Abstract
Ascorbic acid (AA) is the principle interferent present in brain extracellular fluid that can inhibit the ability of electrochemical sensors to selectively detect a particular analyte of interest. Considerable efforts have been made in recent times to develop highly selective membrane coatings to counteract the drawbacks associated with AA interference during in vivo monitoring. The primary objective of the work described within was to investigate the long term effect of storing such selective membranes, i.e., Nafion® and Poly-o-phenylenediamine (PPD) under different conditions and how exposing them to repeated calibration protocols compromises the membranes ability to reject AA. Four different modified platinum (Pt) electrodes, Pt-PPD, Pt-Nafion® (5/2), Pt-Nafion® (1/2)-PPD, and Pt-Nafion® (2/1)-PPD, stored at 4 °C demonstrated deterioration of the polymers integrity when exposed to repeated calibrations. On the contrary, exposing the same four electrode types to single calibrations confirmed excellent retention of AA rejection characteristics. Pt-PPD electrodes were then exposed to varying storage conditions and calibrated against AA on day 1, day 56 and day 168. Storing the Pt-PPD electrodes at 4 °C/N2 saturated glass container demonstrated retention of AA rejection characteristics after day 168. These results have clearly elucidated the optimum storage conditions for Pt-Nafion® and Pt-PPD modified electrodes.
Highlights
The mammalian brain has a large number of possible interfering species present at relatively high concentrations, (e.g., ascorbic acid (AA), uric acid (UA), and neurotransmitters including dopamine (DA) and serotonin (5-HT))
N2 saturated phosphate buffer saline (PBS) solution at 4 °C: Polymer modified electrodes were stored in a glass cell containing mLs of PBS which had been purged for 20 min prior to storage
There is essentially no change in AA detection with the maximum current recorded at 1000 μM AA on day 56 (24 ± 2 pA, n = 4) demonstrating no significant difference (p > 0.05) from day 1 (19 ±4 pA, n = 4)
Summary
The mammalian brain has a large number of possible interfering species present at relatively high concentrations, (e.g., ascorbic acid (AA), uric acid (UA), and neurotransmitters including dopamine (DA) and serotonin (5-HT)). Nafion®, has been widely utilised as a permeable membrane in electrochemical sensor design demonstrating excellent permselective characteristics against anionic species including AA [3,4,5,6]. Electropolymerisation of the monomer o-phenylenediamine (o-PD) in pH 7.4, deposits PPD in its insulating form onto the electrode surface [12] This insulating layer appears to provide selectivity by limiting access of larger molecules such as AA, DA, etc. It is critical that these selectivity characteristics remain intact throughout long-term storage, allowing the researcher to manufacture and calibrate sensors on separate days to in vivo implantation. This forms the basis of the work described within. Investigations were undertaken to determine the effect of various different storage conditions on the longevity of Pt electrodes modified with various combinations of Nafion® and PPD layers
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