ELECTROCHEMICAL SENSORS BASED ON ELECTRODES COATED WITH LAYERED DOUBLE HYDROXIDES

Layered Double Hydroxides (LDHs) have general formula [M(II)M(III)(OH)] x+ (A n-a1-xbx2x/n )mH 2 O, where x ranges between 0.2 and 0.5, depending on the nature of the M a and M b metals [1]. These materials are attractive due to their anionic exchange behaviour which makes them useful in several fields, such as catalysis, separation technology, and medicine. When M a (II) is a transition metal, like Ni or Co, undergoing a reversible redox reaction between the oxidised and reduced states, the material displays improved charge transport properties, especially in alkaline solution [2]. For most LDHs applications in the field of electrochemical sensors, it is required to coat a metal or a carbon-based substrate with a stable thin film. Previously, we have described a simple and rapid electrochemical procedure to deposit, with good reproducibility, well adherent thin films of Ni/Al or Co/Al LDHs on conductive surfaces [3]. In this contribution we report two analytical applications of cobalt-based LDH modified Pt electrodes. The former concerns the development of a potentiometric pH sensor, the latter the amperometric detection of salicylic acid (SA). The performances of the coated electrode as pH sensor are summarized in Figure 1. It shows, as a function of time, the recorded potential and the corresponding pH value, simultaneously recorded with a conventional glass electrode, in a dynamic experiment where the pH of a universal buffer solution is increased in the range 5.5-12 by dropwise addition of KOH. The graph highlights the excellent behaviour of the new sensor, as to response time (5 s for all the investigated pH values) and signal stability. The response is linear, with a slope of -76.2 + 0.6 mV/pH in the pH interval between 2 and 14, and does not suffer from memory effects.