J Mater Sci (2012) 47:6297–6304 DOI 10.1007/s10853-012-6550-8
Grafting of gold nanoparticles and nanorods on plasma-treated polymers by thiols Alena Reznickova • Zdenka Kolska Jakub Siegel • Vaclav Svorcik
•
Received: 18 January 2012 / Accepted: 3 May 2012 / Published online: 26 May 2012 Ó Springer Science+Business Media, LLC 2012
Abstract Grafting of gold nanoparticles and nanorods on the surface of polymers, modified by plasma discharge, is studied with the aim to create structures with potential applications in electronics or tissue engineering. Surfaces of polyethyleneterephthalate and polytetrafluoroethylene were modified by plasma discharge and subsequently, grafted with 2-mercaptoethanol, 4,40 -biphenyldithiol, and cysteamine. The thiols are expected to be fixed via one of –OH, –SH or –NH2 groups to reactive places on the polymer surface created by the plasma treatment. ‘‘Free’’ –SH groups are allowed to interact (graft) with gold nanoparticles and nanorods. Gold nano-objects were characterized before grafting by transmission electron microscopy and UV–Vis spectroscopy. X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and electrokinetic analysis (zeta potential determination) were used for the characterization of polymer surface at different modification phases. It was proved by FTIR and XPS measurements that the thiols were chemically bonded on the surface of the plasma-treated polymers, and they mediate subsequent grafting of the gold nanoobjects. On the surfaces, modified polymers were indicated some objects by AFM, size of which was dramatically larger in comparison with that of original nanoparticles and nanorods. This result and the other results of UV–Vis spectroscopy indicate an aggregation of deposited gold nano-objects. A. Reznickova (&) J. Siegel V. Svorcik Department of Solid State Engineering, Institute of Chemical Technology in Prague, 16628 Prague 6, Czech Republic e-mail:
[email protected] Z. Kolska Faculty of Science, J.E. Purkyne University, Usti nad Labem, Czech Republic
Introduction Design of functional nanomaterials is of current interest because of a variety of potential applications ranging from chemistry to biological sciences. Metal and semiconductor nanoparticles exhibit interesting size- and shape-dependent properties [1]. The nanosized metal particles are emerging as important type of colorimetric reporters because of their extinction; coefficients are several orders of magnitude larger than those of organic dyes [2], and the transition of the nanoparticles from dispersion to aggregation leads to a distinct change in color [3–5]. The phenomenon is termed surface plasmon absorption, and the color change upon aggregation is due to the coupling of the plasmon absorbances as a result of their proximity to each other [6]. Significant progress has been made in recent years in the development of functional nanomaterials by designing monolayer-protected metal clusters [7–9]. Metal nanoparticles play an important role in different areas of science, such as nanoelectronics, nonlinear optics, biological labeling, oxidation catalysis [10], and tissue engineering [11]. Nanoparticles themselves also provide a pragmatic approach to multiscale engineering, functioning as ‘‘building blocks’’ of regular shape and size for the fabrication of larger structures [12]. Spherical gold nanoparticles (AuNP, size of 5–20 nm in diameter) exhibit an intense red color due to the surface plasmon (SP) absorption with the absorption coefficients in the range of 108–1010 dm3 mol-1 cm-1 [13]. These recognition properties rely on the functional groups immobilized on the AuNP surface [14]. It was proved earlier that chemically grafted AuNP increases adhesion of gold nanolayer [15] (applications in electronics) and adhesion and proliferation of living cells on polymer substrates [11] (tissue engineering).
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For most of the applications mentioned above, it is necessary that the metal nanoparticles are tightly adhered at the substrate surface. The suitable approach represents chemical binding of nano-objects on reactive sites on the polymer surface. Due to the polymer inertness and nonreactivity, its surface must be activated to create chemical reactive positions on substrate prior to nanoparticle grafting. Relatively, easy and straightforward method for polymer surface activation is plasma treatment. Plasma treatment of polymers leads to cleavage of chemical bonds (C–H, C–C, C–F, and C–O) [16, 17] to fragmentation of the polymer chains and creation of low mass fragments. Also free radicals, conjugated double bonds, and excessive oxygen containing groups (carbonyl, carboxyl, hydroxyl, and ester) are produced on the polymer surface. A few of the volatile degradation products are released, and polymer surface is ablated [16, 17]. Activated polymer surface (radicals, double bonds, and polar oxygen groups) can be grafted with various groups or molecules, e.g., with biomolecules (aminoacid—e.g., glycine, bovine serum albumin, or colloidal carbon particles [18]), polyethyleneglycol [19], or thiols [15, 20]. Cleavage of the molecular chains facilitates dramatically surface solubility of initially insoluble polymer in common solvents, e.g., water and methanol [21, 22]. In this study, the surfaces of polyethyleneterephthalate (PET) and polytetrafluoroethylene (PTFE) were modified by plasma discharge and subsequently grafted with different thiols (mercaptoethanol, biphenyldithiol, and cysteamine). Thiols are expected to be fixed via one of their functional groups –OH, –SH, or –NH2 to reactive sites created by the preceding plasma treatment. The remaining ‘‘free’’ –SH group is then allowed to interact with gold nanoparticles or gold nanorods. The main goal of this study is to examine the effect of the plasma treatment and the thiol grafting on the binding of gold nano-objects on the polymer surface. Gold nano-objects (nanoparticles and nanorods) were characterized by transmission electron microscopy (TEM) and ultraviolet–visible spectroscopy (UV–Vis). Surface chemistry and structure of polymers (pristine, plasma modified, and grafted) were studied by Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), electrokinetic analyses, and by UV–Vis spectroscopy.
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The samples were modified in direct (glow, diode) Ar? plasma on Balzers SCD 050 device under the following conditions: gas purity 99.997 %, flow rate 0.3 l s-1, pressure 10 Pa, electrode distance 50 mm and its area 48 cm2, chamber volume approx. 1000 cm3, and plasma volume 240 cm3. Exposure time was 120 s, discharge power 8.3 W, and the treatment was accomplished at room temperature [20]. Immediately after the plasma treatment, the samples were inserted into water solution (2 wt%) of 2-mercaptoethanol (Fig. 1a, ME), methanol solution (5 9 10-3 mol l-1) of biphenyl-4,40 -dithiol (Fig. 1b, BPD), and into water solution (2 wt%) of cysteamine (Fig. 1c, CYST) for 24 h. To coat the polymers with the gold nano-objects, the plasma-treated polymers with grafted thiols were immersed for 24 h into freshly prepared colloidal citrate stabilized solution of Au nanoparticles (AuNP) [14, 15], or Au nanorods (AuNR, 0.1 mol l-1 water solution of cetyltrimethylammonium bromide). Gold nano-objects before grafting were characterized by TEM (see Fig. 2a, b) and UV–Vis spectroscopy (Fig. 3). The average diameter of the spherically shaped AuNPs, electrostatically stabilized with citrate, was about 18 nm. The wavelength of the surface plasmon absorbance at 518 nm [23], corresponds well with the average diameter estimated by TEM. The visible extinction spectrum of the AuNRs solution shows two maxima, the first one around 520 nm corresponding to the transverse plasmon oscillation band and the second, more pronounced one, around 805 nm corresponding to the longitudinal plasmon oscillation [24]. Finally, the samples were immersed into distilled water and then dried under N2 flow. Whole procedure can be seen from diagram in Fig. 4. Diagnostic techniques The PET and PTFE samples (pristine, modified, or grafted) were analyzed using the following methods. UV–Vis absorption spectra were recorded using a Varian Cary 400 SCAN UV–Vis spectrophotometer (PerkinElmer Inc., USA). The solutions were kept in 1 cm quartz cell. Reference spectrum of solvent (water) was subtracted from all spectra. Data were collected from 200 to 900 nm with 1-nm data step at the scan rate of 600 nm min-1 [23]. The UV–Vis absorption spectra were measured on PET samples, pristine or modified by the plasma discharge and grafted with thiols and gold nanoparticles, too. Reference
Experimental Materials, plasma modification and grafting Biaxially oriented PET (density 1.3 g cm-3, 23 lm foil, Goodfellow Ltd., UK) and PTFE (density 2.2 g cm-3, 25 lm, Goodfellow Ltd., UK) were used in this study.
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Fig. 1 Molecular structure of a 2-mercaptoethanol, b biphenyl-4, 40 -dithiol, and c cysteamine
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Fig. 2 TEM images of the Au nanoparticles (a) and nanorods (b) used in this study (Color figure online)
Fig. 3 UV–Vis spectra of solutions of gold nanoparticles (AuNP, red) and gold nanorods (AuNR, black) (Color figure online)
Fig. 4 Scheme of polymer modification by plasma treatment (R-chemical reactive groups, e.g., free radicals, double-bonds, and oxygen groups), grafting with thiols (–SH mercapto group), and then with gold nano-objects (Color figure online)
spectrum of pristine PET was subtracted from the spectra of modified samples. No UV–Vis spectra were measured on PTFE because of its too low transparency. TEM images of gold nano-objects were obtained on JEOL JEM-1010 (Jeol Ltd., Japan) instrument operated at 80 kV.
The changes of chemical structure were examined by FTIR on Bruker ISF 66/V spectrometer (Bruker Corp., USA) equipped with a Hyperion microscope with ATR (Ge) objective. The different FTIR spectra, which are presented in this study, were determined from the FTIR spectra, measured either on (i) plasma-treated and biphenyldithiol-grafted samples or on (ii) plasma treated, biphenyldithiol grafted, and grafted with gold nanoparticles or nanorods samples [25]. Concentrations of O, C, F, Au, and S in the modified surface layer were measured by XPS 24 h after the grafting with Au nanoparticles and nanorods. Omicron Nanotechnology ESCAProbe P spectrometer (Omicron Nanotechnology GmbH, Germany) was used to measure photoelectron spectra (typical error of 10 %). Exposed and analyzed area had dimension 2 9 3 mm2. X-ray source was monochromated at 1486.7 eV with step size 0.05 eV. The O(1s), C(1s), F(1s), S(2p), and Au(4f) peaks were studied. The spectral evaluation was carried out by CasaXPS 2.2.99 program (supplied by the Omicron Nanotechnology GmbH, Germany) [15]. Electrokinetic analysis (determination of zeta potential) of all samples was accomplished on SurPASS Instrument (Anton Paar GmbH, Austria). Samples were studied inside the adjustable gap cell in contact with the electrolyte (0.001 mol dm-3 KCl). Our zeta potential determination of planar samples is based on the measurement of the streaming potential dU and the streaming current dI as a function of continuously increasing pressure dp of electrolyte circulated through the measuring cell containing the solid sample. The relationships between zeta potential and the streaming potential dU or the streaming current dI are linear with the slope dU/dp, resp. dl/dp [26, 27]. More detailed principles of this determination are presented in the study of Luxbacher et al. [26]. For each measurement, a pair of polymer foils with the same top layer was fixed on two sample holders (with a cross section of 20 9 10 mm2 and gap between 100 lm) [28, 29]. All samples were measured four times at constant pH (pH 6.0) value with the relative error of 10 %. Since the results obtained by both
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the methods within the experimental errors are identical, we present only the streaming current data. Surface morphology of modified samples was examined by AFM using VEECO CP II setup (phase mode) (Bruker Corp., USA). Si probe RTESPA-CP with the spring constant 0.9 N m-1 was used. By repeated measurements of the same region (2 9 2 lm2 in area), we certified that the surface morphology did not change after five consecutive scans [20].
Results and discussion PET and PTFE were modified in Ar plasma discharge, and the chemical reactive groups created on the polymer surface were used for (i) grafting of mercaptoethanol (ME), biphenyldithiol (BPD), or cysteamine (CYST). In the second reaction step, created interlayer (i) was used for (ii) grafting of gold nano-objects (nanoparticles (AuNP) and nanorods (AuNR)). FTIR spectroscopy was used for the characterization of chemical composition of plasma-modified PET and PTFE samples. Relevant parts of difference FTIR spectra are shown in Fig. 5. In Fig. 5a, the differential FTIR spectra of the PET, plasma treated and grafted -S–Au d with biphenyldithiol and gold nanoparticles or nanorods are shown. The bands at 749, 791, and 815 cm-1 correspond to group on the samples grafted with gold nanoparticles, and the bands at 750, 780, and 815 cm-1 correspond to the same group on the samples grafted with gold nanorods. Similar spectra from PTFE are shown in Fig. 5b. In this case, the bands at 790 and 850 cm-1 correspond to absorption maximum of the –S–Au group in the sample grafted with Fig. 5 Differential FTIR spectra of: a plasma-treated PET, grafted with biphenyldithiol (BPD) and then with gold nano-objects (nanoparticles (AuNP, red) and gold nanorods (AuNR, black)) and b plasma-treated PTFE; grafted with biphenyldithiol (BPD) and then with gold nanoobjects (nanoparticles (AuNP, red) and gold nanorods (AuNR, black)) (Color figure online)
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gold nanoparticles. The band at 806 cm-1 is assigned to the same –S–Au group but on the sample grafted with nanorods. The FTIR results prove that the dithiol was chemically bonded to the plasma-modified polymer surface, and it can mediate subsequent binding of gold nanoobjects (see also [16]). The compositions of PET and PTFE surface layers (8–10 monolayers thick) of pristine, plasma treated, thiols grafted, and coated with Au nano-objects were investigated using XPS method. Typical XPS spectrum, e.g., for PET treated in plasma, grafted with cysteamine and then with gold nanoparticles is shown in Fig. 6. From Fig. 6, the observed shift of Au(4f) peak of about 1.5 eV relative to that from the bulk Au at 83.9 eV supports the idea that the Au nanoparticles are bonded via cysteamine to the PET surface. Atomic concentrations of C(1s), O(1s), F(1s), S(2p), and Au(4f) in pristine and modified PET and PTFE are summarized in Tables 1 and 2 (analytic XPS lines are shown in brackets). It is well known that the plasma treatment results in oxidation of polymer surface layers and creation of oxygen-containing groups (carbonyl, carboxyl, hydroxyl, and ester) [16, 17]. The plasma treatment of PET leads to only minor increase of the oxygen concentration, which is reduced by subsequent grafting with thiols and gold nano-objects. The reduction is the most pronounced on the PET samples grafted with biphenyldithiol (see Table 1). The presence of sulfur is proved after grafting with all thiols used. The highest sulfur concentration on the samples grafted with biphenyldithiol is due to the presence of two – SH groups in its molecule. Final grafting with gold nanoobjects reduces the observed sulfur concentration little bit. The concentration of bonded gold objects varies in broad range from 0.1 to 3.3 at.%, but no systematic trend can be
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to the increase of amount of polar oxygen groups on polymer surface [16]. This conclusion is obvious from the XPS results presented in Tables 1 and 2. This increase is being less pronounced on PTFE. Observed discrepancy may be due to the well-known higher ablation rate of PTFE [16, 21]. After thiols grafting, the zeta potential changes due to the presence of new functional groups (–SH, –OH or –NH2) incorporated on surface. These groups are able to dissociate in liquid surrounding because of negatively charged surface [25, 30]. Subsequent grafting of gold nanoobjects leads to further decrease of zeta potential owing to presence of metal nanostructures on surface [25, 31]. This induces increasing of both, the surface conductivity and surface roughness. From Fig. 7, it is also clear that this effect is much more significant for gold nanoparticles grafting in comparison with Au nanorods. The most pronounced decrease of zeta potential is observed in the cases of PET and PTFE after dithiol (BPD) and mercaptoethanol (ME) grafting with subsequent grafting of Au nanoparticles. It can be explained by the presence of higher amount of Au nanoparticles. These results are well supported by XPS data too (Tables 1 and 2). Extraordinary results are obtained after grafting of PTFE with cysteamine, which lead to dramatic increase of the zeta potential. This could be due to ‘‘free’’ –NH2 groups on polymer surface. Therefore, it follows that cysteamine may be bonded to activated PTFE surface via either –NH2 or –SH group. The presence of nitrogen group leads to increase of positive surface charge, and also the zeta potential increases. Moreover, the same effect may also be responsible for much lower concentration of bonded gold nano-objects found on PTFE (see Table 2). The 2D AFM images, taken in phase mode of pristine PET, PET treated by plasma discharge and grafted by cysteamine or mercaptoethanol (CYST, ME) and then with gold nanoparticles and nanorods (AuNP and AuNR), are shown in Fig. 8. We present here only the images of the PET samples
Fig. 6 XPS spectrum of the plasma-treated PET grafted with cysteamine and gold nanoparticles. The C(1s), O(1s), N(1s), S(2p), and Au(4f) peaks are present (Color figure online)
identified. Similar phenomena are observed on PTFE too (see Table 2). Plasma treatment results in oxidation of the PTFE surface layer. The observed concentration of incorporated oxygen does not change significantly after grafting of plasma-treated PTFE with thiols and gold objects. The surface concentration of fluorine is reduced by plasma treatment and much more strongly by subsequent grafting with thiols and gold objects. Gold concentration varies from 0.1 to 5.1 at.%. Chemical structure of polymer films is expected to influence their electrokinetic potential substantially. Zeta potentials measured on pristine and grafted PET and PTFE are shown in Fig. 7. The zeta potential is affected by several other factors such as surface roughness and morphology, surface polarity (wettability), and electrical conductivity of the polymer surface [16]. For both polymers, the zeta potential increases after the plasma treatment due Table 1 Element concentrations determined by XPS measurements on pristine PET (23 lm), plasma-treated PET (for 120 s) and PET grafted with thiols (biphenyldithiol—BPD, cysteamine—CYST, and mercaptoethanol—ME) and then with Au nano-objects (nanoparticles—AuNP and nanorods—AuNR)
Sample
Element concentration (at.%)
PET
C(1s)
O(1s)
S(2p)
N(1s)
Au(4f)
Pristine
73.7
26.3
–
–
–
Plasma treated
67.0
33.0
–
–
–
Plasma/BPD Plasma/BPD/AuNP
73.8 81.1
18.5 12.7
7.7 6.1
– –
– 0.1
Plasma/BPD/AuNR
82.2
12.1
5.0
–
0.7
Plasma/CYST
71.6
20.9
3.1
4.4
–
Plasma/CYST/AuNP
70.1
21.4
2.3
2.9
3.3
Plasma/CYST/AuNR
75.5
19.9
1.8
2.7
0.1
Plasma/ME
72.0
26.8
1.2
–
–
Plasma/ME/AuNP
77.4
21.7
0.6
–
0.3
Plasma/ME/AuNR
80.5
16.4
1.0
–
2.1
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6302 Table 2 Elements concentrations determined by XPS measurements on pristine PTFE (25 lm), plasma-treated PTFE (for 120 s), and PTFE grafted with thiols (biphenyldithiol—BPD, cysteamine—CYST, and mercaptoethanol—ME) and then with Au nano-objects (nanoparticles—AuNP and nanorods—AuNR)
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Sample
Element concentration (at.%)
PTFE
C(1s)
O(1s)
F(1s)
S(2p)
N(1s)
Au(4f)
Pristine
33.4
–
66.6
–
–
–
Plasma treated
39.8
5.2
55.0
–
–
–
Plasma/BPD
78.0
6.2
4.7
11.1
–
–
Plasma/BPD/AuNP
60.5
4.9
29.7
4.4
–
0.5
Plasma/BPD/AuNR
58.8
4.5
33.0
3.0
–
0.6
Plasma/CYST
48.9
5.4
39.2
3.4
3.1
–
Plasma/CYST/AuNP
44.3
5.1
45.4
2.7
2.3
0.2
Plasma/CYST/AuNR
57.4
5.9
31.2
2.4
3.0
0.1
Plasma/ME
41.2
8.0
49.0
1.8
–
–
Plasma/ME/AuNP
52.8
6.7
35.1
0.3
–
5.1
Plasma/ME/AuNR
50.1
5.6
40.6
1.0
–
2.6
Fig. 7 Zeta potential determined on pristine (pristine) and modified a PET and b PTFE. The polymer foils were plasma treated (plasma), plasma treated and grafted with (i) thiols (biphenyldithiol— BPD, cysteamine—CYST and mercaptoethanol—ME) and then (ii) grafted with Au nanoobjects (nanoparticles—AuNP and nanorods—AuNR) (Color figure online)
grafted with nanoparticles (Fig. 8b) and nanorods (Fig. 8c) and exhibiting the highest gold concentration in XPS analysis. PET was chosen because of its markedly lower surface roughness after the plasma treatment in comparison with PTFE [16]. It is quite surprising that objects are observed with size dramatically higher than the original size of Au nanoparticles and nanorods (see Fig. 2). It may therefore be concluded that a significant aggregation of gold nanoobjects takes place during nano-objects grafting. The size of gold nanoparticles is known to affect the plasmon absorption in UV–Vis spectra. The position of the surface plasmon resonance peak (plasmon absorption) moves toward higher wavelength with increasing nanoparticles size [13, 32]. The effect was observed earlier after thermal annealing of thin Au nanolayers leading to size changes in layer structure [33]. Differential UV–Vis absorption spectra of PET treated by plasma, grafted by
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cysteamine and mercaptoethanol (CYST, ME), and then with gold nano-objects (AuNP, AuNR) are shown in Fig. 9. Prominent broad surface plasmon resonance peak in the region ranging from about 550 to 850 nm is observed on PET grafted with nanoparticles. Similar but much weaker peak is observed also on the PET grafted with nanorods. From comparison with nano-object UV–Vis spectra in Fig. 3, one can conclude that in the course of grafting, significant aggregation, especially of nanoparticles, occurs. This conclusion is in agreement with previous results of AFM imaging (compare with Fig. 8).
Conclusions The main goal of this study was deposition of gold nanoobjects on the surface of polymers modified by plasma
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Fig. 8 AFM images, taken in phase mode, of a pristine PET, b PET treated by plasma and grafted with cysteamine and then with Au nanoparticles, c PET treated by plasma and grafted with mercaptoethanol and then with Au nanorods (Color figure online)
on ‘‘free’’ –SH groups. Also the results from XPS spectroscopy confirm the creation of gold nano-object–thiol layer on the polymer surface. Rather large objects observed on AFM images show that a significant aggregation of the deposited nano-objects takes place during grafting procedure. The aggregation was confirmed also by UV–Vis measurements. Grafting with thiols and gold nano-objects generally leads to a decrease of the zeta potential. Strong increase of the zeta potential observed on PTFE sample, plasma treated and grafted with cysteamine, may be due to partial binding of cysteamine molecule on PTFE surface via –NH2 group. This conclusion was confirmed by much lower concentrations of bonded gold nano-objects found on PTFE determined by XPS measurement.
Fig. 9 Differential UV–Vis spectra of plasma-treated PET grafted with cysteamine or mercaptoethanol (CYST or ME) and then with gold nano-objects (AuNP or AuNR) (Color figure online)
Acknowledgements This study was supported by the GA CR under the Project P108/12/G108.
References discharge and subsequent grafting with different thiol mediators. The systems studied may have an application potential in electronics or tissue engineering. The successful binding of thiols on the plasma-treated polymers was proved by FTIR measurements. The observed signal of S–Au group in different FTIR spectra gives evidence of binding of gold nano-objects, nanoparticles, and nanorods,
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