Preparation of Micro Gold Devices on Poly (dimethylsiloxane) Chips

Sep 21, 2009 - Poly(dimethylsiloxane) (PDMS) is one of the most attractive materials for fabrication of various micro analytical chips due to its supe...
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Anal. Chem. 2009, 81, 8649–8653

Preparation of Micro Gold Devices on Poly(dimethylsiloxane) Chips with Region-Selective Electroless Plating Zhenxia Hao, Hengwu Chen,* and Dan Ma Institute of Microanalytical Systems, Department of Chemistry, Zijin’gang Campus, Zhejiang University, Hangzhou 310058, China A novel protocol for fabrication of micro gold devices on poly(dimethylsiloxane) (PDMS) substrates was developed on the basis of region-selective electroless plating. The layout of a micro gold device was first photochemically patterned onto the PDMS surface through a UV induced poly(acrylic acid) (PAA) grafting process. The carboxylic moieties on the grafted PAA served as the scaffold for a series of wet chemical reactions that led to the immobilization of gold nanoparticles in the UV-exposed region, where electroless plating then occurred under the catalysis of the nanoparticles. Gold devices fabricated with such a protocol could tolerate the Scotch tape test and survive in a repeated bending-straightening test. They also showed good stability in acidic and alkaline solutions, possessed almost the same electrochemical properties as a standard gold disk electrode, and allowed thiolcompounds to form a perfect self-assembled monolayer on their surfaces. The fabricated micro gold electrode was demonstrated to be suitable as the integrated amperometric detection element in a full PDMS micro electrophoresis chip. Poly(dimethylsiloxane) (PDMS) is one of the most attractive materials for fabrication of various micro analytical chips due to its superior properties.1,2 Despite the fact that micro metal devices are frequently required to be integrated onto the PDMS substrates for various applications, it is rather difficult to prepare them directly on PDMS substrates because the low surface energy of PDMS renders the adhesion between the deposited metal layer and PDMS substrate quite weak.3-5 An adhesive layer of Ti or Cr was reported to ensure the effective adhesion.3,6-8 The intent * To whom correspondence should be addressed. E-mail: [email protected]. Fax: +86-571-88273572. Tel: +86-571-88206773. (1) McDonald, J. C.; Whitesides, G. M. Acc. Chem. Res. 2002, 35, 491–499. (2) McDonald, J. C.; Duffy, D. C.; Anderson, J. R.; Chiu, D. T.; Wu, H. K.; Schueller, O. J. A.; Whitesides, G. M. Electrophoresis 2000, 21, 27–40. (3) Schmid, H.; Wolf, H.; Allenspach, R.; Riel, H.; Karg, S.; Michel, B.; Delamarche, E. Adv. Funct. Mater. 2003, 13, 145–153. (4) Pangule, R. C.; Banerjee, I.; Sharma, A. J. Chem. Phys. 2008, 128, 234708. (5) Meacham, K. W.; Giuly, R. J.; Guo, L.; Hochman, S.; Deweerth, S. P. Biomed. Microdevices 2008, 10, 259–269. (6) Bowden, N.; Brittain, S.; Evans, A. G.; Hutchinson, J. W.; Whitesides, G. M. Nature 1998, 393, 146–149. (7) Baek, J. Y.; An, J. H.; Chio, J. M.; Park, K. S.; Lee, S. H. Sens. Actuators, A 2008, 143, 423–429. (8) Loo, Y. L.; Willett, R. L.; Baldwin, K. W.; Rogers, J. A. Appl. Phys. Lett. 2002, 81, 562–564. 10.1021/ac901539n CCC: $40.75  2009 American Chemical Society Published on Web 09/21/2009

of this approach was to prepare electronic or optical devices which were used in dry conditions. A relatively popular method for preparation of metal devices on PDMS substrate was the pattern transfer technique known as microcontact printing. In this method, the self-assembled monolayer (SAM) of a heterofunctional thiocompound linked the patterned gold film and the PDMS substrate and consequently, strengthened the adhesion of the metal devices on the PDMS surface.9-12 It has been reported that such electrodes suffered the problem of delamination when it was used as the detection element in electrophoresis10 and that cracks would be generated on the metal film upon bending the PDMS substrate.12 Electroless plating is a cost-effective tool for the production of high quality metal layers on either conductive or nonconductive substrates in wet chemistry laboratories. Although quite a lot of work has been reported on the preparation of micro metal devices on glass,13,14 thermo-plastic polymers,15,16 and SU-8 epoxy17 with region-selective electroless plating, only very limited work on this topic for PDMS substrate has been published. Thus, using a novel multiphase laminar flow patterning (LFP) technique to restrict the region to be plated, Whitesides’s group18 and more recently Gao et al.19 electrolessly prepared micro silver electrodes inside the PDMS channels. However, it is challenging for the LFP-based electroless plating to prepare micro metal devices with complicated and fine structures. By combining the UV-induced poly(acrylic acid) (PAA) grafting20,21 with electroless gold plating, we recently developed (9) Loo, Y. L.; Willett, R. L.; Baldwin, K. W.; Rogers, J. A. J. Am. Chem. Soc. 2002, 124, 7654–7655. (10) Lee, K. J.; Fosser, K. A.; Nuzzo, R. G. Adv. Funct. Mater. 2005, 15, 557– 566. (11) Atmaja, B.; Frommer, J.; Scott, J. C. Langmuir 2006, 22, 4734–4740. (12) Lim, K. S.; Chang, W. J.; Koo, Y. M.; Bashir, R. Lab Chip 2006, 6, 578– 580. (13) Hilmi, A.; Lutong, J. H. T. Anal. Chem. 2000, 72, 4677–4682. (14) Yan, J. L.; Du, Y.; Liu, J. F.; Cao, W. D.; Sun, X. H.; Zhou, W. H.; Yang, X. R.; Wang, E. K. Anal. Chem. 2003, 75, 5406–5412. (15) Kong, Y.; Chen, H. W.; Wang, Y. R.; Soper, S. A. Electrophoresis 2006, 27, 2940–2950. (16) Hao, Z. X.; Chen, H. W.; Zhu, X. Y.; Li, J. M.; Liu, C. J. Chromatogr., A 2008, 1209, 246–252. (17) Dai, W.; Wang, W. J. Microsyst. Technol. 2008, 14, 1745–1750. (18) Kenis, P. J. A.; Ismagilov, R. F.; Whitesides, G. M. Science 1999, 285, 83– 85. (19) Gao, Y. X.; Chen, L. W. Lab Chip 2008, 8, 1695–1699. (20) Hu, S. W.; Ren, X. Q.; Bachman, M.; Sims, C. E.; Li, G. P.; Allbritton, N. L. Anal. Chem. 2004, 76, 1865–1870. (21) Wang, Y. L.; Lai, H. H.; Bachman, M.; Sims, C. E.; Li, G. P.; Allbritton, N. L. Anal. Chem. 2005, 77, 7539–7546.

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Figure 1. Schematic procedure for fabrication of micro gold patterns on PDMS surface. (A) PDMS prepolymer (b) cast against a template (a); (B) cured PDMS sheet to be treated with benzophenone solution; (C) spreading AA monomer solution (c) over the benzophenone treated PDMS surface with a photomask (d); (D) exposing to UV light (e) for photo polymerization of PAA on the PDMS surface; (E) wet chemical reactions leading to the formation of gold nanoparticle catalyst centers (f); (F) micro gold film (g) deposited by regionselective electroless gold plating.

a novel region-selective electroless plating protocol for fabrication of micro gold devices on PDMS substrates. This paper presents the main results of the work. EXPERIMENTAL SECTION Chemicals and Apparatus. The Sylgard 184 silicone kit was from Dow Corning. Acrylic acid (99.5%, purified via distillation at reduced pressure before use) and benzophenone (99%) were from Acros Organics (Morris Plains, New Jersey). A 0.25 M Na3Au(SO3)2 stock solution used for electroless gold plating was procured from Changzhou Institute of Chemical Research (Changzhou, China). Procedure for Fabrication of Micro Gold Devices on PDMS. The whole procedure is schematically illustrated in Figure 1 and is described as follows. Preparation of PDMS Substrates. PDMS substrates were prepared by molding Sylgard 184 PDMS prepolymer (10:1 mixture of base/curing agent) against a flat glass slide after degassing. The slide was then heated at 75 °C for 1.5 h to cure the PDMS. Selective Grafting of PAA on the PDMS Surface. The procedure for PAA grafting was adopted from the previous report21 with a few modifications. Briefly, the cured PDMS substrate was first immersed into a 10 wt % benzophenone solution (acetone/water ) 65:35) for 1 min. Immediately after the benzophenone-treated PDMS substrate was rinsed with copious water, an appropriate volume of monomer solution (10 wt % acrylic acid with 0.5 wt % benzyl alcohol and 0.5 mM sodium periodate) was pipetted onto the PDMS surface. A chromium-glass photomask was then slowly lowered over the solution, resulting in a uniform layer of the monomer solution between the mask and the PDMS substrate. The mask/PDMS assembly was exposed to the UV-light from a high pressure mercury lamp (125W, the out glass bulb was removed) for 25 min at a lamp-to-mask distance of 6 cm. During the UV irradiation process, the temperature of the PDMS sample was maintained at 32 ± 1 °C. The PAA-grafted PDMS was rinsed with copious water to remove the residual monomer solution and the nongrafted PAA. Activation of the Sites To Be Plated. The PAA-grafted PDMS substrate was immersed in a 0.36 M ethylenediamine solution 8650

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containing 50 mM 1-(3-dimethyl-aminio-propyl)-3-ethylcarbodiimide hydrochloride (EDC) for 1 h to aminate the carboxylic groups on the grafted PAA. After washing, the aminated sample was immersed in 1 mM HAuCl4 aqueous solution for 1 h. Then, it was treated with a 0.1 mM NaBH4 solution for 3 min so as to reduce the adsorbed Au(III) ions to gold nanoparticles. Whenever needed, the activated PDMS sheet was sonicated in a 0.5 M KSCN solution for 3 min to remove any nonspecifically adsorbed gold species from the masked region of the PDMS surface. This additional step could prevent nonspecific overplating in the following plating process. The activated PDMS substrate was then washed with water and dried in a warm air stream. Electroless Gold Plating. The activated PDMS substrate was placed into a gold plating bath (containing 8 mM Na3Au(SO3)2, 0.125 M Na2SO3, and 0.6 M formaldehyde) at room temperature. Micro gold patterns would gradually form in the PAAgrafted region of the PDMS substrate in about 40 min. The plated PDMS substrate was then rinsed with water and dried in a warm air stream. RESULTS AND DISCUSSION Method Development. Region-selective electroless plating makes it possible to fabricate various metal devices onto both conductive and nonconductive substrates with simple facilities available in wet chemistry laboratories. One of the prerequisites for region-selective electroless plating is that the region to be plated should be activated by adsorption of nanoparticles of such metals as Ag, Au, and Pd.13-17 These metal particles serve as the catalyst centers that accelerate the reaction between the plating metal ions and the reducing agent in the electroless plating bath. As a result, the plating metals would merely deposit onto the activated region rather than nonspecifically precipitate from the bulk solution. Zhang et al.22 reported that gold nanoparticles could be formed on a PDMS surface upon simply immersing the PDMS sheet into a HAuCl4 solution overnight. Referring to this report, we initially attempted to exploit such formed nanoparticles to catalyze the electroless gold plating. However, experiments revealed that the pink-colored gold nanoparticles formed in this way were inactive in promoting the electroless gold plating. Presumably, such formed nanoparticles were embedded into the PDMS sheet to a certain extent or covered by the residual nonpolymerized species diffused from the PDMS bulk so that they could not make contact with the electroless plating bath and, consequently, failed to catalyze the reduction of the Au(I) in the bath. In our previous reports, a novel photodirected electroless plating protocol was developed for fabrication of micro gold electrodes on polycarbonate (PC)15 and poly (ethylene terephthalate)(PET)16 sheets. This protocol was based on the UV photolysis of the polymers, leading to the formation of carboxylic groups on the UV exposed surface of the polymer sheets. These carboxylic groups served as a primary scaffold for the immobilization of gold nanoparticle catalysts (seeds) required by electroless gold plating. However, preliminary tests revealed that this protocol did not work for (22) Zhang, Q.; Xu, J. J.; Liu, Y.; Chen, H. Y. Lab Chip 2008, 8, 352–357.

Scheme 1. Chemical Reactions Involved in the Activation Process

PDMS, probably because the UV photolysis of PDMS usually generated hydroxyl groups rather than carboxylic groups.11,23 Recently, Allbritton’s group reported a novel approach for region-selective grafting of PAA onto a PDMS surface20,21 by means of benzophenone-mediated photopolymerization. Considering that the grafted PAA contains carboxylic groups which might be exploited as the primary scaffold for immobilization of gold nanoparticles, we adopted this procedure to introduce the carboxylic moieties onto the PDMS surface. It was observed that the PAA-grafted region was highly hydrophilic and the grafted pattern protruded a little above the PDMS surface (3-5 µm measured with a profile meter). An attenuated total reflection Fourier transformation infrared (ATR-FT-IR) spectrum of the PAAgrafted region clearly showed a strong absorption band around 1710 cm-1 that could be assigned to the stretching vibration band of carboxylic groups (see spectrum b in Figure S1 in the Supporting Information). After PAA grafting, the PDMS substrate was then subjected to a series of wet chemical reactions including amination of the carboxylic groups with ethylenediamine in the presence of EDC, coordination of Au(III) by the amine groups of the conjugated ethylenediamine, and reduction of Au(III) with NaBH4 to form gold seeds. ATR-FT-IR measurements revealed that after amination new absorption peaks appeared at 1552 cm-1 and 1638 cm-1 that could be assigned to the carbonyl stretching vibration of amide groups. Also, it was noted that the broad absorption band appeared above 3000 cm-1, which could probably be caused by the stretching vibration of the N-H bond in the grafted free amine groups (see spectrum c in Figure S1 in the Supporting Information). After the aminated PDMS was sequentially treated with a Au(III) solution and a NaBH4 solution, the UV irradiated region turned slightly gray, indicating the formation of gold nanoparticles. The chemistry of the whole activation process was diagrammatically shown in Scheme 1. Upon immersing the sample PDMS into a gold plating bath, Au(I) ions were reduced, under the catalysis of the gold seeds immobilized in the PAA-grafted region, to Au(0) with formaldehyde on the basis of the following reaction 2Au(I) + HCHO + 3OH- ) HCOO- + H2O + 2Au(0) Thus, gold particles gradually deposited onto the PAA-grafted region, and gold film patterns could be plated within 40 min at room temperature. The plated gold patterns appeared not as shiny as those electrolessly plated on PC and PET surfaces. This might be due (23) Efimenko, K.; Wallace, W. E.; Genzer, J. J. Colloid Interface Sci. 2002, 254, 306–315.

to the relatively high roughness of the gold film prepared on the PAA grafted PDMS surface: a root mean square (rms) of 363 nm was indicated by the AFM measurement (Figure S2 in the Supporting Information), and the high roughness of the gold film was obviously caused by that of the grafted PAA layer. Figure 2 shows the pictures of the typical micro gold devices fabricated with the recommended procedure. Figure 2c is a SEM picture showing an array of gold lines (10 µm in width). It should be pointed out that the chromium-glass photomask used for UV grafting was fabricated via a mother-mask of printed PET film and that uncollimated UV lights were used for the PAA grafting. It is reasonable to assume that finer gold patterns will be achieved when a more accurate photomask and collimated UV light source are employed. Performances of the Electrolessly Plated Micro Gold Devices. Adhesion Strength. A Scotch tape test24 was employed to qualitatively assess the adhesion strength between plated gold films and PDMS substrates. In this test, the Scotch tape was first firmly pressed onto the gold layer and then quickly peeled off. All of the gold devices plated on the PDMS substrates survived in the Scotch tape test, indicating the satisfying adhesion strength of the electrolessly plated electrodes. The mechanical strength of the samples against repeated folding/straightening was also tested. After enduring hundreds of repeated folding/straightening tests, no fracture was observed in the gold devices prepared on a 1 mm thick PDMS substrate. After the folding/straightening test, the gold devices could also tolerate the Scotch tape test. The plated devices also showed good stability when used in electrolyte solutions. No delamination of the fabricated gold electrodes was observed after they had been used as the working electrodes in the electrochemical detections for dozens of hours. Furthermore, no damages occurred after the gold patterns had been immersed in 0.2 M NaOH or 0.2 M HCl solution for over 20 h. Electrochemical Performances. Cyclic voltammetry (CV) was performed with the model analyte of K4Fe(CN)6 to examine the electrochemical properties of the electrodes. The voltammogram obtained with the electrolessly plated gold micro electrode agreed well in peak potentials with that obtained with a commercial gold disk electrode (Figure S3 in the Supporting Information), indicating the excellent electrochemical properties of the plated gold electrodes. SAM Formation. Hexadecanethiol was involved to evaluate the quality of the electrolessly plated gold films, because a perfect SAM of hexadecanethiol formed on a gold surface would block (24) Baker, L. A.; Zamborini, F. P.; Sun, L.; Crooks, R. M. Anal. Chem. 1999, 71, 4403–4406.

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Figure 3. Electropherograms for three consecutive runs of 100 µM DA and 100 µM CA obtained using a full PDMS microfluidic chip with an integrated gold working electrode for amperometric detection. The injection channel was 8 mm long, and the separation channel was 26 mm with an effective separation length of 23 mm. Na2HPO4/ KH2PO4 (20 mM, pH 7.0) was used as the running buffer. High voltages for injection and separation were 500 V and 800 V, respectively. The working electrode was aligned 30 ( 10 µm from the outlet of the separation channel, and a detection potential of 0.8 V (vs Ag/AgCl reference electrode) was employed.

Figure 2. Images of gold patterns fabricated on PDMS surfaces. (a) Picture of a micro gold electrode array prepared on a PDMS sheet of 2.5 cm × 3 cm. (b) Microscope image of the sensing part of the micro gold electrode used in electrophoresis detection. The size of the narrow sensing part was 50 µm (width) × 6 mm (length). (c) SEM image of an array of straight gold lines of 10 µm width and 90 µm spacing between each of the neighboring lines.

Faradaic processes on the electrode.25 On the basis of the area of the CV reduction peaks obtained in a 0.1 mol L-1 H2SO4 before and after the SAM formation (Figure S4 in the Supporting Information), a SAM coverage of 99.61 ± 0.26% (n ) 5) was estimated. This result indicates the good surface properties of the electrolessly deposited gold films and demonstrates the potential of the prepared micro gold electrodes to be further tailored to various biosensors with the help of the SAMs technique. Application. PDMS has been widely used to prepare micro electrophoresis chips in the past one and half decades. When integrated electrochemical detection was adopted, the required sensing electrodes were usually prepared on a glass plate that was subsequently bonded to a PDMS sheet with a channel (25) Hou, Z. Z.; Abbott, N. L.; Stroeve, P. Langmuir 1998, 14, 3287–3297.

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network to form a PDMS/glass hybrid chip.14,26 Such hybrid chips can only support inhomogeneous EOF, which might deteriorate the separation efficiency. Therefore, the present method was applied to prepare a full PDMS chip composed of an electrodeintegrated PDMS cover sheet and a channel-structured PDMS sheet. Despite the electrode protruding (3-5 µm) on the flat PDMS cover sheet, the two pieces of PDMS sheets could be reversibly sealed due to the elastic property of PDMS material. No solution leakage occurred during electrophoresis running, and no electrode damage was observed after continuously running electrophoresis for more than 10 h. The full PDMS chip showed good analytical performance. With dopamine (DA) and catechol (CA) being the model analytes, the relative standard deviation (RSD) of detected peak-height signals for DA and CA were 2.1% and 2.4%, respectively (each at 100 µM, n ) 21). The separation efficiency achieved at the applied electric field of 308 V/cm was 2.1 × 104 plates/m (H ) 4.7 × 10-5 m for DA), which was significantly higher than that (8600 plates/m) obtained at approximately similar electric fields (250 V/cm) with a PDMS/ glass hybrid chip.14 The uniform EOF supported by the homogeneous PDMS channels is one of the reasons for the improvement in the separation efficiency. Figure 3 shows the typical recorded electropherogram traces for three consecutive runs of a standard solution containing DA and CA. This PDMS chip demonstrated the suitability of the electrolessly plated micro gold electrode for electrochemical detection coupled with chipbased electrophoresis. CONCLUSION The PAA layer region selectively grafted on the PDMS surface provides the active carboxylic moieties which can be used as the scaffold for immobilization of metal nanoparticle catalysts necessary for region-selective electroless plating. The prepared micro (26) Martin, R. S.; Gawron, A. J.; Lunte, S. M.; Henry, C. S. Anal. Chem. 2000, 72, 3196–3202.

gold devices show strong adhesion to the PDMS substrate and excellent electrochemical properties and allow the SAMs of thiolcompounds to be perfectly formed on their surface. The resolution of the prepared micro gold devices is no worse than 10 µm, which could be improved if finer photomasks and collimated UV lights are used. Besides the excellent adhesive and mechanical strength the fabricated devices possess, one attractive advantage of this protocol lies in its feasibility in ordinary chemistry laboratories without the need of a clean room and expensive facilities. With a few modifications, the protocol can be extended to fabrication of micro devices of other metals, such as silver, copper, platinum, nickel, etc., onto PDMS surfaces or into the inner wall of PDMS micro channels as well.

ACKNOWLEDGMENT This work is funded by the National Natural Science Foundation of China (Project No. 20675072 and 20890020) and the National Basic Research Program of China (973 Program, Project No. 2007CB714502). SUPPORTING INFORMATION AVAILABLE Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org. Received for review July 10, 2009. Accepted August 29, 2009. AC901539N

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