Di-(2-ethylhexyl) dithiophosphoric acid surface protected gold nanoparticles: micellar synthesis, stabilization, isolation, and properties

Gold nanoparticles (AuNPs) were synthesized in the organic solution by means of the reduction of HAuCl4 by hydrazine in reverse micelles of oxyethylated surfactant Triton N-42, with decane as the dispersion medium. To isolate the powder of particles, the micelles were destroyed with chloroform in the presence of di-(2-ethylhexyl) dithiophosphoric acid as a surface protecting agent. According to the results of several experiments, the yield is within the limits of 90–98%, calculated for gold. The obtained preparations are dark blue hydrophobic powders containing aggregated but not agglomerated gold nanoparticles, as well as microcrystals (∼0.08–0.2 μm) of NaCl. The powders get re-dispersed in weakly polar organic solvents with the formation of colloidal solutions. The shape of the nanoparticles is spherical. Their nuclei are gold single crystals with a narrow size distribution; their diameter (dAu) is about two times as large as the diameter of the aqueous nucleus (dc) of initial micelles: dAu=7.7± 1.4 nm (dc=3.6 nm) and 8.8±1.5 nm (4.6 nm). The preparations were studied by means of dynamic light scattering, atomic force microscopy, transmission electron microscopy, UV–vis spectroscopy, IR spectroscopy, X-ray powder diffraction, and thermogravimetric and elemental analyses. In the case of the particles with dAu=8.8 nm, the product is a mixture of AuNPs and the salt with the molar ratio Au/NaCl≈1:4.54, while the gross composition of AuNPs per one gold atom is estimated as Au(C16H34O2PS2Na 2N2H4)0.16 with the number of gold atoms in one particle ∼21,000.


Introduction
Gold nanoparticles (AuNPs) are very important for advanced materials and nanotechnology first of all due to their electron, optical, and catalytic properties [1][2][3][4][5]. Among a number of the methods of their preparation, rather widespread one is chemical reduction of gold species in solutions including homogeneous [6,7], heterogeneous [8][9][10], and colloidal [11,12] systems. Synthesis in reverse micelles is one of the efficient methods of obtaining nanoparticles with a narrow size distribution due to the possibility of purposeful adjustment of micelle structure through dosed solubilization of the disperse aqueous phase (DAP) [12][13][14][15].
Previously, we studied the kinetics of growth and oxidative dissolution of gold and silver nanoparticles in the reverse micelles of surfactants [16][17][18][19][20][21]. The particle formation occurs inside the polar cavities of reverse micelles during the reduction of the ionic species of metals present in DAP. The isolation of particles from the colloidal solution in the form of the powdered solid phase is achieved due to the destruction of micelles with a polar solvent. The problem of the conservation of the initial size of nanoparticles during the isolation process is a very urgent task, so the isolation of the particles is performed in the presence of a protective agent preventing their agglomeration. Different types of protective reagents for gold nanoparticles are known, in particular citrate ion [6,7], alkylthiols [22][23][24], alkylthioacetates [25], organic amines [26,27], and phosphines [28].
The goal of the present work was to develop a method of synthesis, stabilization, and isolation of AuNPs from micellar solutions of Triton N-42 with the conservation of initial size, as well as examination of some physiochemical characteristics of the resulting preparations. The protective reagents for stabilization were AOT (intermediate protective reagent) and di-(2-ethylhexyl) dithiophosphoric acid (major stabilizing agent). Di-(2-ethylhexyl) dithiophosphoric acid had not been used previously for particle stabilization, but it is well known as a selective extracting agent for nonferrous and noble metals [29][30][31][32].
Crystalline HAuCl 4 ⋅3H 2 O was synthesized from metal Au according to the conventional procedure [33].  (Table 1).  Examination of the nanoparticle preparation by means of atomic force microscopy (AFM) was performed in the semi-contact mode with the scanning probe microscope Solver Pro (NT-MDT) in the air. The samples were prepared by depositing a drop of the colloidal solution of the preparation in toluene on the substrate made of mica and by subsequent evaporation of the solvent.
The morphology and size of individual nanoparticles in preparations were studied with the transmission electron microscope JEM-2010 (JEOL) with the accelerating voltage of 200 kV. To prepare the sample, a drop of the solution of particle preparation in toluene treated with ultrasound was dried on the substrate made of 200-mesh copper grid covered with a "holey" carbon film. Local energy-dispersive X-ray spectroscopy (EDX) analysis of the preparations was performed with EDAX spectrometer (EDAX Co) with Si-Li detector.
X-ray powder diffraction (XRD) data were obtained with a DRON-3M diffractometer (R=192 mm, CuK α radiation, Ni-filter, scintillation point detector with amplitude discrimination) at the room temperature over the range 5-60°2 θ with step 0.03°in 2θ and 1 s counting time per step. The samples were prepared by grinding the powder in agate mortar with heptane and further deposition of this suspension on the polished side of a standard sample holder. Coherent scattering region for AuNPs was estimated using the Scherrer equation. The calculation was performed for the diffraction peak (111) of gold taking into account FWHM of the reference sample-Si.
The UV-visible spectra of the solutions were recorded with SHIMADZU UV-1700 spectrophotometer.
The IR absorption spectra within the range 4,000-400 cm −1 were recorded with a Fourier transform spectrometer SCIMITAR FTS 2000 in a thin layer between KBr glass plates for the liquid reagent C 16 H 34 O 2 PS 2 H and in KBr tablets for the nanoparticle preparation.
Chemical CHN analysis of the preparations was made with the EURO EA 3000 instrument. Analysis for gold and sodium after the dissolution of the weighed portion of preparation in aqua regia, evaporation, and transfer into the solution of 2 M HCl were carried out by means of atomic absorption in airacetylene flame using Hitachi Z-8000 instrument.
Thermogravimetric examination of the preparation was carried out using with thermobalance TG 2009 F1 Iris (NETZSCH) in a corundum crucible under the atmosphere of helium at the gas flow rate of 35 mL/min and heating rate of 10°C/min.

Results and discussion
Isolation of nanoparticles by means of double sequential stabilization Destruction of micelles by chloroform without adding AOT causes rapid agglomeration of AuNPs even before DTPA is introduced. In the presence of chloroform, adsorption on particle surface is likely to be much weaker for nonionic TN-42 than for anionic surfactant AOT that provides intermediate stabilization of particles during micelle destruction and then is replaced by DTPA, which is a stronger protective reagent for chalcophilic gold (Fig. 1).
It was established by means of non-aqueous electrophoresis that, during the reduction of gold by hydrazine in TN-42 micelles, the nanoparticles have a positive surface charge increasing linearly with an increase in the concentration of the reducing agent [37]. During the reduction of Au III and Ag I by hydrazine in AOT micelles, the charge of Au and Ag NPs is positive, too, and it increases with the dilution of AOT solution in decane by chloroform [14,38]. On the basis of the results reported in the present work and those obtained previously, we can propose the following hypothetical mechanism of double sequential stabilization. Chloroform is a known reagent destroying the micelles of oxyethylated surfactants in alkanes even in insignificant concentrations; it is used for concentrating precious metals in the form of salt solutions [39] and NPs [40]. In the case of NPs, chloroform desorbs a part of the molecules of nonionic TN-42 from the surface of AuNPs, which causes a decrease in the osmic and steric components of disjoining pressure and causes subsequent coagulation and agglomeration of NPs [37]. The presence of AOT in chloroform causes more rapid diffusion of the surface-active anion to the positively charged nanoparticle and subsequent adsorption due to the electrostatic attraction. The protective surface layer of surfactant on a nanoparticle is partially recovered, and coagulation is suppressed. Diffusion of uncharged DTPA molecules is slower, and adsorption at the initial stage is due only to Van der Waals attraction. However, with time, DTPA molecules start to form strong, preferably donor-acceptor bonds with AuNPs due to sulfur atoms possessing high affinity to chalcophilic gold [33], and displace AOT from the surface layer. Thus, AOT provides intermediate stabilization of AuNPs. The surface DTPA layer in the organic environment containing chloroform protects nanoparticles only from agglomeration but does not protect them from coagulation, so as time goes by, large aggregates (floccules) of NPs undergo sedimentation and get isolated as the powder. The presence of chemisorbed surface layer of DTPA (with the hydrophobic layer of hydrocarbon groups turned outward) creates the possibility for gold nanoparticle to get peptized in nonpolar solvents. Note that our considerations have a hypothetical character; a more extensive investigation of the mechanism of double sequential stabilization is a part of our future work.
Properties of the preparation The AuNPs powder is hydrophobic and gets re-dispersed in weakly polar organic solvents, in particular in toluene, with the formation of blue colloidal solutions. The color and absorption spectrum of the solution exhibiting a broad band with the maximum at about 650 nm (Fig. 2) provide evidence of particle aggregation [41]. During re-dispersion through manual agitation, aggregates with a size up to d ha =600±50 nm are detected by means of DLS in the colloidal solution of the preparation; under the ultrasonic treatment of the solution with moderate intensity the size decreases to d ha =240± 20 nm. A similar trend is observed also by means of AFM. The images of the films (Fig. 3a, b) obtained as a result of the evaporation of toluene solution of the preparation on mica surface before ultrasonic treatment display the aggregates of ellipsoid shape; their major and minor axes are estimated as d 1 =225±15 and d 2 =200±18 nm, respectively, for the number of measurements N=102 (Fig. 3c, d). After ultrasonic treatment of the solution, the size of aggregates decreased to 40-50 nm. Thus, aggregation of particles in the preparation does not lead to their agglomeration and merging.
The IR spectra of DTPA and the AuNPs preparation are presented in Fig. 4. The spectra contain the bands characteristic of dialkyldithiophosphoric acids [42][43][44]: the bands of asymmetric bending vibrations of CH 3  The XRD data show that two crystal phases are present in the preparations: metal Au and NaCl (Fig. 5). The source of Cl − ions is the initial form AuCl 4 − , while the source of Na + is AOT during the isolation of AuNPs. Judging from broadening of diffraction peaks, the gold phase is ultrafine. The coherent scattering region for AuNPs in the preparation obtained at V s /V o =0.02 was estimated to be at a level of 3-4 nm. For the preparation of AuNPs obtained at V s /V o =0.02, elemental analysis was carried out. The results are presented in Table 2. According to the gold content of the preparation 37%, the yield of the product calculated for the metal is estimated on the basis of several experiments to be within the range 90-98%. The presence of nitrogen in the preparation is explained by the use of hydrazine as the reducing agent in the synthesis of AuNPs. Because of a large excess of the reducing agent, it is assumed that the major form of nitrogen in the preparation is directly N 2 H 4 .
The TGA data are in satisfactory agreement with the elemental analysis. Three steps of thermolysis of AuNPs are observed during heating the sample of the preparation in helium atmosphere (Fig. 6a). The first step relates to the temperature range of 90-210°C, and the weight loss of  (2) AuNPs powder re-dispersed in toluene 2.1% is observed, which corresponds to the detachment of hydrazine. Its calculated content is equal to 1.9% (Table 2). At the second stage (210-430°C) and the third one (430-735°C), the weight losses are 3.1% and 7.1%, respectively. The total weight loss at the second and the third stages (10.2%) can be related to the detachment of the protective reagent; its mass fraction in the preparation is estimated to be 10.6% as suggested by the data of elemental analysis. Therefore, the whole DTPA of the AuNPs sample is completely desorbed at 210-735°C. According to XRD data, the product of thermal decomposition at 735°C is a mixture of the phases of metal Au and NaCl (Fig. 6b). When heated above 735°C weight loss is observed due to sublimation of NaCl. The residual mass is 35.1%, which roughly corresponds to the Au content in the product (37%).
Characterization of AuNPs One can clearly see the chain structure of the aggregates in the TEM micrograph (Fig. 7a,  b), which is a characteristic of the ensembles of nano-  Table 1). The EDX spectrum of individual AuNPs (Fig. 7c), in addition to the signals from C and Cu, which are due to the substrate, exhibits intense signals of Au (lines AuM α and AuL α ), as well as the elements present in the protective reagent C 16 H 34 O 2 PS 2 H: O (OK α line), P (PK α line), and S (SK α line). In addition to AuNPs, the TEM micrograph contains separate cubic microcrystals of the salt, with the edges 80-150 nm long (Fig. 7g) Their EDX spectrum (Fig. 7h) contains the signals of Na (NaK α line) and Cl (ClK α line). The salt crystals create a strong background, so the signals of Na and Cl manifest themselves in the EDX spectrum of gold particles (Fig. 7c). Thus, the results of TEM and EDX analysis confirm the XRD data concerning the presence of the dispersed NaCl phase in the preparations of AuNPs.
One can see in the high-resolution TEM (HRTEM) micrograph that the nuclei of nanoparticles are single crystals (Fig. 7d). Interplanar spacings are equal to 2.35Å and relate to the distance d 111 of metal gold (Fig. 7e, f). The absence of twinning structures in AuNPs, as well as the narrow size distribution of the particles (17-18%), confirms our idea of the kinetic mechanism according to which the growth of AuNPs occurs in separate micelles independently and synchronously as a result of the addition of new gold atoms to the particles but not due to the addition of particles from neighboring micelles [45].
The average values of AuNPs nucleus diameter, determined by means of DLS in the micellar solution of TN-42 during synthesis, are in good agreement with TEM data for individual particles in the preparations (Table 1). So, the change of the protective reagent for the particles from TN-42 sequentially for AOT and then for DTPA during the isolation of the powder allows one to conserve the size of AuNPs nucleus, obtained during the micellar synthesis. At the same time, under the chosen synthesis conditions, the size of AuNPs is almost two times as large as the size of the water nuclei of initial micelles. A similar phenomenon was observed previously by other researchers [46]. Therefore, the micelles of oxyethylated surfactants like TN-42 are not rigid templates but undergo structural rearrangements while the particles get formed.
On the basis of the results of XRD, IR, ED and elemental analysis, stoichiometry of the molecules of protective reagent, and the principle of electric neutrality, it may be concluded that the preparation is a mixture of AuNPs and the salt with the molar ratio Au/NaCl≈1:4.54, and the gross composition of AuNPs per one gold atom is estimated as Au(C 16 H 34 O 2 PS 2 Na 2N 2 H 4 ) 0.16 ; the number of gold atoms in one particle is ∼21,000 in the case when the metal nucleus has a diameter d Au =8.8 nm. The total number of gold atoms in a particle was determined as N t = V Au /V a , where V Au =(1/6)·π·d Au 3 =357 nm 3 is the volume of gold nanoparticle and V a =0.016962 nm 3 is the atomic volume of gold. The resulting value N t =21,036 agrees with the calculated value of N t calc for the "magic" particle with completely filled 18 atomic layers having the fcc structure.

Conclusions
We present the synthesis method for AuNPs in reverse micelles of nonionic oxyethylated surfactant TN-42 with the subsequent isolation of nanoparticles as powder. To isolate nanoparticles from the micellar solution, the micelles were destroyed with a polar solvent, chloroform, in the presence of a surfactant, AOT (intermediate stabilizing agent, and a protective reagent, DTPA (the final surface stabilizing agent), which possesses high affinity to gold due to S-containing functional groups. The developed method of double sequential stabilization allows one to isolate AuNPs from micelles without changing the size of particle nuclei. The yield of the product with respect to gold is 90-98%. The resulting preparations contain AuNPs and NaCl microcrystals. The NPs consist of the metal nucleus and the protective shell. The nuclei are gold single crystals having the spherical shape and a narrow size distribution with the average diameters 7.7±1.4 and 8.8±1.4 nm, which are two times as large as the diameters of aqueous nuclei of initial TN-42 micelles 3.6 and 4.6 nm. The shell is composed of chemisorbed C 16 H 34 O 2 PS 2 − anions that have Na + cations as the counter-ions; in addition, due to their oxygen atoms, they participate in hydrogen bonding with two N 2 H 4 molecules. Assuming monolayer adsorption, there is approximately one C 16 H 34 O 2 PS 2 − anion per one surface gold atom.
The preparation is a hydrophobic powder of aggregated but not agglomerated AuNPs peptizing in weakly polar organic solvents with the formation of colloid solutions. These solutions can be used to deposit the layers of AuNPs on various substrates and to impregnate porous matrices.