Detection of pure chemical vapors in a thermally cycled porous silica photonic crystal

The condensation and evaporation of vapors of isopropanol, heptane, and cyclohexane in mesoporous silica photonic crystals are monitored by optical reflection spectroscopy as a function of sensor temperature. The spectral position of the stop band shifts to the red upon analyte adsorption, and it shifts to the blue as the sensor is heated and analyte evaporates from the porous nanostructure. The hysteresis of the optical response as the temperature of the sensor is cycled between 25 and 80 °C is characteristic of each analyte for partial pressures between 0 and 7.5 Torr. These characteristic hysteresis loops allow identification of the three analytes. The temporal response of the sensor is studied as a function of heating rate and analyte concentration in a flowing stream of analyte vapor, and it is compared with the equilibrium adsorption isotherms of the sensor. The ability of the temporal data to identify the analytes is attributed to differences in diffusion and adsorption properties of each analyte within the mesoporous silica sensor.     

A miniature gas analyzer made by integrating a chemoresistor with a microchannel

A microfluidic channel is integrated with a tin oxide-based generic gas sensor on a PMMA (polymethyl methacrylate) substrate to fabricate a miniature gas analyzer. The analyte gas diffuses along the air-filled channel to affect the sensor installed in a microcavity positioned at the end of the channel. Analyte diffusion rates, experimentally estimated based on the temporal responses received from the sensor, are connected to the analyte's interactions with the channel walls as well as its diffusivity in air. The analyte-related information is extracted from the recorded responses and used for analyte recognition. A single PMMA channel of 80 μm × 3 mm × 50 mm dimensions facilitates the correct classification of single component contaminants each introduced in a wide concentration range in air. The device is also shown to identify 15 ppm of 2-butanol in air contaminated with 1500 ppm of 1-butanol. The gas analyzer fabricated based on this concept is durable, inexpensive, handheld and suitable for a variety of applications.     

Gas sensing mechanism in chemiresistive cobalt and metal-free phthalocyanine thin films

The gas sensing behaviors of cobalt phthalocyanine (CoPc) and metal-free phthalocyanine (H2Pc) thin films were investigated with respect to analyte basicity. Chemiresistive sensors were fabricated by deposition of 50 nm thick films on interdigitated gold electrodes via organic molecular beam epitaxy (OMBE). Time-dependent current responses of the films were measured at constant voltage during exposure to analyte vapor doses. The analytes spanned a range of electron donor and hydrogen-bonding strengths. It was found that, when the analyte exceeded a critical base strength, the device responses for CoPc correlated with Lewis basicity, and device responses for H2Pc correlated with hydrogen-bond basicity. This suggests that the analyte-phthalocyanine interaction is dominated by binding to the central cavity of the phthalocyanine with analyte coordination strength governing CoPc sensor responses and analyte hydrogen-bonding ability governing H2Pc sensor responses. The interactions between the phthalocyanine films and analytes were found to follow first-order kinetics. The influence of O2 on the film response was found to significantly affect sensor response and recovery. The increase of resistance generally observed for analyte binding can be attributed to hole destruction in the semiconductor film by oxygen displacement, as well as hole trapping by electron donor ligands.     

Development and Application of Novel Thin-Film Spectroelectrochemical Sensors Possessing Three Modes of Selectivity*

A thin-film spectroelectrochemical sensor design employing three modes of selectivity is described. Selectivity is achieved through (1) partitioning of the analyte into a chemically selective film, (2) electrochemical cycling of the analyte over a given potential window, and (3) absorbance of one of the redox states of the analyte at the chosen analytical wavelength. Optimization of the sensor is described with respect to both improved selectivity and sensitivity, as well as its response to a number of different chemical species. Lastly, application of the sensor for determination of ferrocyanide, Fe(CN)^4- ₆, in both radioactive waste simulant and actual waste storage tank contents is given.     

Protein biosensors based on biofunctionalized conical gold nanotubes

There is increasing interest in the concept of using nanopores as the sensing elements in biosensors. The nanopore most often used is the alpha-hemolysin protein channel, and the sensor consists of a single channel embedded within a lipid bilayer membrane. An ionic current is passed through the channel, and analyte species are detected as transient blocks in this current associated with translocation of the analyte through the channel-stochastic sensing. While this is an extremely promising sensing paradigm, it would be advantageous to eliminate the very fragile lipid bilayer membrane and perhaps to replace the biological nanopore with an abiotic equivalent. We describe here a new family of protein biosensors that are based on conically shaped gold nanotubes embedded within a mechanical and chemically robust polymeric membrane. While these sensors also function by passing an ion current through the nanotube, the sensing paradigm is different from the previous devices in that a transient change in the current is not observed. Instead, the protein analyte binds to a biochemical molecular-recognition agent at the mouth of the conical nanotube, resulting in complete blockage of the ion current. Three different molecular-recognition agents, and correspondingly three different protein analytes, were investigated: (i) biotin/streptavidin, (ii) protein-G/immunoglobulin, and (iii) an antibody to the protein ricin with ricin as the analyte.     

Understanding the dynamics of signal transduction for adsorption of gases and vapors on carbon nanotube sensors

Adsorption dynamics and their influence on signal transduction for carbon nanotube-based chemical sensors are explored using continuum site balance equations and a mass action model. These sensors are shown to possess both reversible and irreversible binding sites that can be modeled independently. For the case of irreversible adsorption, it is shown that the characteristic response time scales inversely with analyte concentration. It is inappropriate to report a detection limit for this type of sensor since any nonzero analyte concentration can be detected in theory but at a cost of increasing transduction time with decreasing concentration. The response curve should examine the initial rate of signal change as a function of analyte concentration. Conversely, a reversible sensor has a predefined detection limit, independent of the detector geometry with a characteristic time scaling that becomes constant in the zero analyte concentration limit. A simple analytical test is presented to distinguish between these two mechanisms from the transient response of a nanotube sensor array. Two systems appearing in the literature are shown to have an irreversible component, and regressed surface rate constants for this component are similar across different sensor geometries and analytes.     

Amperometric Enzyme‐Based Biosensor for Direct Detection of Formaldehyde in the Gas Phase: Dependence on Electrolyte Composition

An enzymatic sensor detecting the analyte formaldehyde directly from the gas phase is under investigation. In contrast to existing systems, it enables the quantification of the analyte without prior sampling or accumulation and thus can be used as an online system to monitor the formaldehyde concentration in ambient air. The amperometric sensor depends on the enzymatic conversion of the analyte using formaldehyde dehydrogenase from P. putida [EC. 1.2.1.46] as the recognition element. It shows a linear response curve up to 15 ppm, with a detection limit of 0.03 pm (S/N=3). In order to optimize the sensor performance the electrolyte composition within the sensor was varied with respect to pH value, buffer concentration and the addition of Ca²⁺ and Mg²⁺ ions. To elucidate the influence of the mediator and the enzyme on the sensor performance the stability and activity of the electrochemical mediator and the enzyme alone was examined separately in these different electrolytes.     

Position-independent chemical quantitation with passive 13.56-MHz radio frequency identification (RFID) sensors

Recently, we have demonstrated an attractive approach to adapt conventional radio frequency identification (RFID) tags for multianalyte chemical sensing. These RFID sensors could be very attractive as ubiquitous distributed remote sensor networks. However, critical to the wide acceptance of the demonstrated RFID sensors is the analyte-quantitation ability of these sensors in presence of possible repositioning errors between the RFID sensor and its pickup coil. In this study, we evaluate the capability for such position-independent analyte quantification using multivariate analysis tools. By measuring simultaneously several parameters of the complex impedance from such an RFID sensor and applying multivariate statistical analysis methods, we were able to compensate for the repositioning effects such as baseline signal offset and magnitude of sensor response to an analyte.     

Single-use phosphorimetric sensor for the determination of nalidixic acid in human urine and milk

A single-use phosphorimetric sensor to determine the germicide nalidixic acid is proposed. The sensing action is based on the absorption of the analyte into the sensing zone and the subsequent measurement of the phosphorescence intensity emitted by the analyte fixed in the sensor. This plane drop sensor is made up of a 3 x 1.6 cm sheet of the polyester Mylar as solid support, and a circular film 5 mm in diameter and 20 microns in thickness, formed by poly(vinyl chloride) and tributyl phosphate as the plasticizer, adhered to its surface. The sensor is introduced for 2 h into the sample solution, after which it is dried and the phosphorescence intensity is measured directly at lambda ex = 332 nm, lambda em = 412 nm, with a delay time of 0.15 ms and a gate time of 10 ms, under a dry nitrogen stream. The characteristic parameters of the construction of the sensing zone and of the processes of fixing the analyte along with the emission of phosphorescence were studied. The applicable concentration range was from 60 to 1500 ng ml-1, with a detection limit of 20 ng ml-1 and a precision of 2% expressed as relative standard deviation. The method was applied to the determination of nalidixic acid in milk and human urine with recoveries ranging between 96.0 and 103.7%. The calibration process was carried out by applying a mathematical method of finite elements that expresses the analytical signal as a function of the analyte concentration and equilibration time between the sensor and the sample solution.     

Use of spatiotemporal response information from sorption-based sensor arrays to identify and quantify the composition of analyte mixtures

Linear sensor arrays made from small molecule/carbon black composite chemiresistors placed in a low-headspace volume chamber, with vapor delivered at low flow rates, allowed for the extraction of new chemical information that significantly increased the ability of the sensor arrays to identify vapor mixture components and to quantify their concentrations. Each sensor sorbed vapors from the gas stream and, thereby, as in gas chromatography, separated species having high vapor pressures from species having low vapor pressures. Instead of producing only equilibrium-based sensor responses that were representative of the thermodynamic equilibrium partitioning of analyte between each sensor and the initial vapor phase, the sensor responses varied depending on the position of the sensor in the chamber and the time since the beginning of the analyte exposure. The concomitant spatiotemporal (ST) sensor array response therefore provided information that was a function of time, as well as of the position of the sensor in the chamber. The responses to pure analytes and to multicomponent analyte mixtures comprised of hexane, decane, ethyl acetate, chlorobenzene, ethanol, and/or butanol were recorded along each of the sensor arrays. Use of a non-negative least-squares (NNLS) method for analysis of the ST data enabled the correct identification and quantification of the composition of two-, three-, four-, and five-component mixtures from arrays using only four chemically different sorbent films. In contrast, when traditional time- and position-independent sensor response information was used, these same mixtures could not be identified or quantified robustly. The work has also demonstrated that, for ST data, NNLS yielded significantly better results than analyses using extended disjoint principal components modeling. The ability to correctly identify and quantify constituent components of vapor mixtures through the use of such ST information significantly expands the capabilities of such broadly cross-reactive arrays of sensors.     

Resistive-pulse studies of proteins and protein/antibody complexes using a conical nanotube sensor

There is increasing interest in using nanopores in synthetic membranes as resistive-pulse sensors for molecular and macromolecule analytes. In general, this method entails measuring current pulses associated with translocation of the analyte through the nanopore sensor element. A key challenge for this sensing paradigm is building selectivity into the protocol so that the current pulses for the target analyte can be distinguished from current pulses for other species that might be present in the sample. We show here that this can be accomplished with a protein analyte by adding to the solution an antibody that selectively binds the protein. We demonstrate this concept using bovine serum albumin (BSA) and a Fab fragment from a BSA-binding polyclonal antibody. Because the complex formed upon binding of the Fab to BSA is larger than the free BSA molecule, the current-pulse signature for the BSA/Fab complex can be easily distinguished from the free BSA. Furthermore, the BSA/Fab pulses can be easily distinguished from the pulses obtained for the free Fab and from pulses obtained for a control protein that does not bind to the Fab. Finally, we also show that the current-pulse signature for the BSA/Fab complex can provide information about the size and stoichiometry of the complex.     

Minimum-step immuno-analysis based on continuous recycling of the capture antibody

Most immuno-analytical systems employ antibodies that do not readily dissociate upon binding to its partner antigen (i.e., target analyte; α2-macroglobulin as a model) and, thus, either need to be disposed of after one-time use or be reused after binding has been reset. To achieve a minimum-step analysis, an antibody that is capable of rapidly reversible binding with high affinity to an antigen was investigated in this study. This antibody was immobilized on the surface of a label-free sensor, which was combined with microfluidic channels, to demonstrate its applicability. The antibody was successively reused without a regeneration step under physiological conditions, offered specific analysis in the serum medium, and detected the analyte at concentrations as low as 0.1 ng mL(-1), which could further be enhanced by 100-fold. The sensor response reached 95% equilibrium after 8.3 and 14.9 min in average on each dose level for the concentration increase and decrease, respectively. The dynamic range covered a 5 logarithmic analyte concentration. Since the sampling size was in the nanolitre to millilitre range per day under the conditions used and the sensor may retain a long shelf-life, it could potentially be used in a clinical setting for long-term, on-line monitoring of diseases.     

Molecular imprinting in chemical sensing – Detection of aromatic and halogenated hydrocarbons as well as polar solvent vapors

The technique of molecular imprinting – a novel tool of sensor material synthesis – can be successfully combined with mass-sensitive transducers. Imprinted polymers are prepared in presence of inert analytes acting as molecular templates. The remaining imprint allows the molecular recognition of an analyte due to host-guest-interactions. The sensor properties of devices coated with imprint polymers can be tuned to the analyte by variation of the polymerization solvent and the amount of cross-linker added. A further sensitivity enhancement can be achieved by rising the resonant frequencies of QMBs and SAW resonators, since the signal to noise-ratio increases in an approximately linear manner with the oscillation frequency.     

Polyglycerol based coatings to reduce non-specific protein adsorption in sample vials and on SPR sensors

Coatings based on dendritic polyglycerol (dPG) were investigated for their use to control nonspecific protein adsorption in an assay targeted to analyze concentrations of a specific protein. We demonstrate that coating of the sample vial with dPG can significantly increase the recovery of an antibody after incubation. First, we determine the concentration dependent loss of an antibody due to nonspecific adsorption to glass via quartz crystal microbalance (QCM). Complementary to the QCM measurements, we applied the same antibody as analyte in an surface plasmon resonance (SPR) assay to determine the loss of analyte due to nonspecific adsorption to the sample vial. For this purpose, we used two different coatings based on dPG. For the first coating, which served as a matrix for the SPR sensor, carboxyl groups were incorporated into dPG as well as a dithiolane moiety enabling covalent immobilization to the gold sensor surface. This SPR-matrix exhibited excellent protein resistant properties and allowed the immobilization of amyloid peptides via amide bond formation. The second coating which was intended to prevent nonspecific adsorption to glass vials comprised a silyl moiety that allowed covalent grafting to glass. For demonstrating the impact of the vial coating on the accuracy of an SPR assay, we immobilized amyloid beta (Aβ) 1-40 and used an anti-Aβ 1-40 antibody as analyte. Alternate injection of analyte into the flow cell of the SPR device from uncoated and coated vials, respectively gave us the relative signal loss (1 − RU_uncoated/RU_coated) caused by the nonspecific adsorption. We found that the relative signal loss increases with decreasing analyte concentration. The SPR data correlate well with concentration dependent non-specific adsorption experiments of the analyte to glass surfaces performed with QCM. Our measurements show that rendering both the sample vial and the sensor surface is crucial for accurate results in protein assays.     

Label‐free Electrochemiluminescent Enantioselective Sensor for Distinguishing between Chiral Metallosupramolecular Complexes

Chiral molecular recognition of DNA is important for rational drug design and for developing structural probes of DNA conformation. Developing a convenient and inexpensive assay for sensitive and selective identification of DNA‐specific binding compounds with rapid, easy manipulation is in ever‐increasing demand. Here, we present a “turn‐on” and label‐free electrochemiluminescent (ECL) biosensor for distinguishing chiral metallosupramolecular complexes based on DNA three‐way junction formation selectively induced by the analyte. The fabricated ECL sensor shows excellent performance in the chiral discrimination of two enantiomers with an enantioselective recognition ratio of up to 4.4. More importantly, as a “turn‐on” detection system, the ECL chiral sensor does not suffer from false positives and limited signal range of “signal‐off” systems. Therefore, this concept may provide a new insight into the design of efficient sensors for distinguishing chiral molecules and for investigating the interactions between DNA and small molecules.     

Downhill protein folding modules as scaffolds for broad-range ultrafast biosensors

Conformational switches are macromolecules that toggle between two states (active/inactive or folded/unfolded) upon specific binding to a target molecule. These molecular devices provide an excellent scaffold for developing real-time biosensors. Here we take this concept one step beyond to build high-performance conformational rheostat sensors. The rationale is to develop sensors with expanded dynamic range and faster response time by coupling a given signal to the continuous (rather than binary) unfolding process of one-state downhill folding protein modules. As proof of concept we investigate the pH and ionic-strength sensing capabilities of the small α-helical protein BBL. Our results reveal that such a pH/ionic-strength sensor exhibits a linear response over 4 orders of magnitude in analyte concentration, compared to the 2 orders of magnitude for switches, and nearly concentration-independent microsecond response times.     

Determination of Nitroaromatics Using a Double-Layer of Gelatin Nanofibers and a Pyrene-Doped Polystyrene Membrane

ABSTRACT

A double-layered sensor was prepared by an electrospinning method using pyrene, polystyrene, and gelatine to prepare a pyrene-polystyrene/gelatin/glass device for the determination of nitroaromatics. The pyrene- polystyrene top layer, which was an electronspun membrane from a mixture of the starting materials in 3:1?N, N-dimethylformaide:tetrahydrofuan, served as the sensing layer for detecting 2,4-dinitrotoluene. The bottom layer was the gelatin electrospun membrane that served as an intermediate between the top layer and the glass slide. The dinitrotoluene analyte was transferred into the sensing layer above and below due to the porosity of the gelatin layer. With a large number of hydroxyl and amino groups, the gelatin layer formed hydrogen bonds with the 2,4-dinitrotoluene molecules, which caused the enrichment of the analyte around the gelatin layer. Additionally, single-layer pyrene-polystyrene/glass sensor was also prepared for control measurements. The quenching efficiency of the pyrene-polystyrene/gelatin/glass sensor was 76.7% at equilibrium obtained within 6 minutes, while the monolayer sensor pyrene–polystyrene/glass device provided 65.8% quenching efficiency within 6 minutes. The gelatin layer played an important role in the superior performance of the double-layered sensor.     

A Novel Potentiometric Detection Strategy for the Determination of Amlodipine Besylate Based on Functionalized Particles

A novel potentiometric strategy based on functionalized magnetite nanoparticles and microparticles were compared with the classical potentiometric strategy. This strategy provided nano‐ and microsized particles that were highly dispersed and coated with ionophore and plasticizer to promote an in situ cooperative ion‐pairing interaction between the ionophore and the analyte present in inner solution of sensor membrane, compared to the classical technique. Three amlodipine (AML) sensors were constructed using functionalized nanoparticles in sensor 1; microparticles in sensor 2, as ionophores, and the polymeric membrane ionophoric property in sensor 3.     

Effect of the Nature of the Dopant on the Response of a Sensor Array Based on Polyaniline

The effect of the nature of the dopant on the response of a sensor array based on films of polyaniline (PAn) under the influence of the vapor of various organic solvents was studied. It was established that the main factors determining the magnitude of the response of PAn films are the morphology of the films and the accepting power of the analyte molecules (in the case of "standard" acid dopants) and also the possibility of additional donor–acceptor interaction between the analyte molecules and the dopant (in the case of heteropoly acid dopants). It was shown that with heteropoly acids as dopants of PAn it is possible to increase substantially the selectivity of the response of the sensor array.