Production of Ferro-Silicon
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Ferro-silicon (Fe-Si) is a ferro-alloy having iron (Fe) and silicon (Si) as its main elements. The ferro-alloy normally contains Si in the range of 15 % to 90 %. The usual Si contents in the Fe-Si available in the market are 15 %, 45 %, 65 %, 75 %, and 90 %. The remainder is Fe, with around 2 % of other elements like aluminum (Al) and calcium (Ca).
Fe-Si is produced industrially by carbo-thermic reduction of silicon dioxide (SiO2) with carbon (C) in the presence of iron ore, scrap iron, mill scale, or other source of iron. The smelting of Fe-Si is a continuous process carried out in the electric submerged arc furnace (SAF) with the self-baking electrodes.
Fe-Si (typical qualities 65%, 75% and 90% silicon) is mainly used during steelmaking and in foundries for the production of C steels, stainless steels as a deoxidizing agent and for the alloying of steel and cast iron. It is also used for the production of silicon steel also called electrical steel. During the production of cast iron, Fe-Si is also used for inoculation of the iron to accelerate graphitization. In arc welding Fe-Si can be found in some electrode coatings.
The ideal reduction reaction during the production of Fe-Si silicon is SiO2+2C=Si+2CO. However the real reaction is quite complex due to the different temperature zones inside the SAF. The gas in the hottest zone has a high content of silicon mono oxide (SiO) which is required to be recovered in the outer charge layers if the recovery of Si is to be high. The recovery reactions occur in the outer charge layers where they heat the charge to a very high temperature. The outlet gas form the furnace contains SiO2 which can be recovered as silica dust. The formation liquid Si goes through several intermediate reactions. This is described later in the article. The main characteristic features of the production of Fe-Si can be summarized in the following three points.
The schematic flow sheet for the production process of Fe-Si is given at Fig 1.
Fig 1 Schematic flow sheet of the production process for Fe-Si
Raw materials
Fe-Si is produced by smelting Fe containing materials and Si containing materials usually in a SAF. Fe is in the form of iron ore, steel scrap or mill scale and Si is normally in the form of quartzite lumps. These are combined with carbonaceous material such as coal or petroleum coke and a bulking agent such as wood chips. Quartzite is the source for Si in the carbo-thermic process. The purity of quartzites is usually lower than for other types of quartz deposits and but is normally suitable for the production of Fe-Si.
Furnaceability is a common international industrial quality term used for quartzite. The quartzite is having good furnaceability when all its chemical and physical criteria are such so as to make it an appropriate silica raw material for the production of Fe-Si with high content of Si at high rates of process performance. The absolute quality requirements of the quartzite raw material are those which are necessary to achieve for the process to be optimized and include (i) chemistry, (ii) material size (typically 10 mm to 150 mm), (iii) mechanical strength, (iv) thermal strength, and (v) softening properties.
Chemistry and size are the most common specifications used by all Fe-Si producers for specifying quartzite. The requirements for the chemistry are related to the content of impurity elements especially elements such as Al, Ca, titanium (Ti), boron (B), and phosphorus (P). Normally, elements more noble than Si (e.g. Al and Ca) end up in the product, whereas the volatile components go into the off-gas. However, the reactions in the furnace are much more complicated than that, and the distribution of the elements in the raw materials also determines where the elements go. Some elements, especially alkalis such as sodium (Na) and potassium (K) can actually lower the melting point of quartzite. Generally, the requirements for the raw materials are connected to the requirements of the products. Fe-Si production usually has requirements that allows for higher contents of the most difficult elements.
The sizing requirements can vary for the different plants and it ranges from 10 mm to150 mm. However, some producers have specifications for narrower sizing. Some Fe-Si producers focus on, or measure the mechanical strength and thermal strength, although these are usually not included in the specifications to the supplier. Additionally, some producers focus on the softening properties of the quartzite. Further, additional requirements can be defined by the individual producer, according to what is most optimal for the specific operation.
The mechanical properties of quartzite affect the size reduction of the raw materials during production in the mine, transport and storage before charging. The generated fine material creates problems for the carbo-thermic process since it can lower the permeability of the charge and obstruct the gas flow from the lower parts of the furnace to the upper parts where SiO gas reacts with the unreacted C in the charge to form SiC, which is an important reaction in the furnace. Additionally, some of the SiO gas condensates and forms a sticky mixture of SiO2 and liquid Si. Loss of SiO gas through off gas channels and lowered Si recovery can be due to the low permeability of the charge.
Fines are defined by two different criteria. In this context, fines are defined as material less than 2 mm size, which is the most critical for the process. Fines less than 2 mm lower the permeability of the charge. Fines can also be defined as the material of lump size below specifications (e.g. -10 mm). As for the mechanical properties, the thermo-mechanical properties is mainly related to the generation of fines, however, in this case, the fines generation occur inside the furnace as bad thermo-mechanical properties results in disintegration of the quartzite as a result of the extreme heat in the furnace. Ideally, the lumpy quartzite is to keep its original size as it moves down through the charge, until the quartzite starts to soften and melt in the lower parts of the furnace near the cavity wall.
Although, most of the quartzite is likely to disintegrate to a certain degree, it is not to be pulverized and generate too much fines that lowers the permeability of the charge as described above. This size reduction can also, in extreme cases, results in a popping effect where in some cases fragments of quartzite can be thrown up into the air. Quartzite with low thermal stability which disintegrates within the charge can also contribute to slag formation in the furnace.
The softening properties of the quartzite are another side of the thermo-mechanical properties. The softening temperature, or softening interval, is the temperature on which the quartzite starts to melt. This is lower than the melting point of quartzite at deg C. The softening temperature is to be as close to the melting temperature of quartzite as possible to achieve the ideal process where quartzite move down to the cavity walls before it starts to melt and droplets of molten quartzite drip from the cavity wall into the cavity, where Si forming reactions take place. Alkali elements (and to a lesser extent alkali earths) are known to affect the melting temperature of the quartzite. It is to be noted that the quartzite which starts to soften or even melt too high up in the furnace, creates a sticky mass, which agglomerates with other particles and become electrically conductive and alter the electric paths in the furnace and even reduce the power of the arc.
Submerged arc furnace for production of Fe-Si
Commercial grade of Fe-Si with Si content of 15 % is generally produced in the blast furnace (BF) lined with acidic fire bricks. Fe-Si with higher Si content is normally produced in SAF. The size of a SAF producing Fe-Si is given in term of electric load and varies from 1-2 MVA to more than 40 MVA. The sizes of SAFs typically consist of upto 10 meters in diameter and 3.5 meters in depth. Electrical energy is supplied through 3 phase alternating current (AC) by the three electrodes submerged deep in the charge. The specific energy consumption is typically up to 9 MWh &#; 10 MWh (megawatt hour) per ton of produced Fe-Si (75 % Si). To operate efficiently and reduce unit fixed cost, a SAF is required to run continuously, 24 hours per day.
Necessary heat for the highly endothermic reactions of SiO2 reduction is generated direct in the charge of the SAF charge as a result of current flow by resistive heating, and by arc heating which burns in the gas chambers located near the electrodes tip. The internal structure of the furnace and temperature distribution in the reaction zones have a close relationship with the proportions of the heat generated in the furnace on the principle of resistance heating and arc heating. One of the most important structural elements of the Fe-Si furnace are immersed in the charge self-baking &#;Soderberg&#; electrodes which bring electricity required for the process. Burning of electric arc and temperature conditions of the reaction zones has a close relationship with the position of electrodes tips in the furnace. The current heats part of the charge to around deg C in the hottest part. At this high temperature the SiO2 is reduced to molten Si.
The temperature distribution of reaction zones are not subject to direct measurements, but to provide the correct electrical and temperature conditions of the process it is necessary to systematically carry out electrodes slipping. The optimal position of the electrodes leads to the minimization of economic indicators of the process. In periods of good and stable operation of the SAF in the reaction zones are conditions for the continuous evolution of new products of the SiO2 reduction. This process has a cyclical nature and it is associated with melting and periodic penetration of liquid SiO2 inside the arc chambers.
SAF has a hood at the upper part of the furnace which directs the hot gases through a chimney to a gas cleaning system. The raw materials namely quartzite, Fe bearing materials, and C bearing materials are transported on conveyor belts and stored separately in day bins. The raw materials in the form of the mixture batch consisting of quartzite, C reducers, and carriers of Fe are weighed, combined in the required proportions, mixed and charged into the furnace through charging tubes. These tubes are located with outlets towards the electrodes. The number of tubes surrounding the electrodes differs from furnace to furnace. The charged material is at the same level as the floor outside the furnace surrounded by a hood that has stoking gates at different sections and these sections can be opened during a stoking period.
Production process of Fe-Si
The raw materials are charged into the furnace from the top. High?current, low?voltage electricity is delivered through a transformer and into the furnace through C electrodes. The process is very energy?intensive, requiring around 9,000 kWh to 10,000 kWh (kilowatt hours) of electricity to produce one ton of 75 % Fe-Si.
SAF used for the production of Fe-Si is usually operated in cycles with stoking, charging and tapping as the main operations. During stoking, the thin crust on top of the charge is broken and old charge is pushed towards the electrode. The new charge is then laid on top of the old charge.
The stoking charging cycle is an operational cycle. The stoking is carried out by a special moving machine equipped with a stoking rod which is mounted in front of the machine. The unevenly charged burden can be distributed with the machine through the stoking gate. Old charged material at the surface is distributed towards the electrodes where depressions have formed around the electrodes. These depressions are formed by the hot reactions zone in the cavity.
In the furnace, the charge is heated to around deg C. At that temperature, the quartzite combines with the C in the reductants forming carbon monoxide (CO) gas and releasing Si, which forms an alloy with molten Fe. Molten Fe-Si accumulates in the bottom of the furnace. Trace element content of the raw materials (including quartzite reduction materials and electrodes) is carried to the product.
Periodically, around at equal time intervals liquid ferro-alloy is tapped into the ladle, through one of the tap holes in the furnace lining. The tapholes are located in the transition between the side and bottom lining of the furnace. The number of tap holes varies from furnace to furnace. The tap hole is usually opened mechanically and closed with special clay mixture.
The off gases are passed through a gas cleaning plant for removal of the dust the main content of which is amorphous condensed SiO2. This dust is generally used as filler material in concrete, ceramics, refractories, rubber and other suitable applications. A furnace produces around 0.2 tons to 0.4 tons of SiO2 dust per ton of ferro-alloy. The cleaned off gas mainly contains CO, sulphur di oxide (SO2), carbon di oxide (CO2), and oxides of nitrogen (NOx). The heat of the gases can be recovered in the waste heat recovery system.
The reactions
The production process of Fe-Si consists of a high temperature process where SiO2 is reduced with C to Si and CO (g). The overall reaction of the process is based on the carbo-thermal reaction which is idealized as the reaction given below.
SiO2(s) + 2C(s) = 2Si (l) + 2CO (g) Delta H at deg C = 687 kJ/mol
The Fe-Si furnace is normally divided into two zones namely (i) an inner hot zone, and (ii) an outer colder region. Si is produced in the inner zone. The equilibrium condition for the production of Si is given by the following reaction.
SiO (g) + SiC (s) = 2Si (l) + CO (g)
The temperature for the production of Si is around deg C. Then the equilibrium pressure of SiO for the above reaction at 1 atmosphere is 0.5 atmospheres. For getting a high recovery of Si, this SiO is to be recovered in the colder parts of the furnace. The SiO is recovered by a reaction with the C or by condensation. The SiO which is not recovered is lost as SiO2 dust.
The ability of a C material to react with SiO is called the reactivity. In case of high reactivity, much of the C reacts with SiO to form SiC in the outer zone. If the reactivity is low, free C can reach the inner zone. Then less Si and more of SiO and CO are produced. Because of the low reactivity in the outer zone, more of SiO condenses. Since the condensation supplies heat, there is a limit for the condensation. When the limit is exceeded then SiO leaves the furnace. If the reactivity is low, the C balance in the charge is required to be reduced to avoid SiC deposits. In such a case the recovery of Si decreases.
In practical operation there is always some silicon loss in the gas. This is mainly due to a loss of the gas species SiO. The SiO burns together with CO in excess air above the charge. A more accurate description of the process is more complex and involves many intermediate reactions and complicates the situation vastly from what the above reaction describes. The internals of a SAF can be divided into a high temperature (around deg C) and lower temperature (less than deg C) zone, where different reactions dominate. In the high temperature zone around the electrode tip, the following reactions occur.
2SiO2 (s, l) + SiC(s) = 3SiO (g) + CO (g) Delta H at deg C = kJ/mol
SiO2 (s, l) + Si (l) = 2SiO (g) Delta H at deg C= 599 kJ/mol
SiO (g) + SiC (s) = Si (l) + CO (g) Delta H at deg C = 167 kJ/mol
The slowest of these three are probably the SiO (g) producing reactions which consume a major part of the electrical energy developed. Si can be produced through reaction at temperatures above deg C. The SiO gas travels upwards in the furnace and is recovered either by reaction C with the material as given below or by condensation where the temperature is sufficiently low (less than deg C). The last two reactions given below are reversible.
SiO (g) + 2C (s) = SiC (s) + CO (g) Delta H at deg C = -78 kJ/mol
3SiO (g) + CO (g) = 2SiO2 (s, l) + SiC (s) Delta H at deg C = - kJ/mol
2SiO (g) = SiO2 (s, l) + Si (l) Delta H at deg C = &#; 606 kJ/mol
The last two condensate producing reactions are strongly exothermic and are the main factor how heat is transported upwards in the furnace. The equilibrium conditions for the other reactions are shown in Fig 2.
Fig 2 Partial pressure of SiO (g) in equilibrium with SiO2, SiC and C
At the top of the furnace charge, the temperature can vary between deg C to deg C. Typical industrial silicon yield is around 85 % in a well operated furnace. SIC forming reaction is the preferred SiO recovery reaction above deg C. Below this temperature, SiO gas is generally captured by the last two condensate producing reactions. The temperature has a great effect on the equilibrium conditions for these reactions. If the temperature at the top is around deg C (partial pressure of SiO=0.1 atm.) and the main SiO recovery goes through condensation, then the yield of Si is around 80 %.
Refining and casting of Fe-Si
Impurities in the liquid ferro-alloy like Al and Ca can be removed by oxygen (O2) and air while the alloy is in molten stage in the ladle before casting. The liquid ferro-alloy can be tapped from the furnace into a refractory lined steel ladle.
Liquid Fe-Si is poured from the ladles into large, flat cast iron moulds. The moulds are prepared by adding a layer of Fe-Si fines on the mould surface. The cast material is removed from the moulds when it has cooled down to a level where the material strength is high enough to be removed and stacked in piles for further cooling. After cooling and solidification, the Fe-Si is crushed and screened to produce the required lump sizes. In the process of crushing, some fines are generated. Such fine material can be further ground to a powder, combined with a binder, and formed into briquettes. The melt can also be granulated.
All grades of Fe-Si are produced using essentially the same process, but certain additional steps are required to produce higher?purity grades of Fe-Si. Such grades are produced using raw materials containing lower amounts of impurities. In addition, refinement of the liquid Fe-Si to remove unwanted impurities and the addition of special alloying elements occur in the ladles. This further processing to produce higher purity Fe-Si is known as ladle metallurgy. Specialty grade 15 % Fe-Si for dense medium application is typically produced by remelting 75 % Fe-Si with steel scrap in an electric arc furnace and casting into a high?pressure water spray.
(275K)
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The data presented in this study are available on request from the corresponding authors.
The combination of photothermal and magnetic functionalities in one biocompatible nanoformulation forms an attractive basis for developing multifunctional agents for biomedical theranostics. Here, we report the fabrication of silicon&#;iron (Si-Fe) composite nanoparticles (NPs) for theranostic applications by using a method of femtosecond laser ablation in acetone from a mixed target combining silicon and iron. The NPs were then transferred to water for subsequent biological use. From structural analyses, it was shown that the formed Si-Fe NPs have a spherical shape and sizes ranging from 5 to 150 nm, with the presence of two characteristic maxima around 20 nm and 90 nm in the size distribution. They are mostly composed of silicon with the presence of a significant iron silicide content and iron oxide inclusions. Our studies also show that the NPs exhibit magnetic properties due to the presence of iron ions in their composition, which makes the formation of contrast in magnetic resonance imaging (MRI) possible, as it is verified by magnetic resonance relaxometry at the proton resonance frequency. In addition, the Si-Fe NPs are characterized by strong optical absorption in the window of relative transparency of bio-tissue (650&#;950 nm). Benefiting from such absorption, the Si-Fe NPs provide strong photoheating in their aqueous suspensions under continuous wave laser excitation at 808 nm. The NP-induced photoheating is described by a photothermal conversion efficiency of 33&#;42%, which is approximately 3.0&#;3.3 times larger than that for pure laser-synthesized Si NPs, and it is explained by the presence of iron silicide in the NP composition. Combining the strong photothermal effect and MRI functionality, the synthesized Si-Fe NPs promise a major advancement of modalities for cancer theranostics, including MRI-guided photothermal therapy and surgery.
Keywords:
laser ablation in liquids, composite nanoparticles, photothermal therapy, magnetic resonance imaging, silicon, iron silicide, theranostics
To improve the quality of healthcare and advance toward highly selective personal medicine, novel efficient and minimally invasive biomedical approaches that could be promptly translated into clinical practice are still required. Methods of nanotechnology could satisfy such a demand as they offer novel advanced tools for diagnostics and therapy, especially in the field of cancer [1,2,3]. Indeed, newly emerging nanomaterials can not only provide a mechanism for the passive targeting of tumors based on the enhanced permeability and retention (EPR) effect [4], but also offer a number of functionalities for imaging or therapy based on the intrinsic physicochemical properties of these nanomaterials. As an example, the imaging functionality can arise from the photoluminescent [5,6], absorption/scattering [7], photoacoustic [2] or magnetic [8] responses of nanoparticles (NPs). The therapeutic functionality of NPs can arise from their ability to sensitize the effect of local heat release under external stimuli, e.g., photo [3] or magnetic field [9] excitation, which can be used for local hyperthermia of cancer cells, leading to their death.
Nanomaterials combining high biocompatibility and bioimaging and/or therapy potentials are of particular interest for biomedical tasks. Here, nanostructured silicon (nano-Si) occupies a unique niche, as it is not only biocompatible, but also biodegradable. Indeed, in biological tissues nano-Si dissolves into orthosilicic acid; then, products of its dissolution are safely excreted from the organism via renal clearance [10,11]. In addition, being group IV semiconductors, Si-based NPs can exhibit a series of unique imaging and therapy functionalities, including room-temperature photoluminescence [5] and non-linear optical response [12] for bioimaging, light-induced generation of singlet oxygen for photodynamic therapy [13], infrared irradiation-induced [14,15] and radiofrequency-radiation-induced [16] hyperthermia for cancer therapy. Iron (Fe)-based nanostructures represent another promising nanomaterial for biomedical applications [17]. Iron oxide NPs are also biocompatible and biodegradable, although their use in vivo has some tolerance limits due to possible residual toxicity of decaying Fe-based compounds [18]. It is important that iron oxide NPs exhibit prominent physical properties, which makes them an ideal candidate for both magnetic resonance imaging (MRI) [8,19] and magnetic-hyperthermia-induced therapy [20,21].
The combination of silicon- and iron-based compounds in a single nanoformulation presents a promising approach to developing a dual-modal magnetic-semiconductor nanoplatform for cancer theranostics. In this case, one could profit from a combined signal of Si and Fe components, e.g., to simultaneously enable efficient MRI contrast and photothermal therapy options [21,22]. However, there are severe limitations in the fabrication of Si-Fe structures suitable for biomedical use. Conventional methods for the synthesis of such structures are based on wet chemistry routes, but these methods typically involve non-biocompatible chemicals and toxic by-products, which drastically complicates the use of synthesized nanomaterials in biological systems [23]. Alternative pathways are based on methods of &#;dry&#; plasma synthesis [24,25]. In particular, we recently reported the fabrication of Si-Fe composite NPs with a variable content of Fe by an arc-discharge plasma-ablative method. Polycrystalline Si powder mixed with 0.2&#;10% Fe was evaporated at °C in an arc-discharge reactor, and then cooled down to form NPs [24,25]. Our tests also showed that the NPs with the content of Fe exceeding 2.5% could provide a strong shortening of both the longitudinal and transverse proton relaxation times in magnetization relaxometry, which made the achievement of high MRI contrast possible in experiments in vivo [25]. However, an increase in toxicity was observed when the concentration of nanoparticles was larger than 50 μg/mL [25,26], which can complicate the biological prospects of these NPs [26].
Here, we explore the use of methods of pulsed laser ablation in liquids (PLAL) to engineer Si-Fe NPs, which could combine magnetic and photothermal functionalities. Based on a natural formation of nanoclusters during the action of laser radiation on a solid target, PLAL profits from clean and essentially non-equilibrium conditions of nanostructure growth, which makes the formation of non-toxic complex multi-component nanoformulations possible [27]. As we have shown in previous studies, owing to the much lower amount of energy required to ablate a unity of material, femtosecond (fs) laser ablation is especially efficient in controlling the size characteristics of formed nanomaterials, from single spherical NPs [28,29,30] to complex multi-functional core-satellite or core-shell nanoarchitectures [31,32,33]. The choice of this technique for the engineering of Si-Fe nanoformulations is justified by specific nanofabrication conditions: (i) the easy preparation of different compositions of NPs, which can be obtained only in the form of colloidal solutions; (ii) the nearly spherical shape of Si-Fe NPs (iii) the inherent stability of prepared colloidal solutions of bare (ligand-free) NPs and (iv) the possibility of the minimization of residual toxic by-products.
In this study, the fs laser technique of PLAL was applied to ablate a composite target fabricated from hot-pressed powders of silicon and iron silicide. Physicochemical and photothermal properties of the obtained NPs were thoroughly analyzed. The obtained results demonstrate that the prepared silicon&#;iron composite NPs are very promising for the projected multimodal theranostic applications.
Silicon (Si, 99.998%, CAS No. -21-3) and iron (Fe, 99.99%, CAS No. -89-6) polycrystalline powders, as well as acetone (CH3COCH3, 99.9%, CAS No. 67-64-1), were purchased from Sigma-Aldrich (St. Louis, MO, USA). Mili-Q water was used as a solvent.
Targets were prepared by using spark plasma sintering (SPS) of mixed polycrystalline microgranular powders of high-purity silicon and iron in the atomic ratio of 2:1 using a Labox 650 (Sinter Land Inc., Kyoto, Japan) apparatus.
Si-Fe NPs were synthesized using methods of femtosecond laser ablation of the prepared targets immersed in acetone, according to the procedure described in previous studies [34]. Briefly, the composite target was vertically fixed in a quartz cuvette at 3 mm from the side wall. The laser beam was focused through the side wall on the surface of the target using a 100 mm working distance F-theta objective. The ablation was conducted using a femtosecond Yb: KGW laser Teta 10 (Avesta, Moscow, Russia) with a wavelength of nm, pulse repetition rate of 100 kHz and pulse energy of 30 μJ. The laser beam scanned the surface of the target at 5 m/s using a galvanometric scanner. The cuvette was filled with 60 mL of acetone. The synthesis duration was 30 min. The concentration of Si-Fe NPs in the colloidal solution was estimated as 1.2 mg/mL, which was determined by weighing the target before and after synthesis.
For further studies, Si-Fe NPs were transferred from acetone to water by centrifugation (Microspin 12, BioSan, Riga, Latvia) at a relative centrifugal force of × g for 10 min, followed by mixing with de-ionized water to obtain the NP concentration of approximately 1 mg/mL. Additionally, pure Si NPs were synthesized by femtosecond ablation of a crystalline Si target in water.
The size, morphology and semi-quantitative chemical composition of formed NPs were examined by transmission electron microscopy (TEM) using a Jeol JEM- microscope (JEOL Ltd., Tokyo, Japan) operating at 200 kV. The chemical composition was characterized by an energy-dispersive spectrometry (EDS) detector X-act, Aztech X-Max 100 (Oxford Instruments, High Wycombe, UK), attached to the TEM system. Samples were prepared by dropping 20 μL of the Si-Fe NPs solution onto a Cu grid, which was covered by a carbon film, and subsequent drying under ambient conditions. The size (diameter) distribution of NPs was plotted after measuring at least 250 NPs using ImageJ software (version 1.53t).
Hydrodynamic sizes and ζ-potentials were measured for 10-fold diluted aqueous colloids of Si-Fe NPs by using a Zetasizer Nano ZS (Malvern Instruments, Malvern, UK).
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Raman spectra of dried Si-Fe NPs deposited on stainless steel foil were recorded under a 633 nm laser excitation with 1 mW power using a Confotec MR350 micro-Raman spectrometer (SOL Instruments, Minsk, Belarus). The laser beam was focused with a 50× microscope objective, and the acquisition time was 30 s. The measurements were carried out under ambient conditions. Special attention was given to temperature control in order to prevent overheating of the samples during measurements.
An X-ray diffraction (XRD) study was performed by using a Radian (NTC Expert center, Moscow, Russia) apparatus with a Cu Kα line (λ = 0. nm) under ambient conditions.
Electron paramagnetic resonance (EPR) spectra of dried Si-Fe NPs were measured at room temperature by using a X-band EPR spectrometer CMS Adani (Linev Systems Inc., Conroe, TX, USA), with the center frequency of a rectangular TE102 cavity of f = 9.4 GHz and magnetic field of B &#; 0.7 T. Effective g-factors of the samples were calculated with respect to a reference sample of a,g-bisdiphenyline-b-phenylallyl (BDPA) with the g-factor of 2..
Photothermal properties were evaluated by irradiating aqueous solutions of Si-Fe and Si NPs (1 mL, 1 mg/mL) by a CW semiconductor laser operating at 808 nm with the power of 384 mW under ambient conditions. The collimated laser beam was 2 mm in diameter and the optical path length of a cuvette was 10 mm. Temperature was measured by a platinum thermometer with an accuracy of 0.01 °C and a 15 Hz rate. To avoid inhomogeneous photoheating, we used a magnetic stirrer with a frequency of 110 rpm, which allowed us to condition a uniform temperature of the whole volume during the recording of heating&#;cooling transitions. The photothermal conversion efficiency was determined from the temperature transitions according to an analysis proposed in Refs. [35,36].
Optical extinction spectra were recorded in the range from 350 to nm using a UV752P spectrophotometer (Wincom Ltd., Changsha, China) in quartz cuvettes with a 10 mm optical path length. The optical measurements were carried out for aqueous suspension of Si-Fe and Si NPs with a 0.1 mg/mL concentration.
Longitudinal and transverse proton relaxations were investigated by a Minispec NMR (Bruker, Billerica, MA, USA) relaxometer equipped with a 20 MHz probe, at 40 °C.
Size and morphology of the synthesized NPs were analyzed by electron microscopy. A typical TEM image ( a) shows a conglomerate of dried Si-Fe NPs. The observed aggregation of the NPs is caused by the sample preparation technique and does not represent their colloidal properties. NPs with different transparencies for electron beams are clearly observable, suggesting that the synthesized nanocomposites do not have a uniform distribution of Si and Fe.
Open in a separate windowThe results of the statistical analysis of TEM images are shown in b. One can see that the laser-synthesized Si-Fe NPs exhibit a broad size distribution from 10 to 150 nm. The size distribution can be fitted by bimodal Gaussian and log-normal functions: for populations with mean sizes of 20 and 90 nm, these are log-normal and Gaussian function fits, respectively. The size distribution of Si NPs can also be fitted by a log-normal function (Figure S1) with the mean size of 30 nm. One can see from c and Figure S1 that all laser-synthesized NPs are mostly spherical. A high-resolution TEM imaging ( d) demonstrates that the synthesized NPs have both nanocrystalline and amorphous areas (dark area inside NPs in c). The coexistence of amorphous and nanocrystalline phases in prepared Si-Fe NPs can be related to the non-equilibrium conditions of PLAL when the fast cooling does not allow complete crystallization. The latter is also dependent on the composition of NPs. As for the size distribution, we suggest that the two populations could appear due to the phenomenon of the metal-stimulated growth of Si NPs at iron-rich surface regions. This effect has already been reported for similar Si-Fe NP formed by dry plasma-ablative synthesis in Ar plasma jets [25].
The chemical composition of the Si-Fe NPs was measured by the EDS technique. An area containing a large number of NPs ( a) was analyzed to obtain an averaged composition of NPs. As is shown in b, the EDS spectrum of the Si-Fe NPs consists of characteristic Fe lines, including Kα (6.404 keV), Kβ (7.058 keV), Lα (0.703 keV) and Lβ (0.525 keV). In addition, the spectrum contains a characteristic Kα (0.525 keV) line of oxygen and Kα (1.74 keV) line of Si. In general, this spectrum illustrates a dominant contribution of signals from Si, Fe and O with and atomic % of 64, 18 and 18, respectively. Additional EDS peaks in b can be attributed to contributions of carbon and copper from the TEM grid.
Open in a separate windowThe spatial distribution of silicon, iron and oxygen atoms within a Si-Fe NP is shown in c. Here, one can see the homogeneous distribution of all elements in the composite NP. The obtained composite is likely composed of fractions of nanocrystalline silicon, iron silicide, amorphous silicon and iron oxides.
a shows the results of the structural characterization of dried Si-Fe NPs and that of the ablation target by Raman spectroscopy. One can see that the Raman signal from the target only has peaks associated with the crystalline Si lattice, while other phases related to iron silicide or iron oxides cannot be detected. On the other hand, the presented Raman spectrum from Si-Fe NPs has more features, as it contains a sharp peak at 516 cm&#;1 and a broad band centered at 490 cm&#;1, which are attributed to the Si nanocrystalline and amorphous Si phases, respectively [37]. Note that the recorded shift in Raman peak of crystalline Si from 520 cm&#;1 to 516 cm&#;1 ( b) for the spectra obtained from the ablation target and Si-Fe NPs, respectively, indicates that Si nanocrystals have sizes of a few nanometers, while the size of NPs is tens of nanometers. Therefore, laser-ablated Si-Fe NPs have a polycrystalline structure. Additional peaks in the Raman spectrum of NPs can be attributed to the iron oxide (Fe2O3) and iron silicide (β-FeSi2) [38,39].
Open in a separate windowThe XRD spectra from Si-Fe NPs and the Si-Fe target are shown in b. One can easily find that the main peaks in the spectrum from the target correspond to the crystalline Si lattice, while weak peaks at diffraction angles of 35.22 and 44.45 deg. can be attributed to two phases of iron silicide (α-FeSi2 and β-FeSi2). The peak near 56 deg. could be associated with either Si or β-FeSi2. The spectrum of Si-Fe NPs contains similar peaks, but in this case, a well resolvable peak of α-FeSi2 at 35.22 deg is absent. It should also be noted that the XRD technique does not reveal the presence of crystalline iron oxides, which means that the iron oxides are mostly in an amorphous phase.
a shows a typical DLS spectrum from Si-Fe NPs in an aqueous suspension and an image of the suspension in a plastic cuvette (inset). The mean hydrodynamic diameter of the NPs is approximately 100 nm. A slightly larger value measured by DLS compared to the physical size (measured by TEM) can be explained by the formation of a hydration layer near the NP&#;s surface. Moreover, the sensitivity of the DLS technique is highly size-dependent and a small fraction of large NPs can completely mask more-abundant smaller NPs. The large hydrodynamic size can also originate from the aggregation of NPs, but this supposition is not confirmed by a high stability of the NP&#;s size during their storage under ambient conditions for several months. The high colloidal stability of the laser-synthesized NPs is obviously due to an electrostatic stabilization as a result of charging of their surface during the synthesis. This supposition is confirmed by a high value of the ζ-potential (&#;28 ± 5 mV), which exceeds the stability threshold at room temperature.
Open in a separate windowOptical extinction spectra from the colloidal solutions in the visible&#;near-IR range are shown in b. One can see that the spectrum from Si-Fe NPs has a peak near 400 nm with a long tail extending to the IR, while the spectrum from Si NPs has a broad peak centered near 540 nm, which could be attributed to the activation of dielectric Mie resonances in Si NPs. Optical extinction is higher for Si-Fe NPs than for Si NPs in the whole measured range. Photo heating&#;cooling curves for both NP types under the irradiation of an 808 nm laser are shown in c. The maximal achievable temperature increment for the Si-Fe NPs was approximately four-fold higher than that for Si NPs (3.75 ± 0.05 and 0.85 ± 0.05 K, respectively). Both types of NP demonstrated a high photostability under near-IR irradiation and did not change their photothermal properties in multiple heating&#;cooling cycles.
To evaluate the photothermal conversion efficiency of Si-Fe NPs, we analyzed the temperature transitions during and after the photoexcitation ( c). It was assumed that water is nearly transparent for the employed IR laser radiation and the mass fraction of NPs in the studied solutions with an NP concentration of 1 mg/mL was very small, i.e., 0.1%. Then, the temperature change in a solution with dispersed NPs can be described by the following thermal balance equation [36]:
Cw·mw·dTdt=Qin&#;Qout ,
(1)
where mw and Cw are the mass and specific heat capacity of the photoheated solution, respectively, dTdt is the rate of temperature change vs. time, Qin is the input heat energy during photoexcitation and Qout is the heat dissipation from the solution to the external environment.
By neglecting the light absorbance and reflectance by water and walls of the plastic cuvette, the input heat energy during photoexcitation can be expressed by the following way [36]:
Qin=(I0&#;It)·η,
(2)
where I0 is the incident laser power, It is the laser power transmitted through the solution of NPs and η is the photothermal conversion efficiency of NPs.
At the same time, the heat dissipated to the external environment can be described by the following expression [36]:
Qout=Cw·mw·ΔTτ,
(3)
where τ is the cooling time due to the heat dissipation to the external environment and ΔT is the temperature growth relative to the initial temperature.
The value of η can be obtained from Equations (2) and (3) under steady-state conditions, i.e., Qin&#;Qout=0, and it can be expressed as follows:
η=Cw·mw·ΔTmax(I0&#;It)·τ ,
(4)
where ΔTmax is the maximal temperature growth.
c shows that the cooling decay can be fitted by a single exponential function with the decay time of approximately τ= 3.3 ± 0.1 min for both Si-Fe and Si NPs. This fact indicates a negligible effect of dispersed NPs on the heat dissipation under our experimental conditions.
By using Equation (4) to treat the results shown in d, the photothermal conversion efficiencies for Si-Fe and Si NPs reach 33 ± 5% and 10 ± 3%, respectively.
The initial photoheating rate of the solution with Si-Fe NPs is approximately 1.45 ± 0.06 K/min and it is only 0.35 ± 0.05 K/min for Si NPs, as shown in d. A higher heating rate of the former NPs agrees with their larger photothermal efficiency. Note that the initial photoheating rate can be used to estimate the photothermal conversion by NPs. Indeed, the initial growth of temperature during photoexcitation can be expressed from Equation (1) in the following way:
dTdt&#;(I0&#;It)·ηCw·mw
(5)
The photothermal conversion efficiencies estimated via Equation (5) by using the initial temperature growth were found to reach approximately 42 ± 4 and 14 ± 2% for Si-Fe and Si NPs, respectively. These values are close to the estimations obtained from dependences of maximal temperature growth by using Equation (4). The observed difference between values of η obtained by using the analysis of initial and steady-state parts of temperature transitions can be explained by both an uncertainty in the determination of the initial time interval and a possible contribution of additional heat losses, e.g., evaporation and air conversion flows, which can affect the maximal temperature during prolonged photoheating.
The revealed high efficiency of the photothermal conversion of Si-Fe NPs under laser irradiation at 808 nm, which is 3.0&#;3.3 times larger than that of Si NPs, is obviously determined by both their high optical absorbance and a relatively low light scattering efficiency of Si-Fe NPs in comparison with pure Si NPs.
As is shown in a, the EPR spectrum of Si-Fe NPs exhibits two resonance modes. The dominant broad mode with a g-factor of approximately g1 &#; 2.00 can be related to Fe3+ ions in the iron oxide inclusions [40]. A narrow central mode of the EPR spectrum is characterized by g2 = 2. and can be attributed to silicon dangling bonds at the Si/SiO2 interface [41,42]. Using a reference sample, the density of paramagnetic centers in the dried Si-Fe NPs is estimated to be spin/g.
Open in a separate windowA large number of paramagnetic centers in Si-Fe NPs leads to the effect of a spin&#;spin relaxation, which results in a shortening of proton magnetization times in aqueous suspensions of those NPs. b,c shows transitions of the proton magnetization of pure water and aqueous suspensions of Si-Fe NPs. One can see that both the longitudinal ( b) and transverse proton magnetization ( c) become faster in the presence of Si-Fe NPs. Such data allow us to estimate a specific relaxation rate, i.e., relaxivity, which can be calculated according to the following expression:
r1,2=R1,2/C,
(6)
where C is the mass concentration of NPs in aqueous suspension, and R1 and R2 are the relaxation rates for the longitudinal and transverse proton magnetizations, respectively, related to NPs.
Equation (5) allows us to estimate the longitudinal relaxivity of r1 = 1.7 g&#;1s&#;1L for the prepared Si-Fe NPs in aqueous suspension, while the transverse relaxivity demonstrates an even more remarkable value of r2 = 54 g&#;1s&#;1L. The observed predominant shortening of the transverse relaxation time is typical for NPs, as it was earlier reported for Si-Fe NPs obtained by plasma-ablative synthesis [24,25]. Note that the corresponding values for laser-ablated pure Si NPs are r1 = 0.2 g&#;1s&#;1L and r2 = 1.4 g&#;1s&#;1L, which indicate the major role of iron ions in the shortening the proton relaxation times.
Thus, we showed that the femtosecond laser-ablative technique can be used to fabricate non-agglomerated spherical Si-Fe NPs with two size populations, with a mean size of 20 nm and 90 nm. The NP population with a mean size of 20 nm looks especially promising for projected biomedical applications. Indeed, this size provides a sufficiently large surface area for functionalization by biopolymers and targeting molecules. In addition, Si-Fe NPs of this size can still profit from efficient cell internalization via endocytosis mechanisms, including mechanisms of clathrin-mediated and caveolin-mediated endocytosis. On the other hand, large NPs (100 nm in size or more) can be also useful for biomedical applications, although they cannot internalize as efficiently into cells via mechanisms of receptor-mediated endocytosis and they have a reduced blood circulation time due to the increased elimination rate of large NPs by the mononuclear phagocyte system.
The prepared Si-Fe NPs are partially oxidized, which is typical for nanomaterials prepared by PLAL synthesis [29,34]. The oxidation of Si-based NPs can potentially occur at different stages during the synthesis [27]. Laser ablation in liquids is a complex process, which still does not have complete theoretical description. However, it is widely accepted that this process starts from energy absorption by the target material, followed by a material ejection in the form of hot plasma consisting of individual atoms and the smallest clusters. The energy exchange between the laser-induced plasma and surrounding liquid leads to the formation of a so-called cavitation bubble, which contains liquid molecules, products of their decomposition and dissolved gases, along with target material ions and clusters. NPs start to grow from the formed nanoclusters inside the bubble; therefore, dissolved molecular oxygen and oxygen from the decomposed liquid molecules can potentially oxidize the forming NPs. The cavitation bubble stage is followed by a slow growth and a surface oxidation at steady-state physical and chemical conditions [27]. The transformation of Si to SiO2 has a negative effect on the optical and photothermal properties for projected biomedical applications. Therefore, we minimized the oxidation phenomena by performing the synthesis in acetone. This strategy has already proved its efficiency in decreasing the oxidation rate of laser-synthesized nanomaterials [29,34].
The obtained results clearly demonstrate that nanocomposites combining Si and other chemical elements (Fe in the present study) can possess a very strong optical absorption in the visible and NIR spectral ranges, as compared to pure Si NPs. Moreover, in this study, for the first time, we measured the photothermal conversion efficiency of laser-synthesized Si-Fe NPs. The conversion coefficient was found to reach 33&#;42% at a 808 nm excitation radiation, which is located in the center of the window of relative transparency of biological tissues (650&#;950 nm). It should be noted that this value is higher than the relevant parameter for some popular nanoscale photothermal sensitizers, including Cu9S5 NPs [43] and bismuth sulfide nanorods [44], which makes the laser-ablated Si-Fe NPs a very promising candidate for potential applications in photohyperthermia treatment and photoacoustic imaging ( ). At the same time, the photoheating coefficients obtained for Si and Si-Fe NPs can be compared with similar values for gold nanoparticles (Au NPs) in Ref. [36]. The photothermal conversion efficiency for Au NPs is in the range of 60&#;80% for a 532 nm laser. If we use a laser with a wavelength near the absorption maximum for these particles, it is possible to achieve the same results as for gold NPs. Despite this opportunity, we applied laser irradiation in the transparency range of biological tissues, which corresponds better to conditions of photothermal treatment in biomedical applications. In addition, we observed the enhancement of photothermal conversion efficiency for the Si-Fe NPs compared to the Si NPs, which encourages further work on the assessment of photothermal properties of composite NPs. In particular, it would be interesting to study dependences of the photoheating efficiency on both the NP size and concentration of Fe in the composition of Si-Fe NPs, wavelength of photoexcitation, and power density of laser radiation.
In addition to important quantitative results in determining the photothermal conversion efficiency of Si-Fe NPs, two methods for determining this physical parameter were considered. A standard method to perform the task is based on the measurement of the maximal heating temperature, while an alternative approach addresses the use of the heating rate parameter at the initial stage of the time dependence. The first method allowed us to estimate that the conversion efficiency was approximately 33 ± 5%. On the other hand, the second method, based on the assessment of initial stage of the photoheating transition (i.e., the initial heating rate), provided the value of photothermal conversion efficiency of approximately 42 ± 4%. While both values are similar within the experimental error bars, the latter approach seems to be faster and more convenient to obtain the conversion efficiency, especially for semiconductor NPs (Si, Ge, Si-Fe, etc.), whose optical properties are temperature-dependent and/or are not stable during a prolonged photoheating.
The transverse proton relaxivity of laser-ablated Si-Fe NPs (r2 = 54 g&#;1s&#;1L) appears to be several times larger than that of Si-Fe NPs prepared by dry ablative synthesis in Ar plasma jets [24,25]. This fact indicates a high potential of the PLAL method to fabricate efficient NP-based contrast agents (CAs) for MRI. Considering the composition of our Si-Fe NPs, their mean molar mass is approximately 31 g, while the molar transverse relaxivity is approximately mM&#;1s&#;1. This is significantly larger than the corresponding values of clinically approved Gd-based CAs [48] and prospective CAs based on superparamagnetic iron oxide (SPIO) NPs [46,47] ( ). Therefore, the observed strong shortening of the transverse relaxation time, mediated by laser-synthesized Si-Fe NPs, opens up opportunities for their application as CAs for T2 contrasting.
As for the biocompatibility of S-Fe NPs, it can be noticed that similar NPs prepared in the Ar plasma jets [24,25] and by PLAL [49] demonstrate a very low level of cytotoxicity in different human cells in vitro [48]. The low toxicity of Si-Fe NPs can be explained by a low toxicity of laser-synthesized pure Si and Fe NPs [11,33].
Silicon&#;iron composite NPs were successfully synthesized by fs laser ablation in acetone, followed by their transfer to water. The synthesized aqueous solutions consisted of non-agglomerated, uniform, spherical Si-Fe NPs, while their size distribution contained two populations with the mean sizes of 20 and 90 nm. TEM and EDS studies revealed a composite structure of Si-Fe NPs, which was characterized by a nearly spatially homogeneous distribution of silicon, iron and oxygen atoms over the volume of individual NPs. Raman spectroscopy and X-Ray diffraction analysis revealed a crystalline Si phase in the composite Si-Fe NPs, as well as the presence of iron oxide and iron silicide features.
The photothermal experiments with Si-Fe NPs under irradiation with CW infrared laser (808 nm) demonstrated a superior photothermal conversion efficiency compared to the laser-ablated pure Si NPs (33&#;43% for Si-Fe NPs and 10&#;14% for Si NPs). This fact encourages a further assessment of the photothermal properties of Si-Fe NPs as a promising sensitizer of photothermal therapy for biomedical applications, as well as a contrast agent in tasks of photoacoustic imaging. Here, one has to consider the behavior of the photothermal conversion efficiency on the concentration of NPs in the suspension, the crystal structure, the photoexcitation wavelength, the power density of the laser radiation and the size of Si-Fe NPs.
Moreover, high relaxation rates of the protons (the longitudinal relaxivity of r1 = 1.7 g&#;1s&#;1L and the transverse relaxivity of r2 = 54 g&#;1s&#;1L) in the presence of Si-Fe NPs show promise for their potential theranostic applications, e.g., as CAs in MRI diagnosis and the MRI-guided photothermal therapy of tumors. Since NPs based on Si and Fe were found to be biodegradable and have low toxicity, one can expect a biodegradability option for the fabricated laser-ablated Si-Fe NPs, although this option has yet to be confirmed in further studies in vivo. We believe that the combination of optical, photothermal and MRI contrasting properties, and a high biological safety and potential biodegradability makes laser-ablated composite Si-Fe NPs a very promising multifunctional agent for cancer theranostic applications.
TEM analysis was performed using the equipment of the Center of Shared Research Facilities (MIPT).
The following supporting information can be downloaded at: https://www.mdpi.com/article/10./nano/s1, Figure S1: (A) Typical TEM image of Si NPs; (B) Size distribution of Si NPs obtained from the image in panel (A), where the red curve gives a fit by the log-normal function.
Click here for additional data file.(275K, zip)The optical part of the research was funded by the Ministry of Science and Higher Education of the Russian Federation (contract FSWU--). The authors gratefully acknowledge the financial support from the Ministry of Science and Higher Education of the Russian Federation (Agreement No. 075-15--606). The composite target preparation was supported by the Russian Science Foundation (grant No. 21-12-). The Innovation Protection Fund (program UMNIK-21 contract No. GU/) funded the study of photoheating.
V.Y.T. and A.V.K. conceived the research. A.A.P., S.M.K. and A.V.K. designed the laser fabrication experiments. A.A.P., G.V.T. and A.M.P. performed the fabrication and characterization of NPs. A.A.B. performed the experiments with photoheating and evaluation of quantitative values. A.Y.K., V.S.B. and M.V.S. investigated the magnetic properties of the NPs. Y.V.K. and A.A.B. investigated the optical properties of the NPs. A.V.S., V.S.V. and A.Y.K. obtained and analyzed the TEM images. V.Y.T., A.A.P., A.V.K. and A.A.B. prepared the manuscript using data from the co-authors. A.A.B., V.Y.T. and A.A.P. analyzed the heating data. V.V.K. created the target for obtaining the NPs using the method of laser ablation. V.Y.T., A.V.K., A.A.P. and A.A.B. wrote the manuscript with comments from all authors. All authors analyzed, discussed and obtained the data. All authors have read and agreed to the published version of the manuscript.
The data presented in this study are available on request from the corresponding authors.
The authors declare no conflict of interest.
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