Voltammetric pH Measurements in Unadulterated Foodstuffs, Urine, and Serum with 3D-Printed Graphene/poly(lactic Acid) Electrodes

Abstract

The pH of a system is a critical descriptor of its chemistry – impacting reaction rates, solubility, chemical speciation, and homeostasis. As a result, pH is one of the most commonly measured parameters in food safety, clinical, and environmental laboratories. Glass pH probes are the gold standard for pH measurements, but suffer drawbacks including frequent recalibration, wet storage of the glass membrane, difficulty in miniaturization, and interferences from alkali metals. In this work, we describe a voltammetric pH sensor that uses a 3D-printed graphene/poly(lactic acid) filament electrode that is pretreated to introduce quinone functional groups to the graphene surface. After thoroughly characterizing the pretreatment parameters using outer-sphere and inner-sphere redox couples, we measured pH by reducing the surface-bound quinones, which undergo a pH-dependent 2e<sup>–</sup>/2H<sup>+</sup> reduction. The position of the redox peak was found to shift –60 ± 2 mV pH<sup>-1</sup> at 25 ºC, which is in excellent agreement with the theoretical value predicted by the Nernst Equation (–59.2 mV pH<sup>-1</sup>). Importantly, the sensors did not require the removal of dissolved oxygen prior to successful pH measurements. We investigated the impact of common interfering species (Pb<sup>2+</sup> and Cu<sup>2+</sup>) and found that there was no impact on the measured pH. We subsequently challenged the sensors to measure the pH of unadulterated complex samples including cola, vinegar, serum, and urine, and obtained excellent agreement compared to a glass pH electrode. In addition to the positive analytical characteristics, the sensors are extremely cheap and easy to fabricate, making them highly accessible to a wide range of researchers. These results pave the way for customizable pH sensors that can be fabricated in (nearly) any geometry for targeted applications using 3D-printing.

Novel and Highly Sensitive Electrochemical Sensor for the Determination of Oxytetracycline Based on Fluorine-Doped Activated Carbon and Hydrophobic Deep Eutectic Solvents

“…Plots of the peak potential (E p ) and pH (Figure S4D) revealed that the linear relationship can be expressed as follows: E p = −0.0690pH + 0.9099 (R 2 = 0.9988). It is clearly seen that the linear slope, 69.0 mV pH −1 , is close to the theoretical value, 59.2 mV pH −1 , 58 implying that the numbers of protons and electrons transferred in the reaction of OTC were equal.…”

An Environmentally Friendly Three-Dimensional Printed Graphene-Integrated Polylactic Acid Electrochemical Sensor for the Quality Control of Biofuels

“…45 In addition, the NaOH activation can induce pH-sensitivity in the G/PLA 3D-printed electrode response. 46 Thus, the electrochemical behavior of the electrode in the presence/absence of TBHQ (40 μmol L −1 ) was studied in a buffered solution controlling the pH from 2.0 to 7.0 using BR buffer, by square wave voltammetry (SWV) as shown in Figure 3 (Figure 3A− F). For the G/PLA 3D-printed electrode, the increase in pH results in peak shifting to lower potential with current decrease.…”

Exploring different carbon allotrope thermoplastic composites for electrochemical sensing

“…Whilst these are easy to make, it is difficult to make smaller and complex geometry electrodes. Therefore, recently thermoplastics such as poly (meth-methacrylate) [19][20] and poly lactic acid (PLA) [21][22] have been more widely used as binders and studies have shown these can be patterned and molded to make sub-millimeter sized electrodes 20,[23][24][25] . Further developments have been achieved through the utilization of carbon composite thermoplastics in 3D printing, which has provided the scope to fabricate electrodes of complex geometries [26][27][28][29][30] .…”