The study of the development of microbial biofilms is crucial to find strategies that allow their control and eradication. As a result of their metabolic activities, microbes secrete bioactive chemical compounds for intercellular communication, as well as metabolic by-products that can also greatly influence the development and composition of biofilms. For instance, microbial fermentation processes often lead to the production of acids that can lower the local pH significantly. Thus, non-invasive and simultaneous monitoring of extracellular bioactive metabolites and physicochemical factors (e.g. pH) can provide valuable information regarding the mechanisms that regulate the biogenesis, composition, and function of microbial communities. Herein, we report a SERS hybrid substrate consisting in plasmonic Au@Ag@mSiO2 nanorattles embedded within an agar matrix for pH sensing in bacterial colonies. A key element of the SERS substrate is the mesoporous silica shell encapsulating single plasmonic nanoparticles, which furnishes colloidal stability and molecular sieving capabilities, thereby realizing robust, sensitive, and reliable measurements. This multifunctional SERS substrate enabled us to efficiently perform spatiotemporal non-invasive detection and imaging of pH changes in colonies of Escherichia coli. In addition, we demonstrate the applicability of the Au@Ag@mSiO2 nanosensors for the detection of pyocyanin, a quorum sensing regulated metabolite, in Pseudomonas aeruginosa liquid culture without any sample pretreatment. The reported SERS-based approach may be used as a is a powerful tool for deciphering the role of extracellular metabolites and physicochemical factors in microbial community dynamics and interactions.

Plasmonic nanorattles for in situ SERS imaging of cellular metabolism in microbial colonies

ACS Fall 2021

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