Umeå University's logo

Spores of species belonging to the Bacillus cereus sensu lato (s.l.) group are common contaminants in food processing environments due to their ability to adhere to surfaces and resist cleaning procedures. These spores are equipped with pilus-like endospore appendages (ENAs), which are believed to promote surface adhesion. We investigated the role of ENAs in spore adhesion to abiotic surfaces using a wild-type (WT) Bacillus paranthracis strain and isogenic mutants lacking ENAs or an intact exosporium. WT spores expressing both short and long ENAs (S+L+) adhered significantly more to stainless steel (SS) and polypropylene (PP) compared to bald spores (S-L-) and spores of an exosporium-deficient mutant (Delta exsY), whereas adhesion to polystyrene (PS) and glass was not significantly affected by the presence of ENAs. The Delta exsY mutant also showed the lowest adhesion across all tested surfaces, a pattern similarly observed for vegetative cells. The strongest adhesion to PP was observed when both fiber types were present. A clear trend also emerged: on PP, WT remained adhered for at least an hour, while bald spores tended to detach within that time. Under saline conditions and at different pH levels, bald spores adhered strongly to SS. However, in the presence of a non-ionic surfactant or a concentrated protein solution, WT spores adhered more. Our results highlight the crucial role of ENAs in B. cereus spp. spore adhesion to industrially relevant surfaces, providing mechanistic insight into spore persistence. These insights support the design of surface treatments to prevent contamination, spoilage, and foodborne illnesses.IMPORTANCEBacteria belonging to the Bacillus cereus sensu lato group represent a persistent challenge in food production due to their highly resilient endospores (spores), which withstand cleaning, disinfection, and food processing. Understanding spore adhesion is essential for designing effective surface treatments that reduce chemical use, enhance food safety and quality, and minimize environmental impact. This study underscores the important role of endospore appendages (ENAs) in spore adhesion to common materials in food processing and laboratory environments. Wild-type spores expressing both S-ENA and L-ENA adhered significantly more than mutants lacking ENAs or the exosporium, highlighting ENAs as potential targets for disrupting spore adhesion. Time-dependent adhesion assays on polypropylene revealed strong, sustained attachment by wild-type spores, contrasting with weaker, transient adhesion by ENA-depleted mutants. These findings offer valuable insights into B. paranthracis spore adhesion dynamics, guiding the development of tailored cleaning protocols to improve contamination control and sustainability.

Light is a central part of how we perceive and interact with the world, shaping the colors ofour surroundings, the images we capture, and the signals we use to communicate. Its behavior is influenced not only by its intrinsic properties but also by the materials and structures it encounters. At familiar scales, this is expressed through everyday phenomena such as reflection, refraction, and absorption. When these interactions are controlled at the nanoscale, however, new effects emerge, as materials can be structured to manipulate light in ways that do not occur in bulk systems. These possibilities define the field of nanophotonics, within which plasmonics has emerged as a versatile approach that uses the collective motion of electrons in metals to concentrate light at the nanoscale, with applications ranging from molecular disease detection to solar–thermal energy conversion.

Part of this thesis explores how structured surfaces can be used to control light through plasmonic nanostructures, with particular attention to surface-enhanced Raman scattering (SERS). To provide reliable and accessible sensing platforms, two contrasting metasurfaces were developed using gold-coated polymer and diamond substrates. The polymer platforms, fabricated by nanoimprint lithography, offered low-cost and large-area reproducibility, whereas the diamond substrates, grown by chemical vapor deposition, provided recyclable bases with enhanced thermal stability. In a complementary direction, surface oxidation of stainless steel was employed to form thin oxide films of controlled thickness, which exploited interference effects to improve solar absorption and demonstrated how even simple materials can be structured for cost-effective energy applications.

The second part of the thesis focuses on biosensing, with particular attention to the detection of bacterial spores, which represent a persistent challenge in both public health and industry. Spores are dormant bacterial cell types that can survive extreme conditions that normally eliminate pathogens, including heat, radiation, disinfectants, and nutrient deprivation. Their resilience allows them to remain viable for long periods and to reactivate when conditions improve, making them a source of contamination and disease in food production, healthcare environments, and even biodefense, where species such as Bacillus anthracis are of concern. In this thesis, a SERS-based method was developed using gold nanorods to identify spores of Bacillus thuringiensis through the spore-specific biomarker calcium dipicolinate (CaDPA), enabling qualitative detection at concentrations as low as 10³ spores/mL. Applicability was further examined in a case study with milk spiked at 10⁶ spores/mL, where detection after dilution confirmed that the method can operate in a complex food matrix. Complementary work investigated the adhesion of Bacillus paranthracis spores to industrially relevant surfaces, showing that filament-like endospore appendages promote attachment to stainless steel and polypropylene while having less influence on glass or polystyrene. Together, these studies provide both a method for spore detection and insights into spore–surface interactions, offering knowledge relevant to contamination control in food and related industries.

The work presented in this thesis provides perspectives on the design of plasmonic sensing substrates, the use of thin oxide films for solar absorption, and the detection and adhesion of bacterial spores on technological surfaces. Taken together with the theoretical background and practical considerations developed throughout the thesis, these studies demonstrate how plasmonic and nanophotonic concepts can be translated into functional platforms and applied across different contexts.

Ljus är en grundläggande del av hur vi uppfattar och samspelar med vår omvärld. Det påverkar färgerna i vår omgivning, de bilder vi tar och de signaler vi använder för kommunikation. Ljuset formas inte enbart av sina inre egenskaper, utan också av de material och strukturer det möter. På vardagliga skalor tar detta sig uttryck i fenomen som reflektion, brytning och absorption. När interaktionerna däremot styrs på nanonivå uppstår nya effekter, eftersom material kan designas för att manipulera ljus på sätt som inte är möjliga i bulkform. Detta utgör grunden för nanofotonik, inom vilket plasmonik har etablerat sig som en mångsidig metod. Genom att utnyttja elektronernas kollektiva rörelser i metaller kan ljus koncentreras på nanoskalan, med tillämpningar som sträcker sig från molekylär sjukdomsdiagnostik till sol–termisk energikonvertering.

En del av denna avhandling undersöker hur strukturerade ytor kan användas för att styra ljus via plasmoniska nanostrukturer, med särskilt fokus på ytförstärkt Ramanspridning (SERS). För att skapa tillförlitliga och lättillgängliga sensorer utvecklades två olika typer av metaytor baserade på guldöverdragna polymer- respektive diamantsubstrat. Polymerplattformarna, framställda med nanoimprintlitografi, erbjöd en kostnadseffektiv och storskalig tillverkningsmetod, medan diamantsubstraten, odlade genom kemisk ångdeposition, gav återanvändbara baser med hög termisk stabilitet. I ett kompletterande spår användes kontrollerad ytoxidation av rostfritt stål för att framställa tunna oxidfilmer. Dessa utnyttjade interferenseffekter för förbättrad solabsorption och illustrerade hur även enkla material kan struktureras för billiga och effektiva energilösningar.

Avhandlingens andra del fokuserar på biosensorik, med särskild betoning på detektion av bakteriella sporer, en långvarig utmaning inom både folkhälsa och industri. Sporer är vilande cellformer som kan överleva extrema förhållanden som normalt eliminerar patogener, inklusive värme, strålning, desinfektionsmedel och näringsbrist. Deras motståndskraft gör att de kan förbli livskraftiga under lång tid och återaktiveras när förhållandena förbättras. Därmed utgör de en riskkälla inom livsmedelsproduktion, vårdmiljöer och även inom bioförsvar, där arter som Bacillus anthracis är särskilt relevanta. I denna avhandling utvecklades en SERS-baserad metod med guldnanostavar för att identifiera sporer av Bacillus thuringiensis via den sporespecifika biomarkören kalciumdipikolinat (CaDPA). Metoden möjliggjorde kvalitativ detektion vid koncentrationer så låga som 10³ sporer/mL. Metodens praktiska användbarhet testades vidare i en fallstudie med mjölk som spätts till 10⁶ sporer/mL, där detektion efter utspädning bekräftade att tekniken fungerar även i en komplex livsmedelsmatris. Kompletterande experiment undersökte hur sporer av Bacillus paranthracis fäster vid industriellt relevanta ytor. Resultaten visade att filamentliknande utskott på sporerna underlättade vidhäftning till rostfritt stål och polypropen, medan effekten var mindre uttalad på glas och polystyren. Tillsammans ger dessa studier både en metod för sporesdetektion och fördjupad kunskap om spore–ytinteraktioner, med relevans för kontroll av kontaminering inom livsmedelsindustrin och angränsande områden.

Sammanfattningsvis presenterar denna avhandling nya perspektiv på utformning av plasmoniska sensorsubstrat, användning av tunna oxidfilmer för solabsorption samt studier av detektion och adhesion av bakteriella sporer på teknologiska ytor. Tillsammans med den teoretiska bakgrunden och de praktiska överväganden som behandlas visar resultaten hur nanofotoniska och plasmoniska koncept kan omsättas i funktionella plattformar med tillämpningar i skilda sammanhang.

Bacterial spores are highly resilient and capable of surviving extreme conditions, making them a persistent threat in contexts such as disease transmission, food safety, and bioterrorism. Their ability to withstand conventional sterilization methods necessitates rapid and accurate detection techniques to effectively mitigate the risks they present. In this study, we introduce a surface-enhanced Raman spectroscopy (SERS) approach for detecting Bacillus thuringiensis spores by targeting calcium dipicolinate acid (CaDPA), a biomarker uniquely associated with bacterial spores. Our method uses probe sonication to disrupt spores, releasing their CaDPA, which is then detected by SERS on drop-dried supernatant mixed with gold nanorods. This simple approach enables the selective detection of CaDPA, distinguishing it from other spore components and background noise. We demonstrate detection of biogenic CaDPA from concentrations as low as 10³ spores/mL, with sensitivity reaching beyond CaDPA levels of a single spore. Finally, we show the method’s robustness by detecting CaDPA from a realistic sample of fresh milk mixed with spores. These findings highlight the potential of SERS as a sensitive and specific technique for bacterial spore detection, with implications for fields requiring rapid and reliable spore identification.

Solar collectors have the potential for significant climate change mitigation by substituting heat produced with fossil fuels. To achieve this, collectors with highly efficient solar absorbers are essential. Carbon nanotubes are highly absorbing, sustainable, cheap, and thermally stable, making them a promising material for solar absorbers. However, achieving a high solar absorptance and low thermal emittance (solar selectivity), while maintaining good thermal stability and scalability is challenging. Here, we present a selective coating based on multi-walled carbon nanotubes and silica (SiO₂). A water-based dispersion enabled by carboxyl functionalization of the carbon nanotubes (CNT_F) is spray coated on a stainless steel (SS) substrate and fixated with sol-gel dip coated silica. The SS/CNT_F/SiO₂ surface exhibits an optical selectivity dependent on CNTF area load and with 0.83 gCNT m⁻² a solar absorptance and thermal emittance of 0.94 and 0.40, respectively, is achieved. The coating also demonstrates excellent thermal stability, with an estimated lifetime of >25 years at working temperatures ≤222°C. All together, we show that by using scalable and cheap technology, concurrent with sustainable materials and a simple structural design, we can manufacture a coating that exhibits properties suitable for low-to-mid-temperature applications. Our study highlights the potential of carbon-based solar absorbers.

Introduction: Polymethyl methacrylate is a polymer commonly used in clinicaldentistry, including denture bases, occlusal splints and orthodontic retainers.

Methods: To augment the polymethyl methacrylate-based dental appliances incounteracting dental caries, we designed a polymer blend film composed ofpolymethyl methacrylate and polyethylene oxide by solution casting and addedsodium fluoride.

Results: Polyethylene oxide facilitated the dispersion of sodium fluoride,decreased the surface average roughness, and positively influenced thehydrophilicity of the films. The blend film made of polymethyl methacrylate,polyethylene oxide and NaF with a mass ratio of 10: 1: 0.3 showed sustainedrelease of fluoride ions and acceptable cytotoxicity. Antibacterial activity of all thefilms to Streptococcus mutans was negligible.

Discussion: This study demonstrated that the polymer blends of polyethyleneoxide and polymethyl methacrylate could realize the relatively steady release offluoride ions with high biocompatibility. This strategy has promising potential toendow dental appliances with anti-cariogenicity.

The use of antireflective coatings to increase the transmittance of the cover glass is a central aspect of achieving high efficiencies for solar collectors and photovoltaics alike. Considering an expected lifetime of 20–30 years for solar energy installations, the durability of the antireflective surfaces is essential. Here, a novel antireflective SiO₂ coating with a hexagonally ordered closed pore structure, produced with an aerosol-based sol-gel method is benchmarked against two commercial coatings; produced with acid etching and sol-gel roll coating. The optical and mechanical properties together with contact angle characteristics were evaluated before and after various durability tests, including climate chamber tests, outdoor exposure, and abrasion. Compared to the commercial antireflective coatings with open pore structures, the novel coating performed in parity, or better, in all tests. Based on the results of humidity freeze and industrial climate chamber tests, it appears that the coating with closed pore structure has a better ability to prevent water adsorption. Additionally, the closed pore structure of the coating seems to minimize the accumulation of dirt and deposits. The abrasion and cleanability test further confirm the advantages of a closed pore structure, showcasing the coating's mechanical durability. While the coatings exhibit similar hardness and reduced elastic modulus, the closed pore coating proves to be even harder after undergoing the industrial climate chamber test, but also slightly more brittle, as indicated by the probability of crack initiation. In summary the closed pore structure is well suited for tempered and arid climates, making it a truly competitive alternative to existing antireflective coatings.

Plasmonic nanometasurfaces/nanostructures possess strong electromagnetic field enhancement caused by resonant oscillations of free electrons, and has been extensively applied in biosensing, nanophotonic and photocatalysis. However, fabrication of uniform nanostructured metasurfaces by conventional methods is complicated and costly, which mitigates a wide-spread use of this technique in ubiquitous applications. Here, we present a facile and scalable method to fabricate an active nanotrench plasmonic gold substrate. The surface comprises sub-10 nm plasmonic nanogaps and their formation is assisted by a pre-fabrication of nano-imprinted polymer nano-well arrays. The plasmonic metasurface is optimized to maximize the density of the nano-trenches by tuning the substrate material, imprinting procedure and film deposition. We show that the surface Raman enhancement due to plasmonic resonances correlates well with trench density and reach a meritorious enhancement factor of EF > 105 over large surfaces.

We further show that the electric field strength at the nanotrench features are well explained by finite element method simulations using COMSOL Multiphysics. The plasmonic substrate is transparent in the visible spectrum and conductive. In combination with a scalable bottom-up fabrication the plasmonic metasurface opens up for a wider use of the sensitive and reliable SERS substrate in applications such as portable sensing devices and for future internet of things.

In this report we present XPS data for five amino acids (AAs) (tryptophan, methionine, glutamine, glutamic acid, and arginine) with different side chain groups measured in solid state (powder form). The theoretically and experimentally obtained chemical structure of AAs are compared. Here, we analyse and discuss C 1 s, N 1 s, O 1s and S 2p core level binding energies, FWHMs, atomic concentrations of the functional groups in AAs. The experimentally obtained and theoretically calculated ratio of atomic concentrations are compared. The zwitterionic nature of methionine and glutamine in solid state was determined from protonated amino groups in N 1s peak and deprotonated carboxylic groups in the C 1s spectrum. The obtained XPS results for AAs well correspond with previously reported data.

Low volatility nerve agents such as VX, Tabun, and Cyclosarin are highly lethal tohumans even in trace amounts. Their lethality has caused them to see use in terror-ism and chemical warfare, creating a need for rapid and accurate methods to detect,treat, and counteract nerve agents. Surface-enhanced Raman spectroscopy (SERS)is a promising method for detecting trace amounts of nerve agents and other harmfulchemicals. However, the choice of solvent has a significant effect on the SERS signalthat can be achieved from a given sample; an especially important factor to considerwhen a low limit of detection (LOD) is paramount. In this work, we have exploredand evaluated the suitability of some common solvents for use in SERS detectionon gold nanopillar SERS substrates. We evaluated the SERS spectra of VX, Tabun,and Cyclosarin in water, ethanol, acetonitrile, dichloromethane, and n-hexane anddetermined the LOD for the different analyte-solvent combinations. Using DFT, weevaluated solvent-induced variations in the electrostatic potential of the nerve agentmolecules. These variations in turn impact how well the molecules interact with theSERS surface and thus the strength of the SERS signal. Using contact angle mea-surements and SEM, we found that water exerts the strongest tangential force on theSERS substrate and therefore has the largest impact on its nanomorphology. Lastly,we performed Raman mapping to better understand the impact of solvent-inducedchanges to the substrate nanomorphology and surface concentration, and its practi-cal effect on the SERS characteristic of the substrate. Overall, we found that thereare large differences in LOD between the different solvents. VX exhibited an LOD0.2 ppm in water, compared to 8.7 ppm and 6.3 ppm in ethanol and acetonitrilerespectively. DCM and n-hexane were found to be largely ineffective as solvents. Theeffectiveness of water as a solvent was found to likely stem from its relatively highmolecular polarity, and the impact of the water droplet on the nanopillar morphologyof the SERS substrate causing the formation of SERS hotspots. Overall, the resultspresented herein provide a starting point that can be consulted in the design of futureSERS detection assays.