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Effect of free oxygen radical anions and free electrons in a Ca12Al14O33 cement structure on its optical, electronic and antibacterial properties

The aim of this work was to investigate the effect of free oxygen radicals and free electrons in a Ca12Al14O33 (C12A7) cement structure on the optical, electronic and antibacterial activity of this material. Ca12Al14O33 was successfully fabricated via rapid heating to high temperatures by high frequency electromagnetic induction. Ca12Al14O33 cement samples were characterized using XRD and UV-Vis-DRS spectroscopy. The morphology and chemical composition of the samples were also investigated using SEM and EDS techniques. The presence of free oxygen radicals (O2−ions) in the insulating structure of Ca12Al14O33 was confirmed using Raman spectroscopy showing a spectrum peak at 1067 cm−1. The excitation of free electrons in the Ca12Al14O33 cement was indicated by UV-Vis absorption spectra at 2.8 eV and an optical energy gap of 3.5 eV, which is consistent with the first-principles calculations for the band energy level. The effects of free oxygen radicals and free electrons in the Ca12Al14O33 structure as antibacterial agents against Escherichia Coli (E. coli) and Staphylococcus Aureus (S. aureus) were investigated using an agar disk-diffusion method. The presence of O2− anions as a reactive oxygen species (ROS) at the surface of Ca12Al14O33 caused inhibition of E. coli and S. aureus cells. The free electrons in the conducting C12A7 reacted with O2 gas to produce ROS, specifically super oxides (O2−), superoxide radicals (O2•-), hydroxyl radicals (OH•) and hydrogen peroxide (H2O2), which exhibited antibacterial properties. Both mechanisms were active against bacteria without effects from nano-particle sized materials and photocatalytic activity. The experimental results showed that the production of ROS from free electrons was greater than that of the free O2− anions in the structure of Ca12Al14O33. The antibacterial actions for insulating and conducting Ca12Al14O33 were different for E. coli and S. aureus. Thus, Ca12Al14O33 cement has antibacterial properties that do not require the presence of nano-particle sizes materials or photocatalysis.


Introduction
Currently, the use of antimicrobials to promote public health is a research topic with much attention [1,2,3,4]. The goal of such research is to identify materials with antibacterial properties capable of inhibiting or killing various bacteria through mechanisms not limited to photocatalytic and nano-particle effects [4,5]. Antibacterial materials have attracted significant attention due to their very interesting application in preventing bacterial growth on smart building bathroom and kitchen walls [4]. There are four types of antibacterial activity: (1) cation elution, (2) pH effects, (3) electrostatic interactions between the surfaces of bacteria with nano-size particles, and (4) the effects of active oxygen species [4,6,7]. In addition to these, photocatalytic activity, electrostatic interaction, cellular internalization of nanoparticles and production of reactive oxygen species (ROS) also have antimicrobial activity [4,7,8,9]. ROS effects are predominantly used for antibacterial activity since it is an easy mechanism to employ [7,8]. ROS are comprised of hydroxyl radicals (OH ), hydrogen peroxide (H 2 O 2 ) and superoxide ions (O 2 -) [4,8]. These materials cause death by contacting bacterial membranes and directly damaging their surfaces [4,7]. ROS species can be generated from the reaction of free particles (free electrons and holes) in the course of photocatalytic activities [9,10,11]. The photocatalytic effect causes generation of free electrons and holes in materials using phonon energy corresponding to energy gap of the materials. There have been many reports that titanium dioxide (TiO 2 ) [12] and zinc oxide (ZnO) [10] can generate ROS through photocatalysis. Practically, TiO 2 and ZnO used phonon energy at 3.1 eV [13] and 3.2 eV [7], respectively, to photocatalytically produce ROS. They are produced from O 2 and H 2 O adsorption promoting a reaction of free electrons and holes at the surfaces of materials to produce OH À , H 2 O 2 and O 2 -. Alternatively, ZnO [7,8,9,14] nanoparticles cause death of bacterial cells through electrostatic interactions between bacterial surfaces with nanoparticles in the absence of light, thereby damaging cell membranes. TiO 2 and ZnO act through photocatalysis and nanoparticle effects to produced antibacterial activity.
Recently, it has been reported that Ca 12 Al 14 O 33 cement exhibits free oxygen radical O À2 ions in a vacant cage structure [15,16,17,18]. The Ca 12 Al 14 O 33 cement structure is linked by calcium, aluminum and oxygen atoms forming empty nanometer-sized cages within the structure [19,20], as shown in Fig. 1. A unit cell of insulating Ca 12 Al 14 O 33 is comprised of two molecules occupying 12 crystallographic nano-cages while presenting a 4 þ charge at the cage wall, represented as [Ca 24 Al 28 O 66 ] 4þ [21]. The two cages in this unit cell support electrical neutrality by entrapping two free oxygen ions (O À2 ) in cages referred to as an extra-framework [21]. The Ca 12 [21,22]. Hayashi et al. [23] showed that the process of preparing Ca 12 Al 14 O 33 cement in a dry oxygen atmosphere at temperature 1350 C could produce both O À and O 2 -(as ROS species). Lu et al. [24] reported that the present of O À and O 2 resulted in antibacterial activity. Nevertheless, antimicrobial materials should be effective under various conditions and not limited to photocatalytic activity or nano-particle effects. Furthermore, Hayashi et al. [23] reported that free O À and O 2 could be replaced by free electrons in a conducting Ca 12 Al 14 O 33 cement, represented as Ca 12 Al 14 O 33 :e À . It has been reported that this material has distinct optical and electrical properties. However, to best of our knowledge, there are no reports of the effect and mechanisms of free electrons in the nano-cage structure of Ca 12 Al 14 O 33 cement on antibacterial activity. This work aims to investigate the effect of free electrons in the nanocage structure of Ca 12 Al 14 O 33 cement on the optical, electronic and antibacterial activities of these materials. The developed material presenting free electrons in a nano-cage structure was characterized. Conducting Ca 12 Al 14 O 33 cement with free electrons was rapidly prepared by heating insulator cement inside a carbon crucible at high temperatures using high frequency electromagnetic induction. The mechanism and effect of free electrons and free oxygen radicals in the nano-cage structure of these materials on its optical, electronic and antibacterial properties were also investigated. Their antibacterial activities against gramnegative E. coli and gram-positive S. aureus are reported. Moreover, the mechanism of the antibacterial action of free electrons and free oxygen radicals in the nano-cage structure of Ca 12 Al 14 O 33 cement is described.

Chemicals
Calcium carbonate (CaCO 3 , 99% Sigma-Aldrich), alumina powder (Al 2 O 3 , 99.9% Sigma-Aldrich) and ethanol (95%) were used as the starting raw materials. All chemicals were used as received with no further purification. The as-prepared CAO@1200C powder was used as a starting material for fabrication of conducting Ca 12 Al 14 O 33 cement. A mass of 100 g of CAO@1200C powder was placed in a carbon crucible with a carbon cap. Then, the carbon crucible was transferred into the middle of a Cu induction coil. High frequency electromagnetic induction heating was done using an induction coil (Model: TH-60AB (90 A, 3 phase, 380 V, 50-60 kHz)). The temperature was determined using an IR detector (Model: SENTEST (NS50PH1FF), accuracy class:2.0) focused on the surface of the carbon crucible. The CAO@1200C powder was rapidly heated from room temperature to sintering temperatures of 1350 C, 1450 C and 1550 C with a 40 sec holding time (referenced as the CAO@1350C, CAO@1450C and CAO@1550C samples, respectively). Finally, the samples were cooled by natural convection to room temperature.

Preparation of the cement pellets
For pellet fabrication, the obtained CAO@1350C, CAO@1450C and CAO@1550C powders were subjected to uniaxial compression and pressed into disc-shaped pellets that were 10 mm in diameter and 2-3 mm thick. Then, the antibacterial activity of these pellets was tested.

Characterization
The lattice parameters were determined using an X-ray diffractometer (XRD), (Rigaku, Miniflex Cu K-alpha radiation), with a 2θ scanning range from 10 to 80 o and step interval of 0.02 o . Absorption spectroscopy was also done using a UV-Vis Spectrometer (Perkin Elmer, Lamda 950). A scanning electron microscope (SEM), JSM5800LV, JEOL, Japan with energy dispersive X-ray spectroscopy (EDX) (Oxford ISIS 300) was used to measure and confirm the morphologies of all the cement particles and bacteria, along with the elemental composition of the cement samples.

First-principles calculations
A first-principles approach was employed with the density of states of Ca 12 Al 14 O 33 :2O 2cement and Ca 12 Al 14 O 33 :4e À cement using the Vienna Ab initio Simulation Package (VASP) [25]. The pseudopotential used in this work was based on the Projector Augmented Wave (PAW) approach [26]. The PAW valence states were 3s and 3p, 4s, 3s and 3p, and 2s and 2p for Ca, Al and O, respectively. In this work, the Ceperley-Alder form of the exchange-correlation functional [27], which is the local density approximation (LDA), was used to determine the electronic density of states of both the Ca 12 Al 14 O 33 :2O 2and Ca 12 Al 14 O 33 :4e À cements. A 600 eV plane-wave cutoff energy and 5 Â 5 Â 5 K-point sampling of the Brillouin zone were used for all calculations. The HSE06 hybrid functional was chosen to determine the density of the Ca 12 Al 14 O 33 :4e À states.

Property measurements
The vibration mode of atomic bonding was evaluated using Fouriertransform infrared spectroscopy (FTIR), (Bruker, Senterra). The optical properties of the samples were investigated using a diffused reflectance UV-Visible spectrometer, (DRS) (Perkin Elmer, Lambda 950). Optical measurements were used to determine the absorption coefficient spectra of the specimens at room temperature.

Antibacterial property testing
The antibacterial properties of the CAO@1350C, CAO@1450C and CAO@1550C samples were tested using an agar disk-diffusion method against a gram-negative bacterium, Escherichia coli (E. coli) (ATCC 25922) and a gram-positive bacterium, Staphylococcus aureus (S. aureus) (ATCC 25923). All samples were pellet shaped with diameter of 10 mm. E. coli and S. aureus were cultivated on Muller Hinton agar at 37 C for 24. Then, cells of E. coli and S. aureus were suspended in a 0.85%NaCl solution and the cell suspension was adjusted to 0.5 McFarland (1 Â 10 8 CFU/mL). Subsequently, the E. coli and S. aureus cell suspensions were swabbed onto Muller Hinton agar. After drying, CAO@1350C, CAO@1450C and CAO@1550C sample pellets were placed on the agar surfaces. Then, the agar plates were incubated (Contherm, Scientific Ltd., New Zealand) at 37 C for 24 h in a dark incubation chamber. Finally, the inhibition zones on the agar plates were photographed and the widths of inhibition zones reported. Inhibition of E. coli and S. aureus was confirmed using scanning electron microscopy (SEM), JSM5800LV, JEOL, Japan.

Characterization of Ca 12 Al 14 O 33 cement as a starting material
The CAO@1200C sample was prepared via a solid-state reaction at 1200 C for use as a starting material for fabrication of conducting Ca 12 Al 14 O 33 cement. Fig. 2 Fig. 3 (a) presents a schematic of an electromagnetic induction heating system for synthesizing the samples fabricated in the course of this work. The starting CAO@1200C powder was loaded into a carbon crucible. The carbon crucible was wrapped with a Cu induction coil. Cooling water was circulated in a Cu coiled tube to protect against overheating on opration of themagnetic induction heating. An IR detector was used to measure the temperature of the carbon crucible and this information used as feedback to control the power of the induction heater. The carbon crucible was rapidly heated from room temperature to 1350 C, 1450 C and 1550 C, with a holding time of 40 seconds for sintering. Then, the samples were rapidly cooled to room temperature. Fig. 3 (b) shows an electromagnetic induction heating time of approximately 1 minute for sintering. Fig. 4 shows the XRD patterns of the synthesized CAO@1350C, CAO@1450C and CAO@1550C samples prepared by heating in a carbon crucible by high frequency electromagnetic induction. The XRD results showed that the patterns of the sintered CAO@1350C and CAO@1550C samples matched the JCPDS#09-0413 file as Ca 12 Al 14 O 33 cement phase [28,29,30,31,32]. These results confirmed the synthesis of a Ca 12 Al 14 O 33 cement phase in the CAO@1350C and CAO@1550C samples. In contrast, the XRD patterns of the synthesized CAO@1450C sample exhibited a different XRD pattern pattern that matched the JCPDS#09-0413 file. This result was due to formation of a glass Ca 12 Al 14 O 33 cement phase as corresponding to that observed in a previously published XRD pattern [33]. This implied that the preparation process produced a glass phase of Ca 12 Al 14 O 33 cement at a temperature of 1350 C. Thus, this confirmed that all samples formed phases of Ca 12 Al 14 O 33 cement during high frequency induction heating. However, the CAO@1350C, CAO@1450C and CAO@1550C samples did not exhibit 2θ XRD peaks at 26.5 , 10.8 and 25.2 . These reference graphite, graphene oxide, and reduced graphene oxide, respectively. These results confirmed that graphite from the crucible did not dissolve into the samples [34]. Fig. 5(a-k) presents images CAO@1350C, CAO@1450C and CAO@1550C samples fabricated at 1350 C, 1450 C and 1550 C, respectively, and measurement of their electric resistances using a multimeter. Fig. 5 (a) shows a white colored powder sample. This result reveals that the sample did not form a single crystal phase at this temperature. The resistance of the sample was very high as shown in Fig. 5 (b) and (c). Fig. 5 (d) presents a yellow colored sample that formed from a single crystal in the crushed powder sample (Fig. 5 (e)). A single crystal phase was produced that had a very high resistance as shown in Figs. 5 (f) and (g). Fig. 5 (h) shows a greenish-black colored sample with a cement from a single crystal type forming a green colored crushed powder ( Fig. 5 (i)). This sample, sintered at 1550 C, was electrically conductive as shown in Figs. 5 (j) and (k), leading to the observation that sintering at temperatures higher than 1450 C produced a single crystal form of Ca 12 Al 14 O 33 cement. Only the CAO@1550C sample displayed electrical conductivity after its formation at 1550 C. The green color of this sample was not the same as the CAO@1350C and CAO@1450C samples implying a different mechanism for fabrication of the CAO@1550C sample than the CAO@1350C and CAO@1450C samples.

Absorption coefficients
The differences between the CAO@1550C, CAO@1350C and CAO@1450C cement phases were investigated from their optical absorption coefficients measured using UV Visible Spectrometry. Fig. 6 shows absorption coefficients in the UV spectrum of the prepared CAO@1200C starting powder, and the synthesized CAO@1350C, CAO@1450C and CAO@1550C samples at room temperature over the range of 1.6 eV-6 eV. The absorption spectra of the prepared CAO@1200C sample had the highest absorption peak at 4.1 eV. This was characteristic of the electrically insulating Ca 12  Additionally, the absorption spectra of the CAO@1550C sample displayed its two highest absorption peak positions at 2.8 eV and 1.5 eV. The first peak at 2.8 eV presented electrons transitioning from the occupied cage level (an F þ -like center level due to a relaxation time) to the framework conduction band (FCB), as previously reported [28,29]. The energy at 2.8 eV was due to the inter-cage transition energy for the free electrons in cavity-cage structure. The second peak at 1.5 eV displayed an energy level from the F þ -like center level to the cage-conduction band (CCB). A second peak energy level was reported in range of 0.4-1.5 eV [28,29,30,31]. This energy level was too large for the empty cage and an electron with less energy to occupy the cage. The two peak positions at 2.8 and 1.5 eV were characteristic of conducting Ca 12    respectively, which is good agreement with earlier reports [32]. The CAO@1200C sample, i.e., starting powder, as well as the CAO@1350C and CAO@1450C samples, displayed a light color without free electrons in the structure. The CAO@1550C powder had a green color, with approximately 2Â10 20 cm À3 of free electrons. Additionally, the CAO@1200C, CAO@1350C and CAO@1450C samples were electric insulators while the CAO@1550C sample was electrically conductive. The CAO@1200C starting powder, as well as the CAO@1350C and CAO@1450C samples, were white colored, indicating that they were transparent in the visible light region (1.6-3.2 eV), as well as acting as electrical insulators. These results illustrated that the CAO@1200C, CAO@1350C and CAO@1450C samples were electric insulators as was the insulating Ca 12 Al 14 O 33 :O cement owing to free oxygen radicals in their cavity-cages. The greenish-black color of the CAO@1550C sample indicated light absorption in the region of 1.5-2.8 eV. This confirmed that sintering at 1550 C by rapid induction heating could convert electrically insulating Ca 12 Al 14 O 33 :O cement to conducting Ca 12 Al 14 O 33 :e À cement. This process can replace free oxygen radicals with free electrons in an electron-doped process inside a nano-cage Ca 12 Al 14 O 33 cement structure.

SEM and EDS analysis
The imagery in Fig. 7

. Raman spectroscopy analysis
Raman spectroscopy [34,35,36,37] was used to characterize the molecular structure and bonding of the fabricated materials. Fig. 9 shows Raman spectra of the prepared CAO@1200C sample as well as the synthesized CAO@1350C, CAO@1450C and CAO@1550C samples. The absorption bands for the Ca 12 Al 14 O 33 cement structure have normal lattice properties due to vibrations of Al 3þ , Ca 2þ and oxygen ions in the region from 50 to 3000 cm À1 [37]. In previous work [37], Raman peaks between 200 and 1000 cm À1 were ascribed to the lattice framework of the Ca 12 Al 14 O 33 cement structure resulting from Al 3þ ions in a tetrahedral structure. The Raman spectra peaks at 330, 510, and 773 cm À1 of the CAO@1350C, CAO@1450C and CAO@1550C samples corresponded to peaks of the CAO@1200C sample. The Raman peak at 330 cm À1 was caused by the oxygen (O 2-) framework due to vibrations of Ca[AlO 4 ] and Ca-O bonding. The two bands at 510 cm À1 and 773 cm À1 indicated bending vibrations the Al-O-Al linkages and Al-O stretching vibrations, respectively, for a lattice structure with the oxygen and aluminum atoms in a symmetric framework with an Al-O [AlO 4 ] 5sub-structure [38]. The band at 178 cm À1 appeared on the CAO@1200C and CAO@1350C sample and was identified as characteristic of the lattice framework Additionally, the formation of superoxide (O 2 À ) ions occurred via the following chemical reaction in Eq. (3) [33]: Formation of O 2 2À ions can be described by Eq. (4) [39]: and O À ions were not observed in the nano-cage cavity of the CAO@1550C sample. This implied that the CAO@1550C sample was completely converted from insulating Ca 12 Al 14 O 33 cement to its conducting form. This mechanism successfully replaced free oxygen ions in a cavity-cage with free electrons via the reaction in Eq. (5): This mechanism required free radical carbon ions (C 2

2À
) at a reaction temperature of 1550 C. In this process, the C 2 2À ions were generated from the carbon crucible that reacted with O 2 À ions when it was heated to 1550 C. Then, the reaction removed free oxygen O 2 À ions and injected free electrons (e À1 ) into the nano-cage structure. No peaks at 1870 cm À1 were observed for the CAO@1550C sample. Kim et al. [33] reported Raman band spectra at 1870 cm À1 , ascribing them to C 2 2À ions. C 2

2À
ions were dissolved into the CAO@1550C sample from the C 2 2À atmosphere of the carbon crucible and were released out of the nano-cages during cooling. In summary, the extra-framework O 2 2À and O 2 À ions present in the CAO@1200C, CAO@1350C and CAO@1450C samples were responsible for producing an insulating Ca 12 Al 14 O 33 cement. The CAO@1550C sample, i.e., Ca 12 Al 14 O 33 :e À cement, had free electrons in a nano-cage structure that allowed for electrical conduction.

Optical properties
The absorption coefficients shown in Fig. 6 present the optical properties of the CAO@1200C, CAO@1350C, CAO@1450C and CAO@1550C samples. They were used to calculate to optical energy gap (E g ) following the relationship in Eq. (6) [20]: where hυ denotes the photon energy, E g represents the direct optical gap, and 1/2 is a value for the allowed direct transition type. Thus, the allowed direct optical gap can be calculated from Eq. (7): where A is a constant. The optical energy gap (E g ) was fitted to a straight line to intercept the photon energy (hυ) axis. Fig. 10 shows

First-principles calculations
First-principles calculations for evaluating the optical properties of Ca 12 Al 14 O 33 were performed as previously outlined [40,41]. The electronic properties of insulating Ca 12 Fig. 11 (a) clearly shows a peak located at 0.3 eV below the Fermi level corresponding to the 2p states of extraframework 2O 2ions. Additionally, the energy gap in this case is the difference between the energies of the highest states of the framework valence band (FVB) and the lowest states of the CCB. In this case, we found that the energy gap is approximately 4.1 eV. This result was close to the energy gap of the experimental results for CAO@1200C (3.9 eV), CAO@1350C (4.1 eV) and CAO@1450C (4.0 eV) as shown Fig. 10   The electronic density of states of Ca 12 Al 14 O 33 :e À was determined using the local density approximation (LDA) scheme shown in Fig. 11 (b). It is well established that electrical conductivity is directly related to the electron states at the Fermi level. A large number of electron states at the Fermi level energy, E F (the highest occupied state), which is at 0 eV, results in high electrical conductivity.
According to the density of states of Ca 12 Al 14 O 33 :e À , it can easily be seen that there are CCB states at the Fermi level. This result implies that electron conduction comes from the CCB, in this case. Hence, Ca 12 Al 14 O 33 :e À can be electrically conductive. However, the energy gap obtained from LDA functional analysis was always smaller than the experimentally derived energy gap. To improve the accuracy of our calculations, a hybrid functional was used. For Ca 12 Al 14 O 33 :4e À , we determined the electronic density of states using the Heyd-Scuseria-Ernzerh of 06 (HSE06) hybrid functional shown in Fig. 11 (c). The electronic density of states calculated from the HSE06 hybrid functional looks similar to that determined from the LDA functional. Moreover, we found that the FVB states shifted to slightly lower energy levels relative to the FVB of LDA. Hence, the energy gap increased in this case. The energy difference between the highest FVB state and the lowest CCB state was approximately 4.7 eV. Additionally, the energy required for the electronic transition from FVB to FCB was rather high, 7.4 eV. Thus, it is impossible for electrons to be excited from the FVB to the FCB.
The energy difference between the highest FVB state and the lowest CCB state was approximately 3.7 eV-4.7 eV. This result was close to the experimentally derived energy gap for CAO@1550C (3.5 eV). The energy gap between FCB and CCB was about 0.42 eV, representing the metallic bands of Ca 12 Al 14 O 33 :e À that are characteristic of the cage-like structures with no extra-framework oxygen species.. According to Fig. 11 (b) and (c), only electron states of the CCB exist at the Fermi level. Consequently, electronic conduction was observed in this material because of the free electrons in the cage. The above results are consistent with previous work [42,43]. From Figs. 6, 10, and 11, the energy diagram for insulating Ca 12 Al 14 O 33 :2O 2and conducting Ca 12 Al 14 O 33 :e À cements obtained from both experiments and calculations [28,30,43] is summarized in Fig. 11(d).
Moreover, the experimental and calculation results showed that the insulating Ca 12 Al 14 O 33 :2O 2cement had a direct optical gap of approximately 4.0 eV for the electronic transition from the FVB to the CCB. For conducting Ca 12 Al 14 O 33 :e À , the experimental and calculational results showed the direct transition gap from FVB to CCB was around 3.5-3.7 eV. This information indicated that it is impossible for electron excitations to occur between the FVB to FCB due to a rather large energy gap (3.5 eV). The conducting Ca 12 Al 14 O 33 :e À cement displayed metallic behavior when it absorbs external energy at around 2.8 eV, corresponding to the energy level from CCB to FCB for free electron movement between cages. This energy (2.8 eV) corresponds to the absorption peak position at 2.8 eV that results in the greenish-black color of the sintered CAO@1550C sample shown in Fig. 5. Additionally, electrons become free electrons when their energy becomes greater than the work function energy (E vac ), which is 2.4 eV [31,44]. The broad absorption band at 2.8 eV for the CAO@1550C sample is attributed to the transition to intra-cage s-to-p forms of electrons trapped in the cages. The absorption band at 1.5 eV was due to the transition of inter-cage s-to-s charge transfer from an electron trapped in a cage to a vacant neighboring cage [38]. The transition of free electron movement of the CAO@1550C sample (Ca 12 Al 14 O 33 :e À cement) was supported by oxygen gas adsorbed at the surfaces of the materials to produce free radicals [45] according to the reaction Eq. (8):

Schematic diagram for fabrication of conducting Ca 12 Al 14 O 33
A conducting Ca 12 Al 14 O 33 :e À cement was successfully fabricated via a sintering process starting with insulating Ca 12 Al 14 O 33 :O 2cement inside a carbon crucible using high frequency induction heating to 1550 C. A schematic presenting the mechanism for converting insulating Ca 12 Al 14 O 33 :O 2cement to conducting Ca 12 Al 14 O 33 :e À cement via this process is shown in Fig. 12. Fig. 12(a) shows the starting Ca 12 Al 14 O 33 :O 2cement powder loaded inside a carbon crucible. The Ca 12 Al 14 O 33 :O 2structure is a linkage of calcium, aluminum and oxygen atoms consisting of empty nano-sized cages containing of free oxygen ions (O À2 ), loosely bound to a lattice framework, inside cages as an extra-framework. Fig. 12(b) presents the process for producing conducting Ca 12 Al 14 O 33 :e À cement by rapid heating carbon via electromagnetic induction heating to 1550 C in a carbon crucible. The thermal energy (Q) in the carbon crucible was obtained from eddy currents and induced electromagnetic power [46] via the relationship Eq. (9): where σ(T) is electrical conductivity as a function of temperature(S/m), ω is angular frequency (rad/sec), and B is the magnetic vector potential.
At 1550 C, an active carbon radical (C 2 2À ) species was generated from the heated carbon for producing a C 2 2À atmosphere inside the crucible [31,33,34]. Then, the C 2

2À
ions reacted with free oxygen ions in the cavity-cages of the Ca 12 Al 14 O 33 :O 2cement powder to produce CO or CO 2 gas, leaving an electron in the cavity-cage [34].
This process replaced free oxygen ions with free electrons by reacting active carbon C 2 2ions with free oxygen ions. First, carbon C 2

2À
ions were produced inside the crucible. Then, the C 2 2À gas reacted with free oxygen species (O 2-) in cavity-cage at the surface layer of the insulating Ca 12 Al 14 O 33 :O 2cement powder to produce CO or CO 2 gas and ejected free electrons into the cavity-cages via the reaction in Eq. (10), This resulted in formation of two free electrons in the cage obtained from O 2À ions. Finally, the reaction ran to completion when the insulting  ) atmosphere inside the carbon crucible at high temperature [34].

Anti-bacterial activities of Ca 12 Al 14 O 33 cements
Anti-bacterial activities of the as-fabricated CAO@1350C, CAO@1450C and CAO@1550C samples were investigated using two different bacterial species: (1) gram-negative E. coli and (2) gram-positive S. Aureus as shown in Figs. 13 and 14, respectively. The antibacterial properties of the samples were tested using an agar disk-diffusion method. Agar plates were first seeded with the test bacteria, followed by placed on the agar surfaces in a dark incubation chamber at 37 C for 24 h. Both the insulating Ca 12   structural differences in the bacterial cell walls. E. coli and other gramnegative bacteria have an outer cell membrane [47,48]. This provides E. coli with more resistance to reactive oxygen species (ROS). The SEM results show rod and spherical-shaped cells in the growth zones of E. coli and S. aureus in Figs. 13 (a) and (b), and Figs. 14 (a) and (b), respectively. Neither E. coli nor S. aureus grew in the inhibition zones shown in the SEM images (Figs. 13 (f), (g), (h), and Figs. 14 (f), (g) and (h), respectively). These results confirm that both bacterial species were inhibited to varying degrees by the CAO@1350C, CAO@1450C and CAO@1550C cements.
The mechanism to inactivate these bacteria was likely due to the effect of reactive oxygen species (ROS) such as super oxides (   and O À ions) with concentrations up to 2 Â 10 20 cm À1 [24]. Clearly, the presence O 2 À species at the surface of the samples can cause inactivation of E. coli and S. aureus cells, as reported earlier [24], and this mechanism requires no photocatalytic effect.
In the case of conducting Ca 12 Al 14 O 33 :e À , as shown in Fig. 9 Fig. 16. These active oxygen species are extremely reactive, oxidizing and decomposing the organic substances of the bacteria. This implied that the conducting Ca 12 Al 14 O 33 :e À cement displayed antibacterial activity with no photocatalytic effect. The reaction generating the ROS species occurred via redox reactions [8] as follows Eqs. (11), (12), (13), (14) and (15): CAO(e À ) þ H 2 O 2 → OH þ OH À Superoxide anions (O 2 À ) were first generated from O 2 gas in the atmosphere reacting with caged free electrons (Ca 12 Al 14 O 33 :e À (cage)) at the surface of conducting Ca 12 Al 14 O 33 :e À cement following Eq. (11). Then, H 2 O 2 was generated as depicted in Eq. (12). H 2 O 2 reacted with caged free electrons in conducting Ca 12 Al 14 O 33 :e À cement, along with O 2 À , and O 2 À to generate OH radicals according to the reactions in Eqs. (13), (14), and (15). In this manner, conducting Ca 12 Al 14 O 33 :e À cement inhibited both E. coli and S. aureus through the action of nano-caged free electrons in (Ca 12 Al 14 O 33 :e À (cage). Fig. 16 schematically shows a proposed mechanism for the conducting Ca 12 Al 14 O 33 :e À cement's antibacterial activity. In summary, in the first mechanism of antibacterial activity, bacteria near the surface of this material were inactivated by OH at the surfaces of cement materials. Testing confirmed that the Ca 12 Al 14 O 33 :e À cement displayed antibacterial action against E. coli and S. aureus. Neither effect involved photocatalytic activity. These cement materials can be used in smart antibacterial walls for operation rooms and hospital wards, restaurants, nurseries, and in homes. There are also applications in HVAC and food processing.

Conclusions
Antibacterial Ca 12 Al 14 O 33 material was successfully prepared by a rapid heating of insulating Ca 12 Al 14 O 33 powder in a carbon crucible to a high temperature by high frequency electromagnetic induction heating. The CAO@1550C (Ca 12 Al 14 O 33 :e À ) cement sample formed a conducting phase in the Ca 12