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.

Hydrazine (N 2 H 4 ) 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, surfaceenhanced 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 N 2 H 4 , such as NiOx [5], TiO 2 [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 Co 9 S 8 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][16][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 N 2 H 4 , 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 1446.82A mM cm by < 2 s. {tetrakis [4-(4-(5-  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 Pd 2+ 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) Pd 2+ as 'fish' (the captured agent) to enrich Pd 2+ 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.

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 Al 2 O 3 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
PdCl 2 was gotten from Tianjin Yingda Sparseness and Noble Reagent Chemical factory (Tianjin, China).N 2 H 4 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 Na 2 HPO 4 and KH 2 PO 4 and adjusting the pH value with 0.1 M H 3 PO 4 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 PdCl 2 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 Al 2 O 3 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 PdCl 2 solution for 10 mins, to gather Pd 2+ 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.

"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 (Pd 2+ ); Coordination reaction took place between 5-PADAT and Pd 2+ , leading to the concentration of Pd 2+ onto the GCE; Cyclic voltammetry was employed to reduce Pd 2+ 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-Pd 2+ ' capture mode is a bifunctional sensor for metal ions and hydrazine hydrate at environmental pollutants determinant.

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

Electrocatalytic oxidation of N 2 H 4
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.
Amperometric curve is a much more sensitive method for quantitative analysis in electrochemistry.Effect of applied potential for the determination of N 2 H 4 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 N 2 H 4 , 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 4 shows the amperometric current -time curves to study the performance of the fabricated electrodes on successive addition of N 2 H 4 .When N 2 H 4 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 N 2 H 4 , 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.

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.

Conclusions
In the present work, Pd 2+ 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 Pd 2+ .The coordination reaction of 5-I-PADAT with Pd 2+ enriched saturated adsorption of Pd 2+ on the surface, then reduced to Pd 0 by electrochemical process.The bait function of 5-I-PADAT successfully captured enough Pd 2+ lead to ubiquitously distribution of Pd 0 nanospheres on the surface.Compering with PdNPs/GCE, the fabricated N 2 H 4 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 Pd 2+ by 5-I-PADAT base on 'fishbait' capture mode; (2) Steric effect of nonbonding sites effectively suppressed agglomeration.The 'fishbait' 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.

Figure 3 .
Figure 3. Amperometric curves obtained upon the addition of an aliquot concentration of N H 4 into a continuous stirring of 0.1 M, pH 7.0 PBS under different potentials.

Figure 4 .
Figure 4. Amperometric curves of PdNPs/GCE and PdNPs-5-I-PADAT/gce obtained upon the addition of an aliquot concentration of N 2 H 4 into a continuous stirring of 0.1 M, pH 7.0 PBS. 6