"Soft and rigid" dithiols and Au nanoparticles grafting on plasma-treated polyethyleneterephthalate
© Švorčík et al; licensee Springer. 2011
Received: 4 August 2011
Accepted: 25 November 2011
Published: 25 November 2011
Surface of polyethyleneterephthalate (PET) was modified by plasma discharge and subsequently grafted with dithiols (1, 2-ethanedithiol (ED) or 4, 4'-biphenyldithiol) to create the thiol (-SH) groups on polymer surface. This "short" dithiols are expected to be fixed via one of -SH groups to radicals created by the plasma treatment on the PET surface. "Free" -SH groups are allowed to interact with Au nanoparticles. X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR) and electrokinetic analysis (EA, zeta potential) were used for the characterization of surface chemistry of the modified PET. Surface morphology and roughness of the modified PET were studied by atomic force microscopy (AFM). The results from XPS, FTIR, EA and AFM show that the Au nanoparticles are grafted on the modified surface only in the case of biphenyldithiol pretreatment. The possible explanation is that the "flexible" molecule of ethanedithiol is bounded to the activated PET surface with both -SH groups. On the contrary, the "rigid" molecule of biphenyldithiol is bounded via only one -SH group to the modified PET surface and the second one remains "free" for the consecutive chemical reaction with Au nanoparticle. The gold nanoparticles are distributed relatively homogenously over the polymer surface.
KeywordsPET plasma treatment dithiols and gold nanoparticles grafting XPS FTIR zeta potential AFM
The long-term research field of our scientific group is the modification of polymer surfaces, i.e. preparation of chemically active groups or species (e.g. radicals, conjugated double bonds, oxygen containing and other functional groups) on the polymer surface with the aim to increase the polymer surface "attractivity" for applications in tissue engineering and electronics [1–5].
There are several techniques, such as plasma discharge or irradiation with UV-light or ions, for modification of polymer surface [6, 7]. A common feature of all these approaches is a degradation of the polymer macromolecule chains and often an increase in the nanoscale surface roughness. In our preliminary experiment, the polyethylene surface morfology was modified by Ar plasma discharge and subsequent etching of short molecular polymer fragments in water . Another important phenomenon is a formation of free radicals and their subsequent reaction with oxygen from the ambient atmosphere. The newly formed oxygen-containing chemical functional groups render the material surface more wettable and increased wettability may facilitate the adsorption, e.g. cell adhesion receptors [7, 8]. Another interesting property of radiation-modified polymers is the formation of conjugated double bonds between carbon atoms and increased electrical conductivity of the material which may support their colonization with living cells higher or adhesion of subsequently deposited metals [9, 10].
The non-toxicity of gold is related to its well-known stability, non-reactivity and bioinertness. In addition, the gold can easily react with thiol (-SH) derivates giving Au-S bond formation. So that gold nanoparticles can be attached to the radicals, created on the polymer surface by plasma discharge or irradiation with UV-light or ions, by chemical reactions via -SH group [9–12].
In this work, the surface of the polyethyleneterephthalate (PET) was modified by plasma discharge and subsequently grafted with dithiol to introduce -SH groups. Dithiol is expected to be fixed via one of -SH groups to radicals created by the preceding plasma treatment on the polymer surface. The other "free" -SH group is alloved to interact with gold nanoparticle. The main goal of this study is to examine the effect of the plasma treatment and dithiol grafting on the binding of the gold nanoparticles to the polymer surface. Surface properties of the plasma-modified PET are studied by different experimental techniques: X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), electrokinetic analysis were used for the characterization of surface chemistry of the modified polymer and atomic force microscopy (AFM) for the study of surface morphology and roughness of treated polymers and "vizualization" of Au nanoparticles.
Materials and polymer modification
The present experiments were performed on biaxially oriented PET (density 1.3 g cm-3, 50-μm foil, Goodfellow Ltd., Huntingdon, UK). PET was modified by Ar plasma in Balzers SCD 050 (Balzers Union AG, Darmstadt, Germany) at room temperature and under the following conditions: gas purity was 99.997%, flow rate 0.3 l s-1, pressure 10 Pa, electrode distance 50 mm, its area 48 cm2, chamber volume approximately 1, 000 cm3, plasma volume 240 cm3, discharge power 8.3 W, treatment time 180 s.
Properties of the PET samples-pristine or modified by the plasma treatment, by the etching and grafting with dithiol and Au nanoparticles were studied using various methods.
The changes of chemical structure were examined by FTIR on Bruker ISF 66/V spectrometer equipped with an Hyperion microscope with ATR (Ge) objective. The difference FTIR spectra, which are presented, were calculated as a difference of FTIR spectra measured on sample of PET plasma treated + etched in methanol and (1) plasma treated and grafted in solution of bihenyldithiol or (2) plasma treated + grafted in solution of biphenyldithiol + Au nanoparticles.
Electrokinetic analysis (zeta potential) of pristine and modified polymer samples was determined by SurPASS Instrument (Anton Paar, Austria). Samples were placed inside a cell with adjustable gap in contact with the electrolyte (0.001 mol dm-3 KCl). For each measurement, a pair of samples with the same top layer was fixed on two sample holders (with a cross-section of 20 × 10 mm2 and gap in between 100 μm) [15, 16]. All samples were measured four times at a constant pH value with the relative error of 10%. For the determination of the zeta potential the streaming current and streaming potential methods were used and the Helmholtz-Smoluchowski and Fairbrother-Mastins equations were applied to calculate zeta potential [11, 15, 16].
Atomic contents of oxygen (1 s), carbon (1 s), sulphur (2 s) and gold (4f) in the surface layer of the modified polymer was determined from XPS spectra  recorded using an Omicron Nanotechnology ESCAProbeP spectrometer . The results were evaluated using CasaXPS programme. Before the measurement, the samples were stored 2 weeks under standard laboratory conditions.
Surface morphology and roughness of pristine and modified PET were examined by AFM using VEECO CP II setup (both of tapping and phase modes). Si probe RTESPA-CP with the spring constant 0.9 N m-1. By repeated measurements of the same region (1 × 1 μm2 in area), we certified that the surface morphology did not change after five consecutive scans. The mean roughness value (R a) represents the arithmetic average of the deviations from the centre plane of the sample.
Results and discussion
Chemical structure of plasma-modified and -grafted surface
Plasma treatment leads to cleavage of chemical bonds (C-H, C-C and C-O) . The bond breaking leads to fragmentation of the polymer chain, to ablation of polymer surface layer and to creation of free radicals, conjugated double bonds and excessive oxygen containing groups . Activated polymer surface can be grafted with thiol groups. The binding of the molecules is mediated by free radicals, present on the surface of the plasma-treated PET. The binding on new double bonds has not been proved . Cleavage of the molecular chains facilitates solubility of the initially insoluble polymer in common solvents, e.g. water .
Atomic concentrations of C (1s), O (1s), S (2s) and Au (4f)
Atomic concentrations of elements in at. %
Pristine PET 
From the results presented in Table 1 and Figure 2, it is apparent that Au nanoparticles are grafted only on the PET surface previously activated by biphenyldithiol. This can be explained by the concept that "flexible" molecule of ethanedithiol is bonded to activated polymer surface by both of -SH groups, while the more "rigid" molecule of biphenyldithiol is grafted only via one of -SH groups and the second one is "free" for chemical reaction with Au nanoparticle.
Surface morphology and homogeneity of Au nanoparticles on the modified PET
Surface morphology of pristine and modified PET was studied by AFM method. AFM images of pristine PET, PET-treated by plasma, plasma treated + etched in (1) methanol, (2) solution of ED and (3) BFD, plasma treated and grafted with BFD + Au nanoparticles are shown in Figure 4. The different scales of individual images were chosen to emphasize the changes in the surface morphology. From Figure 4, it is evident that the modification of PET by above-mentioned procedures has no significant effect on its surface roughness R a. The R a value "slightly" increases after the plasma treatment, surface etching and grafting with ED, BFD and gold nanoparticles. However the changes in the PET surface morphology are clearly visible. The change in surface morphology after the plasma treatment can be explained by preferential ablation of PET amorphous part of polymer. . It can be asssumed, that the low-mass oxidized structures are preferentially dissolved in methanol and in ED and BFD solutions . More significant change in the surface morphology after gold nanoparticles grafting is apparent. The "pyramidal" structures, relatively "homogeneously" spread on the polymer surface, can be due to the presence of the gold nanoparticles. Their "non-globular" shape in probably caused with the convolution of the tip with the sample's surface.
The gold nanoparticles homogenously distributed over the polymer surface could have a positive effect on the interaction with living cells, the effect which could be interesting for tissue engineering  The presence of gold nanoparticles may also facilitate adhesion of other gold structures to polymeric substrates, which can be useful for electronics .
The presence of the -SH groups, as same as the gold nanoparticles on the grafted polymers was proved by XPS, FTIR, electrokinetic analysis and AFM methods. The gold nanoparticles are distributed relatively homogenously over the PET surface; this finding may be of importance for the future application of gold-polymer structures in tissue engineering and electronics.
This work was supported by the GA CR under the projects 106/09/0125 and 108/10/1106, Ministry of Education of the CR under program LC 06041, and AS CR under the projects KAN200100801 and KAN400480701. The authors thank to Mr. P. Simek from ICT for a part of experimental work.
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