Surface Enhanced Raman Scattering: Applications and methods Brandon Scott CHEM 4930 04-30-2011

Introduction to Raman CV Raman observes the Raman effect using a telescope pointed at the sun

Why Gold? – Free Electron Metals The Group IA and IB elements have a free s electron configuration. Both groups exhibit resonance. Group IA is chemically “very” reactive and not practical for SERS . Group IB are “noble metals” they are very unreactive. SERS Metals

Localized Surface Plasmon Resonance Theory Source E field Metal Oxide Local Field Explanation + - + - + - The electric field next to the metal oxide particle is less than the original field. + - Gold Particle The electric field next to the gold nanoparticle is greater than original field. This is a resonance induced by energy from the local field producing a larger local field. + - + - + - + - + - - +

Surface Enhanced Raman Scattering I  m molecule adsorbed onto surface particle - m a = E + m = electric field-induced dipole moment - + + Surface-enhanced Raman scattering (SERS) enhances the Raman signal of molecules adsorbed to a rough metal surface by several orders of magnitude, making it a valuable tool for trace analyte detection. Gold nanoparticles (NPs), ranging from 30-50 nm in diameter, are easily synthesized in the laboratory and have a long shelf-life. Alone, these particles have virtually no Raman signal, but when tagged with any Raman-sensitive molecule there is an enormous molecular signal enhancement. + + a = polarizability E = electric field surface particle

Nanoparticle Synthesis Gold nanoparticles were synthesized using a known reaction method1. 20 mg of HAuCl4•3H2O was added to 200 mL of hot H2O with rapid stirring and the solution was brought to a rapid boil. 1.2 mL of aqueous sodium citrate solution (1% w/v) was added to the gold solution and the reaction flask was covered with a watch glass. After 20 min, the heat was turned off and the violet reaction product was cooled to room temperature while stirring.

Single nano-particle 60kx 200nm 100nm 100kx 50nm

Nile blue signal enhancement using NaCl: 400 450 500 550 600 650 700 750 Fig 1: Raman spectra of colloids in 1.6 µM nile blue (blue); colloids in 1.6 µM nile blue and 31.3 mM NaCl (red).

SERS Limitations The addition of analyte/NaCl leads to NP instability and eventual precipitation of Au. Spectroscopic analysis with a narrow beam leads to a high limit of detection. Laser Beam Nanoparticles Sample

Solution: Shelled Raman Reporters A known method for coating gold nanoparticles with amorphous silica3-8 was tested using the synthesized colloids. Thiol species form a strong bond to metal nanoparticles2 which leads to a strong SERS signal. It is unaffected by the silation reaction process, making it a suitable tag for core-shell particles. Stable, shelled Raman reporters can be implemented into immunoassays for indirect detection of analytes using antibody-antigen interactions

Shelled Raman Reporter Synthesis Used 1.2 mM BPE in isopropyl alcohol (IPA) to tag NOV colloids. [BPE] = 5 uM Added 4 mL of tagged colloids, 500 uL ammonium OH, and 16 uL TEOS to 16 mL IPA. After 1 h stirring, the solution was centrifuged at 7100 RPM for 10 min. The supernatant was poured off and the pellet was suspended in 1 mL H2O. This solution was centrifuged again and the pellet was suspended in 0.5 mL H2O. Analyte-specific antibodies were adsorbed to the shell surface.

Dynamic Light Scattering results: Colloids were analyzed before and after shelling. Shelled Raman reporters exhibit a larger diameter than bare NPs with little change to the polydispersity.

Shelled colloids maintain SERS BPE signal: 200 400 600 800 1000 1200 1400 1600 1800 Raman Spectra of unshelled NPs in 5 µM BPE(red, integration time: 2 sec) and shelled BPE-tagged NPs (green, integration time: 4 sec ).

Coated NPs + PhSH are stable: 200 400 600 800 1000 1200 1400 1600 Raman spectra of thiophenol-tagged coated NPs taken on 10/1 (blue), 10/8 (green), and 10/29 (red).

Paramagnetic Pull-Down Assays Analyte binding coating Raman coating Analyte shell SERS Active Nanoparticle Paramagnetic Particle B

Paramagnetic Assay Results: Adjusted BPE signal intensity Percent Rabbit Serum added to solution Graph shows SERS marker peak intensity vs. Rabbit serum concentration. Shelled Raman reporters and paramagnetic particles were coated in polyclonal anti-rabbit antibody.

Assay Multiplexing A Binding Events B C Reagent 1 - Paramagnetic Particle Antigen 1 Reagent 2 - SERS Reporter Multiplexing Antigen 2 Reagent 3 - SERS Reporter Antigen 3 Reagent 4 - SERS Reporter

Future works Cholera, LPS, CN, melamine detection Solid/hollow Si-Au-tag-Si particles

References Frens, G. Nature (London), Physical Science, 1973, 241 (105), 20. 2. Carron, K., Environ. Sci. Technol., 1992, 26 (10), 1950-1954. 3. Lu, Y., Nano Letters, 2002, 2 (7), 785-788. 4. Jana, N. R.; Earhart, C.; Ying, J. Y., Chem Mater 2007, 19, 5074-5082. 5. Stober, W.; Fink, A.; Bohn, E., Journal of colloid and interface science 1968, 26, 62-69. 6. Liz-Marzan, L. M.; Giersig, M.; Mulvaney, P., Langmuir 1996, 12, 4329-4335. 7. Hall, S. R.; Davis, S. A.; Mann, S., Langmuir 2000, 16, 1454-1456. 8.  Natan, M. J. Surface enhanced spectroscopy-active composite nanoparticles. 2003

Acknowedgements Special thanks to Dr. Keith Carron, Virginia Schmit, Patrick Johnson’s laboratory for access to the Brookhaven Instruments ZetaPALS, and INBRE for project funding.