Elsevier

Chemical Physics Letters

Volume 620, 20 January 2015, Pages 139-143
Chemical Physics Letters

Editor's Choice
Surface enhanced Raman spectroscopy of polycyclic aromatic hydrocarbons and molecular asphaltenes

https://doi.org/10.1016/j.cplett.2014.12.014Get rights and content

Highlights

  • SIOM substrates enable rapid acquisition of SERS spectra of complex mixtures.

  • SERS is critical to obtain Raman spectra from asphaltenes because of fluorescence.

  • We present the first known SERS spectra of individual asphaltene molecules.

  • DFT calculations attempt to match spectra with corresponding PAH structures.

  • The combination of theory and experiment advances asphaltene characterization.

Abstract

We describe, for the first time to the best of our knowledge, the acquisition of surface enhanced Raman spectra (SERS) of asphaltenes. SERS is an especially sensitive probe for aromatic carbon making it ideal to investigate the enigmatic polycyclic aromatic hydrocarbons (PAHs) of asphaltenes, the heaviest, most aromatic components of crude oil. SERS spectra of a known PAH model compound and of asphaltene samples are compared to density functional theory (DFT) calculations of PAH structures. This combination of experimental and theoretical methods represents an advance in the characterization of asphaltenes and other complex mixtures.

Introduction

Analysis of complex chemical mixtures is very challenging and requires many diverse methods for proper understanding. Petroleum asphaltenes [1], [2], [3] represent one of the most complex chemical mixtures and are only slowly revealing their true nature. Asphaltenes have been subjected to Fourier transform ion cyclotron resonance mass spectroscopy yielding ultrahigh resolution elucidating the elemental composition of individual constituents in the mixture but this is applicable to only part of the asphaltene sample [4]. Laser desorption–laser ionization mass spectroscopy (L2MS), while much lower in resolution, is applicable to virtually all of the asphaltene sample and largely resolves the asphaltenes molecular weight debate [5], [6] yielding results in agreement with molecular diffusion measurements of time-resolved fluorescence spectroscopy and fluorescence correlation spectroscopy [7], [8]. Asphaltenes have been subjected to many sophisticated spectral analysis methods. Sulfur X-ray absorption near edge structure (XANES) and nitrogen XANES methods [9], [10] have helped delineate asphaltenes heteroatom chemistry while carbon X-ray Raman spectroscopy has been instrumental in elucidating the aromatic carbon type (within the Clar representation) of asphaltenes [11]. Pump–probe spectroscopy results on asphaltenes with comparison to molecular orbital calculations are consistent with a polycyclic aromatic hydrocarbon (PAH) centroid of seven fused rings [12] in agreement with singlet state spectroscopy and expectations codified in the Yen-Mullins model of asphaltene molecular and nano-colloidal species [1], [2], [3]. The proposed asphaltene molecular architecture with one relatively large, central PAH and with peripheral alkane substituents is also obtained by comparison of absolute cross sections of small angle X-ray scattering and small angle neutron scattering of asphaltenes [13], [14].

Interfacial properties of asphaltenes are of considerable interest for many purposes such as enhanced oil recovery of conventional oil reservoirs as well as understanding newly emerging oil production in unconventional reservoirs where interfacial science dominates. Sum frequency generation (SFG) spectroscopy has been utilized to perform the only direct measurement of asphaltene molecular orientation at oil–water interfaces [15]. Recent pendant-drop experiments of oil–water interfaces have obtained universal curves which were then analyzed by the Langmuir equation [16], [17]. The resulting determination of the molecular contact area at the interface for asphaltenes is in close agreement other studies [1], [2], [3], [7], [8], [12], [13] presuming the in-plane asphaltene PAH orientation and out-of-plane alkane peripheral substituents, exactly as determined in the SFG experiments [15]. 13C NMR results [18], [19] support a single, large PAH per asphaltene molecule rather than a molecular architecture of many cross-linked single aromatic rings or small PAHs. These architectures are quite distinct and would give rise to very different interfacial properties and aggregation characteristics. While the methods described above have yielded progress in understanding the structure and properties of asphaltenes, they are based on instruments that are in general bulky and complex. A method for the analysis of asphaltene samples employing instrumentation that is simpler and with smaller footprint would not only be valuable for fundamental studies, but could possibly be used in the field to characterize unknown samples. In Raman spectroscopy, vibrational spectra are measured using equipment satisfying these criteria. Indeed, at the time of writing, handheld systems are available [20]. This method faces the challenge however that the Raman scattering cross sections are comparatively small. In surface-enhanced Raman scattering (SERS), the field enhancements provided by metallic nanostructures are employed to boost these cross-sections. An added advantage is the reduction of fluorescence. The characterization of PAHs by SERS has been demonstrated [21], [22]. In this work, we investigate SERS as a means for characterizing asphaltene samples. We begin by measuring the SERS spectrum of a model PAH that is believed to be structurally similar to asphaltene. This spectrum is shown to be in good agreement with the Raman spectrum predicted by quantum chemistry calculations (density functional theory, DFT) of the model compound based on its known structure. We then present, for the first time to the best of our knowledge, the measured SERS spectra of asphaltene samples. The possible molecular structures of the measured samples are obtained by performing DFT calculations of a variety of candidate molecules, and choosing those that match the experimental spectra best. We anticipate that our method could play an important role in understanding not only asphaltenes, but also other carbonaceous materials important in energy production such as kerogen and bitumen in unconventional gas and oil production.

Section snippets

Experimental

Recently Wang et al. [23], described the development and characterization of a large area (4″ wafer) SERS substrate using standard sputtering and evaporation techniques that exhibits huge (∼4 × 107) local field enhancements. This substrate consists of silver nanoparticle islands formed over a silver mirror, with a SiO2 spacer layer, and is termed a ‘SIOM metasurface’. Figure 1 shows a representation of this subtrate, which is used for the SERS experiments in this letter. The substrates were

Theoretical

The experimental studies were complemented by quantum chemical calculations (DFT) of the model compound BPTI. In addition, Raman spectra of a large number of PAH structures were calculated to match molecular structures with the molecular asphaltene SERS spectra.

Density functional calculations were carried out for geometry optimization, simulation of electronic structure and vibrational spectra using the Gaussian 09 program package [24]. Calculations of molecular structures of the different PAHs

Results and discussion

Figure 2 shows a comparison of the SERS spectrum obtained from the SIOM substrate (prepared with BPTI) with the calculated Raman spectrum for the BPTI molecule (inset). Major features of the SERS spectrum are reproduced in the DFT calculation. The prominent bands at 1623, 1400 and 1325 cm−1 are a combination of CC stretching and CH bending modes.

When comparing SERS spectra with single molecule gas phase DFT calculations we point out that chemical interactions between the analyte and the Ag

Conclusions

The development of SIOM substrates with nanoparticle silver allows for the rapid acquisition of SERS spectra. The sensitivity and huge enhancements of SIOM substrates enables Raman spectroscopy to be performed on large PAH molecules which would be otherwise inaccessible. In turn, this enables a new method of investigating the many components of a complex chemical mixture such as asphaltenes. Here, asphaltene PAHs are investigated and several different spectra are matched with plausible proposed

References (29)

  • U. Bergmann et al.

    Chem. Phys. Lett.

    (2003)
  • P. Leyton

    Vib. Spectrosc.

    (2008)
  • D. Michalska et al.

    Chem. Phys. Lett.

    (2005)
  • O.C. Mullins

    Annu. Rev. Anal. Chem.

    (2011)
  • A.G. Marshall et al.

    Acc. Chem. Res.

    (2004)
  • A.M. McKenna

    Energy and Fuels

    (2013)
  • Q. Wu et al.

    J. Am. Soc. Mass Spectrom.

    (2013)
  • A.E. Pomerantz et al.

    J. Am. Chem. Soc.

    (2008)
  • H. Groenzin et al.

    J. Phys. Chem. A

    (1999)
  • A.B. Andrews et al.

    J. Phys. Chem. A

    (2006)
  • G.N. George et al.

    J. Am. Chem. Soc.

    (1989)
  • S. Mitra-Kirtley et al.

    J. Am. Chem. Soc.

    (1993)
  • T. Klee

    Energy Fuels

    (2011)
  • Cited by (22)

    • Multifunctional magnetic Fe<inf>3</inf>O<inf>4</inf>/GO/Ag composite microspheres for SERS detection and catalytic degradation of methylene blue and ciprofloxacin

      2022, Journal of Alloys and Compounds
      Citation Excerpt :

      Nowadays, surface enhanced Raman scattering (SERS) spectroscopy appears as a highly promising analytical technique in food safety, environmental monitoring and security owing to its special ability of no destruction, immensely sensitive and fast analysis [1–5].

    • Correlative nano-spectroscopic imaging of heterogeneity in migrated petroleum in unconventional reservoir pores

      2021, Fuel
      Citation Excerpt :

      These relatively broad, flat bands of enhancement across the spectra may suggest that asphaltene fractions have such significant chemical and structural complexity that a multitude of overlapping features from ring structures, heteroatoms, and sample three-dimensionality produces an unresolvable baseline in far-field infrared data [66]. Similar observations have been made when applying Raman spectroscopy to isolated asphaltene rings [67]. Recognizing this heterogeneity between asphaltene and maltene in far-field spectra, even if subtle, provides a framework for nano-scale analysis of in-situ petroleum.

    • A review of spatially resolved techniques and applications of organic petrography in shale petroleum systems

      2021, International Journal of Coal Geology
      Citation Excerpt :

      This approach does suffer from the inability to control the deposition of the metallic nanoparticles necessary for SERS generation and so would not be appropriate for the study of highly heterogeneous materials such as petroliferous shale and mudrocks. In the context of petroleum systems, a few researchers have applied SERS to characterize asphaltene components separated from crude oils (Alabi et al., 2015; Andrews et al., 2015; Bowden and Taylor, 2019); however, these studies lack a spatial component and so are not considered further in the context of this review. Similar to the preceding discussion on AFM-IR spectroscopy, atomic force microscopy has been combined with surface enhanced Raman scattering in tip enhanced Raman spectroscopy (TERS).

    • Trace detection of polycyclic aromatic hydrocarbons in environmental waters by SERS

      2020, Spectrochimica Acta - Part A: Molecular and Biomolecular Spectroscopy
      Citation Excerpt :

      To improve the sensitivity and avoid interferences from complex matrices, simple pretreatment methods were also developed for the purification and/or enrichment of samples [24,25]. Owing to the weak interaction of the aromatic rings of PAHs with SERS-active surfaces such as Ag or Au, considerable efforts were made to strengthen this interaction by functionalizing these surfaces with long-chain alkanes, calixarenes, cyclodextrin derivatives, and humic acids [26–32]. However, either the functionalization of the SERS active substrate or the detection process has a hard-to-handle protocol.

    • Au nanoparticles grafted on Fe<inf>3</inf>O<inf>4</inf> as effective SERS substrates for label-free detection of the 16 EPA priority polycyclic aromatic hydrocarbons

      2016, Analytica Chimica Acta
      Citation Excerpt :

      In recent years, substrates with specific geometries have been of particular interest for researchers working with PAHs. Novel SERS substrates with ultra-high enhancement factors, such as Au coffee ring [14], Au on nickel 3D foam [15], Au on TiO2 nanotube arrays [16], Au NPs on porous polymer [17], Ag nanoparticle islands with a SiO2 spacer layer [18], and Au NPs on alginate gel [19], have been proposed for PAH detection. In most previous reports, a stringent and complicated protocol is a prerequisite for the successful synthesis of substrates, and the substrates were tailored for detecting a specific class of PAHs.

    View all citing articles on Scopus
    View full text