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Fiber bundled probe for highly sensitive sensor for zinc ion detection using a concave mirror

Fiber bundled probe for highly sensitive sensor for zinc ion detection using a concave mirror

Zinc ion is one of important trace elements that people need to stay healthy. Of the trace elements, this element is second only to iron in its concentration in the body. Zn2+ ions play a significant role in fundamental physiological processes as cofactors for metalloproteins and are regulators of enzyme function in neural signal transmission, and gene expression. It is needed for the body’s defensive (immune) system to properly work. It plays a role in cell division, cell growth, wound healing, and the breakdown of carbohydrates. Zinc is also needed for the senses of smell and taste. During pregnancy, infancy, and childhood the body needs zinc to grow and develop properly. Zinc also enhances the action of insulin.

The information on the zinc ion concentration near the injured brain region is helpful for medical intervention by neuro- surgeons. To collect such information, the zinc ion sensor has to be inside the brain. It can work together with intracranial pressure detection or stand-alone. In any case, the zinc ion sensor has to satisfy at least three criteria: i. The size has to be small for the minimally invasive surgical application. ii. The sensor has to be biocompatible and release no chemicals to the brain. iii. The sensor must respond to the zinc ion in the physiological or pathophysiological range of concentration and remains working for at least a few hours. Previously, zinc ions sensor has been reported using surface plasma resonance of surface-modified gold, which requires a complex spectroscopy system. A high sensitivity zinc ions detection was also reported based on electrical approach with extended gate-AlGaN/GaN. However, the electrical approach has a complicated fabrication process. The combination of the optical fiber and the hydrogel doped with probe molecules appears to be a promising approach for such three criteria. One of the suitable probe molecules is meso 2,6-Dichlorophenyltripyrrinone (TPN-Cl2).

The general challenge for the probes in the hydrogel is the resolution and reproducibility with a low concentration of zinc ions. So far there are few reports on the solid-state zinc ion sensor working at the physiological or pathophysiological conditions. The basal zinc ion concentration in the brain extracellular compartment is extremely low (~10-8 M), but it can increase dramatically under pathological conditions. On stimulation by lipopolysaccharides (LPS) or oxygen/glucose deprivation (OGD), mimicking brain inflammation or ischemia, the concentration can reach the level of 10 -6 M. For medical applications, the working concentration of the sensor must therefore cover the range of 10-6 to 10-5 M. In previous reports for the hydrogel-based zinc ion sensor one-time detection is observed for 10- 7 and 10-6 M, but quantitatively such detection is not reproducible for several independent periods of the same concentrate Reliable detection was obtained only for concentration over 10-5 M. Such poor detection and resolution limit are related to the photo-degradation of the probe molecules under the intense excitation light. The sensitivity needs to be improved for at least one order of magnitude in the intracranial detections for brain insult patients.

During recent years, fiber optic sensors have gained escalating interest in the field of biosensor applications due to their potential to monitor analytes in real-time and in situ with minimum interruption to sample with higher sensitivity and desire selectivity. For instance, fiber optic sensors based on surface plasmonic resonance (SPR) and micro-bottle resonator have been proposed and demonstrated for formaldehyde liquid sensing. In this paper, we demonstrate a bundled optical fiber-based sensor for measuring zinc ion concentration in liquid. The proposed sensor is based on optical non-destructive and on-site measurement, which is obtained using a very simple detection scheme based on collecting the intensity of laser light reflected from the concave mirror, which is placed inside the liquid sample. Compared to the free space reflective configuration, the use of bundled fiber increases the sensitivity of the sensor due to the better control of transmitting and receiving light. 630 nm laser is used in this work due to the stability and low cost of the laser.

We have successfully demonstrated a new fiber-optic sensor using a bundled fiber probe to measure the concentration of zinc ion solution based on measuring the reflection of laser light from a concave mirror. We observed that the intensity of reflected light increases proportionally to the increase of zinc concentration. The sensor is very stable and responds linearly to the concentration of zinc ion liquid, which was placed on the concave mirror surface. Our sensing technique is simple and low cost. It could be applied to detect zinc ion concentration in a non-invasive and non-destructive way. The resolution, linearity and sensitivity of the sensor are obtained at 0.046%, 92 % and 4.56 mV/%, respectively. The proposed sensor has potential to be further developed towards a portable device, which is suitable for applications in many fields.

Author: Prof. Dr. Moh. Yasin, M.Si.

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