Conference Agenda

Polybetaine nanobrushes are widely used as inert platforms for label-free biosensing due to their resistance to non-specific interactions. Despite being considered cationic or electrically neutral, polybetaines can exhibit negative zeta potential (ZP) ​​at pHs above their isoelectric point (pI). To clarify whether negative zeta potential effectively contributes to surface interactions, we examined three types of nanobrushes deposited on a planar gold substrate: two polybetaines – poly(carboxybetaine methacrylamide) (pCBMAA) and poly(sulfobetaine methacrylamide) (pSBMAA), and hydrophilic poly[N-(2-hydroxypropyl) methacrylamide] (pHPMAA) which carries no ionic group. All three brushes exhibit well defined pI and negative surface ZP at pHs above their pI. The pH dependence of the interactions of these brushes with anionic dextran sulfate (DS) and cationic poly[(N-trimethylammonium)ethyl methacrylate] (PTMAEMA) was monitored by infrared reflection spectroscopies (IRRAS, GAATR). DS adsorbs to pCBMAA strongly and only weakly to pSBMAA at pHs below their pI but can adsorb slightly to both polybetaines even at pHs above their pI. This is due to the displacement of their carboxylate or sulfo groups from interaction with the quaternary ammonium cation by the DS sulfate groups. However, DS does not adsorb to pHPMAA at any pH, and PTMAEMA does not adsorb to any of the brushes, regardless of pH. These findings highlight that zeta potential determinations alone may not be sufficient to predict electrostatic interactions, as the apparent negative charge does not necessarily translate into a functional surface charge influencing macromolecular interactions.

Polaritons are half-light half-matter quasiparticles, resulting from the strong coupling between excitons and cavity modes. Polaritons hold great potential in modifying physical and chemical properties of materials. We show that by coupling molecular excitons in aggregates to surface plasmon polaritons, the resulting exciton-polaritons show enhanced propagation distance to 10 micrometers, much longer than the diffusion length of excitons of less than 1 micrometer. This is due to the unique feature of polaritons, i.e., a high group velocity that is roughly half the speed of light, benefiting from the light component in polaritons. Therefore, even though the lifetime of surface plasmons is only tens of femtoseconds, the propagation distance can be as long as 10 micrometers. We demonstrate that the propagation distance can be manipulated by engineering the lifetime of the surface plasmon modes.

In the second part, we will discuss the ultrafast carrier dynamics of polaritons, which is vital for their optoelectronic applications. Self-hybridized polaritons were formed in a 170 nm thick WSe2 flake on a gold substrate without top mirror, which is absent in a 13 nm thick WSe2. We find that, under different pump excitation energies, significantly different carrier dynamics of lower polariton, upper polariton, and exciton reservoir can be observed. For above bandgap excitation, carriers will be firstly relaxed to exciton reservoir, followed by scattering to lower and upper polaritons in the time scale of 1-100 ps. For low energy excitation, which exclusively excites lower polaritons, reversed scattering from lower polariton to exciton reservoir is seen on the same time scale of 1-100 ps. After that, inter-band relaxation occurs on a longer time scale of hundreds of picoseconds. These results unravel the carrier dynamics of polaritons and exciton reservoir, specifically the important role of inter-state scattering in carrier relaxation in strong coupling systems.

Both, laser induced breakdown spectroscopy (LIBS) and Raman scattering categorically follow the same workflow in which a non-resonant laser is focused tightly onto a sample target while the characteristic emission of the irradiated spot is collected and spectrally analyzed to obtain specific information on the samples’ composition. While LIBS spectra contain the elemental information, Raman spectra holds the specific details about chemical bonds, i.e. molecular information. This pairing of seemingly identical hardware demands on the input demands to obtain highly orthogonal complementary knowledge about the targets’ composition appears a textbook example of spectrochemical synergy. However, in common applications hardware demands regarding lasers (pulsed for LIBS, continuous wave for Raman) and spectrometer ranges (UV for LIBS, NIR for Raman) strongly differ. Also, standard sampling parameters ultimately dictating the scanning pattern in mapping experiments like spot sizes and exposure times are typically two orders of magnitude apart for the two techniques. For instance, 100 µm is a common crater size in LIBS while 1 µm is typical for the Raman irradiation area, LIBS signals are intense and lead to a meaningful spectrum in the matter of tens of milliseconds (ms) while Raman spectra are often accumulated over seconds.

This presentation shows that meaningful compromises in both techniques are not enough to bring the two methods together. However, when applying uncommon and innovative hardware concepts, a single instrumental setup can be realized, which allows to record LIBS and Raman spectra with a common repetition rate and exposure time on the order of ms, both sharing the same laser, scanning pattern and spectrometer. This union was achieved by using ultraviolet diode pumped solid sate lasers in combination with spatial heterodyne spectrometers. While the first are highly flexible in terms of their low prices, their unique quantum efficiency and their flexibility in terms of pulse behaviour, the latter combines the best of traditional diffraction with the light throughput of interferometry. To showcase this novel approach, samples from the lifecycle of lithium-ion batteries are studied. These samples range from pristine electrodes via cycled batteries with early defects all the way to end of life black mass.

The ability to “take aim” and accurately manipulate gas-phase ions is critical to ensure the proper function of ion-based analytical techniques, such as mass spectrometry. Traditionally, ions are focused, gated, redirected, and separated with ion optics, which function analogously to light optics. These components rely on the behavior of charged analytes within electric and/or magnetic fields. However, due to both the complexity of individual devices and the function-specific design of ion optics, there is no single, user-friendly, multi-purpose component for the manipulation of ions at and above rough vacuum pressures. Here, we utilize both charge and fluid behaviors of gas-phase ions at atmospheric pressure as the basis for a novel class of ion optics. Acoustic ion manipulation (AIM) is a newly discovered phenomenon, whereby, in one manifestation, gaseous ions are controlled by dynamic pressure regions in an ultrasonic standing acoustic wave. Inside an acoustic resonator, stable and unstable areas, referred to as nodes and antinodes, respectively, repeat at regular intervals within the standing acoustic wave, which allows a single standing wave to focus or deflect an ion beam based on alignment. As such, a standing acoustic wave is able to perform four classes of ion optic functionality (i.e. focusing, gating, redirection, and separation) without any additional changes to the acoustic field.

In this work, we demonstrate the function of a standing acoustic wave as an ion optic at atmospheric pressure in the absence of external electric fields. Experimentally, beams of ions were generated with plasma-based or electrospray-based ionization sources and were aligned with the inlet of a mass spectrometer. A standing acoustic wave was generated in the ion beam path and was moved so that either a node or antinode would interact with the gas-phase ions before detection. When a node was present in the ion flow, a signal enhancement of 2-3 times was observed due to acoustic focusing. In contrast, when the antinode was aligned with the ion beam, the standing wave acted as a gate and ion signal decreased by more than 99.9%. This decrease is attributed to the deflection of the ion beam away from the mass spectrometer inlet. Characterization of the ion-beam profile with an IonCCD showed a 2-mm displacement along the axis of acoustic propagation upon the ions interacted. This change in position reflected the structure of the standing acoustic wave, where nodes are located λ/4 above and below the center of the antinode, with λ indicating the wavelength of sound. As such, antinodes redirect ion beams to preferentially pass through the stable regions within the acoustic field. Lastly, ion-specific properties such as mass-to-charge ratio, collisional cross section, or charge state lead to ion separation by impacting the efficacy of acoustic-ion interactions. In one example, ions with lower mass-to-charge ratios were more effectively displaced than larger species. Additionally, higher charge-state protein ions were more susceptible to acoustic deflection as compared to lower charge-state counterparts. The development of an acoustically based ion optic could have far-reaching impacts on mass spectrometry, as well as ex vacuo methods for ion control.

Laser-Induced Breakdown Spectroscopy (LIBS) is typically regarded for its simplicity and applicability for the direct analysis of samples of any kind, including liquids. However, these matrices are challenging and often require the development of more or less intricate sampling strategies and/or instrumental modifications in order to reach the required levels of stability, reproducibility and sensitivity. Hence, its application to liquids remains scarce in comparison to the vast variety of solid samples focused works.

On the other hand, it is well known that halogens pose a challenge of their own when determined via OES techniques, as their resonant emission lies within the VUV. Aside from signal enhancement approaches (such as double-pulse LIBS), a common approach to halogen determination is the indirect detection through halide molecule emission resulting from the recombination of the halogen of interest and an alkali-earth metal element, such as calcium.

Previous works [1,2] have demonstrated the feasibility of halogen determination (F, Cl) in aqueous liquid matrices by depositing the sample on a Ca-containing target (calcium carbonate pellets, paper) online via nebulization of the former, with limits of detection varying from 5 ppm (F) to 200 ppm (Cl). Real samples such as mouthwashes were successfully evaluated, but a particularly interesting case was the study of an aqueous dilution of a fluorosurfactant, where total fluorine content could be determined with similar results as those obtained from solutions of an inorganic fluorine salt. Such outcome implies that prior knowledge of the organic/inorganic state of halogen compounds might not be critical in LIBS analyses (as it is the case, for example, in ionic chromatography). These results motivated the investigation focused on organic samples, particularly focusing on gasolines. The research focused on halogen determination in liquid samples of inorganic and organic form will be overviewed in this talk.

[1] C. Méndez-López et al. (2023) J. Anal. At. Spectrom., 38, 80-89.

[2] C. Méndez-López et al. (2023) Opt. Laser Technol., 164, 109536.