The integration of carbon nanotubes in conjunction with a chemical or biological recognition element into a semiconductor field-effect device (FED) may lead to new (bio)chemical sensors. In this study, we present a new concept to develop field-effect-based sensors, using a light-addressable potentiometric sensor (LAPS) platform modified with layer-by-layer (LbL) films of single-walled carbon nanotubes (SWNTs) and polyamidoamine (PAMAM) dendrimers. Film growth was monitored for each layer adsorbed on the LAPS chip by measuring current-voltage (I/V) curves. The morphology of the films was analyzed via atomic force microscopy (AFM) and field-emission scanning electron microscopy (FESEM), revealing the formation of a highly interconnected nanostructure of SWNTs-network into the dendrimer layers. Constant current (CC) measurements showed that the incorporation of the PAMAM/SWNT LbL film containing up to 6 bilayers onto the LAPS structure has a high pH sensitivity of ca. 58 mV/pH. The biosensing ability of the devices was tested for penicillin G via adsorptive immobilization of the enzyme penicillinase atop the LbL film. LAPS architectures modified with the LbL film exhibited higher sensitivity, ca. 100 mV/decade, in comparison to ca. 79 mV/decade for an unmodified LAPS, which demonstrates the potential application of the CNT-LbL structure in field-effect-based (bio)chemical sensors.

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The assembly of carbon nanotubes (CNTs) into nanostructured films is attractive for producing functionalized hybrid materials and (bio‐)chemical sensors, but this requires experimental methods that allow for control of molecular architectures. In this study, we exploit the layer‐by‐layer (LbL) technique to obtain two types of sensors incorporating CNTs. In the first, LbL films of alternating layers of multi‐walled carbon nanotubes (MWNTs) dispersed in polyaminoamide (PAMAM) dendrimers and nickel phthalocyanine (NiTsPc) were used in amperometric detection of the neurotransmitter dopamine (DA). The electrochemical properties evaluated with cyclic voltammetry indicated that the incorporation of MWNTs in the PAMAM‐NT/NiTsPc LbL films led to a 3‐fold increase in the peak current, in addition to a decrease of 50 mV in the oxidation potential of DA. The latter allowed detection of DA even in the presence of ascorbic acid (AA), a typical interferent for DA. Another LbL film was obtained with layers of PAMAM and single‐walled carbon nanotubes (SWNTs) employed in field‐effect‐devices using a capacitive electrolyte–insulator–semiconductor structure (EIS). The adsorption of the film components was monitored by measuring the flat‐band voltage shift in capacitance–voltage (C –V) curves, caused by the charges from the components. Constant‐capacitance (ConCap) measurements showed that the EIS‐PAMAM/SWNT film displayed a high pH sensitivity (ca. 54.5 mV/pH), being capable of detecting penicillin G between 10–4 mol L–1 and 10–2 mol L–1, when a layer of penicillinase was adsorbed atop the PAMAM/SWNT film. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The semiconductor field-effect platform is a powerful tool for chemical and biological sensing with direct electrical readout. In this work, the field-effect capacitive electrolyte-insulator-semiconductor (EIS) structure - the simplest field-effect (bio-)chemical sensor - modified with citrate-capped gold nanoparticles (AuNPs) has been applied for a label-free electrostatic detection of charged molecules by their intrinsic molecular charge. The EIS sensor detects the charge changes in AuNP/molecule inorganic/organic hybrids induced by the molecular adsorption or binding events. The feasibility of the proposed detection scheme has been exemplarily demonstrated by realizing capacitive EIS sensors consisting of an Al-p-Si-SiO2-silane-AuNP structure for the label-free detection of positively charged cytochrome c and poly-d-lysine molecules as well as for monitoring the layer-by-layer formation of polyelectrolyte multilayers of poly(allylamine hydrochloride)/poly(sodium 4-styrene sulfonate), representing typical model examples of detecting small proteins and macromolecules and the consecutive adsorption of positively/negatively charged polyelectrolytes, respectively. For comparison, EIS sensors without AuNPs have been investigated, too. The adsorption of molecules on the surface of AuNPs has been verified via the X-ray photoelectron spectroscopy method. In addition, a theoretical model of the functioning of the capacitive field-effect EIS sensor functionalized with AuNP/charged-molecule hybrids has been discussed.

The control of molecular architecture provided by the layer-bylayer (LbL) technique has led to enhanced biosensors, in which advantageous features of distinct materials can be combined. Full optimization of biosensing performance, however, is only reached if the film morphology is suitable for the principle of detection of a specific biosensor. In this paper, we report a detailed morphology analysis of LbL films made with alternating layers of single-walled carbon nanotubes (SWNTs) and polyamidoamine (PAMAM) dendrimers, which were then covered with a layer of penicillinase (PEN). An optimized performance to detect penicillin G was obtained with 6-bilayer SWNT/PAMAM LbL films deposited on p-Si-SiO 2 -Ta 2 O 5 chips, used in biosensors based on a capacitive electrolyteinsulator-semiconductor (EIS) and a light-addressable potentiometric sensor (LAPS) structure, respectively. Field-emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM) images indicated that the LbL films were porous, with a large surface area due to interconnection of SWNT into PAMAM layers. This morphology was instrumental for the adsorption of a larger quantity of PEN, with the resulting LbL film being highly stable. The experiments to detect penicillin were performed with constant-capacitance (ConCap) and constant-current (CC) measurements for EIS and LAPS sensors, respectively, which revealed an enhanced detection signal and sensitivity of ca. 100 mV/decade for the field-effect sensors modified with the PAMAM/SWNT LbL film. It is concluded that controlling film morphology is essential for an enhanced performance of biosensors, not only in terms of sensitivity but also stability and response time.

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a b s t r a c tThe toxicity of chromium ions was investigated using mammalian cell cultures on impedance sensors as well as physiological in vitro sensor systems. The performance of commercially available systems like the 2500 Analyzing System (Bionas), xCELLigence (Roche) and Cytosensor Microphysiometer (Molecular Devices) was compared with a novel CMOS-based impedance-to-frequency converter device. Cell-based sensor systems are shown to be powerful tools to detect Cr(VI) pollutions within several hours in the range of multinational drinking water regulations. The ability to distinguish between toxic Cr(VI) and non-toxic Cr(III) species is one advantage of these integral sensor systems. Impedance only devices are not sufficient for the fast detection of toxic chromium species as rapid cellular changes occur only in the respiration system and the cell physiology.

A concept for a new generation of an integrated multi‐functional biosensor/actuator system is developed, which is based on biomolecular logic principles. Such a system is expected to be able to detect multiple biochemical input signals simultaneously and in real‐time and convert them into electrical output signals with logical operations such as OR, AND, etc. The system can be designed as a closed‐loop drug release device triggered by an enzyme logic gate, while the release of the drug induced by the actuator at the required dosage and timing will be controlled by an additional drug sensor. Thus, the system could help to make an accurate and specific diagnosis. The presented concept is exemplarily demonstrated by using an enzyme logic gate based on a glucose/glucose oxidase system, a temperature‐responsive hydrogel mimicking the actuator function and an insulin (drug) sensor. In this work, the results of functional testing of individual amperometric glucose and insulin sensors as well as an impedimetric sensor for the detection of the hydrogel swelling/shrinking are presented.