Using Reactands in CMOS-based Calorimetric Sensors: New Functional Materials for Electronic Noses

Abstract

One of the major targets for public health is the safety and quality of products addressed to human consumption which may be characterized by their taste and smell. Electronic noses have been developed in order to yield information on these basic parameters by using solid state gas sensors in combination with conventional polymers for the detection of various analytes. [1][2][3][4][5][6][7][8] However, due to their still insufficient selectivity, current gas sensors do not allow one to detect all single compounds or a combination of analyte molecules on a complex background. This is due to the fact that, in most cases, the recognition of the analytes is unspecific and relies on a simple sorption/desorption process of the analyte in the metal oxide semiconductor material or in a polymer coating. [1][2][3][4][5][6][7][8] To overcome this limitation, new functional materials have to be developed in order to design sensors with improved selectivity and sensitivity and to give a fast analysis. In this article, we describe the first use of reversible chemical reactions for the development of CMOSbased calorimetric microsensors with enhanced selectivity. The recognition is based on the reaction of the trifluoroacetyl group of a chemosensor molecule with the amino group of 1-butylamine to form a hemiaminal. Since the chemosensor combines both the properties of a chemical reagent with those of a selective ligand, it is termed "reactand". So far, we have already used optical sensors based on chromogenic and fluorogenic reactands to detect alcohols, 9 aldehydes 10 and amines. 11 However, optical sensors are more difficult to miniaturize than CMOS-based devices because they require light sources, waveguides and detectors. Furthermore, the use of CMOS-based microsensors provides response times in the range of seconds rather than minutes. Consequently, reactandbased functional materials are a step towards more selective microsensors intended to be used in electronic noses and tongues. ExperimentalIn order to determine the reaction enthalpy and entropy of the chromogenic reactand 4-N,N-dioctylamino-4′-trifluoroacetylazobenzene (ETH T 4001) in organic solvents, the absorbance spectra were recorded on a Specord S100 spectrometer (Zeiss, Germany) in a temperature range from 293 to 343 K at intervals of 10 K. Upon the addition of 1-butylamine to ETH T 4001 in toluene or acetonitrile, the trifluoroacetyl form of the dye was converted into the hemiaminal (Fig. 1) and a shift of the maxima from around 490 nm to around 430 nm occurred. Different amounts of 1-butylamine were added to evaluate the equilibrium constants for the reaction of ETH T 4001 with the amine at different temperatures. The spectroscopic changes of ETH T 4001 were then used to calculate the equilibrium constants of the chemical reactions at different temperatures, allowing the determination of the reaction entropy and enthalpy via the van't Hoff plot and the Gibbs-Helmholtz equation. 12 The dipole moments of the trifluoroacetyl and hemiaminal form of the reactand we...

Chromogenic and Fluorogenic Reactands: New Indicator Dyes for Monitoring Amines, Alcohols and Aldehydes

“…Reactands may be used in optical artificial noses [34] but the development of more selective calorimetric and capacitive microsensors for electronic noses is promising as well, because chemical reactions yield changes in both the reaction enthalpy and the dipole moment [ 15]. The covalent immobilisation of reactands on AFM or SNOM sensor surfaces may be beneficial for a selective recognition of functional groups with a high local resolution.…”

“…However, these values represent both the time for an exchange of sample solutions in the flow cell and the true response of the reactand in the layer. Gas-phase measurements with sensor layers using a CMOS-based calorimetric transduction of the chemical reaction have shown that the reaction of ETHT 4001 with dry !-butylamine in plasticised PVC can be as fast as 3 s for forward and 15 s for reverse response [15].…”

Chromo‐ and Fluororeactands: Indicators for Detection of Neutral Analytes by Using Reversible Covalent‐Bond Chemistry

“…Reactands may overcome this limitation and may be used for the development of more selective and sensitive calorimetric and capacitive microsensors, because chemical reactions can provide changes in reaction enthalpy as well as in dipole moment. [41] Chemically reactive functional groups may also be introduced into conjugated polymers. The resulting materials might be used for the investigation of intra and intermolecular energy transfer (due to a change in the electron acceptor strength of the functional groups via chemical reactions) as well as for the preparation of supramolecular conjugates with chemically complementary macromolecules.…”