Sonophysical cost effective rapid indigenous preparation of aluminium particles via exfoliation of aluminium foil

Abstract

A rapid sonophysically aided process of obtaining aluminium particles from commercial aluminium foil has been attempted for the first time. Through the cavitation-enabled exfoliation of the foil, particles were obtained following seiving in size ranges of >125 μm, 25–50 μm and <25 μm. The prepared particles were tested for their bacterial compatibility/toxicity against Escherichia coli and Streptococcus mutans. Reports on the effect of aluminium on bacterial cells are highly limited and no data exists on the influence of aluminium particles and its bacterial interactions. Moreover, no data on the size-dependent bactericidal activity of aluminium is available either. This work fills in these voids and confirms that size-dependent toxicity of Al particles exists below 25 μm.

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Introduction

Aluminium was discovered by Hans Oersted in 1825. Its name is derived from the Latin name for alum, ‘alumen’, meaning bitter salt. Aluminium is the most abundant metal in the Earth's crust (8.1%), but is rarely found freely occurring in nature. It is usually found in minerals such as bauxite and cryolite. After iron, aluminium is now the second most widely used metal in the world. Commonly present in the form of oxides and silicates, aluminium is the most abundant metal in the Earth's crust, representing 8.8% of its weight (The Merck Index, 1989). Aluminium is a light metal with a density of 2.78 g cm³ and is an excellent conductor of heat and electricity.

Commercially produced aluminium is extracted by the Hall–Héroult process. The properties of aluminium that contribute to its potential applications include: low density and therefore low weight, high strength, superior malleability, easy machining, excellent corrosion resistance and good thermal and electrical conductivity. Aluminium naturally generates a protective oxide film when exposed to air; the film consists of aluminum oxide.^1,2 This film is responsible for its corrosion resistant properties. Surface treatments such as anodising, painting or lacquering have been found to further improve this property.³

Aluminum finds use in a huge variety of products including cans, foils, kitchen utensils, window frames, beer kegs and aeroplane parts. This is because of properties such as; low density, no toxicity, high thermal conductivity, excellent corrosion resistance and ability to be easily cast, machined and formed. It is also non-magnetic and non-sparking. It is the second most malleable metal and the sixth most ductile. It is often used as an alloy because aluminium itself is not very strong. Alloys with copper, manganese, magnesium and silicon are lightweight but strong. They find promising roles in the construction of airplanes and other forms of transport.⁴

Aluminium is a good electrical conductor and is often used in electrical transmission lines. It is cheaper than copper. The European, Japanese and American building industries use approximately over a million ton of aluminium every year. Aluminium is found everywhere, in roof and wall claddings, windows and doors, stairs and railings, roof frames, scaffolding, greenhouses and home extensions.

Metal and metal oxide bulk material and nanomaterial toxicity have been a rising environmental/health concern. Nanomaterials are becoming highly reputed for their antibacterial and antimicrobial activities. There are numerous reports on the antibacterial effects^5,6 of various nanoparticles, that have been summarized in some recent reviews.^7–10 Antimicrobial nanostructured materials range from metallic and metal oxide nanoparticles to semiconductors, polymers, and carbon-based materials. The mechanisms of toxicity depend on composition, surface modification, intrinsic properties, and the bacterial species. The exact mechanisms of toxicity against various bacteria are still not entirely understood. It is reported that nanoparticles are able to attach to the membrane of bacteria by electrostatic interaction and disrupt the integrity of the bacterial membrane.¹¹ Nanotoxicity is generally triggered by the induction of oxidative stress by free radical formation, that is, the ROS, following introduction of nanomaterials. Thus, with raising environmental concerns, every synthesized micro/nano material is required to be assessed of its biological toxicity.

As far as aluminium is concerned, it has no known biological role. In its soluble +3 form, it is reported to be toxic to plants. Crops can absorb Al³⁺ leading to lower yields. Our bodies merely absorb small quantities of the aluminium from food, way below any toxicity levels. Aluminium can accumulate in the body, and a link with Alzheimer's disease (senile dementia) has been speculated yet unproven.^12–14 In the following paper, we demonstrate a simple, rapid novel sonophysical method of preparation of aluminium particles from commercial aluminium foil. For the first time we apply the sonication technology to produce aluminium particles from a cost effective source like foil. The particles were characterized using FESEM and EDAX and their bacterial toxicity/compatibility assessed. This work presents first time information on the size dependant interaction of Al particles with bacterial cells, while previous authors have solely focussed on aluminium oxide and its interaction alone.

Materials and methods

All the chemicals used in the study, unless specified as otherwise, were all of analytical grade. Millipore water was used for all experiments. The aluminium source material was commercial foil purchased from a Korean supermarket. A JAC-2010 Ultrasonicator (KODO Technical Research Co., Ltd [Korea]), which is a water bath type sonicator, equipped with an ultrasonic power of 300 W and frequency of 60 Hz was used. 1 g of the foil was cut out and placed in the bottom of the bath sonicator and sterile water was added. Sonication was carried out at room temperature for 10 min. The water with the particles was collected and sieved through 125 μm, 50 μm and 25 μm pore sized sieves (Testing Sieve, Chung Gye Sang Gong Sa, Seoul, Korea). Three varying sized particles were collected after sieving, Al particles >125 μm (Al-2), Al particle 50–25 μm (Al-3) and particles <25 μm (Al-4). Al-1 was the aluminium foil cut into 3 mm sized (breadth) strips which was considered to be the control, or the aluminium source material used as such without any sonication. The codes Al-1, Al-2, Al-3 and Al-4 will thus be used in the Results section.

The particles were characterized using FESEM (JEOL, JSM-5410LV) at an accelerating voltage of 2 kV. Energy dispersive X-ray (EDX) was used to derive the compositional analysis of the Al particles. X-ray diffraction (XRD) studies were conducted using D8 Discover with GADDS (Bruker, Germany). The % yield was calculated using the formula % yield = (actual yield (in grams)/source material (in grams)) × 100.

The bacterial compatibility/toxicity was assessed by testing the four Al particles (Al-1, Al-2, Al-3 and Al-4) against two pathogenic bacteria. Two bacteria strains S. mutans 11823 (ATCC 25175) and E. coli (ATCC 11234) were purchased from Korean culture centre of microorganisms, Seoul, South Korea and used in this study. 0.01 g L⁻¹ and 0.1 g L⁻¹ concentrations of Al-1, Al-2, Al-3 and Al-4 were incubated with E. coli and S. mutans overnight and the bacterial growth was measured at 600 nm using ND-1000 v 3.3.1 spectrophotometer, (Nanodrop Technologies, Inc., Wilmington, US). The total viable count, indicating the number of bacteria that survived after the Al particle interaction, was enumerated my plate count method. The TVC was represented as cfu mL⁻¹ (colony forming unit per mL).¹⁵

Results and discussion

In an ultrasonicator a piezoelectric transducer generates ultrasound which propagates through a liquid via mechanical vibration. The ultrasonic principle is operational when introducing high power ultrasound into a liquid medium. Sound waves are transmitted in the fluid and create alternating high-pressure (compression) and low-pressure (rarefaction) cycles, with differing rates depending on the frequency. During the low-pressure cycle, high-intensity ultrasonic waves create small vacuum bubbles or voids in the liquid. When the bubbles attain a volume at which they can no longer absorb energy, they collapse violently during a high-pressure cycle. This phenomenon is termed as cavitation.¹⁶ Cavitation occurs at various locations of the tank. The spots are unevenly distributed and described as ‘hot spots’. It is based on this phenomenon that the aluminium foil test is deviced. It is a standard procedure used to test the functionality or efficiency of the ultrasonicator. A thin aluminum or tin foil is placed at the bottom of a water filled ultrasonic tank. After sonication, single erosion marks are visible due to cavitation at hot spots. This results in single perforated spots and holes in the foil at the cavitational hot spots sites. Due to the low energy and the uneven distribution of the ultrasound within the tank, the erosion marks occur only spot-wise. This test is done to verify the efficiency of the instrument.

In the following work we have used the ultrasonic phenomenon of cavitation and perforation of aluminium for the preparation of Al particles or dust from commercial aluminium foil. Fig. 1 gives the schematic work flow followed in the study. 1 g sheets of Al foil were placed in the bottom of the sonication tank and subjected to sonication for 10 min. Within seconds it was observed that perforations appeared on the foil and in minutes particles started separating out. At the end of 10 min the entire sheet was exfoliated to debris of various sizes. We also attempted prolonging the sonication time to achieve obtaining uniform sized particles, which never could be accomplished, owing to the uneven distribution of the ultrasonic energy (which is typical of bath type ultrasonicators). The particles obtained after sieving are shown in Fig. 1, as Al-2, Al-3 and Al-4. Al-1 was Al foil (which is the source material from which the particles were formed) cut to strips without sonication, which served as the control, to compare against the Al material formed on sonication. The process of particle formation from the Al foil is due to the cavitation effect described above. Previous authors¹⁷ have reported that materials subjected to sonication impact energy could show elastic deformation, plastic deformation, or even fracturing. Tomlinson et al.¹⁸ have demonstrated that pure alumina showed intergranular mode fractures due to cavitation erosion. The erosion products in our case are the Al particles.

Fig. 1 Schematic showing flow of work and particles obtained during the process.

The sieves aided in separating out the particles in three size ranges >125 μm (Al-2), 25–50 μm (Al-3) and <25 μm (Al-4). Fig. 2 gives the morphology of the particles as observed using FESEM. Al-2 consisted of a range of particles, as observed from Fig. 2(a-1). No distinct morphology was observed. Varying sizes and varying morphologies were obtained via the cavitation aided exfoliation of the foil. Fig. 2(b-1) and (c-1) show the morphologies of the Al-3 and Al-4 particles, the narrowing down of their sizes is evident, marginal uniformity in the morphology is observed in Al-4. The CLSM optical images demonstrate the size reduction from Al-2 (a-2) to Al-4 (c-2).

Fig. 2 FESEM micrographs of (a-1) Al-2 and (b-1) Al-3 and (c-1) Al-4 and CLSM optical micrographs of the (a-2) Al-2, (b-2) Al-3 and (c-2) Al-4 (inset showing enlarged view of the particles).

The chemical nature of particles was confirmed by XRD (Fig. 3). XRD patterns exhibited strong diffraction peaks at 38.52°, 44.76°, 65.14°, 78.26° and 99.11° indicating pure Al phase.¹⁹ EDX was also conducted on the source foil (Fig. 4A) and on the sonophysically synthesized particles (Al-3) (Fig. 4B). As observed from the EDX spectra, Al and O peaks are the major constituents of the foil and the particles, with the Al composition higher than O, which is typical of Al phase. Sonication did not lead to any changes in the chemical composition of the material. The composition is the same as the foil, with the percentage of Al exceeding the percentage of oxygen, as seen from the inset of Fig. 4B in case of the Al particles too. This confirms that the material is still aluminium and has not transformed to aluminium oxide (in which case the oxygen percentage will be higher than that of Al).²⁰

Fig. 3 XRD results showing peaks confirming the nature of the synthesized particles to belong to pure Al phase.

Fig. 4 EDX graph of (A) Al-1 and (B) Al-3 particles, inset showing compositional details.

Thus, the process of particle formation is merely sonophysical and not sonochemical. Aluminium when exposed to the atmosphere or to oxygen sources would readily form aluminium oxide. The foil too has thin aluminium oxide films on its surface. This accounts for the oxygen content showing up in the EDX analysis.

The yield% of the respective particles from the source foil was calculated using the formula mentioned in the materials section. Al-3 was highest compared to Al-2 and Al-4 with a yield% of 45, followed by Al-2 which had a yield% of 35 and the least was Al-4 at 20 (Fig. 5).

Fig. 5 Graph showing % yield of the various Al particles from the foil via the sonophysical method.

The bacterial compatibility/toxicity of the prepared Al particles was assessed. Bioavailability of a material is defined as a measure of its potential to interact with biological systems and cause a response.²¹ The bioavailability of aluminium in soils and its concentration in waters is reported to be very low because of its adsorption to mineral surfaces, its association with organic matter and the insolubility of the hydroxide complexes that form when the pH is near neutrality. Soil contains aluminium in a variety of forms such as aluminosilicate minerals, as well as exchangeable or soluble Al.²² For soil microorganisms Al is a ubiquitous component of the environment, and although poorly understood, the relationship between the two may extend back to the origin of life itself. It is known that when Al becomes more soluble it becomes potentially toxic to plants²³ and microorganisms.²⁴ Although there is much information on the possible mechanisms of Al toxicity to plants.²⁵ Al toxicity to microorganisms has received little attention.^26,27 Also, with respect to Al dust or Al particles and their interactions with bacterial systems no standard reports exist. The oxide of aluminium, aluminum oxide, also known as alumina, is a well-proven and well studied since it is a biocompatible ceramic that has been used as a dental restorative material for many years. Aluminum oxide is an inert compound of aluminum and oxygen. It is formed by oxidation of aluminum and oxygen to form Al₂O₃, a non-metal or pure ceramic. In the manufacture of dental ceramics, care is taken to ensure that all the aluminum is converted to the inert alumina compound throughout the manufacturing process. There are no bodily functions that react with the alumina, hence the excellent biocompatibility.²⁸ On the other hand, aluminium oxide is well reputed for its bacterial toxicity. The activity of nanoscale alumina against B. subtilis, E. coli and P. fluorescens was examined and compared with its bulk counterpart. The nanoparticles had higher toxicity than the bulk materials at the concentration studied and it was observed that P. fluorescens was the most sensitive.²⁹ Similar results were also obtained by Balasubramanyam et al.³⁰ who observed that Salmonella sp. exhibited higher mutagenicity towards alumina nanoparticles compared to the bulk. In their study, Jiang et al. ruled out the role of aluminum ions in nanoparticle mediated toxicity. They observed that the nanoparticles attached to the surface of the bacteria due to surface charge (bacterial surface was negative while alumina nanoparticles were positive at the pH studied), leading to cell death. The toxicity of alumina nanoparticles on microalgae has been studied recently by Sadiq et al.³¹ They investigated the difference in toxic response of micron sized and nano sized alumina towards Scenedesmus sp. and Chlorella sp. A growth inhibitory effect of the nanoparticle was observed against both the species and an evident decrease in the chlorophyll content was also observed in the cells treated with nanoparticles. Once again interaction of the nanoparticles with the cell surface was suggested as the possible mechanism for the observed toxicity. Of the few scattered reports that exist regarding studies testing the toxicity of Al, none of them report the testing of Al particles. All the studies mentioned below use Al compounds amended into the culture medium (not Al particles) to assay the Al toxicity. Guida et al. (1921), studied the aluminium toxicity towards E. coli, they observed that growth inhibition was markedly dependent on pH, only at pH 5.4 and 6.6.³² Aluminium concentrations lower than 100 μM (ref. 33 and 34) or even 50 μM (ref. 35 and 36) are inhibitory to the growth of Bradyrhizobium spp. Appanna et al. (1994) found that a P. fluorescens strain was able to tolerate up to 50 μM Al.^37,38 Binding of aluminium to the cell wall of Staphylococcus aureus³⁹ and the cyanobacterium A. cylindrica⁴⁰ has been reported. Inhibitory effects of aluminium on growth, photosynthesis and nitrogen fixation by A. cylindrica has also been reported.⁴¹ Competition with iron and magnesium, and binding to DNA, membranes or cell walls were considered to be the main toxic effect of aluminium in microbes.⁴² No information exists with respect to bacterial toxicity/compatibility of Al particles.

We have checked the interaction of the prepared Al particles with bacterial strains to confirm their bacterial compatibility/toxicity. Fig. 6A shows the results of the spectrophotometric method of bacterial growth evaluation after incubation with the Al particles. The results show that 0.01 g L⁻¹ concentrations of the Al-2, Al-3 and Al-4 particles showed no toxicity against E. coli. Marginal toxicity was observed at concentrations of 0.1 g L⁻¹ with respect to Al-2 and Al-3. But, significant inhibition/toxicity (two order of magnitude reduction) was observed in case of Al-4 particles against E. coli. With respect to Al-1, which was the control, where Al strips cut from the source foil material used to make the particles were used, no toxicity was observed even at saturated concentrations of 0.1 g L⁻¹. These results were further confirmed by the plate count method (Fig. 6B), which evidenced the same trend observed in the spectrophotometric method. Only Al-4 particles showed bacterial toxicity, while all others were compatible. Fig. 7A and B display the results from the testing the Al particles against S. mutans. In this case too, it was observed that the Al-1, Al-2 and Al-3 particles were compatible and showed no toxicity, while the Al-4 particles showed significant toxicity (two orders of magnitude).

Fig. 6 (A) Bacterial toxicity/compatibility assessment of the synthesized particle against E. coli using spectrophotometric method. (B) Bacterial toxicity/compatibility assessment of the synthesized particle against E. coli using plate count method.

Fig. 7 (A) Bacterial toxicity/compatibility assessment of the synthesized particle against S. mutans using spectrophotometric method. (B) Bacterial toxicity/compatibility assessment of the synthesized particle against S. mutans using plate count method.

The Al-4 particles were sized <25 μm, airborne Al particles which are smaller sized are always been a concern. The size variation was the only difference between the Al-4 particles and the rest. Smaller the size, easier the penetrate ability of the particles. We have previously reported the size dependant toxicity of platinum nanoparticles.⁴³ Al is known to interact with membrane phospholipids and cellular DNA; hence their ability to cross the cell barrier plays a vital role in enabling these interactions. Yet another report raises concern in the ability of nanoparticles to enter human skin barrier owing to their small size and has verified this based on its results.⁴⁴

Our results show that the sonophysical preparation of Al-particles from Al foils did not change the compatibility of the source foil material, the Al-2, Al-3 particles showed similar bacterial compatibility as that of Al-1. The toxicity exhibited by Al-4 is a consequence of size dependant properties and not changes in the chemical composition. Bulk quantities of Al particles/dust of the desired size (based on the application) can be made in large scale via this rapid methodology.

Conclusion

Using the sonophysical exfoliation approach, we reported the successful cost effective preparation of Al particles from commercial Al foil. The process was rapid and a good yield was obtained. Al-2 and Al-3 particles which were in the size range of 125–25 μm showed excellent bacterial compatibility, while the Al-4 particles which were <25 μm in size showed significant bacterial toxicity. The study revealed that a size dependant alteration in bacterial toxicity definitely does exist with respect to Al particles.

Acknowledgements

This work was supported by the KU Research Professor Program of Konkuk University.

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