Deep eutectic solvents
(DESs) formed by hydrogen bond donors and
acceptors are a promising new class of solvents. Both hydrophilic
and hydrophobic binary DESs readily absorb water, making them ternary
mixtures, and a small water content is always inevitable under ambient
conditions. We present a thorough study of a typical hydrophobic DES
formed by a 1:2 mole ratio of tetrabutyl ammonium chloride and decanoic
acid, focusing on the effects of a low water content caused by absorbed
water vapor, using multinuclear NMR techniques, molecular modeling,
and several other physicochemical techniques. Already very low water
contents cause dynamic nanoscale phase segregation, reduce solvent
viscosity and fragility, increase self-diffusion coefficients and
conductivity, and enhance local dynamics. Water interferes with the
hydrogen-bonding network between the chloride ions and carboxylic
acid groups by solvating them, which enhances carboxylic acid self-correlation
and ion pair formation between tetrabutyl ammonium and chloride. Simulations
show that the component molar ratio can be varied, with an effect
on the internal structure. The water-induced changes in the physical
properties are beneficial for most prospective applications but water
creates an acidic aqueous nanophase with a high halide ion concentration,
which may have chemically adverse effects.
With the advent of flexible electronics, the old fashioned and conventional solid‐state technology will be replaced by conductive inks combined with low‐cost printing techniques. Graphene is an ideal candidate to produce conductive inks, due to its excellent conductivity and zero bandgap. The possibility to chemically modify graphene with active molecules opens up the field of responsive conductive inks. Herein, a bioresponsive, electroactive, and inkjet‐printable graphene ink is presented. The ink is based on graphene chemically modified with selected enzymes and an electrochemical mediator, to transduce the products of the enzymatic reaction into an electron flow, proportional to the analyte concentration. A water‐based formulation is engineered to be respectful with the enzymatic activity while matching the stringent requirements of inkjet printing. The efficient electrochemical performance of the ink, as well as a proof‐of‐concept application in biosensing, is demonstrated. The versatility of the system is demonstrated by modifying graphene with various oxidoreductases, obtaining inks with selectivity toward glucose, lactate, methanol, and ethanol.
Graphene oxide (GO) coated electrodes provide an excellent platform for enzymatic glucose sensing, induced by the presence of glucose oxidase and an electrochemical transduction. Here, we show that the sensitivity of GO layers for glucose detection redoubles upon blending GO with chitosan (GO +Ch) and increases up to eight times if covalent binding of chitosan to GO (GO−Ch) is exploited. In addition, the conductivity of the composite material GO−Ch is suitable for electrochemical applications without the need of GO reduction, which is generally required for GO based coatings. Covalent modification of GO is achieved by a standard carboxylic activation/amidation approach by exploiting the abundant amino pendants of chitosan. Successful functionalization is proved by comparison with an ad-hoc synthesized control sample realized by using non-activated GO as precursor. The composite GO−Ch was deposited on standard screen-printed electrodes by a dropcasting approach. Comparison with a chitosan-GO blend and with pristine GO demonstrated the superior reliability and efficiency of the electrochemical response for glucose as a consequence of the high number of enzyme binding sites and of the partial reduction of GO during the carboxylic activation synthetic step.
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