Ligand exchange based paraoxon imprınted QCM sensor

In the present work, a paraoxon imprinted QCM sensor has been developed for the determination of paraoxon based on the modification of paraoxon imprinted film onto a quartz crystal combining the advantages of high selectivity of the piezoelectric microgravimetry using MIP film technique and high sensitivity of QCM detection. The paraoxon selective memories have formed on QCM electrode surface by using a new metal–chelate interaction based on pre-organized monomer and the paraoxon recognition activity of these molecular memories was investigated. Molecular imprinted polymer (MIP) film for the detection of paraoxon was developed and the analytical performance of paraoxon imprinted sensor was studied. The molecular imprinted polymer were characterized by FTIR measurements. Paraoxon imprinted sensor was characterized with AFM and ellipsometer. The study also includes the measurement of binding interaction of paraoxon imprinted quartz crystal microbalance (QCM) sensor, selectivity experiments and analytical performance of QCM electrode. The detection limit and the affinity constant (K_affinity) were found to be 0.06 μM and 2.25 × 10⁴ M^? 1 for paraoxon [MAAP–Cu(II)–paraoxon] based thin film, respectively. Also, it has been observed that the selectivity of the prepared paraoxon imprinted sensor is high compared to a similar chemical structure which is parathion.     

Ethanol and LPG sensing characteristics of SnO<Subscript>2</Subscript> activated Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> thick film sensor

The sensing response of pure and SnO₂ activated Cr₂O₃ to ethanol vapours and liquefied petroleum gas (LPG) has been investigated. Fine particles of commercial chromium oxide powder were selected and deposited as thick film to act as a gas sensor. The sensor surface has been activated by tin dioxide, on surface oxidation of tin chloride. The concentration of tin chloride solution, used as activator, was varied from 0 to 5% and its effect on gas response, selectivity and operating temperature has been studied. It was found that response to ethanol vapours significantly improved, whereas response to LPG remained unaffected. Moreover, operating temperature remains unchanged both for LPG and ethanol vapours.     

Effect of iodine solutions on polyaniline films

Polyaniline (PANI) emeraldine-base films have been exposed to iodine solutions. The interaction between the films and the iodine solution was studied using the quartz-crystal microbalance (QCM) technique and the UV-visible absorption spectroscopy. The iodine-treated film of emeraldine base was subjected to dedoping process using 0.1 M ammonia solution. The resulting film was exposed again to the previously used iodine solution. Iodine was found to play multiple roles: the ring-iodination of PANI film, the oxidation of PANI to pernigraniline base, and iodine doping to PANI salt. A sensor based on PANI-coated electrode of QCM was developed to monitor the presence of iodine in solution.     

Flexible carbon nanotube sensors for nerve agent simulants

Chemiresistor-based vapour sensors made from network films of single-walled carbon nanotube (SWNT) bundles on flexible plastic substrates (polyethylene terephthalate, PET) can be used to detect chemical warfare agent simulants for the nerve agents Sarin (diisopropyl methylphosphonate, DIMP) and Soman (dimethyl methylphosphonate, DMMP). Large, reproducible resistance changes (75-150%), are observed upon exposure to DIMP or DMMP vapours, and concentrations as low as 25?ppm can be detected. Robust sensor response to simulant vapours is observed even in the presence of large equilibrium concentrations of interferent vapours commonly found in battle-space environments, such as hexane, xylene and water (10?000?ppm each), suggesting that both DIMP and DMMP vapours are capable of selectively displacing other vapours from the walls of the SWNTs. Response to these interferent vapours can be effectively filtered out by using a 2?μm thick barrier film of the chemoselective polymer polyisobutylene (PIB) on the SWNT surface. These network films are composed of a 1-2?μm thick non-woven mesh of SWNT bundles (15-30?nm diameter), whose sensor response is qualitatively and quantitatively different from previous studies on individual SWNTs, or a network of individual SWNTs, suggesting that vapour sorption at interbundle sites could be playing an important role. This study also shows that the line patterning method used in device fabrication to obtain any desired pattern of films of SWNTs on flexible substrates can be used to rapidly screen simulants at high concentrations before developing more complicated sensor systems.     

Synthetic chemoreceptive membranes. Sensing bitter or odorous substances on a synthetic lipid multibilayer film by using quartz-crystal microbalances and electric responses

Specific adsorptions of bitter or odorous substances on a synthetic lipid multibilayer matrix (2C18N+2C1/PSS-) were detected by observing frequency changes of a multibilayer-coated quartz-crystal microbalance (QCM). Partition coefficient (P) and diffusion constants (D) of these substances in the lipid matrix could be obtained quantitatively by using the QCM method. There were good correlations between partition coefficients of various bitter or odor substances to the synthetic multibilayer film on the QCM and the intensity of bitter tastes or olfactory receptions in humans: the stronger the intensity of a bitter substance or odorant, the greater the adsorption on the lipid matrix. This indicates that the lipid-coated QCM acts as a sensitive and selective sensor for bitter taste and odor. Electric responses (changes of membrane potential and membrane resistance) of the 2C18N+2C1/PSS-film occurred consecutively by the adsorption of these substances. The bitter or odor substance showing the stronger intensity induced membrane potential change in lower concentrations. It was found that bitter substances having sterically bulky molecular structures adsorb on the surface of the lipid matrix, and the phase-boundary potential of the membrane is thereby changed. On the contrary, odor substances with relatively small or slender structures can penetrate into the lipid matrix and cause reduction of the membrane resistance (the increase of ion permeability). The selective adsorption behavior of bitter and odor substances by molecular shapes was confirmed by adsorption studies of simple C9-10 hydrophobic alcohols having various molecular structures.     

Synthesis and characterization of plasma-polymerized tert-butylacrylate films

Plasma polymerized tert-butylacrylate (pp-t-BA) film was prepared using tert-butylacrylate monomer under 100 Pa of vapor pressure with varying RF power of 10-250 W and continuous wave RF power of 13.56 MHz. The deposition rates of pp-t-BA films were determined using quartz crystal microbalance (QCM) method. The chemical structure of pp-t-BA films was characterized using FT-IR, contact angle and XPS techniques. The gas sorption properties of pp-t-BA were measured using a QCM sensor array. Results showed that deposition rates of pp-t-BA film were proportional to the polymerization time at 100 Pa of monomer pressure under the same RF power. The deposition amount of pp-t-BA films increased gradually with increasing RF power of 30-150 W. Increasing the RF power on plasma polymerization decreased the amount of ester group in pp-t-BA films. Sensitivity of gas sorption on pp-t-BA films is related to the RF power of polymerization.     

Alloy induced inhibition of fatigue crack growth in age-hardenable Al–Cu Alloys

Environmental fatigue crack propagation (EFCP) is alloy-inhibited in under aged Al–Cu–Mg and peak aged Al–Cu–Li alloys stressed in pure aqueous chloride solution. Counter to H diffusion and H-embrittlement rate limited step considerations which predict fatigue crack growth rate (da/dN) to be either independent of fatigue loading frequency (f) or increase with decreasing f, da/dN declines with decreasing f. The mechanism for such alloy induced inhibition and decreasing da/dN with decreasing f is reduced crack tip H production and uptake due to stabilization of the native aluminum passive film resulting from: (1) dissolution of anodic Cu-containing GP zones or precipitates (Al₂CuLi or Al₂CuMg) by dealloying, (2) crack surface Cu enrichment, and (3) enhanced crack wake cathodic reaction kinetics on Cu enriched sites that increase crack solution pH. Peak aged Al–Zn–Mg–Cu alloy 7075 does not exhibit alloy induced inhibition because the predominant anodic phase, Mg(Cu,Zn)₂, does not provide a source for Cu surface enrichment. Alloy induced inhibition is similar to ion-assisted inhibition by molybdate or chromate addition into bulk chloride solution which provides an alternate path to stabilize a crack tip passive film. Both alloy-induced and ion-assisted inhibition of EFCP are promoted by reduced f and crack tip strain rate which favor repassivation of the crack tip passive film over film rupture.     

CO sensing with SnO2 thick film sensors: role of oxygen and water vapour

The paper investigates the gas response of nanocrystalline SnO₂ based thick film sensors upon exposure to carbon monoxide (CO) in changing water vapour (H₂O) and oxygen (O₂) backgrounds. The sensing materials were undoped, Pt- and Pd-doped SnO₂. We found that in the absence of oxygen, the sensor signal (defined as the ratio between the resistance in the background gas, R₀ and the resistance in the presence of the target gases, R, namely R₀/R) have the highest values. These values are higher for doped materials than for the undoped ones. The presence of humidity increases dramatically the sensor signal of the doped materials. In the presence of oxygen, the sensor signal decreases significantly for all sensor materials. The results indicate that there is a competitive adsorption between O₂ and H₂O related surface species and, as a result, different sensing mechanisms can be observed for CO.     

A hybrid humidity sensor using optical waveguides on a quartz crystal microbalance

In this study, slab and ridge optical waveguides (OWGs) made of fluorinated polyimides were deposited on a quartz crystal microbalance (QCM), and hybrid sensors using OWG spectroscopy and the QCM technique were prepared. Polyvinyl alcohol (PVA) film with CoCl₂ was deposited on the OWGs, and the characteristics of humidity sensing were investigated. A prism coupler was used to enter a He–Ne laser beam (λ = 632.8 nm) to the slab OWG. The output light intensity markedly changed due to chromism of the CoCl₂ as a result of humidity sorption, and this change was dependent on the incident angle of the laser beam to the slab OWG. During the measurement of output light, the QCM frequency was simultaneously monitored. The humidity dependence of the sensor with the slab OWG was also investigated in the range from 15 to 85%. For the sensor with the ridge OWG, white light was entered by butt-coupling, and the characteristics of humidity sensing were investigated by observing the output light spectrum and the QCM frequency.     

Surface chemistry and film growth during TiN atomic layer deposition using TDMAT and NH3

Surface chemistry and film growth were examined during titanium nitride (TiN) atomic layer deposition (ALD) using sequential exposures of tetrakis-dimethylamino titanium (TDMAT) and NH₃. This ALD system is shown to be far from ideal and illustrates many potential problems that may affect ALD processing. These studies were performed using in situ Fourier transform infrared (FTIR) techniques and quartz crystal microbalance (QCM) measurements. Ex situ measurements also analyzed the properties of the TiN ALD films. The FTIR studies revealed that TDMAT reacts with NH_x* species on the TiN surface following NH₃ exposures to deposit new Ti(N(CH₃)₂)_x* species. Subsequent NH₃ exposure consumes the dimethylamino species and regenerates the NH_x* species. These observations are consistent with transamination exchange reactions during the TDMAT and NH₃ exposures. QCM studies determined that the TDMAT and NH₃ reactions are nearly self-limiting. However, slow continual growth occurs with long TDMAT exposures. In addition, the TiN ALD growth rate increases progressively with growth temperature. The resistivities of the TiN ALD films were ?10⁴ μΩ cm and the densities were ?3 g/cm³ corresponding to a porosity of ∼40%. The high porosity allows facile oxidation of the TiN films and lowers the film resistivities. These high film porosities will seriously impair the use of these TiN ALD films as diffusion barriers.     

A real time detection of the ovarian tumor associated antigen 1 (OVTA 1) in human serum by quartz crystal microbalance immobilized with anti-OVTA 1 polyclonal chicken IgY antibodies

In this study, we developed a sensitive quartz crystal microbalance (QCM) sensor which employs the anti-ovarian tumor associated antigen 1 (OVTA 1) IgY polyclonal chicken antibodies on the crystal surface for the detection of OVTA 1 in human serum. The anti-OVTA 1 IgY antibodies were anchored on the thiol-activated sensor surface using 1-ethyl-3[3-dimethylaminopropyl] carbodiimide hydrochloride/N-hydroxysuccinimide (EDC–NHS) coupling. The responses for different concentrations of anti-OVTA 1 antibody were also studied and 40 μg/mL was favorable for homogenous antibody coverage. The QCM sensor was used to monitor both immobilization and the target binding affinity. The anti-OVTA 1 antibody based QCM has allowed the competent detection of OVTA 1 in a high linear range of 0.5–10 μg/mL. The total time needed to complete the detection was as short as 5–6 h. The regeneration studies demonstrated that the proposed sensor was reusable up to 9 cycles with a slight loss in binding affinity. The detection of OVTA 1 in human serum allows a potential exploitation of the anti-OVTA 1 polyclonal antibody based QCM immunoassay for the screening of OVTA 1 antigen.     

Plasticization effects on environmental craze microstructure

Environmental crazes were grown in thin films of polystyrene (PS) using the homologous series of alcohols, CH₃OH, C₂H₅OH, n-C₃H₉OH and n-C₄H₉OH. The films were bonded to copper grids, strained to below the minimum crazing strain in air and exposed to the vapour of the various alcohols. The craze microstructure, as measured quantitatively by transmission electron microscopy, varies significantly along the series. The craze fibril volume-fraction, V _f, decreases monotonically from 0.25 for methanol, which depresses the glass transition temperature, T _g, of PS to 91° C, to 0.09 for n-butanol, which depresses T _g of PS to 71° C. All these slowly-growing vapour crazes thicken by drawing more fibrillar material from the craze surfaces rather than by fibril creep. The large decrease in V _f along the series of alcohols cannot be due to a change in the chain-entanglement molecular weight, as a result of swelling by the alcohols, but must result rather from an easier slippage of molecular entanglements in the drawing glassy fibrils. The large decrease in V _f from methanol to butanol crazes must also enhance the nucleation of cracks within these crazes, as evidenced by the ten-fold decrease in the environmental fatigue life of PS along the series from methanol to butanol.     

Flexible single walled nanotube based chemical sensor for 2,4-dinitrotoluene sensing

In this work, we have attempted to fabricate flexible single walled carbon nanotube based sensor for detection of 2,4-dinitrotoluene (DNT) an explosive chemical. For analyte sensing study, the flexible sensor is fabricated by vacuum filtration method. These fabricated gas sensors are characterised by SEM and Raman spectroscopy. The sensor response is investigated toward the explosive chemicals which have NO₂ group in their molecular structure. The fabricated sensor is able to detect the traces of DNT at room temperature. The sensor gives 0.28–0.32% repeatable response to 0.22 ppm of DNT. The response of sensor increases with increase in the vapour concentration of the DNT vapours.     

Mechanical Response of <Superscript>4</Superscript>He Films Adsorbed on Graphite with a Quartz Tuning Fork

We have carried out quartz-crystal microbalance (QCM) experiments of 32 kHz quartz tuning fork for ⁴He films adsorbed on Grafoil, and have measured the temperature dependence of the resonance frequency and Q value for various areal densities. It was found that the frequency does not decrease exactly in proportion to the areal density. This means that the film still undergoes decoupling partly, although it is strongly suppressed from that of 5 MHz QCM measurements. Above the three-atom thick film, the decoupling due to the superfluidity of the overlayer is observed. In addition, we found that the competition between the superfluidity and the slippage takes place for a large oscillation amplitude. From the comparison with 5 MHz QCM measurements, it is concluded that the acceleration of substrate plays an important role in the slippage.     

Protection of polymer from atomic-oxygen erosion using Al2O3 atomic layer deposition coatings

Thin films of Al₂O₃ grown using atomic layer deposition (ALD) techniques can protect polymers from erosion by oxygen atoms. To quantify this protection, polyimide substrates with the same chemical repeat unit as Kapton^® were applied to quartz crystal microbalance (QCM) sensors. Al₂O₃ ALD films with varying thicknesses were grown on the polyimide substrates. The ALD-coated polyimide materials were then exposed to a hyperthermal atomic-oxygen beam. The mass loss versus oxygen-atom exposure time was measured in situ by the QCM. Al₂O₃ ALD film thicknesses of ∼ 35 Å were found to protect the polymer from erosion.     

Indium oxide thin film based ammonia gas and ethanol vapour sensor

A sensor for ammonia gas and ethanol vapour has been fabricated using indium oxide thin film as sensing layer and indium tin oxide thin film encapsulated in poly(methyl methacrylate) (PMMA) as a miniature heater. For the fabrication of miniature heater indium tin oxide thin film was grown on special high temperature corning glass substrate by flash evaporation method. Gold was deposited on the film using thermal evaporation technique under high vacuum. The film was then annealed at 700 K for an hour. The thermocouple attached on sensing surface measures the appropriate operating temperature. The thin film gas sensor for ammonia was operated at different concentrations in the temperature range 323–493 K. At 473 K the sensitivity of the sensor was found to be saturate. The detrimental effect of humidity on ammonia sensing is removed by intermittent periodic heating of the sensor at the two temperatures 323K and 448 K, respectively. The indium oxide ethanol vapour sensor operated at fixed concentration of 400 ppm in the temperature range 293–393 K. Above 373 K, the sensor conductance was found to be saturate. With various thicknesses from 150–300 nm of indium oxide sensor there was no variation in the sensitivity measurements of ethanol vapour. The block diagram of circuits for detecting the ammonia gas and ethanol vapour has been included in this paper.     

Quartz crystal microbalance and spectroscopy measurements for acid doping in polyaniline films

Abstract

We investigated the doping of thin polyaniline (PANI) films, prepared by the chemical oxidation of aniline, with different acids. The initial step in the investigation is the preparation of PANI films from aqueous hydrochloric acid solution. This is followed by dedoping with ammonia to obtain a PANI base, which is subsequently doped with strong acids (e.g. hydrochloric, sulfuric, phosphoric and trichloroacetic acids) and with a weak acid (acetic acid). The dopant weight fraction (w), which is connected with the gain of mass during the doping of PANI, was determined in situ using a quartz crystal microbalance (QCM). The behavior of PANI upon doping with different anions derived from strong acids indicates that both proton and the anion uptake into the polymer chains occur sharply, rapidly, completely, and reversibly. However the uptake in the case in acetic acid is characterized by slow diffusion. The doping was studied at different concentrations of acetic acid. A second cycle of dedoping–redoping was also performed. The kinetics of the doping reaction is dominated by Fickian diffusion kinetics. The diffusion coefficients (D) of the dopant ions into the PANI chains were determined using the QCM and by UV–Vis absorption spectroscopy in the range of (0.076–1.64)× 10^?15 cm² s^?1. It was found that D in the second cycle of doping is larger than that evaluated from the first cycle of doping for high concentrations of acetic acid. D for the diffusion and for the dopant ion expulsion from the PANI chains was also determined during the redoping process. It was found that D for acetic acid ions in the doping process is larger than that calculated for the dedoping process.     

Investigation of gas-sensing properties of copper phthalocyanine films

In this work, the sensor response of thin films of octasubstituted copper phthalocyanine derivative (CuPcR₈) with bulky substituents on R = –N(SO₂C₆H₅–CH₃)CH₂(CH₂)₈CH₃ to the vapour of benzene, chloroform, ethanol and butanol were investigated by surface plasmon resonance technique. The application and some advantages of total internal reflection ellipsometry method for determination of the organic vapours are also demonstrated.     

Ternary Excess Molar Enthalpies of Alcohols with Methyl t-Butyl Ether at T=298.15 K

Ternary excess molar enthalpies H ^E _m at 298.15 K and atmospheric pressure measured in a flow microcalorimeter are reported for the methanol+2-propanol+methyl t-butyl ether (MTBE) and ethanol+1-propanol+MTBE systems. The ternary results have been correlated by means of a polynomial equation and used to construct constant excess enthalpy contours. Furthermore, the results have been compared with those calculated from a UNIQUAC associated-solution model taking into account molecular association of alcohols and solvation between unlike alcohols and alcohols with MTBE using only binary information.     

Ethanol gas sensing properties of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-doped ZnO thick film resistors

The characterization and ethanol gas sensing properties of pure and doped ZnO thick films were investigated. Thick films of pure zinc oxide were prepared by the screen printing technique. Pure zinc oxide was almost insensitive to ethanol. Thick films of Al₂O₃ (1 wt%) doped ZnO were observed to be highly sensitive to ethanol vapours at 300°C. Aluminium oxide grains dispersed around ZnO grains would result into the barrier height among the grains. Upon exposure of ethanol vapours, the barrier height would decrease greatly leading to drastic increase in conductance. It is reported that the surface misfits, calcination temperature and operating temperature can affect the microstructure and gas sensing performance of the sensor. The efforts are, therefore, made to create surface misfits by doping Al₂O₃ into zinc oxide and to study the sensing performance. The quick response and fast recovery are the main features of this sensor. The effects of microstructure and additive concentration on the gas response, selectivity, response time and recovery time of the sensor in the presence of ethanol vapours were studied and discussed.