Thermal degradation kinetics of polyimide containing 2,6-benzobisoxazole units

A novel polyimide (PI) based on 2,6-bis(p-aminophenyl)-benzo[1,2-d;5,4-d′]bisoxazole has been synthesized via a conventional two-stage procedure with bis(ether anhydrides) (HQDPA). The intermediate poly(amic acid) had inherent viscosities of 1.70 dl/g and could be thermally converted into light yellow polyimide film. The resulted polyimide showed excellent thermal stability, and the glass transition temperatures (T_g) were above 283 °C, the 5% weight loss temperature of the polymer was at 572 °C in N₂. The thermal degradation of the polyimide was studied by thermogravimetric analysis (TGA) in order to determine the actual reaction mechanisms of the decomposition process. The activation energy of the solid-state process was determined using Flynn-Wall-Ozawa method, which does not require knowledge of the reaction mechanism, which resulted to be 361.36 kJ/mol. The activation energy of different mechanism models and pre-exponential factor (A) were determined by Coats-Redfern method. Compared with the value obtained from the Ozawa method, the actual reaction mechanism obeyed nucleation and growth model, Avrami-Erofeev function (A₃) with integral form g(X) = [−ln(1−X)]³.     

Synthesis and characterization of electroactive copolymer with phenyl-capped aniline tetramer in the main chain by oxidative coupling polymerization

A novel electroactive alternating copolymer, with fixed conjugated length of oligoaniline (phenyl-capped aniline tetramer) in the main chain, was successfully synthesized via oxidative coupling polymerization. The structure of the copolymer was systematically studied by Fourier-transform infrared (FTIR) spectra, NMR, elemental analysis (EA), UV-vis spectra and X-ray powder diffraction (XRD). And its electrochemical behavior was studied by cyclic voltammetry (CV) measurement. It was found that the obtained copolymer bearing phenyl-capped aniline tetramer segments had a reversible electrochemical property in the cyclic voltammetry, and the copolymer was oxidized to its emeraldine oxidation state and then to the pernigraniline oxidation state, which was same as that of polyaniline. Moreover, the thermal properties of the copolymer were evaluated by thermogravimetric analysis (TGA). The electrical conductivity of the obtained copolymer was about 1.43 × 10⁻⁷ S cm⁻¹ at room temperature.     

Structural aspects of high temperature thermosets - Bismaleimide/propargyl terminated resin system-polymerization and degradation studies

Bispropargyl ether of bisphenol-A (BPEBPA), 4,4′-bismaleimido diphenyl ether (BMIE) and a blend consisting 60 mol% of BPEBPA and 40 mol% of BMIE are prepared. The materials are structurally characterized by FTIR. The curing characteristics of the monomers are measured by FTIR and DSC. The results indicated that BPEBPA-BMIE blend has low ΔH_cure (J g⁻¹) for the thermal polymerization and the whole temperature window for the exothermic curing reaction is shifted to lower temperature compared to BPEBPA. Borchardt and Daniels method is used to study the cure kinetics of the materials. The thermal curing of BMIE requires activation energy of 156.0 kJ mol⁻¹ whereas BPEBPA needs slightly higher activation energy (177.2 kJ mol⁻¹). From the TG studies, it can be concluded that the cured BPEBPA exhibits higher thermal stability than the cured BMIE due to the more complex network structure that are formed during thermal polymerization of BPEBPA. Dharwadkar and Kharkhanavala equation is employed to calculate the activation energy needed for the thermal degradation of the thermally cured materials. BPEBPA shows much higher activation energy (65.5 kJ mol⁻¹) for thermal degradation indicating the higher thermal stability over the other two materials (BMIE: 42.5 kJ mol⁻¹ and BPEBPA-BMIE blend: 46.9 kJ mol⁻¹). The isothermal degradation of cured materials is effected in nitrogen atmosphere for constant time interval (10 min). The detailed analysis of the degradation products by GC-MS revealed the formation of phenols and several substituted phenols. This finding hints that the competitive C-C and C-O scissions of the chromene ring units formed via the Claisen rearrangement of the aryl propargyl ether system present in BPEBPA is operative.     

Thermoelectric properties of polycrystalline La1−xSrxCoO3

Thermoelectric properties of polycrystalline La_1−xSr_xCoO₃, where Sr²⁺ is substituted in La³⁺ site in perovskite-type LaCoO₃, have been investigated. Sr-doping increases the electrical conductivity (σ) of La_1−xSr_xCoO₃, and also decreases the Seebeck coefficient (S) for 0.01?x?0.40. A Hall coefficient measurement reveals that the increase in electrical conductivity arises from increases in both carrier concentration and the Hall mobility. The decrease in the Seebeck coefficient is caused by a decrease in carrier effective mass as well as increase in carrier concentration. The highest power factor (σS²) is 3.7×10⁻⁴ W m⁻¹ K⁻² at 250 K for x=0.10. The thermal conductivity (κ) is about 2 W m⁻¹ K⁻¹ at 300 K for 0?x?0.04, and increases for x?0.05 because of an increase in heat transport by conductive carrier. The thermoelectric properties of La_1−xSr_xCoO₃ are improved by Sr-doping, and the figure of merit (Z=σS² κ⁻¹) reaches 1.6×10⁻⁴ K⁻¹ for x=0.06 at 300 K (ZT=0.048). For heavily Sr-doped samples, the thermoelectric properties diminish mainly because of the decrease in the Seebeck coefficient and the increase in thermal conductivity.     

Sensitive DNA biosensor improved by Luteolin copper(II) as indicator based on silver nanoparticles and carbon nanotubes modified electrode

A novel and sensitive electrochemical DNA biosensor has been developed for the detection of DNA hybridization. The biosensor was proposed by using copper(II) complex of Luteolin C₃₀H₁₈CuO₁₂ (CuL₂) as an electroactive indicator based on silver nanoparticles and multi-walled carbon nanotubes (Ag/MWCNTs) modified glassy carbon electrode (GCE). In this method, the 4-aminobenzoic acid (4-ABA) and Ag nanoparticles were covalently grafted on MWCNTs to form Ag/4-ABA/MWCNTs. The proposed method dramatically increased DNA attachment quantity and complementary ssDNA detection sensitivity for its large surface area and good charge-transport characteristics. DNA hybridization detection was performed using CuL₂ as an electroactive indicator. The CuL₂ was synthesized and characterized using elemental analysis (EA) and IR spectroscopy. Cyclic voltammetry (CV) and fluorescence spectroscopy were used to investigate the interaction between CuL₂ and ds-oligonucleotides (dsDNA). It was revealed that CuL₂ presented high electrochemical activity on GCE, and it could be intercalated into the double helices of dsDNA. The target ssDNA of the human hepatitis B virus (HBV) was quantified in a linear range from 3.23 × 10⁻¹² to 5.31 × 10⁻⁹ M (r = 0.9983). A detection limit of 6.46 × 10⁻¹³ M (3σ, n = 11) was achieved.     

Phase behavior of biodegradable amphiphilic poly(l,l-lactide)-b-poly(ethylene glycol)-b-poly(l,l-lactide)

This study describes the miscibility phase behavior in two series of biodegradable triblock copolymers, poly(l-lactide)-block-poly(ethylene glycol)-block-poly(l-lactide) (PLLA-PEG-PLLA), prepared from two di-hydroxy-terminated PEG prepolymers (M_n = 4000 or 600 g mol⁻¹) with different lengths of poly(l-lactide) segments (polymerization degree, DP = 1.2-145.6). The prepared block copolymers presented wide range of molecular weights (800-25,000 g mol⁻¹) and compositions (16-80 wt.% of PEG). The copolymer multiphases coexistance and interaction were evaluated by DSC and TGA. The copolymers presented a dual stage thermal degradation and decreased thermal stability compared to PEG homopolymers. In addition, DSC analyses allowed the observation of multiphase separation; the melting temperature, T_m, of PLLA and PEG phases depended on the relative segment lengths and the only observed glass transition temperature (T_g) in copolymers indicated miscibility in the amorphous phase.     

ROMP of t-butyl-substituted ferrocenophanes affords soluble conjugated polymers that contain ferrocene moieties in the backbone

A substituted ferrocenophane, 1,1′-((1-tert-butyl)-1,3-butadienylene)ferrocene, was synthesized and polymerized via ring-opening metathesis polymerization (ROMP) to give soluble high molecular weight polymers with ferrocenylene units in the backbone. The monomer readily underwent polymerization upon exposure to a tungsten-based metathesis initiator, W(CHC₆H₄-o-OMe)(NPh)[OCMe(CF₃)₂]₂ (THF), to give high molecular weight polymers (M_w=ca. 300,000). The molecular weights could be varied systematically by adjusting the monomer-to-catalyst ratio. UV/vis spectra revealed a bathochromic shift for the polymer, consistent with enhanced conjugation compared to the monomer. The polymer exhibited thermal properties similar to oligomeric poly(ferrocenylene). Cyclic voltammetry of the polymer suggested that the iron centers are coupled electronically. Upon doping with I₂ vapor, the polymers displayed semiconducting properties (σ=10⁻⁵ S cm⁻¹). Theoretical calculations were used to evaluate the nature of the bonding in these and related polymers.     

Solution-processable transparent polyimides with low coefficients of thermal expansion and self-orientation behavior induced by solution casting

A fluorinated tetracarboxylic dianhydride (amide-type TA-TFMB) was prepared from trimellitic anhydride chloride and 2,2′-bis(trifluoromethyl)benzidine (TFMB). A chemically imidized polyimide (PI) derived from TA-TFMB and TFMB was rather soluble in various solvents. Solution casting of this PI (TA-TFMB/TFMB) led to a flexible, non-turbid, and seemingly almost colorless PI film with a high T_g of 328 °C and a considerably low coefficient of thermal expansion (CTE) of 9.9 ppm K⁻¹ which results from significant in-plane chain orientation induced during solution casting. The self-orientation mechanism is discussed. The properties of TA-TFMB/TFMB were compared with those of some relevant systems. The results suggest that an electron-withdrawing effect of the 2,2′-CF₃ substituents of TA-TFMB and a twisted conformation of the central biphenyl moiety greatly contribute to the suppressed coloration of the TA-TFMB/TFMB film. The use of a TA-TFMB counterpart (ester-type TA-TFBP) was effective for further enhancing the transparency owing to reduced charge-transfer interaction. However, the thermal properties of TA-TFBP/TFMB were not always satisfactory. Copolymerization using 2,3,6,7-naphthalenetetracarboxylic dianhydride led to a PI film with an increased T_g of 277 °C and a very low CTE of 12.6 ppm K⁻¹ without significant decreases in the transparency and the solubility. Thus, this work proposes promising candidates as novel heat-resistant plastic substrate materials in display devices.     

Electrosyntheses of free-standing poly(dibenzo-18-crown-6) films in boron trifluoride diethyl etherate on stainless steel electrode

High-quality free-standing poly(dibenzo-18-crown-6) (PDBC) films with a conductivity of 4.1 × 10⁻² S cm⁻¹ and good thermal stability were synthesized electrochemically on stainless steel electrode by direct anodic oxidation of dibenzo-18-crown-6 (DBC) in pure boron trifluoride diethyl etherate (BFEE). In this medium, the oxidation potential onset of DBC was measured to be only 0.98 V vs. SCE, which was much lower than that in acetonitrile + 0.1 mol L⁻¹ Bu₄NBF₄ (1.45 V vs. SCE). PDBC films obtained from this medium showed good redox activity and stability in BFEE. The structural characterization of PDBC was performed using UV-vis, FTIR spectroscopy. The results of quantum chemistry calculations of DBC monomer and FTIR spectroscopy of PDBC films indicated that the polymerization mainly occurred at C₍₄₎ and C₍₅₎ positions). Fluorescent spectral studies indicated that PDBC was a blue light emitter. To the best of our knowledge, this is the first report on the electrodeposition of free-standing PDBC films.     

Surface structure of thin asymmetric PS-b-PMMA diblock copolymers investigated by atomic force microscopy

Asymmetric poly(styrene-b-methyl methacrylate) (PS-b-PMMA) diblock copolymers of molecular weight M_n = 29,700 g mol⁻¹ (M_PS = 9300 g mol⁻¹M_PMMA = 20,100 g mol⁻¹, PD = 1.15, χ_PS = 0.323, χ_PMMA = 0.677) and M_n = 63,900 g mol⁻¹ (M_PS = 50,500 g mol⁻¹, M_PMMA = 13,400 g mol⁻¹, PD = 1.18, χ_PS = 0.790, χ_PMMA = 0.210) were prepared via reversible addition-fragmentation chain transfer (RAFT) polymerization. Atomic force microscopy (AFM) was used to investigate the surface structure of thin films, prepared by spin-coating the diblock copolymers on a silicon substrate. We show that the nanostructure of the diblock copolymer depends on the molecular weight and volume fraction of the diblock copolymers. We observed a perpendicular lamellar structure for the high molar mass sample and a hexagonal-packed cylindrical patterning for the lower molar mass one. Small-angle X-ray scattering investigation of these samples without annealing did not reveal any ordered structure. Annealing of PS-b-PMMA samples at 160 °C for 24 h led to a change in surface structure.     

Thermophysical properties of SrHfO3 and SrRuO3

Polycrystalline samples of strontium series perovskite type oxides, SrHfO₃ and SrRuO₃ were prepared and the thermophysical properties were measured. The average linear thermal expansion coefficients are 1.13×10⁻⁵ K⁻¹ for SrHfO₃ and 1.03×10⁻⁵ K⁻¹ for SrRuO₃ in the temperature range between 423 and 1073 K. The melting temperatures T_m of SrHfO₃ and SrRuO₃ are 3200 and 2575 K, respectively. The longitudinal and shear sound velocities were measured by an ultrasonic pulse-echo method at room temperature in air, which enables to evaluate the elastic moduli and Debye temperature. The heat capacity was measured by using a differential scanning calorimeter, DSC in high-purity argon atmosphere. The thermal diffusivity was measured by a laser flash method in vacuum. The thermal conductivities of SrHfO₃ and SrRuO₃ at room temperature are 5.20 and 5.97 W m⁻¹ K⁻¹, respectively.     

X-ray diffraction study on the thermal expansion behavior of cellulose Iβ and its high-temperature phase

Oriented films of cellulose prepared from algal cellulose were hydrothermally treated to convert them into highly crystalline cellulose Iβ. The lateral thermal expansion behavior of the prepared cellulose Iβ films was investigated using X-ray diffraction at temperatures from 20 to 300 °C. Cellulose Iβ was transformed into the high-temperature phase when the temperature was above 230 °C, allowing the lateral thermal expansion coefficient of cellulose Iβ and its high-temperature phase to be measured. For cellulose Iβ, the thermal expansion coefficients (TECs) of the a- and b-axes were α_a = 9.8 × 10⁻⁵ °C⁻¹ and α_b = 1.2 × 10⁻⁵ °C⁻¹, respectively. This anisotropic thermal expansion behavior in the lateral direction is ascribed to the crystal structure and to the hydrogen-bonding system of cellulose Iβ. For the high-temperature phase, the anisotropy was more conspicuous, and the TECs of the a- and b-axes were α_a = 19.8 × 10⁻⁵ °C⁻¹ and α_b = −1.6 × 10⁻⁵ °C⁻¹, respectively. Synchrotron X-ray fiber diffraction diagrams of the high-temperature phase were also recorded at 250 °C. The cellulose high-temperature phase is composed of a two-chain monoclinic unit cell, a = 0.819 nm, b = 0.818 nm, c (fiber repeat) = 1.037 nm, and γ = 96.4°, with space group = P2₁. The volume of this cell is 4.6% larger than that of cellulose Iβ at 30 °C.     

A novel planar miniaturized potentiometric sensor for flow injection analysis of nitrates in wastewaters, fertilizers and pharmaceuticals

A novel all-solid-state miniaturized nitrate sensor is developed, characterized and used for flow injection analysis (FIA) of nitrates in various samples. The sensor incorporates silver bis(bathophenanthroline) nitrate [Ag(bath)₂NO₃] as an electroactive material in a plasticized PVC membrane. The sensing membrane (3 mm × 5 mm) is immobilized on a wafer polyimide microchip (size 13.5 mm × 3.5 mm) to offer a planar miniaturized design easily used in a single channel wall-jet flow injection system. Under hydrodynamic mode of operation (FIA) the sensor displays fast response, high sensitivity, long term stability and good selectivity for NO₃⁻ in the presence of many common associated anions. The calibration slope is 55.1 ± 0.1 mV decade⁻¹ over the concentration range 1.0 × 10⁻¹ to 1.0 × 10⁻⁶ mol L⁻¹, the lower detection limit is 0.05 μg mL⁻¹, the working pH is 2-9,and the output is 70-90 samples h⁻¹. Validation of the assay method reveals good performance characteristics and suggests application for routine determination of NO₃⁻ in industrial wastewaters, fertilizers and pharmaceuticals. The results agree fairly well with data obtained by the standard spectrophotometric methods.     

Electrochemistry and scanning electron microscopy of polyaniline/peroxidase-based biosensor

An amperometric biosensor was prepared by in situ deposition of horseradish peroxidase (HRP) enzyme on a polyaniline (PANI)-doped platinum disk electrode. The PANI film was electrochemically deposited on the electrode at 100 mV s⁻¹/Ag-AgCl. Cyclic voltammetric characterization of the PANI film in 1 M HCl showed two distinct redox peaks, which prove that the PANI film was electroactive and exhibited fast reversible electrochemistry. The surface concentration and film thickness of the adsorbed electroactive species was estimated to be 1.85×10⁻⁷ mol cm⁻² and approximately 16 nm, respectively. HRP was electrostatically immobilized onto the surface of the PANI film, and voltammetry was used to monitor the electrocatalytic reduction of hydrogen peroxide under diffusion-controlled conditions. Linear responses over the concentration range 2.5×10⁻⁴ to 5×10⁻³ M were observed. Spectroelectrochemistry was used to monitor the changes in UV-vis properties of HRP, before and after the catalysis of H₂O₂. The biosensor surface morphology was characterized by scanning electron microscopy (SEM) using PANI-doped screen-printed carbon electrodes (SPCEs) in the presence and absence of (i) peroxidase and (ii) peroxide. The SEM images showed clear modifications of the conducting film surface structure when doped with HRP, as well as the effect of hydrogen peroxide on the morphology of biosensor.     

Synthesis, crystal structure and thermal behavior of Sr3B2SiO8 borosilicate

Single crystals of Sr₃B₂SiO₈ were obtained by solid-state reaction of stoichiometric mixture at 1200 °C. The crystal structure of the compound has been solved by direct methods and refined to R1=0.064 (wR=0.133). It is orthorhombic, Pnma, a=12.361(4), b=3.927(1), c=5.419(1) Å, V=263.05(11) Å³. The structure contains zigzag pseudo-chains running along the b axis and built up from corner sharing (Si,B)−O polyhedra. Boron and silicon are statistically distributed over one site with their coordination strongly disordered. Sr atoms are located between the chains providing three-dimensional linkage of the structure.The formation of Sr₃B₂SiO₈ has been studied using annealing series in air at 900-1200 °C. According powder XRD, the probe contains pure Sr₃B₂SiO₈ over 1100 °C. The compound is not stable below 900 °C. In the pseudobinary Sr₂B₂O₅-Sr₃B₂SiO₈ system a new series of solid solutions Sr_3−xB₂Si_1−xO_8−3x (x=0-0.9) have been crystallized from melt. The thermal behavior of Sr₃B₂SiO₈ was investigated using powder high-temperature X-ray diffraction (HTXRD) in the temperature range 20-900 °C. The anisotropic character of thermal expansion has been observed: α_a= −1.3, α_b=23.5, α_c=13.9, and α_V=36.1×10⁻⁶ °C⁻¹ (25 °C); α_a= −1.3, α_b=23.2, α_c=5.2, and α_V=27.1×10⁻⁶ °C⁻¹ (650 °C). Maximal thermal expansion of the structure along of the chain direction [0 1 0] is caused by the partial straightening of chain zigzag. Hinge mechanism of thermal expansion is discussed.     

Adsorption of arsenic(III) into modified lamellar Na-magadiite in aqueous medium—Thermodynamic of adsorption process

Synthetic Na-magadiite sample was used for organofunctionalization process with N-propyldiethylenetrimethoxysilane and bis[3-(triethoxysilyl)propyl]tetrasulfide, after expanding the interlayer distance with polar organic solvents such as dimethylsulfoxide (DMSO). The resulted materials were submitted to process of adsorption with arsenic solution at pH 2.0 and 298±1 K. The adsorption isotherms were adjusted using a modified Langmuir equation with regression nonlinear; the net thermal effects obtained from calorimetric titration measurements were adjusted to a modified Langmuir equation. The adsorption process was exothermic (Δ_intH=−4.15-5.98 kJ mol⁻¹) accompanied by increase in entropy (Δ_intS=41.32-62.20 J k⁻¹ mol⁻¹) and Gibbs energy (Δ_intG=−22.44−24.56 kJ mol⁻¹). The favorable values corroborate with the arsenic (III)/basic reactive centers interaction at the solid-liquid interface in the spontaneous process.     

Electrical detection of deoxyribonucleic acid hybridization based on carbon-nanotubes/nano zirconium dioxide/chitosan-modified electrodes

A novel and sensitive electrochemical DNA biosensor based on nanoparticles ZrO₂ and multi-walled carbon nanotubes (MWNTs) for DNA immobilization and enhanced hybridization detection is described. The MWNTs/nano ZrO₂/chitosan-modified glassy carbon electrode (GCE) was fabricated and oligonucleotides were immobilized to the GCE. The hybridization reaction on the electrode was monitored by differential pulse voltammetry (DPV) analysis using electroactive daunomycin as an indicator. Compared with previous DNA sensors with oligonucleotides directly incorporated on carbon electrodes, this carbon nanotube-based assay with its large surface area and good charge-transport characteristics increased DNA attachment quantity and complementary DNA detection sensitivity. The response signal increases linearly with the increase of the logarithm of the target DNA concentration in the range of 1.49 × 10⁻¹⁰ to 9.32 × 10⁻⁸ mol L⁻¹ with the detection limit of 7.5 × 10⁻¹¹ mol L⁻¹ (S/N = 3). The linear regression equation is I = 32.62 + 3.037 log C_DNA (mol L⁻¹) with a correlation coefficient value of 0.9842. This is the first application of carbon nanotubes combined with nano ZrO₂ to the fabrication of an electrochemical DNA biosensor with a favorable performance for the rapid detection of specific hybridization.     

Investigation of Ba fully occupied A-site BaCo0.7Fe0.3−xNbxO3−δ perovskite stabilized by low concentration of Nb for oxygen permeation membrane

BaCo_0.7Fe_0.3−xNb_xO_3−δ (BCFN, x = 0–0.2) were prepared by the conventional solid state reaction process. The crystal structure, electrical conductivity and oxygen desorption property were studied by X-ray diffraction (XRD), different thermal analysis (DTA), four-terminal direct current conductivity and oxygen temperature programmed desorption (O₂-TPD), respectively. At x = 0.08–0.20, BCFN have a cubic perovskite structure, while it exhibits the hexagonal structure for x = 0.00 and the mixed phases of cubic perovskite with trace amount of hexagonal for x = 0.05. BCFN shows good structure stability in 5%H₂ + Ar reducing atmosphere, and it is enhanced with the increased Nb-doping content. The electrical conductivity of BCFN increases with increasing temperature and decreases with the Nb substitution content for iron. BCFN exhibits a p-type semiconductor and obeys the thermally activated small polarons hopping mechanism. The oxygen fluxes increase with the working temperature and the COG flow rate, but decrease with increasing Nb content. The flux of BCFN (x = 0.08) with 1.0 mm thickness membrane reaches 25.77 ml min⁻¹ cm⁻² at 875 °C, higher than most of the reported materials.     

Synthesis and properties of thermally cross-linkable main-chain azobenzene polymers containing diacetylene moieties

A series of main chain azobenzene polymers containing diacetylene moieties with different lengths of the spacer {-[CC-CH₂-O-C₆H₄-OCO-(CH₂)_m-O-C₆H₄-NN-C₆H₄-O-(CH₂)_m-OCO-C₆H₄-O-CH₂-CC]_n-, where m = 3, 6, 11} were synthesized by oxidative coupling polymerization. These polymers had molecular weights of 17,600-68,600 and polydispersity indices of 1.2-1.8 as determined by gel permeation chromatography using polystyrene as a standard. Their structures and properties were characterized and evaluated with NMR, FT-IR, X-ray diffraction (XRD), thermogravimetry (TG), differential scanning calorimetry (DSC) and nonlinear optical (NLO) analyses. All the polymers could be cross-linked at the elevated temperatures due to the polymerization reactions of the diacetylene groups in the polymer backbone, and the cross-linked polymers showed dramatically modified properties, such as thermal stability and solvent resistance. The third-order nonlinear susceptibilities of the cross-linked polymers were evaluated by means of the Z-scan technique and calculated to be 3.60 × 10⁻⁹, 2.73 × 10⁻⁹, 2.28 × 10⁻⁹ esu, respectively, whereas the un-cross-linked polymers showed no obvious NLO property.     

Enhanced electrochemical performance at screen-printed carbon electrodes by a new pretreating procedure

A new method for the pretreatment of screen-printed carbon electrodes (SPCEs) by two successive steps was proposed. In step one, fresh SPCEs were soaked into NaOH with high concentration (e.g. 3 M) for tens to hundreds of minutes, and the resulted electrodes were called as SPCE-I. In step two, SPCE-I were pre-anodized in low concentration of NaOH, which were designated as SPCE-II. The pretreated electrodes showed remarkable enhancement in heterogeneous electron transfer rate constant (k⁰) increased from 1.6 × 10⁻⁴ cm s⁻¹ at the fresh SPCE to 1.1 × 10⁻² cm s⁻¹ at SPCE-I for Fe(CN)₆^3−/4− couple. The peak to peak separation (ΔE_p) in cyclic voltammetry was reduced from ca. 480 to 84 mV, indicating that the electrochemical reversibility was greatly promoted, possibly due to the removing of polymers/oil binder from the electrode surfaces. The electroactive area (A_ea) of the electrode was increased by a factor of 17 after pretreatment in step one. Further analysis by the electrochemical impedance method showed that the electron transfer resistance (R_ct) decreased from ca. 2100 to 1.4 Ω. These pretreated electrodes, especially SPCE-II, exhibited excellent electrocatalytic behavior for the redox of dopamine (DA). Interference from ascorbic acid (AA) in the detection of DA at SPCE-II could be effectively eliminated due to the anodic peak separation (190 mV) between DA and AA, which resulted from the functionalization of the electrode surface in the pretreatment of step two. Under optimum conditions, current responses to DA were linearly changed in two concentration intervals, one was from 3.0 × 10⁻⁷ to 9.8 × 10⁻⁶ M, and the other was from 9.8 × 10⁻⁶ to 3.3 × 10⁻⁴ M. The detection limit for DA was down to 1.0 × 10⁻⁷ M.