A Review of Current Research into the Biogenic Synthesis of Metal and Metal Oxide Nanoparticles via Marine Algae and Seagrasses

Today there is a growing need to develop reliable, sustainable, and ecofriendly protocols for manufacturing a wide range of metal and metal oxide nanoparticles. The biogenic synthesis of nanoparticles via nanobiotechnology based techniques has the potential to deliver clean manufacturing technologies. These new clean technologies can significantly reduce environmental contamination and decease the hazards to human health resulting from the use of toxic chemicals and solvents currently used in conventional industrial fabrication processes. The largely unexplored marine environment that covers approximately 70% of the earth’s surface is home to many naturally occurring and renewable marine plants. The present review summarizes current research into the biogenic synthesis of metal and metal oxide nanoparticles via marine algae (commonly known as seaweeds) and seagrasses. Both groups of marine plants contain a wide variety of biologically active compounds and secondary metabolites that enables these plants to act as biological factories for the manufacture of metal and metal oxide nanoparticles.

In recent years a convergence between biological based technologies, green chemistry, and nanotechnology has taken place. The objective of this convergence is to create new materials and manufacturing processes that reduce or eliminate the use of hazardous substances [1]. An important aspect of nanotechnology is the synthesis of nanometer scale materials and the direct control of particle morphology and dimensions during formation. Nanometer scale materials have at least one dimension less than 100 nm and can have a wide variety of geometric shapes such as plates, sheets, tubes, wires, and particles. Numerous studies have shown that nanometer scale materials exhibit unique chemical, physical, electronic, optical, thermal, mechanical, and biological properties that significantly differ from their bulk scale counterparts [2, 3]. These unique properties result from the extremely small size, shape, and size distribution. In addition, because of their small size, nanomaterials can act as bridge between bulk scale materials and molecular structures [4]. In terms of composition, nanomaterials can be broadly classified into two types, namely, organic and inorganic. Organic nanomaterials are carbon based, while inorganic nanomaterials include noble metals (gold, platinum, and silver), magnetic materials (iron oxide Fe3O4), and semiconductors such as titanium dioxide and zinc oxide.

The physiochemical properties of bulk scale materials are largely understood and recent discoveries have focused on materials existing in the region between the atomic scale and the much larger bulk scale. These discoveries arise from the significant increase in the surface area to volume ratio when bulk scale materials are reduced into much smaller amounts. The larger surface area to volume ratio found in nanometer scale materials is the major contributing factor that influences the physiochemical properties [5]. Thus, at the nanometer scale there are significant changes in both surface chemistry and chemical reactivity [6]. These unique and novel size-dependent properties have led to nanoparticles being promoted for a wide range of applications. Typical applications include biosensors [7, 8], catalysts [9–11], environmental remediation [12–14], labelling for immunoassays [15, 16], hyperthermia treatment of tumors [17–19], antibacterial drugs [20, 21], and vector delivery of therapeutic drugs for cancer treatments [22–24]. However, studies have also shown the properties of manufactured nanoparticles are heavily dependent on specific parameters such as particle size, morphology, and size distribution [25]. Consequently, it is very important to control both particle size and shape during manufacture. Thus, effective control during production enables the customization of nanoparticles for specific applications.

The manufacture of nanoparticles can be broadly defined into two approaches. The first is the top down approach and involves a material undergoing significant size reduction via physical or chemical processes [26, 27]. During size reduction, the resulting particle size, shape, and surface structure are heavily dependent on the technique used. Unfortunately, size reduction also tends to introduce surface imperfections that can significantly impact the overall physicochemical properties of the fabricated nanoparticle. The second, bottom up approach builds nanoparticles via the assembly of atoms, molecules, and smaller particles or monomers [28, 29]. Unfortunately, many of the chemical and physical processes used in both approaches suffer from several drawbacks such as low material conversion rates, are technically complex, have high energy requirements, and are generally expensive. Furthermore, many of these processes employ hazardous chemicals such as reducing agents, organic solvents, and nonbiodegradable stabilizing agents. For example, processes such as chemical precipitation and pyrolysis often result in toxic chemical species being deposited on the surface of newly formed nanoparticles. The presence of surface contaminants makes these nanoparticles unsuitable for clinical and biomedical applications [30, 31]. Because of the drawbacks associated with conventional manufacturing processes there has been a growing interest in developing new ecofriendly production technologies based on the principles of green chemistry [32]. Accordingly, research in recent years has focused on manufacturing nanomaterials via nanotechnology-based processes that promote the principles green chemistry and reduce or totally eliminate the use of hazardous chemicals. Thus, ecofriendly green nanotechnology-based processes for the manufacture of nanoparticles have attracted considerable interest worldwide [33, 34]. To emphasise this alternative approach, recent research has focused on using biological entities to synthesize a wide variety of nanoparticles. Biosynthesis via unicellular and multicellular biological entities such as actinomycetes [35], bacteria [36], fungus [37], marine algae [38], plants [39], viruses [40], and yeast [41] offer alternative ecofriendly approaches for producing nanoparticles. Each of these biological entities, to varying degrees, can perform as natural biofactories for producing particular nanoparticles. Each of the biological entities has active molecules and compounds that can act as reducing agents and stabilizing agents to synthesize nanoparticles with diverse sizes, shapes, compositions, and physicochemical properties [42].

The biologically diverse marine environment covers around 70% of the earth’s surface and is largely unexplored. Recent studies have shown that several marine plants have the ability to perform as biofactories for the production of nanoparticles [43, 44]. In particular, interest in producing new and effective medicines from natural marine plant sources has revealed that several forms of marine algae [45] have the potential to produce a variety of nanoparticles. A number of these nanoparticle types have been found to be effective antimicrobial agents [46, 47]. Marine algae contains a diverse range of different species, which are generally classified into two groups, namely, microalgae and macroalgae. Microalgae species such as phytoplankton survive suspended in the water column, while macroalgae (commonly known as seaweed) are plant-like organisms that can range in size from a few centimeters up to several meters in length. For example, the giant kelp grows up from the seafloor to form vast underwater forests. Seaweeds have adapted to living in a variety of habitat, ranging from small tidal rock pools close to shore or living several kilometers offshore in seawater depths capable of receiving sufficient light to promote photosynthesis. Algae are broadly classified into three groups based on the algal body or thallus pigmentation. The color groups are brown algae (phaeophytes), green algae (chlorophytes), and red algae (rhodophytes) [48]. Brown and red algae are predominantly marine based, with some species of red algae being found in water depths where light levels are extremely low, while green algae is found in both marine and freshwater environments. Figure 1 presents a selection of marine algae and seagrasses found in coastal waters along the West Australian coastline.