Endosomal pH Monitored by SERS

Classical metallic NPs can form a SERS tag for pH sensing. SERS studies on Ag NP clusters with 4-MBA showed the pH sensitivity in the range of 6-8 (TaUey et al. 2004). Kneipp and co-workers (Kneipp et al. 2007) analysed the pH sensitive SERS of 4-MBA on aggregated Au NPs in order to determine and to image the pH values in subceUular structures in the pH range from 6.8 to 5.4. The results are shown in Fig. 6.5. The NIH/3T3 ceUs were incubated 4.5 h with the Au NP tag (Au NPs and 4-MBA). Numerous Au NPs have accumulated in the ceU, enabling pH 

Many SERS pH sensors required high laser power ( 10 mW) and long collection time (about 10 s) and thus they were not suitable for scanning living cells. Moskovits and co-workers showed pH dependence of SERS spectra using 4-MBA-functionatized NP clusters and mapped local pH in HeLa cells (Pallaoro et al. 2010). Ag clusters were linked with bifunctional hexamethylenediamine molecules and encapsulated by PVP to prevent aggregation. The SERS-active 

6 Nanopipette for SERS intracellular studies. SERS spectra from the cell nucleus upper spectram) and cytoplasm middle spectrum). The bottom spectrum black line) was collected from the nanopipette tip before insertion 785 nm excitation was used. The spectra are offset for clarity. Inset SEM image of the nanopipette tip covered with Au NPs and their insertion to HeLa cell (adapted with permission from Vital et al. 2009. Copyright 2009 Amaican Chemical Society) 

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