Photonic Crystal Optrode Sensor for Detection of Pb^2+ in High Ionic Strength Environments

We developed an optrode sensing device that utilizes a polymerized colloidal array (PCCA) photonic crystal material. This array diffracts light in the visible spectral region due to the periodic spacing of colloidal particles. The PCCA changes diffraction wavelength due to binding of Pb^2+ to an 18-crown-6 ether molecular recognition agent. This optrode consists of a probe assembly that contains the PCCA Pb^2+ sensing film. An inexpensive, commercial diode array spectrometer and a fiber-optic reflectance probe monitors the wavelength of light back diffracted by the PCCA. Liquid inlet and outlet connections are provided to introduce the sample solution and to exchange out nonbinding ions. In low ionic strength solutions, diffraction wavelength shifts are actuated by the binding of the Pb^2+ to the crown ether to immobilize the Pb^2+ counterions. In these low ionic strength solutions, a Donnan potential forms to cause an osmotic pressure, which swells the PCCA in proportion to the number density of bound Pb^2+. This Donnan potential disappears at high ionic strengths. Thus, no response of the PCCA occurs. Our optrode design allows for the fast removal of nonbound ions from the PCCA by washing with pure water. Since the bound Pb2+ ions have a slow off rate from the crown ether, the bound Pb^2+ PCCA diffraction transiently red shifts during washing, directly in proportion to the sample Pb^2+ concentration. This transient diffraction red-shift can be used to quantitatively determine Pb^2+ concentrations in high ionic strength solutions such as bodily fluids.