Construction of an amperometric biosensor for xanthine via supramolecular associations

Xathine oxidase was chemically modified with β-cyclodextrin-branched carboxymethylcellulose and further supramolecularly immobilized on a gold electrode, previously coated with a monolayer of 1-adamantanyl residues. The electrode was employed for constructing an amperometric biosensor device, which showed linear response (poised at +700 mV vs. Ag/AgCl) toward xanthine concentration between 300 μM and 10.4 mM at pH 7.0. The biosensor reached 95% of steady-state current in about 14 s and its sensitivity was 8.2 mA/M cm². The enzyme electrode retained 93% of its initial activity after 3 weeks of storage at 4 °C in 50 mM sodium phosphate buffer, pH 7.0. The supramolecular nature of the immobilization approach was confirmed by cyclic voltammetry.     

Amperometric enzyme biosensor for hydrogen peroxide via Ugi multicomponent reaction

A novel strategy based on the Ugi multicomponent reaction was employed for immobilizing horseradish peroxidase on sodium alginate-coated gold electrode. The electrode was employed for constructing an amperometric biosensor device using 1 mM hydroquinone as electrochemical mediator. The electrode showed linear response (poised at −300 mV vs Ag/AgCl) toward H₂O₂ concentration between 70 μM and 8.8 mM at pH 7.0. The biosensor reached 95% of steady-state current in about 12 s and its sensitivity was 33.8 mA/M cm². The electrode retained full initial activity after 30 days of storage at 4 °C in 50 mM sodium phosphate buffer, pH 7.0.     

A structure-based analysis of the vibrational spectra of nitrosyl ligands in transition-metal coordination complexes and clusters

The vibrational spectra of nitrogen monoxide or nitric oxide (NO) bonded to one or to several transition-metal (M) atom(s) in coordination and cluster compounds are analyzed in relation to the various types of such structures identified by diffraction methods. These structures are classified in: (a) terminal (linear and bent) nitrosyls, [M(σ-NO)] or [M(NO)]; (b) twofold nitrosyl bridges, [M₂(μ₂-NO)]; (c) threefold nitrosyl bridges, [M₃(μ₃-NO)]; (d) σ/π-dihaptonitrosyls or “side-on” nitrosyls; and (e) isonitrosyls (oxygen-bonded nitrosyls).Typical ranges for the values of internuclear N–O and M–N bond-distances and M–N–O bond-angles for linear nitrosyls are: 1.14–1.20 Å/1.60–1.90 Å/180–160° and for bent nitrosyls are 1.16–1.22 Å/1.80–2.00 Å/140–110°. The [M₂(μ₂-NO)] bridges have been divided into those that contain one or several metal–metal bonds and those without a formal metal/metal bond (M?M). Typical ranges for the M–M, N–O, M–N bond distances and M–N–M bond angles for the normal twofold NO bridges are: 2.30–3.00 Å/1.18–1.22 Å/1.80–2.00 Å/90–70°, whereas for the analogous ranges of the long twofold NO bridges these are 3.10–3.40 Å/1.20–1.24 Å/1.90–2.10 Å/130–110°. In both situations the N–O vector is approximately at right angle to the M–M (or M?M) vector within the experimental error; i.e. the NO group is symmetrical bonded to the two metal atoms. In contrast the threefold NO bridges can be symmetrically or unsymmetrically bonded to an M₃-plane of a cluster compound. Characteristic values for the N–O and M–N bond-distances of these NO bridges are: 1.24–1.28 Å/1.80–1.90 Å, respectively. As few dihaptonitrosyl and isonitrosyl complexes are known, the structural features of these are discussed on an individual basis.The very extensive vibrational spectroscopy literature considered gives emphasis to the data from linearly bonded NO ligands in stable closed-shell metal complexes; i.e. those which are consistent with the “effective atomic number (EAN)” or “18-electron” rule. In the paucity of enough vibrational spectroscopic data from complexes with only nitrosyl ligands, it turned out to be very advantageous to use wavenumbers from the spectra of uncharged and saturated nitrosyl/carbonyl metal complexes as references, because the presence of a carbonyl ligand was found to be neutral in its effect on the ν(NO)-values. The wide wavenumber range found for the ν(NO) values of linear MNO complexes are then presented in terms of the estimated effects of net ionic charges, or of electron-withdrawing or electron-donating ligands bonded to the same metal atom. Using this approach we have found that: (a) the effect for a unit positive charge is [plus 100 cm^?1] whereas for a unit negative charge it is [minus 145 cm^?1]. (b) For electron-withdrawing co-ligands the estimated effects are: terminal CN [plus 50 cm^?1]; terminal halogens [plus 30 cm^?1]; bridging or quasi-bridging halogens [plus 15 cm^?1]. (c) For electro donating co-ligands they are: PF₃ [plus 10 cm^?1]; P(OPh)₃ [?30 cm^?1]; P(OR)₃ (R = alkyl group) [?40 cm^?1]; PPh₃ [?55 cm^?1]; PR₃ (R = alkyl group) [?70 cm^?1]; and η⁵-C₅H₅ [?60 cm^?1]; η⁵-C₅H₄Me [?70 cm^?1]; η⁵-C₅Me₅ [?80 cm^?1]. These values were mostly derived from the spectra of nitrosyl complexes that have been corrected for the presence of only a single electronically-active co-ligand. After making allowance for ionic charges or strongly-perturbing ligands on the same metal atom, the adjusted ‘neutral-co-ligand’ ν(NO)*-values (in cm^?1) are for linear nitrosyl complexes with transition metals of Period 4 of the Periodic Table, i.e. those with atomic orbitals (…4s3d4p): [ca. 1750, Cr(NO)]; [1775,Mn(NO)]; [1796,Fe(NO)]; [1817,Co(NO)]; [ca. 1840, Ni(NO)]. Period 5 (…5s4d5p): [1730 Mo(NO)]; [—, Tc(NO)]; [1745,Ru(NO)]; [1790,Rh(NO)]; [ca. 1845, Pd(NO)]. Period 6 (…6s4f5d6p), [1720,W(NO)]; [1730,Re(NO)]; [1738,Os(NO)]; [1760,Ir(NO)]; [—, Pt] respectively. Environmental differences to these values, e.g. data taken in polar solutions or in the crystalline state, can cause ν(NO)* variations (mostly reductions) of up to ca. 30 cm^?1.Three spectroscopic criteria are used to distinguish between linear and bent NO groups. These are: (i) the values of ν(¹⁴NO) themselves, and (ii) the isotopic band shift – (IBS) – parameter which is defined as [ν(¹⁴NO)–ν(¹⁵NO)], and, (iii) the isotopic band ratio – (IBR) – given by [ν(¹⁵NO/ν¹⁴NO)]. The former is illustrated with the ν(¹⁴NO)-data from trigonal bipyramidal (TBP) and tetragonal pyramidal (TP) structures of [M(NO(L)₄] complexes (where M = Fe, Co, Ru, Rh, Os, Ir and L = ligand). These values indicate that linear (180–170°) and strongly bent (130–120°) NO groups in these compounds absorb over the 1862–1690 cm^?1 and 1720–1525 cm^?1-regions, respectively. As was explicitly demonstrated for the linear nitrosyls, these extensive regions reflect the presence in different complexes of a very wide range of co-ligands or ionic charges associated with the metal atom of the nitrosyl group. A plot of the IBS parameter against M–N–O bond-angle for compounds with general formulae [M(NO)(L)_y] (y = 4, 5, 6) reveals that the IBS-values are clustered between 45 and 30 cm^?1 or between 37 and 25 cm^?1 for linear or bent NO groups, respectively. A plot of IBR shows a less well defined pattern. Overall it is suggested that bent nitrosyls absorb ca. 60–100 cm^?1 below, and have smaller co-ligand band-shifts, than their linear counterparts.Spectroscopic ν(NO) data of the bridging or other types of NO ligands are comparatively few and therefore it has not been possible to give other than general ranges for ‘neutral co-ligand’ values. Moreover the bridging species data often depend on corrections for the effects of electronically-active co-ligands such as cyclopentadienyl-like groups. The derived neutral co-ligand estimates, ν(NO)*, are: (a) twofold bridged nitrosyls with a metal–metal bond order of one, or greater than one, absorb at ca. 1610–1490 cm^?1; (b) twofold bridged nitrosyl ligands with a longer non-bonding M?M distance, ca. 1520–1490 cm^?1; (c) threefold bridged nitrosyls, ca. 1470–1410 cm^?1; (d) σ/π dihaptonitrosyl, [M(η²-NO)], where M = Cr, Mn and Ni; ca. 1490–1440 cm^?1. Isonitrosyls, from few examples, appear to absorb below ca. 1100 cm^?1.To be published DFT calculations of the infrared and Raman spectra of complexes with formulae [M(NO)_4?n(CO)_n] (M = Cr, Mn, Fe, Co, Ni, and n = 0, 1, 2, 3, 4, respectively) are used as models for the assignments of the ν(MN) and δ(MNO) bands from more complex metal nitrosyls.     

MgO polyhedral nanocages and nanocrystals based glucose biosensor

MgO polyhedral nanocages and nanocrystals, synthesized by non-catalytic simple thermal evaporation process, were used to fabricate high-sensitive amperometric glucose biosensor which showed a high and reproducible sensitivity of 31.6 μA μM^?1 cm^?2 with a response time less than 5 s, linear dynamic range from 1.0 to 9.0 μM and correlation coefficient of R = 0.9993. The detection limit of fabricated biosensor (based on S/N ratio = 3) was estimated to be 68.3 ± 0.02 nM. To the best of our knowledge, this is the first report which demonstrates the use of MgO nanostructures for the fabrication of glucose biosensor; hence, this work opens a new way to utilize MgO nanostructures as an efficient electron mediator to fabricate efficient glucose biosensors.     

Ferrocene-modified Fe3O4@SiO2 magnetic nanoparticles as building blocks for construction of reagentless enzyme-based biosensors

A novel amperometric glucose biosensor was developed by entrapping glucose oxidase (GOD) in chitosan (CS) composite doped with ferrocene monocarboxylic acid-modified magnetic core-shell Fe₃O₄@SiO₂ nanoparticles (FMC-AFSNPs). It is shown that the obtained magnetic bio-nanoparticles attached to the surface of a carbon paste electrode (CPE) with the employment of a permanent magnet showed excellent electrochemical characteristics and at the same time acted as mediator to transfer electrons between the enzyme and the electrode. Under optimal conditions, this biosensor was able to detect glucose in the linear range from 1.0 × 10⁻⁵ to 4.0 × 10⁻³ M with a detection limit of 3.2 μM (S/N = 3). This immobilization approach effectively improved the stability of the electron transfer mediator and is promising for construction of biosensor and bioelectronic devices.     

Sensitive lactate determination based on acclimated mixed bacteria and palygorskite co-modified oxygen electrode

A sensitive bacteria biosensor was prepared for the detection of trace lactate. The sensitive bioelement, Lactobacillus bulgaricus and Streptococcus thermophilus mixed cultrue, and palygorskite, a perfect matrix for bacteria, was co-immobilized on the surface of oxygen electrode. The biosensor possesses fine selective specificity, good sensitivity and longer operational life time, which were due to the mutual help relationship of symbiotic bacteria and 240 days acclimation with lactate as the carbon source. Hydrodynamic amperometry, an advanced electrochemical method, is suitable for on-line monitoring the concentration change of dissolved oxygen that is closely accompanied with the metabolism of lactate. Electrochemical data show that the current is very sensitive to the changes of the concentration of lactate. The response current was linear with lactic acid concentration in the range from 0 to 300 μmol L^? 1, where the response time is no more than 240 s (R = 0.9952), and the sensitivity was 1.87 mA mol^? 1 L. Experiments show the biosensor is also very useful for long time on-line monitoring of lactate, such as fermentation progress.     

Study on the effects of cefotaxime on intracellular Ca^2＋ in human peripheral lymphocytes by fluoremetry

Characteristic of Fura-2-Ca~(2 )interaction was studied based on the fluorescence technique.The apparent dissociation constants(K_d)of the Fura-2-Ca~(2 )complex were determined at different temperature.The effect of cefotaxime(CEFA)on intracellular Ca~(2 )concentration([Ca~(2 )]_i)was discussed by using a ratiometric fluorescence dye Fura-2 as a probe.The basal[Ca~(2 )]_i in resting humanperipheral lymphocytes was 100 7 nmol/L but after treatment with cefotaxime,the changes of[Ca~(2 )]_i were observed in differentconditions.In the concentration range of 1-30 μmol/L of cefotaxime[Ca~(2 )]_i increased,as a result of releasing intracellular Ca2 stores.Higher concentration of cefotaxime(50-500 μmol/L)stimulated to decrease of[Ca~(2 )]_i.     

Electrochemical Measurement of Immobilized Onion Leaves PPO in Agar-Abelmoschus Escucentus Gum

Onion leaves monophenol monooxygenase and o-diphenol oxidase were extracted by salt and solvent method followed by purification on affinity column using natural affiant. For the fabrication of enzyme-based biosensor, composite matrix of natural biopolymers agar-Abelmoschus escucentus gum was used to immobilize monophenol monooxygenase and o-diphenol oxidase. L-tyrosine and L-dopa were used as substrate for the measurement of phenol. Monophenol monooxygenase and o-diphenol oxidase oxidizes phenolic substrate to the corresponding quinone to allow convenient low-potential detection of phenolic analyte. In the enzymatic catalysis, reduction of quinone to phenol was also observed using (vs.Ag/AgCl) at a platinum electrode. The V_max for tyrosine and dopa is 102.4 and 108.2 μM/litre/min where as K_m is 11.26 X 10^-3 mM and 11.90 X 10^-3 mM respectively. The biosensor exhibited good performance in terms of reusability, linearity, sensitivity, fabrication, simplicity, shelf-life and operational stability. Thus, the biosensor is suitable for the analytical quantification of phenols.     

Glucose biosensor based on new carbon nanotube-gold-titania nano-composites modified glassy carbon electrode

In this paper, a novel biosensor was prepared by immobilizing glucose oxidase （GOx） on carbon nanotube-gold-titania nanocomposites （CNT/Au/TiO2） modified glassy carbon electrode （GCE）. SEM was initially used to investigate the surface morphology of CNT/Au/TiO2 nanocomposites modified GCE, indicating the formation of the nano-porous structure which could readily facilitate the attachment of GOx on the electrode surface. Cyclic voltammogram （CV） and electrochemical impedance spectrum （EIS） were further utilized to explore relevant electrochemical activity on CNT]Au/TiO2 nanocomposites modified GCE. The observations demonstrated that the immobilized GOx could efficiently execute its bioelectrocatalytic activity for the oxidation of glucose. The biosensor exhibited a wider linearity range from 0.1 mmol L-1 to 8 mmol L^-1 glucose with a detection limit of 0.077 mmol L^- 1.     

Selective colorimetric assay of cyanide ions using a thioamide-based probe containing phenol and pyridyl groups

The selective assay of cyanide ions with a thioamide compound (HNPTU) containing phenol and pyridine as a chemosensor is reported using absorbance changes in a buffered aqueous solution (50 mM HEPES, pH 7.4) containing ethanol. Upon treatment with cyanide ions, the colorless solution of HNPTU turned yellow. No significant changes were observed with other comparable anions, such as F⁻, Cl⁻, Br⁻, I⁻, and CH₃COO⁻. The color change of HNPTU upon treatment with CN⁻ was maintained even in the presence of the comparable monovalent anions. The complex stability constant (K_a = 2.6 × 10³) for the stoichiometric 1:1 complexation of HNPTU with cyanide ions was obtained based on absorbance titrations. The interaction of HNPTU with cyanide ions was proposed to be deprotonation, as shown by NMR and Cu(II) treatment experiments.     

Pt-polyaniline nanocomposite on boron-doped diamond electrode for amperometic biosensor with low detection limit

Boron-doped diamond electrodes covered with a nanostructured Pt nanoparticle-polyaniline composite have been fabricated and employed as sensitive amperometric sensors with low detection limit. A highly conductive boron-doped diamond thin film (BDD) was prepared by chemical vapor deposition, and its morphology was characterized by scanning electron microscopy and transmission electron microscopy. The nanostructured composite layer was grown on the BDD electrode by electrochemical deposition of polyaniline and Pt nanoparticles. Glucose oxidase (GOx) was then adsorptively immobilized on the modified BDD electrode. The biosensor displays a large surface area, high catalytic activity of the Pt nanoparticles, efficient electron mediation through the conducting polymer, and low background current of the electrode. The biosensor exhibits an excellent response to glucose, with a broad linear range from 5.9 μM to 0.51 mM, a sensitivity of 5.5 μA·mM^?1, a correlation coefficient (R) of 0.9947, and a detection limit of 0.10 μM. The apparent Michaelis-Menten constant (K _M ^app ) and the maximum current density of the electrode are 4.1 mM and 0.021 mA, respectively. This suggests that the immobilized GOx possesses a higher affinity for glucose at the lower K _M ^app , and that the enzymatic reaction rate constitutes the rate-limiting step of the response.     

Looking glass inhibitors: both enantiomeric N-benzyl derivatives of 1,4-dideoxy-1,4-imino-d-lyxitol [a potent competitive inhibitor of α-d-galactosidase] and of 1,4-dideoxy-1,4-imino-l-lyxitol [a weak competitive inhibitor of α-d-galactosidase] inhibit naringinase,an α-l-rhamnosidase competitively

Benzhydryl protection by diphenyldiazomethane of an alcohol in enantiomeric base-sensitive ribonolactones allows short efficient syntheses of 1,4-dideoxy-1,4-imino-d-lyxitol (DIL) and of 1,4-dideoxy-1,4-imino-l-lyxitol (LIL). DIL showed potent [K_i = 0.13 μM]—and LIL showed weak [K_i = 113 μM]—competitive inhibition of α-d-galactosidase. Both enantiomers N-benzyl-DIL [K_i = 64 μM] and N-benzyl-LIL [K_i = 13 μM] were moderate competitive inhibitors of naringinase, an α-l-rhamnosidase.     

Synthesis,characterizations of silica-coated gold nanorods and its applications in electroanalysis of hemoglobin

Gold nanorods (GNRs) were synthesized by a seed–mediated growth approach followed by TEOS polymerization leading to the formation of silica layer surrounding the gold nanorod core. TEM images showed that the silica-coated gold nanorods (GNRs@SiO₂) were dispersed with an average aspect ratio of 3.1 for the GNRs cores and a uniform thickness of the silica shell. The core/shell nanocomposites were further used as efficient supports for the immobilization of hemoglobin (Hb) to fabricate a novel biosensor. The immobilized Hb showed an enhanced electron transfer for its heme Fe(III) to Fe(II) redox couple. This biosensor showed an excellent bioelectrocatalytic activity towards H₂O₂ with a linear range from 8.0 × 10⁻⁷ to 6.1 × 10⁻⁵ M, and the detection limit was 6.0 × 10⁻⁸ M at 3σ. The apparent Michaelis–Menten constant of the immobilized hemoglobin was calculated to be 0.13 mM.     

Amperometric glucose sensor based on glucose oxidase immobilized on gelatin-multiwalled carbon nanotube modified glassy carbon electrode

We investigated the direct electrochemistry of glucose oxidase (GOx) at gelatin-multiwalled carbon nanotube (GCNT) modified glassy carbon electrode (GCE). GOx was covalently immobilized onto GCNT modified GCE through the well known glutaraldehyde (GAD) chemistry. The immobilized GOx showed a pair of well-defined reversible redox peaks with a formal potential (E⁰′) of ? 0.40 V and a peak to peak separation (ΔE_p) of 47 mV. The surface coverage concentration (Г) of GOx in GCNT/GOx/GAD composite film modified GCE was 3.88 × 10^? 9 mol cm^? 2 which indicates the high enzyme loading. The electron transfer rate constant (k_s) of GOx immobilized onto GCNT was 1.08 s^? 1 which validates a rapid electron transfer processes. The composite film shows linear response towards 6.30 to 20.09 mM glucose. We observed a good sensitivity of 2.47 μA mM^?1 cm^? 2 for glucose at the composite film. The fabricated biosensor displayed two weeks stability. Moreover, it shows no response to 0.5 mM of ascorbic acid (AA), uric acid (UA), acetaminophen (AP), pyruvate (PA) and lactate (LA) which shows its potential application in the determination of glucose from human serum samples. The composite film exhibits excellent recovery for glucose in human serum at physiological pH with good practical applicability.     

Kinetic spectrophotometric method for trace determination of thiocyanate based on its inhibitory effect

A kinetic spectrophotometric method for the determination of thiocyanate, based on its inhibitory effect on silver(I) catalyzed substitution of cyanide ion, by phenylhydrazine in hexacyanoferrate(II) is described. Thiocyanate ions form strong complexes with silver(I) catalyst which is used as the basis for its determination at trace level. The progress of reaction was monitored, spectrophotometrically, at 488 nm (λ_max of [Fe(CN)₅PhNHNH₂]^3?, complex) under the optimum reaction conditions at: 2.5 × 10^?3 M [Fe(CN)₆]^4?, 1.0 × 10^?3 M [PhNHNH₂], 8.0 × 10^?7 M [Ag⁺], pH 2.8 ± 0.02, ionic strength (μ) 0.02 M (KNO₃) and temperature 30 ± 0.1 °C. A linear relationship obtained between absorbance (measured at 488 nm at different times) and inhibitor concentration, under specified conditions, has been used for the determination of [thiocyanate] in the range of 0.8–8.0 × 10^?8 M with a detection limit of 2 × 10^?9 M. The standard deviation and percentage error have been calculated and reported with each datum. A most plausible mechanistic scheme has been proposed for the reaction. The values of equilibrium constants for complex formation between catalyst–inhibitor (K_CI), catalyst–substrate (K_s) and Michaelis–Menten constant (K_m) have been computed from the kinetic data. The influence of possible interference by major cations and anions on the determination of thiocyanate and their limits has been investigated.     

A novel method for Co(Ⅱ) and Cu(Ⅱ) analysis by capillary electrophoresis with chemiluminescence detection

Based on the fact that some metal ions can catalyze the chemiluminescence(CL)reaction of luminol with K_3Fe(CN)_6,a novel capillary electrophoresis CL method was developed for the determination of Co(Ⅱ)and Cu(Ⅱ).The separation was carried out with a 10 mmol/L sodium acetate solution containing 0.8 mmol/L luminol and 2.0 mmol/Lα-HIBA(adjusted to pH 4.8 by HAc solution).The post-capillary reagent was 2.0 mmol/L K_3Fe(CN)_6 which was adjusted to pH 13.0 by NaOH solution.Under the optimum conditions,the detection limits(S/N=3)for Co(Ⅱ)and Cu(Ⅱ)were 7.5×10~(-11)mol/L and 7.5×10~(-9)mol/L,with the linear range of 7.5×10~(-9)mol/L to 1.0×10~(-6)mol/L and 7.5×10~(-8)mol/L to 5.0×10~(-5)mol/L, respectively.     

Synthesis and molecular modeling study of Cu(Ⅱ) complexes derived from 2-(diphenylmethylene)hydrazinecarbothioamide derivatives with cholinesterase inhibitory activities

Thiosemicarbazones of 2-amino-5-chlorobenzophenone and 3-aminobenzophenone（L¹-L⁴） have been synthesized and their Cu（Ⅱ） complexes（1-4） were afforded via coordination with cupric chloride.All these compounds were characterized by UV-vis and IR spectroscopy together with CHN elemental analysis.NMR spectroscopy was also applied to characterize the ligands.In vitro chohnesterase inhibitory assays for the complexes（1-4） showed IC₅₀ values less than 10μmol/L,with complex 1 exhibiting the most activity,IC₅₀=2.15μmol/L and 2.16μmol/L for AChE and BuChE,respectively. Molecular modeling simulation revealed the binding interaction template for complex 1 with the AChE and BuChE receptors.In DPPH assay,the complexes also showed more in vitro antioxidant activities in comparison to their parent ligands.     

Novel poly (neutral red) nanowires as a sensitive electrochemical biosensing platform for hydrogen peroxide determination

Poly (neutral red) nanowires (PNRNWs) have been synthesized for the first time by the method of cyclic voltammetric electrodeposition using porous anodic aluminum oxide (AAO) template and were examined by scanning electron microscopy (SEM) and transmission electron microscope (TEM). Moreover, horseradish peroxidase (HRP) was encapsulated in situ in PNRNWs (denoted as PNRNWs–HRP) by electrochemical copolymerization for potential biosensor applications. The PNRNWs showed excellent efficiency of electron transfer between the HRP and the glassy carbon (GC) electrode for the reduction of H₂O₂ and the PNRNWs–HRP modified GC electrode showed to be excellent amperometric sensors for H₂O₂ at −0.1 V with a linear response range of 1 μM to 8 mM with a correlation coefficient of 0.996. The detection limit (S/N = 3) and the response time were determined to be 1 μM and <5 s and the high sensitivity is up to 318 μA mM⁻¹ cm⁻².