Preparation and characterization of DNA-aromatic ... · Fourier-transform infrared spectroscopy (FTIR) and UV-VIS spectroscopy were used to characterize the presence of specific chemical

Preparation and characterization of DNA-aromatic surfactant complexes for optoelectronic applications

Ting-Yu Lin, Chia-Yun Chang, Chien-Hsiang Lien, Yi-Wen Chiu, Wei-Ting Hsu,

Che-Hsuan Su, Yu-Sheng Wang, and Yu-Chueh Hung*

Institute of Photonics Technologies, National Tsing Hua University, No. 101, Section 2, Kuang-Fu Road, Hsinchu, Taiwan 30013

ABSTRACT

DNA biopolymer has emerging as a promising material in photonic applications. In this paper, we present the preparation and characterization of a series of DNA-surfactant complexes based on aromatic surfactants, including vinylbenzyltrimethylammonium chloride, benzyltrimethylammonium chloride, and phenyltrimethylammonium chloride. Fourier-transform infrared spectroscopy (FTIR) and UV-VIS spectroscopy were used to characterize the presence of specific chemical groups in the materials. These synthesized DNA complexes show high transparency from 400nm to 1100nm. These materials can be spin casted into thin films from nm to um and the morphology was examined by SEM and AFM. Thermal property was characterized by thermogravimetric analysis. Conductivity was examined to investigate the effect of aromatic surfactants on the electrical properties of DNA complexes. In addition, the photoluminescence and lasing properties for DNA-aromatic surfactants with rhodamine dyes were investigated to study the amplified spontaneous emission where the ASE emission wavelength, lasing threshold, and gain were presented and discussed. The results were compared with DNA complex with single chain aliphatic surfactant complex (DNA-cetyltrimethylammoniumchloride).

Keywords: biopolymer, conductivity, amplified spontaneous emission

1. INTRODUCTION Over the last few years, much research attention has been paid to use deoxyribonucleic acid (DNA) as one kind of biopolymer for numerous applications in optoelectronic devices. DNA has several unique features and has been demonstrated its promise in organic light emitting devices (OLEDs)1-2, organic field effect transistors (OFETs)3,4, and organic lasers5,6. In order to make DNA soluble in organic solvents for device fabrication, extra procedures are required to modify the DNA molecules. A common approach is to bind the DNA phosphate backbone with surfactants to form DNA-surfactant complex. Device implementation based on DNA-surfactant complex up to date is mainly focused on single chain aliphatic surfactants, such as cetyltrimethylammonium chloride (CTMA). Due to the intrinsic property of aliphatic chain in CTMA, however, these DNA complexes normally possess high resistivity7 and may limit the implementation horizon of DNA biopolymer in optoelectronic devices.

In this study, we utilized different type of surfactants, i.e. surfactants with aromatic rings including vinyl-benzyltrimethylammonium chloride (VBTMA), benzyltrimethylammonium chloride (BTMA) and phenyl-trimethylammonium (PTMA), for synthesizing novel DNA complexes. A series of experiments were carried out to investigate the effect of aromatic surfactant on the optical and electrical properties of DNA complexes. In addition, the photoluminescence and lasing properties for DNA-aromatic surfactants with rhodamine dyes were investigated to study the amplified spontaneous emission (ASE) where the ASE emission wavelength, lasing threshold, and gain were presented and discussed in the following sections.

*Email: [email protected]

Organic Photonic Materials and Devices XIII, edited by Robert L. Nelson, François Kajzar, Toshikuni Kaino, Yasuhiro Koike,Proc. of SPIE Vol. 7935, 79350E · © 2011 SPIE · CCC code: 0277-786X/11/$18 · doi: 10.1117/12.874228

Proc. of SPIE Vol. 7935 79350E-1

Downloaded from SPIE Digital Library on 17 Jan 2012 to 140.114.195.186. Terms of Use: http://spiedl.org/terms

2. MATERIAL SYNTHESIS Deoxyribonucleic acid (DNA) from salmon testes (D1626) was purchased from Sigma-Aldrich. The surfactants, cetyltrimethylammonium chloride (CTMA), vinybenzyltrimethylammonium chloride (VBTMA), benzyl-trimethylammonium chloride (BTMA), phenyltrimethylammonium chloride (PTMA) were purchased from Alfa Aesar or ACROS Organics and used as received. The molecular structures of the aforementioned surfactants were illustrated in Figure 1.

2.1 Preparation of DNA-CTMA complexes

CTMA solution was prepared by dissolving commercially available CTMA in DI water at room temperature to the final concentration of 1x10-6 mol/L. DNA was fragmented by sonication to proper base pair and the resulting molecular weight of the fragmented DNA was around 1500kDa. DNA solution was then prepared by dissolving fragmented DNA in DI water at room temperature. The DNA solution was added dropwisely to the CTMA solution and white precipitate (DNA-CTMA complex) was observed immediately. The solution was stirred for additional 4 hours at room temperature. The reaction mixtures were centrifuged at 300 rpm for 120 minutes. The precipitate was washed thoroughly with water and dried in a vacuum oven overnight at 40 °C.

2.2 Preparation of DNA-VBTMA, DNA-BTMA, and DNA-PTMA complexes

Similar procedures were applied for the preparation of DNA-VBTMA, DNA-BTMA and DNA-PTMA complexes except for the purification step. Centrifugal filters (30000 MW cut-off) were used for filtering off the excess of surfactants. The concentrations of VBTMA, BTMA, and PTMA solutions were adjusted according to the molecular weight of the corresponding molecules to the final concentration of 1x10-6mol/L and the final products were dissolved in ethanol.

N+

Cl-

N+Cl-

N+

Cl-

d) phenyltrimethylammonium chloride (PTMA)

N+

Cl-

b) vinybenzyltrimethylammonium chloride (VBTMA)

a) cetyltrimethylammonium chloride (CTMA)

c) benzyltrimethylammonium chloride (BTMA)

Figure 1. The molecular structures of various surfactants used in this study.

3. MATERIAL CHARACTERIZATION 3.1 Fourier-transform infrared spectroscopy

The synthesis complexes were characterized by IR spectroscopy. Figure 2 shows the IR spectra of DNA and synthesized DNA complexes measured by a PerkinElmer Spectrum 100. It can be observed that the characteristic absorption peaks of DNA and surfactants were shown in the IR spectrum of the DNA complex of each kind, which implies the success synthesis of materials. IR spectroscopy was performed in all synthesized DNA complexes to verify the synthesis and Figure 2 shows the IR spectra for DNA-CTMA and DNA-BTMA.



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wave number (cm-1)

DNA

CTMA

DNA-CTMA

C-N C=OC-O1052962 1231

1647C-O PO2

�

1088

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DNA BTMA DNA-BTMA

C-N C=OC-O1052962 1231 1647

C-O PO2�

1088

Figure 2. The FTIR spectra of (a) DNA, CTMA, and DNA-CTMA complex and (b) DNA, BTMA, and DNA-BTMA

complex.

3.2 Optical properties

Figure 3 shows the optical properties of the synthesized DNA-complex measured by a PerkinElmer UV/VIS Spectrometer (Lambda 35). Figure 3(a) shows the absorption spectra of synthesized DNA complexes. For each DNA complex, below 300nm the complex shows absorption from both DNA and surfactants, whereas high transmission is observed as shown in Figure 3(b) in the full visible regime and NIR, which is an advantageous feature for DNA biopolymer as a host material. Refractive indices of the synthesized DNA complexes were measured by an ellipsometer, where an index of refraction ~1.54 at 500 nm was obtained for synthesized DNA complexes.

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Figure 3. (a) Absorption spectra and (b) transmission spectra of the synthesized DNA complexes.

3.3 LUMO/HOMO levels

The HOMO levels of synthesized DNA complexes were measured by a photoelectron spectrometer (Riken Keiki AC-2) and LUMO levels were retrieved by the bandgap energy. Analyzed results were summarized in Table 1, that indicates synthesized DNA complexes show similar HOMO/LUMO levels.

(a) (b)

(a) (b)



Table 1. HOMO/LUMO levels of the synthesized DNA complexes.

Material HOMO (eV) LUMO (eV)

DNA-CTMA 5.7 1.634

DNA-BTMA 5.97 1.904

DNA-VBTMA 5.7 1.531

DNA-PTMA 5.79 1.724

3.4 Morphology of films

Scanning electron microscope (JEOL, JSM-5600) was used to analyze the morphology of these synthesized DNA complexes, as showed in Figure 4. DNA complexes solution was spun with apropos speed on the silicon substrate and dried on the hotplate at 80 °C for1 hour. The typical SEM image of DNA-CTMA is presented in Figure 4(a), where uniform morphology without serious defect is observed. SEM images of DNA-VBTMA and DNA-BTMA are shown in Figure 4(b) and (c), respectively. All images suggest uniform films can be formed from synthesized DNA complexes.

Figure 4. SEM images(X1300) of various DNA-surfactant thin films (a) DNA-CTMA (b) DNA-VBTMA (c) DNA-BTMA

Atomic force microscopy (Veeco/DI, NanoScope IIIa) was used to confirm the surface roughness of the synthesized DNA complexes, as shown in Figure 5.The results show that DNA complexes can form uniform thin films with a surface roughness within nanometers by spin-coating technique.

Figure 5. AFM images of DNA-surfactant thin films (a) DNA-CTMA (b) DNA-VBTMA.

(a) (b) (c)

(a) (b)



3.5 Thermal property

Thermogravimetric analysis (TA Instruments, SDT Q600) was used to characterize the thermal properties of these synthesized DNA complexes, as shown in Figure 6. All material durability showed upon 200°C.

Figure 6. TGA analysis of DNA-CTMA,DNA-VBTMA, and DNA-BTMA.

3.6 Conductivity of synthesized DNA complexes

The electrical properties of the organic materials are critical parameters for optoelectronic device application. The measurement was performed by a coplanar electrode configuration where parallel metal electrodes were deposited on the surface of organic thin films8. Electrodes were deposited by a thermal evaporator with a thickness of 150nm. The samples were encapsulated by clean glass and then removed from the glove box. The resistivity was determined by the current-voltage (I-V) curves measured between parallel contacts, using a Keithley B1500A semiconductor device analyzer at room temperature. In order to dilute the environmental influence on the materials and measurement, we repeated the experiment several times with the same fabrication condition. The conductivities of the synthesized DNA complexes were summarized in Figure 7.The conductivities deviated from batch to batch, implying the sensitive nature of such measurement, and yet a tendency can be observed from the results. In Figure 7(a), it shows that conductivities of DNA-BTMA, DNA-VBTMA and DNA-PTMA were larger than that of DNA and DNA-CTMA, which indicates the aromatic rings in DNA complexes may promote electrical conduction. To further investigate the effect of surfactant concentration on the material conductivity, different concentration mixtures of surfactant for DNA complexes were investigated. Surfactant BTMA was added in DNA complex synthesis with different ratio. Samples with stoichiometric ratio (DNA:BTMA) of 1:1, 1:3, 1:5, 1:7, and 1:10 were prepared and the conductivities were measured, as shown in Figure 7(b). It is observed that the conductivity of the DNA complex increases with a larger amount of surfactant, which implies a positive correlation between the concentration of surfactant and material conductivity. Part of the 1:5 sample solution was on purpose treated with a longer centrifuge process to decrease the concentration of surfactant, and it was indeed reflected by a lower conductivity of the sample as <5X denoted in Figure 7(b). Possible mechanism may be explained in two aspects. First the additional unbound surfactants in the material cause the molecules certain degree of extrusion. Therefore, the double helix with pi-pi stacking, along with the aromatic rings contributed from surfactants surrounding DNA, forms tunnels suitable for carrier transportation9. Another possible mechanism may be explained by extra ionic molecules that act as dopants for increased conduction10.



10-7

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Figure 7. (a) The conductivities of different materials (b) The conductivities of DNA-BTMA with different stoichiometric ratio.

4. AMPLIFIED SPONTANEOUS EMISSION DNA has been used as a template to assemble metal ions and intercalate dyes and small molecule in many studies11-13. It is well known that many fluorescent dyes can be easily intercalated into the helices of DNA, whereby the intensity of the fluorescence is greatly enhanced14. Herein, we investigated the optical and lasing properties of rhodamine dyes incorporated in DNA complex films5. Dye doped DNA complex thin films were fabricated by spin coating onto glass substrates with an average thickness of ~1μm.

First, we measured the absorption spectrum and fluorescence intensity of the dye-doped DNA complex, as shown in Figure 8. An absorption peak near 530 nm in Figure 8(a) was observed owing to the rhodamine dyes. Figure 8(b) shows the photoluminescence of the dye-doped DNA complexes, where slight shifts in peak wavelength were observed for different surfactant types.

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.)

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DNA-CTMA DNA-BTMA DNA-VBTMA

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.)

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555

Figure 8 (a) Absorption spectra and (b) photoluminescence spectra of the synthesized dye-doped DNA complexes

The amplified spontaneous emission properties of the films were characterized by variable-stripe-length technique15. The sample was pumped by a frequency-doubled 532nm Nd:YAG laser (Spectra-Physics) with a pulse width of 7ns and a repetition rate of 10Hz. The laser pulses were focused on the sample by a cylindrical lens, and the pumping area was adjusted by a slit. Light emitted from the edge of the film was collected by a fiber-coupled spectrometer. Intensity of the excitation was changed by a neutral density filter. The experimental setup is depicted in Figure 9.

(a) (b)

(a) (b)



Figure 9 Experimental setup of ASE measurement. M : mirror, P : polarizer, λ/2 : half wave plate, C : convex lens, CL :

cylindrical lens, S : sample, F : optical fiber, OSA : optical spectrum analyzer

400 500 600 700 800 900

Nor

mal

ized

inte

nsity

Wavelegnth (nm)

DNA-CTMA DNA-VBTMA

Figure 10. The ASE of dye-doped DNA-CTMA and dye-doped DNA-VBTMA.

The emission intensity increases as increasing the pumping intensity, accompanied by spectral narrowing. The ASE spectra of DNA-CTMA-Rh6G and DNA-VBTMA-Rh6G were shown in Figure 10. The FWHM of the emitted spectrum of ~ 10nm can be observed under high excitation. The threshold was estimated by monitoring the peak ASE intensities under different excitation. The threshold was 2.01 mJ/cm2 for DNA-CTMA-Rh6G and 1.14 mJ/cm2 for DNA-VBTMA-Rh6G. Gain coefficient was derived from the ASE intensity versus different pumping lengths, where a slit was used to change the length of the pumping area. By this configuration, the gain coefficient of DNA-VBTMA-Rh6G was 6.33 cm-1, whereas the gain coefficient of DNA-CTMA-Rh6G was 2.25 cm-1. From the measurement results, it implies that DNA-VBTMA complex exhibits higher optical gain compared to the DNA-CTMA counterpart. Possible reasons may be attributed to less aggregation of dyes or fluorescence enhancement resulted from molecular interaction between dyes in different DNA complex systems. Further investigation is required to clarify the mechanism of such enhancement.

5. CONCLUSION In this work, we present the synthesis, preparation and characterization of DNA biopolymers based on surfactants with aromatic rings. The characterization shows that DNA complexes formed with aromatic surfactants exhibit high transparency in the visible/NIR regime, and good morphology of films can be obtained using spin-coating technique. The electrical conductivities of DNA complexes based on aromatic surfactants are higher than that of DNA-CTMA, and a positive correlation between the concentration of aromatic surfactant and material conductivity was observed. This technique may be used to tune the electrical properties of DNA biopolymer. The DNA complexes also used in the preparation of dye-doped DNA biopolymer systems and amplified spontaneous emission was observed. By optimizing the fabrication procedures and material selection, DNA biopolymer may pave the way toward highly efficient organic optoelectronic devices.



6. ACKNOWLEDGMENT The authors would like to acknowledge financial support from National Science Council. (NSC98-2221-E-007-023).

REFERENCES

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[2] Sun, Q., Subramanyam, G., Dal, L., Check, M., Campbell, A., Nalk, R., Grote, J. G., and Wang, Y., "Highly efficient quantum-dot light-emitting diodes with DNA-CTMA as a combined hole-transporting and electron-blocking layer," ACS Nano, 3, 737-743 (2009).

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[7] Steckl, A. J., "DNA – a new material for photonics?" Nature Photonics, 1, (2007). [8] Okahata, Y., Kobayashi, T., Tanaka, K., and Shimomura, M., "Anisotropic electric conductivity in an aligned

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nanowires on a DNA template," Appl. Phys. Lett., 78, 536-538 (2001). [13] Braun, E., Eichen, Y., Sivan, U., and Gdalyahu, B. Y., "DNA-templated assembly and electrode attachment of a

conducting silver wire" Nature, 391, 775-778 (1998). [14] Yu, Z., Li, W., Hagen, J. A., Zhou, Y., Klotzkin, D., Grote, J. G., and Steckl, A. J., "Photoluminescence and

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Preparation and characterization of DNA-aromatic ... · Fourier-transform infrared spectroscopy (FTIR) and UV-VIS spectroscopy were used to characterize the presence of specific chemical