Fluorescent detection of ascorbic acid based on the emission wavelength shift of CdTe quantum dots

Abstract

We present a wavelength shift-based fluorescence detection method for ascorbic acid (AA) using CdTe quantum dots (QDs). Upon addition of ascorbic acid, the fluorescence emission peak of CdTe QDs red shifts linearly with the increase of the AA concentration, and the red shift speed could be improved by increasing the diameter of the QDs. The mechanism has been attributed to the AA-induced size change of the QDs. This AA-dependent red shift of fluorescence emission has been used to detect and quantify AA. Dynamic range and detection limit of this fluorescence probe are found to be 10–250 μmol/L and 1.3 μmol/L, respectively. Interference studies indicate that the AA could be specifically detected by using this wavelength shift-based fluorescence probe, and the QDs with larger diameter have better anti-interference performance. This fluorescence detection method is based on the wavelength shift of the emission peak, which is different from those reported traditional emission intensity-based fluorescent probes. Thus this fluorescence probe has the advantages of higher accuracy and convenient operation, which provides an effective strategy for design of wavelength shift-based fluorescence sensing method.

Introduction

As a kind of basic vitamins and anti-oxidant in biological systems, ascorbic acid (AA) plays an important role in many life processes, such as free radical scavenging, human disease prevention, immunity improvement, cell development and so on [1], [2], [3], [4], [5]. The abnormal concentration levels (including lack and excessive intake) of ascorbic acid in biological body can induce some diseases, such as scurvy, stomach convulsion, cancer, diabetes mellitus and many other chronic diseases [2], [5], [6], [7], [8]. Therefore, the sensitive quantitative detection of ascorbic acid is most important in the pharmaceutical analysis, food industry and diagnostic application. In recent years, many methods have been developed for the detection of AA, including electrochemistry [9], chemiluminescence [10], high-performance liquid chromatography (HPLC) [11], colorimetric method [12] and so on. Compared with these reported methods, fluorescent spectroscopic analysis has attracted more attentions because of their simplicity, convenience and excellent sensitivity [5].

Because of the high quantum yield, excellent photo stability and size-dependent fluorescent frequency, fluorescent quantum dots (QDs) have attracted tremendous interest and have been widely used as fluorescence probe for fluorescent spectroscopic analysis and detection. Recently, several efficient QDs-based fluorescent probes have been designed for the AA determination [13], [14], [15], [16], [17]. Li et al. reported a Fe³⁺-functionalized carbon QDs for the highly sensitive and selective detection of AA [1]. Because of the fluorescence resonance energy transfer (FRET), the fluorescence of the functionalized carbon QDs can be sensitively turned on by AA to give an “on–off–on” fluorescence response through the oxidation–reduction reaction between Fe³⁺ and AA. By using this fluorescence probe, AA could be detected with a detection limit of 9.1 nmol/L, and the linear range spans a scope from 0.1 to 50 μmol/L. Based on the AA-induced fluorescence recovery of the quenched carbon QDs/Fe³⁺ complex, Gong et al. also reported an “off–on” approach for the AA detection using carbon QDs as a fluorescent probe [18]. In the study of Duan et al., nitrogen–sulfur co-doped carbon QDs-based fluorescence probe was developed for sensitive detection of AA in the presence of Cu²⁺ [2]. Because of the AA-induced reducing of Cu²⁺ to Cu⁺, the fluorescence of nitrogen–sulfur co-doped carbon QDs could be quenched through electron or energy transfer. The linear response of AA concentration range was from 0.057 to 4.0 μmol/L with a detection limit of 18 nmol/L. Zhu et al. reported a sensitive and selective AA detection with highly photoluminescent nitrogen-doped carbon nanoparticles via solid-state synthesis [19]. The mechanism of the fluorescence suppression was attributed to the synergistic action of the inner filter effect and the static quenching effect. Liu et al. reported a “switch-on” fluorescent sensing of AA in food samples based on carbon QDs–MnO₂ probe [20]. Due to the AA-induced destruction of MnO₂, the MnO₂ nanosheets–induced fluorescence quenching of carbon QDs was destroyed, which leads to fluorescence recovery of the QDs. The detection limit for AA was 42 nmol/L with a concentration linear range of 0.18–90 μmol/L. Liu et al. reported a kind of graphene QDs-based fluorescent probe for turn-on sensing of ascorbic acid in the presence of copper ions [5]. Due to the AA-induced reducing of Cu²⁺ to Cu⁺, the Cu²⁺–induced fluorescence quenching of graphene QDs was destroyed, which inhibits the fluorescence quenching and then leads to fluorescence recovery of the QDs. The linear response range of AA concentration was obtained from 0.3 to 10 μmol/L, and the detection limit was 94 nmol/L. In the report of Li et al., a novel and efficient carbon QDs-based fluorescence probe has been studied for monitoring of cerebral AA in brain microdialysate [21]. Compared with other turn-on fluorescent methods using fluorophore-nitroxide probes for AA detection, this fluorescence probe has a lower detection limit of ∼ 50 nmol/L, and a wider linear range from 100 nmol/L to 20 μmol/L. Chen et al. reported a facile chemical redox strategy to modulate the surface chemistry of CdTe QDs for developing a turn-on fluorescent method to detect AA in biological fluids [22]. In this CdTe QDs-based fluorescence probe, the KMnO₄ serves as the quencher and AA serves as the target analyte. Upon the addition of AA, the quenched fluorescence of the CdTe QDs could be recovered due to the reduction of CdTeO₃/TeO₂ on the surface of the QDs to CdTe. In this sensing method, the linear response of AA concentration range was from 0.3 to 10 μmol/L with a detection limit of 74 nmol/L. Protein-modified gold nanoclusters have also been used as fluorescent probe for sensitive turn-off detection of AA [23]. Because of the AA-induced oxidation state change of gold nanoclusters, a linear relationship between the fluorescence quenching intensity and the AA concentration can be obtained in the range of 1.5–10 μmol/L. Similarly, a fluorescent sensor based on ovalbumin-modified gold nanoclusters has also been used for sensitive detection of AA [24]. The fluorescence intensity could be quenched linearly with the logarithm of AA concentration over the range of 1.0–100 mmol/L with a detection limit of 0.5 nmol/L. Liu et al. reported the detection of AA based on water-soluble CuInS₂ QDs [25]. The detection mechanism was attributed to the AA-induced fluorescence enhancement of mercaptopropionic acid-capped CuInS₂ QDs. The relationship between the fluorescence intensity and AA concentration was linear in the range of 0.25–200 μmol/L. In the report of Durán et al., a flow system using water-soluble CdSe/ZnS QDs modified with β-cyclodextrin was developed for AA detection [26]. The mechanism was based on the quenching effect produced by AA on the fluorescence intensity of surface-modified QDs. The relationship between the fluorescence intensity and AA concentration was linear in the range of 2–100 mg/L. By using a novel near-infrared-far red dual emission fluorescence nanohybrid of gold nanoclusters-PbS-QDs, the ratiometric detection of AA has been fully proved for fruit internal quality assessment, in vitro cellular imaging, and in vivo imaging in nude mice [27]. Recently, Ganiga and Cyriac reported an ascorbic acid sensor based on cadmium sulphide quantum dots [28]. When diphenylcarbadiazone was converted back to diphenylcarbazide in the presence of AA, the fluorescence recovery of CdS QDs takes place. By using this fluorescence recovery, AA could be detected with a dynamic range of 60–300 nmol/L and a detection limit of 2 nmol/L.

Although the fluorescence properties of quantum dots have been widely used to detect ascorbic acid, those previous reports are focused in the intensity change (turn on or turn off) of the fluorescence. As we know, the stability of the fluorescence intensity is not good, which is easy to be affected by the instrument condition and environment. Although the fluorescence intensity variation is much wider than the wavelength variation, the wavelength has the quality of accuracy, which is hardly affected by other interferences such as particle density and background noise. Thus compared to fluorescence intensity, the wavelength shift of the fluorescence peak has a better accuracy. However, the detection method based on wavelength shift of the fluorescence peak has seldom been reported. In this study, we report a wavelength shift-based fluorescence detection method for AA using CdTe QDs. This fluorescence probe has good sensitivity and selectivity. The performance of this fluorescence detection could be further improved by changing the particle size of the QDs.