The effect of organic matter humification (aromaticity and oxidation degree) on structural and nanomorphological characteristics of humic nanocomposites of metallic platinum

Background: It has been previously found that humic substances (HSs) can serve as the environmentally benign non-toxic agent for the preparation of magnetic and noble metals nanoparticles that are increasingly used in biomedicine. The structure of HSs and, hence, their synthetic potential depend on the source of their origin. Thus, humification character, determined by conditions and duration of complex transformation of organic remains in HSs can evidence their structure, in particular their aromaticity and oxidation degree. The incorporation of platinum nanoparticles in a shell of HSs allows obtaining aggregate-stable nanomaterials with directly controlled structural and nanomorphological characteristics, which combine the properties of platinum nanoparticles (selective cytotoxicity, anti-inflammatory activity, etc. And a complex of biological properties of HSs (antioxidant, immunomodulatory and anti-inflammatory activity. At the same time, the expression of valuable properties of platinum nanoparticles can be varied directly by changing their nanomorphological characteristics that strongly depend on the conditions of synthesis, in particular on the HSs type which is used for the synthesis.

Aim: To study the effect of humification (aromaticity and oxidation degree) of three types of humic substances extracted from natural Mongolia sources (therapeutic muds of Gurvan Nuur Lake, brown coal of Baganuur coal deposit and shale of Shine Hudag deposit) on structural and nanomorphological characteristics of Pt⁰-containing nanocomposites produced on their basis.

Methods: Nanocomposites with Pt⁰ content 1.2-6.5% were synthesized by reduction of hydrogen hexachloroplatinate with HSs in an aqueous-alkaline medium upon heating (90 °C). The yield of nanocomposites varied depending on type of HSs in the range 75-96%. The structure, composition and morphology of obtained nanocomposites were characterized by complex of modern physical and chemical methods (elemental analysis, XRD, SEM, TEM, IR- and optical-spectroscopy, DLS).

Results: The platinum-containing nanocomposites have been synthesized using the reduction and stabilizing potentials of HSs with different degrees of humification (aromaticity and oxidation degree) and isolated from three natural Mongolian sources. The effect of functional composition, including oxidation and aromaticity degrees of HSs on the yield, quantitative and nanomorphological characteristics of produced platinum nanocomposites has been found. It is established that under the same reaction conditions platinum nanoparticles with face-centered cubic lattice and the smallest average size (9-15 nm) are formed from humic substances extracted from coal, while HSs derived from mud and shale afford the nanoparticles of larger size (18-28 and 16-26 nm respectively). The increase of platinum content in nanocomposites enlarges their average size, decreases their aggregate stability, as well as augments oxidation degree of HSs macromolecules. The reduction of platinum from precursor to the zero-valent state occurs due to oxidation of the phenolic and alcoholic hydroxyl groups, which are major components of HSs, as well as carbonyl groups. At the same time, HSs cover individual platinum nanoparticles, making them water soluble and preventing their aggregation.

Conclusion: Thus, we have synthesized a series Pt⁰-containing nanocomposites containing 1.2-6.5% with use as reducing and stabilizing matrix of natural available HSs with different humification characteristics (aromaticity and oxidation degree) and isolated from three natural Mongolia sources It is found that obtained nanocomposites are formed in form of platinum nanoparticles mainly of spherical shape and size of 3-42 nm distributed in a matrix of HSs. Polyfunctional composition of HSs is determined by conditions of their transformation and degree of humification, which provides their reducing and stabilizing properties during the synthesis of platinum metallic nanocomposites. It is established that Pt⁰ nanoparticles with the smallest average size and the highest aggregate stability are generated from HSs-coal with the lowest oxidation degree. These data contribute significantly to understanding of possible control of the nanomaterials synthesis and prediction of their properties.