Anal. Chem. 2006, 78, 7588-7591
Fabrication of Polypyrrole-Based Nanoelectrode Arrays by Colloidal Lithography Andrea Valsesia, Patrı´cia Lisboa, Pascal Colpo, and Franc¸ ois Rossi*
European Commission, Joint Research Center, Institute for Health and Consumer Protection, Via E.Fermi, 1 TP203, 21020 Ispra, Varese, Italy
This paper describes a novel technique to produce polypyrrole-based nanoelectrodes for electrochemical detection purpose. The fabrication process relies on the creation of patterned nanotemplates i.e., nanometric gold spots surrounded by an electrically insulating material (SiOx). From these templates, polypyrrole nanopillars are grown by classical electrochemical methods. Atomic force microscopy demonstrates that polypyrrole grows selectively inside the gold nanotemplates. The electrochemical characterization by cyclic voltammetry showed a sigmoidal-shaped voltammogram characterizing the typical nanoelectrode array behavior. The use of nanoelectrode arrays or ultramicroelectrode arrays for the electrochemical detection of biological or chemical species presents many advantages as compared to classical macroelectrodes. Advantages such as higher mass sensitivity, increased mass transport rate, and lower influence of the solution resistance have already been underlined 1,2 Conductive polymers, such as polypyrrole (PPy), used as electrode materials present additional advantages such as high electrical conductivity,3 good stability, and excellent biocompatibility. Furthermore, PPy constitutes an ideal platform for the detection of biological species since it can be functionalized with specific biological-relevant chemical groups4-6 directly during the growth process, creating both a hybrid conductive and a functional layer. This paper describes a novel technique to produce polypyrrolebased nanoelectrodes for electrochemical detection purpose. The nanoelectrodes are fabricated using nanosphere lithography. This method consists of the self-assembly of monodisperse nanoparticles that are used nanomasks during etching and deposition operations. This technique is widely use to produce nanotopography over large area surfaces for Plasmonic-based sensors,7 for * To whom correspondence should be addressed. E-mail:
[email protected]. (1) Tu, Y.; Lin, Y.; Ren, Z. F. Nano Lett. 2003, 3, 107-109. (2) D.Arrigan, W. M. Analyst 2004, 129, 1157-1165. (3) Yasuzawa, M.; Nieda, T.; Hirano, T.; Kunugi, A. Sens. Actuators, B: Chem. 2000, 66, 77. (4) Lisboa, P.; Gilliland, D.; Ceccone, G.; Valsesia, A.; Rossi, F. Appl. Surf. Sci. 2005, 252, 4397-4401. (5) Grosjean, L.;L. Cherif, B.; Mercey, L E.; Roget, A.; Levy, Y.; Marche, P. N.; Villiers, M. B.; Livache, T. Anal. Biochem. 2005, 347, 193-200. (6) Haddour, N.; Cosnier, S.; Gondran, C. J. Am. Chem. Soc. 2005, 127, 57525753. (7) Haes, A. J.; Zou, S.; Schatz, G. C.; Duyne, R. P. V. J. Phys. Chem. B 2004, 108, 6961-6968.
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instance. It presents many advantages: it is relatively inexpensive and it is a parallel fabrication technique, allowing the production of a large number of samples during the same process. In our recent publications, we demonstrated the capability of using this technique to produce chemical 2-D and 3-D nanopatterns in combination with plasma processing.8-10 EXPERIMENTAL SECTION Polystyrene (PS) Nanomask Formation. The formation of the PS nanomask was carried out in the following way: first, a microdrop (drop volume, 5 µL) of PS bead suspension (average diameter 500 ( 50 and 1000 ( 100 nm, 1% dispersed in a 1:1 water/ethanol solution, purchased from Sigma-Aldrich) was deposited on 1 × 1 cm2 gold surface with the spin-coater off. The bulk volume of the drop was removed by a micropipet, to obtain a very thin layer of PS bead suspension on the surface. The sample was spun at an optimized speed favoring slow evaporation of the liquid, which allows the organization of the PS beads in a hexagonal crystal lattice. By adjusting the spin-coating acceleration to reach this speed, it is possible to create macroscopic homogeneous areas covered by monolayered nanobeads of a few square millimeters with a surface coverage ranging from 70 to 100%. Atomic Force Microscopy (AFM). AFM experiments were carried out in intermittent contact mode on a Solver instrument equipped with a Smena head (NT-MDT). The cantilever used was a standard silicon cantilever (NT-MDT) with a resonance frequency of 150 kHz, equipped with a tip characterized by a nominal radius of curvature of 10 nm and a cone angle of 22°. The images quantitative analysis (objects analysis) was carried out by the SPIP software provided by Image Metrology A/S. The average value for the bottom of the cones was calculated by averaging the diameters of the objects detected with a height lower than a threshold (T1) calculated as the rms roughness inside the holes (see Figure 2b and d). The average value for the top side of the cone was calculated by averaging the diameters of the objects detected with a height lower than a threshold (T2) calculated as the rms roughness in the SiOx matrix (see Figure 2b and d). The standard deviation is considered as the error. All the statistics have been carried out considering at least 50 objects. (8) Valsesia, A.; Colpo, P.; Lisboa, P.; Lejeune, M.; Meziani, T.; Rossi, F. Langmuir 2006, 22, 1763-1767. (9) Valsesia, A.; Colpo, P.; Meziani, T.; Bretagnol, F.; Lejeune, M.; Meziani, T.; Rossi, F.; Bouma, A.; Garcia-Parajo, M. Adv. Funct. Mater. 2006, 16, 1242-1246. (10) Valsesia, A.; Colpo, P.; Silvan, M. M.; Meziani, T.; Ceccone, G.; Rossi, F. Nano Lett. 2004, 4, 1047-1050. 10.1021/ac0609172 CCC: $33.50
© 2006 American Chemical Society Published on Web 09/26/2006
Figure 1. Scheme of the process for the creation of the nanotemplate (a) Plasma etching of the PS beads, reduction of the size; (b) PE-CVD deposition of SiOx layer; (c) mechanical removal of the beads.
Cyclic Voltammetry (CV) and Polypyrrole Growth. CV was performed with hexacyanoferrate(II/III). The CV measurements were carried out using an Autolab-PGSTAT30 (Ecochemie) with a graphite rod as counter electrode, a saturated calomel electrode (SCE) as reference electrode, and the analyzed surfaces as working electrode. A degassed aqueous solution of KCl (0.5 M)/ K3Fe(CN)6 (5 mM) was used as electrolyte. The growth process of PPy has been carried out in the same apparatus described before using the nanostructured Au/SiOx surface (0.07 cm2) as working electrode. The deposition of the PPY was done under galvanostatic control (0.2 mA) from an aqueous electrolyte solution of pyrrole (0.05 M) and lithium perchlorate (0.05 M). RESULTS AND DISCUSSION The basic idea for the production of nanostructured PPy arrays relies in the fabrication of a gold nanotemplate where the PPy is given electrochemically. The process of fabrication of the nanotemplate is illustrated in Figure 1.
First a monolayer of hexagonal-packed PS beads is created on the gold surface. Then the PS beads are etched in a highdensity inductively coupled plasma reactor, using oxygen as etching gas. The final size of the beads is controlled by the etching duration. Then a 60-nm-thick layer of SiOx is deposited by means of plasma-enhanced chemical vapor (PE-CVD) deposition onto the etched PS colloidal mask. The homemade plasma reactor used for the deposition was inductively coupled, described and characterized in ref 11. This layer is deposited from hexadimethilsiloxane precursor mixed with O2 and Ar gases to control the molecular fragmentation in the plasma. This PE-CVD deposited SiOx has insulating properties for thickness 10 nm deposited on gold, as demonstrated by electrochemical measurements (not shown). Finally, to create the chemical nanopattern, the etched beads are mechanically removed by an ultrasonic bath leaving some gold nanoholes in the SiOx matrix. Depending on the pristine diameter of the beads (500 and 1000 nm), two patterns characterized by nanoholes with different sizes and spacing have been produced. The nanostructured surfaces have been characterized by AFM (Figure 2). In Figure 2a and c, the topography for the 500 and 1000-nm masking beads nanostructured surface are respectively shown. The profiles for the nanoholes (Figure 2b and d) reproduce the fingerprint of the bead after the plasma etching. The profile of the nanohole, for both the 500 and 1000-nm patterns, is characterized by a conical shape with the bottom side shorter that the upper one. The bottom side size for the 500-nm and the 1000nm holes is respectively 54 ( 15 and 121 ( 16 nm wide as measured by the pore analysis tool provided by the SPIP software, while the top side of the truncated cone is respectively 227 ( 5
Figure 2. (a) AFM picture of the 500-nm nanoholes, (b) height profile along the line in (a), (c) AFM picture of the 1000 nm nanoholes, and (d) height profile along the line in (c).
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Figure 3. (a) AFM height image for the 500-nm holes. (b) Corresponding phase image.
Figure 4. Cyclic voltammograms in K3Fe(CN)6 with bare gold (solid line), Au-SiOx 500 µm (dot line), and Au-SiOx 1000 nm (dashed line). Scan rate of 10 mV/s.
and 600 ( 10 nm. This particular shape for the nanohole is due to the isotropic SiOx deposition, which fills the areas located between the bead and the gold. This is confirmed by the phase image (Figure 3b), evidencing the presence of spots with a different phase located at the center of the holes. Such an evident phase contrast is plausibly due to the presence of gold only at the central part of the bottom of the nanohole, while the surrounding areas are made by SiOx. To assess the average size of these gold nanospots, CV with hexacyanoferrate(II/III) was performed and compared with bare gold. The bare gold surface presents the typical voltammogram with redox peaks whereas the nanostructured surface (1000 nm) voltammogram is characterized by sigmoidal shape (Figure 4) typical of a nanoelectrode behavior. This shape is related to the presence nanoarrayed electrodes on the surface, which increase the mass transport rate, resulting in the presence of a steadystate current (Ilim). The Ilim is related to the shape and the size of the nanoelectrodes. In our case, AFM analysis of the nanopatterned surface reveals that the shape of the electrodes is similar to the one of the recessed disk model.2 Therefore. the Ilim is given by 12
π I(recessed) ) 4nFDCa lim 4L +π a
(1)
Where n is the number of electrodes, F the Faraday constant, D 7590
the diffusion coefficient of the electrolyte, C the concentration of the electrolyte, L the thickness of the SiOx walls, and R the radius of the electrodes. (recessed) Using the values for Ilim calculated from the cyclic voltammograms in Figure 4 following the method described in ref 13, it is possible to extract the theoretical values of a from eq 1 (Table 1). The radius calculated by the recessed disk model for both samples is in good agreement with the radius of the bottom side of the nanoholes measured by the AFM. The nonperfect sigmoidal shape of the CV curves is due to the partial overlapping of the diffusion fields, due to the relative short distance between the nanoelectrodes. One can notice that, in the CV curves in Figure 4, the curve related to the 1000-nm pattern is closest to a perfect sigmoid with respect to the curve related to the 500-nm pattern; this is due to a reduction of the overlapping of the diffusion fields. The overlapping should be further reduced by using beads characterized by a bigger diameter. In fact, the electrode array fabrication technique presented here is not in principle limited by the colloidal mask size. The previously described nanostructured Au/SiOx surface has been used as a nanotemplate to electrochemically grow PPy nanopillars. For a given current of the deposition, the thickness of the PPy grown on the surface is controlled by the deposition time. At a current density of 2.9 mA/cm2, for low deposition time (t < 3 min), the PPy growth driven by the current flowing through the gold nanoareas is done exclusively inside the nanohole forming isolated nanopillars whereas for longer deposition time (t . 3 min) the PPy pillars tend to merge and form PPy homogeneous films. PPy nanopillars have been observed for both the 500-nm and the 1000-nm patterns. The isolated PPy nanopillars grown on the 500nm pattern are shown in the AFM picture in Figure 5. The average dimensions of the PPy nanopillars have been extracted from the AFM pictures by the object analysis tool provided by the SPIP software: the average diameter of the nanopillars was calculated to be 60 ( 3 and 171 ( 10 nm for the 500-nm and for the 1000-nm patterns, respectively. The values for
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(11) Meziani, T.; Colpo, P.; Rossi, F. Plasma Sources Sci. Technol. 2001, 10, 276-283. (12) Zoski, C. G.; Simjee, N. Anal. Chem. 2004, 76, 62-72. (13) Koehne, J.; Li, J.; Cassell, A. M.; Chen, H.; Ye, Q.; Ng, H. T.; Han, J.; Meyyappan, M. J. Mater. Chem. 2004, 14, 676-684.
Table 1. Values for the Geometrical and Electrochemical Parameters of the Gold Nanoelectrode Arrays sample
n
(recessed) Ilim (A)
4FDC (A/m)
L (m)
500 nm 1000 nm
2.2 × 107 6.4 × 105
7.0 × 10-4 5.7 × 10-5
1.5 × 10-3 1.5 × 10-3
8 × 10-8 8 × 10-8
a
a (nm) 58 114
a (nm) (AFM) 54 ( 15 121 ( 16
electrodes/cm2a 3 × 108 9 × 106
Electrode density.
Figure 5. AFM picture of the nanopillars of PPy grown inside the 500-nm template.
the diameter of the nanopillars are in agreement with the estimated size of the gold nanoholes. The electrochemical characteristics of the PPy nanopillar array were evaluated and compared with an uniform PPy surface by CV using the same experimental conditions as above. The uniform PPy surface presents the typical redox shape, whereas the PPy nanopillar surface voltammograms present a sigmoidal shape (Figure 6).This results show that our approach is suitable for the fabrication of nanoelectrode arrays made by PPy. The voltammograms do not have a perfect steady-state shape and present differences between the forward and backward scans. This can be again attributed to the overlap of the diffusion layers of the nanoelectrodes in some areas of the surface where the nanopillar distribution is not uniform. Nanoelectrode arrays are known for having low double layer capacitance and consequently low RC time constant, which results in fast mass transport. For these reasons, they are suited for use in fast electrochemical measurements providing high sensitivity because they have the ability to subtract the double layer charging current and the measurements are performed in a few seconds.15 (14) Lee, H. J.; Beriet, C.; Ferrigno, R.; Girault, H. H. J. Electroanal. Chem. 2001, 502, 138-145. (15) Wang, J. Analytical Electrochemistry, 2nd ed., Wiley-VCH: New York, 2000. (16) Lee, K. B.; Kim, E. Y.; Mirkin, C. A.; Wolinsky, S. M. Nano Lett. 2004, 4, 1869-1872.
Figure 6. Cyclic voltammograms in K3Fe(CN)6 with uniform PPy (solid line), PPy nanopillars 500 nm(dot line), and PPy nanopillars 1000 nm (dashed line). Scan rate of 10 mV/s.
CONCLUSIONS A novel technique to produce PPy-based nanoelectrodes was explored. By creating a nanotemplate characterized by gold nanoholes separated by insulating SiOx matrix, nanopillars of PPy were successfully grown inside the nanoholes, creating a nanoelectrode array. These arrays were studied by CV using hexacyanoferrate and the typical sigmoidal-shaped voltammogram of nanoelectrodes was obtained. The PPy-based nanoelectrode is a very promising electrochemical sensing platform for the design of a new generation of analytical devices. It has the potential to enhance the detection sensitivity thanks to its nanoelectrode behavior. Furthermore, by functionalizing the PPy and then creating biological active/nonactivies region at nanoscale, this sensitivity could be drastically improved as already shown in the literature.8,16 ACKNOWLEDGMENT The authors thank A. Crippa and S. Malfara′ for the technical support in the electrochemical measurements. Special thanks to D. Arrigan and V. Beni, Tyndall Institute, Cork, Ireland, for the fruitful discussions about the voltammograms. Received for review May 18, 2006. Accepted August 10, 2006. AC0609172
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