Fig. 1. Schematic diagram to control the refractive index of DNA thin solid film by denaturation. Denaturation is activated by adding NaOH in DNA aqueous solution precursors, which is irreversibly immobilized in thin solid film to change the refractive index (ds, double stranded; ss, single stranded).

Fig. 2. DNA thin solid film fabrication process. (a) DNA aqueous solution and DNA-NaOH aqueous solution where the denaturation is reversible. (b) O2 plasma treatment on Si/SiO2 substrate to make hydrophilic surfaces. (c) Dispensing aqueous solution precursors on the substrate. (d) Spinning and solidification by water evaporation. (e) Single-stranded DNAs are maintained in the thin solid film to achieve the irreversible denaturation.

Fig. 3. (a) UV/visible spectra of DNA aqueous solution with various NaOH concentrations in the precursor solutions. Here we used 0.15 wt. % DNA. (b) Hyperchromicity near λ=260  nm was clearly observed by increasing the NaOH concentration.

Fig. 4. (a) FTIR absorption spectra of solid freestanding DNA films made from DNA aqueous solutions with various NaOH concentrations. (b) Spectral shift of the cytosine vibration peak as a function of NaOH concentration in the DNA precursor solutions.

Fig. 5. (a) The refractive indices of DNA thin solid film in the spectral range from 380 to 900 nm for various NaOH concentrations in precursor solutions. (b) The refractive indices of DNA thin solid film as a function of NaOH concentration in the precursor solutions.

Fig. 6. Thermally induced changes in the refractive index and the film thickness of DNA thin solid film with various NaOH concentrations in precursor solutions. (a) Refractive index at λ=633  nm as a function of temperature in the first temperature cycle; (b) refractive index at λ=633  nm as a function of temperature in the second cycle. (c) Film thickness as a function of temperature in the first cycle; (d) film thickness as a function of temperature in the second cycle.