A 'Frugal' EGFET Sensor for Waterborne H2S

Hydrogen sulphide (H2S) is a toxic gas soluble in water, H2Saq, as a weak acid. Since H2Saq usually originates from the decomposition of faecal matter, its presence also indicates sewage dumping and possible parallel waterborne pathogens associated with sewage. We here present a low footprint ('frugal') H2Saq sensor as an accessible resource for water quality monitoring. As a sensing mechanism, we find the chemical affinity of thiols to gold (Au) translates to H2Saq. When an Au electrode is used as a control gate (CG) or floating gate (FG) electrode in the electric double layer (EDL) pool of an extended gate field effect transistor (EGFET) sensor, EGFET transfer characteristics shift along the CG voltage axis in response to H2Saq. We rationalise this by the interface potential from the adsorption of polar H2S molecules to the electrode. The sign of the shift changes between Au CG and Au FG, and cancels when both electrodes are Au. The sensor is selective for H2Saq over the components of urine, nor does urine suppress the sensor's ability to detect H2Saq. Electrodes can be recovered for repeated use by washing in 1M HCl. Quantitatively, CG voltage shift is fitted by a Langmuir-Freundlich (LF) model, supporting dipole adsorption over an ionic (Nernstian) response mechanism. We find a limit-of-detection of 14.9 nM, 100 times below potability.

A Combined Solution and Solid-State Study on the Tautomerism of an Azocalix[4]arene Chromoionophore

Azocalixarenes functionalized with cation binding sites are popular chromoionophores due to the ease of synthesis and the large complexation-induced shifts of their absorption band that originate from an azo-phenol-quinone-hydrazone tautomerism. Despite their extensive use, however, a thorough investigation of the structure of their metal complexes has not been reported. We describe herein the synthesis of a new azocalixarene ligand (2) and the study of its complexation properties with the Ca²⁺ cation. Through a combination of solution (¹H NMR and UV-vis spectroscopies) and solid-state (X-ray diffractometry) techniques, we demonstrate that metal complexation induces a shift of the tautomeric equilibration towards the quinone-hydrazone form, while deprotonation of the complex results in the reversion to the azo-phenol tautomer.

Porous Polymers Containing Metallocalix[4]arene for the Extraction of Tobacco-Specific Nitrosamines

We designed porous polymers with a tungsten-calix[4]arene imido complex as the nitrosamine receptor for the efficient extraction of tobacco-specific nitrosamines (TSNAs) from water. The interaction between the metallocalix[4]arene and the TSNA, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (nicotine-derived nitrosamine ketone, NNK) was investigated. We found that the incorporation of the nitrosamine receptor into porous polymers increased their selectivity toward NNK over nicotine. The polymer with an optimal ratio of calixarene-containing and porosity-inducing building blocks showed a high maximum adsorption capacity of up to 203 mg/g toward NNK under sonication, which was among the highest values reported. The adsorbed NNK could be removed from the polymer by soaking it in acetonitrile, enabling the adsorbent to be reused. A similar extraction efficiency to that under sonication could be achieved using the polymer-coated magnetic particles under stirring. We also proved that the material could efficiently extract TSNAs from real tobacco extract. This work not only provides an efficient material for the extraction of TSNAs but also offers a design strategy for efficient adsorbents.

Host-Guest Complexes

Host-guest complexes, also known as inclusion complexes, are supramolecular structures [...].

Non-Enzymatic Phenylboronic Acid-Based Optode Membrane for Glucose Monitoring in Serums of Diabetic Patients and in the Culture Medium of Human Embryos

Monitoring glucose levels is important not only for diabetics, but also for tracking embryonic development in human embryo culture media. In this study, an optochemical sensor (glucose-selective polymer membrane) was fabricated for the determination of glucose in serum from diabetic patients and the culture media of human embryos. The optode membranes were formulated using polyvinyl chloride (PVC) as the polymer matrix and 4',5'-dibromofluorescein octadecyl ester (ETH 7075) as the chromoionophore. The sensitivity of the optode membranes was optimized using two different plasticizers (tricresyl phosphate-TCP and nitrophenyloctyl ether-NOPE) and three ionophores (nitrophenylboronic acid-NPBA, trifluorophenyboronic acid-TFPBA, 4'-nitrobenzo-15-crown-5) and tested for glucose detection. The best optode membrane was formulated from 49.5% PVC, 49.5% TCP, 1% NPBA, and 1% ETH 7075. It showed a linear dynamic range of 10^-3 M to 10^-1 M, with a detection limit of 9 × 10^-4 M and a response time of 2 min. The detection mechanism involves H-bonding between NPBA and glucose, which was confirmed by Fourier transform infrared (FTIR) and nuclear magnetic resonance (NMR). The reaction also involves the formation of boronate esters in basic media with deprotonation of the chromoionophore (ETH 7075), leading to a decrease in UV-Vis absorbance at λmax = 530 nm. The membrane optode was used for glucose determination in synthetic culture medium, commercial embryo culture medium (GLOBAL^® TOTAL^® W/HEPES), and serum from normal and diabetic patients, showing good accuracy and precision of the optode.

A fully water-soluble Calix[4]arene probe for fluorometric and colorimetric detection of toxic hydrosulfide and cyanide ions: Practicability in living cells and food samples

Although hydrosulfide and cyanide anions play important roles in daily life that they are available in a lot of foods. However, their excess amounts contaminate water, land, and food and cause serious problems to human health. Herein, we introduce a water-soluble macrocyclic sensor based-on Calix[4]arene (MPI-Calix[4]) with dual response sites for fluorescence recognizing cyanide (CN^-) and hydrogen sulfide (HS^-) under longwave light. MPI-Calix[4] exhibits a high selectivity and sensitivity in the detection of CN^- and HS^-, where the limits of detection were as low as 0.115 and 8.12 μmol/L, respectively. The cell imaging studies shows that this probe can be easily used in the detection of CN^- and HS^- on living cells. Full understanding of these results paved a fruitful system to improve an applicable analytical process for food safety and quality.

Alkali metal ion binding using cyclic polyketones

Cyclic oligoketones composed of 3,3-dimethylpentane-2,4-diones showed crown ether-like alkali metal ion binding behavior with association constants up to 1.7 × 10⁴ M^-1 in chloroform/acetonitrile (v/v, 9/1). The binding properties have been used for catalysis in the Finkelstein reaction in a low-polarity solvent. Furthermore, novel ion-binding hosts were generated by terminal functionalization of linear polyketones.

Molecular Recognition by Pillar[5]arenes: Evidence for Simultaneous Electrostatic and Hydrophobic Interactions

The formation of inclusion complexes between alkylsulfonate guests and a cationic pillar[5]arene receptor in water was investigated by NMR and ITC techniques. The results show the formation of host-guest complexes stabilized by electrostatic interactions and hydrophobic effects with binding constants of up to 10⁷ M⁻¹ for the guest with higher hydrophobic character. Structurally, the alkyl chain of the guest is included in the hydrophobic aromatic cavity of the macrocycle while the sulfonate groups are held in the multicationic portal by ionic interactions.

Metallocalix[4]arene Polymers for Gravimetric Detection of N-Nitrosodialkylamines

N-Nitrosamines are found in food, drugs, air, water, and soil. They pose a significant risk to human health because of their carcinogenicity; consequently, materials that can be used to selectively and sensitively detect nitrosamines are needed. In this work, we designed and synthesized two polymers bearing calix[4]arene or 4-tert-butylcalix[4]arene tungsten-imido complexes (PCalixH and PCalixtBu) as N-nitrosodimethylamine (NDMA) receptors. The interaction between metallocalix[4]arene monomers/polymers and NDMA was confirmed by ¹H NMR and IR spectroscopy. Single-crystal X-ray analysis further revealed that the host-guest interaction is based on binding of the terminal oxygen of NDMA to tungsten within the calixarene cavity. Gravimetric detection of NDMA was performed on a quartz crystal microbalance (QCM) in air. Both polymers show responses to NDMA, with PCalixtBu exhibiting a low theoretical limit of detection of 5 ppb for NDMA. The sensor also shows high selectivity toward NDMA and moderate humidity tolerance. This work provides a sensitive sensor for detection of NDMA and also offers a class of new, selective, and efficient NDMA receptors for the future design of NDMA sensors and NDMA extraction materials.

A New Benzothiazin-functionalized Calix[4]arene-based Fluorescent Chemosensor for the Selective Detection of Co2+ Ion

Calixarenes, which have a great place in supramolecular chemistry, have become the most prominent macrocyclic compounds in synthetic organic chemistry due to their easy synthesis and functionalization. In this study, p-tert-butyl calix[4]arene dihydrazide derivative was synthesized and then reacted with 3-oxo-3,4-dihydro-2 H-benzo[b][1,4] thiazin-2-ylideneacetyl chloride to prepare new calixarene based chromophore compound 4. The structure of the synthesized compound was elucidated by spectroscopic methods such as ¹H NMR ¹³C NMR and FT-IR spectroscopy. Chromogenic and fluorescence properties of compound 4 were evaluated. It was observed from both studies that compound 4 was Co²⁺ selective and shows fluorescence Switched-off behavior. Stoichiometry, binding constant and the detection limit were calculated. The stoichiometry between compound 4 and Co²⁺ was found to be 1:1. The binding constant value (K) was calculated as 666.67 M^- 1 using Benesi-Hildebrand equation, while the detection limit for Co²⁺ ion was calculated as 0.0465 µM.

Supramolecular and suprabiomolecular photochemistry: a perspective overview

In this perspective review article, we have attempted to bring out the important current trends of research in the areas of supramolecular and suprabiomolecular photochemistry. Since the spans of the subject areas are very vast, it is impossible to cover all the aspects within the limited space of this review article. Nevertheless, efforts have been made to assimilate the basic understanding of how supramolecular interactions can significantly change the photophysical and other related physiochemical properties of chromophoric dyes and drugs, which have enormous academic and practical implications. We have discussed with reference to relevant chemical systems where supramolecularly assisted modulations in the properties of chromophoric dyes and drugs can be used or have already been used in different areas like sensing, dye/drug stabilization, drug delivery, functional materials, and aqueous dye laser systems. In supramolecular assemblies, along with their conventional photophysical properties, the acid-base properties of prototropic dyes, as well as the excited state prototautomerization and related proton transfer behavior of proton donor/acceptor dye molecules, are also largely modulated due to supramolecular interactions, which are often reflected very explicitly through changes in their absorption and fluorescence characteristics, providing us many useful insights into these chemical systems and bringing out intriguing applications of such changes in different applied areas. Another interesting research area in supramolecular photochemistry is the excitation energy transfer from the donor to acceptor moieties in self-assembled systems which have immense importance in light harvesting applications, mimicking natural photosynthetic systems. In this review article, we have discussed varieties of these aspects, highlighting their academic and applied implications. We have tried to emphasize the progress made so far and thus to bring out future research perspectives in the subject areas concerned, which are anticipated to find many useful applications in areas like sensors, catalysis, electronic devices, pharmaceuticals, drug formulations, nanomedicine, light harvesting, and smart materials.

Isolation of a Nitromethane Anion in the Calix-Shaped Inorganic Cage

A calix-shaped polyoxometalate, [V₁₂O₃₂]⁴⁻ (V12), stabilizes an anion moiety in its central cavity. This molecule-sized container has the potential to control the reactivity of an anion. The highly-reactive cyanate is smoothly trapped by V12 to form [V₁₂O₃₂(CN)]⁵⁻. In the CH₃NO₂ solution, cyanate abstracts protons from CH₃NO₂, and the resultant CH₂NO₂⁻ is stabilized in V12 to form [V₁₂O₃₂(CH₂NO₂)]⁵⁻ (V12(CH₂NO₂)). A crystallographic analysis revealed the double-bond characteristic short bond distance of 1.248 Å between the carbon and nitrogen atoms in the nitromethane anion in V12. ¹H and ¹³C NMR studies showed that the nitromethane anion in V12 must not be exchanged with the nitromethane solvent. Thus, the V12 container restrains the reactivity of anionic species.

Calix[n]arene/Pillar[n]arene-Functionalized Graphene Nanocomposites and Their Applications

Calix[n]arenes and pillar[n]arenes, which contain repeating units of phenol and methane, are class of synthetic cyclic supramolecules. Their rigid structure, tunable cavity size, flexible functionalization, and rich host-guest properties make them ideal surface modifiers to construct functional hybrid materials. Introduction of the calix[n]arene/pillar[n]arene species to the graphene may bring new interesting or enhanced physicochemical/biological properties by combining their individual characteristics. Reported methods for the surface modification of graphene with calix[n]arene/pillar[n]arene utilize either covalent or non-covalent approaches. This mini-review presents the recent advancements in the functionalization of graphene nanomaterials with calix[n]arene/pillar[n]arene and their applications. At the end, the future outlook and challenges for the continued research of calix[n]arene/pillar[n]arene-functionalized graphene nanohybrids in the development of applied nanoscience are thoroughly discussed.

New protective ligands for atomically precise silver nanoclusters

Atomically precise silver nanoclusters (NCs) have emerged as a hot topic attracting immense research interest. Protecting ligands are needed for direct capping on cluster surfaces in order to prevent aggregation and to stabilize NCs. It has been demonstrated that protective ligands are critical to determining the sizes, structures and properties of silver NCs. The past decades have witnessed conventionally used organic ligands (thiolates/selenols, phosphines and alkynyls) and inorganic ligands (chalcogens and halogens) being extensively used to passivate NC surfaces. However, only in the most recent years have new-type protecting ligands beyond the conventional ones begun to be introduced in the protecting sphere of new functional silver NCs. The present Frontier article covers the most recent examples of some new protective agents for well-defined silver NCs. We describe four classes of novel silver NCs stabilized by newly-developed surface ligands, namely, nitrogen-donor organic ligands, oxygen-donor inorganic ligands, metalloligands and macrocyclic hosts, paying attention to the synthesis, structures and properties of these silver NCs. This Frontier article will hopefully attract more cluster scientists to explore more freshly ligated atomically precise silver NCs with novel structures and properties in the years ahead. The literature survey in this review is based on publications up to February 2020. Some suggestions for future directions in this field are also given.

Controlled nitrite anion encapsulation and release in the molecular cavity of decamethylcucurbit[5]uril: solution and solid state studies

Nitrite anion encapsulation was realized using molecular cavitands of decamethylcucurbit[5]urils as molecular receptors.

Biological and related applications of pillar[n]arenes

Pillar[n]arenes are a new class of synthetic supramolecular macrocycles streamlined by their particular pillar-shaped architecture which consists of an electron-rich cavity and two fine-tuneable rims. The ease and diversity of the functionalization of the two rims open possibilities for the design of new architectures, topological isomers, and scaffolds. Significantly, this emerging class of macrocyclic receptors offers a unique platform for biological purposes. This review article covers the most recent contributions from the pillar[n]arene field in terms of artificial membrane transport systems, controlled drug delivery systems, biomedical imaging, biosensors, cell adhesion, fluorescent sensing, and pesticide detection based on host-guest interactions. The review also uniquely describes the properties of sub-units that make pillar[n]arenes suitable for biological applications and it provides a detailed outline for the design of new innovative pillar-like structures with specific properties to open up a new avenue for pillar[n]arene chemistry.

Intermolecular chemo- and regioselective aromatic C-H amination of alkoxyarenes promoted by rhodium nitrenoids

Intermolecular aromatic C(sp²)-H amination promoted by neutral rhodium nitrenoids has been developed. The reactions proceeded with various oxygen-substituted arenes (1.5 equiv.) in a chemo- and regioselective manner. The aromatic C(sp²)-H amination took place at the para position of the oxygen substituent in the presence of benzylic C(sp³)-H bonds and/or C(sp³)-H bonds α to ethereal oxygen.

Reaction-based ratiometric fluorescent probe for selective recognition of sulfide anions with a large Stokes shift through switching on ESIPT

Ratiometric fluorescent probe BNPT has been synthesized and characterized for S²⁻ sensing via ESIPT mechanism.

Structures of potassium calix[4]arene crown ether inclusion complexes and application in polymerization of rac-lactide

Reaction of 1,3-dipropoxy-p-tert-butyl-calix[4]arene (L(1)H2) with KN(SiMe3)2 afforded a one-dimensional (1D) chain complex [K2L(1)]n (1). Upon reaction with 1 equivalent 18-crown-6, complex 1 can convert to complex [K2(18-crown-6)L(1)] (2) which possesses a sandwich structure. Treatment of two calix[4]arene-crown ligands of 1,3-dihydroxy-p-tert-butyl-calix[4]arene-crown-5 (L(2)H2) and 1,2-dihydroxy-p-tert-butyl-calix[4]arene-crown-5 (L(3)H2) with KN(SiMe3)2 gave the dinuclear complex [K2L(2)] (3) and the mononuclear complex [K(THF)L(3)H] (4), respectively. Complexes 1-4 were all characterized by single-crystal X-ray diffraction techniques. The variable temperature (1)H NMR spectrum indicates there is a quick rotation equilibrium of the two phenoxy groups in complex 3. In addition, complexes 1-4 have been tested for the ring-opening polymerization (ROP) of rac-lactide and the results showed that complexes 2 and 3 are highly active for the ROP of rac-lactide. The obtained polymers displayed low dispersity values (Đ) and the molecular weights are close to the calculated ones. Furthermore, complexes 2 and 3 show moderate isoselectivities of Pm = 0.67 and Pm = 0.73, respectively.

Supramolecular nanocapsules from the self-assembly of amphiphilic calixarene as a carrier for paclitaxel

An amphiphilic calixarene was synthesized as a supramolecular vesicle for paclitaxel with sustained release behavior and enhanced anticancer activity.

Calixarene-functionalized graphene oxide composites fixed on glassy carbon electrodes for electrochemical detection

Four composite materials were prepared by grafting calixarene derivatives onto the surfaces of graphene oxide (GO) via covalent functionalization to yield covalently functionalized graphene oxides (CFGOs).

Interfacial binding of divalent cations to calixarene-based Langmuir monolayers

The interactions of Langmuir monolayers produced through the self-assembly of an amphiphilic p-carboxycalix[4]arene (1) with a series of divalent, fourth-period transition metals, at the air-water interface, were investigated. Changes in the interfacial behavior of 1 in response to the presence of CuCl2, CoCl2, MnCl2, and NiCl2 were studied by means of Langmuir compression isotherms and Brewster angle microscopy (BAM). The measurements revealed that the self-assembly properties of 1 are significantly affected by Cu(2+) ions. The interactions of 1-based monolayers with Co(2+) and Cu(2+) ions were further investigated by means of synchrotron radiation-based X-ray reflectivity (XRR), X-ray near-total-reflection fluorescence (XNTRF), and grazing incidence X-ray diffraction (GIXD). XNTRF and XRR analyses revealed that the monolayer of 1 binds more strongly to Cu(2+) than Co(2+) ions. In the presence of relatively high concentrations of Cu(2+) ions in the subphase (1.4 × 10(-3) M), XNTRF exhibited anomalous depth profile behavior and GIXD measurements showed considerably strong diffuse scattering. Both measurements suggest the formation of Cu(2+) clusters contiguous to the monolayer of 1.

Introduction of luminescent rhenium(i), ruthenium(ii), iridium(iii) and rhodium(iii) systems into rhodamine-tethered ligands for the construction of bichromophoric chemosensors

Several classes of rhenium(i), ruthenium(ii), iridium(iii) and rhodium(iii) complexes tethered with a rhodamine moiety have been synthesized and characterized, and their photophysical and ion-binding properties were investigated.

The phenanthroimidazole-based dizinc(ii) complex as a fluorescent probe for the pyrophosphate ion as generated in polymerase chain reactions and pyrosequencing

The phenanthroimidazole-based dizinc(II) complex is an efficient fluorescent probe for the pyrophosphate ion (PPi) in water with a very low detection limit, and also used to detect PPi released from DNA polymerization chain reaction.

Structural requirements for anti-oxidant activity of calix[n]arenes and their associated anti-bacterial activity

Treatment of neural cells with calix[n]arenes featuring sulphonate moieties and linked to Ag nanoparticles results in reduced reactive species. For Gram+ bacteria there is an inverse correlation between anti-bacterial activity and ROS reduction whereas for Gram- bacteria only calix[6]arenes bearing O-alkyl sulphonate functions act as ROS inhibitors and anti-bacterial agents.

Biocomputing, Biosensing and Bioactuation Based on Enzyme Biocatalyzed Reactions

The focus of this review paper is on the design and implementation of smart ‘Sense-and-Treat’ systems using enzyme-biocatalytic systems. These systems were used to perform biomolecular computing and they were functionally integrated with signal responsive materials aiming towards their biomedical use. Electrode interfaces, functionalized with signal-responsive materials, find applications in biocomputing, biosensing, and, specifically, triggered release of bioactive substances. ‘Sense-and-Treat’ systems require multiple components working together, including biosensors, actuators, and filters, in order to achieve closed-loop and autonomous operation. In general, biochemical logic networks were developed to process single biochemical or chemical inputs as well as multiple inputs, responding to nonphysiological (for concept demonstration purposes) and physiological signals (for injury detection or diagnosis). Actuation of drug-mimicking release was performed using the responsive material iron-cross-linked alginate with entrapped biomolecular species, responding to physical, chemical or biochemical signals.

Spectral study and protein labeling of inclusion complex between dye and calixarene sulfonate

The host-guest inclusion complex of calix[6]arene sulfonate (SCA6) with thiazole orange (TO) formed in aqueous solution was studied. Absorption and fluorescence techniques were used for the analysis of this inclusion complex. The addition of calixarene sulfonate leads to a decrease in both absorption and fluorescence intensity of the dye, indicating that the inclusion complex was formed. Simultaneously, the inclusion phenomenon of another cyanine dye, Cy3, with calixarene sulfonate was investigated. The stability constant of the two complexes was determined, and the results were compared. The water solubility of TO dye was increased in the presence of calixarene sulfonate, and further protein labeling experiments suggested that this TO-SCA6 complex can act as a fluorescent probe for labeling of biomolecules.

Calix[4,6,8]arenesulfonates functionalized reduced graphene oxide with high supramolecular recognition capability: fabrication and application for enhanced host-guest electrochemical recognition

Reduced graphene oxide (rGO) modified with three kinds of water-soluble p-sulfonated calix[4,6,8]arene sodium (SCn: SC4, SC6, SC8) were successfully prepared by using a simple wet chemical strategy. Three obtained SCn-rGO nanocomposites were characterized by Fourier transform infrared spectroscopy, Ultraviolet-visible spectroscopy, static contact angle measurement, thermogravimetric analysis, scanning electron microscope and electrochemical impedance spectroscopy, which confirmed that different amount of SCn molecules had been effectively loaded onto the surface of rGO, and the water-dispersity and stability of SCn-rGO increased with the increase of the value of n in SCn (n = 4, 6, 8). More significantly, cyclic voltammetry measurement showed that the SCn-rGO could exhibit high supramolecular recognition and enrichment capability and consequently displayed excellent electrochemical response toward four probe molecules (biological and organic dye molecules). Especially, SC8-rGO exhibited an excellent electrochemical performance for dopamine with high current densities of 73.04 mA mM(-1) L cm(-2), broad linear range (1 × 10(-8) to 2.1 × 10(-5) M) and very low detection limit (8 × 10(-9) M) at a signal-to-noise ratio of 3.

Imino-phenolic-pyridyl conjugates of calix[4]arene (L1 and L2) as primary fluorescence switch-on sensors for Zn2+ in solution and in HeLa cells and the recognition of pyrophosphate and ATP by [ZnL2]

Pyridyl-based triazole-linked calix[4]arene conjugates, viz. L(1) and L(2), were synthesized and characterized. These two conjugates were shown to be selective and sensitive for Zn(2+) among the 12 metal ions studied in HEPES buffer medium by fluorescence, absorption, and visual color change with the detection limit of ~31 and ~112 ppb, respectively, by L(1) and L(2). Moreover, the utility of the conjugates L(1) and L(2) in showing the zinc recognition in live cells has also been demonstrated using HeLa cells as monitored by fluorescence imaging. The zinc complexes of L(1) and L(2) were isolated, and the structure of [ZnL(1)] has been established by single-crystal XRD and that of [ZnL(2)] by DFT calculations. TDDFT calculations were performed in order to demonstrate the electronic properties of receptors and their zinc complexes. The isolated zinc complexes, viz. [ZnL(1)] and [ZnL(2)], have been used as molecular tools for the recognition of anions on the basis of their binding affinities toward Zn(2+). [ZnL(2)] was found to be sensitive and selective toward phosphate-bearing ions and molecules and in particular to pyrophosphate (PPi) and ATP among the other 18 anions studied; however, [ZnL(1)] was not sensitive toward any of the anions studied. The selectivity has been shown on the basis of the changes observed in the emission and absorption spectral studies through the removal of Zn(2+) from [ZnL(2)] by PPi. Thus, [ZnL(2)] has been shown to detect PPi up to 278 ± 10 ppb at pH 7.4 in aqueous methanolic (1/2 v/v) HEPES buffer.