Preparation of voltammetric biosensor for tryptophan using multi-walled carbon nanotubes

A New Electrochemical Method to Determine Tryptophan in Fruit Juices: Development and Validation

Tryptophan (Trp) is an essential amino acid usually found in fruit juices. Its determination is necessary for food companies because of its relation to human health. In this work, a new electrochemical method based on sonogel–carbon electrodes (SNGCEs) was developed and validated using an ultra performance liquid chromatography (UPLC) method as a reference method for the determination of Trp in fruit juices. Cyclic voltammetry (CV), chronoamperometry, and differential pulse voltammetry (DPV) techniques were applied to investigate the oxidation of Trp on a previously polarized SNGCE surface in a Britton–Robinson (BR) buffer solution at pH 3.6. The operating conditions for electroanalysis were optimized using a Box–Behnken design (BBD), obtaining an oxidation peak for Trp at 0.749 V. The linear range for this method was from 0.1 to 5 mg/L. The intraday and interday precision, expressed as a relative standard deviation (RSD), were 3.1% and 2.7%, respectively. The average recovery was 99.01%, and the limit of detection and quantitation were 0.33 and 1.09 mg/L, respectively. Therefore, from the quality analytical parameters obtained, it can be concluded that the new electrochemical method can be successfully used for the routine analysis of Trp in fruit juices. As far as we are concerned, this is the first time that a methodology for Trp determination was performed in this kind of real food matrices.

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.

An aniline trimer-based multifunctional sensor for colorimetric Fe3+, Cu2+ and Ag+ detection, and its complex for fluorescent sensing of L-tryptophan

The detection of metal ions and amino acids by the aniline oligomer-based receptor has not been reported yet, to the best of our knowledge. In this study, an efficient multifunctional cation-amino acid sensor (CAS) with aniline moiety and chiral thiourea binding site was synthesized by the reaction of aniline trimer and (S)-(+)-1-phenyl ethyl isothiocyanate. CAS can sense Fe³⁺, Cu²⁺, Ag⁺ ions, and L-tryptophan. These results can be recognized by the naked eye. The appropriate pH range for the quantitative analysis of Fe³⁺, Cu^2+, and Ag⁺ by CAS in DMSO/water (30 vol% water) was evaluated. The interaction between CCS and metal ions was analyzed by ¹H NMR titration. The detection limits of CAS for the Cu²⁺, Ag^+, and Fe³⁺ were 0.214, 0.099, and 0.147 μM, respectively. Moreover, the CASCu²⁺ complex can act as a turn-on fluorescence sensor for L-tryptophan. On the contrary, there is no response upon the addition of other amino acids, such as L-histidine, L-proline, L-phenylalanine, L-threonine, L-methionine, L-tyrosine, and L-cystine to CASCu²⁺ complex.

Novel microfluidic graphene oxide-protein amperometric biosensor for detecting sulfur compounds

Sulfur compounds are essential for many industries and organisms; however, they cause serious respiratory problems in human beings. Therefore, determination of sulfur concentration is of paramount importance. The research approach in the field of detecting contaminants has led to smaller systems that provide faster and more effective ways for diagnosis purposes. In this study, a novel portable amperometric graphene oxide-protein biosensor platform is investigated. The main characteristic of this structure is the implementation of a microfluidic configuration. With albumin metalloprotein as the biorecognition element, graphene oxide was synthesized and characterized by transmission electron microscopy and Fourier-transform infrared spectroscopy (FTIR). Albumin protein was stabilized on the surface of graphene oxide by the application of the N-(3-dimethylamionpropyl)-N-ethylcarbodiimide hydrochloride/N-hydroxysuccinimide method. The stabilization was confirmed by FTIR and electrochemistry analyses. The calibration curve of sulfur concentration was determined. When the graphene oxide-protein complex was stabilized by nephion on the surface of the microfluidic system, the response time reduced to 50 Sec, which is a relatively faster response among the similar studies and validated the significant effect of the microfluidic system. The nanosystem had an optimized pH of 7.4 and exhibited high sensitivity in determining sulfide. The results confirm that the portable graphene oxide-protein nanosystem has a fast and accurate response in detecting sulfide.

A new diclofenac molecularly imprinted electrochemical sensor based upon a polyaniline/reduced graphene oxide nano-composite

A diclofenac (DCF)-imprinted polymer, composed of polyaniline, reduced graphene oxide (rGO) and triphenylamine, as cross linker, was synthetized. This composite was identified by using SEM and FT-IR techniques. The prepared DCF-imprinted polymer (MIP) was used for modification of carbon paste electrodes (CPEs) to fabricate a selective DCF electrochemical sensor. Electrochemical behavior of DCF on the investigated sensor and the optimization of the parameters affecting the DCF determination were screened by cyclic voltammetry (CV). The cyclic voltammogram of DCF showed an anodic peak current at about 0.5 V (vs. SCE). The calibration curve for DCF determination was obtained by applying the investigated sensor as working electrode in differential pulse voltammetry (DPV). A linear increase in the anodic peak current was observed in the range 5-80 mg L^-1 of DCF. The corresponding limit of detection was calculated to be 1.1 mg L^-1. The relative standard deviations of the inter- and intra-day analysis of DCF presented by the method were found to be as 2.43% and 2.47%, respectively. The selectivity of the investigated sensor was evaluated by its use for determination of DCF in the binary solutions containing DCF/glucose, DCF/urea and DCF/ascorbic acid. It was shown that the fabricated electrode can be successfully used for analysis of DCF in pharmaceutical and urine samples.