Results and discussion
As shown in Scheme 1, the multifunctional composite polymer fibers were obtained via the in situ, photochemical reduction of Ag NCs in electrospun polymer fibers. To prepare a nanoparticle composite bulk material, naturally, many ex situ, bottom-up methods based on assembly strategy were considered, in which nanoparticles are treated as building blocks and a suitable assembly approach is required. However, these ex situ assembly methods are impracticable for Ag NCs since excessive surface modifications may cause the aggregation of NCs, resulting in fluorescence quenching. Therefore, the growth of metal NCs by an in situ reaction in bulk materials is proposed.
We first electrospun PMMA–PMAA nanospheres to prepare a kind of carboxylic group-rich polymer fibers. PMMA in this template only works for a relative hydrophobic matrix and a three-dimensional solid polymer network, which can protect and stabilize the produced Ag NCs as well [43]. Fig. 1a shows a photo of the PMMA–PMAA electrospun fiber mat. It forms a circular white membrane with a diameter of about 12 cm and the fibers are disordering wires with a width of about 600 nm (Fig. 1b). These fibers have abundant carboxylic groups (from PMAA). The role of carboxylic groups in preparing Ag NCs has been investigated in previous work [18], [33] and [43]. When synthesizing Ag NCs in aqueous phase, carboxyls can coordinate with Ag(I) ions, becoming a host of Ag(I). Also, it owns reducibility in a light-induced reaction and additionally protects the produced Ag NCs as a ligand. The FT-IR spectrum of electrospun polymer fibers (Fig. 1c) proves the existence of carboxyl groups due to the absorption band at 1730 cm− 1, which is attributed to the stretching vibration of carboxylate. The absorption bands at 1155 cm− 1 and 2930–3010 cm− 1 are related to the stretching vibration of formate esters and hydroxyl groups.