Simple phase transfer of nanoparticles from aqueous to organic media using polymer colloids as carriers

Abstract

A new simple and versatile technique for the phase transfer of nanoparticles from water to water-immiscible organic solvents is presented. The proposed method is based on using charged colloids as carriers for the transfer of nanoparticles with opposite surface charge. The method has been successfully applied for the transfer of SrFe₁₂O₁₉ nanoplatelets and CdTe nanoparticles.

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Colloidal nanoparticles have received significant attention from scientists in the fields of catalysis, medicine, optics, information storage and so on. Colloids can be synthesised in both hydrophilic and hydrophobic media, but the subsequent transfer of already formed nanoparticles to different environments for further applications presents a substantial challenge that is far from being resolved. Some general techniques for the phase transfer of nanoparticles from a polar to a non-polar environment and vice versa have been reported in recent years.¹

Ligand exchange is a widespread phase transfer method. There are numerous examples of its effective application for the phase transfer of Au, Ag, ZnO, CdS, PbS, Zn_xCd_1−xS and CdSe@CdZnS nanoparticles with different sizes and shapes.^2–7 The ligand exchange technique can perfectly maintain the small particle size and monodispersity of nanoparticles compared with other phase transfer methods. An interesting approach with ligand exchange as one of the stages in the phase transfer procedure has been proposed by Park et al.⁸ Metal nanoparticles with different sizes, shapes and chemical environments in water were transferred to chloroform by centrifugation following functionalization by thiol-containing molecules. The approach proposed requires that the organic solvent is of a higher density than water. For other media, solvent exchange procedure should be applied.⁸ An unfavourable feature of ligand exchange is that the modification of the particle surface needed for the phase transfer can lead to blocking of the particle active sites, which are necessary for catalysis and other applications.⁹ A simple phase transfer method of hydrophilic nanoparticles to organic media without surface modification has been suggested by Bai et al.¹⁰ The method is based on the encapsulation of nanoparticles in gel microparticles and the subsequent exchange of water swollen in hydrogel particles with organic solvents. It is worth noting that the transfer efficiency (TE) is reduced with increasing nanoparticle size due to slower diffusion of nanoparticles into hydrogel microparticles. Similarly, a mesoporous silica nanocomposite has been used for the reversible transfer of Pd(OAc)₂, Pd nanoparticles and organic molecules between an organic phase and water.¹¹ The transfer of various species between phases has been realized by changing of pH.

Here, we report a simple and versatile method of the phase transfer of nanoparticles with different shapes and a broad size range from water into water-immiscible organic solvents. The method is based on the use of the water suspension of charged polymer (polystyrene (PS)) microspheres as carriers for nanoparticles with opposite electric charge for their transfer to non-polar organic solvents. In order to demonstrate the universality of the method, hard magnetic strontium M-type hexaferrite (SrM) nanoplatelets and luminescent CdTe spherical nanoparticles were transferred to non-polar solvents (styrene, toluene, benzene and chloroform). It should be noted that manipulation of the suspensions of hard magnetic nanoparticles, such as SrM nanoplatelets, is a challenge because of the strong magnetic attractive interactions between particles. Therefore, we have chosen SrM nanoparticles as an interesting model for testing the proposed phase transfer method.

Scheme 1 illustrates the typical procedure of the phase transfer of positively charged hydrophilic particles to non-polar organic solvents using the suspension of SrM nanoplatelets. A water suspension of PS microspheres is added to a water suspension of the particles, followed by intensive mixing. The non-polar solvent is then added to the mixture and stirred for about a minute using a vortex agitator. After a short period of time, the fractionation of the organic and water phases is observed in the final suspension. The nanoparticles are found in the non-polar solvent. The experimental details are described in the ESI.†

Scheme 1 Schematic view of the phase transfer of strontium M-type hexaferrite nanoplatelets from water (denoted as “w”) to the organic (denoted as “o”) phase. The photographs illustrate various stages of the phase transfer procedure.

The suggested mechanism of phase transfer includes the electrostatic adhesion of positively charged hydrophilic particles on the surface of negatively charged PS microspheres, the transfer of uncharged aggregates “SrM nanoplatelets–PS microsphere” to the non-polar solvent, followed by the dissolution of PS (Scheme 1). Finally, a stable suspension of transferred particles in the non-polar solvent is obtained. The PS chains in the final suspension may play a role of a stabilizing agent.

The phase transfer of SrM nanoplatelets with an average diameter of D = 35 nm and an average thickness of h = 6 nm (Fig. 1a) was performed by PS microspheres with D = 545 nm (Fig. 1b). A scanning electron microscope (SEM) image of the dried mixture of the suspensions of SrM nanoplatelets (zeta potential ζ = +39 mV) and PS microspheres (ζ = −29 mV) is shown in Fig. 1c. It is clear that SrM nanoplatelets adhere to the surface of the PS microspheres due to the opposite surface charges. The sizes and zeta potentials of the different types of particles described in this study are listed in Table 1.

Fig. 1 (a) Transmission electron microscopy (TEM) image of the dried aqueous suspension of SrM nanoplatelets. (b) SEM image of the dried aqueous suspension of PS microspheres. (c) SEM image of the dried mixture of the aqueous suspensions of SrM nanoplatelets and PS microspheres.

Table 1 Characteristics of colloidal particles

Particles material	D or D × h, nm	ζ, mV
PS	545 ± 30	−29 ± 5
	80 ± 5	−40 ± 8
SrM	35 ± 15 × 6.0 ± 2.0	+39 ± 5
CdTe	4.4 ± 1.0	+45 ± 5

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In the range from 400 to 800 nm the absorption spectra of the toluene and water based SrM suspensions are similar (Fig. S2 in ESI†), which allows us to use the following formula for determination of transfer efficiency of SrM nanoplatelets to organic solvents:

where A_o and A_w are the optical densities at λ = 550 nm of the suspensions of SrM nanoparticles in organic solvent and water, respectively, which are proportional to the concentration of the SrM, and V_o and V_w are the volumes of the organic solvent and water used for the transfer process, respectively. It is important to note that this formula differs from the frequently used equation for TE calculations¹² which does not take into account the particles assembled at the interface between the aqueous and organic phases.

The transfer efficiency curve for SrM nanoplatelets versus the concentration of PS microspheres is presented in Fig. 2a. The TE reaches a maximum (88%) at a concentration of PS microspheres of ca. 0.17 wt% and then decreases. It seems that a low concentration of PS microspheres is not enough for the phase transfer of all SrM nanoplatelets in the organic solvent. Otherwise, a high concentration of PS microspheres leads to the steric constraints for the transfer of uncharged aggregates “SrM nanoplatelets–PS microsphere” to the organic solvent due to the excess of PS microspheres. The organic solvent used for the proposed phase transfer method should possess non-miscibility with water and be able to dissolve the particles used as carriers (PS). In this study, SrM nanoplatelets were successfully transferred to chloroform, benzene, toluene and styrene (Fig. 2b). It is worth mentioning that nanoparticles can be transferred to organic phase using a PS in other form, e.g. as a thin film. In this case, TE value is lower than for suspensions of PS microspheres because of the lower surface area of the carrier. In this study, for 1 cm² PS film consisted of the close-packed and sintered PS microspheres with D = 545 nm (Fig. S1 in ESI†), the TE value was equal to 22%. The experimental details of the preparation and characterization of the PS film are described in ESI.†

Fig. 2 (a) Dependence of the transfer efficiency of SrM nanoplatelets from water to toluene on the concentration of PS microspheres. (b) Photos of the colloidal suspensions of SrM nanoplatelets in different organic solvents.

According to the statistical analysis of several TEM micrographs (Fig. S3 in ESI†) the upper bound for the degree of SrM nanoplatelets aggregation in organic solvent is of ca. 35%. We cannot exclude the possibility of the aggregation of SrM nanoplatelets during drying of suspension on TEM grid or partial aggregation of SrM nanoplatelets in initial colloidal suspension in water. In order to better characterise the colloidal suspensions of SrM nanoparticles in organic solvents, optical spectra were recorded in the range of 400–800 nm under an applied external magnetic field (170 Oe) using polarised light (Fig. 3).

Fig. 3 Wavelength dependences of optical density A of the toluene based SrM suspension with an applied magnetic field of H = 170 Oe parallel (solid line) and perpendicular (dashed line) to the electric vector E of the incident polarised light.

The magnetic field was perpendicular to the wave vector of light during all measurements. The optical density at the perpendicular orientation of the applied magnetic field (H) and the electric vector (E) of the incident light (A_⊥) is higher than at parallel orientation (A_∥). The magneto–optical properties of water-based colloidal solutions of SrM nanoplatelets have been reported previously.^13,14 According to the theory described in ref. 15 and 16 the ratio R = A_⊥/A_∥ is affected by particle shape, the degree of the aligning of particles and the optical properties of the particles and medium. Ratio R is equal to 1 in the case of colloidal suspensions of isotropic particles or aggregates. Value R for the suspension of SrM nanoplatelets in water (1.81 at 550 nm) is slightly higher than for toluene based suspension (1.75 at 550 nm). Most probably, the change of R value after the phase transfer from water to toluene is caused by the decrease in difference between the refractive indexes of SrM particles (3.0)¹⁷ and the liquid media (1.33 for water¹⁸ and 1.50 for toluene¹⁸). Thus, there is no significant change in the particle aspect ratio, i.e. no aggregation process occurs during the phase transfer of the SrM nanoparticles.

CdTe nearly spherical nanoparticles were chosen as another model object for phase transfer from a water medium to water-immiscible organic solvents. CdTe nanoparticles are quite different in size and shape compared to SrM nanoplatelets. For the phase transfer of CdTe quantum dots with an average diameter of D = 4.4 nm and zeta potential ζ = +45 mV, a suspension of PS microspheres with D = 80 nm and ζ = −40 mV was used. Fig. 4a shows a photograph of the suspensions of CdTe nanoparticles under UV irradiation (λ = 365 nm) in water and benzene. The photoluminescence emission spectra of CdTe nanoparticles in water and benzene are given in ESI (Fig. S4).† It is clear that luminescence is observed in both colloidal solutions, which confirms the successful phase transfer of the particles from water to organic solvent. A TEM image of the dried drop of benzene based CdTe colloidal solution (Fig. 4b) shows that most (87%) of the CdTe nanoparticles after phase transfer are not aggregated.

Fig. 4 (a) Photograph of the suspensions of CdTe nanoparticles in water and benzene under UV irradiation (λ = 365 nm). (b) TEM micrograph of the dried colloidal solution of CdTe nanoparticles in benzene.

In conclusion, a simple and versatile way of the phase transfer of different types of nanoparticles with various shapes and sizes from water to water-immiscible organic solvents was reported. A water suspension of charged PS microspheres was used as carriers for nanoparticles (35 nm × 6 nm SrM nanoplatelets and 4.4 nm CdTe quantum dots) with opposite electric charge to their transfer to non-polar organic solvents. In this study, the concentration of PS microspheres in water is a critical factor for influencing the transfer efficiency of nanoparticles. The TE at the optimum concentration of PS microspheres exceeds 85%. The absence of significant aggregation process during the phase transfer was confirmed by UV-Vis spectroscopy and TEM analysis. The reported findings open a novel approach for phase transfer and will find many applications both in fundamental and applied sciences.

Acknowledgements

The work is supported by the Russian Science Foundation (Grant No. 14-13-00809). We would like to thank E. V. Samsonova (Lomonosov Moscow State University) for providing the PS microspheres with an average diameter of 80 nm. The authors are grateful to S. S. Abramchuk and D. S. Koshkodaev (Lomonosov Moscow State University) for the TEM measurements and A. A. Eliseev (Lomonosov Moscow State University) for the help in the construction of the transmission spectroscopy setup. Some parts of the experiment were carried out using the scientific equipment purchased by M. V. Lomonosov Moscow State University Program of Development.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Description of experimental and characterisation procedures. See DOI: 10.1039/c6ra19636e

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