The invention provides a method of analyzing a liquid sample, comprising the steps of: passing near-infrared light through the sample, wherein the light travels in metal nanoparticles-added microfiber that is immersed in the sample; obtaining spectral information across a wavelength range of about 750 nm to about 1800 nm; and qualitatively identifying, and optionally quantitatively characterizing, one or more analytes in the sample. Near-infrared spectrophotometer carry out the method is also provided by the invention.