Selective detection of dopamine on poly(indole-5-carboxylic acid)/tyrosinase electrode

We report here on a simple tyrosinase (TYR) modified electrode designed through the covalent bonding of the enzyme with poly (indole-5-carboxylic acid) (PIn5COOH) conducting polymer. This electrode was applied to the amperometric detection of dopamine (DA) in the presence of ascorbic acid (AA), uric acid (UA) and their mixtures, in the concentration range and ratios similar to those found in blood serum. Our experiments demonstrate that the presence of these interferents (AA, UA) does not affect the selectivity of such electrode towards dopamine with linear concentration dependence in the range of 0.5–20 μM, depending on the experimental conditions, however its sensitivity depends on the type and amount of interferent present. The lower limit of detection of DA in the presence of high AA (1000 fold) or UA (500 fold) concentration was found to be 0.1–0.5 μM. The sensitivity for DA detection is 6.2 A/M cm² with UA and 2.3 A/M cm² with AA present as interfering agents. For both interferents present in the ratio 12.5:1 (AA:UA), the sensitivity drops to the value of ca. 1.3 A/M cm². The Michaelis–Menten (K_M) constant and I_max values were evaluated, showing improved electrode sensitivity towards dopamine as judged from the decrease of the Michaelis–Menten constant.     

Functional gold nanorod particles on conducting polymer poly(3-octylthiophene) as non-enzymatic glucose sensor

The immobilization of surface-functionalized self-assembled monolayer (SAM) gold nanoparticles onto poly(3-octylthiophene) (POT) was achieved by the cooperation of hydrophobic forces. SAMs were prepared by 11-mercaptoundecanoicacid (MUA), 4-mercaptophenyl boronic acid (MPB), and 1-decanethiol (DT) hydrophobic substrates. Nanoparticles-SAM-POT system was characterized by cyclic voltammetry, SEM, EDAX and contact angle measurements. SAMs (MUA) is closely packed providing effective blocking of the underlying platinum electrode and preventing a ferrocyanide molecule from penetrating. However, potential scanning was applied at SAMs (MUA) modified electrode on which electron penetrating holes or defects were occured. Since SAMs (MPB) is poorly packed according to SAMs (MUA), ferrocyanide molecules could penetrate to SAMs (MPB) modified electrode surface. POT-Au-SAM (MPB) electrode was used for glucose determination as potentiometric non-enzymatic glucose sensor. The analytical performance was evaluated and linear calibration graphs were obtained in the concentration range of 5-30 mM glucose including the level of human blood glucose.     

Swelling and morphological properties of poly(vinyl alcohol) (PVA) and poly(acrylic acid) (PAA) hydrogels in solution with high salt concentration

Understanding the swelling properties of hydrogels and how they affect the hydrogel's morphology is of fundamental importance in the development of hydrogel-based artificial muscles, bio-actuators, sensors and other devices. In this paper, the swelling behavior of PVA-PAA hydrogel films in saline water and in buffer solutions of different pH values was investigated. It was observed that the swelling factor of the hydrogel decreases when the ionic strength of the solvent solution increases. Scanning Electron Microscopy (SEM) revealed structures with different pore shapes and sizes depending on the type of solution used for hydration. In saline water, Energy Dispersive X-Ray (EDS) analysis indicated the formation of NaCl crystals within the polymeric network. Finally, the PVA-PAA hydrogel was used as an actuator to strain a fiber Bragg grating sensor, thus providing an indirect measurement of the pH value of the surrounding solution.     

Effects of molten poly(3-hydroxybutyrate) on crystalline morphology in stereocomplex of poly(L-lactic acid) with poly(D-lactic acid)

Effects of poly(3-hydroxybutyrate) (PHB) on crystalline morphology of stereocomplexing capacity of poly(L- and D-lactic acid) (PLLA and PDLA) were studied by differential scanning calorimetry (DSC), polarizing-light optical microscopy (POM), atomic-force microscopy (AFM) and wide-angle X-ray diffraction (WAXD). When crystallized at high T_c (130 °C or above), morphology transition of stereocomplexed PLA (sc-PLA) occurs from original well-rounded Maltese-cross spherulites to dendritic form in blends of high PHB contents (50 wt.% or higher), where PHB acts as an amorphous species. Microscopy characterizations show that morphology of sc-PLA in PHB/sc-PLA blends crystallized at T_c = 170 °C no longer retain original complexed Maltese-cross well-rounded spherulites; instead, the spherulites are disintegrated and restructured into two types of dendrites: (1) edge-on feather-like dendrites (early growth) and (2) flat-on wedge-like crystal plates (later growth) by growing along different directions and exhibiting different optical brightness. The concentration and/or distribution of amorphous PHB at the crystal growth front, corresponding to variation of the slopes of spherulitic growth rates, is a factor resulting in alteration and restructuring of the sc-PLA spherulites in the blends. Despite of spherulite disintegration, WAXD result shows that these two PHB-induced dendrites still retain the original unit cells of complexes, and thus these two new dendrites are sc-PLA.     

Salt-, pH- and temperature-responsive semi-interpenetrating polymer network hydrogel based on poly(aspartic acid) and poly(acrylic acid)

In this study, a novel salt-, pH- and temperature-responsive semi-interpenetrating network (semi-IPN) hydrogel, composed of poly(aspartic acid) (PAsp) and poly(acrylic acid) (PAAc), was prepared. PAsp/PAAc semi-IPN hydrogel being ionic in nature, the swelling behavior was significantly influenced by various swelling medium. The structure of the triply responsive hydrogel was studied by Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM), and the salt-, temperature- and pH-sensitivities were investigated through measuring equilibrium swelling ratios in various environmental solutions. The results indicate that there is a structure of polyelectrolyte complex in the hydrogel, and that the responsive behaviors of this hydrogel to alternating changes in inorganic salt (different physiological bio-fluids), pH and temperature are improved because of the incorporation of PAsp. In addition, during the repeatable swelling and shrinkage period, the semi-IPN hydrogel shows suitable mechanical strength. The salt-, pH- and temperature-responsive hydrogel will have wider applications in biomedical areas.     

Electrochemical characterization of poly(eriochrome black T) modified glassy carbon electrode and its application to simultaneous determination of dopamine, ascorbic acid and uric acid

A polymerized film of eriochrome black T (EBT) was prepared on the surface of a glassy carbon (GC) electrode in alkaline solution by cyclic voltammetry (CV). The redox response of the poly(EBT) film at the GC electrode appeared in a couple of redox peak in 0.1 M hydrochloride and the pH dependent peak potential was −55.1 mV/pH which was close to the Nernst behavior. The poly(EBT) film-coated GC electrode exhibited excellent electrocatalytic activity towards the oxidations of dopamine (DA), ascorbic acid (AA) and uric acid (UA) in 0.05 mM phosphate buffer solution (pH 4.0) and lowered the overpotential for oxidation of DA. The polymer film modified GC electrode conspicuously enhanced the redox currents of DA, AA and UA, and could sensitively and separately determine DA at its low concentration (0.1 μM) in the presence of 4000 and 700 times higher concentrations of AA and UA, respectively. The separations of anodic peak potentials of DA-AA and UA-DA reached 210 mV and 170 mV, respectively, by cyclic voltammetry. Using differential pulse voltammetry, the calibration curves for DA, AA and UA were obtained over the range of 0.1-200 μM, 0.15-1 mM and 10-130 μM, respectively. With good selectivity and sensitivity, the present method provides a simple method for selective detection of DA, AA and UA in biological samples.     

Hetero-stereocomplex formation between substituted poly(lactic acid)s with linear and branched side chains,poly(l-2-hydroxybutanoic acid) and poly(d-2-hydroxy-3-methylbutanoic acid)

Differential scanning calorimetry and wide-angle X-ray diffractometry first revealed the formation of hetero-stereocomplex (HTSC) between biodegradable, optically active, and isotactic poly(2-hydroxyalkanoic acid)s having different chemical structures and opposite configurations, i.e., l-configured substituted poly(lactic acid) (PLA) [poly(l-2-hydroxybutanoic acid), P(l-2HB)] with linear side chains (ethyl groups) and d-configured substituted PLA [poly(d-2-hydroxy-3-methylbutanoic acid), P(d-2H3MB)] with branched side chains (isopropyl groups) in solution and in bulk from the melt. The melting temperature of P(l-2HB)/P(d-2H3MB) HTSC crystallites was 197–204 °C, which is much higher those of P(l-2HB) and P(d-2H3MB) homo-crystallites (100–101 °C and 158–165 °C, respectively). The interplain distances and crystalline lattice sizes of P(l-2HB)/P(d-2H3MB) HTSC crystallites were respectively larger and smaller than those of P(l-2HB)/P(d-2HB) and P(l-2H3MB)/P(d-2H3MB) homo-stereocomplexes. The HTSC formation of substituted PLA with opposite configurations reported in the present study will provide a versatile way to prepare poly(2-hydroxyalkanoic acid)-based biodegradable materials having a wide variety of physical properties and biodegradability.     

Effect of boric acid on the stabilization of poly(acrylonitrile-co-itaconic acid)

The effect of boric acid on the stabilization of poly(acrylonitrile-co-itaconic acid) (P(AN-co-IA)) was studied in detail by differential scanning calorimetry (DSC) incorporating with thermogravimetry (TG) in air and nitrogen. In nitrogen atmosphere the ionic cyclization of the doped P(AN-co-IA) copolymer was badly retarded by boric acid, however, the free radical cyclization was little influenced. In air atmosphere boric acid was found to weaken the first exothermic peak corresponding to dehydrogenation and cyclization reactions, and enhance the second exothermic peak corresponding to the oxidative reactions resulting in a great expansion of the heat evolved. The activation energy of stabilization was calculated with Kissinger method, the results indicated that boric acid seemed to be effective to reduce the activation energy. TG studies showed that boric acid could reduce the weight loss during stabilization, which could be attributed to the crosslinking of polymer chains caused by the reaction between boric acid and the hydroxyl groups.     

Rheology control by modulating hydrophobic and inclusion associations in modified poly(acrylic acid) solutions

The rheology of modified poly(acrylic acid) (PAA) solutions can be tuned by controlling the inclusion interactions between α-cyclodextrins and alkyl hydrophobes. We demonstrate three modes of control: (1) using free cyclodextrins (CD) to displace hydrophobe-hydrophobe association in hydrophobically modified poly(acrylic acid) (HMPAA) polymers—which reduces fluid viscosity, (2) using competitive inclusion interactions where stronger SDS:CD binding can be used to ‘unmask’ CD:hydropobe inclusion interactions—which increases viscosity, and (3) employing HMPAA inclusion interactions with CD groups grafted to PAA chains (CDPAA)—which produces higher viscosities than purely hydrophobic association systems at the same concentration. The inclusion association between alkyl side-group in HMPAA and CD, either free or grafted onto PAA, obeys a 1-to-1 stoichiometry at low polymer concentrations (<1 wt%). In contrast to purely hydrophobically associating polymers, the CD:hydrophobe interaction is only binary, and, therefore, these associated networks should be ideal model systems to test theoretical predictions for associative fluids.     

Study on the effect of dispersion phase morphology on porous structure of poly (lactic acid)/poly (ethylene terephthalate glycol-modified) blending foams

A methodology for blending foam of poly (lactic acid) (PLA)/poly (ethylene terephthalate glycol-modified) (PETG) was proposed. PLA/PETG blends were prepared through a melt blending method, using multiple functionality epoxide as reactive compatibilizer. The effects of blending ratio and compatibilizer content on the dispersion morphology, molecular structure, mechanical properties, and rheological behavior of PLA/PETG blends were studied. Then PLA/PETG blends were foamed using supercritical CO₂ as physical blowing agent, and their porous structure, pore size, as well as pore density were investigated. The results showed that the mechanical properties and rheological parameters such as melt strength and melt elasticity, as well as the porous structure of the foams dispersion morphology of PLA/PETG blends were affected strongly. The melt elasticity of PLA/PETG blends increased with increasing compatibilizer content. Dispersion phase morphology of PLA/PETG blends also had a significant effect on the pore density of all the samples. The results indicated that homogeneous and finer porous morphology of PLA/PETG foams with high expansion ratio could be achieved with a proper content of compatibilizer in the blends.     

The phase diagram of poly(4-hydroxybenzoic acid) and poly(2,6-hydroxynaphthoic acid) and their copolymers from X-ray diffraction and thermal analysis

Homopolymers and copolymers of 4-hydroxybenzoic acid (HBA) and 2,6-hydroxynaphthoic acid (HNA) have been studied with differential scanning calorimetry and temperature-resolved wide angle X-ray diffraction. All polymers have more than one disordering transition between the glass transition (between 400 and 430 K) and decomposition (between 710 and 750 K). The first transition in PHBA at 616-633 K is from orthorhombic rigid crystals to a conformationally disordered pseudo-hexagonal phase (condis phase). The two higher transitions are first, a further disordering process to a hexagonal condis crystal, and then a change to an anisotropic melt (liquid crystal) at about 800 K, with increasing decomposition above 750 K. In PHNA, orthorhombic crystals change above 600 K to an orthorhombic condis crystal structure, which go to an anisotropic melt at 750 K, and subsequent decomposition. In addition, using empirical entropy rules that account for the changes during the transitions from the crystal to the disordered mobile phases, an effort is made to understand the disorder and mobility, and to arrive at a non-equilibrium phase diagram of the copolymer system. The existence of a single, but up to 200 K wide, glass transition and remaining high crystallinity of the copolyesters, indicate partial solubility of the repeating units in all phases. The new data are compared to and brought into agreement with the large number of prior measurements and often unclear interpretations.     

Formation of banded and non-banded poly(l-lactic acid) spherulites during crystallization of films of poly(l-lactic acid)/poly(ethylene oxide) blends

In this study, a series of poly(l-lactic acid) (PLLA)/poly(ethylene oxide) (PEO) blends with different PLLA concentrations was prepared. Films of these blends crystallized with and without a coverslip were characterized by the presence and absence of banded structures, respectively. This difference in morphology was observed because the PEO component of the blends was oxidized at a high temperature (125 °C) in air without the protection of a coverslip. X-ray photoelectron spectroscopy (XPS) results showed that the surface of the blends crystallized in nitrogen without a coverslip contained mostly PLLA while the surfaces of the same blends crystallized under a coverslip contained only a moderately higher concentration of PLLA than their bulks. The migration of PLLA to the surface of the blends during crystallization in nitrogen when no coverslip was used was due to its low surface tension. Phase images obtained using atomic force microscopy (AFM) indicated that the banded structures consisted of valleys and ridges, which were in fact flat-on and edge-on lamellae, respectively. A detailed time-of-flight secondary ion mass spectrometry (ToF-SIMS) examination suggested that PLLA and PEO were located mainly on the surfaces of the ridges and valleys, respectively.     

Kinetics of crystal nucleation of poly(L-lactic acid)

The kinetics of crystal nucleation of poly(L-lactic acid) (PLLA) has been analyzed by fast scanning chip calorimetry in a wide temperature range between 313 and 383 K, that is, between temperatures about 30 K below and 40 K above the glass transition temperature. The relaxed melt was rapidly cooled to the analysis temperature and subsequently aged between 10⁻¹ and 10⁴ s, to permit formation of nuclei. The number of formed crystal nuclei has been probed by analysis of the crystallization rate at 393 K. The nucleation rate is maximal at 370–375 K and decreases monotonously with decreasing temperature in the analyzed temperature range. The observation of a monomodal dependence of the nucleation rate on temperature points to predominance of a single nucleation mechanism in the analyzed temperature range, regardless nucleation occurs in the glassy or devitrified amorphous phase. The collected data suggest that nuclei formation at ambient temperature requires a waiting time longer than about 10⁸ s. The performed study is considered as a quantitative completion of nucleation-rate data available for PLLA only at temperatures higher than 360 K, suggesting that the nucleation mechanism is independent on temperature in the analyzed temperature range between 313 and 383 K.     

The synthesis of poly (lactic acid)-fulvic acid graft polymer and its effect on the crystallization and performance of poly (lactic acid)

ABSTRACT

The poly (lactic acid)-fulvic acid graft polymer (PLA-FA) was synthesized with lactic acid and fulvic acid (FA). The optimum parameters were determined by orthogonal experiment. X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy exhibited that FA was successfully grafted onto PLA. Then, PLA/PLA-FA composites were prepared with PLA-FA as fillers by melt blending. The structure characterization and performance tests demonstrated that PLA-FA effectively enhanced the comprehensive performance of PLA composites. The rheological analysis demonstrated that PLA-FA had plasticization effect. The non-isothermal crystallization kinetics demonstrated that PLA-FA promoted the crystallization rate of PLA composites, improving toughness of PLA composites.     

Temperature- and pH-sensitive membrane formed from blends of poly(vinylidene fluoride)-graft-poly(N-isopropylacrylamide) and poly(acrylic acid) microgels

Temperature- and pH-responsive membranes prepared from blends of poly(vinylidene fluoride)-graft-poly(N-isopropylacrylamide)(PVDF-g-PNIPAM) copolymer and poly(acrylic acid) (PAA) microgels in N,N-dimethylformamide (DMF) solution by phase inversion method. PAA microgels help PNIPAM chains largely enrich onto membrane surface. Furthermore, adding PAA microgels increases the porous size, porosity and hydrophilic property of the blend membrane. The membranes show temperature-sensitivity between 30 and 35 °C, and pH-sensitivity between pH 3 and 5 on permeating aqueous solutions. Meanwhile, the blend membranes keep good antifouling property even if one of the hydrophilic components becoming hydrophobic in response to temperature or pH stimuli, which is superior to single-sensitive PVDF membrane.     

Solid-state NMR study on the structure and mobility of the noncrystalline region of poly(3-hydroxybutyrate) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate)

The noncrystalline structures of poly(3-hydroxybutyrate) (PHB) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) copolymers were studied by variable temperature solid-state wide-line ¹H NMR and solid-state high-resolution ¹³C NMR spectroscopy. It is found that at room temperature there exists a rich and rigid component in the noncrystalline region of PHB and PHBV. The content of this component decreases with the increase in 3-hydroxyvalerate content in PHBV and with the increase in temperature. The brittleness of PHB may be partly attributed to the rigidness of the noncrystalline region at room temperature, while the improvement of the properties of PHBV may come from the enhanced mobility of the noncrystalline region.     

Electrochemical sensors based on platinum electrodes modified with hybrid inorganic-organic coatings for determination of 4-nitrophenol and dopamine

New electrochemical sensors for dopamine (DA) and phenol derivative, based on hybrid inorganic redox material-organic conducting polymers, were developed. Hybrid inorganic-organic coatings based on Prussian blue, polyazulene, poly(3,4-ethylenedioxythiophene) (PEDOT) and poly(3-[(E)-2-azulene-1-yl)vinyl]thiophene) have been prepared by electrochemical methods in various configurations onto Pt substrate. Dopamine and 4-nitrophenol (4-NP) have been determined by square wave voltammetry using both mono and bilayer modified electrodes. The modified electrodes exhibited a linear response over wide range of 4-nitrophenol concentrations from 30 to 90 μM, with a detection limit of 8.23 μM (s/n = 3). A linear trend for the responses for dopamine concentrations ranging from 2 to 100 μM, with a sensitivity of 0.116 μA/μM, has been also obtained.     

Melting parameters of poly(glycolic acid)

Equilibrium melting temperature T_m⁰, heat of fusion ΔH_f, and entropy of fusion ΔS_f of poly(glycolic acid) (PGA) was determined by using Clapeyron-Clausius equation. Equilibrium melting temperature T_m⁰ was 504.6 K which was determined by Hoffman-Weeks plots. The pressure dependence of T_m⁰ was determined by high pressure DTA up to 150 MPa. Volume change ΔV_f at melting was determined by using dilatometer. Heat of fusion in PGA was 183.2 (J g⁻¹), which is very close to the value reported by Chujo et al. who determined it by using T_m depression in copolymer with poly(lactic acid). ΔS_f of PGA was 0.363 (J g⁻¹ K⁻¹), which is about twice that of PLA, and the reason was discussed on the basis of the elastic modulus below T_m.     

Effect of nucleation and plasticization on the crystallization of poly(lactic acid)

In this paper, different strategies to promote PLA crystallization were investigated with the objective of increasing the crystalline content under typical polymer processing conditions. The effect of heterogeneous nucleation was assessed by adding talc, sodium stearate and calcium lactate as potential nucleating agents. The PLA chain mobility was increased by adding up to 10 wt% acetyl triethyl citrate and polyethylene glycol as plasticizers. The crystallization kinetics were studied using DSC analysis under both isothermal and non-isothermal conditions. The isothermal data showed that talc is highly effective in nucleating the PLA in the 80-120 °C temperature range. In the non-isothermal DSC experiments, the crystallinity developed upon cooling was systematically studied at cooling rates of 10, 20, 40, and 80 °C/min. The non-isothermal data showed that the combination of nucleant and plasticizer is necessary to develop significant crystallinity at high cooling rates. The nucleated and/or plasticized PLA samples were injection molded and the effect of mold temperature on crystallinity was determined. It was possible to mold the PLA formulations using mold temperatures either below 40 °C or greater than 60 °C. At low temperature, the molded parts were nearly amorphous while at high mold temperatures, the PLA formulation with proper nucleation and plasticization was shown to achieve crystallinity levels up to 40%, close to the maximum crystalline content of the material. Tensile mechanical properties and temperature resistance of these amorphous and semi-crystalline materials were examined.     

Ultra-fine polyelectrolyte fibers from electrospinning of poly(acrylic acid)

Ultra-fine polyelectrolyte fibers have been generated from electrospinning of poly(acrylic acid) in aqueous and DMF solutions. The fiber diameters ranged from 80 to 500 nm and increased with increasing solution concentrations and electrospinning voltages. The fibers generated from the aqueous solutions were more homogeneous in sizes, especially when NaCl or NaOH was added. Higher voltages in electrospinning of the aqueous solutions also resulted in fibers with larger heat capacity in the glass transition region, and higher dehydration temperatures. These polyelectrolyte fibers could be rendered water-insoluble by incorporating β-cyclodextrin (at 20 wt% of PAA) in the aqueous solution, then heat-induced crosslinking was performed at 140 °C for 20 min. The resulting hydrogel fibers showed strongly pH-responsive swelling behaviors.