Fluorescent methods are widely used for the sensing of compounds due to their fast, convenient, sensitive, and reliable detection of analytes. Since traditional organic fluorophores have several potential drawbacks such as low water solubility and photobleaching, fluorescent nanomaterials are being actively developed currently in this area, because they can finely disperse in water and provide excellent spectroscopic properties such as brighter fluorescence, wider selections of excitation and emission wavelengths, and higher photostability [1]. As representative fluorescent sensing methods using nanoparticles (NPs), fluorescence resonance energy transfer (FRET) [2] and metal enhanced fluorescence (MEF) [3] are well known in the field of chemical sensing.

Semiconductors quantum dots (QDs) are often used as FRET donors [2]. Recent progress has, for example, led to the development of a FRET assay by Katayama et al. to monitor protein kinase (PKA) activity using CdSe/ZnS QDs and dye-labeled peptides [4]. CdSe/ZnS QDs modified with molecules, which can capture phosphate groups strongly and selectively, were used to interact with dye-labeled phosphorylated peptides, leading to FRET from QDs to dyes. This ratiometric assay showed quantitative capability and enabled the measurement of PKA activity. In addition, cadmium-free QDs such as carbon and silicon are used for this objective from the viewpoints of environment and biocompatibility. Chen et al. developed a fluorometric aptasensor for the determination of Escherichia coli O157:H7 (E. coli O157:H7) based on the FRET effect between aminated carbon QDs and graphene oxide [5]. This method showed a good linear relationship with concentrations of E. coli O157:H7 in the range 102–107 cells mL−1, with the detection limit of 89 cells mL−1. Han et al. reported on the rapid determination of Sunset Yellow (SY) in soft drinks using FRET from silicon QDs to SY [6]. This approach had a good linear relationship in the range of the SY concentration of 0.050–14.0 μg mL−1 and the detection limit is 0.023 μg mL−1. Also, metal NPs work as FRET acceptors based on light absorption originated from the localized surface plasmon resonance (LSPR) [2]. Shaghaghi et al. attained the sensitive determination of pantoprazole based on FRET between fluorescent terbium complexes and silver (Ag) NPs [7]. The concentration range of the good linear relationship and the limit of detection were 10–7–10–5 M and 7.2 × 10–8 M, respectively.

Metal NPs interact with proximal fluorophores at an appropriate distance and increase their quantum yields. This phenomenon is called MEF, which is attributable to the interaction between fluorophores and LSPR generated from metal NPs [3]. In recent years, MEF-based methods have been frequently applied to improve the sensitivity [8]. Shiigi et al. achieved the high-throughput, highly sensitive fluorescent analysis using an enzyme-linked immunosorbent assay that employed a metal NPs-immobilized 96-well plate [9]. The fluorescence intensity of each well was measured with a plate reader after naturally drying the aqueous solution containing fluorophores in a dark room. Despite an immobilized amount of fluorophore, the fluorescence intensity increased linearly upon increasing the amount of Ag NPs. The Ag NP-immobilized plate showed the best MEF effect among all plates immobilized by metal NPs (Ag, Au, and Pd NPs) examined in that study. When the Ag NPs-immobilized plate was used to detect biomolecules and bacteria, both the fluorescence intensity and the detection limit were improved by more than 100 times compared with those of the unmodified 96-well plates [10].

Particularly in the biological field, near-infrared (NIR) fluorescent sensing is desirable nowadays due to the reduction of autofluorescence from biological tissues and the further tissue penetration depth compared to those with UV- and visible lights. Therefore, the technical innovation of NIR-emitting QDs and NIR-dye-doped NPs will rapidly proceed from now on in the chemical sensing.

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