Understanding the Citric Acid–Urea Co–Directed Microwave Assisted Synthesis and Ferric Ion Modulation of Fluorescent Nitrogen Doped Carbon Dots: A Turn On Assay for Ascorbic Acid

Abstract

Herein, nitrogen doped carbon dots (NCDs) were synthesised from citric acid and urea via a previously reported microwave assisted route. The NCDs shows emission maximum at 500 nm on excitation at 400 nm. The fluorescence of NCDs decreases slightly with increase in basicity of solution up to pH 7.5 and then increases again after pH 8.5, along with a blue‐shift in tested alkaline pH. This pH dependent blue‐shift indicates the presence of both carboxyl↔carboxylate and phenol↔phenolate prototropic equilibrium in NCDs. Due to the special interaction of these phenolates and carboxylates on NCDs surface with di‐ or tri‐ valent heavy transition metal ions; it is demonstrated that ferric ion (Fe3+ ion) can quench the fluorescence of NCDs. This Fe3+ induced static quenching of NCDs is a collaborative effect of inner filter effect, aggregation and ferromagnetism. However, Ascorbic acid (AA) can recover the fluorescence of Fe3+ quenched NCD with detection limit as low as 96 μM. This detection strategy has good selectivity towards AA over other antioxidants, saccharides, proteins and neurotransmitters. Furthermore, (spiked) human serum and (spiked) human urine were analysed and found good recovery percentage.

A rich gallery of carbon dots based photoluminescent suspensions and powders derived by citric acid/urea

“…S11 ) is governed by the peaks at 160.2 ppm and 155.9 ppm, stemming from urea and biuret, respectively. The 13 C-NMR spectrum also displays peaks in the range 25–35 ppm and 150–210 ppm, confirming the presence of both sp 3 and sp 2 hybridized carbon atoms 57 , while the additional signal at 151 ppm indicates the presence of CYA 36 , 61 . The UV spectra of F-series are governed by the peaks at 250, 270, 320, 420 nm, consistent with the presence of CTA and HPPT (Supplementary Information, Fig.…”

“…In this work particular emphasis was given to the detection of melamine traces, a possible product of urea’s thermolysis 61 that is known to impose health risks to humans. To that end, a commercially available melamine test kit was used to confirm that the concentration of melamine in the powder was below 5 ppm.…”

A Rich Gallery of Carbon Dots based Photoluminescent Suspensions and Powders Derived by Citric acid/urea

“…S10) is governed by the peaks at 160.2 ppm and 155.9 ppm, stemming from urea and biuret, respectively. The 13 C-NMR spectrum also displays peaks in the range 25-35 ppm and 150-210 ppm, confirming the presence of both sp 3 and sp 2 hybridized carbon atoms [56], while the additional signal at 151 ppm indicates the presence of CYA [32,61]. The UV spectra of F-series are governed by the peaks at 250, 270, 320, 420 nm, consistent with the presence of CTA and HPPT (Supplementary Information, Fig.…”

“…S10). In this work particular emphasis was given to the detection of melamine traces, a possible product of urea's thermolysis [61] that is known to impose health risks to humans. To that end, a commercially available melamine test kit was used to confirm that the concentration of melamine in the powder was below 5 ppm.…”

Carbon Nanodots Derived from Urea and Citric Acid in Living Cells: Cellular Uptake and Antioxidation Effect

“…The photoluminescence features of the U-CNDs are reported to be a cooperative contribution from a few photoemissive centers, i.e., graphite core, surface states, and molecular fluorophores. − The blue or green emissions are associated with small molecular residues linked to the surface of the U-CNDs depending on the synthetic conditions. The origin of the blue fluorescence is attributed to the presence of bonded citrazinic acid or its derivatives and n−π* transitions at the dots’ edge when the CNDs are synthesized in a sealed reactor. , The green fluorescence of the CNDs may be contributed from the bonded 4-hydroxy-1 H -pyrrolo­[3,4- c ]­pyridine-1,3,6­(2 H ,5 H )-trione (HPPT) or its derivatives at the surfaces of the dots synthesized in an open vessel reactor (solvent-free) reported by Kasprzyk et al On the basis of the fluorescence property, researchers have reported their roles as optical probes for cell imaging and for the selective detection of hypochlorite, peroxynitrite, ferricyanide, or ascorbic acid at μM level. ,− …”