PEDOT:PSS–MODIFIED PLATINUM MICROELECTRODES FOR MEASUREMENTS IN AQUEOUS MEDIA: EFFECT OF POLYMER SURFACE AREA ON LONG-TERM ANODIC PEAK CURRENT STABILITY

Contamination of drinking water by hazardous agents is becoming a serious global threat, so it is necessary to develop more efficient sensing technologies for applications in liquid media. The limited working lifetime of electrochemical biosensors, especially when measurements are made continuously in liquid media, remains an unsolved challenge. We studied the effect of PEDOT:PSS surface area on platinum microelectrodes with respect to electrode ability to conduct reversible ion-to-electron transduction in liquid media. Electropolymerization of 3,4-ethylenedioxythiophene:poly(styrene sulfonate) EDOT:PSS to poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) was conducted on microplatinum electrodes 5 and 10 mm long using a galvanostatic mode. Cyclic voltammetry was used to determine capacitive peak current; higher peak current indicates higher redox capacitance. Field-emisison scanning-electron microscopy was used to study the surface morphology of the PEDOT:PSS transucer layer after measurement in liquid media. The anodic capacitive peak currents did not differ significantly between the two electrodes at day one (~0.20 mA); however, peak current decreased by ~ 20% and ~ 80% at day six for 10- and-5 mm electrode lengths, respectively. The results imply that PEDOT:PSS surface area plays a role in transduction of PEDOT:PSS in aqueous media.

ABSTRACT: Contamination of drinking water by hazardous agents is becoming a serious global threat, so it is necessary to develop more efficient sensing technologies for applications in liquid media. The limited working lifetime of electrochemical biosensors, especially when measurements are made continuously in liquid media, remains an unsolved challenge. We studied the effect of PEDOT:PSS surface area on platinum microelectrodes with respect to electrode ability to conduct reversible ion-to-electron transduction in liquid media. Electropolymerization of 3,4ethylenedioxythiophene:poly(styrene sulfonate) EDOT:PSS to poly (3,4ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) was conducted on microplatinum electrodes 5 and 10 mm long using a galvanostatic mode. Cyclic voltammetry was used to determine capacitive peak current; higher peak current indicates higher redox capacitance. Field-emisison scanning-electron microscopy was used to study the surface morphology of the PEDOT:PSS transucer layer after measurement in liquid media. The anodic capacitive peak currents did not differ significantly between the two electrodes at day one (~0.20 mA); however, peak current decreased by ~ 20% and ~ 80% at day six for 10-and-5 mm electrode lengths, respectively. The results imply that PEDOT:PSS surface area plays a role in transduction of PEDOT:PSS in aqueous media.

INTRODUCTION
The ability to evaluate food quality and freshness, as well as water quality, requires sensors that can operate in aqueous media. Most sensors available in the market are not for long-time use in aqueous media, limiting their potential [1][2][3]. Nowadays, the most common type of sensing technology for detection of biological agents is electrochemical biosensors, owing to their fast measurement, simple construction, low-cost fabrication, real-time measurement capability, and applicability to a wide range of sample types [4]. By definition, an electrochemical biosensor is a device for measuring the concentration of a biological analyte, with conversion of a biochemical signal to an electrical signal via an electrochemical transducer [5]. The transducer is often a conductive polymer with reversible ion-storage capacity.
A challenge that remains unsolved when PEDOT:PSS is used as a transducer layer in electrochemical biosensors, is its limited working lifetime when electrical measurements are made in liquid media. PEDOT:PSS is highly hydrophilic and unstable in liquid media because of the presence of the water-soluble PSS chain [8], which has a tendency to degenerate in an aqueous medium; with time, it can be easily peeled off from the electrode surface, leading to a decrease in biosensor performance [9] and limiting PEDOT:PSS application as the transducer layer in biosensors.
Considerable research has been performed to improve the longevity in liquid media of biosensors using PEDOT:PSS. Different techniques such as dip-coating and drop-casting have been used to deposit PEDOT:PSS on the working-electrode surface; techniques of PEDOT:PSS deposition can affect the quality of the transducer layer. A binder has been utilized in the deposition techniques in order to increase adhesion of PEDOT:PSS to an electrode surface [9] [10]. To fabricate PEDOT:PSS transducers that are stable with long lifetime in liquid media, good adhesion is required between the transducer and the electrode surface. In this preliminary work, we studied the effect of the PEDOT:PSS surface area on anodic capacitive peak current. Field-emission scanning-electron microscopy (FESEM) was used to verify PEDOT:PSS adhesion to the platinum electrode surface after electrode measurement in aqueous media.

Electropolymerization of PEDOT:PSS on a Platinum Microelectrode (µPtE)
Electropolymerization of EDOT monomer was performed under galvanostatic conditions (scan rate 100 µA/s, current 100 µA, and potential 400 mV) using a threeelectrode cell and pocketSTAT in an aqueous solution of 0.5 ml EDOT, 1 ml LiClO 4 (0.1 M), and 1 ml NaPSS in 50 ml distilled water. The EDOT:PSS solution was stirred for 24 hours before electropolymerization. Two µPtEs, one 10-mm long and one 5-mm long, were used; the choice of lengths was determined by availability for purchase.

Electrochemical Characterization of µPtE/PEDOT:PSS
Cyclic voltammetry (CV) was performed in 0.1 M potassium ferricyanide (K 3 Fe(CN) 6 ) at a scan rate of 100 µV/s to evaluate the capacitive peak current of the PEDOT:PSS transducer layer on a platinum microelectrode (µPtE).

Capacitive Peak Current of PEDOT:PSS Transducer in Liquid Media
CV was performed in 0.1 M potassium ferrocyanide K₄[Fe(CN)₆] solution to assess the capacitive peak current of PEDOT:PSS on µPtEs (µPtE/PEDOT:PSS) 10 mm and 5 mm long for six consecutive days against a control (bare µPtE). Figures 1 and 2 show the CV curves at days one, two, three, and six and Fig. 3 shows the values of anodic peak current from all days in comparison to the control.  The anodic capacitive peak current for both coated µPtEs was ~ 0.2 mA during the first day of measurement (day one); however, after six days the anodic peak current of the 5-mm µPtE had decreased by ~80% to 0.045 mA, while peak current for the 10-mm µPtE had decreased by only 20% (from 0.195 mA to 0.143 mA), indicating that the 10-mm µPtE/PEDOT:PSS maintains redox activity, possibly as a result of the larger surface area that provides a stronger adhesion between PEDOT:PSS and the µPtE surface.

Surface Morphology of µPtE/PEDOT:PSS
Because the 10-mm electrode maintained anodic capacitive peak current with only a ~ 20% reduction, we utilized this electrode for chronoamperometry measurements in buffer solution; most electrochemical biosensors operate in this mode. We characterized the surface of the electrode using FESEM to test the surface morphology of the electrodes after 24 hours of measurement in 0.1 M phosphate buffered saline (PBS).  shows FESEM images of PEDOT:PSS before and after the chronoamperometry measurement. Before measurement, the PEDOT:PSS structure on the modified µPtE was observed to be globular (Fig. 4 (A-B)), suggesting an irregular amorphous shape, common to PEDOT material. After measurement, the PEDOT:PSS structure lost the globular shape and became irregular as if ruptured as a result of prolonged measurement in liquid media (Fig 4 (C-D)). However, more studies are required on PEDOT:PSS morphology in liquid media.

CONCLUSION
PEDOT:PSS was successfully deposited on a µPtE via electropolymerization. Electrodes with larger surface area maintained anodic peak current over a period of 6 days. FESEM results imply that the structure of electropolymerized PEDOT could affect current measurement in liquid media.