Water Quality Monitoring: All-Solid-State 3D-Printed Sensors for Phosphate Detection in River Water
Authors: Erukainure Frank Efe , Ali Azahar 
Published: August 2024

INTRODUCTION

High phosphate ( PO4
3- or P) levels in surface water cause eutrophication, which negatively impacts aquatic life and causes an economic loss of ~$2.2 billion per year. Currently, no low-cost sensors exist for long-term continuous monitoring of phosphate in water. Existing meters (HANNA colorimeter, and HORIBA) can detect P-levels but can neither perform continuous monitoring nor detect low concentrations. A paper-based colorimeter sensor for phosphate detection was demonstrated [1], however, this sensor takes a longer time to test. A Fabry−Perot phosphate sensor was introduced [2], but it is not a field-deployable sensor. Although many sensors have been developed to detect P-level [3 - 5], these sensors cannot detect a low concentration of P-level. This is an important concern as river water has a P-level below 1 ppm. As such, the applicability of existing devices in practical scenarios is not suitable for reliable measurement.
Thus, we are motivated to develop lithography-free, low-cost, and solid-state 3D-printed sensors for the ultra-sensitive detection of phosphate in river water. Our 3D-printed sensors are innovative compared to traditional fabrication techniques in the literature. We have characterized, calibrated, validated, and analyzed river samples and the results are superior to existing state-of-the-art P-sensors. Our 3D-printed P sensors showed an analytical sensitivity of 0.001 ppm of PO4
3- in river water within a detection range of 0 – 10 ppm and a detection time not exceeding 30 seconds.

DEVICE FABRICATION

First, we made a 3D design (CAD model) of the sensor (Fig. 1a). The sensor has one working electrode (WE) and a reference electrode (RE) having diameters of 8 and 3 mm, respectively. The CAD model was then exported to the 3D printer software, and the software was programmed based on the desired material and parameters. Fig.1b illustrates the 3D printing setup. The material used for the printing was high-temperature resin (an epoxy polymer). The resulting sensor structure was removed from the printing build platform, washed in an isopropyl alcohol bath, and dried. The sensor was then cured by treating it with UV light at 80 °C. A 100 nm-thick layer of gold (Au) was then coated on the sensor with a Kapton shadow mask and using an electron-beam evaporator. Then the RE was coated with an Ag/AgCl ink and dried at 80 °C for 2 hours. Fig.1d depicts the fabricated sensor with its Au electrode upon which poly(3-octylthiophene) (POT) and phosphate ion-selective membrane (p-ISM) were prepared and drop-coated on the WE in layers. Fig.1c summarizes the detection principle for phosphate ion exchange at different electrode layers.

RESULTS AND DISCUSSION

Fig.1e shows the sensor characterization based on electrochemical impedance spectroscopy (EIS). The charge transfer resistance (RCT) of the EIS curve was evaluated using its semicircle diameter. The RCT observed for the Au, POT/Au, and ISM/POT/Au electrodes were 263.67, 652.64, and 893.13 Ω, respectively. The p-ISM coating suppressed the electrochemical current in the ISM/POT/Au electrode due to the negatively charged buffer solution containing ferro/ferricyanide. Further, the 3D-printed sensor was calibrated using known standard solutions and the potential between the WE and RE decreased while we increased the phosphate concentration (Fig.1f – g). The sensor showed a slope of 37.4 mV/dec and an analytical sensitivity of 0.001 ppm. We believe that the high sensitivity of our sensor is due to the large surface area of our 3D-printed sensor structure which has wrinkled polymer surfaces that enhance more PO4
3- ion exchange. Our sensor also showed an excellent selectivity (Fig.1h) with various interfering ions.
Real water samples were collected from five locations at the Rappahannock River in eastern Virginia, USA. Fig.1i shows the open circuit potentiometry (OCP) for all five river water samples. Fig.1j shows the comparison between our sensor readings and those of a commercial phosphate meter (purchased from Hanna Instruments Inc.). Here, we observed a close relation between the two readings while our sensor showed a higher sensitivity, suggesting the usefulness of our sensor in practical scenarios. Our sensor achieved excellent detection of low concentrations of P-levels in river water. In the future, our 3D-printed sensors will be deployed into river water for continuous monitoring of the P-levels in the Rappahannock River.

REFERENCES

[1] V. Choudhary and L. Philip, Microchemical Journal, 2021,171, p. 106809.
[2] J. Zhu et al., ACS Sens, 2020, 5:5, pp. 1381–1388.
[3] R. V Manurung, et al., IOP Conf Ser Mater Sci Eng, 2019, 620:1, p. 012093.
[4] M. Sarwar, J. Leichner, G. M. Naja, and C.-Z. Li, Microsyst Nanoeng, 2019, 5:1, p. 56.
[5] M. M. Grand et al., Front. Mar. Sci., 2017, 4:255, doi: 10.3389/fmars.2017.00255.

Figure 1