"RaMassays": Synergistic Enhancement of Plasmon-Free Raman Scattering and Mass Spectrometry for Multimodal Analysis of Small Molecules

Abstract

SiO₂/TiO₂ core/shell (T-rex) beads were exploited as "all-in-one" building-block materials to create analytical assays that combine plasmon-free surface enhanced Raman scattering (SERS) and surface assisted laser desorption/ionization (SALDI) mass spectrometry (RaMassays). Such a multi-modal approach relies on the unique optical properties of T-rex beads, which are able to harvest and manage light in both UV and Vis range, making ionization and Raman scattering more efficient. RaMassays were successfully applied to the detection of small (molecular weight, M.W. <400 Da) molecules with a key relevance in biochemistry and pharmaceutical analysis. Caffeine and cocaine were utilized as molecular probes to test the combined SERS/SALDI response of RaMassays, showing excellent sensitivity and reproducibility. The differentiation between amphetamine/ephedrine and theophylline/theobromine couples demonstrated the synergistic reciprocal reinforcement of SERS and SALDI. Finally, the conversion of L-tyrosine in L-DOPA was utilized to probe RaMassays as analytical tools for characterizing reaction intermediates without introducing any spurious effects. RaMassays exhibit important advantages over plasmonic nanoparticles in terms of reproducibility, absence of interference and potential integration in multiplexed devices.

Figure 1. The RaMassay concept.

3D T-rex colloidal crystals are exploited as a highly efficient, background-free matrix for mass analysis and all-dielectric substrate for plasmon-free SERS.

Figure 2. RaMassay at work: Raman and mass detection of caffeine and cocaine solutions.

Examples of combined plasmon-free SERS (left) and SALDI/MS (right) detection of small molecules, caffeine (M. W.: 194.19 Da, on the top) and cocaine (M.W.: 303.36 Da, on the bottom). The Raman spectra acquired with T-rex beads are compared to reference spectra from analyte solutions with the same concentration (10⁻³ M) deposited on a glass microscope slide. The SALDI/MS spectra acquired with T-rex beads are compared with those acquired using a conventional organic matrix, isovanillin, indicated by I.

Figure 3. SERS-SALDI/MS mutual assistance: SALDI disambiguation of structurally -related drugs.

Example of RaMassay that takes advantage of SALDI/MS to discriminate between amphetamine (blue SERS and SALDI/MS spectra) and ephedrine (red SERS and SALDI/MS spectra, both 10⁻³ M solutions), two structurally-related drugs with similar Raman spectra but different molecular weight.

Figure 4. SERS-SALDI/MS mutual assistance: SERS detection of two regioisomers.

Example of RaMassay that takes advantage of plasmon-free SERS to discriminate between theobromine (blue SERS and SALDI/MS spectra) and theophylline (red SERS and SALDI/MS spectra, both 10⁻³ M solutions), two caffeine metabolites which have the same molecular weight (regioisomers) and, therefore, cannot be discriminated by SALDI/MS.

Figure 5. Analysis of reaction intermediates/products without metal interference.

Ramassay utilized to detect L-tyrosine and L-DOPA 10⁻³ M solutions. The addition of a second OH- group improves the anchorage on T-rex and promotes charge-transfer between the target molecule and TiO₂ shell, which determines a strong enhancement of the Raman spectrum of L-DOPA (blue spectrum) in comparison to L-tyrosine (red spectrum). This effect, combined to the precise SALDI/MS (right) identification of the molecular targets can be exploited for monitoring intermediates and products of various chemical reactions in solution without the interference of any metallic enhancers.