Rapid and sensitive detection of selective 1,2-diaminobenzene based on facile hydrothermally prepared doped Co3O4/Yb2O3 nanoparticles

In this approach, the performance of a newly developed sensor probe coated with low-dimensional Co3O4/Yb2O3 nanoparticles (NPs) in rapidly detecting 1,2-diaminobenzene was evaluated by an electrochemical technique. The sensor probe was fabricated by depositing a very thin layer consisting of synthesized Co3O4/Yb2O3 NPs using a 5% Nafion conducting binder onto a glassy carbon electrode (GCE). The facile hydrothermally prepared Co3O4/Yb2O3 NPs were totally characterized by conventional methods such as FTIR, UV-vis, TEM, XPS, EDS, and XRD analyses. The fabricated chemical sensor probe was found to exhibit long-term activity, stability in electrochemical response, good sensitivity (5.6962 μAμM-1cm-2), lowest detection limit (0.02±0.001 pM), and broad linear dynamic range (0.1 pM to 0.01 mM). The observed performances suggest that the newly introduced sensor could play an efficient role in detecting 1,2-diaminobenzene especially in healthcare and environmental applications on a broad scale.


Introduction
Generally, 1,2-diamino benzene (1,2-DAB) is an organic amine and industrially very important chemical intermediate. It has extensive usage in producing medicine, auxiliaries, pesticides, dyes, pigments, photosensitive materials and, so on [1]. 1,2-DAB has also been reported [2,3] as a carcinogenic and mutagenic substance causing hazardous effects in the human body through inhalation, ingestion, eye contact and, so on. Heavy exposure to1,2-DAB can often cause cancer and damage liver, respiratory, and digestive systems of the human body. The ACGIH (American Conference of Governmental Industrial Hygienists) has identified 1,2-DAB as a serious hazardous substance that causes environmental pollution [4]. Therefore, it drives the need to search for a sustainable method to detect 1,2-DAB efficiently. Currently, various numerical methods including liquid chromatography [5][6][7], spectrophotometry [8,9], capillary electrolysis [10], etc are widely used in detecting 1,2-DAB. The technical advantages of these methods include good sensitivity, high selectivity, and long-term stability. However, these approaches have different drawbacks including complicated detection process, specialized instrumentation, laborious, high cost, and so on [11]. To ensure the real-time detection of this hazardous toxin, the electrochemical method is most effective. Due to the attractive electrical and optical properties, the various nanostructured metal oxides such as Pd-Rh nano-frames [12], Au@Pt core/shell nanorods [13], Fe 3 O 4 magnetic nanoparticles [14], and tungsten carbide nanorods [15] have been recognized as potential sensing elements in the chemical sensor for detecting 1,2-DAB. The chemical sensor fabricated by Fe 3 O 4 doped functionalized multiwall carbon nanotubes composite (Fe 3 O 4 @f-MWCTNs) is reported for its efficient performance with the sensitivity of 2.8002 mAmM -1 cm -2 as well as the detection limit of 50.0 μM [16]. Similarly, Fe-MIL-88-H 2 O 2 -OPD based DAB (diaminobenzene) chemical is reported for its good performance with 50.0 nM-30.0 μM LDR and 46.0 nM DL [17].
In the present study, Co 3 O 4 /Yb 2 O 3 NPs/Binder/GCE based on a novel electrochemical sensor is introduced for detecting 1,2-DAB in an optimized buffer system successfully. Firstly, the slurry of Co 3 O 4 /Yb 2 O 3 NPs in ethanol is prepared and then deposited a very thin layer onto GCE with an additive of 5% Nafion binder. This fabricated sensor is applied to detect selective 1,2 DAB through the implementation of a reliable electrochemical method by using the electrochemical approach. It is worth mentioning that the development of Co 3 O 4 /Yb 2 O 3 NPs based GCE sensor seems to be promising technology especially for detecting toxicant to the safety of the environment.

Hydrothermal process to prepare Co 3 O 4 /Yb 2 O 3 NPs
The facile hydrothermal process is applied to prepare the Co 3 O 4 /Yb 2 O 3 NPs. The hydrothermal is an efficiently used method to synthesis nano-doped material. This process basically consists of three major steps. These include (i) hydroxides co-preparation in aqueous medium, (ii) precipitation drying, and then (iii) calcining in the muffle furnace. For executing the experimentation, ytterbium chloride (YbCl 3 ) and cobalt chloride (CoCl 2 ) are dissolved in 100.0 mL de-ionized water. This mixing process is done in 250.0 mL conical flask. Since the hydrothermal process works in an alkaline medium, 0.1 M NH 4 OH solution is added dropwise for adjusting the pH at 10.5 under continuous magnetic stirring. The ions Co +2 and Yb +3 are co-precipitated with the state of Co(OH) 2 /Yb 3 (OH) 2 . The formed metal hydroxides are separated and then carefully washed with di-ionized water as well as ethanol. Then the separated precipitate is dried in an oven at a temperature of 105˚C. Subsequently, the dried precipitate is placed inside the furnace to calcine at 500˚C temperature for 6h. In the presence of oxygen, this calcination process transforms the metal hydroxides into Co 3 O 4 /Yb 2 O 3 NPs. The obtained sample is then ground to particles at nano level by using a motor. The possible reaction schemes are listed below:

Reactions occurred in muffle furnace
Due to the presence of several metal ions in the reaction medium, the precipitation of metal ions as metal hydroxides are dependent on the product solubility (Ks) of the corresponding metal hydroxides. It is noted that the values of product solubility (Ks) for Yb(OH) 3 and Co (OH) 3 are 1.0x10 -22 and 5.92x10 -15 respectively [18]. As the addition of 0.1 M NH 4 OH solution dropwise in the reaction medium, the concentration of OHis increased gradually. Therefore, Yb(OH) 3 starts to precipitate first due to its lower value of product solubility (Ks). It forms Yb (OH) 3 crystal nuclei and then starts the aggregation. However, as the pH keeps increasing, the Co(OH) 3 starts precipitating at a certain pH value. These precipitations are then absorbed on crystallites of Yb(OH) 3 . Similar growth patterns of nano-materials are reported in literature [19][20][21]. The calcined Co 3 O 4 /Yb 2 O 3 NP is then ground in a mortar to make a powder sample for full of characterization. The prepared Co 3 O 4 /Yb 2 O 3 NPs are applied to detect the selective 1,2-DAB through an electrochemical approach at room temperature.

GCE fabrication by using Co 3 O 4 /Yb 2 O 3 NPs
A slurry/paste of Co 3 O 4 /Yb 2 O 3 NPs was prepared by mixing ethanol in order to prepare a thin layer coated GCE. The prepared GCE was then dried at ambient temperature. A drop of 5% ethanolic emulsion of Nafion (so-called conducting binder) was added during the coating process to enhance the binding strength between GCE and used NPs. It is worth mentioning that Nafion has been reported in the literature for its capability to improve conductivity, stability, and electron transfer rate of the electrode [22,23]. For drying the produced conducting film entirely, the fabricated electrode was put inside an oven at 34.0˚C temperature. At the next stage, the prepared Co 3 O 4 /Yb 2 O 3 NPs/binder/GCE and 1.5 mm dia Pt-wire were used as working and counter electrode respectively for making an electrochemical cell. The 1,2-DAB solution was used as a target analyte in the developed chemical sensor. The sensitivity (Sen) of the sensor was determined from the calibration curve that represents current vs. concentrations. Similarly, DL and LDR were determined from noise and sensitivity. The used Keithley electrometer (6517A, USA) consisting of two electrode systems supplies voltage in developing electrochemical curve. Throughout this chemical experimentation, the solution with 0.1 PBS was kept constant to 10.0 mL in glass beakers.

Analysis of structural and optical properties
The obtained FT-IR spectra of Co 3 O 4 /Yb 2 O 3 NPs are presented in Fig 1A. The FT-IR spectrum of Co 3 O 4 /Yb 2 O 3 NPs is displayed two distinctive peaks at 552 and 654 cm -1 , which originate from metal-oxygen stretching vibration. The bands at 552 and 654 cm -1 are associated with Co +3 -O and Co +2 -O vibration respectively [24,25]. The adsorption peaks at wavenumber 1407, 1625, and 3306 cm -1 are more likely attributed to the stretching vibration mode of O-H due to the adsorption of water from the environment [26][27][28]. The photo-electronic sensitivity through UV-vis analysis of prepared Co 3 O 4 /Yb 2 O 3 NPs was conducted at 290-800 nm wavelength. Due to the adsorption of visible light radiant energy, the electrons of synthesized NPs are transmitted from the low-level to high-level, which resulted in UV-vis spectra [29]. The NPs. The obtained UV absorption band is in good agreement with the literature [30][31][32]. As per Eq (6), the measured band-gap energy is found to be 4.04 eV.
Where, E g : band-gap energy; λ max : maximum absorbed wavelength [33,34]. The X-ray diffraction (XRD) analysis was conducted on Co 3 O 4 /Yb 2 O 3 NPs in the range of 10 to 80 degrees at the scanning speed of 2 / min. The obtained XRD spectrum is presented in Fig 1C which

PLOS ONE
Detection of 1,2-diaminobenzene with Co3O4/Yb2O3 nanoparticles As per general practice, the crystal size of synthesized NPs is measured from XRD diffraction patterns by using Eq (7).
Here, λ: wavelength, β: width at half corresponding to the highest intense peak, and θ: diffraction angle [40]. The measured crystal size by using Eq (7) is found to be 27.03 nm.

Analysis of binding energy
The prepared NPs of Co 3 O 4 /Yb 2 O 3 were also analyzed by the XPS technique. When Co 3 O 4 / Yb 2 O 3 NPs are scanned using XPS, the valence electron from outer orbit transmits from lowenergy level to high-energy level owing to the kinetic energy of the X-ray beam absorbed. This practice is utilized to determine the atomic composition as well as oxidation compounds present in the tested sample [41][42][43]. The obtained XPS spectra are presented in Fig 2A for Co2p, Yb4d, and O1s. The Co2p core level spectrum presented in Fig 2D shows two dominating peaks of Co2p3/2 and Co2p1/2 at 781 eV and 796 eV respectively along with satellite peaks. These two dominating sharp peaks can be recognized to verify the existence of Co +3 . The observed satellite peaks of Co2p3/2 and Co2p1/2 are at 786 eV and 802 eV respectively. These two peaks are more likely ascribed to the presence of Co +2 in the prepared Co 3 O 4 /Yb 2 O 3 NPs [44][45][46][47][48]. Thus, the presence of Co2p spin orbitals demonstrates the co-existence of Co(II) and Co(III) on the synthesized Co 3 O 4 /Yb 2 O 3 NPs. The O1s peak at 530 eV as shown in Fig 3C is [49][50][51]. Similarly, the Yb4d peak at 186 as is ascribed to Yb +3 -O -2 bonds in Yb 2 O 3 as illustrated in Fig 3B [52-54].  Fig 3A and 3B clearly shows that the prepared Co 3 O 4 /Yb 2 O 3 nanomaterials are spherical in shape [55][56][57]. The elemental analysis of Co 3 O 4 /Yb 2 O 3 NPs as per EDS test presented in Fig 2C and 2D shows O 14.8%, Yb 63.4%, and Co 21.9%. Besides this, there is no other visible peak and therefore it can be concluded that the prepared Co 3 O 4 /Yb 2 O 3 NPs consist of cobalt, ytterbium, and oxygen only [58,59].

Applications: Detection of 1,2-DAB by Co 3 O 4 /Yb 2 O 3 NPs
The conductive binder (5% Nafion) was used during the fabrication of the sensor probe through the deposition of a thin layer of Co 3 O 4 /Yb 2 O 3 NPs on GCE. The use of conductive binder successfully improved its conductivity, stability, and electron transfer rate. The prepared Co 3 O 4 /Yb 2 O 3 NPs/binder/GCE based chemical sensor was then employed to detect 1,2-DAB selectively in optimized aqueous buffer solution. In electrochemical detection of 1,2-DAB by electrochemical method, current versus potential was determined on the thin film of Co 3 O 4 /Yb 2 O 3 NPs by fixing the holding time for 1 s. Scheme 1 presents the possible reaction mechanism of 1,2 DAB. It is worth mentioning that enrichment of electrons is observed during the determination of sensing performance. Thus, the conductivity of the sensing medium of 1,2-DAB is increased. As a result, amplified electrochemical responses as illustrated in Fig  5A are [16,60,61]. This is why the electrochemical responses are observed to be more significant with the increase of 1,2-DAB concentrations. This demonstrates that the electrochemical responses are directly proportional to the concentration of 1,2-DAB. This is in good agreement with the results reported elsewhere [62]. The pictorial representation of the Co 3 O 4 /Yb 2 O 3 NPs modified electrode in detecting of 1,2-DAB is demonstrated in Scheme 1.
The possible oxidation reaction of 1,2-DAB is presented by reaction (8) below.
The hydrothermally prepared Co 3 O 4 /Yb 2 O 3 NPs are not similarly active to all the phosphate buffer systems in applied electrochemical method. Therefore, it is very important to optimize the buffer solution for obtaining the maximum electrochemical responses. The fabricated Co 3 O 4 /Yb 2 O 3 NPs/binder/GCE based sensor probe was examined in different buffer solutions with pH ranging from 5.7 to 8.0 T he tested chemical sensor probe was maximum  Fig 4B. Among others, 1,2-DAB was found to exhibit the maximum electrochemical responses. The key analytical performance of a sensor is denoted by the response time. This test was conducted in 1.0 nM concentration for 1,2-DAB to optimize in buffer solution. The highly applicable value of the response time as per observation from Fig  4C is 11.0 sec. Reproducibility is another important sensor performance parameter. The data on reproducibility is presented in Fig 4D after conducting the test in 1.0 nM concentration of 1,2-DAB and pH of 7. The responses were observed to be outstanding even after washing the working electrode in each trail. At the next stage, RSD (relative standard deviation) was calculated. It was found to be 2.26 at +.8 V potential. Fig 5 shows the graphical presentation of current versus the potential for 1,2-DAB detection at the concentration ranging from 0.1 mM to 0.1 mM. The observed responses corresponding to the concentration are found to be distinguishable. The resulted plot shows the regression coefficient value (r 2 = 0.9745) at +1.0 V potential. The estimated sensitivity and LDR (Linear dynamic range) are found to be 5.6962 μAμM -1 cm -2 and 0.1 pM-0.01 mM respectively. The detection limit (DL) is calculated as 0.02 ± 0.001 pM (signal to noise ratio of 3). Fig 5A demonstrates that the electrochemical response varies with the concentration of 1,2-DAB which is in good agreement with the finding reported in the literatures [63][64][65]. A few 1,2-DAB chemicals are absorbed in the very beginning of the sensing performance of 1,2-DAB with the Co 3 O 4 /Yb 2 O 3 NPs/binder/GCE sensor probe. The absorption of 1,2-DAB mainly occurs when oxidation starts progressively onto the working electrode surfaces [66]. In the meantime, the analyte concentration on the surface increases and covers a larger part of the surface. It indicates a gradual increase of reaction rate on the Co 3 O 4 /Yb 2 O 3 NPs/binder/ GCE surface. However, the surface reaction rate and corresponding current density reach a  Table 1 shows the comparison of chemical sensors fabricated with various nanostructured metals by electrochemical methods.

Real sample analysis
The newly developed Co 3 O 4 /Yb 2 O 3 NPs/binder/GCE based chemical sensor was justified for its applicability within the practical field through employing it in detecting 1,2-DAB with verities of real samples collected from the various sources. To confirm its wide range of applicability, the tested real samples were collected from different sources including extracts from PC baby-bottles, PC water bottles, PVC-made food packing bags, and so on. The collected samples are initially filtered. Finally, the samples are used to detect by fabricated Co 3 O 4 /Yb 2 O 3 NPs/ binder/GCE sensor probe using an electrochemical method in room conditions. The obtained results as presented in Table 2, are found to be quite satisfactory and acceptable.

Conclusions
Finally, nanoparticles of Co 3 O 4 /Yb 2 O 3 were prepared hydrothermally and analyzed by various conventional methods, such as UV-vis, FTIR, FESEM, EDS, XRD and XPS. A selective chemical sensor probe was successfully prepared through depositing a very thin layer of Co 3 O 4 / Yb 2 O 3 NPs onto GCE. During deposition process, a drop of Nafion was added for improving its fabrication process to stick the powder NPs onto GCE surface. The fabricated sensor probe was then employed in detecting the target toxic 1,2-DAB in phosphate buffer system. The 1,2-DAB chemical sensor is exhibited satisfactory results for various analytical performing factors including detection limit, selectivity, linearity, response time, reproducibility, repeatability, stability, and so on. The obtained sensitivity and LDR (linear dynamic range) are observed to be 5.6962 μAμM -1 cm -2 and 0.1 pM-0.01 mM respectively. The DL (detection limit) is found to be 0.02 ± 0.001 pM at the point where the signal to noise ratio is 3. The test results are obtained from various real sample sources demonstrated that the fabricated sensor is highly reliable and suitable in detecting toxins by an electrochemical process on a broad scale. Thus, a sensor probe with binary doped nanostructure material is applied and useful for the safety of environmental and health care fields on a large scale.