Nanoparticles: An Overview of Preparation

Abstract

Nanoparticles ranging below several 10 nm such as metals, semiconductors and metal oxides are of great interest for a wide variety of applications in the field of information, energy, environmental and medical technologies due to their unique size, composition and structure. In this review article synthesis method of nanosized particles is briefly described including their application in today’s world. The preparation and synthesis of nanoparticles is carried out by different methods such as polymerization, preformed polymers or ionic gelation etc. These methods can be useful in certain methods such as drugs can be entrapped in the polymer matrix, encapsulated in a nanoparticle core, surrounded by a shell- like polymer membrane, chemically conjugated to the polymer, or either it may be bound to the particle’s surface by adsorption.

Introduction

A technology that measures, manipulate or incorporate materials with a critical dimensions between 1-100 nm in size [1,2], also whose application exploits properties distinct from bulk macroscopic systems from which they arise is referred as Nanotechnology that deals with nano-meter sized objects. Nanotechnology will be developed at several levels of materials, devices and a system including development at nanomaterial level is the most advanced at present, both in scientific knowledge and in commercial applications through nanoparticles.

Nanoparticles are nanosized colloidal structures manufactured from biocompatible and biodegradable polymers that ranges in size from 10 to 1000 nm composed of synthetic or semisynthetic polymers [3,4]. Such polymeric nanoparticles can modify the activity of drugs, delay and control of drug release, and increase the drug adhesivity. On the basis of preparation methods, nanostructures can be obtained from Nanospheres of solid core spherical particulates which are nanometric in size containing drug embedded within the matrix or take up onto the surface of nanomaterials membrane, while in Nanocapsules drugs are embedded to a cavity surrounded by a unique polymer membrane.

Significantly, technological advancement of nanometer-sized particles is expanding by the variety of chemical compositions with advancement in their sturctures and applications. In field of synthesis of different types of nanoparticles, polymeric nanoparticles [5-10] (PNP’s) playing an important role in field of electronics [11], photonics, conducting materials, sensors, medicine, biotechnology, therapeutics, pollution control and environmental technology, drug delivery [12-15] as nanoparticles are known for delivery of drugs, proteins and DNA to their target cells and organs that plays a vital role in medication and treatment of various diseases [16]. The nature, size and optical properties of nanoparticles make them easier to enter, translocate and damage living microorganisms or the target cell as nanoparticles can penetrate physiological barriers and travel within the circulatory systems of a host.

Techniques Of Preparation

The appropriate method for the preparation of nanoparticles depends on the characteristics of polymer and the drug that is to be used in Nano preparations therefore in order to achieve the properties of interest the mode of preparation plays a vital role. Different techniques employed in preparation [17-20] and synthesis of nanoparticles is discussed below:

Solvent Evaporation

Solvent evaporation (Figure 1) was the first method that was developed for the preparation of nanoparticles, in this technique the polymer solutions [21] were prepared in volatile solvents and emulsions [22] were formulated by employing dichloromethane and chloroform, but now it is replaced with ethyl acetate that shows a much better toxicological profile to obtain polymeric particles less than 500 nm in size. During the preparation, emulsion is converted into a nanoparticle suspension on evaporation of the solvent, after that the solution is allowed to diffuse through the continuous phase of the emulsion to carry out conventional mode of methods i.e. single emulsions e.g., oil-in-water (o/w) and double emulsions [22] e.g., (water-in-oil)-in-water, (w/o)/w. Such type of methods utilize high-speed homogenization or ultrasonication, followed by evaporation of the solvent [23,24], either by continuous magnetic stirring at room temperature or under reduced pressure resulting in the formation of solidified nanosized particle collected by ultracentrifugation followed by washing to remove surfactants and at last the product is lyophilized.

Figure 1: Solvent evaporation technique.

Single emulsion and Double emulsion has been widely used for pharmaceutical applications [25] to obtain clinically applicable drug delivery systems, encapsulation of various hydrophilic and hydrophobic anticancer drugs, anti-inflammatory drugs, antibiotic drugs, proteins and amino acids and their applications in theranostics. In solvent evaporation technique [26], efforts are being made in clinics to develop more specific, individualized therapies for various diseases, and to combine diagnostic and therapeutic capabilities into a single agent.

Nanoprecipitation

Nanoprecipitation (Figure 2) is a facile, mild, and low energy input process to carry out polymeric nanoparticles synthesis [27] which is also termed as solvent displacement [28,29] method. The process of preparing involves preformed polymer of organic solution (acetone, ethanol, or methanol) and then in the presence or absence of surfactant [30] the organic solvent is allowed to diffuse generally using polymer Poly-Lactic Acid (PLA).

Figure 2: Nanoprecipitation technique.

The polymer PLA of intermediate polarity is allowed to dissolved in a water-miscible solvent, resulting in formation of nanospheres [1,2] and the solution is injected into an aqueous solution containing stabilizer as a surfactant as to result the formation of nanoparticles due to interaction between the water and the organic solvent. The nanoparticles synthesised through the process are of submicron size (<210 nm) with of low polydispersity. Biodegradable nanocarriers [31-35] such as lipid or polymer based nanoparticles that were designed to enhance the efficacy of nanoparticles and reduce the toxic effects of drugs [36] that results from therapeutic delivery of drugs for treatment of diseases. The Nanoprecipitation, without using surfactant of hydrophobic compounds in a non-solvent solutions leads to scattering of nanoparticles with effect of nanosized particles and such process is termed as “Ouzo” effect.

Emulsification Diffusion

Emulsification or solvent diffusion (ESD) technique (Figure 3) is the modification of solvent evaporation method which utilizes water miscible solvent [37-40] and a small amount of water immiscible organic solvent due to the spontaneous diffusion of immiscible solvents that generate turbulence [41] between the two phases results the formation of nanosized particles. The formation of nanoparticles depends only on the diffusion of the solvent of the dispersed phase [42,43] and the formation of nanospheres or nanocapsules [46], according to the oil-to-polymer ratio in which an aqueous solution containing stabilizer [44] successfully leads to solvent diffusion to the external phase[45] of the solution for nanoparticle formation.

Figure 3: Emulsification diffusion technique.

ESD presents many advantages such as high encapsulation efficiencies [47], no homogenization required, high batch-to-batch reproducibility [48], ease of scale-up, simplicity, narrow size distribution [49,50]. As drug loaded nanoparticles can be prepared by ESD technique [52], thus hydrophobic or hydrophilic drugs [51] can be used for medical and electronical importance. Similarly, several other nanoparticles such as mesotetra porphyrin-loaded PLGA (p-THPP) nanoparticles [52-57], doxorubicin-loaded PLGA nanoparticles, plasmid DNA-loaded PLA nanoparticles, coumarin-loaded PLA nanoparticles, indocyanine [58-60] can also be used for a number of applications.

Salting Out

The salting out (Figure 4) is modification of emulsification solvent diffusion technique [61-65] in which water miscible solvent is separated from aqueous solution through salting out process where, initially polymer and drug are dissolved in a solvent such as acetone, then it emulsifies into an aqueous gel consisting a salting-out agent in it as electrolytes such as magnesium chloride, calcium chloride, and magnesium acetate, or non- electrolytes such as sucrose. Importance of technique depends upon the type of salting out agent used, as it play an important property of encapsulating efficiency [66] of the drugs because the solvent and the salting out agent [67,68] are then eliminated by cross-flow filtration.

Figure 4: Salting out technique.

Dialysis

Dialysis is a simple and effective method for the preparation of small, narrow-distributed nanoparticles synthesis [69,70] in which polymer is dissolved in an organic solvent and placed inside a dialysis tube with proper molecular weight cut off and the displacement of solvent inside the membrane is followed by the progressive aggregation of polymer [71] due to a loss of solubility and the formation of homogeneous suspensions [72] of nanoparticles. The mechanism of dialysis [73] (Figure 5) is similar to Nanoprecipitation [74] whereas, it is based on the use of a physical barrier, specifically dialysis membrane [75] or common semi permeable membranes that allow the passive transport of solvents to slow down the mixing of the polymer solution with a non-solvent; the dialysis membrane [76,77] contains the solution of the polymer.

Figure 5: Dialysis or osmosis based method for nanoparticles preparation.

Supercritical Fluid Technology (SCF)

Supercritical fluid (Figure 6) is defined as a solvent at a temperature above its critical temperature, at which the single phase regardless of pressure moreover; the technology has been used as an alternative to prepare biodegradable [78] micro and nanoparticles because supercritical fluids [79] are environmentally safe. Supercritical CO₂ is most widely used as supercritical fluid because of its mild conditions, non-toxicity, non-flammability where this fluid along with dense gas technology [80] are expected to offer an interesting and effective technique of particle production [81], avoiding most of the drawbacks of the traditional methods (Figure 6). This technique is environmentally friendly, suitable for mass production and is more expensive.

Figure 6: Experimental set up for Nanoparticles preparation of Supercritical fluid solution.

To attain the desired properties for a particular application of nanoparticles, suitable polymer nanoparticles must be designed, which can be done during the polymerization of monomers [82] and processes for the production of NPs through the polymerization of monomers are discussed below.

Polymerization in Emulsion

The term Emulsion is defined as a mixing of two or more totally or partially immiscible liquids obtained in the presence or absence of a surface active agent [83] basically depending on the type of dispersed phase and of the dispersion medium. Emulsion technique is one of the fastest methods for nanoparticle preparation [84] and this major emulsion polymerization technique is classified into two categories, based on the use of an organic or aqueous continuous phase. The major emulsion polymerization method includes conventional emulsion polymerization, surfactant-free emulsion polymerization, as well as mini- (or nanoemulsions) and microemulsions [85] polymerizations which differ from the kinetically and thermo-dynamically different emulsion behaviours.

The continuous organic phase [86,87] technique involves the dispersion of monomer into an emulsion or inverse microemulsion, or into a material in which the monomer is not soluble (nonsolvent) as a result, polyacrylamide nanospheres [88] were produced by this method. In the aqueous continuous phase the monomer is dissolved in a continuous phase that is usually an aqueous solution, and the surfactants or emulsifiers are not needed thus, the polymerization [89,90] process can be initiated by different mechanisms resulting the synthesis of nanoparticle of submicron size.

Miniemulsion polymerization (Figure 7) is generation of Nanoemulsions [91,92] using high-energy methods and employing low molecular mass compound as co-stabilizer prior initiating polymerization by formulation the synthesis in water, monomer mixture, co-stabilizer, surfactant, and initiator. The successful synthesis of NP’s is achieved by low water solubility of reactants and the slower polymerization kinetics than emulsification. Microemulsion [93] is a thermodynamically and spontaneous stable system prepared with a high quantity of surfactant and characterized by an interfacial tension at the oil/water interface close to zero that depicts, the main difference between microemulsion and miniemulsion polymerization method. Nanoparticles synthesized through microemulsion results in smaller particle size (<80 nm) than in miniemulision processes.

Figure 7: Miniemulsion Polymerization Method. (a) Pre-emulsification (b) Nanoemulsification using a high shear device (c) Formation of polymeric nanoparticles upon addition of initiator.

Interfacial Polymerization

Interfacial polymerization (Figure 8) is characterized by the polycondensation of monomers [94] at the droplet interface leading to the generation of mainly nanocapsules [95] instead of nanospheres as the method involves step polymerization of two reactive monomers or agents, which are dissolved respectively in two phases (i.e., continuous and dispersed phase), and the reaction takes place at the interface of the two liquids. Finally, nanometer-sized hollow polymer particles were synthesized by employing interfacial cross-linking reactions as polyaddition and polycondensation or radical polymerization [96] effective formulation of nanocapsules through polymerization of monomers.

Figure 8: Different strategies of Interfacial Polymerization. (1) Monomer introduction onto oily phase (2) Monomer in aqueous phase (3) Monomers introduction in the oily and aqueous phase (4) Polymerization using a membrane reactor device.

Oil-containing nanocapsules [97] were obtained by the polymerization of monomers at the oil/water interface of a very fine oil-in-water micro- emulsion alternatively, water-containing nanocapsules can be obtained by the interfacial polymerization of monomers in water-in-oil micro-emulsions. In this method the organic solvent, which was completely miscible with water, served as a monomer vehicle and the interfacial polymerization of the monomer [98] was believed to occur at the surface of the oil droplets that formed during emulsification results in formation of Oil-containing nanocapsules whereas the polymer formed locally at the water-oil interface and precipitated to produce the nanocapsule shell [99] produce water-containing nanocapsules.

Controlled/Living radical polymerization(C/LRP)

The principal methods of controlled/living radical polymerization [100,101] are nitroxide-mediated polymerization (NMP), atom transfer radical polymerization (ATRP), and reversible addition and fragmentation transfer chain polymerization (RAFT) that relies only on pure reactions as to ignore terminations aroused due to impurities. Several kinds of polymeric nanoparticles [102] have been synthesized by this technique such as polybutyl acrylate, polystyrene, or polymethyl methacrylate and even block copolymers.

The controlled/living radical polymerization has emerged as a new field in the recent past years helped by the industrial production of hydrophilic polymeric nanoparticles [100] that were designed specifically for biomedical applications and for environmental concern with the development of ‘‘green chemistry’’. This technique also have some drawbacks due to the lack of control over the molar mass, molar mass distribution, end functionalities and macromolecular architecture, which are due to unavoidable fast radical–radical termination reactions during the preparation of nanoparticles.

Living and Controlled radical polymerization differ differently as Living radical polymerization developed by Ostu, et al. in 1982, where they referred organic disulfide initiator with chain transfer and termination as initiator-transfer agent terminator (iniferter) and Controlled radical polymerization developed by Michael Szwarc in 1956 empolyes living anionic polymerization of styrene with an alkali metal for further preparation of nanoparticles.

Ionic Gelation or Coacervation Of Hydrophilic Polymers

In ionic gelation technique, Figure 9 also known as ion induced gelation in which PNPs are prepared by using biodegradable hydrophilic polymers such as chitosan, gelatin and sodium alginate in which ionic gelation refers to the material undergoing transition from liquid to gel due to ionic interaction conditions at room temperature. Chitosan, a natural linear biopolyaminosaccharide that is used for the synthesis of nanosized particles under which involves the mixture of two aqueous phases, one is polymer chitosan, and the other is a poly anion sodium tripolyphosphate. Finally, the positively charged amino group of chitosan interacts with negative charged groups of tripolyphosphate to form Coacervates resulting in the formation of nanosized particles by employing emulsion cross-linking technique. These microparticles formed due to electrostatic interaction between two aqueous phases can be characterized using FTIR (Fourier Transform Infrared) spectrum.

Figure 9: Ionic gelation method.

Conclusion

The main goal of this review article was to reflect techniques for the preparation and synthesis of organic & bioorganic NP’s. Nanoparticles present a highly attractive platform for a diverse array of biological application as it was observed that the preparation of nanoparticles is a state-of-art technology that emerges in recent past years.

The technique for the preparation of NP’s are more challenging as an important challenge is to obtain materials with well-defined structures and morphologies including wide range of physical, chemical, biological, physiological factors and conditions that must be taken into account for successful preparation and biofunctionalization of nanoparticles for a given biomedical application. A better fundamental acknowledgement about the processes, mechanism & techniques for NP’s preparation should be the subject of an intensive research in next decades because nanoparticles have therapeutic potential at both research and clinical levels.

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