Thioflavine T-induced charge neutralization assembly of AuNPs for colorimetric sensing of thallium

https://doi.org/10.1016/j.snb.2022.132437Get rights and content

Highlights

  • ThT, a G-quadruplex dye, was found to induce charge screening assembly of AuNPs.

  • A colorimetric approach was developed for facile Tl+ detection in environmental water samples.

  • The method was explore for real-time monitoring of Tl+ leakage from coal mine during raining water soaking.

Abstract

Thallium (Tl) is regarded as one of the 129 so-called “priority pollutants”, but its rapid optical sensing is largely unexplored. In this work, an AuNP-based label-free colorimetric sensing scheme was developed for Tl+. Specifically, thioflavine T (ThT) was found to induce charge neutralization aggregation of AuNPs, resulting in stable and intense blue color of AuNPs. Due to the higher affinity of PS2. M (DNA)-AuNPs over PS2. M-ThT, PS2. M DNA would protect AuNPs from aggregation by ThT. Since the affinity of PS2. M DNA-Tl+ was even higher than that of PS2. M DNA-AuNPs, in the presence of Tl+, PS2. M DNA would be first captured by Tl+ and restricted to be adsorbed onto AuNPs, resulting in ThT-induced aggregation of AuNPs for colorimetric sensing of Tl+. The proposed colorimetric sensing offered a limit of detection (LOD, 3σ) of 3.2 nM, which was lower than the maximum permitted amount of Tl+ in drinking water regulated by EPA (10 nM). The accuracy of the proposed method was first verified through analysis of six water samples. Furthermore, the proposed colorimetric system was also explored for real-time monitoring of Tl+ leakage from coal mine during raining water soaking, and maximum Tl+ amounts occurred at about 12–24 h.

Graphical Abstract

ThT-induced charge screening assembly of AuNPs was harvested for colorimetric detection of Tl+ in combination with G-quadruplex recognition.

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Introduction

Due to its extreme toxicity [1], [2], [3], thallium is regarded as one of the 129 so-called “priority pollutants” (US Environmental Protection Agency, EPA). Although the abundance of thallium is very low, the environmental thallium burden is increasing, due to the related mining as well as its particular use in superconductive industry. EPA has mandated that Tl content in normal drinking water should be lower than 2 ppb (10 nM) [4]. At present, atomic spectroscopic approaches are predominant for Tl+ detection [5], [6], [7], [8], [9], but the volatility at high temperatures and high ionization potential makes the accurate detection of Tl challenging. It is thus urgent to develop highly sensitive analytical probes for thallium detection [10], [11], [12].

DNA is structurally programmable and has been widely used as receptors for various metal ions in the form of aptamer or DNAzyme [13]. G-quadruplex is a special secondary structure of nucleic acids packed by G-quartets (a noncanonical base pairing motif with four guanine bases forming a cyclic and coplanar hydrogen bonding array) [14]. The special structure of G-quadruplex provides excellent host for a series of metal ions [15], [16], [17]. Interestingly, it was found that Tl+ could also bind with G-quadruplex [18], [19], [20], thus providing a new biological receptor for Tl+. For example, Liu et al. developed two fluorescent probes for Tl+ based on fluorophore-labeled G4 [21], [22]. Therefore, G-quadruplex can be explored for selective recognition of Tl+ in optical design [23].

Due to the distance-dependent color and extremely high extinction coefficients, gold nanoparticles (AuNPs) have widely been explored for colorimetric sensing [24], [25], [26], [27]. To achieve aggregation of AuNPs (color change from red to blue), cross-linking strategy is typically adopted [28], [29], but salt-induced charge screening (non-crosslinking) assembly of AuNPs features greater simplicity [30], [31], [32], [33], [34]. However, the aggregated AuNPs induced by salt is not stable, making quantitative measurement difficult. Previously, our group proposed a charge neutralization strategy for highly stable assembly of AuNPs with positively charged nucleic acid-staining dye SYBR Green I (SG) [35]. Considering that the G-quadruplex dyes are also typically positively charged [36], and Tl+ can be also trapped by G-quadruplex, we thus envision whether the charge neutralization strategy of AuNPs can be further extended for Tl+ sensing.

Herein, upon screening a series of G-quadruplex-incorporating dyes, we found that thioflavin T (ThT, with similar structure as SG) could indeed induce charge neutralization assembly of AuNPs. Due to the higher affinity of PS2. M (DNA)-AuNPs over PS2. M-ThT, PS2. M DNA would protect AuNPs from aggregation by ThT. Since the affinity of PS2. M DNA-Tl+ was even higher than that of PS2. M DNA-AuNPs, in the presence of Tl+, PS2. M DNA would be first captured by Tl+ and restricted to be adsorbed onto AuNPs, resulting in ThT-induced aggregation of AuNPs for colorimetric sensing of Tl+. The aggregated AuNPs induced by ThT was quite stable, the color change could be thus easily read out with a cell phone. Besides, such sensing approach for Tl+ was not only highly sensitive, but also exhibited favorable interference tolerance. The developed colorimetric probe was successfully explored for detection of Tl+ in environmental water samples and Tl+ leakage from coal mine during raining.

Section snippets

Materials and instruments

All reagents were at least analytical grade. DNA oligonucleotides were obtained from Sangon Biotechnology Co. (Shanghai, China). The sequences of PS2. M and CS-PS2. M (5′→3′) are GTG GGT AGG GCG GGT TGG and CCA ACC CGC CCT ACC CAC, respectively. The standard stock solution of Tl+ was purchased from National Research Center for Standard Substances (Beijing, China). Citrated-capped AuNPs were prepared according to the previous method [37]. Detailed information about other reagents and instruments

Assembly of AuNPs by ThT

First, ThT-induced aggregation of AuNPs was investigated. As shown in Fig. 1 A, a deep blue color was emerged upon addition of 1.3 μM ThT to the solution of AuNPs (13 nm, both commercial and lab-synthesized), with a simultaneous decrease of the plasmonic absorption peak at 520 nm and aggregated AuNPs absorption at 680 nm. Transmission electron microscopy (TEM) investigations indicated that AuNPs were well dispersed in solution (Fig. 1B), but quickly aggregated after the addition of ThT (Fig. 1

Conclusion

In summary, we found here that similar to SYBR Green I, ThT could also induce charge neutralization assembly of AuNPs. In combination with G-quadruplex-based dye inclusion and metal ion recognition, a colorimetric sensing system was developed for Tl+. The proposed assay offered an LOD of 3.2 nM, which is lower than the maximum permitted amount of Tl+ in drinking water regulated by EPA. The accuracy of the proposed method was verified through analysis of water samples, the results of which

CRediT authorship contribution statement

Fengjie Lei: Conceptualization, Methodology, Investigation, Formal analysis, Writing − original draft. Ziyi Ye: Methodology, Investigation, Data curation, Validation. Zhen Dong: Methodology, Investigation, Data curation, Validation. Xinfeng Zhang: Supervision, Project administration, Writing − review & editing, Resources. Peng Wu: Supervision, Project administration.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Nos. 21874093 and 21475013), the Sichuan Science and Technology Project (Nos. 2021YFH0124), and the Open Research Fund of School of Chemistry and Chemical Engineering, Henan Normal University (2022A02). Detailed characterizations supported by the public Platform of Analytical & Testing Center, Sichuan University, are greatly appreciated.

Fengjie Lei is a M. A. candidate of analytical chemistry at Chengdu University of Technology. Her research focuses on optical sensing.

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      First of all, to probe the Tl+-induced potential structure transformation of G-rich DNA to G4, it is desired to select an appropriate fluorescent probe. Since there were already several G-rich DNAs that could be induced to form G4 [14–17], we thus checked whether the interaction between Tl+ and G4 DNA could be probed with the existing G4 dyes. A previously reported G4 DNA, PW17 [36], was chosen here as the model.

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    Fengjie Lei is a M. A. candidate of analytical chemistry at Chengdu University of Technology. Her research focuses on optical sensing.

    Ziyi Ye is a M. A. candidate of analytical chemistry at Chengdu University of Technology. Her research focuses on optical sensing.

    Zhen Dong is a PhD candidate of analytical chemistry at Sichuan University. His research focuses on ThT and G-quadruplex.

    Xinfeng Zhang is now a professor at Chengdu University of Technology. His research interests are AuNP-based colorimetric and chemiluminescent sensing for environmental and geological samples.

    Peng Wu received his PhD from Nankai University in 2011. Now he is a professor in Analytical & Testing Center and College of Chemistry. His research interests are mainly spectroscopy and nanomaterials for analytical chemistry, with over 70 publications on the leading journals of analytical chemistry and related.

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