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Sensors and Actuators B 181 (2013) 92– 98

Contents lists available at SciVerse ScienceDirect

Sensors and Actuators B: Chemical

journa l h o me pa ge: www.elsev ier .com/ locate /snb

tacked graphene nanoplatelet paper sensor for protein detection

aryn L. Heldta,∗, Amy K. Sieloff a, Joshua P. Merillata, Adrienne R. Minericka,ulia A. Kinga, Warren F. Pergerb, Hiroyuki Fukushimac, Jeffri Narendrac

Department of Chemical Engineering, Michigan Technological University, 1400 Townsend Drive, Houghton, MI, USADepartment of Electrical and Computer Engineering, Michigan Technological University, 1400 Townsend Drive, Houghton, MI, USAXG Sciences, Inc., 815 Terminal Road, Lansing, MI, USA

r t i c l e i n f o

rticle history:eceived 8 November 2012eceived in revised form 15 January 2013ccepted 18 January 2013vailable online 28 January 2013

eywords:arbonrotein

a b s t r a c t

Carbon-based sensors have shown great potential to revolutionize protein diagnostic tools, includingthe ability to detect pathogens and biomarkers. Many different microdevices have been fabricated usingcarbon nanotubes and graphene nanoplatelets. However, creating devices on the nanoscale can be dif-ficult and expensive. Stacked graphene nanoparticle composites (hereafter called graphene paper) are aconvenient and novel technology that has not been previously reported as a carbon-based sensor plat-form. Graphene paper is inexpensive and available on a macroscale, making device construction simple.Additional advantages include that paper composition and additives can be manipulated to increase thesensitivity and eventually the selectivity of proteins. We have created a microdevice sensor that detects

icrodeviceiosensorab-on-a-chip

proteins in solution by measuring the surface electrical resistivity of graphene/cellulose composite paperas a function of protein concentration. Four different proteins were tested for their ability to change thesurface resistivity of the graphene paper and there was a clear correlation between the molecular weightof the protein and the equilibrium dissociation constant calculated by fitting the protein adsorption datato the Langmuir isotherm model. This result implies that the dissociation constant is likely a function of

the size of the protein.

. Introduction

Sensor technology is developing rapidly toward smaller devicesith extremely sensitive detecting platforms. These sensors have

he potential to detect proteins and pathogens for quick and inex-ensive disease diagnose and/or biological responses in vivo or

n vitro [1]. Novel sensors could revolutionize the detection ofiruses and biomarkers for diseases like hepatitis [2], cancer [3]nd Alzheimer’s disease [4]. Carbon, in a multitude of conforma-ions, has recently been shown to be a sensitive platform for proteinbsorption, and therefore, could be applied to many medical appli-ations. Carbon nanotubes (CNTs) [5,6], carbon black [7,8], andraphene [9,10], can detect protein adsorption via the disruption ofhe electron-carrying crystalline network. However, there are dis-dvantages to each of these existing sensing platforms. CNTs are

xpensive and are produced with low yield. Carbon black does notave a layered carbon structure that is likely necessary for efficientlectron transfer and sensitive protein detection. Stacked graphene

Abbreviations: BSA, bovine serum albumin; LYS, chicken egg white lysozyme;IB, bovine fibrinogen; HEM, bovine hemoglobin; SR, electrical surface resistivity;NT, carbon nanotube; FET, field-effect transistor.∗ Corresponding author. Tel.: +1 906 487 1134; fax: +1 906 487 3213.

E-mail address: heldt@mtu.edu (C.L. Heldt).

925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.snb.2013.01.041

© 2013 Elsevier B.V. All rights reserved.

nanoplatelets are relatively easy to synthesize, but they can bedifficult to construct into microdevices due to their small size.

These nanoscale carbon sources have been engineered intocarbon-based sensors, which have been tested for their ability todetect an array of compounds. A standard electrode has been madefrom carbon black/polymer composites to measure the concentra-tion of organic compounds in air [11]. Both CNTs and grapheneelectrodes have been shown to detect human antibodies in solution[12]. Field-effect transistors (FETs) have demonstrated an abilityto detect proteins with either CNTs or graphene via electrical net-work disruption mechanisms [5,13,14]. FETs have been shown tobe sensitive to protein charge [15], as well as the pH [9] and flowrate [16] of a solution. Conductivity changes have also been deter-mined with an alternating current CNT FET [17]. A CNT sensormodified with aptamers detected the binding of protein biomark-ers using impedance spectroscopy [18], while direct adsorption ofions to layers of graphene detected biologically relevant ion con-centrations [19]. This demonstrates that the change in electricalproperties of carbon can be used to measure protein and organiccompound concentrations. Thus, innovations in nanoscale carbonsources for devices, as described in this manuscript, can be used to

measure solution parameters and improve human health-relateddiagnostics.

Graphene is a single layer of two-dimensional aromatic macro-molecules with an sp2 bonded network of carbon atoms. Graphene

nd Actuators B 181 (2013) 92– 98 93

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Table 1Protein properties.

Protein Molecular weight(kDa)

Isoelectric point Hydrophobicitya

(×103)

BSA 66 4.7b −4.2LYS 14 11.3c −4.6HEM 15 6.8d 5.5FIB 340 5.8e −5.6

a Hydrophobicity was calculated by assigning each amino acid a hydrophobic-ity value, as given by Eisenberg [37] with positive representing more hydrophobicand negative representing more hydrophilic amino acids. Hydrophobicity is theweighted average of the individual amino acid hydrophobicity.

b [42].

C.L. Heldt et al. / Sensors a

s the basic building block of natural and synthetic graphite. CNTsonsist of a single or multiple layers of graphene that are formedn the shape of a tube. Hence, CNTs and graphene have similarlectrical and chemical properties. Stacked graphene nanoplateletsroduced from natural or synthetic graphite are much more readilyvailable and less expensive than CNTs. The abundance of graphenever CNTs has made it a good replacement for CNTs, while main-aining the desired properties needed for a carbon-based proteinensor. When graphene and CNT sensors were directly compared,hey produced similar results [20].

Graphene can be difficult to work with due to its tendency toggregate in water [21]. This is due to the high hydrophobicity ofraphene. In order to increase the processability of graphene andncrease its ability to be a sensitive detection platform, grapheneybrid materials have been explored. A hybrid graphene and goldanoparticle electrode was developed to increase the sensitivity ofNA detection [22]. A hybrid graphene electrode that containedhitosan was able to detect glucose [23]. A chitosan/grapheneensor also demonstrated sensitivity and selectivity for sulfateeducing bacteria [24]. This demonstrates the power of hybridraphene materials and suggests the potential for other organicnd inorganic hybrid graphene materials for biosensing.

Graphene paper is a novel technology that supports stackedraphene nanoplatelets within a polymer sheet, creating a hybridaterial. It has not been previously reported as the sensing

lement in a carbon-based sensor. Graphene paper is inexpen-ive and easily tailored for porosity and surface chemistry. Largeheets can be manufactured and easily cut to size, thus facilitat-ng device construction. Graphene/cellulose paper has also shownreat promise in creating light weight, flexible, and mechanicallytrong supercapacitors [25]. The addition of 0.5 wt% graphene oxideo microcellulosic paper increased the mechanical strength of theaper [26] and decreased water and gas permeation in regener-ted cellulose films [27]. The unique properties of carbon-basedensors combined with the mechanical and electrical properties ofraphene/cellulose papers has provided an opportunity to create

unique sensor that is sensitive to changes in protein concen-ration, but is also mechanically stable, which is useful for future

icrodevice manufacturing.In this work, we have created a simple, two-terminal, hybrid

raphene paper sensor. By measuring the change in electricalurface resistivity (SR) (=1/surface electrical conductivity) of araphene paper sensor, a change in either protein concentration orolecular weight could be detected. This sensor is unique in that

t uses graphene paper, fabricated on the macroscale and manipu-ated on the microscale, facilitating microdevice sensor fabrication.

. Materials and methods

.1. Materials

Three different graphene paper composites from XG SciencesLansing, MI) were originally screened. Each paper containedGnP® graphene nanoplatelets [28]. xGnP® samples were madey heating graphite intercalated compounds (GICs). The GICs wereabricated by intercalating mixtures of acids into natural graphiteakes followed by thermal processing. After treatment, theseraphite flakes showed significant expansion due to the vaporiza-ion of the intercalated acids in the graphite galleries. The expandedraphite flakes were then pulverized to a specific size. The averagehickness and size of the xGnP® was controlled by changing the

rocess conditions. The xGnP® samples used in this research hadn average thickness of 5–10 nm and an average size of 25–50 �m.he carbon content of xGnP® was 95–96% while the oxygen contentas 4–5%.

c [43].d [44].e [45].

The first paper composite consisted of an xGnP®/cellulose blend.The cellulose was copy paper consisting of 60–65 wt% cellulose,30–35 wt% CaCO3 and the remaining parts were chemical binders.This paper was about 150 �m thick with a porosity of 40%. Thesecond paper composite was approximately 250 �m thick with aporosity of 40% and composed of xGnP® layered on a porous, non-conducting NEXUS® PET (polyethylene terephthalate) polymerveil, hereafter referred to as porous polymer paper. The graphenelayer had a low SR and the polymer layer was electrically insu-lating. The third paper composite consisted of a very thin layer(about 2–5 �m) of xGnP® on thin nonporous Mylar PET polymersheet, hereafter referred to as nonporous polymer paper. Onlythe first paper contained cellulose, the other two papers were agraphene/PET hybrid material.

The proteins in this study, bovine serum albumin, BSA (Sigma,St. Louis, MO), chicken egg white lysozyme, LYS (CalBioChem,Billerica, MA), bovine fibrinogen, FIB (Sigma, St. Louis, MO), andbovine hemoglobin, HEM (Sigma, St. Louis, MO), were used asreceived. Physical property data for each protein can be found inTable 1. Monobasic potassium phosphate was purchased from VWRInternational (Radnor, PA). Solutions were made with water thatwas purified with a NanoPure water system (Thermo Scientific,Waltham, MA) to a resistance of >18 M� and filtered with a 0.2 �msyringe filter (Nalgene, Rochester, NY) prior to use. All proteinswere in 10 mM phosphate buffer adjusted to pH 7.2 with NaOHor HCl (VWR, Radnor, PA), as needed.

2.2. Methods

2.2.1. Sensor constructionFor each graphene paper sample, a 2–3 mm wide by 3.5 cm long

strip was cut from a larger paper sheet using a straight edge andscalpel. The ends of the strip were adhered to a standard microscopeglass slide (75 mm × 25 mm) using two-part fast drying epoxy. Cop-per wire (∼30 AWG) was bent into an ∼1 mm fish hook shape,dipped in silver epoxy (Circuitworks® 60 min conductive epoxy)and positioned on top of the graphene paper strip with an innergap of 1 cm. The wires were bent away from the slide at a 90◦ angleand glued in place with the two-part fast drying epoxy. A picture ofthe paper microdevice slide can be found in Fig. 1. A minimum ofthree paper microdevices were tested for each different paper andprotein combination.

2.2.2. Sensor testingThe copper wire leads of the sensor were connected with alli-

gator clips to a Keithley 2400 sourcemeter (Cleveland, OH) and

tested at 23 ◦C following the ASTM D4496 method. The voltagewas adjusted between ±0.006 V and the amperage measured of thedry graphene paper at three different voltages, with two readingsbeing positive voltages and one being negative, allowing for the

94 C.L. Heldt et al. / Sensors and Act

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ig. 1. Graphene paper microdevice. The graphene paper was cut into a strip–3 mm in width and affixed to a glass slide. Copper leads were attached directly tohe paper 1 cm apart with silver epoxy.

alculation of SR using Eq. (1). This procedure was repeated afterddition of 10 �L of 10 mM phosphate buffer, followed by 10 �L ofhe lowest concentration protein up to the highest concentration ofrotein. Amperage values were in the microamp range when mea-ured for voltages ranging from −0.003 to +0.006 V. A new deviceas used for each trial.

R = Vw

IL(1)

he units of SR are �/sq. The other variables in Eq. (1) are defineds: V is voltage, w is the width in cm of the graphene paper, I is theurrent in amps and L is the length in cm of the graphene paper

etween the copper leads, which was kept constant at 1 cm.

In the first stage of paper testing, we examined the change inhe SR at high protein concentrations as a way to screen differentraphene/polymer papers. In the second stage of paper testing, the

able 2raphene paper response to buffer.

Paper Dry SR (�/sq) Wet SR (�/sq)

Cellulosic 15.2 124

Porous 16.6 33.5

Nonporous 5.57 6.45

ig. 2. Screening of graphene papers. (A) Graphene papers containing different polymellulosic paper had the largest change in SR with respect to protein concentration comespectively. The remaining experiments were completed with cellulosic paper. (B) Grapo detect changes in BSA concentration. The 50% graphene/cellulose paper had the largesdditional proteins. The error bars represent the standard deviation of three or more uniq

uators B 181 (2013) 92– 98

xGnP®/cellulose blend paper was altered by removing surfactantsand increasing the graphene content to 50 wt% and 70 wt%.

3. Results

We have created a simple, two-terminal sensor out of hybridgraphene paper, as shown in Fig. 1. A two-terminal device wasselected for its simplicity. It eliminated cross-talk, which is asso-ciated with four-terminal devices [29]. However, it increases thechance of parasitic or contact resistance [30]. We did not inde-pendently explore the contact resistance in this work, althoughthe aggregate copper wire, conductive epoxy, and graphene paperresistance was initially measured. Resistance of the copper wirewas ∼10−6 �/cm (<10 cm of wire was used for the constructionof each device), while the conductive epoxy was <10−3 �/cm[31]. These resistances were considered negligible compared tothe aggregate resistances measured, which ranged from 60 to260 �/cm (units converted from those found in Table 2 using Eq.(1)). The contact resistance created by an improper contact betweenthe copper wire and the graphene paper was reduced by using silverepoxy.

3.1. Screening of graphene paper with differing supports

Three different types of graphene paper were tested for theirability to change SR in response to a change in protein concentra-tion. Each type of graphene paper was tested by measuring the SR ofthe dry paper, the SR of the paper plus the buffer, and then by addingincreasing concentrations of protein without washing betweenadditions. Any liquid that remained on top of the paper after mea-suring the SR was removed with a pipette between samples. Eachpaper had a different physical response to the aqueous solution,as shown in Table 2. The paper containing cellulose absorbed and

swelled in the presence of water. The porous polymer paper alsoabsorbed water, but did not swell. The nonporous polymer paperdid not absorb water. This physical response was also apparentin the change in SR from the dry to wet state, shown as a ratio

Ratio (wet SR/dry SR) Swelled Adsorbed water

8.2 Yes Yes2.0 No Yes1.2 No No

er supports were tested for their ability to detect changes in BSA concentration.pared to the porous and nonporous polymer papers, NEXUS Veil and Mylar PET,

hene papers containing different amounts of cellulose were tested for their abilityt change in SR with respect to BSA concentration. This paper was chosen to exploreue device readings obtained without normalization of the data.

nd Actuators B 181 (2013) 92– 98 95

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Fig. 3. Langmuir isotherm fitting. (A) The Langmuir isotherm equation was lin-

concentration by oxidation of tyrosine was able to detect BSA con-centrations down to 3.5 �M [36]. A two-terminal measurement ofconductivity changes was able to detect as low as 1 �M of ions witha multilayer graphene sensor [19]. The graphene/cellulose sensor

Table 3Equilibrium dissociation constants.

Protein Kd (�M)

C.L. Heldt et al. / Sensors a

n Table 2. Although the cellulosic and the porous polymer paperad similar dry SR values, their wet SR values were significantlyifferent. It is likely that the cellulose swelled and separated thetacked graphene nanoplatelets, reducing the graphene contacts inhe paper and therefore increasing the SR.

The change in SR of the different papers as a function of BSAoncentration can be found in Fig. 2A. The cellulose paper showedn increasing linear �SR response as the BSA concentration wasncreased. We hypothesize that this was due to the non-specificbsorption of the BSA onto the graphene that hindered electronransport through the material. The other two graphene paperslso had a linear and increasing trend, but their response as aunction of BSA concentration was less substantial, as shown bylope values that were 6-fold (porous polymer paper) and 10-foldnonporous paper) smaller than the cellulosic paper. Regardless ofhe SR measured for the dry paper (see Table 2), the paper withhe highest buffer SR gave the largest change in SR with respecto protein concentration. From this result, we concluded that theraphene/cellulose paper had the greatest potential for detectinghanges in protein concentration, so we explored the cellulosicaper further.

The cellulose/graphene paper contained 33 wt% grapheneanoplatelets and the remaining composition was cellulose papernd surfactants. To investigate the sensitivity of the graphene paperensor, we explored different graphene concentrations (50 wt% and0 wt%) and removed the surfactant to increase the SR. The resultsan be found in Fig. 2B. The 50 wt% graphene paper had the great-st change in SR with respect to protein concentration. The datalso suggested that the graphene paper lost reliability at high pro-ein concentrations, which occurred between 450 and 600 �M BSA.his can be seen by the large error bars at the higher concentra-ions in Fig. 2. This sequence of experiments suggested that thereater the wet resistance of the cellulosic graphene paper, theore sensitive it was to protein concentration. Over the differ-

nt papers we tested (the porous polymer, the nonporous polymer,he 33 wt% graphene/cellulose with surfactant, and the 50 wt% and0 wt% graphene/cellulose without surfactant), the 50 wt% cellu-

osic graphene paper had the greatest wet SR and the greatesthange in SR with respect to protein concentration. Therefore,e studied the absorption of a panel of proteins with the 50 wt%

raphene paper.

.2. Protein absorption with 50% graphene paper

To quantitatively observe the nonspecific absorption of pro-eins to 50 wt% graphene paper, we used a panel of four proteins.hese proteins were chosen to represent a large range of molec-lar weights, isoelectric points and hydrophobicity (see Table 1).rotein binding to the graphene/cellulose paper was tested over

larger concentration range, with a cluster of data points inhe low concentration range. The proteins were tested from 0 to5 �M for BSA, 0 to 350 �M for LYS, 0 to 15 �M for FIB, and 0 to30 �M for HEM. Maximum concentration differed based on pro-ein solubility, or in the case of BSA, it was due to the large errorxperienced for graphene paper devices at high concentrationsFig. 2).

Protein adsorption to the graphene paper was fit to the Langmuirsotherm, shown in Eq. (2), and two parameters were determined:

SRmax and Kd. �SRmax is the maximum change in the surface resis-ivity as saturation is reached and Kd is the equilibrium dissociationonstant. The other variables in Eq. (2) are c, the concentration of the

rotein solution, and �SR, the change in surface resistivity. Fig. 3A

s an example of the plot of c/�SR vs. c, and the fit line used toetermine �SRmax and Kd. The data was normalized by �SRmax inq. (3), where � = �SR/�SRmax. This has been shown as a method

earized, fit to the data, and plotted for one device with BSA binding. (B) Thenormalized data is plotted along with the fit of the Langmuir isotherm parametersfor one device.

to normalize other carbon-based protein sensors [5,19], and a fit ofthe data from Fig. 3A can be found in Fig. 3B.

C

�SR= C

�SRmax+ Kd

�SRmax(2)

C

�= C + Kd (3)

The binding of all of the proteins tested was fit to the Langmuirisotherm, as shown in Fig. 4 with the Kd and standard deviationsshown in Table 3. This model has been shown to be valid for thebinding of BSA to both graphene nanoplatelets [9] and CNTs [5]. Thecalculated value of Kd of 7 �M is in good agreement with the valueof 3.6 �M found for a sprayed graphene FET [32]. FETs are thoughtto increase the sensitivity of carbon-based biosensors. This demon-strates that the two-terminal graphene paper biosensor created hasa similar dissociation constant as an FET, which is a four-terminaldevice. Cellulose has been shown to have little adsorption to BSA[33,34] or LYS [35], so it is not likely participating in the binding orSR measured for the graphene paper sensor. A measurement of BSA

BSA 7 ± 4LYS 30 ± 10HEM 17 ± 2FIB 1.5 ± 0.5

96 C.L. Heldt et al. / Sensors and Actuators B 181 (2013) 92– 98

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a globular or spherical protein). Replacing r with the surface area(A) of a circle (A = �r2) [i.e. the surface area of the graphene cov-ered by the protein], then MW is proportional to area to the threehalves power, A3/2, demonstrating that the change in Kd is likely

ig. 4. Detection of proteins. The normalized surface resistivity and the fit to theepresent the standard deviation of three or more unique device readings obtained

escribed here was tested as low as 0.2 �M and was able to detect change in BSA concentration at this level. Further, the grapheneaper sensor is much simpler than the tyrosine oxidation method,hich requires denaturable enzymes [36]. It is also much simpler

han an FET, which requires a voltage sweep, instead of the discreteoltage measurement technique described here.

We followed the same methodology to study the binding of FIBnd HEM and the results are reported in Fig. 4C and D. The cal-ulated Kd values can be found in Table 3. There was an order ofagnitude difference in the equilibrium binding coefficient for the

ifferent proteins tested. The lowest Kd was 1.5 �M for FIB. Thetandard deviation for some of the proteins is a little high, but thiss likely due to variations in the graphene paper, which we are cur-ently addressing, and due to the fact that we only tested threeevices.

. Discussion

Proteins with a range in molecular weight, isoelectric point, andydrophobicity (Table 1) were used to determine the sensitivityf cellulosic and polymeric graphene papers for the detection ofrotein concentration. This range of protein properties allowed forn analysis of the mechanism of protein binding. There was not aorrelation between the calculated Kd and isoelectric point. It haseen shown that when protein charge differs, the slope of a currents. protein concentration plot can change sign for a graphene oxideET that is modified to specifically bind protein [15]. However, ourroteins at pH 7.2 were either positively charged (BSA and FIB),

egatively charged (LYS), or neutral (HEM) and the response of SRo protein concentration was positive for all proteins. This demon-trates that charge is likely not playing a role in the detection ofhese proteins with our two-terminal graphene paper microdevice.

muir isotherm equation for (A) BSA, (B) LYS, (C) FIB and (D) HEM. The error barsalculated Kd values can be found in Table 3.

The hydrophobicity of each protein was calculated based on theweighted average of the hydrophobicity of each amino acid in theprotein sequence [37]. It appears that the different Kd values arenot based on the inherent hydrophobicity of the proteins.

One correlation we determined was between molecular weight(MW) and the Kd, as shown in Fig. 5. The solid line is a powerlaw fit line with a relationship of Kd proportional to MW−3/2.MW = 4/3�NA�r3, where � is the density of the protein, NA is Avo-gadro’s number and r is the radius of the protein (assuming it is

Fig. 5. Equilibrium constant as a function of molecular weight. Equilibriumconstants for all four proteins plotted against molecular weight demonstrated thatthe graphene paper sensor is able to determine the size or molecular weight of aprotein. A = 1330.

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ue to the different sizes of the proteins. We hypothesize that thed is sensitive to the surface area of graphene that is covered by therotein. The smaller the protein, the less graphene surface area cov-red, which we measured by a larger Kd. It has been documentedhat proteins denature in contact with hydrophobic surfaces [38,39]nd this is likely the mechanism of protein interaction that resultsn a measurable SR change with protein concentration.

The selectivity of the graphene paper sensor was not specificallyxplored. However, each protein was detected in sub-micromolaroncentrations. This demonstrates that this type of sensor is sen-itive at low protein concentrations. If increased sensitivity isequired, FETs have been shown to be sensitive in the nanomo-ar range [9], a sandwich enzyme assay with CNTs has been able toetect antibodies in the picomolar range [40], and gold nanoparticleonjugates have been shown to detect antibodies in the femtomo-ar range [41]. These techniques are some of the effective ways toncrease the sensitivity for other nanoscale carbon sources. How-ver, these techniques also increase the cost of the device anddd complexity fraught by reproducibility issues and operationalhallenges, which are less conducive to point-of-care devices. Ourevice can rapidly take a single SR reading in seconds, unlike FETs,r antibody or enzyme assays. In the future, we plan to explorehanges in paper composition for increased sensitivity to detectow protein concentrations. Chemical modifications will also bexplored to create a selective and reusable sensor. The ability toune paper properties with minimal increases in cost or opera-ional complexity is an advantage that graphene paper has overther carbon-based sensing platforms.

. Conclusions

A graphene paper microdevice has been shown to have thebility to detect 0.2 �M and greater protein concentrations andiscern molecular weight by measuring a change in the SR ofraphene paper. Different cellulosic and polymer papers wereested for their ability to be sensitive to protein concentration, and

cellulosic paper containing 50 wt% graphene demonstrated theost sensitive response. An increase in protein concentration was

ccompanied by an increase in the SR of the graphene paper sensor.our different proteins were tested for their ability to change the SRf the graphene paper, and there was a clear correlation betweenhe molecular weight of the protein and the Kd calculated by fittinghe protein adsorption data to the Langmuir isotherm. We hypoth-size that the change in SR was likely due to the protein adsorptiono the graphene paper.

cknowledgements

Financial support from the Department of Chemical Engineeringt Michigan Tech is greatly appreciated. Funding was also receivedrom the MTU-REF Research Seed Grant and the Michigan Spacerant Consortium.

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Biographies

Caryn Heldt received her Ph.D. in Chemical Engineering from North Carolina StateUniversity in 2008. She is currently an Assistant Professor at Michigan Tech whereher research lab focuses on the removal, purification, and detection of pathogensand other protein molecules.

Amy Sieloff is pursuing her B.S. in Chemical Engineering at Michigan Tech.

Joshua Merillat is pursuing his B.S. in Chemical Engineering at Michigan Tech.

Adrienne Minerick received her Ph.D. in Chemical Engineering from the Univer-sity of Notre Dame in 2003. She is currently an Associate Professor at MichiganTech where her primary area of research is electrokinetics with a focus on medicalmicrodevices, blood cell dynamics, and point-of-care diagnostics.

Julia King is currently a Professor in Chemical Engineering at Michigan Technologi-cal University. She received her M.S. in Chemical Engineering from the University ofWyoming in 1987, and her Ph.D. in Mechanical Engineering from the University ofWyoming in 1989. She worked for 10 years in industry (Exxon and Conoco) beforejoining MTU in September 1996. Dr. King’s research is in the area of compositematerials, typically using various carbon fillers in polymers.

Warren Perger received his M.S. in Electrical Engineering from the University ofWisconsin-Madison in 1981 and his Ph.D. in Physics from Colorado State in 1987. Heis a Professor at Michigan Tech University with research interests in electromagneticinteractions with matter and modeling of crystalline materials.

Hiroyuki Fukushima received his Ph.D. in Materials Sciences from Michigan StateUniversity in 2003. After receiving his Ph.D., he became a research specialist atMSU where he conducted graphene related research projects. His research effortsgave technical foundations and lead to a new company called XG Sciences, Inc.,where he is one of the co-founders. Now he is a senior scientist at the com-pany and coordinating the research activities in composites, papers, inks, andsensors.

Jeffri Narendra received his Ph.D. in Electrical Engineering from Michigan StateUniversity in 2010. His research interests are in the design, simulation, fabrica-tion, characterization, and applications of electronic materials and devices, utilizingmicrowave and plasma technologies. He is currently a Senior Manufacturing andProcess Engineer at XG Sciences, Inc.