Jürgen Popp’s research while affiliated with Leibniz Institute of Photonic Technology and other places

Accurate and reliable bacterial identification at the genus and species levels is essential for effective clinical diagnostics. Pathogens such as Clostridium perfringens, Bacillus cereus, Clostridioides difficile, and Paraclostridium sordellii pose...

Using theoretical and experimental transmission, transflection, and attenuated total reflection (ATR) spectra, we investigated how well corresponding absorbance spectra correlate with true absorbance, defined as the absorption index function multiplied by the wavenumber, using poly(methyl methacrylate) layers on CaF2, Si, and gold substrates. To improve correlation, the substrate spectrum is often subtracted from the sample spectrum. A typical example is layers on CaF2, where this approach is sufficient to establish a strong linear correlation. However, in many cases, the substrate spectrum is not a suitable reference for removing unwanted physical contributions, such as substrate-related effects. One such example is layers on Si substrates, where high reflectance causes the spectrum to be dominated by interference fringes. Instead of using the spectrum of an uncoated substrate, one must use the spectrum of a substrate with a non-absorbing layer that has the same refractive index in the transparency region between the MIR and visible spectral regions. For ATR spectra, a simple multiplicative correction based on the wavelength dependence of the penetration depth significantly increases the Pearson coefficient, though not to levels high enough for spectral recognition. To achieve higher accuracy, the Poor Man’s ATR Correction can be employed. For transflection spectra, all relatively simple methods generally fail, and only methods that ultimately determine the optical constant function show promise for success.

Cell death programs such as apoptosis and ferroptosis are associated with aberrant redox homeostasis linked to lipid metabolism and membrane function. Evidence for cross-talk between these programs is emerging. Here, we show that cytotoxic stress channels polyunsaturated fatty acids via lysophospholipid acyltransferase 12 into phospholipids that become susceptible to peroxidation under additional redox stress. This reprogramming is associated with altered acyl-CoA synthetase isoenzyme expression and caused by a decrease in growth factor receptor tyrosine kinase (RTK)-phosphatidylinositol-3-kinase signaling, resulting in suppressed fatty acid biosynthesis, for specific stressors via impaired Akt-SREBP1 activation. The reduced availability of de novo synthesized fatty acids favors the channeling of polyunsaturated fatty acids into phospholipids. Growth factor withdrawal by serum starvation mimics this phenotype, whereas RTK ligands counteract it. We conclude that attenuated RTK signaling during cell death initiation increases cells’ susceptibility to oxidative membrane damage at the interface of apoptosis and alternative cell death programs.

The year of 2024 marks the 50th anniversary of the discovery of surface-enhanced Raman spectroscopy (SERS). Over recent years, SERS has experienced rapid development and become a critical tool in biomedicine with its unparalleled sensitivity and molecular specificity. This review summarizes the advancements and challenges in SERS substrates, nanotags, instrumentation, and spectral analysis for biomedical applications. We highlight the key developments in colloidal and solid SERS substrates, with an emphasis on surface chemistry, hotspot design, and 3D hydrogel plasmonic architectures. Additionally, we introduce recent innovations in SERS nanotags, including those with interior gaps, orthogonal Raman reporters, and near-infrared-II-responsive properties, along with biomimetic coatings. Emerging technologies such as optical tweezers, plasmonic nanopores, and wearable sensors have expanded SERS capabilities for single-cell and single-molecule analysis. Advances in spectral analysis, including signal digitalization, denoising and deep learning algorithms, have improved the quantification of complex biological data. Finally, this review discusses SERS biomedical applications in nucleic acid detection, protein characterization, metabolite analysis, single-cell monitoring and in vivo deep Raman spectroscopy, emphasizing its potential for liquid biopsy, metabolic phenotyping, and extracellular vesicle diagnostics. The review concludes with a perspective on clinical translation of SERS, addressing commercialization potentials and the challenges in deep tissue in vivo sensing and imaging.

The spatiotemporal assessment of tissue dynamics after the introduction of disruptive factors is crucial for evaluating their impact and for developing effective countermeasures. Here, we report a 4D-spatiotemporal imaging approach using second harmonic generation (SHG) imaging microscopy, enabling an advanced time-resolved analysis of three-dimensional tissue features. This is of particular interest as topical administration of drugs during spinal surgeries is a standard practice for preventing and treating postoperative complications like infections. Local drug concentrations on tissue are high in these scenarios, and given the dura’s role as a protective barrier for the brain and spinal cord, potential drug-induced damage should be evaluated critically. By employing 4D-SHG imaging, we gained detailed insights into changes in dimensional properties of thin section samples, namely, width, height, and volume, as well as into alterations within the hierarchic structure of collagen. The latter thereby allowed us to postulate a mode of action, which we attributed for the herein investigated samples to the pH of the formulation.

When Beer's law is interpreted through electromagnetic theory, it becomes clear that assuming a linear relationship between molar concentration and the imaginary part of the dielectric function is more accurate than using the absorption index function. A similar relationship holds true for attenuated total reflection (ATR) absorbance. When the negative logarithm of the reflectance is expanded into a series and truncated after the linear term, the approximation proves more accurate when based on the imaginary part of the dielectric function. Moreover, ATR correction schemes that utilize the low absorption approximation or the Bertie–Eysel formalism with this imaginary part, rather than the absorption index, tend to converge more quickly and provide more accurate results, particularly for stronger oscillators across an extended range of oscillator strength. Therefore, correction schemes for ATR spectra should prioritize the imaginary part of the dielectric function rather than the absorption index function when analyzing scalar and isotropic media.