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Capacitive field-effect sensors modified with a multi-enzyme membrane have been applied for an electronic transduction of biochemical signals processed by enzyme-based AND-Reset and OR-Reset logic gates. The local pH change at the sensor surface induced by the enzymatic reaction was used for the activation of the Reset function for the first time.
Capacitive field-effect electrolyte-insulator-semiconductor sensors consisting of an Al-p-Si-SiO2 structure have been used for the electrical detection of unlabelled single- and double-stranded DNA (dsDNA) molecules by their intrinsic charge. A simple functionalization protocol based on the layer-by-layer (LbL) technique was used to prepare a weak polyelectrolyte/probe-DNA bilayer, followed by the hybridization with complementary target DNA molecules. Due to the flat orientation of the LbL-adsorbed DNA molecules, a high sensor signal has been achieved. In addition, direct label-free detection of in-solution hybridized dsDNA molecules has been studied.
Miniaturized setup, compatibility with advanced micro- and nanotechnologies, and ability to detect biomolecules by their intrinsic molecular charge favor the semiconductor field-effect platform as one of the most attractive approaches for the development of label-free DNA chips. In this work, a capacitive field-effect EIS (electrolyte–insulator–semiconductor) sensor covered with a layer-by-layer prepared, positively charged weak polyelectrolyte layer of PAH (poly(allylamine hydrochloride)) was used for the label-free electrical detection of DNA (deoxyribonucleic acid) immobilization and hybridization. The negatively charged probe single-stranded DNA (ssDNA) molecules were electrostatically adsorbed onto the positively charged PAH layer, resulting in a preferentially flat orientation of the ssDNA molecules within the Debye length, thus yielding a reduced charge-screening effect and a higher sensor signal. Each sensor-surface modification step (PAH adsorption, probe ssDNA immobilization, hybridization with complementary target DNA (cDNA), reducing an unspecific adsorption by a blocking agent, incubation with noncomplementary DNA (ncDNA) solution) was monitored by means of capacitance–voltage and constant-capacitance measurements. In addition, the surface morphology of the PAH layer was studied by atomic force microscopy and contact-angle measurements. High hybridization signals of 34 and 43 mV were recorded in low-ionic strength solutions of 10 and 1 mM, respectively. In contrast, a small signal of 4 mV was recorded in the case of unspecific adsorption of fully mismatched ncDNA. The density of probe ssDNA and dsDNA molecules as well as the hybridization efficiency was estimated using the experimentally measured DNA immobilization and hybridization signals and a simplified double-layer capacitor model. The results of field-effect experiments were supported by fluorescence measurements, verifying the DNA-immobilization and hybridization event.