Comparing electron- and hole transporting semiconductors in ion sensitive water- gated transistors

Abstract

We present a systematic study comparing different solution-processed semiconductors in cation-sensitive water-gated thin film transistors (WGTFTs): A hole transporting semiconducting polymer (rrP3HT), and an electron-transporting precursor-route metal oxide (ZnO). To allow comparison, we used the same ionophore to sensitise the gate contact for both semiconductors. We find both organic hole transporter and inorganic electron transporter have their relative merits, and drawbacks, in ion-sensitive WGTFTs. Hole transporting rrP3HT WGTFTs show low hysteresis under water-gating and give super-Nernstian sensitivity. However, rrP3HT responds to ionic strength in water even when WGTFTs are not sensitised, compromising selectivity. Electron transporting ZnO WGTFTs show higher mobility, but also stronger hysteresis, and sub-Nernstian response. However, ZnO WGTFTs show little response to ionic strength when not sensitised. We rationalise the super-versus-sub-Nernstian sensitivities via a capacitive amplification/attenuation effect. Our study suggests that the optimum semiconductor material for ion-selective WGTFTs would be a precursor-route inorganic hole transporting semiconductor.

Sub-nanomolar detection of cesium with water-gated transistor

“…An important sub-genre of WGTFTs are the ion-selective WGTFTs, first introduced by List-Kratochvil [5]. So far, the sensitiser in such devices always was an organic 'ionophore', for example a crown ether [6], calixarene [7,8], or valinomycin [9]. Typical 'target' ions are K + , Na + , Li + , Ca 2+ , Mg 2+ [10].…”

“…Typical 'target' ions are K + , Na + , Li + , Ca 2+ , Mg 2+ [10]. The ionophore is often introduced into the WGTFT within a plasticised PVC membrane, for example in a 2-chamber design [5] similar as in classical electrochemical potentiometry [11,12], or by direct application of the membrane onto the gate contact [6], or the semiconducting channel [9], of the WGTFT. Also, membrane-free ion sensitive WGTFTs have been demonstrated [8] where the ionophore is incorporated into the semiconducting channel.…”

“…WGTFT sensors are typically formed using solution-processed semiconductors, e.g. semiconducting polymers [3,6,8], precursor-route metal oxides [6], or carbon nanotubes [9]. Selective binding of waterborne ions in the gating water to the ionophore leads to a membrane potential, V M , and consequential shift in V th .…”

“…Selective binding of waterborne ions in the gating water to the ionophore leads to a membrane potential, V M , and consequential shift in V th . Quantitatively, threshold shift follows a Nikolsky-Eisenman law [6,13], i.e. Nernstian (linear on a logarithmic concentration scale) at high ion concentrations (c >> c st ), but flatlining below a concentration c st , hence giving a limit-of-detection L D st :…”

“…Wherein z is the valency of the cation (z = 1 for alkaline metals), and c ref >> c st is the ion concentration in a reference solution. c st depends on ion and ionophore, but typically is in the range 100 nM to 1µM [6,14,15]. Strictly speaking eq.…”