Engineering Gold Nanoparticle Interaction by PAMAM Dendrimer

Article pubs.acs.org/JPCC

Engineering Gold Nanoparticle Interaction by PAMAM Dendrimer Taraknath Mandal, Chandan Dasgupta, and Prabal K. Maiti* Centre for Condensed Matter Theory, Department of Physics, Indian Institute of Science, Bangalore 560012, India S Supporting Information *

ABSTRACT: Bare faceted gold nanoparticles (AuNPs) have a tendency to aggregate through a preferred attachment of the [111] surfaces. We have used fully atomistic classical molecular dynamics simulations to obtain a quantitative estimate of this surface interaction using umbrella sampling (US) at various temperatures. To tune this surface interaction, we use polyamidoamine (PAMAM) dendrimer to coat the gold surface under various conditions. We observe a spontaneous adsorption of the protonated as well as nonprotonated PAMAM dendrimer on the AuNP surface. The adsorbed dendrimer on the nanoparticle surface strongly alters the interaction between the nanoparticles. We calculate the interaction between dendrimercoated AuNPs using US and show how the interaction between two bare faceted AuNPs can be tuned as a function of dendrimer concentration and charge (pH-dependent). With appropriate choice of the dendrimer concentration and charge, two strongly interacting AuNPs can be made effectively noninteracting. Our simulation results demonstrate a strategy to tune the nanoparticle interaction, which can help in engineering self-assembly of such nanoparticles.

1. INTRODUCTION Dendrimers have become a very useful material in modern nanotechnology because of their unique properties such as ordered structure, porosity, and ability to provide reaction sites at uniformly distributed terminal groups.1−5 They have been used as a stabilizer of metallic nanoparticles,6−10 and they can also serve as a molecular (nano) filter, as shown by Crooks et al.11 One of the most important applications of dendrimer− metal nanoparticles composites is using them as a catalyst in chemical reactions. However, the catalytic property of these composites depends strongly on the metal core as well as on the terminal groups of the dendrimer.12 In this sense, polyamidoamine (PAMAM) dendrimers are more useful because many other derivatives of this family can easily be synthesized by modifying the terminal groups.13−15 Structure and dynamics of PAMAM dendrimer under varying solution conditions have been studied in great detail in recent years.16−22 PAMAM dendrimer−AuNP composite has found many applications in medicine23 because of the lower toxicity24 of gold and easy methods such as UV irradiation6 and chemical reduction10 of tetrachloroaurate salts of PAMAM dendrimer to synthesize these composites. In recent years, self-assembly of AuNPs mediated by both polymer11,25−35 and biopolymer36−51 molecules has attracted a considerable amount of research interest. Self-assembly techniques are used to control the interparticle spacing, which has a strong influence on optical,25 electronic, and magnetic52 properties of the nanocomposites. However, control over the organization of the nanocomposites into a desired network requires a complete understanding of © XXXX American Chemical Society

the interactions between the nanoparticles and dendrimers as well as of the inter-nanocomposite interactions. Here, we study the interaction between two bare faceted AuNPs in aqueous media and give a quantitative estimate of this interaction as a function of temperature using US. We observe a similar kind of oriented attachment of AuNPs as that observed in recent experiments.53 Then, we report the interaction of PAMAM dendrimer with a faceted AuNP. In the presence of dendrimers in solution, dendrimer−AuNP composite is formed by spontaneous absorption of the dendrimers on the AuNP surface. The interactions between these composites are significantly different from those of the bare AuNPs. We calculate the potential of mean force (PMF) between dendrimer-wrapped AuNPs using US and demonstrate how one can tune the interaction between them by varying the concentration of the dendrimer. To the best of our knowledge, this is the first fully atomistic simulation study of the interaction between dendrimer−nanoparticle composites. The rest of the paper is organized as follows: In Section 2, we give the details of simulation methodology including system building along with the force field used. In Section 3, we discuss the results obtained from MD simulations. Finally, in Section 4, we provide a summary of the main results and conclusions. Received: February 2, 2013 Revised: May 30, 2013

A

dx.doi.org/10.1021/jp401218t | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

Article

2. METHODOLOGY We have used truncated octahedral AuNP with face-centered cubic crystalline structure, as shown in Figure 1. This

protonated PAMAM dendrimers of second generation (G2) to study the interaction between the dendrimer-AuNP composites. We have used GAFF63 to describe the intermolecular interactions for dendrimers. Note that recently we have validated the GAFF for dendrimer modeling.64,65 Fully atomistic molecular dynamics simulations are performed at constant pressure and constant temperature (NPT) ensemble with 1 fs time step. Periodic boundary condition is applied in all three directions and the long-range electrostatic interaction is computed using the particle−particle mesh Ewald method.66 Bonds involving hydrogen are kept fixed using SHAKE67 algorithm. We use the US68 method to calculate the PMF between the AuNPs and the dendrimer-coated AuNPs. The PMF is calculated considering the center-to-center distance of nanoparticles/nanocomposites as the reaction coordinate. A harmonic potential is used as the biasing potential in the US. The biasing potential is used to restrain the distance between the centers of masses of the nanoparticles/nanocomposites while they are allowed to rotate freely. Nanoparticles/nanocomposites are initially kept at large distance; then, they are gradually brought close to each other with 1 Å window gap. At each window, the structures are equilibrated for 1−3 ns and the resulting structures are used as the starting configurations for the next window. Weighted histogram analysis method (WHAM) is used to calculate the PMF from the US results.

Figure 1. Initially built faceted AuNP. Different facets are also shown in the Figure.

nanoparticle has eight [111] and six [100] surfaces. The initial configuration of the nanoparticle is generated using Cerius254 (version 4.9). Cerius2-generated AuNPs are solvated in a TIP3P55 water box using the xleap module of AMBER package.56 The size of the water box was chosen to ensure that the systems have at least 10 Å solvation shell in all directions. The total system contains 19 962−99 659 atoms depending on the different studies. The details of the system sizes are given in Table 1. The Lennard-Jones potential is used to describe the interaction between the gold atoms. Interaction parameters for gold are taken from the recent work of Agrawal et el.57 Recently, these potential parameters have been successfully used to study various properties of DNAfunctionalized AuNPs58,59 as well as crystal structure of DNA-linked60 AuNP . These simulations provided results consistent with some of the recent experimental results61,62 on similar system and demonstrated the accuracy of the gold interaction parameters. We have used both nonprotonated and

3. RESULTS AND DISCUSSION Interaction between Two Bare Faceted Gold Nanoparticles. Figure 2a shows the PMF between two bare faceted AuNPs along the center of mass (COM) separation at three different temperatures. The PMF value starts to decrease when the distance between the centers of masses of two nanoparticles is lower than ∼21 Å, and the minimum of the PMF is nearly at 16.3 Å where these two nanoparticles touch each other. This high decrease in the PMF value indicates that the bare AuNPs are strongly interacting with each other. As the two nanoparticles approach each other, they do not face any energy barrier suggesting that two nanoparticles can aggregate spontaneously. As a result of the aggregation process, when one nanoparticle gets attached with the other one, the total surface area of the combined system decreases and hence the

Table 1. Simulation Details of Different Systems Used in This Study different systems PMF calculation between two bare AuNPs in water AuNP + one protonated G2 PAMAM dendrimer AuNP + one nonprotonated G2 PAMAM dendrimer AuNP + two protonated G2 PAMAM dendrimer AuNP + two nonprotonated G2 PAMAM dendrimer PMF calculation between AuNP and protonated G2 PAMAM dendrimer PMF calculation between AuNP and nonprotonated G2 PAMAM dendrimer PMF calculation between two AuNPs coated with single protonated G2 PAMAM dendrimer. PMF calculation between two AuNPs coated with single nonprotonated G2 PAMAM dendrimer. PMF calculation between two AuNPs coated with two protonated G2 PAMAM dendrimers. PMF calculation between two AuNPs coated with two nonprotonated G2 PAMAM dendrimers.

number of atoms in AuNP

number of dendrimer atoms

number of counterions

402 201 201 201 201 201

532 516 1064 1032 532

16

201

516

402

1064

402

1032

402

2128

402

2064

B

32 16

32

64

number of water molecules

total number of atoms

6520 8421 8008 14065 12848 8787

19962 26012 24741 43492 39777 27110

7604

23529

17042

52624

12708

39558

32355

99659

20369

63573



Article

Figure 2. (a) PMF between two bare AuNPs at different temperatures. (b) Snapshot of the aggregated AuNPs at 345 K. The [111] facets get attached to each other. (c) Change in internal energy as a function of the angle between the [111] surfaces. E0 is the energy of the AuNPs when the angle between the [111] faces is 0°. (d) Snapshots of the two degenerate lowest energy configurations. The upper one corresponds to the configuration when relative angles between the faces are 0/120°. The lower configuration represents the configuration when the angles between the faces are 60/180°. Note that the configuration obtained at 345 K from the PMF is similar to the upper configuration.

surface energy of the entire system also decreases. This gain in surface energy is the driving force for the aggregation process. During their approach to each other, the particles rotate freely to attain the favorable orientation. Interestingly, we find that in all cases one of the [111] facets of one nanoparticle gets attached to one of the [111] facets of the other one, as shown in Figure 2b. From the snapshots shown in Figure S1 in the Supporting Information, we observe69 that at 300 and 330 K temperature, although the nanoparticles are attached by the [111] surfaces, they are not perfectly (crystallographic) aligned. In contrast, at higher temperature (345 K), the nanoparticles are perfectly aligned while attaching to each other through their [111] facets. We have also calculated the number of Au atoms in the contact region at different temperatures and found that the numbers of atoms in the contact region are 27 and 28 at 300 and 330 K, respectively. However, at 345 K, because of better alignment of the [111] facets, we get a larger number of atoms (38) in the contact region. As a result, the surface energy gain and hence the PMF value is also higher at 345 K temperature. The configurations obtained at 300 and 330 K are possibly metastable states, which require an extra energy to attain the most stable state. The details of the enthalpy and entropy contributions to the total free-energy change are given in Tables 2 and 3. To evaluate the change in enthalpy, we calculated the internal energy of the AuNPs for two cases: (1) when the AuNPs are well-separated and (2) when they are at the minimum position of the PMF. We have also calculated the change in PV terms for these two cases and found it to be very

Table 2. Enthalpy and Entropy Contributions to the FreeEnergy Changes of Bare AuNPs at Different Temperatures temperature (K)

ΔG (kcal/mol)

ΔH (kcal/mol)

TΔS (kcal/mol)

300 330 345

−175 ± 1.64 −321 ± 1.36 −475 ± 1.50

−647 ± 15 −592 ± 14 −812 ± 15

−472 ± 15 −271 ± 14 −337 ± 15

small. These values were averaged over 1000 configurations taken from the last 1 ns of simulation. Finally, to calculate the entropy change, we subtracted the enthalpy change from the PMF value. Note that the large change in the free energy over a small temperature range is mainly because of the decrease in the contact surface area at higher temperatures. The interaction of the water molecules with AuNPs may also play an important role in the observed temperature dependence. At higher temperature, the density of water molecules is lower, which may help the AuNPs to interact with each other more strongly. In Figure 2c, we have shown the changes in internal energy of the AuNPs as a function of the relative angle between the two [111] surfaces. We find that there are two degenerate lowest energy configurations, as shown in Figure 2d. The configuration obtained from the PMF calculation at 345 K is one of these two degenerate configurations and is one of the most stable states. From the change in PMF value, we calculate the surface energy of the AuNP. The surface energy is calculated by dividing the PMF value by twice of the [111] surface area. The surface energy of the [111] surface is 2.76 kcal/mol-Å2 at 345 K temperature. This value is higher than the previously reported C



Article

Table 3. Enthalpy and Entropy Contributions to the Free-Energy Changes of Various AuNP−Dendrimer Systems system PMF PMF PMF PMF

between between between between

AuNP and nonprotonated G2 dendrimer AuNP and protonated G2 dendrimer two AuNPs coated with single nonprotonated G2 dendrimer. two AuNPs coated with single protonated G2 dendrimer.

value of 1.8 kcal/mol-Å2 obtained from ab initio calculation70 and 1.34 kcal/mol-Å2 obtained from molecular dynamics study.71 The difference in surface energy might be an effect of the aqueous media considered in our calculation, whereas the previous study was done in vacuum. To understand this favorable oriented attachment, we have calculated the number of water molecules per unit area in the perpendicular directions of [111] and [100] surfaces. The results are shown in Figure 3. The sharp peak near the surface

ΔG (kcal/mol)

ΔH (kcal/mol)

TΔS (kcal/mol)

−18 ± 0.08 −7 ± 0.09 −17 ± 0.08 −9 ± 0.09

−136 ± 22 −141 ± 22 −54 ± 28 −159 ± 30

−118 ± 22 −134 ± 22 −37 ± 28 −150 ± 30

surface energy is larger in this preferred attachment direction. In general, the surface energy of the [111] surface (1.34 kcal/ mol-Å2)71 is lower than that of the [100] surface (1.65 kcal/ mol-Å2)71 but the [111] surface has much larger area (86.5 Å2) than the [100] surface (33.3 Å2), and hence the effective decrease in surface energy for this preferred attachment is also larger. Spontaneous Adsorption of PAMAM Dendrimer on the Gold Nanoparticle Surface. Recent experiments have shown that the PAMAM dendrimer can be used to control the interactions between the AuNPs.26 So it is interesting to study the adsorption of PAMAM dendrimer on a bare AuNP and understand how that modulates the nanoparticle interaction. In this section, we discuss the interactions of the AuNP with both protonated and nonprotonaed PAMAM dendrimers of generation 2. The 2-D drawing and atomistic structures of a G2 nonprotonated PAMAM dendrimer are shown in Figure 4. The initial structures of the PAMAM dendrimers are built using the in-house developed Dendrimer Builder Tool-Kit.64 These structures were then equilibrated in aqueous media for 25 ns. These equilibrated dendrimers were then placed close to the AuNP. The initial configurations of the nanoparticle-dendrimer system are shown in Figures 5a and 6a for the nonprotonated and protonated case, respectively. The dendrimer gets adsorbed on the AuNP surface over a several nanosecond long MD simulation. Figures 5b and 6b show instantaneous snapshots of the dendrimer-AuNP composites structure after 30 ns long MD simulation. The terminal amine groups of the dendrimer are more interactive with the AuNP surface than the core region of the dendrimer. To scrutinize the conformational changes in the dendrimers, we have calculated (Figure 7a) the radius of gyrations (Rg) of the AuNP and the dendrimer−nanoparticle combined systems separately. We observe that the Rg value of

Figure 3. Number of water molecules per unit area along the perpendicular direction of [111] and [100] surfaces.

reflects the hydrophobic nature of the gold surface.72 Note that the water density near the [111] surface is lower than that near the [100] surface. Thus when the [111] surfaces of two nanoparticles approach each other, a smaller number of water molecules are required to be pushed away from the intermediate region, which makes these surfaces more favorable for the attachment. Another reason is that the total gain in

Figure 4. Chemical (source: http://journals.iucr.org) and atomistic structures of generation two (G2) nonprotonated PAMAM dendrimer. D



Article

Figure 5. (a) Initial configuration of the AuNP and single nonprotonated G2 PAMAM dendrimer at the beginning of the simulation. (b) Instantaneous snapshot of the dendrimer-wrapped AuNP after 30 ns long MD simulation.

Figure 6. (a) Initial configuration of the AuNP and single protonated G2 PAMAM dendrimer at the beginning of the simulation. (b) Instantaneous snapshot of the dendrimer-wrapped AuNP after 30 ns long MD simulation.

Figure 7. (a) Radius of gyration (Rg) of the bare AuNP and the AuNP-dendrimer composite for G2 protonated (g2p) and nonprotonated (g2np). (b) Radial distribution function (g(r)) of the AuNP in the presence and absence of PAMAM dendrimer. Black line corresponds to the AuNP without the dendrimer adsorption and the red line is for the AuNP after the dendrimer adsorption.

adsorption. The RDF in Figure 7b shows that the structure remains perfectly crystalline after the adsorption, which means that the interaction between the dendrimer and the AuNP is not strong enough to drive any conformational change in the AuNP. The very fact that the dendrimer does not affect the internal structure of the AuNP but modifies only the surface interaction, makes it a very promising material for nanoscale

the combined system of the AuNP and nonprotonated dendrimer has decreased significantly from the Rg value of 10.55 Å of the nonprotonated dendrimer alone. This decrease in Rg implies a significant adsorption of the dendrimer on the AuNP surface. To investigate the effect of this adsorption on structure of the AuNP, we have also calculated the radial distribution function (RDF) of the AuNP before and after the E



Article

Figure 8. Instantaneous snapshots of the AuNP-dendrimer composite for two G2 PAMAM dendrimer after 30 ns equilibration for (a) nonprotonated and (b) protonated case, respectively.

Figure 9. (a) PMF between the two single G2 PAMAM dendrimer-wrapped AuNP composites for both protonated and nonprotonated dendrimer. Instantaneous snapshots of the AuNP−dendrimer composite at the minimum position of the PMF (b) for nonprotonated dendrimer and (c) protonated dendrimer.

nanocomposites in Figure 9a. Compared with the interaction energy of −175 kcal/mol at 300 K between the two bare AuNPs, single G2 dendrimer-wrapped AuNPs have significantly less interaction energy on the order of −17 kcal/mol for nonprotonated dendrimer and −8 kcal/mol for protonated dendrimer. Note that the adsorbed dendrimers have decreased the strength of the bare AuNP interaction significantly. Figure 9b,c shows the snapshots of the AuNP composites corresponding to the minimum position of the PMF. We find that part of the dendrimer that is not adsorbed on the surface of one AuNP embraces the bare part of the surface of the second AuNP. Thus both protonated and nonprotonated dendrimers act as a linker between the AuNPs in these composites. Interestingly, when the dendrimer concentration is increased, that is, the AuNP is wrapped by two G2 PAMAM dendrimers (either protonated or nonprotonated), the interaction behavior between these two kinds of composites is different. From Figure 10, we see that two nonprotonated dendrimers adsorbed on the nanoparticle surface decrease the interaction strength further, making these composites almost noninteracting. On the other hand, the interaction between two protonated

engineering. We observe that a single dendrimer covers only a small fraction of the AuNP surface. To increase the coverage of the dendrimers on the nanoparticle surface, we have studied the interaction of an AuNP with multiple dendrimers. The equilibrated snapshots of the AuNP composites with two G2 dendrimers after 30 ns long MD simulation are shown in Figure 8. It will be shown later that by controlling the number of adsorbed dendrimers (or dendrimer coverage) we can control the AuNP interaction. Interaction between the Dendrimer-Nanoparticle Composites. In the previous section, we have shown that a dendrimer wraps an AuNP over nanosecond time scales. We have also seen that two AuNPs have an interaction that depends on the surface orientation as they approach each other. For example, (111) surfaces are more favorable for the interaction between two AuNPs. How does the dendrimer adsorption on the gold surface influence such interaction? To investigate the effects of the adsorbed dendrimers on the AuNP interactions, we have also studied the interaction between two dendrimer-wrapped AuNP nanocomposites. We have shown the PMF between two single dendrimer-coated AuNP F



Article

unwrapped branches of the other one. Hence the interaction strength should be equivalent to that of the two dendrimers only. In Figure 11b, we observe that the interaction between two protonated dendrimers is repulsive and the interaction between two nonprotonated dendrimers is attractive, but the strength of the interaction is negligible compared with the interaction between two bare AuNPs. Hence, the interaction between double nonprotonated dendrimer-coated composites is also very weakly attractive and that of double protonated dendrimer-coated composites is repulsive (Figure 10). Note that even if a single dendrimer is present at the nanoparticle surface, the two AuNPs cannot come as close as in the case of the bare AuNPs. Figures 9a and 10 show that at the equilibrium positions, the distances between two AuNPs are approximately 3.0 and 4.2 nm for single nonprotonated and double nonprotonated dendrimer-coated composites, respectively. At this large distance, the two AuNPs are not interacting with each other, as shown in Figure 2a, and hence the interaction strength between the AuNP composites is so small. Effect of Gold Force Field. Recently, another set of LJ parameters73 was developed for Au atoms that reproduce the density, surface energy, and various elastic modulus of the Au metal in quantitative agreement with the available experimental data. To check the dependence of the Au FF parameter, we have recalculated the PMF between the bare AuNPs and that of between the single nonprotonated dendrimer-coated AuNPs, where the gold interaction parameters are important. The PMF for the bare AuNPs at T = 300 and 345 K are shown in Figure 12a. Note that the nature of the interaction is qualitatively the same as the previous results, as obtained using the Agrawal LJ parameter. However, there is a decrease in the PMF of bare AuNPs because of the smaller well depth of the gold−gold interaction parameter of Heinz et al. We also observe that the minimum of the PMF profiles for bare AuNPs move slightly toward the right because of the larger σ value of the gold. We see similar quantitative changes for the interaction between the dendrimer-coated AuNPs, as shown in Figure 12b: there is a reduction of the PMF by ∼5 kcal/mol in going from the Agrawal LJ parameter set to the Heinz parameter set. So changing the gold FF can make a quantitative difference between the PMF values but qualitative trends remain the same. Using different FF and comparing their results is certainly an important exercise, and we plan to take that up as a separate study in the near future.

Figure 10. PMF between the two double G2 PAMAM dendrimerwrapped AuNP composites for protonated and nonprotonated case. Figure S7 in the Supporting Information shows the instantaneous snapshot of the AuNP−dendrimer composite at the minimum position of the PMF.

dendrimer-coated composites is repulsive. To understand the reason behind this strong decrease in the interaction between two AuNP composites, we have calculated the PMF between the dendrimer and a bare AuNP and the PMF between the dendrimers, separately. The results are shown in Figure 11. From Figure 11a, we see the interaction strengths between the bare AuNP and the dendrimer are −17 and −8 kcal/mol for the nonprotonated and protonated case, respectively. These results are consistent with the interaction behavior between single dendrimer coated composites. When a single dendrimer wraps the nanoparticle, most of the nanoparticle surface remains bare. The free (unattached to the nanoparticle) branches of the other single dendrimer-coated composite wraps the bare part of the first nanoparticle. Thus the interaction strength between two single dendrimer coated composites should be similar to the interaction between the dendrimer and the bare AuNP. Indeed that is the case, as we observe in Figures 9a and 11a. Note that the nonprotonated dendrimer is more interactive than the protonated one. This difference in interaction strength indicates that the −NH2 group might be more interactive with gold surface than the −NH3 group with the same. The situation changes when the nanoparticle is covered by two dendrimers. The surface coverage of the two G2 dendrimer-coated composite is much larger than that of the single dendrimer coated composite. When two such composites approach each other, the free branches of one composite interact with the

Figure 11. (a) PMF between the single AuNP and single G2 PAMAM dendrimer for the protonated and nonprotonated dendrimer cases. (b) PMF between the two G2 PAMAM dendrimers for the nonprotonated and protonated cases. G



Article

Figure 12. (a) Comparisons of the PMF between the bare AuNPs obtained from Agrawal et al. and Heinz et al. parameters. Comparisons are done at 300 and 345 K temperature. Snapshot at the top shows the configuration of AuNPs at the minimum of the PMF obtained from Heinz et al. parameter at 345 K. (b) Comparisons of the PMF between the single nonprotonated dendrimer-coated AuNPs obtained from Agrawal et al. and Heinz et al. parameters at 300 K. Snapshot at the top shows the configuration at the minimum of the PMF obtained from the Heinz et al. parameter.

4. SUMMARY AND CONCLUSIONS In conclusion, we have used all-atom MD simulations to study the interaction between two faceted AuNPs in aqueous media. Our results show that these nanoparticles are strongly interacting and this interaction strength increases with increase in temperature. We observe that the bare nanoparticles have a tendency to aggregate through a preferred attachment of the [111] surfaces. Lower water density near the [111] surfaces compared with the water density near the [100] surfaces and larger gain in surface energy make the [111] surfaces more favorable for attachment. We use G2 PAMAM dendrimer as a coating material to tune this interaction. We show that both the protonated and nonprotonated dendrimers can be used for tuning this interaction. When the nanoparticle is coated by a single G2 PAMAM dendrimer, the dendrimer wraps a small part of the surface of the nanoparticle. The rest of the nanoparticle surface remains bare. When two such dendrimerwrapped AuNP composites approach each other, the free dendrimer branches of one composite wrap the bare surface of the nanoparticle of the other composite. As a result, two AuNPs cannot come as close to each other as in the bare nanoparticle case. Hence the interaction strength decreases significantly. When the dendrimer concentration is increased so that the nanoparticle is coated by two G2 PAMAM dendrimers, the free branches of the dendrimers of one composite interact with the free dendrimer branches of the other. So the interaction strength between two such composites decreases further as the interaction strength between two nonprotonated G2 dendrimer is lower than that between the AuNP surface and the dendrimer. As the interaction between two protonated dendrimers is repulsive because of the positive charges of the dendrimers, the interaction between double protonated dendrimer-coated nanoparticles is also repulsive. Thus our results elucidate the nature and the strength of the interaction between AuNPs and effects of PAMAM dendrimers on this

interaction. We demonstrate that for a given dendrimer generation, by controlling the charge and the dendrimer density, one can tune the interaction between the AuNPs gradually and make them noninteracting or repulsive. These findings can help us in engineering the self-assembly of AuNP, which is currently under investigation in our laboratory.

■

ASSOCIATED CONTENT

S Supporting Information *

Snapshots of the aggregation process of single-dendrimercoated AuNPs. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We acknowledge financial support from MONAMI, Indo-EU project. T.M. thanks Council of Scientific and Industrial Research (CSIR), India for a fellowship.

■

REFERENCES

(1) Tomalia, D. A.; Naylor, A. M.; Goddard, W. A. Starburst Dendrimers - Molecular-Level Control of Size, Shape, SurfaceChemistry, Topology, and Flexibility from Atoms to Macroscopic Matter. Angew. Chem., Int. Ed. Engl. 1990, 29, 138−175. (2) Frechet, J. M. J. Functional Polymers and Dendrimers Reactivity, Molecular Architecture, and Interfacial Energy. Science 1994, 263, 1710−1715. (3) Zeng, F. W.; Zimmerman, S. C. Dendrimers in Supramolecular Chemistry: From Molecular Recognition to Self-Assembly. Chem. Rev. 1997, 97, 1681−1712.

H



Article

(4) Bosman, A. W.; Janssen, H. M.; Meijer, E. W. About Dendrimers: Structure, Physical Properties, and Applications. Chem. Rev. 1999, 99, 1665−1688. (5) Esumi, K.; Torigoe, K. Preparation and Characterization of Noble Metal Nanoparticles Using Dendrimers As Protective Colloids. Prog. Colloid. Polym. Sci. 2001, 117, 80−87. (6) Esumi, K.; Suzuki, A.; Aihara, N.; Usui, K.; Torigoe, K. Preparation of Gold Colloids with UV Irradiation Using Dendrimers As Stabilizer. Langmuir 1998, 14, 3157−3159. (7) Balogh, L.; Tomalia, D. A. Poly(amidoamine) DendrimerTemplated Nanocomposites. 1. Synthesis of Zerovalent Copper Nanoclusters. J. Am. Chem. Soc. 1998, 120, 7355−7356. (8) Zhao, M. Q.; Sun, L.; Crooks, R. M. Preparation of Cu Nanoclusters within Dendrimer Templates. J. Am. Chem. Soc. 1998, 120, 4877−4878. (9) Endo, T.; Yoshimura, T.; Esumi, K. Synthesis and Catalytic Activity of Gold-Silver Binary Nanoparticles Stabilized by PAMAM Dendrimer. J. Colloid Interface Sci. 2005, 286, 602−609. (10) Garcia, M. E.; Baker, L. A.; Crooks, R. M. Preparation and Characterization of Dendrimer-Gold Colloid Nanocomposites. Anal. Chem. 1999, 71, 256−258. (11) Crooks, R. M.; Zhao, M. Q.; Sun, L.; Chechik, V.; Yeung, L. K. Dendrimer-Encapsulated Metal Nanoparticles: Synthesis, Characterization, And Applications to Catalysis. Acc. Chem. Res. 2001, 34, 181− 190. (12) Oh, S. K.; Niu, Y. H.; Crooks, R. M. Size-Selective Catalytic Activity of Pd Nanoparticles Encapsulated within End-Group Functionalized Dendrimers. Langmuir 2005, 21, 10209−10213. (13) Aoi, K.; Itoh, K.; Okada, M. Globular Carbohydrate Macromolecule Sugar Balls 0.1. Synthesis of Novel Sugar-Persubstituted Poly(Amido Amine) Dendrimers. Macromolecules 1995, 28, 5391− 5393. (14) SayedSweet, Y.; Hedstrand, D. M.; Spinder, R.; Tomalia, D. A. Hydrophobically Modified Poly(amidoamine) (PAMAM) Dendrimers: Their Properties at the Air-Water Interface and Use as Nanoscopic Container Molecules. J. Mater. Chem. 1997, 7, 1199− 1205. (15) Li, Y. J.; Dubin, P. L.; Spindler, R.; Tomalia, D. A. ComplexFormation between Poly(dimethyldiallylammonium chloride) and Carboxylated Starburst Dendrimers. Macromolecules 1995, 28, 8426− 8428. (16) Maiti, P. K.; Cagin, T.; Wang, G. F.; Goddard, W. A. Structure of PAMAM dendrimers: Generations 1 through 11. Macromolecules 2004, 37, 6236−6254. (17) Maiti, P. K.; Cagin, T.; Lin, S. T.; Goddard, W. A. Effect of Solvent and pH on the Structure of PAMAM Dendrimers. Macromolecules 2005, 38, 979−991. (18) Lin, S. T.; Maiti, P. K.; Goddard, W. A. Dynamics and Thermodynamics of Water in PAMAM Dendrimers at Subnanosecond Time Scales. J. Phys. Chem. B 2005, 109, 8663−8672. (19) Maiti, P. K.; Goddard, W. A. Solvent Quality Changes the Structure Of G8 PAMAM Dendrimer, A Disagreement with Some Experimental Interpretations. J. Phys. Chem. B 2006, 110, 25628− 25632. (20) Maiti, P. K.; Messina, R. Counterion Distribution and ZetaPotential in PAMAM Dendrimer. Macromolecules 2008, 41, 5002− 5006. (21) Maiti, P. K.; Li, Y. Y.; Cagin, T.; Goddard, W. A. Structure of Polyamidoamide Dendrimers up to Limiting Generations: A Mesoscale Description. J. Chem. Phys. 2009, 130, 144902. (22) Maiti, P. K.; Bagchi, B. Diffusion of Flexible, Charged, Nanoscopic Molecules in Solution: Size and pH Dependence for PAMAM Dendrimer. J. Chem. Phys. 2009, 131, 214901. (23) Bielinska, A.; Eichman, J. D.; Lee, I.; Baker, J. R.; Balogh, L. Imaging {Au-0-PAMAM} Gold-Dendrimer Nanocomposites in Cells. J. Nanopart. Res. 2002, 4, 395−403. (24) Panyala, N. R.; Pena-Mendez, E. M.; Havel, J. Gold and NanoGold in Medicine: Overview, Toxicology and Perspectives. J. Appl. Biomed. 2009, 7, 75−91.

(25) Srivastava, S.; Frankamp, B. L.; Rotello, V. M. Controlled Plasmon Resonance of Gold Nanoparticles Self-Assembled with PAMAM Dendrimers. Chem. Mater. 2005, 17, 487−490. (26) Frankamp, B. L.; Boal, A. K.; Rotello, V. M. Controlled Interparticle Spacing through Self-Assembly of Au Nanoparticles and Poly(amidoamine) Dendrimers. J. Am. Chem. Soc. 2002, 124, 15146− 15147. (27) Daniel, M. C.; Ruiz, J.; Nlate, S.; Blais, J. C.; Astruc, D. Nanoscopic Assemblies between Supramolecular Redox Active Metallodendrons and Gold Nanoparticles: Synthesis, Characterization, And Selective Recognition of H2PO4−, HSO4−, and Adenosine-5′triphosphate (ATP(2-)) Anions. J. Am. Chem. Soc. 2003, 125, 2617− 2628. (28) Schmid, G.; Baumle, M.; Beyer, N. Ordered Two-Dimensional Monolayers of Au-55 Clusters. Angew. Chem., Int. Edit. 2000, 39, 181− 183. (29) Zehner, R. W.; Lopes, W. A.; Morkved, T. L.; Jaeger, H.; Sita, L. R. Selective Decoration of a Phase-Separated Diblock Copolymer with Thiol-Passivated Gold Nanocrystals. Langmuir 1998, 14, 241−244. (30) Bockstaller, M. R.; Lapetnikov, Y.; Margel, S.; Thomas, E. L. Size-Selective Organization of Enthalpic Compatibilized Nanocrystals in Ternary Block Copolymer/Particle Mixtures. J. Am. Chem. Soc. 2003, 125, 5276−5277. (31) Shenhar, R.; Jeoung, E.; Srivastava, S.; Norsten, T. B.; Rotello, V. M. Crosslinked Nanoparticle Stripes and Hexagonal Networks Obtained via Selective Patterning of Block Copolymer Thin Films. Adv. Mater. 2005, 17, 2206−2210. (32) Boal, A. K.; Ilhan, F.; DeRouchey, J. E.; Thurn-Albrecht, T.; Russell, T. P.; Rotello, V. M. Self-Assembly of Nanoparticles into Structured Spherical and Network Aggregates. Nature 2000, 404, 746−748. (33) Cui, H. G.; Chen, Z. Y.; Zhong, S.; Wooley, K. L.; Pochan, D. J. Block Copolymer Assembly via Kinetic Control. Science 2007, 317, 647−650. (34) Nie, Z. H.; Fava, D.; Kumacheva, E.; Zou, S.; Walker, G. C.; Rubinstein, M. Self-Assembly of Metal-Polymer Analogues of Amphiphilic Triblock Copolymers. Nat. Mater. 2007, 6, 609−614. (35) Ofir, Y.; Samanta, B.; Rotello, V. M. Polymer and biopolymer mediated self-assembly of gold nanoparticles. Chem. Soc. Rev. 2008, 37, 1814−1823. (36) Mirkin, C. A.; Mucic, R. C.; Storhoff, J. J.; Letsinger, R. L. DNAInduced Assembly of Gold Nanoparticles: A Method for Rationally Organizing Colloidal Particles into Ordered Macroscopic Materials. Abstr. Pap. Am. Chem. Soc. 1996, 212, 249−INOR. (37) Deng, Z. X.; Tian, Y.; Lee, S. H.; Ribbe, A. E.; Mao, C. D. DNAEncoded Self-Assembly of Gold Nanoparticles into One-Dimensional Arrays. Angew. Chem., Int. Ed. 2005, 44, 3582−3585. (38) Alivisatos, A. P.; Johnsson, K. P.; Peng, X. G.; Wilson, T. E.; Loweth, C. J.; Bruchez, M. P.; Schultz, P. G. Organization of ‘Nanocrystal Molecules’ Using DNA. Nature 1996, 382, 609−611. (39) Loweth, C. J.; Caldwell, W. B.; Peng, X. G.; Alivisatos, A. P.; Schultz, P. G. DNA-Based Assembly of Gold Nanocrystals. Angew. Chem.-Int. Edit. 1999, 38, 1808−1812. (40) Aldaye, F. A.; Sleiman, H. F. Dynamic DNA Templates for Discrete Gold Nanoparticle Assemblies: Control of Geometry, Modularity, Write/Wrase and Structural Switching. J. Am. Chem. Soc. 2007, 129, 4130−4131. (41) Li, H. Y.; Park, S. H.; Reif, J. H.; LaBean, T. H.; Yan, H. DNATemplated Self-Assembly of Protein and Nanoparticle Linear Arrays. J. Am. Chem. Soc. 2004, 126, 418−419. (42) Niemeyer, C. M.; Ceyhan, B. DNA-Directed Functionalization of Colloidal Gold with Proteins. Angew. Chem.-Int. Edit. 2001, 40, 3685−3688. (43) Zhang, J. P.; Liu, Y.; Ke, Y. G.; Yan, H. Periodic Square-Like Gold Nanoparticle Arrays Templated by Self-Assembled 2D DNA Nanogrids on a Surface. Nano Lett. 2006, 6, 248−251. (44) Le, J. D.; Pinto, Y.; Seeman, N. C.; Musier-Forsyth, K.; Taton, T. A.; Kiehl, R. A. DNA-Templated Self-Assembly of Metallic Nanocomponent Arrays on a Surface. Nano Lett. 2004, 4, 2343−2347. I



Article

(67) Andersen, H. C. Rattle - a Velocity Version of the Shake Algorithm for Molecular-Dynamics Calculations. J. Comput. Phys. 1983, 52, 24−34. (68) Torrie, G. M.; Valleau, J. P. Non-Physical Sampling Distributions in Monte-Carlo Free-Energy Estimation - Umbrella Sampling. J. Comput. Phys. 1977, 23, 187−199. (69) The snapshots of the aggregated nanoparticles at different temperatures are shown in the Supporting Information. (70) Needs, R. J.; Mansfield, M. Calculations of the Surface Stress Tensor and Surface-Energy of the (111) Surfaces of Iridium, Platinum and Gold. J. Phys.., Condes. Matter 1989, 1, 7555−7563. (71) Shim, J. H.; Lee, B. J.; Cho, Y. W. Thermal Stability of Unsupported Gold Nanoparticle: A Molecular Dynamics Study. Surf. Sci. 2002, 512, 262−268. (72) Leff, D. V.; Brandt, L.; Heath, J. R. Synthesis and Characterization of Hydrophobic, Organically-Soluble Gold Nanocrystals Functionalized with Primary Amines. Langmuir 1996, 12, 4723−4730. (73) Heinz, H.; Vaia, R. A.; Farmer, B. L.; Naik, R. R. Accurate Simulation of Surfaces and Interfaces of Face-Centered Cubic Metals Using 12−6 and 9−6 Lennard-Jones Potentials. J. Phys. Chem. C 2008, 112, 17281−17290.

(45) Wang, G. L.; Murray, R. W. Controlled Assembly of MonolayerProtected Gold Clusters by Dissolved DNA. Nano Lett. 2004, 4, 95− 101. (46) Nakao, H.; Shiigi, H.; Yamamoto, Y.; Tokonami, S.; Nagaoka, T.; Sugiyama, S.; Ohtani, T. Highly Ordered Assemblies of Au Nanoparticles Organized on DNA. Nano Lett. 2003, 3, 1391−1394. (47) Patolsky, F.; Weizmann, Y.; Lioubashevski, O.; Willner, I. AuNanoparticle Nanowires Based on DNA and Polylysine Templates. Angew. Chem., Int. Ed. 2002, 41, 2323−2327. (48) Ganguli, M.; Babu, J. V.; Maiti, S. Complex Formation between Cationically Modified Gold Nanoparticles and DNA: an Atomic Force Microscopic Study. Langmuir 2004, 20, 5165−5170. (49) Srivastava, S.; Verma, A.; Frankamp, B. L.; Rotello, V. M. Controlled Assembly of Protein-Nanoparticle Composites through Protein Surface Recognition. Adv. Mater. 2005, 17, 617−621. (50) Loo, L.; Guenther, R. H.; Lommel, S. A.; Franzen, S. Encapsidation of Nanoparticles by Red Clover Necrotic Mosaic Virus. J. Am. Chem. Soc. 2007, 129, 11111−11117. (51) Verma, A.; Srivastava, S.; Rotello, V. M. Modulation of the Interparticle Spacing and Optical Behavior of Nanoparticle Ensembles Using a Single Protein Spacer. Chem. Mater. 2005, 17, 6317−6322. (52) Frankamp, B. L.; Boal, A. K.; Tuominen, M. T.; Rotello, V. M. Direct control of the magnetic interaction between iron oxide nanoparticles through dendrimer-mediated self-assembly. J. Am. Chem. Soc. 2005, 127, 9731−9735. (53) Halder, A.; Ravishankar, N. Ultrafine Single-Crystalline Gold Nanowire Arrays by Oriented Attachment. Adv. Mater. 2007, 19, 1854−1858. (54) A graphical molecular modelling program developed by Accelrys, Inc. (55) Jorgensen, W. L.; Chandrasekhar, J.; Madura, J. D.; Impey, R. W.; Klein, M. L. Comparison of Simple Potential Functions for Simulating Liquid Water. J. Chem. Phys. 1983, 79, 926−935. (56) Pearlman, D. A.; Case, D. A.; Caldwell, J. W.; Ross, W. S.; Cheatham, T. E.; Debolt, S.; Ferguson, D.; Seibel, G.; Kollman, P. Amber, a Package of Computer-Programs for Applying Molecular Mechanics, Normal-Mode Analysis, Molecular-Dynamics and FreeEnergy Calculations to Simulate the Structural and Energetic Properties of Molecules. Comput. Phys. Commun. 1995, 91, 1−41. (57) Agrawal, P. M.; Rice, B. M.; Thompson, D. L. Predicting Trends in Rate Parameters for Self-Diffusion on FCC Metal Surfaces. Surf. Sci. 2002, 515, 21−35. (58) Lee, O. S.; Schatz, G. C. Molecular Dynamics Simulation of DNA-Functionalized Gold Nanoparticles. J. Phys. Chem. C 2009, 113, 2316−2321. (59) Lee, O. S.; Schatz, G. C. Interaction between DNAs on a Gold Surface. J. Phys. Chem. C 2009, 113, 15941−15947. (60) Ngo, V. A.; Kalia, R. K.; Nakano, A.; Vashishta, P. Supercrystals of DNA-Functionalized Gold Nanoparticles: A Million-Atom Molecular Dynamics Simulation Study. J. Phys. Chem. C 2012, 116, 19579− 19585. (61) Xu, J.; Craig, S. L. Influence of the Extent of Hybridization on the Hydrodynamic Radius of DNA-Functionalized Gold Nanoparticles. Langmuir 2007, 23, 2015−2020. (62) Park, S.; Hamad-Schifferli, K. Evaluation of Hydrodynamic Size and Zeta-Potential of Surface-Modified an Nanoparticle-DNA Conjugates via Ferguson analysis. J. Phys. Chem. C 2008, 112, 7611−7616. (63) Wang, J. M.; Wolf, R. M.; Caldwell, J. W.; Kollman, P. A.; Case, D. A. Development and Testing of a General Amber Force Field. J. Comput. Chem. 2004, 25, 1157−1174. (64) Maingi, V.; Jain, V.; Bharatam, P. V.; Maiti, P. K. Dendrimer Building Toolkit: Model Building and Characterization of Various Dendrimer Architectures. J. Comput. Chem. 2012, 33, 1997−2011. (65) Maingi, V.; Kumar, M. V. S.; Maiti, P. K. PAMAM DendrimerDrug Interactions: Effect of pH on the Binding and Release Pattern. J. Phys. Chem. B 2012, 116, 4370−4376. (66) Hockney, R. W.; Eastwood, J. W. Computer Simulation Using Particles; Institute of Physics Publishing: Bristol, 1988. J


Engineering Gold Nanoparticle Interaction by PAMAM Dendrimer

Recommend Documents