Enhanced Absorption and Fluorescence quenching methods for the quantitative analysis of Cr(VI) ions using avocado seed-derived carbon quantum dots as pseudo-derivatising reagents.

Abstract

Water pollution by chromate emissions or leachates, especially from ferrochrome mining and the downstream processing industries, is prevalent and pervasive, and requires selective, ultra-sensitive, rapid, and low-cost methods for routine detection and quantification. Herein, we report the enhanced absorption and fluorescence methods for the quantitative analysis of chromium(VI) ions using avocado seed-derived carbon quantum dots as pseudo-derivatising reagents. Sulfur- and nitrogen-doped green carbon dots (S, N-CDs) were hydrothermally synthesised from an avocado seed powder extract and were characterised using TEM, SEM-EDX, FT-IR, and XPS. The S, N-CDs selectively detect Cr(VI) by the enhanced absorption at Cr(VI)’s charge transfer band at 375 nm as well as fluorescence quenching at 420 nm. Both modes were used for nanomolar detection of the latter in the presence of other metal ions at a 100-fold higher concentration. Mn(VII) ions strongly interfere with the Cr(VI) responses. At pH 9, the enhanced absorption method showed a linear correlation with the [Cr(VI)] range of 0.5 - 1500 ppb, yielding estimated LOD and LOQ values of 0.14 ppb and 0.49 ppb, respectively. The quenching of the S, N-CDs fluorescence linearly varied with the [Cr(VI)] in the 20 - 1500 ppb concentration range, giving estimated LOD and LOQ of 5.9 and 25 ppb. Both methods showed good recovery (89 - 99 %) for ppb levels of Cr(VI) spiked in river water samples. They could detect Cr(VI) in a contaminated laboratory waste (positive control) with good comparability and accuracy compared to the ICP-OES result. Thus, these two methods, employing the S, N-FCDs derived from avocado seeds as pseudo-sensing reagents, can be used for the quantitative detection of Cr(VI) ions at ppb levels, surpassing conventional calorimetric detection of Cr(VI) after a derivatising step.