In-situ potentiometry and ellipsometry: a promising tool to study biofouling of potentiometric sensors

Transcription

1 In-situ potentiometry and ellipsometry: a promising tool to study biofouling of potentiometric sensors Grzegorz Lisak 1,2,3, *, Thomas Arnebrant 2,3, Andrzej Lewenstam 1,4, Johan Bobacka 1, Tautgirdas Ruzgas 2,3 1 Johan Gadolin Process Chemistry Centre, c/o Laboratory of Analytical Chemistry, Åbo Akademi University, FIN Åbo-Turku, Finland 2 Department of Biomedical Sciences, Faculty of Health and Society, Malmö University, SE Malmö, Sweden 3 Biofilms - Research Center for Biointerfaces, Malmö University, SE Malmö, Sweden 4 Faculty of Materials Science and Ceramics, AGH University of Science and Technology, Krakow, Poland *corresponding author: grzegorz.lisak@gmail.com Tel: S-1

2 Table of contents: 1. Surface characterization of conditioned K -ISEs, used in in-situ potentiometry and ellipsometry, before and after fouling by BSA: Scanning electron microscopy. Contact angle measurements. 2. Potentiometric characteristics (off ellipsometry) of conventional solid-contact electrodes (gold or glassy carbon coated with immobilized 6-(ferrocenyl)hexanethiol or poly(3,4- ethylenedioxythiophene) polystyrene sulfonate coated with potassium selective membrane) before and after fouling by BSA: Potentiometric sensitivity. Low detection limit. Potential drop due to adsorption of BSA. S-2

3 Surface characterization of conditioned K -ISEs. Field Emission Gun Scanning Electron Microscopy (FEG-SEM, Leo 1530, Zeiss) was used on K -ISM electrode that was cut in two pieces to investigate the thickness (from a cross section) and morphology after conditioning in 10 2 mol dm 3 PBS (1st piece) and after conditioning in 10 2 mol dm 3 PBS and biofouling by 1 mg ml 1 BSA for 30 min (2nd piece). Before the measurements both substrates were vigorously washed with deionized water. The contact angle ( c ) measurements were done using a goniometer with inbuilt camera and software to process the images (Kruss, DSA100, Hamburg, Germany). The contact angle was measured soon after the water droplet was in contact with the substrate surface, n= 3. Measurements were performed on a pure gold (Au), thiolated gold (Au/FT), unconditioned and conditioned in 10 2 mol dm 3 PB -ISEs (Au/FT/K -ISM) and K -ISEs after BSA adsorption (Au/FT/K -ISM:BSA). Figure S1. SEM images of: (a) conditioned Au/FT/K -ISM electrode in 10 2 mol dm 3 PBS for 2 h and (b) the same electrode after biofouling of the ISM by 1 mg ml 1 BSA during 1800 s adsorption time. Inserts: The optical images captured in real-time contact angle measurements when the droplet contacted the electrode (t= 0 s). The SEM images of an electrode (Au/TF/K -ISM) before and after BSA biofouling are shown in Figure S1 a and b. The images show a lack of uniformity for the thickness of the K -ISM (ca 0.1 μm) that was prepared by dip-coating. The change in the morphology of the same K -ISM by the BSA was observed by SEM (Figure S1 b). It shows that the K -ISM which had been in contact with BSA exhibited different surface morphology, namely possessed additional morphological features, than the same membrane before adsorption of the protein. Furthermore, the surface of the K -ISM was slightly more hydrophilic after addition of BSA (Inserts in Figure S1 a and b). In S-3

4 comparison, the contact angle of a water droplet on the bare gold surface was c = 4.1 ± 1.1 degrees, on the gold surface covered with 6-(ferrocenyl)hexanethiol it was c = 70.0 ± 4.1 degrees and on unconditioned K -ISM it was c = 87.7 ± 3.8. After conditioning of the K -ISM in 10 2 mol dm 3 PBS for 2 h, the surface became more hydrophilic ( c = 77.7 ± 1.9). Potentiometric characteristics (off ellipsometry) of conventional solid-contact electrodes. Potentiometric measurements with four different conventional types of solid-contact K -ISEs were performed (without ellipsometry), namely: 1- gold electrode with FT and approx. 3 m thick ISM (Au/FT/3 m K -ISM), 2- gold electrode with FT and approx. 60 m thick ISM (Au/FT/60 m K -ISM), 3- glassy carbon electrode with poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT(PSS - )) and approx. 3 m thick ISM (GC/PEDOT(PSS - )/3 m K - ISM) and 4- glassy carbon electrode with PEDOT(PSS - ) and approx. 60 m thick ISM (GC/PEDOT(PSS - )/60 m K -ISM). The gold substrate electrodes were prepared according to the procedure described for electrodes used in ellipsometric measurement, while PEDOT(PSS ) was electropolymerised according to the procedure described elsewhere. 1,2 Before the measurements all electrodes were conditioned in 10 3 mol dm 3 KCl for 24 h. Potentiometric measurements were done in quiescent solution by measuring the EMF of the potentiometric cell for 120 s in a series of solutions: to mol dm 3 KCl and to mol dm 3 PBS. The response of all ISEs was always investigated by increasing tenfold the activity of potassium in the measuring solutions and measuring the EMF against single junction Ag/AgCl/3 M NaCl reference electrode. The potentiometric characteristic (slope and low detection limit) of the K -ISEs was determined before and after addition of BSA to the solution. Furthermore, the EMF was measured as a function of time during biofouling (after addition of BSA). The open circuit potential of the potentiometric cell was followed by an EMF16 Interface (Lawson Labs Inc., USA). The activity coefficients were calculated according to the Debye-Hückel approximation. The measurements were performed at room temperature, o C. The potential drop caused by the adsorption of BSA at the surface of the electrodes as well as the Nernstian response and low detection limit of ISEs before and after biofouling by BSA were summarized in Table S1. Interestingly, the magnitude of the potential drop of the ISEs after contact with BSA was clearly related to the thickness of the K -ISM. As shown in Table S1, the potential of the K -ISEs with thinner membranes (3 μm) were more affected by the adsorption of S-4

5 BSA compared to the K -ISEs with thicker membranes (60 μm). Similarly, the potentiometric response in KCl and PBS solutions was closer to Nernstian for thicker membranes. Moreover, the addition of BSA resulted in a slight decrease in the slopes of the calibration curves. The Au/FT/3 m K -ISM showed a sub-nernstian response already before contact with BSA (Table S1). Table S1. Potentiometric characteristics of K -selective electrodes in off ellipsometry potentiometric measurements, using the following electrodes: 1- Au/FT/3 m K -ISM, 2- Au/FT/60 m K -ISM, 3- GC/PEDOT(PSS - )/3 m K - ISM, 4- GC/PEDOT(PSS - )/60 m K -ISM. The measurements were performed in KCl and PBS solutions before* and after** the biofouling by BSA. K -ISEs -ISEs in KCl* -ISEs in KCl** LDL* LDL** -ISEs PBS* -ISEs PBS** E ads ± 0.9 < ± ± ± 0.5 < ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 0.05 The Nernstian response in most cases was only slightly decreased after addition of BSA. The K - ISEs based on glassy carbon covered with electropolymerized PEDOT(PSS ) performed better than the K -ISEs based on gold electrodes with 6-(ferrocenyl)hexanethiol self-assembly monolayer. This indicates that in further studies the application of PEDOT(PSS ) as ion-toelectron transducer would be preferable. However, since PEDOT(PSS ) partially absorb light at the wavelength used in null ellipsometry further studies should be performed by means of in-situ potentiometry and spectroscopic ellipsometry at a wavelength where PEDOT(PSS ) does not absorb light. The performance of the K -ISEs in the clinically relevant concentration range, 3-6 mmol L 1, was found to be rather stable but a small decrease in the slope was observed after contact with protein (BSA). S-5

6 REFERENCES (1) J. Bobacka, Anal. Chem. 1999, 71, (2) M. Guzinski, G. Lisak, T. Sokalski, J. Bobacka, A. Ivaska, M. Bochenska, A. Lewenstam, Anal. Chem. 2013, 85, S-6