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16 Νοε 2013 (πριν από 3 χρόνια και 6 μήνες)

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Surface
-
Enhanced Raman Scattering
(SERS)

Laser
Excitation

(
w
0
)

Raman
Scattering

(
w
0

w
vib
)

m

=
a
E

Chemical

Electromagnetic

Haynes, McFarland, and Van Duyne,
Anal. Chem.,
77
, 338A
-
346A (2005).


Enhancement

Factor (EF)

=

s
SERS

s
剡浡n

SERS Enhancement Mechanisms

Chemical Mechanism
:

Laser excites (a) new electronic states arising from

chemisorption or (b) shifted or broadened adsorbate

electronic states yielding a resonance condition.



Short range (1
-
5
Å
)



No roughness requirement



Contributes EF ~ 10
2



10
4


Electromagnetic Mechanism
:

LSPR induces large electromagnetic fields at roughened

metal surface where molecules are adsorbed.



Long range (2
-
4 nm)



Affected by all factors determining LSPR



Contributes EF > 10
4


Localized Surface Plasmon Resonance

Non
-
resonant

Resonant

1)
Resonant
l

is absorbed

2)
EM fields localized at nanoparticle surface

Metal Salt

Reducing Agent

Capping Agent

Metal Colloids

coated with

Reducing Agent/Capping Agent

heat or


sonication

h
n



Aged

Metal Colloids

Noble Metal Nanoparticles

50 nm

0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
400
500
600
700
800
Absorbance
Wavelength (nm)
Master
Au_1spin_5min_7500xg.Master.Absorbance
Au_3spin_5min_7500xg.Master.Absorbance
D = 24.8
±

4.1 nm

Noble Metal Nanoparticles

Nanostructured Substrates

http://pubs.acs.org/cgi
-
bin/article.cgi/ancham
-
a/0000/77/i17/pdf/905feature_vanduyne.pdf

L
ocalized
S
urface
P
lasmon
R
esonance

The resonance results in (1) wavelength
-
selective
extinction and (2) enhanced EM fields at the surface.

Spectral location of the LSPR is dependent upon particle
size, shape, composition, and
dielectric environment
.

2 3 3/2
2 2
24
ln(10) ( 2 )
m
I
R m I
Na
Extinction
 

l   
 

 
 
 
Resonant Rayleigh Scattering

Light that is elastically scattered due to the LSPR can
be used as an
in situ

monitor of nanoparticle optical
properties.



The primary benefit of scattering spectroscopy is that
the single
-
to
-
noise ratio is much higher than
extinction spectroscopy when examining single
nanoparticles.

20
m
m

Measuring the LSPR

Wavelength (nm)

S

c

a

t

t

e

r

i

n

g



I

n

t

e

n

s

i

t

y

4

0

0

5

0

0

6

0

0

7

0

0

8

0

0

9

0

0

0

2

0

0

4

0

0

6

0

0

8

0

0

1

0

0

0

4

0

0

5

0

0

6

0

0

7

0

0

8

0

0

9

0

0

0

2

0

0

4

0

0

6

0

0

8

0

0

1

0

0

0

S

c

a

t

t

e

r

i

n

g



I

n

t

e

n

s

i

t

y

Wavelength (nm)

4

0

0

5

0

0

6

0

0

7

0

0

8

0

0

9

0

0

0

2

0

0

4

0

0

6

0

0

8

0

0

1

0

0

0

S

c

a

t

t

e

r

i

n

g



I

n

t

e

n

s

i

t

y

4

0

0

5

0

0

6

0

0

7

0

0

8

0

0

9

0

0

0

2

0

0

4

0

0

6

0

0

8

0

0

1

0

0

0

S

c

a

t

t

e

r

i

n

g



I

n

t

e

n

s

i

t

y

LSPR Dielectric Response

Blue = N
2

(1.000), Green = Methanol (1.329), Red = 1
-
Propanol
(1.385), Purple = Chloroform (1.446), Orange = Benzene (1.501)

450

500

550

600

650

700

0

20

40

60

80

100

Wavelength (nm)

I

n

t

e

n

s

i

t

y

A

510.2

574.2

588.0

600.8

611.9

1

1.2

1.4

1.6

480

520

560

600

640

Refractive Index

l

m

a

x



(

n

m

)

B

l

max

= 203.1*RI + 306.5

Single Nanoparticle Sensing


Proof of Concept

Wide
-
field Image

Biosensing with Single Nanoparticles

Streptavidin:

60kDa Tetrameric Protein

~4 nm x 4 nm x 5nm

Biotin:

Vitamin H

K
a

= 10
14

M
-
1

http://relic.bio.anl.gov

Biosensing with Single Nanoparticles

450

500

550

600

0

400

800

1200

508.0

520.7

Wavelength (nm)

I

n

t

e

n

s

i

t

y

Dl
max

= +12.7 nm


Conc = 10 nM SA


~700 SA molecules

Monitoring Biotin
-
Streptavidin Binding with Single Nanoparticles

Comparison of Unenhanced and Enhanced
Raman Spectra

Relationship between the LSPR and Laser
Wavelength

l
max

= 690 nm


1575 cm
-
1

band


N = 13 points


Range = 475


700
nm


High = 662 nm


EF = 1.9 x 10
7

Locating Landmines with SERS

Sylvia, J. M. et al.
Anal. Chem.
2000
, 72,
5834.

l
ex

= 785 nm

Roughened Au

Locating Landmines with SERS

Sylvia, J. M. et al.
Anal. Chem.
2000
, 72,
5834.

115 mW

30 sec

TNT

DNT

DNB

Are you getting the concept?

Why hasn’t surface
-
enhanced Raman replaced normal

Raman completely? In other words, why would someone

do a normal Raman scattering experiment?

Partition Layers to Detect Non
-
Traditional
Analytes with SERS


Kyle Bantz

Polychlorinated Biphenyls (PCBs)


Present in soil, water, and air worldwide


Classified as persistent organic pollutants


Analysis traditionally done by GC
-
MS


Complicated by low

concentrations and

multiple congeners


Prohibitively difficult

for
in situ

environmental

analysis


Partitioning PCBs

Other Experiments


Use band intensities
and shifts to
understand the
partitioning
mechanism


Environmental
interferants not a
problem


Substrates can be
reused


Are you getting the concept?

Kyle fabricates a new Ag SERS substrate and wants to calculate the EF.
She doses the substrate with benzenethiol (packing density = 6.8 x 10
14

molecules/cm
2
), and measures the spectrum with a laser spot size of
1.26 mm
2
. The intensity of the 1000 cm
-
1

shift ring breathing stretch is

35793 adus. For her standard, she puts undiluted benzenethiol (
r

=
1.073 g/cm
3
) into a cuvette with a known probe volume of 0.00413 mL
and measures a 1000 cm
-
1

shift band intensity of 36364 adus. What is
the EF (assuming that the laser power and collection time were the

same for both measurements)?

Single Molecule SERS

S. Nie, et. al.
Science

1997,
275
, 1102
-
1106

EF=

10
14
-
10
15

!

K. Kneipp, et. al.
Phys. Rev. Lett.

1997,
78
, 1667
-
1670

Hyper Raman and Surface
-
Enhanced
Hyper Raman Spectroscopy

With focused, pulsed laser, you can induce a non
-

linear interaction:

1 1
...
2 6
ind
E E E E E E
m a  
      
Incident:
n
0

Scattered: 2
n
0
, 2
n
0

±

n
1

Selection Rules:

All IR active modes are also hyper
-
Raman active

Some hyper
-
Raman active modes are neither IR

or Raman active

Coherent Anti
-
Stokes Raman

Phase
-
match
w
p

and
w
s

in a four wave mixing process:


Incident:
n
p
,
n
s

Scattered: 2
n
p

-

n
s

CARS Advantage:

The
n
AS

signal beam is spatially and temporally removed

from the fluorescence signal.

1 1
...
2 6
ind
E E E E E E
m a  
      
I
CARS

~ I
p
2
I
s

Large signal when:


n
p
-
n
s

=
D

D