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Langmuir 2008, 24, 9110-9117
Inkjet-Printed Thiol Self-Assembled Monolayer Structures on Gold: Quality Control and Microarray Electrode Fabrication Ina Rianasari,† Lorenz Walder,*,† Malte Burchardt,‡ Izabella Zawisza,‡ and Gunther Wittstock*,‡ Institute of Chemistry, Center of Interface Science (CIS), UniVersity of Osnabru¨ck, Barbarastrasse 7, D-49069 Osnabru¨ck, Germany, and Department of Pure and Applied Chemistry and Institute for Chemistry and Biology of the Marine EnVironment, Center of Interface Science (CIS), Faculty of Mathematics and Science, Carl Von Ossietzky UniVersity of Oldenburg, D-26111 Oldenburg, Germany ReceiVed March 6, 2008. ReVised Manuscript ReceiVed May 15, 2008 Laterally structured, self-assembled monolayers (SAMs) of different thiols (HS-R-X, R ) (CH2)3-16, X ) -CH3, -COOH, -NH2) on gold have been prepared by inkjet printing. The printer is a modified, low-cost desktop printer (Epson Stylus Photo R200), the ink is a 1 mM solution of the thiol in ethanol/glycerol (6:1). The quality of inkjetprinted large area SAMs obtained in this study is between that of a layer self-assembled from a thiol solution and that obtained by soft lithography, according to cyclic voltammetry, electrochemical impedance spectroscopy, scanning electrochemical microscopy (SECM), and polarization-modulated Fourier transform infrared reflection-absorption spectroscopy (PM IRRAS). For the first time, simultaneous printing of two different thiols in a single print job as an alternative to sequential printing and backfilling is demonstrated. The smallest structures consisting of conductive disks of 40 µm diameter were analyzed as single spots by SECM and as random array electrodes with different average disk-disk distance. Conductive band electrodes with variable bandwidth (300 µm to 1 cm) are presented, as well as a pH switchable band structure. As compared to stamping, inkjet printing allows for simultaneous multiple thiol printing in a single print job with the resolution limited only by the droplet size and the precision of the translation stage.
I. Introduction Inkjet printing (IJP) technique has caught much interest for lateral structuring of flat surfaces in the range of several tens to several hundred micrometers. The advantages of IJP over other techniques, such as stamping or lithography, are computer controllability, flexibility, and high throughput. IJP has been applied in different research areas, for example, display technology and electronic circuitry,1–3 combinatorial studies of new materials,4,5 and patterning biological compounds.6–8 The structures produced by IJP are constructed from droplets controlled by a drop-on-demand (DOD) system. The DOD system combined with a precise translation stage allows controlling the density and placement of the droplet on the substrate. The resolution of low priced desktop inkjet printers used for photographs is remarkable, and their use s after modifications s for structuring surfaces with biological> materials has been reported.7,8 Notably, the smallest accessible drop size of the * Corresponding authors: Professor Dr. Lorenz Walder, University of Osnabru¨ck, e-mail
[email protected], tel +49 541 9692495, fax +49 541 9693308; Professor Dr. Gunther Wittstock, Carl von Ossietzky University of Oldenburg, e-mail
[email protected], tel +49 441 7983971, fax +49 441 798 3979. † University of Osnabru¨ck. ‡ Carl von Ossietzky University of Oldenburg. (1) Guo, T.-F.; Chang, S.-C.; Pyo, S.; Yang, Y. Langmuir 2002, 18, 8142– 8147. (2) Hebner, T. R.; Wu, C. C.; Marcy, D.; Lu, M. H.; Sturm, J. C. Appl. Phys. Lett. 1998, 72, 519–521. (3) Moeller, M.; Asaftei, S.; Corr, D.; Ryan, M.; Walder, L. AdV. Mater. 2004, 16, 1558–1562. (4) de Gans, B. J.; Schubert, U. S. Macromol. Rapid Commun. 2003, 24, 659–666. (5) Mohebi, M. M.; Evans, J. R. G. J. Comb. Chem. 2002, 4, 267–274. (6) Bietsch, A.; Zhang, J.; Hegner, M.; Lang, H. P.; Gerber, C. Nanotechnology 2004, 15, 873–880. (7) Goldmann, T.; Gonzalez, J. S. J. Biochem. Biophys. Methods 2000, 42, 105–110. (8) Pardo, L.; Wilson, W. C.; Boland, T. Langmuir 2003, 19, 1462–1466.
standard desktop inkjet printers is in the same range as that of highly priced DOD systems used in material sciences, that is, 2-3 pL corresponding to an ∼40 µm spot diameter. This is only surpassed by new prototype developments claiming to have reached a minimal drop volume of ∼0.02 pL corresponding to a spot diameter of ∼2 µm.9,10 The formation of thiol alkane self-assembled monolayers (SAMs) on gold is a process occurring preferentially under equilibrating condition (immersion of the substrate in a dilute solution of the thiol) and requiring sometimes hours to reach commensurate packing.11 This is in contrast to the time scale available for SAM formation in soft lithography, or for inkjetprinted structures (typically 30 s to 1 min). Some studies suggested that the SAM quality obtained by stamping is comparable to that obtained from immersion, if a sufficiently high thiol concentration (100 mM) is used on the stamp.12,13 Only a few studies have addressed the problem of thiol quality resulting from the IJP technique.6,14,15 Thus, Pardo et al. found from IR and contact angle studies that the inkjet-printed SAMs have a similar quality as those produced by microcontact printing (µCP), and Bietsch et al. found almost perfect circular SAM disk structures for inkjetted single droplets of ethanolic thiol solutions after development by etching techniques.8,14 (9) Wang, Y.; Bokor, J. J. Micro/Nanolith. MEMS MOEMS 2007, 6, 043009. (10) Wang, Y.; Bokor, J.; Lee, A. Proc. SPIE-Int. Soc. Opt. Eng. 2004, 5374, 628–636. (11) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559–3568. (12) Larsen, N. B.; Biebuyck, H.; Delamarche, E.; Michel, B. J. Am. Chem. Soc. 1997, 119, 3017–3026. (13) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1103–1169. (14) Bietsch, A.; Hegner, M.; Lang, H. P.; Gerber, C. Langmuir 2004, 20, 5119–5122. (15) Sankhe Amit, Y.; Booth Brandon, D.; Wiker Nathan, J.; Kilbey, S. M. Langmuir 2005, 21, 5332–5336.
10.1021/la800711m CCC: $40.75 2008 American Chemical Society Published on Web 07/11/2008
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Figure 1. Microscopic view of ink jetted original Epson ink on photo paper using (a) 1%, (b) 5%, (c) 10%, (d) 30%, (e) 50%, and (f) 100% color density on a surface of ∼2.6 mm2.
There are many reports on structured SAMs consisting of different thiols and prepared by stamping/backfilling or multiple stamping, but only a few reports exist on the use of IJP or IJP/ backfilling as an alternative, for example, for the fabrication of macroscopic gradients of two thiols, to stabilize a SAM structure of one thiol within another SAM, or to address spatially separated cantilevers.6,8 Interestingly, the inherent advantage of IJP, that is, the creation of structures from two or more thiols in a single print job, has not found much attention so far.8 In the this study we compare the quality of homogeneous SAMs on gold produced (i) by IJP, (ii) by stamping, and (iii) by immersion (under equilibrating conditions), using cyclic voltammetry, impedance measurements, scanning electrochemical microscopy (SECM), polarization-modulated Fourier transform infrared reflection-absorption spectroscopy (PM IRRAS), and optical techniques. We present simultaneous printing of two alkane thiols on gold, by using the multichannel option of the piezo print head as an alternative to the backfilling technique. We studied the size and quality of thiol disks made from 3 pL droplets consisting of a “conductive thiol” (i.e., a nonpassivating thiol) within an environment of “insulating thiols” (i.e., a passivating thiol). We demonstrate the possibility of producing small band microelectrodes as well as random arrays of disk microelectrodes using the IJP technique. Finally, we report on the fabrication and characterization of patterns with pH-dependent permeabilities.
II. Materials and Methods Materials. Mercaptopropionic acid (HS-(CH2)2-COOH, Fluka), hexadecanethiol (HS-(CH2)15-CH3, Fluka), 11-amino-1-undecanethiol (AUT, HS-(CH2)11-NH2, Dojindo), 11-mercaptoundecanoic acid (HS-(CH2)10-COOH, Aldrich), potassium hexacyanoferrate(III) (K3[Fe(CN)6], Fluka), 1-(ferrocenyl)ethanol (C12H14FeO, Fluka), ferrocenylmethanol (C11H12FeO, ABCR), sodium sulfate (Na2SO4, Roth), Na2HPO4 · 2H2O (Scharlau), NaH2PO4 · 2H2O (Fluka), sodium chloride (NaCl, Sigma) and
potassium chloride (KCl, Merck), absolute ethanol, and glycerol (C3H8O3, Riedel de Haen) were of analytical grade and used as received. Printer Modification. An Epson Stylus Photo R200 inkjet printer with the option to print on flat substrates (labels on CDs) was modified to print chemicals on electrodes instead of ink on CD labels. For details, see Supporting Information. Thiol Ink. Unless otherwise stated, a mixture of ethanol/glycerol of 6:1 (v/v) containing the thiol at 1 mM concentration was used. The ink viscosity under this condition was 2.5 cPa s, as measured with an Ubbelohde viscosimeter (averaged over three measurements). Graphical Software. Corel Draw X3 running on a PC was used to layout the SAM structure and to launch the printing job. All structures were drawn using the rectangle tool with the outline function (stroke) set to zero. In order to fill a structure with a single thiol, the structure was filled with pure color at 100% color density, that is, cyan, magenta, or yellow, with the color ink replaced by the corresponding thiol solutions in the cartridge. At 100% color density the attainable surface concentration of the alkanethiol is 1 × 10-9 mol cm-2 for a 1 mM thiol solution concentration. In order to get well-separated disks of thiols on gold with an average distance of ∼250 µm, we used the yellow channel with the color density of 1%. For further details and calibration, see Supporting Information. Gold Electrodes. Sputtered gold (200 nm) on glass plates (1.1 mm thick) were obtained from Ssens (Hengelo, NL) and cut into 1 cm × 2.5 cm electrodes. Prior to use, the electrodes were exposed to air plasma (PDC-32G, Harrick Scientific, Ithaca, NY) for 10 min. They were then immersed in ethanol for 10 min and dried under a stream of Ar. Used electrodes were cleaned prior to the plasma treatment by exposing them to a Piranha solution s a mixture of H2O2/H2SO4 (1:3 v/v) s at room temperature for about 2 min. Caution: This mixture reacts Violently with all organic material. The solution has to be handled with extreme care to aVoid personnel injury and property damage. Homemade gold electrodes were prepared by evaporating ∼0.5 nm Cr (g99.7%, Goodfellow) and ∼50 nm Au (g99.98%, Goodfellow) on cleaned, cut glass slides (Menzel Gla¨ser) in a Tectra Minicoater (Tectra). For IR measurements, 0.7 µm of Cr and 200 nm of Au have been evaporated. Monolayer Preparation. Immersion. The gold electrodes were immersed in a 1 mM ethanolic thiol solution overnight at room temperature, washed with EtOH, and dried under a stream of Ar. Stamping. A flat polydimethylsiloxane (PDMS) surface (prepared from Sylgard 184 Elastomer Kit (Dow Corning)) was wetted with 1 mM ethanolic hexadecane thiol solution for 3 min and blown dry with Ar. The stamp was gently pressed on the gold surface and remained there for 1 min. It was then rinsed with EtOH and dried with Ar. Inkjet Printing (IJP). The thiol inks were printed at 100% color density with the modified Epson stylus Photo R200, and each print was visually controlled for print head clogging. (i) For printing nonstructured homogeneous SAMs, the yellow cartridge channel was used as the thiol reservoir. (ii) For printing structures using two thiols, the cyan and the magenta cartridges were used as thiol reservoirs. (iii) For printing structures using one thiol in the printing step, followed by backfilling by immersion, we used (a) in the case of the band electrodes the yellow cartridge and (b) in the case of “diluted” disks of a thiol the yellow cartridge at 1, 4, or 10% color density. After modification, the electrode was washed with ethanol and dried under a stream of argon. Electrochemical, Spectroscopic, and Optical Methods. Cyclic Voltammetry. The measurements were performed in a three-electrode system using a PGSTAT 20 from AUTOLAB controlled by GPES (software version 4.9, ECO Chemie 1995). The modified gold electrode was used as a working electrode (active area 0.5-1 cm2),
Table 1. Color Density, Dot-Dot Distance, and Surface Coverage color density (%) av dot-dot center distance (µm) surface coverage (%)
100 multiple overlap 100
50 partial overlap 80
10 80 23
4 110 8.9
1 390 1.9
9112 Langmuir, Vol. 24, No. 16, 2008 the reference was an Ag|AgCl (Methrohm, 6.0724.140, separated by a salt bridge), and the auxiliary electrode was a Pt wire. Unless otherwise stated, the scan rate was 100 mV s-1. Electrochemical Impedance Spectroscopy (EIS). The EIS measurements were performed with the same potentiostat and setup as described for cyclic voltammetry. A single sine wave excitation with 10 mV amplitude at the dc potential of the redox couple (0.2 V vs Ag/AgCl) was applied. The spectra were recorded from 100 or 10 kHz down to 10 mHz. The impedance data were analyzed by the fit programs incorporated in the FRA package using the equivalent circuits Rs(C[Rct Zw]) and Rs(Q[Rct Zw]).16 Scanning Electrochemical Microscopy (SECM). SECM experiments were carried out on three different home-built positioning systems (Ma¨rzha¨user,17 Mechonics, OWIS18). SECM measurements were performed in a three-electrode cell consisting of a Pt UME as working electrode, a Pt wire as auxiliary electrode, and a silver wire as quasi-reference electrode. Ferrocenylmethanol (1 mM) in 0.1 M Na2SO4 was the redox system unless otherwise stated. Data representation and analysis have been described earlier.19 PM IRRAS. All the PM IRRAS spectra were recorded with a Bruker Vertex 70 spectrometer with the polarization modulation set (PMA 50) equipped with the photoelastic modulator and demodulator (Bruker, Ettlingen, Germany; Hinds Instruments, USA). All spectra were recorded with a resolution of 2 cm-1. The PEM maximum efficiency was set for the half-wave retardation at 2900 cm-1 for analysis of the CH stretching bands and at 1500 cm-1 for the CH bending modes. The spectra in the CH stretching region contain 6000 and in the CH bending region 10000 averaged spectra. The angle of incident light was set to 80°. All IR spectra were collected in a dry air atmosphere. The PM IRRAS spectra were processed using the OPUS software (Bruker). Optical Microscopy. A Biolux 654 optical microscope was used for the optical analysis of the ink printed on paper. An additional Peltier element was fitted to the top of the substrate holder, to study thiol structures on gold optically by specific water vapor condensation according to the hydrophilicity/hydrophobicity of the thiols present. The surface was illuminated from the side by a focused LED lamp for maximum optical contrast.
III. Results and Discussion III.1. Quality of a Homogeneous, Inkjet-Printed Monolayer. The used printer shoots the ink as droplets (∼3 pL). Each droplet is accompanied by a slightly smaller satellite droplet as typical for Epson (Figure 1).20 Upon arrival on the surface the droplet and its satellite transform into a pair of small disks with a diameter of typically ∼45 µm. For the thiol solution printed onto the gold substrate, we find the same double disks or ellipsoidal structures of ∼40 µm × 80 µm diameter if the disks coalesce (for details of the single droplet structure see next section). The area density of ink disks on paper can be adjusted from well-separated to overlapping disks by the “color density” function within the graphical software Corel Draw X3 (Figure 1). Partial overprinting starts in the range of 30% color density (Figure 1d); at 100% all the area is extensively overprinted. With thiol on gold instead of ink on paper, the same general trend is expected, but overprinting starts earlier because the thiolon-gold disks are slightly larger than the ink-on-paper disks (for a discussion of the thiol single droplets or disks, see next section). From a calibration of the volume jetted per cm2 at 100% color density, that is, under overprinting conditions, we calculate that the applied average surface concentration, Γ, of the alkyl thiol is 1 × 10-9 mol cm-2, for the used concentration of the alkyl (16) Boukamp, B. A. Solid State Ionics 1986, 20, 31–44. (17) Nunes Kirchner, C.; Hallmeier, K. H.; Szargan, R.; Raschke, T.; Radehaus, C.; Wittstock, G. Electroanalysis 2007, 19, 1023–1031. (18) Wilhelm, T.; Wittstock, G. Microchim. Acta 2000, 133, 1–9. (19) Wittstock, G.; Asmus, T.; Wilhelm, T. Fresenius’ J. Anal. Chem. 2000, 367, 346–351. (20) Beeson, R. IS&T Non-Impact Printing Conference, San Diego, CA, 2002.
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Figure 2. Cyclic voltammograms of 1 mM K3[Fe(CN)6] in 0.1 M KCl at a bare gold electrode (O) and at HS-(CH2)15-CH3 modified electrodes prepared by IJP (9), µCP (0), and immersion (∆).
thiol of 1 × 10-3 mM in the “thiol ink”. This value is slightly larger than the theoretical Γ value for an equilibrated, selfassembled thiol monolayer (Γ ) 7.6 × 10-10 mol cm-2).21 Any excess nonbonded thiol as well as the glycerol used for the viscosity adjustment is washed away after printing. The exposition time of the gold substrate to the thiol droplet solution (ethanol/ glycerol ) 6:1 (v:v)) under IJP conditions is in the range of 1 min (evaporation of the ethanol), that is, much less than claimed for reversible SAM formation but in the same time domain as for stamping techniques.13 In a comparative study we have checked the quality of HS-(CH2)15-CH3 SAMs on gold prepared (i) by inkjet printed at 100% color density, (ii) by traditional immersion technique, and (iii) by the stamping method Figure 2. The CVs of a 1 mM K3[Fe(CN)6] at the unmodified gold electrode, and at the HS-(CH2)15-CH3 modified electrode prepared by immersion, by micro-contact printing (µCP), and by IJP are shown in Figure 2. The peak height of the Fe(II) oxidation at the bare electrode is ∼100 times higher as compared to that of µCP HS-(CH2)15-CH3 modified electrode. No waves are observed the electrodes modified by inkjet printing and by immersion. However, the ohmic behavior points to a higher charge transfer resistance for the layer obtained by immersion as compared to IJP. From the overall CV responses, it is seen that HS-(CH2)15CH3 on the inkjet printed electrode reaches almost the insulation quality electrode prepared by immersion. There is no evidence for the formation of pinholes of micrometer size acting as microelectrodes in the case of IJP and immersion, but the small CV wave observed for µCP could be due to such inhomogeneity.11 Figure 3 shows the Nyquist plots obtained from electrochemical impedance spectroscopy of an inkjet-printed monolayer of HS-(CH2)15-CH3 on gold, as compared to the responses of a HS-(CH2)15-CH3 SAM modified electrode obtained by immersion and that of a bare gold electrode. We have analyzed the impedances using the Randles model and its modification with the parallel capacity C replaced by the constant phase element Q to fit the experimental data, that is, Rs(C[Rct Zw]) and Rs(Q[Rct Zw]), respectively. Fitting was slightly better with the more complicated Rs(Q[Rct Zw]) model, but the resulting charge transfer resistance values are very similar in both models (Table 2). The diameter of the semicircles in the impedance plane depends strongly on the modification method. It follows the sequence immersion > IJP > µCP. The corresponding charge transfer resistances (Rct) have been calculated. The Rct values of IJP- and (21) Dubois, L. H.; Nuzzo, R. G. Annu. ReV. Phys. Chem. 1992, 43, 437–463.
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constants κ were transformed to effective first-order heterogeneous rate constants keff by eq 2
keff ) κDFeMeOH ⁄ rT
(2)
By use of the values presented in Table 2 and eq 1 the coverage is 0.997, 0.9996, and 0.99997 for µCP, IJP, and immersion methods, respectively. In addition, the capacitance C calculated from the Randles model is approximatley a factor of 2 larger for the µCP- and IJP-modified electrodes as compared to the one prepared by immersion. As C is a linear function of 1/dm (dm ) modifier layer thickness), the thiol layers produced by both printing methods extend in the average to approximately onehalf of the height of the ideal layer produced by the immersion method. SECM has already been widely used to probe the permeability of SAMs.18,24–31 Their insulating quality can be estimated from the heterogeneous reaction rate constant obtained from SECM approach curves.26,27 Here, SECM approach curves to the differently prepared SAMs of HS-(CH2)15-CH3 on gold have been recorded using ferrocenylmethanol as mediator, which is expected to penetrate into small defects of the HS-(CH2)15-CH3 monolayer due to its hydrophobicity. The approach curves were evaluated according to Cornut and Lefrou (see Supporting Information IV).32 The extracted normalized heterogeneous rate
where keff is the effective heterogeneous rate constant, DFcMeOH ) 7.25 × 10-6 cm2 s-1 is the diffusion coefficient of ferrocenylmethanol, and rT ) 13.44 µm is the radius of the active part of the UME (see Supporting Information IV). For the SAMs prepared by immersion of the gold electrode in a 1 mM HS-(CH2)15-CH3 in ethanol (Figure 4, curve 3) and by IJP of 3 mM HS-(CH2)15-CH3 in ethanol/glycerol (Figure 4, curve 4), only slightly increased currents could be observed as compared to an approach curve to glass (Figure 4, curve 1 and 2) yielding keff in the range of 10-5 cm s-1. This indicates that the SAMs prepared by immersion or IJP have a very low defect density in agreement with the CV and EIS data shown above. Approach curves to the SAM prepared by µCP reveal higher currents (Figure 4, curve 5, keff ) 5 × 10-4 cm s-1), that is, a higher defect density than those prepared by immersion or IJP. The packing and arrangement of the monolayers was further characterized by PM IRRAS. Figure 5a shows the PM IRRAS spectra in the CH stretching modes region of HS-(CH2)15-CH3 monolayers on a gold surface prepared by immersion, IJP, and µCP methods. The IR spectra are composed of four absorptions corresponding to νas(CH3), νas(CH2), νs(CH3), and νs(CH2).33,34 The intensity does not fall to zero on the low energy side of the νas(CH2) band. This weak and broad spectral feature was assigned to Fermi resonance (FR) between the νs(CH2) and the overtones of the methylene bending mode.35 The shoulder at 2932 cm-1 is assigned to the FR between the νs(CH3) and the overtones of the asymmetric methyl bending modes.35 Table 3 summarizes band positions and their full width at half-maximum (fwhm) in all investigated HS-(CH2)15-CH3 monolayers. The positions of the methylene stretching modes are sensitive to the phase and conformation of the hydrocarbon chain.35,36 Hydrocarbons chains of different compounds with disordered arrangements (gauche and trans) of the hydrocarbon chains show fwhm of 18-21 cm-1 for the νas(CH2) and 12-15 cm-1 for the νs(CH2)37,38 The observed fwhm (Table 3) are smaller than these ranges for disordered alkyl chains. Together with the band position, this indicates the existence of the hydrocarbon chains of the SAM in the solid state with all-trans conformation. Similar results were reported for various long-chain n-alkane thiols selfassembled on the gold surface.11,39–43 While the intensities of the methylene stretching modes in the HS-(CH2)15-CH3 monolayer prepared by immersing and IJP are comparable, the intensities of these bands are slightly higher in monolayers formed by µCP (Figure 5a). Since the peak positions and fwhm show that all monolayers are in the solid state in all-trans conformation, the surface concentration of thiols is approximately the same in all three monolayers and the
(22) Diao, P.; Guo, M.; Tong, R. J. Electroanal. Chem. 2001, 495, 98–105. (23) Sabatani, E.; Cohenboulakia, J.; Bruening, M.; Rubinstein, I. Langmuir 1993, 9, 2974–2981. (24) Boldt, F.-M.; Baltes, N.; Borgwarth, K.; Heinze, J. Surf. Sci. 2005, 597, 51–64. (25) Cannes, C.; Kanoufi, F.; Bard, A. J. Langmuir 2002, 18, 8134–8141. (26) Forouzan, F.; Bard, A. J.; Mirkin, M. V. Isr. J. Chem. 1997, 37, 155–163. (27) Liu, B.; Bard, A. J.; Mirkin, M. V.; Creager, S. E. J. Am. Chem. Soc. 2004, 126, 1485–1492. (28) Mansikkama¨ki, K.; Ahonen, P.; Fabricius, G.; Murtomaki, L.; Kontturi, K. J. Electrochem. Soc. 2005, 152, B12-B16. (29) Oyamatsu, D.; Kanaya, N.; Hirano, Y.; Nishizawa, M.; Matsue, T. Electrochemistry (Tokyo, Jpn.) 2003, 71, 439–441. (30) Ufheil, J.; Boldt, F. M.; Bo¨rsch, M.; Borgwarth, K.; Heinze, J. Bioelectrochemistry 2000, 52, 103–110. (31) Wittstock, G.; Schuhmann, W. Anal. Chem. 1997, 69, 5059–5066. (32) Cornut, R.; Lefrou, C. J. Electroanal. Chem. 2008, 1009–1021.
(33) Casal, H. L.; Mantsch, H. H. Biochim. Biophys. Acta, ReV. Biomembr. 1984, 779, 381–401. (34) Fringeli, U. P. Z. Naturforsch., C: J. Biosci. 1977, 32, 20–45. (35) MacPhail, R. A.; Strauss, H. L.; Snyder, R. G.; Elliger, C. A. J. Phys. Chem. 1984, 88, 334–341. (36) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86, 5145–5150. (37) Zawisza, I.; Lipkowski, J. Langmuir 2004, 20, 4579–4589. (38) Bin, X.; Zawisza, I.; Lipkowski, J. Langmuir 2005, 21, 330–347. (39) Atre, S. V.; Liedberg, B.; Allara, D. L. Langmuir 1995, 11, 3882–3893. (40) Bensebaa, F.; Ellis, T. H.; Badia, A.; Lennox, R. B. Langmuir 1998, 14, 2361–2367. (41) Bertilsson, L.; Liedberg, B. Langmuir 1993, 9, 141–149. (42) Evans, S. D.; Goppert-Berarducci, K. E.; Urankar, E.; Gerenser, L. J.; Ulman, A.; Snyder, R. G. Langmuir 1991, 7, 2700–2709. (43) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558–569.
Figure 3. Electrochemical impedance spectroscopy of unmodified (O) and HS-(CH2)15-CH3 modified electrodes: IJP (9), µCP (0) and immersion (4) in 1 mM K3[Fe(CN)6]/0.1 M KCl. E(dc) ) 0.2 V vs Ag|AgCl.
immersion-modified electrodes are comparable to those reported in literature for medium and short immersion time, both spectra show no Warburg lines, indicating good insulation.22,23 On the other hand, the Rct value of µCP-modified electrode is lower than those of IJP and immersion, and a Warburg line at lower frequencies is observed. These facts give again evidence of pinholes in the µCP monolayer. The surface coverage of a monolayer (θ) can be estimated according to eq 1
1 - θ ) Rct0 ⁄ Rct
(1)
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Table 2. Results from Electrochemical Impedance Measurements Rs(C[Rct Zw]) -2
bare golda HDT µCP HDT IJP HDT immersion a
Rs(Q[Rct Zw]) -2
-2
Rct (kΩ cm )
C (µF cm )
Rct (kΩ cm )
Y0 (Ω cm-2 sn)
n
0.281 98.7 667 10800
16.2 1.06 0.995 0.52
0.339 116.48 674 11300
8.2 × 10-6 0.81 × 10-6 2.23 × 10-6 0.95 × 10-6
0.87 0.95 0.993 0.987
The data were obtained from extended frequency window (100 kHz to 10 mHz).
Figure 4. SECM approach curves to glass (2) and to HS-(CH2)15-CH3modified gold (3-5), prepared by immersion (3), prepared by IJP (4), and prepared by µCP (5). Solid lines: fitted theoretical curves according to Cornut and LeFrou32 (curves 3-5) and to Amphlett and Denuault68 (cuves 1 and 6). Curve 6 is given for comparison representing regeneration of the mediator at a diffusion-limited rate at the sample. Experimental conditions: rT ) 13.4 µm, VT ) 0.625 µm s-1, RG ) 10, 1 mM ferrocenylmethanol in 0.1 M Na2SO4.
Figure 5. The PM IRRAS spectra of HS-(CH2)15-CH3 SAM on Au prepared by: immersion (1), IJP (2), and µCP (3) in the CH stretching region. Table 3. Positions and fwhm of CH Stretching and Bending Modes in HS-(CH2)15-CH3 SAMs Prepared by Immersion, IJP, and µCP HS-(CH2)15-CH3 monolayer immersion
µCP
IJP
band
position/ cm-1
fwhm
νas(CH3) νs(CH3) νas(CH2) νs(CH2) δ(CH2)
2964 2878 2918 2850 1470
12 10 12 10 10
2964 2878 2918 2850 1470
11 9 12 10 10
δas(CH3)
1459
8
1459
8
position/ cm-1
fwhm
position/ cm-1
fwhm
2964 2878 2919 2851 1471 1464 or 1470 1458
12 10 13 10 4 8 10 6
orientation of the methylene groups is uniform within the molecules. Intensity variations can then result from different angles between the transition dipole moments and the surface normal.44 The νas(CH2) and νs(CH2) transition dipole moments are orthogonal to each other and orthogonal to the hydrocarbon
chain. If the chain inclines toward the surface, the angle between the surface normal and the transition dipol moments of νas(CH2) or νs(CH2) decreases, leading to a larger intensity of the band. This is seen for the νas(CH2) band of the SAM prepared by µCP and is in line with the expectation from SECM and electrochemical experiments that this monolayer is less ordered (contains more defects) and allows for a larger average inclination of the chains. The CH bending modes between 1450 and 1475 cm-1 are sensitive to interchain interactions hydrocarbon chain packing in crystals and monolayer assemblies.37,45–50 The number and positions of the δ(CH2) modes depend on the way of monolayer preparation (Table 3). A single absorption for the δ(CH2) centered at 1468-1469 cm-1 was found in monolayers prepared by immersion and IJP indicating an amorphous or a hexagonally packed layer. SAMs prepared by µCP showed different appearances (single or split methylene bending mode) and occasionally changed during the measurement. This phenomenon requires further studies that are beyond the scope of this communication on IJP samples. III.2. Inkjet-Printed SAM Structures. Inkjet-printed structures are build from aligned single droplets. In this paragraph we present first studies on the optimization of the size and corner definition of conductive single disks prepared by the IJP method. In the second part, we look at larger structures made from aligned thiol disks. In order to study the single disks, we printed areas using 1% color density, that is, thiol spots corresponding to the ink disks on paper shown in Figure 1a. We used mercaptopropionic acid (HS-(CH2)2-COOH) for the dots, and we embedded them in a SAM consisting of HS-(CH2)15-CH3, using either sequential printing/backfilling or simultaneous printing from two channels. The length of the HS-(CH2)2-COOH molecule is short enough to allow electron transfer whereas HS-(CH2)15-CH3 passivates the surface electrochemically as shown in the previous section. Such permeation and blocking behavior provide the contrast for imaging with SECM.18,24–31 The quality of inkjet-printed objects in the 50 to several 100 µm range is influenced by different parameters, such as (i) the droplet size, (ii) the spreading of the droplet after arrival on the gold surface, (iii) the diffusion of thiol molecules from printed to naked gold areas via the gas phase or via surface gliding after evaporation of the solvent, as well as (iv) by the precise positioning of a series of droplets on the substrate. The droplet size (i) is (44) Lippert, R. J.; Lamp, B. D.; Porter, M. D., Specular Reflection Spectroscopy. In Modern Techniques in Applied Molecular Spectroscopy; Mirabella, F. M., Ed.; John Wiley and Sons, Inc.: New York, 1998; pp 83-126. (45) Cameron, D. G.; Casal, H. L.; Gudgin, E. F.; Mantsch, H. H. Biochim. Biophys. Acta, Biomembr. 1980, 596, 463–467. (46) Cameron, D. G.; Gudgin, E. F.; Mantsch, H. H. Biochemistry 1981, 20, 4496–4500. (47) Casal, H. L.; Mantsch, H. H.; Cameron, D. G.; Snyder, R. G. J. Chem. Phys. 1982, 77, 2825–2830. (48) Koyama, Y.; Yanagishita, M.; Toda, S.; Matsuo, T. J. Colloid Interface Sci. 1977, 61, 438–445. (49) Pelletier, I.; Laurin, I.; Buffeteau, T.; Desbat, B.; Pezolet, M. Langmuir 2003, 19, 1189–1195. (50) Snyder, R. G. J. Mol. Spectrosc. 1961, 7, 116–144.
Inkjet-Printed Thiol SAM Structures on Gold
Figure 6. Single inkjet disks prepared at 1% color density: (a) optical micrograph of Epson color on paper; (b) optical micrograph of HS-(CH2)10-COOH/glycerol ink on gold (washing step omitted); (c-e) SECM images of HS-(CH2)2-COOH ink on gold backfilled with HS-(CH2)15-CH3 using [HS-(CH2)2-COOH] ) 0.5 mM (c), 1 mM (d), 3 mM (e). Experimental conditions: rT ) 5.5 µm, VT ) 5.5 µm s-1 (c), 5.1 µm s-1 (d), 7.1 µm s-1 (e), redox system 1 mM ferrocenylmethanol in 0.1 M Na2SO4, d ≈ 8-10 µm.
controlled by the piezo of the Stylus Photo R200 and is in the range of 3 pL. Its shape resembles that of a flying comet. During flight the front meniscus part may be broken up into a primary droplet while the tail part becomes a smaller droplet, that is, a satellite.51,52 The Epson printer shoots exclusively such twins of droplets with the original ink and this happens also with thiols in ethanol/glycerol.20 When an ejected pair of droplets impacts the surface, the liquid spreads outward reaching its final diameter. Paper has a great in-depth absorbability which is not the case for gold. This leads inherently to a larger spreading diameter on gold as compared to paper. Besides the excess energy of the droplet and the in-depth absorbability properties of the substrate, the viscosity and surface tension of the ink also affect the processes.52 The ink viscosity was adjusted by adding glycerol to ethanol as a cosolvent. Optimum results (good droplet formation and small spreading diameter) were achieved with an ethanol/glycerol mixture of 6:1 (v/v) leading to a viscosity of 2.5 cPa s (slightly below the value of 3.9 cPa s measured for the original Epson ink). Besides constraining the droplet spreading, the glycerol addition makes it possible to image the droplets optically. It can be visualized in the streak of light under the microscope, if after printing the washing step is omitted (Figure 6b). Glycerol appears as ∼10 µm sized droplets at the periphery of the ellipsoidal disks. If compared to the original Epson ink droplets on photopaper consisting of a pair of droplets (Figure 6a), the shape of the ethanol/glycerol ink on gold suggests that the pair has coalesced leading to the ellipsoidal shape observed in Figure 6b. Droplets created with different thiol concentrations in the ink have been (51) Notz, P. K.; Basaran, O. A. J. Fluid Mech. 2004, 512, 223–256. (52) Park, H.; Carr, W. W.; Zhu, J. Y.; Morris, J. F. AIChE J. 2003, 49, 2461–2471.
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Figure 7. IJP structures. (a, b) Optical micrograph of 300 µm band structures, (a) Epson ink on paper and (b) HS-(CH2)10-COOH SAM line embedded in HS-(CH2)15-CH3 on gold prepared in a single print job developed by water vapor condensation. (c) SECM image of a 300 µm band structure of HS-(CH2)11-NH2 embedded in HS-(CH2)15-CH3. (d) SECM image of three HS-(CH2)2-COOH lines of 500 µm width at 1000 µm spacing embedded in HS-(CH2)15-CH3. rT ) 13.5 µm (c), 45 µm (d), VT ) 4.2 µm s-1 (c), 16.7 µm s-1 (d), d ) 14 µm (c), 20 µm (d), redox system 1 mM K4[Fe(CN)6] in 0.1 M phosphate buffer (pH 2) (c), and 0.5 mM ferrocenylmethanol in 0.1 M phosphate buffer and 0.1 M NaCl (d).
analyzed by SECM. With 0.5 and 1 mM thiol concentration, pairs of separate droplets could be observed (Figure 6, panels c and d). The size of the spots (extracted from profiles, fwhm, see Supporting Information VI) is approximately 30 µm for the 0.5 mM ink and 40-50 µm for the 1 mM ink, and their separation (peak to peak distance) is 70 or 95 µm, respectively. At an ink concentration of 3 mM, there is only one larger, ellipsoidal spot visible (Figure 6e). The sizes of the optically observed glycerol ellipsoid in Figure 6b and the ellipsoidal spot observed with SECM for 3 mM ink are comparable, whereas the pairs of spots observed with SECM for 0.5 and 1 mM ink (30 µm and 40-50 µm diameter, respectively) (Figure 6, panels c and d) resemble those of original ink on paper (Figure 6a). As mentioned before, for glycerol we observe a coffee stain effect, but this is not the case for the thiol in the same droplet.14,53,54 Thus, thiol absorption from the ink onto the gold seems to be fast as compared to the lateral spreading of the ethanol/glycerol. This is nicely reflected by the series of SECM measurements with varying thiol concentrations (Figure 6c-e). In other words, thiol SAM disk diameters comparable to the impact diameter of the droplet can be achieved for well-adjusted minimum thiol concentrations. Having optimized the droplet and thiol disk formation on gold, we studied the possibility to fabricate inkjetted SAM structures. In Figure 7b we present an optical micrographs of a 300 µm (53) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Nature 1997, 389, 827–829. (54) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Phys. ReV. E 2000, 62, 756–765.
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broadband of HS-(CH2)10-COOH embedded in HS-(CH2)15CH3 on gold (prepared in a single print job, developed by water condensation), as compared to the same band, but using Epson color ink printed on photopaper (Figure 7a). The band edge definition in panels a and b of Figure 7 is similar, both showing arcs resulting from overlapping 40 µm HS-(CH2)10-COOH and HS-(CH2)15-CH3 disks. The rough surface within the structure in Figure 7b is an artifact of the “water vapor condensation development”, as can be seen from the SECM measurement (Figure 7c). There is no large current variation within the regions, but a wavy contour line between the HS-(CH2)2-COOH and HS-(CH2)15-CH3 modified regions. Notably, the structure in Figure 7b was prepared in a single print job using two channels for the two thiols. III.3. Fabrication of Electrochemical Devices: Band Microelectrodes and Microelectrode Arrays. The results described so far enabled us to try some obvious applications. Here, we show the performance of microelectrode arrays with variable average spot distances and that of a band microelectrode with variable bandwidth, both prepared by IJP. Microelectrodes are defined as electrodes of a size similar to or smaller than the diffusion layer thickness.55 An interesting characteristic of microelectrodes is their increased sensitivity due to amplification of the mass transport at the electrode interface which is related to the radial mass transport at the electrode perimeter. Arrays of regular or random microelectrodes show similar phenomena as long as the diffusion layers do not overlap. Different fabrication techniques for the production of microelectrode arrays are known, that is, e-beam lithography,56 photolithography,57,58 electrochemical selective desorption,59 laser ablation,60 screen printing,61 and soft lithography.62,63 However, there exists no report on the construction of microelectrode arrays by IJP so far. IJP of thiols under our conditions is limited to smallest structures of 40 µm diameter or ellipsoidal structures of 40 × 80 µm because of the satellite formation discussed above. In Figure 8 we present the overlaid electrochemical responses ferrocenylethanol at random microelectrode disk arrays with active spots consisting of HS-(CH2)2-COOH at 100, 50, 10, 4, and 1% color density. In this series the average dot-dot distance (neglecting the existence of twin dots) changes from overlapping (100 and 50%) to ∼80 µm (10%), 110 µm (4%), and 390 µm (1%), and the surface coverage decreases from 100%, via >80, 23, 8.9 to 1.9% (Figure 1 and Table 1). The cyclic voltammograms (CVs) are typical for a microarray electrode dominated at least partially by radial diffusion: (i) the peak current ratio does not reflect the active surface ratio, for example, ipa(a)/ipa(d) is ∼1.4 but Γ(a)/ Γ(d) is ∼11, and (ii) the ipa/ipc ) 1 is fulfilled only for Γ ) 100% coverage but decreases to an immeasurable low value (55) Stulik, K.; Amatore, C.; Holub, K.; Marecek, V.; Kutner, W. Pure Appl. Chem. 2000, 72, 1483–1492. (56) Rodriguez, B. B.; Schneider, A.; Hassel, A. W. J. Electrochem. Soc. 2006, 153, C33-C36. (57) Ordeig, O.; Banks, C. E.; Davies, T. J.; del Campo, F. J.; Munoz, F. X.; Compton, R. G. Anal. Sci. 2006, 22, 679–683. (58) Revzin, A. F.; Sirkar, K.; Simonian, A.; Pishko, M. V. Sens. Actuators, B 2002, 81, 359–368. (59) Nishizawa, M.; Sunagawa, T.; Yoneyama, H. J. Electroanal. Chem. 1997, 436, 213–218. (60) Tender, L. M.; Worley, R. L.; Fan, H. Y.; Lopez, G. P. Langmuir 1996, 12, 5515–5518. (61) Craston, D. H.; Jones, C. P.; Williams, D. E.; Elmurr, N. Talanta 1991, 38, 17–26. (62) He, H. X.; Li, Q. G.; Zhou, Z. Y.; Zhang, H.; Li, S. F. Y.; Liu, Z. F. Langmuir 2000, 16, 9683–9686. (63) Wilbur, J. L.; Kumar, A.; Kim, E.; Whitesides, G. M. AdV. Mater. 1994, 6, 600–604.
Rianasari et al.
Figure 8. Cyclic voltammograms of 1 mM ferrocenylethanol in 0.1 M KCl at HS-(CH2)2-COOH random array microdisk electrodes printed at different color density/average center dot-dot distance/surface coverage: (a) 100%/overlapping/100%, (b) 50%/partially overlapping/ 80%, (c) 10%/80 µm/23%, (d) 4%/110 µm/8.9%, (e) 1%/360 µm/1.9% and backfilled with HS-(CH2)15-CH3.
Figure 9. Voltammograms of ferrocenylethanol (1 mM) in 0.1 M KCl at 100 mV s-1on band electrodes prepared by IJP using two-channel printing 1 mM HS-(CH2)2-COOH in EtOH/glycerol (channel 1) and HS--(CH2)15-CH3 in EtOH/glycerol (channel 2), band widths: 300 µm (a), 500 µm (b), 1 mm (c), 3 mm (d), 5 mm (e), and 10 mm (f).
for Γ ) 1.9%.64 Studies over a broad range of scan rates and disk densities did not reveal a pure radial diffusion case with the expected plateau current independent of scan rate.65 However, this is probably related to the size of the disks as compared to their distance and to the fact that many distances are active, that is, the constant distance between the disks that constitute a pair and the variable twin-twin distance. Thus the CVs presented can be assigned to Compton’s category 4 or mixed category 3 and 4 depending on the average twin-twin distance.64 A series of single band electrodes with different band widths (10 mm, 5 mm, 3 mm, 1 mm, 500 µm and 300 µm) were prepared similar to the one presented in Figure 7, panels a and b, and again using two channels for printing the conductive HS-(CH2)2COOH and the insulating HS-(CH2)15-CH3. The CVs of ferrocenylethanol at the band electrodes are shown in Figure 9. The currents in the CVs are reported as current densities with respect to the active area. The voltammogram of the 300 µm band electrode shows the highest current density and comes (64) Davies, T. J.; Compton, R. G. J. Electroanal. Chem. 2005, 585, 63–82. (65) Finklea, H. O.; Snider, D. A.; Fedyk, J.; Sabatani, E.; Gafni, Y.; Rubinstein, I. Langmuir 1993, 9, 3660–3667.
Inkjet-Printed Thiol SAM Structures on Gold
Figure 10. SECM line scans at different pH values across a HS-(CH2)11-NH2 SAM pattern backfilled with HS-(CH2)15-CH3: curve 1 (filled symbols), pH 2.8 (open configuration); cuve 2 (open symbols), pH 10.6 (closed configuration). Experimental parameters: rT ) 5.5 µm, VT ) 7.14 µm s-1 (curve 1) and 11.1 µm s-1 (curve 2), d ≈ 10 µm, working solution: 1 mM K3[Fe(CN)6] in 0.1 M KCl, pH adjusted with 0.1 M HCl or 0.1 M NaOH, respectively.
closest to the sigmoidal wave shape, expected for microelectrodes, while the response of the 1 mm band electrode is identical with that of the macroelectrode. III.4. IJP Construction of a Band Electrode with pH Switchable Electron Transfer Rate. Having shown the feasibility of the microelectrode array construction by IJP, and holding in mind the possibility of using different thiols in a single print job, another application becomes accessible in a very economic way, that is, electrochemical sensing. The socalled “ion gating mechanism” is based on electrostatic interactions of the protonable terminal group of the SAM with the redox-active probe ions in solution.66,67 Boldt et al. have demonstrated the use of SECM approach curves in order to probe the pH-dependent permeability of SAMs.24 In the following we exemplify the pH response of an HS-(CH2)11-NH2-modified, 500 µm wide band electrode toward the redox probe ion [Fe(CN)6].3 The band electrode was created by IJP, while the remaining area was backfilled with HS-(CH2)15-CH3. At low pH, the ammonium-terminated HS-(CH2)11-NH3+ SAM is positively charged and permeable for [Fe(CN)6]4-/3-. The [Fe(CN)6]4 generated at the UME can thus be reoxidized at the gold electrode leading to an increased current at the UME. The HS-(CH2)15CH3 SAM blocks the mediator regeneration almost completely (66) Degefa, T. H.; Schoen, P.; Bongard, D.; Walder, L. J. Electroanal. Chem. 2004, 574, 49–62. (67) Schoen, P.; Degefa, T. H.; Asaftei, S.; Meyer, W.; Walder, L. J. Am. Chem. Soc. 2005, 127, 11486–11496. (68) Amphlett, J. L.; Denuault, G. J. Phys. Chem. B 1998, 102, 9946–9951.
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independent of pH. A SECM line scan across the structure clearly shows the different permeability of the SAMs (Figure 10, curve 1). If the pH is changed to 10.28, the amino groups of the H2N-C11H22-SH SAM are deprotonated, the permeability of the SAM drops, and the contrast in the SECM line scan recorded at the same tip-to-sample separation d is strongly reduced (Figure 10, curve 2). The HS-(CH2)11-NH2 SAM is a less effective barrier for [Fe(CN)6]4-/3- than the HS-(CH2)15-CH3 SAM. This may be due to the length of the hydrocarbon chain (C11 compared to C16) and/or to the different head groups NH2 and CH3. It is in agreement with earlier reports showing incomplete blocking of the same mediator by the H2N-C11H22-SH SAM on macroscopic electrodes.66
IV. Conclusions Inkjet printing shows very interesting properties for the preparation of patterned thiol SAMs on gold: (i) all structures can be prepared using a PC program, and the print command starts the production of the structured SAM; (ii) the quality of the resulting SAM as judged by electrochemical passivation and PM IRRAS is between that achieved by µCP and self-assembling from solution; (iii) different thiols (as many as reservoirs/channels available) can be printed in a single print job; (iv) all structures are build from juxtaposed/overlapping disks; (v) the disk size is governed by the droplet volume (∼2-3 pL for current desk top ink jet printers, corresponding to ∼50 µm thiol disks) and spreading of the thiol ink on gold. (vi) Thiol adsorption occurs at a similar rate as lateral spreading. Thus the thiol disk size can be controlled by the thiol concentration and by viscosity-adjusting additives. (vii) Random microarrays of conductive spots with different spot density as well as band electrodes with different width can be prepared easily. (viii) Functional patterns such as a 500 µm pH-switchable broadband electrode embedded in a passivating SAM environment are also relatively easily prepared. Acknowledgment. I.R. thanks the Federal State of Niedersachsen and the Graduate college 695 for financial support. I.Z. is supported by DFG Project ZA 543/1. We thank Professor H. O. Finklea for fruitful discussions. Supporting Information Available: Ink calibration, printer modification, CVs, EIS, SECM approach curve analysis, selection of mediator for SECM experiments, and extracted line profiles of single droplet SECM images. This material is available free of charge via the Internet at http://pubs.acs.org. LA800711M
