flash” preparation of strongly coupled metal nanoparticle

“Flash” Preparation of Strongly Coupled Metal Nanoparticle Clusters with Sub-nm Gaps by Ag+ Soldering: Toward Effective

Plasmonic Tuning of Solution-Assembled Nanomaterials

Miao Liu,a Lingling Fang,a Yulin Li,a Ming Gong,b An Xuc and Zhaoxiang Deng*a

a CAS Key Laboratory of Soft Matter Chemistry & Collaborative Innovation Center of Suzhou Nano Science and Technology, Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, China. Email: [email protected] b Engineering and Materials Science Experiment Center, University of Science and Technology of China, Hefei, Anhui 230027, China c Key Laboratory of Ion Beam Bioengineering, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei, Anhui 230031, China

Experimental Details

Chemicals

Fish sperm DNA (FSDNA), NaCl and AgNO3 were purchased from Bio Basic Inc. (BBI,

Canada). Bis(p-sulfonatophenyl)phenylphosphine dihydrate dipotassium salt (BSPP) and sodium

tetrachloropalladate trihydrate (Na2PdCl43H2O) were obtained from Strem Chemicals

(Newburyport, MA, USA). 4-mercaptopyridine was from Acros Organics. Chloroauric acid

tetrahydrate (HAuCl44H2O) and hexachloroplatinic acid hexahydrate (H2PtCl66H2O) were

bought from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Sodium citrate tribasic

dihydrate was purchased from Sigma. All reagents were used as received without further

purifications.

Nanoparticle synthesis

Citrate-capped AuNPs with diameters of 5.5 and 13.3 nm were synthesized following literature

methods.[1-2] Other nanoparticles used in this work were prepared according to the following

procedures.

24.1 nm AuNPs[3]

13.3 nm AuNPs were used as nucleation seeds to grow 24.1 nm AuNPs. Immediately after the

synthesis of 13.3 nm AuNPs (50 mL), the solution was cooled to 90 C. 660 μL of sodium citrate

(60 mM) and 660 μL of HAuCl4 (25 mM) were sequentially added (time delay of ~2 min between

the two additions) to this solution, with the mixture being maintained at 90C under vigorous

stirring for 30 min. By repeating the additions of sodium citrate and HAuCl4 up to 5 times, AuNPs

with a diameter of 24.1 nm were obtained.

37.5 nm AuNPs [3]

Immediately after the synthesis of 24.1 nm AuNPs (56 mL), 660 μL of 25 mM HAuCl4 was

injected. After an incubation at 90 C under stirring for 30 min, 30 mL of the reaction solution was

extracted, which was then diluted by mixing with 28 mL of deionized water and 2 mL of 60 mM

sodium citrate. The above mixture was heated to 90 C in about 30 min before an injection of 1

mL of 25 mM HAuCl4. The same heating and HAuCl4 addition were repeated one more time, with

the final solution being heated at 90 C for another 30 min to produce 37.5 nm AuNPs.

30 nm PtNPs[4]

The synthesis of 30 nm PtNPs started with a preparation of 5 nm PtNP seeds. Briefly, 46.4 mL

of deionized water was heated to boiling before the addition of 72 μL of H2PtCl6 (193.1 mM)

Electronic Supplementary Material (ESI) for Chemical Science.This journal is © The Royal Society of Chemistry 2016

under strong stirring. 1 min after adding H2PtCl6, 1.1 mL of a solution containing 1% sodium

citrate and 0.05% citric acid was added. 0.5 min later, 550 μL of a freshly prepared solution

containing sodium borohydride (0.08%), sodium citrate (1%) and citric acid (0.05%) was

introduced. The solution was kept boiling for 10 min before being cooled down to room

temperature to obtain 5 nm PtNP seeds. To obtain 30 nm PtNPs, 1 mL of the 5 nm PtNP seeds was

diluted with 29 mL of deionized water at room temperature. Afterwards, 93.2 μL of H2PtCl6

(193.1 mM) and a 0.5 mL solution containing sodium citrate (1%) and L-ascorbic acid (1.25%)

were sequentially introduced. The mixture was slowly heated (~10C/min) to boiling and

maintained under this condition for about 40 min to produce 30 nm PtNPs.

29 nm Au@PdNPs[5]

2.47 mL of deionized water was pre-cooled in an ice-water bath. 80.6 μL of BSPP-capped 13.3

nm AuNPs (0.248 μM), 95.4 μL of Na2PdCl4 (100 mM), and 123.1 μL of sodium citrate (100 mM)

were sequentially added to the pre-cooled water. Afterwards, 1.23 mL of ice-cooled L-ascorbic

acid (100 mM) was slowly added dropwise to the above solution under continuous stirring. The

reaction lasted 45 min to obtain 29 nm Au@PdNPs.

BSPP ligand exchange

BSPP was added to a freshly prepared nanoparticle solution to reach a concentration of 0.2

mg/mL (for 5.5 and 13.3 nm AuNPs) or 1 mg/mL (for 24.1 and 37.5 nm AuNPs, 30 nm PtNPs,

and 29 nm Au@PdNPs). The solution was kept standing at room temperature for 8 h followed by

a centrifugation and redispersion of the nanoparticles in deionized water.[6]

Ag+ triggered self-assembly of discrete nanoparticles

Au, Pt, or Au@Pd nanoparticles were dispersed in 0.5×TBE (Tris, 44.5 mM; EDTA, 1 mM;

boric acid, 44.5 mM; pH 8.0) buffer containing 1 mg/mL FSDNA and appropriate amount of

AgNO3 (0.5 to 20 mM). The solution was kept at room temperature for a soldering reaction to take

place. In the case of AuNPs, the color of the solutions changed from red to purple (or blue) within

seconds (see a supporting video clip).

Agarose gel purification of nanoparticle oligomers

A mixture of nanoparticle clusters was loaded in a 3% (for 5.5 and 13.3 nm AuNPs) or 1.5%

(for larger AuNPs, PtNPs, and Au@PdNPs) agarose gel, which was run in 0.5×TBE at 13 V/cm

for an electrophoretic separation. To recover a desired oligomer, the corresponding gel band was

cut out and simply soaked in a 0.5×TBE buffer for several hours to elute the product. If an electric

field is applied to accelerate the elution,[6] the whole process including sample preparation, gel

separation, and product elution will be finished in1-2 hours.

Ultrasonication-based stability test

Purified AuNP dimers were dispersed in water, followed by a sonication at 12% of the full

power (130 W) of a VCX 130PB probe-type sonicator (Sonics & Materials Inc., USA). A

sonication probe with a diameter of 3 mm was used.

X-ray Photoelectron Spectroscopic (XPS) characterization

A sample for XPS analysis was prepared by spotting 30 μL of an AuNP solution onto a 1×1

cm2 square of aluminum foil. The sample was allowed to dry in air. This spotting-and-drying

process was repeated three times to ensure enough sample coverage. XPS measurements were

carried out on an ESCALAB 250 high performance electron spectrometer using Al Kα (1486.6 eV)

radiation.

Samples for XPS analyses

5 nm AuNPs

5 nm AuNPs were used considering their larger surface area which can give stronger XPS

signals of surface adsorbed species. 100 μL of BSPP-exchanged 5 nm AuNPs (4 μM) was

centrifuged at 21500 g for 60 min. The supernatant was discarded, and the precipitate was

re-dissolved in 100 μL water. This process was repeated up to 5 times to remove excess BSPP in

the solution. The final AuNP solution was divided into two 100 μL parts for further experiments

(one as a control, and the other for an Ag+ treatment).

Ag+ and FSDNA treated 5 nm AuNPs

To one part of the above-obtained AuNPs was added 4 μL of 100 mM AgNO3. The resulting

mixture was kept at room temperature for up to 30 min before being subjected to a centrifugation

at 21500 g for 30 min to remove dissolvable Ag+-containing species in the supernatant. The

precipitate was dispersed in 100 μL of a FSDNA solution (1 mg/mL). After 30 min, the solution

was centrifuged at 21500 g for 1 h, with the precipitate being dissolved in 100 μL water. This

process was repeated one more time to remove free DNA and other dissolvable species.

TEM characterization

Transmission electron microscopy (TEM) images were taken on a JEM-2100F field emission

TEM operated at an electron acceleration voltage of 200 kV. Samples for TEM imaging were

prepared on carbon-coated copper grids. The sample droplets were allowed to stay on the TEM

grids for 5 minutes before being removed by gently touching with a piece of filter paper.

Spectroscopic characterizations

Optical extinctions were measured on a Hitachi U-2910 UV-Vis spectrophotometer. Raman

scattering was measured at room temperature with a low-volume liquid cell (40-100 µL) on a

portable Ocean Optics Raman system equipped with a Maya 2000 CCD detector and two incident

lasers (532 and 671 nm).

Reference:

[1] Slot, J. W.; Geuze, H. J. Eur. J. Cell. Biol. 1985, 38, 87-93.

[2] Frens, G. Nat. Phys. Sci. 1973, 241, 20-24.

[3] Bastús, N. G.; Comenge, J.; Puntes, V. Langmuir 2011, 27, 11098-11105.

[4] Bigall, N. C.; Härtling, T.; Klose, M.; Simon, P.; Eng, L. M.; Eychmüller, A. Nano Lett. 2008,

8, 4588-4592.

[5] Hu, J. W.; Zhang, Y.; Li, J. F.; Liu, Z.; Ren, B.; Sun, S. G.; Tian, Z. Q.; Lian, T. Chem. Phy.

Lett. 2005, 408, 354-359.

[6] Deng, Z. X.; Tian, Y.; Lee, S. H.; Ribbe, A. E.; Mao, C. D. Angew. Chem. Int. Ed. 2005, 44,

3582-3585.

92 90 88 86 84 82 800

20000

40000

60000

80000

100000

Cou

nts

Binding energy /eV

Au4f

174 172 170 168 166

8600

8800

9000

9200

9400

9600

9800

Cou

nts

Binding Energy /eV

S2p

140 138 136 134 132 130 12810400

10500

10600

10700

10800

10900

11000

11100

Co

unts

Binding energy /eV

P2p

406 404 402 400 398 396 394

14600

14800

15000

15200

15400

15600

Co

unts

Binding energy /eV

N1s

Figure S1. High resolution XPS spectra for Au, S, P, and N elements of BSPP-exchanged 5 nm gold nanoparticles.

The S and P elements came from the BSPP ligands on the AuNPs.

92 90 88 86 84 82 800

10000

20000

30000

40000

Co

unts

Binding energy /eV

Au4f

173 172 171 170 169 168 167 166 165

6850

6900

6950

7000

7050

7100

7150

7200

Cou

nts

Binding energy /eV

S2p

142 140 138 136 134 132 130 128 126

6100

6200

6300

6400

6500

6600

6700

Cou

nts

Binding energy /eV

P2p

406 404 402 400 398 396 394

12600

12800

13000

13200

13400

13600

13800

14000

14200

Cou

nts

Binding energy /eV

N1s

Figure S2. High resolution XPS spectra for Au, S, P, and N elements of BSPP-exchanged 5 nm gold nanoparticles

after Ag+ treatment and FSDNA adsorption. A decrease of S signal implied a partial desorption of BSPP

ligands from the AuNPs. The N signal came from the DNA bases of surface-adsorbed FSDNA on the AuNPs.

The P signal could be weakened due to a desorption of BSPP but then enhanced by the phosphate backbone

of adsorbed FSDNA (the overall intensity of the P signal thus did not show a big change).

136 135 134 133 132 131 130 129 128

Cou

nts

/ a.

u.

Binding Energy (eV)

AuNP BSPP

P2p scan

144 142 140 138 136 134 132 130 128 126

Cou

nts

/ a.u

.

Binding Energy / eV

Monomer FSDNA

P2p scan

144 142 140 138 136 134 132 130 128 126

Cou

nts

/ a.u

.

Binding Energy / eV

Dimer FSDNA

P2p scan

174 172 170 168 166

Cou

nts

/ a.u

.

Binding Energy / eV

AuNP BSPP

S2p scan

180 176 172 168 164 160 156

Cou

nts

/ a

.u.

Binding Energy (eV)

monomer

S2p scan

180 176 172 168 164 160 156

Co

unts

/ a.

u.

Binding Energy / eV

Dimer

S2p scan

Figure S3. High resolution XPS P2p (upper panels) and S2p (lower panels) spectra of BSPP-exchanged 13 nm

AuNPs as well as monomeric and dimeric products isolated from Ag+ soldered (in the presence of FSDNA) 13 nm

AuNPs by agarose gel electrophoresis. XPS data of BSPP and FSDNA have also been measured for comparisons.

400 500 600 700 8000.0

0.2

0.4

0.6

0.8

1.0

13.3 nm 1mer 13.3 nm 3mer 13.3 nm 4mer

Ext

inct

ion

Wavelength /nm Figure S4. (a,b) TEM images of purified 13.3 nm AuNP trimers and tetramers; (c) Visible extinction spectra of

monomeric, trimeric, and tetrameric 13.3 nm AuNPs after Ag+ soldering.

(A) (B)

(C)

Stored for 0, 1, 3, 7, 14, 30 days

Figure S5. Agarose gel electrophoretic stability examination of 5.5 nm, 13.3 nm, 24.1 nm, and 37.5 nm AuNP

dimers after being stored in water at room temperature over one month.

Figure S6. Agarose gel electrophoresis of AuNP dimers treated by strong sonications for different periods. Dimers

with a larger size were more susceptible to such a mechanical disruption.

(A) (C)

(B) (D)

(5.5 nm AuNP dimer)

(13.3 nm AuNP dimer)



400 500 600 700 800 900 1000

0.0

0.2

0.4

0.6

0.8

1.0

Ext

inct

ion

Wavelength /nm

24.1 nm dimer 24.1 nm dimer+H

2O

2

37.5 nm dimer 37.5 nm dimer+H

2O

2

Figure S7. Gel electrophoresis (A) and visible extinction spectra (B) of 24.1 nm and 37.5 nm AuNP dimers after

being etched with 10% H2O2 for 30 min to remove possibly existing Ag(0) species. No changes were caused by

the H2O2 treatment.

24.1 nm dimer 37.5 nm dimer

H2O2: + +

(A)

(B)

(A)

Control Ag+ K+ Mg2+ Ca2+ Cu2+ Ni2+ Zn2+

10 mM 10 mM 1 mM 1 mM 10 mM 2 mM 4 mM

(B)

Control Ag+ K+ Mg2+ Ca2+ Cu2+ Ni2+ Zn2+

10 mM 10 mM 1 mM 1 mM 10 mM 2 mM 4 mM

Figure S8. Agarose gel electrophoresis of 13.3 nm AuNPs after being treated by different metal ions in the

presence of FSDNA. Samples in (b) corresponded to the same samples in (a) after a precipitation (by

centrifugation) and redispersion in water before gel electrophoresis. It is clearly seen that only Ag+ treated samples

formed stable and sharp gel bands.

Ag+ +

AuNP Ag+ Na+ K+ Mg2+ Ca2+ Cu2+ Ni2+ Zn2+

5 4 0.1 0.05 0.1 0.1 0.1 mM

Ag+ +

AuNP Ag+ Na+ K+ Mg2+ Ca2+ Cu2+ Ni2+ Zn2+

10 10 1 1 10 2 4 mM

Figure S9. 2% Agarose gel electrophoresis showing that coexisting metal ions at relatively low concentrations

(upper panel) promoted Ag+ soldering efficiency of AuNPs, while high concentrations of coexisting cations (lower

panel) led to large AuNP aggregates that did not enter the gel.

[Ag+]/mM: 0 0.01 0.04 0.05 0.07 0.1 0.2 0.5 1 2 3 10 16

Figure S10. Agarose gel electrophoresis of 13.3 nm AuNPs after being treated by different concentrations of Ag+

in the absence of FSDNA.

Lanes: 1 2 3

Figure S11. Agarose gel electrophoresis showing that Ag+-induced aggregates (lane 2) of BSPP-capped 13.3 nm

AuNPs (lane 1) could be re-dispersed into discrete gel bands (lane 3) after an addition of FSDNA.

400 500 600 700 800 900 10000.0

0.2

0.4

0.6

0.8

1.0

1.2

AuNP DimerN

orm

aliz

ed

Ext

inct

ion

Wavelength / nm

0 mM 20 mM 40 mM 80 mM 100 mM 120 mM 140 mM 160 mM 200 mM 240 mM

[NaNO3](A-1)

10 100 1000 10000

0

5

10

15

20

25

Inte

nsi

ty

Dh / nm


AuNP Dimer[NaNO

3]

(A-2)

400 500 600 700 800 900 1000 1100 1200

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Nor

ma

lize

d E

xtin

ctio

n

Wavelength / nm

0 mM 20 mM 40 mM 80 mM 100 mM 120 mM 140 mM 160 mM

[NaNO3]BSPP-AuNP

(B-1)

10 100 1000 10000

0

5

10

15

20

25

30

35

Inte

nsi

ty

Dh / nm


[NaNO3]

BSPP-AuNP(B-2)

400 500 600 700 800 900 1000 1100

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Nor

ma

lized

Ext

inct

ion

Wavelength / nm


[KNO3]AuNP Dimer

(C-1)

10 100 1000 10000

0

5

10

15

20

25

30

Inte

nsity

Dh / nm


[KNO3]AuNP Dimer(C-2)

400 500 600 700 800 900 1000 1100

0.0

0.2

0.4

0.6

0.8

1.0

1.2

No

rma

lize

d E

xtin

ctio

n

Wavelength / nm


BSPP-AuNP [KNO3]

(D-1)

10 100 1000 10000

0

5

10

15

20

25

30

35

40

Inte

nsity

Dh / nm


[KNO3]BSPP-AuNP

(D-2)

Figure S12. UV-Vis (left panels) and DLS (right panels) data reflecting the salt resistivity of Ag+ soldered dimers

(A, C) in contrast to BSPP-capped AuNPs (B, D) against Na+ (A, B) and K+ (C, D). These data revealed an even

better stability of the dimer product against Na+ and K+ due to surface-adsorbed FSDNA.

400 500 600 700 800 900

0.0

0.4

0.8

1.2

No

rma

lized

Ext

inct

ion

Wavelength / nm

H2O

PBS( 15 mM NaCl) PBS( 25 mM NaCl) PBS( 35 mM NaCl) PBS( 55 mM NaCl) PBS( 75 mM NaCl) PBS( 95 mM NaCl) PBS(135 mM NaCl) PBS(175 mM NaCl)

13.3 nm Dimer(A-1)

10 100 1000 10000

02468

10121416182022242628

AuNP Dimer

Inte

nsity

Dh / nm

H2O

PBS( 15 mM NaCl) PBS( 25 mM NaCl) PBS( 35 mM NaCl) PBS( 95 mM NaCl) PBS(135 mM NaCl) PBS(175 mM NaCl)

(A-2)

400 500 600 700 800 900 1000 1100 1200 1300

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Nor

ma

lize

d E

xtin

ctio

n

Wavelength / nm

H2O

PBS( 15 mM NaCl) PBS( 25 mM NaCl) PBS( 35 mM NaCl) PBS( 75 mM NaCl) PBS( 95 mM NaCl) PBS(135 mM NaCl)

BSPP-AuNP(B-1)

10 100 1000 10000

0

5

10

15

20

25

30

35

40

Inte

nsi

ty

Dh / nm

H2O

PBS( 15 mM NaCl) PBS( 25 mM NaCl) PBS( 35 mM NaCl) PBS( 75 mM NaCl) PBS( 95 mM NaCl) PBS(135 mM NaCl)

BSPP-AuNP(B-2)

Figure S13. UV-Vis and DLS data reflecting the stability of Ag+ soldered dimers (A) in comparison with

BSPP-capped AuNPs (B) in a PBS buffer (pH 7.4) containing 8.2 mM Na2HPO4, 1.8 mM KH2PO4, and different

amounts of NaCl. These data revealed a much better stability of the dimer product in a saline buffer due to the

presence of surface-adsorbed FSDNA.

5 10 15 20 25 30 35 40500

550

600

650

700 Simulation, Longitudinal Experiment, Longitudinal Simulation, Transverse Experiment, Transverse

Pea

k po

sitio

n /n

m

Diameter /nm

Figure S14. Comparisons of plasmonic peak positions between experiments and simulations for 5.5 nm, 13.3 nm,

24.1 nm, and 37.5 nm AuNP dimers.

Figure S15. TEM images showing the approaching and coalescence of two AuNPs in a dimer structure under

electron beam irradiation in a zoom-in and zoom-out cycle.

400 500 600 700 800 900 1000

0.0

0.2

0.4

0.6

0.8

1.0

No

rma

lize

d E

xtin

ctio

n

Wavelength / nm

0.0 M 4-MPy 0.1 M 4-MPy 1.0 M 4-MPy 5.0 M 4-MPy 10 M 4-MPy

Figure S16. Visible extinction spectra accounting for the good stability of 37.5 nm AuNP dimers in the presence

of 4-MPy at concentrations close to that for SERS measurements.

400 600 800 1000 1200 1400 16000

1000

2000

3000

4000

5000

1110

cm

-1

Inte

nsity

/a.u

.

Raman shift /cm-1

AuNP monomer AuNP dimer

1020

cm

-1

Figure S17. Raman spectra of 4-MPy (10 M) in the presence of 37.5 nm AuNP monomers and dimers measured

with a 532 nm incident laser.

0 500 1000 1500 2000 25000

1000

2000

3000

4000

5000

6000

Inte

nsi

ty (

a.u

.)

Raman shift (cm-1)

H2O

500 mM 4-MPy

0 500 1000 1500 2000 25000

1000

2000

3000

4000

5000

6000

Inte

ns

ity

(a.u

.)

Raman shift (cm-1)

500 mM Py

Figure S18. Normal Raman spectra of 0.5 M 4-MPy (A) and pyridine (B) aqueous solutions excited by a 670 nm

laser.

(A) (B)

Figure S19. 2D views of electric field distributions for 5.5 (A), 13.3 (B), 24.1 (C), and 37.5 nm (D) gold

nanoparticle dimers obtained through near field calculations with DDASCAT 7.3. The top panels show electric

field intensity profiles along center-to-center track lines of the dimers. A 671 nm incident radiation polarized along

Y axis and propagating along X axis was assumed. Lattice spacing of 2a/160 was used for all simulations, where

2a represents nanoparticle diameter.

|E|/

|E0|

|E|/

|E0|

|E|/

|E0|

|E|/

|E0|

Figure 20. 2D views of electric field distributions for 5.5 (A), 13.3 (B), 24.1 (C), and 37.5 nm (D) gold

nanoparticle dimers calculated by DDASCAT 7.3. A 532 nm incident radiation polarized along Y axis and

propagating along X axis was assumed.

(A) (B)

(C) (D)


nanoparticle monomers calculated with DDASCAT 7.3. A 671 nm incident radiation polarized along Y axis and



nanoparticle monomers calculated with DDASCAT 7.3. A 532 nm incident radiation polarized along Y axis and


(A) (B)

(C) (D)

(A) (B)

(C) (D)

13.3 nm AuNP dimer 24.1 nm AuNP dimer

w/o Ag+ +Ag+ +10xAg+ w/o Ag+ +Ag+ +10xAg+

Figure S23. Gel electrophoresis showing that as-purified AuNP dimers were stable in an Ag+ solution with a

concentration equal to that used for Ag+ soldering. The dimers got further reacted with Ag+ and precipitated in a

10 concentrated Ag+ solution.

Table S1. TEM-based statistical analysis indicating a high purify of gel-isolated dimers.

Size/Material Monomer counts

(Nmo)

Dimer counts

(Ndi)

Trimer counts

(Ntr)

Dimer yield

[2Ndi/(Nmo+2Ndi+3Ntr)]

5.5 nm/Au 17 245 3 95%

13.3 nm/Au 6 239 2 98%

24.1 nm/Au 19 229 3 94%

37.5 nm/Au 19 243 6 93%

29 nm/Au@Pd 23 226 7 91%

30 nm/Pt 10 204 1 97%

flash” preparation of strongly coupled metal nanoparticle

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