Luminescence turn-on/off sensing of biological iron by carbon dots in transferrin

Abstract

Iron is a key nutrient as well as a potential toxin for almost all living organisms. In mammalian cells, serum transferrin (Tf) is responsible for iron transport and its iron overload/deficiency causes various diseases. Therefore, closely regulated iron homeostasis is extremely essential for cellular metabolism. In the present article we report the pH-dependent luminescence turn-on/off sensing of bound Fe(3+) ions of serum Tf by carbon dots (CDs) with the help of photoluminescence (PL) spectroscopy, FTIR spectroscopy, dynamic light scattering (DLS), circular dichroism (CD) and PL imaging techniques. At physiological pH (7.4), the intrinsic luminescence of CDs gets quenched in the presence of Tf as a consequence of ground-state association, which is driven by favorable electrostatic interactions between negatively charged CDs (-25.45 ± 1.23 mV) and positively charged Fe(3+) ions of Tf. The estimated detection limit of Tf by CDs at physiological pH is found to be 1.82 μM (signal-to-noise ratio of 3), which is much lower than the in vivo plasma concentration of Tf (∼ 25-35 μM). Various thermodynamic parameters have been evaluated by using the van't Hoff equation. Importantly, the secondary structure of Tf remains unaltered upon association with CDs. However, at pH 3.5, no such luminescence quenching of CDs has been observed in the presence of Tf due to the lack of ground-state interactions between positively charged (+17.63 ± 0.84 mV) CDs and Tf. Furthermore, the results from UV-Vis and far-UV CD measurements revealed a significant conformational change of Tf at pH 3.5 relative to pH 7.4, which triggers the subsequent release of bound iron from Tf. PL microscopy of individual CD revealed significant luminescence quenching at the single particle level, which further supports the non-emissive ground-state complexation at pH 7.4. Our present results show that these chemically synthesized water-dispersed CDs have the ability to selectively sense the bound iron from released iron of Tf without any conformational perturbation and hence they can be used as potential biological iron sensors as well as luminescent markers for the detection of iron deficiency/overload in biological macromolecules.

Carbon Dot-Mediated Capillary Electrophoresis Separations of Metallated and Demetallated Forms of Transferrin Protein

“…However, the slope for holo-Tf is −0.048 RFU/µM, revealing a direct proportionality between the extent of fluorescence quenching of the CDs signal and the concentration of holo-Tf. In work by Bhattacharya and colleagues [39] a similar effect was characterized as static quenching via their steady-state and time-resolved photoluminescence measurements at pH 7.4. Based on estimated thermodynamic parameters of the CD-Tf association determined from quenching measurements performed at various temperatures, they concluded that the observed quenching was a result of the electrostatic interaction between CDs and the Fe 3+ ions associated with holo-Tf, not the amino acid residues.…”

“…There are four possible conformations of Tf, depending on the number and position of bound Fe 3+ ions: (i) holo-Tf (fully metallated), (ii) single Fe 3+ bound only to the C-lobe or (iii) only to the N-lobe (partially metallated), and (iv) apo-Tf (demetallated). The Tf receptor is overexpressed on proliferating cancer cells, but not normal cells; therefore, Tf is a promising carrier protein for targeted drug delivery and therapy for cancerous cells [31,32,33,34,35,36,37,38,39]. The ability to separate the different conformations of Tf (fully metallated, partially metallated, and demetallated), is important because potential drug molecules may have different affinities for the different conformations of Tf.…”

Fluorescent carbon nanoparticles obtained from charcoal via green methods and their application for sensing Fe3+ in an aqueous medium

“…Hydrophilicity, easy preparation and environmental friendliness are some of their additional notable features . Optical imaging and sensing are potential areas of their application . These properties and applications make them distinct from and superior to commonly used organic fluorophores and quantum dots.…”

“…[12] Optical imaging and sensing are potential areas of their application. [13][14][15][16][17][18][19] These properties and applications make them distinct from and superior to commonly used organic fluorophores and quantum dots.…”

Thiosemicarbazide chemical functionalized carbon dots as a fluorescent nanosensor for sensing Cu2+and intracellular imaging

“…[4][5][6][7][8] In addition, they are extremely detrimental to living organisms for example algae, bacteria, fungi and viruses. In recent years, carbon dots, a new class of carbonaceous and fluorescent nanomaterials with spherical shape and size below 10 nm, 27 have attracted much attention for their unique characteristics, like much more sources of raw materials, easily synthetic method, high quantum yield, excellent optical and chemical stabilities, easy surface-functionalization, low toxicity, good water-solubility and biocompatibility, 28,29 and have been applied to determine many substances, for instance, DNA, nitrite, hypochlorite, ascorbic acid, 6-mercaptopurine, superoxide anion, pH, H 2 S, metal ions, etc. The traditional measurement methods of Cu 2+ include colorimetric method, capillary electrophoresis, atomic absorption spectroscopy (AAS), inductively coupled plasma mass spectrometry (ICP-MS), surface-enhanced Raman scattering, polarography, chemiluminescent method, electrochemical method and so on, 10-20 but these methods require time-consuming pretreatments, tedious procedures, organic reagents, complex and expensive instrumentations, etc.…”