Fabrication of selective and sensitive chemical sensor probe based on ternary nano-formulated CuO/MnO2/Gd2O3 spikes by hydrothermal approach

In this approach, thin spikes (NSs) of ternary nano-formulated mixed CuO/MnO2/Gd2O3 were synthesized by the hydrothermal approach for efficient detection of 3-methoxyphenyl hydrazine (3-MPHyd) chemical from various environmental samples. The NSs were systematically characterized by using XPS, EDS, TEM, FTIR, UV/vis, and XRD. The fabricated NSs onto the glassy carbon electrode (GCE) was successfully applied for the selective and sensitive detection of 3-MPHyd in the phosphate buffer system (PBS), which displayed the highest sensitivity, good selectivity with ultra-trace detection limit, high stability, good reproducibility, and quick response time. The real environmental samples were tested for validation from stand point of the ternary doped nanomaterials for sensing in the practical applications using by electrochemical method.

Hydrothermally synthesis of CuO/MnO 2 /Gd 2 O 3 NSs. The inorganic salts copper chloride (CuCl 2 ), manganese chloride (MnCl 2 ), gadolinium chloride (GdCl 3 ) and alkali ammonium hydroxide (NH 4 OH) were used to prepare CuO/MnO 2 /Gd 2 O 3 NSs by solvo/hydrothermal method at low temperature. The solvothermal process was widely used efficient method to fabricate nanomaterials of metal oxides, and the resultant guest or doped metal oxides are smaller in size as well as phase formation. Following this method, 100.0 mL of 0.10 M CuCl 2, 100.0 mL of 0.10 M MnCl 2 , 100.0 mL of 0.10 M GdCl 3 and 100.0 mL of 0.10 M NH 4 OH were prepared in a different four 200.0 mL beaker with de-ionized water and resultant solutions were kept with continuous magnetic stirring. Another 250.0 mL of conical flask was taken and 50.0 mL of each prepared metallic salt solution was added. Then the mixture was shacked with continuous magnetic stirring onto the hot plate. To obtain the co-precipitation of metal hydroxides, the prepared 0.10 M NH 4 OH was added slowly and at the pH value 10.5, all metal hydroxides were precipitated out in conical flask. Then the total solution was kept at 80 °C on the hot plate with continuous magnetic stirring around 6 h. As-prepared participate of metal hydroxides were washed thoroughly by de-ionized water and kept it to dry at room condition for overnight. Consequently, the powdered sample was heated for calcination at 510 °C for 6 h. Under higher temperature, the metal oxides is transform to Scientific Reports | (2020) 10:20248 | https://doi.org/10.1038/s41598-020-76662-6 www.nature.com/scientificreports/ crystalline metal oxide i.e., CuO/MnO 2 /Gd 2 O 3 nanostructure shapes, which contains the higher metallic-ions. The prepared material was properly grained into fine powder of nano-sized materials for details characterization.
The following reactions may happen: In the aqueous medium: In furnace: The Ks was low (Ks = 2.2 × 10 -20 in Cu(OH) 2 , 2.0 × 10 -13 for Mn(OH) 2 and 1.88 × 10 -23 for Gd(OH) 3 30 . Metal ions were precipitated out quantitatively as various oxides. The crystal formation was happened initially, where an aggregation to the Gd(OH) 3 was started. In the reaction system, pH was continued to enhance, then the Cu(OH) 2 was started to precipitate, which was re-aggregated onto the Gd(OH) 3 crystallites. Further increasing of pH, Mn(OH) 2 is also participated out to form aggregation with other two metal hydroxides. The formation of NSs is similar with the previously reported article 31 . The synthesized NSs were characterized in terms of elemental composition, crystallinity, optical property, morphology, structure, and functional properties. Later, CuO/MnO 2 / Gd 2 O 3 NSs were applied to detect 3-MPHyd by reliable electrochemical method at room conditions. This is the first time, the produced CuO/MnO 2 /Gd 2 O 3 NSs were implemented for the selective determination of 3-MPHyd for environmental safety by electrochemical method.

Fabrication of CuO/MnO 2 /Gd 2 O 3 /Nafion/GCE sensor probe.
The ternary doped materials based on the NSs of CuO/MnO 2 /Gd 2 O 3 was successively implemented to determine the target environmentally unsafe 3-MPHyd in reaction medium. To prepare the working electrode for 3-MPHyd detection, the ethanolic slurry of CuO/MnO 2 /Gd 2 O 3 NSs was put onto the GCE. Platinum wire (Pt-wire) was used as a counter electrode. The dispersed materials was attached between NSs of ternary metal oxides and GCE by air dry initially. It was fabricated on the flat GCE and dried in air for complete thin-film formation. Later, after drying completely, 1.0 μL of 5.0% Nafion (ethanolic) was dropped onto the fabricated electrode surface and waited until dry it completely. Here, nafion is used as a chemical glue for the stable attachment of ternary materials onto the surface of flat-GCE. Then the dried fabricated electrode was used as working electrode in this investigation. The electrochemical cell was composed by CuO/MnO 2 /Gd 2 O 3 /binders/GCE sensor probe as working electrode. The fabrication scheme is presented in the Fig. 1. The target analyte 3-MPHyd was used to prepare the solution in di-ionized water on the concentration range from 1.0 mM to 1.0 pM and this formulated solutions of 3-MPHyd (lower to higher concentration) were investigated into electrochemical cell (chemical material). Then the linear- → Cu(OH) 2 /Mn(OH) 2 /Gd(OH) 3(aq) ↓ + NH 4 Cl (aq) .  www.nature.com/scientificreports/ ity was calculated from the linear plot by using regression co-efficient (r 2 ). The other analytical properties of 3-MPHyd chemical sensor such as LDR and LOD were estimated according to ratio of 3 N/S. where λ is wavelength (1.5418 Å) and β is width at half, according to the apex peak, and θ is the diffracting edge 37 . Here, the determined crystallinity was 43. Binding energy analysis. The XPS is defined in Fig. 4 are also investigated to evaluate the chemical composition, electronic and valance states of prepared CuO/MnO 2 /Gd 2 O 3 NSs. As it is shown the full spectrum ( Fig. 4a), Cu2p, Mn2p Gd3d and O1s core level of spin orbitals are investigated. The observed O1s peak at 530.0 eV, which was presented in Fig. 4b and assigned to the O 2−42,43 . The Cu2p spin orbital ruptures into Cu2p 3/2 and Cu2p 1/2 as depicted in Fig. 4c. Here, it displays the high resolution spectrum of Cu 2p, separated into Cu 2p 3/2 and Cu 2p 1/2 at 930.5 eV and 952.1 eV, respectively ( Fig. c1 and c2). The distance between these Cu 2p main peaks positions is 21.6 eV, which agrees well with previous reports about CuO spectrum. It is also denoted to the existence of Cu 2+ chemical state as an indication of the formation of CuO, which is matched to the reported literatures [44][45][46][47][48][49][50][51][52] . Moreover, additional confirmation of CuO state was seen with the broad satellite peaks at a higher binding energy than the main peaks. The main peak of Cu 2p 3/2 at 930.5 eV was accompanied by satellite peaks on the higher binding energy side at 939.2 eV, 941.5 eV and 943.1 eV, which suggests the existence of CuO. From the full spectrum, we can clearly see that the main peak of Cu 2p 1/2 at 952.1 eV, which also confirms the presence of CuO. The XPS spectrum is also exhibited the two major peaks of Mn2p orbital and the resultant spectrum of Mn2p is represented in Fig. 4d. As evaluated, the spin energy of Mn2p 3/2 (641.2 eV) and Mn2p 1/2 (653.1 eV) are also the adjacent position with the reported data for MnO 2 45,46 , which is presented separately in Fig. 4d1,d2. In Gd3d spectra (Fig. 4e), two peaks are found, where the binding-energy of strong peak at ~ 1186.0 eV is respon- www.nature.com/scientificreports/ The pictographic representation (Fig. 5a) and mechanism (Fig. 5b) of the CuO/MnO 2 /Gd 2 O 3 NSs modified electrode of 3-MPhyd chemical material is depicted in Fig. 5. As clarified in Fig. 5c, the electrochemical signalling  www.nature.com/scientificreports/ data was illustrated with prepared NS of CuO/MnO 2 /Gd 2 O 3 , which is exhibited the higher current response compared to pure CuO and MnO 2 . Here, the oxidation reaction of 3-MPHyd onto surface of CuO/MnO 2 /Gd 2 O 3 NSs/Nafion/GCE into the buffer system is proposed and presented below according to Eq. (12). According to the electrochemical oxidation process, targeted 3-MPHyd molecule is oxidized, hence released the electrons. Thus it is increased the electrochemical response with the CuO/MnO 2 /Gd 2 O 3 NSs/Nafion/GCE sensor probe in the electrochemical system during measurement of resultant current. As a result, in contact with the CuO/MnO 2 / Gd 2 O 3 NS surface, the target analyte 3-MPHyd is directly oxidized by releasing two electrons onto the sensor surface of CuO/MnO 2 /Gd 2 O 3 NSs/Nafion/GCE probe, which is measured during the electrochemical measurement at room conditions. During the oxidation of 3-MPHyd, the resultant current is significantly increased by producing ammonia, water, and carbon dioxide into the electrochemical process.

Results and discussion
The synthesized CuO/MnO 2 /Gd 2 O 3 NSs/binder/GCE is not equally given electrochemical response in the full range in buffer system. The invented working electrode was investigated in alkaline and acidic media, and it was observed that the chemical material was exhibited the maximum electrochemical response in pH 7.0. The pH optimization performance is illustrated in Fig. 6a. To obtain the selectivity, the fabricated working electrode based on CuO/MnO 2 /Gd 2 O 3 /binder/GCE was performed in presence of various toxins such as 2,4-DNP (2,4-dinitrophenol), 3,4-DAT, pyridine, BH, 3-CP, THF, methanol, 3-MPHyd, and AH. As it is depicted in Fig. 6b, 3-MPHyd was displayed with the highest electrochemical responses. Also the most important analytical characteristic of chemical material is the ability to reproducible performance repeatedly. This performance of sensor was executed in 0.1 nM concentration of 3-MPHyd solution. The outstanding reproducibility was observed, which is shown in Fig. 6c.
The calculated RSD (relative standard deviation) is 1.10%, which is measured at + 1.0 V. The response time with the fabricated working electrode based on CuO/MnO 2 /Gd 2 O 3 /binder/GCE is 9.25 s. It was evaluated under the certain amount of 0.1 nM of 3-MPHyd solution, which is shown in Fig. 6d. The fabricated sensor is very fast response towards the target analyte with CuO/MnO 2 /Gd 2 O 3 /binder/GCE sensor probe by electrochemical method. After the 9.25 s, signal become sensor response become stable and flat, due to the saturation of contact surface with target analyte. The fabricated materials CuO/MnO 2 /Gd 2 O 3 /binder/GCE was studied and (12) www.nature.com/scientificreports/ compared in presence of various hydrazine derivatives (Fig. 7a). It was found that 3-MPHyd shows the highest electrochemical response (Fig. 7a) compared to blank solution (without 3-MPHyd), only hydrazine and phenylhydrazine derivatives in the identical conditions. Additionally, a control experiment has been performed with the only GCE, GCE/Nafion, and GCE/Nafion/CuO/MnO 2 /Gd 2 O 3 electrodes in the identical conditions in presence of target 3-MPHyd chemical, which is presented in the Fig. 7b. It is observed that the CuO/MnO 2 / Gd 2 O 3 fabricated glassy carbon electrode is showed the highest electrochemical current compared to only GCE and GCE/Nafion electrodes. The materials fabricated CuO/MnO 2 /Gd 2 O 3 /binder/GCE sensor probe intra-day reproducibility (Fig. 8a) and inter-day validity (Fig. 8b) have been also studied and presented in Fig. 8. According to these studies, the sensor probe is reproduced almost the similar response in the same day in different measurements in the identical conditions. On the other hand, fabricated CuO/MnO 2 /Gd 2 O 3 /binder/GCE sensor probe is exhibited the almost similar reproducible responses in different inter-day measurement in the identical conditions, which is presented in the Fig. 8b.
As indicated in Fig. 9a, the electrochemical response of CuO/MnO 2 /Gd 2 O 3 NSs/binder/GCE electrode is the maximum at the lowest concentration of 3-MPHyd. To execute this performance, a range of 3-MPHyd solution was prepared based on the concentration (full range 1.0 mM to 1.0 pM) and electrochemical measurement was carried out in the range from 0.0 to + 1.5 V. The linearity (r 2 : 0.9919) of the calibration plot (Fig. 9b,c was drawn as current versus concentration of 3-MPHyd). The analytical parameters have been calculated from the calibration plot (Fig. 9b)    www.nature.com/scientificreports/ with CuO/MnO 2 /Gd 2 O 3 NSs were increased with increasing of the target 3-MPHyd concentration. In presence of higher concentration of target analyte, the resultant current is gradually increased due to the oxidation of 3-MPHyd during electrochemical process with CuO/MnO 2 /Gd 2 O 3 NSs. During the addition of analyte into the electrochemical solution, the oxidation current is increased gradually until 0.1 nM. After that the current response is found stable until 1.0 pM. No significant increase of current is occurred. A comparison between the electrodes fabricated on binary MnO/CuO, and ternary combinations of CuO/MnO 2 /Gd 2 O 3 NSs were studied, and it was found that CuO/MnO 2 /Gd 2 O 3 NSs/binder/GCE electrode was exhibited the highest electrochemical responses (Fig. 9d).
The possible reaction mechanism of the capturing under optimum condition is depicted in Fig. 5. The metal oxides nanomaterials are investigated as potential materials with various electroanalytical methods [57][58][59] . The significant application of CuO/MnO 2 /Gd 2 O 3 NSs materials was employed in the capturing of environmentally toxic compound. The CuO/MnO 2 /Gd 2 O 3 NSs materials are nontoxic, long-term stability, consistence, high electrochemical activity, nontoxicity and easy-to-use. The electrochemical method for the CuO/MnO 2 /Gd 2 O 3 NSs is considerably changed during the adsorption of 3-MPHyd as the target agent. Here, Table 1, it is presented about the materials for capturing of hydrazine and their derivatives by various electrochemical approaches [60][61][62][63][64][65][66][67][68][69][70][71][72] . In this nano-formulated ternary CuO/MnO 2 /Gd 2 O 3 NS materials have accomplished great deal of attention owing to their structural, chemical, optical, electrochemical, and morphological properties in terms of large-active surface area, high-stability as well as good porosity, and permeability [73][74][75][76][77][78] . This method has numerous benefits including easy and facile preparation, accurate control of the reactants temperature, easy to handle, one-step reaction, and high-porosity as well as porous natures [79][80][81][82] . Finally, this mixed CuO/MnO 2 /Gd 2 O 3 NSs material have also attracted substantial attention owing to their impending applications in fabricating chemical devices, opto-electronics, electro-analytical, selective detection of chemical and biochemical assays, hybrid-composites, electron-field emission sources for emission exhibits, and biochemical detections etc.
Real sample analysis. The ternary CuO/MnO 2 /Gd 2 O 3 NS materials fabricated electrode probe is potentiality depended on the real sample treating and others feasibility parameters [62][63][64][65] . To measure the 3-MPHyd in www.nature.com/scientificreports/ the real environmental sample with various concentrations, the fabricated material based on CuO/MnO 2 /Gd 2 O 3 NSs was used to detect in the industrial effluent (collected from the Jeddah Industrial Area, Saudi Arabia) and extracted samples. The collected industrial effluent was initially filtered to remove the floating large particles and then filtered sample was directly used for analysis. The extracted sample from plastic baby bottle, plastic water bottle, and PVC food packaging bags were also filtered and analysed with CuO/MnO 2 /Gd 2 O 3 NSs/Nafion/GCE sensor probe by electrochemical method. The analysis report is presented in the Table 2. The results clarified that the proposed NSs has high possibility to selective detection of 3-MPHyd significantly and efficiently.

Conclusions
In this study, selective 3-MPHyd chemical material based on nano-formulated CuO/MnO 2 /Gd 2 O 3 spike was fabricated and reported in details. The NSs of transition metal oxides were prepared by the hydrothermal approach in alkaline phase at low-temperature. The fabricated nanospikes were totally characterized by using FTIR, UV-Vis,