Towards biosensing via fluorescent surface sites of nanoparticles: Fluorescence spectroscopy of silicon clusters in water, ethanol and isopropanol

The suitability of silicon nanoparticles of 1 nm in diameter for fluorescent sensing was investigated. Silicon nanoparticles were produced in a cluster beam and co-deposited with a beam of vapourised liquids (water, ethanol, isopropanol) onto a cold substrate. Melting of the frozen cluster-ice mixture yielded an aqueous suspension which emitted strong fluorescence in the deep blue spectral range when exposed to UV light. The fluorescence wavelength of the strongest peak was found to correlate with the dipole moment of the solvent molecules which allowed us to derive the transition energy for an isolated nanoparticle. The strong solvent sensitivity showed that the fluorescence originated from a surface state. A second fluorescence peak showed almost no sensitivity to different solvents, hence the peak was attributed to a transition within the bulk–volume of the nanoparticles. Our findings establish that silicon nanoparticles may serve as highly specific bio-sensors in living organism.