A Poly(vinyl alcohol)Carbon-Black Composite Film

Experimental

A detailed procedure for growing LC templated PEDOT films on ITO is given elsewhere [10].Absorption spectra were collected on a Cary 500UV/VIS/NIR spectrometer in double beam mode with a lin-early polarizing filter (Edmund Optics A43-785)mounted on a rotat-ing goniometer in the sample beam.Spectra were normalized to the baseline absorption of the filter and the ITO substrate.Substrates for four-probe conductivity measurements were photolithographically patterned using Clariant AZ1518photoresists and the exposed ITO was removed with HCl.Patterned substrates were cleaned and poly-merized as above.After washing and removal of the LC template,PEDOT films were cast with a 20wt.-%solution of a thermosetting poly(amide-imide)(42827-2,Aldrich)in 1-methyl-2-pyrrolidinone (NMP),heated to 150 C for 2h;the resulting film was lifted off,transferring the PEDOT film from ITO to the insulating polyamide substrate.For multi-domain film conductivity measurements,Au elec-trodes were e-beam evaporated through a shadow mask across narrow strips of PEDOT.For single domain measurements,probes were con-tacted directly to the PEDOT film (Fig.3a).A Keithley 2400source meter was used to measure I±V curves for LC templated and non-templated PEDOT,and both exhibited ohmic behavior.Uncertainty values given are a 95%confidence interval for the mean based on ten samples.Film thickness was measured with a Tencor P10profilometer.30nm thick PEDOT films for XPS were polymerized on Au coated Si substrates and studied using an Omicron ESCA Probe XPS with an Al K a source operating at 14kV .Binding energies were calibrated using the Au 4f peak (84.0eV).After background subtraction,relative atomic composition was determined by integrating the C 1s (286.0eV),S 2p (163.7eV)and Cl 2p (200.4eV)peaks and multiplying by instru-ment sensitivity factors determined for C,S,and Cl from known stan-dards.

Received:October 3,2003

Final version:December 27,2003

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A Poly(vinyl alcohol)/Carbon-Black Composite Film:A Platform for Biological Macromolecule Incorporation**

By Lawrence L.Brott ,Sophie M.Rozenzhak ,

Rajesh R.Naik ,Shawn R.Davidson ,Ronald E.Perrin ,

and MorleyO.Stone *

Protein responses to stimuli can be extremely difficult to detect due to the fact that the secondary and tertiary structure conformational changes tend to be minute,often on an ang-strom scale.Therefore,a challenge exists with respect to mea-suring and real-time coupling of these molecular-scale changes into a format that can be easily integrated with exist-ing device architectures.Most studies of biomolecules focus on amplifying structural changes through light diffraction,[1]changes in light polarization,[2]changes in fluorescence,[2]or through flexing of microsensor cantilevers.[3]One limit to these techniques is that they require a stable environment

C O M M U N I C A T I O N S

DOI:10.1002/adma.200305641

Adv.Mater.2004,16,No.7,April 5

[*]Dr.M.O.Stone,Dr.L.L.Brott,S.M.Rozenzhak,Dr.R.R.Naik,

S.R.Davidson,R.E.Perrin

Materials and Manufacturing Directorate Air Force Research Laboratory 3005Hobson Way

Wright-Patterson Air Force Base,OH 45433-7702(USA)E-mail:morley.stone@https://www.360docs.net/doc/468391073.html,

[**]This research was supported by the Air Force Office of Scientific

Research (AFOSR)and by the Defense Advanced Research Projects Agency (DARPA).

with large supporting equipment,all of which do not promote a mobile platform.Here we describe the development of a technique that integrates a biological sensing agent with tradi-tional and compact synthetic signal processing to yield a prac-tical and functional device.To achieve this goal,it is necessary to incorporate this biomolecule into solid-state technology while also preserving its bioactivity.The fabrication of a bio-mimetic solid-state thermal sensor described herein is based on the thermosensitive protein TlpA.[2]The sensitivity of this proteinaceous sensor rivals that of current uncooled inorganic sensors,yet operates under ambient atmosphere and tempera-ture.The detector is fabricated by incorporating the protein into a highly plasticized polymer matrix doped with a conduc-tive carbon black,so that any change in the protein's confor-mation is immediately translated as a change in the polymer's electrical resistance.

The TlpA gene of Salmonella encodes an autoregulatory repressor protein that makes use of the coiled-coil motif to sense temperature changes.[4]The molecular basis for thermo-sensing in TlpA is the dynamic coiled-coil to monomer struc-tural transition that is directly coupled to differences in tem-perature.[2,4]At low temperatures,TlpA assumes a coiled-coil formation that is capable of binding to sequence-specific DNA.However,warmer temperatures promote unfolding of TlpA,and these random-coil monomers are unable to remain bound to DNA.[5]It has been demonstrated that this tempera-ture-dependent folding/unfolding is rapid and reversible.[2,4] TlpA represents a novel class of molecules that has adapted the coiled-coil motif to function,coupling its transcriptional activity with protein folding in response to temperature cues. Recent work with the Drosophila protein Engrailed indicated that thermally induced structural refolding kinetics were occurring on a microsecond timescale(18l s).[6]Due to the structural and functional similarity between Engrailed and TlpA,these proteins could represent a novel class of biomac-romolecules adapted to perform rapid thermal sensing.A truncated version of the TlpA protein(43kDa),TlpA8 (12kDa),was used in this research,since it retains the ther-mal-sensing properties of the native protein while benefiting from higher solubility.[7]

While the folding behavior of the TlpA8protein can be monitored either by CD spectroscopy or fluorescence,[2]there was a need to develop a reliable technique that monitors the protein's behavior without large supporting equipment with an ultimate goal of fabricating a protein-based thermal-imag-ing array.Nature has an extensive list of thermal detectors, from bacteria[4]to beetles[8]and snakes,[9]that outperform commercial devices yet operate under physiological and often ambient conditions.[10]Therefore it was reasoned that by using a biomimetic approach,several problems of current inorgan-ic-based thermal sensors could be avoided,such as a need for cryogenic temperatures and expensive fabrication processes. To achieve this goal,it was proposed that the thermoprotein TlpA8be added to a highly plasticized polymer film doped with electrically conductive carbon-black particles.As the protein is heated and its structural conformation changes,the carbon-black particles would be redistributed,thereby chang-

ing the electrical resistance of the device.By using a large per-centage of plasticizer,the protein would still have the freedom

to unfold and refold while the polymer matrix would provide enough structural support for solid-state device development.

Some additional criteria of this system would be that the poly-

mer and plasticizer both be water soluble and that the final polymer film would have a low glass transition(T g)tempera-

ture.Poly(vinyl alcohol)(PVA)was used because of its solu-

bility in water and the ease in which films could be cast from

an aqueous solution,whereas poly(ethylene glycol)(PEG)

was used as a plasticizer because of its flexibility and size,

which would prevent it from migrating to the surface of the polymer film.[11,12]The conductive carbon black would serve

two purposes:it would provide the necessary electrical con-ductivity to the polymer film while also acting as a thermal absorber.In order to maximize the sensitivity,the amount of

carbon black added to the polymer would have to be carefully controlled to fall within the percolation region of the system.

In summary,we have prepared a thermal-imaging array based

upon polymer,protein,carbon black,and plasticizer.

The suspension of carbon black in polymers as a way to cre-

ate sensors has been used extensively in electronic noses where organic vapors swell polymer/carbon-black films,affecting

their conductivity.[13,14]Response time for these devices is im-

proved through the addition of a plasticizer,which increases

the diffusivity of the solvent vapor through the sample.[15]

While there are similarities between the chemical and thermal detectors,the nature of the response is inherently different.

The chemical sensor's signal is driven primarily by a solvent's diffusivity through a solid,whereas the thermal sensor's re-

sponse is driven by thermal radiation being focused onto the sensor.This difference leads to drastic differences in reaction

times with the thermal sensor having a response time of milli-seconds and the chemical sensor having a responsiveness of minutes.Other similar devices are self-resetting circuit break-

ers,which rely on a polymer/carbon-black wire to expand as ex-

cess current flows through it,thereby disrupting the percolation pathways of the conductive material and ultimately opening

the circuit.[16]These circuit breakers require large temperature differences to activate and are relatively slow,and therefore

are not optimized to detect the minute temperature fluctua-

tions necessary in thermal imaging.However,the one feature

that makes the research described below unique is the reliance

on a biomolecule to enhance the sensing properties of a device.

A typical polymer solution is made by first dispersing the

carbon black in a detergent/acetone solution.[17]This suspen-

sion is then added dropwise to a heated aqueous PVA solu-

tion and the acetone is removed.The solution is cooled and a

PEG solution containing TlpA8is added and stirred until it appears uniform.Because the protein is being further diluted

by an aqueous-based polymer solution,there is reason to

believe that the protein will be homogeneously dispersed throughout the polymer network.Films are made by applying

a250l m coating onto a circuit board with electrodes,and

then drying at40 C for2h(see Fig.1a).When fabricating an

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array of detectors,individual pixels are cut using laser https://www.360docs.net/doc/468391073.html,ser micromachining is a technique in which the polymer film can be quickly cut to linewidths of 15l m using a UV laser with little damage to the underlying substrate.[18]A completed 8 8array (25mm 2)is shown in Figure 1b.If desired,a flexible protective UV acrylate overcoat can then be applied to shield the chip from fluctuations in humidity.Chips were tested under ambient temperature and pressure with the thermal signal being focused onto the chip using a germanium lens.The chips were first allowed to establish a stable baseline and subsequently heated for 10s for a change in temperature of 0.3±0.4 C at the surface of the sensor and then allowed to recover.Three types of chips were tested:a control without any protein,one with TlpA8,and a third with collagen,a protein with a similar structure to TlpA,without its refolding characteristics (see Fig.2a).One of the most striking features that uniformly occurs in all three samples is the way the resistance of the films decreases when heated,a trait that is counter-intuitive,though not uncommon.This negative temperature coefficient (NTC)is seen in low T g sam-ples and indicates a redistribution and re-agglomeration of carbon-black particles that enhances conductivity as the sample is heated.[19,20]If desired,crosslinking the sample will eliminate the NTC.[21,22]However,when a crosslinked control formulation was tested,the signal was completely quenched.Additionally,the size of the change in resistance is remark-

able,even without the benefit of the protein.While the maxi-mum wavelength response is still being explored,the trans-mission of the germanium lens suggests that it would be between 7±11l m (see Fig.2b).However,a conservative esti-mate of the sensitivity of the TlpA8/polymer film would sug-gest that at room temperature,the film has a d X /d T of at least 15%,as compared to vanadium oxide,which has a d X /d T of ~3%.[23]Furthermore,the addition of TlpA8enhances the signal sensitivity,whereas the addition of collagen does not.This enhancement improves linearly as additional thermo-protein is added to the formulation (data not shown).How-ever,a further 3%increase in d X /d T through the addition of 4.3wt.-%TlpA8is significant,which highlights the sensitivity and potential for this technique.From a purely materials per-spective,TlpA8can be thought of as an enhanced plasticizer.When compared to the plasticizer PEG,which becomes more energetic and increases its free volume when heated,TlpA8does so in a much more exaggerated way,as indicated in pre-vious experiments analyzing changes in protein secondary structure as a function of temperature.[2]This unfolding would undoubtedly greatly increase the free volume and conse-quently enhance the redistribution of the carbon-black parti-cles.

Another notable feature of Figure 2a is the recovery of the signal to its original baseline after the stimulus is removed.Both proteins enhance the recovery and their baselines return to their original values,unlike the control.The control is rely-ing only on the entanglement of the PVA matrix to return the film to its original configuration;however,the protein is more forceful in its memory of its desired conformation and is therefore more active in its refolding.TlpA and TlpA8are truly unique in their recovery properties.Rapid,fully revers-ible recovery has been observed with TlpA,even after numer-ous heating/cooling cycles.[2]It would appear that this remark-able re-naturation property has been captured within the polymer network of this device.Future improvements may take advantage of this behavior by directly coupling TlpA to the polymer matrix in order to further enhance the recovery properties,though indiscriminate crosslinking also has the potential to destroy the protein's activity (unpublished obser-vations).Furthermore,the TlpA8protein also has a pro-nounced effect on the rate of recovery.This sample requires 20.5s (±2.8s,n =3)to recover 85%of its original baseline,whereas the collagen and control samples need 24.9s (two of the three samples did not ever fully recover)and 31.2s (±14.5s),respectively.Finally,if one were to define system noise based on the standard deviation of the baseline signal,it is notable that the TlpA8chip exhibits the least noise when compared to collagen and the control (0.02%±0.01,0.15%±0.07,and 0.06%±0.01variation,respectively).

While research continues to optimize the conditions of incorporating additional amounts of protein into the polymer films,it has become apparent that this material is ideal for a lightweight uncooled thermal-imaging device.One of the larg-est impediments for inorganic IR imagers is their need for either cryogenic temperatures or,at best,thermally stabilized

C O M M U N I C A T I O N

Figure 1.a)Single sensors;the chip on the right is protected with a UV-curable poly(butadiene)±dimethacrylate overcoat.b)A 25mm 28 8pro-tein-based polymer array.

temperatures.The great advantage of the TlpA8/PVA system

is that it requires no special treatment;six-month old films are

nearly as sensitive as when they were first fabricated even without special handling.Additionally,these chips are just as effective at moderately elevated tem-peratures (￡30 C)as they are at room temperature (~21 C).Because of these qualities,a prototype 8 8imaging ar-ray was fabricated using the protein/polymer formulation with the pixels de-fined through laser micromachining.By converting the signals into a gray-scale image,the heat from a battery-operated flashlight can be tracked over the array from over 6m away and can distinguish between three separate heat sources (see Fig.3).

What began as a project to monitor the conformational changes of a ther-mal protein grew into research aimed at

creating a biomimetic material for ther-mal imaging.Perhaps the greatest promise of this research is its flexibility;while it is difficult to alter the funda-mental properties of inorganic sensors,organic-based materials are easily modi-fied and conformal in nature.Likewise,large and uniform arrays are inexpen-sive to fabricate,opening the possibility of mass-produced,disposable thermal detectors.Because this sensor is poly-mer-based,lightweight,and conformal,we envision miniaturized detectors that can be integrated into clothing,distrib-uted adisposableosensor networks,medical diagnostics,and anywhere weight and cost might be an issue.

Experimental

Carbon black (0.600g,Cabot XC72)was suspended in acetone (15mL)and detergent

(54.0l L,Triton X-100)by sonicating for

10min in a bath sonicator and an additional 10min with a probe sonicator.The carbon-black suspension was slowly added dropwise to a hot aqueous poly(vinyl alcohol)(3.750g,98±99%hydrolyzed,M w 85000±146000,Al-drich,in 25mL distilled water)solution so that the acetone flash-evaporated.Once the

acetone was completely removed,the mix-ture was cooled to room temperature.

Poly(ethylene glycol)(15.0l L,M n 200,

Sigma)was stirred into an aqueous solution

of purified protein (167.0l L of either 20mg mL ±1TlpA8or 46mg mL ±1collagen

(type III from calf skin,Sigma),which was then added to 0.333g of the polymer/carbon-black mixture and mixed until homogeneous.

A 250l m film of the polymer/protein solution was applied to an electrode (Surfboard 6004,Capital Advanced Technologies)using a drawdown bar.The chips were dried in a 40 C gel dryer before being

trimmed to size.The final dry film thickness was ~35l m and has a surface resistivity of roughly 1G X /square.The chip was placed at the

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0.89

0.900.910.920.930.940.950.960.970.980.991.001.010

Time (seconds)

R / R o

Germanium Lens Transmission

020

6080

0510152025Micrometers % T r a n s m i s s i o n

Figure 2.a)Representative baseline-corrected,normalized heating curves for control,collagen-,and TlpA8-containing

chips.b)Infrared

transmission of the germanium lens.b)

a)Figure 3.a)Schematic of the 8 8array detecting heat from three independent heat sources,and b)a screen capture from an 8 8detector sensing the three heat sources.

focal point of a germanium lens which directed thermal energy from a heat source 30cm away onto the circuit board while also filtering out extraneous visible light.A multimeter (Keithley,model 2700)was used to record the changes in resistance of the films.

Film percentages by weight:PVA (60.0%),PEG (24.3%),XC72(10.0%),TlpA8(4.3%),and X-100(1.4%).

Received:July 3,2003

Final version:January 16,2004

[1]R.R.Naik,L.L.Brott,S.M.Kirkpatrick,M.O.Stone,Proc.

SPIEDInt.Soc.Opt.Eng.2001,4590,115.

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2001,16,1051.

[3]R.McKendry,J.Zhang,Y.Arntz,T.Strunz,M.Hegner,H.P .Lang,

M.K.Baller,C.Ulrich,E.Meyer,H.J.Guntherodt,C.Gerber,Proc.Natl.Acad.Sci .USA 2002,99,9783.

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7408.

[8]D.X.Hammer,J.Seigert,M.O.Stone,H.G.Rylander I I I

,A.J.

Welch,J.Insect Physiol.2001,47,1441.

[9]N.Fuchigami,J.Hazel,V .V .Gorbunov,M.Stone,M.Grace,V .V .

Tsukruk,Biomacromolecules 2001,2,757.

[10]A.L.Campbell,R.R.Naik,L.Sowards,M.O.Stone,Micron 2002,

33,211.

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49,989.

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Large-Area Electric-Field-Induced Colloidal Single Crystals for Photonic Applications**

By Anand Yethiraj ,*Job H.J.Thijssen ,Alan Wouterse ,and Alfons van Blaaderen

Materials with a periodic modulation of refractive index on the (sub)-micrometer scale interact strongly with light and can exhibit a photonic bandgap,the optical analogue of the electronic bandgap in semiconductors.[1]Colloidal suspensions of monodisperse micro-spheres that self-organize,analogously to atomic crystals,into periodic structures with the lowest free energy,are promising as three-dimensional photonic materi-als.[2±6]However,growing large single-domain colloidal crys-tals without an overlying fluid layer is difficult.We present a technique to grow millimeter-scale (3mm 0.5mm)electric-field-induced colloidal single crystals,and a polymerization process that immobilizes them,allowing drying and reversal of the refractive-index contrast.A 70V mm ±1(rms)electric field switches the crystal structure from close-packed to body-centered tetragonal (bct).Lower values increase the area of single-domain close-packed crystals and the preference for face-centered cubic (fcc)packing over hexagonal close-packed (hcp).Intermediate fields produced mixed crystals with a lower fcc layer and a connected upper bct layer.

Most photonic applications require periodic structures with a low filling fraction of the high dielectric-constant compo-nent.This can be achieved by preparing wet colloidal crystals with a high particle volume fraction and ainverting the con-trastoby drying the crystal,re-infiltrating it with a high-index material,and ultimately removing the solid spheres by etching or burning.[2]High-volume fraction colloidal crystals can be made by allowing colloids in suspension to sediment in gravity and densify.When colloids interact with each other as hard spheres or as slightly charged spheres (with a hard core plus a repulsive inter-particle interaction)the resulting equilibrium structure is an fcc crystal.However,for hard spheres,the

C O M M U N I C A T I O N

DOI:10.1002/adma.200306192

Adv.Mater.2004,16,No.7,April 5

______________________

[*]Dr.A.Yethiraj,[+]J.H.J.Thijssen,A.Wouterse,

Prof.A.van Blaaderen

FOM Institute for Atomic and Molecular Physics

Kruislaan 407,1098SJ Amsterdam (The Netherlands)and Soft Condensed Matter,Debye Institute,Utrecht University Princetonplein 5,NL-3584CC Utrecht (The Netherlands)E-mail:yethiraj@chem.ubc.ca;A.vanBlaaderen@phys.nu.nl [+]Present address:Department of Chemistry,2036Main Mall,

Vancouver,British Columbia,Canada V6T 1Z1,Canada.

[**]We acknowledge useful discussions with Alexander Moroz and

Benito Groh,Jacob Hoogenboom for synthesizing the silica spheres,Dirk Vossen for his help in obtaining the SEM images and C.J.Wisman for help with the electronics.This work is part of the research program of the aStichting voor Fundamenteel Onderzoek der Materie (FOM)o,which is financially supported by the aNeder-landse organisatie voor Wetenschappelijke Onderzoek (NWO)o.Supporting Information is available online from Wiley Interscience or from the authors.