2013-anti-fouling pedot-pss modification on glassy carbon electrodes for continuous_2
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have been applied since the 1970s by the US Air Force [9]. Over
several decades, 3 wt% TCP has been added into commercial jet
engine oils, such as Mobil Jet Oil [10]. TCP could be mixed with the
breathable air entering the airliner cabin during engine oil leakage
[11]. Although the most toxic ortho-isomer of TCP is intentionally
excluded from these fluids [12], alternatively used meta- and
para-isomers can have a potentially severe effect on human health
mainly through inhalation of aerosols and dermal adsorption.
The most commonly used methods for detecting TCP include:
gas chromatography and mass spectrometry GCMS [13], highperformance liquid chromatography (HPLC) [14] or thin layer
chromatography (TLC) [15]. Gas chromatography measure-
ment can be based on flame photometric detector (GC/FPD),
nitrogenphosphorus sensitive detector (GC/NPD) [16]. Although
they are able to detect TCP below ppm level and with high selectiv-
ity, these devices have several drawbacks, such as large size, high
cost, complexity, and a need for highly trained operators, which
make their use on aircraft impractical. Alternatively, electrochemi-
cal sensors have the advantages oflow price, highsensitivity, small
size, ease of operation, and rapid response. A portable real-time
electrochemical sensor with linear response for ppb level ofTCP
in gas has been recently reported by our group [17,18]. Since TCP
itself is electrochemically inactive, it was converted to electro-
active cresol for detection. However, the sensor system suffered
from electrode fouling which limited its repetitive usage in long
term applications. A long lasting electrode is needed for on-site
continuous monitoring, so electrode surfaces must be modified to
minimize or eliminate fouling.
Electrode fouling is a common problem during electrochem-
ical analysis of phenolic compounds, such as cresol, and causes
decay in signal response during repetitive measurements [1927].
The fouling is caused by the formation ofa passive polymeric film
on the electrode surface [23,25,28,29]. Upon anodic oxidation of
cresol, phenoxy radicals form, which then couple to form dimers
or oligomers, and finally, a polymeric film deposits on electrode
surface [30]. This polymeric film is tough, thermally stable, and
so chemically inert that little oxidation or hydrolysis will occur in
either acidic or basic media [26]. The films are characterized by
low permeability [31] and strong adhesion to the electrode, whichblocks the surface, and yields electrode fouling [1922,24]. Various
surface treatments and modifications have been used to reduce or
even avoid electrode fouling by preventing polymeric substrates
from absorbing onto the surface of electrode [32,33]. For instance,
a special compound called sodium 3,5-dibromo-4-nitroso benzene
sulfonate (DBNBS) wasused as an anti-foulingagent against the for-
mationofthecresolpolymericfilm[33]. TheDBNBSmoleculereacts
with the oxidized radical ofcresol to form a compound which does
not adhere to the surface ofthe electrode and consequently pre-
ventsthe fouling effect. However, thisapproachis difficult to realize
in practical applications. Overall, the DBNBS should be present in
the buffer solution in stoichiometric concentration with the ana-
lyte of interest, which makes its application for real world samples
problematic.
Having advantages ofhigh electronic conductivity and porosity[34], conducting polymers have attracted considerable interests in
recent years. Modification ofconductive polymer for preventing
electrode fouling has been reported by many researchers [3538].
Poly(3,4-ethylenedioxythiophene) (PEDOT) is one of the widely
used conductive polymers. Patra and Munichandraiah [34], Heras
et al. [39], and Lupu et al. [40] applied PEDOT on electrodes
through electropolymerization for detection of phenolic com-
pounds. The surfactant poly(sodium-4-styrenesulfonate) (NaPSS)
was additionally used during PEDOT electropolymerization to
avoid the problems of[41]: (1) the low solubility of thiophene
structures in water, (2) the oxidation potentials higher than that of
water, and (3) and the water-catalyzed formation of thienyl cation
radicals which can activate concomitant reactions and prevent the
formation of the main polymer [42]. Moreover, the hydrophobic
hydrocarbon residues ofPSS exhibit strongaffinityfor PEDOT, while
the hydrophilic sulphonic groups are oriented toward or even pro-
trude into the solution and may hence induce poor adhesion of
the fouling polymeric film [39,43]. NaPSS also induces the forma-
tionof a permeable and less compact polymer network, and yields
high (ionic) conductivity andpermeability to thefouling polymeric
film. This makes the permeation ofphenolic compounds through
the film and oxidation inside possible, ensuring electroneutrality[39,43]. In the present work, PEDOT:PSS has been used to mod-
ify the commonly used glassy carbon electrode [44,45] to develop
a sensor for continuous monitoring of TCP in gas and has been
investigated.
2. Materials and methods
2.1. Reagents, solutions, and instruments
All aqueous solutions were prepared using de-ionized
Milli-Q water (18m cm). 3,4-Ethylenedioxythiophene (EDOT)
(SigmaAldrich, MO), poly(sodium-4-styrenesulfonate) (NaPSS)
(SigmaAldrich, MO, MW70,000), and lithium perchlorate
(LiClO4) (SigmaAldrich, MO) were used for electro-synthesis of
PEDOT:PSS [39]. The stock 20mM tri-p-cresyl phosphate (p-TCP)
(SigmaAldrich, MO) solution was prepared in methanol (BDH-
VWR) and diluted to obtain TCP samples with concentrations of
interest. All TCP samples were converted to cresol with the aid of
alkaline catalyst. Alkaline catalyst was made from NaOH (Fisher,
NJ) and neutral aluminum oxide (Al2O3) (SigmaAldrich, MO) and
packed in Pipet filter tips (USA Scientific, Inc., 20L leveled filter
tips). p-Cresol (Acros Organics, NJ, 99+%) was dissolved in 0.2M
Na2HPO4/0.2 M KH2PO4/10m M NaCl buffer (0.4 M phosphate
buffer,pH = 6.67). Na2HPO4, KH2PO4, and NaCl were obtained from
SigmaAldrich, MO.
2.2. Electrodes and electrochemical sensor
All amperometric experiments were performed with CH 93Instruments (CH1910B) Bi-Potentiostat. A desktop computer was
used to collect the data. Flow injection analysis (FIA) was carried
out using a unicell electrode set (BASi, IN) and a switch injec-
tion unit (Valco Instruments Co. Inc.) with a 50Lsample loading
loop. The flow rate was maintained at 20mL/min by using a single
syringe pump (KD Scientific, MA). The unicell electrode set con-
sists of one glassy carbon working electrode cell (2mm ), one
electrode set including the stainless steel counter electrode and
Ag/AgCl reference electrode, and one circular gasket (BASi, IN). A
coating solution was applied on the Ag/AgCl reference electrode
surface before detection as per instruction from the company. The
working electrode was polished with alumina powder (BUEHLER,
IL, 1, 0.3, and 0.05m in order). 10mM NaCl was contained in all
solutions to maintain the potential ofreference electrode. 0.64 V
vs. Ag/AgCl/10mM NaCl was applied in amperometry. In batchmode, a glassy carbon electrode (3mm ), Pt counter electrode,
and Ag/AgCl/3M KCl reference electrode (BASi, IN) were used.
2.3. Electropolymerization of PEDOT:PSS onglassy carbon
electrode
The electropolymerization ofmonomer of EDOT was performed
under amperometric conditions in aqueous solution. The solution
contained 5 mM EDOT (0.7108g/L), 0.1M NaPSS (20.62g/L), and
0.1M LiClO4 (10.64 g/L). The unicell glassy carbon electrode was
polished with alumina as mentionedin Section 2.2, rinsedand soni-
cated with de-ionized water in an ultrasonic bath for 5 min, flushed
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Fig. 1. Schematicrepresentativeof TCPsamplingsystem. TCPwas heated to 230 C inoil bathand gasified along withN 2 bubbling, and entered thealkalinecatalyst column
set insidean automatic TCP conversion box. In theelectronicallycontrolled TCPconversion box, as shown in thedottedsquarein above figure, TCPwas hydrolyzed for later
detection. The control of the automatic TCPconversion box is described in Ref. [17].
with ethanol and water, and dried with N2 gas. 0.5mC of charge
was applied on the glassy carbon electrode with 2mm diameter
in order to obtain the thickness ofinterest ofelectropolymeriza-
tion layer, unlessotherwisestated. The potential was maintainedat
0.95Vvs.Ag/AgCl/3MKCl. Thecolorofelectrodesurfaceturnedyel-lowafter modification. For comparison, different amount ofcharges
have been applied including 0.3, 1, 1.3, 2, 4, and 20mC on the same
electrode.
2.4. Automatic sampling system forhydrolysis of TCP samples
and TCP in engine oils
Since p-TCP has a very low saturated vapor pressure at room
temperature (1104 mmHg0.01Paat20 C,100Paat220 C,
1000Pa at 260 C, [4446]), the TCP samples in gas phase are not
readily available. A system was built in our lab for TCP sampling
and is shown schematically in Fig. 1. 0.5mL of TCP methanol solu-
tions with concentrations of10, 20, 30, 50, 100, 200, and 300M
were prepared from the20 mM TCPstock solution.N2was bubbledat a flow rate of 1.1L/min through the container with TCP for
5min at room temperature and then 5 min at 230 C (temperature
controlled with an oil bath) to make sure all TCP was evaporated.
The total N2 volume in 10min was 11 L 0.5mol (considered at
room temperature), therefore, TCP samples with 10, 20, 30, 50,
100, 200, and 300 ppb in gas phase were realized. Each sample was
flown along with N2 through the alkaline catalyst column where
it was hydrolyzed, following the dotted (red) arrow path in Fig. 1.
The alkaline catalyst column was prepared by packing a 100mg of
mixture of NaOHand Al2O3(1:10wt) inside.The preparation ofthe
catalyst is described in more detail in Ref. [17]. In the electronically
controlled TCP conversion box (dotted square in Fig. 1), valvesturned after 10min ofgas flow and the pumpworked topush 3mL
0.4M phosphate buffer solution to wash out TCP hydrolyzate for
detection at the sensor, following the solid (blue) arrow path in
Fig. 1 (its method of control was mentioned in Ref. [17]).
Engineoil samples were preparedby dissolving thesamples into
methanol in 2L/0.5 mL ratio and with the same procedure as TCP
samples. Oil BP274 does not include TCP, Mobil Jet Oil II includes
13%, and BP2380 includes 15%TCP [47]. BP274oil samples spiked
with TCP in different concentrations were also prepared, sampled,
and measured.
3. Results and discussion
3.1. Diffusion controlled cresol oxidation
The oxidation process of cresol on a bare glassy carbon working
electrode was studied with cyclic voltammetry (CV). In the CV
curve (Fig. 2), current increased quickly at the beginning in each
potential scan due to the charging current, followed by a flat line.
Starting from around0.4 V, current increasedagainwhichindicates
the onset of oxidation of cresol. The oxidation was restricted to
Fig. 2. Cyclicvoltammetry results ofcresoloxidationon bare glassycarbon electrode. (a)Firstcycleof cyclic voltammetryof 100M cresolin 0.4M phosphatebuffer solution
at scan rates varying from 10 to 200mV/s. scan rates increase along the arrow direction. Range of potential: 0800mV vs. Ag/AgCl/3M KCl. (b) Calibration of the relation
between thepeakcurrent density andsquare root ofthe scan rate. Thecoefficientof determination R2 indicates the variabilityfrom linear fit. Glassy carbonworkingelectrode
(3 mm) was usedfor all of detections and polished between uses.
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Fig. 3. Observationsof fouling on glassy carbon electrode from SEMand EDS. (a) SEMimage of Au sputtering coated bare electrode, (b) SEM image of fouled electrode, and
(c) EDS results of thebare and fouledelectrodes.
the vicinity of electrode surface and reduced the concentration
of oxidizable cresol in this area. Once the concentration gradient
existed between the vicinity of electrode surface and the bulk
solution, diffusion occurred which yielded a mass flow ofcresol
from bulk to electrode surface. In this batch mode, migration was
reduced to negligible levels by addition ofa high concentration of
phosphate ions and convection was avoided by preventing stirring
and vibrations, which made diffusion the only path for mass
transfer. We define a diffusion layer where linear concentration
gradient exists. The thickness of the diffusion layer increased with
time which would flatten the concentration profile. In contrast,
the potential continued to scan positively and decreased the
concentration of oxidizable cresol in the vicinity of electrode
surface, which steepen the concentration profile. The slope of con-
centration difference was intensified; when the effect ofpotential
scan dominated first, so did the diffusion, and thus the current
increased which yields the left shoulder of CV peak. Potential scan
kept going positively and the concentration of oxidizable cresol
in the vicinity decreased approaching zero. By the point when
vincinity concentration dropped relatively slower, since it already
approached zero, the diffusion layer thicken effect dominated
and thus the slope of concentration difference reduced. The
current started to decrease, meanwhile, an IV peak was created
[48].
Fig. 4. PEDOT:PSSmodification of glassy carbonelectrodes. (a) Amperometric results of 4 electrodesapplied with0.5 mC of charge in EDOT:PSS solution, (be)optical images
ofthese 4 electrodes after modification with 0.5mC of charge, (fi) opticalimages of 4 electrodes after modification with 1, 1.3, 2, and 4mC of charge, (jl) SEM images of
electrode surfaceapplied with 0.3, 0.5, and 2 mC of chargein EDOT/PSS solution.
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Fig. 5. Amperometric results of 10M cresol on bare and modified electrodes. (a) Initial amperometric peaks obtained from successive injections of 10M cresolin 0.4M
phosphate buffer. Two experiments have been carriedout on bare electrodeand fiveon modified electrodes, two examples of which are shown here. (b) Calibration of peak
current vs. peaks. Note that the first peak ineachcase has been normalized to 100%. Unicell glassycarbonelectrodes with 2 mm diameter were used.
The results of CV experiments demonstrating the change of the
IV at different scan rates are shown in Fig. 2. 100M p-cresol
in 0.4M phosphate buffer was used as the analyte solution, and
the potential was scanned in the range of 00.8V vs. Ag/AgCl/3M
KCl with scan rates of10, 20, 50, 80, 100, 120, 150, and 200 mV/s.
Following the higher scan rate, the whole IV cycle expanded due
to the increased charging current. The peak current also increased
(Fig. 2a). Fig. 2b shows the calibration ofcurrent density vs square
root of scan rate. The linear relation indicates the oxidation pro-
cess of cresol is limited by the diffusion to electrode surface, while
the electron transfer at electrode/solution interface is relatively
quick [41,49]. The diffusion controlled property ofcresol oxidation
ensures thatcurrent signalis proportional to the bulkconcentration
inCV as well as amperometry mode.
Lookingat thereversal scan curve in Fig. 2a,we note theabsence
of reduction peak which indicates the process was not reversible.
Cresol was oxidized to radicals, followed by the radical coupling
and the formation of inert dimers or oligomers. A stable polymericlayerformed,finally, andbroughtthe electrode fouling [30]. Indeed,
while this fouling layer could be removed by physical polishing, it
is inert to chemicals and cannot be oxidized until a potential ofup
to 3V is applied [50].
3.2. Microscopy images of electrode fouling
The formation offouling layer was studied by SEM, EDS, and
AFM. For SEM imaging, the electrodes were sputter coated prior to
analysis. One electrode was left polished, while the other one was
fouled by 10 injections of 10M cresol in 0.4MPB buffer in amper-
ometry mode. The polished electrode and fouled electrode were
comparedin SEMimages (Fig.3aand b).Onlysputter coatedAu was
seenon polished electrode,but additionalstructures wereobserved
from the surface of fouled electrode. EDS shows the existence ofoxygen on thefouledelectrode, which could come from the fouling
products of cresol (Fig. 3c). AFM images also confirm the existence
of fouling layer on electrode surface (Results not shown). The sur-
face profile of fouled electrodes varied in the range of 030 nm
while the variation in the bare electrode was less than 15nm.
3.3. Controlled PEDOT:PSS modification onglassy carbon
electrode
To reduce the fouling from oxidation of cresol, PEDOT:PSS
modification was applied on glassy carbon electrode via elec-
tropolymerization. One important parameter for modification is
the reproducibility. To show the ability to control modifica-
tion, 4 unicell glassy carbon electrodes (2mm ) were immersed
in EDOT/NaPSS solution and applied 0.5 mC of charge (poten-
tial= 0.95V in single-potential amperometry mode). Fig. 4ae
shows the results ofthese modifications. The amperometry pro-
files were almostthe same forthese 4 electrodesas shownin Fig.4a,
and all of these 4 electrodes had the same color after modification
Fig. 6. Amperometric results of injections of 0.210M cresol in 0.4M phosphate
buffer. (a) Two representative sets of continuously amperometric results. Different
concentrations of cresol samples were injected in random order to avoid poten-
tial system error and by this way electrode fouling could be seen more clearly.
(b) Calibration curve of detection of cresol with different concentrations on modi-
fied electrode. The coefficient of determination R2 indicates the variation of linear
fit. Error bars are marked as bars above and below current symbols. Unicell glassy
carbon electrodes with 2 mm diameter were used.
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which indicates the uniform and controllable configuration. Addi-
tionally, 4 more electrodes were coated with 1, 1.3, 2, and 4mC
of charge. The color of modified electrode surface changed with
different amounts ofcharge (Fig. 4fi).
SEM was also carried out to compare the electrode surfaces
modified with 0.3, 0.5, and 2 m C of charge. Images from left to
rightcorrespondto increasinglymore charge(Fig.4jl). During SEM
imaging, the configuration was seen to be uniform over the whole
electrode surface and so representative SEM images in small areas
can be produced. Fig. 4jl illustrate the change in morphology andparticle size of the PEDOT layer. Cracks formed on the PEDOT:PSS
layer prepared by applying 2 mC of charge
3.4. Detection of cresol with modified electrodes
The controlled PEDOT:PSS modification described above was
used to reduce fouling effect in the amperometric detection of
cresol. 10M cresol in 0.4M phosphate buffer was injected suc-
cessively on both modified (left side ofFig. 5a) and un-modified
electrodes (right side of Fig. 5a). The peak currents decayed in
both cases after repeated exposure to cresol, but magnitude of
the decrease was much lower in the modified electrode. For com-
parison, the peak height was normalized to the first peak in each
case being normalized to 100%. The successive peaks were seri-ously decayed on bare electrode but much less fouling occurred
on modified electrode (Fig. 5b). The comparison indicates that for
continuous on-site monitoring ofTCP, the bare electrode is not a
good candidate. Although thoroughly polishing the electrode could
remove the fouling layer, this would require trained personnel and
adds complexity to sensor operation. Modified electrodes, how-
ever, overcome this limitation.
With modified electrodes, a series of cresol samples with con-
centrations of 0.2, 0.5, 1, 2, 5, 7.5, and 10M were detected in
amperometry mode 3 times at each concentration). Two represen-
tativeexamplesofinitialamperometric results are shownin Fig.6a.All samples were injected in random order to avoid system error
andto show foulingclearer since signal from a samplewoulddecay
more if a higher concentration was previously injected. The cali-
bration curve is shown in Fig.6b. A linear relationship between the
peak current and the concentration ofcresol was obtained, con-
firmed by the R2 being very close to unity. The error bars which
were contributed mainly from electrode fouling were too small to
be distinguished from the data symbols. Therefore, thedetection of
0.210M cresol was reliable with the modified electrode.
3.5. Detection of TCP with modified electrodes
To show the ability of detecting TCP samples, several TCP
samples with 10, 20, 30, 50, 100, 200, and 300p pb were gasi-fied, hydrolyzed, and detected with PEDOT:PSS modified electrode.
Fig. 7. Detectionof TCPon bare/modified electrode. (a,left)Calibrationof detectionof TCPsamples with concentrationsof 10,20, 30, 50,100, 200, and300 ppbon PEDOT:PSS
modified electrode, (a, right) Calibration of detection of 30, 50, 100, and 300ppb TCP on bare electrode. 10 and 20ppb were not detected and compared with modified
electrode, (b) responsesfrom modified electrode were normalized to 100%and the error bars andsignals from bare electrodewere relatively calibrated. RSD wascalculated
from the ratio of Stdev (standard deviation) to Avg (average). Unicell glassy carbon electrodes with 2mm diameter were used. Note that the bare electrode was polished
after measurement of each setof 4 samples, while modified electrode was not polished through three sets of measurements.
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Experiments have been carried out three times. The calibration of
allthree experiments is shown in theleft frame ofFig. 7a. Errorbars
indicate the variation between all three experiments. A linear rela-
tionship was observed between current signal and concentration
of sample in the range of 50300ppb. For comparison, the corre-
sponding results from bare electrodes are shown in the right side
ofFig. 7a, excluding 10 and 20ppb TCP. Although a linear relation
wasalsoobserved, theerror bars were much biggerdue toelectrode
fouling. Note that the bare electrode was polished after measure-
ment of each set of 4 samples. The fouling effect would be muchmore serious if the electrode were left unpolished through the
whole measurement, which was done for modified electrode. To
observe the comparison clearly, the responses from modified elec-
trodeswerenormalized to100% andthe signalsfrombare electrode
and error bars were relatively calibrated (Fig. 7b). Comparing the
error bars andRSD values from modifiedelectrode with those from
bare electrode,modified electrode wasseen to have much less foul-
ing. Indicated by these results, the sensing system with PEDOT:PSS
modifiedelectrode is able to continuouslymonitor TCPin gasphase
in the concentration range of50300ppb. It should be mentioned
that the lower point of the linear range for the PEDOT:PSS modi-
fied electrode (50 ppb) is comparable with others work. De Nola
reported the LOD of1.4 pg/20 nL(70ppb, TCP in isohexane) using
gas chromatography [13].
Previously, our group reported the detection ofTCP with bare
glassy carbon electrodes with a linear range of5300 ppb [51].
However, the bare electrodes required thorough polishing before
each measurement, which would make impossible its usage for
continuously monitoring TCP on the aircraft.
3.6. Detection of hydrolysates from engine oils
Finally, to test the ability ofthis system to determine TCP in
air from real samples, such as samples from jet engine oils, 2L
of commercially available BP Turbo Oil 274, BP Turbo Oil 2380, and
MobileJet Oil II weremixed in 0.5 mLof methanol and were usedto
prepare the hydrolysate samples with the same procedure as TCP
hydrolysates described above. The results ofoil hydrolysates with
Fig. 8. Detectionof commercial engineoil andoil spikedhydrolysatesamples.All samples were measured randomly 3 times.(a) Results of hydrolysate samples from engine
oils BP 274, Mobil, and BP 2380with modified electrode; (b) with unmodified electrode. (c) Calibration of the concentrations of TCP in the oilsamples, and the results from
modified electrode and unmodified electrode were compared with the claimed values by manufactures. (d) Results of hydrolysate samples from oil BP 274 spiked with
0200 ppbTCP with modified electrode;(e) with unmodifiedelectrode. (f)Comparisonof theresults between themodifiedand unmodifiedelectrode. RSD(relative standard
deviation) was calculated the same as Fig. 7.
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Biographies
Xiaoyun Yang received the B.S. degree in Chemistry from Tongji University,Shanghai, Chinain 2006, followedby 2-years studying for Masters degree in Physicsdepartment. Currently, he is pursuing his PhD degree in Materials Engineering atAuburn University.
Jeffrey Kirsch received his B.S. in Materials Engineering at Auburn University,Auburn, Alabama in2011. Currently, heis pursuing hisMasters Degree in MaterialsEngineering at Auburn University.
Dr.EricV.Olsenisthe Directorof theClinicalResearch at KeeslerAFB Mississippi. Hereceived hisPh.D.in Biological Sciences andM.S. in Microbiologyfrom Auburn Uni-versity. His research interests include PCR assay development, piezoelectric-basedbiosensor systems andbio-preservation techniques. Heis a Lt.Colonelin the UnitedStates AirForce with over 20 years of service.
Dr.JeffreyW. Fergus received his B.S. degree in Metallurgical Engineering fromtheUniversity of Illinoisin 1985andhis Ph.D. degreein MaterialsScienceand Engineer-ing from the University of Pennsylvania in 1990. He was a post-doctoral researchassociate in theCenter for Sensor Materialsat theUniversity of Notre Dame and, in1992,joined the Materials Engineering program at Auburn University, where he iscurrently a professor. His research interests are generally in high-temperature andsolid-state chemistry of materials, including electrochemical devices (e.g. chemicalsensors andfuel cells)and thechemicalstability of materials(e.g.hightemperatureoxidation).
Dr.AlexL. Simonianisa Professorof MaterialsEngineeringat AuburnUniversityanda Biosensing Program Directorat NSF. Hereceivedhis M.S. inPhysics from theYere-van State University (Armenia, USSR), a Ph.D. in Biophysics and a Doctor of Sciencedegree in Bioengineering from the USSR Academy of Science. His current researchinterests are primarily in the areas of bioanalytical sensors, nano-biomaterials andfunctional interfaces.
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