SERS active pH-nanosensor with tunable properties

The extraordinary optical properties of gold nanoparticles (AuNPs), together with their rewarding chemical stability and ease of functionalization, make them an invaluable platform to develop ultrasensitive and molecular specific chemical sensors. The huge amplification of the spectroscopic signal of molecules located at the metal interface, arising from the confinement of strong electromagnetic fields on the AuNPs surface, results in a remarkable increase of the sensitivity of vibrational spectroscopies. In particular, Surface Enhanced Raman Spectroscopy (SERS) emerged as a powerful analytical tool with detection limits lowered down to the single molecule recognition. In this framework, we developed and characterized a plasmonic pH-nanosensor by conjugating AuNPs with the pH-sensitive molecular probe 4-mercaptobenzoic acid (4MBA), which shows a SERS signal depending on its protonation degree. pH is indeed a key target parameter in a wide field of applications, ranging from environmental science to industry to biomedicine. Exposing the AuNP-4MBA nanosensors to solutions at varying pH, we identified the dynamic range of sensitivity as a function of the relative intensity of selected pHdependent SERS bands. From the comparison of the obtained calibration curve with a standard acid-base titration curve of the free molecule, we enlightened that the pKa of the molecule shifts to higher values when it is measured by SERS at the interface of the plasmonic nanostructure. In particular, for AuNPs with a diameter of 60 nm, the pKa value results around pH 6, making this system suitable for pH measurements in physiological environment, at the single cell level. Proceeding from this, we will explore the possibility of tuning the 4MBA acidic properties by varying the AuNPs size as well as the core material, with the final aim toreach a modulation of the working point of the nanosensor, depending on the system of interest.