Biosensors and Bioelectronics 41 (2013) 65–70

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Biosensors and Bioelectronics

0956-56

http://d

n Corr

E-m

journal homepage: www.elsevier.com/locate/bios

DNA sensor based on vapour polymerised pedot films functionalised withgold nanoparticles

Elaine Spain, Tia E. Keyes, Robert J. Forster n

School of Chemical Sciences, National Centre for Sensor Research, Dublin City University, Dublin 9, Ireland

a r t i c l e i n f o

Article history:

Received 29 March 2012

Received in revised form

20 June 2012

Accepted 21 June 2012Available online 30 June 2012

Keywords:

Pedot

Vapor-phase polymerization

Conducting polymer

DNA detection

Enzyme biosensor

Mastitis

63/$ - see front matter & 2012 Published by

x.doi.org/10.1016/j.bios.2012.06.046

esponding author.

ail address: robert.forster@dcu.ie (R.J. Forster

a b s t r a c t

Poly-3,4-ethylenedioxythiophene, PEDOT, films have been deposited on gold electrodes using poly-

merization from the vapor-phase in which the surface is first covered with a Fe (III) tosylate oxidant

and then exposed to 3,4-ethylenedioxythiophene, EDOT, vapor. Gold nanoparticles were then electro-

deposited to give a nanocomposite material, PEDOT-AuNP. Thiolated capture strand DNA, that is

complementary to the sequence associated with the pathogen S. aureus that causes mammary gland

inflammation, was then immobilized onto the gold nanoparticles and the underlying gold electrode.

The target oligo was then hybridized to the capture strand DNA. A probe strand, labeled with horse

radish peroxidase, was then hybridized to the target. The concentration of the target was determined

by measuring the current required to reduce hydroquinone oxidized during the regeneration of the HRP

label. Semi-log plots of the pathogen DNA concentration vs. faradaic current are linear from 150 pM to

1 mM and pM concentrations can be detected without the need for molecular, e.g., PCR or NASBA,

amplification.

& 2012 Published by Elsevier B.V.

1. Introduction

The detection of low concentrations of nucleic acids withoutchemical amplification of the target using PCR or NASBA wouldrepresent a significant advance in healthcare impacting areasfrom the rapid detection of pathogen sub-type in infectiousdiseases to cancer diagnosis and prognosis through miRNAprofiling. Nanoparticles functionalized with nucleic acids repre-sent a powerful approach to creating complex hierarchical nanos-tructures as well as proving useful for the highly sensitivedetection of disease biomarkers.(Mirkin et al., 1996) Conductingpolymer and their composites with metal nanoparticles, nano-composites, are attractive materials for the development ofelectrochemical sensors since the nanoparticles provide a con-venient surface onto which capture strand DNA can be immobi-lized while simultaneously increasing the conductivity allowinghybridization to occur throughout the 3-D film.(Selvaganeshet al., 2007) A variety of methods for the preparation of thesecomposites have been described, including electrochemicaldeposition of nanoparticles onto electrodes previously coatedwith a conducting polymer.(Selvaganesh et al., 2007)

In this contribution, we describe the use of gold nanoparticlesand PEDOT to create nanocomposites and use them to develop ahigh sensitivity DNA hybridization assay. The nanocomposite has

Elsevier B.V.

).

a large surface area for capture DNA immobilization, good con-ductivity and excellent porosity leading to excellent signal-to-noise current ratios even for low concentrations of DNA. Cyclicvoltammetry, Raman and ultraviolet visible near infrared spectro-scopy have been used to characterize the nanocomposite as wellas the hybridization of DNA. Scheme 1 illustrates the detectionapproach that is based on vapor phase polymerized PEDOT ontowhich gold nanoparticles have been chemically grown to give ananocomposite material, PEDOT–AuNP. Here, the target is a DNAsequence from the specific bacteria, Staphylococcus aureus, thatcause mastitis (mammary gland inflammation). To date, identifi-cation of mastitis is based purely on clinical signs e.g. udderswelling, irritation and fever but at this stage milk production isalready compromised.(Keefe, 1997; Blum et al., 2008; Fournieret al., 2008) The analytical performance is characterized in termsof the dynamic range and limit of detection and compared to theparent polymer.

2. Experimental

2.1. Materials

3,4-ethylenedioxythiopene (97%) was obtained from Merckand was fractionally distilled under a nitrogen atmosphere intoa darkened vessel and subsequently kept at -18 1C in the darkprior to use. Baytron CB40, otherwise known as ferric para-toluene sulfonate in 40% 1-butanol, Fe (III) tosylate, was used as

Scheme 1. Systematic showing the stepwise creation of PEDOT-AuNP nanocom-

posite functionalized with capture strand DNA. From top to bottom: 1. Following

vapour polymerization of PEDOT and incorporation of AuNPs onto a gold disc

working electrode, the DNA was immobilized onto the modified electrode in 3

steps. 2. The capture oligo (3’ thiolate) was immobilized by immersing the

working electrode in a 1 mM oligo solution for 5 hours. 3. Hybridization of target

oligo to the capture strand. 4. Hybridization of HRP labelled probe DNA to the

target strand.

E. Spain et al. / Biosensors and Bioelectronics 41 (2013) 65–7066

received from H.C. Starck. Denhardt’s hybridization solution(Z 99.5%), for DNA probe assembly and tetrachloride goldaureate (HAuCl4 Z99.99%) was used as received from SigmaAldrich. The quantity of enzyme labeled DNA present on themodified electrode surface was measured in 0.1 M phosphatebuffer saline (PBS), containing 0.1 M KCl and 1.8 mM hydroqui-none. All other chemicals were purchased from Sigma Aldrich andused as received unless otherwise stated. All aqueous solutionswere prepared using Milli-Q water. The oligonucleotides werepurchased from Eurogentec&TM and their purity was 498%. Thetarget oligonucleotide has been underlined for capture and probecomplementary sequences. The base mismatches have been high-lighted in bold for clarity. The base sequences are as follows

Capture: 50- CGG-CAG-TGT-TTA-TCA -30—SHTarget: 50–TGA-TAA-ACA-CTG-CCG-TTT-GAA-GTC-TGT-TTA-

GAA-GAA-ACT-TA-30 0

Probe: 50Horseradish peroxidase-TA-AGT-TTC-TTC-TAA-ACA-GAC-30

1 Base Mismatch: 50–TGC-TAA-ACA-CTG-CCG-TTT-GAA-GTC-TGT-TTA-AAA-GAA-ACT-TA-30 0

3 Base Mismatch: 50–TGC-TAA-ACA-CTG-CCG-TTT-GAA-GTC-TGT-TTA-GAT-GAA- ATA-TA-30 0

2.2. Instrumentation

A three-electrode electrochemical cell was used throughout ata temperature of 2272 1C. The working electrode was a 2 mmdiameter planar gold disk. It was polished with a nylon cloth with1 mM diamond polish and thoroughly rinsed with milli-Q waterand ethanol before sonication in milli-Q water for 5 min. Voltam-metry in acid was used to determine the surface roughness factor.The counter electrode was a large area coiled platinum wire and asilver/silver chloride (Ag/AgCl, KCl (satd)) acted as reference.

Absorption spectra (300 to 1100 nm) of thin polymer filmssupported on ITO-glass were recorded using Shimadzu UV-1601spectrophotometer. Raman spectroscopy of polymer films wereperformed on a Jobin Yvon Horiba HR800 connected to a CCDdetector. The laser beam (He–Ne laser) with 632.8 nm excitingradiation utilizing a 300-line grating, was focused on the sampleby a 100x lens. Baseline correction and smoothing were performedusing Lab Spec software.

2.3. Procedures

2.3.1. Synthesis of PEDOT

Vapor phase polymerization, VPP, consists of three key steps:oxidant deposition, monomer polymerization and residual oxi-dant removal.(Groenendaal et al., 2001) Fe (III) tosylate in a 40%aqueous solution of 1-butanol was used as the oxidizing agent.The catalyst was deposited by drop casting 250 mL of the 0.85 Moxidant directly onto the electrode surface and allowing thesolvent to evaporate. When the coating was almost dry, butbefore the Fe (III) tosylate forms crystals, the samples were putin an oven at 120 1C for approximately 2 min. until the solventevaporated and a dark yellow film formed. The electrode wasexposed to the EDOT monomer vapor for approximately 1 h. atroom temperature in a sealed chamber. Polymerization occurs asthe EDOT monomer vapor comes into contact with the oxidantlayer on the substrate forming a PEDOT film. The modifiedelectrode was then removed from the chamber, washed inethanol or methanol to remove any residual oxidant and driedquickly under a hot air blower (40 1C) for approximately 20 sec-onds. The electrode was then allowed to cool to room tempera-ture before any further modification or analysis.

2.3.2. Synthesis of PEDOT-AuNP

A 3 mM HAuCl4 in aqueous 0.1 M KCl was used to deposit goldnanoparticles onto the PEDOT modified working electrode byapplying a fixed potential of �0.273 V for 3 min.

2.3.3. DNA probe immobilization and hybridization

Step 1: The capture oligo (30 thiolate) was immobilized byimmersing the working electrode (unmodified, PEDOT modifiedor PEDOT-AuNP modified) in a 1 mM oligo solution prepared in1 M Denhardt’s Buffer. After 5 hours the electrode was rinsedwith deionized water for 15 s to remove loosely bound oligo. Step2: The target oligo was hybridized to the capture surface at 37 1Cin hybridization buffer for 90 min. Following hybridization, themodified electrode was rinsed thoroughly with buffer. Step 3: TheHRP-labeled probe oligo was hybridized to the target by immer-sing the modified electrode in a 1 mM solution of the enzymelabeled oligo for 90 min at 37 1C. Finally, it was thoroughly rinsedand dried in a nitrogen stream.

2.3.4. Electrochemical detection of S. aureus target

The quantity of the Horse Radish Peroxidase, HRP, labeledprobe DNA present on the surface was determined using a1.8 mM hydroquinone redox mediator dissolved in phosphatebuffer saline (0.1 M KCl). This solution was thoroughly deoxyge-nated using argon. Due to the sensitivity of the hydroquinone tophotodegradation, the solution was prepared daily and the cellwas wrapped in tinfoil to prevent photochemical degradation.Chronoamperometry experiments, at �0.4 V vs. an Ag/AgClreference electrode were used to measure the current associatedwith reducing the benzoquinone produced by the hydrogenperoxide that was generated by the HRP.

Fig. 2. Cyclic voltammograms of unmodified gold electrodes (lower curve), and

electrodes modified with PEDOT (G¼1.870.1�10–9 mol cm�2, middle curve)

and PEDOT–AuNP (upper curve) in 0.1 M H2SO4. The scan rate is 100 m V s�1. The

working electrode is a 2 mm diameter gold disk and the potentials are referenced

with respect to a Ag/AgCl, KCl (satd) reference.

E. Spain et al. / Biosensors and Bioelectronics 41 (2013) 65–70 67

3. Results and discussion

3.1. Voltammetry of PEDOT and PEDOT-AuNP films

Depositing gold nanoparticles onto the surface of the PEDOTfilm can improve the conductivity of the polymer film as well asincreasing the surface area available for binding thiol terminatedcapture DNA. Fig. 1 shows SEM images of the PEDOT films formedby vapor polymerization before (Fig. 1A) and after (Fig. 1B)electrodeposition of the gold nanoparticles. The PEDOT films arecontinuous and exhibit significant surface roughness as well asporosity which is desirable for amplification of DNA binding. Thetopography changes somewhat after nanoparticle electrodeposi-tion with a more nodular film being observed. These changesmost likely arise from the application of a negative potential so asto reduce the gold salt as well as the presence of the goldparticles.

The active surface area of the gold can be determined bycycling the potential in acidic electrolyte so as to create andsubsequently reduce a gold oxide monolayer on the exposedunderlying electrode and the PEDOT bound nanoparticles. Fig. 2illustrates cyclic voltammograms for the unmodified electrode,PEDOT and PEDOT–AuNP modified electrodes when cycled in0.1 M H2SO4. The bare electrode exhibits well-defined wavesassociated with gold oxide formation and reduction (Shen et al.,2005) at potentials of þ1.38 V and þ0.94 V, respectively. Thecalculation of the electrochemical area and surface roughnessfactor is given below in Eqs. 1–3, where AG is the geometric area,A is the electrochemical area, Ap is the area under the peak ofinterest and R.F. is the roughness factor. The charge passed duringthe reduction of a monolayer of gold oxide is 390 mC cm�2 and,following correction for double layer charging, the oxide reduc-tion peak yields an area of 0.037 cm2 corresponding to a rough-ness factor (ratio of the microscopic to geometric areas) of 1.2.

AG ¼ pr2 ð1Þ

A¼Ap

390mCcm�2ð2Þ

R:F:¼AG

Að3Þ

After poly(3,4-ethylenedioxythiophene) deposition onto theworking electrode surface by vapor phase polymerization, theoxide reduction and oxidation peak potentials shift by approxi-mately –0.120 V and �0.02 V, respectively. Significantly, thecharge under the oxide reduction peak increased following PEDOTdeposition corresponding to an apparent gold area of 0.067 cm2

(apparent roughness factor of 2.1). This result suggests that eitherthe catalyst or PEDOT deposition causes some restructuring of theelectrode surface. The PEDOT surface coverage, G, as measured

Fig. 1. SEM of PEDOT films deposited from the vapor phase before (A) and after

(B) electrodeposition of gold nanoparticles. For both images the scale bar

represents 100 mm. The accelerating voltage is 5 kV in both cases.

using voltammetry in acetonitrile containing 0.1 M LiClO4 assupporting electrolyte is 1.870.1�10–9 molcm�2.

In the second stage, reduction of Au3þ leads to the formationof gold nanoparticles, predominantly on the PEDOT surface.Current–time transients followed classical nucleation and growthdynamics and the upper limit on the mass of gold deposited wasestimated using Faraday’s law from the charge passed assuming a100% faradaic efficiency.

Fig. 2 shows that the PEDOT–AuNP film exhibits significantlyhigher peak currents and a smaller peak-to-peak potentialseparation when compared with the unmodified electrode, whichis consistent with the formation of gold nanoparticles. (Serafınet al., (2011)) Significantly, deposition of the gold nanoparticlesincreases the gold surface area by a factor of approximately four.The creation of this additional area for DNA binding ought toincrease the sensitivity of DNA detection. For systems of this kind,scanning electron microscopy reveals that the surface of thePEDOT is coated with nanoparticles with radii between approxi-mately 50 and 120 nm that are randomly distributed across theentire surface. (Spain et al., 2011)

3.2. UV–vis-NIR spectroscopy

The combination of an unusually low oxidation potential aswell as a relatively low band gap, gives PEDOT unique electro-chemical and spectroscopic properties which are not found inother conducting polymers.(Zotti et al., 1996) When PEDOT isoxidized, polarons exist and the material has a radical cationstructure. The free radical is highly reactive and capable ofgenerating a new covalent bond which may couple with another

Fig. 3. UV–vis spectra of PEDOT films grown by vapor phase polymerization (thin

line) and after gold nanoparticle electrodeposition (bold line) onto ITO glass

electrode.

E. Spain et al. / Biosensors and Bioelectronics 41 (2013) 65–7068

free radical to form a bipolaron to give two cations.(Kim et al.,1997) PEDOT exhibits a strong sky blue color when in theconducting state. Fig. 3 illustrates UV–Vis spectra of the vaporphase polymerized PEDOT film before and after nanoparticledeposition. For PEDOT, the absorption peak centrered at 306 nmis characteristic of a p–p* transition, suggesting that most of thepolymer is in an oxidized, polaronic state.(Cho et al., 2010;Ouyang et al., 2004) The PEDOT film also exhibits a broadbipolaron absorption between 600 and 1300 nm which is oftenreferred to as a ‘‘free carrier tail’’.(Zheng HuaJing et al., 2010;Pacios et al., 2007) Broad absorption bands in the 800 nm regionare attributed to polarons and/or bipolarons or electrons that aredelocalized on the PEDOT chains and that charge hopping isfacilitated.(Kim et al., 2009) The absorption bands at 408 nmrange are assigned to transitions from the valence band to theuppermost bipolaron band.(Cho et al., 2010) In contrast, thedominant feature in the spectrum of the PEDOT–AuNP nanocom-posite is a band centrered around 666 nm which suggests that thepolymer structure is in a bipolaron state (Sakmeche et al., 1996).In addition, strong absorbance is observed in the 400–500 nmregion. This band has been previously assigned to the excitontransitions of the main chain of PEDOT(Pacios et al., 2007) butthere may also be a contribution from the surface plasmonresonance of the gold nanoparticles. The dominance of thesebands is consistent with the relatively high surface coverage ofnanoparticles revealed by voltammetry in acidic electrolyte aswell as the large extinction coefficient of gold nanoparticles.

3.3. Raman microscopy

The voltammetry in acidic electrolyte clearly demonstratesthat the area available for immobilizing DNA capture strands issignificantly higher for the PEDOT–AuNP nanocomposite. How-ever, given our focus on using enzyme based electrocatalysis toamplify the response obtained for the capture of a small numberof DNA copies, the conductivity of the composite is as importantas the available gold area if DNA detection throughout the fullfilm thickness is to be achieved. Raman spectroscopy combinedwith electrochemical doping studies, allows doping inducedstructural changes to be identified.(Zykwinska et al., 2003) Thelinear conformation orients the neighboring thiophene rings ofthe PEDOT chain into almost the same plane (Zykwinska et al.,2003) causing the p-electrons to be delocalized over the wholechain. PEDOT with its alkylendioxy substituents in the 3 and4 position prevents a,b coupling and ensures a linear chainformation. The electron donating oxygen atoms lower the oxida-tion potential compared to thiophene and this stabilizes theoxidized polymer.(Zykwinska et al., 2003) Fig. 4 illustrates theRaman spectrum of the vapor polymerized PEDOT film. The mostintense band at 1422 cm�1 is assigned to the symmetric stretch-ing (Ca–Cb) mode of the aromatic CQC band while a less intenseband at 1526–1546 cm�1 is assigned to the CQC antisymmetricstretching (Ca–Cb) vibration. Other weaker bands are assigned tothe stretching mode of the single C–C bond (Cb–Cb) at 1363 cm�1

and to the stretching mode of the C–C inter-ring bond (Ca–Ca) at1237 cm�1.(Sakmeche et al., 1996)

Following deposition of gold nanoparticles an intense mode at1449 cm�1 is observed that is attributed to the symmetricstretching vibration of CaQCb and a weak peak at 1473 cm�1 isassigned to the antisymmetric stretching vibration of CaQCb.This single peak at 1473 cm�1 can be compared to the PEDOTband centrered at approximately 1422 cm�1 in the absence ofAuNPs. This band is actually the combination of three peaks at1422, 1526 and 1546 cm�1 and the single peak found after AuNPdeposition indicates a transformation of the resonant structure ofthe polymer upon incorporation of the metal particles from a

mixture of benzoid and quinoid (Scheme 2) in the parent poly-mer, to a mostly quinoid structure for the composite.(Semaltianoset al., 2010) Other peaks observed at 1321, 1208, 1058, 958,675 cm�1 are due to CbQCb stretching, C–C inner ring stretching,C–O–C deformation and symmetric C–S–C deformation, respec-tively. Two resonant structures have been proposed for PEDOT-benzoid and quinoid. In the case of the benzoid structure thereare two conjugated p-electrons on the CaQCb, whereas there isno conjugated p-electron on the Ca–Cb bond for the quinoidstructure. Overall, these results indicate that the PEDOT withinthe nanocomposite is in a highly doped state.(Harish et al., 2009)Raman spectroscopy indicates that the PEDOT film exists as aquinoid structure after gold nanoparticle deposition. The AuNPspresent within the PEDOT allow for more facile electron transferthroughout the polymer network which would in turn improvethe redox switching rate for electrochemical sensors in compar-ison to the parent PEDOT polymer. Significantly, this resultindicates that the PEDOT within the nanocomposite exists in ahighly oxidized conducting state and that there is a significantelectronic interaction between the two components of thenanocomposite.

3.4. DNA detection

The hybridized target DNA can be conveniently detected bymonitoring the reduction of hydroquinone that mediates electrontransfer to the HRP labeled probe strand. As shown in Eq. (1),

Fig. 4. Raman spectra of the vapor polymerized PEDOT film before (thin line) and

after AuNP deposition (bold line). The substrate is an ITO electrode. Excitation was

via a He-Ne laser at 632.8 nm.

Scheme 2. General structures of PEDOT in the (A) benzoid form and

(B) quinoid form.

E. Spain et al. / Biosensors and Bioelectronics 41 (2013) 65–70 69

hydrogen peroxide is a substrate of HRP which catalyzes thereduction of H2O2 yielding the oxidized form of the enzyme. TheHRP is then re-reduced by H2Q to give BQ which is then reducedat the electrode to give a measurable current.(Spain et al., 2011)

HydroquinoneþH2O2��!HRPbenzoquinoneþH2O

benzoquinoneþ2Hþ þ2e���!�0:4Vvs:Ag=AgClhydroquinoneð4Þ

Here, 200 mM H2O2 was added to the cell and the system andallowed to equilibrate for 10 min before the current was mea-sured. The electrode response, Di, is defined as the difference incurrent before and after the addition of H2O2.

Fig. 5 shows the dependence of Di on the semi-log of the DNAconcentration for the detection of the DNA from the pathogen S.

aureus that causes mammary gland inflammation using a baregold electrode after AuNP electrodeposition as well as films ofboth PEDOT and the PEDOT–AuNP nanocomposite. Significantly, alinear response is observed for each system for concentrationsranging from 150 pM to 1 mM DNA. The observation that Diincreases linearly with log [DNA] rather than [DNA] suggests thatthe current response is influenced by the concentration of theHRP co-reactant, H2O2, as well as the DNA concentration. Thecurrent at a fixed DNA HRP concentration increases from 0.05 mAto 0.4 mA ongoing approximately from 0.2 mM to 4 mM H2O2.However, the signal-to-noise ratio that ultimately controls theanalytical performance (LOD) did not increase significantly and allsubsequent measurements were performed using 200 mM H2O2.Moreover, using a low H2O2 concentration minimizes the like-lihood of DNA damage.

The sensitivity increases approximately eighty fold for thePEDOT film compared to the unmodified electrode. Significantly,even at a DNA concentration of 150 pM the signal-to-noise ratio isat least ten. Irrespective of the DNA concentration, electrodeposi-tion of the gold nanoparticles increases the absolute magnitude ofthe current observed, e.g., where the DNA concentration is 1 mM,Di is approximately four fold higher for the nanocomposite thanfor the parent polymer. The absolute magnitude of the currentobserved for the nanocomposite also nearly doubled in size whencompared to the AuNPs in the absence of PEDOT. This increase insignal arises due to the larger quantity of capture strand DNA thatcan be immobilized in the presence of the gold nanoparticles.Moreover, the analytical sensitivity increases by a factor of almostfour following nanoparticle deposition. However, in trace analysisthe key issue is the signal-to-noise ratio (S/N). In all cases thereproducibility is excellent even at low DNA concentrations andat 150 pM the signal-to-noise ratio is at least ten. Significantly,where the capture oligo or the target oligo were omitted, the Divalue observed is of the order of a few nanoamps., i.e., less than0.1% of that observed in the presence of a 150 pM concentrationof the target DNA. This result suggests that non-specific adsorp-tion of the capture or probe strand is negligible.

The selectivity of the sensor was also investigated using atarget DNA sequence that contained a single mismatch. Signifi-cantly, the differential current observed for this 1 base mismatchDNA sequence was a factor of four smaller than that found for thecomplementary DNA. Moreover, Staphylococcus epidermidis(S. epidermidis), which has 3 base mismatches produced aresponse only 6% of that found for the full matching oligonucleo-tide. The results show that a three base mismatch showedexcellent discrimination between the nucleic acid sequencesand the system is robust with respect to false positives. This issignificant since S. epidermidis is often mistaken for S. aureus andits presence incorrectly associated with mastitis.

4. Conclusion

The detection of infectious species and genetic mutation at lowconcentrations can lead to a reliable diagnosis before any symp-toms of a disease appear. Here we demonstrate that PEDOT filmsformed by vapor phase polymerization and subsequently functio-nalized with gold nanoparticles and capture strand DNA leads to ahighly sensitive DNA detection approach. This vapor phase poly-merization approach is highly compatible with mass manufactureof low cost, high sensitivity biosensors. The sensitivity of thenanocomposite film is more than one eighty times greater thanthat found for a bare electrode. This sensitivity, coupled to adynamic range of more than four orders of magnitude and

Fig. 5. Dependence of the difference in current before and after addition of H2O2 on log[DNA] for an unmodified electrode (m),AuNP (K), vapor polymerized PEDOT (~)

and vapor polymerized PEDOT following AuNP deposition (’). The DNA sequence is specific to the S. aureus pathogen. The applied potential was �0.40 V. Where error

bars are not visible, they are smaller than or comparable to, the size of the symbols.

E. Spain et al. / Biosensors and Bioelectronics 41 (2013) 65–7070

picomolar limits of detection for DNA from the pathogen S. aureus,makes these nanocomposite materials very attractive for sensordevelopment. Moreover, the approach show excellent ability todiscriminate against DNA sequences that contain basemismatches.

Acknowledgment

This material is based upon works supported by the ScienceFoundation Ireland under Grant no. 10/IN.1/B3021.

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