Photoluminescence modulation of silicon nanoparticles via highly ordered arrangement with phospholipid membranes
Graphical abstract
Introduction
The strongly size-, shape- and surface-dependent optical properties of semiconductor nanoparticles (NPs) promise them as tunable emitters in applications such as illumination, imaging, and single-photon sources [[1], [2], [3], [4], [5]]. However, modulation of the NP photoluminescence (PL), including emission wavelength, monochromaticity, radiative lifetime, and photostability, etc., is still challenging to date [[6], [7], [8]]. Much efforts have been made to regulate the PL property of NPs via, for example, shell coating with a foreign material and ligand decoration by means of physical adsorption or covalent binding [[9], [10], [11]]. For example, the quantum yield and radiative lifetime of the colloidal CdSe NPs can be significantly modulated via surface reconstruction during synthesis or by introducing photooxidation with laser irradiation [12]. The stability of CdSe NPs was also found to be significantly influenced by the phase of the surrounding phospholipid molecules (i.e. liquid or gel phase) [13]. Through ion doping, the excitation intensity of BCNO (stands for the elements of B, C, N and O) nanocrystals can be improved and the emission spectra of them can be red shifted by up to 45 nm [14]. Moreover, the PL of Si nanocrystals has been successfully tuned across the entire visible spectral region via surficial functionalization without changing particle size [8]. However, the practical applications of the engineered NPs are somewhat impeded due to the complicated manipulation procedures, difficulties in purification after ligand functionalization, and/or easy aggregation of NPs. In this regard, self- or directed-assembly of NPs into well-ordered structures provides a promising solution to these problems [[15], [16], [17]]. Controllability in the structures, e.g. the inter-NP distance and the spatial organization of NPs, gives the NP assembling a unique advantage for PL modulation due to the coupling effect between adjacent particles and/or between particles and surrounding medium [7,11]. Specifically, plasmonic Au/Ag nanoparticles have been co-assembled with CdSe NPs. A precise control over the separation between the NPs and the plasmonic surface has been observed to exert an obvious influence on the emission spectra and lifetime of the NPs due to the exciton-plasmon coupling effect [[18], [19], [20]]. This method would bring in heterogeneous particles to the system. On the other hand, the introduction of a flexible and biocompatible scaffold for NP dispersion broadens the use of the NP-based composites in the field of bionanotechnology [21,22]. Many strategies including substrate-directed NP deposition and polymer- (as well as ligand, DNA and protein) modulated NP organization (in one-dimensional chains, two-dimensional layers or three-dimensional scaffold), have been intensively investigated [23,24]. However, challenges still lie in assembling NPs into a defect free, ordered and controllable structure, especially on a large scale. Moreover, all the works mentioned above focused on the modulation of PL properties of NPs including their quantum yields, emission wavelengths and radiative lifetimes. However, to the best of our knowledge, little work has been done to improve the PL monochromaticity of NPs.
In recent years, successful synthesis of small (2–5 nm) and water-dispersible silicon NPs (Si NPs), which have strong fluorescence, robust photostability and favorable biocompatibility, promises the fabrication of such composites for applications, especially those PL-related, in biosensing, long-term bioimaging, disease diagnosis and monitoring, to name a few [[25], [26], [27], [28]]. On the other hand, phospholipids have unique properties such as favorable biocompatibility, modifiable structure for biofunctionalization, and the ability to self-assemble into various phases including micelles, vesicles, liposomes, hexagons and mono-/multi-layers. Functionalization of nanostructures, e.g. nanoparticles, colloidal crystals, and even nanoporous (gold or alumina) surfaces, with phospholipids, further broadens their applications for biosensors and/or drug delivery, etc [29,30]. Therefore, phospholipids have been regarded as one of the most representative biological molecules and promised attractive advantages for biocomposite fabrications [13,[31], [32], [33]].
In this work, three types of water-dispersed Si NPs with different PL properties were synthesized. Phospholipids were used to organize the Si NPs into composite membranes with controlled arrangement. Based on a facile co-assembling method, a type of multi-lamellar composite lipid membranes was fabricated, in which NPs were sandwiched between adjacent layers with high in-plane ordering. On the basis of this, the PL properties of the NPs were efficiently modulated with a significantly improved monochromaticity, blue-shifted emission and shorter radiative lifetime; meanwhile, the Si NPs within membranes maintained robust photostability under UV irradiation.
Section snippets
Materials and machines
1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), 1,2-dioleoyl-sn-glycero-3-phospho-(1′-rac-glycerol) (DOPG), 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP) and 1,2-di-(10Z,12Z-tricosadiynoyl)-sn-glycero-3-phosphocholine (Diyne-PC) were purchased from Avanti Polar Lipids. The molecular structures of the lipids are shown in Fig. S1 (Supplementary information). (3-Aminopropyl)-trimethoxysilane (APS, 97%) and 1,8-naphthalimide were purchased from Sigma-Aldrich. Trisodium citrate dehydrate
Preparation of Si NPs
Characterizations of the three types of Si NPs are shown in Fig. 1 and Table S1. Under irradiation at 365 nm, the Si NPs exhibit blue, turquoise, and yellow colors, and accordingly are named as Si-Blue, Si-Turquoise and Si-Yellow, respectively. Under ambient light, the Si NP dispersions are transparent, light yellow or pale brown, respectively (Fig. 1a). These Si NPs can be stably dispersed in water for more than three months in ambient environment. Zeta-potential test indicates an almost
Conclusions
In this work, we fabricated a type of lipid/NP multi-lamellar nanocomposite membranes based on the co-assembly of lipids and small-sized (2.2–3.0 nm) Si NPs. Within the membrane, a long-scale ordered distribution of NPs was achieved with controllable inter-particle separation, and the PL properties of NPs were efficiently modulated. XRR, GIXRD and TEM measurements confirmed the existence of multilayered structure and ordered distribution of the NPs within layers. PL test demonstrated a
Author contributions
B.Y., K.Y. and Y.Q.M. designed the research. J.J.L. and B.S. performed the experiments. K.Y. performed MD simulations. B.Y. and K.Y. co-wrote the paper. All the authors analyzed and discussed the results and commented on the manuscript.
Notes
The authors declare no competing financial interest.
Acknowledgements
This work was financially supported by the National Science Foundation of China (Nos. U1532108, 21422404, 21374074, 21774092, and 21728502). B.Y. and K.Y. thank the support of the Natural Science Foundation of Jiangsu Province of China (Nos. BK20171207 and BK20171210). The authors sincerely thank Prof. Yao He (Soochow University) for the help in Si NP synthesis. The authors also thank the Small Angle X-ray Scattering Station (BL16B1) and X-ray Diffraction Station (BL14B) at Shanghai Synchrotron
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These authors contributed equally to this work.