Nanoparticle/Cellulose-Based Surface Enhanced Raman Scattering Hybrid Sensors
Authors: Roth Stephan Volkher , Chen Qing , Brett Calvin Jay , Müller-Buschbaum Peter , Soederberg Daniel 
Published: August 2024

Surface-Enhanced Raman Scattering (SERS) sensors are versatile biosensors for detecting very low concentrations of molecules. Their molecular and chemical sensitivity crucially depends on the tailoring of their nanostructure. These sensors rely on noble metal or oxide cluster layers, with their crystallinity and nanostructure carefully tuned to enable a maximum of plasmonic hot spots and enabling charge transfer. Typically, such sensors are in thin film geometry. In order to optimize their nanostructure and optical properties of the metal or oxide clusters, cellulose nanofibrils (CNFs) as template for nanoparticle-biopolymer hybrid materials are readily employed due to their high mechanical strength and ability to establish nanoporous, scalable thin films via spray deposition.[1] In order to characterize their nanostructure during synthesis of the hybrid nanoparticle-CNF material, grazing incidence X-ray scattering (GIXS) is ideally suited due to its ability to observe the nanostructural changes during farication and optimization of the sensor materials. The resulting nanostructure is then directly correlation to the sensor capability of the hybrid materials. As a first example, we use the catalytic properties of CNF during thermal decomposition of silver nitrate as precursor for fabrication of size-tuned silver clusters on CNF to tune their optical band-gap. The correlation between hot-spot size and silver nanoparticle size for optimizing the SERS sensitivity is elucidated using GIXS, showing for the first time the optimum hot spot size.[2] In order to facilitate an equal distribution of hot-spots, we make use of the three-dimensional nanoporous structure of the CNF thin films by adding shell-free silver nanoparticles in aqueous solution, again applying spray deposition.[3] The nanoporosity allows for reducing drastically agglomerations of silver nanoparticles, detrimental to hot-spots. At the same time, the spectral position of the plasmon resonance is tuned by the amount of CNF as structural matrix. As a final example we extend these results to novel semiconductor metal oxide nanomaterial (SMON)-based sensors.[4] Here, titanium oxide is deposited via atomic layer deposition (ALD) on the nanoporous CNF network, preserving its three-dimensional morphology. We optimize the nanostructure, crystallinity and rutile-to-anatase ratio in order to obtain superior sensitivity of the hybrid titania/CNF thin films as SMON SERS substrates. The improvement of SERS activity relies on the cooperative modulation of the CNF network and the crystalline states of the deposited titania. To summarize, we present new routes for SERS sensors based on plasmonic and charge transfer properties of hybrid materials by tailoring the hybrid nanostructure.

[1] Brett, C. J.; Ohm, W.; Fricke, B.; Alexakis, A. E.; Laarmann, T.; Körstgens, V.; Müller-Buschbaum, P.; Söderberg, L. D.; Roth, S. V. Nanocellulose-Assisted Thermally Induced Growth of Silver Nanoparticles for Optical Applications. ACS Appl. Mater. Interfaces 2021, 13 (23), 27696–27704. https://doi.org/10.1021/acsami.1c07544.

[2] Santoro, G.; Yu, S.; Schwartzkopf, M.; Zhang, P.; Koyiloth Vayalil, S.; Risch, J. F. H.; Rübhausen, M. A.; Hernández, M.; Domingo, C.; Roth, S. V. Silver Substrates for Surface Enhanced Raman Scattering: Correlation between Nanostructure and Raman Scattering Enhancement. Appl. Phys. Lett. 2014, 104 (24), 243107. https://doi.org/10.1063/1.4884423.

[3] Chen, Q.; Brett, C. J.; Chumakov, A.; Gensch, M.; Schwartzkopf, M.; Körstgens, V.; Söderberg, L. D.; Plech, A.; Zhang, P.; Müller-Buschbaum, P.; Roth, S. V. Layer-by-Layer Spray-Coating of Cellulose Nanofibrils and Silver Nanoparticles for Hydrophilic Interfaces. ACS Appl. Nano Mater. 2021, 4 (1), 503–513. https://doi.org/10.1021/acsanm.0c02819.

[4] Chen, Q.; Betker, M.; Harder, C.; Brett, C. J.; Schwartzkopf, M.; Ulrich, N. M.; Toimil‐Molares, M. E.; Trautmann, C.; Söderberg, L. D.; Weindl, C. L.; Körstgens, V.; Müller‐Buschbaum, P.; Ma, M.; Roth, S. V. Biopolymer‐Templated Deposition of Ordered and Polymorph Titanium Dioxide Thin Films for Improved Surface‐Enhanced Raman Scattering Sensitivity. Adv. Funct. Mater. 2022, 32 (6), 2108556. https://doi.org/10.1002/adfm.202108556.