Polydopamine-based functional composite particles for tumor cell targeting and dual-mode cellular imaging
Graphical abstract
Introduction
Imaging technology, one of the most intuitive ways for pathological changes research in lesions, is an essential part of cancer clinical protocols, which could furnish morphological, structural, metabolic and functional information [1]. Fluorescence (FL) technique is a commonly used imaging modality in cancer research due to its high spatial resolution, high speed, intuitive observation and ease of manipulation [2], [3]. Nevertheless, fluorophore molecules in living cells may suffer from photo bleaching. What's more, the background fluorescence interferences also influence the FL image quality of the biological samples [3]. Surface-enhanced Raman scattering (SERS) imaging techniques, as an alternative method to FL imaging, may make up for those disadvantages due to its high stability under exposure to photo and super sensitivity. Moreover, SERS imaging can provide abundant vibrational information of molecules [4], whose “fingerprint” information could mark specific molecules and monitor their changes. However, SERS imaging normally adopts the acquirement mode of point by point and then is constructed by computer, which is time-consuming. Accordingly, it is imperative to integrate the merit of fluorescence and Raman signal into composite nanoparticles for multimodal imaging [5].
Composite nanoparticles have been widely applied in bio-sensing, drug delivery, and disease diagnosis [6]. Recently, composite nanoparticles possessing FL and SERS tags have been considerably investigated in the fields from biological imaging to nanoparticle-cell interaction monitoring [7], [8]. Functional composite particles with FL-SERS dual-mode imaging capability could improve not only the imaging velocity but also spatial resolution. By using CVD method, Chen and co-workers reported the Graphene-isolated-Au-nanocrystal nanostructures to realize multimodal cell imaging [9]. However, the CVD method requires large-scale and expensive instruments, which in turn restricts their real application. Cui and co-workers [10], [11], [12] reported numerous silica-based FL-SERS dual-mode composite to realize cell imaging, but incorporated probes/tags were prone to leach/dissociate from silica particles [13]. Therefore, it is highly desirable to develop simple, fast and low-cost methods to synthesize stable composite nanoparticles with favorable biocompatibility and excellent multimodal imaging capacity.
Dopamine (DA), a bioactive molecule that contains both catechol and amine functional groups, can self-polymerize at alkaline conditions to form polydopamine (PDA) [14]. PDA is a kind of bio-mimetic melanin-like material which processes excellent biocompatibility for in vivo study [15]. Lee et al. proved that PDA spheres could serve as an active template for the convenient fabrication of multiple nanostructures, eg. PDA/Fe3O4 core/shell spheres [16], Au@PDA nanoparticles [17], and PDA@Ag nanoparticles [18]. In these cases, because of the reductive and adsorptive nature of PDA, no surface modification is further required for binding the desired molecules or nanoparticles via active groups of –OH and –NH2, which makes the functionalization process simpler and more convenient than common templates, e.g. PS spheres [19], [20] or SiO2 spheres [21], [22]. Moreover, biomacromolecules can be easily decorated on PDA surfaces via Michael addition/Shiff base mechanism [23].
As an essential nutrient for cell growth, iron is transported into cells by iron-carrying holotransferrin (holo-Tf) via a transferrin receptor (TfR)-mediated endocytosis pathway [24], [25]. As is known, TfR is the ubiquitous human cell surface glycoprotein related to cell proliferation [26]. To maintain fast metabolism/proliferation, the malignant tissues have higher expression of TfRs than in normal tissues because of the higher iron demand for faster cell growth and division [27]. Consequently, Tf can be potentially utilized as a cell marker for tumor detection. Tf–TfR interaction has been employed as a potential efficient pathway for cellular uptake of contrast agent, drugs and genes [28], [29], [30]. Therefore, the incorporation of Tf can enhance the cellular uptake of dual-mode composite particles.
In this work, we report the fabrication of gold/polydopamine-Methylene Blue@Bovine Serum Albumin-glutaraldehyde-Transferrin (Au/PDA-MB@BSA-GA-Tf)-based SERS/FL dual mode functional composite particles for SERS imaging and FL imaging. Considering that gold nanoparticles (Au NPs) process numerous unique properties such as ease of functionalization, good stability, biocompatibility [31], [32], [33] and excellent SERS performance [34]. Herein, Au NPs were produced in situ by the abundant catechol and amine groups in PDA and then adhered on PDA spheres in a tight fashion. Additionally, the PDA spheres processed high-efficient adsorption capacity for methylene blue (MB) due to the synergistic effect of electrostatic interaction and π-π stacking [35]. MB could play both roles as a Raman reporter and a fluorescence label [36]. It should be stated that MB is an inexpensive phenothiazine dye with low toxicity, which has been approved for clinical use [37], [38]. Bovine serum albumin (BSA) is the most popular material for surface coating and protection for SERS probes [39]. This dual-mode functional composite particles showed three merits: First, they could be easily synthesized in a massive way. Second, they were biocompatible imaging reagent with FL/SERS dual mode imaging capability. Third, they showed excellent stability in physiological (pH 7.4 PBS buffer) condition. The acquired SERS/FL signals were strong enough for tracking composite particles in living cells.
Section snippets
Materials
Tris(hydroxymethyl)aminomethane hydrochloride (Tris, 99%), dopamine hydrochloride (DA, 98%), and methylene blue (MB, 95%) were purchased from Aladdin (Shanghai, China). Ethanol (≥ 99.7%) were obtained from Rich-joint chemical Reagent Co.,Ltd (Shanghai, China). Chloroauric acid (HAuCl4, 99%) was purchased from Sinopharm Chemical Reagent Co.,Ltd (Shanghai, China). Trisodium citrate (Na3Cit, ≥ 99%), glutaraldehyde (GA, 50%), Transferrin (Tf, ≥ 98%), Dulbecco's Modified Eagle's Medium (DMEM) and
Preparation of Au/PDA-MB@BSA-GA-Tf NPs
The general synthetic process of the targeting composite particles is presented in Scheme 1. In brief, the whole process involved: (1) the synthesis of PDA spheres; (2) by utilizing the reducing property of abundant active catechol (-OH) and amine group (-NH2) on the PDA spheres surface, the gold ions can be reduced to form Au/PDA NPs; (3) the loading of MB in Au/PDA NPs, endowing NPs with Raman and fluorescence properties; (4) an outer layer of BSA/GA is formed outside the above composite
Conclusions
In summary, we developed a simple method to synthesize Au/PDA-MB@BSA-GA-Tf NPs with dual-modal imaging capability, in which PDA spheres play an important role in gold ion reducing, AuNPs binding and MB adsorption. Positively charged MB molecules as a drug model can adsorb on negatively charged PDA spheres via electrostatic interaction and π-π stacking and act as both Raman reporter and fluorescence source. Tf was used to enhance the cellular uptake of composite particles. The whole fabrication
Acknowledgement
This work was supported by the National Natural Science Foundation of Shanghai Municipality (grant number 14ZR1430200) and the National Natural Science Foundation of China (grant number 21475088).
Novelty statement
Polydopamine (PDA)-based composite particles with both tumor cell targeting and fluorescence (FL) / surface-enhanced Raman scattering (SERS) dual-modal imaging capabilities are reported for the first time. PDA spheres act multiple roles as active template, gold nanoparticles (AuNPs) binding agent, gold ion reducing agent as well as methylene blue (MB) adsorption agent, simultaneously.
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