Immobilized sorption microprobes as a tool for small-size colorimetric sensing

Miniaturized analytical procedures and tools have gained significant attention due to their advantages of low consumption of analyzed samples and reagents, low cost and high greenness. Within this field, a concept of immobilized sorption microprobes is proposed and tested. Each microprobe represents an individual sorbent particle included in an array attached to a holder via an adhesive layer. The analytical response of food dyes adsorbed from a solution on the microprobes can be directly measured using high-resolution scanning followed by processing the digital image in available software. The microprobes based on γ-Al₂O₃ and hypercrosslinked polystyrene, which differ in their polarity, were studied. An array of distanced microprobes shows higher color intensity and 3–5 times better sensitivity compared to a dense array where they are located side-by-side. The γ-Al₂O₃ microprobes were successfully applied for the micro-colorimetric determination of Ponceau-4R and Fast Green FCF synthetic food dyes in an energy drink (at the level of 2.5 µg mL⁻¹) and capsules of a pharmaceutical formulation (at the level of 6 µg per capsule cap) with the recovery within 101–115 %.