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In this study, we developed an odor sensor system using chemosensitive resistors, which outputted multichannel data. Mixtures of gas chromatography stationary materials (GC materials) and carbon black were used as the chemosensitive resistors. The interaction between the chemosensitive resistors and gas species shifted the electrical resistance of the resistors. Sixteen different chemosensitive resistors were fabricated on an odor sensor chip. In addition, a compact measurement instrument was fabricated. Sixteen channel data were obtained from the measurements of gas species using the instrument. The data were analyzed using machine learning algorithms available on Weka software. As a result, the sensor system successfully identified alcoholic beverages. Finally, we demonstrated the classification of restroom odor in a field test. The classification was successful with an accuracy of 97.9%.

In this study, we have fabricated multichannel odor sensor using chemosensitive resistors. The chemosensitive resistors were made from complex of carbon black and gas chromatography stationary materials (GC materials). The electrical resistance of the chemosensitive resistor changed by sensing gas species. We have fabricated an odor sensor chip, which had 16 types of chemosensitive resistors. In addition, we developed a measurement instrument with compact size. The odor sensor chip was embedded in the instrument to construct an odor sensor system. The sensor system outputted the data of 16 channels if sensing gas species. The data have been analyzed using machine learning algorithms that were available on software Weka. As a result, it was successful to identify alcohol beverages by sensing their odor using the sensor system.

The detection and recognition of metabolically derived aldehydes, which have been identified as important products of oxidative stress and biomarkers of cancers; are considered as an effective approach for early cancer detection as well as health status monitoring. Quartz crystal microbalance (QCM) sensor arrays based on molecularly imprinted sol-gel (MISG) materials were developed in this work for highly sensitive detection and highly selective recognition of typical aldehyde vapors including hexanal (HAL); nonanal (NAL) and bezaldehyde (BAL). The MISGs were prepared by a sol-gel procedure using two matrix precursors: tetraethyl orthosilicate (TEOS) and tetrabutoxytitanium (TBOT). Aminopropyltriethoxysilane (APT); diethylaminopropyltrimethoxysilane (EAP) and trimethoxy-phenylsilane (TMP) were added as functional monomers to adjust the imprinting effect of the matrix. Hexanoic acid (HA); nonanoic acid (NA) and benzoic acid (BA) were used as psuedotemplates in view of their analogous structure to the target molecules as well as the strong hydrogen-bonding interaction with the matrix. Totally 13 types of MISGs with different components were prepared and coated on QCM electrodes by spin coating. Their sensing characters towards the three aldehyde vapors with different concentrations were investigated qualitatively. The results demonstrated that the response of individual sensors to each target strongly depended on the matrix precursors; functional monomers and template molecules. An optimization of the 13 MISG materials was carried out based on statistical analysis such as principle component analysis (PCA); multivariate analysis of covariance (MANCOVA) and hierarchical cluster analysis (HCA). The optimized sensor array consisting of five channels showed a high discrimination ability on the aldehyde vapors; which was confirmed by quantitative comparison with a randomly selected array. It was suggested that both the molecularly imprinting (MIP) effect and the matrix effect contributed to the sensitivity and selectivity of the optimized sensor array. The developed MISGs were expected to be promising materials for the detection and recognition of volatile aldehydes contained in exhaled breath or human body odor.

The paper reports on development of a novel method for fabrication of highly sensitive spherical surface acoustic wave (Ball-SAW) devices for odor-sensing. The proposed method is based on functionalization of the Ball-SAW devices’ surface by means of chemisorption of self-assembled lipopolymers. Once functionalized, the devices’ surface can be further modified by means of physisorption of various lipid derivatives. More traditional techniques used for sensing-film deposition on Ball-SAW devices (such as AC electrospray) cause a considerable attenuation of the acoustic wave propagation after many circulations. The method proposed here is relatively simple and additional attenuation due to coating is negligible even if the amount of the coating is the same as that in the traditional technique. This makes for a significant enhancement of sensitivity to odorants as compared to the sensors coated with traditional techniques.

The new prototype of odor recorder based on real-time mass spectrometry was developed. Based on this developed system, a set of sensor outputs with high selectivities to the odor components are easily obtained from the available m/z's so that a large number of odor components can be used with minimized collinearity. Moreover, the gas-phase target odor can be measured and reproduced in real-time with wide dynamic range based on the improved odor blending method. In recording process, the mass patterns of all odor components were used as the database data and the nonnegative least squares method was performed to estimate recipe of the target odor. It was found that this system could accurately estimate the recipe of the target odor within 25 s using only small set of m/z's and could reproduce the odor with the same impression of smell as the target one. Thus, it was confirmed that the recording wide range of odor in real time could be realized based on this developed odor recorder.

Biological olfaction is a powerful system enabling acquisition and processing of various chemical information from environment. Vast significance of the sense of smell is reflected in attempts to create instrumental techniques mimicking the biological system—artificial/machine olfaction. Following the biological systems, the artificial olfaction relies on arrays of gas sensors with broad specificities to odorants. Arguably, among available gas-sensing technologies, the most suitable choices for artificial olfaction are acoustic wave sensors, including quartz crystal microbalance (QCM) gas/odor sensors. The short review herein presents basic information on organization and principles of biological and artificial olfaction systems as well as several methods for fabrication of biomimetic or bioinspired (QCM) sensors for artificial olfaction. Copyright © 2009 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Search for novel odor-sensing materials is vital for the general field of artificial olfaction using acoustic-wave based sensors such as QCMs. Here we present recent result of our ongoing study on application of pegylated lipids as sensing film supports for QCM odor-sensors. The method presented herein is based on chemisorption of lipopolymer- or lipid-lipopolymer cushion and subsequent physisorption of selected, amphiphilic gas chromatography (GC) materials. The GC materials physisorb onto the supports owing to hydrophobic interaction of their tails with lipid moieties of the lipopolymeric supports. The method allows for fabrication of hybrid QCM odor-sensors with high sensitivity to odorants without much acoustic loss, despite the liquid or semi liquid state of the GC materials used. The sensors are also capable of very good discrimination among odorant samples at various concentrations.

An odor sensing system using a quartz crystal microbalance (QCM) sensor array and pattern recognition technique has been for years a main research topic in our group. For the general field of artificial olfaction using acoustic-wave based sensors such as QCMs it is vital to search for novel sensing materials. Here we present recent results of our ongoing study on application of pegylated lipids as coatings for QCM odor-sensors. The method presented herein is based on self-assembling of lipids and lipid-derivatives on the QCM surfaces. The disulphide-terminated lipids and lipopolymers are co-chemisorbed onto gold electrodes of QCM sensors by simple immersion in ethanolic solutions. This creates porous supports onto which additional layers of lipopolymers are physisorbed. The method allows for fabrication of lipopolymeric QCM odor-sensors with enhanced sensitivity to odorants, capable of very good discrimination among odorant samples—according to the functional group of an odorant.

We report application of the PEGylated lipids (PEG lipopolymers) containing disulphide as supports for sensing films for quartz crystal microbalance (QCM) odor sensors. The materials are binding covalently to the surface of gold QCM sensor electrode creating self-assembled cushion. Additional amounts of lipid or lipid-derived materials can be physisorbed on that chemisorbed coating. Both processes do not require much labor and can be performed with minimum instruments in a simple process. The sensors fabricated with the chemisorbed supports are more sensitive to the tested odorants than their non-supported counterparts. Enhanced sensitivity is derived from higher fluidity of the supported films in comparison to the non-supported ones. Discrimination capability among odorants is also better for the sensors with chemisorbed supports than for the non-supported ones—there are no overlaps between the sample groups and the samples are clustered closely within the groups. Overall, the studied supported sensors introduce interesting properties that can be utilized in the odor sensing systems using QCM sensors.

In this report, we improve a sensitivity of odor sensing system using ball SAW devices. Although, we reported the three-flavor identification using four ball SAW devices with different sensing films, the concentrations of the fruits flavors had to be high in the previous study. Thus several optimizations were performed here. First, we optimized the coating parameters of AC electrospray method. Second, we optimized sensing film thickness. Thirdly, we used Savitzky-Golay filter for smoothing response waveforms. After the improvement, we could detect butanol of 90ppb below human threshold. Finally we found that the response speed of ball SAW devices are superior to that of QCM with comparable sensitivity.

A new prototype of odor recorder with higher-order sensing capability was proposed. This system was developed using a preconcentrator with variable temperature so that higher-order sensing based upon the different desorption property of each odor component can be achieved. The enriched data were used for odor recording process using a five-level quantization method to estimate recipes of the target odors. In order to investigate the fundamental capability of this system, recipe estimations of the target odors were performed using four odor components even including the two with high similarities in sensor response patterns. The recipes of target odors were appropriately estimated using the proposed odor recorder while the previous method without a preconcentrator was not successful. This proposed system is useful to enhance the robustness of recipe estimation against collinearity.

We report on improved ultrasonic atomizer method of depositing the sensing films on quartz crystal microbalance (QCM) sensors. The main objectives of the present work were minimizing the sensor-to-sensor response variation, improving the sensing film stability, and reducing amount of materials used during coating (reducing time of coating). In order to achieve the goals, we have redesigned and refabricated large parts of the atomizer system reported previously. The main principle of the operation—deposition of the fine mist generated by the ultrasonic device remained unchanged. The largest modifications encompass introduction of the new, sealed deposition chamber, considerable reduction of the flow resistance, and control over the flow rates of the mist and air streams in the system. The paper reports the new configuration of the ultrasonic atomizer method, optimization of the atomizer system performance (deposition parameters) and the results of improved quality of sensors fabricated using the new system.

An odor recorder is a system to record and reproduce odors. In order to record wide a range of odors, a number of available odor components are required in an odor recorder. Although many odor components were available in a previously developed odor recorder using an automatic sampler, there is still a collinearity problem caused by the lack of pattern separation among odor components. Then, we propose a new odor recorder with a wider recording range based on both transient and steady-state sensor responses in recording process. In this system, time constants are extracted from QCM sensor responses using the auto regressive (AR) model and are used together with conventional steady-state frequency shifts as variables to discriminate among odor components. Based on this proposed method, the pattern separation among odor components can be improved and up to 13 odor components can be used with a minimized collinearity problem. Recipes of five target odors with different impressions were successfully estimated based on the same set of these 13 odor components and sufficient similarities of impressions between all approximated odors and their target ones were confirmed by a human panel.

Lipids play an outstanding role in biological chemoreception schemes. The odorant–lipid interactions can be used as a mean of detecting the odorant molecules. A sensible way to exploit the interaction is coating the quartz crystal microbalance (QCM) sensors with natural or synthetic lipids or lipid-derived materials. Interesting candidates for such coatings are reported in the present paper—the synthetic lipids grafted with poly(ethylene glycol) (pegylated lipids, PEG lipopolymers). Two such materials grafted with PEG chains 2000 and 5000 were used for coating the QCMs. The obtained lipopolymer sensors were compared with pure lipid, pure polymer and lipid–polymer blend sensors. The results revealed very good discrimination power of the fabricated lipopolymer sensors for the range of odorants used in the study. The sensors have high sensor-to-sensor reproducibility and characteristics distinctly different from those for the remaining sensors used.

Odor sensing system using quartz crystal microbalance (QCM) sensor array and pattern recognition technique has been for a long time an important research topic. Research of novel sensing materials for QCM odor sensors is vital for realization of artificial olfaction and related devices such as odor recorder. Herein we study quartz crystal microbalance (QCM, 20 MHz, AT-cut) sensors coated with lipopolymers with polyethylene glycol (PEG) as a tether. The tether's molecular weights were 1000, 2000 and 5000. In addition, we fabricated QCM sensors coated with PEGs of molecular weights 1000, 2000 and 4000. The fabricated sensors' properties were evaluated during experiments of exposures to vapors of alcohols, esters and acids. From the obtained results it is clear that the tether's length (molecular weight) is an important factor influencing the resulting material's sensing properties. Sensititivity patterns of the lipopolymeric sensors were clealrly different from the ones for respective polymers. The obtained sensors seem to have a good capability to discriminate among odor samples according to the functional group of an odorant.

Mimicking the biological olfaction, large odor-sensor arrays can be used to acquire a broad range of chemical information, with a potentially high degree of redundancy, to allow for enhanced control over the sensitivity and selectivity of artificial olfaction systems. The arrays should consist of the largest possible number of individual sensing elements while being miniaturized. Chemosensitive resistors are one of the sensing platforms that have a potential to satisfy these two conditions. In this work we test viability of fabricating a 16-element chemosensitive resistor array for detection and recognition of volatile organic compounds (VOCs). The sensors were fabricated using blends of carbon black and gas chromatography (GC) stationary-phase materials preselected based on their sorption properties. Blends of the selected GC materials with carbon black particles were subsequently coated over chemosensitive resistor devices and the resulting sensors/arrays evaluated in exposure experiments against vapors of pyrrole, benzenal, nonanal, and 2-phenethylamine at 150, 300, 450, and 900 ppb. Responses of the fabricated 16-element array were stable and differed for each individual odorant sample, proving the blends of GC materials with carbon black particles can be effectively used for fabrication of large odor-sensing arrays based on chemosensitive resistors. The obtained results suggest that the proposed sensing devices could be effective in discriminating odor/vapor samples at the sub-ppm level.

The paper reports on development of a method for stabilization of coatings for QCM odor sensors that use liquid GC stationary phase materials. The liquid materials tend to be easily removed from the sensor's surface when coated directly onto its surface. The proposed method employs chemisorbed lipopolymers and lipids to form a matrix into which the liquid GC materials are encapsulated. The fabrication method is relatively simple as the sensing films are formed in a sorption process from a single solution. The obtained experimental results show significantly enhanced stability of the fabricated sensing films. The obtained sensors reproducibly show high sensitivities to vapors of selected odorants with detection limits in sub-ppm range. They also showed a good capability to discriminate among the odorant samples.

We present application of a novel method for fabrication of highly sensitive spherical surface acoustic wave (ball-SAW) odor sensors, using self-assembling of lipopolymers, lipids and GC (gas chromatography) materials. The method allows good control over the thickness of the odor-sensitive layer - an important feature for sensitivity enhancement of the ball-SAW sensors. The fabrication method consists of (i) chemisorption of a lipid-derivative(s), and (ii) physical sorption of GC stationary-phase materials inside the lipid-derivative's matrix. Ball-SAW sensors fabricated using this method have very high sensitivity to n-butanol in both dry- and humidified-air with detection limits well below human threshold.

Application of quartz crystal microbalance (QCM) sensors for odor sensing has been an important research topic. Proper performance of odor sensing systems with QCM sensors requires good discrimination among odor samples. Recently we have been studying pegylated lipopolymers - composed of lipid and poly(ethylene glycol). (PEG) The materials allowed for very good discrimination among odorants of different functional groups. Here we study lipopolymers with various molecular weight of PEG moiety. Our results indicate that the PEG chain length is an important factor influencing lipopolymer's sensing properties.

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