Electrochemical characterization of self assembled monolayers on flexible electrodes

Electrochimica Acta 65 (2012) 159–164

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Electrochemical characterization of self assembled monolayers on flexible electrodes Erika Scavetta a,∗ , Antonio Giuseppe Solito a , Monia Demelas b,c , Piero Cosseddu b,c , Annalisa Bonfiglio b,c a b c

Department of Physical and Inorganic Chemistry, University of Bologna, Viale Risorgimento 4, 40136 Bologna, Italy Department of Electrical and Electronic Engineering, University of Cagliari, Piazza d’Armi, 09123 Cagliari, Italy CNR-Institute of Nanoscience, Via Campi 213/A, 41125 Modena, Italy

a r t i c l e

i n f o

Article history: Received 27 October 2011 Received in revised form 9 January 2012 Accepted 9 January 2012 Available online 16 January 2012 Keywords: Self assembled monolayers Flexible electrode Gold PET Electrochemical impedance spectroscopy

a b s t r a c t Self assembled monolayers of amine- and carboxylic acid terminated thiols have been chemisorbed on gold electrodes deposited on flexible polyethylene terephthalate substrates. These devices, being flexible, low cost, and highly resistant could find application in several fields such as the design of implantable biomedical devices and disposable light-weight sensors. Four different molecules have been investigated, namely cysteamine, 12-mercaptododecanoic acid, aminothiophenol, and 3-mercaptopropionic acid, each bearing either an amino or a carboxylic terminal group and different chain lengths. These molecules have been chosen since the SAM they can form could find application as such or constitute the basis for a further modification step. The modified surfaces have been characterized by electrochemical techniques and the surface pKa values of the terminal groups have been estimated by impedance titration. © 2012 Elsevier Ltd. All rights reserved.

1. Introduction The development of new analytical methods able to fulfill the need of rapid in situ analysis is a topic of great interest. These methods should be sensitive and accurate, and also applicable to the detection of various kinds of substances [1]. In situ detection devices and on-body wearable sensors are interesting since they can be used in the monitoring of people’s health [2,3] and for a wide range of healthcare, food industry, and sport applications. To this aim Wu et al. developed a flexible PDMS amperometric sensor which displays very good performances for the detection of H2 O2 [4]. Polyethylene terephthalate (PET), a plastic widely used in industry, has recently been used as the supporting substrate for sensing devices. In addition to being flexible, which allows its use for advanced implantable biomedical devices, PET is low-cost and does not show any interference with electrochemical signals, so it can be used as a substrate for highly sensitive, accurate, and disposable light-weight sensors.

∗ Corresponding author. Tel.: +39 0512093256; fax: +39 0512093690. E-mail address: [email protected] (E. Scavetta). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2012.01.033

A recent example of a PET application has been proposed by Wakamatsu et al. who prepared self-organized single-walled carbon nanotube thin films with a honeycomb structure on PET [5]. Recent advances in organic electronics have demonstrated that it is possible to fabricate whole electronic circuits including passive and active elements on top of PET substrates [6]. In order to build chemical sensors it is necessary to design interfaces sensitive towards a selected analyte. To this aim, chemically modified interfaces such as alkanethiol self-assembled monolayers (SAMs) on gold have attracted considerable scientific interest. In fact, not only they can provide a method for creating well-defined surfaces with controllable chemical functionalities, but they also allow the control of the electrode–solution interface and consequently of the electron transfer properties of the electrode itself [7,8]. SAMs are important components in many examples of nanotechnology products; they can assemble onto surfaces of any shape, geometry and dimension and thus constitute a highly versatile method to control the interface properties. Moreover, SAMs allow the addition of various functionalities to electrodic surfaces and their connection to more complex systems [9]. Owing to these reasons research in this area is still very active since SAMs are more than a specific class of compounds but a flexible concept with virtually unlimited applications [10,11]. SAMs can be used to prevent corrosion [12,13], to modify wetting properties

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[14], to develop nanodevices for electronics [15] and to generate molecular scale electrical junctions [9]. Very dynamic research areas are now those related to the use of SAMs to design surfaces coated with biologically active functionalities [16] and to the development of ‘switchable’ SAMs, which can change their properties when subjected to an external stimulus [17]. It is well-known that the functionalization with alkanethiols bearing either carboxylic (COOH) or amino (NH2 ) terminal groups allows the synthesis of charged templates similar to a Langmuir film. In electrolyte solutions, interactions between these groups and counterions of the electrolyte should affect the packing, ordering, and stability of SAMs [18]. Since amino and carboxylic groups, can change their charge depending on the solution pH, they can be exploited for the development of pH sensors [19,20]. Such property facilitates the direct electron transfer of biological molecules, as proposed by Bowden in his works [21], or the conferral of selectivity to the electrode, as reported by Shervedani et al., who used an Au-cysteamine SAM electrode to detect dopamine in the presence of a high concentration of ascorbic acid [22]. Moreover these kinds of SAMs can be the first step for a subsequent surface modification based on the chemical reactivity of amino and carboxylic groups. There are several examples of application in the literature. Cysteamine monolayers on gold electrodes, for instance, have been used to immobilize enzymes such as peroxidase [23,24], glutathione reductase [25], fructose dehydrogenase [26], glucose dehydrogenase apoenzyme [27], and glucose oxidase [28] or other kinds of sensitive molecules like EDTA, to develop amperometric devices selective towards Pb2+ or Cu2+ [29]. This work describes the formation of SAMs of amino or carboxylic acid terminated thiols on flexible PET-Au substrates to be used for the development of OFET based chemo sensors. Four different molecules have been studied, 2-aminoethanethiol (cysteamine, CA), 12-mercaptododecanoic acid (12-MDA), 4-aminothiophenol (4-ATP), and 3-mercaptopropionic acid (3-MPA), having different chain lengths and bearing either an amino or a carboxylic terminal group. Aminothiophenol has been selected as a model molecule to study the behavior of high-electron-density SAMs. The modified surfaces have been characterized by atomic force microscopy and by electrochemical techniques since they are suitable for the investigation of the packing degree of SAMs. In particular, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) have been employed to measure the influence of the charge of the acidic or basic terminal group of SAMs on the amperometric response for a negatively charged electroactive probe (e.g. Fe(CN)6 3− ). The redox response of the probe molecule changes with the solution pH, as well as the extent of the repulsion between the probe itself and the terminal group. Impedance spectroscopy has been employed since it describes more explicitly the faradaic or nonfaradaic behavior, such as currents due to diffusion, doublelayer charging, resistance of the solution (Rs ), and charge transfer (Rct ), which all occur in the SAMs [30–32]. The influence of substrate bending on the electrical parameters and on the electrochemical performances of the SAMs modified electrodes has been studied in order to assess the applicability of our devices to the construction of flexible sensors.

(3-MPA) from Sigma–Aldrich. All the reagents were of analytical grade and were used without further purification. All aqueous solutions were prepared with deionized water, processed through a MilliQ (Waters) water purification system (UPP). All the aminothiol and thio carboxylic acid solutions were freshly prepared under nitrogen atmosphere and stored in the dark, to avoid photo-oxidation. 2.2. Preparation of bare PET-gold electrodes Before use the PET substrates (175 ␮m thick) were cleaned in an ultrasonic bath in acetone first, then in 2-propanol, rinsed in deionized water and finally dried with a Nitrogen flux. Then a parylene C buffer layer (1.5 ␮m thick) was deposited by Chemical Vapour Deposition on the PET surface since our tests have proven that it significantly improves gold adherence to the substrate and its resistance to chemical treatments. Finally gold electrodes (∼90 nm thick) have been realized by thermal evaporation in a high-vacuum system (1 × 10−5 mbar) through a round shaped (6 mm diameter) shadow mask. The electrical contact between the gold disk and the potentiostat was realized by a copper wire glued using a silver paste. 2.3. Apparatus All the electrochemical tests were carried out in a single compartment, three-electrode cell. Electrode potentials were measured with respect to an aqueous saturated calomel electrode (SCE). A Pt wire was used as a counter electrode and the working electrode was the PET-Au electrode under investigation. An Autolab PGSTAT100 equipped with px1000 modulus for pH control (Ecochemie, Utrecht, The Netherlands) potentiostat/galvanostat interfaced with a personal computer was used in all the cyclic voltammetric (CV) measurements. Electrochemical impedance spectroscopy (EIS) measurements were performed with a CHInstruments Mod. 660A, controlled by a personal computer via CHInstruments software. The temperature was kept constant using a thermostat HAAKE D8. The tests were performed in 0.01 M buffer (phosphate or borate depending on the desired pH) solutions containing 0.5 mM Fe(CN)6 3−/4− as a redox probe, at a potential of 0.17 V versus SCE. The investigated frequency range was 100 mHz–10 kHz. The ionic strength was kept constant by adding 0.1 M KCl to all the buffered solutions. The impedance spectra were plotted in the form of complex plane diagrams (Nyquist plots) and the experimental data were simulated by using the software developed by Boukamp using Randle’s equivalent circuit. The morphology of the electrode surfaces, before and after SAM formation, were investigated by means of a SPM RESOLVER PRO by NT-MDT in semicontact mode. The effect of substrate bending on the resistance and on the electrochemical behavior of the functionalized and not functionalized electrodes was investigated. The resistance of the polished electrode before, during and after bending at a selected angle for 5 min, was measured using a Keithley 2636 SourceMeter. The influence of the bending induced stress on the electrochemical behavior of the SAM modified surfaces was studied by CV, placing the electrode into the three-electrode cell described above just after bending.

2. Experimental

2.4. Preparation of SAMs on gold electrodes

2.1. Chemicals

Before use, the electrodes were electrochemically polished by cycling from 0.0 to +1.5 V versus SCE at 1 V s−1 in 0.5 M H2 SO4 until reproducible cyclic voltammograms were obtained (usually 500 cycles were sufficient). The roughness factor was evaluated from the ratio of real to geometric surface area, which can be

K2 HPO4 , KH2 PO4 , and KOH were purchased from Fluka; H2 SO4 , H3 BO3 , Cysteamine (CA), 12-Mercaptododecanoic acid (12MDA), 4-aminothiophenol (4-ATP), and 3-mercaptopropionic acid

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Table 1 Conditions employed for SAMs formation. Thiol

CA

12-MDA

4-ATP

3-MPA

Concentration

24 mM, aqueous solution 16 h PET-Au-CA

10 mM, aqueous ethanol solution (ethanol/water ratio 75:25) 5h PET-Au-MDA

10 mM, aqueous ethanol solution (ethanol/water ratio 75:25) 5h PET-Au-ATP

10 mM, aqueous solution

calculated by integration of the cathodic peak observed during the CV at 0.05 V s−1 in 0.5 M H2 SO4 , assuming 386 ␮C cm−2 is the charge for the reduction of the gold oxide (AuO) monolayer [33]. The SAMs modified electrodes were prepared by soaking the clean electrodes in the solution containing the thiol molecule at room temperature, in the darkness, and under nitrogen atmosphere. Table 1 summarizes the conditions employed for the SAMs modified surfaces preparation. The electrodes were then removed from the solution, washed thoroughly with water and ethanol, and used for the electrochemical tests.

16 h PET-Au-MPA

-4

1.0x10

-5

5.0x10

0.0 -5

I/A

Dipping time Electrode name

-5.0x10

-4

-1.0x10

-4

-1.5x10

-4

-2.0x10

3. Results and discussion

-4

-2.5x10

-4

3.1. Bare electrodes characterization Atomic Force Microscopy (AFM) was employed to obtain information on the morphology and thickness of the gold layer evaporated on the PET substrate. Fig. 1a shows the AFM images obtained for a 4 ␮m × 4 ␮m portion of the gold surface; the surface appears homogeneous with the presence of gold nanoparticles of about 50 nm. The corrugation was calculated in terms of Root Mean Square (RMS) Roughness. The analysis was performed over different portions of the sample, obtaining an average value of 5.8 ± 0.8 nm (depending on the observed area which was varied from 4 ␮m2 up to 140 ␮m2 ). AFM was also employed to estimate the average thickness of the gold layer, which resulted 89 nm (see Fig. 1b). Typical voltammograms recorded with five different PET-Au electrodes in 0.5 M H2 SO4 are shown in Fig. 2; the CVs are highly reproducible to confirm that the structure and the area of the gold layers are well controlled. The CV was used to estimate the geometric area of the gold surfaces and to evaluate the reproducibility of the gold layer preparation. The method is based on the electrochemically induced generation of a gold oxide monolayer on gold and on the following measurement of the charge corresponding to the reduction of this monolayer [33]. The area under the cathodic peak on the voltammogram is proportional to the real area of the gold surface and is therefore an indication of the surface roughness.

-3.0x10

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

E vs. SCE/V Fig. 2. CV recorded in 0.5 M H2 SO4 with 5 different PET-Au electrodes. Potential scan rate: 0.05 V s−1 .

The roughness factor result, calculated by the ratio of the real area to the geometric area, was 2.26 ± 0.11. 3.2. SAMs characterization The morphology of the SAM coated electrodes was again investigated by AFM. The RMS roughness of the PET-Au-CA and PET-Au-MDA electrode resulted 4.7 ± 0.5 nm and 4.8 ± 0.5 nm, respectively. No significant variation in morphology was noticed in comparison with the unmodified surfaces, hence this technique does not seem to be suitable to prove the monolayer formation, probably due to the features of the gold surface. Therefore CV and EIS were used to further investigate these molecular self-assemblies formed on gold and their ability to change the interfacial charge as a function of the solution pH, which is certainly needed for the development of pH sensors. The electrochemical tests for all the functionalized surfaces were carried out in the conditions described in Section 2. Fig. 3

Fig. 1. AFM micrograph of the gold layer surface (a); three dimensional image of the PET-Au interface cross section and the corresponding profile (b).

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Fig. 3. Nyquist plots obtained at 0.17 V in the frequency range 100 mHz–10 kHz (A) and CV curves recorded at 0.05 V s−1 (B) for a PET-Au-ATP electrode in a 0.5 mM Fe(CN)6 3− solution at the pH shown in the legend.

shows typical CV and EIS plots recorded for ATP functionalized surfaces in 0.5 mM Fe(CN)6 3− solution at different pHs. The responses are a function of the solution pH since the charge at the interface of the coated electrode changes when the solution pH is changed, due to the presence of the amino groups. In particular, moving towards a more basic pH results in an increase of the charge transfer resistance and a corresponding decrease in the faradic current (in the CV), since a negative redox probe is employed. Similar trends can be observed for amino- and carboxylic acid-functionalized surfaces. The Rct values are lower for amino terminated groups and follow the order MDA > MPA > ATP > CA; this sequence reflects not only the nature and the charge of the terminating group, but also the features of the monolayer (in terms of thickness and chemical identity of the molecules) since it is known that ATP and CA are able to form less dense layers than MDA and MPA, thus exploiting a lower blocking effect. As far as 4-ATP is concerned, even lower Rct values were expected since it has been proved that it forms a poorly dense monolayer [34]. Komura demonstrated, employing a positive redox probe (Ru(NH3 )6 3+ ), that 4-ATP hardly affects its charge transfer resistance as it has a high standard heterogeneous constant. Our

apparently high Rct values could then be explained with the intrinsic characteristics of the redox probe here employed; Fe(CN)6 3− is negatively charged and therefore the electron transfer reaction is inhibited by the aromatic ring, in agreement with Ganesh, who compared the ATP barrier properties towards Fe(CN)6 3− and Ru(NH3 )6 3+ [35]. For each molecule, the CV and EIS measurements have been repeated five times. In Fig. 4, we show the plots of the measured mean values of Rct and ipc versus the pH of the solution. The amperometric measurements confirm the effective immobilization of the molecules to the PET-Au surface. All the curves are sigmoidal in shape and show a flex at different values depending on the pKa of the amino or carboxylic terminal groups. The Rct versus pH plots have therefore been employed to evaluate the pKa of the different molecules, with the following results: 7.3 for CA, 7.2 for ATP, 6.1 for 12-MDA, and 5.6 for 3-MPA. These values are in good agreement with the literature data obtained by electrochemical techniques [22] and other kinds of measurements such as contact angle measurements [36], chemical force titration using atomic force microscopy [37], and quartz crystal microbalance [38]. Nevertheless, it should be taken into account that the surface pKa is dependent on the surface coverage, the

Fig. 4. Plots of the faradic current resulting from the CV experiments (black line) and of Rct resulting from EIS tests (blue line) as a function of the solution pH for PET-Au-CA (A); PET-Au-ATP (B); PET-Au-MDA (C) and PET-Au-MPA (D).

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ATP CA MDA MPA Rct(pH8)/Rct(pH4)

density of the monolayer [39] and the ionic strength of the solution. Thus, it is not easy to compare the data obtained with the values in the literature. For example, for 3-MPA, our pKa value is in agreement with the value found by Kim et al. since it falls in between that reported in their work for a dense monolayer (pKa = 6.0) in a high ionic strength solution (0.5 M) and that found for a lower density monolayer (pKa = 5.2). However, our value is lower than that obtained by Schweiss et al. [40] who found a pKa of 6.62; in this case, the higher value is determined by the low ionic strength solution (10−4 M) employed. As a matter of fact, the lower is the ionic strength of the solution, the stronger are the interactions among the carboxylic groups of the monolayer and, as a result, the larger is the shift of the pKa towards positive values. An interesting datum is that, as reported in the literature [41], when carboxylic terminated groups are employed, the chain length affects the pKa value – the longer the chain length, the higher the pKa value. To test the stability of the SAMs, the EIS measurements at pH 4 and pH 8 have been repeated for one week and the ratio of Rct at pH 8 to Rct at pH 4 has been plotted as a function of the utilization day. The devices have been stored at 4 ◦ C in water solution when not in use. From the plot reported in Fig. 5, we can deduce that all the monolayers retain their properties for approximately 5 days. Finally, in order to fully exploit the mechanical properties of the flexible structures herein reported, it is necessary that the electrical behavior and the functionality of the final device is not affected by mechanical deformations. In order to investigate the reproducibility and reliability of the fabricated flexible devices, the electrical and electrochemical responses of gold electrodes upon mechanical stress (Fig. 6A) have been examined. Mechanical deformation has been induced in the fabricated samples by bending the devices

163

0

1

2

3

4

5

6

7

8

9

day Fig. 5. Plot of Rct (pH = 8)/Rct (pH = 4) as a function of the day of utilization of the electrode.

at different bending radii, varying from 1.9 down to 0.3 cm. Considering that, for a uniaxial deformation as the one here imposed, the surface strain can be calculated as ε = d/2R where d is the substrate thickness and R is the bending radius we have estimated that the induced surface strain ranges from 0.45 to 2.5%. Very interestingly, as it can be observed from Fig. 6B no significant variations in the electrode resistivity can be observed. Similar investigations have been performed in order to evaluate the influence of mechanical deformation on the behavior of the SAM modified electrode. Fig. 6C shows CV curves recorded for a CA functionalized surface in

Fig. 6. Photo of a PET-Au electrode during the inward bending at a curvature radius of 5 mm (A); resistance variations recorded for Au-PET electrodes before, during and after mechanical deformations at different bending radii (B); CV curves recorded at 0.05 V s−1 for a PET-Au-CA electrode in a 0.5 mM Fe(CN)6 3− solution.

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a 0.5 mM Fe(CN)6 3− solution before and after bending at two different radii, 1.9 cm and 0.3 cm. Very interestingly, the CV response of the functionalized device does not seem to be affected by the mechanical stress, and this is certainly a clear indication that this approach can be employed for the fabrication of flexible electronic devices. 4. Conclusions We have successfully developed flexible PET-Au electrodes depositing a gold film on PET substrates by thermal evaporation. The electrode preparation procedure is highly reproducible as results from the morphological and electrochemical characterization in H2 SO4 . The gold layer surface appears homogeneous and is characterized by nanoparticles with a mean dimension of 50 nm. The result of the roughness factor, calculated by cyclic voltammetry, is 2.26 ± 0.11. The flexible devices have been used as a basis to develop self assembled monolayers, of four different molecules either amino- or carboxylic-terminated, namely, cysteamine, 12-mercaptododecanoic acid, aminothiophenol, and 3mercaptopropionic acid, all chosen for their potential use in the development of sensing devices. The functionalized surfaces have been characterized by AFM and electrochemical techniques. Both the CV tests and the EIS experiments, carried out in the presence of Fe(CN)6 3− redox probe, in buffer solution at different pHs, confirm the successful formation of the SAMs and allow us to calculate the surface pKas of the different thiols, with the following results: 7.3 for CA, 7.2 for ATP, 6.1 for 12-MDA, and 5.6 for 3-MPA, all in agreement with the data in the literature. The stability of SAMs, evaluated by EIS tests, may be quantified in five days and is not influenced by mechanical deformations for surface strains as high as 2.5%; as a consequence the reported structures can be considered suitable for the development of flexible sensors. References [1] O.D. Renedo, M.A. Alonso-Lomillo, M.J.A. Martinez, Talanta 73 (2007) 202. [2] S. Coyle, Y. Wu, K.T. Lau, D. De Rossi, G. Wallace, D. Diamond, MRS Bull. 32 (2007) 434.

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