Detection of Lactate via Amperometric Sensors Modified With Direct Electron Transfer Enzyme Containing PEDOT:PSS and Hydrogel Inks

In this letter, we present amperometric sensors based on direct electron transfer (DET) enzyme for the detection of lactate, which is an important medical parameter present in blood and interstitial dermal fluid (ISF). For measurement in blood, we present a planar screen-printed biosensor with carbon working electrodes, whereas for the intended measurement in ISF, we investigated platinum-metallized epoxy microneedle sensors. On both sensor types, a bioink was applied, consisting of a DET enzyme mixed with poly(3,4-ethylenedioxythiophene) polystyrene sulfonate. As a second layer, a hydrogel layer is deposited to hold the enzyme on site. Local modification of the platinum microneedle sensors was performed by non-contact spotting. The developed modification enables the detection of lactate at a potential of 0 V with response times of 500–700 s. For carbon sensors, a limit of detection of 0.12 mM lactate was determined, and two linear ranges of 0.3–5 and 10–50 mM were observed with sensitivities of 319 and 9.6 nA/(mm 2 ·mM), respectively. For locally modified platinum microneedle sensors, two linear ranges of 0.3–2.5 and 5–30.5 mM were observed with sensitivities of 322.5 and 3.7 nA/(mm 2 ·mM), respectively. Given the low sensitivities in the higher concentration range, saturation for carbon sensors and locally modified platinum microneedle sensors starts at 10 and 5 mM lactate, respectively. Thus, both sensors allow sensitive measurements in the lower concentration range. Current densities at saturating lactate concentration are higher on freshly prepared carbon electrodes with 1.80 μA/mm 2 (10 mM) compared to platinum microneedle electrodes with 0.75 μA/mm 2 (10 mM) with full electrode modification. For platinum microneedle electrodes with optimized, dried local microneedle modification, a current density of 0.95 μA/mm 2 (5 mM) was measured. Detection of lactate in whole blood was demonstrated on carbon sensors, showing increasing currents after exercise, correlating with higher blood lactate levels, measured with a test strip reference system.