Electrochemical sensors now play an important role in analysis and detection of nucleic acids. In this work, we present a novel double-signal technique for electrochemically measuring the sequence and length of the d(CAG)n repeat. The double-signal technique used an electrochemical molecular beacon (a hairpin DNA labeled with ferrocene), which was directly modified on the surface of a gold electrode, while a reporter probe (a DNA sequence labeled with horseradish peroxidase) was hybridized to the target DNA. First a simple single-signal sensor was characterized in which d(CAG)n repeats were detected using a short reporter DNA strand labeled with horseradish peroxidase. To obtain a reliable signal that was dependent on repeat number, a double-signal biosensor was created in which the single strand capture DNA in single-signal sensor was replaced by an electrochemical molecular beacon labeled with ferrocene. When the hairpin DNA hybridized to the target–reporter DNA complex, it opened, resulting in a decreased ferrocene current. Both electrochemical biosensors exhibited high selectivity and sensitivity with low detection limits of 0.21 and 0.15 pM, respectively, for the detection of d(CAG)n repeats. The double-signal sensor was more accurate for the determination of repeat length, which was measured from the ratio of signals for HRP and ferrocene (H/F). A linear relationship was found between H/F and the number of repeats (n), H/F = 0.1398n + 9.89788, with a correlation coefficient of 0.974. Only 10 nM of target DNA was required for measurements based on the value of H/F in the double-signal technique. These results indicated that this new double-signal electrochemical sensor provided a reliable method for the analysis of CAG trinucleotide repeats.Keywords: CAG trinucleotide repeat; double-signal technique; electrochemical sensors; ferrocene; HRP;

A novel bifunctional nanoprobe was designed and used in an electrochemical sensor to rapidly detect CGG trinucleotide repeats. The bifunctional nanoprobe was synthesized using carboxyl-functionalized Fe3O4 magnetic nanoparticles as carriers of nucleic acids recognition molecule and electron mediator-ferrocene derivatives. Both the small molecules with the terminal amino group were immobilized on the surface of the magnetic nanoparticles via an amide bond, which resulted in double functions of recognition and electrochemical activity. An electrochemical sensor based on the new bifunctional nanoprobe was established for CGG trinucleotide repeat detection. After the target DNA was captured by the sensor, the recognition molecule on the surface of the bifunctional nanoprobe could interact with the G base of the CGG trinucleotide repeat selectively. This allowed the bifunctional nanoprobes to be enriched on the modified electrode surface, which resulted in a remarkable electrochemical signal. CGG trinucleotide repeat can be easily discriminated from other trinucleotide repeats by the sensor. Furthermore, these results demonstrated that the new sensor exhibited an excellent selectivity to the Fragile X syndrome disease biomarker CGG repeat DNA. The proposed method is simple and rapid, the target DNA is label-free and has excellent repeatability for the detection of the CGG trinucleotide repeat because of the double function of the designed nanoprobe. This sensing strategy will provide great promise for the early diagnosis of neurodegenerative diseases associated with trinucleotide repeats.

New ferrocene naphthyridine derivatives were successfully synthesized. The di-substituted ferrocene derivative FecDN behaves as a selective visual chemosensor for mercury ions, exhibiting a rapid and clearly visible precipitation with good selectivity (precipitation) and sensitivity (0.01 μM). The interaction of the ferrocene derivatives and metal ions was investigated by ultraviolet–visible spectroscopy (UV–vis), electrochemical methods, 1H NMR, as well as infrared (IR), and energy dispersive X-ray spectroscopy (EDX). The application of FecDN in the detection and removal of mercury ions is very simple, rapid, and intuitive.Graphical abstract