Colorimetric Sensor for Online Accurate Detection of Breath Acetone.

Breath acetone (BrAce) is a validated biomarker of lipid oxidation and has been extensively studied for many applications, such as monitoring ketoacidosis in diabetes, guiding ketogenic diet, and measuring fat burning during exercise. Although many sensors have been reported for BrAce measurement, most of the contributions tested only synthetic or spiked breath samples, because of the complex components of human breath. Here, we show that online accurate detection of BrAce can be achieved using a colorimetric sensor. The high selectivity is enabled by the specific reaction between acetone and hydroxylamine sulfate, and the sensor has a high agreement with a reference instrument in ketosis monitoring. We anticipate that the colorimetric acetone sensor can be applied to various health-related applications.

Moving Electrons Purposefully through Single Molecules and Nanostructures: A Tribute to the Science of Professor Nongjian Tao (1963-2020).

Electrochemistry intersected nanoscience 25 years ago when it became possible to control the flow of electrons through single molecules and nanostructures. Many surprises and a wealth of understanding were generated by these experiments. Professor Nongjian Tao was among the pioneering scientists who created the methods and technologies for advancing this new frontier. Achieving a deeper understanding of charge transport in molecules and low-dimensional materials was the first priority of his experiments, but he also succeeded in discovering applications in chemical sensing and biosensing for these novel nanoscopic systems. In parallel with this work, the investigation of a range of phenomena using novel optical microscopic methods was a passion of his and his students. This article is a review and an appreciation of some of his many contributions with a view to the future.

Gradient-Based Colorimetric Sensors for Continuous Gas Monitoring.

Colorimetry detects a color change resulted from a chemical reaction or molecular binding. Despite its widespread use in sensing, continuous monitoring of analytes with colorimetry is difficult, especially when the color-producing reaction or binding is irreversible. Here, we report on a gradient-based colorimetric sensor (GCS) to overcome this limitation. Lateral transport of analytes across a colorimetric sensor surface creates a color gradient that shifts along the transport direction over time, and GCS tracks the gradient shift and converts it into analyte concentration in real time. Using a low cost complementary metal-oxide semiconductor imager and imaging processing algorithm, we show submicrometer gradient shift tracking precision and continuous monitoring of ppb-level ozone.

Assessing metabolic rate and indoor air quality with passive environmental sensors.

The present work introduces the use of environmental sensors to assess indoor air quality (IAQ) in combination with human biometrics. The sensor array included temperature, relative humidity, carbon dioxide, and noise monitors. The array was used in a classroom as well as in a vehicle cabin to assess the carbon dioxide production rate of individuals in a closed ventilation environment. Analysis of carbon dioxide production allowed for the quantification of the average metabolic rate of the group of individuals in the classroom, and for one individual in the vehicle cabin. These results yielded a mere 5% difference from the values assessed using commercial metabolic rate instruments, and averaged values from epidemiological studies. The results presented in this work verify the feasibility of determining an individual's metabolic rate using passive environmental sensors; these same sensors are able to provide a metric of IAQ that helps characterize the safety of the environment in which the individual is present.

Adsorption Thermodynamic Analysis of a Quartz Tuning Fork Based Sensor for Volatile Organic Compounds Detection.

A volatile organic compounds (VOC) sensor based on molecularly imprinted polymer (MIP) modified quartz tuning fork (QTF) has been developed. In this paper, the stability of the modified sensor as a function of the MIP composition, and the temperature effect of the analyte adsorption on the sensing transduction mechanism are evaluated. By mixing MIP and PS together, the stability was improved. A target analyte, o-xylene, was chosen as the VOC model to study the sensor response in a temperature range of 6-40 °C. Langmuir model fitted adsorption isotherms were used for thermodynamic analysis. The changes in the sensitivity of the QTF sensor to temperature rendered different behaviors. For a freshly modified QTF sensor, the adsorption response increased with increasing temperature, while for an aged QTF sensor, the adsorption response decreased with increasing temperature. The results indicated that the enthalpy change of the MIP and PS composition sensing material changes from positive to negative over the course of aging. The characterization of the reaction enabled the definition of sensor calibration conditions and stable sensor performance in field testing conditions.

A Novel Wireless Wearable Volatile Organic Compound (VOC) Monitoring Device with Disposable Sensors.

A novel portable wireless volatile organic compound (VOC) monitoring device with disposable sensors is presented. The device is miniaturized, light, easy-to-use, and cost-effective. Different field tests have been carried out to identify the operational, analytical, and functional performance of the device and its sensors. The device was compared to a commercial photo-ionization detector, gas chromatography-mass spectrometry, and carbon monoxide detector. In addition, environmental operational conditions, such as barometric change, temperature change and wind conditions were also tested to evaluate the device performance. The multiple comparisons and tests indicate that the proposed VOC device is adequate to characterize personal exposure in many real-world scenarios and is applicable for personal daily use.

Acetone as biomarker for ketosis buildup capability--a study in healthy individuals under combined high fat and starvation diets.

BACKGROUND: Ketogenic diets are high fat and low carbohydrate or very low carbohydrate diets, which render high production of ketones upon consumption known as nutritional ketosis (NK). Ketosis is also produced during fasting periods, which is known as fasting ketosis (FK). Recently, the combinations of NK and FK, as well as NK alone, have been used as resources for weight loss management and treatment of epilepsy. METHODS: A crossover study design was applied to 11 healthy individuals, who maintained moderately sedentary lifestyle, and consumed three types of diet randomly assigned over a three-week period. All participants completed the diets in a randomized and counterbalanced fashion. Each weekly diet protocol included three phases: Phase 1 - A mixed diet with ratio of fat: (carbohydrate + protein) by mass of 0.18 or the equivalence of 29% energy from fat from Day 1 to Day 5. Phase 2- A mixed or a high-fat diet with ratio of fat: (carbohydrate + protein) by mass of approximately 0.18, 1.63, or 3.80 on Day 6 or the equivalence of 29%, 79%, or 90% energy from fat, respectively. Phase 3 - A fasting diet with no calorie intake on Day 7. Caloric intake from diets on Day 1 to Day 6 was equal to each individual's energy expenditure. On Day 7, ketone buildup from FK was measured. RESULTS: A statistically significant effect of Phase 2 (Day 6) diet was found on FK of Day 7, as indicated by repeated analysis of variance (ANOVA), F(2,20) = 6.73, p < 0.0058. Using a Fisher LDS pair-wise comparison, higher significant levels of acetone buildup were found for diets with 79% fat content and 90% fat content vs. 29% fat content (with p = 0.00159**, and 0.04435**, respectively), with no significant difference between diets with 79% fat content and 90% fat content. In addition, independent of the diet, a significantly higher ketone buildup capability of subjects with higher resting energy expenditure (R(2) = 0.92), and lower body mass index (R(2) = 0.71) was observed during FK.

A Novel Real-time Carbon Dioxide Analyzer for Health and Environmental Applications.

To be able to detect carbon dioxide (CO2) with high accuracy and fast response time is critical for many health and environmental applications. We report on a pocket-sized CO2 sensor for real-time analysis of end-tidal CO2, and environmental CO2. The sensor shows fast and reversible response to CO2 over a wide concentration range, covering the needs of both environmental and health applications. It is also immune to the presence of various interfering gases in ambient or expired air. Furthermore, the sensor has been used for real-time breath analysis, and the results are in good agreement with those from a commercial CO2 detector.

A pocket-sized metabolic analyzer for assessment of resting energy expenditure.

BACKGROUND & AIMS: The assessment of metabolic parameters related to energy expenditure has a proven value for weight management; however these measurements remain too difficult and costly for monitoring individuals at home. The objective of this study is to evaluate the accuracy of a new pocket-sized metabolic analyzer device for assessing energy expenditure at rest (REE) and during sedentary activities (EE). The new device performs indirect calorimetry by measuring an individual's oxygen consumption (VO2) and carbon dioxide production (VCO2) rates, which allows the determination of resting- and sedentary activity-related energy expenditure. METHODS: VO2 and VCO2 values of 17 volunteer adult subjects were measured during resting and sedentary activities in order to compare the metabolic analyzer with the Douglas bag method. The Douglas bag method is considered the Gold Standard method for indirect calorimetry. Metabolic parameters of VO2, VCO2, and energy expenditure were compared using linear regression analysis, paired t-tests, and Bland-Altman plots. RESULTS: Linear regression analysis of measured VO2 and VCO2 values, as well as calculated energy expenditure assessed with the new analyzer and Douglas bag method, had the following linear regression parameters (linear regression slope LRS0, and R-squared coefficient, r(2)) with p = 0: LRS0 (SD) = 1.00 (0.01), r(2) = 0.9933 for VO2; LRS0 (SD) = 1.00 (0.01), r(2) = 0.9929 for VCO2; and LRS0 (SD) = 1.00 (0.01), r(2) = 0.9942 for energy expenditure. In addition, results from paired t-tests did not show statistical significant difference between the methods with a significance level of α = 0.05 for VO2, VCO2, REE, and EE. Furthermore, the Bland-Altman plot for REE showed good agreement between methods with 100% of the results within ±2SD, which was equivalent to ≤10% error. CONCLUSION: The findings demonstrate that the new pocket-sized metabolic analyzer device is accurate for determining VO2, VCO2, and energy expenditure.

Online sample conditioning for portable breath analyzers.

Various innovative chemical sensors have been developed in recent years to sense dangerous substances in air and trace biomarkers in breath. However, in order to solve real world problems, the sensors must be equipped with efficient sample conditioning that can, e.g., control the humidity, which is discussed much less in the literature. To meet the demand, a miniaturized mouthpiece was developed for personal breath analyzers. A key function of the mouthpiece is to condition the humidity in real breath samples without changing the analyte concentrations and introducing substantial backpressure, which is achieved with optimized packing of desiccant particles. Numerical simulations were carried out to determine the performance of the mouthpiece in terms of various controllable parameters, such as the size, density, and geometry of the packing. Mouthpieces with different configurations were built and tested, and the experimental data validated the simulation findings. A mouthpiece with optimized performance reducing relative humidity from 95% (27,000 ppmV) to 29% (8000 ppmV) whereas retaining 92% nitric oxide (50 ppbV to 46 ppbV) was built and integrated into a hand-held exhaled nitric oxide sensor, and the performance of exhaled nitric oxide measurement was in good agreement with the gold standard chemiluminescence technique. Acetone, carbon dioxide, oxygen, and ammonia samples were also measured after passing through the desiccant mouthpiece using commercial sensors to examine wide applicability of this breath conditioning approach.

Novel monitor paradigm for real-time exposure assessment.

A wearable monitor that can reliably, accurately, and continuously measure personal exposure levels of various toxicants would not only accelerate the current environmental and occupational health and safety studies, but also enable new studies that are not possible with the current monitoring technology. Developing such a monitor has been a difficult challenge, and requires innovative sensing science and creative engineering. We have developed, built, and tested a wearable monitor for real-time detection of toxic hydrocarbons and acids in the environment. The monitor is low-cost, accurate, and user friendly. In addition, it can communicate wirelessly with a cell phone in which the monitoring results can be processed, displayed, stored, and transmitted to a designated computer. We have validated the functions and performance of the monitor, and carried out field tests with workers involving waste management, fire overhaul, and floor-cleaning activities, as well as with first- and second-hand smokers. The averaged exposure levels are in agreement with those determined by the standard NIOSH methods. The monitor provides accurate and real-time exposure assessment for the workers involving different activities. The real-time and continuous monitoring capability makes it possible to correlate the exposure levels with different activities and changes in the microenvironments. The monitor provides unprecedented real-time information that will help advance occupational safety and environmental health studies. It may also be used to better protect workers from occupational overexposure to toxic molecules.

Ultrasensitive detection of nitrogen oxides over a nanoporous membrane.

A nitrogen oxide (NO(x); x = 1, 2) optical sensor with an extremely low detection limit in the range of fractions of ppbV (0.3 ppbV for 20 s sample injection) is presented. Phenylenediamine derivatives are utilized as molecular probes in the solid state on a nanoporous membrane to produce a miniaturized and low cost sensing platform for use as a wearable personal monitor.

A hybrid nanosensor for TNT vapor detection.

Real-time detection of trace chemicals, such as explosives, in a complex environment containing various interferents has been a difficult challenge. We describe here a hybrid nanosensor based on the electrochemical reduction of TNT and the interaction of the reduction products with conducting polymer nanojunctions in an ionic liquid. The sensor simultaneously measures the electrochemical current from the reduction of TNT and the conductance change of the polymer nanojunction caused from the reduction product. The hybrid detection mechanism, together with the unique selective preconcentration capability of the ionic liquid, provides a selective, fast, and sensitive detection of TNT. The sensor, in its current form, is capable of detecting parts-per-trillion level TNT in the presence of various interferents within a few minutes.

Hybrid separation and detection device for analysis of benzene, toluene, ethylbenzene, and xylenes in complex samples.

We present a hybrid system for rapid detection and analysis of benzene, toluene, ethylbenzene, and xylenes (BTEX). The system combines selective and sensitive sensing elements with a fast and miniaturized chromatographic separation method. The sensing elements are an array of microfabricated quartz crystal tuning forks modified with selective molecularly imprinted polymers, and the separation method uses optimized short columns. The high sensitivity and selectivity of the sensing elements together with the help of the separation provides fast detection and analysis of BTEX in real samples containing highly concentrated interfering agents without preconcentration or heating of columns. The low cost, low power consumption, and small size of the hybrid device are particularly suitable for occupational health, industrial safety, and epidemiological applications.

A hybrid electrochemical-colorimetric sensing platform for detection of explosives.

A highly selective, sensitive, and low-cost hybrid sensing platform is developed based on extraordinary properties of explosives in an ionic liquid and an integrated electrochemical and colorimetric approach.

Real-time ozone detection based on a microfabricated quartz crystal tuning fork sensor.

A chemical sensor for ozone based on an array of microfabricated tuning forks is described. The tuning forks are highly sensitive and stable, with low power consumption and cost. The selective detection is based on the specific reaction of the polymer with ozone. With a mass detection limit of ∼2 pg/mm(2) and response time of 1 second, the sensor coated with a polymer sensing material can detect ppb-level ozone in air. The sensor is integrated into a miniaturized wearable device containing a detection circuit, filtration, battery and wireless communication chip, which is ideal for personal and microenvironmental chemical exposure monitoring.

Detection of lectin-glycan interaction using high resolution surface plasmon resonance.

We report the real-time and label-free detection of direct disaccharide binding to a lectin using a differential surface plasmon resonance detection method that allows for measurement of nanomolar concentrations of disaccharides.

Frequency-coded chemical sensors.

We have developed a new method to intelligently sample analytes and introduce the analytes to sensors. The method automatically adjusts sampling duration according to the sensors' response to the analytes and converts the amplitude of the sensor output to a frequency output, giving us another opportunity to reduce noise in the signal. It also addresses some of the common sensor issues such as response time, saturation, chemical dynamic range, and sensor protection, saving precious detection time, protecting sensors, and enabling sensitive sensors built for low-concentration detection to be used for high-concentration detection as well. We have put together a system using a tuning fork chemical sensor as a sample sensor to demonstrate the feasibility and benefits of the new sensing technique.

Charge transport in mesoscopic conducting polymer wires.

In an attempt to reconcile transport in aniline oligomers with that observed in bulk polyaniline, we constructed meso-scale (60 nm) molecular junctions bridged by polyanion-stabilized polyaniline (PANI) strands. Junctions were characterized by their conductance versus electrochemical potential (G-E) and current versus voltage (I-V) characteristics, In contrast to bulk polyaniline, sharp peaks were seen in the G-E data, and these gave rise to negative differential resistance in the I-V curves, behavior much like that observed in aniline oligomers. The width of the conductance peaks increased with the amount of polymer deposited in the junction. In contrast to oligomers, the peaks in the meso-scale devices displayed a large hysteresis. The absolute conductance of the junctions is far too high to be consistent with transport along isolated chains, suggesting that a fundamental charge carrying unit is something morphologically more complex than a single polymer molecule.

Electrical detection of hepatitis C virus RNA on single wall carbon nanotube-field effect transistors.

We report the unambiguous detection of a sequence of Hepatitis C Virus (HCV) at concentrations down to the fractional pM range using Single Wall Carbon Nanotube (SWNT) Field Effect Transistor (FET) devices functionalized with Peptide Nucleic Acid (PNA).