Silicon nanowires (SiNWs) formed by metal(silver)-assisted chemical etching of lowly boron-doped (100)-oriented single crystalline silicon substrates in hydrofluoric acid solutions are investigated by means of the electron microscopy and optical spectroscopy (absorption and reflection measurements, photoluminescence spectroscopy and imaging). The growth rate of SiNWs is found to depend nonlinearly on the time of etching. The formed SiNW arrays demonstrate a strong decrease of the total reflectance below 1% in the full visible and near infrared region between 300 and 1000 nm and the samples show the similar optical properties as “Black Silicon”, which can be used as antireflection coating in photovoltaic applications. The prepared SiNWs exhibit efficient photoluminescence in the spectral region of 600-1000 nm and it is explained by the radiative recombination of excitons confined within nanostructured sidewall of SiNWs. The excitons luminescence is also observed in aqueous suspensions of SiNWs, whose application in bio-imaging is demonstrated in vitro.