Tumor is one of the growing global health care problems, with more than one million new diagnoses arose each year. It is a polygenic disease with genetic changes caused by disorders of related genes, with high morbidity and mortality [1]. At present, tissue biopsy is the most commonly used means of diagnosing cancer in clinical practice and is widely used to characterize tumor types. However, tissue biopsy is an invasive sampling method, and during the sampling process, it is likely to cause cancer misidentification due to the heterogeneity of the tissue itself or the failure to sample at the location of the lesion. In addition, real-time monitoring of tumors cannot be carried out due to the fact that biopsies themselves cannot be repeatedly sampled. Therefore, tissue biopsy technology still faces more challenges in terms of operating cost, diagnostic accuracy, and efficient treatment of cancer [2]. In order to overcome the above problems, some researchers have developed a new type of non-invasive test, called liquid biopsy. The assay is based on analysis of circulating tumor DNA (ctDNA), circulating tumor cells and other tumor-derived substances in plasma. They are derived from tumor tissue and exist in the blood, which can reflect information such as tumor development and drug resistance, and guide individualized precision treatment [3, 4]. Detection of ctDNA in liquid biopsy can reveal a variety of epigenetic alterations such as single nucleotide variation, copy number variation, methylation, and so on. It carries the information code of tumor tissue, is a good tumor detection biomarker, has high accuracy and safety in tumor detection, so it has attracted much attention and has been widely studied [5, 6].
In recent years, molecular biology techniques have developed rapidly, promoting the research and detection of ctDNA [7]. At present, the main types of methods for detecting ctDNA are PCR-based methods, including real-time fluorescent quantitative PCR (qPCR), digital PCR (dPCR), amplification refractory mutation system PCR (ARMS-PCR). The other is based on next-generation sequencing (NGS) method. The shortcomings of PCR-based techniques and NGS method are that they requires high cost and are time-consuming. In addition to this, several new technologies, especially nanotechnology-based biosensors, have great potential in detecting ctDNA, providing a simple and high-performance general strategy for low-cost clinical applications, mainly including electrochemical biosensors, fluorescent biosensors, surface enhanced Raman scattering biosensors, colorimetric/resonance light scattering biosensors, etc [8]. Particularly, electrochemical biosensing methods have received a lot of attention because of their high sensitivity, wide determination range, low cost, quick response, easy to miniaturize and carry. To improve the detection competence, multiple functional nanomaterials have been adopted for the building of electrochemical identification platforms, such as metal nanomaterials, nanostructured metal compounds, carbonaceous nanomaterials, transition metal sulfides nanosheets, and so forth [9–13].
As an emerging two-dimensional material, black phosphorus has a unique honeycomb structure, which can be prepared by the phase conversion process of white phosphorus or red phosphorus in a high temperature and high pressure environment, and is one of the allotropes of elemental phosphorus [14, 15]. The two-dimensional black phosphorus nanosheets (BPNSs) obtained after stripping of the bulk black phosphorus have layer-like structures similar to graphene, and the phosphorus atoms in the layer are connected by SP3 hybrid covalent bonds, the difference is that graphene has only one bond, and there are two P-P bonds of different lengths in the black phosphorus sheet layer structure. The unique sheet structure of two-dimensional black phosphorus has high carrier mobility and adjustable band gap, which shows great application potential in many fields such as energy storage, catalysis, optoelectronic devices, biomedical and sensing [16–19]. In particular, due to its high specific surface area and fast electron transfer speed, two-dimensional black phosphorus has shown great application prospects in the fields of electrochemical sensing. However, due to the chemical instability of two-dimensional black phosphorus, its separate application in the field of electrochemical sensing has been less studied. Two-dimensional black phosphorus are commonly combined with other functional materials to effectively improve the chemical stability and obtain excellent electrochemical sensing performance [20–23]. For example, Kim et al. developed a sensitive and rapid bovine viral diarrhea virus detection system using few layer black phosphorus modified with gold nanoparticle [20]. Jia et al. employed Ni2P nanocrystallines anchored on BPNSs for highly selective and sensitive non-enzymatic electrochemical detection of L-cysteine over glutathione [21].
Xanthurenic acid (XTA) is one of the metabolites of tryptophan, and when vitamin B6 is deficient, the content of XTA in the urine increases. As a new type of conductive polymer, poly-xanthurenic acid (PXTA), has been utilized to identify the bioactive substances on account of its non-toxicity and splendid redox activity [24, 25]. In this work, we report for the first time that XTA was electropolymerized on the pre-prepared BPNSs surface to acquire a novel polymer nanocomposite, which provided a favourable platform for self-signal electrochemical identification of ctDNA in peripheral blood. The obtained PXTA/BPNSs nanocomposite revealed wonderful internal electrochemical activities to denote the process of DNA immobilization and hybridization. As far as we know, such strategy has not been previously proposed for the measurement of ctDNA in liquid biopsy.