Proposed Method for Label-Free Separation and Infrared Spectroscopy of Carbonyl-Containing Micro- and Nanoparticles Using Mid-Infrared Optical Force

Separation and spectroscopy are essential and complementary techniques in molecular analysis, including gas/liquid chromatography, electrophoresis, flow cytometry, and vibrational spectroscopies, where independent modalities are required for separation and spectral characterization. In many cases, separated materials require spectroscopic analysis, whereas characterized components in a mixture may need further separation. Here, we present a mid-infrared optical force technique in which spectroscopic differences alone can be directly utilized to separate materials based on their molecular species and structures without labeling. In particular, we demonstrate an optical manipulation of micro- and nanospheres via a tunable mid-infrared laser, where their velocity, induced by optical force, at different wavenumbers closely match the Fourier-transform infrared spectra of the constituent material. The mid-infrared laser covers the spectral range of the vibrational mode of carbonyl bonds in the particles: we successfully demonstrate the selective manipulation of PMMA (poly(methyl methacrylate)) and TPM (3-(trimethoxysilyl)propyl methacrylate), which contain the same carbonyl bonds but in different surrounding environments. The experimental results agree with optical force calculations based on the finite-difference time-domain simulation. This constitutes the first direct evidence that the velocity is proportional to infrared absorbance at different wavenumbers, enabling precise reconstruction of infrared absorbance spectra from measured velocities. We believe that the proposed method enables a versatile particle separation and characterization across a wide range of materials, e.g., cells, nucleic acids, viruses, proteins, and potentially down to molecules, as the mid-infrared region is home to the molecular vibrational modes in a vast array of compounds.