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Synthesis, physical properties and antibacterial activity of Ce doped CuO: a novel

nanomaterial

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2014 J. Phys. D: Appl. Phys. 47 355301

(http://iopscience.iop.org/0022-3727/47/35/355301)

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Journal of Physics D: Applied Physics

J. Phys. D: Appl. Phys. 47 (2014) 355301 (7pp) doi:10.1088/0022-3727/47/35/355301

Synthesis, physical properties andantibacterial activity of Ce doped CuO:a novel nanomaterialTariq Jan1, Javed Iqbal1, Qaisar Mansoor2, M Ismail2, M Sajjad HaiderNaqvi3, Asma Gul4, S Faizan-ul-Hassan Naqvi3 and Fazal Abbas1

1 Laboratory of Nanoscience and Technology, Department of Physics, International Islamic UniversityIslamabad, Pakistan2 Institute of Biomedical and Genetic Engineering (IBGE), Islamabad, Pakistan3 Department of Biochemistry, University of Karachi, Karachi, Pakistan4 Departments of Bioinformatics and Biotechnology, International Islamic University Islamabad, Pakistan

E-mail: javed.saggu@iiu.edu.pk

Received 20 April 2014, revised 13 June 2014Accepted for publication 4 July 2014Published 7 August 2014

AbstractCuO nanostructures doped with Ce at different concentration levels have been synthesized viaa simple co-precipitation technique. The prepared samples have been characterized by x-raydiffraction, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy(FTIR) and UV–visible absorption spectroscopy. Structural studies exhibit the presence of amonoclinic structure of CuO for undoped and Ce doped samples without any additionalimpurity phases. SEM images have revealed the rod-like morphology with an averagediameter of 30 nm for undoped CuO. FTIR results of undoped and Ce doped CuOnanostructures have further confirmed the formation of monoclinic CuO. The optical band gapcalculated from the Tauc relation has been observed to be 2.48 eV for undoped CuOnanostructures, which is found to decrease down to 2.2 eV with the increase in the Ce dopinglevel. This tuning in the optical band gap may be attributed to the merging of the impurityband with the conduction band of CuO. The Ce doping induced effects on the antibacterialactivity of the CuO nanostructures have been examined by recording the growth curves ofbacteria in the presence of prepared nanostructures. It has been observed that S. aureusbacterium may be completely eradicated by the application of Ce doped CuO nanostructures.Finally, the cytotoxicity analysis has shown that the synthesized undoped and Ce doped CuOnanostructures are biocompatible and non-toxic towards the human cell line SH-SY5Y cells.

Keywords: Ce doped CuO, nanostructures, chemical synthesis, physical properties,antibacterial activity

(Some figures may appear in colour only in the online journal)

1. Introduction

Hospital acquired infections can cause patient mortality orforce a huge burden on hospitals due to the extended periods ofhospitalization. The increase in hospital acquired infections isstrongly associated with the high level of resistance acquiredby various bacteria to the antibiotics. The antibiotics resistantbacteria pose a serious threat to public health as reported bythe World Health Organization (WHO) [1]. Life threatening

infections caused by antibiotic resistant bacteria have emergeddue to the extensive use and abuse of antibiotics in the last twodecades [2]. This has prompted an urgent need to develop andintroduce novel antibacterial agents.

In this context, inorganic nanoparticles have showngreat potential because of their excellent catalytic, opticaland antibacterial characteristics [3]. Silver nanomaterialis the most studied antibacterial agent having remarkableantibacterial activity against a wide range of bacteria [4].

0022-3727/14/355301+07$33.00 1 © 2014 IOP Publishing Ltd Printed in the UK

J. Phys. D: Appl. Phys. 47 (2014) 355301 T Jan et al

However, the cytotoxicity of silver nanomaterial for healthycells limits its application as an antibacterial agent [5]. Thenext option is to explore inorganic metal oxides nanoparticlessuch as TiO2 [6], ZnO [7], CuO [8], SiO2 [9], SnO2 [10], MgO[11] and CeO2 [3], which have shown significant antibacterialproperties. Among these metal oxides, CuO is an interestingmultifunctional narrow band gap p-type semiconductor havingtremendous physiochemical properties [12]. CuO can bemore readily mixed with polymers as compared to other metaloxides. The polymers mixed with CuO nanostructures canbe used for dressing wounds and coating hospital implants.This will effectively enhance the wound healing rate andcontrol the spread of infectious diseases [13]. Variousstudies have demonstrated the antibacterial activity of CuOnanostructures [3, 8]. Several mechanisms underlying theantibacterial activity of CuO nanostructures have been reportedsuch as adherence to the bacterial cell, release of Cu ions andreactive oxygen species (ROS) generation on the surfaces ofnanostructures [13]. The ROS generation is reported to besignificantly increased by the creation of defects in the metaloxide structure [14]. Selective metal doping into a metal oxidematrix can lead to the creation of a large amount of structuraldefects (such as oxygen vacancies and interstitial defects) [15].Only a few studies are available in the literature about thedoping induced effects on the antibacterial activity of CuOnanostructures. From those findings it has been reported thatthe doping of Zn in CuO nanostructures can lead to higher ROSproduction due to the structural defects thereby increasing theirantibacterial activity [13, 16]. However, the Ce doping inducedeffects on the antibacterial activity of CuO nanostructures hasnot yet been properly reported. In the current study, undopedand Ce doped CuO nanostructures have been synthesized bya facile chemical method and their antibacterial activity andcytotoxicity have been investigated.

2. Materials and methods

2.1. Synthesis of the nanostructures

Undoped and Ce doped CuO nanostructures were synthesizedvia a chemical co-precipitation route. Appropriate ratios ofcopper chloride (CuCl2 · 5H2O) and cerium nitrate (CeNO3)

were dissolved in distilled water to obtain 0, 2, 4 and 6%Ce doped CuO. Acetic acid was added as a surfactant tocontrol the size and morphology. After 20 min of vigorousstirring, an aqueous solution of sodium hydrooxide (1M)was added to adjust the pH value to be approximately at10. The obtained solutions were again vigorously stirredat 80 ◦C. After 1 h of stirring, the solutions were cooleddown to room temperature naturally. The precipitateswere collected, washed with distilled water repeatedly anddried at 80 ◦C in an electric oven. The dried sampleswere annealed in an electric furnace at 300 ◦C for 2 h toobtain a highly crystalline material. The annealed sampleswere characterized for various physical properties usingdifferent techniques such as x-ray diffraction (XRD) by aPANalytical x-ray diffractrometer (Model: 3040/60 X’PertPRO made in the Netherlands), scanning electron microscopy

Figure 1. XRD patterns of the prepared samples.

(SEM) (JEOL, SEM, Model JSM6490LA), Fourier transforminfrared (FTIR) spectroscopy using a NICOLET 6700 FTIRspectrometer (made by THERMO SCIENTIFIC, in the USA)and UV–visible spectroscopy via a PerkinElmer Lambda 200UV/VIS/NIR absorption spectrometer.

2.2. Antibacterial activity and cytotoxicity analysis

The antibacterial activity of the prepared nanostructures wasassessed against: Gram-negative E. coli and Gram-positiveS. aureus by an antibacterial assay technique. The cytotoxicityof the undoped and Ce doped CuO nanostructures wasexamined on the human cell line SH-SY5Y Cells, which werepurchased from ATTC (Manassas, VA, USA). The detailedexperimental procedures of bacterial strains and healthy cellsculturing, their exposure to nanostructures and data extractionwere kept similar to those reported by us earlier [7].

3. Results and discussions

3.1. Structural and morphological investigations

The structural characteristics of the prepared samples havebeen investigated through XRD. Figure 1 depicts the XRDpatterns of undoped and Ce doped CuO samples. From theXRD data, only diffraction peaks of the CuO monoclinicstructure can be found. No characteristic peaks are observedrelated to the secondary phases of ceria, hence confirming theformation of single phase CuO monoclinic structure for dopedand undoped samples. The most intense peak i.e. (1 1 1),systematically shifts towards lower 2θ values with Ce doping(shown in the inset of figure 1), which reveals the successfuldoping of Ce ions into the host matrix.

The morphologies of the prepared undoped and Cedoped CuO samples have been investigated through SEM.Figures 2(a)–(d) shows the SEM images of (a) undoped,(b) 2%, (c) 4% and (d) 6% Ce doped CuO samples. TheSEM images reveal the formation of rod-like nanostructureshaving an average diameter of 30 nm in case of undoped

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Figure 2. SEM images of (a) undoped, (b) 2% Ce, (c), 4% Ce and (d) 6% Ce doped CuO samples.

CuO. Interestingly, upon Ce doping, the morphology of CuOnanostructures is transformed into sheet-like nanostructures.The thickness of the sheet-like nanostructures is observedto increase gradually with the increase in Ce dopingconcentration. This morphological transformation may beattributed to the successful incorporation of Ce ions intothe CuO matrix. The chemical reactivity of an elementsignificantly increases with the decrease in the Pauling electro-negativity [16, 17]. The Pauling electro-negativity of Cu (1.9)is much higher as compared to that of Ce (1.1). Hence, Cedoping may enhance the growth rate of CuO nanostructuresand cause morphological transformation.

3.2. Vibrational and optical investigations

The presence or absence of vibrational modes on the surfacesof the prepared nanostructures has been investigated usingFTIR analysis. Figure 3 shows the FTIR spectra of undopedand Ce doped CuO nanostructures. FTIR spectra of CuOnanostructures show a single characteristic strong peak at529 cm−1, associated with Cu–O stretching vibrations ofmonoclinic CuO, which match very well with the reportedvalues [18]. With the Ce doping, this mode undergoes asystematic blue shift towards 537 cm−1. The blue shift in theIR band with Ce doping can be linked with the variation insurface defects [19]. Furthermore, vibrational modes relatedto Cu2O and CeO2 are not detected. This confirms that all the

Figure 3. FTIR spectra of prepared nanostructures.

prepared samples are comprised of the purely monoclinic CuOphase. Thus, the FTIR analysis confirms the XRD results.

The Ce doping induced effects on the optical propertiesof CuO nanostructures have been explored using UV–visibleabsorption spectroscopy. The absorption spectra of undopedand Ce doped CuO nanostructures have been recorded in thewavelength range of 200–800 nm, and are shown in figure 4.

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Figure 4. UV–visible absorption spectra of the synthesizednanostructures. The inset of the figure shows the direct band gapenergy estimation of undoped and Ce doped CuO nanostructures.

The absorption spectra of undoped CuO nanostructures showa broad absorption peak in the UV region at 380 nm. TheCuO nanostructures doped with 2% and 4% Ce show a slightred shift in the absorption peak, whereas the 6% Ce dopedsample exhibits no shift as compared to the undoped CuOnanostructures. Moreover, it is observed that undoped CuOstrongly absorbs the light in the UV region and weakly inthe visible region. However, all Ce doped CuO samplesexhibited significant light absorption in the visible region. Thisremarkable absorption of light in the visible region and the redshift in the absorption edge may be attributed to the followingreasons: (1) the Ce doping may reduce the band gap of CuOand the electrons from the valence band can be excited tothe conduction band by absorbing visible light, and (2) theinteraction between the 4f electrons of Ce and the conductionband electrons of CuO [20].

The optical band gaps of the undoped and Ce dopedCuO nanostructures have been estimated using the followingrelation [21];

(αhν)n = B(hν − Eg). (1)

In this relation, α is the absorption coefficient, hν is thephoton energy, B is a constant, Eg represents the band gapenergy and n can either take up value 2 for a direct bandgap or 1/2 for an indirect band gap. Here, n is taken tobe 2 for calculation of the direct band gap energies of theprepared nanostructures. The band gap energies of undopedand Ce doped CuO nanostructures have been calculated byplotting (αhν)2 versus hν and extrapolating the linear part ofthe curves to the energy axis. The direct band gap energyof undoped CuO nanostructures is observed to be 2.48 eV,which is found to decrease down to 2.2 eV with Ce doping,as shown in the inset of figure 4. This may be explainedas follows: the Ce dopant may produce a distinct impurity

band into the band gap of CuO by its 4f5d electrons localizedstates, which may broaden due to the overlap of the wavefunctions of the adjacent dopants electrons [22, 23]. Thisimpurity band can merge with the bottom of the conductionband at the sufficient dopant concentration level. The mergingof the impurity band with the conduction band may cause thereduction of the band gap [24]. Such discussions are recentlyreported for Ce doped ZnO and GaN [25, 26]. Furthermore,the decrease in the band gap may also be associated withthe charge transfer between the conduction or valence bandelectrons of CuO with 4f5d electrons of Ce4+ ions. Thischarge transfer may create trapping levels in the Ce dopedCuO nanostructures, which can decrease the band gap ofCuO [27].

3.3. Antibacterial study

Ce doping induced effects on the antibacterial activity ofCuO nanostructures have been investigated. Figures 5(a)and (b) represents the growth curves of E. coli and S. aureusin the presence of colloidal suspensions of undoped and Cedoped CuO nanostructures. Undoped CuO nanostructureshave delayed the growth of E. coli for 4 h and decreasedit afterwards. The growth inhibition capability of CuOnanostructures against E. coli is observed to increasesignificantly with the increase in the Ce doping level. The6% Ce doped CuO nanostructures inhibit almost 90% growthof E. coli, as shown in figure 5(a). The prepared undopedand Ce doped CuO nanostructures are found to be moreeffective against S. aureus thanE. coli. The 65% growth ofS. aureus is inhibited by the application of undoped CuOnanostructures and is found to increase significantly with Cedoping. It is found that the multi-drug resistant S. aureusbacterium can be completely eradicated by the applicationof the 6% Ce doped CuO nanostructures, as shown infigure 6(b).

Several mechanisms of action have been proposed for theinhibition of bacteria by metal oxides nanostructures such asthe direct electrostatic interaction between nanostructures andbacteria cell walls, the photoactivation of metal oxides andthe generation of reactive oxygen species (H2O2, hydroxylradicals and singlet oxygen) or the release of metal ions inthe suspension [13, 28]. Here, a question arises of why theantibacterial activity of CuO nanostructures is increased withthe increase in the Ce dopant concentration. This enhancementmay have two reasons. (1) The Ce doping may enhancethe ROS generation capability of CuO nanostructures. CuOnanostructures can be activated by white light to generateelectron–hole pairs. Upon reaction with water molecules,these holes can generate ROS via the mechanism discussedby Shah et al [29]. But the problem is the recombination ofelectron–hole pairs, which can reduce the ROS generation. Inthe case of Ce doped CuO samples, the probability of electron–hole pair recombination may be reduced due to the trapping ofelectrons by Ce ions via the following process [27]:

Ce4+ + e− → Ce3+ (2)

Ce3+ + O2 → O•2 + Ce4+. (3)

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Figure 5. Growth curves for (a) E. coli (b) S. aureus bacterial strain in the presence of undoped and Ce doped CuO nanostructures.

Figure 6. Effect of undoped and Ce doped CuO nanostructures onSH-SY5Y Cells.

Hence, the enhancement in the antibacterial activity of CuOnanostructures with Ce doping may be credited to the electronstrapped in the Ce sites. The trapping of electrons will lead tomore ROS generation, resulting in higher antibacterial activityof Ce doped CuO nanostructures. (2) The antibacterial activityof CuO nanostructures is also associated with the release ofCu2+ ions in the solution [30]. With the substitution of Ce4+

ions for Cu2+ ions in the host matrix, there is a possibility thatsome Cu2+ ions will be accommodated on the interstitial sites.So it is logical to expect the release of more Cu2+ ions in thesolution, because ions can easily be released from interstitialsites compared to from native sites. Furthermore, Ce4+ ionsmay also be released in solution in the case of Ce doped CuOnanostructures. Thus, Ce doping will enhance the release ofCe4+ and Cu2+ ions leading to the higher antibacterial activityof Ce doped CuO nanostructures.

3.4. Cytotoxicity

Nanomaterials can be used as potential antibacterial agentsonly if they exhibit toxicity towards bacteria but not towardshealthy human cells. Thus, understanding the cytotoxicityof the nanomaterials is essential before their application inthe biomedical sector. Therefore, the cytotoxicity of theprepared nanostructures has been investigated on human cell

line SH-SY5Y cells. SH-SY5Y cells have been treated withsynthesized undoped and Ce doped CuO nanostructures for24 h and number of viable cells were recorded under anoptical microscope. The prepared nanostructures have nosignificant effect on the viability of a healthy cell line, asshown in figure 6. To support these results, phase contrastmicroscopy has been used to evaluate the effects of synthesizednanostructures on the morphology of the SH-SY5Y cells. Itcan be seen from figure 7 that both undoped and Ce dopedCuO nanostructures have altered the cell morphology to someextent. But no significant effects have been observed on the cellcount. Hence, the prepared nanostructures may be consideredbiosafe and biocompatible towards the tested cell line. Thesefindings suggest that Ce doping has significantly enhancedthe antibacterial activity of CuO nanostructures both againstGram-negative E. coli and Gram-positive S. aureus bacteriabut have a non-toxic nature towards SH-SY5Y cells.

4. Conclusions

We have synthesized undoped and Ce doped CuOnanostructures via a simple chemical co-precipitationtechnique. The XRD results have shown the formation ofa single phase monoclinic structure of CuO for all samples.The SEM images have revealed the formation of rod-like CuOnanostructures with an average diameter of 30 nm. The FTIRspectra of undoped and Ce doped CuO nanostructures haveshown a single characteristic strong peak associated with theCu–O stretching vibrations of monoclinic CuO. The band gapenergy of CuO nanostructures has been found to significantlydecrease with Ce doping. The Ce doped CuO nanostructureshave been observed to be more effective than the undopedCuO nanostructures both against Gram-negative E. coli andGram-positive S. aureus bacteria. Interestingly, it has beenfound that S. aureus bacterium can be completely eradicatedby application of Ce doped CuO nanostructures. Cytotoxicityresults have demonstrated that all the prepared nanostructuresare biosafe and biocompatible.

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Figure 7. Phase contrast microscopy images of the SH-SY5Y cells after treatment with the prepared nanostructures (circles showmorphological variation).

Acknowledgments

This work was funded by the HEC Project IPFP (Grant No:PM-IPFP/HRD/HEC/2011/3386) and funding for HEC PhDScholar (Tariq Jan) under the supervision of Dr Javed IqbalSaggu.

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