Electrostatic Assembly of Functional and Macromolecular Ferricinium

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Electrostatic Assembly of Functional and Macromolecular Ferricinium Chloride-Stabilized Gold Nanoparticles Roberto Ciganda,†,‡ Haibin Gu,§ Ricardo Hernandez,*,‡ Ane Escobar,∥ Angel Martínez,∥ Luis Yates,∥ Sergio Moya,∥ Jaime Ruiz,† and Didier Astruc*,† †

ISM, UMR 5255, University of Bordeaux, Talence 33405 Cedex, France Facultad de Quimica, Universidad del Pais Vasco, Apdo 1072, 20080 San Sebastian, Spain § Key Laboratory of Leather Chemistry and Engineering of Ministry of Education, Sichuan University, Chengdu 610065, P. R. China ∥ CICbiomaGUNE Unidad Biosupeficies, Paseo Miramon 182 Edif C, 20009 San Sebastian, Spain ‡

S Supporting Information *

ABSTRACT: Substituted ferrocenes with various stereoelectronic effects including a ferrocene-terminated dendrimer in ether reduce aqueous HAuCl4 to gold nanoparticles (AuNPs) by interfacial electron transfer. The dependence on the stirring speed plays a crucial role, and the stereoelectronic influences on the reaction rates are dramatic. With a ferrocene-containing polymer, the reaction is conducted using an homogeneous THF/water medium, also forming AuNPs. Fully stable functional, dendritic and polymeric ferricinium chloride-stabilized AuNPs are obtained with core sizes between 13 and 35 nm, an optimal size range for potential biomedical applications. Finally the ferricinium coating of the Au nanoparticles is replaced by a more electron-rich ferricinium derivative by exergonic redox reaction with the corresponding ferrocene derivative.

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INTRODUCTION The interaction between macromolecules and gold nanoparticles (AuNPs) has attracted considerable attention from nanoscience researchers in particular for nanomaterials,1−5 biomedical,6−9 plasmon-related optical,10−13 and catalytic applications.14−17 Indeed among the modes of synthesis of AuNPs,2,18−24 both the Turkevitch−Frens synthesis with citrate as Au(III) reductant and stabilizer18−20 and Brust−Schiffrin method using the NaBH4 reductant and a thiolate stabilizer22−24 that are the currently used methods most often involve macromolecular stabilizers. Many other reductants are known, and the AuNP stabilization mode generally involves macromolecules combined with AuNP coordination.2 Reduction of Au(III) by neutral single-electron transfer reagents25−30 has rarely been used in AuNP synthesis.31 Such redox reagents are stable under both redox forms and behave as electron reservoirs32−34 to provide electrostatic stabilization. Ferrocene derivatives are typical examples of readily available reagents that are stable under both oxidized and reduced forms. The reduced Fe(II) form can be oxidized by Au(III) and contribute to the stabilization of AuNPs as parts of thiol stabilizers or otherwise either under the ferrocene or ferricinium form.35−40 In the present Article, various substituted, more or less bulky ferrocenes including a polyferrocene-terminated dendrimer and a ferrocene-containing polymer41 are compared as reducing agents of Au(III) and AuNP stabilizers. The results show dramatic stereoelectronic effects on Au(III) and an excellent electrostatic stabilization of large AuNPs by the ferricinium, substituted ferricinium, and dendritic and polymeric ferricinium derivatives. Consequences of these results include potential © XXXX American Chemical Society

applications of AuNP syntheses of appropriate size for biomedical and sensing uses. In a recent communication, it was indicated that ferrocene (Fc or FcH) in ether reduced HAuCl4 in water at room temperature or 1 °C upon stirring in air at low ferrocene and Au III concentrations.40 In the present article, this reaction is extended to various substituted ferrocenes in order to investigate stereoelectronic effects on the feasibility, rate and conditions of the reaction including new interferrocene redox chemistry. This extension includes macromolecular ferrocenecontaining materials, i.e. a ferrocene-terminated dendrimer and a ferrocene-containing polymer. First let us start by briefly recalling the main aspects of the reaction of ferrocene itself that led to the formation of ferricinium chloride-stabilized AuNPs. Transmission Electron Microscopy (TEM) of these AuNPs showed quite large core size of 16−19 nm, which corresponds to an average of 18 000 Au atoms in each AuNP core (Figure 1). Its UV−vis spectrum showed both the surface plasmon band (SPB) at 545 nm characteristic of AuNPs2 and ferricinium absorption at 600 nm42 (Figure 2). This stoichiometric reaction of aqueous HAuCl4 with 3 equiv of ferrocene (eq 1) resulted from interfacial exergonic electron transfer between AuIII (E0 AuIII/Au0 = 0.93 V vs H+/H2; 0.76 V vs SCE)43 and ferrocene (E0 FeIII/FeII = 0.545 V vs FeCp*2; 0.475 vs SCE):29,30,44,45 Received: December 5, 2016

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DOI: 10.1021/acs.inorgchem.6b02850 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. TEM of ferricinium chloride-stabilized AuNPs.

Figure 3. XPS spectrum of the ferricinium chloride-stabilized AuNPs.

clean reaction monitored by UV−vis. spectroscopy with an isosbestic point. This reaction proceeded with modest efficiency due to the large AuNP size, however.52−56 Addition of water-soluble thiols yielded the thiolated (RS)n AuNPs with a new SPB at 534 nm (R = PEG 550) respectively 541 nm (RS = L-cysteinyl) with the unchanged core size of 26 nm. When the reaction between ferrocene in ether and aqueous HAuCl4 was carried out in the presence of poly(ethylene glycol) (PEG 5000) or poly(N-vinylmethylpyrrolidone), TEM showed that the AuNPs had a core size of 25 ± 3 nm (or only 13 ± 3 nm).

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RESULTS AND DISCUSSION Reaction of Ferrocene with HAuCl4. We have now investigated the substrate concentration effect on the reaction and found that, when the ferrocene and HAuCl4 concentrations were increased ([HAuCl4·3H2O] > 10−4 mol/L) in view of conducting a higher-scale reaction, the AuNPs precipitated and agglomerated. Meanwhile, however, the concentration could be enhanced 5-fold without precipitation/agglomeration when the reaction was conducted without stirring at rt. This concentration increase of the reactants under analogous conditions without agglomeration reached 10-fold at 1 °C. Thus, an orange solution containing 5 mg ferrocene in 5 mL ether reacted completely with the colorless solution of 3.5 mg HAuCl4·3H2O in 15 m water (i.e., ([HAuCl4·3H2O] = 10−3 mol/L) according to the stoichiometric reaction of 3 equiv ferrocene per mol HAuCl4·3H2O at rt in 30 min without precipitation or agglomeration to provide a colorless ether solution and a purple AuNP solution in water. The reaction at 1 °C with 10 mg ferrocene with 7 mg of AuCl4·3H2O in the same amount of solvent took 2 h to reach completion, and no precipitation or agglomeration was observed. The surface plasmon band (SPB)2 was found at 547 nm for AuNPs obtained at rt and 542 nm for the AuNPs obtained at 1 °C. In both cases, the ferricinium band was observed at 600 nm.42 The water from this purple AuNPs was evaporated to dryness, and the solid residue redissolved in water without agglomeration or change of the SPB position, indicating the robustness of these AuNPs. The reaction was also extended to a monophasic system when the aqueous HAuCl4 solution was added to a THF solution of ferrocene. In this case, the AuNP formed in 30 s seemingly with some agglomeration/precipitation witnessed by the appearance of a black suspension. In this case, however,

Figure 2. UV−vis spectrum of the ferricinium-stabilized AuNPs showing the surface plasmon band at 545 nm and the ferricinium absorption at 600 nm.

n[H+aqAuCl4 −] + 3nFc → [3nFc+, 4nCl−, nH+aq , Au n] (1)

The pH of the aqueous HAuCl4 solution increased from 2 before the reaction to 5 afterward in the case ferrocene. Consequently, it was suggested that an average of 18000 hydronium cations were trapped by the electron-rich AuNP surface, the 72000 chloride anions being weakly coordinated to this AuNP core. The steric protection of the AuNP core inhibiting aggregation was ensured by an average of 54000 ferricinium cations around the core. X-ray photoelectron microscopy (XPS) indicated that the proportion was 80% of Au0 and 16% of AuI, which was higher than the 8% surface coverage for an 18 nm core and corresponded to approximately two layers (Figure 3). This result confirmed proton coverage of the AuNP surface resulting from partial “special-type” oxidative addition of the hydronium onto the surface Au atoms, increasing their oxidation state.46 On the other hand, XPS literature data on AuNPs synthesized with strong reducing agents and nitrogen ligands indicate that the surface atoms are only in the Au0 state.47−49 These AuNPs were reduced by NaBH4, giving back ferrocene in the ether layer and in the aqueous layer AuNPs stabilized by a polymer of [B(OH)4]−Na+ in water, and Cl−.50,51 A test of the reactivity of the AuNP surface activity was the catalysis of 4-nitrophenol reduction by excess NaBH4, a B


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Inorganic Chemistry the intense plasmon band was still found at 545 nm (548 nm when the reaction was conducted at 1 °C) indicating formation of large AuNPs besides the agglomeration phenomenon. This feature highlights the advantage of the biphasic reaction in the case of ferrocene. On the other hand for ferrocene-containing polymers the situation was different, and no black precipitate formed under analogous homogeneous conditions because of the slow reaction rate (vide infra). Reaction of Ferrocene Derivatives with HAuCl4. A variety of mono- and polynuclear ferrocene derivatives (Figure 4)41,48 reacted similarly in the biphasic ether/water system. The

Table 1. Reactions of Substituted and Macromolecular Ferrocene Derivatives with HAuCl4 in Ether/Water and SPB and TEM of the AuNPs Obtained ferrocenesa FcH (1) FcH, 1 °C FcHd FcHd, 1 °C FcHe, 1 °C FeCp*Cp (6) FeCp*2 (7) CH3COFc (2) CH3COFc, 1 °C (C5H4Ac)2Fe (3) PhCOFc (4) CH3CH2Fc (5) PEG-FcH PVP-FcH nona-Fcf (8) Fc-polymerg (9)

reaction time

SPB (nm)

TEM (nm)

E1/2b (V)

Rc (rpm)

± ± ± ± ± ± ± ± ±

0.545 0.320 0.000 0.800 -

1250 1250 0 0 0 50 50 1250 1250

10−30 s 60−90 s 30 min 2h 2h 1h 2h 10−30 s 90−120 s

545 542 547 542 545 530 545 540 537

18 21 45 30 35 18 27 20 10

7 4 15 20 15 4 20 4 3

15 min

521

15 ± 4

1.040

1250

30 min 30−35 min 10−30 s 10−30 s 3h 2h

556 550 540 529 537 539

27 ± 5 13 ± 5 25 ± 3 13 ± 3 34 ± 7 15 ± 3

0.810 0.500

1250 1250 1250 1250 0 1250

0.555 0.620

a

See Figure 4 for the ferrocene structures; Fc = FeC10H9- (ferrocenyl); Cp = η5-C5H5; Cp* = η5-C5Me5; reactions at 22 °C unless noted otherwise. The reaction time corresponds to the time when colors or SPB band stop changing. bThese E1/2 values of the various ferrocene substrates were (re)measured in this work for comparison under identical conditions vs FeCp*2 +/0 as internal reference (22 °C; CH2Cl2, [n-Bu4N][BF4], 0.1 M) and are consistent with values reported in the literature.29,45,47,48 cRate per min (rpm) of stirbar rotation. d5-times more concentrated solution. e10-times more concentrated solution. f Dendrimer 8. gPolymer 9 (homogeneous reaction in THF/H2O).

Figure 4. Various ferrocenes forming mostly monodispersed AuNPs upon reaction with HAuCl4 in ether/water. The orange or red Fe(II) derivatives are represented in red color, and the Fe(III) derivatives are represented in blue or green color throughout the paper, although the purple AuNP color dominates (see Figures 1-3). With polymer 9, the reaction was conducted in a homogeneous THF/H2O medium.

variations of the stirring-dependent reaction rates as a function of the ferrocene substituents and bulk were enormous (Table 1) and sometimes remarkably indicative of the differences in mechanisms. The stirring rate also was a key factor depending on the ferrocene bulk. Indeed for bulky and macromolecular ferrocene derivatives, an excessive stirring rate resulted in AuNP aggregation. The values reported in Table 1 were obtained in the absence of aggregation/precipitation. The reaction of HAuCl4 with acetylferrocene also proceeded within seconds in spite of the apparent isoergonicity based on the redox potentials. This reaction was also driven by the overall irreversible Au(III) reduction, producing monodisperse 20 ± 4 nm-core-sized AuNPs (see the TEM in Figure 5). In contrast, ethylferrocene that should have been easier to reduce than ferrocene and especially than acetylferrocene based on the oxidation potentials (Table 1) also reacted to form AuNPs. This reaction was dramatically slower than with the latter, however, taking more than 30 min at 22 °C. The

Figure 5. TEM of acetylferricinium chloride-stabilized AuNPs formed upon reaction of acetylferrocene with HAuCl4..

enormous counterintuitive reaction rate difference if the oxidation potentials are considered can only be taken into account by the coordination of the oxygen atom of the acetyl group to Au(III) provoking an inner-sphere rate-limiting electron transfer.57,58 On the other hand ferrocene and ethylferrocene can only transfer an electron by an outer-sphere mechanism.59 Very remarkably, this inner-sphere process with coordination in the case of acetylferrocene resulted in high quality of monodisperse acetylferricinium-coated AuNPs (Figure 5). Even diacetylferrocene reacted in the same way in 15 min only, although the reaction looked endergonic by 0.2 V based on the redox potential. This illustrates the favorable role of the irreversibility of the Au(III) reduction shifting the redox C


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Inorganic Chemistry

monophasic THF/H2O medium upon mixing the aqueous HAuCl4 solution with the THF solution of the polymer 9 (Figure 7). The reaction monitored by progressive color

equilibrium, which to some extent compensates the lack of driving force. The rate difference between the reactions of ferrocene and ethylferrocene also shows the crucial role of bulk (i.e., distance) in outer-sphere electron transfer. The bulk effect also interfered when the inner-sphere electron transfer was sterically inhibited. This later effect was evidenced by comparing the reaction rates of acetylferrocene and benzoyl ferrocene, the later reaction also requiring 30 min. Analogously, 1,2,3,4,5-pentamethylferrocene,60 although it is more electron-rich than ferrocene (see Table 1), reacted slowly in 1 h, and it gave monodispersed 18 ± 1 nm-core AuNPs. With decamethylferrocene,61,62 the AuNPs formed tended to aggregate when the reaction was carried out under usual stirring conditions. The successful reaction took about 2 h with very slow stirring to avoid aggregation, and the AuNP obtained were polydispersed (vide infra, however). With the nonaferrocene dendrimer 8,63−65 the reaction had to be carried out without stirring (in contrast to the other reactions) to avoid aggregation. It proceeded very slowly in 3 h upon diffusion and cleanly gave dendrimer-stabilized AuNPs of 33−36 nm size in which the AuNPs were surrounded by the small 1 nm-sized dendrimer molecules (Figure 6). Finally the known amidoferrocene polymer 9 containing an average number of 50 ferrocene units synthesized by ring opening metathesis polymerization with Grubbs’ 3rd generation catalyst was prepared as reported.66 This polymer 9 is not soluble in ether; thus the reaction with HAuCl4 was conducted in the

Figure 7. Homogeneous reaction of the amidoferrocene polymer 9 with HAuCl4 in THF/H2O (top) and TEM of the resulting ferricinium chloride polymer-stabilized AuNPs (15 ± 3 nm; bottom).

change from orange to deep carmine red took 2 h, giving stable poly(amidoferricinium)-stabilized AuNPs for which the plasmon band was observed at 539 nm (compare with ferricinium monomer-stabilized AuNPs in THF/H2O at 542 nm). In this case, the electron-withdrawing amido group of 9 decreased the driving force for electron transfer, and the polymer bulk also slowed down the reaction leading to the formation of the AuNPs. The polymer frame is an excellent AuNP stabilizer, however, associating both the electrostatic and polymeric stabilization. The AuNPs obtained were monodisperse (Figure 7, bottom), this monodispersity resembling that obtained with acetylferrocene 2. This is probably due to a related inner-sphere reaction initially involving a likely Au(III)-amido interaction. This enormous rate difference among all the ferrocene derivatives is reminiscent of the problem of distant electron transfer in proteins and other biological systems that require a facilitating redox mediator for adequate efficiency.67 This series of reactions indicates that the redox potential, polarity, size, and shape of the ferrocene derivative play significant or crucial roles in determining the reaction rate, AuNP size, and AuNP dispersity. Nevertheless it is remarkable that with such a large variety of simple ferrocene compounds, stable, clean, and water-soluble ferricinium-AuNPs are always obtained, most of the time with low polydispersity. The extreme experimental simplicity in open air and ambient

Figure 6. Reaction of the ferrocene dendrimer 8 (see Figure 4) with HAuCl4 and formation of ferricinium dendrimer-stabilized AuNPs. D


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Inorganic Chemistry

formation of larger AuNPs than with the latter. A rather large range of AuNP sizes have been obtained, between 13 and 34 nm (Table 1). Changing the reaction temperature, presence and nature of a polymer or ferrocene substituents could indeed modulate the AuNP size. This size range is particularly useful for the formation of AuNPs toward biomedical usage.2,6−9 The analogy of AuNP sizes obtained with ferrocenes and a large variety of natural organisms suggest that with the latter the reaction of HAuCl4 proceeds likewise according to a SET mechanism concerning the primary redox step. This also implies degradation of these biomaterials, thus involving AuNP stabilization by partly degraded biomaterials. After the reactions, the ferrocene derivatives were recovered by addition of NaBH4 to the reaction mixture, and the colorless ether phase then turned to orange/yellow, while the AuNPs remained electrostatically stabilized by the anions Cl− and [BR4R4−n ′ ]− (R = H; R′ = OH) (Scheme 1). The latter formed

conditions, the rapidity of this method of AuNP synthesis, the monodispersity of the AuNPs obtained, and the flexibility of AuNP design concerning the size and nature of the stabilizing entity make this method very competitive and attractive. Redox Reactions between Acetylferricinium-AuNPs and Other Ferrocenes. Finally, taking advantage of the distinct redox potentials of the ferrocene derivatives caused by the various electronic effects of the ferrocene substituents (Table 1), stoichiometric redox reactions were conducted between acetylferricinium-AuNPs and other more electron-rich ferrocenes (Figure 8). For instance acetylferricinium-AuNPs

Scheme 1. Synthesis of the 26-nm-Core-Sized AuNPthiolates (R = PEG 550 or RS = L-Cysteinyl) from Ferrocene Involving Subsequent in Situ Reaction with NaBH4 then Thiola

Figure 8. Successive redox reactions in ether/water at r.t. with the stoichiometry 1Au:3Fc-derivative (top). See TEM of AuNP (19−22 nm) stabilized by [CH3COFc]+ in Figure 5. TEM of AuNP (20−23 nm) stabilized by [FcH]+ in the 2nd reaction (bottom left) and AuNP (20−23 nm) stabilized by [FeCp*2 ]+ in the 3rd reaction (bottom right).

a

X = H, OH.

a H-bonded supramolecular network with water molecules, resulting from the slow hydrolysis of borohydride, and these AuNPs were fully stable for weeks in solution.51 Oswald ripening68,69 then led to a AuNP core size increase from 20−22 nm to 25−27 nm, and the AuNPs were even more monodispersed than before NaBH4 addition.

(prepared from HAuCl4 and acetylferrocene in ether/water) instantaneously reacted with ferrocene upon stirring. This reaction led to a color change of the ether solution from yellow (ferrocene) to orange (acetylferrocene). Meanwhile in the aqueous phase the SPB slightly changed (see SI), and the AuNP core size changed from 19−22 nm to 20−23 nm. Likewise decamethylferrocene reduced ferricinium-AuNPs to ferrocene and decamethylferricinium-AuNPs (Figure 8 and SI). Reduction of HAuCl4 was thus somewhat slower with ferrocene (and even considerably slower with bulky ferrocenes) than with the classic reducing agent NaBH4, resulting in the

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CONCLUSION In conclusion, a novel, well-defined and simple biphasic method of synthesis of AuNPs with various and controlled sizes using ferrocene derivatives in ether as reductants of aqueous HAuCl4 has been disclosed. It has been extended to a large variety of ferrocenes containing electron-withdrawing or electron-releasing substituents and large variations of steric bulk from simple E


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Inorganic Chemistry

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ferrocenes to dendritic and polymeric ferrocenes. Caution is necessary in stirring conditions when the steric bulk of the ferrocene substituents considerably slows down the redox process. The reaction was conducted in THF/water with a ferrocene-containing polymer that was insoluble in ether. The reaction stands as a new method of stabilization of AuNPs that is readily followed up by thiolate complexation if desired. This method contrasts with the usual NaBH4 reduction of Au(III) yielding ill-defined polyborane-stabilized AuNPs. Of great interest is this simple reaction of ferrocene compounds in terms of monodisperse AuNPs, outer-sphere versus innersphere redox mechanism and disclosure of the hydronium contribution of AuNP stabilization. The application of the reaction to ferrocene-containing macromolecules of dendritic or polymeric type involves a synergy between electrostatic and macromolecular AuNP stabilization that opens a route to AuNP-containing redox active macromolecules. The reaction also helps rationalizing a large number of AuNP stabilizations by plants and bio-organisms. 70−73 The concept might potentially be extendable to other transition metal ions.

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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.inorgchem.6b02850. Synthetic procedures, reaction schemes and photographs, UV−vis spectra, and TEM of AuNPs (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Luis Yates: 0000-0002-0411-4932 Sergio Moya: 0000-0002-7174-1960 Didier Astruc: 0000-0001-6446-8751 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This article is dedicated to the memory of Prof. Fritz Vögtle, a brilliant chemist. Financial support from Gobierno Vasco (R.C., postdoctoral scholarship), the Universidad del Pais̈ Vasco, the Universities of Bordeaux, and Sichuan, Chengdu, China, CIC biomaGUNE, and the CNRS is gratefully acknowledged.

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Inorganic Chemistry

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Electrostatic Assembly of Functional and Macromolecular Ferricinium

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