Ratiometric sensing of CO2 in ionic liquid modified ethyl cellulose matrix

A novel optical dual sensor based on a coaxial electrospinning method for simultaneous sensing of oxygen and ammonia

The coaxial electrospinning method is widely used in a wide range of applications, including medical devices and sensing technology. This study proposes a novel optical dual sensor for simultaneous detection of oxygen (O2) and ammonia (NH3) based on coaxial electrospinning method to produce core-shell fiber membrane doped fluorescent dyes. The O2 (core) and NH3 (shell) sensitive dye membranes were successfully fabricated using coaxial electrospinning method by dissolving a polymer matrix, cellulose acetate (CA), with platinum (II) meso-tetrakis (pentafluorophenyl) porphyrin (PtTFPP) and Eosin-Y, respectively. The optical dual sensor was illuminated by an UV LED to monitor the intensity change and wavelength shift in the presence of selected analyte gases. The experimental data show that the sensitivities of optical dual sensor were found to be 6.4 and 3.2 for O2 and NH3, respectively. The response and recovery times of O2 and NH3 sensing probes were measured to be 12 s/29 s and 65 s/66 s, respectively. Also, when exposed to NH3 gas gradually from 0 to 500 ppm, the wavelength shift data of Eosin-Y was started at 569.5 nm, 573.9 nm, 578.4 nm, 579.4 nm, 580.8 nm, and 582.2 nm, respectively. In applications, the proposed optical dual sensor based on coaxial electrospinning method can detect O2 and NH3 gases simultaneously.

An emission based optical CO2 sensor fabricated on grating-like TiO2 substrates using HPTS

TiO2 is a widely used material in various applications, including photo-catalysis, sensing, and dye-sensitized solar cells. In this work, we presented a stable and sensitive CO2 sensor working on principle of emission signals of the HPTS dye immobilized on TiO2 substrates whose effectiverefractive indicesin the parallel and perpendicular grating orientations evidenced earlier. The HPTS exhibited 8.4-fold enhanced emission signal on the offered substrates with respect to the previously reported intensities recorded for the polymeric supports. The emission peaks of the HPTS exhibited CO2 induced intensity quenching when excited at 466 nm. Calibration sensitivity of the offered composites has been tested exposing the sensor materials to varying concentrations of the CO2 between 0.0 and 100.0 % pCO2 and correlating the measured fluorescence intensity with the corresponding CO2 levels. The dye revealed enhanced CO2 induced response exhibiting the I0/100 values of 11.9, 13.3, 11.2 and 6.5. We also recorded bi-exponential excited state lifetimes for the HPTS on four different test materials both in the absence and presence of the CO2. The emission based variations were followed as the analytical signal due to higher CO2-induced relative signal changes than that of the lifetime based variations. Herein we used the ion pair form of the HPTS in a protective chemical microenvironment and obtained considerable long term stability extending to16 mounts which can be attributed to the presence of the 1-butyl-3-methylimidazolium tetrafluoroborate as an internal buffering system in the sensing composition as well as the inert and robust structure of the substrate. The extremely high optical response, advantage of excitation with blue LEDs and long-term stability observed on grating-like TiO2 surfaces make the proposed system a promising design for the quantification of CO2 for further applications.

Effect on Improving CO2 Sensor Properties: Combination of HPTS and γ-Fe2O3@ZnO Bioactive Glass

8-Hydroxypyrene-1,3,6-trisulfonic acid (HPTS) dye, a fluorescent dye often used as a pH indicator, is embedded within the bioactive glass matrix and undergoes changes in its fluorescent properties when exposed to carbon dioxide (CO2). The aim of the current study is to investigate the use of bioactive glass (BG) particles containing γ-Fe2O3@ZnO to enhance the CO2 sensitivity of HPTS. X-ray diffraction, Fourier transform infrared, scanning electron microscopy, and photoluminescence spectroscopies were used to characterize the sol-gel synthesized powders. The sensing slides were prepared in the form of a thin film by immobilizing the fluorescent dye and γ-Fe2O3@ZnO-based additives into the poly(methyl methacrylate) matrix. The addition of γ-Fe2O3@ZnO nanoparticles with bioactive glass additives to the HPTS improves the performance characteristics of the sensor, including the linear response range, relative signal variation, and sensitivity. Meanwhile, the CO2 sensitivities were measured as 10.22, 7.73, 16.56, 17.82, 19.58, and 42.40 for the undoped form and M, M@ZnO, 5M@ZnO-BG, 10M@ZnO-BG, and 20M@ZnO-BG NP-doped forms of the HPTS-based thin films, respectively. The response and recovery times of the HPTS-based sensing slide along with 20M@ZnO-BG NPs have been measured as 44 and 276 s, respectively. The γ-Fe2O3/ZnO-containing BG particle-doped HPTS composites can be used as a promising sensor agent in the detection of CO2 gas in various fields such as environmental monitoring, medical diagnostics, and industrial processes.

Improvement of the CO2 Sensitivity: HPTS-Based Sensors along with Zn@SnO2 and Sn@ZnO Additives

Fluorescent pH-sensitive indicator dye, 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS), has become known as a preferred alternative for continuous and accurate monitoring of dissolved and/or gaseous CO2 in chemistry, medical, and biochemical research. The objective of this work is to enhance the HPTS dye's CO2 sensitivity in the presence of Zn@SnO2 and Sn@ZnO additive particles. Sol-gel synthesized metal oxide semiconductors (MOSs) were characterized using XRD, XPS, and SEM. The fluorophore dye and the MOS additives were embedded in the ethyl cellulose (EC) polymeric matrix to prepare the sensing thin films. The steady-state and decay kinetic measurements of the HPTS-based composites were obtained by PL spectroscopy for the concentration ranges of 0-100% p[CO2]. As expected, the addition of MOSs improves the sensor characteristics, specifically its CO2 sensing ability, linear response range, and relative signal change compared to the free form of HPTS. The CO2 sensitivities of the HPTS-based thin films were found at 17.6, 23.2, and 40.9 for the undoped, Zn@SnO2,-doped, and Sn@ZnO-doped forms of the HPTS, respectively. Additionally, the response and recovery times of the HPTS-based sensor agent with Sn@ZnO were measured as 10 and 460 s, respectively. The obtained results demonstrate that materials composed of HPTS with MOSs are potential candidates for CO2 sensors.

Carbon Dioxide Sensing-Biomedical Applications to Human Subjects

Carbon dioxide (CO2) monitoring in human subjects is of crucial importance in medical practice. Transcutaneous monitors based on the Stow-Severinghaus electrode make a good alternative to the painful and risky arterial "blood gases" sampling. Yet, such monitors are not only expensive, but also bulky and continuously drifting, requiring frequent recalibrations by trained medical staff. Aiming at finding alternatives, the full panel of CO2 measurement techniques is thoroughly reviewed. The physicochemical working principle of each sensing technique is given, as well as some typical merit criteria, advantages, and drawbacks. An overview of the main CO2 monitoring methods and sites routinely used in clinical practice is also provided, revealing their constraints and specificities. The reviewed CO2 sensing techniques are then evaluated in view of the latter clinical constraints and transcutaneous sensing coupled to a dye-based fluorescence CO2 sensing seems to offer the best potential for the development of a future non-invasive clinical CO2 monitor.

Tuning CO2 sensing properties of HPTS along with newly synthesized coordination polymers (CPs)

Coordination polymers (CPs), new functional hybrid materials, have received important attention due to their structural features and many different applications such as gas storage, catalysis, energy storage, small molecule adsorption, luminescence, and chemical sensors. In this study, two newly synthesized metal-organic frameworks (MOFs); [Mn₂(µ₄-dmg)-(µ₄-dmg)(µ-bipy)]_n (CP1) and [Mn₂(µ₄-dmg)(µ₄-dmg)(µ-dpeten)]_n (CP2); were used for sensing of gaseous carbon dioxide with a spectrofluorometric method with a dye, 8-hydroxypyrene-1, 3, 6-trisulfonic acid (HPTS) as matrix additive material for the first time. The ion-pair form of the HPTS was used in the polymethyl methacrylate matrix as a thin film form. When the HPTS based sensing slides along with the CPs, resulted in many advances such as high relative signal change and larger linear response range, improved sensor dynamics, and higher sensitivity with respect to the additive-free form. More specifically, in the presence of silver nanoparticles (AgNPs), the variation in relative signal intensities of CP1 and CP2 doped HPTS sensing agents were measured as 60 and 40% for the concentration range of 0-10% pCO₂, respectively. The aim of this study is to enhance the CO₂ response of HPTS with doped CPs due to their useful attributes and potential applications containing selective gas absorption.

Usage of thiocyanate-based ionic liquid as new optical sensor reagent: Absorption and emission based selective determination of Fe (III) ions

In this work, the ionic liquid 1-butyl-3-methylimidazolium thiocyanate ([BMIM][SCN]) was evaluated for the first time for its probable usage as new optical sensor reagent for the determination of several metal ions. The ionic liquid exhibited a selective and sensitive response to iron ions in acidic aqueous solutions among all of the tested metal ions. The ([BMIM][SCN]) was encapsulated in ethyl cellulose (EC) matrix in the form of continuous thin films. The effect of [BMIM][SCN] concentration and pH to iron response, the fluorescence quantum yield, the absorption, emission and excitation based characteristics of the ionic liquid in presence of Fe³⁺ and Fe²⁺ ions were investigated in both EC and [BMIM][SCN]/aqueous buffer solution mixtures. As a result, the highly sensitive, selective and rapid responding optical sensor reagent which does not need any time-consuming extraction, oxidation and reduction procedures was presented for the distinguishing determination of Fe³⁺ and Fe²⁺ in both aqueous solutions and solid thin film matrix. The ionic liquid exhibited a better emission and absorption based response for Fe³⁺ ions when compared with the Fe²⁺ ions. The molar absorptivity constant in presence of ionic liquid-based SCN^- was enhanced 10 times to 1.21 × 10⁴ L mol^-1 cm^-1 for Fe³⁺ ions in the solution phase. Linear absorption and emission-based calibration graphs were obtained for a wide concentration range of 8.0 × 10^-8-6.2 × 10^-4 M and 8.0 × 10^-8-6.2 × 10^-5 M for Fe³⁺, respectively. Limit of detection (LOD) values for absorption and emission-based methods were 2.48 × 10^-5 and 2.4 × 10^-8, respectively. The reaction is instantaneous and absorbance remains stable for over 4 months.

Hierarchical TiO2:Cu2O Nanostructures for Gas/Vapor Sensing and CO2 Sequestration

We investigate the effect of high-surface-area self-assembled TiO₂:Cu₂O nanostructures for CO₂ and relative humidity gravimetric detection using polyethylenimine (PEI), 1-ethyl-3-methylimidazolium (EMIM), and polyacrylamide (PAAm). Introduction of hierarchical TiO₂:Cu₂O nanostructures on the surface of quartz crystal microbalance sensors is found to significantly improve sensitivity to CO₂ and to H₂O vapor. The response of EMIM to CO₂ increases fivefold for 100 nm-thick TiO₂:Cu₂O as compared to gold. At ambient CO₂ concentrations, the hierarchical assembly operates as a sensor with excellent reversibility, while at higher pressures, the CO₂ desorption rate decreases, suggesting possible application for CO₂ sequestration under these conditions. The gravimetric response of PEI to CO₂ increases by a factor of 3 upon introduction of a 50 nm TiO₂:Cu₂O layer. The PAAm gravimetric response to water vapor also increases by a factor of 3 and displays improved reversibility with the addition of 50 nm TiO₂:Cu₂O structures. We found that TiO₂:Cu₂O can be used to lower the detection limits for CO₂ sensing with EMIM and PEI and lower the detection limits for H₂O sensing with PAAm by over a factor of 2. Coarse-grained and all-atom molecular dynamics simulations indicate the dissociative character of ionic liquid assembly on TiO₂:Cu₂O interfaces and different distributions of CO₂ and H₂O molecules on bare and ionic liquid-coated surfaces, confirming experimental observations. Overall, our results show high potential of hierarchical assemblies of TiO₂:Cu₂O/room temperature ionic liquid and polymer films for sensors and CO₂ sequestration.

Dual pO2/pCO2 fibre optic sensing film

A fibre optic multi-sensor has been developed for biomedical sensing applications using a tip coating solution sensitive to both oxygen and carbon dioxide. An oxygen sensitive phosphorescence quenching complex based on platinum octaethylporphyrin (PtOEP) was combined with a carbon dioxide sensitive phosphorescence compound based on 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (HPTS). When excited by blue light (470 nm), the resultant coating had two fluorescent peaks at 515 nm (green) and 645 nm (red) which responded to partial pressure of CO₂ and O₂ respectively. The sensor was tested in vitro and shown to be able to measure CO₂ and O₂ simultaneously and in real time, with calibration constants of 0.0384 kPa^-1 and 0.309 kPa^-1 respectively. The O₂ sensitive peak received some overlap from the 515 nm peak (0.38% of peak intensity) as well as some cross-sensitivity (maximum, 5.1 kPa pCO₂ gave a measurement equivalent to 0.43 kPa of O₂, a ratio of 0.08 : 1). However, these effects can be subtracted from measurements and no significant cross-sensitivity or overlap was seen in CO₂ measurements from O₂. This novel compound presents great potential for use in medical sensors and we expect it to be important to a wide range of future applications.

Ionic liquids on optical sensors for gaseous carbon dioxide

This work presents a study on the influence of eight different ionic liquids (ILs) in the composition of dry membranes used for gaseous CO₂ optical sensing. The presence of CO₂ causes a displacement of a colorimetric pH indicator toward its acid form that increases the emission intensity of the luminophore by an inner filter process. The influence of ILs in the membrane on the stability and dynamic behavior-usually the main drawbacks of these sensors-of the membranes is studied. The characterization of the different membranes prepared was carried out and the discussion of the results is presented. In all cases, the response and recovery times improved considerably, with the best case being response times of only 10 s and recovery times of 48 s, compared to response and recovery times of 41 and 100 s, respectively, for membranes without IL. The useful life of the detection membranes is also considerably longer than that of membranes that do not include IL, at least 292 days in the best case. The sensing membrane without luminophore and only containing the pH indicator is proposed for the color-based measurement of CO₂ using a digital camera for possible use in food-packaging technology. Graphical abstract ᅟ.

Conjugated Polyelectrolyte-Based New Strategy for in Situ Detection of Carbon Dioxide

A conjugated polymer centered on fluorene and 2,1,3-benzothia-diazole (PFBT) is prepared for sensing CO₂ in situ with high sensitivity and low background. Upon introducing CO₂, the weaker electrostatic repulsion and stronger hydrophobic interactions between neighboring PFBT molecules enhance the interchain contacts compared to that without CO₂, leading to the energy transfer from fluorene to 2,1,3-benzothia-diazole sites and the emission color shift from blue to green, which is sensitive to sensing CO₂ in atmospheric air with a content of ∼400 ppm. Importantly, PFBT is employed to monitor photosynthesis and respiration upon cycling day and night in situ.

Polyoxy-Derivatized Perylenediimide as Selective Fluorescent Ag (I) Chemosensor

Recent investigations indicated that same concentrations of the ionic silver have harmful effects on aquatic life, bacteria and human cells. Herein we report chemosensory properties of N,N ^' -Bis(4-{2-[2-(2-methoxyethoxy)ethoxy] eth- oxy}phenyl) -3,4:9,10-perylene tetracarboxydiimide (PERKAT) towards ionic silver. The dye doped sensing agents were prepared utilizing ethyl cellulose (EC) and poly (methylmethacrylate) (PMMA) and then forwarded to electrospinning to prepare sensing fibers or mats. The PERKAT exhibited bright emission in embedded forms in EC or in the solvents of N,N-Dimethylformamide (DMF), Dichloromethane (DCM), Tetrahydrofurane (THF) and in the mixture of DCM/ethanol. The PERKAT exhibited selective and linear response for ionic silver in the concentration range of 10^-10 - 10^-5 M Ag (I) at pH 5.5. Detection limits were found to be 2.6 × 10^-10 and 4.3 × 10^-11 M, in solution phase studies and PERKAT doped sensing films, respectively. Cross sensitivity of the PERKAT towards pH and some metal ions was also studied. There were no response for the Li⁺, Na⁺, K⁺, Ca²⁺, Ba²⁺, Mg²⁺, NH₄⁺, Ni²⁺, Co²⁺, Cu²⁺,Pb²⁺, Al³⁺, Cr³⁺,Mn²⁺, Sn²⁺, Hg⁺, Hg²⁺, Fe²⁺ and Fe³⁺ in buffered solutions. To the best of our knowledge, this is the first study investigating silver sensing abilities of the PERKAT.

Highly CO2sensitive extruded fluorescent plastic indicator film based on HPTS

A HPTS-based, very CO₂-sensitive plastic film is reported which is yellow, and fluoresces green in the absence of CO₂, and colourless and fluoresces blue in the presence of CO₂.

Ionic Liquid-Based Optical and Electrochemical Carbon Dioxide Sensors

Due to their unusual physicochemical properties (e.g., high thermal stability, low volatility, high intrinsic conductivity, wide electrochemical windows and good solvating ability), ionic liquids have shown immense application potential in many research areas. Applications of ionic liquid in developing various sensors, especially for the sensing of biomolecules, such as nucleic acids, proteins and enzymes, gas sensing and sensing of various important ions, among other chemosensing platforms, are currently being explored by researchers worldwide. The use of ionic liquids for the detection of carbon dioxide (CO₂) gas is currently a major topic of research due to the associated importance of this gas with daily human life. This review focuses on the application of ionic liquids in optical and electrochemical CO₂ sensors. The design, mechanism, sensitivity and detection limit of each type of sensor are highlighted in this review.

Methods and approaches of utilizing ionic liquids as gas sensing materials

Gas monitoring is of increasing significance for a broad range of applications in the fields of environmental and civil infrastructures, climate and energy, health and safety, industry and commerce. Even though there are many gas detection devices and systems available, the increasing needs for better detection technologies that not only satisfy the high analytical standards but also meet additional device requirements (e.g., being robust to survive under field conditions, low cost, small, smart, more mobile), demand continuous efforts in developing new methods and approaches for gas detection. Ionic Liquids (ILs) have attracted a tremendous interest as potential sensing materials for the gas sensor development. Being composed entirely of ions and with a broad structural and functional diversity, i.e., bifunctional (organic/inorganic), biphasic (solid/liquid) and dual-property (solvent/electrolyte), they have the complementing attributes and the required variability to allow a systematic design process across many sensing components to enhance sensing capability especially for miniaturized sensor system implementation. The emphasis of this review is to describe molecular design and control of IL interface materials to provide selective and reproducible response and to synergistically integrate IL sensing materials with low cost and low power electrochemical, piezoelectric/QCM and optical transducers to address many gas detection challenges (e.g., sensitivity, selectivity, reproducibility, speed, stability, cost, sensor miniaturization, and robustness). We further show examples to justify the importance of understanding the mechanisms and principles of physicochemical and electrochemical reactions in ILs and then link those concepts to developing new sensing methods and approaches. By doing this, we hope to stimulate further research towards the fundamental understanding of the sensing mechanisms and new sensor system development and integration, using simple sensing designs and flexible sensor structures both in terms of scientific operation and user interface that can be miniaturized and interfaced with modern wireless monitoring technologies to achieve specifications heretofore unavailable on current markets for the next generation of gas sensor applications.

A simple BODIPY-aniline-based fluorescent chemosensor as multiple logic operations for the detection of pH and CO2gas

4-Aniline BODIPY dye was developed as a highly sensitive fluorescent chemosensor for the detection of pH and CO₂gas.

Copper ion sensing with fluorescent electrospun nanofibers

In this work, the use of electrospun nanofibrous materials as highly responsive fluorescence quenching-based copper sensitive chemosensor is reported. Poly(methyl methacrylate) and ethyl cellulose were used as polymeric support materials. Sensing slides were fabricated by electrospinning technique. Copper sensors based on the change in the fluorescence signal intensity of fluoroionophore; N'-3-(4-(dimethylamino phenly)allylidene)isonicotinohydrazide. The sensor slides exhibited high sensitivities due to the high surface area of the nanofibrous membrane structures. The preliminary results of Stern-Volmer analysis show that the sensitivities of electrospun nanofibrous membranes to detect Cu(II) ions are 6-20-fold higher than those of the continuous thin films. By this way we obtained linear calibration plots for Cu(II) ions in the concentration range of 10(-12)-10(-5)M. The response times of the sensing slides were less than 1 min. Stability of the employed ionophore in the matrix materials was excellent and when stored in the ambient air of the laboratory there was no significant drift in signal intensity after 6 months. Our stability tests are still in progress.

Ratiometric fluorescence-based dissolved carbon dioxide sensor for use in environmental monitoring applications

The focus of this work is on the development and characterisation of a fluorescence-based ratiometric sol-gel-derived dissolved carbon dioxide (dCO(2)) sensor for use in environmental monitoring applications. Fluorescence-based dCO(2) sensors are attractive as they facilitate the development of portable and low-cost systems that can be easily deployed outside the laboratory environment. The sensor developed for this work exploits a pH fluorescent dye 1-hydroxypyrene-3,6,8-trisulfonic acid, ion-paired with cetyltrimethylammonium bromide (HPTS-IP), which has been entrapped in a hybrid sol-gel-based matrix derived from n-propyltriethoxysilane along with the liphophilic organic base. The sensor spot deposited on a cover slip has been interrogated with a robust, ratiometric optical probe that combines effective fluorescence excitation and detection and thus facilitates the production of a highly sensitive sensor system using low-cost optoelectronic components. The probe design involves the use of dual-LED excitation in order to facilitate ratiometric operation and uses a silicon PIN photodiode. HPTS-IP exhibits two pH-dependent changes in excitation bands, which allows for dual excitation ratiometric detection as an indirect measure of the dCO(2). Such measurements are insensitive to changes in dye concentration, leaching and photobleaching of the fluorophore and instrument fluctuations unlike unreferenced fluorescence intensity measurements. The performance of the sensor system is characterised by a high degree of repeatability, reversibility and stability. Calculated limit of detection for the sensor was 35 ppb. The sensor probe was used to monitor dCO(2) levels in a laboratory-based aquatic habitat, and the expected diurnal pattern was clearly visible. The influence of temperature, biofouling and photobleaching on sensor performance has been also investigated.

Phosphorescent sensing of carbon dioxide based on secondary inner-filter quenching

A study of different strategies to prepare phosphorescence-based sensors for gaseous CO(2) determination has been performed. It includes the characterization of different configurations tested, a discussion of the results obtained and possibilities for the future. The optical sensor for gaseous CO(2) is based on changes in the phosphorescence intensity of the platinum octaethylporphyrin (PtOEP) complex trapped both on oxygen-insensitive poly(vinylidene chloride-co-vinyl chloride) (PVCD) membranes and PVCD microparticles, due to the displacement of the alpha-naphtholphthalein acid-base equilibrium with CO(2) concentration. A secondary inner-filter mechanism was tested for the sensor and a full range linearized calibration was obtained by plotting (I(100)-I(0))/(I-I(0)) versus the inverse of the CO(2) concentration, where I(0) and I(100) are the detected luminescence intensities from a membrane exposed to 100% nitrogen and 100% CO(2), respectively, and I at a defined CO(2) concentration. The different configurations tested included the use of membranes containing luminophore and pH-sensitive dye placed on two opposite sides of a transparent support to prevent the observed degradation of the PtOEP complex in the presence of the tetraoctylammonium hydroxide (TOAOH) phase transfer agent, which produced better results regarding stability and sensitivity. The CO(2) gas sensor based on PtOEP homogeneous membranes presented better properties in terms of response time and sensitivity than that based on PtOEP microparticles. With a detection limit of 0.02%, the response time (10-90% maximum signal) is 9 s and the recovery time (90-10%) is 115 s. The lifetime of the membranes for CO(2) sensing preserved in a 94% RH atmosphere and dark conditions is longer than at least 4 months.