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To be useful in optoelectronic devices and sensors, a platform comprising stable fluorescence materials is essential. Here we constructed quantum dots (QDs) embedded DNA thin films which aims for stable fluorescence through the stabilization of QDs in the high aspect ratio salmon DNA (SDNA) matrix. Also for maximum luminescence, different concentration and configurations of core- and core/alloy/shell-type QDs were embedded within SDNA. The QD-SDNA thin films were constructed by drop-casting and investigated their optoelectronic properties. The infrared, UV-visible and photoluminescence (PL) spectroscopies confirm the embedment of QDs in the SDNA matrix. Absolute PL quantum yield of the QD-SDNA thin film shows the ~70% boost due to SDNA matrix compared to QDs alone in aqueous phase. The linear increase of PL photon counts from few to order of 5 while increasing [QD] reveals the non-aggregation of QDs within SDNA matrix. These systematic studies on the QD structure, absorbance, and concentration- and thickness-dependent optoelectronic characteristics demonstrate the novel properties of the QD-SDNA thin film. Consequently, the SDNA thin films were suggested to utilize for the generalised optical environments, which has the potential as a matrix for light conversion and harvesting nano-bio material as well as for super resolution bioimaging- and biophotonics-based sensors.
Deoxyribonucleic acid (DNA) molecules are commercially available at relatively low cost and have unique polymer characteristics, and so have potential as functional biomaterials in various fields, such as biophotonics, bioelectronics, and biosensors. A thin film made of DNA shows low optical loss and high transparency1. These unique and superior optoelectronic characteristics of DNA are useful for solid state devices as a charge transport layer in functional devices and a host medium for luminophores in photonics. Cationic-modified DNA material has been used as a selective charge transport layer in light-emitting diodes (LEDs)2, organic light-emitting diodes (OLEDs)3, 4 and organic field-effect transistor devices (OFETs)5, 6. DNA materials have been used as hosts of fluorescent dye7 and metal ions8 by doping and resulted in amplified emissions in lasing9–11, nonlinear behaviour in optics12 and also in wave-guiding applications13.
Although DNA itself has important physical, chemical, and biological characteristics, DNA as a template for hosting functionalised materials14, 15 such as nanoparticles, di- and tri-valent ions, fluorescent dyes, carbon-based materials, proteins and drugs might be superior in terms of specific functionality enhancement and reliable output due to its specific binding affinity to the functionalised guest materials and ability to be modified. The presence of the hydrogen bonding by DNA molecules amplifies the fluorescence emissions of a given luminophore16. Among the available luminophores, DNA molecules with fluorescent dyes and metal ions have been used to study optical characteristics. However, other fluorescence materials; i.e. quantum dots (QDs) which have unique optoelectric characteristics in terms of stable fluorescence, large Stoke shift, quantum confinement and semiconducting nature, warrant investigation.
QDs are fluorescent semiconducting nanoparticles with confined electrons at a scale of a few nanometres that exhibit a quantum size effect17, 18. QDs enable mimicking of natural biomaterials and so could replace conventional electronics and photonics materials19–21. In biological research, QDs have potential as biomarkers for site-specific drug delivery and medical imaging22. For use in biological applications, QDs must be soluble in aqueous liquids with high colloidal stability, which requires an optimised surface ligand exchange process with appropriate surface ligands23. QDs and DNA materials have different optoelectronic characteristics in different phases; i.e. the colloidal and condensed matter (thin film) phases. Although conjugation of QDs to DNA in the colloidal phase has been reported to facilitate florescence imaging and sensing applications24, 25, DNA and QDs in the condensed matter phase (in the form of a thin film) has been the subject of few studies.
To be useful ‒ e.g. overcome concentration-dependent luminescence quenching26, performance inconsistency, non-biocompatibility, and structural and photo instability ‒ in optoelectronic devices and sensors, a stable platform (production of thin films with easily tunable thickness) using stable fluorescence materials (efficient dispersibility in a given platform) is essential. Here we developed a natural salmon DNA (SDNA) thin film (as a stable template) with QDs (photo stable inorganic luminophores) dispersed in aqueous and organic phases and investigated the optoelectronic characteristics of QD-embedded SDNA thin films. Two QD luminophore configurations ‒ core-type QD (QDC) and core/alloy/shell-type QD (QDA) ‒ were synthesised by the one-pot hot-injection method. As-synthesised QD surface were replaced by hydrophilic ligands (for aqueous compatibility). Here the core/shell configuration was adopted in order to maintain the photoluminescence quantum yield (PLQY) and avoid structural instability in an aqueous phase. An appropriate amount of surface functionalised QDs was mixed with SDNA molecules in an aqueous phase to fabricate thickness-controllable QD-embedded SDNA thin films by a drop-casting method.
The synthesised QDs were evaluated by high-resolution transmission electron microscopy (HRTEM), scanning transmission electron microscopy (STEM), high-angle annular dark-field (HAADF) elemental-mapping (for verification of the internal structures of QDs), atomic force microscopy (AFM) (for visualisation of the topological profile), Fourier transform infrared (FTIR) spectroscopy (for confirmation of the presence of ligands on QDs), and UV-vis and PL spectroscopies (for evaluation of their optical characteristics). We also investigated the QD-embedded SDNA thin films by the FTIR spectrum to confirm the QD and SDNA configuration, UV-vis and PL spectra to assess their photonic characteristics and current‒voltage (I‒V) measurements to evaluate their electrical properties.
A QD is a relatively more stable fluorescent material than the conventionally available fluorescent dye molecules. Emission of QDs is tunable across the entire visible range by means of bandgap engineering. Among the QDs (normally dispersed in an organic phase), cadmium-selinium (CdSe) binary QDs (core type, QDC) have a high PLQY up to 85%27. However core-type QDs in an aqueous phase exhibit a fivefold reduction in PLQY due to the surface modification required for stabilisation in water28. The core/alloy/shell configuration of QD (QDA) was synthesised by one-pot method, and exhibited enhanced structural stability compared to core/shell-type QDs (multi-pot preparation required and relatively low structural stability due to a high lattice mismatch)29.
QD synthesis was shown in Supplementary Figure S1, and discussed in the methods section. QDC and QDA structures were formed in the absence and presence of Zn and S, respectively. Schematic diagrams of QDC and QDA are shown in Fig. 1a. The TEM and STEM images (Fig. 1b) show a QDA size of ~7nm with moderate distribution, and HAADF images show the single-particle internal elemental distribution and mapping. The single particle elemental mapping of Cd, Se, Zn, and S shows that the alloy QD configurations (QDA) are in agreement with the energy dispersive spectrum (EDS) and quantitative atomic and weight percentage data (Supplementary Figure S2).
Figure 1c shows QD surface modification by ligand exchange process (discussed in detail in the Methods section) in which the pre-synthesised oleic acid (OA, C18H34O2) capped hydrophobic ligands are replaced by 3-mercaptopropionic acid (MPA, C3H6O2S) capped hydrophilic ligands, to facilitate homogeneous dispersion of the QDs in an aqueous phase. The ligand exchange method used was simple and efficient; mixing of MPA (as a phase-transfer agent) with target QDs. A representative AFM image of QDA in an aqueous phase is shown in Fig. 1c. The average hydrodynamic diameter (obtained in a colloidal phase) of QDA was 9.68±0.2nm, which was slightly larger than the TEM value (obtained in a dry phase) due to inclusion of surface ligands in the aqueous phase (Supplementary Figure S3).
The synthesised QDs were analysed by FTIR spectrometry (see the Methods section) to confirm ligand attachment on the QD surface. QDs capped with an OA ligand (QD-OA) and MPA (QD-MPA) are compatible with chloroform and deionized (DI) water solvents, respectively. Figure 2a shows the FTIR absorbance peaks of QDC-OA (organic), QDA-OA (organic), QDC-MPA (aqueous), and QDA-MPA (aqueous). The absorbance characteristic peak at 2,900cm−1 is attributable to C-H stretching vibration of OA ligand attached to QDC-OA and QDA-OA; similarly, the absorbance characteristic peaks at 1,400 and 1,750cm−1 indicate the carboxylate symmetric and asymmetric stretching vibrations of MPA ligand attached to QDC-MPA and QDA-MPA. The peak assignments are in good agreement with a previous report (Supplementary Table S1)30. Figure 2b shows the UV-vis absorption (Abs., solid line) and PL excitation (Exi., dotted) spectra of colloidal QDC-OA, QDA-OA, QDC-MPA, and QDA-MPA. Solid lines represent the first (second) excitonic peak maximum at 556nm (456) and 588nm (560) for QDC-OA and QDA-OA, respectively. QDC-MPA and QDA-MPA dispersed in an aqueous phase are blue-shifted by ~4nm due to shortening of the ligand length on the QD surface by the ligand exchange process ‒ a longer OA ligand (hydrophobic) was replaced by a shorter MPA (hydrophilic) to enhance the aqueous dispersibility. The dotted line represents the PL excitation of QDC and QDA samples; the overall PL excitation spectra maximum was observed at 370nm. The second and higher excitation peaks of absorbance and PL excitation peaks have some resemblance (shown by vertical dotted lines in Fig. 2b). However the PL excitation peaks have excitation maximum at ~370nm and falls but absorbance peaks keep heading towards maximum without fall at shorter wavelength.
Fluorescence emission of colloidal QDC and QDA were obtained by PL spectroscopy (discussed in detail in the Methods section), exciting at 370nm; fluorescence emissions were found at 590 and 608nm for QDC-OA (absolute PLQY: 7%) and QDA-OA (PLQY: 61%) dispersed in an organic solvent, respectively (Fig. 2c). QDC-MPA (PLQY: 2%) and QDA-MPA (PLQY: 13%) dispersed in an aqueous medium showed red-shifted emissions from 590 to 614 and 608 to 611nm, respectively due to the ligand-exchange process. The normalized PL spectra of QDs show smooth curves with full-width at half-maximum (FWHM) of about 35 and 38nm for QDC-OA and QDA-OA, respectively. Also, the ligand-exchange process alters the PL FWHM from 35 to 39nm (increased) and 38 to 35nm (decreased) for QDC-MPA and QDA-MPA, respectively; these narrow PL emission characteristics of QDs would facilitate sensing applications (e.g. bio-labelling and diagnostic). Figure 2d shows the luminescence of organic (QDC-OA, QDA-OA) and aqueous (QDC-MPA, QDA-MPA) QDs; luminescence under UV light with excitation wavelengths of 254 and 365nm clearly shows excitation wavelength-dependent emission characteristics (compared to ambient light), which visually confirm the importance of the optical characteristics of the synthesised QD (i.e. QDC and QDA).
The volumetric ratio of SDNA to QD (SDNA:QD) was varied for a reasonable amount of QD (hereon all the QD’s samples were dispersed in DI water which have MPA ligand) embedding condition within the condensed thin film. By using one weight-percentage (1 wt.-%) of SDNA and 1.0 optical density (OD) of QDs (which correspond to first excitation peak’s absorbance value of QD, obtained by UV-vis spectrophotometer), we prepared a homogeneous 20µL QD-embedded SDNA colloidal mixture of six SDNA:QD ratios of 16:0 (20µL of SDNA without QDs, served as a reference), 16:1 (18.8µL of SDNA with 1.2µL of QDs, named QD10), 16:2 (17.8µL SDNA with 2.2µL QDs, QD20), 16:3 (16.8µL SDNA with 3.2µL QDs, QD30), 16:4 (16.0µL SDNA with 4.0µL QDs, QD40), and 16:5 (15.2µL SDNA with 4.8µL QDs, QD50). The mixtures were dropped on an O2 plasma-treated quartz substrate. The fabrication and sample preparation procedures are described in Fig. 3a, the Methods section, and Supplementary Table S2.
To evaluate the optical characteristics of a QD and SDNA mixture, the absorbance of SDNA thin films with different concentrations (controlled by the volume of QDs) of QDCs and QDAs was determined (Fig. 3b,c). The absorbance peaks that indicate the binding environment and chemical functional groups of DNA molecules and the corresponding characteristic peaks were assigned accordingly (Supplementary Table S4)15. Based on the absorbance measurements, strong (weak) characteristic peaks were found at 780, 960, 1010, 1055, 1083, 1230, and 1652 cm−1 (828, 895, 1372, 1416, 1488, 1604, 1693, and 3357cm−1). The absorbance bands were divided into three regions: 3600–3000cm−1 for water OH stretching, 1800–1300cm−1 for nucleotide bases, and 1250–600cm−1 for sugar and phosphate groups. The decrement of absorbance intensities was observed for both QDC- and QDA-embedded SDNA thin films because of the increment of QDs (caused decrement of SDNA concentration) within the SDNA matrix. Interestingly, either an appearance or a disappearance of a certain absorbance peak was not observed, suggesting that the QDs in a SDNA matrix did not significantly alter the binding environment of DNA molecules. Thus the SDNA thin film acts as a suitable host for QD luminophores.
The SDNA thin film, particularly that with QDAs, showed significant luminescence under UV light. The photographs in Fig. 3d show the luminescence of QDC(A)-embedded SDNA thin films at excitation wavelengths of 254 and 365nm. The excitation-dependent emission characteristics confirm the optical quality of QD-embedded SDNA thin films. Due to the differences in QD configuration (shown in Fig. 1a,b) and shell passivation between QDC- and QDA-embedded SDNA thin films, different emissions were observed (i.e. brighter with QDA-embedded SDNAs than QDCs). The ZnS shell on the QDs (QDAs) prevents chemical degradation and eliminates the dangling bonds, which improves the passivation of QDAs. Consequently, QDA-embedded SDNA thin films have a higher probability of radiative relaxation of photons. Interestingly, the QDA-SDNA thin film shows the 22% PLQY compare to QDA without SDNA in aqueous phase, which has only 13%. This indicates the 70% boost of PLQY efficiency due to the high aspect-ratio of SDNA duplex which eventually contributed for better passivation of QDA surface. A previous finding also supports the fluorescence intensity increase up to the order of 3 when the virtually non-fluorescent dye complexes stabilized in DNA31. In contrast, QDC (CdSe core without a shell) undergo chemical degradation and dangling bonds (caused trap states), hence QDC-embedded SDNA thin films (PLQY: 1.2%) have a lower probability of radiative relaxation.
DNA has a large transparency window and the maximum absorption peaks are determined by the conjugated π electrons in the phenyl rings32. We fabricated QD-embedded SDNA thin films on a quartz rather than a glass substrate to avoid background absorbance in the visible region (a quartz substrate has negligible absorbance in the wavelength range 200–800nm). Absorption spectra of QD-embedded SDNA thin films were obtained by UV-vis spectrophotometer (discussed in detail in the Methods section). The absorbance maxima of pristine SDNA was found at 260nm, as expected (dotted lines in Fig. 4a,d) and were the peak intensity maximum altered due to the QD embedment within the SDNA matrix and changes in the film thickness (Supplementary Figure S4). We controlled the net amount of QDs (with a fixed OD of 1.0) within the SDNA matrix by altering the SDNA to QD volume ratio (16:1, 16:2, 16:3, 16:4, and 16:5). Based on the absorbance analysis, the first and second (higher Stoke shift than first excitation) excitonic maxima of the QDC(A)-embedded SDNA thin films were found at 558nm and 462 (dotted lines in Fig. 4b) (588 and 563nm, Fig. 4e) respectively, and the first and second absorbance maximum peaks suggested the band edge transition probability of the QDC(A)-embedded SDNA thin films (electronic state transition from the ground to higher excitation states).
The absorbance increased (roughly proportionally) with increasing QDC(A) volume ratio within the SDNA matrix, which indirectly indicated the presence of a different net amount of QDC(A) within the SDNA thin film. A comparison of absorbance maximum intensities between the QDCs in the SDNA thin film (average of the first and second absorbance maximum peak intensities was 0.023; a horizontal dotted line in Fig. 4c) and SDNAs (average first absorbance maximum peak intensity was 3.182; a horizontal dotted line in Fig. 4c) yielded a ~136-fold difference, whereas maximum peak intensities between the QDAs in the SDNA (0.070; dotted line in Fig. 4f) and SDNAs (3.132) differed by ~45-fold. The absorbance difference (ca. threefold) between QDCs and QDAs in SDNA might be due to the presence of additional alloys and the shell configuration in QDAs. The absorbance characteristics indicated the composition of QD-embedded SDNA thin films and predicted the transition probabilities of band edges33.
A stable luminophore improves lasing gain and photonic device stability10, 34 – the performance and life time of the device are determined by the non-degradation and photostability of the material. Although a photonic device comprising a Rhodamine-6G-doped DNA complex showed amplified spontaneous emission35, replacement of a stable and highly sensitive QD luminophore with DNA might increase the device life time and performance further, facilitating their use in biophotonics. We fabricated QDA-embedded SDNA thin films and evaluated their PL characteristics. The PL characteristics of QDA-embedded SDNA thin films fabricated using QD concentrations [QD] of OD 0.5 and 1.0 are shown in Fig. 5a,b. The PL emission area and CIE (International Commission on Illumination) coordinate shifts increased with altering of the SDNA to QDA volume ratio. The QDA-embedded SDNA thin films were excited at a wavelength of 370nm. PL spectra for OD 0.5 (1.0) showed red-shifted emission from 612 to 614nm (613 to 616nm); hence the CIE coordinate colour-shifts were from 0.621 to 0.642 (0.638 to 0.645) along the x-axis and from 0.371 to 0.354 (0.357 to 0.351) along the y-axis (Supplementary Table S5).
Radiative and polarised emissions probability ‒ proportional to the integrated PL emission area – from the QDA-embedded SDNA thin films increased roughly linearly while altering the volume ratio of SDNA to QDs (Fig. 5c,d). Thus the PL emission sensitivity of QDA-embedded SDNA thin films could be easily controlled by altering the volume ratio of the QDA luminophore. SDNA functions as a superior host that can organise and stabilise the QDA luminophores in a 3D matrix and significantly reduces luminophore self-quenching. Also, SDNA interacts with the QDA luminophore surface and provides a superior passivation environment, which enhances the probability of radiative and polarised emission through the modified quantum states of QDA-SDNA interactions within the matrix. The normalised PL spectra indicated slight emission shifts and Gaussian-like narrow emission (FWHM, ~35nm), indicating the discrete band edge emissions of QDA-embedded SDNA thin films (Fig. 5e,f). The PL emission characteristics of the QDA-embedded SDNA thin film can be implied as a photo-induced signal in photonic-based sensors and be acknowledged as a change of bio-environment in a biological transducer via the emission shift.
Current-voltage (I-V) measurements (discussed in detail in the Methods section) were performed to evaluate the charge carrier transport properties of QDC(A)-embedded SDNA thin films, which are essential for electronics applications. QDC(A)-embedded SDNA thin films were fabricated on a glass substrate by altering the SDNA to QDC(A) volume ratio, and silver electrodes were deposited as the probe contacts. The electrical properties were analysed using a semiconductor parameter analyser; the data was shown in Fig. 6a,b for QDC-embedded SDNA thin films and Fig. 6c,d for QDA-embedded SDNA thin films. The output I was obtained by the preset input V in the range −5V to +5V. Insets show resistance (R=V/I) as a function of the SDNA to QD volume ratio at a fixed V=5V. The R ranged from a few tens of MΩ (pristine SDNA thin film) to a few hundred (QD-embedded SDNA). The SDNA thin films without QDs showed a lower R (26.9 MΩ) than the QD-embedded SDNA thin films. R increased gradually with increasing the QDC(A) volume ratio to 87.9 MΩ (131.0 MΩ). Although QDs are semiconductors, the measured I decreased with increasing [QD], possibly due to the limitation of packing of QDs within the SDNA matrix, thin film’s thickness variation and the increment of interfacial barriers between the QDs and the SDNA.
QDs (core- and core/alloy/shell-type) ‒ synthesized by fast, simple hot-injection method and surface functionalized by mass-action principle ‒ were dispersed in organic and aqueous media, a salmon DNA thin film containing QDs was prepared, and its optoelectronic characteristics were investigated. The internal configuration and shell passivation of the QDs was extremely sensitive and responsible for its high fluorescence emission and PLQY. No chemical interaction between the QDs and SDNA molecules was evident from the FTIR absorbance bands, also the measured PLQY of QDA aqueous solution was 13% where the QDA-SNDA thin film was 22%, suggesting that the SDNA matrix could function as an efficient host for QD luminophores. UV-vis absorbance peak intensities confirmed the presence of QDs (with differentiation of a QD configuration) within the SDNA matrix, which increased with increasing [QD]. The photoluminescence of SDNA thin films with various [QD] showed narrow Gaussian-like fluorescence with a discrete bandgap. A monotonic increase in the integrated emission area and slight red-shift of the emission (confirmed by the CIE chromaticity diagram) controlled by various [QD] were observed. Current-voltage measurements of QD-embedded SDNA thin films were performed. The SDNA thin films without QDs showed lower resistance than QD-embedded SDNA thin films due to the limitation of packing of QDs within the SDNA matrix and the increment of interfacial barriers between the QDs and SDNA. QDs (stable luminophores) in a SDNA thin film template (an efficient host) have been suggested to have promise as a functionalised platform for multifunctional devices and highly sensitive sensors for bioimaging and biophotonics.
Cadmium oxide (CdO, 99.99%), zinc acetate (Zn(Ac), 99.9%), selenium (Se), sulphur (S), trioctylphosphine (TOP, 90%), oleic acid (OA, 90%), and 1-octadecene (ODE, 90%) were used for QD synthesis (Sigma-Aldrich, St. Louis, USA). CdO (1.60mM), Zn(Ac) (4.00mM) and OA 5mL were placed in a 200mL three-neck round-bottom flask. A temperature sensor was inserted into the flask and heated to 150°C with stirring at 450rpm. Nitrogen gas (N2) was flowed into the flask and an N2 atmosphere was maintained inside the flask throughout the experiment to prevent chemical oxidation. Fifteen millilitres of ODE were injected into a flask at 150°C and the system was heated to 310°C at a rate of 15°C/min. At 310°C, 3mL of stock solution; i.e. Se, S+TOP (prepared by dissolving Se (0.40mM) and S (1.33mM) in 3mL of TOP solvent) were injected into the system rapidly and cooled to 280°C followed by incubation (15min) to enable QD growth and surface reconstruction (Supplementary Figure S1). Core (CdSe)-type QDs (QDC) and core/alloy/shell (CdSe/Cd1−xZnxSe1−ySy/ZnS)-type QD (QDA) structures were formed in the absence and presence of Zn and S, respectively (Fig. 1a).
QDAs dispersed in chloroform (CF) at 5mg/mL were drop-casted on a copper mesh grid of a high-resolution transmission electron microscope (HRTEM), followed by drying for 1h at 60°C. The grid with QDAs was placed inside the HRTEM (Cs-Corrected/EDS/EELS, JEM ARM 200F, JEOL Corp., Tokyo, Japan) under an ultra-vacuum for 3h to remove residue and moisture prior to high-angle annular dark-field (HAADF) elemental mapping (Fig. 1b and Supplementary Figure S2).
Purified QDs dispersed in CF were placed in a vial (containing 5mg QDs dispersed in 1mL CF), followed by addition of 0.2mL of MPA. The mixture (QD+CF+MPA) was vortexed for 2min to achieve homogeneity. The MPA-treated QD residues were decanted by centrifugation for 5min at 8,000rpm. After removal of excess solvent, an equal amount of buffer (ammonia) was added as a co-stabilising agent. Finally, the QDs were dispersed in deionised (DI) water (Fig. 1c).
For AFM imaging, 5µL of QDs dispersed in the DI water were dropped on a mica substrate attached to a metal puck using instant glue. Then, 30µL of DI water were pipetted onto the mica and a further 30µL were dispensed into the SiN AFM tip (NP-S10, Veeco Inc., NY, USA). AFM images were obtained using a Multimode Nanoscope III (Veeco Inc., NY, USA) in liquid-tapping mode (Fig. 1c and Supplementary Figure S3).
To prepare the SDNA solution (Chitose Institute of Science and Technology, Hokkaido, Japan), 0.1g of purified SDNA are dissolved in 10mL of DI water (the resulting solution has a concentration of one weight-percentage (wt.-%) SDNA in DI water) followed by the magnetic stirring (with 1,000rpm for 10h at room temperature) to achieve a homogeneous mixture of SDNA in solvent (Fig. 3a).
SDNA solution (1 wt.-%) with a volume of 160 μL was pipetted into an Eppendorf test-tube. Two different QD concentrations ([QD], measured using an UV-vis absorption spectrometer), optical densities (OD) of 0.5 and 1.0 (which correspond to the first excitation maximum peak in the absorbance spectra) were prepared. QD solutions of OD=0.5 or 1.0 (0, 10, 20, 30, 40, and 50 μL) were added to a test-tube containing SDNA solution, followed by vortex mixing for 5min and then incubation for 24h at room temperature to obtain a homogeneous QD-embedded SDNA solution (Fig. 3a and Supplementary Table S2).
Glass (for PL and I–V) and quartz (for absorbance and FTIR) with a size of 5×5mm2 were treated with O2 plasma for surface modification. The O2 plasma treatment introduced silanol groups to the substrate. This changed the net charge from neutral to negative, resulting in a change from a hydrophobic to a hydrophilic substrate. An O2 plasma cleaner (Plasma Processing System, CUTE–1MP/R, Gyeonggi, Korea) was used in this study. The O2 plasma process was performed under the following conditions: 50W power, 5×10−2 Torr base pressure, 47 SCCM oxygen flow rate, 7.8×10−1 Torr working oxygen pressure, and plasma generation time of 5min for glass and 10min for quartz substrates. Following O2 plasma exposure, 20 μL of QD-embedded SDNA solution were drop-casted and allowed to dry for a day to fabricate a QD-embedded SDNA thin film (Fig. 3a).
The thickness of pristine SDNA and QD-embedded SDNA thin films was measured using a surface profilometer (Alpha-Step D-500, KLA-Tencor Corp., CA, USA) and a field emission scanning electron microscope (FE-SEM) (JSM 7401F, JEOL Corp., Tokyo, Japan). The measured average thickness of the thin films was 4.5µm for SDNA, 4.3µm for a QD-embedded SDNA thin film with a 10µL QD volume at a given SDNA volume of 160µL and 1.6µm for QD-embedded SDNA with a 50µL QD volume (Supplementary Figure S4).
The FTIR spectra (the TENSOR 27 spectrometer with a detector MIR_ATR (ZnSe), Bruker Inc., MA, USA) of the QDs in solution (Fig. 2a) and a QD-embedded SDNA thin film (Fig. 3b,–c) were obtained in the range 3,600 to 600cm−1. Thirty-two scans were co-added and averaged at a resolution of 4cm−1. The data in the FTIR spectra used in the present study were analysed by subtracting the background spectrum produced by a bare substrate (Supplementary Tables S1, S4).
A spectrophotometer (Cary 5G, Varian, CA, USA) was used to conduct optical absorbance measurements of QDs in solution (Fig. 2b) and a QD-embedded SDNA thin film (Fig. 4) obtained in the visible and UV regions in the range 800 to 200nm. The spectrophotometer was equipped with two light sources: a deuterium arc lamp (for near-infrared and visible) and a quartz W-halogen lamp (for UV). The instrument has two detectors: a cooled PbS detector (for near-infrared) and a photomultiplier tube (for visible and UV). The spectrophotometer measures the frequency-dependent light intensity passing either through a vacuum or through the sample.
The PLQY of QD solution (ODs of ~0.07 and 0.5) and QDC(A)-embedded SDNA thin films were measured by absolute method using an integration sphere (C9920-20, Hamamatsu Photonics, Japan). The PL spectra were recorded in the wavelength range of 200–950nm at the fixed excitation wavelength of 350nm and corrected based on system response (Supplementary Table S3).
The PL excitation and emission spectra of the QDs in solution (Fig. 2b,c) and a QD-embedded SDNA thin film (Fig. 5) were measured at room temperature using a Xe-arc lamp equipped with a flurometer (FS-2, Scinco, Seoul, Korea) with a power of 25W. The excitation spectra were obtained at a fixed emission wavelength of 615nm, while the emission spectra were measured by exciting the samples at a fixed excitation wavelength of 370nm.
The electrical property of a QD-embedded SDNA thin film on glass was determined using a semiconductor parameter analyser (4200–SCS, Keithley Instruments Inc., OH, USA) in the dark at room temperature. Silver pastes (as metal electrodes with a channel gap distance of ~1 mm) were placed on a QD-embedded SDNA thin film prior to measurement (Fig. 6).
This work was supported by the National Research Foundation (NRF) of Korea funded by the Korea government, the Ministry of Science, ICT & Future Planning (MSIP) and the Ministry of Education, Science and Technology (MEST) (2014R1A2A1A11053213, 2016R1D1A1B03933768, and 2017R1D1A1B03035053).
V.A. and S.R.D. conceived and designed the experiments with contributions from H.K.C. and S.H.P.; V.A., S.R.D. and M.R.K. collected the data and all authors contributed to the data analysis. V.A. and S.R.D. wrote the first version of the paper and all authors revised the manuscript. H.K.C. and S.H.P. supervised the research.
The authors declare that they have no competing interests.
Velu Arasu and Sreekantha Reddy Dugasani contributed equally to this work.
Electronic supplementary material
Supplementary information accompanies this paper at doi:10.1038/s41598-017-11797-7
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Ho Kyoon Chung, Email: ude.ukks@gnuhckoh.
Sung Ha Park, Email: ude.ukks@krapahgnus.