Luminescent platinum complex in solid films for optical sensing of oxygen

Analyhca Chtmrca Acta, 262 (1992) 21-32 Elsewer Science Pubhshers B V , Amsterdam

21

Luminescent platinum complex in solid films for optical sensing of oxygen Xlang-Ming Ll * and Kwok-Ym Wong Department of Applzed Bwlogy and Chemrcal Technology, Hong Kong Polytechnrc, Hunghorn, Kowloon (Hong Kong) (Recewed 4th October 1991, rewed

manuscript

recewed 7th January

1992)

Abstract A strongly lummescent platmum complex embedded m various sohd films was tested for eta characterlstlcs m optlcal oxygen sensmg m the gaseous state The effects of the munobdlzation procedures and the matnx materials on the degree of lummescence oxygen quenching of the complex were studled A model that combmed the quenchmg kmetics and the solublhty equation of oxygen m polymers IS proposed to correlate the decrease m lummescence mtenslly with the quencher concentration Thu model could explain the common feature of negatwe devlatlons from the Stem-Volmer equation of fluorophores bemg quenched m polymer films Keywords Chemdununescence, Kmetlc analysis, OptIcal sensor, Oxygen, Platinum complex, Polymer film

Oxygen concentration 1sclosely related to many important chemical and blochemlcal reactions Therefore, the development of various oxygen sensors has created strong mterest m the fields of analytlcal chemistry, blosensors and blomedlcal research Optlcal sensing of oxygen based on the prmclple of lummescence quenching IS one of the important areas m the development of oxygen sensors and it offers several distinct advantages Unhke the Clark electrode [l], it does not consume oxygen, which makes It to be Immune to the interference of flow condltlons It has unique sultablhty for mmlaturlzatlon as a consequence of the development of optical fibres and supporting devices A flbre-optic probe was developed by Peterson et al [2] m 1984 for in vivo measurement of oxygen partial pressure A smular flbre-optic sensor was developed for intravascular blood gas momtormg usmg fluorescent organic dyes [3] The fluorescent chemicals used m oxygen sensmg are mostly polycycllc aromatic hydrocarbons [2,4-61, although Ru(bpy)f + (bpy = 2,2’-blpyndme) and

its analogues have also been used [7-101 A review of the theory and practice of optxal oxygen sensors was given by Surge [ill In order to achieve high sensltlvlty of the sensor, the lummescent dye chosen should have a high quantum yield and a slgmficant decrease m lummescence mtenslty m the presence of oxygen These lummescent chemicals should usually be munoblhzed m polymer films for the construction of reusable sensors The procedures for lmmoblhzatlon and the matenals used have strong effects on the performance of the sensor m terms stablhty and sensltlvity In addition to Ru(bpy)f + , the platinum dnner tetralus(pyrophosphlto)dlplatmate(I0 {[Pt2(popj414-, pop = P205H:-) IS well known for its mtense lummescence and very high quantum yield at room temperature [12,13] The intense emission at about 515 nm was attrlbuted to phosphorescence from the 3AZsstate The quenching of phosphorescence of [Pt,(pop),14- by oxygen m water was studled by Peterson and Kalyanasundaram [14] and normal lmear Stem-Volmer behavlour was observed The

ooO3-2670/92/$05 00 0 1992 - Elsevler Science Pubhshers B V All rights reserved

28

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T~/T value (70 = hfetlme of the platinum com-

ionic polyelectrolyte 2051 A (Doni) (10 mg ml-‘) The solution was nuxed wth sillcone-rubber m a volume ratio of 1 2 (3) The platmum complex was dissolved m delomzed water (2 mg ml-‘) containing the catlomc polyelectrolyte 2042C (Doni) (10 mg ml-‘) The solution was mured with slhconerubber m a volume ratio of 1 2 (4) The platinum complex was dissolved (2 mg ml-‘) m a mixed solvent (0 2 ml of water, 1 ml of toluene, 1 ml of ethanol, 0 5 ml of tetrahydrofuran> The solution was mixed with silicone-rubber in a volume ratio of 1 2 (5) The platinum complex was dissolved (2 mg ml-‘) m water containing the ion-exchange resin Sephadex DEAE-A25 (Pharmaaa) (30 mg ml-‘) Silicone-rubber was diluted with equal volume of toluene The platinum solution was mixed with Tween 80 and the silicone-rubber solution m a volume ratio of 2 1 4 In order to make reproducible films, a piece of metal sheet (thickness 0 3 mm) with a central hole was placed on the top of a glass slide to form a mould A small droplet of the paste-like mixed solution was placed m the central hole of the mould The volume of the solution added should be kept smaller than the void volume of the mould Another piece of glass slide (with a PTFE film coating on its surface to prevent stlckmg) was placed on the top of the mould and pressed the paste mto a uniform film m the mould The cover-slide with PTFE film coating could be removed after 24 h The polymer film coated on the bottom glass slide had the same thickness as that of the metal sheet The five mnced solutions listed above were coated on mdlvldual glass slides and allowed to solidify at room temperature

plex m deaerated water and T = hfetnne 111aerated water) was found to be 7 6 In this work, (Bu,N),[Pt,(pop),l was used as the sensing dye for optical oxygen measurements The effects of the nnmoblllzatlon procedures and materials on the performance of optical oxygen sensing, and the mechanism of oxygen quenching m polymer films, which usually showed negative deviations from the Stern-Volmer equation [lo], were studied

EXPERIMENTAL

Mater&s K 2PtCI, was purchased from Johnson-Matthey (Bu4N&[Pt2(pop&] was synthesized from K,PtCl, and phosphorous acid (H,PO,) and purified as described [151 Polfivmyl alcohol) (PVA, mol wt 14000) was obtained from BDH Slhcone-rubber (RTV 732) was obtained from VersoChem All other chemicals were of analytlcalreagent grade and were used as received Immobdtzatron of the platmum complex (Bu4N)4[Pt2(pop)4] was soluble m water and had very low solublhtles m many orgamc solvents In order to lmmoblhze the platinum complex m PVA, the platinum complex was first dissolved m ethanol-delomzed water (1 + 1, v/v) at a concentration of 2 mg ml-’ PVA was then added to the solution at a concentration of 5 mg ml-’ The final solution was coated on a glass slide and dned at room temperature In order to mvestlgate the effect of PVA on the oxygen quenching, another sample was prepared with the same solution as stated above, but m addltlon mixed with silicone-rubber m a volume ratio of 1 3 The followmg procedures were also applied to nnmoblhze the platinum complex mto slhcone rubber (1) The platinum complex was dissolved m deionized water (2 mg ml-‘) and mixed with Tween 80 surfactant (Atlas Chemical Industries) and s&cone-rubber m a volume ratio of 2 1 4 (2) The platinum complex was dissolved in deionized water (2 mg ml-‘) containing the an-

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Wang/Anal

Chum Acta 262 (1992) 27-32

Instrumentatwn Luminescence intensity measurements were made with a Model LS-5 luminescence spectrometer (Perkm-Elmer) coupled with a mlcrocomputer All the samples were excited at 368 nm, which was the excitation wavelength producmg maxnnum emlsslon intensity at 509 nm Two gas flow meters, mdlvldually calibrated by a volumetric method, were utilized to measure the relative flow-rates of oxygen (99 9% purity, Hong Kong

X-M 0 and K-Y Wong/AnaL Chm Acta 262 (1992) 27-32

29

Qxygen) and nitrogen (99 9% purity, Hong Kong Oxygen) The oxygen and mtrogen gases were mixed m a l-m long tube and then fed mto a flow cell m which the polymer film with the unmoblhzed lummescence platinum complex was exposed to the mixed gas stream, and the glass slide was facmg the excitation light m the spectrometer The oxygen concentration (%) was calculated by dlvldmg the oxygen flow-rate by the sum of the oxygen and mtrogen flow-rates All the measurements were made at room temperature (25 * 2’0

Equation 2 denotes the excltatron of molecule A, formmg a photoexclted molecule A* A* can return to its ground state by releasmg a photon P, as shown m Eqn 3, or colhde mth a quencher Q to form an excited complex, as shown m the forward reaction m Eqn 4 The reverse reaction of Eqn 4 represents a non-effectwe colhslon with the quencher Equation 5 denotes the excited complex returning to the ground state with nonradiative release of energy E, m the form of heat energy Based on the above kmetlc model, the followmg kmetlc equations can be obtained d[A*]/dt

= k,[A] - k,[A*] - k,[A*][Q]

THEORETICAL

+ It 1s well known that lummescence quenchmg methods of analysis are based on decreases m the emlsslon intensity or the lummescence lifetimes m the presence of a quencher In a hquld medium, the relatlonshlp between enusslon mtenslty and quencher concentration can be correlated by the Stem-Volmer equation Z,/Z = 1+

K&Q]

(1)

where Z IS emlsslon intensity, Z,, 1s emlsslon mtenslty m the absence of quencher, K,, 1s the Stem-Volmer quenching constant and [Q] 1s the concentration of the quencher In a solid medmm, such as m polymer films, a plot of Z,/Z versus [Q] often shows a negative devlatlon from the linear Stem-Volmer equation [8,10] Here we propose a kmetlc model for the lummescence quenching whrch gives the same result as the Stem-Volmer equation Considering the dlssolutlon of a gas m a polymer shows a negative devlatlon from Henry’s law, a modified Stem-Volmer equation 1s then proposed which can fit the lummescence quenching data m a polymer medmm very well The kmetlc model for lummescence quenching can be presented as follows A+hv2

A*

(2)

A*2A+P

(3)

A* + Qj$(AQ)*

(4)

(AQ)*3A+Q+E

(5)

d[(AQ)*]/df

b[CAQ,*l = k,[A*][Q]

(6) -k&IQ)*]

- k&AQ)*

1

[A,1 = [Al + [A*] + [*l Z = d[P]/dt = k,[A* ]

(7)

(f3 (9)

where square brackets denote concentration, t 1s time, Z 1s the luminescence intensity and [A,] 1s the total concentration of the lummescence molecule At the steady state, d[A*]/dt

=0

(10)

and d[(AQ)*]/dt

=0

(11) Based on Eqns 6-11, the lummescence mtenslty can be represented by Z=W,[A,l/(ZG

+UQl)

(12)

where K, = k, + k, and K, = k, + (k, - k_,)k, /(k_3 + k,) In the absence of a quencher, [Q] = 0, and Z, = Wz[A,l/ZG Dlvldmg Eqn 13 by Eqn 12 gives Z,/Z= 1+

W,/WQl

(13)

(14)

Although Eqn 14 1s the same as the SternVolmer equation, it gives the kmetlc meaning of the Stem-Volmer constant, 1e , K, 1s equal to I&/K, If one expects a lmear plot of Z,/Z versus partial pressure of oxygen m the gas phase, Paz, it nnphes the assumption that the dissolution of

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Wang/Anal Chun Acta 262 (1992) 27-32

oxygen m the polymer film obeys Henry’s law at low concentration [Ql =

(15)

W’o,

where K, is the Henry’s law constant However, it IS not uncommon that the dlssolutlon of a gas m a polymer shows a negative devlatlon from Henry’s law [16] The solublhty of a gas m a polymer can be written as the sum of two terms

[Ql= [Q,l + [Qsl

(16)

where [Q,] represents the ordinary dlssolutlon and [Q,] denotes sorption m the sinks, e g , mlcrocavltles or holes It was proposed [16] that

[Q,l = KhP02 IQ,1= WP,J( 1+ bPo2)

(17)

(18)

where KS 1s the hole saturation constant, b denotes the hole affinity constant and PO, is the partial pressure of the gas m the gas phase Combrmng Eqns 16-18 mth Eqn 14 gives Z,/Z= 1 +APo,+BPo*/(l

+bPo2)

(19)

where A = KZKh/Kl and B =K,K,b/K, Equation 19 IS the modified Stern-Vohner equation used m polymer media, which fits the expenmental data very well

RESULTS AND DISCUSSION

The emlsslon spectrum of (Bu4N)4[Pt2(pop)4] inside sdlcone-rubber, as shown m Rg 1, has a maximum at about 509 nm which closely resembles that of aqueous solutions of the complex This green emlsslon has been attributed to phosphorescence from the 3A2g state [17] The emlsslon intensity 1s quenched m the presence of oxygen The ratlo of lo/Z,, of the platinum complex lmmoblhzed m polymer films 1s directly related to the sensitivity to oxygen, where Zloo denotes the emission intensity m a 100% oxygen environment at 1 atm The experimental results for Z,/Z,, for the platmum complex m various polymer films are listed in Table 1 Based on the results m Table 1, Nos 1 and 2, one can infer that the high solublhty of oxygen m

Emssmn

Wave Length (nml

Rg 1 Enussion spectra of the platmum complex (excltatlon wavelength 368 nm)

silicone-rubber 1181 slgmflcantly increases the sensltlvlty to oxygen quenching In cases 3-7, all the basic supports are &cone-rubber The uses of different additives m lmmoblhzatlon are mtended to distribute the hydrophlhc platmum complex uniformly m the hydrophobic slhconerubber However, some of the additives, such as catlomc polyelectrolyte 2042C and ion-exchange resin DEAEAX, show strong mhlbltlon of the oxygen quenching Wolfbels et al [7] had notlced similar quenching mhlbltlon when Ru(bpy):+ was bound to an ion-exchange membrane such as Naflon They attributed this to the lack of moblllty of the complex on electrostatic bmdmg Although some workers have proposed that a fracTABLE I SensltnQ to oxygen quenctnng of platmum complex-polymer systems a

No

Solvent/ad&tlves/polymer

7

Ethanol-water (l+ l)/PVA As No 1 +&zone-rubber Water/Tween 8O/sdicone-rubber (1) b Water/2051A/skcone-rubber (2) b Water/2042A/sdicone-rubber (3) b Water, ethanol, THF, toluene/ sdicone-rubber (4) b Water/DEAE-A25/sdicne-rubber (5) b

~o/h 109 2 12 171 206 111 2 22 102

a Lummescence mtensitles were measured at 509 nm b The numbers m parentheses denote the lmmobdlzatlon conchtlons under Expertmental

31

X-M Ll and L-Y Wong/Anal Chrm Acta 262 (1992) 27-32

tlon of the fluorophores mslde polymer are maccesslble to the quenchers [10,19], the fraction of excited molecules bemg quenched at any instance depends on the concentration of the fluorophores and the quenchers, the relative velocity of these two kmds of molecules m the polymer matrur and the molecular orientation effect during colhslon Whereas the non-linear Stem-Volmer response can be explained by a multiple-ate model [10,19,20], the mode1 proposed here, which combmes the quenching kmetlcs and the solublhty equation of oxygen m a polymer, can explam the common feature of a negative deviation from the Stern-Volmer equation of fluorophores m polymer films equally well The platinum complex-polymer system No 6 m Table 1, which had the highest sensltlvlty to oxygen quenching, was tested for the varlatron of IO/Z at different oxygen concentrations Equation 19, the modified Stern-Volmer equation for oxygen quenching m a polymer medium, was used to fit the experimental data usmg a non-linear regression program in a microcomputer The results are given m Table 2 The three best fit parameters m Eqn 19 estlmated by the non-linear regression program are TABLE 2 Vanatlon of I,/1 O,(%)

00 193 2 78 4 11 604 105 169 25 9 367 444 478 64 2 723 803 909 1000

at Merent oxygen concentrations a Error (%)

&/I Experimental

Calculated using Eqn 19

10 107 109 1 13 1 17 126 132 146 160 171 179 191 198 2 05 2 13 222

10 1062 1086 1121 1 166 1254 1358 1479 1607 1691 1728 1896 1977 2056 2 159 2 246

00 -076 -037 -082 -034 -044 +287 +132 +043 -108 -348 -072 -0 15 +027 +134 +117

a Platmum complex-polymer system No 6 m Table 1

3

Fig 2 Comparison of (A) the expenmentai data (No 6 m Table 1) and the fittmg of (a) the moddied Stem-Volmer equation (Eqn 19) and (b) the Stern-Volmer equation (Eqn 14)

A 309156 atm-‘, B=25911 atm-’ and b= 6 8409 atm-’ It 1snoted that the total pressure is 1 atm Therefore, the partial pressure of oxygen 1s numerically equal to the percentage of oxygen divided by 100 The experlmental results and the results calculated by Eqn 19 are presented m Fig 2, curve a The standard devlatlon 1s 0 023 In companson, the experimental data were also fitted by the Stem-Volmer equation, 1e , Eqn 14 and the results are shown m Fig 2, curve b The standard devlatlon of using the Stem-Volmer equation 1s 0 0945, which 1s more than four tunes larger than that using the modified Stem-Vohner equation, i e , Eqn 19 The dynamic response of the luminescence mtenslty on swltchmg from 100% nitrogen to 100% oxygen was also investigated usmg the platmum complex-polymer system prepared accordmg to the conditions of No 6 m Table 1 The results are shown m Fig 3 The excitation and emlsslon wavelengths are 368 and 509 nm, respectively The film thickness IS about 0 3 mm The response time or recovery time 1s less than 20 s Because the excited [Pt2(pop)J4-* molecule 1sa stronger reductant and oxidant than its ground state 1131, potential mterferents include easily reduced or oxldlzed species The physlologlcally important gas carbon dloxlde has no effect at any

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Wong/AnaL

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Acta 262 (1992) 27-32

The authors thank Hong Kong Polytechmc and the Umverslty and Polytechnic Grant Committee of Hong Kong for financial support

1

MAX40000

REFERENCES

MIN=OOO 0

20

40

60

60 Tme

100

120

140

160

160

ISKI

Fig 3 Dynamic responses of emission Intensity of the platmum complex-polymer system (No 6 m Table 1) to changes m oxygen content (excltatlon at 368 nm, emlsslon at 509 nm)

concentration Halocarbons such as chloroform can cause detectable quenchmg of the emlsslon at concentrations higher than 1000 ppm, which IS consistent wth a previous report [211 that alkyl and aryl halides can quench the phosphorescence of [Ptz(pop),,14- m solution Sulphur dloxlde can also cause mterference However, m samples normally encountered It 1s present only m trace amounts and the effect 1s mslgmficant A detailed study of the response of this sensmg film towards various substrates will be reported m a subsequent paper Although [Pt,(pop),14- IS known to undergo hydrolysis m alkaline solution [22l, (Bu,N),[Pt,(pop),] mslde a s&cone-rubber film IS stable for several months with no dlmmutlon m humnescent intensity Although the present system IS not superior to that of Ru(Rh#he@ (Ph,phen = 4,7-dlphenyl-l,lO-phenanthrohne) [81, It IS expected that [Pt,(pop),14-, bemg coordmatlvely unsaturated, might be a good sensing material for optical measurement of quenchers that can bmd reversibly to the complex Studies of this aspect are in progress

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