Cation Sensor Research Articles

Hydrazone derivative of 2-hydroxyquinoline-3-carbaldehyde: an efficient anionic and cationic sensor

A hydrazone (1) based on 2-hydroxyquinoline-3-carbaldehyde was synthesized and its anion and cation detection ability were studied. It could detect both fluoride in acetonitrile selectively among anions and copper ions in semiaqueous medium among cations. The addition of fluoride ion to the acetonitrile solution of the receptor produced a sharp colour change from light yellow to bluish green. The corresponding UV–vis measurements showed a red shift of the band of receptor 1 for fluoride and a blue shift of the band for copper ions. The fluorescence intensity of the receptor 1 got quenched with both fluoride and copper ions. The detection limits for both the ions are in order of micromolar level. The practical applications of fluoride detection were extended to oral care products.

Nanocomposite cross-linked conjugated polyelectrolyte/MWCNT/poly(pyrrole) for enhanced Mg2+ ion sensing and environmental remediation in real samples

Conjugated polyelectrolyte/MWCNT/poly(pyrrole) (CPE-K/MWCNT/PPY) composites have been prepared by a simple solution mixing technique. Synthesis of PPY/CPE/MWCNT composites was characterized by scanning electron microscopy and thermogravimetric analysis. These nanocomposites of CPE-K/MWCNT/PPY were deposited onto a glassy carbon electrode (GCE) in order to fabricate a reliable Mg2+ cationic sensor. A plot called the calibration curve is produced from the current as a function of concentration of Mg2+ ion. This plot was found to be linear in the range 0.1 nM to 0.01 mM, which is defined as the linear dynamic range. According to the surface area of the GCE (0.0316 cm2) and the slope of the calibration curve, a high sensitivity (19.4968 µA µM–1 cm–2) of the Mg2+ ion sensor was calculated. A significant lower limit of detection (97.71 ± 4.89 pM) was obtained in association with a signal to noise ratio of 3. Sensor parameters such as reproducibility, long-time stability, and short response time were measured and found to be quite satisfactory. This new approach to Mg2+ detection with high sensitivity is suggested for application as a novel multifunctional sensing system. Besides, it was found to be very effective to analyze real environmental samples.

Gamma irradiation of graphene quantum dots with ethylenediamine: Antioxidant for ion sensing

Due to the low consumption of chemicals, the absence of toxic residual side products, the procedure simplicity and time-saving aspects, gamma irradiation offers advantages over the classical chemical protocols. We successfully employed gamma irradiation in order to introduce N-atoms in Graphene Quantum Dots (GQDs). By irradiating GQDs water dispersions in the presence of isopropyl alcohol and ethylenediamine, at doses of 25, 50 and 200 kGy, we attached amino groups onto GQDs in a single synthetic step. At the same time, a chemical reduction is achieved, too. Selected conditions induced incorporation of N-atoms within GDQs atomic lattice (around 3 at%), at all applied doses. Additionally, the C-atoms percentage was highly increased, from 63 to 79 at% or higher. The zeta potential of dots changed from −34.6 to +9.1 mV, due to the modification of functionalizing groups localized at the surface. Produced chemical changes lead to the desired alteration of the GQDs optical properties, such as an increased photoluminescence intensity, a higher photoluminescence quantum yields (from 2.07 to 18.40%) and a narrowing of the spectral features in the emission spectra. The ability of gamma-irradiated GQDs to quench free radical species was investigated and positively assessed; additionally, non-enzymatic optical detection of Cu(II) ions using GQDs as a sensor was studied and the detection limits are herein reported. These results suggest that GQDs can be potentially applied as smart photoluminescent sensors for metal cations.

A New Cr3+ Electrochemical Sensor Based on ATNA/Nafion/Glassy Carbon Electrode.

A new electrochemical sensor of metal cation in an aqueous solution based on homobifunctional tridentate disulfide Schiff base and named 1,1′-(-((disulfanediylbis(2,1-phenylene))bis(azaneylylidene))bis(methaneylylidene))bis(naphthalene-2-ol) (ATNA) was easily obtained quantitatively from the condensation reaction of 2-hydroxy-1-naphthaldehyde and 2-aminothiophenol, and then fully characterized by spectroscopic techniques for structure elucidation. The molecular structure of ATNA was also confirmed by a single-crystal X-ray diffraction study to reveal a new conformation in which the molecule was stabilized by the O–H…N type intramolecular hydrogen bonding interactions in both moieties. The ATNA was used as a selective electrochemical sensor for the detection of chromium ion (Cr3+). A thin film of ATNA was coated on to the flat surface of glassy carbon electrode (GCE) followed by 5 % ethanolic Nafion in order to make the modified GCE (ATNA/Nafion/GCE) as an efficient and sensitive electrochemical sensor. It was found to be very effective and selective against Cr3+ cations in the company of other intrusive heavy metal cations such as Al3+, Ce3+, Co2+, Cu2+, Ga3+, Hg2+, Mn2+, Pb2+, and Y3+. The detection limit at 3 S/N was found to be 0.013 nM for Cr3+ ions within the linear dynamic range (LDR) (0.1 nM–10.0 mM) of Cr3+ ions with r2 = 0.9579. Moreover; this work instigates a new methodology for developing the sensitive as well as selective electrochemical toxic cationic sensors in the field of environmental and health care.

SYNTHETIC METHODS, PROPERTIES AND APPLICATION OF CONDENSATION PRODUCTS OF 1,8-NAPHTHALIC ANHYDRIDES WITH 1,2-DIAMINES (A REVIEW)

The literary data concerning preparation methods and spectral properties of cyclocondensation products of 1,8-naphthalic anhydrides with aromatic and aliphatic 1,2-diamines and their use as luminescent materials have been summarized. The reaction of 1,8-naphthalic anhydrides with ortho-phenylenediamine results in the formation of 1,8-naphthoylene-1’,2’-benzimidazoles, which are widely used as materials for luminescent flaw detection, for synthesis of polymers with luminescent color, in organic light emitting diodes, as luminescent sensors for cations of various metals. Moreover, their cytotoxic activity is studied in relation to cells of various tumors. Acenaphthoquinone or 8-formyl-1-naphthoic acid can also be used as starting materials for synthesis of 1,8-naphthoylene-1’,2’-benzimidazoles. The spectral properties of 1,8-naphthoylene-1’,2’-benzimidazoles can be varied by the substituent introduction to naphthalene or/and phenyl moieties leading to luminescent materials with blue, green-yellow or red luminescence. Reaction of 4-substituted naphthalic anhydrides with o-phenylenediamine results in mixture of two constitutional isomers due to nonequivalence of two carbonyl groups in starting anhydride. The isomer ratio remains unchanged provided that syntheses are carried out under the same conditions, therefore the luminescent properties of such isomer mixtures are constant. 1,8-Naphthoylene-1’,2’-benzimidazoles with electron donating groups in positions 4 or 5 of naphthalene ring can be successfully obtained by nucleophilic aromatic substitution of halogen by amine or alcoholate. The introduction of electron donating substituents to positions 4 or 5 of naphthalene ring results in bathochromic shift of absorption and emission maxima compared to unsubstituted 1,8-naphthoylene-1’,2’-benzimidazole, corresponding shift is 10-20 nm larger for 5-isomer than for 4-isomer. There are few examples of applying amines other than o-phenylenediamine for cyclocondensation with naphthalic anhydrides, for instance naphthalene-2,3-diamine and ethylenediamine were used as starting materials.

The Performance of Various SWCNT Loading into CuO–PMMA Nanocomposites Towards the Detection of Mn2+ Ions

The well-dispersed CuO doped poly(methyl methacrylate) (CuO–PMMA) nanocomposites (NCs) with a successful loading of SWCNT as carbon nanofillers with different percentages (2%, 5%, and 10%) were fabricated effectively via the ex-situ polymerization technique. XRD, SEM–EDX, SEM-Map, TEM, AFM, Raman, and FTIR were carried out to confirm the formation of CuO–PMMA NCs, as well as the formation of CuO–PMMA–SWCNT NCs on the surface of CuO–PMMA sheet nanocomposite. The morphology of both CuO nanoparticles and SWCNT on the PMMA sheet, as well as agglomeration, concentration, roughness profiles, and size distribution, were considered during the experiments. BET surface area was carried out to determine the effect of various percentages of SWCNT loading on CuO–PMMA NC on the BET surface area. Furthermore, thermogravimetric analysis was conducted to evaluate the thermal behavior. Interestingly, the thermal stability of the NCs was inferred to significantly enhance with a proper amount of SWCNTs. EDX and elemental mapping of Cu, C, and O confirmed the presence of SWCNTs and CuO on the PMMA polymer matrix. In this approach, various loadings of SWCNT were performed into the CuO–PMMA–SWCNT NCs for the selective detection of metal ions using the electrochemical method. It exhibited the best performance in the detection of particular Mn2+ cation to 2% of SWCNT loading into NCs. The Mn2+ cationic sensor analytical response was investigated by CuO–PMMA–SWCNT (2%) NCs with glassy carbon electrode (GCE) coated uniform thin film using conductive nafion (5% nafion) binder. The sensor showed linear electrochemical responses over the concentration range of 0.1 nM to 0.01 mM identified through the calibration curve, which is known as the linear dynamic range. The sensor analytical performance sensitivity (88.16 µA µM−1 cm−2) and detection limit (92.67 ± 4.63 pM) were calculated from the slope of the calibration curve. The proposed Mn2+ ion sensor based on CuO–PMMA–SWCNT (2%) NCs/binder/GCE was found satisfactory in terms of reproducibility, linear dynamic range detection limit, sensitivity, stability, and response time. In real environmental samples, it also showed substantial performances during detection. This methodology can be inferred safely as a unique approach to develop metal ion sensors using inorganic-carbon NCs on GCE.

ArkaeVision VR Game: User Experience Research between Real and Virtual Paestum

The design of a virtual reality (VR) cultural application is aimed at supporting the steps of the learning process-like concrete experimentation, reflection and abstraction—which are generally difficult to induce when looking at ruins and artifacts that bring back to the past. With the use of virtual technologies (e.g., holographic surfaces, head-mounted displays, motion—cation sensors) those steps are surely supported thanks to the immersiveness and natural interaction granted by such devices. VR can indeed help to symbolically recreate the context of life of cultural objects, presenting them in their original place of belonging, while they were used for example, increasing awareness and understanding of history. The ArkaeVision VR application takes advantages of storytelling and user experience design to tell the story of artifacts and sites of an important cultural heritage site of Italy, Paestum, creating a dramaturgy around them and relying upon historical and artistic content revised by experts. Visitors will virtually travel into the temple dedicated to Hera II of Paestum, in the first half of the fifth century BC, wearing an immersive viewer–HTC Vive; here, they will interact with the priestess Ariadne, a digital actor, who will guide them on a virtual tour presenting the beliefs, the values and habits of an ancient population of the Magna Graecia city. In the immersive VR application, the memory is indeed influenced by the visitors’ ability to proceed with the exploratory activity. Two evaluation sessions were planned and conducted to understand the effectiveness of the immersive experience, usability of the virtual device and the learnability of the digital storytelling. Results revealed that certainly the realism of the virtual reconstructions, the atmosphere and the “sense of the past” that pervades the whole VR cultural experience, characterize the positive feedback of visitors, their emotional engagement and their interest to proceed with the exploration.

New conjugate of bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetate with naphthalimide as a fluorescent sensor for calcium cations

New 4-methoxy-1,8-naphthalimide derivative equipped with bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetate receptor moiety at the imide nitrogen atom showed a fluorescent response to Ca2+ cations due to the formation of complexes of 1:1 ligand–metal composition.

Hybrid poly(ether-arylidene-ether-sulphone)s derivatives for divalent cobalt ion detection

A new set of hybrid poly(ether-arylidene-ether-sulphone)s containing diarylidenecycloalkanone moieties in the polymers main chains has been synthesized in good yields by solution poly-condensation well known polymerization method. The chemical structures of all precursors, monomers and model compound were confirmed by both elemental and spectral data. More particularly, a modified electrode of Co+2 cationic sensor was prepared by the coating of a glassy carbon electrode with synthesized copolymer 8e as a thin layer. The fabricated cationic sensor was displayed long-term stability, enhanced electrochemical activity, good sensitivity, shorter response time, broad linear dynamic range and lower detection limit. A calibration curve is plotted as current versus concentration of Co+2 ions. The cationic sensor sensitivity is calculated from the slope of calibration curve as 12.8165 µA nM−1 cm−2. The detection limit (0.74 ± 0.04 nM) is also estimated from the signal to noise ratio of 3. Therefore, the development of Co+2 cationic sensor might be a novel effort with hybrid sulphone polymers by electrochemical method to ensure the sustainability in health care sector.

Chemical Sensing and Chemoresponsive Pumping with Conical-Pore Polymeric Membranes.

Synthetic membranes containing asymmetrically shaped pores have been shown to rectify the ionic current flowing through the membrane. Ion-current rectification means that such membranes produce nonlinear current–voltage curves analogous to those observed with solid-state diode rectifiers. In order to observe this ion-current rectification phenomenon, the asymmetrically shaped pores must have pore-wall surface charge. Pore-wall surface charge also allows for electroosmotic flow (EOF) to occur through the membrane. We have shown that, because ion-current is rectified, EOF is likewise rectified in such membranes. This means that flow through the membrane depends on the polarity of the voltage applied across the membrane, one polarity producing a higher, and the opposite producing a lower, flow rate. As is reviewed here, these ion-current and EOF rectification phenomena are being used to develop new sensing technologies. Results obtained from an ion-current-based sensor for hydrophobic cations are reviewed. In addition, ion-current and EOF rectification can be combined to make a new type of device—a chemoresponsive nanofluidic pump. This is a pump that either turns flow on or turns flow off, when a specific chemical species is detected. Results from a prototype Pb2+ chemoresponsive pump are also reviewed here.

Heteroleptic Pt(II)-dithiolene-based Colorimetric Chemosensors: Selectivity Control for Hg(II) Ion Sensing.

Hg2+ ions can accumulate in the natural environment and in organisms, where they cause damage to the central nervous system. Therefore, the detection of Hg2+ ions is essential for monitoring environmental contamination and human health. Herein, we demonstrate a simple method for tuning chemosensor signal ratios that significantly increased chemosensor selectivity for Hg2+ detection. Selectivity tuning was accomplished for chemosensors of the type (diphosphine)Pt(dmit), bearing the two different terminal groups 1,2-bis(diphenylphosphino)ethane (dppe) and 1,2-bis[bis(pentafluorophenyl)phosphino]ethane) (dfppe) due to the modulation of specific intermolecular interactions between the dmit ligand and Hg2+ ion. The structure exhibited a large pseudo-Stokes shift, which was advantageous for the internal reference signal and for eliminating potential artifacts. Straightforward chain-end manipulation enabled the tuning of chemosensor properties without additional chemical alterations. Based on these findings, we propose a new platform for improving the selectivity and sensitivity of colorimetric cation sensors. The results of this study will facilitate the designing of organic materials whose certain properties can be enhanced through precise control of the materials’ chemical hybridization by simple functional end-group manipulation.

Low cost, high sensitivity detection of waterborne Al3+ cations and F− anions via the fluorescence response of a morin derivative dye

Morin dye is known as a cheap and readily available selective ‘off → on’ fluorescent sensitiser when immobilised in a phase transfer membrane for the detection of Al3+ ions. Here, a morin derivative, NaMSA, which readily dissolves in water with good long-term stability is used in conjunction with a fibre optic transducer with lock-in detection to detect Al3+ in drinking water below the potability limit. The combination of a water soluble dye and the fibre optic transducer require neither membrane preparation nor a fluorescence spectrometer yet still display a high figure-of- merit. The known ability to recover morin-based Al3+ cation sensors selectively by exposure to fluoride (F−) anions is further developed enabling a complementary sensing of either fluoride anions, or aluminium cations, using the same dye with a sub-micromolar limit-of-detection for both ions. The sensor performance parameters compare favourably to prior reports on both aqueous aluminium and fluoride ion sensing.

N-(O-methoxyphenyl)aza-15-crown-5-ether derivatives: Highly efficient and wide range nonlinear optical response based cation recognition

In this work, we demonstrate the recognition of cations by complexation of N-(O-methoxyphenyl)aza-15-crown-5-ether derivative (TZC) with alkali (Li+, Na+, K+), alkaline earth (Mg2+, Ca2+) and transition metal cations (Ni2+, Zn2+). The doping of mono- and divalent metal cations are used to significantly enhance the first hyperpolarizability values as compared to TZC. By utilizing density functional (DFT) methods, the parent molecule and metal-doped complexes have been optimized using B3LYP-D3/6-31G* level of theory. The amplitudes of first hyperpolarizability βtot have been obtained CAM-B3LYP and M06-2X functional for all studied systems. Interestingly, the atomic number, size, and charge of the metal cations and the O- and N-atoms of the crown-ether are found to be the major factor that gives rise to higher NLO response properties. From the natural bond orbital (NBO), localized orbital locator (LOL) and interactional energy analyses, alkali metal cations possessing a higher degree of ionic characteristics, whereas, the alkaline earth and transition metal cations possessing significant covalent characters. Furthermore, the frontier molecular orbital analysis and the TD-DFT studies reveal that the transition energies as a function of metal size and charge are responsible for remarkably enhanced and distinctive βtot amplitudes as large as TZC-Li+ 10,158 a.u., TZC-Mg2+ 21,948 a.u. and TZC-Ni2+ 25,483 a.u. Additionally, the electrostatic potential maps (ESP) and the difference of dipole moments (Δμ) from the ground to the first excited state have also been carried out to understand the abilities of these systems as highly efficient and selective metal cation sensors.

Reduced texaphyrin: A ratiometric optical sensor for heavy metals in aqueous solution.

We report here a water-soluble metal cation sensor system based on the as-prepared or reduced form of an expanded porphyrin, texaphyrin. Upon metal complexation, a change in the redox state of the ligand occurs that is accompanied by a color change from red to green. Although long employed for synthesis in organic media, we have now found that this complexation-driven redox behavior may be used to achieve the naked eye detectable colorimetric sensing of several number of less-common metal ions in aqueous media. Exposure to In(III), Hg(II), Cd(II), Mn(II), Bi(III), Co(II), and Pb(II) cations leads to a colorimetric response within 10 min. This process is selective for Hg(II) under conditions of competitive analysis. Furthermore, among the subset of response-producing cations, In(III) proved unique in giving rise to a ratiometric change in the ligand-based fluorescence features, including an overall increase in intensity. The cation selectivity observed in aqueous media stands in contrast to what is seen in organic solvents, where a wide range of texaphyrin metal complexes may be prepared. The formation of metal cation complexes under the present aqueous conditions was confirmed by reversed phase high-performance liquid chromatography, ultra-violet-visible absorption and fluorescence spectroscopies, and high-resolution mass spectrometry.

A built-in self-calibrating luminescence sensor based on RhB@Zr-MOF for detection of cations, nitro explosives and pesticides.

A RhB@Zr-MOF composite with dual-emission properties was successfully constructed, which comprises a zirconium-based metal–organic framework and the luminescent dye molecule, Rhodamine B (RhB), embedded via the encapsulation method. The fluorescence intensity ratio of the two emissions was found to be ca. 370 nm/590 nm for RhB@Zr-MOF. The fluorescence intensity values of the two emissions of RhB@Zr-MOF can also be affected by the structures of analytes containing different organic groups. Due to the existence of the dual-emission properties in RhB@Zr-MOF, the relative fluorescence intensity of the emission peaks was introduced as a detection index instead of absolute fluorescence intensity. RhB@Zr-MOF, which possesses the characteristics of a built-in self-calibrating fluorescence sensor, was investigated for detecting cations, nitroaromatics and pesticides. Aside from high sensitivity and selectivity, recyclability is the most important property for sensing pesticides. This work shows that RhB@Zr-MOF can maintain its stability after 5 cycles of detecting nitenpyram, with LOD of 0.2 μM. These results demonstrate that dye@MOFs with dual-emission properties can be employed as multifunctional fluorescence sensors for different types of analytes, and that RhB@Zr-MOF provides a new paradigm for analyte sensing.

One pot synthesis of selective Al (fluorometric) and Hg (colorimetric) dual cation sensor

Amino substituted 5H-thiazolo[4,3-b]-l,3,4-thiadiazole was synthesized by using equimolar amount of an aldehyde, thioglycolic acid and thiosemicarbazide in presence of sulphuric acid at −20 °C in good yield. Various spectral studies of the compound confirm the expected chemical structure. The purified heterocycle was investigated for its ion recognizing abilities with various cations and anions. UV–Vis studies reveal that only ions of Al and Hg forms coordination with the synthesized compound. Hg2+ produce distinguished colour change from colourless to yellow, but in the case Al2+ it did not produce any significant colour change to naked eye but it showed pronounce turn-on emission on contact with Al ion at room temperature. The selectivity and sensitivity of the ion sensor are also investigated using UV–Vis and Fluorescence spectroscopy.

Proof of principle of a purine D-A-D' ligand based ratiometric chemical sensor harnessing complexation induced intermolecular PET.

A comprehensive photophysical study of a series of purines, doubly decorated at C2 and C6 positions with identical fragments ranging from electron acceptor to donor groups of different strengths, is presented. The asymmetry of substitutions creates a unique molecular D-A-D' structure possessing two independent electronic charge transfer (CT) systems attributed to each fragment and exhibiting dual-band fluorescence. Moreover, the inherent property of coordination of metal ions by purines was enriched due to a presence of nearby triazoles used as spacers for donor or acceptor fragments. New molecules present a bidentate coordination mode, which makes the assembly of several ligands with one metal cation possible. This property was exploited to create a new concept of a ratiometric chemical fluorescence sensor involving the photoinduced electron transfer between branches of different ligands as a mechanism of fluorescence modulation.

The synthesis and application of (E)-N'-(benzo[d]dioxol-5-ylmethylene)-4-methyl-benzenesulfonohydrazide for the detection of carcinogenic lead.

In this study, noble ligands of (E)-N′-(benzo[d]dioxol-5-ylmethylene)-4-methyl-benzenesulfonohydrazide (BDMMBSH) were prepared via a simple condensation method using benzo-[d][1,3]-dioxole carbaldehyde, benzenesulfonylhydrazine (BSH), and 4-methyl-benzenesulphonylhydrazine (4-MBSH) in good yield, which were crystallized in acetone, EtOAc, and EtOH. The BDMMBSH derivatives were characterized using different spectroscopic techniques, such as 1H-NMR, 13C-NMR, FTIR, and UV-Vis spectroscopy, and their crystal structures were analyzed using the single crystal X-ray diffraction method (SCXRDM). Subsequently, the BDMMBSH compounds were used for the significant detection of the carcinogenic heavy metal ion, lead (Pb2+), via a reliable electrochemical approach. A sensitive and selective Pb2+ sensor was developed via the deposition of a thin layer of BDMMBSH on a GCE with the conducting polymer matrix Nafion (NF). The sensitivity, LOQ, and LOD of the proposed sensor towards Pb2+ were calculated from the calibration curves to be 2220.0 pA μM−1 cm−2, 320.0 mM, and 96.0 pM, respectively. The validation of the BDMMBSH/GCE/NF sensor probe was performed via the selective determination of Pb2+ in spiked natural samples with a satisfactory and rational outcome.

Near field non-invasive electrophysiology of retrotrapezoid nucleus using amperometric cation sensor

Central chemoreception is the process whereby the brainstem senses blood gas levels and adjusts homeostatic functions such as breathing and cardiovascular tone accordingly. Rodent evidence suggests that the retrotrapezoid nucleus (RTN) is a master regulator of central chemoreception, in particular, through direct sensation of acidosis induced by CO2 levels. The oscillatory dynamics caused by pH changes as sensed by the RTN surface and its relationship to the fluctuations in cation flux is not clearly understood due to the current limitations of electrophysiology tools and this article presents our investigations to address this need. A cation selective sensor fabricated from polypyrrole doped with dodecyl benzenesulfonate (PPy (DBS)) is placed over RTN in an ex-vivo en bloc brain and changes in cation concentration in the diffusion limited region above the RTN is measured due to changes in externally imposed basal pH. The novelty of this technique lies in its feasibility to detect cation fluxes from the cells in the RTN region without having to access either sides of the cell membrane. Owing to the placement of the sensor in close proximity to the tissue, we refer to this technique as near-field electrophysiology. It is observed that lowering the pH in the physiological range (7.4–7.2) results in a significant increase in cation concentration in the vicinity of RTN with a median value of ~5 μM. The utilization of such quantifiable measurement techniques to detect sub-threshold brain activity may help provide a platform for future neural network architectures. Findings from this paper present a quantifiable, sensitive, and robust electrophysiology technique with minimal damage to the underlying tissue.

Inkjet-printed pH-independent paper-based calcium sensor with fluorescence signal readout relying on a solvatochromic dye.

A challenge for paper-based cation sensors relying on classical carrier-based ion-selective optodes (ISOs) is their pH-cross response caused by the use of H+-sensitive chromoionophores as optical signal transducers. This work demonstrates fully pH-independent fluorescence-based calcium detection with a paper-based plasticizer-free ISO. To achieve a pH-independent assay, a solvatochromic dye (SD) instead of a traditional H+-sensitive chromoionophore has been applied to the paper-based ISO by means of inkjet printing technology. The detection principle depends on an ionophore-driven phase-transfer ion-exchange reaction between target cations and the positively charged SD, which no longer involves H+ in the optical signal transduction process. The developed paper-based ISOs with the SD resulted in Ca2+ concentration-dependent response curves not affected by the sample pH (pH 6.0, 7.0, and 8.0). The dynamic range obtained for Ca2+ detection was from 10-5 to 1 mol L-1 with a detection limit of 19.3 μmol L-1. Additionally, excellent selectivity derived from the used ionophore has been confirmed. As a simple practical application, the determination of Ca2+ in mineral water has been achieved without the pH-buffering process required for conventional cation-exchange ISOs.