**2.1. General principles**

This section describes the principles of the IMS in ionic exchange matrices. This technique takes advantatges of the in-situ approach (above mentioned), and has a wide range of application because of the multiple metal-polymer existing possibilities. In this sense, even if the number of polymers is reduced to those with ion exchange capacities, the multiple possibilities remain and a different number of polymer-nanocomposites can be obtained. Figure 7 shows the multiple possibilities of the IMS methodology.

**Figure 7.** Scheme of the multiple possibilities of Ion-Matrix Synthesis.

The general principles of IMS, valid for any kind of polymer matrix and type of nanoparticle, are based on:


These guidelines are only achievable if NPs precursors can properly be immobilized in the polymeric matrix. In this sense, Ion Exchange matrices are the perfect template to retain the ionic species, either metal cations, anions or any kind of coordination compound.

To illustrate this approach, two anion charged groups as sulfonic group (SO3- ) can efficiently interact with the metal cation (M12+) which afterwards can undergo a chemical reaction (precipitation, reduction, etc.) which finally will yield to the formation of the MNP.

The same can be done for an anion exchange matrix bearing a functional group such as a quaternary ammonium (–NR4+), capable to immobilize metal complexes (i.e. [CoCl4]2-)or other anions (i.e. BH4- ).

Figure 8 illustrate the two main consecutive stages which rule the IMS technique: (i) the immobilization of the metal ion or complex (in a Cation Exchange Matrix, CEM) or immobilization of the reductant (in an Anion Exchange Matrix, AEM) and (ii) the reduction of the metal ion inside the matrix.

**Figure 8.** Monometallic MNPs preparation inside a polymeric ion-exchange matrix by IMS. Green spheres represent the M12+ cation, and the black ones the MNPs obtained after the reduction.

These two stages may be described by the following equations (equations 3-4 and 5-6) considering the presence of strong acid groups (2R-SO3- ) in the CEMs and the presence of strong basic groups (R-R3N+) in the AEMs. M1 represents a divalent metal and R the organic radical.

For CEMs:

42 Ion Exchange Technologies

**2.1. General principles** 

**Figure 6.** Lustre Pottery. Fatimid-sherds excavated from Fustât; (a) the sample and (b) sample with an

This section describes the principles of the IMS in ionic exchange matrices. This technique takes advantatges of the in-situ approach (above mentioned), and has a wide range of application because of the multiple metal-polymer existing possibilities. In this sense, even if the number of polymers is reduced to those with ion exchange capacities, the multiple possibilities remain and a different number of polymer-nanocomposites can be obtained.

orientation that corresponds to the diffraction angle and lustre shining is observed.

Figure 7 shows the multiple possibilities of the IMS methodology.

**Figure 7.** Scheme of the multiple possibilities of Ion-Matrix Synthesis.

$$2\left(\text{R}-\text{SO}\_3\text{ Na}^+\right) + \text{M}\_1^{2+} \leftrightarrow \left(\text{R}-\text{SO}\_3^-\right)\_2\text{M}\_1^{2+} + 2\text{ Na}^+ \tag{3}$$

$$\left(\text{R}-\text{SO}\_3^{-}\right)\_2\text{M}\_1^{2+} + 2\text{ NaBH}\_4 + 6\text{H}\_2\text{O} \rightarrow 2\left(\text{R}-\text{SO}\_3^{-}\text{Na}^{+}\right) + 7\text{H}\_2 + 2\text{B}\left(\text{OH}\right)\_3 + \text{M}\_1^{0\text{-}} \tag{4}$$

For AEMs:

$$\text{R}-\text{(R}\_{3}\text{N)}^{+}\text{Cl}^{-}+\text{NaBH}\_{4} \leftrightarrow \text{R}-\text{(R}\_{3}\text{N)}^{+}\text{BH}\_{4}^{-}+\text{NaCl}\tag{5}$$

$$\begin{aligned} 2\left(\mathbf{R} - \left(\mathbf{R}\_3\mathbf{N}\right)^+\mathbf{B}\mathbf{H}\_4^{-\top}\right) + \left[\mathbf{M}\_1\right]^{2+}\mathbf{C}\mathbf{I}\_2 + 6\mathbf{H}\_2\mathbf{O} &\to 2\left(\mathbf{R} - \left(\mathbf{R}\_3\mathbf{N}\right)^+\mathbf{C}\mathbf{I}^{-\top}\right) + \\ &+ 7\mathbf{H}\_2 + 2\mathbf{B}\left(\mathbf{O}\mathbf{H}\right)\_3 + \mathbf{M}\_1^{\;0} \end{aligned} \tag{6}$$

A deeper look in the second stage reveals that the MNPs formation (equations 4 and 6) is, indeed, a combination of an ion-exchange reaction and a reduction reaction, accordingly the reduction of the metal ions to the zero-valent metal takes place in the solution boundary, close to the ion exchange groups:

For CEMs:

$$\left(\text{R}-\text{SO}\_3^{-}\right)\_2\text{M}\_1^{2+} + 2\text{ Na}^+ \leftrightarrow 2\left(\text{R}-\text{SO}\_3^{-}\text{Na}^+\right) + \text{M}\_1^{2+} \tag{7}$$

$$\text{M}\_1\text{}^{2+} + 2\text{ BH}\_4^- + 6\text{H}\_2\text{O} \rightarrow 7\text{H}\_2 + 2\text{B(OH)}\_3 + \text{M}\_1^{0\text{}}\tag{8}$$

For AEMs:

$$\text{R}-\text{\text{(}R\_3N)}^{+}\text{BH}\_4^- + \text{Cl}^- \leftrightarrow \text{R}-\text{\text{(}R\_3N)}^{+}\text{Cl}^- + \text{BH}\_4^- \tag{9}$$

$$\text{M}\_1\text{}^{2+} + 2\text{ BH}\_4^- + 6\text{H}\_2\text{O} \rightarrow 7\text{H}\_2 + 2\text{B(OH)}\_3 + \text{M}\_1^{0\text{}}\tag{10}$$

Additionally, as it can be seen from equations 7 and 9 in both cases the matrix is regenerated after the second stage of the IMS, so it is possible to apply consecutive IMS cycles to increase the total loaded metal [32] or to obtain bimetallic nanoparticles (core-shell, alloys or coresandwich).

**Figure 9.** Bimetallic core-shell MNPs preparation. Black spheres represent the MNPs obtained after he first loaing-reduction cycle, blue spheres the M22+ cations and the pink ones are the final core-shell MNPs.

In Figure 9, there is an schematic representation for the preparation of core-shell NPs by coating the monometallic MNPs obtained after the first cycle with a secondary functional metal shell. As follows, the formation of core-shell MNPs (M1-M2, represented as M2@M1 where M2 is the coating and M1 is the core) allows modification of charge and functionality, improves the stability or combines the properties of both metals to make their future applications more efficient. The final activity of the nanocomposite is determined by the properties of the shell metal, although in some cases the properties of the metal core may add an additional advantage to the final nanocomposite (e.g. magnetic core).

To better understand the procedure shown in Figure 9, let us consider the reactions corresponding to the IMS of PSMNPs inside the parent polymeric matrix after the first loading-reduction cycle for example in the CEM:

$$2\left(\text{R}-\text{SO}\_3^-\text{Na}^+\right) + \text{M}\_1^{\text{0}} + \text{M}\_2^{\text{2+}} \leftrightarrow \left(\text{R}-\text{SO}\_3^-\right)\_2\text{M}\_2^{\text{2+}} + \text{M}\_1^{\text{0}} + 2\text{ Na}^+ \tag{11}$$

$$\begin{aligned} \left(\text{R}-\text{SO}\_3^-\right)\_2\text{M}\_2\text{}^{2+} + \text{M}\_1\text{}^0 + 2\text{NaBH}\_4 + 6\text{H}\_2\text{O} &\to 2\left(\text{R}-\text{SO}\_3^-\text{Na}^+\right) + \\ &+ 7\text{H}\_2 + 2\text{B}\{\text{OH}\}\_3 + \text{M}\_2\text{@M}\_1 \end{aligned} \tag{12}$$

According to some authors [33], the second metal ion can act as an oxidizing agent towards the core-metal (M10) resulting in the oxidation of the first metal by the following transmetallation reaction:

$$\text{M}\_2\text{}^{2+} + 2\text{M}\_1\text{}^0 \rightarrow \text{M}\_1\text{}^{2+} + \text{M}\_2\text{}^0 \oplus \text{M}\_1\text{}^0 \tag{13}$$

As it can be seen, in the general IMS procedure described before, there are always two species bearing the same charge: the matrix and the reducing agent (in CEMs) or the metal ion (in AEMs). This means that there is an electrostatic repulsion between the matrix and one of the species mentioned that impede the penetration inside the polymeric matrix, referred to as Donnan-exclusion effect [34, 35].

The Donnan-exclusion effect is based on the exclusion (inability to deeply penetrate inside the polymer) of co-ions when the sign of their charge coincides with that of the polymer

**Figure 10.** Donnan Exclusion Effect.

44 Ion Exchange Technologies

close to the ion exchange groups:

For AEMs:

For CEMs:

For AEMs:

sandwich).

 <sup>2</sup> <sup>0</sup> 31 4 2 3 2 <sup>1</sup> <sup>3</sup> <sup>2</sup> R SO M 2 NaBH 6H O 2 R SO Na 7H 2B OH M (4)

R R N Cl NaBH R R N BH NaCl <sup>3</sup> 4 34

A deeper look in the second stage reveals that the MNPs formation (equations 4 and 6) is, indeed, a combination of an ion-exchange reaction and a reduction reaction, accordingly the reduction of the metal ions to the zero-valent metal takes place in the solution boundary,

> 2 2 3 1 3 1 <sup>2</sup>

R R N BH Cl R R N Cl BH 3 4 3 4

2 0

Additionally, as it can be seen from equations 7 and 9 in both cases the matrix is regenerated after the second stage of the IMS, so it is possible to apply consecutive IMS cycles to increase the total loaded metal [32] or to obtain bimetallic nanoparticles (core-shell, alloys or core-

**Figure 9.** Bimetallic core-shell MNPs preparation. Black spheres represent the MNPs obtained after he first loaing-reduction cycle, blue spheres the M22+ cations and the pink ones are the final core-shell MNPs.

2 0

2 R R N BH M Cl 6H O 2 R R N Cl

2 3 4 12 2 3

(5)

R SO M 2 Na 2 R SO Na M (7)

1 42 2 <sup>1</sup> <sup>3</sup> M 2 BH 6H O 7H 2B OH M (8)

(9)

1 42 2 <sup>1</sup> <sup>3</sup> M 2 BH 6H O 7H 2B OH M (10)

7H 2B OH M

2 1 3

0

(6)

functional groups. Consequently, ion penetration inside the matrix is balanced by the sum of two driving forces acting in opposite directions: the gradient of the ion concentration and the Donnan-effect itself. The result of these two driving forces is the formation of the MNPs mainly near the surface of the polymer matrix (see Figure 10).

Regarding the final application of the nanocomposite, this is a really suitable distribution, since MNPs remain maximally accessible for substrates of interest such as chemical reagents or bacteria.
