Absorption-based optical-fibre oxygen sensor

152

Sensors und Acruators B, 7

Absorption-based F. Baldini,

( 1992)

752-757

optical-fibre oxygen sensor

M. Bacci, F. Cosi and A. Del Bianco

Isrituto di Ricerca sulle Onde Elettromagnetiche

de1 CNR,

Viu Panciatichi

64. 50127 Florence (Italy)

Abstract A model of an optical-fibre sensor for the detection of oxygen is described. Its working principle is based on the absorbance change of a bis( histidinato)cobalt( II) solution at 1 = 408 nm as a function of the molecular oxygen concentration in the surrounding environment. The sensor is able to detect very low oxygen concentrations and it is potentially suitable for the detection of oxygen in hostile environments or during production processes in which oxygen must be completely absent.

Introduction

The detection of molecular oxygen, both in solution and in the gaseous phase, is of great importance for industrial processing and for environmental and biomedical analysis. The possibility of continuous monitoring of oxygen concentration via optical-fibre technology is very attractive because of the traditional advantages provided by optical fibres (e.g., geometrical versatility, easy handling, high miniaturization, the absence of electrical contacts and the possibility of remote sensing). At present, optical detection of oxygen is performed by using its capability of quenching the fluorescence emitted by organic fluorophores [I]. Although this method might offer high sensitivity, it is subject to the interference of other substances that give rise to competitive quenching with oxygen. Hence, our attention has been devoted to the search for chemical compounds, the absorption of which undergoes changes in the presence of oxygen. An optical-fibre sensor which exploits the absorption change of haemoglobin in the presence of oxygen has already been developed, but the lifetime of the probe is very short due to the biological characteristics of the optical reagent [2]. Since, when they are used as reagents for optical-fibre oxygen sensors, certain natural compounds are characterized by very poor stability [2], our attention has been focused on synthetic substances. Some organometallic compounds are able to bind reversibly with molecular oxygen, 0925-4005/92/$5.00

and for this reason they are known as oxygen carriers. These substances are transition metal complexes [3], the absorption spectrum of which may change during the oxygenation process, depending on the oxidation state of the central atom. This work is concerned both with a thorough spectrophotometric analysis in order to choose the best organometallic compound as the oxygen transducer and with the development of the optical-fibre sensor.

Experimental

Materials and optical instrumentation Tetrahydrofuran (THF), benzene, nitrobenzene, ethyl ether (Et,O), dichloromethane (CH,Cl,) and toulene were supplied by Fluka. Tributylphosphine (PBu,), trimethylphosphine ( PMe3), dimethylphenylphosphine (PMe,Ph), triethylphosphine ( PEt3) and tricyclohexylphosphine ( PCy,) were supplied by Fluka and used as they were; only PBu3 was purified by distillation in an argon because the infrared spectrum atmosphere, showed the presence of some phosphine oxide that might alter the reaction. Manganese chloride (MnCl,) was supplied by BDH and manganese iodide (MnI,) was prepared by adding 1.Ol mole of iodine to 1 mole of metallic manganese in dried Et*0 under argon. The brown precipitate of MnI, was filtered, washed with Et,O, dried and kept in an argon atmosphere. Cobaltous per@ 1992 -

Elsevier

Sequoia.

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reserved

153

chlorate hexahydrate (Co( C104)26H20) and Lhistidine (His) were supplied by Fluka. A Celgard microporous hollow-fibre membrane (type X20 400) supplied by Hoechst AG, was used as the probe. Spectrophotometric analysis was carried out using both a Perkin-Elmer spectrophotometer (model 552) and a Guided Wave spectroanalyser (model 200) equipped with optical fibres. Absorption spectra in solution were recorded either by using anaerobic cuvettes (Hellma) coupled with the Perkin-Elmer spectrophotometer or by using a three-neck flask coupled with the probe of the Guided Wave spectrophotometer. Solid-state reflectance spectra were recorded by using an external integrating sphere connected with the Perkin-Elmer spectrophotometer: the analyte was placed in an appropriately designed cell located over the hole of the integrating sphere. In each case, the percentage of oxygen contained in the atmosphere surrounding the oxygen carrier was rigorously controlled. Optical-fibre measurements were carried out by using the optical-fibre sensor, the block diagram of which is shown in Fig. 1. An optical-fibre coupler (fibre core diameter: 200 urn) is used in order to connect the source and the detector to the probe. The light of a halogen lamp is modulated by a mechanical chopper and filtered by a monochromator so as to select the working wavelength. The light is coupled to one branch of the coupler, and is driven to the optrode through

objective

chopper

the common branch. The other branch of the coupler is then connected to a photodetector. A second fibre (80 urn core diameter silica-silica fibre) directly connects the source to a second photodetector, with the aim of eliminating the fluctuations of the lamp. Data are collected by using a personal computer. Synthesis of oxygen carriers

A large number of oxygen carriers are reported in the literature [3], but not all of them are suitable for an optical revelation of oxygen owing to their poor spectral properties or to the very slow oxygenation process. Four kinds of oxygen carriers showing a colour change during the oxygenation process were selected: the so-called Vaska’s complex (IrCl( CO)( PPh3)J [4], bis( salicylaldehyde)ethylenediimminecobalt( II) (Co( salen)) [ 51, bis( histidinato)cobalt( II) (Co( His)Z) [ 61, and manganese complexes of the form Mn(PR,),Y, except (where PRx is a tertiary phosphine triphenylphosphine (PPh3), x may assume values 1 or 2 and Y is a halogen [7]). None of these was found on the market and, for this reason, synthesis was carried out following, if possible, the methods described in the literature. Co (Hisk

A solution of lo-’ mole of His in 70 ml of phosphate buffer at about pH 7.5 (histidine is more reactive in neutral or lightly alkaline environments) and a solution of 5 x lop3 mole of CO(C~O,)~*~H~O in 50 ml of phosphate buffer, previously deoxygenated, were mixed, in an argon flux, yielding a pink solution of Co(His)*. Manganese compounds

Y coupler

____t

oxygen flux ---+

==IL

optrode Fig. 1. Block diagram

of the oxygen sensor model.

+!!/J

The synthesis of manganese compounds was extremely difficult, due both to the complexity of the operations to be followed and the lack of details reported in the literature. Since an extreme sensitivity to contamination with water was reported in the literature, all the solvents used were accurately dried and deoxygenated, allowing them to reflux in an argon atmosphere on proper desiccant agents immediately before their use. Mn(PBu,)C1,. Equal amounts of PBu, and MnCl, (4 x 10d3 mole) were added to a mixture of

50 ml of toluene and 50 ml of dichloromethane. Although the solution was stirred for five days, no precipitate but only the persistence of the manganese halide was observed. However, while trying to filter under argon, a colour change was observed, indicating some product of the reaction in solution. The same result was observed in THF. These poor results were blamed both on an extremely low yield of the reaction and on an excessive sensitivity to oxygen; for this reason the following attempts to synthesize these compounds were performed by changing the ligand, halide and stoichiometric proportions in order to reduce the affinity to oxygen. Mn(PMe,), I*. By adding 8 x lo-’ mole of pure PMe, to an Et20 solution of MnI, (4 x low3 mol of MnI, in 50 ml of Et,O), white crystals of Mn(PMe,),I, began to precipitate quickly. In an argon flux the colour of the crystals changed to light green. The complex is very soluble in THF, giving a green solution; on bubbling oxygen into the solution, the colour changes rapidly to deep blue. Unfortunately, the original colour cannot be restored, which indicates a lack of reversibility of the oxygenation process. Mn(PMe,Ph)212. By adding 8 x lop3 mole of pure PMe,Ph to an Et20 solution of MnI, (4 x 10e3 mole of MnI, in 50 ml of EtZO), a redorange solution containing a small quantity of a light-brown viscous liquid is observed. After two days in an argon atmosphere the colour changed to light green. By allowing the solution to rest for a few more days, a colour change to light pink was observed; some clear crystals began to precipitate, but the number of crystals obtained was too low to perform any spectrophotometric analysis. Mz(PC~~)~I,. By adding 8 x lop3 mole of pure PCy, to an Et20 solution of MnI, (4 x lop3 mole of MnI, in 50 ml of Et20), a rapid formation of light-brown needle-like crystals was obtained. No change of colour was observed when they were exposed to oxygen: this fact can be attributed to the steric hindrance of the ligand molecules, which keep oxygen away from the manganese atom.

of Mn12 in 50 ml of Et20), light-pink crystals were rapidly produced. Solutions of Mn(PEt,),I, in toluene and THF rapidly changed their colour when exposed to oxygen; the original colour was restored after flushing with argon. Hence, Mn(PEt,),I, seemed to be the best manganese compound for our purpose. Vaska’s complex and Co(salen) were prepared according to the indications found in the literature [4,51.

Results and discussion A thorough spectrophotometric study ried out on Vaska’s complex, Co( salen), and Mn(PEt,),I, in order to determine suitable one for optical oxygen detection: was necessary because no information point was available. Co (Hislz

Figure 2 shows absorption spectra of Co(His)* in phosphate buffer at about pH 7.5 in equilibrium with different concentrations of molecular oxygen in the surrounding atmosphere. The formation of a band is apparent at ;1 = 408 nm during the oxygenation process, due to a charge transfer between oxygen and metal electronic levels. Moreover, Co(His)z shows a good reversibility at room temperature and atmospheric pressure.

0.0

L

300

400

500

600

700

800

900 h

By adding 8 x lop3 mole of pure PEt3 to an Et,0 solution of MnI, (4 x lop3 mole Mn(PEt,)J,.

was carCo(His)z the most this step on this

Fig. 2. Absorption spectra of Co(His), in equilibrium oxygen concentrations in the surrounding atmosphere.

boo @ml

with different

755

......

w7w aluminium reflector

Fig. 4. Sketch

h (nm) Fig. 3. Reflectance spectra ent oxygen concentrations:

of Mn(PEt,),I, in the presence (a) 0, = OX, (b) O2 = lO”A

of differ-

Figure 3 shows the reflectance spectra of the solid complex Mn(PEt,),I,. Curve (a) refers to the compound in an argon atmosphere; curve (b) refers to the same compound after exposure to an atmosphere composed of 10% oxygen and 90% argon. Many bands are changed by the formation of the manganese-oxygen bond, due to the change of symmetry of the molecule from pseudo-tetrahedric to trigonal bipyramidal. However, it should be emphasized that, even if this compound is extremely sensitive to oxygen, it is also highly sensitive to water: the least trace groups, of humidity oxidizes the phosphine blocking the reactivity of this compound toward oxygen. Vaska ‘s complex

When a solution of Vaska’s complex in initially deoxygenated nitrobenzene (cont. = 1 x lop3 M) is exposed to an atmosphere of 95% argon and 5% oxygen, a strong change in the absorbance at 1 = 440 nm is apparent; however, it should be emphasized that the oxygenation process is very slow (several hours). Another attempt, using DMF as a solvent, led to analogous results: the response time was shorter, but still long enough to make Vaska’s complex unsuitable for the realization of an optical sensor. Co(salen)

Solid Co(salen) has very different reflectance spectra when in the deoxygenated and in the fully

Celgard membrane 0.d. 400 pm)

of the optrode

optical fibre without jacket

for oxygen

detection.

oxygenated form, showing a large change of reflectance for 1 > 700 nm. This is due to the shift of an infrared band (at Iz = 1.24 pm) towards longer wavelengths. This compound shows very good reversibility of the oxygenation process, but, unfortunately, only at high temperatures and low pressures. On the basis of these spectrophotometric studies, Co( His)2 was selected as reagent for the oxygen optical-fibre sensor, because: (i) its synthesis is very easy; (ii) it is characterized by good sensitivity and stability; (iii) it presents a good reversibility of the oxygenation process at room temperature and atmospheric pressure. Optical-Jbre

measurements

The optrode, coupled with the optical-fibre sensor described above and shown in Fig. 4, is made with a Celgard capillary (length = 3 mm, internal diameter = 400 pm), closed at one end with an aluminium cap as a reflector. Celgard was chosen for the hydrophobic membrane because at the moment is seems to be one of the most efficient for guaranteeing a fast and complete diffusion of oxygen inside the probe. The membrane capillary is filled with the Co( His), solution operating in a dry-box in an inert atmosphere. Then, the common branch of the Y-coupler is inserted into the capillary and the junction is sealed with black silicon in order to obtain a waterproof assembly and to avoid parasitic reflections. The probe obtained is inserted in a glass tube, sealed with a 1 cm thick silicon layer, and inserted in a flow cell where it is possible to control the composition of the internal atmosphere. A typical response curve of the sensor is shown in Fig. 5, where the ratio I/I,, versus time is reported: I and I,, are the intensities of the light coming from the optrode and directly from the monochromator, respectively. The probe is in contact with a pure argon atmosphere;

l/l, ,/

0.75

I?

0.50

u 0

25

50

seconds(xlO*') Fig. 5. Response curve of the sensor for a O-+ 100% step in oxygen concentration

(the arrow

shows when the oxygen concentration

is

changed).

sufficient to obtain the maximum response of the sensor: oxygen concentrations higher than the threshold level ( z 2 x 10e2 ppm) will cause the same change in the r/1, ratio. On the other hand, different concentrations of oxygen in the ambient surrounding the probe will give rise to different diffusion gradients through the membrane, and consequently will cause different response times. Hence by measuring the response time, the measurement of oxygen concentration is possible. It is apparent that this sensor is potentially useful to detect oxygen wherever its presence must be carefully avoided: explosive environments and industrial processes in controlled atmosphere are only two examples.

Conclusions

the arrow indicates when pure oxygen begins to flow. The response time of such a sensor, defined as the time needed to have a signal variation from 10 to 90% of the final value, is about 2 min. At this point it is important to outline that there is a threshold level which represents the maximum quantity of oxygen that can be bound with a fixed concentration of Co(His)* in solution: actually the number of Co(I1) ions that are able to interact with oxygen inside the probe is well defined and can easily be calculated taking into account the dimensions of the probe. Moreover, the threshold level is a function of the solubility of oxygen; hence it depends on the temperature and pressure, the measurement of which will be considered in view of the final use of the sensor. The calculation of the threshold level is straightforward and is based upon the fact that oxygen bridges between cobalt ions so that each mole of cobalt binds two moles of oxygen: hence the knowledge of the Co(His), concentration is sufficient to know the maximum concentration of oxygen inside the probe which is required to obtain a full response. Quite obviously information about temperature and pressure is also necessary, because the quantity of oxygen that diffuses inside the probe depends on these parameters. From these considerations, it is possible to estimate that a concentration of 2 x lo-* ppm of oxygen in the atmosphere surrounding the probe at 25 “C and 760 Torr is

The present study allowed Co( His)* to be identified as a reagent, working on the basis of light absorption, for oxygen detection via optical fibres. In this way it is possible to avoid the interferences that often characterize optical fibre sensors that exploit the quenching of the fluorescence emitted by organic fluorophores. The optical-fibre sensor developed here is characterized by a small optrode which allows very low concentrations of molecular oxygen to be detected, hence the sensor appears to be particularly suitable in industry where it is often important to detect extremely low quantities of oxygen. An open problem is still the response time of the sensor when low oxygen concentrations must be detected. For this reason, two lines of research are to be followed: the identification on the market of different types of membranes characterized by a better permeability to oxygen than Celgard and the possibility of immobilizing the oxygen carrier on a solid support so as to change completely the structure and geometry of the probe.

Acknowledgements

This work has been developed and supported by the CNR Finalized Program on Materials and Devices for Solid State Electronics.

References 1 0. S. Wolfbeis and M. J. P. Leiner, Recent progresses in optical oxygen sensing, Optical Fibres in Medicine, Vol. III (Proc. SPIE 906), Los Angeles, CA, Jun. 1988, pp. 42-48. 2 Z. Zhujun and W. R. Seitz, Optical sensor for oxygen based on immobilized hemoglobin, Anal. Chem., 58 (1986) 220-227. 3 R. W. Erskine and B. 0. Field, Reversible oxygenation, Struct. Bonding (Berlin), 28 (1976) I-50. 4 ic. Vrieze, J. P. Collman, C. T. Sears and M. Kubota, Trans-chloro

carbonyl bis( triphenylphosphine)iridium, Inorg. Synth., XI ( 1968) 101-104. 5 D. J. Aymes and M. R. Paris, Molecular uptake by a solid Co(H) complex. Its synthesis and kinetic oxygenation, J. Chem. Educ., 66 ( 1989) 854-856. 6 J. Simplicio

and R. G. Wilkins, tion of bis(histidinato)cobaIt(II) 89 (1967)

The kinetics of the rapid interacwith oxygen, J. Am. Chem. Sot.,

6092-6095.

7 C. A. McAuliffe,

Manganese( II) phosphine complexes: analogues proteins, Oxygen wd Life, 2nd BOC Priesley Conference, Birmingham. UK. 1980, pp. I 19- 13 I. of transport