Fine-scale in-situ measurement of lead ions in coastal sediment pore water based on an all-solid-state potentiometric microsensor

Methods for in-situ measurements of heavy metal ions in coastal sediment pore water to elucidate fine-scale biogeochemical and environmental processes are highly required but have rarely been reported. In this work, an all-solid-state lead-selective microelectrode (Pb²⁺-ISμE) based on a poly(3,4-ethylenedioxythiophene)-poly(sodium 4-styrenesulfonate) (PEDOT/PSS) modified gold wire with a diameter of 14 μm has been fabricated. The proposed Pb²⁺-ISμE is capable of in-situ measurement of Pb²⁺ in coastal sediment pore water at millimeter depth intervals. The Pb²⁺-ISμE shows a Nernstian response for Pb²⁺ within the activity range of 2.1 × 10^-9-2.1 × 10^-4 M (S = 28.1 ± 1.3 mV/dec, R² = 0.998) in 0.5 M NaCl, and the detection limit is 6.4 × 10^-10 M. By lowering the microelectrode into a coastal sediment core with a micro-manipulator, the proposed Pb²⁺-ISμE allows the direct measurement of the vertical distribution profile of Pb²⁺ in the pore water. The in-situ measurement of Pb²⁺ using the microsensor could avoid the problems of sample handling. Moreover, the detection system can be extended to assess the vertical distribution profiles of other heavy metal ions in sediment pore water by using different ion-selective microelectrodes.