Analytical characterization of bioactive fluoropolymer ultra-thin coatings modified by copper nanoparticles

Abstract

Copper–fluoropolymer (Cu-CFx) nano-composite films are deposited by dual ion-beam sputtering. The extensive analytical characterization of these layers reveals that inorganic nanoparticles composed of Cu(II) species are evenly dispersed in a branched fluoropolymer matrix. In particular, X-ray photoelectron spectroscopy has been employed to study the surface chemical composition of the material and to assess how it changes on increasing the copper loading in the composite. Transmission electron microscopy reveals that the copper nanoclusters have a mean diameter of 2–3 nm and are homogeneously in-plane distributed in the composite films. Electrothermal atomic absorption spectroscopy has been used to study the kinetics of copper release in the solutions employed for the biological tests. The Cu-CFx layers are employed as bioactive coatings capable of inhibiting the growth of target microorganisms such as Saccharomyces cerevisiae, Escherichia coli, Staphylococcus aureus, and Lysteria. The results of the analytical characterization enable a strict correlation to be established among the chemical composition of the material surface, the concentration of copper dissolved in the microorganisms broths, and the bioactivity of the nano-structured layer.

Appendix

Curve-fit parameters employed for XPS data treatment

Full width at half maximum values (eV) as a function of spectral region and metal loading

φ	C1s	F1s	O1s	Cu2p3/2a
				Photoelectron peaks (1 and 2)	Shake-up (1)	Shake-up (2 and 3)
0.05	2.29	2.66	–	–	–	–
0.15	2.34	2.82	2.40	3.64	3.18	3.12
0.25	2.13	2.71	2.40	3.82	3.82	2.90

1. ^aPhotoelectron peaks 1 and 2 are relevant to signals falling at BE=934.0 eV and BE=935.8 eV. Shake-up signals 1, 2, and 3 are relevant to peaks falling at BE=939.2 eV, BE=941.0 eV and BE=943.4 eV, respectively

The curve-fit function (gl) takes into account the mixed Gaussian–Lorentzian character of the peak shape as reported in the following:

$$ {\text{gl}} = \frac{1} {{\left( {1 + \frac{{m\left( {x - {\text{center}}} \right)^2 }} {{b^2 }}} \right){\text{e}}^{\frac{{\left( {1 - m} \right)\ln \left( 2 \right)\left( {x - {\text{center}}} \right)^2 }} {{b^2 }}} }} $$

The tail function is:

$$ {\text{tail}} = \left( {{\text{CT}} + {\text{ETH}} \times {\text{e}}^{\left( {x - {\text{center}}} \right){\text{ET}}} } \right) $$

and the total function is:

$$ {\text{FitFunc}} = h \times \left[ {{\text{gl}} + \left( {1 - {\text{gl}}} \right) \times {\text{tail}}} \right] $$

where x is an energy variable, h the peak height, m the Lorentzian/Gaussian merge factor as Lorentzian percentage center (peak center), b the peak width, CT the tail constant, ETH the exponential tail height, and ET the exponential tail.

Generally speaking, curve-fitting was carried out without the use of tails (CT=0, EHT=0, ET=0), except for the cupric oxide component of the Cu2p_3/2 and O1s spectral regions. For these signals, after analysis of a CuO standard, the parameters reported in the table were used.

Tail parameters used for curve-fitting in the O1s and Cu2p_3/2 regions

	CT	EHT	ET
CuO	0.01	0.0384	0.2
CuO	0.005	0.05	0.05