NiTe Magnetic Semiconductor Nanorods for Optical Limiting and Hydrogen Peroxide Sensor

The hydrothermal technique was used to make nickel telluride nanorods (NiTe NRs) utilizing ascorbic acid and cetrimonium bromide (CTAB) as reducing agents. Temperature dependent magnetic study for NiTe NRs shows a ferromagnetism behavior. Under 532 nm laser excitation, the obtained materials had a better optical limiting property, with a two photon absorption coefficient of 6.6 × 10− 10 m/W and an optical limiting of 2.44 J/cm2 at 200 µJ. NiTe NRs modified electrode shows a excellent hydrogen peroxide electrocatalytic activity with reproducibility, repeatability and durability. It displays an outstanding sensitivity of 6.35 µAµM− 1 cm− 2 and a detection limit of 6 nM. In the presence of interfering species such as dopamine, uric acid, ascorbic acid, glucose, and folic acid, the electrode has a high level of selectivity. A real sample analysis for NiTe NRs sensor has been established in human serum and rat brain serum showed good recoveries.


Introduction
Hydrogen peroxide (H 2 O 2 ) is a key intermediate biomolecule in biological systems, as well as in the industrial, pharmacological, and clinical areas [1]. Reactive oxygen species (ROS) such as H 2 O 2 are involved in a variety of disorders including Parkinson's disease, Alzheimer's disease, cardiovascular disease, and cancer [2,3]. The detection of H 2 O 2 is unable to avoid in industrial and biological systems. Several analytical approaches have been used for the fast and exact determination of H 2 O 2 such as a spectrophotometer, photometry, fluorimerty, fluorescence, chemiluminescence, and electrochemical [4,5]. Among them, the electrochemical method has higher advantages such as low cost, portability, ease to use, fast response, high selectivity, reliability, and also consume less power [6]. The electron transfer between H 2 O 2 and the electrode can be enhanced by enzymes. The immobilized enzymes changed the electrode as a result of denaturation, resulting in poor bioactivity, stability, and reproducibility [7,8]. The non-enzymatic detection of H 2 O 2 is still important, and several reactive substances have been suggested for the generation of inexpensive sensitivity, and selectivity catalysts.
The oxidation/reduction of H 2 O 2 was monitored using a variety of transition metal oxide and sulfide compounds placed on the surface of electrodes. Telluride-based materials have gotten a lot of interest as a possible candidate for electron transfer ability among transition metal chalcogenide materials. Wan et al. have synthesized NiTe nanowires with thorny nanostructure for the detection of H 2 O 2 and glucose with a detection limit of 25 nM over a linear range of 0.1-0.5 µM and 0.42 µM over a linear range of 1-50 µM by a colorimetric detection system respectively [9]. CdTe NRs show a sensitivity of 1326 µA mM cm −2 with a detection limit of 0.1 µM with a rapid response time of 5 s towards H 2 O 2 [10]. Detection of dopamine has been reported for Pt-doped CoTe nanoflake with a sensitivity of 0.3422 µAnM −1 cm −2 and the temperature-dependent magnetization peaked at 2% Pt-doped CoTe and thereafter reduced as the Pt concentration was increased [11]. Ni 3 Te 2 have been synthesized using the hydrothermal and electrodeposition method by Amil et al. The prepared Ni 3 Te 2 electrode shows a sensitivity of 41.615 mA mM −1 cm −2 with a low LOD of 0.43 μM in a range between 0.01 and 0.8 mM in electrodeposited method. And, in hydrothermal method, its displays high sensitivity of 35.213 mA mM −1 cm −2 from 0.01 μM to 0.8 mM with a LOD as low as 0.38 μM. The intrinsic features of Ni 3 Te 2 for glucose sensing through direct electro-oxidation were confirmed by the electrode's good selectivity, reproducibility of current response, and long-term functional stability [12]. A non-enzymatic sensor based on NiTe nanorods has been developed for the detection and quantification of hemoglobin (Hb) from anemic pregnant patients using a single approach [13]. Seed mediated growth approach followed by TEOS polymerization leading to the formation of a silica layer surrounding the Au core displayed the excellent bioelectrocatalytic activity towards H 2 O 2 with a linear range of 8 × 10 -7 to 6 × 10 -5 M and the limit of detection was 6 × 10 -8 M at 3σ [14]. Pd/SBA-15 modified electrode showed an excellent response to the reduction of H 2 O 2 and the linear range of 1.8 to 119.3 μM with a LOD of 0.8 μM [15].
Because of its electric, magnetic, and thermodynamic properties, NiTe antiferromagnetic with nickel arsenide type (NiAs) crystal structure has received a lot of interest. Bhat et al. [16] fabricated NiTe nanostructures electrode displays onset potential of − 422 mV towards hydrogen evolution reaction and over the potential of 679 mV towards oxygen evolution reaction. The remarkable performance of NiTe nanosheets on nickel foam electrode for the oxygen evolution reaction activity with over the potential of 262 mV to derive 10 mA cm −2 in an alkaline medium has been reported by Wang et al. [17]. NiTe and CoTe nanoparticles showed electrocatalytic oxidation toward uric acid with a limit of detection of 0.095 and 0.875 µM respectively [18]. In the present work, the hydrothermal process of NiTe nanostructures for non-enzymatic hydrogen peroxide biosensor have been developed. A highly sensitive, efficient and cost-effective enzymeless H 2 O 2 sensing has been taken advantages using NiTe NRs. An amperometric H 2 O 2 sensor has been fabricated using NiTe NRs modified electrode and its applicability has been demonstrated in pharmaceutical samples.

Synthesis of NiTe NRs
S o d i u m t e l l u r i d e ( Na 2 Te O 3 ) , n i cke l a c et a t e (Ni(CH 3 CO 2 ) 2 ·2H 2 O) (Sigma Aldrich) and ascorbic acid (C 6 H 8 O 6 ), cetyltrimethylammonium bromide (CTAB) (Merck) and deionized water from a Milli-Q-ultra pure (18.2 MΩ cm −1 ) were used as the starting materials. 14.2 mM C 6 H 8 O 6 was dissolved in 40 mL deionized water in the hydrothermal technique. After then, 0.82 mM CTAB was added. The above solution was then treated with 1.88 mM Na 2 TeO 3 and 1.88 mM (Ni(CH 3 CO) 2 ·2H 2 O), producing a white TeO 2 precipitate almost immediately. The resulting solution was then mixed with 40 mL deionized water and transferred to a 100 mL Teflon-lined stainless steel autoclave, which was sealed and kept at 180 °C for 24 h in an oven before being allowed to cool to room temperature in air. The resultant powder was collected by centrifugation, washed with deionized water and ethanol for several times then dried at room temperature [19].

Material Characterization
Energy-dispersive X-ray (EDX) mapping was done using HORIBA EMAX X-ACT (Sensor + 24 V = 1-6 W, resolution at 5.9 keV) and the elemental analysis was carried out by using XPS (Shimadzu ESCA 3100). On a Lakeshore 7410S vibrating sample magnetometer, magnetic measurements were taken at a low temperature (VSM). The SQUID magnetometer was used to measure low-temperature magnetization in both zero-field-cooled (ZFC) and field-cooled (FC) protocols. After chilling the sample to 5 K at a constant field of 500 Oe, the FC M-H measurements were obtained. The Z-scan approach was used to investigate nonlinear optical properties using an Nd:YAG laser (532 nm, 9 ns, 10 Hz). Before laser excitation, CdTe nanopowder was stirred continuously for 1 h and placed in a 1 mm quartz cuvette with a transmittance of 65%. Cyclic voltammetry, amperometric response and electrochemical impedance spectroscopy (EIS) studies were carried out using a CHI 1205A workstation. Informed consent was obtained from all individual participants included in the study. Rat serum was collected from the sigma company.

Fabrication of Electrodes
The electrochemical investigations were performed out in a three-electrode setup with a working electrode made of glassy carbon electrode (GCE), a reference electrode made of Ag/AgCl, and a counter electrode made of Pt wire. Drop casting was used to prepare the modified electrodes on the GCE surface. The GCE surface was first pre-cleaned by cycling between 1.0 and 1.2 V in 0.10 M PB (pH 7) for 10 cycles at a sweep rate of 50 mV s −1 . 7 μL NiTe was then placed on the pre-cleaned GCE and dried at room temperature. Scheme 1 shows the schematic diagram of the nonenzymatic H 2 O 2 sensor application of NiTe NRs produced by the green synthesized hydrothermal method. It represents the decomposition of H 2 O 2 into H 2 O. NiTe NRs enhanced the electron transfer between the electrode and electrolyte.  Te 3d region and c Ni 2p region [22]. Ni 2p is best suited with two spin-orbits and a satellite using Gaussian fit. The peak fitted at the binding energy of 856.1 eV in Ni 2p3/2 and 874.5 eV in Ni 2p1/2 is assigned to Ni 2+ in the Ni 2p spectrum, as illustrated in Fig. 2c [23]. The satellite peaks appeared at 861.7 eV and the energy difference between the two peaks is 18.4 eV [21]. Figure 3a presents the magnetization versus applied field (M-H) curves recorded at temperature dependence VSM for the NiTe sample in the field range of − 10,000 to + 10,000 Oe under various isothermal magnetization temperatures (100 K to 300 K). The saturation magnetization (Ms) and coercivity (Hc) values of NiTe NRs measured at room temperature are 0.0008 emu g −1 and 158 Oe. The magnetic responses of NiTe expose the paramagnetic behavior at high field and soft ferromagnetism at low magnetic field respectively. NiTe x nanorods exhibit between diamagnetism and paramagnetism based on the experimental study have been reported by Lie et al. [24].

Magnetic Properties NiTe NRs
Magnetization temperature (M-T) curves for zero-fieldcooled (ZFC) and field-cooled (ZC) observations with temperature collected under a 500 Oe applied magnetic field. The solution was cooled to 2 K in the absence of a magnetic field, then a 500 Oe external field is introduced, and the behavior of the material is also observed while heating from 2 and 300 K. (Fig. 3b). The behavior is observed in the case of NiTe samples that demonstrate the paramagnetic nature. In both ZFC and FC curves, the magnetization increases with decreasing temperature. According to the literature, the temperature dependence of magnetization illustrates the transition temperature at about 20 K for NiTe shows that the Curie temperature of the system is much higher than 400 K [25]. For the prepared NiTe sample, the magnetic transformation occurs at 32 K. In the present work, there is no magnetic transition in the prepared samples due to the existence of paramagnetic nature. Room temperature ferromagnetism and superparamagnetic blocking behavior have been observed in NiTe [26].

Nonlinear Properties of NiTe NRs
Third-order nonlinear behavior was observed using the Z-scan approach. Z-scan is the most common method for identifying third-order nonlinear properties such nonlinear index of refraction and absorption coefficient since it is the simplest and uses the most accurate single beam approach. The polarization of the material by the intense laser light falling on it causes a nonlinear response in NiTe NRs. The material's third-order NLO performance is a critical physical trait for a variety of applications, involving optical switching and limiting. One of the most fundamental methods to determine third-order nonlinearity in a medium is the Z-scan method. An open-aperture Z-scan approach with a nanosecond pulse with a wavelength (λ) was used to calculate the nonlinear absorption coefficient (β) in the present work.
A NiTe dispersion media was placed between the source and the detector to estimate the transparency for that specific area (z). The convex lens focused a laser source with varied energies of 100 µJ, 150 µJ, and 200 µJ on the specimen, and the sample was shifted to both sides of the focus. In Fig. 4a, the observations are plotted, and it is clear that the material has reverse saturation absorption (RSA). When the materials are optically pumped at 532 nm, the electrons are stimulated to trapping levels of impurity or crystal defects with long lives (2.3 eV). Because intermediate trapping levels have longer durations, the likelihood of resonance two-photon transition increases.
Through numerically fitting the open aperture Z-scan curve with typical nonlinear equations, the origin of the Genuine TPA in metal tellurides is uncommon, whereas nonlinearity is linked to free carrier absorption (FCA). There is a presence of TPA with FCA in the produced samples, which has a noticeable absorbance at 532 nm. Figure 4 has been used to calculate the optical limiting value (b). Nonlinear scattering, in addition to nonlinear absorption, plays a significant role in the optical limiting mechanism at ns laser excitation [27]. Cheng et al. revealed that the optical limiting property of graphene-PbS nanohybrids under 4 ns laser excitations is probably a combination of FCA and nonlinear scattering [28]. In this scenario, the repetition rate is 0.25 s, which allows bubbles to quickly develop and generate nonlinear scattering. The repetition rate in our case (9 ns) is 0.11 s, which is insufficient for forming nonlinear scattering centres. As a result, the 2PA induced free FCA for the prepared material is the source of nonlinearity. The value of and optical limiting thresholds is calculated from all of these computations and is displayed in Table 1

Electrocatalysis of H 2 O 2 Reduction
Figure 5a displays the CV recorded for unmodified and NiTe NRs modified GCE measured at a scan rate of 50 mV s -1 in the potential range of 0 to − 0.8 V. In the supporting electrolyte comprised 50 µM H 2 O 2 , an enhanced electrocatalytic ability (24 folds higher) and fast electron transfer is found for NiTe NRs/GCE than unmodified GCE. Moreover, the over potential observed at NiTe NRs/GCE is lower (540 mV) than that of unmodified GCEs. The NiTe NRs improved electrocatalytic activity can be attributed to a strong synergetic effect and adequate tailoring. Furthermore, by a variety of interactions, the edge planes of NiTe NRs can provide additional sites for electrocatalysis. The influence of scan rate on H 2 O 2 reduction is investigated by using different scan rates ranging from 10 to 100 mV s −1 , with oxidation increasing as the scan rate increases (Fig. 5b). The cathodic peak current increases linearly as H 2 O 2 concentration rises. The linearity of the graph between the cathodic peak current and the square root of the scan rate indicates that the reduction process is diffusion regulated (Fig. 5c). The CV achieved at NiTe NRs/GCE in 0.1 M PBS (pH 7.0) having varied amounts of H 2 O 2 ranging from 50 to 350 μM is shown in Fig. 5d. The oxidation peak current is increased linearly the same as the H 2 O 2 concentration increases.

Amperometric Determination of H 2 O 2
Amperometric detection can be utilized to detect electroactive solutes, particularly those that are easily reduced or oxidized. A potential is applied between a working and reference electrode, and where solutes having to pass over the working electrode are reduced or oxidized, current flows, this is the detection method's principle. Figure 6a shows the amperometric response of a NiTe NRs modified electrode (rotation speed = 1200 RPM) to 50 s intervals of H 2 O 2 injections into PB (pH 7.0) (Eapp = − 0.56 V). The reactions were well-defined and rapid, and the steadystate current was attained in less than 5 s. The linear range is 35 nM-1950.12 µM (Fig. 6b) and the limit of detection (LOD) is 6 nM. The sensitivity is calculated as 6.35 μA µM -1 cm -2 . The previously reported H 2 O 2 sensor and the LOD, pH and linear range are summarized in Table 2.
The electrode exceptional sensing performance is demonstrated by a low limit of detection at the nanomolar level.
The sensor properties are higher than the NiTe NRs integrated electrodes that are currently available. The interference experiment was used to test the sensor's selectivity in the presence of possible interfering substances. Figure 7 depicts the electrode's amperometric response to (a) 5 μM of H 2 O 2 , (b) 5 μM of dopamine, (c) 5 μM of uric acid, (d) 5 μM of ascorbic acid, (e) 5 μM of glucose and (f) 5 μM of folic acid, the electrode quickly responds to H 2 O 2 . Since it is insensitive to other species also, the electrode recognizes and detects H 2 O 2 in a pool of many other biological analytes.

EIS spectra for NiTe NRs
The EIS recorded at GCE (a) NiTe NRs and (b) in 0.1 M KCl adding 5 mM Fe(CN) 6 3−/4− is shown in Fig. 8. The experimental data (inset to Fig. 8) was fitted using the Randles equivalent circuit model, in which R s , R ct , C dl , and Z w denote the electrolyte resistance, charge transfer resistance, double layer capacitance, and Warburg impedance, respectively. Semicircles indicate the parallel combination of R ct and C dl

Durability, Reproducibility and Repeatability
The NiTe NRs/GCE electrode material was measured every week in order to assess the sensor longevity and determine the electrode's storage stability. The electrode was maintained at 4 °C while not in use. Even after 9 weeks of continuous usage, the electrode was able to preserve 96.7% of its initial current, indicating excellent durability. Measurements at five separate electrodes in PB (pH 7.0) towards 10 M H 2 O 2 yielded an RSD of 3.52% for the repeatability trials. Five repeated measurements of the modified electrode were used to calculate the repeatability; the RSD was 3.53%. As a result, it is understood that the sensor's repeatability and reproducibility are within acceptable limits.

Real-Time Applicability
The fabricated sensor practical feasibility has been proved in human blood serum and rat brain serum. Before the analysis, the genuine materials were diluted in PBS (pH 7). For optimal experimental circumstances, NiTe NRs were used in the amperometric tests. Table 3 lists the results of the recovery tests. The sensor's average recovery value is obtained to be 97.6-101.4%. The results show that the suggested NiTe NRs/GCE has a lot of potential for real-time H 2 O 2 sensing.

Conclusion
To make NiTe NRs, a simple hydrothermal approach has been reported. XPS confirms the binding energy of Ni 2+ and Te 2− . The ferromagnetic character of NiTe NRs is revealed by the effect of temperature of magnetization, which suggests a transition temperature of roughly 32 K. The prepared materials show a better optical limiting threshold of 2.44 J/cm 2 and two-photon absorption coefficient of 6.6 × 10 -10 m/W at 200 µJ. With a large working range (0.035-1950.12 µM) and a low detection limit, the NiTe NRs film modified electrode is a very sensitive non-enzymatic H 2 O 2 amperometric sensing substrate (6 nM). The sensor's selectivity, repeatability,