TUNING COLOURIMETRIC AND FLUOROMETRIC GAS SENSORS FOR CARBON-DIOXIDE

The basic theory behind conventional colourimetric and fluorimetric optical sensors for CO2 is examined and special attention is given to the effect on sensor response of the key parameters of initial base concentration and dye acid dissociation constant, K(D). Experimental results obtained in aqueous solution using a variety of different dyes and initial base concentrations are consistent with the predictions made by the theoretical model. A series of model-generated pK(D) versus %CO2 curves for different initial base concentrations allow those interested in constructing an optical CO2 sensor to readily identify the optimum dye/initial base combination for their sensor; the response of the sensor can be subsequently fine-tuned through a minor variation in the initial base concentration. The model and all its predictions appear also to apply to the new generation of plastic film CO2 sensors which have just been developed.

EQUILIBRIUM STUDIES ON COLORIMETRIC PLASTIC FILM SENSORS FOR CARBON-DIOXIDE

The equilibrium responses of three new colorimetric plastic film sensors for CO2 as a function of % CO2 and temperature are described. The results fit a model in which there is a 1:1 equilibrium reaction between the deprotonated form of the dye (present in the film as an ion pair) and CO2. The 0-50% and 0-90% response and recovery times of each of these films when exposed to an alternating atmosphere of air and 5% CO2 are determined and in two cases are typically less than 3 s. The shelf life of the films is long (many months); however, prolonged use of the films leads to the permanent generation of the protonated form of the dye over a period of 20-100 h. A possible cause of this latter effect is discussed.

Molecular Logic Gates and Luminescent Sensors Based on Photoinduced Electron Transfer

The competition between Photoinduced electron transfer (PET) and other de-excitation pathways such as fluorescence and phosphorescence can be controlled within designed molecular structures. Depending on the particular design, the resulting optical output is thus a function of various inputs such as ion concentration and excitation light dose. Once digitized into binary code, these input-output patterns can be interpreted according to Boolean logic. The single-input logic types of YES and NOT cover simple sensors and the double- (or higher-) input logic types represent other gates such as AND and OR. The logic-based arithmetic processors such as half-adders and half-subtractors are also featured. Naturally, a principal application of the more complex gates is in multi-sensing contexts.

Solid-state sodium-selective sensors based on screen-printed Ag/AgCl reference electrodes

The potentiometric and AC impedance characteristics of all solid-state sodium-selective electrodes based on planar screen-printed Ag/AgCl electrodes are described. Two solid-state designs have been investigated. The first was based on the deposition of a sodium-selective PVC membrane directly on top of a screen-printed Ag/AgCl electrode, The second design included a NaCl doped hydrogel layer, between the PVC and Ag\AgCl layers. The hydrogel provides a mechanism to relieve any blockage to charge transfer occurring when PVC membranes are used directly on top of Ag/AgCl and also improves adhesion between the two layers. Results suggest the electrodes display Fast ion exchange kinetics, low noise and drift. The performance compares favorably to that of a conventional ion-selective electrode with internal filling solution.

Building Stone Condition Monitoring Using Specially Designed Compensated Optical Fiber Humidity Sensors

This paper presents the design and implementation of a novel optical fiber temperature compensated relative humidity (RH) sensor device, based on fiber Bragg gratings (FBGs) and developed specifically for monitoring water ingress leading to the deterioration of building stone. The performance of the sensor thus created, together with that of conventional sensors, was first assessed in the laboratory where they were characterized under experimental conditions of controlled wetting and drying cycles of limestone blocks, before being employed “in-the-field” to monitor actual building stone in a specially built wall. Although a new construction, this was built specifically using conservation methods similar to those employed in past centuries, to allow an accurate simulation of processes occurring with wetting and drying in the historic walls in the University of Oxford.