Liquid Cladding Mediated Optical Fiber Sensors for Copper Ion Detection.

Vien Tran,Than Nguyen,Won Yoon,Nhu Tran,Heongkyu Ju

doi:10.3390/mi9090471

Abstract

We present a label-free optical fiber based sensor device to detect copper ions (Cu2+) in water. A multimode optical fiber, with its polymer cladding removed along a 1-cm length, is used for the optical sensor head, where the injected Cu2+ in the liquid phase acts as a liquid cladding for the optical mode. The various Cu2+ concentrations modulate the numerical aperture (NA) of the liquid cladding waveguide part. The degree of NA mismatch between the liquid cladding and solid cladding guided parts gives rise to an optical power transmittance change, forming the sensing principle. The presented liquid cladding fiber sensor exhibits a minimum resolvable refractive index of 2.48 × 10−6. For Cu2+ detection, we functionalize the sensor head surface (fiber core) using chitosan conjugated ethylenediaminetetraacetic acid (EDTA) which captures Cu2+ effectively due to the enhanced chelating effects. We obtain a limit of detection of Cu2+ of 1.62 nM (104 ppt), which is significantly lower than the tolerable level in drinking water (~30 µM), and achieve a dynamic range of 1 mM. The simple structure of the sensor head and the sensing system ensures the potential capability of being miniaturized. This may allow for in-situ, highly-sensitive, heavy metal sensors in a compact format.

Highlights

Copper ion (Cu2+) is a transition metal that is critically hazardous for the fundamental physiological system of human beings
Numerous methods have been considered as tools for the sensitive detection and monitoring of Cu2+, in-vivo and in-vitro, such as inductively coupled plasma (ICP) [8,9], near-infrared up-conversion chemodosimeters, which directly detect Cu2+ in vivo [10], electrochemical [11], fluorescence [12,13], and colorimetric [14,15]
There is a variety of optical platforms for heavy metal detection, such as surface plasmon resonance (SPR) [17,18,19], optical interferometric sensors [20,21], and surface-enhanced Raman scattering (SERS)-based optical fiber [22]

Summary

Introduction

Copper ion (Cu2+) is a transition metal that is critically hazardous for the fundamental physiological system of human beings. Injection of different concentrations of liquid analyte, which have different refractive indexes, cause the waveguide numerical aperture (NA) to change across the interface between solid and liquid cladding waveguide parts This leads optical power transmittance to change at the fiber sensor output as a function of the injected analyte concentration, forming the principle of a highly sensitive refractometer. We demonstrate a liquid-cladding modulated fiber sensor of only 1 cm in length (much shorter than the 5-cm length in Reference [24]) that still shows a similar sensitivity in the refractive index resolution, and put this sensor platform to use for detecting heavy metal ions, such as Cu2+, in water by additional immobilization of ligand layers. This optical sensor device shows the potential capacity of in-situ sensitive detection of Cu2+ in water in a compact format

Materials and Reagents

Findings

Fiber Sensor Surface