Bio-recognitive photonics of a DNA-guided organic semiconductor

ARTICLE Received 27 May 2015 | Accepted 19 Nov 2015 | Published 4 Jan 2016 DOI: 10.1038/ncomms10234 OPEN Bio-recognitive photonics of a DNA-guided organic semiconductor Seung Hyuk Back1,*, Jin Hyuk Park2,*, Chunzhi Cui2,* & Dong June Ahn1,2,3 Incorporation of duplex DNA with higher molecular weights has attracted attention for a new opportunity towards a better organic light-emitting diode (OLED) capability. However, biological recognition by OLED materials is yet to be addressed. In this study, specific oligomeric DNA–DNA recognition is successfully achieved by tri (8-hydroxyquinoline) aluminium (Alq3), an organic semiconductor. Alq3 rods crystallized with guidance from single-strand DNA molecules show, strikingly, a unique distribution of the DNA molecules with a shape of an ‘inverted’ hourglass. The crystal’s luminescent intensity is enhanced by 1.6-fold upon recognition of the perfect-matched target DNA sequence, but not in the case of a single-base mismatched one. The DNA–DNA recognition forming double-helix structure is identified to occur only in the rod’s outer periphery. This study opens up new opportunities of Alq3, one of the most widely used OLED materials, enabling biological recognition. 1 KU-KIST Graduate School of Converging Science and Technology, Korea University, Seoul 02841, Korea. 2 Department of Chemical and Biological Engineering, Korea University, Seoul 02841, Korea. 3 Center for Theragnosis, Biomedical Research Institute, Korea Institute of Science and Technology, Seoul 02792, Korea. * These authors contributed equally to this work. Correspondence and requests for materials should be addressed to D.J.A. (email: ). NATURE COMMUNICATIONS | 7:10234 | DOI: 10.1038/ncomms10234 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms10234 N ovel display materials have gained keen attraction recently in the fields of electronics and photonics research especially owing to the rapid evolution of smart communication devices1–3. Among the various display materials available, organic semiconductors or metal-organic compounds are considered to be very promising, and they have therefore been intensely investigated4–6. An alumina quinoline, tri (8-hydroxyquinoline) aluminium (Alq3), first reported approximately three decades ago, which emits in the green and blue spectra, is a material of central interest7–9. Alq3 is currently used in a multitude of organic light-emitting diodes (OLEDs)10–13 that are used in various displays. Since it was first reported, enormous improvements have been made in its light emission efficiency, to provide higher display quality14–18. One peculiar approach incorporates a biological material into the light-emitting device, often called a BioLED19. An example is DNA in the form of a thin film introduced within a conventional electroluminescent cell incorporating an Alq3 layer20. Utilized in the device were double-strand DNAs (dsDNAs) extracted from natural organisms and complexed with cationic surfactants; the device provides B30-fold increase in luminescence intensity21. This phenomenon was attributed to the contribution of the DNA layer to the electron blocking effect, thus reducing significant loss of electrons and enhancing electron–hole recombination in the cell20. Luminescent dyes entrapped within dsDNA thin films reported22 also exhibited higher intensity owing to less nonradiative relaxation. This novel capability of DNA is noteworthy as a gadget in light-emitting devices. The value of the devices can be recognized even higher as they incorporate the ‘bio-recognition function’. Current BioLEDs now face a new journey to the realm of biological recognition. To this end, this study presents a critical step endowing an OLED material with a biological recognition function. We demonstrate for the first time that only specific DNA–DNA recognition triggers photoluminescent enhancement reflected by Alq3, the most widely used OLED material. Results Optical properties analyses of DNA-guided Alq3 rods. We first observed the characteristic alteration when Alq3 particles incorporating single-strand DNA (ssDNA) moieties interacted with specific target DNA (tDNA) molecules. Crystallization of Alq3 has been conventionally executed with the aid of surfactants and recently become successful using ssDNA molecules only23. With guidance from ssDNA, we fabricated prismatic hexagonal rod crystals composed of Alq3. In this study, the oligomeric ssDNA used for crystal guidance was a 27-mer sequence of anthrax lethal factor. Figure 1a shows a schematic illustration of the recognition of specific tDNA by the light-emitting Alq3 rod crystallized by ssDNA. Figure 1b,c provide colour charge-coupled device (CCD) images of the ssDNA-guided Alq3 (ssDNA-Alq3) rods before and after treatment with tDNA molecules, respectively. We observed the ssDNA-Alq3 rods emitting green luminescence. Interestingly, the intensity of the green luminescence of the ssDNA-Alq3 rods was markedly enhanced after interaction with specific tDNA molecules. For quantitative analysis of the intensity enhancement, we measured the photoluminescence (PL) spectra of the Alq3 rods. As shown in Fig. 1d, a broad PL peak was observed at B512 nm when samples were excited with a laser at 365 nm, which corresponds to the main absorption band of Alq3. The PL spectra were yellowish-green, composed of both a and d phases8,9,24. After interaction with specific tDNA molecules, the PL peak intensity increased B1.6-fold, which is concordant with the results of the CCD analysis. Interestingly, when treated with single-base (1-mer) mismatched tDNA molecules that are less specific, the Alq3 rods showed little enhancement of PL intensity. In addition, PL excitation (PLE) spectrum analysis confirmed the enhancement of PL intensity, as shown in Fig. 1e. The intensity with excitation at 365 nm and emission at 512 nm was clearly higher following treatment with specific target molecules. Crystal structure analyses upon interaction with DNA. To further explore the PL enhancement of the Alq3 rods after interaction with specific tDNA molecules, we selected four crystal samples of ssDNA-Alq3, ssDNA-Alq3 treated with specific tDNA, ssDNA-Alq3 treated with 1-mer mismatched tDNA and dsDNAAlq3 (dsDNA-guided Alq3 rods crystallized by the use of dsDNA molecules from the start). X-ray diffraction (XRD) patterns were observed, as shown in Fig. 2a, to examine structural changes in the Alq3 crystals. The XRD pattern of the ssDNA-Alq3 rod showed typical a-phase peaks for Alq3 at 11.40° and 12.81°, along with a d-phase peak at 11.79°. Hence, the ssDNA-Alq3 rods fabricated in this study contain both a- and d-phases8,9,25–27, which is consistent with the yellowish-green luminescence PL intensity (a.u.) 5,000 ssDNA Specific tDNA 3,000 2,000 Initial After treatment with 1-mer mismatched tDNA 1,000 0 400 Excitation wavelength (nm) After treatment with specific tDNA 4,000 500 600 Wavelength (nm) 50 (...truncated)