Design and fabrication of gold microelectrode arrays for SERS-based chemical sensing
Creators: name: Mehla, Sunil
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 340307
subject: Structure and dynamics of materials
classificationCode: 340309
subject: Theory and design of materials

description: Chemical sensors play a pivotal role in monitoring different aspects of our environment, gathering information from industrial systems, quality control for consumer and industrial goods, detection and prevention of diseases and maintenance of public safety and security. In the design and fabrication of chemical sensors, control of the topological features of the receptor material such as morphology, particle size, orientation, aspect ratio, order of arrangement and specific surface area plays a critical role in governing their performance for analyte reception and signal transduction. This thesis investigated design and fabrication of gold microelectrode arrays for detection of the Raman label, 4-aminothiophenol (ATP), which finds wide application in binding and detection of different biomarkers such as mycotoxin aflatoxinB1, endocrine disrupting compound biomarker vitellogenin, H1 influenza protein and mercury ions in liquid.

Gold is widely investigated as the preferred receptor material in optical, and surface-enhanced Raman scattering (SERS) based chemical sensors because of its desirable properties such as chemical inertness, corrosion resistance, biocompatibility, ability to selectively bind with specific functional groups, and plasmon excitation. For fabrication of sensors with gold as the receptor material electrodeposition is one of the most facile, versatile, low-cost, and scalable method that allows deposition of nanostructured films with good control over their composition and morphology. However, an enormous drawback of the electrodeposition method for sensor fabrication is the poor uniformity in morphology and orientation of nanostructures across the sensor surface because these differences directly impact sensors characteristics such as limit of detection, linearity, sensitivity, and uniformity in sensor response. Thus, electrochemical growth of uniform and functionally active gold nanostructures using electrodeposition is a formidable challenge. Patterning of gold films into an ordered arrangement of repeatable units, called arrays, results into intriguing optical properties which are governed by the geometrical structure of the array in addition to the composition and properties of individual units of the array. Thus, investigation of the fundamental relationship between the array architecture and the extent of plasmon mediated optical processes such as SERS and photocatalysis is of enormous interest and relevance to the chemical sensing community.

To address these challenges, this thesis investigated the design and fabrication of gold microelectrode arrays with systematically engineered topological features for detection of chemical analytes using surface-enhanced Raman scattering. A novel two-stage fabrication method was developed involving an initial fabrication of electrically anisotropic disc arrays and subsequent electrodeposition of gold nanostructures selectively on the lateral surfaces. Thus, the composition, geometrical structure, and arrangement order of the array in the XY plane were controlled through the first step whereas the growth, morphology, and size of the gold nanostructures in the Z direction were controlled through the second step. Gold nanoring microelectrode arrays with a rough surface were obtained from lateral electrodeposition on Ni-SiO2 disc arrays and gold sunflower microelectrode arrays with dendritic nanostructures on the lateral surfaces were obtained from lateral electrodeposition on Au-SiO2 disc arrays. FEM simulations were employed to understand the role of current density and electric field distributions in governing the morphology and orientation of laterally electrodeposited gold nanostructures. Based on experimental data and FEM simulations it was posited that the growth of gold nanostructures occurs along the electric field lines on the lateral surfaces of electrically-anisotropic discs. For gold nanoring arrays this led to curling of the inner surface of the gold rings towards the centre of the ring whereas the outer surface of the gold ring continued to expand radially outwards with increase of electrodeposition times. This unique curling led to trapping of the dielectric SiO2 film between two reflecting gold surfaces which is called a Fabry-Perot cavity. Similarly, electrochemical growth of gold dendrites originated perpendicular to the lateral surfaces, but the dendrites bent upwards and oriented vertically with increase of electrodeposition times.

Gold nanoring arrays exhibited direct excitation of multiple plasmon resonances in the spectral region from 400 – 2000 nm under normal incidence which were readily tunable through the control of ring thickness. The excitation of multiple plasmon resonances was assigned to the interaction between surface plasmon resonance and Fabry-Perot cavities, an optical phenomenon which is called Fabry-Perot resonance. The facile control over different aspects of plasmon resonances such as the number of plasmon resonances, their relative intensities and position of the resonance maximum made gold nanoring arrays promising for optical sensing and plasmon applications. In contrast, gold sunflower arrays exhibited anti-reflection properties in the visible spectrum and near-infrared region making them interesting for visible/infrared light harvesting and reflection scavenging applications.

The SERS performance of fabricated gold nanoring arrays was evaluated in terms of the detection limit for ATP, relative enhancement in SERS intensities compared to the background, uniformity of SERS response across the surface and relative intensities of the different vibrational peaks in the SERS spectra. Gold nanoring arrays exhibited moderate SERS activities and among the fabricated gold nanoring arrays the best SERS performance was exhibited by gold nanoring array of the highest thickness (969 nm, GNR5) because its second order resonance overlapped with the Raman laser (785 nm). GNR5 exhibited a detection limit of 1 mM for ATP, an enhancement of SERS intensities by ~19 folds compared to the background and a relative standard deviation in SERS intensities of 46.27 % across the sensor surface. Comparatively, gold sunflower arrays exhibited significantly higher SERS activities which were governed by morphology of the gold dendrites on their lateral surfaces. The optimal morphology for best SERS performance was identified as the branched dendritic morphology. For the best performing gold sunflower array, a detection limit of 0.5 nM for 4-ATP, enhancement in SERS intensities by ~1.7 × 106 folds compared to the background and relative standard deviation in SERS response of 6.72% across the sensor surface was identified. The fabricated microelectrode arrays could be reused for SERS measurements after surface regeneration using low pressure oxygen plasma treatment.