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Much effort was made in the literature to put pencil-drawing as an attractive way to fabricate electrochemical sensors. The pencil drawing is one of the easiest, cost-effective, unique single-step methods for fabricating paper-based devices without necessitating any expensive instrumentation by merely transferring the carbon by exfoliation abrasion on paper. Additionally, much has to be done to improve the quality of this transfer carbon to achieve a suitable sensing platform for electrochemical measurements. Hence, this review article aims to bring the new approaches developed in the last five years to circumvent some fabrication problems and enhance this “carbonization” step for electrochemistry detection using paper-based sensors. We will start the discussion on the carbon materials moving to the substrate and finally discuss the strategies to produce the conductive tracks and enhance their properties for sensing applications.

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Forensic studies are extremally important to investigate suspected adulterations of consumable products, such as Viagra®. This report describes the determination of sildenafil citrate (SC) in commercial and adulterated tablets based on square-wave voltammetry (SWV) measurements using disposable stencil-printed carbon electrodes. The conductive ink used for the manufacture of integrated electrodes was produced by combining graphite powder and glass varnish. To promote a reusable strategy for limiting the geometric area of the electrodes, a 3D-printed holder was constructed. Detailed morphological and electrochemical characterization studies revealed well-defined graphite flakes incorporated on the polymeric substrate and a faster heterogeneous electron-transfer rate constant (Ks = 1.3 × 10–3 cm s–1). Based on the analytical performance, a linear behavior was observed in a SC concentration range from 1 to 20 µmol L–1 with limit of detection equal to 0.2 µmol L–1. The selectivity of the proposed method was evaluated and the presence of potentially interfering compounds like phosphate, lactose, paracetamol and tadalafil and no difference higher than 15% was observed. The analysis of SC was performed in commercial and seized tablets and the achieved values were 50 ± 1 mg for Viagra® tablet, 54 ± 1 mg for generic formulations 38 ± 1 mg for seized tablet. In addition, the proposed method offered satisfactory accuracy (98.2 – 102.0%) no noticeable matrix effect. Lastly, considering the achieved results, the use of stencil-printed carbon electrodes and SWV has demonstrated to be a powerful and robust analytical tool for forensic investigations.