Method for evaluating the permeability of a thin membrane

Abstract: A method of evaluating the permeability of a thin membrane comprising the steps of mixing solutions having substances that when reacted, produce chemiluminescense. The resulting emitted light is measured by a photodetesctor and correlated to the permeability of the membrane.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a method of evaluating the permeability of a thin membrane, and more particularly, it relates to a method of evaluating the permeation of substances through thin membranes and the walls of microcapsules in the chemical and pharmaceutical industries. Description of the Related Art Controlling the permeation of substances through thin membranes and the walls of microcapsules is one of the basic techniques that have been adopted in the chemical and pharmaceutical industries. Designing a thin membranes or microcapsule, and designing optimum thin membranes or microcapsules suitable for a specific purpose is an essential prerequisite for the practical application of this technique. The permeability of thin membranes can be evaluated by several methods including analysis by gas chromatography, volumetric measurement of a permeant gas, and gravimetric measurement of a desiccant whose weight will change with absorption of the permeant moisture. Another method may be adopted in which a dye is allowed to permeate through a thin membrane and then measuring the resulting change in the light absorbance of the dye, or in the intensity of fluorescence is measured, or in which an electrolyte is allowed to permeate through the membrane and the small amount of permeation is measured in terms of the change in electrical conductivity caused by permeation of the electrolyte. These prior art methods, however, have a common problem in that they are all designed to determine the total amount of permeation within a certain period of time, and therefore are incapable of evaluating the permeability of a thin membrane at very small time intervals. In addition, for example the minimum concentration of a substance that can be detected is on the order of 10 -9 mole/ℓ in the measurement of light absorbance. Moreover, these methods require a considerably long time to detect the time-dependent change in an extremely small amount of permeant. Under these circumstances, the present inventors previously proposed a method of measuring a very small amount of permeant by utilizing chemiluminescence (Japanese Patent Application No. 280497/86, now Japanese Unexamined Publication No. 63-133039). In this method, two dissimilar substances are juxtaposed across a thin membrane and one substance is allowed to diffuse and permeate through the membrane until it mixes with the other substance and the resulting luminescence is measured with a photodetector to thereby evaluate the permeability of the membrane to the first substance. This method, which utilizes chemiluminescence, is capable of more sensitive detection of the permeant than the other methods and has the added advantage of enabling real-time measurement of the time-dependent change in the amount of permeation and two-dimensional observation. However, the combinations of substances that can be mixed with each other to produce chemiluminescence are limited to hydrogen peroxide/luminol, adenosine triphosphate (ATP), luciferase, and a few other examples. This restriction of the substances for chemiluminescence has made it impossible to evaluate the permeability of a thin membrane for many other kinds of substances. SUMMARY OF THE INVENTION It is an object of the present invention to provide a method that is capable of evaluating the permeability of a thin membrane for various kinds of substances with high sensitivity and at a fast rate. Additional objects and advantages of the invention will be set forth in the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims. To achieve the foregoing objects, and in accordance with the present invention, a solution comprising a first substance and a solution comprising a mixture of a chemiluminescent substance and a second substance that reacts with the first substance , with the resulting reaction product further reacting with the chemiluminescent substance , are juxtaposed across a thin membrane. The first substance is allowed to diffuse and permeate through the thin membrane until it contacts the mixture solution comprising the second substance and the chemiluminescent substance. The first substance reacts with the second substance, and that reaction product then reacts with the chemiluminescent substance to produce chemiluminescence. The light emitted is measured by photodetector. In the second method of evaluating the permeability of a thin membrane in accordance with the present invention, a solution comprising a first substance that catalyzes chemiluminescence and a solution containing a chemiluminescent substance that produces chemiluminescence in the presence of said castalyst are juxtaposed across the thin membrane to be evaluated. The first substance diffuses and permeates through the thin membrane to react with the solution containing a chemiluminescent substance and the resulting chemiluminescence is measured with a photodetector. In the third method of evaluating the permeability of a thin membrane in accordance with the present invention, a solution comprising a fluorescent substance and a solution comprising a plurality of reactive substances that produce an excited intermediate capable of transferring energy to the fluorescent substance are juxtaposed across the thin membrane to be evaluated. The fluorescent substance diffuses and permeates through the thin membrane, and the emission of light produced by the combination of the chemiluminescent reaction and the energy transfer is measured with a photodetector. BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate a presently preferred embodiment of the invention and, together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain the principles of the invention. Fig. 1 depicts an apparatus for measuring the permeability of a thin membrane by a method of the present invention; Fig. 2 depicts an apparatus for measuring the permeability of the skin of a microcapsule by a method of the present invention; Fig. 3 depicts the chemiluminescence intensity profile with time lapse in an example of the present invention; and Fig. 4 depicts the time-dependent change in the integration of the values of chemiluminescence intensity of the example of Fig. 3. DESCRIPTION OF THE PREFERRED EMBODIMENT When the first substance is glucose, glucose oxidase is preferably used as the first catalyst. Several preferred combinations of the first substance and the first catalyst are listed below: cholesterol/cholesterol oxidase; cholesterol ester/cholesterol ester hydrase or cholesterol ester oxidase; neutral fat/lipase and glycerol oxidase phospholipid/phospholipase D and choline oxidase; free fatty acid/acyl-CoA synthetase and acyl-CoA oxidase; Urea/Urease, glutamate dehydrogenase and glutamate oxidase; ammonia/glutamate dehydrogenase and glutamate oxidase; uric acid/uricase; creatine/creatininase, creatinase and sarcosine oxidase; lactic acid/lactate oxidase butyric acid/butyrate oxidase; pyruvic acid/pyruvate oxidase; amino acid/amino acid oxidase; and inorganic phosphorus/pyruvato oxidase. Each precursor of the first substance, for example, starch in the case of glucose, can also be used and the permeability of a membrane by that precursor can, of course, be evaluated by the method of the present invention. The substances listed above are employed in aqueous solution and the second substance, which reacts with the first substance in the presence of the first catalyst, is dissolved oxygen in the aqueous solution. Any substances can be used as the first and second substances as long as they react with each other to generate hydrogen peroxide, and the combinations that can be employed in the present invention are not limited to those listed above. If the reaction product is hydrogen peroxide, a chemiluminescent substance that emits light upon reaction with hydrogen peroxide is also used. For example, luminol, lucigenin or lophine is used as a chemiluminescent substance. As the second catalyst, peroxidase, Co 2+ , Co 3+ , Fe (CN) 6 3- , SbCl 6 -, Fe 2+ , Cu 2+ , hemin, Cu(NO 3 ) 2 , CuCl 2 , salt of Co 2+ or the like is used. Thus, the first catalyst catalyzes the oxidation reaction of the first substance, which is oxidized to generate hydrogen peroxide, and the second catalyst catalyzes the reaction between the chimiluminescent substance and the so generated hydrogen peroxide to produce luminescence. The first and second substances may be such that they react to generate adenosine triphosphate, (ATP). In this case, the first substance is creatinic acid, phosphenol pyruvate kinase, etc., and the second substance is creatine kinase/adenosine diphosphate (ADP), pyruvate kinase/ADP, etc. Luciferin, etc, are used as a chemiluminescent substance which emits light upon reaction with ATP. Various thin membranes can be evaluated for their permeability, including packaging films, reverse osmotic membranes, dialysis membranes, ultrafiltration membranes, microfiltration membranes, etc. These membranes may be made of polyethylene, polyvinyl alcohol, cellulose acetate, regenerated cellulose, and various types of polymer alloys. The shape of thin membranes is not limited in any particular way, but they may be the skins of microcapsules, laminates, tubes, hollow fibers, etc. The method of the present invention for evaluating the permeability of a thin membrane by detecting chemiluminescence has been described above primarily with reference to the permeation of substances that are capable of inducing the generation of hydrogen peroxide. It should be noted, however, that the method of the present invention is also applicable to the measurement of permeability for the following substances. (1) Substances Catalyzing Chemiluminescent Reaction In this case, chemiluminescence, although slight in amount, will generally be produced even in the absence of a catalyst to increase the background emission level. Such slight amount, however, is almost negligible for the purpose of evaluating the permeability of a thin membrane by trace metal ions. Examples of chemiluminescent substances that can be used in this instance are solutions in which aqueous hydrogen peroxide is mixed with luminol, lucigenin, lophine, 1, 10-phenanthroline, siloxene, etc. Catalysts whose permeation through the thin membrane can be measured are metal ions such as Fe 2+ , Cr 3+ , Co 2+ , C u 2+ , Mn 2+ , Cd 2+ , Zn 2+ , etc. (2) Fluorescent Substances The key factor is the steady-state formation of an excited intermediate. With a highly viscous solvent system such as a glycerin solvent being selected, the reactants will react slowly to form an excited intermediate. The quantitative analysis is not as satisfactory as in the case of other chemiluminescent systems (e.g. light emission from the above substances (1) and luminol) but the advantage of this system lies in the extreme breadth of the range of permeants since luminescence can be produced from many kinds of fluorescent substances. Examples of the fluorescent substances include anthracene, pyrene, rhodamine B, uric acid, catecholamines, folic acid, thyroxine quinones, etc., and a plurality of reactive substasnces that generate an excited intermediate capable of transferring energy to the fluorescent substance may be examplified by a mixture of hydrogen peroxide and an oxalic diester such as bis (2,4-dinitrophenyl) oxalate or bis (2,4,6-tricholorophenyl) oxalate. The light emitted by the reactions described above is measured with a photodetector such as a photomultiplier tube, an image intensifier, an infrared sensor or a one-dimensional sensor. The present invention provides a first substance, which diffuses and permeates through a thin membrane under test and contacts the second substance in a mixture with the chemiluminescent substance, thereby forming the reaction product such as hydrogen peroxide. This reaction product further reacts with the chemiluminescent substance in the mixture to produce chemiluminescence. Similar chemiluminescence can be produced even when a chemiluminescence catalyzing substance is selected as the first substance, which diffuses and permeates through the membrane to make contact with a substance that produces chemiluminescence with the aid of the first substance catalyst. When a fluorescent substance is selected as the first substance, it diffuses and permeates through the membrane until it contacts a plurality of reactive substances that generate an excited intermediate capable of transferring energy to the fluorescent substance. This system is also capable of producing luminescence. The quantity of emitted light is correlated to the amount of the first substance that permeates through the thin membrane. The time-dependent change in light emission can be measured by a photon-counting method using a photomultiplier tube, an image intensifier, an infrared sensor, or a one-dimensional sensor. The results of measurement provides a basis for estimating a very small change in the trace amount of permeant. In addition, varying the combination of the first and second substances will enable the permeability of the membrane to be evaluated for many kinds of substances. Several examples of the present invention are described hereinafter with reference to the accompanying drawings. Example 1 Fig. 1 is a sketch of an apparatus for measuring the permeability of a thin membrane by the method of the present invention. The apparatus comprises the membrane under test 1, cells 2 and 3, which are respectively charged with solutions 4 and 5 of the first substance and the mixture containing the second substance and the chemiluminescent substance, a photodetector 6 and a dark box 7. The solution of the first substance was a 0.01 % aqueous solution of glucose, and the solution of the mixture was an aqueous solution having a mixture of glucose oxidase (1 mg), luminol (2 mg) and peroxidase (1 mg) contained in 10 cc of a 0.1 M phosphate buffered (pH 7) solution. The thin membrane under test was a commercial cellulose dialysis membrane (fractionation mol. wt. 1,500) having a thickness of 35 µm. The cell 2 containing the solution of first substance 4 and the cell 3 completely filled with the aqueous solution of mixture 5 were placed carefully in the dark box 7 in such a way that the open bottom of the cell 2 would face the open top of the cell 3 through the thin membrane 1. After placing the photodetector 6 above the cell 2, the dark box was totally isolated from outside light. The glucose in the cell 2 permeated, through the membrane 1, into the aqueous solution of mixture, where it reacted with dissolved oxygen in the mixture solution in the presence of the glucose oxidase to generate hydrogen peroxide. The hydrogen peroxide thus generated oxidized luminol, as catalyzed by peroxidase, to emit light. The light emission was measured with the photodetector 6 to estimate the amount of glucose that permeated through the membrane 1. Example 2 Fig. 2 is a sketch of an apparatus for measuring the permeability of the skin of a microcapsule by a method according to the present invention. Numeral 8 represents the microcapsule under test, and the same numerals as those used in Fig. 1 are identical to those elements as shown in Fig. 2. An aqueous solution of the first substance, which was the same as that used in Example 1, was put into the cell 2, and the microcapsules confining an aqueous solution of the same mixture as that used in example 1 were submerged in the aqueous solution of the first substance. The cell 2 was placed carefully, in the dark box 7. After placing the photodetector 6 above the cell 2, the dark box was totally isolated from outside light. As the glucose in the aqueous solution of the first solution permeated through the walls of the microcapsules, the emission of faint light was measured with the photodetector 6. Example 3 Fig. 3 shows the chemiluminescence intensity versus time profile obtained in Example 1 using a photomultiplier tube as the photodetector. The relationship between chemluminescence intensity and glucose permeation rate may be expressed by: I CL = V. C . CL . PM . DET .N A Where I CL : chemiluminescence intensity (cps); V: glucose permeation rate (mol -1 );  C : the efficiency of chemical reaction for hydrogen peroxide generation:  CL : the efficiency of luminol's luminescence;  PM : the quantum efficiency of the photomultiplier tube;  DET : the light yield of the photomultiplier tube; and N A : Avogadro's number. Equation 1 shows that the intensity of chemiluminescence is proportional to the permeation rate of a permeant. Assuming  C = 0.01,  CL = 10 -4 ,  PM = 0.1 and  DET = 0.01, one can estimate from equation 1 that the permeation rate at the steady state reached after 80 minutes would be on the order of 4 x 10 -11 mol S -1 . Fig. 4 shows the time-dependent change in the integration of the values of chemiluminescence intensity shown in Fig. 3. The result is related to the time-dependent change in the total amount of glucose that permeated through the thin membrane. Assuming that glucose is transported through the membrane by diffusion of the type governed by Fick's law, the delay time ( in Fig. 4 required for the steady state to be reached is given by: =ℓ 2 /6D where ℓ: membrane thickness D: the diffusion coefficient of permeant (glucose) within the membrane (for detailed discussion, see J. Crank, The Mathematics of Diffusion, Oxford University Press, p. 51, 1975). Substituting 32 min and 35 µm into  and ℓ, respectrively, D = 1 x 10 -9 cm 2 s -1 for the case shown in Fig. 4. This value is considerably smaller than the diffusion coefficient of a low-molecular weight substance in the solution (≃10 -5 cm 2 s -1 ). This is probably due to the long pathway of glucose diffusion through the membrane and to the interaction of glucose molecules with the membrane of other substances. Example 4 Permeation Test on Ferric ion Ferric oxalate was used as the first substance that catalyzes chemiluminescence, and luminol and aqueous hydrogen peroxide were used as chemiluminescence substances. The chemiluminescent substances were mixed, fixed in gelatin and overlaid with a thin membrane to be tested. A solution of the ferric oxalate was poured above the membrane. Before the ferric ions permeated through the membrane, chemiluminescence, although weak, was already observed by a photon counting method. It was also observed that the intensity of chemiluminescence increased greatly as the ferric ions permeated through the membrane. Example 5 Permeation Test On Anthracene Anthracene was used as a fluorescent substance, and bis (2,4-dinitrophenyl) oxalate and hydrogen peroxide were used as a plurality of reactive substances that would generate an excited intermediate capable of transferring energy to the fluorescent substance. The excited intermediate in this case was therefore dioxetane dione. In order to ensure slow progress of the reaction, a mixed solvent system composed of dimethyl sulfoxide, water and glycerin was used. Increasing the rate of glycerin will enable the excited intermemdiate to be generated at a lower rate. Gelatin or agar may be used as a fixing agent as required. Chemical luminescence from anthracene was observed as anthracene permeated through the thin membrane under test.

Claims: A method of evaluating the permeability of a thin membrane comprising the steps of: preparing a first solution comprising a first substance; preparing a second solution comprising a mixture of a chemiluminescent substance and a second substance that reacts with said first substance; juxtaposing said first and second solutions across said membrane to be evaluated; allowing said first substance to diffuse and permeate across said membrane wherein said first substance will react with said second substance to form a third substance and said third substance will react with said chemiluminescent substance; and determining the permeation rate of said first substance by measuring the emission of light produced by the chemiluminescent reaction by a photodetector. The method of claim 1, wherein said third substance comprises hydrogen peroxide. The method of claim 2, wherein reaction of said first substance and said second substance is catalyzed by a first catalyst, and the combination of said first substance and said first catalyst is selected from any one of the following combinations; cholesterol/cholesterol oxidase; cholesterol ester/cholesterol ester hydrase or cholesterol ester oxidase; neutral fat/lipase and glycerol oxidase phospholipid/phcspholipase D and choline oxidase; free fatty acid/acyl-CoA synthetase and acyl-CoA oxidase; urea/urease, glutamate dehydrogenase and glutamate oxidase; ammonia/glutamate dehydrogenase and glutamate oxidase; uric acid/uricase; creatine/creatininase, creatinase and sarcosine oxidase; lactic acid/lactate oxidase; pyruvic acid/pyruvate oxidase; amino acid/amino acid oxidase; and inorganic phosphorus/pyruvate oxidase. The method of claim 2, wherein said chemiluminescent substance comprises any one of the substances which react with hydrogen peroxide to emit chemiluminescent light. The method of claim 4, wherein said chemiluminescent substance comprises a substance selected from the group consisting of luminol, luciferin (lucigenin) and lophine. The method of claim 2, wherein reaction of said third substance and said chemiluminescent substance is catalyzed by a second catalyst comprising peroxidase. The method of claim 1, wherein said first substance comprises Adenosin triphosphate (ATP). The method of claim 7, wherein said first substance is creatinic acid, or phosphoenol pyruvate kinase, and said second substance is creatine kinase/adenosine diphosphate (ADP), or pyruvate kinase/ADP. The method of claim 8, wherein said chemiluminescent substance comprises luciferine. A method of evaluating the permeability of a thin membrane comprising the steps of: preparing a first solution comprising a first substance for catalyzing a chemiluminescent reaction; preparing a second solution comprising a chemiluminescent substance for producing chemiluminescence in the presence of said first substance; juxtaposing said first and second solutions across said membrane to be evaluated; allowing said first substance to diffuse and permeate across said membrane wherein said first substance will catalyze said second substance resulting in a chemiluminescent reaction; and determining the permeation rate of said first substance by measuring the emission of light produced by the chemiluminescent reaction by a photodetector. The method of claim 10, wherein said second solution comprises a solution of aqueous hydrogen peroxide mixed with luminol, luciferin, lophine, 1,10-phenanthroline, or siloxene. The method of claim 10, wherein said first substance comprises a metal ion selected from the group consisting of Fe 2+ , Cr 3+ , Co 2+ , Cu 2+ , Mn 2+ , Cd 2+ , or Zn 2+ . A method of evaluating the permeability of a thin membrane comprising the steps of: preparing a first solution comprising a fluorescent substance; preparing a second solution comprising a plurality of reactive substances that produce an excited intermediate which is capable of transferring energy to said fluorescent substance; juxtaposing said first and second solutions across said membrane to be evaluated; allowing said first substance to diffuse and permeate across said membrane; and determining the permeation rate of said first substance by measuring the emission of light produced by the chemiluminescent reaction by a photodetector. The method of claim 13, wherein said fluorescent substance comprises a substance selected from the group consisting of anthracene, pyrene, rhodamine B, uric acid, catecholamine, folic acid, and thyroxine quinones. The method of claim 13, wherein said excited intermediate capable of transferring energy to said fluorescent substance is a mixture comprising hydrogen peroxide and an oxalic diester. The method of claim 15, wherein said oxalic diester is bis (2,4-dinitrophenyl) oxalate or bis (2,4,6-trichlorophenyl) oxalate. The method of any of claims 1, 10 or 13, wherein said photodetector is a photomultiplier tube, an image intensifier, an infrared sensor, a one-dimensional sensor, or a two-dimensional sensor. The method according to any of claims 1, 10 or 13, wherein said thin membrane is the wall of a microcapsule, packaging film, reverse osmotic membrane, dialysis membrane, ultrafiltration membrane, or microfiltration membrane.

Inventors: HIRAMATSU MITSUO, ITO TOSHIAKI, MURAKI HIROJI, HIRAMATSU, MITSUO, ITO, TOSHIAKI, MURAKI, HIROJI, HIRAMATSU, MITSUO C/O HAMAMATSU PHOTONICS, ITO, TOSHIAKI C/O HAMAMATSU PHOTONICS, MURAKI, HIROJI C/O HAMAMATSU PHOTONICS
Applicants: HAMAMATSU PHOTONICS KK, HAMAMATSU PHOTONICS K.K.
Application year: 1989
Main classification: G01N15
IPC codes: G01N15, G01N21, G01N13