Application of cobalt oxide nanostructured modified aluminium electrode for electrocatalytic oxidation of guanine and single-strand DNA

Abstract

A novel electrochemical biosensor for the selective and sensitive determination of guanine and single-strand DNA (ssDNA) has been developed by the electrodeposition of cobalt oxide nanoflowers (CoOx) on an aluminium electrode (Al). The modified aluminium electrode showed an excellent intensification of the guanine oxidation response in ssDNA. The morphological characteristics, phase composition and electrochemical properties of the modified electrode were studied by SEM, XRD and electrochemical impedance studies. The effects of scan rate, pH, and concentration of ssDNA and guanine on the response of the sensor were investigated. Detection limits of 4 and 450 nM were obtained for guanine and ssDNA, respectively. Simplicity of fabrication, excellent electrocatalytic ability, high stability and selectivity of the modified electrode were achieved.

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1. Introduction

DNA analysis plays an ever-increasing role in numerous areas related to human health such as diagnosis of infectious diseases, genetic mutations, drug discovery, forensics and food technology. For this purpose, numerous assay techniques are available; however, primarily, sequencing is achieved through the tagging of a DNA sample with a fluorescent compound.¹ As an option, the use of electrochemistry may provide a cheaper and a cost-effective method of DNA assay. Both the purine bases (guanine and adenine) are of special interest to electrochemistry due to the relative ease with which they can be oxidized, and thus allow the quantification of a DNA sample. Guanine is an important component present in DNA and is the most readily oxidized form of the four nucleic acid bases.^2–4 It is believed that guanine plays a key role in the oxidation of DNA by various types of oxidants and free radicals.

Semiconductor nanostructures,^5,6 especially metal oxide nanostructures, with unique properties and applications in many areas, configured as electronic devices have been utilized as universal devices for ultra-sensitive direct electrical detection of biological and chemical species. Sensing behavior is one of the most important and well-known properties of metal oxide sensors, which usually demonstrate much higher sensitivity to their chemical environment than other chemical/biosensors for their sensitivity, selectivity and stability. The sensing mechanism of metal oxide materials is primarily regulated by the electrically and chemically active oxygen vacancies on the oxide surfaces. In this example, two kinds of sensing responses have been observed.^7,8 First, electrons in metal oxides are withdrawn and effectively depleted from the conduction band upon the adsorption of charge-accepting molecules, such as NO₂ and O₂, at vacant sites, leading to a reduction of conductivity. Second, in an oxygen-rich environment, chemical molecules, such as CO and H₂, react with surface adsorbed oxygen and release the captured electrons back to the channel, resulting in an increase in metal oxide conductance.

Cobalt oxides (CoOx) are versatile materials in emerging fields such as clean energy, biomaterials, and catalysis.⁹ Nanostructured cobalt oxide hydroxide (CoOOH) and Co₃O₄ materials have remarkable electrochemical properties for applications in batteries, fuel cells, sensors, etc.^10–12 due to their excellent electrocatalytic activity. For many of these applications, high surface areas and conductivities of materials are crucial. Electrodeposition as a simple and easy procedure for the fabrication of CoOx films is very interesting.^13,14 Recently, various forms of cobalt oxide nanomaterials have been applied to construct chemical sensors or biosensors based on the electrocatalytic ability of the cobalt oxide redox couple for oxidation. Electrocatalytic oxidation of hydrogen peroxide,¹⁵ hydroquinone,¹⁶ glutathione,¹⁷ glucose¹⁸ and arsenic¹⁹ has been reported on electrodeposited CoOx nanostructures on various electrodes. Due to the biocompatibility of cobalt oxide nanostructures, the proposed nanomaterials have also been used to immobilize biomolecules such as FAD, cholesterol oxidase and hemoglobin.^20–22

Until today, the most widely used electrodes for the determination of guanine have been glassy carbon²³ and carbon paste electrodes.²⁴ Aluminium is the cheapest metal possessing unique properties such as excellent thermal and electrical conductivity, low density, light weight, etc. By changing the surface of aluminium,^25,26 an assortment of novel sensors can be constructed, which can even compete with carbon nanotube (CNT)-modified electrodes.

Based on the comprehensive literature, it was found that no work has reported the fabrication of a chemical or electrochemical sensor using an Al electrode deposited with CoOx nanoflowers for the detection of biomolecules. For the first time, we are reporting mechanistic studies on the electrocatalytic oxidation of guanine and single-strand DNA (ssDNA) with a simply prepared cobalt oxide nanostructure modified aluminium electrode. The probable analytical application of the modified electrode was assessed, and it was used for the voltammetric detection of guanine in the nanomolar concentration range. The present study extends the application of cobalt oxide nanomaterials to the detection of important biomolecules using an electrochemical method.

2. Experimental

2.1. Reagents

Guanine and double-stranded DNA (dsDNA) from calf thymus were obtained from Sigma. Cobalt(II) nitrate, sodium nitrate, potassium chloride, potassium hexacyanoferrate and all other reagents were obtained from Merck, India and were used without further purification. Double-distilled deionized water was used throughout the study. A 5 μg mL⁻¹ solution of guanine was prepared daily by dissolving appropriate amounts of guanine in 100 mL of alkali media (NaOH 0.1 M). This solution was diluted to appropriate concentrations, and its pH was adjusted by the addition of acetic acid.

2.2. Preparation of ssDNA samples

Thermally denatured dsDNA was produced according to the previous report.²⁷ In brief, native calf thymus dsDNA samples were dissolved in water, and then the solution was heated in a boiling-water bath (100 °C) for about 10 min. Finally, the solution was rapidly cooled in an ice bath. In general, thermal denaturation involves the rupturing of hydrogen bonds, disturbance of stacking interaction, but not the breakage of a covalent bond. Therefore, thermally denatured dsDNA may act as ssDNA. The obtained solution was diluted to an appropriate concentration daily using phosphate buffer solution (pH 7.2).

2.3. Modified electrode fabrication

An aluminium sheet (purity 99.99%, thickness 0.3 mm) was used as a substrate for the preparation of CoOx nanoflowers. Before electrodeposition, aluminium was annealed at 450 °C for half an hour, and then it was etched in 5% sodium hydroxide for 2 min to remove the native barrier layer. After etching, the electrode was rinsed in distilled water. Then, the Al electrode was chemically polished with a mixture of concentrated sulphuric, nitric and phosphoric acids. Prior to electrodeposition, the electrode was dipped in phosphoric acid for 5 min to remove the oxide layer.

2.4. Formulation and characterization of CoOx array films

The electrodeposition of cobalt oxide was performed in a standard three-electrode glass cell at 20 °C using the aluminium electrode as the working electrode, a saturated calomel electrode (SCE) as the reference electrode and Pt foil as the counter electrode. The precursor films²⁸ were electrodeposited from an aqueous solution containing 0.6 M Co(NO₃)₂ and 0.05 M NaNO₃ using a CHI 760c electrochemical workstation. The electrodeposition experiment was carried out at a constant potential of −1.0 V vs. calomel electrode for various time periods. After electrochemical deposition, the electrode was washed with deionized water, dried at 85 °C and then annealed at 250 °C in air for 1 h to transform Co(OH)₂ to Co₃O₄.

2.5. Instrumentation

The electrodeposition of CoOx nanoflowers and electrochemical measurements were performed on a CHI 760C electrochemical workstation (CH Instrument Inc., USA). Voltammetric experiments were carried out by utilizing a schematic three-electrode system with aluminium having a diameter of 3.0 mm as the working electrode, a Pt coil as the auxiliary electrode, and calomel as the reference electrode. A Hitachi SU-70 was employed for field emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD) experiments. All the experiments were performed at room temperature.

3. Results and discussion

The formation of CoOx nanoflowers on the aluminium electrode by electrodeposition may involve the following mechanism. The electrodeposition process of the Co(OH)₂ precursor film would include an electrochemical reaction and precipitation, followed by annealing and can be stated as:²⁹

NO₃⁻ + H₂O + 2e⁻ → NO₂⁻ + 2OH⁻ (1)

Co²⁺ + 2OH⁻ → Co(OH)₂ (2)

3Co(OH)₂ → Co₃O₄ + 2H₂O (3)

3.1. SEM and XRD patterns of CoOx/Al

Fig. 1 shows the SEM image of a typical sample composed of several uniform flower-like architectures approximately 500 nm in diameter. The detailed morphology of CoOx/Al is shown in Fig. 1b, which reveals that the full structure of the nanoflowers is built from several dozen nanopetals with smooth surfaces. These nanopetals are connected to each other through the center to form 3D flower-like structures. The figure also shows that the petals are porous because of the removal of water molecules at high temperature.³⁰ In this morphology, triangular-shaped porous structures are also found, which are formed by the merging of approximately three or four nanopetals (inset Fig. 1b). These triangular-shaped porous CoOx nanoflowers placed at different angles have high surface areas and easily attract particles from the environment, forming a conduit to exchange electrons.

Fig. 1 (A and B) Different magnifications of the SEM image of CoOx/Al. (C) XRD pattern image of CoOx nanostructure.

The surfaces of the petals of the CoOx nanoflowers are very smooth, probably due to Ostwald ripening. The morphology of the CoOx nanoflower depends on several factors, including crystal-face attraction, electrostatic and dipolar fields associated with the aggregate,³¹ van der Waals forces,³² hydrophobic interactions,³³ and hydrogen bonds.³⁴

The XRD pattern of the as-deposited CoOx nanoflowers on the aluminium electrode is shown in Fig. 1c. The diffraction peaks of CoOx nanoflowers can be indexed as spinel cubic Co₃O₄ phase (JCPDS no. 42-1467), indicating that the precursor Co(OH)₂ is transformed to the Co₃O₄ nanoflower. The sharp peaks confirm the polycrystalline nature of the CoOx nanoflower.

3.2. Electrochemical characterization of the modified electrodes

Electrochemical impedance spectroscopy (EIS) provides detailed information on the changes in the surface properties of modified electrodes. The impedance spectra include a semicircular portion and a linear portion. The semicircular diameter at higher frequencies corresponds to the electron-transfer-limited process or electron-transfer resistance (R_et), and this resistance controls the electron transfer kinetic process of the redox probe on the electrode interface. The linear portion at lower frequencies corresponds to the diffusion process. The typical Nyquist diagrams of equivalent [Fe(CN)₆]^3−/4− at the bare Al and CoOx/Al electrodes are illustrated in Fig. 2. The R_et of the bare Al electrode is estimated to be 994 Ω cm⁻². Later, a further decrease in the R_et (519 Ω cm⁻²) is observed due to the electrodeposition of the CoOx nanoflowers, implying that the presence of CoOx nanoflowers plays a significant role in accelerating the transfer of the electrons, thus decreasing the resistance of the CoOx/Al to Fe(CN)₆^4−/3−. These effects indicate that CoOx nanoflowers were modified successfully on the surface of aluminium and significantly enhanced conductivity.

Fig. 2 Nyquist diagrams of (a) bare aluminium, (b) CoOx/Al recorded in 5.0 mM [Fe(CN)₆]^3−/4− containing 0.1 M KCl.

3.3. Electrocatalytic oxidation of guanine and ssDNA at the surface of CoOx/Al-modified aluminium electrode

The electrochemical reduction and oxidation of natural nucleic acids is irreversible and occurs at highly negative and positive potentials, respectively. The oxidation of guanine is irreversible and occurs at a highly positive potential on conventional electrodes. This nucleic acid base roughly shows an oxidation peak at 0.9–1.0 V at different surface electrodes.^35–37 Since the oxidation peak of guanine is close to a well-defined oxidation peak of CoOx appearing in the phosphate buffer medium, we expected an electrocatalytic mechanism initiated by the electrochemical oxidation of the reduced form of the CoOx existing at the surface of the electrode and then completed by the chemical oxidation of guanine, which also regenerates the reduced form of the CoOx; therefore, this system can be used for the electrocatalytic oxidation of guanine. To exhibit the electrocatalytic activity of CoOx nanoflowers toward the oxidation of guanine, the voltammetric behavior of guanine was investigated at the surface of bare and CoOx-modified aluminium electrode. Fig. 3A shows the cyclic voltammograms of CoOx nanoflower-modified aluminium electrode in a 0.25 M phosphate buffer solution in the absence (curve a) and the presence (curve b) of 0.5 μM guanine. As shown, for the CoOx/Al electrode (curve a), a low redox response obtained in the absence of guanine can be seen in the potential range 0.5 to 1.1 V. After the addition of guanine at the CoOx/Al electrode (curve b), the oxidation current of cobalt oxide nanoflowers was considerably increased due to the electrocatalytic oxidation of guanine. Similar results were obtained via the oxidation of ssDNA, as shown in Fig. 3B. In this figure, the cyclic voltammogram indicates the signal of the CoOx-modified aluminium electrode in 0.25 M phosphate buffer solution. Fig. 3B shows the cyclic voltammogram related to 25 μM ssDNA (the signal of 25 μM ssDNA on the surface of the bare electrode was subtracted). The anodic peak current of CoOx/Al increased due to the presence of guanine or ssDNA, whereas the cathodic peak current of this CoOx decreased. The anodic peak potential for the oxidation of guanine at the CoOx-modified aluminium electrode is about 0.791 V. Therefore, an enhancement of peak current is achieved in this system, which clearly demonstrates the occurrence of an electrocatalytic process. As shown in Fig. 3, for the bare Al electrode (curve c), no significant redox response is obtained in the electrocatalytic oxidation of guanine and ssDNA.

Fig. 3 (A) Cyclic voltammograms of CoOx-modified Al electrode (a) in the absence and (b) in the presence of 0.5 μM guanine in 0.25 M phosphate buffer solution. (B) Cyclic voltammograms of CoOx-modified Al electrode (a) in the absence and (b) in the presence of 25 μM ssDNA guanine in 0.25 M phosphate buffer solution. (c) Cyclic voltammogram of 0.5 μM and 25 μM ssDNA guanine at the surface of the bare Al electrode, scan rate of 100 mV s⁻¹ in 0.25 M phosphate buffer solution.

3.4. Effect of CoOx deposition time on electrocatalytic property of CoOx/Al electrode

The film thickness of the porous thin film-modified electrode disturbs both the kinetics of the electrode processes and the mass transfer mechanism via diffusion through the porous film,^38–41 and thus plays a predominant role in the voltammetric response of these electrodes toward different analytes. The electrodeposition technique was chosen to produce thin, porous films of CoOx nanoflowers on the Al electrode surface because the layer thickness may be easily modulated. The attained effect of the amount of deposited CoOx nanoflowers on the response current was investigated and the corresponding results are shown in Fig. 4. The response current of the CoOx/Al to the addition of 1 μM guanine increased with increasing the duration of the CoOx deposition time from two to four minutes, showing that the modification of Al with thin films of CoOx nanoflowers results in a significant intensification of the guanine oxidation response. However, when the CoOx deposition time is more than four minutes, the response current decreased. This might be linked to a decrease in the actual “working” surface area of the electrode, resulting from excess deposition when larger volumes of CoOx nanoflowers might be aggregated on the electrode surface. In the light of this possibility, the deposition time of four minutes was chosen for further sensor optimization and study.

Fig. 4 Effect of electrodeposition time of CoOx/Al on peak current in the presence of 0.5 μM guanine.

3.5. Effect of pH

To assess optimum pH and to evaluate the ratio of electrons and protons involved in the anodic oxidation of guanine on the surface of the modified electrode, the experiments were carried out at various pH values. Accordingly, as shown in Fig. 5, the electrocatalytic activity of the modified electrode toward guanine oxidation is pH-dependent. In the pH range 12–5, the modified electrode shows electrocatalytic activity but higher peak currents are observed at pH 6. This pH was selected as the most favourable for determining the experiments. In addition, peak potentials are shifted to higher positive potentials with decreasing pH values and no peak current was observed at pH values below 4 due to the disintegration of the cobalt oxide film. As illustrated, both peak current and potentials are dependent on pH values of the buffer solution.

Fig. 5 Effect of pH on peak potential and peak current of CoOx/Al-modified electrode in 0.25 M phosphate buffer solution containing 0.5 μM guanine.

3.6. Kinetics of electrocatalytic oxidation of guanine on modified electrode

Fig. 6a shows the cyclic voltammograms of a 0.5 μM guanine solution at different scan rates. The peak current (I_p) for the anodic oxidation of guanine is proportional to the square root (ν^1/2) of the scan rate (Fig. 6b), suggesting that the process is controlled by the diffusion of analyte as expected for a catalytic system. It can be noted in Fig. 6a that by increasing the scan rate, the peak potential for the catalytic oxidation of guanine moves to more positive values and the plot of the peak current vs. square root of scan rate deviates from linearity (at ν > 200 mV s⁻¹), suggesting a kinetic limitation in the reaction between the redox sites of the cobalt oxide nanostructures and guanine. Based on these findings, the following catalytic framework describes the reaction sequence in the oxidation of guanine by cobalt oxide nanostructures, which is comparable to those reported previously.^42,43

Co₃O₄ + 4H₂O + 4OH⁻ → 12CoOOH + 4e⁻ (4)

Fig. 6 (a) Cyclic voltammograms of the CoOx/Al-modified electrode in 0.25 M phosphate buffer solution (pH 6) containing 0.5 μM guanine at scan rates of 10–100 mV s⁻¹. (b) Plot of anodic peak current vs. square root of scan rate. (c) Plot of anodic peak potential vs. log ν.

A plot of log I versus η (overpotential), recognized as a Tafel plot, is useful for evaluating kinetic parameters.⁴⁴ The Tafel slope can be obtained by a method according to the following equation, which is valid for a completely irreversible diffusion-controlled process.⁴⁵

On the basis of this equation, the slope of the E_p versus log ν plot (Fig. 6c) is b/2 = ∂E_p/∂ log ν, where b indicates the Tafel slope. Thus, b = 2 × 58 = 116 mV s⁻¹. This slope yields a value of 2.71 for (1 − α)n_a, which indicates a one-electron transfer as the rate-limiting step by assuming a transfer coefficient of α = 0.3.

The number of electrons in the overall reaction (n) can also be obtained from the slope of I_p vs. ν^1/2 according to the following equation,⁴⁶

I_p = 2.99 × 10⁵n[n_α(1 − α)]^1/2AC_sD^1/2ν^1/2 (7)

where α is the charge transfer coefficient (calculated from the Tafel slope) and all the other symbols have their conventional meaning. The total number of electrons involved in the anodic oxidation of guanine is 3.79. A four-electron transfer in the oxidation of guanine at other modified electrodes was reported.^36,42,43

3.7. Analytical application

The DPV technique is one of the most sensitive and high-resolution techniques compared to the CV technique to examine the electrochemical behavior of reactant molecules, which are bound to the electrode surface. Fig. 7a displays the different concentrations of guanine, ranging from 50 nM to 10 μM, at a CoOx-modified Al electrode obtained by differential pulse voltammetry (DPV). As can be seen in Fig. 7b, the linear dynamic range for guanine is I_pa (μA) = 0.53C_guanine (nM) + 101.54 (R² = 0.995). Deviation from linearity was observed for more concentrated solutions due to the adsorption of guanine or its oxidation product on the electrode surface. By using 3S_b in the calibration equation, we calculated the detection limit concentration. A detection limit of 4 nM and sensitivity of 0.53 μA μM⁻¹ were achieved.

Fig. 7 DPV of various concentrations of (a) free guanine over the range of 50 nM–10 μM in 0.25 M phosphate buffer solution. Pulse amplitude: 0.05 V. Pulse width: 0.05 s. Pulse period: 0.2 s. (b) Calibration curve obtained from these voltammograms.

The relevance of the modified electrode in biological samples was assessed by measuring guanine in ssDNA of calf thymus. Fig. 8a shows the DPV of various concentrations of ssDNA, ranging from 5 to 55 μM in PBS (pH 6). As can be seen, the acid-denatured ssDNA gives a well-defined peak due to the oxidation of guanine residues. As shown in Fig. 8b, the linear dynamic range for guanine is I_pa (μA) = 2C_ssDNA (μM) + 101.54. To determine the detection limit for DNA, the abovementioned procedure was performed at the peak potential of DNA, and a detection limit and sensitivity of 450 nM (based on 3S_b) and 2 μA μM⁻¹, respectively, were estimated for ssDNA. The detection limit, linear calibration range and sensitivity of the modified electrode for guanine detection are comparable to and even improved than those obtained by using other modified electrodes (Table 1).

Fig. 8 DPV of various concentrations of (a) guanine in ssDNA over range of 5–65 μM in 0.25 M phosphate buffer solution. (b) Calibration curve obtained from these voltammograms.

Table 1 Analytical factors of different modified electrodes for guanine detection

Electrode	Analytical method	Selectivity	E (V)	Linear dynamic range	Detection limit (nM)
a Differential pulse voltammetry.b Cyclic voltammetry.c Square-wave voltammetry.d Multi-walled carbon nanotube.
Redox polymer-modified indium tin oxide^35	Amperometry	Guanine	0.65	8.0 nM–100 μM	5.0
Cobalt(II) phthalocyanine-modified carbon paste electrode^50	DPV^a	Guanine	0.920	—	550
β-Cyclodextrin-incorporated carbon nanotube-modified electrode^42	DPV	Adenine and guanine	0.79	200 nM–20 mM	200
Cobalt hexacyanoferrate-modified carbon paste electrode^51	CV^b	Guanine	0.9	0–4 μg mL^−1	340
Polythionine/Au-nanoparticles/MWCNT^d modified electrode^52	DPV	Adenine and guanine	0.7	50 nM–5 mM	10
Ionic liquid/carbon nanotube/Au nanoparticle composite film^53	DPV	Adenine and guanine	0.7	8 nM–2 mM	5
Nanostructured platinum-modified glassy carbon electrode^23	SWV^c	Guanine	0.82	0.1–500 μM	31
Cobalt oxide nanostructure-modified aluminium electrode (present study)	DPV	Guanine	0.79	50 nM–10 μM	4

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After each measurement the modified electrode was washed thoroughly with distilled water. The reproducibility of the CoOx/Al electrode was estimated by comparing the oxidation peak current obtained for 10 determinations of a 5 × 10⁻⁶ mol L⁻¹ guanine solution at pH = 6. The relative standard deviation (RSD) of 3.2% (n = 10) revealed a good reproducibility of the method. Storage stability is a vital parameter for the assessment of sensor performance, which was occasionally tested over 90% of its initial value after 45 days.

3.8. Interference study and selectivity

Fig. 9 shows the electrocatalytic oxidation of guanine in ssDNA at a cobalt oxide nanostructure-modified aluminium electrode. The selective detection of guanine in the presence of several interfering compounds potentially existing in biological liquids is a very advantageous feature for modified electrodes. To highlight the selectivity of the proposed electrochemical sensor, the interferences of different compounds were examined during DPV response for guanine. In the present work, Fig. 10a shows the interference effects of 1 mM ascorbic acid (AA), 0.5 mM uric acid (UA), and 1 μM dopamine (DP) and Fig. 10b shows that the other purine base, adenine, thymine, and cytosine, were tested on the DPV response of 1 μM guanine. No adjustments in the response current of guanine were observed in the presence of AA, UA, or DP solutions or the mixtures of all these. In the mixture of all these compounds, by using the modified electrode, four well-defined waves with very good resolution were obtained. Among these interferences, adenine, thymine and cytosine showed no response but AA, UA, and DP showed the oxidation process in the selected potential range. Consequently, this modified electrode can be utilized for the detection of guanine in the presence of other substances. This may be due to the fact that guanine is the most easily oxidizable base in DNA due to its lowest potential^47,48 and it also tends to be oxidized to form stable radical cations.⁴⁹ The obtained results demonstrated the satisfactory selectivity of the proposed CoOx nanoflower-modified Al electrode toward the electrocatalytic oxidation of guanine at a remarkable reduced over-voltage. Thus, the proposed electrode could be a practical sensor for the determination of guanine in an assortment of various common oxidizable species without separation.

Fig. 9 Graphical representation of the electrocatalytic oxidation of guanine in ssDNA at a cobalt oxide nanostructure-modified aluminium electrode.

Fig. 10 (a) DPV for the determination of 0.6 μM guanine (●) in 0.25 M PBS (pH 7) at CoOx/Al in the presence of 1 mM AA ■ curve), 0.1 mM UA (▲curve), 10μM DP (◆ curve). (b) DPV for the determination of 0.6 μM guanine in 0.25 M PBS (pH 7) at CoOx/Al in the presence of other purine base adenine (A - ◆ curve), thymine (T - ● curve), cytosine (C - ■ curve).

4. Conclusions

In this paper, a porous and stable CoOx nanoflowers with high surface area were obtained on an Al electrode by electrodeposition, and were employed as a biosensor for the oxidation of guanine and ssDNA in phosphate buffer medium for the first time. We demonstrated that the well-defined oxidation peak potential of CoOx appearing in the phosphate buffer medium can electrocatalyze and dramatically improve the oxidation signal of guanine and ssDNA. The relationship between current response and guanine concentration is linear in the concentration range of 50 nM–10 μM. The detection limits of guanine and ssDNA are 4 nM and 450 nM, respectively. This modified electrode can be utilized as a sensitive and reproducible voltammetric sensor for guanine detection in a wide pH range at reduced overpotentials. The fabricated CoOx/Al not only exhibited strong catalytic oxidation activity towards guanine but also provided selective detection in the presence of another purine base. Furthermore, the modification procedure is simple and the Al electrode is less expensive compared to other electrodes and more expedient than those used for other guanine sensors. Therefore, our present study may present an alternative for the creation of a nanostructured modified aluminium electrode for the electrochemical detection of biomolecules.

Acknowledgements

One of the authors Jayachandran Silambarasan thanks the Council of Scientific & Industrial Research (CSIR), New Delhi, for the award of a Senior Research Fellowship (9/810 (0021)/2013-EMR-I) and UGC networking resource centre for providing visiting fellowship and Prof. T. P. Radhakrishnan, School of Chemistry, University of Hyderabad, India for providing facilities and counselling.

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