Utilizing the interfacial reaction of naphthalenyl thiosemicarbazide-modified carbon dots for the ultrasensitive determination of Fe (III) ions

https://doi.org/10.1016/j.saa.2019.117485Get rights and content

Highlights

  • A novel nanosensor has been successfully constructed for highly effective detection of Fe3+.

  • The detection limit of the nanosensor is as low as 1.68 nM.

  • The nanosensor realized visualized monitoring of Fe3+ levels in living cells.

  • The multicolor emission of the NSTC-CDs makes it an ideal biolabeling agent.

Abstract

Since thiosemicarbazide contains numerous nitrogen and sulfur atoms in its structural formula that enhance its strong coordinating abilities with metal ions, it is always selected as the mother molecule for the design of metal-ion sensors. In this report, a thiosemicarbazide derivative (4-naphthalenyl-3-thiosemicarbazide (NTSC)) was synthesized via a single step process and covalently conjugated onto the surfaces of carbon dots (CDs). The modified CDs demonstrated excellent monodispersity, good photostability, and tunable luminescence properties. More importantly, the CDs retained a highly specific Fe3+ recognition capacity in contrast to other competing metal ions. Fe3+ can efficiently quench the fluorescence of CDs even at fairly low concentration (30 μM) with a detection limit as low as 1.68 nM. The fluorescence quenching kinetics are likely to involve static quenching, which is caused by specific interactions between NTSC-CDs and Fe3+ toward the formation of a ground state complex. Due to their excellent optical performance, low toxicity, and good biocompatibility the NTSC-CDs may be applied to the imaging and monitoring of Fe3+ in bacillus subtilis. In effect we successfully fabricated an effective fluorescent nanosensor for both the quantitative detection of Fe3+ in aqueous solutions, and its real-time imaging in vivo.

Graphical abstract

The synthesis procedures for the CDs-based nanosensor and applications in detection and bioimaging.

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Introduction

As essential metal ions in the human body, Fe (III) ions play critical roles in many physiological processes, including oxygen transport and metabolism, transcription regulation, and electronic transmission [1]. However, excessive or deficient Fe3+ can often lead to various diseases, such as anemia, diabetes, Parkinson's disease, Alzheimer's disease, and cancer [2]. Therefore, the selective and sensitive detection of Fe3+ ions in biological and environmental samples are essential for human health and environmental protection. Currently, there are numerous methods for the detection of Fe3+ such as spectrophotometric methods [3], cathodic striping voltammetry [4], flow injection analysis [5], flame atomic absorption spectrometry (FAAS) [6], inductively coupled plasma mass spectrometry (ICP-MS) [7], ion chromatography [8] and fluorescence method [[9], [10], [11], [12], [13], [14]], etc. For these techniques, fluorescence methods based on organic dyes [15,16], metal nanoclusters/nanoparticles [17,18] and semiconductor quantum dots [19,20] have been greatly developed because of their high fluorescent efficiency, intrinsic sensitivity, operational ease, and simple equipment. However, organic fluorophores are commonly restricted by relatively complex synthetic processes, photobleaching, and narrow excitation spectrum. Fluorescent probes based on metal nanoclusters/nanoparticles and semiconductor quantum dots have the potentials to address these problems on account of their broad excitation band, narrow emission band, photostability, and large Stokes shift. Nevertheless, poor water-solubility, non-biocompatibility, high toxicity, and high cost have limited their utility for typical analyses. In recent years, carbon dots, as a new class of fluorescent nanomaterials, have attracted significant attentions owing to their remarkable optical stability, ease of functionalization, low toxicity, good solubility, and biocompatibility [21,22], and their applications covered bioimaging, fluorescent ink, electrocatalytic oxygen reduction reaction, photocatalytic degradation of organic pollutants and detection of metal ions [[23], [24], [25], [26], [27], [28], [29]]. Hence, many metal-ion probes have been emerged now [[30], [31], [32], [33]]. Yet, fluorescence probes based on pristine CDs suffer from low fluorescence quantum yields, poor specificity, and low sensitivity. To further broaden the applications of CDs, their modification with functional groups has been shown to enhance the detection of specific target analytes. Zhao et al. [34] decorated carbon dots with polyethylene imine to obtain highly luminescent CDs, with a quantum yield of 42.5%. When applied to the detection of Fe3+ in tap water and river water, the RSD was less than 3.0%. Qu et al. [35] conjugated TPEA ([Tris(pyridin-2-ylmethyl) ethane- 1,2-diamine]) on the surfaces of CDs to fabricate a carbon dots-based fluorescence probe for the biosensing of Cu2+ ions in living cells. The probe exhibited good selectivity and a low detection limit. Zhu et al. [36] selected AE-TPEA ([N-(2-aminoethyl)-N,N,N-tris(pyridin-2-ylmethyl)ethane-1,2-diamine]) as a specific Cu2+ receptor and bonded it with a CdSe@CDs nanohybrid for the determination of Cu2+ ions in living cells, with a detection limit of 1 μM, etc. However, these probes still required tedious raw material synthesis, the use of toxic metals and organic matter, involved complex modified processes, and lacked suitable detection limits.

Thiosemicarbazide possesses strong coordinating abilities with metal ions and is often employed as the mother molecule for the design of metal-ion sensors. In our previous work, we employed thiosemicarbazide to decorate CDs to fabricate a novel Cu2+ fluorescent probe [37]. For this study, we synthesized 4-naphthalenyl-3-thiosemicarbazide through the introduction of a large aromatic group-naphthyl into the thiosemicarbazide structure to functionalize the carbon dots, with the synthesis process shown in Scheme 1. Following their functionalization, the carbon dots exhibited controlled fluorescent emission, as well as good fluorescent stability and water-solubility. In contrast to other metal ions, Fe3+ can strongly quench the fluorescence of CDs, and a good linear calibration curve was achieved between △F (△F = F0  F) and Fe3+ concentrations, ranging from 0 to 0.20 μM, with a detection limit as low as 1.68 nM, which mean that a novel nanosensor has been successfully constructed for highly effective detection of Fe3+. By reason of its low cytotoxicity, good biocompatibility, and cell membrane permeability, the nanosensor realized visualized monitoring of Fe3+ levels in living cells. Furthermore, the multicolor emission of the NSTC-CDs also gives us much space to pick out the wavelength for observation in vitro, which makes it an ideal biolabeling agent.

Section snippets

Materials

1-naphthalene isothiocyanate (97%) was purchased from Xiparker Chemical Co. (Zhengzhou, China). Isopropyl alcohol was obtained from Tianli Chemical Reagent Co. (Tianjin, China). Hydrazine hydrate (85%) was supplied by Aladdin (Shanghai, China). EDTA and common metal ions were purchased from Sinopharm Chemical Reagent Co. (Shanghai, China). The other reagents were all of analytic grade. The ultrapure water used in the experiment were obtained from a Milli-Q ultrapure water system (Millipore,

Characterization of NTSC-CDs

EDTA is a water-soluble chelating reagent that is rich in carboxyl and amino groups, thus the preparation of the CDs using EDTA as the precursor had many of these groups' resident on their surfaces. As the recognition moiety for the proposed nanosensor, NTSC was tethered to the surface of the CDs through the reaction of carboxylic groups of the pristine CDs with primary amine groups of the compound NTSC under the catalysis of EDC and NHS. TEM characterization (Fig. 1a1) revealed that the

Conclusions

In conclusion, a novel Fe3+ nanosensor was successfully developed through the facile conjugation of NTSC onto the surfaces of CDs via carbodiimide chemistry. The nanosensor was highly specific and sensitive for the determination of Fe3+ in real samples, with a detection limit as low as 1.68 nM, which was much lower than the established threshold for Fe3+ ions in drinking water (5.4 × 10−6 M). The detection mechanism was speculated to be a static quenching process. Additionally, thanks to its

Acknowledgements

We gratefully acknowledge the financial support from the National Natural Science Foundation of China (U1704170 and 21172056), Key Scientific Research Project of Henan Ministry of Education (20A610005), Key Programs of Science and Technology Department of Henan Province (182102310848 and 182102310103), PCSIRT (IRT1061), the Program for Innovative Research Team at the University of Henan Province (2012IRTSTHN006), and analytical support from the Instrumental Analysis Center of Tsinghua

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