Highly sensitive shape dependent electro-catalysis of TNT molecules using Pd and Pd–Pt alloy based nanostructures

Abstract

This study demonstrates the fabrication of a highly sensitive electrochemical sensor designed for the quantification of 2,4,6-trinitrotoluene (TNT). The devised sensor system relies on the electro-catalytic reduction of TNT molecules achieved at the surface of palladium (Pd) and palladium-platinum (Pd–Pt) alloy nanostructure modified glassy carbon electrodes (GCEs). The electrode based reduction was studied in a competitive manner with Pd nanocubes (Pd NCs) in comparison to Pd hollow nanospheres (Pd HNS) and Pd–Pt alloy nanostructures (Pd–Pt NA). The experiments revealed Pd NCs to possess high catalytic capability in comparison to their other competitors where relatively greater signal sensitivity suggested the importance of shape-dependence electro-catalysis. The Pd NCs based sensor was found to be highly sensitive towards TNT molecules with detection limits up to 0.01 ppm and a working window of 0.1–7.0 ppm. Moreover, the Pd NCs based sensor demonstrated excellent selectivity in the presence of other common nitro-aromatic compounds. In addition, the excellent recoveries obtained in the real matrix environment (tap water) further promise the real-time application of the developed sensor for trace level detection of TNT.

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1. Introduction

The applications of interdisciplinary nano-electroanalytical chemistry has enabled development of highly sensitive sensor systems.¹ The excellent synergy of electrochemistry with nano-dimensional conductive materials possessing unique surface architectures have provided researchers with a new platform for the diverse electroanalytical investigations.² Such investigations allow not only the determination of any specific target molecule at trace levels, but also enable sensing with minimum interference from the sample matrix. In this regard, the detection of trace amounts of explosive compounds such as 2,4,6-trinitrotoluene (TNT) is a subject of high interest due to the associated security threat and environmental concerns.³ The substantial production and excessive usage of TNT for civil purposes is known to severely contaminate the environment.⁴ Thus, it is important to monitor the trace level concentration of such aromatic compounds both for the environmental and security reasons.⁵ The conventional strategies utilized for the determination of TNT involve application of Raman, mass spectrometry (MS), nuclear magnetic resonance spectroscopy (NMR), fluorescence and surface plasmon resonance (SPR) spectroscopy.^5–8 Such techniques, although sufficient for the quantification are associated with restraints like long, expensive protocols, tedious sample preparation processes and expensive instrumentation making them an unsuitable candidate for a diverse range of applications.⁹ In contrast to this, the electrochemical approach is a more simple, sensitive, rapid and yet an inexpensive protocol. In addition, it has the benefit of miniaturization which can enable production of portable and lab to field sensor systems.

The electro-chemical approach for quantification of TNT affords an explicit advantage of selectivity based on the TNTs finger print reduction peaks.¹⁰ However, the direct reduction of TNT over the bare electrode is quite poor which leads to the decline in the sensitivity, stability and selectivity of the sensors.⁹ Improvement in the sensitivity of such sensors have been achieved through the usage of nanomaterials with greater surface area and enhanced conductivities. In this context, variety of nanomaterials ranging from carbon nanotubes to polymer/silica composite material have been explored previously.^9,11 Recently, graphene assisted noble metals have attracted considerable attention in terms of the sensitivity improvement. Similarly, Lu. X., et al. (2011)¹² reported the application of dispersive Ag nanoparticles on the functionalized graphene for the electrochemical sensing of nitrobenzene. Further, Yuan. C.-X., et al. (2014)¹³ investigated the potential of three-dimensional porous Pt–Pd nanoparticles towards the electrochemical detection of di-nitrobenzene. Although, size of nanomaterial plays a crucial role in the electro-catalytic reaction however, the dependence of electro-activity has also been linked with the shape and structure of the nanomaterial.¹⁴ Unlike the size dependent electro-catalysis, the shape dependent catalytic activity is a highly complicated interplay between exposed surface crystal facets, active sites, and competitive adsorption between anions and reactants which can either promote or suppress the reaction of interest. In this regard, Bansal. V., et al. (2010)¹⁵ investigated the shape dependence electro-catalysis of hydrazine using silver nanoprisms, nanospheres and nanocubes. The reported experimental results suggested dominant catalytic activity of Ag nanoprisms compared to its other competitors. Recently, Zhang. R., et al. (2015)¹⁰ reported the potential capability of Pt–Pd concave cube shaped nanostructures associated with graphene nanoribbons towards the reduction of TNT. Thereof, the sensitivity was attributed to the synergetic effort of graphene and electron rich Pt–Pd nanocubes.

Since it is widely accepted that the shape of a particular nanomaterial can largely influence electro-catalysis whereas electron rich metal like Pd and Pt can actively participate in the reduction of nitro-groups. This study explores the shape dependent potential capability of Pd nanostructures relative to Pd–Pt alloy for the electrochemical reduction of TNT molecules. The comparatively higher electro-catalytic potential of Pd nanocubes enabled development of highly selective and sensitive sensor system with detection level of 0.01 ppm. The successful quantification of TNT from real matrix (tap water) further reflected the analytical capability of the devised sensor.

2. Materials and methods

2.1 Chemicals and materials

Analytical grade 2,4,6-trinitrotoluene (TNT) (C₇H₅N₃O₆), 2-nitrotoluene (2-NT), 4-nitrotoluene (4-NT) (C₇H₇NO₂) 2,4-dinitrotoluene (2,4-DNT), 2,6-dinitrotoluene (2,6-DNT) (C₇H₆N₂O₄), phosphate buffer solution (PBS) (0.1 M), ascorbic acid (AA), ethylene glycol (EG), poly(vinyl pyrrolidone) (PVP, M_n ≈ 55 000), potassium tetrachloroplatinate(II) (K₂PtCl₄), silver nitrate (AgNO₃, 99%), sodium sulfide (Na₂S, 99%), sodium tetrachloropalladate(II) (Na₂PdCl₄) were purchased from Sigma Aldrich, Germany. Potassium bromide (KBr) and potassium chloride (KCI) were purchased from Merck. The Nafion® solution (5%) was prepared in isopropanol (C₃H₈O) (Merck) and was utilized as an electro-active polymer. The glassy carbon electrodes (GCE, 3 mm in diameter) were obtained from CH-Instrument Company. The stock solution of 1 mg/100 mL of TNT was prepared carefully in a ventilating hood and was successively diluted to obtain the desired concentration range 0.1–7.0 ppm. All the aqueous solutions were prepared with de-ionized (DI) water (18.2 MΩ cm^−​1 at 25 °C).

2.2 Instrumentation

UV-vis spectroscopy (Shimadzu), transmission electron microscopy (TEM) (Titan™, FEI) and X-ray diffraction (XRD) (Bruker) was used to explore the optical, morphological and compositional characteristics of as-synthesized Pd and Pd–Pt alloy nanostructures. The sensing studies were carried using electrochemical workstation (E-760 USA) consisting a conventional three electrode based system housing silver/silver chloride (AgCl) and platinum (Pt) wire as reference and auxiliary electrode. The modified electrodes were used as working electrodes in the discussed electrochemical studies.

2.3 The synthesis of nanostructures

2.3.1 The synthesis of palladium nanocubes (Pd NCs). In a typical experiment, 8.0 mL of an aqueous solution containing 105 mg of PVP, 60 mg of AA, and 600 mg KBr were vortexed in at 80 °C for about 10 min. After complete homogenization, the mixture was introduced 3.0 mL of an aqueous solution containing 57 mg of Na₂PdCl₄. The reaction mixture was then allowed to swirl at 80 °C for 3 h.

2.3.2 The synthesis of palladium hollow nanosphere (Pd HNS). The Pd HNS was synthesized by using galvanic replacement method. The fabrication was achieved using Ag nanocubes as template (size ∼ 40 nm), synthesized according to our previous study.¹⁶ Briefly, 20 mL of pre-prepared Ag nanocubes were washed using centrifugation (12 500 rpm for 15 min) to remove the excess surface bound PVP. The Ag nanocubes were then re-dispersed in 20 mL of de-ionized (DI) water followed by the addition of Na₂PdCl₄ (0.02 g/10 mL) at slow rate of 0.5 mL per 5 min under constant stirring (300 rpm). The formation of Pd HNS was followed by monitoring the disappearance of characteristic SPR peak of Ag nanocubes.

2.3.3 The synthesis of palladium–platinum nanoalloy (Pd–Pt NA). The Pd–Pt alloy nanostructures were also synthesized using galvanic replacement approach. Briefly, pre-washed 20 mL of Ag nanocubes (∼40 nm) were heated up to the boiling point under constant stirring. The solution was then introduced 3.0 mL of 0.14 mmol Na₂PdCl₄ slowly until the corresponded SPR peak of Ag cubes disappeared. The obtained solution was kept under stirring for 30 min to achieve formation of palladium layer onto the outer surface of Ag nanocubes. The solution was injected 3.0 mL of 0.14 mmol K₂PtCl₄, which replaced the rest of the silver atoms inside the cube resulting in formation of Pd–Pt alloy nanostructures.

2.4 The modification of glassy carbon electrode (GCE) with Pd and Pd–Pt nanostructures

Prior to the modification, the glassy carbon electrodes (GCEs) were thoroughly polished with alumina slurry (0.5 and 0.02 μM) followed by ultrasonic treatment in de-ionized and ethanol water to obtain a mirror shiny surface. The modification was achieved simply by drop casting 8 μL of Pd NCs, Pd HNS and Pd–Pt NA over the surface of pre-cleaned GCEs. The electrodes were then dried under gentle N₂ stream followed by casting 4 μL of Nafion® (5%) layer over the top of modified layer to restrict material erosion from the electrode surface. The modified electrodes were then finally dried before being utilized as working electrodes. The modified electrodes were denoted as Pd NCs/GCE, Pd NHS/GCE and Pd–Pt NA/GCE throughout the manuscript for the easiness of readers.

3. Results and discussions

3.1 Material characterization

The as-synthesized nanostructures were evaluated for their morphological characteristics using TEM imaging. Fig. 1(a–c) shows the captured TEM images of each representative sample. As seen, both Pd NCs and hollow HNS possess distinctive structural features with uniform size and monodisperse nature. The average size was determined to be 25 and 50 nm for Pd NCs and Pd HNS respectively. The synthesized Pd–Pt NA was observed to possess interesting morphological features. Although, the structure resembles cube but was noted to possess multi-pod interfaces. The average size of Pd–Pt NA was determined to be 30 nm. The compositional characteristics of the as-synthesized nanostructures were studied using XRD analysis. Fig. 2 presents the recorded XRD patterns of Pd NCs, Pd HNS and Pd–Pt NA. The Pd NCs and Pd HNS displayed peaks with corresponding reflections at 40.1, 46.6, 68.1 and 82.1 indexed to (111), (200), (220) and (311) crystal planes of pure Pd metal. However, the enhanced intensity for (111) reflection of Pd NCs compared to Pd HNS indicates the promoted growth along (111) plane of Pd nanocubes.¹⁸ The XRD pattern for Pd–Pt NA consists the representative reflection of both Pd and Pt indexed to FCC crystal structure of pure metals as referenced against ICCD card no 04-0802 and 46-1043 respectively.¹⁷

Fig. 1 TEM images of (a) Pd NCs (b) Pd HNS and (c) Pd–Pt NA with inset figures representing the size interpretation of individual nanostructures.

Fig. 2 The XRD pattern for Pd NCs, Pd HNS and Pd–Pt NA.

3.2 The electrochemical behavior of modified electrodes towards TNT

The electrochemical behavior of the modified electrodes i.e. Pd NCs/GCE, Pd HNS/GCE and Pd–Pt NA/GCE against TNT was studied using square wave-voltammetry (SWV) as a primary mode of investigation. The response of each modified electrode was noted in the presence of TNT (2.5 ppm) with 0.1 M PBS buffer solution (pH 6.0) containing 0.3 M KCl solution. The observed behavior was accessed in comparison with both bare and Nafion® coated GCE electrodes to ascertain the active role of nanostructures towards the reaction of interest. The developed SWV profile is shown in Fig. 3. As expected, the bare and Nafion® coated GCEs exhibited insignificant response in the presence of TNT. Whereas, well-defined reduction peaks were noted for all the modified electrodes. Interestingly, Pd NCs modified electrode demonstrated higher reduction current density with more positive (near to zero) potential compared to its other competitors. The order of electrode behavior in terms of their sensitivity is as follow: Pd NCs < Pd HNS < Pd–Pt NA.

Fig. 3 The SWV behavior of Pd NCs/GCE, Pd HNS/GCE and Pd–Pt NA/GCE in the presence of 2.5 ppm TNT in comparison with bare and Nafion coated GCE.

The well-defined peaks are attributed to the step-wise reduction of TNT to hydroxylamine and amine group respectively.⁹ Although much higher catalytic potential has been anticipated for structures other than nanocubes based on their high surface energy atoms with lower lattice co-ordination numbers. Herein, the enhanced response of Pd NCs in comparison to Pd HNS and Pd–Pt NA may be attributed to the dominant growth of Pd NCs along the (111) crystal facets. Similar scenario was observed by Zhang. R., et al. (2015)¹⁰ where cube shaped alloy of Pt–Pd supported by graphene demonstrated high sensitivity towards TNT. In addition, Ding. H., et al. (2010).¹⁸ also emphasized that Pd (111) surface is more catalytic-active when utilized for electrochemical reactions. Thus, in this case the observed disagreement between the current densities and reduction potentials of modified electrodes signify the importance of nanomaterials shape in the electro-catalytic reduction of TNT. The small difference in the current densities noted for Pd NCs and Pd HNS might be associated with shape-rounding effect.¹⁹ Whereas, the smaller current density noted for Pd–Pt NA relative to Pd NCs and HNS might be related to the association of higher Pt metal content at the corner of the multi-pod structure, which may have resulted during the galvanic replacement reaction of Pd layer from the Ag nanocubes.

The step wise reduction of TNT molecules using Pd NCs/GCE is depicted in Fig. 4. In general, charge transfer interactions should occur between the Pd rich surface (electron rich) of the cube and nitro-groups of TNT molecule (electron deficient), allowing homogenous charge distribution over the nitro-groups. However, with the reduction of first nitro-group, the parent symmetry of TNT molecules would change, distorting the homogenous charge distribution. This is reflected from the characteristic SWV profile (Fig. 3) where the first reduction peak appeared at more positive (−0.31 V) than the second peak at −0.47 which was more positive than third peak at −0.59 V. This difference in the noted reduction potential for each nitro-group of TNT indicate the planar approach of TNT molecules over the Pd nanocubes. Considering the relatively higher sensitivity of Pd NCs, the Pd NCs/GCE was selected as champion electrode for further optimization and quantification of TNT.

Fig. 4 A general view of step wise electro-catalytic reduction of each nitro-group of TNT using Pd NCs/GCE.

3.3 The optimization of developed sensor system

To achieve the best possible current density for the devised sensor system (Pd NCs/GCE), crucial parameters such as pH, supporting electrolyte concentration, accumulation time and deposition volume of the nano-catalyst were elaborately studied with 2.5 ppm of TNT solution as constant parameter. Initially, pH of the system was optimized in range from 3 to 8. The corresponding current densities for the three representative peaks of TNT plotted against the pH with bar-graph depicting the observed variation in current densities is shown in Fig. 5(a and b). The highest current densities were obtained at pH 6.0 which was then selected as the optimum pH for the entire electrochemical studies. The influence of supporting electrolyte (KCl) was also studied in concentration range of 0.1–0.5 M (Fig. 5(c and d)). The observed trend indicates an initial increase up to 0.3 M thereafter current density was observed to decline at higher concentrations. The observed behavior of supporting electrolyte at higher concentration may be associated with the adsorption of supporting electrolyte's ions on the electrode surface which might block the active site of the electro-catalyst resulting in the decrement of measured current response.²⁰ Thus, 0.3 M was considered as the optimum concentration for the supporting electrolyte.

Fig. 5 The optimization of Pd NCs/GCE in terms of (a) pH in the range 3–8 (b) inset bar-graph reflecting current variation with change in pH (c) concentration of supporting electrolyte (KCl) in the range 0.1 to 0.5 M (d) bar-graph depicting current vs. KCl concentration (e) accumulation time from 0 to 240 s and (f) effect of Pd NCs deposition amount on the generated current signal for the three representative peaks of TNT.

The variation in current density for Pd NCs/GCE against accumulation time was noted in range of 0–240 s with 60 s interval as shown in Fig. 5(e). A rapid increment in the observed current density was noted with pre-concentration up to 180 s followed by gradual rise until the saturation point reached at 240 s. To enhance the sensitivity of the obtained signal pre-concentration time of 240 s were considered adequate for the developed sensor. The variation in the observed current density was also measured against the Pd NCs deposition volume in range of 3–15 μL (Fig. 5(f)). An initial advancement of peak current was noted until the deposition volume reached 8 μL which may be attributed to the greater availability of active sites. However, at much higher volumes current density was noted to decrease. This might be associated with the inability of the TNT molecules to reach the actives sites, embedded in much deeper layers of the deposited catalyst at higher volumes. Thus, 8 μL was taken as an optimum deposition volume of Pd NCs during electrochemical reduction of TNT.

3.4 The quantification of TNT

The analytical evaluation of the devised sensor system was carried using SWV as the principal mode of measurement. The quantification was carried in range of 0.1–7.0 ppm with Pd NCs/GCE as working electrode besides 0.1 M (pH 6.0) PBS and 0.3 M KCl solution. The recorded SWV curves for the corresponding range are shown in Fig. 6. The calibration was plotted for current densities corresponding to the peak “A” at −0.31 V against the selected concentration range. The selection of peak A was based on the fact that it followed good linearity compared to peak B and C (data not shown) in lower concentration range. The coefficient of determination (R²) (0.9978) was determined from the linear fit analysis (Fig. 5(b)) with LOD and LOQ values estimated to be 0.01 ppm and 0.08 ppm (S/N = 3). The LOD and LOQ were calculated as three (3σ/slope) and ten times (10σ/slope) the standard deviation of the blank signal (without TNT) divided by the slope of the plotted calibration respectively.

Fig. 6 The SWV curves recorded for Pd NCs/GCE with TNT (a) calibration in the range of 1 to 7.0 ppm (b) inverted profile of peak A (−0.31 V) selected for quantification in desired concentration range with inset representing the corresponding linear fit analysis.

The obtained analytical characteristics were compared with other reported sensor in the area of interest. The comparative data is presented in Table 1. As seen, the described sensor have satisfactory LOD values with good working linear range in comparison to various electrode based system for TNT detection.

Table 1 Analytical characteristics of the developed sensor in comparison with various electrode based sensor systems for TNT quantification

Methods	LOD (ppm)	Linear range (ppm)	Reference
a VO2: vanadium oxide.b IL-GNs: ionic liquid-graphene hybrid nanosheets.
VO2 modified electrode^a	0.01	0.1–1	23
IL-GNs^b	0.04	0.03–1.5	24
Nitrogen-doped graphene	0.02	0.12–2	25
TiO2/nano-Pt particles	0.2	2–5	26
Pd NCs/GCE	0.01	0.1–7.0	This work

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3.5 The selectivity and stability of the developed sensor

The selectivity of the developed sensor was evaluated in conjunction with different molecules with similar chemical characteristics such as nitrobenzene (NB), 2-nitrotoulene (2-NT), 4-nitrotoulene (4-NT), 2,4-dinitrotoluene (2,4-DNT) and 2,6-dinitrotoluene (2,6-DNT). Fig. 7(a and b) shows the SWV waves obtained for the said molecules (2.5 ppm) using Pd NCs/GCE as working electrode. The observed behavior of Pd NCs/GCE for these molecules suggested the dependence of cathodic peak on the number and arrangement of associated nitro-groups. As observed, each interferent exhibits distinct reduction peaks which may be explained on the basic of dissimilar charge localisation associated with the chemically similar molecules such as 2-NT, 4-NT, 2,4 and 2,6-DNT. This disagreement in the associated charge localisation caused by the steric hindrance of neighboring methyl groups strongly affects the electron transfer reaction, which is then reflected as the difference in peak potentials during reduction reaction.^10,21,22 Since the noted reduction potentials for selected interferences are different from the corresponding reduction potential of TNT thus, the characteristic peak potential (−0.31 V) can be used for selective determination of TNT molecules. To further ascertain the selectivity of developed sensor, amperometric measurement was carried at the characteristic reduction potential (−0.31 V) in the presence of above mentioned interferents with concentration equal to TNT. The current vs. time graph is shown in Fig. 7(c). As expected, the only change in the current response against time was noted with the addition of TNT, whereas the addition of other nitro-aromatic compounds had negligible effect on the measurement as observed from the linear portion of curve. This experiment further reflects the capability of Pd NCs/GCE to distinguish TNT from other nitro-aromatic compounds.

Fig. 7 (a) The response of Pd NCs/GCE for NB, 2 NT and 4 NT (b) 2,4 DNT, 2,6 DNT (c) amperometric graph for 2.5 ppm TNT in presence above mentioned interferents and (d) stability of Pd NCs/GCE after 100 cycles in 2.5 ppm TNT solution with 0.1 M PBS (pH 6.0) and 0.3 KCl.

The stability of the developed sensor system was studied by measuring 100 cycles of Pd NCs/GCE within 2.5 ppm TNT and 0.1 M PBS (pH = 6.0) containing 0.3 M KCl. Fig. 7(d) displays the variation in current density obtained before and after completion of 100 cycles. The insignificant variation in the recorded current with small RSD value (<1.5%) signifies the excellent stability of the developed sensor.

3.6 The determination of TNT in real matrix

To evaluate the capability of developed sensor for its practical applications, the sensor was tested in real matrix environment to assess the analytical reliability. The standard addition method was adopted with addition of 1.5, 2.5 and 3.5 ppm TNT in untreated tap water with 0.1 M PBS (pH 6.0) and 0.3 M KCl solution. The prepared samples were then analyzed using protocol mentioned in Section 3.2. The excellent agreement (98–99%) (Table 2) suggest analytical robustness of the developed sensor with promising applicability for real sample determination.

Table 2 Recoveries of TNT from tape water using Pd NCs/GCE sensor^a

Samples^n	Added TNT	Detected TNT	Recovery (%)
a n = no. of replications = 3.
Sample 1	1.5	1.45 ± 0.01	96
Sample 2	2.5	2.58 ± 0.05	103
Sample 3	3.5	3.49 ± 0.03	99.7

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4. Conclusion

In this study, sensor system based on Pd NCs modified GCE was constructed for highly sensitive and selective determination of TNT compound in aqueous environment. The study explored the sensing capability of Pd NCs in competition with Pd HNS and Pd–Pt NA. The experimental observations led to the conclusion that the electro-catalytic reduction of TNT molecule was greatly influenced by the shape and composition of nanostructures under investigation. The Pd NCs were observed to exhibit greater sensitivity compared to Pd–Pt NA which had lower sensitivity compared to Pd HNS. This excellent electro-catalytic potential of Pd NCs enabled sensitive quantification of TNT up to 0.01 ppm within the detection window of 0.1–7.0 ppm. Moreover, the sensor possessed good analytical capability to distinguish TNT from other nitro-aromatic compounds. Besides the excellent selectivity, the successful quantification TNT from real the environmental matrix (tap water) further reflects the analytical capability of the devised sensor for practical applications.

Acknowledgements

The authors would like to acknowledge the research fellowship from TUBITAK (BIDEB-2216) and Genetic and Bioengineering Department, Fatih University Istanbul for their facilities during this research.

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