Facile synthesis of carbon dots from wheat straw for colorimetric and fluorescent detection of fluoride and cellular imaging

Highlights

• Fluorescent CDs were prepared by a green hydrothermal method from wheat straw.

• The obtained CDs showed excitation-dependent emission and excellent biocompatibility.

• The CDs can be used as colorimetric and fluorescent dual-model probes for sensing F⁻.

• The CDs can be applied for bioimaging and intracellular sensing of F⁻.

Abstract

Colorimetric and fluorescent detection of F⁻ have attracted enormous interest owing to their simplicity, low-cost and high selectivity. However, traditional colorimetric and fluorescent sensors mainly based on the insoluble and toxic organic molecules, which is not favorable for sensing F⁻ in water media and living cells. In this work, we designed fluorescent carbon dots (CDs) with excellent water solubility and good biocompatibility as a colorimetric and fluorescent dual-model probe for the detection of F⁻. The CDs were prepared by a green, one-step hydrothermal strategy from wheat straw without any additives and surface passivation. The obtained CDs exhibited a bright blue fluorescence, special response to F⁻ and low cytotoxicity. More interestingly, a significant color change from light yellow to red can be observed by the naked eye upon addition of F⁻ ions to the CDs solution probably due to the formation of hydrogen bonding between CDs and F⁻. Besides, the fluorescence of CDs also can be selectively quenched by F⁻ with the detection limit of about 49 μM. Additionally, the CDs are also applied to intracellular imaging and sensing of F⁻ in living cells. This strategy may provide a new method for the detection of F⁻ in water media and biological systems.

Introduction

Fluoride plays a key role in human health and animal activities. Appropriate fluoride can be used to prevent dental caries and treat osteoporosis [1]. However, excessive intake of fluoride may cause serious skeletal fluorosis and induced liver and kidney damage. More seriously, a high level of fluoride ion may disorder the synthesis of protein and DNA, leading to the damage of the immune system and even death [2]. Therefore, developing effective methods for sensing of fluoride anion is urgently needed.

Currently, a variety of methods, such as ion-selective electrodes, ion chromatography, spectrophotometry, colorimetric and fluorometric method, have been developed for sensing F⁻ [[3], [4], [5], [6], [7], [8]]. Among them, colorimetric and fluorometric methods have gained extremely interest due to their advantages of simplicity, low cost, high selectivity and sensitivity [9,10]. At present, the colorimetric and fluorescent chemosensors for sensing F⁻ mainly focus on organic receptors such as anthracene [11], azo [12], benzimidazole [13], urea [14,15], naphthalimide [16,17], Schiff base [18,19] and so on. However, almost all these receptors have some disadvantages such as high toxicity, complicated procedures or insoluble in water. Besides, the above chemosensors are not suitable for sensing F⁻ in living cells due to their poor biocompatibility. Thus, it is essential to develop a novel chemosensor with good water solubility and excellent biocompatibility for sensing F⁻ in living cells.

Recently, fluorescent carbon dots (CDs) have gained more attention for optical sensing owing to their outstanding fluorescent feature, good photostability, excellent biocompatibility and facile synthesis [[20], [21], [22], [23], [24], [25], [26]]. Many efforts have been devoted to developing fluorescence probes based on carbon dots for sensing various metal ions such as Fe³⁺, Cu²⁺, Hg²⁺, Cr(VI) and Ag⁺ and so on [[27], [28], [29], [30], [31], [32], [33], [34], [35], [36]]. Up to now, however, utilizing carbons dots as a probe for sensing F⁻ is rarely reported. For instance, U. Baruah et al. designed an F⁻ detection probe based on CDs modified with β-cyclodextrin, and they found that the fluorescence intensity of CDs increased gradually with the addition of F⁻ [37]. Liu and co-workers reported a Zr(H₂O)₂EDTA functionalized carbon dots as a fluorescent probe for detection of F⁻ based on the competitive ligand reactions between the CDs and F⁻ coordinated to Zr(H₂O)₂EDTA [38]. However, to the best of our knowledge, there is no report for colorimetric detection of F⁻ based on carbon dots as receptors.

Herein, we reported fluorescent carbon dots (CDs) as a colorimetric and fluorescent dual-model probe for the detection of F⁻ in water media (Scheme 1). The carbon dots can be prepared by hydrothermal treatment of wheat straw without any additives or post-synthetic surface passivation. The obtained CDs exhibited bright fluorescent emission, excellent dispersibility in water and good biocompatibility. Moreover, a significant color change from light yellow to red along with the fluorescent quenching can be observed by the naked eye upon addition of F⁻ ions into the CDs solution. Additionally, the obtained CDs also can be utilized as a fluorescent probe for cellular imaging and detection F⁻ in living cells. Therefore, this simple, economical technique for detection of F⁻ will have prospective applications in the detection of F⁻ and cell imaging.