An Optical Sensor Based on Polyvinyl Benzyl Malonate Cross-Linked with Divinyl Benzene Dispersed in a Hydrogel Membrane for Detection of Some Heavy Metals
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In previous work we have developed a dicarboxylate functionalized polymer that demonstrated chemical sensing. It showed good response to pH changes as well as to varying concentrations of copper and calcium ions. Our recent investi- gations showed interesting results upon testing the functionalized sensing polymer on heavy metals. This sensor is composed of microspheres of polyvinyl benzyl malonate lightly-cross-linked with divinyl benzene dispersed in a hy- drogel membrane. The response of the optical sensor is based on the interaction between the metal cations with the de-protonated functional group. The polymer, thus, undergoes shrinking as a result of neutralization of adjacent negative charges on the back-bone of the polymer. This causes significant changes in the optical properties of the sensing ele- ment. The optical changes were measured as absorbance vs. wavelength as the sensing membrane is exposed to solu- tions of varying concentrations of heavy metal ions. The sensor showed significant increase in absorbance up to a con- centration of 5 × 10–3 M to the following metal ions: Ni2+, Zn2+, and Cd2+. Furthermore, the studied capacity of the deri- vatized microspheres showed close values to Ni2+, Zn2+, Cd2+ (1.20, 1.09, 1.08 mmol/g respectively). These kinds of properly functionalized polymers appear to be suitable, versatile sensing elements for the detection of low concentra- tions of heavy metal ions. In addition, all of the tested heavy metals showed a similar value of the equilibrium formation constant, (log Kf1 is 2.63). In contrast, the sensor showed no significant response to varying concentrations of K+ and Mg2+ metal ions.