Design and characterisation of SERS nanoprobes responsive to pH

Abstract

Numerous pH-sensitive SERS nanoprobes have already been reported in the literature. However, not all of them give a very strong signal while being able to be uptaken by the cells for biochemical applications. This work is carried out to find the balance between the strongest possible SERS signal and the possibility of introducing SERS nanoprobes into cells to determine intracellular pH. The objectives of this work are therefore to design and characterise pH-sensitive SERS nanoprobes, which will give the highest SERS signal, while maintaining the nanoparticles non-aggregated. Au@Ag-4MBA-PEG SERS nanoprobes have been successfully synthesised and characterised, and did not aggregate in complex media relevant to biological applications. Calibration curves in different media showed that these SERS nanoprobes were sensitive to the medium pH and that the pKa of the 4MBA molecule in the SERS probes slightly depended on the medium composition. From these calibration curves, the pH in tissue homogenates, with specific pH values for validation purposes, were estimated. The pH values obtained using the pH-sensitive Au@Ag-4MBA-PEG SERS nanoprobes agreed with the expected pH values, which is promising for biomedical applications where small changes of pH could now be monitored in microsections of the tissues. Tests were also carried out on cells and demonstrated that the nanoprobes can be included in the cytoplasm of cells. However, the calibration curves developed in aqueous solution could not be applied to determine the intracellular pH in fixated cells yet.