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Ultrasensitive molecular sensor using N-doped graphene through enhanced Raman scattering

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Science Advances  22 Jul 2016:
Vol. 2, no. 7, e1600322
DOI: 10.1126/sciadv.1600322
  • Fig. 1 HRTEM and STM images of as-transferred NG.

    (A and B) HRTEM images of monolayer and bilayer NG sheets on TEM grids. The inset of (A) is the corresponding selected-area electron diffraction pattern of NG. (C to E) Typical STM images of NG sheets synthesized using different doping parameters: (C) NH3, 850°C; (D) NH3, 800°C; and (E) NH3, 750°C. (F and G) Models of (F) N2AA and (G) N1 nitrogen doping configurations. (H) Model of PG without any nitrogen doping. The triangles in (C) and the circles in (D) indicate the STM double N substitution configuration, and the arrow in (D) indicates the STM single N substitution.

  • Fig. 2 Probing the uniformity of GERS effect within the graphene surfaces.

    (A and B) Raman mapping of the intensity ratio between the 1650 cm−1 peak of RhB and the G band (I1650/IG) plotted for (A) PG and for (B) NG. The average ratio of I1650/IG in PG is ca. 0.7, whereas that in NG is ca. 2. For the PG sample, around 70% of the scanned area is green-colored, whereas for the NG sample, this ratio reaches around 85%, thus demonstrating a very good uniformity across the samples and a signal enhancement for NG when compared to PG.

  • Fig. 3 Resonant Raman scattering effect on NG sheets probed for different dye molecules.

    (A to C) Excitation laser energies of 2.54, 2.41, 2.18, and 1.92 eV are used to test the GERS effect of NG sheets with (A) RhB, (B) CRV, and (C) MB molecules. The probe molecule concentrations are all 5 × 10−5 M. The peaks marked with “◆,” “●,” and “★” are the major Raman signals from RhB, CRV, and MB molecules, respectively. arb., arbitrary.

  • Fig. 4 Laser dependence of the graphene 2D band frequency for various dye molecules on NG.

    (A to C) Linear dispersion of the graphene 2D band of NG with (A) RhB, (B) CRV, and (C) MB molecules. (D) Graphene 2D band intervalley DR process, which explains the linear dispersion.

  • Fig. 5 Comparison of the enhanced Raman scattering effect between NG and PG sheets when probing RhB with different concentrations.

    (A to G) Raman signals of RhB molecules on PG and NG sheets are shown here with (A) 5 × 10−5 M, (B) 5 × 10−6 M, (C) 5 × 10−7 M, (D) 5 × 10−8 M, (E) 5 × 10−9 M, (F) 5 × 10−10 M, and (G) 5 × 10−11 M RhB concentrations. The laser excitation line is 2.41 eV, and the integration time is 10 s for all cases, where the arrows indicate graphene G and D bands. (H) Raman intensity ratio between the strongest RhB peak (1650 cm−1) and the graphene G band when NG (red curve) and PG (black curve) are used as a sensing substrate.

  • Fig. 6 Probing the interaction between various fluorescence molecules with PG and NG through UV-visible transmission measurements.

    (A to D) UV-visible transmittance spectra of (A) RhB-quartz, PG, NG, RhB-PG, and RhB-NG; (B) CRV-quartz, PG, NG, CRV-PG, and CRV-NG; (C) MB-quartz, PG, NG, MB-PG, and MB-NG; and (D) MB, CRV, and RhB solutions. The absorption valleys related to the HOMO-LUMO gap of the RhB, CRV, and MB are denoted by the rose, lavender, and blue box, respectively.

  • Fig. 7 DOS of the clusters representing the adsorbed organic dyes on PG and NG.

    (A to F) DOS for RhB (A and B), CRV (C and D), and MB (E and F) are shown. The filled areas are the PDOS projected on the dyes. Vertical dashed lines indicate the position of the system’s Fermi energy, EF. Horizontal arrows and values indicate the HOMO-LUMO gaps (in electron volts) and the energy HOMO-EF (in electron volts).

  • Fig. 8 Simulated Raman spectra of the dye molecules.

    (A) Simulated configuration when RhB, CRV, and MB molecules lay on top of NG sheets. (B to D) Simulated Raman spectrum when (B) RhB, (C) CRV, and (D) MB are on top of NG sheets. Black curves are the spectra of the cations, and colored curves are the spectra of neutral molecules in the same geometry of the cations.

  • Table 1 Ultrasensitive molecular sensor.

    Comparison of the performance of different graphene samples as GERS substrates for molecular sensing. R6G, rhodamine 6G; PPP, protoporphyrin; CuPc, copper phthalocyanine.

    Sensing materialType of grapheneDetection levelLaser lineReference
    RhBCVD N-doped graphene5 × 10−11 M514 nmThis work
    R6GCVD N-doped graphene1 × 10−8 M514 nmThis work
    PPPCVD N-doped graphene1 × 10−8 M647 nmThis work
    R6GExfoliated graphene8 × 10−10 M514 nm(5)
    PPPExfoliated graphene2 × 10−8 M633 nm(5)
    R6GExfoliated graphene1 × 10−5 M514 nm(6)
    PPPExfoliated graphene1 × 10−5 M633 nm(6)
    CuPcExfoliated graphene1 × 10−6 M633 nm(12)
    CRVExfoliated graphene5 × 10−7 M514 nm(27)
    RhBMildly reduced GO5 × 10−8 M514 nm(29)
    MBExfoliated graphene1 × 10−4 M647 nm(25)
    CuPcExfoliated graphene on Au1 Å on top633 nm(28)
    CuPcExfoliated graphene3 Å on top633 nm(26)
    PPPExfoliated graphene3 Å on top633 nm(26)

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/2/7/e1600322/DC1

    Supplementary Materials and Methods

    fig. S1. NG sheets on different substrates and their typical Raman spectra with different synthesis conditions.

    fig. S2. Raman mapping of PG and NG showing monolayer coverage.

    fig. S3. XPS spectra (C1s and N1s) of NG synthesized at 850°C.

    fig. S4. Probing-enhanced Raman scattering effect between NG and PG sheets for different dye molecules.

    fig. S5. Comparison of Raman spectra with bare SiO2/Si substrates and NG for probing different dye molecules at their resonant condition.

    fig. S6. Enhancement factors for three different dye molecules between NG and PG.

    fig. S7. Comparison of GERS and SERS by applying sputtered Au nanoparticles as SERS substrates for comparison.

    fig. S8. Testing the molecular sensibility of the NG with different laser energies.

    fig. S9. Photos of RhB in ethanol solution with different concentrations.

    fig. S10. AFM images of NG samples with different concentrations of RhB.

    fig. S11. Raman spectrum of 5 × 10−12 M RhB on NG.

    fig. S12. Probing GERS effect between NG and PG for additional molecules such as R6G and PPP.

    table S1. Calculated HOMO-LUMO gap, adsorption data, and resonant Raman laser excitation energy for each molecule.

  • Supplementary Materials

    This PDF file includes:

    • Supplementary Materials and Methods
    • fig. S1. NG sheets on different substrates and their typical Raman spectra with different synthesis conditions.
    • fig. S2. Raman mapping of PG and NG showing monolayer coverage.
    • fig. S3. XPS spectra (C1s and N1s) of NG synthesized at 850°C.
    • fig. S4. Probing-enhanced Raman scattering effect between NG and PG sheets for different dye molecules.
    • fig. S5. Comparison of Raman spectra with bare SiO2/Si substrates and NG for probing different dye molecules at their resonant condition.
    • fig. S6. Enhancement factors for three different dye molecules between NG and PG.
    • fig. S7. Comparison of GERS and SERS by applying sputtered Au nanoparticles as SERS substrates for comparison.
    • fig. S8. Testing the molecular sensibility of the NG with different laser energies.
    • fig. S9. Photos of RhB in ethanol solution with different concentrations.
    • fig. S10. AFM images of NG samples with different concentrations of RhB.
    • fig. S11. Raman spectrum of 5 × 10−12 M RhB on NG.
    • fig. S12. Probing GERS effect between NG and PG for additional molecules such as R6G and PPP.
    • table S1. Calculated HOMO-LUMO gap, adsorption data, and resonant Raman laser excitation energy for each molecule.

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