《Table 2.Thermo-Optic Coefficient of DNA Thin Solid Film at a Wavelength of 633 nm for Various NaOH

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《Irreversible denaturation of DNA: a method to precisely control the optical and thermo-optic properties of DNA thin solid films》

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Variation of the refractive index with the external temperature，the thermo-optic effect，is a very fundamental mechanism to add novel functionalities in photonic devices providing spectral tuning，switching，and sensing applications.Implementing a temperature-controlled unit inside a Wollam ellipsometry system，we measured the changes in the refractive index and the thickness of DNA films made from NaOH-DNA precursor solutions for various NaOH concentrations.The results are summarized in Fig.6 in the temperature range from 40°C to 90°C.The heating/cooling rate was about10°C∕min，which was controlled by a Peltier thermoelectric device.We observed a linear correlation between the decrease of the refractive index and the temperature increase，which is consistent with prior reports on a negative thermo-optic coefficient，dn/d T，of DNA thin films[9–11].We also observed a monotonic decrease of the film thickness as the temperature increased，and after the first cycle，the film thickness slightly decreased due to additional drying of the DNA film，which is also consistent with prior reports[10，11].We repeated measurements in successive temperature cycles and observed no further significant changes in the thermo-optic coefficients and the film thickness within experimental errors after the second cycle.We summarized the dn/d T measurements of the DNA thin films atλ?633 nm for various NaOH concentrations in the precursor solutions in Table 2.The difference in jdn∕d T j between the first cycle and the second cycle was due to water evaporation within the DNA thin film in the first cycle，which is consistent with prior DNA-based films[11].In repeated temperature cycling experiments，we did not observe any hysteresis in the refractive index and thickness，which indicated that the DNA films were thermally stable and the melting temperature of the film would be above 90°C.We observed that the magnitude of dn/d T of our denatured DNA thin films was significantly higher than that of DNA-CTMA（cetyltrimethylammonium）thin film，which has recently been measured to be dn/d T～3.57×10-4°C-1[9]for a film thickness of 60 nm.The addition of NaOH and the subsequent increase in the level of DNA denaturation especially showed a good linear correlation with the magnitude of the thermo-optic coefficient，which could be attributed to both the random coil structure of ss-DNA and NaOH ingredients in the DNA thin films.Note that the magnitude of dn/d T of these denaturated DNA thin films is also larger than those of conventional thermo-optic polymers[39]，which can provide a definite advantage in various device applications.