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Disposable sensors based on the degradation of thin films as a result of an enzymatic reaction have been developed into efficient enzyme detectors. Film degradation has traditionally been monitored using surface plasmon resonance (SPR), quartz crystal microbalance (QCM), or classical ac impedance measurements. The enzyme detection principle has now been integrated with an array technology derived from a recently developed impedance imaging technique, scanning photoinduced impedance microscopy (SPIM). SPIM is based on photocurrent measurements at field-effect structures. The material under investigation is commonly deposited onto a semiconductor--insulator substrate. In this work, field-effect capacitors were replaced by hydrogenated amorphous silicon (a-Si:H) n-i-p photodiode structures, which have recently been shown to be suitable for SPIM measurements with good lateral resolution. To demonstrate the feasibility of SPIM for the characterization of biosensor arrays, polymer dots of the inert polymer cellulose acetate and an [alpha]-chymotrypsin-sensitive poly(ester amide) were deposited onto a-Si:H n-i-p/Si[O.sub.2] structures and their enzymatic degradation was monitored using a laser scanning setup.

Scanning photoinduced impedance microscopy (SPIM) is an impedance imaging technique, which is based on photocurrent measurements at electrolyte--insulator-semiconductor (EIS) and metal--insulator--semiconductor (MIS) field-effect structures. The material to be investigated has to be deposited on top of the insulator (E/I or M/I interface). The lateral resolution of SPIM is limited by the lateral diffusion of minority charge carriers. Therefore, it would be advantageous if semiconductors with a short diffusion length of charge carriers such as amorphous silicon could be employed. However, field-effect capacitors fabricated using amorphous silicon suffered from a large number of interface states, high leakage currents through the insulator, and small photocurrents. In this work, field-effect capacitors were replaced by amorphous hydrogenated silicon photodiode structures (a-Si:H p-i-n/Si[O.sub.2] or n-i-p/Si[O.sub.2]) as this was expected to result in higher photocurrents and eliminate the necessity of a high-quality insulator. The photodiode structures were shown to be suitable for SPIM measurements. The resolution of photocurrent measurements was found to depend strongly on the frequency of the modulated light and the doping concentration of the amorphous silicon layer closest to the insulator. An equivalent circuit model was developed to simulate this behavior.

Microelectrode arrays having eight 10-[micro]m-diameter gold microelectrodes arranged on a gold-covered Si chip were designed and characterized. The chips prove useful for the detection of single-nucleotide mismatches in unlabeled and prehybridized DNA by electrochemical impedance spectroscopy.

Thin films of alternating DNA and rare earth ion Eu3+ layers from dilute aqueous solutions were fabricated onto quartz substrates and silicon wafers through the layer-by-layer (LbL) self-assembly technique. UV-visible spectroscopy shows that a uniform layer of DNA can be fully adsorbed onto each alternate Eu3+ layer. Microscopic FTIR spectra show Eu3+ interacts with both the phosphate groups and nitrogenous bases of DNA, and the formation of [DNA/Eu]n films induces a change of the conformation of the DNA secondary structure to a certain extent. Various parameters affecting the DNA or Eu3+ loading into the composite film were investigated with emphasis on the effect of the pH and ionic strength of the DNA solution used for the film preparation. Atomic force microscopy was utilized to observe the morphologies of the DNA in the films obtained at two different pH values. Small molecules, such as alpha-tenoyltrifluoroacetone (TTA), Hoechst 33258 (Hoe), and ethidium bromide (EB), are all observed to interact with Eu3+ or DNA in the [DNA/Eu]n films. The [DNA/Eu]n films incorporated with these molecules show different fluorescent characteristics, and the fluorescence intensity of the films versus the bilayer number has a good linear relationship, confirming the potential for creating a different luminescence ability of the multilayer by controlling the number of DNA/Eu bilayers.