Ultrasensitive and Fast Determination of Fulvic Acid in Sapropel and in the Techirghiol Lake Water

Sapropel and Techirghiol Lake water are an excellent source of organic substances like fulvic acid, which can be extracted and used in the pharmaceutical industry. On-site determination of fulvic acid from lake water and sapropel is valuable for the possibility of exploring the sapropel and water as it is (can serve as daily quality control) for therapeutic purposes, or it can be taken to specialised laboratories for the extraction of fulvic acid, followed by its utilisation in the pharmaceutical industry. An ultrasensitive stochastic sensor based on reduced graphene oxide paste decorated with gold and palladium nanoparticles and modified with quinine was designed, characterised, and validated for the determination of fulvic acid in sapropel and also in the Techirghiol Lake water. The sensor can be used on a wide concentration range, from 5.00 fg mL⁻¹ to 5.00 μg mL⁻¹, with a high sensitivity (1.97 × 10⁸s⁻¹ g⁻¹ mL). High recovery values (>99.00%) were recorded for the determination of fulvic acid in sapropel and in the Techirghiol Lake water. Validation of the proposed sensor and screening method for fulvic acid is done versus an HPLC method. The on-site measurements with the ultrasensitive stochastic sensor will contribute to the reliable determination of the quality of sapropel and water in real time.