Ultrarapid Cationization of Gold Nanoparticles via a Single-Step

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Ultra-rapid Cationization of Gold Nanoparticles via a Single-step Ligand Exchange Reaction Yohei Ishida, Jun Suzuki, Ikumi Akita, and Tetsu Yonezawa Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02226 • Publication Date (Web): 08 Aug 2018 Downloaded from http://pubs.acs.org on August 18, 2018

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Ultra-rapid Cationization of Gold Nanoparticles via a Single-step Ligand Exchange Reaction Yohei Ishida*, Jun Suzuki, Ikumi Akita, Tetsu Yonezawa* Division of Material Science and Engineering, Faculty of Engineering, Hokkaido University, Kita 13 Nishi 8, Kita-ku, Sapporo, Hokkaido 060-8628, Japan.

ABSTRACT We propose a novel method for the ultra-rapid cationization of gold nanoparticles with diameters ranging from several to a hundred nanometers via a single-step ligand exchange reaction of citrate-protected anionic gold nanoparticles with cationic thiol ligand. This reaction was performed only in an aqueous medium via a single step from citrate to cationic thiol, therefore enables a rapid preparation of cationic Au NPs without a contamination of organic solvent or lipid-soluble molecules. The cationization was successfully completed within 20 min without changes in the core diameter and optical characteristic.

1 Environment ACS Paragon Plus

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INTRODUCTION Metallic nanoparticles, particularly gold nanoparticles (Au NPs), are attracting considerable interest owing to their high stability and interesting optical properties, dominated by the localized surface plasmon resonance (LSPR).

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interest and are potentially applicable to catalysis,

Such Au NPs are of intrinsic scientific 4-6

chemical sensing,

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optical imaging,1

biolabeling,3 etc. Importantly, the properties and applications of Au NPs strongly rely on their size, shape, and surface functionalization with organic ligands. Although many Au NPs protected by neutral or anionically charged organic ligands have been reported,1 there are relatively few reports on the preparation of cationic Au NPs. 10-12 In some special areas, such as biological signaling, particle transportation, and cellular toxicity, cationic-ligand-protected NPs can be indispensable owing to the good affinity of the cationic ligand for the biomaterials. 13-15 For example, Elci et al.14 studied how surface-ligand charge affects the biodistribution of 2-nm Au NPs in vivo, and showed that the liver and spleen accumulated higher levels of positively charged Au NPs than neutral or negatively charged ones. This knowledge is important for the intentional design of cationicligand-protected NPs suited to specific nano-biological applications. Cationic Au NPs have been traditionally synthesized by directly reducing gold salts in the presence of cationic ligands10,11,16. The size control and monodispersity of the resulting NPs are typically poor using. The Brust–Schiffrin method using two-liquid phases17,18 is superior in terms of monodispersity; however, NPs with diameters greater than 5 nm are difficult to synthesize, leading to weak intensity of LSPR. Larger NPs show stronger LSPR because of the approximately cubic dependence of the absorption intensity on the particle size.19 The strong and narrow LSPR absorption observed with larger Au NPs is desired for applications based on plasmonic characteristics such as sensing.


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Ligand exchange reaction of neutral or anionic Au NPs with cationic ligands can offer better size control and monodispersity of the resultant cationic Au NPs. The two-step ligand exchange reaction20 of citrate-protected anionic Au NPs was reported that involves a neutral amine ligand and toluene solvent in the first step and subsequently transfer into aqueous solution via the exchange with cationic thiols. This two-step method offers high monodispersity, reflecting the uniformity of initial citrate-Au NPs in the size range smaller than 20 nm. The use of organic solvent (toluene) and lipid-soluble amine molecules, which are generally toxic for biomedical applications, limits its practical application owing to the necessity of repeated purification processes. If an easy and facile approach for obtaining cationic Au NPs with diameters ranging from a few to hundred nanometers with high monodispersity can be developed via a simple approach without the use of toxic organic solvents or lipid-soluble molecules, the applications of cationic Au NPs such as in nanobiomedical or Coulombic self-assembled systems21 will be accelerated and broadened. The purpose of this study was to explore a novel method for the ultra-rapid cationization of Au NPs with diameters reaching a hundred nanometers and high monodispersity via a single-step ligand exchange reaction of citrate-protected Au NPs with cationic thiol ligand (11mercaptoundecyl)-N,N,N-trimethylammonium bromide (MUTAB) in an aqueous solution.

EXPERIMENTAL SECTION Chemicals Five different sized (6.2 - 99.7 nm, Figure S1) citrate-protected Au NPs, (11mercaptoundecyl)-N,N,N-trimethylammonium bromide (MUTAB) were purchased from Aldrich. The citrate-Au NPs were purified by ultrafiltration (14000 rpm, 45 min) using an ultrafiltration filtering tube (NMWL 3000 Da, Amicon), and then redispersed in 500 µL of deionized water before use. Cl– type anion-exchange resin (Amberlite IRA400J, ion exchange


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capacity (eq L–1) ≧ 1.4) was purchased, and cleaned with 0.1 M HCl overnight before use. The deionized water used in experiments was prepared by ELGA Purelab system (> 18.2 MΩ). Synthesis of cationic MUTAB-Au NPs by single-step ligand exchange reaction. MUTAB (3 mg, 9.19 × 10–6 mol) was dissolved in 500 µL of deionized water and Cl– type anion-exchange resin (1 g) was mixed. Citrate-Au NPs (500 µL) was added with under stirring at room temperature for 20 min. Analysis Absorption spectra were recorded in UV-visible range with UV-1800 (Shimadzu). Spectra were recorded in quartz cells with 1 mm optical pass. Transmission electron microscopy (TEM) images were taken with JEM-2000FX (JEOL). Samples were prepared by dropcasting Au NPs dispersion on carbon-coated copper grids, then dried under vacuum overnight before observation. Part of the dispersion of Au NPs (pH 7) was used for measuring ζ potential using an Otsuka ELSZ-2 at room temperature. Nuclear magnetic resonance (NMR) analyses were performed with JNM-ECS400 (JEOL) operating at 400 MHz. Before NMR measurements, obtained MUTAB-Au NPs were purified by a centrifugation (14000 rpm, 45 min) with ultrafiltration filteration tube (NMWL 3 KDa, Amicon) twice. The Au NPs were then dried and redispersed into D2O. To avoid signal broadening in MUTAB-Au NPs, the Au NPs were etched with potassium cyanide before measurements.

RESULTS AND DISCUSSION A single-step ligand exchange reaction involves an anion-exchange resin as illustrated in Figure 1. In a typical condition, anionic citrate-protected Au NPs (diameter of 6.2 nm, 9.09 × 10−8 mol L−1, purchased from Aldrich), which were purified using an ultracentrifugation filtering tube (pore size: 3000 Da) before use, were added to the solution


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containing MUTAB (3 mg, 9.19 × 10−6 mol) with 1 g of Cl– type anion-exchange resin (Amberlite IRA400J,) under stirring at room temperature. The dispersion was maintained for 20 min under stirring before analyses. This single-step ligand exchange process enables a rapid preparation of cationic Au NPs without a contamination of organic solvent or lipid-soluble molecules in a short period. We attempted to apply this method for various sizes of Au NPs from 6.2 to 99.7 nm as summarized in Figure S1 in the Supporting Information. Details of the experimental conditions are summarized in the Supporting Information. Importantly, the presence of an anion-exchange resin in this scheme is crucial to avoid the aggregation of Au NPs owing to electrostatic interaction between cationic MUTAB and anionic citrate-protected Au NPs. The single-step ligand exchange reaction without an anion-exchange resin resulted in the formation of aggregated Au flakes larger than ca. 500 nm and the absorption maximum largely shifted as shown in Figure 1.


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Figure 1. (a) Schematic representation of single-step ligand exchange reaction of anionic citrate-protected Au NPs with cationic MUTAB ligand under the presence of an anionexchange resin in water. (b) Absorption spectra of starting citrate-Au NPs (black), Au NPs obtained after the single-step ligand exchange reaction without (blue) and with (red) anionexchange resin. (c) TEM image of the Au NPs obtained after the single-step ligand exchange reaction without an anion-exchange resin

Figure 2 shows the UV/Vis absorption spectra of five different-sized Au NPs before and after the single-step ligand exchange reaction. The LSPR maximum (λmax) depends on the size of the particles, and the smaller the size of Au NPs, the shorter is λmax. As shown in Figure S2, when the particle size changes by aggregation, λmax shifts to red. λmax of Au NPs in all cases was almost constant, whereas a large spectral shift was observed under the absence of the anion-exchange resin examined (Figure S2). These results demonstrate that the single-


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step ligand exchange reaction was successfully achieved without a change in diameters of NPs.

Figure 2. Absorption spectra of the five different-sized Au NPs before (dashed curve) and after (solid curve) the single-step ligand exchange reaction

To ensure that the diameter of Au NPs remained constant throughout the ligand exchange process, we obtained transmission electron microscopy (TEM) images and determined the particle size distributions (Figure 3). The average diameter and monodispersity did not change before and after the single-step ligand exchange reaction, as shown in Figure 3 and Figure S1 (initial citrate-Au NPs), and as summarized in Table 1. The larger NPs tend to gather without a change in the core diameters by the evaporation of the solvent as similarly observed for citrate-protected NPs in Figure S1 in the Supporting Information. These TEM results together with the absorption spectra suggest a successful ligand exchange without the agglomeration of Au NPs. 1

H-nuclear magnetic resonance (1H-NMR) measurement was conducted to confirm

the complete ligand exchange from citrate to MUTAB (Figure S3). The obtained Au NPs were etched via a reaction with potassium cyanide and the detached free ligands in the solution were analyzed in D2O using the method outlined in the Supporting Information. The 1

H-NMR results only showed the signals of MUTAB, and those of citric acid were not


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observed. We therefore obtained the MUTAB-stabilized Au NPs, and free citrate was successfully removed by the anion-exchange resin as illustrated in Figure 1. The optimum amount of anion-exchange resin in this protocol was roughly estimated by using 9.1-nm citrate-protected Au NPs as an example. Upon decreasing the amount of anion-exchange resin used from 1 g to 100 mg, λmax in absorption spectra and TEM sizes were constant (Figure S4). With 10 or 1 mg of anion-exchange resin, λmax shifted to 528 nm from the initial value of 522 nm.

50 nm

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30 20 10 0

30 40 50 60 70 80 90 Diameter / nm

70 80 90 100 110 120 Diameter / nm

Figure 3. TEM images and the corresponding size distribution histograms of the fivedifferent-sized Au NPs after the single-step ligand exchange reaction

As estimated from the ion exchange capacity of resin (1.4 eq L–1), 10 mg of the resin (= 1.4 × 10–5 L) has sufficient capacity (2.0 × 10–5 eq) to exchange all the citrate molecules on Au NP surfaces (1 × 10−8 mol, see the calculation procedure in the Figure S4).22 The bead-like shape of an anion-exchange resin limits the exchange rate of citrates owing to slow diffusion. ζ potential was measured for Au NPs before and after the single-step ligand exchange reaction. The ζ potentials of the initial citrate-Au NPs ranged from –22.0 to –31.0 mV, and the values changed to +23.3 to +26.4 mV after the ligand exchange as summarized in Table 1.


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These values are of the same magnitude as the reported values for cationic Au NPs with quaternary ammonium ligands (+10 to +60 mV). 20,23,24 Finally, to study the time required for the completion of the reaction, we measured ζ potentials as a function of the reaction time using 6.2-nm Au NPs as an example (Figure 4). ζ potential reached +20.9 mV after approximately 1 min. The value gradually increased with time, and reached +25.0 mV after 20 min, which is equal to the equilibrium after 1 day. These results demonstrate that the ultra-rapid cationization of Au NPs with diameters of 6 to 100 nm was successfully achieved, whereas several hours of reaction time, a toxic organic solvent, and lipid-soluble molecules are necessary for the two-step ligand exchange reaction reported previously.20 In addition, the obtained cationic Au NPs showed good colloidal stability; the absorption spectrum, particle size, and monodispersity remained the same after they were maintained at least for 3 months at 4 °C (Figure S6). 30 Zeta potential / mV

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20 10 0 -10 -20 -30 0

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Time / min

Figure 4. ζ potential of 6.2-nm Au NPs as a function of reaction time. The dotted red line denotes the zeta potential (+25.0 mV) after 1 day.

Table 1. Summary of the data; TEM diameters, absorption maxima (λmax), and ζ potentials of Au NPs before (upper, in the bracket) and after (lower) the single-step ligand exchange reaction


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Diameter / nm

(6.2 ± 0.9) 6.1 ± 1.0

(9.1 ± 0.8) 9.1 ± 0.8

(17.5 ± 2.1) 17.6 ± 1.7

(54.2 ± 6.9) 54.3 ± 7.6

(99.7 ± 8.9) 100.0 ± 7.4

λmax / nm

(521) 523

(524) 522

(523) 523

(535) 535

(567) 567

ζ potential / mV

(–22.8) +25.0

(–29.0) +26.4

(–27.8) +23.3

(–25.6) +25.0

(–31.9) +24.4

CONCLUSION In conclusion, we succeeded in the ultra-rapid cationization of Au NPs with high monodispersity and scalable particle size (6–100 nm) via a single-step ligand exchange reaction. This reaction was performed only in an aqueous medium via a single step from citrate to cationic thiol, therefore enables a rapid preparation of cationic Au NPs without a contamination of organic solvent or lipid-soluble molecules. We believe that the novel ultrarapid cationization method presented here will facilitate future applications of cationic NPs such as in nano-biological systems.

SUPPORTING INFORMATION This material is available free of charge on the ACS Publishing website. Experimental details, TEM images, 1H-NMR and absorption spectra. (PDF)

AUTHOR INFORMATION E-mail * [email protected] (Y. I.)


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[email protected] (T. Y.) Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS Y.I. acknowledges financial support from Building of Consortia for the Development of Human Resources in Science and Technology, Ministry of Education, Culture, Sports, Science and Technology, Japan, JSPS KAKENHI Grant Number 18K14070, Kurata Grant awarded by the Hitachi Global Foundation, Mayekawa Houonkai Foundation, Murata Science Foundation, and Nippon Sheet Glass Foundation for Materials Science and Engineering. Partial financial supports by Kakenhi (18H01820 to TY) from JSPS and by the Cooperative Research Program of “Network Joint Research Center for Materials and Devices” (20181111 to TY), Japan are gratefully acknowledged.

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Ultrarapid Cationization of Gold Nanoparticles via a Single-Step

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