Coordination Chemistry Directed Recognition Reaction - American

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10100

J. Phys. Chem. C 2008, 112, 10100–10107

Nanoparticle Organization by a Co(II) Coordination Chemistry Directed Recognition Reaction Iuliana E. Sendroiu,† David J. Schiffrin, and Jose´ M. Abad* Centre for Nanoscale Science, Chemistry Department, UniVersity of LiVerpool, LiVerpool L69 7ZD, United Kingdom ReceiVed: March 19, 2008; ReVised Manuscript ReceiVed: April 24, 2008

A new strategy for the self-organization of bimetallic structures using an inorganic chemistry coordination reaction through the ligand shells of silver and gold nanoparticles is demonstrated. Silver nanoparticles functionalized with thioctic acid and nitrilotriacetic-cobalt(II) as the metal center were reacted with gold nanoparticles functionalized with imidazole moieties used as the Co(II) specific ligand. TEM and UV-vis spectroscopy demonstrate the attachment through the ligand shells of the particles. 1. Introduction Nanoparticles (NPs) are key building blocks for new materials fabrication due to their unusual size-dependent properties.1 Different approaches have been followed for their self-assembly into two- and three-dimensional structures for the construction of devices and sensors. A central issue to achieve this is the chemical recognition mechanism of the constituent nanoscale objects and the consequent rules of self-organization. Directed nanoparticles organization using biomolecules2 and driven by biospecific interactions such as those in DNA,3 peptides,4 biotinstreptavidin,5 and antibody-antigen systems6 have been previously investigated. The self-assembly of thiol-functionalized nanoparticles into macroscopic materials has also been previously realized using two-dimensional crystallization,7 covalent bonding,8 chemical cross-linking by bifunctional,9 tri- and tetradentate ligands,10 metal ions,11 and polymers as ligands, the latter through electrostatic or hydrogen-bonding interactions.12 Metal complexes with accessible coordination sites have found wide use in molecular recognition13 since ligand coordination to metal ions can provide large binding energies in comparison with water to give stable and well-defined structures from aqueous media. Importantly, ligand-metal specificity could allow the rational assembly of components into supramolecular structures. A new linking strategy that employs metal complexes containing a metal center with labile ligands as a binding site to construct bimetallic nanoscopic structures is described in the present work. A directed coordination linking strategy as a new organization concept is demonstrated in the present work by employing Ag and Au nanoparticles of different sizes as building blocks. The method developed is described in Scheme 1. It is based on the use of a metal ion coordination reaction when the metal center and the ligand are present in the ligand shells of different nanoparticles. In contrast to other strategies, such as the use of complementary ssDNA functionalized nanoparticles,3 recognition is achieved through the coordination chemistry of the nanoparticles capping shells. As an example, the reaction between Au nanoparticles modified with a mixed * E-mail: [email protected]. † Chemistry Department, University of California Irvine, Irvine, CA 92697.

monolayer of 6,8-dithioctic acid (TA) and 1-(11-mercaptoundecyl) imidazole, and Ag nanoparticles capped with a monolayer of TA modified with a nitrilotriacetic-Co(II) complex (NTACo(II)) has been investigated. This complex contains two labile H2O ligands that are easily displaced by the unshared electron pair on N3 of the neutral imidazole molecule due to the binding affinity for these coordination sites in the Co(II) complex.13a,b Coordination to a transition metal center has been extensively used in the past for biological chromatographic separations.13c–e In the present work, an imidazole ligand was bound to a gold nanoparticle (AuNP) through thiolate groups and therefore, a chelate effect would be expected when two imidazole groups are coordinated to the cobalt ion (Scheme 1). It is proposed that this effect increases the affinity of the recognition reaction in a manner similar to that occurring in the well-known technique of metal affinity chromatography.13c–e Thus, the NTACo(II) complex acts as a specific binding center for imidazolefunctionalized gold nanoparticles. The nanoparticles investigated were stabilized with the same thiol, thioctic acid, and the experiments were carried out at a pH in which the ligand was fully ionized. The reason for this choice was to ensure that the main component of the capping layer was the same in both particles studied with the main difference in the incorporated chemical recognition groups, Co(II) and imidazole. It was also desirable to avoid hydrogen bonding between the ligand shells of the particles that could obscure the ligand coordination reaction to the transition metal ion. Two different metals were chosen for the cores in the expectation that this could provide additional information, through plasmon coupling, on the structure of the aggregates formed in solution.14 2. Experimental Section All chemicals were obtained from commercial sources and used as received. MilliQ water (Millipore) was used throughout. 2.1. Synthesis of Silver Nanoparticles Capped with a TA: TA-ANTA-Co(II) Mixed Monolayer. Silver nanoparticles were synthesized in an organic phase following the procedure described in Reference 7b. In a typical synthesis, silver ions were transferred to toluene by contacting an aqueous AgNO3 solution (5 mL, 15 mM, Aldrich 99.99%) with tetra-decylammonium bromide (TDABr) in toluene (10 mL, 23 mM, Fluka) and finally reduced by contact with aqueous sodium borohydride

10.1021/jp802401x CCC: $40.75  2008 American Chemical Society Published on Web 06/14/2008

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SCHEME 1: Reaction Strategy for Assembly of NTA-Co(II) Silver and Imidazole Modified Gold Nanoparticlesa

a

Im ) imidazole.

(2 mL, 0.4 M, Aldrich) for 20 min. An unusual feature of using TDABr as a phase-transfer reagent was that transfer could be achieved without major precipitation of silver bromide. After reduction, the organic phase was separated and washed once with 0.1 M sulfuric acid to remove excess borohydride, once with 1 M sodium carbonate and five times with water. Thioctic acid derivatization was carried out by overnight incubation in a 0.1 M solution of TA in toluene. The carboxylic acidterminated nanoparticles thus formed were insoluble in toluene and precipitated. The material was separated by centrifugation, washed several times with toluene and once with 1-propanol to remove reaction byproduct. The purified nanoparticles were redissolved with brief sonication in 20 mM aqueous HEPES (N-(2-hydroxyethyl)piperazine-N′-(2-ethanesulfonic acid), Sigma) at pH 10. The metal center used was the amino-nitrilotriacetic-Co(II) complex (ANTA-Co(II)). The ANTA-Co(II) complex was prepared by reaction of NR,NR-bis(carboxymethyl)-L-lysine hydrate (38 mM ANTA, Fluka) with cobalt(II) chloride (40 mM, Sigma) in 20 mM HEPES in aqueous solution. Typically, 10 mg of ANTA were dissolved in 1 mL of HEPES buffer and 9.5 mg of cobalt(II) chloride was added to form the complex. The cobalt excess was precipitated by increasing the pH to 10 and the hydroxide removed by filtration through a 0.2 µm membrane (PTFE, Amicon). Activation of the carboxylate groups of the Ag-TA nanoparticles (200 nM of particles) and subsequent amidation of the NHS-esters with 20 mM of ANTACo(II) complex was performed overnight in a single step in the presence of 2 mM N-hydroxysuccinimide (NHS, Fluka) and 2 mM 1-ethyl-3-[3′-(dimethylamino)propyl]carbodiimide (EDC, Sigma) in 20 mM HEPES buffer (pH 7.5). Further purification of the Au-TA-ANTA-Co(II) assembly was carried out by ultrafiltration through low-adsorption hydrophilic 30 000 NMWL cutoff membranes (regenerated cellulose, Amicon) by centrifugation at 4000 rpm and using 20 mM HEPES buffer (pH 8.0) as diluent. This membrane retained the nanoparticles assemblies and allowed their separation from the solution components. 2.2. Synthesis of Gold Nanoparticles Capped with a Thioctic Acid (TA):HSC11-Imidazole Mixed Monolayer. Aqueous sodium citrate stabilized Au nanoparticles with a mean diameter of (13 ( 1) nm were synthesized following welldocumented procedures.15 A dispersion of these nanoparticles (5 mL, 20 nM in particles) was reacted by addition of a solution containing 6,8-dithioctic acid (TA, Sigma, 250 µL of a 0.2 M ethanolic solution), HSC11-imidazole thiol (Prochimia, 2.5 µL of a 0.2 M ethanolic solution) and 70 µL of 1 M aqueous NaOH (the final pH of the mixture was 9.5) to achieve a stabilizing

mixed monolayer on the nanoparticles surface. The nanoparticles were unstable for concentrations of the imidazole thiol above 0.5 mM and slowly precipitated. This indicates that the imidazole thiol displaces the absorbed citrate anions from the particles surface more rapidly than TA, leading to deprotection and aggregation. This question is further discussed in Section 3.2. Purification of the nanoparticles to remove excess reactants and reaction byproduct was carried out using several cycles of ultrafiltration through low-absorption hydrophilic Centricon filters with YM-100K MW retention membranes by centrifugation at 4000 rpm employing a 20 mM borate buffer (pH 9.5) as diluent. As control experiments (see below), the synthesis of nanoparticles capped only with TA was carried out under similar conditions as above but without the addition of HSC11-imidazole thiol. 2.3. Au/Ag Nanoparticle Assemblies. In a typical preparation, Ag nanoparticles capped with the TA-ANTA-Co(II) complex (20 nM) were reacted by mixing with Au nanoparticles capped with the TA/imidazole thiol (2 nM) in 20 mM borate buffer (pH 9.5) solution in an argon atmosphere. 2.4. Nanoparticles Characterization. Fourier transform infrared spectroscopy (FTIR) measurements were carried out by evaporation to dryness of nanoparticles dispersions on a polished CaF2 window. The IR spectra were recorded in a Nicolet 860 FTIR spectrometer equipped with an MCT detector and a purge gas system for removal of CO2 and H2O (Whatman). The IR spectra were averaged over 1024 scans; the spectral resolution was 2 cm-1. The spectra were blank-subtracted and baseline-corrected using OMNIC software from Nicolet. Transmission electron microscopy was carried out in a FEI Tecnai G2 electron microscope using an acceleration voltage of 120 kV. The samples for examination by TEM were prepared by evaporation of a drop of the nanoparticle solution onto carbon films supported on standard copper grids. Mean particle size and standard deviations were determined from measurements on at least 100 particles. AFM measurements were performed to confirm the sizes measured by TEM. In this case, the nanoparticles were fixed to a nonanedithiol SAM on a gold film support. Electrochemical reductive desorption of TA: HSC11imidazole AuNPs (2 nM) was carried out with an Autolab potentiostat, PGSTAT 10 (Echo Chemie, The Netherlands), in deaerated 0.5 M KOH aqueous solution using a Au disk electrode (0.0707 cm2) as working electrode. The reference was a saturated calomel electrode (SCE) and all potentials are referred to this electrode. A platinum wire was used as the counter electrode.

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Figure 1. FTIR spectra of: (A) (C10H21)4N+Br--stabilized silver nanoparticles. The peak at 1630 cm-1 is associated with sulfate originating from the acid washing; (B) TA-capped silver nanoparticles after transfer to water; (C) Silver nanoparticles capped with a TA:TAANTA-Co(II) mixed monolayer.

3. Results and Discussion 3.1. AgNPs Capped with a TA:TA-ANTA-Co(II) Mixed Monolayer. Silver nanoparticles were first prepared in toluene using a two-phase reaction.7b,16 Syntheses carried out in an organic phase yield good monodispersity and higher particle concentrations than water-based preparations. The characteristic surface plasmon band (SPB) at 418 nm (Figure S1, Supporting Information) was observed for this dispersion in toluene. FTIR spectroscopy (Figure 1A) shows the presence of the phase-transfer reagent (C10H21)4N+Br-, TDABr) in the reaction product with absorbances at 2954, 2920, 2850, 1469, 1379, and 1340 cm-1. These corresponded to vibrational modes νasy(-CH3), νasy(-CH2), νsym(-CH2), δ (-CH2), δasy (-CH3), and δsym (-CH3), respectively, characteristic of the methylene chains of TDABr.17a–c The nanoparticles present in the toluene phase were capped with TA by addition of a solution of TA in toluene immediately after preparation, since the silver dispersion was not stable for extended periods. The dithiolane sulfur atoms in TA chemisorb strongly on the silver surface displacing the adsorbed phasetransfer reagent and causing precipitation due to the consequent polarity change of the stabilizing ligand. The TA functionalized particles were separated by filtration and then dissolved in HEPES buffer at pH 10. Ag metal colloids synthesized in an organic phase have been separated by transfer to an aqueous medium using an aqueous capping ligand,18 and Doty et al.19 recently reported the use of a thioalkylated poly(ethylene glycol) to stabilize Ag nanoparticles. In view of their lower pKa,20 the use of short-length thioacids has also been investigated for nanoparticle transfer from an aqueous to an organic phase. The colloids, however, are unstable which may be a consequence of the known tendency of very short chain alkanethiolates to form disordered structures.21 By contrast, the use of thioctic acid (TA) as a ligand for AuNPs as described above provides excellent stability to the nanoparticle dispersions.22 FTIR measurements of the TA-capped nanoparticles (Figure 1B) showed the characteristic bands associated with TA vibrational modes. Vibrations at 2924, 2879, 1610, and 1354

Sendroiu et al. cm-1 correspond to the vibrational modes νasy(-CH2), νsym(-CH2), νasy(-CO2-), and νsym(-CO2-), respectively.17a–c The band at approximately 3310 cm-1 correspond to the ν(-OH) vibration due to the presence of residual water, probably strongly attached to the carboxylate groups. The R4N+Br- bands were completely absent demonstrating the complete exchange of TDABr by thioctic acid on the clusters. TEM and AFM characterization of the Ag-TA nanoparticles (Figure 2) shows a narrow particle size distribution with an average diameter of (5.2 ( 0.5) nm. From elemental analysis, the sulfur to silver molar ratio found was 0.1. Considering that one nanoparticle of 5.2 nm contains approximately 4350 atoms (calculated for a face-centered cubic structure) and that 1100 atoms are at the surface, the number of TA molecules estimated was approximately 217 per particle, equivalent to a 43% coverage of the surface Ag atoms.23 Elemental analysis also confirmed the absence of N and Br thus providing additional support to the FTIR results. Figure 3 shows the UV-vis spectra of the aqueous colloidal solution of TA stabilized AgNPs at different pHs. The characteristic plasmon band for silver can be observed at 402 nm. The shift of this band to lower wavelengths compared with the results in toluene is a consequence of the changes in both the dielectric permittivity of the ligand shell and of the refractive index of the solvent.24 The aqueous silver nanoparticles were very stable at pH 8 and solutions were unaltered when kept for over a year under ambient conditions. Aggregation occurs for pH values less than 6.6 due to protonation of the terminal carboxylate groups on the nanoparticles, a value similar to previous observations of the pKa of TA attached to Au.25 The pKa of TA in solution is approximately 5 and the observed higher pKa value for these functionalized NPs is in accordance with the increase observed for ω-mercaptoacids when attached to Au surfaces as compared with the value in solution.25 As shown in Figure 3, protonation induces particle aggregation and the emergence of a longitudinal plasmon absorption band due to the optical coupling of the metallic cores plasmons.26 The pH stability range of solutions of TA-capped AgNPs was adequate for further covalent modification with the aminonitrilotriacetic-Co(II) complex (ANTA-Co(II)) by activation of the terminal ω-carboxylic groups of the capping ligand following a previously described procedure.22a This leads to silver nanoparticles containing a mixed capping layer of Co(II)-terminated ligand and unreacted TA, the latter providing electrostatic stabilization in slightly alkaline media due to ionization. The ANTA-Co(II) complex was covalently attached to TA terminated Ag nanoparticles and purified from the original complex by successive steps as described in the Experimental Section. The FTIR spectra of TA-capped AgNPs after ANTACo(II) assembly (Figure 1C) displays the characteristic amide bands together with vibrational modes associated with the NTA and TA carboxylate groups. The band at 1656 cm-1 is assigned to the amide I CO stretch and those at 3304, 2924, 2855, 1357, and 1261 cm-1 correspond to ν(-NH), νasy(-CH2), νsym(-CH2), νsym(-CO2-), and ν(-CO) vibrational modes, respectively, confirming the further functionalization of the -COOH termination with the ANTA-Co(II) complex. Figure S2 (Supporting Information), compares the UV-visible spectra of Ag nanoparticle solutions before and after attachment of the ANTA-Co2+ complex to the TA nanoparticles ligand shell. The absorbance of the Ag nanoparticles dispersion changed significantly on modification with the ANTA-Co2+ complex (Supporting Information Figure S2, curves a and b). The spectrum measured after the purification procedure shows

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Figure 2. Imaging of TA-derivatized silver nanoparticles. (A,B) TEM pictures of the nanoparticles from their aqueous solution; (C) AFM image in MAC mode of the nanoparticles attached to a dithiol-modified gold support; and (D) histogram from images (A) and (B).

Figure 3. UV-visible spectra of silver nanoparticles capped with thioctic acid in 20 mM HEPES buffer at different pHs. The pH values are indicated in the figure.

a broadening at 512 nm of the usual silver NP plasmon band. This broadening is due to the absorbance of the ANTA-Co2+ complex, which shows an absorption band centered at 512 nm (Supporting Information Figure S2, curve c). This spectral information was employed to monitor the purification of the modified nanoparticles (see section 2.1) by measuring the spectra of the filtrates after each purification step (Figure S3, Supporting Information). The purification procedure was repeated until the filtrate showed no absorption between 200 and 820 nm indicating the absence of unattached Co(II) complex. It can be concluded that the broadening at 512 nm is only a result of the absorbance of the Co(II) -terminated ligand shell of the Ag-TA nanoparticles. The presence of cobalt in the nanoparticle preparation (attached ANTA-Co(II)) was quantified by atomic emission spectroscopy (AES-ICP). A Ag/Co molar ratio of 0.038 was found from which the number of cobalt atoms estimated per silver particle was approximately 165, indicating that approximately 70% of the TA molecules on the surface of the nanoparticles were modified with the Co(II) complex by the synthetic procedure followed. All these results confirm the functionalization of the nanoparticles with the cobalt(II) complex.

Figure 4. FTIR spectra of gold nanoparticles capped with a mixed TA:HSC11-imidazole layer. A 100:1 thiol ratio was employed.

3.2. Gold Nanoparticles Containing Imidazole-Terminated Ligands. Citrate-stabilized Au particles were prepared by the aqueous citrate reduction method.15 TEM of the nanoparticles (Figure S4, Supporting Information) showed a narrow particle size distribution with an average diameter of (13 ( 1) nm. They were capped with a mixed TA:HSC11-imidazole layer by reaction with different TA:HSC11-imidazole molar ratios in solution. The FTIR spectrum of the [TA:HSC11-imidazole]-capped nanoparticles (Figure 4) exhibits characteristic bands associated with the vibrational modes of TA, which in most cases overlap with those associated with the imidazole group. As listed in Table 1, the peaks at 1565, 1504, 1345, and 1228 cm-1 correspond to the vibrational modes of the imidazole ring, in agreement with the assignments reported in ref.17a The bands at 1729, 1662, 1630, and 1392 cm-1 were assigned to CdO stretching modes and those at 2919, 2849 cm-1 correspond to νasy(-CH2), νsym(-CH2), respectively.17b,c The peak at 3282 cm-1 is associated with ν(-NH) and ν(-OH) stretching vibrations.

10104 J. Phys. Chem. C, Vol. 112, No. 27, 2008 TABLE 1: Infrared Band Positions and Assignments for Gold Nanoparticles Capped with a Mixed TA:HSC11-Imidazole Layer band position (cm-1) 1228 1345 1392 1504, 1565 1630 1662 1729 2849, 2919 3982

assignment Overtone RN-H Imidazole ring Imidazole ring breathing. In-plane RC-H deformation Symmetrical stretching of -CO2In-plane imidazole ring deformation Stretch CdO vibration. imidazoliumcarboxylate ion-pair. Stretch CdO vibration. Carboxylatecarboxylate interactions. Stretch CdO vibration for protonated -CO2H Symmetrical and asymmetrical ν(-CH2) ν(-NH) and ν(-OH) stretching vibrations

The surface composition of the nanoparticle thiol layer was estimated by electrochemical reductive desorption using cyclic voltammetry.27 Typical results are shown in Figure S5 (Supporting Information) for nanoparticles modified with a mixed TA-HSC11 imidazole monolayer made by reacting with a 100:1 molar ratio solution in toluene of the two thiols. Two desorption peaks at approximately -1.07 and -0.99 V are visible. Figure S5b (Supporting Information) shows a detail of the deconvolution of this voltammetric wave using Gaussian functions.28,29 From the results in Figure S5b (Supporting Information), the peak potentials corresponding to these two processes are -0.99 and -1.095 V. The desorbed species at each of these potentials was identified by comparison with the electrochemical desorption of self-assembled monolayers of TA and HSC11-imidazole on a bulk gold electrode, as shown in Figure S6 (Supporting Information). The desorption peak potential of TA from a SAM on Au was -1.04 V, close to previous observations at approximately -1.0 V vs SCE,25 whereas that of 1-(11mercaptoundecyl) imidazole occurred at the more negative potential of -1.13 V. The more negative desorption potential for the imidazole thiol is probably due to the longer hydrocarbon chain compared with TA. The desorption potential becomes more negative the longer the chain, due to chain-chain interactions.30 Additionally, the values of the desorption potentials for a SAM are approximately 40 mV more negative than for the nanoparticles. This is not unexpected considering the higher packing density in a SAM that results in a greater stabilization of the thiols due to lateral interactions.27a The difference between the desorption potential of the two thiols investigated is similar for the SAM and for the nanoparticle case, -90 and -105 mV, respectively. The similarity in the difference in desorption potentials gives additional confirmation that the technique employed to assess the composition of the capping layer is appropriate. From elemental analysis, the S/Au molar ratio was 0.050, and considering that a 13 nm nanoparticle contains approximately 69000 atoms (estimated for a face-centered cubic structure) and 7300 atoms are at the surface,23 the number of S atoms per particle was 3450. From the integrated voltammetric waves shown in Figure S5b (Supporting Information), the TA/ HSC11-imidazole charge ratio was 6.8:1 from which each AuNP had approximately 1500 and 345 molecules of thiols, respectively. Considering that two imidazole groups can bind to each Co(II) center, the number of imidazole groups present is comparable with the number of previously calculated free Co(II) attachment centers present on the Ag nanoparticles of 165. The 100:1 thiol ratio employed for the functionalization of the Au nanoparticles provided good stability and sufficient

Sendroiu et al. coverage with ligands specific to the Co(II) complex. The use of lower molar TA: HSC11-imidazole ratios gave unstable sols. There is, however, a large difference between the ligand composition on the functionalized nanoparticles surface and that present in the solution. For example, for a 100:1 ratio of TA to HSC11-imidazole in the solution, the composition ratio on the nanoparticle ligand shell was 6.8:1 (see above). The origin of this large difference is related to the kinetics of attachment. TA contains a disulfide bond that must be broken before attachment. The rate of formation of the gold-thiol bond to form a SAM is much slower from disulfides than for single thiols as demonstrated by Bain et al.30 who found a selectivity ratio of 75:1 for their attachment on gold from mixtures of thiols and disulfides in solution. Although the selectivity will be altered by the presence of steps and plane boundaries on the surface of the nanoparticles, the rate of attachment from the disulfide is clearly slower than that of the single imidazole thiol giving a selectivity ratio of 15:1 for the results shown in Figure S5 (Supporting Information). 3.3. Ligand Shell Reaction of NTA-Co(II) Capped AgNPs and Imidazole-Terminated AuNPs. The possibility of using a coordination chemistry approach for the specific binding of nanoparticles was investigated using the coordination reaction between the ligand shells of the Au and Ag nanoparticles described above. The reaction was conducted by mixing their aqueous solutions at room temperature (Scheme 1) at pH 9.5. This pH was chosen to ensure that the unsaturated N3 pyridine nitrogen of imidazole was in its unprotonated form. The ImH+ cation has a pKa of 6.99,13b,31 and therefore, the assembly reaction would not be driven by electrostatic interactions between -NH+ and -COO- groups at this pH. Considering the difference in dimensions between the silver and gold particles, each one of the latter should be able to bind more than one silver particle, and for this reason, different molar ratios of AuNPs and AgNPs were studied. A 1:10 ratio was found optimal to form assemblies without experiencing rapid precipitation. Immediately after mixing, the solution had a clear yellow-pink color, but after one hour, the color changed to a blurred violet shade, characteristic of nanoparticle aggregation in solution. Complete aggregation was observed after leaving the mixture for 12 h at room temperature to give a macroscopic violet precipitate and a clear supernatant. The course of the reaction was followed by TEM and UV-visible spectroscopy. Examples of the structures of the assemblies formed after one hour of reaction are shown in Figure 5, indicating the formation of binary nanoscale networks. Ordered networks are observed in which the larger gold particles are generally surrounded by the smaller silver nanoparticles. In some cases, the strong Au-Au interaction (see Supporting Information Figure S7 for an example of this) results in a displacement of the Ag NPs from the bridging region. The rapid formation of extended binary networks demonstrates the reactivity of the ligand shells. The ligand shell reaction results also in large changes in the UV-vis spectra as shown in Figure 6. Trace (a) shows the spectra recorded immediately after mixing the nanoparticle dispersions. The two absorption bands at 406 and 526 nm are characteristic of the surface plasmon bands (SPBs) of silver (trace (c)) and of gold nanoparticles (trace (d)) and the initial spectrum of the mixture is a superposition of that of the two components. After 1 h of reaction (trace (b)) a shift as well as a broadening of the gold SPB with the emergence of a new band at approximately 576 nm is observed. In addition, the 406 nm band decreases due to the decrease in the concentration of

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Figure 6. UV-visible spectra of: (a) A solution containing TAimidazole thiol functionalized AuNPs and TA-ANTA-Co(II) functionalized AgNPs recorded just after mixing the nanoparticle dispersions; (b) the same solution after 1 h of reaction; (c) TA-imidazole AuNPs; (d) TA-ANTA-Co(II) AgNPs. The measurements were carried out in 20 mM borate buffer, pH 9.5. The Ag and Au concentrations were adjusted so that they were the same as for the individual nanoparticle dispersions to avoid sample dilution effects (see Experimental Section).

Figure 7. UV-visible spectra for a solution containing: (a) TAimidazole thiol functionalized AuNPs and TA functionalized AgNPs; (b) TA functionalized AuNPs and TA-ANTA-Co functionalized AgNPs recorded after just mixing the nanoparticle dispersions (dashed line) and after 3 h of reaction (solid line) in 20 mM borate buffer, pH 9.5.

Figure 5. TEM micrographs of Au/Ag nanoscale networks prepared by the imidazole-Co(II) reaction at the ligand shells one hour after mixing dispersions of the Au and Ag functionalized nanoparticles. See Experimental Section for details. The darker particles are Au.

free silver particles, an effect that has been previously noted for gold nanoparticles undergoing controlled aggregation.14 The new 576 nm band results from the presence of particles assemblies in solution that absorb at wavelengths characteristic of longitudinal plasmon bands.14 The wavelength of the longitudinal SPB associated with the formation of aggregates depends on interparticle separation up to distances of ∼8 nm.14 Au/Au particles plasmon coupling would not be expected if these were uniformly surrounded by silver particles due to the long separations involved. The coupling of metal core plasmons for the silver nanoparticles results in the appearance of a longitudinal-like resonance band absorption, accompanied by a considerable decrease in the intensity of the transversal plasmon band (see Figure 3). The 576 nm band cannot be, however, entirely attributed to silver-silver plasmon coupling, since the silver transversal SPB only shows a slight decrease in intensity.

The various possible plasmon interactions among Au/Au, Ag/ Ag, or Ag/Au nanoparticles cannot be separated unequivocally and a quantitative analysis of the various coupled plasmon resonances is beyond the scope of the present work. These results confirm, however, the formation in solution of the structures proposed in Scheme 1 and that the results shown in Figure 5 are not an artifact caused by electron irradiation during the TEM measurements. To ascertain the specificity of the coordination linkage proposed, different control experiments were carried out using a similar 1:10 Au/Ag molar ratio and similar reaction conditions as described above but changing the groups present in the ligand shell. The first control consisted of mixing silver nanoparticles capped with TA, yet without the attachment of the NTA-Co(II) complex, with AuNPs modified with TA:HSC11-imidazole. Figure 7a shows that the initial absorbance (solid line) remained unaltered and no changes were observed in the SPB for Au and Ag after three hours of reaction (dashed line). In the second control experiment, silver nanoparticles capped with TA and the NTA-Co(II) complex were mixed with AuNPs modified with TA but without HSC11-imidazole. Again, no spectral changes between the initial absorbance (solid line) and after three hours of reaction (dashed line) were observed (Figure 7b). These control experiments demonstrate that binding in solution occurs only when the ligand shells include the complementary ImCo(II) recognition groups.

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Figure 8. TEM micrographs of: (a) TA-imidazole thiol functionalized AuNPs and (b) TA functionalized AgNPs. The samples were prepared after one hour of mixing the Au and Ag functionalized nanoparticles.

The specificity of the structures formed by the reactivity of the ligand shells was further confirmed by TEM as shown in Figure 8. Figure 8a,b corresponds to a solution containing both imidazole thiol Au NPs and TA functionalized Ag nanoparticles, the same as indicated in Figure 7a. The distribution of Au and Ag NPs is completely unrelated to each other. The images from this control experiment demonstrate that, indeed, the Co(II) complex represents the essential linkage in the assembly reaction and the TEM images and the UV-visible spectra indicate that the labile ligand groups of the NTA-Co(II) complex provide a recognition center for imidazole to produce the hybrid material. The kinetics of formation of the assemblies can be readily controlled by altering the Au coverage with imidazole. Figure 9 shows the UV-vis spectra at different times for the assembly of TA-imidazole thiol functionalized AuNPs and TA-ANTACo functionalized AgNPs when the reaction was performed with AuNPs modified with five times lower concentration of imidazole thiol (500:1 ratio of TA to HSC11-imidazole) compared with the results shown in Figure 6. It is observed that, in this case, the solution assembly process becomes slower showing that the assembly is dependent on the imidazole group concentration on the AuNPs. The spectral changes observed after 6 h (Figure 9f) were very similar to those observed in Figure 6 for 1 h thus providing additional evidence that the assembly occurs with the participation of the imidazole group. Furthermore, when the same reaction was performed with an imidazole concentration one order of magnitude lower (5000:1), the particle networks were stable for several weeks in solution. It was possible to dissociate the structures in solution by protonation of the imidazole group with decreasing pH to values less than its pKa (6.99), but aggregation occurs in this pH range due to protonation of the terminal carboxylate groups on the nanoparticles.

Figure 9. (A) UV-visible spectra for (a) TA-imidazole thiol functionalized AuNPs synthesized using a 500:1 thiol ratio; (b) TA-ANTACo functionalized AgNPs; and (c) sum of the (a) and (b) spectra. (B) to (F) Comparison of the sum of the absorption spectra of single functionalized Au and Ag nanoparticles in solution (dash line) at different time intervals after mixing the nanoparticle dispersions: (B) 5 min; (C) 1, (D) 3, (E) 4, and (F) 6 h. The measurements were carried out under the same conditions described in Figure 6.

4. Conclusions

specific spatial organization of structures constructed from functionalized nanoparticles.

The importance of this work is to show the possibility of employing the coordination chemistry properties of transition metal complexes attached to the ligand shell of nanoparticles for their self-assembly into nanostructures. We have also demonstrated the synthesis of silver and gold nanoparticles appended with a metal center and imidazole moieties, respectively, as building blocks. The Ag-TA particles prepared are remarkably stable compared with other thiol-capped silver nanoparticles. It is also suggested that the directionality imposed by the atomic orbitals of the metal center in coordination compounds could provide the interesting possibility of achieving

Acknowledgment. J.M.A. acknowledges a postdoctoral fellowship from Fundacio´n Ramo´n Areces. The financial support from the EU Research Training Network SUSANA (Supramolecular Self-Assembled Interfacial Nanostructures, contract HPRN-CT-2002-00185) is gratefully acknowledged. We are grateful to Prof. V. M. Ferna´ndez and Dr. T. Doneux for providing FTIR measurements facilities, to Dr. Stijn F.L. Mertens for useful discussions, to the Liverpool EM unit for support in the use of TEM instrumentation and to Mr. Stephen Apter for help in the analyses.

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