A Multifunctional Microgripper Capable of Simultaneous Single Cell Manipulation and Associated Ion Sensing
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A Multifunctional Microgripper Capable of Simultaneous Single Cell Manipulation and Associated Ion Sensing Rachael Daunton1,2, Andrew J. Gallant2, Ritu Kataky1 and David Wood2 1 Department of Chemistry, Durham University, South Road, Durham DH1 3LE UK 2 School of Engineering and Computing Sciences, Durham University, South Road, Durham DH1 3LE UK ABSTRACT The successful modification of the tips of a cellular microgripper into ion selective electrodes capable of sensing calcium ions at concentrations as low as 8x10-5 M is described. The modification involves applying the process of adding the components of all solid state ion selective electrodes. Specifically, poly(3,4-ethylenedioxythiophene) (PEDOT) is added to a gold electrode protruding from the microgripper tip; this is then coated with a poly(vinyl chloride) PVC based calcium selective membrane. Excellent Nernstian response was observed from our devices, with calibration slopes of 29.5 ± 2.5 mV/dec. INTRODUCTION Detecting the changes in intra- and extra-cellular ion concentration associated with cell signaling has always been of great interest, with many different methods being utilized. Potentiometric sensors are particularly advantageous in this area due to their comparatively small size, portability, low energy consumption and low cost [1]. Ion selective electrodes (ISEs) fall into this category. Conventional ISEs contain an internal filling solution between the electrical contact and the selective membrane [2]. While these show good detection limits they are difficult to miniaturize and often have durability issues due to leaking of the internal solution, as well as issues associated with the aging of the internal electrolyte. This led to the development of all solid-state ion selective electrodes (ASSISEs). Due to the potential instability occurring when the selective membrane was directly applied to the electrical contact, a transducer material was added. This ion-to-electron transduction process occurs asymmetrically (Figure 1), so an intermediate layer with sufficiently high redox capacitance is required to minimize the polarizability of the solid contact. This led to the inclusion of conducting polymers (CPs) to the ASSISE design. Solution
Membrane
Solution
Electrical contact
Membrane
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Electron Ion
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Ion
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Figure 1 – Schematic showing the (a) symmetrical ion movement in a conventional ISE and (b) the asymmetrical ion-to-electron movement in an ASSISE.
While there are many types of ASSISEs, the most popular rely on glassy carbon, gold or platinum as the electrical contact; poly(3,4-ethylenedioxythiophene) (PEDOT) as the CP and poly(vinyl chloride) (PVC) as the membrane base [1]. The choice in the components within the
ion selective membrane (ISM) is complex and will not be discussed here; however a plasticizer, free ionophore and ionic site must be included within the membrane base [3]. The plasticizer is needed to set the membrane; the ionophore is the sensing element and determines the ISE selectivity, (it can be either charge
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