Insights for applying N,S-doped carbon dots as a fluorescent nanoprobe for estimation of some nitro-calcium channel blockers

Abstract

A facile and simple one-step hydrothermal approach was adopted for fabrication of N and S co-doped carbon quantum dots probe (NSCDs) by using thiosemicarbazide as a dopant and citric acid as a precursor. The prepared NSCDs with a high quantum yield of 0.58 were characterized using UV-visible spectroscopy, IR spectroscopy and high-resolution transmission electron microscopy. The as-obtained NSCDs could be deemed as an effective fluorescent nanosensor for the determination of some anti-hypertensive nitro-calcium channel blockers (Nitro-CCBs) including nicardipine (NIC), nifedipine (NIF) and nimodipine (NIM) whether in pure form or in their pharmaceutical formulations. Measurements of NSCD emission intensity were performed at 416 nm after being excited at 345 nm. Nitro-CCBs could induce quenching in the native fluorescence of NSCDs due to the inner filter effect and static quenching mechanism. The studied compounds were investigated within linear detection range of (10.0-100.0 µM) for NIC, (5.0-60.0 µM) for NIF and (5.0-60.0 µM) for NIM. Correlation coefficients are greater than or equal to 0.9998 and detection limits are ranged between 0.55 and 1.86 µM. The proposed method was extended to estimate the studied compounds in different pharmaceutical samples with high % recoveries ranging from (97.95 to 101.28%) and low % relative standard deviation values (less than 2%). Validation of the developed spectrofluorimetric method was done along with the International Council of Harmonization requirements.

Keywords: inner filter effect; nicardipine; nifedipine; nimodipine; nitrogen/sulfur carbon quantum dots.

Figure 1.

Structural formulae of nicardipine hydrochloride (a), nifedipine (b) and nimodipine (c).

Figure 2.

(a) Fluorescence spectra of the NSCDs at different excitation wavelengths. (b) Typical HRTEM images of the NSCDs at two different magnifications. (c) FT-IR spectrum of NSCDs.

Figure 3.

Fluorescence emission spectra of the NSCDs in aqueous solution upon addition of various concentrations of NIC (a) from top to bottom: 0 µM, 10.0 µM, 30.0 µM, 50.0 µM, 70.0 µM, 80.0 µM, 100.0 µM; NIF(b) from top to bottom: 0 µM, 5.0 µM, 10.0 µM, 20.0 µM, 30.0 µM, 40.0 µM,50.0 µM, 60.0 µM; NIM (c) from top to bottom: 0 µM, 5.0 µM, 10.0 µM, 20.0 µM, 30.0 µM, 40.0 µM,50.0 µM, 60.0 µM.

Scheme 1.

General procedure for synthesis and application of NSCDs.

Figure 4.

A co-plot showing the the great overlap between UV–Vis absorption spectra of NIC, NIF and NIM and fluorescence excitation of the NSCDs.

Figure 5.

Suppressed efficiency of observed and corrected fluorescence of NSCDs after addition of different concentrations of NIC (a), NIF (b) and NIM (c).

Figure 6.

Stern–Volmer plots between F⁰/F and concentration of NIC (a), NIF (b) and NIM (c).

Scheme 2.

Schematic illustration of the quenching mechanisms of NSCDs with NIC, NIF and NIM.