Functionalized Mesoporous Polymers with Enhanced Performance as Reference Electrode Frits

Abstract

Mesoporous glasses (∼10 nm pores) and macroporous polymers (∼1 μm pores) are often used as frits in the fabrication of aqueous reference electrodes. These frits function as salt bridges that allow for electrical contact between the sample and reference solutions while slowing cross contamination of the two solutions. Unfortunately, mesoporous glass and macroporous polymer frits used for these purposes inherently result in sample-dependent potentials or in rapid cross contamination of the sample and reference solutions, respectively. To address these issues, we synthesized mesoporous polymer frits, with much smaller pore sizes (∼10 nm) and electrically neutral hydrophilic pore walls. These monoliths were prepared from a bicontinuous, microphase-separated, and cross-linked block polymer precursor, that is, poly­(lactide)-b-poly­(isoprene)-b-poly­(styrene-co-divinylbenzene), PLA-b-PI-b-P­(S-co-DVB). The PLA serves as a selectively etchable sacrificial block, the PI provides latent reactive sites on the pore walls, and the P­(S-co-DVB) forms the mechanically robust matrix. Subjecting the PI repeat units in the monoliths to epoxidation and subsequent hydrolysis reactions renders the pore walls hydrophilic and uncharged, permitting the use of the polymer as a porous frit material in reference electrodes with aqueous electrolyte solutions. This monolith chemistry allows for reference electrodes with reduced flow rates that approach those of mesoporous glass frits and thus mitigated cross contamination. Moreover, reference electrode potential variations are reduced across a large range of electrolyte concentrations.

Recent progress in the development of improved reference electrodes for electrochemistry

“…These frits maintained a low flow rate (0.007 ± 0.003 µL/h) and exhibited no hysteresis effects while decreasing the charge screening effect in low ionic strength solutions by a factor of 3. 23 While restricting the volumetric flow is generally desirable for reference electrodes, especially in long-term measurements, restricting the flow of bridge electrolyte through a frit junction too much can result in contamination of the bridge electrolyte, as shown in Figure 2. In this example, after just 5 min, diffusion of SCNfrom the sample solution into frits filled with FeCl3 solutions of higher concentration is evidenced by the appearance of a red color resulting from the formation of the complex Fe[(SCN)(H2O)5] 2+ .…”

Potentiometric Sensing

“…Another way to solve the charge screening problem may be to use a nanoporous material with electrically neutral hydrophilic pore walls in contrast to glass. Bühlmann and co-workers explored this approach as well and reported on the synthesis of a mesoporous polymer frit with corresponding characteristics and pore sizes as small as 10 nm . These monoliths were prepared from a bicontinuous, microphase-separated, and cross-linked block polymer precursor, that is, poly­(lactide)- b -poly­(isoprene)- b -poly­(styrene- co -divinylbenzene).…”

“…Buḧlmann and coworkers explored this approach as well and reported on the synthesis of a mesoporous polymer frit with corresponding characteristics and pore sizes as small as 10 nm. 3 These monoliths were prepared from a bicontinuous, microphaseseparated, and cross-linked block polymer precursor, that is, poly(lactide)-b-poly(isoprene)-b-poly(styrene-co-divinylbenzene). The charge screening effect was tested across a large range of electrolyte concentrations.…”

Critical Comparison of Reference Electrodes with Salt Bridges Contained in Nanoporous Glass with 5, 20, 50, and 100 nm Diameter Pores

“…The measurement of stable and reproducible reference potentials is an area of interest for many but is not without its complications. [1][2][3][4][5][6][7][8][9][10][11][12][13][14] Consistent and constant reference potentials are particularly difficult to measure in very dilute aqueous solutions, a problem of concern especially for accurate pH measurements of environmental importance. 9,[15][16][17][18] In many cases, salt bridges are used to provide for electrical contact between reference electrodes and samples.…”

“…4 Reference electrodes comprising polymeric frit materials with much smaller pore sizes are feasible but at this point require custom synthesis and are not easy to functionalize with a hydrophilic surface that does not bind certain ions specifically. 7 To this end, we tested in the work presented here commercially available porous glass frits with pore diameters of 1, 5, 20, 50, and 100 nm. The frits were used to separate a 3.0 M KCl bridge electrolyte solution from different types of sample solutions.…”