analysis of lignin by surface enhanced raman spectroscopy
TRANSCRIPT
Presented by: Umesh AgarwalInstitution: USFS FPL
Analysis of Lignin by Surface Enhanced Raman Spectroscopy
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)
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
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……
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
0.04
0.05
0.06
0.07
0.08
0.09
11501200125013001350140014501500155016001650
Intensity reproducibility and standard deviation
10 SERS spectra
Significant SD
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
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
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
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
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
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