A novel approach for the selective determination of tryptophan in blood serum in the presence of tyrosine based on the electrochemical reduction of oxidation product of tryptophan formed in situ on graphite electrode

C-undecylcalix[4]resorcinarene Langmuir-Blodgett/Porous Reduced Graphene Oxide Composite Film as a Electrochemical Sensor for the Determination of Tryptophan

In this study, a composite film was developed for the electrochemical sensing of tryptophan (Trp). Porous reduced graphene oxide (PrGO) was utilized as the electron transfer layer, and a C-undecylcalix[4]resorcinarene Langmuir-Blodgett (CUCR-LB) film served as the molecular recognition layer. Atomic force microscopy (AFM), transmission electron microscopy (TEM), Raman spectroscopy, scanning electron microscopy (SEM), and electrochemical experiments were employed to analyze the characteristics of the CUCR-LB/PrGO composite film. The electrochemical behavior of Trp on the CUCR-LB/PrGO composite film was investigated, revealing a Trp linear response range of 1.0 × 10-7 to 3.0 × 10-5 mol L-1 and a detection limit of 3.0 × 10-8 mol L-1. Furthermore, the developed electroanalytical method successfully determined Trp content in an amino acid injection sample. This study not only introduces a rapid and reliable electrochemical method for the determination of Trp but also presents a new strategy for constructing high-performance electrochemical sensing platforms.

MoS2/S@g-CN Composite Electrode for L-Tryptophan Sensing

L-tryptophan (L-TRP) is an essential amino acid responsible for the establishment and maintenance of a positive nitrogen equilibrium in the nutrition of human beings. Therefore, it is vital to quantify the amount of L-tryptophan in our body. Herein, we report the MoS2/S@g-CN-modified glassy carbon electrode for the electrochemical detection of L-tryptophan (L-TRP). The MoS2/S@g-CN composite was successfully synthesized using an efficient and cost-effective hydrothermal method. The physical and chemical properties of the synthesized composite were analyzed using powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and energy-dispersive X-ray analysis (EDX). The crystallite size of the composite was calculated as 39.4 nm, with porous balls of MoS2 decorated over the S@g-CN surface. The XPS spectrum confirmed the presence of Mo, S, O, C, and N elements in the sample. The synthesized nanocomposite was further used to modify the glassy carbon (GC) electrode (MoS2/S@g-CN/GC). This MoS2/S@g-CN/GC was used for the electrochemical detection of L-TRP using cyclic voltammetry (CV) and differential pulse voltammetry (DPV) techniques. For the purpose of comparison, the effects of the scanning rate and the concentration of L-TRP on the current response for the bare GC, S@g-CN/GC, MoS2/GC, and MoS2/S@g-CN/GC were studied in detail. The MoS2/S@g-CN-modified GC electrode exhibited a rational limit of detection (LoD) of 0.03 µM and a sensitivity of 1.74 µA/ µMcm2, with excellent stability, efficient repeatability, and high selectivity for L-TRP detection.

Electrochemical sensor based on molecularly imprinted copolymer for selective and simultaneous determination of riboflavin, dopamine, and L-tryptophan

This research shows the exact detection of riboflavin (RF), dopamine (DA), and L-tryptophan (Trp) through molecularly imprinted polymer (MIP) based on the electropolymerization method. MIP was placed on the surface of the glassy carbon electrode (GCE) by electropolymerization of monomers such as catechol and para-aminophenol, in the presence of all three analytes. The introduced sensor was investigated using field emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM), Fourier-transform infrared spectroscopy (FTIR), and electrochemical methods, for example, electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and differential pulse voltammetry (DPV). The MIP/GCE performs well in terms of selectivity, reproducibility, repeatability, and stability. This sensor revealed good linear ranges of 0.005-500 μM for RF, 0.05-500 μM for DA, and 0.1-250 μM for Trp with limits of detection (LOD) as 0.0016 μM, 0.016 μM, and 0.03 μM for RF, DA, and Trp, respectively. The modified GCE was successfully applied to detect RF, DA, and Trp in serum and milk samples with satisfactory results.

Electrochemical Amino Acid Sensing: A Review on Challenges and Achievements

The rapid growth of research in electrochemistry in the last decade has resulted in a significant advancement in exploiting electrochemical strategies for assessing biological substances. Among these, amino acids are of utmost interest due to their key role in human health. Indeed, an unbalanced amino acid level is the origin of several metabolic and genetic diseases, which has led to a great need for effective and reliable evaluation methods. This review is an effort to summarize and present both challenges and achievements in electrochemical amino acid sensing from the last decade (from 2010 onwards) to show where limitations and advantages stem from. In this review, we place special emphasis on five well-known electroactive amino acids, namely cysteine, tyrosine, tryptophan, methionine and histidine. The recent research and achievements in this area and significant performance metrics of the proposed electrochemical sensors, including the limit of detection, sensitivity, stability, linear dynamic range(s) and applicability in real sample analysis, are summarized and presented in separate sections. More than 400 recent scientific studies were included in this review to portray a rich set of ideas and exemplify the capabilities of the electrochemical strategies to detect these essential biomolecules at trace and even ultra-trace levels. Finally, we discuss, in the last section, the remaining issues and the opportunities to push the boundaries of our knowledge in amino acid electrochemistry even further.

Sulfur and nitrogen co-doped graphene quantum dot-assisted chemiluminescence for sensitive detection of tryptophan and mercury (II)

A simple one-step thermal treatment to prepare strong fluorescent sulfur and nitrogen co-doped graphene quantum dots (SN-GQD) using citric acid and l-cysteine as precursors was developed. The ultra-weak chemiluminescence (CL) from the reaction of hydrogen peroxide (H₂ O₂ ) and periodate (IO₄ ^- ) was significantly enhanced by SN-GQD in acidic medium. The enhanced CL was induced by excited-state SN-GQD (SN-GQD*), which was produced from the transfer energy of (O₂ )₂ * and ¹ O₂ to SN-GQD and recombination of oxidant-injected holes and electrons in SN-GQD. In the presence of tryptophan (Trp), the CL intensity of the SN-GQD-H₂ O₂ -KIO₄ system was greatly diminished. This finding was used to design a novel method for determination of Trp in the linear range 0.6-20.0 μM, with a limit of detection (LOD) of 58.0 nM. Furthermore, Hg²⁺ was detectable in the range 0.1-9.0 μM with a LOD of 64.0 nM, based on its marked enhancement of the SN-GQD-H₂ O₂ -KIO₄ CL system. The proposed method was successfully applied to detect Trp in milk and human plasma samples and Hg²⁺ in drinking water samples, with recoveries in the range 95.7-107.0%.

Facile sonochemical synthesis of perovskite-type SrTiO3 nanocubes with reduced graphene oxide nanocatalyst for an enhanced electrochemical detection of α-amino acid (tryptophan)

In this paper, perovskite-type SrTiO₃ nanocubes decorated reduced graphene oxide is synthesized by sonochemical method. The as-synthesized SrTiO₃@RGO nanocomposite was confirmed by XRD, TEM, SEM, elemental mapping and electrochemical technique. Furthermore, surface morphological and X-ray diffraction studies revealed the formation and high loading of SrTiO₃ nanocubes on reduced graphene oxide matrix. The SrTiO₃@RGO nanocomposite modified electrode shows an excellent electrochemical detection towards of amino acid (tryptophan). The developed sensor was showed a wide linear range from 30 nM to 917.3 µM and detection limit is 7.15 nM. Furthermore, the sensitivity was calculated to be 9.11 µA µM^-1 cm². In addition, the proposed modified sensor is exhibited good selectivity, stability, reproducibility and repeatability. The SrTiO₃@RGO catalyst modified electrode was successfully applied to tryptophan analysis in biological samples.

A Simple and Efficient Molecularly Imprinted Electrochemical Sensor for the Selective Determination of Tryptophan

In this paper, a tryptophan (Trp) molecularly imprinted chitosan film was prepared on the surface of an acetylene black paste electrode using chitosan as the functional polymer, Trp as the template molecule and sulfuric acid as the crosslinking agent. The surface morphologies of non-imprinted and imprinted electrodes were characterized by scanning electron microscopy (SEM). The formation of hydrogen bonds between the functional polymer and the template molecule was confirmed by infrared spectroscopy. Some factors affecting the performance of the imprinted electrode such as the concentration of chitosan, the mass ratio of chitosan to Trp, the dropping amount of the chitosan-Trp mixture, the solution pH, and the accumulation potential and time were discussed. The experimental results show that the imprinted electrode exhibit good affinity and selectivity for Trp. The dynamic linear ranges of 0.01–4 μM, 4–20 μM and 20–100 μM were obtained by second derivative linear sweep voltammetry, and the detection limit was calculated to be 8.0 nM. The use of the imprinted electrode provides an effective method for eliminating the interference of potentially interfering substances. In addition, the sensor has high sensitivity, reproducibility and stability, and can be used for the determination of Trp in pharmaceutical preparations and human serum samples.

Ta2O5/rGO Nanocomposite Modified Electrodes for Detection of Tryptophan through Electrochemical Route

l-tryptophan is one of the eight kinds of essential amino acids for sustainable human life activity. It is common to detect the concentration of tryptophan in human serum for diagnosing and preventing brain related diseases. Herein, in this study, GCE (glassy carbon electrode) modified by Ta₂O₅-reduced graphene oxide (-rGO) composite (Ta₂O₅-rGO-GCE) is synthesized by the hydrothermal synthesis-calcination methods, which is used for detecting the concentration of tryptophan in human serum under the as-obtained optimal detection conditions. As a result, the obtained Ta₂O₅-rGO-GCE shows larger electrochemical activity area than other bare GCE and rGO-GCE due to the synergistic effect of Ta₂O₅ NPs and rGO. Meanwhile, Ta₂O₅-rGO-GCE shows an excellent response to tryptophan during the oxidation process in 0.1 M phosphate buffer solution (pH = 6). Moreover, three wide linear detection range (1.0-8.0 μM, 8.0-80 μM and 80-800 μM) and a low limit of detection (LOD) of 0.84 μM (S/N = 3) in the detection of tryptophan are also presented, showing the larger linear ranges and lower detection limit by employing Ta₂O₅-rGO-GCE. Finally, the as-proposed Ta₂O₅-rGO-GCE with satisfactory recoveries (101~106%) is successfully realized for the detection of tryptophan in human serum. The synthesis of Ta₂O₅-rGO-GCE in this article could provide a slight view for the synthesis of other electrochemical catalytic systems in detection of trace substance in human serum.

Electrochemical Sensor for Rapid and Sensitive Detection of Tryptophan by a Cu2O Nanoparticles-Coated Reduced Graphene Oxide Nanocomposite

In this paper, a nanocomposite of cuprous oxide and electrochemically reduced graphene oxide (Cu₂O–ERGO) was prepared by a simple and low-cost method; hereby, a new method for the electrochemical determination of tryptophan (Trp) by this composite modified glassy carbon electrode (GCE) is proposed. The prepared materials and modified electrodes were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and cyclic voltammetry (CV). The results showed that Cu₂O–ERGO/GCE had good electrocatalytic activity for Trp. The effects of supporting electrolyte, scanning rate, accumulation potential, and accumulation time on the determination of Trp were studied. Under the optimum experimental conditions, Trp was quantitatively analyzed by square-wave voltammetry (SWV). The oxidation peak current of Trp had a good linear relationship with its concentration in the range of 0.02–20 μM, and the detection limit was 0.01 μM (S/N = 3). In addition, the modified electrode has high sensitivity, good repeatability, and long-term stability. Finally, the proposed method has been successfully applied in the determination of Trp concentration in practical samples.

CVD graphene as an electrochemical sensing platform for simultaneous detection of biomolecules

The development of electrochemical biosensors for the simultaneous detection of ascorbic acid (AA), dopamine (DA), uric acid (UA), tryptophan (Trp), and nitrite ([Formula: see text]) in human serum is reported in this work. Free-standing graphene nanosheets were fabricated on Ta wire using the chemical vapor deposition (CVD) method. CVD graphene, which here served as a sensing platform, provided a highly sensitive and selective option, with detection limits of AA, DA, UA, Trp, and [Formula: see text] of 1.58, 0.06, 0.09, 0.10, and 6.45 μM (S/N = 3), respectively. The high selectivity of the electrode is here explained by a relationship between the bandgap energy of analyte and the Fermi level of graphene. The high sensitivity in the oxidation current was determined by analyzing the influence of the high surface area and chemical structure of free-standing graphene nanosheets on analyte adsorption capacity. This finding strongly indicates that the CVD graphene electrode can be used as a biosensor to detect five analytes in human serum.

A miniaturized electrochemical toxicity biosensor based on graphene oxide quantum dots/carboxylated carbon nanotubes for assessment of priority pollutants

The study presented a sensitive and miniaturized cell-based electrochemical biosensor to assess the toxicity of priority pollutants in the aquatic environment. Human hepatoma (HepG2) cells were used as the biological recognition agent to measure the changes of electrochemical signals and reflect the cell viability. The graphene oxide quantum dots/carboxylated carbon nanotubes hybrid was developed in a facile and green way. Based on the hybrid composite modified pencil graphite electrode, the cell culture and detection vessel was miniaturized to a 96-well plate instead of the traditional culture dish. In addition, three sensitive electrochemical signals attributed to guanine/xanthine, adenine, and hypoxanthine were detected simultaneously. The biosensor was used to evaluate the toxicity of six priority pollutants, including Cd, Hg, Pb, 2,4-dinitrophenol, 2,4,6-trichlorophenol, and pentachlorophenol. The 24h IC₅₀ values obtained by the electrochemical biosensor were lower than those of conventional MTT assay, suggesting the enhanced sensitivity of the electrochemical assay towards heavy metals and phenols. This platform enables the label-free and sensitive detection of cell physiological status with multi-parameters and constitutes a promising approach for toxicity detection of pollutants. It makes possible for automatical and high-throughput analysis on nucleotide catabolism, which may be critical for life science and toxicology.

Development of a disposable and low-cost electrochemical sensor for dopamine detection based on poly(pyrrole-3-carboxylic acid)-modified electrochemically over-oxidized pencil graphite electrode

In this study, preparation of a single-use electrochemical sensor for the selective and sensitive determination of dopamine (DOP) was investigated by electrochemical polymerization of pyrrole-3-carboxylic acid on electrochemically over-oxidized pencil graphite electrode (p(P3CA)/EOPGE). Cyclic voltammetry measurements of Fe(CN)₆^4-/3- indicated that the electrochemically over-oxidized PGE (EOPGE) showed superior electron transfer characteristics according to bare PGE. The ionized carboxyl groups found in the structure of poly(pyrrole-3-carboxylic acid) (p(P3CA)) showed high affinity towards positively charged DOP. The combination of the advantages of EOPGE and p(P3CA) in p(P3CA)/EOPGE led to a synergistic effect on the electrochemical oxidation of DOP. The effects of experimental variables on the voltammetric performance of the p(P3CA)/EOPGE were examined by preparing the electrodes at different conditions. The p(P3CA)/EOPGE showed high selectivity towards DOP by discriminating its oxidation potential from the common interfering substances such as ascorbic and uric acids. The p(P3CA)/EOPGE showed linear responses in the electrochemical oxidation of DOP between the concentration values of 0.025µM and 7.5µM. Detection limit was determined as 0.0025µM according to signal to noise ratio (S/N: 3). Analytical application of p(P3CA)/EOPGE was successfully tested in the determination of DOP in blood serum and pharmaceutical samples.

A tryptophan assay based on the glassy carbon electrode modified with a nano-sized tryptophan-imprinted polymer and multi-walled carbon nanotubes

A new nanostructured molecularly imprinted polymer (nano-MIP), possessing tryptophan compatible cavities, was synthesized using 1-(allyloxy)-4-nitrobenzene as an innovative functional monomer.

Nanomolar simultaneous determination of tryptophan and melatonin by a new ionic liquid carbon paste electrode modified with SnO2-Co3O4@rGO nanocomposite

This work describes the development of a new sensor for simultaneous determination of tryptophan and melatonin. The proposed sensor was an ionic liquid carbon paste electrode modified with reduced graphene oxides decorated with SnO₂-Co₃O₄ nanoparticles. The voltammetric oxidation of the analytes by the proposed sensor confirmed that the electrooxidation process undergoes a two-electron/one-proton reaction for melatonin and a two-electron/two-proton reaction for tryptophan in diffusion-controlled processes. Moreover, based on the excellent electrochemical properties of the modified electrode, a sensitive voltammetric method was used for individual and simultaneous determination of melatonin and tryptophan in the aqueous solutions. Under the optimized experimental conditions, a linear response obtained in the range of 0.02 to 6.00μmolL^-1 with detection limits of 4.1 and 3.2nmolL^-1 for melatonin and tryptophan, respectively. The prepared sensor possessed accurate and rapid response toward melatonin and tryptophan with a good sensitivity, selectivity, stability, and repeatability. Finally, the applicability of the proposed sensor was verified by evaluation of melatonin and tryptophan in various real samples including human serum and tablet samples.

An electrochemical sensor for determination of tryptophan in the presence of DA based on poly(l-methionine)/graphene modified electrode

Electrocatalytic oxidation of l-Trp at poly(l-methionine)/graphene composite film modified glassy carbon electrode (PLME/GR/GCE) were investigated by differential pulse voltammetry.

Reusable potentiometric screen-printed sensor and label-free aptasensor with pseudo-reference electrode for determination of tryptophan in the presence of tyrosine

Analysis of L-tryptophan (Trp) in biological samples has great importance for biomedical studies. Amino acid tyrosine (Tyr) that usually coexist with Trp in biological fluids can significantly interfere with reliable determination of Trp. In the current study, we demonstrate the development of two ultra-sensitive electrochemical sensor and label-free aptasensor for selective analysis of Trp in biological samples (i.e., cow's milk and human plasma, saliva and urine samples). In addition, without using AgCl/KCl, an Ag pseudo-reference screen printed electrode (Ag-PR-SPE) was exploited as a reference electrode. To prepare the engineered Trp sensor/aptasensor, a gold SPE was first modified with multiwall carbon nanotube (MWCNT-AuSPE) and then armed with Trp aptamer molecules (Apt-MWCNT-AuSPE). The prepared sensors were characterized using constant current-potentiometric stripping analysis (CC-PSA) and electrochemical impedance spectroscopy (EIS). The MWCNT-AuSPE and Apt-MWCNT-AuSPE were compared with respect to the linear detection range, limit of detection (LOD), accuracy, precision, repeatability. MWCNT-AuSPE and Apt-MWCNT-AuSPE demonstrate fast near-Nernstian response for PSA of Trp over the concentration ranging from 1.0 × 10(-9) to 2.0 × 10(-4) mol L(-1) and 1.0 × 10(-11) to 1.0 × 10(-4) mol L(-1) with detection limits of 3.6 × 10(-10) mol L(-1) and 4.9 × 10(-12) mol L(-1), respectively. Common interfering species present in the biological fluids (i.e., tyrosine, uric acid, ascorbic acid) showed no effects on the determination of Trp using CC-PSA. MWCNT-AuSPE and Apt-MWCNT-AuSPE represented well reproducibility and great precision with relative standard deviation (RSD) of 2.9% and 5.3% respectively. In comparison with the MWCNT-AuSPE, Apt-MWCNT-AuSPE provided higher sensitivity, selectivity and accuracy of Trp detection in real samples. Based on these findings, we propose the developed Apt-MWCNT-AuSPE as a simple detection method for analysis of Trp in biological samples.

A novel strategy to analyze L-tryptophan through allosteric Trp repressor based on rolling circle amplification

Rolling circle amplification (RCA) has been considered as a powerful tool for nucleic acids detection. Here, a novel repressor-RCA-based method for L-tryptophan (L-Trp) detection was developed. This method utilizes the specific interaction between the RCA circular template and the Trp repressor protein (TrpR) involved in trp operon of Escherichia coli (E. coli). In the absence of L-Trp, the TrpR protein could not bind to the RCA template, and the RCA process can be continued. When L-Trp is present, the activated TrpR will bind to the operon sequence on the RCA template and inhibit the RCA reaction. Thus, the concentration of L-Trp is correlated directly with the fluorescent RCA signals. We succeeded in detecting L-Trp in a single step in simple homogeneous reaction system. The detection limit was estimated to be 0.77 μM (S/N=3) with good linearity. The method can unambiguously distinguish L-Trp from other 19 standard amino acids and L-Trp analogs. This strategy is also promising for detecting many small molecules such as other amino acids and carbohydrates.

The application of l-tryptophan functionalized graphene-supported platinum nanoparticles for chiral recognition of DOPA enantiomers

A carbon-based nanocomposite surface was prepared to develop a simple strategy for electrochemical chiral analysis.

Supramolecular p-sulfonated calix[4,6,8]arene for tryptophan detection

Numerous techniques have focused on the ability of p-sulfonated calix[n]arene to form complexes with tryptophan. Scanning electron microscopy and Fourier transform infrared spectroscopy were utilized to study the organization and molecular structure of different layers of the electrode surface. Scanning electron microscopy results showed that SC4A displayed a cubic structure whereas SC6A and SC8A displayed dendrite structures. The electrochemical properties and potential complex formation between SCnA and tryptophan were characterized by cyclic voltammetry and electrochemical impedance spectroscopy. Cyclic voltammetry experiments showed that the gold electrode was successfully functionalized by self-assembled cysteamine and SC4A. Electrochemical impedance spectroscopy results showed the observation of the tryptophan–SCnA interaction and indicated that SC4A had the highest sensitivity to tryptophan and allowed 2.04 μg L−1 tryptophan to be detected. Electrochemical impedance spectroscopy analysis and molecular modeling calculation confirmed that SC4A has higher tryptophan sensitivity than SC6A and SC8A.