Aqueous mercuric ion detection with microsphere optical resonator sensors

We have developed a novel optical sensor based on fused silica microsphere resonators for mercuric ion Hg(II) detection in an aqueous environment. The whispering gallery mode (WGM) of the sphere is the surface mode. Therefore, its spectral position shifts in response to the binding of Hg(II) to the sphere surface. In addition, the WGM features very high Q-factor (>10). This light circulation effect significantly increases the light interaction with the analytes on the sphere surface, resulting in an enhanced sensor sensitivity. In our experiment, we add Hg(II) to water in which the sensor is immersed. The sensorgram is obtained by monitoring the WGM position in real time. Our experimental results show that a detection limit of 50 ppb is achievable when Hg(II) ions bind to the thiol group on the sphere surface with nearly 1:3 stoichiometry. Controlled experiments with Zn(II) are also performed, and virtually no spectral shift in the WGM is observed. INTRODUCTION Optical microsphere resonators [1-14] have been demonstrated as an excellent technical solution for bio/chemical sensors. Several label-free applications have been demonstrated, including the detection of protein and DNA molecules [9,12-13], as well as refractometric measurements of solutions [10-11]. Microspheres are attractive as sensors because of their small size, low optical power requirement, and high light-matter interaction. Light of resonant wavelengths propagates around the surface of the microsphere cavity via total internal reflection in the form of whispering gallery modes (WGM). Microspheres with a Q-factor of greater than 10 have been demonstrated [14]. This high degree of optical confinement results in excellent light matter interaction, as photons may circulate the sphere hundreds to thousands of times on average. Thus, a microsphere with sub-millimeter dimensions can have an interaction length on the order of tens to hundreds of centimeters, while only consuming a sample volume on the order of microliters. The light-matter interaction at the surface of the sphere is a result of the evanescent field of the WGM. As the light propagates around the surface of the sphere, an evanescent field protrudes into the medium beyond the sphere surface, as illustrated in Figure 1. As a result, the spectral position of the WGM is sensitive to the refractive index beyond the surface of the sphere. A change in refractive index of the surrounding solution will cause a shift in the spectral position of the WGM, as will the binding or removal of molecules at the sphere surface. Therefore, quantitative bio/chemical sensing can be performed by measuring the spectral shift of a WGM. We utilize these capabilities of optical microsphere resonators to demonstrate the detection of the presence of mercuric ions (Hg(II)) in water. This design provides the advantages of simplicity, lower cost, faster measurement time, and compactness, as compared to the standard method of cold vapor atomic fluorescence detection [15]. Other simple optical techniques, including surface plasmon resonance (SPR) [16-19], have already been demonstrated for Hg(II) detection. Detection limits of 0.1 ppm and nanomolar have been achieved respectively by a commercialized SPReeta system and by a lab SPR system with the assistance of split-field photodiode detection technology [18-19]. However, due to its small size, low sample consumption, and high light-matter interaction, the microsphere resonator may ultimately prove to be a better concept for heavy metal detection. 0 r0 Captured molecule