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Fate of antibiotic resistance genes and bacteria under sequentialredox conditions within biofilm reactors
Get PDFPhD ThesisAntibiotic resistance (AR) is a major health threat to global populations. However, mortal infections are most profound in Low-Middle-Income Countries (LMICs) where wastewater treatment is not universal and rarely precedes urbanisation. Therefore, reducing waste- and water-borne AR exposures through improved wastewater treatment is a high priority; however, few small-scale and economical technologies are available for application in LMICs.
This thesis studied low-energy, sponge-core bioreactors, called Denitrifying Downflow Hanging Sponge (DDHS) systems, as a technology reducing AR genes and bacteria from domestic wastewater. The technology uses sequential redox conditions (i.e., aerobic-anoxic), an option previously shown to enhance AR reduction in wastewater ecosystems. Here, DDHS systems were co-optimised for total nitrogen (TN) and AR genes removal, using a 20% influent wastewater bypass (by volume of total influent) to enhance denitrification in the second-stage anoxic unit. Under such conditions, removals of 2.0 to 3.0 log AR genes, >79% carbon and 71% TN were achieved. Subsequent 16S rRNA amplicon sequencing and microbiome characterisation indicated the wastewater bypass positively influenced resident microbial communities, especially increasing reactor biodiversity (Shannon diversity index for 0% bypass = 5.92 ± 0.05 and 20% bypass = 6.15 ± 0.03), which in turn, translated to improved overall treatment performance.
To better explain AR fate in the DDHS reactors, independent experiments assessed the impact of different redox conditions on relative transmission of AR gene-bearing plasmids. Biofilm and liquid phase samples from aerobic, anoxic and anaerobic bioreactors were collected and assessed for in situ horizontal gene transfer, tracked using a fluorescent-labelled AR plasmid assay (developed here) from a recombinant E. coli host added to the systems at seeding concentrations of 106 cells/mL. Overall, plasmid hosts disappeared more rapidly in the aerobic bioreactors (2.0 log net reduction; final concentrations = 4.4 ~ 4.7 x104 cells/mL after 72 hours) and survived much longer in oxygen-free systems, especially in anaerobic biofilms (1.0 log net reduction; final concentrations = 1.6 ~ 2.7 x 105 cells/mL after 72 hours). However, evident conjugal transfer of the AR plasmid was limited in native biofilm communities.
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Final work tested DDHS systems at pilot-scale in Southern Malaysia to operationalise and validate the technology for field application. A semi-optimised configuration was developed, effectively removing C and TN (respective percentage load removal at 55% and 53%; satisfying local discharge standards), micropollutants, and reducing AR genes by 1.0 to 2.0 log from the wastewater community. Promising field results warrant further development of future prototypes to fuel the uptake of the DDHS technology, especially for LMIC applications.AstraZeneca UK and the ‘Engineering and Physical Sciences Research Council’ (EPSRC
Investigating the therapeutic potential of ATR, CHK1 and WEE1 inhibitors in cervical cancer
Get PDFM. D. Thesis.Introduction: Cervical cancer is the 4th most common cause of cancer-related death in
women. It is caused by infection with high-risk HPV (HR-HPV). Current therapy with cisplatin
and radiotherapy acts by damaging DNA. The DNA damage response (DDR), critical for
survival following endogenous and therapeutic DNA damage, comprises signalling to cell
cycle checkpoints and DNA repair. HR-HPV inactivates p53 and pRB and thereby the GI/S
checkpoint, making cervical cancer an ideal target for inhibition of intra-S and G2/M cell
cycle checkpoints. This thesis directly compares the efficacy of inhibitors of the S and G2/M
checkpoints: ATR, CHK1 and WEE1 as single agents and as sensitisers to cisplatin and ionising
radiation in cervical cancer cell lines
Methods: A panel of 6 cervical cancer cell lines with different histopathology and HPV status
were used. DDR protein expression and inhibition of ATR, CHK1 and WEE1 by VE-821, PF477736 and MK-1775, respectively were measured by Western blot. Checkpoint proteins
were measured in a TMA of human cervical cancer by IHC. Cytotoxicity of inhibitors and
combinations with cisplatin or IR was determined by clonogenic assay. Cell cycle analysis
with propidium iodide was used to investigate cell cycle changes.
Results: The expression of DDR and checkpoint proteins varied in both cell lines and the
TMA. There was a modest spectrum of sensitivity to cisplatin, IR and the inhibitors but the
rank order was different for each agent, which was not related to the levels of DDR proteins
in general but low ATM was associated with VE-821 sensitivity. The inhibitors were used at
fixed concentrations for chemo- and radio-sensitisation studies: 1 µM VE821, 50 nM PF477736 and 100 nM MK-1775 reflecting their relative target inhibition potency and intrinsic
cytotoxicity. Greater sensitisation was observed with cisplatin than IR, with VE-821 having
the greatest and MK-1775 the least effect. Cisplatin caused S-phase accumulation that was
reduced by the kinase inhibitors in 4/6 cell lines but increased in the other 2. The effects
were more marked for VE-821 and PF-477736 vs MK-1775 and were not related to cisplatin
sensitisation.
Conclusions: Cytotoxicity and sensitisation effects were not explained by protein expressions
or enzyme inhibition. The effect of the inhibitors on cisplatin-induced S-phase arrest varied
across the cell line panel and did not correlate with sensitisation data. Analysis was
hampered by the size of the panel and their similarity. Further work with a larger, more
diverse panel of cell lines is required before the mechanisms and potential biomarkers of
response to ATR, CHK1 and WEE1 inhibition in cervical cancer can be identified.Northern Cancer Care and Research Society
(NCCRS
Physical Unclonability Framework for the Internet of Things
Get PDFPh. D. ThesisThe rise of the Internet of Things (IoT) creates a tendency to construct unified architectures
with a great number of edge nodes and inherent security risks due to centralisation.
At the same time, security and privacy defenders advocate for decentralised solutions
which divide the control and the responsibility among the entirety of the network nodes.
However, spreading secrets among several parties also expands the attack surface.
This conflict is in part due to the difficulty in differentiating between instances of the
same hardware, which leads to treating physically distinct devices as identical. Harnessing
the uniqueness of each connected device and injecting it into security protocols can provide
solutions to several common issues of the IoT. Secrets can be generated directly from this
uniqueness without the need to manually embed them into devices, reducing both the risk
of exposure and the cost of managing great numbers of devices.
Uniqueness can then lead to the primitive of unclonability. Unclonability refers to
ensuring the difficulty of producing an exact duplicate of an entity via observing and
measuring the entity’s features and behaviour. Unclonability has been realised on a physical
level via the use of Physical Unclonable Functions (PUFs). PUFs are constructions
that extract the inherent unclonable features of objects and compound them into a usable
form, often that of binary data. PUFs are also exceptionally useful in IoT applications
since they are low-cost, easy to integrate into existing designs, and have the potential to
replace expensive cryptographic operations. Thus, a great number of solutions have been
developed to integrate PUFs in various security scenarios. However, methods to expand
unclonability into a complete security framework have not been thoroughly studied.
In this work, the foundations are set for the development of such a framework through
the formulation of an unclonability stack, in the paradigm of the OSI reference model. The
stack comprises layers propagating the primitive from the unclonable PUF ICs, to devices,
network links and eventually unclonable systems. Those layers are introduced, and work
towards the design of protocols and methods for several of the layers is presented.
A collection of protocols based on one or more unclonable tokens or authority devices
is proposed, to enable the secure introduction of network nodes into groups or neighbourhoods.
The role of the authority devices is that of a consolidated, observable root of
ownership, whose physical state can be verified. After their introduction, nodes are able
to identify and interact with their peers, exchange keys and form relationships, without
the need of continued interaction with the authority device.
Building on this introduction scheme, methods for establishing and maintaining unclonable
links between pairs of nodes are introduced. These pairwise links are essential for
the construction of relationships among multiple network nodes, in a variety of topologies.
Those topologies and the resulting relationships are formulated and discussed.
While the framework does not depend on specific PUF hardware, SRAM PUFs are
chosen as a case study since they are commonly used and based on components that
are already present in the majority of IoT devices. In the context of SRAM PUFs and
with a view to the proposed framework, practical issues affecting the adoption of PUFs in
security protocols are discussed. Methods of improving the capabilities of SRAM PUFs
are also proposed, based on experimental data.School of Engineering Newcastle Universit
The role of molten-salt distribution in dual-phase ceramic molten-salt membranes
Get PDFPh. D. ThesisDual-phase ceramic molten-salt membranes offer high permselectivity for carbon
dioxide at high temperature (> 400 °C) under continuous operation and offer a reduced
environmental footprint compared to current carbon dioxide separation technologies.
However, these membranes have not been developed with consideration to controlling
the distribution of the molten carbonate phase within their ceramic supports, which can
decrease molten carbonate effective thickness and increase surface area for carbon
dioxide desorption and feasibly improve flux. Furthermore, the highly wetting nature of
molten salts on ceramic supports could be exploited to spread and self-heal cracks
developed during long-term operation attributed to the brittle nature of ceramic
supports.
Here, carbon dioxide permeation was initially investigated in a model dual-phase
membrane, where the support did not contribute to the permeation mechanism so that
the permeation of carbon dioxide was restricted to the molten phase alone. Surfaceexchange reactions were found to be rate-limiting on carbon dioxide flux between 450
– 750 °C, whereas bulk-diffusion limitations occurred above 750 °C. Thus, carbon
dioxide flux can be increased in the surface-exchange-limited region by increasing the
available surface area for carbon dioxide desorption (chapter 4).
Subsequently, an asymmetric hollow fibre with high and tailorable surface area for
carbon dioxide desorption was used as the membrane support. Asymmetric hollowfibre supports (widely used in polymeric and ceramic membrane systems) comprise
two distinct porosity domains: micro-channels, conically shaped with an open entrance
on the lumen/permeate-side surface (pore size: 2 – 30 μm), and a porous microstructured packed-pore network (pore size: 0.05 – 0.5 μm), located between microchannels and at the shell/feed-side surface of the hollow-fibre supports. Hence, the
permeate-side surface area of the hollow fibre supports consists of the projected
surface areas of the micro-channels and the areas between them, whereas the feedside surface is the projected surface area of a cylinder. So far, the molten phase has
been infiltrated into both porosity domains of the hollow fibre supports in an
uncontrolled way, sacrificing gaseous mass transfer advantages of the micro-channels
and decreasing the available interfacial area between gaseous carbon dioxide and
molten carbonate from the projected surface areas of the micro-channels and the
areas between them to that of a cylinder. A new carbonate infiltration method was
developed in this work, aiming to control the incorporation and distribution of the molten
phase inside the packed-pore network of the hollow-fibre supports alone. As the
infiltration targeted the incorporation of the carbonates in the packed-pore network
alone, leaving the micro-channels unblocked, the interfacial area between molten
carbonate and carbon dioxide increased (chapter 3).
In the controlled-infiltrated hollow-fibre membranes the porous micro-channels
remained unblocked, increasing the surface area for carbon dioxide desorption to that
of the sum of the micro-channels surface areas. In the uncontrolled-infiltrated
membranes the surface area for carbon dioxide desorption was ~5 times smaller as
the surface area of the micro-channels was blocked. The increase in surface area in
the membranes developed by the controlled-infiltration method showed an 8-fold flux
improvement (0.036 ml min-1 cm-2) at 600 °C compared to membranes where the
molten-salt distribution, and available surface area, was uncontrolled (0.004
ml min-1 cm-2) (chapter 4).
Finally, molten carbonate wetting on the ceramic surface, was exploited to self-heal
cracks in membrane supports. A membrane system with an incorporated sacrificial
crack-inducing material demonstrated that the molten phase can spread and self-heal
the catastrophic crack created upon removal of the crack-inducing material. The
permselectivity of the membrane was restored after self-healing occurred,
demonstrating the first autonomous and intrinsic self-healing membrane (chapter 5).ESPRC, Newcastle Universit
Small and high-temperature electrical machine for vehicle applications
Get PDFPhD ThesisInterior permanent magnet (IPM) motors are a very promising design alternative in comparison with other types of electrical motors. Even though the price of rare-earth magnets has become a severe concern, IPM motors are gaining increasing attention due to their high torque density and excellent field weakening performance. Therefore, researchers have attempted to reduce the use of magnet materials but, at the same time, maintain the output performance of IPM motors. One solution is to reduce the size of the machine, which will also reduce the amounts of all materials used. Generally, a small scale is a profound advantage. Still, it may constitute a deficiency from the thermal point of view by contributing to higher loss density and problems operating at a higher temperature. An IPM motor may fail due to winding failure or the demagnetization of permanent magnets. It is crucial to make sure that these motors can run safely.
The purpose of this study is to develop a new electrical machine for automotive applications that is smaller in size with minimised use of magnets and which meets all requirements. The focus is on design alterations to reduce the size of the motor. Furthermore, high-temperature materials are used to ensure that the motor can work safely, even in hotter conditions.
A comparison is conducted on the performance of different sizes of the motor using finite element analysis in attempting to reduce the usage of the magnet material. Then, the temperature and heat transfer exposure of IPM motors are predicted by applying thermal modelling. Research is also conducted to find the most suitable material for smaller IPM motors to run at higher temperatures. Besides using neodymium-iron-boron as magnet material for an existing IPM motor, this study also analyses an IPM motor with samarium-cobalt, which has advantages in terms of higher temperature operation. The characteristics of IPM motors equipped with distributed and concentrated winding for automotive applications are also considered, and a proper motor winding is proposed. Two prototype IPM motors with different sizes and magnet materials are built and tested to validate the finite element analysis results. The first machine is developed using the same materials as in the existing Nissan Leaf machine, while the second is designed using high-temperature materials.
The most suitable size of a smaller IPM motor is ultimately determined, which can reduce the usage of permanent magnet and other material, but which maintains the output
performance of existing IPM motors. The design allows significant weight and size reductions in comparison with existing PM motors due to the use of high-temperature materials, which makes this electrical motor the right candidate for traction drive applications. The motor also satisfies all safety requirements
Homomorphic encryption in algebraic settings
Get PDFPhD ThesisCryptography methods have been around for a long time to protect sensitive data. With
data sets becoming increasingly large we wish to not only store sensitive data in public
clouds but in fact, analyse and compute there too. The idea behind homomorphic encryption
is that encryption preserves the structure and allows us to perform the same
operations on ciphertext as we would on the plaintext. A lot of the work so far restricts
the operations that can be performed correctly on ciphertexts. The goal of this thesis is
to explore methods for encryption which should greatly increase the amount of analysis
and computation that can be performed on ciphertexts.
First of all, we will consider the implications of quantum computers on cryptography.
There has already been research conducted into quantum-resistant encryption methods.
The particular method we will be interested in is still classical. We are assuming these
schemes are going to be used in a post-quantum world anyway, we look at how we can use
the quantum properties to improve the cryptosystem. More speci cally, we aim to remove
a restriction that naturally comes with the scheme restricting how many operations we
can perform on ciphertexts.
Secondly, we propose a key exchange protocol that works in a polynomial ideal setting.
We do this so that the key can be used for a homomorphic cryptography protocol. The
advantage of using key exchange over a public key system is that a large proportion of the
process needs to be carried out only once instead of needing a more complicated encryption
function to use for each piece of data. Polynomial rings are an appropriate choice of
structure for this particular type of scheme as they allow us to do everything we need. We
will examine how we can perform computation correctly on ciphertexts and address some
of the potential weaknesses of such a process.
Finally after establishing a fully homomorphic encryption system we will take a more
in-depth look at complexity. Measuring the complexity of mathematical problems is, of
course, crucial in cryptography, but the choice of measure is something we need to consider
seriously. In the nal chapter we will look at generic complexity as its gives us a good feel
for how di cult the typical instances of a problem are to solve.Engineering and Physical Sciences Research Council, Centre
for Doctoral Training in Cloud Computing for Big Dat
Hybrid dye-sensitised photocathodes for photoelectrochemical hydrogen evolution
Get PDFPhD ThesisNanostructured materials offer new, cost-efficient and sustainable opportunities for converting sunlight into electricity and fuels. The complexity of photoelectrochemical water splitting needs thorough understanding of the system and new ideas to overcome the limiting factors in the process. My work has been focused on investigating new hybrid dye-sensitised photocathodes for photoelectrochemical water splitting and developing a system that enables efficient photoelectrochemical H2 production.
Firstly, the effect of the experimental environment to photoelectrochemical performance of the dye-sensitised NiO photocathode was investigated. For that, an optimal electron acceptor for photocurrent generation was chosen and applied in all experiments, while changing the experimental conditions. The effect of electrolyte composition, the pH of the electrolyte and the concentration of the electron acceptor were studied. It was observed that the highest photocurrent was obtained with 4,4’-dithiodipyridine electron acceptor with 5 mM concentration in pH 3 aqueous electrolyte. After determining the optimal conditions for photocurrent generation using the most robust dye available, the system was tested for H2 evolution with electrodeposited Pt catalyst. In addition, the NiO photocathode was tested with another dye with a polymeric structure and with a commercially available benchmark organic dye. Both systems were studied photoelectrochemically under the optimal conditions determined previously and also tested for H2 evolution with electrodeposited catalysts on the surface. The highest photocurrents were observed with the commercially available benchmark organic dye with an electron acceptor and H2 evolution was also shown on the same system with catalyst on the surface.
Secondly, a new approach to increase the faradaic efficiency of dye-sensitised photocathodes for H2 evolution from water was investigated using integrated photocatalysts. Superiority to previously reported photocathodes was demonstrated, producing photocurrent densities of 30–35 μA cm-2 at an applied bias of -0.2 V vs. Ag/AgCl over 1 hour of continuous white light irradiation, resulting in the generation of 0.41 mmol h-1 cm-2 of H2 with faradaic efficiencies of up to 90%. Surface analysis of the photocathodes before and after photoelectrocatalysis revealed that the photocatalyst was photochemically stable, highlighting the benefits of the approach towards robust, hybrid solar-to-fuel devices.
Furthermore, CuCrO2 as an alternative material to NiO was studied as a photocathode for photoelectrochemical H2 evolution. The aim was to show that the post-calcination of the photocathodes under N2 atmosphere changes the surface chemistry of the CuCrO2 photocathodes which translates into superior stability and efficiency. The origin of these properties was discussed with the help of X-ray photoelectron spectroscopy surface analysis. Stable H2 production on the CuCrO2 photocathode from aqueous buffer solution was shown and the efficiency of the process was further increased with the use of inorganic cobalt co-catalyst in solution. In addition, successful sensitisation of the CuCrO2 photocathode with an organic dye was demonstrated resulting in considerable increase in photocurrent when using an electron acceptor in solution. An additional study was made where Fe was added to the CuCrO2 to form CuCr0.5Fe0.5O2 mixed metal delafossite. The aim of this study was to see how the Fe3+ addition to the delafossite crystal lattice influences the photoelectrochemical performance and stability of the material.
Finally, the photoelectrochemical cell was modelled and studied by building a simplified model in COMSOL Multiphysics and changing the exchange current density value on the electrode-electrolyte boundary based on experimental data obtained from photoelectrochemical measurement with different systems presented in this thesis. Results obtained from the model were discussed along with discussion on future optimisation possibilities of the model to increase the output of meaningful data
Governance, decision-making and publicness in marine space
Get PDFPh. D. ThesisThis research explores the relationship between the publicness of the sea and the
process through which marine development is assessed and consented within
England’s Marine Spatial Planning (MSP) marine licensing framework. This is
undertaken through the use of additional research questions which explore the
publicness of marine governance frameworks and the marine development decision
making process. The publicness of the sea is also defined and explored. The research
uses a case study approach to explore these questions. This is supplemented by
analysis of marine policy and legislation, quantitative marine licensing data and
additional application case examples to contextualise the case study within the marine
licensing decision making process. The main case study focuses on the Goodwin
Sands intertidal and subtidal sandbank complex located in the South East Inshore
Marine Plan area. By focusing on this area, and a controversial marine licence
application for development activity within it, this research explores the multiple
representations which produce meaning and value within the Goodwin Sands. The
conceptual framework applies the Production of Space thesis to the case study area
to help to understand the conflicts present within the case study marine licence
application.
The results show that when the representations of a proposed marine development
space within application assessment and support documents are limited to categories
within the Environmental Impact Assessment process there is a risk of major objection
from local publics. The Goodwin Sands is a social space produced through the
relationship between its unique physicality, its historical and contemporary uses and
its mythology and legend. Whilst these are not material planning considerations,
having regard for them within development decision making would help to mitigate
objection through working collaboratively with local publics. This research emphasises
the need to engage in meaningful public consultation during development consent
processes within MSP. This includes identifying affected publics and gaining an
understanding of the social and cultural landscape which are given expression through
marine development consent objection.ESR
Parametric estimation of non-minimum phase switch mode power converters
Get PDFPhD ThesisNowadays, switching mode power converters (SMPCs) are widely used in many applications. The advanced control technique for converters, such as adaptive control is also spread-used in many converter control scheme designs. System identification as a tool for estimating the converter operating conditions, and providing the information to the controller is a key technique for these applications; and parametric estimation, which is part of the system identification technique, is an advanced identification technique which can allow on-line system identification and adaptive control design. However, most of the research over the past decades has only covered parametric estimation of buck converters and there is barely anything about boost converters or other non-minimum phase converters. The reason behind this is that the parametric estimation results of non-minimum phase converters are not fitted to the calculated model weights, especially for the numerator weights of the model transfer function. Thus, the controller gains cannot be determined correctly by the wrong estimated model weights. It has been a big problem in the application of parametric estimation for decades. In this research, a modelling method which is based on trailing-edge PWM off-time sampling (TEOS) is introduced in order to address this problem.
The objective of this research is to develop an approach to resolve the existing accuracy problems of non-minimum phase SMPC parametric estimation. The problem, which has existed for decades, is that commonly used state-space averaged model numerator weights are not fitted to the non-minimum phase SMPC parametric estimation results. There are several possible ways to address this problem, including modification of converter modelling, modification of parametric estimation mechanism, or with the help of compensators. In this research, the TEOS modelling method has been verified by both simulation and practical experiment to provide the best-fit model weights for the parametric estimation of buck converters and boost converters; and it has also been verified, by simulation, to be used for buck-boost converter parametric estimation, which has opened up great possibilities for its use on other non-minimum phase converters. The experimental results have shown that the proposed modelling approach has improved the accuracy of parametric estimation for boost converters by more than 20% compared with the commonly used state-space averaged modelling approach.
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In addition, the TEOS model will also present a thorough inspection of the relationships between system parameters (load resistance, capacitance and inductance) and the model transfer function parameters, which can then realise the sensor-less on-line system parameters estimation or monitoring. This function is also a novel approach to the area of system component monitoring.
In this thesis, the reason behind the problem of non-minimum phase converter parametric estimation is analysed for the first time. The system parametric estimation of three converters (buck, boost and buck-boost) were tested with on-line simulation and off-line experimental tests for both the averaged model and the proposed model. Then, system parameters estimation was also tested for the buck converter and boost converter in the simulation and practical experiment. In addition, the platform setup, the interface build between the Matlab Simulink and the Code Composer Studio (CCS), the settings of the Digital Signal Processor (DSP) TMS320F28335, the parameters design of boost SMPC, and the design of the Printed Circuit Board (PCB) schematics and layout are also presented in this thesis. The outcome of the research should be able to further benefit many applications of advanced control systems, fault detection, and system component monitoring
Amperometric gas sensor technologies
Get PDFPh.D. Thesis.This project investigated the electrochemistry of carbon monoxide and various volatile organic
compounds (VOCs primary alcohols, ketones and esters) with the aim of understanding the details of
the operation of amperometric CO sensors and of developing VOC sensors of similar performance. The
electrochemical devices studied were based on commercial CO sensors (supplied by Alphasense Ltd)
and specially modified Alphasense sensor devices designed to measure VOCs.
In amperometric response to carbon monoxide, all of the tested sensors (kindly provided by
Alphasense) showed an increase in current flowing within the sensor. This increase in current was
directly proportional to the concentration of analyte within the zero-air carrier gas. The large
capacitance of the devices prevents the use of standard potentiodynamic techniques to interrogate
the mechanism. Instead we employed concentration-step experiments at constant potential using a
gas flow system under digital mass flow control.
A diffusion-based model for the sensors was derived and solved. Custom modelling software using
nonlinear least squares was developed to fit the experimental data and to derive estimates of the
apparent membrane/diffusion layer thicknesses and the apparent diffusion coefficient of the analyte.
The standard CO-AF sensors whilst exposed to carbon monoxide were calculated as having (at 20°C)
layer thicknesses (L) on the order of 10-1
cm, this is commensurate with the physical distance from the
top face of the sensor to the electrolyte boundary of approximately 0.3 (±0.1) cm. It also gave CO
diffusion coefficients (D) on the order of 10-3
- 10-2
cm2
s
-1 (at 20°C). These values suggest currentlimiting diffusion in the gas phase rather than the thin liquid layer (D approximately 10-6
cm2
s
-1
)
covering the working electrode. Increasing the thickness of the semi-permeable membrane in the
standard CO-AF sensor gave an increase in L, thus validating the diffusion model.
In response to VOCs, the CO-A1 sensors (kindly provided by Alphasense) only gave a significant
amperometric response to alcohol and aldehyde functional groups. This is in line with the current
literature and was hypothesised to be due to those functional groups being relatively easily
chemisorbed and subsequently electrooxidised at platinum under aqueous acidic conditions. The
unresponsive VOCs were thought to be due to a combination of lack of solubility and high oxidation
potential. Again, the sensors showed directly proportional current responses to the concentration of
VOC analyte within the carrier gas.
At all temperatures the CO-A1 sensors showed layer thicknesses (L) on the order of µm or smaller, and
VOC diffusion coefficients (D) were in the 10-8
cm2
s
-1
range or smaller. Thus, indicating that the rate
limiting step for the detection of alcohols and aldehydes was within the 5M sulfuric acid electrolyte
solution or perhaps a kinetic barrier, e.g., for dissolution at the air/electrolyte interface.
Finally, some initial studies on alternative electrolytes for amperometric gas sensors were carried out.
The motivation for this is that the standard electrolyte (5M H2SO4) is corrosive and hygroscopic. The
choice of electrolyte for amperometric devices is not simple because it must have a negligible rate of
evaporation in air over long periods (1-2 years). Most aqueous electrolytes are unsuitable and only a
few organic electrolytes (propylene carbonate) are both non-volatile and non-toxic. An alternative is a
polyionic hydrogel; polyacrylate was chosen and the electrochemistry of some simple redox
compounds were investigated. However, the hydrogels ultimately proved unsuitable for the long-term
requirements of the sensors due to weight loss over time.Alphasense Ltd