Three-Electrode on-Chip Sensors for Voltammetric Detection of Trace Metals in Natural Waters

Abstract

This paper reports on the fabrication and electrochemical characterization of newly designed three-electrode on-chip devices for in situ voltammetric detection of trace metals in marine environment. Sensitivity values higher than those previously reported in literature were obtained for cadmium (Cd(II)), lead (Pb(II)), copper (Cu(II)), zinc (Zn(II)) [1] and arsenite (As(III)) [2]. The key novelty of these devices is the on-chip integration of a reference electrode (RE) towards the miniaturization of the voltammetric in situ profiling system [3].Compared to the microdisc arrays reported by Belmont et al. [1], [4], our sensors also differ in number of iridium (Ir) microdiscs (190 vs. 100), center to center distance of the microdiscs, as well as deposition/patterning techniques used during the fabrication. The sensors were fabricated on silicon wafers passivated with silicon nitride (Si3N4). Ir deposited by sputtering was patterned by Ion Beam Etching, followed by a Si3N4layer deposited by Plasma-Enhanced Chemical Vapor Deposition and subsequently patterned by Deep Reactive-Ion Etching to define the microelectrodes and bonding pads. Ir electrodes were used as either counter electrode (CE) without further modification or as a seed layer for the fabrication of the working and reference electrode (WE, RE). The WE was obtained by plating Ir microdisc array with mercury or gold nanoparticles for direct quantification of respectively cadmium, lead, copper , zinc [5] and arsenite [2], the RE was realized by plating the Ir with silver (Ag) by galvanostatic electrodeposition and chloridizing Ag to silver chloride (AgCl). A schematic view of the sensor is shown in Figure 1 as well as a picture of the microfabricated device. The electrode surface topography and cleanness were analyzed by Scanning Electron Microscopy (SEM) showing an excellent quality of the Si3N4 layer patterning in terms of geometry definition and absence of fabrication residue (as seen in Figure 2). The material properties and the electrochemical behavior of the Ir-based electrodes were characterized by cyclic voltammetry: the integrity of the thin-film Ir layer was assessed in degassed 1 M H2SO4(Figure 3a) and the absence of shielding effect was demonstrated in 1 mM potassium ferricyanide at different scan rates from 1 mV/s to 50 mV/s, as shown by the S-shaped current response in Figure 3b. The measured diffusion controlled steady-state current (118.9±2.24 nA) was found to be in good agreement with the theoretical value calculated for a recessed geometry (119.5 nA). Afterwards SU-8 was patterned by photolithography around the WE and RE to serve as containment ring for an antifouling agarose gel membrane. The pseudo-reference Ag/AgCl RE was tested in seawater showing a potential of about 37 mV against an Ag/AgCl/3 M KCl RE stable over more than 2 months as shown in Figure 4, and in synthetic solutions with different chloride concentrations giving a potential slope of 50.1 mV/decade at 25 °C. The use of an Ionic Liquid (IL) poly(vinyl chloride) (PVC) membrane [6] is currently under investigation in order to develop an all-solid-state RE towards a versatile use in brackish water as well. The first Square Wave Anodic Stripping Voltammetry (SWASV) measurements were carried out in synthetic solutions of Pb(II), Cu(II) and Cd(II) or As(III) against an external RE. Figure 5 shows the SWASV measurements obtained for different concentrations of Pb(II), Cd(II) and Cu(II) in 0.1 M NaNO3 background electrolyte for 600 s deposition time. Registered sensitivity values were 2.73 nAnM-1 min-1, 1.23 nAnM-1 min-1, 1.29 nAnM-1 min-1 and 0.6 nAnM-1 min-1for Pb(II), Cu(II), Cd(II) and As(III), respectively. SWASV measurements against on-chip RE are currently under investigation.