Gold nanocomposite assemblies using functionalized Ru(II)-polypyridyl complexes

Abstract

Ru(II)-polypyridyl complexes with various surface anchoring functional groups were prepared at room temperature and utilized as capping and engineering agents to attain surface functionalized gold nanocomposites (Au NCs) with unique morphologies without structure directing templates or functionalized thin films. To corroborate the morphological features obtained by the place-exchange methodology, functionalized Ru(II)-polypyridyl complexes with different surface binding groups such as thiol, ketone, hydrazone and carboxylic acid groups were studied. Au nanocubes, chain and random assemblies were attained upto the microscale level using these Ru(II)-polypyridyl complexes with functional groups acting as Au NP surface binding molecular clips. The morphological and optical changes on the Au NP surface before and after functionalization were studied using electron microscopies (HRTEM with EDS, STEM, SEM), UV-vis and photoluminescence (PL) spectroscopy.

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1. Introduction

The controlled organization of gold nanoparticles (Au NPs) using a supramolecular approach represents a novel route to obtain functionalized nanostructured composite materials.^1,2 The placement of functionalized metal complexes at the periphery of the Au NP surface leads to the formation of a metal–molecule interface with fascinating properties in catalysis, sensors and optoelectronic devices.^3–5 The ability of Au NPs showing structural dependent visible range localized surface plasmon resonance (LSPR) makes them prominent nanomaterials for many opto-plasmonic applications.⁶ Additionally, the surface bound molecular interface on Au NPs acts as a novel heterostructure module to create highly functionalized hybrid conjugates with exceptional properties which are not seen individually.⁷ Therefore, the surface binding of Ru(II)-metal based chromophores on Au NPs attracted much attention for single molecule probing Raman spectroscopic studies and biologically important processes. For example, Au NPs surface was functionalized with cis-(4-aminothiophenol)-bis(bipyridyl)(chloro)ruthenium(II) complex and the resulting hybrid was utilized for binding and controlled release of biologically important nitric oxide (NO) molecule in the presence of light.⁸ Recently, Theil et al. demonstrated the influence of surface-bound Ru(II)-polypyridyl complexes on Au nanopeanuts surface and showed surface enhanced Raman scattering (SERS) at metal–molecule interface.⁹ Elmes et al.¹⁰ prepared water soluble Ru(II)-polypyridyl complexes and functionalized them on Au NPs surface. The resulting surface functionalized supramolecular nanostructures possessed high DNA binding ability and were proved as promising cellular imaging agents. Based on the affinity of functional groups present in the surface capping molecules, the surface chemistry of Au NPs could be modified in a controlled fashion with various binding modes such as covalent and electrostatic interactions. Generally, the redox-active metal centered molecules were attached onto Au and Ag NPs using various functional groups such as thiol, dithiol, amine, pyridine and carboxylate group.^11–15 Ru(II)-polypyridyl complex possessing amine (–NH₂) functional group was prepared for the stabilization of Au NPs surface and the resulting functional nanocomposite (NC) material showed excellent electrochemiluminescence (ECL) activity.¹⁶ The surface functionalization strategy followed in these studies clearly imparted the key role of immobilization of transition-metal complexes over Au NPs with specific functional groups for targeted applications and understanding of metal–molecule interface chemistry. Based on newly emerging properties through surface functionalization methodologies, we have designed various heteroleptic Ru(II)-polypyridyl complexes and demonstrated the importance of hybrid nanostructures on various surfaces (glass/ITO/silica/Ag NP).¹⁷ Literature survey reveals that there are only few reports available on generation of Au NPs one-dimensional (1D) chains such as functionalized polymer templates, O₂ plasma etched latex spheres and thin film of polymers as structure directing materials. But the reports based on the functionalized Ru(II)-polypyridyl complexes as self-templating and structure directing agent for Au NCs chain assembly are found to be scarce.¹⁸ In the present work, we have exploited the controlled surface functionalization of Au NPs to produce templateless, functionalized Au NCs assemblies as chains and nanocubes by place-exchange methodology. The specific role of each Ru(II)-polypyridyl complexes binding on Au NP surface led to tuning of Au NP SPR position. Furthermore, surface modified Au NCs morphologies were studied in detail using various electron microscopic techniques (TEM coupled EDS, STEM, SEM) and optical studies were performed using UV-vis spectroscopy, photoluminescence (PL) spectroscopy.

1.1 Materials and methods

Hydrogen tetrachloroaurate trihydrate (HAuCl₄·3H₂O, ACS grade 49.5% metal basis), silver nitrate (AgNO₃, 99+%), trisodiumcitrate dihydrate (99%), 4-aminothiophenol (4-ATP, 97%) were received from Alfa Aesar, Massachusetts (USA). 2,2′-Bipyridine (2,2′-bpy, 99%), RuCl₃·xH₂O (99.98%) were received from Sigma-Aldrich, 1,10-phenanthroline (1,10-phen, 99%) from Spectrochem, hydrazine monohydrate (N₂H₄·H₂O, 80%) from Loba Chemie, acetic acid (99.7%) from Speckpure, diethyl ether (98%) from Thomas Baker, acetonitrile (HPLC grade) from Rankem (India). 4,5-Diazafluoren-9-one (dafo),¹⁹ cis-bis(2,2′-bpy)₂RuCl₂·2H₂O and cis-bis(1,10-phen)₂RuCl₂·2H₂O,²⁰ Ru(2,2′-bpy)₂(IPBA)·2PF₆ (5) and Ru(1,10-phen)₂(IPBA)·2PF₆ (ref. 21) (6) (where IPBA = 4-(1H-imidazo[4,5-f][1,10]phenanthrolin-2-yl)benzoic acid), Ru(1,10-phen)₂(dafo)·2PF₆ (ref. 22) (4), citrate capped Au NPs²³ were prepared as mentioned in the literature. All aqueous solutions were prepared using milliQ water unless specified elsewhere.

1.2 Characterizations

Surface functionalized Au NCs morphological features were studied using high resolution transmission electron microscopy (HRTEM, TECNAI G² T30) working at 300 kV accelerating voltage coupled with EDS facility. Fast fourier transform (FFT) pattern was obtained using the same. Scanning transmission electron microscopic high angle annular dark field (STEM-HAADF) images were captured using the same. Scanning electron microscopy (SEM) images taken using FEI Quanta 200-3D dual beam ESEM with resolution of 3 nm at 30 kV accelerating voltage. Mass spectra were collected using Autoflex III Smartbeam MALDI-time-of-flight instrument from Bruker Daltonics (Bremen, Germany), equipped with a solid-state laser (λ = 355 nm) and spectra produced from 200 laser shots in positive ion mode using matrix 2,5-dihydroxybenzoic acid with ACN/TFA [(0.1%) 2 : 1 v/v] and the electrospray ionization mass spectra (ESI-MS) collected using Micromass (KC-455) UK limited instrument with appropriate carrier solvents. ¹H NMR spectra were collected using a Jeol ECX 400 MHz spectrometer operating at 400 MHz field strength. Single crystal X-ray diffraction data were collected using an Oxford Xcalibur single crystal diffractometer equipped with a single wavelength enhanced X-ray source with MoKα radiation, 4-circle κ goniometer and Sapphire-3 CCD detector. The IR spectral data were recorded using KBr pellets on a Shimadzu IR435 spectrometer (400–4000 cm⁻¹) and on PerkinElmer FT-IR spectrometer (ZnSe crystal, 600–4000 cm⁻¹). Elemental analysis was performed on an Elementar Analysensysteme GmbH varioEL V3.00. UV-visible absorption spectra of all samples were recorded using Jasco V-670 spectrophotometer at room temperature. Photoluminescence studies were carried out using Lab Ram HR 800 instrument with laser excitation wavelength 488 nm.

1.3 Synthesis of Ru(2,2′-bpy)₂(4-ATP)₂·2PF₆ (ref. 24) (1)

23.9 mg (4.6 mmol) of cis-bis(2,2′-bpy)₂RuCl₂·2H₂O was taken in 10 mL of EtOH : water (1 : 1 v/v) solvent mixture and kept under N₂ gas bubbling for 10 min. To the above reaction mixture, 10.4 mmol of 4-aminothiophenol (4-ATP) in 10 mL of EtOH : water (1 : 1 v/v) solvent was added at once and refluxed for 14 h. The reaction mixture was cooled to room temperature after refluxing and filtered out using a Whatman paper followed by sintered crucible. To the filtrate, saturated aqueous NH₄PF₆ solution was added for Cl⁻ ion exchange. The resulting reddish brown coloured solid was collected and washed with copious amount of water, then with diethyl ether. Yield: 22.3 mg (70%), UV λ_max (CH₃CN, ε mol⁻¹ cm⁻¹): 252 nm (47 909), 284 nm (60 436), 418 nm (10 552); MALDI-TOF MS (m/z, [M]): 954.03 calc./947.974 obser.

The similar reaction condition was followed for synthesis of Ru(1,10-phen)₂(4-ATP)₂·2PF₆ (2) using cis-bis(1,10-phen)₂RuCl₂·2H₂O instead of cis-bis(2,2′-bpy)₂RuCl₂·2H₂O. Yield: 33.2 mg (66%), UV λ_max (CH₃CN, ε mol⁻¹ cm⁻¹): 221 nm (79 468), 263 nm (81 584), 466 nm (10 174); EA: (calc.) C, 43.16; H, 3.02; N, 8.39; S, 6.40 (found) C, 43.77; H, 3.19; N, 9.40; S, 6.15.

1.4 Synthesis of Ru(1,10-phen)₂(dah)·2PF₆ (ref. 25) (3) (dah = 4,5-diazafluorenone hydrazone)

To 25.6 mg (0.027 mol) of Ru(1,10-phen)₂(dafo)·2PF₆ (4) in 10 mL methanol, 0.22 mol of hydrazine hydrate was dissolved in methanol (10 mL) added at once under magnetic stirring. To the above reaction mixture, few drops of acetic acid was added and kept under N₂ gas bubbling, followed by reflux for 3 h. After reflux, the reaction mixture was cooled to room temperature filtered out. The title compound was obtained by recrystallization repeatedly in acetonitrile/diethyl ether solvent mixture. Yield: 32 mg (75%). UV λ_max (CH₃CN, ε mol⁻¹ cm⁻¹): 222 (72 097), 261 (81 642), 445 (16 165). FT-IR (ν, cm⁻¹): 559 (s), 717 (s), 826 (s), 1200 (m), 1290 (m), 1415 (s), 1566 (m), 1690 (m), 2938 (w), 3353 (w, –NH₂), 3645 (w). EA: (calc.) C, 61.22; H, 3.52; N, 16.32 (found) C, 61.07; H, 3.74; N, 17.32.

1.5 Sample preparation for SEM and photoluminescence (PL) studies

All Ru(II)-polypyridyl complexes functionalized Au NCs were centrifuged prior to morphological and optical studies to make free from loosely bound Ru(II)-polypyridyl complexes and washed in ACN : H₂O (1 : 1 v/v) solvent mixture twice and then coated over cleaned silicon wafers in order to make thin layer of functionalized Au NCs.

2. Results and discussion

Citrate capped Au NPs surfaces were modified and functionalized using Ru(II)-polypyridyl complexes with different surface binding functional groups (Fig. 1). In order to observe optical changes in the SPR position of Au NPs, optical overlapping resulting from Ru(II)-polypyridyl complexes was eliminated by adding them in both reference and sample cuvettes during UV-vis absorption measurements in all cases. Ru(II)-polypyridyl complexes were prepared by incorporating two 4-ATP molecules(molecule I and II) with aromatic thiol (–SH) functional group, which binds through covalent bonding with Au NP surface via soft–soft (Au–S) interactions.²⁶

Fig. 1 Schematic representation of molecular structure of Ru(II)-polypyridyl complexes bearing surface binding thiol (I and II), hydrazine (III), ketone (IV), and carboxylic acid (V, VI) functional groups.

The spherical shape of citrate capped Au NP was confirmed by UV-vis spectroscopy (Au NPs λ_max = 522 nm, Fig. 2A). The surface modifications on citrate capped Au NPs were carried out by the addition of molecule I (5 μL per addition). Step-wise addition of molecule I changed the surface chemistry of Au NP, which is evident by UV-vis spectroscopy studies (Fig. 2B). Optimal conditions (1–12 × 10⁻⁷ M) were used to form surface functionalized one-dimensional Au NCs chains through place-exchange methodology. As citrate capped Au NPs get functionalized with molecule I, functionalized Au NCs get close to each other and thus lead to one-dimensional plasmon coupling with many number of nearby Au NPs.²⁷ In this case, bathochromic shift of Au NPs SPR band was observed from 522 nm to 528 ± 3 nm and a new SPR band started to appear at 591 nm and then shifted upto 650 nm at 12 × 10⁻⁷ M concentration (Fig. 2B). Beyond 12 × 10⁻⁷ M, addition of molecule I destabilized the Au NCs colloidal solution and hypochromic shift was observed (13–18 × 10⁻⁷ M). The appearance of new SPR band is an evidence for the formation of chain-like assembly of Au–Ru(II) polypyridyl NCs from homogeneous dispersion of citrate capped Au spherical NPs.¹⁸ Excessive addition of molecule I (>2 × 10⁻⁵ M) at once to citrate capped Au NPs led to rapid aggregation of Au NP–Ru(II)-polypyridyl complex conjugates.²⁷ To confirm this molecular binding induced Au NCs assembly, surface functionalization was again carried out with 1.3–26 × 10⁻⁷ M molecule II having structurally rigid 1,10-phenanthroline instead of 2,2′-bipyridine. Changes in SPR band at 524 ± 2 nm and a new plasmon band at 625 nm were observed. In this case, the red shift in Au NPs SPR position was quite controlled than that in molecule I (Fig. 2C). These comparative UV-vis studies confirmed that the molecular steric effect resulting from 1,10-phen ligand in molecule II might played a key role in the controlled bathochromic shift during surface functionalized Au NCs formation.

Fig. 2 (A) UV-vis spectra showing SPR position of citrate capped Au NPs (20 μL dispersed in 2.5 mL water) before (pink) and after surface functionalization with molecule I (purple) and inset: digital image showing visible colour change before (pink) and after surface functionalization (purple); (B) step-wise Au NPs SPR tuning upon surface functionalization with molecule I (5 μL, 1–18 × 10⁻⁷ M); (C) sequential Au NPs SPR tuning upon surface functionalization with addition of molecule II (5 μL, 1.3–26 × 10⁻⁷ M).

Molecule I induced Au NCs assembly into chain like morphology as confirmed by TEM imaging and the changes in modified Au NPs surface as shown in HRTEM image (Fig. 3 and S3‡). Here, the consecutive displacement of citrate molecules and covalent binding of molecule I did not affect the spherical shape of Au NPs. The presence of molecule I on Au NP was differentiated from citrate capped Au NPs using HRTEM imaging in which optically transparent nano-thin molecular layer was observed as bright layer (indicated by arrow) (Fig. 3B). In this molecular based methodology, Au NCs chain having length of >0.8 μm was achieved, as evident by STEM and TEM images (Fig. 4A and B). EDS profile of these Au NCs chain showed the characteristic peaks for the presence of Ru, S, Au elements on functionalized Au NCs surface (Fig. 4C).

Fig. 3 (A) Representative TEM image of citrate capped spherical Au NPs; (B) HRTEM image of Au–Ru(II)-polypyridyl NCs chain with 1–12 × 10⁻⁷ M molecule I, inset showing microscale self-assembly view; (C) and (D) TEM views of large scale functionalized Au NCs chain assembly.

Fig. 4 (A) STEM image showing the microscale one-dimensional (1D) chain assembly of Au NCs; (B) TEM image showing >0.8 μm length of Au NCs; (C) EDS micrograph showing the surface bound Ru, S elements from Ru(2,2′-bpy)₂(4-ATP)₂ on Au nanocomposite surface.

Further, to determine the effect of functional group and molecular structure, molecule III with –NH₂ functional group was used for Au NCs formation (1–17 × 10⁻⁷ M, 5 μL per addition) (Fig. S4 and S5‡). The presence of lone pair in molecule III induced weak covalent interactions on citrate capped Au NP surface²⁸ which led to mild bathochromic shift in SPR position from 522 nm to 529 nm. Additionally, a new SPR band at 609 nm appeared, which could be assigned to functionalized Au NCs formation (Fig. 5A).

Fig. 5 (A) UV-vis spectra of Au–Ru(II)-polypyridyl NC formation with molecule III showing the structural π-conjugation effect on Au NPs SPR; (B) lack of π-conjugation in molecule IV induces mild red shift in Au NPs SPR; (C) electrostatic binding of molecule V on citrate capped Au NPs; (D) electrostatic binding of molecule VI on citrate capped Au NPs surface.

There was no shift at 609 nm on further addition of molecule III upto the concentration 5 × 10⁻⁶ M. This indicates requirement of covalently bonding functional group (–SH) (as in molecule I) for nano-chains formation. Furthermore, to find out the effect of ketone functionality, the binding studies of molecule IV (5.0 × 10⁻⁶ M) over citrate capped Au NPs were performed and a progressive red shift of SPR band from 522 nm to 531 nm without the formation of any new SPR band was observed (Fig. 5B). Here, the surface modified Au NCs morphology changed from spherical shape to Au NC cubes along with random aggregates (Fig. 6 and S8‡).

Fig. 6 (A–C) Representative TEM images showing the step-wise formation of Au–Ru(II)polypyridyl cubic NCs from spherical shaped Au NCs assembly with molecule IV; (D) SEM image showing functionalized Au–Ru(II)polypyridyl NC cubes having 2 μm diameter.

This indicates the ability of ketone functionality present in molecule IV to stabilize the surface crystal planes of Au–Ru(II) polypyridyl NCs like polyvinylpyrrolidone (PVP) at room temperature.²⁹

Additionally, Ru(II)-polypyridyl complexes (molecule V and VI) having carboxylate (–COO⁻) functional group and extended π-conjugation were prepared and their surface binding and stabilizing ability was studied on citrate capped Au NPs (1–16 × 10⁻⁷ M, 3 μL per addition). In both these molecules V and VI, the presence of positive charge on Ru(II)-center made the electrostatic binding over citrate capped Au NPs as like [Ru(bpy)₃]²⁺ dye molecule.³⁰

This non-covalent binding of molecule V led to shift in Au NPs SPR towards visible region with a new band at 605 nm (Fig. 5C), which is assigned to the formation of randomly assembled Au NCs (Fig. 7). The random assembly was observed as a short nanoscale assembly within length of ≤200 nm. The morphological features obtained correlated with each functionalized Ru(II)-polypyridyl molecules utilized for Au NPs surface functionalization. The same trend of binding curves was observed for molecule VI also (1–14 × 10⁻⁷ M, 3 μL per addition) and showed bathochromic shift in UV-vis studies with new SPR band at 610 nm which was assigned to random assembly of Au NCs (Fig. 5D and 8). In all surface functionalization process, addition of positively charged Ru(II)-polypyridyl complexes to negatively charged citrate capped Au NPs facilitated strong surface binding and the resulting colour change confirmed the displacement of electrostatically stabilized citrate molecules from Au NPs surface.³¹ However, the presence of –COO⁻ group with extended π-system in molecule V and VI seem to be poor in forming assembled one-dimensional chains due to rigid molecular structure and electrostatic repulsion by negatively charged –COO⁻ group. These UV-vis binding studies clearly demonstrated difference between covalent and electrostatic assemblies of functionalized Au NCs with different morphologies.

Fig. 7 Representative TEM images showing formation of randomly assembled Au–Ru(II)-polypyridyl complex NCs aggregates with 1–16 × 10⁻⁷ M molecule V.

Fig. 8 Representative TEM images showing formation of randomly assembled Au–Ru(II)-polypyridyl complex NCs aggregates with 1–14 × 10⁻⁷ M molecule VI.

2.1 Photoluminescence (PL) studies

Photoluminescence (PL) properties of surface functionalized Au NCs were studied by exciting the samples at laser excitation of 488 nm. In this study, enhanced photoluminescence (PL) was noted for Au NCs of molecule II (λ_em = 642 nm) and molecule IV (λ_em = 660 nm) (Fig. 9). But, there was no significant photoluminescence (PL) from Au NCs of molecule V and VI. These emission studies of surface functionalized Au NCs highlight the importance of covalently modified Au NCs surface for altering photo-physical properties in contrast to electrostatically functionalized Au NCs.

Fig. 9 Photoluminescence of Au–Ru(II)polypyridyl NCs functionalized with (a) molecule II, (b) molecule V, (c) molecule VI and (d) molecule IV (λ_ex = 488 nm).

3. Conclusions

Surface modification and functionalization of citrate capped Au NPs were performed via place-exchange methodology using various functionalized Ru(II)-polypyridyl complexes as surface binding agents. The effect of functional groups such as thiol, ketone, hydrazone and carboxylic acid present in Ru(II)-polypyridyl complexes were studied in detail and showed great influence on the formation of surface functionalized Au NCs with unique optical properties and morphological features. Furthermore, surface functionalized Au NCs were obtained as chains via templateless bottom-up assembly by molecule I through covalent bonding. On the other hand, weakly covalent binding ketone functional group from molecule IV, converted spherical Au NPs to functionalized cubic Au NCs, but electrostatic binding of molecule V and VI generated only surface functionalized Au NC random assemblies. Additionally, photoluminescence (PL) studies also showed the importance of covalently surface functionalized Au NCs than electrostatically surface functionalized Au NCs. This work highlighted and pioneered the importance of molecular design and engineering of molecules to achieve Au NC chains and other unique morphologies for photonics and optoplasmonic device fabrications³² simply without any structure directing templates.

Acknowledgements

NV acknowledges financial support from DST Nanomission (SR/NM/NS-12/2010), Govt. of India, India and CSIR-NIPER, Mohali for providing DST funded HRTEM facility. NV also thanks Velayudam, Pondiaraj, Manjunath for TEM, SEM images and Selvaprakash for mass spectrometry. MC thanks CSIR, India for the award of Senior Research Fellowship.

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Footnotes

† Dedicated to late Dr Tarkeshwar Gupta.

‡ Electronic supplementary information (ESI) available: Fig. S1: mass profile of Ru(2,2′-bpy)₂(4-ATP)₂·2PF₆ complex (I). Fig. S2: FT-IR spectral bands of Ru(1,10-phen)₂(4-ATP)₂·2PF₆ complex (II). Fig. S3: additional HRTEM images, SAED and FFT pattern of surface functionalized Au–Ru(II)polypyridyl NCs by Ru(II)(2,2′-bpy)₂(4-ATP)₂·2PF₆ molecule (I). Fig. S4: UV-vis spectra of, (A) 10⁻⁵ M Ru(2,2′-bpy)₂(4-ATP)₂·2PF₆ (I); (B) Ru(1,10-phen)₂(4-ATP)₂·2PF₆ (II). Fig. S5 UV-vis spectra of, (A) 10⁻⁵ M Ru(1,10-phen)₂(dafo)·2PF₆ (IV); (B) Ru(1,10-phen)₂(dah)·2PF₆ (III) in acetonitrile. Fig. S6 FT-IR spectral bands of, (A) Ru(1,10-phen)₂(dafo)₂·2PF₆ (IV) complex; (B) Ru(1,10-phen)₂(dah)·2PF₆ (III) complex. Fig. S7 fluorescence emission profile of 10⁻⁵ M Ru(1,10-phen)₂(dah)·2PF₆ (III) in acetonitrile. Fig. S8 SEM image and EDS analysis of functionalized Au NC cubes with random aggregates. Fig. S9 ORTEP diagram of Ru(1,10-phen)₂(IPBA) molecule (VI) with thermal ellipsoids. CCDC 935100. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra11516k

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