Chemical Identification of Single Ultrafine Particles Using Surface-Enhanced Infrared Absorption

The interest in the plasmonics field has skyrocketed as a result of technical progress that has allowed accurate design of nanostructures. Another motivation for spiked interest has been as a result of the possibility of concentrating electromagnetic fields in deeply subwavelength volumes, which opens up endless applications for these elements. For example, the plasmonic near-field enhancement of nanostructures has been used to increase the sensitivity of photodetectors, to probe local chemical reactions, to enhance the efficiency of solar cells, and can even be helpful in cancer therapy.

Currently, the most promising applications in chemical sensing are based on refractive index sensing, surface-enhanced Raman spectroscopy (SERS), and surface-enhanced infrared (IR) spectroscopy (SEIRA) with resonant plasmonic nanostructures. When conducting SEIRA measurements, molecular vibrational signals are enhanced by coupling the vibrational excitation to the plasmonic one via the strong electromagnetic field of the plasmonic excitation. This method has been demonstrated over and again as the most feasible measurement technique so far. Unfortunately, much less attention has so far been given to the possibility of using SEIRA for the detection and characterization of nanometer-sized particles, such as ultrafine dust particles.

Recently, University of Heidelberg researchers: Dr. Christian Huck, Michael Tzschoppe (PhD candidate), Dr. Frank Neubrech, and Professor Annemarie Pucci together with Rostyslav Semenyshyn (PhD candidate) at University of Stuttgart conducted a study where they experimentally demonstrated that single ultrafine dust particles down to a diameter of 34 nm could be chemically characterized by SEIRA measurements utilizing tailored bowtie nanoapertures. Remarkably, by employing experimental IR spectra and near-field simulations for the plasmonic resonance, they were able to provide design rules for the optimum bowtie geometry for the detection of ultrafine dust particles with diameters down to 15 nm. Their work is currently published in the research journal, Physical Review Applied.

Their approach was based on plasmonic resonances of bowtie-shaped gold apertures that were designed to extraordinarily enhance the material specific phononic excitations of a nanometer-sized silica particle. Experimental IR spectra and near-field simulations for the plasmonic resonance were carried out. Towards the end, numerical simulations were effected so as to confirm the experimental findings and also determine the optimal antenna design.

The authors observed that the bowtie geometry was specifically suited for single-particle spectroscopy, as it combined the advantage of an intense electromagnetic hot spot, the size of which could be adjusted to the particle dimension, with easy positioning of ultrafine dust particles inside that hot spot. Additionally, the numerical simulations helped show that a detection limit in terms of a particle diameter of less than 20 nm could be achieved, which corresponds to a ratio of the diameter to the vacuum wavelength below 0.002.

In summary, the Christian Huck et al study demonstrated a SEIRA platform based on bowtie nanoapertures for the detection and chemical characterization of ultrafine dust particles with diameters well below 100 nm. Generally, they were able to show that bowtie nanoapertures offer clear advantages with regard to the detection of such small particles by plasmonic enhancement, because such nanoapertures provide a high electromagnetic field strength at the center of the nanostructure. Altogether, the approach presented here offers the possibility of analyzing infrared bands from tiniest particles and thus paves the way toward SEIRA-based devices that can sense ultrafine dust.