**2.2 Carbon nanoparticles**

Carbon nanoparticles, also named colloidal carbon, can be visually detected in a qualitative or semi-quantitative manner and thus being used as colored labels. Compared to GNPs, carbon nanoparticles have several excellent properties, e.g., high stability, nontoxicity, ease-of-preparation, and ease-of-modification [26]. The dark black color of carbon dots endows a high signal-to-noise ratio, allowing sensitivities below the low picomolar range even by visual inspection [27].
