Non-enzymatic sensors built with graphite and metal oxide nanoparticles

The clinical detection of biomolecules such as glucose and uric acid is performed using analytical techniques such as ultraviolet-visible spectrophotometry, which requires a pretreatment step in the sample to avoid interferents that cause errors in quantification. To avoid the pretreatment step, which can sometimes be cumbersome, several working groups have recently made efforts to develop electrochemical sensors capable of measuring the levels of biomolecules in physiological samples such as blood, sweat, saliva and even tears, without the need for pretreatment.

But what is an electrochemical sensor? The most commonly used definition describes them as devices capable of detecting and/or quantifying electroactive species and translating the information into an analytical signal. Their operation involves (Figure 1) two basic components: recognition element (capable of interacting with or selectively recognizing the electroactive species present in the sample) and transducer (element that converts the response signal from the receiver into a processable electrochemical signal).

Figure 1. Representation of the elements of an electrochemical sensor (image prepared by the authors).

The most commonly used recognition elements in the construction of electrochemical sensors are enzymes (giving rise to enzymatic sensors); for example, the enzyme uricase is used for the detection of uric acid. Unfortunately, enzymes have disadvantages (instability due to changes in temperature, pH, pressure, humidity and they are expensive materials due to their complex purification processes), leading to errors in the measurement.

To overcome the limitations of enzymes, efforts have focused on replacing enzymes with inert materials to develop functional non-enzymatic electrochemical sensors capable of measuring the levels of biomolecules in physiological samples, an example of these inert materials are metallic nanoparticles (NPs).

Nanoparticles of noble metals such as gold, silver and platinum are inert materials widely used in the development of non-enzymatic sensors due to their excellent catalytic and conductive properties; however, their high costs and low availability are disadvantages that complicate their use in this task.

Recently, metal oxide NPs have shown good results in the task of enzyme substitution and their use in the detection of biomolecules, due to their electrical and catalytic properties, but mainly to their low costs and easy acquisition.

In addition, metal oxide NPs provide electrochemical sensors with long-term stability under environmental conditions without losing their functionality, compared to enzymatic sensors.

Incorporation into the sensors is straightforward, mainly in carbon paste electrodes. Figure 2 illustrates the fabrication of an unmodified (2 A) and modified (2 B) carbon paste electrode. To make a modified carbon paste electrode, in a first step, adequate quantities of graphite, nanoparticles and mineral oil (binder) are mixed until a homogeneous paste is obtained; then, the paste is introduced into a plastic tube avoiding air bubbles, then an electrical connector is placed at one end and, finally, the electrode surface is renewed using a white sheet with high purity (operation that is repeated after each reading).

Figure 2. Representation of the processing of (A) unmodified and (B) modified carbon paste electrodes (image prepared by the authors).

In addition to their easy modification, the modified carbon paste electrodes present a further advantage, the ability to combine the properties of graphite (electrical and mechanical) and those provided by the metal oxide nanoparticles in the oxidation process of biomolecules, facilitating the electrochemical oxidation of biomolecules.

These modified sensors have been tested using electrochemical techniques, such as differential pulse voltammetry, which allows relating the oxidation of biomolecules to an electrical signal.

In the academic area of Chemistry of the Institute of Basic Sciences and Engineering and in the Natural Resources Corps (UTSEV-CA-01), electrochemical methodologies based on differential pulse voltammetry and non-enzymatic carbon paste electrodes modified with metal oxide nanostructures are being developed and optimized. For example, for the quantification of glucose and uric acid in human urine samples they used iron III oxide and bismuth III oxide nanoparticles, respectively.

The results indicate that we will soon have on the market non-enzymatic sensors modified with metal oxide nanoparticles to quantify glucose and uric acid levels in physiological samples that will allow preventive actions to be taken to avoid health complications caused by abnormal levels of these biomolecules.