Recent Advances in Electrochemical and Optical Sensors for Detecting Tryptophan and Melatonin

Copper(II) phthalocyanine as an electrocatalytic electrode for cathodic detection of urinary tryptophan

Herein, we introduce a novel method for tryptophan detection via a reduction reaction facilitated by its interaction with a copper(II) phthalocyanine (CuPc) electrocatalytic electrode. This method addresses challenges associated with the susceptibility of the oxidation response to interference from various species when measuring tryptophan in bodily fluids. The reduction currents exhibit a linear increase with tryptophan concentrations in two ranges: 0.0013-0.10 mM and 0.10-1.20 mM, with the sensitivities of 14.7 ± 0.5 μA mM-1 and 3.5 ± 0.1 μA mM-1, respectively. The limit of detection (LOD, 3SB/m) is determined to be 0.39 μM. The sensor exhibits excellent reproducibility, with the relative standard deviation of <5%. Application of the sensor to authentic urine samples yields a % recovery of 101 ± 4%.

Highly sensitive detection of tryptophan based on Schiff base reaction and surface-enhanced Raman spectroscopy

In this study, a simple, rapid and sensitive method combining surface-enhanced Raman spectroscopy and Schiff base reaction was developed for the detection of tryptophan. This method does not require product separation to obtain a significant Raman signal of the derivatized product, and the derivatization reaction can be controlled by experimental parameters such as reaction temperature, time, concentration of derivatization reagent and concentration of sodium nitrite. The characteristic peak of the derivative of tryptophan (1620 cm-1) was selected for quantitative analysis, and the intensity of the characteristic Raman spectrum peak showed a linear relationship with the concentration of tryptophan (10-8-10-4 mol/L) in the range of with a correlation coefficient R2 of 0.9922. This assay combines surface-enhanced Raman spectroscopy and Schiff base reaction, which is characterized by high sensitivity and easy operation, and has good application prospects in the detection of tryptophan in food.

Development of an impedimetric sensor for susceptible detection of melatonin at picomolar concentrations in diverse pharmaceutical and human specimens

Our investigation aimed to create and manufacture an electrochemical impedance sensor with the purpose of improving the detection efficiency of melatonin (ME). To achieve this objective, we employed gold nanoparticles coated on polydopamine formed in glassy carbon electrodes (AuNPs/PDA/GCE) as a means to enhance the sensor's capabilities. A novel approach employing the signal-off strategy and electrochemical impedance spectroscopy (EIS) technique was utilized to determine ME. When the AuNPs/PDA/GCE electrode was immersed in a buffered solution containing ME, and the oxidation current of AuNPs was recorded, it was observed that the oxidation current of AuNPs decreased upon the introduction of ME molecules. The decrease in electrical current can be ascribed to the inhibitory impact of ME molecule adsorption on the electrode surface with applying -0.2 V for 150 s in acetate buffer solution (ABS) (pH, 5) through various mechanisms, which hinders the electron transfer process crucial for AuNPs oxidation. Consequently, by utilizing EIS, various concentrations of ME were quantified spanning from 1 to 18 pM. Moreover, the ME sensor achieved an impressive detection limit of 0.32 pM, indicating its remarkable sensitivity in detecting low concentrations of ME. Importantly, these novel sensors demonstrated exceptional attributes in terms of sensitivity, specificity, stability, and repeatability. The outstanding performance of these sensors, coupled with their desirable attributes, establishes their considerable potential for a wide range of practical applications. These applications encompass various fields such as clinical diagnostics, pharmaceutical analysis, environmental monitoring, and industrial quality control, where accurate and sensitive detection of ME is of utmost importance.

Measuring tryptophan dynamics using fast scan cyclic voltammetry at carbon fiber microelectrodes with improved sensitivity and selectivity

Despite the fact that tryptophan (Trp) is an essential amino acid that humans typically obtain through diet, there are several interesting tryptophan dynamics at play in the body. Quantifying and understanding these dynamics are crucial in studies of depression, autism spectrum disorder, and other disorders that involve neurotransmitters directly synthesized from tryptophan. Here we detail the optimization of waveform parameters in fast scan cyclic voltammetry at carbon fiber microelectrodes to yield four-fold higher sensitivity and six-fold higher selectivity compared to previously reported methods. We demonstrate the utility of our method in measuring (1) exogenous Trp dynamics from administration of Trp to PC-12 cells with and without overexpression of tryptophan hydroxylase-2 and (2) endogenous Trp dynamics in pinealocyte cultures with and without stimulation via norepinephrine. We observed interesting differences in Trp dynamics in both model systems, which demonstrate that our method is indeed sensitive to Trp dynamics in different applications.

Methylene blue molecularly imprinted polymer for melatonin determination in urine and saliva samples

A highly sensitive and rapid electrochemical sensor was developed for detecting melatonin using a molecularly imprinted polymer (MIP) with methylene blue as the functional monomer and melatonin as the template. The MIP was synthesized via a simple electropolymerization process that did not require an initiating reagent. The sensor demonstrated good selectivity for melatonin against common interferences such as lactate, cytosine, cytidine, urea, ascorbic acid, creatine, creatinine, serotonin, and tryptophan. Melatonin detection was achieved at a potential of 0.60 V vs. Ag/AgCl with a sensitivity of 138.8 ± 4.7 µA µM‒1 in the linear range 0.097 - 200 µM and a limit of detection of 29 nM (3SB/m). The sensor exhibited excellent reproducibility and repeatability for both within (intra) and between (inter) electrodes (%RSD < 3% for n = 3). The sensor was applied to authentic urine and saliva samples with recoveries of 103 ± 1% and 102 ± 1%, respectively.

Copper oxide nanoflowers/poly-l-glutamic acid modified advanced electrochemical sensor for selective detection of l-tryptophan in real samples

The main objective of this research work is to develop a low-cost sensor to detect l-tryptophan (L-tryp) in real sample medium based on a modified glassy carbon electrode. For this, copper oxide nanoflowers (CuONFs) and poly-l-glutamic acid (PGA) were used to modify GCE. The prepared NFs and PGA coated electrode was characterized using field emission scanning electron microscope (FE-SEM) with energy dispersive X-ray (EDX) and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. Furthermore, the electrochemical activity was performed by cyclic voltammetry (CV), differential pulse voltammetry (DPV) and electrochemical impedance spectroscopy (EIS). The modified electrode showed excellent electro-catalytic activity towards L-tryp detection in PBS solution at neutral pH 7.0. Based on the physiological pH condition, the proposed electrochemical sensor can detect L-tryp concentration with a linear range of 1.0 × 10^-4-8.0 × 10^-8 molL^-1 with having a detection limit of 5.0 × 10^-8 molL^-1 and sensitivity of 0.6μA/μMcm². The selectivity of L-tryp was tested with a mixture of salt and uric acid solution at the above conditions. Finally, this strategy demonstrated excellent recovery value in real sample analysis like milk and urine.

Melatonin in different food samples: Recent update on distribution, bioactivities, pretreatment and analysis techniques

Melatonin (MLT) plays a significant role on maintaining the basic physiological functions and regulating various metabolic processes in plentiful organisms. Recent years have witnessed an increase in MLT's share in global market with its affluent functions. However, the worrisome quality issues and inappropriate or excessive application of MLT take place inevitably. In addition, its photosensitive properties, oxidation, complex substrate concentration and trace levels leave exact detection of MLT doubly difficult. Therefore, it is essential to exploit precise, sensitive and stable extraction and detection methods to resolve above questions. In this study, we reviewed the distribution and bioactivities of MLT and conducted a comprehensive overview of the developments of pretreatment and analysis methods for MLT in food samples since 2010. Commonly used pretreatment methods for MLT include not only traditional techniques, but also novel ones, such as solid-phase extraction, QuEChERS, microextraction by packed sorbent, solid phase microextraction, liquid phase microextraction, and so on. Analysis methods include liquid chromatography coupled with different detectors, GC methods, capillary electrophoresis, sensors, and so on. The advantages and disadvantages of different techniques have been compared and the development tendency was prospected.