Over the last decade, single-molecule optical microscopy has become the gold-standard approach to decipher complex molecular processes in cellular environments. [1-3] Single-molecule fluorescence microscopy has several advantages such as ease of application, high sensitivity, low invasiveness and versatility due the large number of available fluorescent probes. It bears however some drawbacks related to the poor photostability of organic dye molecules [4] and auto-fluorescent proteins [5-7] and and to the relatively large size of semiconductor nanoparticles in the context of live cell applications. [4,8,9] The overall size of the functional biomarkers is a general issue for any imaging approach because of steric hindrance effects in confined cell regions. Small red-shifted nano-emitters that are highly photostable are not currently available, while they would combine the best physical and optical penetration properties in biological tissues. Although single-molecule absorption microscopy was early used to detect single-molecules [10] at cryogenic temperatures, it is only with the advent of photothermal microscopy [11,12] that practical applications of absorption microscopy were developed in single-molecule research. Photothermal imaging (PhI)
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