Langmuir 2004, 20, 7351-7354
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Chemical Lithography by Ag-Nanoparticle-Mediated Photoreduction of Aromatic Nitro Monolayers on Au Kwan Kim* and Inhyung Lee Laboratory of Intelligent Interfaces, School of Chemistry and Molecular Engineering, Seoul National University, Seoul 151-742, Korea Received June 10, 2004. In Final Form: July 19, 2004 Patterned, amine-terminated monolayers can be fabricated from 4-nitrobenzenethiol (4-NBT) monolayers simply by irradiating under ambient conditions with visible laser after spreading Ag nanoparticles onto selected regions of the 4-NBT monolayers on Au. Au nanoparticles were adsorbed selectively onto the amine groups produced by photoreaction, and polyaniline was found to grow exclusively at the amine groups when electrochemical polymerization was conducted using the patterned substrate as the working electrode. These observations clearly support our previous contention that Ag nanoparticles can act as moderate photoelectron emitters.
1. Introduction Two-dimensional nano- to mesoscale structures for use in laboratory studies and commercial applications can be realized by controlling chemical functionalities at the nanoscale. Site-selective alternations of surface chemistry provide a means of engineering surfaces that can be used in sensitive optoelectronic and biomimetic devices and sensors. Patterning of the self-assembled monolayers (SAMs) is an excellent strategy for preparing templates that possess variable surface chemical properties. Modification of SAMs can be conducted either by conventional1 or state-of-art destructive lithographic processes,2 including atomic-beam3 and proximal-probe methods.4 Recently, nondestructive processes have also been reported. For instance, chemical reactions can be induced in the terminal groups of organosilane by a catalytically active transitionmetal-coated atomic force microscope (AFM) tip.5,6 A terminal group of organothiol monolayers on gold can also be converted using low-energy electron beams.7 We have recently demonstrated that patterned binary monolayers can be prepared on silver by inducing a surface-induced photoreaction for a SAM of benzyl phenyl sulfide (BPS) on Ag.8 Irradiation by an argon ion laser (514.5 nm) of a selected region of BPS SAMs on silver led to the formation of benzenethiolate monolayers. The unilluminated sulfides were thereafter replaced with thiols or carboxylic acids by a self-assembly process. The method can produce very robust binary monolayers, but there is one inherent difficulty in that the region of benzenethiolate layers develops nanosized vacancies owing to the removal of the decomposition product, that is, a benzyl radical. A new system was, thus, needed that could preserve the * To whom all correspondence should be addressed: phone +822-8806651; fax +82-2-8891568; e-mail
[email protected]. (1) Collins, R. J.; Bae, I. T.; Scherson, D. A.; Sukenik, C. N. Langmuir 1996, 12, 5509. (2) Lercel, M. J.; Craighead, H. G.; Parikh, A. N.; Seshadri, K.; Allara, D. L. Appl. Phys. Lett. 1996, 68, 1504. (3) Thywissen, J. H.; Johnson, K. S.; Younkin, R.; Dekker, N. H.; Berggren, K. K.; Chu, A. P.; Prentiss, M.; Lee, S. A. J. Vac. Sci. Technol., B 1997, 15, 2093. (4) Liu, G.-Y.; Xu, S.; Qian, Y. Acc. Chem. Res. 2000, 33, 457. (5) Blackledge, C.; Engebretson, D. A.; McDonald, J. D. Langmuir 2000, 16, 8317. (6) Maoz, R.; Cohen, S. R.; Sagiv, J. Adv. Mater. 1999, 11, 55. (7) Eck, W.; Stadler, V.; Geyer, W.; Zharnikov, M.; Go¨lzha¨user, A.; Grunze, M. Adv. Mater. 2000, 12, 805. (8) Lee, I.; Han, S. W.; Kim, C. H.; Kim, T. G.; Joo, S. W.; Jang, D.-J.; Kim, K. Langmuir 2000, 16, 9963.
overall structural integrity of the primary monolayer even after the surface-induced photoreaction. We found that nitro groups on aromatic SAMs on silver could be converted to amino groups under irradiation with visible light, thereby facilitating the preparation of patterned binary monolayers on silver with selectable surface chemical properties.9 Site-specific chemical reactions and patterned crystal nucleation were demonstrated using the patterned monolayers. Unfortunately, however, these visible laserinduced surface reactions hardly take place for SAMs assembled on substrates other than Ag. For the abovementioned lithographic strategy to function as a general tool for preparing patterned binary monolayers, the limitations on the type of substrate to be used have to be overcome. Very recently, we have discovered that Ag nanoparticles in physical contact with organic films can induce a photolytic reduction of the organic moiety simply by irradiating it with a visible laser under ambient conditions.10 This indicates that Ag nanoparticles can act as moderate photoelectron emitters. It also suggests that Ag nanoparticles can be used in preparing patterned monolayers on diverse substrates. In this work, we confirmed the feasibility of this process by preparing amine-groupterminated patterned monolayers on Au using Ag nanoparticles as photoreducing catalysts via visible laserinduced photoreaction. The prospect of real applications is considered to be very high because fabrication of such patterned monolayers on engineered substrates is essential for the development of molecule-based optoelectronic and biomimetic devices and sensors. 2. Experimental Section We purchased 4-nitrobenzenethiol (4-NBT) and 4-aminobenzenethiol (4-ABT) from Aldrich and used them as received. Other chemicals, unless specified, were reagent grade, and ultrapure water (resistivity greater than 18.0 MΩ‚cm) was used when preparing the aqueous solutions. The Au substrates were prepared by resistive evaporation of titanium and gold at about 10-6 Torr on batches of glass slides. SAMs of 4-NBT or 4-ABT were prepared by immersion of the gold substrates in 1 mM ethanolic solution of adsorbate for about 3 h. To prepare a silver sol solution, a silver plate (Aldrich, >99.99%) was placed at the bottom of a glass vessel filled with 3 mL of triply distilled water and then irradiated using a focused (9) Han, S. W.; Lee, I.; Kim, K. Langmuir 2002, 18, 182. (10) Kim, K.; Lee, I.; Lee, S. J. Chem. Phys. Lett. 2003, 337, 201.
10.1021/la0485677 CCC: $27.50 © 2004 American Chemical Society Published on Web 07/30/2004
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beam of a Nd:YAG laser (Continuum Surelite II-10, at 1064 nm). The typical pulse energy, repetition rate, pulse duration, and irradiation time were 50 mJ, 10 Hz, 6 ns, and 30 min, respectively. The final concentration of the solution, consisting of Ag nanoparticles with an average diameter of 20 nm, was estimated to be ∼1 nM. Au sol was prepared following the recipes of Lee and Miesel.11 Briefly, 95 mL of chlorauric acid (HAuCl4) solution containing 5 mg of Au was brought to boil. A solution of 1% sodium citrate (5 mL) was then added to the HAuCl4 solution under vigorous stirring, and boiling was continued for ∼30 min. For selective spreading of Ag nanoparticles onto the 4-NBT SAMs on Au, the so-called µ-molding technique was employed;12 that is, a Ag nanoparticle solution was allowed to flow by capillary action into the holes of a prepatterned poly(dimethylsiloxane) (PDMS) stamp. To induce visible laser-induced photoreaction, the Ag-nanoparticle-spread 4-NBT SAMs were irradiated, after removing the PDMS stamp, with a 20 mW Ar+ laser at 514.5 nm (Melles-Griot model 351MA520). After removing the Ag nanoparticles by sonication in water, we performed a modification reaction with stearic acid (STA) by overnight incubation of the laser-irradiated sample in 0.01 M STA solution in N,N-dimethylformamide containing 0.02 M 1-ethyl-3-(3-(dimethylamino)propyl)carbodiimide as a coupling reagent.13 As an alternative route, the laser-irradiated sample was electrochemically treated in aniline solution using a CH Instrument model 600A potentiostat. The AFM images of the prepared films on Au were obtained using a Digital Instrument model Nanoscope IIIa scanning probe microscope. Raman spectra of the films were obtained by using a Renishaw Raman system model 2000 spectrometer equipped with an integral microscope (Olympus BH2-UMA). The 514.5nm line from a 20 mW Ar+ laser (Melles-Griot model 351MA520) or the 632.8-nm line from a 17 mW He/Ne laser (Spectra Physics model 127) was used as the excitation source. Raman scattering was detected with 180° geometry with a Peltier cooled (-70 °C) charged-coupled device camera (400 × 600 pixels). The typical laser power at the sampling position was 0.2 mW.
3. Results and Discussion Our earlier observation of a visible laser-induced nitroto-amine conversion was made by surface-enhanced Raman spectroscopy (SERS) for 4-NBT SAMs assembled on a SERS-active Ag substrate.9 In this work, we have examined first of all whether such a photoreaction occurs on Ag regardless of its detailed surface roughness. We irradiated 4-NBT SAMs assembled on a vacuum-evaporated Ag substrate (root-mean-square roughness determined by AFM was ∼1.5 nm) with an Ar+ laser (17 mW, spot size ∼ 1 µm) for 30 min, but any photoreaction was barely discerned using Raman spectroscopy, because of the SERS-inactive characteristics of the Ag substrate. No clear contrast was obtained either in AFM (and lateral force microscope) images taken to judge the occurrence of the nitro-to-amine conversion. The amide coupling reaction was, thus, conducted using STA to obtain a much clearer AFM image, but it turned out that visible laser-induced nitro-to-amine conversion did not occur favorably on a smooth Ag substrate. Referring to our recent observation,10 we have, thus, attempted to induce nitro-to-amine conversion of the 4-NBT SAMs on a smooth Ag substrate using Ag nanoparticles laid thereon as photoreduction catalysts. Initially, Ag nanoparticles were spread onto selective regions of the 4-NBT SAMs using a micropipet, and then the Ag substrate was exposed to an Ar+ laser (at 514.5 nm) for 30 min; both the diameter of the spread region of Ag nanoparticles and the spot size of the Ar+ laser were ∼1 (11) Lee, C. P.; Miesel, D. J. Chem. Phys. 1982, 86, 3391. (12) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998, 37, 550. (13) (a) Katz, E.; Itzhak, N.; Willner, I. Langmuir 1993, 9, 1392. (b) Checkik, V.; Crooks, R. M.; Stirling, C. J. M. Adv. Mater. 2000, 12, 1161.
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Figure 1. AFM image of 4-NBT SAMs on smooth Ag acquired after STA coupling to the amine group produced by Ar+ laserinduced photoreaction in the presence of overlaid Ag nanoparticles.
µm. Thereafter, the Ag nanoparticles were removed by sonication in water, and then the amide coupling reaction was conducted using STA. Figure 1 shows the AFM image obtained after amide coupling, and the image clearly indicates that the visible laser-induced nitro-to-amine conversion does occur by the mediation of Ag nanoparticles. As mentioned in the introductory section, the Ar+ laserinduced surface reaction does not take place when 4-NBT is assembled on a Au substrate; No reaction is detected either from SERS or by an amide-coupling reaction. Accordingly, referring to the above experiment conducted on a smooth Ag substrate, we have attempted to induce a nitro-to-amine conversion on Au using Ag nanoparticles as photoreduction catalysts, and its feasibility has been examined by taking SERS spectra. We can report that although the vacuum-evaporated, smooth Au substrate is SERS-inactive, the SERS spectrum of 4-NBT can be obtained via a silver nanoparticle-mediated electromagnetic (EM) field enhancement mechanism, once Ag nanoparticles are overlaid onto the SAMs; the Ag-nanoparticles-to-Au-substrate coupled EM enhancement is, thus, operative. Figure 2a shows the SERS spectrum of 4-NBT on gold acquired in this way with a 632.8-nm line of a He/Ne laser as an excitation source. The overall spectral feature is nearly the same as that of 4-NBT on silver obtained with spinning to minimize the occurrence of photoreaction (data not shown). The prominent peak at 1345 cm-1 is due to the symmetric stretching vibration of the nitro group [νs(NO2)], and other distinct peaks in Figure 2a can be attributed to either the C-C stretching mode, that is, at 1573 cm-1, or the in-plane C-H bending modes, that is, at 1110 and 1082 cm-1. This clearly indicates that photoreaction of 4-NBT is not noticeably induced by irradiating it with 632.8-nm laser light. Noticeable SERS spectral changes take place, however, as can be seen in Figure 2b, when an Ar+ laser at 514.5 nm is used as the excitation source. The νs(NO2) peak is no longer identified in Figure 2b, and several new peaks appear, for instance, at 1436, 1392, 1192, and 1143 cm-1. This observation is exactly the same as that for 4-NBT adsorbed on SERS-active Ag and then irradiated with the 514.5 nm radiation. As reported in the previous work,9 the two peaks at 1436 and 1392 cm-1 can be assigned to
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Figure 2. SERS spectra of 4-NBT SAMs on Au taken after spreading Ag nanoparticles onto the SAMs: (a) with the 632.8nm line of a He/Ne laser and (b) with 514.5-nm line of an Ar ion laser. (c) Authentic SERS spectrum of 4-ABT on Ag taken using 514.5-nm excitation.
the C-C stretching plus the in-plane C-H bending modes of 4-ABT, while the 1192 and 1143 cm-1 peaks are attributed to the in-plane C-H bending modes of 4-ABT; for reference, in Figure 2c is shown the SERS spectrum of 4-ABT itself adsorbed on Ag. This clearly indicates that photoreaction of 4-NBT can indeed be induced even on Au, by using Ag nanoparticles as photoreduction catalysts, and the reaction product must be 4-ABT. (The nitro group can in fact be reduced to other products such as nitroso, hydroxylamine, and azo compounds,14 but no peak due to any such compounds is identified at all in our SERS spectrum. The exclusive formation of the amine group is probably due to the catalytic effect of Ag nanoparticles.) Although Ag nanoparticles are physically in contact with the SAMs, a thin water film is presumed to form in the interface for ready Ag-to-SAM electron transfer. Patterned organic monolayers may then be produced even on gold by irradiating with an Ar+ laser after spreading Ag nanoparticles only at desired regions of the 4-NBT SAMs. Ag nanoparticles are, thus, spread selectively onto the 4-NBT SAMs on Au via the µ-molding process. After removing the PDMS mold, the Au substrate was moved at a rate of 0.1 µm/s under exposure of an Ar+ laser light. After that, the Ag nanoparticles were removed by sonication in water. We then immersed the Au substrate in an aqueous Au nanoparticle solution, expecting that the Au nanoparticles would be immobilized exclusively on the amine-terminated region. A typical AFM image obtained after the process is shown in Figure 3. The image clearly shows that Au nanoparticles were immobilized only at specific regions of the monolayer. The dimensions of the pattern closely matched the actual sizes of the PDMS pattern used as a template for the Ag nanoparticle array. This proves that the present method can serve as a general tool for the manufacture of amine-functionalized, patterned monolayers on various engineered solid substrates. In fact, similar amine-functionalized, patterned monolayers can also be fabricated via electron-beam lithography, but that method requires the ultrahigh vacuum system while our method can readily be conducted under ambient conditions. (14) Tomilov, A. T.; Mairanovskii, S. G.; Fioshin, M. Y.; Smirnov, V. A. Electrochemistry of Organic Compounds; Halstead: New York, 1972.
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Figure 3. AFM image of Au nanoparticles immobilized on an amine-terminated substrate. See text.
Figure 4. AFM image acquired after electrochemical polymerization of aniline using the patterned, amine-functionalized substrate as the working electrode.
The formation of patterned, amine-terminated SAMs was confirmed further by subsequent electrochemical reactions conducted in a 0.01 M aniline solution in 0.5 M HClO4 using the patterned SAMs as a working electrode. It is well-known that surface-adsorbed 4-NBT is reduced at below -0.650 V while surface-adsorbed 4-ABT is oxidized at above 0.730 V versus saturated calomel
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electrode (SCE). In contrast, aniline in the solution phase is oxidized at above 1.05 V.15 This suggests that polyaniline may be grown preferentially at the sites of patterned 4-ABT by holding the electrode potential at ∼0.730 V. At the latter potential, neither the precipitation of polyaniline in the solution phase nor the growth of polymers at the adventitious defect sites of the 4-ABT SAMs can occur, because the potential is too low for aniline cation radicals to form. On these grounds, the potential of the patterned SAMs was cycled between -0.200 and +0.760 V versus SCE at a scan rate of 0.100 V‚s-1 for 15 min using a platinum spiral wire as the counter electrode. In Figure 4, a tapping mode AFM image of the resulting electrochemically treated SAMs is shown. The AFM image clearly reveals selectively grown polyanilines which are 3-µm wide and ∼50-nm high. In summary, we have demonstrated that a site-selective reduction can be induced for 4-NBT SAMs by irradiating with an Ar+ laser after spreading Ag nanoparticles onto (15) Hayes, W. A.; Shannon, C. Langmuir 1998, 14, 1099.
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the desired regions of the SAMs. The reaction takes place via the photoelectron transfer from the Ag nanoparticles to the SAMs. Because the substrate itself is a passive material, even a dielectric substrate such as a silicon wafer can be used as the substrate of patterning. The Ag particlemediated photoreaction is also nearly independent of the extent of flatness of the substrate onto which SAMs are assembled. Although the finest resolution that can be achieved at the moment is of the order of few micrometers, a rapid improvement of drop-on-demand devices may allow the ready fabrication of even the submicrometer-scaled patterns in the near future. It is hoped that the present method turns out to be a cost-effective, environmentally friendly means of fabricating patterned, amine-functionalized monolayers. Acknowledgment. This work was supported by the Ministry of Science and Technology of the Republic of Korea (Nano Project No. M10213240001-02B1524-00210). LA0485677
