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Sol-gel materials under gamma rays   Print  E-mail
Written by Editor  
Sunday, 21 March 2004

Gamma rays which are generated by radioactive materials, have the smallest wavelengths of any other wave in the electroma­gne­tic spectrum. They are very penetrating and can kill living cells. Therefore, they are used in medicine to kill cancerous cells or in food industry to sterilization. Gray is the SI unit of absorbed dose of radiation. The gray is the absorbed dose when the energy per unit mass imparted to matter by ionizing radiation is one joule per kilogram (1 Gy = 1 J/kg).

In our research, influence of gamma rays on organic molecules entrapped in sol-gel matrices is investigated. Tetraethoxy- and tetra­methoxysilan (TEOS and TMOS) are used to obtain the matrices. The compounds enclosed in porous silicate bulks are dyes with strong absorption band in the visible region (metal phthalocyanines, complexes of ruthenium(II) or safranine). Dyes are dissolved in water or organic solvent and added to the sol. The obtained matrix shows very good transmission properties in the investigated region even after irradiation. The translucent bulks are exposed to 60Co radiation source at room temperature, at a dose rate of 15,8 Gy/min. For comparison, the solutions of the dyes are also irradiated.

When dye dissolved in solvent is exposed to gamma radiation, the decay of absorbance is observed. The rate of the decay depends among others on a kind of solvent, exposure time, concentra­tion of the dye and the level of oxygen. When, for example, magnesium(II) phthalocyanine (MgPc) dissolved in dimethyl formamide (DMF) is exposed to irradiation, gamma rays cause the complex decomposition. This effect is registered as the discoloration of the solution and the decay of the absorption. The complex has three absorption bands in the visible region (~ 604, 642 and 670 nm). The fast decrease in the absorbance is observed for all of them (fig.1). The initial blue color almost disappears at doses of about 1 kGy (for the initial concentration of the dye 0,62 μM). The decay of absorbance at 670 nm versus dose or gamma irradiation is exponential. Neodymium bisphthalocyanine (NdPc) in DMF has two absorption bands in the visible region (~630 and 670 nm) which can be assigned to the monomeric and dimeric form of the phthalocyanine. The fall in absorption depends on the solvent. For the complex in DMF, at the beginning of irradiation, the longer-wavelength band increases and the other decreases its intensity. The monomer : dimer ratio changes during irradiation. Probably the dimer form degradation is faster. In DMF : H2O solution the ratio is constant at the beginning. The faster decrease in the dimer form seems to appear in the later stage of radiation. In one case the decay of absorbance versus dose or gamma irradiation is exponential, in the other case, it is linear.

Gamma rays also changes the properties of silicate matrix. It becomes nontranspa­rent in the blue region of electromagnetic spectrum at big doses. The absorption shifts from 270 to 400−450 nm for 1000 kGy. The bulks, from transparent and colorless, becomes yellowish. The changes are quite similar for bulks from TEOS and TMOS.

The absorption spectra of phthalocyanines in silicate matrix are different as compared to the solutions. For MgPc, there is a broad band with maxima at 610, 680 and 750 nm. The decay of absorption intensity under gamma rays is much slower in the xerogel (fig.2). It slows a lot especially in the later stage of irradiation. Even up to radiation dose of 1100 kGy, the bulk samples are still color and the sol­­‑gel matrix has very good transmission properties in the investigated region. Further research were difficult because the bulk started to disintegrate.

For NdPc2 in the sol‑gel matrix the broad absorption band consists of at least three overlapping bands. They also can be assigned to the monomeric and aggregated forms of the Pc. First doses of radiation causes fast degradation. After abut 40 kGy the degradation is much slower. The blue shift, broaden and increase of the band intensity is observed. The longer-wavelength band is assigned to monomer. It is possible that faster degradation of this form causes appa­rent shift and the absorption increase. The change in the monomer↔dimer equilibrium is also possible. The variation in the absorbance depends on the concentration of the dopant. For different concentrations, the shift and absorbance growth appear at the beginning or in the later stage of irradiation. To apply the sol‑gel matrix with organic compounds as a gamma radiation sensor, it should show proper dependence of absorbance on gamma dose. Only in some cases the dependence is linear or, more often, exponential. Such relationship can be found only for some range of doses. Usually, the straight line can be drawn for small doses and the curve line for larger doses. Perhaps some molecules could be applicable to the dosimeters purpose. For instance, safranine which shows linear decay in a big range from 0 to 40 kGy (fig.3) or tris(1,10‑phe­nantroline)ruthenium(II) with exponential decay from 0 to 150 kGy.

As a result of interaction between matter and ionising radiation, radicals and solva­ted electrons are formed. They can later react with molecules in the environment. The radiation induced transformation of complex compounds are:

  • redox processes leading to the change of oxidizing degree of central ion which changes the coordination number and causes the molecule decomposition;
  • radicals effect (reactions with radicals);
  • reactions with excited molecules of solvents.

But the final products of the decomposition of many molecules are unknown. The radicals formed under high-energy radiation can be responsible for the degradation. The macrocyclic rings are especially sensitive to radicals attack. The cleavage of carbon−carbon, carbon−hydrogen and carbon−nitrogen bonds is possible in the dyes molecules.

The decrease in the dye decomposition rate in the sol‑gel matrix might be related to lower concentration of free radicals formed in it upon gamma radiation. Another possibility is that the matrix­induced change of the macrocycle complex surroundings occurs which might increase the dye stability. The main conclusions of the research are:

  • Gamma rays cause decomposition of organic molecules entrapped in sol-gel matrix.
  • The dye decomposition rate is lower in the sol-gel matrix in comparison with dyes in solutions.
  • The linear or exponential decay in absorbance occurs only in a particular range of doses.
In the xerogel less radicals can be formed or the matrix increases the dye stability.


Fig.1

Fig.1. Absorption spectra of magnesium phthalocyanine in DMF before and after gamma irradiation.


Fig.2

Fig.2. Absorption spectra of magnesium phthalocyanine in the sol­gel matrix before and after gamma irradiation.


Fig.3

Fig.3. Linear absorption intensity changes (at 519 nm) versus dose of gamma irradiation for safranine in xerogel.

Last Updated ( Sunday, 21 March 2004 )
 
 
   
     

 
 
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