Method for bonding an analyte-sensitive dye compound to an addition-cure silicone and use in the manufacture of sensor apparatus

Abstract: A method of bonding an analyte-sensitive polynuclear aromatic indicator substance to addition cure silicone to form a sensor element, comprises the steps of adding butyllithium and tetramethylene diamine to the indicator substance in a solvent system comprising tetrahydrofuran to generate a mixture containing the carbanion of the indicator substance, adding a bromoalkene to the carbanion mixture to form a functionalized indicator substance, reacting the functionalized indicator substance with polymethyl-hydrosiloxane to form an indicator-bound silicone liquid, and cross-linking this indicator-bound silicone liquid with vinyl-terminated polysiloxane to form said sensor element the method being characterised in that said solvent system comprises a mixture of tetrahydrofuran with hexamethylphosphoramide thereby greatly improving the efficiency of the method.

Description: The present invention relates generally to the manufacture of means for the measurement of concentrations of analytes in a fluid or gaseous mixture, and more specifically, to a more efficient method for making an improved analyte-sensing optode, more specifically an oxygen-sensing optode, in which the indicator substance is covalently bonded onto a matrix. As is explained in our european Patent Application EP-A-0 413 114 priority date 16.08.89, publication date 20.02.91 fiber-optic based oxygen sensing apparatus has proliferated over the years due to its numerous applications in the medical, chemical and environmental fields. Many such oxygen sensors rely on the phenomenon of fluorescence quenching as a means for determining the presence of oxygen in a liquid or gaseous mixture and this approach has been quite attractive for such devices due to the high sensitivity that can be achieved. Systems and instruments implementing fluorescence quenching techniques utilize an encapsulated oxygen-quenchable fluorescent dye that is placed within a gas permeable matrix. The matrix is usually made from a polymer or similar substance. The dye/matrix element, generally referred to as a sensor element or optode, can be applied to the tip of an optical fiber using wellknown techniques in the art. A light source with appropriate filtering system provides a select wavelength or wavelengths of light which is propagated down the optical fiber and excites the dye. The fluorescence signal, induced by the excitation energy, travels back through the same optical fiber and is collected by a photodetector near the proximal end of the fiber. Using techniques known in the art, the intensity of the fluorescence of the dye, which is a function of the oxygen level in the sample, can be transduced into a partial pressure of oxygen. While many sensor elements or optodes have been developed for use with oxygen measuring devices, there are inherent problems associated with them that are detrimental to the accuracy of the measurements. For example, it is sometimes a difficult task to immobilize the fluorescent dye in a gas permeable matrix because of the chemical incompatibility between the dye and matrix. Many of the more widely used oxygen fluorescent dyes are polynuclear aromatic compounds which, because of their high degree of symmetry, usually have low solubility in organic materials. As a result, the fluorescent dyes have a tendency to leach through the permeable matrix into the solution or gas mixture that is being tested. Various approaches for creating an operable sensor element include adsorbing the dye on inorganic or organic solid supports, dispersing the dye in the matrix by way of organic solvents, and covalently bonding the dye on porous glass. While useful under some circumstances, many of these techniques still have serious drawbacks if the dye is chemically incompatible with the polymer matrix. Furthermore, these dyes still have a tendency to leach out, particularly when in contact with a sample that includes a substance that has similar properties to the dye/polymer matrix being used for the optrode. Unfortunately, such substances include blood proteins and many organic solvents, which are often the samples being tested. As a result of the leaching of the dye during use, the sensing element may have to be routinely replaced to ensure the accuracy of oxygen measurements. Moreover, symmetrical dye molecules that are free to move within a polymer matrix tend to agglomerate which results in changes in fluorescent properties. Accordingly, those concerned with the development and use of oxygen sensing devices have long recognized the need for an improved method for creating a sensor element that will not leach when placed in the sample solution or during storage and will not suffer dye agglomeration over time. Preferably, the improved method should produce a dye/matrix that can be readily affixed to the end of an optical fiber or other similar device in a single step. Moreover, the sensor made according to the method should be relatively inexpensive to manufacture and should provide accurate oxygen measurements. It would also be advantageous if the method were able to provide relatively high yields of quality sensors. There is described and claimed in our european Patent Application EP-A-0 413 114 a method for bonding an analyte-sensitive indicator substance to addition-cure silicone to form a sensor element which method comprises functionalizing the analyte-sensitive indicator substance to obtain a linker arm with an isolated multiple bond; hydrosilylating the functionalized indicator substance with a polymethylhydrosiloxane; and cross-linking the resulting compound with a vinyl-terminated polysiloxane. As is explained in our european patent application EP-A-0 413 114 the manufacture of economic quantities of fluorescent sensors to be used in combination with optical fiber systems to measure blood oxygen levels has become very important as the use of such sensors has increased. In particular, methods to manufacture sensors which resist leaching, are stable over time and result in high yields from a given quantity of materials are very advantageous. The present invention provides these benefits in a new and novel process which in particular results in economically advantageous yields of sensors which are resistant to leaching of the dye from the matrix. The present invention provides a development of the method described and claimed in our european Patent Application EP-A-0 413 114 and which unexpectedly gives a much greater yield of the sensor material. According to the present invention there is provided a method of bonding an analyte-sensitive polynuclear aromatic indicator substance to addition cure silicone to form a sensor element, comprising the steps of adding butyllithium and tetramethylene diamine to the indicator substance in a solvent system comprising tetrahydrofuran to generate a mixture containing the carbanion of the indicator substance, adding a bromoalkene to the carbanion mixture to form a functionalized indicator substance, reacting the functionalized indicator substance with polymethylhydrosiloxane to form an indicator-bound silicone liquid, and cross-linking this indicator-bound silicone liquid with vinyl-terminated polysiloxane to form said sensor element whereby said solvent system comprises a mixture of tetrahydrofuran with hexamethylphosphoramide. Preferably the analyte-sensitive polynuclear aromatic indicator substance is selected from coronene, perylene, decacyclene and benzoperylene, and the bromoalkene is preferably selected from 3-bromo-1-propene, 4-bromo-1-butene, 6-bromo-1-hexene and 8-bromo-1-octene, 8-bromo-1-octene being particularly preferred. In the inventive solvent system of the present invention an advantageous proportion of tetrahydrofuran to hexamethylphosphoramide is about 80:20 but other proportions also give beneficial yields. The other steps of the method of the invention are generally the same as described in our european Patent Application EP-A-0 413 114. As is explained in our european Patent Application EP-A-0 413 114 the dye, when contained in the polysiloxane according to the present invention exhibits high sensitivity of the fluorescent quenching function in the presence of oxygen, which makes it extremely advantageous for use in optical systems that measure concentrations of oxygen gas. The dye/matrix structure results in the formation of a cross-linked silicone rubber with the dye covalently bonded to it. The cross-linked silicone can be made to assume various advantageous characteristics by substitutions on the vinyl-terminated polysiloxanes. Viscosity and physical strength of the silicone can also be varied by changing the molecular weight of the polysiloxane. As a result, the dye of the sensor of the present invention is less susceptible to leaching or agglomeration. A sensor element made in accordance with the method of the present invention can be more accurate than prior art sensors. Other features and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying diagrams and equations, which illustrate, by way of example, the principles of the invention. Oxygen sensors which utilize fluorescent dyes, the output of which is quenched in proportion to the concentration of oxygen in a sample to be evaluated are well known in the art. The present invention is embodied in a new and more efficient method for bonding an oxygen-sensitive indicator substance to an addition-cure silicone to form a sensor element that is particularly resistant to leaching of the indicator substance through the polymer matrix. The oxygen-sensitive fluorescent indicator substance is typically a polynuclear aromatic compound. The matrix may be an addition-cure silicone which provides a gas-permeable substance that is hydrophobic and prevents leaching of the indicator dye out of the matrix, since the dye is bonded to the silicone. The polynuclear aromatic dye compound used for sensors has a high degree of symmetry that results in its low solubility in organic materials. As a result, it has been found to be advantageous to first functionalize the dye to lower its degree of symmetry, thereby increasing its solubility in the polymer matrix. The functionalization step provides a linker arm with a terminal multiple bond. This linker arm helps minimize steric interaction between the dye and the polymer. Moreover, the linker arm provides a means to locate the multiple bond remote from the conjugated aromatic systems. In practice, it has been found that the length of the linker arm may be from n = 0 to n = 500 and may advantageously have a length of n = 1 to 22. The following examples are included for further understanding of the invention. It should be understood that these examples are included for the purpose of illustration but are not intended to limit the scope of the present invention. EXAMPLE 1 This is an example of the first and inventive step of the method of the present invention. Coronene is used as the polynuclear aromatic compound. In a dry, 1.0 liter round bottom flask, 1.0 g of coronene (3.3 mmol) is dissolved in an 80/20 solution of tetrahydrofuran (THF)/hexamethylenephosphoramide (HMPA). The solution is then cooled to -50°C over a period of 15 minutes. To this solution is added 5.0 ml of TMEDA (33.1 mmol) and 16.5 ml of n-Butyllithium (33.1 mmol). The dark blue solution is stirred at -50°C for 30 minutes. After 15 minutes of stirring, 1.20 ml of 8-bromo-1-octene (10.0 mmol) is added to the mixture. The temperature is then maintained at -45 to -55°C for three hours. The reaction mixture is then allowed to warm up to room temperature overnight. Isopropyl alcohol (100 ml) is added to quench the remaining n-BuLi. The reaction mixture is then rotary evaporated to remove the THF. The resulting brown residue is dissolved in toluene and washed seven times with deionized water (7 x 200ml) to remove the HMPA. The remaining organic layer is separated, dried over MgSO₄, and rotary evaporated to dryness. The resulting brown liquid is purified on an DNAA silica gel column with a mobile phase of 90/10 hexane/ CH₂Cl₂. The product fractions are then collected and rotary evaporated to dryness. A silica gel prep plate is run, with 90/10 hexane/CH₂Cl₂ as the mobile phase, to remove any impurities. The product band was excised from the prep plate and washed with 200ml of CH₂Cl₂. The CH₂Cl₂ was rotary evaporated to yield 700mg of the fluorescent green product. NMR data (CDCl₃) : 3.31 (dd, 2H, Ar-CH₂) 4.92 (dd, 2H, =CH₂) 5.76 (m, 1H, -CH=) 7.81 - 8.91 (m, 11H, aromatic protons) An important benefit derived from the use of a cosolvent of THF and HMPA in this embodiment of the first and inventive step of the method is the improved yield of the process over a method utilizing THF only. It has been found that this process provides a yield 35 times greater than a corresponding process utilizing THF only ( e.g. , 70% yield compared to 2%), thereby dramatically and unexpectedly improving the economy and efficiency of the process. This fact, when combined with the other benefits of the method, results in important advantages from the use of the invention in the manufacture of fluorescent intravascular sensors. Equation 1 below describes the reaction: As is explained in our european Patent Application EP-A-0 413 114 the second step of the method, the hydrosilylation of the functionalized polynuclear aromatic compound with polymethylhydrosiloxane, can be performed by several well - known methods. The following formula discloses a typical polymethylhydrosiloxane: Where R = H, CH₃, CH₂ CH₂CF₃, CH₂ (CH₂) n CH₃ and phenyl Polymethylhydrosiloxanes are available with different degrees of Si-H substitutions and different R group functionalities. The amount of Si-H substitutions will determine the number of dye molecules that can be bonded to the polymer. The type of R group on the polymethylhydrosiloxane will determine the refractive index of the polymer and the solubility of the dye molecule in the silicon reaction mixture. The higher the solubility of the dye in the polymer, the more likely the dye will bond. EXAMPLE 2 In a first preferred embodiment of the hydrosilylation step of the present invention, a reaction mixture which consists of 15 mg of 8-octenyl coronene (4x10-2 mmol), 1 g of polymethylhydrosiloxane (50-55% SiH, 8 mmol) and 1 drop of 10% platinic acid was heated to 120°C under an inert atmosphere for 16 hours (See EQ 2 below)-with the solid dye compound slowly solubilizing into the polymer. At the end of the reaction period, the dye-bound silicone liquid was filtered through Celite to remove impurities. This scheme provides flexibility to vary the dye concentration by changing the percent Si-H substitution on the polymethylhydrosiloxane. The refractive index and physical strength of the polymer can be changed by varying R. The dye/polymer is extremely stable when stored in a dark and cool environment. EXAMPLE 3 In a second preferred embodiment of the hydrosilylation step, a reaction mixture which consists of 5 - 100 mg of 8-octenyl coronene, 1.0 g of polymethylhydrosiloxane (50 - 55% Si-H), 200 mg of cyclohexane (20% of the polymethylhydrosiloxane) and 1 - 20 drops of 20% platinum/vinylsiloxane catalytic complex in cyclohexane, was heated to 80 - 130°C for 2 - 15 hours under an inert atmosphere. At the end of the reaction period, the cyclohexane was removed by purging with N₂ and the resulting mixture was filtered through a celite/glass wool plug to remove impurities. As is also described in our european Patent Application EP-A-0 413 114 the final step of the method, the cross-linking of the polysiloxane with the hydrosilylated polynuclear aromatic compound, can also be accomplished using known techniques in the art. For example, a mixture of dye/polyhydrosiloxane and vinyl-terminated polysiloxane when heated in the presence of platinum catalyst, results in the formation of a cross-linked silicone rubber with dye covalently bonded to it. The cross-linked silicone can take on different characteristics by changing substitutions on the vinyl-terminated polysiloxanes. Viscosity and physical strength of the silicone can also be varied by changing the molecular weight of the polysiloxane. EXAMPLE 4 In a first preferred embodiment of the final step of the process, 10 mg of dye/polyhydrosiloxane and 50 mg of vinyl-terminated polymethylphenylsiloxane with 5 ppm of platinum catalyst is mixed in an aluminum dish. The silicone mixture is degassed under a vacuum and was applied to the optical fiber. The mixture is then cured by heating the fiber tip in an oven at 100°C for 1 hour. EXAMPLE 5 In a second preferred embodiment of the final step of the process, 10mg of dye/polyhydrosiloxane is mixed with 90 mg of hexane. To this mixture is added 80 mg of vinyl-terminated polymethylphenylsiloxane. The mixture is then stirred and promptly applied to the optical fiber. The mixture is cured by heating the fiber tip in an oven at 100° C for 1 hour. In order to evaluate the performance of a sensor formed according to the method of the invention, an optical fiber including a quantity of the cured matrix at its distal end was connected to an instrument which provides means to activate and read the output of the sensor. The fiber tip was placed in a saline solution and 7% O₂ gas was introduced. When the sensor was irradiated with 380nm light, the emission at 440nm yielded a normalized voltage of 2.857V. When the concentration of O₂ was increased to 20%, the resulting voltage was 1.934V. A voltage of 2.852V was registered when the O₂ level was reverted back to 7% showing reversibility and no hysteresis. The sensing tip was also placed in media such as methylene chloride, methyl alcohol, isopropyl alcohol, silicone liquids, and bovine blood. No noticeable drop in signal was observed. From the above examples, it is evident that the present invention provides a very efficient chemical method for covalently bonding a polynuclear aromatic dye to an addition-cure silicone. While several particular forms of a method in accordance with the present invention have been described, it will also become apparent to those skilled in the art that various modifications may be made without departing from the scope of the invention. Accordingly, it is not intended that the invention be limited, except as by the appended claims.

Claims: A method of bonding an analyte-sensitive polynuclear aromatic indicator substance to addition cure silicone to form a sensor element, comprising the steps of adding butyllithium and tetramethylene diamine to the indicator substance in a solvent system comprising tetrahydrofuran to generate a mixture containing the carbanion of the indicator substance, adding a bromoalkene to the carbanion mixture to form a functionalized indicator substance, reacting the functionalized indicator substance with polymethyl-hydrosiloxane to form an indicator-bound silicone liquid, and cross-linking this indicator-bound silicone liquid with vinyl-terminated polysiloxane to form said sensor element whereby said solvent system comprises a mixture of tetrahydrofuran with hexamethylphosphoramide. A method as claimed in Claim 1 wherein said analyte-sensitive polynuclear aromatic indicator substance is selected from coronene, perylene, decacyclene and benzoperylene. A method as claimed in Claim 1 wherein said bromoalkene is selected from 3-bromo-1-propene, 4-bromo-1-butene, 6-bromo-1-hexene and 8-bromo-1-octene. A method as claimed in Claim 1 wherein said analyte-sensitive polynuclear aromatic indicator substance is selected from coronene, perylene, decacyclene and benzoperylene, and said monobromoalkene is selected from 3-bromo-1-propene, 4-bromo-1-butene, 6-bromo-1-hexene and 8-bromo-1-octene. A method as claimed in any one of the preceding claims wherein the solvent system is a solution of tetrahydrofuran and hexamethylphosphoramide in a proportion of about 80:20. A method as claimed in any one of the preceding claims wherein the bromoalkene is 8-bromo-1-octene. Use of a method as claimed in any one of the preceding claims in the manufacture of fiber-optic based oxygen-sensing apparatus.

Inventors: DIVERS GEORGE, HUI HENRY, SOIKOWSKI CARMEN, DIVERS, GEORGE, HUI, HENRY, SOIKOWSKI, CARMEN
Applicants: PURITAN BENNETT CORP, PURITAN-BENNETT CORPORATION
Application year: 1991
Main classification: G01N31
IPC codes: G01N31