The method for detecting a biomolecule by surface-enhanced Raman spectroscopy is a spectroscopic method of detecting a virus in a biological sample. A target substrate is prepared by depositing nanoparticles of a metal on a substrate of silicon nanowires. The nanoparticles may be gold, silver or a combination thereof. The far-field Raman spectra of at least one biomolecule associated with the virus are obtained. The biological fluid sample to be tested is then applied on the target substrate, and the surface-enhanced Raman spectroscopy spectra of the biological fluid sample on the target substrate are obtained. The far-field Raman spectra of the at least one biomolecule are compared against the surface-enhanced Raman spectroscopy spectra of the biological fluid sample on the target substrate.