Different types of algal-synthesized NPs and its antibacterial activity.
A nanoparticle can be defined as a small object that behaves as a whole unit in terms of its transport and properties. Nanoparticles are sized between 1 and 100 nm in diameter. Nanoparticles can act against the microbes in multiple ways, and the microbes are less likely to develop resistance against nanoparticles because it requires multiple gene mutations. The large surface-to-volume ratio of nanoparticles, their ability to easily interact with other particles, and several other features make them attractive tools in various fields. Nanoparticles are widely used various fields such as electronics, cosmetics, biomedical, and biotechnology. Nanoparticles can be synthesized by physical methods such as attrition, pyrolysis, and using some wet chemical methods. The physical and chemical methods have various drawbacks such as high cost of production, require high energy input and generation of toxic by-products. To overcome this, several biological methods are employed in the synthesis of nanoparticles. The biological methods are generally cost effective, nontoxic, and ecofriendly. This chapter focuses on the methods involved in algal-synthesized nanoparticles and its applications.
- Green synthesis
- Antibiotic activity
Nanotechnology is a vibrant and developing area of science, engineering, and technology accomplished at the nanoscale level. The products of nanotechnology are nanoparticles or nanomaterials (NPs), lying in the range of 10−9 m and having dimensions of 1–100 nm. NPs are categorized into three types: natural nanoparticles, incidental nanoparticles, and engineered nanoparticles . The large surface-to-volume ratio of nanoparticles, their ability of easy interaction with other particles, and several other features make them as an attractive tool in various fields. NPs are widely used in electronic, cosmetic, biomedical, and biotechnological applications. The efficient crystallographic and physiochemical properties of NPs make nanotechnology as an excellent area to focus. The synthesis of NPs can be achieved by some physical methods and chemical methods. The traditional and commonly used method for nanoparticles synthesis is wet method. In chemical synthesis, nanoparticles are grown in a liquid medium containing various reactants particularly reducing agents such as sodium borohydride , potassium bitartartarate , methoxypolyethylene glycol , or hydrazine . Some stabilizing agents such as sodium dodecyl benzyl sulfate  or polyvinyl pyrrolidone  are added to the reaction mixture to prevent the agglomeration of metallic nanoparticles. Most commonly used chemical methods are chemical reduction , electrochemical techniques , and photochemical reactions in reverse micelles . Commonly used physical methods are attrition and pyrolysis. Attrition involves grinding of the particles by a size-reducing mechanism. The particles are then air-classified, and oxidized nanoparticles are recovered. Pyrolysis involves burning of the precursor by passing them through an orifice at high pressure. The ash obtained is air classified to recover the oxidized nanoparticles . Chemical methods are of low cost for high volume, and their major drawbacks include contamination from precursor chemicals, use of toxic solvents, and generation of hazardous by products, and the demerits of physical methods are low production rate, high cost of production, and high energy consumption . There is need for replacing the toxic ingredients with environmentally safe method for synthesizing NPs. To overcome this, researchers are focusing on employing biological method for the synthesis of nanoparticles. They are generally cost effective, nontoxic, and ecofriendly . So far, several plant extract , bacteria , fungi , enzymes , and algae  have been used for the synthesis of NPs. To our surprise, an emerging trend of synthesizing NPs using algae is developing in the recent years.
Algae are economically and ecologically important group of photosynthetic organism. They are unicellular or multicellular organisms dwelling in different environment such as freshwater, marine water, or surface of moist rocks [16–18]. Algae are categorized as microalgae (microscopic) and macroalgae (macroscopic). They play a key role in medical, pharmaceutical, agriculture, aquaculture, cosmetics applications. Algae are valuable source for various commercial products such as natural dyes and biofuels [19–22]. Till now, for the biosynthesis of metallic NPs, different group of algae such as Chlorophyceae, Phaeophyceae, Cyanophyceae, Rhodophyceae, and others (diatoms and euglenoids) have been used . The ability of algae to accumulate metals and reduce metal ions makes them the superior contender for the biosynthesis of nanoparticles. Furthermore, algae are relatively convenient and easy to handle, along with several other advantages such as synthesis at low temperature with greater energy efficiency, less toxicity, and risk to the environment. In physical and chemical method, different commercially available surfactants were used as templates and capping agents in NPs synthesis with different morphologies. Removal of the residual components becomes a major issue. Considering this utilization of naturally eco-friendly methods having been developed which involves the synthesis of NP using different biological sources which could naturally modify the shape or size of a crystal with superior quality .
Among the biological materials, algae are called as ―bionanofactories‖ because both the live and dead dried biomasses were used for the synthesis of metallic nanoparticles . Several algae such as
2. Types of nanoparticles
There are two different types of NPs, inorganic NPs and organic NPs. The inorganic NPs include metal and metal oxides, which are potent antibacterial agents  (Figure 1). Metal oxide nanoparticles such as silver (Ag), iron oxide (Fe3O4), titanium oxide (TiO2), copper oxide (CuO), and zinc oxide (ZnO) are certain examples of inorganic NPs. Organic NPs includes poly-ɛ-lysine, quaternary ammonium compounds, cationic quaternary polyelectrolytes, N-halamine compounds, and chitosan . Organic nanoparticles are generally less stable at high temperatures. Due this reason, inorganic nanoparticles are more preferred as antimicrobial polymers .
2.1. Inorganic nanoparticles
So far, there are different types of inorganic metals and metal oxide NPs, which have been studied. Some important examples are detailed (Figure 2)
Silver nanoparticle (AgNP) is the most widely used antimicrobial agent against many bacteria, fungi, and viruses . The antimicrobial activity AgNP was found to be size dependent, and larger particles are less active than smaller one against many pathogens in both
2.1.2. Titanium oxide
Titanium oxide (TiO2) is found to be effective against both Gram-positive/Gram-negative bacteria, viral, and parasitic infections [48, 49]. They are photocatalytic; their toxicity can be induced by visible light, or UV light, generates ROS . TiO2 is an effective bactericidal agent and a potent sporicidal agent against wide range of bacteria .
2.1.3. Zinc oxide
ZnO nanoparticles (ZnONPs) are another broad spectrum antibacterial agent, based on concentration and size of the NPs, and they are effective against methicillin-sensitive
2.1.4. Iron oxide
Iron oxide is generally inactive in their bulk form. Reducing their size to nanoscale makes them a potential antimicrobial agent. Iron oxide nanoparticles-coated surfaces prevent the adhesion and colonization of Gram-positive and Gram-negative bacteria .
As compared to Ag, Au nanoparticles are less effective and lack antimicrobial properties when used alone but found to be effective when used in combination with antibiotics such as ampicillin [57, 58], vancomycin , and lysozyme (an antibacterial enzyme) . The Au nanoparticles can also be used in combination with nonantibiotic molecules such as amino substituted pyrimidines  and citrate, which induces the generation of ROS and mutations, hence used in cancer therapy .
2.1.6. Copper oxide
Despite copper oxide (CuO) nanoparticles are used as antibacterial agents, they are less effective than that of Ag and ZnO. So a comparatively higher concentration is required to get desired results. But some bacteria are more susceptible to CuO than Ag. For example,
2.1.7. Magnesium oxide
Magnesium oxide (MgO) nanoparticles are efficient antimicrobial agent exhibiting bactericidal activity against both Gram-positive and Gram-negative bacteria, spores and viruses. The MgO NPs can be prepared from available and economical precursors. Along with membrane disruption and ROS generation, it also inhibits the essential enzymes of bacteria [50, 67].
2.1.8. Nitric oxide
Nitric oxide (NO) nanoparticles are highly reactive antibacterial agent. Similar to other nanoparticles, the activity of NO is also size dependent [68, 69]. The mode of inhibition is by the production of reactive nitrogen species (RNS) rather than ROS. They are effective against MRSA
2.2. Organic nanoparticles
Some of the well-known examples of organic NPs are discussed below (Figure 3).
Poly-ɛ-lysine, a cationic homopeptide of L-lysine is effective against Gram-positive bacteria and spores of
2.2.2. Quaternary ammonium compounds
Quaternary ammonium compounds are well known disinfectants and their antimicrobial property dependents on the chain length. The positively charged moieties of the compounds are attached to the negatively charged bacterial membrane by weak electrostatic interaction, followed by the insertion of hydrophobic tail of the compound in to the bacterial hydrophobic membrane core leading to the denaturation of structural proteins and enzymes .
2.2.3. Cationic quaternary polyelectrolytes
They are synthesized from methacrylic monomers such as 2-(dimethylamino) ethyl methacrylate and majority of them are derivatives of acrylic and methacrylic compounds. These molecules possess a wide range of biological applications due to their structural flexibility through the alteration of hydrophobicity, molecular weight, surface charge and other factors .
2.2.4. N-halamine compounds
N-halamine compounds are formed by the halogenation of imide amide or amine groups with one or more nitrogen–halogen covalent bonds. These are high stable compounds releasing free active halogen groups slowly in to the environment leading to the inhibition or inactivation of the microbial cells .
Chitosan NPs are biocompatible, nontoxic, and have the ability to act as absorption enhancer. These characteristics make the chitosan nanoparticles as an effective antimicrobial agent with broad spectrum activity against a wide range of bacteria, fungi and viruses. The antibacterial activity of chitosan nanoparticles depends on several factors such as pH and the nature of solvent [76, 77]. The use of chitosan along with metal nanoparticles is not feasible since chitosan reduced the activity of metal nanoparticles such as Zn. It can be used in combination with antibiotics [76, 78]. Even though some studies state that the interaction of cells with chitosan lead to membrane destabilization, followed by lysis and cell death, the detailed mode of action is unclear .
2.3. Synthesis of NPs using algae
The abundance and ease of availability of algae make them good and worthwhile sources for the synthesis of metallic nanoparticles . Synthesis of nanoparticles using algae can be performed in three important steps, (i) preparation of algal extract in water or in an organic solvent by heating or boiling it for a certain duration, (ii) preparation of molar solutions of ionic metallic compounds and (iii) incubation of algal solutions and molar solutions of ionic metallic compounds followed either by continuous stirring or without stirring for a certain duration under controlled conditions [10, 30]. The synthesis of NPs is dose dependent and it is also related to the type of algae used. There are a variety of biomolecules responsible for the reduction of metals which include polysaccharides, peptides, and pigments. Stabilizing and capping the metal nanoparticles in aqueous solutions is done by proteins through amino groups or cysteine residues and sulfated polysaccharides . Synthesis of nanoparticles using algae takes comparatively shorter time period than the other biosynthesizing methods [10, 30]. So far, several seaweeds (
2.4. Application of algal-synthesized NPs
The biomedical application of algal-synthesized NPs is significantly becoming more important due to their antibacterial, antifungal, anti-cancer, and wound healing activity. They are given (Figure 4).
2.4.1. Antibacterial activity
Algal-synthesized NPs are known to possess efficient antibacterial activity (Figure 5; Table 1). Brown alga (
AgNPs synthesized using
|Algae||NPs||Size||Shape||Intracellular (IC) or extracellular (EC)||Pathogens||References|
|CuO||5–45 nm||Spherical and elongated||IC|||
|Ag||4 and 6 nm||Spherical||IC|||
|Ag||5–25 nm||Spherical and triangle|||
|Au||60 nm||Triangle, rectangle & square||IC|||
The aqueous extract of red marine algae
|Silver nanoparticles||[112, 113]|
|Bismuth oxide aqueous colloidal nanoparticles||[114, 115]|
|Nano-oil formulation from |||
|Nano-emulsion (detergent, oil, and water) in combination with cetylpyridinium chloride|||
|Silver- and gold-incorporated polyurethane, polycaprolactam, polycarbonate, and polymethylmethacrylate|||
|Silver nanoparticles in combination with nystatin and chlorhexidine|||
|Silver nanoparticle and 12-methacryloyloxydodecylpyridinium bromide (MDPB)||Dental plaque microcosm biofilms||[120, 121]|
oxide/oleic acid core/shell nanostructures
|Zinc and copper oxide nanoparticles|||
|Zerovalent bismuth nanoparticle|||
|Dextran sulfate nanoparticle complex containing ofloxacin and levofloxacin|||
|PEG-stabilized lipid nanoparticles loaded with terpinen-4-ol|||
|Magnesium fluoride nanoparticles||[128–130]|
|Yttrium fluoride nanoparticles|||
|Iron oxide/oleic acid in combination with essential oil from |||
|Gold nanoparticles and methylene blue|||
|Starch-stabilized silver nanoparticles|||
|Iron oxide–oleic acid nanofluid|||
|Quaternary ammonium polyethylenimine nanoparticles||Oral biofilms|||
|Zinc oxide nanoparticles, chitosan nanoparticles, and combination of both|||
2.4.2. Antifungal activity
Algal-synthesized NPs were used as efficient antifungal agents. Only countable number of work has been carried out in this aspect. This includes the synthesis AgNPs using the aqueous extract of red seaweed
2.4.3. Anticancer activity
In a work done by Boca et al.  synthesized chitosan-coated silver nano-triangles (Chit-AgNPs) were used as a photothermal agents against a line of human nonsmall lung cancer cells (NCI-H460) . In another work, AgNPs (10 nm) were synthesized using
2.4.4. Other applications
Algal-synthesized NPs are explored in certain other area of applications, which include the synthesis of spherical palladium nanocrystals via aqueous Na2 [PdCl4] solution using the photosynthetic reaction within
2.5. Future application of algal-synthesized NPs
2.5.1. Antibiofilm agents
The use of nanoparticles as antibiofilm agents is an emerging area of research. Due to the extensive use and misuse of antibiotics, many of the pathogens acquired resistance toward multiple drugs. As the bacteria are less likely to develop resistance against nanoparticles, they can be used as a promising therapeutic agent against biofilms. Nanoparticles have the ability to penetrate EPS and the cell membranes (Figure 6). Silver nanoparticles were found to be more prevalent than the other ones, and they exhibit antibiofilm activity against both Gram-positive and Gram-negative pathogens. In a work done by LewisOscar et al. . Chemical synthesis of CuNPs was performed by one-pot synthesized method and used for biofilm inhibition against
The zero-valent selenium and tellurium NPs synthesized using
2.5.3. Lipid nanoparticles
There are possibilities for the production of lipid nanoparticles with the help of lipid-rich marine organisms such as algae, fungi, and bacteria . Lipid nanoparticles can be synthesized from the organisms through heating to liquefy fatty acids; incorporating active agents of pharmacological and cosmetics importance; adding a hot surfactant; and stirring or homogenized under high pressure by ultrasound. These can be used in the production of food stuffs, cosmetics, and medicines .
Algal-synthesized NPs can be explored in biosensing applications. Such as, AuNPs has been proved as an important tool for hormone (HCG) detection in pregnant women urine sample . Platinum (Pt) NPs act as a novel biosensor with high sensitivity for the determination of adrenaline for the treatment of allergies, heart attack, asthma, and cardiac surgery . Synthesis of nanoscale Au–Ag alloy prepared using chloroplasts exhibited high electrocatalytic activity for 2-butanone at room temperature which can be developed as a tool for detecting cancer at early stages .
The developing era of nanoscience is a renowned gift for the development of science all over the world. Despite numerous studies conducted over the last decade, there are still considerable gaps in our knowledge about the biotechnological potential of green-synthesized nanoparticles. Furthermore, the precise basis of their antibiotic and antibiofilm activity has yet to be defined. However, the toxicity of nanoparticles to eukaryotic cells is a legitimate concern and still remains uncharacterized. One way of avoiding this potential drawback might be to target green-synthesized nanoparticles to the specific site of an infection so that toxic nanoparticles concentrations are localized. In addition, improvements in the way that green-synthesized nanoparticles are incorporated into medical devices could increase their efficacy and diminish any side effects, but considerable research effort is still required to perfect this technology.
Financial support provided to Dr. C.N by the DST INSPIRE faculty scheme is gratefully acknowledged (DST/Inspire Faculty Award/2012 [IFA12- LSPA13]). The authors gratefully acknowledge DBT [BT/PR7005/PBD26/357/2012 dated: 26.03.2015 and BT/PR6619/PBD/26/310/2012] for their financial assistance, and FIST program provided by the DST-FIST scheme [SR/FST/LSI-523/2012] is gratefully acknowledged.