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
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
(ηPL‑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.