Abstract:
The optical fiber pH microsensor includes an optical fiber having a portion of the surface of a light conducting core covered with a layer of a pH sensitive dye material. The dye material is covalently bonded to a polymeric matrix which is in turn covalently bonded to the optical fiber core to prevent leaching of the indicator dye material during extended use. The dye material is crosslinked in situ over the tip of the optical fiber to yield a hydrophilic, ion permeable pH sensor which can be used intravascularly to monitor blood

Description:
This a continuation of application Ser. No. 07/902,402, filed Jun. 19, 1992, now U.S. Pat. No. 5,277,872, which is a continuation of application Ser. No. 598,137, filed Oct. 16, 1990, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention is generally directed to chemical and biochemical quantitative analysis, and more specifically concerns an optical fiber sensor for measuring pH in a fluid or gaseous mixture. 
     2. Description of Related Art 
     In modern medicine, measurement of acidity (pH) in the blood has become an important factor in the determination of the respiratory status of a patient. Although electrodes have been developed which are capable of measuring pH in fluids, they are of limited use in measurement of in vivo blood pH levels. Optical sensors for taking intravascular measurements of acidity and other blood analytes such as oxygen and carbon dioxide show promise for in vivo measurement of blood pH. Such optical pH sensors typically include a fluorescent indicator dye, such as fluorescein or hydroxypyrenetrisulfonic acid (HPTS), placed over the tip of an optical fiber and a membrane cover over the dye which is permeable to the hydronium ions to be measured. The dye fluoresces when exposed to a certain wavelength of light conducted to it by the optical fiber. In practice, a pH sensor is fabricated by immobilizing a pH sensitive dye into a matrix attached to the distal end of the fiber. The dye is typically capable of existing in two forms, an anionic or base form, and a protonated or acid form. The two forms are each excited by a different frequency, but fluoresce at the same frequency, with the output responsive to excitation at the appropriate different frequencies being proportional to the pH of the sample to which the sensor is exposed. In this manner, measurement of the intensity of fluorescence of the indicator dye can be related to pH. 
     Optical absorbance indicator dyes, such as phenol red, have also been utilized in optical pH sensors. In this type of pH sensor, green and red light are emitted from one end of the optical fiber into the dye material, passing through the dye to be reflected back into another optical fiber. The green light is absorbed by the base form of the indicator, and the red light is not absorbed by the indicator, so that it may be used as an optical reference. The ratio of green to red light can thus be related to pH. 
     One approach to construction of optical fiber sensors involves the attachment of a dye filled porous glass to the tip of the optical fiber, such as by an adhesive. Another approach has involved the application of sensing material directly to the tip of the optical fiber. Another approach has involved the attachment of a sleeve which contains the dye indicator sensing material immobilized in a hydrophilic polymeric matrix by entrapment or by ionic interactions over the tip of the optical fiber. However, such sensors allow the indicator dye to leach out over extended time periods. Leaching of the indicator dye results in increasingly inaccurate blood pH measurements. Other covalently bonded sensors known in the art have either not been capable of attachment to the end of the optical fiber, or have been merely cast over the tip of the fiber without being crosslinked or covalently attached to the fiber. 
     There remains a need for a fiber optic pH sensor which provides covalent linkages between the dye and matrix, and between the matrix and the optical fiber, to prevent leaching of the indicator material during periods of extended use of the sensor in measuring blood pH intravascularly. It would also be desirable to allow for control of the concentration of dye in the final sensor matrix, and to allow for uniform application of the sensor matrix over a wide range of sensor thicknesses. 
     SUMMARY OF THE INVENTION 
     Briefly and in general terms, the present invention provides a new and improved optical fiber pH microsensor which includes a pH sensitive dye material covalently bonded to a polymeric matrix, which is in turn covalently bonded to the surface of the core of the optical fiber to prevent leaching of the indicator dye material during extended use. The dye material is crosslinked in situ over the tip of the optical fiber to yield a hydrophilic, ion permeable pH sensor which can be used intravascularly to monitor blood pH. 
     Because the dye is attached to a stable polymer which is completely miscible with the crosslinking component, the exact concentration of the dye in the final sensor material can be quantified and closely controlled by use of the invention. Control of the viscosity and dilution of the polymer and choices of the solvents used, including various combinations of cosolvents, allow for uniform application of the sensor material over a wide range of thicknesses of the sensor. The nature of the crosslinking polymer also allows for formation of the sensor with a closed cell polymer or an open cell material, so that the response time and molecular exclusion parameters of the sensor may be suitably adjusted. 
     Other aspects and advantages of the invention will become apparent from the following detailed description and the accompanying drawings, which illustrate, by way of example, the features of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective diagram of a fiber optic sensor system utilizing the sensor of the invention for monitoring blood pH levels; and 
     FIG. 2 is an enlarged, cross-sectional schematic diagram of the fiber optic sensor. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The extensive application of long term intravascular blood pH sensors utilizing fluorescent dyes immobilized on the distal ends of optical fibers has been limited by a number of problems. Among the problems is the leaching of dye indicator materials inadequately immobilized in the chemical sensing area of optical fiber pH sensors during extended periods of monitoring of blood pH levels. This has resulted in inaccurate long term intravascular measurement of blood pH by this method. According to the present invention, an optical fiber pH microsensor is prepared by covalently bonding the dye material to the polymeric matrix, and covalently bonding the crosslinked polymer to the tip of the optical fiber. 
     As is shown in the drawings, which are provided for purposes of illustration, the invention is embodied in an optical fiber pH microsensor which may be used for intravascular monitoring of blood pH levels, and a method for making the pH microsensor. As is illustrated in FIG. 1, in such a system a light source 2 provides an output light beam 4 that is passed through a dichroic mirror 30 and focused by a lens system 6 into a connector 8 of an optical fiber 10, which carries the light beam to a sensor module 12 at a distal end of the optical fiber. The light source preferably includes one or more excitation filters 14, actuated and controlled by stepper motor 16, for controlling the wavelength range of the light provided to the sensor module. Sensor module 12 is adapted to be placed in a fluid 18, such as blood, for quantitative measurement of a chemical parameter of the fluid, such as pH. The sensor could, of course, be adapted to detect concentrations of gases, such as oxygen or carbon dioxide, drugs, or other blood constituents. 
     As is illustrated in FIG. 2, the optical fiber sensor module is generally formed from an optical fiber having a light conducting core 20, such as glass, and an outer cladding material 22 having a refractive index such that light conducted by the core is substantially retained in the core material. A length of cladding on the distal end of the optical fiber is removed, leaving an exposed distal tip of the core. The exposed distal tip, preferably primed to provide sites for covalent attachment of a polymeric matrix, is coated with the polymeric matrix 24, which is preferably a hydrophilic polymer covalently bonded to one or more indicator dyes which are known to fluoresce in response to irradiation with light of various wavelength ranges. The polymeric matrix is preferably a polyether polyisocyanate, such as HYPOL-2002 made by W. R. Grace &amp; Co., covalently bonded in a polyether polyamine form to HPTS. 
     A coat of reflective material 26 is also preferably provided over the dye containing sensing matrix, to retain and reflect both the irradiating light and the fluorescence emissions from the dye indicator. The reflective coating is preferably a mixture of about 50% by weight titanium dioxide in a polyether polyisocyanate, such as HYPOL-2002 diluted to 37% in acetone. Ten percent water by weight is utilized to initiate crosslinking. In certain applications, an exterior coating or sheath 28 may be used to further facilitate or protect the optical fiber assembly. 
     The output optical fiber 10 may also carry light fluoresced from the dye indicators via a dichroic mirror 30 to emission filters 32 which may be actuated by stepper motor 34 and the fluorescent light beam 36 upon a detector array 38. Similarly, the portion of the light 35 beam 4 that passes through the dichroic mirror 30 may be focused by a suitable lens 40 upon a reference detector array 42, which allows measurement of the excitation signal strength. The electrical output of the detectors is fed through cables 44 to a computer 46, such as an IBM PC, which receives the electrical output of the detectors and determines the blood analyte being monitored, such as pH. The computer is preferably programmed to determine the pH based upon the specific measurement of fluorescence intensity represented by the electrical output signal received by the computer, according to an algorithm based upon signal outputs from measurements from samples with known pH levels. The output of the computer may be indicated on a meter 48 or another suitable readout device. 
     The method of making the optical fiber pH microsensor involves hydrolyzing the hydrophilic polymer, whic is preferably a polyether polyisocyanate, such as HYPOL-2002, preferably in the presence of an alakaline base and butanone, to form the polyether polyamine, HYPOL-polyamine, and carbon dioxide gas, as shown in equation (I) below: ##STR1## 
     The HYPOL-polymaine is then reacted with a sulfonyl chloride form of the indicator dye, preferably acetyoxy-HPTS-SO2Cl to covalently bond the dye to the HYPOL-polymaine, forming HYPOL-polyamine-HPTS, as shown in equation (II) below: ##STR2## facilitating uniform application of the sensor material over a wide range of thicknesses of the sensor. 
     In order to prepare an optical fiber for application of the dye sensor material, a portion of the cladding at the end of the optical fiber is removed to expose the glass core. The exposed surface of the glass core is primed by treating it with an isocyanatosilane, for example, isocyanatopropyltriethoxysilane, to provide sites for covalent attachment of the polymer to the fiber. The HYPOL-polyamine-HPTS is diluted with HYPOL-2002 and a common solvent such as acetone, as desired, to form a dye mixture, ready for application to the optical fiber, that is stable for several days if stored under anhydrous conditions. By controlling the viscosity of the uncured polymer matrix material, a desired thickness of matrix material may be applied. In practice, it has been found that a variety of solvents of the matrix may be used to alter both the thickness of the matrix applied and the cured properties of the matrix. For example, acetone, methanol, or ethanol may be used in greater proportions as a solvent if relatively thin coatings are desired, while polyvinylpyrrolidone in DMI may be used in greater proportions for thicker coatings. Similarly, glycerol, polyols, and hydroxyethyl methacrylate may be used as a matrix modifier in various proportions to alter the resilience and strength of the cured matrix. 
     When it is desired to apply the dye mixture to the exposed surface of the glass core of the optical fiber, the HYPOL-2002/HYPOL-polyamine-HPTS solvent mixture is mixed with approximately 10% water by weight to initiate cross-linking, and the mixture is then applied to the exposed tip of the fiber. The applied mixture is then allowed to cure at room temperature for approximately one hour to form the pH sensing matrix, as shown in Equation III below: ##STR3## 
     After the sensing matrix is completely solidified, the coating of reflective material 26 may be applied over the sensing matrix. The cured dye sensor matrix is preferably coated with a reflective material comprising approximately 50% TiO2 in HYPOL-2002, which serves to provide protection, optical isolation and reflection of both the excitation and fluorescence emission light. 
     From the foregoing it will be appreciated that the invention provides an optical fiber pH microsensor which will prevent the problems of leaching of dye indicator materials during extended periods of intravascular monitoring of blood pH. It is significant that the optical fiber microsensor is prepared by covalently bonding the dye material to the polymeric matrix, and covalently bonding the crosslinked polymer to the tip of the optical fiber. As will be readily appreciated, the principles of the invention are applicable to other types of optical fiber microsensors such as blood oxygen and carbon dioxide sensors, in which similar problems of inaccuracies of analyte measurements have resulted from the leaching of dye indicator materials during extended periods of use of the sensors, particularly in intravascular monitoring of blood analytes. 
     While particular forms of invention have been illustrated and described, it will be apparent that various modifications can be made without departing from the spirit and scope of this invention. Accordingly, it is not intended that the invention be limited, except as by the appended claims.