Nobel Metal Nanotags Active in SERS for Sensing and Imaging (NSF Grant CMMI 1136287)

Polina Pinkhasova,1 Tsengming Chou1, Jin Park1, Svetlana Sukhishvili2 and Henry Du1

1Department of Chemical Engineering and Material Science, Stevens Institute of Technology, Hoboken, NJ 07030, USA 2Department of Chemistry, Chemical Biology, and Biomedical Engineering, Stevens Institute of Technology, Hoboken, NJ 07030, USA

Introduction

We have developed nanotags encapsulated in an

intricate Au-Ag nanostructure that is active in surface

enhanced Raman scattering (SERS). Hollow Au-Ag

alloy nanoshells with a tunable and porous wall were

synthesized by galvanic replacement reaction, and

were subsequently loaded with thiocyanate (SCN-)

tag molecule. The open structure of the nanoshells

was filled with Ag via citrate reduction, closing the

pores and sealing SCN- inside the nanoparticles. The

resultant nanotags were individually SERS-active,

yielding linear correlation between the SERS intensity

of the encapsulated SCN- and the number of

nanoparticles.

Figure 1. TEM image of nanoshell (left) and nanotag

(right) filled with SCN- and citrate reduced Ag.

Synthesis and

Characterization

Figure 2. UV-visible absorption spectra obtained for

aqueous dispersions of (1) citrate reduced Ag colloids

with localized surface plasmon resonance (LSPR) of

408 nm, and (2-6) galvanic displacement reaction of

porous alloyed Ag-Au nanoshells with molar ratios of

20, 10, 8, 4 and 2.85 of colloidal Ag to HAuCl4

solution with LSPR ranging from 445 to 725 nm.

Corresponding TEM images of (1) Ag colloids with

average size of 47 ± 6 nm and zeta-potential of -48 ±

8 mV and (5) Ag-Au nanoshells with average size of

56 ± nm and zeta-potential of -17 ± 7 mV.

.

Figure 5. SERS spectra of (A-1) nanotags loaded with

SCN- both within and on the surface of the nanostructure,

(A-2) nanotags treated with 1 M NaCl that displaced

surface-adsorbed SCN-, (A-3) 200 ppb of 1,2-di-(4-

pyridyl)ethylene (BPE) obtained using nanotags after (A-

2). (B-1) SCN- adsorbed at the surface of Ag

nanoparticles, (B-2) after subsequent treatment with 1 M

NaCl. Spectra (1) and (2) were acquired at an excitation

wavelength of 633 nm (4 mW and 20 s). The

characteristic bands for BPE at ~1636 cm-1 and ~1210

cm-1 correspond respectively to δ(C-N) and ν(C=N), the

band at ~2120 cm-1 corresponds to SCN-.

Figure 6. (A) Adsorption isotherm of BPE. θ = I/ISCN- or

θ = I/Imax, where I is the SERS intensity of the

characteristic band at 1636 cm-1 that is proportional to the

amount of adsorbed BPE, ISCN- is the SERS intensity of

the reference SCN- inside the nanotags, and Imax is the

SERS intensity at saturated surface coverage of BPE as a

function of the solution concentration. (B) Linear

correlation between SERS intensity of SCN- band and

nanoparticle coverage density as supported by SERS and

SEM measurements.

Conclusions

⟡ We have developed individually SERS-active nanotags

that uniquely function both as a reporter and ultra-

sensitive platform for SERS measurements of analytes of

interest.

⟡ Our nanotags have the potential to overcome the

current challenges in quantitative SERS measurements

as well as further advance SERS-based method for a

wide range of applications such as trafficking of

nanoparticles in cellular systems.

Figure 3. EDX, TEM with elemental mapping, and

UV-visible absorption spectra of nanostructures at

various stages of the nanotag synthesis process. In

the UV-visible spectra, the blue curve corresponds to

porous Ag-Au nanoshells with LSPR at 668 nm and

atomic composition of 53% Au and 47% Ag, the red

curve refers to nanoshell with partially closed core

with LSPR at 511 nm and atomic composition of 32%

Au and 68% Ag, and the green curve shows nanotags

filled with SCN- and citrate reduced Ag which

nucleates and grows inside the core with atomic

composition of 14% Au and 86% Ag and LSPR at 443

nm.

SERS and Imaging

Figure 4. SERS spectrum of nanotags with

characteristic SCN- vibration at ~2120 cm-1 and the

corresponding SEM and Raman images. SERS

spectrum and Raman image were acquired at an

excitation wavelength of 785 nm (20 mW and 20 s).

Inset in SERS spectrum illustrates the use of

poly(allylamine hydrochloride) to immobilize the

nanotags on planar substrates via electrostatic

interactions.

SCN-

Ag+ + citrate

400 500 600 700 8000.0

0.9

1.8

2.7

Ab

so

rba

nce

Wavelength, nm

1

2

3

4 56

1 3Ag(s) + AuCl- 4 (aq) → Au(s) + 3Ag+(aq) + 4Cl-(aq)

1000 1500 2000 2500 3000

Inte

nsity, (a

.u.)

Wavenumber, (cm-1)

21

20

cm

-1 S

CN

-

SERS substrate

1.6 2.4 3.2 4.00

4500

9000

13500

Co

un

ts

Energy, (keV)

Ag

Au

20 nm

A

1.6 2.4 3.2 4.0

Energy, (keV)

Ag

Au20 nm

B

1.8 2.4 3.0 3.6

Energy, (keV)

Au

Ag

20 nm

C

400 500 600 700 800

0.9

1.8

Absorb

ance

Wavelength, nm

(C) 668 nm

(B) 511 nm

(A) 443 nm

800 1600 2400

Inte

nsity, (a

. u

.)

Wavenumber, (cm-1)

1) nanotags loaded w/ SCN-

2) surface treated w/ 1M NaCl

3) BPE 200 ppb at the surface

21

20

12

10

16

36

A

800 1600 2400

Wavenumber, (cm-1)

1) SCN- at the surface of Ag nanoparticles

2) Ag surface treated with 1 M NaCl

B

21

20

(S

CN

- )control

0.5 0.6 0.9 2 5 7.5 100.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

T

he

ta

[BPE] x 10-6, M

(1) BPE normalized to SCN- [I

BPE/I

SCN]

(2) BPE [I/Imax

]

A

20 40 60 80 100

3x105

5x105

6x105

8x105

9x105

2100

are

a o

f S

NC

- pea

k

number of nanoparticles / um2

B

Inte

nsity, a

. u

.

Wavenumber, (cm-1)

21

20

(S

CN

- )