Stochastic dot microsensors based on the complexes of Mn(III) with porphyrins and modified diamond or carbon pastes were employed for the assay of dopamine in pharmaceutical and biological samples. The advantage of using such sensors for the assay of dopamine is the possibility of qualitative and quantitative assay of dopamine in one run of the experiment. The covered linear concentration range for these sensors was between 10 −10 and 10 −3 mol/L with high sensitivities. The proposed dot sensors were used reliably (RSD < 1%) for the assay of dopamine from different samples for more than 8 months, with a recovery higher than 93.00%.The first stochastic sensors were designed by Hladky and Haydon 1 in 1970 and were based on granomycin. The advantages of stochastic sensors are that they can monitor individual binding of the molecules in the nanochannels characterized through t on as well as unbinding events characterized through t off ( Figure 1). t on is a measure of the quantity of the analyte in the sample (1/t on = f(concentration)), and t off is a signature of the analyte to be determined. 2 The mechanism of the response of such sensors is very complex; there are binding and unbinding events, redox processes, traveling through the nanochannels and while the nanochannel is blocked by the analyte it is losing its conductivity -this being illustrated by the value of the intensity of the current which is getting to zero (Figure 1). Dopamine (DA) is a very important substance for the body. The ability to determine dopamine concentration in vivo would allow mapping the dopamine concentration in the brain. Low levels of DA indicate the presence of diseases such as Parkinson's, schizophrenia and Huntington's. 3,4 Medication based on DA is supplied with precaution as it can cause the rising of blood pressure. Reliable methods of analysis are a need for the assay of dopamine in biological fluids and its pharmaceutical formulations. Therefore, different methods of analysis were proposed for the assay of dopamine: electrochemical methods based on GC electrodes, 5,6 gold-palladium binary system based electrodes, 7 plastic membrane based electrodes, 8 screen-printed electrodes, 9 modified carbon 10,11 and diamond paste; 10 capillary zone electrophoresis, 12 and HPLC. 13 Dot microsensors represents a new concept in the technology of microsensors which combined with the stochastic concept 14 enhanced the capability of sensor for qualitative and quantitative analysis of single molecules of biomedical importance. In this paper we proposed 6 stochastic dot microsensors based on the complexes of Mn III with (I) 5,10,15,20-tetratolyl-21H,23H-porphyrin, (II) 5,10,15,20-tetraphenyl-21H,23H-porphyrin and (III) 5,10,15,20-tetrakis(3,4-dimethoxiphenyl)-21H,23H-porphyrin and carbon and/or diamond paste for the assay of dopamine in pharmaceutical products and urine. The selection of Mn III complexes with porphyrins as electroactive compounds for the design of the microsensors was done based on the nanostructured channels formed by their mo...

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Different porphyrins were used for the design of seven carbon paste and seven diamond paste based microelectrodes, which were employed for the determination of ascorbic acid in pharmaceutical and beverages samples using differential pulse voltammetry (DPV). The limits of detection lie between 1.1 × 10-14 and 5.1 × 10-7 M while the sensitivities were between 3.07 pA/M and 1285.18 A/M. Ascorbic acid was recovered reliable from pharmaceutical and beverages samples in percentages higher than 92.00% and 91.50%, respectively. The surface of the microelectrodes can easily be renewed by simple polishing, obtaining a fresh surface ready for use in a new assay.

Eight disposable stochastic dot sensors based on porphyrins and modified diamond or carbon pastes were employed for the assay of ascorbic acid in pharmaceutical, beverages, and biological samples. The advantage of using such sensors for the assay of ascorbic acid is the possibility of qualitative and quantitative assay of ascorbic acid in one run of the experiment. The covered linear concentration range for these sensors was between 10 À14 and 10 À5 mol/L with high sensitivities. The proposed dot sensors were used reliably (RSD < 1%) for the assay of ascorbic acid from different samples for more than 6 months, with a recovery higher than 92.00%.