DOI: 10.1002/chem.201406396
Communication
& Surfaces and Interfaces
Sequential Surface Modification of Au Nanoparticles: From Surface-Bound AgI Complexes to Ag0 Doping Eva-Corinna Fritz,[a] Corinna Nimphius,[a] Albrecht Goez,[b] Sebastian Wrtz,[a] Martin Peterlechner,[c] Johannes Neugebauer,[b] Frank Glorius,*[a] and Bart Jan Ravoo*[a] Abstract: Gold nanoparticles (Au NPs) with tailor-made structures and properties are highly desirable for applications in catalysis and sensing. In this context, surface modifications of Au NPs are of particular relevance. Herein, we present a sequential surface modification of Au NPs with AgI coordination complexes, which can be converted into Ag0-doped Au NPs by simple ligand-exchange reaction. The key innovative element of this surface modification is a multifunctional bioxazoline-based ligand that brings coordinated AgI into close proximity to the particle surface.
It is an elemental goal of materials science to design and synthesize materials with tailored structure and properties and gold nanoparticles (Au NPs) are particularly interesting due to their unique and tunable chemical, electronic, and optical properties.[1–5] The optical properties of Au NPs are dominated by the surface plasmon resonance (SPR), which is induced by light irradiation and results in the oscillation of the conduction band electrons of the particle surface relative to the nuclei.[1, 4, 6, 7] The oscillation frequency, and thus the plasmon absorption, is highly dependent on the composition,[4, 8–10] size,[1, 11] and shape[12, 13] of the NPs, as well as their surface charge[14] and the interparticle distance.[15] The nature of the surface metal atoms has a great influence on the chemical and physical properties of Au NPs, which is why the modification of the particle surface is highly interesting for applications in catalysis[1, 16–18] or sensing.[19–21] More recently, surface modification by the immobilization of metal–ligand complexes intro-
duced a new field, which combines the functionality, chemical stability, and straightforward synthesis of Au NPs with the structural variety and reactivity of coordination chemistry.[22–25] The immobilization of metal complexes on the particle surface can be achieved by multifunctional bridging ligands that simultaneously bind to the surface atoms and coordinate the metal ion(s).[26, 27] The metal complex itself can either pre-assemble before the particle synthesis, replace an already immobilized ligand on the particle surface or assemble after the ligand is immobilized on the particle surface.[22–30] Typically, the immobilized ligand is anchored in such a way that the coordinated metal is located on the surface averted side of the ligand. However, a binding mode that is nearly unexplored coordinates the metal ion at the side of the ligand that is facing the surface.[29] Herein, we present a multivalent ligand design to functionalize Au NPs with AgI complexes, which can optionally undergo a subsequent reduction by simple ligand exchange resulting in a surface confined Ag0 doping of the Au NPs (Figure 1 a). This approach constitutes an alternative strategy to the preparation of core–shell metal nanoparticles by underpotential deposition or seeded growth.[10, 31–33] The ligand enabling the sequential surface modification is designed in such a way that coordinated AgI is brought into close proximity to the particle surface. On a molecular level, a rigid bioxazoline (BIOX)[34] is equipped with four aliphatic thioethers to bind to the Au surface and stabilize the Au NPs. The synthesis of BIOX Au NPs occurs in two steps. First, didodecyl sulfide-stabilized Au NPs (DS Au NPs) were synthesized
[a] E.-C. Fritz, Dr. C. Nimphius, Dr. S. Wrtz, Prof. F. Glorius, Prof. B. J. Ravoo Organisch-Chemisches Institut Westflische Wilhelms-Universitt Mnster Corrensstrasse 40, 48149 Mnster (Germany) E-mail: [email protected] [email protected] [b] A. Goez, Prof. J. Neugebauer Organisch-Chemisches Institut and Center for Multiscale Theory and Computation Westflische Wilhelms-Universitt Mnster Corrensstrasse 40, 48149 Mnster (Germany) [c] Dr. M. Peterlechner Institut fr Materialphysik Westflische Wilhelms-Universitt Mnster Wilhelm-Klemm-Strasse 10, 48149 Mnster (Germany) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201406396. Chem. Eur. J. 2015, 21, 4541 – 4545
Figure 1. (a) Schematic overview of the synthesis and the sequential surface modification of BIOX Au NPs; (b) TEM images; and (c) SPR bands of DS Au NPs (black) in comparison to BIOX Au NPs (red).
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Communication according to the Brust protocol[35] in a ratio of Au/S 1/1.3. According to transmission electron microscopy (TEM) images (Figure 1 b) and dynamic light scattering (DLS) analysis, the obtained DS Au NPs have a spherical shape and display an average core diameter of 4.96 1.27 nm and a hydrodynamic diameter of 5.62 nm in toluene (Figure S14 in the Supporting Information). The minor difference in size can result from the solvation shell around the particle, which is also taken into account in DLS measurements. The maximum absorption of the surface plasmon band of DS Au NPs is located at 525 nm (Figure 1 c). In a subsequent ligand-exchange reaction, the weakly binding DS is replaced by the tetravalent BIOX, which was prepared in a five-step synthesis (see the Supporting Information). Released DS and excess BIOX were removed by centrifugation to give BIOX Au NPs with an average core diameter of 5.62 1.17 nm and a hydrodynamic diameter of 5.62 nm in toluene. The SPR band of BIOX Au NPs is located at 524 nm. A comparison of the size and the optical properties before and after the ligand exchange strongly indicates that the Au NPs are stable throughout the ligand-exchange reaction, and no aggregation occurred. When stored in the fridge as solid or concentrated solution (5 mg mL 1 in toluene), BIOX Au NPs remain stable for weeks. The complete exchange of the ligands was confirmed by 1H NMR spectroscopy, because the obtained chemical shifts and integrals are consistent with the neat BIOX ligand (see the Supporting Information). A proportion of the gold core to the ligand shell of BIOX Au NPs of 57.07 to 42.93 wt % was ob-
tained from thermogravimetric analysis (TGA). According to a literature-known formula,[1] the obtained TEM and TGA data result in an average molecular weight of approximately 1.89 106 g mol 1 per BIOX Au NP, approximately 5500 Au atoms and approximately 800 BIOX ligands per particle. Details concerning the calculation are shown in the Supporting Information. The surface modification by metal coordination of the BIOX Au NPs was achieved by the simple addition of AgI in form of silver nitrate to give AgI@BIOX Au NPs. The surface-bound AgI can be visualized qualitatively by an element line scan of a single particle (Figure 2 a), as well as an elemental mapping by using energy-dispersive X-ray spectroscopy (EDX; Figure S17 in the Supporting Information). Regarding the sample preparation, it should be noted that after adding AgI to the BIOX Au NPs and five minutes incubation time, unbound AgI was removed by centrifugation. TEM images of AgI@BIOX Au NPs display that the addition of AgI did not affect the spherical shape of the BIOX Au NPs (Figure 2 b). Because the surface-coordinated AgI could be detected with TEM, the addition of AgI increased the average core diameter to 6.07 0.95 nm. Both TEM experiments were performed under the same settings, which justifies that the rather small increase of 0.45 nm is significant. On the other hand, the hydrodynamic diameter remains unchanged after AgI coordination, which can be explained due to the fact that the metal coordination occurs inside the BIOX cavity and thus does not affect the solvation shell (Figure S15 in the Supporting Information). Furthermore,
Figure 2. (a) EDX spectroscopic element profile of one representative AgI@BIOX Au NP; (b) TEM image of AgI@BIOX Au NPs; (c) surface plasmon resonance spectrum of AgI binding to a BIOX self-assembled monolayer (red arrow: AgI application, 50 mL min 1, 10 min, 2 mm in milli Q water, green arrow: rinsing with milli Q water); (d) XPS N1 s spectra of BIOX Au NPs (red line) and AgI@BIOX Au NPs (green line); (e) XPS Ag3d spectra of AgI@BIOX Au NPs (green line) in comparison to bulk Ag (grey line) and neat silver nitrate (yellow line); and (f) SPR band of BIOX Au NPs during stepwise AgI coordination (red line: BIOX Au NPs 0.05 mg, green line: final AgI addition of 200 nmol). Chem. Eur. J. 2015, 21, 4541 – 4545
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Communication the binding process of AgI to the BIOX ligand can be monitored in real time by surface plasmon resonance spectroscopy (Figure 2 c). To this end, a self-assembled monolayer (SAM) of BIOX ligands on a planar Au surface was prepared, and a solution of AgI was applied to that surface for ten minutes. The SPR shift can be directly correlated to a binding of AgI to the BIOX ligand, which is stable upon rinsing with pure solvent for at least one hour. The coordination of AgI to the bioxazoline ligand can be monitored by X-ray photoelectron spectroscopy (XPS). Comparing the N1 s XPS spectra before and after AgI addition, a signal shift towards higher binding energies from 399.1 to 399.8 eV can be detected, which can be correlated to AgI withdrawing electron density from the nitrogen atoms (Figure 2 d). Based on the EDX analysis of AgI@BIOX Au NPs, the final number of AgI binding to BIOX Au NPs could be determined. It is important to mention that an excess of AgI compared to the BIOX ligand was added, and remaining unbound AgI was removed before EDX analysis. The spectra were recorded at different sample positions over an estimated surface area of 50 50 nm to give information about the sample homogeneity. A representative spectrum is shown in Figure S18 in the Supporting Information. Resulting from the detected elemental composition of Au and Ag, which amounts to 85.05 atom % Au and 14.95 atom % Ag (each 1.65 atom %), and the previously determined number of Au atoms, the final number of AgI binding per BIOX Au NP is approximately 960 or 1.18 AgI per BIOX ligand. These results clearly support a specific 1:1 coordination of AgI by the BIOX ligand on the particle surface.
XPS also clearly indicated that the oxidation state of AgI remains unchanged upon surface modification. To this end, the Ag3d spectra of AgI@BIOX Au NPs were compared with neat silver nitrate and bulk silver (Figure 2 e). The detected binding energy of the AgI@BIOX Au NPs (374.6 and 368.6 eV) is in very good agreement with the binding energy of neat silver nitrate (374.8 and 368.8 eV). The influence of coordinated AgI on the optical and electronic properties of the BIOX Au NPs was investigated by UV/ Vis spectroscopy and XPS. UV/Vis spectra showed a pronounced enhancement of the surface plasmon band of the BIOX Au NPs upon stepwise AgI addition, although only a minor redshift from 524 to 525 nm was observed (Figure 2 f). Usually, a drastic increase of the plasmon band can be correlated to an increased particle size, which can be ruled out in this case, because TEM and DLS (Figure 2 b and Figure S15 in the Supporting Information, respectively) indicate that AgI coordination causes no considerable changes in the particle size. Furthermore, metal coordination usually results in a pronounced shift of the wavelength but not in the absorbance intensity. Evidently, AgI coordination in close distance to an Au NP surface results in unique and interesting properties. An exact mechanism how AgI affects plasmon absorption and thus the oscillating electron density of the BIOX Au NPs is not known yet. Regarding the electronic properties, XPS spectra of Au4 f with and without AgI coordination revealed a shift towards lower binding energies from 88.0 and 84.3 eV to 87.7 and 84.0 eV (Figure 3 a). Therefore, positively charged AgI caused an increase in electron density of the Au surface.
Figure 3. (a) XPS Au4f spectra of BIOX Au NPs (red line), AgI@BIOX Au NPs (green line), and Ag0@DT Au NP; (b) DFT-optimized structure of the truncated BIOX ligand on an Au(111) surface with AgI placed inside the BIOX cavity; (c) dynamic light scattering of DS Au NPs during stepwise AgI addition (black line: DS Au NPs 0.5 mg, green line: final AgI addition of 500 nmol); (d) EDX element profile of one representative DT Au NP upon AgI addition; and (e) bright field (BF) TEM (left) and corresponding energy-filtered-transmission electron microscopic image (EFTEM, right) of Ag0@DT Au NPs. Chem. Eur. J. 2015, 21, 4541 – 4545
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Communication The experimental findings described above are supported by DFT calculations, which revealed an energetically favored AgI coordination by the BIOX ligand and reduced electron density near the nitrogen atoms upon AgI coordination. First, the adsorption geometry of the BIOX ligand on Au(111) in vacuo was optimized with the four alkyl chains truncated to methyl groups to simplify the calculations. Because the surface curvature of the BIOX Au NPs is negligible in relation to their core size, an optimization on a planar surface was deemed sufficient. Although several surface types are available on NPs, the minimum-energy Au(111) surface can be regarded as exemplary, if not representative. The lowest-energy structure of the adsorbed BIOX ligand found in this study features all four sulfur atoms coordinating in an atop fashion, and the obtained binding energy is 290.7 kJ mol 1 (Figure S26 in the Supporting Information). Starting from this structure, two positions concerning the binding of AgI to the surface of the BIOX Au NP were considered, either inside (Figure 3 b) or outside (Figure S28 in the Supporting Information) the BIOX cavity. Further geometry optimization with AgI resulted in similar distances of the ion to the Au(111) surface in both scenarios, but a pronounced difference in energy of 78.2 kJ mol 1 in favor of AgI coordination to the nitrogen atoms inside the BIOX cavity. Furthermore, atomic partial charges were calculated by using a Hirshfeld analysis. Upon insertion of AgI into the BIOX cavity, decreased electron density was found near the nitrogen atoms (change in partial charges for nitrogen: + 0.025/ + 0.026 electrons). A density increase occurred near the gold atoms spatially close to the AgI binding site (change in partial charges for the three closest gold atoms: 0.015/ 0.027/ 0.028 electrons). This supports the binding energies obtained from the XPS measurements. Note that no solvent effects have been considered in our calculations. Further details concerning the calculations can be found in the Supporting Information. Reference experiments applying Au NPs not providing a ligand equipped with a metal docking site showed neither an enhancement of the plasmon absorption upon AgI addition, nor a binding of AgI to the Au surface, which supports the relevance of our ligand design for a specific surface functionalization by metal coordination. Two types of reference Au NPs stabilized by densely packed aliphatic thioethers as weak binding ligands (dodecyl sulfide, DS Au NPs) and thiols as strong binding ligands (1-dodecanethiol, DT Au NPs) were analyzed. Upon AgI addition, the DS Au NPs aggregate within few minutes, which can be observed even by eye due to the significant color change from a red brown solution to grey precipitates, as well as with DLS (Figure 3 c) and UV/Vis spectroscopy (Figure S10 in the Supporting Information). Apparently, unlike the BIOX ligand, the DS ligand is too weak to shield the Au surface from AgI, which is known to undergo metal exchange with surface atoms resulting in particle aggregation.[19] The DT Au NPs remain stable in the presence of AgI, and although a slight influence on the plasmon absorption was visible (Figure S11 in the Supporting Information), EDX investigations can clearly exclude any AgI binding to the particles (Figure 3 d). TEM images before and after the AgI addition to DT Au NP are shown in the Supporting Information. Chem. Eur. J. 2015, 21, 4541 – 4545
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Starting from the AgI complexes on the surface of BIOX Au NPs, a reductive ligand-exchange reaction can be performed to give a Ag0 doping of Au NPs. The doping procedure merely involves the addition of an excess of DT to AgI@BIOX Au NPs resulting in both the ligand replacement of the BIOX ligand by the stronger binding thiol and the Ag reduction giving Ag0@DT Au NPs. No additional reducing agent was necessary to obtain the Ag0 doping. The close proximity of coordinated AgI to the Au surface with increased electron density is sufficient to enable a simultaneous reduction during ligand exchange by DT. The exchange reaction can be monitored by 1 H NMR. The binding energy of Ag3d of the Ag0@DT Au NPs (374.1 and 368.1 eV) measured by XPS is in accordance with the binding energy of bulk silver (374.3 and 368.3 eV, see Figure S21 in the Supporting Information). Regarding the electronic properties of Ag0@DS Au NPs, the initial binding energy of the Au4f signal of 88.1 and 84.4 eV is regained after Ag0 reduction (Figure 3 b). According to TEM images, the average core diameter of the Ag0@DT Au NP is 5.81 1.02 nm, which is slightly but not significantly smaller compared with the AgI@BIOX Au NP (Figure S7 in the Supporting Information). Regarding the optical properties, the addition of DT to the AgI@BIOX Au NPs immediately shifted the position of the plasmon band from 524 to 527 nm, which becomes constant at 538 nm after one day, when the ligand exchange reaction was completed (Figure S13 in the Supporting Information). Energy filtered TEM (EFTEM) images of the Ag0@DT Au NPs showed a fairly homogeneous Ag distribution across the particle surface, which supports that no clustering of Ag occurred during the reduction, and the final surface modification resulted in a surface confined Ag0 doping (Figure 3 e). In conclusion, based on the design of a multifunctional BIOX ligand, we demonstrated a simple method for a two-step postsynthetic surface modification of Au NPs by AgI coordination and surface confined Ag0 doping. AgI coordination occurred at close distance to the particle surface, so that the surface plasmon band was significantly enhanced: a phenomenon, which is reported herein for the first time. Our concept also allows a subsequent reductive ligand exchange by a stronger binding ligand, which results in a homogeneously distributed Ag0 doping of the Au NPs. It is hoped that the ligand-directed metal doping described herein may give rise to tailor-made NPs for catalysis and sensing.
Experimental Section The synthesis of the BIOX ligand, DS Au NPs, BIOX Au NPs, and DT Au NPs, as well as the particle characterization by UV/Vis, TEM, and DLS techniques, are described in the Supporting Information. Furthermore, metal coordination experiments giving AgI@BIOX Au NPs and the reference experiments by using DS Au NPs and DT Au NPs investigated by UV/Vis, DLS, EDX, XPS, and NMR methods can be found in the Supporting Information. Details regarding the DFT calculations are also listed in the Supporting Information.
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Communication Acknowledgements We gratefully acknowledge the Deutsche Forschungsgemeinschaft (SFB 858) for financial support. Keywords: doping · gold · metal coordination · nanoparticles · surface modification [1] M. C. Daniel, D. Astruc, Chem. Rev. 2004, 104, 293 – 346. [2] S. Link, M. A. El-Sayed, J. Phys. Chem. B 1999, 103, 8410 – 8426. [3] C. J. Murphy, T. K. Sau, A. M. Gole, C. J. Orendorff, J. Gao, L. Gou, S. E. Hunyadi, T. Li, J. Phys. Chem. B 2005, 109, 13857 – 13870. [4] L. M. Liz-Marzn, Langmuir 2006, 22, 32 – 41. [5] R. Sardar, A. M. Funston, P. Mulvaney, R. W. Murray, Langmuir 2009, 25, 13840 – 13851. [6] K. Saha, S. S. Agasti, C. Kim, X. Li, V. M. Rotello, Chem. Rev. 2012, 112, 2739 – 2779. [7] K. L. Kelly, E. Coronado, L. L. Zhao, G. C. Schatz, J. Phys. Chem. B 2003, 107, 668 – 677. [8] S. Link, Z. L. Wang, M. A. El-Sayed, J. Phys. Chem. B 1999, 103, 3529 – 3533. [9] P. Mulvaney, M. Giersig, A. Henglein, J. Phys. Chem. 1992, 96, 10419 – 10424. [10] M. Chirea, S. S. E. Collins, X. Wei, P. Mulvaney, J. Phys. Chem. Lett. 2014, 5, 4331 – 4335. [11] P. N. Njoki, I. I. S. Lim, D. Mott, H. Y. Park, B. Khan, S. Mishra, R. Sujakumar, J. Luo, C.-J. Zhong, J. Phys. Chem. C 2007, 111, 14664 – 14669. [12] Y. Xia, N. J. Halas, MRS Bull. 2005, 30, 338 – 348. [13] I. O. Sosa, C. Noguez, R. G. Barrera, J. Phys. Chem. B 2003, 107, 6269 – 6275. [14] X. Qu, Z. Peng, X. Jiang, S. Dong, Langmuir 2004, 20, 2519 – 2522. [15] H. Lange, B. H. Jurez, A. Carl, M. Richter, N. G. Bastffls, H. Weller, C. Thomsen, R. von Klitzing, A. Knorr, Langmuir 2012, 28, 8862 – 8866. [16] Nanoparticles and Catalysis (Ed. Didier Astruc), Wiley-VCH, Weinheim, 2008.
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[17] P. Hervs, M. Prez-Lorenzo, L. M. Liz-Marzn, J. Dzubiella, Y. Lub, M. Ballauff, Chem. Soc. Rev. 2012, 41, 5577 – 5587. [18] C. Richter, K. Schaepe, F. Glorius, B. J. Ravoo, Chem. Commun. 2014, 50, 3204 – 3207. [19] A. Labande, J. Ruiz, D. Astruc, J. Am. Chem. Soc. 2002, 124, 1782 – 1789. [20] P. D. Beer, D. P. Cormode, J. J. Davis, Chem. Commun. 2004, 414 – 415. [21] S. Y. Lin, S. W. Liu, C. M. Lin, C. Chen, Anal. Chem. 2002, 74, 330 – 335. [22] J. D. E. T. Wilton-Ely, Dalton Trans. 2008, 25 – 29. [23] E. R. Knight, N. H. Leung, A. L. Thompson, G. Hogarth, J. D. E. T. WiltonEly, Inorg. Chem. 2009, 48, 3866 – 3874. [24] H. X. Liu, X. He, L. Zhao, Chem. Commun. 2014, 50, 971 – 974. [25] F. C. M. Leung, A. Y. Y. Tam, V. K. M. Au, M. J. Li, V. W. W. Yam, ACS Appl. Mater. Interfaces 2014, 6, 6644 – 6653. [26] H. E. Toma, V. M. Zamarion, S. H. Toma, K. Araki, J. Braz. Chem. Soc. 2010, 21, 1158 – 1176. [27] D. J. Lewis, Z. Pikramenou, Coord. Chem. Rev. 2014, 273 – 274, 213 – 225. [28] S. Wang, W. S. Sim, Langmuir 2006, 22, 7861 – 7866. [29] D. J. Lewis, T. M. Day, J. V. MacPherson, Z. Pikramenou, Chem. Commun. 2006, 1433 – 1435. [30] M. Friederici, I. Angurell, M. Seco, O. Rossell, J. Llorca, Dalton Trans. 2011, 40, 7934 – 7940. [31] M. L. Personick, M. R. Langille, J. Zhang, C. A. Mirkin, Nano Lett. 2011, 11, 3394 – 3398. [32] B. Rodrguez-Gonzlez, A. Burrows, M. Watanabe, C. J. Kiely, L. M. LizMarzan, J. Mater. Chem. 2005, 15, 1755 – 1759. [33] A. K. Samal, L. Polavarapu, S. Rodal-Cedeira, L. M. Liz-Marzan, J. PerezJuste, I. Pastoriza-Santos, Langmuir 2013, 29, 15076 – 15028. [34] For the related family of bioxazolines and N-heterocyclic carbenes derived thereof, see: a) G. Altenhoff, R. Goddard, C. W. Lehmann, F. Glorius, Angew. Chem. Int. Ed. 2003, 42, 3690 – 3693; Angew. Chem. 2003, 115, 3818 – 3821; b) S. Wrtz, C. Lohre, R. Frçhlich, K. Bergander, F. Glorius, J. Am. Chem. Soc. 2009, 131, 8344 – 8345. [35] M. Brust, M. Walker, D. Bethell, D. J. Schiffrin, R. Whyman, J. Chem. Soc. Chem. Commun. 1994, 801 – 802. Received: December 8, 2014 Published online on February 4, 2015
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Communication
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Sequential Surface Modification of Au Nanoparticles: From Surface-Bound AgI Complexes to Ag0 Doping Eva-Corinna Fritz,[a] Corinna Nimphius,[a] Albrecht Goez,[b] Sebastian Wrtz,[a] Martin Peterlechner,[c] Johannes Neugebauer,[b] Frank Glorius,*[a] and Bart Jan Ravoo*[a] Abstract: Gold nanoparticles (Au NPs) with tailor-made structures and properties are highly desirable for applications in catalysis and sensing. In this context, surface modifications of Au NPs are of particular relevance. Herein, we present a sequential surface modification of Au NPs with AgI coordination complexes, which can be converted into Ag0-doped Au NPs by simple ligand-exchange reaction. The key innovative element of this surface modification is a multifunctional bioxazoline-based ligand that brings coordinated AgI into close proximity to the particle surface.
It is an elemental goal of materials science to design and synthesize materials with tailored structure and properties and gold nanoparticles (Au NPs) are particularly interesting due to their unique and tunable chemical, electronic, and optical properties.[1–5] The optical properties of Au NPs are dominated by the surface plasmon resonance (SPR), which is induced by light irradiation and results in the oscillation of the conduction band electrons of the particle surface relative to the nuclei.[1, 4, 6, 7] The oscillation frequency, and thus the plasmon absorption, is highly dependent on the composition,[4, 8–10] size,[1, 11] and shape[12, 13] of the NPs, as well as their surface charge[14] and the interparticle distance.[15] The nature of the surface metal atoms has a great influence on the chemical and physical properties of Au NPs, which is why the modification of the particle surface is highly interesting for applications in catalysis[1, 16–18] or sensing.[19–21] More recently, surface modification by the immobilization of metal–ligand complexes intro-
duced a new field, which combines the functionality, chemical stability, and straightforward synthesis of Au NPs with the structural variety and reactivity of coordination chemistry.[22–25] The immobilization of metal complexes on the particle surface can be achieved by multifunctional bridging ligands that simultaneously bind to the surface atoms and coordinate the metal ion(s).[26, 27] The metal complex itself can either pre-assemble before the particle synthesis, replace an already immobilized ligand on the particle surface or assemble after the ligand is immobilized on the particle surface.[22–30] Typically, the immobilized ligand is anchored in such a way that the coordinated metal is located on the surface averted side of the ligand. However, a binding mode that is nearly unexplored coordinates the metal ion at the side of the ligand that is facing the surface.[29] Herein, we present a multivalent ligand design to functionalize Au NPs with AgI complexes, which can optionally undergo a subsequent reduction by simple ligand exchange resulting in a surface confined Ag0 doping of the Au NPs (Figure 1 a). This approach constitutes an alternative strategy to the preparation of core–shell metal nanoparticles by underpotential deposition or seeded growth.[10, 31–33] The ligand enabling the sequential surface modification is designed in such a way that coordinated AgI is brought into close proximity to the particle surface. On a molecular level, a rigid bioxazoline (BIOX)[34] is equipped with four aliphatic thioethers to bind to the Au surface and stabilize the Au NPs. The synthesis of BIOX Au NPs occurs in two steps. First, didodecyl sulfide-stabilized Au NPs (DS Au NPs) were synthesized
[a] E.-C. Fritz, Dr. C. Nimphius, Dr. S. Wrtz, Prof. F. Glorius, Prof. B. J. Ravoo Organisch-Chemisches Institut Westflische Wilhelms-Universitt Mnster Corrensstrasse 40, 48149 Mnster (Germany) E-mail: [email protected] [email protected] [b] A. Goez, Prof. J. Neugebauer Organisch-Chemisches Institut and Center for Multiscale Theory and Computation Westflische Wilhelms-Universitt Mnster Corrensstrasse 40, 48149 Mnster (Germany) [c] Dr. M. Peterlechner Institut fr Materialphysik Westflische Wilhelms-Universitt Mnster Wilhelm-Klemm-Strasse 10, 48149 Mnster (Germany) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201406396. Chem. Eur. J. 2015, 21, 4541 – 4545
Figure 1. (a) Schematic overview of the synthesis and the sequential surface modification of BIOX Au NPs; (b) TEM images; and (c) SPR bands of DS Au NPs (black) in comparison to BIOX Au NPs (red).
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Communication according to the Brust protocol[35] in a ratio of Au/S 1/1.3. According to transmission electron microscopy (TEM) images (Figure 1 b) and dynamic light scattering (DLS) analysis, the obtained DS Au NPs have a spherical shape and display an average core diameter of 4.96 1.27 nm and a hydrodynamic diameter of 5.62 nm in toluene (Figure S14 in the Supporting Information). The minor difference in size can result from the solvation shell around the particle, which is also taken into account in DLS measurements. The maximum absorption of the surface plasmon band of DS Au NPs is located at 525 nm (Figure 1 c). In a subsequent ligand-exchange reaction, the weakly binding DS is replaced by the tetravalent BIOX, which was prepared in a five-step synthesis (see the Supporting Information). Released DS and excess BIOX were removed by centrifugation to give BIOX Au NPs with an average core diameter of 5.62 1.17 nm and a hydrodynamic diameter of 5.62 nm in toluene. The SPR band of BIOX Au NPs is located at 524 nm. A comparison of the size and the optical properties before and after the ligand exchange strongly indicates that the Au NPs are stable throughout the ligand-exchange reaction, and no aggregation occurred. When stored in the fridge as solid or concentrated solution (5 mg mL 1 in toluene), BIOX Au NPs remain stable for weeks. The complete exchange of the ligands was confirmed by 1H NMR spectroscopy, because the obtained chemical shifts and integrals are consistent with the neat BIOX ligand (see the Supporting Information). A proportion of the gold core to the ligand shell of BIOX Au NPs of 57.07 to 42.93 wt % was ob-
tained from thermogravimetric analysis (TGA). According to a literature-known formula,[1] the obtained TEM and TGA data result in an average molecular weight of approximately 1.89 106 g mol 1 per BIOX Au NP, approximately 5500 Au atoms and approximately 800 BIOX ligands per particle. Details concerning the calculation are shown in the Supporting Information. The surface modification by metal coordination of the BIOX Au NPs was achieved by the simple addition of AgI in form of silver nitrate to give AgI@BIOX Au NPs. The surface-bound AgI can be visualized qualitatively by an element line scan of a single particle (Figure 2 a), as well as an elemental mapping by using energy-dispersive X-ray spectroscopy (EDX; Figure S17 in the Supporting Information). Regarding the sample preparation, it should be noted that after adding AgI to the BIOX Au NPs and five minutes incubation time, unbound AgI was removed by centrifugation. TEM images of AgI@BIOX Au NPs display that the addition of AgI did not affect the spherical shape of the BIOX Au NPs (Figure 2 b). Because the surface-coordinated AgI could be detected with TEM, the addition of AgI increased the average core diameter to 6.07 0.95 nm. Both TEM experiments were performed under the same settings, which justifies that the rather small increase of 0.45 nm is significant. On the other hand, the hydrodynamic diameter remains unchanged after AgI coordination, which can be explained due to the fact that the metal coordination occurs inside the BIOX cavity and thus does not affect the solvation shell (Figure S15 in the Supporting Information). Furthermore,
Figure 2. (a) EDX spectroscopic element profile of one representative AgI@BIOX Au NP; (b) TEM image of AgI@BIOX Au NPs; (c) surface plasmon resonance spectrum of AgI binding to a BIOX self-assembled monolayer (red arrow: AgI application, 50 mL min 1, 10 min, 2 mm in milli Q water, green arrow: rinsing with milli Q water); (d) XPS N1 s spectra of BIOX Au NPs (red line) and AgI@BIOX Au NPs (green line); (e) XPS Ag3d spectra of AgI@BIOX Au NPs (green line) in comparison to bulk Ag (grey line) and neat silver nitrate (yellow line); and (f) SPR band of BIOX Au NPs during stepwise AgI coordination (red line: BIOX Au NPs 0.05 mg, green line: final AgI addition of 200 nmol). Chem. Eur. J. 2015, 21, 4541 – 4545
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Communication the binding process of AgI to the BIOX ligand can be monitored in real time by surface plasmon resonance spectroscopy (Figure 2 c). To this end, a self-assembled monolayer (SAM) of BIOX ligands on a planar Au surface was prepared, and a solution of AgI was applied to that surface for ten minutes. The SPR shift can be directly correlated to a binding of AgI to the BIOX ligand, which is stable upon rinsing with pure solvent for at least one hour. The coordination of AgI to the bioxazoline ligand can be monitored by X-ray photoelectron spectroscopy (XPS). Comparing the N1 s XPS spectra before and after AgI addition, a signal shift towards higher binding energies from 399.1 to 399.8 eV can be detected, which can be correlated to AgI withdrawing electron density from the nitrogen atoms (Figure 2 d). Based on the EDX analysis of AgI@BIOX Au NPs, the final number of AgI binding to BIOX Au NPs could be determined. It is important to mention that an excess of AgI compared to the BIOX ligand was added, and remaining unbound AgI was removed before EDX analysis. The spectra were recorded at different sample positions over an estimated surface area of 50 50 nm to give information about the sample homogeneity. A representative spectrum is shown in Figure S18 in the Supporting Information. Resulting from the detected elemental composition of Au and Ag, which amounts to 85.05 atom % Au and 14.95 atom % Ag (each 1.65 atom %), and the previously determined number of Au atoms, the final number of AgI binding per BIOX Au NP is approximately 960 or 1.18 AgI per BIOX ligand. These results clearly support a specific 1:1 coordination of AgI by the BIOX ligand on the particle surface.
XPS also clearly indicated that the oxidation state of AgI remains unchanged upon surface modification. To this end, the Ag3d spectra of AgI@BIOX Au NPs were compared with neat silver nitrate and bulk silver (Figure 2 e). The detected binding energy of the AgI@BIOX Au NPs (374.6 and 368.6 eV) is in very good agreement with the binding energy of neat silver nitrate (374.8 and 368.8 eV). The influence of coordinated AgI on the optical and electronic properties of the BIOX Au NPs was investigated by UV/ Vis spectroscopy and XPS. UV/Vis spectra showed a pronounced enhancement of the surface plasmon band of the BIOX Au NPs upon stepwise AgI addition, although only a minor redshift from 524 to 525 nm was observed (Figure 2 f). Usually, a drastic increase of the plasmon band can be correlated to an increased particle size, which can be ruled out in this case, because TEM and DLS (Figure 2 b and Figure S15 in the Supporting Information, respectively) indicate that AgI coordination causes no considerable changes in the particle size. Furthermore, metal coordination usually results in a pronounced shift of the wavelength but not in the absorbance intensity. Evidently, AgI coordination in close distance to an Au NP surface results in unique and interesting properties. An exact mechanism how AgI affects plasmon absorption and thus the oscillating electron density of the BIOX Au NPs is not known yet. Regarding the electronic properties, XPS spectra of Au4 f with and without AgI coordination revealed a shift towards lower binding energies from 88.0 and 84.3 eV to 87.7 and 84.0 eV (Figure 3 a). Therefore, positively charged AgI caused an increase in electron density of the Au surface.
Figure 3. (a) XPS Au4f spectra of BIOX Au NPs (red line), AgI@BIOX Au NPs (green line), and Ag0@DT Au NP; (b) DFT-optimized structure of the truncated BIOX ligand on an Au(111) surface with AgI placed inside the BIOX cavity; (c) dynamic light scattering of DS Au NPs during stepwise AgI addition (black line: DS Au NPs 0.5 mg, green line: final AgI addition of 500 nmol); (d) EDX element profile of one representative DT Au NP upon AgI addition; and (e) bright field (BF) TEM (left) and corresponding energy-filtered-transmission electron microscopic image (EFTEM, right) of Ag0@DT Au NPs. Chem. Eur. J. 2015, 21, 4541 – 4545
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Communication The experimental findings described above are supported by DFT calculations, which revealed an energetically favored AgI coordination by the BIOX ligand and reduced electron density near the nitrogen atoms upon AgI coordination. First, the adsorption geometry of the BIOX ligand on Au(111) in vacuo was optimized with the four alkyl chains truncated to methyl groups to simplify the calculations. Because the surface curvature of the BIOX Au NPs is negligible in relation to their core size, an optimization on a planar surface was deemed sufficient. Although several surface types are available on NPs, the minimum-energy Au(111) surface can be regarded as exemplary, if not representative. The lowest-energy structure of the adsorbed BIOX ligand found in this study features all four sulfur atoms coordinating in an atop fashion, and the obtained binding energy is 290.7 kJ mol 1 (Figure S26 in the Supporting Information). Starting from this structure, two positions concerning the binding of AgI to the surface of the BIOX Au NP were considered, either inside (Figure 3 b) or outside (Figure S28 in the Supporting Information) the BIOX cavity. Further geometry optimization with AgI resulted in similar distances of the ion to the Au(111) surface in both scenarios, but a pronounced difference in energy of 78.2 kJ mol 1 in favor of AgI coordination to the nitrogen atoms inside the BIOX cavity. Furthermore, atomic partial charges were calculated by using a Hirshfeld analysis. Upon insertion of AgI into the BIOX cavity, decreased electron density was found near the nitrogen atoms (change in partial charges for nitrogen: + 0.025/ + 0.026 electrons). A density increase occurred near the gold atoms spatially close to the AgI binding site (change in partial charges for the three closest gold atoms: 0.015/ 0.027/ 0.028 electrons). This supports the binding energies obtained from the XPS measurements. Note that no solvent effects have been considered in our calculations. Further details concerning the calculations can be found in the Supporting Information. Reference experiments applying Au NPs not providing a ligand equipped with a metal docking site showed neither an enhancement of the plasmon absorption upon AgI addition, nor a binding of AgI to the Au surface, which supports the relevance of our ligand design for a specific surface functionalization by metal coordination. Two types of reference Au NPs stabilized by densely packed aliphatic thioethers as weak binding ligands (dodecyl sulfide, DS Au NPs) and thiols as strong binding ligands (1-dodecanethiol, DT Au NPs) were analyzed. Upon AgI addition, the DS Au NPs aggregate within few minutes, which can be observed even by eye due to the significant color change from a red brown solution to grey precipitates, as well as with DLS (Figure 3 c) and UV/Vis spectroscopy (Figure S10 in the Supporting Information). Apparently, unlike the BIOX ligand, the DS ligand is too weak to shield the Au surface from AgI, which is known to undergo metal exchange with surface atoms resulting in particle aggregation.[19] The DT Au NPs remain stable in the presence of AgI, and although a slight influence on the plasmon absorption was visible (Figure S11 in the Supporting Information), EDX investigations can clearly exclude any AgI binding to the particles (Figure 3 d). TEM images before and after the AgI addition to DT Au NP are shown in the Supporting Information. Chem. Eur. J. 2015, 21, 4541 – 4545
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Starting from the AgI complexes on the surface of BIOX Au NPs, a reductive ligand-exchange reaction can be performed to give a Ag0 doping of Au NPs. The doping procedure merely involves the addition of an excess of DT to AgI@BIOX Au NPs resulting in both the ligand replacement of the BIOX ligand by the stronger binding thiol and the Ag reduction giving Ag0@DT Au NPs. No additional reducing agent was necessary to obtain the Ag0 doping. The close proximity of coordinated AgI to the Au surface with increased electron density is sufficient to enable a simultaneous reduction during ligand exchange by DT. The exchange reaction can be monitored by 1 H NMR. The binding energy of Ag3d of the Ag0@DT Au NPs (374.1 and 368.1 eV) measured by XPS is in accordance with the binding energy of bulk silver (374.3 and 368.3 eV, see Figure S21 in the Supporting Information). Regarding the electronic properties of Ag0@DS Au NPs, the initial binding energy of the Au4f signal of 88.1 and 84.4 eV is regained after Ag0 reduction (Figure 3 b). According to TEM images, the average core diameter of the Ag0@DT Au NP is 5.81 1.02 nm, which is slightly but not significantly smaller compared with the AgI@BIOX Au NP (Figure S7 in the Supporting Information). Regarding the optical properties, the addition of DT to the AgI@BIOX Au NPs immediately shifted the position of the plasmon band from 524 to 527 nm, which becomes constant at 538 nm after one day, when the ligand exchange reaction was completed (Figure S13 in the Supporting Information). Energy filtered TEM (EFTEM) images of the Ag0@DT Au NPs showed a fairly homogeneous Ag distribution across the particle surface, which supports that no clustering of Ag occurred during the reduction, and the final surface modification resulted in a surface confined Ag0 doping (Figure 3 e). In conclusion, based on the design of a multifunctional BIOX ligand, we demonstrated a simple method for a two-step postsynthetic surface modification of Au NPs by AgI coordination and surface confined Ag0 doping. AgI coordination occurred at close distance to the particle surface, so that the surface plasmon band was significantly enhanced: a phenomenon, which is reported herein for the first time. Our concept also allows a subsequent reductive ligand exchange by a stronger binding ligand, which results in a homogeneously distributed Ag0 doping of the Au NPs. It is hoped that the ligand-directed metal doping described herein may give rise to tailor-made NPs for catalysis and sensing.
Experimental Section The synthesis of the BIOX ligand, DS Au NPs, BIOX Au NPs, and DT Au NPs, as well as the particle characterization by UV/Vis, TEM, and DLS techniques, are described in the Supporting Information. Furthermore, metal coordination experiments giving AgI@BIOX Au NPs and the reference experiments by using DS Au NPs and DT Au NPs investigated by UV/Vis, DLS, EDX, XPS, and NMR methods can be found in the Supporting Information. Details regarding the DFT calculations are also listed in the Supporting Information.
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