ppb-level heavy metal ion detection by electrochemistry-assisted nanoPorous silicon (ECA-NPS) photonic sensors

In this paper, we present an ultra-sensitive photonic refractive index (RI) sensor based on nanoporous silicon (NPS) one-dimensional (1D) microcavity structures for detecting heavy metal ions in water. Consisting of two distributed Bragg reflectors (DBR) symmetrically placed about a defect layer fabricated by successive electrochemical etchings (ECE), the 1D microcavity is employed to enhance the sensitivity of the sensors. Although these RI sensors possess the property of high sensitivity to RI change and high resolution due to high porosity and narrow resonance peak, respectively, however their selectivities to analytes are poor, and limit of detections (LOD) are also restricted owning to passive interaction between analytes and sensors. To solve these problems, we propose a new procedure for heavy metal ion detection that offers good selectivity and ultra-low LOD while keeping the advantages in sensitivity and resolution of the NPS sensor. In the procedure, an electrochemical reduction process is applied to actively/selectively pre-concentrate and reduce metal ions on the pore surfaces of the NPS that results in the improvement in selectivity and LOD. This ECA-NPS based photonic sensor was applied to detect cadmium ions in deionized water revealing a sensitivity of 342 nm/RIU and 0.152 ppb of LOD in DI water. LOD as low as 1.16 ppb with a selectivity over sodium, potassium, magnesium, calcium and nickel etc, has also been demonstrated with a longer reduction time of 20 min for 10 ppb cadmium ions spiked in lake water.