**2. Inter-Matrix synthesis in ion exchange matrices**

40 Ion Exchange Technologies

stability against aggregation and oxidation while nanoparticles prepared in the absence of

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).

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

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

c

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

8kT <sup>=</sup> <sup>η</sup> k (1)

polymers are prone to quick aggregation and oxidation.[18, 23]

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

interaction in PSMNP systems versus non-stabilized MNP dispersions. [24]

the Boltzman constant, and T is the temperature):

can be given by the following equation:

The ion-exchange synthesis of Metal Nanoparticles (MNPs) refers to a group of methods which can be generally classified as Inter-Matrix Synthesis (IMS) technique. The main feature of IMS is the dual function of the matrix, which allows the stabilization of the MNPs to prevent their uncontrollable growth and aggregation and provides a medium for the synthesis.

It is noteworthy that IMS was essentially the first method employed by the humans to incorporate nanoparticles inside inorganic materials. In this sense, one of the oldest nanocomposite materials found is the "Lycurgus cup" which dates back to the late 4th century B.C.[26] and it is made of a sort of glass that changes its colours depending on the incident light: in reflected light the glass turns green, but when the light is shone directly through it, it turns red due to the presence of small amount of Ag-Au-MNPs with the diameter of approximately 70 nm. It is remarkable that Greco-Roman techniques have been used up to modern times: related recipes were described by Arabian authors during the medieval period [27], during the Renaissance as practical application of alchemical knowledge[28], and by modern chemists, from the Encyclopedie of Diderot and d'Alembert [29] through to the present day.

Another example is the "lustre pottery"[30] employed at the same time in Asian an European countries by a simple two-step procedure:


Examples of this ceramics are showed in Figure 6.

Although, as it has been shown above, humanity had used the properties of the nanoscale materials for a long time, the fundamentals of the scientific Nanoscience and Nanotechnology studies did not appear since the middle of the XIXth. As an example, in 1949 the first communication of the IMS is published by Mills and Dickinson.[31] In this pioneered publication it is described the preparation of the anionic resin containing Cu-MNP ("colloid copper") and the use of this nanocomposite material for the removal of oxygen from water due to its interaction with copper MNPs. Since that, a great number of researchers focused their efforts to the development of a new class of ion-exchange materials, combining ion-exchange and redox properties (known also as "redoxites" or "electron-ion exchangers").

**Figure 6.** Lustre Pottery. Fatimid-sherds excavated from Fustât; (a) the sample and (b) sample with an orientation that corresponds to the diffraction angle and lustre shining is observed.
