Preparation of Highly Stable and Photoluminescent Cadmium‐Free InP/GaP/ZnS Core/Shell Quantum Dots and Application to Quantitative Immunoassay

Abstract

Indium phosphide (InP) quantum dots (QDs) are ideal substitutes for widely used cadmium‐based QDs and have great application prospects in biological fields due to their environmentally benign properties and human safety. However, the synthesis of InP core/shell QDs with biocompatibility, high quantum yield (QY), uniform particle size, and high stability is still a challenging subject. Herein, high quality (QY up to 72%) thick shell InP/GaP/ZnS core/shell QDs (12.8 ± 1.4 nm) are synthesized using multiple injections of shell precursor and extension of shell growth time, with GaP serving as the intermediate layer and 1‐octanethiol acting as the new S source. The thick shell InP/GaP/ZnS core/shell QDs still keep high QY and photostability after transfer into water. InP/GaP/ZnS core/shell QDs as fluorescence labels to establish QD‐based fluorescence‐linked immunosorbent assay (QD‐FLISA) for quantitative detection of C‐reactive protein (CRP), and a calibration curve is established between fluorescence intensity and CRP concentrations (range: 1–800 ng mL−1, correlation coefficient: R2 = 0.9992). The limit of detection is 2.9 ng mL−1, which increases twofold compared to previously reported cadmium‐free QD‐based immunoassays. Thus, InP/GaP/ZnS core/shell QDs as a great promise fluorescence labeling material, provide a new route for cadmium‐free sensitive and specific immunoassays in biomedical fields.

Sensitive Immunoassay Based on Biocompatible and Robust Silica-Coated Cd-Free InP-Based Quantum Dots

“…After that, the InP/GaP/ZnS QD@SiO 2 nanoparticles were coupled with CRP antibodies as the fluorescent probes for the CRP quantitative detection by the QD-based fluorescence-linked immunosorbent assay (QD-FLISA). At last, the fabricated InP/GaP/ZnS QD@SiO 2 –FLISA exhibited a high sensitivity of 0.9 ng mL –1 for CRP, which was more than a 3.2-fold improvement over those of previously reported PMAH-coated InP/GaP/ZnS QDs and was close to that of state-of-the-art silica-encapsulated Cd-based QDs …”

“…The LOD was a ratio of three standard deviations (SDs) of 10 individual measurements to the slope ( K ) of the lowest concentration calibration plot (LOD = 3 SD/ K ), which was a significant parameter for evaluating the sensitivity of the immunoassay. Compared with our previous InP/GaP/ZnS QD@PMAH–FLISA, the key achievement in the InP/GaP/ZnS QD@SiO 2 –FLISA was that the detection sensitivity increased 3.2 times to 0.9 ng mL –1 . This indicated that the SiO 2 shell played a critical role in the high PL QY of InP/GaP/ZnS QD@SiO 2 and the InP/GaP/ZnS QD@SiO 2 with large surface area coupled with more antibodies, both of which had the benefit for improving the detection sensitivity.…”

“…InP-based QDs are anticipated to be one of the best ideal replacements for Cd- and Pb-based QDs, due to their being free of heavy metal ions, low toxicity, and being environmentally friendly. , Nevertheless, the stability and PL QY of the InP-based QDs are still not up to those of the Cd- and Pb-based QDs, despite some tremendous advances been achieved in the synthesis of InP QDs. ,, Most recently, InP-based quantum dots light-emitting diodes (QLEDs) have witnessed milestone progress with the improvement of the synthesis technology, and the performance can compare favorably with those of the state-of-the-art Cd-based QLEDs. ,,− However, the progress of InP-based QDs in biological detection is still lagging behind because the high-quality water-soluble InP-based QDs are difficult to synthesize. Although we have recently reported that the as-prepared high-quality InP/GaP/ZnS QDs are used to quantitatively detect C-reactive protein (CRP) antigens, the stability and centrifugal separation of InP/GaP/ZnS QDs are hard to control in some cases, and the detection sensitivity is still lower than that of the Cd-based QDs. , To further improve the detection sensitivity, it is imperative to adopt a more effective method to protect the fluorescence of the InP/GaP/ZnS QDs and improve the colloid stability in aqueous solution.…”

“…, tetramethyl orthosilicate, tetraethyl orthosilicate (TEOS), etc. ] in the reverse microemulsion method. ,− , This has indeed been experimented for both trioctylphosphine oxide-capped InP/ZnS QDs and oleylamine-capped InP/ZnS QDs. , However, the surface of InP-based QDs is prone to water and oxygen contamination, which results in a strong reduction of the PL QY after transferring from the organic form to the aqueous phase. , Comparing with the aforementioned surface ligands, 1-dodecanethiol (DDT) or 1-octanethiol (OT) represents another important type of ligand, which can effectively passivate the dangling bonds of the QD surface and therefore enables InP/GaP/ZnS QDs with superior photo-physical properties. ,,, However, due to the strong binding between this organic ligand and QD surface, the ligand exchange essentially failed, and the DDT-capped CdTe QDs cannot be coated in a silica sphere via the prevailing silica-encapsulated method. , Recently, a different silica-coating technique has been developed, which is reached via dispersing the synthesized QDs into a silica matrix to form a nanocomposite system. − Although the intrinsic high PL QY of the oil-soluble QDs is well maintained, it suffers from a severe QD aggregation, being detrimental for biological applications. − Therefore, it is strongly appealing to experimentally develop an efficient silica coating technique to simultaneously retain the superior PL QY and monodispersity of the oil-soluble QDs, which is certainly beneficial for a photo-stable and high-sensitivity biological detection utilizing an environmentally more friendly QD systems.…”

“…Herein, for the containing thiol-capped hydrophobic InP QDs, we describe a silica coating method to prepare high-quality InP/GaP/ZnS QD@SiO 2 nanoparticles without the PL QY sharp decrease. First, the high-quality oleic acid (OA) and OT-capped hydrophobic InP/GaP/ZnS QDs with the PL QY of 72% were synthesized by our recent work . Second, the pre-silanization of hydrophobic QDs with the waterless and ammonia-free hydrolysis of TEOS and subsequent reverse microemulsion method was adopted to obtain the high-quality InP/GaP/ZnS QD@SiO 2 nanoparticles.…”