Nano Structured and Pyrolytic Deposited Carbon Electrodes for Detection of Nitroaromates – a Comparison
Authors: Guth Ulrich , Riedel Johanna 
Published: May 2020

Introduction
Nitroaromatics like 2,4,6-trinitrotoluene (TNT) and 2,4-dinitrotoluene (DNT) are long-lasting organic pollutants in natural water and soil caused mainly by the military usage. Nano structured electrodes are established in detection of such compounds in water by linear sweep voltammetry (DP) [1]. On the other hand, it could be shown that screen printed carbon electrodes [2] and such made of pyrolytic deposited carbon are also suited for that purpose [3]. Both kind of electrodes were investigated regarding the determination of nitroaromatic compounds in aqueous solution. The aim of this paper is to compare the performance of both electrodes in comparison to commercial electrodes respecting sensitivity, selectivity and long-term stability.
Experimental
Electrodes using carbon nano-tubes and gold nano-particles on printed carbon are commercially available (DropSens, Spain DRP-110CNT-GNP). Screen printed electrodes (SPCE) of 3mm in diameter were industrially like produced on an alumina substrate with a commercial carbon ink (DuPont) carbon paste. Pyrolytic graphite layer were deposited by high temperature decomposition (1050 °C) of aromatic vapor for 15 min with high reproducibility (PyGE). Solid state analyses as XRD and SEM analysis show crystalline carbon with a sp2 hybridization (Fig. 1). The setup of the electrodes is seen in Fig. 2. All experiments were performed in a three electrode arrangement using platinum counter electrode and Ag/AgCl,Cl--reference electrode in a buffer solution. Voltammetric experiments were performed by a PalmSens potentiostate (Palm Instruments, NL). DRP-110CNT-GNP were investigated by linear sweep voltammetrie. SPCE and PyGE were measured in differential pulse voltammetry mode (DPV). The parameter for the DPV were selected as follows: scan rates of 15 mV∙s-1 to 20 mV∙s-1, amplitude of 50 mV and step height of 5 mV
Results and discussion
The potentials of the single reduction peak of nitro-aromates can be clearly separated (Fig. 3) for both kind of electrodes, SPCE and PyGE. The nitroaromates trinitrotoluene (TNT) and dinitrotoluene (DNT) are reduced and dopamine is oxidized in DPV mode (Fig.4). The peak currents are proportional to the TNT concentrations with a sensitivity of 2.7 nA/ppb for SPCE (Fig.5) and 0.78 for PyGE (Fig. 6) at the characteristic peak potential of -0.59 V (R²=0.99). Commercial electrodes DRP-110CNT-GNP made of carbon with gold nano particles require a special treatment in cyclo-voltammertic mode up to stable measuring conditions. The sensitivity of 2.8 nA/ppb is high compared to PyGEs. The relative errors in measuring cycles (35 %) are very much higher than those for PyGEs (5 %). PyGE do not need a special pretreatment before use.
Screen printed and pyrolytic deposited carbon electrodes can be applied in air saturated aqueous solutions like in field application. For both electrodes, SCPE and PyGE, the oxygen reduction peak do not interfere with the reduction of the nitro groups. The manufacturing tolerances become smaller in the row DRP-110CNT-GNP, SPCE, PyGE. PyGEs can be produced in few steps with very small deviations. They can stored in air or in water over years without a loss of performance. Commercial electrodes are very complex in setup and obviously not so easy to produce in proper quality. Furthermore, their signals are remarkably influenced by oxygen so that measurements in air are not possible.
References
[1] http://www.dropsens.com/en/screen_printed_electrodes_pag.html, updated Nov. 26th, 2019
[2] J. Riedel, M. Berthold, U. Guth, Pyrolytic deposited graphite electrodes for voltammetric sensors: an alternative to nano structured electrodes, Sensors & Actuators: A. Physical (2016) 212-215, DOI information: 10.1016/j.sna.2016.01.046
[3] J. Riedel, M. Berthold, U. Guth, Electrochemical determination of dissolved nitrogen-containing explosives, Electrochim. Acta 128 (2014) 85–90; DOI: 10.1016/j.electaca.2013.11.067

Fig 1- SEM images of pyrolytic graphite with onion-like structure of concentric layers; cross-section with height profile
Fig. 2.- Commercial electrode (DRP-110CNT-GNPE) in a three electrode arrangement, above, screen printed carbon electrode (SPCE), below
Fig. 3 - DPV responses of screen printed carbon electrodes: background und baseline subtracted at 909 ppb of 2,4,6-trinitrotoluene, 2,4-dinitrotoluene, 2-nitrotoluene, 3-nitrotoluene and 4-nitrotoluene, respectively (argon purged)
Fig 4 - DP diagrams of pyrolytic deposited carbon electrode for nitro aromates (TNT and DNT) at a in salt containing solutions and dopamine in buffer solution
Fig. 5. - Determination of TNT at bare SPCE in 0.1 M acetate buffer solution (purged with argon) with increasing TNT-concentration; DPV conditions: scan rate: 15 mV∙s-1

Fig. 6 - Peak current vs. potential for pyrolytic deposited carbon electrode at different TNT concentration in air saturated aqueous solution. (peak current increases with increasing TNT concentration)