Electrochemical Detection of Tryptophan Metabolites via Kynurenine Pathway by Using Nanocarbon Films

Abstract

We studied nanocarbon film electrodes with the aim of detecting tryptophan metabolites via the kynurenine pathway. The nanocarbon films were formed by using unbalanced magnetron sputtering, and they exhibited superior electrode properties including a wide potential window and a low background current as a result of the sp 3 -containing structure and ultraflat surface. These properties allowed us to detect certain tryptophan metabolites such as kynurenic acid (KYNA), which has a relatively high oxidation potential. We also investigated the effect of the sp 2 /sp 3 ratio of the nanocarbon film as regards the electrode activity in relation to target molecules. We found that the sp 2 /sp 3 ratio played important roles in both widening the potential window and obtaining superior electrode performance for the metabolites. The nanocarbon film with a high sp 3 content was beneficial as regards the electrode performance with respect to the detection limit and sensitivity. Compared with conventional carbon-based electrodes, the nanocarbon film electrode with a high sp 3 content exhibited higher electrode activity against KYNA while maintaining a low background current. Computational experiments revealed that the theoretical oxidation potential (E ox ) value for some targets coincided with that obtained in electrochemical experiments using our nanocarbon film electrode.

Electrochemical Determination of Kynurenine Pathway Metabolites—Challenges and Perspectives

“…Owing to high E ox , electrochemical determination of Kyna can be also problematic. Such an assumption can be supported by the data obtained by Kato et al, who calculated the theoretical electrochemical E ox of QA and Kyna at nanocarbon film electrode to be equal 2.36 V and 1.40 V, respectively (50 mM phosphate buffer pH 7.0 as the supporting electrolyte) [40]. To note, in DPV at GCE in typical supporting electrolytes for potentials >+1.2 V, the background current increases and baseline drift is observed.…”

“…A boron-doped diamond electrode (BDDE) exhibits wider electrochemical windows for aqueous media [90] and a lower background current magnitude than GCE [42], making BDDE more suitable for Kyna detection. Nanocarbon film electrode with a high sp 3 content exhibits a higher S/N ratio against Kyna compared with GCE or BDDE, but estimated LOD in phosphate buffer pH 7.0 (0.4 µM) [40] is not sufficient to detect this metabolite in many biological matrices (Table 2). This electrode can also be applied for quantification of other KP metabolites (with QA being an exception) [40], but LODs are not impressive either (Table 1).…”

“…Nanocarbon film electrode with a high sp 3 content exhibits a higher S/N ratio against Kyna compared with GCE or BDDE, but estimated LOD in phosphate buffer pH 7.0 (0.4 µM) [40] is not sufficient to detect this metabolite in many biological matrices (Table 2). This electrode can also be applied for quantification of other KP metabolites (with QA being an exception) [40], but LODs are not impressive either (Table 1). All the above suggest that new electrode materials with high electrocatalytic activity or other solutions should be exploited to allow direct electrochemical detection of QA and Kyna at low physiologically relevant levels.…”

Disposable Molecularly Imprinted Polymer-Modified Screen-Printed Electrodes for Rapid Electrochemical Detection of l-Kynurenine in Human Urine

“…Other techniques, such as gas chromatography, immunoassays, and, more recently, sensors, have also been used. Papers that describe sensors technique for l -Kyn detection without using chromatographic processes have been published starting from 2017 and show various advantages, especially in terms of high sensitivity and the potentiality to reduce the size of the system to obtain a portable device [ 12 , 13 , 14 , 15 , 16 , 17 ]. Among them, electrochemical sensors have been demonstrating interesting successes and/or potentiality, such as an elevated sensitivity, rapid response, reduced size of apparatus, simple installation, easy sample preparation, and in situ analysis.…”

Establishing a quantitative fluorescence assay for the rapid detection of kynurenine in urine

“…There has been some research in electroanalytical quantification of kynurenine which has produced sensitive chemosensors and simplified the sample preparation and processing. [20][21][22] However, whilst electroanalytical methods can produce sensitive measurements in point of care formats such as glucose sensing, there is still some challenges to address before mass clinical testing. 23 Ideally, a useful kynurenine sensor would have highthroughput capabilities allowing for rapid results from multiple patient samples.…”