Highly sensitive, uranium based UV detectors. [View all]

[NNadir]

I am fascinated by the remarkable chemistry of the actinide elements because of the interesting chemistry of the 5f orbitals.

(One of the "actinides," thorium, strictly has limited or no 5f chemistry, although its considered an actinide nonetheless, for convenience.)

One of the interesting things about the actinides, all of which are radioactive, is that they are excellent shielding materials for high energy radiation, owing to the fact that they have so many electrons - uranium, for example has 92 - making it possible for them to have many electronic transitions, and because they are massive, their inner electrons can absorb very high energy radiation to emit "Auger electrons."

Thus I was fascinated by a paper published recently in the wonderful - if overly dense - journal ACS Appl. Mater. Interfaces, specifically, this one: Highly Sensitive Detection of UV Radiation Using a Uranium Coordination Polymer, published by scientists at the Key State Laboratory of Radiation Medicine and Protection and the School for Radiological and Interdisciplinary Sciences and a few other institutions in Sozhou, China. (Wang et al, ACS Appl. Mater. Interfaces, 2018, 10 (5), pp 4844–4850)

(The Chinese government doesn't hate science quite as much as our government hates it, which is why they are going to eat us alive in the 21st century.)

Here's the introductory text from the paper:

Ultraviolet radiation is widely used in chemical industries, such as curing and photolithography, sterilization, surface modification technique, and so forth,1?3 but can exhibit either positive or negative impacts on human health. For instance, UV radiation is crucial for assisting human skin to produce vitamin D that is necessary in physiological processes.4 Excessive doses of UV radiation, however, impose great damage on the human body and may result in the development of cutaneous malignant melanoma (CMM) and non-melanoma skin cancer (NMSC),5 leading to premature skin-aging and eye disorders. 6,7 Besides these physical impacts, developing efficient UV photodetectors is also highly desirable in automotive, aerospace, environmental, and biological researches.7 Currently, various techniques have been developed to detect UV radiation both qualitatively and quantitatively. The most developed semiconductor photodetectors, including metal?semiconductor? metal (MSM) detectors,8 PIN photodiodes detectors,9 p? n junction diodes,10 and Schottky barrier detectors,11 often suffer from several disadvantages, such as sophisticated synthesis and manufacturing procedure, not being able to measure the accumulated UV dosage as well as high defect density in the material. The latter greatly lowers the detection sensitivity and efficiency.12

The authors propose a uranium based detector for the following reasons:

Uranium, the most critical 5f element in the nuclear fuel cycle, is chosen in this work as the metal center based on the following considerations. First, depleted uranium is an abundant long-half-life radioactive byproduct of the nuclear power industry that receives limited studies in luminescent coordination polymer systems compared with other metals. Second, uranyl luminescence originating from the HOMO? LUMO transition of hybridized molecular orbitals often exhibits brighter emission and more efficient absorption of UV light than trivalent lanthanides owing to the non Laporte forbidden nature which greatly extends the detection limit.34 Third, given the 5f/6d orbitals of uranyl are deeply involved in coordination, the luminescence is highly sensitive to the coordination environment, which affords more opportunities for developing detection ability (i.e., more efficient energy transfer).35

LUMO here refers to the "lowest unoccupied molecular orbital" and HOMO to the "lowest occupied molecular orbital." Transitions between molecular orbitals (or in some cases atomic orbitals), defined by quantum mechanics, determine the properties of radiation absorption and emission, not only at high energy levels such as those observed for UV, X-rays, and gamma radiation, but also in the visible range: Color is a function of these effects.

The authors synthesize a "MOF" - a "metal organic framework" - a class of materials that has been the subject of vast amounts of research in recent years. This particular framework is built from uranium atoms, nitroisophtalic acid and dimethylformamide.

Here's a graphic describing the structure of this framework:



The caption:

Figure 1. Crystal structure depictions of 1, where hydrogen atoms are omitted for clarity: (a) coordination environment of uranium(VI); (b) asymmetric unit of [UO2(L)(DMF)]; (c) 1D metal–organic chain of 1 composed of 5-nitroisophthalic acid linked asymmetric units; (d) pseudolayered structure comprising 1D chains coalescing due to ?···? interactions. Atom colors: U = green, O = red, C = black, N = blue

Although the molecules luminescence nicely, after long term irradiation, the intensity of the luminescence fades:



The caption:

Figure 2. (a) UV dosage dependent luminescence spectra of 1 performed on a single crystal to show the quenching effect under 365 nm UV light. (b) Correlation between the quenching ratio and radiation dosage. Inset is the correlation between D/[(I0 – I)/I0 %] and the UV dosage. (c) is the corresponding luminescence photographs of a single crystal after receiving continuous UV radiation.

Surprisingly however, this effect seems not to relate to structural degradation of the molecular organic framework, which demonstrates remarkable structural integrity even upon irradiation with higher energy wavelengths, to wit, x-rays and gamma rays, as is shown in the XRD (X-ray diffraction pattern) graphics shown:



The caption:

Figure 3. (a) PXRD patterns for samples irradiated with UV, 100 Gy X-ray and 100 kGy ?-ray radiation. (b) EPR spectra of 1 before and after UV, 100 Gy X-ray, 100 kGy ?-ray radiations.

The EPR (Electron Paramagnetic Resonance) spectra clearly shows the persistence of free radicals, thought to reside on the dimethylformamide ligand:



The caption:

Figure 4. (a) Optimized geometry structure and bond parameters of ground state DMF molecule. (b) Optimized geometry structure, bond parameters (left), and net spin density (right) of triplet DMF· radical. (c, d) Simulated radical-free and radical-bearing coordination structures of the fragment, named as uranyl-5-NIPA-DMF and uranyl-5-NIPA-DMF·, respectively. Bond parameters are labeled below each structure.

I love this last graphic, because one doesn't get to look at electron density diagrams of molecular orbitals resulting from the mixing of f orbitals all that much:



The caption:

Figure 5. Density of states (DOS) of (a) the isolated uranyl molecule, (b) the uranyl-5-NIPA-DMF complex, and (c) the uranyl-5-NIPA-DMF· complex. The gray-filled and empty areas below DOS curves indicate the occupied and unoccupied states, respectively. For each DOS, the lowest unoccupied U(5f) orbital is normalized at 0 eV for convenience.

The authors thus conclude:

In summary, a highly stable uranium coordination polymer was successfully synthesized through solvothermal method that exhibits superior sensing property. The intrinsic luminescence of 1 could be quenched by UV which makes it suitable for monitoring UV radiation. The radical-induced quenching mechanism confirmed by EPR, X-ray crystallography, and DFT calculations studies corroborates this property of 1...

... This work provides us new opportunity for searching powerful UV responsive materials by taking advantage of efficient UV light asbsorber (uranyl) as metal center. We further noticed that many other uranyl hybrid materials constructed from different types of ligands and solvents may exhibit similar properties, which can be therefore fine-tuned by varying uranyl coordiation enviorments, crystal structures, and chemical constituents (e.g., light sensitizer), and the systematic investigations are in progress. We also believe this work offers new insight into methods in which depleted uranium may be reused for beneficial purposes.

It's a fine paper, but I will note that my preferred use for depleted uranium is as a precursor to plutonium as a nuclear fuel.

The interesting thing for me about this paper is the stability of this framework in a high radiation field. This suggests it's use as a "breathable" nuclear fuel, albeit one that would operate in a thermal spectrum, thus of use in thorium derived U-233 systems as opposed to plutonium breeding systems.

An interesting paper I think.

Have a nice weekend.