Immobilization of silver nanoparticles on polyethylene terephthalate
© Reznickova et al.; licensee Springer. 2014
Received: 3 April 2014
Accepted: 6 June 2014
Published: 16 June 2014
Two different procedures of grafting with silver nanoparticles (AgNP) of polyethylene terephthalate (PET), activated by plasma treatment, are studied. In the first procedure, the PET foil was grafted with biphenyl-4,4′-dithiol and subsequently with silver nanoparticles. In the second one, the PET foil was grafted with silver nanoparticles previously coated with the same dithiol. X-ray photoelectron spectroscopy and electrokinetic analysis were used for characterization of the polymer surface at different modification steps. Silver nanoparticles were characterized by ultraviolet-visible spectroscopy and by transmission electron microscopy (TEM). The first procedure was found to be more effective. It was proved that the dithiol was chemically bonded to the surface of the plasma-activated PET and that it mediates subsequent grafting of the silver nanoparticles. AgNP previously coated by dithiol bonded to the PET surface much less.
Immobilization of microspheres and nanoparticles (NPs) onto the surface of organic polymers provides fascinating opportunities for the design of smart heterostructures . In addition to size, shape, and size uniformity, control of dispersion of NPs is a key parameter to minimize the loss of properties related to the nanosize regime .
Silver nanoparticles (AgNPs or nanosilver) have attracted increasing interest due to their unique physical, chemical, and biological properties compared to their macroscaled counterparts . AgNPs have distinctive physicochemical properties, including a high electrical and thermal conductivity, surface-enhanced Raman scattering, chemical stability, catalytic activity, and nonlinear optical behavior . These properties make them of potential value in inks, microelectronics, and medical imaging . Besides, AgNPs exhibit broad-spectrum bactericidal and fungicidal activity  that has made them extremely popular in a diverse range of consumer products, including plastics, soaps, pastes, food, and textiles, increasing their market value . To date, nanosilver technologies have appeared in a variety of manufacturing processes and end products. Nanosilver can be used in a liquid form, such as a colloid (coating and spray) or contained within a shampoo (liquid), and can also appear embedded in a solid such as a polymer master batch or be suspended in a bar of soap (solid). Nanosilver can also be either utilized in the textile industry by incorporating it into the fiber (spun) or employed in filtration membranes of water purification systems. In many of these applications, the technological idea is to store silver ions and incorporate a time-release mechanism. This usually involves some form of moisture layer that the silver ions are transported through to create a long-term protective barrier against bacterial/fungal pathogens [7, 8].
Materials and modification
Biaxially oriented polyethylene terephthalate (PET, density 1.3 g cm-3, 23-μm foil, supplied by Goodfellow Ltd., Huntingdon, UK) was used in this study. The samples were treated in Ar+ plasma on a Balzers SCD 050 device: the exposure time was 120 s, and the discharge power was 8.3 W. The plasma treatment was accomplished at room temperature. More detailed description of the plasma modification can be found in .
Immediately after the plasma treatment, the samples were inserted into a methanol solution of biphenyl-4,4′-dithiol (BPD, 4.10-3 mol l-1). Silver nanoparticles (AgNPs) were obtained using a similar process of AgNO3 reduction to that reported by Smith et al. . Thiols are expected to be fixed via one of their functional -SH group to reactive sites created by the plasma-activated polymer surface . The remaining ‘free’ -SH group is then allowed to interact with AgNPs . Coating of polymers with AgNP*s was accomplished by two procedures (illustrated in Figure 1): (A) the plasma-treated polymer, grafted with BPD, was immersed into freshly prepared solution of silver nanoparticles (in what follows denoted as AgNP); and (B) the plasma-treated polymer was exposed to a solution of silver nanoparticles, previously coated with BPD (AgNP*) for 24 h. Finally, the samples were immersed into distilled water and then dried under N2 flow.
For characterization of silver nanoparticles, transmission electron microscopy (TEM) images of silver nanoparticles (AgNP and AgNP*) were obtained on a JEOL JEM-1010 (JEOL Ltd., Tokyo, Japan) instrument operated at 80 kV. UV-vis absorption spectra of nanoparticles were recorded using a Varian Cary 400 SCAN UV-vis spectrophotometer (PerkinElmer Inc., Waltham, MA, USA). The solutions were kept in 1-cm quartz cell. Reference spectrum of the solvent (water) was subtracted from all spectra. Data were collected in the wave region from 350 to 800 nm with 1-nm data step at the scan rate of 240 nm min-1.
Different techniques were used for characterization of the modified polymer surface. Concentrations of C(1s), O(1s), S(2p), and Ag(3d) atoms in the modified surface layer were measured by X-ray photoelectron spectroscopy (XPS). An Omicron Nanotechnology ESCAProbe P spectrometer (Omicron Nanotechnology GmbH, Taunusstein, Germany) was used to measure photoelectron spectra (typical error of 10%).
Electrokinetic analysis (zeta potential) of all samples was accomplished on SurPASS Instrument (Anton Paar GmbH, Graz, Austria) to identify changes in surface chemistry and polarity before and after individual modification steps. Samples were studied inside the adjustable gap cell with an electrolyte of 0.001 mol l-1 KCl, and all samples were measured eight times at constant pH = 6.0 and room temperature (error of 5%). Two methods, streaming current and streaming potential, were used to evaluate measured data, and two equations, Helmholtz-Smoluchowski (HS) and Fairbrother-Mastins (FM), were used to calculate zeta potential .
Surface morphology was examined by atomic force microscopy (AFM) using a Veeco CP II setup (tapping mode) (Bruker Corporation, Billerica, MA, USA). Si probe RTESPA-CP with a spring constant of 0.9 N m-1 was used. By repeated measurements of the same region (2 × 2 μm2 in area), we proved that the surface morphology did not change after five consecutive scans.
Results and discussion
Element concentrations of C, O, S, and Ag determined by XPS in surface polymer layer
Element concentration (at.%)
The systems studied may have potential application, e.g., in medicine as prevention of creation of bacterial biofilm .
Two different procedures were used for coating of PET surface with silver nanoparticles. Both procedures are based on the surface activation of PET by Ar plasma discharge and use of dithiol as binding reagent between silver nanoparticles and plasma-modified PET surface. XPS results confirmed creation of a silver nanoparticle-thiol layer (in the case of AgNP) on the PET surface. Rather large objects observed on AFM images show that a significant aggregation of deposited AgNPs takes place during the grafting procedure. Grafting with thiols and gold nano-objects generally leads to a decrease of the zeta potential. We achieved higher concentration of silver nanoparticles by deposition on PET grafted beforehand with dithiol.
This work was supported by GACR under projects 14-18149P (A.R.) and P108/12/G108.
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