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Title: The development of disposable screen-printed amperometric sensors for monitoring VSC production in oral malodour
Author: Spencer, Paul St. John
ISNI:       0000 0004 2720 6017
Awarding Body: University of the West of England, Bristol
Current Institution: University of the West of England, Bristol
Date of Award: 2002
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A prototype gas sensor comprising of screen-printed carbon electrodes modified with cobalt phthalocyanine was constructed for the purposes of detecting volatile sulphur compounds (VSC) in oral malodour. For hydrogen sulphide (H2S) detection, hydrodynamic voltammetry showed the optimum applied potential to be +600 mV using a supporting electrolyte buffered to a pH value of 12.0. The prototype was shown to detect H2S at concentrations as low as 100 ppb. The sensor was not subject to interference by carbon dioxide and the sensor response to methyl mercaptan (CH3SH) was low (10% when compared to H2S at 10 ppm). Ethanol at concentrations above 5% showed background interference in the non-Faradaic current of the sensor. This was differentiated from the Faradaic (analytical) response, which remained proportional to the concentration of the H2S. The response to H2S was shown to be rapid, reversible and proportional to a range of concentrations from 0.1 ppm to 18 ppm. The prototype sensor was placed in the headspace of flasks containing buffer, substrate and washed cell suspensions of oral microbial species, Peptostreptococcus micros ATCC 33270, Fusobacterium nucleatum ATCC 10953 and Porphyromonas gingivalis W50. The sensor gave a characteristic response commensurate with the evolution of H2S. Control assays (cells without substrate and substrate without cells) gave no response. It was established that the assay showed linearity with time and amount of enzyme. Cell suspension assays were used to determine Km values and to investigate the putative effects of inhibitors. All the test species were capable of transforming cysteine substrate to H2S with Km values of 0.031, 0.304, and 0.165mM for P. micros, F. nucleatum and P. gingivalis respectively. The Km value obtained for P. micros using glutathione substrate was extremely low at 0.000348mM showing that this species has a uniquely high capacity to form H2S from this substrate. The putative inhibitor, lactate, at 10 mM was ineffectual against P. gingivalis and F. nucleatum but gave >50% inhibition of H2S production against P. micros. The oxidising agents, sodium chlorite (100 - 250 ppm) and hydrogen peroxide (250 - 500 ppm) were highly active but flavour oil mix was inactive even at the highest concentration tested (0.0 I % W/v). It was shown that flavour oils combined with ethanol did not interfere with the analytical signal of the sensor. The in vitro assay was capable of detecting significant differences between tongue scrape samples (one sample from a low and one from a high odour individual, matched in terms of cell density). A comparison of the amperometric device with the halimeter using standard H2S concentrations in vitro showed a high correlation (R2 = 0.949). However, the relationship between the amperometric sensor when applied to volunteers (n=21) showed a weaker relationship (R = 0.33) with the halimeter demonstrating a higher degree of variation. By placing the prototype sensor into a tube inserted into the mouth it was possible to continuously monitor the levels of VSC both before and after a cysteine mouthrinse. The output was shown to fluctuate (mean = 700ppb H2S equivalent). This increased threefold (to 2200 ppb H2S), following a cysteine mouthrinse. The study has demonstrated a variety of applications for use of this type of sensor in oral malodour research, which.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available