Determination of Manganese in Human Whole Blood with Indium Tin Oxide Electrochemical Point-of-Care Sensor

Abstract

Manganese (Mn) is one of the most common elements in the earth’s crust, and its compounds are widely used in consumer products including gasoline, cell phone batteries, fertilizer, pigments, fireworks, etc. Although a trace metal in the human body and under tight homeostatic control, elevated levels of Mn due to occupational or environmental exposure can cause serious adverse health effects, such as manganism (a Parkinsonian-like syndrome), liver cirrhosis, and Behcet disease. The normal range of Mn in blood is 4.7-18.3 µg/L (5-20 ppb), and values greater than 36 µg/L correlate with disease. The current method for analyzing Mn in blood is via inductively coupled plasma mass spectrometry (ICP-MS) in centralized labs, which requires a venipuncture sample, can take 3-6 months for analyses, and can be costly. Electrochemical analysis can offer an attractive alternative, leading to developing a point-of-care system for Mn monitoring in blood, with the advantages of being inexpensive and easily miniaturized. Herein, we report the first Indium Tin Oxide (ITO) electrochemical microsensor capable of accurately measuring Mn levels in human blood with cathodic stripping voltammetry (CSV). As shown in Fig. 1 (a), the ITO microsensor was developed by integrating a screen-printed carbon auxiliary electrode and an Ag/AgCl reference electrode on an ITO coated glass slide (1 cm × 4 cm). A circular ITO film with 3 mm diameter was exposed and used as the working electrode. To start electrochemical analysis, blood sample was first microwave digested with nitric acid and hydrogen peroxide to release Mn2+ bound to the proteins. The acid-digested blood sample was then titrated with sodium hydroxide to pH 5. Square wave cathodic stripping voltammetry (SWCSV) with optimized parameters was carried out to construct the calibration curve in the digested & pH adjusted blood sample with the Mn concentration ranging from 1.2 ppb to 25 ppb (confirmed by ICP-MS). The voltammogram in Fig. 1(b) shows an increasing trend of peak height and area with Mn concentration; and the calibration curve in Fig. 1(c) shows a sensitivity of 38.6 nC/ppb and linearity of 0.997 from the sensor’s measurements. We used the same stripping parameters, while applying a 3-point standard addition method, to determine the Mn concentration in two blood samples. The concentrations of 10.5 ppb and 26.3 ppb from our measurements lead to an average accuracy of 90% in comparison to the ICP-MS results of 8.9 ppb and 27.6 ppb. Four 150 µl sample droplets with a total volume of 600 µl were used in the measurement. Considering the 10X dilution from the blood digestion and pH adjustment process, only 60 µl of whole blood is needed to determine the Mn level with our approach. The favorable results and the small blood volume required in this work suggests the feasibility of point-of-care monitoring Mn in blood and is expected to be very useful for monitoring Mn exposure in vulnerable populations like children. Figure 1