M. F. Reynolds and co-workers from the Cornell University and the University of Chicago, USA, have recently discovered the transition metal di-chalcogenide (TMDs) based molecular pixel array, for real time detection of redox activity that occurs inside a molecule.
This new method determines redox activity through photoluminescence (PL) imaging of redox molecules, using the two-dimensional semiconducting material array. For redox-active molecular analysis in given time and space, is extremely important for the sake of understanding of the chemical as well as the biological systems, and to further develop new scientific technologies. The research is published in the journal Science Advances.
![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.](https://d36nqgmw98q4v5.cloudfront.net/image-handler/picture/2019/11/%40F1.jpg "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. 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. See https://advances.sciencemag.org/content/5/11/eaat9476
Additionally, the traditional schemes, based on the optical detection methods are though scalable and noninvasive, but in comparison to electrical detection methods, they possess slower response. Further, most of the fluorescent molecular materials, for redox determinations, gets degraded in the presence of intense brightness over a period.
TMDs like MoS2 act as two dimensional (2D) semiconducting materials having band gap corresponding to the visible region of the electromagnetic spectrum. The deeper investigations on TMDs based materials reveals that the monolayer of MoS2 behaves as direct band gap semiconducting material, having an appreciably reasonable photoluminescence (PL) efficiency. These materials have gained much attention recently. Furthermore, the PL of TMDs have also been explored in context to their response to chemical doping, electrostatic gating, changes in pH, and also the defects. However, there are rare reports highlighting the incorporation of this TMDs sensitivity, for exploration as PL based chemical or biological sensors. Previous studies only used ion intercalation schemes in biological sensors for cell viability measurements, optically. Various studies also analyzed the transfer of charge between MoS2 and different electrolytes, documenting the dependence of charge transfer rates on illumination intensity and back-gate voltages.
The most curious activity of TMDs, which has not been previously explored, is the optical detection of redox molecules with spatial resolution, which is currently explored by scientists and that too at a at micrometer level. Current approaches using this TMDs activity (specially resolved redox molecules detection) include arrays of microelectrodes, ion-sensitive field-effect transistor arrays, altered complementary metal-oxide-semiconductor (CMOS) camera detectors, and scanning electrochemical microscopy (SECM). There are various optical detection methods viz scanned photocurrent, porous silicon, and surface Plasmon techniques, and some others using fluorescent molecules and nanomaterials. Organic fluorescent molecules can be extensively used to detect a diverse range of redox molecules having high spatial and temporal resolution, however the technique face problem of photo bleaching which generate the interest of researchers to use photoluminescent nanomaterials in chemical sensing applications. Current research showed that MoS2 pixel arrays as a powerful class of sensors to detect redox-active molecules. Typically, the patterned arrays obtained from MoS2 squares have been used to detect changes in redox concentrations with spatial resolution of micrometer-scale and at 10-ms temporal resolution.
Reynolds et al showed that PL of “pixel” arrays of MoS2 monolayer can image temporal as well as spatial changes in redox molecular concentrations. Because of dependence of photoluminescence of MoS2 on doping, variations in the local chemical potential substantially modifies the PL of MoS2, with a high sensitivity of the order of 0.9 mV/ √HZ on a 5 µm × 5 µm pixel, and at better than parts per-hundred variations in redox molecular concentration and down to nano molar concentrations at 100-ms frame rates. Thus, this technique provides us a completely new strategy for studying the biomolecules and the chemical reactions, with a smart two-dimensional screen of this TMD material.
These MoS2 pixels could also be employed in a wide range of environments and from microfluidic systems to optical fibers. This renders the material as a highly attractive platform for redox sensing, for a wide range of applications.
"This work demonstrates a previously unknown class of 2D fluorescent sensors for the detection of redox-active species. The sensor is shot noise limited, with a sensitivity of 10% in a 30-Hz bandwidth at a 5 µm×5µm pixel and detection limits down to nanomolar concentrations. Improvements to the PL efficiency could increase this sensitivity by another one to two orders of magnitude", Reynolds added.
Next generation materials based on these TMDs may include the molecular functionalization of MoS2 for targeted molecular detection. ‘’The fast, all-optical detection of chemical potentials and ionic densities using the PL of a 2D material has great potential for monitoring various chemical and biological systems, such as hydrogen evolution reactions and neurological activity’’ M. F. Reynolds added. Flexibility, chemically inert, and easily transferrable nature, makes MoS2 an attractive target for redox sensing, that can be easily incorporated into a broad range of environments and systems.
Reference: MoS2 pixel arrays for real-time photoluminescence imaging of redox molecules, M. F. Reynolds, M. H. D. Guimarães, H. Gao, K. Kang, A. J. Cortese, D. C. Ralph, J. Park and P. L. McEuen, Science Advances 08 Nov 2019: Vol. 5, no. 11, eaat9476, DOI: 10.1126/sciadv.aat9476, https://advances.sciencemag.org/content/5/11/eaat9476
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