Preconcentration and determination of trace Hg(ii) using ultrasound-assisted dispersive solid phase microextraction

Defect rich molybdenum disulfide (MoS₂) nanosheets were hydrothermally synthesized and their potential for ultrasound assisted dispersive solid phase microextraction of trace Hg(ii) ions was assessed. Ultrasonic dispersion allows the MoS₂ nanosheets to chelate rapidly and evenly with Hg(ii) ions and results in improving the precision and minimizing the extraction time. The multiple defect rich surface was characterized by X-ray diffraction and high-resolution transmission electron microscopy. The surface charge of intrinsically sulfur rich MoS₂ nanosheets and their elemental composition was characterized by zeta potential measurements, energy dispersive spectroscopy, and X-ray photoelectron spectroscopy. The cracks and holes on the basal planes of MoS₂ led to diffusion of the Hg(ii) ions into the interior channels. Inner-sphere chelation along with outer-sphere electrostatic interaction were the proposed mechanism for the Hg(ii) adsorption onto the MoS₂ surface. The experimental data showed good selectivity of MoS₂ nanosheets towards Hg(ii) adsorption. The systematic and constant errors of the proposed method were ruled out by the analysis of the Standard Reference Material (>95% recovery with <5% RSD). The Student''s t-test values for the analyzed Standard Reference Material were found to be less than the critical Student''s t value at 95% confidence level. The limit of detection (3S) was found to be 0.01 ng mL⁻¹. The MoS₂ nanosheets were successfully employed for the analysis of Hg(ii) in environmental water samples.

Hg(ii) ion adsorption onto an MoS₂ surface.     

A multifunctional composite Fe3O4/MOF/l-cysteine for removal,magnetic solid phase extraction and fluorescence sensing of Cd(ii)

Fe₃O₄/MOF (metal organic framework)/l-cysteine was synthesized and applied for the removal of Cd(ii) from wastewater. The adsorption kinetics and isotherms were investigated, and the results indicated that the adsorption obeyed the pseudo-second-order kinetic model and Langmuir isotherm. The maximum adsorption capacity was calculated to be 248.24 mg g⁻¹. Fe₃O₄/MOF/l-cysteine was further applied to determine trace amounts of Cd(ii) in real water samples using ICP-AES (inductively coupled plasma-atomic emission spectroscopy) based on magnetic solid-phase extraction (MSPE). The determination limit was 10.6 ng mL⁻¹. Additionally, Fe₃O₄/MOF/l-cysteine can also be used as a fluorescent sensor for “turn-off” detection of Cd(ii), and the detection limit was 0.94 ng mL⁻¹.

Fe₃O₄/MOF (metal organic framework)/l-cysteine was synthesized and applied for the removal of Cd(ii) from wastewater.     

Adjunctive N-Acetyl-l-Cysteine in Treatment of Murine Pneumococcal Meningitis

Despite antibiotic therapy, acute and long-term complications are still frequent in pneumococcal meningitis. One important trigger of these complications is oxidative stress, and adjunctive antioxidant treatment with N-acetyl-l-cysteine was suggested to be protective in experimental pneumococcal meningitis. However, studies of effects on neurological long-term sequelae are limited. Here, we investigated the impact of adjunctive N-acetyl-l-cysteine on long-term neurological deficits in a mouse model of meningitis. C57BL/6 mice were intracisternally infected with Streptococcus pneumoniae. Eighteen hours after infection, mice were treated with a combination of ceftriaxone and placebo or ceftriaxone and N-acetyl-l-cysteine, respectively. Two weeks after infection, neurologic deficits were assessed using a clinical score, an open field test (explorative activity), a t-maze test (memory function), and auditory brain stem responses (hearing loss). Furthermore, cochlear histomorphological correlates of hearing loss were assessed. Adjunctive N-acetyl-l-cysteine reduced hearing loss after pneumococcal meningitis, but the effect was minor. There was no significant benefit of adjunctive N-acetyl-l-cysteine treatment in regard to other long-term complications of pneumococcal meningitis. Cochlear morphological correlates of meningitis-associated hearing loss were not reduced by adjunctive N-acetyl-l-cysteine. In conclusion, adjunctive therapy with N-acetyl-l-cysteine at a dosage of 300 mg/kg of body weight intraperitoneally for 4 days reduced hearing loss but not other neurologic deficits after pneumococcal meningitis in mice. These results make a clinical therapeutic benefit of N-acetyl-l-cysteine in the treatment of patients with pneumococcal meningitis questionable.     

Au–Ag core–shell composite nanoparticles as a selective and sensitive plasmonic chemical probe for l-cysteine detection in Lens culinaris (lentils)

The present work reported is a simple and selective method for the colorimetrical detection of l-cysteine in Lens culinaris (or lentils) using Au–Ag core–shell (Au core Ag shell) composite nanoparticles as a chemical probe. The phenomenon is based on the color change of composite nanoparticles from yellowish brown to light blue, followed by a shift of the localized surface plasmon resonance (LSPR) absorption band in the UV-visible region (i.e., 200–800 nm) with the addition of l-cysteine into the solution of bimetallic nanoparticles. The mechanism for the detection of l-cysteine is based on the electrostatic interaction of the metal ion with the thiol group of the amino acid, which causes the red shift of the LSPR band at 685 nm. The size distribution, morphology, composition and optical properties of the Au–Ag core–shell composite nanoparticles were characterized by transmission electron microscopy (TEM), dynamic light scattering (DLS), energy dispersive X-ray diffraction (EDX), UV-visible spectrophotometer and Fourier transform infrared spectroscopy (FTIR) techniques. An excellent linearity range for the present method was observed in the range of 20–140 μg mL⁻¹ with a limit of detection at 1.95 μg mL⁻¹ and correlation coefficient (R²) of 0.986. A good% recovery of 4.0% showed the selectivity of the method for l-cysteine determination from sample matrices. The advantageous features of the present method are being simple, rapid, low cost and selectivity towards the determination of l-cysteine in lentils.

The present work reported is a simple and selective method for the colorimetrical detection of l-cysteine in Lens culinaris (or lentils) using Au–Ag core–shell (Au core Ag shell) composite nanoparticles as a chemical probe.     

The structural and luminescence properties of plexcitonic structures based on Ag2S/l-Cys quantum dots and Au nanorods

A technique of obtaining plexitonic structures based on Ag₂S quantum dots passivated with l-cysteine (Ag₂S/l-Cys QDs) in the presence of Au nanorods passivated with cetyltrimethylammonium bromide molecules (Au/CTAB NRs) with controlled luminescence properties has been developed. The structural and luminescence properties of Ag₂S/l-Cys QDs with Au/CTAB NRs are studied. The effect of plasmonic Au/CTAB NRs on IR trap state luminescence (750 nm) is considered. It has been found that the direct interaction between the components of the plexcitonic nanostructure leads to a significant luminescence quenching of Ag₂S/l-Cys QDs, with the luminescence lifetime being constant. This is the evidence for photoinduced charge transfer. The spatial separation of the components of plexcitonic nanostructures due to the introduction of a polymer – poly(diallyldimethylammonium chloride) (polyDADMAC) provides a means to change their mutual arrangement and achieve an increase in the IR trap state luminescence intensity and a decrease in the luminescence lifetime from 7.2 ns to 4.5 ns. With weak plexcitonic coupling in the nanostructures [Ag₂S QD/l-Cys]/[polyDADMAC]/[Au/CTAB NRs], the possibility of increasing the quantum yield of trap state luminescence for Ag₂S QDs due to the Purcell effect has been demonstrated. In the case of formation [Ag₂S QD/l-Cys]/[polyDADMAC]/[Au/CTAB NRs] a transformation of shallow trap state structure was established using the thermostimulated luminescence method.

A technique of obtaining plexitonic structures based on Ag₂S quantum dots passivated with l-cysteine (Ag₂S/l-Cys QDs) in the presence of Au nanorods passivated with cetyltrimethylammonium bromide molecules (Au/CTAB NRs) with controlled luminescence was developed.     

A green l-cysteine modified cellulose nanocrystals biosorbent for adsorption of mercury ions from aqueous solutions

Using a green biosorbent to remove toxic mercury ions from aqueous solutions is a significant undertaking. In the present study, a novel biosorbent, l-cysteine modified cellulose nanocrystals (Lcys-CNCs), was prepared by functionalizing high surface area cellulose nanocrystals with l-cysteine through periodate oxidation and reductive amination reaction. Lcys-CNCs were characterized by FT-IR, ¹³C CP-MAS NMR, elemental analysis, XPS, zeta potential and SEM. As cellulose nanocrystals are the natural nanomaterial, and l-cysteine contains strong mercury chelating groups, Lcys-CNCs show excellent adsorption capacity for mercury ions. The experimental conditions such as pH, contact time, and initial mercury ion concentration are discussed. The pseudo-second order model can describe the removal kinetics of Hg(ii) more accurately than the pseudo-first order model. The adsorption isotherm study of Hg(ii) followed the Langmuir model of monolayer adsorption. The maximum uptake capacity of Lcys-CNCs was determined to be 923 mg g⁻¹. Lcys-CNCs can remove mercury ions with 93% removal efficiency within 5 min from a 71 mg L⁻¹ solution. For Cd(ii), Pb(ii), Cu(ii) and Zn(ii) ions, Lcsy-CNCs can selectively adsorb Hg(ii) ions and the removal efficiency is 87.4% for Hg(ii). This study suggests Lcsy-CNCs are a green and highly efficient biosorbent for adsorption of mercury ions from aqueous solutions.

A green biosorbent, l-cysteine modified cellulose nanocrystals, was successfully synthesized and applied to adsorb mercury ions from aqueous solutions.     

Establishment of an In Vitro d-Cycloserine-Synthesizing System by Using O-Ureido-l-Serine Synthase and d-Cycloserine Synthetase Found in the Biosynthetic Pathway

We have recently cloned a DNA fragment containing a gene cluster that is responsible for the biosynthesis of an antituberculosis antibiotic, d-cycloserine. The gene cluster is composed of 10 open reading frames, designated dcsA to dcsJ. Judging from the sequence similarity between each putative gene product and known proteins, DcsC, which displays high homology to diaminopimelate epimerase, may catalyze the racemization of O-ureidoserine. DcsD is similar to O-acetylserine sulfhydrylase, which generates l-cysteine using O-acetyl-l-serine with sulfide, and therefore, DcsD may be a synthase to generate O-ureido-l-serine using O-acetyl-l-serine and hydroxyurea. DcsG, which exhibits similarity to a family of enzymes with an ATP-grasp fold, may be an ATP-dependent synthetase converting O-ureido-d-serine into d-cycloserine. In the present study, to characterize the enzymatic functions of DcsC, DcsD, and DcsG, each protein was overexpressed in Escherichia coli and purified to near homogeneity. The biochemical function of each of the reactions catalyzed by these three proteins was verified by thin-layer chromatography (TLC), high-performance liquid chromatography (HPLC), and, in some cases, mass spectrometry. The results from this study demonstrate that by using a mixture of the three purified enzymes and the two commercially available substrates O-acetyl-l-serine and hydroxyurea, synthesis of d-cycloserine was successfully attained. These in vitro studies yield the conclusion that DcsD and DcsG are necessary for the syntheses of O-ureido-l-serine and d-cycloserine, respectively. DcsD was also able to catalyze the synthesis of l-cysteine when sulfide was added instead of hydroxyurea. Furthermore, the present study shows that DcsG can also form other cyclic d-amino acid analogs, such as d-homocysteine thiolactone.     

Synthesis and stereocomplex formation of enantiomeric alternating copolymers with two types of chiral centers,poly(lactic acid-alt-2-hydroxybutanoic acid)s

Stereocomplex (SC) formation was reported for the first time for enantiomeric alternating copolymers consisting of repeating units with two types of chiral centers, poly(lactic acid-alt-2-hydroxybutanoic acid)s [P(LA-alt-2HB)s]. l,l-Configured poly(l-lactic acid-alt-l-2-hydroxybutanoic acid) [P(LLA-alt-l-2HB)] and d,d-configured poly(d-lactic acid-alt-d-2-hydroxybutanoic acid) [P(DLA-alt-d-2HB)] were amorphous. Blends of P(LLA-alt-l-2HB) and P(DLA-alt-d-2HB) were crystallizable and showed typical SC-type wide-angle X-ray diffraction profiles similar to those reported for stereocomplexed blends of poly(l-lactic acid) and poly(d-lactic acid) homopolymers and of poly(l-2-hydroxybutanoic acid) and poly(d-2-hydroxybutanoic acid) homopolymers, and of l,l-configured poly(l-lactic acid-co-l-2-hydroxybutanoic acid) [P(LLA-co-l-2HB)] and d,d-configured poly(d-lactic acid-co-d-2-hydroxybutanoic acid) [P(DLA-co-d-2HB)] random copolymers. The melting temperature values and melting enthalpy values at 100% crystallinity for stereocomplexed solvent-evaporated and precipitated P(LLA-alt-l-2HB)/P(DLA-alt-d-2HB) blends were correspondingly 187.5 and 187.9 °C, and 98.1 and 91.8 J g⁻¹. Enantiomeric polymer blending of P(LLA-alt-l-2HB) and P(DLA-alt-d-2HB) can confer crystallizability by stereocomplexation and the biodegradable materials with a wide variety of physical properties and biodegradability are highly expected to be prepared by synthesis of alternating copolymers of various combinations of two types of chiral α-substituted 2-hydroxyalkanoic acid monomers and their SC crystallization.

Stereocomplex formation was reported for alternating copolymers of chiral α-substituted 2-hydroxyalkanoic acids which can be utilized for preparation of biodegradable materials with a variety of physical properties and biodegradability.     

Mechanistic study on substitution reaction of a citrato(p-cymene)Ru(ii) complex with sulfur-containing amino acids

The reactions of a dichloro(p-cymene)ruthenium(ii) dimer, [RuCl₂(p-cymene)]₂, with citric acid and sulfur-containing amino acids gave only [Ru(L)(p-cymene)]-type complexes (L = citrate (Cit), l-penicillaminate (l-Pen), S-methyl-l-cysteinate (S-Me-l-Cys) and l-methioninate (l-Met)) in aqueous solutions at various pHs and molar ratios of the reactants, where Cit and the amino acids act as a tridentate ligand. These sulfur-containing amino acid complexes with bound nitrogen, oxygen and sulfur atoms and η⁶-p-cymene take S absolute configuration around Ru(ii) selectively, having the α-proton oriented in the opposite direction from the Ru(ii) center. The concentration dependences of the observed pseudo-first-order rate constants were provided for the substitution reactions of the citrato complex, [Ru(Cit)(p-cymene)], with a large excess of the sulfur-containing amino acids at various temperatures at pH 7.3, where solvolysis path was observed for S-Me-l-Cys and l-Met as an intercept but not for l-Pen. The activation parameters for the substitution reactions by the direct attack of the amino acids were changed significantly, indicating that the reaction mechanism varies sensitively with the amino acids from an associative mechanism to an interchange one. The pH dependences of the rate constants of the substitution reactions suggest that the carboxylate group is an attacking group for S-Me-l-Cys and l-Met under neutral conditions and the thiol group of l-Pen acts as an entering group constantly at any pH showing a considerably smaller activation energy compared with S-Me-l-Cys and l-Met. Differences in stabilities of the amino acid complexes were obtained from the equilibrium constants for the substitution reactions between the amino acids. These results indicate that the activation energies for the substitution reactions of the citrato complex with the amino acids are moderately correlated with the stabilities of the formed amino acid complexes.

Thorough kinetic study revealed characteristics of the reaction mechanism for arene ruthenium(ii) complexes with bio-related ligands.     

Surfactant-assisted hydrothermal synthesis of rGO/SnIn4S8 nanosheets and their application in complete removal of Cr(vi)

To solve the problem of contamination of hexavalent chromium (Cr(vi)), visible-light-driven graphene-based ternary metal chalcogenide nanosheets (rGO/SnIn₄S₈) were synthesized via a one-pot surfactant-assisted hydrothermal method for the photoreduction of Cr(vi). Characterizations demonstrated that SnIn₄S₈ nanosheets were uniformly distributed on the surface of rGO and the as-synthesized nanosheets exhibited excellent photocatalytic activity under visible light. In addition, the effects of pH, concentration of critic acid, holes and electron scavengers on the reduction of Cr(vi) were systematically investigated. It was found that 50 mg L⁻¹ of Cr(vi) could be completely removed within 30 min at pH 2 when citric acid served as a hole scavenger. Kinetic studies showed that the photocatalytic reduction of Cr(vi) processes obeyed the pseudo first order model. Further study indicated that the Cr(iii) species was immediately adsorbed onto the surface of the rGO/SnIn₄S₈ nanosheets after photocatalytic reduction of Cr(vi). Additionally, recycling results suggested that rGO/SnIn₄S₈ nanosheets possessed high recycle ability and stability after repeated use (5 times). This effective and promising work might provide a new strategy for the photoreduction of Cr(vi) and complete removal of chromium from effluent through the novel photocatalyst rGO/SnIn₄S₈.

Fabrication of visible-light-responsive photocatalyst (rGO/SnIn₄S₈) for photoreduction of Cr(vi) and adsorption of Cr(iii).     

pH-responsive polymeric vesicles from branched copolymers

A new type of branched copolymer, poly(l-lactide)₂-b-poly(l-glutamic acid) (PLLA₂–PLGA), based on polypeptide PLGA is synthesized by the ring-opening polymerization (ROP) of N-carboxyanhydride of γ-benzyl-l-glutamate (BLG–NCA) with amino-terminated PLLA₂–NH₂ and subsequent deprotection. The branched copolymer is characterized by ¹H NMR, FTIR and GPC measurements. The self-assembly of the copolymers in aqueous media has been systematically discussed. A pyrene probe has been used to demonstrate the aggregated formation of PLLA₂–PLGA in solution by measuring the critical micelle concentration (cmc). The morphology and size of the micelles have further been studied by transmission electron microscopy (TEM), dynamic light scattering (DLS) and field emission scanning electron microscopy (ESEM). We demonstrated that the Rh of the vesicle is depending on solution pH and salt concentration. The vesicles show good stability with remained shapes and sizes during the lyophilizing process. These vesicles have great potential in the application of drug delivery.

A new type of branched copolymer, poly(l-lactide)₂-b-poly(l-glutamic acid), based on polypeptide PLGA is synthesized by the ring-opening polymerization of N-carboxyanhydride of γ-benzyl-l-glutamate with amino-terminated PLLA₂–NH₂ and subsequent deprotection.     

Integrated optical and electrochemical detection of Cu2+ ions in water using a sandwich amino acid–gold nanoparticle-based nano-biosensor consisting of a transparent-conductive platform

In this paper, an optical-electrochemical nano-biosensor was introduced for measuring Cu²⁺ ion concentrations in water. A multi-step procedure was used to fabricate the transparent-conductive biosensor platform consisting of an l-cysteine–gold nanoparticle-based sandwich structure. First, colloidal gold nanoparticles (GNPs) were synthesized according to the Turkevich–Frens method with some modifications and then functionalized with l-cysteine molecules (GNP/l-cys). Then, cyclic voltammetry was preformed in buffered solutions containing HAuCl₄·3H₂O for gold nanoparticle electrodeposition on cleaned ITO glasses. The GNP-electrodeposited ITO glasses (ITO/GNPs) were thermally treated in air atmosphere for 1 hour at a temperature of 300 °C. Following the procedure, the gold nanoparticles on ITO/GNPs substrates were functionalized with l-cysteine to prepare ITO/GNPs/l-cys substrates. Finally, the sandwich-type substrates of ITO/GNPs/l-cys⋯Cu²⁺⋯l-cys/GNPs were fabricated by accumulation of Cu²⁺ ions using an open circuit technique performed in copper ion buffer solutions in the presence of previously produced colloidal GNP/l-cys nanoparticles. The effective parameters including GNP/l-cys solution volume, pre-concentration pH and pre-concentration time on the LSPR and SWV responses were investigated and optimized. The fabricated transparent-conductive platforms were successfully assessed as a nano-biosensor for detection of copper ions using two different methods of square wave voltammetry (SWV) and localized surface plasmon resonance (LSPR). As a result, the proposed biosensor showed a high sensitivity, selectivity and a wide detectable concentration range to copper ions. The total linear range and the limit of detection (LOD) of the nano-biosensor were 10–100 000 nM (0.6–6354.6 ppb) and below 5 nM (0.3 ppb), respectively. The results demonstrated the potential of combining two different optical and electrochemical methods for quantitation of the single analyte on the same biosensor platform and obtaining richer data. Also, these results indicated that the developed LSPR-SWV biosensor was superior to many other copper biosensors presented in the literature in terms of linear range and LOD. The developed nano-biosensor was successfully applied in the determination of trace Cu²⁺ concentration in actual tap water samples.

The transparent-conductive platforms of ITO/GNPs/l-cys⋯Cu²⁺⋯l-cys/GNPs were fabricated for quantitation of Cu²⁺ ions in water samples using combined LSPR and SWV methods.     

Remarkable diastereomeric effect on thermoresponsive behavior of polyurethane based on lysine and tartrate ester derivatives

This study describes the long-distance diastereomeric effect on thermoresponsive properties in water-soluble diastereomeric polyurethanes (PUs) composed of an l-lysine ethyl ester diisocyanate and a trimethylene glycol l-/d-tartrate ester, which have differences in spatial arrangements of the ethyl esters in the mirror image. The PUs based on l-lysine and l-/d-tartrate ester, named l-PU and d-PU, were synthesized with various number average molecular weights from 4700 to 13 100. In turbidimetry, l-PU showed a steep phase transition from 100%T to 0%T within about 10 °C at 4 g L⁻¹, whereas d-PU did not change completely to 0%T transmittance even at 80 °C at 4 g L⁻¹. In addition, the thermoresponsive properties of l-PU were less affected by concentration than those of d-PU. This long-distance diastereomeric effect on thermoresponsive behavior between l-PU and d-PU appeared in common among 6 samples with 4700 to 13 100 number average molecular weight. In the dynamic light scattering experiments at each transmittance, the hydrodynamic diameter (D_h) of l-PU increased up to 1000 nm, while the D_h of d-PU remained almost at 200–300 nm. The C O stretching vibration of FT-IR spectra showed that d-PU has more hydrogen-bonded ester groups than L-PU. Thus, we speculated that the difference in the retention of polymer chains in the micelle to promote intermicellar bridging generates the long-distance diastereomeric effect.

The long-distance diastereomeric effect on thermoresponsive properties in a polyurethane system consisting of chiral monomers was reported.     

Highly selective synthesis of d-amino acids from readily available l-amino acids by a one-pot biocatalytic stereoinversion cascade

d-Amino acids are key intermediates required for the synthesis of important pharmaceuticals. However, establishing a universal enzymatic method for the general synthesis of d-amino acids from cheap and readily available precursors with few by-products is challenging. In this study, we constructed and optimized a cascade enzymatic route involving l-amino acid deaminase and d-amino acid dehydrogenase for the biocatalytic stereoinversions of l-amino acids into d-amino acids. Using l-phenylalanine (l-Phe) as a model substrate, this artificial biocatalytic cascade stereoinversion route first deaminates l-Phe to phenylpyruvic acid (PPA) through catalysis involving recombinant Escherichia coli cells that express l-amino acid deaminase from Proteus mirabilis (PmLAAD), followed by stereoselective reductive amination with recombinant meso-diaminopimelate dehydrogenase from Symbiobacterium thermophilum (StDAPDH) to produce d-phenylalanine (d-Phe). By incorporating a formate dehydrogenase-based NADPH-recycling system, d-Phe was obtained in quantitative yield with an enantiomeric excess greater than 99%. In addition, the cascade reaction system was also used to stereoinvert a variety of aromatic and aliphatic l-amino acids to the corresponding d-amino acids by combining the PmLAAD whole-cell biocatalyst with the StDAPDH variant. Hence, this method represents a concise and efficient route for the asymmetric synthesis of d-amino acids from the corresponding l-amino acids.

An efficient one-pot biocatalytic cascade was developed for synthesis of d-amino acids from readily available l-amino acids via stereoinversion.     

Electrochemical sensor using poly-(l-cysteine) functionalized CuO nanoneedles/N-doped reduced graphene oxide for detection of lead ions

A highly sensitive and selective electrochemical sensor modified with poly-(l-cysteine)/CuO nanoneedles/N-doped reduced graphene oxide (l-Cys/NN-CuO/N-rGO) has been prepared for the testing of trace Pb²⁺. The electrochemical performance of this proposed sensor was investigated using electrochemical impedance spectroscopy (EIS). Based on the excellent electrochemical properties of NN-CuO/N-rGO as well as the specific complexation of natural substance l-cysteine with Pb²⁺, the l-Cys/NN-CuO/N-rGO was applied as a voltammetric biosensor for the determination of trace Pb²⁺ at pH 5.0. Under the optimum experimental conditions, the voltammetric peak current was linear with the Pb²⁺ concentration over the range from 0.001 to 5.0 nM and 5.0 to 1000 nM, respectively, with a low detection limit for Pb²⁺ concentration on the biosensor of 8.0 × 10⁻⁵ nM (S/N = 3). The significant sensitivity, selectivity, and electron conductivity of this l-Cys/NN-CuO/N-rGO modified electrode have also been studied. The specific detection of Pb²⁺ in water samples was also carried out.

A highly sensitive and selective electrochemical sensor modified with poly-(l-cysteine)/CuO nanoneedles/N-doped reduced graphene oxide (l-Cys/NN-CuO/N-rGO) has been prepared for the testing of trace Pb²⁺.     

A novel surface plasmon resonance sensor based on a functionalized graphene oxide/molecular-imprinted polymer composite for chiral recognition of l-tryptophan

Herein, a novel surface plasmon resonance (SPR) sensor based on a functionalized graphene oxide (GO)/molecular-imprinted polymer composite was developed for the chiral recognition of l-tryptophan (l-Trp). The composite''s recognition element was prepared via a facile and green synthesis approach using polydopamine as both a reducer of GO and a functional monomer as well as a cross-linker for molecular imprinting. The composite was characterized via Fourier transform infrared spectroscopy, scanning electron microscopy, X-ray diffraction, and Raman spectroscopy. After attaching the composite onto the gold surface of an SPR chip, the sensor was characterized using contact-angle measurements. The sensor exhibited excellent selectivity and chiral recognition for the template (i.e., l-Trp). Density functional theory computations showed that the difference in hydrogen bonding between the composite element and l-Trp and d-Trp played an important role in chiral recognition.

Novel SPR sensor for chiral recognition of l-tryptophan using a functionalized graphene oxide/molecularly-imprinted polymer composite as a recognition element.     

Presence of δ-(l-α-Aminoadipyl)-l-Cysteinyl-d-Valine in Fermentations of Penicillium chrysogenum

Cultures of Penicillium chrysogenum, growth with [³⁵S]sulfate or labeled amino acids, were examined by ion-exchange chromatography for possible peptidic precursors of penicillin. A sulfur-containing compound, present in both the mycelial extracts and the culture filtrates, was eluted at the location of the synthetic lld-tripeptide δ-(l-α-aminoadipyl)-l-cysteinyl-d-valine. Since this compound was also labeled when the cultures were incubated with dl-[6-¹⁴C]α-aminoadipic acid, l-[3,3′-³H]cystine, or dl-[1-¹⁴C]valine, its identity with the synthetic lld-tripeptide can be accepted. No δ-(l-α-aminoadipyl)-l-cysteine or lll-tripeptide were detected. The implications of these findings for tripeptide and penicillin biosynthesis are discussed.     

Optimizing the precursor of sulfur source for hydrothermal synthesis of high performance CdS for photocatalytic hydrogen production

Although the CdS photocatalyst has been extensively investigated, a rational hydrothermal synthesis route is still required to prepare highly active CdS for H₂ evolution reaction (HER). To optimize the precursor of the sulfur source, three prevalent organic sulfur sources of thiourea (TA), thioacetamide (TAA) and l-cysteine (l-Cys) were used for hydrothermal synthesis of CdS. Their effects on the crystallographic structure, morphology, optical property, band structure, and photocatalytic HER performance of the products were then investigated systematically. The results indicated that hexagonal branched dendritic structure CdS (S-TA) could be produced in TA solution and showed the highest HER activity due to the branched 1D structure, the smallest interfacial electron transfer resistance and the most negative conduction band bottom (E_cb). Whereas in TAA, spherical CdS (S-TAA) with a mixed phase of hexagonal and cubic was obtained. The mixed phase structure and the more positive E_cb of S-TAA lead to a considerably lower HER activity than that of S-TA. Poorly crystallized hexagonal CdS nanoparticles (S-Cys) were prepared in l-Cys and showed the lowest HER performance as its E_cb is very near to H⁺ reduction potential. Thus, compared to T-AA and l-Cys, TA is a more suitable sulfur source for hydrothermal preparation of highly active CdS for HER.

The precursor of sulfur source for hydrothermal synthesis of high performance CdS for photocatalytic hydrogen production has been optimized.     

d-Cysteine supplementation partially protects against ferroptosis induced by xCT dysfunction via increasing the availability of glutathione

Glutathione (GSH) is synthesized from three amino acids and the overall process is highly dependent on the availability of l-cysteine (l-Cys). GSH serves as an essential cofactor for glutathione peroxidase 4 (Gpx4), which reduces phospholipid hydroperoxides. The inactivation of Gpx4 or an insufficient supply of l-Cys results in the accumulation of lipid hydroperoxides, eventually leading to iron-dependent cell death, ferroptosis. In this study, we investigated the anti-ferroptotic properties of d-cysteine (d-Cys) under conditions of dysfunction in cystine transporter, xCT. l-Cys supplementation completely rescued ferroptosis that had been induced by the erastin-mediated inhibition of xCT in Hepa 1-6 cells. Upon d-Cys supplementation, the erastin-treated cells remained completely viable for periods of up to 24 h but eventually died after 48 h. d-Cys supplementation suppressed the production of lipid peroxides, thereby ferroptosis. The addition of d-Cys sustained intracellular Cys and GSH levels to a certain extent. When Hepa 1-6 cells were treated with a combination of buthionine sulfoximine and erastin, the anti-ferroptotic effect of d-Cys was diminished. These collective results indicate that, although d-Cys is not the direct source of GSH, d-Cys supplementation protects cells from ferroptosis in a manner that is dependent on GSH synthesis via stimulating the uptake of l-Cys.     

Bismuth subsalicylate,a low-toxicity catalyst for the ring-opening polymerization (ROP) of l-lactide (l-LA) with aliphatic diol initiators: synthesis,characterization, and mechanism of initiation

The ring-opening polymerization (ROP) of l-lactide (l-LA) was induced by the catalytic action of bismuth subsalicylate (BiSS) using linear aliphatic diols [HO(CH₂)_nOH, where n = 2, 3, 4, 5, 6, and 8] as initiators and chain transfer agents. The theoretical and experimental degree of polymerization (DP) in all samples of α,ω-hydroxy telechelic poly(l-lactide) (HOPLLAOH) had a good agreement in all samples, an effect attributed to the interaction of BiSS with HO(CH₂)_nOH inducing a transfer reaction. HOPLLAOH was synthesized and characterized by a range of analytical techniques, confirming the insertion of methylene groups from the initiator into the main chain of the polyester. The glass-transition temperature (T_g) of HOPLLAOH was found to be proportional to the number of methylene groups present in the diol. Various parameters regarding the ROP of l-LA were studied, such as temperature, time of reaction, amount of catalyst, and the nature of the diols. A kinetic study of the reaction allowed the determination of the rate constants (k) and activation energy (E_a). A mechanism of initiation is proposed based on a computational study using density functional theory (DFT), evidencing the role of the alkyl diol as an initiator, producing an alkoxide (Bi–OROH). This species then acts as a nucleophile, attacking the carbonyl group, inducing its insertion, and ultimately completing the ring-opening of l-LA.

Bismuth subsalicylate (BiSS) acted as a catalyst in the ring-opening polymerization of l-lactide (l-LA) in the presence of alkyl diols as initiators.