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Enhanced Local and Nonlocal Photoluminescence of Organic Rubrene Microrods using Surface Plasmon of Gold Nanoparticles: Applications to Ultrasensitive and Remote Biosensing Hyung Suk Hwang,† Seong Gi Jo,† Jubok Lee,‡ Jeongyong Kim,*,‡ and Jinsoo Joo*,† †

Department of Physics, Korea University, Seoul 136-713, Republic of Korea Department of Energy Science, Sungkyunkwan University, Suwon 440-746, Republic of Korea

‡

S Supporting Information *

ABSTRACT: Nonlocal photoluminescence (PL) signal transfer through semiconducting nanostructures has been intensively studied for its potential applicability in photonic circuits, optical communications, and optical sensing. In this study, organic semiconducting rubrene microrods (MRs) were synthesized and hybridized with functionalized gold nanoparticles (Au-NPs) to optimize both their optical and biosensing properties. The steadystate local PL intensity of the rubrene MR was considerably enhanced by the Au-NPs’ hybridization due to the energy-transfer effect from the surface plasmon (SP) coupling. It was clearly observed that the nonlocal PL signal-transfer efficiency of rubrene/ Au-NPs hybrid MRs drastically increased along crystalline axes with the aid of the SP effect. The coupling of exciton polaritons in the luminescent rubrene MR with the SP as well as the scattering effect contribute to the variation of the exciton decay rate, resulting in a change in the PL signal-transfer efficiency for the hybrid MRs. The enhancement of the local and nonlocal PL emission of the rubrene/Au-NPs hybrid MRs was applied to ultrasensitive and remote biosensing. We observed PL signal transfer of fluorescent-dye attached DNA along the MR and successfully detected target-DNA with a concentration of 100 picomole using rubrene/Au-NPs/probe−DNA hybrid MR.

1. INTRODUCTION

Crystalline organic semiconductors and their nanostructures consisting of π-conjugated small molecules are excellent systems for optical and/or electrical signal transfer because of their strong π−π intermolecular interactions and delocalization of π-electrons. They can efficiently generate more excitons through optical excitation, they are characterized by the efficient propagation of polaritons, and they exhibit anisotropic energy transport properties. These properties indicate possible applications to photonic integration, optoelectronic communication, and nanoscale optical sensors.9−11 Rubrene (5,6,11,12-tetraphenyltetracene), an oligoacene derivative consisting of a tetracene backbone with four substituted phenyl groups, has been known for its excellent performance as a p-type organic semiconductor with a relatively high carrier mobility of 10−40 cm2/(V s).12 The π−π interactions and local ordering between neighboring rubrene molecules play an important role in charge transport and their optical properties for optoelectronic device applications. Rubrene microstructures and nanostructures show anisotropic luminescent characteristics,13 which can be useful for optical

The propagation of photons and surface plasmon (SP) polaritons is the major mechanism of active optical-signal transfer for low-dimensional semiconductors and metal nanostructures, respectively. Exciton polaritons in organic semiconducting optical materials are formed due to strong coupling between Frenkel excitons and photon-phonon transverse waves (i.e., polaritons),1 and can propagate well through the major transfer axis.2 This results in the nonlocal photoluminescence (PL) signal transfer of the active optical nanostructure. The transfer mechanism of the PL signal of the inorganic and organic semiconducting nanostructures differs from the total reflection of conventional optical fibers using insulating glass, polystyrene, and polymethacrylate.3−5 The propagation of optical signals of the coupled excitons with polaritons can be suffered from inelastic scatterings with phonons, impurities, and boundaries. Therefore, the decay characteristics of PL signal transfer have been commonly observed in semiconducting optical materials or their nanostructures.6 The surface plasmon (SP) coupling with metal nanostructures can be adopted to improve the nonlocal PL signal-transfer rate as well as the local PL intensity of semiconducting nanostructures.7,8 © XXXX American Chemical Society

Received: March 10, 2016 Revised: May 7, 2016

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DOI: 10.1021/acs.jpcc.6b02503 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. Schematic chemical structures of (a) rubrene molecule and (b) (11-mercaptoundecyl)tetra(ethylene glycol) functionalized Au-NP. (c) TEM image of the hybrid MR of rubrene and Au-NPs. The small white spots represent the Au-NPs. (d) X-ray diffraction pattern of rubrene MR. (e) Normalized UV−vis absorption spectrum of functionalized Au-NPs (black curve) and PL spectrum of the rubrene MR (red curve).

signal-transfer efficiencies along the crystalline axes of the rubrene MR have been considerably enhanced for the hybrid rubrene MRs with Au-NPs. The PL signal-transfer rates as a function of the propagation distance for the rubrene MR and the hybrids of rubrene/Au-NPs have been compared to investigate the relationship of the nonlocal PL emissive characteristics with the SP effect. To demonstrate the applicability of such a hybrid system, ultrasensitive and remote biosensing was designed and successfully performed to observe the nonlocal PL signal transfer of the fluorescent Cy5 dye attached probe-DNA (p-DNA) (i.e., remote biosensing) and the ultrasensitive detection of target-DNA (t-DNA) with a concentration of 100 picomole (pM).

signal transfer. In addition, the electronic structures, charge transport, and optical properties of rubrene at the nanoscale have been studied with a view toward using these materials in organic-based optoelectronics and photonics nanodevices.14−17 Dhakal and co-workers reported on the increase of the PL intensity of the rubrene nanofibers embedded with gold nanoparticles (Au-NPs) prepared with the electrospinning method.18 They did not, however, report on the nonlocal PL signal transfer and applications. In our study, the crystallinity of organic rubrene materials has been improved through crystal growth using the physical vapor transfer (PVT) method. This resulted in the enhancement of the optical signal-transfer efficiency for photonic applications. Along these lines, we designed and fabricated the hybrids of crystalline organic rubrene microrods (MRs) attached with Au-NPs. It was necessary to functionalize the surface of Au-NPs to achieve easy hybridization and the homogeneous distribution of NPs on the surface of the MR. The decoration of the functionalized Au-NPs on the surface of rubrene MRs contributes to inducing the efficient energy transfer through SP coupling effect, resulting in the increase of the local and nonlocal PL emissive characteristics.19,20 In this paper, we report upon the unique signal-transfer characteristics of the enhanced PL spectra along the hybrids of organic rubrene MR decorated with Au-NPs. The local PL intensity of the rubrene MR increased with hybridizing Au-NPs because of the energy transfer in SP coupling. The nonlocal PL

2. EXPERIMENTAL SECTION 2.1. Synthesis of Rubrene MR and Hybrids with AuNPs. In order to grow rubrene MR, the rubrene powder purchased from Sigma-Aldrich Co. was placed on the heating zone in the middle of a homemade furnace used for vaporization. The source temperature was 310−320 °C and the flow rate of N2 gas was varied from 50 to 100 cm3/min. To prevent oxidation, pure N2 gas was continuously flowed during the growth and cooling process. The width and thickness of the rubrene MRs were about 3.0 μm and 0.50−2.0 μm, respectively, as estimated by analysis of optical microscope (BK51M-N53MF2&DP73-SET, OLYMPUS) and an atomic B


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The Journal of Physical Chemistry C force microscope (AFM, Albatross, NanoFocus Inc., Korea) images. The length of the MRs was about 70−100 μm. The Au-NPs with (11-mercaptoundecyl)tetra(ethylene glycol)-functionalized groups were purchased from Sigma-Aldrich Co.21 The hydrophilic (11-mercaptoundecyl)tetra(ethylene glycol) Au-NPs (2 wt %) were dispersed in deionized water with the ratio of 1:50. The Au-NPs solution was precisely and locally dropped onto the surface of the rubrene MR using a nanopipette (Eppendorf Reference Pipette Ultra Micro 0.1−2.5 μL, EPPENDORF). The hybrid MRs of rubrene/Au-NPs were dried in a vacuum chamber for 8 h at 80 °C. The p-DNAs (27 mer) attached with Cy5 dye were purchased from Genotech Co. The t-DNAs (27 mer) were purchased from Bioneer Co. 2.2. Measurements and Characterization. For the surface morphology and structural properties of the pristine rubrene MR and its hybrid MRs, field-emission transmission electron microscope (FE-TEM; JEM-3010, JEOL) images and X-ray diffraction patterns (XRD; Rigaku Model D/MAX-2500 V/PC) were obtained. For assessing the nanoscale luminescence properties, PL images and spectra of the single rubrene MR and its hybrid MRs were obtained using a homemade laser confocal microscope (LCM) built around an inverted optical microscope (Axiovert 200, Zeiss GmbH). The 514 nm line of an unpolarized argon (Ar) ion laser was used for LCM PL excitation. The laser power and acquisition time incident on the sample for the LCM PL spectra were at 1.2 μW and 47.5 ms, respectively, for the same experimental conditions. The spot size of the focused laser beam on the sample was estimated to be about 500 nm for solid-state PL measurements on a nanometer scale. The detailed methods for the LCM PL spectra have been reported previously.22 For the LCM PL mapping, the synchronized input laser and output detector were scanned along the direction of the horizontal and ordinate axes of the samples (128 × 64 pixels). The laser power and acquisition time on the samples for the LCM PL mapping experiments were fixed at 1.2 μW and 47.5 ms, respectively. Then, 8192pixel data (one LCM PL spectrum was included in each pixel) were obtained for the mapping images. The LCM PL mapping images of the samples were analyzed using WITec project software. The time-resolved PL decay curves for the exciton lifetimes of the pristine and hybrid MRs were measured using a fluorescence lifetime microscope system (MicroTime 200, PicoQuant GmbH, GN 003; λex = 470 nm; time resolution = 0.13 ns; confocal images resolution ≈ 200 nm). The power of the laser used and photocounts were 1 μW and 104, respectively.

Figure 2. LCM PL spectra of the single pristine rubrene MR (black and dotted curve) and hybrid MR of rubrene/Au-NPs (red and bold curve). Insets: Magnified LCM PL mapping images of portions of the single pristine rubrene MR (bottom) and the hybrid MR of rubrene/ Au-NPs (top). The color scale bar represents the intensity of PL.

3. RESULTS AND DISCUSSION 3.1. Structural and Steady-State Optical Characteristics. Figure 1a shows the schematic chemical structure of a rubrene molecule.13 The rubrene MRs for this study were epitaxially grown by the PVT method.23 Figure 1b shows the schematic chemical structure of the functionalized Au-NP. Because of the functional group, the Au-NPs were homogeneously dispersed and easily hybridized with the rubrene MRs. Figure 1c shows the transmission electron microscope (TEM) image of the hybrid MR of rubrene and Au-NPs. The small white spots in Figure 1c represent the Au-NPs. We observed the homogeneous distribution of Au-NPs on the surface of the rubrene MR. The number density of Au-NPs on the surface of the MR was estimated to about 150/100 × 100 nm2. Figure 1d shows the X-ray diffraction (XRD) pattern of the rubrene MR. We observed the sharp XRD crystalline peaks corresponding to

Figure 3. Time-resolved PL mapping images with color scale bar (right-hand sides) of (a) the single pristine rubrene MR and (b) the single hybrid MR of rubrene/Au-NPs. (c) Time-resolved PL decay curves of the pristine rubrene MR (black curve) and the hybrid MR of rubrene/Au-NPs (red curve).

the (002), (010), (006), and (020) planes.24 The results indicate the crystalline property of the rubrene MR fabricated by the PVT method. It is noted that the rubrene nanofiber in C


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Figure 4. (a) Schematic illustration of the PL signal transfer experiment for the hybrid MR of rubrene/Au-NPs using an LCM system. (b) Output LCM PL spectra with different excitation locations (i.e., differenct propagation distances) along the single pristine rubrene MR. (c) Output LCM PL spectra with different excitation locations along the hybrid MR of rubrene/Au-NPs. The numerical values in the insets represent the propagation distance.

the previous study18 showed the relatively low-crystalline nature compared to our rubrene MR, because the rubrene nanofibers were made through the electrospinning method. The rubrene nanofiber18 is a hexagonal crystalline structure, while our rubrene MR is an orthorhombic crystalline structure.24 The ultraviolet and visible (UV−vis) absorption spectrum (black curve) of the (11-mercaptoundecyl)tetra(ethylene glycol) functionalized Au-NPs is shown in Figure 1e. The UV−vis absorption peak of Au-NPs was observed at λ = 513 nm due to the SP absorption. The laser confocal microscope (LCM) PL spectrum (red curve) of the single rubrene MR was also shown in Figure 1e. Because of the overlap of the SP absorption of the Au-NPs and the PL spectrum of the rubrene MR, efficient energy transfer is expected through the SP coupling. The optical absorption peaks of the rubrene MRs were observed at 432 nm (2.87 eV), 462 nm (2.68 eV), 491 nm (2.52 eV), and 525 nm (2.36 eV)

corresponding to 1La-1A transitions (see Figure S1 in Supporting Information (SI)); these observations agreed with previous results for the rubrene crystal.25 Figure 2 shows the LCM PL spectra of the single pristine rubrene MR (black and dotted curve) and the hybrid MR of rubrene/Au-NPs (red and bold curve). The main LCM PL peak of the MR was observed at 568 nm, corresponding to the crystalline c-axis that is M-axis polarized band of a short tetracene backbone in the rubrene moleclues.26 The shoulder peaks at 600 and 642 nm, corresponding to b- and a-axes, respectively, were also observed. The intensity of the LCM PL peak at 568 nm of the pristine rubrene MR was about 20 000 photon counts. With the same experimental conditions, the LCM PL intensity of the rubrene MR was drastically enhanced to about 39 000 photon counts (i.e., an increase of 1.95 times) after the hybridization of Au-NPs. The LCM PL mapping images of the pristine rubrene MR portion and the Au-NPs D


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Figure 5. (a) Output PL intensity along the crystalline c-axis (λem = 568 nm) as a function of propagation distance of the single pristine rubrene MR (open markers and black line) and the hybrid MR of rubrene/Au-NPs (solid markers and red line). (b) LCM PL mapping images of the pristine rubrene MR (left) and the hybrid MR of rubrene/Au-NPs (right) during the PL signal-transfer experiment. The arrows indicate the scanning direction of the input-focused laser. Output PL intensity along the crystalline (c) b-axis (λem = 600 nm) and (d) a-axis (λem = 642 nm), as a function of propagation distance for the pristine single rubrene MR (open markers and black line) and the hybrid MR of rubrene/Au-NPs (solid markers and red line).

hybridized portion in the same rubrene MR are compared and shown in the insets of Figure 2. The color scale bar in the inset of Figure 2 represents the intensity of LCM PL spectra. As the color scale bar changes from violet to red, the LCM PL intensity increases. We clearly observed that the LCM PL intensity of the rubrene/Au-NPs hybrid part (top inset of Figure 2) was stronger than that of pristine rubrene MR without Au-NPs (bottom inset of Figure 2), confirming the LCM PL spectra. Our analysis indicates that this result is due to the efficient energy-transfer effect through the SP coupling between the Au-NPs and rubrene MR. The LCM PL spectra of the single pristine rubrene MR and hybrid MR of rubrene/Au-NPs were decomposed, corresponding to the rubrene crystalline axes (see Figure S2 in SI). From the analysis of the area of the decomposed spectra, we estimated that the increase ratio of the LCM PL intensity was to be about 1.52, 2.00, and 2.11 times along the crystalline c-, b-, and a-axes, respectively. The crystalline c-axis of the rubrene MR corresponds to the direction of thickness. This suggests that the energy transfer through the SP coupling due to the hybridization of Au-NPs efficiently contributed to the crystalline b- and a-axes, which exhibit stronger π−π interaction due to their relatively shorter intermolecular distance. From the

height analysis of the intensity of the decomposed PL spectra, similar results were obtained quantitatively. To investigate the variation of the exciton lifetime (τ) for the rubrene MR after Au-NPs hybridization, we performed timeresolved PL experiments. The bandpass filter at 570 nm was used for the experiments. Figure 3a,b displays the time-resolved PL mapping images with the color scale bar. The time-resolved PL mapping image of the pristine rubrene MR was much more reddish, indicating a longer exciton lifetime. Figure 3c shows the normalized PL decay curves of the pristine rubrene MR (black curve) and the hybrid MR of rubrene/Au-NPs (red curve). Different PL decay characteristic curves correlate with different exciton decay natures in the rubrene MR after the AuNPs hybridization. The amplitude-weighted average lifetime (τavg) of the MRs was estimated by using an equation, τavg = ΣAiτi/ΣAi, where Ai and τi represent the amplitude and lifetime of the ith exciton component, respectively.27 The best fitting time-resolved PL decay curves were obtained from multiexponential fitting: y(t) = ΣAi exp(−t/τi). From the results shown in Figure 3c, τavg of the pristine rubrene MR was estimated to be about 0.60 ns. The τavg of the hybrid MR of rubrene/Au-NPs was estimated to be about 0.24 ns. The decrease in the τavg of the rubrene MR after the decoration with the Au-NPs could arise from the scattering effect. The excitons E


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Figure 7. LCM PL spectra of the rubrene/Au-NPs/p-DNA hybrid MR (black curve) and rubrene/Au-NPs/p-DNA+t-DNA (100 pM) hybrid MR (red curve). Insets: Magnified LCM PL mapping images of the part of rubrene/Au-NPs/p-DNA hybrid MR (bottom inset) and of rubrene/Au-NPs/p-DNA+t-DNA (100 pM) hybrid MR (top inset).

middle of the MR, that is, the nonlocal PL emission. The output spectra of the PL signal transfer along the single pristine rubrene MR and hybrid MR of rubrene/Au-NPs are shown in Figure 4b,c, respectively. For both MRs, the decay characteristics of the output intensity of PL spectrum were observed. The main LCM PL peak at 568 nm was initially dominant, and the peak was filtered out through the reabsorption process during signal propagation. The output of the PL signal transfer of both the pristine and hybrid MRs mainly occurred at about 600 nm, corresponding to the b-axis, as shown in Figure 4b,c, indicating the major optical signal transfer occurred along the baxis with strong π−π intermolecular interaction. The signal-transfer characteristics of PL spectra were analyzed by using the decomposed PL peaks at 568, 600, and 642 nm, corresponding to the crystalline c-, b-, and a-axes, as shown in Figure 5. Along each crystalline axis, the output PL intensity of the pristine rubrene MR, I(x), decreased as the propagation distance x increased. The intensity equation, I(x) = I0 exp(−αx), was used to fit the data. For the crystalline c-axis, the PL decay constant α of the pristine rubrene MR was estimated to be 0.091 μm−1, while that of the rubrene/Au-NPs hybrid was estimated to be 0.052 μm−1 (Figure 5a). The results indicate that the hybrid MR exhibits better PL signal-transfer characteristics because of the assistance of the SP coupling effect through the Au-NPs hybridization. When the Au-NPs were attached on the surface of the rubrene MR, the propagation loss could occur due to the scattering of guided light by the particles. However, light scattering effect depends on the radius (r) of the NP (∝r6).29 The r of Au-NPs was less than 5 nm and light scattering effect was very weak in our case. The SP coupling effect through the Au-NPs for the PL signaltransfer was relatively stronger than the scattering effect. More efficient PL signal transfer with the assistance of the SP coupling effect was confirmed by the LCM PL mapping images (see Figure 5b). Higher LCM PL intensity along the hybrid MR was observed. For the crystalline b- and a-axes, the similar behaviors of PL signal transfer were observed, as shown in Figure 5c,d, respectively. For the crystalline b-axis, the α-value of the pristine rubrene MR was estimated to be 0.060 μm−1, while that of the rubrene/Au-NPs hybrid was estimated to be

Figure 6. LCM PL spectra with the decomposed PL characteristic peaks corresponding to the crystalline axes and Cy5 dye for (a) the rubrene/p-DNA hybrid MR and (b) the rubrene/p-DNA+t-DNA hybrid MR. Insets: magnified LCM PL mapping images of corresponding samples.

on the surface of the rubrene MR could be scattered by the collective oscillation of surface electron (i.e., surface plasmons) of the Au-NPs. In Figure 3c, the fast decay component can be attributed to the nonradiative recombination lifetime, and the slower decay component is from a radiative recombination lifetime.28 3.2. Nonlocal PL Signal-Transfer Characteristics. Figure 4a shows a schematic illustration of the experimental setup of nonlocal PL signal transfer for the hybrid MR of rubrene/AuNPs using the LCM system. The excitation point from the focused input laser was movable along the axial direction of MR, while the position of detection (output) was fixed at the end of the MR, as shown in Figure 4a. Both the excitation laser and the detector using the LCM system were placed normally to the MR. We clearly observed the PL signal transfer at the end of the MR when the excitation laser was focused on the F


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Figure 8. Output LCM PL spectra with different excitation locations (i.e., different propagation distance) from (a) the single rubrene/p-DNA hybrid MR and (b) the rubrene/p-DNA+t-DNA hybrid MR. The numerical values in the insets represent the propagation distance. Output PL intensity as a function of propagation distance along the crystalline (c) c-axis (λem = 566 nm), (d) b-axis (λem = 592 nm), (e) a-axis (λem = 635 nm), and (f) Cy5 dye (λem = 680 nm) for the rubrene/p-DNA (open markers and black line) and rubrene/p-DNA+t-DNA (solid red markers and red line) hybrid MRs.

0.041 μm−1 (Figure 5c). The diffusion characteristic lengths (l) were estimated to be about 17 and 24 μm for the pristine and hybrid MRs, respectively. The results agree with the effective PL propagation distance (about 25 μm) obtained from finitedifference time-domain (FDTD) simulation for the pristine MR (see Figure S3 in SI). For the crystalline a-axis, the α-value of the pristine rubrene MR was estimated to be 0.045 μm−1, while that of the rubrene/Au-NPs hybrid was estimated to be 0.033 μm−1 (Figure 5d). The PL signal transfer along the crystalline

a- and b-axes was more efficient when compared with that along the c-axis, which originated from the different intensities of π−π intermolecular interaction. Based on the results, the enhancement of the PL signal transfer along the rubrene MR was achieved by the decoration of Au-NPs in terms of the energy-transfer effect of the SP coupling. Similar results from PL signal transfer were also observed for different batches of the hybrid rubrene/Au-NPs MRs (see Figures S4 and S5 in SI). G


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The Journal of Physical Chemistry C 3.3. Applications to Ultrasensitive and Remote Biosensing. To demonstrate the applicability of the rubrene MR and its hybrid system to biosensing, we designed the ultrasensitive and remote biosensing experiments in which the oligonucleotides labeled with Cy5 (fluorescence dye) were attached to the MR. The Cy5 fluorescent-dye-labeled oligonucleotides were prepared in distilled water. The sequence used here was 27-mer anthrax lethal factor DNA sequence: 5′Cy5-ATC CTT ATC AAT ATT TAA CAA TAA TCC-Amino (C6). The Cy5-labeled oligonucleotide molecules have main PL peaks at about 680 nm. The LCM PL spectra of the rubrene/p-DNA (49.1 nM) and the rubrene/p-DNA+t-DNA (50 nM) hybrid MRs are displayed in Figure 6 with the decomposed PL peaks corresponding to the crystalline axes and Cy5 dye. The overall LCM PL intensity of the rubrene/p-DNA+t-DNA hybrid MR (Figure 6b) is clearly higher (about 1.4 times) than that of the rubrene/p-DNA hybrid MR (Figure 6a). This is confirmed by the results of LCM PL mapping in the insets of Figure 6. From the magnified LCM PL images for the rubrene/p-DNA and the rubrene/p-DNA+t-DNA (the insets of Figure 6a,b, respectively), the luminescent intensity of the rubrene was considerably enhanced after the coupling with the t-DNA. This can be analyzed in terms of the fluorescence chain reaction (FCR).30,31 The PL peak of the Cy5 fluorescent dye in the rubrene/p-DNA hybrid MR was observed at about 680 nm. From the decomposed PL peaks corresponding to the a-axis, baxis, c-axis, and the Cy5 dye, the relative PL intensities increased about 1.63, 1.35, 1.87, and 1.66 times, respectively, after the coupling with the t-DNA with a concentration of about 50 nM. Therefore, we successfully detected t-DNA by using the hybrid MR of rubrene/p-DNA. For ultrasensitive biosensing, we utilized the lower concentration of t-DNA (100 pM) and the concept of PL enhancement of rubrene MR through the energy transfer of the SP coupling with Au-NPs. The p-DNA attached with Cy5 dye were mixed with Au-NPs, and then the mixed solution was coated on the surface of the rubrene MR. After drying rubrene/ Au-NPs/p-DNA (49.1 nM) hybrid MR, the t-DNA with a concentration of 100 pM were partially drop on the hybrid MR. Figure 7 shows the LCM PL spectra of the rubrene/Au-NPs/pDNA (49.1 nM) (black curve) and the rubrene/Au-NPs/pDNA+t-DNA (100 pM) hybrid MRs (red curve). The LCM PL intensity of the rubrene/Au-NPs/p-DNA hybrid MR was enhanced (about 1.3 times) by the attachment of the 100 pM tDNA, caused by the FCR effect. The insets of Figure 7 show the LCM PL mapping images of corresponding samples. The LCM PL intensity of the rubrene/Au-NPs/p-DNA MR increased with the attachment of the 100 pM t-DNA. Therefore, we successfully performed the ultrasensitive tDNA detection down to 100 pM using the rubrene/Au-NPs/ p-DNA hybrid MR. For remote biosensing, the nonlocal PL signal-transfer characteristics along the hybrid MRs of rubrene/p-DNA (49.1 nM) and the rubrene/p-DNA (49.1 nM)+t-DNA (50 nM) were investigated by using the LCM system, as shown in Figure 8a,b. The experimental conditions for the PL signaltransfer were the same as those for the rubrene/Au-NPs hybrids in Figure 4. We detected the PL signal-transfer at the ends of the rubrene/p-DNA+t-DNA hybrid MRs when the excitation laser was focused on the middle positions of the MR, that is, the nonlocal PL emission. The decay characteristics of the output intensity of PL peaks were also observed (Figure 8).

The PL signal-transfer characteristics for the rubrene MRs hybridized with p-DNA or p-DNA+t-DNA are similar to those of the hybrid MR of the rubrene/Au-NPs. However, the distinguishable signal-transfer characteristics of the Cy5 dye PL peak at 680 nm along the rubrene MR were observed as shown in Figure 8. Using the decomposed PL peaks corresponding to the crystalline c-, b-, a-axes, and the Cy5 dye (680 nm) peaks, the signal-transfer characteristics of PL spectra were analyzed in terms of the decay characteristic equation, I(x) = I0 exp(−αx), as shown in Figure 8c−f. The values of the decay constant (α) of the rubrene/p-DNA+t-DNA hybrid MRs corresponding to the crystalline c-, b-, a-axes, and the Cy5 dye (the solid markers and their best fitting lines in Figure 8) are 0.10, 0.069, 0.0078, and 0.057 μm−1, respectively, which are higher than those (0.078, 0.046, 0.0047, and 0.027 μm−1, respectively) for the rubrene/p-DNA hybrid MRs (indicated by the open markers and their best fitting lines in Figure 8). The higher decay constants of the rubrene/p-DNA+t-DNA hybrid MRs are due to the scattering effect from the additional coupling of t-DNA on the rubrene surface. We successfully observed the nonlocal PL signal transfer of the Cy5 dye attached to p-DNA or p-DNA +t-DNA through the rubrene MR. The results suggest the possibility of remote biosensing through the nonlocal PL signal transfer along the rubrene MR.

4. CONCLUSION Organic semiconducting rubrene MRs were hybridized with functionalized Au-NPs to optimize both their optical and biosensing properties. The local PL intensity of the rubrene MR was considerably enhanced by the Au-NPs’ hybridization due to the energy-transfer effect from the SP coupling. The nonlocal PL signal-transfer efficiency of rubrene/Au-NPs hybrid MRs increased along crystalline axes with the aid of the SP effect. The enhancement of the local and nonlocal PL emission of the rubrene/Au-NPs hybrid MRs has been applied to ultrasensitive and remote biosensing. The PL signal transfer of fluorescent-dye attached DNA along the MR was observed, suggesting the possibility of remote biosensing. We successfully detected target-DNA with a concentration of 100 pM using rubrene/Au-NPs/probe-DNA hybrid MR.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b02503. Normalized UV−vis absorption spectrum of pristine rubrene MRs. LCM PL spectra with the decomposed PL characteristic peaks. FDTD simulation of the pristine rubrene MR for the effective PL propagation distance. LCM PL mapping image. Output PL intensity along the crystalline axis. Energy dispersive X-ray (EDX) spectra of the rubrene/Au-NPs. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: (J.K.) [email protected]. Fax: +82-31-299-4279. Tel: +82-31-299-4054. *E-mail: (J.J.) [email protected]. Fax: +82-2-927-3292. Tel: +82-2-3290-3103. H


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The Journal of Physical Chemistry C Author Contributions

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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Center for Advanced MetaMaterials (CAMM), funded by the Ministry of Science, ICT and Future Planning as Global Frontier Project (CAMM2014M3A6B3063710). The authors thanks to Professor Q-Han Park at Korea University for useful discussion.

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