**4. Application**

the particles' size from the absorption spectra. UV-Vis absorption spectra are a simple, express method to study the formation of oxides in PLAL of a large number of metals (Ce, Ti, Cu, Zn, Mg, etc.), because a characteristic peak of exciton absorption appears in the spectrum.

258 Applications of Laser Ablation - Thin Film Deposition, Nanomaterial Synthesis and Surface Modification

Raman spectroscopy allows evaluation of the particles' structure both in powders and in dispersions. For example, this method makes it possible to distinguish between three iron oxide forms, magnetite, maghemite and hematite [18], that are difficult to recognize by other methods, including X-ray diffraction. **Figure 7** shows the Raman spectra of iron oxide nanoparticles obtained by PLAL of a metallic iron target in water. Freshly obtained magnetite with superparamagnetic properties (curve 1) during heating or irradiation by laser easily transforms into hematite (curve 2). Also, shifts and widening of bands in Raman spectra allow one to reveal the structure, including amorphous materials [19], defects and NP size [20, 21].

Fluorescence spectroscopy is used for the study of defective states of NPs of many oxides in dispersions and powders. For example, spectra and kinetics of fluorescence give information

**Figure 8.** SEM images of CuO nanoparticles obtained at various conditions: drying of water dispersion with the subsequent annealing at 500°C (a), vacuum drying of water dispersion with addition of 1% (weight) hydrogen peroxide (b)

Obviously, classic methods for determining particles' dimensional characteristics are important and are also used for the study of the NPs obtained by the PLAL method. A typical example of such a study is presented in **Figure 8**, which shows microphotographs of copper (II) oxide crystal powders. An important feature of using different solvents and precursors in PLAL is the ability to control not only the composition and structure of the particles but also their size and morphology. Additional treatments of the particles in the dispersions obtained by PLAL and different methods of nanopowder preparation from them (deposition, thermal and vacuum drying, subsequent annealing) allow, for example, synthesizing nanomaterials with different characteristics but the same chemical composition. Copper (II) oxide CuO powders in **Figure 8** are obtained in different experimental conditions of PLAL and further treatments: drying of water dispersion with the subsequent annealing at 500°C (a), vacuum drying of water dispersion with addition of 1% (weight) hydrogen peroxide (b) and drying of water dispersion with addition of 0.01 M of nitric acid followed by annealing at 300°C (c). In the first case, the oxidation of Cu2O occurs; in the second case, CuO was obtained directly in the dispersion during the PLAL; in the last version, NPs of Cu2NO3(OH)3 were a result of ablation and after

and drying of water dispersion with addition of 0.01 M of nitric acid followed by annealing at 300°C (c).

on different oxygen defects in zinc oxides [22], titanium oxide [23], tin oxide [24], etc.

The scope of application of metal oxide NPs is extremely wide and varied. However, for the oxide nanoparticles obtained by PLAL, there are two most promising applications: scientific research and biomedicine. In both cases, there is no demand for large quantities of nanomaterials and the disadvantage of the PLAL connected with the relatively low productivity is immaterial. Instead, the important advantages of PLAL are express preparation of a wide range of nanomaterials in laboratory conditions for scientific research and "pure" nanoparticles directly in the form of colloidal solutions for biomedicine.

Thus, in our laboratory, oxide nanoparticles obtained by PLAL are used in the study of catalytic processes for searching for the structure and composition of the most effective catalysts for various chemical and photochemical processes. In Ref. [25], the results of the study of catalytic CO oxidation reaction for CeO2-Pd composites obtained by PLAL of Ce and Pd targets in various solvents are presented. The change of the synthesis parameters of PLAL allows control of the size, chemical composition and superficial properties of obtained particles and hence modification of the catalytic properties. In addition to the cerium dioxide, we used Al2O3, SnO, SnO2, CuO and CuO obtained by PLAL as carriers and catalysts. Composites based on zinc oxide, titanium dioxide and copper oxide nanoparticles were used for the study of photocatalytic processes.

Nanocolloids obtained by PLAL are suitable material for the study of the influence of nanomaterials on the environment. On the one hand, inert enough particles of noble metals can be used to define their migration paths in different ecosystems and, on the other, to investigate the toxicity. Authors of Ref. [26] used several types of nanoparticles including particles obtained by PLAL to study their influence on different aquatic organisms.

Search for tools and methods of protection from antibiotic-resistant pathogenic bacteria strains is an actual problem of modern medicine. The use of nanoparticles as antibiotics that does not cause an adaptation effect among bacteria is one of the promising approaches. In this regard, a new area of research in biomedicine has appeared and the term "nano-antibiotics" [27] has been invented. Effects of zinc oxide nanoparticles obtained by PLAL on pathogenic bacteria were examined in Ref. [28] compared to silver nanoparticles and classic antibiotics. Then, we continued research using NPs of Cu2O that are more movable and have the best antibacterial properties compared to NPs of ZnO. **Figure 9** shows a picture of cotton fabric samples with ZnO and Cu2O particles deposited from water dispersions obtained by PLAL; the composites obtained were covered by *Escherichia coli* bacteria. The figure illustrates the suppression of bacteria growth on fabrics with nanoparticles, while bacteria multiply on the test sample.

**Figure 9.** *E. coli* growth inhibition by zinc oxide and copper oxide nanoparticles.

Among other applications in biomedicine, the prospects of using CeO2, TiO2 and ZnO obtained by PLAL to protect the skin from UV radiation (sunscreen) are worth noting. Lately, CeO2 has attracted more and more attention in this field [29] because its toxicity is much lower than standard TiO2 and its photocatalytic effects are lower as well.

The new direction that has developed effectively in nanomedicine in the last decade is the use of magnetic nanoparticles for modern theranostics—to deliver drugs, for contrasting of pathological object and in magnetic therapy. The use of PLAL for obtaining nanoparticles with special magnetic properties based on oxides of 3d metals directly in the biologically compatible liquids, their functionalization by gold particles and biopolymers have great prospects for the applications mentioned above.
