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Chemical Sensors Using Thin Film Xanthene Dyes

Xanthene dyes coated on polymer films are effective sensors for explosives (Zhang and Euler, 2016) but the mechanism(s) of action are not established. As shown in Figure 1 below, the sensor system has a xanthene dye coated onto a polymer, composed of polymethylmethacrylate (PMMA) or poly(vinylidene fluoride) (PVDF). One project is to determine the photophysical properties of thin films of selected xanthene dyes to understand the limits of sensitivity for this sensor system.

Figure 1. The components of the sensor (top) are a mechanical substrate, a polymer layer, and a fluorophore. Polymers used are PMMA and PVDF. Fluorophores are the xanthene dyes Rh6G (rhodamine 6G), Rh560 (rhodamine 560), SRhB (sulforhodamine B), Rh640 (rhodamine 640), and Fl548 (fluoroscein 548).
Figure 1. The components of the sensor (top) are a mechanical substrate, a polymer layer, and a fluorophore. Polymers used are PMMA and PVDF. Fluorophores are the xanthene dyes Rh6G (rhodamine 6G), Rh560 (rhodamine 560), SRhB (sulforhodamine B), Rh640 (rhodamine 640), and Fl548 (fluoroscein 548).

One example is the properties of a thin film of rhodamine 6G (Rh6G) on glass (Chapman, et al, 2016). The absorption and emission properties show a striking change as the thickness of the Rh6G thin film increases. As shown below, the normalized absorption spectra show a new peak and the normalized emission spectra shift to lower energy as the film thickness increases.

Figure 2A. Absorption spectra of Rh6G on glass as a function of thickness.
Figure 2A. Absorption spectra of Rh6G on glass as a function of thickness.
Figure 2B. Emission spectra of Rh6G on glass as a function of thickness.
Figure 2B. Emission spectra of Rh6G on glass as a function of thickness.

 

 

 

 

 

 

 

 

This indicates that the Rh6G is aggregating in the excited state as the films become thicker. The structure of the Rh6G film evolves from isolated, independent molecules when the films are less than one monolayer to interacting aggregates for thicker films. This is shown schematically in the cartoon below. How this impacts the ability of the Rh6G to act as a chemical sensor is still under investigation.

 

Figure 3. Cartoon showing the structural changes of Rh6G on a smooth glass surface as a function of surface density.
Figure 3. Cartoon showing the structural changes of Rh6G on a smooth glass surface as a function of surface density.

Control of Surface Morphology in Polymer Thin Films

As shown in Figure 1, one of the critical components in the sensor system we are developing is a layer of a polymer film. One role of the polymer is to be a conduit for internal reflections that improve the efficiency of the use of the incident light. The other role of the polymer layer is to increase the nominal surface area occupied by the fluorophore. We discovered that, for some polymers, we could create periodic wrinkle patterns by simple spin-coating.

When PMMA is spin-cast onto a smooth glass substrate,a wrinkle pattern is formed across the entire polymer surface, as shown in the micrographs in Figure 4.

Figure 4. Optical micrographs of PMMA films spin-cast from toluene using solution concentrations shown below each image.
Figure 4. Optical micrographs of PMMA films spin-cast from toluene using solution concentrations shown below each image.

The periodicity of the wrinkles is on the order of many tens of microns. This can be quantified by measuring the surface topology, as shown in Figure 5.

Figure 5. Line scan of PMMA (3% w/v)
Figure 5. Line scan of PMMA (3% w/v)

The periodicity is clearer in the line scan, showing a wavelength of about 50 μm and an amplitude of about 30 nm. The amplitude is small enough that it is possible that molecules trapped in valleys could experience quantum confinement effects, although this hypothesis has not been tested, yet.

Polystyrene (PS) also can be spin-cast into wrinkle patterns, as shown in Figure 6. The molecular weight of the PS strongly influences the structure of the wrinkles.

Figure 6. Wrinkle patterns of PS as a function of polymer molecular weight.
Figure 6. Wrinkle patterns of PS as a function of polymer molecular weight.

As the polymer molecular weight increases from 1000 g/mol to 350,000 g/mol the wrinkle wavelength increases from ~65 μm to ~115 μm. Concurrently, the wrinkle amplitude increases from ~10 nm to ~100.  We also will be investigating the role of solution concentration and spin-casting rotational velocity.