Web of Science, ISC, CAS, Google Scholar

Document Type : Original Article


1 Department of Chemistry, Graduate University of Advanced Technology, Kerman, Iran

2 Environment Department, Institute of Science and High Technology and Environmental Sciences, Graduate University of Advanced Technology, Kerman, Iran


In this work, three dimensional NiO nanowrinkles (3D NiO-NWs) were prepared and used as electrode materials to modify the surface of a glassy carbon electrode (3D NiO-NWs/GCE). Then, differential pulse voltammetry (DPV), cyclic voltammetry (CV) and chronoamperometry (CHA) were employed to determine the electrochemical response of theophylline on as-fabricated sensor. The electrochemical theophylline oxidation was elevated on the modified electrode. The peak current on the modified electrode in phosphate buffer solution (PBS, 0.1 M, pH=7.0) showed a linear elevation with an increase in the theophylline concentration (0.1-900.0 µM), with a narrow detection limit of 0.03±0.001 µM.

Graphical Abstract

Electrochemical Sensing of Theophylline using Modified Glassy Carbon Electrode


Main Subjects


1,3-Dimethylxanthine, or theophylline, is a member of xanthine family, with a great influence on respiration. Theophylline is extensively administered to manage different asthmatic problems as a bronchodilator agent. Theophylline acts as a successful respiratory stimulant to treat neonatal apnea, pediatric acute asthma, signs of acute and chronic asthma, bronchospasm [1], chronic obstructive pulmonary disease, and emphysema in adults. The acceptable limit of theophylline has been set at 5-20 μg/mL in adults for effectual treatment [2]; exceeding this range may be associated with adverse events, such as tachycardia, vomiting, central nervous system irritation and seizures. Theophylline, similar to other derivatives of methylated xanthine, is not only a competitive nonselective phosphodiesterase inhibitor [3], but also is a nonselective adenosine receptor antagonist [4]. Accordingly, there is a need for development of a facile analytical protocol to sensitively detect the theophylline. The existing techniques for the quantification of theophylline include spectrophotometry [5], high-performance liquid chromatography [6], chemiluminescence [7], surface-enhanced raman scattering [8], and gas chromatography–mass spectrometry [9]. All these methods, in addition to many advantages, suffer from some disadvantages such as complexity and cost- and time-consuming requisites. Among these, the electrochemical systems have aroused widespread attentions owing to rapidity, excellent sensitivity, time saving, negligible energy consumption, adorable reliability, affordability and ease of use [10-20].

The electrochemical sensors can have potent controllable properties through chemically surface modification of inert electrode substrates [21-28]. In operation, redox active sites move electrons between the analyte solution and the electrode substrates, mostly with a remarkable decrease in overpotential of activation. Moreover, chemically modified electrodes are less prone to oxide formation and surface fouling when comparing with inert electrode substrates.

Nanotechnology has enormous potential for providing innovative solutions to a wide range of applications [29-33]. Development of nanoscience and nanotechnology has allowed trials to apply different nanomaterials for the fabrication of chemically modified electrodes. In recent years, metal oxide nanoparticles (NPs), metallic (like silver and gold) NPs), and carbon nano-structures, like graphene and carbon nanotubes, alone or in combination have been utilized for electrode surface modification [34-40].

A metal oxide possessing great electrocatalytic response, as a p-type NiO semiconductor, has
a band-gap energy between 3.6 and 4 eV [41]. They have special merits, such as impressive electro-activity, exceptional electrochromic efficiency, admirable electro-conductivity, huge surface area, cost-effective fabrication process, and a broad range of modulation. Accordingly, NiO has been a successful electron mediator in the construction of various electrochemical sensing systems [42-44].

In this paper, the analytical application of a GCE modified with 3D NiO-NWs is proposed for determining theophylline. The modified electrode could remarkably enhance the electrochemical responses of theophylline and improve the sensitivity of theophylline detection.

Materials and Methods

Instruments and chemicals

PGSTAT-302N Autolab potentiostat/galvanostat (Eco Chemie, Netherlands) was a device to perform all electrochemical tests whose data were analyzed by General Purpose Electrochemical System (GPES) software. The phosphate buffer solutions within the PH value of 2.0 to 9.0 were prepared by the orthophosphoric acid and relevant salts (KH2PO4, K2HPO4, K3PO4). The pH values were measured by a digital pH meter (Metrohm type 713) equipped with a combined glass electrode in all solutions. The analytical grade of chemicals such as theophylline and other reagents were from Sigma-Aldrich and used as received.

Preparing the electrode

The 3D NiO-NWs were used for the modification of a GCE to prepare 3D NiO-NWs/GCE. For this purpose, 3D NiO-NWs suspension (1 mg/mL) in deionized water was prepared and 4 μL of the 3D NiO-NWs suspension drop was cast onto the GCE surface and allowed to dry completely in ambient conditions.

Results and Discussion

Electrochemical behaviour of theophylline at the various surface of electrodes

The electrochemical behavior of theophylline is proportional to the aqueous solution pH (Scheme 1). Hence, it appears necessary to optimize the solution pH to reach the best electrocatalytic oxidation of theophylline. Therefore, DPV was used to explore the electrochemical response of theophylline on the surface of 3D NiO-NWs/GCE in 0.1 M PBS at various pH values (2-9). According to the results, the highest peak current was related to the pH value of 7 (the optimum one) for theophylline oxidation on the 3D NiO-NWs/GCE surface.

The electrochemical performance of 3D NiO-NWs/GCE in comparison with bare GCE was studied by the CV technique in exposure to 200.0 μM of theophylline at 50 mV/s in PBS (0.1 M). The CVs of all as-fabricated electrodes in this study can be observed in Figure 1. The 3D NiO-NWs/GCE voltammetric behaviour (curve b) exhibited the relatively strongest and most characteristic anodic peak at 850 mV, sequentially. The bare GCE voltammetric behaviour (curve b) exhibited the relatively weak oxidation peak with a less intense at 1000 mV, sequentially. Consequently, the 3D NiO-NWs/GCE has obviously better electrocatalytic behaviour than the bare GCE towards the theophylline with the relatively strong current response.

The effects of the scan rate

The CV method was used to explore the influence of scan rate on the oxidation process of theophylline (100.0 μM) in 0.1 M PBS, as shown in Figure 2. An elevation in the scan rate (10-600 mV/s) enhanced the oxidation peak current of theophylline, in addition to gradually shifting the anodic potentials to more positive values. Figure 2 (inset) emphasizes the proportionality of the anodic response current linearly to the scan rate square root, which shows controlled-diffusion processes for the electrocatalytic oxidation of theophylline on 3D NiO-NWs/GCE.

Figure 1: CVs captured for 3D NiO-NWs/GCE (a) and bare GCE (b) in PBS (0.1 M, pH = 7.0) in exposure to 200.0 μM of theophylline at 50-mV/s the scan rate

Figure 2: CVs captured for 3D NiO-NWs/GCE in PBS (0.1 M, pH = 7.0) in exposure to 100.0 μM of theophylline at various scan rates; (1): 10, (2): 30, (3): 70, (4): 100, (5): 200, (6): 300, (7): 400, (8): 500, and (9): 600 mV/s, sequentially; Inset: changes in anodic peak currents against ν1/2

Chronoamperometric measurements

Chronoamperometry was considered for the investigation of theophylline electro-oxidation at the potential of 900 mV and various analyte concentrations in PBS (pH = 7.0), as seen in Figure 3. The current response of electroactive theophylline was described under a diffusion-limited electrocatalytic process using Cottrell’s equation:

I = nFAD1/2Cbπ-1/2t-1/2

In this equation, n stands for the number of electron transfer exchanged per reactant molecule, F for the Faraday constant, Cb for sunset yellow concentration (mol/cm3), and D for diffusion coefficient (cm2/s). The plot of I versus t−1/2 provided various linear curves for different theophylline contents of 0.1-1.0 mM (Figure 3, inset A). The slope of each straight line against the theophylline content eventually made it possible to calculate the overall slope of the best-fit line (Figure 3, inset B). At last, the overall slope in the Cottrell’s equation was utilized to estimate the mean D value, which was 1.6±0.02 ×10−6 cm2/s.

Figure 3: Chronoamperograms for 3D NiO-NWs/GCE in PBS (0.1 M, pH = 7.0) for different theophylline contents; (1): 0.1, (2): 0.4, (3): 0.7, and (4): 1.0 mM of theophylline; Insets: (a) Plots of I against t−1/2 from chronoamperograms 1 to 4. (b) Plot of straight lines slope against theophylline content

Calibration plot and limit of detection

The theophylline content was measured by the DPV technique. The DPVs captured for 3D NiO-NWs/GCE at various theophylline contents in PBS (0.1 M) are shown in Figure 4. There was a stepwise enhancement in the theophylline oxidation current by gradually increasing the theophylline contents, meaning the applicability of 3D NiO-NWs/GCE for electrochemically sensing the theophylline. Figure 4 (inset) represents the alterations in the oxidation signal on the 3D NiO-NWs/GCE as a function of various theophylline contents (0.1-900.0 µM), having a low limit of detection of 0.03 μM.

Also, the detection limit, Cm, of theophylline was obtained using the following equation:


In the above equation, m is the slope of the calibration plot (0.0049 μA/ μM) and Sb is the standard deviation of the blank response, which is obtained from 20replicate measurements of the blank solution. The detection limit is 0.03±0.001 µM. The comparison of the results for the detection of theophylline with different modified electrodes in the literatures is listed in Table 1.

Figure 4: DPVs captured for 3D NiO-NWs/GCE in PBS (0.1 M, pH = 7.0) in exposure to various theophylline contents; (1): 0.1, (2): 5.0, (3): 10.0, (4): 30.0, (5): 70.0, (6): 100.0, (7): 200.0, (8): 300.0, (9): 400.0, (10): 500.0, (11): 600.0, (12): 700.0, (13): 800.0, and (14): 900.0 µM of theophylline; Inset: plot of peak current as a function of various theophylline contents (0.1-900.0 µM)

Table 1: Comparison of the determination of theophylline between 3D NiO-NWs/GCE and some previous works reported in the literature


Analytical methods

Linear range



A Nafion/multi-wall carbon nanotubes (MWNTs) composite film-modified glassy carbon electrode


8⋅0 × 10–8–6⋅0 × 10–5 M

2⋅0 × 10–8 M


Urchin-like CdSe microparticles modified glassy carbon electrode


1.0-700.0 µM

0.4 µM


boron-doped diamond electrode


2.0-380.0 µM

1.45 µM



0.91 µM

manganese oxide nanoparticles/multiwalled carbon nanotube nanocomposite modified glassy carbon electrode


0.1-20.0 µM

0.01 µM


MnO2nanosheets/ionic liquid-functionalized grapheme/ glassy carbon electrode


1.0-220.0 µM

0.1 µM


Tungsten trioxide nanoparticles_multiwall carbon nanotubes/ glassy carbon electrode


0.025-2.6 µM

0.008 µM


poly-sulfosalicylic acid film decorated pure carbon fiber/ glassy carbon electrode


0.6-137.0 µM

0.2 µM




0.1-900.0 µM

0.03 µM

This Work



In summary, a GCE was modified using 3D NiO-NWs to provide a sensitive electrochemical sensor in the theophylline sensing. Based on the findings, the 3D NiO-NWs/GCE possessed a potent electrocatalytic performance toward theophylline. In the optimized circumstances, the linear range was broad from 0.1 to 900.0 μM for theophylline and the limit of detection was 0.03±0.001 μM.


The authors wish to thank Graduate University of advanced technology, Kerman, Iran for finantioal support of this research.



This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Authors' contributions

All authors contributed toward data analysis, drafting, and revising the paper and agreed to be responsible for all the aspects of this work.

Conflict of Interest

We have no conflicts of interest to disclose.


Hadi Beitollahi



Effat Sharifi Pour, Maryam Ebrahimi, Hadi Beitollahi. Electrochemical Sensing of Theophylline using Modified Glassy Carbon Electrode, Chem. Methodol., 2022, 6(7) 560-568


URL: http://www.chemmethod.com/article_150089.html

[1]. Kawai M., Kato M., Methods Findings Experiment. Clinical Pharmacology, 2000, 22:309 [Google Scholar]
[2]. Shetti N.P., Malode S.J., Nayak D.S., Bagihalli G.B., Reddy K.R., Ravindranadh K., Reddy C.V., A novel biosensor based on graphene oxide-nanoclay hybrid electrode for the detection of Theophylline for healthcare applications. Microchemical Journal, 2019, 149:103985 [Crossref], [Google Scholar], [Publisher]
[3]. Peters-Golden M., Canetti C., Mancuso P., Coffey M.J., Leukotrienes: underappreciated mediators of innate immune responses. The Journal of Immunology, 2005, 174:589 [Crossref], [Google Scholar], [Publisher]
[4]. Daly J.W., Jacobson K.A., Ukena D., Adenosine receptors: development of selective agonists and antagonists. Progress in clinical and biological research, 1987, 230:41 [Google Scholar], [Publisher]
[5]. Goicoechea H.C., Olivieri A.C., de la Peña A.M., Determination of theophylline in blood serum by UV spectrophotometry and partial least-squares (PLS-1) calibration. Analytica chimica acta, 1999, 384:95 [Crossref], [Google Scholar], [Publisher]
[6]. Chen P., Shen J., Wang C., Wei Y., Preparation of magnetic molecularly imprinted polymers based on a deep eutectic solvent as the functional monomer for specific recognition of lysozyme. Microchimica Acta, 2018, 185:1 [Crossref], [Google Scholar], [Publisher]
[7]. Zhou M.X., Guan C.Y., Chen G., Xie X.Y.,Wu S.H., Determination of theophylline concentration in serum by chemiluminescent immunoassay. Journal of Zhejiang University SCIENCE B, 2005, 6:1148 [Crossref], [Google Scholar], [Publisher]
[8]. Liu P., Liu R., Guan G., Jiang C., Wang S., Zhang Z., Surface-enhanced Raman scattering sensor for theophylline determination by molecular imprinting on silver nanoparticles. Analyst, 2011, 136:4152 [Crossref], [Google Scholar], [Publisher]
[9]. Saka K., Uemura K., Shintani-Ishida K., Yoshida K. I., Acetic acid improves the sensitivity of theophylline analysis by gas chromatography–mass spectrometry. Journal of Chromatography B, 2007, 846:240 [Crossref], [Google Scholar], [Publisher]
[10]. Nassar A.M., Salah H., Hashem N., Khodari M., Assaf H.F., Electrochemical Sensor Based on CuO Nanoparticles Fabricated From Copper Wire Recycling-loaded Carbon Paste Electrode for Excellent Detection of Theophylline in Pharmaceutical Formulations. Electrocatalysis, 2022, 13:154 [Crossref], [Google Scholar], [Publisher]
[11]. Beitollahi H., Dourandish Z., Tajik S., Ganjali M.R., Norouzi P., Faridbod F., Application of graphite screen printed electrode modified with dysprosium tungstate nanoparticles in voltammetric determination of epinephrine in the presence of acetylcholine. Journal of Rare Earths 2018, 36:750‏ [Crossref], [Google Scholar], [Publisher]
 [12]. Karimi-Maleh H., Darabi R., Shabani-Nooshabadi M., Baghayeri M., Karimi F., Rouhi J, Karaman C.,  Determination of D&C Red 33 and Patent Blue V Azo dyes using an impressive electrochemical sensor based on carbon paste electrode modified with ZIF-8/g-C3N4/Co and ionic liquid in mouthwash and toothpaste as real samples. Food and Chemical Toxicology 2022, 11:2907 [Crossref], [Google Scholar], [Publisher]
[13]. Guan Q., Guo H., Xue R., Wang M., Wu N., Cao Y., Yang W., Electrochemical sensing platform based on covalent organic framework materials and gold nanoparticles for high sensitivity determination of theophylline and caffeine. Microchimica Acta, 2021, 188:1 [Crossref], [Google Scholar], [Publisher]
[14]. Beitollahi H., Garkani-Nejad, F., Voltammetric determination of vitamin B6 (pyridoxine) at a graphite screen-printed electrode modified with graphene oxide/Fe3O4@ SiO2 nanocomposite. Russian Chemical Bulletin, 2018, 67:238 [Crossref], [Google Scholar], [Publisher]
[15]. Killedar L.S., Shanbhag M.M., Shetti N.P., Malode S.J., Veerapur R.S., Reddy K.R., Novel graphene-nanoclay hybrid electrodes for electrochemical determination of theophylline. Microchemical Journal, 2021, 165:106115 [Crossref], [Google Scholar], [Publisher]
[16]. Karimi-Maleh H., Shojaei A.F., Tabatabaeian K., Karimi F., Shakeri S., Moradi R., Simultaneous determination of 6-mercaptopruine, 6-thioguanine and dasatinib as three important anticancer drugs using nanostructure voltammetric sensor employing Pt/MWCNTs and 1-butyl-3-methylimidazolium hexafluoro phosphate. Biosensors and Bioelectronics, 2016, 86:879 [Crossref], [Google Scholar], [Publisher]
[17]. Karimi-Maleh H., Khataee A., Karimi F., Baghayeri M., Fu L., Rouhi J., Boukherroub R., A green and sensitive guanine-based DNA biosensor for idarubicin anticancer monitoring in biological samples: A simple and fast strategy for control of health quality in chemotherapy procedure confirmed by docking investigation. Chemosphere, 2022, 291:132928 [Crossref], [Google Scholar], [Publisher]
[18]. Tajik S., Dourandish Z., Garkani-Nejad F., Aghaei Afshar A., Beitollahi H., Voltammetric determination of isoniazid in the presence of acetaminophen utilizing MoS2-nanosheet-modified screen-printed electrode. Micromachines, 2022, 13:369 [Crossref], [Google Scholar], [Publisher]
[19]. Shikandar D.B., Shetti N.P., Kulkarni R.M., Kulkarni S.D., Silver-doped titania modified carbon electrode for electrochemical studies of furantril. ECS Journal of Solid State Science and Technology, 2018, 7:Q3215 [Google Scholar], [Publisher]
[20]. Raoof J.B., Ojani R., Beitollahi H., Hosseinzadeh R., Electrocatalytic oxidation and highly selective voltammetric determination of L-cysteine at the surface of a 1-[4-(ferrocenyl ethynyl) phenyl]-1-ethanone modified carbon paste electrode. Analytical Sciences, 2006, 22:1213 [Crossref], [Google Scholar], [Publisher]
[21]. Tajik S., Askari M.B., Ahmadi S.A., Garkani-Nejad F., Dourandish Z., Razavi R., Beitollahi H., Di Bartolomeo A., Electrochemical sensor based on ZnFe2O4/RGO nanocomposite for ultrasensitive detection of hydrazine in real samples. Nanomaterials, 2022, 12:491 [Crossref], [Google Scholar], [Publisher]
[22]. Poo-arporn Y., Pakapongpan S., Chanlek N., Poo-arporn R.P.,  The development of disposable electrochemical sensor based on Fe3O4-doped reduced graphene oxide modified magnetic screen-printed electrode for ractopamine determination in pork sample. Sensors and Actuators B: Chemical, 2019, 284:164‏ [Crossref], [Google Scholar], [Publisher]
[23]. Tajik S., Afshar A.A., Shamsaddini S., Askari M.B., Dourandish Z., Garkani-Nejad F., Bartolomeo A.D., Fe3O4@MoS2/rGO Nanocomposite/Ionic Liquid Modified Carbon Paste Electrode for Electrochemical Sensing of Dasatinib in the Presence of Doxorubicin. Industrial & Engineering Chemistry Research, 2022 [Crossref], [Google Scholar], [Publisher]
[24]. Karimi-Maleh H., Karimi F., Orooji Y., Mansouri G., Razmjou A., Aygun A., Sen F.,  A new nickel-based co-crystal complex electrocatalyst amplified by NiO dope Pt nanostructure hybrid; a highly sensitive approach for determination of cysteamine in the presence of serotonin. Scientific reports, 2020, 10:11699 [Crossref], [Google Scholar], [Publisher]
[25]. Tajik S., Beitollahi H., Shahsavari S., Garkani-Nejad F., Simultaneous and selective electrochemical sensing of methotrexate and folic acid in biological fluids and pharmaceutical samples using Fe3O4/ppy/Pd nanocomposite modified screen printed graphite electrode. Chemosphere, 2022, 291:132736 [Crossref], [Google Scholar], [Publisher]
[26]. Esfandiari-Baghbamidi S., Beitollahi H., Mohammadi S.Z., Tajik S., Soltani-Nejad S., Soltani-Nejad V.,  Recent advances in applications of voltammetric sensors modified with ferrocene and its derivatives. ACS omega, 2013, 34:1869‏ [Crossref], [Google Scholar], [Publisher]
[27]. Eren T., Atar N., Yola M.L., Karimi-Maleh H., A sensitive molecularly imprinted polymer based quartz crystal microbalance nanosensor for selective determination of lovastatin in red yeast rice. Food chemistry, 2015, 185:430 [Crossref], [Google Scholar], [Publisher]
[28]. Miraki M., Karimi-Maleh H., Taher MA., Cheraghi S., Karimi F., Agarwal S., Gupta V.K., J. Voltammetric amplified platform based on ionic liquid/NiO nanocomposite for determination of benserazide and levodopa. Journal of Molecular Liquids, 2019, 278:672 [Crossref], [Google Scholar], [Publisher]
[29]. Toufani M., Kasap S., Tufani A., Bakan F., Weber S., Erdem E., Synergy of nano-ZnO and 3D-graphene foam electrodes for asymmetric supercapacitor devices. Nanoscale, 2020, 12:12790 [Crossref], [Google Scholar], [Publisher]
[30]. Mohanraj J., Durgalakshmi D., Rakkesh R.A., Balakumar S., Rajendran S., Karimi-Maleh H., Facile synthesis of paper based graphene electrodes for point of care devices: A double stranded DNA (dsDNA) biosensor. Journal of Colloid and Interface Science, 2020, 566:463 [Crossref], [Google Scholar], [Publisher]
[31]. Al Sharabati M., Abokwiek R., Al-Othman A., Tawalbeh M., Karaman C., Orooji Y., Karimi F., Biodegradable polymers and their nano-composites for the removal of endocrine-disrupting chemicals (EDCs) from wastewater: a review. Environmental Research, 2021, 202:111694 [Crossref], [Google Scholar], [Publisher]
[32]. Karimi-Maleh H., Karaman C., Karaman O., Karimi F., Vasseghian Y., Fu L., Mirabi A., Nanochemistry approach for the fabrication of Fe and N co-decorated biomass-derived activated carbon frameworks: a promising oxygen reduction reaction electrocatalyst in neutral media. Journal of Nanostructure in Chemistry, 2022, 1 [Crossref], [Google Scholar], [Publisher]
[33]. He Q., Shi J., Mesoporous silica nanoparticle based nano drug delivery systems: synthesis, controlled drug release and delivery, pharmacokinetics and biocompatibility. Journal of Materials Chemistry, 2011, 21:5845 ‏[Crossref], [Google Scholar], [Publisher]
[34]. Irannezhad F., Seyed-Yazdi J., Hekmatara S.H., Electrochemical sensing platform for simultaneous detection of 6-mercaptopurine and 6-thioguanine using RGO-Cu2O/Fe2O3 modified screen-printed graphite electrode. Journal of Electrochemical Science and Engineering, 2022, 12:47 ‏[Crossref], [Google Scholar], [Publisher]
[35]. Beitollahi H., Tajik S., Asadi M.H., Biparva P., Application of a modified graphene nanosheet paste electrode for voltammetric determination of methyldopa in urine and pharmaceutical formulation. Journal of Analytical Science and Technology, 2014, 5:1 ‏‏[Crossref], [Google Scholar], [Publisher]
[36]. Karimi-Maleh H., Sheikhshoaie M., Sheikhshoaie I., Ranjbar M., Alizadeh J., Maxakato N.W., Abbaspourrad A., A novel electrochemical epinine sensor using amplified CuO nanoparticles and an-hexyl-3-methylimidazolium hexafluorophosphate electrode. New Journal of Chemistry, 2019, 43:2362‏‏ [Crossref], [Google Scholar], [Publisher]
[37]. Azizpour Moallem, Q., Beitollahi H., Electrochemical sensor for simultaneous detection of dopamine and uric acid based on a carbon paste electrode modified with nanostructured Cu-based metal-organic frameworks. Microchemical Journal, 2022, 177:107261‏ [Crossref], [Google Scholar], [Publisher]
[38]. Patil S.M., Pattar V.P., Nandibewoor S.T., A microfabricated potentiometric sensor for metoclopramide determination utilizing a graphene nanocomposite transducer layer. Analytical and Bioanalytical Chemistry, 2016, 6:265‏ [Crossref], [Google Scholar], [Publisher]
[39]. Alavi-Tabari S.A., Khalilzadeh M.A., Karimi-Maleh H., Simultaneous determination of doxorubicin and dasatinib as two breast anticancer drugs uses an amplified sensor with ionic liquid and ZnO nanoparticle. Journal of electroanalytical chemistry, 2018, 811:84 [Crossref], [Google Scholar], [Publisher]
[40]. Jahani P.M., Electrocatalytic determination of levodopa in presence of cabergoline using carbon paste electrode modified with graphene quantum dots/2-chlorobenzoyl ferrocene/ionic liquid. Journal of Electrochemical Science and Engineering, 2022, 12:81‏ [Crossref], [Google Scholar], [Publisher]
[41]. Raeisi-Kheirabadi N., Nezamzadeh-Ejhieh A., Aghaei H., Electrochemical amperometric sensing of loratadine using NiO modified paste electrode as an amplified sensor. Iranian Journal of Catalysis, 2021, 11:181 [Google Scholar]
[42]. Wang J.M., Shao D., Jiang L.L., Li H.X., Gao Y.J., Rao S.Q., Yang Z.Q., Synthesis of Rod-like NiO–Co3O4 Composites for Sensitive Electrochemical Detection of Hydrogen Peroxide. Journal of Analysis and Testing, 2021, 1 [Crossref], [Google Scholar], [Publisher]
[43]. Yi W., Li Z., Dong W., Han C., Guo Y., Liu M., Dong, C., Three-Dimensional Flower-like Nickel Oxide/Graphene Nanostructures for Electrochemical Detection of Environmental Nitrite. ACS Applied Nano Materials. 2022, 5:216 [Crossref], [Google Scholar], [Publisher]
[44]. Xu Z., Li R., Zhao S., Zhangsun H., Wang Q., Wang L., Combine etching-doping sedimentation strategy and carbonization to design double-deck petal-like NiO/CoO nanoporous carbon composite for methyl parathion detection. Chemical Engineering Journal,  2021, 426:131906‏ [Crossref], [Google Scholar], [Publisher]
[45]. Yang S., Yang R., Li G., Li J., Qu L., Voltammetric determination of theophylline at a Nafion/multi-wall carbon nanotubes composite film-modified glassy carbon electrode. Journal of Chemical Sciences, 2010, 122:919 [Crossref], [Google Scholar], [Publisher]
[46]. Yin H., Meng X., Su H., Xu M., Ai S., Electrochemical determination of theophylline in foodstuff, tea and soft drinks based on urchin-like CdSe microparticles modified glassy carbon electrode. Food Chemistry, 2012, 134:1225 [Crossref], [Google Scholar], [Publisher]
[47]. Cinková K., Zbojeková N., Vojs M., Marton M., Samphao A., Švorc Ľ., Electroanalytical application of a boron-doped diamond electrode for sensitive voltammetric determination of theophylline in pharmaceutical dosages and human urine. Analytical Methods, 2015, 7:6755 [Crossref], [Google Scholar], [Publisher]
[48]. Yang Y. J., Li W., High sensitive determination of theophylline based on manganese oxide nanoparticles/multiwalled carbon nanotube nanocomposite modified electrode. Ionics, 2015, 21:1121 [Crossref], [Google Scholar], [Publisher]
[49]. Zhuang X., Chen D., Wang S., Liu H., Chen L., Manganese dioxide nanosheet-decorated ionic liquid-functionalized graphene for electrochemical theophylline biosensing. Sensors and Actuators B: Chemical, 2017, 251:185 [Crossref], [Google Scholar], [Publisher]
[50]. Rezvani S. A., Soleymanpour A.,  Application of a sensitive electrochemical sensor modified with WO3 nanoparticles for the trace determination of theophylline. Microchemical Journal, 2019, 149:104005 [Crossref], [Google Scholar], [Publisher]
[51]. Duan Y., Wang A., Ding Y., Li L., Duan D., Lin J., Liu J., Fabrication of poly-sulfosalicylic acid film decorated pure carbon fiber as electrochemical sensing platform for detection of theophylline. Journal of Pharmaceutical and Biomedical Analysis, 2021, 192:113663 [Crossref], [Google Scholar], [Publisher]