Highly sensitive alkane odour sensors based on functionalised gold nanoparticles

We deposit dense, ordered, thin films of Au-dodecanethiol core/shell nanoparticles by the Langmuir-Schafer (LS) printing method, and find that their resistance at ambient temperature responds selectively and sensitively to alkane odours. Response is a rapid resistance increase due to swelling, and is strongest for alkane odours where the alkane chain is similar in length to the dodecane shell. For decane odours, we find a response to concentrations as low as 15 ppm, about 600 times below the lower explosive limit. Response is weaker, but still significant, to aromatic odours (e.g. Toluene, Xylene), while potential interferants such as polar and/or hydrogen-bonding odours (e.g. alcohols, ketones, water vapour) are somewhat rejected. Resistance is weakly dependent on temperature, and recovers rapidly and completely to its original value within the error margin of measurement. (C) 2011 Elsevier B.V. All rights reserved

An electrical characterisation system for the real-time acquisition of multiple independent sensing parameters from organic thin film transistors

The presence of multiple independent sensing parameters in a single device is the key conceptual advantage of sensor devices based on an organic thin film transistor (OTFT) over simple organic chemiresistors. Practically, however, these multiple parameters must first be extracted from the electrical characteristics of the OTFTs and, thus, they are not immediately apparent. To exploit the advantage of OTFT sensors, we require a measurement technology to extract these parameters in real time. Here, we introduce an efficient, cost-effective system that is a faster and more compact alternative to the expensive and cumbersome laboratory-based instruments currently available. The characterisation system presented here records the electric behaviour of OTFTs in the form of its “saturated transfer characteristics” multiple times per second for virtually unlimited periods of time, with the option to multiplex up to 20 devices in parallel. By applying a bespoke algorithm to the measured transfer characteristics, the system then extracts, in real time, several underlying transistor parameters (on- and off-current, threshold voltage, and charge carrier mobility). Tests were conducted on the example of a poly(thieno[3,2-b]thiophene) (PBTTT) OTFT exposed to ethanol vapour. The system extracts the underlying OTFT parameters with very low noise without introducing apparent correlations between independent parameters as an artefact

A new precursor route to semiconducting Zinc Oxide

We demonstrate a new precursor route towards solution- processed films of the II-VI semiconductor Zinc oxide (ZnO). Spray pyrolysis of aqueous solutions of the Zinc salt Zinc chloride (ZnCl2) onto a substrate heated to at least 250 oC gives films that are insoluble in water, display an absorption edge at 365 nm, and when electrically gated display thin film transistor characteristics similar to ZnO films derived via the established Zinc Acetate (ZnAc) precursor route; we therefore identify it as ZnO. Control experiments attempting spray pyrolysis of aqueous Zinc sulfate solutions, and delayed pyrolysis of cold- sprayed and dried ZnCl2 films, do not lead to semiconducting films. Formation of ZnO from an aqueous Zinc salt requires the simultaneous presence of Zinc ions, Chloride ions, and water, at the time of pyrolysis. We therefore suggest the actual ZnO precursor is the ZnClxH2O(4−x) species that forms when ZnCl2 dissolves in water [The Journal of Chemical Physics 39, 3436 (1963)]. The reported process is easy to upscale for large area ZnO coatings, e.g. on window panes for thermal control, as no organic solvent vapours are released

Electrochemical gating of a hydrophobic organic semiconductor with aqueous media

We show that even the highly hydrophobic semiconducting polymer poly [2,5-bis(3-tetradecylthiophen-2-yl) thieno[3,2-b]thiophene] (PBTTT) can operate in organic electrochemical transistor (OECT) rather than field effect (OFET) mode when gated with aqueous media. The required bulk penetration of anions into the semiconducting film within the electrochemical window of water is here enabled by the choice of anion, namely, picric acid (PA). OECT mode operation in PBTTT films or its analogues had previously been seen only when gated with solid electrolytes with a larger electrochemical window, or for PBTTT analogues with hydrophilic sidechains. We assign the ability of PA anions to penetrate bulk PBTTT from their similarity to PBTTT solvents (chlorinated benzenes), in the sense that they both display an electron-deficient π electron system. In control experiments, we confirm that OECT mode is not observed when gating PBTTT with another organic acid (acetic acid) that does not display such π electron system, and that the gating mechanism indeed is electrochemical doping rather than a charge transfer mechanism. OECT mode is observed only at rather high PA concentrations in an aqueous gate medium (50 mM or more), but when it is, it leads to conductivities of ≈80 S/cm, slightly larger than for a PBTTT analogue with hydrophilic side chains gated with chloride anions

Adaptive and sensitive fibre-optic fluorimetric transducer for air- and water-borne analytes

A sensitive fibre optic fluorescence intensity meter has been designed and built as a transducer to detect quenching of conjugated polymer fluorescence with minimum adjustment between air- and waterborne analytes. Only generic, commercially available parts including optical fibres, solvents, airbrush, standard optical and electronic parts, and a digital lock-in amplifier have been used, avoiding the need for a fluorescence spectrometer. To test the instrument, optical fibres were sensitised with the generic fluorescent poly(phenylene-vinylene) derivative MDMO-PPV and exposed to a variety of vapour pressures, and concentrations in water, of the nitroaromatic explosive 2,4 dinitrotoluene (DNT). We establish dimensionless Stern-Volmer constants (KSV) and limit-of-detection (LoD) for air- and water-borne DNT as KSV(air) = 1.4 × 107 vs. KSV(water) = 5.8 × 106 and LoD(air) = 10.9 ppb and LoD(water) = 56 ppb. These LoDs compare favourably to prior reports. We consider our study of the MDMO-PPV/DNT system as a successful test of our transducer design and recommend its wider use

A membrane- free cation selective water- gated transistor

Sensors for the detection of waterborne cations are of great practical interest, and chemistry has synthesised a formidable catalogue of cation selective complexation agents (‘ionophores’) as selective sensitisers (e.g. 9-13, 15-19, 28, 29). Current ionophore- based sensors separate the complexation of the cation by the ionophore, and the transduction of complexation into an electrical signal, into separate components. We here unite both functions into a single, sensitised semiconducting layer of a water- gated organic thin film transistor (OTFT). The resulting OTFT transduces waterborne cations into an electrical signal with same selectivity, sensitivity, and limit of detection as established sensors at much simplified preparation and operation. This opens a new route to apply the ‘ionophore’ family of functional organic materials in practical cation sensors

Transport Properties of Highly Aligned Polymer Light-Emitting-Diodes

We investigate hole transport in polymer light-emitting-diodes in which the emissive layer is made of liquid-crystalline polymer chains aligned perpendicular to the direction of transport. Calculations of the current as a function of time via a random-walk model show excellent qualitative agreement with experiments conducted on electroluminescent polyfluorene demonstrating non-dispersive hole transport. The current exhibits a constant plateau as the charge carriers move with a time-independent drift velocity, followed by a long tail when they reach the collecting electrode. Variation of the parameters within the model allows the investigation of the transition from non-dispersive to dispersive transport in highly aligned polymers. It turns out that large inter-chain hopping is required for non-dispersive hole transport and that structural disorder obstructs the propagation of holes through the polymer film.Comment: 4 pages, 5 figure

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

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 capacitative amplification/attenuation effect. Our study suggests that the optimum semiconductor material for ion- selective WGTFTs would be a precursor- route inorganic hole transporting semiconductor

Monolithic solder-on nanoporous Si-Cu contacts for stretchable silicone composite sensors

We report a method of creating solderable, mechanically robust, electrical contacts to interface (soft) silicone-based strain sensors with conventional (hard) solid-state electronics using a nanoporous Si-Cu composite. The Si-based solder-on electrical contact consists of a copper-plated nanoporous Si top surface formed through metal-assisted chemical etching and electroplating, and a smooth Si bottom surface which can be covalently bonded onto silicone-based strain sensors through plasma bonding. We investigated the mechanical and electrical properties of the contacts proposed under relevant ranges of mechanical stress for applications in physiological monitoring and rehabilitation. We also produced a series of proof-of-concept devices, including a wearable respiration monitor, leg band for exercise monitoring and Squeeze-ball for monitoring rehabilitation of patients with hand injuries or neurological disorders, to demonstrate the mechanical robustness and versatility of the technology developed, in real-world applications

Water-gated transistor using ion exchange resin for potentiometric fluoride sensing

We introduce fluoride-selective anion exchange resin sorbents as sensitisers into membranes for water-gated field effect transistors (WGTFTs). Sorbents were prepared via metal (La or Al)-loading of a commercial macroporous aminophosphonic acid resin, PurometTM MTS9501, and were filled into a plasticised poly(vinyl chloride) (PVC) phase transfer membrane. We found a potentiometric response (membrane potential leading to WGTFT threshold shift) to fluoride following a Langmuir–Freundlich (LF) adsorption isotherm with saturated membrane potential up to ~480 mV, extremely low characteristic concentration c1/2 = 1/K, and picomolar limit of detection (LoD), even though ion exchange did not build up charge on the resin. La-loading gave a superior response compared to Al-loading. Membrane potential characteristics were distinctly different from charge accumulating sensitisers (e.g., organic macrocycles) but similar to the Cs+ (cation) selective ion-exchanging zeolite mineral ‘mordenite’. We propose a mechanism for the observed threshold shift and investigate interference from co-solutes. Strong interference from carbonate was brought under control by ‘diluting’ metal loading in the resin. This work sets a template for future studies using an entirely new ‘family’ of sensitisers in applications where very low limit of detection is essential such as for ions of arsenic, mercury, copper, palladium, and gold