Morphological and Optoelectronic Characteristics of Double and Triple

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Morphological and Optoelectronic Characteristics of Double and Triple Lanthanide Ion-doped DNA Thin Films Mallikarjuna Reddy Kesama, Sreekantha Reddy Dugasani, Sanghyun Yoo, Prathamesh Chopade, Bramaramba Gnapareddy, and Sung Ha Park ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02880 • Publication Date (Web): 11 May 2016 Downloaded from http://pubs.acs.org on May 15, 2016

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ACS Applied Materials & Interfaces

Morphological and Optoelectronic Characteristics of Double and Triple Lanthanide Ion-doped DNA Thin Films

Mallikarjuna Reddy Kesama†, Sreekantha Reddy Dugasani†, Sanghyun Yoo, Prathamesh Chopade, Bramaramba Gnapareddy, Sung Ha Park*

Department of Physics and Sungkyunkwan Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, Suwon 16419, Korea

†

These authors contributed equally to this work.

KEYWORDS: DNA lattice, DNA thin film, Lanthanide ion, Co-doping, Photoluminescence

ABSTRACT Double and triple lanthanide ion (Ln3+)-doped synthetic double crossover (DX) DNA lattices and natural salmon DNA (SDNA) thin films are fabricated by the substrate assisted growth and dropcasting methods on given substrates. We employed three combinations of double Ln3+-dopant pairs (Tb3+–Tm3+, Tb3+–Eu3+, and Tm3+–Eu3+) and a triple Ln3+-dopant pair (Tb3+–Tm3+–Eu3+)

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with different types of Ln3+, (i.e., Tb3+ chosen for green emission, Tm3+ for blue, and Eu3+ for red), as well as various concentrations of Ln3+ for enhancement of specific functionalities. We estimate the optimum concentration of Ln3+ ([Ln3+]O) wherein the phase transition of Ln3+-doped DX DNA lattices occurs from crystalline to amorphous. The phase change of DX DNA lattices at [Ln3+]O and a phase diagram controlled by combinations of [Ln3+] were verified by atomic force microscope measurement. We also developed a theoretical method to obtain a phase diagram by identifying a simple relationship between [Ln3+] and [Ln3+]O that in practice was found to be in agreement with experimental results. Finally, we address significance of physical characteristics—current for evaluating [Ln3+]O, absorption for understanding the modes of Ln3+ binding, and photoluminescence for studying energy transfer mechanisms—of double and triple Ln3+-doped SDNA thin films. Current and photoluminescence in the visible region increased as the varying [Ln3+] increased up to a certain [Ln3+]O, then decreased with further increases in [Ln3+]. In contrast, the absorbance peak intensity at 260 nm showed the opposite trend, as compared with current and photoluminescence behaviors as a function of varying [Ln3+]. A DNA thin film with varying combinations of [Ln3+] might provide immense potential for the development of efficient devices or sensors with increasingly complex functionality.

1. INTRODUCTION Structural DNA nanotechnology is an emerging interdisciplinary research field that provides methods to construct various dimensional nanostructures with nanometer scale precision for technological applications.1 Recently, DNA molecules having unique features—programmability of base sequences and self-assembly of dimensional nanostructures—have begun to be used as an engineering material for the ready development and construction of periodic and aperiodic


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nanostructures, especially in the field of bionanotechnology.2-4 Nanomaterials (e.g., metallic, magnetic, and semiconducting nanoparticles; carbon-based nanomaterials; organic/inorganic molecules; and di/tri-valent ions with specific functionalities) have found highly promising and numerous applications in physical, chemical, and biological devices and sensors.5-7 A synthetic double crossover (DX) tile (4 nm × 12 nm in width and length shown in Figure S1 and Tables S1, S2 in Supporting Information)–based DNA lattice has two repeating DX tiles, and this DX DNA lattice forms a relatively larger domain size with a simple annealing method.8 The salmon DNA (SDNA) duplex, which can be readily produced in large quantities at low cost, can be used as a host material that interacts with various functionalized nanomaterials.9,10 Among a huge number of available nanomaterials, rare earth metallic ions (i.e., lanthanide ion Ln3+) have unique properties (e.g., water soluble, non-toxic, photo-stable) and functionalities (e.g., electromagnetic characteristics, long-lived luminescence).11 It is well known that Ln3+, having specific electrical and fluorescent properties, can be readily intercalated and bound into the helices of negatively-charged DNA molecules, whereby the conductivity and fluorescence of Ln3+-doped DNA are expected to be enhanced. Among Ln3+, terbium ion (Tb3+, chosen for green emission), thulium (Tm3+, for blue), and europium (Eu3+, for red) demonstrate noticeable luminescence behaviours in the visible spectrum, and they can interact directly with DNA molecules without any modification.12-21 Although there are a limited number of reports on single Ln3+-doped DNA complexes, DNA complexes doped with more than two different types of Ln3+ (as the functionality of double and triple Ln3+-doped DNA complexes can be selectively tuned with different combinations of Ln3+) are rarely discussed. In this paper, we discuss: (i) the construction of the synthetic DX DNA lattices (via the substrate assisted growth (SAG) method) and SDNA thin films (through drop-casting) doped



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with three combinations of double Ln3+-doped pairs (Tb3+–Tm3+, Tb3+–Eu3+, and Tm3+–Eu3+) and a triple Ln3+-doped pair (Tb3+–Tm3+–Eu3+) at various concentrations of Ln3+ ([Ln3+]); (ii) the estimation of the optimum concentration of Ln3+ ([Ln3+]O), wherein the phase transition of Ln3+doped DX DNA lattices occurs from crystalline to amorphous; and (iii) the significance of physical characteristics of double and triple Ln3+-doped SDNA thin films.22-25 The phase change of DX DNA lattices at [Ln3+]O and a phase diagram controlled by combinations of [Ln3+] were verified by atomic force microscope (AFM) measurement. Additionally, current, absorption, and photoluminescence (PL) were analyzed by the semiconductor parameter analyzer, the ultravioletvisible (UV-Vis.) absorption spectroscopy, and PL spectroscopy, respectively, in order to show the feasibility of applications in the fields of nanoelectronics, biophotonics, and biosensors.

2. EXPERIMENTAL PROCEDURE We used O2 plasma processing for surface cleaning and functional preparation of a substrate (either glass or fused silica, size 5 × 5 mm2), as this process provides a simple, effective, and safe method of surface cleaning and modification. The O2 gas is energized by direct current, creating energetic species in the plasma, which is then brought in contact with the surface of the substrate; an energy transfer to the surface follows to break down organic bonds of contaminants and to functionalize the silanol groups. By altering the silanol group, the glass substrate obtains an enhanced hydrophilic characteristic for better adhesion of DNA. A commercially available O2 plasma processing system (CUTE-1MP/R, Gyeonggi, Korea) was used under the following process parameters: base pressure 5 × 10-2 Torr, process pressure 7.5 × 10-1 Torr, power 45 W, O2 flow rate 45 SCCM, and processing time of 5 minutes for each side of the substrate (see 1st row in Figure 1).


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The synthetic oligonucleotides of DNA molecules, purified through high performance liquid chromatography, were purchased from Bioneer (Daejeon, Korea). For SAG and Ln3+ predoping, individual DX DNA strands, O2 plasma treated substrate, as well as the appropriate amount of Tb(NO3)3.6H2O, Tm(NO3)3.5H2O, and Eu(NO3)3.5H2O (Sigma Aldrich, USA) were added into an AXYGEN-tube containing 250 µL of a physiological 1× TAE/Mg2+ buffer [40 mM Tris, 20 mM Acetic acid, 1 mM EDTA (pH 8.0), and 12.5 mM magnesium acetate]. The sample tube is placed in the Styrofoam box with 2 L of boiling water, and then cooled down slowly (about 24 hours) from 95 to 25 oC for hybridization. During the course of annealing process, the individual DX DNA tiles are formed in solution (by hydrogen bonds between base pairs) and are bound onto a given substrate (through electrostatic interaction between DNA molecules and a substrate); DX DNA lattice growth follows. Eventually, fully covered polycrystalline DX DNA lattices are formed on the substrate at 50 nM of DX DNA strands (1st row in Figure 1). Ln3+-doping with the pre-annealing method was performed to verify the structural stability of the DX DNA lattices at a given [Ln3+], in order to find the optimum concentration of Ln3+ (≡ [Ln3+]O) and to monitor the phase change from crystalline to amorphous (1st row in Figure 1 and Figure S2). Additionally, we conducted post-annealing Ln3+-doping; after the DX DNA lattices are formed on a substrate, the appropriate amount of Ln3+ was added (followed by 24 hours incubation at room temperature) in order to maintain crystalline phase even at slightly higher than [Ln3+]O (Figure S3 in Supporting Information). For preparation of the SDNA solution, an enzyme isolation processed SDNA (Chitose Institute of Science and Technology, Hokkaido, Japan) of 0.1 g dissolved in 10 mL of de-ionized water was placed on a magnetic stirrer with 800 rpm for 10 hours at room temperature to obtain 1% weight of homogeneous SDNA solution. For fabrication of the Ln3+-doped SDNA thin film,



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0.5% weight SDNA solution was incubated with varying [Ln3+]. The Ln3+-doped SDNA solution of 20 µL was drop-casted on the plasma treated glass (for I–V) or fused silica (for absorbance and PL) substrates and allowed to dry naturally for 24 hours (2nd row in Figure 1). For AFM imaging, the surface-assisted grown DX DNA sample was placed on a metal disc with the help of instant glue. 30 µL 1× TAE/Mg2+ buffer was added onto the substrate and another 20 µL of 1× TAE/Mg2+ buffer was dispensed into the AFM tip (NP-S10, Veeco Inc., USA). The AFM images were obtained with a Multimode Nanoscope (Veeco Inc., USA) in the fluid tapping mode (3rd row in Figure 1). The electrical measurement of the Ln3+-doped SDNA thin film was performed at room temperature using a semiconductor parameter analyzer (4200-SCS, Keithley Instruments Inc., USA). Silver pastes (serving as electrodes) were applied on the surface of the Ln3+-doped SDNA thin film to make a channel of approximately 1 mm width (3rd row in Figure 1). A spectrophotometer (Cary 5G, Varian, USA) was used to conduct the optical absorbance measurement of the Ln3+-doped SDNA thin film on fused silica in the visible and UV regions (wavelength between 800 and 190 nm). The spectrophotometer was equipped with two light sources: a deuterium arc lamp (near-infrared and visible) and a quartz W−halogen lamp (UV). The instrument also had two detectors: a cooled PbS detector (near-infrared) and a photomultiplier tube (visible and UV). The spectrophotometer measures the frequencydependent light intensity passing either through a vacuum or through the sample (3rd row in Figure 1). The PL and excitation spectra of the Ln3+-doped SDNA thin film were measured at room temperature by using the Xe-arc lamp equipped flurometer (FS-2, Scinco, Seoul, Korea) with a 25 W power. The excitation spectra were obtained at fixed emission wavelengths λem (545 and


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615 nm), while the emission spectra were measured by exciting the samples at the 297 nm wavelength (3rd row in Figure 1).

3. RESULTS AND DISCUSSION In order to construct multiple Ln3+-doped DX DNA lattices without structural deformation (due to the presence of excess Ln3+ ), it was essential to estimate [Ln3+]O (the concentration at which phase transition of Ln3+-doped DX DNA lattices is about to occur from crystalline to amorphous). Compared to single doping, multiple Ln3+-doping (e.g., double and triple) into DNA facilitates the provision of functionality with intrinsic characteristics, tunability by different combinations, and variety in applications. In a previous study, we explored DNA complexes doped with single Ln3+ (terbium (Tb3+), europium (Eu3+), or thulium (Tm3+)), investigated the structural stabilities of DX DNA lattices controlled by [Ln3+], and developed a simple method for identifying [Ln3+]O in DNA complexes.26 Although we expected different [Ln3+]O with different ions, structural phase transitions of DX DNA lattices with single Ln3+ revealed optimum concentrations at slightly above 1 mM for all Ln3+, meaning that 1 mM applied to [Tb3+]O, [Eu3+]O, as well as [Tm3+]O (up to this concentration, crystalline lattices were guaranteed). Based upon the single Ln3+-doped DX DNA lattices with [Ln3+]O, the crystalline region with multiple Ln3+-doping onto DX DNA lattices can be easily predicted by a simple relationship between [Ln ] and [Ln ] ,

[ ]

[ ]

[ ]

[ ]

+ [ + · · · + [ ≤ 1, where N indicated Ln3+ ] ]

species. For example, a condition of [Tb3+] ≤ [Tb3+]O has to be met for the crystalline phase of DX DNA lattices for single Tb3+ doping. For double Tb3+ and Eu3+-doping, and

[ ]

[ ]

[ ]

[ ]

[ ]

[ ]

+ [ ] ≤ 1,

+ [ ] > 1 (where [Tb3+]O = [Eu3+]O = 1 mM) were expected for crystalline phase



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[ ]

and for amorphous, respectively. A theoretically expected line (magnitude of slop =

[ ]

) in

a phase diagram of Tb3+ and Eu3+-doped DX DNA lattices is shown in Figure 2(a), and this line separated crystalline and amorphous phases of DX DNA lattices. Similarly expected lines in the other two combinations, Tb3+–Tm3+ and Tm3+–Eu3+, are shown in Figure 2(b, c), respectively. Although it was not marked in Figure 2(d) for simplicity, a crystalline triangular surface can be easily assigned by a condition,

[ ]

[ ]

[ ]

[ ]

+ [ ] + [ ] = 1.

Figure 2 shows phase diagrams with representative AFM images of double and triple Ln3+-doped DX DNA lattices. The AFM data (more than 5 samples were tested) were used to check the structural formation and stability with various combinations of Ln3+-doped DX DNA lattices fabricated by pre-annealing SAG on a glass substrate (detailed sample preparation was discussed in experimental procedure). We fabricated double Ln3+-doped DX DNA lattices with the three combinations of Ln3+: Eu3+, Tm3+, and Eu3+, varied from 0.2 to 0.8 mM by increments of 0.2 mM at fixed 0.4 mM of each Tb3+, Tb3+, and Tm3+, respectively. In these cases, phase transition from crystalline to amorphous occurs at 0.6 mM ≡ [Eu3+]O, 0.6 ≡ [Tm3+]O, and 0.4 ≡ [Eu3+]O, respectively. Similarly, triple Ln3+-doped DX DNA lattices with the 4 combinations (Eu3+ varied at fixed Tb3+ (0.2 mM) and Tm3+ (0.2); Tb3+ (0.2) and Tm3+ (0.4); Tb3+ (0.2) and Tm3+ (0.6); and equal concentrations 0.3 mM and 0.4 of Tb3+, Eu3+, and Tm3+) of Ln3+ are discussed as well. Phase transitions occurred at 0.6 mM ≡ [Eu3+]O, 0.4 ≡ [Eu3+]O, and 0.2 ≡ [Eu3+]O; and 0.3 ≡ [Ln3+]O of each [Ln3+], respectively (Figure S2 in Supporting Information). The light blue surface area represented in Figure 2 indicates crystalline domains of DX DNA lattices containing various combinations of [Ln3+]. From the representative AFM images, the crystalline and amorphous phases of DX DNA lattices were clearly noticeable. Yellow dotted


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lines in crystalline phase indicated Ln3+-doped DX DNA lattice boundaries and insets are noisefiltered reconstructed images by fast Fourier-transformation. In amorphous phase, periodicity of DX DNA lattices was not visible, which was also confirmed by fast Fourier-transformation. As seen in our discussion of phase conditions, the Ln3+-doped DX DNA lattices were not properly formed at above 1 mM of combined [Ln3+]. Although formation of DNA duplex with different Ln3+ were dependent upon the ionic radius, the oxidation state of the ions, as well as buffer conditions such as pH and humidity,25-27 the phase transition of the Ln3+-doped DX DNA lattices from crystalline (within the tolerance of formation up to [Ln3+]O) to amorphous (beyond the tolerance of formation above [Ln3+]O) might be mainly due to the Ln3+ binding to DNA molecules at proper (intercalating between bases and binding at phosphate backbones) and improper (e.g., interstitial binding between neighboring base pairs) sites, respectively. In the combination of Tm3+–Eu3+ doping shown in Figure 2(c), there was a slight discrepancy between expected (0.6 mM ≡ [Eu3+]O) and experimental (0.4) values, while overall phase lines obtained from experimental data for double and triple Ln3+-doping into SDNA were in good agreement with expected analytical values. The natural SDNA duplexes extracted from salmon by using an enzyme isolation procedure provide a ready fabrication methodology with relatively low cost, having potential usage in a variety of physical and biological applications. Although we used synthetic DX DNA lattices (~2.0 nm of thickness) with various [Ln3+] in order to find [Ln3+]O by evaluation of topological phase changes, physical characterizations (e.g., conductivity, absorption, and PL) were conducted with multiple Ln3+-doped SDNA thin films (thickness, ~5.0 µm) in order to obtain the significance of multiple Ln3+ doping with consistent and reproducible results.



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We performed current–voltage (I–V) measurement using double and triple Ln3+-doped SDNA thin films, with characteristic results shown in Figure 3. The Ln3+-doped SDNA samples were prepared on a plasma-treated glass substrate by using the drop-casting method, followed by the electrical measurement with a coplanar electrode configuration. We prepared double Ln3+doped SDNA thin films with the 3 combinations of Ln3+ (i.e., Eu3+, Tm3+, and Eu3+ varied at fixed 0.4 mM of each Tb3+, Tb3+, and Tm3+) and triple with the 3 combinations (Eu3+ varied at fixed Tb3+ (0.2 mM) and Tm3+ (0.2); Tb3+ (0.2), and Tm3+ (0.4); and equal concentrations 0.3 mM and 0.4 of Tb3+, Eu3+, and Tm3+). Most of I curves—voltage swept from –3 to 3V—of multiple Ln3+-doped SDNA thin films represented rectifying behaviors that might be possible due to the hopping of charge carriers through the doped Ln3+. Interestingly, we observed I offsets at 0 V—many other reports also observed them, measured via pristine DNA28-31—for all SDNA thin films that might be a result of the negative charge, charge storage capacity, and charge trapped/detrapped characteristics of DNA molecules. After doping, Ln3+-doped SDNA thin films showed noticeable increments in conductance up to 5 times higher than that of non-doped SDNA. From I–V measurement, maximum I occurred at a certain [Ln3+] in a SDNA thin film that was heavily related with [Ln3+]O. Resistance (R) as a function of varying [Ln3+] of multiple Ln3+-doped SDNA thin films at three different fixed V (1, 2, and 3 V) were shown in the inserted graphs, Figure 3. For instance, Eu3+ varied from 0.2 to 0.8 mM at fixed Tb3+ 0.4 mM, while minimum R (i.e., maximum I at a fixed V) occurred at 0.6 mM ≡ [Eu3+]O (the optimum concentration of Eu3+ at a given condition), which agreed perfectly with our topological analysis evaluated through DX DNA lattices discussed in Figure 2. Although [Ln3+]O obtained from DX DNA lattices by phase transition analysis and acquired from SDNA thin films via minimum R were slightly different at two combinations of [Ln3+]—(i) 0.4 and 0.6 mM of [Eu3+]O at fixed


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Tm3+ shown in Figure 3(c), and (ii) 0.3 and 0.4 mM of [Ln3+]O at equal concentrations of each Ln3+ in Figure 3(f)—overall [Ln3+]O evaluated by topological phase change showed good agreement with I–V analysis. Up to the [Ln3+]O, Ln3+ were placed in appropriate sites of DNA (enhancing conductivity), but when further increasing [Ln3+], Ln3+ might coordinate at improper interstitial locations of the DNA (reducing).32-34 We also studied the absorption of multiple Ln3+-doped SDNA thin films by UV-Visible spectroscopy in order to understand the modes of binding of Ln3+ to SDNA. Figure 4(a‒e) shows distinct absorbance characteristics of pristine and various combinations of Ln3+-doped SDNA (i.e., Eu3+ varied at fixed Tb3+; Tm3+ varied at fixed Tb3+; Eu3+ varied at fixed Tm3+; Eu3+ varied at fixed Tb3+ and Tm3+; and individual Tb3+, Tm3+, Eu3 at fixed concentration) and Figure 4(f) represents absorbance peak intensity with respect to varying [Ln3+] at a fixed wavelength. Interaction of double and triple doped [Ln3+] in SDNA led to altered absorption intensity (related with hyperchromism and hypochromism) and band shift (bathochromism and hypsochromism) with respect to pristine SDNA.35-38 In the Ln3+ intercalation mode between the base pair of SDNA, decreasing (hyperchromic effect) and increasing (hypochromic) absorption intensities were clearly observable (Figure 4(f)) due to the presence of Ln3+, causing a change in the double helical structure of DNA. From the observation, absorption intensity was gradually decreased as varying [Ln3+] increased at a given condition, and it showed minimum intensity at [Ln3+]O. This observation means that DNA duplexes should almost certainly maintain their structures without much distortion up to [Ln3+]O. At slightly above [Ln3+]O, Ln3+-doped SDNA duplexes were not very stable, leading to hypochromism. Structural instability of SDNA thin films by Ln3+ interaction on the DNA molecules would be predicted by a small band red-shift (< 10 nm,



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bathochromic effect) above [Ln3+]O due to the excess [Ln3+]. At [Ln3+] ≤ [Ln3+]O, noticeable band shifts either to the red (bathochromism) or to the blue (hypsochromism) were not observed, which indicated structural stability of SDNA. Lastly, PL characteristics of multiple Ln3+-doped SDNA thin films were studied in order to understand energy transfer mechanisms between the SDNA and dopants (Ln3+). The PL requires suitable excitation energy for exciting an electron from a singlet ground state to excited state upon the absorbance of photon energy. The excited electron prefers to relax through an internal conversion from a higher vibrational energy level of an excited state to its lower levels of that excited state, called a non-radiative energy transfer. Then, an electron relaxation occurs from a singlet excited state to a triplet excited state through intersystem crossing. The PL arises from the release of a photon upon relaxation of the electron from the triplet excited state to the ground state. The electron transition within the excited singlet state (through an internal conversion process) and then to the triplet state (through intersystem crossing) followed by the emissive states can be obtained from the absorption spectra of the DNA molecules in the near UV region. The excitation spectra revealed the appreciable intense peak around 297 nm due to the significant f–d transition of Ln3+, indicating the proper excitation wavelength λex. Figure S4 in Supporting Information shows the excitation spectra of definite double Ln3+-doped SDNA thin films (i.e., [Tb3+ ] = 0.4 mM at [Eu3+] = 0.6, and [Tm3+ ] = 0.4 at [Eu3+] = 0.6) that had been obtained by monitoring a green emission of Tb3+ at 545 nm, and red emission of Eu3+ at 615 nm, respectively. Generally, energy transitions from a singlet excited state to emissive 5D3, 5D4 states of bound Tb3+; 5D1, 5D0 states of bound Eu3+; and 1D2, 1G4 states of bound Tm3+ in the single Ln3+-doped SDNA thin films were expected to take place from the lowest triplet excited state


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and its corresponding ground states, 7FJ (J = 6–0) for Tb3+; 7FJ (J = 0–6) for Eu3+; and 3FJ (J = 0–4) and 3HJ (J = 0–6) for Tm3+.26 The emission spectra of double and triple Ln3+-doped SDNA thin films without and with various combinations of [Ln3+] at a fixed λex, 297 nm (obtained from fixed λem, 545 nm for Tb3+– Eu3+, Tb3+–Tm3+, Tb3+–Tm3+–Eu3+, and 615 nm for Tm3+–Eu3+ dopant combinations) are shown in Figure 5. The emission spectra of double Tb3+–Eu3+-doped SDNA were shown in Figure 5(a). Although the intensities of Tb3+ emission at 490 nm (5D4→7F6), 545 nm (5D4→7F5), 587 nm (5D4→7F4), and 653 nm (5D4→7F2) revealed relatively less variations, Eu3+ emissions at 615 nm (5D0→7F2), 654 nm (5D4→7F3), and 698 nm (5D0→7F4) were noticeably enhanced, which meant variation of [Eu3+] influenced significantly at these Ln3+ combinations. The emission spectra of Tb3+–Tm3+-doped SDNA were shown in Figure 5(b). Interestingly, the emission intensity of Tb3+ increased with increasing [Tm3+] up to [Tm3+]O, followed by reduction with further increasing [Tm3+]. Improvement of the emission efficiency for Tb3+ with no appreciable peaks of Tm3+ might be due to the combination of Tm3+ and Tb3+, in which each served as a sensitizer and an activator (allowing energy transition from Tm3+ to Tb3+). Figure 5(c) represents the emission spectrum of Tm3+–Eu3+-doped SDNA. The emission intensity at λem = 615 nm (5D0→7F2) significantly increased along with [Eu3+] up to [Eu3+]O. In this combination, Tm3+ and Eu3+ each served as a sensitizer and an activator (allowing efficient energy transfer from Tm3+ to Eu3+). Consequently, the combination of Tm3+–Eu3+-doped SDNA can be an efficient route to improve the red emission of Eu3+ because of the 4f–5d transition of Tm3+. The emission spectra of triple Tb3+–Tm3+–Eu3+-doped SDNA were shown in Figure 5(d, e). In both cases, [Eu3+] varied at fixed [Tb3+] and [Tm3+]. The intensities of Tb3+ and Eu3+ increased along with [Eu3+] up to [Eu3+]O, and reduced further, while the energy transitions from



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Tm3+ to Tb3+ and Eu3+ caused further enhancement in the emission intensity. Figure 5(f) shows the emission spectra of the individual (single) Tb3+, Tm3+, Eu3 and equal (triple) [Ln3+]-doped SDNA. Overall characteristic peaks of individual Ln3+ with 1 mM ([Ln3+]O) and equal [Ln3+] of each Ln3+ with 0.3 mM ([Tb3+]O = [Tm3+]O = [Eu3+]O) showed maximum intensities at specific λem, demonstrating good agreement with [Ln3+]O analysis. Increasing PL intensity as [Ln3+] increases up to [Ln3+]O could be possible by energy transition from an excited state of Ln3+-doped SDNA to an emissive state of bound Ln3+. Energy transitions at [Ln3+] ≤ [Ln3+]O and [Ln3+] > [Ln3+]O were expected to take place from the lowest triplet excited state to the ground state in Ln3+-doped SDNA thin film via direct transition because of proper Ln3+ binding on DNA molecules, and from Ln3+ to the SDNA through cross relaxation due to the presence of nonspecifically binding excess Ln3+, respectively. Such energy transitions resulted in enhancement and suppression of the luminescence in Ln3+-doped SDNA thin films.26,39,40

4. CONCLUSION We have fabricated the various concentrations and combinations of double and triple Ln3+-doped DX DNA lattices and SDNA thin films on a given substrate by the SAG and drop-casting methods, respectively. We estimated [Ln3+]O of each combination of double and triple Ln3+doped DX DNA lattices, and we further completed 2-dimensional (for double Ln3+-doping) and 3-dimensional (triple) phase diagrams based on careful analysis of AFM data. We also proposed an analytical method to complete a phase diagram by knowing a simple relationship between [Ln3+] and [Ln3+]O, which showed good agreement with experimental results. Furthermore, we studied the electrical and optical properties of double and triple Ln3+-doped SDNA thin films.


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The current increased with increasing [Ln3+] up to [Ln3+]O, and then decreased with further increasing [Ln3+]; whereas the absorbance peak intensity at 260 nm decreased with increasing [Ln3+] up to [Ln3+]O, and then increased with increasing [Ln3+]. The PL spectra of Ln3+-doped SDNA thin films showed enhanced intensities at characteristic emission wavelengths of green (for Tb3+ at 545 nm) and red (Eu3+ at 615 nm) in visible region due to the effective energy transitions between DNA molecules and multiple Ln3+. Selection of different types of Ln3+, various [Ln3+], and different combinations of Ln3+ on either a DNA lattice or a DNA thin film was an effective method to control the specific physical properties and an efficient platform to develop the functional electro-photonic devices and sensors.

ASSOCIATED CONTENT Supporting Information. The sequence and schematics of double-crossover DNA tiles, additional AFM images of double and triple Ln3+-doped DX DNA lattices, and the photoluminescence excitation spectra of double Ln3+-doped SDNA thin films. This material is available free of charge via the internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Author Contributions †

These authors contributed equally to this work.



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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) (2013R1A1A2061731, 2014R1A2A1A11053213, 2012M3A7B4049801, and CAMM2014M3A6B3063707) funded by the Ministry of Science, ICT & Future Planning (MSIP) of the Korean government. Competing financial interests: The authors declare no competing financial interests.

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FIGURES


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Figure 1. Schematic diagrams of sample preparation, experimental setups, and symbolic results. (1st row) Ln3+-doped DX DNA lattices through the substrate assisted growth method. Two or three different types of Ln3+ are added along with individual DX DNA strands and an O2 plasmatreated glass substrate in order to grow Ln3+-doped DX DNA lattices on the substrate during the course of annealing. The representative coordination sites of Ln3+—base pairs and phosphate backbones—in a DX DNA tile are shown. (2nd row) Ln3+-doped SDNA thin films constructed by the drop-casting method. The SDNA solution is prepared by magnetic stirring followed by incubation with various [Ln3+]. The SDNA solution with two or three different Ln3+ is dropped on an O2 plasma-treated glass substrate for fabrication of a Ln3+-doped SDNA thin film. (3rd and 4th rows) Experimental setups and corresponding characteristic results. AFM, I-V, absorption, and PL measurements are conducted to study the structural stability of DX DNA lattices with various [Ln3+], the electrical characteristics of Ln3+-doped SDNA thin films, the quantitative



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analysis of Ln3+ binding to SDNA, and the energy transfer mechanism of Ln3+ in SDNA, respectively.

Figure 2. Phase diagrams with representative AFM images of double and triple Ln3+-doped DX DNA lattices. Double Ln3+-doped DX DNA lattices with the 3 combinations of Ln3+: (a) Tb3+ 0.4 mM fixed and Eu3+ varied from 0.2 to 0.8 mM by increment of 0.2 mM marked with either a circle (crystalline phase) or a cross (amorphous). Phase transition occurs from crystalline (up to 0.6 mM ≡ [Eu3+]O, the optimum concentration of Eu3+ at a given condition) to amorphous (at 0.8 mM of [Eu3+]). For clarity, the region of crystalline phase was marked as light-blue; (b) Tb3+ 0.4 mM fixed and Tm3+ varied. Phase transition is observed between 0.6 (≡ [Tm3+]O) and 0.8 mM of [Tm3+]; (c) Tm3+ 0.4 mM fixed and Eu3+varied. Phase transition happens between 0.4 (≡ [Eu3+]O) and 0.6 mM of [Eu3+]. An expected phase slope is obtained by the following condition, [Ln ] + 3+ 3+ [Ln and Ln ! ] = 1 mM where Ln ! are two different types of Ln . (d) Triple Ln -doped DX DNA lattices with the 4 combinations of Ln3+: Tb3+ 0.2 and Tm3+ 0.2 mM are fixed and Eu3+ is varied from 0.2 to 0.8 mM by increment of 0.2 mM (blue). Phase transition from crystalline to amorphous occurs between 0.6 (≡ [Eu3+]O) and 0.8 mM of [Eu3+]); Tb3+ 0.2 and Tm3+ 0.4 mM


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are fixed and Eu3+is varied from 0.2 to 0.6 mM (green). Phase transition is obtained between 0.4 (≡ [Eu3+]O) and 0.6 mM of [Eu3+]); Tb3+ 0.2 and Tm3+ 0.6 mM are fixed and Eu3+ is varied from 0.2 to 0.6 mM (yellow). Phase transition happened between 0.2 (≡ [Eu3+]O) and 0.4 mM of [Eu3+]); Equal concentrations 0.3 and 0.4 mM of three different Ln3+ are tested as well (pink). Phase transition from crystalline to amorphous occurs between 0.3 (≡ [Ln3+]O) and 0.4 mM of each [Ln3+]. Blue, green, yellow, and pink dots show each combination of [Ln3+]O and are placed on the plane of crystalline phase (light-blue). In AFM images (scan size 1 μm × 1 μm), yellow dotted lines in crystalline phase indicate Ln3+-doped DX DNA lattice boundaries and insets (scan size 100 nm × 100 nm) are noise-filtered reconstructed images by fast Fourier-transformation. In amorphous, no periodic DX DNA tiles are visible.

Figure 3. Current–Voltage (I–V) characteristics of double and triple Ln3+-doped SDNA thin films. I–V of SDNA without Ln3+ and with various combinations of [Ln3+], i.e., (a) Tb3+ 0.4 mM fixed (labeled as Tb 0.4), Eu3+ varied from 0.2, 0.4, 0.6 and 0.8 mM (Eu 0.2, Eu 0.4, Eu 0.6, and Eu 0.8, respectively); (b) Tb3+ 0.4 mM fixed (Tb 0.4), Tm3+ varied from 0.2, 0.4, 0.6, and 0.8 mM (Tm 0.2, Tm 0.4, Tm 0.6, and Tm 0.8); (c) Tm3+ 0.4 mM fixed (Tm 0.4), Eu3+ varied from 0.2, 0.4, 0.6, and 0.8 mM (Eu 0.2, Eu 0.4, Eu 0.6, and Eu 0.8); (d) Tb3+ and Tm3+ 0.2 mM fixed (Tb 0.2 and Tm 0.2), Eu3+ varied from 0.2, 0.4, 0.6, and 0.8 mM (Eu 0.2, Eu 0.4, Eu 0.6, and Eu 0.8); (e) Tb3+ 0.2 and Tm3+ 0.4 mM fixed (Tb 0.2 and Tm 0.4), Eu3+ varied from 0.2, 0.4, and 0.6 mM (Eu 0.2, Eu 0.4, and Eu 0.6); and (f) individual Ln3+ (Tb3+, Tm3+, Eu3) with 1 mM (Tb 1,



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Tm 1, and Eu 1) and equal concentrations of each Ln3+ with 0.3 (Tb 0.3, Tm 0.3, and Eu 0.3) and 0.4 mM (Tb 0.4, Tm 0.4, and Eu 0.4). The insets show resistance as a function of varying [Ln3+] at three different fixed V, i.e., 1, 2, and 3 V.

Figure 4. Absorbance characteristics of double and triple Ln3+-doped SDNA thin films. Absorbance of pristine and various combinations of Ln3+-doped SDNA, i.e., (a) Tb3+ 0.4 mM fixed, Eu3+ varied from 0.2, 0.4, 0.6, and 0.8 mM; (b) Tb3+ 0.4 mM fixed, Tm3+ varied from 0.2, 0.4, 0.6, and 0.8 mM; (c) Tm3+ 0.4 mM fixed, Eu3+ varied from 0.2, 0.4, 0.6, and 0.8 mM; (d) Tb3+ and Tm3+ 0.2 mM fixed, Eu3+ varied from 0.2, 0.4, 0.6, and 0.8 mM; and (e) individual Tb3+, Tm3+, Eu3 with 1 mM. (f) Absorbance peak intensity with respect to varying [Ln3+] at a fixed wavelength of 260 nm obtained from (a–d).


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Figure 5. Photoluminescence characteristics of double and triple Ln3+-doped SDNA thin films. Emission spectra of SDNA without Ln3+ and with various combinations of [Ln3+], i.e., (a) Tb3+ 0.4 mM fixed, Eu3+ varied from 0.2, 0.4, 0.6, and 0.8 mM; (b) Tb3+ 0.4 mM fixed, Tm3+ varied from 0.2, 0.4, 0.6, and 0.8 mM; (c) Tm3+ 0.4 mM fixed, Eu3+ varied from 0.2, 0.4, 0.6, and 0.8 mM; (d) Tb3+ and Tm3+ 0.2 mM fixed, Eu3+ varied from 0.2, 0.4, 0.6, and 0.8 mM; (e) Tb3+ 0.2 and Tm3+ 0.4 mM fixed, Eu3+ varied from 0.2, 0.4, and 0.6 mM; and (f) individual Tb3+, Tm3+, Eu3 with 1 mM and equal concentrations of each Ln3+ with 0.3 and 0.4 mM (some overlapped peaks are magnified for clarity).



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Table of Contents Graphic


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Morphological and Optoelectronic Characteristics of Double and Triple

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