Employing Label‐free Electrochemical Biosensor Based on 3D‐Reduced Graphene Oxide and Polyaniline Nanofibers for Ultrasensitive Detection of Breast Cancer BRCA1 Biomarker

A label‐free DNA biosensor based on three‐dimensional reduced graphene oxide (3D‐rGO) and polyaniline (PANI) nanofibers modified glassy carbon electrode (GCE) was successfully developed for supersensitive detection of breast cancer BRCA1. The results demonstrated that 3D‐rGO and PANI nanofibers had synergic effects for reducing the charge transfer resistance (R_ct), meaning a huge enhancement in electrochemical activity of 3D‐rGO‐PANI/GCE. Probe DNA could be immobilized on 3D‐rGO‐PANI/GCE for special and sensitive recognition of target DNA (1.0×10^?15–1.0×10^?7 M) with a theoretical LOD of 3.01×10^?16 M (3S/m). Furthermore, this proposed nano‐biosensor could directly detect BRCA1 in real blood samples.     

Graphene/Poly(vinylferrocene) Composite Based Amperometric Biosensor for L‐lysine Determination

A novel and sensitive amperometric biosensor for L‐lysine determination based on a glassy carbon electrode (GCE) modified with graphene (GR) and redox polymer poly(vinylferrocene) (PVF) was constructed. L‐lysine‐α‐oxidase was immobilized onto the modified GCE by a glutaraldehyde/bovine serum albumin cross‐linking procedure. SEM, CV and EIS were used for the characterization of the surface morphology and stepwise fabrication processes of PVF/GR composite. Optimal composition of the biosensor and experimental conditions that affect the performance of the biosensor are discussed. The effect of buffer pH on biosensor response was studied in detail over a wide pH range. L‐lysine biosensor displayed a linear range of 9.9×10⁻⁷ ‐ 3.1×10⁻⁴ M with a low detection limit of 2.3×10⁻⁷ M and K_M ^app value of 0.4 mM. The L‐lysine biosensor was tested using pharmaceutical sample and cheese with satisfactory results.     

Electro‐oxidized Monolayer CVD Graphene Film Transducer for Ultrasensitive Impedimetric DNA Biosensor

We report the effect of electrochemical anodization on the properties of monolayer graphene as the main aim of this research and consequently using the resulting label‐free impedimetric biosensor for DNA sequences detection. Monolayer graphene was grown by chemical vapor deposition (CVD) with methane as precursor on copper foil, transferred onto a glassy carbon electrode and electrochemically anodized. Raman spectroscopy and X‐Ray photo electron spectroscopy revealed enhancement of defect density, roughness and formation of C−O−C, C−O−H and C=O functional groups after anodization. Amine‐terminated poly T probe was linked covalently to the carboxylic groups of anodized graphene by the zero‐length linker to fabricate the impedance‐based DNA biosensor. The anodized graphene electrode demonstrated a superior performance for electrochemical impedance detection of DNA. The DNA biosensor showed a large linear dynamic range from 2.0×10⁻¹⁸ to 1.0×10⁻¹² M with a limit of detection of 1.0×10⁻¹⁸ M using electrochemical impedance spectroscopy (EIS) method. Equivalent circuit modeling shows that DNA hybridization is detected through a change in charge transfer resistance.     

Amperometric Biosensor for Detection of Phenolic Compounds Based on Tyrosinase,N‐Acetyl‐L‐cysteine‐capped Gold Nanoparticles and Chitosan Nanocomposite

A novel biosensor was fabricated based on the immobilization of tyrosinase and N ‐acetyl‐L ‐cysteine‐capped gold nanoparticles onto the surface of the glassy carbon electrode via the film forming by chitosan. The NAC‐AuNPs (N ‐acetyl‐L ‐cysteine‐capped gold nanoparticles) with the average size of 3.4 nm had much higher specific surface area and good biocompatibility, which were favorable for increasing the immobilization amount of enzyme, retaining the catalytic activity of enzyme and facilitating the fast electron transfer. The prepared biosensor exhibited suitable amperometric responses at −0.2 V for phenolic compounds vs. saturated calomel electrode. The parameters of influencing on the working electrode such as pH , temperature, working potential were investigated. Under optimum conditions, the biosensor was applied to detect catechol with a linear range of 1.0 × 10⁻⁷ to 6.0 × 10⁻⁵ mol•L⁻¹ , and the detection limit of 5.0 × 10⁻⁸ mol•L⁻¹ (S /N =3). The stability and selectivity of the proposed biosensor were also evaluated.     

Electrogenerated Chemiluminescence Ethanol Biosensor Based on Carbon Nanotube‐Titania‐Nafion Composite Film

Mesoporous titania‐Nafion composite doped with carbon nanotube (CNT) has been used for the immobilization of tris(2,2′‐bipyridyl)ruthenium(II) (Ru(bpy)₃²⁺) and alcohol dehydrogenase on an electrode surface to yield a highly sensitive and stable electrogenerated chemiluminescence (ECL) ethanol biosensor. The presence of CNT in the composite film increases not only the sensitivity of the ECL biosensor but also the long‐term stability of the biosensor. The present biosensor responds linearly to ethanol in the wide concentration ranges from 1.0×10^?5 M to 1.0×10^?1 M with a detection limit of 5.0×10^?6 M (S/N=3). The present ECL ethanol biosensor exhibited higher ECL response compared to that obtained with the ECL biosensor based on the corresponding composite without CNT. The present CNT‐based ECL biosensor showed good long‐term stability with 75% of its initial activity retained after 2 weeks of storage in 50 mM phosphate buffer at pH 7.0.     

A Novel Enzymatic Glucose Biosensor and Nonenzymatic Hydrogen Peroxide Sensor Based on (3‐Aminopropyl) Triethoxysilane Functionalized Reduced Graphene Oxide

In the present study, a novel enzymatic glucose biosensor using glucose oxidase (GOx) immobilized into (3‐aminopropyl) triethoxysilane (APTES) functionalized reduced graphene oxide (rGO‐APTES) and hydrogen peroxide sensor based on rGO‐APTES modified glassy carbon (GC) electrode were fabricated. Nafion (Nf) was used as a protective membrane. For the characterization of the composites, Fourier transform infrared spectroscopy (FTIR), X‐ray powder diffractometer (XRD), and transmission electron microscopy (TEM) were used. The electrochemical properties of the modified electrodes were investigated using electrochemical impedance spectroscopy, cyclic voltammetry, and amperometry. The resulting Nf/rGO‐APTES/GOx/GC and Nf/rGO‐APTES/GC composites showed good electrocatalytical activity toward glucose and H₂O₂, respectively. The Nf/rGO‐APTES/GC electrode exhibited a linear range of H₂O₂ concentration from 0.05 to 15.25 mM with a detection limit (LOD) of 0.017 mM and sensitivity of 124.87 μA mM⁻¹ cm⁻². The Nf/rGO‐APTES/GOx/GC electrode showed a linear range of glucose from 0.02 to 4.340 mM with a LOD of 9 μM and sensitivity of 75.26 μA mM⁻¹ cm⁻². Also, the sensor and biosensor had notable selectivity, repeatability, reproducibility, and storage stability.     

Rate constants for the reactions of OH radicals and Cl atoms with Di‐n‐Propyl ether and Di‐n‐Butyl ether and their deuterated analogs

Using relative rate methods, rate constants for the gas‐phase reactions of OH radicals and Cl atoms with di‐n‐propyl ether, di‐n‐propyl ether‐d₁₄, di‐n‐butyl ether and di‐n‐butyl ether‐d₁₈ have been measured at 296 ± 2 K and atmospheric pressure of air. The rate constants obtained (in cm³ molecule⁻¹ s⁻¹ units) were: OH radical reactions, di‐n‐propyl ether, (2.18 ± 0.17) × 10⁻¹¹; di‐n‐propyl ether‐d₁₄, (1.13 ± 0.06) × 10⁻¹¹; di‐n‐butyl ether, (3.30 ± 0.25) × 10⁻¹¹; and di‐n‐butyl ether‐d₁₈, (1.49 ± 0.12) × 10⁻¹¹; Cl atom reactions, di‐n‐propyl ether, (3.83 ± 0.05) × 10⁻¹⁰; di‐n‐propyl ether‐d₁₄, (2.84 ± 0.31) × 10⁻¹⁰; di‐n‐butyl ether, (5.15 ± 0.05) × 10⁻¹⁰; and di‐n‐butyl ether‐d₁₈, (4.03 ± 0.06) × 10⁻¹⁰. The rate constants for the di‐n‐propyl ether and di‐n‐butyl ether reactions are in agreement with literature data, and the deuterium isotope effects are consistent with H‐atom abstraction being the rate‐determining steps for both the OH radical and Cl atom reactions. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 425–431, 1999     

Electrochemical DNA Biosensor Based on Magnetite/Multiwalled Carbon Nanotubes/Chitosan Nanocomposite for Bacillus Cereus Detection of Potential Marker for Gold Prospecting

A label‐free DNA biosensor based on magnetite/multiwalled carbon nanotubes/chitosan (Fe₃O₄/MWCNTs‐COOH/CS) nanomaterial for detection of Bacillus cereus DNA sequences was fabricated. Negatively charged DNA was electrostatically adsorbed onto materials by protonation of positively charged chitosan under acidic conditions. The electrode surface and hybridization process were carried out by cyclic voltammetry (CV), differential pulse voltammetry (DPV), and electrochemical impedance spectroscopy (EIS). Under optimal conditions, the biosensor showed a good linear relationship between peak currents difference (ΔI) and logarithm of the target DNA concentration (Log C) ranging from 2.0×10⁻¹³ to 2.0×10⁻⁶ M with a detection limit of 2.0×10⁻¹⁵ M (signal/noise ratio of 3). The biosensor also revealed an excellent selectivity to three‐base, completely mismatched and completely matched DNA. This is a simple, fast and friendly method with a low detection limit for the detection of Bacillus cereus specific DNA compared with previously reported electrochemical DNA biosensor. Furthermore, the DNA biosensor may lead to the development of a technology for gold prospecting in the wild.     

Selective Electroanalysis of Ascorbic Acid Using a Nickel Hexacyanoferrate and Poly(3,4‐ethylenedioxythiophene) Hybrid Film Modified Electrode

The mixed‐valent nickel hexacyanoferrate (NiHCF) and poly(3,4‐ethylenedioxythiophene) (PEDOT) hybrid film (NiHCF‐PEDOT) was prepared on a glassy carbon electrode (GCE) by multiple scan cyclic voltammetry. The films were characterized using atomic force microscopy, field emission scanning electron microscopy, energy dispersive spectroscopy, X‐ray diffraction, and electrochemical impedance spectroscopy (AC impedance). The advantages of these films were demonstrated for the detection of ascorbic acid (AA) using cyclic voltammetry and amperometric techniques. The electrocatalytic oxidation of AA at different electrode surfaces, such as the bare GCE, the NiHCF/GCE, and the NiHCF‐PEDOT/GCE modified electrodes, was determined in phosphate buffer solution (pH 7). The AA electrochemical sensor exhibited a linear response from 5×10⁻⁶ to 1.5×10⁻⁴ M (R²=0.9973) and from 1.55×10⁻⁴ to 3×10⁻⁴ M (R²=0.9983), detection limit=1×10⁻⁶ M, with a fast response time (3 s) for AA determination. In addition, the NiHCF‐PEDOT/GCE was advantageous in terms of its simple preparation, specificity, stability and reproducibility.     

Electrochemical Biosensing Platforms Using Phthalocyanine‐Functionalized Carbon Nanotube Electrode

Iron‐phthalocyanines (FePc) are functionalized at multi‐walled carbon nanotubes (MWNTs) to remarkably improve the sensitivity toward hydrogen peroxide. We constructed a highly sensitive and selective glucose sensor on FePc‐MWNTs electrode based on the immobilization of glucose oxidase (GOD) on poly‐o‐aminophenol (POAP)‐electropolymerized electrode surface. SEM images indicate that GOD enzymes trapped in POAP film tend to deposit primarily on the curved tips and evenly disperse along the sidewalls. The resulting GOD^@POAP/FePc‐MWNTs biosensor exhibits excellent performance for glucose with a rapid response (less than 8 s), a wide linear range (up to 4.0×10^?3 M), low detection limits (2.0×10^?7 M with a signal‐to‐noise of 3), a highly reproducible response (RSD of 2.6%), and long‐term stability (120 days). Such characteristics may be attributed to the catalytic activity of FePc and carbon nanotube, permselectivity of POAP film, as well as the large surface area of carbon nanotube materials.     

A microfluidic chip‐based fluorescent biosensor for the sensitive and specific detection of label‐free single‐base mismatch via magnetic beads‐based “sandwich” hybridization strategy

A novel microfluidic chip‐based fluorescent DNA biosensor, which utilized the electrophoretic driving mode and magnetic beads‐based “sandwich” hybridization strategy, was developed for the sensitive and ultra‐specific detection of single‐base mismatch DNA in this study. In comparison with previous biosensors, the proposed DNA biosensor has much more robust resistibility to the complex matrix of real saliva and serum samples, shorter analysis time, and much higher discrimination ability for the detection of single‐base mismatch. These features, as well as its easiness of fabrication, operation convenience, stability, better reusability, and low cost, make it a promising alternative to the SNPs genotyping/detection in clinical diagnosis. By using the biosensor, we have successfully determined oral cancer‐related DNA in saliva and serum samples without sample labeling and any preseparation or dilution with a detection limit of 5.6 × 10^?11 M, a RSD (n = 5) < 5% and a discrimination factor of 3.58–4.54 for one‐base mismatch.     

Low‐Background,Highly Sensitive DNA Biosensor by Using an Electrically Neutral Cobalt(II) Complex as the Redox Hybridization Indicator

An electrically neutral cobalt complex, [Co(GA)₂(phen)] (GA=glycollic acid, phen=1,10‐phenathroline), was synthesized and its interactions with double‐stranded DNA (dsDNA) were studied by using electrochemical methods on a glassy carbon electrode (GCE). We found that [Co(GA)₂(phen)] could intercalate into the DNA duplex through the planar phen ligand with a high binding constant of 6.2(±0.2)×10⁵ M ^?1. Surface studies showed that the cobalt complex could electrochemically accumulate within the modified dsDNA layer, rather than within the single‐stranded DNA (ssDNA) layer. Based on this feature, the complex was applied as a redox‐active hybridization indicator to detect 18‐base oligonucleotides from the CaMV35S promoter gene. This biosensor presented a very low background signal during hybridization detection and could realize the detection over a wide kinetic range from 1.0×10^?14 M to 1.0×10^?8 M , with a low detection limit of 2.0 fM towards the target sequences. The hybridization selectivity experiments further revealed that the complementary sequence, the one‐base‐mismatched sequence, and the non‐complementary sequence could be well‐distinguished by the cobalt‐complex‐based biosensor.     

An Organic Sol‐Gel Film as Modifier to Construct Biosensor

A new amperometric biosensor for hydrogen peroxide (H₂O₂) was developed by adsorbing hemoglobin (Hb) on an organic sol‐gel film. The organic sol‐gel was prepared using resorcinol and formaldehyde as monomers. This sol‐gel film shows a biocompatible microenvironment for retaining the native activity of the adsorbed Hb. The direct electron transfer between Hb and electrode is achieved. Hb adsorbed on the film shows an enzyme‐like catalytic activity for the reduction of H₂O₂. The reduction peak currents are proportional linearly to the concentration of hydrogen peroxide in the range of 6×10^?8 to 3.6×10^?6 M, with a detection limit of 2.4×10^?8 M (S/N=3). This research enlarges the applications of organic sol‐gel materials in biosensor field.     

Sensitively Electrochemical Sensing for Sequence‐Specific Detection of Phosphinothricin Acetyltransferase Gene: Layer‐by‐Layer Films of Poly‐L‐Lysine and Au‐Carbon Nanotube Hybrid

An electrochemical DNA sensing film was constructed based on the multilayers comprising of poly‐L ‐lysine (pLys) and Au‐carbon nanotube (Au‐CNT) hybrid. A precursor film of mercaptopropionic acid (MPA) was firstly self‐assembled on the Au electrode surface. pLys and Au‐CNT hybrid layer‐by‐layer assembly films were fabricated by alternately immersing the MPA‐modified electrode into the pLys solution and Au‐CNT hybrid solution. Cyclic voltammetry was used to monitor the consecutive growth of the multilayer films by utilizing [Fe(CN)₆]^3?/4? and [Co(phen)₃]^3+/2+ as the redox indicators. The outer layer of the multilayer film was the positively charged pLys, on which the DNA probe was easily linked due to the strong electrostatic affinity. The hybridization detection of DNA was accomplished by using methylene blue (MB) as the indicator, which possesses different affinities to dsDNA and ssDNA. Differential pulse voltammetry was employed to record the signal response of MB and determine the amount of the target DNA sequence. The established biosensor has high sensitivity, a relatively wide linear range from 1.0×10^?10 mol/L to 1.0×10^?6 mol/L and the ability to discriminate the fully complementary target DNA from single or double base‐mismatched DNA. The sequence‐specific DNA related to phosphinothricin acetyltransferase gene from the transgenically modified plants was successfully detected.     

Organometallic complexes with biological molecules. XIV. Biological activity of dialkyl and trialkyltin(IV) [meso‐tetra(4‐carboxy‐ phenyl)porphinate] derivatives

The effects of several organotin(IV) meso‐tetra(4‐carboxyphenyl)porphinate] derivatives with the general formula (R₂Sn)₂TPPC and (R₃Sn)₄TPPC (R = Me, Bu, Ph) were tested in vivo on ascidian embryonic development. Embryos at the two‐cell stage were incubated in 1 × 10⁻⁵ or 1 × 10⁻⁷ M solutions of various compounds. The ligand, [meso‐tetra(4‐carboxyphenyl)porphine] (H₄TPPC) was toxic at 1 × 10⁻⁵ M , because development was blocked at an early gastrula stage, whereas 1 × 10⁻⁷ M H₄TPPC allowed the eggs to develop up to the larva stage. The most toxic among the tested compounds was tributyltin(IV) [meso‐tetra(4‐carboxyphenyl)porphinate], (Bu₃Sn)₄TPPC, since the fertilized eggs were unable to divide into two cells, even at a concentration of 1 × 10⁻⁷ M . To correlate this embryonic arrest with the metabolic pathway, and especially to understand why cellular organelles first underwent chemical damage, 10⁻⁵ and 10⁻⁷ M (Bu₃Sn)₄TPPC‐cultured fertilized eggs were tested for DNA, RNA, protein, glucose, lipid and ATP contents, comparing the values obtained with those of control culture fertilized egg contents. The higher concentration (1 × 10⁻⁵ M ) reduced the content of all the tested compounds, but the lower one (1 × 10⁻⁷ M ), even if still unable to allow cleavage, reduced only the lipids and the ATP contents. A hypothesis concerning initial damage to mitochondrial membrane is proposed. Copyright © 2000 John Wiley & Sons, Ltd.     

Poly(pyrrole‐co‐pyrrole‐2‐carboxylic acid)/Pyruvate Oxidase Based Biosensor for Phosphate: Determination of the Potential,and Application in Streams

A biosensor based on conductive poly(pyrrole‐co‐pyrrole‐2‐carboxylic acid) [Poly(Py‐co‐PyCOOH)] copolymer film coated gold electrode was developed for the quantitative phosphate determination. Enzyme pyruvate oxidase was immobilized chemically via the functional carboxylated groups of the copolymer. The potential to be applied which is deficiency of phosphate biosensor studies for precise phosphate detection was clarified by using differential pulse voltammetry technique. Performance of the sensing ability of the biosensor was improved by optimizing cofactor/cosubstrate concentrations, polymeric film density and pH. The biosensor showed a linearity up to phosphate concentration of 5 mM, operational stability with a relative standard deviation (RSD) of 0.07 % (n=7) and accuracy of 101 % at ?0.15 V (vs. Ag/AgCl). Detection limit (LOD) and sensitivity were calculated to be 13.3 μM and 5.4 μA mM^?1 cm^?2, respectively by preserving 50 % of its initial response at the end of 30 days. It's performance was tested to determine phosphate concentrations in two streams of Zonguldak City in Turkey. Accuracy of phosphate measurement in stream water was found to be 91 %.     

Amperometric Ethanol Biosensor Based on Carbon Nanotubes Dispersed in Sol–Gel‐Derived Titania–Nafion Composite Film

Composite solution of sol–gel‐derived titania and perfluorosulfonated ionomer (Nafion) was used as a solubilizing agent for multiwalled carbon nanotubes (CNT) as well as an encapsulation matrix for alcohol dehydrogenase (ADH) for the fabrication of a highly sensitive and stable amperometric ethanol biosensor. ADH was immobilized within a thin film of CNT–titania–Nafion composite film coated on a glassy carbon electrode. Because of the mesoporous nature of the CNT–titania–Nafion composite film, the present biosensor exhibited remarkably fast response time within 2 s. The presence of CNT in the composite film increases not only the sensitivity of the ethanol biosensor but also the long‐term stability of the biosensor. The present biosensor responds linearly to ethanol in the wide concentration ranges from 1.0×10^?5 M to 3.0×10^?3 M with the sensitivity of 51.6 mA M^?1cm^?2. The present biosensor showed good long‐term stability with 75% of its activity retained after 4 weeks of storage in 50 mM phosphate buffer at pH 7.0.     

Two‐photon excitation and optical limiting in polyfluorene derivatives

Fluorene‐based polymers are widely known materials due to a combination of features such as photoluminescence and electroluminescence, oxidative stability, and film‐forming ability. However, studies reporting nonlinear optical properties in this class of conjugated polymer are scarce. Here, we report a new class of polyfluorene derivatives poly(9,9′‐n‐dihexyl‐2,7‐fluorenedilvinylene‐alt‐1,4‐phenylenevinylene), poly(9,9′‐n‐dihexyl‐2,7‐fluorenedilvinylene‐alt‐2,5‐thiophene), and poly[(9,9‐di‐hexylfluorenediylvinylene‐alt‐1,4‐phenylenevinylene)‐co‐((9,9′‐(3‐t‐butylpropanoate) fluorene‐1,4‐phenylene)] displaying high two‐photon absorption (2PA) in the spectral range from a 490 to 1100 nm. The 2PA cross‐section peak values for these materials are as high as 3000 Göppert Mayer (1 GM = 1 × 10^?50 cm⁴ s/photon), which is related to the high degree of conjugation along the polymer backbone. The polymers that were used in this study presented a strong two‐photon luminescence and also displayed optical limiting behavior, which, in combination with their well‐established properties, make them highly suitable for nonlinear optical devices. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 50: 148–153, 2012     

Carbon Nanotubes‐Based Amperometric Cholesterol Biosensor Fabricated Through Layer‐by‐Layer Technique

A carbon nanotubes‐based amperometric cholesterol biosensor has been fabricated through layer‐by‐layer (LBL) deposition of a cationic polyelectrolyte (PDDA, poly(diallyldimethylammonium chloride)) and cholesterol oxidase (ChOx) on multi‐walled carbon nanotubes (MWNTs)‐modified gold electrode, followed by electrochemical generation of a nonconducting poly(o‐phenylenediamine) (PPD) film as the protective coating. Electrochemical impedance measurements have shown that PDDA/ChOx multilayer film could be formed uniformly on MWNTs‐modified gold electrode. Due to the strong electrocatalytic properties of MWNTs toward H₂O₂ and the low permeability of PPD film for electroacitve species, such as ascorbic acid, uric acid and acetaminophen, the biosensor has shown high sensitivity and good anti‐interferent ability in the detection of cholesterol. The effect of the pH value of the detection solution on the response of the biosensor was also investigated. A linear range up to 6.0 mM has been observed for the biosensor with a detection limit of 0.2 mM. The apparent Michaelis‐Menten constant and the maximum response current density were calculated to be 7.17 mM and 7.32 μA cm^?2, respectively.     

Fabrication of Nanocomposite Containing Naphthoquinone and Nanogold Supported on Poly(2,6‐pyridinedicarboxylic acid) Film for Voltammetric Determination of N‐Acetyl‐L‐Cysteine

A modified electrode was fabricated by grafting of poly (2,6‐pyridinedicarboxylic acid) film (PDC) by electropolymerization of 2,6‐pyridinedicarboxylic acid on the glassy carbon electrode (GCE). Then, gold nanoparticles (NG) and 1,2‐naphthoquinone‐4‐sulfonic acid sodium (Nq) were immobilized on the PDC/GCE to prepare Nq/NG/PDC/GCE by immersing electrode into NG and Nq solution, respectively. The Nq species on NG/PDC/GCE could catalyze electrooxidation of N‐acetyl‐L ‐cysteine (NAC) with lowering the over potential by about 600 mV. This method used for detection of NAC in dynamic range from 4.0×10^?6 M to 1.30×10^?4 M with a detection of limit (2σ) 8.0×10^?7 M.