Microemulsions are self-aggregated colloidal systems that provide a controllable system with a promising application as nanoreactors: they can act as pools within which the properties of the nanoparticles can be controlled without difficulty. So in this chapter, I will deal with the metal NPs synthesized by the microemulsion method. This method allows in some cases to control the properties of size, shape, and crystal structure of the metallic NPs, thus generating with the same reagents a series of seeds of different shapes and sizes. The control of the reaction time, the temperature, and the reaction conditions will give us a production of different geometries that will find different applications in large range of research fields.
- microemulsion method
- colloidal suspensions
During the last decades, self-assembly of micro- and nano-materials attracted a widespread mindfulness because of special properties when comparing with bulk counterparts, because of mechanical and physicochemical property changes at micro- and nanoscale. I will focus on a particular three-phase diagram proportion that is known as the microemulsion system. It is thermodynamically stable and isotropic dispersion of two immiscible liquids, where a surfactant stabilizes micro-domains . It includes different structures and configurations corresponding to different performances of self-assembled colloidal systems. Then I will focus on the assembled structure of nanoparticles. To be able to obtain these structures by microemulsion method, it must be known making a good use of the properties of the microemulsion systems, in which the composition of these structures will determine properties as important as the size of the microdroplets. That ultimately will determine the reaction system of our synthesis of nanoparticles.
The microdroplet environment offers a controllable medium to obtain NPs with tailored size, shape, and structural properties.
2. Metal nanoparticles (MNPs)
2.1 Anionic surfactants
Here I focus on MNPs synthesized by microemulsion method using the anionic surfactant, reviewing literature that employed this surfactant type I found a very noticeable research.
Microemulsion method was used by Kon-No et al. ; they prepared two microemulsions, one them solubilizing FeCl3 aqueous solution in AOT/cyclohexane and the other aqueous NH3 in AOT/cyclohexane; both were mixed joint (see AOT chemical structure in Figure 1). To obtain the formation of Fe3O4 in the micelle, authors added a FeCl2 solution to the mixture of two microemulsions. The solution showed stability during 2 years. A diluted solution of the colloid obtained was characterized by TEM to determine the crystal structure and size distribution by electron micrography. Then the authors obtained magnetic measurements together with an extended analysis to determine detailed surface magnetic properties that allowed them to conclude that (i) distribution function of colloidal particles is a log-normal type, coincidentally with results obtained by gas evaporation technique, (ii) the magnetic interaction between the colloidal particles is negligible, (iii) the colloidal system showed superparamagnetism about 50K, (iv) paramagnetic Fe ions potentially are present on a particle surface, and (v) the temperature dependence of the spontaneous magnetization linearly decreases with the increase of temperature.
López-Quintela and Rivas  reported a simple and powerful method to obtain ultrafine particles by means of chemical reactions in microemulsions. The reaction happened within the nanodroplets of the microemulsion. Varying droplets’ radii authors were able to control particle dimension. They reported that the proper selection of microemulsion components would ease the exchange of reagents between the “transition dimers” formed by two droplets of the microemulsions; for this to happen, an important alteration in the local curvature is necessary, so the selection of adequate surfactants that shows a radius of curvature close to their natural radius will facilitate the opening of channels [4, 5, 6, 7, 8, 9], particle formation, and growth. Once the particles are formed, the surfactant particles act as surface agents, limiting the future growth of the particles. The sizes of the microemulsion droplets can be tuned between 5 and 50 nm by changing the relation of the components of the microemulsion (e.g.,
Mann et al. reported  that interfacial activity could be used to combine nanoparticle synthesis and self-assembly to produce complex organized materials rising from the interaction of tensioactive molecules attached to specific nanoparticle crystal faces. Authors demonstrated this principle by controlling the [Ba2+]:[CrO42−] inside of droplets of the microemulsion that tunes the fusing of microdroplets and reverse micelles. It allowed them to produce linear chains, rectangular superlattices, and long filaments, as a function of reactant ratio. They carried out the synthesis and self-assembly of NPs by converting sodium AOT to Ba(AOT)2 by direct precipitation .
Wang et al.  reported by the first time a new microemulsion method to synthesize magnetite NPs. The novelty of their method founded on the cost-effective use of a single microemulsion. The authors prepared the microemulsion by solubilizing NaOH solution into DBS/ethanol/toluene system (DBS, sodium dodecylbenzene sulfonate), and then ferric and ferrous salts with molar ratio (2:1) were made into a mixed solution. A volume of this mixture was drop-wise added to microemulsion under vigorous stirring. The mixed system turned black immediately. After the mixed system was stirred during ½ h and then left undisturbed during 5 h, a two-layer system appeared. The upper layer was black, which contained the NPs, and lower layer was discarded. Following a magnetic separation technique, the magnetite NPs were extracted and then washed several times in ethanol/water solution (1:1) to obtain magnetite NPs.
2.2 Cationic surfactants
Here I focus on MNPs synthesized by microemulsion method using the cationic surfactant, examining literature that employed this surfactant type I found very outstanding research.
Chin and Yaacob  reported the synthesis of magnetic iron oxide nanoparticles by preparing water-in-oil microemulsion system. Authors investigated two different volumetric ratios of Fe2+ and OH− (1:1 and 2:1). Crystallized, physical, and magnetic sizes of magnetic iron oxide nanoparticles were in the superparamagnetic size range (XRD, TEM, and AGM). Superparamagnetic behavior was warranted by hysteresis loop nonappearance in magnetization curve at room temperature. MIONPs were obtained by Massart’s method too. The saturation magnetization of MIONPs obtained by
The preparation of iron oxide NPs was carried out according to the following route. Ferrous chloride salt solution was prepared (0.2 M); it was steady by adding a few drops of hydrochloric acid (0.5 M). Afterward, the microemulsion was set by melting HTAB in n-octane; subsequently 1-butanol was added followed by FeCl2 aqueous salt solution. The system was gradually shaken till a clear microemulsion suspension was attained. By repeating above method a microemulsion containing 0.25 M NaOH solution was produced.
These microemulsion systems were next fused at 1:1 vol. Instantaneously, a dark green solid was produced before it turned to black. The NPs were gathered and later cleaned repeatedly with acetone and de-ionized water and then they were dried at room temperature which is called S1. The same route was followed to yield a second sample with primary [Fe2+] = 0.1 M. [NaOH] was kept at 0.25 M. The volumetric proportion of microemulsion containing Fe2+ to microemulsion containing OH− was 2:1. The sample was called S2 . To compare, by Massart’s procedure, S3 has been obtained . Magnetic iron oxide NPs (MIONPs) were effectively formed at r.t. with microemulsion method. The MIONPs showed spherical shape. From the results of TEM, AGM, and XRD, the crystallite, physical, and magnetic sizes of the MIONPs were <10 nm. The mean physical size for S1, S2, and S3 is 6.5, 4.2, and 8.7 nm, respectively. The rise of the total number of microemulsion produced smaller particles. Particles produced by microemulsion procedure were smaller in size and showed higher saturation magnetization, compared to the particles obtained by Massart’s procedure.
Shen et al.  prepare CTAB-stabilized gold nanowires and nanoparticles with a networked structure and different shapes using in situ
With the decrease of
Sharma et al.  reported a detailed and complete study on the growth kinetics of iron oxalate nanorods as they formed throughout several days inside the water pool of CTAB microdroplets (MDs) in isooctane. Authors underlined the novelty of this experimental study on nanostructure preparation in microemulsion-based reactions that could explain the key role of droplet interplay in the reaction kinetics. DLS, TEM, and FCS characterization was completed throughout some days to follow the whole growth kinetics of iron oxalate nanorods beginning from their particle nucleation. Considering the FCS data with an appropriate kinetic model, the droplet-fusion time or dimer lifetime 28 μs was obtained along with the droplet melting rate and the equilibrium constant of the chemical reaction. The droplet association rate exhibited a noteworthy time dependency that straight relates it to the growth mechanism. Combining FCS, DLS, and TEM, three different periods in the complete nanorod growth kinetics appeared: (i) a prolonged nucleation-dominant NP growth period, then (ii) a short period where isotropic NPs shifted to anisotropic growth to form nanorods, and (iii) finally the period where droplet-fusion-assisted elongation of nanorods was proved. The detailed methodologies discussed in this study could be applied to understand the growth kinetics of nanostructures, which are required for various applications in nanoscience and nanotechnology.
2.3 Nonionic surfactants
Here I review the literature about the use of nonionic surfactants as stabilizer/emulsifier of the microemulsion involved on the synthesis of MNPs by microemulsion method.
Sanchez-Dominguez et al.  reported a novel and direct approach to synthesize inorganic NPs at ambient conditions using
Capek reviewed water-in-oil microemulsion method to prepare metal NPs . This technique allows preparation of ultrafine metal particles ranging in the size diameter between 5 and 50 nm. Author reviewed the previous literature on particle synthesis of various metals such as silver, copper, cadmium, cobalt, nickel, cadmium, and gold in the reverse microemulsion systems. The precursor metal salts and reducing agents are mainly water-soluble molecules, and consequently the MNP nuclei formation progresses in the water microdroplets. The rate of the nuclei formation of the particle depends on the percolation degree of microemulsion droplets. Effects of stabilizer nature and concentration, the oil phase nature, reducing agent, and additive on the NP synthesis are shortened and assessed. The impact of numerous factors such as temperature, the metal salt nature, the incident light, and reaction conditions were also revised.
Tan et al.  reported the microemulsion application to prepare silica-coated iron oxide NPs using nonionic surfactants (Triton X-100, Igepal CO-520, and Brij-97, their chemical structure can be observed in Figures 4, 5, 6, respectively) for the preparation of microemulsions. Iron oxide NPs are formed by co-precipitation. TEM, XRD, and SQUID magnetometry were used to assess both uncoated and silica-coated FeO NPs. Particles displayed magnetic properties near to that of superparamagnetic materials. By using this route, magnetic NPs ranging 1–2 nm and with very monodispersed in size (percentage std. <10%) were obtained. Base-catalyzed hydrolysis and the polymerization reaction of tetraethyl orthosilicate (TEOS) in microemulsion were used to produce a uniform silica coating of 1 nm enclosing the naked NPs.
Khiev et al.  reported the synthesis of regular shape and monodispersed nickel sulfide (NiS) NPs were made in
The typical microemulsion used in their synthesis had a composition of 28, 56, and 16 wt% of tetradecane/1-butanol (see chemical structure in Figure 7) and of aqueous solution containing Ni(NO3)2 and Na2S, respectively. The hastened fine particles achieved from centrifugation were cleaned with absolute ethanol and distilled water leastwise fivefold so as to wash away the excess tensioactive byproducts and unreacted reagents. The products were dried in vacuum oven for 16 h at 50°C till a constant weight was attained. It was denoted that the size of the NPs enhanced with the reactants’ concentration due to the fusion of nuclei. The presence of quantum confinement effect was apparent for the resultant NPs as the likely bandgap energy exhibited noteworthy increase. Sugar-ester-based nonionic microemulsion offers proper micro-medium to formulate NPs with narrow size distribution and high uniformity.
Pine-Reyes and Olvera reported  the use of human and environmentally friendly
Sample M10 with 15% ZnAc, 55% surfactant, and 30% oil exhibits the smallest average particle size, 31.2 nm, and a distribution of 41.2 nm. SEM and TEM micrographs have demonstrated a pseudo-spherical morphology. XRD analysis confirms that samples presented a hexagonal wurtzite phase with preferential growth direction . The results showed that this method is a suitable pathway for ZnO NPs synthesis. Moreover, it is a simple, direct, and cost-effective method for obtaining nanostructured materials with small particle size and narrow size distribution particle.
3. Silica nanoparticles
Here we focus on a metalloid and the use of microemulsion method to obtain silica NPs. Yamauchi et al.  reported the synthesis of ultramicroparticles of silica by means of
Shantz et al. reported  synthesis of silicalite-1 nanocrystals in AOT-based microemulsion. They demonstrated that anionic microemulsions drive to essentially distinct crystal patterns than the nonionic  or cationic microemulsions formerly explored. The authors concluded that AOT and SDS anions synthesized by microemulsion method were employed to crystallize silicalite-1 nanocrystals with well-defined shapes and structures. Box, disk-like, spherical, and twinned nanocrystals were found under distinct experiential circumstances. It was also shown that tensioactive uniqueness, TPAOH content, and the presence of salts and co-surfactant influence the morphology and crystallinity of silicalite-1. The crystal size and shape contrasted in all cases than those prepared in default of the microemulsion. The crystal morphology could be regulated by adjusting the interplay among surfactant and zeolite surfaces.
4. Outlooks and perspectives
Here I focus on the state-of-the-art research on nanotechnology that involves the microemulsion method.
Chen et al.  reported the synthesis of CdS QDs by means of a chemical reaction between amide of cadmium acetate dehydrate and thioacetamide by using a microemulsion-based hydrothermal method. Their properties could be tailored through the use of Emulsifier OP and CTAB, which yield a usual cubic phase and a scarce hexagonal phase, apiece. Authors explored a possible mechanism involving the critical role of surfactant in the formation of crystal structure. A direct dependence of the crystal size that regularly increased with the increase of temperature is also found, and the appearance of red shift in the absorption and emission peaks confirmed the quantum confinement effect. All the desired properties of CdS QDs synthesized by this route denote the chance of the preparation of high-quality QDs under the appropriate reaction conditions.
First, aqueous solutions 0.005 M of cadmium acetate and thioacetamide were set by melting in deionized water with the aid of magnetic stirrer. Next, two reverse microemulsions (MCd and MS) with different aqueous phases were prepared. Either MCd or MS enclosed three mutual constituents in the volume ratio of 3:5:20, that is, a surfactant of CTAB or emulsifier OP, co-surfactant of n-butyl alcohol, and a continuous oil phase of n-hexane. Once jointly mixed under constant stirring at r.t. during 30 min, it was moved into a 100 mL Teflon-lined autoclave with hydrothermal treatment at 70–120°C for 13 h, succeeded by chilling at r.t. readily. The yellow precipitants were harvested from the synthesis medium by centrifugation, cleaned with anhydrous ethanol and deionized water sometimes and then dried at 70°C, to finally obtain CdS QDs.
The crystal structure was characterized by XRD, obtaining OP at 2θ values of 26.5, 43.9, and 52.1° which agree with the (1 1 1), (2 2 0), and (3 1 1) planes of cubic structure. The morphology was analyzed with a transmission electron microscope, denoting that at 70°C, the mean crystal size of CdS QDs is much tinier (≈3 nm), and afterward it achieved 5.9 nm, when the temperature was increased to 120°C. It noticeably proved the influence of reaction temperature on the crystal size of the as-prepared CdS QDs. The optical properties of samples were characterized by the UV-4100 spectrophotometer and FLS920 steady-state and transient fluorescence spectrometer. In addition, infrared spectroscopy was recorded.
Wei et al.  reported the synthesis of the core-shell NaGdF4@CaCO3-PEG NPs were carried out through a facile microemulsion method with using NaGdF4 NPs as templates . Concisely, 5 mL NaGdF4 NPs, 10 mL of cyclohexane, 3.45 mL of Triton X-100, and 3.2 mL of 1-hexanol were added to a flask and combined carefully, followed by annexing 800 μL of a 30 mM solution of CaCl2 to obtain a well-dispersed
In this work, a core-shell nanoparticle of NaGdF4@CaCO3-PEG was designed as an activable MR/US dual-modal imaging contrast for cancer diagnosis, which is activated by the acidic environment. Authors used the coating of NaGdF4 with a layer of hydrophobic CaCO3 to limit water availability, thus quenching the sphere Gd3+ relaxation effects, with the aim of achieving this OFF/ON responsive MR imaging behavior. At acidic aqueous solution, CaCO3 was melted to produce CO2 bubbles, which is applied to obtain US signal. While a robust MRI augmentation could be triggered on dissolution of CaCO3 and discharge of the earlier quieted NaGdF4 in the aqueous solution. In vivo results confirmed the heavy dual-modal magnetic resonance/ultrasonic imaging capabilities of NaGdF4@CaCO3-PEG at the tumor area with an acidic environment. Authors expected that their findings might deliver a novel sight for approaches to developing NPs with reactive dual-modal imaging skills. The here-described proof-of-concept nanoparticles with pH-triggered magnetic resonance/ultrasonic dual-modal imaging improvement may assist as a useful guide to advance various molecular imaging strategies for cancer diagnosis in the future.
Majumder and Roy reported the synthesis of mesoporous CeO2 nanospheres  with noticeably raised surface area. It was ready using reversed micelles by a water-in-oil microemulsion method. The structure and semiconducting properties of the NPs are accurately explored using TEM, FESEM, XRD, and UV-Vis. Despite after high-temperature burning, the structural holding of the nanomaterial was evident by EM. Comparing to those of other sensors of the same type, outstanding performances in terms of sensitivity, response-recovery times, and selectivity were found on the distribution of undoped CeO2 nanospheres for the detection of low-ppm CO. These CO sensors showed 52% sensitivity with a reply time of only 13 s. The sensor parameters were analyzed as a function of both gas concentration and temperature. In addition to that on the scalable and cost-effective synthesis of CeO2 nanospheres, authors also reported on the fabrication of packaged CO sensors, which could be potentially used for industrial and environmental monitoring purposes. To carry out the material synthesis, all of the chemicals (analytical grade) were applied without further treatment. Primarily, a microemulsion was formed by merging n-hexane, n-butyl alcohol, and diethyl ether in the weight ratio 3:2:1, energetically stirred till the mix turns out to be clear. The quantity of CTAB to reach critical micelle concentration (CMC) was calculated applying the standard conductometric method, where a sequence of microemulsions were examined changing the [CTAB] between 0.2 and 1.5 g. Then, 10 mL of 1 M aqueous solution of Ce(NO3)3 including 0.65 g of CTAB was annexed to the mix, and then it was agitated again till it showed transparency. Concurrently, another aqueous solution of 10% (w/v) NH4OH was prepared. The above solutions were merged strongly till it forms a colloidal suspension. The material was gathered by centrifugation at 6000 r.p.m. during 30 min and consecutively washed using deionized water in an ultrasonic bath during 2 h. Lastly, the collected material was dried under vacuum and calcined at 600°C during 4 h.
Wang et al. reported the synthesis and applications of new biomimetic hybrid nanoplatform with theranostic agents . Authors reported the synthesis of this nanoplatform; it was out by the next method: Silica NP core was used to encapsulate fullerene (C60) by a reverse microemulsion method. First, hexanol, Triton X-100, cyclohexane, and deionized water were jointly mixed. A total volume of 2 mL of fullerene in toluene (2 mg mL−1) was then added into the mixture. Subsequently, 60 mL of ammonium hydroxide solution (28 wt%) and 100 mL TEOS were serially annexed, and the sample was rattled at 800 r.p.m. using a mini-stir bar during 24 h at r.t. Finally, the sample was annexed in 30 mL of ethanol to cease the reaction. The obtained sample was centrifuged at 13,800 × g during 10 min to attain fullerene-inserted silica C60S-NPs. After twofold cleaning with deionized water and ethanol, the NPs were suspended in 3 mL of ethanol and followed by the addition 20 mL of APTMS. The mixture was stirred using a mini-stir bar (200 r.p.m., during 12 h) at room temperature to acquire the APTMS-coated C60S, C60S-A NPs. Lastly, the sample of C60S-A NPs was merged with DPPC liposomes and shacked for 5 h (r.t.) to harvest the phospholipid bilayer-coated C60S, LC60S-NPs. The LC60S NPs were gathered by centrifuging at 13,800 × g for 10 min. The DPPC liposomes were prepared by hydrating a thin DPPC film. Subsequently sonication for 2 min using a Branson 450 Sonifier, the sample of DPPC (1,2-dihexadecanoyl- sn-glycero-3-phosphocholine) liposomes (≈100 nm) was filtered throughout a 0.2 mm filter earlier mixing with the solution of C60S-A nanoparticles. I carried out a research on re-dispersion and self-assembly of C60 pointed special performance and arrangement depending on the solvent nature .
Authors concluded that both the encapsulation efficiency (EE) and minimal drug loading content (LC) of theranostic agents are vital calibers of multifunctional NPs for drug transfer. To attain great EE, low drug-to-NP provision ratios have frequently been applied for encapsulation, which outcomes in low drug LC. Authors designed a eukaryotic cell-like nanoplatform or EukaCell, which can be applied to attain great EE (≈100%) and LC (up to ≈87%) of theranostic agents (DOX and ICG). The release of the encapsulated drug can be exactly controlled using NIR laser irradiation to minimize the potential side effects. With the biomimetic eukaryotic cell-like configuration, the EukaCell had an extended half-life and strong stability in blood circulation and could favorably accumulate in tumor following intravenous injection. Finally, the drug-laden EukaCell displayed excellent safety and efficacy for cancer therapy. Authors demonstrated the tremendous potential of their EukaCell for providing theranostic agents to detect and treat cancer.
Tianimoghadam and Salabat reported the formulation of new microemulsion to prepare thiol-functionalized AuNPs . The microemulsion system was obtained by mixing a yellow aqueous solution of 0.03 M of HAuCl4 and a 0.05 M solution of TOAB/toluene. The same microemulsion containing 0.4 M of NaBH4 was prepared independently and drop-wised under energetic stirring into the microemulsion enclosing metal ions. The formation of AuNPs was confirmed by the appearance of a stable light-ruby-red color resulting in microemulsion when the reduced Au(III) ions changed. To form a thiol monolayer surrounding the AuNPs, dodecanethiol (17 mg) was added while kept stirring during 1 h. To eliminate all residual TOAB surfactants (see TOAB chemical structure in Figure 10) and thiols from the AuNP surfaces, the AuNPs were pipetted into a 10 mL vial along with 3 mL ethanol and then centrifuged. This procedure was sixfold repeated, after which the dodecanethiol-stabilized AuNPs were then resuspended in toluene. The stable nano-colloid system of Au/toluene was then prepared for characterization. To confirm the size distribution and formation of the AuNPs, TEM techniques and UV-Vis absorption were used. The absorption maximum was found to be 524 nm, which is a shift typical for spherical AuNPs. This surface plasmon band agrees to 3–4 nm particles. According to the Mie theory, the purification procedure removed the low-molar-mass impurities without essentially varying the structure of the NPs. The AuNPs were basically spherical and monodispersed with diameters around 3–4 nm.
Authors discussed an easy method to prepare dodecanethiol-stabilized AuNPs through a new microemulsion TOAB cationic surfactant-based system. This approach was related with the liquid-liquid phase method. Both AuNPs arranged by different procedures were studied with UV-Vis spectroscopy and TEM imaging, denoting good steadiness. The TEM images of the thiol-capped AuNPs ready by the microemulsion method showed a more acute monodispersity related to that prepared using the Brust method. Since all the methods could be carried out in one step, the microemulsion procedure was faster and easier.
As this review chapter has stated, microemulsion method must be taken into account when the aim of the researcher is to prepare well-controlled, narrow-sized, monodispersed NPs.
The influence of microemulsion components has been reviewed; not only the ratio between aqueous phase/surfactant and surfactant/metal precursor but also the influence of the nature of the surfactant—ionic, cationic, and nonionic surfactants—will offer different self-assembled systems. It was underlined that hydrodynamic radius (
Dr. A. Cid acknowledges the postdoctoral contract granted to CIA research group by FEDER Funds at Physical Chemistry Department Universidade de Vigo and the Unidade de Ciências Biomoleculares Aplicadas-UCIBIO which is financed by national funds from FCT/MEC (UID/Multi/04378/2013) and co-financed by the ERDF under the PT2020 Partnership Agreement (POCI-01-0145-FEDER-007728). This chapter is dedicated to Professor Julio Casado Linarejos, who was a great support during my formation as scientist.
Conflict of interest
Author does not declare conflict of interest.