CA2183455A1

CA2183455A1 - Solid state sensors

Info

Publication number: CA2183455A1
Application number: CA002183455A
Authority: CA
Inventors: Misa V. Jovanovic; David Reed Markle
Original assignee: Misa V. Jovanovic; David Reed Markle; Biomedical Sensors, Ltd.
Current assignee: Biomedical Sensors Ltd
Priority date: 1994-02-16
Filing date: 1995-01-18
Publication date: 1995-08-24
Also published as: AU1390995A; US5511547A; DE69516272D1; ATE191790T1; DE29580807U1; MX9603433A; KR970701348A; JPH09502528A; ZA951219B; DE69516272T2; EP0745220A1; EP0745220B1; WO1995022759A1; IL112595A0; KR0171549B1; JP2821026B2

Abstract

A stabilized bio-inert sensor for the determination of an analyte, especially pO2, pCO2 and pH, in a medium which comprises a chemical indicator sensitive to the analyte in association with a stabilizing substrate formed from a polymer which is inert to the medium and analyte and does not affect the sensitivity of the indicator, which polymer is a cross-linked, solid silicone rubber formed from a silicone carbinol having a molecular structure compatible with said indicator.

Description

2 1 8 3 4 5 5 P~ . S

Solid State Sensors This invention relates to solid state sensors. More particularly the invention is eoneemed with stabilked solid state sensors for the d~""i" n of cvllce"t,, of gases, espeeially pO, and pCO2, and also, with an ~,u,uru~ .t~ ~ndieator, for the cl"t,t""i" ) of pH, in liquid media, sueh as human blood 1û The measurement in blood of PO2, PCO2, and pH is important during sursiealproeedures, post-operaUvely, and during l l , , under intensive eare, numerous sensor deviees for the measurement and monitoring of sueh parameters are diselosed in the art. A sensor deviee, hereinafter referred to as a sensor, for d~t~llllillil,~ the cu" .~ ., of an analyte in a liquid medium, typieally eomprises an indieator having a ,II~G.lt:liati-, whieh ehanges in the presenee of the analyte in r ' ' ~ with a sultable earrier or substrate which also acts as a ll~lallliaa;ull line for conveying a signal lI:,Ultlaell -.L;~ of said change to a suitable detector. For example, the use of pyrenebutyric aeid as a fluoreseent indieator for the v~t~""i" ~ of the cu~
of oxygen in blood is known and Uhe use of this sensor in eonjunetion with an optieal fiber, wherein the fluoreseent indieator is enelosed within a selecUvely permeable membrane, is diselosed in U.S. Patent No. 4,476,870.
A sensor utilizing a fluoreseent indieator and adapted to funetion ..~ tul;l~
in a biological e:ll/i.vllllle:llt should possess at least four ,llal lull:lialk,a: sensitivity, short response time, stability and bio-inertness.
Sensitivity depends upon the quantum efticiency of the fluoreseent ~ndieator, the eul l i :l Ill cll;ul1 of the indicator present in the sensor and availability of the indicator to the substance, i.e. ion or gas, it must sense. Thus a sufticient amount of indicator must be available to produce a meaningful fluorescent response. However, if indicatormolecules are too close together there occurs a type of behavior whieh is frequently detrimental to the sensor p-, tu""C.l l-,e, this behavior is known as eximer fluoreseenee.
Therefore, for a given indication there is an optimum indicator Cùll~,._.ltl " n for maximum sensltivity.
A further problem which must be solved in the constnuction of a fluorescent sensor is the availability of the indicator to the environment to be sensed. If the subjeet ions or gas cannot reach the indicator molecules the indicator will not respond to the presence or absence of said ions or gas. This problem is clearly related to the p~", ty of the strUclUre in which the indicator molecules are ~mhe~ rl . _ , .. . . . . .

Wo ss/227sg ` 2 1 8 3 4 5 5 Also related to permeability is the response time. If the substance to be sensed(i.e. ions or gas) diffuses very slowly through the structure the response time of the sensor will be cu""u~ Ion9 which 9reatly reduces its usefulness.
A sensor for blood gas or blood pH should be capable of use over a period of 5 many hours or days. r,a "' , of a sensor which is used in vivo is clumsy and inefficient or even impossible. Thus, the stability of the sensor is a key factor in .l~t~ its utility. A common problem in existing fluorescent sensor design is thegraduai ioss of the indicator from the sensor. This not only reduces the sensitivity, thereby creating instability in the sensor's indication even at constant cu, ,u~"t, , of 1û the substance being sensed but also releases a chemicai indicator into the blood stream. A device which releases chemical substances into the blood stream can not be considered to be bio-inert. As used herein, the term ~bio-inert~ is defined to mean that ul lalaule,l i~ti,, of a device, i.e. a sensor, whereby any and all chemicai substances which are part of the device are so securely bonded to the structure of the device thd 15 they are not released or leached away from the device under normal operating conditions.
In the prior art the problem of leaching of the indicating substance from the sensor, which is inherent when small molecules are embedded in a polymer matrix, was generally addressed by enveloping or embedding the indicdor in ~ seiectiveiy permeabie membrane.
in practice, the problem manifests itseH as a progressive loss of sensitivity of the sensor as the indicator is lost; this requires a continual re-calibration of the sensor.
The stated prior art al Ia~ lllea ~t does not completely solve the problem, since a portion of the indicating substance is still leached from the sensor. Thus, the problem of re-calibration still remains, and, moreover, the reieased indicator goes into the patients iJluo iall.,_.ll.
Acc~, di, Inl~, it is desirable to provide a sensor which is more stable in the sense that the indicator is not leached or washed away therefrom upon contact with body fluids.
The desired stability may be achieved, according to U.S. Patent No. 5,019,350 by providing a sensor ~or the d~ , of the cui,~ t,. , of a dissolved substance in an aqueous medium comprising an optical fiber havins a distai end to which is stably bonded an adherent, water-insoluble or9anic polymer having a plurality Wo 95l227s9 2 1 8 3 4 5 5 . ~11~ t

-3-of fluorescent or3anic substituents, which may be the same or different, covalently bonded to said polymer through ester or amide linkages.
The c~"~bi~ of polymer and fluorescent organic substituents forms a fiuorescent polymeric indicator, examples of which are indicators for PO2. pH and pCO2.
U.S. Patent No. 5,262,037 discloses an eléullu~.llellli~,~l sensor for the ie~l l l l;l l.~'-JI~ of the partial pressure of oxygen in a ~loo~i~t~ u~., .. This eléull v~.l lel l li-,6i sensor for PO2 may be used in ~,u,,,~il, n with a pH sensor and a pCO2 sensor toform a multi-parameter sensor. In such a multi-parameter sensor the pH sensor and PC02 sensor preferably are made in ~ , ddl~ce with the disciosure in U.S. Patent No.
10 4,889,407 which provides an opticai waveguide sensor for de~ell"i"i,~g an analyte in a medium, which sensor comprises an optical waveguide having a portion to be brought into contact with the medium, said portion having a pluraiity of celis arranged in an ~rray which substantially covers the cross-sectional area of the waveguide, each of said cells containing an indicator sensitive to said anaiyte. The preferred waveguide is an 15 opticai fiber and indicators disclosed include absorption indicators for pH, such as phenol red and fiuorescent indicators, such as B-~,.,l.. " ' .-,,~e for pH or pCO2, and pyrene butyric acid for PO2. In preparing the sensor the indicator is deposited in the celis of the optical fiber in the form of a gel or solid by immersing the portion of the fiber containing the cells in a solution of the indicator and a~J,clu~ le gel-forming 20 ingredients, subjecting the immersed fibers to a vacuum so that the cells are evacuated to allow ingress of the solution and curing the gel so thd the indicator is retained in the cells in a stable manner.
PCT Application Publication No. WO 91/05252 discloses a carbon dioxide monitor which comprises a substrate having thereon an indicding member comprising 25 an intimate mixture of a llall~ .ll poiymer vehicle, and an indicator material which undergoes a color change on exposure to carbon dioxide. The indioator materiai comprises a salt of an indicator anion and a lipophilic organic quatemary cation.
US Patent No. 6,ûû5,572 discloses a detector for the detellllil '~.. of carbon dioxide in respiratory gases and a method for detel Il l;l lil lg the proper piacement of an 30 intubation device in a patient's trachea. The carbon dioxide detector comprises a pH-sensiUve dye, a solid phase support and a phase transport enhancer for enhancing a reaction between H2CO3 and the pH-sensitive dye.

W09512Z759 ~ 2 1 û34 55 U.S. Patent No. 4,728,499 discloses a c~",~i" ' , rapid response device for the detection of carbon dioxide in a gas mixture comprising an enclosure with a bcula,u~llt window having mounted therein an indicator cu,,,,uù~ comprising a cnrrier to which is flxedly attached an indicating element including a cl ,,u,, ,oy_";~. pH-sensitive indicdor which changes color when the cu"~ "t,, " , of carbon dioxide in the surrounding t~llua~ exceeds 2%. The device is used to determine the oorrect placement of an ~,, iut,c,. ll_~i catheter.
It has now been found that the stability and pt:,lul",~,~.~ of sensors for the d~ l " ,i".-t;~ " of an analyte in a medium, particularly for the d~t~l " ,i, r~ of PO2~ PCO2 1û and pH in both liquid and gaseous media, is greatly enhanoed when an cl,u,u,u,u,;..t~, indicator is used in association with a polymeric silicone crlrbinûl as described hereinaRer.
In ~cc~,.Ic~ ,e with the present invention there is provided a stabilized bio-inert sensor for the d~tt~llill " , of an analyte in a medium which comprises a chemicai 15 indicator sensitive to the anaiyte in association with a stabilizing substrate formed from a polymer which is inert to the medium and analyte and does not aflect the sensitivity of the indicator, which polymer is a cross-linked, solid silicone nubber forrrled from n silicone carbinol having a molecular structure compatible with said indicator.
The polymeric silicone rubber used in the sensor of the present invention is 2û preferably one of the following two types:-1. A silicone carbinol llu~lu,uuly.~ having the formula:-CH3 / R \ ~H3 CH3--gl--o~SI-- ) - I--CH3 (I) \ C H2 OH ~ C H3 wherein i~ is methyl or phenyl, z in an Integer from 1 to 2û and n is an integer from 2 to 500;
2. A carbinol siloxane copolymer having the formula:-~ W0 95122759 ' 2 1 8 3 4 5 5 r~
C H3--5 1 -0 ~ 5 ~--O ~T~ 51--0~ S I--C H 3 ( I ~ ) wherein each R is methyl or phenyl and the Rs may be the same or different, z is ~n integer from 1 to 20, and each of x and y is an linteger wherein the sum of x and y is from 2 to 500.
A particularly preferred hu,,,u~.ùly.,,tl of fonmula (I) is a methyl silicone carbinol 10 hu,,,u~.~,ly.,,~l of the fonmula:-CH3 / CH3 \ CH3 CH3--Sl--tsl--O ~ I--CH3 (III) ::
15 CH3 \(CH2)~ ~1 CH3 wherein n is an integer from 2 to 500.
A prefenred copolymer of fonmula (Il) is a dimethyl/methyl carbinol siloxane copolymer of the fommula:-20 CH3 ~ CH3 \ /CIH3 \ CH3 CH3--Sl-0----S~--0--_--Sl--0-- Sl--CH (I
CH3 CH3 (CHz~s CH3 3 \ CH20H
wherein each of x and y is a positive whole integer and the sum of x and y is from 2 to 500.
The above linear polymers have unique features which may be attributed to the carbinol side chains.
As used herein the tenn "carbinol- is a generic term to describe silicones with diflennt alcohol side-chains. When cross-liked the carbinol groups form a IIJll.,ul~uL.iu, gas-permeable silicone matrix for the analyte-sensitive indicator associated therewith. A particularly preferred indicator is an oxygen-sensitive nuthenium woss/227~9 ~ ~ 2 1 ~3455 indicator described hereinafter. Another preferred indicator is a carbon dioxide-sensitive indicator such as phenol red.
The linear siiicone poiymer used as the substrate in the sensor according to theinvention is preferably prepared by protecting a terminaliy unsaturated long-chain 5 alcohol which is then used in the i,~d~ reaction (also referred to as a h~dlv~ reaction) of a selected i,;~ e. The finai step is de,ulvte~_tiul~ of the alcohol-OH groups.
Reactions with similar chemistry to that of the above reaction arQ known in the art. For example, a i,jJ~, ' , reaction is disclosed in U.S. Patent No. 3,122,522.
10 However, prior to the present invention, the type of silicone polymer made ina~,cu, v~ .ce with the stated reaction has not been used with Gl ,al~tv ~ , ~ indicators to make the unique sensors described herein. A particular ~d~ tclv,vvus feature of the novel sensors of the present invention is the manner whereby the silicone carbinols are cross-linked to produce solid silicone rubber-like polymers which may be tailored to be 15 combined with diflerent indicators to form the desired sensors Some of the unique and advcultc~u,t:ous features of the materials used in the sensors of the invention are:-a) long carbinol side-chains behave as surfactants to ~soiubilize~ the polar materials which are not solubie in water. In essence, they replace plastisizers in 20 plasticized solid matrices which are the key elements forthe PO2 and pCO~ sensors that are currently on the market. These solubilizing side-chains are particulariy compatible with polar transition metal complexes which contain the non-poiar ll~ u~,~.,Lvl~ and unsaturated i,Jdlvuc,Lùl~ ligands.
b) The size and length of the carbinol side-chains may be varied to change 25 the l"u"ul,vivgy of the cross-iinked solid pûlymer. The same polymer also may be made less l,ldluvl~oLic by shortening the carbon chain length and very hydrophilic (even water-soluble) when z in Formula (I) or (Il) is less than 4. The ).j~,v~-l,/' ty of the polymer also may be controlled by combining the iong chain and short chain alcohol group in the same linear ~ ,ol~,.,v c) The stability of the silicone polymers and p,v,uuly,,,~., is greatly enhanced by the pol~ ' ~e back bone. The poiymers have a high resistance to a variety of chemical agents and remain intact on long term exposures to water vapor and biological ~ases. The stability under gamma irradiation is cu-, ,,v. c~LIv, if not better ~ w0 9s~227s9 '~ t ' 2 1 8 3 4 5 5 r ~

than, that of PVA. Some conversion of free aicohol groups to ether linka3es may occur due to the formation of 0- and H- radicais.
d) The linear siiicone i~le,Uul~ as well as the final polymers haYe no adYerse a~fects and show no leaohing of ~ndioator elements as dt~ u~ t~i by 6 preliminary cytotoxiclty studies.
Preferred ~I~liJû~ llta of the inYention are pa~ticulariy describe hereinafter.
1. PolYurethane t,YPe cure sYstem:
A ~production friendly' silicone nubber matrix which ;"~.ul,uul :~,., an oxygen-sensitiYe nuthenium indicator (i.e. tris (4,7-diphenyl-1, 1 0-ul ,~, ,u, Itl 1,, " ~e) ruthenium (Il) 10 chloride) and an alcohol polymer side-chain as the solubilizing e".i.u"",~:,lt (for the indicator) is prepared as foliows:- The selected ~" ~,uulyl "~ 'y "' ,c, linear polymer to which alcohol side-chains are attached through a silicone-carbon bond) is reacted with a difunctl'onai isocyanate in the presence of a catalyst to fomm a silicone matrix in which some of the alcohol groups are cross-linked through the urethane type linkage 15 as indicated in the followin3 equation:-o C
-g I--O-S l O_g 1- N C a t a l y s t -g I--O-S 1-O--S l-~CH2)zl (CH2)z ~Cross ' (CH2)z (CH2)z CH20H Ch20H ~l~er O=CO-NH Ch20H
N ~1 C (CL ri 15( 5 , O ~
NH .
O~C-o ( CH2)~
-O--S i_O-S i The cross-linker may be any one of a number of c~,,,,,,c,u;_:, aYailable A preferred cataiyst is dibutyltin dilaurate (D3TDL) haYing the fommula woss/227s9 - 21 83455 r~

[CH3(CH2)3]2 Sn[02C(CH2)l0CH3]2 The low molecular weight silicone carbinol (in this and all other compounds which follow it is to be understood that carbinol is a C10 alcohol unless spec-lfied othenAlise) is combined with the nJthenium indicator which was previously dissolved in 5 methylene chloride; the two solutions are mixed and the methylene chloride is blown off by purging the mixture with air. The use of a solvent to presolubilize the nuthenium indicator before addins it to the silicone carbinol is only for the sake of Cu~ .,Cu and to speed up the mixing process. Altematively, the indicator may be dissolveddirectly in the silicone carbinol(s) by ~ " G~UI 1' " ~i.
The clear, deep red polymer precursor mixture is then combined with an a,u~luuli_t~ difunc1ional isocyanate and cured in an oven at 65C. The cure time may be shortened by substituting a more reactive aromatic isocyanate for the one currently used and/or by adding more catalyst (i.e. tolylene ~ .cy~...t~, or TDI, with a tin catalyst such as DBTDL, cures the silicone carbinol at room temperature in 2 to 10 1 5 minutes).
A typical polymer precursor mixture consists of the following:
80-90% Silicone Cl 0-carbinol 25mg/g of prepolymer Ruthenium indicator pr~snl~' " ' in methylene chloride and treated as described above 10-20% di~uc~,al,t~ cross-linker 0.01-2% cahlyst (optional - to speed up the cure) ~tris (4,7-dyphenyl-1,10-,ul~t",a"tl"- ': ,e) ruthenium (Il) chloride Typical recipe for a thermal cure system:
900mg low molecular weight silicone C1O-carbinol h~l-lopoly~
259 Ru indicator (chloride) ,~ e :' in methylene chloride is added ând mixed with the silicone carbinol 100mg isophorone diisocyanate (IPDI), 10% by weight-100ml 1% of DBTDL catalyst solution in methylene dichlor~de (effectively 0.01% in overall catalyst COI~Ct "I, " ,) 30 ~polymer precursor + cross-linker considered to be 100% of total weight; the weight of ruthenium indicator is not included and the weight of the catalyst is not considered rls part of total weight unless it exceeds 1% of total weight.

~ W095/22759 2183455 r~"~
The working life of the above mixture is 60 to 90 minutes; this may be extended by the .~,~,u~v~ i..h choice oS the eross-linker and the cataiyst.
2. Polvether tyPe cures:
In this elllvvvil"_.~. it has surprisingly been discovered that eer.ain silieone6 carbinols may be eured (to form solid silieone nubbers) in the presenee of a tin eataiyst (DBTDL), iJli,aiJIIII~ by aicohol side-ehains reaeting with eaeh other and formlng ether linkages.
--S l-0-S l-0- - l- a t a~ s t( hea t )- l-0-C; 1-0- l-(CH2)z ( H2~Z CD (~H2~z ( H2)z CH20H CH20H o CH20H
( H2)z _r 1-0--S i-Sueh solids have been prepared with high moleeular weight silieone e~rbinol 10 I,v", r ').., with tin eataiyst cv,~c~,ltl . of 2 0.5% (by wt.). Low moleeular weight silieone e.3rbinol l~v".v,~vl~."~,, failed to produce 3 solid at low DBTDL
cvl~c.:"~ ~s. When making the oxygen sensor by this method, it is essentia. to keep the tin eatalyst .,v, ,.""'~ . , low (not to exceed 2%) in order to prevent another s~de reaetion from taking place. (At high cv~ . r~s of tin catalyst the solid films darken 15 and their oxygen sensing capabilities drop, presumably because some oxidation-reduetion takes place between the tin cataiyst and the complexed nJthenium (Il) ~on).
The low moiecular weight silicone carbinols may be used to prepare a solid state pCO2 sensor by the above method.
3. Alcohol Droteetinq arouPs (delayed cure systems):
As an extension of the above e~ c~ t~ and in order to better control the curing process, .t is desirable to have silicone carbinol(s) wlth blocked -OH groups, which, when Vi,,41~ 'i, may undergo the s3me type of chemistry described above.
Such proteeting groups may serve as ehemical and thermai switches for a ehosen F '~ ~ ' - l process.

WO 95/227~i9 . 2 1 8 3 4 5 5 P~
~o.
One such example involves the use of lli",~ ' / derivatives tTMS) which may be ci_,u, ul~ul~d ~n situ to give the free -OH groups and initiate the poly,"~, i ' ..
Low and medium molecular weight TMS-protected silicone carbinols (which are also ,1 l: . ' ' for making the ~,u~ u~, ii"~ carbinols) and hydrolysed TMS groups 5 have been prepared by exposing the polymer precursor mixtures to HCI vapor.
--51-0-51-0- 1- + Hi(HCI gas~ -- i-0-Si-0-Si-2)Z (-H2)z (~H )z (CH2)z 0-TMS OTMS oH2 OH
The reaction then proceeds as in methods 1 or 2 above, (or any other type of cure which involves the aicohol -OH groups).
The reaction may be conducted by piacing filled fiber sensors (made *om 10 polymethyl ",~tl,~ t~, PMMA, fibers) in an enclosed chamber filled with hydrogen chlcride gas or by simply keeping the filled sensors above a dish containing COI~C~"t, ~ .d aqueous HCI. The ~i~,u, utc~.tiu, of the alcohol groups readily takes pl~ce at or belcw room temperature. In this oase, it is aiso desirable to us fast cureconditions tTDI cross-linker with the cataiyst) in order to minimize the problems whlch 15 frequently accu~,u~u~y long cure F ~
The only drawb~ck to this approach is that once the i~ul~ is Initiated on the surface of the poiymer mixture, further p !~ - is controlled by tha rate of TMS group hydrolysis (i.e. diffusion rate of HCI gas through the polymer v~hich has ~iready fcrmed). The poly (methyl 1~ l~ti ~- ~1 ) fiber itself should not be affected by 20 exposure to the HCI vapor since the conditions for its hydroiysis are quite rigorous (i.a.
PMMA surface etching takes place in hot alcoholic KOH or cu, ,-_~"~ ~.t.~i suNurio ac~d).
The fiber cladding should also remain intact since it is composed of a ,II~,,,;-,_,:, inert periiuorinated IIJ~i~U~LUII~
Other alcohol protecting groups may be used as candidates for a controlled 25 delayed cure prooess.

4. UV-cure sVstems:
Severai concepts utilized in thermal cure systems were extended to the silicone carbinols which may be UV-cured. Partiai substitution of the alcohol groups with WO 95122759 ` - 2 1 8 3 4 5 5 r~
. "~ .tu groups provides the means for SUch a system; linear chain extensions and cross-linking taking place by a free-radicai Ill~ulla~);all- throuah the ~ UIa~
groups.
-SI-O~ + UV lnl tlat~ --Si-O-Si-O-~i-,H2)zl ( H2>Z (CH2~Z (CH2)z `C''O H ~oo - `C` H
C I~C H2 ~C Hz O~C--O
(CHz)z --O--S i--O-S i-6 uv-initiator is benzoin iso-butyl ether or any other benzoin derivative currently on the market.
Another approach is to paltiaily esterify silicone carbinol -OH groupâ wlth ...~tl.aGr~" acid. Such systems have been tested with encouraging resultâ. For instance, pcl~ lI.yl C,O-carbinol (37-40%) copolymer when 10 completely esterified with ~ groups produoed a hard solid when it was Irradiated with UV-radiation ~n the presence of a liquld uv-in~tiator (benzoin ~-butyj ether). Several low molecular weight silicone carbinols with low ~ tl.wr~' content (up to 7.5%) under the same conditions produced gels or very soft solids.
Another method involves the preparabon of a low viscosity silicone ClO carb~noi 15 ~Ivl.;v~,Gl~ . pa~tially esterified with ..I~U...c.~' groups which can be uv-cured to a gas permeable solid and still have a sufflcient number of 'free' carbinol side-chains le~t to solubilize the n~thenium indicator and provide greater flexibillty and elasUcity to the cured polymer. (Cross-linking through the carbon-carbon bonds usually gives morebrittle solids than when the cross-linking is done through the urethane groups, or 20 polyether linkages R-O-R. A content of about 20% of Illc,tlla.,l~' groups on the carbinol l,v,llu,~,ùl~ , is generally enough to produce such a solid, leavmg theremaining 80% of free carbinol chains t~ ~soften up~ the polymer).
The main i;~ad~a.~ta~es of this approach are the handling and storage of methaorylated silicones which potenUally may have short sheN life (~

~ ^
WO95122759 ~ = - 21 83455 r~

taking place prematurely if these materials are not kept at a low temperature and away from sun light). Another drawback is that this type of chemistry is not applicable to the production of solid sdate pCO2 sensors since any radicws which are produced on exposure to the uv-radidion will destroy the pH indicator.
Some of the ~d~ ~tclyes of this process include automation ot the process ~nd setding off the cure when desired. There may also be add'dional adv~ yes for mak~ng the mulUple sensors on a single fiber such as combinin3 Uhe cure method currently used to make pH sensors with that of making the free radicw initiated PO2 sensor.

5. Phctolabile Protectinq grour~ chemistrv:
As a special case of alcohol protecting groups and in conlurlction with that described in 3 and 4 above, special Col~aidl:l ' I was given to using the alcohol -OH
protecting groups which may be oleaved pl~ by exposure to uv-radiation.
Such groups have been used ~lc~escf~ in sugar chemistry and to a lesser extent ~n geneUc and DNA research.
In another ~:" ILod;l, IC "t ortho-nitrobenzyl ethers are chosen as prime protectl'ng groups since, upon irradiation d ~ 320 nm. for ten minudes, they give a quanUtaUve hydrolysis of benzyl groups and a 'free~ alcohol. The reaction may be Icl~ ,a~l.t~d by the following equation:
CH2X CH2-O-R O=C~
R-OH t ~N2 -- ~ 2 uv ROH ~ ~NO
20 wherein ROH is an alcohol or sugar and X is ch~orine or bromine.
The same type of chemistry may be extended to the silicone carbinol. Thus, ~
C~O-alcohol may be alkylated with 2-nitrobenzyl bromide and the resulting nrdrobenzyl ether hydrolysed by exposing it to high intensity uv-radiation (as evidenced by the ~ ,_wc",.,e of the -O-H stretching vibration in the i.r. spectrum of the crude product).
The nitrobenzyl protected silicone carbinol is prepared directly by wkylating 'dw'dh the 2-n'drobenzyl halide or by first alkylating the unsaturated alcohol and then doing the 1 " i" " " 1 reaction of the . ' ~,be~ Jl protected alcohol with l Iy.ll, "' -2 1 ~ 3 4 S 5 Wo gs/227ss r~

ThQ ~ mixture contaming the nitrobenzyl-protected silicone carbinoi, nJthenium indicator, and TDI with the tin catwyst may then be irradiated wHh uv-radiation when the sensor fibers ara filled to set off the shndard, themmaw curepolyurethane solid fommation --S i-O--S i-O-g I-- uv --g i-O--S i-O-g ~- CHONo (CH2)zl (CHz~z (CH2)2 (CH2)Z
O O bH OH

The above reaction may be followed by any of the avwlable cl.t..,i,t,;~s described above.
The by-product, ortho~ e stays ~n the matrlx but ~t should not ~dversely affect the polyurethane cure. The nitrate esters may be used as wcohol10 protecting groups which also may be cleaved ,ul~utu1~1H.wy. The irradiat~on and subsequent d ~ ~c ~~ ,r ~ ~ thereof will produce only gaseous by-products wnd may be better suHed for the production of the finw polymer.
The following Example illustrates the IJIePW~ i~, of an indicator-polymer c~,,..L,;I . for a pO~ sensor according to the invention.

In the final polymer matrix the desired co ,., .,t. . of oxygen-sensH~ve indicator is 25mg of ir.d;.,~.t .~ w.. of polymer. The polymer is made up of 80%silicone carbinol and 2û% IPDI (I~v~,l ....,..~ di;..ocy~r ) cross linker. A stock solut~on of the indicator + the silicone carbinol may be made up and kept indefinHely. The 20 recipe for the stock solution is as follows.
1 ) The we~ght of a 16 x 125 mm test tube and r~n ~ r ' ' holding beaker was recorded.
2~ 4 grams of silicone carbinol was introduced into the test tube.
3) In a separate beaker 0.125 grams of ruthenium (chloride fomm) indicator was added to 1.25 cc. of m~ hylene chloride and completely dissolved.

2i 83455 wo 95/227s9 ~

4) The indicator solution was added to the silicone carbinol and mixed well.
This was done by sucking the mixture in and out of a disposable pipette.

6 6) An air source was connected to the disposable Up of a 200 micro liter pipette. The tip of the pipette was placed into the test tube with the p~ ..,e,/i,.-li.._tur mixture and air was bubbled through the mixture until dl the methylene chloride was blown off as determined by weighing, 10 Next a 0.01% solution of DBTDL (dibutyltin dilaurate) catalyst was prepared as follows:-1) A 1% solution was prepared by adding Add 100 micro liters ofdibutyltin dilaurate (DBTDL) to 10 ml. of methylene chloride, 15 2) 100 micro liters of the 1% solution was added to 10 ml. of methylene chloride to make the 0.01% solution.
The final sensor was formed by filling the cells in an optical fiber made in ~ccu, d~ ,ce w'lth the procedure disclosed in U.S. Patent No. 4,889,407 as described above. When the cells of the optical fiber sensors were ready to be filled the polymer was prepared 20 as follows.
1) The weight of the 10x75 mm test tube and c.~,,u,u,~ u holding breaker was recorded.
2) 0.4 grams of stock solution (Si-carb + indicator was introduced into test tube.
3) 1 cc of methylene chloride was added to test tube and mixed well with a disposable pipette.
30 4) 0.1 grams of IPDI was added to test tube and mixed well with same disposable pipette as above.
5) 500 micro liters of the 0.01% D~TDL solution was added to the test tube nnd mixed well with the same disposable pipette as above.
6) Methylene chloride was removed by purging the mixture with air. The mixture was weighed to determine when all the methylene chloride was removed.
It took 20 to 30 minutes to blow off all the solvent. After this time the mixture 40 was pipetted into the cells of the PO2 fibers for 60 to 90 minutes. At this time the mixture became too thick to work with. After the fibers are filled, the polymer is cured in an over at 65C for 24 hours. Actually, curing may take place after 2 to 3 hours but will certainly be completed in 24 hours.

~ Wo 95122759 2 1 8 3 4 5 5 r~

The procedure described in the above ~xample also may be used to prepare a pCO2 sensor. In a preferred c",~udi",~"l a carbon dioxide-sensitive indicator is ~ommed from an ionic complex of an Gl y~. Iù,~ll 1-'- quatemary ammonium cation and the .~nionofas~ ,ul Itl~al~;.l dye,forexamplel~t,c.v-,lj,~"",u,~iumhydroxideandPhenol 5 Red. The indicator is il ,~.-,, ,uu,_'.d into a silicone carbinol polymer matrix suitable for fillin3 the cells of an optical fiber as described above. The puly~ ,/i"--;c~ ., matrix provides a system which exhibits rapid response to changes in carbon dioxide ~.ul ICel 1~
A mulU-p.~rameter sensor comprising an élie,t~u~ ",;~.al sensor for the 1û ~,t..",i, " ~ of PO2. as disclosed in U.S. Patent No. 5,262,û37, in cu,,,L;, )n wlth a pH sensor and a pCO2 sensor, made in 6CCUl ~ -e with the disclosure in U.S. Patent No. 4,889,4û7 is currently manufactured by Biomedical Sensors Ltd., the assignees of the present invention. The technclogy of the present invention may be applied to an optica flber structure in 6cc~dG~Ice with the disclosure in U.S. Patent No. 4,889,4û7 15 to prepare an optical fiber PO2 sensor which may replace the el~:-,l,ucl,~,,,;-,d. PO2 sensor used in the aforesaid multi-parameter sensor. ikewise the pCO2 and pH
sensors may be made in clccul ial1ca with the present invention and the resulUng multi-parameter sensor, analogous in other respects to the sensor currently on the market has a number of 6dv~ ,I.~es.
2û For example, in the ,"u,~ of pCO2 sensors according to techniques disdosed in the prior art films containing the indicator are cast by alowing large amounts of solvent to evaporate. Such a casting technique does not facilitate the filling of the cells in the optical fiber because of volume loss from the solvent evauu~ -A~ II), the polymers used to form the films are not very permeable to c.srbon dioxide and will provide a fast response only if the films are very thin and highly plasticized. Another problem with prior art tedhnology is that the polymers and . are ll ,... l ' . sources of acids or bases and thus shift the pH range over which the sensor responds to pCO2. For ~,, " - ,~ such as the d~t~,-";" , of the correct placement of an ~,,dutl~ l tube this latter problem is not of great 30 si~u,,, ~ ,ue since the film-based sensor is used primarily as a color switch. However, - in a preferred ~:",IJO~ I of the present invention, i.e. a muUi-parameter sensor .u~ uu~ ~ta;i in an invasive catheter for d~l~""i"i,)g the Cul ~ tl 'iù., of analytes in a patient's blood, the color change is used as part of an analytical measurement system .

Wo ss/227sg ' 2 1 8 3 4 5 5 P~l/~, 5 ~ ~ ~

and any changes in the acidic and/or basic c~l,,,uul,t,,.U. of the polymer matrix which shffl the CO2-sensitive ran5e of the matrix will cu,,,,u,u,,,i~e the accuracy of the measurements made by the sensor.
In attempts to solve the above problem a number of known silicone nubbers 5 were examined but it was found that the indicator complex inhibited the cure of all the neutral cure silicones i".. 'il I~ However, the problem was solved by uslng the silicone carbinol polymer of the present invenUon. The polymer of the present invention has no volume loss during curing, provides a neutral matrix with p~" - t~
u l l~a~ t~liatk.s similar to silicone and pCO2 sensors made therewith provide a rapid 1û response to changes in CO2 cu"~.~"L, ~. Most ai~U,l " ,t`~, the cure of the system is not affected by Uhe presence of the indicator complex.
A number of pCO2 sensors have been prepared in âcuul d~ ,- e with the present invention and the ~ u""...,~.e thereof has been very good. Depending upon the cross-linking the 9û% response times vary from 75 seconds to 36û seconds with a 15 typical modulation of 55%. This compares favorably with current sensors which have a response time of 1 5û to 2ûû seconds and a modulation range of 25% to 55%.
In addition to its use in a multi-parameter optical fiber sensor as described h",~i,. . ~ve by iliCul,uuld~iull of a suitable substrate, a CO2 sensor prepared with a silicone carbinol polymer according to the present invenUon may be used in a device 2û for the ~ " "i" ~ of the placement of an tll nlut~ ~ .l ,aal tube in the trachea of patient, for example a device analogous to that disclosed in U.S. Patent No. 4,728,499.
Regarding the PO2 sensor described herein and illustrated in the Example, the preferred indicator tris (4,7-diphenyl-1,10-ul,t,~c,,,Ll,,, ,e) ruthenium (Il) chloride was used because of its desirable physical properties and history of application in oxygen 25 sensing technology. It has the high extinction coefflcients for the two charge-transfer bands seen in the visible spectrum. The excitation wavelength is compatible with the blue LEDs currently on the market. The l~""i"~:a~.~"~ e quantum yields are quite high (n~ û.5) and at times may double (approach unity) when the indicator is ir,cc"~ u, in the polymer matrix. 5esides having intense Yisible ~aui ,uLiul la this indicator also has 30 a very intense fluorescence with a remarkable Stoke's shift of n~ 140 nm. Thus, i,l~,l~,t",-,e from reflected backgrcund light is minimal.
Both the lulllilICa~ e quantum yields and lulllille,ac~ efflciency are in,l~:ue, ,.1_"L of the excitation wavelength. The radiative oxygen quenching constants WO 95/22759 2 1 8 3 4 5 5 P~
-17~
are quite high, increasing the sensitivity towards the analyte of interest (O,). Non-radiative quenching constants change in diflerent media (sclvents, polymers) but are never signlficant enough to overccme the radiative emissions.
The preferred indicator also is thermally, chemically and pl~ul.J~ e" ,i.,.,l'y stable.
6 Photo-bleaching over any extended period of time is minimal (particularly wlth the low energy light sources nommally used) and this greatly increases the lifetime of the sensor.
Cx~elilllel~t~ using gamma-radidion for sterilization indicated that about 20 to 25% of the indicator is décOl I ,p"sed, but this compares favorably with Icss previously observed with other indicators.
It is to be understood that other PO2 and PCO2 indicators may be used in the sensors acccrding to the invention. For example, suitable indicators for PO2 delellllill " ~areplatinummeso-tetra(pentafluorophenyl)porphineandplatinummeso-tetraphenyl porphine; and suitable pC02 indicators are cresol red and thymoi blue.

Claims

1. A stabilized, bio-inert sensor for the determination of an analyte in a medium which comprises a chemical indicator sensitive to the analyte in association with a stabilizing substrate formed from a polymer which is inert to the medium and analyte and does not affect the sensitivity of the indicator, which polymer is a cross-linked, solid silicone rubber formed from a silicone carbinol having a molecular structure compatible with said indicator.

2. A sensor according to claim 1, in which the polymer is a silicone carbinol homopolymer having the formula:- (I) wherein R is methyl or phenyl, z in an Integer from 1 to 20 and n is an integer from 2 to 500.

3. A sensor according to claim 2, in which the polymer is a methyl silicone carbinol homopolymer of the formula:- (III) wherein n is an integer from 2 to 500.

4. A sensor according to claim 1, in which the polymer is a carbinol siloxane copolymer having the formula:- (II) wherein each R is methyl or phenyl and the Rs may be the same or different, z is an integer from 1 to 20, and each of x and y is an integer wherein the sum of x and y is from 2 to 500.

6. A sensor according to claim 4, in which the polymer is a dimethyl/methyl carbinol siloxane copolymer of the formula:- (IV) wherein each of x and y is an integer and the sum of x and y is from 2 to 500.

6. A sensor according to claim 1 for the determination of the concentration of oxygen in a liquid medium, in which the indicator is the oxygen-sensitive fluorescent indicator tris(4,7-diphenyl-1,10-phenanthroline)ruthenium (II) chloride.

7. A sensor according to claim 1 for the determination of the concentration of carbon dioxide in a liquid or gaseous medium, in which the indicator is an ionic complex of an organophilic quaternary ammonium cation and an anion of a sulfonephthalein indicator.

8. A sensor according to claim 7, in which the indicator is an ionic complex of tetraoctylammonium hydroxide and phenol red.