A Novel and Simplified Procedure for Patterning Hydrophobic and

Sep 2, 2006 - LAAS-CNRS, 7 Avenue du Colonel ROCHE, 31077 TOULOUSE Cedex 4, France, ICMMO-LCBB, UMR-CNRS 8124, Bât 420, 5 rue Georges Clemenceau, Uni...
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Langmuir 2006, 22, 8346-8352

A Novel and Simplified Procedure for Patterning Hydrophobic and Hydrophilic SAMs for Microfluidic Devices by Using UV Photolithography Eric Besson,*,† Anne-Marie Gue,† Jan Sudor,† Hafsa Korri-Youssoufi,‡ Nicole Jaffrezic,§ and Jacques Tardy| LAAS-CNRS, 7 AVenue du Colonel ROCHE, 31077 TOULOUSE Cedex 4, France, ICMMO-LCBB, UMR-CNRS 8124, Baˆ t 420, 5 rue Georges Clemenceau, UniVersite´ Paris-Sud, 91405 Orsay, France, CEGELY, Ecole Centrale de Lyon, Baˆ timent H9, 36 aVenue Guy de Collongue, 69134 ECULLY Cedex, France, and LEOM, Ecole Centrale de Lyon, 36 aVenue Guy de Collongue, 69134 ECULLY Cedex, France ReceiVed December 6, 2005. In Final Form: June 28, 2006 This work describes how selective patterning of hydrophobic and hydrophilic areas inside microchannels of microfluidic devices can be achieved by combining well-known chemical protocols and standard photolithography equipment (365 nm). Two techniques have been performed and compared. The first technique is based on the preparation of selfassembled monolayers of photocleavable organosilane and the second one on photoassisted grafting (365 nm) of self-assembled monolayers (SAMs) on a silicon or glass substrate. In the first case, we begin with monolayers carrying an o-nitrobenzyl function (hydrophobic area) that is photochemically cleaved, revealing a carboxylic acid group (hydrophilic area). The problem is that the energy necessary to cleave this monolayer is too high and the reaction time is more than 1 h with 50 mW/cm2 irradiation flux. To overcome this practical disadvantage, we propose another approach that is based on the thiol-ene reaction with benzophenone as photoinitiator. In this approach, a monolayer of mercaptopropyltrimethoxysilane (MPTS) is prepared first. Subsequently, a hydrocarbon chain is photografted locally onto the thiol layer, forming a hydrophobic surface while the reminding unmodified thiol surface is oxidized into sulfonic acid (hydrophilic area). We demonstrated the feasibility of this approach and synthesized high-quality self-assembled monolayers by UV grafting with an irradiation time of 30 s at 365 nm (50 mW/cm2). The modified surfaces have been characterized by contact angle measurements, X-ray photoelectron spectroscopy (XPS), AFM, and multiple internal reflection infrared spectroscopy (MIR-FTIR). The difference in the contact angles on the hydrophilic and hydrophobic surfaces reached a remarkable 77°. We have also demonstrated that this method is compatible with selective surface grafting inside microfluidic channels.

Introduction The area of micro total analysis systems (µTAS), also called “lab on a chip” or miniaturized analysis systems, is growing rapidly. A microfluidic device typically consists of channels, reservoirs, pumps, and valves to create integrated systems for chemical analyses, syntheses, and bioassays. The advantages of this technology lie in the consumption of a small amount of the samples and reagents, fast processing, and high throughput compared to macroscopic systems. A lab on a chip generally consist of micro or nanochannels fabricated in a solid support, such as silicon, glass, or polymer.1,2 However, when the system is miniaturized to the submillimeter scale, the surface-to-volume ratio increases dramatically, and surface properties of microchannels, especially wetting, have significant effects on the liquid’s behavior. Indeed, to control wetting in microfluidic devices appears to be a very powerful approach for liquid handling; however, it entails a fine-tuning of the channel surface properties. * To whom correspondence should be addressed. Tel: 05 61 33 64 65. Fax: 05.61.33.62.08. E-mail: [email protected]. † LAAS-CNRS. ‡ ICMMO-LCBB. § CEGELY. | LEOM. (1) Reyes, D. R.; Iossifidis, D.; Auroux, P. A.; Manz, A. Anal. Chem. 2002, 74, 2623. (2) Reyes, D. R.; Iossifidis, D.; Auroux, P. A.; Manz, A. Anal. Chem. 2002, 74, 2637.

Patterning hydrophobic and hydrophilic regions inside microchannels typically requires modifying the surface in selected areas first and then aligning and bonding substrates to form microchannel networks. Recently, Beebe et al. has reported on a patterning of surface free energies inside microchannels by the use of self-assembled monolayers (SAMs) in combination with either multistream laminar flow or photolithography.3,4 It was shown that the patterned surfaces in such microfluidic channels play the role of a flow guide. For instance, aqueous solutions flow only along the hydrophilic pathways when the pressure is maintained below a critical value and the gas-liquid interface becomes pinned precisely at the boundary between the hydrophilic and hydrophobic areas. As a result, a microfluidic structure is formed in which the liquid circulates in microchannels with virtual side walls. This approach can also be interesting for studying interactions between different phases or for flow actuation in micro- and nanodevices. The monolayers prepared from organosilane compounds have attracted much attention because of their technological applications, such as in electronic devices and biomedical materials5-11 and because they can form resistant coatings on various materials. (3) Zhao, B.; Moore, J. S.; Beebe, D. J. Anal. Chem. 2002, 74, 4259. (4) Zhao, B; Viernes, N. O. L.; Moore, J. S.; Beebe, D. J. J. Am. Chem. Soc. 2002, 124, 5284. (5) Ulman, A. Chem. ReV. 1996, 96, 1533. (6) Kim, H. I.; Graupe, M.; Oloba, O.; Koini, T.; Imaduddin, S.; Lee, R. T.; Perry, S. S. Langmuir 1999, 15, 3179.

10.1021/la053303l CCC: $33.50 © 2006 American Chemical Society Published on Web 09/02/2006

Patterning Hydrophobic and Hydrophilic SAMs

In this paper, we describe a novel and simple method for producing hydrophilic/hydrophobic patterns on Si(100) or glass surfaces by UV irradiation. Our goal was to use this novel surface chemistry to develop microchannels with virtual walls. More over, we show that our surface chemistry allows for a fast grafting, surface modification inside a microchannel, and it is also compatible with standard photolithography apparatus. We tested and compared two different approaches. First, the flexible method described by Beebe et al.3 based on photolithography in combination with photocleavable SAMs was studied. Second, we used the silane coupling agent 3-mercaptopropyltrimethoxysilane (MPTS) as the functional molecule that was subsequently UV grafted with an alkene. MPTS was selected because it can be obtained commercially and has already been used to make functional monolayers,12,13 and the nonreacted thiol groups (-SH) can easily be oxidized into sulfonic acid (-SO3H).14,15 The sulfonate groups provide both a high degree of surface acidity and a hydrophilic character. Experimental Section Synthesis of Decyl 4-(11-Trichlorosilyl-1-oxoundecyloxymethyl)3-nitrobenzoate. Decyl 4-(Bromomethyl)-3-nitrobenzoate, 1. The 1-decanol (2 g, 12.6 mmol) 1 was added to a solution of 4-(bromomethyl)-3nitrobenzoic acid (2 g, 6.9 mmol) in 30 mL of dichloromethane followed by 1,3-dicyclohexylcarbodiimide (2.42 g, 11.73 mmol) and 4-(dimethylamino)pyridine (0.1 g, 0.82 mmol). The mixture was stirred at room temperature for 24 h. The product was collected after filtration and was dried in a vacuum; it appeared as a brown oil. The crude product was purified by column chromatography with silica gel using a mixture of CH2Cl2-petroleum ether (1:1 v/v). The yield of a pure ester (white solid) was 2.5 g, 72%: 1H NMR (250 MHz, CDCl3) δ (ppm) 8.59 (d, 1H, Ar), 8.16 (d, 1H, Ar), 7.6 (d, 1H, Ar), 4.78 (s, 2H, CH2Br), 4.3 (t, 2H, -COOCH2-), 1.7 (m, 2H, OCOCH2CH2), 1.2 (m, 16H, 8 CH2), 0.81 (t, 3H, CH3); MS (ES+) m/z 422 [M + Na]+. Decyl 4-(1-Oxo-10-undecenyloxymethyl)-3-nitrobenzoate, 2. Undecylenic acid (0.92 g, 5 mmol) was diluted in DMF, and the ester 1 (2 g, 5 nmol) and NaHCO3 (0.5 g, 6 mmol) were added to the solution. The mixture was stirred at 90 °C for 3 h and then at room temperature for 48 h. The DMF was removed on a rotary evaporator. Dichloromethane was added to the mixture and the organic phase was washed subsequently with 0.1 M HCl solution, two times with water, and finally with saturated sodium chloride solution. The organic layer was then dehydrated using sodium sulfate, and the organic solvent was evaporated. The crude product was purified by chromatography on silica gel using mixture CH2Cl2petroleum ether (2:1 v/v). The yield was 1.1 g, 44%: 1H NMR (250 MHz, CDCl3) δ (ppm) 8.7 (d, 1H, J ) 7.3, Ar), 8.28 (d, 1H, J ) 7.3, Ar), 7.69 (d, 1H, J ) 7.3, Ar), 5.7 (m, 1H, CHd), 5.54 (s, 2H, ArCH2O), 4.93 (t, 2H, J ) 7.5, dCH2), 4.35 (t, 2H, J ) 7, -COOCH2-), 2.42 (t, 2H, J ) 7.8, COCH2), 2.01(m, 2H, dCHCH2-), 1.75 (m, 2H, OOCCH2CH2), 1,64 (m, 2H, OCOCH2CH2), 1.3 (m, 24H, 12 CH2), 0.83 (t, 3H, CH3); MS (ES+) m/z 526 [M + Na]+. Decyl 4-(11-Trichlorosilyl-1-oxoundecyloxymethyl)-3-nitrobenzoate, 3. We added, to a round-bottom flask under the protection (7) Sugimura, H.; Nakagiri, N. J. Am. Chem. Soc. 1997, 119, 9226. (8) Chang, Y.; Frank, C. W. Langmuir 1998, 14, 326. (9) Faverolle, F.; Attias, A. J.; Bloch, B; Audebert, P.; Andrieux, C. P. Chem. Mater. 1998, 10, 740. (10) Jeon, N. L.; Finnie, K.; Branshaw, K.; Nuzzo, R. G. Langmuir 1997, 13, 3382. (11) Onclin, S.; Ravoo, B. J.; Reinhoudt, D. N. Angew. Chem., Int. Ed. 2005, 44, 6282. (12) Hong, H. G.; Jiang, M.; Sligar, S. G.; Bohn, P. W. Langmuir 1994, 10, 153. (13) Kin Lok Cheung, M.; Trau, D.; Yeung, K. L.; Carles, M.; Sucher, N. J. Langmuir 2003, 19, 5846. (14) Balachander, N.; Sukenik, C. N. Langmuir 1990, 6, 1621. (15) Collins, R. J.; Sukenik, C. N. Langmuir 1995, 11, 2322.

Langmuir, Vol. 22, No. 20, 2006 8347 of N2, 1 mL of propanol, 1 g of 2, 4 mL of trichlorosilane, and 36.6 mg of hydrogen hexachloroplatinate(IV) hydrate. The solution was then refluxed for 12 h under N2 atmosphere. The resulting product was rotary evaporated and placed under high vacuum during 2 h to remove the excess of trichlorosilane: 1H NMR (250 MHz, CDCl3) δ (ppm) 8.7 (d, 1H, J ) 7.3, Ar), 8.28 (d, 1H, J ) 7.3, Ar), 7.69 (d, 1H, J ) 7.3, Ar), 5.54 (s, 2H, ArCH2O), 4.35 (t, 2H, J ) 7, -COOCH2-), 2.42 (t, 2H, J ) 7.8, COCH2), 1.75 (m, 2H, OOCCH2CH2), 1,64 (m, 2H, OCOCH2CH2), 1.5 (m, 2H, SiCH2), 1.3 (m, 28H, 14 CH2), 0.83 (t, 3H, CH3); MS (ES+) m/z 638 [M + H]+. Formation of SAMs of 3. Silicon wafers (〈100〉, n-type) were piranha-cleaned, rinsed with deionized water, and dried over a nitrogen stream. The wafers were then functionalized with compound 3 (10-2 M) in trichloroethylene/hexadecane (2/1) mixture. The grafting reaction was performed in a 20-mL bottle under an inert atmosphere at room temperature for 17 h without stirring. The wafers were successively washed with trichloroethylene and ethanol in an ultrasonic bath. Synthesis of Thiol-Terminated SAMs. Silicon wafers (〈100〉, n-type) or glass slides were piranha-cleaned, rinsed with deionized water, and dried over a nitrogen stream. All reagents were used as received and stored under nitrogen atmosphere. A 10-mL aliquot of MPTS in trichloroethylene solution (10-3 M) was added to a 20-mL clean bottle under a N2 atmosphere. A maximum of three precleaned substrates were added to each bottle and sealed. The samples were incubated at room temperature for 17 h. The modified substrates were then removed from the solution and rinsed with trichloroethylene and ethanol under ultrasonic stirring (3 min) to remove the adsorbed organosilane molecules. Only freshly prepared MPTS monolayers were used for UV irradiation in order to minimize their oxidation with air. UV Grafting Procedure. All experiments were carried out with an Olympus fluorescent microscope at 20 °C. UV light from the source passed through a near-UV filter cube (U-MNU2, type BP 360-370 nm) with a band-pass of 360-370 nm. The light intensity was 50 mW/cm2. Substrates were soaked in the grafting solution and insolated for a given time. Then they were rinsed with trichloroethylene under ultrasonic stirring (3 min) to remove the excess alkene and the adsorbed polymer (homopolymerization product). Oxidation of Thiol-Terminated SAMs. The oxidation of the -SH groups to the sulfonated monolayers had been carried out by dipping the substrates into a solution of 30% H2O2/CH3COOH (1:5) between 10 min to 1 h, washed subsequently with ethanol and deionized water, and dried in air. Substrate Analysis. Samples were analyzed by ellipsometry, water contact angle measurements, multiple internal reflection Fourier transform infrared (MIR-FTIR) spectroscopy, atomic force microscopy (AFM), and X-ray photoelectron spectroscopy. Contact Angle Measurements. The quality of the monolayers was first analyzed by measuring the contact angles. Advancing and receding contact angles were measured using a Cahn dynamic contact angle analyzer (DCA-322). The contact angle liquids used were distilled water and hexadecane. Each contact angle measurement was repeated three times, and between each measurement the sample was dried under a stream of nitrogen. Ellipsometry. The thickness of the monolayers on polished silicon wafers was measured using a Rudolph AutoEL-II fixed angle, singlewavelength ellipsometer. The incident angle of the He-Ne laser beam was 70°. Multiple samples were prepared, including blanks, and thickness was calculated from the optical constants using a refractive index of 1.50 for the monolayer, 1.46 for silica, and 3.83 for silicon. XPS Measurement. X-ray photoelectron spectra were obtained using an S-Probe Surface Science Instrument scanning XPS system with a focused monochromatic Al KR X-ray (1486.7 eV) source for excitation and a spherical section analyzer. The X-ray beam angle was 71° with the normal to the sample. All spectra were recorded at a base pressure of 5 × 10-9 Torr and calibrated on a bond energy of aliphatic C 1s at 285.0 eV.

8348 Langmuir, Vol. 22, No. 20, 2006 Scheme 1. Synthesis of Trichlorosilyl 1-Oxoundecyloxymethyl)-3-nitrobenzoate

MIR-FTIR Spectroscopy. An S-polarized IR beam coming from a Bruker IFS 55 FTIR spectrometer was directed on the coupling area of an input prism that ensured optical tunneling inside the wafer. After being internally reflected, the IR beam is coupled out of the wafer by the second prism and focused onto the liquid N2 cooled HgCdTe detector. Spectra have been acquired using 75-mm propagation distance between 1500 and 4000 cm-1, due to the silicon absorption, which enabled measurement below 1500 cm-1. Details of the experimentation are described elsewhere.16

Results and Discussion 1. Formation of Photocleavable SAMs. 1-1. Synthesis of Decyl 4-(11-Trichlorosilyl-1-oxoundecyloxymethyl)-3-nitrobenzoate. The o-nitrobenzyl group is known as an easy photoremovable group17 and has largely been used as protecting group in solidphase synthesis.18 Photocleavable SAMs by incorporating the o-nitrophenyl groups and the silane groups have also been synthesized.3,4 The synthesis of organotrichlorosilane bearing the photoremovable o-nitrophenyl groups designed for the formation of SAMs was obtained with three reaction steps, as shown in Scheme 1. The first step was the trans-esterification reaction of 4-(bromomethyl)-3-nitrobenzoic acid with 1-decanol in the presence of 1,3-dicyclocarbodiimide as coupling agent and with a small amount of 4-(dimethylamino)pyridine to form the decyl 4-(bromomethyl)-3-nitrobenzoate (yield of 72%). The decyl 4-(1-oxo10-undecenyloxymethyl)-3-nitrobenzoate was then obtained (40% yield) following a procedure described by McKee et al.,19 i.e., reaction of decyl 4-(bromomethyl)-3-nitrobenzoate with undecylenic acid in the presence of sodium hydrogen carbonate. Hydrosilation of alkenes was performed using the known procedure20 with trichlorosilane in the presence of Speier catalysis, hydrogen hexachloroplatinate(IV) hydrate (H2PtCl6‚6H2O), to give the desired product, which was confirmed by RMN and mass spectroscopy. The second step in the synthesis of decyl 4-(11-trichlorosilyl1-oxoundecyloxymethyl)-3-nitrobenzoate was made using the procedure described by McKee et al.19 that resulted in high yield and did not require the use of HMPA as compared to the procedure described by Beebe et al.3,4 1-2. Formation of SAMs. (16) Rochat, N.; Troussier, A.; Hoang, A.; Vinet, F. Mater. Sci. Eng. C 2003, 23, 99. (17) Pillai, V. N. R. Synthesis 1980, 1. (18) Rich, D. H.; Gurwara, S. K.; J. Am. Chem. Soc. 1975, 97(6), 1575. (19) McKee, J. A.; Miller, M. Bioorg. Med. Chem. Lett. 1991, 1, 3. (20) Tilley, T. D. In The Chemistry of Organic Silicon Chemistry; Patai, S., Rappoport, Z., Eds.; John Wiley: Chichester, 1989; p 1415.

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In this approach, the formation of decyl 4-(11-trichlorosilyl1-oxoundecyloxymethyl)-3-nitrobenzoate SAMs on a silicon wafer is the first step. After 17 h of silanization in octadecane, the water contact angle (101°) confirms that the surface is hydrophobic, and the surface morphology with AFM measurement shows that the layer is relatively dense with a root-meansquare (rms) roughness of 0.42 nm. We noticed that the use of other organic solvents (toluene, pure trichloroethane) gives contact angle values smaller than 90°, indicating that octadecane is crucial for the organization of the monolayer. The thickness of the layer of 3.1 nm (ellipsometry measurements) is somewhat smaller than the calculated value (3.5 nm). That indicates that the molecules are inclined and perhaps slightly folded up21,22 because of the size of the chains. 1-3. Photochemical Cleavage. The second step is the photochemical cleavage of the nitrobenzyl function23 (Scheme 2). The 2-nitrobenzyl functionality is the most widely used photoremovable protecting group for application in synthesis,24,25 photolithography (DNA microarrays),25,26 and biochemistry (“caged compounds”).27 It is based on the photochemically induced photoisomerization of o-nitrobenzyl alcohol derivatives into o-nitrosobenzaldehyde.28 It has been shown23 that, during the photosolvolysis, solvent and pH affect the kinetics of the cleavage. We tested different solvents (1,4-dioxane, methanol, water) and different pH values (1, 7, 12), but the best conditions were those chosen by Beebe et al.3 Upon exposure to UV irradiation (67 mW/cm2) in 0.1 M solution of HCl in methanol, the o-nitrobenzyl-oxygen bond in SAMs (on silicon) was cleaved in 30 min. The contact angle decreased from 101° to 57°, which is characteristic for carboxylic acid monolayers.29 Despite these results, this chemistry was not efficient and was inadequate with our photolithography apparatus in terms of reaction time. Indeed, when we decreased the power of UV light to 50 mW/cm2, or took glass as substrate, the irradiation time had to be increased to 90 min. The exigent cleavage of the hydrophobic SAMs developed by Beebe et al.3 prompted us to develop an alternative approach for surface patterning compatible with standard photolithography apparatus. 2. Formation of Hydrophobic and Hydrophilic SAMs by Thiols Reaction. 2-1. Introduction. The second strategy is based on the reaction known in the polymer field:30 thiol-ene reaction using the benzophenone (BP) as photoinitiator31 for the formation of hydrophobic surface. After formation of mercaptopropyl SAMs on silicon wafers or glass, the substrates were UV grafted with aliphatic chains. Under UV treatment, benzophenone abstracted hydrogen from the thiol to generate surface radicals and semipinacol radicals.32,33 (21) Wasserman, S. R.; Tao, Y.; Whitesides, G. M. Langmuir 1989, 5, 1074. (22) Duchet, J.; Chapel, J. P.; Chabert, B.; Gerard, J. F. Macromol. 1998, 31, 8264. (23) Il′ichev, Y. V.; Schworer, M. A.; Wirz, J. J. Am. Chem. Soc. 2004, 126, 4581. (24) Guillier, F.; Orain, D.; Bradley, M. Chem. ReV. 2000, 100, 2091. (25) Nakanishi, J.; Kikuchi, Y.; Takarada, T.; Nakayama, H.; Yamaguchi, K.; Maeda, M. J. Am. Chem. Soc. 2004, 126, 16314. (26) Pirrung, M. C. Angew. Chem., Int. Ed. Engl. 2002, 41, 1276. (27) Pelliccioli, A. P.; Wirz, J. J. Photochem. Photobiol. Sci. 2002, 1441. (28) Bochet, C. G. J. Chem. Soc., Perkin Trans 1 2002, 125. (29) Faucheuxa, N.; Schweissb, R.; L.utzowa, K.; Wernerb, C.; Groth, T. Biomaterial 2004, 25, 2721. (30) Hoyle, C. E.; Lee, T. Y.; Roper, T. J. Polym. Sci. Part A: Polym. Chem. 2004, 42, 5301. (31) Allen, N. S.; Hardy, S. J. J. Appl. Polym. Sci. 1991, 42, 1169. (32) Decker, C.; Thi Viet, T. Macromol. Chem. Phys. 1999, 200, 1965. (33) Wilderbeek, H. T. A.; Van der Meer M. G. M.; Bastiaansen, C. W. M.; Broer, D. J. J. Phys. Chem. B 2002, 106, 12874.

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Scheme 2. Photocleavage of SAMs of p-Nitrobenzylsilane under UV 365 nm.

Subsequently, in the presence of an ene function, the grafting takes place. Scheme 3 shows a schematic of the reaction procedure. Scheme 3. Simplified Schematic Overview of the Photoinitiated Thiol-Ene Free-Radical Polymerization Mechanism

To graft the aliphatic chains on the MPTS-coated SiO2 substrates, the surfaces were irradiated using UV light (365 nm) for desired time periods in the presence of the benzophenone and the alkenes. The grafted surfaces were then cleaned in an ultrasonic bath in trichloroethylene and ethanol. Contact angle was used to study the quality of hydrophobic SAMs on glass or silicon. The first group tested for the grafting reaction with alkene was the octadecylvinyl ether (0.2 M in 1,4-dioxane, 2 wt % benzophenone, irradiation time 15 min) but without positive results (may be due to the homopolymerization of the alkene). Then the second product tested was the hexadecene. The exposure time was fixed to 15 min, and the results are reported in the Table 1. In diluted conditions, with three different solvents and 5% Table 1. Influence of the Experimental Parameters on Water Contact Angle

Thiol-ene additions were first discovered by Posner in 1905, and the basic chemistry, polymerization mechanism, and photoinduced reactivity of thiols with various enes have been extensively investigated.34 The thiol-ene reaction proceeds by the addition of the thiols to the double bonds via a free radical chain reaction mechanism. These studies clearly demonstrated that thiol-ene mixtures exhibit significant advantages, including rapid reaction, little or no oxygen inhibition, formulation latitude due to the large number of enes that react with thiols, and a good addition to many different substrates35. With this approach, the thiol surfaces can easily be patterned with an aliphatic chain and become hydrophobic. On the other hand, the surfaces not grafted with aliphatic chains can subsequently be oxidized and become hydrophilic (sulfonic acid groups). 2-2. Formation of Hydrophobic SAMs. Three different ene functions were tested for the formation of hydrophobic surfaces by thiol-ene reactions octadecylvinyl ether, hexadecane, and octadecyl acrylate (ODA) derivativesas it is known that the reactivity of alkenes toward thiol (RS‚) depends on the structure of the alkene groups.36 (34) Lee, T. Y.; Roper, T. M.; Jonsson, E. S.; Guymon, C. A.; Hoyle, C. E. Macromolecules 2004, 37, 3606.

substrate silicon

glass a

concn (mol/L) 10-2

(toluene) 10-2 (hexadecane) 10-2 (ethanol) without solvent without solvent without solvent without solvent

BPa

water contact angle (deg)

5 5 5 1 5 10 5

70 69 70 85 101 100 67

The percentage in weight of benzophenone relative to hexadecene.

mol of photoinitiator, there is a weak modification of the contact angle, but it is not very significant. Without solvent and under the same conditions, the contact angle values indicate that the surfaces have a hydrophobic character. We can notice that beyond 5% mol of photoinitiator (PI), the effect of PI levels off. To sample the grafting quality, the layer was soaked in an oxidizing solution (30% H2O2/HOAc 1:5). After 30 min, the contact angle decreased to 85° (loss of 15°). Compared to a reference prepared from octadecyltrichlorosilane (OTS), the (35) Roffey, C. G. Photopolymerisation of Surface Coating; Wiley-Interscience, 1982; p 157. (36) Kim, Y. B.; Kim, H. K.; Choi, H. C.; Hong, J. W. J. Appl. Polym. Sci. 2005, 95, 342.

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Figure 1. Tapping mode AFM images of (a) silicon substrate, (b) MPTS monolayer on silicon oxide, and (c) ODA UV-grafted on MPTS. Table 2. Water Contact Angles Obtained for Different Conditions and Substrate during the Photografting at 365 nm between the Thiol Layer and Octadecylacrylate

substrate glass

silicon a

BPa

solvent

10 10 10 2 2 2 2 2 2 2 1 0.5 0.5 0.5 0.5

methanol trichloroethane 1,4-dioxane 1,4-dioxane

concn monomer (mol/L)

reaction time (s)

water contact angle (deg)

0.1 0.1 0.1 0.1 0.01 0.1 0.1 0.1 0.2 0.2 0.1 0.5 0.5 0.5 0.5

900 900 900 900 900 600 300 60 60 900 900 900 60 30 120

80.1 57.7 65.6 90.9 60.6 86.6 92.2 65.2 74.7 101.1 94.6 106.3 105.9 105.6 112.2

Figure 2. MIR-FTIR spectrum of the MPTS monolayer on silicon dioxide.

The percentage in weight of benzophenone relative to ODA.

contact angle reached 100° in the same time period (the original value was 110°). It can be concluded that the layer obtained by UV irradiation with hexadecene is slightly less dense and more prone to the oxidation than the reference layer (OTS). Then we studied the grafting of the octadecyl acrylate. Different solvents and conditions were tested, such as the photoinitiator concentration, the relative amount of the methacrylate monomer, and the reaction time; it is known that both could influence the thiol-ene reaction versus methacrylate homopolymerization.37 The results are summarized in Table 2. With two different substrates and using a short irradiation time and lower concentration of both initiator and monomer, both silicon wafer and glass substrates give the contact angles of about 112° and 105°, respectively. This suggests that the film is relatively dense. The resulting layer is extremely robust and withstand harsh chemical treatment with an oxidizing solution of 30% H2O2/ CH3COOH (1:5 v/v) and shows similar behavior as the OTS monolayer. After 30 min of exposure, contact angles remain at the original values. During the irradiation, we observed an increase in viscosity of the liquid phase that indicates the formation of a homopolymer in the solution. The homopolymerization can be reduced by decreasing the concentration of the monomer without really changing the surface grafting. After irradiation, the excess (37) Lecamp, L.; Houllier, F.; Youssef, B.; Bunel, C. Polymer 2005, 42, 2727.

Figure 3. MIR-FTIR spectrum of the (a) propylsulfonic acid surface obtained after oxidation of the (b) MPTS monolayer and (c) after UV grafting of ODA.

of homopolymer is easily removable by washing the substrate with trichloroethylene and ethanol under ultrasonic stirring. We employed atomic force microscopy (AFM) to image the surface topography before and after the formation of the MPTS and the aliphatic monolayers. Tapping mode AFM images showed the characteristic features of silicon surfaces (Figure 1a, root-mean-square roughness of 0.3 nm). MPTS-covered surface images showed that the topography of the monolayers (Figure 1b) reproduces the features of the nongrafted substrate with a rms roughness of 0.45 nm. A similar trend was also observed for ODA UV grafted on MPTS for which rms value is 0.55 nm (Figure 1c). FTIR spectroscopy was used as a quality control tool for routine

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Figure 4. XPS S 2p spectra (a) MPTS monolayer (b) after UV grafting of ODA and (c) propylsulfonic acid surface obtained after oxidation of MPTS monolayer.

monitoring of the build-up process of the layers. In the first attempt we analyzed the FT-IR spectra of the SiO2 surface modified with MPTS. Figure 2 shows an FTIR spectrum of the MPTS layer. Figure 2 shows a large absorption band located in the region between 3500 and 3100 cm-1, which could be attributed to O-H vibration stretching associated with the presence of water, SiOH, or methanol. On the other hand, in the 2800 and 3000 cm-1 range, different absorption bands are present that can be attributed to the C-H valence vibration of the aliphatic groups of the layers. An intensive band at 2977 cm-1 (Figure 2) indicates that a large amount of residual methoxy groups from mercaptopropyltrimethoxysilane precursor remains on the surface and that the hydrolysis is not completed. The shift in wavenumber for the methylene stretching modes [νs(CH2) 2856 cm-1, νa(CH2) 2926 cm-1], along with the increased bandwidth of these peaks, indicates a high density of defects and a random orientation of the alkyl chains on the surface. This result is not surprising with a C3 chain length silane. Generally, the stretching vibration band of the thiol S-H was observed with FT-IR experiment at 2570 cm-1, but here we observe a weak band at 2500 cm-1 attributed to S-H bond.38-40 Figure 3 gives interesting information about the quality of the hydrocarbon monolayer. It shows that even if the quality of MPTS monolayer is not perfect we can still obtain a hydrocarbon monolayer with good quality and density. Two groups of absorptions can be distinguished in Figure 3 (region c). Bandwidths and peak positions of the H-C-H stretch bands, at 2917 (νas(CH2)) and 2850 (νas(CH2)) cm-1, and methyl group of octadecyle, at 2944 (νa(CH3)) cm-1, and the CH2 peak intensity increase (compared to MPTS layer) indicate an organization of densely packed hydrocarbon chains.41 The band at 1733 cm-1 can be unambiguously assigned to the CdO stretch of the ester groups. Ellipsometric measurements after UV grafting revealed a thickness of about 2.7 ( 0.2 nm, which is lower than the expected value (3.1 nm) and confirm that the first layer of MPTS is incomplete or nonstructured. The layer thickness has been measured for different exposure times (30, 60, and 90 s), and the same values were obtained. The elemental composition of the surface under study was determined by XPS. The XPS spectra of the MPTS layer (Figure (38) Petoral, R. M.; Uvdal, U. J. Phys. Chem. B 2005, 109, 16040. (39) Kang, T.; Park, Y.; Yi, Y. Ind. Eng. Chem. Res. 2004, 43, 1478. (40) Iwata, T.; Tokutomi, S.; Kandori, H. J. Am. Chem. Soc. 2002, 124, 11840. (41) Liu, Y. J.; Navasero, N. M.; Yu, H.-Z. Langmuir 2004, 20, 4039.

4a) shows the sulfur peak (S2p) at 164.0 eV, which indicates that all of the surface sulfurs exist either as thiol or disulfide.42 Figure 4b corresponds to the UV-grafted ODA layer and is relatively similar to Figure 4a. Figure 4c is more interesting, as it shows the apparition of the sulfur peak (S2p) detected at 169 eV can be attributed to the sulfur group of the sulfonic acid. The same figure also shows the decrease of the sulfur peak at 164.0 eV associated with the thiol or disulfide groups. We can conclude that a majority of thiol was converted into sulfonic acid43 under the engaged oxidation conditions. 3. Selective Patterning of Surfaces with Hydrophilic and Hydrophobic Areas. Surfaces patterned with well-defined hydrophobic and hydrophilic areas of small dimensions can be successfully applied for wetting-driven liquid handling of submicroliter volumes, operations that might be difficult to perform with classical approaches. We have developed a novel photochemical approach for surface patterning that has been validated with two examples: (i) the surface patterning inside microfluidic devices to construct channels with virtual walls and (ii) the patterning of planar surfaces for fabrication of biochip arrays. 3-1. Selective Patterning inside Microchannels. To demonstrate that direct patterning inside channels is feasible, simple microstructures have been manufactured. Channels 500 µm to 1 mm wide, 250 µm deep, and 8 cm long were fabricated in 4 in. silicon wafers using deep reactive ion etching (DRIE) and then sealed with a Pyrex plate by an anodic bonding technique. Inlet and outlet holes were machined in the Pyrex lid prior to the bonding step. In these chips, the large channels acted as reservoirs that confined the reactive solution during the UV grafting step. First, the channels were piranha-cleaned, washed with water, and dried over a nitrogen stream. Then, a solution of MPTS in trichloroethylene solution (10-3 M) was flowed through the microchannels under a N2 atmosphere. The samples were incubated at room temperature for 17 h to allow the formation of a MPTS monolayer. The modified substrates were then rinsed with trichloroethylene and ethanol under ultrasonic stirring (3 min). Freshly prepared MPTS monolayers were used for UV irradiation. The Pyrex/silicon wafers were filled with a grafting solution (0.5 M octadecyl acrylate, 0.5% benzophenone) in 1,4-dioxane. Then, they were positioned on a Karl Su¨ss MA6 mask aligner and irradiated with UV light (20 mW/cm2, 365 nm) through a (42) Lenigk, R.; Carles, M.; Ip, N. Y.; Sucher, N. J. Langmuir 2001, 17, 2497. (43) Cano-Serrano, E.; Blanco-Brieva, G.; Campos-Martin, J. M.; Fierro, J. L. G. Langmuir 2003, 19, 7621.

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Figure 5. Images of microchannels of different geometries after UV grafting.

3-2. Selective Patterning of Biochip Arrays. DNA arrays are topical owing to their ability to yield information on nucleic acid levels and sequences faster, simpler, and cheaper than traditional methods.44 We demonstrated that our method might be useful for the fabrication of biochip arrays. MPTS-functionalized silicon wafers were irradiated through a photomask to fabricate hydrophilic, regularly spaced spots on hydrophobic surfaces. The results are represented in Figure 6. The formation of regularly spaced water drops after dipping the wafer in water can be seen in the Figure 6. The spot diameters are about 100-200 µm, and they are spaced by 1 mm. The hydrophilic, thiol-containing spots, can be easily reacted with disulfide-modified oligonucleotides through the thiol/disulfide exchange.45

Conclusions

Figure 6. Images of spots on modified silicon wafers after UV grafting.

mask. The irradiation time was adjusted to 120 s. Various patterns were realized on the same substrate and are shown in the Figure 5. Acetone was introduced inside the channels by capillarity. Figure 5 shows that acetone flow was clearly delimited to nonexposed areas, indicating that selective modification inside channels has been performed. The hydrophilic channel width is 300 µm, while the micromachined channels are 1 mm wide. We observed that these monolayers allow guiding of the flow along 90° angles or curved virtual channels. We also noticed that edges of the flow pathways were not well defined. This may be a consequence of the diffraction effects during the UV grafting, as the distance between the photomask and the bottom of the silicon channel was about 300 µm. Noting that due to the short lifetime of the benzophenone radicals in their triplet state (80120 µs), the distance diffused by the photoinitiator radicals (D ∼ 10-5 cm2/s) is negligible (∼0.35 µm) compared to the size of the surface defects (ca. a few micrometers). We conclude that mainly the irradiation process has to be improved in order to overcome this effect.

In this work, a simple and effective procedure for preparing hydrophobic and hydrophilic SAMs under UV (365 nm) treatment on silicon or glass has been developed. This novel approach has been employed for the fabrication of virtual microfluidic channels and biochip arrays by using standard UV photolithography. The thiol-ene reaction is known in polymer chemistry, but this is the first example, to our knowledge, of using this chemistry for preparing SAMs. Layers prepared with this procedure are highly hydrophobic and well organized. On the other hand, the conversion of the nonreacted thiol groups into sulfonic acid makes such surfaces highly hydrophilic and less reactive than, for example, carboxylic acid modified surfaces.3 As demonstrated here, the use of this novel chemical approach for the formation of surfaces with high hydrophilic/hydrophobic contrast (77°) allows for fabrication of (i) microfluidic devices with surface-directed liquid flow and (ii) simple biochip arrays. The next step is to focus on the surface patterning through sophisticated masks and optics and to evaluate this novel chemical approach for micro- and nanofluidic applications. Acknowledgment. This work has been supported by the Program “Nanoscience, Nanotechnologies” of the French Ministe`re de la la Recherche. The authors would like to thank N. Rochat, F. Vinet, and F. Martin from CEA-LETI for performing IR and XPS measurements. LA053303L (44) Pirrung, M. C. Angew. Chem., Int. Ed. 2002, 41, 1276. (45) Rogers, Y. H.; Jiang-Baucom, P.; Huang, Z. J.; Bogdanov, V.; Anderson, S.; Boyce-Jacino, M. T. Anal. Biochem. 1999, 266, 23.