Effect of Temperature on the Direct Synthesis of Gold Nanoparticles

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Effect of Temperature on the Direct Synthesis of Gold Nanoparticles Mediated by Poly(dimethylaminoethyl methacrylate) Homopolymer Grigoris Mountrichas,* Stergios Pispas, and Efstratios I. Kamitsos Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vassileos Constantinou Avenue, 11635 Athens, Greece S Supporting Information *

ABSTRACT: The one-pot synthesis of gold nanoparticles (AuNPs) mediated by poly(dimethylaminoethyl methacrylate) homopolymer, without the use of a reducing agent, has been studied under different temperatures. The obtained results show that the higher is the temperature, during nanoparticle synthesis, the faster is nanoparticle creation and the more uniform are the produced nanoparticles. The kinetic studies on the nanoparticles synthesis were performed mainly by light scattering techniques. The obtained results indicate that at elevated temperatures the synthesis takes place in three steps, while at lower temperatures the followed synthetic pathways are more complex. Finally, the development of gold nanoparticles was also followed by UV−vis absorption measurements, indicating a good agreement between the two techniques.

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INTRODUCTION Gold nanoparticles (AuNPs) have been proven to be one of the most useful types of nanoparticles in the field of advanced nanomaterials.1−3 Catalysis, sensing, medicine, electronics, and optics are some of the myriad of potential application fields where the utilization of AuNPs has been proposed. Most of such applications rely on the electronic properties of Au nanoparticles, which are sensitive to size, shape, and immediate environment of nanoparticles.4,5 Therefore, a large variety of synthetic routes have been followed in order to synthesize the desired AuNPs in terms of size and shape.5,6 The majority of these techniques use sodium borohydride or sodium citrate in the presence of low molecular weight additives which act as stabilizers or provide surface chemistry control.7 The use of polymers in the synthesis of AuNPs can provide interesting features in terms of the colloidal stability and communication of AuNPs with the environment as well as for controlling the mechanical properties of the final bulk hybrid NP/polymer nanomaterial.8 Moreover, the use of copolymers can lead to structures with well-defined chemical environment around the AuNPs as well as with controlled spatial distribution of nanoparticles within the bulk nanomaterial, e.g., with predetermined interparticle distances. In addition, the stimulus responsive character of some macromolecular chains can be used toward the synthesis of advanced smart hybrid nanomaterials.9−12 One of the most appealing features is that the use of certain polymers can play a crucial role in the in situ synthesis of AuNPs, leading to the controlled growth of nanoparticles without the need of additional reducing agents. The use of polymer chains both as reducing agents and stabilizers offers great advantages in terms of simplicity during nanoparticles synthesis, i.e., one-pot synthesis, use of aqueous © 2014 American Chemical Society

and/or organic media, and limited postsynthesis work-up. To date, several AuNPs synthesis approaches utilizing polymers have been proposed, including the use of Pluronics13−18 and polyacrylates with amino groups.19−21 In both of the mentioned cases, polymer chains tend to chemisorb onto the nanoparticles during their formation offering the desired colloidal stabilization. Even though the use of polymers for the reduction/ stabilization of AuNPs has been studied by several research groups, there is still a wide gamut of basic questions that have to be answered concerning both the effect of environmental parameters on the AuNPs synthesis and the mechanism involved in the formation of nanoparticles. This work presents a rather detailed study of the effect of temperature on the synthesis of AuNPs in the presence of amine-containing homopolymer. The nanoparticles were synthesized in the presence of a homopolymer with a constant concentration and polymer/gold ions ratio. The homopolymer utilized is the poly(dimethylaminoethyl methacrylate) (PDMAEMA) which has one tertiary amino group on each repeating unit (Scheme 1). The kinetics of the AuNP formation has been correlated with the reaction temperature in a Scheme 1. Chemical Structure of PDMAEMA

Received: June 9, 2014 Revised: September 1, 2014 Published: September 5, 2014 22754

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preparation conditions were placed on copper grids, and excess of water was blotted away. The samples were allowed to dry in air and imaged directly under the microscope since metallic nanoparticles give enough contrast for observation.

straightforward way using light scattering techniques in order to record structure formation in real time.

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EXPERIMENTAL PART Materials. All reagents where purchased from Aldrich and where used without any further purification. The used water is distilled with conductivity lower than 1.5 μS/cm. The experiments of the AuNPs synthesis were performed in a carefully washed, with aqua regia, glassware. Polymer Synthesis. The synthesis of PDMAEMA has been realized through conventional free radical polymerization. In particular, dimethylaminoethyl methacrylate (10 g), dioxane (50 mL), and recrystallized AIBN (0.25 g) were placed in a round-bottom flask. The solution was degassed through three freeze−thaw cycles under vacuum. Following these cycles, the flask was sealed under vacuum using a flame torch. The solution was placed in an oil bath at 60 °C for 24 h in order to perform the polymerization. Subsequently, the polymer was isolated through precipitation in hexane and was dried under vacuum. The molecular weight of the PDMAEMA homopolymer is 40K (by SEC against polystyrene standards), and the polydispersity index is equal to 2.2. Synthesis of Au Nanoparticles. The synthesis of gold nanoparticles was realized in the presence of PDMAEMA. In particular, a dilute solution of HAuCl4 (45 μL, 10 mM) was added to an aqueous polymer solution (3 mL, 0.1 mg/mL) at pH = 3. At this pH PDMAEMA is predominately in its protonated form. The formation of AuNPs took place instantly, without the addition of any reducing agent. Measurements. The formation of AuNPs was probed by a number of complementary techniques. In particular, light scattering, UV−vis spectroscopy, and transmission electron microscopy were used in order to study the growth of the nanoparticles. For the in-situ measurements with light scattering, the mixed solution of PDMAEMA and gold chloride anions was placed in the thermostated cell holder of the light scattering instrument pre-equilibrated at the desired temperature, and the measurements were started immediately. In the case of UV−vis in-situ measurements, the solution temperature was set and spectra were recorded during frequent time intervals. Instrumentation. Light scattering measurements were performed on a ALV/CGS-3 compact goniometer system (ALV GmbH, Germany), using a JDS Uniphase 22 mW He− Ne laser, operating at 632.8 nm, interfaced with a ALV-5000/ EPP multitau digital correlator with 288 channels and a ALV/ LSE-5003 light scattering electronics unit for stepper motor drive and limit switch control. Subsequent measurements of 30 s were recorded every 60 s in an automatic way at an angle of 90°. The viscosity and the refractive index of the medium at each temperature were automatically introduced during the analysis of the measurement by the instrument software. The viscosity of the solutions was measured by capillary viscometry, while the refractive index of the particles was considered as constant. The data were evaluated by both cumulants and CONTIN analysis of the respective correlation functions. In particular, the polydispersity of the system was calculated using the cumulant analysis, while the hydrodynamic radii were evaluated following the CONTIN analysis. The UV−vis absorption measurements were performed on a PerkinElmer (Lambda 19) spectrometer. Finally, TEM measurements were performed on a JEOL 6300 at 200 kV. AuNPs/PDMAEMA solution droplets obtained under different

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RESULTS AND DISCUSSION The synthesis of AuNPs in the presence of polymer chains with tertiary amino functional groups, namely poly(ethylene oxide)b-poly(2,3-dihydroxypropyl methacrylate)-b-poly[2(diisopropylamino)ethyl methacrylate], has been also studied previously in terms of pH and Au concentration, both in absolute values and relative to the amino groups/Au ions ratio ([N]/[Au]).21 The results obtained in that work indicate that the synthesis is performed through the unprotonated tertiary amino groups, while a [N]/[Au] ratio higher than 3.5 is needed in order to obtain well-defined nanoparticles.21 In this work the polymer is present in its protonated form, and nanoparticle synthesis has been perfomed at different temperatures, revealing the great influence that this physicochemical parameter plays in such systems. The followed synthesis protocol relies on the addition of a predetermined amount of tetrachloroauric acid into an aqueous solution of PDMAEMA at pH 3. The addition of auric acid in combination to the low solution pH value causes the protonation of the tertiary amino groups of PDMAEMA. However, due to the dynamic and reversible nature of the protonation process, the existence of some unprotonated amino groups cannot be neglected, and these groups can act as the reducing agents against the gold ions, as has been proposed by Arms and co-workers.20 The [N]/[Au] ratio of all the performed experiments was kept equal to 4 in order to ensure the formation of nanoparticles with welldefined surface plasmon resonance. The synthesis of AuNPs has been conducted at different temperatures, and their formation has been followed by light scattering. Light scattering data from the preparation of AuNPs at 60 °C are presented in Figure 1. The variation of scattered light intensity, I, size polydispersity, and hydrodynamic radius, Rh, as a function of time indicates the existence of three different steps toward the synthesis of the nanoparticles. The first step should be an incubation period, where apparently there is no formation of Au nanoparticles. During this incubation time the recorded intensity and polydispersity index remain constant, while a small increase of the hydrodynamic radius is observed, in comparison with the radius of the net polymer which is about 10 nm. The abovementioned results imply that most probably AuCl4− ions form electrostatic complexes with polymer chains, which are rather random in size, leading to increased polydispersity index (PDI) and of rather low mass. This step is very crucial since it is responsible for the initial location of Au ions in the vicinity of the polymeric chains via interaction with the amino groups. During the next step, the formation of gold nanoclusters is recorded as has been previously reported in the literature.21 The increase of the intensity should be directly connected to the formation of Au metal nanoparticles. In particular, intensity increased by a factor of several hundred during this nanoparticle development stage. The results can be correlated either to the increasing number of the AuNPs or to the increasing size of the gold clusters. In both cases the high refractive index of AuNPs contributes to the scattered intensity increase. It has to be noted that kinetic results are reproducible (see Supporting Information, Figure S1). Simultaneously, the polydispersity index is rapidly decreased, since more uniform entities develop 22755

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Figure 1. Gold nanoparticles formation kinetics at 60 °C as followed by light scattering. The scattered intensity, I (up), the polydispersity index (middle), and hydrodynamic radius, Rh (down), are recorded as a function of time.

Figure 2. Gold nanoparticles formation kinetics at 50 °C as followed by light scattering. The scattered intensity, I (up), the polydispersity index (middle), and hydrodynamic radius, Rh (down), are recorded as a function of time.

in solution. In parallel the mean hydrodynamic radius also decreases, pointing to the formation of rather small clusters stabilized with few PDMAEMA chains. The presented results probably are correlated to the Au ions reduction during this step. The reduction of Au ions leads to the decomposition of the PDMAEMA/AuCl4 complexes formed in the first step as well as to decreasing of PDI. It is noted that the size of the formed nanoparticles in this step should be quite small and cannot be recorded using light scattering techniques, especially in the presence of the polymeric chains. The third step is characterized by a step increase of the intensity of scattered light, while PDI and Rh are only slightly increased. Probably this step is correlated with a kind of nanoparticles aging, since it is also connected with changes of the surface plasmon resonance, as it will be discussed later. This aging probably has to do with changes in the cluster size and size/conformational transformation of the Au/PDMAEMA hybrid nanoparticles. After this time window, i.e. the time for the three steps, the particles remain stable, and no changes are observed in terms of light scattering intensity, PDI, and hydrodynamic radius. It is worth noting that the particles obtained following the abovementioned protocol are stable for several months after their preparation. Moreover, it has to be noted that it is challenging to perform measurements at aqueous solutions of nanoparticles,22 since the dust can increase the polydispersity and the size of the polymer chains is comparable with that of nanoparticles. However, the quality of light scattering measurements reported here allows for the observation of the characteristic trends in the systems under study. A similar kinetic profile has been also observed for the same experiment at lower temperature, as shown in Figure 2 for the synthesis of AuNPs at 50 °C. The main difference with the results at 60 °C is that the time needed to accomplish the

reaction is much longer. The time window for the reaction is about 260 min, in contrast with the previously recorded time of 60 min at the higher temperature. It is noted that all the individual steps are longer at lower temperature; however, the same trends are recorded. Moreover, the light scattering intensity is much higher than before, although solutions with the same concentration were used. In addition, the hydrodynamic radius is also higher during the first step. These observations indicate that the lower temperature leads probably to the formation of larger complexes between the polymer chains and the chloroaurate ions. A somehow different situation is observed in the case that the experiment is run at 40 °C. In this case, during the first step of the reaction no significant changes are observed, in contrast to previous experiment, where the formation of complexes was recorded. However, the formation of complexes takes place with a delay, during the second step. Probably, the lower kinetic energy of the reacting species leads to the observed delay at the reduced temperature. It is observed that the lower is the temperature, the higher is the size of the complexes formed initially. Furthermore, it is worth to note that the formation of complexes takes place simultaneously with the appearance of gold clusters. Following the above result, a portion of Au ions seems to be reduced by the tertiary amino groups, while another part of the ions seems that it is still involved in the formation of complexes. In addition to the above observation, the incubation time for the synthesis of the first Au clusters is much larger in comparison to that recorded at elevated temperatures. Figure 3 shows that the lower is the reaction temperature, the higher is the recorded scattered light intensity, as has been also noted before. The incubation time and the scattered light intensity are even much higher as the reaction temperature is reduced further to 22756

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Moreover, after about 1 day, precipitation is observed. Interestingly, the scattering intensity of the solution keeps increasing after precipitation, indicating that, probably, the origin of the intensity increase is mainly the size of the particles and not their number. The precipitate has a dark blue color and is attributed to large gold particles, which cannot be stabilized by the polymer chains, while it cannot be redispersed even with extensive bath sonication. The overall kinetic and structural data from light scattering are recorded in Table 1. The results indicate that the higher is the temperature, the smaller is the size of the produced colloid and the smaller also its size polydispersity. Moreover, the reaction time (i.e., time for the completation of steps I−III), in order to produce the final hybrid colloid, is reduced at elevated temperatures, while the same trend is also observed for the incubation time. It is worth mentioning that the overall reaction time follows the Arrhenius equation (Figure 5), with a quite low activation energy which is only 0.021 eV. Finally, the recorded hydrodynamic radius of the final hybrid polymer/ metal nanoparticles is increased from 10.2 to 45.8 nm as temperature decreases from 60 to 30 °C. The changes in the nanoparticle size are probably correlated with the results of the light scattering intensity, where an immense increase is observed as the temperature decreases. The size of the hybrid nanoparticles seems to play a critical role on the abovementioned parameters. The general outcome of the presented results is that the reaction temperature is a very decisive parameter for the development of hybrid nanoparticles as small changes, in the order of few degrees, can dramatically change the characteristics of the final product. Moreover, the trend is that the higher is the temperature, the faster is the reaction and the smaller in size is the final hybrid colloid. Moreover, comparing the results at elevated temperatures, i.e. at 50 and 60 °C, the size of the obtained nanoparticles is similar, leading to the conclusion that at this temperature range the temperature plays an important role in the nucleation step, while it is less important for the growth of the nanoparticles, similarly to what has been already proposed.23 In line with light scattering results are the UV−vis absorption spectra of the formed Au nanoparticles (Figure 6 and Figure S2 in Supporting Information). The obtained results indicate that the higher is the temperature, the higher is also the energy where the maximum surface plasmon resonance is recorded. The position of the plasmon resonance has been correlated with the nanocluster size of the particles.24 The size of the nanoparticles is calculated to be in the range between 13 and 90 nm for reaction temperature 60 and 30 °C, respectively.24 It is worth mentioning that at intermediate temperatures the appearance of longitudinal surface plasmon resonance is observed, which is typically connected with the existence of elongated nanostructures.25 The effect of parameters, like the polymer structure, on the UV−vis spectrum of the formed Au nps has been previously discussed.20 The lower stabilization capability of a polymer has been related with the broadening of the absorption spectrum. In line with the previous results, the formation of large particles that cannot be stabilized by the polymer chains lead to structures with modified surface plasmon resonance, comparing with that of the monodisperse spherical nanoparticles. Finally, it has to be noted that the reduced absorbance of the nanoparticles formed at 30 °C is assigned to the partial precipitation of the sample after about 1 day of reaction.

Figure 3. Gold nanoparticles formation kinetics at at 40 °C as followed by light scattering. The scattered intensity, I (up), the polydispersity index (middle), and hydrodynamic radius, Rh (down), are recorded as a function of time.

30 °C (Figure 4). However, the three-step pattern that has been described before is not the case at this temperature.

Figure 4. Gold nanoparticles formation kinetics at 30 °C as followed by light scattering. The scattered intensity, I (up), the polydispersity index (middle), and hydrodynamic radius, Rh (down), are recorded as a function of time. 22757

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Table 1. Kinetic and Structural Parameters Determined by Light Scattering Data for the Formation of AuNPs in the Presence of PDMAEMA at Different Temperatures temp (°C)

incubation time (min) (step I)

60 50 40 30

17 77 158 750

Imax (Kcounts) 360 2061 6825 16500

± ± ± ±

10 25 60 150

Rh (nm)

PDI

reaction time (min) (steps I−III)

10.2 11.6 26.2 45.8

0.23 0.46 0.49 0.41

60 265 600 2110

Figure 7. Simultaneous monitoring of AuNPs formation (at 60 °C) followed by absorption spectroscopy (left axis) and light scattering (right axis). Inset: time dependence of the absorption maximum from UV−vis spectroscopy.

Figure 5. Arrhenius plot of the synthesis of AuNPs at different temperatures in the presence of PDMAEMA homopolymer.

scattering. The results clearly demonstrate the simultaneous increase of both light scattering intensity and surface plasmon resonance absorption. Moreover, the shift that is observed at the surface plasmon resonance is probably correlated with the transaction from the nucleation to the growth of the nanoparticles, as has been also previously suggested.21 The quality and size uniformity of the AuNPs formed at elevated temperature (60 °C) have been also studied through transmission electron microscopy (TEM) (Figure 8 and Figure

Figure 6. UV−vis absorption spectra of the AuNPs formed at different temperatures.

The results presented so far demonstrate clearly that (i) the reaction temperature strongly affects the structural and optical properties of the obtained polymer/Au hybrid nanoparticles and (ii) the higher is the reaction temperature, the faster is the Au nanoparticle creation, and the smaller and better defined in terms of size polydispersity are the formed AuNPs and hybrid PDMAEMA/Au nanoparticles. In order to investigate further the formation of AuNPs at elevated temperature, the correlation between surface plasmon resonance development and light scattering intensity increase during the formation of AuNPs has been studied by a parallel experiment of UV−vis absorption spectroscopy (Figure 7). The development of gold nanoparticles at elevated temperature (60 °C) is followed both by absorption measurements and by light

Figure 8. TEM (left) and HR-TEM(right) images of Au NPs synthesized at 60 °C in the presence of PDMAEMA.

S3). The recorded images indicate the successful formation of uniform spherical Au nanoparticles decorated with PDMAEMA. Moreover, the use of high-resolution TEM (HR-TEM) clearly demonstrates the crystalline nature of the nanoparticles as well as that they are surrounded by a protected PDMAEMA layer. Notably, the size of 13 nm calculated by surface plasmon resonance is similar to the one measured using electron microscopy. Moreover, it is comparable with the result from light scattering, leading to the conclusion that the stabilizing polymer chains have a minor contribution to hydrodynamic 22758

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radius, most probably due a flat conformation of the surface adsorbed chains.

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CONCLUSIONS Summarizing, the synthesis of gold nanoparticles without the aid of any reducing agent can been realized in the presence of a PDMAEMA homopolymer bearing tertiary amino groups. However, small and well-defined nanoparticles can be only obtained at elevated temperatures. The role of reaction temperature was found to be significant with respect to the structure and optical properties of the final product. In particular, decrease of the reaction temperature seems to lead to much slower reaction times and larger hybrid nanoparticles. The followed method for the synthesis of AuNPs at elevated temperatures is a very appealing approach since it gives wellcrystallized small Au nanoparticles, while it combines synthetic simplicity (one-pot synthesis, no work-up) with a fast procedure at temperatures that are easily accessible for a production line. Finally, it seems that the study of AuNPs synthesis at even higher temperature is a promising challenge for future research work.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

Gold nanoparticles formation kinetics at 60 °C for three different runs, simultaneous monitoring of AuNPs formation by absorption spectroscopy and light scattering at 50 and 40 °C, and TEM images of Au NPs synthesized at 50, 40, and 30 °C. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*E-mail [email protected]; Tel (+30)222107273821 (G.M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a Marie Curie European Reintegration Grant within the 7th European Community Framework Programme FP7/2007-2013/under REA grant agreement no. [276980]. Dr. A. Bakandritsos is acknowledged for the TEM images.

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dx.doi.org/10.1021/jp505725v | J. Phys. Chem. C 2014, 118, 22754−22759