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In a recent study published in the ACS Sustainable Chemistry and Engineering Journal, researchers used metal-free cellulose nanofiber polymer substrates instead of graphene oxide substrates in surface-enhanced Raman spectroscopy (SERS). Organic semiconductor materials to analyze organic porphyrin-type molecules.
Research: SERS enhancement of porphyrin-type molecules on metal-free cellulose substrates. Photo Credit: Igor Petrushenko/Shutterstock
Due to the distributed growth of self-assembled structures on cellulose nanofibers (CNF) and the formation of disordered 3D clusters of porphyrin-type molecules, the polymer substrate increases the detection limit to 10-5 M.
In Raman spectroscopy, photons are incident on a sample, and their molecular bond energy is measured by the scattering of photons. The Raman spectrum intensity peak is sharp for metal bonds and weak for non-metal bonds. Therefore, in order to inspect organic substances, metal substrates are usually used.
However, the enhancement of the surface of the substrate can reduce the reliance on the metal substrate to examine porphyrin molecules, which is of great significance in medical diagnosis, safety, forensics, and environmental monitoring.
(a) The chemical structure of the cellulose molecule and (b) the porphyrin molecule used in the experiment. (c) SEM image of cellulose nanofibers on a silicon substrate. Compared with the background spectrum of CNF and the spectrum of porphyrin in solution, the normalized UV-vis absorption spectrum of (d) TMPyP and (e) TPP porphyrin deposited on CNF at a concentration of 10-3 M. (f) The FTIR spectrum of CNF is compared with the spectrum of sample porphyrin molecule (10-4 M TMPyP) on CNF and powder form. Image source: Fularz, A et al., ACS Sustainable Chemistry and Engineering
Traditionally, there are two methods to enhance the substrate in SERS, namely. Electromagnetic enhancement by localized surface plasmon resonance (LSPR) using metal nanoparticles, and chemical enhancement by charge transfer between the substrate and analyte molecules.
Although chemical enhancement is a relatively weak process, surface enhancement based on metal/surface plasma lacks biocompatibility and inertness, and high material and manufacturing costs. Recently, semiconductor-based substrates are being studied as an alternative to these two methods.
In addition, cellulose-based organic polymer materials combined with silver (Ag) and gold (Au) nanoparticles have shown promising stretch due to their biocompatibility, flexibility, large surface area, and large nanoparticle retention capabilities. Man signal enhancement.
In this study, cellulose nanofibers and TEMPO-oxidized cellulose nanofibers (TEMPO-CNF) were dispersed in distilled (DI) water to the desired concentration, and then sonicated for 20 minutes to ensure uniform fiber distribution.
Subsequently, the cellulose acetate powder and the hydroxypropyl cellulose powder were dissolved in acetone and deionized water, respectively, and then the cellulose was drop-cast on the silicon substrate and dried to obtain the cellulose substrate. Similarly, graphene oxide (GO) and silver nanoparticles (AgNPs) are also dispersed in deionized water and then deposited on a silicon substrate.
The normalized SERS spectra of 10-3 M (a) TMPyP and (b) TPP on CNF, AgNPs, and Si show shifts in peak positions. Confocal fluorescence microscopy images of (c) TMPyP on CNF, (d) TMPyP and (e) CNF only show the difference in porphyrin structure. Image source: Fularz, A et al., ACS Sustainable Chemistry and Engineering
Subsequently, the probe molecule solution was prepared by dissolving powders of several nanoporphyrin molecules as analytes in deionized water and/or dichloromethane (CH2Cl2) at an initial concentration of 10-2 or 10-3 M. These porphyrin-type molecules include mesotetraphenylporphyrin (TPP), meso-tetra(N-methyl-4-pyridyl) porphine tetrachloride (TMPyP), meso-tetra(N-methyl-4- Pyridyl) porphyrin tetratoluene sulfonate (TMPyP), 5,10,15,20-tetra(4-trimethylaminophenyl) porphyrin tetra(p-toluene sulfonate) (TMAP), meso Tetra(4-sulfophenyl) porphyrin tetrasodium salt dodecahydrate (TPPS) and mesotetra(4-carboxyphenyl) porphyrin (TCPP) methyl green (zinc chloride salt), Victoria blue B (VBB) and Methyl Violet 2B (MV2B).
Then, all the probe molecule solutions are diluted in their respective solvents to a standard fixed concentration of 10-4 M, and then the solubility is checked visually at room temperature.
A scanning electron microscope (SEM) image of a substrate synthesized with a high-concentration cellulose dispersion shows a thick layer of randomly distributed nanofibers. The fiber consists of several smaller basic fibril bundles with a diameter of about 3 nm.
The presence of cellulose disrupts the TMPyP nanorod assembly process and produces 3D clusters, which causes the structure of the adsorption layer to change from order to disorder, the peak position in the Raman spectrum changes, and the Raman spectrum increases.
(a) Proposed band diagram showing possible charge transfer between CNF substrate and TMPyP molecule. The SERS spectra of porphyrin on different types of cellulose-based substrates are compared with silver nanoparticles and silicon (b) 10-4 M TMPyP and (c) 10-3 M TPP. (d) Normalized SERS spectra of TMPyP on CNF and (e) GO were used to determine the detection limit. (f) The 10-5 M TMPyP spectrum on CNF highlights the low detection limit for metal-free substrates. Image source: Fularz, A et al., ACS Sustainable Chemistry and Engineering
In addition, the Cl− counterion of TMPyP is attracted to the positively charged end of the -OH group in the cellulose structure, resulting in a stable dipole moment and electric field in the porphyrin adsorption layer, thereby enhancing the intensity and transferring the spectrum Raman Peak.
In addition, the SERS signal intensity of TMPyP containing Cl− ions is lower than that of PTSA, indicating that the addition of cellulose has a more significant effect on TMPyP/Cl− molecules, and when Cl − uses counterions, PTSA is only 5.9 times higher, confirming that The structure of the morpholine affects the SERS spectrum.
Researchers have developed a cellulose-based polymer substrate containing silver and gold nanoparticles to enhance the surface of the substrate, thereby realizing high-resolution Raman spectroscopy signals for organic molecules such as porphyrin molecules. It is of great significance in application.
Due to the electric field and dipole moment generated in the porphyrin adsorption layer, TMPyP showed the highest enhancement result. Compared with graphene and other non-surface plasmon SERS substrates, CNF substrates show promising characteristics, thus indicating that this method is feasible for future Raman spectroscopy substrate manufacturing.
Fularz, A., Almohammed, S. and Rice, J., SERS enhancement of porphyrin-type molecules on metal-free cellulose substrates, ACS Sustainable Chemistry and Engineering, 2021, https://pubs.acs.org/doi/ 10.1021/acssuschemeng.1c06685
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Bismay is a technical writer living in Bhubaneswar, India. His academic background is engineering, and he has extensive experience in content writing, journal review, and mechanical design. Bismay holds a master's degree in materials engineering and a bachelor's degree in mechanical engineering, and is passionate about science, technology and engineering. Outside of work, he likes online games and cooking.
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