Tunable near white light photoluminescence of lanthanide ion (Dy³⁺, Eu³⁺ and Tb³⁺) doped DNA lattices

Abstract

For more than two decades, structural DNA nanotechnology has been investigated, yet researchers still have not clearly determined the functional changes and the applicability of DNA structures resulting from the introduction of a variety of ions. Lanthanide ions, such as Dy³⁺, Eu³⁺ and Tb³⁺, are interesting rare earth ions that have unique characteristics applicable to photonics. Here, we have constructed lanthanide ion doped double-crossover DNA lattices, a new class of functional DNA lattices, grown on a silica substrate. Deformation-free lattices were fabricated on a given substrate, and dopant ions were introduced to study their photoluminescence characteristics. The photoluminescence of the lanthanide ion-doped DNA lattices exhibited broad emission spectra in the visible region and a tendency of near white light emission composed of various colours. The intensity of the distinct spectral lines produced by the photoluminescence increased as the doping concentration of the ions reached the critical point, and the intensity then decreased with a further increase in the ions. Photoluminescence quenching was also observed when the excitation wavelength increased. These phenomena are the result of energy transfer between the DNA and the dopant ions. Finally, we make use of chromaticity diagrams to identify the colour coordinates of the luminescence produced by the lanthanide ion-doped DNA lattices, and this information may be useful to construct efficient bio-photonic devices or sensors in the future.

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The motivation behind fabricating structural lanthanide ion (Ln³⁺)-doped DNA (Ln³⁺-DNA) lattices is to control and improve the physical properties of the DNA molecules, particularly for applications in the field of optoelectronics. Herein, we have selected the excellent luminescence in the visible region for Ln³⁺, such as dysprosium ions (Dy³⁺), europium ions (Eu³⁺), and terbium ions (Tb³⁺). Artificially designed DNA nanostructure is one of the most promising biomaterials because selective bindings of Ln³⁺ into the DNA molecules allows for the transfer of functionality, which makes it possible to utilize such molecules in a variety of applications in science and engineering. In addition, Ln³⁺ can be used to transform DNA molecules into luminescent probes. DNA is an efficient host of functional materials¹ in the sense that programmable DNA nanostructures can be used as templates to align various materials, making Ln³⁺-DNA lattices as one of good candidates to produce bio light emitting diodes, biophotonics and biosensors. DNA is an important bio-macromolecule² that offers specific binding sites for a wide range of functional molecules and metal ions.^2–12 Although DNA luminescence can be enhanced by attaching fluorescent dyes, the size of these dyes is much larger than Ln³⁺, and their attachment into DNA molecules often requires modifying specific sites to produce ligands in the DNA sequences. In addition, fluorescent dyes produce an inherent toxicity and also exhibit aggregation and relatively less binding to DNA at a low resolution as compared to Ln³⁺.

Previous reports have focused on enhancing the luminescence of selective cations attached to single- and double-stranded DNA in a solution phase. Tb³⁺ enhances the luminescence of single-strand DNA to detect single base pair mismatches in duplex sequences,¹³ and the fluorescence sensitivity of Eu³⁺-doped protein and DNA molecules¹⁴ has also been previously reported. Tb³⁺ cations enhance the fluorescence as a consequence of an interaction with DNA because Tb³⁺ has the appropriate electronic transition to serve as an acceptor for long distance energy transfer (ET) from near ultraviolet absorbing nucleic acid bases.¹⁵ When DNA was used as an electron blocking layer in organic light emitting diodes, improved electroluminescent characteristics have been observed in both green and blue emissions.¹⁶ Such devices adopting DNA molecules as an electron blocking layer exhibit an appreciable luminescence when compared to conventional organic light emitting diodes without the DNA layer. Altogether, the dynamic admixtures of artificially designed and sequence programmable DNA nanostructures with Ln³⁺ will provide important luminescence applications with unique characteristics in terms of sensitivity, precision, and signal response. Here, we report on a simple but efficient fabrication method for Ln³⁺-DNA lattices to enhance the luminescence of DNA lattices. Ln³⁺-DNA lattices may serves as DNA detectors by controlling the ion doping concentration of Ln³⁺ as well.

In the present study, we utilize two-dimensional double-crossover (DX) DNA nanostructures^17,18 consisting of two repeating DX tiles: DX1 and DX2. A unit DX tile is organized into two DX junctions where two parallel duplexes are tied up. The two types of DX tiles – each with a width of 4 nm and length of 12 nm – that were used to construct the DX DNA lattices on the given substrate are shown in Fig. 1a. Each tile consisted of four strands, and the complementary sticky end pairs are shown as S# and S#′ in the sequence drawings (Fig. 1a). The DX lattices were fabricated on a silica substrate via substrate-assisted growth (SAG)^19–24 (for more details, see ESI†). The DNA strands are dissolved with a certain amount of Ln³⁺ in a common physiological 1× TAE/Mg²⁺ buffer (40 mM Tris, 20 mM acetic acid, 1 mM EDTA (pH 8.0), and 12.5 mM magnesium acetate). These samples were stable for hybridization at up to a certain critical concentration, which is referred to as an optimum concentration C_o. The DX lattices with Ln³⁺ concentration higher than C_o are not properly hybridized to form crystalline structures due to the presence of excess Ln³⁺ introduced before annealing in the test tube. Instead they form amorphous and aggregated structures without a periodic crystalline form (data not shown). To avoid them, we adopted a post-annealing procedure where Ln³⁺ was added after hybridization in order to maintain deformation-free DX lattices, even at a concentration slightly higher than C_o. The experimental data indicated that, by coincidence, all tested ions – Dy³⁺, Eu³⁺, and Tb³⁺ – had the almost same value of 1 mM for C_o. The schematics of the DX lattices and the possible representative Ln³⁺ coordination sites in the DNA molecules are shown in Fig. 1b and c, respectively. The yellow circles in the schematics (Fig. 1c) represent all the possible coordination sites of the Ln³⁺ in the DNA molecules.

Fig. 1 Schematic diagrams of DX sequence map, DX lattice and Ln³⁺ coordination sites in DNA. (a) DNA sequences of two double-crossover (DX) tiles. Each tile consists of four strands, and each strand arrow points to the 3′ end. The complementary sticky-end pairs are shown as S# and S#′ (violet). (b) An illustration of a DX lattice. (c) Schematic diagram of the representative Ln³⁺ coordination sites into the DX duplex. Characteristic AFM images of (d–g) pristine, dysprosium ion (1 mM) doped, europium ion (1 mM) doped, and terbium ion (1 mM) doped DNA lattices, labelled as DNA, Dy 1, Eu 1, and Tb 1, respectively. The insets in the AFM images are noise-filtered images reconstructed via fast Fourier transform showing clear periodicities in the DX lattices. The yellow dotted lines in the AFM images reveal the lattice boundaries.

For consistency, other experimental parameters remained fixed during the annealing process, that is, the annealing time to control the thermal energy remained at 24 hours, the substrate was set to 5 × 5 mm² to determine the total electrostatic interaction between the substrate and DNA molecules, and the sample volume in a test tube was set to 250 μL. On the other hand, the concentration of Ln³⁺ was adjusted as a control parameter, with distinct concentrations of 0, 0.5, 1, 2, and 4 mM of Dy³⁺, Eu³⁺, and Tb³⁺ into DX lattices. The different concentrations are indicated as Dy 0, Dy 0.5, Dy 1, Dy 2, and Dy 4 for the Dy³⁺-DNA lattices; Eu 0, Eu 0.5, Eu 1, Eu 2, and Eu 4 for the Eu³⁺-DNA; and Tb 0, Tb 0.5, Tb 1, Tb 2, and Tb 4 for the Tb³⁺-DNA. We conducted atomic force microscopy (AFM) to verify that the Ln³⁺-DNA lattices had formed well on the given substrate. Fig. 1d–g show AFM images of both pristine DNA and of Ln³⁺-DNA lattices with a critical concentration of each Dy 1, Eu 1, and Tb 1 with deformation-free DX structures with the respective Ln³⁺ coordination. The insets in the AFM images present noise-filtered images reconstructed via fast Fourier transformation that show clear periodicities in the DX lattices. The yellow dotted lines in all of the AFM images indicate the DX lattice boundaries.

The excitation spectra were captured at fixed emission wavelengths (λ_em) 466, 538, and 560 nm for pristine DNA; at 464, 558, and 608 nm for Dy 1; at 526, 539, and 586 nm for Eu 1; and at 539, 574, and 593 nm for Tb 1, as shown in Fig. 2a–d. These excitation spectra clearly reveal intense peaks in the range of 350–450 nm, which indicates a proper range of excitation wavelengths (λ_ex) between 350 and 450 nm. The considerable ET can be explained by using the energy level (EL) diagrams shown in Fig. 2e–h. The photoluminescence requires the appropriate excitation energy to excite an electron to a singlet excited state (S₁) from the singlet ground state (S₀) upon absorbing the photon energy. A preferable approach for relaxation is an internal conversion (IC) in which relaxation from a higher vibrational energy level of an excited state occurs toward the zero level of that excited state. Such a mechanism is referred to as nonradiative energy transfer (NRET).

Fig. 2 Excitation spectra and representative energy level diagram of pristine DNA and Ln³⁺-DNA lattices. Excitation spectra of (a–d) pristine, dysprosium ion (1 mM) doped, europium ion (1 mM) doped, and terbium ion (1 mM) doped DNA lattices at various emission wavelengths. (e–h) The representative energy level diagrams for pristine, dysprosium ion (1 mM) doped, europium ion (1 mM) doped, and terbium ion (1 mM) doped DNA lattices show the various excitation (↑) and conceivable emission (↓) wavelength transitions. Here the red dotted regions indicate the energy transfer (ET) from DNA molecules to Ln³⁺, and the violet dotted regions indicate the energy transfer from Ln³⁺ to DNA through cross relaxation (CR). (Abbreviation note: IC = internal conversion, VEL = vibrational energy levels, ISC = inter-system crossing, ET = energy transfer, CR = cross relaxation, NRET = nonradiative energy transfer.)

The inter-system crossing (ISC) is a type of NRET that occurs when an electron relaxes to the triplet excited state (T₁) from the singlet excited state. Photoluminescence then arises from the release of a photon upon relaxation of the electron from the triplet excited state. The EL diagrams shown in Fig. 2e–h depict the possible emission process for the pristine DNA and Dy³⁺, Eu³⁺, and Tb³⁺-DX lattices with indications of the various excitation (↑) and conceivable emission (↓) wavelength transitions. DNA molecules are well known to exhibit absorption in the range of 250–280 nm, thus ET occurs through an IC process within the excited singlet state, then from the excited singlet state to the triplet state through the ISC process, and finally to emissive states.¹³ At up to C_o for each Ln³⁺, the ET from the excited states of the DNA to the emissive ⁴I_15/2 and ⁴F_9/2 states of the bound Dy³⁺, to the emissive ⁵D₁ and ⁵D₀ states of bound Eu³⁺, and to the emissive ⁵D₃ and ⁵D₄ states of bound Tb³⁺ are expected to take place from the lowest triplet excited state in Ln³⁺-DNA lattices. Above C_o for each ion, the ET might occur from Ln³⁺ to the DNA through cross relaxation (CR) (violet dotted lines in Fig. 2e–h) due to the excess Ln³⁺ binding nonspecifically to DNA molecules. Such an ET results in the enhancement of the luminescence in Ln³⁺-DNA lattices.

The photoluminescence spectra of the Ln³⁺-DNA lattices as a function of Ln³⁺ concentration with a λ_ex of 350 nm at room temperature are shown in Fig. 3. The corresponding columns indicate the photoluminescence spectra of the same samples under various λ_ex's of 375, 400, 425, and 450 nm. These emission spectra present distinct emission peaks and intensities. The results obviously differentiated the Ln³⁺ doping levels in the DNA molecules. With various λ_ex and concentrations of Dy³⁺ in the Dy³⁺-DNA lattices, the photoluminescence spectra exhibit emissions from the DNA singlet excited state ET to the Dy³⁺ in the visible spectral regions correspond to the ⁴I_15/2 excited state to ⁶H_15/2 ground state, and ⁴F_9/2 excited state to the ⁶H_15/2 ground state as well as to the ⁶H_13/2,11/2 states. The ET pathway of the emission spectral lines is shown in Fig. 2e and f. Here, the red dotted circle and the violet dotted line indicated the ETs from DNA to Dy³⁺ (≤C_o of Dy³⁺) and from Dy³⁺ to DNA (>C_o of Dy³⁺) in the Dy³⁺-DNA lattices, respectively. The characteristic spectral lines of Dy³⁺ were previously identified at 485 and 580 nm in different host.²⁵ The blue lights at 464 and 485 nm were observed due to the magnetic dipoles allowing ⁴I_15/2 → ⁶H_15/2, and ⁴F_9/2 → ⁶H_15/2 transitions, respectively. A relatively light doping of Dy³⁺ into DNA lattices, compared to C_o, exhibits broad green spectral line at 558 nm. This green luminescence has been associated with the presence of specific ion bindings into DNA which allowed ⁴F_9/2 → ⁶H_13/2 transitions at 558 nm. Interestingly, ⁴F_9/2 → ⁶H_11/2 spectral transition at ∼580 nm (yellow) was strongly sensitive to the ion concentrations which might be due to the CR between DNA and Dy³⁺.

Fig. 3 Emission spectra of Ln³⁺-DNA lattices under various excitation wavelengths. The emission spectra of (a) dysprosium ion doped, (b) europium ion doped, and (c) terbium ion doped DNA lattices with various ion concentrations of 0, 0.5, 1, 2, and 4 mM under excitation wavelengths of 350, 375, 400, 425, and 450 nm.

Fig. 3b shows the variation in the photoluminescence spectra of the Eu³⁺-DNA lattices as the ion concentration increases. The emission spectra consist of several emission lines originating from the transitions of the emissive state ⁵D₁ to the ground state ⁷F₁ and of ⁵D₀ to the ground state ⁷F_J (J = 0, 1, 2, 3) in the 4f⁶ configuration of Eu³⁺. Fig. 2e and g show the ET pathway of the emission spectral lines, and here, the red dotted line indicates the ET from DNA to Eu³⁺ (≤C_o of Eu³⁺), and the violet dotted line indicates the ET from Eu³⁺ to DNA (>C_o of Eu³⁺) in Eu³⁺-DNA lattices through CR. The emission intensities increased with the increase of the ion concentration up to C_o 1 mM of Eu³⁺, and then decreased after the further increase of the ion concentration. The characteristic Eu³⁺ peaks were observed at 573, 586, and 615 nm, which are similar as those that were previously reported.²⁶ At lower concentrations of Eu³⁺ into the DNA lattices, a green emission band at 539 nm was exhibited, which allowed the emission transition of ⁵D₁ → ⁷F₁. At higher concentrations, the relative intensities of ⁵D₀ → ⁷F₀, ⁵D₀ → ⁷F₁, ⁵D₀ → ⁷F₂, and ⁵D₀ → ⁷F₃ transitions were quite sensitive to the concentration of Eu³⁺ in a manner similar as that for Dy³⁺. The photoluminescence quenching in the Eu³⁺-DNA lattices might depend upon λ_ex and Eu³⁺ concentration. With the given excitation of light, Eu³⁺ can interact with neighbouring ions through various magnetic dipole–dipole, electric dipole–dipole, electric dipole–quadrupole, and electric quadrupole–quadrupole interactions.

Fig. 3c shows the photoluminescence spectra of the Tb³⁺-DNA lattices as a function of Tb³⁺ concentration. At λ_ex of 350 nm, the spectral intensities of the emissions increased as the ion concentration increased up to 2 mM, and then decreased. The blue and green emissions (lower) and yellow and red emissions (higher) were dominant at lower and higher concentration of Tb³⁺ into DNA lattices relative to C_o. Under various λ_ex and Tb³⁺ concentration, the Tb³⁺-DNA lattices showed emissions from the DNA singlet excited state ET to the Tb³⁺ in the visible spectral regions, which corresponded to the ⁵D₃ excited state to the ⁷F_4,3 ground states; and from ⁵D₄ excited state to the ⁷F₆ ground state and the ⁷F_5,4,3 levels. As for Dy³⁺ and Eu³⁺, the ET pathway of the emission spectral lines is shown in Fig. 2e and h. The red dotted line indicates the ET from DNA to Tb³⁺ (≤C_o of Tb³⁺), and violet dotted line indicates the ET from Tb³⁺ to DNA (>C_o of Tb³⁺) in the Tb³⁺-DNA lattices. The characteristic emission peaks for Tb³⁺ could be observed at 438, 464, 486, 532, 575, 630 nm, which match quite well with those reported in previous studies.²⁶ The emission transitions that were allowed were ⁵D₃ → ⁷F₄ (blue) at 438 nm, ⁵D₃ → ⁷F₃ (blue) at 464 nm, ⁵D₄ → ⁷F₆ (blue) at 486 nm, ⁵D₄ → ⁷F₅ (green) at 532 nm, ⁵D₄ → ⁷F₄ (yellow) at 575 nm, and ⁵D₄ → ⁷F₃ (orange) at 630 nm. The lightly-doped Tb³⁺-DNA lattices exhibited a high intensity with sharp blue and broad green spectral lines at 486 and 532 nm. The intensity gradually decreased by reducing the given excitation energy of the Ln³⁺-DNA lattices, which could be due to the fact that the energy was not sufficient for ET from donor to acceptor states.

The variations in the photoluminescence intensities and the spectral line positions were a result of the different transitions in the Ln³⁺-DNA lattices manifesting that Ln³⁺ ions were well coordinated into DNA. At Ln³⁺ concentrations higher than C_o, more crucial description was often needed due to inhomogeneous and nonspecific bindings that occurred as well. These undesired bindings to the DNA lattices led to slight distortions in the DNA lattice structures, and consequently decreased the intensity of the luminescence. The intensities of the blue spectral lines were found to heavily rely upon the critical C_o of Ln³⁺. They gradually increased at up to C_o, which was 1 mM for Dy³⁺ and Eu³⁺ and 2 mM of Tb³⁺, and a decrease in intensity followed as the ion concentrations further increased, which indicated high sensitivity of the luminescence to the concentration of the Ln³⁺ dopant and determined the controllable optical properties of the DNA lattices. In addition, a significant blue luminescence resulted from the dominant blue transition in the DNA lattices when the Ln³⁺ were optimally coordinated with homogeneous and specific bindings at C_o. Furthermore, the luminescence in the green to orange spectral region was found to be stronger for lightly doped DNA lattices than for highly doped ones. This means that further quenching of the luminescence had to be taken into account as a result of the differences in the ion concentration. When the excitation spectra were monitored in the fixed blue to orange emission spectral region, the emissions were observed to be preferentially excited through higher excited states of the 4fⁿ configuration, i.e., 4f⁹ of Dy³⁺, 4f⁶ of Eu³⁺ and 4f⁸ of Tb³⁺.

Overall, the photoluminescence spectra produced broad emissions which might be due to the extremely thin thickness (∼0.6 nm) of the Ln³⁺-DNA lattices. The known diameter of the DNA duplex is of ∼1.9 nm, but the DNA duplex on a substrate has a diameter of 1.2 ± 0.2 nm in a buffer solution and 0.6 ± 0.2 nm in a dry state due to the presence of electrostatic attraction between the DNA molecules and the substrate.^27,28 Our observations indicate that the Ln³⁺-DNA lattices have a significant luminescence in a wide range of wavelengths where the Dy³⁺ yellow lines merge with broad blue-green emissions, Eu³⁺ red lines merge with broad green-orange emissions, and Tb³⁺ green lines merge with broad blue-yellow emissions. The ET has an advantage in that it improves Ln³⁺ emissions produced as a result of the transfer from an excited state of the donor bases to the emissive state of the Ln³⁺ acceptors. The efficiency of the ET can be controlled by adjusting the Ln³⁺ concentration, the excitation energy, and the proper coordination sites of the ions, i.e., the base pairs and the phosphate backbones.^13,15,29 Interestingly, divalent metal ion bindings to DNA have been previously described by a number of researchers. Eichhorn et al.³⁰ suggested that the metal ion binding sites in the DNA molecule were phosphate groups and electron donor atoms on heterocyclic bases. If zinc ion (Zn²⁺) doped DNA duplexes were prepared through a freeze-dry method, the chemical configuration of the Zn²⁺ revealed covalent bonding of Zn²⁺ with nitrogen atoms in the base-pairs due to the extremely dry condition.⁸ Similarly Lee et al. reported intercalation of the Zn²⁺ between the base-pairs in order to study the cooperative conformational change in the DNA duplexes.³¹

To better understand the origin of the photoluminescence via Ln³⁺-DNA lattices, we analyzed the resulting photoluminescence spectra through Gaussian and Lorentzian functions. The typical Gaussian fittings of the UV-visible photoluminescence spectra exhibits various peaks located at 471, 527, 580, 620, 635, 651, and 717 nm for Dy 4; 566, 572, 616, 630, 655, and 710 for Eu 2; 438, 464, 486, 532, 575, and 630 nm for Tb 1, as shown in Fig. 4a–c. Fig. 4d–f show the position of the intense emission peaks, as well as the height, area, and full width at half maximum (FWHM), as a function of the Ln³⁺ concentration under an λ_ex of 400 nm. Appreciable red shift is observed in the peak position as the ion concentration increases, which is a strong evidence of Ln³⁺ coordination into the DNA. The peak height and area of the Dy³⁺-DNA lattices increase as the ion concentration increases, and the FWHM values increase to up to 1 mM and then decrease. For the Eu³⁺-DNA and Tb³⁺-DNA lattices, the peak height, area, and FWHM values increase as the ion concentration increases up to 1 mM of Eu³⁺ and 2 mM of Tb³⁺, and then decrease as the concentration increases further. The bending tendency of the FWHM values and the associated critical doping level of the Ln³⁺-DNA lattices can be explained as follows: the Ln³⁺ prefer to intercalate between the base-pairs and to bind with the phosphate backbone up to C_o, forming long chains that serve as luminescent arrays inside the duplex without disturbing the original DNA structures much. An excess amount of Ln³⁺ (Ln³⁺ concentration >C_o) into the DNA causes them to bind to improper and indiscriminate sites, which in turn creates stress on the helical properties of the DNA.

Fig. 4 Typical Gaussian fitting and emission analysis of Ln³⁺-DNA lattices. Gaussian fitting of (a) dysprosium ion (4 mM) doped, (b) europium ion (2 mM) doped, and (c) terbium ion (1 mM) doped DNA lattices under specific excitation wavelengths of 400 nm. Representation of intense emission peak position, height, area, and full width at half maximum (FWHM) as function of the Ln³⁺ concentration of (d) dysprosium ion doped, (e) europium ion doped, and (f) terbium ion doped DNA lattices. These values are extracted from the emission spectra under excitation wavelengths of 400 nm using Gaussian and Lorentzian functions.

In general, the trichromatic system of colorimetry recommended by the Commission Internationale de l'éclairage (CIE) (1931) is used to specify the colour of light. The CIE standards provide two coordinates along the x and y directions that are related to the emission wavelength and the energy distribution of the light. Fig. 5a–c show the colour coordinate chromaticity diagrams of Dy³⁺-DNA, Eu³⁺-DNA, and Tb³⁺-DNA lattices as a function of the Ln³⁺ concentrations under typical λ_ex, such as 375, 350, and 350 nm, respectively. Fig. 5d–f show the colour coordinates in the CIE 1931 chromaticity diagram of the Ln³⁺-DNA lattices as a function of λ_ex at a critical concentration of each ion into DNA. The diagram indicates colour changes from blue to orange as the ion concentration varies. Interestingly, the emission quite close to the white light can be possible by obtaining the appropriate mixture of visible transitions of the Ln³⁺.

Fig. 5 Possible colour coordinates controlled by various Ln³⁺ concentration and excitation wavelength. Chromaticity diagram of (a) dysprosium ion doped, (b) europium ion doped, and (c) terbium ion doped DNA lattices with various Ln³⁺ concentrations under particular excitation wavelengths such as 375, 350, and 350 nm, respectively. The chromaticity coordinates on the CIE 1931 chromaticity diagram of (d) dysprosium ion doped, (e) europium ion doped, and (f) terbium ion doped DNA lattices with critical concentrations of each ion under various excitation wavelengths.

The activation of ET phenomena through variations in Ln³⁺ concentration permits an excellent control of emission chromaticity across the CIE 1931 diagram, e.g., (x, y) colour coordinates from (0.154, 0.178) to (0.423, 0.462), (0.401, 0.464), and (0.411, 0.465) can be obtained for Dy³⁺-DNA, Eu³⁺-DNA, and Tb³⁺ DNA, respectively, at a fixed λ_ex of 350 nm. These variations in colour coordinates for the Ln³⁺-DNA lattices are mainly a result of the different intensities of blue, green, yellow, and orange spectral lines. The ET between the host DNA molecules and the Ln³⁺ cations originate from the Ln³⁺-DNA emitting center. The ability to tune the colour coordinates by controlling the Ln³⁺ concentration in the DNA nanostructures can be useful for practical applications due to the simple fabrication process. Even though the colour coordinates are also slightly controllable by adjusting λ_ex, the colour efficiency and yield might be reduced because the emission intensities were much suppressed at longer λ_ex than at shorter ones, as shown in Fig. 3.

Conclusions

In conclusion, we studied the photoluminescence of the ultra thin Ln³⁺-DNA lattices as a function of both λ_ex and Ln³⁺ concentration. By adding Ln³⁺ into DNA, the luminescence colour of the spectral lines varies from blue to orange. At a particular concentration of each Ln³⁺, the intensity of the photoluminescence is enhanced due to the homogeneous and specific bindings to DNA via intercalation between base-pairs and binding on phosphate backbones. The coordination of Ln³⁺ into DNA has originated as an alternative to overcome some of the limitations of the traditional fluorescent labels, such as organic dyes or quantum dots. The ultimate advantage of adopting DNA lattices combined with Ln³⁺ ions is the ability to control the spectrum of the luminescence with Ln³⁺ concentrations in precision. The insight gained from this study opens the door to searching for new functional DNA nanostructures for practical applications towards the fabrication of bio-photonic devices and biosensors with better sensitivity, efficiency, and photo-stability.

Acknowledgements

This research was supported by the Basic Science Research Program (NRF-2013R1A1A2061731) for S.R.D. and by the Nano Material Technology Development Program (NRF-2012M3A7B4049801 & NRF-2012M3A7B4049804) of the National Research Foundation of Korea (NRF) for S.H.P. & C.K., through funding from the Ministry of Education, Science and Technology (MEST) and the Ministry of Science, ICT & Future Planning (MSIP) in Korea.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra07360j

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