pH-Responsive mixed-thiol-modified surface of roughened GaN: A wettability and SERS study

Chemical etching of gallium nitride (GaN) crystals produces various morphological patterns such as ‘nanowhiskers’. Due to the microscopic roughness etched GaN surfaces may be converted via further chemical functionalization into superhydrophobic materials or substrates for surface-enhanced Raman scattering (SERS). Here, properties of Au-sputtered GaN nanowhiskers functionalized with two mixed thiols: aliphatic non-polar 1-butanethiol (C4SH) and aromatic ionizable 4-mercaptobenzoic acid (4-MBA) are characterized. Changing the ratio of two thiols results in large variations of water droplet contact angle. Coinciding sigmoidal pH-dependencies of the contact angle and relative spectral SERS intensities assigned to ionized 4-MBA species are observed for surfaces with approximately 1:2 M 4-MBA:C4SH ratio. Stability of 4-MBA+C4SH layers was probed through thiol-exchange with a non-ionizable aromatic thiol: 2-naphthalenethiol (2NT). While the SERS characteristics of (4-MBA+C4SH)-Au/GaN platform exposed to 2NT acquires new spectral features the layer maintains pH-responsive wettability indicating the presence of 4-MBA possibly conserved through the π-stacking interaction with 2NT. Our results demonstrate that deposition of multicomponent thiol layers on nanowhiskers could facilitate development of pH-responsive self-cleaning sensors based on GaN. The results are discussed in light of mechanisms underlying superhydrophobicity and SERS.