Abstract
Surface-enhanced IR absorption (SEIRA) microscopy was used to reveal main chemical and physical interactions between Staphylococcus aureus bacteria and different laser-nanostructured bactericidal Si surfaces via simultaneous chemical enhancement of the corresponding IR-absorption in the intact functional chemical groups. A cleaner, less passivated surface of Si nanoripples, laser-patterned in water, exhibits much stronger enhancement of SEIRA signals compared to the bare Si wafer, the surface coating of oxidized Si nanoparticles and oxidized/carbonized Si (nano) ripples, laser-patterned in air and water. Additional very strong bands emerge in the SEIRA spectra on the clean Si nanoripples, indicating the potential chemical modifications in the bacterial membrane and nucleic acids during the bactericidal effect.
Highlights
The nanopatterned Si spots indicated that weak homogeneous ablation of the wet Si wafers across the focused Gaussian beam was enhanced by strong nanoscale surface plasmon-mediated ablation, providing the polarization-dependent directionality of the generated 1D-nanoripples (Figure 1b–d)
The corresponding minimal ripple period in air approaches to Λ(air) ≈ 0.9 μm at the same normal orientation regarding the laser polarization (Figure 1d). These nanoripples are tunable in their period, exhibiting considerable (30%–40%) reduction of their periods versus the cumulative laser exposure N—from Λ(H2 O) ≈ 0.18 μm and Λ(CS2 ) ≈ 0.16 μm at N = 100 until Λ(H2 O) ≈ 0.12 μm and
Surface topography and chemical composition of the nanopatterned samples were characterized by means of a scanning electron microscope (SEM) JEOL 7001F, equipped by an energy-dispersion x-ray spectroscopy (EDX) module INCA (Oxford Instruments, UK) for chemical micro-analysis at the 10 keV kinetic energies of electrons
Summary
Nanostructured surfaces of different materials exhibit unexpected and unprecedented bactericidal effects regarding pathogenic bacteria, which is usually related either to photochemical reaction of gaseous oxygen with composing nanocrystallites, yielding in reactive oxygen species and singlet oxygen [1,2,3,4,5], or to mechanical reaction of bacterial membranes to sharp surface nanoroughness [6,7,8].in the case of Si a broad variety of its nanomorph–nanospike [6], nanosheets [7] and nanoparticles (NPs) [9] were demonstrated to be bactericidal.surface-enhanced Raman scattering (SERS) and IR-absorption (reflection) (SEIRA/R)was performed on nanostructured Si surfaces, where linear- and non-linear optical enhancement could be achieved via plasmonic enhancement of local electric fields in heavily doped Si [10], magnetic dipolar resonance [11] or chemical charge-transfer processes. Less-sensitive mid- and far-IR (typical wavelengths ~3–25 microns) SEIRA(R) probing involves the micro-scale interfacial region of the IR electrical near fields, being highly beneficial for characterization of specific internal elements in viruses and bacteria [17,18]. As a result, it is SEIRA(R) that potentially holds promise as a very important spectroscopic modality in characterizing key-enabling interactions of pathogenic bacteria and bactericidal nanostructured surfaces
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