Micro sensing of pH levels in biological samples by graphene-based Raman spectroscopy

Graphene provides a unique way for sensing local pH level of substances, with important implications in the monitoring of cellular metabolic activities where protonic excretion occurs in suitable conditions [1]. Doping modifications of graphene,  induced by the contact of the graphene sheet with different pH solutions were investigated by micro-Raman spectroscopy in order to develop a pH biosensor. A small amount of liquid (of the order of few microliters) was dropped on a graphene surface and the Raman response collected by using He-Ne laser light excitation. Depending on the doping level, the energy of the Raman  G mode of graphene significantly changes in the range of 1580-1610 cm^-1. The Raman response of graphene exposed to known pH aqueous solutions was investigated in order to provide a calibration curve for the sensor. In the aim to test the developed biosensor with  real biological systems, the pH values of cell culture media in different conditions were evaluated. The obtained results suggested that the proposed biosensing scheme could be adopted also for monitoring more complex biological systems as a single cell at the micrometer scale.

[1]   G. L. C. Paulus et al. “A graphene-based physiometer array for the analysis of single biological cells” Sci. Reports 4 (2014) 6865.