High-Purity Separation of Gold Nanoparticle Dimers and Trimers

Transcription

1 -Supporting Information- High-Purity Separation of Gold Nanoparticle Dimers and Trimers Gang Chen, Yong Wang, Li Huey Tan, Miaoxin Yang, Lee Siew Tan, Yuan Chen and Hongyu Chen* Division of Chemistry and Biological Chemistry and Division of Chemical and Biomolecular Engineering, Nanyang Technological University, Singapore Experiment section All chemical regents were obtained from commercial suppliers and used without further purification. Hydrogen tetrachloroaurate(iii) hydrate (HAuCl 4 H 2 O), 99.9% (metal basis Au 49%) was purchased from Alfa Aesar; amphiphilic diblock copolymer polystyrene-block-poly(acrylic acid) (PS 154 -b-paa 60, M n =16000 for the polystyrene block and M n =4300 for the poly(acrylic acid) block, M w /M n =1.15) was obtained from Polymer Source, Inc.; cesium chloride, 99.9% was purchased from Applichem. Deionized water (resistance > 18.2 MΩ cm -1 ) was used in all of our experiments. All other chemicals were purchased from Aldrich. Copper specimen grids (200 mesh) with formvar/carbon support film (referred to as TEM grids in the text) were purchased from Electron Microscopy Sciences. UV-vis spectra were collected on a Cary 100 UV-vis spectrophotometer. TEM images were collected from a JEM-1400 Transmission Electron Microscopy (JEOL) operated at 120 kv. Preparation of TEM Samples. (NH 4 ) 6 Mo 7 O 24 was used as the negative stain in all TEM images reported in this paper, so that the polymer shells appear white against the stained background. TEM grids were treated with oxygen plasma in a Harrick plasma cleaner/sterilizer for 1 min to improve the surface hydrophilicity. A sample solution was carefully mixed with stain solution ([(NH 4 ) 6 Mo 7 O 24 ] = 6.8 mm) on the surface of a plastic Petri dish; the hydrophilic face of the TEM grid was then placed in contact with the sample solution. A filter paper was used to wick off the excess solution on the TEM grid, which was then dried in air for 5 min. Synthesis of AuNPs. AuNPs were prepared following literature procedures 1 by sodium citrate reduction of HAuCl 4, with small modifications. In a representative synthesis, a 250 ml flask was charged with 100 ml HAuCl 4 (0.01% w.t. in H 2 O). This solution was heated to 100 C with vigorous stirring for 30 minutes, and then 3.5 ml of sodium citrate (1% w.t. in H 2 O) was added quickly to the solution. After heating the solution for an additional 60 minutes, a deep-red AuNP solution was obtained. The average particle diameter was measured to be 14.6 ± 1.6 nm from TEM pictures using ImageJ ( last visited Feb 1 st, 2009). The concentrations of the as-synthesized AuNP solution were estimated from the total amount of Au used during the synthesis, the density of Au, and the average size of AuNPs. Synthesis of AuNP The following synthetic details for AuNP are sufficient to reproduce the as-synthesized sample mentioned in the main text. The related discussions will be published elsewhere in a separate topic in the near future. In a typical reaction, a AuNP solution (d av = 15 nm, ml, 4.75 nm) was concentrated to ~10 μl by centrifugation at g for 15 min. To the deep red suspension collected at the bottom of eppendorf tube, 600 μl DMF was added, followed by 2-naphthalenethiol in DMF S1

2 (20 μl, 3 mg/ml). The mixture was then added to a vial which contains NaOH (10 μl, 2.5 mm), NaCl (4 μl, 0.1 M), H 2 O (66 μl, so that V NaCl + V H2O + V AuNP = 90 μl), and DMF (200 μl), then incubated at 60 C for 2 h to induce slow aggregation. Linear aggregates of 15 nm AuNPs were generated at this stage. Different linear chain length can be simply tuned by changing [NaCl] and the incubation time. To encapsulate the linear aggregates, PS 154 -b-paa 60 solution (80 μl, 8 mg/ml in DMF) and 100 μl H 2 O were added in sequence to the above mixture; the rate of aggregation significantly slowed down upon polymer addition. The total volume of the final mixture solution was 1100 μl, where the DMF/H 2 O volume ratio was 4.5; [PS 154 -b-paa 60 ] = 28.5 μm. The mixture was heated to 110 C for 2 h and then allowed to slowly cool down in the oil bath to room temperature. This resulting solution contained AuNP empty polymer micelles, DMF, NaOH, NaCl, and 2-naphthalenethiol. To isolate the nanoclusters from the excess reactants, the product solution was diluted (300 μl diluted by 4200 μl water), divided into individual eppendorf tubes, and then centrifuged at g for 30 min to remove the supernatant. The purplish-red solution at the bottom of centrifuge tubes was diluted by aq. NaOH (100 μl, ph=10), and further purified by repeating the above purification steps. The collected product was again diluted by ph 10 NaOH, giving a stable solution that contained mainly AuNP Purification by differential centrifugation. A stock solution was made by dissolving excess CsCl in aq. NaOH (0.1 mm, ph 10); this solution has a density of 1.91 g cm -3 and contained ~61.8% CsCl by weight. Solutions of lower concentration were prepared by diluting this stock solution using ph 10 NaOH. For convenience, the volume change during dilution is ignored, and the solutions were referred to in the text according to the dilution ratio, such as 10.8% ( 10 dilution) and 40.6% ( 2 dilution) CsCl solutions. The sucrose solutions of different concentration were directly prepared by dissolving appropriate amount of sucrose in water. For example, 50 g sucrose in 50 ml water gave 50% w.t. sucrose. In a typical experiment, a step gradient was created directly in an eppendorf tube (polystyrene, 1.5 ml). To make a 10.8%+61.8% gradient, 600 μl 10.8% CsCl solution was added to the centrifuge tube first, then 600 μl 61.8% CsCl solution was carefully added below the 10.8% layer by inserting a pipet to the bottom of the centrifuge tube. Alternatively, 10.8% CsCl could be directly added on top of 61.8% CsCl, although this method is generally more difficult to control. The purified AuNP (~ 50 μl, ph 10) was then carefully layered on top of the density gradient, and the tube was centrifuged at 8.5k rpm (~5800 g) for a pre-set time (e.g. 20 min). References (1) Frens, G. Nat. Phys. Sci. 1973, 241, 20. S2

3 Figure S1. Large-area view of sample a1 (of Figure 1). S3

4 Figure S2. Large-area view of sample b1 (of Figure 1). S4

5 Figure S3. Large-area TEM images of a dimer sample, after 2 nd purification (b2 of Figure 1). S5

6 Figure S4. Large-area TEM images of a trimer sample, after 2 nd purification (c3 of Figure 1). S6

7 Figure S5. High-resolution images of those shown in Figure 2 (same labels were used), with additional supplementary images (SA-SC and SF-SG); (SD-SE) A control experiment that loaded AuNPn@PSPAA in 200 μl water gave broad bands that were difficult to separate; (SH-SI) Differential centrifugation using 20% wt sucrose solution. S7

8 Figure S6. Normalized total AuNP volume (proportional to weight) distribution of monomers, dimers, trimers, and tetramers. The radii of AuNPs (327 data points) were measured from AuNP (d AuNP = ~15 nm) using ImageJ ( and then converted to volume (V = 4πr 3 /3) of monomers. Volume of dimers was created from all possible permutations of two monomers (327 2 = data points); volume of trimers was created from permutations of monomer + dimer (327 3 = data points, from which points were randomly sampled); and volume of tetramers was created from permutations of two dimers (using randomly sampled 2139 dimer points, to give = data points). The frequency of the volume data in these data sets was analyzed to give the total volume distribution of the individual nanoclusters, which was then normalized. The above image shows the significant overlap of trimer weight distribution with that of dimers and tetramers. Due to the large population of dimers in the as-synthesized solution, heavy-weight dimers could easily co-separate with the trimers. S8