Abstract
In this study, we grafted bromo-terminated poly(N-isopropylacrylamide) (PNIPAAm) brushes onto thin gold films deposited on silicon, and then reacted with NaN3 to produce azido-terminated PNIPAAm brushes. A probe sequence of single-stranded DNA (ssDNA) with a 4-pentynoic acid succinimidyl ester unit was grafted onto the azido-terminated PNIPAAm brushes through a click reaction, resulting in the formation of block copolymer brushes. The PNIPAAm-b-ssDNA copolymer brushes formed supramolecular complexes stabilized by bio-multiple hydrogen bonds (BMHBs), which enhanced the proton transfer and thereby decreased the resistivity of the structures. In addition, the optimal operation window for DNA detection ranges from 0 to 0.2 M of NaCl concentration. Therefore, the specimens were prepared in the PBS solution at 150 mM NaCl concentration for target hybridization. The supramolecular complex state of the PNIPAAm-b-ssDNA copolymer brushes transformed into the phase-separated state after the hybridization with 0.5 ng/µL of its target DNA sequence owing to the competition between BMHBs and complementary hydrogen bonds. This phase transformation of the PNIPAAm and probe segments inhibited the proton transfer and significantly increased the resistivity at 25 °C. Moreover, there were no significant changes in the resistivity of the copolymer brushes after hybridization with the target sequence at 45 °C. These results indicated that the phase-separated state of the PNIPAAm-b-ssDNA copolymer brushes, which was generally occurred above the LCST, can be substantially generated after hybridization with its target DNA sequence. By performing the controlled experiments, in the same manner, using another sequence with lengths similar to that of the target sequence without complementarity. In addition, the sequences featuring various degrees of complementarity were exploited to verify the phase separation behavior inside the PNIPAAm-b-ssDNA copolymer thin film.
Highlights
BackgroundDNA is an interesting molecular material that shows a long rod-like duplex structure with base-pair (bp) stacking
DNA is an interesting molecular material that shows a long rod-like duplex structure with base-pair stacking.The base separation and the diameter of the duplex were 3.4 and 20 Å respectively
We have found that bio-multiple hydrogen bonds (BMHBs) between PNIPAAm and adenine (A) can be exploited to change the morphology, crystalline structure, and temperature-responsiveness of PNIPAAm merely by varying the concentration of A [16]
Summary
DNA is an interesting molecular material that shows a long rod-like duplex structure with base-pair (bp) stacking. The sample was rinsed with DMF followed by methanol and DI water before drying in the stream of N 2 After these reactions, the wafers were placed in a Soxhlet apparatus to remove any non-grafted materials and dried under N 2 prior to use. A solution of the 4-pentynoic acid succinimidyl ester-conjugated probe sequence (100 nmol) in phosphate buffer (PB, pH 7.2; 20 mM, 2.5 mL) was added to purged glass vessel containing a grafted azido-PNIPAAm sample and DMF (2.5 mL) to perform the click reaction. Assessment of BMHBs through effective equilibrium constant of layers (Ke) HB between the PNIPAAm and probe segments at temperatures below and above the LCST led to respective complexation and decomplexation resulted in changes to the thicknesses of the copolymer brushes and thermoresponsive behavior of the surfaces. The resistivity of each copolymer film was measured at 25 and 80 °C and the voltage sweeps were recorded from − 0.2 to + 0.2 V [11, 24]
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