Nano Res
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Electronic Supplementary Material
Separation of Gold Nanorods Using Density Gradient
Ultracentrifugation
Shuai Li§, Zheng Chang§, Junfeng Liu (), Lu Bai, Liang Luo, and Xiaoming Sun ()
State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China
§
Contributed equally to this work
Supporting information to DOI 10.1007/s12274-011-0128-7
Experimental Section
Materials
Hydrogen tetrachloroaurate(Ⅲ) hydrate (HAuCl4·4H2O, 99.9%) was purchased from Sinopharm Chemical Reagent
Co. Cetyltrimethylammonium bromide (CTAB, 99%) was obtained from Sigma. Silver nitrate (AgNO3, 99.8%),
ascorbic acid (C6H8O6, 99.7%), ethylene glycol (C2H6O2, 96%), and sodium borohydride (NaBH4, 98%) were purchased
from Beijing Chemical Works. Rhodamine 6G (R6G) was obtained from Aladdin Chemistry Co. Ltd. All chemicals
were used as received without further purification. All of the solutions were prepared with high purity water.
Synthesis of gold nanorods
Gold nanorods were prepared by a seed-mediated growth method in cetyltrimethylammonium bromide (CTAB)
solutions in the presence of AgNO3, as previously described [1]. An aqueous gold seed solution was prepared
by adding a freshly prepared NaBH4 solution (0.6 mL, 0.01 mol/L) to a solution composed of 50 μL of 0.05 mol/L
HAuCl4 and 9.95 mL of 0.1 mol/L CTAB, stirred for 2 min and then kept at 30 °C for 2 h. The NR growth
solution was prepared by mixing 19.8 mL of 0.1 mol/L CTAB with 150 μL of 0.01 mol/L AgNO3 solution, 200 μL
of 0.05 mol/L HAuCl4, and 115 μL of 0.1 mol/L ascorbic acid, with continuous stirring.
Au NRs were obtained by adding 24 μL of the gold seed solution into the growth solution and mixed by
capping the reaction vessel and slowly inverting it two times. Then the growth solution was maintained at 30 °C
without stirring for over 12 h. The product was separated by centrifugation and washed with high purity water,
and then dispersed into 2 mL high purity water.
Density gradient preparation
Ethylene glycol (EG) solutions with a suitable density distribution were used to prepare the density gradient,
since EG solutions gives appropriate capability to disperse Au NRs. CTAB was added to all the gradient solutions
to inhibit the aggregation of Au NRs. In a typical procedure, 0.365 g of CTAB was dissolved in 100 mL of water to
make a 0.01 mol/L CTAB solution. A four-layer step gradient was made using 50%, 60%, 70%, 80% concentration
(by volume) of EG in 0.01 mol/L CTAB solution. For instance, a volume ratio of CTAB solution:EG = 2:8 was used
to make the 80% solution. A step gradient was created directly in Beckman centrifuge tubes (polycarbonate, inner
diameter 15 mm, length 90 mm) by adding layers to the tube with decreasing density (i.e., lower EG concentration).
Address correspondence to Junfeng Liu, [email protected]; Xiaoming Sun, [email protected]
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Nano Res
To make a 50% + 60% + 70% + 80% gradient, 2 mL of 80% EG/CTAB solution was first added to the centrifuge
tube, and then 2 mL 70% EG/CTAB solution was slowly layered above the 80% layer. The subsequent layers were
made following the same procedure and resulted in a density gradient along the centrifuge tube. To reduce the
diffusion between layers, the preparation time of the density gradient layers was minimized and the the density
gradient solutions used for density layers were cooled in a refrigerator to decrease the molecular diffusion.
Separation of Au nanorods
Typically, 0.8 mL of a solution of Au NRs was ultrasonicated for 10 min and layered on top of the gradient prior
to ultracentrifugation. The typical centrifugation condition was 10 min at 10,000 r/min (SW65 Rotor, Beckman
Coulter) at 15 °C. Calibrated micropipettes were then used to manually extract 400 μL fractions at various positions
along the centrifuge tube after ultracentrifugation, for characterization.
Characterization of separated nanorods
The separated NRs were further purified by centrifuging at 12,000 r/min for 10 min, removing and discarding the
supernatant, and redispersed in high purity water. This washing process was repeated several times to ensure
removal of excess CTAB and EG from the Au nanorods. Absorbance spectra were acquired using a UNICO
UV-2802PC/PCS spectrophotometer with samples dispersed in water in quartz cuvettes with a 1 cm path length.
Nanorods were imaged by transmission electron microscopy (TEM) using a Hitachi H-800 TEM operated at
200 kV. Images were digitally acquired and used for particle size and shape evaluation. TEM samples were
prepared by drop-casting aqueous dispersions onto carbon-coated copper 230 mesh TEM grids. A Jobin Yvon
LabRAM HR Raman microscope was utilized to collect Raman spectra using a laser at λ = 633 nm. For SERS
studies, Au films were prepared by drop-casting colloidal solutions of Au NRs of varying aspect ratio onto silicon
wafers and then used as substrates. 5 μL of 10–6 mol/L aqueous R6G solution was dropped onto the different Au
films. The morphology of the Au films was characterized by field-emission scanning electron microscopy (SEM)
using a JEOL JSM6335 instrument operated at 20 kV.
Figure S-1 (a) and (b) TEM images, and (c) length histograms, and (d) aspect ratio histograms of the original Au NRs before separation
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Figure S-2
(a) TEM images, (b) length histograms, and (c) aspect ratio histograms of Au NR fractions
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Nano Res
Table S-1
*
The NR dimensions and UV–vis peaks of different fractions
TSP (nm)*
LSP (nm)*
Peak width at half
height of LSP (nm)
Fraction
Length (nm)
Diameter (nm)
Aspect ratio
5
39.0
10.3
3.8
510
805
208
7
45.7
13.3
3.5
510
760
179
9
46.4
14.3
3.3
510
734
168
12
49.4
15.9
3.2
512
712
149
15
48.4
16.2
3.1
515
705
138
17
42.1
18.4
2.5
531
695
—
20
31.1
15.9
2.0
534
—
—
Original Au
NR sample
46.5
14.8
3.3
514
718
183
TSP denotes the position of the transverse surface plasmon band; LSP denotes the position of the longitudinal surface plasmon band
Reference
[1] Nikoobakht, B.; El-Sayed, M. A. Preparation and growth mechanism of gold nanorods (NRs) using seed-mediated growth method.
Chem. Mater. 2003, 15, 1957–1962.