whose one of the ends forms a sensor head. The coating and the cladding of a fiber are removed in an area of the head, so that a core (1.7) lies freely and the area is covered with a thin pH-sensitive layer (1.2) of copolymer that has high refractive index and forms a micro environment for an immobilized coloring material, where the layer reaches a refractive index of the core.

Description

field of use the invention

The The invention relates to a fiber optic pH sensor whose sensor signal through the inner strength of the measuring medium is influenced only to a small extent.

State of technology

optical Chemosensors (optodes) are known in principle. you will be as is known to carry out used chemical-analytical measurements, for example for pH measurement where, because of external electrical Fields working with otherwise preferred conventional potentiometric Glass membrane electrodes not possible is. Such optodes are often realized in the way that Dyes on or in vehicles be immobilized. As an example, here are the anchoring of dye molecules in a matrix of PVC, the anchoring of dye molecules in a sol-gel layer with a silane as the basic substance, the anchoring of dye molecules in one Matrix from a hydrogel or the covalent attachment of dye molecules to a Cellulose acetate layer or listed on a porous glass membrane. Also are already fiber optic sensors for determining the pH value known by measuring the extinction. There are optodes on the Based on fluorescence or luminescence changes. colorimetric Optodes, on the other hand, are optically more structured due to the requirement and more precise beam paths marked in Optodenkopf what high manufacturing requirements presupposes. Their advantage is mainly that no monochromatic and coherent radiation sources, about laser diodes, and no high-sensitivity receivers, like Photomultiplier, needed become. As is known, such components are relatively expensive and need an elaborate one Electronics.

Of the Previous state of optical chemosensors in general as well as different embodiments of colorimetric optode designs are for example in H. Baltes, W. Göpel, J. Hesse: Sensors Update, Vol. 1, Section 1: Opto-Chemical and Opto-Immuno-Sensors, Weinheim: VHC-Verlag 1996, further in G. Boisde, A. Harmer: Chemical and Biochemical Sensing with Optical Fibers and Waveguides, Boston: Artech House Inc., 1996 in detail explains so that here dispenses with the presentation of further details can be.

It is also known, the cladding of an optical waveguide, running as Multimode fiber with step index profile, through an immobilization matrix too substitute.

The principle of substitution of the cladding by a chemically sensitive layer has already been indicated for multimode fiber based colorimetric optical pH sensors (H. Lehmann, G. Schwotzer, P. Czerney and R. Willsch: Intrinsic Fiber-Optic pH Sensor using NIR Dyes Immobilized in the Fiber Cladding by Sol-Gel Techniques, Europt (r) ode III, Zurich, 1996; BD MacCraith, CM McDonagh, G. O'Keeffe, AK McEvoy, T. Butler, FR Sheridan: Sol-gel coatings for Sensors and Actuators B 29 (1995), 51-57, W. Fichtner, H. Kaden, R. Müller: Low-loss fiber optic probe for absorbance measurements. DE 19524207 ; R. Müller, M. Berthold, H. Kaden, B. Schweder, P. Czerney: Low-attenuation optical chemosensor with extended pH range using new, covalently bound indicator dyes. Dresdner Beiträge zur Sensorik, Volume 16, Dresden: web Universitätsverlag, 2002, p. 171-173). Here, different principles of action are to be distinguished.

To H. Lehmann et al. (see above) is applied to the core of a silica glass fiber a sol-gel layer with trapped indicator dye applied. B. D. Mac Craith et al. (see above) use an analogous experimental setup. The resulting physical effect is evanescent Absorption. In geometrical optics, the total reflection is known as punctual reflection of the light at the interface of denoted optically denser to an optically thinner medium. Already for a long time It is known that the incident beam is in a visually thinner medium penetrates a two-phase system and there wavelength selective can be absorbed to get back into the optically denser medium to return.

The above-mentioned sol-gel layer acts as a visually thinner medium and has a smaller refractive index than the fiber core. The conventional light pipe is retained in the portion of the fiber acting as a sensor element, the absorbance being detected in the sensitive cladding as evanescent absorption. In G. Steiner, Investigations on the application of evanescent field spectroscopy in Biomesstechnik, Dissertation Dresden University of Technology, Verlag Shaker, Aachen 1993, it is stated that the evanescent absorption in comparison to absorption by Lambert-Beer for the direct irradiation of a layer is very weak, resulting in high geometric lengths for the modified fiber section in the arrangements described by H. Lehmann et al. (see above) and BD Mac Craith et al. (see above). Overall, the sensitivity of such arrangements is too low to build optical pH sensors, the can be easily used for field measurements.

Of the Invention of Fichtner et al. (see above) is based on that Light rays in an arrangement with an optical cylinder and applied thereto sensitive layer so that they form an interface from the lateral surface of a cylinder and a sensitive layer, pass through and through Total reflection at the interface sensitive layer / aqueous Analyte be fed back into the optical cylinder. Thereby experience the light beams independently from an evanescent field, a wavelength-selective dependent on the dye used Absorption. In the fiber core and in the optical cylinder, respectively only then all run Coupled light beams in the manner described above in the sensitive layer, when the refractive index of the sensitive layer is greater than that of the core.

is the refractive index of the sensitive layer is slightly smaller than that of the core fiber, depending on the led Mode in the fiber light rays by total reflection at the interface fiber core / sensitive Layer led or by total reflection at the interface sensitive layer / aqueous analyte guided.

In DE 199 04 938 a fiber optic pH sensor based on a polymethyl methacrylate optical waveguide segment is presented (polymethylmethacrylate-lat, abbreviated PMMA below). The sensitive layer used is a PVC immobilization matrix. The cladding of the fiber was replaced in the range of 6 to 20 mm of a fiber end by the immobilization matrix. As a result, the geometrical geometrical geometries for the light guide with total reflection at the interface immobilizing matrix / aqueous analyte are sufficient that the immobilization matrix of PVC is optically denser than the aqueous analyte and optically at least as dense as the fiber core of PMMA (n _PMMA = 1.49 ). The refractive index of the immobilization matrix of PVC with bis (2-ethylhexyl) adipate as the plasticizer fraction is 1.49.

Fiberoptic Draw pH sensors based on a PMMA fiber optic segment itself by relatively simple manufacturability and the suitability as Disposable sensor off. Among other things, it is very easy to do that faces to polish such a fiber plan.

The in DE 199 04 938 given sensor based on a PVC membrane shows at the coating length 8 mm and the layer thickness of 1.5 microns the sensitivity of 0.083 absorbance units per pH (measuring range 3.4 pH units, λ = 680 nm). For the ionic strength 20 mM, the pK value is determined to be 8.8 and for 100 mM the pK value 8.2. The lifetime of the sensor is about three months.

In Efforts have recently been made to use optical pH sensors with immobilization matrices that show the influence of ionic strength reduce the characteristic of the sensor. This is related to a change Microenvironment for the individual dye molecules together. Especially those present in the immobilization matrix Charge has a serious impact.

By M. Berthold et al., PH measurement in media of different ionic strength with optical sensors, 2nd Conference for Interior Analysis (CIA) 2003, Berlin 2003, hydrogels based on polyurethanes are used for the immobilization of acidochromic dyes. Hydrous polyurethane hydrogels, however, have a refractive index which is less than 1.49, so when using this immobilization matrix for a pH sensor according to this patent DE 199 04 938 the sensitivity decreases. The reasons for this have also been explained in detail above.

Of the Thus, the state of the art as a whole is that compact low-attenuation absorption-based pH optodes, on a segment of a multimode optical fiber with a partially cladding substituted by a sensitive layer, with a technically usable sensitivity high cross sensitivities across from the ionic strength exhibit. Is through the use of modified immobilization matrices the ionic strength influence on reduces the characteristic, there is a drop in sensitivity.

problem

task The invention is a low-loss fiber-optic absorption-based pH sensor to create the same time a high sensitivity, a high long-term stability and has a slight influence on the sensor characteristic by the ionic strength.

description

• – dem modifizierten Segment eines PMMA-Lichtwellenleiters,
• – einer pH-sensitiven Schicht,
• – einer Verspiegelung und
• – einer Abdeckschicht, wie im weiteren erläutert wird.

The invention relates to a pH optode which solves the above-mentioned problem. The pH optode according to the invention consists of the following constituent parts:

• The modified segment of a PMMA optical waveguide,
• A pH-sensitive layer,
• - a mirroring and
• - A cover layer, as will be explained below.

The Optodenköpfe consist of 5 to 20 cm long, modified segments of PMMA optical fibers with a core diameter of 0.5 ... 1 mm. An important characteristic of the new optical pH sensors is that in the range 6 ... 20 mm the cladding 1-1 ( 1 ) through a pH-sensitive layer 1-2 is substituted. The end face on the side of the coating is with a mirror layer 1-3 Mistake. At the in 1 left, not mirrored face 1-4 can be coupled by means of a fiber optic bundle certain wavelengths. In another application, the returned radiation is directed to a detector.

aim This arrangement is the electromagnetic radiation so to lead, that the light rays are the interface fiber core / sensitive layer happen and sensitive by total reflection at the interface Layer / aqueous Analyzer be returned to the nucleus. As a result, the light rays experience independent of an evanescent field a wavelength selective Absorption.

The preparation of such a pH optode is done in the following manner: In a first step, the two faces of a fiber segment 5 to 20 cm in length are polished. Thereafter, one of the two fiber ends is provided with a mirror layer of gold, aluminum or titanium. Subsequently, in the area behind the mirror layer, the coating 1-5 of the optical waveguide to a length of 8 ... 22 mm away. Then the mirror layer with a protective layer 1-6 covered by an epoxy resin. Another manufacturing step involves removing the cladding in a range of 6 ... 20 mm in length in the coating-free region of the fiber core 1-7 , This is done in a mixture of air and tetrahydrofuran (THF) at its saturation vapor pressure. The cladding decomposes and dissolves from the fiber core. This happens after a reaction time of the gas mixture of 50 ... 60 s. In this exposure time, the fiber core itself is not attacked. The detached remnants of the cladding remaining on the fiber core are removed with a soft cloth soaked with THF. Subsequently, the attachment of the shaft takes place 1-8 made of stainless steel using rubber molding technology and use of cold-curing epoxy resin 1-9 , Finally, the application of the sensitive layer is carried out. The procedure for this step is explained in detail below.

The particular innovative achievement lies in the combination of the The following presented pH-sensitive layer with the optode construction described above. The pH-sensitive layer has the decisive, sufficiently high Refractive index and simultaneously forms a microenvironment for the immobilized Dye that has a low ionic strength influence on the characteristic caused by the sensor. A particular advantage of the invention is that the dye can be covalently immobilized.

To prepare the pH-sensitive layer is 2-hydroxyethyl methacrylate 2-1 (abbreviated HEMA) as a monomer, Benzoinmethylether used as a crosslinker and an ethyl methacrylate modified Styrylacridinfarbstoff 2-2 ( 2 ). The variable x in FIG 2-2 indicate different chain lengths that can be used in modifying the styrylacridine dye.

According to the invention suitable monomer mixtures, hereinafter referred to as cocktail, may be composed in the following manner (in percent by weight):
1 to 3% of an ethylmethacrylate-modified styrylacridine dye ( 2-2 in 2 ) 1 to 3% benzoin methyl ether
98 to 94% 2-hydroxyethyl methacrylate.

The coating of the optodes prepared according to the above procedure is carried out in the following manner ( 3 ): It becomes a gas-tight container 3-1 with a quartz glass tube 3-2 connected. The container is purged with nitrogen via this tube. In the container are arrangements 3-3 , which are located with the area to be coated within the quartz glass tube, in rotation (about 2 ... 10 U / min) offset about its longitudinal axis. A UV lamp 3-4 (6 ... 10 watts, 254 ... 366 nm) is adjusted at a distance of 3 ... 5 cm next to the quartz glass tube. The fiber with the fiber cladding surface to be coated is dipped into the cocktail and pulled out of the mixture at the speed of 0.3 ... 0.7 mm / s. Thereafter, the prepared assembly is immediately inserted into the gas-tight container, set in rotation, and the UV lamp is turned on. The irradiation time is three to five hours. The polymer formed as a result of the polymerization process due to the energy input 2-3 is also in 2 shown.

After that the optode becomes the container taken and in air at ambient temperature for 24 h with light stored. After a subsequent Conditioning of the sensitive area of the optode over 48 h in a buffer solution of pH 7.5 ... 8.5, the optode is ready to measure.

advantageous effect

The advantageous effect of the invention is that the absorption-based pH optodes according to the invention have a low attenuation at a sensitivity suitable for industrial applications and have a high long-term stability. Furthermore, a slight influence of the characteristic by the internal strength occurs. The combination of low attenuation and high sensitivity results from the fact that primarily the total reflection at the interface sensitive layer / aqueous sample is utilized. On the other hand, high sensitivity and low influence on the characteristic due to the internal strength are brought about by the fact that the pH-sensitive layer embodied according to the invention has a high refractive index and at the same time forms a microenvironment for the immobilized dye which results in a low ionic strength influence on the characteristic curve of the sensor. Another advantage is that the dye is covalently immobilized. It is well known that pH optodes whose effective dye is covalently immobilized have higher long-term stabilities than pH optods based on adsorptive dye immobilization.

• – Durch die große Ähnlichkeit der Brechzahlen von PMMA und dem als sensitive Schicht eingesetzten Copolymerisat entstehen an der Grenzfläche zwischen diesen beiden Materialien nur sehr geringe optische Verluste.
• – Infolge der besonders dichten, optisch verlustfreien Zusammenfügung der beiden genannten Werkstoffe, die durch die Polymerisation der sensitiven Schicht auf dem Kern der Faser herrührt, findet auch nach längerer Einwirkung wässriger Phasen (über Wochen) auf die Anordnung keine Ablösung der sensitiven Schicht statt.

The interface PMMA / poly-HEMA according to the arrangement according to the invention provides the following advantages:

• - Due to the great similarity of the refractive indices of PMMA and the copolymer used as a sensitive layer only very small optical losses occur at the interface between these two materials.
• - Due to the particularly dense, optically loss-free assembly of the two materials mentioned, which results from the polymerization of the sensitive layer on the core of the fiber, no detachment of the sensitive layer takes place even after prolonged exposure to aqueous phases (over weeks) on the arrangement.

optodes based on PMMA fiber optic segments are particularly lightweight produce and can find use as disposable sensor.

example

The invention will be explained in more detail below with reference to an embodiment. The optode consists of a 15 cm long segment of the "standard PMMA fiber" from "TORAY Industries, Inc." (Tokyo, Japan) The fiber core consists of high-purity PMMA [diameter (∅) 1 mm, n _K = 1.491 (20 ° C, standard pressure, 589.3 nm), numerical aperture (NA) = 0.5.] The cladding also consists of PMMA, whereby the refractive index was lowered by fluorine doping. ∅ 2.2 mm), the right side of the coating is 11 mm long, the cladding is 8 mm on the right side, and the right side of the fiber is sputtered with a layer of gold (layer thickness) 150 nm) The gold layer is covered with 0.5 mm ³ SMD adhesive (Heraeus GmbH, Hanau, product designation PD 944) The adhesive acts as a protective layer.

Of the Cladding-free area of the fiber core [lateral surface; 8 mm long] is with a 2 micron thick coated sensitive layer; the thickness specification refers to this on the non-watered state. The stem consists of a 60 mm long stainless steel tube (5 × 1 mm; Steel grade 1.4571). The stem starts 10 mm behind the one made of gold produced mirror layer. Located between shaft and coating cold curing Epoxy resin.

Of the Cocktail for the sensitive layer consists of 2.83% modified with ethyl methacrylate Styrylacridine dye designated 6- [2 - ((E) -2-acridin-9-yl-vinyl) -5-diethylaminophenoxy] -hexanoic acid 2- (2-methyl-acyloyloxy) -ethyl ester, further 2.83% benzoin methyl ether and 94.34% 2-hydroxyethyl methacrylate.

Of the from the o. g. Cocktail resulting monomer film was through Dipping method using the pulling speed of 0.5 mm / s applied to the fiber core freed from cladding. The polymerization time was four hours; it was carried out under a nitrogen atmosphere. The UV lamp (CARL ROTH GmbH & Co KG, 8 watts, 366 nm) was adjusted at a distance of 4 cm. The optode was while the polymerization at 2 U / min in rotation. The resulting Layer thickness is 2 μm. To The storage of the coated optode over 4 h in air was the sensitive Area of the optode 48 h in a phosphate buffer (internal thickness 0.1 M; pH = 8.5). These operations were under light exclusion carried out at room temperature.

Of the thus designed sensor shows at 680 nm a sensitivity of 0.06 Extinction units per pH unit over the measuring range of 4 pH units. For the ionic strength of 20 mM, the pK value of 5.8 and for 100 mM, the pK value of 5.9 was determined. The lifetime of the sensor is about 6 months. These values are considerably cheaper than those with optodes achievable on the basis of PMMA fiber segments and off the literature known results with PVC or polyurethane hydrogel membranes as a sensitive layer.

1-1

cladding

1-2

pH-sensitive layer

1-3

mirror layer

1-4

Not mirrored face

1-5

coating

1-6

Protective layer,

1-7

fiber core

1-8

shaft

1-9

epoxy resin

2-1

2-hydroxyethyl methacrylate

2-2

With Ethyl methacrylate modified styrylacridine dye

2-3

polymer

3-1

gas-tight casing

3-2

quartz glass tube

3-3

optode

3-4

UV lamps

Claims (7)

Fiber optic probe for pH measurement, which functions on the basis of light absorption, consisting of a segment of a multimode optical waveguide, wherein one of the two ends of this multimode optical waveguide forms the sensor head, characterized in that in the region of the sensor head both the coating and the Cladding of the fiber is removed so that the core is exposed and that this area is coated with a thin sensitive layer of a copolymer which is polymerized on the fiber and consists of more than 80% of poly (2-hydroxyethyl methacrylate), wherein the layer contains an immobilized dye and at least reaches the refractive index of the core of the fiber. Fiber-optic probe for pH measurement according to claim 1, characterized in that a Styrylacridinderivat covalently to poly (2-hydroxyethyl methacrylate) is attached. Fiber-optic probe for pH measurement according to claims 1 and 2, characterized in that preferably 6- [2- (trans) -2-acridin-9-yl-vinyl) -5-diethylaminophenoxy] hexanoic acid 2- (2-methyl-acryloyloxy) ethyl ester covalent on poly (2-hydroxyethyl methacrylate) is connected. Fiber-optic probe for pH measurement according to claims 1 to 3, characterized in that the chain length of the aliphatic grouping - (CH ₂ ) _m - in the hexanoic acid radical of the styrylacrinedtine derivative 6 [2- (trans) -2-acridine mentioned in claim 3 -9-yl-vinyl) -5-diethylamino-phenoxy] -hexanoic acid 2- (2-methyl-acryloyloxy) ethyl ester not only m = 5, but optionally m = 1 to 18. Fiber-optic probe for pH measurement according to claims 1 to 4, characterized in that as a fiber segment a plastic fiber optic cable, preferably with a core material of PMMA, is used. Fiber-optic probe for pH measurement according to claims 1 and 5, characterized in that the end face on the side of the sensor head of the Probe is polished and mirrored and this mirror layer through a gold layer is generated. Fiber-optic probe for pH measurement according to claims 1 to 6, characterized in that at input and decoupling of Light radiation over the face the plastic fiber optic segment without Verspieglung the light rays in the modified part of the plastic optical waveguide segment the broad agreement the refractive index of the fiber core and sensitive layer, the interface fiber core / sensitive Layer without refraction and without reflection losses can happen and at the interface sensitive layer / aqueous Analyte totally reflected and returned to the nucleus and thereby a wavelength-selective Experience absorption.