Limits...
Bio-benchmarking of electronic nose sensors.

Berna AZ, Anderson AR, Trowell SC - PLoS ONE (2009)

Bottom Line: The comparison also highlights some important questions about the molecular nature of fly ORs.The comparative approach generates practical learnings that may be taken up by solid-state physicists or engineers in designing new solid-state electronic nose sensors.It also potentially deepens our understanding of the performance of the biological system.

View Article: PubMed Central - PubMed

Affiliation: CSIRO Entomology and CSIRO Food Futures Flagship, Canberra, Australian Capital Territory, Australia.

ABSTRACT

Background: Electronic noses, E-Noses, are instruments designed to reproduce the performance of animal noses or antennae but generally they cannot match the discriminating power of the biological original and have, therefore, been of limited utility. The manner in which odorant space is sampled is a critical factor in the performance of all noses but so far it has been described in detail only for the fly antenna.

Methodology: Here we describe how a set of metal oxide (MOx) E-Nose sensors, which is the most commonly used type, samples odorant space and compare it with what is known about fly odorant receptors (ORs).

Principal findings: Compared with a fly's odorant receptors, MOx sensors from an electronic nose are on average more narrowly tuned but much more highly correlated with each other. A set of insect ORs can therefore sample broader regions of odorant space independently and redundantly than an equivalent number of MOx sensors. The comparison also highlights some important questions about the molecular nature of fly ORs.

Conclusions: The comparative approach generates practical learnings that may be taken up by solid-state physicists or engineers in designing new solid-state electronic nose sensors. It also potentially deepens our understanding of the performance of the biological system.

Show MeSH
A comparison between the absolute sensitivities of MOx sensors (PA/2, SY/GH) and dOR35a.Log-concentration response curves show the sensitivity of dOR35a (blue), PA/2 (green) and SY/GH (red) to 1-octanol (A) and 1-octen-3-ol (B). MOx sensor responses are measured as fractional changes in resistance. dOR35a responses were recorded by measuring calcium-stimulated Fluo4 fluorescence intensity for receptors transiently expressed in Sf9 cells. Error bars represent the standard errors of the means and, for the MOx sensors, these are too small to be visualised.
© Copyright Policy
Related In: Results  -  Collection


getmorefigures.php?uid=PMC2712691&req=5

pone-0006406-g006: A comparison between the absolute sensitivities of MOx sensors (PA/2, SY/GH) and dOR35a.Log-concentration response curves show the sensitivity of dOR35a (blue), PA/2 (green) and SY/GH (red) to 1-octanol (A) and 1-octen-3-ol (B). MOx sensor responses are measured as fractional changes in resistance. dOR35a responses were recorded by measuring calcium-stimulated Fluo4 fluorescence intensity for receptors transiently expressed in Sf9 cells. Error bars represent the standard errors of the means and, for the MOx sensors, these are too small to be visualised.

Mentions: One of the attractive features of MOx sensors is their high sensitivity to volatile odorants [1], which can confer a detection threshold comparable to that of humans, at least for some volatiles [4]. However, depending on the receptor and the volatile, the detection limits of biological receptors vary widely. In order to compare the sensitivities of different types of sensors it is necessary to relate their responses to known concentrations of the same odorants. When recording from the living fly, limitations of the stimulus presentation generally preclude knowledge of odorant concentrations. It is common practice to use standard dilutions of a stock odorant solution in lieu of defined absolute concentrations. Concentrations at the receptor vary depending on the vapour pressure of the odorant and the degree of extra dilution introduced in the airflow system and/or in diffusing from the insect cuticle to the receptor. The alternative approach pursued here, heterologous functional expression of insect ORs, permits known concentrations of odorants to contact the receptors in the liquid phase and enables direct measurements of OR sensitivity [18]. Measurements with the electronic nose are more straightforward because a predetermined concentration of odorant can be presented to the sensors. However, as far as we can ascertain, sensitivity of MOx sensors has not previously been stated in terms of an EC50 value at the sensor surface. Instead a limit of detection is determined and generally this relates to the level of analyte in the sample. The latter approach is ideal for determining the utility of MOx sensors for a particular task, however it is sub optimal for comparing the absolute sensitivities of different types of sensors. In the past, therefore, it has not been possible to make a rigorous comparison of the sensitivities of MOx sensors and biological ORs. In this study we have directly compared the EC50 values of MOx sensors. We surveyed the responses of all sensors to 89 compounds from the 110 odorant test panel for which we could obtain vapour pressure data. We selected the dOR35a receptor and MOx sensors PA/2 and SY/GH from a Fox 3000 electronic nose as representative receptors to perform a rigorous comparison because these sensors respond strongly to some of the same compounds. dOR35a is slightly unusual in that it is the only member of its class known to be expressed in one of the coelonic sensilla, which otherwise house a new class of variant ionotropic receptors [19]. In all other respects, dOR35a behaves as a normal member of its class. 1-octanol was selected as a test odorant because it gives the highest vapour pressure-adjusted response for dOR35a, generating an EC50≈23 nM for dOR35a (Figure 6A, Table 1). This compound was also detected as sensitively as any other by the MOx sensors (data not shown). Nevertheless, MOx EC50 values were 0.2–0.4 µM i.e. 10–20 fold higher than for dOR35a. 1-octen-3-ol was also selected, as representative of compounds that generate moderate responses in both types of sensors (Figure 6B, Table 1), in both the dOR35a and MOx sensors and elicited EC50 responses in the low micromolar range from both types of sensor. The EC50 values observed here represent a conservative upper limit for insect ORs. Using the same assay conditions, certain Drosophila [18] and lepidopteran [20], [21] OR/odorant combinations generate EC50 values as low as 14–16 pM to some odorants. It seems that, although the affinities of this group of MOx sensors for odorants overlap the working range for ORs, the affinities of some ORs may extend to at least four log units more sensitive than for the MOx sensors considered here. However, if the comparison were made in terms of limits of detection the relative sensitivity of the MOx sensors would be substantially better, due to their very low noise levels (Figure 6).


Bio-benchmarking of electronic nose sensors.

Berna AZ, Anderson AR, Trowell SC - PLoS ONE (2009)

A comparison between the absolute sensitivities of MOx sensors (PA/2, SY/GH) and dOR35a.Log-concentration response curves show the sensitivity of dOR35a (blue), PA/2 (green) and SY/GH (red) to 1-octanol (A) and 1-octen-3-ol (B). MOx sensor responses are measured as fractional changes in resistance. dOR35a responses were recorded by measuring calcium-stimulated Fluo4 fluorescence intensity for receptors transiently expressed in Sf9 cells. Error bars represent the standard errors of the means and, for the MOx sensors, these are too small to be visualised.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2712691&req=5

pone-0006406-g006: A comparison between the absolute sensitivities of MOx sensors (PA/2, SY/GH) and dOR35a.Log-concentration response curves show the sensitivity of dOR35a (blue), PA/2 (green) and SY/GH (red) to 1-octanol (A) and 1-octen-3-ol (B). MOx sensor responses are measured as fractional changes in resistance. dOR35a responses were recorded by measuring calcium-stimulated Fluo4 fluorescence intensity for receptors transiently expressed in Sf9 cells. Error bars represent the standard errors of the means and, for the MOx sensors, these are too small to be visualised.
Mentions: One of the attractive features of MOx sensors is their high sensitivity to volatile odorants [1], which can confer a detection threshold comparable to that of humans, at least for some volatiles [4]. However, depending on the receptor and the volatile, the detection limits of biological receptors vary widely. In order to compare the sensitivities of different types of sensors it is necessary to relate their responses to known concentrations of the same odorants. When recording from the living fly, limitations of the stimulus presentation generally preclude knowledge of odorant concentrations. It is common practice to use standard dilutions of a stock odorant solution in lieu of defined absolute concentrations. Concentrations at the receptor vary depending on the vapour pressure of the odorant and the degree of extra dilution introduced in the airflow system and/or in diffusing from the insect cuticle to the receptor. The alternative approach pursued here, heterologous functional expression of insect ORs, permits known concentrations of odorants to contact the receptors in the liquid phase and enables direct measurements of OR sensitivity [18]. Measurements with the electronic nose are more straightforward because a predetermined concentration of odorant can be presented to the sensors. However, as far as we can ascertain, sensitivity of MOx sensors has not previously been stated in terms of an EC50 value at the sensor surface. Instead a limit of detection is determined and generally this relates to the level of analyte in the sample. The latter approach is ideal for determining the utility of MOx sensors for a particular task, however it is sub optimal for comparing the absolute sensitivities of different types of sensors. In the past, therefore, it has not been possible to make a rigorous comparison of the sensitivities of MOx sensors and biological ORs. In this study we have directly compared the EC50 values of MOx sensors. We surveyed the responses of all sensors to 89 compounds from the 110 odorant test panel for which we could obtain vapour pressure data. We selected the dOR35a receptor and MOx sensors PA/2 and SY/GH from a Fox 3000 electronic nose as representative receptors to perform a rigorous comparison because these sensors respond strongly to some of the same compounds. dOR35a is slightly unusual in that it is the only member of its class known to be expressed in one of the coelonic sensilla, which otherwise house a new class of variant ionotropic receptors [19]. In all other respects, dOR35a behaves as a normal member of its class. 1-octanol was selected as a test odorant because it gives the highest vapour pressure-adjusted response for dOR35a, generating an EC50≈23 nM for dOR35a (Figure 6A, Table 1). This compound was also detected as sensitively as any other by the MOx sensors (data not shown). Nevertheless, MOx EC50 values were 0.2–0.4 µM i.e. 10–20 fold higher than for dOR35a. 1-octen-3-ol was also selected, as representative of compounds that generate moderate responses in both types of sensors (Figure 6B, Table 1), in both the dOR35a and MOx sensors and elicited EC50 responses in the low micromolar range from both types of sensor. The EC50 values observed here represent a conservative upper limit for insect ORs. Using the same assay conditions, certain Drosophila [18] and lepidopteran [20], [21] OR/odorant combinations generate EC50 values as low as 14–16 pM to some odorants. It seems that, although the affinities of this group of MOx sensors for odorants overlap the working range for ORs, the affinities of some ORs may extend to at least four log units more sensitive than for the MOx sensors considered here. However, if the comparison were made in terms of limits of detection the relative sensitivity of the MOx sensors would be substantially better, due to their very low noise levels (Figure 6).

Bottom Line: The comparison also highlights some important questions about the molecular nature of fly ORs.The comparative approach generates practical learnings that may be taken up by solid-state physicists or engineers in designing new solid-state electronic nose sensors.It also potentially deepens our understanding of the performance of the biological system.

View Article: PubMed Central - PubMed

Affiliation: CSIRO Entomology and CSIRO Food Futures Flagship, Canberra, Australian Capital Territory, Australia.

ABSTRACT

Background: Electronic noses, E-Noses, are instruments designed to reproduce the performance of animal noses or antennae but generally they cannot match the discriminating power of the biological original and have, therefore, been of limited utility. The manner in which odorant space is sampled is a critical factor in the performance of all noses but so far it has been described in detail only for the fly antenna.

Methodology: Here we describe how a set of metal oxide (MOx) E-Nose sensors, which is the most commonly used type, samples odorant space and compare it with what is known about fly odorant receptors (ORs).

Principal findings: Compared with a fly's odorant receptors, MOx sensors from an electronic nose are on average more narrowly tuned but much more highly correlated with each other. A set of insect ORs can therefore sample broader regions of odorant space independently and redundantly than an equivalent number of MOx sensors. The comparison also highlights some important questions about the molecular nature of fly ORs.

Conclusions: The comparative approach generates practical learnings that may be taken up by solid-state physicists or engineers in designing new solid-state electronic nose sensors. It also potentially deepens our understanding of the performance of the biological system.

Show MeSH