Novel hydrazine hydrate sensor based on”fish-bait” capture mode

ABSTRACT

Coordination reaction is a usual way for the concentration of metal ions in aqueous solution. In this work, 5-(5-iodine-2-pyridine azo)-2,2 amie toluene (5-PADAT) was firstly casting onto carbon glass electrode (GCE), play a role of ‘bait’; then, immersed 5-PADAT modified GCE into immersed aqueous solution containing order of magnitude ppm Palladium ions (Pd²⁺, play a role of ‘fishes’); ‘Fishes’ were captured by ‘bait’ with the reaction process of 5-PADAT and Pd²⁺, leading to the concentration of Pd²⁺ onto the GCE; Cyclic voltammetry was employed to reduce Pd²⁺ in situ, formed Pd nanoparticles (PdNPs) modified GCE. It can be used to detect the oxidation of hydrazine hydrate at a very low potential, 0.1 V.

KEYWORDS:

• “Fish-bait” capture mode
• coordination reaction
• pyridine azo reagent
• hydrazine hydrate
• metal ions

Hydrazine (N₂H₄) is employed as a starting material for many derivatives used in chemical industry and pharmaceutical field. Because of its hyper toxicity, the U.S. Environmental Protection Agency has classified hydrazine as an irritant and group B2, probable human carcinogen. There are many analytical methods used for the determination of hydrazine which include: spectrophotometry, chromatography, fluorescence, surface-enhanced Raman spectroscopy, chemiluminescence, electrochemistry and et al [1]. The electroanalytical methods [2] are the simplicity, the rapidity and the low cost, which offer the opportunity for portable, cheap and rapid methodologies. It is observed that a large proportion of the work developing electrochemical sensors for hydrazine focus on the use of metallic nanoparticles and some other surface modifications. Precious metal nanoparticles, such as AuNP [3]，PdNPs [4] were used. Bard explored that precious metal nanoparticles freshly oxide product play as active site. Metal oxide also can be used as catalysts for electrochemical oxidation of N₂H₄, such as NiOx [5], TiO₂ [6]. The oxidizing capacity lies in not only the physical and chemical properties, but also the geometric configuration of the constructed electrochemical platform. The fabrication of core-shell structure transition metal sulphides Co₉S₈ promotes the charge, mass transfer, and protects the nanocrystals from damage [7].

Palladium nanoparticles in the form of a layer on the surface of an electrode reduced overpotential of oxidation of hydrazine arising from the increased surface area of the interface and partly from an increased catalytic activity of the nanoparticles [8]. Coordination reaction involves inorganic, analytical, organic, physicochemical and many other fields [9]. Coordination reaction has an important effect on the structure of nanomaterials [10]. With the booming of materials science, it has been used in the preparation of functionalized nanomaterials [11,12]. Natural species can precisely manipulate the complexation between metal ions and biomacromolecules to achieve various functions [13]. Simulate the process，Ding prepared functionalized gold nanoparticles clusters，using the coordination between protein and gold nanoparticles. A double-signal mercaptans sensor with electrochemical blocking and fluorescence was constructed base on it. Enzymes, rich in nitrogen atoms in their amide and amine groups, were suggested to form complexes with copper ions via coordination interaction. Ligand molecule can serve as a glue to drive the nucleation of primary nanoparticles, which contribute to the formation of the nanoflowers [14].

The coordination sites, coordination environment and conformation, influences structures of formed nanomaterial. The hydrophilic or hydrophobic surrounding affects the catalytic performance. The choice of coordination agents determined the final catalytic properties of materials. Our group devote themselves to synthesizing pyridine azo reagents with controllable selectivity towards different metal ions and applying them for the determination of heavy metal pollution in water [15–17]. Organic p – n bilayer functionalized Ag can improve electrocatalytic oxidation of hydrazine [18]. Mohiuddin [19] constructed Co(OH)₂ nanoparticles decorated benzaldehyde-functionalized graphene (denoted as RGO/DHB/Co(OH)2) electrochemical sensor for selective determination of N₂H₄, which showed broad linear concentration range from 5 to 1700 M at a lower limit of detection of 0.165 M with good sensitivity of 1446.82 A mM cm by < 2 s. {tetrakis [4-(4-(5-chloro-1 H-benzo[d]imidazol-2-yl) phenoxy phthalocyaninato] Co (II) (CoTPc)}(CoMPc) -{tris tert butyl phenoxy mono [4-(4-(5-chloro-1 H-benzo[d]imidazol-2-yl) phenoxy phthalocyaninato]} Co (II)(rGONS) gave catalytic rate constant and LOD of 1.37 × 10⁶ M⁻¹ s⁻¹ and 0.82 μM respectively for electrocatalytic detection of hydrazine, due to low symmetry [20]. Coordination reaction showed better selective in electrocatalytic detection of hydrazine. Whether selectivity and the sensitivity for the determination of hydrazine will be significantly enhanced by the modified pyridine azo reagents on electrode as trapping agent strategy?

The rational design of catalytically active sites in porous materials is essential in electrocatalysis. In this study, we try to fabricate novel electrochemical hydrazine hydrate sensor based on the coordination reaction between metal ions and pyridine azo reagents, to realize the removal and reuse of Pd²⁺ in water. The designing strategy is ‘kill two hawks with one arrow’. The construction method is designed as follows: (1) Take pyridine azo chromogenic agent, 5-(5-iodine-2-pyridine azo)-2, 4-diaminotoluene (5-I-PADAT) as ‘bait’ (the trapping agent), and modify it onto the surface of electrode; (2) Immersed it into an aqueous solution containing extremely low concentration (ppm level) Pd²⁺ as ‘fish’ (the captured agent) to enrich Pd²⁺ on the electrode surface; (3) PdNP modified electrode will be prepared by in situ electro-reduction; (4) The electrocatalytic oxidation behaviour of the modified electrode towards hydrazine hydrate will be studied.

Experimental

Apparatus and measurements

T1810 UV – visible spectrophotometer: Beijing spectrometer automation technology co., LTD， was used for the exploration of coordination mechanism of 5-PADAT and Palladium(II). Scanning electron microscopic (SEM) measurements were carried out on a scanning electron microscope (JEOL, JSM–6700F) at 15 kV. GCE of 3 mm diameter, before use, was first polished to a mirror – like with 1.0, 0.3 and 0.05 μm of Al₂O₃ slurry on a polish cloth, and rinsed with double – distilled water, then sonicate treated in ethanol and double-distilled water for 5 min, respectively. All electrochemical experiments were carried out on a CHI660 e electrochemical workstation (Shanghai CH Instrument Co. Ltd., China) using a three electrode system with a GGE as the working electrode, a saturated calomel electrode (SCE) as reference electrode and a platinum electrode as counter electrode. DL–180 ultrasonic cleaning machine (35 KHz, Zhejiang Haitian Electron Instrument Factory, China) was used to dissolve and form homogeneous solution. All the electrochemical experiments were conducted at room temperature (25 ± 2°C).

Reagents

PdCl₂ was gotten from Tianjin Yingda Sparseness and Noble Reagent Chemical factory (Tianjin, China). N₂H₄ was got from Luoyang Hao Hua Chemical Reagent Ltd (Tianjin, China). 5-I-PADAT was synthesized in our Laboratory by Huo Yanyan. All other chemicals were of analytical reagent grade and doubly distilled water was used in all the experiments. A 0.1 M sodium phosphate buffered saline (PBS) with various pH values were prepared by mixing stock standard solutions of Na₂HPO₄ and KH₂PO₄ and adjusting the pH value with 0.1 M H₃PO₄ and NaOH.

The preparation of nano palladium modified electrode

The preparation of 5-I-PADAT solution

Take 0.3450 g solid of 5-I-PADAT into little ethyl alcohol, heating with thermostatic water bath under string below 75°C. After 5 min, let the solution cool down and fill it to volume of 1 L with ethyl alcohol. Keep in dark place for use.

The preparation of palladium solution

Dissolve 0.0018 g PdCl₂ with 1:1 hydrochloric acid aqueous solution, heating to boiling, then let the solution cool down and fill it with distilled water to volume of 1 L. Keep in 4°C for use.

Electrodeposition of palladium onto electrode

GCE of 3–mm diameter, before use, was first polished to a mirror-like with 1.0, 0.3 and 0.05 mm Al₂O₃ slurry on a polish cloth, and rinsed with double-distilled water, then sonicated in ethanol and double – distilled water for 5 min, respectively.

Dilute 5-I-PADAT 10 times with ethyl alcohol. In the first stage, 10 μL of 5-I-PADAT solution was cast onto the surface of GCE by using a syringe to prepare 5-I-PADAT/GCE. Refrigerate and let dry naturally; Then, immerse 5-I-PADAT/GCE in PdCl₂ solution for 10 mins, to gather Pd²⁺ onto the surface of the GCE; Build a three-electrode system in the above solution, CV at the scan rate of 50 mV/s for 20 scans in the above solutions under voltage range of −1.2 V ~0.2 V to get PdNPs–5-I-PADAT/GCE. For comparison, PdNPs/GCE was prepared withouting the modification of 5-I-PADAT.

Results and discussion

“Fish-Bait” capture mode

Nitrogen atoms of pyridine azobenzene reagent are used as coordinating atoms to react with nitrophilic metals. Introducing halide atoms at 5-bit of pyridine rings, the coordination ability with metal ions become stronger, due to reduce the electron excitation energy of the molecule. Lone pair electrons of halide atoms in pyridine ring are easy to produce π-conjugation with the large bond of benzene ring. The unfilled d orbital of metal ions at transition zone can accept electronic from rich electronic of nitrogen at pyridine rings, azo groups and amine groups at adjacent sites. Pyridine azobenzene reagent modified electrode can capture metal ions at transition zone in aqueous solution through coordination reaction, just like the bait on the hook abduct fishes from water. The strategy of enriching metal ions onto the electrode surface is defined as ‘fish-bait’ capture mode.

In this work, 5-(5-iodine-2-pyridine azo)-2,2 amie toluene (5-PADAT) was firstly casting onto carbon glass electrode (GCE), play a role of ‘bait’. Then, immersed 5-PADAT modified GCE into immersed aqueous solution containing order of magnitude ppm Palladium ions (Pd²⁺); Coordination reaction took place between 5-PADAT and Pd²⁺, leading to the concentration of Pd²⁺ onto the GCE; Cyclic voltammetry was employed to reduce Pd²⁺ in situ, formed Pd nanoparticles (PdNPs) modified GCE, as shown in Scheme 1. Spectrophotometry was employed to investigate the optimal condition of coordination reaction. Hydrochloric acid of 1 mol/L, 5-PADAT of 1 × 10⁻³ mol/L and reaction time of 10 mins are used in the following section. Novel hydrazine hydrate sensor based on ’5-PADAT-Pd²⁺’ capture mode is a bifunctional sensor for metal ions and hydrazine hydrate at environmental pollutants determinant.

Scheme 1. Schematics of the fabrication process of the PdNPs-5-PADAT/GCE.

Characterizations of the PdNPs–5-I-PADAT/gce

Scanning electron microscopy (SEM) can clearly see the condition of the electrode surface. Nanospheres with diameter of 40 ~ 50 nm were scattered over the matrix, as shown in Figure 1(a). The low loading efficiency of Pd²⁺ through physical absorption leads to the formation PdNPs on PdNPs/GCE were sporadically distributed and inconformity. With the introduction of 5-I-PADAT as bait, the loading efficiency of Pd²⁺ through coordination reaction was enhanced, which leads to coverage uniformity of the formation PdNPs on PdNPs–5-I-PADAT/GCE. The nanospheres of PdNPs with diameter of 100 ~ 120 nm are densely and uniformly distributed on the electrode, as shown in Figure 1(b). The capture effect of 5-I-PADAT leads to Pd²⁺ load capacity became saturated on the surface.

Figure 1. SEM imagines of PdNPs/GCE (a) and PdNPs–5-I-PADAT/gce(b).

Electrocatalytic oxidation of N₂H₄

As shown Figure 2, remarkable oxide peaks are displayed at the potential range around 0.2 V. The results indicated the direct oxide of hydrazine hydrate were took place at the surface of the PdNPs–5-I-PADAT/GCE. The relative lower potential for oxidation can reduce the interference in the solution. With the increase of concentration, the current are increases significantly. Thus, hydrazine hydrate electrochemical sensor with high sensitivity was successfully constructed here.

Figure 2. Cvs of PdNPs–5-I-PADAT/gce in 0. 1 mol/L, pH 7.0 PBS at scan rates of 0.1 V/s with the adding of different N₂H₄ concentration (from a to m: 0, 0.0662, 0.130, 0.198, 0.269, 0.349, 0.433, 0.520, 0.615, 0.721, 0.864, 1.02, 1.2 μmol/L).

Amperometric curve is a much more sensitive method for quantitative analysis in electrochemistry. Effect of applied potential for the determination of N₂H₄ based on the PdNPs–5-I-PADAT/GCE was investigated at 0.05 V, 0.10 V and 0.15 V. With the addition of same concentration of N₂H₄, at the potential of 0.05 V and 0.10 V showed the some ratio current changes, as shown in Figure 3(a,b). The higher potential displays better stability than the lower, however. As displayed in Figure 3(c), when the work potential increase to 0.15 V, both of current change and stability are attenuation. And, PdNPs/GCE under the similar condition at 0.10 V showed terrible behaviours. Moreover, the baseline current of the signal became unstable, as shown in Figure 3(d). The results indicated that (1) PdNPs–5-I-PADAT/GCE show a much better oxidation behavior than PdNPs/GCE; (2) 0.10 V is the optimum for PdNPs–5-I-PADAT/GCE. 0.1 V was finally chosen as the applied potential throughout all the amperometric measurements.

Figure 3. Amperometric curves obtained upon the addition of an aliquot concentration of N₂H₄ into a continuous stirring of 0.1 M, pH 7.0 PBS under different potentials.

Figure 4 shows the amperometric current – time curves to study the performance of the fabricated electrodes on successive addition of N₂H₄. When N₂H₄ was added into the stirring 0.1 M, pH 7.0 PBS, quick responses to the substrate happened both at the PdNPs–5-I-PADAT/GCE and PdNPs/GCE. The proposed PdNPs–5-I-PADAT/GCE got the best amperometric response as expected. Comparing with PdNPs/GCE upon the addition of the same concentration gradient of N₂H₄, the PdNPs–5-I-PADAT/GCE showed about ten times higher response. The storage stability of the modified electrode was further investigated. The amperometric measurements were measured using the same electrode and it retained above 95% of its initial response stored at 4°C after 30 days. These results displayed that the sensor had a good stability.

Figure 4. Amperometric curves of PdNPs/GCE and PdNPs–5-I-PADAT/gce obtained upon the addition of an aliquot concentration of N₂H₄ into a continuous stirring of 0.1 M, pH 7.0 PBS.

Interference experiment

To apply sensor for determining hydrazine hydrate in environmental water samples, the influence of some substances on determining hydrazine was examined. Various interferent – to–analyte ratios of these ions caused less than±4% relative error for a hydrazine hydrate concentration of 10 μmol/L. The results were listed in Table 1. It is shown that most of the examined cations and anions do not interfere with hydrazine hydrate determination. The response of the proposed sensor was essentially selective.

Table 1. Effect of interfering substance on the detecting of 1 μmol/L hydrazine hydrate. Ratio denotes the ratio of the concentration between the interfering substance and hydrazine, i.e. [ion]/[N₂H₄].

Species	Maximum tolerable concentration ratio
K+,Na+,Mg^2+,Al^3+,Ca^2+,Mn^2+,Pb^2+,NO3-,Cl-,Br-,PO4^2-	100
Glucose, urea	100
Ag+,Cu^2+	80
Fe^2+,Fe^3+,Zn^2+	10
Thiocarbamide	10

Conclusions

In the present work, Pd²⁺ was successfully captured by 5-I-PADAT onto the GCE. Unpaired electron of azo groups and nitrogen atoms at pyridine ring can be packed to unfilled d orbital of Pd²⁺. The coordination reaction of 5-I-PADAT with Pd²⁺ enriched saturated adsorption of Pd²⁺ on the surface, then reduced to Pd⁰ by electrochemical process. The bait function of 5-I-PADAT successfully captured enough Pd²⁺ lead to ubiquitously distribution of Pd⁰ nanospheres on the surface. Compering with PdNPs/GCE, the fabricated N₂H₄ sensor based on PdNPs–5-I-PADAT/GCE, exhibited much higher sensitivity at very low applied potential. This was mainly due to: (1) The concentration of Pd²⁺ by 5-I-PADAT base on ‘fish-bait’ capture mode; (2) Steric effect of nonbonding sites effectively suppressed agglomeration. The ‘fish-bait’ capture mode as a novel sensing concept will pave the way for the synthesis of novel nanomaterials with unique morphology and properties, which will be employed to design novel electrochemical sensors. Further works are on our schedule.

Acknowledgments

The authors gratefully acknowledge the financial support of this project by by National Science Foundation of Shaanxi province for new young star of science and technology (No. 2018KJXX−090); National Science Foundation of Shaanxi province (NO. 2020JQ − 886, 2018JM3036); Scientific research plan projects of Department of Education in Shaanxi Province (NO.20JK0867); Xi’an Science and Technology Plan Project (22 G×FW0112); Dr. Startup funds from Xi’an University (No. 06005017) and the development transformation of key disciplines in Shaanxi provincial–analytical chemistry (No.09009001). The Youth Innovation Team of Shaanxi Universities (Environmental Pollution Monitoring and Control Innovation Team, 51).

Disclosure statement

No potential conflict of interest was reported by the authors.