Preparing a Dye-Containing Polymer

The invention relates to a method for the preparation of a dye-containing polymer to be used for the spectroscopic analysis of samples. Such dye containing polymers are used, for example, in fiber optical sensors for the determination of the pH value or of the concentration of oxygen or of the concentration of carbon dioxide in the blood of a patient.

A method of that kind is known from U.S. Pat. No. 4,194,877. In the known method an emulsion polymerization of acrylamide, bisacrylamide and a dye is per formed. In this known method, the dye/acryl/polymer composition is produced in the form of small spheres. Unfortunately, the small spheres produced by this method can have very different sizes and can have highly variable dye concentrations. This is particularly disadvantageous in applications (e.g. in pH measurements) where the dye-containing polymer needs to be as homogeneous as possible.

In accordance with the disclosed preferred embodiment of the invention, a method is presented for preparing dye-containing polymer particles having sufficiently uniform chemical and physical properties that they are useful in spectroscopic analysis. The dyed polymer is prepared as bulk polymer so that a chemically, optically and mechanically homogeneous material is obtained. Thin slices of the bulk material are analyzed to determine whether the optical, chemical and physical properties of the bulk gel are suitable for use, in powder form, in analytical sensors. The bulk polymer is ground by methods suitable for producing polymer particles which have the same optical, mechanical, and chemical properties as the bulk polymer from which they have been produced. In this manner, the properties of the dyed polymer are determined by the properties of the bulk polymer. These particles are then used in a spectroscopic sensor. It is a particular advantage of this method that, since the optical and mechanical properties of the polymer particles are determined by the properties of the bulk dye-containing polymer, those polymers that do not meet the requirements of the respective application can be easily detected and rejected. Thus, it can be ensured, for example, that, for different sensors, the respective dye-containing polymer fillings have substantially the same properties.

According to one embodiment of the invention, the thin sections, for analysis in a spectrometer, are cut from the same block of bulk dye-containing polymer as is ground to produce the polymer particles. The thin sections can be cut by use of a vibrating knife or by freezing the gel and then cutting the slices with microtomes.

In a second embodiment of the invention, a set of parallel plates are immersed in the monomer during polymerization. Thin coatings of the polymer form on the surfaces of these plates during the polymerization process. These thin coatings are used as thin slices of the polymer for use in analyzing the bulk polymer. Because these thin slices are formed from the same monomer source as is formed the bulk polymer, analysis of these thin polymer slices will also characterize the bulk polymer.

The most favorable method of grinding the bulk polymer to produce polymer particles depends on the consistency of the respectively produced polymers. In one embodiment of the invention, very soft materials are passed through a sieve to form an initial set of particles. This method is particularly suitable for solvatgels, e.g. for hydrogels. Desolvatization of the particles substantially reduces their volumes, resulting in very small particles having substantially equal sizes. A decrease in volume up to a factor of five hundred is possible. Brittle polymers can be pulverized to produce the powdered polymer.

The particles to be used in a sensor can be sorted according to their size in order to achieve a more uniform size distribution. This can be achieved by centrifuging, by sifting through a sieve or by air separation.

In FIG. 1 is presented a flow diagram of the steps used to produce dye-containing gel particles having the same chemical, physical and optical properties. In step 11, a monomer source containing a dye is polymerized to form a bulk polymer. In step 12, thin sections of the polymer are formed for testing to determine the optical, chemical and physical properties of the polymer. Particles of polymer are then formed from the bulk polymer.

An empty test tube 180 mm long and 18 mm in diameter is evacuated in a splinter shield (e.g. in a plexiglass tube) to test it for vacuum stability. Specimen substrates 76 mm long and 26 mm wide are each divided into four equal parts and are carefully cleaned with HCl/HN03, H2O (water) and acetone. These substrates are exposed to dimethyl dichlorosilane vapor to make them hydrophobic. An alternating stack of specimen supports and 150 micrometer thick cover glass plates are tied together with a kevlar cord.

4.25 grams of acrylamide p.A., 90 mg of N,N'-methylene bisacrylamide p.A. and 50 mg of high purity phenol red are dissolved in 10 ml deionized water and 0.4 ml of a 6% (weight percentage) solution of water-temed ("temed" means tetramethyl ethylene diamine) and this solution is cooled to 0 degrees centigrade. Nitrogen gas is bubbled through this, solution to assist in dissolving 50 mg of (NH.sub.4).sub.2 S.sub.2 O.sub.8 (p.A. ammonium persulphate) in the solution. The bundled stack of glass slides is immediately immersed into the solution. Nitrogen gas is introduced several times via a three way cock (Czako cock) and vacuum is applied to the solution, whereby the solution is brought to a boil, injecting the solution between the glass plates. Finally, nitrogen gas is introduced under a very low overpressure and the test tube is brought to 40 degrees Centigrade. After about a minute the solution gels. After an hour of synthesis time the test tube is cooled to zero degrees centigrade and is shattered. Most of the gel has formed into a clear intensively colored soft-elastic cylindrical block of bulk polymer. The stack of glass plates is separated under water, thereby producing 150 micrometer thick intensively yellow colored gel slices which are chemically, optically and mechanically homogeneous and which have an optically perfect smooth surface. The optical, chemical and mechanical properties of the gel slices are the same as for the cylindrical block of bulk polymer.

After storage in water, the gel slices contain a lot of water so that the gel in these slices is in an expanded, hydrated state. These slices are put between two specimen supports with pH 12 buffer, and their thickness is reproducibly set to within 5 micrometers by means of a micrometer screw. The average thickness is approximately 200 micrometers. For the determination of the optical properties, detachable 500 micrometer (sample space) quartz cuvettes are utilized. The gel slices are measured in buffer pH 12, whereby, from the extinction spectrum, the following parameters are determined:

lambda: the exact wavelength of the absorption peak of the bonded dye;

E.sub.m : the extinction factor at wavelength lambda of the gel slice of thickness d;

E.sub.s : the specific extinction factor of the gel material; where

and where e is the extinction coefficient of the bonded dye and c is the concentration of the dye. The precision of the value of E.sub.s substantially depends on the precise thickness measurement of the gel slice. In calculating the concentration c of the phenol red from this relationship, the extinction coefficient e of the bonded dye can be approximated by the value of the extinction coefficient of the free dye.

Gel particles of definite size are produced in the following way. The cylindrical block of gel in the expanded state is comminuted to pieces of about 5 mm size, for example by crushing. These pieces are scraped with a metal spatula with excess water through a sieve. The sieve can be, for example, an analysis sieve having a mesh size of 20 micrometers. With this method of comminuting, it is easily possible to produce particles having a diameter of 20 micrometers or smaller in the hydrated state. After drying, the volume of these particles is reduced to about a tenth of their expanded volume. These small particles are then used to fill a sensor.

It should be understood that the invention is not limited to sensors for determining the pH value of samples, but can also be used for the production of other types of sensors such as sensors for measuring carbon dioxide concentration (PCO.sub.2 sensors) or oxygen concentration (PO.sub.2 sensors).