The SEM images (500 × magnifying) of the three Silver film deposition samples are presented in Figure 5. The silver layer was not only deposited on the fabric surface. It penetrated through the interstices between the twisted fibers that formed the yarns from the fabric texture. In this way, the deposition produced a composite structure with new properties. The ionic state of the silver was the intended goal due to its antibacterial and antifungal properties and, for that reason, the study of the interaction of silver plasma plume with the citric acid from the support was important.

The images in Figure 5 evidence the droplets of silver resulted during the laser deposition. EDS analysis showed an elemental composition on the thin layers as presented in Table 1 and in the spectra of Figure 6a–c. The analyzed areas were of 0.185 mm² on the surfaces presented in Figure 5b–d) for data in Table 1.

The PLD technique is based on laser ablation when the solid material of the target is transformed into liquid, gas and plasma. Plasma, also known as ionized gas [18,19,20,21,22,23,24,25,26], is a state of matter where ions, electrons, atoms and clusters exist at the same time. The predominant content in the Silver of the deposited thin film is noted in Table 1 This is in good accordance with the results of elemental composition of the ablated area on the target. It implies that the silver ions should arrive on the support surface and react with the citric acid. Because some of the silver was already derived from the ablation that occurred in liquid state, as per the temperature plots obtained from the COMSOL simulation of silver ablation and depositions shown in the work of A. Cocean et al., 2017 [27], the reaction rate was not expected to be high. Nevertheless, other phenomena can improve the active state of the film and increase its predisposition to subsequent reactions when in contact with biological agents such as bacteria and fungi. Regardless, this was not the experimental subject for this study, but it was where the idea started. Therefore, in order to determine whether the reaction took place, the powder of the deposited layers was collected by scraping it off the surface of the samples, and the FTIR results are presented in Figure 7. In order to avoid the possibility that silver ions could occur from oxidized areas or from Ag₂S that may form, in time, on the target surface when in contact with the atmosphere, borax was incorporated on the target (as presented in the Section 2). It followed that silver ions from oxides and/or sulfides had been converted into atoms before the PLD process, but also during PLD if any remaining oxygen traces were present in the deposition chamber. This provides further evidence that the only source of silver ions that will react with the citric acid is the plasma obtained via ablation during which ions, atoms and clusters co-exist. For a better evaluation of the FTIR spectra of the samples that were experimentally obtained, they were also compared with the spectra of the hemp fabrics used as supports for silver deposition, namely HTND (hemp fabric) and HTNS (hemp fabric impregnated with the supersaturated citric acid aqueous solution). The FTIR spectra (Figure 7) show the changes that citric acid undergoes as a result of silver deposition. The transformation of carboxyl groups into carboxylate ions can be observed in the FTIR spectra, and the formation of silver citrate is indicated.

In this regard, for both types of hydroxyl groups assigned to carboxyl from citric acid (free: 3500 cm⁻¹; H-bounded: 3283 cm⁻¹ [28,29]), there is evidence that they were transformed into ionized COO⁻ groups, which were involved in ionic bonds with Ag⁺, as indicated by the carbonyl bands from 1638 cm⁻¹ to 1760 cm⁻¹ [28,29].

The bands from about 3500 cm⁻¹ were the alcoholic groups [28,29] of the citric acid spectrum (CA), but they were also found in the sample of PLD film spectra for citrate when the peak became broader due to hydrogen bonding. This caused the silver ions from the plasma that was produced via ablation, that still existed when arriving on the substrate surface, to enter into a reaction with the citric acid, and thus, may have formed trisilver citrate as well as also mono- and disilver citrate (Figure 8). However, the physico-chemical process was more complex than only a citric reaction with silver plasma ions. The presence of intermolecular H-bonds between the hydroxyl groups of the citrate is indicated in the broad band at 3500 cm⁻¹ in the sample E spectrum. Furthermore, it has been established that the partially ionized oxygen from the carbonyl groups will enter into hydrogen bonds with hydroxyl groups from other molecules, but also intramolecular bonds, as the peak from 1638 cm⁻¹ could indicate [28,29]. H-bonding could also have occurred between the citrate and cellulose (Figure 9b). The spectrum of sample E (hemp impregnated with citric acid before deposition) evidences the formation of citrate. As for the silver that was deposited directly on the citric acid (sample F), the spectrum indicates that some of the citric acid had reacted with silver plasma ions, while some still existed as citric acid, or alternatively, only part of the carboxylic groups had been transformed into carboxylates (Figure 9c).

Due to the environmental conditions in the vacuum chamber, it was possible for a further ionized structure of the silver citrate to form, which may have interacted through ionic and H-bonding and formed aggregates and complex structures, the most evident being for the citrate formed on the glass slab. The very broad band of the F (Ag Film/CA/Glass) spectra between 3560 and 2598 cm⁻¹ indicates that both the carboxyl and carboxylate groups coexisted [28,29], meaning that part of the citric acid was transformed into citrate and part remained as citric acid. The broad band also indicates H-bonds, intermolecular ionic bonds and other intermolecular interactions, such as Van Der Waals interactions. Such a model may be depicted as in Figure 9a–c. Van Der Waals interactions between ionized and/or partially ionized atoms would have taken place in both films (the E-film on the hemp treated with citric acid and the F-film that was deposited on the citric acid). The adsorption of silver atoms on the carbonyl groups was also part of the complex interactions in the thin film system (Figure 9b,c).

The reactions, namely the intermolecular and intramolecular interactions, that may occur upon the impact of silver plasma with the substrate surface containing citric acid are schematically presented in Figure 8 and Figure 9a–c. Further studies of this process of interaction between the silver plasma ions and the citric acid molecules during pulsed laser deposition, and its mechanism, may lead to a technique that can produce silver layers that are active against bacteria as well as being useful for other applications where an ionic state of silver and/or other metals is required. As noticed when analyzing the FTIR spectrum for silver deposited, by means of the PLD technique, on the citric acid, not all of the citric acid was transformed into citrate; this was because the quantity of the citric acid was excessive compared to the quantity of silver ions formed in the plasma. Based on this observation, a method to measure the quantity of the ions of metals that arrive on the substrate, and further, a PLD method of “titration”, could be developed to analyze different intermediary compounds formed during the travelling of plasma on the path from the target to the substrate.

Regarding the sizes and shapes of the nanoparticles, UV-Vis spectral analysis was performed on sample F (Ag Film/CA/Glass). For the analysis, material was scraped from the upper part of the layer and was deposited on a glass slide (Ag Film/CA/Glass), with care taken not to scrape the glass directly. The material was dissolved in 2 mL of distilled and deionized water. The distilled and deionized water was used for comparison. Because the amount of material taken from the thin layer for UV-Vis analysis was very small and the dilution was high (imposed by the 2 mL cuvette volume), the signal obtained for absorption was of low intensity. However, peaks that, in the literature, were assigned to various sizes of silver nanoparticles, as well as silver citrate, can be observed. Thus, the UV-Vis spectrum (Figure 10) shows a succession of 284 nm, 353 nm, 412 nm, 443 nm peaks and a wide band between 487 nm and 543 nm. The peak at 284 nm could be related to the formation of (Ag⁺)₃/citrate complexes [30], probably in the form of silver clusters and/or ultra-small silver nanoparticles [31]. Furthermore, in the same spectrum, a narrow band at 353 nm and a wide band between 487 and 543 nm appeared, possibly caused by the fusion of heated nanoparticles as a result of laser exposure. The spectral footprint of the fused nanoparticles is suggested by the wide band (between 487 and 543 nm) of the longitudinal oscillation mode of the surface plasmon and the narrow band at 353 nm of the transverse oscillation mode [32,33]. The fused nanoparticle formation could also be assigned to the broader bands, at 3500 cm⁻¹, of the FTIR spectra of samples E (AgFilm/CA/Hemp) and F(AgFilm/CA/Glass), but without neglecting other forms of aggregation, such as those described and presented in Figure 9.

Two other peaks at 412 nm and 443 nm can be attributed to the formation of triangular nanoparticles, with dimensions of around 40 nm [34], and nanoparticles with uneven shapes between 60 and 70 nm [35].

Citrate molecules act as both a capture ligand for silver particles and as a photoreducing agent for silver ions [36,37]. It is known that under the action of light radiation, citrate photoresists silver ions at the surface of previously formed silver seeds. The growth of nanoparticles with different morphologies is due to the rates of citrate reduction on certain faces of silver crystallites [34]. Silver–citrate complexes formed as a result of pulsed laser deposition indicate a possible application of the developed material as an antimicrobial layer [38].