Silicon Transducers Research Articles

Thermoreflectance-based thermometry of silicon thin films with resonantly enhanced temperature sensitivity.

We demonstrate a thermoreflectance-based thermometry technique with an ultimate temperature resolution of 60 µK in a 2.6 mHz bandwidth. This temperature resolution was achieved using a 532 nm-wavelength probe laser and a ∼1 µm-thick silicon transducer film with a thermoreflectance coefficient of -4.7 × 10-3 K-1 at room temperature. The thermoreflectance sensitivity reported here is over an order-of-magnitude greater than that of metal transducers, and is comparable to the sensitivity of traditional resistance thermometers. Supporting calculations reveal that the enhancement in sensitivity is due to optical interference in the thin film.

The influence of illumination and ionic strength of a solution on the formation of biosensor structure based on a silicon substrate and glucose oxidase molecules

We report on a change in parameters of a sensitive layer of field effect biosensor based on a silicon transducer and glucose oxidase (GOx) enzyme molecules. Change in the parameters of the enzyme layer occurs due to the white-light illumination of a Si substrate during the GOx adsorption. It was found that the addition of certain value of NaCl salt concentration, which depends on the presence of a polyethylenimine buffer layer on the semiconductor surface, led to a more prominent effect of photo-assisted adsorption of GOx molecules.

Toward Multi-Parametric Porous Silicon Transducers Based on Covalent Grafting of Graphene Oxide for Biosensing Applications.

Graphene oxide (GO) is a two-dimensional material with peculiar photoluminescence emission and good dispersion in water, that make it an useful platform for the development of label-free optical biosensors. In this study, a GO-porous silicon (PSi) hybrid device is realized using a covalent chemical approach in order to obtain a stable support for biosensing applications. Protein A, used as bioprobe for biosensing purposes, is covalently linked to the GO, using the functional groups on its surface, by carbodiimide chemistry. Protein A bioconjugation to GO-PSi hybrid device is investigated by atomic force microscopy (AFM), scanning electron microscopy (SEM), water contact angle (WCA) measurements, Fourier transform infrared (FTIR) spectroscopy, steady-state photoluminescence (PL), and fluorescence confocal microscopy. PSi reflectance and GO photoluminescence changes can thus be simultaneously exploited for monitoring biomolecule interactions as in a multi-parametric hybrid biosensing device.

Label-free detection of the IL-6 and IL-8 interleukines through monolithic silicon photonic chips and simultaneous dual polarization optics

A silicon microphotonic chip with ten monolithically integrated interferometers and an equal number of light sources is employed for the label-free and multiplexed detection of two proteins of the cytokine family. The chip implements broad-band Mach-Zehnder interferometry and is used for the detection of the IL-6 and IL-8 interleukins. The silicon transducer is made through mainstream silicon technology and integrates avalanche diode based white light LEDs and monomodal waveguides forming Mach-Zehnder interferometers. The light spectrum is modulated through the interferometer and the spectrally resolved interference fringes are monitored by a digital spectrometer. Biomolecular built-up on the sensing arm causes pattern shifts that are evaluated through Fourier Transform. The microsystem level set-up includes microfluidic components, direct contacting of the chip contact pads through spring loaded pins and alignment guides and a spectrometer. The dual polarization optics allows the simultaneous derivation of the TE and TM sensitivity plots and the calculation of the refractive indices of either protein.

Microwave complex permittivity of voltage-tunable nematic liquid crystals measured in high resistivity silicon transducers

A split post dielectric resonator is used to determine the effect of AC bias voltage on the microwave complex permittivity of nematic liquid crystals. High resistivity silicon transducers separated by 100 μm are bridged by nematic liquid crystals and their properties determined. The in-plane permittivity of liquid crystals can be effectively tuned (change of effective permittivity of up to 8%) by increasing the AC bias voltage from 0 to 6 V. Using high resistivity silicon allowed us to obtain tunable dielectric stacks with relatively small dielectric losses at microwave frequencies.

Multifrequency Interrogation of Nanostructured Gas Sensor Arrays: A Tool for Analyzing Response Kinetics

This paper presents a unique perspective on enhancing the physicochemical mechanisms of two distinct highly sensitive nanostructured metal oxide micro hot plate gas sensors by utilizing an innovative multifrequency interrogation method. The two types of sensors evaluated here employ an identical silicon transducer geometry but with a different morphological structure of the sensitive film. While the first sensing film consists of self-ordered tungsten oxide nanodots, limiting the response kinetics of the sensor-chemical species pair only to the reaction phenomena occurring at the sensitive film surface, the second modality is a three-dimensional array of tungsten oxide nanotubes, which in turn involves both the diffusion and adsorption of the gas during its reaction kinetics with the sensitive film itself. By utilizing the proposed multifrequency interrogation methodology, we demonstrate that the optimal temperature modulation frequencies employed for the nanotubes-based sensors to selectively detect hydrogen, carbon monoxide, ethanol, and dimethyl methyl phosphonate (DMMP) are significantly higher than those utilized for the nanodot-based sensors. This finding helps understand better the amelioration in selectivity that temperature modulation of metal oxides brings about, and, most importantly, it sets the grounds for the nanoengineering of gas-sensitive films to better exploit their practical usage.

Interferometric porous silicon transducers using an enzymatically amplified optical signal

This paper describes a novel transducer principle converting biomolecular binding events into changes in porous silicon structural colour. An enzyme-catalysed reaction, horseradish peroxidase (HRP) mediated oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB), induced an amplified optical detection signal on an antibody-functionalised porous silicon film, which manifested as significant changes in the Fabry–Pérot fringe pattern. Exposing porous silicon to non-oxidised TMB and oxidised TMB products and measuring the resulting effective optical thickness (EOT) changes of the porous silicon layer, an amplified signal from porous silicon was found, caused by an intermediate radical cation produced during the enzymatic-catalysed oxidation of TMB. Finally, it was shown that this system could be used to detect a human immunoglobulin (IgG) at 0.2 μg/ml. This work may lead to the development of new biosensors where porous silicon acts as the sensing matrix and transducer element.

Applications of porous silicon thin films in solar cells and biosensors

An overview of the applications of porous silicon (PS) thin films, as antireflection coatings (ARC) in silicon solar cells and transducers in biosensors, is presented. The reflectance spectra of PS films have been compared with other conventional ARCs (such as SiNx TiO2/MgF2 and ZnS), and optimal PS ARC with minimum reflectance has been obtained. The implementation of PS into an industrially compatible screen-printed (SP) solar cell by both the electrochemical etching (ECE) and chemical etching (CE) methods are reviewed. Porous silicon films, formed via ECE for short anodization times, on textured n+ emitter ofc-Si solar cell having SP front and back contacts, lead to improvements in the performance of solar cells and demonstrate their viability in industrial applications.

Integration of a Chemical‐Responsive Hydrogel into a Porous Silicon Photonic Sensor for Visual Colorimetric Readout

The incorporation of a chemo-responsive hydrogel into a 1D photonic porous silicon (PSi) transducer is demonstrated. A versatile hydrogel backbone is designed via the synthesis of an amine-functionalized polyacrylamide copolymer where further amine-specific biochemical reactions can enable control of cross-links between copolymer chains based on complementary target-probe systems. As an initial demonstration, the incorporation of disulfide chemistry to control cross-linking of this hydrogel system within a PSi Bragg mirror sensor is reported. Direct optical monitoring of a characteristic peak in the white light reflectivity spectrum of the incorporated PSi Bragg mirror facilitates real-time detection of the hydrogel dissolution in response to the target analyte (reducing agent) over a timescale of minutes. The hybrid sensor response characteristics are shown to systematically depend on hydrogel cross-linking density and applied target analyte concentration. Additionally, effects due to responsive hydrogel confinement in a porous template are shown to depend on pore size and architecture of the PSi transducer substrate. Sufficient copolymer and water is removed from the PSi transducer upon dissolution and drying of the hydrogel to induce color changes that can be detected by the unaided eye. This highlights the potential for future development for point-of-care diagnostic biosensing.

Gas sensing properties of MWCNTs decorated with gold or tin oxide nanoparticles

Gas sensors based on oxygen plasma functionalised MWCNTs and plasma-treated nanotubes decorated either with gold nanoclusters or tin oxide nanoparticles were evaluated for the detection of NO2, CO and ethylene. The sensor active layers were deposited by airbrushing onto micro-machined silicon transducers. Sensitivity, linearity, selectivity, response and recovery times and humidity effect were studied. XPS and TEM were employed to analyse the gas sensitive films. Among the different sensors tested, those based on tin oxide decorated MWCNTs showed the highest sensitivity to NO2 (at ppb level) and the lowest humidity cross-sensitivity when operated at room temperature.

Surfactant cytotoxicity assay based on a silicon transducer

A cytotoxicity assay for surfactants was developed using a potentiometric silicon transducer and Lactobacillus plantarum (NBRC3070), and the results are reported here. L. plantarum was used as the test microorganism for assaying the cytotoxicity of the surfactants. The extracellular pH change resulting from the metabolic pathway was measured by a light-addressable potentiometric sensor. This potentiometric sensor system measured the acidification of the L. plantarum medium; acidification resulting from metabolic activity was inhibited by the toxic activity of the surfactant. Results indicated that the anionic surfactant did not exhibit inhibitory activity in either cell growth or extracellular acidification until 0.2 mM. By contrast, the inhibition activity of cationic surfactants toward cellular growth and metabolic activity was greater than that of anionic surfactants. These results are consistent with those from the growth inhibition assay.

Selective Detection of Live Pathogens via Surface-Confined Electric Field Perturbation on Interdigitated Silicon Transducers

Detection of physical changes of cells is emerging as a new diagnostic approach to determine their phenotypical features. One of such changes is related to their viability; live (viable) cells are more voluminous than the dead ones, and monitoring this parameter in tissue cells becomes essential in fields such as drug discovery and hazard evaluation. In the area of pathogen detection, an analytical system capable of specifically detecting viable cells with the simple sample preparation and detection process would be highly desirable since live microorganisms can rapidly increase their numbers even at extremely low concentration and become a severe health risk. However, current sensing strategies cannot clearly determine the viability of cells, and hence they are susceptible to false-positive signals from harmless dead pathogens. Here we developed a robust electronic immunoassay that uses a pair of polycrystalline silicon interdigitated electrodes for the rapid detection of pathogens with high specificity for live cells. After bacterial cells were specifically anchored to the surface of the antibody-modified electrode, the characteristic geometry of the transducer enables the selective detection of viable cells with a limit of detection of 3 x 10(2) cfu/mL and an incubation time of only 1 h. The CMOS compatible fabrication process of the chip along with the label-free, reagent-less electronic detection and the easy electrode regeneration to recycle for another impedance measurement make this approach an excellent candidate for oncoming economical in-field viable-cell detection systems, fully integrable with sophisticated signal processing circuits.

Capacitive Sensing of Amino Acids Using Caliraxene‐Coated Silicon Transducers

AbstractThe possibility of using capacitance and flat band voltage as measurable quantities for determining amino acids that are neither electroactive nor with strong UV‐vis absorption has been explored. The sensors were fabricated by immobilizing calixarene derivatives on Si/SiO2/Si3N4 transducers. The measurements were made in sulfuric acid media of ca. pH 1 and in physiological buffer of pH 7.4. The different calixarene derivatives showed varying sensitivities to the amino acids ranging from 8 to 137 mV/decade.

Progress in electroacoustic transducer research

Manfred Schroeder always had a vivid interest in the field of electroacoustic transducers. During his time as Director of Acoustics Research at Bell Laboratories, work on electret transducers was started by Jim West and the author in 1962 and the activities were extended over the years to electret materials, directional microphones, and microphone signal processing. Schroeder himself made some interesting contributions to this field, such as the design of directional microphones based on gradients of various orders with toroidal and unidirectional directivity patterns. Some of these devices were implemented with electret transducers. Work on directional microphones at Bell Labs continued as the author went to the Darmstadt University of Technology. There, studies on silicon transducers were initiated, with the first silicon condenser microphone described in 1983. Recently, a completely new and promising variety of piezoelectric transducers was implemented, consisting of charged cellular polymers. This is a new modification of the electret transducer principle developed in the 1960s at Bell Laboratories.

Laser-scanned silicon transducer (LSST) as a multisensor system

A multisensor with a four-sectional membrane based on two Li- and two K-ionophores with different selectivities has been developed based on the laser-scanned silicon transducer (LSST), where the responses of the sensor sections have been measured by scanning the potentials at different points on the membrane surface. An artificial neural network (ANN) was employed for the analysis of mixed solutions. It has been demonstrated that by applying this hybrid sensor as a laser-scanned multisensor system, the concentration regions for the simultaneous determination of lithium and potassium ions in complex solutions have been enlarged significantly. It has been shown that using such a kind of multisensor system makes it possible to overcome the restriction of the selectivity in comparison with single sensors.

Impedance-based detection of DNA sequences using a silicon transducer with PNA as the probe layer.

Electrochemical impedance measurements were used for the detection of single-strand DNA sequences using a peptide nucleic acid (PNA) probe layer immobilized onto Si/SiO2 chips. An epoxysilane layer is first immobilized onto the Si/SiO2 surface. The immobilization procedure consists of an epoxide/amine coupling reaction between the amino group of the PNA linker and the epoxide group of the silane. A 20-nucleotide sequence of PNA was used. Impedance measurements allow for the detection of the changes in charge distribution at the oxide/solution interface following modifications to the oxide surface. Due to these modifications, there are significant shifts in the semiconductor's flat-band potential after immobilization and hybridization. The results obtained using this direct and rapid approach are supported by fluorescence measurements according to classical methods for the detection of nucleic acid sequences.

The double K +/Ca 2+ sensor based on laser scanned silicon transducer (LSST) for multi-component analysis

In the present work a double ion sensor based on a laser scanned semiconductor transducer (LSST) for the simultaneous determination of K +- and Ca 2+-ions in solutions has been developed. Specially elaborated ion-sensitive membrane compositions based on valinomycin and calcium ionophore calcium bis[4-(1,1,3,3-tetramethylbutyl)phenyl] phosphate (t-HDOPP-Ca) were deposited as separate layers on a silanized surface of the Si/SiO 2/Si 3N 4-transducer. The proposed multi-sensor exhibits theoretical sensitivities and the detection limits of the sensor were found to be 2×10 −6 mol l −1 for K + and 5×10 −6 mol l −1 for Ca 2+. The elaborated double sensor is proposed for the first time as a prototype of a new type of multi-sensor systems for chemical analysis.

Transfer of mixed protein–fatty acid LB films onto Si/SiO2 substrates. Influence of the surface free energy

In order to use mixed enzyme–fatty acid Langmuir–Blodgett (LB) films for the preparation of glucose biosensors based on silicon transducers, the transfer of such mixed LB layers on Si/SiO2 substrates is studied in this paper. The kinetics of the adsorption of the glucose oxidase (GOx) at the air–water interface is studied as a function of the behenic acid surface concentration. The pressure–area isotherms of pure behenic acid film and of mixed enzyme–behenic acid film at the air–water interface are compared, showing an insertion of the enzyme in the behenic acid film and a smooth phase transition for this mixed film.The influence of the surface free energy of the Si/SiO2 substrate is studied in detail. Three surface states are compared: untreated thermal silica, hydroxylated silica and silanized silica. The higher transfer coefficients of the mixed LB film are obtained with silanized silica which presents the lower values of the electron donor component of the surface free energy. The transferred mixed LB films are characterized by AFM and FTIR techniques.

The Hybrid K+/Ca2+ Sensor Based on Laser Scanned Silicon Transducer for Multi-component Analysis

In the present work a hybrid ion sensor based on the light-addressable potentiometric sensor (LAPS) principle for the simultaneous determination of K and Ca-ions in solutions has been developed. Specially elaborated ion-sensitive membrane compositions based on valinomycin and calcium ionophore t-HDOPP-Ca were deposited on a silanized surface of a Si/SiO2/Si3N4-transducer. The hybrid multi-sensor exhibits a sensitivity of 57±1 mV/pK and 27±1 mV/pCa in the concentration ranges from 10 to 10 mol/l. The detection limits of the sensor was found to be 2⋅10 mol/l for K and 5⋅10 mol/l for Ca. The elaborated hybrid multi-sensor is proposed as a basic element for realizing a multi-sensor analyzer, which is referred to as “electronic tongue”.

Piezoresistive effect in metal–semiconductor–metal structures on p-type GaN

We report on a strong piezoresistive effect in metal–semiconductor–metal structures fabricated on p-type GaN. The maximum measured gauge factor was 260, which is nearly two times larger than for piezoresistive silicon transducers. We attribute this large sensitivity to applied strain to the combination of two mechanisms: (i) a high piezoresistance of bulk p-GaN and (ii) a strong piezoresistive effect in a Schottky contact on p-GaN. The obtained results demonstrate that GaN-based structures can be suitable for stress/pressure sensor applications.