5

Low-Dimensional Systems: Nanoparticles

C.M. Miyazaki*
A. Riul Jr.**
*    Federal University of São Carlos, Center of Sciences and Technology for Sustainability, Sorocaba, São Paulo, Brazil
**    “Gleb Wataghin” Institute of Physics, State University of Campinas, Campinas, São Paulo, Brazil

Abstract

Nanoparticles have served an important role in the development of many fields, such as catalysis, biotechnology, and sensor development. This chapter introduces new basic knowledge about nanoparticle production techniques, describes top-down and bottom-up syntheses processes, and discusses the physicochemical aspects and stabilization processes of these techniques. The interesting properties of this material are discussed, with a succinct description of the main characterization techniques, such as X-ray diffraction, transmission electron microscopy and atomic force microscopy. Nanoparticles are extensively applied to different fields; this chapter presents breakthroughs in biosensor, catalysis and biomedical applications.

Keywords

nanoparticles
synthesis
top-down
bottom-up
nanoparticle stabilization
nanoparticle properties
nanoparticle characterization
nanoparticle applications
nanoparticles in biosensors
nanoparticles in catalysis

5.1. Introduction

Nanoparticles have been extensively explored because of their unique properties, which are dependent on their size and shape and enable their application in an extensive variety of fields, such as, cancer treatment and diagnosis [1–3]. Nanoparticles also enable the exploitation and production of existing materials with novel properties that, combined with advances in the synthesis and characterization processes, facilitate the development of new products, such as fabrics, dyes, cosmetics, and sports products.
In the 4th century, the Romans employed metal nanoparticles (silver and gold) to produce colorful glasses. The Lycurgus cup is a classic example, with its variable coloration according to the incident light, which ranges from green to red. In the mid-18th century, photography was discovered based on the production of light-sensitive silver nanoparticles. In 1857, Michael Faraday synthesized and performed experiments with a colloidal suspension of gold via the reduction of [AuCl4]- salt using phosphorus as a reducing agent [4]. In 1951, Turkevitch et al. described a detailed syntheses study, using transmission electron microscopy (TEM), of the preparation of gold nanoparticles using different reducing agents to elucidate the processes of nucleation, growth and agglomeration of nanoparticles, and the synthesis of monodisperse and reproducible suspensions [4]. Richard Feynman, who was awarded the 1965 Nobel Prize in Physics, predicted the technological potential of nanostructures, which suggests that the manipulation of individual atoms can provide a new material with new properties, and their application in electronic circuits on a nanometric scale for more powerful computers, as well as the manipulation of biologic systems. Currently, many examples and applications show that Feynman’s vision was ahead of his time [5].
Nanoparticles are defined as materials with sizes that range from 1 to 100 nm in at least one of their dimensions [6]; by this definition, nanotubes, fullerene, and nanothreads are also considered to be nanoparticles. However, this chapter focuses on zero-dimensional nanostructures. The term cluster (or nanocluster) will be extensively applied to define colloidal particle aggregates and may sometimes refer to nanoparticles.
The synthesis of nanoparticles involves two different approaches: the top-down method and the bottom-up method; these approaches will be detailed in Section 5.2. The important requirements for the practical application of nanoparticles are as follows: nanometric dimension, uniform size distribution, morphology, chemical composition, and identical crystal structure [7]. In addition to advancements in the synthesis of nanoparticles, the need for characterization techniques that can demonstrate the physico-morphological characteristics of these nanometric entities is evident. Currently, a range of tools are available to investigate these characteristics, including electronic microscopy and X-ray diffraction (XRD) techniques and spectroscopy.
Given the extent of the content that involves synthesis, properties, and applications of nanoparticles, this chapter will focus on the main synthesis processes, with a simplified approach to the nanoparticle properties, describe the main characterization methods, and indicate some applications in the literature.

5.2. Synthesis Methods

As previously mentioned, the unique properties of a nanoparticle are dependent on its shape and size, which are dependent on the synthesis process that is employed for the fabrication of nanoparticles. The process in which nanoparticles derive from atomic precursors that aggregate to form a cluster and subsequently form a nanostructure is referred to as a bottom-up process. The process in which nanoparticles are obtained by the physical wear of a larger volume is referred to as a top-down process. The main top-down (mechanical attrition and lithography) and bottom-up (chemical synthesis via sol–gel and reduction of metal salts) techniques are described in the next section.

5.2.1. Top-Down Methods

The mechanical attrition and lithography are the most well-known top-down methods for the production of nanometric structures, in which the former is commonly applied in the industry for large-scale production and the latter is a more sophisticated technique for the production of electronic and optical devices.

5.2.1.1. Mechanical Friction

Mechanical friction is extensively applied by the metallurgical industry for the production of new alloys and mixtures with different properties due to the incorporation of defects in the crystal lattice of a metal [8,9]. This technique enables syntheses that are impossible via traditional fusion routes, such as, the production of uniform dispersions of ceramic particles in a metal matrix or metal dispersions with different melting points [8,10]. Mechanical friction also allows the solubility of immiscible binary systems due to the segregation of solute in the grain boundaries [9].
In the high-energy grinding process, particles with diameters near 50 μm are placed together with steel balls or tungsten binary composites inside sealed chambers and subjected to intense agitation (Fig. 5.1). The high energy of the grinding process can be obtained by applying high-frequency and low-amplitude vibration [8]. The grain size decreases with the grinding time until a constant value is obtained depending on the melting point of the material. The high-energy grinding process enables the creation of mono- or multicomponent nanoparticles at the industrial scale; however, contamination by the grinding medium is a major disadvantage [9]. Lam et al. produced Si nanoparticles via the milling process with stainless steel balls. The reaction on the solid phases between graphite and silicon oxide (C + SiO2 → Si + CO2) produces nanoparticles with a 5-nm Si nucleus and 1-nm external layer of amorphous silicon oxide [11]. Details about the process of nanoparticle production by mechanical friction are provided by Koch (1993).
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Figure 5.1 Schematic of the ball-milling process using steel balls.

5.2.1.2. Lithography

The lithographic process consists of the transfer of patterns to the desired substrate and is extensively applied for the production of electronic and optical devices due to the high resolution obtained [12]. Briefly, this process consists of the deposition of a photoresist, exposure of its specific areas using a mask, the exposure of the resist and consequently, the transfer of the desired pattern.
The first step includes the deposition of the resist on a substrate, using the controlled rotation of the substrate by a technique denominated as spin-coating [13]. The second step includes the irradiation of the photoresist using a polymeric material that undergoes chemical structural alterations when irradiated, such as, the rupture of the polymeric chain (positive resist) or the formation of cross-links (negative resist). In the case of the positive resist, the irradiated part will be dissolved during the exposure process due to the breakdown of the macromolecule into smaller parts; for the negative resist, the formation of cross-links render the irradiated parts insoluble [12].
Depending on the type of radiation that is used to sensitize the resist, such as ultraviolet, X-ray, electron or ion beam, different resolutions can be obtained. Patterns of up to 5 nm can be obtained with an electron beam, whereas 30-nm patterns are obtained using X-ray [12]. After the desired model is created, the transfer can be performed via the corrosion or the lift-off metallization process. The chemical corrosion involves the protection of certain areas by the pattern created in the resist and the corrosion of the other areas by specific chemical solution or plasma with reactive radicals [12]. Metal structures can be created by metallization followed by lift-off, that is, the substrate receives the resist with the desired pattern, and the metal is evaporated on top of it. The metal is fixed at the substrate-free zone, and the resist is removed using an appropriate solvent, leaving only the desired metal pattern, as illustrated in Fig. 5.2.
image
Figure 5.2 Schematic of the lift-off process for the production of metal patterns.
Lithography is an extensive process, including various details. For a better understanding of the technique, refer to Dobisz et al. (1996) and Hilleringmann (2005).

5.2.2. Bottom-Up Methods

In the bottom-up process, the starting points are the atomic or ionic precursors that unite to form larger particles. Initially, the discussion will cover the physicochemical aspects of the process of nanoparticle production in solution, the nanoparticle stabilization in suspensions, the synthesis of metal nanoparticles via metal reduction and the synthesis of oxide nanoparticles via the sol–gel method. Although we will only discuss the techniques based on chemical synthesis, other bottom-up methods, such as molecular beam epitaxy, chemical vapor deposition, pulse laser deposition, laser ablation and sputtering deposition, are extremely important in the semiconductor industry. However, these sophisticated techniques are dependent on specific equipment for temperature and pressure control; details are provided in Refs. [14–16]. We also emphasize the high control and quality of nanoparticles that are obtained by physical methods in relation to size and stoichiometric composition [17,18]. The metal nanoparticles should be protected against the oxidative effects of the environment for their subsequent technological applications. We also emphasize the complexity that is required by these experimental setups, which justify the selected descriptions of the chemical methods discussed in this paper.
For each application, the synthesis conditions should be well controlled to ensure that the product has well-defined characteristics, such as equivalent dimension (uniform size distribution), shape and morphology, and similar composition and crystal structure between particles to prevent the formation of agglomerates [7].

5.2.2.1. Physicochemical Aspects of Nanoparticle Formation

The mechanism of nanoparticle formation in a liquid phase can be explained by nucleation and growth phenomena [7,19]. According to the theory developed by Lamer [20], particle formation consists of the following steps:
1. increase in monomer concentration in the solution until supersaturation and the start of nuclei formation;
2. continuous aggregation of monomers in the nucleus, which causes a gradual decrease in the monomer concentration in the solution; and
3. stabilization of the surface of the resulting particles by surfactants.
If the solute concentration increases to the limit of solubility or the temperature decreases to a point that enables phase transformation, the system will have a high Gibbs free energy. To minimize the energy of the system, the solute aggregates and the variation of the Gibbs free energy per unit volume (∆Gv) of the solid phase is given by Eq. 5.1, which is dependent on the solute concentration [7].

∆Gv=−KTΩlnCC0

image(5.1)
where C is the solute concentration, C0 is the solubility, K is the Boltzmann constant, T is the temperature and Ω is the atomic volume. Consider the supersaturation σ = (C – C0)/C0:

∆Gv=−KTΩln(1+σ)

image(5.2)
From Eq. 5.2, when σ = 0, the Gibbs free energy will be zero and supersaturation will not occur. If C > C0, the free energy will be negative and the nucleation process will be spontaneous.
A spherical nucleus of radius r can be assumed, with the Gibbs free energy given in Eq. 5.3 [7]:

∆μv=43πr3∆Gv

image(5.3)
In addition to the decrease in free energy, the total energy of the system will also be influenced by the appearance of the surface energy ∆μs. Thus, Eq. 5.4 indicates that the reduction in the free energy of the system is influenced by the appearance of the surface energy, which is expressed as 4πr2γ [7].

∆G=∆μv+∆μs=43πr3∆Gv+4πr2γ

image(5.4)
where γ is the surface energy per unit area. The formed nuclei should have a minimum radius to be stable; otherwise, they dissolve and return to their volume [7,19]. This minimum radius is referred to as the critical radius r*, and the nuclei with r ≥ r* will act as nucleation centers for the formation of particles. Fig. 5.3 illustrates the nucleation and growth processes.
image
Figure 5.3 Schematic of the nucleation and growth process by LaMer. Modified from V.K. LaMer, R.H. Dinegar, Theory, production and mechanism of formation of monodispersed hydrosols, J. Am. Chem. Soc. 72 (1950) 4847–4854 [20].
After nucleation starts, the concentration of the species in solution and the free energy of the system decreases [7,21]. The concentration decreases to values below a specific concentration when the nucleation stops, and the growth process continues until an equilibrium concentration Cs is attained. When a nucleus is formed, the growth process immediately starts, with larger particles that increase at the expense of the smaller particles by a process named Ostwald ripening, which reduces the surface energy. Above a minimum concentration, the nucleation and growth process simultaneously occur but at different rates.

5.2.2.2. Nanoparticle Stabilization

Nanometric particles have a wider surface area and tend to form aggregates to minimize their surface energy. The agglomeration process can initiate during synthesis; thus, the use of different types of surfactants and stabilizers has been explored [22]. To minimize the surface energy of the particles and prevent the agglomeration process, two paths can be followed: (1) electrostatic repulsion between covered particles, which is effective in diluted aqueous or polar organic systems, and (2) steric effects, which are active in aqueous and nonaqueous systems, and dispersions at high concentration; however, they are less sensitive to impurities or additives when compared with electrostatic repulsion [4,22].
For example, when suspended, gold nanoparticles that are synthesized via sodium citrate are surrounded by an electron double layer, which is formed by the adsorbed citrate, chloride ions and cations that are attracted to the surface. In this case, any disturbance in the system, such as an increase in ionic force, may cause the formation of agglomerates. The protection by steric effects includes the adsorption of molecules, such as polymers, surfactants and other types of ligands, on the surface of nanoparticles. Polymers are commonly applied and selected according to the solubility of the precursor in the polymeric solution and the ability to stabilize the product [4]. Some natural polymers, such as chitosan [23,24] and cyclodextrins [25,26], as well as synthetic polymers, such as PVP (polyvinylpyrrolidone) [27–29] and PVA (polyvinyl alcohol) [30,31], have been employed.

5.2.2.3. Sol–Gel Synthesis

Sol–gel synthesis is extensively employed for the production of colloidal dispersions of oxides and the production of core-shell nanostructures, which is also useful for the production of inorganic materials and organic–inorganic hybrids, particularly oxides [7,32]. The precursor can be an inorganic metal salt (acetate, chloride, nitrate, and sulfate) or a metal organic species, such as a metal alkoxide [33]. The precursor can be dissolved in an aqueous or organic solvent, and a catalyst is added to promote the hydrolysis and condensation reactions [7].
A typical sol–gel production process consists of the use of metal alkoxide as a precursor in an aqueous system that undergoes hydrolysis and condensation. The alkoxides are precursors for silica, alumina, titanium, and zirconium [32]. With hydrolysis, the metal alkoxide is transformed into sol (colloidal dispersion of particles in a liquid) and a gel after condensation [33]. The hydrolysis and condensation steps occur sequentially and in parallel with the condensation step, which frequently causes the formation of aggregates of oxides or metal hydroxides with incorporated or linked organic groups. These organic groups may derive from incomplete hydrolysis or may be introduced as organic nonhydrolyzable ligands [7].
The synthesis in an aqueous medium has the disadvantage of different reactivities of metal alkoxides, which hinder the control of the composition and its homogeneity in the synthesis of multicomponent oxides [33,34]. The nonaqueous synthesis can be divided into synthesis by surfactant control or by synthesis controlled by solvents. In the first case, the precursor is commonly injected into a heated solvent that contains surfactants, with the surfactant molecules avoiding agglomeration, which produces reasonable colloidal stability in organic solvents [34]. Zeng et al. produced MnFe2O4 magnetic nanoparticles with different sizes by controlling the stabilizers/Fe ratio. Depending on the proportion, the produced particles presented a spherical, polyhedron or cubic shaped [35].
In the case in which the presence of surfactants is undesirable, for example, when it affects the catalytic activity, the synthesis is performed via the solvent-control path. Ba et al. produced monodisperse tin oxide nanoparticles with a diameter of 3.5 nm. For the synthesis, a tin chloride solution was dripped into a benzyl alcohol solvent under agitation at 100°C for 24 h. The precipitate was collected, centrifuged, and suspended in tetrahydrofuran (THF) to form a suspension of transparent nanoparticles that are stable even in the absence of surfactants without the formation of agglomerates [36]. Additional information about the sol–gel hot-injection method is provided in Ref. [37].

5.2.2.4. Metal Reduction

The reduction of salts to obtain colloidal suspensions of metal nanoparticles is the most common synthesis technique [4], with the use of an extensive variety of metal salts, surfactants, and reducing agents. The following materials can be applied as metal precursors: oxides, nitrates, chlorides, acetates, and acetyl ketones [38]. The following materials can be employed as reducing agents: hydrides (such as sodium borohydride), dehydrogenating gases, citrates, ascorbic acid, hydrazine, and ethylene glycol. Depending on the reduction potential of the reducing agents, a reaction may occur at room temperature or at high temperatures [19].
The reduction of gold by citrate is one of the most common examples of metal reduction. In 1951, Turkevitch demonstrated the synthesis of nanoparticles in the scale of 20 nm via the reduction of HAuCl4 in an aqueous medium with sodium citrate [39]. Yonezawa et al. proposed a modification of the synthesis via citrate using 3-mercaptopropionate as a stabilizer. The nanoparticle size can be controlled by varying the stabilizer/precursor ratio [40]. Zhu et al. produced functionalized Au nanoparticles with carboxylic groups via reduction with citrate using 2-mercaptosuccinic acid [41], in which the product was more stable to pH changes.
Nanoreactors have been applied to improve the control of the synthesis processes. The dendrimers are extensively employed because they are macromolecules of tree-like morphology with regularly spaced ramifications, three-dimensional structures and an abundance in surface groups [42]. The advantages of their application are as follows: (1) uniform structure and composition; (2) the stabilization and encapsulation of nanoparticles inside the dendrimers without the formation of agglomerates; (3) the encapsulation of nanoparticles by steric effects; thus, a large part of the particle surface is available to participate in the reactions; and (4) the peripheral groups of dendrimers can be adapted for the control of solubility, which helps in surface adsorption [43]. Fig. 5.4 illustrates the synthesis of metal nanoparticles inside the structure of a dendrimer via reduction by borohydride.
image
Figure 5.4 Schematic of the synthesis of metal nanoparticles via reduction by borohydride inside a dendrimer. R.M. Crooks, M. Zhao, L. Sun, V. Chechik, L.K. Yeung, Dendrimer-encapsulated metal nanoparticles: synthesis, characterization, and applications to catalysis, Acc. Chem. Res. 34 (2001) 181–190 [43].
Polyamidoamine (PAMAM) is one of the most employed nanoreactors because it produces very small nanoparticles with diameters less than 4 nm. This type of method has been applied to the fields of biosensors and catalysis and the fabrication of electronic devices. Crespilho et al. developed an enzymatic biosensor for the detection of glucose. Initially, PAMAM-Au was synthesized by reduction of KAuCl4 solution with formic acid in the presence of G4 PAMAM. PAMAM-Au were alternated with poly(vinylsulfonic acid) (PVS) on ITO electrodes using the self-assembly technique [layer-by-layer (LbL)] to form bilayers. The hexacyanoferrate was electrodeposited, which modified the gold nanoparticles, and the glucose oxidase enzyme was subsequently immobilized on the self-assembled film to form the biosensor [44].

5.3. Properties

As predicted by Feynman, new phenomena govern the characteristics of materials that affect their properties at the nanoscale, which can be properly explored in many applications. When the size of a nanoparticle or nanocrystal is smaller than the wavelength of the incident radiation, its de Broglie wavelength will be comparable to its diameter. Consequently, the conduction electrons can become trapped in the nanoparticles; this effect is known as quantum trapping, which induces alterations in the bandgap and in the electron energy levels of the material. This effect is more pronounced in semiconductor nanoparticles due to an increase in the bandgap that is caused by a decrease in their size, which generates interband transitions that cause electron displacements to higher frequencies. In a simplistic manner, different electronic configurations exist compared with the configurations observed in the same materials at the macroscopic scale. The control of the nanostructure size generates control of the wavelength of the scattered light and, consequently, of the color of the observed sample [6,7,45].
A nanoparticle with a diameter of 10 nm, for example, contains ∼10% of its atoms at the surface, which become more active than the atoms in the volume due to the number of bonds that the volumetric atoms make with the closest neighbors. Consequently, the surface atoms in the nanoparticles have more electron energy levels due to the imperfections or active sites caused by the bonds that are not completed. When an electromagnetic wave is incident on the nanoparticle, these surface conduction electrons interact with the electric field of the incident wave, which induces the polarization of the free electrons in relation to the ions of the crystal structure of the material. These free electrons start to oscillate together from one side to the other side at the surfaces of the nanoparticles. When the incident radiation frequency approaches the oscillation frequency of the conduction electrons (dipolar resonance), strong energy absorption is induced at the surfaces of the nanoparticles (region in which the oscillation occurs), which is known as surface plasmon absorption (which is dependent on the nanoparticle size and shape). To illustrate this phenomenon, Fig. 5.5 (top panels) presents the micrographs of the seeds (a) that originate the rod-shaped gold nanoparticles (b–e) with different aspect ratios (length divided by width). The bottom panels show the optical spectra that correspond to each suspension [46].
image
Figure 5.5 Transmission electron micrographs (top), optical spectra (left) and photographs (right) of gold nanorod aqueous solutions of different aspect ratios (length divided by width). Scale: 500 nm for (a) and (b); 100 nm for (c–e). C.J. Murphy, T.K. Sau, A.M. Gole, C.J. Orendorff, J. Gao, L Gou, et al., Anisotropic metal nanoparticles: synthesis, assembly, and optical applications, J. Phys. Chem. B. 109 (2005) 13857–13870 [46].
The macroscopic metal materials have a continuous conduction band, and the nanoparticles have discrete electronic states that are dependent on their size. In nanometric scale, a significant percentage of the material surface is exposed, which intensifies the surface and interface effects, which, consequently, also produce a significant variation in their magnetic properties [45].
A magnetic field can be created due to the electron movement inside the material, whereas a magnetic model can arise due to the spin orientations inside the sample. Because small magnetic particles tend to form monodomains in the range of 20–2000 nm, the magnetic behavior of the majority of experimental systems can be attributed to the contributions of the effect of size and the interactions among the nanoparticles. The correlation between the nanostructure and its magnetic properties has isolated nanometric particles that generally have small interactions with magnetism due to the spin polarized tunneling effect and oscillatory coupling between magnetic/nonmagnetic multilayers and magnetoresistance. Conversely, volumetric materials with nanoscale structures (nanostructured films), whose magnetic properties are predominantly caused by the interactions involved in these systems, are observed. However, when the nanoparticle size is reduced to a size smaller than the critical diameter, the formation of domains is energetically unfavorable, and the variations in the magnetization are not caused by the movement of domains but by the coherent rotation of spins, which is frequently affected by thermal fluctuations (the system starts to present paramagnetic behaviors) [45]. An example that is often cited in studies is the possibility of the application of magnetic nanoparticles for cancer treatment. The nanoparticles are encapsulated with specific chemical agents that bond in tumor regions; with the control of magnetization, they agitate, which heats the affected region by a hyperthermia process and selectively destroys cancer cells [19].

5.4. Characterization Methods

Various experimental techniques can be employed for the characterization of nanoparticles, such as spectroscopic methods (UV-vis and infrared), XRD, TEM, scanning electron microscopy, and atomic force microscopy (AFM). Some methods and examples of nanoparticle characterization by XRD, TEM, and AFM are presented in the next section.

5.4.1. X-Ray Diffraction

The XRD technique enables the investigation of the crystallinity degree and the size of the crystallites of different materials, including nanoparticles [47], according to the spaces between adjacent atomic planes [48]. With the incidence of X-rays of wavelength λ, with the incidence angle θ on a crystal, they can pass with no interruptions or interact with the atoms of the crystal lattice of the material. Bragg’s law (Eq. 5.5) indicates that diffracted X-rays of various intensities, which represent a specific interplanar distance d in the lattice, can be detected when X-rays of known wavelength and incidence angle are directed to a crystal lattice.

nλ=2 dsenθ

image(5.5)
Scanning at different angles and a constant wavelength generates a diffraction pattern [48]. The determination of the unit cell size and interplanar atomic distances can be evaluated by the angle position of the peaks in the diffractogram, and the atomic arrangement inside each unit cell can be associated with the relative intensities of these peaks [49].
XRD has also been explored in the estimation of nanoparticle size with Scherrer’s equation (Eq. 5.6), which relates the full width at half maximum (FWHM) of the diffraction peak with the crystallite size D [7].

D=kλw cosθ

image(5.6)
where λ is the wavelength of the incidence photons (nm), θ is Bragg’s reflection angle, w is the FWHM and k is a constant that is dependent on geometrical factors [50]. The widening of the peaks represents crystallites of increasingly smaller sizes [51]; thus, nanoparticles present wide diffraction peaks that differ from the materials at the macroscopic scale, which present fine lines in the diffractograms. Note that Scherrer’s equation can be applied to crystallites of average size in the range of approximately 100–200 nm; above these dimensions, the diffraction peak width can be affected by other factors than the crystallite size [52].
Borchert et al. investigated the nanoparticle size of CoPt3 via three different methods: TEM, small-angle X-ray scattering (SAXS) and XRD. The correctly adapted Scherrer’s equation for the near-spherical geometry of CoPt3 nanoparticles provides diameters that are highly consistent with the diameters obtained by the other methods: 8.5 nm by TEM, 8.11 by SAXS and 8.4 with the adapted Scherrer’s equation (Eq. 5.7), which yields a difference less than 5%.

D=430.9λw cosθ

image(5.7)
The method for the determination of the average crystallite size by XRD is indirect, and its use is preferably combined with microscopic techniques. With the appropriate care in the experiments and analyses, the results have been consistent with the values determined by direct methodologies. Details about Scherrer’s equation in the investigation of nanoparticle size are provided in Refs. [50,53].

5.4.2. Transmission Electron Microscopy

TEM is one of the most prevalent techniques for the nanoparticle size and shape analysis. Electrons are generally accelerated at 100 KeV or higher energy values, displaced on a thin layer of the sample, and elastically or inelastically scattered. The inelastic scattering occurs in heterogeneous regions, such as intergrain boundaries, defects, and density variations, and cause scattering effects that produce differences in the intensities of the transferred electrons [7]. The wavelength of the electron beam limits the image resolution, which may attain the Angstrom scale depending on the equipment.
Yonezawa and Kunitake, for example, produced gold nanoparticles that were stabilized in 3-mercaptopropionate reduced via citrate. The TEM investigation provided a detailed analysis of the form and size distribution (Fig. 5.6). Particles presented spherical shape and smaller dimensions when the stabilizer/gold ratio was higher [40]. From the analysis of the histograms, a narrow size distribution was observed in greater stabilizer/gold ratio, and a broad variation in size was observed when the ratio was 0.1.
image
Figure 5.6 Transmission electron microscopy (TEM) and size distribution of gold nanoparticles that were stabilized in 3-mercaptopropionate reduced via citrate, with different stabilizer/gold ratios. T. Yonezawa, T. Kunitake, Practical preparation of anionic mercapto ligand-stabilized gold nanoparticles and their immobilization, Colloids Surf. A 149 (1999) 193–199 [40].

5.4.3. Atomic Force Microscopy

AFM is an important tool for the investigation of the surface topography and roughness at the atomic level [54]. A flexible probe that is sensitive to an interaction force is displaced with a tracking pattern on the surface of a solid sample. The force that acts between the probe and the sample surface causes small deflections in the cantilever, which are detected by optical systems [54]. Surface scanning with a continuous pattern xy, which is performed with a cantilever with a tiny tip that moves up and down along the z-axis according to the topographic change of the surface can be translated by a computer into a topography image [54].
Three modes can be employed for AFM image acquisition: contact, noncontact and intermittent. The contact mode is not adequate for soft surfaces, such as biological samples and polymers, because the tip will be in constant contact with it. In some cases, the damage can be avoided by the intermittent mode, in which the tip is periodically in contact with the sample surface for a short period of time. In the noncontact mode, the tip fluctuates over the surface at a few nanometers of distance [54].
Sruanganurak et al. investigated the modification of the surface of natural latex rubber to minimize the friction effect, which is the main problem of latex application in gloves. Poly(methyl methacrylate) (PMMA) nanoparticles were deposited by the self-assembly technique (LbL) on the natural rubber substrates that were treated with polyacrylamide (natural rubber grafted with polyacrylamide, NR-g-PAAm). The AFM analysis that was employed in the intermittent mode (tapping mode) enabled the analysis of the surface morphology and roughness. In Fig. 5.7, Cs represents the surface coverage, which is expressed by Cs (%) = (N/Nmax)*100, where N is the number of adsorbed particles per unit area and Nmax is the maximum number of adsorbed particles in the same area, assuming hexagonal close packing [55].
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Figure 5.7 Investigation of the average surface roughness (Ra) of NR-g-PAAm covered with poly(methyl methacrylate) (PMMA) particles, with different Cs values. A. Sruanganurak, K. Sanguansap, P. Tangboriboonrat, Layer-by-layer assembled nanoparticles: a novel method for surface modification of natural rubber latex film, Colloids Surf. A 289 (2006) 110–117 [55].
Chartarrayawadee et al. employed the AFM technique to confirm the anchorage of platinum nanoparticles on graphene sheets, as illustrated in Fig. 5.8. The composite was successfully applied for the catalysis in the reactions of solar cells, which are sensitized by dyes, and the generation of hydrogen from acid. According to the topological analysis of the cross-section, the heights of the clusters in the graphene-platinum composite varies from 3 to 6 nm, whereas the diameter varies from 40 to 100 nm. With the amplification of the topological image of a cluster [dashed rectangle in (B)], its dimension and shape can be observed [56].
image
Figure 5.8 Atomic force microscopy (AFM) image of (A) cast film of graphene–platinum colloidal suspension, (B) topology of the cross-section at the line showed in the image in (A), and (C) expanded image of the topology of the cross-section of a cluster [represented by the dashed rectangle in (B)]. Adapted from W. Chartarrayawadee, S.E. Moulton, D. Li, C.O. Too, G.G. Wallace, Novel composite graphene/platinum electro-catalytic electrodes prepared by electrophoretic deposition from colloidal solutions, Electrochim. Acta 60 (2012) 213–223 [56].

5.5. Applications

5.5.1. Biosensors

Nanoparticles can exert many functions in applications such as biosensors, which favor the immobilization of biomolecules and the catalysis of electrochemical reactions, in addition to an increase in the charge transfer and biomolecule labeling [57]. The immobilization of biomolecules in the volume of a material can cause the denaturation or loss of bioactivity; however, the use of nanoparticles enables the immobilization and the preservation of the biocompatibility. The use of nanoparticles with catalytic properties produces high-sensitivity sensors, which attain limits of detection lower than 2 fM [57].
Molecular recognition is one of the most important points of biosensor selectivity because some biological entities can be recognized and linked to one another with high selectivity and specificity [7]. These biological entities include antibodies, oligonucleotides, and enzymes. The antibodies—proteins of the immune system—recognize a virus as an intruder and bond to viruses to destroy them. For example, antibodies and oligonucleotides can easily bond to the surfaces of nanoparticles via thiol–Au bonds in the case of gold nanoparticles or covalent bonds at silanized surfaces via biotin–avidin bonds, in which avidin is bonded to the material surface [7].
Gold nanoparticles have been applied for the amplification of the analytical signal due to its ease of synthesis, narrow size distribution, efficient surface modification by thiols and other ligands and biocompatibility [58]. The metal nanoparticles can be guided to specific regions using the antigen-antibody recognition in the ligand–receptor interactions, for example, by bonding to cancer cells [19].
Biosensors that are based on molecular recognition, such as the antigen–antibody interaction, are referred to as immunosensors [59]. Li et al. developed a fast-detection immunosensor for Escherichia coli. The gold electrode was treated to receive -NH2 functional groups and after, alternated nanoparticle layers of gold and the chitosan–MWNT–thionine composite were produced via the LbL technique. Voltammetric readings guarantee the modification of electrodes, and the sensitivity and stability of the sensor are related to the amount of thionine mediator [60]. Subsequently, anti-E. coli O157:H7 was immobilized, followed by a period of incubation for 60 min in bovine serum albumin. The sensor proved to be efficient for the detection of E. coli in milk and water samples.

5.5.2. Catalysis

Since the surface atoms of nanoparticles are more reactive than the volumetric atoms of nanoparticles, they have sites with missing bonds, which increases the activity of nanoparticles and provides additional electronic states and a higher reactivity [6].
Fig. 5.9 illustrates the increase in surface area with the decrease in the volume size of the nanoparticles and the increase in surface area, which are proportional to its reactivity and catalytic activity.
image
Figure 5.9 Increase in surface area, which is proportional to its reactivity and catalytic activity.
The versatility of the nanoparticles enables their use as homogeneous and heterogeneous catalysts. In the first case, only the nanoparticle is employed; in the second case, it can be anchored or deposited on a substrate. Nanoparticles exhibit excellent performance as catalysts in many areas, including dehydrogenation, halogenation, oxidation, reduction, decomposition, and electron transfer reactions, and the catalytic efficiency is dependent on their shape, composition and size [6].
For example, the catalysis of oxidoreduction in fuel cells, in which a suitable catalyst is required for efficient redox reactions, can be cited. Metal colloidal particles, especially platinum, are very interesting due to their catalytic action in methanol oxidation reactions and oxygen reduction, which are responsible for the energy generation in direct methanol fuel cells (DMFC) [61]. Platinum nanoparticles have been applied with graphene as LbL films, and their catalytic activities were analyzed by cyclic voltammetry [62]. Graphene was modified with an ionic liquid to obtain a positively charged suspension, whereas the platinum nanoparticles were stabilized in citrate with negative charges. The LbL film that electrostatically formed presented high electrocatalytic activity in the reduction of oxygen [62]. The production of composite catalysts with more than one metal (generally Pt alloys) is interesting for applications in methanol oxidation in DMFCs. Yola et al. synthesized catalysts composed of Pt nanoparticles and bimetal particles (contain gold and silver with platinum) that were anchored in functionalized graphene oxide for the oxidation of methanol. The formed Au-Pt/GO was superior than other tested structures, which indicates a wide active surface, high electrocatalytic activity, and high tolerability to contamination by carbon monoxide (product of the methanol oxidation reaction) [63]. To reduce the mass of Pt that is required by the electrode and reduce the cost of energy generation, Zhao et al. synthesized core-shell-structured nanoparticles, an Au nucleus and a shell composed of Pt and Cu alloy to obtain a larger electrochemically active area that is superior to other structures (PtCu/C, AuPtCu/C and commercial Pt/C) due to the synergic effects between the nanostructured core and the shell with uniform dispersion [64].
The synthesis process by the Rampino and Nord method, for example, is extensively applied for the production of an aqueous suspension of platinum nanoparticles. This method involves K2PtCl4 and polyacrylic acid solution that receives argon gas and is reduced by hydrogen gas [65]. Hao-Lin et al. proposed the modified synthesis of platinum nanoparticles with Nafion. The particles presented a size of approximately 4 nm, were adsorbed by the surfaces of the carbon nanotubes and employed as catalysts in proton exchange membrane fuel cells, which demonstrate a superior performance [66]. Other researchers applied in situ polymerization with platinum [67,68] for the modification of Nafion membranes to target the application in DMFCs. The in situ polymerization of pyrrole was performed on the Nafion membrane, followed by the reduction of platinum salt to form nanoparticles. The methanol permeability (one of the main problems in DMFCs) decreased with the time of pyrrole monomer impregnation [67].

5.5.3. Magnetic Nanoparticles in Biomedicine

The quantum effect and the large surface area of the magnetic nanoparticles produce a material with altered magnetic properties, which presents a superparamagnetic phenomenon because each particle is considered to be a single magnetic domain [1]. Factors such as the biocompatibility and possible functionalization of the surfaces of magnetic nanoparticles elevate the potential for their use in biomedical applications.
The magnetic nanoparticles can be synthesized and encapsulated by polymers, which enables the functionalization of the surface according to the objective. Thus, different biological molecules, such as antibodies, proteins, and target ligands, can bond to the surfaces of these nanoparticles by amide or ester chemical bonds [1]. Among the most investigated applications are the controlled release of pharmaceuticals, applications in hyperthermia, imaging by nuclear magnetic resonance and the separation and selection of molecules [3,69].
The iron oxides are the most investigated materials, with an emphasis on maghemite (γ-Fe2O3) and magnetite (Fe2O3). The possibility of nanoparticles to be controlled by an external magnetic field enables their conduction through specific body parts and favors the release of medication in certain regions. The use of nanometric systems enables the direct delivery of the drug to the cells or specific tumor regions surrounded by a healthy tissue [70]. Peptides and proteins can bond to nanoparticles, permeate membranes and enable intracellular drug delivery [1]. When subjected to an external magnetic field of alternated frequency, the magnetic nanoparticles cause the heating of the location where they are located (magneto hyperthermia, as previously mentioned). Since tumor cells are more sensitive to a significant temperature increase than normal cells, they are destroyed at temperatures between 41 and 47°C [1,69].
Tissue regeneration is another well-explored area because stem cells have significant potential for substituting degenerated cells or repairing damaged tissue [1]. Superparamagnetic nanoparticles can be associated with stem cells to be transported to the location of interest. Many proteins and growth factors can be associated with these nanoparticles and, therefore, are distributed in the damaged tissue, where they can act to regenerate the affected area.

5.6. Final Considerations

The unique properties of nanoparticles have been extensively explored by the most diverse fields, which corroborates the prediction by Feynman regarding the potential of nanostructures. The changes in the physical and chemical properties of the nanoparticles are related to the increase in the number of surface atoms in relation to the volume. Similarly, the electronic properties are altered by the formation of discrete energy levels instead of the conventional band structure for volumetric materials. This chapter discussed typical synthesis processes for the production of metal and oxide nanoparticles according to the need of application. Additionally, the improvements in the processes of material characterization during the last decade significantly contributed to the investigation of nanoparticles. Although various studies have been performed, additional exploration is needed.

List of Symbols

C Solute concentration

C0 Solubility

Cs Surface coverage

d Interplanar distance

D Particle diameter

σ Concentration of supersaturation

∆Gv Gibbs free energy of volume

∆μv Surface energy

γ Surface energy per unit area

k Constant dependent on geometric factors

K Boltzmann constant

λ Wavelength

N Number of particles adsorbed per unit area

Nmax Maximum number of particles adsorbed per unit area

nm Nanometers

Ω Atomic volume

r Radius of spherical nucleus

r* Critical radius

Ra Roughness average

T Temperature

θ Incidence angle in relation to the considered plane

w Full width at half maximum

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