Development of a Miniaturized Dissolved Oxygen Sensor with Anti-Biofouling Coating for Water Monitoring

The consumption of dissolved oxygen (DO) in water is indicative of aqueous organic content and therefore contamination in water¹. Optical DO sensors consist of an oxygen sensitive fluorescence membrane on which oxygen can cause the quenching effect; the quenched fluorescence intensity is proportional to oxygen concentration². These optical DO sensors are highly sensitive and long-term stable; however, the complex optical design and optical components make the cost always more expensive than electrochemical DO sensors. In order to reduce the cost, microfabricated optical DO sensing device is developed³; nevertheless, the sensitivity is sacrificed owing to the small sensing area resulting from miniaturization. Conventional optical DO sensing setup has single signal excitation site which contributes to the fact that the devices cannot provide sufficient signal intensity when being miniaturized (Fig. 1 (a)). In the light of this, we designed a multi-reflection (MR) optical DO sensor for overall sensitivity enhancement. Experimental results showed that the sensitivities can be increased by more than 3 times in MR sensors. In another aspect, biofouling is one of the major challenges of all water sensors⁴; therefore, we grafted polyethylene glycol (PEG) on the raw materials (polydimethylsiloxane, PDMS) of DO sensitive membrane. Reduction of protein adsorption (first step of biofouling) was observed as a result.

Experiments Results and Discussions

The optical scheme of MR optical DO sensor is shown in Fig. 1 (b).  The fluorescence DO sensitive membrane was immobilized in a reservoir with total reflective (gold) surface at its bottom. A 455 nm laser was used as light source for generating fluorescence. A fluorescence-choosing band-pass filter is placed on top of the reservoir for not only preventing the excitation light from detected by a CCD sensor but also reflecting the excitation light into the reservoir again for MR purpose. After excitation, the generated fluorescence from membrane was collected by a convex lens and read by a commercially available spectrometer. The synthesis of DO sensitive membrane is guided by the following procedure: first, the PDMS elastomer (Dow Corning) was mixed with crosslinker at 10 to 1 weight ratio. Then 1 mg of the luminescent dye (Tris(bipyridine)ruthenium(II) chloride, Sigma Aldrich) was added to the polymer solution and stirred until uniformly mixed. The mixture was spin-coated on a three-inch silicon wafer at 4000 rpm for 30 second to obtain a membrane with 20 µm thickness. The DO sensing results suggest that the sensitivity is increased three times when using MR device as compared with the conventional setup.

Contact angles of PDMS surfaces before and after PEG grafting are shown in Fig. 2 (a). The results show that the contact angle of PDMS is reduced significantly after PEG grafting (from 106.4 to 24.3), which illustrates the facts that the hydrophobicity of PDMS is reduced and that the protein (hydrophobic) absorption is restricted as showed in Fig. 2 (b).  

Conclusion 

The MR DO sensing device shows three times higher sensitivity as compared with signal excitation optical DO sensing setup. The result suggests that the multi-reflection device is a good replacement of miniaturized optical DO sensor. PEG grafting results in our experiments also show good anti-protein absorption capacity which may be effective in eliminating the biofouling progress.   

Reference

1.           U.S. Environmental Protection Agency. in Volunteer Estuary Monitoring: A Methods Manual(U.S. Environmental Protection Agency) 9–3 (Office of Water, Washington, DC, 2006).

2.           McDonagh, C., Maccraith, B. D. & McEvoy, a K. Tailoring of sol-gel films for optical sensing of oxygen in gas and aqueous phase. Analytical chemistry 70,45–50 (1998).

3.           McEvoy, A., McDonagh, C. & MacCraith, B. Dissolved oxygen sensor based on fluorescence quenching of oxygen-sensitive ruthenium complexes immobilized in sol–gel-derived porous silica coatings. Analyst 121,785–788 (1996).

4.           Flemming, H.-C. Biofouling in water systems--cases, causes and countermeasures. Applied microbiology and biotechnology 59, 629–40 (2002).