Design and fabrication of an alternating dielectric multi-layer device for surface plasmon resonance sensor

An alternating dielectric multi-layer device was fabricated and tested in the laboratory to show that dielectric mirrors of alternating high/low refractive index materials, based on the design of distributed Bragg reflector (DBR) for vertical cavity surface emission lasers (VCSELs), can be used in designing SPR biochemical sensors. The thickness, number of layers, and other design parameters of the device used were optimized using optical admittance loci analysis. The proof-of-concept device was fabricated with a symmetrical structure using Au/(SiO₂/TiO₂)₄/Au.Using a 632 nm-wavelength light source on a BK7 coupling prism, our laboratory tests showed that, under water, there was an 11.5° shift in resonant peak position towards the critical angle (from 74° in a conventional single-layer Au film), and a 3.25 times decrease in FWHM (the half-peak width). Our design also resulted in a wider dynamic range of up to a 1.50 refractive index unit (RIU), compared to 1.38 RIU in a conventional single-layer Au film. Using glucose solutions in ddH₂O, the calculated resolution was 1.28 × 10⁻⁵. The calculated intensity sensitivity was 10 000 a.u./RIU, about twice the improvement over the conventional single-layer Au film.     

A novel multisensing optical approach based on a single phthalocyanine thin films to monitoring volatile organic compounds

This paper reports the use of optical trasduction techniques to characterise solid state chemo-optical sensors prepared by Langmuir–Schäfer technique (LS) in thin film form based onto Cu(II) tris-(2,4-di-t-amylphenoxy)-(12-hydroxy-1,4,7,10-tetraoxadodecyl)-phthalocyanine macromolecules CuPcOH as active layers. The study consists in the UV–vis optical absorption monitoring of the active LS layers in the presence of specific five volatile organic compounds (VOCs) mixed in dry air in controlled atmosphere; in particular tert-butylamine, methanol, ethanol, hexane and ethyl acetate, all analytes of interests in the food quality control. The UV–vis spectra have been monitored by recording the dynamic variation in the integral of the absorbance curves in well defined spectral regions: 300–400 nm, 550–600 nm, 600–640 nm, 640–700 nm, covering the whole spectrum and centred around the typical absorption bands of phthalocyanine thin films. This simultaneous UV–vis four channel monitoring allowed to use only one active layer as sensing element where each selected spectral region generates independent sensors. The dependency of the above mentioned outputs towards the analytes has been discussed. A base optical characterisation of the investigated LS thin films has been performed.     

Investigations of prussian blue films using surface plasmon resonance

A fiber-optic based surface plasmon resonance (SPR) instrument is used to characterize electrodeposited prussian blue (PB) films. PB films are deposited potentiostatically or galvanostatically onto the gold sensing surface. When reduced to prussian white (PW), the film undergoes a reversible refractive index change that is observed using SPR. The strong absorbance of PB films within the sensing region of the instrument inhibits quantitative characterization of these films; however qualitative characterization has been achieved. The PB coated SPR probe can be used as an optical transducer of electrochemical phenomena. A PB film is deposited on the gold SPR sensing surface; the probe is submerged in 0.1 M KNO₃ in a tube terminated with a porous vycor frit. The isolated PB half-cell and a AgCl-coated Ag wire are placed in a beaker containing 0.1 M KNO₃ and connected by a variable battery. Addition of chloride to the bulk solution changes the potential of the AgCl wire, which induces a change in the PB film that can be observed using SPR. A change in chloride concentration of 40 μM can be detected using this system.     

Electrical properties of nickel oxide thin films for flow sensor application

In this work, Ni oxide thin films, with thermal sensitivity superior to Pt and Ni thin films, were formed through annealing of Ni films deposited by a r.f. magnetron sputtering. The annealing was carried out in the temperature range of 300–500 °C under atmospheric conditions. Resistivity of the resulting Ni oxide films were in the range of 10.5 μΩ cm/°C to 2.84 × 10⁴ μΩ cm/°C, depending on the extent of Ni oxidation. The temperature coefficient of resistance (TCR) of the Ni oxide films also depended on the extent of Ni oxidation; the average TCR of Ni oxide resistors, measured between 0 and 150 °C, were 5630 ppm/°C for the 300 °C and 2188 ppm/°C for 500 °C films. Because of their high resistivity and very linear TCR, Ni oxide thin films are superior to pure Ni and Pt thin films for flow and temperature sensor applications.     

Development of an optical fibre reflectance sensor for copper (II) detection based on immobilised salicylic acid

Immobilized salicylic acid onto XAD-2 (styrene–divinylbenzene cross-linked copolymer) has been attempted in this study as a reagent phase for the development of an optical fibre copper (II) sensor. The measurements were carried out at a given wavelength of 690.27 nm since it yielded the largest divergence different in reflectance spectra before and after reaction with the analyte element. The optimum response was obtained at pH 5.0. The linear dynamic range of Cu(II) was found within the concentration range of 1.0–2.0 mmol L⁻¹ with its LOD of 0.5 mmol L⁻¹. The sensor response from different probes (n = 9) gave an R.S.D. of 8.4% at 0.55 mmol L⁻¹ Cu(II). The effect of interfered ions at 1:1 molar ratio of Cu(II):foreign ion was also studied in this work.     

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.     

Gas sensing properties of ZnO thin films prepared by microcontact printing

In situ patterned zinc oxide (ZnO) thin films were prepared by precipitation of Zn(NO₃)₂/urea aqueous solution and by microcontact printing of self-assembled monolayers (SAMs) on Al/SiO₂/Si substrates. The visible precipitation of Zn(OH)₂ from the urea containing Zn(NO₃)₂ solution was enhanced by increasing the reaction temperature and the amount of urea. The optimized condition for the ZnO thin films was found to be the Zn(NO₃)₂/urea ratio of 1/8, the precipitation temperature of 80 °C, the precipitation time of 1 h and the annealing temperature of 600 °C, respectively. SAMs are formed by exposing Al/SiO₂/Si to solutions comprising of hydrophobic octadecylphosphonic acid (OPA) in tetrahydrofuran and hydrophilic 2-carboxylethylphosphonic acid (CPA) in ethanol. The ZnO thin film was then patterned with the heat treatment of Zn(OH)₂ precipitated on the surface of hydrophilic CPA. The ZnO gas sensor was exposed to different concentrations of C₃H₈ (5000 ppm), CO (250 ppm) and NO (1000 ppm) at elevated temperatures to evaluate the gas sensitivity of ZnO sensors. The optimum operating temperatures of C₃H₈, CO and NO gases showing the highest gas sensitivity were determined to be 350, 400 and 200 °C, respectively.     

Orientation and thickness dependence of electrical and gas sensing properties in heteroepitaxial indium tin oxide films

Heteroepitaxial indium tin oxide (ITO) films were grown on three differently oriented yttria-stabilized zirconia (YSZ) substrates ((1 0 0), (1 1 0), (1 1 1)) by rf magnetron sputtering, and their structural characteristics and electrical and gas sensing properties were investigated. The initially formed ITO exhibited an island structure on the very thin layer and became a continuous film after the prolonged deposition. The heteroepitaxial relationships between ITO films and YSZ substrates were confirmed by X-ray diffraction, pole figure, and high resolution transmission electron microscopy (HRTEM). The chemical composition, determined by X-ray photoelectron spectroscopy (XPS), was slightly different at early stage depending on the substrate orientation, but it became similar after the longer deposition. Hall measurements indicated that the electrical resistivity of ITO films decreased with increasing the deposition time (or film thickness) irrespective of the film orientation. The ITO film deposited on (1 1 0) YSZ for 10 s showed the highest electrical resistivity. The gas sensor fabricated from the ITO film on (1 1 0) YSZ deposited for 10 s showed the highest NO₂ gas response at relatively low temperature (100 °C), which was attributed to the higher Sn concentration and higher surface roughness of that film.     

Mesoporous nanoparticle TiO2 thin films for conductometric gas sensing on microhotplate platforms

Mesoporous TiO₂ nanoparticle thin films were prepared on MEMS microhotplate (μHP) platforms and evaluated as high-sensitivity conductometric gas sensor materials. The nanoparticle films were deposited onto selected microhotplates in a multi-element array via microcapillary pipette and were sintered using the microhotplate. The films were characterized by optical and scanning electron microscopies and by conductometric measurements. The thin films were evaluated as conductometric gas sensors based on the critical performance elements of sensitivity, stability, speed and selectivity. The nanoparticle films were compared with compact TiO₂ films deposited via chemical vapor deposition (CVD) and the nanoparticle films were found to demonstrate higher sensitivity to target analytes. The nanoparticle films were also stable with regard to both baseline conductance and signal response over 60 h of continuous operation at high temperatures (up to 475 °C). Sensor response times were evaluated and the TiO₂ nanoparticle films showed fast responses to the presence of analyte (≈5 s) and a response-time dependence on the analyte concentration. Control of the sensor operating temperature, an inherent benefit of the microhotplate platform, was employed to demonstrate the selectivity of the nanoparticle films.     

Gas sensing behavior of polyvinylpyrrolidone-modified ZnO nanoparticles for trimethylamine

Gas sensors based on polyvinylpyrrolidone (PVP)-modified ZnO nanoparticles with different molar ratios of Zn²⁺: PVP were prepared by a sol–gel method. Morphology of the sensors was characterized by field emission-scanning electron microscopy (FE-SEM), which indicated that the sensor with a molar ratio of Zn²⁺: PVP = 5:5 showed uniform morphology. Moreover, the sensor exhibited fairly excellent sensitivity and selectivity to trimethylamine (TMA). The response and recovery time of the sensor were 10 and 150 s, respectively. Finally, the mechanism for the improvement in the gas sensing properties was discussed.     

Evaluation of response characteristics of resistive oxygen sensors based on porous cerium oxide thick film using pressure modulation method

In order to reduce the response time of resistive oxygen sensors using porous cerium oxide thick film, it is important to ascertain the factors controlling response. Pressure modulation method (PMM) was used to find the rate-limiting step of sensor response. This useful method measures the amplitude of sensor output (H(f)) for the sine wave modulation of oxygen partial pressure at constant frequency (f). In PMM, “break” response time, which is minimum period in which the sensor responds precisely, can be measured. Three points were examined: (1) simulated calculations of PMM were carried out using a model of porous thick film in which spherical particles are connected in a three-dimensional network; (2) sensor response speed was experimentally measured using PMM; and (3) the diffusion coefficient and surface reaction coefficient were estimated by comparison between experiment and calculation. The plot of log f versus log H(f) in the high f region was found to have a slope of approximately −0.5 for both porous thick film and non-porous thin film, when the rate-limiting step was diffusion. Calculations showed the response time of porous thick film was 1/20 that of non-porous thin film when the grain diameter of the porous thick film was the same as the thickness of non-porous thin film. At 973 K, “break” response time (t_b) of the resistive oxygen sensor was found by experiment to be 109 ms. It was concluded that the response of the resistive oxygen sensor prepared in this study was strongly controlled by diffusion at 923–1023 K, since the experiment revealed that the slope of plot of log f versus log H(f) in the high f region was approximately −0.5. At 923–1023 K, the diffusion coefficient of oxygen vacancy in porous ceria (D_V) was expressed as follows: D_V (m²s⁻¹) = 5.78 × 10⁻⁴ exp(−1.94 eV/kT). At 1023 K, the surface reaction coefficient (K) was found to exceed 10⁻⁴ m/s.     

Liquid crystalline phthalocyanine spun films for organic vapour sensing

Vapour sensing properties of spin-coated films of mesogenic octa-substituted phthalocyanine derivatives MPcR₈, where M = Ni(II), R = –S(CH₂)₁₁CH₃, –SCH(CH₂OC₁₂H₂₅)₂, –S(CH₂CH₂O)₃CH₃ were investigated using spectroscopic ellipsometry, Raman spectroscopy and surface plasmon resonance (SPR) technique. Solvent molecules with saturated CC bonds such as chloroform are found to have formed hydrogen bonds with alkyl chains of the substituents of phthalocyanine molecules. The sensor response to aromatic compounds such as benzene is, on the other hand, believed to be the consequence of their π–π interaction with the conjugated phthalocyanine ring.     

Moisture sensor at glass/polymer interface for monitoring of photovoltaic module encapsulants

A sensor developed for measurement of water concentration inside glass/polymer encapsulation structures with a particular application area in accelerated aging of photovoltaic module encapsulants is described. An approximately 5 μm thick porous TiO₂ film applied to a glass substrate with a conductive coating acts as the moisture-sensitive component. The response is calibrated with weather chamber experiments for sensors open to the environment and with diffusion experiments for sensors laminated under an encapsulant. For the interpretation of diffusion experiment results, a transport model describing the diffusion of water across the polymer/TiO₂ interface is developed. The logarithm of AC resistance shows a linear dependence on water concentration in both open and encapsulated calibration. The first measurable response from an encapsulated 3.5 mm × 8 mm size sensor is obtained when approximately 10 μg of water has entered the film. Implications of the calibration results for sensor usage in accelerated aging tests are discussed.     

Fluorescence detection of nitrogen dioxide with perylene/PMMA thin films

Thin films of polymethylmethacrylate (PMMA) doped with perylene provide selective, robust and easily prepared optical sensor films for NO₂ gas with suitable response times for materials aging applications. The materials are readily formed as 200 nm thin spin cast films on glass from chlorobenzene solution. The fluorescence emission of the films (λ_max=442 nm) is quenched upon exposure to NO₂ gas through an irreversible reaction forming non-fluorescent nitroperylene. Infrared, UV–VIS and fluorescence spectroscopies confirmed the presence of the nitro adduct in the films. In other atmospheres examined, such as air and 1000 ppm concentrations of SO₂, CO, Cl₂ and NH₃, the films exhibited no loss of fluorescence intensity over a period of days to weeks. Response curves were obtained for 1000, 100 and 10 ppm NO₂ at room temperature with equilibration times varying from hours to weeks. The response curves were fit using a numerical solution to the coupled diffusion and a nonlinear chemical reaction problem assuming that the situation is reaction limiting. The forward reaction constant fitted to experimental data was k_f∼0.06 (ppm min)⁻¹.     

Flexible vapour sensors using single walled carbon nanotubes

Thin, strongly adhering films of single-walled carbon nanotube bundles (SWNT) on flexible substrates such as poly(ethyleneterephthalate) (PET) were used for vapour sensing (hexane, toluene, acetone, chloroform, acetonitrile, methanol, water, etc.). These sensors are extremely easy to fabricate using the line patterning method. For example, ‘4-probe’ sensor patterns are drawn on a computer and then printed on overhead transparency (PET) sheets. These PET patterns were coated with films of electronically conductive SWNT bundles (1–2 μm thick) by dip-coating in aqueous surfactant-supported dispersions and mounted in glass chambers equipped for vapour sensing. Experiments conducted under saturated vapour conditions in air showed sensor responses that correlated well with solvent polarity [E_T(30) scale]. Similar results were obtained under controlled vapour conditions (no air) at 10,000 ppm. Control experiments using films of carbon black on PET (Aquadag-E^®), also prepared by the line patterning method, showed very little response to vapours under identical experimental conditions. The sensors are very flexible, e.g., they can be bent to diameters as small as 10 mm without significantly compromising sensor function.     

Development of a mercury optical sensor based on immobilization of 4-(2-pyridylazo)-resorcinol on a triacetylcellulose membrane

A new optical sensor for mercury(II) ions is developed based on immobilization of 4-(2-pyridylazo)-resorcinol (PAR) on a triacetylcellulose membrane. Chemical binding of Hg²⁺ ions in solution with a PAR immobilized on the triacetylcellulose surface could be monitored spectrophotometrically at 525 nm. The optode shows excellent response over a wide concentration range of 5–3360 μM Hg(II) with a limit of detection of 1.5 μM Hg(II). The influence of factors responsible for the improved sensitivity of the sensor were studied and identified. The response time of the optode was 20 min for a stable solution, and was 15 min for a stirrer solution. The influence of potential interfering ions on the determination of 5 × 10⁻⁵ M Hg(II) was studied. The sensor was applied for determination of Hg(II) in water samples.     

Design and performance of a microcantilever-based hydrogen sensor

This paper describes the design of, and the effects of basic environmental parameters on, a microelectromechanical (MEMS) hydrogen sensor. The sensor contains an array of 10 micromachined cantilever beams. Each cantilever is 500 μm wide×267 μm long×2 μm thick and has a capacitance readout capable of measuring cantilever deflection to within 1 nm. A 20-nm-thick coating of 90% palladium–10% nickel bends some of the cantilevers in the presence of hydrogen. The palladium–nickel coatings are deposited in ultra-high-vacuum (UHV) to ensure freedom from a “relaxation” artifact apparently caused by oxidation of the coatings. The sensor consumes 84 mW of power in continuous operation, and can detect hydrogen concentrations between 0.1 and 100% with a roughly linear response between 10 and 90% hydrogen. The response magnitude decreases with increasing temperature, humidity, and oxygen concentration, and the response time decreases with increasing temperature and hydrogen concentration. The 0–90% response time of an unheated cantilever to 1% hydrogen in air is about 90 s at 25 °C and 0% humidity.     

Fabrication and characterization of polyaniline-based gas sensor by ultra-thin film technology

Pure polyaniline (PAN) film, polyaniline and acetic acid (AA) mixed film, as well as PAN and polystyrenesulfonic acid (PSSA) composite film with various number of layers were prepared by Langmuir–Blodgett (LB) and self-assembly (SA) techniques. These ultra-thin films were characterized by ultraviolet–visible (UV–VIS) spectroscopy and ellipsometry. It is found that the thickness of PAN-based ultra-thin films increases linearly with the increase of the number of film layers. The gas-sensitivity of these ultra-thin films with various layers to NO₂ was studied. It is found that pure polyaniline films prepared by LB technique had good sensitivity to NO₂, while SA films exhibited faster recovery property. The response time to NO₂ and the relative change of resistance of ultra-thin films increased with the increase of the number of film layers. The response time of three-layer PAN film prepared by LB technique to 20 ppm NO₂ was about 10 s, two-layer SA film was about 8 s. The mechanism of sensitivity to NO₂ of PAN-based ultra-thin films was also discussed.     

All-optical hydrogen sensor based on a high alloy content palladium thin film

Optical reflectance measurements were performed to determine the hydrogen response characteristics of 20 nm thick Pd_0.6Au_0.4 films. The response time and signal change characteristics were determined as a function of hydrogen concentrations ranging from 0.05% to 4% in a balance of dry CO₂ free air. The detection limit was determined to be 0.05%, with a corresponding response time of 130 s, while at 4% hydrogen concentrations the response time was 5 s at ambient temperatures. A linear decrease of both the signal change and response time was measured within an operating temperature range between 25 °C and 100 °C for a 1% hydrogen in air gas mixture. The sensor response dependence of the Pd_0.6Au_0.4 film with a change in humidity was determined between ambient levels and 95% relative humidity (RH). While the signal change was independent of humidity the response time increased due to water adsorption on the Pd alloy sensing layer. A similar increase in response time was shown for 100 ppm of background CO mixed with 1% hydrogen in nitrogen at room temperature. At an elevated operating temperature of 80 °C, 100 ppm of CO did not affect the sensor response towards 1% hydrogen in a balance of nitrogen. Reliability tests have been performed over a 1-year time period and the sensing specifications have not drifted beyond 2% and 13% of the calibrated signal change and response time, respectively. A response time on the order of seconds and the proven stability of the high alloy content Pd thin film demonstrate the promising attributes of this material for use in an all-optical hydrogen sensor.     

Spin-valve planar Hall sensor for single bead detection

The planar Hall effect (PHE) sensor with a junction size of 3 μm × 3 μm for a single micro-bead detection has been fabricated successfully using a typical spin-valve thin film Ta(5)/NiFe(16)/Cu(1.2)/NiFe(2)/IrMn(15)/Ta(5) nm. The PHE sensor exhibits a sensitivity of about 7.2 μV Oe^?1 in the magnetic field range of ±7 Oe approximately. We have performed an experiment to illustrated the possibility of single micro-bead detection by using a PHE sensor. A single micro-bead of 2.8 μm diameter size is secluded from 0.1% dilute solution of the Dynabeads^® M-280 dropped on the sensor surface and is located on the sensor junction by using a micro magnetic needle. The comparison of the PHE voltage profiles in the field range from 0 to 20 Oe in the absence and presence of a single micro-bead identifies a single Dynabeads^® M-280, the maximal signal change as large as ΔV ～ 1.1 μV can be obtained at the field ～6.6 Oe. The results are well described in terms of the reversal of a basic single domain structure.