Fixation and Shrinkage of Polyurethane Molecular Stamps used for the synthesis of DNA Microarray

Transcription

1 Fixation and Shrinkage of Polyurethane Molecular Stamps used for the synthesis of DNA Microarray Z C Liu, Q G He, P F Xiao, JXTang,NYHe,ZH Lu Presented at the 8th International Conference on Electronic Materials (IUMRS-ICEM 2002, Xi an, China, June 2002) 433

2 Fixation and Shrinkage of Polyurethane Molecular Stamps used for the synthesis of DNA Microarray Z.C. Liu* Q.G. He P.F. Xiao J.X. Tang N.Y. He* Z.H. Lu* (Key Laboratory for Molecular and Bio-molecular Electronics of Ministry of Education, Nanjing , China) Abstract The shrinkage of polyurethane stamps used for the in situ synthesis of DNA microarrays via molecular stamping method was studied with Micron XYZ Scope. It was found that the polyurethane stamp fixed on the epoxy resin modified glass strongly and showed minimal linear shrinkage. The linear shrinkage of the whole polyurethane stamp and that of each feature of polyurethane stamp were controlled within % and 0.309%, respectively, which were due to the strong van der Waals forces and hydrogen bonds between polyurethane and epoxy resin. It was also confirmed by scanning electron microscope that the polyurethane stamp fixed on the epoxy resin modified glass replicated the patterns of motherboard with a high fidelity. All these underlay the synthesis of DNA microarray through molecular stamping method. Keyword: Polyurethane molecular stamp; fixation; shrinkage 1. Introduction Polyurethane elastomers are synthetic elastomers with many advantageous properties, which have led to their use in a wide range of applications [1-4]. In recent study by the present authors polyurethane was found to be idea candidate for molecular stamps [5] used for DNA microarray synthesis via molecular stamping method [6]. DNA microarray synthesis using molecular stamps is in fact a kind of solid phase reaction with microcontact printing [7] and combinatorial experiment. The solutions of monomers are spun onto the patterned surface of stamps and the patterned surface is pressed onto the designed substrate such as 434 *Correspondence author. Tel: ; fax: address: liuzhengchun@seu.edu.cn; nyhe@seu.edu.cn; zhlu@seu.edu.cn

3 aminosilane modified glass slides and the monomers remained on stamps are allowed to react with these amino groups. According to the documents [6], a set of stamps with different patterns is required to overlap accurately during the synthesis of DNA microarrays. So a set of stamps with high fidelity patterns from motherboards is required to keep the verity of features. However, polyurethane will usually shrink about 2% in linearity during solidification [8], which is a disaster for the large-scale stamp with micrometer grade pattern to iteratively print going with patterns overlapping exactly. If it is used as the molecular stamp for the synthesis of DNA microarrays, cross contamination of reaction monomers will happen. Besides, the stamp will depart from its adhering substrate and accordingly deform if improper fixation methods are employed. All these problems relevant to the elastomer will affect the fabrication and the application of molecular stamp. In this paper, we studied the fixation and shrinkage of polyurethane molecular stamp, which laid a foundation for the synthesis of DNA microarray via molecular stamping method. 2. Experimental details 2.1. Materials Epoxy resin was obtained from ALTECO CHEMICAL PTE LTD., Singapore. Unless otherwise noted, all materials were obtained from commercial suppliers and used as received Fabrication of PU stamps 4 different glass substrates (Glass I, II, III and IV) used for the substrates of polyurethane stamps have been prepared according to the following methods. Commercially obtained glass slides were firstly washed with acetone, immersed in washing liquid (acidic Cr (VI) solution) for 30 min, rinsed carefully with distilled water, finally dried under nitrogen flow to produce Glass I. One side of Glass coated with a thin film of epoxy resin, which turned Glass into Glass if the resin did not solidify before use or Glass if the resin solidified before use. Commercially obtained ground glass slides were treated using the way of preparing Glass I to produce Glass IV. 435

4 According to the method previously reported [5], 4 kinds of polyurethane stamps fixed on the above- mentioned substrates were transferred from a stamp motherboard, having a 5 7 array of foursquare features with each feature being 50 µm in side size with center-to-center spacing of 100 µm and a feature thickness of 30 µm Characterization The features on motherboard and polyurethane stamp were measured with Micron XYZ Scope from NISSHO SEIMISU KOGAKU CO. LTD. Scanning electron microscopy images were obtained using Hitachi-650 (Japan) scanning electron microscope. An accelerating potential of 20 kv was used for the analysis of sample. 3. Results and discussion 3.1. Shrinkage of polyurethane stamps The shrinkage of polyurethane is related to its thickness, width and the raw materials and conditions chosen to produce it. The optimized experimental conditions were used for the preparation of polyurethane [5], including prescription, reaction temperature and time. Accordingly, we discussed here only the effects of different fixation methods on the shrinkage of definite polyurethane stamps used for DNA microarrays synthesis via molecular stamp method, with m in thickness and m in feature height. Experimental results told that there was an obvious discrimination among the fixations of polyurethane stamps. Polyurethane stamp would not be peeled from the Glass unless the stamp was destroyed. Contrarily, polyurethane stamps were easily peeled off other glass substrates without being disfeatured. The variations from motherboard to polyurethane stamps were accurately measured with Micron XYZ Scope. Table 1 lists in detail the features side values of polyurethane stamp and its corresponding motherboard. Considering there were a lot of ways introducing errors, we measured the every feature on the stamp (5 7 arrays, 35 features altogether) to decrease the error. And considering there were too large numbers of raw 436

5 data, we list in Table 1 only the results from the polyurethane stamp fixed on Glass, while the data from the other stamps were not shown. In order to compare the functions of these four substrates, we analyzed all raw data with statistical method and the results are list in Table 2. It is clearly shown that the features shrinkage of stamp on Glass averaged only 0.309%, while about 2% on the other three substrates approximating with that of bulk polyurethane [8]. At the same time, there was a feasibility for the features lying in different regions of a stamp to show a very large difference of shrinkage and this was the fact on Glass, Glass and Glass, where the linear shrinkage shift from 0.183% to 2.42%, 0.443% to 2.64% and 0.935% to 2.12%, respectively. It is interesting that this phenomenon was not observed on Glass, where the largest shrinkage was only 0.789% (from m to m), as shown in Table 1. We cared about not only the shrinkage of single features but also that of a whole stamp. If the overall shrinkage of stamps could not be accurately controlled, both deformation and shift of stamps would take place easily too. To our satisfaction, it is shown in Table 2 that the overall linear shrinkage of stamp on Glass was only % (3.4 µm per 1.0 cm), largely less than others (more than 1%, 0.1 mm per 1.0 cm). All these could be explained by the following way. According to the theory of intermolecular interaction, two kinds of molecules with similar polarity attract each other strongly. Epoxy resin, a sort of glass adhesive owing the solubility parameter about 20.9 (M Pa) 1/2 approximating with the [19.9(M Pa) 1/2 ] of polyurethane [9] was chosen to adhere polyurethane stamp here. Due to the similar properties of epoxy resin to polyurethane, the epoxy resin molecules on the Glass II would migrate toward polyurethane layer and the polyurethane molecules would migrate toward resin film too during solidification. There were strong van der Waals forces between the molecules and hydrogen bonds between the amino groups of polyurethane and hydroxyl groups of epoxy resin, which merged the two parts into a whole phase. To remember this, the epoxy resin is excellent glass glue. So the glass substrate, resin and polyurethane formed pseudo-sandwich structure, which was very stable and long-standing. It was of great importance for the fixation of polyurethane stamp 437

6 and the prevention of stamp distortion. On the other hand, the features stood out of the stamp surface about 30 µm, then conglutination function laid effect on vertical direction while little effect on horizontal direction. In this way, the features of the stamp on the Glass were self-existent and showed very little and homogenous linear shrinkage. Although there were hydroxyl groups on the Glass which formed a positive element for the adhesion of polyurethane stamp to the glass substrate, the hydroxyl groups are so short that they can not rivet with curing polyurethane molecules. The similar results lay in the Glass although the specific surface area was augmented by convoluted folds formed through mechanical ground. Glass was coated with epoxy resin as well as Glass. But, since the resin has solidified prior to use, not only the epoxy resin molecules were prohibited from moving into the polyurethane molecules as well as the reverse procedure, but also the amounts of hydroxyl groups on the substrate surface reduced due to cohesive action of molecules. Accordingly, the features shrinkages of stamps on Glass, Glass and Glass were very big and inhomogeneous; the linear shrinkage of single feature and that of a whole stamp followed the next order: Glass Glass Glass Glass, as shown in Table Scanning electron microscopy To further confirm the feasibility of fixing polyurethane stamp with epoxy resin, the photoresist motherboard and the corresponding polyurethane stamp fixed on epoxy resin modified glass substrate were examined using scanning electron microscope (SEM). The SEM micrographs, as shown in Figure 1 and Figure 2, show that the polyurethane stamp replicated the patterns of the motherboard very well and presented negative patterns with square microposts. During the process no considerable change in geometry could be observed and the patterns of the motherboard were transferred with a high fidelity. That is to say, epoxy resin modified glass slide is a good substrate for the fabrication of top-quality polyurethane molecular stamp. 438

7 4. Conclusions Micron XYZ Scope was used to check the shrinkage of polyurethane molecular stamp fixed on 4 different glass substrates. The data were analyzed with arithmetic average method. It is shown from the results that the epoxy resin modified glass substrate held fast the polyurethane molecular stamp, which drove the features on the stamp to shrink very little. It was also confirmed by SEM that polyurethane stamp on the epoxy resin modified glass replicated the patterns of motherboard with a high fidelity. Accordingly no dislocation will happen when this kind of polyurethane molecular stamp is used to fabricate DNA microarray with 50 µm in features side and 100 µm in center-to-center spacing through molecular stamping method. Acknowledgements We acknowledged with gratitude the financial supports of 973 National Key Fundamental Research Project, and National Natural Science Foundation of China References [1] W.Y. Chiu, P.S. Wang, T.M. Don, Polym. Degrad. Stab. 66 (1999) [2] St.Oprea, S.Vlad, A.Stanciu, Eur.Polym. J. 36 (2000) [3] S. Desai, I.M. Thakore, B.D.Sarawade, S.Devi, Eur.Polym. J. 36 (2000) [4] R.S.Kane, D. E.Ingber, G. M.Whitesides, Biomaterials 20(1999) [5] Z.C. Liu, Q.G. He, P.F. Xiao, N.Y. He, Z.H. Lu, B. Liang, in: Micromachintg and Microfabrication Process Technology and Devices, Proc. SPIE 2001, Vol. 4601, (Nanjing, China, 2001) [6] P.F. Xiao, N.Y. He, Z.C. Liu, Q.G. He, Z.H. Lu, Chinese Sci. Bull. - [7] Y. Xia, G. M.Whitesides, Angew. Chem. Int. Ed. 37(1998) [8] Y.J. Huang, C.J. Chu, J.P. Dong, J. Appl. Polym. Sci. 78 (2000) [9] P. H. Sun, C.Y. Lin, Eur. Polym. J. 33(1997)

8 Table 1. Data of the features of motherboard and the corresponding polyurethane stamp fixed on glass substrate No. R (10-4 m) R s (10-4 m) S % L (10-4 m) L s (10-4 m) S % R and L refer to length and width, respectively, of features of motherboard; R s and L s refer to length and width, respectively, of features of polyurethane stamp transferred from motherboard S%=100 (R R s )/R or 100 (L L s )/L. 440

9 Table 2. The comparison of linear shrinkage of polyurethane stamps fixed on different glass substrates Average linear shrinkage of a whole stamp (%) Figure 1. SEM of photoresist motherboard ( 150) Figure 2. SEM of polyurethane stamp fixed on epoxy resin modified glass substrate ( 150) 441