Adsorption of some biologically significant compounds, such as a pirimicarb (PIR) pesticide, avidine (AV) protein, and 7-N nucleotide, at a chemically modified Au electrode of a quartz crystal piezoelectric transducer (QCPT) was investigated by using piezoelectric microgravimetry with an electrochemical quartz crystal microbalance (EQCM). This adsorption was utilized for determination of the compounds under the batch or flow injection analysis (FIA) conditions.
PIR was determined by using QCPT modified with a molecularly imprinted polymer (MIP) thin film. The MIP film was prepared by a UV light polymerization of a mixture of suitable functional and cross-linking monomers, polymerization initiator, and PIR template. Next, the template was washed out leaving molecular cavities in the MIP film of the size and shape of the template molecule. Then, PIR was determined (limit of detection 0.22 mM and linear concentration range 0.36 to 1.5 mM) by measuring the frequency change, Δf. FIA peaks rather than steps of Δf vs. time were formed indicating lack of chemical interactions between PIR and MIP.
AV was determined by using QCPT coated with a polypyrrole-biotin film prepared by electropolymerization of pyrrole under cyclic voltammetry conditions (CV) in the presence of biotin. The growth of the biotin-modified poly(pyrrole) film was monitored by simultaneous measurement of the CV and piezoelectric microgravimetry curves with the use of an electrochemical quartz crystal microbalance (EQCM) under quiescent solution conditions. Steps rather than peaks were present in the FIA curve due to consecutive injections of the AV samples. These steps were smaller the larger was the injection number indicating that AV interacted chemically with the biotin sites of the poly(pyrrole) film and that these sites were gradually saturated.
A 7-N nucleotide was determined by virtue of hybridization with a single strand of complementary deoxyribonucleic acid (ssDNA). For that purpose QCPT was modified with self-assembled monolayers (SAMs) of ssDNA. Two modification procedures were adopted, i.e., (i) a direct procedure using ω-thiolated DNAss to form SAMs and (ii) an indirect procedure using ω-thiolated mercaptoundodecanoic acid (MUA) esterified with AV for SAMs; then, ssDNA modified with biotin was attached to the immobilized MUA by the AV-biotin non-covalent bonds. The FIA steps and peaks were obtained for ssDNA immobilized according to the (i) and (ii) procedure, respectively, after injecting samples of 7-N nucleotide.