Pyrene-Appended Boronic Acids on Graphene Foam Electrodes Provide Quantum Capacitance-Based Molecular Sensors for Lactate
Highlights
- The Adsorption of a pyrene-appended receptor molecule onto Gii causes significant capacitance changes (when positively charged pyridinium group is attached close to the Gii surface).
- Gii provides a great platform for sensing mechanism based on capacitance changes.
- The use of quantum capacitance in sensing can open new opportunities in wireless sensing but more in-depth research is still needed.
Abstract
Molecular recognition and sensing can be coupled to interfacial capacitance changes on graphene foam surfaces linked to double layer effects and coupled to enhanced quantum capacitance.
3D graphene foam film electrodes (Gii-Sens; thickness approximately 40 μm; roughness factor approximately 100) immersed in aqueous buffer media exhibit an order of magnitude jump in electrochemical capacitance upon adsorption of a charged molecular receptor based on pyrene-appended boronic acids (here, 4-borono-1-(pyren-2-ylmethyl)pyridin-1-ium bromide, or abbreviated T1). This pyrene-appended pyridinium boronic acid receptor is employed here as a molecular receptor for lactate. In the presence of lactate and at pH 4.0 (after pH optimization), the electrochemical capacitance (determined by impedance spectroscopy) doubles again. Lactic acid binding is expressed with a Hillian binding constant (Klactate = 75 mol−1 dm3 and α = 0.8 in aqueous buffer, Klactate = 460 mol−1 dm3 and α = 0.8 in artificial sweat, and Klactate = 340 mol−1 dm3 and α = 0.65 in human serum).
The result is a selective molecular probe response for lactic acid with LoD = 1.3, 1.4, and 1.8 mM in aqueous buffer media (pH 4.0), in artificial sweat (adjusted to pH 4.7), and in human serum (pH adjusted to 4.0), respectively. The role of the pyrene-appended boronic acid is discussed based on the double layer structure and quantum capacitance changes. In the future, this new type of molecular capacitance sensor could provide selective enzyme-free analysis without analyte consumption for a wider range of analytes and
complex environments.
Introduction
Boronic acids provide a potent class of molecular chemical receptors with selectivity for a range of analytes including glucose [1] and α-hydroxy-carboxylic acids.[2] Although commonly employed in fluorescence assays, [3] boronic acids have also been attached or assembled at electrochemical sensor surfaces,4 but they have never been employed directly to sense via
electrochemical (quantum) capacitance responses. A related “varactor”-based quantum capacitor device with an adsorbed boronic acid on graphene was shown to respond to glucose binding.5 A varactor (or varicap) is a device with potential dependent capacitance. The capacitance in the varactor response was linked to double layer structure changes upon glucose binding, affecting the electronic states within the graphene. Boronic acids embedded in functional microgels6 have been employed for detection of lactate in sweat. Here, a special boronic acid with a positive pyridinium charge bound closely to the graphene electrode surface is introduced.
A precursor molecule with protected boronic acid is synthesized, attached to a graphene surface, and then employed for the sensor by neopentylglycol release in contact with aqueous media. In recent work, it was shown that T1 can be adsorbed onto graphene foam film electrodes for the voltammetric detection of glucose9 or for the detection of lactic acid. [10] In these studies, a redox-active polymer was employed to modulate Faradaic current responses related to analyte binding in a polymer indicator displacement assay (PIDA). Here, it is reported that the interfacial capacitance response of 3D-graphene foam can be employed directly for sensing (without any Faradaic current consuming analyte), even in complex matrices and without the need for a redox polymer indicator.