Incorporation of N and O into the Shell of Silicon Nanoparticles Offers Tunable Photoluminescence for Imaging Uses

Silicon nanoparticles (SiNPs) show tunable photoluminescence (PL), water-dispersibility, high photostability, and low cytotoxicity, thus constituting promising candidates for bioimaging applications. Because SiNP PL depends finely on particles' crystallinity and surface composition, specific tuning of PL properties has remained elusive. Herein, using steady and time-resolved PL studies, absorbance spectroscopy, and electrochemical techniques, we have deeply analyzed the origin of the PL of SiNPs obtained from a wet chemical synthesis procedure based on the oxidation of Zintl salts in dimethyl formamide (DMF). Obtained SiNPs, surface-functionalized with propylamine terminal groups, were amorphous and 2.8-3.7 nm in size. The photophysical evidence, together with XPS and FTIR spectroscopy, supported a core-shell structure of the nanoparticles consisting of a silicon core surrounded by a 0.7-1.25 nm-thick oxidized silicon shell containing low concentrations of trapped iminium siloxyl ions Cl-(CH3)2N+=CH-O-Si or related compounds. The introduction of N-functionalities in the nanoparticle shell was assigned to the reaction of Si-Cl and Si-H bonds formed during synthesis, with DMF. The use of increasing amounts of NH₄Cl in the synthesis procedure led to more oxidized shell structures of SiNPs. It is suggested that the presence of an oxidized silicon shell containing trapped iminium siloxyl ions provided a high density of localized states capable of quenching the core-state emission and of being themselves populated by absorption of visible light. Moreover, it was experimentally confirmed that emission preferentially takes place from localized states introduced by O-functionalities with a high quantum efficiency (η_PL-trap≅ 1). As fluorophores, the obtained SiNPs display tunable PL emission and an important red-edge shift, allowing the selection of the PL by changing the excitation wavelength without modification of its chemical composition and size, thus meeting the needs of various types of biosensing methods.