Control of pH-Responsiveness in Graphene Oxide Grafted with Poly-DEAEMA via Tailored Functionalization

Abstract

Polymer-grafted nanomaterials based on carbon allotropes and their derivatives (graphene oxide (GO), etc.) are typically prepared by successive reaction stages that depend upon the initial functionalities in the nanostructure and the polymerization type needed for grafting. However, due to the multiple variables involved in the functionalization steps, it is commonly difficult to predict the properties in the final product and to correlate the material history with its final performance. In this work, we explored the steps needed to graft the carboxylic acid moieties in GO (COOH@GO) with a pH-sensitive polymer, poly[2-(diethylamino)ethyl methacrylate] (poly[DEAEMA]), varying the reactant ratios at each stage prior to polymerization. We studied the combinatorial relationship between these variables and the behavior of the novel grafted material GO-g-poly[DEAEMA], in terms of swelling ratio vs. pH (%Q) in solid specimens and potentiometric response vs. Log[H⁺] in a solid-state sensor format. We first introduced N-hydroxysuccinimide (NHS)-ester moieties at the -COOH groups (GO-g-NHS) by a classical activation with N-ethyl-N'-(3-dimethylaminopropyl)carbodiimide (EDC). Then, we substituted the NHS-ester groups by polymerizable amide-linked acrylic moieties using 2-aminoethyl methacrylate (AEMA) at different ratios to finally introduce the polymer chains via radical polymerization in an excess of DEAEMA monomer. We found correlated trends in swelling pH range, interval of maximum and minimum swelling values, response in potentiometry and potentiometric linear range vs. Log[H⁺] and could establish their relationship with the combinatorial stoichiometries in synthetic stages.

Keywords: DEAEMA; graphene oxide; pH-sensitive materials; potentiometry; radical polymerization; swelling ratio.

Figure 1

Grafting degree of graphene oxide (GO) depending on the reactant ratios at different stages: (a) N-hydroxysuccinimide (NHS)-ester amount introduced after varying N-ethyl-N′-(3-dimethylaminopropyl)carbodiimide (EDC) against the molar content of COOH per sample of GO (COOH@GO), for an NHS:EDC molar ratio = 2 in all the cases. (b) Material history in terms of graft content for a selected ratio EDC:COOH@GO of 4 mol/mol at Stage I, subsequently modified with amide-linked polymerizable 2-aminoethyl methacrylate (AEMA) moieties under a ratio of 24 mmol/g of AEMA vs. the departing material GO-g-NHS at Stage II, and its further polymerization with a five-fold excess in weight of 2-(diethylamino)ethyl methacrylate (DEAEMA) against GO-g-AEMA in Stage III. (c) Amount of poly[DEAEMA] chains incorporated to GO after Stage III for a fixed five-fold excess in weight of DEAEMA vs. GO-g-AEMA, but a variable ratio in AEMA vs. GO-g-NHS at Stage II and an initial molar ratio EDC:COOH@GO = 4 mol/mol at Stage I; in (c), a preliminary potentiometric evaluation of the material in a flexible solid-state sensor format showed a positive correlation between polymer grafting degree and electrochemical response against Log[H⁺].

Figure 2

Performance evaluation for the nine different families of GO-g-poly[DEAEMA] obtained by combinatorial variations in reactant ratios. (a) Response surface curve for average potentiometric response against Log[H⁺]; (b) response surface curve for the difference between maximum and minimum average swelling ratios (Δ[%Q] = %Q_max – %Q_min) for the same families in (a); (c) average potentiometric response values against Log[H⁺] including 1 s.d. in error bars, and their comparison with Δ[%Q]; (d) average potentiometric linear ranges for response vs. pH including 1 s.d. in error bars, and their comparison with the swelling pH ranges and routes at which minimum-to-maximum average swelling ratios occur. Average values and s.d. are for N = 3; blank materials in (c,d) are GO-g-AEMA.

Figure 2

Performance evaluation for the nine different families of GO-g-poly[DEAEMA] obtained by combinatorial variations in reactant ratios. (a) Response surface curve for average potentiometric response against Log[H⁺]; (b) response surface curve for the difference between maximum and minimum average swelling ratios (Δ[%Q] = %Q_max – %Q_min) for the same families in (a); (c) average potentiometric response values against Log[H⁺] including 1 s.d. in error bars, and their comparison with Δ[%Q]; (d) average potentiometric linear ranges for response vs. pH including 1 s.d. in error bars, and their comparison with the swelling pH ranges and routes at which minimum-to-maximum average swelling ratios occur. Average values and s.d. are for N = 3; blank materials in (c,d) are GO-g-AEMA.