Fluorescent recognition of Fe3+ in acidic environment by enhanced-quantum yield N-doped carbon dots: optimization of variables using central composite design

Abstract

A versatile synthetic approach for development of highly fluorescent nitrogen-doped carbon dots (n-cDs) from carboxymethylcellulose in the presence of linear polyethyleneimine (Lpei) has been developed. According to single factor method, central composite design incorporated with response surface methodology matrix was applied to find and model optimal conditions for the temperature (220-260 °C), duration (1-3 h) and LPEI weight (0.5-1.5%). The statistical results show that duration was the most significant parameter for efficient carbonization conversion rate in comparison with temperature and Lpei weight. the reduced cubic model (R 2 = 0.9993) shows a good correlation between the experimental data and predicted values. the optimal variables were temperature of 260 °C, duration of 2 h and LPEI weight of 1%. Under these conditions, quantum yield of up to 44% was obtained. The numerically optimized N-CDs have an average size of 3.4 nm with graphitic nature owing to the abundant amino species incorporated into the carbon core framework. the blue-green n-cDs possess emission dependent upon the solvent polarity, wide pH stability with enhanced emission in an acidic environment. Impressively, the N-CDs show long-shelf-life for up to 1 year with no noticeable precipitation. the n-cDs were able to recognize a high concentration of fe 3+ ions with a detection limit of 0.14 μM in acidic solution owing to the special coordination for Fe 3+ to be captured by electron-donating oxygen/ amino groups around n-cDs. Moreover, the n-cDs can also be used as a new kind of fluorescent ink for imaging applications. Carbon dots (CDs) are the latest member of fluorescent carbon nano-sized family. Typically, CDs are nearly spherical-shaped nanoclusters with sizes of less than 10 nm and consist of amorphous or crystalline cores with sp 2 carbon atoms 1. Since the first established work 2 , CDs have attracted a considerable focusing in the fields of wastewater treatment, photocatalysis, bioimaging, cancer therapy and chemical sensing 3,4. This is owing to their feature of having outstanding optical properties, including excellent biocompatibility, tuneable photoluminescence, negligible toxicity, ease of production and resistance to photobleaching in comparison to QDs counterparts 5-7. It is well known that QDs, for instance, tend to be decayed in the biological environment leading to a serious toxicity concern 8 .

An Overview of the Recent Developments in Carbon Quantum Dots—Promising Nanomaterials for Metal Ion Detection and (Bio)Molecule Sensing

“…The possible mechanism is explained through the higher affinity of the Fe 3+ ions for the oxygen/nitrogen electron donors, resulting in the coordination and covalent bonds formation between the atoms. The new formed bonds resulted in the migration of the excited electrons to the vacant d-orbitals of Fe 3+ , leading to non-radiative recombination of the excitons (e-h) causing the PL quenching [86]. The significance in the Fe 3+ and in many other ions sensing at low pH was also emphasized by Yang et al [87].…”

“…It is well known that ferric ions exist in the biological systems and in the environment, but the excessive exposure to these ions could induce severe disease, such as kidney failure, liver damage, etc. [86]. Therefore, efficient methods for the monitoring of Fe 3+ in biofluids and the environment could be useful for analytical practice.…”

“…The novel CQDs-based sensors for the detection of Fe 3+ ions are listed in Table 3. A report by Issa et al [86] includes preparation of the N-CQDs from carboxymethylcellulose (CMC) and polyethyleneimine (LPEI) by hydrothermal synthesis at 260 • C for 2 h (Figure 3). The product of N-CQDs was purified by a filtration and dialysis (1kDa) procedure.…”

“…Typically, Fe 3+ ions using CQDs were analyzed under natural conditions, and mostly, the PL intensity of N-CQDs is unstable under these conditions. In the study by Issa et al [86], the detection of Fe 3+ ions in acidic environment was enabled by the presence of -OH, -COO − , and -NH functional groups around the N-CQDs edge, and these groups can act as a bridge to bind the nanoprobe with any surrounding analytes. The developed method showed a wide linear range from 1 to 400 µM, with a coefficient of determination of R 2 = 0.9933.…”

“…Schematic illustration of the synthesis of N-CQDs using CMC and LPEI as precursors; their application in Fe 3+ sensing and as fluorescent ink. Reprinted from Ref [86]…”