Cesium ion detection by terahertz light

Recent developments in terahertz technologies provide new tools for analysis, inspection, and nondestructive sensing. If a heavy atom is encapsulated in a cage of a porous material, the atom should vibrate slowly and resonate with a low-frequency terahertz light. From this perspective, a cyanide-bridged metal framework is a suitable system because it contains many cages that can adsorb Cs ions. Herein we show the vibration mode of a Cs ion in a cage of a cyanide-bridged metal framework. First-principles phonon mode calculations and terahertz time-domain spectroscopy (THz-TDS) measurements indicate that the vibration mode of a Cs ion in a cyanide-bridged manganese-iron framework is at 1.5 THz, which is significantly apart from other lattice vibrations. Taking advantage of this feature, we develop a THz-light detection method for Cs ions, which is useful for non-contact sensing of Cs ions in dangerous environments or harmful circumstances.

First-principles electronic structure calculations.
First-principles phonon mode calculations.
Elemental analyses of the samples recollected from Cs solutions.

Figs. S4 and S5
Supplementary Movie S1: The concept of non-contact sensing of heavy atoms using THz-light. Schematic illustration of a heavy atom encapsulated in a cage is initially shown. The heavy atom in the cage vibrates slowly and resonate with a low frequency THz light. This phenomenon can be applied as effective detection method for non-contact sensing of heavy atoms such as Cs.
Supplementary Movie S2: Calculated phonon mode of the Cs vibration in the cyanide-bridged metal framework. The crystal structure of the cyanide-bridged manganese-iron framework is initially shown. Turquoise, green, navy, and light blue spheres represent Mn, Fe, C, and N atoms respectively. Then Cs ions (orange spheres) are adsorbed at the interstitial sites of the framework and vibrate in the cages, displaying the atomic movements of the calculated phonon mode at 1.3 THz.
Supplementary Movie S3: Calculated phonon mode of the transverse translational mode of the Mn-N≡C-Fe framework. This movie shows the atomic movement of the phonon mode at 5.5 THz assigned to the transverse translational mode of the Mn-N≡C-Fe framework. Orange, turquoise, green, navy, and light blue spheres denote Cs, Mn, Fe, C, and N atoms, respectively. Supplementary Movie S4: Calculated phonon mode of the transverse librational mode of the Mn-N≡C-Fe framework. This movie shows the atomic movement of the phonon mode at 12.4 THz assigned to the transverse librational mode of the Mn-N≡C-Fe framework. Orange, turquoise, green, navy, and light blue spheres indicate Cs, Mn, Fe, C, and N atoms, respectively. Supplementary Movie S5: Calculated phonon mode of the C≡N stretching mode of the cyanide-bridged metal framework. This movie shows the atomic movement of the phonon mode at 65.6 THz assigned to the C≡N stretching mode of the manganese-iron metal framework. Orange, turquoise, green, navy, and light blue spheres represent Cs, Mn, Fe, C, and N atoms, respectively.

Supplementary Movie S6: THz-TDS measurement system for detection of the Cs vibration mode.
A diagram of the THz-TDS measurement system is initially shown. THz pulse is irradiated into the sample, and the transmitted THz pulse is detected in the time domain to obtain a temporal wave form. The irradiated and transmitted temporal waveforms are Fourier transformed to obtain the absorption spectrum.

Supplementary Movie S7: Cesium ion adsorption by cyanide-bridged metal framework.
In this movie, a schematic illustration of the cyanide-bridged metal framework capturing the Cs ions (orange spheres) from a cesium ion containing solution is shown. The amount of Cs adsorbed in the metal framework can be monitored by THz light (orange lines).
Supplementary Movie S8: Mechanism for the high Cs adsorption capacity of the cyanide-bridged manganese-iron framework. When Cs + ion is adsorbed to cyanidebridged manganese-iron framework, the Fe 3+ is reduced to Fe 2+ to maintaining charge neutrality of the framework. CsMnFe was prepared by reacting an aqueous solution of cesium chloride (0.3 mol dm −3 ) and manganese chloride (0.1 mol dm −3 ) with an aqueous solution of potassium ferricyanide (0.1 mol dm −3 ) according to the modified method based on our previous report (Dalton Trans. 5046 (2006)