Self-Assembly of Silver Metal Clusters of Small Atomicity ... · Self-Assembly of Silver Metal Clusters of Small Atomicity on ... Platinum electrode was electrochemically cleaned

Self-Assembly of Silver Metal Clusters of Small Atomicity on Cyclic Peptide Nanotubes

Miguel Cuerva,‡ Rebeca Garcia-Fandino,† Carlos Vazquez-Vazquez,‡ M. Arturo Lopez-Quintela,‡ Javier Montenegro,*,† and Juan R. Granja*,†

†Center for Research in Biological Chemistry and Molecular Materials (CIQUS), Organic Chemistry Department, University of Santiago de Compostela (USC),

Santiago de Compostela 15782, Spain ‡Technological Research Institute (IIT), Physical Chemistry Department, University of

Santiago de Compostela (USC),Santiago de Compostela 15782, Spain

ACS NANO

October 6, 2015

VOL. 9 , NO. 11 , 10834–10843

BySHRIDEVI S BHAT

21/05/2016

Background

Chem. Soc. Rev., 2012, 41, 6023–6041

Classes of cyclic peptides that assemble into nanotubes through β-sheet interactions

(a) Cyclic peptides with α-alt(D,L) residues; (b) cyclic peptides with β-residues; (c) cyclic peptides with α- and γ-residues and; (d) self assembling heterocyclic peptides by incorporation

of ε-amino acids. In all cases, the N–H and COO- hydrogen bond donors and acceptors are aligned with tube length.

Subnanometric samples, containing exclusively Ag2 and Ag3 clusters, were synthesized for the first time by kinetic control using an electrochemical technique without the use of surfactants or capping agents.

To achieve the formation of stable, small Ag clusters, it is required that: a) the current densities should be much lower than those normally achieved using supporting electrolytes, requiring the use of miliQ water with no added electrolytes for the synthesis; b) the concentration of Ag ions must be kept at very low levels, ensuring that the clusters will grow very slowly.

After the electrochemical synthesis, it is also very important to remove any excess of Ag ions which have not been reduced because the presence of such ions causes the Ag cluster solution to become unstable, and further growth of clusters takes place until all Ag ions have been taken up.

Angew. Chem. Int. Ed., 2015, 54, 7612 –7616

A) AFM picture of Ag-AQCs deposited on mica (mean squareroughness ~150 pm) and B) AFM height profiles measured throughthe red and green lines depicted in (A), plotting distance (x axis)against height (y axis). C) UV/Vis absorption spectrum of Ag-AQCsdispersions in water. D) Fluorescence emission spectra (λex=230 nm)of the Ag-AQCs cluster dispersions. IF=fluorescence intensity.

Angew. Chem. Int. Ed., 2015, 54, 7612 –7616

Set of -1 peaks of Agn clusters identified by ESI-Mass Spectrometry in Ag-AQCs samples containing 0.1% formic acid and NaCl. High-resolution analysis ofseveral prominent -1 ions (left) and corresponding simulations (right) show excellent match in both, m/z values and isotopic distributions. All detected species contain Agclusters with stable close shell structures (1S2): A, [Ag2 Cl(H2O)]-

1; B, [Ag2(HCOO)Cl2Na2(H2O)]-1; C, [Ag3(OH) (HCOO) ClNa]-1; D, [Ag3(OH )(HCOO) Cl2Na2]-1.

Angew. Chem. Int. Ed., 2015, 54, 7612 –7616

Introduction

Subnanometric noble metal clusters, composed by only a few atoms, behave like molecular entities and display magnetic, luminescent and catalytic activities.

Noncovalent interactions of molecular metal clusters, lacking of any ligand or surfactant represent an unexplored, clean and innovative opportunity for the adjustment of the spatial position and the molecular properties of these metal clusters.

In the recent years many techniques have been developed in order to achieve the arrangement of nanoparticles over different templates or surfaces.

However, noncovalent interactions (van der Waals forces and/or π-stacking and hydrophobic effects) have not yet been considered and applied as experimental tools to influence and manipulate subnanometric metal(0) clusters (SNMCs) at the molecular level.

In this paper

It is shown that uncharged molecular-like SNMCs, smaller than 10 Å, can undergo genuine noncovalent interactions with other supramolecular entities.

These interactions have been applied to prepare self-assembled hybrid architectures of neat silver clusters (Ag3) aligned on top of cyclic peptide nanotubes over long distances (μm).

Experimental and computational evidences are presented to confirm the sensitivity of SNMCs toward noncovalent interactions and their application for the precise positioning of molecular metal clusters at the nanoscale level.

Results and Discussion

Figure 1. Proposed model for the coupling between SCPNs and Ag3 clusters and general AFM images for hybrid modelstructures.(a) Structure of CP-1.(b) Expected model for the attachment of an SCPN to an anionic surface. The cationic arginine (pH ∼2.5) interacts with the mica and the hydrophobic interactions of the pyrene residues enhance nanotube self-assembly.(c) Incubation of nanotubular arrays with silver clusters aqueous solutions (enriched in Ag3) results in spontaneous cluster deposition on SCPNs (0.8 x 2.5 μm2). (d) General AFM topography micrographs of pure nanotubes with average heights of 2.3 nm (1.65 x 5 ∼μm2).

(e) Metal cluster alignment showing punctuated increases of heights (between 0.5 and 1 nm)depicting cluster self-assembly and aggregation on peptide nanotubes (5x5 μm2).

Figure 2. AFM micrographs, including topographic profiles (red lines) for increasing concentrations (and/or repeated depositions) of Ag3 clusters over SCPNs. (a) Barely SCPNs deposited over mica (Grade V-1 muscovite) from CP-1 aqueous solutions (100 μM, pH 2.5). The profile along the red line shows a ∼continuous average height of 2.3 nm. (b) Silver cluster solutions enriched in Ag∼ 3 (10 μg/mL in water) self-assembled on SCPNs. The white arrows indicate height jumps below 450 pm. The corresponding profile line depicts two height jumps of 200 and 500 pm from the typical 2.3 nm height of individual SCPNs. (c) Detail on cluster aggregates (2-3 nm) formed along peptide nanotubes. Aggregates were most frequently observed at high cluster concentration ([Ag3] = 100 or 1000 μg/mL in water). (d) SCPNs shielded with Ag3 clusters with an average height of 6.0 nm. Complete coverage was observed when depositions of high concentrated cluster solutions (100 or 1000 μg/mL) were repeated for 2 or 3 times. All micrographs are 1x1 μm2.

Figure 3. Pyrene fluorescence crude emission (a) and normalized emission (b) of CP1 solutions titrated with increasing volumes of H2O and silver clusters. The fluorescence emission was obtained by pyrene excitation (λex = 340 nm) and recording the emission spectra between 350 and 600 nm. (a) Pyrene crude fluorescence emission of CP-1 aqueous solution (800 μM) after titration with equal volumes of H2O and Ag3 clusters (300 nM) shown in black and orange respectively: (a) 0, 750, 1700 μL and summary at λem = 373 nm of complete titration.(b) Normalized (at 373 nm) pyrene fluorescence emission of CP-1 aqueous solution after titration with H2O and Ag3 clusters (300 nM) : 0, 750, 1700 μL and normalized summary at λemission = 470 nm of complete titration. In (b) the normalization of the crude fluorescent emission data was at I1 = 373 nm and in the titration summary the initial maximum was normalized to 1.

Figure 4. Computational chemistry. (a) DFT calculations [B97D3/6-31+G(d,p) and LanL2dz for Ag] for one Ag3 cluster and two pyrene molecules. (b) Snapshot structure at t = 10 ns after MD simulation of the SCPN in the presence of single Ag3 (200 mM) showing preferential cluster accumulation along the hydrophobic pyrene side of the nanotube. (c) Detail of a snapshot of the surroundings of one Ag3 cluster self-assembled in the middle of a peptide nanotube (10 CPs) after 10 ns of MD simulations. The graph depicts the distances between the centers of mass of Ag3 and pyrene moieties in the computed ensemble during the simulation time. Each CP ring of the SCPN is represented by the same color in the tubular model and in the 2D graph.

Conclusion

The sensitivity of molecular-like subnanometric metal (0) clusters toward noncovalent interactions is reported.

These interactions are applied for the synergic self-assembly of Ag3 and silver clusters of small atomicity with peptides nanotubes.

Topographic micrographs of concomitant or sequential deposition of SCPNs and Ag3 over mica revealed a clear preference of silver SNMCs to be deposited and aligned on top of the peptide nanotubes.

Fluorescence studies of solutions of SCPNs titrated with the Ag3 disclosed the presence of the self-assembled supramolecular structures where each member of the ensemble stabilizes the other in aqueous solution.

Computational calculations with DFT and molecular dynamics supported the proposed pyrene/cluster type of interactions.

• Applications of this new hybrid material.• Can modified CP monomer units be used as protecting agent for cluster synthesis?

Can this be a substrate for spray synthesis?

Thank You

Ag clusters dispersed in water were purchased from Nanogap, Spain (NGAP AQCAg-1102-W) with the following specifications given by the supplier:

Product form: water solution; Color: greenish, clear; Concentration: 1mg/L (determined by FAAS); Density: 1,0 g/ml; Storage: refrigerated (4ºC-8ºC) and protected from light; Size

distribution: mainly Ag3 with some amounts of Ag2 (determined by MassSpectroscopy); Observations: contains NaCl (<11mM) and Ag+ ions (< solubility

product of AgCl), samples are free from any surfactant or other chemical compound.

Ag3 clusters

Cluster (Ag-AQCs) synthesisSynthesis of small silver clusters without any surfactant was carried out by a modificationof a previously reported electrochemical method. In a typical synthesis, a three electrodechemical cell was used with two foil electrodes (2.5 cm2 surface area) of Ag (asworking electrode) and Pt (as counter electrode) and a hydrogen reference electrode. A2V constant voltage was applied for 1200s in N2 deaerated MiliQ water at 25 ºC. Prior tothe synthesis, silver electrode was polished with sand paper (600 grid, Wolfcraft)followed by alumina (≈ 50 nm, Buehler), washed thoroughly with MiliQ water andsonicated. Platinum electrode was electrochemically cleaned by cyclic voltammetry in1M MeOH/1M NaOH solution followed by cyclic voltammetry in 1M H2SO4. After thesynthesis, remaining Ag+ ions were removed by addition of NaCl and subsequentprecipitation and filtration. Purified samples are then concentrated at 35ºC using a rotaryevaporator and named as Ag-AQCs samples.