Tuning the Sensing Properties of N and S Co-Doped Carbon Dots for Colorimetric Detection of Copper and Cobalt in Water

Abstract

In this study, nitrogen and sulfur co-doped carbon dots (NS-CDs) were investigated for the detection of heavy metals in water through absorption-based colorimetric response. NS-CDs were synthesized by a simple one-pot hydrothermal method and characterized by TEM, STEM-coupled with energy dispersive X-ray analysis, NMR, and IR spectroscopy. Addition of Cu(II) ions to NS-CD aqueous solutions gave origin to a distinct absorption band at 660 nm which was attributed to the formation of cuprammonium complexes through coordination with amino functional groups of NS-CDs. Absorbance increased linearly with Cu(II) concentration in the range 1-100 µM and enabled a limit of detection of 200 nM. No response was observed with the other tested metals, including Fe(III) which, however, appreciably decreased sensitivity to copper. Increase of pH of the NS-CD solution up to 9.5 greatly reduced this interference effect and enhanced the response to Cu(II), thus confirming the different nature of the two interactions. In addition, a concurrent response to Co(II) appeared in a different spectral region, thus suggesting the possibility of dual-species multiple sensitivity. The present method neither requires any other reagents nor any previous assay treatment and thus can be a promising candidate for low-cost monitoring of copper onsite and by unskilled personnel.

Keywords: N and S co-doped carbon dots; colorimetry; divalent copper; optical sensing; water quality.

Figure 1

FTIR spectra of the dried samples: as-prepared NS-CDs sensing solution (blue curve), NS-CDs + Cu(II) (black), and NS-CDs + Fe(III) (red).

Figure 2

Micrographs of NS-CDs. TEM of single NS-CDs (a) or small clusters (b) observed before the interaction with Cu(II) ions. The white arrows highlight the single NS-CDs showing a diameter of approximately 5 nm. Large clusters are observed upon interaction with the metal ions (c). STEM micrograph of the same area confirming the presence of NS-CDs clusters after interaction (d).

Figure 3

EDS chemical analysis of NS-CDs. TEM of NS-CDs clusters after interaction with Cu(II) ions (a); and related EDS maps (b,c). The elemental analysis shows the presence of the ions indicating that Cu(II) was complexed with the NS-CDs. Other elements such as carbon and oxygen were detected along the NS-CDs clusters likely due to the carbon layer of the grid where the sample was deposited. Also note the presence of other metals such as gold likely due to the gold grid supporting the carbon layer (see Section 2).

Figure 4

UV–Vis absorption (a); and fluorescence (b) spectrum of NS-CDs sensing solution.

Figure 5

Selective response of UV–Vis absorption spectra of as-prepared NS-CDs sensing solution upon the addition of different HM ions at a concentration of 100 µM. Note that As(III) is present as the anion AsO₃³⁻ in water while Cr(VI) is present as CrO₄²⁻ and Cr₂O₇²⁻, which is in contrast to all the other metals, which are present as cations.

Figure 6

(a) UV–Vis absorption spectra of the as-prepared NS-CDs sensing solution upon the addition of Cu(II) ions at different concentrations; (b) calibration curve.

Figure 7

Selective response (red bars) and interference from other HMs (blue bars) in the as-prepared NS-CD sensing solution.

Figure 8

Selective response of UV–Vis absorption spectra of pH-optimized NS-CDs sensing solution upon the addition of different HM ions at a concentration of 100 µM. Note that As(III) is present as the anion AsO₃³⁻ in water while Cr(VI) is present as CrO₄²⁻ and Cr₂O₇²⁻, which is in contrast to all the other metals, which are present as cations.

Figure 9

(a) UV–Vis absorption spectra of the pH-optimized NS-CDs sensing solution upon the addition of Cu(II) ions at different concentrations; (b) calibration curve.

Figure 10

Interference from other HMs (blue bars) in the pH-optimized NS-CDs sensing solution.