Tailoring the Conductivity of Polypyrrole Films Using Low-Energy

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Tailoring the Conductivity of Polypyrrole Films Using Low-Energy Platinum Ion Implantation Marsilea Adela Booth,†,‡ Jérôme Leveneur,†,§ Alexsandro Santos Costa,† John Kennedy,§,⊥ and Jadranka Travas-Sejdic*,†,⊥ †

Polymer Electronics Research Centre (PERC), University of Auckland, Private Bag −92019, Auckland, New Zealand Institute of Environmental Science Research Ltd. (ESR), Private Bag 92-021, Auckland, New Zealand § National Isotope Centre, GNS Science, 30 Gracefield Road, Lower Hutt, New Zealand ⊥ MacDiarmid Institute for Advanced Materials and Nanotechnology, New Zealand ‡

S Supporting Information *

ABSTRACT: Low-energy platinum ions were implanted with 15 keV under normal incidence into synthesized conducting polymer films with the aim to improve film conductivity and to demonstrate the use of implanted platinum in a simple sensing design. Conductivity measurements, cyclic voltammetry, and Raman spectroscopy were performed on samples both before and following ion implantation. Results display an optimum fluence of ion implantation for which polypyrrole films implanted with 2 × 1016 at. cm−2 display and retain enhanced conductivity compared with nonimplanted samples. X-ray photoelectron spectroscopy (XPS) and scanning electron microscope−energy-dispersive Xray spectroscopy (SEM-EDS) confirmed that implanted platinum is present mainly as Pt0 and indicated that the depth and amount of ion implantation are in agreement with a simulated implantation profile. Raman spectroscopy showed a surface-enhanced Raman spectroscopy (SERS) effect with platinum’s presence. The advantageous increase in conductivity can be rationalized by two chemical modifications to the polymer upon high-fluence implantation: (1) an increase in the number of charge carriers (dications) within the polymer and (2) the presence of elemental platinum metal and its synergistic effect on conductivity. A simple DNA sensor was constructed on the basis of polypyrrole/Pt0 films where Pt0 was able to serve as anchoring points for DNA attachment as well as an enhancer of the film’s conductivity. This enabled a DNA sensor capable of successful detection of cDNA, and a good discrimination of noncDNA, thus opening a way to direct electrochemical biosensing on the basis of ion implanted highly conducting polymer films.

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ranging from batteries and solar cells to biosensing.8,9 Polypyrrole (PPy) in particular has been targeted for application in biosensors, which have the potential to aid progress in medical diagnostics and forensic investigations.9,10 Previous studies on ion-implanted and swift heavy irradiated (∼Me V/u energy) conducting polymers, targeted for a range of applications such as p or n type junctions, capacitors, and actuators,7,8,11 have reported a range of outcomes from decreases to increases in conductivity.2,4,7,8,11−15 The energy, type, and fluence (number of ions deposited) of implanted ions affect the changes in conductivity observed. Increases in conductivity are reported and are explained by increases in ionic and charge carrier concentrations within the films, crosslinking of polymer chains and facilitated hopping of charge carriers within the polymer,4,8,16,17 and light reordering of polymer chains.12 Meanwhile, decreases in conductivity are thought to arise from damage to the conducting polymer rings (as revealed through X-ray photoelectron spectroscopy (XPS)

INTRODUCTION Ion implantation provides a means to modify mechanical, physical, and chemical properties of materials.1 It works through the controlled incorporation of atoms of one material into another material thereby bringing about physical and chemical property changes. A variety of effects are observed from the radiation caused by ion implantation, such as an increase in conductivity, damage to the polymer backbone, amorphization, and damage or loss of polymer counterions1−5 as well as the presence of the foreign chemical species within the sample. Current industries utilize this for semiconductor device fabrication6 demonstrating the value and ease of implementation of the technique. Many existing research directions involve implantation of metals into nonconducting polymers.5,7 By utilizing a conductive precursor, a smaller quantity of noble metal may be required to enhance conductivity thereby decreasing the cost of the composite. To this cause, intrinsically conducting polymers (ICPs) may fulfill the role of the conductive precursor material. With some mechanical properties of plastics, flexibility, control over growth, low-cost, and the ability to conduct upon oxidation, ICPs offer versatility in applications © 2012 American Chemical Society

Received: January 19, 2012 Revised: March 20, 2012 Published: March 21, 2012 8236

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assuming a pristine C4H3N PPy film of 1.5 g cm−3 density.28 They revealed a projected range of between 20.5 and 22 nm below the PPy surface depending on the implanted fluence (Figure 1). The fluences employed provided a Pt peak

and X-ray diffraction (XRD) data analysis), disruption of the conducting π-system, and loss or damage to the polymer counterions.2,3 Other characteristics affected by ion implantation include surface morphology, crystallinity, mechanical stiffness, electrochemical features, mechanical work outputs, and stability.2,4,7,12,18 Essentially, any ion can be used for ion implantation1 with the type of ion implanted affecting sample properties.19 To this end, many different ions are used, such as hydrogen,13 alkali ions,11 and argon.2 In choosing a specific ion for implantation, for example, platinum (Pt) ions, enhanced characteristics of the PPy films may be targeted, for example, conductivity,20,21 catalytic activity,22,23 and biosensing abilities.24 Implantation allows for incorporation of conducting, metal Pt particles permanently within the PPy framework without the requirement of reducing agent. With control over the implantation fluence, the amount of noble metal implanted can be selected. It has been shown that a higher concentration of metal ions (gold) ensures close proximity between the particles as well as larger sizes of metal clusters (in polydimethylsiloxane (PDMS)),5 which in turn influence electrical contact across the film and hence conductivity.21 Similarly, here, inclusion of Pt within the film is investigated for the first time using ion implantation methodology and is expected to enhance PPy conductivity with only a small amount of noble metal implantation demonstrating a benefit over full Pt or impregnated Pt surfaces. The research herein investigates the changes in PPy films after the implantation of Pt ions. Specifically, the conductivity of films is compared for implanted and nonimplanted films as well as for composition and electrochemical properties. Finally, application of films as DNA sensors is examined. Improvements in the properties of PPy films may bring PPy one step closer toward real world use in biosensing applications.

Figure 1. Dynamic-TRIM simulations of Pt implantation at 15 keV with various fluences in PPy.

concentration varying between 2 and 34 at.%. To limit the influence of changes induced by exposure to high vacuum conditions on the data analysis, all samples were kept for the same length of time inside the vacuum chamber regardless of the implantation duration. Control samples of pristine PPy were included in the study in which no Pt ions were implanted; however, they were exposed to the same vacuum cycle as the implanted samples. Characterization of the Polymer Films. Scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy (EDS) measurements were performed with a Philips XL30S FEG (field emission gun) scanning electron microscope equipped with a SiLi (lithium drifted) EDS detector with a Super Ultrathin Window, which was able to measure both secondary electron (SE) and backscattered electron (BSE) images. Transmission electron microscope (TEM) images were captured with a Phillips CM12 TEM and a Gatan model 792 BioScan camera. The films, embedded in resin, were thinly sliced in order to view a cross section. Raman spectra were collected using a Reinshaw Raman spectrometer (system RM-1000) and a 785 nm laser excitation wavelength. Cyclic voltammetry (CV) and electrical impedance spectroscopy (EIS) of the polymer films were performed using a threeelectrode setup consisting of a Ag/AgCl reference electrode, a platinum counter electrode, and a working electrode of glassy carbon electrode and PPy film. EIS measurements were performed in PBS solution containing 5.0 mM [Fe(CN)6]3−/4− (1:1, mol:mol) at an applied bias potential of 230 mV. Data analysis was performed using the EC Lab Express Z fit function (V5.40, Bio-Logic). The dc conductivity of the PPy films was measured using a Jandel four-point probe instrument. A current of 100 μA was passed through the films while the voltage was measured. X-ray photoelectron spectroscopy (XPS) was recorded on a Kratos Axis Ultra DLD (Kratos Analytical, Manchester, United Kingdom) using monochromatic Al Ka X-rays (1486.69 eV) with X-ray power of 150 W. Data analysis was performed using CasaXPS using Kratos’ relative sensitivity factors. DNA Sensing Using Implanted Polymer Films. Oligonucleotides (ODNs) were purchased from Alpha DNA with the following sequences: Hairpin probe ODN 5′-HS-C6-

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EXPERIMENTAL SECTION Full experimental details are provided in the Supporting Information. In short, lithium perchlorate, propylene carbonate, phosphate-buffered saline (PBS, pH 7.4), and monomer pyrrole were obtained from Aldrich. Pyrrole was distilled and was kept under nitrogen atmosphere prior to use at −20 °C. All aqueous solutions were made with Milli-Q water (18.2 MΩ cm). Propylene carbonate solutions were deoxygenated prior to use through nitrogen bubbling for 10 min. Remaining chemicals were of reagent grade or better and were used as supplied. Electrochemical growth of the polymer films was performed by applying a constant potential of 0.85 V (vs Ag/AgCl (3 M NaCl)) from a solution containing 0.1 M LiClO4 dopant and 0.1 M pyrrole monomer in propylene carbonate. The films (possessing an area of approximately 1 × 1 cm) were secured to a nonconductive surface for implantation experiments. Implantation of the Polymer Films with Platinum Ions. PPy films were implanted with 15 keV Pt ions under normal incidence in high vacuum (6 × 1015 at. cm−2 arises because of overlap with the specific combination of energy and fluence31 providing radical formation as well as a high enough Pt concentration to overcome the percolation threshold5 and to enhance conductivity. In these experiments, relatively low energy ions are used (keV range); therefore, the dominant process is expected to be elastic binary collisions generally producing fragments, excited species, and radicals.31 An increase in generated radicals explains the increase in conductivity at high ion fluence through the increased cross-linking within the polymer,4 the formation of unsaturated groups in the molecular structure, and the increased conductivity through Pt metal contribution.8,31 Although conductivity is high for films implanted with 2 × 1016 at. cm−2, this begins to decrease over time because of air exposure and polymer degradation as do conductivities of all samples to different extents, Figure 3B. However, after 15 days, the sample implanted with the highest Pt concentration still possesses the highest conductivity. Raman Spectroscopy of Films. To further investigate the effect of ion implantation on the conductivity of PPy films, Raman spectroscopy was performed. Raman spectroscopy is able to provide molecular-level information which is highly sensitive to changes within the measured substrates. Spectra obtained for all PPy films indicate characteristic bands of oxidized PPy, see Figure 4. As can be immediately observed, a striking increase in signal intensity arises from the Pt implanted samples when compared to that of pristine PPy. We suggest that this phenomenon is a feature of the surface-enhanced Raman spectroscopy (SERS) effect,32,33 which is expected to result from the interaction between PPy nitrogen atoms and neighboring Pt particles hence suggesting the presence of Pt particles. The nucleation of a Pt precipitate is expected during ion implantation which increases with the implanted concentration. There are three main areas of interest for comparison of the spectra, namely, the band arising from the CC backbone stretching of the PPy at 1580−1601 cm−1, bands at 1050−1083 cm−1 arising from the C−H in-plane deformation, and the band at 937−939 cm−1 indicating ring deformation.34,35 The latter two bands and their neighboring bands can be considered composed of two adjacent bands: one arising from dication deformations (937−939 cm−1 and 1083 cm−1) and one arising

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RESULTS AND DISCUSSION SEM-EDS and TEM Characterization of Films. Following ion implantation, the visual coloration of the films changed as a gradient function of ion dose, and a metallic sheen was observed for all implanted films. Physical and chemical properties of the films were further investigated using SEMEDS. The presence of Pt in the film is evidenced by EDS spectra, Figure 1 of the Supporting Information. The SE and BSE images of a cross section of PPy film implanted with 2 × 1016 Pt at. cm−2 are seen in Figure 2A and B. Because of the

Figure 2. Cross-sectional images of a PPy film implanted with 2 × 1016 at. cm−2 as a captured (A) SE image and (B) BSE image. The BSE image shows the distribution of Pt (white areas) within the PPy film. Cross-sectional TEM image of (C) pristine PPy and (D) a PPy film implanted with 6 × 1015 at. cm−2 showing the implanted Pt as a higher contrast dark layer. In all EMs, the arrows indicate the implanted side of the films.

large atomic number, the Pt signal is observed with a brighter intensity and a higher contrast. The depth profile of Pt implantation can be clearly observed in the cross section, where the maximum implanted Pt ion concentration is found very close to the polymer surface as expected by the 21 nm depth target. This is corroborated by TEM images as shown in Figure 2C where the implanted Pt is clearly visible as a highly contrasted layer just below the polymer surface. Conductivity of Films. Conductivity of the PPy films was investigated as seen in Figure 3A where conductivity is displayed as the percentage of conductivity remaining after various levels of ion implantation. Calculations of conductivity 8238

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Figure 3. (A) Remaining conductivity of PPy films as a function of the number of implanted ions. Error bars show the standard deviation from the six conductivity measurements. A line is drawn to guide the eye. (B) Graph showing the stability of sample conductivity for PPy films: (a) PPy film with 6 × 1015 at. cm−2 implanted Pt ions, (b) pristine PPy, and (c) with 2 × 1016 at. cm−2 implanted Pt ions. The lines serve to guide the eye.

length. The low conjugation length is explained by the collisions during ion implantation producing fragments within the polymer.31 We infer from the Raman spectra that although dication species are spaced intermittently within the highly Pt implanted PPy film, the increased concentration throughout the film provides high conductivity. Electrochemical Characterization of Films. Following the increase in conductivity observed for implanted films, the electrochemical properties of the films were investigated. Cyclic voltammetry (CV) of the polymer films indicates similar CV shape with very broad oxidation (between 0 and 0.3 V) and reduction peaks (between 0.3 to −0.4 V attributed to the ClO4− dedoping of the film33), Figure 2 of the Supporting Information. An increase in charge is calculated for implanted PPy (158 mC cm−2) as compared to nonimplanted PPy (154 mC cm−2) showing a beneficial effect of ion implantation on electrochemical activity perhaps because of fewer ohmic losses. Raman spectroscopy was performed on films after CV cycling (data not shown). Results indicate some degradation of both films by shifts in characteristic bands. Degradation can be explained by irreversible reactions to the PPy backbone that occur because of nucleophilic attack of species such as OH− ions.36 One degradation pathway can be monitored through analysis of the conjugation length in the films estimated by the benzoid/quinoid band at ∼1600 cm−1, which shifted from 1580 to 1607 cm−1 for the nonimplanted PPy and from 1601 to 1605 cm−1 for the PPy implanted with 2 × 1016 Pt at. cm−2. This suggests that conjugation length is better maintained by implanted PPy than by pristine PPy. XPS Characterization of Films. To further investigate the interactions between the implanted Pt atoms and the polymer film, XPS was employed. Survey spectra indicate all expected elements for each sample with no Pt contamination in nonimplanted samples, Figure 3 of the Supporting Information. To examine the implantation of Pt ions, a depth profile of a sample implanted with 2 × 1016 at. cm−2 was performed. This was done by combining a sequence of ion gun etch cycles (in our case, argon ions were used) interspersed with XPS measurements from the unveiled surface. The concentrations for each element through the etched layer are seen in Figure 5. Survey spectra were captured for each cycle as core level measurements of PPy during sputtering are unadvisable because of the possibility of polymer structural changes during sputtering.3 Results revealed that the PPy surface has a high oxide concentration as expected from atmosphere exposure and degradation. The Pt profile concentration begins low at the PPy surface but increases to a maximum at about 500 s of etch

Figure 4. Raman spectroscopy results for (a) pristine PPy film, (b) PPy film with 6 × 1015 at. cm−2 implanted Pt ions, and (c) 2 × 1016 at. cm−2.

from radical cation deformations (∼983 cm−1 and 1050 cm−1).34 By comparing the ratios of these two bands (Table 1), information can be gathered about the major species Table 1. Band Intensity Ratios and Positions for the Different Pt Ion Implantation Fluences Pt implanted (at. cm−2) 0 1 3 6 1.2 2

× × × × ×

1015 1015 1015 1016 1016

(I1083 + I937)/(I1050 + I983)

Raman band position of ring deformation (cm−1)

Raman band position of C−C stretch (cm−1)

1.17 1.17 1.09 1.07 1.17 1.21

934 938 938 936 937 939

1580 1582 1601 1596 1587 1601

present in the PPy films. The results suggest that the sample implanted with 2 × 1016 at. cm−2 has a higher proportion of dications (or bipolarons) than pristine PPy, which in turn has a higher proportion of dications than PPy implanted with 6 × 1015 at. cm−2 correlating with conductivity measurements. Despite the high number of dication species within the polymer film, implanted films appear to have low conjugation lengths. The band located at 1580−1601 cm−1 is able to provide information about the benzoid or quinoid structures and, hence, conjugation length within the film. It has been shown that the lower the wavenumber of this band, the higher the conjugation length within the polymer.32 From this, it can be seen that highly implanted samples have a low conjugation length, while pristine PPy retains the highest conjugation 8239

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metal, while peaks 3 and 4 at 74.13 and 77.46 eV, respectively, indicate that the remaining 6.0 ± 0.3% of the Pt is in the form of Pt(II).41 Meanwhile, samples implanted with 6 × 1015 at. cm−2, Figure 6B, display a Pt(0) concentration of 80 ± 3% and a Pt(II) concentration of 20 ± 3%. The oxygen Pt−O signal at 530 eV42 also shows a higher contribution for a film with 6 × 1015 at. cm−2 than for a film with 2 × 1016 at. cm−2 implanted ions, Table 2. The majority of Pt present in both samples exists in metallic form having been either precipitated as a result of the energy of ion implantation and immiscibility of Pt in PPy or reduced by either the PPy film or through other pathways, such as environmental iron and silicon contamination. The Pt 4f7/2 peaks (72.1 and 72.6 eV for films implanted with 2 × 1016 at. cm−2 and 6 × 1015 at. cm−2, respectively) are higher than that of bulk Pt (Pt 4f7/2 = 71.2 eV)23,40 though this can be explained by interactions between the Pt atoms and PPy, such as electron shifts causing decreased electronic charge density on the Pt.43 Interactions between metals and ICPs have been observed, for example, exhibited as metal/ICP complexes.44 Such interactions may aid in the conductivity along the polymer as conducting links, when present at high concentrations such as in a sample implanted with 2 × 1016 Pt at. cm−2, can contribute to high conductivity. The presence of more Pt metal with increasing fluence is also in good agreement with the formation of metallic precipitates. Such precipitation during implantation has also been observed with gold metal nanoclusters in PDMS.5 These precipitates are likely to stay in the form of particles as the implanted concentration is not high enough to lead to the formation of a Pt layer. Biosensing Ability of Films. To investigate the properties of Pt implanted films as biosensors, a sensor platform was developed to selectively detect target DNA sequences. The higher nominal and retained conductivity of PPy after Pt implantation will provide improved response when using electrochemical detection. Additionally, the simplicity of the sensor platform removed the need for complicated functionality of polypyrrole either before or following polymerization in contrast to most literature on biosensors using polypyrrole.10,9 The DNA sensing ability of Pt implanted films was demonstrated using polymer films implanted with a fluence of 2 × 1016 at. cm−2. A sensor was constructed using thiolated hairpin probe ODN and thiolated mPEG attached to the Pt particles within the film, Scheme 1.24,29 Thiolated mPEG is used in order to modulate probe surface density.29 Electrical impedance spectroscopy (EIS) is a sensitive and informative technique which can be used to detect target ODN. Data was modeled using a Randles equivalent circuit labeled as Rs + Q2/ (Rct + W2), where R represents resistors for solution resistance (Rs) and charge-transfer resistance (Rct), Q2 represents a constant phase element, and W2 represents a Warburg impedance element.9,10,33 Impedance measurements were performed before and after incubation with target ODN, both complementary ODN and noncomplementary. As can be seen

Figure 5. Depth profile of a PPy film implanted with 2 × 1016 at. cm−2 after etching with argon. Concentrations are based on integrated survey spectra taken after every 30 s of sputtering.

cycles to ca. 18 at.% of the sample complementing SEM results. The Pt profile is in good agreement with the profile expected from simulations, Figure 1. The observed differences in the concentration are likely to originate from the presence of additional species in the film (such as silicon contamination) and corresponding changes in the PPy density. Ar sputtering may also affect the depth profile because of the different sputtering yields dependent on the different densities and structures in the film. The amount of chlorine remains stable throughout the depth investigated suggesting that ClO4− doping is fairly homogeneous through the depth of the film. This data beautifully displays the distribution of the Pt through the PPy within the measured film. XPS core surveys allow in-depth analysis of the elemental state and composition of the films. Carbon core level deconvolution analyses of the PPy films describe the species present, namely, Cα bonds observed at 285 eV; Cβ bonds observed at 283 eV; C−OH, CN, and C−N+ bonds at 286 eV; CO and CN bonds at 287 eV; and COOH and COO− bonds at 289 eV.2,37−39 In comparing pristine films with implanted films, the major changes are a slight increase in Cβ bond contribution for films implanted with 2 × 1016 at. cm−2 perhaps because of graphitic carbon2 and an increase in the COOH (carbonyl) signal at 289 eV for implanted films, see Table 2. The level of carbonyl defects within the polymer help to explain the observed conductivity measurement in which the lowest conductivity is for a film with 6 × 1015 at. cm−2 implanted ions. The Pt deconvoluted peaks, Figure 6, indicate the Pt species present in the implanted films. For the purposes of analysis, it was assumed that the doublet components were of equal halfwidths, were positioned 3.33 eV in separation, and had an intensity ratio of 3:4 (5/2:7/2).40 The Pt 4f spectra reveal that samples are composed of a mixture of Pt metal (Pt(0)) and Pt(II) species. For the highest implanted sample (2 × 1016 at. cm−2), Figure 6A, peaks 1 and 2 at 72.1 and 75.4 eV, respectively, indicate that 94.0 ± 0.3% of the Pt exists as Pt

Table 2. XPS Data from Pristine PPy and PPy Implanted with 6 × 1015 and 2 × 1016 Pt at. cm−2 a # Pt (at. cm−2)

Cα 285 eV

Cβ 283 eV

C−OH, CN, C−N+ 286 eV

CO, CN+ 287 eV

COOH, COO− 289 eV

CO eV 531 eV

C−OH 532−533 eV

Pt−O 530 eV

0 6 × 1015 2 × 1016

69 ± 1 72 ± 1 68 ± 1

19.3 ± 0.9 18.6 ± 0.3 22.2 ± 0.4

8.0 ± 0.3 2.2 ± 0.7 5.2 ± 0.5

3.0 ± 0.3 2±2 2.6 ± 0.4

0.97 ± 0.07 4.3 ± 0.7 2.1 ± 0.4

11.0 ± 0.3 15.4 ± 0.9 26 ± 3

86.3 ± 0.5 71 ± 1 62.8 ± 0.8

0.0 14 ± 1 11 ± 3

a

Values are given as a percentage of the total C 1s or O 1s peak with the error as % st dev as calculated using Monte Carlo simulations. 8240

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Figure 6. XPS detailed spectrum and analysis of the Pt 4f region for a PPy film implanted with (A) 2 × 1016 Pt at. cm−2 and (B) 6 × 1015 Pt at. cm−2.

Scheme 1. (A) PPy film implanted with Pt ions with hairpin probe ODN attached preventing some redox species from approaching the electrode surface. (B) After hybridization, the double-stranded ODN formation allows redox species to approach the surface, and hence, redox reactions occur more readily

charge-transfer resistance can be normalized against the Rct value for the film prior to target ODN incubation (Rct0) giving Rct/Rct0.9 The decrease in Rct after target incubation can be explained by the unfolding of hairpin probes upon hybridization, Scheme 1B. Hairpin probes occupy a larger area as compared to single-stranded probes, and hence, following hybridization and unfolding the uncovered surface provides access to the Fe(CN)63−/4− decreasing charge-transfer resistance. In the case of noncomplementary ODN, the slight decrease in Rct observed is suggested to arise from nonspecific interactions. The large concentration of noncomplementary target used (5 mM) should be noted and indicates that the sensor possesses good selectivity.

in Figure 7 insets i and ii, a significant decrease in the chargetransfer resistance (Rct) occurs after incubation of a probemodified PPy surface with complementary target ODN, while only a small change occurs after incubation with noncomplementary target ODN, Figure 7 inset iii. Changes in

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CONCLUSIONS The implantation of Pt ions within PPy conductive films causes interesting changes within the polymer behavior and character. The implanted fluence affects the resulting properties. Implantation of ions at a fluence range of 1 × 1015 to 6 × 1015 at. cm−2 causes defects in the polymer backbone through consequences such as disruption of the conducting π backbone and production of further defects (e.g., carbonyl functionalities) within the polymer. This in turn impinges on the conductivity of the film, whereby a decrease in conductivity is observed. When a high fluence is applied (2 × 1016 at. cm−2), defects are still present, though their effect is overpowered by the synergistic effect of the increased Pt concentration as well as by the increase in charge-carrier concentration importantly resulting in increased conductivity. In addition, the presence of Pt metal precipitates resulting from the large implanted

Figure 7. Normalized changes in the charge-transfer resistance for a probe ODN incubated PPy film implanted with 2 × 1016 Pt at. cm−2 after incubation with different ODN sequences and concentrations (A) 50 μM complementary target ODN, (B) 5 μM complementary target ODN, and (C) incubation with 5 mM noncomplementary ODN. Insets show the corresponding electrical impedance spectra measured in 5 mM Fe(CN)64−/3− redox couple in PBS for (a) probemodified polymer and (b) incubation with (i, ii) complementary target ODN and (iii) noncomplementary target ODN. 8241

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(14) Schiestel, S.; Banniza, P.; Wolf, G.; Edinger, K. Nucl. Instrum. Methods Phys. Res., Sect. B 1996, 116, 164−167. (15) Himanshu, A.; Bandyopadhayay, S.; Sen, P.; Nath Mondal, N.; Talpatra, A.; Taki, G.; Sinha, T. Radiat. Phys. Chem. 2011, 80, 414− 419. (16) Kishore, S.; Gavade, C.; Singh, D.; Singh, N. Radiat. Eff. Defects Solids 2011, 166, 606−614. (17) Wasserman, B. Phys. Rev. B 1986, 34, 1926−1931. (18) Banerjee, S.; Kumar, A. Radiat. Eff. Defects Solids 2011, 166, 598−605. (19) Ramola, R.; Alqudami, A.; Chandra, S.; Annapoorni, S.; Rana, J.; Sonkawade, R.; Singh, F.; Avasthi, D. Radiat. Eff. Defects Solids 2008, 163, 139−147. (20) Goswami, L.; Sen Sarma, N.; Chowdhury, D. J. Phys. Chem. C 2011, 115, 19668−19675. (21) Costa, A. S.; Li, K.; Kilmartin, P. A.; Travas-Sejdic, J. AIP Conf. Proc. 2009, 1151, 123−125. (22) Jeon, S. S.; Kim, C.; Ko, J.; Im, S. S. J. Phys. Chem. C. 2011, 115, 22035−22039. (23) Cen, L.; Neoh, K.; Cai, Q.; Kang, E. J. Colloid Interface Sci. 2006, 300, 190−199. (24) Ferreira, V. C.; Melato, A. I.; Silva, A. F.; Abrantes, L. M. Electrochem. Commun. 2011, 13, 993−996. (25) Islah-u-din; Fox, M. R.; Martin, H.; Gainsford, G. J.; Kennedy, J.; Markwitz, A.; Telfer, S. G.; Jameson, G. B.; Tallon, J. L. Chem. Commun. 2010, 46, 4261−4263. (26) Markwitz, A.; Kennedy, J. Int. J. Nanotechnol. 2009, 6, 369−383. (27) Biersack, J. Nucl. Instrum. Methods Phys. Res., Sect. B 1987, 27, 21−36. (28) Salmon, M.; Diaz, A.; Logan, A.; Krounbi, M.; Bargon, J. Mol. Cryst. Liq. Cryst. 1982, 83, 265−276. (29) Kjällman, T. H. M.; Peng, H.; Soeller, C.; Travas-Sejdic, J. Analyst 2010, 135, 488−494. (30) Chan, J. Four Point Probe Manual. http://physlab.lums.edu.pk/ images/c/c6/Four_point_probe.pdf (accessed Nov 29, 2011). (31) Giovanni, M. Nucl. Instrum. Methods Phys. Res., Sect. B 1990, 46, 295−305. (32) Lee, H. T.; Liu, Y. C.; Lin, L. H. J. Polym. Sci., Polym. Chem. 2006, 44, 2724−2731. (33) Chen, W.; Li, C. M.; Chen, P.; Sun, C. Electrochim. Acta 2007, 52, 2845−2849. (34) Duchet, J.; Legras, R.; Demoustier-Champagne, S. Synth. Met. 1998, 98, 113−122. (35) Furukawa, Y.; Tazawa, S.; Fujii, Y.; Harada, I. Synth. Met. 1988, 24, 329−341. (36) Yamato, H.; Ohwa, M.; Wernet, W. J. Electroanal. Chem. 1995, 397, 163−170. (37) Malitesta, C.; Losito, I.; Sabbatini, L.; Zambonin, P. J. Electron Spectrosc. 1995, 76, 629−634. (38) Hu, C. C.; Lin, X. X. J. Electrochem. Soc. 2002, 149, A1049− A1057. (39) Yao, C.; Li, X.; Neoh, K.; Shi, Z.; Kang, E. J. Membr. Sci. 2008, 320, 259−267. (40) Handbook of X-ray photoelectron spectroscopy; Chastain, J., Ed.; Perkin-Elmer: Eden Prairie, Minnesota, 1992. (41) Casella, I. G.; Desimoni, E. Electroanalysis 1996, 8, 447−453. (42) Peuckert, M.; Bonzel, H. Surf. Sci. 1984, 145, 239−259. (43) Sen, F.; Gökagaç, G. J. Phys. Chem. C 2007, 111, 5715−5720. (44) Tsakova, V. J. Solid State Electrchem. 2008, 12, 1421−1434.

concentration, evidenced through XPS analysis, can explain the increase in Raman signal through a SERS process. Furthermore, films implanted with 2 × 1016 at. cm−2 display and retain higher conductivity over time with air exposure as compared to pristine PPy. These enhanced properties of PPy/Pt0 are observed with relatively small amounts of noble-metal inclusion and are advantageous for biosensing applications utilizing electrochemical detection where the implanted Pt provides means for excellent attachment of probe ODN. Hence, a simple ODN sensor was demonstrated which was able to selectively distinguish between complementary target ODN and noncomplementary ODN using EIS as a detection technique. The benefits of enhanced conductivity and the presence of Pt0 demonstrated after ion implantation of Pt into PPy are not limited to advances in DNA sensing but may be beneficial for a vast scope of PPy applications, including energy storage systems, catalytic processes, and chemical and biological sensing.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed experimental details, an EDS spectrum for a PPy film implanted with 2 × 1016 at. cm−2, cyclic voltammograms of samples, and survey XPS data. This information is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: + 64 9 373 7599 ext. 88272. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Institute of Environmental Science and Research Ltd. and the University of Auckland for their financial support. We thank Dr. Colin Doyle (XPS) and Dr. Adrian Turner (TEM) for their expertise.

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REFERENCES

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dx.doi.org/10.1021/jp300682q | J. Phys. Chem. C 2012, 116, 8236−8242