br Luminescent imaging that can be double checked
Luminescent imaging that can be “double-checked” by two emission signals is desirable to provide more reliable information for accurate and precise detection of cancer cells. For developing a “double-check” luminescent probe, the general requirement is to select two emissive moieties with the properties as: (i) high photostability to enable reliable signals to be recorded over excitation; (ii) large Stokes shifts to allow minimum crosstalk between excitation and emission spectra; (iii) dis-tinct emissions to ensure detection with high sensitivity; and (iv) long-
Scheme 1. Schematic diagram for the preparation of the nanoprobe [email protected] for cancer cell-targeted “double-check” luminescence imaging. (A) Preparation of the core–shell nano-particles, [email protected]; (B) conjugation of FA onto the surface of the core–shell nanoparticles to aﬀord the nanoprobe [email protected];
(C) cancer cell recognition through FA-FR binding;
(D) “double-check” imaging of FR overexpressed cancer cells at steady-state luminescence and TGL modes.
lived emissions to facilitate the time-domain detection and imaging, i.e., time-gated luminescence (TGL) detection and imaging [9,27]. Taking these criteria in consideration, luminescent lanthanide com-plexes with appropriate photochemical/physical properties could be a favorable emissive moiety for preparing “double-check” luminescent probes for detection and imaging of cancer cells [35–42].
Herein we report a FA-functionalized dual-emissive nanoprobe based on core–shell silica nanoparticles, [email protected], for specific recognition and “double-check” luminescence imaging of cancer cells. As shown in Scheme 1, after 5-carboxyte-tramethylrhodamine (CTMR) was covalently doped in silica core, the shell was engineered by conjugating a Eu3+ complex having long-lived luminescence, BHHBCB-Eu3+ , within the silica matrix (Scheme 1A). Then folic (−)-Apomorphine (FA) that has high aﬃnity to folate receptor (FR) [43–45], a membrane-anchored cancer cell biomarker, was covalently bound onto the surface of silica nanoparticles to aﬀord the dual-emis-sive nanoprobe [email protected] (Scheme 1B). The experi-mental results reveal that the nanoprobe is low cytotoxic with excellent luminescent properties, and can target FR-overexpressed cancer cells through the FA-FR binding interaction (Scheme 1C). These features enabled the nanoprobe to be potentially used for the “double-check” imaging of FR-overexpressed cancer cells at steady-state luminescence and TGL modes (Scheme 1D).
2. Materials and methods
2.1. Materials and physical measurements
Tetraethyl orthosilicate (TEOS), (3-aminopropyl)triethoxysilane (APTES), CTMR, and 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenylte-trazolium bromide (MTT) were purchased from Sigma-Aldrich. FA-PEG2000-COOH was obtained from Ponsure Biotechnology Ltd., Shanghai, China. BHHBCB was synthesized according to the previous method . N-Hydroxysuccinimide (NHS) and 1-(3-
dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC) were obtained from Aladdin, China. Deionized water was used throughout. Unless otherwise stated, all chemical materials were purchased from commercial sources and used without further purification. HeLa cells (human cancer cells) were provided by Dalian Medical University, and L-02 cells (human fetal hepatocyte cells) were obtained from School of Pharmaceutical Science and Technology, Dalian University of Tech-nology.
The morphology and size of nanoparticles were characterized using a JEOL JEM-2000EX transmission electron microscope (TEM). The particle dynamic size distribution and zeta potential were measured on a Nano Zeta-Sizer (Malvern instruments). FT-IR spectra were measured on a Nicolet iS10 (Thermo Fisher Scientific Inc., USA) spectrometer at a resolution of 4 cm−1 for 32 scans. Steady-state luminescence and TGL spectra were measured on a Perkin Elmer LS 50B luminescence spec-trometer. All luminescence images were acquired on a custom modified luminescence microscope. Confocal luminescence cell images were re-corded on an OLYMPUS FV-1000 microscope (Ex 405 nm, Em 540–640 nm).