Crown ether decorated silicon photonics for safeguarding against lead poisoning

Abstract

Lead (Pb²⁺) toxification is a concerning, unaddressed global public health crisis that leads to 1 million deaths annually. Yet, public policies to address this issue have fallen short. This work harnesses the unique abilities of crown ethers, which selectively bind to specific ions. This study demonstrates the synergistic integration of highly-scalable silicon photonics, with crown ether amine conjugation via Fischer esterification in an environmentally-friendly fashion. This realizes an integrated photonic platform that enables the in-operando, highly-selective and quantitative detection of various ions. The development dispels the existing notion that Fischer esterification is restricted to organic compounds, facilitating the subsequent amine conjugation for various crown ethers. The presented platform is specifically engineered for selective Pb²⁺ detection, demonstrating a large dynamic detection range, and applicability to field samples. The compatibility of this platform with cost-effective manufacturing indicates the potential for pervasive implementation of the integrated photonic sensor technology to safeguard against societal Pb²⁺ poisoning.

Fig. 1. Concept of the Crown Ether/SiP platform for Pb²⁺ ion detection.

a 3-D illustration of the photonic Pb²⁺ ion sensor based on the crown ether-decorated SiP platform. The functionalization performed in the sensing region is indicated. For the sake of clarity, the 20 nm SiO₂ deposited on top of the waveguides in the sensing region is not indicated. Information is provided in Fig. 3a. b The zoomed-in illustration of the sensing region, where the binding between the Pb²⁺ ions and crown ether functional layer is depicted. The technology sub-layers in the sensing region are labeled: SiP, Fischer esterification, and the crown ether functional layer. c Micrograph image of the Pb²⁺ ion sensor, where the sensing arm and scale bar (500 μm) are indicated d The Pb²⁺ photonic sensor assembly, consisting of the photonic chip and a microfluidic chamber. e Elucidated operating principle of the photonic Pb²⁺ ion sensor. The inset shows the exemplary applications that the ion detection platform can be extended to.

Fig. 2. Photonic design of the Pb²⁺ ion sensor.

a Simulation of the number of supported TE optical modes in the slot waveguides as a function of strip and slot width. b Sensor surface sensing FoM as a function of strip and slot width. c The comparison of two proposed splitting Mach-Zehnder architectures (see Supplementary Note 5) in terms of the power asymmetry required of the splitter; condition 1 (${\mathrm{S}}_1={\mathrm{S}}_2,{\mathrm{S}}_1^{\prime }={\mathrm{S}}_2^{\prime },{\mathrm{S}}_{1/2}^{\prime }\ne 0.5$), condition 2 (${\mathrm{S}}_1^{\prime }\ne 0.5,{\mathrm{S}}_2^{\prime }=0.5$). Top-down electric field distribution of the d, asymmetrical adiabatic tapered splitter, and e, adiabatic strip-slot converter, where the structures of the components are outlined.

Fig. 3. The development of the Crown Ether/SiP functionalization process.

a The developed crown ether/SiP functionalization process, described in four steps; the 2D chemical structure of the DBTDA crown ether is illustrated. XPS narrow spectra analysis of the b, N 1 S, c, C 1 S, d, O 1 S regions of the photonic chips, before and after functionalization.

Fig. 4. Analysis of the crown ether functional layer selectivity via XPS.

Normalized narrow scan XPS spectra at the photonic chip surface by subtracting the spectra prior to ion interaction from that of after ion interaction (normalization). The ions tested are a, Na⁺, b, K⁺, c, Mg²⁺, d, Li⁺, e, Zn²⁺, f, Ca²⁺, g, Fe²⁺, h, Cu²⁺, i, Al³⁺, j, Sn²⁺, k, Cd²⁺, and l, Pb²⁺ at 100 ppb, in DI water. The pH of all the analyte is maintained at 6.8 (see Methods).

Fig. 5. Experimental characterization of Pb²⁺ sensor performance.

a Wavelength spectrum of the sensor, when DI water is applied into the sensor assembly (as shown in Fig. 1d). b Calibration curve of the sensor when exposed to reference Pb²⁺ concentration of 0, 5, 25, 125, 625, 2625, 12625, 62625, 262625 ppb via a cumulative testing approach (see Methods). Error bars pertaining to six independent measurements (n = 6) are indicated, where the bottom and top edges of the boxplot correspond to the 25^th and 75^th percentile of the data. The minimum and maximum points of the data are indicated by the extension to which the whiskers of the boxplot are extended to. The mean of the data is indicated by the red point. c A set of optical fringe minima corresponding to the tested concentrations in the calibration curve (Fig. 5b). d Fig. 5c, zoomed-in at concentrations of 0, 5, 25 ppb. Validation of the calibration curve (Fig. 5b) at reference concentrations of e, 80 ppb, f. 1 ppb and g, 10 ppb, h, 62 ppm. For the detection of Pb²⁺ concentration via the sensor in Fig. b–h, the pH of the analyte is maintained at 6.8 (see Methods). DI water is used.

Fig. 6. Selectivity performance of the Pb²⁺ ion photonic sensor against.

a Cd²⁺, and b, K⁺ at reference concentrations of 15 ppb in DI water where no shifts in the interferometric spectra indicative of ion binding is observed. c The detection performance of the Pb²⁺ photonic ion sensor is evaluated at reference Pb²⁺ concentrations of 15 ppb in DI water. The pH of the analyte is maintained at 6.8 (see Methods).

Fig. 7. Sensor pH analysis and deployment in field samples.

a Pb²⁺ XPS analysis of the crown ether functional layer across a pH range of 2−8. b Optical fringe minima corresponding to 15 ppb of Pb²⁺ reference concentrations in DI water, with pH of 5, 6, 6.8 and 8. Optical fringe minima corresponding to the detection of Pb²⁺ via the photonic sensor in c, tap, and d, lake, and e, sea water. f XPS analysis of the crown ether functional layer when the sensor is subjected to DI, tap, lake and seawater.