Higher-Order Chemical Sensing

Direct measurement of extracellular electrical signals from mammalian olfactory sensory neurons in planar triode devices

An artificial nose was developed to mimic aspects of sensory transduction of the peripheral mammalian olfactory system. We directly cultured and differentiated rat olfactory sensory neurons (OSNs) on indium-tin oxide electrodes of planar triode substrates without a coupling agent. Direct voltage (~50 μV) and current (~250 nA) signals were measured simultaneously when OSNs on the planar triode substrates were exposed to odorant mixtures. The response signals were sensitive to the concentration of the odorant mixture, with a typical lifetime, shape, and adaptation profile as seen in responses upon repeated stimulation in vivo. We found that the rising time to the peak current was ~161 ms, while the signal back to baseline was in 1.8 s, which are in agreement with the natural intracellular electrophysiological responses. These results provide the first evidence that mature OSNs grown in a planar triode device are able to detect direct electrophysiological responses to odorants.

Tin oxide nanowire sensor with integrated temperature and gate control for multi-gas recognition

The selectivity of a chemiresistive gas sensor comprising an array of single-crystalline tin oxide nanowires (NWs) is shown to be greatly enhanced by combined temperature and gate voltage modulation. This dual modulation was effected by a novel microsensor platform that consisted of a suspended nitride membrane embedded with independently addressable platinum heater and back-gate structures. The sensor was evaluated in a chemical vapor exposure test consisting of three volatile organic compound (VOC) analytes in an air background; VOC concentrations ranged from 20 μmol/mol to 80 μmol/mol. During the exposure test, the temperature and gating conditions of the NW sensor were modulated in order to induce variations in the sensor's analyte response behavior. By treating these temperature- and gate-dependent analyte response variations as an identifying "fingerprint," analyte identification was achieved using a statistical pattern recognition procedure, linear discriminant analysis (LDA). Through optimization of this pattern recognition procedure, a VOC recognition rate of 98% was obtained. An analysis of the recognition results revealed that this high recognition rate could only be achieved through the combined modulation of temperature and gate bias as compared to either parameter alone. Overall, the highly accurate VOC analyte discrimination that was achieved here confirms the selectivity benefits provided by the utilized dual modulation approach and demonstrates the suitability of miniature nanowire sensors in real-world, multi-chemical detection problems.

Characterization of a clinical olfactory test with an artificial nose

Clinical olfactory tests are used to address hyposmia/anosmia levels in patients with different types of olfactory impairments. Typically, a given test is employed clinically and then replaced by a new one after a certain period of use which can range from days to several months. There is a need to assess control quality of these tests and also for a procedure to quantify their degradation over time. In this paper we propose a protocol to employ low-cost artificial noses for the quantitative characterization of olfactory tests used in clinical studies. In particular, we discuss a preliminary study on the Connecticut Chemosensorial Clinical Research Center Test kit which shows that some odorants, as sensed by an artificial nose, seem to degrade while others are potentiated as the test ages. We also discuss the need to establish a map of correspondence between human and machine olfaction when artificial noses are used to characterize or compare human smell performance in research and clinical studies.

Label-free electrical detection of pyrophosphate generated from DNA polymerase reactions on field-effect devices

We introduce a label-free approach for sensing polymerase reactions on deoxyribonucleic acid (DNA) using a chelator-modified silicon-on-insulator field-effect transistor (SOI-FET) that exhibits selective and reversible electrical response to pyrophosphate anions. The chemical modification of the sensor surface was designed to include rolling-circle amplification (RCA) DNA colonies for locally enhanced pyrophosphate (PPi) signal generation and sensors with immobilized chelators for capture and surface-sensitive detection of diffusible reaction by-products. While detecting arrays of enzymatic base incorporation reactions is typically accomplished using optical fluorescence or chemiluminescence techniques, our results suggest that it is possible to develop scalable and portable PPi-specific sensors and platforms for broad biomedical applications such as DNA sequencing and microbe detection using surface-sensitive electrical readout techniques.

Preoxidation for colorimetric sensor array detection of VOCs

A disposable preoxidation technique that dramatically improves the detection and identification of volatile organic compounds (VOCs) by a colorimetric sensor array is reported. Passing a vapor stream through a tube packed with chromic acid on silica immediately before the colorimetric sensor array substantially increases the sensitivity to less-reactive VOCs and improves the limits of detection (LODs) ~300-fold, permitting the detection, identification, and discrimination of 20 commonly found indoor VOC pollutants at both their immediately dangerous to life or health (IDLH) and permissible exposure limit (PEL) concentrations. The LODs of these pollutants were on average 1.4% of their respective PELs.

Rapid identification of bacteria with a disposable colorimetric sensing array

Rapid identification of both species and even specific strains of human pathogenic bacteria grown on standard agar has been achieved from the volatiles they produce using a disposable colorimetric sensor array in a Petri dish imaged with an inexpensive scanner. All 10 strains of bacteria tested, including Enterococcus faecalis and Staphylococcus aureus and their antibiotic-resistant forms, were identified with 98.8% accuracy within 10 h, a clinically important time frame. Furthermore, the colorimetric sensor arrays also proved useful as a simple research tool for the study of bacterial metabolism and as an easy method for the optimization of bacterial production of fine chemicals or other fermentation processes.

An optoelectronic nose for the detection of toxic gases

We have developed a simple colorimetric sensor array (CSA) for the detection of a wide range of volatile analytes and applied it to the detection of toxic gases. The sensor consists of a disposable array of cross-responsive nanoporous pigments whose colors are changed by diverse chemical interactions with analytes. Although no single chemically responsive pigment is specific for any one analyte, the pattern of color change for the array is a unique molecular fingerprint. Clear differentiation among 19 different toxic industrial chemicals (TICs) within two minutes of exposure at IDLH (immediately dangerous to life or health) concentration has been demonstrated. Quantification of each analyte is easily accomplished based on the color change of the array, and excellent detection limits have been demonstrated, generally below the PELs (permissible exposure limits). Identification of the TICs was readily achieved using a standard chemometric approach, i.e., hierarchical clustering analysis (HCA), with no misclassifications over 140 trials.

Single-nanobelt electronic nose: engineering and tests of the simplest analytical element

Electronic instruments mimicking the mammalian olfactory system are often referred to as "electronic noses" (E-noses). Thanks to recent nanotechnology breakthroughs the fabrication of mesoscopic and even nanoscopic E-noses is now feasible in the size domain where miniaturization of the microanalytical systems encounters principal limitations. Here we describe probably the simplest and yet fully functioning E-nose made of an individual single-crystal metal oxide quasi-1D nanobelt. The nanobelt was indexed with a number of electrodes in a way that each segment of the nanobelt between two electrodes defines an individual sensing elemental "receptor" of the array. The required diversity of the sensing elements is "encoded" in the nanobelt morphology via longitudinal width variations of the nanobelt realized during its growth and via functionalization of some of the segments with Pd catalyst. The proposed approach represents the combined bottom-up/top-down technologically viable route to develop robust and sensitive analytical systems scalable down to submicrometer dimensions.

Enhanced sensitivity volatile detection with low power integrated micromechanical resonators

Very high aspect (length/thickness) ratio doubly clamped beams with integrated piezoelectric transducers are demonstrated for low power sensing of volatiles. The described approach allows for high yield fabrication of a dense array of suspended resonators that can be individually functionalized. Upon polymer coating, the resonators become highly sensitive to absorption of volatile compounds, allowing for detection at ppm-level concentrations of low-molecular weight analytes. The determined sensitivity enhancement is due to the significant contribution of vapor absorption-induced polymer swelling that results in axial stress formation in length restricted high aspect ratio beams.

A colorimetric sensor array for identification of toxic gases below permissible exposure limits

A colorimetric sensor array has been developed for the rapid and sensitive detection of 20 toxic industrial chemicals (TICs) at their PELs (permissible exposure limits). The color changes in an array of chemically responsive nanoporous pigments provide facile identification of the TICs with an error rate below 0.7%.

Discrimination of complex mixtures by a colorimetric sensor array: coffee aromas

The analysis of complex mixtures presents a difficult challenge even for modern analytical techniques, and the ability to discriminate among closely similar such mixtures often remains problematic. Coffee provides a readily available archetype of such highly multicomponent systems. The use of a low-cost, sensitive colorimetric sensor array for the detection and identification of coffee aromas is reported. The color changes of the sensor array were used as a digital representation of the array response and analyzed with standard statistical methods, including principal component analysis (PCA) and hierarchical clustering analysis (HCA). PCA revealed that the sensor array has exceptionally high dimensionality with 18 dimensions required to define 90% of the total variance. In quintuplicate runs of 10 commercial coffees and controls, no confusions or errors in classification by HCA were observed in 55 trials. In addition, the effects of temperature and time in the roasting of green coffee beans were readily observed and distinguishable with a resolution better than 10 degrees C and 5 min, respectively. Colorimetric sensor arrays demonstrate excellent potential for complex systems analysis in real-world applications and provide a novel method for discrimination among closely similar complex mixtures.

Adaptive microsensor systems

We provide a broad review of approaches for developing chemosensor systems whose operating parameters can adapt in response to environmental changes or application needs. Adaptation may take place at the instrumentation level (e.g., tunable sensors) and at the data-analysis level (e.g., adaptive classifiers). We discuss several strategies that provide tunability at the device level: modulation of internal sensing parameters, such as frequencies and operation voltages; variation of external parameters, such as exposure times and catalysts; and development of compact microanalysis systems with multiple tuning options. At the data-analysis level, we consider adaptive filters for change, interference, and drift rejection; pattern classifiers that can adapt to changes in the statistical properties of training data; and active-sensing techniques that can tune sensing parameters in real time. We conclude with a discussion of future opportunities for adaptive sensing in wireless distributed sensor systems.

A colorimetric sensor array of porous pigments

The development of a low-cost, simple colorimetric sensor array capable of the detection and identification of toxic gases is reported. This technology uses a disposable printed array of porous pigments in which metalloporphyrins and chemically-responsive dyes are immobilized in a porous matrix of organically modified siloxanes (ormosils) and printed on a porous membrane. The printing of the ormosil into the membrane is highly uniform and does not lessen the porosity of the membrane, as shown by scanning electron microscopy. When exposed to an analyte, these pigments undergo reactions that result in well-defined color changes due to strong chemical interactions: ligation to metal ions, Lewis or Brønsted acid-base interactions, hydrogen bonding, etc. Striking visual identification of 3 toxic gases has been shown at the IDLH (immediately dangerous to life and health) concentration, at the PEL (permissible exposure level), and at a level well below the PEL. Identification and quantification of analytes were achieved using the color change profiles, which were readily distinguishable in a hierarchical clustering analysis (HCA) dendrogram, with no misclassifications in 50 trials.

Electrochemical biosensors at the nanoscale

The general mechanism of chemical sensing is based on molecular recognition linked to different transduction strategies based on electrochemical, optical, gravimetric or thermal effects that can convert the signal to digital information. Electrochemical sensors support accurate, fast, and inexpensive analytical methods with the advantages of being easily embedded and integrated into electronics, and having the greatest potential impact in the areas of healthcare, environmental monitoring (e.g. electronic noses), food packaging and many other applications (E. Bakker and Y. Qin, Anal. Chem., 2006, 78, 3965). Nanoscale electrochemical biosensors offer a new scope and opportunity in analytical chemistry. The reduction in the size of electrochemical biosensors to nanoscale dimensions expands their analytical capability, allowing the exploration of nanoscopic domains, measurements of local concentration profiles, detection in microfluidic systems and in vivo monitoring of neurochemical events by detection of stimulated dopamine release (R. Kennedy, L. Huang, M. Atkinson and P. Dush, Anal. Chem., 1993, 65, 1882). This article reviews both state of art developments in electrochemical nanosensing, and the industrial outlook.

Detecting chemical hazards with temperature-programmed microsensors: overcoming complex analytical problems with multidimensional databases

Complex analytical problems, such as detecting trace quantities of hazardous chemicals in challenging environments, require solutions that most effectively extract relevant information about a sample's composition. This review presents a chemiresistive microarray-based approach to identifying targets that combines temperature-programmed elements capable of rapidly generating analytically rich data sets with statistical pattern recognition algorithms for extracting multivariate chemical fingerprints. We describe the chemical-microsensor platform and discuss its ability to generate orthogonal data through materials selection and temperature programming. Visual inspection of data sets reveals device selectivity, but statistical analyses are required to perform more complex identification tasks. Finally, we discuss recent advances in both devices and algorithms necessary to deal with practical issues involved in long-term deployment. These issues include identification and correction of signal drift, challenges surrounding real-time unsupervised operation, repeatable device manufacturability, and hierarchical classification schemes designed to deduce the chemical composition of untrained analyte species.

Chemically responsive nanoporous pigments: colorimetric sensor arrays and the identification of aliphatic amines

A general method has been developed for the preparation of microspheres of nanoporous pigments, their formulation into chemically responsive pigment inks, and the printing of these inks as colorimetric sensor arrays. Using an ultrasonic-spray aerosol-gel synthesis from chemically responsive dyes and common silica precursors, 16 different nanoporous pigment microspheres have been prepared and characterized. New colorimetric sensor arrays have been created by printing inks of these chemically responsive pigments as primary sensor elements; these arrays have been successfully tested for the detection, identification, and quantitation of toxic aliphatic amines. Among 11 structurally similar amines, complete identification of each analyte without confusion was achieved using hierarchical cluster analysis (HCA). Furthermore, visual identification of ammonia gas was easily made at the IDLH (immediately dangerous to life or health), PEL (permissible exposure limits), and 0.1 PEL concentrations with high reproducibility.

A colorimetric sensor array for detection and identification of sugars

Molecular recognition of sugars and a practical method to detect and discriminate among a large number of such similar analytes remain substantial scientific challenges. We report here a low-cost, simple colorimetric sensor array capable of identification and quantification of sugars and related compounds. Fifteen different monosaccharides, disaccharides, and artificial sweeteners were differentiated without error in 80 trials. Limits of detection at pH 7.4 for glucose were <1 mM, which is below physiologically important levels.