Designing and Fabricating 3D Printed Substrates for SERS Sensing

Surface-enhanced Raman spectroscopy (SERS) is an ultra-sensitive analytical technique for obtaining the vibrational features of trace-level analytes adsorbed on SERS substrates, consisting of plasmonic nanostructures made up of gold and silver nanoparticles. In combination with the portable Raman spectrometers, SERS technique is a promising next-generation point-of-care chemical and biological sensing application. Developing portable SERS sensing has a wide range of applications in both real-time chemical and biological analysis and fundamental mechanistic studies even in remote locations. Fabrication of SERS substrates involves the formation of periodically arrayed plasmonic substrates that must meet the demand for low costs, signal uniformity, reproducibly responsive plasmonic platforms, and the need for reliable characterization of substrates. Despite these advantages, factors like high cost, reliance on single-use Ag/Au enhancing substrates and non-reproducibility are some of the major challenges that need to be addressed. A viable approach to address this issue is to develop multifunctional SERS substrates, which in addition to providing SERS enhancement, are equipped with photocatalytic functionality that allows the substrate to be regenerated for reusability. While there are many complex methods for fabricating SERS substrates, there has been a recent shift towards the development of simple, low-cost fabrication methods that are based on additive manufacturing (3D printing). However, functionalizing the 3D printing substrates is challenging due to their chemical inertness, surface roughness and variation in chemical composition. The development of smart multifunctional SERS sensors on 3D printed substrates to obtain recyclable SERS substrates by a combination of SERS and photocatalytic regeneration sets the primary objective of the thesis. Functionalization of the 3D printed structures using the plasmonic nanostructures and semiconductor photocatalysts by conventional chemical methods was not found to be successful due to the rough surface as well as the chemical inertness of the substrates. Therefore, a unique strategy of functionalization was developed that involves soot as a template, which is derived from the combustion of candle wax and natural oils including eucalyptus, clove and camphor oil. To graft the TiO2/Ag functional layer onto the surface, a three-stage functionalization strategy was developed. Firstly, candle soot was deposited on the surface using the flame deposition technique. This was followed by depositing TiO2 onto the soot surface by using chemical vapour deposition of TiO2 precursors and calcination to remove the soot template to obtain the fractal-structured TiO2. The third stage of functionalization involved the silver deposition on these fractal-structured TiO2 surfaces using the electroless deposition technique. The resultant TiO2/Ag multi-functional layer has the unique morphology of metal-semiconductor Schottky-junction throughout the surface that combines the photocatalytic activity and the SERS activity. The presence of silver nanoparticles and their uniform dispersion on the high surface area TiO2 template was found to enhance the vibrational spectra of the adsorbed RhB molecules and exhibit a high SERS enhancement factor. The adsorbed RhB molecules can be removed completely from the surface by the excellent photocatalytic activity of TiO2, upon exposure to UV light. This eventually regenerated the clean surface, which can restore the SERS activity of the surface and was shown that this substrate can be recycled. This unique functionalization approach of making Ag/TiO2 functional was demonstrated for its potential to be an excellent photocatalyst and recyclable Surface-Enhanced Raman Scattering (SERS) substrates. The amount of soot formed, its adherence with the substrate and its surface area depending on the nature of the organic fuel used for combustion. Three different natural oils including eucalyptus, clove and camphor oil were used to produce three different kinds of soot on silicon substrates. The amount of soot formed and its surface area using these oils were found to be higher than the soot formed from the burning of the candle. Subsequent functionalization of soots derived from these three oils using TiO2 and Ag nanoparticles led to the formation of fractal structured titania-silver nano-surfaces, which were studied for photocatalysis and SERS sensing. The soot derived from the eucalyptus oil was found to have a high surface area and the resultant TiO2/Ag nanostructures tend to have high SERS enhancement factors and self-cleaning characteristics. The selective Laser Melting (SLM) technique was used to print metal brush substrates, which were functionalized using multi-functional nanostructures on the soot template, CVD and ELD techniques. Soot derived from eucalyptus oil was considered the choice of soot to be deposited on the SLM metal structures. Firstly, Inconel, an alloy of nickel, chromium and molybdenum were printed as brushes on a base plate. These printed materials were calcined at 500 oC to promote the formation of oxides on the surface that enables the functionalization of soot. Interestingly, the nickel atoms from the printed structures tend to migrate to the soot and TiO2 nanostructures which aid in modulating their photocatalytic activity. SERS sensing of Rhodamine B was observed, and the enhancement factor was calculated to be higher than the silicon substrate. Moreover, the photocatalytic cleaning of the substrate under UV light led to the regeneration of the substrate, which showed reproducible SERS enhancement. However, the photocatalytic activity was not improved significantly compared to the silicon substrates. To enhance photocatalytic activity and SERS sensing capability, SLM printed Ti brushes were used as the migration of Ti from the printed structures into soot and TiO2 and its interaction with silver nanostructures will positively contribute towards functionalization. These printed Ti brushes were calcined at 500 oC to promote the formation of oxide on the surface that enables the functionalization of soot. Like the previous results, the titanium atoms from the printed structures tend to migrate the soot and TiO2 nanostructures that modulate their photocatalytic activity and enhance the SERS capability. Overall, this present work demonstrated fractal structured TiO2/Ag materials functionalization on 3D printed metal structures for the first time and the application of multi-functional material as promising candidates for recyclable SERS substrates. These materials have a broad scope in the given field of research and offer a new generation of 3D-printed chemical and biological sensors for any portable Raman applications and point-of-care clinical analysis.