Presented by: Umesh AgarwalInstitution: USFS FPL

Analysis of Lignin by Surface Enhanced Raman Spectroscopy

Micro-Raman

SERS UVRR

FT-Raman

ConventionalRaman

Lignin

Outline

• What is SERS• Mechanisms – surface plasmon and CT• Near-IR lignin SERS• Spectral characteristics• Carbohydrate contribution• CT mechanism - XPS analysis• Lignin models• Applications• Conclusions

What is surface enhanced Raman scattering or SERS

Raman effect is greatly enhanced when a molecule is close to a

rough metal surface.

(Nano- and micro-particles)

Benzoic acid

What is the origin of the enhancement?What is the origin of the enhancement?

1. Electromagnetic mechanism – 106 to 1013

enhancement2. Charge transfer (CT) mechanism – up to

100 times enhancement

surface plasmonssurface plasmons

Electromagnetic mechanismElectromagnetic mechanism

Second mechanism Second mechanism is charge transferis charge transfer

Ag

analyte

SERS

CT mechanism

SERS contribution due to polarizability modification of the adsorbed molecule

• Ag, Au, Cu, Li, K, and Na have shown SERS effect

• Dielectric properties of the noble metals most suited to plasmon resonance

• Light scattering efficiencyElocal = E0/[ε(ω) + 2]

Why only particles of certain metals

Electromagnetic mechanismElectromagnetic mechanism……

Shape dependence of the surface plasma excitations

shape

Au nanorods with increasing aspect ratios

size

aggregated vs.

unaggregated

aggregation

Another example and enhancement factor

calculation

Muconic acid

1

10

100

1000

10000

100000

1000000

10000000

1 10 100 1,000 10,000 100,000 1,000,000 10,000,000

r a t io Ag/ a c i d

Enhancement factorP

eak

heig

ht ra

tioCompared to the intensity of an equivalent # of molecules in solution

SERS of lignin

Spruce MWL -Normal vs. SERS

reproducibility

same spot

0.04

0.05

0.06

0.07

0.08

0.09

11501200125013001350140014501500155016001650

Intensity reproducibility and standard deviation

10 SERS spectra

Significant SD

as little as 10 μ gram of ligninby SERS

Sensitivity

Chemical modification due to adsorption on Ag ?

190 180 170 160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 ppm

1D 13C NMR

post adsorption

ppm

10 9 8 7 6 5 4 3 2 1 ppm

180

160

140

120

100

80

60

40

20 2D NMR

Lignin SERS - dilution effects

lignin

Ag

Modes of lignin aggregation

Packing density

Spectral characteristics

• Shifted band frequencies• Altered band intensities• Appearance of new bands• Most bands belong to the aromatic unit• Bands enhanced only in the region 250-

1750 cm-1 region• Relaxation of selection rules

Lignin in Wiley-milled wood

Aggregates of silver (a) and wood (b) particles (SEM)

WMW

MWL

Normal WMW

No carbohydrate contribution

Lignin in unbleached KP

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

250 750 1250 1750 2250 2750 3250

cm-1

Inte

nsity

, a.u

.

SERS Unbleached kraft pulp

Control pulp

100 nm Ag

Mechanisms operating in lignin SERS

Diffuse reflectance excitation spectra (near-IR)

0.39

0.395

0.4

0.405

0.41

0.415

0.42

0.425

40005000600070008000900010000

cm-1

KM

Uni

ts

2- 3.5 μ Ag particles

1. EM mechanism

9398 cm-1 or 1064 nm

Evidence of CT complex

2. Charge Transfer2. Charge Transfer

Ag

lignin

CT mechanism

Binding Energy (eV)

N(E)

Min: 0 Max: 176091

385 382 379 376 3 73 370 36 7 364 361 358 355

Only Ag particles

XPS

3d 3/2 and 3d 5/2 electron emission from silver

Binding Energy (eV)

N(E)

Min: 0 Max: 47217

385 382 379 376 3 73 370 36 7 364 361 358 355

XPS

MWL + Ag

3d 3/2 and 3d 5/2 electron emission from silver

R1

R2

R3

R4

SERS of lignin models

40 lignin models studied

COCH3

OCOCH3

COOH OCH3OH COH

> >>

COOH COH

> > >

• Electron withdrawing groups on or conjugated groups

• Multiple electron-withdrawing groups

• Differences between G and S

SERS strengthening

Applications

1Sensor – lignin detection in cotton linters

0.025

0.035

0.045

250 750 1250 1750

Wavenumber/cm-1

Ram

an In

tens

ity/A

rbitr

. Uni

ts

420 641

780928

1265 1493 1582

0

0.1

0.2

0.3

250 650 1050 1450 1850

Wavenumber/cm-1

Inte

nsity

/Arb

itr. U

nits B Spruce

Aspen

1272

1319

586

1308

1270

G- vs. S-MWL SERS

2

0

0.1

0.2

0.3

0.4

0.5

0.6

250 450 650 850 1050 1250 1450 1650 1850

Wavenumber/cm-1

Inte

nsity

/Arb

itr. U

nits

WMW 40 meshMWLKraft pulp

3Differences in lignin SERS of wood, MWL and Unbl. KP

Conclusions

• SERS of MWL, WMW and Unbl. KP were obtained

• Lignin can be selectively detected without separation

• As little as 10 μ gram of lignin can be detected• Lignin is chemisorbed on Ag• G- and S-lignins were easily distinguished• Models work - insights on structural attributes

important in SERS

Rick ReinerJim BeecherSally RalphTom Kuster

Published Online: May 6, 2009 J. Raman SpectroscopyDOI: 10.1002/jrs.2294