**4. Synthesis of nanoparticles in scCO2-based microemulsions**

Nanoparticles with tunable size and high monodispersity present great interest for biomedical application, in particular to be used in bioimaging or as drug delivery systems. Synthesis of inorganic and organic NPs in scCO2 as green alternative to the physical and chemical traditional methods suffer from the same difficulties in controlling the nanoparticle growth step and to ensure narrow size distribution of the final product. One of the most efficient routes developed to fabricate monodisperse nanoparticles is the synthesis performed in heterogeneous media, i.e., in microemulsions or in blue emulsions, in order to ensure the nucleation and growth of nanoparticles in the restricted domain of liquid nanoreactors (oil or aqueous droplets). Microemulsions, as liquid-liquid colloidal systems consisting of nanometric droplets with size ranging from few nanometers to 100, stabilized with surfactant and cosurfactant mixture, are considered the most suitable reaction media for the synthesis of organic and inorganic nanoparticles with tunable size and morphology. Oil-in-water microemulsions are usually used for the fabrication of hydrophobic polymeric nanoparticles, while water-in-oil ones are used as reaction media for inorganic nanoparticles, especially for metal NPs obtained from reduction of metallic salt. All advantages of the controlled synthesis in water-in-oil microemulsions could be preserved using water-in scCO2 microemulsions, since the general structure of the colloidal system is the same (**Figure 7**).

However, the stabilization of miniemulsions and the formation of thermodynamically stable microemulsions in supercritical CO2 are challenging tasks, due to the specific properties of carbon dioxide. Supercritical carbon dioxide possesses peculiar properties such as very high diffusivity and low viscosity, different from usual oily phases in the microemulsion composition; thus special requirements are

**Figure 7.** *The schematic view of structure and stabilization of water droplets in W/O and W/scCO2 microemulsions.*

**169**

*Synthesis and Functionalization of Nanoparticles in Supercritical CO2*

aggregation and leads to the stabilization of microemulsion [21].

needed for the surfactants that enable the formation of W-in-scCO2 microemulsion: (i) enhanced absorbability at water/scCO2 interface, (ii) enhanced surface activity in lowering water/scCO2 interfacial tension, and (iii) strong repulsive interaction (steric effect) between CO2-philic groups, which reduces the probability of droplet

Many researches have been performed addressing the problem of surfactant solubility in supercritical and dense CO2, in order to evidence their ability to stabilize water-in-CO2 microemulsions, and surfactants with special designed molecular

The early studies [22] conclude that conventional theories in molecular design

for surfactant with superior efficiency in lowering interfacial tension at W/O interfaces are inapplicable to the W-scCO2 ones, because CO2-philicity could not be directly compared with oleophilicity. Considerable efforts have been made to find the most favorable chemical structures able to ensure adequate solubility of the surfactant in supercritical CO2, together with better adsorption at W-scCO2 interface. Highly branched hydrocarbon surfactants were found to possess good solubility, but unfortunately they are not efficient enough as stabilizer, compared to AOT sodium salt, a commercially available cost-effective common surfactant widely used for the formulation of water-in-oil microemulsions. The typical surfactants exhibit very different Hildebrandt solubility parameters for carbon dioxide; thus their solubility in CO2 is very low, explaining their lack in stabilizing supercritical CO2 emulsions

Based on the data evaluated [22], an interesting family of surfactants were synthesized as AOT analogues AO-VAc bearing vinyl-acetate oligomeric chains and AOK bearing *t-*butyl and carbonyl groups, grafted to promote CO2-philicity. Those syntheses are very demanding, and the difficult fabrication of industrial scale quantities limits the extension of the new compounds in application such as preparation of W/scCO2 microemulsions as reaction media. Sagisaka et al. [23] conducted a systematic study on the influence of the hydrophobic tail structure on the efficiency of fluorinated surfactants to stabilize water-in- supercritical CO2 microemulsions. Since anionic hydrocarbon surfactant AOT sodium salt is the most successful surfactant in obtaining W/O microemulsions, fluorinated surfactants with similar structures were investigating, and the results suggest that fluorinated analog bis(1*H*,1*H*,2*H*,2*H*-heptadecafluorodecyl)-2 sulfosuccinate sodium salt is the most effective in the formulation of water-in-scCO2 microemulsion, allowing a

In order to increase the region of microemulsion in the phase diagram of the water-supercritical CO2-surfactant system, novel amphiphilic compounds were synthesized with a more flexible molecule due to the presence of linkers. A series of fluorocarbon derivatives having different fluorocarbon chain lengths F(CF2)n with n = 4,6,8, and 10 and oxyethylene groups (CH2CH2O)m/2 (where m = 2 and 4) introduced into the molecule as spacers were developed [23]. The analysis of phase diagram of quaternary systems water-scCO2-*n*FS(EO)*m* surfactants suggests that an

0 is the maximum water-to-surfactant molar

0 = 32 (W c

*DOI: http://dx.doi.org/10.5772/intechopen.89353*

structure were synthesized for this purpose [21].

and microemulsions.

water solubility up to W c

ratio in W/O or W/scCO2 microemulsions).

### *Synthesis and Functionalization of Nanoparticles in Supercritical CO2 DOI: http://dx.doi.org/10.5772/intechopen.89353*

*Advanced Supercritical Fluids Technologies*

between drug and polymer inside the alginate beads. The PAM release curves show

The encapsulation of PAM in alginate beads was improved by impregnation in the presence of compressed carbon dioxide at high temperature. The release of ionic drug from drug delivery beads obtained by impregnation of PAM in supercritical CO2 (7.5 MPa, 40°C) is controlled by diffusion of active substances. The alginate beads with encapsulated PAM in subcritical conditions (7.5 MPa, 40°C) release

The release of PAM from alginate beads can be controlled by changing the initial

Nanoparticles with tunable size and high monodispersity present great interest for biomedical application, in particular to be used in bioimaging or as drug delivery systems. Synthesis of inorganic and organic NPs in scCO2 as green alternative to the physical and chemical traditional methods suffer from the same difficulties in controlling the nanoparticle growth step and to ensure narrow size distribution of the final product. One of the most efficient routes developed to fabricate monodisperse nanoparticles is the synthesis performed in heterogeneous media, i.e., in microemulsions or in blue emulsions, in order to ensure the nucleation and growth of nanoparticles in the restricted domain of liquid nanoreactors (oil or aqueous droplets). Microemulsions, as liquid-liquid colloidal systems consisting of nanometric droplets with size ranging from few nanometers to 100, stabilized with surfactant and cosurfactant mixture, are considered the most suitable reaction media for the synthesis of organic and inorganic nanoparticles with tunable size and morphology. Oil-in-water microemulsions are usually used for the fabrication of hydrophobic polymeric nanoparticles, while water-in-oil ones are used as reaction media for inorganic nanoparticles, especially for metal NPs obtained from reduction of metallic salt. All advantages of the controlled synthesis in water-in-oil microemulsions could be preserved using water-in scCO2 microemulsions, since the

that the release mechanism depends on the drug impregnation conditions.

PAM according to swelling and erosion of an amorphous biopolymer.

**4. Synthesis of nanoparticles in scCO2-based microemulsions**

general structure of the colloidal system is the same (**Figure 7**).

However, the stabilization of miniemulsions and the formation of thermodynamically stable microemulsions in supercritical CO2 are challenging tasks, due to the specific properties of carbon dioxide. Supercritical carbon dioxide possesses peculiar properties such as very high diffusivity and low viscosity, different from usual oily phases in the microemulsion composition; thus special requirements are

conditions of impregnation in the presence of CO2.

**168**

**Figure 7.**

*The schematic view of structure and stabilization of water droplets in W/O and W/scCO2 microemulsions.*

needed for the surfactants that enable the formation of W-in-scCO2 microemulsion: (i) enhanced absorbability at water/scCO2 interface, (ii) enhanced surface activity in lowering water/scCO2 interfacial tension, and (iii) strong repulsive interaction (steric effect) between CO2-philic groups, which reduces the probability of droplet aggregation and leads to the stabilization of microemulsion [21].

Many researches have been performed addressing the problem of surfactant solubility in supercritical and dense CO2, in order to evidence their ability to stabilize water-in-CO2 microemulsions, and surfactants with special designed molecular structure were synthesized for this purpose [21].

The early studies [22] conclude that conventional theories in molecular design for surfactant with superior efficiency in lowering interfacial tension at W/O interfaces are inapplicable to the W-scCO2 ones, because CO2-philicity could not be directly compared with oleophilicity. Considerable efforts have been made to find the most favorable chemical structures able to ensure adequate solubility of the surfactant in supercritical CO2, together with better adsorption at W-scCO2 interface. Highly branched hydrocarbon surfactants were found to possess good solubility, but unfortunately they are not efficient enough as stabilizer, compared to AOT sodium salt, a commercially available cost-effective common surfactant widely used for the formulation of water-in-oil microemulsions. The typical surfactants exhibit very different Hildebrandt solubility parameters for carbon dioxide; thus their solubility in CO2 is very low, explaining their lack in stabilizing supercritical CO2 emulsions and microemulsions.

Based on the data evaluated [22], an interesting family of surfactants were synthesized as AOT analogues AO-VAc bearing vinyl-acetate oligomeric chains and AOK bearing *t-*butyl and carbonyl groups, grafted to promote CO2-philicity. Those syntheses are very demanding, and the difficult fabrication of industrial scale quantities limits the extension of the new compounds in application such as preparation of W/scCO2 microemulsions as reaction media. Sagisaka et al. [23] conducted a systematic study on the influence of the hydrophobic tail structure on the efficiency of fluorinated surfactants to stabilize water-in- supercritical CO2 microemulsions. Since anionic hydrocarbon surfactant AOT sodium salt is the most successful surfactant in obtaining W/O microemulsions, fluorinated surfactants with similar structures were investigating, and the results suggest that fluorinated analog bis(1*H*,1*H*,2*H*,2*H*-heptadecafluorodecyl)-2 sulfosuccinate sodium salt is the most effective in the formulation of water-in-scCO2 microemulsion, allowing a water solubility up to W c 0 = 32 (W c 0 is the maximum water-to-surfactant molar ratio in W/O or W/scCO2 microemulsions).

In order to increase the region of microemulsion in the phase diagram of the water-supercritical CO2-surfactant system, novel amphiphilic compounds were synthesized with a more flexible molecule due to the presence of linkers. A series of fluorocarbon derivatives having different fluorocarbon chain lengths F(CF2)n with n = 4,6,8, and 10 and oxyethylene groups (CH2CH2O)m/2 (where m = 2 and 4) introduced into the molecule as spacers were developed [23]. The analysis of phase diagram of quaternary systems water-scCO2-*n*FS(EO)*m* surfactants suggests that an optimum length of the fluorocarbon tail is required for the formation of W/scCO2 microemulsion with increased amount of water included. The maximum water/ surfactant ratio Wc 0 is obtained for the CO2-philic tail with 12–14 Å length, corresponding to the surfactants with eight carbon atoms and two oxyethylene groups or six carbon atoms and four oxyethylene groups.

These fluorinated surfactants with specific chemical structure are quite difficult to obtain, and their use is restricted in the "green chemistry" methods due to the high toxicity. In this respect, other classes of surfactants were investigated, in particular polymeric derivatives. Block copolymers have been extensively studied regarding their ability to act as stabilizer of water droplets in scCO2 since J. de Simone reports for the first time in 1994 the solubility of fluorinated polymers in liquid and scCO2. Chemicals such as fluoropolymers, polysiloxanes, and some poly(vinyl esters) derivatives prove to be CO2-philic polymers but with reduced surface activity. In the last decades, the progresses in the synthesis techniques allow the fabrication of a large variety of block copolymers with CO2-philic moieties and specially designed structures, such as amphiphilic block, amphiphilic comb copolymers, or gradient copolymers bearing linear or branched chains, such as poly(1,1,2,2-tetrahydroperfluorooctyl methacrylate)-based copolymers with either poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) or poly(oligo(ethylene glycol) methacrylate) (POEGMA) as hydrophilic blocks or 1,1,2,2-tetrahydroperfluorodecyl tetrahydroperfluorodecyl acrylate and vinyl benzoic phosphonic acid [24]. A remarkable strategy to enhance the surfactant efficiency in scCO2 media is based on the synergistic effect obtained by ion-pairing cationic and anionic surfactants. Sagisaka et al. [25] report the properties of short tail, unbranched fluorinated surfactants with sulfate ionic groups and their cationic analogues synthesized by Verdia [26].

Individual compounds, for example, [C6F13S] and [C6F13mim], both exhibit very low solubility in CO2 and produce limited water solubility; even in water they have modest surface activity. The anionic and cationic pair of surfactants with similar hydrophobic tail used together in adequate molar ratio produces a spectacular synergistic effect, which enable the formulation of water-in-scCO2 microemulsion with significant water content (W0 max up to 50).

Metal nanoparticles (Ag, Au, and Cu) are particularly interesting for many industrial applications, due to their size- and shape-dependent unique optic and electric properties. In the last decades, applications of metal nanoparticles in nanomedicine, in emerging fields such as in vivo bioimaging, innovative cancer therapy, and diagnostics evolve in solution with enormous positive impact in the health-care system. The large-scale applicability of these nanomaterials is still restricted due to the concern about their intrinsic biocompatibility and the difficulties to remove toxic residual precursors and solvents during the synthesis, in particular regarding synthesis in W/O microemulsion as heterogeneous reaction media. Thus, new methods to synthesize metal or metal oxide nanoparticles using "green chemistry" approach as alternative to classic technologies have been investigated. The first reported synthesis of silver nanoparticles in CO2 microemulsion was reported in the late 1990s. The results were promising in terms of the quality of the nanopowder obtained, with additional advantage of fine-tuning the nanoparticle size. W/O microemulsions, as heterogeneous reaction media allow the synthesis of

**171**

cal methods.

microemulsion.

*Synthesis and Functionalization of Nanoparticles in Supercritical CO2*

nanoparticles with desired average size through the specific mechanism of precipitation in arrested environment, are defined by the water droplets. The radius of the water nanoreactor could be tuned in a facile way by changing water/surfactant

strated that the use of scCO2 microemulsion as reaction media exhibits additional advantage in continuous tuning of the Ag nanoparticle size. The nanoparticles were prepared in scCO2 microemulsion with different water/surfactant molar ratios

The same strategy is used to obtain tunable size of quantum dots, cadmium sulfide and zinc sulfide [28]. Similarly, Cu nanoparticles have been obtained [29] by reduction of the copper precursor (copper nitrate) dissolved in the water droplets of microemulsion after injection of CO2-soluble reducing agents such as *N*,*N*,*N*,*N*-tetramethyl-*p*-phenylenediamine or sodium cyanoborohydride. The scCO2 microemulsion used as reaction media was formulated with a mixed surfactant system consisting in 12.8 mM AOT and 25.3 mM fluorinated derivative (perfluoropolyether-phosphate) at a water-to-surfactant ratio *W*([H2O]/[AOT]) = 12. Nanoparticles with average size of 5–15 nm were obtained, according to the type of

Polymeric nanoparticles could also be obtained through polymerization processes in scCO2 microemulsion, using adapted strategy from emulsion/microemulsion polymerization with common oily phases. The preparation of polyamide nanoparticles was reported by Ohde et al. [30], using a scCO2 microemulsion stabilized with a mixture of commercially available hydrocarbon and fluorinated surfactants (AOT and perfluoropolyether-phosphate PFPE–PO4). The NPs were prepared by using polymerization of acrylamide monomer in the presence of potassium persulfate, both dissolved in the water core of scCO2 microemulsions with identic composition. The reaction is performed in homemade interconnected high-pressure vessels that allow the equilibration of separate scCO2 microemulsions containing monomer and initiator and mixing them at the desired time. The polymerization was completed very quickly, in less than 1 minutes at 25°C, 20 MPa,

Metallic nanoparticles deposited on the surface of the graphenes or carbon nanotubes are hybrid material with particular interest in various domains, for example, catalysis, energy saving, modified electrodes in biosensing, etc. The classic methods employed suffer from various drawbacks, from high amount of organic reagents to the lack of dimension control. Deposition of metal nanoparticles on the carbon nanotubes using scCO2 microemulsion as reaction media was proposed as green alternative to wet chemistry or other technologies of deposition using physi-

Shimizu et al. [31] report the preparation of hybrid electrocatalyst consisting in Pt NPs deposited on multiwalled carbon nanotubes in three different media: waterin-supercritical CO2 microemulsion, supercritical CO2 fluid, and water-in-hexane

As it is shown in **Figure 8,** the reaction in scCO2 microemulsion leads to a better dispersion of Pt NPs on the surface of nanotubes that is obtained in homogeneous

Among the many techniques of processing and synthesis in fluid carbon dioxide phases, the use of water-in-scCO2 microemulsions is one of the most effective in

) in the composition of microemulsion. Fernandez et al. [27] demon-

), that is, different water nanopool dimensions, stabilized with surfactant AOT fluorinated analogue at 10 mM concentration. The change of fluid phase density is obtained by simply adjusting temperature and pressure conditions. For the scCO2 microemulsion with lower water content (W = 6) very fine tuning was achieved, producing silver nanoparticles with average size ranging from 1.9 to 9.3 nm for a

*DOI: http://dx.doi.org/10.5772/intechopen.89353*

variation of density from 0.96 up to 0.80 g/mL.

the reduction agent and rate of the reagent addition.

scCO2 phase and conventional W/O microemulsion.

and in less than 30 s at 50°C, 15 MPa.

ratio (W0

(W0 c c

*Synthesis and Functionalization of Nanoparticles in Supercritical CO2 DOI: http://dx.doi.org/10.5772/intechopen.89353*

*Advanced Supercritical Fluids Technologies*

six carbon atoms and four oxyethylene groups.

cationic analogues synthesized by Verdia [26].

with significant water content (W0 max up to 50).

surfactant ratio Wc

optimum length of the fluorocarbon tail is required for the formation of W/scCO2 microemulsion with increased amount of water included. The maximum water/

sponding to the surfactants with eight carbon atoms and two oxyethylene groups or

These fluorinated surfactants with specific chemical structure are quite difficult to obtain, and their use is restricted in the "green chemistry" methods due to the high toxicity. In this respect, other classes of surfactants were investigated, in particular polymeric derivatives. Block copolymers have been extensively studied regarding their ability to act as stabilizer of water droplets in scCO2 since J. de Simone reports for the first time in 1994 the solubility of fluorinated polymers in liquid and scCO2. Chemicals such as fluoropolymers, polysiloxanes, and some poly(vinyl esters) derivatives prove to be CO2-philic polymers but with reduced surface activity. In the last decades, the progresses in the synthesis techniques allow the fabrication of a large variety of block copolymers with CO2-philic moieties and specially designed structures, such as amphiphilic block, amphiphilic comb copolymers, or gradient copolymers bearing linear or branched chains, such as poly(1,1,2,2-tetrahydroperfluorooctyl methacrylate)-based copolymers with either poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) or poly(oligo(ethylene glycol) methacrylate) (POEGMA) as hydrophilic blocks or 1,1,2,2-tetrahydroperfluorodecyl tetrahydroperfluorodecyl acrylate and vinyl benzoic phosphonic acid [24]. A remarkable strategy to enhance the surfactant efficiency in scCO2 media is based on the synergistic effect obtained by ion-pairing cationic and anionic surfactants. Sagisaka et al. [25] report the properties of short tail, unbranched fluorinated surfactants with sulfate ionic groups and their

Individual compounds, for example, [C6F13S] and [C6F13mim], both exhibit very low solubility in CO2 and produce limited water solubility; even in water they have modest surface activity. The anionic and cationic pair of surfactants with similar hydrophobic tail used together in adequate molar ratio produces a spectacular synergistic effect, which enable the formulation of water-in-scCO2 microemulsion

Metal nanoparticles (Ag, Au, and Cu) are particularly interesting for many industrial applications, due to their size- and shape-dependent unique optic and electric properties. In the last decades, applications of metal nanoparticles in nanomedicine, in emerging fields such as in vivo bioimaging, innovative cancer therapy, and diagnostics evolve in solution with enormous positive impact in the health-care system. The large-scale applicability of these nanomaterials is still restricted due to the concern about their intrinsic biocompatibility and the difficulties to remove toxic residual precursors and solvents during the synthesis, in particular regarding synthesis in W/O microemulsion as heterogeneous reaction media. Thus, new methods to synthesize metal or metal oxide nanoparticles using "green chemistry" approach as alternative to classic technologies have been investigated. The first reported synthesis of silver nanoparticles in CO2 microemulsion was reported in the late 1990s. The results were promising in terms of the quality of the nanopowder obtained, with additional advantage of fine-tuning the nanoparticle size. W/O microemulsions, as heterogeneous reaction media allow the synthesis of

0 is obtained for the CO2-philic tail with 12–14 Å length, corre-

**170**

nanoparticles with desired average size through the specific mechanism of precipitation in arrested environment, are defined by the water droplets. The radius of the water nanoreactor could be tuned in a facile way by changing water/surfactant ratio (W0 c ) in the composition of microemulsion. Fernandez et al. [27] demonstrated that the use of scCO2 microemulsion as reaction media exhibits additional advantage in continuous tuning of the Ag nanoparticle size. The nanoparticles were prepared in scCO2 microemulsion with different water/surfactant molar ratios (W0 c ), that is, different water nanopool dimensions, stabilized with surfactant AOT fluorinated analogue at 10 mM concentration. The change of fluid phase density is obtained by simply adjusting temperature and pressure conditions. For the scCO2 microemulsion with lower water content (W = 6) very fine tuning was achieved, producing silver nanoparticles with average size ranging from 1.9 to 9.3 nm for a variation of density from 0.96 up to 0.80 g/mL.

The same strategy is used to obtain tunable size of quantum dots, cadmium sulfide and zinc sulfide [28]. Similarly, Cu nanoparticles have been obtained [29] by reduction of the copper precursor (copper nitrate) dissolved in the water droplets of microemulsion after injection of CO2-soluble reducing agents such as *N*,*N*,*N*,*N*-tetramethyl-*p*-phenylenediamine or sodium cyanoborohydride. The scCO2 microemulsion used as reaction media was formulated with a mixed surfactant system consisting in 12.8 mM AOT and 25.3 mM fluorinated derivative (perfluoropolyether-phosphate) at a water-to-surfactant ratio *W*([H2O]/[AOT]) = 12. Nanoparticles with average size of 5–15 nm were obtained, according to the type of the reduction agent and rate of the reagent addition.

Polymeric nanoparticles could also be obtained through polymerization processes in scCO2 microemulsion, using adapted strategy from emulsion/microemulsion polymerization with common oily phases. The preparation of polyamide nanoparticles was reported by Ohde et al. [30], using a scCO2 microemulsion stabilized with a mixture of commercially available hydrocarbon and fluorinated surfactants (AOT and perfluoropolyether-phosphate PFPE–PO4). The NPs were prepared by using polymerization of acrylamide monomer in the presence of potassium persulfate, both dissolved in the water core of scCO2 microemulsions with identic composition. The reaction is performed in homemade interconnected high-pressure vessels that allow the equilibration of separate scCO2 microemulsions containing monomer and initiator and mixing them at the desired time. The polymerization was completed very quickly, in less than 1 minutes at 25°C, 20 MPa, and in less than 30 s at 50°C, 15 MPa.

Metallic nanoparticles deposited on the surface of the graphenes or carbon nanotubes are hybrid material with particular interest in various domains, for example, catalysis, energy saving, modified electrodes in biosensing, etc. The classic methods employed suffer from various drawbacks, from high amount of organic reagents to the lack of dimension control. Deposition of metal nanoparticles on the carbon nanotubes using scCO2 microemulsion as reaction media was proposed as green alternative to wet chemistry or other technologies of deposition using physical methods.

Shimizu et al. [31] report the preparation of hybrid electrocatalyst consisting in Pt NPs deposited on multiwalled carbon nanotubes in three different media: waterin-supercritical CO2 microemulsion, supercritical CO2 fluid, and water-in-hexane microemulsion.

As it is shown in **Figure 8,** the reaction in scCO2 microemulsion leads to a better dispersion of Pt NPs on the surface of nanotubes that is obtained in homogeneous scCO2 phase and conventional W/O microemulsion.

Among the many techniques of processing and synthesis in fluid carbon dioxide phases, the use of water-in-scCO2 microemulsions is one of the most effective in

**Figure 8.**

*TEM images of hybrid catalyst Pt-CNT synthesized in various media by (a) water-in-supercritical CO2 microemulsion, (b) water-in-oil microemulsions, (c) direct deposition in supercritical CO2 (adapted with permission from ref. [31]).*

terms of control on the size, size distribution, and shape of the final product. The stabilization of such reaction media with low amount of affordable and nontoxic surfactants remains the major bottleneck and limits the extension of this method.
