Carbon dots-embedded fluorescent molecularly imprinted photonic crystals hydrogel strip for accurate and selective detection of rutin in Sophora japonica products

Highlights

• A CDs-embedded fluorescent molecularly imprinted photonic crystals hydrogel (FMIPH) strip was developed.

• The CDs-embedded FMIPH strip exhibited stable and reproducible fluorescence response to rutin.

• The CDs-embedded FMIPH strip had sensitive and selective recognition performance to rutin.

• The strip supplied an effective strategy for detecting rutin in functional foods.

Abstract

This study aimed to develop a fluorescent molecularly imprinted photonic crystals hydrogel (FMIPH) strip based on carbon dots (CDs) and inverse opal photonic crystals (PCs) for the detection of the active ingredient rutin in Sophora japonica products. The FMIPH-based sensing system was constructed using rutin as template molecules, 4-vinylpyridine as functional monomers, ethylene glycol dimethacrylate as the cross-linker, and inverse opal PCs with three-dimensional macroporous structure as the skeleton. The prepared CDs, SiO₂ microspheres, PCs model, and FMIPH were characterized in detail, and the prepared CDs-embedded FMIPH strip exhibited stable and sensitive reproducible fluorescence response, and selective recognition capability to rutin. The linear fluorescence response of CDs-embedded FMIPH to rutin was obtained in the concentration range of 2.5–40 μg mL⁻¹ (R² = 0.9876) with a limit of detection of 2.3 μg mL⁻¹ and a limit of quantitation of 7.2 μg mL⁻¹. The CDs-embedded FMIPH strip obtained satisfying recoveries (81.7–95.6 %) with a relative standard deviation less than 3.1 % (n = 3) in spiked samples, having a good agreement with the results from high-performance liquid chromatography (R² = 0.9945). This portable strip could be used for rapid identification and quantitative detection of rutin in complex matrices, showing broad prospects in the quality evaluation and grading of functional foods.

Introduction

Rutin is an active ingredient in Flos Sophorae Immaturus (a “medicinal and food homologous” food), which is considered as a flavonoid derivative with a wide range of sources. Rutin flavonoids have been widely used in clinical treatment, dietary regulation or the development of functional foods due to their free-radical scavenging, immunomodulatory, anti-oxidation, anti-inflammatory, antibacterial and other functions [1], [2]. In the field of food and medicine, Flos Sophorae Immaturus is the main source of rutin flavonoids. It is even an important functional food of import and export standards [3], [4]. The content of rutin in Flos Sophorae Immaturus is closely related to varieties, producing areas and environmental conditions, which can be used as a reference standard to evaluate the quality of Sophora japonica and its products. Therefore, the quantitative analysis of the iconic active ingredient rutin is helpful to the quality classification and evaluation of functional foods such as Sophorae japonica products, which is more conducive to the formulation and revision of relevant import and export standards.

Currently, the strategies for determining the content of rutin flavonoids are mainly based on large-scale analytical instruments, such as high-performance liquid chromatography (HPLC) [5], [6] and high-performance liquid chromatography-mass spectrometry (HPLC-MS) [7], [8], capillary electrophoresis [9], [10] and electrochemical method [11], [12]. Although these methods are efficient, accurate and can almost cover the detection of most ingredients in foods, the shortcomings of expensive analytical instruments, relatively cumbersome testing process, and time-consuming process restrict their application in rapid screening, and even real-time online detection. Fluorescence sensing has the characteristics of high sensitivity, fast speed, and easy operation, having been one of the research hotspots and applied in various detection fields [13], [14], [15]. At present, the fluorescence signal for the analysis mainly comes from organic fluorescent dyes, metal nanoclusters, carbon dots (CDs), and other nanoscale materials [16], [17], [18]. These materials could be further applied to the development of miniaturized and integrated fluorescence sensing and detection devices. The CDs have excellent optical properties, good biocompatibility and low cytotoxicity, and can be combined with other materials to prepare nanocomposites, which have been widely used in the study of fluorescence sensing [19], [20].

Identification of trace targets accurately and effectively in complex matrices is necessary to analyze ingredients in food samples. Molecularly imprinted polymers (MIPs) are a kind of artificially synthesized biomimetic host-guest complex that can specifically recognize targets, which have been widely used in the purification of food matrices and the enrichment and identification of trace targets [21], [22], [23]. The focus of current research is to maintain the specific recognition ability of MIPs for the targets, while solving the problem of mass transfer barriers in the process of molecular recognition, which is vital in improving the detection efficiency of food. The inverse opal photonic crystals (PCs) have a three-dimensional macroporous structure that can provide more mass transfer channels for the molecularly imprinted polymerization system, which is more suitable for improving mass transfer efficiency than its Bragg diffraction performance [24], [25], [26]. This is beneficial to the rapid diffusion of the targets, thereby increasing the mass transfer speed in the highly crosslinked system. Fluorescent molecularly imprinted materials using inverse opal PCs as the skeleton not only have high porosity, but also can form specific recognition sites in the imprinting system [27], [28]. This can increase the number of specific recognition sites and their spatial arrangement order to a certain extent, which is beneficial to improve the selective recognition ability of fluorescent imprinted materials.

In this study, a CDs-embedded fluorescent molecularly imprinted photonic crystal hydrogel (FMIPH) with a specific recognition ability for rutin was prepared using inverse opal PCs as the skeleton. A portable strip for rapid fluorescence detection of rutin was further developed. The fabricated CDs-embedded FMIPH strip exhibited good fluorescence response and selective recognition ability to rutin, providing an effective and promising strategy for the quality evaluation and grading of functional foods.