Organic dye-doped deoxyribonucleic acid (DNA)-surfactant complex films for dye lasers were fabricated by immersing non-doped complex films into a solution of hemicyanine dye. The threshold pumping intensity for amplified spontaneous emission was found to be 0.3 mJ/cm2, the value was one order smaller than those obtained for the samples made by conventional methods. Durability under pumping was also significantly improved and laser oscillation under optical excitation was observed. Dye concentration of the final products was estimated to be 10 wt% and there were no deformation of the samples, suggesting that dye molecules in the complex did not necessarily intercalate in DNA strand but replace with surfactant molecules.
© 2014 Optical Society of America
Deoxyribonucleic acid (DNA)-surfactant complex is a poly-ion complex composed of anionic DNA polymer and a cationic surfactant [1–3]. One of the most important characteristics of the complex is an ability to incorporate fluorescent organic dyes with high doping concentration without significant quenching, then making it suitable for light emitting devices. Furthermore, fluorescence intensity was sometimes enhanced in the complex, compared to the case when the identical dye was embedded in conventional polymers as poly(methyl methacrylate) (PMMA) or in solutions . Taking advantage of this property, amplified spontaneous emission (ASE) and laser oscillation were realized in several kinds of organic dyes doped in the DNA-complexes [4–9]. Most demonstrations of the light amplification were made in thin films of μm thickness under optical pumping, because high doping concentration made it possible to excite molecules effectively even in such short interaction lengths.
Thin films of the complexes were usually fabricated with a well-known standard method. At first, DNA-surfactant (usually, cetyltrimethylammonium or CTMA was used) complex was synthesized by mixing two aqueous solutions dissolving each of ingredient, spontaneously obtaining the complex precipitates insoluble to water. After drying, the complex was dissolved into alcohols or a mixture of chloroform and alcohol along with appropriate amount of organic dyes . Films can be formed by casting or spin coating from the solution.
Although most of the complexes under investigation have been prepared with this standard manner, some alternative techniques were proposed in order to overcome a drawback; the standard method demands a solvent common to the complex and dye. Several years ago, we have prepared the DNA-surfactant-dye complexes with water soluble dyes via a procedure similar but the order of which processes were exchanged . In this method, both DNA and the dye were dissolved at first in water to promote the interaction (or intercalation if possible) between the dye and DNA, then it was mixed with surfactant solution obtaining the final precipitates. The films made by this method showed improvement of durability as a laser active media, compared with the counterpart prepared with the conventional way. Recently, You et al. proposed a simple alternative doping method which was applied to water soluble dyes . In their study, DNA-CTMA complex powder was immersed into aqueous solutions of ionic dyes for a while, and they confirmed visually or by a spectroscopic way that significant amount of the dye were bound to the complex powder via ion exchange process.
These recent results urge us to fabricate the complex films with high optical quality with a simpler method as well as to reconsider the interaction mode between dye and DNA. In early studies, fluorescence enhancement induced by the addition of DNA complex has been attributed to intercalation of the dye into DNA double strand from the analogy with the case of ethidium bromide [12,13]. However, it has been known that there were several interaction modes other than intercalation like binding inside the minor groove or electrostatic adherence to the outer rim of the strand. For bare DNA, interaction mode depends strongly on the size, structure, ionic character, concentration and electronic state of molecule and also on environmental parameters as pH [12,14–17]. On the other hand, there have been few studies on the structure of DNA-surfactant complexes and no direct proof for intercalation . You el al. also found that anionic dyes as well as cationic ones were also incorporated into the complex through the immersion method, suggesting the importance of ion exchange or formation of ionic aggregates composed of the dye, surfactant and DNA. In such a case, DNA is presumed to play a role of template for the complex formation . Although these studies do not provide a direct answer for the structural problem, but do suggest the importance of ion-exchange process for the complex growing.
In this paper, we describe an approach that is an extension of the method developed by You et al., and show its applicability to thin film and bulk samples. In particular, we used an acetonic solution to which film samples were immersed, because the complex was insoluble to acetone as well as water. Therefore, the available types of dyes were no longer restricted to water-soluble materials, but greatly expanded to those having more hydrophobic character. We applied the method to fabricate DNA-surfactant complex films doped with a hemicyanine derivative, 4-[4-(dimethylamino)stylyl]-1-dococylpyridium bromide (DMASPDB, sometimes referred as Hemi22). DMASPDB has been often used to demonstrate ASE and lasing in DNA-complexes [4,18,19]. We will show that higher dye concentration can be achieved by the method and the performance of ASE was greatly enhanced, compared to preceding results. The durability under continuous laser irradiation was also remarkably improved, and the fact was quite promising since the poor stability under optical pumping has been one of the most severe drawbacks to realize practical solid-state dye lasers or optical amplifiers.
In the next section, the preparation method for dye-doped complex films and basic optical characteristics are described, and experimental details for optical measurements are given in section 3. Experimental results for optical absorption spectra, ASE threshold, ASE spectral characteristics, durability test and demonstration of lasing by a dynamic grating are given in section 4 as well as the discussion for the molecular binding mode to DNA. Summary is addressed in the last section.
2. Sample preparation and basic characteristics
Molecular structures of CTMA and the hemicyanine DMASPDB are given in Fig. 1. Non-doped DNA-CTMA complex was synthesized by the method given in our preceding works using DNA-Na (Ogata Material Science Laboratory) and CTMA chloride (Aldrich) [3–5,9,10]. Thin films of the complex were made on a glass substrate with a spin coating method (2,000 rpm, 15 sec.) from an ethanol solution with concentration of 60 g/l. Thickness of the films was evaluated to be 1.1 μm by a surface profiler. After drying at 80°C for 3 hours, the samples were immersed into an acetone solution of DMASPDB (3.3 x 10−5 M). We observed the coloring of the films accompanied by discoloring of the solution after one day, the fact indicated that significant amount of the dye were adsorbed to the film. Figure 2 gives pictures showing the identical adsorption process for the same complex powder and the dye solution. The picture (a) expresses the color of starting dye solution, and picture (b) taken 3 hours later shows that the solution became almost transparent and the powder have clear orange color. The picture of film prepared by the immersion method is given in the left side of Fig. 2(c). For comparison, we also fabricated a dye-doped film with the conventional method, that is, spin coating from an ethanol solution containing both the complex and dye with the molar ratio of 20:1 (here, we count one base pair of DNA as a molecule unit) , corresponding to 2.5 wt% of the dye. The difference of color between two cases can be easily observed from the picture in Fig. 2(c) which must be due to the concentration difference, because the thickness of the films was the same for both films. Considering the value of 2.5 wt% for the conventionally prepared film, the dye concentration obtained by the immersion method was estimated to be 10 wt% from the absorption peak magnitudes (Fig. 3).Therefore, the ratio of the dye to the complex base pair must be 1:5. The value was remarkably high because it has been difficult to dope the hemicyanine dye up to this amount by the usual mixing method due to precipitation of dye crystallites resulted from poor solubility in alcohol solvents which were commonly used for the usual fabrication process .
It is intriguing to determine the dye concentration distribution along the depth direction, since it will be important for applications, and also give an insight into interaction mechanism. Because the thickness of the films under study was too thin to find out the distribution along the vertical direction, we prepared a thick (> 500 μm) free-standing plate from the same pristine complex by a simple casting method, and soaked it into the same hemicyanine solution. Leaving it for 3 days, we found from microscope observation at a cut cross section that the dye have penetrated entirely inside the sample. Therefore, we could expect that the dopant distributed uniformly in much thinner samples, also showing the applicability of the immersion staining process even to bulky-shaped samples. It might be possible to investigate the adsorption dynamics by monitoring the absorption spectra for films and solutions during immersion. Such process, however, must strongly depend on volume of the complex and dye as well as their concentration. Unfortunately, we do not have enough experimental results to disclose at the present time.
In order to evaluate the efficacy as a laser medium, we pumped the films with a pulsed laser in the experimental apparatus depicted in Fig. 4, observing spectral narrowing and directional light emission induced by the excitation. We used second harmonics from a ns Nd3+:YAG laser (Surelite II, Continuum, repetition rate: 10Hz, pulse duration: 7ns) as a pumping source, of which beam was focused by a cylindrical lens forming a stripe shape on a sample held on a holder. The light beam was incident from the normal angle, and size of the stripe was adjusted to be about 1 x 5 mm2. We collected the light emitted from an edge of the film by a lens system since the amplification occurred mainly along the longitudinal direction of the stripe. The collected light was transferred to a fiber-coupled spectrometer (Ocean Optics U4000) to register its spectrum and relative intensity. In order to determine more precise threshold value, the pump and emission intensities for every pulse were monitored with two photodiodes (S1223, Hamamatsu Photonics), and non-averaged values for every pulse event were stored with a boxcar integrator (SRS, SR250), avoiding the influence from pulse-to-pulse fluctuation. For this measurement, the spectrometer in Fig. 4 was replaced with another photodiode connected to the boxcar integrator. The incident energy was varied in the range of 0.05 ~50 mJ/cm2 by rotating a variable neutral density filter disk by hand. All experiments were conducted in ambient atmosphere at room temperature.
In order to investigate the lasing characteristics, a dynamic grating was formed by interference of two beams radiated from the source. The optical setup was almost the same to that used in our preceding studies, where a ns laser beam was split into two parts then being led to the sample surface from symmetric directions [9,18]. Half cross sectional angle between interfering beams was adjusted to be around 38.5 – 40.0 deg. to satisfy Bragg’s condition for the second order diffraction. Oscillation spectra were obtained by the same spectrometer described above.
4. Results and discussion
Typical emission spectra from both films under pumping with the intensity above their thresholds are also shown in Fig. 3. Their spectral widths were estimated to be 24 and 14 nm for the films made by the conventional and the immersion methods, respectively, and these widths were apparently narrower than those of usual fluorescence, being an evidence of ASE. The narrowing was obtained with much weaker excitation intensity when the immersion-made sample was employed compared to the other case. And the emission peak wavelength for the sample was shifted about 20 nm to longer wavelength side although little differences were found in the absorption spectral shapes. Such deviation can be explained in terms of the concentration of the incorporated dye. ASE peak wavelength is usually located in the region where the gain of stimulated transition has the maximum amplitude, but there would be an overlapping of absorption and emission band. Therefore, ASE is usually observed more or less in red side of the emission maximum. Among our samples, immersion-made films gave stronger absorption due to higher concentration, leading to the peak emission in much longer wavelength side.
To evaluate accurate oscillation threshold values for both films, we recorded the correlation between the excitation and emission intensities by using the apparatus described in the previous section. Results in Fig. 5 indicate superlinear increase of emission intensity above 0.3 mJ/cm2 for the sample prepared by immersion, while the other gives a value more than one order greater. As known from preceding studies, the threshold value was comparable to the lowest records obtained in DNA systems [4,6]. The reason for the threshold reduction would be mainly attributed to the increase of the concentration, although more detailed study is required to clarify it.
For practical uses as laser medium, durability under optical pumping was one of the most important requirements for candidate materials. We made durability test under identical pumping intensity for the two samples. ASE emission magnitude was monitored for 1 hour, while the films were continuously excited with intensity of 10 mJ/cm2 at 10 Hz. Results shown in Fig. 6 indicate that the emission intensity from the immersion-made film gradually decreased to 70% of the initial value after 1 hour (3.6 x 104 pulses). Compared with the other case of which intensity reduced by half in 0.9 hour (3.3 x 104 pulses), durability was known to be improved significantly by employing the new fabrication method. Because the immersion-made film had lower threshold, the same experiment could be done with lower excitation intensity. When it was pumped with 3 mJ/cm2, the emission intensity decreased only to 80% after 3 hr (~105 pulses). Durability over 1 hour was one of the best results observed in this class of laser media [10,20–23].
The reason for lifetime improvement for immersion-made sample would be related to the initial 10 min. behaviors shown in Fig. 6. Emission intensity shown in Fig. 6(a) decayed almost constantly all along the test duration, but the other showed much faster degradation in the same 10 minutes. Such initial difference suggests the difference in dye molecule numbers which are well ‘stabilized’ or ‘isolated’ from environment through ion-exchange process. However, some direct measurements as in situ monitoring for absorption during pumping would be required to investigate it, because ASE intensity does not always reflect the number of molecules proportionally. We will leave it for a future study.
Many methods have been employed to implement feedback structures for realizing laser emission in organic gain media. For DNA complex system, lithographic technique, imprinting formation, dynamic gratings formed inside or outside of the media, and several other techniques have been successfully applied to demonstrate lasing [6–10]. In this study, we formed a dynamic and periodic modulation of the gain by the interference of two pumping beams in the immersion-made sample. Laser oscillation was observed around 620 nm under the pumping with the intensity above 3 mJ/cm2 as depicted in Fig. 3. The value was relatively high compared to the best preceding example made for DNA complex systems , because the optical setups was not well optimized.
Our study supports the conjecture for the interaction mode of the dye-doped DNA-complexes glanced in the introduction. How dyes interact with DNA chain in the complex is still unknown, although many scientists including us had suggested the intercalation of the dye molecules into DNA double strand as confirmed for ethidium bromide . Estimation of dye concentration for our sample gave higher value than that predicted from the intercalation mechanism. Because dye ratio per DNA base pair might depends on the type of molecule and experimental condition [12,24], it might be difficult to determine upper concentration limit. For cyanine dyes, recent studies indicated that about ten base pairs can accept one molecule [14,15]. One rational interpretation for our case is that cationic dye molecules can replace with the surfactants via ion-exchange process during film immersion. Cationic dye as well as surfactant will attach the outside of DNA strand by electrostatic binding, and being in balance among the composing elements. Preservation of high optical quality after the immersion also supports the conjecture, because intercalation usually induces unwinding of DNA strand leading to macroscopic deformation of the films. We need to remark that such an ion exchange processes are not restricted to cationic dyes, but also anionic dyes could be incorporated into the complex with the same method . In this case, the dye could not simply take over the surfactant position, but will form a different type of complex in which the dye will indirectly interact with DNA through the surfactant. Indeed, our preliminary result showed that anionic Eosin Y (a kind of water soluble xanthene dye) can be doped into DNA-CTMA from its aqueous solution. These facts prompt reconsideration for the role of surfactants, urging us to recognize that they must not be only an inert matrix. Recently, strong dependence of the ASE spectrum on the surfactant type was shown by one Taiwanese group . Optimized selection of dye and surfactant, that is not restricted by common solvents, would be expected to improve the performance of laser devices.
Capability of staining thick samples indicates rapid diffusion of molecule into the complex. The penetration condition must depend not only on the type of the complex and usual physical conditions but also on parameters associated to the sample history as residual solvent. Such a deep penetration without any sample deformation may justify the ion exchange mechanism for the doping.
In summary, we prepared DNA-surfactant-dye complex laser medium by immersing non-doped DNA complex films into hemicyanine dye solution, obtaining final complex with higher concentration of the dye than given by conventional mixing preparation. The complex films showed ASE under pumping with a green pulsed laser, giving a threshold value several ten times smaller than those for the devices made by the usual way. Laser emission was also achieved by the feedback from a dynamic grating made inside the medium. Improvement of durability of the material was also confirmed by the continuous pumping of the sample, showing that 70% of the initial intensity was maintained after one hour. These results indicate that the devices fabricated with the immersion method will be promising for application to compact solid-state dye lasers.
T.S. wishes to thank Toshifumi Chida for technical instruction.
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