Abstract
A novel approach to polymeric Ru(II)-diimine luminescent O2 sensors is described. The Ru(II)-diimine, tris(4,7-diphenyl-1,10-phenanthroline)ruthenium(II) dichloride ([Ru(dpp)3](2+)), is first ion-paired to the surface of heterogeneous TiO2 particles, rendered negatively charged due to the alkali nature of the aqueous solution, to produce an O2 sensitive pigment with a strikingly high oxygen sensitivity (i.e. PO2 (S = 1/2) = 0.002 atm, where PO2 (S = 1/2) is defined as the amount of oxygen required to reduce the initial, oxygen free luminescence by 50%), and a rapid response to oxygen. The pigment is extruded in low density polyethylene (LDPE) to produce a thin (60 μm), flexible, O2 sensing plastic film, with an O2 sensitivity (PO2 (S = 1/2) = 0.84 atm) comparable to the more traditional homogeneous lumophore ion-pair based O2 sensor ink films reported in the literature.
Highlights
I0/I 1⁄4 s0/s À (1 1⁄4 KSV[O2]) (2)The detection and quanti cation of oxygen is of great importance in many industries and practical applications, not least environmental monitoring, medical applications, and more recently, food packaging.[1,2,3] Research into O2 detection has increasingly moved away from the more traditional Clark electrode[4] and GC analysis, which is limited by its expense and bulkiness, towards optical detection
These quenching parameters reveal the pigment to be extremely sensitive to oxygen, more so than the homogeneous, ion-pair, ink-based O2 sensors reported in Table 1; the calculated PO2 (S 1⁄4 1/2) (0.002 atm) for the pigment is ten times less than that of the Bacon and Demas[12] silicone-based sensor
A study of the variation in luminescence intensity upon switching the gas stream alternately from O2 to Ar was carried out and the results are illustrated in Fig. 5, from which values for tY50 and t[50 were calculated to be 6 s and 14 s, respectively. These results show that the heterogeneous lumophore ion-pair O2 sensitive lm has a much reduced O2 sensitivity compared to the pigment alone most probably due to the low O2 permeability in low density polyethylene (LDPE) (2.3 Â 1010 cm[3] cm cmÀ2 sÀ1 cm HgÀ1)
Summary
The detection and quanti cation of oxygen is of great importance in many industries and practical applications, not least environmental monitoring, medical applications, and more recently, food packaging.[1,2,3] Research into O2 detection has increasingly moved away from the more traditional Clark electrode[4] and GC analysis, which is limited by its expense and bulkiness, towards optical detection. These lms suffer problems of low luminescence and a tendency for the lumophore to undergo self-quenching of luminescence when its concentration exceeds a critical value.[11] Given these difficulties, other means of encapsulating [Ru(dpp)3]2+ into a lipophilic polymer, such as silicone, were investigated subsequently, and most O2 sensors utilise this lumophore in a form in which it is electrostatically bound as an ion-pair to a lipophilic anion, such as perchlorate, dodecyl sulphate (DSÀ), tetramethylsilypropansulfonate (TSPSÀ) or tetraphenyl borate (Ph4BÀ). Such heterogeneous O2 sensors overcome the problems associated with the more traditional, homogeneous ion-pair lumophore O2-sensitive inks, such as lm curling[12] and the need for a solid, largely in exible support (such as glass or Mylar).[11,12,15,16]
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