Preparation of yttrium hexacyanoferrate/carbon nanotube/Nafion nanocomposite film-modified electrode: Application to the electrocatalytic oxidation of l-cysteine

An yttrium hexacyanoferrate nanoparticle/multi-walled carbon nanotube/Nafion (YHCFNP/MWNT/Nafion)-modified glassy carbon electrode (GCE) was constructed. Several techniques, including infrared spectroscopy, energy dispersive spectrometry, scanning electron microscopy and electrochemistry, were performed to characterize the yttrium hexacyanoferrate nanoparticles. The electrochemical behavior of the YHCFNP/MWNT/Nafion-modified GCE in response to l-cysteine oxidation was studied. The response current of l-cysteine oxidation at the YHCFNP/MWNT/Nafion-modified GCE was obviously higher than that at the bare GCE or other modified GCE. The effects of pH, scan rate and interference on the response to l-cysteine oxidation were investigated. In addition, on the basis of these findings, a determination of l-cysteine at the YHCFNP/MWNT/Nafion-modified GCE was carried out. Under the optimum experimental conditions, the electrochemical response to l-cysteine at the YHCFNP/MWNT/Nafion-modified GCE was fast (within 4 s). Linear calibration plots were obtained over the range of 0.20–11.4 μmol L⁻¹ with a low detection limit of 0.16 μmol L⁻¹. The YHCFNP/MWNT/Nafion-modified GCE exhibited several advantages, such as high stability and good resistance against interference by ascorbic acid and other oxidizable amino acids.     

The reduction of l-cystine hydrochloride at stationary and rotating disc mercury electrodes

The kinetics of l-cystine hydrochloride reduction have been studied at a mercury-plated copper rotating disc electrode (RDE) and at a stationary mercury disc electrode (SMDE) in 0.1 mol dm⁻³ HCl at 298 K. The reduction of the disulphide is irreversible and hydrogen evolution is the major side reaction. In contrast to steady state electrode kinetic studies at a mercury drop electrode (which shows a well-defined limiting current), the mercury-plated Cu RDE shows overlap between disulphide reduction and hydrogen evolution. These effects are attributable to strong reactant adsorption with a calculated surface coverage close to 100%. A Tafel slope of −185 mV per decade is found with a cathodic transfer coefficient of 0.32 and a formal rate constant of 6.7 × 10⁻⁹ m s⁻¹. The relative merits of steady state voltammetry at a mercury-plated copper RDE and linear sweep voltammetry at the SMDE are discussed, as is the mechanism of l-cysteine hydrochloride formation.     

High molecular weight poly(l-lactide) and poly(ethylene oxide) blends: thermal characterization and physical properties

The miscibility of high molecular weight poly(l-lactide) PLLA with high molecular weight poly(ethylene oxide) PEO was studied by differential scanning calorimetry. All blends containing up to 50 weight% PEO showed single glass transition temperatures. The PLLA and PEO melting temperatures were found to decrease on blending, the equilibrium melting points of PLLA in these blends decreased with increasing PEO fractions. These results suggest the miscibility of PLLA and PEO in the amorphous phase. Mechanical properties of blends with up to 20 weight% PEO were also studied. Changes in mechanical properties were small in blends with less than 10 weight% PEO. At higher PEO concentrations the materials became very flexible, an elongation at break of more than 500% was observed for a blend with 20 weight% PEO. Hydrolytic degradation up to 30 days of the blends showed only a small variation in tensile strength at PEO concentrations less than 15 weight%. As a result of the increased hydrophilicity, however, the blends swelled. Mass loss upon degradation was attributed to partial dissolution of the PEO fraction and to an increased rate of degradation of the PLLA fraction. Significant differences in degradation behaviour between PLLA/PEO blends and (PLLA/PEO/PLLA) triblock-copolymers were observed.     

Gentamicin sulphate release from lost foam wollastonite scaffolds using poly(dl-lactide-co-glycolide) acid

A study on the gentamicin sulphate release from lost foam wollastonite scaffolds using poly(dl-lactide-co-glycolide) acid (PLGA) was performed. Scaffolds were made through the lost foam technique using two different evaporative-patterns: poly(methyl-methacrylate) (PMMA) spheres and polyurethane (PUR) sponge. With the aim to control the rate of the gentamicin sulphate delivery, poly(dl-lactide-co-glycolide) acid was used. The porous scaffolds were gentamicin sulphate loaded by different ways. All methods showed the same gentamicin sulphate release pattern: a high rate of release in the first 24 h followed by a slow rate for a period longer than 300 h. The compression strength of the PLGA filled pores scaffolds after a degradation test was measured and compared with that of empty pores scaffolds, having as a result a notable increase in the compression strength. A hemolysis test with human blood was made to each delivery system. With only one exception, every system showed a hemolysis lower than 5%, proving to be hemocompatible. The scaffolds made by using PMMA spheres showed a better behavior in the drug release process, as well as higher mechanical properties.     

Three-dimensional electrochemical microfabrication of n-GaAs using l-cystine as a scavenger

The confined etchant layer technique (CELT) was used to fabricate complex three-dimensional (3D) microstructures on gallium arsenide (n-GaAs). The design of an appropriate chemical etching system is needed in order to realize successful microfabrication. In this study, Br₂ was electro-generated at the mold surface and used as an efficient etchant for n-GaAs. The use of l-cystine as a scavenger to replace the toxic scavenger H₃A_SO₃ was explored. The resolution of the fabricated microstructure depended strongly on the composition of the electrolyte, and especially on the concentration ratio between l-cystine and KBr. A well-defined and polished Pt micro-cylindrical electrode with a diameter of ∼50 μm was employed as one kind of mold for CELT. By inspecting the deviation of the sizes of the etching spots from the real diameter of the microelectrode, the thickness of confined etchant layer (CEL) can be estimated and thus the composition of electrolyte can be optimized for better etching precision. By choosing an appropriate concentration ratio between l-cystine and KBr, complex microstructures were fabricated successfully on n-GaAs. The etched patterns on n-GaAs were approximately negative copies of the mold, and the precision of duplication could easily reach the submicrometer scale, which was better than that achieved with H₃A_SO₃. The experimental results indicated that l-cystine is a good scavenger for microfabrication on n-GaAs by CELT. This technique avoids severe pollution of the environment, which will help to extend its future application in industry.     

Reversibility of the l-cysteine/l-cystine redox process at physiological pH on graphite electrodes modified with coenzyme B12 and vitamin B12

We report on the electrocatalytic activity of immobilized coenzyme B₁₂ and vitamin B₁₂ (as aquocobalamin) for the electrooxidation of l-cysteine and their effects on the electrochemical reversibility of the l-cysteine/l-cystine redox couple, a crucial biological system. Cyclic voltammograms of coenzyme B₁₂ adsorbed on a graphite electrode show that upon the reductive elimination of the 5′-deoxyadenosyl group from the cobalt center, at approximately −1.1 V, the electrochemical response of the modified electrode becomes similar to that of aquocobalamin. The electrochemically pretreated coenzyme B₁₂ shows a high electrocatalytic activity for the electro-oxidation of l-cysteine at physiological pH that has never been observed before with the commonly used metallophthalocyanine catalysts. Also, its activity is slightly higher than that exhibited by aquocobalamin.     

Synthesis and characterization of poly(l-glutamic acid)-block-poly(l-phenylalanine)

Poly(γ-benzyl l-glutamate)-block-poly(l-phenylalanine) was prepared via the ring opening polymerization of γ-benzyl l-glutamate N-carboxyanhydride and l-phenylalanine N-carboxyanhydride using n-butylamine·HCl as an initiator for the living polymerization. Polymerization was confirmed by ¹H-nuclear magnetic resonance spectroscopy and matrix assisted laser desorption ionization time of flight mass spectroscopy. After deprotection, the vesicular nanostructure of poly(l-glutamic acid)-block-poly(l-phenylalanine) particles was examined by transmission electron microscopy and dynamic light scattering. The pH-dependent properties of the nanoparticles were evaluated by means of ζ-potential and transmittance measurements. The results showed that the block copolypeptide could be prepared using simple techniques. Moreover, the easily prepared PGA-PPA block copolypeptide showed pH-dependent properties due to changes in the PGA ionization state as a function of pH; this characteristic could potentially be exploited for drug delivery applications.     

Electrochemical detection of l-cysteine using a boron-doped carbon nanotube-modified electrode

A boron-doped carbon nanotube (BCNT)-modified glassy carbon (GC) electrode was constructed for the detection of l-cysteine (L-CySH). The electrochemical behavior of BCNTs in response to l-cysteine oxidation was investigated. The response current of L-CySH oxidation at the BCNT/GC electrode was obviously higher than that at the bare GC electrode or the CNT/GC electrode. This finding may be ascribed to the excellent electrochemical properties of the BCNT/GC electrode. Moreover, on the basis of this finding, a determination of L-CySH at the BCNT/GC electrode was carried out. The effects of pH, scan rate and interferents on the response of L-CySH oxidation were investigated. Under the optimum experimental conditions, the detection response for L-CySH on the BCNT/GC electrode was fast (within 7 s). It was found to be linear from 7.8 × 10⁻⁷ to 2 × 10⁻⁴ M (r = 0.998), with a high sensitivity of 25.3 ± 1.2 nA mM⁻¹ and a low detection limit of 0.26 ± 0.01 μM. The BCNT/GC electrode exhibited high stability and good resistance against interference by other oxidizable amino acids (tryptophan and tyrosine).     

Tailor-made gold brush nanoelectrode ensembles modified with l-cysteine for the detection of daunorubicine

A new functionalized l-cysteine surface modified 3D gold brush nanoelectrode assembly BNEE (l-cys/BNEEs) was prepared. The BNEEs consisted of gold nanowires 100 nm in diameter and up to 400 nm in length fabricated by template synthesis in track etched polycarbonate membranes. The nanowires were exposed by controlled chemical etching of the membrane and were then modified by coating l-cys on the surface of the exposed gold nanowires. The morphology of the BNEEs was imaged by scanning electron microscopy and the real active area of BNEEs was determined by electrochemical impedance spectroscopy. The redox of daunorubicine (DNR) at the l-cys/BNEEs exhibited absorption-controlled characteristics and higher current activity than that at l-cys surface modified 2D disk NEEs (l-cys/DNEEs). The square wave voltammetry technique was employed to detect DNR. The detection limit was 1.0 × 10⁻⁸ M (s/n = 3). The linear detection concentration range of DNR was from 2.5 × 10⁻⁸ to 4.0 × 10⁻⁷ M.     

Electrocatalytic oxidation of l-cysteine with a stable copper-cobalt hexacyanoferrate electrochemically modified carbon paste electrode

A stable composition of hybrid copper-cobalt hexacyanoferrate (Cu-CoHCF) film was electrodeposited on a carbon paste electrode (CPE). There are a few reports for using this hybrid as a mediator, but all of them require almost 12 h conditioning time before usage. Contrary to previous reports this electrode does not require any conditioning and can be used immediately after film formation. The electrocatalytic activity of this film was investigated and showed a good electrocatalytic effect for oxidation of l-cysteine (Cys) in phosphate buffer solution (PBS) in pH range of 1-7. A linear range of 6 μM to 1 mM of Cys and an experimental detection limit of 5 μM of Cys were obtained using cyclic voltammetry method. The diffusion coefficient of Cys and catalytic rate constant for electrocatalytic reaction were also calculated. The major problem reported in electro oxidation of Cys is poisoning of electrode surface with reaction product, but in this study oxidation of Cys had no significant fouling effect on the modified electrode surface for the concentrations below 0.5 mM of Cys.     

Studies on the oxidation reaction of l-cysteine in a confined matrix of layered double hydroxides

The amino acid l-cysteine (l-Cys) was intercalated into a MgAl layered double hydroxide (LDH), and its oxidation reaction by hexacyanoferrate (III) (Fe(CN)₆³⁻) in the confined region between sheets of LDH has been studied in detail. Based on the measurement results of XRD, Raman and FT-IR, it was found that the interlayer l-Cys was oxidized to cystine by Fe(CN)₆³⁻. Furthermore, the kinetics of this reaction was investigated in batch mode. The influences of initial Fe(CN)₆³⁻concentration, l-Cys-LDH quantity and reaction temperature on the interlayer oxidation reaction have been studied, respectively. The reaction follows a diffusion-controlled mechanism represented by Crank-Ginstling and Brounshtein kinetic model with the apparent activation energy of 29.93 kJ/mol. Therefore, this layered material may have prospective application as a novel “molecular reactor” for confined chemical reactions.     

Electrochemical reduction of Zn(II) ions on l-cysteine coated gold electrodes

Zn(II) ions have been selectively bound to the l-cysteine coated gold electrode in the form of a four-coordinated complex. Voltammograms of the Zn complex on the l-cysteine coated gold electrode showed a cathodic wave at ca. 0.05 V in the pH 7.54 phosphate buffered saline. The charge transfer coefficient and rate constant for the reduction of this Zn complex were 0.65 and 0.003 s⁻¹, respectively. The complexation of Zn(II) ions with l-cysteine on the gold electrode resulted in the maximum surface coverage of the Zn complex of 0.35 nmol cm⁻² and the Gibbs energy change of −27.6 kJ mol⁻¹. The cathodic peak current, influenced by the types of the end functional groups in thiols, the preconcentration time, and pH values of the supporting electrolyte, was linear with the concentration of Zn(II) ions in the range of 5.0 nM to 5 μM with a detection limit of 2.1 nM. The proposed voltammetric method was utilized successfully to detect the concentration of Zn(II) ions in hairs.     

Melting behavior of poly(l-lactic acid): X-ray and DSC analyses of the melting process

To clarify the melting behavior of poly(l-lactic acid) (PLLA), the wide-angle X-ray diffraction patterns of the isothermally crystallized PLLA samples (ICSs) were successively obtained during heating. We have already suggested the discrete change in the crystallization behavior of PLLA at a crystallization temperature (T_c) of 113 °C (= T_b) and formation of two crystal modifications for the ICSs obtained in the temperature range T_c ≤ T_b and T_c ≥ T_b. It was elucidated from the change in the X-ray diffraction pattern that the phase transition from the low-temperature crystal modification (α′-form) to the high-temperature one (α-form) occurred in a range 155-165 °C for the ICSs(T_c ≤ T_b), and that the crystal structure for the ICSs(T_c ≥ T_b) did not change. Recrystallization during heating, which is the origin of the multiple melting behavior, was proved by the increase in the diffraction intensity before steep decrease due to the final melting. A temperature derivative curve of the X-ray diffraction intensity almost coincided with the DSC melting curve.     

Bifunctional redox flow battery: 2. V(III)/V(II)-l-cystine(O2) system

A new bifunctional redox flow battery (BRFB) system, V(III)/V(II)—l-cystine(O₂), was systematically investigated by using different separators. It is shown that during charge, water transfer is significantly restricted with increasing the concentration of HBr when the Nafion 115 cation exchange membrane is employed. The same result can be obtained when the gas diffusion layer (GDL) hot-pressed separator is used. The organic electro-synthesis is directly correlated with the crossover of vanadium. When employing the anion exchange membrane, the electro-synthesis efficiency is over 96% due to a minimal crossover of vanadium. When the GDL hot-pressed separator is applied, the crossover of vanadium and water transfer are noticeably prevented and the electro-synthesis efficiency of over 99% is obtained. Those impurities such as vanadium ions and bromine can be eliminated through the purification of organic electro-synthesized products. The purified product is identified to be l-cysteic acid by IR spectrum. The BRFB shows a favorable discharge performance at a current density of 20 mA cm⁻². Best discharge performance is achieved by using the GDL hot-pressed separator. The coulombic efficiency of 87% and energy efficiency of about 58% can be obtained. The cause of major energy losses is mainly associated with the cross-contamination of anodic and cathodic active electrolytes.     

Crystallization and morphology of stereocomplexes in nonequimolar mixtures of poly(l-lactic acid) with excess poly(d-lactic acid)

Crystallization of nonequimolar compositions of poly(d-lactic acid) with low-molecular-weight poly(l-lactic acid) (PDLA/LM_w-PLLA) blends leads to formation of various fractions of stereocomplexed PLA (sc-crystallites) and homocrystallites (PDLA or PLLA). For the PDLA/LM_w-PLLA blends within the composition window of LM_w-PLLA content between 30 and 50 wt%, only sc-crystal exists and no homocrystal is present. On the other hand, for PDLA/LM_w-PLLA blends with excess PDLA, e.g. PDLA/LM_w-PLLA = 90/10, atomic-force microscopy (AFM) characterization on various stages of crystallization of sc-PLA crystal with PDLA homocrystal shows a repetitive stacking of excess PDLA on pre-formed sc-PLA crystal serving as crystallizing templates. The crystallization initially begins with string-like (fibril-like) PDLA lamellae, followed with PDLA aggregating on sc-PLA crystal into a bead-on-string crystal, then growing to thicker irregularly-shaped dough-like lamellae. Repetitive growth cycle from strings to bead-on-string lamellae continues on top of the dough-like lamellae as new substrates, until ending impingement of the PDLA spherulites.     

Thermal degradation of poly(l-lactide): effect of alkali earth metal oxides for selective l,l-lactide formation

To achieve the feed stock recycling of poly(l-lactide) (PLLA) to l,l-lactide, PLLA composites including alkali earth metal oxides, such as calcium oxide (CaO) and magnesium oxide (MgO), were prepared and the effect of such metal oxides on the thermal degradation was investigated from the viewpoint of selective l,l-lactide formation. Metal oxides both lowered the degradation temperature range of PLLA and completely suppressed the production of oligomers other than lactides. CaO markedly lowered the degradation temperature, but caused some racemization of lactide, especially in a temperature range lower than 250 °C. Interestingly, with MgO racemization was avoided even in the lower temperature range. It is considered that the effect of MgO on the racemization is due to the lower basicity of Mg compared to Ca. At temperatures lower than 270 °C, the pyrolysis of PLLA/MgO (5 wt%) composite occurred smoothly causing unzipping depolymerization, resulting in selective l,l-lactide production. A degradation mechanism was discussed based on the results of kinetic analysis. A practical approach for the selective production of l,l-lactide from PLLA is proposed by using the PLLA/MgO composite.     

Spherulite growth of l-lactide copolymers: Effects of tacticity and comonomers

The spherulite growth behavior and mechanism of l-lactide copolymers, poly(l-lactide-co-d-lactide) [P(LLA-DLA)], poly(l-lactide-co-glycolide) [P(LLA-GA)], and poly(l-lactide-co-ε-caprolactone) [P(LLA-CL)] have been studied using polarization optical microscopy in comparison with poly(l-lactide) (PLLA) having different molecular weights to elucidate the effects of incorporated comonomer units. The incorporation of comonomer units reduced the radius growth rate of spherulites (G) and increased the induction period of spherulite formation (t_i), irrespective of the kind of comonomer unit. Such effects became remarkable with the content of comonomers. At a crystallization temperature (T_c) of 130 °C, the disturbance effects of comonomers on the spherulite growth decreased in the following order: d-lactide>glycolide>ε-caprolactone, when compared at the same comonomer unit or reciprocal of averaged l-lactyl unit sequence length (l_l). The t_i estimation indicated that the glycolide units have the lowest disturbance effects on the formation of spherulite (crystallite) nuclei. The PLLA having the number-average molecular weight (M_n) exceeding 3.1×10⁴ g mol⁻¹ showed the transition from regime II to regime III at T_c=120 °C, whereas PLLA with the lowest M_n of 9.2×10³ g mol⁻¹ crystallized solely in regime III kinetics and the copolymers excluding P(LLA-DLA) with 3% of d-lactide units crystallized solely according to regime II kinetics. The nucleation and front constant for regime II and III [K_g(II), K_g(III), G₀(II), and G₀(III), respectively] estimated with each (not with a fixed for high-molecular-weight PLLA) decreased with increasing the amount of defects per unit mass of the polymer for crystallization, i.e. with increasing the comonomer content and the density of terminal group through decreasing the molecular weight.     

pH-dependent self-assembly of amphiphilic poly(l-glutamic acid)-block-poly(lactic-co-glycolic acid) copolymers

A novel biodegradable AB-type diblock copolymer poly(L-lactic- co-glycolic acid)-block-poly(l-glutamic acid) (PLGA-b-PGA) was synthesized by a macromolecular coupling reaction between carboxyl-terminated PLGA and amino-terminated poly(γ-benzyl-glutamate) (PBLG) and the subsequent elimination of the protecting benzyl group. The structures of PLGA-PGA and its precursors were confirmed by Fourier transform infrared spectroscopy (FT-IR), ¹H nuclear magnetic resonance (¹H NMR) spectroscopy and gel permeation chromatography (GPC). This synthetic strategy simplified a former synthesis process of polypeptide-poly(l-lactic acid)(PLA); by using this new synthetic route the molecular weight and block ratio of PLGA-PGA could be easily controlled by adjusting the chain length of PLGA/PGA. The pH sensitivity and self-assembly behavior of PLGA-PGA copolymer were investigated by environmental scanning electron microscopy (ESEM), transmission electron microscopy (TEM) and dynamic light scattering (DLS). The results showed that the copolymer exhibited high pH responses, and the morphologies of the copolymer aggregates underwent four stages orderly with the pH increase (pH = 3-9): a disorganized form, micelles, semi-vesicles with thick walls and vesicles. Such a pH-dependent self-assembly process of the copolymer is promising for drug control release and bio-applications.     

Dielectric study on dynamics and conformation of poly(d,l-lactic acid) in dilute and semi-dilute solutions

Fractionated samples of d,l-poly(lactic acid) (PLA) were prepared and the dielectric normal mode relaxation was studied for dilute and semi-dilute solutions of the PLA in a good solvent benzene. Results indicate that in the dilute regime the normal mode relaxation time is proportional to [η]M_w in agreement with the Rouse-Zimm theory, where [η] and M_w denote the intrinsic viscosity and weight average molecular weight, respectively. The dielectric relaxation strength which is proportional to the mean square end-to-end distance 〈r²〉 increases with increasing M_w with the power of 2ν, where ν is the excluded volume parameter determined from [η]. The relaxation time in the semi-dilute regime increases with increasing concentration C due to increases of the entanglement density and the friction coefficient. The relaxation time corrected to the iso-friction state agrees approximately with the dynamic scaling theories. The relaxation strength decreases with increasing concentration indicating that 〈r²〉 decreases on account of the screening of the excluded volume effect. The concentration dependence of 〈r²〉 agrees approximately with the scaling theory proposed by Daoud and Jannink.     

pH affected migrational transport of ferrocene derivative of l-cysteine in aqueous solutions: Voltammetric studies

The mass transport of biologically-active l-cysteine derivatized with the ferrocenyl group (FcCH₂Cys) was investigated voltammetrically at a microelectrode in aqueous solutions under the conditions of varying content of supporting electrolyte and at different pH values. By varying the pH conditions one could obtain samples containing differently ionized forms of the l-cysteine derivative, from a monovalent cation via a neutral form (zwitterion) to a monovalent anion. Due to the acid-base equilibrium the obtained solutions were, in fact, mixtures composed predominantly of either anionic or cationic, or neutral species of FcCH₂Cys, respectively. Under the conditions of low ionic support the mass transport of these forms is differently affected by the migrational contribution. The results obtained experimentally were in good agreement with the theoretical predictions. In the calculations it was assumed that the FcCH₂Cys forms, coexisting in the solution, contribute independently to the steady-state transport-limited current. It was also assumed that the diffusion coefficients of the FcCH₂Cys forms were equal. This was validated by the voltammetric measurements at the supporting electrolyte excess (purely diffusional conditions). The diffusion coefficients of different forms of FcCH₂Cys are very similar and the average diffusion coefficient is (5.35 ± 0.05) × 10⁻¹⁰ m²/s.The studies clearly show that the variation in the conditions of pH or/and concentration of electrolyte can change the transport rate of l-cysteine even by several tens percent.