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.

Water pollution by chromate wastes is pervasive and requires selective and sensitive low-cost methods for 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 by various techniques. The S, N-CDs selectively detect Cr(VI) by enhanced absorption at Cr(VI)'s charge transfer band at 375 nm and 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 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 contaminated laboratory waste (positive control) with good comparability and accuracy compared to the ICP-OES result. Thus, these two spectrochemical methods 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.