Gravimetric Sensors Research Articles

GEOSTATISTICS FOR AIR QUALITY MAPPING: CASE OF BAGUIO CITY, PHILIPPINES

Abstract. Mapping of air quality are often based on ground measurements using gravimetric and air portable sensors, remote sensing methods and atmospheric dispersion models. In this study, Geographic Information Systems (GIS) and geostatistical techniques are employed to evaluate coarse particulate matter (PM10) concentrations observed in the Central Business District of Baguio City, Philippines. Baguio City has been reported as one of the most polluted cities in the country and several studies have already been conducted in monitoring its air quality. The datasets utilized in this study are based on hourly simulations from a Gaussian-based atmospheric dispersion model that considers the impacts of vehicular emissions. Dispersion modeling results, i.e., PM10 concentrations at 20-meter interval, show that high values range from 135 to 422 μg/mm3. The pollutant concentrations are evident within 40 meters from the roads. Spatial variations and PM10 estimates at unsampled locations are determined using Ordinary Kriging. Geostatistical modeling estimates are evaluated based on recommended values for mean error (ME), root mean square error (RMSE) and standardized errors. Optimal predictors for pollutant concentrations at 5-meter interval include 2 to 5 search neighbors and variable smoothing factor for night-time datasets while 2 to 10 search neighbors and smoothing factors 0.3 to 0.5 were used for daytime datasets. Results from several interpolation tests indicate small ME (0.0003 to 0.0008 μg/m3) and average standardized errors (4.24 to 8.67 μg/m3). RMSE ranged from 2.95 to 5.43 μg/m3, which are approximately 2 to 3% of the maximum pollutant concentrations in the area. The methodology presented in this paper may be integrated with atmospheric dispersion models in refining estimates of pollutant concentrations, in generating surface representations, and in understanding the spatial variations of the outputs from the model simulations.

Manufacturing Simple and Inexpensive Soil Surface Temperature and Gravimetric Water Content Sensors

Manufacturing Simple and Inexpensive Soil Surface Temperature and Gravimetric Water Content Sensors

Manufacturing Simple and Inexpensive Soil Surface Temperature and Gravimetric Water Content Sensors.

Quantifying temperature and moisture at the soil surface is essential for understanding how soil surface biota respond to changes in the environment. However, at the soil surface these variables are highly dynamic and standard sensors do not explicitly measure temperature or moisture in the upper few millimeters of the soil profile. This paper describes methods for manufacturing simple, inexpensive sensors that simultaneously measure the temperature and moisture of the upper 5 mm of the soil surface. In addition to sensor construction, steps for quality control, as well as for calibration for various substrates, are explained. The sensors incorporate a Type E thermocouple to measure temperature and assess soil moisture by measuring the resistance between two gold-plated metal probes at the end of the sensor at a depth of 5 mm. The methods presented here can be altered to customize probes for different depths or substrates. These sensors have been effective in a variety of environments and have endured months of heavy rains in tropical forests as well as intense solar radiation in deserts of the southwestern U.S. Results demonstrate the effectiveness of these sensors for evaluating warming, drying, and freezing of the soil surface in a global change experiment.

A Microfluidic MEMS-Microbalance Platform With Minimized Acoustic Radiation in Liquid.

In this article, the microfluidic channels that deliver liquid to a microscale thin-film piezoelectric-on-silicon (TPoS) gravimetric resonant sensor are incorporated into the backside of the silicon-on-insulator (SOI) wafer on which the resonator is fabricated. Specifically, a microwell is embedded at the bottom of the disk -shaped TPoS resonator, while a very thin layer of parylene covering the backside of the resonator and the microwell forms an isolation layer between the liquid and the top device-layer features. In this way, the liquid is in contact with the backside of the resonator, while the device-defining trenches and the electrical connections to the resonator stay clear, thus mitigating the acoustic energy loss and undesirable feedthroughs. The impact of the parylene layer thickness on a few symmetric ( S ) and antisymmetric ( A ) Lamb wave modes of the resonator is experimentally studied, and the performance of such modes in the liquid is characterized by filling the microwells through a PDMS-based microfluidic channel. The parylene layer, while marginally affecting the resonator in the air, is found to substantially enhance its performance in the liquid media. Strong resonance peaks with high quality factors ( Q ) are observed for the S modes, among which Q values above 400 are recorded for a specific mode named S (4, 2) (among the highest ever reported). This article can potentially facilitate the realization of highly stable and sensitive resonant mass sensors (i.e., microbalance) for real-time applications. Additionally, the effect of the acoustic energy radiation in the form of evanescent shear and longitudinal waves in liquid on the Q and resonance frequency of the disk resonators is experimentally validated.

Fabrication and characterisation of piezoelectric screen-printed in plane resonant microcantilevers used as gravimetric sensors

Screen-printed microcantilever transducers of different geometries (1 × 1 × 0.1 mm3 to 8 × 2 × 0.1 mm3) are fabricated and characterised. The microcantilevers are made of PbZr0.52Ti0.48O3 (PZT) which allows them to be self-actuated and self-readout. A polyester sacrificial layer is used in the releasing of the beam and decomposes entirely during the firing step of the PZT at 900 °C without further intervention. This process gives resonators with quality factors as high as 2000 and a densification equal to 93.8% of the bulk PZT. Experimental measurements testing the transducers as gravimetric sensors give mass sensitivities of 85 Hz μg−1 with a corresponding limit of detection of 70 ng at a resonant frequency of 680 kHz.

Highly sensitive CMUT-based humidity sensors built with nitride-to-oxide wafer bonding technology

In this paper, we present highly sensitive capacitive micromachined ultrasonic transducer (CMUT) based gravimetric humidity sensors that are fabricated with nitride-to-oxide wafer bonding technology. In contrast to conventional wafer bonding CMUT processes that use expensive silicon-on-insulator (SOI) wafers to produce resonating membranes, the proposed process employs low-pressure chemical vapor deposition (LPCVD) silicon nitride as the membrane material. The proposed process provides thinner and lighter membranes, and thus more sensitive CMUT resonators. Furthermore, additional benefits such as reduced fabrication complexity and more controllable membrane thickness are possible. Ten MHz CMUTs were designed, fabricated and functionalized by graphene oxide (GO) with three different concentrations. The device uniformities and resonance frequencies were characterized by a laser vibrometer and a vector network analyzer. Humidity sensing performance of the sensors was analyzed through the resonance shifts in a relative humidity (RH) range of 11.3%–93.6%. The CMUT coated with the highest concentration of GO (0.5 mg/ml) exhibited the highest sensitivity of up to 15.3 kHz/%RH. Decent repeatability and a short response/recovery time were achieved by all the CMUT sensors. This study demonstrates that the CMUT humidity sensors that are realized with nitride-to-oxide bonding technology can be an excellent candidate for humidity sensing due to their high sensitivity and CMOS-compatible fabrication process.

Impact of FBAR design on its sensitivity as in-liquid gravimetric sensor

In contrast to quartz crystal microbalances, the sensitivity of thin film bulk acoustic wave resonators (FBARs) is strongly dependent on all films comprising the multilayered structure. Previous studies proved that placing low acoustic impedance materials at the sensing surface of longitudinal-mode FBARs operating in air can enhance their mass sensitivity by modifying the energy trapped at the sensing surface. Here we investigate if the in-liquid sensitivity to density-viscosity and mass of shear-mode AlN-based solidly mounted resonators displays a similar dependence on the device configuration. By using the finite element simulation method accompanied by experimental verifications we demonstrate that for a given value of the resonant frequency, the sensitivity can be boosted by a proper design of the devices. The results can be of application to in-liquid physical sensors or to gravimetric chemical or biological sensors.

Development of a highly sensitive humidity sensor based on the capacitive micromachined ultrasonic transducer

In this paper, we present a highly sensitive capacitive micromachined ultrasonic transducer (CMUT) based resonant gravimetric humidity sensor using graphene oxide as the sensing material. The CMUT transducer with a diameter of 2 mm and an operating frequency of 12 MHz was fabricated by the direct wafer bonding technique. To validate the design, both the laser vibrometer and the vector network analyzer were adopted to characterize the resonant properties of the fabricated device. A graphene oxide thin film was then coated as the water vapor absorbing layer on the resonating membranes of the CMUT. The humidity sensing performance of the CMUT humidity sensor was evaluated through the electrical impedance analysis over a relative humidity (RH) range of 22.5% to 93.6%. Frequency shifts caused by the added mass of the absorbed water molecules was observed in the measurements. The CMUT humidity sensor exhibited a sensitivity of up to 11.5 kHz/%RH in the tested humidity range and showed a good repeatability and a rapid response/recovery. Benefit from the microelectromechanical (MEMS) technology and the water-sensitive graphene oxide, the developed CMUT humidity sensor is expected to be a strong candidate for miniaturized, highly sensitive, and reliable humidity sensors.

Full-SAW Microfluidics-Based Lab-on-a-Chip for Biosensing

Many approaches to diagnostic testing remain decades old. Well-established biosensing technologies (e.g., enzyme-linked immunosorbent assays and radio-immunoassays) typically cannot fulfill the requirements of portability and ease of use necessary for point-of-care purposes. Several alternatives have been proposed (e.g., quartz-crystal-microbalances, electrochemical sensors, cantilevers, and surface-plasmon-resonance sensors) but often lack high performance or still necessitate bulk ancillary instruments to operate. Here we present a highly sensitive, versatile, and easily integrable microfluidic lab-on-a-chip (LoC) for biosensing, fully based on surface acoustic waves (SAWs). By using ultra-high-frequency resonator-biosensors, we show that it is possible to perform highly sensitive assays in complex media. This all-electrical readout platform is benchmarked with the biotin-streptavidin binding in presence of non-specific binding proteins (serum albumin) at physiological concentration. The benchmark experiments were performed with the idea of mimicking a biological fluid, in which other molecular species at high concentration are present together with the analytes. We demonstrate that this LoC can detect sub-nanomolar concentrations of analytes in complex media. As a comparison with similar acoustic-wave-based systems, this full-SAW platform outperforms the standard commercial gravimetric sensors (i.e., quartz-crystal-microbalances) and the more common Love-SAW biosensors. This full-SAW LoC could be further developed for the detection of biomarkers in biological fluids.

Light-Activated Hybrid Nanocomposite Film for Water and Oxygen Sensing.

Oxygen and water vapor sensing properties are investigated in metal-oxide-hybrid polymer nanocomposite thin films generated by infiltration synthesis, which incorporates molecular ZnO into the matrix of SU-8 polymer, a common negative-tone photoresist. The hybrid thin films display 20-fold higher gravimetric responses to oxygen and water vapor than those of control ZnO thin films in the dark. An additional 50-500% enhanced responses are detected under UV irradiation. The overall enhanced gravimetric response in the hybrid film is attributed to the ZnO molecules distributed in the polymer matrix, whereas the UV enhancement is explained by the light-induced, reversible generation of hydrophilic fluoroantimonic acid from triarylsulfonium hexafluoroantimonate photoacids, which leads to the increased surface potential and adsorption energies for oxygen and water. A gravimetric sensor based on a series of ZnO-infiltrated SU-8 films under UV excitation enables 96% accurate classification of water and oxygen environment with sub 10 mTorr detection limits. The results demonstrate UV-induced fully reversible surface hydrophilicity of ZnO/SU-8 hybrid nanocomposites.

Combined colorimetric and gravimetric CMUT sensor for detection of benzyl methyl ketone

The detection of benzyl methyl ketone (BMK) is of interest as it is a common precursor for the synthesis of (meth)amphetamine. Resonant gravimetric sensors can be used to detect the mass and hereby the concentration of a gas while colorimetric arrays typically have exceptional selectivities to a target analyte. We present a sensor system consisting of a capacitive micromachined ultrasonic transducer (CMUT) and a colorimetric array for detection of BMK. The CMUT is used as a resonant gravimetric gas sensor where the resonance frequency shift due to mass loading of the plate. A single local oxidation of silicon (LOCOS) step was used to define the cavities which were sealed with a Si3N4 plate with a thickness of 100 nm, resulting in a resonance frequency of 38.8 MHz and a mass sensitivity of 28.3zgHz×μm2 or 8.5 × 103 cm2/g This sensitivity is, to our knowledge, the best reported as compared with the state of the art of CMUTs. The CMUTs were functionalized with the same dyes used to fabricate the colorimetric arrays. While both the CMUT and the colorimetric array showed selectivity between BMK and acetone and water, the highest selectivity was achieved with the colorimetric array. Here the analyte BMK resulted in the only signal (color change), while acetone and water did not give any signal. Furthermore, the mass of the BMK was found as a function of time. Thus, the combination of a colorimetric array and a CMUT resulted in a sensor system with a selectivity between BMK, acetone and water and in addition a quantitative value for the mass.

Filament Assisted Chemical Vapor Deposited organosilicate as chemical layer for nanometric hydrocarbon gas sensors

The fabrication of gravimetric gas sensors based on Nano Electro Mechanical System (NEMS) requires the use of a chemical sensitive layer that is uniformly deposited by a solvent-free deposition technique. In this work, we show than an organosilicate layer deposited by Filament-Assisted Chemical Vapor Deposition using methyltriethoxysilane presents very high affinity toward hydrocarbon gas. High partition coefficients toward toluene and pentane were obtained on Quartz Crystal Microbalance sensors after being annealed at 400 °C. It is shown that the hydrophobic nature of this material composed of a SiOSi backbone with methyl groups bonded to silicon, combined with the presence of isolated ethoxy groups and microporosity, allow a very high sensibility in comparison to others SiOCH. Furthermore, the functionalization of NEMS-based gas sensors was demonstrated. This material, deposited using a CMOS compatible technique, constitutes a promising solution for the development of a complete and portable multi-gas analysis system.

Ni-MOF-74 as sensing material for resonant-gravimetric detection of ppb-level CO

For the first time, ultrasensitive detection of 10 ppb CO has been realized using metal organic framework (MOF) crystals of Ni-MOF-74 as gravimetric-type sensing material with resonant microcantilever sensors. Due to the specific interaction between CO molecules and the active transitional metal centers of Ni2+ in the Ni-MOF-74 crystal, the Ni-MOF-74 material with ultra-high surface area and nanoporous structure possesses large absorption capacity to CO gas. By loading the Ni-MOF-74 material onto the resonant microcantilever, the formed gravimetric sensor exhibits high-performance sensing to CO at trace level. The experimentally obtained limit of detection (LOD) of the sensor to CO reaches finer than 10 ppb (parts per billion in volume), which indicates an extremely high CO-detection sensitivity of the sensor. Besides the excellent sensitivity, the sensor also shows remarkable repeatability and long-term stability. Eight kinds of interfering gases are tested and a fine selectivity of the sensor is verified. This work indicates that MOF materials such as Ni-MOF-74 are promising in gravimetric-type gas sensing applications.

Toward optimized SiOCH films for BTEX detection: Impact of chemical composition on toluene adsorption

Abstract Volatile Organic Compounds are a cause of concern for human health. It is particularly the case for BTEX compounds (Benzene, Toluene, Ethylbenzene and Xylenes). Gravimetric sensors are good candidates for their detection but they have to be functionalized by a sensitive film to become active. In this work, organosilicate thin films (SiOCH) deposited by plasma enhanced chemical vapor deposition are investigated. This work aims at understanding the role of their chemical composition on gas adsorption and to propose an optimized material for BTEX detection. Through the synthesis and characterization of various SiOCH thin films, the role of hydrophobicity, carbon content and specific chemical bonding is highlighted. This led to an optimized film presenting both high affinity (partition coefficient toward toluene higher than 15000) and rapid temporal response.

Highly Selective Polypyrrole MIP-Based Gravimetric and Electrochemical Sensors for Picomolar Detection of Glyphosate.

There is a global debate and concern about the use of glyphosate (Gly) as an herbicide. New toxicological studies will determine its use in the future under new strict conditions or its replacement by alternative synthetic or natural herbicides. In this context, we designed biomimetic polymer sensing layers for the selective molecular recognition of Gly. Towards this end, complementary surface acoustic wave (SAW) and electrochemical sensors were functionalized with polypyrrole (PPy)-imprinted polymer for the selective detection of Gly. Their corresponding limits of detection were on the order of 1 pM, which are among the lowest values ever reported in literature. The relevant dissociation constants between PPy and Gly were estimated at [Kd1 = (0.7 ± 0.3) pM and Kd2 = (1.6 ± 1.4) µM] and [Kd1 = (2.4 ± 0.9) pM and Kd2 = (0.3 ± 0.1) µM] for electrochemical and gravimetric measurements, respectively. Quantum chemical calculations permitted to estimate the interaction energy between Gly and PPy film: ΔE = −145 kJ/mol. Selectivity and competitivity tests were investigated with the most common pesticides. This work conclusively shows that gravimetric and electrochemical results indicate that both MIP-based sensors are perfectly able to detect and distinguish glyphosate without any ambiguity.

Revealing humidity-enhanced NH3 sensing effect by using resonant microcantilever

High performance NH3 sensor is of great importance in various application fields such as environmental protection and early diagnosis of stomach cancer. Herein, carboxyl group functionalized mesoporous silica nanoparticles (C-MSNs) with ultrahigh specific surface area are developed as high performance mass-type sensing material for trace-level NH3 detection. By using inkjet printing technology, C-MSNs material is precisely loaded on our lab-made integrated resonant microcantilever to construct a mass-type gas sensor. Based on the specific acid-base interaction between Brønsted acidic C-MSNs material and basic NH3 molecules, our sensor exhibits satisfactory NH3 sensing performance. The experimentally observed detection limit of our sensor reaches parts per billion (ppb) level. Exposed to NH3 with identical concentration, our sensor outputs a higher response in wet air than that of in dry atmosphere. In real air, hydrate NH3 molecule (i.e. NH3·xH2O) has a larger molecular weight than anhydrous NH3 in absolute dry experimental atmosphere. Therefore, our gravimetric sensor exhibits a higher response in real air. In order to in-depth elucidate the humidity-enhanced sensing effect, the micro-gravimetric sensing data obtained in wet air and in dry experimental environment are compared together. Based on our quantitatively measurement data, the molecular formula of hydrate NH3·xH2O existing in wet air (with 50–80% relative humidity) can be obtained as NH3·2∼2.4H2O.

Biofouling Removal and Protein Detection Using a Hypersonic Resonator.

Nonspecific binding (NSB) is a general issue for surface based biosensors. Various approaches have been developed to prevent or remove the NSBs. However, these approaches either increased the background signals of the sensors or limited to specific transducers interface. In this work, we developed a hydrodynamic approach to selectively remove the NSBs using a microfabricated hypersonic resonator with 2.5 gigahertz (GHz) resonant frequency. The high frequency device facilitates generation of multiple controlled microvortexes which then create cleaning forces at the solid-liquid interfaces. The competitive adhesive and cleaning forces have been investigated using the finite element method (FEM) simulation, identifying the feasibility of the vortex-induced NSB removal. NSB proteins have been selectively removed experimentally both on the surface of the resonator and on other substrates which contact the vortexes. Thus, the developed hydrodynamic approach is believed to be a simple and versatile tool for NSB removal and compatible to many sensor systems. The unique feature of the hypersonic resonator is that it can be used as a gravimetric sensor as well; thus a combined NSB removal and protein detection dual functional biosensor system is developed.

A Rayleigh surface acoustic wave (R-SAW) resonator biosensor based on positive and negative reflectors with sub-nanomolar limit of detection

A label-free sub-nanomolar Rayleigh surface acoustic wave (R-SAW)-based resonator biosensor is demonstrated for biomolecular detection in liquid after drying. The biosensor comprises two interdigital transducers for R-SAW generation and two positive and negative reflectors to confine the acoustic energy in the sensitive area. We benchmark this biosensor against biotin-streptavidin binding, which is a standard, well-known model for a variety of biosensing processes. The experiments demonstrate a limit of detection of 104pM and a normalized sensitivity of −296m2kg−1. As a comparison with similar acoustic-wave based systems, both sensitivity and limit of detection are better than that of standard commercial gravimetric sensors (i.e., quartz-crystal-microbalances) and generally better than that of more common Love-SAW biosensors. Our biosensor has a dynamic range potentially compatible with several health- and safety-related assays, among all cancer biomarker detection.

Gravimetric sensors operating at 1.1\u2009GHz based on inclined c-axis ZnO grown on textured Al electrodes

Shear mode solidly mounted resonators (SMRs) are fabricated using an inclined c-axis ZnO grown on a rough Al electrode. The roughness of the Al surface is controlled by changing the substrate temperature during the deposition process to promote the growth of inclined ZnO microcrystals. The optimum substrate temperature to obtain homogeneously inclined c-axis grains in ZnO films is achieved by depositing Al at 100 °C with a surface roughness ~9.2 nm, which caused an inclination angle of ~25° of the ZnO c-axis with respect to the surface normal. Shear mode devices with quality-factors at resonance, Qr and effective electromechanical coupling factors, {{boldsymbol{k}}}_{{bf{eff}}}^{{bf{2}}}, as high as 180 and 3.4% are respectively measured. Mass sensitivities, Sm of (4.9 ± 0.1) kHz · cm2/ng and temperature coefficient of frequency (TCF) of ~−67 ppm/K are obtained using this shear mode. The performance of the devices as viscosity sensors and biosensors is demonstrated by determining the frequency shifts of water-ethanol mixtures and detection of Rabbit immunoglobin G (IgG) whole molecule (H&L) respectively.

Quantitative evaluation of the responses of a gravimetric gas sensor based on mesoporous functionalized silica: Application to 2,4-DNT and TNT detection

The literature reports several attempts to predict the responses of gas sensors using as only input the molecular structures of the analyte and sensing material. However, these are restricted to relatively simple and homogeneous materials. The present paper reports for the first time the semiquantitative estimation of the sensing responses of quartz crystal microbalance sensors based on a complex material, taking relative humidity into account. The sensing material is methylated mesoporous silica, and the analytes are vapors of the nitroaromatic compounds 2,4-dinitrotoluene (2,4-DNT) and trinitrotoluene (TNT). Our approach is based on molecular dynamics simulations. The amount of vapor (water, 2,4-DNT or TNT) adsorbed on the sensing surfaces is obtained as a function of their partial pressure by simulating adsorption isotherms at high temperature and extrapolating the results to ambient conditions. Furthermore, a correction factor is introduced to account for the role of residual water within the mesoporous silica. The order of magnitude of the simulated sensing responses to 2,4-DNT and TNT is found to be consistent with experimental data.