Highly Sensitive Electrochemical Sensor for the Detection of Anions in Water Based on a Redox-Active Monolayer Incorporating an Anion Receptor.

In the present work, gold electrodes were modified using a redox-active layer based on dipyrromethene complexes with Cu(II) or Co(II) and a dipodal anion receptor functionalized with dipyrromethene. These modified gold electrodes were then applied for the electrochemical detection of anions (Cl-, SO42-, and Br-) in a highly diluted water solution (in the picomolar range). The results showed that both systems, incorporating Cu(II) as well as Co(II) redox centers, exhibited highest sensitivity toward Cl-. The selectivity sequence found for both systems was Cl- > SO42- > Br-. The high selectivity of Cl- anions can be attributed to the higher binding constant of Cl- with the anion receptor and the stronger electronic effect between the central metal and anion in the complex. The detection limit for the determination of Cl- was found at the 1.0 pM level for both sensing systems. The electrodes based on Co(II) redox centers displayed better selectivity toward Cl- anion detection than those based on Cu(II) centers which can be attributed to the stronger electronic interaction between the receptor-target anion complex and the Co(II)/Co(III) redox centers in comparison to the Cu(II)/Cu(I) system. Applicability of gold electrodes modified with DPM-Co(II)-DPM-AR for the electrochemical determination of Cl- anions was demonstrated using the artificial matrix mimicking human serum.