Plasmonic nanoparticle interactions for high-performance imaging fluorosensors

We demonstrate a nanoparticle based ratiometric fluorosensor suitable for non-invasive high-resolution imaging of complex samples. Coextraction generates a movement of a solvatochromic dye, with a consequent shift from plasmophore coupled emission towards longer wavelength scattering enhanced emission. The nanoparticle doped sensors confirmed superior sensitivity, reversibility, durability and overall image quality.     

The pillar[5]arene-based spun thin films: preparation,characterization, development of optical and mass sensitive sensors for swelling dynamics and gas sensing abilities

Pillar[5]arene (P5)-based materials can be preferable one of the most sensing elements in chemical sensor applications due to their high cavity and their special chiral structure. While the P5-based macrocycle molecules have been utilized as thin-film materials, the reports of chemical sensor application by performing P5 as sensor molecules have been very limited in the available literature. In this report, quinoline P5 (P5-Q) molecules were used to produce thin films via spin coating technique. P5-Q spun films were characterized with Atomic Force Microscopy (AFM) and Ultraviolet–Visible (UV–Vis) spectrophotometer. The gas sensing abilities of these P5-Q spun films were investigated by Quartz Crystal Microbalance (QCM) and Surface Plasmon Resonance (SPR) techniques. In order to illuminate the gas sensing properties of P5-Q spun films, they were prepared as mass-sensitive and optical sensors. These sensors were utilized for its sensing abilities against organic vapours (acetone, methyl alcohol, and ethyl alcohol) by the mechanism of host–guest interaction. The current study also describes the diffusion coefficients of these organic vapors to illuminate the swelling dynamics of P5-Q spun films by performing Fick’s diffusion equation. The responses of P5-based optical (SPR) or mass sensitive (QCM) sensor in terms of the change in reflective intensity or the change in frequency and the values of diffusion coefficients showed that P5-Q molecules can be developed as potential chemical sensor element for acetone vapor compared to alcohol vapors.

     

Towards advanced chemical and biological nanosensors-An overview

This paper reviews recent developments in the design and application of two types of optical nanosensor, those based on: (1) localized surface plasmon resonance (LSPR) spectroscopy and (2) surface-enhanced Raman scattering (SERS). The performance of these sensors is discussed in the context of biological and chemical sensing. The first section addresses the LSPR sensors. Arrays of nanotriangles were evaluated and characterized using realistic protein/ligand interactions. Isolated, single nanoparticles were used for chemosensing and performed comparably to the nanoparticle array sensors. In particular, we highlight the effect of nanoparticle morphology on sensing response. The second section details the use of SERS sensors using metal film over nanosphere (MFON) surfaces. The high SERS enhancements and long-term stability of MFONs were exploited in order to develop SERS-based sensors for two important target molecules: a Bacillus anthracis biomarker and glucose in a serum protein mixture.     

Multi-Analyte Sensing: From Site-Selective Deposition to Randomly-Ordered Addressable Optical Fiber Sensors

In this report we review the progress in the development of imaging fiber chemical sensors. Emphasis is placed on the chemical sensor component and the fabrication of architectures appropriate for multi-analyte sensing, such as optical fiber sensors. Two main approaches in the fabrication of such sensors will be highlighted: first, sensors made with spatially-resolved sensing sites by site-selective polymerization, second, sensors prepared by random distribution of microsphere sensors on an optical imaging fiber containing thousands of μm-scale wells. Examples of each are given.     

Nanoparticles as structural and functional units in surface-confined architectures

The nanoscale engineering of functional chemical assemblies has attracted recent research effort to provide dense information storage, miniaturized sensors, efficient energy conversion, light-harvesting, and mechanical motion. Functional nanoparticles exhibiting unique photonic, electronic and catalytic properties provide invaluable building blocks for such nanoengineered architectures. Metal nanoparticle arrays crosslinked by molecular receptor units on electrodes act as selective sensing interfaces with controlled porosity and tunable sensitivity. Photosensitizer/electron-acceptor bridged arrays of Au-nanoparticles on conductive supports act as photoelectrochemically active electrodes. Semiconductor nanoparticle composites on surfaces act as efficient light collecting systems, and nanoengineered semiconductor 'core-shell' nanocrystal assemblies reveal enhanced photoelectrochemical performance due to effective charge separation. Layered metal and semiconductor nanoparticle arrays crosslinked by nucleic acids find applications in the optical, electronic and photoelectrochemical detection of DNA. Metal and semiconductor nanoparticles assembled on DNA templates may be used to generate complex electronic circuitry. Nanoparticles incorporated in hydrogel matrices yield new composite materials with novel magnetic, optical and electronic properties.     

Smart nano-architectures as potential sensing tools for detecting heavy metal ions in aqueous matrices

The discharge of heavy metal ions into water resources as a result of human activities has become a global issue. Contamination with heavy metal ions poses a major threat to the environment and human health. Therefore, there is a dire need to probe the presence of heavy metal ions in a more selective, facile, quick, cost-effective and sensitive way. Conventional sensors are being utilized to sense heavy metal ions; however, various challenges and limitations like interference, overlapping of oxidation potential, selectivity and sensitivity are associated with them that limit their in-field applicability. Hence, nanomaterial based chemical sensors have emerged as an alternative substitute and are extensively employed for the detection of heavy metal ions as a potent analytical tool. The incorporation of nanomaterials in sensors increases their sensitivity, selectivity, portability, on-site detection capability and device performance. Nanomaterial based electrodes exhibit enhanced performance because surface of electrode at nano-scale level offers high catalytic potential, large active surface area and high conductivity. Therefore, this review addresses the recent progress on chemical sensors based on different nanomaterials such as carbon nanotubes (CNTs), metal nanoparticles, graphene, carbon quantum dots and nanocomposites for sensing heavy metals ions using different sensing approaches. Furthermore, various types of optical sensors such as fluorescence, luminescence and colorimetry sensors have been presented in detail.     

Evaluation of the performance of sensors based on optical imaging of a chemically sensitive layer

Interest in the use of the optical properties of chemical indicators is growing steadily. Among the optical methods that can be used to capture changes in sensing layers, those producing images of large-area devices are particularly interesting for chemical sensor array development. Until now, few studies addressed the characterization of image sensors from the point of view of their chemical sensor application. In this paper, a method to evaluate such performance is proposed. It is based on the simultaneous measurement of absorption events in a metalloporphyrin layer with an image sensor and a quartz microbalance (QMB). Exploiting the well-known behaviour of QMB, comparison of signals enables estimation of the minimum amount of absorbed molecules that the image sensor can detect. Results indicate that at the single pixel level a standard image sensor (for example a webcam) can easily detect femtomoles of absorbed molecules. It should therefore be possible to design sensor arrays in which the pixels of images of large-area sensing layers are regarded as individual chemical sensors providing a ready and simple method for large sensor array development.     

Plasmophore sensitized imaging of ammonia release from biological tissues using optodes

A plasmophore sensitized optode was developed for imaging ammonia (NH(3)) concentrations in muscle tissues. The developed ammonia sensor and an equivalent non plasmophore version of the sensor were tested side by side to compare their limit of detection, dynamic range, reversibility and overall imaging quality. Bio-degradation patterns of ammonia release from lean porcine skeletal muscle were studied over a period of 11 days. We demonstrate that ammonia concentrations ranging from 10nM can be quantified reversibly with an optical resolution of 127 μm in a sample area of 25 mm × 35 mm. The plasmophore ammonia optode showed improved reversibility, less false pixels and a 2 nM ammonia detection limit compared to 200 nM for the non-plasmophore sensor. Main principles of the sensing mechanism include ammonia transfer over a gas permeable film, ammonia protonation, nonactin facilitated merocyanine-ammonium coextraction and plasmophore enhancement. The vast signal improvement is suggested to rely on solvatochroism, nanoparticle scattering and plasmonic interactions that are utilized constructively in a fluorescence ratio. In addition to fundamental medicinal and biological research applications in tissue physiology, reversible ammonia quantification will be possible for a majority of demanding imaging and non imaging applications such as monitoring of low ammonia background concentrations in air and non-invasive medicinal diagnosis through medical breath or saliva analysis. The nanoparticle doped sensor constitutes a highly competitive technique for ammonia sensing in complex matrixes and the general sensing scheme offers new possibilities for the development of artificial optical noses and tongues.     

Interferences in hydride atomization studied by atomic absorption and atomic fluorescence spectrometry

A hydride atomizer able to operate in the flame-in-tube mode and in the miniature diffusion flame mode was used to investigate interferences of arsenic in selenium atomization. A twin-channel continuous flow hydride generator was utilized to eliminate liquid phase interferences. Both atomic absorption and atomic fluorescence detectors (EDL sources) were employed. The miniature diffusion flame can tolerate interferent concentrations up to 70 μg ml⁻¹. The magnitude of interferences in the flame-in-tube atomizer is controlled by the distance between the atomization and detection zones. The best tolerance to interferents, comparable with that in the miniature diffusion flame, was obtained for the minimum distance of the zones. The figures were deteriorated by two orders of magnitude when increasing the distance between the observation and the atomization zones to 50 mm. Also a curvature and rollover of calibration graphs was observed when increasing the distance. The presence of the interferent enhanced substantially the curvature and rollover, so that the magnitude of observed interferences was dependent on the analyte concentration. All the observed interferences and the calibration graph curvature are due to the decay of free analyte atoms by reactions in the free space. The nature of the species formed is discussed. No significant depletion of hydrogen radicals was observed. As demonstrated by measurements in the miniature diffusion flame, the species formed can be reatomized by interaction with hydrogen radicals with an efficiency better than 90%.     

Modeling the analytical response of optical fiber sensors for aromatic compounds determination

In recent years, there has been an increasing interest in the application of optical fiber sensors for in situ monitoring of chemical pollutants, including volatile organic compounds, regarding air quality assurance. In order to enhance the usefulness and applicability of this methodology to environmental analysis, a proper study of the analytical signal and an adequate calibration model are required. This contribution is focused on the model for optical fiber sensors calibration, discussing some problems associated with the estimates of the figures of merit of these analytical systems. We also suggest and discuss a calibration model based on a cumulative symmetric double sigmoidal (SDS) function, as a suitable and general alternative to the more limited and classical linear calibration model.     

Logical regulations of the aggregation/dispersion of gold nanoparticles via programmed chemical interactions

Nanoparticles that respond to various chemical and physical stimuli form the basis for various conceivable applications including sensors, chemical logic, biomedical imaging, and therapies. In this work, we demonstrate that the electrostatic and chemical (complexing and gold-thiol bonding) interactions existing in a gold nanoparticle/Zn(2+)/dithiothreitol-based ternary chemical system is "programmable" and can be utilized to regulate the aggregation and dispersion of nanoparticles via XOR and INHIBIT logics. The resulting solutions alter their colors according to different input combinations because of the well-controlled aggregation or dispersion of plasmonic gold nanoparticles, opening up new possibilities for the developments of advanced sensors and nanobiomedical devices based on the coupling, gating, and signaling of different chemical stimuli.     

Imprinting with sensor development – On the way to synthetic antibodies

Molecular imprinting is an attractive tool for the development of artificial recognition systems. Even non-covalent imprinting provides universal interaction centers for sensoric applications. The coated chemical sensors have high stabilities under harsh conditions in both the gas and liquid phases. With adequate efforts optical and mass-sensitive sensors (quartz crystal microbalance, QCM, surface acoustic wave detector, SAW) are suitable for analysis down to the ppb (nL/L) range. PAHs, isomer VOCs as well as complex oil mixtures are appropriate analytes.     

Optical sensing based on analyte recognition by enzymes,carriers and molecular interactions

Despite the tremendous variety of methods suitable for sensing applications, we face the fact that chemical sensors displaying sensitivity, selectivity and reversibility are still scarce and are mostly confined to low-molecular-weight species. Obviously, it is not the lack of optical (or other) transduction methods that limit the performance of present day sensor desingns, but rather the insufficient selectivity of the recognition process, particularly in the field of sensors for organic and bioorganic species. The use of enzymes, ion carriers and natural or synthetic receptor/carriers which can under go specific interactions with the species to be recognized (such as through hydrogen bonding or charge-transfer interaction) can result in specific recognition and, consequently, sensing. Examples for optical sensing schemes for clinically or biologically important species including enzyme substrates, metabolites, drugs, alkali and ammonium ions and other will be given. In enzyme-based sensors various options exist: depending on which species is immobilized, assays for substrates (such as glucose, ethanol, lactate or creatine), enzymes (such as esterases) or inhibitors (such as organophosphates) can be designed. In addition, the intrinsic optical properties of certain enzymes, coenzymes or metabolites can be utilized for sensing purposes, a fact that presents an interesting alternative to enzyme sensors with chemical transducers.Notwithstanding the selectivity of biocatalytic sensors, their stability and sensitivity is moderate. Bioorganic synthetic molecules which can recognize and reversibly bind other species offer an attractive alternative, particularly in terms of stability. However, quite a different situation is found in such cases because receptors, in contrast to enzymes, do not “digest” their substrates. Hence, while the steady-state response in enzyme-based sensors is a result of kinetic equilibration, substrate binding in non-metabolizing receptors results in thermodynamic equilibration. However, most existing receptors (except antibodies) lack the unique specificity of enzymes. On the other side, new bioorganic molecules and stable receptor/carriers along with polymer materials of proper permeation selectivity can help to overcome current limitations of protein-based systems. Neutral ion carriers, which may be considered as ion receptors, are a useful example of sometimes highly specific recognition/carrier molecules with excellent stability. Unfortunately, no receptor /carrier molecules of similar specificity do exist yet for most other organic and clinical parameters of interest. There is an obvious need for new and stable molecules suitable for specific recognition of low-molecular weight organic species.We will report on the use of such new receptor/carrier molecules, the respective sensor materials, and how the process of recognition can be coupled to optical transduction. Such receptors/carriers also allow other kinds of discriminations: if, for instance, it is enantio-selective (i.e. preferably binds one species out of a pair of optical isomers), a fairly specific recognition of enantiomers of biogenic amines (such as some drugs and biogenic amines) will become possible. Specific examples will also be given of new types of sensors based on recognition by charge-transfer interaction, through-space interaction and hydrogen bonding, with fair specificity for thiamine, penicilline, nitrate, salicylate and cholic acids. Finally, current problems and the significant challenges for sensors research in the 1990s will be discussed.     

Identification of Nanoparticles via Plasmonic Scattering Interferometry

The development of optical imaging techniques has led to significant advancements in single‐nanoparticle tracking and analysis, but these techniques are incapable of label‐free selective nanoparticle recognition. A label‐free plasmonic imaging technology that is able to identify different kinds of nanoparticles in water is now presented. It quantifies the plasmonic interferometric scattering patterns of nanoparticles and establishes relationships among the refractive index, particle size, and pattern both numerically and experimentally. Using this approach, metallic and metallic oxide particles with different radii were distinguished without any calibration. The ability to optically identify and size different kinds of nanoparticles can provide a promising platform for investigating nanoparticles in complex environments to facilitate nanoscience studies, such as single‐nanoparticle catalysis and nanoparticle‐based drug delivery.     

Fluorescence-based fibre optic arrays: a universal platform for sensing

Optical fibres provide a universal sensing platform as they are easily integrated with a multitude of different sensing schemes. Such schemes enable the preparation of a multitude of sensors from relatively straightforward pH sensors, to more complex ones, including artificial olfaction sensors, high-density oligonucleotide arrays, and high-throughput cell-based arrays. Imaging fibre bundles comprised of thousands of fused optical fibres are the basis for an optically connected, individually addressable parallel sensing platform. Fibre optic imaging bundles possess miniature feature sizes (3-10 micron diameter fibres), allowing high-density sensor packing (approximately 2 x 10(7) sensors per cm2). Imaging fibre bundles transmit coherent images enabling combined imaging and sensing, relating the responses monitored by the sensor to observable physical changes. The individual fibre cores can also be selectively etched to form a high-density microwell array capable of housing complementary sized microsensors. The miniature feature sizes facilitate a faster response and more sensitive measurement capabilities. The platform is extremely versatile in its sensing design, allowing the sensing scheme to be tailored to fit the experimental design, whether for monitoring single analytes or more complex multiplexed assays. A number of sensing schemes and applications are described in this review.     

Phthalocyanines as Sensitive Materials for Chemical Sensors

This paper provides a review of phthalocyan- ines suitable for the development of chemical sensors. Phthalocyanines may be utilized for different types of chemical sensors, including in particular electronic conductance sensors [such as semiconductive, field-effect transistor (FET), solid-state ionic and capacitance sensors], mass-sensitive sensors utilizing a quartz crystal microbalance (QCM) and surface acoustic-wave (SAW) sensors, and optical sensors. The phthalocyanines used are discussed in terms of their physical and chemical properties, as well as their sensitivity, selectivity and reversibility towards the detection of NO₂ and organic solvent vapours. The interaction mechanism between phthalocyanine films and analyte molecules is also discussed.     

Recent advances and applications in QDs-based sensors

As a unique nanomaterial, quantum dots (QDs) are not only applied in fluorescent labeling and biological imaging, but are also utilized in novel sensing systems. Because QDs have attractive optoelectronic characteristics, QD-based sensors present high sensitivity in detecting specific analytes in the chemical and biochemical fields. In this review, we describe the basic principles and different conjugation strategies in QD-based sensors. An overview of recent advances and various models of QD-sensing systems is also provided. Furthermore, perspectives for sensors based on QDs are discussed.     

Imprinting with sensor development – On the way to synthetic antibodies

Molecular imprinting is an attractive tool for the development of artificial recognition systems. Even non-covalent imprinting provides universal interaction centers for sensoric applications. The coated chemical sensors have high stabilities under harsh conditions in both the gas and liquid phases. With adequate efforts optical and mass-sensitive sensors (quartz crystal microbalance, QCM, surface acoustic wave detector, SAW) are suitable for analysis down to the ppb (nL/L) range. PAHs, isomer VOCs as well as complex oil mixtures are appropriate analytes. Received: 4 November 1998 / Revised: 29 April 1999 / Accepted: 5 May 1999     

Recent progress in fluorescent and colorimetric sensors for the detection of ions and biomolecules

Optical sensors are widely used in the field of analytical sensing and optical imaging because of their high sensitivity, fast response time, and technical simplicity. This review focuses on recent contributions concerning the ions, neutral molecules and especially tumor micro-environment-related parameters based fluorescent or colorimetric sensors and is organized according to their target classifications.     

Influence of Gold Nanoparticle Film Porosity on the Chemiresistive Sensing Performance

The porosity of 1‐hexanethiol‐functionalised gold nanoparticle films was assessed and utilised as chemiresistor sensors. Electrochemical capacitance measurements showed that the accessibility of electrolytes of different ionic strengths into the pores depended on the thickness of the electric double layer formed. A large variation in capacitance was measured in 0.01–1000 mM NaClO₄, implying a wide pore size distribution. The change in morphology of the nanoparticle films upon storage in air, water and ethanol for two weeks was investigated. There was a significant decrease in the electrochemical capacitance at high electrolyte concentrations for the ethanol‐stored films compared to the freshly‐prepared films suggesting a decrease in the number of small pores of radii in the range of 0.3–3 nm. This was further supported by optical topographical measurements where a decrease in the thickness of ethanol‐stored films was observed relative to the freshly‐prepared films. The porous nature of the nanoparticle films was found to have an effect on the chemical sensing behaviour. When used as chemiresistor sensors, for the detection of heptane in water, the ethanol‐stored films provided larger resistance changes and longer response times. This suggests that the more densely packed ethanol‐stored films provided more sites that enabled film swelling, and that diffusion of the analyte occurred through the narrower water‐filled pores. This demonstrates the effect of different storage conditions on film morphology and subsequently sensor response.