Bio-inspired gas sensing: boosting performance with sensor optimization guided by "machine learning".

The performance of existing gas sensors often degrades in field conditions because of the loss of measurement accuracy in the presence of interferences. Thus, new sensing approaches are required with improved sensor selectivity. We are developing a new generation of gas sensors, known as multivariable sensors, that have several independent responses for multi-gas detection with a single sensor. In this study, we analyze the capabilities of natural and fabricated photonic three-dimensional (3-D) nanostructures as sensors for the detection of different gaseous species, such as vapors and non-condensable gases. We employed bare Morpho butterfly wing scales to control their gas selectivity with different illumination angles. Next, we chemically functionalized Morpho butterfly wing scales with a fluorinated silane to boost the response of these nanostructures to the vapors of interest and to suppress the response to ambient humidity. Further, we followed our previously developed design rules for sensing nanostructures and fabricated bioinspired inorganic 3-D nanostructures to achieve functionality beyond natural Morpho scales. These fabricated nanostructures have embedded catalytically active gold nanoparticles to operate at high temperatures of ≈300 °C for the detection of gases for solid oxide fuel cell (SOFC) applications. Our performance advances in the detection of multiple gaseous species with specific nanostructure designs were achieved by coupling the spectral responses of these nanostructures with machine learning (a.k.a. multivariate analysis, chemometrics) tools. Our newly acquired knowledge from studies of these natural and fabricated inorganic nanostructures coupled with machine learning data analytics allowed us to advance our design rules for sensing nanostructures toward the required gas selectivity for numerous gas monitoring scenarios at room and high temperatures for industrial, environmental, and other applications.

Use of analyte-modulated modal power distribution in multimode optical fibers for simultaneous single-wavelength evanescent-wave refractometry and spectrometry.

A new method is described for the simultaneous determination of absorbance and refractive index of a sample medium. The method is based on measurement of the analyte-modulated modal power distribution (MPD) in a multimode waveguide. In turn, the MPD is quantified by the far-field spatial pattern and intensity of light, i.e., the Fraunhofer diffraction pattern (registered on a CCD camera), that emerges from a multimode optical fiber. Operationally, light that is sent down the fiber interacts with the surrounding analyte-containing medium by means of the evanescent wave at the fiber boundary. The light flux in the propagating beam and the internal reflection angles within the fiber are both affected by optical absorption connected with the analyte and by the refractive index of the analyte-containing medium. In turn, these angles are reflected in the angular divergence of the beam as it leaves the fiber. As a result, the Fraunhofer diffraction pattern of that beam yields two parameters that can, together, be used to deduce refractive index and absorbance. This MPD based detection offers important advantages over traditional evanescent-wave detection strategies which rely on recording only the total transmitted optical power or its lost fraction. First, simultaneous determination of sample refractive index and absorbance is possible at a single probe wavelength. Second, the sensitivity of refractometric and absorption measurements can be controlled simply, either by adjusting the distance between the end face of the fiber and the CCD detector or by monitoring selected modal groups at the fiber output. As a demonstration of these capabilities, several weakly absorbing solutions were examined, with refractive indices in the range from 1.3330 to 1.4553 and with absorption coefficients in the range 0-16 cm-1. The new detection strategy is likely to be important in applications in which sample coloration varies and when it is necessary to compensate for variations in the refractive index of a sample.

Optical time-of-flight chemical detection: absorption-modulated fluorescence for spatially resolved analyte mapping in a bidirectional distributed fiber-optic sensor.

A continuous chemically sensitive optical fiber is used with optical time-of-flight chemical detection (OTOF-CD) for spatially resolved analyte mapping. To enhance signal levels and to improve their reproducibility, two novel principles for signal generation and processing are introduced. In the first, the fluorescene of an analyte-insensitive fluorophore is monitored as a function of the evanescent wave absorption of an analyte-sensitive indicator. The resulting signal levels are well above those encountered in optical time domain reflectometry methods that rely upon backscattering for spatially resolved detection. As a result, the method could significantly expand the range of species that can be detected with absorption reagents used in OTOF sensors. The second method raises signal-to-noise ratios by 3-4.5-fold for measurements made at the far ends of the sensing fiber. It functions by sending probe laser pulses into and monitoring their return sequentially from both ends of the sensing fiber. Because the two pulses provide complementary information, only the first half of each of the collected waveforms is used for analyte quantitation. The introduced concepts were experimentally verified with a distributed sensor constructed from a 40-m-long continuous chemically sensitive optical fiber. This sensing element was produced by immobilization of an ammonia-sensitive absorbing reagent (phenol red) and an analyte-insensitive fluorophore (rhodamine 640) into the original silicone cladding of the plastic-clad silica fiber.

Adapting selected nucleic acid ligands (aptamers) to biosensors.

A flexible biosensor has been developed that utilizes immobilized nucleic acid aptamers to specifically detect free nonlabeled non-nucleic acid targets such as proteins. In a model system, an anti-thrombin DNA aptamer was fluorescently labeled and covalently attached to a glass support. Thrombin in solution was selectively detected by following changes in the evanescent-wave-induced fluorescence anisotropy of the immobilized aptamer. The new biosensor can detect as little as 0.7 amol of thrombin in a 140-pL interrogated volume, has a dynamic range of 3 orders of magnitude, has an inter-sensing-element measurement precision of better than 4% RSD over the range 0-200 nM, and requires less than 10 min for sample analysis. The aptamer-sensor format is generalizable and should allow sensitive, selective, and fast determination of a wide range of analytes.

Optical time-of-flight chemical detection: spatially resolved analyte mapping with extended-length continuous chemically modified optical fibers.

We theoretically evaluate and experimentally verify a novel strategy for spatially resolved analyte mapping over extended remote areas. The approach combines a method for the fabrication of continuous extended-length sensors with optical time-of-flight chemical detection (OTOF-CD). The use of OTOF-CD makes it possible to locate the zones in the fiber where attenuation or fluorescence takes place, to determine the magnitude of these variations, and to relate the magnitude of the variations to the local concentration or concentrations of a single analyte or several analytes. Simulation experiments suggest that OTOF-CD should provide spatial resolution close to its theoretical limit by deconvolution of the returned wave form with all time-dependent experimental variables (laser pulse width, reagent fluorescence lifetime, etc.). The signal-processing technique should be useful for a wide variety of sensors based on absorption, refractive index, or statically and dynamically quenched fluorescence. Experimental results with a model system (a 48-m-long oxygen sensor) compare favorably with those predicted by numerical simulations. Possible experimental difficulties in the realization of these novel sensors are discussed as are ways to overcome them.

Near-ultraviolet evanescent-wave absorption sensor based on a multimode optical fiber.

Fiber-optic near-ultraviolet evanescent-wave sensors have been constructed, and their feasibility for practical applications has been demonstrated. The sensors, used for the detection of ozone near the 254-nm peak of the Hartley absorption band, were fabricated from coiled segments of low-cost multimode plastic-clad silica optical fibers. The sensing sections were produced alternatively by stripping only the protective jacket from the fiber to expose the gas-permeable silicone cladding or by stripping the jacket and the cladding to expose the bare-silica fiber core. Response characteristics are given, including sensitivity to ozone, reversibility, and aging effects. The useful lifetime was unacceptably short for the sensor that employed the bare-silica core, whereas the exposed-cladding sensor demonstrated good stability over the entire two-month period of investigation. The latter, more useful sensor demonstrated a linear response to ozone over the range 0.02-0.35 vol% and a reversible response with a time constant on the order of 1 min. Differences in ozone absorption spectra obtained in the transmission and evanescent-wave modes are discussed. Projected applications of the new exposed-cladding sensor include ozone determination in water-treatment processes and ozone production plants.

Scintillator light source for chemical sensing in the near-ultraviolet.

A novel chemical sensor based on a light source composed of a radionuclide and a scintillator is experimentally evaluated. Proper selection of a radionuclide/scintillator combination permits fabrication of a practical light source emitting in the ultraviolet (UV). Such a UV light source is critical for chemical sensors which utilize UV-excitable chromophores or fluorophores. Unlike conventional gas-filled discharge lamps, the developed UV source is compact, inexpensive, simple in design, stable, and highly reliable, and it does not require an external power source. The utility of the new source was demonstrated through construction of sensors for oxygen. This application was selected for experimental evaluation of the new light source since oxygen sensors have been characterized well with conventional light sources. Although the scintillator light source is less intense than conventional sources, its excellent short- and long-term stability provides a reproducibility of fluorescence measurements of about 0.35% RSD. The stability of the scintillator light source suggests its utility in simple single-beam detection configurations.

Kramers-Kronig analysis of molecular evanescent-wave absorption spectra obtained by multimode step-index optical fibers.

Spectral distortions that arise in evanescent-wave absorption spectra obtained with multimode step-index optical fibers are analyzed both theoretically and experimentally. Theoretical analysis is performed by the application of Kramers-Kronig relations to the real and the imaginary parts of the complex refractive index of an absorbing external medium. It is demonstrated that even when the extinction coefficient of the external medium is small, anomalous dispersion of that medium in the vicinity of an absorption band must be considered. Deviations from Beer's law, band distortions, and shifts in peak position are quantified theoretically as a function of the refractive index and the extinction coefficient of the external medium; the effect of bandwidth for both Lorentzian and Gaussian bands is also evaluated. Numerical simulations are performed for two types of sensing sections in commonly used plastic-clad silica optical fibers. These sensors include an unclad fiber in contact with a lower-index absorbing liquid and a fiber with the original cladding modified with an absorbing species. The numerical results compare favorably with those found experimentally with these types of sensing sections.