Voltammetric determination of hydrogen peroxide at decorated palladium nanoparticles/poly 1,5-diaminonaphthalene modified carbon-paste electrode

In this work, palladium nanoparticles (PdNPs)/p1,5-DAN/ carbon paste electrode (CPE) and p1,5-DAN/CPE sensors have been developed for determination of hydrogen peroxide. Both sensors showed a highly sensitive and selective electrochemical behaviour, which were derived from a large specific area of poly 1,5 DAN and super excellent electroconductibility of PdNPs. PdNPs/p1,5-DAN/CPE exhibited excellent performance over p1,5-DAN/CPE. Thus, it was used for detecting hydrogen peroxide (H2O2) with linear ranges of 0.1 to 250 µM and 0.2 to 300 µM as well as detection limits (S/N = 3) of 1.0 and 5.0 nM for square wave voltammetry (SWV) and cyclic voltammetry (C.V) techniques, respectively. The modified CPE has good reproducibility, adequate catalytic activity, simple synthesis and stability of peak response during H2O2 oxidation on long run that exceeds many probes. Both reproducibility and stability for H2O2 detection are attributable to the PdNPs immobilized on the surface of p1,5-DAN/CPE. The modified CPE was used for determining H2O2 in real specimens with good stability, sensitivity, and reproducibility.

Electrochemical methods have important applications such as sample analysis as well as organic and inorganic synthesis [11].These methods are attractive for biological and environmental analysis because they are cheap, simple, fast, sensitive and selective [12].For example, electrochemical assays were used for H 2 O 2 detection, with high sensitivity and selectivity [13].H 2 O 2 interferes with oxygen and its oxidation peak, which allows adequate detection rather than using the reduction peak [14].The direct oxidation or reduction of H 2 O 2 is inconvenient because of the high overvoltage and slow kinetics of a bare electrode [15].Detectors, sensors and electrode syntheses depend mainly on carbon paste because of the wide voltage window, low price and low background intensity (current) [16].
The hollow polymer nanospheres allowed the dispersion of metal nanoparticles (NPs) [17].The fabricated sensors of metal NPs have many advantages which gives such a response [18].Palladium is a very important rare transition metal that has good catalytic activity and reported hetero-catalytic and electroanalytical behaviours [19].The physical and chemical properties of palladium (Pd) NPs are similar to those of platinum NPs, but their price is lower.PdNPs are ideal building blocks for designing and altering nanoscale structures for specific sensing applications.They have many interesting properties such as electrocatalytic behaviour, high specific surface area, strong adsorption ability, high conductivity, improved electron transfer and reduced overpotential in electrochemical reactions [20][21][22].Incorporating PdNPs in biosensors gave such high catalytic behaviour with good stability [23,24].Electrochemical and some chemical methods have been used to incorporate PdNPs into a conducting polymer matrix [25][26][27].Also, PdNPs could be dispersed in many other polymers [27,28].H 2 O 2 electrooxidation via PdNPs was reported [29][30][31].The direct electrooxidation of the three isomers of dihydroxybenzene at lower potentials and lower detection limits was studied on PdNPs/ poly1,5-diaminonaphthalene modified GC electrodes [32].In addition, many electrochemical sensors, which were based on p1,5-DAN, were previously investigated [33][34][35][36].
Modified CPE has been applied as an electrochemical sensor for the analysis of various biologically important compounds [11,12,[37][38][39][40][41][42][43] due to its low residual current, low cost, relative ease of electrode preparation and regeneration, and the porous surface.To the best of our knowledge, this is the first time to use PdNPs/p1,5-DAN on the surface of CPE for sensitive determination of H 2 O 2 .This approach is considered accessible and ecologically green for p1,5-DAN/CPE and PdNPs/p1,5-DAN/ CPE.PdNPs/p1,5-DAN/CPE have a larger surface area, rapid mass transfer and excellent electron transfer capability compared to p1,5-DAN/CPE, imparting excellent electrocatalytic performance toward H 2 O 2 sensing at 0.05 and −0.2 V using cyclic voltammetry (CV), and −0.12 V for square wave voltammetry (SWV) techniques.
A recent study has developed a non-enzymatic amperometric sensor by stabilizing gold (Au) NPs on a porous titanium dioxide (TiO 2 ) nanotube (NTs) electrode (TiO 2 -NTs).The aggregation was prevented by entrapping AuNPs on TiO 2 NTs.The sensor enhanced the electron transfer rate and the electrical conductivity, and generated a low detection limit of 104 nM [18].Also, nickel oxide NPs modified multiwalled carbon nanotubes were prepared and supported on glassy carbon electrode for H 2 O 2 sensing.The applied electrode showed good stability and reproducibility with LOD of 1.0 µM [44].The modification of glassy carbon electrode with colloidal microcatalyst of PdNPs decorated on polyaniline coated carbon microspheres was applied for H 2 O 2 sensing.This provides a large number of catalytic sites, high electrochemical surface area and excellent electrocatalytic activity toward H 2 O 2 reduction with LOD of 0.7 µM [25].

Preparation of working electrode
The manual blending and mixing of graphite powder with paraffin oil '70 : 30 wt./wt.' was accomplished using a mortar and pestle.Electrical contact was achieved using copper wire inside a glass tube with an internal radius of approximately 1.5 mm that enclosed the paste.After the bare electrode surface was smoothed on white paper, a smooth and shiny surface was observed.The preparation of p1,5-DAN/ CPE from a well-mixed solution of 1.0 M HClO 4 and 1.5 mM 1,5-DAN was employed using CV under the following conditions:15 cycles, potential range 0.0-0.8V and at a scan rate of 0.02 V.s −1 .The modified electrode was placed in a homogeneous mixture of 0.1 M HClO 4 and 2.5 mM PdCl 2 and analysed by CV for 25 cycles at a sweep rate '0.05 V.s −1 ' between '−0.35 and 0.65 V' for '25 cycles', as reported for PdNPs/p1,5-DAN/CPE [4].This method has received widespread attention because it is simple, and it can ensure the high purity of sensor and the selective position of Pd on the surface [23].The response of a modified electrochemical electrode is related to its physical morphology.As shown in figure 1a,b, the surface topographies of the modified electrode were analysed by SEM, which demonstrated significant differences in the surface structure of both p1,5-DAN/CPE and PdNPs/p1,5-DAN/CPE, respectively.

Results and discussion
SEM shows that the morphologies and textures of both layers were not similar (figure 1).The structure of p1,5-DAN (figure 1a) shows whitish grey, spherical, amorphous open structure porous accumulations with a large lumpy shape [36,45].Figure 1b presents PdNPs deposited on p1,5-DAN/ CPE, in which Pd particles appear as white light-grey spherical spots with a mean diameter of approximately 46 nm dispersed on the modified electrode surface.
These data show that PdNPs/p1,5-DAN/CPE is affected by the incorporation of PdNPs into the p1,5-DAN structure.This structure gives a high number of effective active sites that act as supporting sites for the deposition of PdNPs [46].The conducting polymer increased active surface area and the catalytic activity.Figure 1d shows that EDX identified the presence of PdNPs in the polymeric matrix where the PdNPs were dispersed in the p1,5-DAN polymeric matrix at 2.4% for PdNPs/p1,5-DAN/CPE.

Determination of the electroactive surface area
PdNPs/p1,5-DAN/CPE showed a smaller peak voltage separation 'ΔE p = 0.099 V, with a high redox intensity when tested in 1 × 10 −6 mol.cm −3 [Fe(CN) 6 ] −3/−4 ' in CV, which was comparable to the bare CPE (figure 2a).It exhibited better redox kinetics due to better current response, lower oxidation potential and smaller peak to peak separation and marked as quasi-reversible redox process and its redox parameters were enhanced sufficiently because of their high electric conductivity.
Equation (3.1) shows the calculation of the active surface area of the modified electrode using the Randles-Sevcik equation [47]: These results suggest an increase in the active surface area of the modified electrode by 12-fold compared to the bare CPE.The voltage peak difference was approximately 0.192 V where formal potential E 0 was 0.14 V for bare CPE, as shown in figure 2b.After the CPE modification with p1,5-DAN, there is a small decrease of ΔE p ∼0. 16 V with E 0 of 0.124 V (figure 2c), whereas the ΔE p decreased to approximately 0.099 V with E 0 of 0.25 V after the deposition of palladium nanoparticles (PdNPs), as shown in figure 2d [48].ΔE p is inversely proportional to the electron transfer rate [49].
The bare CPE showed less electron transfer at the modified electrode surface.Accordingly, PdNPs/ p1,5-DAN/CPE demonstrated the highest electron transfer rates from the lowest ΔE p ∼ 0.099 V (figure 2d ).Overall, depositing PdNPs on p1,5-DAN/CPE improved the conductivity and electrochemical properties of the catalyst.

Effect of the electrolyte concentration on the redox behaviour of H 2 O 2
The strength of the basic solution influences the H 2 O 2 decomposition rate, where the decomposition rate in a LiOH solution is 4-5 times higher than in distilled water, pH (7.0), as reported by Haines et al. [50].Navarro et al. [51] investigated the optimal H 2 O 2 decomposition in NaOH solutions with pH = (11.5-11.7).H 2 O 2 oxidation in more alkaline solutions was enhanced at lower voltages, as reported by Katsounaros [52].This study examined the effect of the electrolyte concentration using sodium hydroxide solutions.Figure 3a presents the CV traces of 24 µM H 2 O 2 decomposition on the p1,5-DAN/CPE electrode in various NaOH concentrations (0.02-0.5 M) with pH ranges from 12.3 to 13.7.The oxidation current, Ipa, increased from 76 to 118 µA, and the peak voltage difference (ΔE p ) decreased from 0.751 to 0.544 V. Figure 3b

Effect of the pH of NaOH on the redox behaviour of H 2 O 2
The pH of the supporting electrolyte influenced the value of oxidation and reduction peak voltage of H 2 O 2 , suggesting the involvement of protons in the redox reaction [53].According to figure 4a,c, the anodic and cathodic peak currents for redox reaction of H 2 O 2 at the two electrodes, one of palladium   royalsocietypublishing.org/journal/rsos R. Soc.Open Sci.11: 231894 and the other of palladium absence, increased with increasing pH from (12.3 to 13.7) and the maximum current observed at pH (13.7). Figure 4b,d showed that the anodic and cathodic peak potentials presented a dependence on pH throughout the studied range (12.3-13.7),with shifting of anodic and cathodic potential to lower energy.Therefore, pH (13.7) was determined as the optimal pH value for the electrolyte [54].

Effect of the scan rate
PdNPs/p1,5-DAN/CPE could catalyze the H 2 O 2 redox reaction at lower potentials with sufficiently high currents.Figure 5 shows the CV trace of 4.8 µM H 2 O 2 with various sweep rates from 0.025-0.50Vs −1 at PdNPs/p1,5-DAN/CPE.
The square root of the scan rate from 0.1-0.5 Vs −1 and the anodic and cathodic pulse current showed a linear relationship with increasing the scan rate.Hence, the adsorption-controlled process changed to a diffusion-controlled process [32].Thus, the process was reported as diffusion controlled.Figure 6b presents the linear XY graph between the square root of the scan rate and the H 2 O 2 peaks intensity with I pa (µA) = −8.006+ 9.44 v 1/2 (mVs −1 ) and I pc (µA) = −25.43-44.2v 1/2 (mVs −1 ).By applying the for reduction [47], which was recently reported [56].
By plotting the logarithm of the scan rate versus the logarithm of the current intensity, the H 2 O 2 redox process at PdNPs/p1,5-DAN/CPE was determined to be a diffusion-controlled process (figure 6c).The slope was approximately 0.48, indicating that the reaction is diffusion-controlled [57].
A graph of the logarithm of the scan rate, log v and peak voltage (E p ) for the H 2 O 2 redox process at PdNPs/p1,5-DAN/CPE was linear at a high scan rate, as shown in figure 6d.The cathodic voltages shifted to the negative direction, whereas the anodic voltages shifted to the positive direction by the gradual increase in scan rate.

Effect of the amplitude on the redox behaviour of H 2 O 2
The pulse amplitude is a parameter that strongly affects the peak current in square wave voltammetry affecting the sensitivity of the peak.Larger amplitudes have offered a better peak current, but as amplitudes become larger, the background current increases, the peak appears broader and shifts from its proper location it.Thus, if inappropriate amplitudes are used, it can lead to inaccurate results [58]. Figure 7 represents SWV of 19.5 µM H 2 O 2 in 0.5 M NaOH with changing amplitude from (a = 0.02, b = 0.05 to c = 0.07 V) at (a) p1,5-DAN/CPE and (b) PdNPs/p1,5-DAN/CPE.We found that the peak current of H 2 O 2 increased with increasing pulse amplitude from 0.02 to 0.07 V.When the pulse amplitude was higher than 0.05 V, the oxidation peak was wider and broader [33]. .By modification of CPE by only p1,5-DAN/CPE, an anodic peak appeared at E pa = 0.222 V, I pa = 415 µA and a cathodic peak of E pc = −0.322V, I pc = 420 µA.In the case of adding a second layer of palladium nanoparticles onto the polymer film, a well-characterized and enhanced anodic peak was observed with E pa = 0.05 V, I pa = 466 µA, and a cathodic peak of E pc = −0.2V, I pc = −705 µA.The peak-to-peak separation (ΔE) and formal potential were 0.25 and 0.125 V for PdNPs/p1,5-DAN/CPE compared to 0.544 and 0.272 V for p1,5-DAN/CPE.The PdNPs impart very high electrocatalytic activity for H 2 O 2 catalytic oxidation and reduction at low redox potentials compared to p1,5-DAN/CPE.The PdNPs/ p1,5-DAN/CPE improved the CV sensitivity for H 2 O 2 because of the ease of electron transfer in catalytic reactions [59].

Behaviour of square wave voltammetry 'SWV'
The recognition of the background current and low detection limit were examined by SWV [33].This technique was employed for H 2 O 2 oxidation at PdNPs/p1,5-DAN/CPE because of its low contribution to the background current and high current sensitivity.
Figure 9 presents the SWV trace of PdNPs/p1,5-DAN/CPE and p1,5-DAN/CPE in 0.5 M NaOH containing 3.2 µM H 2 O 2 , the oxidation peaks with anodic currents were at −0.12 and 0.115 V for 40.6 and 19.5 µA, respectively.From the anodic peak potential and current, PdNPs/p1,5-DAN/CPE can improve H 2 O 2 detection corresponding to the CV results.Figure 10b presents the CV traces for the H 2 O 2 concentrations in 0.5 M NaOH at PdNPs/p1,5-DAN/ CPE.The inset showed the linear relationship between the redox concentrations and their currents.The linear regression equation for anodic oxidation was I pa (µA) = −5.9+ 7.5 C (µM) with a correlation coefficient of 0.99 where I pc (µA) = −54.8−15.4C (µM) for cathodic reduction.The limit of detection (LOD) and limit of quantification (LOQ) for H 2 O 2 oxidation CV were 5.0 and 16.6 nM, respectively, whereas they were 4.0 and 13.3 nM for its reduction, respectively.The LOD and LOQ values were calculated using equations (3.6) and (3.where s is the blank standard deviation and m is the linear's slope [16].These results suggest that the PdNPs/p1,5-DAN/CPE can effectively mediate electron transfer between the electrode and H 2 O 2 , showing clear catalytic activity toward electrochemical redox determination.This approach can be applied to the determination of a lower H 2 O 2 concentration.The electrochemical H 2 O 2 reduction mechanism was reported previously [60,61] as equations (3.8)-(3.10).
The polymer film (1,5-DAN) has many imino groups (─NH) which have constructed hydrogen bonds leading to a decrease in the hydroxyl bond energy through an O─H ….NH bond [32].Moreover, PdNPs have created more O─H ….PdNPs bonds which helped in the electron transfer [62].
This electron transfer has gained by adsorbed H 2 O 2 on the electrocatalyst surface which produces (OH) ads and OH ─ as in equation (3.8).Subsequently, another electron is received by (OH) ads resulting in H 2 O formation.In all processes, the reaction rate is influenced primarily by two factors : (1) H 2 O 2 adsorption at the electrocatalyst surface and (2) electron transfer from the electrocatalyst to the (OH) ads .Thus, the electrocatalyst must enhance the adsorption and electron transfer to establish the electrocatalytic reduction successfully [63].

Sensitivity ¼ [m] [A] ð3:11Þ
where m is the linear slope and A the electrode active area., respectively, at p1,5-DAN/CPE.However, the values for PdNPs/ p1,5-DAN/CPE were 1.0 nM, 3.32 nM and 45 000 µAmM −1 cm −2 , respectively.The Pd in the hollow polymer matrix was responsible for the excellent performance, because it increases the surface area and provides many active sites for the catalytic reactions that in turn improves the sensitivity, selectivity and conductivity of the sensor [32].

Real samples analysis
This study examined H 2 O 2 in various spiked water sources (canned, underground, tap water) using the very good response electrode PdNPs/p1,5-DAN/CPE.No permissions were required prior to conducting field studies.A 10 ml cell of a real sample of 0.5 M NaOH was used for the spiked sample 0.1 mM H 2 O 2 / 0.5 M NaOH using the standard addition method, while SWV was applied using the PdNPs/p1,5-DAN/CPE.

Conclusion
In this study, the double character of DAN (a hollow film) and PdNPs improved the electrochemical redox response toward H 2 O 2 via increasing the effective surface area and electron transfer.The PdNPs/p1,5-DAN/CPE has become a promising sensor to determine H 2 O 2 in real samples, environmental samples, and in enzymeless catalyzed reactions due to its advantages of high sensitivity, reproducibility, good stability and anti-interference ability against many interferents.The assay was simple and inexpensive for H 2 O 2 determination with a lower detection limit and good precision.royalsocietypublishing.org/journal/rsos R. Soc.Open Sci.11: 231894

Figure 8
Figure 8 shows the CV response of H 2 O 2 , representing a typical CV trace of 78.3 µM H 2 O 2 in 0.5 M NaOH at (a) PdNPs/p1,5-DAN/CPE and (b) p1,5-DAN/CPE, at a sweep rate of 0.05 Vs −1.By modification of CPE by only p1,5-DAN/CPE, an anodic peak appeared at E pa = 0.222 V, I pa = 415 µA and a cathodic peak of E pc = −0.322V, I pc = 420 µA.In the case of adding a second layer of palladium nanoparticles onto the polymer film, a well-characterized and enhanced anodic peak was observed with E pa = 0.05 V, I pa = 466 µA, and a cathodic peak of E pc = −0.2V, I pc = −705 µA.The peak-to-peak separation (ΔE) and formal potential were 0.25 and 0.125 V for PdNPs/p1,5-DAN/CPE compared to 0.544 and 0.272 V for p1,5-DAN/CPE.The PdNPs impart very high electrocatalytic activity for H 2 O 2 catalytic oxidation and reduction at low redox potentials compared to p1,5-DAN/CPE.The PdNPs/ p1,5-DAN/CPE improved the CV sensitivity for H 2 O 2 because of the ease of electron transfer in catalytic reactions[59].

3. 11 .
Stability and reproducibility of the PdNPs/p1,5-DAN/CPEThe stability, reversibility and reproducibility of PdNPs/p1,5-DAN/CPE are important factors for practical applications[80].Figure12presents the CV traces of 4.8 µM H 2 O 2 in 0.5 M NaOH for 30 successive cycles.The antifouling surface of the electrode was indicated by stable redox voltage and peak intensity.In addition to the intra-day precisions, the convenient tools for checking the electrode confirmed its reproducibility.The precision of the assay was tested by examining the sensor behaviour in one determined solution four times.The good precision of the assay was highlighted by the low RSD value of 0.62%.Ten different PdNPs/p1,5-DAN/CPE were prepared for the evaluation of reproducibility, five for intra-day and five for inter-day measurement.Each prepared electrode was used to measure the current intensity response of 4.8 µM H 2 O 2 in 0.5 M NaOH.For the five intra-day fabricated electrodes the RSD of the five current intensities was 0.62% and 0.85% for the five inter-day prepared electrodes.
Table 2 lists the average recoveries of all specimens repeated four times.Data analysis showed that the fabricated modified electrode could determine different H 2 O 2 concentrations in real samples.The good recovery and response, easy preparation, and low cost give a chance for the PdNPs/p1,5-DAN/CPE for its use in industry as a fast detector of H 2 O 2 .CPE containing 4.97 µM H 2 O 2 .Table 3 confirms the absence of interference with H 2 O 2 peak current, only a signal change ≤5%.Thus PdNPs/p1,5-DAN/CPE has good selectivity toward H 2 O 2 determination.