Hydrophilic Metal Nanoparticles Functionalized by 2

Article pubs.acs.org/JPCC

Hydrophilic Metal Nanoparticles Functionalized by 2‑Diethylaminoethanethiol: A Close Look at the Metal−Ligand Interaction and Interface Chemical Structure Iole Venditti,† Giovanna Testa,† Fabio Sciubba,† Laura Carlini,‡ Francesco Porcaro,‡ Carlo Meneghini,‡ Settimio Mobilio,‡ Chiara Battocchio,*,‡ and Ilaria Fratoddi*,† †

Department of Chemistry, Sapienza University of Rome, P.le A. Moro 5, 00185 Rome, Italy Department of Science, Roma Tre University of Rome Via della Vasca Navale 79, 00146 Rome, Italy

‡

S Supporting Information *

ABSTRACT: Hydrophilic gold and silver nanoparticles stabilized with 2-diethylaminoethanethiol hydrochloride (DEA) have been prepared and characterized. AuNPs-DEA and AgNPs-DEA with mean diameter below 10 nm have been characterized by means of dynamic light scattering and fieldemission scanning electron microscopy techniques. Nuclear magnetic resonance (NMR) studies allowed to assess translational mobility, aggregation equilibrium in function of pH variations and presence of chemisorbed and physisorbed thiol molecules; in particular ethyl groups on DEA ligands are free to rotate, suggesting a rather loose packing of the thiols on the nanoparticle surface. NMR results were compared with X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure, and X-ray absorption spectroscopy. The complementary information acquired allowed to obtain information on the interaction at the interface between the organic thiol ligand and metal nanoparticles (NPs) at atomic level as well as on the molecular structure. The influence of the thickness of the functionalizing layer on the stability of NPs has been studied and opened new insight on the local structure surrounding the NPs.

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INTRODUCTION Functionalized metal nanoparticles (MNPs) with average size ranging from units to tens of nanometers are considered emerging materials for advanced applications in catalysis,1,2 optoelectronics,3 sensors,4 and biomedicine5,6 from drug delivery7 to diagnostics.8 In-depth investigations focused on their chemico−physical characteristics, highlighting the role of size and shape dependence of surface plasmon resonance (SPR) and electronic properties on the nanoscale.9−11 Among others, gold and silver nanoparticles (AuNPs and AgNPs) can be stabilized by a variety of ligands with chemical ending functionalities purposely chosen for the specific application.12,13 Citrate-stabilized AuNPs can be considered among the most popular ones, and remarkable studies evidenced the role of citrate concentration in the size and dispersion of AuNPs; that is, a high concentration of citrate usually gives rise to small size NPs, whereas a low concentration leads to larger NPs and aggregation phenomena.14 The use of thiols has been extensively exploited from the pioneering studies on alkanethiols in the Shiffrin−Brust twophase route, particularly suited to fine-tune nanoparticles’ size and shape,15 and the overall mechanism including all of the steps of the AuNPs synthesis has been demonstrated by Raman and nuclear magnetic resonance spectroscopies.16 For example, 3-mercaptobenzoic acid (3-MBA) has been extensively studied, © XXXX American Chemical Society

gaining insights into the formation mechanism of Au nanoclusters.17 The availability of functional thiols opened new perspectives for the achievement of nanoparticles soluble in different environments by using hydrophilic18,19 or hydrophobic thiols;20 bifunctional thiols have been used for the achievement of interconnected networks.21,22 The Shiffrin− Brust method was extended to single-phase systems by selecting thiols soluble in the same solvent as HAuCl4 and avoiding the introduction of phase-transfer agents such as tetraoctylammonium bromide (TOAB). In particular, watersoluble MNPs are of great biomedical interest23−25 and have been obtained by direct synthesis or ligand-exchange reactions. For example, water-soluble AuNPs have been obtained with low size dispersion26 by direct reduction of aqueous solutions of HAuCl4, and ligand exchange approach has been applied to obtain small core-size nanoparticles,27 although the extent of ligand exchange depends on the nature of the incoming ligand. Biocompatibility and toxicity tests have been carried out, giving evidence of the mechanism of their cellular uptake and biodistribution, which mainly depends on the characteristics of the MNPs surface.28,29 AuNPs can be used to selectively Received: February 13, 2017 Revised: March 23, 2017 Published: March 23, 2017 A

DOI: 10.1021/acs.jpcc.7b01424 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

to 1/10 (AgNPs-DEA-10) and 1/4 (AgNPs-DEA-4). The synthetic procedure is herein reported for AgNPs-DEA with Ag/DEA molar ratio = 1/10: DEA (1.997 g, 1.177 × 10−2 mol, in 10 mL of deionized water) is mixed with a water solution of Ag+ ions (0.200 g AgNO3, 1.177 × 10−3 mol, in 10 mL of deionized water), and the suspension was bubbled under nitrogen at room temperature for 10 min. Next, 0.222 g of NaBH4 (1.177 × 10−3 mol) dissolved in 10 mL of deionized water was added. The mixture was allowed to react for 2 h at room temperature under magnetic stirring. After 2 h of stirring at room temperature, a NaOH solution (8 mL, 1 M) has been added; the product has been extracted twice from CHCl3/H2O and recovered from the organic phase. Yield: 37% (AgNPsDEA-10) and 35% (AgNPs-DEA-4). Main Characterizations of AgNPs-DEA. FTIR (ν cm−1, film) = 2968, 2931, 2875 (νCH2, CH3), 1465, δ(CH2), 1382, 1344, 1292 (δ CH2, CH3), 1203, 1136 (νC−N) 684, 657 (νC− S), 725, 501 (νC−C). AgNPs-DEA-10: UV−vis (λmax, nm H2O): 423; DLS: , nm, CHCl3: 8 ± 3; DLS , nm, H2O pH = 2:5 ± 1; DLS , nm, H2O pH = 7:50 ± 10 and 250 ± 20. AgNPs-DEA-4: UV−vis (λmax, nm H2O): 422; DLS: , nm, CHCl3: 5 ± 1; DLS , nm, H2O pH = 2:8 ± 3; DLS , nm, H2O pH = 7:60 ± 30. Main Characterizations of AuNPs-DEA. FTIR (ν cm−1, film) = 2970, 2933, 2872 (νCH2, CH3), 1464, δ(CH2), 1384, 1346, 1294 (δ CH2, CH3), 1197, 1139 ((νC−N) 684, 657 (νC−S), 721, 497 (νC−C). AuNPs-DEA-10: UV−vis (λmax, nm H2O): 521; DLS: , nm, CHCl3: 6 ± 2; DLS , nm, H2O pH = 2:5 ± 2; DLS , nm, H2O pH = 7:90 ± 10 and 550 ± 30;. AuNPs-DEA-4: UV−vis (λmax, nm H2O): 517; DLS: , nm, CHCl3: 5 ± 1; DLS , nm, H2O pH = 2:5 ± 1; DLS , nm, H2O pH = 7:40 ± 10 and 200 ± 20. 1 H NMR (δ, D2O) (DEA pattern I): 0.98 (t (7.4 Hz), Et −CH3), 2.48 (q (7.4 Hz), CH2−2), 2.71 (m, Et CH2), 2.71 (m, CH2−1); (DEA pattern II): 1.08 (m, Et −CH3′), 2.59 (m,CH2−2′), 2.84 (m, Et CH2′), 2.84 (m, CH2−1′). 13C NMR(δ, D2O) (DEA pattern I): 11.0 (Et −CH3), 37.2 (CH2− 2), 47.1 (Et CH2), 52.0 (CH2−1); (DEA pattern II): 11.1 (Et −CH3′), 34.8 (CH2−2′), 47.0 (Et CH2′), 50.9 (CH2−1′). Self-Assembled Monolayers Preparation. Gold-coated silica wafers prepared by growing Au film 4000 Å thick onto Si(111) substrates were cut into slices (ca. 1 cm2) and washed with several organic solvents, that is, acetone and ethanol chloroform, and blown dry with nitrogen. For the preparation of DEA self-assembled monolayer, an aqueous solution of DEA at concentration 1 mg/mL in deionized water, whose pH was fixed at 2, was stirred at 30 °C for 2 h. The mixture was filtered on Celite, and a freshly washed gold substrate was dipped into the solution for 2 h. The obtained multilayer was then rinsed with deionized water to achieve the formation of DEA films in the monolayer thickness regime. Instrumentation. UV−vis. UV−vis spectra were run in deionized water or CHCl3 solutions by using quartz cells with a Varian Cary 100 scan spectrophotometer. FTIR, FIR, and ATR transmission infrared spectra have been acquired by a Bruker Vertex 70 spectrophotometer in the spectral range 4000−400 and 600−200 cm−1 on cast-deposited films from chloroform or water solutions using KRS-5 cells. Size and size distribution of MNPs in aqueous solution have been investigated by means of DLS technique using a Zetasizer Nanoseries Malvern at a temperature of (25.0 ± 0.2) °C. Correlation data have been acquired and fitted by analogy to our previous work.32 Nonnegative least-squares (NNLSs)39 or CONTIN40 algorithms,

deliver therapeutic agents that would otherwise exhibit low solubility,30−32 limited diffusion,33 or compounds that would exhibit poor intracellular penetration (e.g., siRNA).34 In this context, a key role is due to the high density of ligands on the MNPs surface35 and their binding affinity toward bioactive molecules. In this framework, gold and silver nanoparticles stabilized with 2-diethylaminoethanethiol hydrochloride (DEA) have been prepared. The role of the DEA as capping agent has been investigated to identify intermolecular interactions between the thiol ligands and the metal surface. Previous work on AuNPs stabilized with DEA evidenced the affinity of the obtained hydrophilic nanoparticles toward the immobilization of Candida rugosa lipase and the lipolytic activity of the bioconjugate,36 and our interest was to obtain detailed information about the local chemistry, atomic distribution, and molecular structure around metal sites of AuNPs-DEA and AgNPs-DEA. In this work, AuNPs-DEA and AgNPs-DEA have been prepared with different metal/thiol stoichiometric ratios and have been carefully purified to control their size and monodispersity. Colloidal suspensions have been investigated by dynamic light scattering (DLS) and zeta potential to assess size and stability of functionalized MNPs. To assess translational mobility, aggregation equilibrium as a function of pH variations, and the presence of chemisorbed and physisorbed thiol molecules, bidimensional NMR studies have been carried out. The chemical bond between DEA thiol and MNP as well as the thiol/metal interface and thiol molecular and electronic structure were investigated by means of synchrotron radiationinduced X-ray photoelectron spectroscopy (SR-XPS) and nearedge X-ray absorption spectroscopy (NEXAFS) at C Kedge.37,38 To obtain a better insight into the “metallic core”/ “molecular overlayer” interface and DEA molecular and electronic structure and interaction with the different metals, SR-XPS and NEXAFS data were also collected on a selfassembled monolayer of DEA molecules anchored on a “flat” gold surface (Au/Si(111) wafer), providing an appropriate model for the data analysis. Besides, XAS data were also collected at the Au LIII-edge on a AuNPs-DEA sample selected as model, with the aim to obtain more detailed information about the average Au coordination chemistry in AuNPs-DEA.

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MATERIALS AND METHODS Materials. 2-Diethylaminoethanethiol hydrochloride (HSCH 2 CH 2 N(CH 2 CH 3 ) 2 ·HCl, DEA), silver nitrate (AgNO3), tetrachloroauric(III) acid trihydrate (HAuCl4· 3H2O), sodium borohydride (NaBH4), and sodium hydroxide (NaOH) have been used as received (Aldrich reagent grade). Solvents CHCl3 and HCl (Aldrich reagent grade) have been used as received. Deionized water has been degassed for 30 min with argon before use. Free DEA Characterization. 1H NMR (δ, D2O): 1.19 (t (7.4 Hz), Et −CH3), 2.78 (t (7.4 Hz), CH2−2), 3.15 (q (7.4 Hz), Et CH2), 3.24 (t (7.4 Hz), CH2−1). 13C NMR (δ, D2O): 8.0 (Et −CH3), 17.8 (CH2−2), 47.0 (Et CH2), 53.9 (CH2−1). Synthesis and Purification of AuNPs-DEA and AgNPsDEA. AuNPs-DEA samples have been prepared according to literature reports, and in this work samples prepared with Au/S molar ratios equal to 1/10 (AuNPs-DEA-10) and 1/4 (AuNPsDEA-4) have been characterized.36 The synthesis of AgNPsDEA samples is herein reported in detail. By analogy to AuNPsDEA, samples have been prepared with Ag/S molar ratios equal B


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The Journal of Physical Chemistry C Scheme 1. AuNPs-DEA and AgNPs-DEA Reaction and Purification Scheme

supplied with the instrument software, were used to fit correlation data. The zeta potential was calculated from the measured electrophoretic mobility by means of the Smoluchowski equation.41 SR-Induced Spectroscopic Techniques. Synchrotron radiation-induced SR-XPS experiments were performed at the UE52-Multi-Color-PES end-station, mainly used for high-resolution XPS, which is located at the undulator beamline UE52PGM at BESSYII (Berlin, Germany). The beamline is equipped with a ScientaR4000 electron analyzer. The base pressure in the setup was maintained below 10−10 mbar. A photon energy of 500 eV was used for C 1s, S 2p, N 1s, Cl 2p, Au 4f, and Ag 3d spectral regions with energy resolution ΔE = 0.22 eV. Calibration of the energy scale was made referencing all of the spectra to the gold Fermi edge of an Au reference sample, and the Au 4f7/2 signal was always found at 83.96 eV. Curvefitting analysis of the C 1s, N 1s, S 2p, Cl 2p, Au 4f, and Ag 3d spectra was performed using Gaussian curves as fitting functions.42,43 The S 2p3/2,1/2 doublet was fitted by using the same full width at half-maximum (fwhm) for both components, a spin−orbit splitting of 1.2 eV, and a branching ratio (S 2p3/2/ S 2p1/2) of 2. For Au 4f7/2,5/2 and Ag 3d5/2,3/2 doublets, spin− orbit splittings of, respectively, 3.6 and 6.0 eV, branch ratios Au 4f7/2/Au 4f5/2 of 4/3 and Ag 3d5/2/Ag 3d3/2 of 3/2, and the same fwhm values for both spin−orbit components of each signal were used. When several different species were identified in a spectrum, the same fwhm value was used for all individual photoemission bands. Synchrotron-Induced NEXAFS Measurements. Synchrotron-induced NEXAFS measurements were performed at ELETTRA storage ring at the BEAR (Bending Magnet for Emission Absorption and Reflectivity) beamline. The energy range available spans from the visible to soft X-rays (3 eV-1600

eV), and the UHV end station offers a large variety of geometries well-suited for polarization-dependent experiments.44−46 The photon energy and resolution were calibrated and experimentally tested at the K absorption edges of Ar, N2, and Ne. In addition, our carbon K-edge spectra have been further calibrated using the resonance at 285.00 eV assigned to the graphite impurities adsorbed on the mirror.47 The raw C Kedge NEXAFS spectra were normalized to the incident photon flux by dividing the sample spectrum by the spectrum collected on a freshly sputtered gold surface. The spectra were then normalized, subtracting a straight line that fits the part of the spectrum below the edge and assessing to 1 the value at 320.00 eV. Measurements reproducibility was checked: The same spectra were collected in different points of each sample, and different samples of the same material were prepared and tested (at least two for each system). X-ray Absorption Spectroscopy Measurements. X-ray absorption spectroscopy (XAS) measurements at the Au-LIII absorption edge were carried out in transmission geometry at BM23 beamline at the European Synchrotron Radiation Facility (ESRF) (Grenoble, France). Two samples (AuNPs-DEA-10, AuNPs-DEA-4) were investigated, kept at low temperature (20 K) during data collection to reduce the thermal contribution to structural disorder. For each sample, six spectra were collected, and the data were averaged to improve the data statistics. A pure gold foil was measured under the same experimental conditions to be used as reference. Standard procedures48 were used for data normalization and to extract EXAFS (extended Xray absorption fine structure) signal. Theoretical amplitude and phase functions were calculated using FEFF 8.0 program.49 Multishell EXAFS data analysis was carried out following the procedure carefully described in ref 50. For AgNPs, this procedure involves crystallographic constraints among the C


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The influence of metal/thiol molar ratio was not evident in the investigated samples that show a hydrodynamic radius in the range 5−8 nm for AuNPs-DEA and AgNPs-DEA prepared with M/S molar ratios equal to 1/4 and 1/10. Depending on the pH of the solution, nanoparticles can lose charge, and a certain grade of heterogeneity in the surface coverage can result with ammonium−amine ending-group interchange. DLS and zeta potential measurements carried out at pH between 2 and 7 confirmed the stability of the obtained colloids under these conditions (at pH 2, DLS size equals 5 ± 1 and 8 ± 3 nm and zeta potential equals 30 ± 5 and 40 ± 10 mV for AuNPs-DEA4 and AgNPs-DEA-4 samples, respectively; experimental data are reported in Figure SI-4 together with data for M/DEA 1/ 10), whereas at pH higher than 7, aggregation phenomena occurred and DLS data evidenced the instability of the colloidal system, with a general broadening and the presence of two populations with size higher than 50 and 200 nm. Experimental details are reported in Figure SI-5. In Figure 1a, the UV−vis spectra of AuNPs-DEA and AgNPs-DEA samples are reported. Plasmon resonance bands

structural parameters to achieve structural information on local and intermediate scales (next-neighbor shells), providing reliable details of Au−S coordination and NP size.49 Nuclear Magnetic Resonance Measurements. All NMR spectra were recorded in D2O at 298 K on a Bruker AVANCE III spectrometer at 9.4 T operating at the hydrogen frequency of 400.13 MHz and equipped with a Bruker multinuclear zgradient inverse probe head. The 1H spectra were acquired, employing the standard presat pulse sequence for solvent suppression. The scan number is 32 transients, the recycle delay is 9.5 s, the spectral width is 15 ppm, and there are 64K data points for a total acquisition time of 15 s. Bidimensional 1 H−1H TOCSY experiments were acquired with 32 scans, a spectral width of 15 ppm in both dimensions, a data matrix of 8K × 256 data points, a mixing time of 90 ms, and a recycle delay of 2 s. Bidimensional 1H−1H NOESY experiments were acquired with 32 scans, a spectral width of 15 ppm in both dimensions, a data matrix of 8K × 256 data points, a mixing time of 150 ms, and a recycle delay of 2 s. Bidimensional 1 H−13C HSQC experiments were acquired with 32 scans, a spectral width of 15 and 200 ppm for hydrogen and carbon, respectively, a data matrix of 8K × 256 data points, a coupling constant J1CH = 145 Hz, and a recycle delay of 2 s. Pseudo-bidimensional DOSY experiments were acquired with 32 scans, a spectral width of 15 ppm, 16 gradient increments, a diffusion time of 150 ms, a gradient length of 1500 μs, 64K data points, and a recycle delay of 2 s.

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RESULTS AND DISCUSSION AuNPs-DEA and AgNPs-DEA Synthesis and Characterizations. Functionalized noble-metal nanoparticles were prepared using a chemical reduction of HAuCl4 or AgNO3 for AuNPs-DEA or AgNPs-DEA, respectively, in the presence of the DEA thiol ligand, with NaBH4 as the reducing agent (see Scheme 1). A single aqueous phase procedure has been adopted thanks to the water solubility of metal precursors and DEA thiol; in this way, the introduction of phase-transfer agents and consequent purification has been avoided.51,52 The amine−ammonium salt equilibrium of the ending group is the determinant for making the colloids suspendable, either in acidic water or in organic solvents. This equilibrium has been utilized in the purification method by using a basic aqueous solution for the extraction of the nanoparticles in the organic phase. FTIR investigations confirmed the linking of DEA thiol on the MNPs surface and the presence of amine/ammonium ending groups53 (peaks in the range 2600−2400 cm−1, see Figures SI-1 and SI-2) depending on the pH conditions. DEA molecules anchor on the gold nanoparticle surface through the sulfur atom in the SH group (as revealed by the disappearance of the S−H stretch mode at ∼2560 cm−1). In particular, the Au−S vibrations have been found (see Figure SI-3) in the farinfrared region in the range 330−220 cm−1 and assigned to Au−S−C bending modes around 180 cm−1, Au−S stretching modes around 220−280 cm−1, and tangential Au−S stretching modes around 320 cm−1, according to literature reports.54,55 The Ag−S stretching mode was found at 270 cm−1, (see Figure SI-3) in agreement with the values reported in the literature.56 Hydrophilic nanoparticles have been obtained with different metal/thiol molar ratios, and the highly charged ammonium end groups inhibit the interdigitation of chains at acidic pH values, maintaining the colloidal suspensions stable,57 with limited aggregation phenomena.

Figure 1. (a) UV−vis spectra in CHCl3 of AuNPs-DEA (red color) and AgNPs-DEA (blue color) samples. (b) FESEM image of AuNPsDEA. (c) FESEM image of AgNPs-DEA samples.

studied in chloroform occurred at about 518 and 423 nm for AuNPs-DEA and AgNPs-DEA, respectively. The band shifts toward higher wavelengths with a general broadening if spectra are collected in water at pH 7 or under basic conditions (see Figure SI-5). FESEM investigations reported in Figure 1b,c evidenced the presence of nanoparticles with a size of ∼5 nm for functionalized gold and silver nanoparticles, respectively, in good agreement with DLS data obtained at acidic pH. NMR Studies on MNPs-DEA. NMR spectroscopy is a powerful technique for nanoparticle analysis. It is able to identify the species bound to the nanoparticle surface by monodimensional 1H spectra as well as determine their D


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Figure 2. AuNPs-DEA in D2O at pH 7: (a) 1H NMR spectrum and (b) DOSY spectrum.

SI-6a−c. To further characterize the native molecule, in particular to assess its translational mobility, DOSY experiments were acquired (see Figure SI-6c), and the autodiffusion coefficient logarithm of DEA was esteemed to be log(D) = −9.1. This value will be compared with the one measured for the nanoparticles. Because the spectra of AuNPs-DEA and AgNPs-DEA and spectra carried out on 1/10 and 1/4 molar ratio samples present the same spectral patterns, only the AuNPs-DEA-10 will be discussed in the text. From the analysis of 1D 1H NMR spectrum of AuNPs-DEA (Figure 2a), the presence of two different types of DEA ligands can be clearly

rotational mobility by the analysis of bandwidth. It is also able to evaluate the spatial arrangement among the thiols bound to the NP through bidimensional 1H−1H NOESY experiments. Moreover, pseudo-bi-dimensional 1H DOSY experiments are able to determine the autodiffusion coefficient of the molecular species in solution and thus evaluate the translational mobility of the nanoparticles.58 To evaluate the binding of DEA on MNPs, a preliminary study of free DEA was carried out through monodimensional 1 H NMR, and resonance assignment was performed on the basis of bidimensional 1H−13C HSQC, as reported in Figure E


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Figure 3. (a) DOSY spectrum of AuNPs-DEA at pH 2. (b) NOESY spectrum of AuNPs-DEA in D2O.

moiety d from 17.8 to 37.2 ppm. To correctly assign the two structures, DOSY experiments were carried out (Figure 2b). From the spectrum, the DEA pattern I shows an autodiffusion coefficient logarithm log(D) = −10.2, while the pattern II shows log(D) = −9.7. On the basis of these values, it is possible to identify the first patter as the chemisorbed DEA on the basis of its lower translational mobility, while the second one belongs to the physisorbed one. The fact that the second pattern is characterized by both broader resonances as well as by grater translational mobility compared with the chemisorbed

observed, with one of the two characterized by much broader resonances. The assignment was carried out by means of bidimensional 1 H−1H TOCSY and 1H−13C HSQC experiments (see Figure SI-7a,b). The two populations of DEA molecules could be attributed to one covalently bonded to the nanoparticle surface (chemisorbed), while the other is physisorbed on the nanoparticle, as confirmed by HR-XPS data (see next paragraph, and NMR data reported in Tables SI-1 and SI-2). The Au−S bond is confirmed by the change of δ 13C of DEA F


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The Journal of Physical Chemistry C Scheme 2a

a

(a) Hypothesis of nanoparticle aggregation at pH 7 and pH 2. (b) Reconstruction of the relative arrangement of the chemisorbed and physisorbed DEA on the basis of NOESY experiments.

experiments are able to measure interproton distances up to 5 Å and thus are a powerful tool for structure elucidation.59−61 In this NOESY spectrum reported in Figure 3b, correlations between the protons a and a′, between b and b′, and between c and d are observed. The latter correlation is evidence of the rigidity of this molecular moiety and is a further confirmation that DEA bonds to the nanoparticle by its thiol function, as observed in analogue systems.30 The absence of correlations between the protons of the ethyl group of chemisorbed DEA, as well as between the protons of its thioamino ethyl moiety, indicates that the ethyl groups are free to rotate and thus suggests a rather loose packing of the thiols on the nanoparticle surface. The existence of dipolar correlations between the ethyl groups of the chemisorbed and physisorbed DEA can give an insight into their relative arrangement. These contacts, coupled to the absence of correlations between the protons of the thioamino ethyl moiety of the two populations of DEA, allow us to hypothesize (Scheme 2b) that the physisorbed DEA is placed parallel to the chemisorbed DEA with its thiol function directed toward the nanoparticle. This type of arrangement is also in good accord with the XPS experiments reported in the next paragraph. SR-XPS at S 2p and Metal Core Levels: Interaction at the Molecule-Metal Interface in SAMs and MNPs. The

DEA could be explained by the existence of an exchange mechanism between the physisorbed DEA and a small population of free DEA in solution, and as such its diffusion coefficient is an average between the one of free DEA (log(D) = −9.1) and the one of chemisorbed DEA (log(D) = −10.2). With the aim of further confirm this attribution hypothesis in solution, DOSY experiments were carried out on the same AuNPs-DEA sample at pH 7 and at 2 (acidified by HCl). While the first experiment is identical to the one reported in Figure 2b, in the DOSY of AuNPs-DEA at pH 2 (Figure 3a) the two observed autodiffusion coefficients are log(D) = −9.7 and log(D) = −9.2. The first value, attributed to the chemisorbed DEA, is higher than the one measured at pH 7 (which was log(D) = −10.2), and this observation could be explained by the reduction of nanoparticle aggregation due to their greater surface charge, according to Scheme 2a. Regarding the autodiffusion coefficient observed for physisorbed DEA, it is possible to observe that its value of log(D) = −9.2 is very close to the one measured for free DEA (which was log(D) = −9.1), suggesting that the protonation of the amine end-groups into ammonium increases the DEA molecules mobility, which, in turn, affects NP mobility. The interaction of physisorbed and chemisorbed DEA molecules on the MNPs surfaces was investigated in bidimensional 1H−1H NOESY experiments (Figure 3b). These G


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The Journal of Physical Chemistry C high spectral resolution and photon flux of synchrotron radiation-induced XPS are extremely well-suited to investigate the interaction at the interface between small functional molecules and metals, allowing us to probe the chemistry occurring at the headgroup/metal interface in capped nanoparticles as well as in self-assembled monolayers deposited onto “flat” metal surfaces. SR-XPS measurements were carried out at the C 1s, N 1s, S 2p, Cl 2p, and Au 4f/Ag 3d core levels of AuNPs-DEA, AgNPs-DEA, and a self-assembled sample of DEA thiol, anchored on polycrystalline gold and prepared at pH = 2 (SAM). Data acquired on 1/4 and 1/10 molar ratios samples showed the same behavior; as an example, results obtained for 1/4 samples are reported below. S 2p and Au 4f and Ag 3d spectra collected on SAM, AuNPsDEA and AgNPs-DEA are reported in Figure 4; S 2p3/2 and Au 4f7/2−Ag 3d5/2 peak positions expressed as BEs (binding energies), fwhm values, and relative intensities are collected in Table SI-3.

In the literature concerning thiols anchored on gold surfaces a low BE S 2p signal is alternatively attributed to two kinds of sulfur atoms: S 2p3/2 peaks at 161.0 eV are due to atomic S adsorbed on metal surfaces as a result of C−S bond cleavage, for example, in annealed SAMs;66 however, the semiquantitative analysis of C 1s components carried out on SAM-DEA, AuNPs-DEA-4, and AgNPs-DEA-4 is in excellent agreement with the proposed DEA chemical structure (atomic ratios C−C+C−S:C−N = 1:1 theoretically, respectively, 1.2:1.0 for SAM and 1.1:1.0 for AuNPs-DEA-4 and AgNPs-DEA-4, as reported in Table SI-3), suggesting that molecule breaking should be excluded; furthermore, our spectra show the low BE S 2p3/2 signal at a slightly higher BE value, 161.5 eV, coherent with what was already observed by some authors for organic thiols chemisorbed onto metal surfaces.67 This signal is reported as occurring characteristically in SAMs of extremely low degree of coverage,68 obtained, for example, by immersing the substrate in the thiol solution for ∼2 h, as in our SAM preparation procedure. In this interpretation, the S 2p3/2 component at 161.5 eV BE is attributed to S atoms still covalently bonded to the metal surface, as for the “classical” S 2p3/2 peak usually found at 162.4 eV BE, but differently hybridized. In fact, hybridization change is likely to cause a peak shift:69 If we assume that the S 2p3/2 BE for a sp3 S atom is 162.4 eV, then a BE value of ∼161.5 eV (1 eV lower) would be expected for an sp-hybridized S atom.68 It is expected that also the interactions through the sulfur lone pairs may be attributed to multiple S 2p core levels.67 Another interpretation of the low BE signal of S 2p, maintaining the S−C bond intact, is the different chemisorption sites on the metal surface. In fact, the binding of sulfur in a different metal site might cause the S 2p BE shift.66 With reference to Au substrates, sulfur atoms of alkyl thiolates are usually observed to locate on the three-fold hollow site of Au(111) surface in the ordered domains of fully covered SAMs;70,71 the isolated molecules outside the ordered domains could absorb onto other sites, as, for example, bridge or on-top sites. Because the interaction between S orbital and Au surface depends on the site, the site change effect would likely cause a BE shift (low BE peak formation). From the reported literature data it is possible to draw a quite exhaustive picture of the nature of S atoms giving rise to the low BE signal: a sphybridized S atom, strongly interacting with the metal atoms on the substrate surface, in bridge or on-top sites, leading to thiolmetal chemical bonds.67 The preparative procedure followed for our samples, consisting of short contact times between metal and ligand, is in excellent agreement with the hypothesized S 2p 161.5 BE assignment. Au 4f and Ag 3d core-level spectra are reported in the second row of Figure 4 for AuNPs-DEA-4 and AgNPs-DEA-4, respectively. All spectra, as expected, are composed of spin− orbit doublets (Au 4f7/2−Au 4f5/2; Ag 3d5/2−Ag 3d3/2); the more intense component, due to metallic Au(0) or Ag(0) atoms, is usually taken as reference (and reported in Table SI3). Although for SAM-DEA a single Au 4f spin−orbit pair can be observed, with the main Au 4f7/2 signal centered at 83.96 eV, as expected for metallic gold (see Table SI-3), AuNPs-DEA-4 show two couples of spin−orbit peaks. In fact, in SAMs usually only the signal related to metallic-like gold atoms can be observed due to the predominance of bulk atoms with respect to surface atoms that are involved in the S−Au bond. Otherwise, in small nanoparticles such as AuNPs-DEA-4 the surface-to-volume ratio increases, allowing us to observe also

Figure 4. SR-XPS spectra of S 2p and metal (Au 4f, Ag 3d) core levels for samples SAM-DEA, AuNPs-DEA-4, and AgNPs-DEA-4.

As reported in the first row of Figure 4, all S 2p spectra are resolved in six peaks, attributed to three spin−orbit doublets, indicative of three different sulfur species. The B.E. position of S 2p3/2 signal, taken as reference for the S 2p3/2−1/2 spin−orbit pair, usually indicates whether the sulfur atom is covalently bonded to the metal surface. For thiols chemisorbed on metals as well as for thiols covalently bonded to MNP surface, according to literature an S 2p3/2 BE value of nearly 162.4 eV is expected;62,63 on the contrary, S 2p3/2 signals around 163.5 eV are usually assigned to physisorbed thiols or thiolates.64,65 Accordingly, for the S 2p spectra reported in Figure 4 the peak at 162.4 eV BE value is ascribed to sulfur covalently bonded to gold, subsequent to the anchoring of the thiol on the metal atoms on the AuNP surface; the component at higher BE values is assigned to sulfur in the free thiol terminal group of physisorbed molecules on the external AuNP surface. The third signal observable at lower BE values (S 2p3/2 BE value of 161.5 eV) needs some more detailed discussion. H


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NEXAFS C K-edge spectra of SAM-DEA, AuNPs-DEA-4, and AgNPs-DEA-4 recorded at an X-ray incidence angle of 54.7° on the substrate (magic incidence), for which the measured intensity distribution is independent of the molecular orientation, are displayed in the following Figure 5.

the Au 4f signal related to substrate-thiol interface atoms. The bulk atoms signal is still more intense, but the spin−orbit pair at higher BE values (Au 4f7/2 = 84.8 eV) clearly appears as a pronounced shoulder on the measured spectra.72 For Ag 3d spectrum of AgNPs-DEA-4 a completely similar interpretation can be done, as already observed for AgNPs stabilized by organic and organometallic thiol molecules:50 a main signal arising from metallic atoms (Ag 3d5/2 368.0 eV BE) and a secondary pair of spin−orbit components at higher BE, as expected for positively charged metal atoms on NPs surface, interacting with the ligand functionality. The relative contributions of the different S 2p and Au 4f, Ag 3d components, corresponding to relative amounts of S and metal atoms in different configuration (i.e., bulk metal atoms, interface metal atoms interacting with the two kinds of “chemically bonded” S, the two different kinds of S atoms covalently bonded to metal, and free ligands end-groups), can be estimated from the ratio between the respective signal intensities (peak areas, individuated by following a peak-fitting procedure). The calculated intensity ratios, reported in Table SI-3, evidence that both AuNPs-DEA-4 and AgNPs-DEA-4 have a core−shell structure, with DEA molecules chemically bonded to the metal atoms surrounded by a thin overlayer of physisorbed thiols. In more detail, looking at the atomic percent of sulfur species, we observe that the amount of physisorbed ligand (S−H component at ∼163.5 eV) is considerably lower than the chemisorbed one: S−H/S−M (total) is 1/2.9 for AuNPs-DEA-4 and 1/6.1 for AgNPs-DEA-4, suggesting that a compact layer of DEA stabilizes the metallic core and weakly interacts with a thin overlayer of thiol molecules. The analysis of Au LIII edge EXAFS data measured on selected samples gives further details of the average Au atomic structure, NP, size, and Au−S coordination (see below). N 1s and Cl 2p core-level spectra were also collected and analyzed (see Table SI-3 for BEs, fwhm, and relative intensity values). For all samples the N 1s spectrum has a main feature occurring at 399.80 eV, associated with nitrogen atoms belonging to aminic groups. At higher BE values (nearly 401.00 eV BE) a minor component can be observed and attributed to positively charged N atoms, as expected for positively charged amines interacting with Cl− ions, as in R3NH +Cl− moieties.73 Correspondingly, in all chlorine spectra the Cl 2p3/2 main spin orbit component is observed at 197.80 eV, as expected for Cl− ions alike for NH4Cl.74 The atomic ratio between amine-like and charged amine-like nitrogen atoms is about 3/1 in all MNPs-DEA samples, as shown in Table SI-3. N 1s and Cl 2p spectra for ML-DEA and MNPs-DEA-4 samples are reported in Figure SI-8a,b. Near-Edge Absorption Fine Structure Spectroscopy at C K-Edge: Further Insights into the DEA Ligand Molecular Structure. With the aim to further investigate the molecular structure of the DEA ligand interacting with metal NPs, near-edge X-ray absorption fine structure spectroscopy measurements have been performed at C K-edge on AuNPsDEA, AgNPs-DEA, and SAM-DEA. NPs of different metals were compared, an approximation that appears justified by the SR-XPS data, which show a similar behavior for S 2p spectra of different systems: S atoms are the most involved in DEA−metal interaction, and in the first analysis we suppose that the amine groups behavior will be analogous for Au- and Ag-based nanosystems.

Figure 5. NEXAFS C K-edge spectra collected on SAM-DEA, AuNPsDEA-4, and AgNPs-DEA-4 at an X-ray incidence angle of 54.7° on the substrate (magic incidence: no angular dependent effects47).

The peak assignment of the relevant spectral features can be done by comparison with literature data. As a first approach, NEXAFS C K-edge spectra were calibrated (rigid shift of photon energy by ∼1.10 eV for all samples) using literature data on analogous molecules as reference. On this basis, the peak around 285.0 could be associated with the C 1s → π* transition of the graphite impurities on the beamline optics and is used to calibrate the entire spectrum. Molecule-related features appear at photon energy values higher than 286 eV. The first feature, at nearly 286.5 eV, is associated with C 1s → σ* transitions of C−S bonds. The band at 288.5 eV is associated with C 1s → σ* C−H transitions due to the CH2CH3 groups of alkyl side chains, the large broadening at ∼288.7 eV is thought to derive from σ*C−H, and the broad band around 293 eV is assigned to C 1s → σ* C−C excitations of the alkyl groups. AuNPs-DEA spectrum appears different due to the very low intensity of the C 1s → σ* C−H methyl groups feature (288.5 eV). XAS at Au-LIII Edge: A Deeper Insight into the AuNPsDEA Interface Local Structure. XAS is a local chemically selective probe used here to get details of the Au local atomic structure and, in particular, the Au−thiol interactions at the NP surface. In the Figure 6 we show the Au LIII edge EXAFS experimental data and best fit for Au-reference foil and two AuNPs (AuNPs-DEA-10 and AuNPs-DEA-4); the best fit parameters are resumed in Table SI-4. The XAFS spectra of Au-foil and AuNPs are closely similar, but the fine structure oscillations of AuNPs spectra are evidently weaker than Au-foil in the XANES (see Figure SI-9) and in the extended regions (Figure 6). The XAFS signal reduction effect should be ascribed to the reduction of the average coordination number of Au due to the finite size of NPs: The Au atoms close to the NP surface are undercoordinated. The quantitative analysis of Au-LIII edge EXAFS spectra was carried out by a multishell data fitting procedure along the lines described in ref 50, taking into account the Au−Au neighbor I


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assess translational mobility, aggregation equilibrium as a function of pH variations, and the presence of chemisorbed and physisorbed thiol molecules; in particular, ethyl groups on DEA ligands are free to rotate, suggesting a rather loose packing of the thiols on the nanoparticle surface. Dipolar correlations between the ethyl groups of the chemisorbed and physisorbed DEA suggested that physisorbed DEA is placed parallel to the chemisorbed DEA, with its thiol function directed toward the nanoparticle. SR-XPS and NEXAFS studies evidenced the chemical bond between DEA thiol and MNP as well as the thiol/metal interface. Physisorbed and covalently bonded thiols have been identified, and the S 2p3/2 component at 161.5 eV BE can be attributed to sp hybridized S atoms, strongly interacting with the metal atoms on the substrate surface. XAS studies allowed us to define the Au local atomic structure and, in particular, the Au−thiol interactions at the NP surface. The thickness of the functionalizing layer plays a fundamental role in the stability of nanoparticles, and a variation of the local structure as a function of the thickness of the functionalizing layer has been assessed by XAS data. EXAFS data analysis shows that the MNPs have a diameter below 10 nm, in good agreement with microscopy data.

Figure 6. k-weighted Au-LIII edge EXAFS data analysis on Aureference foil (a), AuNPs-DEA-4 (large NP) (b), and AuNPs-DEA-10 (small NP) (c). Experimental data (red symbols) and best fit (black lines) are presented (top curves) along with (vertically shifted) partial contributions of Au−S and Au-fcc structure: AuI to AuIV and AuIV including single (SS) and multiple (MS) scattering contributions (double and triple multiple scattering;50 AuMS describes a multiple scattering path linking two nearest neighbors on adjacent fcc cube faces). The residuals (experimental minus best fit) are also reported for sake of completeness (bottom curves).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b01424. UV−vis, DLS, zeta potential, XPS, and XAS data. (PDF)

contributions up to the fourth fcc coordination shell (Figure 6, Table SI-4). We imposed crystallographic constraints among the parameters to reduce the number of free parameters in the fitting and provide reliable data refinement. This procedure has been first tested on Au-foil EXAFS data to check the reliability and tailor the empirical parameters (S02) and edge energy (E0) that were kept fixed in the analysis of Au-NPs data. The quantitative analysis of AuNPs-DEA data demonstrates a reduction of Au average multiplicity numbers of the several Aufcc coordination shells considered; in addition, a Au−S shell is required (Figure 6, Table SI-4) around 2.3 Å. The multiplicity numbers (Ni) of the neighbors shells are less than in bulk Au (Nibulk) due to under-coordinated atoms close to the NP surface; the loss of coordination relative to the bulk increases roughly linearly with coordination distance, with a slope related to the NP size.50 The EXAFS data analysis (Table SI-4) shows that the AuNPs-DEA-4 have a diameter around 9.0 ± 1.0 nm, while AuNPs-DEA-10 has diameter around 3.6 ± 0.5 nm, in good agreement with microscopy data. The disorder (meansquare relative displacement σ2 parameters) increases for smaller NPs, while Au lattice appears slightly compressed in NPs with respect to the bulk values. We found that the multiplicity of Au−S shell is relatively high (0.3 to 0.7) and cannot be justified assuming only Au−S bonded at sharp NP surfaces; therefore, we must consider an Au−S phase that can originate from a thin shell around the NP Au core (as found in ref 50) or from isolated or very small Au clusters bonded to S. The fraction of Au in Au−S phase is ∼10% in AuNPs-DEA-4, increasing to 20−25% in AuNPs-DEA-10.

ACKNOWLEDGMENTS We gratefully acknowledge the Sapienza University of Rome, Ateneo Sapienza 2016, 2015/C26A15H5J9 and 2015/ C26A15LRMA projects for financial support. We thank HZB for the allocation of neutron/synchrotron radiation beamtime; this project has received funding from the European Union’s Seventh Framework Programme for research, technological development, and demonstration under the NMI3-II Grant number 283883.

CONCLUSIONS AuNPs-DEA and AgNPs-DEA have been prepared with different metal/thiol stoichiometric ratios, 1/4 and 1/10. DLS measurements at pH 2 confirmed dimension lower than 10 nm and monodispersity of the obtained colloids: = 5 ± 1 nm and 8 ± 3 nm for AuNPs and AgNPs. Zeta potential measurements of these colloidal systems verified the stability of functionalized MNPs in water. NMR studies allowed us to

(1) Fang, J.; Zhang, B.; Yao, Q.; Yang, Y.; Xie, J.; Yan, N. Recent Advances in the Synthesis and Catalytic Applications of LigandProtected, Atomically Precise Metal nanoclusters. Coord. Chem. Rev. 2016, 322, 1−29. (2) Saa, L.; Coronado-Puchau, M.; Pavlov, V.; Liz-Marzán, L. M. Enzymatic Etching of Gold Nanorods by Horseradish Peroxidase and Application to Blood Glucose Detection. Nanoscale 2014, 6, 7405− 7409.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (I.F.). *E-mail: [email protected] (C.B.). ORCID

Francesco Porcaro: 0000-0001-6506-1398 Chiara Battocchio: 0000-0003-4590-0865 Ilaria Fratoddi: 0000-0002-5172-0636 Author Contributions

All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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