Phosphorus-Doped Carbon Quantum Dots as Fluorometric Probes for Iron Detection

Abstract

Carbon quantum dots (CQDs), a novel fluorescent nanomaterial, have been extensively employed/explored in various applications, that is, biosensors, bioimaging, nanomedicine, therapeutics, photocatalysis, electrocatalysis, energy storage system, and so forth. In this study, we report the synthesis, characterization, and the application of phosphorus-doped CQDs (PCQDs), synthesized using trisodium citrate and phosphoric acid by the hydrothermal method. The effect of phosphorus doping on optical features and the formation of PCQDs have been explored elaborately by controlling the concentrations of precursors, reaction time, and the temperature. The fluorescent quantum yield for PCQDs was determined to be 16.1% at an excitation/emission wavelength of 310/440 nm. Also, the optical and structural properties of PCQDs were determined by using various spectroscopic and microscopic techniques. Static quenching of fluorescence was determined upon the addition of Fe³⁺ to PCQDs because of the formation of the fluorescent inactive complex (PCQDs-Fe³⁺). Hence, this chemistry leads to the development of a new fluorometric assay for the detection of Fe³⁺. The lower limit of Fe³⁺ detection is determined to be 9.5 nM (3σ/slope), with the linear fit from 20 nM to 3.0 μM (R ² = 0.99). We have validated this new assay in the raw, ejected, and purified water samples of the RO plant by the standard addition method. These results suggest the possibility of developing a new commercial assay for Fe³⁺ detection in blood, urine, and various industrial waste and sewage water samples. Furthermore, recycling the pollutant water into the freshwater using filters that consist of PCQDs offers a great deal.

Scheme 1. Schematic Representation of the Formation of PCQDs

Figure 1

(a) Normalized UV–vis absorbance spectra of different PCQDs, (b) PL spectra of various PCQDs at an excitation wavelength of 310 nm, (c) and (d) PL spectra of PCQD 3 and PCQD 11 at different excitation wavelengths ranging from 240 to 440 nm with an interval of 10 nm.

Figure 2

(a) UV–vis absorbance, (b) PL spectra of PCQDs at pH 2.0 and 12.0, (c) fluorescence decay curve of PCQDs before and after the addition of 5 μM of Fe³⁺, (d) fluorescence intensity vs time plot for the stability of PCQDs at an excitation/emission wavelength of 310/440 nm.

Figure 3

Deconvoluted high-resolution P(2p), C(1s), and O(1s) XP spectra of (a–c) PCQD 1, (d–f) PCQD 3, (g–i) PCQD 11, and (j–l) PCQDs_sp.

Figure 4

(a) TEM image and (b) SAED pattern of PCQDs. The inset of (a) shows the normal distribution histogram for the sizes of PCQDs.

Figure 5

(a) PL spectra of PCQDs in the presence/absence of Fe³⁺ ion concentration ranging from 20 nM to 16 μM and its (b) Stern–Volmer plot, (c) PL intensities of PCQDs in the presence/absence of 500 μM of various metal ions and 5 μM of Fe³⁺ ions at an excitation/emission wavelength of 310/440 nm.

Figure 6

Deconvoluted high-resolution XPS spectra of (a) P(2p), (b) C(1s), (c) O(1s), and (d) Fe(2p) of the PCQDs–Fe³⁺ complex.

Figure 7

(a) TEM image, EDXS color mapping of (b) carbon, (c) oxygen, (d) phosphorus, and (e) iron, (f) overlapped image of (b–e) of the PCQDs–Fe³⁺ complex.