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Sensitive Explosive Vapor Detection with PolyfluoreneLasers

PER

By Ying Yang, Graham A. Turnbull,* and Ifor D. W. Samuel*

Distributed feedback organic semiconductor lasers based on polyfluorene are

shown to be suitable for use as chemical sensors for the detection of

nitroaromatic-based explosive vapors. The laser threshold is increased by a

factor of 1.8 and the slope efficiency is reduced by a factor of 3 after exposure

to the vapor. The sensing efficiency depends strongly on the excitation energy

with a maximum efficiency of 85%. The temporal dynamics of the laser

response to the analyte have been investigated. The laser emission falls to

60% of its initial value in 46 s. A model is developed to offer some insight into

the diffusion of the vapor molecules inside the polymer layer.

1. Introduction

The key characteristics of semiconducting polymers, for instancehigh photoluminescence quantum yield (PLQY), broad emissionspectra across the full visible range, low cost, and ease offabrication, make them attractive materials for lasers,[1–6] LEDs,[7]

and photovoltaic devices.[8,9] Since the first demonstration of thesolution polymer laser in 1992,[10] there have beenmany advancesin laser cavity and material design[11–17] because of their potentialexciting applications in spectroscopy,[18] data communication,[19]

optical switching,[20,21] and chemical sensing.[22,23] Polyfluorenehas been demonstrated to be an attractive lasermediumwith highgain and PLQY, as well as low loss.[24] Using distributed feedbackresonators, polyfluorene lasers give low lasing thresholds andbroad tunability in the blue.[25]

Sensors for explosive vapors are important for environmentalapplications, national security, and human health protection.Conjugated polymers are promising candidates for sensingnitroaromatic-based explosives (e.g., trinitrotoluene, TNT).When they are in contact with nitroaromatic groups, photo-induced electron transfer will occur, leading to quenching of thelight emission.[26,27] The electron transfer requires that the lowestunoccupiedmolecular orbital (LUMO) of the analytemolecule lieslower in energy than the LUMO of the polymer. The sensingprocess is therefore only partly selective and in practice a widerange of nitroaromatic molecules will quench the light emission.However in the context of applications suchas land-minedetection

[*] Dr. G. A. Turnbull, Prof. I. D. W. Samuel, Y. YangOrganic Semiconductor Centre, SUPA, School of Physics andAstronomyUniversity of St AndrewsNorth Haugh, St Andrews KY16 9SS (UK)E-mail: gat@st-andrews.ac.uk; idws@st-andrews.ac.uk

DOI: 10.1002/adfm.200901904

Adv. Funct. Mater. 2010, 20, 2093–2097 � 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Wein

this process forms the basis of an effectivesensorwhere the simultaneous detection ofany quenching nitroaromatic molecule isrequired.[22]

Rose et al.[23] have shown that organicsemiconductor lasers have the potential forsensing explosive vapors. In their work, amicroring laser made from a custom-synthesized PPV derivative was used todetect 2,4-dinitrotoluene (DNT) vapor. Achange of lasing threshold was observed,but the timescale for the laser to respondwas not given. Recently, we have shown thatfluorene-based organic semiconductors

can successfully be used as nitro-aromatic vapor sensors.[28] Inthe present work, we demonstrate that the widely availableprototypical polymer polyfluorene can be used for explosivessensing. We show that the convenient distributed feedback (DFB)laser geometry can be used and that there is a strong change inslope efficiency after exposure to an explosive analyte.Wemeasurethe temporal response of the laser andfind it to be on a timescale ofseconds. We also present a simple model to simulate how theexplosive vapor molecules diffuse into the polymer layer andcompare with its overlap with the transverse waveguide mode ofthe laser.

2. Results and Discussion

2.1. Lasing Threshold and Slope Efficiency

DFB lasers made from a thin film of poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO) on corrugated SiO2 substrates were exposed to vaporsof 1,4-dinitribenzene (DNB) at a concentration of 9.8 ppb innitrogen gas. The lasers were optically pumped with a nitrogenlaser. Figure 1a shows the change of the laser properties betweenpre-exposure, following5-minexposure, and following recovery byevacuating the chamber. We observe that when the polymer laserwas exposed to DNB vapors, the lasing threshold increased by1.8 times from 6.8 to 12.5mJ due to the quenching statesintroduced by the DNBmolecules. The change in the light outputcan be described by a ‘‘sensing efficiency’’ (Fig. 1b), defined as thechange in the intensity of the emission after exposure divided bythe original emission intensity, which depends on the excitationenergy. A maximum sensing efficiency of 82% is achieved whenthe pump energy is close to the threshold of the exposed laser. Thisis 1.2 times higher than the sensing efficiency for fluorescencefrom a planar thin film of the same thickness and exposed to the

heim 2093

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5 10 15 20 25 300.0

0.2

0.4

0.6

0.8

1.0

5 10 15 20 25 3055

60

65

70

75

80

85

pre exposure post exposure post recovery

a)La

ser o

utpu

t / a

.u.

Pump energy / Jb)

Sen

sing

effi

cien

cy /

%

Pump energy / J

Figure 1. a) Power characteristics of a PFO laser before exposure

(threshold energy is 6.8mJ), after 5-min exposure (threshold energy is

12.4mJ), and after recovery (threshold energy is 6.8mJ). b) Dependence of

the sensing efficiency h on the pump energy.

0 500 1000 1500 2000

e9

e10

1400 1500 1600 1700

e9

e10

a) recovery in vacuum

2nd exposure1st exposure

Lase

r out

put /

a.u

.

Time / sb)

Lase

r out

put /

a.u

.

Time / s

Figure 3. a) Laser output dynamics during two exposure and recovery

cycles. b) Details of the second exposure cycle.

2094

same concentration of DNB (Fig. 2). In addition to the change oflaser threshold,we also observed a reduction in the slope efficiencyby a factor of 3, suggesting that the presence of theDNBmoleculesnot only serves as quenchers for the excitons in the PFO layer, butmay also change the surface output coupling efficiency of the laserresonator.Wenote that the change of the slope efficiency is slightlyhigher than the change in lasing threshold. Consequently, inaddition toa change in sensingefficiencynear the lasing threshold,a change in laser slope efficiency canbeusedas a robustmultipointmeasurement for sensitive detection. We observe that followingrecovery in vacuum, flowing nitrogen or air, both the lasingthreshold and slope efficiency can recover to their pre-exposurevalues, indicating the process is reversible.

0.20 0.25 0.30 0.3555

60

65

70

75

80

85

PL

sens

ing

effic

ienc

y / %

Pump energy / J

Figure 2. Dependence of the fluorescence sensing efficiency of a 257-nm-

thick PFO film on pump energy after a 5-min exposure to DNB.

� 2010 WILEY-VCH Verlag GmbH & C

2.2. Exposure Dynamics

In order to investigate the interaction betweenDNBmolecules andthe PFO laser, we monitored the temporal dynamics of the laseremission during exposure and recovery. Figure 3a shows thechanges in laser output during two cycles of exposure and recovery(in vacuum). Figure 3b shows in more detail the decay of thesecond exposure cycle. In Figure 3b, the output emissiondecreased rapidly by 40% within 46 s with a 20% drop in thefirst 14 s. Prolonged exposure to DNB molecules beyond 300 scauses very little further change. A possible explanation for theflattening of the curve following a fast decay is that most of theexcitonsnear the surface of thePFOfilmwere quenched as soon asthe DNB molecules reached the surface region of the PFO film.Further quenching depends on the diffusion speed of DNBmolecules deep into the PFO films, which is much slower incomparison to the surface exciton quenching. The diffusion speedof the explosive vapor molecules into polymer film depends onseveral parameters such as the vapor pressure, the size of the vapormolecules, and the packing of the polymer chains.[27] Therefore,excitons deep in the PFO film still experience radiativerecombination to the ground state because of a reducedconcentration of quenchers and the limited exciton diffusionlength. We also observed that with exposure duration extendedfrom 5min to 2 h, the device was still capable of lasing.

How the DNB molecules are removed from the polymer layerhas a strong impact on the recovery dynamics. When the DNBmolecules are removed from the chamber under a vacuum of10�4mbar, the PFO laser can recover fully within 20 s. Flowingnitrogen gas through the chamber makes the output recover towithin 1/e of the original value in 32 s and recover fully in 200 s,which is comparable induration to theexposureprocess.When the

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85

90

95

100

effic

ienc

y / %

Q=100%;z=49 nm Q=85%;z=71 nm

PFO laser was left in air, it took 3.5 h to recover to 97% of theoriginal state with 3% loss due to the effect of oxidation andphotodegradation. The potential for such a fast response andrecovery time makes PFO lasers promising chemical sensors inpractical applications with high sensitivities.

ER

0 50 100 150 200 250 300

70

75

80

fluor

esce

nce

film thickness d / nm

Figure 5. Simulation of DNB penetration in the PFO film. The experimen-

tal PL sensing efficiencies were measured for films of different thicknesses.

The lines show two theoretical fits to the experiment one assuming

complete quenching (Q¼ 100%, z¼ 49 nm) and one assuming partial

quenching (Q¼ 85%, z¼ 71 nm).

3. Theoretical Simulation

Tounderstandbetter how theDNBmolecules affect lasing,wenextdeveloped a simple model to determine how far the DNBpenetrates into the PFO film. We assume that the molecules haveuniformdensity up to amaximumpenetrationdepth from thefilmsurface, a profile similar to case II diffusion[29] and to neutronscatteringmeasurements of nitroaromatic diffusion in dendrimerfilms.[30] Case II diffusion has previously been observed in thesolvent ingress process in polymers, for instance the transporta-tion of methanol mixed with Rhodmine 6G into poly(methylmethacrylate) (PMMA).[31–33] In our model, the polymer film isdivided into two regions by the penetration depth of the DNBmolecules as shown in Figure 4. In the region without DNBmolecules, the pristine PFOhas a PLQYof 55%. In the regionwithDNB, we assume the DNB will introduce an additional non-radiative decay mechanism, leading to a quenching of theemission by a factor of Q. Therefore, the fluorescence from thepre-exposed and exposed film can be described by Equations (1)and (2), respectively.

Ibefore ¼ AP 1� expð�adÞ½ � (1)

Iafter ¼ AP 1�Qð Þ 1� expð�azÞ½ �

þ AP expð�azÞ � expð�adÞ½ � (2)

In the equations, Ibefore is the emission intensity beforeexposure and Iafter theemission intensity after 5-minexposure.A isa constant and P is the pump beam density. a is the absorptioncoefficient at the pump wavelength, d is the thickness of the PFOfilm, and z is the penetration depth of the DNB molecules in thePFOfilm. Equations (1) and (2) canbeused to calculate the sensingefficiency h:

h ¼ 1�IafterIbefore

¼ Q1� expð�azÞ1� expð�adÞ (3)

The sensing efficiencies h were measured experimentally for aseries of thin films on flat silica substrates with film thickness

z

d

0x

Figure 4. Theoretical model of the penetration of DNB molecules in the

PFO film. The penetration depth of the DNB molecules is z and the

thickness of the PFO film is d. On the right is a sketch of the exciton density

through the film.

Adv. Funct. Mater. 2010, 20, 2093–2097 � 2010 WILEY-VCH Verl

ranging from 30 to 300 nm. In order to compare to the laserproperties in Figure 1, the exposure duration was chosen to be5min. The fitting parameters are the quenching efficiency Q andthe penetration depth of the DNB molecules z. For films ofdifferent film thickness, the penetration depth z should beidentical, or it should be equal to the film thickness if the film isthinner than the penetration depth z obtained for the thickestfilms.

Figure 5 shows the dependence of the fluorescence sensingefficiency on film thicknesses ranging from 270 to 30 nm. Thesquares show the experimental data and the dotted curves are thetheoretical fits assuming Q¼ 100% and Q¼ 85%. In Figure 5,the experimental data shows that the sensing efficiency increasesas the PFO film thickness is reduced to a maximum value of 85%for a 30-nm-thick film. Since all films were exposed for the sameduration, the penetration depth of the DNB molecules should bethe same. However for thin films a larger fraction of the polymerinteracts with the DNB quenchers than for thick films.Consequently, thin films give a larger fractional change in theoutput emission after exposure to DNB vapors. We alsomeasuredthe sensingefficiency for even thinnerfilms, butobservedadrop inthe fluorescence sensing efficiency (to only 53% for a 15-nm-thickfilm). Such thin films of PFO exhibit a significantly shorterfluorescence lifetime thanfilmsof�100-nmthickness,[34]which islikely to cause a reduced sensitivity to the addition of DNBquenchers.

Comparing the data with theory, if we assume 100% quenchingwe calculate an optimized penetration depth of 48 nm, and thecurve fits quite well for films thicker than 70 nm. However, the fitdiverges from the experimental data for thinner films, because100% quenching means that the sensing efficiency should reach100% for films thinner than 49 nm. When partial quenching isassumed, we find an optimum fit for all film thicknesses forQ¼ 85%anda71-nmDNBpenetrationdepth. Thismeans that thePFO films still give 15% of their original emission in the presenceof the DNB quenchers. One possible reason for partial quenchingis that excitons created outside the exciton diffusion range from aDNB molecule cannot be quenched.

The simple model can also be used to predict the thresholdchange in thePFO lasers. The threshold of the laserRth is inverselyproportional to the confinement factor G, defined as the overlap

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between the optical transverse mode of the waveguide and thepopulation inversion distribution in the following equation.[35,36]

1

Rth� G ¼

Rdx¼0

E2ðxÞ expð�2apxÞdxRE2ðxÞdx (4)

E(x) is the waveguided transverse field amplitude profile, ap isthe absorption coefficient at the pump wavelength, and d is thethickness of the film. The presence of theDNBmolecules changesthe distribution of the excitons by introducing quenchers and thushas an impact on the laser threshold, shown by the modifiedconfinement factor:

G ¼

Zz

x¼0

E2ðxÞ expð�2apxÞdx þ ð1�QÞZd

z

E2ðxÞ expð�2apxÞdx

R1

E2ðxÞdx

(5)

From the simulation, the quenched region is near the surfacewith depth z¼ 71 nm and the quenching efficiencyQ of 85%. Thefirst and the second terms in the numerator show the confinementfactor in the unaffected and affected part respectively. For a simpleapproximation, the substrate was treated as flat without corruga-tions. The fractional threshold change predicted from theconfinement factor is 2.1, in comparison to the change of 1.8obtained from experiments in Figure 1.

4. Conclusion

We have demonstrated that the widely available polymerpolyfluorene is an excellent sensor for low vapor pressureexplosive detection of DNB analyte at 9.8 ppb. The convenientDFB laser geometry is shown to give a larger response to the DNBmolecules than the change in fluorescence from thin films of thesame thickness. This is because of the inherent increase in thedirectionality of light emission when the DFB laser is pumpedabove threshold. The sensing efficiency in the DFB laser was ashigh as the optimized sensing efficiency by fluorescence in verythin PFO films. By reducing the thickness of the PFO layer in thelaser, a further enhancement of its detection sensitivity is expected.Furthermore, the reduction in the laser slope efficiency by a factorof 3 gives a robust and sensitive approach to detecting explosiveanalytes. The output dynamics of the exposure and recovery cyclesweremeasured. The laser outputwas quenched to 60%of its initialvalue in 46 s and recovered to within 1/e of its initial value in 32 sunder the flow of clean nitrogen. Using a simple model of thepenetration of the DNBmolecules into the PFO layer, we find thatDNB molecules can quench 85% of the original emission with apenetration depth of 71 nm for a 5-min exposure. The simulationresults also successfully predicted the fractional change in the laserthreshold. Overall, a fast and sensitive interaction betweenpolyfluorene lasers and explosive vapors shows that organic lasersare promising for chemical sensing.

� 2010 WILEY-VCH Verlag GmbH & C

5. Experimental

PFO (AmericanDye Source Inc.) was used as the laser gainmedium. A 257-nm-thick film was spin coated from chloroform (25mgmL�1) onto a 1Dgrating in a silica substrate to form a distributed feedback (DFB) laser. Theperiod of the DFB resonator was chosen to be 268 nm to give feedback (viasecond order diffraction) and surface output coupling (via first orderdiffraction) near 450 nm, the amplified spontaneous emission (ASE) peakof the PFO. The explosive analyte we examined was DNB, which is similarto TNT consisting of nitroaromatics rings, being strongly electronegative,and having low equilibrium vapor pressure of 30 ppb at 25 8C. The vaporpressure used in the exposure is 9.8 ppb and not limited by the detectioncapability of the polyfluorene laser.

For the polyfluorene lasers, a nitrogen laser running at 10Hzwith 500 psoutput pulse at 337 nm was used as the pump source. A series of neutraldensity filters were used to vary the excitation energies. For florescencesensing experiments, the polyfuorene thin films were excited by aPicoQuant laser diode at 390 nm at 80 MHz. The surface emission fromboth laser and plan films was collected through a fiber-coupled charge-coupled device (CCD) spectrograph.

To perform an exposure, the polymer laser or thin films were placedinside a chamber that was connected to a vacuum pump, DNB vaporsupply, and exhaust through three different ports. First, the chamber waspumped to 10�4mbar to remove atmospheric oxygen. The port connectedto the vacuumpumpwas then switched off with the other two ports open tolet the DNB vapors enter into the chamber under a flow of clean nitrogengas. The exposure time was 5min. The fiber-coupled CCD spectrographmonitored the dynamics of the surface emission in steps of between 2 and45 s. For PFO lasers, the change of the laser thresholds and slopeefficiencies after exposure for 5min was also calculated.

The pressure of the DNB vapors used is determined from thefluorescence quenching of the PFO thin films indicated in the followingequation [27]:

FQ � VP expð�DGÞ2h i

Kb (6)

FQ is the fluorescence quenching per unit time, VP is the vapor pressureof the analytes,DG is the exergonicity energy, and Kb is the binding strength

between the analyte molecules and the polymer. For the polymer films ofidentical thickness and exposed to the same analyte for the same amountof time, FQ is proportional to the analyte vapor pressure. To determine theDNB vapor pressure used in the exposure, we compared the fluorescencequenching of the thin film exposed under experimental condition as statedabove with that exposed under pre-filled equilibrium chamber, whichshowed that the fluorescence quenching under flowing DNB vapors in N2

is 3.3 times smaller than that under equilibrium condition, meaning thatthe pressure of the DNB vapor is 9.8 ppb.

To monitor recovery of the luminescence, the ports connected to theDNB vapor and the exhaust were switched off to cut the DNB supply. Threedifferent recovery environments were tested: using vacuum, flowing cleannitrogen gas across the sample, and immersion in clean air. When thepolymer was recovered in vacuum, the port connected to thevacuum pump was turned on to extract DNB molecules from thechamber. In the second case, an extra vacuum chamber port was used toallow clean nitrogen gas to flow into the chamber with a similar flow rate tothat used in the exposure process. For recovery in air, the whole chamberwas removed and the film was left in air unattended.

Acknowledgements

We are grateful to the financial support from the UK Engineering andPhysical Sciences Research Council and the Scottish Universities PhysicsAlliance. We also thank Professor W. L. Barnes of the University of Exeterfor supplying the corrugated silica substrates.

Received: October 8, 2009

Published online: May 25, 2010

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LLPAPER

[1] N. Tessler, G. J. Denton, R. H. Friend, Nature 1996, 382, 695.

[2] F. Hide, M. A. DiazGarcia, B. J. Schwartz, M. R. Andersson, Q. B. Pei,

A. J. Heeger, Science 1996, 273, 1833.

[3] N. Tessler, Adv. Mater. 1999, 11, 363.

[4] M. D. McGehee, A. J. Heeger, Adv. Mater. 2000, 12, 1655.

[5] I. D. W. Samuel, G. A. Turnbull, Chem. Rev. 2007, 107, 1272.

[6] S. Riechel, C. Kallinger, U. Lemmer, J. Feldmann, A. Gombert, V. Wittwer, U.

Scherf, Appl. Phys. Lett. 2000, 77, 2310.

[7] J. H. Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks, K. Mackay,

R. H. Friend, P. L. Burns, A. B. Holmes, Nature 1990, 347, 539.

[8] S. Gunes, H. Neugebauer, N. S. Sariciftci, Chem. Rev. 2007, 107, 1324.

[9] N. S. Sariciftci, L. Smilowitz, A. J. Heeger, F. Wudl, Science 1992, 258, 1474.

[10] D. Moses, Appl. Phys. Lett. 1992, 60, 3215.

[11] H. Yamamoto, H. Kasajima, W. Yokoyama, H. Sasabe, C. Adachi, Appl.

Phys. Lett. 2005, 86, 083502.

[12] T. Matsushima, H. Sasabe, C. Adachi, Appl. Phys. Lett. 2006, 88, 033508.

[13] P. Gorrn, T. Rabe, T. Riedl, W. Kowalsky, F. Galbrecht, U. Scherf, Appl. Phys.

Lett. 2006, 89, 161113.

[14] M. Stroisch, T. Woggon, U. Lemmer, G. Bastian, G. Violakis, S. Pissadakis,

Opt. Express 2007, 15, 3968.

[15] P. Gorrn, T. Rabe, T. Riedl, W. Kowalsky, Appl. Phys. Lett. 2007, 91, 041113.

[16] B. K. Yap, R. Xia, M. Campoy-Quiles, P. N. Stavrinou, D. D. C. Bradley, Nat

Mater, 2008, 7, 376.

[17] C. Gartner, C. Karnutsch, U. Lemmer, J. Appl. Phys. 2007, 101, 023107.

[18] Y. Oki, S. Miyamoto, M. Maeda, N. J. Vasa, Opt. Lett. 2002, 27, 1220.

[19] H. W. Wong, K. S. Chan, E. Y. B. Pun, Appl. Phys. Lett. 2005, 87, 011103.

Adv. Funct. Mater. 2010, 20, 2093–2097 � 2010 WILEY-VCH Verl

[20] S. Perissinotto, G. Lanzani, M. Zavelani-Rossi, M. Salerno, G. Gigli, Appl.

Phys. Lett. 2007, 91, 191108.

[21] D. Amarasinghe, A. Ruseckas, A. E. Vasdekis, G. A. Turnbull,

I. D. W. Samuel, Appl. Phys. Lett. 2008, 92, 083305.

[22] D. T. McQuade, A. E. Pullen, T. M. Swager, Chem. Rev. 2000, 100, 2537.

[23] A. Rose, Z. Zhu, C. F. Madigan, T. M. Swager, V. Bulovic, Nature 2005, 434,

876.

[24] R. Xia, G. Heliotis, Y. Hou, D. D. C. Bradley, Org. Electron. 2003, 4,

165.

[25] G. Heliotis, R. Xia, G. A. Turnbull, P. Andrew, W. L. Barnes, I. D. W. Samuel,

D. D. C. Bradley, Adv. Funct. Mater. 2004, 14, 91.

[26] S. J. Toal, W. C. Trogler, J. Mater. Chem. 2006, 16, 2871.

[27] J. S. Yang, T. M. Swager, J. Am. Chem. Soc. 1998, 120, 11864.

[28] S. Richardson, H. S. Barcena, G. A. Turnbull, P. L. Burn, I. D. W. Samuel,

Appl. Phys. Lett. 2009, 95, 063305.

[29] M. Sanopoulou, D. F. Stamatialis, J. H. Petropoulos,Macromolecules 2002,

35, 1012.

[30] H. Cavaye, A. R. G. Smith, M. James, A. Nelson, P. L. Burn, I. R. Gentle,

S. Lo, P. Meredith, Langmuir, 2009, 25, 12800.

[31] C. Ramos, P. J. Dagdigian, Appl. Opt. 2007, 46, 620.

[32] M. C. J. Large, S. Ponrathnam, A. Argyros, N. D. Pujari, F. Cox, Opt. Express

2004, 12, 1966.

[33] P. A. Drake, P. W. Bohn, Appl. Spectrosc. 1996, 50, 1023.

[34] P. E. Shaw, Ph.D. Thesis, University of St Andrews, UK 2009.

[35] O. Svelto, Principles of Lasers, 4th ed., Plenum Press, New York 1998,

Appendix E.

[36] G. F. Barlow, K. A. Shore, G. A. Turnbull, I. D. W. Samuel, J. Opt. Soc. Am. B

2004, 21, 2142.

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