Milk-sense: a volatile sensing system recognises spoilage bacteria and yeasts in milk

An electronic nose unit including 14 conducting polymer sensors, was used to detect the volatile profiles produced by uninoculated skimmed milk media or that inoculated with bacteria (Pseudomonas aureofaciens, P. fluorescens, Bacillus cereus) or yeasts (Candida pseudotropicalis, Kluyveromyces lactis) when grown for 5 h at 30°C. Using discriminant function analyses (DFA) it was possible to separate unspoiled milk and that containing spoilage bacteria or yeasts. The sensor array used was a useful discriminator of microbial volatile profiles. Quantitative differentiation between three different concentrations of P. aureofaciens (10⁶, 3.5×10⁸, 8×10⁸ CFUs ml⁻¹) was also investigated and showed that the system could effectively differentiate between treatments. Using an initial inoculum of about 10³–10⁴ CFUs ml⁻¹ it was possible to discriminate between unspoiled milk, yeasts and bacterial species (S. aureus, B. cereus and the Pseudomonas spp.) using principal component analyses (PCA), and also between the bacteria, the unspoiled milk, and the two yeasts C. pseudotropicalis and K. lactis with 85% of the data accounted for. The potential for differentiation between four of the five individual bacterial and yeast species was analysed after 5 h growth at 25°C by using a three-layer back propagation neural network (NN) of 46 input sensor parameters. This showed that it was possible to recognise, and differentiate, between species, the butanol and milk medium controls. Cross validation using labelled individual replicates of treatments as unknowns demonstrated that it was possible to differentiate between (a) butanol controls; (b) unspoiled milk medium; (c) S. aureus; (d) K. lactis; (e) C. psuedotropicalis; and (f) B. cereus. The potential for using an electronic nose system for early detection of microbial spoilage of milk-based products is discussed.     

Automatic detection of Salmonella enterica in sprout irrigation water using a nucleic acid sensor

A nucleic acid sensor capable of automated sample and reagent loading, real-time PCR, automated detection, and sample line cleaning was tested. Real-time PCR reactions were performed with Salmonella enterica in autoclaved and spent alfalfa sprout irrigation water. S. enterica boiled cells were detected over a range of approximately 10⁴ to 10⁸ CFU/reaction (rxn). It was possible to generate enough PCR product to visualize a band on a gel at the expected size over approximately five orders of magnitude from 3.2 × 10³ to 10⁸ CFU/rxn. Automated detection experiments yielded correct identification of 9/9 positive control reactions over a range of 10⁴ to 10⁸ CFU/rxn, correctly identified a negative control reaction, and a sample of 3.2 × 10³ CFU/rxn was incorrectly identified as negative. Primer dimers were not seen in positive or negative control reactions with sprout irrigation water, suggesting that it may be possible to improve the detection limit simply by increasing the number of thermal cycles or by lowering the annealing temperature. The system required no interpretation of real-time PCR data by the operator. The entire process of loading, running the PCR, automated data interpretation, and sample line cleaning was completed in under 2 h and 20 min, significantly faster than it would take to ship a sample and have it tested by an independent laboratory.     

Detection of Escherichia coli using CMOS array photo sensor-based enzyme biochip detection system

This work presents optical enzyme detection system based on the CMOS array photo sensor and 1 × 3 polymeric enzyme biochip for detecting Escherichia coli in a one-step procedure. This assay, using 4-methylumbelliferyl-β-d-glucuronide (MUG) as a fluorogenic substrate, had a detection limit of 0.1 U/ml for β-glucuronidase (GUD), which was approximately equal to a cell concentration of 10⁶ CFU/ml of E. coli. MUG was incorporated into lauryl tryptose broth at a final concentration of 100 μg/ml for immediate verification of the presence of E. coli in 1 × 3 polymeric enzyme biochip. The 40 strains of E. coli studied all produced GUD. Of another 36 strains of bacteria tested, one strain (Salmonella choleraesuis subsp. choleraesuis) yielded very small amounts of GUD after 24 h incubation. The optical enzyme detection system was sensitive and rapid.     

Electrochemical genosensor based on modified octadecanethiol self-assembled monolayer for Escherichia coli detection

An electrochemical genosensor based on 1-fluoro-2-nitro-4-azidobenzene (FNAB) modified octadecanethiol (ODT) self-assembled monolayer (SAM) has been fabricated for Escherichia coli detection. The results of electrochemical response measurements investigated using methylene blue (MB) as a redox indicator reveal that this nucleic acid sensor has 60 s of response time, high sensitivity (0.5 × 10⁻¹⁸ M) and linearity as 0.5 × 10⁻¹⁸-1 × 10⁻⁶ M. The sensor has been found to be stable for about four months and can be used about ten times. It is shown that water borne pathogens like Klebsiella pneumonia, Salmonella typhimurium and other gram-negative bacterial samples has no significant effects in the response of this sensor.     

Electrochemical sensor for hydroperoxides determination based on Prussian blue film modified electrode

An electrochemical sensor for hydroperoxides determination was investigated. The sensor was based on the electrocatalytic reduction of hydroperoxides on Prussian blue (PB)-modified glassy carbon electrode. The modified electrode possesses a high electrocatalytic effect towards all studied peroxides with the highest effect obtained with H₂O₂ followed by tert-butyl hydroperoxide (TBH), cumene hydroperoxide (CH) and linoleic acid hydroperoxide (LAH). In addition, the modified electrode showed a good stability and a fast response time (<20 s). The lower detection limits of H₂O₂, TBH, CH and LAH were found to be 10⁻⁷ mol L⁻¹, 2 × 10⁻⁷ mol L⁻¹, 3.5 × 10⁻⁷ mol L⁻¹ and 4 × 10⁻⁷ mol L⁻¹, respectively. The electrochemical sensor was then applied for amperometric determination of peroxide value (PV) in edible oil at an applied potential of 50 mV (vs. Ag/AgCl (1 M KCl)). A good linearity has been found in the range 0.02–1.0 mequiv. O₂/kg, with a detection limit (S/N = 3) of 0.001 mequiv. O₂/kg. The precision of the method (R.S.D., n = 9) for within and between-days is better than 1.9% and 2.7%, respectively at 0.1 mequiv. O₂/kg. The method was successfully applied to the determination of PV in real edible oil samples with an excellent agreement with results obtained with the official standard procedure. The proposed method is accurate, simple, cheap and could be used to control edible oil rancidity with a high sample throughputs (more than 120 samples/h).     

Rapid parallelized and quantitative analysis of five pathogenic bacteria by ITS hybridization using QCM biosensor

We developed a 2 × 5 model quartz crystal microbalance (QCM) DNA biosensor array for detection of five bacteria, which based on hybridization analysis of bacterial 16S-23S rDNA internal transcribed spacer (ITS) region. A pair of universal primers was designed for PCR amplification of the ITSs. The PCR products were analyzed by the biosensor. We used gold nanoparticles to amplify the frequency shift signals. Fifty clinical samples were detected by both the biosensor and conventional bacteria culture method. We found a linear quantitative relationship between frequency shift and logarithmic concentration of synthesized oligonucleotides or bacteria cells. The measurable concentration ranged from 10⁻¹² to 10⁻⁸ M for synthesized oligonucleotides and 1.5 × 10² to 1.5 × 10⁸ CFU/mL for bacteria. The 10⁻¹² M of synthesized oligonucleotides or 1.5 × 10² CFU/mL of Pseudomonas aeruginosa could be detected by the biosensor system. The detection could be completed within 5 h including the PCR amplification procedure. Compared with bacteria culture method, the detection sensitivity and specificity of the biosensor system were 94.12% and 90.91%, respectively. There was no significant difference between these two methods (P = 0.625 > 0.05). The biosensor system provides a rapid and sensitive method for parallelized and quantitative analysis of multiple pathogenic bacteria in clinical diagnosis.     

An electrochemical biosensor for determination of hydrogen peroxide using nanocomposite of poly(methylene blue) and FAD hybrid film

An electrochemical biosensor for determination of hydrogen peroxide (H₂O₂) has been developed by the hybrid film of poly(methylene blue) and FAD (PMB/FAD). The PMB/FAD hybrid film was performed in PBS (pH 7) containing methylene blue and FAD by cyclic voltammetry. Repeatedly scanning potential range of −0.6-1.1 V, FAD was immobilized on the electrode surface by electrostatic interaction while methylene blue was electropolymerized on electrode surface. This modified electrode was found surface confined and pH dependence. It showed good electrocatalytic reduction for H₂O₂, KBrO₃, KIO₃, and NaClO as well as electrocatalytic oxidation for NADH. At an applied potential of −0.45 V vs. Ag/AgCl, the sensor showed a rapid and linear response to H₂O₂ over the range from 0.1 μM to 960 μM, with a detection limit of 0.1 μM and a significant sensitivity of 1109 μA mM⁻¹ cm⁻² (S/N = 3). It presented excellent stability at room temperature, with a variation of response current less than 5% over 30 days.     

Highly sensitive humidity sensor at low humidity based on the quaternized polypyrrole composite film

Polypyrrole composite was fabricated by using chemical polymerization at room temperature, which was further quaternized with 1,4-bromobutane to prepare the humidity sensitive material. The electrical properties of the sensor were measured. The results showed that the quaternized polypyrrole composite film exhibited low impedance at very low humidity; the impedance value was about 780 kΩ at 11% RH and the impedance changed from 10⁶ to 10³ Ω between 11% and 95% RH, which was suitable for detection of low humidity. Moreover, the logarithm of impedance of the polypyrrole composite film varied linearly (R² = 0.9916) with relative humidity in the range of 11–95% RH. Surface and cross-sectional morphology of the films, response–recovery time and hysteresis were also measured and estimated. The sensing mechanism of the sensor was also investigated.     

Direction finding of moving ferromagnetic objects inside water by magnetic anomaly

The remote sensing methods by the use of magnetic anomaly are gaining importance in applications of defense technologies and industrial purposes recently. In this study, it is aimed to determine the remote detection, the variation of characteristic of the voltage in the sensor relative to the motion, the effects of material length, magnetic permeability and direction of motion of the object on this characteristic and to convert them to a useful mathematical expression by using magnetic anomaly of ferromagnetic objects such as submarines moving inside water. For this purpose, first of all, a water tank of 1 m³ is prepared and approximately a homogeneous magnetic field of 10⁻³ T is created within this water tank. Ferromagnetic materials with six different lengths and permeabilities are moved in three different directions relative to the position of the sensor by means of a computer controlled x–y scanner designed for this experiment inside this magnetic field. The magnetic change caused by this motion at the point where the sensor is positioned is detected as the output voltage of the sensor. A mathematical expression is formulated taken into account the variations of the sensor output voltage with respect to the length, magnetic permeability and the direction of motion of the material and it is validated by the experimental results. This study clearly shows that the existence and the direction of motions of ferromagnetic objects with different lengths and magnetic permeabilities inside water can be detected with high accuracy.     

A sensitive and selective sensor for dopamine determination based on a molecularly imprinted electropolymer of o-aminophenol

A simple and reliable method was proposed for preparing a selective dopamine (DA) sensor based on a molecularly imprinted electropolymer of o-aminophenol. The sensor is selective for the determination of DA in the presence of high concentrations of ascorbic acid (AA), with a maximum molar ratio of 1/1000. The molecular imprinted (MIP) sensor was tested by cyclic voltammetry (CV) as well as differential pulse voltammetry (DPV) to verify the changes in oxidative currents of ferricyanide. In optimized conditions, DA at concentrations of 2 × 10⁻⁸ to 0.25 × 10⁻⁶ mol/L could be determined with a detection limit of 1.98 × 10⁻⁹ mol/L (S/N = 3). The MIP sensor showed high selectivity, sensitivity, and reproducibility. Determination of DA in simulated samples of dopamine hydrochloride showed good recovery.     

Potentiometric sensor based on molecularly imprinted polymer for determination of melamine in milk

A polymeric membrane ion-selective electrode for determination of melamine is described in this paper. It is based on a molecularly imprinted polymer (MIP) for selective recognition, which can be synthesized by using melamine as a template molecule, methacrylic acid as a functional monomer and ethylene glycol dimethacrylate as a cross-linking agent. The membrane electrode shows near-Nernstian response (54 mV/decade) to the protonated melamine over the concentration range of 5.0 × 10⁻⁶ to 1.0 × 10⁻² mol L⁻¹. The electrode exhibits a short response time of 16 s and can be stable for more than 2 months. Combined with flow analysis system, the potentiometric sensor has been successfully applied to the determination of melamine in milk samples. Interference from high concentrations of ions co-existing in milk samples such as K⁺ and Na⁺ can be effectively eliminated by on-line introduction of anion- and cation-exchanger tandem columns placed upstream, while melamine existing as neutral molecules in milk of pH 6.7 can flow through the ion-exchanger columns and be measured downstream by the proposed electrode in an acetate buffer solution of pH 3.7.     

Bioluminescent sensor for naphthalene in air: Cell immobilization and evaluation with a dynamic standard atmosphere generator

A dynamic atmosphere generator with a naphthalene emission source has been constructed and used for the development and evaluation of a bioluminescence sensor based on the bacteria Pseudomonas fluorescens HK44 immobilized in 2% agar gel (10⁷ cell mL⁻¹) placed in sampling tubes. A steady naphthalene emission rate (around 7.3 nmol min⁻¹ at 27 °C and 7.4 mL min⁻¹ of purified air) was obtained by covering the diffusion unit containing solid naphthalene with a PTFE filter membrane. The time elapsed from gelation of the agar matrix to analyte exposure (“maturation time”) was found relevant for the bioluminescence assays, being most favorable between 1.5 and 3 h. The maximum light emission, observed after 80 min, is dependent on the analyte concentration and the exposure time (evaluated between 5 and 20 min), but not on the flow rate of naphthalene in the sampling tube, over the range of 1.8–7.4 nmol min⁻¹. A good linear response was obtained between 50 and 260 nmol L⁻¹ with a limit of detection estimated in 20 nmol L⁻¹ far below the recommended threshold limit value for naphthalene in air.     

Modified glassy carbon electrode with multiwall carbon nanotubes as a voltammetric sensor for determination of noscapine in biological and pharmaceutical samples

A simple and highly sensitive method is described for direct voltammetric determination of noscapine in blood and pharmaceutical sample. Glassy carbon electrode with effective method is modified with multiwall carbon nanotubes (MWNTs) to cause activation of multiwall carbon nanotubes structures for electrocatalyzes of noscapine oxidation. The cyclic voltammetric (CV) results indicated that MWNTs remarkably enhances electrocatalytic activity toward the oxidation of noscapine, which is leading to considerable improvement of anodic peak current for noscapine, and allows the development of a highly sensitive voltammetric sensor for detection of noscapine in pharmaceutical and clinical samples. Under the optimum condition, the calibration curve was linear in the concentration range 4.0 × 10⁻⁷–1.0 × 10⁻⁴ mol L⁻¹ with the detection limit of 8.0 × 10⁻⁸ mol L⁻¹ and relative standard deviation (R.S.D.%) lower than 1.0% (n = 5). Finally, some kinetic parameters were determined and multistep mechanism for oxidation of noscapine for first time was proposed.     

Modeling historical fecal coliform loadings to large European rivers and resulting in-stream concentrations

Information on fecal coliform (FC) concentrations in European rivers is scarce. The objective of this study was to identify hotspots of water pollution in Europe and provide information on the different FC sources and their contributions to the loads that lead to concentrations in rivers. Model simulations were carried-out with the large-scale water quality model WorldQual to assess the calculated loads regarding to its associated sources and to further estimate the related in-stream concentration. For the year 1995, model results indicated that FC loadings were higher in central Europe with 500 to above 2000 10¹⁰ cfu km⁻² a⁻¹ than in northern and eastern Europe where loadings ranged between 0 and 200 10¹⁰ cfu km⁻² a⁻¹. Major sources of FC loadings are domestic sewage, followed by scattered settlements (private treatment), urban surface runoff and manure application. Concentrations showed similar regional patterns as loadings, with high concentrations in central Europe and low concentrations in northern and eastern Europe.     

Evaluation of peach quality indices using an electronic nose by MLR, QPST and BP network

In this paper, responses of a gas sensor array were employed to establish a quality indices model evaluating the peach quality indices. The relationship between sensor signals and the firmness, the content of sugar (CS) and acidity of “Dabai” peach were developed using multiple linear regressions with stepwise procedure, quadratic polynomial step regression (QPST) and back-propagation network. The results showed that the multiple linear regression model represented good ability in predicting of quality indices, with high correlation coefficients (R² = 0.87 for penetrating force CF; R² = 0.79 for content of sugar CS; R² = 0.81 for pH) and relatively low average percent errors (ERR) (9.66%, 7.68% and 3.6% for CF, CS and pH, respectively). The quadratic polynomial step regression provides an accurate quality indices model, with high correlation (R² = 0.92, 0.87, 0.83 for CF, CS and pH, respectively) between predicted and measured values and a relatively low error (5.47%, 3.45%, 2.57% for CF, CS and pH, respectively) for prediction. The feed-forward neural network also provides an accurate quality indices model with a high correlation (R² = 0.90, 0.81, 0.87 for CF, CS and pH, respectively) between predicted and measured values and a relatively low average percent error (6.39%, 6.21%, 3.13% for CF, CS and pH, respectively) for prediction. These results prove that the electronic nose has the potential of becoming a reliable instrument to assess the peach quality indices.     

Environmentally friendly disposable sensors with microfabricated on-chip planar bismuth electrode for in situ heavy metal ions measurement

This paper presents an environmentally friendly disposable heavy metal ion sensor for in situ and online monitoring in the nature and physiological systems. The miniaturized sensor chip consists of a non-toxic microfabricated bismuth (Bi) working electrode that replaces the conventional mercury electrodes, an integrated Ag/AgCl reference electrode, a gold counter electrode, and microfluidic channels. In this work, the electrochemical behavior of the Bi working electrode was characterized in several non-deaerated buffer solutions using cyclic voltammetry. The detection and quantification of Pb (II) and Cd (II) were statically performed using anodic stripping voltammetry inside the microchannels, in the Pb (II) concentration range of 25–400 ppb (R² = 0.991) with limit of detection of 8 ppb for 60 s deposition, and in the Cd (II) concentration range of 28–280 ppb (R² = 0.986) with limit of detection of 9.3 ppb for 90 s deposition. Particularly, the applications of this sensor chip have been reported with the examples of in situ measurement of Cd (II) concentration in soil pore and ground water and online direct measurement of Cd (II) concentration in cell culture media in its native environment.     

Simultaneous measurement of Pb, Cd and Zn using differential pulse anodic stripping voltammetry at a bismuth/poly(p-aminobenzene sulfonic acid) film electrode

A novel sensor was developed for simultaneous detection of Pb, Cd and Zn, based on the differential pulse anodic stripping response at a bismuth/poly(p-aminobenzene sulfonic acid) (Bi/poly(p-ABSA)) film electrode. This electrode was generated in situ by depositing simultaneously bismuth and the metals by reduction at −1.40 V on the poly(p-ABSA) modified electrode. Compared with the bismuth film electrode, the Bi/poly(p-ABSA) film electrode can yield a larger stripping signal for Pb, Cd and Zn. Under the optimum conditions, a linear response was observed for Cd and Zn in the range from 1.00 to 110.00 μg L⁻¹ and for Pb in the range from 1.00 to 130.00 μg L⁻¹. The detection limits of Pb(II), Cd(II) and Zn(II) were 0.80, 0.63 and 0.62 μg L⁻¹, respectively. Finally this sensor had been applied to the simultaneous determination of Pb(II), Cd(II) and Zn(II) in river water samples and the results were quite corresponding to the value obtained by atomic absorption spectrometry.     

Packaging effect investigation of WL-CSP with a central opening: A case study on pressure sensors

This study reports the packaging effects of wafer-level chip scale packaging (WL-CSP) with a central opening on piezoresistive pressure sensors. A regular pressure sensor with calculated sensitivity of 3.1 × 10^?2 mVV^?1 kPa^?1 and a sensitive pressure sensor with calculated sensitivity of 32.0 × 10^?2 mVV^?1 kPa^?1 are investigated. A finite element (FE) model validated by experimental measurements is used to explore the sensing characteristics of the pressure sensors. The results show that the output variation of the packaged pressure sensor is dominated by the CTE mismatch not the piezoresistive coefficient change as temperature varies. WL-CSP with small polyimide (PI) thickness and large PI opening produces small packaging induced stress, making it ideal for precision sensing for both regular and sensitive pressure sensors.     

Detection of ammonia in the ppt range based on a composite optical waveguide pH sensor

An optical waveguide (OWG) pH sensor with two thin guiding layers (composite OWG) was fabricated, and its application to sensing extremely low concentrations of ammonia was demonstrated. The highly sensitive element based on a titanium dioxide (TiO₂) film was deposited onto the surface of a potassium ion (K⁺) exchanged glass OWG by RF sputtering. The surface of the TiO₂ film was coated with a thin film of a pH indicator dye (bromothymol blue, BTB) by spin coating. With optimum thickness of BTB film at about 46 nm and of TiO₂ films at 18–20 nm, this system proved to be an extremely sensitive ammonia sensor. The experimental results of the optimum conditions on BTB and TiO₂ film thicknesses were close to theoretically calculated values. The sensor easily detected 1 parts per trillion (ppt) ammonia reversibly, and had a short response time. The present sensor is also characterized by low cost, simple structure and facile fabrication.     

An amperometric ethanol sensor based on a Pd–Ni/SiNWs electrode

A new amperometric ethanol sensor has been developed. The sensor uses the silicon nanowires covered with co-deposited palladium–nickel (Pd–Ni/SiNWs) as the working electrode. The detection of ethanol concentration is based on the response currents resulted from the electro-catalytic oxidation of ethanol. The performance of the sensor was characterized by cyclic voltammetry and fixed potential amperometry techniques. In 1 M KOH solution containing different ethanol concentrations, the sensor shows a good sensitivity of 7.48 mA mM⁻¹ cm⁻² and the corresponding detection limit (signal-to-noise ratio = 3) of 6 μM for cyclic voltammetry. Meanwhile, it also displays a sensitivity of 0.76 mA mM⁻¹ cm⁻² and the corresponding detection limit of 10 μM for fixed potential amperometry. The results demonstrate that the Pd–Ni/SiNWs electrodes are potential as the electrochemical integrated sensors for ethanol detection.