Fluorescent Carbon Dots as Biosensor, Green Reductant, and Biomarker

Abstract

Carbon dots, the celebrated green material among the nanocarbon family, are blessed with several interesting features like biocompatibility, solubility, tunable luminescence, and so forth. Herein, carbon dots derived from Mint leaf extract (M-CDs) via a green method are exploited for versatile applications as a biosensor, reductant, and biomarker. M-CDs are applied for fluorimetric sensing of biologically relevant folic acid through quenching response originating from the inner filter effect, with a limit of detection of 280 nM. The carbon dots were highly selective toward folic acid in a collection of 16 biomolecules. The specificity of carbon dots toward folic acid is explained based on the interaction between the two. Along with sensing, herein, we project M-CDs as a green reducing agent by demonstrating the reduction of Fe(III) and noble metal nanoparticle synthesis from their salt solutions. The particles are found to be significantly non-cytotoxic, as evident from the MTT assay performed on primary H8 cells. The application of M-CDs in multicolor imaging is also illustrated using HeLa cells.

Figure 1

(a) HR-TEM image of M-CDs; the inset shows lattice fringes of M-CDs. (b) Histogram derived from TEM shows particle size distribution. The inset of (b) shows the SAED pattern of M-CDs.

Figure 2

(a) XRD pattern of M-CDs, (b) Raman spectrum of the M-CDs, and (c) FT-IR spectrum of the system.

Figure 3

(a) UV–vis spectrum. (b) PL spectrum of the M-CD solution at 360 nm excitation; the inset shows the color of M-CDs in daylight and UV light (365 nm). (c) PL decay curve of M-CDs. The dependence of luminescence intensity of the M-CDs on (d) irradiation time and (e) ionic concentration (NaCl upto 100 mM).

Figure 4

(a) Emission spectrum of M-CDs with increasing amounts of FA; (b) dependence of relative fluorescence quenching efficiency ((F₀ – F)/F) on FA concentration; (c) plot illustrating the selectivity of the system toward FA over other interfering analytes.

Figure 5

(a) UV absorption spectral behavior of M-CDs alone with water as a reference (black line); the mixture of M-CDs and FA with water as a reference (red line); FA alone with water as a reference (green line); the mixture of M-CDs and FA with M-CDs as a reference (pink line); (b) overlapping of the normalized absorbance spectrum of FA, the excitation spectrum of M-CDs, and the emission spectrum of M-CDs; (c) TCSPC measurements of M-CDs (pink line) and the FA + M-CD mixture (blue line); (d) overlapping of the absorption spectrum of biomolecules and the excitation curve of M-CDs.

Figure 6

(a) Absorption spectrum of Fe³⁺ (red line) and Fe³⁺ plus M-CDs (blue line); the inset shows the photographic image of (a) Fe³⁺ solution (b) Fe³⁺ in the presence of M-CDs. (b) UV–visible spectrum of Fe³⁺(blue line), Fe³⁺ plus phenanthroline (black line), and (c) Fe³⁺ plus 1,10 phenanthroline in the presence of M-CDs; the insets show the corresponding photographic images. (c) Absorption spectra of Fe³⁺ plus 1,10 phenanthroline in 1–10 min in the presence of M-CDs. The images of intensity variations are shown in the inset.

Figure 7

(a) Absorption spectra of AgNPs synthesized (b) HRTEM image of AgNPs, inset shows the lattice fringes at higher magnification. (c) SAED pattern of AgNPs obtained from TEM. (d) Absorption spectra of gold nanoparticles synthesized by using M-CDs as reducing agent.

Figure 8

Fluorescence microscopy image of HeLa cells, control (a) and under excitation wavelengths of 360–380 nm (b), 460–480 nm (c), and 510–590 nm (d). Percentage cell viability of primary H8 cells in the presence of M-CDs of varying concentrations (e).