Micrometer-level naked-eye detection of caesium particulates in the solid state

Abstract

Large amounts of radioactive material were released from the Fukushima Daiichi nuclear plant in Japan, contaminating the local environment. During the early stages of such nuclear accidents, iodine I-131 (half-life 8.02 d) is usually detectable in the surrounding atmosphere and bodies of water. On the other hand, in the long-term, soil and water contamination by Cs-137, which has a half-life of 30.17 years, is a serious problem. In Japan, the government is planning and carrying out radioactive decontamination operations not only with public agencies but also non-governmental organizations, making radiation measurements within Japan. If caesium (also radiocaesium) could be detected by the naked eye then its environmental remediation would be facilitated. Supramolecular material approaches, such as host-guest chemistry, are useful in the design of high-resolution molecular sensors and can be used to convert molecular-recognition processes into optical signals. In this work, we have developed molecular materials (here, phenols) as an optical probe for caesium cation-containing particles with implementation based on simple spray-on reagents and a commonly available fluorescent lamp for naked-eye detection in the solid state. This chemical optical probe provides a higher spatial resolution than existing radioscopes and gamma-ray cameras.

Keywords: 10.11; caesium cation recognition; charge transfer; optical probe; radiocaesium; supramolecular.

Figure 1.

Photographs of fluorescence changes of a mixture of 1 and various carbonate salts after addition of a drop of methanol. (a) Fluorescence change of a powdered mixture of 1 and Cs₂CO₃ under UV irradiation (365 nm) after addition of a drop of methanol. (b) Photographs of a mixture of 1 and various carbonate salts under UV irradiation (365 nm) after addition of a drop of methanol. (c) Photographs of 1 + Cs⁺ on dirt under room light (left) and under UV irradiation (365 nm, right) after spraying with methanol. (d) Photographs of 1 + Cs⁺ particles on filter paper (diameter 110 mm) under UV irradiation (365 nm) after spraying with methanol.

Scheme 1.

Synthesis of the receptors 1 and 2.

Scheme 2.

Synthesis of the receptors 3TEG.

Figure 2.

Detection limit of Cs₂CO₃ by using 1. (a) Photographs and plot of fluorescence emission maxima wavelengths for mixtures between 1 and Cs₂CO₃/K₂CO₃ under UV irradiation (365 nm) after addition of a drop of methanol. (b) Fluorescence spectra of mixtures used to construct the plot in (a). (c) Photographs and emission maxima for mixtures between 1 and Cs₂CO₃/K₂CO₃ under UV irradiation (365 nm) after addition of a drop of methanol.

Figure 3.

Interaction of 1 (a) or its component chromophore moieties (3TEG) (b) with alkali metal cations. (c) Fluorescence spectra of the mixtures revealing the different spectrum in the presence of Cs⁺. (d) In the simple mixture of component chromophores the shift in fluorescence maximum is rather small and is not sufficient for sensing activity. Numbers above the spectra in panels (c) and (d) indicate the wavelength of the fluorescence maximum.

Figure 4.

Photographs (a)–(c) and spectra of fluorescence change (d)–(f) of a mixture of 1 and alkali metal salt under UV irradiation (365 nm) after addition of a drop of methanol. Fluorescence change of a powder mixture of 1 and Cs⁺ under basic conditions after spraying with methanol. Numbers in panels (a)–(c) indicate the wavelength of the fluorescence maximum.

Figure 5.

Photographs of sunflowers and cross sections of their freeze-dried stems after immersion in water or an aqueous solution of potassium or caesium carbonate. The photographs were taken with the stems under UV irradiation (365 nm) after spraying only with methanol and with 1 in methanol.

Figure 6.

Photographs and spectra illustrating the differences in fluorescence emission from (a) 1 and (b) 2 in the presence of carbonate salts of alkali metal cations after spraying with methanol.

Figure 7.

Selected angles of the computed structures of the complexes of 1 with K⁺, Rb⁺ or Cs⁺ and 2 with K⁺, Rb⁺ or Cs⁺. Dihedral angles subtended at the indicated bonds are listed in the table on the right.

Figure 8.

Origin of the donor–acceptor (D–A) character of terphenol. As illustrated in the lower part, in the phenol both the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are situated over the extent of the molecule. Upon deprotonation, HOMO is situated at the electron-rich phenoxide group while the LUMO is situated at the electron-deficient phenyl group remote from the phenol. Experimental and simulated electronic absorption spectra (top-right panel) reveal the accuracy of the model.

Figure 9.

Schematic of the computed structures of the complexes of 1 with K⁺ or Cs⁺. Em stands for emission wavelength.

Figure 10.

Schematic of spatial resolution of radioscopes and caesium sensor 1.