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Improved Photoreaction Yields for Soft Ultraviolet Photolithography in Organothiol Self-Assembled Monolayers Panida Prompinit,† Ammathnadu S. Achalkumar,‡ Xiaojun Han,† Richard J. Bushby,‡ Christoph Wa¨lti,§ and Stephen D. Evans*,† Molecular and Nanoscale Physics, School of Physics and Astronomy, UniVersity of Leeds, Leeds LS2 9JT, U.K., Self-Organizing Molecular Systems (SOMS) Centre, UniVersity of Leeds, Leeds LS2 9JT, U.K.and Institute of MicrowaVes and Photonics, School of Electronic and Electrical Engineering, UniVersity of Leeds, Leeds LS2 9JT, U.K. ReceiVed: August 17, 2009; ReVised Manuscript ReceiVed: October 15, 2009
Patterned surfaces can be obtained by soft UV (365 nm) irradiation of thiol-on-gold SAMs (self-assembled monolayers) terminated with ortho-nitrobenzyl-protected carboxylic acid groupings. However, direct irradiation in air leads to incomplete photolysis (350 nm) holds more promise in terms of control over the quality of the SAM obtained, compatibility with conventional lithography, and compatibility with biological systems. Most soft-UV methods are based on the photodeprotection of ortho-nitrobenzyl derivatives generating a hydroxyl group,18 an amine group,11,12,14 or a carboxylic acid group (Figure 1).13,15,19-21 The disadvantage of the ortho-nitrobenzyl-based methods is that, whereas in solution the yield of this reaction is very high, in the SAM environment the yield is only moderate, since a competing photoreaction takes place where the nitro groups are reduced to amines (see Figure 1).15 Incomplete photodeprotection has been reported by Nakagawa,12 de Campo,11 Whitesides,8 Jonas,22 Fukushima,18 and also in our previous papers.14,15 Although the detailed mechanisms remain a matter for dispute it has been known for several years that the photolysis of ortho-nitrobenzyl compounds is very sensitive to the pH of the medium.23 This has led the group of Besson and Moore to investigate the photolysis of orthonitrobenzylester-terminated silane SAMs under a catalytic adlayer of diluted HCl in methanol.19-21 This acid catalysis leads * To whom correspondence should be addressed. Phone: +44 113 343 3852. Fax: +44 113 343 3900. E-mail:
[email protected]. † Molecular and Nanoscale Physics, School of Physics and Astronomy, University of Leeds. ‡ Self-Organizing Molecular Systems (SOMS) Centre, University of Leeds. § Institute of Microwaves and Photonics, School of Electronic and Electrical Engineering, University of Leeds.
to a cleaner photodeprotection. However, in our experience, it also leads to methyl ester formation and is thus not an ideal way to improve the ortho-nitrobenzyl photolysis yield. In this paper, we report the influence of a range of dilute acid catalyst solutions on the photodeprotection of orthonitrobenzyl-terminated thiol-on-gold SAMs (Figure 1) and compare these results with the photolysis in air.15 We find that using a suitable catalyst leads to photocleavage yields in excess of 90% without significant esterification. Finally, the SAMs that have been photopatterned using our preferred catalyst have been decorated with amine-modified microspheres, though this can be extended to other systems. 2. Experimental Methods Materials. Dichloromethane 99.9% (DCM), hydrogen peroxide (27.5 wt %), 4,4′-dithiodibutyric acid, -[S(CH2)3-CO2H]2 (DTBA), 1 M hydrochloric acid, acetic acid 99%, methanol (MeOH) 99.9%, isopropyl alcohol (IPA) 99.9%, red-fluorescent amine-modified polystyrene latex beads (1.0 µm mean particle size), 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDAC), and N-hydroxysulfosuccinimide sodium salt (sulfo-NHS) were used as received from Sigma-Aldrich. 4,4′dithiodibutyric[5-(1H,1H,2H,2H-perfluorooctyloxy)-4-methoxy2-nitro-benzyl] ester 1 was synthesized as previously described.14,15 Sulfuric acid (98%) was supplied by Fisher Scientific. Glass microscope slides (thickness 0.8 mm) were purchased from Agar and were cut to approximately three-quarters of the original length. Millipore Milli-Q water with a resistivity better than 18.1 MΩ · cm was used throughout. High-purity (99.99%) temper-annealed gold wire (0.75 mm diameter) was supplied by Goodfellow. Substrate Preparation. Glass microscope slides were first cleaned by ultrasonication for 15 min in a 10% solution of Decon90 in Milli-Q water. Each slide was rinsed 10 times in Milli-Q water and then dried under a stream of zero grade nitrogen. The samples were ultrasonicated in dichloromethane for 15 min, removed, and dried in a stream of zero grade nitrogen, rinsed under Milli-Q water, and immersed in piranha
10.1021/jp907950c 2009 American Chemical Society Published on Web 11/04/2009
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Figure 1. Scheme showing the formation of SAM1 on gold surface using reagent 1, and photodeprotection of SAM1 using soft UV (365 nm) irradiation to generate -CO2H-functionalized SAMs. Soft UV irradiation not only leads to photolysis of the ortho-nitrobenzyl (at a rate k1) but also to a competing photoreaction (at a rate k2).15
solution (70:30, v/v, H2SO4/H2O2) for 10 min. The substrates were then rinsed in Milli-Q water, dried under nitrogen, and placed in an Edwards Auto 306 thermal evaporator. A 150 nm gold layer was thermally deposited (at a rate of 0.1 nm s-1) onto a chromium adhesion layer (5 nm thick), at a base pressure of ∼1 × 10-6 mbar. The gold-coated samples were cleaned immediately prior to use by placing them in freshly prepared piranha solution for 2 min, followed by a rinse with Milli-Q water. SAM Adsorption. Compound 1 and DTBA SAMs were formed by immersing the gold-coated slides in 0.5 mM solutions of the corresponding material, in DCM, for 16 h, at 23 °C. The samples were then removed from solution, rinsed with DCM, dried with a nitrogen stream, rinsed with Milli-Q water, and again dried. UV Irradiation. A 365 nm UV lamp (Blak-Ray Model B100 AP) with a nominal power (at the sample) of 3.5 mW cm-2 for 2 h (25.2 J cm-2 UV dose) was used to irradiate the samples, in air and in acid solutions. After the UV irradiation, samples were rinsed with DCM, followed by Milli-Q water, and finally dried under a stream of nitrogen. Microsphere Attachment. Red-fluorescent amine-modified polystyrene latex beads (1 µm diameter) were diluted to a concentration of 2.3 × 107 particles mL-1 in Milli-Q water. Patterned samples were reacted with 0.5 µM EDAC and 2.5 µM sulfo-NHS for 15 min. After being rinsed with water, samples were placed in the microsphere solution for 2 h. Samples were rinsed with water and dried with a stream of nitrogen. Wetting Measurements. Contact angles were measured using a First-Ten-Ångstrom 2000 goniometer under ambient conditions. Milli-Q water droplets were advanced and receded across the surface from a microsyringe needle. Images of at least five advancing and receding droplets were analyzed on both sides of each droplet to give a minimum of five values per surface. X-ray Photoelectron Spectroscopy. Spectra were obtained using a Thermo Electron Corporation ESCA Lab 250 with a chamber pressure maintained below 1 × 10-9 mbar during acquisition. A monochromated Al KR X-ray source (15 kV 150 W) irradiated the samples, with a spot diameter of ∼0.5 mm. The spectrometer was operated in Large Area XL magnetic lens mode using pass energies of 150 and 20 eV for survey and detailed scans, respectively. The spectra were obtained with an electron takeoff angle of 90°. High-resolution spectra were fitted
Figure 2. FT-IRAS spectra of SAMs. (a) Fresh SAM1; (b) SAM1 after soft UV irradiation in 0.1 M HCl in MeOH; (c) fresh DTBA SAM; and (d) DTBA SAM after immersion in 0.1 M HCl in MeOH without UV irradiation.
using the Avantage (Thermo VG software package) peak fitting algorithms. All spectra have been corrected by using C 1s peak at 284.5 eV for charge shifting. Fourier Transform-Infrared Reflection Absorption Spectroscopy. FT-IRAS spectra were obtained using a Bruker IFS66 spectrometer equipped with a liquid-N2 cooled MCT detector. The optical path was evacuated. A p-polarized beam at an incident angle of 80° to the surface normal was used for the FTIR measurements. The spectra were taken at a 2 cm-1 resolution, and 1000 interferograms were coadded to yield spectra of high signal-to-noise ratio. The reference spectrum was taken from a freshly cleaned gold surface. 3. Results and Discussion Figure 2 shows FT-IRAS spectra of SAM1 before and after soft UV irradiation in 0.1 M HCl in MeOH. Figure 2a shows the spectrum of a freshly formed monolayer of SAM1; the peaks have been assigned previously.15 Of particular note are the bands between 1100 and 1250 cm-1 corresponding to the perfluoro side chain, the carbonyl bands at 1735 cm-1 and the aromatic NO2 asymmetric vibration at 1531 cm-1. On exposure to soft UV (365 nm, 3.5 mW cm-2) in acidic solution for 2 h we
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Figure 3. FT-IRAS spectra of SAMs. (a) Fresh SAM1; (b) SAM1 irradiated with soft UV in air for 2 h; (c) SAM1 irradiated with soft UV in 0.1 M HCl in MeOH for 2 h; (d) SAM1 irradiated with soft UV in 0.1 M HCl in IPA for 2 h; and (e) SAM1 irradiated with soft UV in 10% acetic acid in DCM for 2 h.
observe, Figure 2b, the complete disappearance of the CF2 and NO2 stretching bands at 1250 and 1531 cm-1, respectively. Additionally, we observe the presence of two new bands at 1440 and 1749 cm-1 corresponding to the CH3O bending and CdO stretching modes, respectively.24,25 These bands are not seen in the starting material or in a SAM of DTBA, the expected product (Figure 2c). Furthermore, when a DTBA SAM is immersed in 0.1 M HCl in methanol (without UV irradiation), after 2 h, these same bands are observed (Figure 2d). This suggests that photodeprotection of SAM1 under soft UV light in 0.1 M HCl in methanol does occur effectively, but that the CO2H groups on the SAM react (at least in part) to form methyl ester. This is as expected since, although esterification is usually relatively slow in the sterically demanding SAM environment, these are classic conditions for an esterification reaction. By choosing a more sterically hindered alcohol as the solvent (such as isopropanol) esterification of the SAM is expected to be extremely slow. Alternatively, esterification could perhaps be avoided altogether by using an organic acid catalyst in DCM. For a fresh DTBA SAM (Figure 2c), a broad, weak band was expected at 1430 cm-1 but proved difficult to observe in these experiments and also in our previous studies.15 This C-O-H bending mode has been previously observed on high density -COOH terminal groups, for long-chain thiolate SAMs (COOH(CH2)nSH, n g 10) such as 11-mercaptoundecanoic acid (n ) 10)26 and 16-mercaptohexadecanoic acid (n ) 15).27 It seems probable that in the case of short-chain DTBA (n ) 3) molecules more loosely packed SAMs presenting a lower density of COOH terminal groups would be formed and that these might present different orientations, leading to the low intensity of the C-O-H mode. In order to test this hypothesis, we have irradiated SAM1 under different conditions, including in 0.1 M HCl in isopropanol and 10% acetic acid in DCM. FT-IRAS spectra for the various results are shown in Figure 3. Following irradiation in air (Figure 3b) the aromatic NO2 vibration at 1531 cm-1 and the aromatic -OCH3 vibration peak at 1287 cm-1 both show significant reductions, as does the CF2 axial mode at 1220 cm-1. In contrast, the CF2 asymmetric stretching mode (1250 cm-1), perpendicular to the CF2 chains, shows a moderate increase, associated with the change of orientation of the fluorinated
Prompinit et al.
Figure 4. X-ray photoelectron spectra of the C 1s region. (a) Fresh SAM1 on Au; (b) SAM1 after 2 h of soft UV irradiation in air; (c) SAM1 after 2 h of soft UV irradiation in 0.1 M HCl in isopropanol; and (d) DTBA SAM on Au. The dashed lines in all spectra indicate how these data were fitted to Pseudo-Voigt peak functions (FHWM ) 1.2 eV and a Lorenzian-to-Gaussian mix of 3:7).
chains of uncleaved molecules.28,29 The CdO peak is shifted to higher frequency compared to that observed for DTBA (Figure 2c), which is at 1726 cm-1. This indicates less hydrogen bonding in the monolayer and/or possibly ester formation.25 As discussed above, after photolysis in 0.1 M HCl in methanol (Figure 3c), the FT-IRAS shows a new peak at 1440 cm-1, which corresponds to the CH3 symmetric bending mode of a CO2CH3 ester.24,25 In contrast, when the photolysis is carried out in a more sterically hindered alcohol, e.g., in 0.1 M HCl in isopropanol (Figure 3d), or in 10% acetic acid in DCM (Figure 3e), this additional peak does not appear, suggesting that esterification can be avoided using these conditions. This is consistent with the additional steric hindrance associated with the isopropyl group which makes esterification difficult and totally absent in the case of acetic acid. To determine the yield of the photolysis under the various different conditions, XPS measurements were carried out in the C 1s, O 1s, and F 1s regions of the various SAMs. Figure 4a shows the X-ray photoelectron spectra of the C 1s region of SAM1. The bands located at 293.7, 291.5, and 290.7 eV are assigned to CF3, CF2, and CF2CH2 species, respectively.15,30 The band at 289.2 eV is associated with the CdO groups. The peak at 286.4 eV is assigned to the -C-O species and the bands at 285.5 and 284.5 eV are associated with the hydrocarbons CH2CF2 and CH2/CHarom, respectively.30,31 After UV exposure of SAM1 in air for 2 h these main peaks can still be clearly observed (Figure 4b). We found that the total area beneath the C 1s spectrum in this energy range was reduced from 0.047 to 0.022 (counts/s) · eV (Table 1). This is in accordance with our previous work which showed that after the same period of irradiation, at saturation, only about half of the molecules in the SAM had been cleaved to DTBA, and that the competitive reduction of the ortho-nitro group resulted in residual fluorocarbon on the surface (Figure 1).15 After photoreaction in 0.1 M HCl/isopropanol solution, the peaks associated with CF3, CF2, and CF2CH2 species are no longer observable (Figure 4c) and the spectrum approaches that of the DTBA SAM (Figure 4d) with the exception that there is some residue of the starting material peak at 286.4 eV (C-O) and that the CdO peak at
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TABLE 1: XPS Peak Area of the F 1s, C 1s, and O 1s Spectra, As Measured and Normalized to the Au 4f Peak, and Water Contact Angles, of SAM1 before and after Soft UV Irradiation for 2 h peak areas (counts/s) · eV [normalized, %]
water contact angles (deg)
SAM on Au
F 1s
C 1s
O 1s
θAdvancing
θReceding
fresh SAM1 irradiation in air irradiation in 0.1 M HCl in isopropanol irradiation in 0.1 M HCl in methanol irradiation in 10% acetic acid in DCM DTBA SAM after irradiation in 0.1 M HCl in isopropanol fresh DTBA SAM
0.218 [100] 0.111 [51] 0.009 [4] 0.006 [3] 0.020 [9] 0.000 [0] 0.000 [0]
0.047 [100] 0.022 [48] 0.017 [37] 0.013 [29] 0.017 [37] 0.012 [25] 0.012 [25]
0.029 [100] 0.019 [66] 0.006 [22] 0.002 [6] 0.009 [29] 0.008 [28] 0.009 [31]
118 ( 1 99 ( 1 76 ( 3 76 ( 3 82 ( 1 70 ( 2
