Research ArticleChemistry

MoS2 pixel arrays for real-time photoluminescence imaging of redox molecules

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Science Advances  08 Nov 2019:
Vol. 5, no. 11, eaat9476
DOI: 10.1126/sciadv.aat9476
  • Fig. 1 MoS2 PL response due to a change in [Fc+]/[Fc] ratio.

    (A) Bright-field transmitted light optical image of a MoS2 pixel array consisting of 5 μm × 5 μm MoS2 squares and Ti/Au contacted devices. The Pt/Ir electrode used to contact devices and oxidize the ferrocene molecules is shown in the middle of the image. (B) PL image of the same region in (A) excited by the 546-nm peak of a mercury lamp and imaged with a filter centered at 650 nm. The image shown is taken with a 2-s integration time. (C) Schematic of the charge transfer between ferrocene molecules and MoS2. The red shade represents positively charged ferrocene molecules (ferrocenium). (D) PL of MoS2 pixels varying the relative concentrations of ferrocene and ferrocenium.

  • Fig. 2 MoS2 PL response to ionic liquid gating.

    (A) Schematic and circuit diagram of the MoS2 PL measurement as a function of the ionic liquid gate voltage (VLG). (B) MoS2 PL (red, left axis) and source-drain current (blue, right axis) as a function of the ionic liquid gate voltage for a solution of B4NPF6 (100 mM) in acetonitrile. Exposure time for images and data are 50 ms. (C) PL signal from gating of MoS2 device and from MoS2 pixels at different concentrations of ferrocene and ferrocenium. The correspondence between the two curves, using kBT = 25.7 meV, indicates that sweeping the gate potential and changing the chemical potential cause an equivalent response for MoS2. Data are taken with a 100-ms exposure time. Variation in the initial charge density of two different MoS2 samples due to different surface adsorbates likely accounts for the difference in between the PL versus gate voltage curves (B) and (C).

  • Fig. 3 Visualizing ferrocenium diffusion using a MoS2 pixel array.

    (A) Time-lapse PL images of MoS2 pixels (2 μm × 2 μm) after applying a 0.6-V square wave pulse to a working electrode (versus a distant large platinum electrode) located at the top left corner of the image at t = 0 s. The images show the MoS2 pixels lighting up in response to the diffusion of ferrocenium ions. Exposure time for each image is 10 ms. (B) 3D surface plot of the image at 0.64 s, showing the spatial gradient of the PL signal. (C) Average PL value for each MoS2 pixel in the image versus distance from the working electrode at t = 0.64 s. These data are fit with an error function centered at zero with characteristic length-scale R = (4D0t)0.5, where D0 is the diffusion constant for ferrocenium. (D) Values for R2 extracted from each frame versus time. Linear fitting of these data gives D0 = (1.76 ± 0.02) × 10−9 m2/s, which matches well with other values found in the literature for ferrocenium.

  • Fig. 4 MoS2 pixel detector power spectral density.

    Noise power spectra of PL for three sizes of MoS2 redox detectors. The left axis shows photons detected squared per hertz, and the right axis is converted to voltage via a MoS2 gate curve. All curves are taken at 10-ms exposure times. Power spectrum for a 2 μm × 2 μm pixel (A), a 5 μm × 5 μm pixel (B), and a 15 μm × 15 μm MoS2 region (C). The shot noise limit of our setup is shown by the dashed gray lines and corresponds to 1.5 mV/Hz for (A), 0.6 mV/Hz for (B), and 0.2 mV/Hzfor (C). The insets show the PL versus time graphs from which the power spectra were calculated.

  • Fig. 5 MoS2 PL for different ferrocene concentrations.

    (A) Schematic of CV experiment. The initial solution contains only ferrocene, which is oxidized to ferrocenium by applying a potential to a Pt/Ir microelectrode versus a large platinum wire far from the reaction site. The local change in concentration in ferrocenium dopes MoS2, changing the brightness of the PL. (B) Top: Current (Iw) versus voltage (Vw) of the working electrode for different concentrations of ferrocene. Inset: Log-log plot of current at Vw = 1 V versus concentration of ferrocene. Bottom: MoS2 PL versus working electrode voltage for different concentrations of ferrocene. Data are taken with 10-ms exposure time. (C) Effective change in potential on MoS2 plotted against the working electrode current normalized by ferrocene concentration (C0). The effective change in potential is obtained from the MoS2 PL by using the slope of the linear region of the PL versus VLG curve measured in the same device (inset). The experimental points collapse to one curve that is well described by the Nernst equation: E = E′ + (kBT/e) ln(I/C0), with kBT/e = (21 ± 5) mV and E′ = (0.53 ± 0.01) V accounting for a current offset of −1 nA/mM. (D) PL versus electrode voltage for low concentrations of ferrocene. The first response is seen at 10 nM concentrations of ferrocene, likely limited by background concentrations of contaminant redox molecules. Data are taken with 100-ms exposure time.

  • Fig. 6 Flow imaging in microfluidic channels with MoS2 pixels and optical fiber.

    (A) Bright-field image of PDMS channel placed on substrate with MoS2 pixel array and platinum surface electrode, with schematic showing circuit with voltage VW for applying potentials to oxidize ferrocene and syringe for driving flow of solution. (B) PL images MoS2 pixel array during laminar flow. A 1-V pulse is applied to the platinum surface electrode for 1 s to oxidize ferrocene. Images are taken with 100-ms exposure time. (C) Reconstructed confocal PL microscopy images showing MoS2 pixels transferred onto an optical fiber. (D) Plot of chemical potential sensing with light coupled to MoS2 through the optical fiber. As in previous experiments, a pulse is applied to a probe nearby MoS2 to oxidize ferrocene to ferrocenium. MoS2 is illuminated with a 532-nm laser coupled into the optical fiber, and the PL is observed with an optical microscope. Data are taken with 100-ms exposure time. a.u., arbitrary units.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/11/eaat9476/DC1

    Supplementary Materials and Methods

    Fig. S1. Characterization of MoS2 samples.

    Fig. S2. Diagram of the experimental setup.

    Fig. S3. Detection of ions in solutions containing ruthenocene and ruthenocene/ferrocene mixtures.

    Fig. S4. Diffusion current mapping of ferrocene ions.

    Fig. S5. PL decay time for varied Fc concentrations.

    Fig. S6. PL imaging of electroosmotic flow.

    Fig. S7. PL versus dopamine concentration.

    Fig. S8. Pixel-to-pixel variation of PL.

    Movie S1. Visualizing ferrocenium diffusion using a MoS2 pixel array.

    Movie S2. Visualizing laminar flow of ions in a microfluidic channel.

    Movie S3. Visualizing electroosmotic flow of ions in a microfluidic channel.

  • Supplementary Materials

    The PDFset includes:

    • Supplementary Materials and Methods
    • Fig. S1. Characterization of MoS2 samples.
    • Fig. S2. Diagram of the experimental setup.
    • Fig. S3. Detection of ions in solutions containing ruthenocene and ruthenocene/ferrocene mixtures.
    • Fig. S4. Diffusion current mapping of ferrocene ions.
    • Fig. S5. PL decay time for varied Fc concentrations.
    • Fig. S6. PL imaging of electroosmotic flow.
    • Fig. S7. PL versus dopamine concentration.
    • Fig. S8. Pixel-to-pixel variation of PL.
    • Legends for movies S1 to S3

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    Other Supplementary Material for this manuscript includes the following:

    • Movie S1 (.avi format). Visualizing ferrocenium diffusion using a MoS2 pixel array.
    • Movie S2 (.avi format). Visualizing laminar flow of ions in a microfluidic channel.
    • Movie S3 (.avi format). Visualizing electroosmotic flow of ions in a microfluidic channel.

    Files in this Data Supplement:

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