The surface temperature on the paper immediately after the flame deposition was approximately 40–60 °C, although the maximum temperature in the LFS flame is above 2000 °C. The surface temperature depends on, and can be controlled by the speed at which the sample passes through the flame, the distance between flame nozzle and the substrate, the number of passes through the sweeps, and the heat capacity of the surface being coated. After the silver nanoparticle LFS deposition, a greyish color change was induced onto the surface of the samples depending on the number of flame passes. However, a reference sample that passes 30X through a flame without silver precursor did not have any observable color change in the sample surface indicating that the color change is due to the deposition of silver nanoparticles. Since LFS produces nanoparticles by pyrolysis, it is expected that the heat from the flame causes a (partial) melting of the polymer surface, especially if the glass transition temperature is lower than the temperature of the flame at the sample surface. This potentially enhances the adherence and embedding of nanoparticles into the polymer coating layer. Ideally, a partial embedding into the coating layer is preferred since this will improve adhesion and make nanoparticles readily available on the surface for antibacterial action. Embedding nanoparticles a few nanometers below the surface may also have the same effect. The results presented subsequently describe the embedding and its effect on antibacterial properties.
SEM image analysis
SEM imaging showed that nanoparticles are homogeneously distributed on the substrates as seen in Fig. 2. It is also clear that the amount of nanoparticles on a surface depended on the number of passes through the flame. Larger number of flame passes (30X) produced more nanoparticles on the surface compared to less flame passes (1X). The images show that this is true for both paper samples (PE & PET). Particle morphology appears spherical . The average size increased slightly with the number of flame passes, which can be a result of particle–particle sintering due to the induced heat from the flame. Results for the average particle sizes were about 22 nm, 27 nm, and 32 nm for 1X, 10X, and 20X, respectively. About 70–80% of particles on the surface had size between 10 and 35 nm and the maximum particle size was around 100 nm. PE is a softer polymer, whereas PET provides a comparatively harder surface for nanoparticle deposition with glass transition temperatures of − 110 °C and 78 °C, respectively.
For the same number of passes through the flame (e.g. 5X), the SEM images show similar distribution and surface characteristics for both samples. At the same time, smaller nanoparticles are observed on the surface and these appear to have a lower contrast. This may suggest that particles are slightly further away from the surface. Considering that the particles are estimated to be spherical with diameter of about 30 nm on the surface, low contrast particles could be embedded below the coating as a result of heating that resulted from increasing the number of flame passes.
AFM surface analysis
Surface analysis by AFM was a comparison of height, phase and stiffness measurements (Figs. 3, 4). A comparison of the reference PE and PE_10X samples show significant differences in the surface topography. Waviness and the absence of nanoparticles was clearly observed in reference sample. AFM images of PE_10X samples showed nanoparticles on the surface that appeared spherical, similar to observations in SEM images. The maximum height after processing varied for all samples depending on the residual waviness in the sample surface. Figure 3 shows clearly the stiffness contrast between the soft polymer surface and the relatively harder silver nanoparticles. A maximum stiffness of 2.5 GPa, was recorded for the nanoparticles whereas the reference PE without nanoparticles yielded a stiffness of about 1 GPa. Figure 3 compares the height and stiffness images from the same surface location. Some particles are only visible in the stiffness image, i.e. these particles give rise to only a very weak if any height contrast in the topograph. This observation suggests that such silver nanoparticles were embedded below the polymer surface. These embedded nanoparticles (partially or fully) still contribute to the mechanical contrast even if being coated by a polymer layer, and appear only as a gentle sloping in the height profile.
A similar kind of effect was seen in Fig. 4, which shows a phase image corresponding to the height image at a particular location. Certain nanoparticles appear distinctly in the phase image, but are not observed in the height image. This indicates that such particles were fully covered by the polymer coating and therefore contribute to the height profile by a lesser extent. However, the change in mechanical properties due to embedding is clearly observed in the phase image. A line profile at the marked locations in Fig. 4a, b show two clear peaks representing particles in the phase image that are weakly observed in the height image. This comparison of line profiles at the same locations between 0.04 and 0.08 µm confirms the presence of an embedded nanoparticle. The “Appendix” shows full size AFM images from which the line profiles are obtained. Different scenarios that describe nanoparticle embedding and the observations made in line profile of the phase and height images are given in Table 1.
Focused ion beam (FIB) cross-sectional imaging
Results from FIB-SEM imaging, which enables observation of nanoparticles from sample cross-sections, are shown in Fig. 5. The incision on the sample surface showing the cross-section is shown in (a), and a magnification of the interface between the platinum coating and the polymer coating layer in (b). A few embedded nanoparticles are visible in the polymer layer, right below the platinum coating. The estimated maximum depth of embedding is about 30 nm. After cross-sectional imaging, surface characterization was complemented using SEM integrated within the FIB apparatus. Imaging the sample from an angle perpendicular to the surface showed similar nanoparticle-decorated surface as was seen in Fig. 2. However, a 15° tilt in the imaging angle of the surface gave a different perspective of the particle distribution as shown in Fig. 6. The tilted image confirms that some nanoparticles were partially embedded within the coating layer. Here, the embedding and partial embedding is attributed to the thermo-mechanical deformation of the coating layer resulting from the heat provided by the LFS process. A lower glass transition temperature for PE in comparison to PET also facilitated the integration of the nanoparticles into the softened polymer surface. The softened polymer surface acted as a molten plastic that captured the nanoparticles expelled from the nozzle in the LFS coating process.
One can observe from Fig. 6 that some nanoparticles are vertically oriented on the surface forming assemblies similar to nano-pillars. The vertical orientation may have resulted from sintering of particles that settled on top of each other. This effect was more dominant in samples that had a higher number of flame passes. Further analysis of FIB-SEM images revealed that the more nano-pillar structures were found on PE_30X in comparison to PET_30X as shown in Fig. 6. For PE_30X, particles that were embedded within the coating layer appeared to be mixed with the surface of coatings. On the other hand, particles appeared to be mostly partially embedded in PET_30X. There were isolated cases where large particles appeared to be fixed to the surface similar to a meteorite attached within a crater. These could have resulted from excessively large precursor droplets within the LFS flame.
Silver nanoparticles on the surface was quantified in atomic percentage using XPS (Fig. 7). The amount of measured silver on sample surfaces increased in proportion to the number of flame passes, reaching ca. 15–20 at.% for the 30X samples. Both PE and PET samples showed a similar amounts of silver on the surface, with the exception of 30X sample, where the PE sample showed a lower silver amount. Repeated passes through the flame raised the sample surface temperature, thereby (partially) melting the polymer allowing the nanoparticles to penetrate into the surface. This suggests that for the PE_30X sample, a significant amount of nanoparticles were partially or fully embedded in the polymer surface. Since the maximum electron escape depth in XPS is ca. 10 nm, nanoparticles beyond this depth are not detected, which could have resulted in the lower silver amount observed for PE samples. It appears that in the current experiments, the surface temperature did not rise above the glass transition temperature of PET, which is considerably higher than that of PE. It is important to note that the glass transition temperature of the polymer coating is an important parameter to be considered in the embedding process.
Our previous work confirmed that silver nanoparticle coated paper fabricated using the LFS process is antibacterial . In the current study, the focus was to demonstrate nanoparticle embedding while maintaining the antibacterial effect. Nanoparticles that are embedded too deep within the coating layer may hinder the release of silver to the surface, resulting in reduced or no antibacterial effect. Bacterial growth during the testing was ranked as 0, 1, 2, 3, corresponding to no growth, weak growth (0–50 CFU), moderate growth (50–100 CFU), and strong growth (> 100 CFU), respectively . The results in Fig. 8 show that the antibacterial properties of the paper samples were maintained on the polymer coated surfaces. The mechanism of antibacterial action was not studied, but this could be a result of rapturing the cell wall or modification of internal cell processes resulting in bacteria death, as suggested in literature . From the results, it is not clear whether the silver nanoparticles have a bactericidal or bacteriostatic effect. Consequently, further studies are needed to clarify this.
Reference samples had no silver coating on the surface. Therefore, they sustained the growth of both tested bacteria, E. coli and S. aureus. Silver is clearly a more effective antibacterial agent against the gram-negative E. coli as compared to the gram-positive S. aureus. Only some colonies of E. coli were observed on PET-coated paper with the lowest amount of deposited nanoparticles (PET_1X). For the PE-coated paper, and for the PET-coated paper with higher silver nanoparticle deposition amounts, no bacterial growth was observed for E. coli. On the contrary, response of the gram-positive S. aureus to the silver nanoparticles was less pronounced. Only 30X silver deposition amounts showed considerable antibacterial effect against S. aureus for both PE and PET samples. This suggests that silver coatings may be more effective against gram-negative bacteria as compared to gram-positive bacteria. Gram-positive bacteria have a thicker outer covering mainly constituted of peptidoglycan, which makes it difficult for the nanoparticles or silver ions to penetrate the cell easily. Thus, this may be the reason why silver nanoparticles deposited onto the samples at low nanoparticle amounts appeared to be ineffective against S. aureus. Alternatively, the silver amount may have been below the limit required to inhibit S. aureus, especially for coatings less than 30X.