Luteolin (3′,4′,5,7-tetrahydroxyflavone) is an important flavonoid that is naturally present in a variety of plants, e.g., peppermint, green pepper, thyme, and parsley [1
]. In clinical applications, luteolin can be used as a drug to treat many diseases (e.g., respiratory disease, cardiovascular disease, and hyperlipidemia). This is because it has unique biological functions and pharmacological activities, e.g., anti-inflammatory, anti-allergic, anti-ulcer, and anti-oxidation effects, anti-cancer and anti-viral activities, cardiovascular protection, and cataract prevention [4
]. Moreover, recent studies have demonstrated that luteolin can enter the cell nucleus and suppress oxidative damage of deoxyribonucleic acid [9
]. Consequently, the detection and monitoring of luteolin in pharmaceuticals and biological fluids is of great importance for drug quality control, clinical medicine research, and biochemical study.
Up to now, many analytical methods have been developed for the detection of luteolin. These have been basedon various techniques, including high-performance liquid chromatography, gas chromatography, mass spectrometry, capillary electrophoresis, and spectrophotometry [10
]. Although these methods show high sensitivity and selectivity for detecting luteolin, most of them require expensive and sophisticated instruments, complicated operations, and professional operators, which limit their practical applications. In contrast, electrochemical methods are considered as simple, cost-effective, and sensitive techniques for luteolin detection because they can be easily carried out with inexpensive and simple instruments [4
]. Liu et al. investigated the electrochemical behavior of luteolin at a glassy carbon electrode (GCE), based on which they developed an electrochemical sensor for the detection of luteolin with a detection limit of 5.0 nM [4
]. To improve the sensitivity of electrochemical detection of luteolin, recent research has focused on the design of innovative, chemically modified electrodes based on various nanomaterials, such as multi-walled carbon nanotubes, macroporous carbon nanomaterials, graphene-hydroxyapatite nanocomposites, and In2
]. These nanomaterials exhibit excellent electrocatalytic activity toward the redox reaction of luteolin and lead to the amplification of the electrochemical signal. For example, Guo’s group reported a sensitivity-enhanced electrochemical sensor for determining luteolin using macroporous carbon nanomaterial modified GCE with a detection limit of 1.3 nM [17
]. Along with the development of nanoscience, it is still necessary to exploit new nanomaterials with high electrocatalytic activity for further improving the sensitivity of electrochemical luteolin sensors.
Gold nanocages (AuNCs), representing an emerging class of nanosized gold material, have attracted much research interests since being invented by Xia’s group in 2002 [22
]. Due to the unique features of noble metal composition with hollow, porous, and thin-walled structure, AuNCs possess several distinctive properties over commonly used gold nanoparticles, including good chemical/thermal stability, high catalytic activity, strong localized surface plasmon resonance, and excellent controlled release properties [22
]. Such outstanding properties make them very attractive for many applications including catalysis, diagnostics, therapy, and spectral signal enhancement, which enable the constructed devices/methods with unparalleled performance [22
]. For example, by using the reduction of p
-nitrophenol by NaBH4
as a model reaction, Xia et al. demonstrated that AuNCs are catalytically more active than solid gold nanoparticles [26
]. The advantages of AuNCs inspired us to investigate whether it is possible to utilize them as the electrocatalyst to design a new chemically-modified electrode for luteolin sensing. To the best of our knowledge, there has been no report about the employment of AuNCs for the development of electrochemical luteolin sensors.
Herein, we design a new sensitive electrochemical sensor for the determination of luteolin using AuNCs-modified carbon ionic liquid electrode (CILE) as a sensing platform (AuNCs/CILE). CILE is prepared by using 1-hexylpyridiniumhexafluorophosphate (HPPF6
) as the binder, which has been widely reported as the substrate electrode for electroanalysis [27
]. CILE has been proven to have advantages such as high conductivity, anti-fouling ability, and good stability, which are due to the use of high conductive ionic liquid (IL) as the binder and the modifier. The assay is performed by utilizing AuNCs/CILE as the working electrode via a voltammetric method. The electrochemical signal of luteolin can be recorded because luteolin is an electroactive compound [4
]. By monitoring the change in the electrochemical signal, we could quantitatively determine the concentration of target luteolin in samples. Notably, the employment of AuNCs as the electrocatalyst in this sensing system endows AuNCs/CILE with excellent electrocatalytic activity toward the redox reaction of luteolin with high sensitivity for luteolin detection. The study aims to emphasize that AuNCs with superior electrocatalytic activity can be utilized as an alternative to previously used nanocatalysts in electrochemical sensing of luteolin.
Luteolin (≥99%), baicalein (≥99%), and quercetin (≥99%) were purchased from Xi’an Yuquan Biotechnology Co. Ltd. (Xi′an, China). Graphite powder (average particle size 30 μm, >99%) was obtained from Shanghai Huayi Group Huayuan Chem. Industry Co. Ltd. (Shanghai, China). Paraffin liquid was obtained from Tianjin Damao Chemical Reagent Factory (Shanghai, China). HPPF6 (≥99%) was obtained from Lanzhou Yulu Fine Chemical Co. Ltd. (Lanzhou, China). AuNCs (50 µg/mL) were purchased from Nanjing XFNANO Materials Tech. Co. Ltd. (Nanjing, China). All other reagents and chemicals were of analytical grade at least. All aqueous solutions were prepared using deionized (DI) water (18.2 MΩ·cm) obtained from a Milli-Q water purification system (Millipore, Burlington, MA, USA). Phosphate buffered saline (PBS) was used as electrolyte and different pH values were prepared by mixing stock solutions of 0.1 M NaH2PO4 and 0.1 M Na2HPO4, and then the required pH values were adjusted by using 0.1 M H3PO4and 0.1 M NaOH.
All electrochemical measurements including cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were performed on a CHI 1220B electrochemical workstation (Shanghai CH Instrument, Shanghai, China). The electrochemical impedance spectroscopy (EIS) was performed on a CHI 750B electrochemical workstation (Shanghai CH Instrument, Shanghai, China). A conventional three-electrode system was used with AuNCs/CILE as the working electrode, a platinum wire electrode as the auxiliary electrode, and a saturated calomel electrode (SCE) as the reference electrode. Scanning electron microscopy (SEM) was conducted with a JSM-7100F scanning electron microscope (JEOL, Tokyo, Japan) and transmission electron microscopy (TEM) on a JEM-2010F transmission electron microscope (JEOL, Tokyo, Japan).
2.3. Fabrication of AuNC/CILE
In a standard fabrication procedure, CILE was initially prepared as follows [27
]. First, 4.8 g of graphite powder, 2.4 g of HPPF6
, and 1500 µL of liquid paraffin were mixed and heated at 80 °C for 1 h to form a homogeneous carbon paste. A portion of the formed carbon paste was then packed into one end of a glass tube that had a diameter of 4 mm, and a copper wire was inserted through the other end of the tube to establish an electrical contact. Before use, the electrode surface was polished on a weighing paper to obtain a mirror-like surface. Then, 8 µL of 50 µg/mL AuNCs water solution was dropped on the surface of CILE. After being dried at room temperature, the fabricated AuNCs/CILE was stored in a refrigerator at 4 °C for future use.