Integration of a neutral ionophore-based ion pair extraction reaction onto a glass microchip was performed. Since the liquid microspace provides a short molecular diffusion distance and a large specific interracial area of the liquid-liquid interface, novel attractive analytical features arise such as extremely fast ion sensing and ultrasmall reagent solution volume. In contrast to the slow response time of a standard ion optode, in which the response time is basically governed by slow diffusion of ionic species in the viscous polymer membrane, that of the on-chip ion-sensing system is clearly faster due to the short molecular diffusion distance and low viscosity of organic solution. In this case, the organic solution containing a neutral ionophore and a lipophilic pH indicator dye and the aqueous solution containing sample ion were independently introduced into microchannel to form an organic-aqueous interface. Then determination of the ion was performed by thermal lens microscopy at the downstream of the organic phase under continuous-flow conditions. The response time and minimum required reagent solution volume of the on-chip ion-sensing system are about 8 s and 125 nL, respectively, indicating the advantages of using the liquid microspace. Other advantages of the on-chip ion pair extraction system arising from using the liquid microspace and microfluidic system are also discussed in detail.
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