Tip-enhanced Raman scattering on high-energy nanoparticles.
Nanodiamonds are becoming increasingly important in medical technology, for example as a carrier material for active ingredients. As a synthesis precursor, the explosive mixture hexolite has proven to be very efficient. By combining far-field and near-field Raman spectroscopy, it can be shown that core-shell nanoparticles have significantly more homogeneous material properties.
By: TANJA DECKERT-GAUDIG, VOLKER DECKERT.
Nanodiamonds can be synthesized highly efficiently by a controlled detonation of hexolite. They offer a diverse range of applications because they are chemically inert and exhibit high biocompatibility. Thus, they represent an alternative to conventional nanomaterials. Particularly small diamonds with homogeneous size distribution (1-5 nm) can be obtained if the hexolite nanoparticles are already as homogeneous as possible. This precursor can be generated from TNT (trinitrotoluene) and RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine) using the so-called spray flash evaporation (SFE) method. To do this, for example, a TNT-RDX mixture is dissolved in acetone. At this point, it was previously unclear whether co-crystals or defined core-shell structures were generated.
In the present project, hexolite nanoparticles were therefore morphologically and chemically characterized in the far and near field using conventional Raman spectroscopy and tip-enhanced Raman spectroscopy (TERS). Since its development in 2000, TERS has proven to be a suitable analytical method for the chemical structural analysis of surfaces for a wide variety of inorganic, organic and biochemical samples. By combining atomic force microscopy (AFM) and Raman spectroscopy, objects such as carbon nanotubes, proteins, viruses, bacteria, cells, dyes, etc. can be scanned and analyzed with nanometer precision. Nanometer precise scanning and simultaneous Raman spectra can be detected. Since conventional Raman spectroscopy is a specific but not very sensitive method, modified AFM tips are used to amplify the Raman signal from molecules under the tip by several orders of magnitude. In this context, the high spatial resolving power of TERS allows localization/assignment of chemical information on the sample with nanometer accuracy.
In Fig. 1a, three example spectra from the measurements on 15-20 nm small hexolite particles are shown. It is clear that a clear distinction of the components is possible on the basis of marker bands (red – TNT, green – RDX). On all investigated nanoparticles, 80-100% of all spectra showed the TNT characteristics, a clear indication that TNT covers almost the entire particle surface.
In contrast, either only RDX or TNT-RDX mixed spectra were detected on micrometer-sized particles in conventional Raman measurements (see Fig. 1b). The obtained data clearly show the different information obtained in near- and far-field Raman experiments. Due to the shallow penetration of the signal-amplifying field at the TERS tip, the molecules of the shell were selectively detected in these measurements. The far-field measurements, on the other hand, provided averaged information of the whole sample at the laser focus, whose main component is obviously mainly RDX.
Based on the results, it was possible to directly conclude a core-shell structure of the studied hexolite particles, in which an RDX core is surrounded by TNT. The formation of core-shell particles in the SFE process can most likely be attributed to the significantly higher solubility of TNT in the solvent used. Consequently, RDX seed crystals are formed first, on which TNT crystallizes.
The experimental combined approach of near- and far-field Raman spectroscopy presented here can in principle be used for morphological and molecular structural investigation of core-shell particles on the nanometer scale.