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Photopatterned PLED arrays for biosensing applications
Microelectronic Engineering 86 (2009) 1511–1514
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Photopatterned PLED arrays for biosensing applications Maria Vasilopoulou a,*, Dimitra G. Georgiadou a, Leonidas C. Palilis a, Athanasios Botsialas a, Panagiota S. Petrou b, Sotirios E. Kakabakos b, Panagiotis Argitis a a b
Institute of Microelectronics, NCSR Demokritos, Aghia Paraskevi 15310, Greece Institute of Radioisotopes and Radiodiagnostic Products, NCSR Demokritos, Aghia Paraskevi 15310, Greece
a r t i c l e
i n f o
Article history: Received 1 October 2008 Received in revised form 22 January 2009 Accepted 23 January 2009 Available online 1 February 2009 Keywords: Optical sensor Photopatterned multi-colour PLEDs Dye-labeled biomolecules Fluorescent probe
a b s t r a c t A solid-state optical biosensor integrated with a single layer flexible blue Polymer Light Emitting Diode (PLED) as the light source was demonstrated and used for the detection of biomolecules labeled with two different fluorescent dyes. An anti-rabbit IgG antibody labeled with the fluorescent dyes, that was adsorbed on the opposite side of a polyethylene tetraphthalate (PET) flexible substrate, was excited by light emitted from the PLED. Light is then absorbed by the dye-labeled antibody resulting in a red shift of the polymer emission spectrum. Emission originating from the dye is also observed. The emission spectral shift and its efficiency depend primarily on the concentration of the biomolecule solution, the overlap between the emission spectra of the polymer and the absorption spectra of the dye and the PLED emission characteristics. Biomolecules immobilized onto distinct areas of a plastic substrate might then selectively be detected with high sensitivity after reacting with fluorescently labeled counterpart molecules. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction The next trend in sensor technology is the development of all plastic integrated chemical and biological microarrays for environmental and medical applications. In an optical sensor one requires a light source, appropriate sensing chemistry, a means to immobilize the desired sensing molecules and a light detector. Current light sources such as lasers or inorganic light emitting diodes either can not be readily integrated with the other components due to size and operational constraints or because they involve intricate integration procedures [1]. Integration of the sensing element with an organic light emitting diode (OLED) provides a straightforward and simple approach for flexible device architectures [2,3]. OLEDs have been dramatically improved over the past few years [4,5] and they have become advantageous as low-voltage, miniaturized flexible light sources that have sufficient output power in order to be useful for application in fluorescence-based biosensors [6,7]. Flexible devices can generally be fabricated on plastic substrates, like polyethylene tetraphthalate (PET), which combine flexibility, formability and transparency [8]. Increased interest has recently been shown in integrated micro systems, where different colour OLEDs can be used to excite labeled biomolecules aiming to simultaneous detection of different analytes. In this work, we propose an optical biosensor integrated with a single layer flexible PLED for the detection of biomolecules labeled * Corresponding author. E-mail address: [email protected] (M. Vasilopoulou). 0167-9317/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2009.01.063
with different fluorescent dyes. This system concept is versatile and it can easily be miniaturized and integrated with appropriate microfluidics and photodetectors towards microsystem development, offering wavelength tunability, simplicity and low cost [9]. 2. Experimental Poly[2-(6-cyano-6-methyl-heptyloxy)-1,4-phenylene] (BE-120) was purchased from American Dye Source, whereas poly(9-vinyl carbazole) (PVK) and Poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) were purchased from Aldrich and used with no further purification. The AlexaFluor488Ò and R-phycoerythrin labeled anti-rabbit IgG antibodies were purchased from Molecular Probes, Inc. Polymer films were prepared by spin-coating from chloroform solutions on PET substrates. Dye-labeled biomolecules were drop-cast and subsequently adsorbed on the other side of the PET substrate. For absorption spectra a Perkin–Elmer Lambda-16 spectrometer was employed. Fluorescence and excitation spectra were recorded with a Perkin–Elmer LS-50B fluorescence spectrometer. Flexible PLEDs were fabricated on Indium Tin Oxide (ITO) coated PET substrates [10]. Single layer blue emitting devices were based on (BE-120) as the emitting layer, while the multi-colour photopatterned single layer devices were based on the wide bandgap (PVK) doped with the fluorescent dyes 1-(40 -dimethylaminophenyl)-6-phenyl-1,3,5-hexatriene (DMA-DPH), and 4dimethylamino-40 -nitrostilbene (DANS). ITO coated PET substrates were precleaned and treated with oxygen plasma to increase the
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ITO work function and increase its surface energy. Prior to the deposition of BE-120, a 40 nm thick film of PEDOT-PSS was spin coated on ITO in order to improve hole injection and substrate smoothness. Device fabrication was completed with a 250 nm thick aluminum cathode electrode which was deposited by thermal evaporation under high vacuum. All the testing devices have an active area of 12.56 mm2. Next, a thick layer of biomolecules labeled with a suitable fluorescent dye is drop-cast on the other side of the PET substrate to complete the biosensor fabrication. Current density–voltage (J–V) characteristics were obtained using a programmable Keithley 230 Voltage Source and a 195A multimeter. Luminance measurements (in forward direction) and electroluminescence (EL) spectra were recorded simultaneously using a USB 2000-UV–Vis miniature fiber optic spectrometer. A schematic picture of the experimental setup used for the electroluminescence spectral measurements and biomolecule detection is shown in Fig. 3. All measurements were carried out at room temperature under ambient atmosphere without any encapsulation. 3. Results and discussion Fig. 1 shows the absorption and PL spectra of the two different fluorescent dyes that were evaluated (a) the AlexaFluorÒ 488 dye (ALEXA) (absorption maximum: 495 nm; emission maximum: 519 nm) and (b) the R-phycoerythrin dye (PHYCO) (absorption maxima: 546 and 565 nm; emission maximum: 578 nm). Fig. 2 shows the PL spectra obtained before and after the immobilization of the antibodies labeled with either the ALEXA (left) or the PHYCO (right) fluorescent dye on one side of a PET substrate on the other side of which a PVK layer is spin coated. PL spectra demonstrate a red shift from blue to greenish blue after the immobilization of both antibodies. In that case, blue light emitted from PVK was the excitation source for both fluorescent dyes. Thus, since the PL spectrum of the polymer overlaps to some extent with the absorption spectrum of the fluorescent dye, a red shift in the PL after the
100
b
abs em
Relative intensity
Relative intensity
a
immobilization of the labeled biomolecule is expected, which can be attributed to both emission from the fluorescent label of the biomolecule and PVK. A slightly larger red shift was achieved when the ALEXA fluorescent dye was used, due to better overlap of its absorption spectrum with PVK’s PL spectrum. In addition, it should be mentioned that the concentration of the biomolecule solution appears to play an important role on the emission characteristics and efficiency of the fluorescent label. High concentration results in significant self-quenching of its fluorescence emission due to the small Stokes shift. However, as PVK has very low fluorescence efficiency and a large photon population is needed to excite the dye-labeled biomolecule, next we evaluated BE-120 (which is a highly luminescent blue polymer) as the emitting component in blue flexible PLEDs fabricated on a PET substrate in order to improve the biosensor characteristics. A representative structure of these devices along with the corresponding energy diagram of BE-120 and both polymers’ chemical structures are presented in Fig. 3. These blue emitting, single layer flexible PLEDs exhibit good performance characteristics when operating in air as shown in Fig. 4 where their current density–voltage–luminance characteristics are presented. Thus, they could be used as light sources integrated with the sensing element for the development of biosensors based on fluorescence detection. No demonstrate the potential of the developed PLEDs for biosensing applications, the ALEXA dye-labeled antibody was immobilized by adsorption on the opposite side of the PET substrate. Both PL and the PLED’s EL spectrum were recorded for comparison before and after the labeled biomolecule immobilization. As shown in Fig. 5, a small spectral shift from blue to greenish blue is observed in both PL and EL after the immobilization of the labeled biomolecule on the other size of a PET substrate on top of which a BE-120 layer has been spin coated. The main difference however is that in PL the characteristic emission from the ALEXA dye at 530 nm is clearly observed, though only for a low concentration biomolecule solution. Again, the high concentrated
80 60 40 20
abs em
80 60 40 20 0
0 300
100
400
500
600
300
700
400
500
600
700
Wavelength (nm)
Wavelength (nm)
Fig. 1. Normalized absorption and emission spectra of AlexaFluorÒ 488 dye (a) and R-phycoerythrin (b).
1.0
PVK PVK/ALEXA
0.8 0.6 0.4 0.2 0.0 350
400
450
500
550
Wavelength (nm)
600
Normalised PL
Normalised PL
1.0
PVK PVK+phyco_1 PVK+phyco_2
0.8 0.6 0.4 0.2 0.0 350
400
450
500
550
600
Wavelength (nm)
Fig. 2. Photoluminescence spectra before and after the immobilization of an AlexaFluor488Ò (left) or R-phycoerythrin (right) anti-rabbit IgG antibodies in front of a PVK layer.
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M. Vasilopoulou et al. / Microelectronic Engineering 86 (2009) 1511–1514
Vacuum
Cathode (Al)
LUMO (2,7 eV)
Emitting layer-BE 120
4,3 eV
PEDOT-PSS
Transparent anode (ITO) 5eV
PET substrate
5,2eV HOMO (5,8 eV)
Biomolecule Light output
ITO PEDOT
BE-120
Al
-PSS
Optical fiber
Fig. 3. Representative structure and energy diagram of a flexible PLED (based on the wide bandgap emitting polymer BE-120) integrated with the biomolecule sensing element and the corresponding chemical structures of PVK (bottom left) and BE-120 (bottom right).
Current Density (A/m2)
1000
Luminance
800
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0
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4
6
8
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Luminance (cd/m2)
Current Density
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0
biomolecules in the same sample by implementing a spectrally different fluorescent label for the detection of each biomolecule. In addition, the detection of multiple biomolecules might be feasible through employment of appropriately micropatterned PLEDs on the same substrate in combination with different fluorescent labels. In our previous work [9] we demonstrated the definition of all three primary colours Red–Green–Blue (R–G–B) in a single polymer layer PLED by photochemical transformation of the emission properties of certain fluorescent dyes. Blends of PVK with both the green emitter DMA-DPH and the red emitter DANS along with the triphenyl sulfonium salt as a photoacid generator were prepared and PLEDs were fabricated. Initially, red emission was observed from the lower bandgap compound (DANS). By UV exposure we were able to bleach red emission from DANS and to define green emitting areas, since DMA-DPH emits in the green spectral region. With subsequent irradiation and protonation of the green emitter the blue emission of the DMA-DPH’s photochemical product was achieved. Therefore, distinct areas on a PVK layer emitting red, green and blue light (or any other intermediate colour) were defined, by using a photochemical transformation based on photoacid induced emission changes. In Fig. 6, a schematic presentation is shown of how these photopatterned full-colour devices could be used for the detection of multiple biomolecules through employment of the appropriate micropatterned PLEDs on the same substrate in combination with different fluorescently
1200
1200
0
Voltage (V) Fig. 4. Current density–voltage–luminance characteristics of a single layer blue PLED based on BE-120.
dye-labeled biomolecule solution seems to self-quench its fluorescence. Note, that choosing a lower bandgap emitting polymer for the PLED that its PL has a larger overlap with the absorption spectrum of the dye may result in a more significant spectral shift with emission primarily originating from the dye after its excitation. The ability to create photopatterned multi-colour single layer flexible PLEDs on the same substrate is in accordance with the growing trend toward measuring ‘‘everything” in a sample [11]. This would improve the simultaneous detection of different
Normalized PL
0.8 0.6 0.4
0.15 0.1 0.05
0.2 0.0 350
0.2
EL (au)
BE 120 BE 120+ALEXAhigh conc. BE 120+ALEXAintermediate conc. BE 120+ALEXAlow conc.
1.0
400
450
500
550
600
Wavelength (nm)
650
0 350
450
550
650
750
Wavelength (nm)
Fig. 5. PL (left) and EL (right) spectrum before and after the immobilization of the AlexaFluor488Ò anti-rabbit IgG antibody in front of the blue PLED.
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depends strongly on the concentration of the fluorescent dye-labeled biomolecule solution. The quantum efficiency of the emitting polymers as well as the optimization of the PLED device characteristics play also significant role to the high sensitivity of the optical biosensor. Furthermore, the ability to create patterned multi-colour PLEDs onto the same substrate can improve the selectivity for different biomolecules by implementing a spectrally different fluorescent label for each biomolecule. The proposed solid-state PLED arrays are simple to construct, have low cost, low power consumption and are entirely flexible. Acknowledgement Financial support by EU FP6 NoE ‘‘nano2life” is acknowledged. Fig. 6. Schematic representation of the photopatterned three colour PLED device for the simultaneous detection of biomolecules labeled with different fluorescent dyes.
labeled biomolecules. This is the object of an ongoing study in our lab. 4. Conclusions In summary, blue emitting flexible PLEDs based on either PVK or BE-120 were fabricated. An anti-rabbit IgG antibody, labeled with two different fluorescent dyes (ALEXA or PHYCO) was then adsorbed on the PET substrate. The blue light output of the PLED was used to excite the fluorescent dye resulting in a red shift of the initial PLED’s EL spectrum. The size of the spectral shift
References [1] E.J. Cho, F.V. Bright, Anal. Chem. 73 (2001) 3289. [2] V. Savvate’ev et al., Appl. Phys. Lett. 81 (2002) 4652. [3] X. Wang, O. Hofmann, R. Das, E.M. Barrett, A.J. deMello, J.C. deMello, D.D.C. Bradley, Lab Chip 7 (2007) 58. [4] R.H. Friend et al., Nature 397 (1999) 121. [5] S.R. Forrest, Nature 428 (2004) 911. [6] R. Shinar, J. Shinar, J. Appl. Phys. 96 (2004) 2949–2954. [7] C. Adachi, M.A. Baldo, M.E. Thomson, S.R. Forrest, J. Appl. Phys. 90 (2001) 5048. [8] G. Gustafsson, Y. Cao, G.M. Treacy, F. Klavetter, N. Colaneri, A.J. Heeger, Nature 357 (1992) 447. [9] M. Vasilopoulou, G. Pistolis, D. Georgiadou, P. Argitis, Adv. Funct. Mater. 17 (2007) 3477. [10] M. Vasilopoulou, L.C. Palilis, A. Botsialas, D. Georgiadou, P. Bayiati, N. Vourdas, P.S. Petrou, G. Pistolis, N. Stathopoulos, P. Argitis, Phys. Stat. Sol. (c) 5 (2008) 3658. [11] T.D. Harris, Anal. Chem. 72 (2000) 669.
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Photopatterned PLED arrays for biosensing applications Maria Vasilopoulou a,*, Dimitra G. Georgiadou a, Leonidas C. Palilis a, Athanasios Botsialas a, Panagiota S. Petrou b, Sotirios E. Kakabakos b, Panagiotis Argitis a a b
Institute of Microelectronics, NCSR Demokritos, Aghia Paraskevi 15310, Greece Institute of Radioisotopes and Radiodiagnostic Products, NCSR Demokritos, Aghia Paraskevi 15310, Greece
a r t i c l e
i n f o
Article history: Received 1 October 2008 Received in revised form 22 January 2009 Accepted 23 January 2009 Available online 1 February 2009 Keywords: Optical sensor Photopatterned multi-colour PLEDs Dye-labeled biomolecules Fluorescent probe
a b s t r a c t A solid-state optical biosensor integrated with a single layer flexible blue Polymer Light Emitting Diode (PLED) as the light source was demonstrated and used for the detection of biomolecules labeled with two different fluorescent dyes. An anti-rabbit IgG antibody labeled with the fluorescent dyes, that was adsorbed on the opposite side of a polyethylene tetraphthalate (PET) flexible substrate, was excited by light emitted from the PLED. Light is then absorbed by the dye-labeled antibody resulting in a red shift of the polymer emission spectrum. Emission originating from the dye is also observed. The emission spectral shift and its efficiency depend primarily on the concentration of the biomolecule solution, the overlap between the emission spectra of the polymer and the absorption spectra of the dye and the PLED emission characteristics. Biomolecules immobilized onto distinct areas of a plastic substrate might then selectively be detected with high sensitivity after reacting with fluorescently labeled counterpart molecules. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction The next trend in sensor technology is the development of all plastic integrated chemical and biological microarrays for environmental and medical applications. In an optical sensor one requires a light source, appropriate sensing chemistry, a means to immobilize the desired sensing molecules and a light detector. Current light sources such as lasers or inorganic light emitting diodes either can not be readily integrated with the other components due to size and operational constraints or because they involve intricate integration procedures [1]. Integration of the sensing element with an organic light emitting diode (OLED) provides a straightforward and simple approach for flexible device architectures [2,3]. OLEDs have been dramatically improved over the past few years [4,5] and they have become advantageous as low-voltage, miniaturized flexible light sources that have sufficient output power in order to be useful for application in fluorescence-based biosensors [6,7]. Flexible devices can generally be fabricated on plastic substrates, like polyethylene tetraphthalate (PET), which combine flexibility, formability and transparency [8]. Increased interest has recently been shown in integrated micro systems, where different colour OLEDs can be used to excite labeled biomolecules aiming to simultaneous detection of different analytes. In this work, we propose an optical biosensor integrated with a single layer flexible PLED for the detection of biomolecules labeled * Corresponding author. E-mail address: [email protected] (M. Vasilopoulou). 0167-9317/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2009.01.063
with different fluorescent dyes. This system concept is versatile and it can easily be miniaturized and integrated with appropriate microfluidics and photodetectors towards microsystem development, offering wavelength tunability, simplicity and low cost [9]. 2. Experimental Poly[2-(6-cyano-6-methyl-heptyloxy)-1,4-phenylene] (BE-120) was purchased from American Dye Source, whereas poly(9-vinyl carbazole) (PVK) and Poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) were purchased from Aldrich and used with no further purification. The AlexaFluor488Ò and R-phycoerythrin labeled anti-rabbit IgG antibodies were purchased from Molecular Probes, Inc. Polymer films were prepared by spin-coating from chloroform solutions on PET substrates. Dye-labeled biomolecules were drop-cast and subsequently adsorbed on the other side of the PET substrate. For absorption spectra a Perkin–Elmer Lambda-16 spectrometer was employed. Fluorescence and excitation spectra were recorded with a Perkin–Elmer LS-50B fluorescence spectrometer. Flexible PLEDs were fabricated on Indium Tin Oxide (ITO) coated PET substrates [10]. Single layer blue emitting devices were based on (BE-120) as the emitting layer, while the multi-colour photopatterned single layer devices were based on the wide bandgap (PVK) doped with the fluorescent dyes 1-(40 -dimethylaminophenyl)-6-phenyl-1,3,5-hexatriene (DMA-DPH), and 4dimethylamino-40 -nitrostilbene (DANS). ITO coated PET substrates were precleaned and treated with oxygen plasma to increase the
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ITO work function and increase its surface energy. Prior to the deposition of BE-120, a 40 nm thick film of PEDOT-PSS was spin coated on ITO in order to improve hole injection and substrate smoothness. Device fabrication was completed with a 250 nm thick aluminum cathode electrode which was deposited by thermal evaporation under high vacuum. All the testing devices have an active area of 12.56 mm2. Next, a thick layer of biomolecules labeled with a suitable fluorescent dye is drop-cast on the other side of the PET substrate to complete the biosensor fabrication. Current density–voltage (J–V) characteristics were obtained using a programmable Keithley 230 Voltage Source and a 195A multimeter. Luminance measurements (in forward direction) and electroluminescence (EL) spectra were recorded simultaneously using a USB 2000-UV–Vis miniature fiber optic spectrometer. A schematic picture of the experimental setup used for the electroluminescence spectral measurements and biomolecule detection is shown in Fig. 3. All measurements were carried out at room temperature under ambient atmosphere without any encapsulation. 3. Results and discussion Fig. 1 shows the absorption and PL spectra of the two different fluorescent dyes that were evaluated (a) the AlexaFluorÒ 488 dye (ALEXA) (absorption maximum: 495 nm; emission maximum: 519 nm) and (b) the R-phycoerythrin dye (PHYCO) (absorption maxima: 546 and 565 nm; emission maximum: 578 nm). Fig. 2 shows the PL spectra obtained before and after the immobilization of the antibodies labeled with either the ALEXA (left) or the PHYCO (right) fluorescent dye on one side of a PET substrate on the other side of which a PVK layer is spin coated. PL spectra demonstrate a red shift from blue to greenish blue after the immobilization of both antibodies. In that case, blue light emitted from PVK was the excitation source for both fluorescent dyes. Thus, since the PL spectrum of the polymer overlaps to some extent with the absorption spectrum of the fluorescent dye, a red shift in the PL after the
100
b
abs em
Relative intensity
Relative intensity
a
immobilization of the labeled biomolecule is expected, which can be attributed to both emission from the fluorescent label of the biomolecule and PVK. A slightly larger red shift was achieved when the ALEXA fluorescent dye was used, due to better overlap of its absorption spectrum with PVK’s PL spectrum. In addition, it should be mentioned that the concentration of the biomolecule solution appears to play an important role on the emission characteristics and efficiency of the fluorescent label. High concentration results in significant self-quenching of its fluorescence emission due to the small Stokes shift. However, as PVK has very low fluorescence efficiency and a large photon population is needed to excite the dye-labeled biomolecule, next we evaluated BE-120 (which is a highly luminescent blue polymer) as the emitting component in blue flexible PLEDs fabricated on a PET substrate in order to improve the biosensor characteristics. A representative structure of these devices along with the corresponding energy diagram of BE-120 and both polymers’ chemical structures are presented in Fig. 3. These blue emitting, single layer flexible PLEDs exhibit good performance characteristics when operating in air as shown in Fig. 4 where their current density–voltage–luminance characteristics are presented. Thus, they could be used as light sources integrated with the sensing element for the development of biosensors based on fluorescence detection. No demonstrate the potential of the developed PLEDs for biosensing applications, the ALEXA dye-labeled antibody was immobilized by adsorption on the opposite side of the PET substrate. Both PL and the PLED’s EL spectrum were recorded for comparison before and after the labeled biomolecule immobilization. As shown in Fig. 5, a small spectral shift from blue to greenish blue is observed in both PL and EL after the immobilization of the labeled biomolecule on the other size of a PET substrate on top of which a BE-120 layer has been spin coated. The main difference however is that in PL the characteristic emission from the ALEXA dye at 530 nm is clearly observed, though only for a low concentration biomolecule solution. Again, the high concentrated
80 60 40 20
abs em
80 60 40 20 0
0 300
100
400
500
600
300
700
400
500
600
700
Wavelength (nm)
Wavelength (nm)
Fig. 1. Normalized absorption and emission spectra of AlexaFluorÒ 488 dye (a) and R-phycoerythrin (b).
1.0
PVK PVK/ALEXA
0.8 0.6 0.4 0.2 0.0 350
400
450
500
550
Wavelength (nm)
600
Normalised PL
Normalised PL
1.0
PVK PVK+phyco_1 PVK+phyco_2
0.8 0.6 0.4 0.2 0.0 350
400
450
500
550
600
Wavelength (nm)
Fig. 2. Photoluminescence spectra before and after the immobilization of an AlexaFluor488Ò (left) or R-phycoerythrin (right) anti-rabbit IgG antibodies in front of a PVK layer.
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M. Vasilopoulou et al. / Microelectronic Engineering 86 (2009) 1511–1514
Vacuum
Cathode (Al)
LUMO (2,7 eV)
Emitting layer-BE 120
4,3 eV
PEDOT-PSS
Transparent anode (ITO) 5eV
PET substrate
5,2eV HOMO (5,8 eV)
Biomolecule Light output
ITO PEDOT
BE-120
Al
-PSS
Optical fiber
Fig. 3. Representative structure and energy diagram of a flexible PLED (based on the wide bandgap emitting polymer BE-120) integrated with the biomolecule sensing element and the corresponding chemical structures of PVK (bottom left) and BE-120 (bottom right).
Current Density (A/m2)
1000
Luminance
800
800
600
600
400
400
200
200
0
2
4
6
8
10
12
14
16
Luminance (cd/m2)
Current Density
1000
0
biomolecules in the same sample by implementing a spectrally different fluorescent label for the detection of each biomolecule. In addition, the detection of multiple biomolecules might be feasible through employment of appropriately micropatterned PLEDs on the same substrate in combination with different fluorescent labels. In our previous work [9] we demonstrated the definition of all three primary colours Red–Green–Blue (R–G–B) in a single polymer layer PLED by photochemical transformation of the emission properties of certain fluorescent dyes. Blends of PVK with both the green emitter DMA-DPH and the red emitter DANS along with the triphenyl sulfonium salt as a photoacid generator were prepared and PLEDs were fabricated. Initially, red emission was observed from the lower bandgap compound (DANS). By UV exposure we were able to bleach red emission from DANS and to define green emitting areas, since DMA-DPH emits in the green spectral region. With subsequent irradiation and protonation of the green emitter the blue emission of the DMA-DPH’s photochemical product was achieved. Therefore, distinct areas on a PVK layer emitting red, green and blue light (or any other intermediate colour) were defined, by using a photochemical transformation based on photoacid induced emission changes. In Fig. 6, a schematic presentation is shown of how these photopatterned full-colour devices could be used for the detection of multiple biomolecules through employment of the appropriate micropatterned PLEDs on the same substrate in combination with different fluorescently
1200
1200
0
Voltage (V) Fig. 4. Current density–voltage–luminance characteristics of a single layer blue PLED based on BE-120.
dye-labeled biomolecule solution seems to self-quench its fluorescence. Note, that choosing a lower bandgap emitting polymer for the PLED that its PL has a larger overlap with the absorption spectrum of the dye may result in a more significant spectral shift with emission primarily originating from the dye after its excitation. The ability to create photopatterned multi-colour single layer flexible PLEDs on the same substrate is in accordance with the growing trend toward measuring ‘‘everything” in a sample [11]. This would improve the simultaneous detection of different
Normalized PL
0.8 0.6 0.4
0.15 0.1 0.05
0.2 0.0 350
0.2
EL (au)
BE 120 BE 120+ALEXAhigh conc. BE 120+ALEXAintermediate conc. BE 120+ALEXAlow conc.
1.0
400
450
500
550
600
Wavelength (nm)
650
0 350
450
550
650
750
Wavelength (nm)
Fig. 5. PL (left) and EL (right) spectrum before and after the immobilization of the AlexaFluor488Ò anti-rabbit IgG antibody in front of the blue PLED.
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depends strongly on the concentration of the fluorescent dye-labeled biomolecule solution. The quantum efficiency of the emitting polymers as well as the optimization of the PLED device characteristics play also significant role to the high sensitivity of the optical biosensor. Furthermore, the ability to create patterned multi-colour PLEDs onto the same substrate can improve the selectivity for different biomolecules by implementing a spectrally different fluorescent label for each biomolecule. The proposed solid-state PLED arrays are simple to construct, have low cost, low power consumption and are entirely flexible. Acknowledgement Financial support by EU FP6 NoE ‘‘nano2life” is acknowledged. Fig. 6. Schematic representation of the photopatterned three colour PLED device for the simultaneous detection of biomolecules labeled with different fluorescent dyes.
labeled biomolecules. This is the object of an ongoing study in our lab. 4. Conclusions In summary, blue emitting flexible PLEDs based on either PVK or BE-120 were fabricated. An anti-rabbit IgG antibody, labeled with two different fluorescent dyes (ALEXA or PHYCO) was then adsorbed on the PET substrate. The blue light output of the PLED was used to excite the fluorescent dye resulting in a red shift of the initial PLED’s EL spectrum. The size of the spectral shift
References [1] E.J. Cho, F.V. Bright, Anal. Chem. 73 (2001) 3289. [2] V. Savvate’ev et al., Appl. Phys. Lett. 81 (2002) 4652. [3] X. Wang, O. Hofmann, R. Das, E.M. Barrett, A.J. deMello, J.C. deMello, D.D.C. Bradley, Lab Chip 7 (2007) 58. [4] R.H. Friend et al., Nature 397 (1999) 121. [5] S.R. Forrest, Nature 428 (2004) 911. [6] R. Shinar, J. Shinar, J. Appl. Phys. 96 (2004) 2949–2954. [7] C. Adachi, M.A. Baldo, M.E. Thomson, S.R. Forrest, J. Appl. Phys. 90 (2001) 5048. [8] G. Gustafsson, Y. Cao, G.M. Treacy, F. Klavetter, N. Colaneri, A.J. Heeger, Nature 357 (1992) 447. [9] M. Vasilopoulou, G. Pistolis, D. Georgiadou, P. Argitis, Adv. Funct. Mater. 17 (2007) 3477. [10] M. Vasilopoulou, L.C. Palilis, A. Botsialas, D. Georgiadou, P. Bayiati, N. Vourdas, P.S. Petrou, G. Pistolis, N. Stathopoulos, P. Argitis, Phys. Stat. Sol. (c) 5 (2008) 3658. [11] T.D. Harris, Anal. Chem. 72 (2000) 669.