Oxidation Of L-cysteine Research Articles

Synthesis of Polyaniline/BiPr Composite Oxide Nanowires with Enhanced Electrochemical Sensing Performance

Background: Considerable interest has been devoted to electrochemical sensors for the detection of L-cysteine using BiPr-based oxide-modified electrodes due to high specific surface area and good electro-catalytic activity with several oxidation states. The combination of the BiPr composite oxide nanowires with polyaniline (PAn) can promote the electro-catalytic performance towards Lcysteine because PAn can facilitate the electro-catalytic process by enhancing the charge transfer. Methods: PAn/BiPr composite oxide nanowires were obtained via low temperature one-step hydrothermal route. The obtained composite oxide nanowires were analyzed by X-ray diffraction, electron microscopy, and electrochemical methods. Results: Characterization results indicate that amorphous PAn nanoparticles with a size of about 50 nm are homogeneously dispersed at the surface of the BiPr composite oxide nanowires. PAn/BiPr composite oxide nanowire-modified electrode shows an enhanced L-cysteine electro-catalytic activity. PAn promotes electro-catalytic activity of the BiPr composite oxide nanowires. A pair of quasi-reversible cyclic voltammetry (CV) peaks exist at +0.49 V, -0.19 V, respectively. PAn/BiPr composite oxide nanowire modified electrode possesses a linear response in L-cysteine concentration of 0.001-2 mM and detection limit of 0.095 μM, good repeatability, and stability. Conclusion: PAn/BiPr composite oxide nanowires act as effective electro-catalysts for L-cysteine oxidation resulting in the enhancement of the electro-catalytic activity relative to BiPr composite oxide nanowires.

CRISPR/Cas12a triggered SERS and naked eye dual-mode biosensor for ultrasensitive and on-site detection of nucleic acid via cascade signal amplification

Highly sensitive and on-site detection of nucleic acids has always been a critical issue in the field of analytical chemistry. Surface-enhanced Raman scattering (SERS)-based biosensing exhibits huge potential in nucleic acid detection, while the applicability is restricted in trace nucleic acid screening due to the lack of appropriate signal recognition, transducing and amplification technologies. Inspired by the specific recognition of CRISPR/Cas12a and the improved sensitivity through cascade signal amplification, we innovatively proposed a CRISPR/Cas12a triggered SERS and naked eye dual-mode biosensor for ultrasensitive and on-site detection of nucleic acid via cascade signal amplification. Upon the target DNA recognition, the activated CRISPR/Cas12a indiscriminately cleaved substrate ssDNA, leading to the failure of toehold-mediated DNA-strand displacement reaction (TSDR), and triggering hybridization chain reaction (HCR) to assemble numerous G-quadruplex/hemin DNAzyme (GQH DNAzyme) for cascade signal amplification. The generated GQH DNAzyme catalyzed the oxidation of l-cysteine to cystine, perturbing the aggregation of 4-NTP@AuNPs, resulting in significant Raman signal change. On the other hand, GQH DNAzyme catalyzed the oxidation of 2,2′-azino-di-(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS), leading to obvious color change to realize portable naked-eye detection. Through this strategy, target nucleic acid concentration was tactfully transformed into sensitive Raman and portable visualization signals, and the limit of detection were as low as 34.9 aM and 1 pM, respectively. Then, this biosensor was successfully applied to meat adulteration detection, which showed superb selectivity, sensitivity and applicability for on-site detection of trace nucleic acid in complicated food matrix.

Multiple stimuli-switchable electrocatalysis and logic gates of l-cysteine based on P(DEA–co-VPBA) hydrogel films

In this work, poly(N',N-diethylacrylamide-co-4-vinylphenylboronic acid) (P(DEA-co-VPBA)) hydrogel films were successfully assembled on the pyrolytic graphite (PG) electrode by a simple radical polymerization. Reversible thermal, glucose and pH-triggered electrochemical reactions were obtained in the presence of 1,1′-ferrocene dicarboxylic acid (FDA) with electro-redox properties. The thermal response performances of the thin-film electrode were attributed to the PDEA component, the sensitive behaviors of glucose were ascribed to the VPBA component, and the sensitive behaviors of pH were affected by PDEA and VPBA. The electrocatalytic oxidation of l-cysteine by FDA on the hydrogel film electrodes significantly enhanced the difference between the on and off states of the multi-response cyclic voltammetry (CV) signals. This smart polymer hydrogel film electrode with amplified signals could be further used to realize a 4-input/9-output binary logic circuit, which was constructed with temperature, glucose, pH, and cysteine as inputs and different levels of Ipa and Ipc responses as outputs. Furthermore, a 2-to-4 decoder, a 4-to-2 encoder, and a 4-input keypad lock were built on the same platform. This polymer electrocatalysis system provides a novel idea for constructing complex biological computing systems.

A highly efficient hematite photoelectrochemical fuel cell for solar-driven hydrogen production

Hematite has demonstrated to be a promising photoanode for photoelectrochemical (PEC) hydrogen production because of its narrow band gap and outstanding stability. However, the sluggish water oxidation kinetics on hematite photoanode severely hinder the PEC hydrogen production performance. Herein, we report the construction of a Ti-doped porous hematite-based PEC fuel cell utilizing l-cysteine (L-Cys) oxidation to replace difficult water oxidation for efficient H2 production. The strong complexation interaction between hematite and L-Cys confers the direct photooxidation of L-Cys, producing additional electrons to participate in hydrogen production. As a result, the present Ti-doped porous hematite (Ti-PH) photoanode exhibits a current density of up to 4.45 mA cm−2 at 1.23 V vs. RHE under AM 1.5G illumination. This work provides a new strategy to utilize the additional electron contribution from photoinduced oxidation of organic chemicals for high-efficiency PEC fuel cell and solar fuel production.

NMR study of thiosulfate-assisted oxidation of L-cysteine

The reaction of L-cysteine with sodium thiosulfate in aqueous solution at pH 9 affords mainly L-cystine with noticeable amounts of L-cysteine sulfonic anion –O2CCH(NH+)CH SO–. NMR study revealed the formation of intermediate L-cysteine sulfenic acid and L-cysteine S-sulfite, the latter existing in two active forms –O2CCH(NH2)CH2S(=S)O– and –O CCH(NH)CH SSO–.

A self-cascade system based on Ag nanoparticle/single-walled carbon nanotube nanocomposites as an enzyme mimic for ultrasensitive detection of L-cysteine.

L-Cysteine, widely used in medicine and the food industry, is of great essentiality to organisms and the food quality. Given that current detection approaches require exacting lab conditions and tedious sample treatment, there is a pressing demand for developing a method that possesses advantages of user friendliness, prominent performance, and cost-effectiveness. Herein, a self-cascade system was developed for the fluorescence detection of L-cysteine based on the ingenious performance of Ag nanoparticle/single-walled carbon nanotube nanocomposites (AgNP/SWCNTs) and DNA-templated Ag nanoclusters (DNA-AgNCs). The fluorescence of DNA-AgNCs could be quenched on account of the adsorption of DNA-AgNCs on AgNP/SWCNTs by π-π stacking. With the cooperation of Fe2+, AgNP/SWCNTs with oxidase and peroxidase-like activities could catalyze the oxidation of L-cysteine to produce cystine and hydrogen peroxide (H2O2) and then break the O-O bond of H2O2 to generate a hydroxyl radical (·OH), which could cleave the DNA strand into different sequence fragments which subsequently peeled off from the AgNP/SWCNTs, resulting in a "turn-on" fluorescence response. In this paper, AgNP/SWCNTs with multi-enzyme activities was synthesized enabling the reaction to proceed in just one step. The successful preliminary applications for the L-cysteine detection in pharmaceutical, juice beverage, and serum samples indicated that the developed method exhibited great potential in medical diagnosis, food monitoring, and the biochemical field, which also broadened the horizon for follow-up research.

A new electrochemical sensor based on green synthesized CuO nanostructures modified carbon ionic liquid electrode for electrocatalytic oxidation and monitoring of l-cysteine

In this study, we introduced a new carbon composite electrode modified with green synthesized CuO nanostructures (CuO-NSs) for sensitive l-cysteine determination. We applied a cost-effective and green method to synthesize CuO-NSs using Terminalia catappa leaf extract as a reducer and stabilizer without other additives. The CuO nanostructures were characterized by Ultraviolet–visible spectroscopy (UV–vis), X-ray diffraction (XRD), field emission scanning electron microscopy with energy dispersive X-ray spectroscopy (FE-SEM-EDX) and elemental mapping, transmission electron microscopy (TEM), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Electrochemical techniques such as cyclic voltammetry and amperometry were applied to investigate the electrochemical performance of the composite electrode for cysteine (l-Cys) electrooxidation. The composite electrode showed a noticeable shift in the electrooxidation peak potential of l-Cys over the conventional electrodes and a stable and sensitive electrochemical response by providing active sites and effective electron transfer pathways. Effects of various parameters such as pH of the solution, the amount of modifier, and scan rate were investigated on the electrochemical behavior of l-Cys. Under the optimal experimental conditions, the fabricated l-Cys sensor showed a linear relationship over an extensive concentration range of 10.0 µM to 5000.0 µM (R2 = 0.998) with a limit of detection (LOD) of 0.51 µM at physiological conditions (pH = 7.0). In addition, the composite electrode displayed high selectivity, good average reproducibility (RSD = 2.45 %), and applicability in the real sample on the l-Cys detection. The green, fast, simple, and cost-effective synthesis of CuO-NSs and good electrocatalytic activity of the modified electrode reveal good use of the prepared sensor to detect l-Cys.

A facile synthesis route to BiPr composite nanosheets and sensitive electrochemical detection of l-cysteine

The BiPr composite nanosheets have been synthesized by a facile hydrothermal method using cetyltrimethyl ammonium bromide (CTAB). The BiPr composite products were investigated by means of X-ray diffraction (XRD), electron microscopy, X-ray photoelectron spectroscopy (XPS) and electrochemical method. The obtained nanosheets possess poly-crystalline structure and thickness of less than 100 nm. The application of the BiPr composite nanosheets as the glassy carbon electrode (GCE) modified materials for l-cysteine determination was systematically investigated. The BiPr composite nanosheets modified GCE possesses better interface electron transfer properties than bare GCE exhibiting superior electro-catalytic properties for the electrochemical oxidation of l-cysteine. A pair of quasi-reversible redox peaks are observed at –0.75 V and + 0.06 V, respectively. The current response of l-cysteine is linear in the concentration range of 0.001–2 mM with a detection limit of 0.21 μM. The BiPr composite nanosheets modified GCE has considerable stability and reproducibility.

Controllable synthesis of BiPr composite oxide nanowires electrocatalyst for sensitive L-cysteine sensing properties

BiPr composite oxide nanowires with rhombodedral Bi1.35Pr0.65O3, monoclinic Bi2O3 and monoclinic Pr5O9 phases were synthesized via a facile sodium dodecyl sulfate (SDS) assisted hydrothermal route. The obtained nanowires were characterized by x-ray diffraction, electron microscopy, x-ray photoelectron spectroscopy and electrochemical measurements. The BiPr composite oxide nanowires possess poly-crystalline structure, semi-circular tips, diameter and length of 20–100 nm and several micrometers, respectively. SDS is essential for the formation of the BiPr composite oxide nanowires which can be explained by a SDS assisted hydrothermal growth process. Electrochemical impedance spectroscopy shows that the electrons are easier to transfer by the surface of the BiPr composite oxide nanowires modified glassy carbon electrode (GCE) than bare GCE. The BiPr composite oxide nanowires modified GCE possesses good electro-catalytic activity for L-cysteine detection with a pair of quasi-reversible cyclic voltammetry peaks at +0.04 V and –0.72 V for the oxidation and reduction of L-cysteine, respectively. The roles of the scan rate, electrolyte species and L-cysteine concentration on the electrochemical responses of L-cysteine at the nanowires modified GCE were systematically analyzed. The BiPr composite oxide nanowires modified GCE presents a linear response range from 0.001 to 2 mM and detection limit of 0.27 μM, good reproducibility and stability.

Kinetics and Mechanism of Catalyzed Oxidation of L-Cysteine by Salen Schiff Base Complexes of Co(III), Fe (III) and Cr(III)

Kinetics of oxidation of L-cysteine by new series of substituted ONNO-donor salen-type Schiff base complexes of general formula [MIII(L)Cl] (M = Co, Fe, Cr; L = Schiff base ligand) have been studied in aqueous solutions. Measurements were run at constant temperature (25º C), constant ionic strength (0.20 M), and constant pH (7.0) under pseudo-first order conditions, in which the concentration of cysteine is around two orders of magnitude greater than that of metal complex. The observed rate constant was determined by following the change in absorbance of reaction mixture at a predetermined wavelength with time. Results show that the rate of oxidation depends on the type of metal center, with Co(III) complexes were found to have the highest rates due to higher reduction potential of Co(III). The oxidation rate was also found to depend on steric factor and the electron withdrawing / releasing ability of the ligand bound to the metal ion.

Highly active catalyst using zeolitic imidazolate framework derived nano-polyhedron for the electro-oxidation of l-cysteine and amperometric sensing

Herein, N-doped porous carbon nano-polyhedron embedded with Co3O4 (Co3O4-NPCN) was reported for the electro-catalytic oxidation and amperometric detection of l-cysteine. Co3O4-NPCN was synthesized by the two-step redox calcination of zeolitic imidazolate framework (ZIF). Surface morphology characterization revealed that Co3O4-NPCN displayed a uniform size and rhombic dodecahedral shape. Structure and composition analysis found that Co3O4-NPCN was a N-doped carbon polyhedral matrix with hollow and porous structure, and Co3O4 nano-spheres were evenly distributed into the polyhedral matrix. Due to the hollow and porous structure, N-doped carbon matrix and embedded Co3O4 nano-spheres, Co3O4-NPCN performed a remarkable electro-catalysis towards the oxidation of l-cysteine at a very low potential of 0.10 V. A diffusion-controlled l-cysteine oxidation process was observed at Co3O4-NPCN prepared electrode. Accordingly, amperometric method was established for l-cysteine detection with a very fast current response in 2 s, wide linear range of 0.05 μM- 5.2 mM and low detection limit of 6.9 nM. Besides, notable selectivity, repeatability, reproducibility and long-term stability were also achieved. Moreover, Co3O4-NPCN sensor was successfully applied to the l-cysteine detection in human serum samples indicating the practical application of the as-developed sensor.

Formation Mechanism of Cofactor Cys-Tyr in the Cysteine Dioxygenases (CDO and F2-CDO) and Its Influence on Catalysis: A QM/MM Study.

Cysteine dioxygenase (CDO) is a nonheme mononuclear iron enzyme, which catalyzes the oxidation of cysteine to cysteine sulfinic acid. Crystal structure studies of mammalian CDO showed that there is a cross-linked cysteine-tyrosine (Cys-Tyr) cofactor in its active site. Moreover, the formation of the Cys-Tyr cofactor requires the metal cofactor (Fe2+) and O2, and it was previously considered to substantially enhance the catalytic efficiency and half-life of CDO. Recently, a pure human CDO (F2-CDO) without including the Cys-Tyr cofactor was crystalized by the site-directed mutagenesis approach in the anaerobic condition. In this work, to gain insights into the formation mechanism of the Cys-Tyr cofactor and whether it can really promote the catalytic reactivity of CDO, a series of computational models have been constructed, and quantum mechanical/molecular mechanical (QM/MM) calculations have been performed. Our calculation results reveal that WT-CDO and F2-CDO follow different mechanisms for the formation of the Cys-Tyr cofactor. In F2-CDO, the cofactor formation contains the H-abstraction, C-S bond formation, intramolecular F migration, and aromatization of the residue F2Y157, in which the Fe-coordinate dioxygen can be recovered after the formation cofactor; however, in the WT-CDO, the cofactor formation shows some differences. During the reaction, hydrogen peroxide is generated, and the final aromatization requires the assistance of one water molecule. Furthermore, the overall barriers of cofactor formation are always higher than l-cysteine oxidation for both WT-CDO and F2-CDO irrespective of the absence or presence of the cofactor. Thus, we can theoretically confirm that the Cys-Tyr cofactor is not essential for the oxidation activity of CDO, and cofactor formation is just an accompanying reaction but not a prerequisite for the oxidation reaction. These results may provide useful information for understanding the catalysis of CDO.

Electrocatalytic Oxidation of L-Cysteine, L-Methionine, and Methionine–Glycine Using [Oxoiron(IV)–Salen] Ion Immobilized Glassy Carbon Electrode

Electrocatalytic Oxidation of L-Cysteine, L-Methionine, and Methionine–Glycine Using [Oxoiron(IV)–Salen] Ion Immobilized Glassy Carbon Electrode

L-cysteine oxidation on Pt and Au rotating disk electrodes: Insights on mixed controlled kinetics

L-Cysteine (L-Cys) oxidation on gold and platinum exhibits different behaviors due to adsorption phenomena related to the affinity between the S atom present on the L-Cys side chain and the metal sites. The adsorption of S on Au is very strong, which renders the determination of the L-Cys adsorption reaction order and its symmetry coefficient impossible. By contrast, on Pt electrodes, the L-Cys protonation state can be controlled by the electrolyte pH, and it is shown that the Tafel slope for L-Cys oxidation is pH dependent, with the most favorable kinetics being observed at pH = 9. Under this condition, the thiol is deprotonated, and thiolate is present, with L-Cys presenting a nucleophilic character. Differences between the two metals during L-Cys oxidation were also investigated. In rotating disk electrode measurements, the oxidation peak was observed to shift with the rotation rate, highlighting the kinetic activation control over this process and showing that diffusion influences this activation control as well. The combination of triangular potential perturbation and convection techniques allowed a chemometric soft model to be used to investigate reactions that are simultaneously controlled by activation and mass transport.

Fabrication of hierarchical 3D prickly ball-like Co–La oxides/reduced graphene oxide composite for electrochemical sensing of l-cysteine

A novel 3D prickly ball-like Co–La oxides/reduced graphene oxide composite (Co–La oxides/rGO) has been successfully synthesized by a facile thermal evaporation method. Co–La oxides were hierarchical spiny globular structure made up of numerous radially-grown nanowires with the size of about 50 nm in width. Some of the nanowires grow vertically on the surface of rGO nanosheets. GO was thermally reduced to rGO. XPS data showed that the Co3+, Co2+ and La3+ existed in the prepared samples. The possible growth mechanism was tentatively proposed. The application of the 3D prickly ball-like Co–La oxides/rGO as electrode-modified material for electrochemical oxidation of l-cysteine was studied. The prepared prickly ball-like Co–La oxides/rGO modified electrode expressed prominent electrocatalytic ability for the electrochemical oxidation of l-cysteine. Under optimum experimental conditions, the current response of l-cysteine were linear with its concentration in the range of 1.0–888.0 μM with a low detection limit of 0.1 μM (S/N = 3). The sensor was used to evaluate its application for the detection of l-cysteine in the real samples.

INFLUENCE OF L-CYSTEINE AND N-ACETYL-L-CYSTEINE ON REDUCING ACTIVITY OF THIOUREA DIOXIDE IN AQUEOUS SOLUTIONS

The influence of L-cysteine and N-acetyl-L-cysteine on the stability and reducing activity of thiourea dioxide (NH2)2CSO2 (TDO) in the reaction with azo dye Orange II has been studied. The addition of L-cysteine and N-acetyl-L-cysteine leads to the increase in reducing activity of TDO in weakly acidic, neutral and weakly alkaline solutions, but does not influence its reducing activity in strongly alkaline solutions. In alkaline solutions, the rate of reduction of Orange II is increasing, but the influence of additives of thiol acids is decreasing. The activating effect of N-ace-tyl-L-cysteine is weaker than the effect of L-cysteine, that can be explained by the differences of pKa of these acids. Indeed, the reducing activity of thiols depends on the concentration of thiolate-ions in solutions. Since pKa of cysteine (8.30) is significantly less than pKa of N-acetyl-L-cysteine (9.52), concentration of thiolate-ions in cysteine solutions is higher. Therefore, reducing activity of L-cysteine is higher. The influence of L-cysteine and N-acetyl-L-cysteine on the reaction of TDO with Orange II is different from the influence of aminoacid glycine: in neutral solutions glycine decreases the rate of reaction. The interaction of TDO with L-cysteine in weakly acidic, neutral and weakly alkaline solutions is accompanied by the oxidation of L-cysteine to cysteinesulfenic acid and reduction of TDO to thiourea monoxide (NH2)2CSO. Then cysteinesulfenic acid reacts with L-cysteine with formation of cystine. The reaction studied here is the first example of reduction of TDO by sulfur-containing compounds.

High performance electrochemical L-cysteine sensor based on hierarchical 3D straw-bundle-like Mn-La oxides/reduced graphene oxide composite

Abstract A novel 3D straw-bundle-like Mn-La oxides/reduced graphene oxide composite (Mn-La oxides/rGO) has been successfully synthesized by a facile thermal evaporation method. The products were investigated by means of scanning electron microscopy (SEM), energy dispersive X-ray spectrum (EDS), X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and electrochemical method. The possible growth mechanism was tentatively proposed. The application of the 3D straw-bundle-like Mn-La oxides/rGO as electrode-modified material for electrochemical oxidation of L-cysteine was studied. The prepared Mn-La oxides/rGO modified electrode expressed prominent electrocatalytic ability for the electrochemical oxidation of L-cysteine. Under optimum experimental conditions, the current response of L-cysteine were linear with its concentration in the range of 0.5–832.5 μM with a low detection limit of 0.1 μM (S/N = 3). The proposed sensor was used to evaluate its application for the detection of L-cysteine in the real samples.

The 3-His Metal Coordination Site Promotes the Coupling of Oxygen Activation to Cysteine Oxidation in Cysteine Dioxygenase.

Cysteine dioxygenase (CDO) structurally resembles cupin enzymes that use a 3-His/1-Glu coordination scheme. However, the glutamate ligand is substituted with a cysteine (Cys93) residue, which forms a thioether bond with tyrosine (Tyr157) under physiological conditions. The reversion variant, C93E CDO, was generated in order to reestablish the more common 3-His/1-Glu metal ligands of the cupin superfamily. This variant provides a framework for testing the structural and functional significance of Cys93 and the cross-link in CDO. Although dioxygen consumption was observed with C93E CDO, it was not coupled with l-cysteine oxidation. Substrate analogues (d-cysteine, cysteamine, and 3-mercaptopropionate) were not viable substrates for the C93E CDO variant, although they showed variable coordinations to the iron center. The structures of C93E and cross-linked and non-cross-linked wild-type CDO were solved by X-ray crystallography to 1.91, 2.49, and 2.30 Å, respectively. The C93E CDO variant had similar overall structural properties compared to cross-linked CDO; however, the iron was coordinated by a 3-His/1-Glu geometry, leaving only two coordination sites available for dioxygen and bidentate l-cysteine binding. The hydroxyl group of Tyr157 shifted in both non-cross-linked and C93E CDO, and this displacement prevented the residue from participating in substrate stabilization. Based on these results, the divergence of the metal center of cysteine dioxygenase from the 3-His/1-Glu geometry seen with many cupin enzymes was essential for effective substrate binding. The substitution of Glu with Cys in CDO allows for a third coordination site on the iron for bidentate cysteine and monodentate oxygen binding.

Synthesis of cobalt oxide nanoparticles using Cirsium vulgare leaves extract and evaluation of electrocatalytic effects on oxidation of l-cysteine

In this present study, cobalt oxide nanoparticles (Co3O4 NPs) were synthesized using Cirsium vulgare leaves extract and characterized using various techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). These methods confirmed the formation of Co3O4 NPs, and the microscopic technique confirmed the size of the Co3O4 NPs is about 20 nm. Also, a novel modified electrode for determination of l-cysteine was described by carbon paste electrode modified with Co3O4 NPs (CPE/Co3O4 NPs). The presence of Co3O4 NPs markedly enhances the electrocatalytic activity for determination of l-cysteine. Under the selected conditions, the anodic peak current was linearly dependent on the concentration of l-cysteine in the range 0.20–75 μM by the amperometric method. The detection limit (S/N = 3) was also estimated to be 0.07 μM. This modified electrode was a simple, rapid, and effective sensor, and it was successfully applied to determine of l-cysteine in real samples.

Electrocatalytic activity of a push pull Co(II) phthalocyanine in the presence of graphitic carbon nitride quantum dots

Absract This work reports for the first time on the use of a conjugate of graphitic carbon nitride quantum dots (gCNQDs) with a push-pull asymmetrical cobalt phthalocyanine (CoPc) for electrochemical sensing. The nanocomposite is immobilized on a glassy carbon electrode (GCE) surface for the use in l-cysteine electrocatalysis. The nanocomposites were characterized using techniques such as X-ray diffractometry (XRD), Fourier transform infrared (FTIR) spectroscopy, UV-vis spectroscopy, transmission electron microscopy (TEM), energy dispersive X-ray (EDX) analysis, Raman spectroscopy and electrochemical methods. The nanocomposites were immobilized by the drop-dry method, sequentially or when premixed in solution. Good electrocatalytic oxidation of l-cysteine was observed, especially by the sequentially modified electrode surface, with the CoPc on top of gCNQDs. The sensitivity was determined as 3.5 μA.mM-1 and the limit of detection (LoD) as 101.3 μM for GCE-gCNQDs, 0.65 μA.mM-1 and 0.96 μM for GCE-CoPc, 23.41 μA.mM-1 and 0.41 μM for gCNQDs-CoPc (premixed) and 100.5 μA.mM-1 and 0.02 μM for gCNQDs-CoPc (sequential). The electrode surfaces also showed high stability by continuous cyclization.