Encyclopedia
Copper has attractive characteristics: excellent antimicrobial activity, high natural abundance, low cost and the existence of multiple cost-effective synthesis routes for obtaining copper-based materials with tunable characteristics, which favor their incorporation into polymeric membranes.
- copper nanomaterials
- polymeric membranes
1. Overview
Copper is a transition metal element that can be found in different natural underground or rock deposits. This element shows excellent affinity with sulfur and is one of the most common components of sulfide ores such as pyrite, chalcopyrite, cuprite (oxides), and carbonates (malachite) [1][2][88,89]. This material has excellent properties: it is highly ductile and malleable, with high thermal and electrical conductivity. It can also be indefinitely recycled, and it can form alloys to improve mechanical properties, corrosion, and oxidation resistance, allowing extensive applications [3][90]. Moreover, copper has mainly been used due to its effectiveness as an antimicrobial material. Different research lines have thoroughly explored copper given its capacity to inactivate fungi [4][91], bacteria [5][6][92,93], viruses [7][8][94,95], parasites [9][96], and algae [4][91]. The ranking of pure metal cytotoxicity from most potent to least potent can be presented as follows: Cu > Al > Ag > V > Mn > Cr > Zr > Nb > Mo > Ti [10][97].
Overall, the efficacy of the microbicide effect of copper depends on several factors such as its physical form (bulk, nanoparticle, ions, etc.), its chemical state (elemental, copper oxide, etc.), concentration, wet or dry application form, temperature and humidity, and presence of buffer, among others. Copper toxicity can occur through two mechanisms: (1) direct contact killing, which depends on the proximity between the microorganism and Cu-containing surfaces, and (2) toxic effects induced by copper ions produced by copper dissolution [11][12][13][98,99,100].
2. Copper-Based Materials and Relevant Features
Almost 300 different forms of Cu-based materials were registered as antimicrobial products by EPA. For example, Cu/metals/alloys and metal substrates surface-modified with Cu, composites of Cu with polymers and glass, nonmetal substrates surface-modified with Cu, and superhydrophobic surfaces containing Cu have been widely used as cheap and effective materials for sterilizing, textiles and also human tissues for centuries. Moreover, their application in different fields such as in electronics, thermal energy, catalysis, photonics, biosensors and optoelectronics have been reported [14][15][25,115].
There is evidence of a particular interest in producing copper nanoparticles. The synthesis of different metallic and metal oxide copper nanoparticles have been widely studied. Copper NPs can be obtained by several strategies that include physical and chemical methods. For physical methods, the use of sophisticated equipment and technology is necessary, which makes them a relatively complex process. In the case of chemical methodologies, several strategies can be found in the literature due to their ease of control, simplicity of operation, limited equipment requirement and high quality of particles. Chemical methodologies, such as wet chemical reduction [16][116], reverse micelles [17][117], electrochemical and sonoelectrochemical techniques [18][118], vapor deposition [19][119], laser irradiation [20][120], thermal decomposition [21][121], thiol-induced reduction and microemulsions have been reported. In all these cases, it is very important to control the morphology, particle size and shape, surface charge and physicochemical properties of the synthesized nanoparticles [16][116].
Thus, the characteristics and properties of copper nanoparticles can be treated and controlled during their synthesis and adapted to be added on any solid surface, such as polymeric membranes. In general, the characteristics of the nanoparticles (size and shape, among others) can be dependent on the precursor [22][122]. For this reason, their choice is fundamental to obtain the desired features. Thus, copper nanoparticles often entail the reduction of Cu (I) or Cu (II) sources. Copper sulfate (CuSO4), copper acetylacetonate, copper chloride (CuCl2), or copper nitrate (Cu(NO3)2) have been used as a precursor. For wet chemical techniques, commonly used reducing agents are hydrazine, sodium borohydride, ascorbic acid, glucose, and 1,2-hexadecanediol, among others [23][123]. Several capping agents have been employed to stabilize the nanoparticles and control particle size [24][26]. Moreover, these agents could condition the surface chemistry of the nanoparticle to favor a specific functionality and impact on their properties such as hydrophilicity and shape [20][21][120,121]. In addition, for Cu-NPs, the most important challenge for these kinds of studies is to synthesize a stable Cu-NPs, which can be due to rapid oxidation to Cu+2 provoked by air or the aqueous media [25][124]. Therefore, the methodology to obtain these kinds of nanoparticles are performed in non-aqueous media and under inert atmosphere (argon, nitrogen) [24][26].
The size, shape and the surface chemistry of copper nanomaterials to be incorporated in the polymer membrane could exert tremendous impact on the membrane properties [26][27][125,126]. Thus, the incorporation of copper nanomaterials and their used synthesis agents can influence on surface properties of the modified membrane. Some benefits sought are related to the increase of the hydrophilicity, the reduction of surface roughness, and the improvement of charge property to favor the foulant reject from the modified membrane surface. For instance, CuO-NPs have showed hydrophilic character, which means that these oxide nanoparticles could improve in the surface hydrophilicity and/or the water flux of the modified membrane better than hydrophobic Cu-NPs [28][127]. Other aspects such as the size and their shape could also influence the surface-modified membrane, having an impact in the membrane performance. For instance, a different shape changes the exposed crystal facets and hence, the atomic arrangements in each facet could also have an intense effect on its surface properties. Moreover, an increase in surface membrane roughness can be influenced by the size of the incorporated nanoparticles [29][30][31][32][14,21,22,27]. Finally, the membrane surface charge could be altered after modification attributed to the coverage of the membrane surface by positive or negative charged copper NPs [33][32]. Thus, the control of size, shape and the surface chemistry of copper nanomaterials to be used in membrane modification are important aspects to be considered and are mainly dependent on the synthesis method.
On the one hand, metal and metal-based compounds are commonly used to fabricate antimicrobial composite membranes involving copper. Among these metal-based compounds it is possible to mention metal-polymer complex and coordination polymers. The metal-polymer complexes can be obtained on the basis of heteroaromatic polymers, whose backbone was functionalized by units containing functional groups capable of forming coordination bonds with transition metals, particularly copper(I) or copper(II) salts [34][128]. In this way, the linkers anchored to polymer act as chelating arms to coordinate copper ions, promoting the metal-polymer complex formation. Thus, linkers with carboxylic, sulphur and amine groups are desirable.
On the other hand, coordination polymers contain metal ions linked with coordinated organic ligands into an infinite array. Coordination bonds must define this infinite array [35][129]. These compounds have attracted attention because of the different architecture that can be formed and the several physicochemical properties that can be included in a modified membrane. The development of new systems based on copper is strongly studied due to the different characteristics of copper already mentioned. Copper is a versatile type of building block that has been successfully used for the synthesis of coordination polymers in combination with different neutral ligands that can offer appreciable properties. The selection of additives to form copper-based complex materials is very important, since the overall performance of the modified membranes rely only on it. Thus, the use of copper complexing and chelating agents mainly aims at stabilizing the copper on the membrane, controlling the copper ion dissolution and improving hydrophilicity of the membrane surface.
Therefore, different copper-based materials (composites, metal-polymer complex, coordination polymers) have been synthesized to be incorporated into the membrane performance.
