Compact Optical Spectroscopy for Identification of Colloidal Metals (2020)

Optical spectroscopy has provided new capabilities and opportunities in materials science, structural engineering, chemistry, biology, and medicine. The expense of spectroscopic instrumentation can be prohibitive—often in the range of tens of thousands of dollars. It would be useful to have a compact, practical alternative that is more cost-effective in specific cases. One example is the case of detecting colloidal metals in solution, in which the metal is in nanoparticle or microparticle form; normally suspended in a dielectric material or fluid, such as water. In this project, we provide a practical, low-cost alternative to more traditional spectroscopic techniques for identifying metals in colloidal form. The basis for the method is the interaction of light with materials, as described by the plasma frequency and band gaps in metals . Metals reflect electromagnetic radiation whose frequency is below the plasma frequency. A glance at the plasma-frequency expression, ω p = √(( N e q 2 )/(m * ε 0 )), provides an explanation of why the plasma frequency can be a useful tool for identifying a given metal. In this expression, ω p is the plasma frequency in radians per second; N e is the number of electrons per unit volume (numerical volume density); q is the charge of an electron; m * is the effective mass of an electron; and ε 0 is the permittivity of free space. Now, the quantity N e can be found in reference tables, or it can be readily calculated for a particular metal. As a consequence, a prominent spectral output band can be a clear indicator of a metal’s “identity.” When a metal sample is in bulk form, it will have a plasma frequency in the ultraviolet range. As a result, a strong spectral response will be observed at the wavelength corresponding to that frequency (referred to as the plasma wavelength). Now, the plasma frequency does not change when a material is in colloidal form—but, due to interband transitions within the metal, the response wavelength will be at a lower value than the plasma wavelength. This wavelength may be in the visible or near infrared (NIR) region of the spectrum. This is the case with many metals, such as gold, silver, copper, lead, and magnesium. The presence of the response wavelength in the optical region is the basis for the method we are proposing. In the compact system we have developed, dual-convex lenses are used to collimate a white-light LED source, and then focus this light onto a sample. The sample is a glass slide populated with silver, copper, or magnesium particles of micrometer radius. Upon passing through the metal sample, the white light interacts with the sample, causing a variation in the intensities of the white light’s constituent components. This amounts to a variation in intensity as a function of wavelength, which is based on that particular metal’s plasma frequency and interband transitions. In our work, we have focused on the intensities of the red, green, and blue light that emerge from the sample—all three of which can be measured easily, after they have been spatially separated by a diffraction grating. To measure the intensities, we used low-cost optical sensors (photodiode arrays) which provided real-time measurements of the intensities of the red, green, and blue light. We were able to experimentally differentiate among silver, copper, magnesium, and iron using this method. Figure 1