Transcript Document

The most recent application of OLEDs:
Structurally-integrated OLED-based
luminescent chemical & biological sensors
Ruth Shinar
Microelectronics Research Center & ECpE Dept, ISU
Integrated Sensor Technologies, Inc. (ISTI)
Joseph Shinar
All of the aforementioned & ISTI
Support by DOE, NASA,
NSF, & NIH also gratefully acknowledged
Outline
 Photoluminescence (PL)-based sensors: issues, goal
 Approach:
 Structural integration: OLED excitation source and sensing component
* Applications:
Single analyte detection
Multianalyte detection
 Advanced structural integration: OLED/sensing component/thin film-based
photodetector
* Application example: O2 sensing
Joseph Shinar and Ruth Shinar,
“Organic Light-Emitting Devices (OLEDs) and OLED-Based Chemical and
Biological Sensors: An Overview,” J. Phys. D: Appl. Phys. 41, 133001 (2008).
Components of PL-Based Sensors:
Excitation source: lasers, inorganic LEDs, lamps
Sensing element: a porous film with an embedded luminescent dye, surface immobilized
species, or microfluidic channels with recognition elements in solution
Photodetector: photomultiplier tube, Si photodiode
Electronics and readout
Not Integrated
Issues:
 Light-source is either bulky, or the integration with the sensing
element/microfluidics involves intricate design (fibers, lens)
 Sensors are often immobile, costly
 Sensors are limited in use for real-world applications; often used for single
analyte detection
Long-Term Goal:
Back-detection mode
sensing component
band -pass filter
OLED
PD
PD
long-pass filter
OLED
glass
glass
PD
a-(Si,Ge):H photodetector
Front-detection mode
low gap a-(Si,Ge):H PD
long-pass filter
transparent cover
microfluidic wells
OLED
OLED
OLED
OLED
glass
band-pass filter over OLED pixels
OLEDs Advantages for Sensor Applications
 Are simple to fabricate and uniquely simple to integrate
with a sensing component
 Can be easily fabricated in any 2-D shape
 Are compatible with microfluidic structures




Can be fabricated on plastic substrates
Can be operated at an extremely high brightness
Consume little power and dissipate little heat
Cost is expected to drop to a near-disposable level
OLED/Sensing Component Integration
Approach:
structural integration of two
components:
Sensing
Element
SENSOR FILM
GLASS OR PLASTIC SUBSTRATE
TRANSPARENT ANODE
ORGANIC LAYERS
OLEDs are easily fabricated
as an array of pixels
OLED
+
liquid or gas analyte
Back-detection
mode using
an array of
OLED pixels
CATHODE
Basic Structure
--
Luminescent sensor
glass
substrate
EL
cathode
OLED
pixels
PL
EL
Sensor operation modes:
ITO
(1) monitoring changes in I
PL
photodetector
OLED layers
cathode
(2) monitoring changes in t
1. Oxygen Sensor
R. Shinar et al., Anal. Chim. Acta 568, 190 (2006)
Principle of operation:
 An oxygen sensitive dye is embedded in a thin film matrix or dissolved in solution
 Collisions of O2 with the dye result in quenching of the PL intensity I and shortening of
the PL decay time t
Stern-Volmer (SV) equation
I0/I = t0/t = 1 + KSV[O2]
I0 and t0 – unquenched PL intensity and decay time; KSV - Stern-Volmer constant
Sensor operation modes: (1) monitoring changes in I
(2) monitoring changes in t
The need for optical filters and frequent calibration
is eliminated when using the PL decay time mode.
Results for Gas-Phase Oxygen:
Integrated OLED/sensing element; back-detection
Green OLED (Alq3)/Pt octaethylporphyrin (PtOEP) in a polystyrene film
C2H5
C2H5
I0/I = t0/t = 1 + KSV[O2]
C2H5
N
C2H5
N
Pt
N
40
C2H5
C2H5
C2H5
2.5
20
Absorbance
t0/t
C2H5
day 1
day 10
day 20
day 30
30
N
10
0
0
20
40
60
% O2
S [t0/t(100% O2)]~30-50
80
100
2.0
PtOEP
1.5
emission ~635 nm
1.0
0.5
0.0
350
400
450
500
550
Wavelength (nm)
600
Results for Gas-Phase Oxygen:
Integrated OLED/sensing element; back-detection
Rubrene (0.5%)-doped Alq3/Pd octaethylporphyrin (PdOEP) in a polystyrene film
250
200
PdOEP
1.5
0
/
Absorbance
150
100
emission ~645 nm
1.0
0.5
0.0
50
S [t0/t(100% O2)]~240
350
20
40
60
% O2
80
450
500
550
Wavelength (nm)
0
0
400
100
600
Results for Dissolved Oxygen:
Integrated OLED/sensing element; back-detection
Alq3/PtOEP in a polystyrene film or in solution
I0/I = t0/t = 1 + KSV[O2]
14
30
toluene
25
water
12
0.01 mg/mL PtOEP
10
t
/0
t0/t
20
ethanol
15
8
6
4
10
water
5
2
0
0
0
0
20
40
% O2
60
80
100
10
20
30
DO Concnetration (wt. ppm)
40
20.4
Long-term stability
gas-phase oxygen
20.0
19.8
19.6
0
5
10
15
20
25
30
Time (days)
25
24
Long-term stability
dissolved oxygen
PL Lifetime ( ms)
s)
20.2
23
22
21
20
0
10
20
30
Time (days)
40
50
60
2. Recent Improvements in OLED-Based Oxygen Sensors:
Dispersion of TiO2 Particles in the PS:PtOEP Film
Enables use of reduced brightness for enhanced OLED lifetime and reduced
photo-degradation [Zhou et al., Adv. Func. Mater. 17, 3530 (2007)]
A uniquely simple approach, using 360 nm-diameter TiO2 particles.
Enhancement is due to light scattering by the high refractive index particles, & possibly by
voids induced by the particles.
Typically, the sensor films were prepared by drop casting 40-60 mL of the solution
onto the glass substrate; the resulting films were all ~ 8 mm thick
(a)
(b)
SEM images of PS films doped with PtOEP and titania particles:
(a) 2 mg/mL TiO2 in the solution used for film fabrication, (b) 8 mg/mL particles.
In Air
The PL spectra in air, excited at ~535 nm by the Alq3 OLED, of PtOEP:PS doped with
different concentrations of TiO2, measured in reflection geometry.
Gas-Phase
The effect of titania particles on the PtOEP:PS PL decay curves and on the EL of the Alq3
OLED in a 100% gas-phase Ar environment at 295 K. Shown is the intensity measured by
the PD during and following the 50 ms Alq3 OLED pulse at titania particle concentrations
of 0, 1.5, 2, 4, and 8 mg/mL. A 610 nm long-pass filter was placed in front of the PD, so
that only a small fraction of the OLED emission reached the PD. Inset: the PL signal
enhancement vs. the titania particle concentration.
DO in Water
2.0
4
1.0
1.50
(a)
1.00
0.50
0.00
0.5
10
20
30
40
Time (ms)
4
PL Intensity (arb. units)
1.5
100% Ar
O2- saturated water
PL Intensity (arb. units)
2.00
Intensity (arb. units)
Intensity (arb. units)
100% Ar
3
2
3
O2-saturated water
(b)
2
1
0
1
10
20
30
Time (ms)
0.0
0
0
100
200
Time (ms)
300
400
0
2000
4000
6000
Time (ms)
The PL decay curves in Ar- and O2-saturated solutions for: (a) PtOEP and (b) PdOEP with
and
without titania doping; the exponential (Ar) and bi-exponential (O2) fitting are also shown.
Example of sensor design & operation –
Green OLED/PtOEP embedded in polystyrene
Green Alq3 OLED/PtOEP
 The green Alq3 OLED array is behind the PtOEP-film, which is largely confined to a
region in front of the middle two OLED pixels.
 The green emission from these pixels combines with the red PL of the PtOEP dye to
produce the observed yellowish spots.
 The photodetector is located behind the OLED array.
3. Oxidases-Based Multianalyte Sensor Array
(based on the preceding O2 sensor)
Choudhury et al., J. Appl. Phys. 96, 2949 (2004).
glucose + O2
glucose oxidase
H2O2 + gluconic acid
PtOEP
normal level:~100 mg/dL (5.5 mM)
O
ethanol + O2
H2O2 + H3C C H
alcohol oxidase
PtOEP
Legal limit: ~0.1% = 0.1 mg/dL (~2.2 mM)
lactate + O2
H2O2 + pyruvic acid
lactate oxidase
PtOEP
Normal level:
Venous blood: 4.5-19.8 mg/dL (0.56-2.46 mM);
Arterial blood: 4.5-14.4 mg/dL
Multianalyte Sensor in Operation: consecutive sensing using a
single photodetector
R. Shinar et al., SPIE Conf. Proc. 6007, 600710-1 (2005).
OLED pixel pair #
1 2 3 4 5
6
PL Intensity(a. u)
Oxygen
Glucose
Oxygen
t = 100 ms
0.004
0.002
0.000
0.000
0
100 200 300 400 500
Time (ms)
Glucose
t = 83 ms
0.004
0.002
Lactate
0.003
0.006
0.006
Ethanol
0
0.002
0.003
0.001
0.000
0.000
100 200 300 400 500
Time (ms)
Lactate
t = 87 ms
0.002
0.001
100 200 300 400 500 0
Time (ms)
Alcohol
t = 80 ms
0
100 200 300 400 5
Time (ms)
Multianalyte Sensor Array: simultaneous monitoring
Y. Cai et al., Sensors & Actuators B 134, 727 (2008).
 small-size sensor array (e.g., total size
~1.5x1.5 cm2)
OLED
pixel
 sensing element: O2-sensitive dye (PtOEP)
and an analyte-specific oxidase enzyme
Al cathode
silicon photodiode array
ITO anode
 each sensing element is associated with 2
OLED pixels
 sensing of the different analytes in a single
sample is obtained by addressing the
appropriate OLED pixels
 simultaneous sensing of the different analytes
in a single sample is obtained when all OLED
pixels are lit simultaneously, and the PL of
each analyte is monitored by its associated Si
photodiode. A Labview program enables such
simultaneous monitoring via separate
channels.
6 mm
Sensing in Sealed Wells: Assessing the limit of detection
In reactions performed in sealed wells, where there is no replenishing of O2, and
the initial concentration of the analyte does not exceed that of the initial DO
(~0.25 mM in water at 23oC):
100
80
H2O2 + gluconic acid
PL Lifetime (m
Glucose +O2
GOx
0.05 mM
0.1 mM
0.15 mM
0.2 mM
0.25 mM
0.3 mM
0.35 mM
60
40
20
[DO]final = [DO]initial – [analyte]initial
Modified SV equation:
I0/I = t0/t = 1 + KSV×{[DO]initial – [analyte]initial}
0
20
40
60
80
100
120
Reaction Time (sec)
37oC; cell open to air; lactate
monitoring
Multianalyte mixture sensing
23oC; sealed containers
sequential monitoring
simultaneous monitoring
35
circles: ethanol
squares: lactate
triangles: glucose
lactate
glucose
lactate in mixtures
glucose in mixtures
1/t (ms-1)
30
25
20
15
10
0.0
0.1
0.2
0.3
0.4
Analyte Concentration (mM)
Modified SV equation:
LOD ~0.02 mM; dynamic range: 0.25 mM
at 23oC in the final, diluted sample.
I0/I = t0/t = 1 + kSV×{[DO]initial – [analyte]initial}
Simultaneous detection using a 5×5 mm2 Si photodiode array.
4. Bacillus Anthracis (Anthrax) Lethal Factor (LF)
Principle of Operation:
R. T. Cummings et al. Proc. Nat. Acad. Sci. 99, 6603 (2002)
 The LF enzyme, one of three proteins of the Anthrax toxin, a Zn-dependent
metalloprotease, cleaves certain peptides at specific sites.
 Fluorescence detection of LF is therefore possible by using a peptide- based
FluorescenceResonance Energy Transfer (FRET) assay.
FRET-labeled peptide sequence:
(donor)-Nle-K-K-K-K-V-L-P--I-Q-L-N-A-A-T-D-K- (acceptor) G-G-NH2
cleaving site
 Peptides with donor and acceptor on either side of the cleaving site are synthesized
 In this D-A configuration, the fluorescence of the donor is quenched by the acceptor
 Following exposure to LF, the peptide is cleaved; the donor and acceptor are
separated, resulting in an increase in the detected intensity of the donor fluorescence.
Selected examples showing the effect of the concentration of the peptide on the
photoluminescence using 25 nM LF
PL Change (%)
70
37.5 mM
76.5 mM
60
22.5 mM
50
7.5 mM
The maximal increase in the PL
following exposure of the labeled
peptide to LF was by a factor of 2
at 37 oC
40
30
4.5 mM
20
10
3.2 mM
0
0
10
20
30
40
50
60
Time (min)
To improve sensitivity, need to eliminate OLED tail that overlaps the donor emission.
Turn to Ru dye (exc=385 nm, red emission):
bis(2,2’-bipyridine)-4’-methyl-4-carboxybipyridine-Ru N succinimidyl ester-bis(hexafluorophosphate)
& QSY21 quencher.
5. Hydrazine (N2H4)
S. Rose-Pehrsson and G. E. Collins, US Patent 5,719,061)
 Highly toxic but popular NASA monopropellant & common precursor in the
synthesis of some polymers, plasticizers and pesticides
 M. P. 2°C, B. P. 113.5°C, room temp vapor pressure 14.4 torr
 Gov’t recommended exposure limit is 10 ppb for 8 hrs
 Immediately dangerous to life or health at ~50 ppm
 Detection based on reaction of hydrazine with
anthracene 2,3-dicarboxaldehyde (ADA).
H
H
ADA




Reaction product emits at 549 nm (exc= 476 nm)
Signal proportional to N2H4 concentration
Good for air, solution
Fast and very selective response
Photoluminescence Change (%)
Hydrazine
20
20 ms blue OLED pulse
60 ppb hydrazine
15
The limit of detection (LOD) is
10
60 ppb in ~1 min, or ~1 ppb in
5
1 h, exceeding the OSHAallowed limit of 10 ppb over 8
0
0
5
10
15
hrs by a factor of 80
Time (min)
PL change upon ADA exposure to 60 ppb hydrazine in Ar
6. Enhanced Integration: OLED/Sensing Element/Photodetector
Back Detection
band-pass filter over OLED pixels
sensing component
glass
(thin) glass
PD
long-pass filter
(large gap; cuts blue)
OLED
PD
OLED
PD
nanocrystalline or
a-(Si,Ge):H low-gap PD
Envisioned fully integrated OLED/sensing film/thin film PD array in a back
detection configuration. Grounded Al stripes between the OLEDs and PDs will
block the edge EL and the synchronous electromagnetic noise generated by
modulated OLEDs.
Towards complete integration: front detection
long-pass filter
low gap a-(Si,Ge):H PD
transparent cover
wells or
microfluidic
channels
glass/PDMS
Band-pass filter over OLED pixels
OLED
OLED
OLED
glass
OLED
OLED/Sensing Component/Photodetector Integration
R. Shinar et al., J. Non Cryst. Solids 352, 1995 (2006).
long-pass
First Step:
filter
Structural integration of two components:
PD
gas flow
the sensing element and an
sensor
a-Si thin film-based photodetector
530 nm band-pass filter
OLED light
Second Step: single element; front detection
structural integration of three components:
OLED/sensing element/PD; I mode of operation
The three-component integration is attractive due to the potential for
miniaturization of sensor arrays, their fabrication on flexible substrates, and
integration with microfluidics
PDs
 PECVD-grown p-i-n and n-i-p structures, based on a-Si and a-(Si,Ge), or
nc-Si.
 The p-i-n structures were fabricated on an ITO-coated glass substrate; the
ITO was protected from reduction by the hydrogen plasma with a 0.1 mm
thick ZnO layer grown by RF sputtering.
 The composition and thickness of the p & i layers were tuned to increase
the sensitivity at wavelengths matching the emission band of the oxygensensitive dye and to reduce the sensitivity to the OLED background.
 The emphasis was on
(a) improving the O2 detection sensitivity by improving the PDs & reducing
the synchronous OLED-generated electromagnetic noise
(b) understanding factors that affect the speed of the PDs
TUNING of the PDs
The PDs were first tuned for high sensitivity at the PtOEP/PdOEP emission wavelength
(~635/650 nm) and minimal response at the OLED excitation (~535 nm). The a-(Si,Ge)based PDs are preferred due to their match with the dye PL, and lower response at the EL
band. However, their dark current was higher and their speed lower.
0.4
a-(Si,Ge):H
a-Si:H
(0.7 mm)
0.3 mm p-layer
1.6% Ge in p-layer
10% Ge in i-layer
(0.5 mm)
0.15 mm p-layer
2.5
0.3
0.2
PtOEP
2.0
Absorbance
Quantum Efficiency
0.5
emission ~635 nm
1.5
1.0
0.5
0.1
0.0
350
0.0
400
450
500
550
Wavelength (nm)
500
600
700
Wavelength (nm)
800
600
Sensor Film/Photodetector Integration
100% N2
Response (mV)
150
100
air
50
100% O2
500
510
520
530
540
550
560
Excitation Wavelength (nm)
Detection of the photoluminescence of PtOEP embedded in polystyrene by
the a-(Si,Ge):H- based photodetector, using a 600 nm long-pass filter
O2 sensor: lamp-monochromator/
PtOEP-based sensor film/thin-film PD
25
I0/I = t0/t = 1 + KSV[O2]
I0/I
20
15
10
5
S [I0/I(100% O2)]~23
0
0
20
40
60
80
100
% O2
Detection of the photoluminescence of PtOEP embedded in polystyrene
by the a-(Si,Ge):H- based photodetector using a 620 nm band-pass filter to
reduce the background, and therefore increase the sensitivity.
OLED/Sensing Component/Thin-Film Photodetector Integration
Single element: front detection
 Initial integration resulted in low S attributed to a large
background stemming
from the broad OLED EL, and high PDs’ dark current.
 A major reason to the low S is the synchronous electromagnetic
(EM) noise from
the pulsed OLED when using lockin detection of the intensity.
 S improved significantly by shielding the OLED.
 To shield the PD from this EM noise, a grounded 150 nm thick ITO-coated
glass was placed above the OLED.
O2 sensor, I mode operation: OLED/PdOEP:PS/thin-film PD
Examples of SV plots of different sensors, each with the three-component
integration, i.e., unshielded and shielded Alq3 OLED/PdOEP:PS/a-(Si,Ge) PD,
and double-shielded coumarin-doped Alq3 OLED/PdOEP:PS/nc-Si PD
S ~ 47, improved strongly over the
value of ~7 achieved with
unshielded Alq3 OLEDs.
50
double shielded, coumarin-doped Alq3 OLED
40
30
I0 /I
Shielding the PD improved the
response. The best results were
obtained for the double shielded,
coumarin-doped Alq3;
shielded, Alq3 OLED
20
10
0
Towards operation in the t mode
unshielded, Alq3 OLED
0
20
40
60
% Oxygen
80
100
Concluding Remarks
 OLED science and technology is undergoing explosive growth
 Structurally integrated OLED-based sensors are very promising
 Organic photovoltaics & organic field-effect transistors are also
growing at an extremely rapid clip. They deserve a separate
treatment. See Sumit Chaudhary’s ECpE course on Organic
Electronics.