Her Shuang
Toh
,
Adriano
Ambrosi
and
Martin
Pumera
*
Division of Chemistry & Biological Chemistry, School of Physical & Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore. E-mail: pumera@ntu.edu.sg
First published on 20th August 2012
Nitroaromatic compounds have garnered a lot of attention due to their wide applications in military and industrial fields. Electrochemical techniques can be applied for sensing these compounds due to their electrochemical activity. Electrocatalytic reduction of nitroaromatic explosives was shown previously on silver nanoparticles and graphene based surfaces. Here, we demonstrate electrocatalytic reduction of explosives on zinc oxide nanoparticles. Nitroaromatic explosives were tested using glassy carbon electrodes modified with zinc or zinc oxide nanoparticles to study the catalytic effects on the electrochemical reduction of the nitroaromatic compounds with the aim of improving the sensitivity of their detection. A zinc oxide nanoparticle modified electrode has shown favourable improvement in peak height and hence greater sensitivity compared to bare glassy carbon electrodes and zinc nanoparticle modified electrodes.
Traditionally, metals act as reducing reagents by donating electrons. They are often used as catalysts in synthetic reactions.23 Metals such as iron, zinc and tin are popular catalysts in synthetic organic reactions.24,25 Metal nanoparticles, like silver, have also been used to promote the reduction of nitroaromatic compounds.26,27 Other previous works have also employed non-metallic materials with high surface area such as graphene and carbon nanotubes to encourage the adsorption effect, obtaining enhanced reduction signals of nitroaromatic compounds.2,28 Zinc and zinc-based derivatives have been successfully adopted in organic synthetic reactions with the aim of reducing nitro groups specifically.29,30
Here, we investigate the catalytic properties of zinc and zinc oxide nanoparticles, towards electrocatalytic reduction of nitroaromatic compounds aiming to develop a fast and sensitive electrochemical sensor.
2-NT, 2,4-DNT, 2,6-DNT and 4-A-2-NT were prepared in 1000 ppm solution with acetonitrile. CV was performed with a sample solution of 8 mL buffer consisting of 20 ppm of nitroaromatic compound. DPV was performed with a sample solution of 8 mL buffer consisting of 4, 8, 12, 16 or 20 ppm of nitroaromatic compound.
All CV experiments were carried out at a scan rate of 100 mV s−1. All DPV experiments were carried out at a step potential of 5 mV and a modulation amplitude of 25 mV.
![]() | ||
Fig. 1 Cyclic voltammograms of 2-NT (A), 2,6-DNT (B), 2,4-DNT (C), 4-A-2-NT (D) and TNT (E). Conditions: 20 mM borate buffer, pH 9.3; scan rate of 100 mV s−1. Concentration of nitroaromatic compounds: 20 ppm. |
Fig. 1A shows that 2-NT has two reduction signals at (i) −781 mV and (ii) −1166 mV for a bare GCE system. In a zinc nanoparticle modified GCE (Zn-GCE) system, 2-NT has reduction peaks at (i) −801 mV and (ii) −1100 mV. With a zinc oxide nanoparticle modified GCE (ZnO-GCE) system, 2-NT has only one reduction signal at (i) −853 mV. Upon comparing the reduction peak (i), the improvement in the peak height of Zn-GCE was 51% and of ZnO-GCE was 117% when compared to a bare GCE system. Fig. 1B depicts that reduction of 2,6-DNT with a bare GCE system was (iii) −668 mV, (iv) −847 mV and (v) −1230 mV. With Zn-GCE, it was (iii) −674 mV and (iv) −851 mV. With ZnO-GCE, it was (iii) −676 mV and (iv) −938 mV. The height difference in peak (iv) was −27% in Zn-GCE and 288% in ZnO-GCE with regard to a bare GCE system. In Fig. 1C, the reduction of 2,4-DNT with a bare GCE system gave signals occurring at (vi) −623 mV and (vii) −810 mV. With Zn-GCE, signals occurred at (vi) −636 mV and (vii) −834 mV. With ZnO-GCE, signals were observed at (vi) −641 mV and (vii) −882 mV. Compared with the bare GCE peak (vii), the Zn-GCE peak height dropped by 20% while the ZnO-GCE peak height increased by 190%. Fig. 1D shows that 4-A-2-NT with the bare GCE system, Zn-GCE and ZnO-GCE was reduced at peaks (viii) −848 mV, −875 mV and −894 mV respectively. When compared to a bare GCE system, the improvement in peak current was 7% and 34% in Zn-GCE and ZnO-GCE respectively. Fig. 1E depicts that TNT reduction occurred at (ix) −515 mV, (x) −690 mV and (xi) −924 mV in a bare GCE system. TNT reduction signals were at (ix) −516 mV, (x) −703 mV and (xi) −939 mV in the Zn-GCE system. For the ZnO-GCE system, signals were observed at (ix) −531 mV, (x) −719 mV and (xi) −916 mV. Comparing peaks (xi) across all systems, Zn-GCE gave a 12% increase in peak height and ZnO-GCE gave a 333% increment compared to the bare GCE system.
In the ZnO-GCE system, the reduction signal at the most negative potential has a greater peak width and higher peak height when compared to a bare GCE system and a Zn-GCE system. However, the rest of the reduction peak heights in ZnO-GCE were comparable to that in the other two systems. This may be due to the catalytic effect that zinc oxide nanoparticles have on nitroaromatic reduction. The adsorption effect should not be accountable for improvement in performance linked to zinc oxide nanoparticles as only particular peaks were higher than the bare GCE system. On the other hand, Zn-GCE did not improve the system sensitivity by a large margin compared to the bare GCE system.
We performed materials analysis of the bare electrode surface, ZnO NPs modified surfaces before and after the CV voltammetric study. From SEM/EDS morphological and elemental analysis (Fig. 2) it is clear that there are no observable changes in the electrocatalyst surface and this electrocatalyst does not undergo changes. Survey XPS analysis was performed as well to identify the presence of Zn at the surface of the electrode before and after electrochemical experiments (Fig. 3). Spectra of the ZnO NPs modified electrodes present the characteristic signal of Zn2p at binding energy of 1022.1 eV. XPS confirmed the SEM/EDS observation that the catalyst is present at the surface of the electrode and does not undergo any changes.
![]() | ||
Fig. 2 SEM/EDS analysis of the bare electrode surface (A), ZnO NPs modified surface before (B) and after (C) cyclic voltammetric analysis. |
![]() | ||
Fig. 3 XPS analysis of the bare electrode surface (A), ZnO NPs modified surface before (B) and after (C) cyclic voltammetric analysis. |
![]() | ||
Fig. 4 Cyclic voltammograms of 2,4-DNT at different pHs. 50 mM PBS buffer, pH 4.1 (A), 50 mM PBS buffer, pH 7.0 (B), 20 mM borate buffer, pH 9.3 (C) Conditions: scan rate of 100 mV s−1. Concentration of 2,4-dinitrotoluene: 20 ppm. |
For nitroaromatic compounds containing more than one nitro group, the reduction signal with a lower reducing potential has a smaller peak current compared to the reduction signal with a higher reducing potential under acidic conditions. The inverse is observed under basic conditions. It is also noted that an increase in pH results in a general increment of peak potentials.19
![]() | ||
Fig. 5 Concentration dependence of 2-NT (A), 2,6-DNT (B), 2,4-DNT (C), 4-A-2-NT (D) and TNT (E). Conditions: 20 mM borate buffer, pH 9.3; scan rate of 100 mV s−1. |
2-NT | 2,6-DNT | 2,4-DNT | ||||
---|---|---|---|---|---|---|
Electrode | Sensitivity (nA ppm−1) | R 2 | Sensitivity (nA ppm−1) | R 2 | Sensitivity (nA ppm−1) | R 2 |
Bare GCE | 73.6 | 0.975 | 44.2 | 0.975 | 63.5 | 0.938 |
Zn-GCE | 78.7 | 0.975 | 19.5 | 0.951 | 45.8 | 0.985 |
ZnO-GCE | 70.8 | 0.845 | 66.9 | 0.992 | 73.1 | 0.943 |
4-A-2-NT | TNT | |||
---|---|---|---|---|
Electrode | Sensitivity (nA ppm−1) | R 2 | Sensitivity (nA ppm−1) | R 2 |
Bare GCE | 49.1 | 0.926 | 34.2 | 0.973 |
Zn-GCE | 56.6 | 0.988 | 33.1 | 0.991 |
ZnO-GCE | 78.1 | 0.901 | 46.7 | 0.996 |
![]() | ||
Fig. 6 Differential pulse voltammograms of 2-NT (A), 2,6-DNT (B), 2,4-DNT (C), 4-A-2-NT (D) and TNT (E). Conditions: 20 mM borate buffer, pH 9.3; scan rate of 100 mV s−1. Concentration of nitroaromatic compounds: 20 ppm. |
For 2-NT, the calibration curves were plotted based on peak (i). At the same concentration, the peak height is the greatest in ZnO-GCE, then Zn-GCE and followed by the bare GCE system. The slope of the calibration curve is at around 75 nA ppm−1 for all three systems. For 2,6-DNT, peak (iii) was used to plot the calibration curves. At a concentration of 4 ppm, all three systems gave similar signals. However, the slope of the three systems differs greatly and the ZnO-GCE's slope (66.8 nA ppm−1) is 1.5 times greater than that of the bare GCE system (44.2 nA ppm−1) and twice that of Zn-GCE (19.5 nA ppm−1). ZnO-GCE also gave the best R2 value of 0.992 out of the three systems. In the calibration plots of 2,4-DNT, peak (v) was used for calculation. ZnO-GCE (73.1 nA ppm−1) has a slightly steeper slope than the bare GCE system (63.5 nA ppm−1) and ZnO-GCE gave a greater signal than the bare GCE system and Zn-GCE system at all concentrations. Zn-GCE has a gentler slope (45.8 nA ppm−1) and signal height around the bare GCE system. For 4-A-2-NT, the calibration curve was based on peak (vi). ZnO-GCE gives the best slope (78.1 nA ppm−1) and highest signals at all concentrations. However, it also has the lowest R2 of 0.901. The bare GCE system and Zn-GCE system give slopes of 49.1 nA ppm−1 and 56.6 nA ppm−1 respectively. The TNT calibration curve was plotted based on peak (ix). Both Zn-GCE and ZnO-GCE give a R2 greater than 0.99. ZnO-GCE gives the highest slope (46.7 nA ppm−1) out of the three systems (bare GCE system: 34.2 nA ppm−1 and Zn-GCE: 33.1 nA ppm−1). ZnO also performed the best by giving the highest peak height at all concentrations. Generally, ZnO is the system with the steepest slope and highest signals at all concentrations.
This journal is © The Royal Society of Chemistry 2013 |