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Herbicides such as atrazine and humus substances such as fulvic acid are widely used in agricultural sector. They can be traced in surface and groundwater around the agriculture field at concentrations beyond the approved limit due to their mobility and persistence. Bismuth-based photocatalysts activated by visible light are potential materials for removing various organic pollutants from water bodies. These photocatalysts can also be suitable candidates for developing a hybrid membrane with anti-fouling properties. In this study, Bi

Materials science and technology, with the advent of nanotechnology, has brought about innumerable nanomaterials and multi-functional materials, with intriguing yet profound properties, into the scientific realm. Even a minor functionalization of a nanomaterial brings about vast changes in its properties that could be potentially utilized in various applications, particularly for biological applications, as one of the primary needs at present is for point-of-care devices that can provide swifter, accurate, reliable, and reproducible results for the detection of various physiological conditions, or as elements that could increase the resolution of current bio-imaging procedures. In this regard, iron oxide nanoparticles, a major class of metal oxide nanoparticles, have been sweepingly synthesized, characterized, and studied for their essential properties; there are 14 polymorphs that have been reported so far in the literature. With such a background, this review’s primary focus is the discussion of the different synthesis methods along with their structural, optical, magnetic, rheological and phase transformation properties. Subsequently, the review has been extrapolated to summarize the effective use of these nanoparticles as contrast agents in bio-imaging, therapeutic agents making use of its immune-toxicity and subsequent usage in hyperthermia for the treatment of cancer, electron transfer agents in copious electrochemical based enzymatic or non-enzymatic biosensors and bactericidal coatings over biomaterials to reduce the biofilm formation significantly.

In this work, we report a template-free synthesis of graphene oxide nanosphere (GONs) by the laser ablation of graphene in ethanol. The GONs were formed by crumpling of graphene sheet under the infrared nanosecond pulsed laser irradiation. FESEM and HRTEM images of as-prepared GONs were recorded to analyse the morphological characteristics. Raman, X-ray photoelectron and FTIR spectroscopy results confirmed the formation of GONs under the laser ablation. GONs exhibited the particle size ranges from 100 nm to 700 nm and the average particle size was estimated to be around 137 nm by particle size analyser. Photoluminescence study revealed bright blue-green PL emission under the excitation wavelength of 320–420 nm for GONs.

Urea, being a nitrogen fertilizer, is crucial for plant growth but when excessively provided (above biuret 2% levels specified by the World Health Organization), plant characteristics are deeply affected. A real-time sensor to check the presence of excess urea in plants is therefore necessary. Towards this goal, a manganese oxide–reduced graphene oxide composite was synthesized by modified Hummer’s method and precipitation techniques, which was subsequently used as a nano-interface to immobilize urease enzyme for specific detection of urea. The synthesized nanocomposite helped in shuttling of electrons between the redox species and in enhanced electron transfer rate due to their high surface area, vindicated by their structural and morphological characterization using X-ray diffractometer (XRD), scanning electron microscope (SEM), and X-ray photoelectron spectrometer (XPS), and electrochemical characterization using cyclic voltammetry and amperometry, respectively. The fabricated biosensor for urea exhibited a linear range of 5–100 μM with a sensitivity of 9.7 × 10−3 μA μM−1, limit of detection of 14.693 μM, and a response time of 118 s.

Arsenic is a major water contaminant, threat to human health and country’s economy. Arsenic can occur in different oxidation states (−3, 0, +3 and +5) out of which arsenite (As3+) and arsenate (As5+) are highly toxic. Identification and quantification of such toxicant has become a major challenge. Towards this, a highly electro catalytic ruthenium bipyridine – graphene oxide ([Ru(bpy)3]2+-GO) nanocomposite electrode is utilized. The nanocomposite modified screen printed electrode (SPE) shows an enhanced surface area and electron transfer due to the π-π stacking interactions of nanocomposite. Metal-to-ligand charge transfer (MLCT) property of the composite and its role in detecting arsenic species have been studied. Differential pulse voltammetric response of [Ru(bpy)3]2+-GO modified SPE exhibits three oxidation peaks at 0.38, 0.67 and 0.97 V. The challenge of thermodynamic limitation due to inner layer As(0) deposition is overcome. The nanocomposite modified electrode exhibits high catalytic activity towards the oxidation of As(III) and As(V) with the detection limits of 21 and 34 nM in the wide linear range of 0.08–15 µM. The developed sensing element is selective and exhibits good repeatability and reproducibility at a mean response of 32 µA with a relative standard deviation (RSD) of 2.67 and 2.84% respectively. The selective nature of [Ru(bpy)3]2+-GO nanocomposite in quantifying arsenite and arsenate helps in water quality assessment.

Surface coverage parameter of an electrochemical sensor plays a vital role in enhancing the figure of merits of the sensor. Developing a theoretical model for the surface coverage will help to standardize the fabrication of working electrodes used in electrochemical sensors. In this background, a wavelet based spectral algorithm has been developed to model the surface coverage of an arsenic sensor. For the model, Michaelis-Menten constant of fluorine doped cadmium oxide (F-doped CdO) working electrode based arsenic sensor was used as the seed fount. Theoretical analysis for the estimation of surface coverage based on Michaelis-Menten constant with nonlinear reaction-diffusion equation is considered. In order to estimate the Michaelis-Menten constant and maximum current response, the measured current values are linearized with the help of Hanes-woolf plot. Using the Legendre wavelet spectral approach, the nonlinear reaction-diffusion equation is converted into a system of algebraic equations through operational matrix of derivatives. The surface coverage was determined using Legendre wavelets and this method can be determined the desired surface coverage for detecting arsenic in water of specific range.

In this work, a linear sweep voltammetry method has been proposed for the estimation of captan, carbosulfan, 2,3,7,8-tetrachlorodibenzodioxin and pentachlorophenol using ceria nanospheres modified platinum electrode. The reduction peaks of captan and carbosulfan observed in the linear sweep voltammograms overlapped strongly and it was difficult to estimate captan and carbosulfan individually from their mixtures. Gaussian curve fitting algorithm was used to facilitate the resolution of complex linear sweep voltammograms. Quantitative analysis for each of the pesticides in a mixture was studied with the help of linear regression analysis. The analytical performance of the calibrated models was investigated with the aid of relative prediction error (RPE), root mean square error for cross validation (RMSECV) and recovery. The proposed calibrated models were then applied for the estimation of the four pesticides in several ground water samples, where the RPE, recovery and RMSECV of the spiked samples were within the acceptable limit.

Consumption of fruits leads to increase in glucose level in blood for diabetic patients, which in turn leads to peripheral, vascular, ocular complications and cardiac diseases. In this context, a non-enzymatic hybrid glucose biosensor was fabricated for the first time to detect glucose by immobilizing titanium oxide–manganese oxide (TiO2–Mn3O4) nanocomposite and chitosan membrane on to the surface of Pt working electrode (Pt/TiO2–Mn3O4/chitosan). TiO2–Mn3O4 nanocomposite catalyzed the oxidation of glucose to gluconolactone in the absence of glucose oxidase enzyme with high electron transfer rate, good biocompatibility and large surface coverage. Electrochemical measurements revealed the excellent sensing response of the developed biosensor towards glucose with a high sensitivity of 7.073 µA mM−1, linearity of 0.01–0.1 mM, low detection limit of 0.01 µM, reproducibility of 1.5% and stability of 98.8%. The electrochemical parameters estimated from the anodic process were subjected to linear regression models for the detection of unknown concentration of glucose in different fruit samples.

We report for the first time a cyclic voltammetric nitrate sensor with a low detection limit based on the immobilization of zinc oxide nanoparticles on the surface of the platinum working electrode using chitosan membrane. Cyclic voltammetric data demonstrated that zinc oxide nanoparticles can electrochemically reduce nitrate ions to ammonium ions with high conductivity. In order to estimate electroanalytical parameters for each of the nitrate concentrations, Gaussian and Lorentzian curve fitting algorithms were performed on cyclic voltammetric data. Among them, the best analytical performance results were obtained with Gaussian calibration linear model. The zinc oxide modified platinum electrode showed a linear response to nitrate ions over a concentration range from 0.1 to 2.0 mM with a low detection limit and high sensitivity of 10 nM and 39.91 μA/cm2 mM, respectively. The nitrate ion concentrations in drinking water samples were determined using Gaussian calibration linear model and the predicted, added nitrate ion concentration values showed good correlation.

The present study employed laser ablation to obtain fluorescent carbon nanoparticles (CNPs) using inexpensive and easily available Bougainvillea alba flower juice extract as the source. The laser ablation of flower juice extract was carried out using 355 nm wavelength of Nd: YAG pulsed laser for 60 min. HR-TEM studies showed the presence of both larger (about 60 nm) and smaller size (about 10 nm) CNPs and average particle size was found to be about 25 nm. Crystalline structure was also observed in CNPs, whose interplanar lattice spacing was 0.27 nm, indicating the contracted lattice spacing from usual graphitic structure. XRD and Raman analysis showed that the crystalline CNPs correspond to non-perfect graphitic carbon structure. Optical spectroscopic studies revealed the down-conversion of photoluminescence emission at 485 nm under the excitation wavelength of 400 nm. The average fluorescence lifetime of carbon nanoparticles was estimated as 1.1 ns from time-resolved spectroscopy. The CNPs were found to be non-toxic to human vein endothelial cells and internalized into the cells with high intensity up to 24 h. Two distinct PL emissions were observed with the large particles emitting at the longer wavelength localized predominantly in the cytosol and smaller particles emitting at the shorter wavelength concentrated on the nuclear membrane. The results suggest that the CNPs are biocompatible and have the potential for bioimaging applications.

The consumption of melamine and urea contaminated cow milk causes indigestion, acidity, ulcers and kidney stones in humans. In this context, a highly sensitive acetylcholinesterase cyclic voltammetric biosensor based on zinc oxide nanospheres modified Pt electrode has been successfully developed for the simultaneous determination of melamine and urea in cow milk sample. The fabricated bioelectrode showed 100% permeability to the binary mixture of melamine and urea, which in-turn enhanced selectivity. In addition, linear regression models for the estimation of binary mixture of melamine and urea in cow milk were formulated by keeping added melamine and urea as dependent variables and the estimated electrochemical paremeters as independent variables. The prediction performance of linear regression models was validated using %recovery, relative prediction error and root mean square error for cross-validation. The developed Pt/ZnO/AChE/Chitosan bioelectrode detected melamine and urea over a range of 1–20 nM with a limit of detection of 3 pM and 1 pM respectively. The proposed sensor exhibited good recovery (99.96–102.22%), thus providing a promising tool for analysis of melamine and urea in cow milk samples.

Arsenic in water is considered as a global problem affecting human health as well as world’s economy. Intake of toxic arsenic leads to cytological effects leading to cancer and disorders such as black foot disease, hypertension and dermatitis. A low—cost spray deposited nanocauliflower like fluorine doped cadmium oxide (F-doped CdO) thin film based working electrode was developed to detect arsenic with lower detection limit. The structural properties of F-doped CdO thin film exhibited face centered cubic structure with high crystallinity. Morphological characterization of F-doped CdO highlighted the cauliflower like building blocks. F-doped CdO thin film showed a resistivity, sheet concentration and mobility of 0.13 Ω cm, 8.18 × 10 12 cm −2 and 3151 cm 2 V −1 s −1 respectively. F-doped CdO showed a redox peak at −0.10 and 0.05 V ( vs Ag/AgCl ) respectively without the use of arsenite oxidase or arsenate reductase enzymes and mediators. The redox reaction of arsenic was due to the inherent catalytic behaviour of F-doped CdO. The electrocatalytic activity of F-doped CdO helped in detecting arsenic with a detection limit of 4.55 × 10 −3 ppb and a sensitivity of 5.747 × 10 −3 μA ppb −1 . This suggests, F-doped CdO electrocatalyst can replace electrodes such as platinum, gold and glassy carbon electrodes and can be operated at wider potential ranges.

Monocrotophos (MCP) and dichlorvos (DDVP) belong to the group of organophosphate pesticides and are categorized as highly toxic by the World Health Organization. Consumption of oranges containing MCP and DDVP leads to harmful health effects such as DNA damage, apoptosis, cardiotoxicity and cancer. It also causes chronic disorders such as eye irritation, miosis, blurred vision; dizziness, convulsions, dyspnea salivation, abdominal cramps, nausea, diarrhea and vomiting. In this context, an acetylcholinesterase modified Pt working electrode for the simultaneous detection of MCP and DDVP in orange samples was fabricated using zinc oxide nanospheres as a nano-interface. An analytical method for the simultaneous detection of MCP and DDVP was constructed by establishing 24 linear regression models and error analysis was performed to verify the practicability of these models. The developed biosensor achieved a detection limit of MCP and DDVP with detection limits of 0.036 and 0.012 nM respectively. AChE enzyme was successfully reactivated using 2-pyridine aldoxime methiodide. The developed biosensor exhibited good recovery, thus providing a promising tool for analysis of MCP and DDVP.

Graphene oxide nanoparticles (nGOs) were prepared by nanoscond pulsed laser ablation (ns-PLA) of graphene solution at a wavelength of 1064 nm. The High-Resolution Transmission Electron Microscopy (HR-TEM) reveals the average particles size of nGOs as 26 nm with an interplanar spacing of 0.21–0.24 nm indicating the crystalline nature of nGOs. X-ray Diffraction (XRD) analysis exhibited peak at 14.4° suggesting the exfoliation of graphene sheet under the PLA of graphene solution. Raman, Fourier Transform Infrared Spectroscopy (FTIR) and X-ray Photoelectron Spectroscopic (XPS) analysis reveals the oxidation of graphene precursor. Photoluminescence (PL) studies of nGOs showed the excitation-dependent PL emission with peak PL emission intensity in the bluish-green region (455 nm). In-vitro studies indicate that synthesized nGOs at a low and high concentration (1, 5 and 10% v/v) showed no direct cytotoxicity on smooth muscle cells at different time points. Moreover, they do not affect the proliferation rate of smooth muscles cells. Herein, nGOs prepared by PLA was found to be photoluminescent and biocompatible, which can be utilized for bioimaging applications.

The authors describe an amperometric biosensor for the determination As(III) and Cd(II) based on the inhibition of the enzyme acetylcholineesterase (AChE). A platinum electrode was modified with ruthenium(II)-tris(bipyridyl), graphene oxide and AChE and then showed redox peaks at 0.06 and 0.2 V vs Ag/AgCl in the presence of acetylthiocholine chloride (ATChCl). Amperometry unveiled a steady-state turnover rate with the release of thiocholine. In the presence of arsenic(III) and cadmium(II), AChE showed an inhibitive response at 0.214 and 0.233 V vs Ag/AgCl, respectively. The electrode exhibits a detection limit and linear range of 0.03 μM and 0.05–0.8 μM for As(III) and 0.07 μM and 0.02–0.7 μM for Cd(II), respectively. Type of inhibition and inhibition constants induced by As(III) and Cd(II) on the catalytic sites of AChE were determined from Dixon and Lineweaver-Burk plots. The modified electrode was applied to the determination of As3+ and Cd2+ in river, tap and waste water, and the results proved that the method is sensitive and can be an alternative to chromatographic and spectroscopic techniques.

An increase of putrescine content in tiger prawn can be related to the deterioration of its quality. Increase in putrescine consumption causes nausea, vomiting, diarrhea, allergy, and flushing in humans. To address this issue, a glassy carbon electrode modified with diamine oxidase on ceria nanoparticles was fabricated for rapid determination of putrescine in tiger prawn. Based on the amperometric response, calibrated models such as linear, Hill, and Michaelis–Menten functions were constructed. The best performing model was based on Hill analysis, yielding relative prediction error of 0.035, percentage recovery of 113.10, and root mean square error of cross validation of 0.083. All the three models were verified with extract of tiger prawn sample. Among the three models, only the amperometric biosensor based on Hill analysis showed acceptable recovery with low relative standard deviation in tiger prawn sample.

Calcium carbide (CaC2), a carcinogen, is widely used for artificial ripening of mangoes. The usage of CaC2 for mango ripening results in serious health issues like neurological disorders, ulcers, hypoxia, memory loss, etc. Identification of artificially ripened mangoes and quantification of CaC2 in such fruits help in prevention of related health problems. The instability of CaC2 in water makes its detection complex. In addition, the diffusion of calcium hydroxide (Ca(OH)2) and acetylene (C2H2) into mangoes makes it more challenging. In this scenario, an electrochemical CaC2 biosensor with a platinum (Pt) working electrode surface modified by a ceria (CeO2) nano-interface and acetylcholinesterase (AChE) enzyme has been developed. The mechanism involves competitive inhibition of the Pt/CeO2/AChE bioelectrode by the mixture of calcium peroxide (CaO2) and C2H2. Different aspects of electrode immobilization with the nano-interface were probed using cyclic voltammetric and amperometric studies. The fabricated Pt/CeO2/AChE bioelectrode exhibited a LOD of 0.6 nM with a linear range of 1–20 nM and % recovery in the range of 97.89–104.82.

Most of the metal ions are carcinogens and lead to serious health concerns by producing free radicals. Hence, fast and accurate detection of metal ions has become a critical issue. Among various metal ions arsenic, cadmium, lead, mercury and chromium are considered to be highly toxic. To detect these metal ions, electrochemical biosensors with interfaces such as microorganisms, enzymes, microspheres, nanomaterials like gold, silver nanoparticles, CNTs, and metal oxides have been developed. Among these, nanomaterials are considered to be most promising, owing to their strong adsorption, fast electron transfer kinetics, and biocompatibility, which are very apt for biosensing applications. The coupling of electrochemical techniques with nanomaterials has enhanced the sensitivity, limit of detection, and robustness of the sensors. In this review, toxicity mechanisms of various metal ions and their relationship towards the induction of oxidative stress have been summarized. Also, electrochemical biosensors employed in the detection of metal ions with various interfaces have been highlighted.

Progress in the development of phytochemistry has delivered advancement in materials functionality for range of inter/trans-disciplinary application. Here, we investigated the structural functionality of chemically synthesized phytoconstitutent, chalcone (butein) and flavanone (butin). Photoactive and electroactive behavior of butein and butin were comprehensively studied using UV–vis absorbance, photoluminescence and cyclic voltammetric techniques. Surface morphology of the butein and butin powders was characterized from scanning electron microscope at an operating voltage of 10 kV. Significant ultraviolet absorbance property are observed from butein and butin due to the distribution of π → π* and n → π* transitions. Photoluminescence emission spectra of the prepared materials are well resolved at visible region via keto-enol tautomerization and can be influenced by solvent pH. Cyclic voltammetric studies on the prepared materials enabled a direct electron-transfer reaction at gold-screen printed electrode, indicating the feasibility for analytical validation in herbal industries. Existence of multiple electroactive hydroxyl groups makes butein and butin a redox-functional species at electrode interface. Dispersion ability in aqueous and organic solvents makes butein and butin suitable for variety of photochemical applications. This phytochemical material offers new degrees of optical and redox functionality similar to inorganic nanostructures, in addition to inherent bioactivity, that may be advantageous for further biomedical function.

Increased consumption of fried foods such as grilled chicken contains elevated levels of methylglyoxal (MG), which is associated with diabetes mellitus. Hence, in this work, glyoxalase 1(GLO 1) based, zinc oxide (ZnO) flakes interfaced mediator free electrochemical biosensor was developed to detect MG in grilled chicken. ZnO flakes were synthesized by direct precipitation method. X-ray diffractometer and field emission scanning electron microscope were used to study the structural and morphological characteristics of ZnO flakes. The immobilization of GLO 1 on Pt/ZnO flakes modified electrode was confirmed by Fourier transform infrared spectroscopy. Cyclic voltammetric and amperometric studies were carried out using Pt/ZnO flakes/GLO 1 working electrode. The developed biosensor exhibited linear range of 0.6–2.0 µM, sensitivity of 0.281 µA µM−1, LOD of 9 nM with a response time of

A mediator-free electrochemical biosensor with CeO2–PANI nano-interface for sensing histamine using diamine oxidase (DAO) has been developed. CeO2–PANI core–shell nanoparticles were prepared by hydrothermal method. The field emission scanning electron microscopy (FE-SEM) revealed the aggregated spherical morphology of CeO2. The core–shell formation of CeO2–PANI was confirmed with field emission transmission microscopy (FE-TEM). The polycrystallinity of CeO2 and CeO2–PANI was confirmed using X-ray diffraction (XRD). Immobilization of DAO with CeO2–PANI was confirmed with Fourier transform infrared spectroscopy (FT-IR). Electrochemical studies were carried out through cyclic voltammetry and amperometry using modified GCE/CeO2–PANI/DAO as a working electrode, Ag/AgCl saturated with 0.1 M KCl as a reference electrode and platinum (Pt) wire as a counter electrode. The linear range was observed from 0.45 to 1.05 mM with a sensitivity of 724.94 μA cm−2 mM−1. Michaelis–Menten constant was calculated as 0.798 mM. It exhibited limit of detection of 48.7 μM, limit of quantification of 132.4 μM with a response time of

Intake of water contaminated with lead (Pb2+) and mercury (Hg2+) ions leads to various toxic effects and health issues. In this context, an amperometric urease inhibition-based biosensor was developed to detect Pb2+ and Hg2+ ions in water matrix. The modified Pt/CeO2/urease electrode was fabricated by immobilizing CeO2 nanoparticles and urease using a semi-permeable adsorption layer of nafion. With urea as a substrate, urease catalytic activity was examined through cyclic voltammetry. Further, maximum amperometric inhibitive response of the modified Pt/CeO2/urease electrode was observed in the presence of Pb2+ and Hg2+ ions due to the urease inhibition at specific potentials of −0.03 and 0 V, respectively. The developed sensor exhibited a detection limit of 0.019 ± 0.001 μM with a sensitivity of 89.2 × 10−3 μA μM−1 for Pb2+ ions. A detection limit of 0.018 ± 0.003 with a sensitivity of 94.1 × 10−3 μA μM−1 was achieved in detecting Hg2+ ions. The developed biosensor showed a fast response time (

Increased consumption of raw and par-boiled rice results in the formation of methylglyoxal (MG) at higher concentration and leads to complications in diabetic patients. Highly sensitive electrochemical biosensor was developed using glutathione (GSH) as a co-factor with vanadium pentoxide (V2O5) as a nano-interface for MG detection in rice samples. The Pt/V2O5/GSH/Chitosan bioelectrode displayed two well-defined redox peaks in its cyclic voltammograms for MG reduction. This occurred as two electron transfer process where MG gained two electrons from oxidized glutathione disulfide and formed hemithioacetal. The current density response of the fabricated bioelectrode was linear towards MG in the concentration range of 0.1–100 μM with the correlation coefficient of 0.99, sensitivity of 1130.86 μA cm−2 μM−1, limit of detection of 2 nM and response time of less than 18 s. The developed bioelectrode was used for the detection of MG in raw and par-boiled rice samples.

Simultaneous determination of Cd(II), Pb(II), As(III) and Hg(II) metal ions was carried out based on the synergistic effect of graphene oxide (GO) textured with redox active ruthenium(II) bipyridine complex ([Ru(bpy)3]2+). [Ru(bpy)3]2+-GO nanocomposite on the modified gold (Au) electrode acts as an electrocatalyst and favours the sensitive and selective detection of metal ions. Also, it exhibited an enhanced electron transfer rate with a low solution resistance examined by cyclic voltammetry and impedance analysis. The inherent electrochemical and electrocatalytic behaviours of [Ru(bpy)3]2+-GO on gold electrode were demonstrated for simultaneous detection of heavy metal ions in water matrix. The proposed sensor exhibited a higher sensitivity towards Cd(II), Pb(II), As(III) and Hg(II) metal ions with a lowest detection limit of 2.8, 1.41, 2.3 and 1.6nM respectively. The observed detection limits were less than the World Health Organization standards and hence the developed sensor can be deployed for detecting heavy metal ions in water bodies. Simultaneous electrochemical detection of heavy metal ions in river and tap water was carried out using the developed sensor and the observed results were validated with atomic absorption spectroscopy.

Acetylcholinesterase (AChE) enzyme has been predominantly used for the detection of pesticides and metal ions. But, these sensors respond to pesticides as well as metal ions at certain concentration, which results in poor selectivity. Hence in this work, the amount of thiocholine produced during AChE inhibition has been estimated to detect the residual activity of AChE enzyme in-turn to enhance the efficiency of the biosensor. In this context, Pt/ZnO–CeO2/AChE/Chitosan based biosensor has been developed for sensitive voltammetric quantification of thiocholine in AChE. The sensor exhibited enhanced electron transfer rate, good conductivity and biocompatibility. Both the intrinsic and extrinsic parameters were simultaneously optimized using second order polynomial regression to get the best conditions for ATCh determination. Under optimized experimental conditions, the redox peak current was linear over the concentration range of 0.1–1.5 mM with detection and quantification limit of 0.05 and 0.15 μM respectively and the sensitivity of 1.47 μA mM−1.