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Plasmonic Metal NanoparticleFunctionalization forSelective Sensing ofPersistent Pollutants

by SERSLuca Guerrini

Instituto de Estructura de la Materia. CSIC. Serrano, 121. 28006-Madrid. Spain.

SERS roundtable 2007, Poltersdorf, Germany

OUTLINESERS: looking for Hot Spots (HS)Metal Colloids: best active SERS substrateMetal-Adsorbate interactionsDetection of molecules (PAHs) with low

affinity toward the metal surface: Functionalization

Host Molecules:– Calixarenes– Viologen Dications– Alkyl Diamines

Conclusions

Nanostructures for spectroscopy

Enhancement of Spectroscopic Signal

10-28 cm2/mol

10-22 cm2/mol

10-18 cm2/mol

SERSRaman

No Metal Metal as substrate

Cross-section

Surface-enhanced Spectroscopy: Mechanisms

Electromagnetic Mechanism

Charge Transfer Mechanism

SERS, SEIRA, SEF

SERS

E0 (ω0) Sphere: A=1/3 Eint

E0 3E0 Eint = ⎯⎯⎯⎯⎯⎯ ≅ ⎯⎯⎯

1 + ( ε - 1 ) A ε + 2

Edip (ωR) ESC (ωR)

(1) Electromagnetic Mechanism

A: Depolarisation constant depending on nanoparticle size and shape

ε : Dielectric constant ε(ω) = Re[ε(ω)] + Im[ε(ω)]iRe[ε(ω)] ≈ -2 ⇒ Plasmon ResonanceIm[ε(ω)] → 0 ⇒ Minimum Resistivity

Metals: Ag, Au, Cu

d

Localization of electromagneticfield on Interparticle

JUNCTIONSHOT SPOTS

2nm

25nm

80% SERS signal from 0.64% of molecules adsorbed(Le Ru et al. J.Chem.Phys. 125, 204701(2006))

(2) Charge Transfer Mechanism(chemical mechanism)

Otto wrote: “without the EM mechanism there would be no signal, but the chemical mechanism determines what is observed”

NO O

Ag, Au, Cu

• Distortion of the electronic cloud of theadsorbate new molecule or surface complex

• First layer effect

Characteristics of the phenomenon (EM mechanism and CT mechanism)

High sensitivity: cross section increases (EF up to 1014 SMD).Short range effect (>10nm, no SERS effect) .Quenching FluorescenceNew selection rules (EDIP(ω0) has tangential and normal components with different enhancements. Molecule adsorbed with specific orientation with respect to the surface) .

OUTLINESSERS: looking for Hot Spots (HS)Metal Colloids: best active SERS substrateMetal-Adsorbate interactionsDetection of molecules (PAHs) with low

affinity toward the metal surface: Funtcionalization

Host Molecules:– Dithio Calixarenes– Viologen Dications– Alkyl Diamines

Conclusions

--

-

-

---

--

--

-- ---

-- -

-

-

- ----

Obtention: Nucleation

CHEMICAL REDUCTION

Mp+ + Red → M0 + Prod

Red: NaBH4, Na3Cit, NH2OH, NH2NH2

Nucleation:

nM0 → Mn

mMn → Mn*m

Metal Colloids as SERS substrate

• Easy to prepare, broad plasmon resonance in the visible-near IR Region, high stability…

• Empirical knowledge: highly aggregated colloids as the best substrate for obtaining good SERS signals

• Each preparation leads to a distinct distribution

(MACRO): AVERAGE SERS (Semi)Quantitative Analysis• statistical average SERS spectra from an ensemble of scatterers• relatively low Raman enhancement factors (up to ~106)• normally stable and reproducible spectra

(MICRO): ULTRASENSITIVE SERS Trace Detection (SMD)• SERS from highly localized nanoscale interstitial sites in nanostructures • Raman enhancement factors >1010

• fluctuations on reaching submonolayer coverage

Slide glass


affinity toward the metal surface: Funtcionalization

Host Molecules:– Dithio Calixarenes– Viologen Dications– Alkyl Diamines

Conclusions

In wet systems the diffusion of the adsorbate from the bulk to the NP surface is needed.

(Ag-O) OXYGEN-containing Adsorbatesi.e. Benzoic Acids, Catechol

(Ag-N) NITROGEN-containing Adsorbatesi.e. Pyridine

(Ag-S) SULFUR-containing Adsorbatesi.e. Thiols and Dithiocarbamates

(Ag-X) X= halogen i.e R6G

Metal NP-Adsorbate Interactions

Many other molecules are not able to approach the NP surface due to their poor affinity to the metal

PAHs are a group of pollutants with condensed benzene rings formed during the incomplete combustion of coal, oil, and gas or other organic substances such as tobacco or charbroiled meat. Many of them have been reported to be strong carcinogens.

PYRENE (PYR)

POLYCYCLIC AROMATIC HYDROCARBONS


affinity toward the metal surface:Functionalization

Host Molecules:– Dithiocarbamate Calixarenes– Viologen Dications– (Alkyl Diamines)

Conclusions

Functionalization

=

• Good adherence on the metal• High affinity to bind the analyte• No bands overlapping • Inter or Intra-molecular CAVITY

Conditions to be fulfilled by the host molecule:

Calixarenes

OO

OO

OO

OO

HH

Calixarenes (CXs) are synthetic cyclooligomers:• "cup-like" shape• capable of size-selective molecular encapsulation. • by changing the chemical groups of the upper

and/or lower rim, it is possible to modifytheir affinity toward the guest and themetal surface.

Pyrene

Triphe

nilen

e

Anthrac

ene

Benzo

[c]ph

enan

trene

Chrise

ne

Rubice

ne

Dibenz

oanth

racen

e

Corone

ne

SER

S In

tens

ity

1500 1000 500

SERS of Pyrene (10-4 M)

Solid Pyrene

CalixarenesCalixarenes (CXs) are synthetic cyclooligomers:

• "cup-like" shape• capable of size-selective molecular encapsulation. • by changing the chemical groups of the upper

and/or lower rim, it is possible to modifytheir affinity toward the guest and themetal surface.

Pyrene

Triphe

nilen

e

Anthrac

ene

Benzo

[c]ph

enan

trene

Chrise

ne

Rubice

ne

Dibenz

oanth

racen

e

Corone

ne

SER

S In

tens

ity

1500 1000 500

SERS of Pyrene (10-4 M)

Solid Pyrene

OO

NH

OO

HH

SHS

N H

SHS 5 6 7 8 9 10

SER

S in

tens

ity o

f PA

Hs

-log[PAHs]

PYRENE BENZO[C]PHENANTHRENE

CORONENE TRIPHENYLENE

Functionalization

=

• Good adherence on the metal• High affinity to bind the analyte• No bands overlapping • Inter or Intra-molecular CAVITY

Conditions to be fulfilled by the host molecule:

• BIFUNCTIONAL COMPOUNDSPromoting the organization of the metal NPs intocontrolled architectures (inducing HS formation)attract analytes to the region of the so-formed HS

VIOLOGEN DICATIONS (VGD)N

N

CH3

CH3

N

N

N

N

CH3

CH3

+

+

+

+

+

+

DQ2+

MV2+ LG2+

• Disubstituted-4,4’-bipyridilium dication• Bifunctionality• Strong Electron-acceptors charge

transfer interactions with charge donors

1550 1600 1650

1624

1627

1585

1584

1608

1578

1613

Wavenumbers (cm-1)

N

NCH3

H3C

(+)

(+)

N

N(+)

(+)

= Cl-

solid LG

SERS LG 10-6M

solid DQ

SERS DQ 10-4M

Two bands in the 1550-1650 cm-1 regionare sensitive to the dication structure:

1550 1600 1650

Wavenumbers (cm-1)


N

CH3

CH3

N

N

N

N

CH3

CH3

+

+

+

+

+

+

DQ2+

MV2+ LG2+



1550 1600 1650

1624

1627

1585

1584

1608

1578

1613

Wavenumbers (cm-1)

N

NCH3

H3C

(+)

(+)

N

N(+)

(+)

= Cl-

solid LG

SERS LG 10-6M

solid DQ

SERS DQ 10-4M

LG high tendency to form CT complexes both in solid state and adsorbed on AgNP

DQ high tendency to form CT complexes just when adsorbed on AgNP

MV CT transition seems not to take place both in solid state and adsorbed on AgNP


N

CH3

CH3

N

N

N

N

CH3

CH3

+

+

+

+

+

+

DQ2+

MV2+ LG2+



1550 1600 1650

1624

1627

1585

1584

1608

1578

1613

Wavenumbers (cm-1)

N

NCH3

H3C

(+)

(+)

N

N(+)

(+)

= Cl-

solid LG

SERS LG 10-6M

solid DQ

SERS DQ 10-4M

VDG interact with the Ag surfacethrough the formation of a CT complexwith the adsorbed chloride.

LG is the best candidate to formjunctions between NPS.

400 600 800 1000 1200 1400 1600

LG 10-6M

SER

S in

tens

ity

wavenumber (cm-1)

SERS DETECTION OF PYRENE

LG 10-6MPYR 10-6M

difference spectra

Solid PYR

• Changes in relative intensities of Raman bands of PYR (590cm-1)

• Rotation of the acridine rings

1200 1250 1300

SER

S In

tens

ity

LG(1191)

LG(1280)

PYR

Wavenumber (cm-1)

H

H

δ(C-H)

N

N(+)

CH3

(+)

CH3

ν(C-C)ir

LG-PYR interaction takes place through a CT complex

N

N

CH3

H3C(+)

(+)

Upwards view of PYR interaction

LG – PYR INTERACTION

+

N

N

CH3

H3C(+)

(+)

+

PYR 10-6M: SERS intensity variation at different [VGD]

LIMIT OF DETECTION:LG 10-6M, DQ 10-4M, MV 10-6M

6 7 8 9 10

I 590

of P

YR

6 7 8 9 10

-log [PYR]

1064nm

785nm

514nm

3 4 5 6 7 8 9 10

I 590 o

f PY

R3 4 5 6 7 8 9 10

MV2+

DQ2+

LUC2+

-log[VGD]

1064nm

785nm

514nm

MICRO SERS:~ 100 molecules of PYR

6 7 8 9

SER

S in

tens

ity o

f PA

Hs

-log[PAHs]

DQ 10-4M PYR LG 10-6M PYR

DQ 10-4M BcP LG 10-6M BcP

6 7 8 9

SE

RS

inte

nsity

of P

AH

s

-log[PAHs]

DQ 10-4M PYR LG 10-6M PYR

DQ 10-4M BcP LG 10-6M BcP

785nm

HH

H

H

H

HHH

H

H

H

H

HH

HH

HH

H

HH

H

HH

H

HHH

PYRENE BENZO[C]PHENANTHRENE

N

NCH3

H3C

(+)

(+

pyrene

400 500 600 700 800 900 1000

plas

mon

abs

orba

nce

wavelenght (nm)

Absorption Spectra

AgNPLG 10-6M

400 600 800 1000 1200 1400 1600

difference spectra

PYR 10-6MLG 10-6M

PYR 10-5M

SER

S in

tens

ity

wavenumbers (cm-1)

SERS Spectra

HS FORMATION BY VGD

400 500 600 700 800 900 1000

370-390

800-1000

500-600

Plas

mon

Abs

orba

nce

Wavelength (nm)

UV-Vis Spectroscopy

50 nm

500 nm

SEM AFM

370-390assigned to non aggregated NPs

and/or transversal plamons in associated NPs chains

500-600may corresponds to the gaps

between two NPs (Käll et al. J. Raman Spectr. 2006, 36, 510)

800-1000assigned to larger aggregates or

multimers

DIFFERENCE ABSORPTION SPECTRA

HS FORMATION BY VGD

400 500 600 700 800 900 1000

370-390

800-1000

500-600

Plas

mon

Abs

orba

nce

Wavelength (nm)

UV-Vis Spectroscopy (500-600) DIMER ADS. BANDTo understand the formation of HS by VGD we have followed the dimer absorption band, whose position depends on:• the VGD structure• the concentration (DQ)

DIFFERENCE ABSORPTION SPECTRA

480 560 640

556

540 590

N

NCH3

H3C

(+)

(+)

-N

NCH3

H3C

(+)

(+)

d2 < d1

N

N

d1

N

N

d2

LG 10-6 M

DQ 10-4 M

DQ 10-5 M

DQ 10-6 M

Good agreement with the changes that the v(Ag-Cl) band undergoes in SERS spectra

nm

3 4 5 6 7 8 9 100,0

0,5

1,0No

rmal

ized

Abs

orba

nce

(a. u

.) AgNPs/LG AgNPs/DQ

-log[VGD]

0,0

0,5

1,0

3 4 5 6 7 8 9 10

LucigeninDiquatMethyl VIologen

3 4 5 6 7 8 9 10-log[VGD]

SER

S In

tens

ity o

f VG

D

I 590 o

f PY

RNo

rmal

ized

Abs

orba

nce

(a. u


-log[VGD] Plasmon absorbance intensity of AgNPs dimers in the 500-600 nm region at different VGD concentrations

Intensity of the 590 cm-1 band of PYR 10-6M at different VGD concentrations

Intensity of the SERS spectra measured for the main band of LG, DQ and MV at different VGD concentrations

N

NCH3

H3C

(+)

(+

pyrene

• assignment of the plasmon band to the interparticle junctions in dimers

• the VGD are mainly placed in these junctions

• the main part of the pollutant is attracted in these junctions to render a huge SERS signal

The lower SERS effectiveness observed for MV is presumably related to the different adsorption mechanism on metallic surfaces.

0,0

0,5

1,0

3 4 5 6 7 8 9 10

LucigeninDiquatMethyl VIologen

3 4 5 6 7 8 9 10-log[VGD]

SER

S In

tens

ity o

f VG

D

I 590 o

f PY

RNo

rmal

ized

Abs

orba

nce

(a. u


-log[VGD]

CONCLUSIONSEfficiency of VG dications in the formation of highly sensitiveHS localized at junctions between Ag NPs. • VGD act as:

builders of sensitive areas of giant EM fieldenhancement

hosts to selectively interact with pollutant takingadvantage of the formation of intermolecular cavities(PYR detection down to a few molecules!).

• A new approach in the design of functionalized NPs for thedetection of target molecule having low affinity toward the metal surface

Alkyl Diamines as hosts forChlorinated Pesticides

CSIC: Consejo Superior de Investigaciones CientíficasInstituto de Estructura de la Materia.

Santiago Sánchez-Cortés

José Vicente García Ramos

Concepción Domingo

Acknowledgements • We acknowledge project FIS2004-00108 from Dirección General de Investigación,

Ministerio de Educación y Ciencia and Comunidad Autonoma de Madrid project number S-0505/TIC/0191 MICROSERES for financial support.

• Luca Guerrini acknowledges an I3P fellowship from CSIC.

Thank you for your

attention

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