Quantum capacitance as a reagentless molecular sensing element

Abstract

The application of nanoscale capacitance as a transduction of molecular recognition relevant to molecular diagnostics is demonstrated. The energy-related signal relates directly to the electron occupation of quantized states present in readily fabricated molecular junctions such as those presented by redox switchable self-assembled molecular monolayers, reduced graphene oxide or redox-active graphene composite films, assembled on standard metallic or micro-fabricated electrodes. Sensor design is thus based on the response of a confined and resolved electronic density of states to target binding and the associated change in interfacial chemical potential. Demonstrated herein with a number of clinically important markers, this represents a new potent and ultrasensitive molecular detection enabling energy transducer principle capable of quantifying, in a single step and reagentless manner, markers within biological fluid.

Common Principles of Molecular Electronics and Nanoscale Electrochemistry

“…An equivalent nanoscale or quantum RC circuit exists if the molecular junction is in an electrochemical environment, as shown in Figure b. This is only a matter of evaluating the charge relaxation in the presence of an existing ionic environment and the hypothetically quantum point contact, as discussed in the preceding section, is embedded in a dielectric continuum (Figure b) containing ions (the electrolyte itself), so that C μ is now replaced by C μ̅ (the electrochemical capacitance), such as , where C i is the ionic capacitance of the interface­(normally modeled by the double layer phenomenon, see below). A difference between the C μ and C μ̅ depiction of the charging events should be highlighted simply because C i differs from C e , as illustrated in Figure .…”

Redox Capacitive Assaying of C-Reactive Protein at a Peptide Supported Aptamer Interface

“…In recent years, there has been a growing interest in the development of optimized interfaces capable of supporting the detection of clinically relevant analytes, especially those translatable to rapid low cost analyses. − Derived immunosensors ,− (which translate a biological biomarker recognition into a measurable signal) have been based on a variety of electrochemical techniques, such as voltammetry, amperometry, , and electrochemical impedance spectroscopy (EIS). − EIS is a natively spectroscopic and highly sensitive method within which interfacial charge transfer resistance R ct is most commonly assessed as a reporter of analyte recognition. , Recently we introduced electrochemical impedance-derived capacitive spectroscopy ,− as a label-free, reagentless method of mapping the change in interfacial redox capacitance ( C r ) as a transducer, omitting the need for a solution phase redox-probe. ,− The partition of mobile electrons between an electrode and a surface tethered chargeable (redox or quantum) species generates a spectroscopically resolvable capacitance. We have shown that this interfacial signal is fundamentally quantum mechanical in nature − and that the charging signature is highly sensitive to changes in local environment. ,, When incorporated into a redox addressable molecular film capable of selectively binding an analyte, C r can (where the binding site is in close proximity) become a sensitive function of target concentration. To date, such interfaces have been created by covalent immobilization of bioreceptors within a mixed self-assembled monolayer (SAM), one film component being redox active, the second serving as a receptor anchor. ,, …”

Charge transport and energy storage at the molecular scale: from nanoelectronics to electrochemical sensing

“…We first note again that C m is dominated by the charging of accessible redox states and that C m = e 2 (dN/dm), where (dN/dm) represents the 'chemical softness' of the interface' -the response of this DOS to an incident electric field. 45 We have shown (Fig. 6a) that the distribution of this DOS is reversibly responsive to solvent dielectric.…”

“…These two effects, the dispersive dielectric influence of solvent influence and DOS occupancy modulated by a local recognition event, are distinguishable. 45 One can construct an analytical curve by tracking 1/C m as a function of target presence, where variations are linear functions of the logarithm of concentration 45 (see Fig. 10d and 11), and we note that C m responds 21 more sensitively to C q than the much smaller, non-faradaic, Debye component, C e , (here in its C t non faradaic form C t (see Fig.…”

“…In recent work, we have shown that the local integration of receptors to the interface can lead to these capacitive signatures being utilized in derived reagentless sensors. 45 The receptors may be biological or supramolecular in nature and sensing occurs through detectable localized changes in (a) faradaic activity or (b) ionic content (both changing the chemical potential of the interface). The latter is discussed below in the section ''Ionic capacitive sensors''.…”