Measurements of Pulsed 532 nm Laser Breakdown Spectroscopy of Synthesized Magnetite Nanoferrofluid

We describe the results of 532 nm pulse laser-induced breakdown spectroscopy (LIBS) of two samples of magnetite nanoparticles (SPIONs) nanoferrofluid synthesized at room (S1) and elevated temperatures (S2) and at three different laser energy levels and pulse frequency. The size of magnetite nanoparticles, size distribution, magnetic crystalline phase and magnetization were analyzed and measured using transmission electron microscopy (TEM), X-ray diffraction spectroscopy (XRD) and vibrating sample magnetometry (VSM). The SPIONs showed a distribution between 4 22 nm with a peak about 12 nm and saturation magnetization of about 65 emu/g. The Saha-Boltzmann analysis of spectra for medium energy level (1050 mJ) yields plasma temperatures of (3881 ± 200) K and (26,047 ± 200) K for Fe I and OV as the lowest and highest temperatures respectively. A range of corresponding electron density (Ne) of (0.47 6.80) × 10, (0.58 8.30) × 10 and (0.69 9.96) × 10 cm were determined at 860, 1050 and 1260 mJ respectively using the estimated CCD pictures. The results confirmed a higher elements ratio for S1 than S2 and the signal intensity indicated a non-linear behaviour as a function of pulse frequency with the maximum ratio value at 3 Hz. At higher frequency of 6 Hz no such turning point was observed. The highest and lowest temperatures corresponded to Fe I and OV respectively. The LIBS technique can be utilized to study, characterize and determine the elements ratio required in most applications involving the synthesizing process.


Introduction
In recent year much interest has been shown for remote, versatile, fast as well as accurate elemental analysis.Analysis techniques such as laser-ablation inductively coupled mass spectrometry (LA-ICP-MS) [1], X-ray fluorescence spectrometry (XRFS) [2], and secondary ion mass spectrometry (SIMS) [3] are able to resolve smaller element concentration.However, they often require specific sample preparation and under-lab condition operation and have smaller analyzed material capacity per time.Laser-induced breakdown spectroscopy (LIBS), which is considered as a type of atomic emission spectroscopy is based on focusing a short laser pulse on the surface of the sample where the material is ablated via thermal or non-thermal mechanisms.A high density plasma is generated when the laser intensity reaches the breakdown threshold of ≈10 10 Wcm −2 .
The optical emission is then collected and spectroscopically analyzed.It is the different energy levels of each atom that induces different and well quantized energies with narrowband emissions governed by Heisenberg uncertainty principle.When a material is ablated by a short laser pulse, the plume partly contains free electrons, excited atoms and ions, which are produced by strong ionization under an intense external electric field.The spatial free electron density i.e., plasma is produced by multiphoton absorption process where the electrons absorbing the energy of laser beam photons via inverse bremsstrahlung will result in a process known as avalanche or cascade of ionization.When the laser pulse terminates, a few microseconds later the plasma begin to slow down due to collisions with ambient gas and hence cool down during the expansion where the electrons of the atoms and ions at the excited electronic states decay to ground states, hence causing the plasma to emit light with discrete spectral peaks.Both the continuum and discrete line emission indicate the radiative relaxation of different energy levels of the ions and the neutral atoms and that the excited species are mainly generated by ion-electron recombination.The main features of discrete lines characterizing the material are: wavelength, intensity and the shape which depend on structure of the emitting atom and the environment.The plasma temperature can reach to thousands Kelvin.The major advantages of LIBS are: no sample preparation is required, fast, real-time measurements, depth profiling and spatial resolution and relatively simple set up compared to other conventional techniques [4] [5] [6].LIBS has been widely used as a powerful technique for detection and monitoring of traces of elements of matter in all states of solid, liquid and gas in variety of applications such as industrial for metal detection in water [7] [8], analysis of minerals [9], environmental for identification of bacteria [10] [11], and biomedical such as analysis of cancerous tissues [12] [13] and dental caries [14] [15].World Journal of Nano Science and Engineering Interestingly, iron oxides are present in living organism where Fe (II) with an electron configuration of 1s 2 2s 2 2p 6 3s 2 3p 6 3d 6 4s is responsible for transport of oxygen and storage of oxygen by means of haemoglobin and myoglobin.Magnetite (Fe 3 O 4 ) phase containing both Fe (II) and Fe (III) with electron configuration of 1s 2 s 2 2p 6 3s 2 3d 5 has iron ions arranged in octahedral (Oh) and tetrahedral (Th) sites which form the face-cent red cubic unit cell in the presence of oxygen.The structure of this phase can be represented as [Fe] Th [Fe 3+ Fe 2+ ] Oh O 4 .
In the inverse spinel structure of magnetite, 1 4 of the octahedral holes are occupied by Fe 2+ ions while the Fe 3+ ions are equally divided between 1 8 of the tetrahedral holes and 1 4 of the octahedral holes; electron spins of Fe 3+ ions in octahedral holes are aligned anti-parallel to those in tetrahedral holes while the Fe 2+ ions tend to align their spins parallel with those of Fe 3+ ions in adjacent octahedral sites, leading to a net magnetization.When the size of these nanoparticles becomes so small that their dimension can be considered as a single domain, they lack a hysteresis loop and a possession high field irreversibility, high saturation field and extra anisotropy contributions, it is called superparamagnetic nanoparticles (SPION) [16] [17].Since, SPIONs posses remarkable size and morphology-dependent physical and chemical properties and obey the Coulomb's law and are easily controlled by an external magnetic field, thus they can be utilized for various biomedical applications such as hyperthermia [18] [19] drug delivery [20] [21] bioimaging [22] [23] and as nanofluid for heat transfer [24] [25].
However, synthesis and characterization of high-quality magnetic iron oxide nanoparticles (MNPs) has always been a major issue particularly when dealing with ultrafine particles to control their size at the nanometric scale due to the high surface energy.In this view, the elemental analysis of NPs could be very beneficial particularly at the stage of synthesis of NPs such as the ratio of elements or where the information about physicochemical forms of an element are highly regarded because the redox state of an element in solution can be toxic, adsorption behavior, and transport mechanism.Information about various oxidation states of the elements in variety of objects is crucial and challenging in analytical chemistry as for instance, natural waters contain iron in oxidation states +2 and +3 and Fe III content is significantly higher than Fe II content and it strongly affects the solubility of iron in water [26] [27].Also, the iron oxide present in soil could form from an Fe II chloride solution in ambient temperatures and close to neutral pH values [28] and since oxidation leads to the formation of minerals such as goethite, an iron oxyhydroxide containing ferric iron, knowledge of Fe II: Fe III ration is important in this case.The commonly applied analytical methods are normally time consuming and require reagents and often produce inaccurate results in the presence of high Fe III content.Therefore, using LIBS as a non-contact, fast processing, and accurate technique would be de-World Journal of Nano Science and Engineering sirable.Some researchers have employed LIBS to analyze the oxidation state of wine to monitor its quality [29], to measure the concentration of magnetite (Fe 3 O 4 ) nanoparticles (MNPs) in powder form in biocompatible matrix using Nd:YAG laser [30], or to analyze the graphene nanoparticles using 800 nm laser [31].To best of our knowledge, no results are reported in the literature regarding the application of LIBS to magnetite nanoferrofluid synthesized at elevated temperature using Nd:YAG laser second harmonic generation wavelength (532 nm).
Following our recent reports on MNPs [25] [32] [33] [34] [35], our goal in this work is to use pulse 532 nm laser to perform LIBS: 1) to investigate the emission lines of synthesized magnetite nanoferrofluid, 2) to determine the electron density and the temperature of dominant species, 3) to compare the elements ratio obtained at room and elevated temperatures, and 4) to study the effect of laser pulse frequency on elements ratio at different temperatures.as iron source in double distilled water.The mixture of ferric and ferrous solution was then deoxygenated by bubbling N 2 gas and sonicated for 30 minutes to prevent it from oxygenation.An amount of 0.9 M concentration of ammonia solution was used as alkaline source and vigorously stirred under N 2 bubbling at room temperature.The mixture of ferric and ferrous solution was drop wisely added to ammonia solution and magnetically stirred at 1500 rpm for 30 min at ambient temperature.The colour of reaction mixture was immediately changed to back which is characteristic of a typical black nanoferrofluid material.The molar ratio of Fe 2+ :Fe 3+ = 1:2 and complete precipitation of Fe 3 O 4 between 7.5 -14 pH was maintained under a non-oxidizing environment since this would critically affect the physical and chemical properties of the nanosized magnetic particles.The reaction temperature was kept constant at 25˚C for 1 hour then the black precipitated mixture was purified using magnetic separation 5 times and sedimented by centrifugation.The resultant material was dried by freeze dryer for 24 hours.

Characterization
The crystalline phase of SPIONs was determined using x ray diffraction using

Results and Discussion
Figure 1(a) shows the TEM of synthesized SPIONs where polydispersity and some degree of aggregation, which mainly depends on the surface to volume ratio and augmentation of magnetic dipole-dipole interactions can be seen [32].It is known that at room temperature,300 K, the critical size which defines the superparamagnetic region is equal to 27 nm [36] which in our case as seen in The chemical procedure for synthesis of Fe 3 O 4 is as follow: The first step is the hydrolysis of iron salts as ( ) ( ) ( ) In a reactive system, NH 4 OH can be written as NH OH NH OH The interaction between the resultant materials in solid state will lead to the ( ) and the interaction of 4 NH + cations with the resultant anions from hydrolysis of iron salts gives ( ) By increasing the solution of iron salt to NH 4 OH, the ions of Fe 3+ will precipitate as ferrihydrite which is a hydrous ferric oxyhydroxide mineral and then react with Fe 2+ to produce magnetite.It is noteworthy that Ostwald ripening effect (growth of larger particles at the expense of dissolving the smaller ones) and aggregation of NPs can affect the growth phenomena.The synthesis process that contains many intermediate products can influence the nucleation and growth phenomena.Once the alkaline species and iron salts are added together, some cations and anions such as Magnetic properties of MNPs were determined by VSM at room temperature.
Figure 2(b) shows that the synthesized NPs exhibit strong magnetic properties of 63.1 emu/g.However, these values can be lower than the bulk material of magnetite due to breaking of a large number of exchange bonds at the surface [32] [37].Also, a complete reversibility of magnetization process confirms the superparamagnetic behaviour of prepared nanoparticles.Higher concentration of alkaline media increases the probability of non-magnetite layer production (i.e., magnetically dead layer) which in turn decreases the amount of saturation magnetization.Thus, there is a limit that alkaline media can have a positive effect on the saturation magnetization.Using Langevin's equation and magnetic experimental data, the average magnetic particle diameter was calculated [38]: where a M is the magnetic particle diameter, k B is Boltzmann's constant, μ 0 is the vacuum magnetic permeability, and M b is magnetization of bulk magnetite.In our case, a very bright white spark indicating the high temperature plasma with blue shade probably because of ionized oxygen present in the surrounding environment was observed.
It is well-known that the laser wavelength and the pulse energy influence the interaction process where the former governs the amount of energy absorbed by the material and the plasma-material interaction and the latter, mainly controls the amount of ablated mas and the ablation rate.The plasma formation by nanosecond pulses as in our case seen in Figure 3, is determined by two major mechanisms of inverse Bremsstrahlung where the free electrons absorb energy the laser beam during a collision and increases their kinetic energy.It is responsible for heating the plasma electrons and secondly, the photoionization where the excited species and the ground state atoms are excited with high energies.As seen in Figure 4(a), the main transitions at 860 mJ are Fe I, Fe II, Cl III, O II, Cl I, OV and the amplitude of the peaks are relatively small and by increasing the pulse number not a significant difference was observed.A significant saturation has occurred at excitation wavelength and some lines are very close to other transitions such as OV at 654.37 nm to that of hydrogen alpha at 656.28 nm, which might be due to hydrogen in nanoferrofluid.However, the line shown by spectrophotometer software is compared and denoted to the closest with minimum difference.At 1050 mJ, the transition lines are the same but the peaks between 250 -280 nm seem to be more differentiated as delineated by circle in Figure 4(b).This is mainly because at higher temperatures, the ablated material dissociates (i.e., breaks down) into excited ions and atomic species, the amplitude of lines is also slightly increased.At even higher pulse energy of 1260 mJ, Figure 4. LIBS spectra at different laser pulse energy and pulse number.World Journal of Nano Science and Engineering the first pulse caused much stronger lines amplitudes but then again are reduced at the consequent pulses, Figure 4(c).In principle, the loss of line intensity is mainly explained by the interaction of trailing portion of pulse with the plume and ejected particles which acts a shield hence a considerable fraction of energy is attenuated.The Fe III emission lines lie between 400 -500 nm, which are due to 5 D-3 P, 5 D-3 F, 5 D-3 G and 5 D-3 H transitions among levels of the 3d 6 configuration (i.e., once it loses three electrons to form 3 + , it becomes 1s 2 s 2 2p 6 3s 2 3p 6 3d 5 ).
Thus, although there are some Fe III lines, only limited and most probable observable lines are indicated in the Figure 4 due to lack of sufficient space.The details of spectra analysis are given in Table 1 [39].
The central wavelength of line emission λ 0 is defined by the photon energy difference ΔE = E 2 − E 1 where E 1 and E 2 correspond to the energy of photons at longer and shorter detected wavelengths respectively (i.e., lower and upper energy levels): where h ≈ (6.63 × 10 −34 Js) is Planck constant Js and c = 3 × 10 8 ms −1 is the speed  ( ) where n(p) is the species population or density in the excited state, A 21 is the transition probability and 4π represents the solid angle dΩ i.e., isotropic radiation, measured in sr.Since, the line profile p λ correlates the emission coeffi- cient with the spectral line emission coefficient we have: The full width half maximum (FWHM) of the intensity is a characteristic of the line profile, which in turn depends on the broadening mechanism such as Doppler broadening where the profile has a Gaussian shape.
Since, the evaluation of plasma temperature is difficult due to its short lifetime and partly because of high temperature but one can use the Saha-Boltzmann equation for two or more consecutive spectral lines of the same species where the maximum and minimum amplitudes for each element can be used to measure the corresponding temperature [5] Using the Equation ( 8) to determine ΔE and the values given in Table 1 are substituted in Equation (11) to determine the temperature of the species.Using the spectral data at 1050 mJ as the medium energy value, the plasma temperature was determined between 3881 ± 200 K (or 0.33 eV) for Fe I and the maximum of 26,047 ± 200 K (or 2.24 eV) for OV elements.Assuming that all the laser pulse energy is expended for ionization of MNPs, the number of free electrons corresponding to the element can be approximately calculated as Ne − ≈ (Ep /Pi)/eV where Ep and Pi are laser pulse energy and the element ionization potential respectively [40].The results in Figure 5 show a range of electron density spanning between (0.47 -6.80) × 10  1, which effectively changes the electrons number to an order of 10 20 cm −3 in each case.
Generally, the angular distribution of the ablated material from the target can be represented by the equation I(θ) = I 0 cosnθ, where I(θ) is the flux intensity along a direction forming an angle θ with the normal to the target surface.I 0 is initial intensity corresponding to θ = 0 and n is parameter related to the anisotropy of the distribution.When n = 1, it corresponds to a perfectly spherical distribution and n > 1 implies a more directional plume shape as in our case in Figure 5. Calculated electron density of elements as a function of ionization potential.
Figure 1 particularly at higher energy levels.Indeed, the plasma plume is a combination of hot electrons, colder heavier ions and neutral atoms, which results in an inhomogeneous temperature distribution.The interaction of hot plasma with cold region at the interface with the surrounding colder air may destroy the shape of the emitted lines which can appear as a decrease of the spectral line intensity and increase of the width of line at FWHM [40] [41].Also, upon rapid expansion of the plasma both its temperature and recoil pressure decrease which reduce the rate of heat transfer to the surface.Also, according to Serov and Richardson [42] an intense magnetic field due to laser-induced plasma displays a 1/r 2 radial dependence and that such fields (10 9 Wcm −2 ) would significantly affect the electron thermal conduction process.Thus, it has a crucial role in determining the temperature and density distribution within the plasma.It is also interesting to note that the plasma temperature and hence its possible consequent effects can influence the quality of the material surface during the interaction process.For example, the high pressure exerted on the surface by the plasma plume can suppress the vaporization of the material by raising the boiling point of the material well beyond its normal value and as the plasma expands it generates an impulse reaction on the surface thus, radiates part of its thermal energy back to the surface.Consequently, a high temperature is maintained for a short time after the laser pulse has ended.It is expected that the high temperature is transferred to the surface by high velocity electrons via both direct conductivity and radiative transfer [25] [43].
Figure 6 demonstrates the variation of elements ratio with energy for different temperatures at constant frequency of 1Hz where the higher pulse numbers produced a ratio with higher signal intensity.A characteristic behaviour of the results is their repeating systematic zig-zag feature at almost the same values of energy.The ratio of elements for samples synthesized at room temperature (S1) is higher than those at elevated temperature (S2) except in the case of Fe III/OII where the results are almost superimposed.
At higher frequency (3 Hz) shown in Figure 7, the ratio of elements remained   structural integrity with respect to its applications to avoid any possible damage.

Conclusion
Elemental analysis of magnetite (Fe 3 O 4 ) nanoferrofluid was investigated at two different temperatures using 532 nm-based LIBS technique.Using Saha-Boltzmann relation, the transition lines temperature and the corresponding electron density were calculated.It was shown that the constituting elements and their ratio can be identified depending on the synthesizing and optical conditions.The most important outcome of the experiment is that the elemental ratio is significantly lower at elevated temperature than those at room temperature at all laser energy.
Equally, at higher frequency and room temperature no considerable change in the ratio was observed, however, at elevated temperature and 860 mJ, the ratio was increased and reached its maximum value at 3 Hz.This was not observed at 1260 mJ.One practical application where such an approach can be beneficial and plays a key role in number of scientific, engineering and biomedical applications is when one requires to determine and adjust the ratio of Fe II:Fe III or Fe II/Fe II + Fe III during the synthesis process.

Cu
Kα radiation (XRD, λ = 1.5406Å, FK60-40 X-ray diffractometer).Particle size and morphology of these nanoparticles were determined by transmission electron microscopy (TEM, Philips CM-200-FEG microscope, 120 KV).The saturation magnetization of samples was measured by vibrating sample magnetometer (VSM-PAR 155) at 300 K under magnetic field up to 8kOe.The plasma was analyzed using a spectrometer (UV-Vis USB 2000, Ocean Optics) equipped with 2048-pixel linear silicon CCD array (14 μm × 200 μm) with a detector range of 200 -1100 nm, optical resolution of 0.1 -10 nm FWHM connected to com-World Journal of Nano Science and Engineering puter.
Figure 1(b), the NPs distribution covers a range between 4 -22 nm with most dominant size at 12 nm which satisfies the above condition.

Figure 1 .
Figure 1.(a) An example of TEM micrograph of MNPs; (b) Size distribution of synthesized.MNPs at room temperature (25˚C); and (c) At elevated temperature (70˚C).

Figure 3 Figure 3 .
Figure 3 shows the experimental setup where the output of Q-switched Nd:YAG laser (LG-LM1) operating at second harmonic generation wavelength of 532 nm with a pulse duration of 8 ns was focused by a l00 mm lens into a 500 μm spot size on the surface of nanoferrofluid.The generated plasma was then detected by a 600 μm core diameter optical fiber (LIBS-600-6-SR, Ocean Optics) connected to spectrometer (UV-Vis USB 2000, Ocean Optics) for analysis and the results were then displayed on computer.The pictures of LIBS at different laser energy were recorded by a fast CCD Camera (Panasonic Super Dynamic WV-CP450) connected to an optical microscope (Prior-UK).The examples of plasma recorded at different laser energy demonstrate that by increasing the input energy, the corresponding volume of plasma increases.Clearly, the emitted radiation in the visible spectral range originates from atomic and molecular of light.Since, the energy of a transition is characteristic of an element thus, the central wavelength represents the radiating species.The line intensity is quantified by the line emission coefficient in the unit of W (m 2 sr) −1 .

Figure 6 .
Figure 6.Variation of elements ratio for S1 and S2 with pulse energy at 1 Hz.

Figure 7 .
Figure 7. Variation of elements ratio for S1 and S2 with pulse energy at 3 Hz.

Figure 9 .
Figure 9. Variation of Fe II:Fe III and Fe II:Fe II + Fe III for S1 and S2 with frequency at different energy.

Table 1 .
Spectral data of identified elements during LIBS.