Synthesis and electrochemical studies on Cu-TiO2 thin films deposited by spray pyrolysis technique for sensing Uric acid

Document Type : Reasearch Paper

Authors

• Mathi Vathani Arunananthan ¹
• Dhanalakshmi Selvaraj ²
• Prithivikumaran Natarajan ¹

¹ Department of Physics, V.H.N. Senthikumara Nadar College, Virudhunagar, India

² Department of Chemistry, V.H.N. Senthikumara Nadar College, Virudhunagar, India

Abstract

In this study, we report an effective uric acid (UA) electrochemical biosensor using Cu-TiO₂ electrode. UA is a biomedical compound that plays a vital role in human metabolism. The abnormal level of UA leads to several diseases. TiO₂ and Cu-TiO₂ with various concentrations were deposited on glass substrates by spray pyrolysis technique. The structural study shows by X-ray diffraction analysis shows that all the films are in anatase phase with tetragonal structure and confirm the incorporation of Cu ions into TiO₂ lattice. The morphology and chemical composition of TiO₂ and Cu-TiO₂ were characterized by scanning electron microscopy (SEM) and energy-dispersive x-ray spectroscopy (EDS). These studies reveal that the aggregation of particles occurs due to doping and confirm the presence of Cu. The optical analysis was studied by UV-Vis absorption and photoluminescence spectroscopy, which indicates the band gap changes, shift in absorption peak and defects. Cyclic voltammetry (CV) was used to analyse the performance of the Cu-TiO₂ as the electrochemical biosensor. Cu-TiO₂ electrochemical biosensor exhibits good sensitivity, linearity and high stability for the detection of UA.

Keywords

• Cu- TiO2
• Electrochemical studies
• Sensing
• Spray pyrolysis
• Thin films
• Uric acid

20.1001.1.20088868.2019.10.3.1.6

Main Subjects

• Nanofluids and Thin Films

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Full Text

INTRODUCTION

Uric acid (2,6,8-trihydroxypurine, UA) and other oxy-purines are the principal final products of purine metabolism in the human body [1]. UA is biomedically important compound that plays a vital role in human metabolism [2]. Abnormal level of UA in human body causes several diseases including gout, hyperuricemia, lesch-nyan disease, cardiovascular and chronic renal sickness [3-6]. Several methods can be applied to determine the UA concentration including high performance liquid chromatography [7], amperometric [8], fluorometric enzymatic method [9] and electrochemical analysis [6]. In recent years the electrochemical analysis have gained attention in the investigation of important biological molecules and drugs because of their simplicity, cost effectiveness, easy handling and highly sensitive compared to other methods [10]. The objective of the work is to fabricate a metal doped semiconductor electrode material for electrochemical biosensor to sense UA.

Titanium dioxide (TiO₂) has extreme attentions as a potential semiconductor material with wide range of technological applications [11]. It is a widely used semiconductor material for various applications such as dye-sensitized solar cells, water photoelectrolysis, photocatalysis, gas sensors, chemical oxygen detection (COD) sensor and biosensor [12]. Environmental friendly TiO₂ thin films got interest in the field of biosensor due to its good biocompatibility, large surface area and good surface, structural, physical, chemical and optical properties. The immobilizing amount of enzymes, activity of immobilized enzymes and conductivity are the key factors for the sensitivity of biosensors [13-14]. Many researchers have made the synthesis and modification of TiO₂ for enhancing the electrochemical performance. The modification of TiO₂ with doping enhances the charge transfer rate between the electrolyte and the electrode which also enhances the detection performances of the biosensors. Cu²⁺ causes more effective doping with titanium oxide since ionic radii of Cu²⁺ (0.87 Ǻ) and Ti⁴⁺ (0.75 Ǻ) are similar to each other. Hence Cu²⁺ ion can easily substitute Ti⁴⁺ ion in TiO₂ lattice without destroying the crystal structure, thereby stabilizing the anatase phase [15]. Cu doped TiO₂ shows fast electron transfer rate than TiO₂ electrode, hence the sensing property of TiO₂ was found to be enhanced by doping with metals.

Semiconductors in the form of thin films find greater technological importance because of their variety of advantages over bulk crystals [16]. TiO₂ thin films were fabricated by many methods including molecular beam epitaxy, spin coating, electro deposition, RF-magnetron sputtering, pulsed laser deposition (PLD), metal–organic chemical vapour deposition (MOCVD) and spray pyrolysis. Among these spray pyrolysis is a cost effective, simple and efficient technique. This technique has the capability to produce large surface area, high quality adherent films with uniformity, easiness of doping, operates at moderate temperatures (100-500˚C) that opens the possibility of wide variety of substrates, control of thickness, variation of film composition along with thickness and possibility of multilayer deposition [17-18].

In the present work, Cu doped TiO₂ (Cu-TiO₂) thin film electrode material was synthesized by spray pyrolysis technique. The concentration of Cu dopant in TiO₂ thin film was varied in order to optimize the film. Its structural, morphological, optical and electrochemical properties were analyzed and discussed in detail. The linearity and stability of the Cu-TiO₂ thin film electrode was reported. Also, Cu-TiO₂ electrode biosensor was constructed and the sensing performance of UA was studied.

CHEMICALS AND METHODS

The chemicals used for the preparation of TiO₂ and Cu-TiO₂ thin films were titanium tetra-isopropoxide (TTIP, Ti{OCH(CH₃)₂}₄, 97%), ethanol (C₂H₅OH, 99.9%), acetyl acetone (AcAc, CH₃COCH₂COCH₃, 98%), copper acetate dehydrate (C₄H₁₀CuO₆, 97%) and uric acid (C₅H₄N₄O₃, 99%). All the chemicals were bought in analytical grade and used without further purification.

Preparation of precursor solution and Fabrication of TiO₂ and Cu-TiO₂ Thin films

TiO₂ and Cu-TiO₂ thin films were deposited on finely cleaned glass slides by spray pyrolysis technique. Titanium tetra isopropoxide (TTIP) was the Ti precursor material and copper acetate was the material for doping copper. To prepare the solution to make TiO₂ films ethanol was mixed with the TTIP and stirred for 10 minutes and with that acetyl acetone (AcAc) was added as a stabilizer. Then ethanol was added again and stirred for an hour. Here the molar ratio of TTIP, ethanol and AcAc was maintained as 1 : 10 : 1. The prepared solution was sprayed onto well cleaned glass substrate with air as carrier gas, at a substrate temperature of 350 ^°C, spray rate of 4ml/min, pressure of 1 bar keeping the nozzle to substrate distance as 15cm. For uniformity in coatings, the spray head was allowed to move in the X–Y plane using the microcontroller. TiO₂ solution was sprayed for one minute over the glass slide and then the film was put into the muffle furnace for 10 minutes for annealing at 350 °C. This was considered as one coating. The similar process was followed up to 10 coatings. After that the TiO₂ thin film was post annealed at 500 ^°C for one hour. For the preparation of Cu-TiO₂ thin films copper acetate dihydrate was taken as copper source. To prepare Cu-TiO₂ precursor solution, the procedure detailed above to prepare TiO₂ precursor solution was followed and copper acetate was added finally and stirred again for one hour. The coating procedure is same as mentioned above. In order to optimize the optical, structural, surface and electrochemical properties of Cu-TiO₂ films, copper acetate of various concentrations ie., 0.025Wt%, 0.05 Wt%, 0.1 Wt% and 0.5 Wt% were added to the mixture to get four different Cu-TiO₂ thin film samples.

Characterization

The structural study was done using XRD technique by XPERT-PRO diffractometer which was operated at 40 KV and 30 mA using Cukα radiation of wavelength 1.5406 Ǻ. The surface morphology (SEM) and elemental analysis of the TiO₂ and Cu-TiO₂ thin films were studied using ZEISS and VEGA3 TESCON scanning electron microscope. UV-Vis spectroscopic measurements of the films were carried out by using SHIMADZU UV-Vis 1800 Spectrophotometer. Photoluminescence (PL) spectra were obtained using SHIMADZU RF-6000 Series PL spectrophotometer. Electrochemical analysis was carried out using CHI604E electrochemical work station.

RESULT AND DISCUSSION

Structural Analysis

X- ray diffraction patterns obtained for TiO₂ and different concentration of Cu-TiO₂ thin films are shown in Fig. 1(a). The films are polycrystalline in nature and well fitted with the tetragonal crystal structure and the thin films have dominantly anatase crystalline phase and low intensely rutile crystalline phase matched with JCPDS card no 21-1272. For TiO₂ thin film the diffraction peaks of anatase phase with the orientations of (1 0 1), (0 0 4), (1 0 5), (2 0 0) and (2 1 3) and of rutile phase at (211) and (320) are present. For 0.025Wt% Cu-TiO₂ thin film the anatase phase peak intensity slightly increases and rutile phase disappears which may be due to very low concentration of copper. As the concentration of Cu²⁺ is increased to 0.05Wt%, the intensity of anatase peaks gradually increased and rutile peaks reappear, which may be due to the incorporation or replacement of Cu²⁺ atoms into TiO₂ lattice. It indicates the better crystallinity of the film [19]. At 0.1Wt% of Cu- TiO₂ film, the intensity of anatase peaks was higher than that of TiO₂ thin film but lesser than that of 0.05Wt% of Cu- TiO₂ film. Further increase in Cu²⁺ concentration to 0.5Wt% the anatase peaks become weak which may be due to the reorientation effect [20-21] and also rutile phase disappears again. The similar result was observed for Sn doped TiO₂ thin films [22]. Fig. 1(b) reveals that the 0.025Wt%, 0.05Wt% and 0.1Wt% Cu-TiO₂ diffraction peak positions are slightly shifted towards the left compared to that of TiO₂ film. This shift refers to the incorporation or replacement of Cu²⁺ions into the anatase TiO₂. Thus addition of dopant transforms the degree of phase (2θ) of the TiO₂ thin films and the transformation depends upon the Cu²⁺ concentrations [23].

The average crystallite sizes of the TiO₂ thin films were calculated by Scherrer’s formula,

(nm) (1)

where K=0.94 - shape factor, λ - x-ray wavelength of Cukα radiation, θ - Bragg’s angle and β - full width at half maximum of the peak. The average crystalline size of the synthesized TiO₂ film was 31.31nm as calculated from equation 1. The crystalline size increases when Cu²⁺ ions is added upto 0.05Wt% which may be due to replacement of Ti ions but further addition of Cu²⁺ ions decreases the crystalline size of the film which may be due to higher concentration.

The microstrain was calculated using the relation,

(2)

The values of Dislocation density (δ) were calculated using the formula,

(lines/m²) (3)

The strain and dislocation density decrease with the doping and it can be attributed to the incorporation of Cu ions into TiO₂ and difference between the ionic radius of Cu²⁺ and Ti⁴⁺ [19]. 0.05wt% Cu-TiO₂ film has better crystallinity, as the average crystalline size of it was larger and the dislocation density was lowest for it.

Morphological Analysis

Figs. 2(a) & (b) shows the surface morphology with 1kX magnification of TiO₂ and 0.05Wt% Cu-TiO₂ thin films coated on microscopic glass slides. The SEM micrograph of TiO₂ thin film depicts that the particles were fine spherical in shape and distributed all over the substrate. SEM image of 0.05Wt% Cu-TiO₂ thin film illustrates the agglomeration of the particles. Fig. 2 (c) shows the elemental analysis (EDX image) of 0.05Wt% Cu-TiO₂ thin film. It shows the presence of copper, titanium and oxygen in the sample. The absence of other elements indicates the purity of the film. The presence of copper peak from the analysis also reveals that the Cu²⁺ ions are incorporated in Ti⁴⁺ lattice sites [24].

Optical Analysis

UV-Vis spectroscopy analysis

The UV-Vis absorption spectra of the TiO₂ and Cu-TiO₂ thin films are shown in Fig. 3 (a). It shows the absorption is maximum for 0.05Wt% Cu-TiO₂ thin film. The optical band gap (Eg) values for the TiO₂ and Cu-TiO₂ films were calculated using the Tauc’s relation,

(α hν) = A(hν – E_g)ⁿ (4)

Where α is absorption coefficient, ν is light frequency, h is Planck’s constant, A is proportionality constant, E_g is band gap energy and there exponent n depends on type of transition. For a direct-allowed transition n = 2, for an indirect-allowed transition n =1/ 2 and for an indirect forbidden transition n = 3/2. The obtained band gap values for TiO₂ and Cu-TiO₂ thin films from Fig. 3(b) are tabulated in Table 2. Anatase exhibits an indirect band gap that is smaller than its direct band gap [25]. The difference in band gap value may be attributed due to the morphological change and the improvement of crystallinity of anatase phase [26]. The band gap values shown in Table 2 agree well with XRD results which denote the quantum confinement effect [27]. In the UV-Vis absorption spectra, a notable shift to a higher wavelength occurs for 0.1Wt% and 0.05Wt% Cu-TiO₂ thin film when compared to TiO₂ film. It attributes to the incorporation of Cu²⁺ ions into Ti⁴⁺ ions. It lowers the optical band gap of Cu-TiO₂ thin film by increasing its absorption band to visible region [28].

Photoluminescence Analysis

Photoluminescence (PL) spectrum of TiO₂ and different concentration of Cu-TiO₂ thin films are shown in Fig. 4. The violet emission peak obtained at 404 nm (3.07 eV) for TiO₂ film and at 391nm (3.16 eV) for Cu-TiO₂ thin films. Compared to TiO₂ film, the emission band was blue shifted for Cu-TiO₂ thin films. The blue shift of the PL peak was due to Cu doping [29]. The emission peak at 391nm might be due to luminescent centers, such as nano crystals and defects in the film [30]. The change of defect states on the shallow level of the film surface leads to variation in the PL intensity [31]. It is noted that, 0.05Wt% Cu-TiO₂ sample has the lowest intensity of the PL emission peaks, which indicates that the defect is less for the corresponding sample compared to the other samples. This is also confirmed by the XRD results which show that the dislocation density was lowest for 0.05 Wt% Cu-TiO₂ sample.

Electrochemical analysis

The electrochemical analysis with three electrode cell was carried out by using Ag/AgCl (KCl) as reference electrode, glassy carbon electrode (GCE) as counter electrode and TiO₂ and Cu-TiO₂ thin film as working electrode, separately. 0.1M Phosphate buffer solution (PBS) with pH 7 was the electrolyte. In the present work, the electrochemical study was carried out for 0.05Wt% Cu-TiO₂ thin film along with TiO₂ film, as it has better crystallinity, lower optical band gap and lesser crystal defects.

Fig. 5(a) shows the cyclic voltammetry (CV) sweep plots of 0.05Wt% Cu-TiO₂ electrode at the scan rates of 50 mV/s, 75 mV/s, 100 mV/s and 150 mV/s. The anodic peak potentials (E_pa) and cathodic peak potentials (E_pc) for 0.05Wt% Cu-TiO₂ electrode with various scan rates are shown in Table 3. It was observed that while increasing the scan rate, the anodic peak potentials shift towards higher potential and cathodic peak potentials shift towards lower potential which recommends the surface diffusion controlled and quasi reversible process [32]. The anodic peak current (I_pa) and cathodic peak current (I_pc) for 0.05Wt% Cu- TiO₂ electrode for various scan rates are obtained from the Fig. 5(a) and given in Table 3.

It was noted that the anodic current increased and cathodic current decreased with increase in scan rate from 50mV/s to 150mV/s. The comparison between CV sweep plot for TiO₂ and 0.05Wt% Cu-TiO₂ electrode at the scan rates of 150 mV/s was shown in Fig. 5(b). It was noted that the anodic and cathodic peak potential shifts towards lower potential for 0.05Wt% Cu-TiO₂ electrode due to doping effect. Similar report for anodic and cathodic peak potential shift was reported for TiO₂-NTAs and carbon doped TiO₂- NTAs [33]. 0.05Wt% Cu-TiO₂ electrode has high anodic current (I_pa) and cathodic current (I_pc) compared to TiO₂ electrode at 150mV/s. It shows that the electron transfer rate in Cu-TiO₂ is quicker than that of the TiO₂ electrode. Doping of Cu²⁺ ions into TiO₂ induces more oxygen vacancies into TiO₂.lattice [34]. Zhenguang shen et al., reported that oxygen vacancies would bound electrons as free carriers and they introduce the donor level between the conduction and valance bands which would result in increased conductivity for semiconductor oxides [35]. The higher direct electron transfer between the electrode surface and 0.05Wt% Cu-TiO₂electrode gives rise to enhanced voltammetric response [36].

The correlation between scan rates and current for 0.05Wt% Cu-TiO₂ electrode is shown in Fig. 6(a). The anodic peak current I_pa increased linearly with increasing scan rate which signifies the direct electron transfer between the electrode and surface diffusion controlled process [37-38]. Likewise the cathodic peak current I_pc decreases linearly with increase in scan rate. Fig. 6(b) shows the Linearity calibration plot for anodic current Vs scan rate, which is obtained by least square regression method. The regression equation is I_pa (A) = 2E-08x - 2E-07, with a correlation coefficient of 0.992.

The electrochemical response of 0.05Wt% Cu-TiO₂ electrode at 150mV/s Scan rate for 10 cycles is shown in Fig. 7. The stability of the electrode was observed with respect to the number of scanned cycles [39]. With increasing the number of cycles the CV sweep plot remain the same. It signifies that the 0.05Wt% Cu-TiO₂ electrode has good stability which may be used as electrode material to construct biosensors.

Electrochemical response of 0.05 Wt% Cu-TiO₂ for UA sensing

In the sensing process, UA was added in different concentrations ie; 1mM, 3mM, 5mM and 10 mM into 0.1M PBS electrolyte with 0.05 Wt% Cu-TiO₂ film as working electrode.

The comparison CV plot of sensing of different concentrations of UA in 0.1M PBS electrolyte with 0.05 Wt% Cu-TiO₂ electrode is shown in Fig. 8. It is observed from the plot that the 0.05 Wt% Cu-TiO₂ electrode shows a fast and sensitive response for UA. The anodic peak current has been increased and the cathodic peak current decreased upon the successive addition of UA. The current voltage response of 0.05 Wt% Cu-TiO₂ electrode in the presence of UA in PBS solution shows higher response in comparison with PBS solution. The anodic and cathodic peak potential and current are tabulated in Table 4. The linear curve for anodic and cathodic peak current Vs concentration of UA is shown in Fig. 9(a). It reveals that the anodic current increases and cathodic current decreases as the concentration of UA are increased. The excellent linear relationship between the peak current and the concentration of UA signifies that the 0.05Wt% Cu-TiO₂ electrode shows good response as a UA sensor.

The linearity calibration plot for anodic current Vs concentration of UA, which was evaluated by least square regression method, is shown in Fig. 9(b). The regression equation for 0.05Wt% Cu-TiO₂ electrode with various concentration of UA is I_pa (μA) = 4E-07x + 4E-06 and its correlation coefficient is 0.989. The sensitivity of UA of 0.05 Wt% Cu-TiO₂ electrode as obtained from the above plot is 3.81μA mM^-1cm^-2. The linear range of UA detection is 1mM-10mM.

CONCLUSION

Uric acid (UA) biosensor was successfully constructed using Cu doped TiO₂ electrode in an electrochemical cell. TiO₂ thin films and Cu-TiO₂ thin films were effectively prepared by spray pyrolysis technique with different concentration of Cu. The XRD analysis reveals the anatase and also less intense rutile phases of the film with tetragonal structure. The crystalline size was highest and the dislocation density, strain, optical band gap and PL intensity were lowest for the 0.05Wt% Cu- TiO₂ sample. Good crystallinity and electrical, optical and surface properties of 0.05Wt% Cu TiO₂ film show that the film is good for fabrication of biosensor. The electrochemical response studies of Cu-TiO₂ electrode for detection of UA show high stability, with sensitivity of 3.81μA mM^-1cm^-2. The current study implies that the constructed Cu-TiO₂ thin film based electrochemical biosensor act as a potential candidate for application in the detection of UA.

CONFLICT OF INTEREST

The authors declare that there are no conflicts of interest regarding the publication of this manuscript.

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