Emerging Stoke’s Shift-Based Cr(VI) Fingerprint Sensor from Intensely Blue Fluorescent, High Quantum Yield, Pepitas-Derived Carbon Dots

Abstract

Innovative methods to develop a metal ion sensor to detect heavy metal ions have played a pivotal role in protests against environmental pollution. Already reported studies in sensing are based on quenching, which is insufficient for realworld applications. This study aims for the development of a fingerprint sensor based on the green pepitas carbon dot (GPCD) or green pepitas carbon quantum dot from "pepitas" for Cr(VI) metal ion sensing, even below its permissible level (5 μmol = 10 μg/L) through the Λ max red shift mechanism. The uniqueness and advantage of this study is the green synthesis of high quantum yield (57.6%), intensely blue fluorescent, biocompatible, and highly stable environment-friendly N and O co-doped GPCDs from carbonization of highly nitrogenous pepita seeds without any chemical treatment or surface modification, which is used to develop Cr(VI) fingerprint sensors. The spectrochemical studies using UV−vis, PL spectroscopy, Fourier transform infrared, and X-ray photoelectron spectroscopy analysis revealed the presence of hydroxyl-, carbonyl-, and nitrogen-containing groups and also found that carbon, oxygen, and nitrogen are present in very high amounts. Morphological analysis confirmed that the synthesized GPCDs were spherical in shape with an average size of 2 nm. The studies imply that the prepared GPCDs are highly selective toward the sensing of Cr(VI)ion, a deadly toxic heavy metal ion contaminant. The developed novel Cr(VI) ion sensor showed a large bathochromic shift in wavelength from 397 to 431 nm, which is a unique mechanism for Cr(VI) ion sensing that has not been reported so far. The mechanism behind this red/Stoke's shift is the effect of the Cr(VI) ion on solvent polarity, which is a peculiarity of the GPCDs synthesized through a new methodology. The unique fluorescent quenching is due to pyridinic N, NH 2 , OH, and COOH lone pair-metal ion interactions and effective fluorescent resonance energy transfer interactions between GPCDs and highly charged Cr(VI).