Catalysis Communications 16 (2011) 39–44

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Catalysis Communications

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Short Communication

Photocatalytic and AC magnetic-field induced enhanced cytotoxicity of Fe3O4–TiO2

core-shell nanocomposites against HeLa cells

Md. Shariful Islam a, Yoshihumi Kusumoto a,⁎, Md. Abdulla-Al-Mamun a, Yuji Horie b

a Department of Chemistry and Bioscience, Graduate School of Science and Engineering, Kagoshima University, 1-21-35 Korimoto, Kagoshima 890-0065, Japanb Department of Electrical and Electronics Engineering, Graduate School of Science and Engineering, Kagoshima University, 1-21-40 Korimoto, Kagoshima 890-0065, Japan

⁎ Corresponding author. Tel./fax: +81 99 285 8914.E-mail address: kusumoto@sci.kagoshima-u.ac.jp (Y

1566-7367/$ – see front matter © 2011 Elsevier B.V. Alldoi:10.1016/j.catcom.2011.08.039

a b s t r a c t

a r t i c l e i n f o

Article history:Received 25 June 2011Received in revised form 17 August 2011Accepted 17 August 2011Available online 14 September 2011

Keywords:Cancer cellsNanocompositesAC magnetic-fieldPhotoirradiationHyperthermia

Fe3O4–TiO2 core-shell nanocomposites were successfully prepared and characterized by FE-SEM, XRD, UV–visabsorbance spectra, and EDX. The FE-SEM and EDX studies revealed core-shell structure with a particle size ofca. 40–50 nm. We adopted HeLa cells as a model to investigate the thermal-photocatalytic cell killing efficiencyof Fe3O4–TiO2 using 150 μg/mL dose content for 10 min exposure time.We found neither only ACmagnetic-fieldinduction nor only photoirradiation condition could kill the cancer cells up to satisfactory level using Fe3O4–TiO2

nanocomposites. Finally, the results revealed that almost 100% cancer cells were destructed by Fe3O4–TiO2 nano-composites whereas for bare Fe3O4 and bare TiO2, 90% and 56% cancer cells were killed, respectively, under com-bined AC (alternating current) magnetic-field induction and UV–vis photoirradiated conditions.

. Kusumoto).

rights reserved.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Magnetic nanoparticles are currently being investigated becauseof their numerous applications in several fields such as magneticfluids, medicine, magnetic resonance imaging, data storage, sensing,and water remediation [1, 2]. Magnetite and maghemite haveattracted attention in biomedical applications because of their bio-compatibility and low toxicity in the human body [3]. Magnetite andhematite are semiconductors and can catalyze oxidation/reduction re-actions [4, 5]. But it is often necessary to coat their surface with an or-ganic or inorganic shell, in order to protect them from chemicaldegradation or agglomeration according to the environments in whichthey will be used [2]. The coating can also be performed in order toadd new functionalities to the magnetic core. Therefore, the additionof a TiO2 shell to a magnetic core at the nanometer scale may leadto bifunctional nanoparticles which could be applied for exampleas a magnetic photocatalyst. Among various oxide semiconductorphotocatalysts, TiO2 [6–9] has proven to be the most suitable mate-rial for widespread environmental application for its biological andchemical inertness, strong oxidizing power, cost effectiveness, andlong-term stability against photocorrosion and chemical corrosion.Nanostructured TiO2 exhibits superior photocatalytic activity com-pared to conventional bulk materials because of its high surface

area. To enhance its photocatalytic ability, nanostructured TiO2

with various morphologies including nanoparticles, nanofibers,nanostructured films or coatings, and nanotubes have been pre-pared, and much progress on the synthesis of nanostructured TiO2

with excellent catalytic properties has been made [10–13].Fortunately, researchhas been carried out by immobilizing TiO2 onto

variousmagnetic supports, such as magnetite ferrite, barium ferrite andFe3O4–SiO2 particles [14–17]. TiO2 nanoparticles are an excellent mate-rial. It is widely used in water and air purification, disinfection, wastetreatment, and malignant tumor therapy [18–24]. Especially in thefield of malignant tumor therapy, TiO2 has some extra advantages suchas extremely strong oxidation reactionwhich kills tumor cells. Althoughthe photothermal, photocatalytic and hyperthermia cancer cell killing isalready reported by our group [25, 26], we are interested in observingthe enhanced cytotoxicity of Fe3O4–TiO2 core-shell nanocompositesunder combined Ac magnetic-field induction and photoirradiation con-ditions. The hyperthermia takes place at the cellular level and is mainlydue to the heat-induced malfunction of repair processes after radiation-inducedDNAdamage [27]. So, to demonstrate that Fe3O4–TiO2 core-shellnanocomposites havemuchmore advantages than that of traditional one(TiO2) in malignant tumor therapy, we synthesized superparamagneticmagnetite (Fe3O4) nanoparticles and then coated themwith TiO2 to pre-pare the Fe3O4–TiO2 core-shell nanocomposites (NCs) and have appliedthem in cancer therapy.

Because of our interest in the enhanced toxic effect of AC (alternatingcurrent) magnetic-field induction and chemical reactions of Fe3O4–TiO2

particles producedbyUV–vis irradiation,we studied the cancer cell killingeffects with bare Fe3O4, bare TiO2, and Fe3O4–TiO2 NCs under combinedAC magnetic-field induction and UV–vis photoirradiation conditions.

40 M. Shariful Islam et al. / Catalysis Communications 16 (2011) 39–44

WeadoptedHeLa cells as amodel to investigate the thermal-photocata-lytic cancer cell killing efficiency of Fe3O4–TiO2 core-shell NCs underthree distinct conditions (only ACmagnetic-field induction, only photo-irradiation and combined AC magnetic-field induction and photoirra-diation conditions) using a dose of 150 μg/mL for 10-min inductionand irradiation.

2. Experimental

2.1. Synthesis of Fe3O4–TiO2 core-shell nanocomposites

Fe3O4 nanoparticles with diameter of ca. 20 nm were first synthe-sized via the co-precipitation method. In the next step, Fe3O4–C andfinally, Fe3O4–TiO2 core-shell NCs were synthesized by modifying(see Supplemental material) the synthesis method of Li et al. [28].In a typical co-precipitation method, for the synthesis of Fe3O4,FeCl3 (2.6 g) and FeCl2 (1.3 g) were dissolved in nitrogen gas (N2)purged 2.0 M hydrochloric acid solution and magnetically stirredunder a continuous flow of N2. The mixture was heated at 70 °C for30 min and then themixturewas heated for another 5 min under a blan-ket of N2. Ammonia was added drop by drop to precipitate the magneticnanoparticles and the black product formed was treated hydrothermallyat 70 °C for 30 min. All aqueous solutions and suspensions were madeusingnanopurewater (resistivity 18 MΩ cm). The resultingnanoparticleswere subsequently separated from the reaction media under magneticfield and washed three times with nanopure water before drying. Finallythe MNPs were oven dried at 70 °C for 3 h to get Fe3O4. The overall syn-thesis routes of Fe3O4–C and Fe3O4–TiO2 are described by Scheme 1 (seeSupplemental materials).

2.2. Cell culture

HeLa cells were provide by the RIKEN BRC through the NationalBio-Resource Project of the MEXT, Japan and stored in liquid N2 to en-sure the best quality. The mentioned cancer cell line was cultured in aminimum essential medium (MEM) solution with 10% newborn calfserum (NBS) in a humidified incubator with an atmosphere of 5%CO2 in air at 37 °C and the cells were plated at a concentration ofabout 3×105 in 60 mm Petri dishes and allowed to grow for 3 days.Monolayer cultures of cancer cell line (HeLa cells) were maintainedas described by Abdulla-Al-Mamun et al. [29].

3. Characterization

The general structure characterization, including size, size distributionand crystal structure of the as-synthesized Fe3O4–TiO2 core-shell NCswasperformed for all the sampleswithout any size sorting. To further confirmthe crystal structure and overall phase purity, the NCswith different sizeswere examined usingXRDwith CuKα radiation and aNifilter (for details,see Supplemental information). The surface morphology and NCs sizewere determined using a field emission scanning electron microscope(FE-SEM, model Hitachi S-4100H) using accelerating voltage, extractionvoltage and absorbed current 10 kV, 5 kV and 10 μA, respectively. TheFE-SEM sample was prepared by using an Ag sheet. A carbon tape wasattached on the Ag sheet and a small amount of Fe3O4–TiO2 NCs wastaken over that carbon tape and then the sample was coated with Ptto escape from the heavy charges of the magnetic Fe3O4–TiO2 NCs dur-ing analysis. Further, the core-shellmorphology of theNCswas analyzedby a transmission electron microscope (TEM, JEOL JEM-3010 VII TEM)operating at 300 kV. Absorption (reflectance) spectra were recordedon UV–vis absorption (reflectance) spectra (Shimadzu Corporation,UV-2450, Japan). The samples were standardized with barium sulfatecoated glass substrate and its spectrum was used as the baseline. Thespectra of all samples were measured in a wavelength range between240 and 850 nm. The quantitative chemical composition of the Fe3O4–

TiO2 core-shell nanocomposites was also measured using an energy

dispersive X-ray spectrometer (EDX, Philips, XL 30CP) attached to thecold field SEM.

4. In-vitro cytotoxicity and anti-cancer assay

The in vitro cytotoxicity and anti-cancer effect of the nanocompo-sites against the HeLa cell line was evaluated by trypan blue exclusionmethod. In the experiment, the alternative current (AC) magnetic-field was created by using a magnetic oscillator with desired frequencyand strength of 560 kHz and 5.0 kA/m, respectively, and a Xenon lamp(CERMAX 300-W LX300F, USA, ILC) with heat cut-off and band-pass fil-ters (350–600 nm)with an average intensity of 30 mW cm−2 was usedfor the light irradiation on HeLa cells. The light power wasmeasured bya spectroradiometer (Model LS-100, EKO Instrument Co. Ltd.). A tablerotator was used for the Petri dish to ensure the homogeneous light irra-diation on the cells. To investigate and compare the cytotoxicity efficacy,every Petri dish was subjected to apply the three abovementioned treat-ments for 10 min and immediately after the treatment, temperature in-crement for the every dish was measured by a digital thermometer(Sato Keiryoki-250WP II-R).

Cancer cell viability was examined by treating with bare Fe3O4, bareTiO2 and Fe3O4–TiO2 colloidal solution for 24 h incubation in an incubator.To investigate the cytotoxicity of synthesizednanomaterials, one dishwasused as control without any nanomaterials and the otherfive dishesweretreated with different doses, like 0.2, 0.4, 0.6, 0.8 and 1.0 mL of nanocom-posites colloidal solution per milliliter of MEM solution where the actualcolloidal solution concentration was 150 μg/mL. A hemocytometer wasused to estimate the total number of viable cells (by counting cells inthe four 1 mm2 corners of the hemocytometer) and average number ofthe cells per unit volume (mL) of medium was calculated as the sum ofthe counted cell number/3×105. Every treatment was conducted bydoing three-time independent experiments.

5. Results and discussion

5.1. Particles morphology and crystalline phase

Fig. 1 gives the SEM (A) and TEM (B) photographs of (a) bareFe3O4 and (b) Fe3O4–TiO2 core-shell NCs and EDX is shown in Fig. 1(C). According to Fig. 1A (b), Fe3O4–TiO2 NCs with well-definedcore-shell structure are rather monodispersed and the shape ofFe3O4–TiO2 core-shell NCs is definitely globular. The SEM image(Fig. 1A (b)) also clearly shows round whitish structure of the TiO2

shell around the Fe3O4 core. Further, Fig. 1B (b) confirms that Fe3O4

nanoparticles were well-surrounded by TiO2 shell structure whereasFig. 1B (a) approved the bare Fe3O4 nanoparticles without any shellstructure and the shape of Fe3O4–TiO2 core-shell NCs is definitelyglobular (see also the inset in Fig. 1(C)). The core is uniformly coveredby the shell with TiO2 particles. The EDX spectrum of Fe3O4–TiO2 in-dicates the clear presence of Fe, O and Ti components. The strong Tiand O signals in the EDX spectrum indicate that Ti consists of mixedoxides surrounding the outer layer of the Fe3O4 core.

The wide-angle X-ray diffraction (XRD) patterns of (a) bare Fe3O4,(b) bare TiO2, and (c) Fe3O4–TiO2 NCs are shown in Fig. 2A. Comparedwith bare Fe3O4, Fe3O4–TiO2 NCs exhibit similar peaks at 2θ=30.124,35.483, 43.124, and 62.629° which can be attributed to the diffractionpeaks of (220), (311), (400), and (440) planes of Fe3O4 and in comparisonwith bare TiO2, Fe3O4–TiO2 NCs exhibit similar peaks at 2θ=25.625 and55.660° which can be attributed to the diffraction peaks of (101) and(211) planes of TiO2 (JCPDS, PDF, file nos. 01-075-0033 and 00-002-0406), respectively, demonstrating the formation of Fe3O4–TiO2 NCs.

Fig. 2B shows the UV–vis absorption spectra of (a) bare Fe3O4,(b) bare TiO2, and (c) Fe3O4–TiO2. The UV–vis absorption spectra ofFe3O4–TiO2 NCs show the absorption in the visible region rangingfrom 400 to 700 nm which is originated from Fe3O4.

Fig. 1. SEM (A) and TEM (B) images of Fe3O4 (a) and Fe3O4–TiO2 (b) and EDX (C) spectrum of Fe3O4–TiO2.

41M. Shariful Islam et al. / Catalysis Communications 16 (2011) 39–44

5.2. Hyperthermia ability and cell viability

We compared the heat increment and cytotoxicity using bare Fe3O4,bare TiO2, and Fe3O4–TiO2 NCs suspension under combined ACmagnetic-field induction and photoirradiation conditions with a dose of150 μg/mL for exposing time 10 min (Fig. 3A). The heat generated wasevaluated by using bare Fe3O4, bare TiO2, and Fe3O4–TiO2 NCs suspensiondispersed inMEM. The temperature rising of the three nanomaterials sus-pensions against the dose quantity is shown in Fig. 3A.

The highest temperature achieved was 43.8 °C for Fe3O4–TiO2 incontrast to 43.5 and 40.1 °C for bare Fe3O4 and bare TiO2, respectively,whereas Ito et al. [30] achieved 42.0 °C for 30 min exposure timeusing 100 μg/mL concentration of magnetite nanoparticles.

Furthermore, we investigated the hyperthermia ability of Fe3O4–TiO2

core-shell NCs under only ACmagnetic-field induction and only photoir-radiation condition using the same dose and exposure time and the heat

generated by using Fe3O4–TiO2NCs suspension dispersed inMEM in bothof the conditions are shown in Fig. 3A. The temperature achieved was41.5 and 36.9 °C under only ACmagnetic-field induction and only photo-irradiation condition, respectively.

To evaluate the cytotoxicity, cell dishes were incubated for 24 hwitha concentration of 150 μg/mL of nanomaterials andanother dishwithoutany nanoparticles solution (control dish) was also incubated for 24 h.The cancer cell viability under ACmagnetic-field induced andphotoirra-diated conditions for exposing time 10 min for three samples are shownin Fig. 3B. The results revealed that almost 100% (viability 1%) cancercells were destroyed by Fe3O4–TiO2 NCs whereas 90% (viability 10%)and 56% (viability 44%) cancer cells were killed for bare Fe3O4 andbare TiO2, respectively, using the same dose under the same conditions.

The thermal-photocatalytic cancer cell killing efficiency of Fe3O4–

TiO2 core-shell NCs under only AC magnetic-field induction and onlyphotoirradiation condition is shown in Fig. 3B. It is noteworthy that in

Fig. 2. XRD patterns (A) of (a) bare Fe3O4, (b) bare TiO2, and (c) Fe3O4–TiO2 and UV–visabsorption spectra (B) of (a) bare Fe3O4, (b) bare TiO2, and (c) Fe3O4–TiO2. Here, A: an-atase, R: rutile phase for TiO2.

Fig. 3. Comparative hyperthermia ability (A) and cell viability (B) of bare Fe3O4, bareTiO2, and Fe3O4–TiO2 under combined AC magnetic-field induction and photoirradia-tion conditions and under only AC magnetic-field induction or only photoirradiationcondition using Fe3O4–TiO2 NCs.

42 M. Shariful Islam et al. / Catalysis Communications 16 (2011) 39–44

all doses under combined AC magnetic-field induction and photoirra-diation conditions, the cancer cell killing percentage was higher thanonly AC magnetic-field induction and only photoirradiation conditionand finally almost 100% (viability 1%) cancer cells were destroyed byFe3O4–TiO2 NCs under combined conditions using a dose of 150 μg/mLwhereas under only AC magnetic-field induction or only photoirradia-tion condition, the cancer cell killing percentage was 87% (viability13%) and 64% (viability 36%), respectively.

Although only 10% cancer cells can survive after treating with bareFe3O4, it is not negligible because according to the nature of the cancercells even single live cancer cell may be a great threat for the patientwhen its growth is uncontrolled. Cancers derived from epithelial cellsare called carcinoma and it is fundamentally a disease by failure of reg-ulation of tissue growth. Cancer cell growth is different fromnormal cellgrowth. Instead of dying, cancer cells continue to grow and form new,abnormal cells. It can also invade (grow into) other tissues, somethingthat normal cells cannot do. Cancer costs billions of dollars and it isthe secondmost common cause of death in the United States, exceededonly by heart disease. Cancer accounts for nearly 1 out of every 4 deathsin the United States and it caused about 13% of all human deaths world-wide (7.9million), in 2007.With the growth and aging of the population,prevention efforts are important to help reduction of new cancer cases,human suffering, and economic costs [31]. Considering these facts, itcan be logically concluded that Fe3O4–TiO2 NCs preserve the significantrole to kill (almost 100%) cancer cells under combined AC magnetic-field induction and photoirradiation conditions.

So, it is interesting to note that either bare Fe3O4 or bare TiO2 andeither only AC magnetic-field induction or only photoirradiation con-dition could not kill the cancer cells up to the mark, but when Fe3O4–

TiO2 NCs were used under combined AC magnetic-field induction andphotoirradiation conditions, the cell killing potentiality was signifi-cantly increased and finally almost 100% HeLa cells were killed.

5.3. Cell killing mechanisms

Fig. 4 shows microscopic images of HeLa cells under combined ACmagnetic-field induced and photoexcited conditions. Fig. 4A (control)shows that the cell morphology is unchanged. Fig. 4B (with bare TiO2)shows almost no change, Fig. 4C (with bare Fe3O4) shows moderatechange andFig. 4D (with Fe3O4–TiO2NCs) shows severe change. Especial-ly, Fig. 4D shows that the HeLa cells suffer severe hyperthermal andphotocatalytic shock at 10 min exposure time under the same condition.

To the best of our knowledge, themechanism of cell killing can be as-cribed to cumulative effects of hyperthermia and photocatalytic cell-killing. Hyperthermia is a famous tool for cancer therapy nowadays andphotocatalytic cell-killing is also well known and recently we reportedthe mechanism of synergistic photocatalytic and photothermal cancercell killing of Ag–TiO2 and AC magnetic-field induced hyperthermia ofα-Fe2O3 [25, 26]. However, the stability of Fe3O4–TiO2 NCs is an im-portant consideration. In this report, we used Fe3O4 as a core andTiO2 was used as a shell. It was reported that TiO2 is broadly usedas a photocatalyst because it is photochemically stable, nontoxicand economical and its band gap or even doped TiO2 make it one ofthe most promising materials in photochemical generation of

Fig. 4. Microscopic images of (A) control, (B) bare TiO2, (C) bare Fe3O4, and (D) Fe3O4–TiO2 with HeLa (cancer) cells using a dose of 150 μg/mL and exposure time of 10 min undercombined conditions (Magnification 200).

43M. Shariful Islam et al. / Catalysis Communications 16 (2011) 39–44

hydrogen from water in the presence of solar light [29, 36]. Ji et al.reported that the superparamagnetic iron oxide nanocore-Au nano-shells acted as an efficient photothermal mediator [37].

When TiO2 is excited by a photon whose energy is greater than theTiO2 band gap (3.2 eV), an electron from the valence band canbe excitedto the conduction band, thus creating an electron–hole pair. The electronand hole can then, if they do not recombine, reduce or oxidize species inthe electrolyte solution. For instance, the hole oxidizes a water moleculeto yield OH radicals (•OH), and the electron reduces oxygen (O2) to givesuperoxide anion (O−) and H2O2. These reactive oxygen species candrive various chemical reactions.

It is well known that reactive oxygen species such as OH radicals andH2O2 are formed onphotoexcited TiO2 particles inwater solution [32, 33].The formation of OH radicals andH2O2 fromphotoexcited TiO2 particles isillustrated in Fig. 5. OH radicals are formed by the oxidation of water viaReaction (1), and H2O2 is formed by the reduction of oxygen (O2) via Re-action (2).

H2O þ hþ→•OH þ Hþ ð1Þ

Fig. 5. Schematic illustration of the photocatalytic formation of H2O2 and OH radicalson photoexcited TiO2 particles. e−: electron, h+: hole.

O2 þ 2e− þ 2Hþ→H2O2 ð2ÞFurthermore, some of H2O2 might be cleaved into OH radicals by

UV irradiation or by the Fenton reaction [33]. Some of photoinducedelectrons of TiO2 can be transferred to the TiO2 surface and reducethe dissolved O2 easily, because dissolved oxygen is also one of thegood accepters of electrons.

TiO2ðhÞ þ H2O→•OH þ Hþ ð3ÞO2•−þ Hþ→HO2• ð4Þ2HO2•→O2 þ H2O2 ð5ÞH2O2 þ O−

2 →•OHþ OH− þ O2 ð6ÞThe photo-generated holes on the TiO2 surface can react with water

to produce powerful oxidative radicals •OH and HO2• as shown in

photocatalytic reactionmechanism (Reactions (3) and (4)) [29]. The re-active oxygen species such as hydroxyl radicals and hydrogen peroxideare formed on the photoexcited Fe3O4–TiO2 NCs inMEM solution. Thesehighly oxidizing OH radicals and H2O2 can be expected to be toxic tocells [34]. Moreover, it was reported that a photogenerated hole reactswith a water molecule to form •OH and H2O2 which can be expectedto be toxic to cells as shown in Reactions (3)–(6) [32, 35].

6. Conclusions

We synthesized ca. 20 nm Fe3O4 and coated with TiO2 to prepareFe3O4–TiO2 nanocomposites. The characterization revealed the success-ful preparation of Fe3O4–TiO2 nanocompositeswithhyperthermia ability.Finally, it can be concluded that the cancer cell killing efficiency of Fe3O4–

TiO2 nanocomposites was always higher than bare Fe3O4 and bare TiO2

44 M. Shariful Islam et al. / Catalysis Communications 16 (2011) 39–44

and the cell killing capabilitywas significantly enhancedunder combinedAC magnetic-field induced and photoexcited conditions.

Acknowledgment

Thepresentworkwas partly supported byGrant-in-Aid for ScientificResearch (B) (no. 19360367) from Japan Society for the Promotion ofScience (JSPS) and Grant-in-Aid for JSPS Fellows (no. 20·00083).

Appendix A. Supplementary data

Supplementary data to this article can be found online at doi:10.1016/j.catcom.2011.08.039.

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