3D Pen with cotton cartridge for on-site detection of Malachite Green Dye coating on Okra

This study focusses on the fabrication and application of a frugal ‘3D Bio-Pen cassette’ embedded with cotton strand-based ink cartridge, which is capable of performing on-the-spot detection of malachite green, a banned chemical used to extend the shelf-life of vegetables and products of aquaculture. The developed 3D Bio-Pen achieves the detection of dye on the surface of okra samples by two label-free approaches, firstly by utilizing the bleaching action of citric acid ink at dye concentrations as low as 0.1 mg/mL and secondly the frictional resistance caused due to rubbing action of the pen over the okra surface. This is accomplished by the triprotic nature of citric acid which facilitates the conversion of water insoluble leuco malachite green coated on the okra into water soluble malachite green through an acid-base reaction. It agrees with the UV–vis, FT-IR (Fourier Transform Infrared) spectroscopic techniques and computational studies that have been employed to unveil the possible interactions between okra surface, malachite green coating and citric acid ink. The proposed innovation boasts simplicity in fabrication and usage, reproducibility and sensitivity, which are key attributes to support on-site detection applications.