PAMAM Dendrimers as Support for the Synthesis of Gold Nanoparticles

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PAMAM Dendrimers as Support for the Synthesis of Gold Nanoparticles: Understanding the Effect of the Terminal Groups María Belén Camarada J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b08272 • Publication Date (Web): 05 Oct 2017 Downloaded from http://pubs.acs.org on October 5, 2017

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The Journal of Physical Chemistry

PAMAM Dendrimers as Support for the Synthesis of Gold Nanoparticles: Understanding the Effect of the Terminal Groups M.B. Camarada1* 1Centro

de Genómica y Bioinformática, Facultad de Ciencias, Universidad Mayor, Santiago, Chile.

Abstract The interaction between hydroxyl, aldehyde and amino terminated PAMAM G0 dendrimers with gold nanoclusters Aun (n = 2, 4, 6, and 8) was studied theoretically at DFT level. Different coordination sites were explored, including internal and superficial coordination. In the case of amino terminated PAMAM, the most stable complexes exhibited external coordination, while hydroxyl and aldehyde terminated dendrimers did not favor interaction with the terminal groups, showing lower binding energies. Hydroxyl terminated PAMAM as well as its oxidized form, have lesser capacity as stabilizing capping agent of gold nanoclusters, agreeing with previous experimental reports. The vertical first ionization potential, electron affinity, Fermi level, and the HOMO–LUMO gap of PAMAM and Aun– PAMAM G0 complexes were also analyzed.

*e–mail: [email protected]

ACS Paragon Plus Environment


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1. Introduction Metal nanoparticles have attracted great attention because of their plausible use in a wide range of areas such as medicine1, microelectronics2, catalysis3 and biotechnology4. They are the most fundamental component in the fabrication of nanocomposites and nanostructures5. Unlike bulk metals, the physicochemical properties of nanoparticles are size and shape dependent, converting them in unique materials6-8. However, some nanosized particles have a strong tendency to agglomerate, diminishing and affecting their utility9. Stabilizers, also called capping agents, are widely used in the synthesis of metal nanoparticles to inhibit the over growth and aggregation10. Most of them are based on polymers, surfactants and chelating agents, and act as an ‘‘armor’’ around the metal nanoparticles preventing them from agglomeration. One of the systems proposed in the past as pattern and stabilizer for the synthesis of nanoparticles are dendrimers. These hyper–branched polymers, first visualized by Flory11 in 1941 and synthesized four decades later12-15, are globular nanoscale macromolecules characterized by a strictly controlled structure, definite molecular weight, monodispersity, and biocompatibility16-18. Polyamidoamine (PAMAM–NH2) dendrimers are an important class with an ethylenediamine core, repetitive amide functionalities and primary amines at the end of branches. They have been used with increasing frequency as template for the synthesis of nanoparticles19. At neutral and acid pH, primary external amines remain protonated20-21 (PAMAM–NH3+), allowing its unspecific interaction with biological and inorganic interphases22-23. Branch ends of PAMAM type dendrimers, can also be functionalized with other type of groups such as carboxylic acids or hydroxyls (PAMAM– OH), that may impact the reactivity and solubility of the polymer. In recent years, the interests in metal nanoparticles synthetized in the presence of PAMAM dendrimers has resulted in more extensive characterization and thus, better understanding of nanoparticle formation24-37. With the evolution of nanotechnology, interest in the properties and capabilities of neutral metal nanoparticles formed by single metal atoms or alloys, has been progressively increasing. PAMAM dendrimers have been reported as excellent platforms for the synthesis of different type of nanoparticles such as Pt38, Pd23 and Cu39, by chemical reduction. Between metals, gold has attracted great attention in different fields mainly because of its chemical inertness40, facile synthesis41-42, and biocompatibility43. To date, the main subject of several studies has been the production of stable gold nanoparticles (AuNPs) in the presence of PAMAM dendrimers36-37.


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There are several methodologies that have been applied so far for the synthesis of AuNPs. In the case of dendrimer assisted synthesis, the most common method involves the use of a gold salt like HAuCl4, which is mixed with the dendrimer in aqueous solution to able the interaction between gold and the active coordination sites of the polymer. The complexation starts with the covalent interaction of the AuCl4– ions with the terminal groups of

the

dendrimer44

complexation45-46,

through

covalent

bonds,

electrostatic

interactions,

and/or

displacing the water molecules coordinated to these ending sites.

Subsequently, a reducing agent like ascorbic acid, sodium citrate or sodium borohydride is added to reduce the Au+3 centers to metallic gold, as the following semi-reaction shows: AuCl4– + 3é  Au + 4Cl–

(1)

It has been described that low generation dendrimers (G0–G4) generate inter–dendrimeric complexes when used in the synthesis of gold nanoparticles. Because of their opened and flat structure, nanoparticles are formed in the surface of the dendrimer and are subsequently capped by other polymer units. Gold nanoparticles are stabilized by a coordination sphere of polymers rather than inner coordination47-52, as Figure 1 schematically depicts. On the contrary, upper generations of dendrimers adopt a spherical three–dimensional structure and thus, provide internal coordination sites that can act as an effective protective shell for the formation of nanoparticles inside the dendrimer53.

Figure 1: Schematic synthesis of gold nanoparticles using PAMAM G0 dendrimers. Some authors have explored the effect of PAMAM generation and surface end groups on the architecture of the nanoparticles54. Both PAMAM–NH3+ and PAMAM–OH have been applied for the synthesis AuNPs with sizes starting from approximately 2 nm55. Nonetheless, in the specific case of PAMAM–OH, it has been reported that the synthesis of AuNPs is complicated due to the prematurely reduction of AuCl4– ions56. Nanoparticles synthetized with PAMAM–OH dendrimers are prone to aggregation, coalescing almost instantaneously after formation, showing low stability in comparison to PAMAM–NH3+57. In a previous work reported by Esumi and coworkers, it was described the strong capacity of hydroxyl modified dendrimers to spontaneously reduce metals without the addition of any reducing agent58. “Sugar ball” dendrimers, showed a strong reduction power being able to



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reduce spontaneously Au+3 to metal nanoclusters through the several –OH terminal groups that were oxidized to aldehydes. Up to now, there is a lack of studies directed to explain at atomic level the effect of both dendrimers, amino and hydroxyl terminated PAMAM, on the synthesis of gold nanoparticles. Computational studies offer useful tools for the exploration and better understanding of the mechanism of nanocomposite formation. Therefore, the aim of this work is to make a comparative study at neutral pH of the interaction and energetic stabilization of AuNPs formed in the presence of PAMAM G0, –OH and–NH3+ terminated. Taking into account the experimental evidence related to the spontaneous oxidation of the terminal hydroxyl groups in the presence of cationic gold, the oxidized structure of PAMAM–OH with aldehyde terminal groups (PAMAM–CHO) was also considered to compare the strength of the energetic interactions and the effect on the stabilization of gold nanoparticles. In this work, the nature of the interaction between low generation PAMAM G0 dendrimers (–OH, –CHO and–NH3+ terminated) and gold small clusters Aun (n = 2, 4, 6, and 8) was optimized at Density Functional Theory (DFT) level to study the coordination preferences depending on the terminal groups of the dendrimers and the stabilizing effect of each PAMAM G0 after coordination to the metallic centers. Gold nanoclusters were represented by the planar Au2 dimer, the T–shaped, Au4, the triangular Au6, and the triangular pyramid Au8. Different coordination sites at PAMAM G0 were selected, including core and external coordination, in order to identify the most stable coordination site and the charge transferring properties of the different Aun–PAMAM G0 complexes. Ab initio calculations delivered comparative information about the binding geometries and stabilizing capacity of gold nanoclusters by each dendrimer, and also gave details of ionization potential, electron affinity, and charge distribution in the complexes. Additionally, conductivity was explored through the frontier molecular orbitals. Analysis of the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) delivered relevant information about physicochemical properties of the structures. 2. Computational and experimental details Geometries of complexes formed between PAMAM G0 and gold clusters Aun (n = 2, 4, 6, and 8) were fully optimized at the density functional theory (DFT) level. DFT spin restricted and unrestricted calculations were performed using the Gaussian 0959 computational package. Full geometry optimizations of complexation reactions were calculated using the


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Becke’s three parameter nonlocal hybrid exchange potential with the nonlocal correlation functional of Lee, Yang, and Parr (B3LYP)60-62 without any symmetry restriction. The triple– ζ 6–311G(d,p) basis set was set for light atoms (C, H, O, N) along with a relativistic effective core potential basis set with pseudopotentials for the gold atoms, LANL2DZ63. After every optimization, frequency calculations were performed to confirm that the stationary points were minima at the potential energy surface (PES), and to obtain the zero-point energy (ZPE) values. Tight SCF convergence criteria (10−8 a.u.) was used in all calculations. Charge distribution of the intermolecular interactions was calculated using the natural population analysis (NPA)64 method, as implemented in Gaussian 09. Interaction energy (Eint) is defined as the energy difference between the complex and the energies of constituent monomers. Computation of this quantity with finite basis sets introduces a difference known as basis set superposition error (BSSE) because different number of basis functions are used to describe the complex and the monomers for the same basis set. BSSE corrected interaction energies were computed using the Boys– Bernardi counterpoise correction scheme65. The Eint of complexes was computed using Eq. 2: Eint = EAB – EA (AB) – EB (AB)

(2)

where EAB is the total energy of the complex, EA(AB) represents the total energy of PAMAM G0 with ghost atoms in place of gold atoms, and EB(AB) corresponds to the total energy of the gold cluster of the complex with ghost atoms for the rest of the system. According to the Koopmans theorem66, the ionization potential (IP) can be approximated by the negative of HOMO energy, ignoring the relaxation effects of the ionization process. Within the same framework, LUMO energy can be approximated as the electron affinity (EA). Electrochemical measurements A Voltalab (PGZ100) potentionstat/galvanostat electrochemical workstation was used to measure the electrochemical properties of PAMAM samples at ambient temperature (20 °C). A conventional three-compartment cell was employed throughout the work. A platinum electrode was used as working electrode. The counter electrode was a coiled Pt wire of large area, separated from the electrolytic solution by a sintered glass frit. Prior to each experiment, the working electrode was polished to a mirror finish with an aqueous alumina slurry (particle size 0.3 and 0.05 micron) on micro-cloth pads, rinsed thoroughly with water, ultra-sonicated for 5 min in milli-Q water and dried. All potentials quoted are referred to a



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Ag/AgCl (1 M KCl) electrode. Each working solution was purged with high purity argon for 15 min prior to each experiment and the flow was maintained over the solution during the measurements. 3. Results and discussion 3.1. Characterization of PAMAM G0 and Aun–PAMAM G0 complexes The isolated PAMAM G0 structures were fully optimized at B3LYP/6–311G(d,p) level, characterized as stationary point using second derivative calculations and used as reference for the calculation of interaction energies. Several initial conformations were tested to search for alternative local minima. The binding energies of the Aun clusters were calculated to be 0.94, 1.18, 1.49, and 1.49 eV/atom, respectively. PAMAM type dendrimers have some potential coordination sites with high electron density, where a gold cluster could be able to attach and generate a complex67. In this work, the coordination sites were selected according to previous studies related to metal complexes where dendrimers were used as ligand. Among literature, the described chelating dendrimer atoms that participate in complexation with metals are the amide nitrogen atoms68-70, the oxygen atoms belonging to the carbonyl site of the amide group7172

and the terminal functional groups52. The starting geometries of Aun–PAMAM G0

complexes for optimization were generated by placing Aun clusters near the electron–rich sites of the dendrimer. According to Figure 2, four possible coordination sites were considered: (a) the core site involving the two nitrogen atoms from the ethylendiamine center, (b) the terminal site related to the terminal groups: –NH3+, –OH or –CHO, (c) the mixed amide/terminal site corresponding to a branch site and a terminal site, and (d) the amide site.

Figure 2: Schematic structure of PAMAM G0, showing the four possible main coordination sites considered in this study, and the optimized structures of PAMAM G0. The most stable configurations of each dendrimer are depicted in Figure 2. As can be seen, PAMAM–NH3+ adopts an open and extended conformation, due to the repulsion


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between the terminal protonated amino groups. This structure is consistent with previous studies that report higher radius of gyration in the case of charged PAMAM structures in comparison to the neutral state73. PAMAM–OH has a compact conformation because of the internal hydrogen bonds between the amide groups and the terminal hydroxyl moieties, while PAMAM–CHO exhibits a more extended conformation with lesser internal interactions that contribute to the dendrimer stabilization. Gold clusters Aun (n = 2, 4, 6, and 8) were optimized at LANL2DZ level and subsequently coordinated to each of the PAMAM ground state conformations. Starting sites for geometry optimizations without any symmetry restriction of the PAMAM G0 and each gold nanocluster, were selected according to Figure 2. Four stable binding modes of each cluster coordinated at different sites of PAMAM G0 were obtained. The most stable optimized neutral Aun–PAMAM G0 complexes are shown in Figures 3, 4 and 6, in which bond lengths, NBO charges at selected atomic sites and atomic numbering schemes are depicted. 3.2. Amino terminated PAMAM G0 (PAMAM–NH3+) gold complexes The most stable complexes for each Aun with PAMAM–NH3+ are depicted in Figure 3, in which the corresponding bond length parameters and charge are shown. Almost all the most stable complexes exhibited external interaction with the amine site.

Figure 3: Ground state geometries of the Aun/PAMAM–NH3+ complexes at the B3LYP/6– 311G(d,p)//LANL2DZ level. NPA charges (a.u.) for selected atoms are displayed. Bond lengths in Å. H–Bond length and charges are in italics.



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Table 1 summarizes the BSSE corrected and uncorrected binding energies of all complexes. The uncorrected energy is the direct computation of the interaction energy (Eint) of each optimized complex: Eint = EAB – EA – EB, where EAB is the total energy of the complex, EA represents the total energy of PAMAM G0 and EB corresponds to the total energy of the gold cluster. The corrected energy considers the basis set superposition error (BSSE), as Eq. 2 points, and introduces ghost atoms in the calculation to minimize the basis set error. Most of the complexes exhibited Au–HN external coordination, while Au–O binding was present only in a few cases and always accompanied by a hydrogen bond interaction with the –NH group of the amide. The electron density of the carbonyl site is delocalized throughout the conjugated system of the amide group, and therefore, the interaction with gold is less strong compared to the case of terminal amine sites. The depletion of charge at the terminal sites due to the protonated state of the nitrogen atoms, generates a more probable interaction with the electron density of the metal cluster. Table 1: Description of gold/PAMAM–NH3+ complexes. Au–X (X=N, O or H) anchor bond distances dX–Au in Å. NPA derived atomic charges of the anchor atom qx, the bonded gold atom qAu and the total charge of the metal cluster Δqcluster in a.u. BSSE–corrected and – uncorrected (in parenthesis) interaction energy (Eint, kcal·mol-1) for the studied complexes. Au··H represent hydrogen bond interactions and Final site refers to the ending coordination place of gold nanocluster according to Figure 1. Complex

Anchor

Final site

dX–Au

qx

qAu

Δqcluster

Eint

Au2–I

Au1–N

a

2.339

-0.548

-0.016

-0.124

19.134 (23.442)

Au2··HN

d

2.594

0.425

-0.108

Au2–II

Au1–HN

b

2.493

0.438

-0.055

0.059

9.6311 (10.704)

Au2–III

Au1–O

d

2.317

-0.746

0.031

-0.081

8.3425 (12.091)

Au2··HN

d

2.574

0.418

-0.112 -0.064

9.0933 (12.420)

-0.075

21.210 (25.622)

Au2–IV

Au1–O

d

2.342

-0.738

-0.046

Au2··HN

d

2.826

0.431

-0.018

Au1–N

a

2.539

-0.540

-0.141

Au2··HN

d

2.613

0.433

-0.320

Au4–II Au4–III Au4–IV

Au1–HN

b

2.225

-0.740

-0.241

0.117

24.150 (24.956)

Au1–HN

b

2.231

0.420

-0.242

0.114

23.850 (24.615)

Au1··HN

d

2.523

0.420

-0.295

0.015

13.754 (16.393)

Au6–I

Au1··HN

d

2.720

0.432

-0.054

0.012

19.166 (21.738)

Au4–I


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Au6–II

Au1–HN

b

2.614

0.450

0.016

0.037

22.377 (24.614)

Au6–III

Au1–HN

b

2.466

0.438

-0.173

0.024

22.373 (24.613)

Au6–IV

Au1··HN

d

2.721

0.432

-0.054

0.011

19.167 (21.738)

-0.029

20.741(23.651)

0.083

30.131 (32.135)

-0.028

18.016 (20.904)

-0.037

18.216 (21.492)

Au8–I Au8–II Au8–III Au8–IV

Au1··HN

d

2.655

0.422

-0.212

Au2··HN

d

2.935

0.423

-0.212

Au1–HN

b

2.413

0.442

-0.011

Au8–HN

b

2.459

0.436

0.014

Au1··HN

d

2.552

0.426

-0.101

Au8–O

d

2.786

-0.715

0.015

Au1··HN

d

2.576

0.426

-0.107

Au8–O

d

2.760

-0.716

-0.027

In the case of Au2 complexes, the structure Au2–I had the highest Eint and was the only complex with core site coordination, interacting directly with the ethylenediamine group of the dendrimer and being almost 10 kcal·mol–1 more stable than the rest of the Au2 complexes. This evidence indicates that the available lone pair at the nitrogen atoms can generate stronger interactions with gold clusters. Therefore, the electron shared by the lone–pair orbits of these atoms and the gold 5d and 6s orbitals plays an important role in stabilization. Au2–I was also stabilized by a hydrogen bond with one of the N-H sites of the amide group. It has been reported that gold is capable of acting as a hydrogen bond acceptor forming nonconventional N–H··Au hydrogen bonds74-75. Generally, these exceptional H–bonds are classified as ‘anchor–assisted’ H–bonding because systems that contain these unconventional H–bonds are unstable unless the anchor bond is formed. The rest of the Au2 complexes exhibited coordination to the amide, (d) site, with lower stability. In the Au4 group, the most stable coordination occurred at the terminal (b) site, as well as in the Au6 and Au8 complexes. The smaller size of the Au2 cluster probably allows its entrance to inner sites of the dendrimer. However, considering experimental evidence that indicates an average size of gold nanoparticles synthetized with PAMAM–NH3+ of 2 nm54, it is more probable that coordination with bigger nanoclusters will tend to be stablished with the terminal protonated sites. The overall stability ordering of the complexes was: Au8–II > Au4–II > Au6–II > Au2–I (Figure 3). Complex Au2–I was the only of the group with core site coordination, being almost 10 kcal·mol–1 more unstable than Au8–II. The rest of the geometries exhibited external coordination to the cationic amine terminal group, and specifically in the case of



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Au8–II, there was a double coordination to the terminal site that reinforced the interaction, resulting in higher stability. If the total NPA charge of the gold cluster (Δqcluster) is analyzed for the most stable cases i.e. Au8–II, Au4–II, Au6–II and Au2–I, with the exception of Au2–I, there was a charge transfer from the gold cluster to the ligand, evidenced by the positive Δqcluster values, which are the highest of each group. It can be noticed that the largest amount of charge was transferred to the most stable complex (Au8–II), then to Au4–II and finally to Au6–II. This tendency follows the same trend as the interaction energy, indicating that the charge transfer process influences the energetics of the system. In the case of Au2–I, the coordination to the ligand was not related to a zone with depletion of charge and thus, the charge transfer occurred from the ligand to the metal cluster. The higher values of Eint in the Au4–8 clusters coordinated to the outer sphere of the dendrimer indicate that the amine external site is preferred among all coordination places, although the opened structure of the dendrimer. This evidence is in agreement with experimental reports, and suggests that gold nanoclusters will tend to coordinate to the surface groups of PAMAM–NH3+37, 54, 76. A previous work performed at the same level of theory with neutral amino terminated dendrimer (PAMAM–NH2)44, indicated that the coordination of gold nanoclusters was preferred at external sites, with interaction energies relatively close to the values obtained in this study. Therefore, independent of the protonation state of the terminal amine groups, the most stable coordination sites are the terminal groups of the dendrimer, which can surround and stabilize AuNPs. 3.3. Hydroxyl terminated PAMAM G0 (PAMAM–OH) gold complexes The most stable complexes are shown in Figure 4. Table 2 summarizes the BSSE corrected binding energies of all complexes, distances and charge. In the case of Au2 complexes, interaction energy had the following trend: IV>I>III>II. The most unstable complex exhibited external coordination to the hydroxyl terminal group, while the coordination to site (c), with mixed interaction with the amide and hydroxyl site, presented the higher interaction energy, even superior to the interaction with the core site with the nitrogen lone pairs. In addition, Au2–IV showed the shortest bond distance between ligand and gold nanocluster (2.214 Å), reinforcing the interaction.


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Figure 4: Ground state geometries of the Aun/PAMAM–OH complexes at the B3LYP/6– 311G(d,p)//LANL2DZ level. NPA charges (a.u.) for selected atoms are displayed. Bond lengths in Å. H–Bond length and charges are in italics. The most stable interaction among Au4 complexes was stablished again at the mixed amine–OH (c) site, while the external –OH was the less favored. The two complexes with higher Eint, Au4–IV and –I, showed amide–OH coordination. However, Au4–IV presented triple coordination showing the smallest bond distance, reinforcing the interaction in approximately 4 kcal·mol–1 more than Au4–I. Au6 clusters showed lower tendency to coordinate to terminal –OH groups. The preferred site corresponded to the core (Au6–I), where a double coordination to the nitrogens of the ethylenediamine core resulted in the strongest interaction. Complexes Au6–III and –IV showed similar stability, presenting double coordination.

Table 2: Description of gold/PAMAM–OH complexes. Au–X (X=N, O or H) anchor bond distances dX–Au in Å. NPA derived atomic charges of the anchor atom qx, the bonded gold atom qAu and the total charge of the metal cluster Δqcluster in a.u. BSSE–corrected and – uncorrected (in parenthesis) interaction energy (Eint, kcal·mol-1) for the studied complexes. Au··H represent hydrogen bond interactions and Final site refers to the resulting coordination place of gold nanocluster according to Figure 1.



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Complex

Anchor

Final site

dX–Au

qx

qAu

Δqcluster

Eint

Au2–I

Au1–N

a

2.309

-0.550

0.047

-0.176

15.969 (20.518)

Au2–II

Au1–OH

b

2.303

-0.750

0.023

-0.119

10.600 (14.532)

Au2–III Au2–IV

Au1–O

d

2.249

-0.682

0.068

-0.127

13.953 (17.014)

Au1–O

c

2.214

-0.693

0.098

-0.145

18.807 (24.226)

Au2–HO

c

2.397

0.454

-0.243

Au1–O

d

2.204

-0.695

0.187

-0.171

20.370 (24.312)

Au1–OH

b

2.285

-0.738

0.178

-0.198

17.362 (23.256)

Au2··HN

d

2.781

0.420

-0.201

Au4–III

Au1–N

d

2.247

-0.737

0.145

-0.213

16.330 (21.317)

Au4–IV

Au1–O

c

2.203

-0.698

0.185

-0.228

24.568 (32.093)

Au4–OH

c

2.515

-0.748

-0.149

Au4··HN

c

2.600

0.405

-0.149

Au1–N

a

2.487

-0.549

0.082

-0.209

12.863 (17.931)

Au1–N

a

2.640

-0.547

0.082

Au6–II

Au1–OH

b

2.405

-0.762

0.090

-0.095

5.777 (9.630)

Au6–III

Au1–O

d

2.378

-0.684

0.101

-0.192

8.382 (13.926)

Au6–O

d

2.756

-0.641

-0.042

Au1–NH

c

2.470

-0.710

0.077

-0.197

8.143 (14.938)

Au4–OH

c

2.484

-0.760

0.036

Au1–N

a

2. 550

-0.551

0.148

-0.241

10.770 (16.023)

Au1–N

a

2.574

-0.544

0.148

Au1–OH

b

2.524

-0.762

0.122

-0.287

10.587 (19.502)

Au4–OH

b

2.390

-0.787

0.152

Au8–III

Au1–O

d

2.313

-0.700

0.182

-0.181

11.384 (14.772)

Au8–IV

Au1–OH

b

2.523

-0.762

0.122

-0.287

10.580 (19.499)

Au8–OH

b

2.390

-0.787

0.152

Au4–I Au4–II

Au6–I

Au6–IV Au8–I Au8–II

Finally, Au8 complexes exhibited the following trend of Eint: III>I>II>IV. Au8–III presented amide coordination, while Au8–IV, the most unstable system, had interaction with the terminal –OH groups. In all PAMAM–OH complexes, the electronic flux exhibited ligand to metal charge transfer (LMCT), the opposite result to PAMAM–NH3+ most stable complexes, where the electronic density was transferred from gold to the ligand. In the case of Au6 and Au8 nanoclusters stabilized with PAMAM–OH, interaction energies were approximately half of the values observed with PAMAM–NH3+, indicating that the coordination is less favored in the former case. In PAMAM–OH the most unstable coordination site of gold nanoclusters was the terminal hydroxyl group, thus the external coordination was not favored, unlike the case of


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PAMAM–NH3+, where the protonated terminal sites generated the strongest interactions. Coordination at the amide or core site presented higher interaction energies in the case of the most stable complexes of PAMAM–OH. This fact implies that the gold nanoparticles should enter to the cavities of the dendrimer to get better stabilization. However, as already pointed, PAMAM dendrimers of low generation (G0-G4) adopt a flat and opened spatial configuration, being not suitable to coordinate metal centers at the inner cavities. Therefore, in the case of PAMAM–OH, the internal stabilization will be deficient and the capping of the nanoparticles through the terminal hydroxyl groups is less favored in comparison to the –NH3+ groups. As above mentioned, experimental studies have reported inter-dendrimer coordination to gold nanoparticles in lower generations, where dendrimers surround the centers to generate stabilization. Intra-dendrimer coordination is not favored due to the small size of the internal cavities, which are not adequate to host nanoparticles. In the case of PAMAM– NH3+, gold nanoclusters interact directly with the terminal charged groups with the highest energetic stabilization, contrary to PAMAM–OH. Therefore, the latter dendrimer will have lower performance as stabilizer and capping agent of gold nanoparticles through the interaction with terminal groups, agreeing with the experimental evidence that report poor stabilizing capacity of PAMAM–OH for the synthesis of gold nanoparticles, which present low stability and aggregate rapidly56. On the other hand, it has been reported that dendrimers with hydroxyl terminal groups trigger the automatic reduction of cationic gold without the presence of a reducing agent, causing the spontaneous oxidation of the terminal groups of the dendrimer58. If the oxidation and reduction potential is considered for each of the involved species, it is possible to make an approximated prediction of the spontaneity of the redox reaction. Figure 5 shows the cyclic voltammetry profile of dendrimers of generation four (G4) in aqueous solution. As can be seen, the oxidation potential of PAMAM–NH3+ (0.86 V) is lower than PAMAM–OH value (0.97 V).



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Figure 5: Cyclic voltammetry curves for PAMAM G4 –NH3+ and –OH in KCl 0.1 M. Potentials are quoted against Ag/AgCl (1 M KCl) electrode. The reduction reaction of AuCl4- (Eq. 1) has a standard potential of 0.77 V77 vs Ag/AgCl (KCl 1 M) reference electrode. According to these values, with both dendrimers the reduction reaction of gold will occur spontaneously. However, in the case of PAMAM–OH, the resulting potential difference of the AuCl4-/PAMAM–OH couple is higher, indicating that the probability of automatic reduction of gold with hydroxyl terminated PAMAM is greater, causing the oxidation of hydroxyls to aldehyde moieties. According to this analysis and the reported experimental results, the final structure involved in the stabilization of gold nanoparticles is PAMAM–CHO rather the hydroxyl terminated PAMAM. Taking into account this evidence, aldehyde terminated dendrimer was also considered in this work in order to analyze the energetic characteristics of the interaction and stabilization of gold nanoclusters. 3.4. Aldehyde terminated PAMAM G0 (PAMAM–CHO) gold complexes Figure 6 shows the most stable conformations of PAMAM–CHO dendrimer coordinated to each gold nanocluster.


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Figure 6: Ground state geometries of the Aun/PAMAM–CHO complexes at the B3LYP/6– 311G(d,p)//LANL2DZ level. NPA charges (a.u.) for selected atoms are displayed. Bond lengths in Å. H–Bond length and charges are in italics. In the case of Au2, three of the complexes presented coordination to the amide site (d). Complex Au2-II was the only with external coordination and had the lowest interaction energy of the group, as Table 3 shows. Table 3: Description of gold/PAMAM–CHO complexes. Au–X (X=N, O or H) anchor bond distances dX–Au in Å. NPA derived atomic charges of the anchor atom qx, the bonded gold atom qAu and the total charge of the metal cluster Δqcluster in a.u. BSSE–corrected and – uncorrected (in parenthesis) interaction energy (Eint, kcal·mol-1) for the studied complexes. Au··H represent hydrogen bond interactions and Final site refers to the ending coordination place of gold nanocluster according to Figure 1. Complex

Anchor

Final site

dX–Au

qx

qAu

Δqcluster

Eint

Au2–I

Au1–O

d

2.241

-0.684

0.080

-0.147

16.354 (20.078)

Au2–II

Au1–CHO

b

2.314

-0.541

0.054

-0.089

8.986 (11.628)

Au2–III

Au1–O

d

2.236

-0.680

0.080

-0.124

13.741 (17.904)

Au2··HN

d

2.686

0.409

-0.204

Au1–O

d

2.216

-0.689

0.128

-0.140

20.091 (24.622)

Au2··HN

d

2.647

0.409

-0.268

Au4–I

Au1–O

d

2.195

-0.697

0.201

-0.194

22.850 (27.642)

Au4–II

Au1–CHO

b

2.271

-0.570

0.207

-0.138

14.405 (18.791)

Au4–III

Au1–CHO

c

2.292

-0.550

0.155

-0.217

24.791 (32.960)

Au2–IV



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Au4–O

c

2.329

-0.684

-0.008

Au2··HN

d

2.565

0.440

-0.331

Au1–CHO

c

2.292

-0.550

0.155

Au4–O

c

2.328

-0.684

-0.008

Au2··HN

d

2.565

0.440

-0.331

Au6–I

Au1–N

a

2.415

-0.558

Au6–II

Au1–CHO

b

2.510

Au6–III

Au1–CHO

b

Au6–IV

Au1–O

d

Au2··HN

d

Au8–I Au8–II

Au1–O

d

Au1–CHO

b

2.500

-0.541

Au8–III

Au1–O

d

2.365

Au2··HN

d

Au1–O Au2··HN

Au4–IV

Au8–IV

Page 16 of 25

-0.217

24.786 (32.956)

0.041

-0.160

3.902 (8.309)

-0.549

0.096

-0.133

6.452 (11.511)

2.438

-0.568

0.132

-0.135

5.251 (10.387)

2.312

-0.697

0.174

-0.176

14.105 (20.845)

2.991

0.411

-0.100

2.314

-0.697

0.233

-0.220

15.640 (22.534)

0.155

-0.083

4.966 (7.558)

-0.707

0.188

-0.187

7.128 (13.607)

2.881

0.410

-0.140

d

2.314

-0.701

0.231

-0.221

15.748 (22.534)

d

2.990

0.418

-0.019

Structure Au2-IV exhibited the highest Eint value, related to the double coordination to amide sites through carbonyl and hydrogen bond interaction. Moreover, this structure presented greater charge transfer from the ligand to the gold nanocluster and shorter bond distance from gold to the anchor atom of the ligand, related to the higher stability. In the Au4 series, the weakest interaction occurred with the aldehyde terminal group (Au4– II). The rest of the structures presented mixed (c) and amide site (d) coordination. The most stable complex (Au4–III) had triple coordination to the ligand and the highest amount of electron density transferred from the dendrimer. In the case of the group of complexes Au6 and Au8 (-IV), the most stable complexes exhibited double coordination to amide sites, with the most negative Δqcluster values and shortest bond distances. Again, the less favored coordination site were the external aldehydes. As well as PAMAM–OH, the coordination to the terminal groups was the less probable. Unlike PAMAM–NH3+ and as PAMAM–OH, all PAMAM–CHO complexes presented electronic flux from the ligand to the metal. In relation to the interaction energy, PAMAM– CHO showed upper interaction energies, with an average value of 1.8 kcal·mol-1 more, indicating that the oxidized dendrimer PAMAM–CHO has better performance as stabilizer agent in comparison to the hydroxyl terminated dendrimer. Anyway, PAMAM–NH3+ was the best pattern, generating complexes with gold nanoclusters in average 5 kcal·mol-1


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more stable. Both PAMAM–OH and PAMAM–CHO, unlike PAMAM–NH3+, do not promote the external coordination of gold nanoclusters. If the strength of interaction with the terminal site is analyzed (site b), PAMAM–CHO presents the lowest average interaction energy (8.7 kcal·mol-1), being approx. 2.4 kcal·mol-1 less stable than PAMAM–OH. PAMAM–NH3+ has the highest average interaction energy at the external site (21.5 kcal·mol-1), generating complexes with gold nanoclusters much more stable. Thus, in the presence of PAMAM–OH or its oxidized form, gold must enter to the cavities of the dendrimer to get an efficient energetic stabilization. However, low generation dendrimers adopt a 2D flat spatial configuration, not providing inner space for adequate metal coordination. In this way, the capping of the nanoparticles by the terminal groups of several dendrimer units is not favored, leading to a poor effect as anti-agglomeration agents. 3.5. Charge transport analysis Charge transport in molecular systems is an important phenomenon to study molecular devices. Ab initio methodologies offer some tools that can be applied to explore the efficiency of charge transportation. The energy of the Fermi Level (EFL) was computed by taking the average of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energies. The EFL was calculated for each of the most stable complexes. As Table 4 displays, the EFL values of the Aun–PAMAM–OH and – CHO complexes lied between –3.8 eV and –4.5 eV, values very close to the ones reported for neutral amino terminated PAMAM G044. Table 4: Ionization potential IP (eV), Electron affinity EA (eV), dipole moment (, Debye), energy of the Fermi level (EFL, eV), and the HOMO–LUMO Gap (eV) of the PAMAM G0– Aun most stable complexes. 

EFL

8.461

6.104

-10.924

4.926

13.467

9.406

35.362

-11.437

4.061

Au4–II

12.629

9.680

109.363

-11.154

2.949

Au6–II Au8–II

12.641

9.080

134.696

-10.861

3.561

12.245

9.153

144.207

-10.700

3.091

PAMAM–OH

5.446

0.193

7.909

-2.819

5.253

Au2–IV Au4–IV

5.696

2.094

12.933

-3.895

3.602

5.422

2.384

10.500

-3.903

3.037

System

IP

EA

PAMAM–NH3+

13.387

Au2–I


HL–gap


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Page 18 of 25

Au6–I Au8–III

5.708

2.549

9.039

-4.129

3.160

5.785

2.673

17.185

PAMAM–CHO

5.803

1.493

10.129

-4.229 -3.647

3.111 4.311

Au2–IV Au4–III Au6–IV

5.859

2.520

15.472

-4.189

3.339

5.598

2.751

11.628

-4.174

2.848

5.830

3.191

13.625

-4.510

2.639

Au8–IV

5.908

3.134

13.926

-4.521

2.774

In the case of PAMAM–NH3+, values were quite higher, ranging around –11 eV, due to the protonated state of the system. The EFL of the bare optimized PAMAM G0–NH3+, –OH and –CHO calculated at the same level of theory were –10.9, –2.8 and –3.6 eV, respectively. Previous reports estimated the EFL for the bulk Au metal as –5.5 eV when complexed with PAMAM G4–OH78. The value obtained in this study for the biggest cluster Au8 corresponds to –4.229 eV. However, considering the spontaneous chemical reduction of gold by the oxidation of the terminal groups of PAMAM–OH to PAMAM–CHO, the EFL value increases to –4.521 eV, representing a close approximation for the experimental result, considering the approximations: a lower generation dendrimer and a reduced size cluster. It has been reported that the conductivity of molecular systems mainly depends on the frontier orbitals79. From this point of view, frontier orbitals of PAMAM G0 dendrimers and their complexes with gold clusters have been further investigated. The energy gap is compiled in Table 4. The H–L Gap of the free PAMAM–NH3+ was 4.9 eV, lower than the calculated value at the same level of theory for the neutral system44. In the case of PAMAM–OH and PAMAM–CHO, the HL–gap was 5.3 and 4.3 eV, respectively. Therefore, the conducting properties are better in the oxidized system, PAMAM–CHO. The incorporation of gold clusters to all the structures decreased the band gap. PAMAM–CHO– Au showed the lowest values and therefore, better conductivity. The chemical bonding in transition metal complexes is usually described in terms of ionic and covalent interactions between the metal and the ligand. The ionic contribution is associated with the atomic charges, while the covalent effect is reflected in the orbital mixing. In this context, when the qcluster value is analyzed for each case, it is possible to conclude that in PAMAM–OH and PAMAM–CHO occurred an electron transfer from the ligand toward the metal cluster, while in PAMAM–NH3+, the cluster deliver electron density to the ligand due to the deficiency of charge in the terminal amines. Figure 7 shows the isosurface plots of the HOMO and LUMO of the most stable complexes for the biggest gold cluster (Au8) complexed with each dendrimer. In the case of PAMAM–NH3+, it can be


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found that the HOMO and LUMO orbitals are mainly localized on the gold cluster and the terminal protonated amine group.

Figure 7: Isosurface plots of the HOMO and LUMO of the most stable Au8–PAMAM complexes and their energy gap. PAMAM–OH surfaces show frontier orbitals mainly localized on the gold cluster. In the case of PAMAM–CHO, HOMO orbitals were mainly localized on the dendrimer and LUMO orbitals were principally placed on the gold cluster, which suggest that electronic ionization will take place from the dendrimer fragment. On the other hand, from the shape of the frontier orbitals it is possible to suggest that the orbital mixing exhibited by Au8–PAMAM– CHO indicates a higher covalent character in comparison with the other structures. In PAMAM–NH3+ and PAMAM–OH complexes, the binding between the dendrimer and the gold cluster is probably based on electrostatic interactions other than covalent. The investigation of electron affinity and ionization potential of molecules delivers important information of electron and hole transfer phenomena. The predicted electron affinity (EA) and ionization potential (IP) of each PAMAM G0 dendrimer and Aun–PAMAM G0 complexes are resumed in Table 4. PAMAM–NH3+ has higher values of EA in comparison to PAMAM–OH and PAMAM–CHO, due to its protonated state. Some stabilization is achieved by the incorporation of a gold cluster to the structure. The ionization potential is a very important index of conductivity. Again, PAMAM–NH3+ exhibited the highest values. Gold and gold clusters, have electron affinity and ionization potential values that are significantly larger in comparison to dendrimers80-82, being in average 3.3 eV higher than the values associated to PAMAM–OH and PAMAM–CHO. In this sense, these dendrimers could play a crucial role in charge transport process because of its lower ionization potential. PAMAM–CHO IP values are in average higher than PAMAM–OH, related to the higher stability of the complexes.



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4. Conclusions In this work, the interaction between hydroxyl, aldehyde and amino terminated PAMAM G0 dendrimers with gold nanoclusters Aun (n = 2, 4, 6, and 8) was studied theoretically through DFT calculations. Binding geometries, interaction energies, and the charge transferring properties of different Aun–PAMAM G0 complexes were explored. Different coordination sites were selected, including core and external coordination. Previous experimental studies reported inter-dendrimer coordination to gold nanoparticles in lower generation dendrimers, which surround the metallic center to generate stabilization. Intradendrimer coordination was not favored due to the small size of the internal cavities, which are not adequate to host nanoparticles. In this work, amino terminated PAMAM showed higher interaction with gold nanoclusters at the terminal groups. On the contrary, hydroxyl and aldehyde terminated dendrimers showed better internal stabilization of nanoclusters and hence the external coordination was not preferred. This fact implies that the gold nanoparticles should enter to the cavities of the dendrimer to get better stabilization. However, PAMAM of low generations adopt a 2D flat spatial configuration with small size internal cavities, being not adequate to host nanoparticles. On the other hand, the capping of the nanoparticles through a sphere of terminal groups is less favored in the case of PAMAM–OH and its oxidized form. They have lower performance as stabilizer of gold nanoparticles through the interaction with terminal groups of several capping dendrimers, agreeing with the experimental evidence that report poor stabilizing capacity for the synthesis of gold nanoparticles. The unconventional Au··HN type of hydrogen bonding interactions were found to contribute to the stabilization of the most stable Aun–PAMAM G0 complexes. In PAMAM– OH and –CHO complexes, the electronic flux exhibited ligand to metal charge transfer (LMCT), the opposite result to PAMAM–NH3+ most stable complexes, where the electronic density was transferred from gold to the ligand. Aun–PAMAM G0 complexes exhibited smaller HOMO–LUMO gaps and ionization potentials compared to the isolated dendrimer, related to greater conductivity. Acknowledgements The author is grateful to Fondecyt (Initiation Project Nº 11140107) for funding this research and thanks Dr F.D. González-Nilo (Universidad Andrés Bello) for granting access to the Gaussian Computational package.


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PAMAM Dendrimers as Support for the Synthesis of Gold Nanoparticles

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