Photoluminescent Lanthanide-Doped Silica Nanotubes: Sol Gel Transcription from Functional Template

’ INTRODUCTION Since the discovery of carbon nanotubes, hollow nanotubes have attracted considerable attention due to their functional significance and potential applications in nanoscale devices, sensors, and energy storage/conversion. In particular, silica nanotubes raise special interest because of their biocompatibility, confined environment as nanocontainers, and feasibility of chemical modification of outer/inner surfaces and edges. Template-directed sol gel polycondensation of tetraethoxysilane (TEOS) is a representative method to synthesize silica nanomaterials. In the past decade, silica nanotubes as well as nanotube arrays have been fabricated using organogels, hydrogels, carbon nanotubes, and porous alumina membranes with ordered and vertical channel structures as templates. A variety of work have focused on the design of silica nanotubes with different structural parameters, such as size, shape, pore, or wall thickness. Alternatively, the functionalization and application of silica nanotubes are other hot research topics. Particularly, silica nanotubes doped with functional guests have received extensive interests benefiting from the combination of versatile guest functions and high robustness of the silica framework. Shinkai and co-workers utilized a phenanthrolin-appended gelator to create fluorescent silica organic inorganic composite materials through sol gel polycondensation. Jung prepared luminescent silica nanotubes with tunable emission colors by cocondensation of different functional dyes and TEOS in a organogel system. In a similar way, metals can be deposited inside the silica wall of rolled paperlike structures templated by a gemini type gelator in the presence of metal salts. The photoluminescence (PL) properties of trivalent lanthanide ions have aroused tremendous interest because of their broad applications in chemical or biological sensors, medical diagnostics, cell imaging, and thin film devices. The attractive features of lanthanide ions as luminescent materials include linelike emission, high quantum yield, long luminescence lifetime (μs ms range), high photochemical stability, and low long-term toxicity. The loading of lanthanide ions with host materials can offer the advantages of superior mechanical property, better processability, and thermal stability. Recently, B€unzli reported the synthesis of bare and NH2-functionalized silica nanoparticles (NPs) embedded with lanthanide binuclear helicate using a water-in-oil microemulsion technique. NH2-functionalized NPs are conjugated with avidin (NP-avidin) or goat antimouse IgG antibody (NP-IgG) to test them as luminescent biomarkers. Inspired by the above discussion, we are interested in the design of photoluminecent lanthanide-doped silica nanotubes. The silica backbone can not only serve to protect and stabilize the functional entities inside but also allow its optical properties to be probed from outside due to the chemical inertness and optical transparency of silica. On the basis of previous work of metalcholate supramolecular self-assembly, we use lanthanide-cholate hybrid hydrogels as templates to synthesize lanthanidedoped silica nanotubes. The sol gel transcription process is accompanied by the encapsulation of lanthanide ions into silica nanotube walls. The diameter of as-prepared silica nanotubes can be adjusted by changing the temperature in the preparation process. In addition, silica nanotubes with different luminescent color can be tuned by doping different lanthanide ions, such as Tb3þ, and co-doping Tb3þ/Eu3þ, into the soft template. A possible mechanism is proposed to disclose the sol gel transcription process and lanthanide ions incorporation.