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 an azo dye, and an acid is placed in {HEAD ~ pores} of {TAIL ~ 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 an azo dye , and an acid is placed in { HEAD ~ pores } of { TAIL ~ a transparent porous body } to prepare a sensor element
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