New modified mesoporous silica nanoparticles with bimetallic Ni-Zr for electroanalytical detection of dopamine

4, 2022 Abstract In this research, bimetallic nickel-zirconia supported on mesoporous nanoparticles (Ni-Zr/MSN) were successfully synthesized by a simple in situ electrolysis method. Ni-Zr/MSN were well-characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Brunauer-Emmett-Teller (BET) analyzer, field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM). Ni-Zr/MSN were then cast onto a modified glassy carbon electrode (Ni-Zr/MSN/GCE) as dopamine (DA) sensor. Under optimal conditions, the sensor showed a linear concentration relationship in the range of 0.3 µM – 0.1 mM with a limit of detection of 0.13 µM. The relative standard deviation for 0.1 mM DA solution was 2.1 % (n = 5). The presence of excess catechol, saccharose, glycine, lactose, uric acid, and Cr 3+ , Fe 2+ and Na + as interferents was negligible, except for uric acid in 10-fold excess. The analytical recovery of the sensor was successfully demonstrated by the determination of DA in DA-containing medicine and wastewater samples. The results presented herein provide new perspectives on Ni-Zr/MSN as a potential nanomaterial in the development of


Introduction
Dopamine (DA), which is an important chemical found in human bodies, helps cells to transmit impulses to the receptor other than functioning as a catecholamine neurotransmitter [1]. DA helps in regulating the central nervous system and the cardiovascular system, where an abnormal DA level can cause vital problems in the mammalian body [2][3][4]. Abnormal DA level often contributes to schizophrenia, Parkinson's disease, attention deficit hyperactivity disorder, euphoria, and Alzheimer's and Huntington's diseases [1][2][3][4][5][6][7]. Individuals suffering from mental illness are more prone to coronavirus (COVID-19) when they have difficulties comprehending protective measures such as frequent handwashing, social distancing, and isolation than normal persons [8]. Research conducted on 50,750 COVID-19 patients in France by Mohan et al. [9], found that 1.6 % had schizophrenia. The data revealed an alarming increase in the lifetime prevalence of schizophrenia in the West [9]. Hence, a rapid, accurate and simple method to detect DA levels is demanded. Formerly, chromatography [10], fluorometry [11], chemiluminescence analysis [12], colorimetry [13], and electrochemistry [14][15][16] were employed to detect DA. However, various researchers proved that only electrochemical techniques have simple operation and fast response, being at the same time eco-friendly, costeffective, mobile, highly sensitive and highly selective [17,18].
Although various attempts have been made in the development of electrochemical sensors using MSN, there is a lack of studies on the use of MSN modified by a bimetal as an electrochemical sensor for DA detection. Therefore, in this research, a newly synthesized nickel-zirconia doped with mesoporous silica nanoparticles (Ni-Zr/MSN) was successfully fabricated as a modified GCE sensor for DA detection. The performance of the sensor will provide a new perspective on the modified MSN as a potential nanomaterial in the field of electrochemical sensors.

Materials and reagents
All chemicals used in the synthesis of Ni-Zr/MSN and electrochemical sensing experiments were of analytical grade (Merck and Sigma-Aldrich) and used without further purification. Ethanol, methanol, N,N-dimethylformamide, cetyltrimethylammonium bromide, ethylene glycol, ammonium hydroxide, tetraethyl orthosilicate, 3-aminopropyl triethoxysilane and disodium phosphate (Na2HPO4) were purchased from Merck. Monosodium phosphate (NaH2PO4), catechol, glycine, uric acid, lactose, saccharose, chromium (III) chloride (CrCl3), iron (II) sulfate (FeSO4), sodium chloride (NaCl), and dopamine hydrochloride were purchased from Sigma-Aldrich. The distilled deionized water from EASYpure LF, Barnstead, was used to wash synthesized Ni-Zr/MSN and prepare all analytical solutions. The supporting electrolyte was 0.1 M phosphate buffer solution (PBS) prepared by mixing stock solutions of Na2HPO4 and NaH2PO4.

Apparatus
Cyclic voltammetry and differential pulse voltammetry were performed using a Potentiostat Interface 1010B (Gamry, USA). Three-electrode system consisting of Ag/AgCl (3M KCl) reference electrode MF-2052 with (Bioanalytical System, USA), counter electrode of platinum wire, and Ni-Zr/MSN GCE as the working electrode was used in all electrochemical measurements. The pH of a solution was determined using an Orion 720A glass electrode (Mass., USA). The characteristics of Ni-Zr/MSN were studied using a Fourier transform infrared (FTIR) spectrophotometer model Cary 630 (Agilent, USA), an X-ray diffraction (XRD) instrument model Rigaku 600 (MiniFlex, Japan), an Xray photoelectron spectroscopy (XPS) instrument model PHI Quantera II (ULVAC-PHI, Japan), energy-dispersive X-ray spectroscopy (EDX) model SU 8030 UHR (Hitachi, Japan) and a Brunauer-Emmett-Teller (BET) analyzer model Tristar II Plus (Micromeritics, USA). The XRD data were analyzed using Rigaku SmartLab Studio II software. Electrochemical impedance spectroscopy (EIS) measurements were carried out using a potentiostat/galvanostat model Ref 3000 (Gamry, USA). EIS measurements were performed in the frequency range of 1.0 MHz to 1.0 Hz with 5.0 mV of alternating signal amplitude. Finally, the surface morphology of Ni-Zr/MSN was characterized by a Fieldemission scanning electron microscope (FESEM) model SU8030 (Hitachi, Japan) and a transmission electron microscope (TEM) model JEM 2100F (JOEL, Japan).

Synthesis of Ni-Zr/MSN
Co-condensation and sol-gel methods were implemented to synthesize MSN. A mixture of cetyltrimethylammonium bromide, ethylene glycol, and ammonium hydroxide solutions was vigorously stirred for 30 min at 50 °C. Then, 1.2 mmol tetraethyl orthosilicate and 1 mmol 3-aminopropyl triethoxysilane were added to the homogenous mixture. This solution was stirred for another 2 h at 80 °C and dried at 110 °C overnight. A white powder of MSN was collected after being dried and calcined at 550 °C for 3 h to remove impurities.
Preparation of Ni-Zr/MSN was done by in situ electrolysis method where the platinum (Pt, Nilaco) and zirconia/nickel plates were used as the anode and the cathode, respectively. 30 mL of N,Ndimethylformamide (DMF) solution was added to tetraethylammonium perchloride (TEAP), naphthalene, and 1.5 g of MSN. Electrolysis was then performed under continuous stirring at the constant current density of 480 mA cm −2 and 0 °C under a nitrogen atmosphere. The Ni-Zr/MSN collected at the cathode was heated at 85 °C before drying overnight at 110 °C. Finally, the sample was calcined at 550 °C for 3 h.

Preparation of Ni-Zr/MSN/GCE modified electrode
The surface of GCE was first polished using an alumina slurry (0.05 µm) sequentially, washed ultrasonically in ethanol and deionized water before modification with Ni-Zr/MSN. The Ni-Zr/MSN (2.5, 5.0, and 7.0 mg) was mixed in 10 mL DMF solution and ultrasonicated for 30 min and 2.5 µL of the suspension was drop-casted on the GCE surface. The electrode was allowed to dry at room temperature. As a result, Ni-Zr/MSN modified GCE known as Ni-Zr/MSN/GCE was obtained. Similar procedures were applied for the preparation of unmodified GCE (bare GCE) but without the addition of Ni-Zr/MSN.

Results and discussion
Characterization of Ni-Zr/MSN Figure 1(a) shows the FTIR spectrum of Ni-Zr/MSN recorded in the range of 4000 to 400 cm −1 . The absorption band at 1056 cm −1 was assigned to the asymmetric and symmetric stretching vibrations of Si-O-Si. The addition of bimetallic elements (e.g., Ni and Zr) has possibly formed interaction with Si-O-Si group due to desilication [31]. As shown in Figure 1(b), four diffraction peaks representing NiO were observed at 37.3 (003), 43.3 (012), 62.9 (104), and 75.4° (015). A small diffraction peak was also observed at 79.4° (440), attributing to the characteristic peak of Ni4ZrO phase and possibly indicating that existence of this phase facilitates the formation of Ni-Zr alloy.

Figure 1. (a) FTIR spectrum, (b) XRD patterns and (c) XPS spectrum of Ni-Zr/MSN
Next, XPS wide scan analysis, as shown in Figure 1(c), the sample surface consists of Ni, Zr, C, Si, and O with binding energies of Ni2p3, Zr3d, C1s, Si2p, and O1s at 854, 182, 285, 103, and 532 eV, respectively. In the Ni2p3 region, the spectra were fitted with a single deconvoluted peak at 855 eV, which could be assigned to metallic Ni(OH)2 (Ni 2+ ) species. The Zr3d spectra exhibited peaks at 182 and 183 eV for the presence of Zr metallic ZrO2 (Zr 2+ ) and ZrOH (Zr + ) deconvolution, respectively. For the Si2p spectra, only a single deconvolution occurred (i.e., SiO2 (Si 2+ )), which exhibited a peak at 103 eV. Thus, the results confirm that Ni and Zr have been successfully loaded into the MSN. This result implies a d-electron transfer between Ni and Zr in the Ni-Zr/MSN, resulting in the enrichment of electrons on elemental Ni [32]. Figure 2 shows the morphology of Ni-Zr/MSN as characterized by scanning electron microscopy (SEM). Uniformed size distribution of Ni-Zr/MSN could be observed in the range of 25.9-68.8 nm.
Additionally, the SEM image also reveals new trigonal bipyramidal particles corresponding to a possible interaction between Ni and Zr, which was later confirmed by EDX mapping analysis (Figure 3). The analysis confirmed that Ni and Zr are the main components in this trigonal bipyramidal structure. The TEM image (Figure 4) shows that Ni and Zr were doped in the ordered porous framework of spherical MSN, protecting themselves from aggregation. Then, the selected area electron diffraction (Figure 4, inset) spot pattern shows two characteristic diffraction rings, indicating the crystalline nature of Ni-Zr/MSN. The porosity of Ni-Zr/MSN was identified through the BET surface area analysis. Prior to the analysis, Ni-Zr/MSN was degassed at 300 °C for 1 h to obtain more accurate results on the pore size and BET surface area. From the analysis, the Langmuir surface area was 594.43 m 2 g -1 and the micropore volume was 0.03559 cm 3 g -1 , which indicates a good pore size of Ni-Zr/MSN, which allows bimetallic elements to be doped into it as previously reported [33].  Figure 5. As seen, no redox peak current is observed in the absence of DA (curve a), but a significant redox peak current is observed in the presence of DA (curve b). This means that the sensor shows a sensitive response in the presence of DA. The comparison of CV response between Ni-Zr/MSN/GCE and bare GCE in 0.1 mM DA and 0.1 M PBS (pH 7.0) as a supporting electrolyte was also evaluated. As illustrated in Figure 5, the CV peak current at Ni-Zr/MSN/GCE (curve b) indicated higher current reading than bare GCE (curve c) due to the increase of electron transfer rate and conductivity on the electrode surface. The anodic peak current (Ipa) and cathodic peak current (Ipc) of bare GCE were 2.649 and 2.637 µA, respectively. Meanwhile, the redox peak current of Ni-Zr/MSN/GCE increased to 5.789 µA for Ipa and 6.166 µA for Ipc. The peak-to-peak separation (∆Ep) of Ni-Zr/MSN/GCE also decreases to 60.00 mV compared to bare GCE, which has ∆Ep of 240.01 mV. Therefore, the results showed that Ni-Zr/MSN/GCE has good electrocatalytic activity and good selectivity, hence improving the electron transfer rate and electrochemical response [34].  Generally, a diameter of semicircle impedance response at higher frequencies represents the charge transfer resistance, while a linear part at lower frequencies represents the diffusion process [35]. As illustrated in Figure 7b, the Nyquist of the bare GCE exhibits a larger semicircle (curve a) compared to Ni-Zr/MSN/GCE (curve b), indicating lower charge transfer resistance in the interfacial region of Ni-Zr/MSN/GCE. By fitting the Randles equivalent electrical circuit (Figure 7b, inset), charge transfer resistance (Rct) values for bare GCE and Ni-Zr/MSN/GCE were 106.4 and 0.613 k. Additionally, the electron transfer apparent rate constant (kapp) values [36] calculated for bare GCE and Ni-Zr/MSN/GCE were 2.50×10 −8 and 4.34×10 −6 cm s −1 . The high kapp and low Rct values for Ni-Zr/MSN/GCE indicate a fast electron transfer process facilitated by Ni-Zr/MSN [37]. The EIS result correlates with the finding of CV studies.

Effect of pH
The performance of Ni-Zr/MSN/GCE in 0.1 mM DA in 0.1 M PBS at different pH values was investigated in the range of pH 6.0-8.0 using the differential pulse voltammetry (DPV) method. The graph of Ipa versus pH is shown in Figure 8. The oxidation current for DA reached the most optimized state at pH 7.0, which is a neutral buffer state. There was a slight improvement from pH 7.6 to 8.0, but it was still inferior to the current reading for pH 7.0. The decreased peak current beyond pH 7.0 could be attributed to the deprotonation of DA, and at pH below 7.0, the decrease could be due to protonation [38]. Thus, the PBS pH of 7.0 was chosen as an optimized pH for further measurements of the study. Under optimal analysis conditions, the analytical curve of DA was constructed from the DPV quantification carried out at different DA concentrations. From Figure 9(a), the peak voltammogram for DA increased with the increase of DA concentration in the range from 0.3 µM to 0.1 mM. The linear regression equation Ipa (µA) = 28.073 DA + 0.0506 with R 2 of 0.9929 was obtained within this concentration range (Figure 9(b)). The limit of detection of 0.13 µM was estimated using the formula of 3 Sb/m, where Sb is the standard deviation obtained from five measurements of the blank signal and m is the slope of the linear calibration curve. It seems that the results obtained in this work are more worthy than those reported earlier ( Table 1). The reproducibility of Ni-Zr/MSN/GCE was evaluated by five replicate measurements of 0.1 mM DA, and Ni-Zr/MSN/GCE showed good reproducibility where the relative standard deviation of 2.1 % (n = 5) was obtained.

Interference study
The possible interference for DA detection was investigated by adding some organic compounds and inorganic ions that may coexist with DA in real samples. The potential interferents used were catechol, saccharose, glycine, lactose, uric acid, and Cr 3+ , Fe 2+ , and Na + ions. Under optimal conditions, DPV measurements were performed for DA concentration established at 0.1 mM, while concentrations of interferents were kept at 0.1 and 1.0 mM in 0.1 M PBS (pH 7). As shown in Table 2, Ni-Zr/MSN/GCE demonstrated good anti-interference behavior, except for 10-fold excess of uric acid (the criterion for interference was a ±20 % error in the peak height of DA).

Recovery analysis
In an attempt to test the applicability of Ni-Zr/MSN/GCE, a recovery study of the DA-controlled medicine and wastewater samples were performed by the standard addition method. 1 mL of 0.01 mM DA-containing medicine was diluted with 0.1 M PBS and a known amount of DA solution was added after the original concentration of DA in the prepared sample was detected. Next, wastewater samples from two different locations were collected and the residue was filtered. 10 mL of filtered wastewater samples were diluted with 0.1 M PBS and a known amount of DA solution was added. Table 3 shows satisfactory recovery percentages in the range between 96.0 and 107.6 %, indicating that Ni-Zr/MSN/GCE is capable for the quantification of DA in real samples.

Conclusions
The modified mesoporous silica nanoparticles (MSN) with bimetallic nickel-zirconia have been successfully synthesized using electrodeposition technique. Characterization by FT-IR, XRD, XPS, BET, TEM and FESEM surface techniques confirmed the formation of a highly porous system with a uniform particle size distribution of 25.9 and 68.8 nm, and the presence of Ni, Zr, Si and O in the prepared Ni-Zr/MSN nanomaterial. Different amounts of Ni-Zr/MSN were deposited on GCE and the prepared Ni-Zr/MSN/GCE was employed as an electrochemical sensor for DA determination. The CV and EIS studies of the sensor showed an excellent response and increased charge transfer rate of DA oxidation at the electrode-solution interface. The sensor displayed an excellent response in the working DA concentration range of 0.3 µM-0.1 mM, with a limit of detection of 0.13 µM with good reproducibility. The sensor also demonstrated good recovery for the analysis of DA in DA containing medicine and wastewater samples. This current work will help the scientific community to understand and realize a new perspective of modified MSN in the field of electroanalytical chemistry.