Polyacrylate Microspheres for Tunable Fluorimetric Zinc Ions Sensor

A novel concept of optical fluorimetric sensing using polymeric microspheres is explored on example of zinc ions sensors. The novel approach proposed uses the advantage of concomitant presence in a microsphere of two compounds: a receptor, fluorescently silent complexing ligand and an optical transducer, fluorescent compound. Binding of the analyte by the ligand affects its absorption spectrum, leading to decrease of the free ligand absorption and increase of complex absorption band. The decrease of free ligand absorption exposes emission of the transducer, yielding increase in fluorescence intensity on analyte concentration increase. This approach was verified experimentally using Zn2+ as a model analyte, the fluorimetric sensor obtained uses 1-(2-pyridylazo)-2-naphthol (PAN) as analyte sensitive receptor and pyrene as optical transducer. In the absence of zinc ions in the sample emission of pyrene embedded in the spheres was significantly quenched, whereas increase of Zn2+ ions concentration in the sample resulted in dependence of fluorescence intensity on logarithm of zinc ions concentration in extraordinary wide range, from 10(-7) to 0.1 M. The response mechanism was explained by surface accumulation of zinc ion-PAN complex on the microsphere/sample solution interface. It was also shown that introduction of cation-exchanging sites to the microspheres significantly alters the responses pattern leading to high sensitivity over relatively limited concentration range (3-4 orders of magnitude). In the latter case the observed responses can be tuned to occur in chosen concentration range, simply by adjusting sample pH.