|Ph.D Student||Imad Zakharia|
|Subject||Development of a Submicron Optochemical Probe for Near|
Field Scanning Microscopy
|Department||Department of Biotechnology and Food Engineering||Supervisor||Dr. Shimoni Eyal|
|Full Thesis text|
The need for detecting the local chemical environment in confined spaces has led to major advances in the miniaturization of optochemical sensors. Miniaturized fiber-optic sensors reported in literature are fabricated by tapering the end of an optical fiber till submicrometer scales, creating a small aperture at the fiber apex by metal-coating the sides of the tapered region, and then coating the sharp apex with a porous matrix doped with a fluorescent indicator. However, no scanning work has been reported for such probe configuration at solid surfaces, due to various drawbacks of the probe fabrication methods. Therefore, a need for a specially designed scanning system applying miniaturized optochemical probes in which the immobilized sensing matrix is protected against contact forces with the surface was identified.
This research is aimed at developing a submicron scanning optochemical probe which can image two-dimensionally the local pH at solid surfaces. Here, we introduce an advanced approach in the miniaturization of optochemical sensors based on tailoring the distal end of near-field scanning probe microscopy (NSOM) probes into a submicron protective confined space into which the fluorescent sensing matrix is immobilized. Focused ion beam (FIB) milling was applied to create a cavity of submicrometer diameter and nanometric depth at the apex of the NSOM probe. This cavity was filled with xerogel containing a pH-sensitive fluorescent dye, thus turning the NSOM probe into an analytically active probe.
To verify the probe ability to scan the local pH, a model surface based on a microelectrochemical cell was developed for creating pH micrometer-scale micro-gradient. The microcell consisted of a glass cover slide onto which a nanometer thick conductive coating was sputtered into two areas separated by an uncoated microchannel gap of ~50 µm. When the conductive coatings are connected to a DC power supply unit, they behave as electrochemical electrodes and form a pH gradient across the microchannel under an aqueous solution. This pH micro-gradient was scanned two-dimensionally by the developed active NSOM probe showing significantly different fluorescent signals between the low- and high pH microdomains. In addition, the probe was successfully used to scan two-dimensionally a hydroxide ion releasing microdomain and it exhibited high sensitivity to the rate of hydroxide ion release. In these optochemical scans, the developed probe showed an order of magnitude difference in its fluorescent signal between the low- and high-pH microdomains.