Fluorescent Silicon Nanoparticles Featuring Excitation Wavelength-dependent Fluorescent Spectra For Anti-counterfeiting

Fluorescent silicon nanoparticles(SiNPs), known as the most important zero-dimensional silicon nanostructures, exhibit unique optical properties. The past several years have witnessed great advancement of the fabrication of SiNPs. However, for most of the reported strategies, relatively expensive or/and toxic silicon precursors are generally required, and additional organic reagents are often utilized in the synthetic procedures. Furthermore, although scientists have developed a great deal of fabrication techniques to prepare SiNPs, there are scanty information concerning the relationship between the photoluminescence(PL) property of the SiNPs and solvent polarities.By using gramineae plants(rice husks, sugarcane bagasse and wheat straws) as natural reaction precursors, we herein introduce a green synthetic strategy, which is efficacious for facile production of crystalline, fluorescent(quantum yield: ~15%) and small-size(diameter: ~4 nm) SiNPs. The resultant SiNPs possess stable fluorescence during 1-month storage free of any specific protection. More interestingly, the as-prepared SiNPs feature excitation-wavelength-dependent fluorescent properties, whose maximum emission peaks are tunable from ~430 nm to ~550 nm at serial excitation wavelengths ranging from 300 nm to 500 nm, respectively. Taking advantages of unique optical properties, we further explore the prepared SiNPs as a novel kind of fluorescent inks for anti-counterfeiting applications. The patterns and feathers are coated with a drop of the aqueous solution of SiNPs. When illuminated with different excitation wavelengths, clear excitation-wavelength-dependent fluorescence can be observed in the patterns and feathers.Furthermore, we report the SiNPs with an average diameter of 3.8~5.2 nm are prepared in various kinds of solvents with different polarities by microwave irradiation at 160 oC/8 h. Typically, transmission electron microscopy(TEM) analysis of the SiNPs prepared in butanone(ET(30)=41.3) indicates an average diameter of 3.8±0.4 nm. TEM analysis of the SiNPs prepared in dimethylsulfoxide(DMSO, ET(30)=45.1) shows an average diameter of 4.8±0.3 nm. The SiNPs prepared in acetic acid(ET(30)=51.7) have an average diameter of 5.2±0.3 nm as determined from TEM analysis. TEM analysis indicates the SiNPs prepared in glycol(ET(30)=56.3) possess an average particle size for 5.0±0.3 nm. TEM analysis of SiNPs synthesized in water(ET(30)=63.1) shows that the SiNPs are narrowly distributed with diameters in the range of 4.0±0.2 nm. When excited at 420 nm, the PL spectrum of SiNPs synthesized in butanone shows a maximum emission at 480 nm(photoluminescence quantum yield(PLQY): 7%); the PL maximum of SiNPs synthesized in DMSO appears at 510 nm(PLQY: 3%); the PL spectrum obtained from SiNPs synthesized in acetic acid shows a maximum emission centered at 620 nm(PLQY: 8%); the SiNPs synthesized in glycol show a PL maximum at 525 nm(PLQY: 5%); the PL maximum of SiNPs synthesized in water appears at 490 nm(PLQY: 19%). These SiNPs samples maintain stable fluorescence and preserve 87%~95% of the initial intensity after 150 min of UV irradiation.In summary, we demonstrate gramineae plants as low-cost reaction precursors, capable of producing SiNPs with fluorescence. The green synthetic strategy may open up new promising avenues for fabricating SiNPs in a green manner. Furthermore, we make a systematic effort to alter the solvent with different polarities to tune the PL properties of SiNPs, and provide the important information for tuning the PLQY of SiNPs.