In this invention, a mixture of a diazotizing reagent which reacts with nitrous ions to produce a diazo compound, a coupling reagent which couples with a diazo compound to produce {TAIL ~ an azo dye}, and an acid is placed in {HEAD ~ pores} of a transparent porous body to prepare a sensor element 
In this invention , a mixture of a diazotizing reagent which reacts with nitrous ions to produce a diazo compound , a coupling reagent which couples with a diazo compound to produce { TAIL ~ an azo dye } , and an acid is placed in { HEAD ~ pores } of a transparent porous body to prepare a sensor element
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