Preparation And Properties Of Upconversion Fluorescent Silk Fibroin Films

Flexible luminescent films have promising applications in solar cells, display devices, LEDs, implantable devices and so on. Organic dyes, phosphors and quantum dots are often used as luminescent materials. Lanthanide-doped upconversion nanoparticles(UCNPs) are capable of emitting visible light under near-infrared light excitation. As a contrast, luminescent films fabricated with UCNPs are attracting more and more attentions due to their superior physicochemical features such as chemical stability, high photostability, long-lived luminescence, large anti-Stokes shifts, narrow emission bands, deep penetration and so on.Polymer such as poly(methyl methacrylate)(PMMA), polydimethylsiloxane(PDMS), polycarbonate(PC), Polystyrene(PS), polyethylene terephthalate(PET) are often employed to prepare composite films based on UCNPs. However, these synthetic polymer substrates made from non-renewable fossil resources are not friendly to environment. In particular, with the development of biocompatible and implantable optoelectronic devices, biocompatible, biodegrade, sustainable and lightweight substrate materials are urgently needed.Compared to synthetic polymers, silk fibroin(SF) owns excellent optical, biocompatible, implantable, biodegradable and mechanical properties and is easy to be processed into film, gel, fiber, scaffold, sponge and so on. Therefore SF may be the suitable substrate material for UCNPs to prepare the composite fluorescent films which have important applications in wearable, implantable and biodegradable optoelectronic devices.In this paper, NaYF4:Yb, Er nanocrystals are taken as an example to functionalize SF films to prepare fluorescent composite films while SF is used as a flexible substrate material. In order to overcome the defects of low upconversion fluorescence efficiency of NaYF4:Yb, Er nanocrystals, two types of fluorescence enhancement composite films are further fabricated on the principle of plasmon enhancing upconversion luminescence based on surface plasmon resonance(SPR) effect of Au nanoparticles and silver nanowires(AgNWs).(1) β-NaYF4:Yb,Er nanocrystals with the average diameter of ~35 nm are firstly prepared by thermal decomposition. The UC emission spectra of Na YF4:Yb,Er nanocrystals under 980 nm laser excitation shows green and red emissions located around 520, 540 and 660 nm, which arise from the 4f configuration transitions of Er3+ from 2H11/2 to 4I15/2, 4S3/2 to 4I15/2 and 4F9/2 to 4I15/2, respectively. The main upconversion luminescence mechanism in Na YF4:Yb,Er nanocrystals is energy transfer upconversion(ETU).(2) The β-NaYF4:Yb,Er nanocrystals are employed on silicon substrates by spin-coating and embedded into SF aqueous solution to obtain the composite SF-UCNP films. The composite films exhibit a transparency of 88% in the visible region when the concentration of Na YF4:Yb,Er nanocrystals is 10 mg/mL. The Na YF4:Yb,Er nanocrystals are well dispersed in SF matrix without obvious agglomeration. The FTIR, TG and DSC results indicate that the secondary structure of SF films does not change by adding UCNPs. SF-UCNP film displays a visible green emission under 980 nm laser excitation in dark.(3) A simple spin-coating-sputtering-transferring method is employed to prepare plasmon enhancing upconversion luminescent silk fibroin films. The composite films obtain the maximal enhanced fluorescence intensity when the sputtering time is 30 s. After that the fluorescence intensity decreases rapidly because the nonradiative decay dominates which leading to the quenching of upconversion fluorescence. The composite films present a transmittance of 80% and the fluorescence lifetime decreases when the sputtering time is 30 s. The morphology of the composite films is not affected by Au nanoparticles.(4) The isopropanol solution of AgNWs and Na YF4:Yb,Er cyclohexane solution are spin-coated onto a silicon wafer substrate in turn, then embedded into SF aqueous solution to obtain the composite SF-AgNW-UCNP films with excellent optical transparency(≥80%), conductivity(≤20 Ω/sq) and flexibility performance. Furthermore, the composite SF-Ag NW-UCNP films obtain the maximal enhanced upconversion fluorescence due to surface plasmon resonance(SPR) of Ag NWs when the concentration of NaYF4:Yb,Er nanocrystals is 0.5 mg/mL. The fluorescence enhancement factors of SF-Ag NW-UCNP films compared to SF-UCNPs film at 520 nm, 540 nm and 660 nm are 3, 2.7 and 3.38 respectively. Ag NWs not only form conductive networks used as transparent conductors, but also enhance upconversion fluorescence intensity through SPR effects.