Optical Sol-Gel-Based Dissolved Oxygen Sensor: Progress Towards a Commercial Instrument

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Journal of Sol-Gel Science and Technology 13, 207–211 (1998) c 1998 Kluwer Academic Publishers. Manufactured in The Netherlands. °

Optical Sol-Gel-Based Dissolved Oxygen Sensor: Progress Towards a Commercial Instrument C.M. MCDONAGH, A.M. SHIELDS, A.K. MCEVOY, B.D. MACCRAITH AND J.F. GOUIN School of Physical Sciences, Dublin City University, Glasnevin, Dublin 9, Ireland

Abstract. A dissolved oxygen sensor based on fluorescence quenching of the oxygen-sensitive ruthenium complex, [Ru(II)-tris(4,7-diphenyl-1,10-phenanthroline]2+ , which has been immobilized in a porous silica solgel-derived film, is reported. Ormosil sensing films were fabricated using modified silica precursors such as methyltriethoxysilane (MTEOS) and ethyltriethoxysilane (ETEOS) and were dip-coated onto planar glass substrates. Tailoring of the films for dissolved oxygen (DO) sensing is described whereby sensor response is optimized by maximizing film hydrophobicity by the use of the modified precursors. Sensor performance parameters such as limit of detection and sensor resolution are reported. Issues such as dye leaching and photobleaching are discussed. Progress towards a commercial instrument is reported. Keywords:

1.

optical sensors, sol-gel films, dissolved oxygen

Introduction

Dissolved oxygen (DO) sensing is of major importance in environmental, industrial and medical applications. The amount of oxygen dissolved in water is an indication of the quality of the water, and careful control of oxygen levels is important in fermentation processes and in food preparation. A knowledge of oxygen levels in blood is necessary for physiological and other medical studies. Optical DO sensors [1] are attractive as they do not consume oxygen or require stirring unlike the conventional Clark electrode method. Optical oxygen sensing is usually based on collisional quenching of a fluorophore embedded in a support matrix. The quenching process is described by the Stern-Volmer equations: I0 /I = I + K sv pO2

(1)

where I is the fluorescence signal, I0 is the signal in the absence of oxygen, pO2 is the oxygen partial pressure and K sv is the Stern-Volmer constant which is proportional to the oxygen diffusion coefficient in the matrix. The sol-gel process lends itself very successfully to thin film production either by dip or spin coating. In

this laboratory optical sensors for both gas phase and dissolved oxygen have been developed based on solgel coatings incorporating the oxygen-sensitive ruthenium complex Ru(Ph2 phen)2+ 3 , whose fluorescence is quenched in the presence of oxygen [2, 3]. This paper reports further development and performance parameters of a sol-gel-based dissolved oxygen sensor based on ormosil films. Tailoring of the films for DO sensing is discussed and progress towards a commercial instrument is reported.

2.

Film Fabrication and Characterization

Tetraethoxysilane (TEOS), methyltriethoxysilane (MTEOS), ethyltriethoxysilane (ETEOS) and ethanol were purchased from Aldrich. Glass microscope slides were cleaned sequentially using deionized water, methanol, acetone followed by a final deionized water rinse. A typical sol contained precursor, water at pH = 1 (using HCl as catalyst) 2.5 g/l of the ruthenium complex and ethanol. Water : precursor molar ratios of 2 and 4 were used. Sols were stirred magnetically for one hour. Films were dip-coated in a draught-free environment at a speed of 1 mm/s, using a computer-controlled

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Table 1. Gas-phase and DO quenching response of ormosil films.

dip-coating apparatus. Coated films were dried for 17 hours at 70◦ C and then stored in ambient conditions. Sensor instrumentation consisted of a highbrightness blue LED (Ledtronics, USA) with peak wavelength at 450 nm to match the absorption of the ruthenium complex, and a silicon photodiode detector. The coated slide is placed in a sealed cell through which water containing controlled concentrations of oxygen, is circulated. Further details of the system have been published previously [3]. 3. 3.1.

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Results and Discussion Tailoring of Films for DO Sensing

Optical oxygen gas sensors based on TEOS-derived films have been reported by us previously [2]. While TEOS films produce excellent sensor response in gas phase, this is not the case in dissolved phase. The quenching response of the sensor is defined in terms of the parameter Q, where Q DO is given by Q DO = (Ideoxy − Ioxy )/Ideoxy

(2)

where Ideoxy and Ioxy are the intensities in fully deoxygenated and fully oxygenated water, respectively. The quenching response in gas phase, Q G , is similarly defined where the intensity limits are those in 100% nitrogen and 100% oxygen gas. Figure 1 shows the quenching response for R = 2 TEOS and R = 2 MTEOS films in water. It is clear that the TEOS response is greatly reduced relative to that of MTEOS. TEOS-derived silica is known to have a high surface coverage of hydroxyl groups, thus rendering the surface hydrophilic [4]. It is thought that the quenching mechanism for dissolved oxygen involves the oxygen partitioning out of the water and accessing the ruthenium complex in the film in the gas phase. This partitioning effect is much reduced for a hydrophilic film while it is enhanced in the case of a hydrophobic film surface. The addition of modified precursors such as MTEOS or ETEOS to the sol increases the hydrophobicity of these ormosil films by replacing some of the surface hydroxyl groups with alkyl groups which have low affinity for water [5]. The results of a systematic study of the influence of different amounts of organic precursor on the quenching response, both in gas and aqueous phase, are shown in Table 1. It is clear

QG

Q DO

MTEOS/TEOS 1 : 1

93%

56%

MTEOS/TEOS 2 : 1

92%

62%

MTEOS/TEOS 3 : 1

89%

70%

MTEOS

85%

73%

ETEOS/TEOS 1 : 1

92%

72%

ETEOS

87%

80%

that Q DO increases with organic precursor content, the largest value being obtained from ETEOS films which are more hydrophobic than MTEOS films. Q G values, on the other hand, show a decrease in response with increasing organic precursor content. This is consistent with a decrease in gas diffusion coefficient as the larger alkyl groups gradually replace the hydroxyl groups on the film surface. Hence, for gas-phase quenching, the quenching response of ormosil films decreases slightly as a function of organic precursor content due to decreased diffusion coefficients, while for DO quenching, response is vastly increased due to the increase in hydrophobicity. These conclusions are corroborated by FTIR data where the intensities of the water band at 3400 cm−1 and the SiOH band at 910 cm−1 were observed to decrease as a function of increasing MTEOS content for the mixed MTEOS/TEOS films [5, 6]. 3.2.

Sensor Performance

It is clear from Fig. 1(b) that the MTEOS-based DO sensor exhibits good reproducibility and large signal-to-noise ratio. For ETEOS films the quenching response is even higher due to increased surface hydrophobicity as seen in Table 1. Measured response times of the order of 10 s are typical, although the intrinsic response time is expected to be considerably less than this as the measured value includes the contribution of gas delivery and cell filling times. Limits of detection (LOD) and sensor resolution values for MTEOS and ETEOS films fabricated with R = 2, are shown in Table 2. Both calculations were based on three times the relevant standard deviation and could be improved by further averaging. Dye leaching is a common feature of sensors based on entrapped dyes. In this work, two strategies were employed to

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Optical Sol-Gel-Based Dissolved Oxygen Sensor

Figure 1.

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DO quenching response of (a) TEOS films and (b) MTEOS films.

minimize leaching. Leaching can be reduced by increasing the R value. For example, R = 4 MTEOS shows very little leaching (1%) due to the smaller average pore size and resulting improved dye entrapment. The small decrease in quenching response for R = 4 films compared with R = 2 films can be tolerated due to the excellent signal-to-noise ratio. Figure 2 shows a Table 2. DO performance parameters of MTEOS and ETEOS films. LOD

Sensor resolution

R = 2 MTEOS

15 ppb

0.23 ppm

R = 2 ETEOS

5 ppb

0.18 ppm

reduction in leaching from 12% to 1% between R = 2 and R = 4 films. These results are corroborated by other leaching studies carried out in this laboratory on pH-sensitive dyes [7]. The leaching experiment was carried out over 3 days in continuously flowing water. Dye leaching can also be minimized by coating the film with a silicone rubber barrier layer. Preliminary work indicates a reduction in leaching from 12% to 5% for an unoptimized coating as seen in Fig. 2. This rubber coating, as well as minimizing leaching, also serves to prevent fouling. It has also been established that photobleaching of the ruthenium fluorescence does not occur in these films. Having established that ormosil films exhibit excellent quenching characteristics in DO, the long-term quenching stability

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Figure 2.

Dye leaching study of MTEOS films.

Figure 3.

Long-term stability of sensor response for MTEOS and ETEOS films.

was also investigated. Figure 3 shows quenching response over a 6-month period for R = 2 MTEOS and ETEOS films. The stability of the response is clear. A prototype DO sensor based on a dipstick probe configuration is being built in the laboratory, and field tests will be carried out in the near future. A calibration protocol based on single-point calibration is also being implemented. The performance parameters quoted above emphasize the significance of the solgel approach for commercially viable optical sol-gel sensors.

4.

Conclusions

High performance sol-gel based DO sensors have been reported. Tailoring of the films for DO was discussed in terms of the use of ormosil films in order to optimize the hydrophobicity. The feasibility of sol-gel-based optical sensors has been established. The commercial potential of these sensors is enhanced by their compatibility with low-cost optoelectronic components. The sensors exhibit low LODs and high calibration stability. Issues that remain to be addressed include temperature

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Optical Sol-Gel-Based Dissolved Oxygen Sensor

compensation and the scaling up of the fabrication process for large-scale production.

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Eng. 33, 3861 (1994). 3. A.K. McEvoy, C. McDonagh, and B.D. McCraith, Analyst 121, 785 (1996). 4. K. Matsui, M. Tominaga, Y. Arai, H. Satoh, and M. Kyoto, J. Non-Cryst. Solids 169, 295 (1994). 5. P. Innocenzi, M.O. Abdirashid, and M. Gugielmi, J. Sol-gel Sci. Tech. 45(3), (1994). 6. V. Murphy, M.Sc. Thesis, Dublin City University, 1996, unpublished. 7. T. Butler, Ph.D. Thesis, Dublin City University, 1996, unpublished.