Platinum Microscale Sensor for Electrochemical Determination of Manganese
Authors: Kang Wenjing , Rusinek Cory , Bange Adam , Haynes Erin , Heineman William R. , Papautsky Ian 
Published: February 2020

In this work, we report on the determination of Mn2+ using a microfabciated platinum (Pt) microscale sensor and the electrochemical method of cathodic stripping voltammetry (CSV). Compared with the well-established spectroscopic methods (ICP-MS or AAS) which are sensitive yet inconvenient for point-of-care (POC) applications, this disposable sensor utilizes a portable system and it offers accurate and precise measurements of Mn in environmental and biological samples.
Similar to our previous designs, this sensor is fabricated from a 150 nm thick Pt thin film (with 50 nm Ti seed layer) by evaporation on a Pyrex substrate. The sensor contains three electrodes: a Pt working electrode (WE), a Pt auxiliary electrode (AE), and an electroplated Ag/AgCl reference electrode (RE), as shown in Fig.1a. The electrodes are confined by an SU-8 resin protective layer with a 2-mm circular opening to ensure the same electrode surface area. With the dimension of 10 mm × 6 mm × 0.75 mm, the sensor holds a sample droplet of up to 15 µL. An interface (not shown) provides a simple connection to potentiostat.
First, to generally characterize this sensor, we used cyclic voltammetry (CV) in pH 5.5, 0.2 M acetate buffer to determine the potential windows. We also performed CV of 20 ppm Mn and CSV of 100 ppb Mn in the buffer to locate the reduction peaks of Mn. Then through a series of optimizations, we selected 0.7 V and 900 s as preconcentration potential and time, and 10 µL as sample volume. The waveform parameters were based on the Osteryoung settings: 25 mV amplitude, 70 ms period, and 4 mV increment. With these optimized parameters, we calibrated the Pt sensor in 0.22 M, pH 5.5 acetate buffer in the range of Mn from 5 ppb to 50 ppb to bracket the physiologically relevant range in our applications, which is 4−15 ppb. The voltammograms and calibration plot are shown in Fig.1b-c. By using the baseline subtraction approach, we achieved a calculated detection limit as 0.89 ppb (16.27 nM) based on 3σ/slope (n = 6) and a sensitivity as 1.39 nA/nM or 25.38 nA/ppb (177.49 nA/nM/cm2 or 3.23 µA/ppb/cm2 when normalized to WE area).
Based on calibration, we proceeded to demonstrate the determination of Mn in complex matrices. We first analyzed surface water due to its significance in environmental health. We conducted a pilot study to determine Mn in pond water from Burnet Woods, Cincinnati, OH (Oct 21st, 2015), and then performed measurements of well drinking water samples collected by Dr. Haynes’ group from different sites in Marietta, OH (which is the home of a ferromanganese processing plant). CSVs were performed by diluting samples 2× into pH 5 using a pH 5.5, 1 M acetate buffer, and adding three spikes for the standard addition method (10, 20, 30 ppb Mn). From Fig.1d-e, we calculated the Mn concentration in B.W.P.W (3) to be 101.72 ppb (96.4% precision), leading to an accuracy of 92% in comparison with the ICP-MS result of 94.6 ppb. In well water (n=7), the mean accuracy was 89.4% with mean precision of 97.2%, as compared with ICP-MS. A representative result for sample 655-EL is shown in Fig.1f-g, illustrating the determination of 22.43 ppb Mn, as compared with 23.56 ppb measured by ICP-MS. We are currently in the process of optimizing parameters for determination of Mn in blood. The preliminary results suggest that determination of 50-500 ppb of spiked Mn is possible, using hot-block digestion.
In conclusion, we demonstrated the determination of trace Mn in different matrices using an improved Pt electrochemical sensor. The performance of this simple, low-cost Pt sensor is comparable with the conventional ICP-MS method for determination of Mn in water. Ultimately, we believe this sensor system could act as a simple, fast and low-cost alternative for POC applications in local clinics or resource-limited settings.

Acknowledgements

This work was supported by the National Institutes of Health (NIH) awards R01ES022933 and R21ES024717.

Figure 1