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Surface Enhanced Raman Spectroscopy of Organic Molecules on Magnetite (FeO) Nanoparticles 3

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Namhey Lee, P. James Schuck, Peter Silvio Nico, and Benjamin Gilbert J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.5b00036 • Publication Date (Web): 24 Feb 2015 Downloaded from http://pubs.acs.org on March 1, 2015

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The Journal of Physical Chemistry Letters

Surface Enhanced Raman Spectroscopy of Organic Molecules on Magnetite (Fe3O4) Nanoparticles Namhey Lee*a, P. James Schuck b, Peter S. Nico a, Benjamin Gilbert a a

b

Earth Science Division, Lawrence Berkeley National Laboratory Berkeley, CA 94720, USA

Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, CA 91125, USA

Corresponding Author *E-mail: [email protected] (N. L.)

ACS Paragon Plus Environment

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ABSTRACT

Surface-enhanced Raman spectroscopy (SERS) of species bound to environmentally relevant oxide nanoparticles is largely limited to organic molecules structurally related to catechol that facilitate a chemical enhancement of the Raman signal. Here we report that magnetite (Fe3O4) nanoparticles provide a SERS signal from oxalic acid and cysteine via an electric field enhancement. Magnetite thus likely provides an oxide substrate for SERS study of any adsorbed organic molecule. This substrate combines benefits from both metal-based and chemical SERS by providing an oxide surface for studies of environmentally and catalytically relevant detailed chemical bonding information with fewer restrictions of molecular structure or binding mechanisms. Therefore, the magnetite-based SERS demonstrated here provides a new approach to establishing the surface interactions of environmentally relevant organic ligands and mineral surfaces.

TOC GRAPHICS

KEYWORDS: nanoparticle, magnetite, maghemite, rutile organic molecules, surface enhanced Raman spectroscopy


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In many areas of interfacial chemistry, it is important to determine the binding conformation of organic molecules at metal oxide surfaces. For example, insight into sorption geometry is essential to predict the mobility of pollutants in the natural environment, to understand mechanisms of biomineral formation, and to develop dye-sensitized photovoltaic systems. However, there are rather limited techniques available to probe the interactions between organic molecules and oxide surfaces in water that can provide detailed molecular level information. The most sensitive methods use variants of molecular vibrational spectroscopy to determine configuration information, but are typically challenged by low signal strength and/or difficulty in separating interfacial signal from the bulk.

For example, attenuated total

reflectance- Fourier transform infrared spectroscopy (ATR-FTIR) is widely used for this purpose, but is challenged by low sensitivity (low signal-to-background ratios) due to water interference and signals from other aqueous species1, 2. On the other hand, non-linear spectroscopic techniques, such as sum frequency generation (SFG) and second harmonic generation (SHG), are extremely sensitive to small amounts of adsorbed species on interfaces and single crystal surfaces3. However, these techniques can be particularly susceptible to sample damage when studying iron oxides4. Nuclear magnetic resonance (NMR), despite wide availability and advanced data interpretation5, is unfortunately inapplicable to any Fe bearing substrates. Surface enhanced Raman spectroscopy (SERS) is a highly surface-sensitive approach for obtaining Raman data from molecular species adsorbed to SERS-active substrates. In conventional SERS6, the Raman signal at a metallic surface is amplified by the excitation of surface plasmon resonances and local enhancement of the electromagnetic field, and to a somewhat lesser extent by the so-called chemical enhancement that is due to the increase in


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molecular polarizability resulting from molecule-surface binding7, 8. These enhancements have permitted detailed studies of molecular adsorption on metals such as Ag and Au as nanoparticles or nanostructured substrates9-12. However, metal substrates lack relevance for many systems, particularly environmental studies. As noted above, the additional chemical enhancement of a Raman signal can occur when strong binding interactions alter the molecular Raman polarizability. Recently, Hurst et al. discovered that the formation of an interfacial charge-transfer complex can cause high chemical enhancement at a non-conductive oxide surfaces13. This has enabled the use of chemical SERS to monitor the changes in conformation of catechol-containing molecules at rutile surfaces under aqueous conditions14. So far, however, chemical SERS involving non-metallic substrates is largely limited to enediol family molecules such as catechol. As we show below, chemicallyenhanced SERS can take place on a range of oxide material substrates, but it is not exhibited by all molecules. Many fields of interfacial chemistry thus lack a class of SERS-active substrates that possess representative surface structure and chemistry that are not limited to a narrow range of sorbate characteristics. Here, we demonstrate that magnetite, a conducting mixed-valence iron oxide, can act as a SERS substrate capable of electromagnetically enhancing the Raman signal for diverse surface-sorbed organic molecules. We present studies of Raman spectroscopy of small organic molecules on three different metal oxides: rutile (Ti(IV)O2), magnetite (Fe(II/III)2O4), and maghemite (γ-Fe(III)2O3). Maghemite and magnetite are both inverse spinels. As a consequence of Fe2+ ions in magnetite, this material is a conductive semi-metal. Maghemite is formed by the oxidation of magnetite through a solid-state transformation to form a semiconductor that is non-conductive at room


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temperature. While all oxides enable chemical SERS for enediol molecules, only magnetite shows a SERS signal for all molecules studied. Catechol type molecules bind strongly to metal oxide surfaces, readily exchanging neighboring surface oxygen atoms to form a bidentate charge-transfer (CT) complex13, 15-17. As shown in Figure 1, we observe strong SERS signals for pyrocatechol on the three oxide nanoparticles, as previously reported for TiO2. A powder-phase catechol spectrum is also shown for comparison. All Raman measurements were performed using a Horiba Jobin (JY) LabRAM Aramis confocal Raman instrument. Raman spectra were recorded using a 50x objective and 532 nm laser excitation with 5.8 mW incident power. We also attempted Raman measurements with 785 nm laser excitation, however Raman signal was not detectable (before reaching the damage threshold), presumably due to smaller scattering cross-sections and reduced resonance-related effects for this longer-wavelength excitation. All spectra in Figure 1 contain similar peaks and relative peak intensities indicating the same binding geometry of pyrocatechol on different oxide surfaces. The strongest band in the spectra is near 1451 cm-1 (1517 cm-1 for maghemite), which can be assigned to the stretching of the bond between the carbon atoms (C-C) where the oxygen is attached to the particle. Phenolate C-O stretch can be assigned to 1211 cm-1 and the skeletal benzene mode can be assigned to 1105 cm-1, 1255 cm-1, and 1528 cm-1. All the maghemite peaks are shifted approximately equally to higher frequency with respect to the analogous vibrations for magnetite. This likely indicates that there is a non-specific difference in the chemical environment such as a change in interfacial dielectric constant rather than changes in binding geometry on these very similar oxide surfaces. The spectral differences seen below 800 cm-1 are primarily attributed to differences in Raman transitions between the


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three oxides, with magnetite showing sharper, stronger transitions than maghemite in this region18.

Figure 1. Surface enhanced Raman spectra (SERS) of pyrocatechol on maghemite, magnetite and rutile nanoparticles in comparison with the oxide nanoparticles only in suspensions. (top panel). The Raman spectra of powder pyrocatechol and 5 mM of pyrocatecholate are also shown for comparison (bottom panel). The SERS signal originates from the formation of a chargetransfer complex. The broad feature at ~1600 cm-1 in the maghemite trace originates from the oxide (see Figure 2). Raman data for 5 mM solutions of all the dissolved organic molecules are presented in Figure S2. The Raman signal from the 5 mM solution-phase molecules is extremely weak in all cases. Dicarboxylic acids, such as oxalate, also bind strongly to metal oxide surfaces19-23. However, as shown in Figure 2, oxalate adsorbed to maghemite shows a negligible Raman signal indicating that dicarboxylic acids do not form a CT complex on iron oxides. By contrast, oxalate adsorbed on magnetite nanoparticles exhibits a very strong Raman signal, indicating a surface enhancement without the formation of a CT complex. Because of the conductive nature


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of this mixed-valence oxide, we infer the SERS signal is a consequence of electric field enhancement. Conductive metal oxides can exhibit plasmonic behavior, with recent studies showing that plasmon resonance quality factors in metal oxide nanoparticles can rival those of noble metals24-27. UV-vis absorption spectroscopy does not clearly reveal a plasmonic excitation band for magnetite, but shows a broad and strong absorption from the visible to the near-infrared that is not observed for maghemite (Figure S3).

Figure 2. SERS spectrum of oxalate on magnetite nanoparticles. Oxalate does not form a visiblelight charge-transfer complex on iron oxides and hence we infer that the SERS signal originates from an electric field enhancement at the interface of the conductive oxide. No SERS signal is observed for non-conductive maghemite nanoparticles. Comparison between the normal Raman spectrum of sodium oxalate and the SERS spectrum for oxalate adsorbed on magnetite in aqueous solution reveals shifts in the main vibrational bands upon adsorption (Figure 3). The dominant effects of sorption are appearance of new peaks and overall peak broadening. This is consistent with the infrared spectroscopy of organic acids sorbed to metal oxide surfaces28.


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Figure 3. Comparison of the normal Raman spectrum of sodium oxalate powder, the SERS of oxalate adsorption to magnetite and the Raman spectrum for uncoated magnetite. The weak bands at 1350 and 1606 cm-1 in sodium oxalate likely correspond to the IR-active symmetric and asymmetric CO2 bands that are strongly broadened by sorption to metal oxide surfaces22. This is further supported by the relative decreased peak intensity at 871 cm-1, which corresponds to C-COOH stretching. Another noticeable difference is the prominent peak near 250 cm-1 that appears upon adsorption of oxalate on magnetite. Similar low-frequency vibrations of molecules bound to metal surfaces through non-specific interactions were interpreted as librational or vibrational modes of the entire sorbed molecule13. Figure S4 shows the transmission FTIR spectrum of oxalate sorbed to magnetite, which is dominated by overlapping C-O, C-C and C=O vibrational modes that are not clearly distinguishable. In part, this is because the approach is not intrinsically surface sensitive. An advantage of SERS is that it enhances vibrational contributions from near-surface bonds that preferentially probe binding interactions.


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Cysteine is an amino acid that is known to coordinate metal ions and mineral surfaces in the environment. This small molecule can act as a ligand for nanoparticles developed for biomedical applications29, can alter the growth pathway of iron oxides in aqueous solution30 and may be involved in the sensitization of hematite electrodes by bacterial light-harvesting proteins31. Cysteine possesses carboxylic, amine and thiol functional groups that may preferentially interact with solid interfaces. Prior SERS studies have shown that cysteine sorbs strongly to the surface of metals such as Ag via the thiol group forming metal-S-C bonds32. However, the mode of sorption to metal oxides has not been clearly resolved. In previous studies of cysteine adsorption onto magnetite33 or titania34, 35, the strong vibrational bands associated with the carboxylic acid group were relatively unchanged for dissolved versus bound cysteine. This is in strong contrast with the significant changes observed in carboxylic group vibrations for oxalate sorption onto titania. Nevertheless, because the thiol S-H stretching vibration at ~2550 cm-1 was unchanged by sorption, Rajh et al. concluded that sorption involved direct carboxylic acid binding to the surface.

Figure 4. SERS spectra of cysteine on magnetite showing clear SERS signal and cysteine on maghemite with very weak Raman signals.


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Figure 4 compares the Raman signals obtained from cysteine adsorbed on maghemite and magnetite. As for oxalate, a strong Raman signal is observed for sorption to magnetite but not for maghemite. The major feature at 665 cm-1 corresponds to the C-S stretch, as seen for thiols sorbed to silver10, 32, 36. The data possess a striking absence of a signal from the carboxylic acid groups. As shown by Figure 3, if these functional groups were involved in direct bonding to the oxide surface the SERS effect would cause a strong signal from their vibration bands. As no carboxylic acid bands are observed we conclude that cysteine sorbs to magnetite via the thiol group, thereby providing the first experimental determination of the orientation of this molecule on an oxide surface. The discovery that magnetite is an effective SERS substrate considerably extends the scope for the study of interfacial adsorption reactions at metal oxide interfaces by relaxing the requirements for specific molecular structure and binding mechanism. Future work includes theoretical studies that will permit more detailed interpretation of the spectra the mechanism of the SERS. There is likely to be a large range of non-metallic substrates to be considered for SERS analysis, including other conductive minerals such as pyrite or pyrolusite. In addition, recently developed doped metal oxide nanoparticles with strong plasmonic behavior may also provide SERS-active oxide substrates with a diversity of interfacial structures and properties 38

37,

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Experimental Methods Magnetite was synthesized according to the recipe of Vayssiéres et al (1998)39. Fe(NO3)3 and FeCl2 was injected at the rate of 1 mL/min in 2:1 ratio into a 1 mM NaNO3 solution. The pH of the solution was kept constant at 12 by a titrator under an anoxic environment. Magnetite


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nanoparticles precipitated as a black suspension with a spherical shape and a primary particle size around 9 nm, as shown in TEM (Figure S1). Maghemite was prepared by oxidizing the magnetite nanoparticles. The magnetite particle suspension was bubbled with O2 gas in a water bath with a temperature of 60 °C for 48 hours. Maghemite showed a distinctive brown color in contrast to the black magnetite. The phase purity of the magnetite and maghemite materials has been carefully established as described in Katz et al. (2012)40. Ultraviolet-visible absorption spectroscopy was performed under anaerobic conditions using a Ocean Optics USB4000 spectrometer. For spectroscopy studies, 1 mL of each 10 mM of ligand solution and 10 g/L of nanoparticle suspension was mixed to yield 2 mL of mixed suspension.

The mixture was allowed to

equilibrate at least 30 min. but not more than 3 hours. 40 µL of the suspension containing the surface complexes were transferred to glass microscope slides with a single shallow depression and the solution was covered with a coverslip. The pH of the mixed suspensions was circumneutral within 6.5~8.0 range and were used without buffer. Ionic strength was not manipulated in current study, as it did not directly affect the SERS effect. Possible variation in the amount of adsorption due to slight shift in pH between different minerals might affect the intensity of the signal but not SERS signal itself. Raman measurements were performed at the molecular foundry at Lawrence Berkeley National Laboratory using a Horiba Jobin (JY) LabRAM Aramis confocal Raman instrument. Raman spectra were recorded using 532 nm laser excitation with approximately 5.8 mV laser power with a 50x objective. Typical spectra acquisition conditions were 10 accumulations for 10 seconds, total for 100 seconds. Background was subtracted using polynomial fit using software Igor.


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ASSOCIATED CONTENT Supporting Information. Transmission electron microscopy image of magnetite nanoparticles, UV-vis spectra of magnetite and maghemite nanoparticles, and FTIR transmission measurement of oxalate on magnetite. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Email: [email protected] Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT The authors greatly appreciate discussions with Dr. Nicholas Borys. We thank Jennifer Soltis for acquiring TEM images of magnetite nanoparticles. We also wish to express our thanks to Patricia Fox for her assistance with ATR-FTIR data collection. This work was supported by the Laboratory Directed Research and Development program at Berkeley Lab, through the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award Number DE-AC02-05CH11231. Work at the Molecular Foundry was supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.


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