Building Addressable Libraries: A New Tunable Reaction Layer for Use on a Microelectrode Array
Authors: Moeller Kevin D , Graaf Matthew D. , Nguyen Bichlien H. 
Published: February 2020

Microelectrode arrays provide a potentially powerful method form monitoring the interactions between small molecules and biological receptors in “real-time” as the events happen. This is accomplished by placing or building small molecules proximal to the individually addressable electrodes in the array.
Central to this effort is the availability of porous reaction layers that can be used to coat the arrays and provide attachment points for fixing molecules to surface of the electrodes. The porous reaction layers must be stable with respect to time, chemically inert, compatible with both the synthetic and analytical methods to be performed on the arrays, and resistant to non-specific binding events with biological receptors.
One approach to addressing these issues is to take advantage of diblock copolymers with the general structure shown in Figure 1.1The polymer contains a 2-methylacrylate block functionalized with a cinnamate group and a substituted polystyrene block. The polymer can be spin coated onto the arrays and then photolyzed in order to crosslink the cinnamate groups. This provides stability to the surface. The degree of crosslinking allows for control of the pore size in the polymer. The styrene groups can then be used as attachement points for adding molecules to the surface of the array.
The first polymer studied was the 4-bromostyrene derivative (R = Br). The polymer was stable and compatible with a variety of synthetic methods. Both Pd(0) and Cu(I)-based coupling reactions involving nucleophiles and the arylbromide were successful. In addition, the diblock copolymer was compatible with electrochemical impedance experiments, and therefore the analytical studies necessary for monitoring small molecule – receptor interactions. The bromostyrene derived diblocck copolymer was used to conduct a number of “proof-of-principle” studies that helped establish the overall utility of the arrays. The porous reaction layer based on the diblock copolymer was a vast improvement over the sugar-based surfaces originally used as coatings for the arrays. However, use of the bromostyrene based diblock copolymer was not without issues. Its use led to relatively high levels of non-specific binding with protein targets during the analytical studies. What we needed was a new surface that could be tuned to reduce such non-specific interactions.
To that end, a second generation diblock copolymer containing borate ester substituted styrene block (R = B(OR)2) was synthesized. This polymer currently shows great promise. The initial polymer used a pinacol to establish the borate ester. The pinacol borate ester can be used as a handle to covalent attach small molecules to the electrodes in the array in an irreversible fashion. This can be done with the use of Pd(0), Cu(I), or a modified Chan-Lam procedure using Cu(II) (Scheme 1). In addition, the pinacol group can be reversibly exchanged for an alternative diol with the use of acid. The result is a chance to place molecules irreversibly on the surface of the for study while retaining the ability to vary the rest of the surface at will. This allows the surface of the array to be tuned so that non-specific binding events on the array can be minimized.
In the talk to be presented, the chemistry of this new surface will be discussed. Topics to be addressed will include preparation of the polymer, stability studies, compatibility of the polymer with electrochemical signaling efforts, and the development of site-selective synthetic methods that take advantage of the borate esters. In addition, an application of the arrays and the new surface to a biological study will be presented.
References:
1. Hu, L.; Graff, M. D.; Moeller, K. D. J. Electrochem. Soc.
2013, 160, G3020.