Direct optical sensors: principles and selected applications

In the field of bio and chemosensors a large number of detection principles has been published within the last decade. These detection principles are based either on the observation of fluorescence-labelled systems or on direct optical detection in the heterogeneous phase. Direct optical detection can be measured by remission (absorption of reflected radiation, opt(r)odes), by measuring micro-refractivity, or measuring interference. In the last case either Mach–Zehnder interferometers or measurement of changes in the physical thickness of the layer (measuring micro-reflectivity) caused, e.g., by swelling effects in polymers (due to interaction with analytes) or in bioassays (due to affinity reactions) also play an important role. Here, an overview of methods of microrefractometric and microreflectometric principles is given and benefits and drawbacks of the various approaches are demonstrated using samples from the chemo and biosensor field. The quality of sensors does not just depend on transduction principles but on the total sensor system defined by this transduction, the sensitive layer, data acquisition electronics, and evaluation software. The intention of this article is, therefore, to demonstrate the essentials of the interaction of these parts within the system, and the focus is on optical sensing using planar transducers, because fibre optical sensors have been reviewed in this journal only recently. Lack of selectivity of chemosensors can be compensated either by the use of sensor arrays or by evaluating time-resolved measurements of analyte/sensitive layer interaction. In both cases chemometrics enables the quantification of analyte mixtures. These data-processing methods have also been successfully applied to antibody/antigen interactions even using cross-reactive antibodies. Because miniaturisation and parallelisation are essential approaches in recent years, some aspects and current trends, especially for bio-applications, will be discussed. Miniaturisation is especially well covered in the literature.     

Novel nanostructured materials to develop oxygen-sensitive films for optical sensors

Novel nanostructured materials, such as aluminum oxide (AlOOH), silicon oxide (SiO₂) or zirconium oxide (ZrO₂) embedded into PVA, were investigated as potential matrices to incorporate organometallic compounds (OMCs) for the development of optical oxygen-sensitive sensors which make use of the principle of luminescence quenching.In order to assess the benefits and drawbacks of the nanoporous material, the luminescence quantum yield and the Stern-Volmer constants were investigated and compared with the values shown for the same OMCs solubilized in polymer films (polystyrene). Referred to polymer films, the incorporation of the dyes into nanoporous membranes increased the Stern-Volmer constant by more than a factor of 100. Their response time was less than 1 s and the optode membranes were stable at room temperature for at least 9 months. Sterilization by autoclavation and gamma irradiation resulted in a marginal loss in activity. The photostability and sterilizability of the oxygen-sensitive membranes and the performance of the optodes with respect to of different types of metal oxides are discussed in the paper, as well as the influence of the total pore volume (TPV), the pore diameter (PD), the transparency of the film and the geometry of the pores. The OMCs used in this work were: ETH^T-3003 (tris(4,7-bis(4-octylphenyl)-1,10-phenanthroline) ruthenium(II)), N-926 (bis(2-phenylpyridinyl)-N⁴,N⁴,N⁴′,N⁴′-tetramethyl-(4,4′-diamine-2,2′-bipyridine) iridium(III) chlorate), N-833 (tetrabutylammonium bis(isothiocyanate) bis(2-phenylpyridinyl)-iridium(III)) and N-837 (tetrabutylammonium bis(cyanate) bis(2-phenylpyridinyl)-iridium(III)).     

Metal-organic frameworks as materials for applications in sensors

1. Download : Download high-res image (121KB)
2. Download : Download full-size image

     

Photoactive organic-inorganic hybrid materials: From silylated compounds to optical applications

This review summarizes several aspects of type II photoactive organic-inorganic hybrid materials prepared from silylated fluorophores, including their photophysical properties and uses. In this sense, several examples are presented and discussed taking the nature of the silyl derivative into account. Applications as latent fingerprints detection, chemosensors for metal cations, anions, pH, heavy metals, and small organic molecules, as well as recent use as drug delivery systems, bioimaging, organic solar cells, aerogels, and highly fluorescent hybrid materials, are reported and compared to the literature. Also, fluorescent type II organic-inorganic hybrid materials from non-silylated fluorophores, prepared with binding agents, such as 3-(triethoxysilyl)propyl isocyanate (TESPIC), 3-mercaptopropyltriethoxysilane (TMMPS), or 3-isocyanato propyltrimethoxysilane (ICPTES) are also covered in this review.     

Rational materials design of sorbent coatings for explosives: applications with chemical sensors

A series of chemoselective polymers had been designed and synthesized to enhance the sorption properties of polymer coated chemical sensors for polynitroaromatic analytes. To evaluate the effectiveness of the chemoselective coatings, a polynitroaromatic vapor test bed was utilized to challenge polymer coated surface acoustic wave (SAW) devices with different explosive vapors. Dinitrotoluene detection limits were determined to be in the <100 parts per trillion ranges. ATR-FTIR studies were used to determine the nature of the polymer-polynitroaromatic analyte interactions, and confirm the presence of hydrogen-bonding between polymer pendant groups and the nitro functional groups of polynitroaromatic explosive materials.     

Carbon-based materials applied to the development of chemically modified sensors: Trends to environmental applications**

Nowadays, carbon-based materials applied to the development of chemically modified sensors have been highlighted once they can generate methods with high sensitivity, stability, conductivity, accuracy and low cost. Hence, these sensors have been used in environmental monitoring in aneco-friendlyy, sensitive, fast, efficient, inexpensive and robust way. In this review, firstly we described about carbon-based materials and their derivatives, followed by the chemically modified carbon-based sensors manufacturing overview and their applications in environmental analytical chemistry related to inorganic and organic compounds determinations. Future perspectives on trends of the carbon-based materials applications in the sensor modifications are also described.     

Nanoscopic optical sensors based on functional supramolecular hybrid materials

This review highlights how the combination of supramolecular principles and nanoscopic solid structures enables the design of new hybrid sensing ensembles with improved sensitivity and/or selectivity and for the targeting of analytes for which selectivity is hard to achieve by conventional methods. Such ideas are bridging the gap between molecules, materials sciences and nanotechnology. Relevant examples will be detailed, taking into account functional aspects such as (1) enhanced coordination of functionalized solids, (2) enhanced signalling through preorganization, (3) signalling by assembly–disassembly of nanoscopic objects, (4) biomimetic probes utilizing discrimination by polarity and size and (5) distinct switching and gating protocols. These strategies are opening new prospects for sensor research and signalling paradigms at the frontier between nanotechnology, smart materials and supramolecular chemistry.     

Modification of the surface of integrated optical wave-guide sensors for immunosensor applications

Methods have been developed which enable attachment amino and epoxy groups to the surface of integrated optical wave-guide sensors for immunosensor applications. The SiO2-TiO2 surfaces were modified by use of the trifunctional silane reagents gamma-aminopropyltriethoxysilane (APTS) and gamma-glycidoxypropyltrimethoxysilane (GOPS) in organic and/or in inorganic phases. Silanization methods were optimized taking into consideration the concentration of silane reagent used and the temperature and time of reaction. To evaluate the layers formed, immobilization experiments were undertaken on the modified surfaces using the bovine serum albumin (BSA)-anti-BSA IgG antibody model molecule pair. The regenerability of the sensitized surfaces was also studied.     

Focused ion beam sectioning and lift-out method for copper and resist vias in organic low-k dielectrics.

The focused ion beam lift-out technique for scanning electron microscope (SEM) and transmission electron microscope (TEM) sample preparation was shown to be applicable to copper/low-k dielectric semiconductor technology. High resolution SEM, TEM, and scanning transmission electron microscope analyses were performed on metal contacts and resist vias with no evidence of the interface damage or metal smearing commonly observed with mechanical polishing. Ion milling of the sample ex situ to the substrate provided decoration and adjustment of the exposed plane of the section when necessary for SEM analysis.     

Biosensing with polydiacetylene materials: structures, optical properties and applications

Polydiacetylene (PDA) materials are used as a platform for detection of biological analytes such as microorganisms, viruses and proteins. The environmentally responsive chromic and emissive properties of the polymer, combined with self-assembled material formats, make these materials particularly attractive for biosensing applications. A variety of approaches have been used in developing these materials and demonstrating their potential for biological detection. In this feature article we describe different PDA material formats, discuss the optical properties that are the basis for signal generation, and review the use of PDA for biosensing.     

Three polar derivatives of N‐ethylcarbazole: materials for optical applications

Three N‐ethyl­carbazole derivatives have been synthesized and tested for non‐linear optical (NLO) properties. The compounds are 2‐(9‐ethyl‐9H‐carbazol‐3‐yl­methyl­ene)­malono­nitrile, C₁₈H₁₃N₃, (IIIa), 2‐cyano‐3‐(9‐ethyl‐9H‐carba­zol‐3‐yl)­thio­acryl­amide, C₁₈H₁₅N₃S, (IIIb), and 3‐(9‐ethyl‐9H‐carbazol‐3‐yl)‐2‐(4‐phenyl‐1,3‐thia­zol‐2‐yl)­acrylo­nitrile, C₂₆H₁₉N₃S, (V). It was found that the mol­ecules of (IIIa) and (V) are nearly planar, while non‐planarity is more pronounced for (IIIb). Molecules of (IIIa) and (V) exhibit herring‐bone packing motifs. In (IIIb), the mol­ecules form layers coplanar with (01), within which they form centrosymmetric dimers via N—H?S hydrogen bonds.     

Synthesis and mesomorphic behaviour of high polarisability materials for non-linear optical applications

A variety of high polarisability materials have been synthesised with thioester and alkyne linking groups, and isothiocyanato and cyano terminal groups. The synthesis of these materials is presented and the transition temperatures and optical parameters of the materials are discussed and compared to those of the parent ester and non-linked analogues. The thioester and isothiocyanate units provide very high polarisabilities although the isothiocyanato unit leads to reduced mesophase thermal stabilities when compared to analogous cyano-terminated mesogens.     

Electrochemical sensors for real-world applications

Journal of Solid State Electrochemistry -     

Imaging thin films of nanoporous low-k dielectrics: comparison between ultramicrotomy and focused ion beam preparations for transmission electron microscopy.

Ultramicrotomy, the technique of cutting nanometers-thin slices of material using a diamond knife, was applied to prepare transmission electron microscope (TEM) specimens of nanoporous poly(methylsilsesquioxane) (PMSSQ) thin films. This technique was compared to focused ion beam (FIB) cross-section preparation to address possible artifacts resulting from deformation of nanoporous microstructure during the sample preparation. It was found that ultramicrotomy is a successful TEM specimen preparation method for nanoporous PMSSQ thin films when combined with low-energy ion milling as a final step. A thick, sacrificial carbon coating was identified as a method of reducing defects from the FIB process which included film shrinkage and pore deformation.     

Fabrication of new polypyrrole/silicon nitride hybrid materials for potential applications in electrochemical sensors: Synthesis and characterization

In this research, an efficient fabrication process of conducting polypyrrole (PPy)/silicon nitride (Si₃N₄) hybrid materials were developed in order to be employed as transducers in electrochemical sensors used in various environmental and biomedical applications. The fabrication process was assisted by oxidative polymerization of pyrrole (Py) monomer on the surface of Si/SiO₂/Si₃N₄ substrate in presence of FeCl₃ as oxidant. To improve the adhesion of PPy layer to Si₃N₄ surface, a pyrrole-silane (SPy) was chemically bonded through silanization process onto the Si₃N₄ surface before deposition of PPy layer. After Py polymerization, Si/SiO₂/Si₃N₄-(SPy-PPy) substrate was formed. The influence of SPy concentration and temperature of silanization process on chemical composition and surface morphology of the prepared Si/SiO₂/Si₃N₄-(SPy-PPy) substrates was studied by FTIR and SEM. In addition, the electrical properties of the prepared substrates were characterized by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). It was found that the best silanization reaction conditions to get Si/SiO₂/Si₃N₄-(SPy-PPy) substrate with high PPy adhesion and good electrical conductivity were obtained by using SPy at low concentration (4.3 mM) at 90°C. These promising findings open the way for fabrication of new hybrid materials which can be used as transducers in miniaturized sensing devices for various environmental and biomedical applications.     

Nanoscale chemical structure variations in nano-patterned and nano-porous low-k dielectrics: A comparative photothermal induced resonance and infrared spectroscopy investigation

The recent development of the photothermal induced resonance (PTIR) technique has enabled atomic force microscope based infrared (AFM-IR) spectroscopy and imaging to be achieved at the nanometer scale. However, a direct correspondance between PTIR/AFM-IR and more traditional Fourier transform IR (FTIR) spectroscopy has been prohibited for nanometer scale features due to Rayleigh diffraction constraints that limit the latter to few micron spatial resolution. In this regard, we have overcome this challenge by fabricating 1 cm² arrays of 90 nm wide fins in a nano-porous low dielectric constant (i.e. low-k) amorphous hybrid inorganic-organic silicate material using standard nano-electronic fabrication techniques. With these structures, we demonstrate both a general correspondance between AFM-IR, FTIR, and Germanium attenuated total reflection (GATR) IR spectroscopy, as well as differences in the sensitivities that these techniques exhibit to the nanoscale variations in chemical structure induced in the low-k dielectric by the nanopatterning method. To further illustrate the sensitivity of AFM-IR to changes in chemical structure with nanometer resolution, the nanopatterned low-k dielectric was exposed to additional oxidizing plasma ash cleans post patterning. Focusing on the Si-CH₃ deformation band at ∼1275 cm⁻¹, both the AFM-IR, FTIR and GATR measurements show a clear reduction in the concentration of terminal methyl groups in the low-k dielectric as the oxidation potential of the plasma ash clean increased. These results further establish the power of AFM-IR to perform nanoscale IR spectroscopy and demonstrates a stronger correspondance between AFM-IR and well-known micron scale IR techniques such as FTIR and GATR.     

Redundant chemical sensors for calibration-impossible applications

Multiple chemical sensors are used to measure the same analyte simultaneously to determine whether the redundant signals can improve the long-term accuracy and circumvent the need for periodic calibrations. A specific marine chemistry application was investigated where six glass pH electrodes were placed in a synthetic seawater solution for nearly 2 months without recalibration. The pH accuracy was evaluated by comparison with spectrophotometric pH measurements. The standard deviation, t-test and principal-component analysis were used to evaluate the redundant signals. The average signal standard deviation was useful for determining the onset of drift, whereas, the principal-component analysis readily identified specific sensors that were drifting. The sensor signals, shown through t-tests to be outliers, were eliminated from the data set, resulting in a significant improvement in measurement accuracy. After 56 days, the signals from non-drifting and drifting sensors resulted in a pH accuracy of +/-0.012 and +/-0.040, respectively, over a threefold improvement. The residual +/-0.012 inaccuracy was limited by the performance of the remaining sensors, which appeared to drift with similar magnitude and could therefore not be statistically separated. These results indicate that redundant sensors coupled with a principal-component analysis are a potential alternative for situations where calibrations are not feasible.     

Fluoride materials for optical applications: Single crystals, ceramics, glasses, and glass–ceramics

Fluoride compounds have compelling advantages for many optical applications due to their unique combination of low phonon energy, high UV absorption edge energy, and relatively weak crystal field. In this article, we offer a concise review of the current state of the art in fluoride single crystals, ceramics, glasses, and glass–ceramics for optical applications.