Tip-Enhanced Raman Spectroscopy: Developments and Application to the study of double-stranded DNA bundles and polymer-wrapped carbon nanotubes Sébastien Bonhommeau Groupe Spectroscopie Moléculaire, ISM, Bordeaux
Near-field optical microscopes: Our instrument 1) Metal coating on AFM probes (thermal evaporation, sputtering) SiO x /Ag B.S. Yeo et al., Chem. Phys. Lett. 2009, 472, 1-13. AlF 3 /Ag 2) Nanoparticle deposition (Laserassisted growth ) F. Sinjab et al., Opt. Lett. 2012, 37 2256-2258. TERS instrument with on-axis illumination
Near-field optical microscopes: Our instrument Conversion of the beam polarization: the annular (doughnut) mode 1) The localized surface plasmon resonance (LSPR) depends on: the metal nature (gold or silver); the local environment (the strongest resonance is obtained when ε m = -2 εd for 5nm-diameter particles); the particle size (due to the competition between absorption and scattering) the particle shape (nanoprisms, nanocubes ) W.A. Murray and W.L. Barnes, Adv. Mater. 2007, 19, 3771-3782. 2) The lightning rod effect: Non-resonant phenomenon deriving from the geometrical crowding of the electric lines at the narrow ends of a nanostructure (e.g. a metal tip). Non-Resonant Resonant
Polymer-wrapped Single-Walled Carbon Nanotubes: normal Raman spectroscopy Raman intensity (arb. units) RBM 568.2 nm Vibronic excitation 514.5 nm 488.0 nm 457.9 nm D C-C G G' C-H 500 1000 1500 2000 2500 3000 Raman shift (cm -1 ) C C PNES-[(6,5) SWNT] S. Bonhommeau et al., J. Phys. Chem. C 2013, 117, 14840-14849 Raman intensity (arb. units) PNES G band G + C=C PNES 1500 1600 1700 Raman shift (cm -1 ) Raman intensity (arb. units) 457.9nm 488.0nm RBM 514.5nm 568.2nm (6,5) or (7,4) (8,2) (6,4) x10 x2 250 300 350 400 Raman shift (cm -1 )
Intensity/ arb. units Polymer-wrapped Single-Walled Carbon Nanotubes: AFM/Raman coupling 1.0 B 0.8 0.6 0.4 0.2 0 2 4 6 8 10 12 0.0 N A RBM Si IFM I D /I G G band D band 400 800 1200 1600 Raman shift/ cm -1 C E y z x 200 nm D 15 nm 250 300 x/ nm G' band (x3) (x3) 2500 2600 2700 2800 Raman shift/ cm -1 1.0 0.5 0.0-0.5 z/ nm Intensity/ arb. units Observations: AFM AFM lateral spatial resolution of 15nm, consistent with the tip apex diameter; The 1.3nm height is congruent with the presence of a single (6,5) SWNT core having a 0.76nm diameter wrapped by a single PNES strand. Observations: Raman I D /I G ratio of nearly 0.6 reflects the presence of sp 2 disorder after desolvation, since it does not appear for individual PNES-[(6,5) SWNT] in solution. This disorder is unlikely to be due to vacancy, topological or rehybridization defects. Intensities of the G (or 2D) band and the IFM band are also increased compared with the case of SWNT liquid suspensions. As D, G and IFM bands are all double-resonant spectral features, a doubleresonance effect enhanced by the (6,5) van Hove singularity might occur. S. Bonhommeau et al., J. Phys. Chem. C 2013, 117, 14840-14849
Diffraction -limited Raman image Tip-Enhanced Raman scattering: SWNT and polymers Near-field Raman image TER spectra of a single stranded homopolymer of cytosine (poly(c)) (λ = 530.9 nm; P = 1 mw, integration time 15 s) E. Bailo et al., Angew. Chem. Int. Ed. 2008, 47, 1658-1661. TER spectra of a single stranded calf thymus DNA (λ = 568.2 nm; P = 1 mw, integration time 10 s) R. Treffer et al., Beilstein J. Nanotechnol. 2011, 2, 628-637. Raman images of carbon nanotubes (λ = 633 nm; P = 50-200 µw, integration time 10 ms) N. Anderson et al., J. Am. Chem. Soc. 2005, 127, 2533-2537.
OH UV/O 3 1% OTS Hexane O Borosilicate glass Hydrophilic substrate Acetone, Isopropanol, MilliQ water - HCl Hydrophobic substrate CH3 OH Borosilicate glass CH3 CH3 Si Si Si O O O OH OH Borosilicate glass DNA λ-phage Sodium acetate Acetic acid, HCl Combed double-stranded DNA bundles: Generalities Sample Preparation OH O DNA of the λ-phage virus Adenine Guanine Cytosine Thymine Purines Pyrimidines Other molecules on the combed DNA sample Nucleobases: A, G, C, T Backbone: Sugar and Phosphate groups Ethylenediaminetetraacetic acid (EDTA) EDTA Tris(hydroxymethyl)-aminomethane (TrisHCl) Octadecyltrichlorosilane (OTS) S. Najjar et al., J. Phys. Chem. C 2014, 118, 1174-1181
Tip metalization: Combed double-stranded DNA bundles: TERS investigations Experimental details Laser Thickness 20 nm Ag + 5 nm Au Sputtering rate 0.07 nm/s Ar 99.999%, N 2 99.999%, 10-2 mbar TERS λ=568.2 nm Radial polarization P=50 µw Oil immersion objective (NA=1.42) SEM image 100 nm S. Najjar et al., J. Phys. Chem. C 2014, 118, 1174-1181 Performance of the TERS instrument 1 2 3 AFM 200 nm 1191 cm -1 : C/T 1238 cm -1 : EDTA ν s (C-N) Imaging a ds DNA bundle AFM height of 4 nm Lateral size of 5 nm (5 nm steps) Nanowire-like DNA bundles Detecting additional molecules spectral feature at 1238 cm -1 not correlated with the ds DNA bundle. High sensitivity of the technique Intensity/ a.u. Intensity/ a.u. 2500 2000 1500 1000 TERS features Lateral spatial resolution of 5 nm << 19 nm AFM resolution Enhancement factor EF min =(V far field /V near field ).(I near field /I far field ) 600 1 500 2 3 0 1150 1200 1250 Raman shift/ cm -1 5 nm 20 15 10 19 nm AFM 0 0 50 100 Position/ nm 5 AFM height/ nm
Intensity (arb. units) A 1 2 3 * Combed double-stranded DNA bundles: TERS investigations ν 1 ν 2 ν3 400 600 800 1000 1200 1400 1600 1800 Raman shift (cm -1 ) 40 B AFM C ν Raman 9 nm 5 30 ν 4 8 nm ν 20 3 10 nm 2 9 nm 10 nm 10 200 nm 123 ν 1 28 nm AFM 0 0 20 40 60 80 Position (nm) Intensity (arb. units) S. Najjar et al., J. Phys. Chem. C 2014, 118, 1174-1181. R. Treffer et al., Beilstein J. Nanotechnology 2011, 2, 628-637. ν4 ν 5 RS (cm -1 ) AFM height (nm) TERS of ds DNA (cm -1 ) Point 1 Point 2 Spectral description Modes assigned to A, G, T and C nucleobases observed for ds and ss DNA. A few modes assigned to the DNA backbone are only observed for ds DNA TERS can probe DNA hybridization. the hydrophobic (OTS) layer allowing DNA combing is not detected by TERS only the combed DNA is probed, the silanized substrate is not. TERS of ss DNA (cm -1 ) Assignment 755 754 755 754 T (ring breathing) ν 1 766 765 764 - ν s (OPO) - 824 824 - ν a (OPO) - 845 840 - ν a (OPO) ν 2 1056 1057 1052 1051 A ν (N-sugar) ν 3 1194 1196 1190 1191 C/T ν 4 1466 1452 1443 1466 A ν s (C=N) (Py) ν 5
Conclusions and Prospects Our near-field optical microscope allows polymeric samples to be investigated with high lateral spatial resolution and a sensitivity high enough to detect species in minute concentration. However, performing TERS imaging will require much ( 100-1000 fold) faster image acquisition. To attain this objective, the enhancement factor of the TERS probe must be improved upon optimizing plasmon resonance conditions (using dielectric coatings for example). The metal tip may behave as a DNA sensor able to detect specifically hybridized viral DNA, which might be exploited for biochemical/biomedical applications. Nanostructures organized on a functionalized substrate can be investigated without observing spectral contributions from the substrate, which opens the way toward the study of organized polymeric architectures by TERS (DNA origami, ). PNES-[(6,5) SWNTs] can be studied using the AFM/Raman coupled technique or near-field optical microscopes under resonant conditions, but also upon excitation of vibronic transitions. This should be a general property of polymer-wrapped carbon nanotubes.
ISM Bordeaux D. Talaga (Eng.) S. Najjar (PhD) L. Schué (Master) L. Auneau (undergraduate) S. Trainini (undergraduate) V. Rodriguez (Prof.) L. Servant (Prof.) Acknowledgements IRI Lille Y. Coffinier (Dr) R. Boukherroub (Dr) S. Szunerits (Prof.) PLACAMAT Bordeaux E. Sellier (Eng.) Duke university USA M.J. Therien (Prof.) P.Deria (Post-doc) M.G. Glesner (PhD)