Immobilization/hybridization of amino-modified DNA on plasma-polymerized allyl chloride
Introduction
Recently, plasma surface modification processes have played a significant part in the development of many of these highly specific coatings and modifications. They have received considerable attention in product development due to their versatility and flexibility in view of their ecological and economic aspects [1], [2], [3], [4], [5]. Plasma polymerization processes enable the deposition of a substantive variety of functional thin films [6], [7], [8], [9], [10], [11] onto virtually any substrate. However, the application of these deposits often suffers from insufficient adhesion to the substrate and unpredictable swelling behavior when subjected to aqueous solution. Although a number of methods, including pretreatment of the substrate in an activating plasma (Ar or O2) for polymeric substrates [12] or the use of an additional layer covalently binding to the substrate surface on one side and to the plasma polymer on the other side [4], were applied to enhance the adhesion strength between the plasma films and the substrate. Therefore, it will be very attractive to explore plasma-polymerized materials possessing high stability in aqueous solution. For example, one can prepare plasma polymers under the high plasma power or continuous wave (CW) plasma [13].
Despite remarkable publications on a variety of biotechnology applications, ranging from DNA microarrays [14], [15], [16], [17] to biosensors [18], [19], only few reports address the plasma-polymerized films as DNA immobilization/hybridization matrix, The pulsed plasma-generated acryloyl chloride polymers by Calderon and Timmons shows that some biomolecules can be tailored onto the polymer surface [20]. Duman et al. provided a new approach for immobilizing oligonucleotides onto piezoelectric quartz crystal for the preparation of a nucleic acid sensor for following hybridization [21]. Miyachi et al. reported that the hydrophobic immobilization matrix was considered to enhance hybridization accuracy and efficiency, compared with its hydrophilic acetonitrile-plasma-polymerized layers [22]. Furthermore, a biochemical method to immobilize nucleic acids onto a diamond surface has been developed, which is grown using microwave plasma-enhanced chemical vapour deposition [23]. As an alternative candidate, allylamine (ppA) could be applied as one matrix of DNA immobilization/hybridization because of the strong electrostatic adsorption [4], [24]. Surface plasmon enhanced fluorescence spectroscopy (SPFS) could then be used to monitor the hybridization reaction of fluorophore-labeled 15 mer target oligonucleotides from solution. Meanwhile, the relative weak electrostatic interaction between oligonucleotides and plasma films may result in the dissociation of the hybridized double DNA strands continuously running. In the present work, ppAC was proved to show higher stability in aqueous solution and possessed strong adhesion with the silicon wafer. ppAC films were then investigated as the matrix for DNA immobilization.
Section snippets
Plasma polymerization
The plasma polymerization was carried out on the Plasma 80 Plus PECVD system, manufactured by Oxford Instruments Plasma Technology Inc. of Yatton, UK. The radio frequency (RF) generator was operated at a frequency of 13.56 MHz. The plasma deposition process was performed between two parallel plate electrodes of 24 cm in diameter and 6 cm apart. The silicon chips were placed on the ground electrode. Prior to each experiment, the samples were activated by the argon plasma at a settled flow rate of 5
Surface stability and wettability
The water contact angles of ppAC films deposited at 5, 20, and 50 W are shown in Fig. 1(a). It is found that the water contact angle decreases from 65° to 42° with increasing the input power, which indicates that the hydrophilicity of ppAC films tones up. Fig. 1(b) provides the stability of ppAC films in H2O by using the variation of the thickness of films. For low power ppAC deposited at 5 W the thickness reduced by around 11%, while ppAC films deposited at 20 and 50 W became thicker.
It is known
Conclusions
The surface stability, wettability, and the chemical structure of ppAC films were discussed in details using ellipsometry, the static water contact angles, FTIR, and XPS. It was observed that ppAC films of the higher power deposition were much more stable in the aqueous solution than that of the low power. Under high power, however, the possibility of oxidation of ppAC by O2 in air becomes stronger. Furthermore, the chemical structure of ppAC at high power appears more complex.
In addition, DNA
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