Gold nanoclusters-poly(9,9-dioctylfluorenyl-2,7-diyl) dots@zeolitic imidazolate framework-8 (ZIF-8) nanohybrid based probe for ratiometric analysis of dopamine

Highlights

• A novel strategy of encapsulating dual nano-emitters into metal-organic framework (MOF) was proposed

• A “two-in-one” ZiF-8 encapsulated gold clusters-polymer dots (ZiF-8@AuNC-PFO) probe was constructed.

• The aggregation fluorescence enhancement and DA-triggered asynchronous fluorescence quenching were used.

• The dual-emission probe achieved high selective and ratiometric sensing of DA.

Abstract

Ratiometric analysis of dopamine (DA) in complex biological system is urgently desired. In this work, a novel dual-emission fluorescence probe was fabricated by embedding both gold nanoclusters (AuNCs) and poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO) dots into zeolitic imidazolate framework-8 (ZIF-8) (abbreviated as ZIF-8@AuNCs-PFO) and applied to ratiometric analysis of DA. Remarkably, encapsulating AuNCs and PFO dots into ZIF-8 not only achieved an excellent aggregation induced emission (AIE) enhancement effect on AuNCs (5 times increase), but also brought about an unique DA-triggered asynchronous fluorescence changes of AuNCs and PFO dots. The as-prepared probe exhibited excellent performance toward DA in the concentrations range from 0.01 to 10000 μmol L⁻¹ and good selectivity over interfering substances. The detection limit of DA was as low as 4.8 nmol L⁻¹. Furthermore, good stability and practicability of the probe in human serum samples suggesting its great potential for diagnostic purposes. Moreover, the quenching mechanism of AuNCs was intensively studied and summarized as three synergistic processes: (i) electron transfer between AuNCs and DA; (ii) DA-triggered architecture change of ZIF-8; (iii) fluorescence resonance energy transfer (FRET) between AuNCs and polydopamine (PDA), which offered an important theory for ZIF-based fluorescent probes.

Introduction

Dopamine (3,4-dihydroxyphenethylamine, DA), an essential endogenous neurotransmitter for communicating neurologic information between cells, has been identified its critical roles in maintaining physiological functions and dominating the emotions and perceptions of human [1]. The abnormal level of DA in body is closely interrelated with schizophrenia, cardiovascular, Parkinson’s, Alzheimer’s disease and so on [2]. Over the past few decades, considerable efforts have been devoted toward devising various assay methods for DA, including colorimetry [3], high-performance liquid chromatography (HPLC) [4], electrochemical measure [5,6] and optical sensing [[7], [8], [9], [10]]. Electrochemical measure with prominent sensitivity has been regarded as the most promising detection technique of DA [11,12]. Unfortunately, the drawbacks of poor reproducibility and stability have restricted its application in clinical diagnosis. By comparison, optical probes are particularly attractive in real-time, on-site and in vivo assays owing to their advantages of simplicity, visualization, fast response and good reproducibility. Thus far, an enormous amount of fluorescence probes (organic dyes [13], quantum dots (QDs) [14], metal nanoclusters (MNCs) [15], up-conversion nanoparticles (UCNPs) [16] etc.) have been employed for DA detection. Even so, these frequently-used single-signal sensing modes are always affected by the environment variation (e.g. path length, scattering, background light) [17]. In this respect, precise sensing of DA in complex biological system is urgently desired.

Recently, ratiometric assay has inspired extensive studies due to its extraordinary capability regarding the built-in self-calibration of dual readout signals [18]. For instance, Wang’s group utilized AuNCs and the enzyme catalysis reaction to achieve ratiometric fluorescence detection of DA and tyrosinase (TYR) [19]. However, the ratiometric sensing mode could be only achieved in the co-existence of TYR and DA, which is not suitable for TYR-free system. To address this issue, dual nano-emitters based ratiometric probes have been designed, such as QD_S-carbon dots (CDs) [20], CDs-AuNCs [21] and CDs-CuNCs [22]. However, a lack of sufficient selectivity regarding these hybrid architectures was continually encountered, making them susceptible to interference from other substances [1]. In addition, the quantum yield (QY) and stability of dual nano-emitters have been frequently affected by complicated multistep synthesis and coupling process. Hence, actualizing the simple and fast construction of modification-free ratiometric fluorescence probes with high selectivity is of great significance and challenge.

Metal-organic frameworks (MOFs) are a category of materials with confirmed crystalline structures prepared by assembling metal ions with organic ligands. They hold large internal surface area and uniform but tunable cavities, which have been considered to be one of most promising host matrices to encapsulate multifarious guests, such as molecules, peptides, nucleotides and nanoparticles [23]. The programmable organic ligands and metal ions of MOFs can endow them various and tailorable functionality, especially selectivity or chirality [24]. Inspired by this, the employment of MOFs as host matrices to encapsulate two or more fluorescence emitters would open new paths toward ratiometric fluorescence assay with both high accuracy and selectivity. So far, seldom reports on the encapsulation of two nanoparticles into MOFs is available.

Thus, undergoing careful screening and design, this work successfully constructed a “two-in-one” ratiometric fluorescence probe by encapsulatiing AuNCs and poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO) dots into zeolitic imidazolate framework-8 (ZIF-8) (abbreviated as ZIF-8@AuNCs-PFO). As demonstrated in Scheme 1, the AuNCs (orange fluorescence) and carboxyl functionalized PFO dots (blue fluorescence) were initially synthesized by the chemical reduction reaction and nanoprecipitation method, respectively. The ZIF-8@AuNCs-PFO nanoprobe was prepared by a simple self-assembly process of AuNCs, PFO dots, Zn²⁺ and 2-methylimidazole precursors without any chemical coupling. Excitingly, the obtained probe possessed excellent responds and selectivity toward DA. The probe have three fluorescence emission peaks at 438 nm and 465 nm (derived from PFO dots) as well as 600 nm (derived from AuNCs). After it encountered with DA, the fluorescence of AuNCs in ZIF-8@AuNCs-PFO was sharply quenched, while the fluorescence of PFO dots was slightly quenched. Based on the fluorescence quenching difference, the probe achieved ratiometric sensing of DA and exerted brilliant analytical performance.