On–Off–On Fluorescent Carbon Dot Nanosensor for Recognition of Chromium(VI) and Ascorbic Acid Based on the Inner Filter Effect

TLDR

Chromium(VI) is a highly toxic, carcinogenic environmental pollutant, and low‑cost, highly sensitive sensors are urgently needed, while highly luminescent carbon dots have proven effective as fluorescent nanosensors for various analytes. This study demonstrates an on‑off fluorescent carbon‑dot probe that detects Cr(VI) via the inner‑filter effect. The probe operates by Cr(VI) absorbing the CD emission and excitation, quenching fluorescence, and upon reduction to low‑valent chromium the inner‑filter effect is removed, restoring fluorescence; ascorbic acid was used to illustrate this off‑on response. The CD‑based nanosensor offers simplicity, convenience, rapid response, high selectivity, and sensitivity, making it suitable for environmental Cr(VI) detection.

Abstract

Chromium(VI) [Cr(VI)] is considered as a severe environmental pollutant, due to its highly toxic and carcinogenic properties. Therefore, low cost, highly sensitive sensors for the determination of Cr(VI) are highly demanded. It is well-known that highly luminescent carbon dots (CDs) have been successfully applied as fluorescent nanosensors for pH, ions, and molecular substances. In the present work, we have demonstrated an on-off fluorescent CD probe for detecting Cr(VI) based on the inner filter effect (IFE) because the absorption bands of Cr(IV) fully covered the emission and excitation bands of CDs. This CD-based nanosensor provides obvious advantages of simplicity, convenience, rapid response, high selectivity, and sensitivity, which have potential application for the detection of Cr(VI) in the environmental industry. In addition, because Cr(VI) can be reduced to low valent chromium species easily by reductant, resulting in the elimination of the IFE and recovery of CD fluorescence, the CD-Cr(VI) mixture could behave as an off-on type fluorescent probe for reductant. We employed ascorbic acid (AA) as an example molecule to demonstrate this off-on type fluorescent probe.