Polytetrafluorethylene-Au as a substrate for surface-enhanced Raman spectroscopy
© Žvátora et al; licensee Springer. 2011
Received: 23 September 2010
Accepted: 28 April 2011
Published: 28 April 2011
This study deals with preparation of substrates suitable for surface-enhanced Raman spectroscopy (SERS) applications by sputtering deposition of gold layer on the polytetrafluorethylene (PTFE) foil. Time of sputtering was investigated with respect to the surface properties. The ability of PTFE-Au substrates to enhance Raman signals was investigated by immobilization of biphenyl-4,4'-dithiol (BFD) from the solutions with various concentrations. BFD was also used for preparation of sandwich structures with Au or Ag nanoparticles by two different procedures. Results showed that PTFE can be used for fabrication of SERS active substrate with easy handle properties at low cost. This substrate was sufficient for the measurement of SERS spectrum of BFD even at 10-8 mol/l concentration.
Surface-enhanced Raman scattering (SERS) has great potential as an analytical technique based on the surface enhancement of Raman signals of the molecule situated on the metal surface which is nowadays currently used for the detection of various analytes at low concentration . In general, there are two traditional operational mechanisms to describe the overall SERS effect: electromagnetic  and chemical [1, 2] enhancement mechanism. Electromagnetic mechanism lies in the enhancement of local electromagnetic field of incident radiation applied on the molecule which is adsorbed on or situated in the close proximity to rough metal surface. In order to obtain optimal enhancement of Raman signals of the molecule it is necessary to use nanostructured surfaces or nanoparticles of noble metals with suitable physical parameters such as their size, shape, and degree of aggregation . Many different types of SERS substrates, which meet the above requirements have been developed, including roughened electrodes [4, 5], noble metal colloidal nanoparticles [6, 7], silver island films [8, 9], metal film over nanostructured surfaces [10, 11], acid-etched metal foils , and lithographically produced nanoparticle arrays [13, 14]. Plastic substrates are also known . Polymers were commonly used for improved mechanical stability of nanoparticles  and better signal reproducibility via embossing surfaces and lithographic techniques [15, 17]. Polytetrafluorethylene (PTFE) is a polymer with broad potential applications in microelectronics. Another advantage of this material is its high thermal stability and low degradation due to the exposition to a focused laser beam. PTFE foil has great surface roughness with improved adhesive properties of sputtering gold over layer and can be positive for electromagnetic mechanism. Gold over layer can suppress Raman background signal of the PTFE substrate .
Within the experiments described in this study we have prepared suitable SERS active substrates from synthetic polymer foils of PTFE by deposition of Au layers on its surface inside of plasma discharge . Electromagnetic mechanism enhancement was tested on rude PTFE-Au surface and sandwich structures. The fabrication of sandwich structures  was realized by incorporating of self-assembled monolayer of dithiols between the layers of PTFE-Au surface and Au or Ag nanoparticles.
Preparation of gold layer on PTFE foil
The influence of time of sputtering (t = 10, 20, 30, 50, 80, 150 s) and concentration (c = 10-2, 10-3, 10-4, 10-5, 10-6, 10-7, 10-8 mol/l) of used bifunctional compound (biphenyl-4,4'-dithiol) on the intensity of SERS signals was then studied. In order to study the sputtering time gold layers were modified with biphenyl-4,4'-dithiol in methanol solutions (10-2 mol/l). PTFE foil with gold layer was placed into the methanol solution for 12 h. After that the foil was taken out from the solution, washed by pure methanol, and dried on the air. The study of concentration dependence was similar.
Preparation of nanoparticles
Gold nanoparticles (AuNPs) were obtained by citrate reduction of K[AuCl4] described elsewhere . Silver nanoparticles (AgNPs) were obtained using similar process of AgNO3 reduction published by Smitha et al. . Prepared nanoparticles were characterized by TEM and UV-Vis absorption spectroscopy. UV-Vis absorption spectroscopy was carried out using a Varian spectrophotometer, model Cary 400 SCAN, from 200 to 800 nm. The transmission electron microscopy (TEM) images were recorded using a JEOL microscope, model JEM-1010 with accelerating voltage 100 kV.
Preparation of sandwich structures
In the second procedure (Figure 2b), PTFE foil with gold layer modified by biphenyl-4,4'-dithiol was prepared. Then such modified foil was placed into the solution of 2 ml of nanoparticles for 12 h. After that the foil was removed from the solution, washed by pure methanol, and dried on the air.
Raman spectral measurements were performed on Raman NIR Advantage spectrograph DeltaNu with laser excitation line 785 nm, power 100 mW in the range of 100 to 2000 cm-1 with spectral resolution 4 cm-1. Integration time was 20 s and results spectra are average of five measurements. Surface was focused by the NuScope with manual adjustment and field of view was approximately 800 μm at 100 × focal power. All measurements were carried out on two different places from both sides of PTFE foil.
Results and discussion
Properties of prepared gold layers on PTFE foil
The results of measurements of prepared gold layers on PTFE foils are shown in Figure 1. The thickness of gold layer was calculated from the mass difference of foils before and after sputtering procedure. It is clear from the table that the thickness is a linear function of sputtering time. The value of resistance is related to continuity of gold layer ; therefore, when short times are applied the resistance values are very high and the layer is discontinual, while after the applications of longer times the resistance values change to low which means that the layer becomes continual.
Preparation of nanoparticles
SERS measurements on PTFE foils
The analytical enhancement factor of the surface for immobilized BFD (calculated for c = 1 × 10- 8 mol/l); sandwich structures were prepared according to Figure 2b
Type of surface
Analytical enhancement factor
3.89 × 105
9.12 × 105
6.73 × 106
The dependence of the area of the selected peak 1078 cm- 1 in SERS spectra on the concentration of BFD
PTFE-Au-BFD-AuNP (prepared according to Figure 2b)
PTFE-Au-BFD-AgNP (prepared according to Figure 2b)
3.21 × 106
1.44 × 106
1.86 × 106
8.37 × 106
4.09 × 106
6.10 × 106
5.96 × 106
4.12 × 106
5.93 × 106
10.3 × 106
8.91 × 106
8.04 × 106
12.6 × 106
9.06 × 106
4.96 × 106
6.38 × 106
0.709 × 106
6.58 × 106
0.09 × 106
0.218 × 106
1.14 × 106
It was found that the enhancement of Raman signals of BFD is independent on the measured side of PTFE foil due to the transparency of the foil and very thin layer of sputtered gold. Further, reproducibility of foil preparation is very high but the reproducibility of BFD- and NPs-modified foils is lower (RSD = 20%).
In summary, we have demonstrated the possible preparation of SERS active substrate with suitable properties by sputtering deposition of gold layer on the PTFE foil. Such foil is cheap, easy to manipulate with it, and offers the possibility to measure from both sides of PTFE foil. It was found out that optimum of sputtering time is for 30 s and the maximum of SERS signal intensity was achieved at 10-6 mol/l for BFD. With use of sandwich structures of nanoparticles we were able to obtain signal even at 10-8 mol/l. This substrate had the highest analytical enhancement factor (6.73 × 106).
surface enhanced Raman spectroscopy
transmission electron microscopy.
The financial support from the Ministry of Education of the Czech Republic MŠMT 6046137307, the GACR Foundation No. 203/09/0675 and GAAV CR Foundation KAN200100801 is gratefully acknowledged.
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