Nano- and micro- electrodes: applications in the biosensing field

Abstract

Platinum and gold inlaid disk micro- and nano-sized electrodes were prepared using a laser puller. It is the very first time that a similar procedure is fully described for the preparation of gold microelectrodes. Scanning Electron Microscopy, Cyclic Voltammetry, High Speed Chronoamperometry and Scanning Electrochemical Microscopy were used to characterize the electrodes. Radius of platinum and gold tips ranged from 7 nm and 500 nm up to several micrometers, respectively. Platinum probes with radius < 200 nm were employed in high resolution SECM imaging of an array of nanocavities (600–700 nm wide and 400-500 nm deep) prepared by means of nanosphere lithography. The small size of the probe employed in the study allowed resolving the features of the sample, showing the significant capability to do electrochemistry with submicrometer spatial resolution. The small size of these electrodes allowed their application as immunosensors using the steady-state current of a redox probe as transducing principle. In fact, an object having larger size than the electrode blocks the diffusing of the redox probe towards the electrochemical active surface. Indeed, platinum UMEs were coated with Goat-Human Immunoglobulin (hIgG) while anti-hIgG labelled polystyrene microspheres were injected into the electrochemical cell and let settle. Stable step like decreases (~ 2- 5 %) in the UME current were interpreted as binding of a single bead on the electrode surface due to the immunological reaction between hIgG and anti-hIgG. When the microelectrodes are uncoated, no immunological reaction can occur and then no stable step-like decrease should be observed. However, it was noticed that the electrostatic attraction between the electrode and the microspheres could cause the stabilization of the beads on the electrode surface and produce similar stable step-like drops in the current. Finally, UMEs were employed in the preparation of glucose microsensors which were based on the glucose oxidase immobilization and in the anodic oxidation of the enzymatically produced hydrogen peroxide. Indeed, a comparative study of the results obtained with four enzyme immobilization procedures was carried out. The work highlighted that the high concentration of glucose oxidase necessary to have sensitive microsensors hampered the polymerization of pyrrole. On the other hand, the microsensors prepared by entrapment of the enzyme in a polyphenol matrix showed good sensitivity (~ 1 - 2 mAM-1cm-2) and very fast response time (< 4 seconds). Direct glucose oxidase deposition induced by applied potential did not improve the performances compared to the latter case. Finally, sensors prepared by ruthenium electrodeposition in presence of the enzyme showed encouraging results in terms of enhanced sensitivity (~ 10 - 20 mAM-1cm-2) However, a drift in the amperometric signal prevents its analytical use at the moment