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Developing thin polymeric films for trapping, releasing, delivering, and sensing molecules is important for many applications in chemistry, biotechnology, and environment. Hence, a facile and scalable technique for loading specific molecules on surfaces would rapidly translate into applications. This work presents a novel method for the trapping of functional molecules at interfaces by exploiting diazonium salt chemistry. We demonstrate the efficiency of this approach by trapping two different molecules, 4-nitrobenzophenone and paracetamol, within polycarboxyphenyl layers grafted on gold and glassy carbon (GC) and by releasing them in acidic medium. The former molecule was chosen as a proof of concept for its electrochemical and spectroscopic properties, and the latter one was selected as an example of a pharmaceutical molecule. Advantages of the present approach rely on the simplicity, rapidity, and efficiency of the procedure for the reversible, on demand, trapping and release of functional molecules. [ABSTRACT FROM AUTHOR]

How small should nanoparticles be in order to travel freely through the cytosol similar to proteins? Answering this question remains a challenge, because the majority of nanoparticles are relatively large and their size cannot be finely tuned to match that of proteins. Here, poly(methyl methacrylate) copolymers with varied fraction and type of charged groups (carboxylate, sulfonate, and trimethylammonium) are developed, yielding nanoparticles with controlled sizes from 50 to 7 nm through nanoprecipitation. Loading these nanoparticles with a rhodamine dye/bulky counterion pair at 10wt% makes them highly fluorescent. After their coating with polyethylene glycol groups for preventing non‐specific protein binding and microinjection into living cells, the first systematic study of the size effect on diffusion in the cytosol for solid nanoparticles of the same nature is realized. Single‐particle‐tracking data provide evidence for distinct particle sieving in the cytosol, suggesting that only nanoparticles below a critical size of 23 nm exhibit free diffusion and spreading. These findings show the size limitations imposed by intracellular crowding and compartmentalization, which is critical for applications of nanomaterials in the cytosol. The proposed concept of polymer design opens the route to organic nanoparticles of ultrasmall sizes and high loading for bioimaging and drug‐delivery applications. What is the ideal size of nanoparticles (NPs) for intracellular applications? Here, a series of dye‐loaded polymer NPs with sizes from 50 down to 7 nm is synthesized and microinjected into living cells. Single‐particle tracking of these particles suggests that only NPs smaller than a critical size of 23 nm exhibit free diffusion and spreading in the cytosol. [ABSTRACT FROM AUTHOR]