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Optical Chemosensors and Biosensors
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Optical Chemosensors and Biosensors (Closed)

A topical collection in Chemosensors (ISSN 2227-9040). This collection belongs to the section "Optical Chemical Sensors".

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Editor

Dr. Ambra Giannetti

E-Mail Website
Collection Editor
Institute of Applied Physics “Nello Carrara”, CNR-IFAC, Via Madonna del Piano 10, 50019 Sesto Fiorentino, Italy
Interests: biosensors; biophotonics; fluorescence; label-free
Special Issues, Collections and Topics in MDPI journals

Topical Collection Information

Dear Colleagues,

The field of chemo- and bio-sensors, ranging from biomedical/clinical applications to environmental applications and food analyses, has been growing as of two decades. In fact, in all of these fields, there is a growing demand for rapid responses, quality control, usable devices, low-cost analyses, etc. All these features could lead to an improved healthy life, ranging from a more reliable and controlled quality of food and environment to a faster and more specific diagnosis.

The optical detection methods used in chemo- and bio-sensors are based both on label-based or label-free techniques. The former ones make use, for example, of fluorescent or chemiluminescent-based detection systems, while the latter are based on direct optical detection of physical measurands, which are modified by chemical/biochemical reactions, such as the refractive index or the thickness or the density of the sensing layer at the surface where the interaction occurs.

The aim of the Topical Collection is to collect new optical (label-free and label-based) chemo- and bio-sensors studies for biomedical/clinical, environmental applications and food analysis.

Dr. Ambra Giannetti
Collection Editor

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Keywords

  • optical chemical sensors
  • optical biosensors
  • label-based optical sensors
  • label-free optical sensors
  • sensor materials for optical sensing
  • novel optical sensing principles
  • optical sensor arrays

Published Papers (14 papers)

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2022

Jump to: 2020, 2019, 2018

11 pages, 3095 KiB  
Communication
Advanced Mapping of Optically-Blind and Optically-Active Nitrogen Chemical Impurities in Natural Diamonds
by Sergey Kudryashov, Elena Rimskaya, Evgeny Kuzmin, Galina Kriulina, Victoria Pryakhina, Andrey Muratov, Roman Khmelnitskii, Evgeny Greshnyakov, Pavel Danilov and Vladimir Shur
Chemosensors 2023, 11(1), 24; https://doi.org/10.3390/chemosensors11010024 - 27 Dec 2022
Cited by 1 | Viewed by 1289
Abstract
Natural diamonds with a rich variety of optically blind and optically active nitrogen impurity centers were explored at a nano/microscale on the surface and in bulk by a number of advanced chemical and structural analytical tools in order to achieve a comprehensive characterization [...] Read more.
Natural diamonds with a rich variety of optically blind and optically active nitrogen impurity centers were explored at a nano/microscale on the surface and in bulk by a number of advanced chemical and structural analytical tools in order to achieve a comprehensive characterization by establishing enlightening links between their analysis results. First, novel compositional relationships were established between high-energy X-ray photoelectron spectroscopy (XPS) and low-energy Fourier-transform infrared vibrational spectroscopy (FT-IR) signals of nitrogen impurity defects acquired in the microscopy mode at the same positions of the diamond surface, indicating the verification XPS modality for qualitative and quantitative FT-IR analysis of high concentrations of nitrogen and other chemical impurity defects in diamond. Second, depth-dependent spatial distributions of diverse photoluminescence (PL)-active nitrogen defects were acquired in the confocal scanning mode in an octahedral diamond and then for the first time corrected to the related Raman signals of the carbon lattice to rule out artefacts of the confocal parameter and to reveal different micron-scale ontogenetic layers in the impurity distributions on its surface. Third, intriguing connections between local structural micro-scale defects (dislocation slip bands of plastic deformation zones) visualized by optical microscopy and Raman microspectroscopy, and related distributions of stress-sensitive PL-active nitrogen impurity defects in the proximity of these planes inside bulk diamonds were revealed. These findings demonstrate the broad instrumental opportunities for comprehensive in situ studies of the chemical, structural, and mechanical micro-features in diamonds, from the surface into bulk. Full article
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17 pages, 4148 KiB  
Article
Multiparametric Guided-Mode Resonance Biosensor Monitoring Bulk and Surface-Film Variations
by Joseph A. Buchanan-Vega and Robert Magnusson
Chemosensors 2022, 10(12), 541; https://doi.org/10.3390/chemosensors10120541 - 17 Dec 2022
Viewed by 1677
Abstract
A guided-mode resonance (GMR) sensor with multiple resonant modes is used to measure the collection of biomolecules on the sensor surface and the index of refraction of the sensor environment (bulk). The number of sensor variables that can be monitored (biolayer index of [...] Read more.
A guided-mode resonance (GMR) sensor with multiple resonant modes is used to measure the collection of biomolecules on the sensor surface and the index of refraction of the sensor environment (bulk). The number of sensor variables that can be monitored (biolayer index of refraction, biolayer thickness, and bulk, or background, index of refraction) is determined by the number of supported resonant modes that are sensitive to changes in these variable values. The sensor we use has a grating and homogeneous layer, both of which are made of silicon nitride (Si3N4), on a quartz substrate. In this work, we simulate the sensor reflection response as a biolayer grows on the sensor surface at thicknesses from 0 to 20 nm and biolayer indices of refraction from 1.334 to 1.43 RIU; simultaneously, we vary the bulk index of refraction from 1.334 to 1.43 RIU. In the specified span of sensor variable values, the resonance wavelength shifts for 2023 permutations of the biolayer index of refraction, biolayer thickness, and bulk index of refraction are calculated and accurately inverted. Inversion is the process of taking resonant wavelength shifts, for resonant modes of a sensor, as input, and finding a quantitative variation of sensor variables as output. Analysis of the spectral data is performed programmatically with MATLAB. Using experimentally measured resonant wavelength shifts, changes in the values of biolayer index of refraction, biolayer thickness, and bulk index of refraction are determined. In a model experiment, we deposit Concanavalin A (Con A) on our sensor and subsequently deposit yeast, which preferentially bonds to Con A. A unique contribution of our work is that biolayer index and biolayer thickness are simultaneously determined. Full article
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12 pages, 4369 KiB  
Article
A Surface-Scattering-Based Composite Optical Waveguide Sensor for Aerosol Deposition Detection
by Min Zhao, Hongyi Tang, Ziwei Liu, Zhaoyang Tong and Zhimei Qi
Chemosensors 2022, 10(12), 535; https://doi.org/10.3390/chemosensors10120535 - 15 Dec 2022
Viewed by 1242
Abstract
Aerosol is a suspension of fine chemical or biological particles in the air, and it is harmful, easily causing air pollution, respiratory diseases, infrastructure corrosion, and poor visibility. Therefore, the development of advanced optical sensors for real-time detection of aerosol deposition is of [...] Read more.
Aerosol is a suspension of fine chemical or biological particles in the air, and it is harmful, easily causing air pollution, respiratory diseases, infrastructure corrosion, and poor visibility. Therefore, the development of advanced optical sensors for real-time detection of aerosol deposition is of great significance. In this work, a prism-coupled composite optical waveguide (COWG) sensor for aerosol deposition detection based on surface scattering is proposed and demonstrated theoretically and experimentally. The COWG consists of a single-mode slab glass waveguide locally covered with a tapered thin film of high-index metal oxide. The tapered film can greatly enhance the evanescent field through the adiabatic transition of the fundamental transverse electric (TE0) mode between the uncovered and film-covered regions, thereby enabling the COWG to serve as a simple yet highly sensitive evanescent-wave scattering sensor for sensitive detection of aerosol deposition. The COWG with a tapered layer of Ta2O5 was prepared by masked sputtering, aerosol salt particle deposition on the COWG was successfully detected, and the influence of surface water droplets on the COWG sensor performance was analyzed. The experimental results indicate that the sensitivity of the COWG is 30 times higher than that of the bare glass waveguide. Full article
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24 pages, 3705 KiB  
Review
Optical Immunoassays Methods in Protein Analysis: An Overview
by Fabio Rizzo
Chemosensors 2022, 10(8), 326; https://doi.org/10.3390/chemosensors10080326 - 12 Aug 2022
Cited by 8 | Viewed by 5670
Abstract
Immunoassays are analytical tools that attract growing research attention in the field of sensors. Among the different analytical methods, the immunoassays based on optical readout have an important role due to the high sensitivity reached in past years by the instrumentation as well [...] Read more.
Immunoassays are analytical tools that attract growing research attention in the field of sensors. Among the different analytical methods, the immunoassays based on optical readout have an important role due to the high sensitivity reached in past years by the instrumentation as well as by the preparation of new labels. This review aims to give an overview in term of basic concepts and practical examples of the most used optical immunoassays techniques, in order to help readers to choose the most useful techniques for their analyses. Particular emphasis is dedicated to the application of the presented immunoassays on the detection of the SARS-CoV-2 virus. Full article
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2020

Jump to: 2022, 2019, 2018

3 pages, 207 KiB  
Editorial
Optical Chemosensors and Biosensors
by Ambra Giannetti and Markéta Bocková
Chemosensors 2020, 8(2), 33; https://doi.org/10.3390/chemosensors8020033 - 09 May 2020
Cited by 6 | Viewed by 3189
Abstract
The field of chemo- and biosensors, ranging from biomedical/clinical applications to environmental applications and food analyses, has been growing in the last two decades [...] Full article
12 pages, 3510 KiB  
Article
Vertically Coupling ZnO Nanorods onto MoS2 Flakes for Optical Gas Sensing
by Guido Faglia, Matteo Ferroni, Thi Thanh Le Dang, Maurizio Donarelli, Federica Rigoni and Camilla Baratto
Chemosensors 2020, 8(1), 19; https://doi.org/10.3390/chemosensors8010019 - 02 Mar 2020
Cited by 14 | Viewed by 3503
Abstract
Hybrid structures composed of layered one-dimensional (1D) and two-dimensional (2D) materials opened new perspectives and opportunities through the build-up of hetero-junctions with versatile layered structures and led to fascinating fundamental phenomena and advanced devices. We succeeded in depositing by magnetron sputtering vertically aligned [...] Read more.
Hybrid structures composed of layered one-dimensional (1D) and two-dimensional (2D) materials opened new perspectives and opportunities through the build-up of hetero-junctions with versatile layered structures and led to fascinating fundamental phenomena and advanced devices. We succeeded in depositing by magnetron sputtering vertically aligned 1D ZnO nanorods on 2D MoS2 flakes obtained by exfoliation, preserving the structure of the 2D materials. The photoluminescence (PL) optical properties of the hybrid structure were assessed towards developing a contactless optical chemical sensor. Full article
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13 pages, 3600 KiB  
Article
In-Parallel Polar Monitoring of Chemiluminescence Emission Anisotropy at the Solid–Liquid Interface by an Optical Fiber Radial Array
by Simone Berneschi, Cosimo Trono, Mara Mirasoli, Ambra Giannetti, Martina Zangheri, Massimo Guardigli, Sara Tombelli, Elisa Marchegiani, Francesco Baldini and Aldo Roda
Chemosensors 2020, 8(1), 18; https://doi.org/10.3390/chemosensors8010018 - 29 Feb 2020
Cited by 4 | Viewed by 2636
Abstract
Chemiluminescence (CL) detection is widely employed in biosensors and miniaturized analytical devices since it offers high detectability and flexible device design (there are no geometry requirements for the measurement cell, except the ability to collect the largest fraction of emitted photons). Although the [...] Read more.
Chemiluminescence (CL) detection is widely employed in biosensors and miniaturized analytical devices since it offers high detectability and flexible device design (there are no geometry requirements for the measurement cell, except the ability to collect the largest fraction of emitted photons). Although the emission anisotropy phenomenon for an emitting dipole bound to the interface between two media with different refractive index is well known for fluorescence, it is still poorly investigated for CL reactions, in which the excited-state reaction products can diffuse in solution before the photon emission event. In this paper, we propose a simple method for the real-time evaluation of the CL emission anisotropy based on a radial array of optical fibers, embedded in a poly(methyl methacrylate) semicylinder and coupled with a Charge-Coupled Device (CCD) camera through a suitable interface. The polar-time evolutions of the CL emission have been studied for catalyzing enzymes immobilized onto a solid surface (heterogeneous configuration) or free in solution (homogeneous configuration). Evidence of the anisotropy phenomenon is observed, indicating that the lifetime of the excited-state products of the enzyme-catalyzed reactions is shorter than the time required for their diffusion in solution at a distance at which the CL can be considered isotropic. These results open new perspectives in the development of CL-based miniaturized analytical devices. Full article
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14 pages, 2833 KiB  
Article
Towards an Integrated System as Point-of-Care Device for the Optical Detection of Sepsis Biomarkers
by Ambra Giannetti, Cosimo Trono, Giampiero Porro, Claudio Domenici, Mariarita Puntoni and Francesco Baldini
Chemosensors 2020, 8(1), 12; https://doi.org/10.3390/chemosensors8010012 - 11 Feb 2020
Cited by 8 | Viewed by 3058
Abstract
Severe infection and sepsis are a common, expensive, and frequently fatal conditions in critically ill patients. The sepsis diagnosis is not trivial, since it is an extremely complex chain of events involving inflammatory and anti-inflammatory processes, cellular reactions, and circulatory disorders. For these [...] Read more.
Severe infection and sepsis are a common, expensive, and frequently fatal conditions in critically ill patients. The sepsis diagnosis is not trivial, since it is an extremely complex chain of events involving inflammatory and anti-inflammatory processes, cellular reactions, and circulatory disorders. For these reasons, delay in diagnosis and initiation of drug treatments have shown to be crucial for this pathology. Moreover, a multitude of biomarkers has been proposed, many more than for other pathologies. In order to select optimal treatments for the highly heterogeneous group of sepsis patients and to reduce costs, novel multiplexed tools that better characterize the patient and his or her specific immune response are highly desired. In order to achieve the fundament of drastically improved multi-analyte detection and to attain low limits of detection in diagnostics, the area of point-of-care testing (POCT) technology is developing quickly, leading to the production of instruments, the reliability of which is continuously increasing. For this purpose, a selection of two biomarkers—C-reactive protein (CRP) and neopterin (NP)—was studied in this paper and a fluorescence-based integrated optical system, suitable for future POCT applications, was implemented that is capable of performing the simultaneous measurement of the two different biomarkers in replicate. A limit of detection of 10 and 2.1 µg L−1 was achieved for CRP and NP spiked in commercially available human serum, respectively. Moreover, measurements on both biomarkers were also performed on serum samples collected from septic patients. Full article
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8 pages, 472 KiB  
Article
Label-Free Colorimetric Detection of Urine Glucose Based on Color Fading Using Smartphone Ambient-Light Sensor
by Tian-Tian Wang, Kun Guo, Xue-Mei Hu, Jian Liang, Xing-De Li, Zhi-Feng Zhang and Jing Xie
Chemosensors 2020, 8(1), 10; https://doi.org/10.3390/chemosensors8010010 - 27 Jan 2020
Cited by 14 | Viewed by 4980
Abstract
In this work, a label-free colorimetric assay was developed for the determination of urine glucose using smartphone ambient-light sensor (ALS). Using horseradish peroxidase—hydrogen peroxide—3,3′,5,5′-tetramethylbenzidine (HRP-H2O2-TMB) colored system, quantitative H2O2 was added to samples to-be-determined for deepest [...] Read more.
In this work, a label-free colorimetric assay was developed for the determination of urine glucose using smartphone ambient-light sensor (ALS). Using horseradish peroxidase—hydrogen peroxide—3,3′,5,5′-tetramethylbenzidine (HRP-H2O2-TMB) colored system, quantitative H2O2 was added to samples to-be-determined for deepest color. The presence of glucose oxidase in urine led to the formation of H2O2 and the reduction of TMBred. As a result of this, the color of the urine faded and the solution changed from deep blue to light blue. We measured the illuminance of the transmitted light by a smartphone ambient light sensor, and thereby color changes were used to calculate the content of urine glucose. After method validation, this colorimetric assay was practically applied for the determination of urine samples from diabetic patients. Good linearity was obtained in the range of 0.039–10.000 mg/mL (R2 = 0.998), and a limit of detection was 0.005 mg/mL. Our method was had high accuracy, sensitivity, simplicity, rapidity, and visualization, providing a new sensor to be potentially applicable for point-of-care detection of urine glucose. Full article
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2019

Jump to: 2022, 2020, 2018

13 pages, 3839 KiB  
Article
Sequential Detection of Palladium and Chromium Oxyanion by a Fluorescein Based Chemosensor in Mixed Aqueous Media
by Aasif Helal
Chemosensors 2020, 8(1), 4; https://doi.org/10.3390/chemosensors8010004 - 26 Dec 2019
Cited by 4 | Viewed by 3539
Abstract
A new highly selective chemosensor, based on fluorescein-allyloxy benzene conjugate 1, was developed for the sequential detection of palladium and chromium oxyanions in a mixed aqueous media, and was studied by UV-visible and fluorescence spectroscopy. The sensing of palladium ions produces a [...] Read more.
A new highly selective chemosensor, based on fluorescein-allyloxy benzene conjugate 1, was developed for the sequential detection of palladium and chromium oxyanions in a mixed aqueous media, and was studied by UV-visible and fluorescence spectroscopy. The sensing of palladium ions produces a chemodosimetric and ratiometric change in the emission band of 1 from 450 to 525 nm, followed by the sensing of chromate ions by 2 that quenches the emission band at 525 nm in a buffered H2O: DMF solution (9:1, pH = 7.4). The rate constants of palladium and chromate ions were found to be 8.6 × 105 M−1, 2.1 × 105 M−1, and 5.4 × 104 M−1 respectively. The chemosensor 1 has a palladium detection limit of 49 ppb while the sequential detection limit of chromate ions (CrO42− and Cr2O72−) were 127 and 259 ppb. The ratiometric change in the emission is produced due to the deallylation of 1 by palladium to produce 2 that restores the ESIPT (excited state intramolecular proton transfer) of the phenolic ring and enhances the electron transfer (ET) phenomenon from the phenolic group to fluorescein. The sequential binding of chromate ions to 2 inhibits the ESIPT and causes chelation enhanced quenching (CHEQ) of the fluorescence. Full article
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10 pages, 2659 KiB  
Article
Colorimetric Detection of Multiple Metal Ions Using Schiff Base 1-(2-Thiophenylimino)-4-(N-dimethyl)benzene
by Ali Q. Alorabi, Mohamed Abdelbaset and Sami A. Zabin
Chemosensors 2020, 8(1), 1; https://doi.org/10.3390/chemosensors8010001 - 18 Dec 2019
Cited by 39 | Viewed by 9715
Abstract
In this paper, a Schiff base ligand 1-(2-thiophenylimino)-4-(N-dimethyl)benzene (SL1) bearing azomethine (>C=N-) and thiol (-SH) moieties capable of coordinating to metals and forming colored metal complexes was synthesized and examined as a colorimetric chemosensor. The sensing ability toward the metal ions [...] Read more.
In this paper, a Schiff base ligand 1-(2-thiophenylimino)-4-(N-dimethyl)benzene (SL1) bearing azomethine (>C=N-) and thiol (-SH) moieties capable of coordinating to metals and forming colored metal complexes was synthesized and examined as a colorimetric chemosensor. The sensing ability toward the metal ions of Cu2+, Cr3+, Fe2+ Ni2+, Co2+, Mg2+, Zn2+, Fe2+, Fe3+, NH4VO3 (V5+), Mn2+, Hg2+, Pb2+, and Al3+ was investigated in a mixture of H2O and dimethylformamide (DMF) solvent using the UV–Visible spectra monitoring method. The synthesized Schiff base ligand showed colorimetric properties with Cr3+, Fe2+, Fe3+, and Hg2+ ions, resulting in a different color change for each metal that could be identified easily with the naked eye. The UV–Vis spectra indicated a significant red shift (~69–288 nm) from the origin after the addition of the ligand to these metal ions, which may be due to ligand-to-metal charge-transfer (LMCT). On applying Job’s plot, it was indicated that the ligand binds to the metal ions in a 2:1 ligand-to-metal molar ratio. SL1 behaves as a bidentate ligand and binds through the N atom of the imine group and the S atom of the thiol group. The results indicate that the SL1 ligand is an appropriate coordination entity and can be developed for use as a chemosensor for the detection of Cr3+, Fe2+, Fe3+, and Hg2+ ions. Full article
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16 pages, 2300 KiB  
Article
Optical Sensing of Nitrogen, Phosphorus and Potassium: A Spectrophotometrical Approach toward Smart Nutrient Deployment
by Filipe Monteiro-Silva, Pedro A. S. Jorge and Rui C. Martins
Chemosensors 2019, 7(4), 51; https://doi.org/10.3390/chemosensors7040051 - 29 Oct 2019
Cited by 30 | Viewed by 6549
Abstract
The feasibility of a compact, modular sensing system able to quantify the presence of nitrogen, phosphorus and potassium (NPK) in nutrient-containing fertilizer water was investigated. Direct UV-Vis spectroscopy combined with optical fibers were employed to design modular compact sensing systems able to record [...] Read more.
The feasibility of a compact, modular sensing system able to quantify the presence of nitrogen, phosphorus and potassium (NPK) in nutrient-containing fertilizer water was investigated. Direct UV-Vis spectroscopy combined with optical fibers were employed to design modular compact sensing systems able to record absorption spectra of nutrient solutions resulting from local producer samples. N, P, and K spectral interference was studied by mixtures of commercial fertilizer solutions to simulate real conditions in hydroponic productions. This study demonstrates that the use of bands for the quantification of nitrogen with linear or logarithmic regression models does not produce analytical grade calibrations. Furthermore, multivariate regression models, i.e., Partial Least Squares (PLS), which consider specimens interference, perform poorly for low absorbance nutrients. The high interference present in the spectra has proven to be solved by an innovative self-learning artificial intelligence algorithm that is able to find interference modes among a spectral database to produce consistent predictions. By correctly modeling the existing interferences, analytical grade quantification of N, P, and K has proven feasible. The results of this work open the possibility of real-time NPK monitoring in Micro-Irrigation Systems. Full article
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12 pages, 4759 KiB  
Article
Detecting Fingerprints of Waterborne Bacteria on a Sensor
by Yeşeren Saylan, Özgecan Erdem, Nilüfer Cihangir and Adil Denizli
Chemosensors 2019, 7(3), 33; https://doi.org/10.3390/chemosensors7030033 - 25 Jul 2019
Cited by 36 | Viewed by 4022
Abstract
Human fecal contamination is a crucial threat that results in difficulties in access to clean water. Enterococcus faecalis is a bacteria which is utilized as an indicator in polluted water. Nevertheless, existing strategies face several challenges, including low affinity and the need for [...] Read more.
Human fecal contamination is a crucial threat that results in difficulties in access to clean water. Enterococcus faecalis is a bacteria which is utilized as an indicator in polluted water. Nevertheless, existing strategies face several challenges, including low affinity and the need for labelling, which limit their access to large scale applications. Herein, a label-free fingerprint of the surface proteins of waterborne bacteria on a sensor was demonstrated for real-time bacteria detection from aqueous and water samples. The kinetic performance of the sensor was evaluated and shown to have a range of detection that spanned five orders of magnitude, having a low detection limit (3.4 × 104 cfu/mL) and a high correlation coefficient (R2 = 0.9957). The sensor also designated a high selectivity while other competitor bacteria were employed. The capability for multiple usage and long shelf-life are superior to other modalities. This is an impressive surface modification method that uses the target itself as a recognition element, ensuring a broad range of variability to replicate others with different structure, size and physical and chemical properties. Full article
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2018

Jump to: 2022, 2020, 2019

11 pages, 2702 KiB  
Article
Stability and Safety Assessment of Phosphorescent Oxygen Sensors for Use in Food Packaging Applications
by Caroline A. Kelly, Malco Cruz-Romero, Joseph P. Kerry and Dmitri B. Papkovsky
Chemosensors 2018, 6(3), 38; https://doi.org/10.3390/chemosensors6030038 - 06 Sep 2018
Cited by 13 | Viewed by 4357
Abstract
Five types of new solid-state oxygen sensors, four based on microporous polypropylene fabric materials and one on polyphenylene sulphide films impregnated with phosphorescent platinum(II)-benzoporphyrin dye, were tested for their stability and safety in food packaging applications. All these sensors exhibit useful optical signals [...] Read more.
Five types of new solid-state oxygen sensors, four based on microporous polypropylene fabric materials and one on polyphenylene sulphide films impregnated with phosphorescent platinum(II)-benzoporphyrin dye, were tested for their stability and safety in food packaging applications. All these sensors exhibit useful optical signals (phosphorescence lifetime readout) and working characteristics and are simpler and cheaper to produce and integrate into standard packaging materials than existing commercial sensors. When exposed to a panel of standard food simulants and upon direct contact with raw beef and chicken meat and cheddar cheese samples packaged under modified atmosphere, the sensors based on ungrafted polypropylene fabric, impregnated with PtBP dye by the swelling method, outperformed the other sensors. The sensors are also stable upon storage under normal atmospheric conditions for at least 12 months, without any significant changes in calibration. Full article
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