Study on detection of contamination of pure water using silica microsphere

Abstract

Summary form only given. The adsorption of nanoparticles on the surface of a whispering-gallery mode (WGM) cavity causes the resonant wavelength to shift because it changes the effective refractive index of the cavity material. By taking advantage of this effect, WGM cavities can be used for such applications as the highly sensitive detection of nanoparticles and label-free sensing of bio-molecules [1]. The use of an ultrahigh-Q microcavity for single molecule detection has already been reported [2]. In this study, we demonstrate the detection of the contamination of pure water using a silica microsphere. Pure water contamination is usually detected by monitoring its conductance or measuring the residual silica concentration. However, here we conduct a preliminary study on the detection of contamination in water by monitoring the resonant wavelength shift of a WGM microcavity. We observed a large difference in the wavelength shift of the silica microsphere depending on whether we placed the cavity in city water or pure water.Figure 1(a) shows the transmittance spectra of a silica microsphere before and after the adsorption of city water. The spectrum measurement was carried out with a standard tapered fiber transmittance setup after the cavity had been dried in a clean environment. We observed an 89-pm shift in wavelength. The shift was significantly larger than the wavelength fluctuation caused by the thermo-optic effect, which indicates that small particles were adsorbed at the WGM cavity surface. On the other hand, when the cavity was placed in pure water the resonance wavelength did not move significantly as shown in Fig. 1(b). These results clearly indicate that wavelength shift measurement has the potential for use in monitoring the contamination of pure water. Figure 2 shows the resonant spectra we obtained after placing the cavity in pure water [Fig. 2(low)] followed by the adsorption of city water [Fig. 2(middle)]. Even though the cavity resonance shifts to a longer wavelength [Fig. 2(middle)], the resonant wavelength returned close to its original position after the cavity was exposed in ultra-pure water [Fig. 2(low)], which suggests that the particles on the WGM cavity surface were rinsed off. However, interestingly, the Q-factor (equivalent to the dip in the cavity resonance for our case) continued to decrease even after the cavity was placed in the pure water. This indicates both that the attached particles were rinsed away, and that the surface of the cavity itself was modified by the pure water. Since the wavelength is shortening, we believe that the silica is reacting with water, as SiO2+2H2O -> Si(OH)4. And since the speed of this reaction depends on the residual ion concentration in pure water, this effect is a good indicator for evaluating the purity of water. In summary, we demonstrated the wavelength shift caused by the adsorption of contamination at a cavity surface and by the reaction of silica in pure water. We believe these demonstrations constitute the first step towards the monitoring of pure water using WGM cavities.