Optoelectronic properties of DNA thin films implanted with titania nanoparticle-coated multiwalled carbon nanotubes

Rendering the unique features of individual nanoscale constituents into macroscopic thin films remains technologically challenging; the engineering of these constituents habitually compromises their inherent properties. Efficient, environmentally benign, and biodegradable DNA and cetyltrimethyl-ammonium chloride-modified DNA (DNA-CT) thin films (TFs) implanted with titania nanoparticle-coated multiwalled carbon nanotubes (MCNT-TiO2) are prepared by a drop-casting technique. The energy dispersive X-ray spectroscopy studies of DNA and DNA-CT TFs with MCNT-TiO2 identifies various elements (C, O, N, P, Na, and Ti) via quantitative microanalysis. The X-ray photoelectron, Raman, Fourier-transform infrared (FTIR), and UV-visible absorption spectra show changes in the chemical compositions and functional groups associated with binding energies, enhancement of characteristic MCNT-TiO2 Raman bands, and intensity changes and peak shifts of the FTIR and UV-Vis-NIR absorption bands, respectively. The PL spectra indicate an energy transfer in the measured samples, and the quenching of PL indicates a decrease in the recombination efficiency. Lastly, we measure the conductivity, which increased with an increasing concentration of MCNT-TiO2 in the DNA and DNA-CT TFs due to the better connectivity of MCNT-TiO2. By using these materials, the optoelectronic properties of DNA and DNA-CT TFs implanted with MCNT-TiO2 are easily tunable, enabling several engineering and multidisciplinary science applications, such as photonics, electronics, energy harvesting, and sensors.Rendering the unique features of individual nanoscale constituents into macroscopic thin films remains technologically challenging; the engineering of these constituents habitually compromises their inherent properties. Efficient, environmentally benign, and biodegradable DNA and cetyltrimethyl-ammonium chloride-modified DNA (DNA-CT) thin films (TFs) implanted with titania nanoparticle-coated multiwalled carbon nanotubes (MCNT-TiO2) are prepared by a drop-casting technique. The energy dispersive X-ray spectroscopy studies of DNA and DNA-CT TFs with MCNT-TiO2 identifies various elements (C, O, N, P, Na, and Ti) via quantitative microanalysis. The X-ray photoelectron, Raman, Fourier-transform infrared (FTIR), and UV-visible absorption spectra show changes in the chemical compositions and functional groups associated with binding energies, enhancement of characteristic MCNT-TiO2 Raman bands, and intensity changes and peak shifts of the FTIR and UV-Vis-NIR absorption bands, respectively. The PL spectra indica...


I. INTRODUCTION
For several decades, the physical, chemical, and biological properties of deoxyribonucleic acid (DNA) molecules have been extensively studied to better understand their intrinsic characteristics (e.g., self-assembly and molecular recognition) and extrinsic capabilities with other nanomaterials (e.g., modification and scaffolding) in various applications. DNA has a water-soluble, alternating polymer backbone and demonstrates promising characteristics, including high transparency (near UV and visible), small dumping (near infrared), low optical loss, a high dielectric constant, and excellent thermal resistance. These exceptional optoelectronic characteristics are useful for bioelectronics, photonics, and various sensor and device applications. [1][2][3][4][5][6] In contrast, cationic surfactant cetyltrimethylammonium chloride-modified DNA (DNA-CT) has deprived water solubility and is less delicate to water sorption. DNA-CT is readily soluble in various organic solvents that can rapidly evaporate. In light-emitting diodes, DNA-CT mainly has been employed as a charge transport layer. [7][8][9] In addition, DNA can serve as a template for aligning various nanomaterials (e.g., ions, nanoparticles, carbon-based materials, and proteins) to enhance specific functionalities. [10][11][12] It is important to assimilate a greater number of components into macroscopic materials, thereby facilitating the creation of enhanced multifunctional characteristics.
Metal oxide semiconductor-based nanocomposites have been extensively studied owing to their electronic, energyharvesting, and environmental applications. Among various metal oxide materials, zero-dimensional (0D) titanium dioxide (TiO 2 ) nanoparticles, known as titania, are considered to be an interesting material due to their novel optical characteristics, wide band gap semi-conductivity, nontoxic nature, biological and chemical inertness, and photocatalytic activity. [13][14][15][16] Recently, there have been noticeable developments in the creation of functional nanomaterials by integrating TiO 2 with inorganic and organic compounds. 1D carbon nanotubes (CNTs) have outstanding mechanical strength; excellent electrical conduction, thermal stability, and durability; high surface area; and a unique structure. TiO 2 nanoparticle-coated MCNT (MCNT-TiO 2 ) nanocomposites can be used to provide promising and specific properties for environmental, physical engineering, and biological sciences applications. [17][18][19][20][21][22][23][24][25][26][27] Due to their structural reliability, durability, and large surface area, MCNTs can be treated as efficient carriers for TiO 2 nanoparticles.
The development of reliable and scalable assembly techniques to construct various multifunctional devices is important in order to tailor materials for target applications. Although various functionalized nanomaterials (e.g., nanoparticles, ions, and proteins) can be embedded into DNA molecules, DNA implanted with MCNTs has rarely been reported due to the difficulties in constructing complexes with a uniformly distributed 1D nanostructure. MCNTs with TiO 2 nanoparticles allow for additional functionalization in the composite as compared to pristine MCNTs. The optical and electrical characteristics of MCNT-TiO 2 -implanted DNA and DNA-CT complexes, with various concentrations of MCNT-TiO 2 ([MCNT-TiO 2 ]), provide unique and enhanced functionality compared to pristine DNA and DNA-CT.
With the aid of the aforementioned advantages and unique characteristics, we fabricate DNA and DNA-CT TFs implanted with various [MCNT-TiO 2 ] by a drop-casting method. In addition, the MCNT-TiO 2 -implanted DNA (named as MCNT-TiO 2 +DNA) and MCNT-TiO 2 -implanted DNA-CT (MCNT-TiO 2 +DNA-CT) TFs are characterized in terms of their elemental composition (energy dispersive X-ray spectroscopy, EDS), chemical states and chemical functional groups (X-ray photoelectron spectroscopy, XPS), binding interactions and vibration/stretching modes (Raman and Fourier transform infrared (FTIR) spectroscopies), optical absorption (UV-Vis-NIR spectrophotometer), photoluminescence (PL mapper), and electrical properties (semiconductor parameter analyzer).

A. Sample preparation
Initially, we purchased DNA produced from salmon (GEM Corporation, Shiga, Japan) and MCNT-TiO 2 (US Research Nanomaterials, Inc., Houston, TX, USA). The outside diameter, inner diameter, and length of MCNTs are >50 nm, 5-15 nm, and 5-20 µm, respectively, and the diameter of TiO 2 nanoparticles is ∼8 nm. For prepare the MCNT-TiO 2 +DNA and MCNT-TiO 2 +DNA-CT solutions, 0.05 g of DNA and DNA-CT fibers as well as proper amounts of MCNT-TiO 2 are mixed in 5 mL of de-ionized (DI) water and 5 mL of 1-butanol, respectively,

III. RESULTS AND DISCUSSION
The fabrication procedures for the DNA and DNA-CT TFs implanted with TiO 2 -coated MCNT are shown in Figs. 1(a) and 1(b). In order to prepare the DNA solution, DNA fibers were mixed in DI water followed by magnetic stirring. DNA and CT were dissolved separately in DI water to prepare the DNA-CT. Then, CTMA was added slowly into the DNA solution while stirring. After filtering, CTMA-modified DNA fibers in a solid phase were obtained. To prepare the DNA-CT solution, DNA-CT fibers were dissolved in 1-butanol. An appropriate amount of MCNT-TiO 2 was mixed into the DI water (1-butanol) and then allowed by magnetic stirring to attain a consistent solution of MCNT-TiO 2 +DNA (MCNT-TiO 2 +DNA-CT). 20 µL of the MCNT-TiO 2 +DNA (MCNT-TiO 2 +DNA-CT) solution was then drop-casted on a given substrate in order to obtain a ∼2-µm-thick thin film. 43.90, 16.60, 30.94, 4.58, and 3.98%, respectively. Similarly, the atomic weight% of C, N, O, Na, P, and Ti in MCNT-TiO 2 +DNA were found to be 44.08, 1.30, 42.04, 3.58, 3.38, and 5.62%, respectively. As expected, after adding MCNT-TiO 2 into DNA TFs, the atomic weight% of C and O were enhanced but the atomic weight% of N, Na, and P were reduced, as compared to DNA TFs. The EDS spectra provided evidence of the encapsulation of DNA molecules with MCNT-TiO 2 .
The chemical states, chemical functional groups, and elemental compositions of the DNA and MCNT-TiO 2 +DNA TFs, as well as the MCNT-TiO 2 powder, were analyzed using XPS.  29 The XPS survey graphs showed peaks associated with C, O, N, P, Na, and Ti at their characteristic binding energies, thereby confirming the presence of MCNT-TiO 2 in the DNA thin film.
Figs. 2(b)-2(g) show the comparative high-resolution, deconvolved core XPS graphs of C 1s, O 1s, Ti 2p, N 1s, P 2p, and Na 1s for respective samples. These show a shift in the binding energy, changes in the chemical composition, and variations in the peak intensity. The binding energies (chemical groups) of the C 1s spectra were observed at 284. The XPS spectra provided correlative analysis of the characteristic elements in the DNA and MCNT-TiO 2 +DNA TFs. After adding MCNT-TiO 2 into the DNA thin film, the binding energies at 288 and 290.0 eV in the C 1s orbital were suppressed. In the core orbitals of O 1s, Ti 2p, N 1s, P 2p, and Na 1s, a negative shift (with a magnitude of ∼1.5 eV, which occurred due to charge transfer from MCNT-TiO 2 to DNA) in the binding energies as well as significant suppression of the peak intensities (at 536.

ARTICLE
scitation.org/journal/adv vibration modes of sugar and phosphate backbone groups (600 -1250 cm -1 ), stretching and vibration modes of nucleobases (1300 -1800 cm -1 ), and water OH stretching modes (3000 -3600 cm -1 ). 36 The peaks located at ∼780, 835, and 938 cm -1 can be assigned to sugar phosphate vibration, deoxyribose-phosphate, and adenine-thymine base pairs, respectively. The absorption of C−C/C−O of deoxyribose skeletal motion, P−O stretching vibration, and C−O deoxyribose can be found at 963, 1014, and 1050 cm -1 , respectively. The peaks at 1082 and 1220 cm -1 were caused by the symmetric and anti-symmetric stretching vibrations of the phosphate groups, respectively. The absorption of the cytosine in-plane vibration, adenine C7=N stretching, thymine C2=O stretching, and guanine C=O stretching were observed at 1488, 1603, 1653, and 1700 cm -1 , respectively. The other absorption peaks at 1420 and 1373 cm -1 are related to cytosine and guanine, respectively, and the absorption at 3000 − 3600 cm -1 corresponds to N−H, C−H, and water OH stretching modes. Compared to the pristine DNA, the DNA-CT TFs exhibited additional absorption bands at around 1010, 2850, and 2920 cm -1 ; these can be ascribed to C−H, CH 2 , and CH 3 , respectively, due to surfactant modification of DNA with CTMA.
The UV-Vis-NIR spectroscopy was carried out to examine the optical characteristics of a series of DNA and DNA-CT TFs implanted with MCNT-TiO 2 . Measurements were taken in the wavelength range from 190 to 3200 nm under ambient conditions. Fig. 5 shows the absorption spectra of DNA and DNA-CT TFs implanted with various [MCNT-TiO 2 ]. The characteristic absorption peaks of DNA and DNA-CT TFs without MCNT-TiO 2 at wavelengths of 210 and 260 nm were produced by the phosphate backbone and nitrogenous bases, respectively. 37,38 Although apparent splitting of the DNA absorption peaks at 210 and 260 nm was observed at relatively low [MCNT-TiO 2 ], a merged broad absorption peak in the UV region was noticed at high [MCNT-TiO 2 ] due to the interactions between DNA (DNA-CT) molecules and MCNT-TiO 2 . In addition, the characteristic absorption peak intensities of DNA were reduced as [MCNT-TiO 2 ] increased. Due to the presence of MCNT-TiO 2 , two absorption peaks at wavelengths of 925 and 2330 nm were found; these were caused by various functional group interactions between MCNT and TiO 2 nanoparticles. 39,40 Lastly, a broad absorption peak centered at ∼2970 nm may be caused by water OH molecules present in all samples. advantages in various optical and sensor applications (e.g., photodetectors, UV radiation sensors, and IR detectors) because the overall absorption can cover the full spectral range from the UV, visible, and telecommunication window to far-IR regions.
The PL spectra were taken to investigate energy transfer and charge carrier trapping as well as to better understand the exchange of electron-hole pairs in the samples. The PL characteristics of the DNA TFs implanted with MCNT-TiO 2 at a fixed excitation wavelength of 266 nm are depicted in Fig. 6. The PL spectra of the MCNT-TiO 2 +DNA TFs revealed a broad emission in the blue region. The PL spectra also showed a blue shift and PL quenching with increasing [MCNT- TiO 2 ]. In contrast, a remarkable decrease in the electron-hole recombination was anticipated since no characteristic peaks of MCNT-TiO 2 were observed in the PL spectra. The peak center, peak height (inset), and energetic emission area as a function of [MCNT-TiO 2 ] in the DNA TFs, obtained using Lorentz fitting, are displayed in Fig. 6

ARTICLE
scitation.org/journal/adv wavelength; this might be useful in physical devices and sensor applications.
Finally, we studied the electrical characteristics to investigate the electrical transport phenomena of the DNA and DNA-CT TFs implanted with MCNT-TiO 2 ; this was done with a semiconductor parameter analyzer (Fig. 7). The MCNT-TiO 2 +DNA and MCNT-TiO 2 +DNA-CT TFs showed semiconducting behavior, which originated from the semiconducting TiO 2 nanoparticles in the TFs. Although current enhancements in both MCNT-TiO 2 +DNA (∼10 times) and MCNT-TiO 2 +DNA-CT (∼1000 times) TFs were noticed with increasing [MCNT-TiO 2 ], drastic increases in the currents were observed in MCNT-TiO 2 +DNA-CT TFs as compared to the MCNT-TiO 2 +DNA TFs. During sample preparation, MCNT-TiO 2 mixed with DNA-CT better in 1-butanol (lower viscosity) than with DNA in DI water (higher viscosity), which led to better connectivity of MCNT-TiO 2 (especially at higher [MCNT-TiO 2 ]) in the TFs. Consequently, more electrons were transported through MCNT-TiO 2 in the DNA-CT TFs than in the DNA TFs. The resistances of the MCNT-TiO 2 +DNA and MCNT-TiO 2 +DNA-CT TFs with [MCNT-TiO 2 ] at a fixed voltage of 5 V, as obtained from current-voltage curves, are shown in Fig. 7(c). As expected, the resistances monotonically decreased with increasing [MCNT-TiO 2 ] by factors of ∼767 MΩ (for DNA) and ∼5 GΩ (for DNA-CT) per unit wt%.

IV. CONCLUSIONS
In summary, we developed a new method to construct MCNT-TiO 2 -implanted DNA and DNA-CT TFs by a drop-casting method. Furthermore, we studied the elemental composition, chemical states, functional groups, binding interactions, modes of vibration and stretching, absorption, photoluminescence, and electrical properties of these materials. From the EDS spectra, the atomic weight% of C and O were observed to increase, while the atomic weight% of N, Na, and P decreased after the addition of MCNT-TiO 2 in DNA TFs. As expected, the XPS spectra showed peaks associated with C, O, N, P, Na, and Ti at their characteristic binding energies, which confirmed the presence of MCNT-TiO 2 in the DNA TFs. Raman spectra, which revealed the distinct vibrational and stretching modes of the samples, were used to identify the interactions of the functional groups (i.e., MCNT-TiO 2 in DNA and DNA-CT TFs). Intensity changes and peak shifts of the FTIR absorbance, UV absorption, and PL emission as a function of [MCNT-TiO 2 ] suggested the strong interaction between DNA (DNA-CT) and MCNT-TiO 2 via electrostatic and non-covalent bonding interactions. Interestingly, the DNA and DNA-CT TFs with MCNT-TiO 2 showed semiconducting behavior due to the presence of semiconducting TiO 2 nanoparticles in the TFs. Our results suggest that high tunability of the physical and chemical characteristics can be achieved by simple control of the functionalized [MCNT-TiO 2 ] in both DNA and DNA-CT TFs. Due to their unique features, macroscopic DNA and DNA-CT TFs implanted with MCNT-TiO 2 may lead to new research areas in nanomaterials science and engineering.