Facile preparation of size-controlled gold nanoparticles using versatile and end-functionalized thioether polymer ligands

Abstract

At present, thiol ligands are generally used whenever the classical Brust–Schiffrin two-phase method is employed to prepare metal nanoparticles. In general, the previous research was mainly focused on utilizing small molecular thiol compounds or thiol polymers as the stabilizers in organic phase to obtain small sized and uniform gold nanoparticles (Au NPs). Such preparations are usually associated with the problems of ligand exchange on the nanoparticle's surface due to strong Au–thiol interaction. Herein, we report an approach to produce fairly uniform Au NPs with diameters about 2–6 nm using thioether end-functional polymer ligands (DDT–PVAc and PTMP–PVAc) as the capping agents. These nanoparticles are thoroughly characterized using DLS, TEM, UV-Vis spectroscopy and other complementary techniques. The results indicate that multidentate thioether polymeric ligands (PTMP–PVAc) lead to formation of smaller but special ‘multimer’ morphology in organic phase; whereas fairly uniform nanoparticles are produced using monodentate thioether functionalized ligands (DDT–PVAc). Further modification of such polymer ligands to introduce the hydrophilic functionalities realizes the phase transfer of Au NPs from organic to aqueous media.

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Introduction

Gold nanoparticles (Au NPs) have become attractive nanoscale materials due to their unique properties including high catalytic activity,¹ special optical properties^2,3 and the ability of self-assembling,⁴ which have been extensively applied in the drug targeting delivery system,^5,6 device fabrication,^7,8surface-enhanced Raman scattering (SERS),⁹ photothermal cancer therapy,¹⁰ immobilization of proteins for conformational studies,¹¹etc. The properties of these nanoparticles are highly dependent on their size, morphology, and surface chemistry.^12–14 Surface chemistry is particularly important because all the chemical properties of such materials depend on the surface functionality which controls the chemical interaction of any molecules with nanoparticles.¹⁵ It is, therefore, extremely important to understand and control the chemical properties of such materials, in addition to size and shape-dependent physical properties.

Au NPs are commercially available in many forms, and numerous preparative methods are documented in the literature to control their size, shape and surface chemistry. In addition to the Frens–Turkevich method^16,17 of preparing gold hydrosols, the Brust–Schiffrin method¹⁸ is one of the most popular methods to obtain small metal nanoparticles in organic media. In this method, smaller organic thiol molecules control the nucleation and growth of nanoparticles, in the presence of strong reducing agents, and cap them at initial stages to produce metal nanoparticles of smaller size. There have been several reports to control the size of so-produced nanoparticles by controlling the ratio of thiol to gold, but usually are associated with a significant polydispersity, which limits the fundamental applications of Au NPs in electrochemical quantized capacitance charging,¹⁹ thermal gradient optical imaging²⁰ and single molecule optoelectronics.²¹ In the majority of these cases, monodisperse fractions of particles are usually prepared in low yield following cumbersome size separation procedures, such as size exclusion chromatography. But such fractionation methods are usually associated with various issues and do not necessarily lead to monodisperse samples. The availability of a simple, robust protocol for the preparation of uniform Au NPs with controlled surface chemical properties would thus be of broad practical applications in many fields.

In general, the early research work was mainly focused on utilizing small molecular compounds and thiol polymer as the stabilizers in organic phase to obtain small sized and uniform Au NPs.^22–29 Zhang et al.³⁰ compared the effect of preparing Au NPs with long-chain alkanethioacetate (n-tetradecanethioacetate) ligand and n-tetradecanethiol. The size of alkanethioacetate monolayer-protected Au NPs was 4.9 ± 1.2 nm. Additionally, using a thiol-terminated oil-soluble polymer is another necessary and effective way to prepare Au NPs.^31,32Thiol-capped polystyrene macromolecules (PS–SH) were applied to generate polydisperse nanoparticles long before. The two-phase method has been adapted for the preparation of poly(3-octylthiophene-2,5-diyl) stabilized Au NPs with diameters about 1–10 nm by Dammer et al.³³Water-soluble thioether polymers have recently been introduced as attractive ligands to produce monodisperse nanoparticles in aqueous media and offer a very useful system to restrict the size distributions and facile functionalization with other ligands for a variety of applications.^34–36 This technique was successful in the preparation of monodisperse fluorescent Au NPs,³⁷ with diameter approximately 1.1–1.7 nm, and superparamagnetic nanoparticles (Co NPs and Fe₃O₄ NPs).^38,39 Definitely, these water-soluble thioether polymers are the most effective ligands to form small sized and monodisperse Au NPs in water presently. Synthesis of the monodisperse Au NPs in organic phase also has significant prospects in the field of optoelectronics.¹⁹ It is, therefore, instructive for us to synthesize size-controlled Au NPs using thioether polymer ligands, which is rarely reported in organic phase up to now.

Herein, we now report on the design of hydrophobic thioether end-functionalized polymers, which can be used to produce Au NPs in organic media using a modified Brust–Schiffrin method. The significantly size-controlled Au NPs within the range of 2–6 nm were successfully prepared and the effect of thioether polymer ligands structure, molecular weight and concentration on the particle size, morphology and dispersity has also been investigated. It was interestingly observed that multidentate thioether polymeric ligands (PTMP–PVAc) result in the formation of smaller but special ‘multimer’ morphology in organic phase; whereas fairly uniform nanoparticles are produced using monodentate thioether functionalized ligands (DDT–PVAc), through the measurement by DLS, TEM, UV-Vis spectroscopy and other complementary techniques. Additionally, the Brust–Schiffrin method is also associated with the issues of replacing strong interacting thiol molecules on their surface with other ligands. The ligand exchange is usually slow and mostly ends with a mixture of ligands at the surface of nanoparticles.^40–44 The loose interaction of thioether functionality with gold surfaces renders such nanoparticles more prone to post-synthesis functionalization with a variety of hydrophobic ligands to get a fair control over their surface chemistry. Moreover, hydrolysis of such polymeric ligands renders them hydrophilic resulting in their phase-transfer from organic to aqueous media. These easy-to-functionalize Au NPs would thus be very useful for a variety of applications including optoelectronics, catalysis and sensing due to their controllable surface chemistry.

Experimental

Materials

All chemicals were of analytical grade and used as received without any further purification, unless otherwise described. Vinyl acetate ester (VAc, 99%), dodecanethiol (DDT, 99%), hydrogen tetrachloroaurate (HAuCl₄·4H₂O, 99%), cetyltrimethylammonium bromide (CTAB, 99%), sodium borohydride (NaBH₄, 99%), sodium hydroxide (NaOH, 99%), n-butanol, n-hexane, toluene, acetone, anhydrous ethanol, methanol, and tetrahydrofuran (THF) were purchased from National Medicines Corporation Ltd., China. Pentaerythritol tetrakis 3-mercaptopropionate (PTMP, 97%) was obtained from Aldrich.

Synthesis of functional thioether polymer ligands

In a typical polymer synthesis reaction, 50 mL methanol solution containing vinyl acetate ester (VAc, 40.000 g, 0.465 mol), dodecanethiol (DDT, 18.824 g, 0.093 mol) and 2,2′-azobisisobutyronitrile (AIBN, 0.175 g, 0.930 mmol) were added to a four-necked round-bottom flask fitted with a reflux condenser under nitrogen with magnetic stirring. The temperature of the reaction mixture was maintained at 62 °C and refluxed for 5–6 h (Scheme 1a). The solvent was removed by rotary evaporator after the reaction. The viscous products were dissolved into THF and then isolated by precipitating into cold n-hexane. The excess solvent and monomers were removed by evaporation using a vacuum oven set at 40 °C. A fraction of low molar mass polymer, unreacted monomer and some oligomers due to incomplete reaction were removed during the precipitation step. The yield of thioether polymer DDT–PVAc was 15.013 g, 37%. Another thioether end-functionalized polymer was synthesized by the same method, but using PTMP instead of DDT as the chain-transfer agent and the yield of PTMP–PVAc was 13.674 g, 33.7%.

Scheme 1 The reaction scheme for the synthesis of thioether polymer ligands DDT–PVAc and PTMP–PVAc (a) and preparation of Au NPs stabilized by thioether polymer ligands (b).

Preparation of Au NPs by thioether polymer ligands

A solution of cetyltrimethylammonium bromide in toluene (8 mL, 50 mM) was mixed with an aqueous solution of hydrogen tetrachloroaurate (3 mL, 30 mM) and n-butanol (0.5 mL) as a co-surfactant by vigorous stirring. After the complete transfer of CTAB–Au complex into the organic layer, polymer ligand DDT–PVAc (500 mg, or PTMP–PVAc) was added to the mixture. A freshly prepared aqueous solution of sodium borohydride (2.5 mL, 0.4 mol L⁻¹) was slowly added into the system. The reduction was allowed to continue for 5–6 h and then the organic layer was separated (Scheme 1b). The separated layer was evaporated to 5 mL by a rotary evaporator and mixed with 400 mL ethanol to remove excess polymer ligands, the surfactant, etc. The mixture was kept for 4 h at −4 °C and then ethanol was removed by centrifugation. The post-synthesis washing with ethanol was repeated for three times to get fairly clean particles. Finally, the crude product was dissolved into 10 mL toluene. Yield: 399 mg (77%).

Characterization

The measurements of particle size were done using high-performance particle sizer (Zetasizer Nano S) with DLS (Dynamic Light Scattering) and NIBSTM (Non-invasive Back Scatter) technology from Malvern Instruments (Malvern, UK) with effective detection capability from 0.6 to 6000 nm. The infrared spectra (IR) of Au NPs were acquired by the VERTEX 70 FTIR and Raman Spectrophotometer (Bruker, Germany). The viscous specimen was dispersed in appropriate solvent and the solution was carefully poured between two KBr plates. The powder solid samples were also prepared by mixing the samples with pure KBr and then pressed into transparent films for further characterization. Transmission electron microscopic (TEM) micrographs of nanoparticles were taken on a Tecnai G20 microscope (FEI Corp., USA) instrument operated at an accelerating voltage of 200 kV. All samples were dispersed onto the pure carbon-coated copper grids (400 mesh) which were usually used in the TEM detection. The grids were immersed into the toluene solution containing Au NPs and then evaporated the solvent by air drying. ImageJ software (1.40, NIH, USA) was employed to analyze the particle size and the particle size distribution of nanoparticles, and the histograms were prepared by the data obtained from representative micrographs. The typical surface plasma resonance feature of Au NPs was determined with the Lambda 35 UV-Vis Spectrophotometer (Perkin-Elmer, USA). The nanoparticle samples were dispersed in toluene which was used as the reference solvent.

Results and discussion

Synthesis and characterization of thioether end-functionalized polymer ligands

In order to examine the effect of thioether end-functionalized polymer ligands on the morphology and surface properties of Au NPs, polymer ligands were synthesized by the facile chain-transfer radical polymerization method using vinyl acetate ester as a monomer, and DDT and PTMP as the chain-transfer agents (Scheme 1a). Methanol was used as the solvent instead of ethanol because the transesterification could possibly take place between vinyl acetate ester and ethanol at a certain temperature and ethanol, being a stronger radical transfer agent, could influence the molecular weight of PVAc. The high molecular weight of polymer (>15 000) and the slightly wide molecular weight distributions (PDI = 1.6–2.0, Fig. S1 and S2 in the ESI†) were due to the radical polymerization technique. The low product yield was mainly because of the solution polymerization and repeated purification steps. Meanwhile, the experiment should take the appropriate conversion rate into account for the sake of eliminating the phenomenon of auto-acceleration and ensuring the narrow distribution of polymer molecular weight. ¹H NMR spectroscopy was selected to validate the structure of polymer ligands DDT–PVAc and PTMP–PVAc (Fig. S3 and S4 in the ESI†).

Preparation of Au NPs using thioether polymer ligands

A series of polymer ligands with various molecular weights were synthesized by adjusting the amount of chain transfer agent for the purpose of evaluating the influence of the polymer ligands molecular weight on the average size of Au NPs and dispersity (Table 1). It is already known that the amount of chain transfer agent is the primary factor affecting the molecular weight of free radical polymerization. Under the same reaction conditions, molecular weights of samples 1, 2, 3 and 4 were 51 600, 23 150, 19 550 and 16 910 corresponding to the amount of chain transfer agent (DDT) of 2%, 10%, 20%, and 25% respectively. Obviously, the regular change in molecular weights of polymer ligands is in accordance with the properties of free radical polymerization. We prepared Au NPs using the fixed molar concentration of thioether polymer ligands DDT–PVAc (3.5 mM) with various molecular weights to observe the effect on the average size and distribution of Au NPs. Since Zetasizer nano S measures the hydrodynamic diameter, the sizes of Au NPs obtained by DLS (Fig. S7–S9 in the ESI†) were appreciably larger than those obtained using TEM. Fig. 1 demonstrates the average diameter of Au NPs is 6.8 ± 2.3 nm when the molecular weight of polymer is 51 600 g mol⁻¹, comparatively, 5.4 ± 1.1 nm, 3.7 ± 0.5 nm, 3.5 ± 0.6 nm when the molecular weights are 23 150, 19 550 and 16 910 g mol⁻¹, respectively (Fig. S10 in the ESI†). Apparently, the lower molecular weight polymer gives rise to the Au NPs of relatively smaller size. Meanwhile, particle size distributions of Au NPs become narrower apparently which is consistent with the red shift in the UV-Vis spectra (Fig. 2).

Table 1 DDT–PVAc and PTMP–PVAc thioether polymer ligands with a range of molecular weights (Au NPs were prepared by thioether polymer ligands which fixed the concentration at 3.5 mM)

Item	VAc/thiol (mol/mol)	Molecular weight (g mol^−1)			Yield (%)	D (nm)
		M n	M w	PDI
DDT–PVAc1	100/2	51 600	78 320	1.518	34.2	6.8 ± 2.3
DDT–PVAc2	100/10	23 150	44 930	1.941	48.8	5.4 ± 1.1
DDT–PVAc3	100/20	19 550	32 820	1.679	37.4	3.7 ± 0.5
DDT–PVAc4	100/25	16 910	28 300	1.674	22.0	3.5 ± 0.6
PTMP–PVAc	100/2	27 860	39 880	1.431	33.7	3.8 ± 0.8

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Fig. 1 TEM micrographs of Au NPs prepared from four different molecular weights (M_n) of monodentate ligands DDT–PVAc: (a) 51 600 g mol⁻¹, (b) 23 150 g mol⁻¹, (c) 19 550 g mol⁻¹, and (d) 16 910 g mol⁻¹.

Fig. 2 UV-Vis absorption spectra of Au NPs stabilized with monodentate ligands DDT–PVAc in toluene at different molecular weights. Inset: photograph of Au NPs prepared from various molecular weights (M_n) of monodentate ligands DDT–PVAc: (a) 51 600 g mol⁻¹, (b) 23 150 g mol⁻¹, (c) 19 550 g mol⁻¹, and (d) 16 910 g mol⁻¹.

We also employed the same method to prepare Au NPs using DDT ligand in order to compare the size and distribution of nanoparticles prepared by using our thioether end-functionalized polymer ligands as capping agents. From the TEM micrographs (Fig. S11 in the ESI†), it is evident that DDT results in the larger and less uniform nanoparticles while using the same reaction conditions and molar ratios of the capping ligands. In fact, the dispersity of nanoparticles prepared by our polymer ligands is at least comparable or even superior to some recent reports. Thus, the results reveal that thioether polymer ligand DDT–PVAc also has the capability to control the size and size-distributions of Au NPs when employed in two-phase Brust–Schiffrin methods. It is probably because the polymer ligands are more prone to form a ‘net’ as a barrier, compared with smaller molecular weight compounds (DDT, etc.), which will protect the Au NPs efficiently and be more favorable to disperse during the process of growing. However, a slight rise in particle size is observed with the molecular weight of polymer ligands increased. That is because the high molecular weight polymers require more time to diffuse and reach the growing nanoparticle to stop its growth by passivating its surface.^35,45

To examine the effect of concentration of thioether polymer ligands, we prepared a series of Au NPs with various concentrations of ligands targeting the same molecular weight (M_n = 16 910 g mol⁻¹) for each sample. The concentration of thioether polymer ligands DDT–PVAc was capable of influencing and controlling the particle size and the particle size distribution. The investigations by TEM measurements indicate the average particle sizes of Au NPs stabilized by polymer ligand DDT–PVAc range from 3 nm to 10 nm. Fig. 3 shows that when the concentrations of DDT–PVAc used are 1.0 mM, 3.5 mM, 5.0 mM and 8.0 mM, the corresponding diameters of Au NPs are 9.8 ± 2.3 nm, 3.5 ± 0.4 nm, 4.9 ± 0.6 nm and 5.8 ± 1.2 nm respectively (Fig. S12 in the ESI†). DLS results are also in agreement with the TEM results that the diameter of Au NPs decreases initially and then increases. UV-Vis spectroscopy shows that surface plasma resonance absorption peaks for Au NPs first blue shift and then turn back (red shift) which is totally consistent with the Mie theory (Fig. 4).⁴⁶

Fig. 3 TEM micrographs of Au NPs prepared from four different concentrations of monodentate ligands DDT–PVAc: (a) 1.0 mM, (b) 3.5 mM, (c) 5.0 mM, and (d) 8.0 mM.

Fig. 4 UV-Vis absorption spectra of Au NPs stabilized with monodentate ligands DDT–PVAc in toluene at different concentrations. Inset: photograph of Au NPs prepared from four different concentrations of monodentate ligands DDT–PVAc: (a) 1.0 mM, (b) 3.5 mM, (c) 5.0 mM, and (d) 8.0 mM.

Thereafter, the synthesis of Au NPs with four different concentrations of PTMP–PVAc was carried out. The obvious distinction was that the average nanoparticle size did not change regularly and significantly as the polymer concentration increased, which was in contrast to DDT–PVAc ligands. TEM micrographs prove that the sizes of Au NPs, stabilized by multidentate PTMP–PVAc ligand at four concentrations which are 1.0 mM, 3.5 mM, 5.0 mM and 8.0 mM (equal to molar concentration of DDT–PVAc), are 3.9 ± 1.2 nm, 3.8 ± 0.8 nm, 4.0 ± 1.3 nm and 5.8 ± 1.1 nm respectively (Fig. 5 and S13 in the ESI†). However, it is noticeable that the dispersity of Au NPs stabilized by multidentate PTMP–PVAc ligands does not change significantly for each concentration in particular and this is also in accordance with UV-Vis (Fig. 6) and DLS spectra. The appreciable change in the particle size and the particle size distribution is caused by the tetra-dentate end group of PTMP which has a high space steric effect. The multidentate structure has the advantage of forming reticular space which is like a restrained environment to control the morphology, size and dispersity of Au NPs in the system. Therefore, the concentration and molecular structure of linear polymer ligands DDT–PVAc could tailor the size and distribution of Au NPs efficiently, whereas multidentate ligand PTMP–PVAc does not show this property.

Fig. 5 TEM micrographs of Au NPs prepared from four different concentrations of multidentate polymer ligands PTMP–PVAc: (a) 1.0 mM, (b) 3.5 mM, (c) 5.0 mM, and (d) 8.0 mM.

Fig. 6 UV-Vis absorption spectra of Au NPs stabilized with multidentate polymer ligands PTMP–PVAc in toluene at different concentrations. Inset: photograph of Au NPs prepared from four different concentrations of multidentate polymer ligands PTMP–PVAc: (a) 1.0 mM, (b) 3.5 mM, (c) 5.0 mM, and (d) 8.0 mM.

The structure of polymer ligands plays a significant role in influencing the size and shape of Au NPs. We focused on the thioether polymer ligands containing two different thiol chain transfer agents, for example dodecanethiol (DDT) is a monodentate chain transfer agent and pentaerythritol tetrakis 3-mercaptopropionate (PTMP) has a multidentate structure. It is interesting to know that the morphology of Au NPs prepared by DDT–PVAc and PTMP–PVAc had a visible dissimilarity despite of the fact that they were used in equal concentrations. Fig. 3 illustrates the shape of Au NPs stabilized by DDT–PVAc is almost individually spherical; however, the Au NPs stabilized by PTMP–PVAc have special morphology which is just like the agglomeration of the particles, as shown in all concentrations (Fig. 5), even after repeated ultrasonic and centrifugal treatments. These Au NPs present dimers, trimers, tetramer structures, etc. The special morphologies are universal in PTMP–PVAc samples, but in the DDT–PVAc group, there are few similar phenomena. Although there are also few agglomerations of Au NPs in the DDT–PVAc group, it can be found at the condition of lower concentration and larger molecular weights, which is attributed to weak protection.

The possible mechanism of this morphology may be due to the structure of the polymer ligands. PTMP–PVAc polymer ligands, which possess both special thioether and primary thiol functional groups, have more than one site for binding gold (Scheme 2). Compared with PTMP–PVAc polymer ligands, DDT–PVAc has the linear structure which has only one site to combine Au NPs in organic media. ¹H NMR spectrum is a useful analytical tool to determine which sites are able to influence the interaction of polymers with the Au NPs.²⁸ The observation that the proton signal near 1.25 ppm weakened and the signal at 2.55 ppm disappeared after the polymer ligands DDT–PVAc connected with the nanoparticles confirms that the thioether groups interact with the Au NPs (Fig. S3 in the ESI†). Correspondingly, the ¹H NMR spectra of PTMP–PVAc polymer ligand and corresponding PTMP–PVAc –Au NPs are shown in Fig. S4†. The peak near 1.5 ppm which belonged to a thiol proton became broader and weaker after PTMP–PVAc interacted with Au NPs. These noteworthy phenomena may be attributed to binding of –SH sites to the gold surface. In addition, the site of thioether groups had the ability to stabilize Au NPs proved by the changes of proton signals near 2.61–2.70 ppm and 2.72–2.83 ppm after stabilization of the Au NPs. It is worthwhile to notice that such special morphology is not the ordinary agglomeration, but rather the shape of the particles bridged by the multi-site polymer ligands PTMP–PVAc. Thus the UV-Vis spectra and the DLS results show only the properties of particles not the agglomerates, which is the evidence of the formation of the special multimer morphology. On the other hand, PTMP–PVAc has a more complex space structure, leading to the high stereo-hindrance effect which could influence the particles to aggregate together. Moreover, the better stickiness of PTMP–PVAc ligands is one of the factors which could lead to faster pervasion of Au Nps. In comparison, DDT–PVAc has relatively lower steric hindrance and results in formation of individual particles.

Scheme 2 The structure of thioether polymer ligands DDT–PVAc and PTMP–PVAc. The blue parts represent the structure of transfer agent (DDT, PTMP) and the red sections mean the structure of PVAc. The circles of dashed line represent the possible sites for binding Au NPs. The left TEM micrograph is Au NPs prepared by DDT–PVAc (3.5 mM) and the right one is Au NPs prepared by DDT–PVAc (3.5 mM).

Based on the reliable and available data above, monodentate and multidentate thioether polymer ligands play almost discriminative roles in the process of preparing Au NPs. Multidentate PTMP–PVAc polymer ligands resulted in the formation of small size and special ‘multimer’ morphology of Au NPs; however monodentate DDT–PVAc is benefit to tailor the particle size and uniform Au NPs effectively.

Modification of thioether polymer ligands and ligands exchange

In order to confirm the versatile properties of our thioether polymer ligands, the modification process was introduced, which could realize the transition of Au NPs from the organic phase to the aqueous phase. We utilized the classical ester alcoholysis method (see Scheme S1 in the ESI†) in an alkaline catalytic environment. Infrared spectroscopy is a powerful technique for confirming the structural information concerning thioether polymers DDT–PVAc and PTMP–PVAc. In DDT–PVAc group, a strong absorption peak at 1740 cm^−l is just for the stretching vibration peak of C=O (carbonyl group characteristic peak) and the stretching vibration peak of C–O (ester group characteristic peak) is in the vicinity of 1260 cm^−l and 1023 cm^−l (Fig. 7). Furthermore, strong absorption peaks near 3438 cm^−l, are a proof of the successful process, and are due to the large number of hydroxyl groups after the chemical modification.

Fig. 7 FTIR spectra of the modification process of gold nanoparticles prepared with thioether polymer ligands. (a) Polymer ligands DDT–PVAc–Au NPs before the modification. (b) Polymer ligands DDT–PVA–Au NPs after the modification. (c) Polymer ligands PTMP–PVAc–Au NPs before the modification. (d) Polymer ligands PTMP–PVA–Au NPs after the modification.

Au NPs stabilized with polymer ligands DDT–PVAc and PTMP–PVAc had quite different results in the modification process. From the inset of Fig. 8, we can observe that the aqueous phase of DDT–PVA–Au NPs appeared lavender and PTMP–PVAc –Au NPs appeared faint red after the modification process. The red-shift of the plasma resonance peak of DDT–PVAc protected Au NPs indicates that the size has been significantly increased after the modification which is acquired from the UV-Vis spectrum. Whereas the red shift of the spectrum is only 1 nm, showing that the diameter of Au NPs prepared with PTMP–PVAc does not noticeably increase due to modification. The ‘net’ formed by the multidentate PTMP–PVAc polymer ligands could prevent the Au NPs from agglomerating during the modification process and monodentate DDT–PVAc is not in possession of that characteristic on the contrary (Fig. S15 in the ESI†). Otherwise, it was notably found that the abundance of Au NPs abated in the modification and post-treatment processes, as well as the yield of modified ester functional groups was so limited leading to dissolution into the water slowly. Taking the strong alkaline condition into account, Au NPs would appear as agglomerations in the modification process. The alcohol solution of sodium hydroxide used in the process as a catalyst may cause the dramatic changes that affect the surface properties and stability of gold nanoparticles significantly. Further research is essentially needed to systemically optimize the experimental conditions in order to minimize the phenomenon of loss and agglomeration of Au NPs in the ester alcoholysis process.

Fig. 8 UV-Vis absorption spectra of the modification process of thioether polymer ligands to realize the transfer of Au NPs from organic into aqueous phase. Black solid line is DDT–PVAc–Au NPs and black dashed line is DDT–PVA–Au NPs; red solid line is PTMP–PVAc –Au NPs and red dashed line is PTMP–PVA–Au NPs. Inset: photographs of the modification: (a) the modification of DDT–PVAc ligands, upper: DDT–PVAc–Au NPs; lower: water (left) and upper: toluene; lower: DDT–PVA–Au NPs (right). (b) The modification of PTMP–PVAc ligands, upper: PTMP–PVAc–Au NPs; lower: water (left) and upper: toluene; lower: PTMP–PVA–Au NPs (right).

Additionally, ligands exchange was performed to identify the loose interaction between thioether polymer ligands and gold surfaces. We chose the small molecular thiol substance DDT to exchange the thioether ligand DDT–PVAc. The ligand exchange can be proved by ¹H NMR, which could identify the surrounding substance on the surface of Au NPs (Fig. S16 in the ESI†). The results demonstrate that DDT can replace the DDT–PVAc ligands successfully due to the property that thioether has the weaker binding force with the gold surface than thiol molecules, though the diameter becomes slightly larger (hydrodynamic diameter is 8.1 nm) after the ligands exchange (Fig. 9). This unique property enables thioether end-functionalized Au NPs to possess promising biomedical applications owing to the easier replacement of the capping ligands with thiol-containing molecules including proteins and DNA etc.

Fig. 9 DLS spectra of Au NPs stabilized with monodentate ligands DDT–PVAc in toluene (orange) and after the ligand exchange by DDT (green). Inset: photograph of Au NPs stabilized with DDT (left) and DDT–PVAc (right).

Conclusion

In our research, highly stable and size-controlled Au NPs had been prepared by the two-phase method using versatile thioether end-functionalized polymer ligands DDT–PVAc and PTMP–PVAc. The factors such as polymer structure, polymer molecular weight and polymer concentration, which remarkably affect particle sizes and particle size distributions were investigated thoroughly. The results demonstrated that thioether ligands with smaller molecular weight and certain concentration can synthesize fairly uniform Au NPs with smaller diameter of 3.5 nm and thioether ligands could be easily exchanged by other thiol molecules. Multidentate ligand PTMP–PVAc lead to the formation of special ‘multimer’ morphology of Au NPs while the monodentate DDT–PVAc could effectively tailor the size and dispersity of Au NPs. In addition, the characteristic functional group of the polymer ligands was also chemically modified which accomplished the transfer of gold nanoparticles from organic phase into aqueous phase. Finally, thioether polymer ligands such as DDT–PVAc and PTMP–PVAc can be similarly and potentially applied in preparing Pt, Ag, QD nanoparticles. As the core–shell nanoparticles are superior to the single-component, the considerable research of core–shell nanoparticles has become a hot issue recently and thioether end-functionalized polymer ligands are the feasible candidates for the preparation of polymer core–shell nanoparticles, indeed.

Acknowledgements

We thank National Natural Science Foundation of China (No. 20774032, 50973037) for financial support, and the Analysis and Test Center of Huazhong University of Science & Technology for characterizations. BT gratefully acknowledges the award of a program for New Century Excellent Talents in University (NCET-10-0389).

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Footnote

† Electronic supplementary information (ESI) available: Some details of thioether polymer ligands characterization: Gel Permeation Chromatography, ¹H NMR spectroscopy and thermogravimetric analysis; the characterization of Au NPs prepared by thioether polymer ligands: DLS spectra, UV-Visible absorption spectra; the modification of thioether polymer ligands: reactivity of the modification and TEM photograph of the samples. See DOI: 10.1039/c0nr00835d

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