**1. Introduction**

34 Ion Exchange Technologies

[66] Tsujimoto Y, Kitada A, Uemura Y. J, Goko T, Aczel A. A, Williams T. J, Luke G. M, Narumi Y, Kindo K, Nishi M, Ajiro Y, Yoshimura K, Kageyama H. Two-Dimensional *S*

= 1 Quantum Antiferromagnet (NiCl)Sr2Ta3O10. Chem. Mater. 2010;22 4625-4631.

The unusual electrical, optical, magnetic, and chemical properties of metal colloids (better known in nowadays as metal nanoparticles, MNPs) have attracted increasing interest of scientists and technologists during the last decade. In fact, although Nanoscience and Nanotechnology are quite recent disciplines, there have already been a high number of publications that discuss these topics. [1-11] What is more, there are quite new high impact peer-reviewed journals especially devoted to these research fields and there is also a particular subject category "Nanoscience & Nanotechnology" in the Journal Citation Reports from Thomson Reuters.

MNPs can be obtained by various synthetic routes, such as electrochemical methods, decomposition of organometallic precursors, reduction of metal salts in the presence of suitable (monomeric or polymeric) stabilizers, or vapour deposition methods. Sometimes, the presence of stabilizers is required to prevent the agglomeration of nanoclusters by providing a steric and/or electrostatic barrier between particles and, in addition, the stabilizers play a crucial role in controlling both the size and shape of nanoparticles.

In this sense, the development of polymer-stabilized MNPs (PSMNPs) is considered to be one of the most promising solutions to the MNPs stability problem. For this reason, the incorporation of MNPs into polymeric matrices has drawn a great deal of attention within the last decade as polymer-metal nanocomposites have already demonstrated unusual and valuable properties in many practical applications.

The modification of commercially available ion exchange resins and the development of suitable polymeric membranes with metal nanoparticles (MNPs) having certain functionality, such as for example, biocide or catalytic activity has proved to be a theme of

great interest. The main advantage of the nanocomposite ion exchange materials is the location of metal nanoparticles near the surface of the polymer what substantially enhances the efficiency of their biocide and catalytic application.

## **1.1. Metal nanoparticles**

The most commonly accepted definition for a nanomaterial is "a material that has a structure in which at least one of its phases has a nanometer size in at least one dimension." [12] Regarding this definition, it is possible to classify nanoobjects in three groups:


Such materials include porous materials (with porous sizes in the nanometer range), polycrystalline materials (with nanometer-sized crystallites), materials with surface protrusions separated by nanometric distances, or nanometer-sized metallic clusters. Among all of these materials, metal nanoparticles (MNPs) have attracted increasing interest of scientists and technologists during the last decade, due to their unique electrical, optical, magnetic, and chemical properties.

**Figure 1.** Bibliographic analysis based on the search of "nanoparticles" in Scifinder Scholar. Blue represents scientific publications (e.g. books, articles, reviews) and green represents patents.

Within the last two decades a new focus has been initiated to control and to better understand the nanometer-size objects due to the appearance of a new interdisciplinary field, which is known now as Nanoscience and Nanotechnology. This has stimulated a new wave of intensive and more detailed studies of MNPs and various nanocomposites on their base. Figure 1 shows the tendency in publication about this issue, where it is shown that the number of publications has increased exponentially due to their wide applications in different fields such as Medicine, Chemistry, and Physics and so on. Moreover, not only scientific publications have been growing in the last decades but also a huge number of patents have been issued in the last decade.

The main goal of Nanoscience and Nanotechnology is the creation of useful/functional materials, devices and systems through the control of matter on the nanometer length scale and exploitation of novel phenomena and properties (physical, chemical and biological) at that scale. To achieve such goal it is necessary to use a multidisciplinary approach: inputs from physicists, biologists, chemists and engineers are required for the advancement of the understanding in the preparation, application and impact of new nanotechnologies.

### **1.2. General properties**

36 Ion Exchange Technologies

**1.1. Metal nanoparticles** 

great interest. The main advantage of the nanocomposite ion exchange materials is the location of metal nanoparticles near the surface of the polymer what substantially enhances

The most commonly accepted definition for a nanomaterial is "a material that has a structure in which at least one of its phases has a nanometer size in at least one dimension."

Such materials include porous materials (with porous sizes in the nanometer range), polycrystalline materials (with nanometer-sized crystallites), materials with surface protrusions separated by nanometric distances, or nanometer-sized metallic clusters. Among all of these materials, metal nanoparticles (MNPs) have attracted increasing interest of scientists and technologists during the last decade, due to their unique electrical, optical,

**Figure 1.** Bibliographic analysis based on the search of "nanoparticles" in Scifinder Scholar. Blue represents scientific publications (e.g. books, articles, reviews) and green represents patents.

Within the last two decades a new focus has been initiated to control and to better understand the nanometer-size objects due to the appearance of a new interdisciplinary field, which is known now as Nanoscience and Nanotechnology. This has stimulated a new wave of intensive and more detailed studies of MNPs and various nanocomposites on their base. Figure 1 shows the tendency in publication about this issue, where it is shown that the

[12] Regarding this definition, it is possible to classify nanoobjects in three groups:

ii. 2D nanometer-size objects (e.g., nanowires, nanorods and nanotubes) iii. 3D nanometer-size objects (e.g. nanoparticles and/or nanoclusters)

the efficiency of their biocide and catalytic application.

i. 1D nanometer-size objects (e.g., thin films)

magnetic, and chemical properties.

Because of the decrease in the scale of the materials, their behaviour changes in a remarkable form. In fact, the reduction of the bulk materials to a nanometric size induces sizedependant effects resultant from:


**Figure 2.** Schematic representation of the change in the ratio surface/volume between a bulk microsphere and the same microsphere composed by NPs.

This can be illustrated, for example, by the dependence of gold melting point on the size of gold nanoparticles, or by suspensions of Ag nanoparticles with sizes ranging from 40 to 100 nm which show different colours. [13] In addition, there are physical phenomena that do not exist in materials with larger grain sizes, as the general quantum-size effect for optical transitions in semiconductor nanocrystals which occurs in very small nanoparticles (<10 nm) due to the quantum confinement effects inherent in particles of that size.

Is due to all of these new properties that, indeed, research centred on nanoscopic materials have a large field of application which extends from the semiconductor industry, where the ability to produce nanometer-scale features leads to faster and less expensive transistors [6], to biotechnology, where luminescent nanoparticles are extremely interesting as bioprobes. [14] Some other particular examples are catalyst for fuel cells [15] or electrocatalysts used in sensing devices with enhanced properties. But, as a rule of thumb, nanoparticles, due to the large percentage of surface atoms [12, 16], have already made a major impact on the field of surface science, as Catalysis or Biocide treatment.

## **1.3. MNPs Preparation: Stability challenges and stabilization mechanisms**

In general there are two routes for the preparation of MNPs (see Figure 3):-Down and Bottom-Up. The top-down methods are those that reduce the macroscopic particles to the nanoscale. This route is not very suitable to prepare uniform particles of very small sizes. In contrast, with the bottom-up methods it is possible to obtain uniform particles (usually of different shapes and structures). These routes start from atoms that can be added (either in solution or gas phase) to form larger particles.

**Figure 3.** Scheme of Top-Down and Bottom-Up approaches to the synthesis of MNPs.

Overall, a good method to classify the different methods of synthesis of MNPs is by Physical, Physicochemical and Chemical routes (See Figure 4). [13] Many synthetic pathways can be used but the chemical ones are generally cheaper and do not require equipment or instruments as specific as in the case of physical methods the physical methods.

However, the main drawback which still limits the wide application of MNPs is their insufficient stability dealing with their high tendency to self-aggregate. [17] MNPs are so reactive that when they touch each other, they surfaces fuse, what results in a loss of the nanometric size and in their special properties. These features of nanoparticles, in part determined by the conditions of synthesis, create enormous difficulties in their fabrication and application. [18]

**Figure 4.** Physical, Physicochemical and Chemical routes for the preparation of MNPs.

It is noteworthy that NPs can aggregate not only as a result of a further manipulation but also during their growth. A typical mechanism of aggregation is the Ostwald ripening which is a growth mechanism where small particles dissolve, and are consumed by larger particles. [19] So, the average nanoparticle size increases with time, the particle concentration decreases and their solubility diminishes.

Therefore, the stabilization of MNPs is specifically required to:


38 Ion Exchange Technologies

This can be illustrated, for example, by the dependence of gold melting point on the size of gold nanoparticles, or by suspensions of Ag nanoparticles with sizes ranging from 40 to 100 nm which show different colours. [13] In addition, there are physical phenomena that do not exist in materials with larger grain sizes, as the general quantum-size effect for optical transitions in semiconductor nanocrystals which occurs in very small nanoparticles (<10 nm)

Is due to all of these new properties that, indeed, research centred on nanoscopic materials have a large field of application which extends from the semiconductor industry, where the ability to produce nanometer-scale features leads to faster and less expensive transistors [6], to biotechnology, where luminescent nanoparticles are extremely interesting as bioprobes. [14] Some other particular examples are catalyst for fuel cells [15] or electrocatalysts used in sensing devices with enhanced properties. But, as a rule of thumb, nanoparticles, due to the large percentage of surface atoms [12, 16], have already made a major impact on the field of

**1.3. MNPs Preparation: Stability challenges and stabilization mechanisms** 

**Figure 3.** Scheme of Top-Down and Bottom-Up approaches to the synthesis of MNPs.

instruments as specific as in the case of physical methods the physical methods.

Overall, a good method to classify the different methods of synthesis of MNPs is by Physical, Physicochemical and Chemical routes (See Figure 4). [13] Many synthetic pathways can be used but the chemical ones are generally cheaper and do not require equipment or

However, the main drawback which still limits the wide application of MNPs is their insufficient stability dealing with their high tendency to self-aggregate. [17] MNPs are so reactive that when they touch each other, they surfaces fuse, what results in a loss of the nanometric size and in their special properties. These features of nanoparticles, in part

In general there are two routes for the preparation of MNPs (see Figure 3):-Down and Bottom-Up. The top-down methods are those that reduce the macroscopic particles to the nanoscale. This route is not very suitable to prepare uniform particles of very small sizes. In contrast, with the bottom-up methods it is possible to obtain uniform particles (usually of different shapes and structures). These routes start from atoms that can be added (either in

due to the quantum confinement effects inherent in particles of that size.

surface science, as Catalysis or Biocide treatment.

solution or gas phase) to form larger particles.


The successful synthesis of nanoparticles usually involves three steps: nucleation, growth, and termination by a capping agent or ligand (or stabilizing agent) through colloidal forces.[20] These colloidal forces can be classified in three main types as follows: Van der Waals interactions, electrical double-layer interactions, and steric interactions. In addition, hydrophobic and solvation forces may be important. [21]

Some of the mechanisms regarding the stabilization forces have been thoroughly revised in the literature.[13, 22]

Specially the use polymer-assisted fabrication of inorganic nanoparticles is probably one of the most efficient and universal ways to overcome the stability problem of MNPs and to save their properties. Metal nanoparticles synthesized by this approach exhibit long-time stability against aggregation and oxidation while nanoparticles prepared in the absence of polymers are prone to quick aggregation and oxidation.[18, 23]

In this sense, stabilization of MNPs can be done by different strategies. In the ex-situ synthesis, NPs are dispersed after their synthesis in a solid or liquid medium by using different mechanochemical approaches. The problem is that in these cases, the success of the stabilization is limited by the possibility of re-aggregation of the MNPs along the time. On the opposite hand, by the in-situ synthesis, MNPs are grown directly in the stabilizer medium yielding a material that can be directly used for a foreseen purpose. For this reason, in-situ approaches are getting much attention, because of their technological advantages (Figure 5).

**Figure 5.** Schematic comparison between in-situ and ex-situ methodologies.

Despite the methodology employed, it is of crucial importance to understand the processes occurring in polymer interactions with nanoparticles. In this regard, the mechanism of MNP stabilization with polymers can be explained by two approaches which run simultaneously in the system and influence one another: the substantial increase of viscosity of the immobilizing media (the polymer matrix), and the decrease of the energy of particle-particle interaction in PSMNP systems versus non-stabilized MNP dispersions. [24]

In the first approach, the substantial increase of viscosity of the immobilizing media (the polymer matrix), the Coagulation velocity depends on factors as the range of attraction forces, Brownian motion velocity, concentration of colloidal solution, presence of electrolytes… As follows from the Smoluchowsky equation [25], the rate constant of particle coagulation, kc, is inversely proportional to the viscosity of the media, , (here k stands for the Boltzman constant, and T is the temperature):

$$\mathbf{k}\_c = \frac{8\mathbf{k}\,\mathrm{T}}{\eta} \tag{1}$$

The second approach is the decrease of the energy of particle-particle interaction in PSMNP systems versus non-stabilized MNP dispersions. The potential energy of attraction Ur between two spherical particles of radius r and minimum distance lo between their surfaces can be given by the following equation:

Bifunctional Polymer-Metal Nanocomposite Ion Exchange Materials 41

$$\mathbf{U}\_r \approx \frac{\mathbf{A}\mathbf{r}}{12 \ l\_o} \text{ at } \mathbf{r} \gg \mathbf{l}\_o \tag{2}$$

where A is the effective Hamaker's constant with dimensions of energy. The value of A is known to be close to *kT* for polymer particles (e.g 6.310-20J for polystyrene), while for the metal dispersions it is far higher (4010-20 J for silver). [24]
